GB2138137A - Improvements in sensing apparatus - Google Patents

Abstract

To sense a physical parameter a vibratory element (3) is arranged that its natural frequency of vibration varies with the magnitude of the sensed parameter. The element may be a wire or the like elongate element which is subjected to a tensile or transverse force representing the parameter or a diaphragm which is subjected to a transverse force or a piezoelectric material. In the case of an elongate element, vibration may be in a transverse or torsional mode. The frequency of vibration is sensed by transmitting radiation (1) from a control station (CS) to the vibratory element via an optical fibre (2) and arranging the element (3) so that the radiation is modulated thereby. Modulated radiation is returned via an optical fibre (4) to the control station and the frequency of modulation is sensed. For effecting vibration of an elongate element the element may be disposed in a magnetic field (B) and an alternating driving signal applied to the element. The driving signal is generated by a photodiode (9) activated by radiation transmitted from the control-station via an optical fibre (8). For effecting vibration of a diaphragm, an alternating driving signal is applied to an associated electromagnet. The invention is particularly suited to use in hazardous environments. <IMAGE>

Description

SPECIFICATION Improvements in sensing apparatus This invention relates to apparatus for sensing physical parameters.

When measuring physical parameters such as temperature, pressure, differential pressure and flowrate it is an advantage to employ a transducer which generates an alternating output signal having a frequencywhich represents the magnitude of the parameter. The signal can then be transmitted over long distances without substantial degradation through attenuation or losses, is more compatible with digital receiving equipment than is an analogue signal, and is easier to integrate with respect to time.

One proposed transducer which produces an alternating output signal has a sensing element arranged, in use, to be subjected to a force representative of a physical parameter. Avibratory wire is coupled to the sensing element and is subjected to a tension representing the force on the element, and hence representing the magnitude ofthe parameter.

When plucked the wire oscillates at a natural or resonantfrequencygiven by:

(where T = the tension in the wire, L = length of wireandm = mass per unit length).

Variations in the force applied to the sensing element, due to variations in the physical parameter, cause changes in tension in the wire, and hence changes in the resonantfrequency.To enablethe resonant frequency to be sustained and measured, the wire is disposed in atransverse magnetic field, between the poles of a magnet. If an alternating electric current is passed through the wire, by means of a feedback system, oscillations are sustained at the resonantfrequency.

It is an object ofthe present invention to provide apparatus for sensing physical parameters which not only produces an alternating output signal but which can also be used for measurement of physical parameters in hazardous environments.

According to the present invention there is provided apparatusforsensing a physical parameter wherein vibratory means include a vibratory element so arranged that, in use, the element has a natural frequency of vibration which varies with the magnitude ofthe sensed parameter, optical fibre means are provided for conveying radiation to the vibratory means,wherebythe vibratory means, upon receiving radiation from the optical fibre means generate an electrical driving signal which causes the vibratory elementto vibrate at a frequencywhich rapidly becomes equal to the natural frequency, the vibratory elernent and the optical fibre means are arranged so that at least part of the radiation received by the vibratory means is modulated in intensity at the natural frequency by vibration ofthevibratory element, whereby the frequency at which the radiation is modulated represents the magnitude of the sensed parameter, and the optical fibre means are arranged to convey modulated radiation awayfrom the vibratory means.

In apparatus according to the invention for sensing physical parameters such as temperature, pressure, differential pressure and flowrate the vibratory element may have a natural frequency of vibration which varies with the magnitude of a force applied thereto, in which case the element is arranged to be subjected to a force representing the magnitude of the physical parameter. In apparatus for sensing a physical parameter such as the concentration of a gas, however, the vibratory element may have a natural frequency of vibration which varies with the mass of a gas absorbed thereon, in which casethe element is arranged so asto absorb a mass of gas representing the concentration of the gas.

The vibratory element may be a wire, ribbon, helical spring or otherelongate elementwhich, in use, is subjected to a force representing the magnitude ofthe said parameter and applied to the element in a direction lengthwise thereof, thereby to vary the tensile force applied to the element, the element being arranged to effect vibration in a direction transversely of a longitudinal axis thereof.

Alternatively, the vibratory element may be a wire, ribbon orotherelongate element which, in use, is subjected to a force representing the magnitude of the said parameter and applied to the element in a direction transversely of a longitudinal axisthereof, therebyto vary the tensile force applied to the element, and the element is arranged to effect vibration in a direction transversely of the longitudinal axis.

In either case, a temperature compensating member maythen extend generally parallel with the vibratory element, one end ofthe compensating member being secured to a fixed location in the vibratory means, and an opposed end of the compensating member being securedto oneend ofthe vibratoryelement,thelength and coefficient of thermal expansion of the material ofthe compensating member being such that a change in the length of the element, and hence a change in the tensile force applied thereto, as a result of a change in temperature is compensated or substantially compensated by a change in length of the member.

