Encoderless Control of Industrial Servo Drives

High resolution optical encoders are used in industrial servo drives with position and/or speed control for providing the respective feedback signals. In applications with lower accuracy requirements resolvers are used as feedback sensors with respect to lower cost. As a resolver is nothing else than an electrical AC machine of special design, there is the basic idea to operate the servo motor itself like a resolver ¿ in parallel to its original drive operation, of course. High frequency resolver signals are injected to the servo motor without disturbing its original purpose (i.e. the production of torque). The reaction of the servo motor on the high frequency signals can be detected by the current sensors, which are available with respect to the current control loop anyway. A lot of schemes and concepts for encoderless AC drive control providing good operation during standstill are already published. Acceptance by industry, however, is not very extended. The main reasons for hesitations in industry are ¿ additional processing performance being necessary for the controller or signal processor ¿ additional sensors or hardware ¿ parameters to be adjusted for encoderless control The concept for encoderless control presented in this paper has no restrictions with respect to a minimal speed or frequency. Furthermore it is possible to implement it in industrial servo drives without additional hardware requirements and without parameter adjustments.