Compact yet powerful actuators are vital in many robotic applications, particularly small-scale autonomous systems such as bio-inspired microrobots. In recent years, a number of actuation methods have been proposed or applied in a microrobotic context, including piezoelectric , electrostatic , and dielectric elastomer actuators . These actuation methods have the potential to achieve high efficiencies and power densities in very small geometries. Piezoelectric actuators in particular have shown promise in applications with very stringent weight and power density requirements, such as the Harvard Microrobotic Fly (HMF)—a flapping-wing robotic insect capable of liftoff with external power .
In order to produce mechanical output, the actuation methods mentioned above rely on the presence of electric charge on various electrodes in order to either generate high electric fields, as in the case of piezoelectric actuators, or high electrostatic forces, as in the case of electrostatic and dielectric elastomer actuators. Moreover, the geometries of such actuators inherently produce significant electrical capacitance, and therefore high operating voltages are usually necessary to accumulate a sufficient amount of charge on the actuator electrodes, ranging from tens to thousands of volts. For example, the piezoelectric actuators used in the HMF require drive voltages in the range of 200–300V. There are two major challenges in the design of power electronics capable of driving capacitive actuators: generating high voltages from low-voltage sources and recovering unused energy from the actuator.
Most compact energy sources suitable for microrobotic applications, such as lithium batteries, supercapacitors , solar cells , and fuel cells , generate output voltages below 5V. Connecting many such cells in series to obtain high voltage is generally not practical because the packaging overhead causes a significant reduction in energy density. Consequently, the generation of high voltages for HMF actuators requires voltage conversion circuits with step-up ratios ranging from 50 to 100. While there are a number of circuit topologies with high step-up ratios, many of them cannot be easily miniaturized and/or suffer from poor efficiency at the low output power levels common in microrobotic applications. Careful selection and optimization of the conversion circuit is necessary to ensure that heavy, inefficient electronics do not compromise system performance.
In addition to the voltage step-up functionality, the power electronics circuitry must generate a time-varying signal on the input electrodes of the actuator. The second challenge stems from the fact that, depending on the properties of the actuator, the nature of the mechanical load, and the characteristics of the drive signal, only a small fraction of the electrical energy stored in the actuator is converted into useful mechanical output . In order to maximize overall system efficiency, it is highly desirable to both generate an appropriate drive signal and recover as much of the unused energy as possible, which imposes additional requirements on the drive circuitry.
This paper describes promising power electronics circuits that can generate the high, time-varying voltages necessary for the operation of piezoelectric actuators, while meeting the stringent weight requirements of microrobotic systems and maximizing system efficiency. Although the analysis focuses on piezoelectric actuators, many of the concepts described here can easily be adapted to other high-voltage capacitive actuators, such as electrostatic comb drives or dielectric elastomer actuators. This work reviews the electrical properties and drive requirements of piezoelectric actuators (Section II), and presents power electronics circuits applicable to various types and configurations of piezoelectric actuators (Sections III and IV). Experimental realizations of the drive circuits are described (Section V), including applications to milligram-scale microrobots, such as flapping-wing robotic insects.