GB2040131A

GB2040131A - Electrooptic instruments

Info

Publication number: GB2040131A
Application number: GB7941654A
Authority: GB
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1978-12-13
Filing date: 1979-12-03
Publication date: 1980-08-20
Also published as: IL58895A0; JPS5582917A; DE2950209A1; FR2444258A1; IT7928075A0; SE7910055L; IT1127732B; CA1130108A

Abstract

An rf oscillator comprising an electric circuit and an optical leg is used to measure physical variables capable of changing the length of the optical leg. In the circuitry, a beam of amplitude modulated radiation returning from the optical leg is converted to an electric signal, amplified, and used to modulate the drive of an LED which emits a beam of optical radiation. The beam is delivered to and returned from a reflective surface at the end of the optical path by waveguide means, phase shifts in the beam due to actual or effective changes in the pathlength of the optical leg are converted to corresponding shifts in the frequency of the oscillator. Details are given of using this in a fluid barometer, differential pressure sensor, fluid velocimeter, Pitot tube, shift position locator and in temperature sensors. Fig. 2 shows the barometer, where the modulated beam from LED 16 is passed to the reflective surface 60 of liquid 54 by fibre optic waveguide 44, the returning beam being separated by beam-splitter 26 and passed to detector 22 which modulates the LED via amplifier 20; the resonant frequency varies with the length of path 58 and hence with atmospheric pressure. <IMAGE>

Description

SPECIFICATION Electrooptic instruments The present invention relates to instrumentation and more particularly to electrooptic devices for determining changes in the level of a liquid, the dis placement of a surface, and variations in temperature, pressure, fluid velocity and the like.

The advantages of improved methods and apparatus for observing and recording test or operating data are well known. These data might be observations of functioning machinery in a manufacturing plant, recording information from an experi mental program or the detection of pressure orvelocity of some medium which is subject to change. In some applications, the main requirement on the apparatus is one of extreme accuracy since the vari able such as displacement or temperature must be determined with great precision. Systems designed primarily for accuracy often tend to be optical in nature or have some sort of frequency sensitive output. Also, the demand for sensor devices having an output which is compatible with modern digital electronics has increased.Other applications place heavy demands on reliable operation in a region with intense electric or magnetic fields or a noisy electrical background. Still other applications require ruggedness and desirability because of an especially adverse environment and an area with difficult access. The earlier apparatus in this regard tends to be mechanical in nature because of its ruggedness and simplicity while the later systems tend to incorporate electronics for simplification and improved accuracy. Some interesting systems are based on clever applications of optics or sonics or combinations of these approaches. In any event, there seems to be an insatiable demand for cheaper, more accurate, more rugged, reliable sensors particularly those which can interface with digital electronics.

The present invention is predicated on the discovery that a high resolution detection instrument is possible by joining a simple electric circuit and an optical waveguide in a single integrated system having a characteristic resonance frequency and capable of oscillating over a range of radio frequencies.

In essence, the resonant frequency of the circuit becomes a function of a linear dimension in the circuit. The reference optical path length requires a fixed time for a single round trip of an optical pulse and changes to the optical pathlength change the rf frequency of an electrooptic circuit. The target being monitored is oriented to cause a change by a predictably certain amount in the optic path length for each corresponding variation in the characteristic under surveillance. State of the art electronics keeps track of any frequency changes and the characteristic rf signals are readily coupled away and converted to useful forms of information.

A primary object of the present invention is to correlate changes of some characteristic of a medium with the physical state of the medium. The broad objective would include measuring such parameters as pressure, temperature, velocity, displacement and clearance.

According to the present invention, an electrooptic instrument comprises an electric circuit with a waveguide path for optical radiation. A pressure sensitive, temperature sensitive or position sensitive component is employed as one element in an oscil lator having a resonant frequency which is a function of a linear dimension in the optical leg of the oscil lator. Any change in pressure, temperature or position as the case may be causes a change in the time required for the rfamplitude modulated optical car rier frequency to travel one round trip along the optical path. This change in turn causes the resonant rf frequency of the circuit to change.More particularly, the basic circuit comprises a source of amplitude modulated optical radiation, an optical detector and a detection signal amplifier and the circuit communicates with a target through waveguide means.

In one particular embodiment the waveguide is fiber optic and the target is a reflective surface formed on the end of the fiber. The optical radiation can be either coherent or incoherent of any convenient wavelength; both hollow tube or fiber optic waveguides can be employed.

In operation, the oscillator is designed to resonate at radio frequencies. The output from the optical source is amplitude modulated at a suitable radio frequency, reflected off a target surface, directed to the optical detector and converted to an rf electrical signal. The output from the detector is amplified and in turn used to drive the optical source. The charac teristicto be measured is set up such that any changes in this characteristic causes an effective change in the time required for the optical radiation to travel from the source to the detector.

