Design Of Controller

Design of Open Architecture Controller for PC-based NC Machines

Abstract Open Architecture Controller (OAC) is currently a popularly used concept in automation and control industry since computer was widely in system control. The benefits of ease of operation, interfacing, networking, and system diagnostics are possible when machines can combine the technologies of CNC and PC. This paper presents an introduction of a simple controller into existing machine in order to control tool path through program modules and control algorithm from a PC.

1. Introduction Designs of today’s CNC machine are developed with the capabilities such as multi-axis control, error compensation and multi-process manufacture (e.g. combined mill/turn/laser and grinding machines). Programming task is increasingly more difficult to fulfill these capabilities due to the fact that G-code, a 50-years-old language, is still employed by these machine tools. Thus the machines themselves become less adaptable. However, the trend towards open architecture control, based on OSACA and open modular architecture controller (OMAC), where third party software can be used at the controller working within a standard windows operating system, can overcome this problem. Remarkably, the application of software controllers, where PLC logic is embedded in software rather in hardware, presents a recognizable industrial development [1]. The author also mentioned lack of adaptability, portability, and intelligence of modern machine tools using G-code (ISO 6983) for part programming and presented advantages of part programming using STEP-NC which has more open and adaptable architecture, making it easier to integrate with other manufacturing facilities. Developing open architecture computer numerical control result a kind of software architecture that fits in with a general computer and is independent of a control vendors, while the same time, a communication standard among computer hardware, an operating system and application software must be built [2]. An open system, as defined by IEEE (IEEE 1003.0, 1990), provides capabilities that enable properly implemented applications to run on a variety of platforms from multiple vendors, interoperate wit other system applications and present a consistent style of interaction with user. Closed architecture design of many advanced CNC controllers and supporting hardware, such as those

of Siemens, Allen Bradley, Fanuc, and Mitsubishi, make researchers in this area difficult or impossible to incorporate advanced control schemes within the CNC itself [3]. This problem can be eliminated by designing open architecture controllers as a flexible control system which can be attached to a wide variety of machine tool systems. An open architecture controller should be flexible in hardware as it is in software control levels. It must be standard to allow hardware and software developed by any engineer or technician, and its integration with other controllers, cell control systems and high level planning systems. The machine tool open controller should permit the integration of independent application program modules, control algorithms, sensor and computer hardware developed by different manufacturer [4]. An open architecture system can be either hardware or software. The openness of hardware is limited due to the compatibility of software and hardware. The openness of software is usually easily done by adding some module available. Therefore software development is widely done nowadays. 2. Open Architecture Platform In this research the base system is a NovaMill 3-axis milling machine model 1996. Timing belts and ball screw system are used in transmission system transferring the torque from axis motor (stepper motor) to the table. Motors used are bipolar stepper motors having 50-pole rotor, allowing 200steps/rev by using full step control. Commercial stepper motor drivers G201, manufactured by Geckodrive Inc, are equipped with each motors since this product provides space saving, high power output, and easiness to use [5]. The driver accepts clock pulses and direction signals and translates these signals into appropriate phase currents in the motor.

Communication interface of the system is supported by a personal computer, Pentium Pro 200MHz processor, 62MB RAM; through 25-pin parallel port (or LPT port) with +5V output voltage. Data port H378 is assigned as the base address of LPT1 for outputting data on the parallel port’s data line (pin 2 to 9). Pin 25 supplying +5VDC TTL logic for the transformer in the controller is connected from pin 5 on the mouse port which carries +5VDC.

n = 1... N

The software with built-in Graphic User Interface (GUI) in this research is coded in Visual Basic 6.0 environment. To access LPT port in Visual Basic, port.dll library (for Windows 98 only) is needed to add in to the system folder C:\Windows\System.

 x − x A yB − y A z B − z A   N > Max B , ,  ∆f x , y , z ∆f x , y , z ∆f x , y , z   

The incremental movements must be greater than machine resolution

∆f x , y , z in order to ensure tool

movement is done perfectly in each increment.

 x − xA yB − y A z B − z A  ∆f x , y , z ≥ Max B , ,  Or N N   N

Circular arc interpolation is a kind of non linear movement which can be simplified into linear one:

∆f x , y , z ≥ Max ( Rdϕ cos α , Rdϕ sin α ) Since

Max(sin α , cos α ) = 1

⇒ ∆f x , y , z ≥ R ⇒N≤

a −b N

R ( a − b) ∆f x , y , z

Figure 1: Electrical connection 3. Interpolation Algorithms Two common methods in linear multi axis control are serial and concurrent control. Serial multi-axis control is simple by controlling each motor separately either the X-axis or Y-axis, during the operation. However accumulative machining error and jerky movement of the table are still the major problems which can be overcome by the second method. Instead of driving the axes sequentially, concurrent control drives the motor simultaneously so that the tool could move absolutely straight along the line. The algorithm is quite complicated due to difference of the speeds of each motor, meaning that each stepper motor has to start as well as stop its movement at the same time with the other motors.

Figure 2 Circular arc descriptions The Bezier curve is a parametric curve which appears reasonably smooth at all scales as opposed to polygonal lines, which don’t scale nicely. Bezier curves can be interpolate by a set of many linear segments made by Qi(t).

