Asynchronous Control of a Prototype Inchworm Actuator: Control Design and Test Results

Inchworm actuators are innovative mechanisms that offer nanometer-level positioning coupled with extreme dynamic range. Because of this, they have found applications in optical instruments of various types including interferometers, segmented reflectors, and coronagraphs. In this paper, we present two prototypes of flight-qualifiable inchworm actuators developed at the Jet Propulsion Laboratory. These actuators have two sets of brake piezoceramic (PZT) stacks and an extension PZT stack used for mobility. By proper phasing of the signals to these PZTs, a walking gait can be achieved that moves a runner attached via a flexure to the optic to be moved. A model of these devices, based on first principles, is developed as well as an estimation and control scheme for precise positioning. The estimator estimates physical parameters of the device as well as a self-induced motion disturbance caused by the brakes. Simulations and test data are presented that demonstrate nanometer-level positioning precision as well as the cause of variations in the brake-induced disturbance.