Development and Analysis of the Lumped Parameter Model of a Piezo-Hydraulic Actuator
Nasser, Khalil Maurice
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Hybrid actuation is an expanding field in which several systems, such as a mechanical, electrical, hydraulic, pneumatic, and/or thermal, among others, are integrated in order to combine certain aspects of each system, and achieve a better and more efficient performance under certain operating conditions. The concept of piezohydraulic actuation takes advantage of the high force capabilities that piezoceramics have and combines it with the operation at high frequencies, in order to achieve the hydraulic actuation of a system under a specified stroke and force. High frequency rectification translates the low stroke of a piezoelectric stack into a desired amount of stroke per unit time. Thus, the low displacement, oscillatory motion of the piezoelectric device (coupled with a high frequency operation) is translated into a unidirectional motion of a hydraulic cylinder. As part of this research, a benchtop piezohydraulic unit has been developed and the concept of piezohydraulic actuation has been demonstrated. The effective bidirectional displacement of a hydraulic cylinder through the actuation of a piezoelectric stack has been achieved. A lumped parameter model is developed in order to simulate the dynamics of the hydraulic system and of the entire piezohydraulic unit. The model did approximate the response of the piezohydraulic unit under a one-sided operation. Time response analysis is performed through the frequency spectrum comparison of the measured and the simulated data. Then a two-stage cycle simulation is used to model the pumping operation of the unit. Discrepancies were obtained between the model and the actual system for the single-ended piezohydraulic unit, nonetheless, a good approximation has been achieved for the pumping operation of the double-ended unit under certain conditions. Furthermore, several factors have been identified that may limit the operation of the piezohydraulic unit. First, the need of high displacement actuators often comes with the requirement of high voltage operation along with high current consumptions. Thus, the amplifier becomes the first limitation to overcome. Second, is the response of the controlled valves. The highest valve operating frequency and their time response will set the limit on the piezohydraulic unit. And finally, once these limitations are overcome, the unit is eventually limited by the dynamics of the fluid and the hydraulic system itself. Attenuation in the frequency response, or the operation near resonance and the possibility of cavitation, are some of the aspects that eventually will limit the operation of the piezohydraulic unit. A custom made, high displacement stack is used along with a custom made switching amplifier. The current system is being limited by the second factor, the solenoid valves. Nonethelss the analysis performed has addresed the relevant issues required for the design and use of another set of controlled valves. Finally, the eventual limitation from the hydraulic system has been determined through the analysis of the fluid dynamics of the system. The analysis does not account for potential cavitation, and future operations at high frequencies should take it into account.
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