THE ULTRASONIC motor (USM) is a new type of actuator

Transcription

1 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 20, NO. 5, SEPTEMBER A Highly Effective Load Adaptive Servo Drive System for Speed Control of Travelling-Wave Ultrasonic Motor Güngör Bal, Member, IEEE, and Erdal Bekiroğlu Abstract This paper presents a highly effective load adaptive drive system to control the speed of a travelling-wave ultrasonic motor. The motor driver was built based on the two-phase high-frequency inverter using the mechanical resonant frequency of the ultrasonic motor. To digitally control the drive system, a TMS320F243 digital signal processor was adapted to the driver. The developed system includes two feedback loops; speed control loop and feedback voltage-resonant frequency tracking loop. The driving frequency was used as a control input to control the motor. Direct pulse-width modulation (PWM) control was used to obtain the required driving frequency. The developed drive system was experimentally tested under several operating conditions. The obtained results demonstrated the effectiveness of the drive system for high performance drive applications. Index Terms Digital signal processor (DSP), speed control, ultrasonic motors (USMs). I. INTRODUCTION THE ULTRASONIC motor (USM) is a new type of actuator that uses mechanical vibrations in the ultrasonic range as its driving source. Piezoelectric ultrasonic motors (USMs), especially rotary travelling wave type USMs, have important features such as high holding torque, high torque at low speed, quiet operation, no electromagnetic interference, and compact size [1], [2]. The USM is particularly superior in high holding torque and high response characteristics. Thus, USMs have attracted considerable attention for servo-drive applications [3]. On the other hand, USMs have some disadvantages that must be practically improved. The control characteristics of USMs are complicated and highly nonlinear. The exact values of motor parameters cannot be obtained easily and the motor parameters are time-varying due to increase in temperature and changes in motor drive operating conditions such as driving frequency, source voltage, and load torque [4] [6]. In order to overcome with these problems several speed and position control systems were already proposed. Two automatic resonant frequency tracking control methods using inverter-fed USM were presented in [7], which includes sensor and sensorless schemes. A driving circuit was designed Manuscript received February 24, 2004; revised February 2, This work was supported by The Scientific and Technical Research Council of Turkey TUBITAK, under Grant E012. Recommended by Associate Editor M. G. Simoes. G. Bal is with the Faculty of Technical Education, Electrical Department, Gazi University, Ankara 06500, Turkey ( gunbal@gazi.edu.tr). E. Bekiroğlu is with the Faculty of Engineering and Architecture, Department of Electrical and Electronics Engineering, Abant Izzet Baysal University, Bolu, Turkey. Digital Object Identifier /TPEL and a hybrid controller was proposed for USM in [8]. The hybrid controller combines the advantages of variable-structure system and adaptive-model following control. In [9], [10], a simplified mathematical model was proposed for USM and a speed controller based on the model using adaptive control theory was designed. Ferreira and Minotti [11] described inverter-fed USM servo-control implementation with two control strategies from practical point of view. These strategies are a load-adaptive automatic PZT resonant frequency-tracking and a constantvalue velocity amplitude control. A speed tracking servo control system was presented for USM in [12]. This speed control scheme makes use of both driving frequency control loop with variable-gain strategy and the applied voltage control with reduction strategy for speed ripple of USM. A frequency tracking method based on detection of the maximum current proportional to the motor torque by the open-loop frequency scan and seek technique was proposed [13]. A complete model based control for USM was presented in [14]. The control scheme consists of inner control loops with respect to the oscillation system, and outer control loops for torque and speed. A dual mode controller was proposed and implemented for speed tracking control of the travelling-wave ultrasonic motor (TWUSM). Direct PWM control