A primary feature of the present invention is its adaptability to the use of optical wavelength radiation. A waveguide connects the processing circuitry and the sensor or target interfacing apparatus. The frequency of the electronic circuit varies as a function of change in the parameter measured. The overall instrument utilizes an oscillator having an optical and electrical feedback path arranged to be responsive to the time required for the optical radiation to travel one round trip in the optical portion of the circuit. For most applications, variations in the system read out due to environmental conditions can be canceled with apparatus using dual waveguides.

The present invention is especially attractive for sensing characteristics in harsh environments par ticularly those containing a high level of electrical noise in the vicinity of the sensing location. No electrical power or electronics packages need be located at the sensing station. In addition, the requirements for cooling of electronics or mechanical moving parts are eliminated. Also, no power connectors or signal connectors are required at the sensing station.

The overall package is simple, accurate, small and reliable. Further these electooptic devices provide an rf output signal with a characteristic frequency which is proportional to the parameter being measured and thus they are compatible with and interface easily with digital electronic circuitry.

The foregoing and other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of preferred embodiments as illustrated in the accompanying drawing.

Fig. 1 is a schematic diagram of an electrooptic device in accordance with the present invention in a simplified general form; Fig. 2 is a simplified diagram of a more particular embodiment configured as a liquid barometer; Fig. 3 is a simplified diagram of an embodiment configured as a differential pressure sensor; Fig. 4 is a simplified diagram of an embodiment configured to measure the temperature of a fluid; Fig. 5 is a simplified diagram of an embodiment configured to measure the velocity of a fluid in a venturi tube; Fig. 6 is a simplified diagram of an embodiment configured to measure pressure with a pitottube; Fig. 7 is a simplified diagram of an embodiment configured to measure displacement of a rigid element such as a shaft; Fig. 8 is a simplified diagram of an embodiment configured for use with an aneroid barometer; and Fig. 9 is a simplified diagram of an embodiment configured with a bimetallic strip to measure temperature.

An electrooptic oscillator 10 having an electronic loop 12 and an optical waveguide portion 14 in accordance with the present invention is shown schematically in Fig. 1. The electronic loop comprises a source 16 of electromagnetic radiation such as a light emitting diode (LED) or a laser diode (LD), an rf output coupler 18, an amplifier 20 and a photo detector 22. The waveguide portion as shown in this representation comprises an optical waveguide 24 and a beam divider 26 which is partially transmissive at the wavelength of a beam 28 of electromagnetic radiation emitted by the source 16. In the simple form in Fig. 1, the sensor according to the present invention responds to movement of a reflective surface 30 of a target 32.As will become more and more apparent from the following description, this electrooptic oscillator can be made to sense pressure, temperature, flow rate, clearance and a variety of other quantities of interest.

The operation of the system is described with reference to Fig. 1. The beam 28 from the source 16 is delivered through the waveguide 24 to the reflective surface 30 of the target 32. Some of this beam is reflected from the target and delivered to the photodetector. A detector output signal 34 is fed to the amplifier and an amplifier output signal 36 provides a modulated drive to the source 16. Although not shown, power is supplied to the amplifier from an outside source.

Several embodiments of the present invention combine optical waveguides concepts and the basic circuit described with fluid devices to measure temperature, pressure and velocity of a fluid such that the oscillator frequency varies as a function of the characteristic being measured. In a similar man ner optical waveguides in combination with the circuit measure displacements to yield characteristic information without resorting to fluid concepts.

In order to understand the theory supporting the operation of the various embodiments of the present invention described hereinafter, the following explanation is provided. The waveguide carries radiation which is an amplitude modulated incoherent wave to a reflective surface. Light is reflected back through the waveguide and directed to the photodetector which converts the beam to an electric signal, the latter in turn being amplified and used to drive the LED or beam source. This constitutes an electronic oscillator with an optical feedback path.

The frequency of oscillation depends on the amplitude modulation on the beam from the LED and requires that the phase delay time of the rf signal equal an integer number of frequency periods. This oscillating frequency relationship is expressed as f m (1) 70 e where r0 is the round trip phase time delay of the rf modulated signal on the incoherent optical carrier waveguide, Te is the time delay of the rf signal through the electrical circuit portion of the feedback loop, and m is the number of half rf wavelengths stored in the entire loop.

By assuming that the electrical delay time is negligible in comparison to the optical delay time, the Eq. 1 can be simplified to (a) 0 Since 70 equals Liy the Eq. 2 can be transformed to f = where L is the round trip optical pathlength, and c is the light velocity in free space.

If the location of the reflective surface changes by an amount AL the optical length changes by 2do and the frequency of oscillation changes by an amount

With suitable anticipation of the variations in AL expected in particular application, the bandwidth of the rf amplifier as well as m and L for the operating frequency of the amplifier are set. The frequency range forthese systems can vary considerably as long as the bandwidth of the amplifier is not exceeded.

