GB2146120A

GB2146120A - Photoacoustic force sensor

Info

Publication number: GB2146120A
Application number: GB08327814A
Authority: GB
Inventors: Martin Collier; Stuart Michael Mcglade; Philip Edward Stephens
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1983-09-03
Filing date: 1983-10-18
Publication date: 1985-04-11
Also published as: GB2146120B; GB8323685D0; GB2146123A; GB8419491D0; GB2146123B; GB8327814D0

Abstract

A force sensor comprises a body 21 eg of quartz which resonates at a frequency dependent on the force imposed on the crystal. Light pulses on line 3 induce resonance of the body 21 by means of the photo acoustic effect. The resonance causes modulation of light passing from path 4 through the crystal onto path 5. The modulation of the light on path 5 is at a frequency which is the same as the frequency of vibration of the crystal and a feedback path 13, 14, serves to tune the light source 11 to produce light pulses of that frequency. The frequency displayed at 15 is thus representative of the force imposed on the member 21. <IMAGE>

Description

SPECIFICATION A sensor This invention relates to a sensor and is particularly though not exclusively applicable to a force or strain sensor hereinafter called a force sensor. Force sensors are particularly important compared with other types of sensor because a technique for measuring force can easily be adapted to measure other physi cai properties.

The invention arose when considering the design of a force sensor for use in circumstances where electrical cdnnections might cause a fire risk or be subjected to electromagnetic interference.

The invention provides a sensor comprising: a sensor body which has resonant frequency depending on the value of the property to be sensed; photo-acoustic means including a light source, for causing resonation of the body; modulating means for using the resonation of the body to modulate light travelling along a path to and from the body; and means for detecting the modulation of the light and deriving therefrom an indication of the value of the said property.

The term "light" when used in this Specification is to be interpreted as including the range of electromagnetic radiation from infrared to ultraviolet inclusive.

The sensor body can take many different forms.

For example it can be a piezoelectric (e.g.

quartz crystal such as is used in crystal controlled oscillators). These crystals resonate at a well defined frequency dependent on a force imposed on them and are therefore particularly useful for force sensors. A quartz crystal is also transparent to light and can be arranged to modulate light passing through it in time with its oscillations. The modulation means of the invention can thus simply consist of means for passing light through the crystal. It is possible to arrange for almost any material to resonate at a frequency depending on force or strain, which may either be compressive or tensile or torsional or flexural.

There is therefore a vast selection of possible forms which the sensor body may take. If it is transparent the light path may pass through the sensor body. If it is opaque the light path may be reflected off it. The sensor body need not necessarily be solid. A body of gas can also be arranged to resonate at a frequency dependent on properties such as density, temperature, volume and pressure and can therefore be used in sensors for detecting such properties.

The photo-acoustic means for causing resonation of the body preferably includes a light source which is adapted to generate light pulses at a frequency close to the resonant frequency of the body. A feedback path can advantageously be included to adjust the frequency of the light pulses until the amplitude of resonation is a maximum. The frequency giving the maximum amplitude can be assumed to be the frequency of resonation of the body.

The drive for the light source could be designed to repetitively sweep the frequency through a range of frequencies. In such an arrangement the means for detecting the modulation of the light would include a device for detecting the frequency at which the light received reached a maximum intensity. In this form of the invention there would be no need for a feedback from the detecting means to the drive for light source.

An alternative technique would be to use a light source which generates light pulses at a fixed frequency and to use amplitude (or an amplitude dependent function) of the resonation as a measure of the proximity of the resonant frequency of the sensor body to the frequency of the light pulses. This technique is not however preferred since it is subject to inaccuracies introduced by e.g. stray light.

One way in which the invention may be performed will now be described by way of example with reference to the accompanying schematic drawings in which: Figure 1 illustrates a force sensor constructed in accordance with the invention including parts located at remote positions 1 and 2; and Figures 2 to 5 illustrate respective alternative components for utilisation at position 2 of Figure 1.

Referring firstly to Figure 1, the illustrated strain sensor is located as previously mentioned at the two remote positions 1 and 2, these positions being joined by optical links in the form of optical fibres 3, 4 and 5.

At the position 1, a pulsed light source 11 and a continuous light source 1 2 transmit light respectively along light paths 3 and 4 to the location 2.

At the location 2, pulses of light from the light path 3 are incident on a quartz crystal body 21 having the illustrated shape in accordance with the invention described in our copending patent applications 8315565. It includes three arms joined at their ends by cross-pieces. In use the arms vibrate in planes perpendicular to a common plane in which all three arms lie. the centre arm vibrating in antiphase relative to the outer two arms. The centre arm of the crystal body 21 absorbs the pulses of light, each pulse momentarily generating heat which in turn produces expansion and contraction: resulting in the vibration previously described. This is known as the optoacoustic effect. The pulses of light from the light source 11 are arranged to have a frequency of the same order of magnitude as the resonant frequency of the crystal 21.

