Internal Model Control using EWMA for speed control of wedge-type piezoelectric motors

In this article, speed control for a platform driven by a wedge-type piezoelectric motor is investigated. Such a motion control system is subject to disturbance like friction, preload and operating temperature change, so that a predictive linear relation between the applied input voltage and the output platform velocity is difficult to keep. Especially, operating temperature change is an essential problem of using piezoelectric motor, but very few researches introduce the phenomenon in depth and solve the problem. In practical experiment, deterministic drift due to operating temperature change is found and defined as a ramp-function disturbance. Similar to chemical mechanical polishing (CMP) in semiconductor manufacturing process with a linear relation between the input recipe and the output target in a run-to-run (RtR) control structure, exponentially weighted moving average (EWMA) method has been widely used and verified its capability of overcoming systematic change and drift disturbance. Therefore, EWMA method on the basis of RtR control structure is considered to use for speed control purpose. Besides that, it is attempted to map the EWMA method into an internal model control (IMC) to achieve an RtR-IMC adaptive control scheme. Following that, adding a PI compensator in the feedforward path is theoretically proved to be able to deal with the ramp-type disturbance due to operating temperature change. A number of feedback control methods including single EWMA, double EWMA, and RtRIMC_EWMA with a PI compensator design are investigated and examined. From experimental results, double EWMA with two-stage parameter estimation, as expected, is able to deal with the drift noise and outperforms Single EWMA. The proposed RtR-IMC_EWMA with a PI compensator is superior to other methods. Such a new adaptive control scheme is easy to establish and provides flexibility of adding compensator in the feedforward path to enhance the robustness to external disturbance.