A PID (proportional, integral, and derivative) iterative learning controller is proposed for precise tracking control of industrial robots and CNC (computer numerical control) machine tools performing repetitive tasks. The convergence of the output error by the proposed learning controller is guaranteed under a certain condition even when system parameters are not known exactly and periodic disturbances exist. In actual implementation, the distance difference between the desired path and the actual machined path, which is one of the most significant factors in the evaluation of control performance of industrial robots and CNC machine tools, is reduced in machining a circle with depth on the aluminum workpiece as the proposed learning algorithm repeats. Experimental results demonstrate that the proposed PID learning controller can improve machining accuracy when the CNC machine tool performs repetitive tasks in machining.
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