Alternatively, the apparatus may be adapted for measurement of temperature changes, in which case the vibratory element extends generally parallel with an elongate memberofa material having a coefficient of thermal expansion different from the coefficientofthermal expansion ofthe material ofthe element, and each end ofthe element is secured to an adjacent end of the elongate member, whereby a change in temperature causes a differential expansion ofthe element and the member, and hence a change in the tensile force applied to the element.

Alternatively, the vibratory element may be an elongate generally U-shaped element which, in use, is subjected to a force representing the magnitude of the said parameter and applied to the element in a direction lengthwise thereof, the element being arranged to effect vibration in a torsional mode.

Preferably, when the vibratory element is an elongate element, the vibratory means include a magnet, the vibratory element extendstransversely ofthe magnetic field ofthe magnet, and the vibratory means include means for applying the said alternating electrical driving signal to the vibratory element, wherebythevibratory element is caused to vibrate.

The vibratory element may be a wire, ribbon, helical spring or other elongate element which, in use, is subjected to a force representing the magnitude ofthe said parameter and applied to the element in a direction transversely ofthe axis thereof, the element comprises magnetic material, and the elemenu is arranged to effect vibration in a direction transversely ofthe said axis.

Alternatively, the vibratory element may be a sheet of piezoelectric material which, in use, is subjected to a force in a direction transversely ofthe plane ofthe sheet and representing the magnitude ofthe said parameter, electrical contacts are provided on respective opposite faces of the sheet, and the vibratory meansfurtherinclude meansforapplyingtheelec- trical driving signal to the contacts,therebyto cause the elementto vibrate.

Alternatively, the vibratory element may be a sheet of piezoelectric material having a surface adapted to, absorb a mass of gas representing the concentration of gas in which the element is located, the element having a natural frequency of vibration which varies with the mass of gas absorbed thereby, electrical contacts are provided on respective opposite faces of the sheet and the vibratory means further include meansforapplying the electrical driving signal to the contacts, thereby to cause the element to vibrate.

Alternatively, the vibratory means may include a diaphragm which, in use, is subjected to a force representing the magnitude of the said parameter, the diaphragm forms one plate of a capacitor and the spacing between the diaphragm and another plate of the capacitor varying in accordance with changes in the magnitude ofthe said parameter, the vibratory element is a sheet of a piezoelectric material disposed between the diaphragm and the other plate of the capacitor, whereby changes in the spacing between the diaphragm and the said other plate caused changes in the capacitance ofthe capacitorwhich cause changes in the natural frequency of the vibratory element, and means are provided for applying the electrical driving signal between the plates ofthe capacitor.

Alternatively, the vibratory means may comprise two vibratory elementseach comprising aware, ribbon orthe like elongate element, the two vibratory elements are so arranged that a change in the magnitude ofthe sensed parameter causes an increase in the natural frequency of vibration of one element and a decrease in the natural frequency of vibration ofthe other element, the vibratory means generate at least one electrical driving signal which causes each vibratory element to vibrate at its natural frequency, and the vibratory elements and the optical fibre means areso arranged that at least part ofthe radiation is modulated atthe natural frequency of one vibratoryeiementand at leastpartofthe radiation is modulated atthe natural frequencyofthe other element.

Preferably, the apparatus fu rthercomprises a control station, the vibratory means being disposed at a location remote from the control station, and the control station comprises a source of the said radiation, and means for sensing the frequency at which radiation is modulated in intensity',theoptical fibre means extending between the controletation and the remote location and serving toconvey radiation from ths sou rce to the vibratory means and to convey modulated radiation from the vibratory means to the control station.

In this case, the control station preferablyincl udes means for receivingincoming modulated radiation conveyed from the transducervia the optical fibre means and generating an alternating electrical signal offrequency equal tothefrequency atwhich the incoming radiation is modulated, and meansfor applying the electricaf signal to the source, whereby the source generates radiation fortransmis sionviatheoptical fibre means to thetransducer which is modulated atthefrequency of modulation of the incoming radiation, and the vibratory means include means adapted, when the modulated radiation from the source is applied thereto, to generate the said alternating electrical driving signal.

There may then be provided at the control station a secondsource of radiation of constant or substantial- ly constant intensity, radiation from the second source forming the said at least part ofthe radiation received bythevibratory means.

Alternatively, the apparatus further comprises a control station, the said source is a source of radiation of constant or substantially constant intensity, the vibratory means are disposed at a location remote from the control station, the optical fibre means convey radiation from the sourceto the vibratory means, the vibratory element is so arranged relative totheopticalfibremeansthatvibration ofthe element modulates the radiation which is incoming from the source, the vibratory means include means adapted,when radiation modulated by the vibratory element is applied thereto, to generate the said alternating electrical driving signal, the optical fibre means convey modulated radiation from the vibrate tory means to the control station, and the control station includes means for sensing the frequency at which radiation is modulated in intensity.

