Temperature variation effects on electrostrictive actuators and its quasi-static compensation

Abstract

Deformable mirrors are used for changing the wavefront of the reflected light by using one or more actuators behind the mirror to physically deform it. This application is widely used in telescopes to compensate for atmospheric aberrations using a feedback wavefront sensor. Recent advances in lithography process require more precise wavefront control than ever, and deformable mirrors are being considered as a suitable optical unit in the optical column of lithography systems. Using electrostrictive materials for the actuation provides benefits like less hysteresis, no creep, and fast response time. One major limiting factor in the use of such materials is the high sensitivity of strain response to temperature. Due to application constraints a position sensor is not available for the position control. This leads to a requirement of temperature dependent strain-field model in a feedforward control scheme to compensate for the temperature changes and predict the position to sufficient accuracy. In this thesis, different electrostrictive models are explored, and through high frequency impedance measurements and precise quasi-static strain-field experiments, actuator response is analyzed and model parameters are estimated. The parameters are then used in a feedforward control, with temperature sensor, to compensate for the temperature change resulting in decrease of temperature induced strain variation by 75\%-87\%. Furthermore, an optimum voltage region is identified for maximum actuation performance with compensation of temperature induced strain variation. Lastly, a novel temperature self-sensing approach is explored for which a patent application is filed.