GB2210976A

GB2210976A - Displacement sensor utilizing phase-shift of flexural waves

Info

Publication number: GB2210976A
Application number: GB8723871A
Authority: GB
Inventors: Roger Martin Langdon
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1987-10-10
Filing date: 1987-10-10
Publication date: 1989-06-21
Anticipated expiration: 2007-10-10
Also published as: GB2210976B; GB8723871D0

Abstract

A displacement sensor uses a photo-acoustic wave generator 5, 7 to produce transverse acoustic waves in a flange 4 attached to a movable member 1. The relative phases of the waves at the generator 5, 7 and at a phase sensor 15 are compared to provide a measure of the movement of the member 1 relative to another member 2. A computer 18 instructs a frequency controller 9 to set a power supply 8 to a first frequency and a laser diode 7 coupled to an optical fibre 6 heats up a region of the flange 4 provided with a patch of aluminium and generates a flexural wave along the flange. The separation of another region of the flange 4 and another optical fibre 6 varies and a photodetector 13 generates a signal in phase with the flange movement. The phase of this signal is compared at a comparator 15 with that of the supply 18 to generate a first difference signal and this is stored in the computer. This procedure is repeated at a second supply frequency chosen so that two series of calculated possible distances between the fibres 5, 6 contain a common value which is taken as an absolute value. The computer can then keep track of subsequent movement of the body 1 along an axis 3 relative to this absolute value from phase shifts at the two parts of the flange 4. The sensor may be used as an angle sensor on a rotating member with the flange 4 formed circumferentially. <IMAGE>

Description

Optical Displacement Sensors This invention relates to optical displacement sensors.

Optical displacement sensors are preferred in many situations because of their accuracy and immunity to electro-magnetic interference. Generally such sensors use optical inteferometery, Moire fringe techniques or optically read codes. Interferometery and Moire fringe systems have the disadvantage that they can only be used to measure changes in position, not absolute position.

Because of this they must be set to some reference position and the system zeroed every time the system is activated or after any interruption of the power supply.

Optically read codes give a measure of absolute position and so do not require zeroing, however they require a separate reading channel for each bit of position information they provide, if for instance an optically read code system were arranged to measure which of 16 possible positions a moving member was in this would be a four bit signal and would require four reading channels. This need for a plurality of reading channels makes such systems complex and expensive, in addition the code to be read is costly to make.

This invention provides a displacement sensor including a moveable member, an acoustic wave generator and an acoustic wave phase sensor, arranged so that the path length travelled by an acoustic wave from the wave generator to the wave phase sensor is dependent on the position of the moveable member. As the acoustic wave travels along a member it is possible to calculate the distance between the wave generator and the phase detector by comparing the relative phases of the acoustic wave at the two points, however such a measurement will be ambiguous because the number of integral wavelengths between the two points will not be known.

If a second acoustic wave, having a different wavelength to the first, also travels along the member it will be possible to calculate the absolute distance between the two points by comparing the two sets of relative phases; and the use of this technique is a preferred feature of the invention.

One particular embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a first displacement sensor employing the invention, partly as a schematic perspective view and partly in block diagram form; Figure 2 shows a detail of part of a flange for use in the sensor of Figure 1; and Figure 3 shows a second displacement sensor employing the invention, partly in block diagram form, identical parts having the same reference numerals throughout; Referring to Figure 1 a first body 1 made of silicon can move relative to a second body 2 along an axis 3. The body 1 has a rebate formed by an etching process so as to define a flange 4, 1 micron thick and lmm tall. The ends of the flange 4 are coated with areas of vibration absorbtive plastics material 16 and 17.

A first optical fibre 5 is attached to the body 1 adjacent to the flange 4. A second optical fibre 6 is attached to the body 2 and is also adjacent to the flange 4. The position of the fibre 6 along the flange 4 will vary as the body 1 moves relative to the body 2, but the distance of the fibre 5 from the rest position of the flange 4 will remain a constant as the body 1 moves.

A laser diode 7 is supplied with power by an A.C.

power supply 8. The frequency of the A.C. supply 8 is controlled by a frequency controller 9. The modulated light emitted by the laser diode 7 travels along the optical fibre 5 and illuminates a small section of the flange 4.

