GB2184830A

GB2184830A - Optical displacement transducer

Info

Publication number: GB2184830A
Application number: GB08628566A
Authority: GB
Inventors: Roger Alan Edwards; Peter Thomas Gardiner
Original assignee: Smiths Group PLC
Current assignee: Smiths Group PLC
Priority date: 1985-12-18
Filing date: 1986-11-28
Publication date: 1987-07-01
Also published as: GB8628566D0; DE3640519A1; US4761551A; IT8622545A0; FR2591739B1; IT8622545A1; IT1199683B; GB8531149D0; JPS62150117A; GB2184830B; FR2591739A1

Description

GB2184830A 1 SPECIFICATION ter means is moved into or out of the path of

radiation between the source means and the Optical transducers detector means. The boundary may be a straight line. The detector means may include This invention relates to optical transducers. 70 two separate detectors responsive to radiation The invention is more particularly concerned at respective ones of said different wave with transducers for sensing displacement us- lengths.

ing optical means. The optical transducer may include first Optical displacement transducers, for fibre-optic cable means arranged to supply ra example, are well known and generally employ 75 diation from said source means to said filter a light source and receiver (such as provided means, and second fibre- optic cable means ar by the ends of fibre-optic cables), and means ranged to supply radiation from said filter to vary the amount of radiation failing on the means to said detector means. The said sec receiver in accordance with displacement. The ond fibre-optic cable means may be bifurcated means by which the radiation is varied may 80 at its end closer the detector means, and employ a movable mask with an aperture of each bifurcation may extend to a respective variable size, or a neutral density filter, the detector.

density of which varies along its direction of The optical transducer apparatus may in displacement. These transducers can function clude comparator means arranged to receive satisfactorily providing that the intensity of ra- 85 the outputs from detector means in respect of diation failing on the receiver is not varied for the two different wavelengths. The optical any other reason. However, any change in the transducer apparatus may include display radiation emitted by the light source, such as, means arranged to provide a display represen for exaFnple, caused by variations in power tation indicative of failure of the source means supply to the light source will produce errone- 90 when the output of the detector means in re ous displacement readings. spect of the wavelength which normally pro It is an object of the present invention to duces the higher output falls below a predet provide an optical transducer that can be used ermined value.

to alleviate the above-mentioned problems. The said optical radiation source is prefera- According to one aspect of the present in- 95 bly provided by a single source which may be vention there is provided optical transducer a light-emitting diode. The light-emitting diode apparatus including optical radiation source is preferably driven at reduced power.

means adapted to emit radiation within a first Optical displacement transducer apparatus in band of wavelengths, optical detector means accordance with the present invention will arranged to receive radiation from said source 100 now be described, by way of example, with means, filter means having a transmission reference to the accompanying drawings, in characteristic that is substantially constant which:

over the area of the filter means and that Figure 1 shows the apparatus schematically; varies substantially within said first band of Figure 2 is a graph showing an output char- wavelengths, means for effecting displacement 105 acteristic of a light source in the apparatus, of the filter means relative to the path of radi and a transmission characteristic of a filter in ation between the source means and the dethe apparatus; tector means such that the proportion of radi- Figure 3 is a sectional view along the line ation incident on the detector means via the 111-111 of Figure 1 showing the filter in one filter means to that incident directly on the 110 position; detector means varies according to the posi- Figures 3A and 313 show the filter in its tion of the filter means, said detector means extreme positions; being arranged to provide outputs in respect Figure 4 is a graph showing how the output of the quantity of radiation at two different of detectors vary with displacement of the fil wavelengths within said first band such that 115 ter assembly; and by comparison of the outputs at the two dif- Figures 5 and 6 show alternative filter as ferent wavelengths the position of the filter semblies.

means relative to the radiation path can be With reference first to Figure 1, the trans determined. ducer apparatus includes a light source 1 that The wavelengths over which the transmis- 120 supplies optical radiation (which may be vis sion characteristic of the filter means varies ible, infra red or ultra violet) to one end of a substantially are preferably located substan- fibre-optic cable 2. The other end of the cable tially midway along said first band, the said 2 terminates in front of a collimating lens 3 two different wavelengths being located on that produces a substantially parallel beam of opposite side of said first band. The filter 125 radiation that is incident on a similar lens 4 means may be provided by a part only of the located in front of the end of a second fibre area of a neutrally transparent substrate, the optic cable 5. A filter assembly 20 is located substrate being displaceable such that the in the beam of radiation between the two boundary between the area that is neutrally lenses 3 and 4 and is displaceable in a direc transparent and the area that provides the fil- 130 tion transverse to the beam.

2 GB2184830A 2 Radiation from the other end of the second the difference between the intensity of radia- cable 3 is supplied to two separate radiation tion received at the two different wavelengths detectors 7 and 8. This may be achieved by A. and A, for different positions of the filter the bifurcated cable shown or by other con- assembly. The outputs from the two detectors ventional optical means. The detectors 7 and 70 7 and 8 will therefore be substantially identical 8 are each responsive to different wave- in the position shown in Figure 3A but will be lengths A7 and A, respectively; their outputs substantially different in the position shown in are taken as an electrical signal to a compara- Figure 3B.

tor unit 9 which provides an output to an The comparator unit 9 responds to the ratio indicator display or other utilisation means 10. 75 of the outputs of the two detectors 7 and 8 Referring now also to Figures 2 and 3, the and, after appropriate scaling or model match- output response of the light source 1 is ing, produces an output to the display 10 shown in Figure 2, by the continuous line, as representative of position of the filter as a relatively broad band emission characteristic. sembly 20.

