Design_report_final.pdf

Team 3

RoMow Final Design Report ENGR 339/340 May 2015

From Left: Andrew Frandsen, Dustin Brouwer, Nathan Terschak, Jordan Newhof

© 2015, Calvin College and Jordan Newhof, Andrew Frandsen, Dustin Brouwer, Nathan Terschak.

i

Executive Summary Team 3 set out at the beginning of the fall semester of 2014 to find a project that would challenge the team both mechanically and electrically. The team was put into contact with Dr. Yoon Kim, an engineering professor at Calvin College, who had an idea that he wanted to see put into action. His vision for the team was to design a remote control lawn mowing platform compatible with a standard walk-behind lawn mower. His idea for the project was intended only for personal use, so the team decided to broaden the scope outwards a bit. The goal for the project now called RoMow was the first step towards creating a remote controlled lawn mowing system that could compete in the lawn care industry. It would compete with the other remote controlled lawn mowers by being flexible in its uses, being cheaper to produce and by having the necessary safety precautions in place to make the product safe for consumer use. The team worked together as a group of three mechanical engineers and one electrical engineer to design the prototype along with the controls system. The team deemed this project a success because it strengthened the team’s claim that it was indeed feasible to design a remote controlled system that would work with a range of lawn mowers to allow control of the lawnmower from a significant distance. The design was also a success because the design was able to meet the customer specifications along with the team imposed performance requirements. The project was also successful because the expenditures and production costs for the RoMow remained under the $500 budget each team was given. The idea for a remote controlled lawn mowing system is an exciting one and this design and prototype lays the groundwork in terms of a working concept. The team hopes that in the future, a team will pursue this project and implement some of the design changes proposed later in the report.

ii

Table of Content 1

2

Introduction ........................................................................................................................................... 1 1.1

Project Description........................................................................................................................ 1

1.2

Design Norms ............................................................................................................................... 1

1.2.1

Transparency ......................................................................................................................... 1

1.2.2

Caring.................................................................................................................................... 2

1.2.3

Integrity ................................................................................................................................. 2

1.3

Design Team ................................................................................................................................. 2

1.4

Engineering 339/340 ..................................................................................................................... 4

Project Management ............................................................................................................................. 6 2.1

3

4

Work Breakdown .......................................................................................................................... 6

2.1.1

Frame Design ........................................................................................................................ 6

2.1.2

Electronics and Communications.......................................................................................... 7

2.1.3

Safety Mechanisms ............................................................................................................... 7

2.1.4

Electrical Housing and Waterproofing.................................................................................. 7

2.2

Schedule ........................................................................................................................................ 8

2.3

Budget ......................................................................................................................................... 10

2.4

Design Methodology ................................................................................................................... 11

Overall Design Requirements ............................................................................................................. 13 3.1

Customer Defined Specifications................................................................................................ 13

3.2

Team Defined Specifications ...................................................................................................... 13

3.3

Safety Considerations ................................................................................................................. 14

Design Decisions ................................................................................................................................ 15 4.1

Frame .......................................................................................................................................... 15

4.1.1

Frame Structure ................................................................................................................... 15

4.1.2

Material Selection ............................................................................................................... 16

iii

4.1.3

Structural Element Options ................................................................................................. 16

4.1.4

Mower Suspension System ................................................................................................. 17

4.2

5

4.2.1

Initial System Requirements ............................................................................................... 21

4.2.2

Radio System ...................................................................................................................... 21

4.2.3

Electric Motors.................................................................................................................... 25

4.2.4

Batteries .............................................................................................................................. 27

4.2.5

Motor Speed Controller ...................................................................................................... 27

4.2.6

Microcontroller ................................................................................................................... 30

System Design .................................................................................................................................... 34 5.1

Mechanical Design...................................................................................................................... 34

5.1.1

Electric Motors.................................................................................................................... 35

5.1.2

Wheels................................................................................................................................. 35

5.1.3

Frame .................................................................................................................................. 36

5.1.4

Mower Suspension System ................................................................................................. 37

5.1.5

Electrical System Housing .................................................................................................. 37

5.1.6

Safety Components ............................................................................................................. 39

5.2

6

Electrical Systems ....................................................................................................................... 21

Electrical Design ......................................................................................................................... 39

5.2.1

Overview ............................................................................................................................. 39

5.2.2

User Controls ...................................................................................................................... 40

5.2.3

Electrical Interfacing ........................................................................................................... 42

5.2.4

Software Design .................................................................................................................. 47

5.2.5

Safety/Protection Module ................................................................................................... 50

5.2.6

Final System Design ........................................................................................................... 56

Prototype Testing ................................................................................................................................ 57 6.1

Overview ..................................................................................................................................... 57

6.2

Cutting Height Test ..................................................................................................................... 57 iv

6.3

Slope Test.................................................................................................................................... 58

6.4

Speed Test ................................................................................................................................... 58

6.5

Turn Radius Test ......................................................................................................................... 58

6.6

Battery Life Test ......................................................................................................................... 59

6.7

Transmitter Range Test ............................................................................................................... 59

6.8

Emergency Stop Protocols Test .................................................................................................. 59

6.9

FEA Analysis .............................................................................................................................. 60

7

Project Summary ................................................................................................................................. 62 7.1

Future Improvements .................................................................................................................. 62

7.1.1

Aluminum ........................................................................................................................... 62

7.1.2

Fastened Frame ................................................................................................................... 62

7.1.3

Standard Motors .................................................................................................................. 62

7.1.4

Simplified Electronics ......................................................................................................... 63

7.1.5

Robust Wiring and Mounting ............................................................................................. 63

7.1.6

Automation ......................................................................................................................... 63

8

References ........................................................................................................................................... 64

9

Acknowledgements ............................................................................................................................. 65

10

Appendices...................................................................................................................................... 66 10.1

Calculations on Motor Output Power Requirement .................................................................... 66

10.2

Mechanical Hour Log ................................................................................................................. 66

10.3

Nathan Terschak Hour Log ......................................................................................................... 68

10.4

Jordan Newhof Hour Log ........................................................................................................... 70

v

Table of Figures Figure 1. Team 3 ............................................................................................................................................................3 Figure 2. Initial design schedule.....................................................................................................................................8 Figure 3. Mechanical Second Semester Gantt Chart......................................................................................................9 Figure 4. Electrical Second Semester Gantt Chart .........................................................................................................9 Figure 5. Side View of Lawnmower ..............................................................................................................................17 Figure 6. Kickbar Mower Loading Mechanism.............................................................................................................19 Figure 7. Rear Wheel Loading Ramp ...........................................................................................................................20 Figure 8. Front Mower Suspension Clamp ...................................................................................................................20 Figure 9. Remote Control Car Style Transmitter ..........................................................................................................22 Figure 10. Remote Control Aircraft Style Transmitter..................................................................................................23 Figure 11. Turnigy 6x FHSS 2.4GHz Transmitter ..........................................................................................................25 Figure 12. Motor Current Draw Test ............................................................................................................................28 Figure 13. Toro GTS Recycler .......................................................................................................................................34 Figure 14. Lawnmower Dimensions .............................................................................................................................34 Figure 15. Shihlin motor and mounting plate ..............................................................................................................35 Figure 16. Caster assembly ..........................................................................................................................................36 Figure 17. Wheel and motor mounting........................................................................................................................36 Figure 18. L-Shaped Bracket ........................................................................................................................................37 Figure 19. Front and body section assembly................................................................................................................37 Figure 20. Electrical housing construction ...................................................................................................................38 Figure 21. Electrical Communication Flow Chart .........................................................................................................40 Figure 22. R/C Receiver Output Pins ............................................................................................................................42 Figure 23. Transmitter Channels ..................................................................................................................................43 Figure 24. Example of R/C Receiver Signal ..................................................................................................................43 Figure 25. Receiver Signals Experiment Set-up ............................................................................................................44 Figure 26. Oscilloscope Close-up ..................................................................................................................................45 Figure 27. Sabertooth Speed Controller .......................................................................................................................46 Figure 28. ISR Flow Diagram ........................................................................................................................................48 Figure 29. Arduino Main Loop .....................................................................................................................................49 Figure 30. Safety/Protection schematic .......................................................................................................................52 Figure 31. Safety/Protection circuit bread board setup...............................................................................................54 Figure 32. Safety/Protection board design ..................................................................................................................54 Figure 33. Safety/Protection PCB .................................................................................................................................55

vi

Figure 34. Finished safety/protection board................................................................................................................55 Figure 35. Electrical System Block Diagram .................................................................................................................56 Figure 36. Cutting Height Test .....................................................................................................................................57 Figure 37. RoMow FEA Assembly.................................................................................................................................60 Figure 38. Rear Wheel Suspension Displacement ........................................................................................................61 Figure 39. Front Mower Clamp Displacement .............................................................................................................61

vii

Table of Tables Table 1. Team Responsibilities .......................................................................................................................................6 Table 2. Breakdown of Hours Spent.............................................................................................................................10 Table 3. Initial Project Budget......................................................................................................................................10 Table 4. Actual Budget.................................................................................................................................................11 Table 5. Transmitter Range Test 1 ...............................................................................................................................24 Table 6. Transmitter Range Test 2 ...............................................................................................................................24 Table 7. DC Motor Options ..........................................................................................................................................26 Table 8. Motor Current Draw Test Results ...................................................................................................................28 Table 9. Motor Drvier Criteria Weights........................................................................................................................30 Table 10. Motor Driver Decision Matrix.......................................................................................................................30 Table 11. Microcontroller Criteria Weights .................................................................................................................32 Table 12. Microcontroller Decision Matrix ..................................................................................................................32 Table 13. Major Electrical Components .......................................................................................................................39 Table 14. Safety/Protection circuit connections ..........................................................................................................53

viii

1 Introduction 1.1 Project Description Team 3 was approached by Dr. Yoon Kim, an electrical engineering professor at Calvin College, with the idea for a design project which would benefit those who seek to live independently, but lack the ability to do ordinary tasks around the house. He proposed that Team 3 design a remote-controlled lawn mower. The method he proposed was a platform that could be attached to an existing push lawnmower that many homeowners already own. The platform which he proposed is what the team has named RoMow. The team also saw that it could be designed to appeal to more than just those who lack mobility; RoMow would be able to make the task of mowing the lawn easier for everybody and should be designed with both customer sets in mind. The platform would control the direction and speed of the mower by means of a controls system and a set of electric motors meaning that the operator would be able to direct the mower from quite a distance. RoMow should be removable from the mower and should not require major modifications to be made to the original design of the mower in order to be used in conjunction with RoMow. A lawnmower is a dangerous piece of equipment and removing the manual controls (i.e. the handle and throttle) makes it even more dangerous. Team 3 intends to design safeguards to make the RoMow safe to use, even remotely with the goal in mind that this design is taking the first step towards creating a product which could be profitable in the lawn care market. The team’s goal in essence was to take the first step towards the creation of a consumer product. It was to be a proof of concept that it was indeed plausible to design a remote controlled lawn mowing system that could be used with a range of different lawn mower brands, while still being general enough in design that only one style would be needed to cover that range. The hope was that RoMow would be cheaper to produce than a stand-alone RC lawn-mower, and have the necessary safety protocols and precautions to make the product marketable to home and business owners.

1.2 Design Norms As aspiring engineers at Calvin College, Team 3 believes that responsible and thoughtful design should be at the heart the project. The design of RoMow is both functional as a product and expresses the following qualities, which are characteristics of a responsible design.

1.2.1

Transparency

One of the goals for this project was to make RoMow usable with any mower of similar make and model. For this to occur, the design must be done in such a way that is easy for the general public to understand and use. The product should be flexible to various applications that a customer might use a lawnmower for. The customer should know what the RoMow is capable of and how to use it in a safe and effective way. 1

The mechanical design should be sturdy and have the necessary safety features built in so that the system doesn’t fail prematurely or under normal operation conditions. In addition, driving the platform via the remote control transmitter should be intuitive and easy to learn, so that minimal effort is required from the user.

1.2.2

Caring

RoMow is designed for those who cannot easily mow their lawn, so caring is a critical part of the design. The design has to be sensitive to the customers who are going to be using it. It should make mowing the lawn a simple task requiring little to no physical work to accomplish the task. Along with being easy to use, it should also be safe. The platform should make the task of mowing the lawn no more dangerous than it was before, and if possible make it safer. The team focused a good deal of effort to ensure that the design is safe to use by designing proper safety features and through situational testing.

1.2.3

Integrity

The design process of RoMow was also guided by the principle of integrity. This means that the design is a delightful combination of both form and function. Included in this idea is that the design of the RoMow is intuitive for the user to operate. It is a piece of equipment that is both functional and intuitive for the customer. The team’s goal was that there was no gap between expectation and reality.

