We present results from development and testing of lightweight actuators made of the piezoelectric polymer PVDF. The prototype being developed is intended for microgravity applications in space and has been tested aboard NASA's Reduced Gravity Platform. The design has been driven by the requirements for a full 3D environment. Incorporation of additional electrical leads into the actuators themselves may remove the need for a separate umbilical to the suspended experiment. Linear equations describing the displacement of piezoelectric bimorphs were developed and applied to the bellows actuator including the epoxy layer. Properties for the piezoelectric layers were obtained from the literature; properties for the epoxy layer were obtained through ultrasonic testing. To assess the validity of the assumed linearity of the actuator, we conducted nonlinear finite element analysis, which indicated a high degree of linearity on contraction and up to a maximum of 5% deviation on expansion to full deflection (about 6 mm). We have developed and tested a proportional-plus-derivative (PD) control system for use with the actuator in 1D using a novel folded pendulum to simulate a zero-g environment. Passive and active characteristics are both in agreement with theoretical predictions.
We have designed and built piezoelectric polymer actuators in a 'bellows' configuration and have used them in a near-zero-g environment vibrations suppression apparatus. The actuator is based on poly(vinylidene fluoride) (PVDF) sheets produced by AMP and electroded to our specifications. The actuator consists of two bimorphs, each with a double-bend precurvature, glued together at their ends so that the actuator has its thickest air gap at the middle. Each bimorph consists of two sheets glued together. Each sheet is electroded completely on the outside (ground) side, and has three electrode areas on the other side. If the electrode on the middle half is positive, and on the outer two quarters are negative, the bimorph curvature and the actuator length increase; with opposite polarities they decrease. In the vibration isolation application, the box to be isolated has actuators mounted between it and its surrounding enclosure on the vibrating vehicle. Feedback control is provided to change actuator length to compensate for vehicle motions and vibrations. This feedback is provided by accelerometers and by laser diode position sensors. The inherent softness of the actuator provides good passive damping of higher frequencies. So far, a one-dimensional test of the system has been made using a mass on a 'folded pendulum' as a 'weightless' (no restoring force for small displacements) load. Also, a two- dimensional version was flown on NASA's KC-135, which provided 25-second near-zero-g intervals during parabolic flight segments. Our goal is three-dimensional isolation for space vehicle applications.