Calibration circuit for fiber optic proximity instrument
BACKGROUND OF THE INVENTION 
     This invention relates to proximity instruments and, more particularly, to a calibration circuit for a fiber optic proximity instrument having automatic adjustments for changes in the intensity of the reflected light. 
     Fiber optic proximity probes are devices which use a light beam transmitted from a light source by way of a bundle of light conducting optical fibers from a light source to a target. The light is reflected off the target and returned along other optical fibers in the bundle back to a light intensity sensor in the instrument. The intensity of the returning light is a function of the distance between the proximity probe tip and the target, so the output of the light sensor can be a very precise measure of that distance, or more importantly, changes in that distance. 
     Because of variations in certain operating parameters such as surface reflectivity of the target, light intensity output of the light source, transmission efficiency of the fiber optic bundle, and other factors which influence the intensity of the light transmitted, reflected, returned and sensed, it is necessary to readjust the instrument if any one of these factors are changed during operation. Although this readjustment is a simple and fast procedure, and is cause for no concern in most situations, there are certain applications in which the need to readjust the instrument for each use is an unacceptable requirement, for example, where the instrument is in an inaccessible location or where its readings must be available continuously or at unpredictable intervals. In certain of these installations, the instrument probe is permanently mounted relative to a target and the mean gap between probe and target is constant. In such permanent situations it would be necessary to provide correction for errors due to degradation of the optical light path transmission efficiency caused by such factors as optical fiber breakage, debris on the face of the transducer, and variations in the surface reflectivity of the target. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide a calibration circuit for an improved optical proximity measuring instrument, for use in situations wherein the mean gap between the probe and the target is constant, which automatically compensates for changes in the intensity of the light beam. The output sensitivity of the instrument is essentially independent of the target surface reflectivity and it automatically compensates for factors which influence the transmission of light through the system, such as optical fiber breakage and debris on the face of the probe. It also automatically corrects for dimming of the light source over an extended period of time. 
     These and other objects of the invention are attained in an embodiment having a bundle of light conducting optical fibers, a portion of which transmit light from a light source to a target and the remainder of which return the light reflected from the target back to a light sensor. Since the mean gap between the tip of the proximity probe and the target is constant, a light intensity regulator system is employed to maintain the DC level of light as seen by the receiving fibers at a constant value so the desired slope correction can be obtained independently of the surface reflectivity. The time constant of the lamp intensity regulator system is large, e.g. on the order of 1 second, so the vibration displacement of the target can be measured over a broad range of frequency. The reference by which the light intensity is maintained is a voltage source which is set when the instrument is first installed and calibrated in a step-up mode, and is duplicated in an operating mode circuit during operation. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention and its attendant objects and advantages will become better understood by reference to the following description of the preferred embodiment when read in conjunction with the following drawings, wherein: 
     FIG. 1 is a schematic diagram of a fiber optic proximity sensor made in accordance with the invention, and 
     FIG. 2 is a graph showing automatic adjustments in light output to correct for changes in light beam intensity. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings wherein like reference characters designate identical or corresponding parts, and more particularly to FIG. 1 thereof, a fiber optic proximity instrument is shown having a light source such as a lamp 10 powered by a power supply 12 that receives power from the power source 31 and regulated by a lamp regulator 13. Light from the lamp 10 is conducted along one branch 14 of a fiber optic light transmitting cable 16 to the surface of a target 18. The fiber optic light transmitting cable is the same as that shown in FIG. 1 of U.S. Pat. No. 3,327,584 issued to C. D. Kissinger on June 27, 1967, the entire disclosure of which is incorporated by reference herein. 
     Light is reflected from the target 18 and is picked up by the return fibers in the return branch 19 of the light transmitting cable 16, and conducted to a light sensor 20 which generates an electrical signal proportional to the intensity of the light returned along branch 19 of the light transmitting cable 16. The signal from sensor 20 is amplified by a sensor signal amplifier 23 and an output signal amplifier 23, and is conducted to a recording or indicating device such as the oscilliscope 25 to indicate the amplitude and frequency of the vibration of the target 18. 
     The normal operating range of the instrument is along the linear portion of the performance curve A shown in FIG. 2, that is, over a probe-to-target gap range of about 0.003 to 0.009 inches. In use, the instrument probe is fixed to a support and spaced about 0.006 inches from the target. The intensity of the reflected light thus varies linearly with the variation in the gap, within the linear range of the curve. 
     The instrument disclosed in the aforesaid &#39;584 patent must be adjusted prior to each use to assure that it is operating on the linear portion of the characteristic curve, and that the light intensity is correlated with the probe-to-target gap. Otherwise, if the target reflectivity or light transmission efficiency has increased or decreased since the previous use, the characteristic curves B and C will be steeper and shallower, respectively, than the normalized curve A, so the results will be false, not only absolutely but also relative to each other. 
     The adjustment is accomplished by moving the probe toward and away from the target until the position is found which results in the maximum reflected light intensity. The lamp intensity is adjusted to give a reading at the top of the scale. The probe position is then reset to give a reading of about 40% on the scale, which is about midpoint on the linear portion of the curve. 