1.3 Design Team The team is composed of three mechanical engineering students and one electrical engineering student. The team is shown in Figure 1. From the left the members are Andrew Frandsen, Dustin Brouwer, Nathan Terschak, and Jordan Newhof. The team decided on producing a remote controlled lawnmower after Professor Kim, who is serving as client and mentor for the group, introduced the project. It gained attention through its challenging design work both mechanically and electrically.

2

Figure 1. Team 3

Dustin Brouwer Dustin is from Sheboygan, Wisconsin. He is a mechanical engineering student who has a particular interest in controls and electronics. After graduation he hopes to obtain an engineering position in the automotive field. He has been given the responsibility of frame design along with building and testing prototypes. Dustin also helped with the electrical control system and the safety protocols, given his experience with electrical equipment and controls. Andrew Frandsen Andrew is from Naperville, Illinois. He is a mechanical engineering major who thoroughly enjoys designing real world applications with sustainability in mind. Upon graduation, he hopes to obtain a job in renewable energy and to eventually pursue a master’s degree in Business Administration. Andrew has the responsibility of frame design using computer-assisted design (CAD). Building and testing prototypes also falls under Andrew’s list of responsibilities. Jordan Newhof Jordan was born and raised in Grand Rapids, Michigan. He attended Covenant Christian High School in Standale, Michigan and then began the engineering program at Calvin College in the fall of 2011. He is a mechanical engineering student, who enjoys work in thermodynamics and fluid flow especially. He plans to work in the consulting side of mechanical engineering as well as obtain a graduate degree in either Business or Engineering. 3

Jordan is primarily responsible for communication with the client. Client communication is crucial in meeting all the design specifications provided by the client. Jordan is a major contributor in editing and writing project reports and presentations, as well as assisting in the frame design. Nathan Terschak Nathan was born in Fort Wayne, Indiana, briefly lived near Toronto, Ontario at age 11, and now resides permanently west of St. Louis, Missouri. After graduating from Westminster Christian Academy, he began attending Calvin College where he now studies Electrical/Computer engineering and serves as a captain on the swim team. His career goals include finding a job where both hardware and software design skills are applicable. Nathan is responsible for the electrical components of the system. That includes: R/C transmitter and receiver, electric motors, motor control system, and motor drivers. The electrical system is an emphasized part of the project due to the elaborate control system. He also assisted in writing the electrical design portions of this report.

1.4 Engineering 339/340 The Senior Design Project is the deliverable for the capstone class of the Engineering program at Calvin College. Senior capstone consists of both the Engineering 339 and 340 classes. Engineering 339 is taken in the fall of senior year and focuses on many skills developing engineers need to enter the working world. Some of these skills include project management, effective presentation techniques, safety assessment, time management, and group communication. The skills talked about in lecture are then reinforced in the design process, with deliverables due which focus on time management, identification of roles and skills, and on group communication. The skills are also showcased by various presentations throughout the semester, with faculty and employers, and with a Project Proposal Feasibility Study (PPFS). These are not the only items focused on in Engineering 339/340. Many resources are given to the students to assist them in searching for a job and in preparation for the interviewing process. Most importantly, faith, ethics and how they relate to the Christian engineer are focal points in each discussion. This directs the teams to design projects that are focused on helping people, making the world a better place, and glorifying God. In the second semester the students focus much more on their projects than in the first semester. The teams are responsible for their prototypes during this time, along with a design report focusing on the decisions made during the design. During this portion of the class the students are also responsible for developing tests to check their designs for functionality. This is something that hasn’t been stressed before in the curriculum so it poses some new challenges. There are also many opportunities for students to work on their presentation abilities with the multiple progress updates that need to be made to students, professors, 4

advisors, families and friends. Near the end of the semester the students have an opportunity to show off the prototypes and the work done over the past semester at Senior Design Night, to family, friends, and faculty.

5

2 Project Management 2.1 Work Breakdown The design and fabrication of RoMow is quite a large task for a group to undertake especially in just one semester, given the busy lives of the team members. In order for it to go smoothly, the team divided the main tasks up, and assigned one or more to each member. This way all members would have tasks to continue working on, and could take leadership over a small part ensuring that each member was involved. Table 1 below gives a quick overview of the main responsibilities of each member indicating each person’s roles and expertise. Table 1. Team Responsibilities

Team Member

Responsibilities

Jordan Newhof

Client communication Report writing Fabrication Presentation writing

Andrew Frandsen

CAD Mechanical design Fabrication

Dustin Brouwer

Fabrication Mechanical design Electronics assistance

Nathan Terschak

Electrical design Electrical fabrication Report writing

Although each section below talks about a different system of the design, all of them are incorporated together in the final design of RoMow, meaning that all members did have to work with each other to ensure that there was no interference between disciplines.

2.1.1

Frame Design

The design of the frame was the most difficult mechanical aspect of the project. It was difficult given the spatial constraints of the lawn mower and the self-imposed cutting height requirement. The RoMow needed to be able to support the weight of a lawnmower along with two 12V batteries and electrical components, but at the same time needed to support the mower at such a height that the mower could cut down to 2.5”. To put together a design that would be compatible with the lawn mower and the other constraints, the team

6

decided that it would be best to use a 3D CAD program to design the frame along with the wheels, and motors to make sure that the design was possible before prototyping. Andrew was responsible for doing the design work in Autodesk Inventor, while Jordan and Dustin helped with ideas and drawings of how to make the frame meet the requirements, while maintaining relative simplicity. This part of the design needed a few people working on it to ensure that the best possible solution to the problem was attained. The fabrication of the frame was done after the design of it was finalized in Inventor. Andrew and Dustin worked together on this and addressed issues as they arose during the fabrication process. Removing Jordan from the frame design group allowed all members to have enough work to do independently, so that time was not wasted. The details of the frame’s design will be highlighted later on in Section 4.1.

2.1.2

Electronics and Communications

The electronics that went into the design of RoMow took a substantial amount of effort and time to get in order. The speed and direction of RoMow need to be controlled from a substantial distance away. This means that a signal needs to come from a controller and be interpreted by a receiver and microcontroller pair. This microcontroller then outputs a signal which tells the motor drivers what to output, from the battery, to control the speed of each motor which can be used to change speed and direction of RoMow. The safety mechanisms discussed in Section 2.1.3 also need to be incorporated into the electrical system. Nathan Terschak, the electrical engineer of the group, took lead on this part of the project. He also received assistance from Dustin along the way.

2.1.3

Safety Mechanisms

The safety side of this project was handled by Dustin. He was responsible for developing the theory for the safety feature implementations for both the communications system and the mechanical features on the RoMow itself. The safety system is a perfect blend of mechanical and electrical disciplines and he was able to use his previous experience in both areas. He did receive some assistance from Nathan to implement the features into the electrical/controls system, specifically the software portion.

2.1.4

Electrical Housing and Waterproofing

The RoMow is a piece of yard equipment and must be designed to withstand the elements in order to be effective for a long time. The process began by designing a housing arrangement for the batteries, electrical components and wires located above the motors on the front of RoMow. To ensure the RoMow is reliable the team decided to spend some effort working on waterproofing the housing along with the frame to reduce the chance of water damage and rusting. This portion of the work was done by all the mechanical engineering students in the group.

7

2.2 Schedule At the end of Engineering 339 the team developed a tentative schedule for how the design would be conducted in Engineering 340. It turned out that the team did not stick very closely to the initial schedule because there were so many unforeseen tasks and obstacles that would have been difficult to see at that point in time. Shown below is the initial schedule that was posed. As can be seen it varies greatly from final breakdown of tasks. This was because the frame design took much longer than anticipated due to clearance issues with the mower. This is highlighted later on in Section 4.1.

Figure 2. Initial design schedule

The schedule is very aggressive and the team was not able to keep up with it due to design and component malfunction delays. However, this aggressive posture allowed the team to still finish on time, regardless of delays. The team was initially wary of the schedule, but the team acknowledged that delays would happen but wanted to feel pushed throughout the semester. The initial schedule helped the team stay aggressive in the beginning of the semester to keep making progress on the project. The actual schedule is broken up into 2 different Gantt charts, one for electrical-related work and one for work on the mechanical design. In place of prototypes, the team decided to move to a more iterative design approach. This means that minor testing of parts was continuously done along the way negating the need 8

to make multiple prototypes. The final schedule for the mechanical aspects of the project is shown in Figure 3, and the final schedule of electrical work is shown in Figure 4.

Figure 3. Mechanical Second Semester Gantt Chart

Figure 4. Electrical Second Semester Gantt Chart

Each member of the team recorded the amount of hours that they personally spent on each of the tasks in the schedule. Sections 10.2, 10.3, and 10.4 provide tables listing these hours. Adding the totals from each of these tables yields 573 total hours spent on the RoMow project for the entire team. Table 2 shows a breakdown of how many hours were dedicated to mechanical design and fabrication, electrical design and fabrication, and other tasks.

9

Table 2. Breakdown of Hours Spent Type of Work Mechanical Electrical Other Total

Hours 250 125 198 573

Percent 44% 22% 35% 100%

The percentages in this table show that the project ended up being well suited for a team of three mechanical engineers and one electrical engineer.

2.3 Budget Each design team was initially allocated $500 for the completion of the project. It is not uncommon for teams to go over this budget, especially if they are designing a prototype, but it does have to be approved by a faculty advisor. Team 3 forecasted that the budget for this project would be approximately $635. The breakdown of the team’s budget is given in Table 3. Table 3. Initial Project Budget

The actual money spent on the project ended up being $481. The initial budget posed in the PPFS ended up being higher than necessary. The team decided that a new set of batteries were not necessary for a preliminary prototype after testing them. Also it was determined that a transmitter and receiver pair wasn’t necessary because of the benefits of the one the team already had access to (Section 4.2.2.6).

10

Table 4. Actual Budget

Money Used

2.4

Purchase

Project Use

Cost

Toro GTS Recycler

Lawnmower for proof of concept

$80

Hoveround MPV5

Motors, motor drivers, wheels

$350

Arduino

Controls and signal processing

$23

DigiKey Order

Connectors for circuit wiring

$3

DigiKey Order

Terminal Blocks for PCB

$3

DigiKey Order

External shut-off button

$8

Indicator LEDs

Shows power and battery status

$7

Rustoleum Paint

Paint for electrical housing

$7

Total Cost $481

Design Methodology

The design process of Team 3, this semester, can be described in one word: iterative. The goals that the RoMow needed to meet were set out very clearly and the team felt like they would be achievable fairly easily. The team decided that the initial design and prototype would be very simple, with minimal extra features, to make sure that the design would be complete on time while still meeting the requirements. Once the initial requirements were met, additional features were added as time allowed. The design methodology for the mechanical aspects of the project was to finish the design of RoMow in sections. The team determined that the sections would be independent of each other and would be fastened together at the end. Much brainstorming needed to occur before a design was decided upon and many of the ideas the team came up with were scrapped after coordination issues between systems were realized. When the simplest solution to the frame design became evident, a model was begun. The frame design was first completed in Inventor before it was constructed. This was the first part completed because the dimensions of the frame needed to be finalized before the other aspects of the design could be finalized. This is because the electrical components, motors, wires, wheels, and safety components need to fit on and in the frame. During the design of the frame and electrical housing, finite element analysis was run on the high stress areas to ensure that the design was rugged enough to withstand the stresses it would undergo when in operation. During the construction some slight changes were made, to the frame, to fix clearance issues, but for the most part the construction of the frame went very smoothly because of the simple design and attention to detail in the CAD design.

11

The electrical design process was different than the mechanical design. The electrical system that was implemented was much less about locations and clearances and was much more about implementation and communication. The process began with the identification of the required systems and components needed for RoMow. Diagrams were drawn to show flow and logic charts were mapped to correctly program the controls. Implementation was determined through this process. It was necessary to make sure that the communications and controls were working properly the whole way through the design so that when problems occurred they would be due to the last step completed. To make sure this occurred, design steps were made incrementally and tested to ensure the design was working after each step. This approach proved to be wise because the source of the problem was always readily apparent. Like the mechanical design, the electrical part was initially quite simplified to make sure the design goals were met, but was expanded upon once the initial goals were met. The design process will be covered in much more depth in Section 5.

12

3 Overall Design Requirements The design of the RoMow was governed by a set of specifications as given to the team by the customer, Dr. Yoon Kim. The specifications gave the team a clear direction in which to proceed during the design process. Having customer specifications presented Team 3 with a specific set of issues that needed to be designed around.

