EP0224575A1 - Optical pressure sensor - Google Patents

Abstract

The optical phenomena known as reduced and attenuated total internal reflection are used to measure pressure. A ray of light is sent to a thin glass plate (22) via an optical fiber (18). The ray of light reflected inward is conveyed to a photographic cell (28) via a second optical fiber (20). A diaphragm (12), which can be integral with a pressure chamber, is mounted adjacent to the plate (22) and deflected under pressure. As the diaphragm (12) approaches the glass plate (22), the amount of reflected light decreases. In the preferred embodiment, a bulge (12b) of the diaphragm (12) is uniformly covered with a soft metal with a high reflection index such as gold, in order to cause attenuation of all of the reflections. Gold is flattened by applying overpressure to the exposed side of the diaphragm (12) and compressing the bulge (12b) and the glass plate (22) against each other. The change in the amount of reflected light is translated into an analog electrical pressure signal. The optical transducer has good resistance to the hostile environment of internal combustion engines thanks to its insensitivity to electromagnetic interference, vibrations and thermal transients.

Description

OPTICAL PRESSURE SENSOR

Cross-Reference to Related Application

This application is a continuation-in-part of co-pending application Serial No. 756,289, filed July 18, 1985, which in turn is a divisional of co-pending application Serial No. 740,996, filed June 4, 1985. Both referenced applications were filed by Hoffman et al. and are entitled "Optical Pressure Sensor" and assigned to the assignee of the present application.

Background of the Invention

The invention relates generally to the field of pressure measurement and in particular to rugged pressure transducers for use in environments of widely fluctuating temperature and pressure. Although the invention is specifically directed to automotive applications, uses in fields other than automotive, such as petroleum recovery and process control, are actively contemplated.

The present invention is motivated by a long felt need to indicate instantaneous pressure inside the combustion chamber of an internal combustion engine with a reliable, inexpensive device. Optimization of the performance and emission characteristics of internal combustion engines is of vital national importance. In 1981 automobiles in the United States used approximately thirty percent of all petroleum 'based fuels consumed and contributed as much as fifty percent of the hydrocarbons and oxides of nitrogen discharged into the atmosphere nationwide. It is recognized by researchers in the automotive industry that instantaneous pressure of the gases in the combustion chamber of an internal combustion engine, if measured and interpreted properly, yields data concerning combustion and flow processes in the cylinder which can be fed into a microprocessor-based engine control system to improve efficiency and reduce emissions. The benefits to be derived include optimized automatic spark timing, increased knock limit and improved self-test and diagnostic capability. In addition, accurate pressure measurements allow laboratory determination of the onset of pre-ignition, knock, and heat transfer and leakage losses for each given cycle of the engine. This data is essential to research studies characterizing performance of internal combustion engines. Research and development of closed loop control systems are becoming increasingly important as fuel quality declines to lower octane ratings in the future because the use of such fuels necessitates closer engine monitoring in order to prevent misfire, knock and excessive emissions.

The internal comb.ustion chamber presents a hostile environment of electromagnetic noise and sharp fluctuations in temperature and pressure. Pressures and temperatures can be highly stochastic, varying from near ambient conditions to more than 70 atmospheres and 2000°C, respectively. Fluctuations between these extremes occur at a rate of 30 to 40 times per second in a typical automobile engine cylinder. There are no accurate, reliable low cost transducers currently available for combustion chamber pressure monitoring despite widespread agreement among the internal combustion engine research community, both in industry and academia, that the measurement of pressure in the combustion chamber is extremely valuable both as a diagnostic and control parameter.

Cylinder pressure transducers used today by laboratory scientists are almost exclusively of the piezoelectric variety. While piezoelectric transducers have excellent frequency response and can withstand moderate high temperatures and pressures in the combustion engine, they exhibit a number of disadvantages, namely, high cost, sensitivity to high temperatures, charge leakage and sensitivity to electrical noise. Two other types of pressure transducers which have been successfully used by researchers, although not applied in commercial practice, are the balanced pressure indicator and the strain gage transducer. In the balanced pressure indicator a diaphragm reacts to relative cylinder pressure to close an electrical contact. Balanced pressure indicators have been largely abandoned by researchers because they only generate two pressure readings per cycle and have significant time delays. Strain gage transducers ope ate by mechanically deforming a variable resistance element usually arranged in an electronic bridge circuit. Typical strain gages exhibit temperature gradient sensitivity, creep, nonlinearity and poor bandwidth.

Instrumenting an automotive engine with currently available sensors capable of satisfactory performance in closed loop control or diagnostic applications would cost several hundred dollars per sensor. Even at these prices, such sensors exhibit poor survivability under the harsh operating environment of the power plant of an automotive engine. Fuel economy and emission control made possible by such measurements do not justify the cost of present sensors.

