Non-linear electromechanical behaviour of piezoelectric bimorph actuators: Influence on performance and lifetime

Piezoelectric bimorph bender actuators find application number of areas, ranging from automotive to health care. High voltage operation in harsh environments poses ever more stringent demands on functionality and lifetime. In these high performance benders, the trade-off between functionality and lifetime becomes of increasing importance, and a model-based understanding is imperative for further tailoring actuator performance. An important issue in this is the non-linear electromechanical coupling and mechanical poling/depoling processes. PZT materials are known to exhibit non-linear stiffness when strained. This effect is attributed to domain re-orientation and can result in mechanical depoling. In bimorph benders where only one of the PZT members is actuated, the non-linear stiffness effect results in higher deflections than are predicted by linear elastic models. This effect is larger for soft PZT materials than for hard PZT materials. In this work, the non-linear behaviour of piezoelectric bimorphs was characterized by dynamic mechanical thermal analysis (DMTA). Unimorph benders were characterized in different configurations. Two different PZT materials (one soft, one hard) were used. The non-linear stiffness behaviour is included in a model for piezoelectric multimorphs. The multimorph model predictions are compared to measured deflection and blocking force of bimorph benders made from soft PZT material. The model incorporating non-linear stiffness predicts deflection and blocking force values more accurately than the linear model for the situation studied. Attention is paid to the stresses induced during poling of the bimorphs and their effect on performance and lifetime.