GB2246259A - Optical sensor - Google Patents

Abstract

An interrogating light beam is sent (eg down an optical fibre 15) to a sensor 14 (eg an oscillating mirror) to provide an output which is dependant on a property being measured (eg pressure in a tube 13), and the beam also scans a bar code 19 at the sensor to modulate the output with data such as calibration information. A mirror 14 may be mounted on filaments the tension of which affected by a rod attached to a diaphragm 12. A detector 18 responds to pulses of light reflected by the mirror 14 to energise coils which deflect the mirror by means of an attached magnet thereby maintaining the oscillations of the mirror. The light returned from the bar code 19 is reflected by the mirror 14 back down the optical fibre 15. Alternatively the beam from the fibre 15 may be reflected by a fixed diffraction grating and scanned across the bar code and detector by varying the wavelength of the light. <IMAGE>

Description

OPTICAL SENSOR This invention relates to optical sensors.

Utilisation of optical techniques and advanced materials gives optical sensors the potential for improved accuracy and performance in comparison with their electrical counterparts. To fully realise this potential, calibration information that is specific to each sensor is required which will substantially characterise the functional dependence of the sensor output upon the measurand to which the sensor is responsive. Such calibration information may be stored electronically, but if the full hostile environment capability of an optical sensor is to be exploit, this storage should not be at the sensor, but at a remote processing unit in order to obviate the need for electronic components at the sensor, and more particularly the need for an electrical feed fro the processing unit to such components at the sensor.

Nevertheless, interchangeability of optical sensors, which nay be required in the event of sensor failr, would be considerably enhanced if the calibration information could be optical stored in the sensor neat, as in these circumstances access to the processing unt wo'jlt not be necessary, and there is a mucc reduced rise of a sensor being used with calibration information pertaining to another sensor. The present invention -s particularlv concerned With such optical storace o- calibration information.

The optical storage facility provided by the invention can however be used for other purposes, either additional to the storage of calibration information, or in place of it.

According to the present invention there is provided an optical sensor adapted, in response to receipt of an optical interrogation signal, to provide an optical output signal functionally dependent on the magnitude of a measurand to which the sensor is responsive, which sensor includes a bar code which characterises said sensor and can be read by said interrogation signal.

The invention further provides an optical sensor adapted, in response to receipt of an optical interrogation signal, to excite a mechanical oscillator into oscillation whose frequency of oscillation is functionally dependent upon the magnitude of a measurand to which the sensor is responsive, whereby the interrogation signal is scanned across a bar code in which is stored data which substantially characterises said functional dependence to provide an output sin modulated at the oscillator frequency and additionally modulated with the stored data of the bar code.

where follows a description of a pressure sensor embodying the invention in a preferred for. The description refers to the accompanying drawings, in which : ia. 1 is a schematic view of a resonant pressure sensor cevice; Fig. 2 shows a resonator structure or use it te cevice of riq. 1; and Fig. 3 shows the magnetic drive arrangement associated with the resonator of Fig. 2.

Referring to Figs. 1 to 3, the sensor device is disposed within a housing 11 (Fig. 1) sealed by a flexible diaphragm 12 which, in use, is exposed via tube 13 to a source of pressure to be measured. The housing 11 may be evacuated to provide an absolute pressure reference. A strain responsive resonator generally indicated by the reference 14 is mounted within the housing 11, and is coupled to the diaphragm 12 via a push-rod (not shown).

Displacement of the diaphragm 12 in response to a pressure applied thereto applies a corresponding strain to the resonator 14. The resonator includes a torsional oscillatory element, and is driven via an optical signal applied to the arrangement via an optical fibre 15 coupled to a lens assembly 16 mounted on a carrier 17 within the housing 11. Light from the lens assembly 16 is reflected from the resonator 14 to a photodetector 18 also mounted on the carrier 17. Beside the photodetector 18 is mounted a reflective bar code 19. The data content of the bar code contains information substantially characterising the functional relationship between the resonant frequency of the resonator 14 and the pressure applied to the diaphragm 12.When the resonator 14 undergoes torsional oscillation the light from the lens assembly 16 is periodically swept across the bar code. Reflection from the bar code provides a return signal, modulated with the resonator frequency, which is directed by the lens assembly 16 back ito the fibre 15 for transmission to a remote sensor station (not shown). Superimposed o this modulation at the resonator frequency is the modulation prouduced by the pattern of the bar code. The mannen which the resonator is maintained in state cf torsona oscillator by the Input light sic. - ce described celow.

the corresponding strain is applied to the resonator 14. This results in a frequency change characteristic of the pressure. This frequency change is detected as a change in the modulation frequency of the return signal on the optical fibre 15.