Summary of the Invention

Departing from conventional pressure transducer technology, the invention harnesses an optical phenomenon known as frustrated or attenuated total internal reflection as a means of sensing pressure. The result is a transducer with essentially no moving parts, insensitivity to electromagnetic interference, excellent frequency response and thermal stability. A light beam, preferably modulated, is introduced into a transparent medium with an internal reflection boundary preferably via an optical fiber. The internally reflected light ray is carried to a detector such as a photocell preferably via a second optical fiber. A diaphragm juxtaposed with the reflection boundary is deflected under pressure and moves closer to or farther from the surface of the transparent medium to cause a change in the amount of light reflected which is defected by the light detector and translated into a pressure signal. It is highly advantageous for the intruding surface of the diaphragm facing the reflection boundary of the transparent medium to be composed of a reflective material, preferably metal. When reflective rather than transparent or absorbing, the intruding surface causes attenuated total reflection (ATR) as opposed to frustrated total reflection (FTR). The ATR mechanism achieves the total range of attenuation down to extinction of the reflected beam without requiring contact between the intruding surface of the diaphragm and the transparent medium. In one embodiment/ the transparent medium is a thin disc of glass supported inside a metal cylinder with a closed end. In an embodiment for automotive applications, the cylinder is threaded like a spark plug and the closed end is formed by a thin metal or ceramic diaphragm exposed to the internal combustion chamber. In another embodiment, the diaphragm is formed integrally with the chamber wall by boring a cylindrical hole in an engine block or other pressure vessel, leaving a thin piece of metal forming the diaphragm between the pressurized chamber and the bore, and then inserting the optical head comprising the cylindrical fiber holder and glass disc. Preferably a central cylindrical boss is formed on the inner surface of the diaphgram adjacent to the transparent medium at the point where reflection occurs. The face of the boss is preferably coated with a highly reflective metal or metal alloy to induce attenuated total reflection.

The gap width between the diaphragm and optical head is extremely small and must be precisely determined during manufacture. A preferred method of manufacturing an ATR device is to apply a soft reflective metal coating, preferably gold or platinum, to the face of the boss on the diaphragm and to force the surface of the glass disc and boss together by overpressuring the exposed side of the diaphragm, thereby flattening the metal coating. A preferred way of carrying out this method is to first overpressure the diaphragm and then while maintaining the overpressure condition, forcing the optical head against the coated boss with a known force and simultaneously bonding the optical head in place. When the pressure is released, the diaphragm resumes its normal position leaving a predetermined uniform gap width between the precisely parallel faces of the coated boss and transparent medium.

Extremely small variations in the spacing between the cylindrical boss and the glass disc cause marked variation in the amplitude of light received by the light detector. These variations are directly related to the pressure profile during a given cycle of the internal combustion chamber. The use of fiber optics allows the light generating and receiving electrical components to be remote from the engine. In the preferred embodiment the fibers are supported in a cylindrical fiber holding body to which the glass disc is bonded.

Several techniques eliminate the spurious effects of variations in amplitude at the light source and variations in the received signal due to coupling and transmission changes or drift due to temperature and aging. For example, a third fiber or other light pickup means can be arranged to receive dispersed input light which falls outside the region juxtaposed with the boss to form a reference signal not affected by gap width. Alternatively, a second pickup can be arranged to tap into- the input fiber directly, or a second pair of input and output fibers can be arranged as a reference. Finally, the transmission or filtered reception of selected wavelengths can be employed to yield a ratio which is affected by gap width but substantially unaffected by amplitude and coupling factors. Brief Description of the Drawings

FIG. 1 is a sectional view of an optical pressure sensor for an internal combustion chamber according to the invention.

FIG. 2 is a cross-sectional view of the sensor of Fig. 1 taken along lines 2-2.

FIG. 3 is a partial sectional view through the diaphragm of Fig. 1 showing an alternate embodiment thereof.

FIG. 4 is a partial sectional view similar to that of Fig. 3 showing another alternate embodiment of the diaphragm of Fig. 1.

FIG. 5 is a sectional view of another embodiment of the optical pressure sensor according to the invention.

FIG. 6 is a graph comparing frustrated and attenuated total internal reflection.

1

FIG. 7 is a graph of pressure versus crank angle in an internal combustion chamber comparing a prior art sensor with an optical transducer constructed according to the invention.

FIGS. 8-12 are schematic perspective views of respective alternative embodiments with means to eliminate the effects of amplitude and coupling variations and drift.

FIG. 13 is a sectional schematic view of a sensor similar to that of Fig. 1 with a soft reflective metal coating on the face of the boss of the diaphragm being forced against the glass.

FIGS. 14A, 14B and 14C are sectional schematic views illustrating respective stages of a preferred assembly technique for the optical head and diaphragm of the optical pressure sensor according to the invention.

FIG. 15 is a sectional schematic view of an embodiment of the optical pressure sensor according to the invention where the diaphragm is integral with the chamber wall.

