Design Phase Elongation Feedback Sensor PZT Shifting Interferometry

Phase-shifting interferometry suffers from two main sources of error:one of them is PZT nonlinearity. In order to minimize the impact on Phase-shifting interferometry system, capacitive sensor is introduced in PZT elongation feedback loop.Capacitive displacement sensor supplies high accuracy with low demand, and is chosen as the feedback sensor by comparative analysis of various micro-displacement sensors. In this paper, AD7746based signal-conditioning circuit module, which employs a sigma-delta modulator to measure capacitance directly, is designed to handle the output capacitive signal; AD5791based high accuracy voltage source, which has24.8μV output resolution with0.25%full range nonlinearity, is designed to move the PZT of the phase shifter. Based on numerical experiments, the capacitive sensor shows a resolution better than lOnm, and a measurement nonlinear within0.46%. Finally, the capacitive sensor is applied into two-wavelength phase-shifting interferometry system. The nonlinear hysteresis behavior of the existing actuator is tested. And one method, that combined voltage calibration curve and real time sensor data, is proposed to improve the performance of the phase-shifting actuator. The actual phase drift, during the testing process, is calculated from the corresponding interferograms using conventional PCA algorithm, the comparison result demonstrate the validity of the feedback method.