The invention will now be described, by-way of example, with reference to the accompanying draw ings, in which: Figure lisa schernatic drawing of first apparatus for sensing physical parameters accordingtotbe invention; Figure 2 shows in detail a part of avibratory means in the apparatus. of Figure 1; Figures 3,4 and 5 are schematicdrawings of second, third and fourth apparatus according to the invention; Figure 6 is a vibratory element in a further apparatus according to the invention Figure7 is part of a further apparatus according to the invention; Figures 8 and 9 are vibrating elements in further apparatus according to the invention; and Figure loins a vibratory means in a further apparatus according to the invention.

The apparatus shown in Figure 1 of the drawings is suitable for measuring a physical parameter at a location which may have a hazardous environment and which is remote from a control station.

The apparatus includes a transducerT, partly shown in Figure 1, which is disposed in a measuring head at the remote location. Communication between the transducerT and a control station CS is effected via optical fibre means, as hereinafter described.

ThetransducerTincludes a sensing element (not shown) which, in use, is subjected to the physical parameter subject to measurement.

To enable measurementoftheforce acting on the sensing element, the transducer T is provided with vibratory means V including a vibratorywire 3. The wire 3 is taut, is supported at each end 12thereof and is coupled to the sensing element. In use, the wire is subjected to a force applied in a direction lengthwise thereof by the sensing element. This force, which varies the tensile force applied to the wire 3, is representative ofthe magnitude ofthe physical parameter.

As described above, the wire 3 has a natural or resonant frequency of vibration proportional to the square rootofthetensile force in the wire. To effect vibration, the wire 3 is disposed in a transversely extending magnetic field B of a permanent magnet (notshown) and an alternating electrical driving current is applied thereto.

Energy for establishing and maintaining the driving current is derived from radiation emitted by a first light source 7 at the control station CS. The source 7 is a device which can be modulated ataudio frequencies, such asan LED or laser diode. Modulated visible or infra-red radiation from the source7 is conveyed to the vibratory means V by a fibre 8 ofthe optical fibre means.

In the vibratory means V, a photoelectric diode 9 servesto convert modulated radiation from the fibre 8 into an electrical signal offrequency equal to the modulation frequency. The diode 9 is connected to an inputwinding of an impedance matching transformer 10, whose output winding is connected to the wire 3.

For detecting the frequency of vibration of the wire 3, further radiation is transmitted from the control station CS to the remote location and part of that radiation is reflected back to the control station bythe wire, the radiation in each case being conveyed by the optical fibre means. This further radiation is obtained from a second light source 1,which emits radiation of constant or substantially constant intensity. An input fibre 2 is provided for conveying the further radiation from the source 1 to the vibratory means V and an output fibre 4 conveys reflected radiation back from the vibratory means to the control station CS. At the control station there is a second photoelectric diode 5 for receiving the reflected radiation.

Referring to Figure 2 of the drawings, the ends of thefibres 2 and 4which extend intothevibratory meansVare parallel-arranged and in contact The wire 3 extends perpendicularly of each fibre end.

In thecontrol station CS an amplifier 6 is connected to an output ofthe photodetector 5. The amplifier 6 has a first output connected to a frequency counter 11 and a second output connected to the first light source 7.

When the present apparatus is in use, the wire 3 is subjected to a force representing the magnitude of the physical parameter, as mentioned above.

Once the apparatus is in use, a vibration of associated equipment, ambient noise oran electrical impulse applied to the wire 3 as a result of radiation transmitted to the vibratory meansVfrom the source 7 causes the wire to execute a small initial vibration.

During the course ofthis initial vibration the wire makes a small excursion somewhere between the extremes of positions (a) and (c) in Figure 2. As indicated in Figure 2, position (a) is a position in which none of the radiation transmitted along the inputfibre 2 is reflected by the wire 3to the outputfibre 4. At position (b), a small amount of reflected radiation is received by the fibre 4. Finally, bythe time the wire 3 has moved to the position (c), the amount of radiation reflected to thefibre 4 is approaching a maximum value.

Figure 2(d) is a plot of the intensity of radiation reflected to the fibre 4 versus the displacement ofthe wire 3 awayfrom the position shown in Figure 2(a).

Positions on the plot which correspond to positions 2(a), (b) and (c) areshown in Figure 2(d). It will be noted that these positions utilisethe region of highest sensitivity on the rising slope of the characteristic. In practice, the subsequent falling slope may also be used.

In the result, the initial vibration of the wire 3 causes radiation modulated in intensity to be reflected along the fibre 4to the photoelectric diode 5. An alternating electrical signal is then applied to the inputofthe amplifier 6, an amplified version of the signal is appliedtothelightsource,and radiation of mod ulated intensity is transmitted from the light source 7 tothe vibratory means V. In the vibratory means an alternating electrical driving signal is applied to the wire 3 and interaction between the driving signal and the transverse magnetic field B serves to maintain the vibration ofthewire.