Description of the Preferred Embodiments

Fig. 1 shows a sensor designed to indicate pressure in an internal combustion engine. A cylindrical metal housing 10 includes a disk-shaped recessed, integral metal diaphragm 12 at one end. The outer surface 12a of the diaphragm 12 is exposed to the pressures inside the combustion chamber. Housing 10 is externally threaded so as to be sealingly received, diaphragm end fir,st, through a threaded bore (not shown) in the engine block like a spark plug. Inside the housing 10, a coaxially mounted cylindrical block or fiber holder 16 carries a pair of converging optical fibers 18 and 20. The fiber holder 16 may be made of various materials including ceramic material. For example, a powdered ceramic may be molded with the necessary holes for fibers 18 and 20 and then fired. If the material for the fiber holder has a sufficiently lower melting point than the glass fibers, the fibers may be oriented in the fiber holder material before firing. The lower surface 16a of the fiber holder 16 is ground flat with the ends of the fibers 18 and 2U exposed on the surface. A thin coating of glass 22 is applied directly over the fiber holder surface 16a covering the ends of the fibers 18 and 20 to form a glass disk. The preferred thickness of the glass disk is on the order of one millimeter. The converging ends of the fibers 18 and 20 are oriented such that light generated by a modulated light source 24 introduced into the glass disk 22 via input fiber 18 is totally internally reflected at point 26 on the air/glass interface surface 22a of the glass disk 22. The point of reflection 26 ideally lies on the axis of the cylindrical housing 10, and the axes of the proximal ends of fibers 18 and 20 lie in a plane including the cylindrical axis. Light source 24 may be a light emitting diode or incandescent bulb or any other light source although coherent radiation from a laser may be preferable in certain applications. The reflected light is collected via output fiber 20 which has its proximal end aligned with the axis of the reflected ray. A light detector 28, such as a photocell or photodiode, at the distal end of fiber ,20 detects light coveyed by the output fiber 20.

Diaphragm 12 is formed with a preferably integral, cylindrical boss 12b projecting coaxially toward the surface 22a of the glass disk 22 at the reflection point 26. The variable gap 30 between the face of the cylindrical boss 12b and the opposing ■surface 22a of the glass disk should be on the order of one wavelength of light. The face of the cylindrical boss 12b is highly polished to achieve the desired flatness and parallel relationship with the opposed surface of the glass disk 22. The two surfaces that face each other should be polished to attain an extremely fine surface finish on the order of one quarter wavelength or better.

As shown in Figs. 3 and 4, different materials and coatings can be used for the diaphragm 12. In Fig. 3, for frustrated total reflection a metal diaphragm 12' has a ceramic coating 34 facing the high pressure environment and a light absorbing coating of glass or quartz 36 on the face of the cylindrical boss 12b¹. In Fig. 4, for attenuated total reflection the entire diaphragm 12' ' is formed of a ceramic material with the face of the cylindrical boss being, coated with a light reflecting metal layer 37. In the embodiment of Fig." 1, the active face of the boss 12b of the diaphragm 12 may be coated with a reflective metal layer of gold, platinum silver, copper or beryllium copper, for example.

The active face of boss 12b is coated with a reflective metal layer 37 using techniques known to those skilled in the art including electroplating, chemical vapor deposition, electroless or chemical plating, liquid dynamic compaction or other means for depositing a thin metal layer of predetermined thickness on either a metal or a ceramic substrate. The metal coating 37 is preferably a highly reflective metal or metal alloy with a melting point in excess of the temperatures associated with the pressurized chamber or cylinder walls. For example, silver, gold, copper and brass all have m.p. of 90ϋ°C to 1100°C. Steel has a m.p. of approximately 1400-150ϋ°C. The metal coating 37 usually requires polishing or smoothing to form a good reflecting surface. When a soft metal, such as gold, platinum, silver, copper, aluminum, brass or zinc, having a hardness in the range of 2 to 4 on the Mohs scale (Handbook of Chemistry & Physics, 66th Ed. , page F19, (CRC Press, Inc. 1985) , is coated onto the boss 12b' , the surface can be flattened and smoothed by overpressuring the the diaphragm 12' , as shown in Fig. 13. Pressure in the range of approximately 1000 psi is normally exerted on the underside 12a of the diaphragm 12. Applying a pressure of 1200 psi or more forces the diaphragm upwardly towards glass disc 22, forcing the soft metal coating 37 against the glass disc 22 to flatten the coating. By selecting the appropriate metal or metal alloy and pressure, it is possible to produce a smooth, highly reflective coating 37 of uniform gap width parallel to the glass face. The required pressure will, of course, vary according to the thickness, diameter and composition of the diaphragm. The exact conditions required are standardized to insure proper calibration of the pressure transducer. _t

