A servo-control system for detecting micro-displacement based on interferometry

摘要

Enhancing the measurement precision of the micro-displacement has become a significant trend of modern technology. This paper describes a servo-control system that can detect the displacement up to the order of nanometer on the basis of interference and resonance intensifying. The micro-displacement variance of a sample would cause the amplitude change of the microprobe, vibrating at the resonance frequency. By a point-diffraction-interference system, the amplitude change can converted to electrical signal after the photodiode detector. Then, this signal is sent to the feedback control system with PID model. Amplified by the high-voltage circuit, the feedback signal drives the piezoelectric crystal under the sample working stage to flex, keeping the distance constant between the microprobe and the sample. The circuit design possesses compact structure, good noise resistance and high stability. Apply this system to an atomic force microscope (AFM) to scan the sample surface coating aluminum fluoride. The results show that under the condition of 100 kHz resonance frequency, the image quality of sample's microscale topography with this servo-control system is evidently better than that with open-loop system. The operation only costs dozens of seconds to finish scanning 400×400 points and the precision is better than the order of nanometer.