GB2194049A - A photoacoustic measuring device - Google Patents

A photoacoustic measuring device Download PDF

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Publication number
GB2194049A
GB2194049A GB08619890A GB8619890A GB2194049A GB 2194049 A GB2194049 A GB 2194049A GB 08619890 A GB08619890 A GB 08619890A GB 8619890 A GB8619890 A GB 8619890A GB 2194049 A GB2194049 A GB 2194049A
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GB
United Kingdom
Prior art keywords
light
light source
sensor
optical
frequency
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
GB08619890A
Other versions
GB2194049B (en
GB8619890D0 (en
Inventor
Robert Andrew Pinnock
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
General Electric Co PLC
Original Assignee
General Electric Co PLC
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by General Electric Co PLC filed Critical General Electric Co PLC
Priority to GB8619890A priority Critical patent/GB2194049B/en
Publication of GB8619890D0 publication Critical patent/GB8619890D0/en
Publication of GB2194049A publication Critical patent/GB2194049A/en
Application granted granted Critical
Publication of GB2194049B publication Critical patent/GB2194049B/en
Anticipated expiration legal-status Critical
Expired - Fee Related legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
    • G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
    • G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
    • G01D5/268—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light using optical fibres
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01H—MEASUREMENT OF MECHANICAL VIBRATIONS OR ULTRASONIC, SONIC OR INFRASONIC WAVES
    • G01H9/00—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means
    • G01H9/004—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means using fibre optic sensors
    • G01H9/006—Measuring mechanical vibrations or ultrasonic, sonic or infrasonic waves by using radiation-sensitive means, e.g. optical means using fibre optic sensors the vibrations causing a variation in the relative position of the end of a fibre and another element
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L1/00—Measuring force or stress, in general
    • G01L1/16—Measuring force or stress, in general using properties of piezoelectric devices
    • G01L1/162—Measuring force or stress, in general using properties of piezoelectric devices using piezoelectric resonators
    • G01L1/167—Measuring force or stress, in general using properties of piezoelectric devices using piezoelectric resonators optical excitation or measuring of vibrations
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L1/00—Measuring force or stress, in general
    • G01L1/18—Measuring force or stress, in general using properties of piezo-resistive materials, i.e. materials of which the ohmic resistance varies according to changes in magnitude or direction of force applied to the material
    • G01L1/183—Measuring force or stress, in general using properties of piezo-resistive materials, i.e. materials of which the ohmic resistance varies according to changes in magnitude or direction of force applied to the material by measuring variations of frequency of vibrating piezo-resistive material
    • G01L1/186—Measuring force or stress, in general using properties of piezo-resistive materials, i.e. materials of which the ohmic resistance varies according to changes in magnitude or direction of force applied to the material by measuring variations of frequency of vibrating piezo-resistive material optical excitation or measuring of vibrations
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L11/00—Measuring steady or quasi-steady pressure of a fluid or a fluent solid material by means not provided for in group G01L7/00 or G01L9/00
    • G01L11/02—Measuring steady or quasi-steady pressure of a fluid or a fluent solid material by means not provided for in group G01L7/00 or G01L9/00 by optical means
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L9/00—Measuring steady of quasi-steady pressure of fluid or fluent solid material by electric or magnetic pressure-sensitive elements; Transmitting or indicating the displacement of mechanical pressure-sensitive elements, used to measure the steady or quasi-steady pressure of a fluid or fluent solid material, by electric or magnetic means
    • G01L9/0001—Transmitting or indicating the displacement of elastically deformable gauges by electric, electro-mechanical, magnetic or electro-magnetic means
    • G01L9/0007—Transmitting or indicating the displacement of elastically deformable gauges by electric, electro-mechanical, magnetic or electro-magnetic means using photoelectric means

Abstract

A resonant sensor 1 is optically powered and read along a single optical fibre 18. A band stop filter 20 at the frequency of the power signal from a source 12 prevents it from reaching the reading system. The resonance causes modulation of light reflected from the vibrating element 10 and this modulation gives a measure of the parameter being measured, eg a force F. A pulse timing circuit 10 adjusts the modulation frequency of the source 12 to be equal to the resonant frequency of the element 1B and the sensed parameter is displayed at a device 21. <IMAGE>

