The application of laser Doppler technique to vibration measurement and position control
The laser Doppler interferometer reported here was developed to investigate the possibilities of remote vibration and motion measurements. The method is noncontacting and operates with unprepared targets, using the diffusely scattered light to measure the axial component of the motion. A full description of the motion requires both magnitude and direction of the target motion. The magnitude was found by standard heterodyning techniques, mixing light scattered from the target with a part of the original laser output in a controlled manner. A phase quadrature method was used to identify the direction of the target. This differs from the more usual method of frequency offsetting in requiring only passive optical components and therefore being considerably cheaper. This feature is believed to be novel to the LDI reported here. Measurements were recorded for target motions over the range 100 mm. to (c. ) 1 um. Because unprepared and therefore optically rough targets were used the light received by the detectors was not well behaved. This resulted in instability of the sense of motion signal due to loss of either of the detector signals for displacements above 500 um. However this should not be considered an upper limit to the range of the LDI, as serious loss of the sense signal was rare up to (c. ) 25 mm. and measurements were made up to a peak displacement of 200 mm. Correlations with an accelerometer and an LVDT show that the LDI can reliably measure displacement up to a range of 25 mm. with a maximum target velocity of 32 mm/s limited currently be the signal processing. Theoretical resolution with this device is better than 0.08 um. if full use is made of both detected signals.