A Feedback Control System with A Laser Interferometer for Exact and Real-Time Vibration Measurement

Laser interferometer is one of the most sensitive methods for small displacement measurement for scientific and industrial applications. This paper focus on exact and real-time vibration measurement using a PI (Proportional Integral) feedback control system in a Michelson interferometer with a Helium-Neon (He-Ne) Laser light source. The interferometer can measure a vibration of the object in real time by giving the reference PZT a voltage which is generated by the feedback control system using a signal processing circuit system. The sinusoidal phase-modulation frequency is 10 kHz. A multiplier and a low-pass filter (LPF) with a cut off frequency of 400 Hz function to extract a fundamental frequency component of the interference signal. The measured vibration waveform is calculated as the sum of two measured waveforms obtained from the controller output signal and the feedback signal in the feedback system, respectively. This calculation method provides exact measured vibration amplitude with error less than about 8 nanometers at the vibration frequency of 100 Hz. The vibration amplitude measured from only the controller output signal has an error less than about 21 nanometers. The maximum measurable amplitude decreases from 1935 nm to 903 nm when the vibration frequency increases from 50 Hz to 500 Hz. Keywords - Feedback Control System, He-Ne Laser Interferometer, Measurement, Signal, Vibration