Induced-strain multimorphs for microscale sensory actuation design

The paper proposes an analytic model that evaluates the levels of actuation and sensing by microbenders formed of several sandwiched layers (multimorphs) of active/sensitive materials such as piezoelectric (PZT) and shape memory alloys (SMA) interspersed with structural materials which only react to thermal variations. The model is generic, such that any type of induced strain can be incorporated into it, and its prediction gives the deformed (bent) shape, the corresponding tip bending moment, as well as the position of the neutral axis (NA) and the composite bending rigidity. The generic model reduces to the well-known Timoshenko 'bi-metal thermostat' model in the case of two layers of which one is thermally heated. Finite element simulation produces results that are very close to the analytic model predictions for a three-layer (trimorph) bender when all layers are heated. Concrete examples are further studied that can be encountered in small-scale (MEMS) applications, such as an antisymmetric PZT trimorph with either unilateral actuation and sensing or just antisymmetric actuation, a SMA-actuated bimorph and a hybrid PZT-SMA trimorph that can act as a simultaneous sensory actuator microdevice. Numerical simulations are performed for all examples in order to highlight trends in performance as a function of the microcantilever geometry.