Shaft tilt sensitivity of the laser torquemeter

The Laser Torquemeter is an instrument designed to measure time-resolved torque on a rotating shaft. The torquemeter uses two probe laser beams, axially separated on a shaft of known mechanical properties. When the shaft rotates, the backscattered speckle patterns, observed by a photodetector, change continuously but repeat exactly with each revolution of the shaft. The shaft is rotated at a low (ideally zero) torque level and the photodetector signal is recorded. Measurements of shaft twist are obtained by comparing the photodetector signals from a torsionally-loaded rotating shaft with the previously recorded photodetector signal. Reliable operation of the Laser Torquemeter depends on the repeatability of the speckle patterns scattered from the surface of the shaft. If the shaft tilts-pitch and yaw motion-as it rotates, the backscattered speckle pattern moves at a velocity proportional to twice the shaft tilt rate and the similarity between the photodetector output signal and the recorded signal is reduced. To determine typical values of tilt, measurements are taken from the drive shaft of a 4 cylinder diesel engine. In order to design a reliable torquemeter, knowledge is required of how the photodetector output and the stored reference signal de-correlate as a function of shaft tilt. This paper examines several optical configurations and an optimum configuration for achieving resistance to decorrelation due to shaft tilt is recommended. This completes an earlier study(1) which optimised the resistance to decorrelation due to in-plane motion of the shaft. Both issues must be addressed in the design of a robust torquemeter.