Design and calibration of a low-cost open-loop PZT actuator for phase-shifting speckle interferometry

The paper describes a simple low-cost open-loop PZT actuator and the procedures which have been developed for its calibration and alignment in order to obtain a device dedicated to phase-shifting speckle interferometry. A low-voltage multilayer PZT ceramic block was inserted between a mirror and a cube beamsplitter of a Michelson interferometer to be used in a shearometer assembled on a cube 40x40x40 mm. The PZT was driven by a supply voltage output by a 16 bit D/A converter directly connected to the parallel port of a personal computer which guarantees a very accurate output. By the analysis of the interference pattern produced by the Michelson interferometer the calibration curves (for ascending and descending input voltage) were obtained. When the whole hysteresis loop has been identified, a fairly linear working range can be chosen. The device is equipped with three setscrews acting on a leaf-spring for the compensation of the mirror tilting produced by the non uniform expansion of the PZT block. The calibration can be performed using either the interference between two smooth wavefronts (in presence of a sufficiently fine fringe pattern) or the interference between two speckle patterns. The calibration procedure can be consequently carried out directly on any speckle interferometer at the beginning of the measuring session, in this way the environmental sensitivity of the actuator can-be overcome.