3.1 Customer Defined Specifications The following specifications given by the client ensure that the design will be both safe and flexible for multiple applications. They are as follows: ●

The mower can be attached and detached from the platform without modification

●

There shall be a clamp for the mower blade to continually spin

●

Must not remove wheels or modify the mower

●

The platform must carry the mower and be driven by electric motor

●

The platform must have 2 driven wheels minimum

●

The control of the platform should be via wireless remote control (radio frequency)

●

Design must have an emergency stop switch from the controller

3.2 Team Defined Specifications Separate from the customer specifications are the team-imposed performance specifications which the carrying platform shall meet. These specifications include: ●

Traveling on “flat” ground at a minimum speed of 3 mph

●

Traversing a slope of at least 30 degrees

●

Travel a minimum of 90 minutes on a full charge

●

Must be able to cut grass to a length of 2.5”

●

The cutting height must be changeable upwards of 2.5”

These specifications were reached through discussion among the teammates. Team 3 desired to design a product which would be marketable to a broad range of people. To appeal to such people, the previous requirements were decided upon. The speed specification ensures that the RoMow could travel faster than the average person pushing a lawn mower. The slope requirement makes RoMow flexible enough to tackle most lawns, while the battery life ensures one could finish mowing the lawn before the batteries are drained. Finally the team wanted to make sure the mower, that is riding on RoMow, is capable of cutting to a low

13

enough grass height for a standard user. 2.5” was settled upon because it is the lowest recommended height 1 by Michigan State’s School of Turf Science. RoMow is then adjustable upwards to 3.5”.

3.3 Safety Considerations Team 3 was very concerned with the safety of RoMow, both from the operator’s and from the onlooker’s standpoint. The design needed to make sure that the operator had complete control over what the platform was going to do and where it was going to go. The team accomplished this safety goal by implementing safety features which met the following criteria: •

The motors need to shut off if there is a loss of communication

•

The controller has an emergency stop button for the operator

The safety design, which is discussed in Section 5, further describes the way in which the team was able to meet these design criteria.

1

http://www.turf.msu.edu/mowing-lawn-turf

14

4 Design Decisions This section of the design report lays out the major design decisions that the team needed to make this semester and what was decided upon for the final prototype of RoMow.

4.1 Frame 4.1.1

Frame Structure

The team had a decision to make with regards to how the frame of the RoMow should look. RoMow not only has to carry a lawn mower, but also needs to house two electric motors, two 12V batteries, wiring and other electrical components. The team decided that the structural integrity of the frame, as well as a feeling of “completeness” were the most important aspects of the frame. Completeness means that RoMow looks like a complete product, where everything looks neat and in place. The two possible forms examined are outlined below. 4.1.1.1

Platform Structure

This idea was the simpler of the two options. The idea was that the mower would simply sit on top of the frame which is held off the ground by the attached wheels. In its most simple form it would look like a rectangular halo around the mower. The mower would be suspended above the grass by means of the systems mentioned in Section 4.1.4. The mower would be loaded into the frame from the back using a selfcontained loading ramp. The design would be very light but at the same time would not have much lateral support to negate the effects of torsion upon the frame. This is important because as RoMow rolls over uneven ground the frame would get torqued back and forth. This type of design would also be difficult to give a “complete” feel. It would look as though the mower that is suspended above the grass could very easily roll right back out. Also it would be hard to find places to house the electronics, and batteries on a design such as this. 4.1.1.2

Two-tier Structure

In this design scenario, the team imagined the RoMow having a box-like appearance in which the mower sits. The frame would consist of three modular sections. The first section would be a box frame to support the mower frame, which would be loaded from the back using a self-contained ramp as mentioned in the above section. The box is essentially two rectangular halos surrounding the mower frame with upright sections to hold the two halos together. The suspension system that holds the mower would then be fastened to this section along with the caster wheels (the rear set of wheels). The next modular section would be attached to the front of the section that holds the mower. This section would be where the two electric motors are attached and housed. The motors have direct drive shafts so the shafts extend outside of the frame where they attach to the wheels. The top heights of both aforementioned sections match each other 15

and are then fastened together. The third section of RoMow is where the electronics, batteries and wiring would be located. The shape of this section would be the same box frame idea as the first section and it will be fastened on top of the second, with matching base dimensions. 4.1.1.3

Decision

The two tier design was chosen because it is more structurally sound than the previous design because it distributes the torsional ride effects on two levels, and resists deformation with the vertical supports. It also has a much more contained feel than the previous design alternative. This is important to the team because we see this as being something that a customer would not like. The only downside to this design is that it ends up being heavier than the simple platform frame.

4.1.2

Material Selection

The team determined last semester that the frame should be constructed from steel, rather than aluminum, for both strength and cost benefits. Calvin had quite a bit of steel in storage that the team could use but almost no aluminum. Also the weight of the frame was not incredibly important to the team so the major advantage of aluminum turned out to be insignificant.

4.1.3 4.1.3.1

Structural Element Options Extruded Pipe

Extruded pipe provides lightweight yet durable design which can be easily welded or fastened together to create the frame of RoMow. Calvin also had quite a bit of 1”x 1” square tubing for student use meaning that it would be no cost to the team to use. The down side to the tubing is that it can really only be used to create straight-lined forms. 4.1.3.2

Sheet Metal

Sheet metal provides a design that does not need to be welded, but rather manipulated in order to produce the necessary frame requirements. The metal could be bent and cut to reduce the need for welding and fastening. The structural integrity would not be as sure with a frame made of this compared to the steel tubing. Also there was not very much sheet metal available for use, so more would need to be purchased. 4.1.3.3

Decision

It was decided that the extruded pipe should be used for the structural form of RoMow because of the amount of it available to the team. The frame is going was going to take the form of a box so the team thought that this material could be used for a majority of the pieces. However there are some areas where sheet metal was better suited for the application.

16

4.1.4

Mower Suspension System

The suspension that is referred to in this report is the means by which the mower, used with RoMow, is held above the grass and secured in place. The team had multiple ideas of how to meet the cutting height requirement as specified in Section 3.2. The bottom of the frame of the mower needed to be at 2” off the ground, to cut at 2.5”. The wheels of the mower extend below the bottom edge of the frame. A side view of the mower is shown below in Figure 5. These dimensions were important to have accurately measured because the cutting height is the limiting factor of the design.

Figure 5. Side View of Lawnmower

There are four main areas which the suspension system could attach to, that would be able to support the weight of the mower: the front and back edges of the frame, the rounded sides of the frame and underneath the wheels. 4.1.4.1

Wheel based Suspension

The first idea the team came up with, to support the mower, was a suspension system that supported the wheels of the mower slightly above the ground. As was previously stated, the mower cannot be modified in any way and this includes the wheels. This being said, the wheels are the lowest hanging members on the body of the mower, but the height of the mower deck can be changed by adjusting the height of the wheels. The design consists of four cradles mounted on the side of the interior of the frame; one for each wheel. The mower would be loaded from the back, by pushing the mower up the ramp and placing the back two

17

wheels in the rounded cradles and then by lowering the front end into the front two cradles. These cradles would be made from sheet metal that was bent into a rounded shape that matched the contour of the wheel. This would be done to prevent the wheels from moving while RoMow is in operation. These cradles would then be permanently fixed to the interior of the frame by welding. The height of the cut of grass would then be determined by the level the wheels were set to. At the lowest setting the cutting height would be at 2.5” high and then would extend up from there. There are two major issues with this idea and the first arises with the way that the mower would be loaded. There would need to be a runner from the top of the ramp to the cradle so that the wheel could be easily dropped into place. That would not be so hard to do, but it requires quite a bit of physical exertion to remove the back wheels from the cradle when unloading the mower. The operator would have lift and pull the back wheels to disengage them, and then use the back wheels as a pivot to lift out the front wheels. This would be too much of a hassle for an operator to go through. The second big issue is the clearance between the ground and the bottom of the cradle. The cradle is supporting the wheels which only sit 1.25” above the ground when the cutting height is at 2.5”. If the thickness of the sheet metal is 1/4", then the bottom of the cradle is only 1” off the ground. The problem with this arises when the ground isn’t level. The cradles could bump on the ground and cause a lot of shear stress of the welds that hold them the frame or cause it to be bent out of shape. Finally if this design were implemented, there would most likely be clearance issues when built. For these reasons the team moved onto a new idea. 4.1.4.2

Kick-bar Suspension

The second design utilized a mower support underneath the front and back ends of the mower. The front uses two L-shaped bracket, which have a raised edge. These brackets hook onto the front end and securely hold the front end in place because of the design of the front bumper of the Toro mower. The rear is supported by a contact bar that extends across the width of the mower. These supports hang higher than the cradles so the clearance issues discussed before would not be an issue. The loading of the mower would be accomplished using a kick bar system as shown below. This allows the user to release the mower from the operation height by pushing their foot down on the bar which pivots the contact bar under the back end of the mower up 90 degrees, as shown in Figure 6. The kick bar is then locked into a fully downward position and from this point the front wheels can be pivoted upwards. The mower can then be rolled down the collapsible ramp.

18

Figure 6. Kickbar Mower Loading Mechanism

The benefit of this design is that not much physical exertion is needed besides the use of the foot to push the bar down. Some obstacles do arise with this design too. First off, height adjusting would be very difficult to implement without being able to move the pivot point of the push bar mechanism. Changing the wheel height would not help because they would not be in contract with the ground anyways. Secondly, there would be a lot of stress on the front clamps during the unloading process due to the shift of weight when the back end is pivoted upwards. There is also a good chance that the front end would disengage the clamps, and slip off due to the forward motion of the mower when the contact bar sweeps the 90 degree motion. Elongating the clamps to reduce the chance of the mower disengaging, will increase the chances of the mower sliding during operation and bending failure under normal loading. 4.1.4.3

Front Frame Support and Rear Wheel Support

The final design ended up being a combination of the previous two designs using pieces of both designs. This design uses rear wheel platforms and clamps at the front of the frame to support the weight of the mower. The rear wheel platform is similar to the wheel cradle discussed in Section 4.1.4.1. However, instead of using a rounded sheet metal cradle, the platform is a combination of the 1” square steel tube and 1/8” flat sheet metal. The rear wheels can firmly rest upon the platform and be held very close to the ground at the same time. The sheet metal overhangs the end of the tubing so that it could be bent into a rounded shape to allow it to pass over the grass without catching. The structure of the rear wheel support system can be seen in the Figure 7.

19

Figure 7. Rear Wheel Loading Ramp

The mower is loaded from the back just as with the other two designs, by folding down the ramp. The wheels are rolled up the slight incline of the ramp and then are lowered onto the platform by means of the downward slant that is shown in Figure 7. The front end of the mower can then be lowered onto the clamps as seen in Figure 8.

Figure 8. Front Mower Suspension Clamp

This design has multiple benefits which makes it more desirable than the other two designs. First of all, this design allows for the simplest and least physically demanding loading operation. The mower doesn’t need to be pushed up a steeply inclined ramp to be loaded. It also only takes one motion for the mower to be released from the clamps and removed from the RoMow. Secondly, this design gets rid of the clearance issues that were encountered in the first design by making the platform flat rather than trying to follow the contour of the wheel. Using the front clamps keeps the wheels from rolling around on the platforms and also makes the unloading of the mower very simple. The unfortunate part of this design is that the height adjustment needs to be done by altering the height of the rear wheels and by changing the setting of the height on the clamps. This isn’t as intuitive as the team would have hoped for. Also the rear wheel platforms 20

do have the same issue with height off the ground as discussed in the first design, but the hope is that the extra support given by the steel tubing and the rounded edges will keep RoMow from digging into the ground when it is uneven. 4.1.4.4

Decision

The team decided to pursue the design outlined in Section 4.1.4.3 because it would be the easiest of the three to implement and provides the needed flexibility to fit a range of different lawnmowers without encountering clearance issues.

4.2 Electrical Systems 4.2.1

Initial System Requirements

The design of the electrical system for the RoMow begins with the customer requirement that it shall be propelled and steered with the use of two independently driven electric motors. Since the combined weight of the platform and the lawnmower could potentially be as high as 150 pounds, and the prototype will be able to travel in the upwards direction on a 30 degree slope, there will be significant power requirements for the motors. Since the motors must be powered electrically, the system will also need batteries. The specific type will be determined based on the motors the team decides on. The system will also require an electronic module that is responsible for controlling the speed at which the motors spin. Furthermore, the system must contain a radio system that includes a remote control transmitter and a receiver that will be located on the RoMow platform. This radio system will allow for the user to drive the lawnmower remotely. There will also need to be an electronic module that is responsible for deciphering any signals that are being output by the radio receiver. This same module will then have to generate the proper signals to send over to the motor speed controller module.

