Low Speed Control of AC Servo Motors in No-Load Condition Iman Kardan, Alireza Akbarzadeh and Kaveh Kamali Center of Excellence on Soft Computing and Intelligent Information Processing, Mechanical Engineering Department Ferdowsi University of Mashhad Mashhad, Iran imankardan@yahoo.com, ali_akbarzadeh_t@yahoo.com, kaveh.kamali@gmail.com Abstract— Torque mode control of servo motors in low speed - no load condition requires the applied torques to be as much as dry friction forces which are designed to be as low as possible. Therefore, in these cases amplitude of available noises are comparable to control voltages which deteriorate performance of closed loop system. In this paper a simple but effective method is presented for low speed control of AC servo motors in no load conditions. This method uses a common feature of AC motor drives which enables users to specify torque ramp in torque mode control. By setting this parameter to an appropriate value, first control signal is scaled up to reduce the noise effects. Then, drive will apply the control voltage to motor by internally scaling the signal down to its real value. Since drives are usually well shielded, the noise effects will be negligible during the down scaling process. The effectiveness of the proposed method is demonstrated through some experimental tests. Keywords—AC servo motor; Low Speed - no load control; Torque ramp parameter. I. INTRODUCTION Speed control of AC servo motors is a highly demanded task [1]-[4] which can be done through two different approaches of velocity control mode and torque control mode [5],[6]. In the first approach, the built-in velocity control mode of motor drives is used. Because of hardware implementation of control loop, this method is simple and fast which makes it the best choice for industrial applications. However, velocity control mode has a major drawback of fixed control algorithm which makes it unsuitable for applications which requires implementation of different control algorithms especially academic researches. In the second approach, torque control mode of motor drives is used. In this approach an external loop is used to calculate the required torque for reaching the desired velocity. A voltage proportional to the required torque is send to the drive and the drive adjusts motor torque according to the applied voltage. The main advantage of this method is the possibility of implementation of different control algorithms of model free [7] and model based [3], [4] strategies. On the other hand, because of analogue communication between controller and the drive, control signals are more subjected to noises in this method. Regarding basic physical laws, the torque applied by the motor should counteract friction forces and external loads moreover to providing required inertial torques due to acceleration. If the motor has low linear and rotary inertias, the inertial torques will be negligible in low accelerations. Also when the motor operates with a constant speed, there will not be any inertial effects. Neglecting the accelerations, if the external loads on the motor are low, the motor torque should only oppose the friction forces which contain dry and viscous terms. Since viscous frictions get smaller as the motor speed decreases, dry friction forces are the only terms to be counteracted by the motor torque in low speed - no load operations. It is also a common trend that for efficiency considerations, servo motors are designed to have low friction forces. Low speed - no load control of AC servo motors has a wide range of applications. In some usages like micro machining [8] effective inertia and loads are very low and motor is demanded to move very slowly. Moreover, slow motor rotation is an essential part of friction identification process which is largely used in friction compensation algorithms [9], [10]. Such applications, where the motor is required to rotate slowly with low external loads, are commonly called low speed – no load operations which are in the focus of this study. According to the previous discussion it can be concluded that in low speed – no load operations amplitude of the required control voltage is very low and comparable to available noises which can completely ruin the control signal. Hence, the main difficulty of controlling motor velocity in these conditions is to keep noises as low as possible or to somehow reduce the effect of available noises. This paper presents a simple but effective method for speed control of AC servo motors in low speed – no load conditions. The proposed method uses adjustable torque ramp option which is mostly offered by AC motor drives (e.g. [11], [12]). In torque control mode, this option specifies for the drive that incoming control voltage corresponds to what percent of nominal torque. By setting this parameter to an appropriate Proceeding of the 2nd RSI/ISM International Conference on Robotics and Mechatronics October 15-17, 2014, Tehran, Iran 978-1-4799-6743-8/14/$31.00 ©2014 IEEE 245