In this illuminated section of the flange 4 a layer 19 of aluminium is coated onto one side of the flange 4, see Figure 2.

A laser diode 10 is supplied with power by a power supply 11, the light from the laser diode 10 travels along the optical fibre 6 and illuminates a small part of the flange 4. Some of the light from the optical fibre 6 is scattered from the flange 4 and passes back into the fibre 6. Some of this returned light is directed by a Y-coupler 12 into an optical fibre 20 which supplies it to a photodetector 13, producing a signal that is fed to an amplifier 14. The output of the amplifier 14 is fed to a phase comparator 15, which is in turn linked to a computer 18.

In operation the computer 18 instructs the frequency controller 9 to set the frequency of the A.C. power supply 8 to a first frequency. The laser diode 7 emits light modulated at this first frequency and this modulated light passes along the optical fibre 5 and illuminates the flange 4 and the aluminium coating 19.

When light from the laser diode 7 illuminates the flange 4 the flange 4 heats up. Because of their different thermal expansivities the silicon of the flange 4 and the aluminium coating 19 expand differentially and cause the flange 4 to flex.

Because the light'from the laser diode 7 is modulated this flexing will take place at this modulation frequency as the flange 4 heats and cools, as a result a transverse flexure wave at the modulation frequency is transmitted along the flange 4. This flexure wave is absorbed without reflection at each end of the flange by the areas of absorbtive material 16 and 17.

As the transverse flexure wave moves to the section of the flange 4 adjacent to the optical fibre 6 the area of the flange 4 illuminated by the light from the laser diode 10 moves towards and away from the end of the optical fibre 6. As a result the intensity of the light returning down the fibre 6 and then, via the Y-coupler 12, the fibre 20 to the photo-detector 13 will vary in phase with the movement of the flange 4.

The signal produced by the photo detector 13 is in phase with the movements of the flange 4 adjacent to the optical fibre 6 and is amplified by the amplifier 14. This amplified signal is supplied to the phase comparator 15 which compares the phase of this signal with the phase of the drive signal supplied to the laser diode 7 by the A.C.

power source 8 to provide a first phase-difference signal which is supplied to a computer 18 where it is stored in a memory.

The computer 18 then instructs the frequency controller 9 to change the frequency of the A.C. power supply 8 to a second frequency.

In the same manner as described above this generates a transverse flexure wave in the flange 4 at this second frequency. This wave produces a signal in phase with its movements at the photo detector 13 as described above and this phase is compared with the phase of the A.C. power source 8 by the phase comparator 15 to provide a second phase-difference signal to the computer 18.

Each of these two phase differences can be used to calculate a series of possible values (spaced by the appropriate wavelength) for the distance between the part of the flange 4 adjacent the end of the optical fibres 5 and 6. By appropriate choice of frequencies it is possible to ensure that only one distance value appears in both series. The computer identifies this as being the true value and stores it. The A.C. power source 8 then continues to operate at the second frequency.

Because the computer 18 now knows the absolute distance between the two sections of the flange 4 it can keep track of the movements of the two members by calculating how far they have moved from this absolutely measured position, by measuring the changes in the relative phases of the wave at the two parts of the flange. The computer 18 supplies this information to a display 29.

Referring to Figure 3 a system similar to that in Figure 1 is illustrated, except that this system uses a single optical fibre 24 and measures the distance between this optical fibre 24 and an end 21 of the flange 4, allowing the relative positions of the two members 1 and 2 to be found.

In operation the frequency controller 9 sets the A.C.

power supply 8 to provide power at a first frequency to the laser diode 7. This causes the laser diode 7 to produce light modulated at a first frequency. This light passes along an optical fibre 22, through a directional coupler 23 and along the optical fibre 24 and falls on the flange 4. The flange 4 is coated along its entire length with a thin film of aluminium. The differential thermal expansion of the aluminium film and the silicon of the flange 4 produces a flexure wave in the flange at the frequency of modulation of the laser diode 7. As this wave propagates along the flange it reaches the end 21 of the flange 4 and is reflected back along the flange 4 to the area of absorbtive material 16 where it is absorbed.