4.2.2 4.2.2.1

Radio System Component Requirements

The radio system for the RoMow must meet several requirements. First, the system must have a very long range (greater than 1000 ft). In addition, the team determined that the transmitter must have at least 3 channels that are controlled by the analog position of a stick. This is because there will need to be one stick for speed control, one for steering control, and one that can be used to initiate rotating movement. The transmitter will also need at least one channel that is controlled by a toggle switch. This is because the user must have some means of stopping the mower blade using the remote control transmitter. 4.2.2.2

Hobbyist R/C Radio Systems

The team quickly came to the realization that designing and building a remote control radio system was far beyond the scope of this project, especially considering that the team has only one student who has chosen 21

to focus their studies on electrical engineering. This means that the team will need to purchase a transmitter/receiver pair designed for use with remote control vehicles. Luckily, there are many products that have been made for hobbyists that build their own remote control cars and planes. There are two general styles that the transmitters come in. An example of the first style, which is usually associated with remote control cars, is shown in Figure 9. The other style, which is usually associated with remote control aircraft, is shown in Figure 10. Based on the products available, the team found that transmitters of the first style rarely provide 4 channels, which is the minimum requirement for this project. Furthermore, the team decided that the second style provides a more natural feel for controlling the RoMow. Due to these considerations, the search for the R/C radio system was focused on aircraft-style transmitters.

Figure 9. Remote Control Car Style Transmitter

22

Figure 10. Remote Control Aircraft Style Transmitter

4.2.2.3

Tactic TTX403 4-Channel Transmitter

The Tactic TTX403 is a radio transmitter that was designed for mini-sized Aircore airplanes. It offers a 4channel transmission using frequency hopping spread spectrum (FHSS) technology. Like most new radio systems, it operates in the 2.4GHz frequency. 2.4GHz FHSS technology essentially eliminates the possibility of interference, which will be important for this project. It is absolutely essential that the user maintains complete control of the lawnmower platform. If the radio receiver picks up interference, it could lead to unpredictable movement of the RoMow, which could easily turn into an unsafe situation. The Tactic TTX403 sells for $54.98 when the price of the compatible receiver is included. While this transmitter has 4 channels, they are all controlled by position sticks, meaning there are no toggle switches on it. This is not ideal, as the design should have a switch to disable the blade. The specifications for this transmitter do not provide an exact number for the range of operation. In fact, this is the case for most R/C transmitter products. This is most likely because the achievable range can vary greatly depending on what kind of obstructions are present between the transmitter and receiver. A member of the forums on RCGroups.com posted the results of a range test that he and some others performed on three different transmitters. The test was done by driving a boat away from the shore of a lake where the receiver was held stationary. The transmitter was aboard the boat, and as it took off, the testers made sure there were no obstructions between the shore and the boat. Table 5 shows the results of their test.

23

Table 5. Transmitter Range Test 1

Transmitter Name

Range

Spektrum DX5 with 2.4 GHz Spektrum technology

1.8 miles

Futaba 6EX with 2.4 GHz FASST technology

2.1 miles

Futaba 9C 72MHz with FSS synthesizing

2.6 miles

The two transmitters that use 2.4 GHz technology had a range around 2 miles, which is much greater than the requirement for the RoMow. The results of these tests are certainly promising, as the team will likely choose a transmitter with similar technology as the first two in Table 5. Results for a similar test were found on the website bigsquidrc.com. In this test, the writers of the article titled “2.4 GHz Transmitter Shootout – Radio Range” tested five different transmitters by driving down a long empty road. Table 6. Transmitter Range Test 2

Transmitter Name

Range

Futaba 4PKS

1233 ft.

Futaba 3PL

1058 ft.

Traxxas TQ 2.4 with Link

840 ft.

Tactic TTX-240

808 ft.

Spektrum DX3S

620 ft.

The results that this group of testers developed are much less impressive than those of the previously mentioned test. There are several reasons for why this may be the case. It is worth noting that the transmitters in the first test were all aircraft transmitters while the ones in the second test were all remote control car style radios. Either the airplane transmitters were designed to have better range, or the nature of the test provided better results. Since it will be hard to determine what range is possible for a certain transmitter before purchasing, the team made the assumption that an airplane style transmitter will have a good chance of satisfying the range requirement for the RoMow. 4.2.2.4

2.4G CT6B 6-Channel Transmitter and Receiver

The CT6B transmitter provides 6 channels, 4 controlled by stick position, and two that are linked to the state of toggle switches on the top corners of the controller. Like the Tactic, it was designed for R/C helicopters and airplanes, but has the capabilities needed for controlling the RoMow. This product can be purchased for $39.00, and that includes the compatible receiver. This product would work nicely for the first prototype since it satisfies the need for three stick position channels and at least one toggle switch.

24

4.2.2.5

Turnigy 6x FHSS 2.4GHZ Transmitter and Receiver

The Turnigy 6x transmitter offers many of the same capabilities as the CT6B transmitter described above. It transmits over 6 channels, 4 via stick position, and 2 via toggle switches. It also sells for only $29.99, which is very budget friendly. As the title suggests, it uses 2.4 GHz FHSS technology, suggesting that interference would not be an issue. 4.2.2.6

Decision on Radio System

After weighing the pros and cons of each radio system that was found, the team decided to use the Turnigy 6x FHSS 2.4GHz Transmitter and Receiver for the RoMow prototype. The reasoning behind this decision is simple. This radio system meets all of the previously stated system requirements while selling for the lowest price among the team’s options. Figure 11 includes a picture of this transmitter’s sleek design.

Figure 11. Turnigy 6x FHSS 2.4GHz Transmitter

4.2.3 4.2.3.1

Electric Motors Component Requirements

There are two main requirements for the electric motors that will be used to propel the RoMow. The first requirement is related to the type of electrical power source needed to spin the motors, and the second is related to the amount of mechanical power that the spinning can produce. Since they must be powered by batteries, it makes sense to use direct current (DC) motors. Typical input voltages for DC motors are 12 V and 24 V. The output power requirement was determined by calculating the amount of power needed to move the platform at its required maximum speed along the required maximum slope, which is a conservative estimate. These calculations can be found in Section 10.1, titled Calculations on Motor Output Power Requirement. It was determined that 0.8822 horsepower would be enough to satisfy the project’s performance requirement. This means that the platform will need to house two electric motors that are rated 25

at 0.5 horsepower. This way, the motors will be able to provide a total of 1 horsepower allowing for a big enough safety factor that the team is confident that the prototype will meet performance requirements. 4.2.3.2

Research on Pricing

The team began the search for electric motors by searching for off-the-shelf products that would not exhaust the project budget. It was not long before the team realized that finding the type of high-caliber motors needed for this project within budget would not be easy. Five different brands of electric motors that are rated at 0.5 horsepower were discovered by means of an online search. First, a motor made by Century Electric Motors was discovered that meets the power requirement and costs a reasonable $89.00. However, the operating voltage for this model is 115 volts, meaning it would be highly impractical to power by use of batteries. A similar motor made by Smith + Jones was found. It is a general purpose electric motor rated at 1/2 HP and priced at $109.99. This motor must also be powered by a high-voltage AC (alternating current) source, and thus is not suitable for the project. After the above options were discovered, the team refined its search to find 1/2 horsepower motors that are specifically powered by lower voltage DC sources (e.g. batteries). Three brands were found that are all rated at 1/2 horsepower and are all powered by either 12 V or 24 V DC. The options are presented in Table 7. Table 7. DC Motor Options

Brand of Motor

Price

Leeson Motors Metric DC Motor

$335.95

Worldwide Electric PM DC Motor

$251.95

Reelcraft 2260626 Electric Motor

$430.99

Omega OMPMDC12-18-12V-56C

$309.00

With $500 dedicated to the project, even the cheapest option would take up the entire budget. Therefore, the team will need to find a way to obtain two DC motors other than purchasing them new. 4.2.3.3

Decision on Electric Motors

The team decided on using Shihlin 24V DC motors from a Hoveround MPV5 electric wheelchair. These motors are capable of carrying a person of up to 300 pounds not including the weight of the chair itself. Also the wheel chair can travel a maximum of fifteen miles on a full battery charge. The weight that can be carried and the range of these motors with provided batteries insure that the performance requirements can be met. The customer also believed that pursuing this would make the design much easier by removing the need for additional gearing and possibly for motor drivers. The purchase price of the wheel chair was $350 which includes two electric motors which are capable of the performance desired. The purchase of this

26

wheelchair is the most cost effective way of obtaining two motors. The next cheapest option would be around $500.

4.2.4 4.2.4.1

Batteries Component Requirements

Since the RoMow prototype is a moving object, it will need a portable power source, and since it is propelled by electric motors, it makes sense to use batteries as that source. The battery network for the RoMow will need to produce a voltage of 24 V, as well as have the capability of generating as much current as is needed to spin the motors under load. 4.2.4.2

Hoveround Wheelchair Batteries

The electric wheelchair that the team purchased houses two 12 V lead-acid batteries that satisfy the requirements for powering the motors on the RoMow. However, the age of these batteries is unknown, so the amount of energy that can be delivered on one charge may not be sufficient. They are rated to deliver 32 A-h before reaching a fully discharged state. 4.2.4.3

Decision on Batteries

Since typical lead-acid batteries can be up to $100 apiece, slightly lower battery life may not be a strong enough reason to justify purchasing new ones. The team decided to perform a battery life test on the first prototype in order to determine if the batteries from the Hoveround wheelchair would be suitable. The results can be seen in Section 6.6.

4.2.5 4.2.5.1

Motor Speed Controller Component Requirements

Once the team had decided to use the two DC motors as well as the two 12 V batteries from the Hoveround MPV5 Electric Wheelchair, the requirements for a speed controller module were defined. The module must have two separate channels for controlling motor speed, as the two motors will not always run at the same rotational velocity. The module must also be able to operate with an input voltage of 24 V. The last factor that must be considered is the amount of continuous electrical current that the module can handle without overheating. The method the team used to estimate the current draw involved a physical experiment. One electric motor was powered by the two 12 V batteries wired in series. Also wired in series was a multimeter that was set to current-reading mode. When the circuit was connected, the current reading on the multi-meter was taken both with the wheels spinning freely and also as a team member manually applied mechanical resistance to the attached wheel. The picture of the experiment setup is shown in Figure 12.

27

Figure 12. Motor Current Draw Test

The results of the experiment are displayed in Table 8. Table 8. Motor Current Draw Test Results

Load Free Spin Max Manual

Measured Current 2.93 A 7.95 A

The results of this test show that it takes a significant opposing force to the spinning of the motors in order to make them draw 7.95 A. This means that with the assumption that the resistance encountered when propelling the RoMow prototype is lower than the resistance that was caused in this experiment, the current draw from two electric motors of this type under operation will likely be no higher than twice the measured current, or 15.9 A. Since it is unwise to push an electronic module right to its maximum operating current, the team decided that the motor speed controller for this project is required to be rated for operation under 20 A of continuous current, or 10 A per driven motor. 4.2.5.2

Simple-H DC Motor Driver

Several potential motor drivers were discovered through an online search, and the first is called the SimpleH DC Motor Driver, which is made by Robot Power. This module is capable of driving motors that run in the range of 5 V to 28 V, and is rated for an output current of 20 A. It is capable of driving one motor, so two of the boards would be required if chosen. The product sells on RobotShop.com for $49.99. The Simple-H meets all of the component requirements for this project. The decision on whether or not to use it in the design will depend on the pricing for other products that meet requirements.

28

4.2.5.3

Pololu High-Power Motor Driver

Another motor driver that was discovered is the Pololu High-Power Motor Driver 18v25. Pololu is a company that specializes in robotics and electronics. This module, which can control one motor, is rated for a voltage range of 5.5 V to 30 V and a continuous current of up to 25 A without a fan or heat sink. The Pololu motor driver is priced very similarly to the Simple-H at $49.95. Therefore, if either the Simple-H or the Pololu motor driver were to be purchased, the decision would not be based on price. Rather, the Pololu would be chosen between the two because it has a more compact design, while meeting all component requirements. 4.2.5.4

Sabertooth Dual 25A Motor Driver

Through consultation with the team’s customer, Professor Yoon Kim, another alternative for the motor driver module developed. Prof. Kim offered his personal motor driver to be used in the RoMow prototype. This electronic component, called the Sabertooth Dual 25A Motor Driver, is more high-end than the two described above, as it provides two channels for motor control. This means that only one unit would be required, since it can simultaneously control the speed of both of the motors. It is rated for 6-24V and up to 25A continuous operation per channel. It also provides switches that allow the user to choose between several modes of operation. These include analog mode, R/C mode, and serial mode. The selected mode determines how two input signals S1 and S2 are used to set the speed of the two motors. In analog mode, an analog voltage signal controls the motor speed. In R/C mode, a standard R/C pulse signal, which ranges from 1 to 2 milliseconds, is used to set the speed. In serial mode, a microcontroller can be used to send data to the motor driver using the serial protocol to set the speed and direction of the motors. The last important feature that the Sabertooth provides is a 5 V output that can be used to power an R/C receiver and a microcontroller. 4.2.5.5

Hoveround Wheelchair Speed Controller

Another alternative for the decision on the motor driver for the design is the speed controller from the Hoveround Wheelchair’s control system. The wheelchair uses a joystick to provide control to the user, and the circuit board inside the controller contains an 8-pin header that is attached to the joystick output signal header via a removable 8-wire connector. The advantage to using this board as the motor driver is that it comes from the same system as the motors and batteries being used. This means that voltage and current ratings do not need to be known because the fact that the wheelchair is operational proves that the circuit board is capable of driving the motors. In this scenario the motor driver, motors, and batteries can all be viewed as a single module, since they would essentially be taken from the wheelchair unaltered.

