Influence of the Motor Feedback Sensor on Ac Brushless Servo Drive Performances

This paper discusses the influence of the position feedback sensor technology in servo drive applications. Incremental encoders, resolvers, and sinusoidal encoders are considered. Different hardware and software techniques used for the processing of position measurement signals are presented. Advantages and drawbacks are discussed for each solution. The choice of the best suited position feedback sensor for a given motion control application mainly depends on the speed and position control requirements and on the motor type. The dynamic performances and the accuracy of speed and position servo loops are dramatically influenced by the position feedback sensor resolution and accuracy. These points are analyzed in detail in the paper. Stiffness, stability, servo loop bandwidth and response time are considered as performances evaluation criteria for a given servo drive equipped with various position sensors. Rotating and linear AC brushless servo motors are considered. Experimental results are given. 1) INTRODUCTION Due to the development of automation in manufacturing processes, the demands on accurate and fast machine-tools and robots are increasing. In response to these demands, high dynamic AC brushless servo-motors tend to be adopted in motion control systems, and servo drives are expected to deal with a great variety of applications with maximum performances [1]. According to these requirements, high resolution and high accuracy are required for the motor speed and position measurement in order to close the speed and position servo loops with the maximum obtainable bandwidth. Smooth rotation at very low speed is also necessary for the most demanding applications. The particular sensor used depends on the application requirements, however incremental encoders, resolvers and sinusoidal encoders are the most popular. An AC brushless servo drive consists of a permanent magnet synchronous motor (PMSM) equipped with a position sensor mounted on the motor shaft, as presented on figure 1. The position sensor is used for closing the speed and position servo loops and also for the motor currents commutation in order to control the motor torque. The current amplitude is proportional to the desired torque value and the current phase is tied to the rotor position. Figure 1 : Block diagram of an AC brushless servo drive Resolvers are preferred for rotating brushless AC servomotors used in robot applications, because they are very rugged and provide absolute position value suitable for the motor commutation as well as the speed and position feedback for the servo loops. Encoders are mainly used in machine tools applications when a high position accuracy is required for contouring and machining. Encoders are also chosen for direct drive (rotating or linear), because the load is directly mounted on the moving motor part. So, a high number of pulses is necessary in order to get the required position accuracy. However, when incremental encoders are used with brushless AC servomotors, additional Torque control & phase commutation Speed & position control P W M inverter Brushless motor Position sensor Shaft position Set point