An overpressure technique which eliminates the criticality of gap width measurements during assembly is shown in Figs. 14A, 14B and 14C. First, an empty cylindrical housing 10' with diaphragm 12' is sealably inserted through an opening in a pressure vessel 11. The vessel is then overpressurized to a pressure well in excess of the operating range, for example, 12U0 psi where the maximum operating pressure is 1000 psi. Next, the optical head comprising the fiber holder 16 and oonded glass disc 22 is inserted into the cylindrical housing 10' so that the central portion of the glass disc 22 abuts against the soft reflective metal coating 37 on the face of the boss 12b'. A predetermined force is applied to the top of the fiber holder 16. While this force is far less than that applied to the diaphragm 12' , the force applied to the optical head is sufficient to compress and flatten the surface coating 37 on the boss 12b' . While the glass 22 and coated boss are pressed together, the optical head is affixed to the housing 10'. If the fiber holder 16 is made of metal, it may be affixed to the housing 10' by welding at the outer interface 13 as shown in Fig. 14B. If the fiber holder 16 is nonmetallic, an adhesive may be applied before insertion to the cylindrical sidewall -of the fiber holder 16 and activated by microwave or radio frequency electromagnetic energy to bond the optical head in position. After the optical head is affixed to the housing 10' , the pressure is released as shown in Fig. 14C and the diaphragm resumes its normal position. The soft metal face 36 of the boss 12' will be flat and parallel to the opposing glass surface. By carefully observing diaphragm machining tolerances and establishing predetermined overpressure, a predetermined uniform gap width can be achieved.

The technique of Figs. 14A, 14B and 14C can accommodate batch processing by mounting a plurality of sensors through respective openings in a single pressure vessel, inserting the optical heads and forcing them down collectively against their respective diaphragm bosses with a common source of pressure. This technique would enable mass production without having to establish gap width by direct gauging. The technique is also amenable to automated assembly.

Materials for the fiber holder can also be varied. For example, fiber holder 16 can be formed of a metal block with drilled holes, on the order of 0.39 inch for plastic optical fibers, for example. Instead of a glass coating, a separate glass disk 22 can be epoxied directly to the active surface of the fiber holder 16. The optical coupling between the fibers 18 and 20 and the disk 22, however, are improved by bonding the glass layer 22 directly to the fiber ends as shown in Fig. 1 by applying the glass to the fiber holder in the molten state, or by sputtering or other vacuum deposition techniques.

An alternate configuration of the pressure transducer according to the invention is shown in Fig. 5. In this embodiment, the fiber holder block 16' is metal with drilled holes accommodating fibers 18 and 20. A glass disk 22 is bonded to the surface of the fiber holder 16' . A separate cylindrical end plug 38 is mounted in spaced coaxial alignment with the disk 22 and fiber holder 16' inside a coaxial tubular sheath 40. The closed upper end of cylindrical plug 38 forms diaphragm 12 with a cylindrical boss similar to boss 12b in Fig. 1. The lower end of plug 38 is open and has an outside annular- groove 38a permitting, the protruding end of the plug 38 to be tightly clamped to a source of pressure to be tested. Thus, the interior 42 of the cylindrical plug 38 is exposed to the pressurized fluid. Another embodiment of the pressure transducer according to the invention is shown in Fig. 15. In this embodiment, the pressure vessel, for example an engine casing (head or block) 50 is machined with a bore 52 extending inwardly towards the inner chamber wall 54, stopping just short of the pressurized chamber. The thin metal section remaining forms the sensor diaphragm 56 as well as an integral portion of the chamber wall. The optical head 60, consisting of a coaxially-mounted cylindrical fiber holder 62, carrying a pair of converging optical fibers and a thin glass disc 64 is sealingly received, glass face first, through bore 52 in the pressure vessel 50. A boss 56a can be machined and coated with a soft metal and finished as described with reference to Fig. 13, 14A, 14B and 14C. Alternately, the boss 56a, coated or uncoated, may be polished to present a flat, reflective surface.

In the embodiments of Figs. 1, 5 and 15, an increase in the pressure being tested loads the diaphragm and causes it to deflect toward the glass disk 22 thus narrowing the gap 30 between the opposed parallel faces of the glass disk 22 at the point of reflection 26 and the juxtaposed face of the cylindrical boss 12b (Fig. 1). A deflection range of only two microns is sufficient as shown in Fig. 6 to achieve a sizeable variation in the amount of reflected light. Thus the output of light detector 28 is a function of the diaphragm deflection which is in turn a function of the pressure inside the pressurized chamber.

The precise nature of the mechanism by which internal reflection is affected by the presence of another material close to the interface has been a subject of great interest to physicists. It is well known that if a beam of light in a given medium (glass, for example) is incident on the interface between that medium and one of a lower index of refraction (air, for example) at an angle larger than the so called critical angle relative to the normal to said interface, the beam of light will be fully reflected back into the medium with a higher index of refraction (glass). Scientists believe that when light is internally reflected off an interface in this manner, an optical standing wave pattern is created on the opposite side of the interface. It has been demonstrated that at angles of incidence greater than this critical angle, when either an absorbing or reflecting surface is brought into close proximity with the interface in the vicinity of the standing wave pattern, the total internal reflection of the light will be disturbed. The proximity of the extraneous material apparently alters the boundary conditions at the interface, couples energy out of the standing wave and effectively allows a portion of the light to cross the interface. If the intruding surface is absorbent, this disturbance is called frustrated total internal reflection (FTR). If the surface is reflective, it is called attenuated total internal reflection (ATR) .

