GB2186360A - Stress transducer - Google Patents

Abstract

A transducer suitable for monitoring pressure changes in the combustion chamber of an internal combustion engine comprises a source of light (1), or other radiation, arranged to direct a beam of radiation on to an element such as a diaphragm (7) that is arranged to deflect in response to the applied pressure. A detector (4) monitors changes in the phase of the radiation reflected from the element, and generates a signal representing the applied pressure. The phase change may be detected by monitoring movement of interference fringes in mono-or dichromatic light, or by monitoring changes in intensity of reflected radiation from a source of coherent radiation. In the Fig. 4 embodiment the interference is between light reflected from diaphragm 7 and that from light-transparent plate 15, whose rear surface 16 is semi-silvered. <IMAGE>

Description

SPECIFICATION Transducer This invention relates to transducers.

In a paper entitled "An Advanced Engine Control System Using Combustion Pressure Sensors" pre sented at a conference ofthe Institute of Mechanical Engineers in Birmingham, England in October1985, T.

Sasayama et al. disclosed a transducer suitable for monitoring pressure changes within the combustion chamber of an internal combustion engine. The transducer comprises a tube incorporated within a spark piug which can bethreaded through the cylinder head of an internal combustion engine so that one end ofthetube is exposed to gases inside the combustion chamber. This end ofthetube carries a diaphragm, which deflects in response to changes in pressure withinthe combustion chamber. The inner surface of the diaphragm is reflective. Two beams of light are directed on to the reflective surface to form two overlapping illuminated areas.The extentto which the two areas overlap changes as the diaphragm deflects in response to changes in pressure. By monitoring changes in the size ofthe overlapping areas, an analog electrical signal representing the pressure in the combustion chamber can be generated. However, the mathematical relationship between the area of overlap and the applied pressure is relatively complex.

According to the present invention there is provided a transducer comprising meansfor direcfing a beam of electromagnetic radiation on to an elementthat is arranged to move in response to stress applied thereto, and a detector for generating an electrical signal representing the applied stress by detecting phase changes in radiation reflectedfrom the element caused by movement ofthe element.

The relationship between the applied stress and the phase ofthe reflected radiation is relatively simple, and can, in some embodiments ofthe invention, be used to generate a digital signal which is easierto process electronicallythan the analog signal generated in the transducer described in the paper referred to above.

Although the present invention can be used in any kind of stress measurement, it is especially suitable for monitoring pressure changes in the combustion chambers of internal combustion engines. Forthis purpose, the element subjected to stress is preferably a diaphragm arranged to be deflected in response to a pressure difference between its two surfaces.

The radiation, which is conveniently visible light, may be coherent, as for example produced by a diode laser, monochromatic, or polych romantic.

Where the radiation is coherent, the reflected radiation is preferably directed so as to interfere with the incident radiation. The intensity ofthe reflected beam will thus depend upon the relative phases of the incident and reflected beams and will undergo cyclic variations as the element is moved through distances equal to integral multiples ofthewavelength of the incident beam.With such an arrangement, the detector may comprise means for detecting changes in the intensity ofthe reflected radiation. By providing the detector with means for counting the cyclic variations in intensity, a digital signal representing the move- ment of the elements and thus the applied stress, can be generated.

In an alternative embodimentofthe invention, in which the radiation need not be coherent, but is preferably monochromatic, the detector detects movement of interferencefringes in the reflected radiation,formed bytwo coincident beams of radiation reflected from the element, for example bytwo reflective surfaces so arranged that the path length of the radiation between the surfaces changes in response to the applied stress. For example, the element may comprise a plate of material capable oftransmitting the radiation and having awedge-shaped crosssection which moves under the appled stress.Alternatively, one reflective surface may be formed on a diaphragm to which the stress is applied, and which moves under the applied stress, the other reflective surface being formed on a fixed plate of material capable oftransmitting the radiation and arranged between the source of radiation and the diaphragm.

In either case, a change in the path length ofthe beam between the reflective surfaces will cause the interference fringes to move transversely overthe element. The direction of movementwill depend upon whetherthe path length has increased or decreased.

In order to distinguish between positive and negative pressurechangestherefore,thedetectormust be able to distinguish the direction of movement of the fringes. This may be achieved by providing two sensors in the detector each capable of generating a signal which varies according to the intensity ofthe reflected radiation as the interference fringes pass overthem. Provided that the spacing of the sensors is not equal to half the spacing between the fringes or an integral multiple thereof,the direction of movement of the fringes can be determined by comparing the times taken fortwo fringes to travel past both sensors.

Alternatively, if the radiation is polychromatic or comprises two frequencies, the direction of movement ofthe fringes can be distinguished by providing two sensors, each for generating signals which vary according to the intensity of reflected radiation of a respective one oftwo frequencies in the incident radiation.

