Efficiency of Frequency-Rectified Piezohydraulic and Piezopneumatic Actuation

Piezohydraulic and piezopneumatic frequency rectification is analyzed with a steady-state model of work and energy transfer. In this paper, piezohydraulic actuation is defined as the use of incompressible fluid to rectify the oscillatory motion of a piezoelectric device into unidirectional motion; piezopneumatic actuation is the use of a compressible gas such as air. The steady-state analysis is based on the definition of two efficiency metrics. The mechanical efficiency is defined as the ratio of mechanical work transferred to the load to the total mechanical work performed by the actuator, and the electrical efficiency is defined as the ratio of electrical energy expended to the peak energy required. The analysis demonstrates that piezohydraulic rectification is inherently more efficient than piezopneumatic rectification due to the assumption of fluid incompressibility. The mechanical efficiency is 100% when using incompressible fluid (ideal) but drops to below 40% when using compressible gas. The maximum electrical efficiency of piezohydraulic rectification is also four times higher than that of piezopneumatic actuation (29% to 7%). The overall efficiency of the rectification process (defined as the product of the mechanical and electrical efficiencies) is an order of magnitude greater when using incompressible fluid than when using compressible gas (29% to 2.8%). Thus, frequency rectification using compressible gas will require peak energy inputs that are ten times greater than that of incompressible fluid for an equivalent amount of mechanical work output. The significant difference in peak energy requirements is attributed to the reduced coupling between the actuator and the load that occurs due to compressibility of the gas. These results represent an upper bound on the efficiency because they neglect other sources of system losses, such as material hysteresis and frictional losses within the piston and seals of the actuator.