Beyond Performance/Cost Tradeoffs in Motion Control: A Multirate Feedforward Design With Application to a Dual-Stage Wafer System

Motion systems with multiple control loops often run at a single sampling rate for simplicity of implementation and controller design. The achievable performance in terms of position accuracy is determined by the data acquisition hardware, such as sensors, actuators, and analog-to-digital/digital-to-analog converters, which is typically limited due to economic cost considerations. The aim of this paper is to develop a multirate approach to go beyond this traditional performance/cost tradeoff, i.e., to use different sampling rates in different control loops to optimally use hardware resources. The approach appropriately deals with the inherent time-varying behavior that is introduced by multirate sampling. A multirate feedforward control design framework is presented to optimize the tracking of a dual-stage multirate system. The application of the proposed approach to an industrial dual-stage wafer system demonstrates the advantages of multirate control, both in simulations and experiments.