Mechanical and electromechanical nonlinearities of a piezoelectric trimorph type actuator

An experimental study was made of the mechanical and electromechanical nonlinearities of a PZT trimorph type actuator. The actuator consisted of two piezoceramic poled plates, sandwiched on a thin metallic sheet, forming the usual trimorph structure, with one end fixed and the other one free. The experiments were conducted in two steps. In the first step, a force was applied to the free end of the trimorph and its displacement was measured as a function of the applied force either for increasing and decreasing force. The nonlinearities in this case must be purely mechanical and they give information about the structural arrangement of the piezoceramic elements, glue and metallic sheet. For the second step, the actuator was electrically activated with increasing and decreasing DC voltages and the displacement of the free end was determined as a function of applied DC voltage. The nonlinearities in this case are of electromechanical nature and they were the result of the combined effect of intrinsic material nonlinearities and structural arrangement. The experimental results showed that pure mechanical nonlinearities are practically inexistent even for forces up to 0.7 N. The displacement of the free end follows the theoretical predictions. When the actuator was electrically activated the displacement of the free end was entirely hysteretic and the electromechanical nonlinearities were different from the theoretical predictions. The results were discussed in terms of the displacement of ferroelastic-ferroelectric domain walls.