29

4.2.5.6

Decision on Motor Speed Controller

Since all four of the options meet the essential component criteria, a decision matrix was created in order to determine which motor driver would be the best fit for this project. Three criteria were considered when weighing the options. Table 9 displays these criteria and their relative importance to the team as a weight value. Table 9. Motor Drvier Criteria Weights

Criteria Flexibility Cost Repeatability

Weight 3 2 1

Next, the four alternatives were assigned a score between 0 and 10 for each criteria and a total weighted score was computed. The results of this process are displayed in Table 10. Table 10. Motor Driver Decision Matrix

Motor Driver Simple-H Pololu High-Power Sabertooth Dual 25A Hoveround Controller

Flexibility 5 5 9 2

Criteria Cost Repeatability 4 7 4 7 10 4 10 2

Total 71 71 107 46

This decision matrix confirms the team’s initial hypothesis that the Sabertooth motor driver is the best fit for the project. It offers several options for interfacing with a microcontroller which improves flexibility, and it comes at no cost to the team’s budget. The only downside to this option is that it would raise the cost for additional units of the RoMow to be fabricated. However, repeatability is not one of the highest design criteria, as only one unit will be assembled.

4.2.6 4.2.6.1

Microcontroller Component Criteria

The most well-defined requirement for a microcontroller in the design is the number of digital and analog input and output pins the board contains. Since the team decided to purchase a 6-channel transmitter, it was also decided that the microcontroller should be able to make use of all 6 channels. In addition, the microcontroller will need to have two additional pins that can be used as outputs to drive the two input signals S1 and S2 on the Sabertooth motor driver. The board must also contain an additional output pin that

30

can be used to signal the stopping of the mower blade. This means there must be at least six pins that can be used for digital input, and three pins that can be used as digital output. Lastly, there will need to be at least one analog input pin that can be used to check the voltage level on the batteries, so that the system can be disabled before they are discharged so much that they are permanently damaged. Other criteria that will be considered include the amount of memory available for program and data storage, cost, familiarity, development environment, and available libraries and support. Four alternatives were considered for the system microcontroller, and they are described in the following sections. 4.2.6.2

Arduino Uno

The Arduino Uno is one of the most popular microcontrollers currently on the market. It is based on the ATmega328 processing core, contains 14 digital I/O pins (pins that can be used as either an input or an output to the board), and 6 analog input pins. It also contained 32 KB of flash memory, used as program space; 2 KB of SRAM, used to store and manipulate data during runtime; and 1 KB of electrically erasable programmable read-only memory (EEPROM), used for long term storage. Arduino has a rich collection of software libraries and community support that greatly improves program write-ability. It sells for a reasonable $22.69. 4.2.6.3

Raspberry Pi

The Raspberry Pi is the most high-end controller board that the team considered for this project. It uses a 900MHz quad-core ARM Cortex-A7 CPU and contains 40 general purpose I/O pins. It also contains 1GB of RAM. It also has impressive features such as HDMI, Ethernet and USB ports, camera and display interfaces, and even a graphics core. All of these features explain why this product is often referred to as a single board computer. It can run an operating system, and thus can run programs of many different languages just like personal computers. The board sells for $35, but there would likely be other costs involved if chosen. This is because the product is a used like a computer, and thus requires a keyboard, mouse, and monitor to be used to its fullest capability. 4.2.6.4

STM32 Discovery

The STM32 Discovery is a low-cost ($9.88) controller board that uses a 32-bit ARM Cortex M3 core, and contains 128 KB of flash memory and 8 KB of RAM. The exact number of available I/O pins was not clear after looking at the product manual. However, the processing core is a 64-pin quad-flat package, so it is safe to assume there are at least 9 pins that can be used as I/0. This product is not as widely used as the Arduino Uno or the Raspberry Pi, so software libraries and documentation are not as readily available.

31

4.2.6.5

MSP430 LaunchPad

The MSP430 LaunchPad, made by Texas Instruments, is a flash programmer and debugging tool used for development with microcontrollers of the MSP430G2xx line. The board does not actually come with a processor chip attached. It simply provides a compatible DIP socket that allows for the user to choose what microcontroller chip to use. The MSP430G2 microcontrollers that are compatible with this board can contain up to 512 bytes of SRAM and 24 general purpose I/O pins. Since none of the RoMow team members are familiar with this product, the development kit would need to be purchased at a price of $9.99. It is also possible to upload Arduino sketches to the MSP430 chips that come with the development kit, which aids in development familiarity. 4.2.6.6

Decision on Microcontroller

The decision on which of the four microcontrollers to purchase and use in the RoMow prototype was far from obvious, so the team used a decision matrix as a tool to aid in the process. The four most important criteria were each given a weight based on how important to the team they were. This information is displayed in Table 11. Table 11. Microcontroller Criteria Weights

Criteria Memory Cost Familiarity Support

Weight 2 1 2 3

Once the importance of each criterion was agreed upon, each of the four boards was given a score between 0 and 10. By multiplying each criteria score by its corresponding weight and adding them together, an overall score was tallied for each microcontroller alternative. Table 12 shows the decision matrix that was used to compute each score. Table 12. Microcontroller Decision Matrix

Microcontroller Board Arduino Uno Raspberry Pi STM32 Discovery MSP430 LaunchPad

Memory 7 10 8 3

Criteria Cost Familiarity 7 8 5 5 9 2 9 6

32

Total Support 10 8 5 10

156 94 67 144

The decision matrix indicates that the Arduino Uno is the best-suited microcontroller for the team and project. Table 12 reflects the fact that the alternatives with the three lowest scores all had at least one legitimate concern associated with them. The Raspberry Pi was the most expensive product, which is because it offers so many features that are not necessary for this project. The STM32 Discovery was generally an unfamiliar product that lacks development support because it is not well known. Lastly, the MSP430 LaunchPad only supports microcontroller chips with relatively small amounts of RAM. Thus, the Arduino Uno will be purchased and used as the microcontroller for the RoMow because it meets all component requirements and provides sufficient features to meet all relevant component criteria.

33

5 System Design This section of the report lays out the design of RoMow in full, including the selected design options that were referred in Section 4. It also includes what actions were taken where there were not alternatives.

5.1 Mechanical Design RoMow was designed with the goal to create a product which could be used in conjunction with a push lawn mower to allow someone to operate their mower without directly pushing it, but rather by controlling it remotely. It would have been great to design a system that worked with every shape and size of mower out in the market today but that was not feasible. The team instead wanted to focus on designing RoMow to accommodate a broad range of push lawnmowers made by one manufacturer. The manufacturer that was chosen was Toro. The team then purchased a Toro GTS Recycler to use for the design process. The mower is shown in Figure 13 and the critical dimensions of the same mower in Figure 14.

Figure 13. Toro GTS Recycler

Figure 14. Lawnmower Dimensions

34

5.1.1

Electric Motors

It was determined last semester that the necessary motor power to carry the weight of RoMow plus a lawnmower up a slope of 30 degrees was 0.8822 horsepower (Section 10.1). This means that if two motors were to be used each should be a half horsepower. The team decided to use the electric motors from a Hoveround MPV5 electric wheelchair which was purchased last semester. These motors are rated to 24 volts direct current and are produced by Shihlin. These motors are internally geared to 180 rpm, and are intended to directly drive the wheel without any additional gearing. The motors, given the current wheel size, can reach a maximum of 5 mph which is a bit faster than the performance specification of Section 3.2. The motors have four bolt holes for fastening. Two mounting plates were milled to attach the motors to, and these plates were welded to the front section of the frame. Below is a figure showing one of the motors mounted to a mounting plate.

Figure 15. Shihlin motor and mounting plate

5.1.2

Wheels

The wheels used in this design also come from the purchased wheel chair. The team desired a motor/wheel tandem that was direct driven to reduce the amount of moving parts on RoMow. For this reason the drive wheels (wheels attached to the motors) are mounted directly on the drive shaft of the motor. The height of the mounting plate was directly influenced by the size of the wheel, so because of this the front section of the frame was the first part that was completed. The rear wheels of RoMow are not driven by motor so the team selected to use the caster wheels from the MPV5. The wheels in the rear allow the vehicle to be easily steered by varying the speeds of the front motors. The caster assembly from the wheel chair was cut apart to separate the two wheels that were affixed to the same steel bar. The steel bar that was then cut to a desired length and then welded to the back end of the frame (Figure 16) with the bar parallel to the length of RoMow so as to not increase the width of the design too much. The wheels and motors are shown attached to their respective places on RoMow in Figure 17. 35

Figure 16. Caster assembly

Figure 17. Wheel and motor mounting

5.1.3

Frame

As was discussed in Section 4 in detail, the frame of RoMow was constructed out of 1” square steel tubing, except in a few places. The team chose to take the two-tier approach that was discussed in Section 4.1.1.2 because of the structural benefits as well as the cohesive feel of the structure. The team wanted the design be de-constructible so bolts were used where possible so that RoMow could be stored in a box if necessary. The corners of the frame are attached together using L-brackets. One side of the bracket is welded to the tubing while the other side has drilled holes to match holes on the long pieces of the frame. This can be seen in Figure 18. The section of the frame supporting the mower was assembled and then fastened to the front section which houses the motors (Figure 19). The storage for the batteries and electronics was also assembled and then fastened on top of the front section of frame.

36

Figure 18. L-Shaped Bracket

Figure 19. Front and body section assembly

5.1.4

Mower Suspension System

The team decided to pursue a suspension system for the mower which supported the rear wheels and clamped onto the front of the mower frame. The rear wheel support assembly was made using 1” square steel tubing and 3/16” steel sheet. The rear wheels of the mower rest on the sheet metal while the steel tubing provides support and rigidity. It also provides a way of permanently affixing the platform to the frame of RoMow. The plate was rounded, by means of bending, in the primary direction of travel to ensure that the vehicle will move smoothly over the ground and not catch the grass or earth. The rear wheel suspension system holds the rear wheels at a fixed height above the ground while the front suspension is adjustable from 2” to 3” cutting height. The rear wheels can be adjusted by changing the wheel height settings so that the cutting deck is level from front to back.

5.1.5

Electrical System Housing

The electrical system is to be housed in a box shaped container which is fixed on top of the motor assembly. In this box are contained all the electrical components and batteries of the design along with the wiring. 37

The idea of enclosing all these components is so that they are protected from the weather and from debris during operation. The shape was constructed using four uprights made of 1”x 1” steel tubing and then by riveting steel sheet metal on the outside edges of the uprights. The top is enclosed with a cover that hinges and latches for ease of access and security. The cover also overhangs the edges so that rain does not drip in through the top. An aluminum plate is used to separate the motors from the housing and also to support the batteries and components that are housed within. A picture from the building process is shown in Figure 20 to provide the reader with an idea of what it looks like.

Figure 20. Electrical housing construction

The batteries are placed on the aluminum plate, right on top of the rectangular outlines which can be seen in Figure 20. They are secured in place by passing straps through the notches on the plate and then over top of the battery. The motors are mounted just beneath the batteries and their power cables pass through the side of the housing and are then connect to the motor driver. The electrical components (described in detail in Section 5.2) are then mounted in between the two batteries, on a vertical aluminum plate. The mounting is done to keep are the wires in order, to provide proper clearances for heat dissipation, and to keep the electronics from shorting each other. The seams of the housing were filled using silicone caulk to prevent water and debris from entering in. Finally the outside of the housing was painted white to reduce absorptivity which will prevent the box from getting too warm during the summer which could cause distress to the electronics inside.

38

5.1.6

Safety Components

There is a need for safety systems with this project in order to maintain a level of safety for the end user. In order to achieve this level, the team implemented some safety features that are listed below. For an in depth look at how the systems work refer to Section 5.2.5. 5.1.6.1

Remote Control Emergency Stop

The team has decided to design an emergency stop button that is located on the upper right of the controller. This function has been implemented on Ch. 6 of the remote control. To read about the circuitry used for this button refer to section 5.2.5.2. 5.1.6.2

Push Button Engagement

The team decided to put an emergency stop button on the exterior of the status panel. This E-stop is a toggle button that has been wired directly after the batteries, cutting the voltage to the rest of the components causing the motors, mower, and all other electronics to be powered off. This button will also act as the power button for the RoMow, not just as an emergency stop. 5.1.6.3

Safety Components

In order to shut off the lawn mower in an emergency situation, the team has included a safety relay. This relay is a double pole single throw switch that will short the transformer located in the gas mower. Minor modifications were made to the mower in order for this to work, however, only a ratchet and a screwdriver were needed for implementation.

