Robotics

Lego: A Gentle Introduction to Robotics

Linto Antony [email protected]

Lego: A Gentle Introduction to Robotics by Linto Antony Copyright © 2003 by Linto Antony Revision History Revision 1.0 2 Nov 2003 Revised by: LA Lego: A Gentle Introduction to Robotics

Table of Contents Acknowledgement............................................................. i 1. Introduction .................................................................1 What is Lego? .................................................................. 1 What is a Lego Mindstorm? .............................................. 1 LEGO Mindstorms Robotics Invention System.................. 1 Bricks that think ............................................................. 2 2. The Hardware................................................................5 Overview .......................................................................... 5 Hardware ......................................................................... 5 A Brief description of the Programmable Brick ........... 5 RCX........................................................................... 5 IR Transceiver ........................................................... 6 Cables ....................................................................... 6 3. LegOS ...........................................................................9 LegOS .............................................................................. 9 The LegOS kernel............................................................. 9 Starting the kernel..................................................... 9 Timing ..................................................................... 10 Notions of kernel image, user programs, and tasks .. 10 Memory management .............................................. 10 Semaphore facility ................................................... 11 IR Networking .......................................................... 11 4. How to program Lego ..................................................13 Code Examples .............................................................. 13 Hello, World ............................................................. 13 Controlling Motors ................................................... 13 Touch Sensor .......................................................... 14 Light Sensor ............................................................ 15 Basic Threading....................................................... 15 Playing Sound ......................................................... 16 5. Conclusion..................................................................19 A. References ..................................................................21

iii

iv

Acknowledgement This document was prepared to give an overview about the new trend in the Robotic world; Lego Mindstorms. I’m indebtedly grateful to the Head Of The Computer Science DepartmentDr. Agnisarman Namboodiri for his support. I’m grateful to Lect. Sminesh for his valuable guidance and suggestions. I’m also grateful to all other faculty members of Computer Science Department for their support. I gracefully remeber all students of S7 CSE-A.

i

Acknowledgement

ii

Chapter 1. Introduction The Lego Mindstorm Robotic Invention System is one of the most fascinating technological innovations in recent years.The LEGO Group is a major sponsor of research at the Media Lab, and it is fair to say that the LEGO RCX Brick is inspired by the MIT(Massachusetts Institute Of Technology) Programmable Brick and related research. However, the RCX Brick was designed "from the ground up" by the LEGO Group and is not based on any of the technical work in the MIT Programmable Brick. The two devices use different CPUs and have entirely different software systems behind their operation. Lego robotic systems are used to teach programming, artificial intelligence and control theory in some of the best universities in the world - including our own IIT’s (IIT Kanpur is currently coordinating an interesting project called BRICS).

What is Lego? Lego is the world’s largest toy manufacturing company. They make ‘building blocks’ using which kids construct things out of their imagination - the sheer number, variety and ingenious design of the blocks makes it possible to build virtually anything we can think of.

What is a Lego Mindstorm? Building ‘intelligent robots’ is one of the most fascinating of all human endeavours - it is a result of our desire to embed a part of ourselves in inanimate objects and breathe ‘life’ into them. Robot building was confined to university research departments and big industrial houses, as few individuals had the resources to design and build their own systems. Lego, together with researchers at MIT’s Media Lab, changed all that in the year 1998 with the introduction of their ‘programmable brick’. The ‘programmable brick’ is a small Hitachi microcontroller with 3 motor outputs and 3 sensor inputs. This, combined with standard Lego building blocks and additional parts like wheels, pulleys, gears etc (all made of plastic) can be used to build fascinating programmable robotic systems. The user builds the robot, writes a program on the PC and downloads it onto the ‘brick’ - the robot now becomes fully autonomous and moves and acts according to the program stored in the brick.