Line is composed by linear movement of small segments longer than the resolution of the machine called Basic Length Unit (BLU) ∆f x , y , z :

xB − x A N y − yA y (t ) = y (n∆t ) = y A + n. B N z − zA z (t ) = z (n∆t ) = z A + n. B N x(t ) = x(n∆t ) = x A + n.

Figure 3 Intermediate points on the Bezier curve n

Q(t ) = ∑Vi Bi , n (t ) i =0

Figure 4 Repeatability accuracy

n Bi , n (t ) =  (t )i (1 − t ) n − i , 0 ≤ t ≤ 1 i n n!   = , i = 0,..., n  i  i!(n − i )!

The accuracy of circular movement is tested by machining 5 circles which have the designed diameter of 20mm. The diameter D1, D2 and D3 at three different positions are measured.

The smaller the increment of t, the smaller ∆x, ∆y , and the finer the interpolated curve. The algorithm is summarized in three steps as follows:

Mean diameter: 20.018 Variance: 5 2 ∑ ( D − D ) = 0.004 i 5 −1 j = 1 1

σ=

-

Step 1: Define (N-1) intermediate points along the entire path

-

Step 2: Check whether the tool maintains the

So, D = 20.018±0.004

contact along the entire path ∆x, ∆y

Or the error is 18µm ± 4

≥ ∆f x , y , z .

Only the points which satisfy the condition are selected to be intermediate points on the final tool path.

-

D1

Step 3: Tool path forming: connect all the selected Qi(t) in the step 2 to form the final tool path.

D2 D3

4. Experimental results and discussion In this research, basic interpolations previously described are used to generate tool path for 2.5D contour and 2.5D pocket with our milling machine NovaMill. The dimensional accuracy of linear, circular arc and Bezier curve movements were tested via actual machining. The experiments to testify the efficiency of over-cutting method is also presented. The experiments are conducted on aluminum alloy. All dimensions are measured on a Computerized Measurement Microscope which has a resolution of 0.1 micrometer. To test repeatability of the machine, the tool follows the trajectory mad of 10 horizontal lines (15mm) and 10 vertical lines (15mm). The repeatability on Xaxis and Y-axis are given:

Rx = 0.005mm Ry = 0.010mm

Di ∆D i

Circle 1 Circle 2 Circler 3 20.021 20.013 20.0351 2 2 20.009 20.0311 20.0212 5 20.012 20.020 20.0203 5 4 20.014 20.021 20.0255 4 6

Circle 4 Circle 5 20.028 20.0190 6 20.025 20.0090 7 19.995 20.0116 8 20.016 20.0132 7

0.0136

0.0418

0.0191

0.0191

0.0117

Roundness: R=

1 5 ∑ ∆D j = 0.021 5 j =1

To measure the machining accuracy of Bezier curve, 5 experiments are carried out to find the coordinate of 5 different point at t=0, 0.25, 0.5, 0.75, and 1.0. The errors on X-axis and Y-axis are then charted in the figure below. Bezier curve machining accuracy 0.04 0.03

0.027 0.0203

0.02 0.0105

0.01 0

0 1

-0.01

point 2 -0.009 -0.013

3

4

5

Error X Error Y

-0.0196

-0.02 -0.03

-0.033

-0.04

-0.041

-0.05

Figure 5 Machining accuracy of the Bezier curve To explain the results, there are several factors affecting the accuracy and repeatability of the system: -

Mechanical part: the axes are driven by stepper motors which have limitations on accuracy. In order to enhance the accuracy and repeatability, DC servo motor is

recommended because of higher accuracy. The ball screws also have back lash which affects the repeatability of the machining

-

Computer interface: pulse signals sent to drivers may have been lost on LPT port interface. This problem also causes inaccuracy of machining. Since in this research, open-loop control is used, this problem cannot be overcome.

However, the following figures show the ability of the system to machine a 2.5D impeller and a Bezier swept surface which can be further improved by implementation of both hardware and software.

Figure 6 2.5D impeller

Figure 7 Bezier swept surface 5. Conclusion An Open Architecture Controller has been implemented on a milling machine. Hardware and software are integrated into the system. The openness allows the system to be expanded easily. Although tool path generation is limited to 2.5D contours and 2.5D convex pockets, application modules can be easily added in future works. The experiments were conducted to validate the accuracy, repeatability, and machinability of the system. Machining hard material is still a major limitation of the system because the table is currently driven by stepper motors, which can only work at low torque. Next, the work piece size is limited to the working

envelope (229x152x114). Further possible development is suggested to work with both hardware side and software side.

References

1. X. W. Xu, S.T Newman (2006), “Making CNC

2.

3.

4.

5.

machine tools more open interoperable and intelligent – a review of the Technologies”, Computers in Industry, Vol 57, pp. 141-152 Wang Yuhan, Hu Jun, Li Ye (2003), “Study on a reconfigurable model of an open CNC kernel”, Journal of material processing technology, Vol. 138, pp. 472-474 Stephen J. Rober and Yung C. Shin (1995), “Modeling and control of CNC machines using a PC-based open architecture controller”, Mechatronic, Vol. 5, No. 4, pp. 401-420. O. L. Asato, E. R. R. Kato, R. Y. Inamasu, A. J. V. Porto (22-26 November 1999), “Analysis of Open CNC Architecture for Machine Tools”, 15th Brazilian Congress of Mechanical Engineering, Brazil. Manual document for G201 driver, www.geckodrive.com