Low speed control of AC servo motors in no-load condition

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.