Fiber optic position sensing system

Abstract

A fiber optic sensing system for determining the position of an object requires a light source, an optical fiber, a fiber optic splitter, a fiber tip lens, an optical detector and signal processing circuitry. Light emitted by the light source is conveyed via optical fiber and the splitter to the lens and onto an object, such that at least a portion of the light is reflected by the object and conveyed via fiber and the splitter to the detector. Signal processing circuitry coupled to the detector determines the position of the object with respect to the lens based on a characteristic of the reflected light. The system is suitably employed with a hydraulic accumulator having a piston, the position of which varies with the volume of fluid in the accumulator, with the system arranged to determine the position of the piston, from which the volume can be calculated.

We claim: 
     
         1 . A fiber optic sensing system for determining the position of an object, comprising:
 a light source;   optical fiber;   a fiber optic splitter;   a fiber tip lens;   an optical detector; and   signal processing circuitry;   said system arranged such that light emitted by said light source is conveyed via optical fiber and said splitter to said fiber tip lens and onto an object, the position of which is to be determined, and such that at least a portion of said light conveyed onto said object is reflected by said object and conveyed via optical fiber and said splitter to said optical detector;   said signal processing circuitry coupled to said optical detector and arranged to determine the position of said object with respect to said fiber tip lens based at least in part on a characteristic of said reflected light.   
     
     
         2 . The system of  claim 1 , wherein said light source is a laser. 
     
     
         3 . The system of  claim 1 , wherein said fiber tip lens is a collimator. 
     
     
         4 . The system of  claim 1 , wherein said light conveyed onto said object is reflected by means of specular reflection. 
     
     
         5 . The system of  claim 1 , wherein said light conveyed onto said object is reflected by means of retroreflection. 
     
     
         6 . The system of  claim 1 , further comprising a retroreflective surface affixed to said object such that said light conveyed onto said object impinges on said retroreflective surface and is reflected by means of retroreflection. 
     
     
         7 . The system of  claim 1 , wherein said object is a piston or a bladder. 
     
     
         8 . The system of  claim 1 , wherein said light source is arranged to emit pulses of light, said signal processing circuitry arranged to measure the time required for a given pulse to travel from said light source to said object and back to said optical detector, said time varying with the distance of said object from said fiber tip lens. 
     
     
         9 . The system of  claim 1 , wherein said signal processing circuitry is arranged to compare the intensity of said light emitted by said light source with the intensity of said light reflected by said object, the difference between said intensities varying with the distance of said object from said fiber tip lens. 
     
     
         10 . The system of  claim 1 , wherein said system components form a Michelson interferometer in which the light emitted by said light source is split by said fiber optic splitter into a component that is conveyed to said object and a component which is conveyed via an optical fiber to a reflective surface, and such that light reflected by said object and light reflected by said reflective surface are recombined such that the resulting interference pattern varies with the distance of said object from said fiber tip lens. 
     
     
         11 . The system of  claim 10 , wherein said light source is a laser and one of the arms of the interferometer has a fixed length. 
     
     
         12 . The system of  claim 11 , further comprising a phase modulator arranged to modulate the phase of said light emitted by said laser, said signal processing circuitry coupled to said phase modulator and arranged to adjust the frequency and/or the amplitude of said phase modulation such that the position of said object with respect to said fiber tip lens can be determined based on the interference pattern registered on said optical detector. 
     
     
         13 . The system of  claim 12 , wherein said signal processing circuitry is arranged to determine the amplitude of the first harmonic component ω present in the output of said optical detector, and to adjust the frequency and/or the amplitude of said phase modulation such that the amplitude of said first harmonic component is made equal to zero. 
     
     
         14 . The system of  claim 10 , wherein said light emitted by said light source is spectrally broadband with a short coherence length, and the length of one of the arms of the interferometer is arranged to be adjustable. 
     
     
         15 . The system of  claim 1 , further comprising a time-multiplexing means such that the positions of a plurality of objects can be determined using a single light source, single fiber optic splitter, single optical detector, and single signal processing circuit. 
     
     
         16 . The system of  claim 15 , wherein said time-multiplexing means is an optical splitter/coupler or a sequential optical switch. 
     
     
         17 . The system of  claim 15 , wherein said fiber optic splitter is an optical circulator. 
     