Description

SPECIFICATION A sensor This invention relates to a sensor that is optically powered and read.
An example of a known system of this type, is described in our patent specification No. 2,146,120A, and is illustrated in figure 1 of the accompanying drawings.
This known sensor comprises a quartz body 1 having a resonant frequency which is dependant on a force applied to it and which it is desired to sense. A pulsed light source 2 causes the body 1 to resonate by transmitting pulses of light along an optical fibre 3. The resonant frequency of the body 1, and thus the force applied to it, are measured by a continuous wave light source 4 and a photodetector 5. Light from the light source 4 travels along an optical fibre 6, then along a path 7 and is reflected from the body 1 along a path 8. The light then travels along an optical fibre 9 to the photo-detector 5. The amount of light from fibre 6 transmitted to fibre 9 depends on the position of the reflecting surface. Therefore the signal received at the photo-detector 5 is modulated by the vibrational movement of body 1.
The signal from the photo-detector 5 is fed to a pulse timing circuit 10 which adjusts the frequency of modulation of the pulsed light source 2 to make it equal to the resonant frequency of the body 1. A display 11 shows the value of the sensed force which is calculated from the resonant frequency of the body 1.
A problem with this system is the need for three optical fibres extending respectively from the light sources 2 and 4 and from the photodetector 5; and all terminating at the body 1.
This is a problem because the body 1 can be very small and it can be expensive and difficult, and increase the bulk of the sensor, to fix three fibre terminations at precise positions relative to the body 1. Furthermore, the construction and mounting of the body 1 may be such that access to it may not be possible from both sides.
The present invention was made while attempting to produce a sensor without these problems.
This invention provides a sensor comprising: a body having a resonant frequency which is a function of a quantity to be sensed; a light source; a light path arranged to direct light from the light source to the body to cause the body to vibrate through the photoacoustic ef fect; and an optical detector linked to the body by a second light path so that it can detect the vibration; the two light paths sharing a common portion defined by an optical fibre terminating at or adjacent to the body.
It will be apparent that, by employing the invention only one fibre need terminate at the resonant body making the construction of the sensor simpler.
The light source is preferably constructed or adapted to produce pulses of light. This is however not essential. A continuous light source can be used to produce photoacoustic vibration e.g. as described in our co-pending application No. 8530809.
If light from the abovementioned light source were allowed to reach the photo-detector it might cause a false reading. To prevent this a filter can advantageously be placed in the light path between the body and the optical detector, the filter having characteristics selected to prevent light from the light source reaching the optical detector. An alternative possibility would be to use a chopper synchronised with the pulses produced by a pulsed light source.
Some source of light is required to enable the optical detector to "see" the vibration of the body. This could be ambient light but the sensor preferably includes a second light source for this purpose. This second light source is preferably a different frequency to the first light source so that the light produced by it is passed by the previously mentioned filter (if included). Where a second light source is included it is preferably arranged to illuminate the body via a third light path, all three light paths sharing a common portion defined by an optical fibre terminating at or adjacent to the body.
An embodiment of the invention will now be described with reference to figure 2 of the accompanying figures in which: Figure 1 is a schematic representation shown partly as a block diagram and partly as a perspective view of the known sensor previously referred to; and Figure 2 shows in a similar manner a sensor constructed in accordance with the invention.
Referring to Fig. 2 a quartz body 1 is etched to define a cavity 1A having one open side which is bridged by a beam 1B. The beam 1B and the rest of the body 1 is caused to resonate by light from a pulsed laser diode 12 incident on the beam 1B. The resonation occurs at a resonant frequency which is a function of strain imposed on the body by means depicted schematically by the arrow marked F on Fig. 2. The best pulsation frequencies for laser diodes are generally very high. Rather than to attempt to transmit one pulse per oscillation of the quartz body 1 a train of pulses of frequency f are transmitted and this train is modulated at the resonant frequency f2 of the beam 1B where f2 is many times less than fl.
The pulses of light from laser diode 12 travel along an optical fibre 13. These pulses of light cause the body 1 to resonate due to the photo-acoustic effect: i.e. the light causes local heating of the body 1 and the expansion due to this heating causes vibration of the body 1 at the frequency f2. If the frequency f2 is the same as the resonant frequency of body 1, body 1 wilt resonate.
The vibrational movement of body 1 is measured by a photo-detector 5 which "sees" the body using light provided by a continuous wave laser diode 14 operating at a different frequency to, and at a lower power than, the laser diode 12.
The light from the laser diode 14 is fed along an optical fibre 15 to a unidirectional coupler 16 where it is combined with light from the laser diode 12. The combined signals then pass through a second unidirectional coupler 17 and along a fibre 18 which terminates at the body 1.
The body 1 is the sensing part of the sensor and is at a location spaced a considerable distance from a second location containing the rest of the sensor, the two locations being linked only by the single optical fibre 18.
The light from the laser diode 14 is reflected from the body 1 back into the optical fibre 18. The reflected light is modulated in dependence on the movement of body 1.
This modulated light travels back along the optical fibre 18 to the unidirectional coupler 17 where it is split. Part of the modulated light then travels along a fibre 19 and through an optical filter 20, to the photodetector 5.
The filter 20 is a narrow band pass filter centred on the frequency of the laser diode 14 and prevents the pulsed light from laser diode 12 reaching the photo-detector 5 after being reflected from the body 1.
A pulse timing circuit 10 adjusts the modulation frequency of laser diode 12 to make it equal to the resonant frequency of the body 1. A calculation and display system 21 calculates the value of the sensed parameter from the resonant frequency of the body 1 and then displays this calculated value.