1

Chapter 1. Introduction

LEGO Mindstorms Robotics Invention System When LEGO released their robotics series, it caused a huge stir both with children and also with adults. The appeal of bringing your creations alive has been central to LEGOs strategy in the toy market - now, take a good idea and then add a complete new layer of flexibility, a progammable microcontroller, the appeal just soars! Now, take an excellent software package, and a clearly illustrated book to help the user start, and you have a sure winner. Indeed, I’ve never had more fun with a toy in my life. RIS revolves around the RCX microcontroller (shown right). This little beauty can store up to five programs simultaneously, and can have programs downloaded to it via an IR transmitter. It can even communicate with other RCX controllers! In your kit, you get one of these controllers, two touch sensors, two motors, and a light sensor. Also included are a huge variety of LEGO pieces, axes, bolts, wheels, wires and other related building materials. It is then your choice whether to build and program the LEGO challenges, or explore everything yourself. The inventors are not part of a big research university or at some forward-looking computer company, but a loosely knit group of engineers who build gizmos out of Lego bricks. Developed by the Billund-based Danish toy company, some bricks contain microcomputer-controlled sensors and motors, and all are components of a three-year-old product called Lego Mindstorms. Originally intended for use by children at home or in the classroom, Mindstorms has instead enraptured tens of thousands of adults. Engineers, academic researchers, and hobbyists have reverse-engineered its firmware, hacked together new programming environments, and shared recipes for unanticipated ways of connecting it to the outside world.

Bricks that think Lego and the engineering community share a long history. As far back as the early 1980s, automotive engineers reportedly used the brightly colored interlocking bricks and hinges--then years away from built-in microcomputers--to make scale models of proposed vehicle assembly lines. In 1984, the company began working with researchers at the Massachusetts Institute of Technology (MIT), in Cambridge, to adapt its motors, switches, and other parts for computer control. Indeed, the RCX controller that serves as the brain of every Mindstorms robotic gizmo is based on what the company learned by sponsoring the development at the MIT Media Laboratory of a "programmable brick." RCX stands for robotic command explorer. It is an oversized Lego brick [left], a little larger than two stacked decks of playing cards. On the outside are a liquid-crystal display (LCD), four 2

Chapter 1. Introduction control buttons, and six wiring connectors. Inside are a Hitachi H8 central processing unit (CPU) with 32KB of RAM, space for batteries, and an infrared transmitter and receiver, which communicate with a base station plugged into a PC port. Three of the external wiring connectors are outputs, each supplying a user-controlled voltage to a motor, a lamp, or anything else that will run off half a dozen AA batteries. The remaining connectors can read data from a variety of sensors (one light and two touch sensors, all housed in Lego-style bricks, are included with the basic kit, along with two stepper motors). The LCD can display motor settings, sensor readings, or diagnostic information about the status of the RCX and its software. To construct a working gizmo, a Mindstormer typically builds the mechanism out of the RCX, motors, sensors, wire, and the roughly 700 bricks, beams, gears, axles, and wheels included with the basic kit, which lists for US $199. Then, to program the RCX, an aspiring inventor runs a graphical environment provided by Lego for the PC. By connecting blocks representing commands such as READ SENSOR LEVEL or TURN MOTOR ON, he or she creates a program, downloads it to the RCX by an infrared link, and pushes the appropriate control button to start it running. The PC sends the program to the RCX in the form of byte-codes. These are sequences of 8-bit instructions that specify such actions as turning on a motor, waiting for input from a sensor, or jumping to another point in the program. A little more than half the RCX’s 32 KB of RAM is taken up by firmware--downloaded from the PC when the RCX is first powered on--that interprets these byte codes and executes the H8 machine code that actually makes things happen. From these relatively simple parts, hobbyists have managed to build a mind-boggling array of widgets. The list includes robotic trash trucks that pick up and deposit miniature dumpsters, rolling and walking robots, a light-bulb changer, and maze-following robots. Hundreds of Web pages put up by fans around the globe supply the details for building these creations. That Mindstorms is such a hit with adults does not strike Fred Martin as strange. Said the former MIT researcher, who played a pivotal role in the programmable brick development, "It’s the toy I would have wanted to have" when he was younger, and he still wants it today. The goal of MIT’s research, Martin said, was to create "materials that make ideas accessible," and the programmable brick does exactly that for robotics and for computer programming. People who use Mindstorms learn by building something tangible. Above all, Mindstorms makes ideas about robotics accessible to 3