5.2 Electrical Design 5.2.1

Overview

The electrical design process is essentially centered on creating a system that causes the two electric motors to spin in accordance with the controls on the remote control transmitter. This was accomplished using several components that were deemed essential in achieving this goal. For more information on how the team determined what components to use, refer to section 4.2, which deals with high-level electrical design decisions. Table 13 summarizes the products that were chosen to serve as each of the major electrical components needed for the design. Table 13. Major Electrical Components

Component Radio System Microcontroller Motor Drivers Electric Motors

Product Turnigy 6x FHSS 2.4GHZ Arduino Uno Sabertooth Dual 25A Shihlin Motors from Hoveround 39

Note that the radio system is comprised of both a Transmitter (remote-control) and receiver. Using the four products from Table 13, as well as two 12V batteries for power, the goal of propulsion from remote-control was achieved. Figure 21 provides a general idea of how communication flows from one major component to the next.

Figure 21. Electrical Communication Flow Chart

Control must start from the user, who moves the sticks on the transmitter to achieve a desired type of movement of the RoMow. This information is then communicated wirelessly to the paired receiver, which sends corresponding physical signals to the microcontroller. The microcontroller must then decide how the motors should be activated based on these signals. It then sends information to the motor driver module, which distributes the correct amount of power to the DC motors. All of this together results in usercontrolled propulsion of the RoMow platform. This section of the report describes in detail how all of this communication happens using the four devices in Table 13.

5.2.2

User Controls

The team had to determine how the controls on the remote-control transmitter would ultimately drive and steer the RoMow platform. This mainly involves how the user controls the speed and direction in which it 40

moves. Since the user will clearly see this part of the design, it is a very important one. The team kept the design norm of transparency in mind when implementing this aspect of the design, as it should be as intuitive to the user as possible. However, caring was another design norm that was considered as the user controls should promote safe operation. The transmitter that the user will hold has two flip switches, and two position sticks that can move both vertically and horizontally. Both sticks rest in the center horizontal position when released. However, only the right stick returns to the center position vertically when released. The vertical position of the left stick will remain wherever the user leaves it. This is because this stick is commonly used as the throttle control for model aircraft. This is a feature that the RoMow system may be able take advantage of. Some of the questions the team addressed in deciding how the controls would work are the following. •

What type of stick should be used to control speed?

•

Which stick should be used for steering control?

•

Should the platform be capable of operating in reverse, and if so how is it put in reverse?

The team determined that having control of the platform’s speed should be foremost since not having this is the easiest way to lose control of the RoMow. In order to make this intuitive, speed should be based on moving a stick vertically, since this is how nearly all remote-control vehicles work. This leaves the question of whether the left or the right stick should be used. If the left stick is used, then pushing it all the way down should cause the platform to stand still, while moving the stick upwards would gradually increase the speed until the top position is reached. It would then be moving at maximum speed. The advantage of this approach is that speed can be kept constant very easily (because the stick does not have a spring that brings it back to the center), which is important for cutting grass uniformly. If the right stick were used, then the platform should stand still when it is in the center position, and should move in the forward direction as the stick is moved upward, and in the reverse direction as the stick is moved downward. The main advantage of this option is that the user has to be pushing the stick to make the platform move, which is a safer approach. However, a disadvantage of this approach is that the user can very quickly switch from full speed forward to full speed reverse, which could overload the motor drivers. With all of this in mind, the team decided that using the left stick for speed control made a little bit more sense as it allows for easy constant speed control and it prevents motor driver overloading. It was also decided that the user should be able to put the platform in reverse by flipping the left switch on the top of the transmitter. However, a short delay would be present between operating in forward and reverse to prevent overloading the motor drivers. Thus, the user at any time can flip the forward/reverse switch, at which time there will be a 1 second delay before the platform begins moving in reverse according to the position of the throttle (speed) stick. Lastly, the team decided that since the left stick was being used for speed control, the right stick should be used for 41

steering. This allows the user to change the direction of the platform without accidentally adjusting the position of the throttle stick. Since the left switch is used for choosing between forward and reverse, the right switch was used for activating and deactivating the mower blade.

5.2.3 5.2.3.1

Electrical Interfacing Overview

When designing the electrical system for the RoMow, it was very important to keep a clear understanding of how all of the major components interface with one another. This system incorporates several different types of interfacing techniques, some wireless and some wired. These include how the remote-control transmitter wirelessly communicates with the receiver, how the receiver communicates that same information to the Arduino, how the Arduino communicates with the motor drivers to control the motors, and how the motor driver sends the appropriate amount of power to the motors to achieve propulsion.. 5.2.3.2

Transmitter to Receiver

The transmitter and receiver used in the RoMow system communicate using 2.4GHz FHSS as described in Section 4.2.2. The details of how this wireless technology works is not a concern to the design process for the RoMow team, as the radio system was purchased as a single working module. What was important to pay attention to, is how the moving of the four control sticks and the two switches on the transmitter affected the signals being driven on the output pins of the receiver. The control sticks and switches are visible in Figure 11 from Section 4.2.2.6, and a close-up of the receiver output pins is shown in the top right corner of Figure 22.

Figure 22. R/C Receiver Output Pins

42

As can be seen in the figure, there are seven rows of pins that are each 3-wide. Each row of pins corresponds to a specific channel from the transmitter. The first six rows from the bottom up represent channels 1-6 in ascending order. Figure 23 is a snippet from the Turnigy product manual that shows which channel each stick on the transmitter controls.

Figure 23. Transmitter Channels

In addition, the switch on the left side of the transmitter corresponds to channel 5 while the right switch corresponds to channel 6. Within each row of pins there is a signal pin, a positive pin, and a negative pin. The positive and negative pins are the voltage rails that represent the two logic states HIGH and LOW respectively. A low-voltage regulator must be used to drive these pins. The signal pin will contain a signal that switches between these two states with time, in order to represent the position of the stick from the transmitter. In order to determine exactly what these signals might look like, the team conducted an online search. It was found that a standard R/C receiver signal contains a pulse for a duration that normally lasts between one and two milliseconds and is repeated about fifty times each second. Figure 24 shows what these signals look like with time on the horizontal axis, and the voltage level on the vertical axis. Note that this figure was taken from RCArduino.com and contains a typo. It lists the pulse widths as being measured in microseconds, but the units should be milliseconds.

Figure 24. Example of R/C Receiver Signal

43

This figure shows three different waveforms that correspond to different positions of the throttle stick on a transmitter. The first, titled Full Throttle, shows what the signal would look like when the stick is pushed all the way up. It would repeatedly send 2 ms long pulses, and the time in between pulses does not contain any information. It can be seen that at the center position, the pulses will be 1.5 ms in duration, and at the bottom position, the pulse duration will be 1 ms. Put concisely, the position of a stick on the transmitter is directly proportional to the pulse duration of the receiver signal that corresponds to the same channel as the stick. It is likely that the Turnigy radio system being used in this design works this way, but the team decided it would be wise to use an oscilloscope to measure these signals in order to verify this. Figure 25 shows the set up for this experiment.

Figure 25. Receiver Signals Experiment Set-up

The transmitter was powered on and a 5V power supply was used to power the receiver. The channel one signal pin was connected to the scope probe, with the power supply ground and oscilloscope ground also connected. The right stick was then moved in the left to right direction, and the oscilloscope readings confirmed that this radio system works as described above. Figure 26 shows a close-up of the oscilloscope screen when the channel 1 stick was in the center position.

44

Figure 26. Oscilloscope Close-up

As expected, the pulse duration measured by the oscilloscope was right around one and a half milliseconds, 1.504 milliseconds to be exact. It was also confirmed that the signal resulting from pushing the stick all the way to the left was about one millisecond, and the signal resulting from pushing it all the way to the right was about two milliseconds. Note that the trims on the transmitter must be centered for this to be the case. 5.2.3.3

Receiver to Arduino

In order for the output signals of the Turnigy receiver to be of any use, the Arduino Uno must be able to extract information from them. As was discovered in Section 5.2.3.2, the receiver sends pulse signals for which the pulse length is directly proportional to the position of the corresponding stick on the transmitter. The Arduino must then measure the duration of each pulse and store it in some program variable so that the information can eventually be used to drive the electric motors. So, the outputs of the receiver must be connected to I/O pins on the Arduino that will be configured as inputs. In order for the Arduino to determine the pulse duration, it must record the time of a rising edge on the signal, then record the time of the next falling edge on the signal, then compute the difference. There are two ways that this can be done. The Arduino could continually read the signal until it has seen both a change from LOW to HIGH, and a change from HIGH to LOW. This is, however, a very expensive way to accomplish this goal in the sense of computing. A better approach that the team discovered through an article on RCArduino.com takes advantage of the Arduino Uno’s interrupt capabilities. Using an Arduino library called PinChangeInt, the program running can be interrupted whenever an event on an input pin connected to the receiver occurs. The interrupt service routines can each set the values of shared variables that correspond to the input signals from the Turnigy receiver. This will be discussed further in Section 5.2.4 on Arduino programming, but put simply, the Arduino uses interrupts to compute the duration of multiple input signals from the Turnigy 45

receiver, while still focusing the majority of its computing power on the running the algorithms in its main program loop. 5.2.3.4

Arduino to Sabertooth

There were several options for how the Arduino could interface with the Sabertooth speed controller, and this is due to the different ways the speed controller can be configured. The different modes that this controller can be set to were discussed in Section 4.2.5.4. Analog mode is not a good option since a filtering capacitor would have to be used to create an analog output signal from the Arduino. Both R/C mode and serial mode would work fine for the design, but the team chose to use R/C mode, so that the Arduino’s serial communication can be solely devoted to debugging. With this decision, the Arduino must then send signals in the same format as the Turnigy receiver outputs to the Sabertooth controller. Fortunately, the Arduino provides a library called Servo that generates this type of signal. All the RoMow program will have to do is specify the pin to write the signal to and the duration of the pulse in microseconds. 5.2.3.5

Sabertooth to Electric Motors

The Sabertooth electronic speed control facilitates the electrical design of the RoMow by automatically distributing power from the batteries to the motors based on the low power signals sent into it. All that the RoMow design has to do is connect the two leads of each motor to the screw terminals (M1A, M1B, M2A, M2B) of the Sabertooth, as well as send the appropriate signals from the Arduino into the input signal terminals (S1, S2) of the Sabertooth, as shown in Figure 27.

Figure 27. Sabertooth Speed Controller

46

It is also worth noting that the Sabertooth controller provides a regulated 5V output that powers both the Arduino and the Turnigy receiver. The Sabertooth sends pulse-width modulation signals to each electric motor based on the signals S1 and S2. Since the RoMow design uses the Sabertooth in R/C mode, the pulse duration of the input signals, also known as servo signals in this context, is directly proportional to the dutycycle of the PWM signal being sent across the motor terminals. For example, a servo signal of 1500 microseconds will result in a constant 0V across the motor terminals. Then as the duration of the servo signal increases up to 2000 microseconds, the duty cycle of the PWM signal on the motor outputs increases in proportion up to 100% duty cycle. As the servo signal decreases from 1500 microseconds center down to 1000 microseconds, the duty-cycle of the PWM behaves the same way, but in reverse polarity, which means the motors spin in the opposite direction. This feature is what will allow the RoMow to be able to operate in both the forward and reverse direction.

5.2.4

Software Design

Possibly the most critical portion of the electrical system on the RoMow is the software that runs on the Arduino Uno. The goal of this portion of the design was to create a single program written in the Arduino language that would fulfil several objectives. The first objective is to read the pulse length of the output signals on the Turnigy receiver. The next objective is to generate servo signals for each electric motor based on the signals from the receiver. These two objectives are the most important as they are what allow the user to drive the prototype. The software on the Arduino must also monitor the voltage level of the two lead-acid batteries powering the system. Lastly, the Arduino must have an output signal dedicated as a mower blade enable. 5.2.4.1

Receiver Signal Reading

One of the most important tasks that the Arduino must perform is deciphering the output signals of the Turnigy wireless receiver. In Section 5.2.3.3, it was found that the receiver outputs pulse signals that signal the position of the corresponding stick on the transmitter. As mentioned earlier in the report, the RoMow program uses an approach described in an article on RCArduino.com. All six output signals from the receiver were connected to I/O pins in the Arduino. The program keeps both a local and a shared variable representing the current pulse length in microseconds for each channel. Anytime an event occurs on one of the input pins, the interrupt service routine assigned to that pin is executed. The interrupt service routine (ISR) then goes through the sequence shown in Figure 28.

