Reliability of a MEMS torsional ratcheting actuator

A new surface-micromachined actuator, the torsional ratcheting actuator (TRA), has been characterized and stress tested for reliability. The most prominent failure mechanism when running at high speeds (ratcheting frequencies of 333, 1000, or 3000 Hz) is wear on rubbing surfaces leading to adhesion between the ring gear and the alignment guide. We have also observed failure when running at low speeds (5 Hz) due to misalignment of the ring gear, preventing the ratchet pawl from engaging. A wear model was developed which closely predicts the normal force promoting the wear and is in agreement with supporting calculations. We observed multiple ratcheting in the TRA during calibration and solved the equations of motion to confirm that it was due to the moment of inertia of the large ring gear. Damping was also shown to have an effect.