Chapter 1. Introduction people without sophisticated mechanical-engineering skills or access to a machine shop. With Mindstorms, anyone can build computer-controlled mechanisms by simply snapping parts together. Where an earlier generation of hobbyists struggled to build parts to the submillimeter tolerances required for a working robot arm, "Lego parts are made to exact lengths," with an accuracy measured in tens of micrometers.

4

Chapter 2. The Hardware Overview The base system for using the RCX consists of the RCX itself, an infrared transceiver, and a PC. Additional components, such as motors, sensors, and other building elements, combine with the base system to allow the creation of functional autonomous robotic devices. At the core of the RCX is a Hitachi H8 microcontroller with 32K of external RAM. The microcontroller is used to control three motors, three sensors, and an infrared serial communications port. An on-chip, 16K ROM contains a driver that is run when the RCX is first powered up. The on-chip driver is extended by downloading 16K of firmware to the RCX. Both the driver and firmware accept and execute commands from the PC through the IR communications port. Additionally, user programs are downloaded to the RCX as byte code and are stored in a 6K region of memory. When instructed to do so, the firmware interprets and executes the byte code of these programs.

Hardware A Brief description of the Programmable Brick The Programmable Brick, which forms the heart of the robotic system, is nothing but a Hitachi microcontroller and 32KB of RAM, with some motor driver and sensor/IR circuits, packed in a small yellow coloured Lego block. Programs and data are stored on the RAM. When the brick is powered up, it executes code stored in about 16K of ROM. This code either transfers control to an operating system (called ‘firmware’ in Lego Literature) loaded in RAM, or, if firmware is not present, it gets ready to download one through an IR link from the PC. The RAM is battery backed - even when the unit is powered off, the battery keeps powering the RAM.

RCX To remove the circuit board, open the RCX, remove the four screws, remove the IR shield, then separate the front cover from the circuit board. Release the two battery contacts from the battery side, then slide the circuit board free. Do this at your own risk; if you’re not careful there’s a chance you’ll break something. You can also twist the circuit board free without first removing the battery contacts. 5

Chapter 2. The Hardware The negative battery contact is somewhat easier to release than the positive one. By removing the negative contact first, then sliding the circuit board free, you will be able to remove the positive contact from the circuit board side by inserting a small screwdriver in the slot between the contact and the battery case and gently prying the contact outward and upward. The parts, from left to right, starting with the top row: circuit board, front cover; IR shield, screws, battery contacts; battery case, battery cover. Two removable plastic ribs are sitting on the battery case. The top of the circuit board. Two IR LEDs and an IR receiver are on the left, an LCD is in the center, and several capacitors and an adapter socket are on the right. The LCD is covering an LCD controller and a speaker; above and below the LCD are the contacts for the four rubber keys. Twelve clamps rise from the circuit board to mate with the input and output ports on the cover. Flops, the small chip above that is a bank of NAND gates. The large square chip is the microcontroller, the chip below that is a RAM. The large chip in the middle is a bank of flip.

IR Transceiver It is also possible to disassemble the IR transceiver. Simply remove the four screws and lift off the back cover, making sure to move the battery spring out of the way. From left to right, starting with the top row: front cover, back cover; circuit board, screws. The front cover doubles as an IR filter. A switch and a 6.7 ohm resistor are on the left, the two chips in the middle are banks of NAND gates, and the grey circle in the lower right is a voltage regulator. The white rectangle below the chip in the upper middle is a green LED, and hanging off the right side of the chip are the IR receiver and two identical IR LEDs. The DB-9 serial connector is on the opposite side of the circuit board.

