Abstract
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.
Highlights
This article describes the modeling, control system design, and test data for two prototype flight inchworm actuator (FIA) developed at the Jet Propulsion Laboratory (JPL)
By flight-qualifiable it is meant that these mechanisms are designed to survive launch loads, operate in vacuum, and survive the thermal environment of space without degradation in either their performance or lifetime
The FIA actuators are used to position various optical elements used in highly sensitive optical instruments
Summary
This article describes the modeling, control system design, and test data for two prototype flight inchworm actuator (FIA) developed at the Jet Propulsion Laboratory (JPL). This control design in this paper considers compensation of hysteresis in terms of how it effects the scale factor of the extension PZT; in addition, estimation and compensation of the brake disturbance is addressed These features are important since both the scale factor and brake disturbance vary during operation of the FIA inchworm. Encoder measurements taken before and after each step allow for determining the total amount of motion that occurred Some of this motion will be due to the intended step size, but a significant portion of it will be due to unwanted runner motion induced by opening and closing of the brakes and positioning errors of the non-earth brake with the extension PZTs. The brake disturbances are very difficult to predict and are the primary difficulty in trying to control the position of the runner. As we will see in subsequent discussions, asynchronous sampling avoids saturation of the actuator and makes latencies of the plant vanish which are both important features to the performance of the mechanism
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