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US 5,288,995 AGranted
Electrical measurement apparatus using heterodyne phase conversion techniques
AssigneeOptical Metrology Limited
InventorJohn S. Strachan
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AssigneeOptical Metrology Limited
InventorJohn S. Strachan
Estimating the term…
Precision displacement and load measurements dependent upon the intensity or attenuation of a signal, as by use of Moire fringe gratings or photoelastic load cells, are made using speed-of-light ranging techniques based on heterodyne frequency conversion with multiple phase-locking. Coiled optical fibres introduce phase delays in radio-frequency-modulated laser rays which are then compounded in phase by selective signal attenuation and merger. The phase is then measured by high precision circuits involving ceramic filters and translated into position or load data. Machine tool control and strain gauge applications are described. The ceramic filter circuit generates a signal measuring rate of change of displacement as a by-product, thereby facilitating measurements involving rapid motion.
Electrical measurement apparatus in which the condition or position of an optical transducer element is represented by the relative attenuation of the intensities of two optical signals both derived from a common laser source which produces a light ray amplitude-modulated at a radio frequency, comprising (a) circuit path means for dividing an input from the laser source between a plurality of circuit paths each conveying a transmitted signal at the speed of light (b) propagation delay means in at least one of the paths for retarding the respective transmitted signal, whereby the transmitted signals have different phase at the radio frequency (c) an optical transducer element positioned in the paths of the transmitted signals and operative to adjust relative signal intensity in dependence upon the state of the apparatus requiring measurement and as determined by that transducer, (d) circuit means for recombining the transmitted signals after attenuation to provide as a received signal a single electrical output, and (e) signal processing means for measuring the phase of the electrical output relative to the radio frequency modulating signal input controlling the laser and interpreting this to provide the required measurement, the signal processing means incorporating (f) heterodyne conversion means, whereby a phase-locked version of either the signal input or the electrical output is mixed with the other in each of a pair of mixer circuits to produce two throughput signals having an extended time-spaced measure of the phase.
Electrical measurement apparatus in which the load condition producing strain in a photoelastic load cell is measured by transmitting a polarized laser-generated optical signal through the load cell, the apparatus comprising a laser source, circuit means connected to regulate the laser power to amplitude-modulate the laser-generated optical signal at a radio frequency and thereby also produce a reference signal at the radio frequency as a phase reference, optical polarizing filter means positioned in the path of the optical signal output from the load cell for responding selectively to the separate optically polarized ray components of the signal emerging from the load cell, circuit path means for guiding the separate ray components along separate paths, at least one of which has a delay characteristic which retards the phase of the radio-frequency modulation of the transmitted ray component, circuit coupling means for recombining the ray components by merging their signal waveforms, thereby producing an electrical signal having a phase related to the relative intensity of the radio-frequency modulation of the two ray components, and signal processing means for measuring the phase of that produced electrical signal relative to the phase of the reference signal.
Dependencies are inferred from the claim text.