Cables The cable to connect the IR transceiver to your PC is a null modem cable that contains six wires, of which only five are used. With the larger row on top, the holes of each connector are numbered from right to left starting with the top row. Pins inserted into these holes are joined by the cable to pins inserted into the holes of the opposite connector as follows: Pin To Name Description 2 3 RD Receive Data 3 2 TD Transmit Data 5 5 SG Signal Ground

6

Chapter 2. The Hardware 7 8

8 7

RTS Ready To Send CTS Clear To Send

Pin 4 connects to pin 4 but is unused; pins 1, 6, and 9 have no connection. The RTS/CTS signals are not used for flow control. Instead, they are used by the PC to check whether or not the transceiver is connected. The transceiver wires CTS and RTS together; the PC checks for the transceiver by asserting and deasserting RTS. If it sees that CTS tracks RTS, then it assumes that the device sitting on the serial port is the transceiver.

7

Chapter 2. The Hardware

8

Chapter 3. LegOS Remaining part of this document is an introduction to a reasonably powerful embedded operating system called LegOS which makes Lego coding lots of fun for the serious Linux/C hacker. Through some very simple (and interesting!) programming examples, I demonstrate how Operating System concepts like concurrency and multithreading are employed to control real world mechanical systems.

LegOS It is possible to set up a cross development environment under Linux to develop Lego applications. A port of the GNU compiler to the Hitachi controller is available. Once that is installed, you can get a copy of a small embedded operating system called ‘LegOS’ and compile it. The compiled LegOS can be downloaded to the programmable brick by using the command ‘firmdl3’ (which comes with the LegOS distribution). Once the OS (firmware) is downloaded, you can write C programs, compile them into .lx files (that’s the extension which LegOS uses - the C programs can’t be compiled into native Linux format because they are not going to be executed on Linux) and download them onto the Lego brick by using the ‘dll’ command. There are 8 slots available for storing programs on the Lego brick - using an option to the ‘dll’ command, you can store your program in whichever slot you like. Execution of the program is done by first using the ‘Pgm’ button on the Lego brick to choose a program and then pressing the ‘Run’ button. LegOS supports multiple, preemptible threads. A 16 bit timer generates an interrupt once every 1ms or so - the interrupt handler sets motor speed and direction, checks sensor values, looks at whether the time consumed by the currently executing thread has exceeded the set quantum, and if so, stops that thread and chooses a new one for execution. Knowing this much of architectural detail is sufficient to becoming productive with LegOS. Let’s look at some code.

The LegOS kernel The LegOs kernel is monolithic, as all of the source are compiled together in a system binary image. The interface for the user programs are given in a dynamic linker script, which contains all the linker scripts.

9

Chapter 3. LegOS

Starting the kernel The kernel starts when kmain() is called by the ROM. This function initialises the kernel, before it is started in either single tasking or multitasking mode. During kernel initialization, 3 tasks are started. The first is the idle task, which is a dummy task with the lowest priority. The next two tasks implement the dynamic program loading capabilities. They are called packet_consumer() and key_handler() and are run with the highest priority. The packet_consumer() handles the activity on the IR-port, and the key_handler() handles activity on the buttons of the RCX brick. The task manager is then started, and the kernel starts switching between the execution among the 3 tasks. New tasks are started by downloading and starting userprograms.

Timing LegOS is driven by the interrupts from the 16-bit timer, which is configured to make an interrupt every millisecond. The timer interrupt is handled by a ROM function pointed to by the ocia_vector. This vector points to the systime_handler() function, which in turn polls all the various subsystems in turn, by calling their handlers. This means that LegOS is polling instead of interrupts, to communicate with the enviornment.