Itwill be appreciated that a major component in the vibratory movement of the wire 3 is a component having a frequency equal to the natural or resonant frequency of the wire. Assuming a suitable phase relationship between the modulated radiation received bythe photoelectric diode 5 and the modulated radiation emitted by the source 7 it is this natural orresonantfrequencycomponentofvibra- tion which predominates after a few vibrations of the wire 3. From that stage onwards the frequency of vibration ofthe wire 3, the frequency at which the reflected radiation and radiation emitted from the source 7 are modulated, and the frequency of the electrical signals in the control station CS and in the vibratory means V is in each case equal to the natural or resonant frequency of the wire. This frequency is sensed by the frequency counter 11, and from the sensed frequency the tension in the wire 3 is computed by means ofthe above-mentioned formu- la.As also mentioned above, the tension in the wire 3 is representative ofthe physical parameter sensed by the apparatus.

Referring nowto Figure 3, a second apparatus according to the invention differsfrom the apparatus of Figure 1 only in the manner in which vibration of the wire 3 is sensed. In the apparatus of Figure 3 the ends of fibres 2 and 4which extend into the vibratory means V are coaxially arranged and mutually spaced by a short distance. The wire 3 extends through the space between the ends of respective fibres 2 and 4.

Upon vibration,thewire 3 modulates in intensity light which is conveyed to the vibratory means V bythe fibre 2 and returned to the control station CS via the fibre 4. The apparatus of Figure 3 operates therefore by sensing lighttransmitted from fibre 2 to fibre 4, ratherthan light reflected by the wire 3, as is the case in the apparatus of Figure 1.

In a modification of the apparatus of Figures 1 and 3, shown in Figure 4, a source 7 is a device which emits radiation of constant or substantially constant intensity. Radiaton from the source 7 is modulated by an optical chopper24which may be a Kerr cell, a Pockels cell, a Faraday cell, a reciprocating or rotary chopper, or other suitable device.

Modulated radiation from the source 7 and chopper 24 is transmitted via a fibre 8 to a photoelectric diode (notshown) in avibratory meansV.Acurrent generated by the diode is applied to a transformer (notshown) having an outputwinding connected to a wire (also not shown) in the manner described above in connection with Figure 1. The wire vibrates, as also described above.

For sensing vibration ofthewire, radiation of constant intensity from a second source 1 is transmit- ted to the vibratory meansVvia an input fibre 2.

Radiation modulated by vibration ofthewire is returned to a photoelectric diode 5 in the control station CS via an outputfibre 4.

An outputcurrentfrom the diode 5 is appliedto an amplifierand squarer30, whose output is coupled to a control circuit31. An output ofthe circuit 31 is used to control the chopper 24 in such mannerthatthe radiation transmitted from the source 7 to the vibratory means V is modulated atthe vibrational frequency ofthe wire and has a fixed phase relationship with the vibration.

In one form ofthe apparatus shown in Figure4,the source 7 is a white light source and the chopper 24 is a rotary chopper. The chopper 24 is designed to generate also an electrical pulse train offrequency equal to the choppingfrequency. The phase of this pulsetrain is compared with thephase of the output from the circuit 30 in a phase-locked loop. An error signal representing any difference between the phases is applied to the chopper 24 and causes a variation in speed ofthe chopper until the two signals are in phase. When the two phases are locked together,the apparatustracks any changes in the frequencyofvibration ofthewire caused by changes in the tension applied thereto by the sensing element in the transducer.

The apparatus of Figure 4 can be modified by replacing the two light sources 1 and 7 bya single source.

Figu re 5 of the drawings sh ows a further apparatus according to the invention in which a single light source 1 provides radiation partofwhich is used for sensing vibration ofthe wire 3 and part of which acts as a source of energy from which current for driving the wire is derived. There is therefore no Iightsource corresponding to the source7 of Figure 1.

Referring to FigureS, where components corresponding to components of Figure 1 are given corresponding reference numerals, a fibre 2 is again arranged to convey radiation of constant intensity from the source 1 to the vibratory means V and a return fibre 4 serves to convey reflected radiation, modulated in intensity by vibration ofthe wire 3, back to the control station CS. At the control station the modulated radiation is again sensed by a photoelec tric diode 5 connected to an amplifier 6, and an output ofthe amplifier is again connected to a frequency counter 11.

In the apparatus of Figure 5, a photoelectric diode 9 for supplying driving current to the wire 3 faces an output end ofthe inputfibre 2 and thewire extends through the space between the fibre and the diode.

When the apparatus is in use and the wire 3vibrates, the diode 9 receives radiation whose intensity is modulated at a frequency equal to the frequency of vibration ofthewire. As in the apparatus of Figure 1, current from the diode 9 is supplied to an input winding of a transformer 10 having an output winding connected to the wire 3.

The apparatus of Figures 1,4 and Scan be modified by using a single fibre in place ofthetwofibres 2 and 4. A beam splitter is then provided so as to ensure that radiation from the source 1 is directed to thevibratory meansVwhilstreturning, modulated radiation is directed to the diodeS.

