A force-compensated compliant MEMS-amplifier with electrostatic anti-springs

In this paper, an electrostatic compliant mechanical amplifier intended for force-compensated displacement amplification in MEMS sensor applications is described. Usually, mechanical transformers that enhance a small input displacement into a large output displacement generate large forces at the input of the transformer. The microsystem proposed here allows for the reduction and compensation of the input stiffness of the amplifier and any mechanical components connected to it while providing a constant amplification ratio at the same time. The amplifying mechanism features bidirectional electrostatic anti-springs enabling the control of the stiffness by applying a constant DC voltage. The electrode design of the anti-springs and its influence on the force-displacement characteristic, the side instability and the maximal displacement are studied through analytical approaches and supported by FEA and by experiments. Based on the derived models, a compliant electromechanical amplifier is developed, featuring an amplification ratio of 50. For this amplifier the initial input stiffness of 422 N/m could be reduced to 6.8 N/m by applying a voltage of 100 V. As an additional application, we show how the amplifier can be used as a mechanical force sensor with tuneable sensitivity, where the forces at the input are transformed into large output displacements. Through experiments, we show how the sensitivity can be adjusted and increased by a factor of 25 by applying a voltage at the anti-springs.