Research On Key Technology Of Homodyne Laser Interference Vibration Mearurement

With the advantages of simple structure, the light source can be transport in optical fiber and easy traceability of measurement results, homodyne laser vibrometer has been widely used in vibration measurement. As the extension of the application field of homodyne laser vibrometer, diversity of the vibrating surface can result in the variation of the object reflectivity, and the vibration measurement covers a large frequency range from low frequency with big amplitude – to high frequency of tiny vibration, which make homodyne laser vibrometer develop toward a wider frequency range adapting a wider scope of the fluctuation in reflectivity, to meet the requirement of the high precision, high resolution and high speed for vibration measurement.This dissertation "Research on key technology of homodyne laser vibrometer for various reflective surfaces" mainly aimed to address the vibration measuring for various surface with different reflectivity, the big noise under the condition of low reflectivity and high frequency vibration, and the speed and resolution of phase subdivision contradict each other for high frequency vibration measurement. Firstly, a nonlinear error compensation method for various reflective surfaces was proposed in this paper; secondly, FIR digital low-pass filter and Kalman filter were adopted to suppress the nonlinearity and ensure the validity of measurement resolution; finally, a high-speed phase detection unit based on CORDIC algorithm was designed to implement high-speed real-time detection of vibration. Subject of the main research work is as follows:For vibration measurement, the nonlinear error would be difficult to remove due to the fact that the accuracy of vibration measurement can be affect by the change of reflectivity for different vibrating surfaces. With an analysis that how the output signals in a homodyne interferometer were affected by the reflectivity of vibrating surfaces, an enhanced homodyne interference detection configuration without DC offset was designed, for which in addition to the DC disappeared, unequal AC amplitude error and phase delay error are fixed regardless of the intensity in the measuring arm. On this basis, hardware gain-adjusting correction was applied to reduce the unequal amplitude error and phase delay error between quadrature signals was removed by vector calculation in digital electronics, the nonlinearity can be suppressed effectively against the fluctuation in reflectivity of vibrating surfaces.For vibration measurement, the noise caused by for instance stain of measuring surface and electronic crosstalk can have an influence on measurement resolution and measurement accuracy. An effective method was explored to improve the measuring resolution. A FIR digital low-pass filtering for quadrature signal outputting from a homodyne laser vibrometer was used to reduce the high-frequency noise and improve the SNR of quadrature signal. Then a Kalman digital filter was added to remove the high frequency noise dynamically and improve the effective resolution of the ADC, which could ensure the validity of the measurement resolution.Due to a requirement of high-speed, real-time measurement in vibration, a high-speed phase detection technology based on CORDIC algorithm was proposed in this paper. Quadrants of input signals are converted first to meet the requirements of CORDIC algorithm for input angle, and the arctangent algorithm is converted to multistage assembly line iterative operation to obtain the angle information, then the decimal angle subdivision can be implement after post-processing. The integer part of the angle could be obtained with the four subdivision counting and direction discrimination. The integer part, decimal part of phase can be output synchronously by using multistage pipeline, then the phase can be measured accurately.Finally, based on the above methods and techniques, a homodyne laser vibrometer named “UOI_LSV” was developed. The equivalent test experiment results show that under the condition of the reflectivity of vibrating surfaces ranges 10% to 100%, the residual nonlinear error was less than ±0.15 nm with a measuring resolution of 38.6 pm, the maximum measurement speed was 1.582 m/s, the update rate of displacement measurement data is 40 MHz. The direct experimental results indicated that the developed homodyne interferometer could get a displacement measuring resolution of 0.3 nm residual nonlinear error was less than ±1.98 nm. The vibration frequency measurement range was DC ~ 130 KHZ, the deviation of vibration amplitude measurement was less than ±10.3%.