Parameter Design of Digital P-PI Controller for Motor Positioning Systems Using Limited Pole Placement Method

Positioning systems with motors are widely used in industrial use. Many production systems, mechatronics systems or industrial robots use high speed and high precision positioning systems with motors. Generally, the controllers of the positioning systems are proportional-to-proportional-integral (P-PI) controllers. They are often used because they have small overshoots and easy to tune. However, it needs fine-tunings to make settling motions smooth. There may be two reasons why the fine-tunings are needed for P-PI controllers. One is the pole assignment constraint of P-PI controller. Three poles of the P-PI control systems for motor positioning cannot be assigned freely. One of the three poles always stay at a slow position. Another reason is the digital delay when the P-PI controllers are applied to the practical systems. The digital system with the delay becomes vibrational because of the phase delay of the calculation time. It makes the overshoot of the trajectory. In this paper, the influences and solutions of the pole assignment constraint and the digital delay are shown. The pole assignment constraint limits the performance of the closed loop. The feedforward controller is shown to be the solution of this constraint. The digital delay makes the overshoot, but it can be corrected by the limited pole placement (LPP) method proposed by the author. It enables accurate pole-zero assignment without overshoots under the conditions of the digital delay.