Robust Position Control of Ultrasonic Motor using VSS Observer Shogo Odomari 1 , Araz Darba 1 , Kosuke Uchida 1 , Tomonobu Senjyu 1 , Atsushi Yona 1 1 Department of Electrical and Electronics Engineering, University of the Ryukyus, Japan E-mail: b985542@tec.u-ryukyu.ac.jp Abstract— Intrinsic properties of ultrasonic motor (high torque for low speed, high static torque, compact in size, etc.) offer great advantages for industrial ap- plications. However, when load torque is applied, dead- zone occurs in control input. Therefore, sliding mode controller, which is a nonlinear controller, is adopted for ultrasonic motor. The state quantities, such as ac- celeration, speed, and position are needed to apply the sliding mode controller for position control. However, rotary encoder causes quantization errors in the speed in- formation. This paper presents a robust position control method for ultrasonic motor by using Variable Structure System(VSS) observer. The state variables for sliding mode controller are estimated by the VSS observer. Besides, a small, low cost, and good response sliding mode controller is designed in this paper by using a micro computer that is essential in embedded system for the developments of industrial equipments. The effectiveness of the proposed method is verified by experimental results. I. I NTRODUCTION In recent years, ultrasonic motor(USM) is gaining attention as it has good characteristics and is small in size. The drive source of ultrasonic motor is ultrasonic vibration of piezoelectric element. USM is expected to be applied to robot actuator, high precision positioning and medical equipments[1]. The operating principle of an USM has complicated speeds characteristics com- pared to conventional electromagnetic motor which makes it a special kind of motor. However, an USM has dead-zone in its control input with applied load torque[3]. Since H ∞ controller is a linear controller[2], it cannot control the USM with unknown dead-zone. Therefore, sliding mode controller[4]-[6], which is a nonlinear controller, is used for robust position control of USM with unknown dead-zone. To apply sliding mode controller for posi- tion control of USM, state variables such as accel- eration, speed and position of the USM are needed. Speed information detected by a rotary encoder have quantization errors, especially in low speed region. Therefore, to estimate actual rotor speed accurately by a using VSS observer[7] is proposed with the possibility of decreasing quantization error. Essential industrial equipments are developed in small size, lightweight and power-saving technology, by using embedded system. Use of micro computer in embedded system, has advantages(such as it can dis- cretize all processing, can design stable and small size USM D 1 D 2 L 1 S 1 S 2 L 2 S3 S 4 D 4 C 2 C 1 3 D E RE Load MOS FET driver - Micro f φ computer V V a b Personal computer r y y e y e Fig. 1 Drive system of USM. -100 -50 0 50 100 150 Control input V A V B , [V] Time [msec] 0 5 10 15 20 Va Vb Fig. 2 Output voltages of two-phase inverter. circuit in comparison to analog circuit, can construct control system at low cost, can upgrade response, and can make the system easily design easy and change). This paper presents a digital implementation[8]-[9] of a sliding mode controller and a VSS observer by using a micro computer for efficient position control of the USM with unknown dead-zone. The state variables are estimated by the VSS observer and are used in sliding mode controller. The proposed sliding mode controller is found satisfactory. II. SYSTEM CONFIGURATION A. Driving system of USM All configuration of the USM control system used in this study is shown in Fig. 1. The USM used in the experiment is a traveling wave USM(SHINSEI CORPORATION : USR-60). Output voltages of the two-phase inverter are shown in Fig. 2. A traveling wave is formed on the stator surface when impress