As shown in Fig. 6, ATR has an advantageous, minimum reflectivity at about a half wave length spacing between the interface and the intruding surface. FTR, on the other hand, requires contact for extinction of reflection, but has an advantage in that a particular reflectivity corresponds to a single spacing. In the present embodiment, ATR is preferred although the material requirements are somewhat more difficult. ATR requires a reflective metal such as gold, platinum, silver, copper or beryllium copper. Thus, in the embodiment of Fig. 4, the layer 37 would be one of these metals. FTR could be accomplished with a quartz layer (e.g., 36 in Fig. 3) forming the extraneous light absorbing face on the diaphragm.

In tests of an experimental model constructed in accordance with Fig. 5, performance was compared with a prior art Kistler piezoelectric transducer. The results were in good agreement over a large range of pressures as shown in Fig. 7.

Figs. 8-12 illustrate variations which work on the same principle as that of the basic transducer shown in Fig. 1. The added advantage of each of these variations, which may be used alternatively or in various combinations, is that they take into account and tend to eliminate the spurious effects of instability or drift in the brightness of the light source, and variations in the detector output or attenuation in the fibers, connectors or fiber/disk interface caused by temperature or aging. Each of the following techniques develops a reference signal which is combined (e.g., ratioed) with the detected signal to cancel out variations other than those induced by changes in the gap width. The techniques fall into four general categories.

The three fiber scheme is shown in Fig. 8. If the incoming light ray is wide enough or if it is allowed to diverge enough after leaving the incoming fiber 18, the area on the reflective surface on which it is incident may be larger than the boss 12b on the diaphragm 12a.- Thus, part of the reflected light ray will not be susceptible to FTR or ATR because, except for the boss area, the diaphragm 12a is too far away from the disk 22. Consequently, if a reference output fiber 21 (i.e., a third fiber overall) or an alternate light pickup is situated such that it collects only the internally reflected light which is not affected by the diaphragm, the fluctuation in intensity of the source can be detected by monitoring the reference fiber output. For example, ratio circuitry can be used to produce a ratio- of the output and reference fibers 20 and 21 so that variations in output amplitude or attenuation not^' caused by gap width are cancelled cut.

The second technique shown in Fig. 9 uses a light coupling scheme. Almost the same effect of the three fiber scheme can be achieved by coupling some of the light out of the input fiber before it enters the glass disk and subsequently monitoring it for fluctuations in source intensity. Although the coupling scheme is simpler, in this case the reference light coupled out will not take into account effects of variations in attenuation of the interface between the fiber and glass disk due to temperature and aging. In any event, the output of the third fiber, the reference fiber 21' could be used in the same manner . as the third fiber 21 in Fig. 8 in similar compensation circuitry to normalize the magnitude of light conveyed by the output fiber 20.

A four fiber scheme is shown in Fig. 10. In this case, two input fibers 18 and 19 (or other light transmitting mechanisms) and two output fibers 20 and 21 (or other light collection mechanisms) are utilized. Light from a single source is allowed to enter the two input fibers 18 and 19 via a divider (not shown). These fibers are then directed in such a manner that the point of internal reflection for fiber 19, the reference input fiber, is far enough away from the boss 12b so that the amount of internal reflection and light picked up by the reference fiber 21 will not be affected by gap width. The other pair of fibers 18 and 20 operate in the same manner as the basic transducer of Fig. 1. The output of the reference fiber 21 in Fig. 10 would again be used in ratio circuitry or another compensation scheme to cancel out the effects of source and attenuation variations not due to gap width. The techniques of Fig. 8 and Fig. 10 are similar in that variations in the intensity of light collected from a reflection point distant from the boss take into account variations not only in light intensity, but also in attenuation through the media while remaining unaffected by gap width.

A fourth scheme based on multiple colors of light is illustrated in Figs. 11 and 12. The transfer curves of light transmitted versus gap distance are scaled with the wave length of the light. This scaling feature applies to both FTR and ATR. If the ratio of the amount of light received at two different wave lengths is taken, the value will be a function of the gap distance. Dispersion effects will not be removed, so that if the fibers or other optical parts have substantially different characteristics at different wave lengths, the overall transfer curves of the ratio versus gap distance would be affected. If the attenuation due to fiber connections and optics is about the same at the two wave lengths, then the ratio of the detected light at those wave lengths will be only a function of the gap distance and will not be affected by changes in attenuation so long as the source of attenuation affects both wave lengths equally.

Fig. 11 shows a technique in which a plurality of light emitting diodes (LED) 24' emitting light at distinct wave lengths replaces the single light source 24 of Fig. 1. The LED's 24' are arranged to illuminate the input end of fiber 18. The LED's can be pulsed sequentially so that extra logic in detector 28" can differentiate between the wave lengths. One disadvantage of the alternative scheme of Fig. 11 is that the LED's have to be stabilized to avoid variations in their output or their individual brightness must be measured and used to compensate the response of the detector 28'.

