Design and Use of Pre-Stressed Unimorphs for High-Displacement, High-Load Applications

Abstract

The purpose of this research was threefold: to characterize pre-stressed unimorph actuators fabricated by different routes, to investigate 3-D orientation of the polarization vector through the piezoceramic thickness using piezoresponse force microscopy (PFM), and to design a motor to use the strengths of a compliant actuator. Applications such as robotics need high-force, high displacement actuators with potential for scaling.

Pre-stressed unimorphs, typified by THUNDER™ actuators by Face International Corporation, provide larger displacement than traditional unimorph or bimorph actuators because pre-stress introduced during fabrication enhances piezoelectric strain. The fact that these are compliant actuators has important implications for use.

This research showed that bond material and thickness, as well as end conditions all affect actuator performance. Substrate material and thickness relative to that of the ceramic element were shown to have more subtle effects than previously reported. The likely signature of performance enhanced by pre-stress was found in load-displacement test data, which showed that the effect appears to be modified as displacement under load interacts with the original actuator curvature due to pre-stress.

The novel application of PFM showed that orientation of the polarization vector did indeed vary through the actuator thickness. Internal stress bias has a dominant role in determining orientation of the polarization vector, so much so that effects of initial poling were not seen except at a location likely to be a neutral surface. With overall domain orientation generally out of alignment with the poling direction, piezoelectric strain augmented by a large extrinsic contribution can be expected when electric field is applied in the poling direction.

Performance of a linear motor using an inchworm cycle was found to be limited by clamp slip. The passive (unpowered) clamps otherwise had the advantage of simplifying operation. A rotary motor of novel design was tested using several configurations of actuators and other parts. Its chief advantage was that resonant behavior was little affected by load, since actuators and load were indirectly coupled. Characterization yielded a range of torque and speed data, with best performance generally provided by the simplest drive signals and configurations of parts. Design principles allow the motor to generate high torque. Experimental results, although promising, imply that ample opportunity exists to identify and ameliorate performance-limiting factors.