2008

Cylindrical Piezoelectric Fiber Composite Actuators

Cylindrical actuators offer advantages over flat flexible actuators.

The use of piezoelectric devices has become widespread since Pierre and Jacques Curie discovered the piezoelectric effect in 1880. Examples of current applications of piezoelectric devices include ultrasonic transducers, micropositioning devices, buzzers, strain sensors, and clocks. The invention of such lightweight, relatively inexpensive piezoceramic- fiber-composite actuators as macro fiber composite (MFC) actuators has made it possible to obtain strains and displacements greater than those that could be generated by prior actuators based on monolithic piezoceramic sheet materials. MFC actuators are flat, flexible actuators designed for bonding to structures to apply or detect strains. Bonding multiple layers of MFC actuators together could increase force capability, but not strain or displacement capability.

Cylindrical piezoelectric fiber composite (CPFC) actuators have been invented as alternatives to MFC actuators for applications in which greater forces and/or strains or displacements may be required. In essence, a CPFC actuator is an MFC or other piezoceramic fiber composite actuator fabricated in a cylindrical instead of its conventional flat shape. “Cylindrical” is used here in the general sense, encompassing shapes that can have circular, elliptical, rectangular or other cross-sectional shapes in the planes perpendicular to their longitudinal axes. CPFC actuators retain the desirable high strain or displacement and multiplelayer force enhancement capabilities of conventional flat piezoceramic fiber composite actuators. An advantage of the cylindrical over the flexible flat actuators is that the cylindrical shapes impart stiffness, so that unlike the flat actuators, the cylindrical actuators can bear loads even when they are not attached to supporting structures.

Another advantage of the cylindrical over the flexible flat actuators is that displacements of multiple CPFC actuators can be added together. For this purpose, CPFC actuators having different diameters can be assembled in a concentric telescoping arrangement and joined at alternating ends, as shown in the figure. Application of positive drive voltage causes the assembly to elongate in one direction; application of a negative drive voltage causes the assembly to elongate in the opposite direction.

CPFC actuators were first conceived as an improved means of strain-tuning optical-fiber Bragg gratings for applications involving tunable lasers. The ability to add the displacements of multiple selfstiffening CPFC actuators affords greatly enhanced strain-tuning range while still making it possible for strain-tuning assemblies to be compact and lightweight. However, CPFC actuators have potential utility in a broad range of applications beyond those involving tunable lasers. For example, CPFC actuator assemblies might supplant piezoelectric stacks in some applications, particularly those in which lighter weight and enhanced displacement are desirable.

In comparison with CPFC actuators, piezoelectric stacks are heavier and produce much smaller displacements. Displacements produced by piezoelectric stacks can be amplified mechanically, but the mechanisms needed to effect amplification have considerable weight. Other actuators capable of larger displacements include hydraulic or gas piston-cylinder actuators, which are heavier and must be accompanied by supplies of pressurized hydraulic fluids or gases.

This work was done by Sidney G. Allison, Qamar A. Shams and Robert L. Fox of Langley Research Center. For further information, contact the Langley Innovative Partnerships Office at (757) 864- 4015. LAR-17168-1