A Multi-Cellular PZT Actuator/Generator with Tunable Stiffness and Resonant Frequencies

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A skeletal muscle consists of a number of independent muscle fibers, each producing a mechanical output efficiently in a compact body. Collective behaviors of a bundle of muscle fibers exhibit unique features and functionality that today's actuators are unable to produce. Unlike conventional electric motors, for example, muscles are mechanically flexible and adaptable to load conditions. Muscles can vary effective stiffness and output impedance in a wide range. Furthermore, a variety of muscles having a wide range of size, power, and degrees of freedom can be built from the same module, i.e., the muscle fiber. Inspired by this skeletal muscle architecture, this project investigates cellular artificial muscles using actuator materials, in particular, PZT (Lead Zirconate Titanate). Like a muscle fiber, a PZT cellular unit is an independent unit that is designed for optimal efficiency and functionality. Using an effective flexure, each PZT cell can produce large displacement and force comparable to a skeletal muscle fiber. Such an optimal unit is inevitably small to drive a large load. Arranging the PZT cells in series, parallel, and antagonistic configurations, a bundle of PZT units collectively exhibit unique features and functionality that a single bulk PZT actuator cannot produce. These include: A). Variable stiffness: Switching individual cellular units ON or OFF creates a significant change to the aggregate stiffness of the multi-cellular system. B). Tailored force-displacement characteristics: By exploiting the ON-OFF nonlinearity of individual units and superimposing the multitude of the nonlinear functions, we can tailor the aggregate force-displacement curve to task goals and environment conditions. This allows us to physically implement the "work loop" characteristics of animal motion at the actuator level. C). Variable resonant frequencies: The multi-cellular PZT actuator forms a network of mass-spring systems due to the combined effect of a flexure and PZT stack at each unit. This multi d.o.f. system has resonant frequencies at which the output displacement becomes much larger than its static stroke, making cyclic motion, such as flapping and running, very efficient. Furthermore, ON-OFF switching of individual units causes a change to the mass distribution and thereby makes the resonant frequency highly tunable. D). Energy harvesting: The new actuator is completely backdriveable having negligibly small friction. Low friction and tunable resonance capability enables a multi cell array to capture energy at a resonant frequency that is the most effective for the environmentally imposed forcing.