47

Figure 28. ISR Flow Diagram

When the ISR is entered, there are two possible paths of execution. The function checks to see whether the event that caused it to run was a rising edge or a falling edge. If it is a rising edge, then that means it is seeing the beginning of a pulse, so it records the time and stores it in a variable. If the event was a falling edge, then that means it is seeing the end of a pulse, so it subtracts the variable that was set at the beginning of the pulse from the current time, then stores the result in the shared variable that holds the current pulse length of that signal. The routine finishes by setting a flag so that the main program knows to update its local version of the variable. 5.2.4.2

Main Program

The main loop of the Arduino program running on the RoMow is intended to generate the proper output signals to the Sabertooth speed controller, as well as signals to enable the lawnmower engine and to light the red battery status LED. Figure 29 provides a diagram that shows the general flow of execution of this main program.

48

Figure 29. Arduino Main Loop

The first step in the procedure is to determine whether the RoMow is set to move in the forward direction, or the reverse direction. Since this option is set by the user using the channel 5 switch on the left side of the 49

transmitter, the program simply references the variable that stores the current pulse length coming out of channel 5 on the receiver. It then sets an integer variable to 1 if forward is selected, and to -1 is reverse is selected. Next, the program runs through several blocks of code that will eventually determine if the platform should be enabled to move. There are several reasons for why the RoMow could be disabled by this code. The first possible reason is that the transmitter is either off out too far away from the receiver. This is determined by reading the pulse length of the left switch, which automatically goes to 1500 microseconds when the receiver is not getting a signal from the transmitter. Movement of the RoMow is also disabled for one second upon switching from forward to reverse or vice versa. The RoMow can also be disabled for movement when it reads too low of a voltage on the battery input pin. Lastly, when the user sets the right-side switch in the upward position, movement is disabled. This is also when the mower blade is disabled. Otherwise, the mower is enabled to run. This last step in this block is to choose whether or not to light the red battery indicator LED. Next the loop procedure chooses between two paths based on the move-enable variable. If this Boolean variable is set to false, then the program sets both servo signal variables to 1500 before reaching the last block of execution. However, if it is true, then there are three blocks of code to execute before reaching the last block. It starts by setting a velocity variable based on the position of the throttle stick. This variable can range from 0 to 500, as a throttle stick in the bottom position will cause it to be zero, while pushing it upwards will cause increasing values with 500 resulting from pushing it all the way to the top. Next, the program executes a block of code that sets steering adjustments for each motor. It contains two steering adjustment variables, one corresponding to each motor. Based on the position of the steering (right) stick on the transmitter, these two variables are modified. They will eventually be used to slow down the speed of one of the motors to cause the RoMow to turn. This is done by finally setting servo signal variables for each motor using both the velocity variable and the two steering adjustment variables. The last step in the main loop is to write the two servo signal variable values to two servo objects. In the program setup, the two I/0 pins that are connected to the Sabertooth speed controller were assigned to these objects. To summarize, the Arduino repeatedly executes these steps so that the Sabertooth speed controller is always receiving the correct servo signals. Based on the values of these signals, the speed controller will handle the rest. To gain an even better understanding of how the software on the RoMow, visit Team 3’s website and click on the Documents tab. From there, all five development versions of the source code can be downloaded.

5.2.5

Safety/Protection Module

Due to several needed safety features that could be implemented with circuitry, it was decided that a module dedicated to safety and circuit protection should be included in the electrical system design. These features

50

include battery voltage monitoring, a mower blade activation relay, a power status LED, and a battery level status LED. 5.2.5.1

Battery Level Monitoring

In order to protect the health of the two 12-volt batteries on board the RoMow, the team decided it would be necessary to incorporate some kind of monitoring of their state of charge (SoC). There are several ways that SoC can be measured, and with each method comes the tradeoff between simplicity of implementation and the accuracy of the measurement. The simplest way to measure a battery’s SoC is to read its voltage when it is not under load. However, this method is not very accurate because it takes hours for the battery to reach equilibrium again after having its charge disturbed. For the RoMow application the user cannot wait 4 hours in the middle of a mow to see if the batteries can still be used without being recharged. Another method that is used to measure SoC is the use of a hydrometer. A hydrometer measures the specific gravity of the sulfuric acid in the battery cell, which has a correlation with its SoC. This method is more accurate than reading the voltage, but is not practical for the RoMow. It would require too much additional hardware on the prototype as well as time to setup and integrate that hardware. The last common method that is used to measure battery SoC is coulomb counting. In this approach, the SoC is estimated based on the amount of current that has been delivered from the battery since the last full charge. This method would require the purchase of a current meter, and it would also require the Arduino software to be more complex because it is trying to count the amount of charge based on always reading the current. The team decided that given the amount of time and the remaining budget available for this portion of the project, the simplest method was the best choice. Thus, voltage monitoring was chosen. The team decided to implement voltage measuring through a method recommended by Eric Walstra, the team’s industrial consultant. The method he proposed was a two-resistor network that would provide a constant percentage of the voltage coming from the batteries. Since the RoMow operates from two 12V batteries in series, the voltage should be somewhere in the range of 21V to 25V at any given time. Since an Arduino can only read analog voltages up to 5V, this needs to be scaled down. The team chose to use a voltage divider consisting of a 20 kΩ resistor and a 100 kΩ resistor. This produces a voltage that is one-sixth of the actual voltage from the battery network. Since the voltage of a fully charged 12V lead acid battery is 12.7 volts, which means the maximum voltage that will be seen with two batteries in series is 25.4 V. Dividing that number by six produces 4.23 V. Therefore, the voltage level between the resistors with respect to ground can be sent into the analog-todigital converter on the Arduino to produce a reading that corresponds to the SoC of the batteries. 5.2.5.2

Mower Blade Activation

Given that the RoMow was designed with the design norm, caring, in mind, the team decided that giving the user a way to turn off the lawnmower blade remotely would be an important feature. This makes the 51

machine safer to operate since it gives the user more control over the most dangerous part of the assembly. This was done by using an electrical relay where the contacts were connected to the generator on the mower, and the coil was wired in series with a bipolar junction transistor (BJT) that could be activated with an Arduino output pin. 5.2.5.3

Status LEDs

There are two status LEDs that sit on the control panel of the RoMow. There is a green LED that indicates when the machine is powered on, and there is a red LED that indicates when the batteries need to be charged. The green LED is important because it must be easy for the user to know when the machine is powered on. The red LED is important because without it, the user would not know when the batteries need to be charged. The green LED is simply wired with a current limiting resistor, being powered by the batteries. The red LED is wired similarly, but with a BJT added in that allows an Arduino output pin to control when the LED is lit. Deciding when to activate the red LED is then handled by the software running on the Arduino. 5.2.5.4

Schematic Design

An electrical schematic was created that incorporates the necessary circuitry for each of the features mentioned above. This schematic was created in EagleCAD with the intention of designing a printed circuit board for it, and it is shown in Figure 30.

Figure 30. Safety/Protection schematic

There are eleven total connections that this circuit needs to make with other components in the electrical system, as can be seen in the schematic. Table 14 gives descriptions for each of the pins shown in the schematic. 52

Table 14. Safety/Protection circuit connections

Pin Name BAT-1 BAT-2 REL-1 REL-2 LED_BAT1 LED_BAT2 LED_POW1 LED_POW2 ARD-1 ARD-2 ARD-3

Description Positive terminal of battery network Negative terminal of battery network Relay coil Relay coil Cathode of red LED Anode of red LED Cathode of green LED Anode of green LED Arduino input: Analog battery level Arduino output: Mower blade activation Arduino output: Red LED activation

This module must connect to the battery network, the coil of an electrical relay, the terminals of two status LEDs, and three pins on the Arduino Uno. 5.2.5.5

Bread Boarding

Once the schematic for this module was designed, it was constructed on a bread board to verify that it worked as expected. After all of the components were wired according to the schematic, it was verified that the circuit behaved as expected by the team. Figure 31 shows a photo of the safety/protection circuit constructed on a breadboard and connected to the other components of the system.

53

Figure 31. Safety/Protection circuit bread board setup

5.2.5.6

Board Design

Using the schematic shown above and EagleCAD software, a board design was created, which is shown in Figure 32.

Figure 32. Safety/Protection board design

This board was designed to be fabricated using the PCB fabrication lab at Calvin College. This means that the board must contain only one layer of traces. Luckily, the schematic is quite simple, which made creating a one-layer board feasible. In addition, the trace width and the trace spacing was kept at a minimum of 24 mil throughout the design to minimize the risk of board defects after fabrication.

54

5.2.5.7

Board Fabrication

Using the design shown above, a printed circuit board was fabricated using the PCB fabrication lab at Calvin College. As each step of the process was followed carefully, the PCB turned out error free. This is shown in Figure 33.

Figure 33. Safety/Protection PCB

Once the board was printed, holes were drilled so that the through-hole components could be placed onto the board and soldered in. In addition, mounting holes were drilled in all four corners so that the board could be secured to the final prototype. The result of this work is shown in Figure 34.

Figure 34. Finished safety/protection board

55

Once the board was completely assembled, it was wired to each of the external components that it must connect to and its functionality was verified.

5.2.6

Final System Design

The final system design incorporates all of the ideas presented in the former sub-sections of Section 5.2 to produce a working overall system. Figure 35 shows a block diagram of this system, including all of the major components.

Figure 35. Electrical System Block Diagram

56

6 Prototype Testing 6.1 Overview In order to prove the quality of the first RoMow prototype, the team developed a list of tests that it should pass. The list was created with the original performance requirements in mind. The tests that were conducted measure performance aspects of the prototype such as cutting height, traversable slope, maximum speed, turn radius, battery life, and radio frequency signal range. In addition to these tests, it was verified that all intended emergency stop methods were functional, and a finite-element analysis was done on the frame design.

6.2 Cutting Height Test The test for cutting height was very straightforward. The prototype was taken to a yard where the grass was noticeably long. The RoMow was turned on and the lawnmower was also started. A team member controlled the RoMow to make it drive over a patch of the long grass in order to cut it. After cutting a large enough area of grass to work with, the RoMow was turned off. At this point, a member of the team measured several blades of grass, as shown in Figure 36.

Figure 36. Cutting Height Test

It can be seen that the grass has been cut to a height of about 2.5”, which meets the aforementioned performance requirement and falls well within the range of popular cutting heights as mentioned earlier in the report. The mistake with the design was the assumption that the bottom edge of the mower is the height

57

to which the mower cut to. While the bottom edge does meet the 2” requirement the blade height is actually a bit higher resulting in the cutting height 2.5”.

6.3 Slope Test To ensure that the RoMow would have the ability to go up steep inclines the team conducted a slope test. This was done by taking the RoMow to the bottom of a fairly steep hill and sending it back up the hill. The steepest grade that has been tested to date is ten degrees. The performance of the RoMow on this hill suggests the ability to fulfill requirements as it did not slow down. Given more time, the team would do more intensive testing regarding slope climbing capabilities. The team is confident that the propulsion power calculations show that a total of one horse power will be sufficient for the design. The team made conservative estimates concerning mass and then selected motors that surpassed the suggested power requirement.

6.4 Speed Test In order to measure the speed capabilities of the RoMow, the team measured the time taken for it to travel from one point to another. The two points of reference were defined fifty feet apart within a yard of grass. The RoMow was placed about ten feet behind the first point, so that it would have time to reach maximum speed as it passed by. It was then driven at full throttle on the remote control transmitter until the front end crossed the second reference point. A stopwatch was started as the front crossed the first point, then it was stopped as it crossed the second point. The speed was then calculated as follows. 𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 =

50 𝑓𝑓𝑓𝑓 𝑓𝑓𝑓𝑓 = 4.79 = 3.27 𝑚𝑚𝑚𝑚ℎ 10.43 𝑆𝑆 𝑠𝑠

This was a successful result since the performance requirement was an even 3 mph.

6.5 Turn Radius Test The RoMow was programmed in such a way that when the steering control on the transmitter was pushed all the way either right or left, one of the two motors would come to a stop while the other would remain spinning at a speed in accordance with the throttle control. This implements a rotation of the platform about the wheel that is connected to the idle motor. Thus, the turn radius test simply verified that this was indeed the case. A team member moved the throttle up to a moderate speed, then pushed the steering control all the way to the right. When this was done, it was confirmed that the platform rotated about the right motor, and thus was capable of the desired turn radius. An additional feature of the final Arduino program that runs on the RoMow prototype allows for the platform to optionally rotate about the point in the front center

58

between the two electric motors. This is done by pushing the steering control down while doing a sharp turn. It was also verified that this feature was functional on the final prototype.

6.6 Battery Life Test The team originally planned on carrying out a test to see if the prototype was capable of meeting the 90minute continuous operation performance requirement. Upon taking the prototype out for its first general test run, it was discovered that after about 20 minutes of operation, one of the 12 V lead-acid batteries had been completely drained while the other was still showing an open circuit voltage of 12.3 V. In order to safely continue operating, both batteries must be above 11.3 V. If this is not the case, the batteries can be permanently damaged. Since one of the batteries could not last more than 20 minutes, it was decided that in order to meet the performance requirement, new batteries would need to be purchased. Based on the estimated average current draw from the batteries and the amp-hour rating, the team is confident that the 90-minute requirement would be met with new batteries.