Notions of kernel image, user programs, and tasks The kernel is a separetly compiled binary image, which exports a set of symbols in a linker script called legos.lds. The kernel image is located in the beggining of the RAM. User programs are compiled separetly, and linked with the kernel symbol table, creating the relocatable format .lx. The final linking of the absolute are done when downloading the program with dll, after the absolute address of the text segment of the program is known. User programs are downloaded to the part of RAM following the kernel image part. Tasks are started by a user program as threads in the address space of the user program. As user programs are compiled in seperate address spaces, user programs cannot start threads in other user programs.

Memory management LegOS uses a straightforward continuous allocation scheme for 10

Chapter 3. LegOS managing the memory. There is no support in the RCX hardware for advanced memory management schemes such as paging or segmentation. Furthermore, the memory is a scarce resource, so advanced memory management scheme would imply much overhead. The memory is divided into the kernel part and user program part. The kernel code and static kernel data are located in the lower part from address 0x8000 up to address mm_start, which is a global variable. The memory manager has the responsibility of all the memory from address mm_start to address 0xFFFF. The memory is organised in memory blocks which consist of a 4-byte header and an even number of data bytes. The first two bytes of the header contain the process id of the process which has allocated the block. If the block is unallocated, the value is MM_FREE which is defined to be zero. The last two bytes of the header hold the size of the data block following the header. The unit of memory used by the memory manager internally is 2 byte words. As memory is initialized the memory is divided into blocks for different devices and some free blocks. Memory is allocated using the function malloc(). Malloc() will search for the first free block of suitable size. Memory is deallocated by adding the memory block to the adjacent free block suing the free() function.

Semaphore facility LegOS provides a semaphore facility which implements the classical Dijkstra definition of semaphores. The semaphores are counting semaphores. Besides the usual wait and post operations, a non-blocking wait called sem_trywait() and an operation for reading the semaphore value, sem_getvalue() are provided. The wait operation is implemented using the wait_event() function, which puts the process in the sleep mode, and specifies a wake-up function.The wake-up function used with semaphores is sem_event_wait(), which checks the value of the semaphore.

IR Networking LNP is the LegOS Network protocol which is used by user programs to communicate over the IR port. In LegOS two kinds of LNP packets can be used. The first one is called the integrity packets, which contain no addressing information, and is used for broadcasting packets. The second one is the addressing packet, which provides a UDP like service. 11

Chapter 3. LegOS To communicate with a PC running Linux, a daemon called lnpd is used. Lnpd allows multiple clients to connect to an RCX running LegOS. Furthermore a library called liblnp exists, so applications can connect with remote lnpd daemons.

12

Chapter 4. How to program Lego Here in this chapter I have tried out some sample codes that can be run on Lego using LegOS. I have given it over here to innovate you to think freely about different devices and Artificial Intelligence.

Code Examples Hello, World #include #include int main() { /* Space on the LCD display * to print only ‘hello’ (ie,5 char’s).*/ lcd_clear(); cputs("Hello"); while(1); return 0; }

The classical Hello, World. The message is visible on the LCD screen of the Lego Brick.

Controlling Motors #include #include #include #include



int main() { motor_c_speed(40); motor_c_dir(fwd); sleep (10); motor_c_speed(off); return 0; }

The motors can be connected to any of the three input ports A, B or C. The speed can vary from 0 to 255. Only when the direction is set does the motor start turning. The direction can be one of the enumerated values fwd, rev, off or brake. 13

Chapter 4. How to program Lego

Touch Sensor The kit comes with two touch sensors. Here is a small program which monitors the state of the touch sensor connected to sensor input 1. #include #include #include #include



/* When depressed, it is 1, else * it is zero */ int main() { while(1) { lcd_clear(); lcd_int(TOUCH_1); /* 500 milli sec sleep */ msleep(500); } }

Here is another program: #include #include #include #include #define MIN_ENV_SPEED 40 /* Turn off motors when either * touch sensor is touched */ int main() { motor_a_speed(MIN_ENV_SPEED); motor_c_speed(MIN_ENV_SPEED); motor_a_dir(fwd); motor_c_dir(fwd); while(1) { if (TOUCH_1 || TOUCH_2) { motor_a_speed(off); motor_c_speed(off); break; } } return 0; }

14

Chapter 4. How to program Lego Instead of turning off the motors, we can also think of reversing the direction.