It will be appreciated that changes in temperature cause the wire 3 of Figures 1,3, 4and Sto expand or contract,therebycausing a change in thetension of the wire. To compensate fortemperature changes the wire may be arranged coaxiallyofatube of a material having a length and a coefficient of thermal expansion adequate to compensate for the thermal expansion ofthe wire. One end of the tube is provided with a cross-member which serves as an anchorfor one end of the wire. The other is held at a fixed location and the other end ofthewire is secured to the sensing element in the transducer.With this arrangement, a change in the length ofthewiredueto a temperature change is accompanied by a corresponding change in the length ofthetube.

Referring to Figure 6, an apparatussuitablefor measuring temperature changes also has a vibratory wire 3 extending coaxially of a tube 25. In this case, however,thetube25 has a different coefficient of thermal expansion from that of the wire 3 and the wire is secured at each end to a cross-member on the tube. With this arrangement, a change in temperature causes differential expansion between the tube and the wire, causing a change in tension ofthe wire. The change in tension, which represents the temperature change, is detected bysensingthe resulting change in natural orresonantfrequencyofthewire.

To provide overrange protection in the apparatus of Figures 1 to 6, a closed-coil spring or other known form of protection may be provided between the sensing device and the wire 3. Over an operating range of measurementthe spring remains rigid. For a measurementabovethe operating range, however, the coils of the spring open so thatthe wire is protected from excessive tension.

The wire 3 ofthe embodiments described above can be replaced by a ribbon or helical spring coupled to the sensing element and subjected to a tensile forceproportionaltothe magnitudeofthe measurand.Alternatively, a diaphragm may be used as the vibratory element. In this case, a pressure or differential pressure is applied to the diaphragm in orderto alter its resonant frequency.

Figure 7 shows a part of an apparatus where a diaphragm 21 made wholly or partly of a magnetic material is used as the vibratory element of vibrarory means V. In this case the diaphragm 21 is made to vibrate by applying an alternating electrical driving currentto a winding of an electromagnet 13 whose poles are disposed adjacenttothe diaphragm. The driving current is obtained from a photoelectric diode 14, which corresponds to the diode 9 of Figures 1,3,4 and 5. Radiation modulated in intensity at the natural or resonantfrequency of vibration ofthe diaphragm is conveyed to the diode 14via a fibre 1 which extends between the vibratory means V and a control station (not shown).Vibration of the diaphragm is sensed by radiation transmitted between the control station and the vibratory means via fibres 16 and 17.

In the apparatus of Figure 7, the diaphragm 21 is subjected to a force transversely ofthe plane of the diaphragm and, when vibrating, each part of the diaphragm also moves in a transverse direction. It is likewise possible to apply a transverse force representingthe magnitude of a physical parameter to a vibratory element in the form of a wire, ribbon or helical spring, suitably by means of a magnet attached to the sensing element and a piece of magnetic material attached to the element. The transverse force causes the element to assume a 'bowed' shape and subjects the elementto atensile force which varies the natural or resonantfrequency ofvibration of the element.

Further embodiments ofthe invention include a vibratory element which vibrates in a torsional mode ratherthan a transverse mode.

In a first ofthese embodiments, shown in Figure 8a, a vibratory element 35 is formed as an elongated U-shaped member which is rotatable about a longitudinal axis I thereof. Aforce proportional to the parametersubjectto measurement is applied to the member 35 in a direction parallel to the axis I. The member 35 extends transversely of a magnetic field between the poles of a permanent magnet. An alternating current, derived from a photoelectric diode and transformer corresponding to those shown in Figures 1,3 and 5, is applied to the member35 via two terminals, one at the free end of each limb of the 'U'. Passage ofthe alternating current causes the member 35 to be subjected to forces which cause vibratory movement about the axis I.The frequency of this vibratory movement, which is representative ofthe force applied to the member 35, is detected by radiation reflected from, or intermittently transmitted past, a limb ofthe member 35.

In a second rotary embodiment, shown in Figure 8b, a vibratory member 3 takes the form of a wire or ribbon having a magnetic flag 26 attached thereto. An electromagnet 29 is associated with the flag 26.

Alternating cu rrentfrom a photoelectric diode and transformer is applied to the electromagnet 29 and causes the flag 26 to be alternately attracted towards and repelled away from the electromagnet. This causes vibratory movement of the member 3 about the longitudinal axis thereof. This time, vibration of the member3 is detected by means of radiation reflected from or transmitted past the flag.

Figure 9 shows part of a further embodiment of the invention which provides an output which varies more linearly with the tension applied to the vibratory element. In this embodimenttwo vibratoryelements 3, which can be wires, ribbons or springs, are coupled to a sensing element 27 of a transducer. The sensing element 27 can be a diaphragm or it can be some other element which, in use, is subjected to a force which is applied in a direction transversely of a longitudinal axis ofthe element and which has a magnitude representative ofthe magnitude of the physical parameter subject to measurement.

