Abstract: Two beams of laser light are directed onto the surface of a rotary shaft at points (12a, 12b) spaced longitudinally along the shaft from one another. Light backscattered by the optically rough surface of the shaft is detected by means of detectors (18a, 18b) which produce output signals related to the detected intensity. Signal-processing means (20a, 20b) associated with each beam include memory means for storing a reference waveform. The signal-processing means (20a, 20b) provide a signal indicative of the phase of the detected intensity relative to a reference waveform stored in the memory means. A comparator (22) compares the output signals from the signal-processing means (20a, 20b) to provide an indication of torque transmitted through the shaft (12). Alternatively, a single beam and detector may be used to give an indication of angular position of the shaft.

Abstract: A device for monitoring torsional vibrations in a rotary shaft comprises a laser source, a 50% transmission-reflection beamsplitter and a mirror arranged to provide two spaced parallel laser beams and which strike the surface of the shaft at points A and B respectively. Since the shaft surface is moving, light backscattered at the surface is Doppler shifted and since the velocity of the surface at points A and B differs, the frequency of light backscattered at the two points is shifted by different amounts. Light backscattered along the axes of the incident beams is mixed and directed into a detector where it heterodynes. The frequency at which the detected light intensity is modulated is directly proportional to the speed of rotation of the shaft. The device, and the method it embodies, can be used to measure the difference in velocity at two points on any surface undergoing non-uniform, motion and can, therefore, be used generally to characterize vibrational motion.