was applied for both the driving frequency and voltage amplitude [15]. A current source parallel resonant inverter with energy feedback was used to drive USM in [16]. The speed tracking characteristics, especially under the starting and transient conditions with different and varying loads, have not yet been sufficiently investigated from an experimental point of view. The purpose of the present study is to propose a highly effective load adaptive servo drive system for speed control of TWUSM. The developed system includes feedback voltage and resonant-frequency tracking loop addition to the speed control loop. In the study, Shinsei s travelling-wave type USR60 was used. To drive USM, a digitally controlled drive system was developed. Then a TMS320F243 digital signal processor (DSP) was integrated to the drive system successfully. The effectiveness and fast responses of the proposed drive and control system were proved by experimental results for starting, transient and steady-state operating cases. II. TRAVELLING-WAVE ULTRASONIC MOTOR Although several USM types were designed, the rotary TWUSM is the most commonly used ultrasonic motor type. The TWUSM is driven by high frequency two-phase sinusoidal /$ IEEE

2 1144 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 20, NO. 5, SEPTEMBER 2005 Fig. 1. Equivalent circuit model of piezoelectric ceramic. Fig. 3. Speed-frequency characteristics of USM under different load torque. III. DRIVE AND CONTROL SYSTEM OF USM Fig. 2. Piezoelectric disc and electrode arrangement of USM. voltages with 90 phase difference. The speed of USM is controlled by amplitude, frequency and phase difference between the two-phase voltages [17], [18]. The energy conversion of piezoelectric system is based on the piezoelectric element and mechanical vibration system. The electromechanical behavior of a piezoelectric ceramic used in the stator of USM can be modeled by means of equivalent circuit as illustrated in Fig. 1. In Fig. 1,, and are, respectively, equivalent inductance, equivalent resistance, and equivalent capacitance. is applied voltage, is damping capacitance, and represents the dielectric loss of piezoelectric ceramic. To obtain high efficiency in the drive system, USM should be driven at the near frequency that creates resonance between and in the equivalent circuit. The detailed theoretical information and equations of the equivalent circuit and travelling wave can be found in [20]. The electrode arrangement in disc type piezoelectric ceramic is shown in Fig. 2. and signs show the polarized directions. When a positive voltage applied to a segment indicated by, the segment will expand. With a negative voltage it will contract. The reverse occurs for a segment indicated by. In Fig. 2 phase A provides the, and the phase B provides. By driving these two phase with 90 out of phase temporally, a travelling-wave is produced [19]. The feedback electrode is mounted on the piezoelectric disc addition to A and B phases. This electrode generates high frequency ac voltage signal (Vs) when mechanical vibrations are produced on the stator surface by the stator currents. The amplitude of this signal is proportional to the motor speed [20]. The relation between motor speed and driving frequency depends on load torque, which is an important characteristic for the speed control. In Fig. 3, the speed-frequency characteristics of USM are given under different load torque. From the figure, as the load torque increases the speed of motor decreases. For the practical operation of a USM, a specific and individual power supply and high quality semiconductor devices are required. It is difficult to drive the piezoelectric ceramic owing to its high damping capacitance. To drive piezoelectric ceramic easily, resonant frequency approach is commonly used. For this reason, a serial or parallel inductance is connected with each phase of USM to provide resonant frequency. Fig. 4 shows a newly developed drive system of two-phase high-frequency voltage fed serial-resonant inverter of USM. This inverter includes pulsewidth modulation (PWM), pulse frequency modulation (PFM), and hybrid (PWM/PFM) control techniques. and inductances are connected in series with each phase to become resonant with the damping capacitance of USM. Inverter outputs are two-phase high frequency ac voltages with 90 phase difference. The rotating direction is