     
         18 . The system of  claim 1 , wherein said light source is a continuous laser, said system further comprising:
 an electro-optical modulator connected between said fiber optic splitter and said fiber tip lens and arranged to allow pulses of laser light to propagate through said modulator toward said object and reflected pulses of laser light to propagate from said object toward said splitter at a frequency determined by said signal processing circuitry;   said signal processing circuitry coupled to said optical detector and arranged to adjust said frequency so as to maximize the light measured by said detector, said frequency varying with the distance of said object from said fiber tip lens.   
     
     
         19 . The system of  claim 18 , further comprising a semiconductor optical amplifier (SOA) arranged to amplify said reflected pulses of laser light prior to their being conveyed to said optical detector. 
     
     
         20 . The system of  claim 19 , further comprising a mirror arranged to reflect light from said SOA back through the SOA such that said reflected pulses of laser light are amplified twice before being conveyed to said optical detector. 
     
     
         21 . The system of  claim 20 , wherein said electro-optical modulator, said signal processing circuitry, said SOA and said mirror form an electro-optical servo loop which automatically adjusts the pulsing rate of said electro-optical modulator to maximize the signal on said detector and so that light can pass in both directions through said modulator. 
     
     
         22 . The system of  claim 18 , wherein said SOA is arranged to operate in continuous mode. 
     
     
         23 . The system of  claim 18 , wherein said SOA is arranged to operate in a pulsed mode. 
     
     
         24 . The system of  claim 18 , wherein said signal processing circuitry includes a servoed driver circuit which pulses said modulator at a rate commensurate with the return time for the light going to the object and back to the modulator. 
     
     
         25 . The system of  claim 1 , wherein said object is within an accumulator and has an associated position which varies with the volume of fluid contained within said accumulator. 
     
     
         26 . The system of  claim 25 , wherein said object is a piston or a bladder having an associated surface which varies with the volume of fluid contained within said accumulator, said system arranged such that said light conveyed onto said object is conveyed onto said surface. 
     
     
         27 . A fiber optic sensing system for determining the position of an object, comprising:
 a laser;   a phase modulator arranged to modulate the phase of said light emitted by said laser;   optical fiber;   a fiber optic splitter;   a fiber tip lens;   an optical detector; and   signal processing circuitry coupled to said optical detector and to said phase modulator;   said system components arranged to form a Michelson interferometer in which the phase-modulated light is split by said fiber optic splitter into a component that is conveyed to said object and a component which is conveyed via an optical fiber to a reflective surface, and such that light reflected by said object and light reflected by said reflective surface are recombined such that the resulting interference pattern varies with the distance of said object from said fiber tip lens; and   said signal processing circuitry arranged to adjust the frequency and/or the amplitude of said phase modulation such that the position of said object with respect to said fiber tip lens can be determined based on the interference pattern registered on said optical detector.   
     
     
         28 . The system of  claim 27 , wherein said signal processing circuitry is arranged to determine the frequency of the first harmonic component ω present in the output of said optical detector, and to adjust the frequency and/or the amplitude of said phase modulation such that the amplitude of said first harmonic component is made equal to zero. 
     
     
         29 . The system of  claim 27 , further comprising an isolator between said laser and said phase modulator. 
     
     
         30 . The system of  claim 27 , wherein said laser has a coherence length compatible with the differential leg length of said Michelson interferometer. 
     
     
         31 . A sensor for determining the volume of an accumulator, comprising:
 a laser;   optical fiber;   a fiber optic splitter;   a fiber tip lens positioned such that laser light emanating from said lens impinges on a surface, the position of which varies with the volume of an accumulator;   an optical detector; and   signal processing circuitry;   said components arranged such that laser light emitted by said laser is conveyed via optical fiber and said splitter to said fiber tip lens and onto said surface, and such that at least a portion of said light conveyed onto said surface is reflected by said surface and conveyed via optical fiber and said splitter to said optical detector;   said signal processing circuitry coupled to said optical detector and arranged to determine the position of said surface with respect to said fiber tip lens based at least in part on a characteristic of said reflected light.   
     
     
         32 . The sensor of  claim 31 , wherein said surface is a surface of a piston or a bladder. 
     
     
         33 . The sensor of  claim 31 , wherein said accumulator is a hydraulic accumulator.