A variation on the multicolor technique which eliminates the multiplicity of light sources is shown in Fig. 12. A broad band white light source 24' ' , such as an incandescent bulb, illuminates the input fiber 18. On the receiving end, the output of fiber 20 is fed via a beam splitter 32 to two different optical channels. The respective channels are equipped with optical filters 29 and 29' which allow only light of a different selected band of wave lengths to pass to the respective detectors 28' '. For small changes in the brightness of lamp 24' ' in Fig. 12, the ratio of the intensities of the two selected wave lengths will change much less than the absolute change of lamp brightness. Accordingly, the transducer will be essentially immune to errors caused by minor variations in lamp brightness. However, because the effects of gap width are different at different wave lengths, the gap width variations do not cancel out.

The advantages of the optical pressure sensor according to the invention are manifold. The optical fibers allow the electrical components for generating and detecting the light beam to be removed to a location remote from the engine so as to reduce the electromagnetic and thermal effects of the engine on the circuitry. The optical portion of the system is substantially unaffected by electromagnetic interference. Extremely small diaphgram deflections induced- by high pressures result in relatively large signal .changes. Thus, the diaphragm can be made extremely rugged to withstand the harsh operating environment of the automotive engine. The transducer is simpler in design than prior art transducers and inherently more rugged. Thus, it should be possible to manufacture the transducers at lower cost with equal or better performance and survivability.

The type of sensor described above can be mass produced and used in all automotive engines. Measurement of cylinder pressure would allow optimized automatic spark timing over a wide range of speed and . load for best location of peak pressure. Increased knock limit and higher compression ratios would be possible with lower octane requirements and better efficiency. Not only would such a sensor lead to better fuel economy and reduced emissions, but also combustion monitoring would permit a motor vehicle's own microprocessor to anticipate engine difficulties and implement solutions much more effectively than is now possible. It is conservatively estimated that with current engine optimization techniques based on determination of instantaneous cylinder pressure, a five percent fuel savings can be realized in a gasoline engine. This estimate, although conservative, represents very significant savings which would lead to rapid payback of development costs.

The optical pressure sensor has applications to other areas of technology. For example, measurement of oil well pressures would take advantage of the exceptional stability, ruggedness, and natural absolute offset of the optical pressure transducer. In addition, pressurized fluid can be introduced between the diaphragm and disc for differential readings or to adjust the zero setting.

The thickness of the glass disk 22 has been found to have a significant effect on performance. If the disk is too thick, divergence of the beam from the end of the input fiber 18 can cause interference with the reflected beam picked up by the output fiber 20. A very thin disk on the order of one millimeter reduces the effects of divergence. The cylindrical boss on the diaphragm is not essential, however, it reduces and defines the specific area that needs to be polished to a smooth parallel surface. The optical fibers can be multimode or single mode. In fact, the uses of the invention are not limited to fibers. Other means for conveying and projecting a beam of light into the transparent medium can be used. Other means of collecting the reflected rays can be used as well.

Although ceramic materials may have longer life and higher temperature stability, it should be possible to achieve satisfactory results with metal parts which can be easily machined, assuming the coefficients of thermal expansion at the glass/metal interface are compatible. A ceramic coating may be desirable in any case for the surface of the diaphragm exposed to combustion.

The foregoing description and depiction of the preferred embodiment are intended to be illustrative and not restrictive. Many variations and modifications of the configuration and materials used in the preferred embodiment are, of course, possible without departing from the spirit or scope of the invention as defined in the appended claims.

What is claimed is:

Claims

1. An optical pressure sensor, comprising an optically transparent medium having a reflective interface, input means for introducing a beam of light into said medium at an angle such that said beam is internally reflected at said interface, output means for detecting the amount of light from said beam which is reflected at said interface, a diaphragm operatively juxtaposed with said medium close enough to said interface to disturb internal reflection within said medium at said interface, and means for exposing said diaphragm to a pressurized fluid, whereby loading of said diaphragm under pressure deflects said diaphragm and changes the distance between said diaphragm and said interface thereby controlling the amount of light reflected at said interface as a function of pressure.

2. The sensor of claim 1, wherein said diaphragm has one side facing said medium, said one- side being approximately parallel to said interface at the point of reflection.

3. The sensor of claim 2, wherein the other side of said diaphragm is exposed to pressurized fluid.

4. The sensor of claim 2, wherein said interface is substantially flat at the point of reflection and said diaphragm is also flat and parallel to said interface in the region of said point of reflection.

5. The sensor of claim 1, wherein said diaphragm has a boss projecting toward said interface in the region of said point of reflection.

6. The sensor of claim 5, wherein said boss has a face which is flat and parallel to said interface at the point of reflection.