Embodiments ofthe invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure lisa sketch illustrating the principle of operation of a first embodiment ofthe invention; Figure 2 is a cross-section through part of a transducer operating on the principles illustrated with reference to Figure 1; Figure 3 is a graph illustrating the operation ofthe transducerofFigure2; Figure 4 is a sketch illustrating the principle of operation of a second embodiment of the invention; Figure5 is a sketch illustrating the principle of operation of a third embodiment of the invention; The drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

Figure 6 is a sketch illustrating interference fringes produced in the embodiment ofthe invention illus traded in Figure 5; Figure 7 is a graph illustrating electrical signals obtained from the embodiment ofthe invention illustrated in Figure 5; Figure 8 is a sketch illustrating the principle of operationofafourth embodimentoftheinvention; and Figure 9 is a cross-section through a transducer head suitable for use with transducers in accordance with the invention.

The principle of operation of a first embodiment of the invention is illustrated with reference to Figure 1. A beam of coherent light generated in a diode laser 1 is directed through a half-silvered mirror2 on to a reflective surface 3, back along the path ofthe incident beam, and into a detector 4, which comprises a light cell 5 producing a voltage signal proportional to the intensity of light at it detects. The reflected beam interferes with the incident beam so that intensity of the beam atthe detectorwill vary according to the length ofthe path ofthe beam between the reflective surface 3 and the detector 4. Movement of the reflective surface 3 in the direction ofthe arrow D through a distance equal to onewavelength of the incident light will cause a cycle of variation in intensity ofthe beam at the detector.

Figure 2 illustrates a transduceroperating on the principles illustrated in Figure 1. The transducer comprised an externally-threaded tube 6, which may for example be mounted in the cylinder head of an engine to project into the combustion chamber The end ofthetube 6 is closed by a diaphragm 7 of thin sheet metal, which is polished to provide reflective internal surface 3. Coherent Iightfrom the laser is directed on to the surface3 through a fibre-optic bundle 8. Reflected light is directed to the detector 4 through a separate fibre-optic bundle 9. Both fibreoptic bundles are secured rigidly to the tube 6.

Increases in pressure P on the exterior of the diaphragm 7 with timat (iliustrated graphically at(a) in Figure3) will causethe diaphragm to move towards the fibre-optic bundles 8,9, causing the intensity ofthe beam atthe detector, and therefore the output voltage signal V ofthe detector to vary cyclically, as illustrated at (b) in Figure 3. This signal may be processed electronically by means of a zero-crossing detector 9 (see Figure 1 )to produce a train of square-wave pulses, as illustrated in Figure 3(c), which is fed into a counter 10.Provided that the counter is set to count pulses only during a period in which oressure either increases or decreases, eg duringthe compression or combustion strokes of an internal combustion engine, the content ofthe counter will represent the change in pressure applied to the diaphragm 7.

Figure4 illustrates a second embodiment of the invention in which light from a non-coherent, but monochromatic light source 1 is directed through a half-silvered mirror2 into a fibre-optic bundle 8 secured in a threaded tube 6 similar to that shown in Figure 2.The end ofthe tube is closed by a flexible diaphragm 7which has polished reflective internal surface 3. A plate of light-transmitting material 15, the rear surface 16 of which is semi-silvered, is secured in the end ofthetube 6 and is spaced apartfrom the surface 3 by a few wavelengths ofthe incident light Light reflected from the reflective surface 3 and the semi-reflective surface 16 passes back up the fibreoptic bundle and is directed by the half-silvered mirror 2 in to detector 4.The light reflected from the surface 3 ofthe diaphragm interferes with that reflected from the rear surface 16 ofthe plate 15 to form interference fringes. Pressure applied to the diaphragm 7 will deflectthe diaphragm and alterthe relative phases of light reflected from the surfaces 3 and 16, causing the interferencefringesto move acrossthe plate. The intensity ofthe beam at the detector will therefore undergo cyclicvariationscorrespondingto the movement of the interferencefringes. By counting the numberof interference fringes crossing the plate in a manner similar to that used in the first embodiment of the invention, a digital signal representing the change in pressure on the diaphragm can be produced.

Thedirection of movement of the fringes caused by pressure increases will be the reverse ofthatcaused by pressure decreases. Figure 5 illustrates a system for discriminating between the directions of movement ofthe fringes so that increases in pressure can be distinguished from decreases in pressure. In this arrangement, light reflected from the reflective surface 3Of the diaphragm 7 interferes with the light reflected from the semi-silvered surface 16 ofthe plate 15 and is directed by twofibre-optic bundles 20,21 to two sensors 55' in the detector 4. The ends of the fibre optic bundles are spaced apart by a distance lessthan halfthe spacing ofthe interference fringes inthe direction of movement of the fringes.

Figure 6 illustrates three equally-spaced interference fringes 25. 26, u formed on the surface ofthe diaphragm 7. As the diaphragm deforms, the fringes move radially outwardly or inwardly. The movement of each fringe past the fibre-optic bundles 20, 21 will produce a square-wave pulse in the zero-crossing detectors 9, 9'. The combined signals, as detected in a timing circuit 28, are illustrated at (a) in Figure 7for fringes moving outwardly, and at (b) for fringes moving inwardly, the pulsesfrom the signals transmitted by fibre-optic bundle 20 being shaded grey.