     In situations where the mean probe-to-target gap is constant, it is possible to eliminate the need to readjust the instrument each time it is used. Although the readjustment is very simple and takes only a few seconds, it is very desirable to eliminate the necessity for it in situations where the probe is in an inaccessible location or when its readings must be continuously available. An example of such a situation is in rolling element bearing analysis and monitoring applications wherein the probe will normally be permanently installed radially facing the outer bearing race, or when multiple instruments are installed in multiple locations to monitor all the critical bearing in the system so that it would be impractical to readjust the instrument for each use. Rolling element bearing monitoring and analysis uses a fiber optic proximity instrument to measure the amplitude and frequency of surface expansion traveling waves which travel around the bearing race. These surface expansions, caused by the force exerted by the rolling elements as they roll around the race, can have an amplitude on the order of 80 microinches. The sensitivity of this instrument is on the order of 1 microinch, so the precise waveform of the passing traveling wave can be examined, and this examination can reveal much about the condition of the bearing, as described by Gerald J. Philips in &#34;A New Technology for Bearing Performance Monitoring&#34;, NBS Special Publication 436 (1975); and &#34;Bearing Performance Investigations Through Speed Ratio Measurements&#34;, ASLE 78-AM-1A-2 (1978). 
     To attain the self-calibration capability, the signal from the output signal amplifier 23 is also conducted through a switch 24 and an amplifier 26 to the lamp intensity regulator 13. The lamp intensity regulator 13 adjusts the intensity of the lamp 10 to compensate for changes in target surface reflectivity and/or transmission efficiency of the light conducting cable 16 and other long-term factors which would affect the intensity of the light beam returned along the branch 19. The time constant of the lamp intensity regulator 13 is relatively long, for example, on the order of one second, so that only &#34;long term&#34; factors affect the light intensity regulator, but vibration or other moderate or high-frequency movement of the target 18 does not. For this reason, the time constant of the regulator should be at least about ten times greater than the period of the &#34;long term&#34; factors which the instrument is designed to ignore. However, the automatic lamp intensity regulator can be used only in situations where the mean gap between the probe tip and the target 18 remains constant. If the mean gap changes, the lamp intensity regulator 13 will adjust the lamp intensity to produce the same intensity at the light sensor 20 that was produced before the mean gap changed. The instrument would thus incorrectly indicate that the new mean gap between the probe tip and the target was the same as the original mean gap and the slope of the characteristic curve would be incorrect, so the instrument readings would be incorrect relative to one another as well as absolutely. 
     A setup system, shown in FIG. 1, includes a lamp intensity sensor 28 connected through the switch 24 to the lamp intensity regulator 13, a manual lamp intensity control 30, and a potentiometer 32 connected to the amplifier 26. To calibrate the instrument when it is initially installed, the switch 24 is moved from the position illustrated in FIG. 1 to the raised or set-up position in which the bottom lead from the amplifier 26 engages the grounded contact, and the lead from the lamp intensity sensor 28 is connected to the lamp intensity regulator 13. With the switch 24 in this position, the probe is moved to the position at which the amplitude of the output signal on the meter or oscilloscope 25 is a maximum. The light intensity of the lamp 10 is adjusted with the control 30 to give full scale output signal. The position of the probe 17 is readjusted to give a reading of about 40% on the scale, or midway in the linear portion of the curve A. The switch 24 is moved to the position shown in FIG. 1 and the potentiometer 32 is adjusted to give the same reading, about 40% on the scale, that the meter showed before the switch 24 was moved. The instrument is now calibrated and need not be readjusted. 
     In use, dynamic changes in the gap between the target 18 and the tip of the probe 17 will cause a change in the intensity of the light reflected back into the return fibers 19 of the cable 16, and will be reflected as a change in the electrical signal from the sensor 20, which is amplified and displayed or recorded on the device 25. The instrument is designed to ignore, or more exactly, correct for long term changes in reflected light intensity so that only the dynamic changes in gap dimension are displayed on the device 25. Long term changes, that is, those with a time constant of more than one second, include such changes as surface reflectivity of the target which can be caused by heating, lubricants, dirt or surface texture changes. These changes will cause a change in the intensity of the beam reflected back to the receiving fibers 19, which causes a change in the mean signal voltage seen by the regulator 13, which in turn causes a corresponding change in the intensity of the light from the lamp 10 until the mean signal from the sensor has returned to the correct predetermined value. 
     Obviously, numerous modifications and variations of the preferred embodiment are possible in view of this disclosure. For example, the manual adjustment of the lamp 10 during setup could be done automatically with a signal comparator comparing a reference voltage source against the output of the sensor 20, and generating an error signal to a servo to adjust the light intensity until the error signal is reduced to zero. Also, the potentiometer 32 could be replaced with an automatic reference voltage matching circuit that matches the reference voltage set up in the control 30 to eliminate the manual setting of the potentiometer 32. Moreover, the lamp intensity regulator could be replaced with a light limiting device such as an iris controlled by the light sensor 20, in which case the lamp 10 would merely have to maintain a constant light level.