7. The sensor of claim 6, wherein the face of said boss includes a reflective metal.

8. The sensor of claim 1, wherein said diaphragm is made of metal.

9. The sensor of claim 8, wherein said exposed side of said diaphragm is coated with a ceramic material.

10. The sensor of claim 1, wherein said diaphragm is made of a ceramic material.

11. The sensor of claim 10, wherein the side of the diaphragm facing said interface is coated with a reflective metal in the area opposite the beam reflection point of said interface.

12. The sensor of claim 1, wherein said input means includes elongated optical fiber means for conducting light from a remote source to said transparent medium.

13. The sensor of claim 12, wherein said output means includes an elongated optical fiber having one end arranged to receive light reflected from said medium and remote light detecting means at the other end of said output fiber.

14. The sensor of claim 13, further comprising means for substantially aligning the ends of said fibers adjacent to said transparent medium with respect to the beam path at the reflection point of said beam, such that the beam from the input fiber is substantially reflected to said output fiber.

15. The sensor of claim 14, wherein said aligning means includes a body of material having a pair of angled through-holes, said fibers being received through said throughholes in said body, said transparent medium being held in contact with said body.

16. An optical pressure transducer, comprising a cylindrical housing, a fiber holder formed of a cylindrical body of material coaxially received in said housing and having a pair of through-holes converging with respect to each other toward a cylindrical axis, a plate of transparent material coaxially received in said housing and having one surface held against the surface of said fiber holder over said converging through-holes, a pair of optical fibers having their ends received through said through-holes and optically coupled to said plate, said converging through-holes being arranged such that a beam of light introduced through one of said fibers is totally internally reflected at the other surface of said disk and the reflected beam is aligned with and received by the end of the other fiber, a disk shaped diaphragm coaxially mounted in said housing and having one side in gapped juxtaposition with the reflecting surface of said transparent plate, said gap being small enough to disturb the internal reflection of said beam from the surface of said transparent plate, and means for exposing the other side of said diaphragm to pressurized fluid, whereby deflection of said diaphragm changes the amount of light reflected from said transparent medium surface to vary the amount of light reflected therefrom as a function of pressure.

17. The transducer of claim 16, wherein one of said fibers has its other end coupled to a remote light source and the other of said fibers has its other end coupled to a remote light detector.

18. The sensor of claim 17, wherein said tiber holding body is metal, and said through-holes are drilled.

19. The sensor of claim 17, wherein said fiber holding body is made of a ceramic material.

20. The sensor of claim 19, wherein said transparent plate is formed of a coating of transparent material on said body.

21. The sensor of claim 16, wherein said diaphragm has a cylindrical coaxial boss facing the reflection point on said other surface of said transparent plate.

22. The sensor of claim 21, wherein said diaphragm is made of a ceramic material and the face of said cylindrical boss includes a reflecting metal layer.

23. The sensor of claim 16, wherein said diaphragm comprises a metal base material with a ceramic coating on the side exposed to pressurized fluid.

24. The sensor of claim 16, wherein said diaphragm is made of a ceramic- material, the area adjacent to the reflecting point on the side of the diaphragm facing said transparent plate including a reflecting metal layer.

25. The sensor of claim 16, wherein said plate is a disk mounted coaxially in said housing.

26. An optical p essure sensor, comprising an optically transparent medium having a reflective interface, a diaphgram operatively juxtaposed with said medium having a boss projecting toward the reflective interface of said medium and terminating close enough to said interface to disturb internal reflection within said medium at said interface in the region of said boss, input means for introducing rays of light into said medium at an angle such that said rays are internally reflected at said interface both in the region of said boss and also in an area of said interface away from said boss, first output means for detecting the amount of light from said beam which is reflected at said interface in the region of sa,id boss, second output means for detecting the amount of light from said beam which is reflected at said interface from an area away from said boss, and means for exposing said diaphgram to a pressurized fluid, whereby loading of said diaphragm under pressure deflects said diaphragm- and changes the distance between the diaphragm boss and said interface thereby controlling the amount of light reflected at the interface in the region of the boss as a function of pressure such that the output of said first output means is also a function of pressure while the output of said second output means being unaffected by the diaphragm deflection is available as a reference signal.

27. The sensor of claim 26, wherein said input means comprises a single optical fiber.

28. The sensor of claim 26, wherein said input means includes- a pair of commonly illuminated optical fibers.

29. An optical pressure sensor, comprising , an optically transparent medium having a reflective interface, input means for introducing a beam of light into said medium at an angle such that said beam is internally reflected at said interface, output means for detecting the amount of light from said beam which is reflected at said interface, a diaphragm operatively juxtaposed with said medium close enough to said interface to disturb internal reflection within said medium at said interface,, and means for exposing said diaphragm to a pressurized fluid, said input means including means for producing light selectively at a plurality of different wave lengths, whereby loading of said dipahragm under pressure deflects said diaphragm and changes the distance between said diaphragm and said interface thereby controlling the amount of light reflected at said interface as a function of pressure, the effect being different at different wave lengths such that the output of said output means for different wave lengths is available for combination to eliminate variations due to light intensity and attenuation.