Fig. 2 is a schematic representation of a liquid barometer application in accordance with the present invention. Source 16 provides a divergent beam of incoherent light which is typically at near infrared wavelengths. A collimating lens 38 directs the beam to the beam divider 26 which allows approximately half of the beam to pass to a first double pass lens 40 which in turn concentrates the beam onto an entrance 42 of an optical fiber 44. The radiation is guided by the fiber to the fiber exit 46. The radiation exits the fiber with an angle of divergence and passes through a second double pass lens 48 which collimates the beam. The lens 48 is sealably fitted into a risertube 50 which extends upwardly from a base 52.The tube and the base contain a liquid 54 such as mercury with a base surface 56 exposed to atmospheric pressure while a riser volume 58, formed between the lens 48 and a riser surface 60, is maintained at essentially vacuum conditions. Radiation from the fiber exit 46 is reflectedofftheliquidsur- face 60, coupled back into the fiber 44 by the lens 48, returned to the lens 40 and focused onto a detector 22 by a focusing mirror 64 having been reflected by the beam splitter 26. A detector output signal 34 is fed to the amplifier and in turn an amplifier output signal 36 is used to modulate the output radiation 28 from the LED 16.Since the resonant frequency of the oscillator varies as a function of the height of the liquid in the vertical riser, the greater the separation between the liquid surfaces in the riser and the base, the higher the oscillating frequency. Therefore, by measuring the resonant frequency, the separation distance and in turn the atmospheric pressure are determined.

The apparatus arrangement described above in turns of a barometer application is modified slightly to function with a manometer for measuring the pressure differential in a vessel as is shown in Fig. 3.

Two separate oscillators are shown in this embodiment as a means for compensating for variations in the environmental conditions. A sensor circuit 66 and a reference circuit 68 are shown each having components corresponding to those shown in Fig. 2.

In addition, the reference circuit has an end reflector 70 at the tip of the fiber optic. In the event the environment temperature should increase or decrease in an amount to cause the effective optical path length of the fiber in the sensor circuit oscillator to change, the reference oscillator can be used to account for the change. Each oscillator has its characteristics amplifier output signal which is tapped and directed to a mixer 72. The difference or beat frequency signal 74 correlates the frequency change due to variations in the position of the liquid surface. The difference between the pressure insidethe tank 76 and the pressure above the riser liquid surface is simply the product of the density of the liquid 54 in the riser, gravity, and the difference in the height of the liquid, Ah, in the two legs of the system.

Avariation to this apparatus configuration is useful in the measurement of temperature. The two fiber optic waveguides are physically separated and an end reflector 70 is located on each. Both the coefficient of expansion and the index of refraction for fiber optics are linearly proportional to temperature.

Therefore, by immersing one of the fibers in the medium to be monitored and isolating the second fiber from the temperature excursion of this medium, the oscillators will resonate at separate and distinct frequencies. This difference can be correlated directly to temperature variation in the medium being monitored.

The present invention is shown configured as a thermometer in Fig. 4. For simplicity, the oscillator 10 is shown schematically providing collimated radiation which strikes the liquid surface contained in an open-ended tube 78. The tube is supported in a conduit 80 which contains a specimen fluid 82. Variation in the temperature of the specimen fluid causes the liquid in the tube 78 to expand or contract accordingly and with suitable calibration the frequency variation in the sensor oscillator is correlated to the temperature variations in the specimen fluid through the changes in the position of the riser liquid surface 60.

Another application of the present invention is disclosed in Fig. in an embodiment which determines fluid velocity with a venturi tube 84. The oscillator 10 shown schematically is identical to the apparatus shown and discussed in more detail in connection with the embodiment of Fig. 1. The basic idea of the velocity sensor as shown is to measure the difference in height between the fluid level in each leg of the U-shaped manometer tube 86.With this measured variable, the velocity of the fluid passing through the venturi is determined by the relationship

where v is the velocity of the flowing fluid a is the cross sectional area of the venturi tube at the center em is the density of the liquid in the manometer tube e is the density of the fluid g is the gravitation constant Ah is the difference in elevation of liquid in the manometer legs, and A is the cross sectional area of the venturi tube at the inlet.

Still another embodiment of the present invention is shown in Fig. 6 is a Pitottube configuration. In a very conventional manner the free stream pressure is sensed through ports 88 and the stream static pressure is sensed through nose port 90. These pressures are exposed to the two ends of a manometer tube 86 which can be identical to the tube used in connection with the venturi tube. Measurement of the difference in height of the liquid in each of the legs of the tube provides the velocity of the stream moving past the Piton tube according to the relationship

where v is the velocity of the fluid flow over the tube g is the gravitational constant Ah is the difference in elevation of the liquid in the two legs of the manometer m is the density of the liquid in the manometer, and e is the density of the flowing fluid.

