Microprocessors Systems for Motion Control

Copyrigth e IF AC Motion Control for Intelligent Automation Perugia, Italy, October 27-29, 1992

MICROPROCESSORS SYSTEMS FOR MOTION CONTROL. T. OHMAE T.and M. WATANABE

Hitachi Research Laboratory. Hitachi Ltd. 4026 Kuji-cho. Hitachi-shi. Ibaraki-ken 309-12. Japan

Abstract. This paper presents a review of the recent advances in microcomputerbased motion control systems. At first. the trends of microcomputers and systems are described. As a result, it was clarified that multiple microcomputer systems are suitable for motion control. Rolling mill main drives and robots are used as the application examples and their hardware configurations are described. Furthermore. some of the future technical trends of the above systems are also descri bed. Key Words. microcomputer, motion conntrol, rolling mill drives, robots, multiple microcomputer system, digital conntrol, digital signal processor

1. INTRODUCTION

Remarkable progress has been made in large-scale integrated(LSI) circuit technology in the past 20years, and new microcomputers are continually being placed on the market. Improvements in the functions. reliability, size and cost of control equipments can be expected from the use of microcomputers,which are increasingly being applied in many kinds of motion control systems such as roll ing mi 11s, roll ing stocks, elevators, electric vehicles, and robots (1). As the performance of microcomputers is improved, the application to motion control systems is expanded.RecentlY,microcomputers are essential to all motion control systems and many optimized systems are being developed The general configuration of a motion control system is shown in Fig.I.The system has at least one servo system to control the mechanical motion. In the early stages

of microcomputer-based motion control systems, only one microcomputer was used for the sytem control which did not require high-speed processing. As faster microcomputers are being developed. innner control loops, such as the speed and current control loop. can be easily implemented by software processing. However, the internal loop must be executed in a shorter sampling period than the major 100p(2). In order to process such fast-response internal control loops with microcomputer software, a new system configuration must be considered. ,..... ................................... ·······seAVO·Sy·S·m .·····

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From this point, this paper will explain the fully digitalized speed regulator and servo system using microcomputers.At first, historical trends of microcomputers and their application to rolling mill drives are described. As application examples, microcomputer-based motion control systems for rolling mills and robots are shown. Finaly,the future trends are also presented from the view point of the hardware and software configuration.

2. MICROCOMPUTERS Microcomputers have moved from the 4-bit word length of the original 4004 processor from Intel Corp., to 8bits, addressing 28 bits of memory, and then 16bits. Nowadays 32bits of addresses and data are a common figure(l). The trends in the performance of microcomputers are shown in Fig.2. The millions of instruction per second(MIPS) of microcomputers are remarkably increased and nowadays, the processing speed of microcomputers is readily available for practical use. However, the high device cost remains the main problem. Fig.3 shows the execution time of a proportional plus integral algorithm which is frequently used in a motion control system. The figure also shows that the processing speed of the microcomputer is not an important factor . RecentlY,reduced instruction-set conputer (RISC) machines, such as the 80960 from Intel Corp. ,have been developed. These RISC

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type microcomputers are faster than complex instruction-set computer(CISC) machines as shown in Fig.2. In addition to the progress of microcomputers, one-chip microcomputers, such as the H8/500 series from Hitachi Ltd., have employed many kinds of counters and highaccuracy A/Dconverters. The series expansion of the H8/500 microcomputer is shown in Fig.4. These functions are very useful to simplify the hardware circuits of a motion control system. Even now, the functions of peripheral circuits that are included in one-ch ip microcomputers, are continually being expanded and the memory size is also be i ng increased. Furthermore, digital signal processors (DSPs),such as the TMS32025 from Texaslnst. are very suitable for motion control 100

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systems. In the eary stages, DSPs were used in a specialized purpose as a co-processor of a microcomputer, such as the digital filter of a pattern recognition. However,as the memory size and the number of instructions are being increased, DSPs are being adopted in many motion control applications which used the features of the high-speed multiplication and addition.

ments. When the Ac drive system was used to rolling mill main drives, the microcomputerbased speed regulator included additional high-speed microcomputers, such as DSPs, to the 16bits multiple microcomputer system. The reason is that the software processing time needed to implement the vector control algorithm of Ac motor drives was increased. The most advanced drive system for rolling mill main drives has applied a vector controlled GTO inverter AC drives. In the application the multiple microcomputer system wi th both 16bi ts and 32bi ts was employed(4). From these trends, the multiple microcomputer system is commonly used in the recent micro-computer systems for motion control. The main reasons are the low cost and the increase in the diversity of functions to be processed by the microcomputer.

