Actuator fault detection via electrical impedance testing

Lead magnesium niobate (PMN) actuators are electrostrictive actuators with high dynamic range used in deformable mirrors. Actuator fault detection in deformable mirrors typically occurs through optical testing. We developed a nonoptical method for detecting actuator faults via low electric field resonance testing. The low electric field resonance method is standard practice for characterizing piezoelectric materials. The piezoelectric/electrostrictive coefficient couples the electrical and mechanical impedance of the actuator; a change in the mechanical boundaries (force) on the actuator results in a shift of the impedance resonances. We demonstrate experimentally that a PMN actuator can fracture but retain functionality under compression and that the fracture can be detected by measuring the impedance resonances at various bias voltages (various values of tension and compression). A concurrent optical test using a displacement interferometer was used to corroborate the results. We propose the impedance resonance approach as a non-optical fault detection test for in-situ actuators.