Theoretical And Experimental Studies Of Laser Speckle Statistics On Shaft Torque Measurement

In this thesis, theoretical and experimental studies of dual laser beam torque meter based on distribution characteristics of laser speckle reflected from rotate shaft are presented. Laser spackle experiments are carried out by using a HN150 laser beam generator based on speckle statistics theory. According to the test results, the reflected speckle distributions and the statistics characteristics when the measured target in rotation and/or translate moving are discussed, the general expressions of the speckle intensity and its phases in complex plane are deduced. The correlation functions of the speckle intensity related to the laser torque measurement and the spatial characteristic of the detector, which has a fixed acting area, are emphatically discussed, specially the sensitivities of the speckle to the axial vibration and to the rotation of the rotate shaft. Also, the speckle disturbances are studied, for examples the interference due to the length differences of the two laser beam routes, the "pseudo-vibration", the frequency broadening. Then the principle of the laser torque measurement is introduced. When the measured shaft is rotating, the speckle images in any observation plane appear periodically and accurately accompanying periodic rotation, negative the shaft's vibrations. When the shat is twisted under loads, the periodicity, frequencies and amplitudes of the speckle signal are without any variance, but their phase changes. The variable value of the phase presents the shaft twist value, the torque.A method to strength the speckle intensity is presented in the thesis. Using this method, a kind of periodic speckle signal with lager amplitude can be obtained to solve the problem that the speckle signal from using general reflect tape is usually very weak and like white noise. After this, the theory and method on time-frequency analysis are discussed. Based on the theory, the laser detector output signal is analyzed by using the wavelet di-noise method with given threshold value, then using the Wigner-Villy distribution method and using the FFT analyzing. Finally the phases and amplitude of the speckle signal are obtained. This is we need.Following the studies mentioned above, a dual laser beam torque meter is designed and developed by the author, including its consisting and its working principle. Also, a test rig is set up. All the experiments are carried out on it. Through the studies conclusions are drawn as following:(1) The theoretical and experimental results show that the speckle images gathered by using suitable laser detector appear periodically, this signal is proportional to the integrative intensity acting onto the effective area of the detector. This is proved by that the harmonic signal amplitudes at the shaft revolution frequency and double that are approximately same.(2) The statistic characteristics of speckle reflected by the shaft in longitudinally moving and/or rotating dominate the working behavior of the laser torque meter. In laser torque meter system there are two contradictory requirements each other; one is that the inferences to the speckle periodicity due to the shaft axial motion should be deduced as far as possible to minimize the sensitivities caused by the axial movement, another is to maximize the sensitivities caused by the shaft rotation.(3) The studies of the speckle characteristics due to shaft axial movement show that three parameters can be adjusted to maximize the correlation distance. The first is to change the separation from the beam waist to target, this can change both the incident beam's radius of curvature and its diameter which also to change the speckle size. The second is that changing the target-detector separation is also to change the speckle size. The third is that easily adjustable variable is the detector size itself where an increase should provide a corresponding increase. The most importance is the separation between detector and target, which should be less than 0.5m.(4) Speckle disturbances such as the interference due to the length differences of two laser beam routes, the "pseudo-vibration", the frequency broadening influence the measurement accurate of the laser torque meter. Discussed the principles of disturbance carouses of formation mentioned above, some methods to anti-disturbance in laser torque measurement are presented.(5) Some parameters are determined such as the laser beam diameter, the size of the detector, the distance between the detector and the target.(6) A method to strength the speckle intensity is presented to solve the problem that the speckle signal from using general reflect tape is usually very weak, a periodic speckle signals with lager amplitude is obtained. After this, the theory and method on time-frequency analysis are discussed. Based on the theory, the laser detector output signal is analyzed by using the wavelet di-noise method with given threshold value, then using the Wigner-Villy distribution method and using the FFT analyzing. Finally the phases and amplitude of the speckle signal are obtained. This is we actually need.(7) The general principle of laser torque measurement presented in this thesis is proved by author's experiments. The experimental results are satisfactory, and show that this technique can be used in practice application.