Preliminary Study On Vibration Measurement Based On Laser Doppler Technology For Numerical Control Machine Tools

As modern engineering and technology developed rapidly recently, especially in the domain of aviation, spaceflight and manufacture, it is necessary to make high accuracy measurement of vibration. In the area of manufacture,vibration measurement and analysis with laser interferometry has become a key method to identify and determine the authenticity of industrial machine. Laser Doppler technique has many advantages such as high accuracy, good spatial resolution, rapid dynamic response, large measuring range, and non-contact, so it represents the developing direction of vibration measurement.In this paper, the advantages, disadvantages and application range of several non-contact vibration measurement methods are discussed. After reviewing the development history and current status of the laser Doppler technology, the principle of laser Doppler technique is introduced and the Doppler frequency shift formula is presented. The advantages and disadvantages of three optical models are discussed and analyzed.Based on the characteristics of the Doppler signal, the scheme of signal processing is presented. We apply the least mean square (LMS) adaptive filter technology, the frequency spectrum zooming and frequency spectrum correction technology in the signal processing system. Basic principle of LMS adaptive filter and frequency analysis algorithms were expounded first, then spectrum simulation of ideal sinusoidal signal and experiment test were studied based on these algorithms. The simulation results indicate that the LMS adaptive filter technology has effectively improved signal to noise ratio(SNR) and minish the interference of noise; Resolution of frequency spectrum can be improved by frequency spectrum zooming technology and the accuracy of signal processing can be greatly improved by frequency spectrum correction technology. From the accuracy Doppler frequency, it is easy to deduce the speed, acceleration, displacement and vibration information of object. The programs of LMS adaptive filter, frequency spectrum zooming and frequency spectrum correction are written and base on mixed programming technology between LabVIEW platform and MATLAB platform.Experimental systems of the LDV are built to do some experiment research on speed and vibration measurement. Firstly, a differential laser Doppler system is set up for measuring the tangential velocity of a chopper in order to identify the measurement accuracy of the experimental system and the application feasibility of the signal processing technology. The experiment results indicate that the mean relative accuracy of the laser Doppler system using fast Fourier transform(FFT) to process signal is 1.67%, compared with 0.23%, which is using LMS technology, frequency spectrum zooming and correction technology in the signal processing system. Secondly, a reference beam laser Doppler system is built to measure the vibration information of a loudspeaker. The experiment results indicate that this system is suitable for vibration measurement and it is adequate to measure vibration of 20~1000Hz and 1um~1mm. Measurement errors of the LDV are analyzed systematically and methods to reduce the error in various situations are put forward. It is the guidance for us to design the optic system and the circuitry.