Servo control of micro-cylindrical ultrasonic motor using single/dual mode control

Abstract

Cylindrical Ultrasonic Motor (CUSM), is a relatively new kind of piezoelectric actuator, having different type of characteristics from that of conventional electromagnetic motors. In order to choose the most suitable control variable for speed/position control applications, this paper evaluates the dynamic and steady state performance of the CUSM drive system by using three different control variables, namely, the amplitude, the frequency and the phase difference of the excitation signals. In the single mode control, only one control variable is used while the other two are maintained constant. Among the three variables, amplitude control is found to be superior as compared to other control variables. Furthermore, to achieve quick and precise position control, the dual mode control method is proposed in which two control variables are used simultaneously while maintaining the third variable constant. Experimental test results obtained by using a 10mm diameter and 30mm long CUSM verify that the dual mode control is superior to the single mode control. In dual mode control, the combination of Amplitude and Frequency control is the best choice for position control application. I. IntroductionAs USM has many special features such as high holding torque at low speeds and no EMI problems [1], it has drawn particular attention from the mechatronics field. Moreover, for CUSM, because of its cylindrical construction, it can be made with diameter less than 1 mm and still obtain larger torque than ring-type motors. However, since the motor's driving mechanism is by friction between the stator and the rotor, which is nonlinear, it is difficult to come up with an accurate mathematical model of the motor. Therefore, high performance speed/position control of the motor is not trivial.So far, there have been several attempts as reported to control speed/position of ring-type USM in the literature Lin[5]. Also, Wang[6] presents the work on speed controller of CUSM, but the scope is limited to single mode control with amplitude as the control variable only. In this paper, single and dual mode of control are evaluated in order to find a suitable control variable to get the best performance for positioning application. First, because of CUSM's unique structure, its operating characteristics and driving circuit are discussed in Section II. Next, the single mode control with amplitude, frequency and phase difference as control variable is described in Section III. In this part, by changing one control variable at a time while keeping the other two variables fixed, the dynamic and steady state performances of CUSM are com-