controlled by letting or lead. Clockwise (CW) and counter clockwise (CCW) inputs provide direction control signals. In practice, the driving frequency is set to higher value than resonant frequency of mechanical vibration system due to basic operating characteristics of USM [20], [21]. The speed control of USM was achieved by adjusting the driving frequency. The value of the driving frequency was determined by comparing ac voltage of feedback electrode (Vs) with reference dc voltage (Vdc). This reference voltage was produced by low-pass (LP) filter. The produced signal was applied to the switches through opto-isolator, split-phase and voltage-controlled oscillator (VCO) circuits. The switching frequency fs is adjusted by changing the level of reference dc voltage, so the speed of motor is controlled. Reference dc voltage required for comparing process was produced by a low-pass filter circuit. This circuit converts the PWM signal to the reference dc voltage according to the duty cycle of PWM. When the duty cycle of PWM signal is changed, the value of produced reference dc level also changes. So, the switching frequency is adjusted. As a result, the speed of motor is varied to demanded value. The block diagram of the frequency control for high effective speed control of USM is represented in Fig. 5. First, the

3 BAL AND BEKIROĞLU: HIGHLY EFFECTIVE LOAD ADAPTIVE SERVO DRIVE SYSTEM 1145 Fig. 4. Drive system of USM. Fig. 5. Block diagram of speed control scheme. speed of USM is measured and then compared with the reference speed. According to the speed error, speed controller generates appropriate PWM and resulting reference dc signal. Finally, frequency controller compares the reference dc signal and ac feedback signal to produce and apply the necessary frequency value to the motor. The developed control scheme does not require the knowledge of the parameters of the USM. Feedback voltage and resonant frequency tracking signal is taken from feedback piezoelectric sensor mounted on the stator of USM. Speed control signal is taken from an incremental encoder coupled with the motor shaft. The symbols which appear in Fig. 5 are defined as: reference speed; actual speed; speed error; reference dc voltage; ac feedback voltage; switching frequency. In the present study, the USM drive system was controlled by a 16-b fixed point TMS320F243 EVM DSP. The block diagram of TMS320F243 controlled USM drive system is given in Fig. 6. The event manager and general purpose I/O (GPIO) units of DSP were used to control USM. The PWM signal is produced by general purpose timer-1. The GP timer is set in continuous-up count mode to provide asymmetric PWM signal. CW and CCW direction signals are produced by GPIO. The quadrature encoder pulse (QEP) circuit of event manager was used for encoder signals. Encoder used in this study generates 500-ppr signal, two quadrature encoder signals and an index signal. For loading purpose, an electromagnetic brake was used. In order to control USM drive system, a software programme was written in Code Composer and then applied to the TMS320F243 EVM via XDS510PP emulator. When compared to floating-point architecture, fixed-point DSPs offer a limited degree of precision, but they are usually more cost effective and have far superior performance. Q-values are used whenever a fixed-point variable is used to store a floating-point value. In the present study, Q15 format was used since in this format the most significant bit is the signed bit followed by 15 b of fraction. A number in Q15 format has a decimal range between 1 and Fractional number is expressed as where is the fractional number and is binary value of 1 or 0. Photographs of the studied system are given in Figs. 7 and 8. While Fig. 7 shows a photograph of the USM drive and control