The arrangement ofthe elements 3 and 27 is such that movement of the element 27, due to a variation in the magnitude ofthe physical parameter, causes an increase in the tension T1 applied to one ofthe vibratory elements 3 and a decrease in the tension T2 applied to the other element3,thus causing a change in the natural frequency of each vibratory element.

To measure the natural frequency of vibration, radiation of constant or substantially constant intensity is conveyed to each element 3 via a respective optical fibre in the vibratory means of the transducer.

Radiation modulated at the frequency of vibration of each element 3, either by reflection from the element or by movement of the element into and out ofthe path of the radiation, is then collected by afurther fibre associated with that element. An alternating electrical driving current for each vibratory element 3 is obtained bytransmitting modulated radiation from the control station to the transducer and applying this radiation to an LED or laser diode, in the manner of Figures 1,3 and 4, above, or by transmitting radiation of constant or substantially constant intensity, using each vibratory elementto modulatethe radiation, and then applying the modulated radiation to an LED or laser diode, in the manner of FigureS.

The point at which each vibratory element3 is coupled to the sensing element 27 may be earthed. In this case a first alternating driving current is generated and applied to a first element 3 and a second alternating driving current, independent ofthefirst is applied to a second element 3. Alternatively, the sensing element 27 is notearthed and the two vibratory elements 3 may be electrically connected together. In this case a driving current is applied to the series circuit formed by the two elements 3, the current having one component offrequency equal to the natural frequency of vibration of one element and a second component of frequency equal to the natural frequency ofthe other element.

Radiation modulated at each of the two natural frequencies is conveyed from the transducerto the control station via a single optical fibre. At the control station, the difference between the natural frequen cies of the elements 3, fl-f2, is measured and has a substantially linear relationship with the movement ofthe element 27. This can be an advantage where a minimum ofsignal processing is required. The arrangement also provides a positive indication of a wire failure.

In the embodiments described above, an alternating electrical driving current is derived from visible or infra-red radiation transmitted by the optical fibre meanstoa photoelectric device, isappliedto a vibratory element disposed in a transversely extending magnetic field orto an electromagnet whose poles are disposed adjacent to the vibratory element.

Afurther embodiment of the invention is an electrostatic counterpart of these electromagnetic embodiments.

The electrostatic embodiment is an apparatusfor measuring pressure or differential pressure. Included in the apparatus is a vibratory element in the form of a diaphragm which is made of a thin sheet of quartz or other piezoelectric material. The diaphragm is subjected, in use, to the pressure or differential pressure.

A metal contact is provided on each surface of the diaphragm and an alternating electrical driving signal, derived from visible or infra-red radiation transmitted via optical fibre means, is applied to the contacts. The driving signal causesthe diaphragm to vibrate with an amplitude which reaches a maximum value when the frequency of vibration equals the natural or resonant frequency. This natural frequency is dependent upon the mechanical properties of the piezoelectric material and also upon the transverse force applied thereto as a result ofthe pressure or differential pressure. Accordingly, the magnitude of the pressure or differential pressure is determined by causing the diaphragm to vibrate atthe natural frequency and then detecting that frequency by modulating radiation transmitted from a control station, as described in the above embodiments.

Figure 10 shows vibratory means in a transducer of afurtherapparatusaccording to the invention which is also designed for measuring pressure ordifferential pressure.

In the vibratory means of Figure 10, there is a metal diaphragm 40 which is arranged, in use, to be subjected to a transverse force proportional to the pressure or differential pressure. Application ofthis force causes a lateral movement or 'bowing' of a central part of the diaph rag m 40, as shown diagrammaticallyin Figure 10.

The diaphragm 40 is spaced from and generally parallel with a wall 41, which forms part of a housing for the transducer. A capacitor 42, formed by the diaphragm 40 and the wall 41 and also shown diagrammatically in Figure 10, has a capacitance which varies in accordance with changes in the spacing between the diaphragm and the wall, and hence in accordance with the magnitude of the pressureordifferential pressure.

Mounted within the space between the diaphragm 40 and the wall 41 is a vibratory element 43 formed of a thin sheet of quartz or other piezoelectric material.

Means are provided in the vibratory means for converting visible or infra-red radiation transmitted from a control station into an alternating electrical driving signal and applying the driving signal to the capacitor plates 40,41. Application ofthedriving signal to the plates 40, 41 causes the element 43to vibrate at its natural or rnsonantfrequency' In the vibratory means of Figure 10, the vibratory element 43 has a natural or resonantfrequencywhich varies in accordance with changes in the capacitance coupling applied thereto by the capacitor 42. The natural frequency of the element 43 is therefore "tuned" by changes in capacitance ofthe capacitor42 as the diaphragm 40 moves in response to changes in pressure or differential pressure.Accordingly, a measurement ofthe natural frequency ofthe element 43 enablesthe movement ofthe diaphragm 40, and hence the pressure or differential pressure, to be computed The natural frequency is again determined by using thevibration ofthe element43to modulate visible or infra-red radiation transmitted from a control station in the manner described above.