After one fringe has been detected first by one sensor 5 andthen by the other sensor 5', the period T before the detection ofthe next fringe by the first sensor 5 is longer when thefringes are moving in one direction than the other. By measuring these times in the timing circuit 28, the detector can distinguish between pressure increases and pressure decreases.

Figure 8 illustrates afurther embodiment of the invention using an alternative means of distinguishing between pressure increases and decreases. In this arrangement, the light source 1 produces light of two close butdifferentfrequencies A, B. Two sets of interference fringes are generated by interference between the light reflected from the reflective surfaces 3, 16 ofthe diaphragm 7 and the plate 15, the two fringes having different spacings. The resultant light is directed bytwo half-silvered mirrors 2, 2' through two filters 30, 31 which pass light offrequencyA and B respectively into two separate sensors 5, 5'.As pressure on the diaphragm 7 is increased ordecreased, the two sets offringes will move across the diaphragm and produce corresponding cyclic variations in the output voltage signals at the two sensors 5, 5' in the detector 4. The relative phases of the two signa lswil' be different according to the direction of movement ofthe fringes. By passing the output signals from the two detectors through a phase comparator35, as illustrated at 12 in Figure 8, a signal representing increasing or decreasing pressure is produced. This signal is used to switch up/down counters 9, 9' between up-counting and downcounting modes so that the contents ofthe counters 9, 9' represent the deflection ofthe disphragm at any given moment.The content of the two counters is averaged in a network36to provide an output signal which directly indicates the pressure acting on the diaphragm 7.

Figure 9 illustrates one practical construction for a transducer head suitable for use in transducers in accordance with the invention. The transducer head comprises a tube 40 of metal through which runs a fibre-optic bundle 8. One end ofthe tube has a hexagonal head 41 and a threaded boss 42 enabling the fibre-optic bundle 8 to be mechanically and optically connected to a detector (not shown). The external surface ofthe tube 40 adjacent the hexagonal head 41 is threaded and carried a crimped sealing washer44, enabling the transducer head to be threaded into a bore 45 in the cylinder head 46 ofan internal combustion engine. The bore 45 communicates with the combustion chamber ofthe engine.

The other end ofthetube 40 has a shoulder 47. A seal 48, such as a copper sealing washer, seals the shoulder 47 to a complementary shoulder 50 in the bore 45. Thetubeterminates atthe inner end ofthe bore 45 and is closed bya metal diaphragm 7. A plate 15 of light-transmitting material is mounted in the end ofthetube adjacent the inner reflective surface 3 of the diaphragm 7, to form a construction similarto that described with reference to Figure 4. Although the construction illustrated is that of Figure 4, the trans ducer head may be easily modifiedtoform part of a transducer operating on the principles illustrated with reference to any of the preceding drawings.

Claims (14)

1. A transducer comprising means for directing a beam of electromagnetic radiation on to an element that is arranged to move in response to stress applied thereto, and a detector operable in response to radiation reflected from the element for generating an electrical signal representing the applied stress, characterised in that the detector detects phase changes in radiation reflected from the element caused by movement ofthe element.

2. Atransduceraccording to Claim 1 wherein the element comprises a diaphragm arranged to be deflected in response to a pressure difference between its two surfaces.

3. Atransducer according to Claim 1 or Claim 2 wherein the electromagnetic radiation invisible light.

4, Atransducer according to any one of Claims 1 to 3 wherein the electromagnetic radiation is coherent.

5. Atransducer according to any one of Claims 1 to 4wherein the electromagnetic radiation is monochromatic.

6. Atransducer according to any one of Claims 1 to 5 wherein the electromagnetic radiation is coherent, the reflected radiation interferes with the reflected radiation, and the detector detects changes in the intensity of the reflected radiation.

7. Atransducer according to Claim 6 wherein the detector comprised means for counting cyclic varia- tions in the intensity ofthe reflected radiation as the element moves underthe applied stress.

8. Atransducer according to any one of Claims 1 to 5 wherein the detector detects movement of interference fringes in radiation reflected from the element.

9. A transducer according to Claim 8 wherein the element comprises two reflective surfaces positioned to form interference fringes in the radiation reflected from the element and so arranged thatthe path length ofthe radiation between the surfaces changes in response to the applied stress.

10. Atransducer according to Claim 8 or Claim 9 wherein the detector comprises two sensors for generating signals which vary in accordance with the intensity of the reflected radiation, the spacing between the sensors being less than the spacing between the interference fringes or an integral multiple thereof.

11. Atransducer according to Claim 8 or Claim 9 wherein the radiation is of two frequencies and the detectorcomprisestwo sensors each for generating a signal which varies in accordance with the intensity of reflected radiation of a respective one of the two frequencies.

12. Atransducer according to any one of Claims 1 to 71 wherein the radiation is directed on to the elementthrough a fibre-optic bundle.

13. Atransducer according to any one of Claims 1 to 12 wherein the radiation reflected from the surface is directed through a fibre-optic bundle.

14. Atransducer substantially as herein described with reference to any one ofthe embodiments illustrated in the accompanying drawings.