Claims (5)

1. A sensor comprising: a body having a resonant frequency which is a function of a quantity to be sensed; a light source; a light path arranged to direct light from the light source to the body to cause the body to vibrate through the photoacoustic effect; and an optical detector linked to the body by a second light path so that it can detect the vibration; the two light paths sharing a common portion defined by an optical fibre terminating at or adjacent to the body.
2. A sensor as claimed in claim 1 and additionally comprising an optical filter in the second light path between the body and the optical detector, the filtering characteristics of the filter being such as to prevent light from the light source reaching the detector.
3. A sensor as claimed in claim 1 or 2 and additionally comprising a second light source having a different frequency to the first; and a third optical path from the second light source to the body so that the second light source illuminates the body, all three light paths sharing a common portion defined by an optical fibre terminating at or adjacent to the body.
4. A sensor as claimed in any preceding claim in which said light sources are diode lasers.
5. A sensor substantially as illustrated in figure 2 of the accompanying drawings and substantially as described with reference thereto.
GB8619890A 1986-08-15 1986-08-15 A sensor Expired - Fee Related GB2194049B (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
GB8619890A GB2194049B (en) 1986-08-15 1986-08-15 A sensor

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB8619890A GB2194049B (en) 1986-08-15 1986-08-15 A sensor

Publications (3)

Publication Number Publication Date
GB8619890D0 GB8619890D0 (en) 1986-09-24
GB2194049A true GB2194049A (en) 1988-02-24
GB2194049B GB2194049B (en) 1990-04-25

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Family Applications (1)

Application Number Title Priority Date Filing Date
GB8619890A Expired - Fee Related GB2194049B (en) 1986-08-15 1986-08-15 A sensor

Country Status (1)

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GB (1) GB2194049B (en)

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2201776A (en) * 1987-01-24 1988-09-07 Schlumberger Electronics Resonantly vibratable sensors
GB2215053A (en) * 1988-02-13 1989-09-13 Stc Plc Electro-mechanical oscillating transducer devices
EP0345142A2 (en) * 1988-06-02 1989-12-06 Luxtron Corporation Sensors for detecting electromagnetic parameters utilizing vibrating elements
EP0371592A2 (en) * 1988-09-29 1990-06-06 Solartron Group Limited Sensors using vibrating elements
EP0588005A1 (en) * 1992-09-14 1994-03-23 Allen-Bradley Company, Inc. Pilot light interface for small industrial controls
EP0893675A1 (en) * 1997-07-21 1999-01-27 European Atomic Energy Community (EURATOM) Luminous intensity sensor element and light beam modulating method and device employing such a sensor element

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4379226A (en) * 1981-02-02 1983-04-05 Siemens Corporation Method and sensor device for measuring a physical parameter utilizing an oscillatory, light modulation element
EP0090167A2 (en) * 1982-02-22 1983-10-05 The Foxboro Company Fiber-optic sensor for a resonant element
GB2146120A (en) * 1983-09-03 1985-04-11 Gen Electric Co Plc Photoacoustic force sensor

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4379226A (en) * 1981-02-02 1983-04-05 Siemens Corporation Method and sensor device for measuring a physical parameter utilizing an oscillatory, light modulation element
EP0090167A2 (en) * 1982-02-22 1983-10-05 The Foxboro Company Fiber-optic sensor for a resonant element
GB2146120A (en) * 1983-09-03 1985-04-11 Gen Electric Co Plc Photoacoustic force sensor

Cited By (10)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2201776A (en) * 1987-01-24 1988-09-07 Schlumberger Electronics Resonantly vibratable sensors
GB2215053A (en) * 1988-02-13 1989-09-13 Stc Plc Electro-mechanical oscillating transducer devices
GB2215053B (en) * 1988-02-13 1991-09-11 Stc Plc Transducer device
EP0345142A2 (en) * 1988-06-02 1989-12-06 Luxtron Corporation Sensors for detecting electromagnetic parameters utilizing vibrating elements
EP0345142A3 (en) * 1988-06-02 1990-12-27 Luxtron Corporation Sensors for detecting electromagnetic parameters utilizing vibrating elements
EP0371592A2 (en) * 1988-09-29 1990-06-06 Solartron Group Limited Sensors using vibrating elements
EP0371592A3 (en) * 1988-09-29 1991-11-06 Solartron Group Limited Sensors using vibrating elements
EP0588005A1 (en) * 1992-09-14 1994-03-23 Allen-Bradley Company, Inc. Pilot light interface for small industrial controls
EP0893675A1 (en) * 1997-07-21 1999-01-27 European Atomic Energy Community (EURATOM) Luminous intensity sensor element and light beam modulating method and device employing such a sensor element
WO1999005490A1 (en) * 1997-07-21 1999-02-04 European Atomic Energy Community (Euratom) Luminous intensity sensor element and light beam modulating method and device employing such a sensor element

Also Published As

Publication number Publication date
GB2194049B (en) 1990-04-25
GB8619890D0 (en) 1986-09-24

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PCNP Patent ceased through non-payment of renewal fee