3. MICROCOMPUTER-BASED MOTION CONTROL SYSTEM 3.1. Historical trends of application For rolling mill main drives, a very high speed response is demanded from the speed regulator. Therefore, rolling mill drives wi II be used as an appl ication example in the fa Ilowing. The historical trends of microcomputerbased controllers for rolling mill main drives are shown in Fig.5. A fully digi talized speed regulator for rolling mill main drives was developed using a 16bits microcomputer in 1980 and many advantages, such as high-accuracy, low-cost and high-stability in the manufacturing plant, were clarified(3).Since the first production in 1981, the microcomputer-based speed regulator became essential to rolling mill main drives. Recently, the demand for multiple microcomputer systems is increasing rapidly to meet higher accuracy and response requireYEAR ACCURACY (ERROR) RESPONSE

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3. 2. System configuration There are many items that require clarification for a speed regulator that uses a microcomputer. One item is the multiple microcomputer system configuration to ensure a fast-response speed regulator.Typical system configurations are shown in Fig. 6. The optimum allocation of tasks and the sequencing of the different microcomputers is important to achieve a fast-response speed regulator.Especially, a common memory (CM) and a dual port random access memory (DPRAM) are effective for a multiple micro-

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computer based motion controller because of the high speed data communication and lower costs. Furthermore microcomputers which have communication port, such as the TMS320C40 from Texas Inst., are also effecti ve for the simple system configuration. Another item is the I/O processing such as the gate pulse generati on for the PWM and the speed detect ion. The functi on of the gate pulse generation can be divided into two parts. One is to de te rmine the firing time of each semi conductor element and the other is to generate the gate pulse at some determined time. The former functi on needs the complicated calculat ion in the PWM procsessing for AC motor dri ves. The latter needs high-reso luti on fo r less torque and speed fluctuation. For this reason, one-chip microcomputer is being used for AC motor dri ves(5). The calculation of the pulse width is processed by the microcomputer software and the generation of the gate pulse is performed by hardware counters within the one-chip microcomputer. FEATURE S • Each CPU is able 10 use all I/O devices . · The expansion of C PU and

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The performance of the motion control system depends loughly on the detection method of the position, speed and current. The positi on detection can be easily obtained by counting the encoder output pulse train. However, the digital speed and current must be newly developed for a fast-response, high-accuracy motion control system. For examp le, a new speed detection method which realizes a high resolution and accuracy in a short detection time, was proposed(6). The method achieves a high performance using a software di visi on and a hardware counter. On the other hand, the current detecti on method uses an A/D converter or a V/F converter. The motor current which is supplied by the power converter include current ripple components. When an A/D converter is used for the current detection, the detected current is an instantaneous value and is obtained with very little ripple component by paying attention to the swiching operation of the power converter. When a V/F converter and a counter are used for the current detection, the detected current becomes the mean value by counting the V/F converter output pulses generated within a prescribed period of time. 4. APPLICATIONS

• All 1/0 processin g are execut ed al high ·speed. o Th e processi ng load of the main C PU is rela tiv ely light. • The expan sion 01 110 Ihe CPU is easy .

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Fig.6. Comparison between multiple microcomputer systems for motion control.

4.l.Rolling mills AS shown in Fig.7,the GTO inverter system wh ich can provide a unity power factor and reduced harmonics, has been developed for rolling mill main drives(4).Fig. 7 shows the configurat ion of the proposed system to drive an induction motor of several megawatt power. To obtain both reduced current and torque ripples using GTOs with low switching frequenc y, a multiple converter and inverter system is used along with a current balance control in each multiple paralleled ci rcui 1. A GTO inverter and converter is controlled by the mult iple microcomputer based