30. An optical p essure sensor, comprising an optically transparent medium having a reflective interface, input means for introducing a beam of light into said medium at an angle such that said beam is internally reflected at said interface, output means for detecting the amount of light from said beam which is reflected at said interface, a diaphragm operatively juxtaposed with said medium close enough to said interface to disturb internal reflection within said medium at said interface, and means for exposing said diaphragm to a pressurized fluid, said output means having means for detecting the intensity of light- at a plurality of respective wave lengths, whereby loading of said diaphragm under pressure deflects said diaphragm and changes the distance between said diaphragm and said interface thereby controlling the amount of light reflected at said interface as a function of pressure, the effect being different at said respective wave lengths such that the output of said two channels is available for combination so as to eliminate the effect of variations in intensity of the source and attenuation.

31. An optical pressure sensor, comprising an optically transparent medium having a reflective interface, input means for introducing a beam of light into said medium at an angle such that said beam is internally reflected at said interface, output means for ^detecting the amount of light from said beam which is reflected at said interface, a diaphragm operatively juxtaposed with said medium close enough to said interface to disturb internal reflection within said medium at said interface, means for exposing said diaphragm to a pressurized fluid, and means for generating a reference signal unaffected by the distance between said diaphragm and said interface representative of the brightness of light introduced by said input means to cancel spurious variations detected by said output means not due to changes in said distance, whereby loading of said diaphragm under pressure deflects said diaphragm and changes the distance between said diaphragm and said interface thereby controlling the amount of light reflected at said interface as a function of pressure.

32. A method of manufacturing an optical pressure sensor, comprising applying a soft coating to a predetermined thickness on a central portion of one side of a diaphragm, assembling the' diaphragm in predetermined relationship to a transparent medium having a flat face substantially parallel and opposed to the coated central portion of the diaphragm such that the gap between them is not in gaseous communication with the other side of the diaphragm, and overpressuring said other side of the diaphragm so as to force the coated central portion thereof against the face of₍ said transparent medium to promote a uniform flat parallel surface.

33. The method of claim 32, wherein said coating is applied to a raised boss at a central location on said one side of said diaphragm.

34. The method of claim 32, wherein said coating is reflective metal.

35. A method of manufacturing an optical pressure sensor, comprising applying a soft coating to a predetermined thickness on a central portion on one side of a diaphragm, overpressuring the other side of the diaphragm to deflect said diaphragm beyond its operational limit, contacting a flat face of a transparent medium with said coating with a predetermined force to flatten said coating, while contacting said coating with said predetermined force, fixing the position of said transparent medium, whereby when said overpressure is released, said diaphragm assumes a normal position with a uniform gap width between the surface of said coating and the opposed surface of said transparent medium.-

36. The method of claim 35, wherein said step of fixing said transparent medium includes bonding.

37. The method of claim 35, wherein said coating is applied to a raised boss at a central location on said one side of said diaphragm.

38. The method of claim 32, wherein said coating is gold or platinum.

39. An optical pressure sensor, comprising , a block of transparent material having a flat face, a diaphragm sealably mounted substantially parallel to and in spaced juxtaposition with the face of said transparent block and having a central portion with a soft coating flattened by overpressuring said diaphragm and forcing the central portion with the coating and the face of the transparent block together, whereby there is a uniform gap width between the surface of the coating and the opposed face of the transparent block.

40. The sensor of claim 39, wherein said coating is gold or platinum.

41. The sensor of claim 39, wherein said diaphragm has a raised central boss projecting toward said face terminating in a flat surface substantially parallel to said face, said coating being applied to said flat surface of said boss.

42. The sensor of claim 39, wherein said coating is selected from the group of metals and metal alloys having a high index of reflection and a hardness between 2 and 4 on the Mohs scale.

43. The sensor of claim 42, wherein said coating is selected from the group of metals and metal alloys consisting of gold, platinum, silver, brass, aluminum, copper, zinc and combinations thereof.

44. A system for measuring pressure in a pressurized chamber defined by at least one chamber wall, comprising a bore in the chamber wall extending from an outer face thereof inwardly towards said chamber, said bore terminating just before the inner wall of said chamber to define therewith a diaphragm forming an integral part of the chamber wall, means for mounting in said bore a block of transparent material having a flat face in predetermined gapped parallel juxtaposition with said diaphragm, said gap being sufficiently small that excursion of said diaphragm disturbs total internal reflection at the face of said transparent block as a function of diaphragm displacement, and optical means for reflecting a beam of light off the internal side of said face within said transparent block, whereby deflection of said diaphragm as a function of chamber pressure affects the intensity of the reflected beam.

45. The system of claim 44, wherein said optical means includes fiber optical elements for transmitting and receiving the reflected beam respectively.

46. The system of claim 44, wherein the side of said diaphragm in' the bore is formed with a raised central boss having a flat upper surface substantially parallel to the face of the transparent block.

47. The system of claim 46, wherein a coating of reflective metal is applied to the upper surface of said boss facing said transparent block.

48. The system of claim 47, wherein said coating metal is gold or platinum.