Typically the resonator comprises a structure etched from a body of single crystal silicon. A suitable resonator structure is shown in Fig. 2 and comprises a rectangular support frame 21 within which a resonator element 22 is mounted on taut support filaments 23. Flexible hinges 24 are provided in the frame 21 to permit displacement of the frame whereby a corresponding tension is applied to the filaments 23. A lever arm 25 may be provided extending from the frame 2i whereby displacement of the frame by the push-rod (not shown) about the hinges 24 may be effected. The frame also includes a mounting portion 21a adjacent the hinges 24 whereby the resonator is affixed to a support 26.

Displacement of the frame in response to a similar displacement of the diaphragm 12 (Fig. 1) causes a corresponding change in tension in the filaments 23, thus changing the frequency of torsional oscillation of the resonator element 22. A permanent magnet 27 is mounted on the element 22, the field of the magnet being in a direction substantially perpendicular to the plane of the element. The resonator structure is located between the poles 28 (Fig. 3) of a yoke 29 o a magnetically permeable material, a coil 30 being wound around the yoke. The coil 30 is coupled to a capacitor 31 to form a tuned circuit whose frequency is substantially equal to the resonator frequency. In some applications the capacitor 31 may be dispensed with.

The feedback is the untuned aperiodie. This reduces feedback efficency, but allows operation of the resonator over a frequency range considerably wider nan that provided by a tuned circuit. Advantagecusly ts yoke 29 comprises a metallic glass. The resonator element 22 may be coated with gold to improve its optical reflectivity.

In use, continuous wave (CW) light from a light source (not shown), e.g. a semiconductor laser disposed at the remote station (not shown), is directed on to the resonator element 18 via optical fibre 15 provided with its lens assembly 16. Light is reflected from the resonator element at an angle corresponding to the instantaneous position of the resonator element. This reflected light signal is received by the photodetector 18 and is converted to a corresponding oscillatory electrical signal. This signal is fed back to the tuned circuit to induce a corresponding oscillatory signal in the coil 30, the signal being delayed by a phase angle of CU/2 in relation to the resonator oscillation. The magnetic fields of the magnet and coil are mutually perpendicular, The presence of a current in the coil thus applied a torsional couple to the resonator element.This causes a rotation of the element and a consequent reduction in the intensity of light received by the photodetector 18. The reduction of the current allows the resonator element to return towards its rest position. The coil current then increases again to maintain oscillation. The resonant frequency of the LC circuit defined by the coil and the capacitor is chosen to be sliahtly below the resonator frequency. The tt/2 phase lag introduced by the inductance of the coil then ensures maximum coupling to the resonator.

The choice of photodetector depends on the optical wavelength of the light chosen for launch - onto optical fibre 15. or exa s:e, R silicon photodiode may be employed in conAunction it & wavelength of 850mm.

In the sensor just described with reference to Figures 1 to 3 the bar code 19 has been scanned with the aid of a mechanically vibrating element, namely the resonator 14, but it should be understood that the requisite scanning may be achieved by alternative non-mechanical means. Thus by choosing to use monochromatic light to interrogate the bar code, the interrogation signal can be arranged to be reflected by a diffraction grating on to the bar code so that, by sweeping the wavelength of that signal, the angle of reflection is varied and thus caused to sweep across the bar code. It will be appreciated that this type of arrangement can be ued with a wider range of sensor transducer since mechanical vibration is no lonaer a prerequisite.

Claims (7)

CLAIMS.

1. An optical sensor adapted, in response to receipt of an optical interrogation signal, to provide an optical output signal functionally dependent on the magnitude of a measurand to which the sensor is responsive, which sensor includes a bar code which characterises said sensor and can be read by said interrogation signal.

2. An optical sensor adapted, in response to receipt of an optical interrogation signal, to provide an optical output signal functionally dependent on the magnitude of a measurand to which the sensor is responsive, which sensor includes a bar code in which is stored data which substantially characterises said functional dependence and is associated with means adapted to scan the interrogation signal over the bar coade whereby the output signal is additionally modulated with the stored data of the bar code.

3. An optical sensor as claimed in claim 2, wherein the means adapted to scan the interrogation signal over the bar code Includes a mechanical oscillator.

t. An optical sensor as claimed in claim 2, wherein the means adapted to scan the interrogation signal over the bar code includes a diffraction grating.

5. An optical sensor adapted, in response to receipt of an optical interrogation signal, to excite a mechanical oscillator into oscillation whose frequency of oscIllation Is functional y dependent upon tre magnitude of a measurand to which the sensor responsive, whereny the interrogation signal across a car coo in rich is stored data which substantially characterises said functional dependence to provide an output signal modulated at the oscillator frequency and additionally modulated with the stored data of the bar code.

6. An optical sensor substantially as hereinbefore described with reference to the accompanying drawings.

7. A telemetry system wherein at least one sensor as claimed in any preceding claim, is optically coupled with a sensor information processing unit at a location remote from that sensor.