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control system which consists of four 16bits 80196 and three 32bits 80960 microcomputers. Four l6bits microcomputers are assigned to the PWM gate pulse generation which is executed every 0.6ms. Two 32bits microcomputers are used for the current and vector control which are executed every 1.2 ms. A 32bits microcomputer executes the voltage and speed control every lOms. According to the experimental results, the cut off frequency of the speed regulator was 87 rad/s and the power factor was 0.98. From these results, the proposed system shown in Fig.7 was clarified to be effective for rolling mill main drives. 4.2 Robots A new servo system which enables flexible motion control of an industrial robot is shown in Fig.8. The hardware circuit of the DSP-based controller is shown in Fig.9(7).A DSP(TMS320C25) executes all servo processing functions which are 6-axis digital position and speed controllers. The result of the servo processing is converted to a pulse-width-modulated signal and transmitted to the current controller. The current controllers consists of analog circuits because of the necessity of the high-speed processing. A rotary type encoder is used as a position and speed sensor. The detected GTO CONVERTER

encoder pulse is converted by the position/ speed detection LSI. The DSP-based controller implements the position control loop, the speed control loop, the speed detection and the current reference output function. The processing is executed for plural axes in a time sharing manner every lms. The speed control processing is executed for all axes and the position control processing is executed for 2 axes every lms. Then, the sampling period of the position control is 3ms. Experimental results verify the effectiveness of the DSP-based servo system to the 6-axis industrial robot. Precise position control is performed at maximum and minimum speed movements using the proposed servo system. INVERTER R

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Fig.7. Microprocessor-based control system for rolting mill drives

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5. FUTURE

3)Two typical applications of microcomputer-based motion control systems were described. One is the speed regulator using multiple microcomputers for rolling mill main drives. Another is the multi axis servo system using a DSP for robots. 4) The important future subjects for microcomputer-based motion control systems were presented.

TRENDS

Microcomputer systems for motion control are continuously expanding to many industrial applications from robots to rolling mills. They have been directed to innovations and improvements of performance, cost, size and energy savings. The above customer's needs will likely remain true in the future.To achieve these needs,the following must be taken into consideration; I)RISC type microcomputers will be required for high-speed processing, 2)intelligent I/O LSI using ASIC technology will be developed for motion control systems, 3)many kinds of advanced control algorithms will be adopted for the improvement of control performance, 4)software packages will be standardized for motion control system, 5)the distributed system using multiple microcomputers will be commonly used, and 6)fault tolerant computer system will be realized for high-reliability systems using multiple microcomputers.

REFERENCES

I)P.P.Gelsinger et al., "Microprocessors circa 2000," IEEE SPECTRUM, vol. 26, pp. 43 -47,Oct. 1989. 2)T.Konishi et al., "A performance analysis of microprocessor-based control systems applied to adjustable speed motor drives, "IEEE Trans.lnd.Appl.,vol. IA-16,pp.378-387,May/June 1980. 3)T.Ohmae et al., "A microprocessor-controlled fast-response speed regulator with dual mode current loop for DCM drives, "IEEE Trans.lnd.Appl.,vol.IA-16, pp.388-394,May/June 1980. 4)T.Sukegawa et al., "A multiple PWM GTO line-side converter for unity power factor and reduced harmonics," IEEE-IAS annual meeting conf. rec.,pp.279-284, Oct. 1991. 5)K.Kubo et al., "A fully digitalized speed regulator using multimicroprocessor system for induction motor drives." IEEE Trans. Ind. Appl., vol. IA-21, pp. 10011008,July/Aug. 1985. 6)T.Ohmae et al., " A microprocessor-cont rolled high-accuracy wide-range speed regulator for motor drives." IEEE Trans. Ind.Electron.,vol.IE-29,pp.207-211 1982. 7)T.Ohmae et al., "Flexible multi-axis digital servo system." IEEE-IECON'86 conference record, pp.23-27 Nov. 1986

6. CONCLUS IONS

Microcomputer systems for motion control were presented and some of their future trends were also investigated. The results can be summarized as follows. I)The processing speed of microcomputers is remarkably increased. There are many kinds of microcomputers, such as RISC type,one-chip type, and DSPs, which can be utilized for fast-response motion control systems. 2)The historical trends of microcomputerbased speed regulator for rolling mills show that mUltiple microcomputer systems are being commonly used for motion control systems.

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