As the reflected wave passes the optical fibre 24 the distance from the fibre 24 to the end 21 of the flange 4 can be calculated from the phase difference between the reflected wave and the wave being generated.

A power supply 11 powers a laser diode 10 which emits light at a different frequency to the laser diode 7, this light then passes along an optical fibre 25, through directional couplers 23 and 26 and then along the optical fibre 24 to the flange 4. The light is then scattered from the flange 4 and part of it passes back into the optical fibre 24; this returning light will have its amplitude altered as the flange 4 moves towards and away from the optical fibre 24. The light scattered from the flange 4 passes along the optical fibre 24, through the directional couplers 23 and 26 and then along an optical fibre 27 to a filter 28. Light passing through the filter 28 falls on the photodetector 13. The filter 28 is a bandpass filter centred on the frequency of the laser diode 10 and prevents light from the laser diode 7 being received at the photodetector 13.

The amplifier 14, phase comparator 15 and computer 18 operate as before to calculate the distance travelled by the wave from the optical fibre 24 to the end 21 of the flange 4 and back. The computer then calculates the relative positions of the two members 1 and 2 from this and shows this on the display 29.

As described above the frequency control 9 then changes the frequency of the A.C. power supply 8 to a second frequency in order to allow an absolute value of the relative positions of the two members to be calculated.

The system shown in Figure 3 could be used as an angle sensor on a shaft or other rotating member, with the flange formed circumferentially on the rotating member.

Claims

1. A displacement sensor including a moveable member, an acoustic wave generator through the moveable member and an acoustic wave phase sensor, arranged so that the path length travelled by an acoustic wave from the wave generator through the moveable member to the wave phase sensor is dependent on the position of the moveable member.

2. A displacement sensor as claimed in claim 1 in which means are provided to calculate the position of the moveable member from the relative phases of the acoustic wave at the generator and the sensor.

3. A displacement sensor as claimed in claim 1 or claim 2 in which the acoustic wave generator is able to generate acoustic waves at two different frequencies and means are provided to calculate the position of the moveable member from the relative phases at the generator and the sensor of waves at both of these frequencies.

4. A displacement sensor as claimed in claim 1 or claim 2 in which the acoustic wave generator is a photo-acoustic wave generator.

5. A displacement sensor as claimed in any preceding claim in which the phase sensor is an optical sensor.

6. A displacement sensor as claimed in any preceding claim in which the wave generator and the acoustic wave phase sensor operate on the same point on the member, the acoustic wave travelling from the generator and a discontinuity in the member and being reflected back to the sensor.

7. Apparatus for sensing the position of a member comprising : a first transducer for launching a signal in the form of a mechanical wave into the member; a second transducer for receiving the signal from the member after propagation along a path therethrough and means for sensing the delay to the signal caused by the propagtation along the said path ; the arrangment of the transducers in relation to the member being such that the length of the path and thus the delay is dependent on the position of the member.

8. A displacement sensor substantially as shown in and as described with referenc to Figure 3 of the accompanying drawings.

8. A displacements sensor substantially as shown in and and as described with reference to Figures 1 and 2 of the accompanying drawings.

9. A displacement sensor substantially as shown in and as described with referenc to Figure 3 of the accompanying drawings.

Amendments to the claims have been filed as follows 1. A displacement sensor including a moveable member, a photo-acoustic wave generator an acoustic wave phase sensor, arranged so that the path length travelled by an acoustic wave from the wave generator through the moveable member to the wave phase sensor is dependent on the position of the moveable member.

4. A displacement sensor as claimed in any preceding claim in which the phase sensor is an optical sensor.

5. A displacement sensor as claimed in any preceding claim in which the wave generator and the acoustic wave phase sensor operate on the same point on the member, the acoustic wave travelling from the generator and a discontinuity in the member and being reflected back to the sensor.

6. Apparatus for sensing the position of a member comprising : a first transducer for launching a signal in the form of a mechanical wave into the member; a second transducer for receiving-the signal from the member after propagation along a path therethrough and means for sensing the delay to the signal caused by the propagtation along the said path ; the arrangment of the transducers in relation to the member being such that the length of the path and thus the delay is dependent on the position of the member.

7. A displacements sensor substantially as shown in and and as described with reference to Figures 1 and 2 of the accompanying drawings.