Light from the path 4 is directed through the crystal 21 by a lens assembly 22 and is then collected by lens assembly 23 and passed onto the return line 5. The light guiding properties of the crystal 21 are such that they are modified by its resonation thus imposing, on the light path, passing through the crystal, modulations in intensity, these modifications having the same frequency as the frequency of vibration. The modulated light from line 5 is collected by a photo-detector 1 3 at position 1. The resulting electrical signals at the output of photodetector 1 3 are modulated in a manner similar to the optical signals on line 5.The extent of modulation is measured in a pulse timing circuit 14 which integrates the modulus of the a.c. component of the received signal over set periods of time and adjusts the frequency of the light source 11 in such a way as to maximise the result of integration. The frequency of the light source 11 is thus made equal to the resonant frequency of the crystal 21. A display 15 connected to the output of the circuit 14 indicates this frequency. The crystal 21 is subjected to a force as indicated by the arrow on Figure 1 and the strain thus imposed on it affects its resonant frequency. The value displayed at 1 5 can thus be used as a measure of the force on the crystal 21.

Figure 2 shows alternative components for use at the position 2 including an optical element 21A which is subjected to a force to be measured and which is illuminated by the pulses of light from the optical path 3.

Light from path 4 is passed through a polariser P1 and the polarised light passes through the element 21A to another polariser P2 whose plane of polarisation is perpendicular to that of P1. The amount of light passing from optical line 4 to optical line 5 will be dependent on any rotation of the plane of polarisation in the element 21A which changes in time with vibration of the element 21A at a frequency dependent on the force to be measured.

In the arrangement shown in Figure 3 the force to be measured is applied so as to put an optical element 21 B under tensile stress.

Photo-acoustically generated vibrations of the element 21 B, caused by light from line 3, serve to deflect the light path passing from line 4 to line 5 thereby modulating the light on path 5 in time with the vibrations of the element 21 B the frequency of which depends on the force applied.

In Figure 4 a non-transparent vibrating element 21 C is used which is again caused to vibrate by the photo-acoustic effect of light pulses received from line 3. Light from line 4 passes through a fibre-optic coupler C onto lines 4A and 4B from whence it is reflected off the element 21 C and a reflective end surface of the fibre 4B respectively. The reflected light on lines 4A and 4B is mixed in the fibre-optic coupler and the result of mixing the light is applied to a line 4C and thence to a photodetector PD. Flexing of the member 21C during its vibration alters the distance between the end of line 4A and the part of the member 21 C from which reflection occurs. The phase of the light reflected back along line 4A thus changes relative to the phase of the light reflected back along line 4B causing an interference effect on line 4C. The electrical output of the photodetector PD is thus modulated in time with the vibration of the member 21C. It is notable that in this embodiment the output signal appears on an electrical line instead of on the optical line 5 as in the previous embodiments. This may be a disadvantage in some situations.

In Figure 5 the sensor body 21 D is similar to that shown in Figure 1 but, instead of passing light from line 4 through the sensor body, the light is reflected off its surface in the direction of line 5. Small movements of the member 21 D during its resonation cause changes in the direction of the reflected light beam so that a greater or larger amount of it is received by the line 5, thereby producing modulation in the light intensity on line 5.

Claims

1. A sensor comprising: a sensor body which has a resonant frequency depending on the value of the property to be sensed; photoacoustic means, including a light source, for causing resonation of the body, modulation means for using resonation of the body to modulate light travelling along a path to and from the body; and means for detecting the modulation of the light and deriving therefrom an indication of the value of the said property.

2. A sensor according to Claim 1 for sensing force in which the sensor body is a piezoelectrid crystal having a resonant frequency depending on the mechanical force on the crystal.

3. A sensor according to Claim 1 or 2 in which the photo-acoustic means includes a pulsed light source and means for directing the light onto light absorbing matter forming part of or fixed relative to the sensor body.

4. A sensor according to any preceding claim in which at least a part of the sensor body is transparent and in which the modulating means is formed by a portion of the sensor body, through which the light path passes.

5. A sensor according to any one of Claims 1 to 3 in which the modulating means includes a reflective surface portion of the sensor body, and in which the light path includes a part where the light is reflected off the said reflective portion.

6. A sensor according to any preceding claim in which the means for detecting the modulation of the light includes means for measuring the frequency of modulation.

7. A sensor according to any of the Claims 1 to 5 in which the means for detecting the modulation of the light includes means for measuring the amplitude, or a function of amplitude of the modulation

8. A sensor according to Claim 1 and substantially as described with reference to Figure 1 of the accompanying drawing.

9. A sensor according to Claim 1 and substantially as described with reference to Figure 1 modified as shown in Figure 2 of the accompanying drawings.

10. A sensor according to Claim 1 and substantially as described with reference to Figure 1 modified as shown in Figure 3 of the accompanying drawings.

11. A sensor according to Claim 1 and substantially as described with reference to Figure 1 modified as shown in Figure 4 of the accompanying drawings.

1 2. A sensor according to Claim 1 and substantially as described with reference to Figure 1 modified as shown in Figure 5 of the accompanying drawings.