This paper presents a micro heat engine fabricated in silicon micro technology. Its operation principle is based on a cavity filled with a liquid-gas phase-change working fluid that performs a self-controlled reciprocating motion between a heat source and a heat sink. A bistable buckling membrane generates the respective upward and downward driving forces upon expansion and contraction of the working fluid. For prediction of the engine performance a hybrid model is developed. This model predicts an operation frequency of 0.72 Hz and a mechanical output power of 1.29 μW at a temperature difference of 37 K. Loss mechanisms are theoretically explored and ways to enhance the overall engine efficiency are discussed. To verify this model, a functional demonstrator is fabricated. In the experiments, an operation frequency of 0.71 Hz is found at a temperature difference of 37 K.

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The German Artificial Sphincter System project aims at the development of an implantable sphincter prosthesis driven by a piezoelectrically actuated micropump. The system has been designed to be fully implantable, i.e. the power supply is provided by a rechargeable lithium polymer battery. In order to provide sufficient battery duration and to limit battery dimensions, special effort has to be made to minimize power consumption of the whole system and, in particular, of the piezoactuator driver circuitry. Inductive charge recovery can be used to recover part of the charge stored within the actuator. We are going to present a simplified inductor-based circuit capable of voltage inversion across the actuator without the need of an additional negative voltage source. The dimension of the inductors required for such a concept is nevertheless significant. We therefore present a novel alternative concept, called direct switching, where the equivalent capacitance of the actuator is charged directly by a step-up converter and discharged by a step-down converter. We achieved superior performance compared to a simple inductor-based driver with the advantage of using small-size chip inductors. As a term of comparison, the performance of the aforementioned drivers is compared to a conventional driver that does not implement any charge recovery technique. With our design we have been able to achieve more than 50% reduction in power consumption compared to the simplest conventional driver. The new direct switching driver performs 15% better than an inductor-based driver. A novel, whole-system SPICE simulation is presented, where both the driving circuit and the piezoactuator are modeled making use of advanced nonlinear models. Such a simulation is a precious tool to design and optimize piezoactuator drivers.

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