4 1146 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 20, NO. 5, SEPTEMBER 2005 Fig. 6. TMS320F243 controlled USM drive system. Fig. 7. Photograph of USM drive and control system. Fig. 9. Speed responses of USM in 75, 100, 130 rpm. Fig. 8. Photograph of experimental test setup. system, Fig. 8 shows a photograph of an experimental test set-up used to load USM. IV. EXPERIMENTAL RESULTS In this section, the experimental studies were performed to examine the performance of the developed digitally controlled USM drive system for different speed and load cases. Shinsei s travelling-wave type USR60 ultrasonic motor was used in the study. The specifications and parameters belong to this motor are given in the Appendix. First, the experimental results obtained during an acceleration of the unloaded motor are presented in Fig. 9. The experiment was repeated for three different set speeds, 75 rpm, 100 rpm, and 130 rpm, respectively. The motor started its acceleration phase from rest to set speed. The actual motor speed followed its set speed command very closely without overshoot. Second, the motor was accelerated to 120 rpm set speed value with no load, half-load (0.2 Nm) and full-load (0.4 Nm) individually to show that the developed USM drive system is load adaptive. It is very clear from Fig. 10 that a) the motor unloaded, b) loaded with 0.2 Nm, and c) 0.4 Nm can accelerate robustly and rapidly follow the set speed command. The speed response of the motor is quite fast. The motor reaches the demanded set

5 BAL AND BEKIROĞLU: HIGHLY EFFECTIVE LOAD ADAPTIVE SERVO DRIVE SYSTEM 1147 Fig. 10. Speed responses of USM (120 rpm): (a) reference speed, (b) with 0.2 Nm load, and (c) with 0.4 Nm load. speed for each case within only 60 ms without overshoot and oscillation. Besides, the loaded motor works very stable and smoothly as no load case. In this case, the demanded set speed value was specified so as to provide multiple stepwise changing modes as illustrated in Fig. 11(a). The set speed steps are rpm. The Fig. 11. Speed responses of USM: (a) speed reference, (b) actual speed (no load), and (c) actual speed (0.2 Nm). motor speed responses for no-load and 0.2 Nm load were illustrated in Fig. 11(b) and (c), respectively. The exact and rapid speed tracking characteristics can be proved from this reasonable experiment. As seen from the figures, the motor speed is

6 1148 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 20, NO. 5, SEPTEMBER 2005 optimized by TMS320F243 DSP. High-frequency voltage-fed inverter which uses serial-resonant technique was designed to drive USM. The speed of USM was controlled by the driving frequency by detecting resonant frequency and the generated voltage of the feedback electrode mounted on USM. This control system can on-line compensate the variations in the speed characteristics of the motor. The performance of proposed driving and control system was examined by the experimental results. Also, the feasible effectiveness of USM under the transient and steady-state conditions was investigated. The experimental results reveal that the developed drive and control system gives superior speed characteristics for USM under different load conditions. The proposed control scheme is new and very effective for TWUSM. Fig. 12. Speed of USM (100 rpm) (a) without load and (b) with 0.3 Nm step-wise load. changed four times in 400 ms interval. Although the speed reference changes in very short time, the motor follows the reference speed exactly. These results confirm that the motor can successfully accelerate and decelerate under different loads. As a final experiment, Fig. 12 shows the speed regulation characteristics of the USM under the external stepwise applied torque. The speed value is 100 rpm while the motor was running at steady state speed with no-load as shown in Fig. 12(a). The external load torque was stepped up to 0.3 Nm (75% of rated torque) at 3 s and then totally removed at 7s as shown in Fig. 12(b). The USM drive system responded very quickly and the motor delivered the demanded load torque while maintaining the motor speed virtually at its set value. Observing Fig. 12, the proposed drive and control system is greatly stable and precise under instantaneous variations of the stepwise applied load torque. The speed characteristic has not transient perturbation although the load changes suddenly. V. CONCLUSION In this paper, a highly effective speed control of USR60 TWUSM was implemented. This control was achieved and APPENDIX The specifications of driver and USM Drive frequency khz. Drive voltage Vrms. Drive current 53 ma. Rated torque 0.4 Nm. Rated power 4 W. Rated speed 10 rad/s. Holding torque 800 mnm. Rotor inertia kg/m. Weight 0.23 kg. The equivalent circuit parameters of USM. = 9 nf. =20k. = 0.1 H. = 150. = 168 pf. REFERENCES [1] T. Sashida and T. Kenjo, An Introduction to Ultrasonic Motors. London, UK: Oxford, [2] K. Uchino, Piezoelectric ultrasonic motors: Overview, Smart Mater. Struct., vol. 7, pp , [3] T. Senjyu, T. Kashigawi, and K. Uezato, Position control of ultrasonic motors using MRAC and dead-zone compensation with fuzzy inference, IEEE Trans. Power Electron., vol. 17, no. 2, pp , Mar [4] T. Senjyu, H. Miyazato, S. Yokoda, and K. Uezato, Speed control of ultrasonic motors using neural network, IEEE Trans. Power Electron., vol. 13, no. 2, pp , May [5] F. J. Lin, R. J. Wai, and C. M. Hong, LLCC resonant inverter for piezoelectric ultrasonic motor drive, Proc. Inst. Elect. Eng., vol. 146, no. 5, pp , Sep [6] H. Hirata and S. Ueha, Design of a traveling wave type ultrasonic motor, IEEE Trans. Ultrason., Ferroelect., Freq. Contr., vol. 42, no. 2, pp , Mar [7] S. Furuya, T. Maruhashi, and Y. Izuno, Load-adaptive frequency tracking control implementation of two-phase resonant inverter for ultrasonic motor, IEEE Trans. Power Electron., vol. 45, no. 3, pp , Jul [8] F. J. Lin and L. C. Kuo, Driving circuit for ultrasonic motor servo drive with variable-structure adaptive model-following control, Proc. Inst. Elect. Eng., vol. 144, no. 3, pp , [9] T. Senjyu, S. Yokoda, H. Miyazato, and K. Uezato, Speed control of ultrasonic motors by adaptive control with simplified mathematical model, Proc. Inst. Elect. Eng., vol. 145, no. 3, pp , May [10] T. Senjyu, K. Uezato, and H. Miyazato, Adjustable speed control of ultrasonic motors by adaptive control, IEEE Trans. Power Electron., vol. 10, no. 5, pp , Sep

7 BAL AND BEKIROĞLU: HIGHLY EFFECTIVE LOAD ADAPTIVE SERVO DRIVE SYSTEM 1149 [11] A. Ferreira and P. Minotti, High-performance load-adaptive speed control for ultrasonic motors, Contr. Eng. Practice, vol. 6, pp. 1 13, [12] Y. Izuno et al., Speed tracking servo control system incorporating travelling-wave type ultrasonic motor and feasible evaluations, IEEE Trans. Ind. Applicat., vol. 34, no. 1, pp , Jan./Feb [13] K. Kato and T. Sase, Robust resonant frequency tracking control for ultrasonic-motor drive, Electron. Commun. Jpn., pt. 2, vol. 80, no. 3, pp , [14] J. Maas, T. Schulte, and N. Fröhleke, Model-based control for ultrasonic motor, IEEE/ASME Trans. Mechatron., vol. 5, no. 2, pp , Jun [15] K. T. Chau, S. W. Chung, and C. C. Chan, Neuro-fuzzy speed tracking control of travelling-wave ultrasonic motor drives using direct pulsewidth modulation, IEEE Trans. Ind. Applicat., vol. 39, no. 4, pp , Jul./Aug [16] F. J. Lin, R. Y. Duan, and J. C. Yu, An ultrasonic motor drive using a current-source parallel-resonant inverter with energy feedback, IEEE Trans. Power Electron., vol. 14, no. 1, pp , Jan [17] T. Senjyu, S. Yokoda, and K. Uezato, A study on high efficiency drive of ultrasonic motors, Electric Power Compon. Syst., vol. 29, pp , [18] S. W. Chung and K. T. Chau, Servo speed control of travelling-wave ultrasonic motors using pulse width modulation, Electric Power Compon. Syst., vol. 29, pp , [19] N. W. Hagood and A. J. McFarland, Modeling of a piezoelectric rotary ultrasonic motor, IEEE Trans Ultrason., Ferroelect. Freq. Contr., vol. 42, no. 2, pp , Mar [20] G. Bal and E. Bekiroğlu, Servo speed control of travelling wave ultrasonic motor using digital signal processor, Sens. Actuators A, vol. 109, pp , [21] F. J. Lin, Fuzzy adaptive model-following position control for ultrasonic motor, IEEE Trans. Power Electron., vol. 12, no. 2, pp , Mar Güngör Bal (M 99) was born in Denizli, Turkey, on September 20, He received the B.Sc. and M.Sc. degrees in electrical engineering from Gazi University, Ankara, Turkey and the Ph.D. degree in electrical and electronics engineering from Strathclyde University, Glasgow, UK, in Since 1994, he has been with the Department of Electrical Education, Faculty of Technical Education, Gazi University, where he is currently a Professor. His research interests are drive and control of special electrical motors, extended Kalman filters, brushless dc motors, and vector controlled induction motors. electrical motors. Erdal Bekiroğlu was born in Hasankeyf, Turkey, on June 13, He received the B.Sc., M.Sc., and Ph.D. degrees in electrical education from Gazi University, Ankara, Turkey. He was a Research Assistant at Gazi University from 1996 to He is currently an Assistant Professor with the Department of Electrical and Electronics Engineering, Faculty of Engineering and Architecture, Abant İzzet Baysal University, Bolu, Turkey. His research interests are fuzzy logic, digital signal processors, drive, and control of special