Afurtherapparatusaccordingtotheinvention is designed to sense the concentration of a gas by sensing a change in the natural or resonantfrequency of a vibratory element due to reversible absorption of the gas by the element.

In this further apparatus a transducer includes a vibratory element in the form ofthin sheet of piezoelectric crystal having a part of one or both surfaces coated with a compound which absorbs the gas.

Absorption ofthe gas causes a change in the natural or resonantfrequency ofthe crystal, the frequency decreasing as the mass of gas absorbed increases. The crystal is caused to vibrate at its natural frequency by applying an electrical driving signal to contacts on respective opposite faces of the crystal. The driving signal is again derived from visible or infra-red radiation and the frequency of vibration is again sensed bycausingthevibrating crystal to modulate visible or infra-red radiation.

Claims (25)

1. Apparatusforsensing a physical parameter wherein vibratory means include a vibratory element so arranged that, in use, the element has a natural frequency of vibration which varies with the magnitude of the sensed parameter, optical fibre means are provided for conveying radiation to the vibratory means, whereby the vibratory means, upon receiving radiation from the optical fibre means, generate an electrical driving signal which causes the vibratory element to vibrate at a frequency which rapidly becomesequaltothe natural frequency,the vibration element and the optical fibre means are arranged so thatat least part ofthe radiation received bythe vibratory means is modulated in intensity atthe naturaf frequency byeibration of the vibratory element, whereby thefrequency at which the radiation is modulated represents the magnitude of the sensed parameter, and the optical fibre means are arranged to convey modulated radiation away from the vibratory means.

2. Apparatus as claimed in claim t, wherein the vibratory element is a wire, ribbon, helical spring or other elongate element which, in use, is subjected to a force representing the magnitude ofthe said parameter and applied to the element in a direction lengthwise thereof, thereby to vary the tensile force applied to the element, and the element is arranged to effectvibration in a direction transversely of a longitudinal axis thereof.

3. Apparatus as claimed in claim 1, wherein the vibratory element is a wire, ribbon or other elongate element which, in use, is subjected to a force representing the magnitude of the said parameter and applied to the element in a direction transversely of a longitudinal axis thereof, thereby to vary the tensile force applied to the element, and the element isarrangedtoeffectvibration in a direction trans verselyofthelongitudinal axis.

4. Apparatusas claimed in claim 2or3,wherein a temperature compensating member extends generally parallel with the vibratory element, one end ofthe compensating member is secured to a fixed location in the vibratory means, and an opposed end of the compensating member is secured to one end of the vibratory element, the length and coefficient of thermal expansion of the material ofthe compensating member being such that a change in the length of the element, and hence a change in thetensileforce applied thereto, as a result of a change in temperature is compensated or substantially compensated by a change in length of the member.

5. Apparatus as claimed in claim 4, wherein the compensating member istubularand the vibratory element extends coaxially of the member.

6. Apparatus as claimed in claim 2 or3,wherein the apparatus is adapted for measurementoftemperature changes, thevibratory element extends generally parallel with an elongate member of a material having a coefficient of thermal expansion different from the coefficient ofthermal expansion of the material of the element, and each end ofthe element is secured to an adjacent end ofthe elongate member, whereby a change in temperature causes a differential expansion ofthe element and the member, and hence a change inthetensileforce applied to the element.

7. Apparatus as claimed in claim 1, wherein the vibratoryelementisan elongate generally U-shaped elementwhich, in use, is subjected to a force representing the magnitude of the said parameter and applied to the element in a direction lengthwise thereof, the element being arranged to effect vibration in a torsional mode.

8. Apparatus as claimed in any one of Claims 2 to 7, whereinthevibratorymeansincludea magnet,the vibratory element extends transversely ofthe magnetic fieid of the magnet, and the vibratory means include means for applying the said alternating electrical driving signal to the vibratory element, wherebythe vibratory element is caused to vibrate.

9. Apparatus as claimed in claim 1, wherein the vibratory element is a diaphragm orthe like element comprising magnetic material, and the element is arranged to be subjected to aforce in a direction transversely ofthe plane of the element and repre senting the magnitude ofthe said parameter.

10. Apparatusasclaimed in claim 1, wherein the vibratory element is a wire, ribbon, helical spring or other elongate element which, in use, is subjected to a force representing the magnitude of the said parameter and applied to the element in a direction transversely ofthe axis thereof, the element comprises magnetic material, and the element is arranged to effect vibration in a direction transversely of the said axis.

11. Apparatus as claimed in claim 1, wherein the vibratory element is a wire, ribbon or the like elongate element having a magnetic member attached thereto, the element being subjected to a force representing the magnitude ofthe said parameter and applied tothe element in a direction lengthwise thereof, and the element is arranged to effect vibration in a torsional mode.

12. Apparatus as claimed in any one of claims 9to 11 ,wherein the vibratory means include an electromagnet and means for applying the alternating electrical driving signal to a winding ofthe electromagnet, the vibratory element being so arranged relative to the electromagnetthat the application of the driving signal to the winding causes vibration of the vibratory element.

