Effects of corner frequency on bandwidth and resonance amplitude in designing PZT thin-film actuators

In the last decade, lead zirconate titanate oxide (PZT) thin-film actuators have received increasing attention because of their high frequency bandwidth, large actuation strength, fast response, and small size. The PZT film thickness is usually less than several microns as opposed to hundreds of microns for bulk PZT patches that are commercially available. As a result, PZT thin-film actuators pose unique vibration issues that do not appear in actuators with bulk PZT. Two major issues affecting actuator performance are the frequency bandwidth and the resonance amplitude. As an electromechanical device, a PZT thin-film actuator's bandwidth and resonance amplitude depend not only on the lowest natural frequency ωn of the actuator's mechanical structure but also on the corner frequency ωc of the actuator's RC-circuit. For PZT thin-film actuators, the small film thickness implies large film capacitance C and small ωc. When the size of the actuator decreases, ωn increases dramatically. As a result, improper design of PZT thin-film actuators could lead to ωc ≪ ωn substantially reducing the actuator bandwidth and the resonance amplitude. This paper is to demonstrate this phenomenon through theoretical analyses and calibrated experiments. In the theoretical analyses, frequency response functions of a PZT thin-film actuator are obtained to predict 3 dB actuator bandwidth and resonance amplitude for cases when ωc ≪ ωn, ωc ≈ ωn and ωc ≫ ωn. In the experiments, frequency response functions of a fixed–fixed silicon beam with a 1 μm thick PZT film are measured through use of a laser Doppler vibrometer and a spectrum analyzer. The silicon beam has multiple electrodes with a wide range of resistance R and corner frequency ωc. The experimental results confirm that the actuator bandwidth and resonance amplitude are substantially reduced when ωc ≪ ωn.