The laser torquemeter and implications of speckle decorrelation on torque measurement
Torque measurement of a rotating shaft is a method of monitoring machine performance. Steady transmission of mechanical power from the prime mover to the load is vital to avoid gear and bearing wear, shaft fatigue failure, bearing and coupling failure and noise. Mean and fluctuating torque are fundamental quantities of interest. The laser torquemeter is capable of providing a non-contact measurement of time-resolved torque through a driven system. The laser torquemeter depends upon laser speckle produced from coherent light illuminating a diffuse object and the speckle pattern may be used in determining the angular position of a rotating object. When the object rotates the backscattered speckle pattern, which changes continuously but repeats exactly with every revolution, is sampled by a suitably positioned photo detector. The photo detector output signal is periodic and one period is recorded in memory as a reference and the angular position of a shaft can then be determined by a comparison of this recorded reference signal with the current photo detector output signal. The speckle pattern from two axially separated points on the shaft are monitored and under low or, ideally, zero torque the photo detector outputs are recorded into the laser torquemeter electronics. The laser torquemeter then tracks the live photo detector output and determines the angle at the two points on the shaft. Relative angular displacement in the two angle outputs appears when torque is applied and the shaft twists. When the shaft is displaced, for example by vibration, the backscattered speckle pattern changes on the photo detector and the similarity between the recorded, reference signal and the live, current signal is reduced. In this thesis, the cross-correlation of the real-time photodetector output signal and the recorded reference signal as a function of shaft position is examined. The effects of various shaft motions - rotation, axial translation, pitch and yaw, and radial translation are theoretically and experimentally examined and the results can then be used in the design of an optical head for the laser torquemeter. A review of the current torquemeter technology allows for discussion of the broad spectrum of typical torquemeter operating conditions. The optical head of the laser torquemeter may vary significantly for various torque measurement scenarios. A design procedure for the optical head of the laser torquemeter is summarised. The holy grail of torquemeter manufacturers is to produce a cheap, easy to use, robust, accurate, reliable and non-contacting torquemeter. The laser torque meter has great potential to meet these requirements.