High-Speed Displacement Measurement of an Ultrasonic Vibratory Tool Using Heterodyne Interferometry and Lock-In Amplifier

In this paper, we set up a mechanical displacement measurement system for an ultrasonic vibration tool at 20 kHz using an interferometer combined with a point-by-point signal processing system based on the lock-in amplifier method. The optical measurement system is a double-path heterodyne interferometer using a Zeeman frequency-stabilized He–Ne head with a few MHz beat frequency. To obtain such high-speed measurements, the interference heterodyne signals should be captured at a sampling rate of a few tens of MS/s to hundreds of MS/s. The calculations of the LIA algorithm should then be continuously performed in point-by-point architecture for each data point at each time or at the same sampling rate in real time. Experiments were created to clarify the measurability of the system. The experimental results have demonstrated that the system can measure the tool's ultrasonic vibrations with a peak-to-peak amplitude of ~ 100 nm at a vibration frequency of 20 kHz. The theoretical analysis and experimental results in this paper are presented.