6.7 Transmitter Range Test The range of the radio system on the RoMow was measured in the parking lot on the west side of the Calvin College campus. One team member held the remote control transmitter and drove the platform away from them while another team member followed the prototype. Since the software on the RoMow causes it to not move when the transmitter signal is not reaching the receiver, it could be easily determined when the range reached its limit because the platform would no longer move forward. After driving the RoMow nearly all the way across the parking lot, it began to start and stop due to the signal loss. The team member who held the transmitter then counted the number of parking spaces he passed as he walked all the way to the ending position of the RoMow. The prototype was able to travel past 103 parking spots as well as 3 medians. The width of a parking spot was measured to be 9 feet, while each median was measured to be 32 feet wide. Using the equation below, the range was determined to be 1023 feet, which is well past the range in which the user could see the RoMow well. 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 = (103 ∗ 9𝑓𝑓𝑓𝑓) + (3 ∗ 32𝑓𝑓𝑓𝑓) = 1023𝑓𝑓𝑓𝑓

6.8 Emergency Stop Protocols Test

Several emergency stop protocols were tested. First, it was verified that pushing the power button on the side control panel of the RoMow reliably cut power to all of the electronics (including the motors and mower blade). Next, it was verified that flipping the channel 6 switch, which was dedicated as the transmitter emergency stop, on the controller to the up position prevented the mower blade from running

59

as well as the RoMow from moving. Lastly, it was verified that if the remote control transmitter was powered off, the RoMow was not able to move, even if the machine was powered on.

6.9 FEA Analysis One of the main reasons that the team chose to work with steel on this prototype was because of the extreme amount of strength that the material possesses. The team was not worried about failure of the frame or mower suspension components, even under impact loading situations. Although the team felt confident, FEA analysis using Inventor was used to investigate the stresses and displacements in the frame. It was determined that the stresses and displacements that could occur in the design were minimal enough to the point where no additional strengthening needed to be done to the design. The maximum displacement in the design is .003” according to the analysis and is found on the front mower clamp, which was the expected area of highest stress. The highest stress elements in the RoMow are shown in the figures below for the reader’s benefit.

Figure 37. RoMow FEA Assembly

60

Figure 38. Rear Wheel Suspension Displacement

Figure 39. Front Mower Clamp Displacement

61

7 Project Summary Overall, the team felt as though this project was a success. The RoMow prototype designed could be the first step towards creating a product that could be both practical and desirable for consumers. The team was able to space out the workload evenly among the teammates, and over the past semester. The team grew a lot in this past year as each became more familiar with the strengths and weaknesses of each member. This knowledge of one another allowed the team to operate more smoothly as the year went on. The team also realized that it worked much more smoothly if weekly meetings were scheduled and major tasks were broken down into smaller ones. These two things allowed members to be more independent in their work which increased team productivity. In terms of the overall design, the specifications were met adequately, and the customer was happy with the design. The team was also very pleased that it stayed under budget with the design and with the prototype performing as well as it does. Like was mentioned before, RoMow is not a finished product but rather a first step in the development of a product. It was exciting to see that the concept of a flexible lawn mowing system was plausible and the hope is that this project can be developed even further in the coming years.

7.1 Future Improvements The team has few things that it wished could have been accomplished with this prototype that were not. If there was enough time, the team would pursue making the following additions or changes to the RoMow.

7.1.1

Aluminum

If a second prototype was to be constructed, the team would design it using aluminum rather than steel. This would drastically reduce the overall weight of the RoMow which would be very nice from a customer’s point of view. This would make the assembly of RoMow much easier and would reduce the required power to drive the motors. This modification would lengthen the drivable time between charges.

7.1.2

Fastened Frame

The team was able to construct the initial prototype in three sections which were easily disassembled into section. However it was desired that the whole frame be mechanically fastened together rather than by being welded together. This would make RoMow a product which could be purchased and assembled at home by the user. It would also allow for very easy packaging of the RoMow.

7.1.3

Standard Motors

If the design was to be improved upon it would be necessary to find a way to obtain motors to meet the power specifications, from a source besides a used electric wheelchair. The motors would need to be easily 62

obtained from a supplier in order to become a legitimate consumer product. For this reason it would have been nice to have obtained motors with the same specifications and features as the Shihlin motors, but could be purchased individually. Ideally it would be nice for the new motors to be directly driven like the ones on this prototype.

7.1.4

Simplified Electronics

One way the team sees that the RoMow could be improved is by reducing the amount of electrical components purchased off the shelf. This would be done to reduce the price of production and to remove components that have more features than necessary. Some possible components that could be substituted are the Arduino as well as the Sabertooth motor driver. These are expensive components which could be produced in a cheaper manner. The processor on the Arduino is really the only piece that the team really needed for the design and these processors can be purchased for a very low price. If the RoMow did become a product these types of changes would be necessary.

7.1.5

Robust Wiring and Mounting

There were multiple times during the assembly and testing stages where the team came to the realization that the wiring being used to connect the electrical components was too fragile. The pins and wires would come undone from the Arduino and the R/C receiver on frequent basis because the wires were pinned into place rather than by soldering. If a second attempt at electrical assembly was attempted the team would use lower gauge wire and would refrain from using pinned connections wherever possible. Also it would be beneficial to design a mounting plate for the electronics which is easier to access and remove, and is ideally oriented in a horizontal plane to minimize the vibrational effects.

7.1.6

Automation

At the beginning of the design process the team desired to design the RoMow to become autonomous or at least have some memory function so that the user would not have to control the system all the time. However after consulting with Eric Walstra during the fall of 2014, the team decided not to pursue either of these options because of the intensive programming work that would be necessary. This is a feature that the team believes would be a great addition to the RoMow and allow it to become a one-of-a-kind product.

63

8 References "Arduino Alternatives: 5 Microcontrollers You Should Know." Popular Mechanics. N.p., n.d. Web. 12 May 2015. "BU-903: How to Measure State-of-charge." Measuring State-of-charge. N.p., n.d. Web. 12 May 2015. "Michigan State University Turfgrass Scienceturf.msu.edu." Mowing Lawn Turf. N.p., n.d. Web. 12 May 2015. "MSP430 LaunchPad Value Line Development Kit." - MSP-EXP430G2. N.p., n.d. Web. 12 May 2015. "Pololu High-Power Motor Driver 18v25." Pololu High-Power Motor Driver 18v25. N.p., n.d. Web. 13 May 2015. "RC Transmitter Range Test Results (FPV and Related) - RC Groups." RC Groups RSS. N.p., n.d. Web. 12 May 2015. "RCArduino." : How To Read an RC Receiver With A Microcontroller. N.p., n.d. Web. 12 May 2015. "RCArduino." : How To Read Multiple RC Channels. N.p., n.d. Web. 12 May 2015. "Section 2." About Lead Acid Batteries. N.p., n.d. Web. 12 May 2015. "Simple-H 20A, 5V to 28V DC Motor Driver." - RobotShop. N.p., n.d. Web. 13 May 2015. "Tactic TTX403 4-Channel Transmitter." Motion RC. N.p., n.d. Web. 13 May 2015. "2.4 GHz Transmitter Shootout – Radio Range." Big Squid RC. N.p., n.d. Web. 12 May 2015. "Use Arduino Code on a TI Launchpad MSP430." Instructables.com. N.p., n.d. Web. 12 May 2015.

64

9 Acknowledgements The team would like thank a few people for their efforts and advice in this past year. First of all the team would like to acknowledge Prof. Mark Michmerhuizen for being the faculty advisor to the team. The team would like to thank Dr. Yoon Kim who provided the idea for this project as well as much support throughout the design process. Next the team would like to thank Eric Walstra for acting as industrial consultant to the group and helping to narrow the scope of the project while also providing advice on testing procedures. Next the team would like to thank both Phil Jasperse and Chuck Holwerda for their expertise and advice on implementation of mechanical and electrical systems. Finally the team would like to thank fellow classmate Gerrick Hershberger for his assistance with welding and for general fabrication advice.

65

10 Appendices 10.1 Calculations on Motor Output Power Requirement

10.2 Mechanical Hour Log This appendix provides a table of hours spent on the project by Dustin Brouwer and Andy Frandsen, who headed up the mechanical design and assembly. Mechanical Hours Andy

Dustin

Wheelchair Disassembly Controller Motors Casters

0 0 0

1 3 0.5

Motor Mount Electrical Box Halo Back Bar

2 2 2

0 2 2

Frame Design

66

1 3 0.5 4.5 2 4 4

Iteration 1 Iteration 2 Iteration 3

4 4 4 1

4 4 4 1

3

0

8 2 20 3

1

0

1

Cutting Welding

0.5 1

0.5 1

1 2

Cutting Drilling Welding

0.5 1 1

0.5 1 1

1 2 2

2 4 0 2 0 1 1

2 4 0 2 0 0 0

4 8 0 4 0 1 1

Cutting Welding

0 0.5

1 0.5

Cutting Milling Welding Drilling

1 1 0.5 1

0 0 0 0

1 1 0 1 1 0.5 1

Cutting Drilling Welding Adjust 1 Adjust 2

2 3 0.5 1 1

2 3 0.5 1 1

4 6 1 2 2

3

3

6

Casters Motor Housing Motor Mount 1 Motor Mount 2 Crossbar

Brackets

Electrical Housing Sheet Metal Cutting Riviting Hinges Drilling Latch Drilling Riviting Frame Casters

Front Bracket

Halos

Backbar/ramp Cutting 67

Bending Welding Adjusting Drilling

1.5 2 1 1

1.5 2 1 1

3 4 2 2

CEAC Senior Design Night In class Presentations

15 3 7 2

15 3 7 2

30 6

Electrical Troubleshooting Front Guides Painting Testing Client Meetings Rewiring Electrical Testing Assistance Team Meetings

5 0 3 8 10 0 0 14

5 2 3 8 10 3 3 14

10 2 6 16 20 3 3 28

124

126

Miscellaneous Presentations

Total Sum 250

10.3 Nathan Terschak Hour Log This appendix outlines the tasks done by Nathan Terschak, who was the leader of the electrical design of the RoMow. Hours are assigned to each task and the total is computed. Task Hoveround Wheelchair Parts Testing Isolating and Reassembling Wheelchair electronic components Modified joystick connector assembly Joystick pin mapping Hoveround Controller Debugging Testing Electric Motors Battery Charger Experiment Electrical Component Research Research on Motor Drivers Microcontroller Research 68

Hours 3 5 3 3 2 1 6 3

Transmitter/Receiver Research Electrical System Design Block Diagram Draft 1 Block Diagram Draft 2 Block Diagram Draft 3 Safety Circuit Design Safety Circuit Breadboarding Safety Circuit PCB Design Safety Circuit PCB Fabrication Website Development 1.5 hours per week (14 weeks) Other Component Testing Sabertooth Motor Driver Research and Testing Turnigy receiver output experiment Battery Voltage Testing Motor Current Draw Testing Presentations Verbal Presentation 3 Preparation CEAC Presentation Verbal Presentation 4 Report Writing CEAC report editing Final Report Electrical Design Decisions Final Report Electrical System Architecture Choosing and Ordering Parts Arduino Software Development Setup Program Version 1 Program Version 2 Program Version 3 Program Version 4 Program Version 5 Team Meetings and Planning 1 hour per week (14 weeks) Electrical Assembly Receiver to Arduino connectors Wiring in electrical housing Prototype Testing Developing Test Plan Conducting Tests Electrical Repairs Other Small Tasks Total Hours

69

3 3 3 1 3 6 4 6 21 4 2 1 3 1 4 4 1 8 10 5 2 7 2 3 2 1 14 3 4 1 6 5 5 174

10.4 Jordan Newhof Hour Log Task Disassembly & Design Disassembly of wheelchair CAD Design Mechanical Design Meetings Mower Measurements Motor Mounting plates Battery Charger Experiment Assistance with Mech. Fabrication

Hours 1 3 9 1.5 3.5 1 25

Deliverables Team Blurb Project Brief Senior Design Program Description Update Poster Verbal Presentation 1 Verbal Presentation 2 CEAC Review Presentation Ethical Issue Paper Design Night Presentation

3 4 3 2.5 2 2.5 2 4.5 3

Design Report Research necessary report components Initial Report Outline Introduction Project Management Overall Design Requirements Design Decisions System Design Testing Project Summary Review for draft Review revisions Content Addition Final Draft Review

3 3 4.5 2.5 3 6 7 1 4 1.5 1.5 4.5 9

Meetings Team Meetings (14 weeks) Presentation Prep CEAC Review Client Meeting (4 meetings)

14 10 1 3

Total Hours

70

149