A system for sensing the displacement in position of a piece of machinery such as a shaft 92 is shown in Fig. 7 as an indication of the versatility of the present invention both a sensor circuit and a reference circuit are shown adjacent to the capillary tube 94 into which is fitted a shaft 92. The operation of the system is very similar to those involving the use of a liquid surface to reflect the optical energy back to the sensor oscillator. In orderto reflect as much radiation as possible, the shaft usually has a polished face 96.

A sensor system using fiber optics in an aneroid barometer embodiment is shown in Fig. 8. A sealer enclosure 98 with low internal pressure and having a rigid extension 100 contained in a housing 102 is exposed to atmospheric pressure. Since the enclosure can act as a bellows during changes in environmental pressure, the enclosure expands or contracts correspondingly causing the rigid extension to change position with respect to the housing. This position change is sensed in the same manner as has been discussed and the frequency of the oscillator correlated to pressure variation.

Fig. 9 shows the electrooptic oscillator in accordance with the present invention applied to measure temperature in combination with a bimetallic strip 104. The strip is shown supported by end clamps 106 and a rigid extension upon which is targeted the probing radiation of the sensor oscillator. The oscillator is able to record the deviations of position of the bimetallic strip from a reference or unbent position which is known to occur at a given temperature, and with suitable bookkeeping the variations in position of this strip from the reference point are correlated to changes in temperature.

The various embodiments discussed above are not exhaustive of the applications possible with the present invention. For example, the optical source can be either a coherent or incoherent beam which is delivered to the target surface with or without focusing optics depending upon such variables as the intensity of the source, the length of the waveguide and the reflectivity of the target. The waveguides are usually but need not necessarily be fiber optical.

Single fibers or fiber bundles are suitable for the fiberwaveguide depending upon the application requirements; the fibers can be single mode for highest accuracy or multimode. All types of digital electronics are easily interfaced with the signal available through the output coupler rendering the invention very versatile.

Although this invention has been shown and described with respect to preferred embodiments thereof, those skilled in the art should understand that various changes or omissions in the form and detail thereof may be made without departing from the spirit and scope of the invention.

Claims

1. An electrooptic oscillator comprising source means for providing a beam of optical radiation which is amplitude modulated at a radio frequency; waveguide means for delivering the beam to a reflective surface and for returning at least a portion of the beam reflected by the surface over an optical pathlength which is subject to variation; transducer means for converting the returned radiation to an rf signal having a phase shift identical to any phase shift experienced by the optical beam during transit between the source means and the transducer means; signal amplification means for increasing the strength of the electric signal from the transducer sufficiently to drive the source means thereby amplitude modulating the optical radiation from the source at the frequency of the rf electric signal from the transducer; and an rf signal output coupler for sensing the frequency of the electric signal driving the source means and - providing an output signal at the resonant frequency of the oscillator.

2. The invention according to claim 1 wherein the, waveguide means is a fiber optic waveguide.

3. The invention according to claim 2 wherein the waveguide means is a plurality of fiber optics.

4. The invention according to claim 1 wherein the same waveguide delivers the beam to the reflective surface and returns reflective radiation to the transducer means.

5. The invention according to claim 4 including further a beam divider in the path between the source and the reflective surface for intercepting the radiation being returned to the source, and redirecting at least some of this returned beam onto the transducer means.

6. The invention according to claim 1 including means for collimating the beam of optical radiation from the source.

7. The invention according to claim 6 including means for focusing the beam of optical radiation before transmission through the waveguide.

8. The invention according to claim 6 including means for focusing the optical radiation from the waveguide onto the reflective surface.

9. The invention according to claim 1 including further a second electrooptic oscillator having components corresponding to components in the first oscillator wherein the reflective surface which the beam of optical radiation strikes in the second oscillator is on the end of the waveguide, the rf signal from the output coupler of each oscillator is processed in a signal mixer, and an rf signal is provided by the signal mixer at the beat frequency for the resonance frequencies of the two oscillators.

10. The method of operating an electrooptic oscillator including the steps of: providing with a source means a beam of optical radiation which is amplitude modulated at a radio frequency; delivering the beam to a reflective surface and returning at least a portion of the beam reflected by the surface with waveguide means; converting the returned radiation to an rf electric signal with transducer means, the electric signal having a phase shift identical to any phase shift experienced by the optical beam during transit between the source means and the transducer means; increasing the strength of the electric signal from the transducer with signal amplification means suffi ciently to drive the source means; driving the source means with the amplified signal to amplitude modulate the optical radiation from the source means with an rf signal output coupler to provide an output signal at a radio frequency which represents the resonant frequency of the oscillator.

11. An electrooptic oscillator substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

12. A method of operating an electrooptic oscil lator substantially as hereinbefore described with reference to the accompanying drawings.