Energy-Based Comparison of High-Power Commercially Available Induced-Strain -State Actuators

A study of published literature and information from piezoelectric, electrostrictive, and magnetostrictive actuator vendors has been undertaken to establish the mechanical and electrical operating characteristics of the actuators, and to compare them using output energy density criteria. Output energy values of up to 0.666 J can be achieved with off-the-shelf actuators. Energy density per unit volume was found in the range 1.816-7.280 J/dm. Energy density per unit mass was found in the range 0.233-0.900 J/kg. In one isolated case, higher energy densities of 11.9 J/dm and 1.09 J/kg were identified for a small co-fired PMN stack of 0.00481 J total energy. Energy transformation efficiency between input electric energy and output mechanical energy was found to be: 17-27% for adhesively-bonded PZT and PMN stacks, 5-20% for co-fired PZT and PMN stacks, and 67.1% for TERFENOL-D devices. The overall performance of induced-strain actuators based on output energy density criteria was found to vary widely from vendor to vendor, and even from one model to another within the same vendor catalogue list. These variations are attributed to progress being made currently in both the active material technology and in the detailed mechanical construction of inducedstrain actuators based on these materials. INTRODUCTION The use of solid-state induced-strain actuators has experienced a great expansion in recent years. Initially developed for high-frequency, low-displacement acoustic applications, these revolutionary concepts are currently expanding in their field of application in many other areas of mechanical and aerospace design. Compact and reliable, induced-strain actuators directly transform the input electrical energy into output mechanical energy. One application area in which solid-state induced-strain devices have a very promising perspective is that of linear actuation. At the moment, the linear actuation market is dominated by hydraulic and pneumatic cylinders, and by electromagnetic solenoids and shakers. Hydraulic and pneumatic cylinders offer reliable performance, with high force and large displacement capabilities. When equipped with servovalves, the hydraulic cylinders can deliver variable stroke output over a relatively large frequency range. Servovalve-controlled hydraulic devices are the actuator of choice for most aerospace, automotive, and robotic applications. However, a major drawback in the use of conventional hydraulic actuators is the need for a separate hydraulic power unit equipped with large electric motors and hydraulic pumps that send the high-pressure hydraulic fluid to the actuators through hydraulic lines. These features can be a major drawback in certain applications; for example, in the actuation of a servo-tab placed at the tip of a rotating blade, the high-g environment, and the fact that the blade rotates, prohibit the use of conventional hydraulics. In such situations, an electro-mechanical actuation that directly converts electrical energy into mechanical energy is preferred. Conventional electromechanical actuator devices, that are based on electric motors, either deliver only rotary motion or require gearboxes and eccentric mechanisms to achieve linear motion. This route is cumbersome and leads to additional weight being added to that of the device, thus reducing its design effectiveness. Linear-action electro-mechanical devices, such as solenoids and electrodynamic shakers, exist, but are known for their typical low-force performance. The use of solenoids or electrodynamic shakers to perform the actuator duty-cycle of a hydraulic cylinders is not presently conceivable. Solid-state induced-strain actuators offer a viable alternative. Though their output displacement is relatively small, they can produce remarkably high force. Through the use of well-architectured displacement amplification, induced-strain actuators can achieve dynamic output strokes similar to those of conventional hydraulic actuators. Additionally, unlike conventional hydraulic actuators, solid-state induced-strain actuators do not require separate hydraulic power units and long hydraulic lines, and use the much more efficient route of direct electric supply to the actuator site. The development of solid-state induced-strain actuators has entered the production stage, and actual actuation devices based on these concepts are likely to reach the applications market in the next few years. An increasing number of vendors are producing and marketing solid-state actuation devices based on induced-strain principles. However, the performance of the basic induced-strain actuation materials used in these devices, and the design solutions used in their construction, are found to vary from vendor to vendor. This variability aspect presents a difficulty for the application engineer who simply wants to utilize the solid-state inducedstrain actuators as prime movers in their design, and does not intend to detail the intricacies of active materials technology. Recognizing this need, the present paper sets out to perform a comparison of commercially-available induced-strain actuators based on a common criterion: the amount of energy that they can deliver, and the density of this energy per unit volume, unit mass, and unit cost. Additionally, this paper also compares the efficiency with which various induced-strain actuators convert the input electrical energy into output mechanical energy for use in the application. The comparison is done using vendor-supplied information collected in an extensive survey performed over approximately a one-year period. BASIC ASPECTS OF INDUCED-STRAIN ACTUATORS ELECTROACTIVE AND MAGNETOACTIVE MATERIALS Active materials exhibit induced-strain actuation (ISA) under the action of an electric or magnetic field. They are primarily of three types: 1 The acronym ISA is used to signify either an induced-strain actuator, or the induced-strain actuation principle (a) = V uISA Applied voltage Free-displacement Stack of active material layers Intercalated electrodes (b) I uISA Free displacement