13. Apparatus as claimed in claim 1,wherein the vibratory element is a sheet of piezoelectric material which, in use, is subjected to a force in a direction transversely ofthe plane ofthe sheet and represent ingthe magnitude ofthe said parameter, electrical contacts are provided on respective opposite faces of the sheet, and the vibratory means further include meansforapplying the electrical driving signal to the contacts, thereby to cause the element to vibrate.

14. Apparatus as claimed in claim 1, wherein the vibratory element is a sheet of piezoelectric material having a surface adapted to absorb a mass of gas representing the concentration of gas in which the element is located, the element having a natural frequency of vibration which varies with the mass of gas absorbed thereby, electrical contacts are provided on respective opposite faces of the sheet, and the vibratory meansfurther include meansfor applying the electrical driving signal to the contacts, therebyto cause the element to vibrate.

15. Apparatus as claimed in claim 1 ,wherein the vibratory means include a diaphragm which, in use, is subjected to a force representing the magnitude of the said parameter, the diaphragm forms one plate of a capacitor and the spacing between the diaphragm and another plate ofthe capacitorvarying in accordancewith changes in the magnitude of the said parameter, the vibratory element is a sheet of a piezoelectric material disposed between the diaphragm and the other plate ofthe capacitor, whereby changes in the spacing between the diaphragm and the said other plate cause changes in the capacitance ofthe capacitor which cause changes in the natural frequency of the vibratory element, and means are provided for applying the electrical driving signal between the plates of the capacitor.

16. Apparatus as claimed in claim 1, wherein vibratory means comprise two vibratory elements each comprising a wire, ribbon orthe like elongate element, the two vibratory elements are so arranged that a change in the magnitude ofthe sensed parameter causes an increase in the natural frequen cy of vibration of one element and a decrease in the natural frequency of vibration of the other element, the vibratory means generate at least one electrical driving signal which causes each vibratory element to vibrate at its natural frequency, and the vibratory elements and the optical fibre means are so arranged that at least part of the radiation is modulated at the natural frequencyof onevibratory elementand at least partofthe radiation is modulated at the natural frequency ofthe other element.

17. Apparatus as claimed in any one of the preceding claims, wherein the apparatus further comprises a control station, the vibratory means being disposed at a location remote from the control station, and the control station comprises a source of the said radiation, and means for sensing the frequency at which radiation is modulated in intensity, the optical fibre means extending between the control station and the remote location and serving to convey radiation from the sourceto the vibratory means and to convey modulated radiation from the vibratorymeansto the control station.

18. Apparatus as claimed in claim 17, wherein the control station includes means for receiving incoming modulated radiation conveyed from thetransducervia the optical fibre means and generating an alternating electrical signal of frequency equal to the frequency at which the incoming radiation is modulated, and means for applying the electrical signal to the source,wherebythe source generates radiation for transmission via the optical fibre means to the transducerwhich is modulated at the frequency of modulation ofthe incoming radiation, and the vibratory means include means adapted, when the modulated radiation from the source is applied thereto, to generate the said alternating electrical driving signal.

19. Apparatusasclaimed in claim 17,wherein there is provided at the control station a second source of radiation of constant or substantially constant intensity, radiation from the second source forming the said at least part of the radiation received by the vibratory means.

20. Apparatusasclaimed in claim 18, wherein the said at least part ofthe radiation is modulated by reflection bythevibratory elementor by movement ofthe vibratory element into a section ofthe path of the radiation.

21. Apparatus as claimed in any one of claims 2 to 8 and 16, wherein the apparatusfurther comprises a control station, the said source is a source of radiation of constant orsubstantially constant intensity, the vibratory means are disposed at a location remote from the control station, the optical fibre means convey radiation from the source to the vibratory means,the vibratory element is so arranged relative to the optical fibre means that vibration ofthe element modulates the radiation which is incoming from the source, the vibratory means include means adapted, when radiation modulated by the vibratory element is applied thereto, to generatethe said alternating electrical driving signal, the optical fibre means convey modulated radiationfromthevibra- tory means to the control station, and the control station includes means for sensing thefrequency at which radiation is modulated in intensity.

22. Apparatus as claimed in claim 21,wherein the vibratory element is so arranged that the said at least partof the radiation is modulated by reflection by the vibratory element.

23. Apparatus as claimed in claim 21 or22, wherein radiation is conveyed from the source to the vibratory meansvia a firstoptical fibre and modulated radiation is returnedto the sensing means at the control station via the said first optical fibre or via a second optical fibre.

24. Apparatus as claimed in claim 21, wherein the vibratory element is so arranged thatthe said at least partofthe radiation is radiation which is modulated by vibratory movement ofthevibratory element into a section ofthepath of the radiation which extends between first and second optical fibres of the optical fibre means.

25. Apparatusforsensing a physical parameter, the apparatus being constructed, arranged and adapted to operate substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.