Light Sensor The unit comes equipped with one light sensor, which operates in either active or passive mode. In active mode, the light sensor operates under the glow of a small LED - it is ideal to find out reflectivity changes of near objects. In passive mode, the sensor operates under environmental illumination. #include #include #include #include #define MIN_ENV_SPEED 40 int main() { /* My desk shows a reading of * around 87. The dark side of * a floppy around 40 and the * metallic part of a floppy * around 124. */ /* Get into active mode */ ds_active(&SENSOR_2); while(1) { msleep(750); lcd_int(LIGHT_2); } }

Basic Threading A new thread is started by the execi call, which is given the address of the function to be executed as the new thread, an argument count and and argument vector, priority (maximum 20) and a stack size, which is set to a default value. #include #include #include #include #define PRI 10 /* Main and fun1 execute in * parallel */ int c = 0;

15

Chapter 4. How to program Lego int fun1(int argc, char **argv) { while(1) { cputs("Fun1"); msleep(250); } return 0; } int main() { execi(&fun1, 0, 0, PRI, DEFAULT_STACK_SIZE); while(1) { cputs("Main"); msleep(450); } return 0; }

Does the termination of the main thread result in the stopping of the ‘child’ thread? No, that’s what I gather from the output of the following program. It keeps on counting. #include #include #include #include #define PRI 10 /* What happens when main dies? * Child thread does not stop, it * keeps on running. */ int c = 0; int fun1(int argc, char **argv) { while(1) { lcd_int(c++); msleep(700); } return 0; } int main() { execi(&fun1, 0, 0, PRI,DEFAULT_STACK_SIZE); sleep(5); return 0; }

16

Chapter 4. How to program Lego

Playing Sound Playing musical notes is simple. #include #include #include #include #include note_t music[] = { {PITCH_D4, 5}, {PITCH_END, 0} }; int main() { cputs("start"); dsound_play(music); wait_event(dsound_finished, 0); cputs("end"); return 0; }

We define an array of structures, each structure having two fields, a pitch and duration (seconds). The pitch is defined in standard musical notation (and is internally converted to real frequency). The array should end with the structure {PITCH_END, 0}. All the pitches in the array are played, till {PITCH_END, 0}. The standard function dsound_finished returns true only when the hardware finishes playing the tunes. All these examples are to be compiled using a gcc cross compiler for the Hitachi H8/300 platform. The programs are compiled and should be downloaded into Lego using an Infrared port and a command called dll.

17

Chapter 4. How to program Lego

18

Chapter 5. Conclusion The potential of using robots in the classroom has intrigued educators for as long as robots have been a part of our culture. Economic constraints, however, have demanded patience on our part; robots were initially too expensive and too difficult to work for large-scale classroom use. Over time, however, technology becomes better, cheaper, and easier to use. If we are patient, devices originally suited for research labs and highly trained proffesionals may become cheap enough and user-friendly enough for classroom use. Lego Mindstorms have contributed such an innovation in the computing world. Since Robotics has now becoming so important in the world of computers. It has now grown into a seperate discipline in western countries. I hope that the future curriculum of our universities will contain Robotics. Then there will be a sure place for the Lego Mindstorms in our colleges.

19

Chapter 5. Conclusion

20

Appendix A. References •

IEEE Robotics and Automation Magazine.

•

http://www.legomindstorms.com/

•

http://www.generation5.org/

•

www.cs.uu.nl

•

www.usafa.af.mil

21

Appendix A. References

22