The transmission characteristic of the filtering 80 The light source 1 may be a fight-emitting part of the assembly 20 is represented by the diode and is preferably driven at a reduced broken line; this will be seen to have a steep, current such that marginal fluctuations in cur rapidly changing transmission characteristic in rent do not substantially alter the wavelength the region of the peak emission by the light characteristic of its output. Only one light source 1. The peak responses of the two light 85 source is needed, in contrast to some prior detectors 7 and 8 are represented by the two arrangements which can be susceptible to er arrows along the abscissa. These are spaced rors caused by differential changes in output from one another on opposite sides of the of two sources. Because the present appara emission peak of the light source 1 and of the tus depends for displacement measurement on slope of the transmission characteristic of the 90 relative intensities of radiation at different filter 20. wavelengths, changes in either current supply, The filter assembly 20 is of circular shape, operation of the light source, or transmission as seen in Figure 3, although it can be of any of the fibre-optic cables will affect the intensi other shape, and comprises a glass plate 21, ties at the two wavelengths by substantially or other transparent substrate, which has a 95 equal proportions, thus having little effect on filter coating 23 on one half. The other half of the ratio. This contrasts with apparatus relying the plate 21 is preferably neutrally transparent, on, for example, displacement of a graduated only the coated region 23 having the neutral density filter, since such apparatus will transmission characteristic shown in Figure 2. be susceptible to amplitude variations resulting The boundary 24 between the coated and un- 100 from any other cause.

coated regions is a straight line. The filter as- In previous apparatus, where one extreme sembly 20 is displaceable transversely to the of displacement results in a zero or substan light beam, and to the boundary 24, between tially zero intensity output from the detector, it the positions shown in Figures 3A and 313 may not be possible to distinguish between through that shown in Figure 3. In Figures 3, 105 such an extreme displacement and failure of 3A and 313 the section of the light beam is the light source. In the present apparatus, represented by the broken circle 30. In one however, because the intensity of radiation re extreme, shown in Figure 3A, the light beam ceived by one detector 7 does not vary sub passes entirely through the transparent region stantially with displacement, the output of that of the filter assembly 20, whilst in the other 110 detector is normally higher. It is therefore pos extreme of displacement, shown in Figure 3B, sible to monitor for a source failure by moni the light beam passes entirely through the re- toring the output of that detector and indicat gion 23 coated with the filter material. At dising failure on the display 10 when that output placements between these two extremes vary- falls below a predetermined value.

ing portions of the beam are filtered and unfil- 115 The apparatus may be used to measure the tered. displacement or position of an object directly, Turning back to the curves shown in Figure by coupling the filter assembly to the object, 2 and to the graph shown in Figure 4, it will which may be via gearing or linkage. Alterna be clear that there will be very little change of tively, the apparatus may be used to measure attenuation in the intensity of radiation reach- 120 other variables. For example, pressure can be ing the detector 7 upon displacement of the measured by coupling the filter assembly to a filter assembly 20, because the peak response Bourdon tube or a diaphragm. Temperature of the detector 7 lies in a high transmission could be measured by coupling the filter as region of the filter. Because, however, the sembly to an expansible member, such as a peak response of the other detector 8 lies in 125 bimetallic strip.

a low transmission region of the filter, the The filter assembly 20 could take other intensity of radiation reaching it will vary to a forms such as shown in Figures 5 and 6. In greater extent according to what portion of Figure 5, the boundary 124 between the the light beam passes through the filtering re- coated region 123 and the uncoated region of gion 23 of the assembly 20. Figure 4 shows 130the assembly 120 is a straight line that is 3 GB2184830A 3 inclined to the axis of displacement of the area that is neutrally transparent and the area filter: this allows a greater travel for the as- that provides the filter means is moved into or sembly 120 between its two extremes of po- out of the path of radiation between the sition. The boundary between the coated and source means and the detector means.

uncoated regions could have other profiles 70 4. Optical transducer apparatus according to

Claims

and, for example, may be stepped or curved, Claim 3, wherein the boundary is a straight to give the desired output relationship. Figure line. 6 shows a rotatable filter assembly 220, the 5. Optical transducer apparatus according to light beam being attenuated to a degree ac- any one of the preceding claims, wherein the cording to the angle of the assembly. 75 said detector means includes two separate de- It may not be necessary to use two sepa- tectors responsive to radiation at respective rate detectors to detect radiation at the two ones of said different wavelengths. different wavelengths. Instead, for example, a 6. Optical transducer apparatus according to single detector comprising a stack of two any one of the preceding claims, including first photodiodes, may be used. The top photodi- 80 fibre-optic cable means arranged to supply ra ode provides an output of the intensity at one diation from said source means to said filter wavelength peak and transmits radiation at the means, and second fibre- optic cable means ar other wavelength to an underlying photodiode ranged to supply radiation from said filter which is responsive to radiation at that wave- means to said detector means. length. 85 7. Optical transducer apparatus according to The output from the comparator 9 may be Claim 5 and 6, wherein the said second fibresupplied to some form of utilisation means optic cable means is bifurcated at its end other than a display. For example, if the filter closer the detector means, and wherein each assembly is displaced by temperature, the util- bifurcation extends to a respective detector. isation means may be a temperature control 90 8. Optical transducer apparatus according to unit. Many other forms of utilisation means any one of the preceding claims including can be used. comparator means arranged to receive the outputs from detector means in respect of the CLAIMS two different wavelengths.

1. Optical transducer apparatus including op- 95 9. Optical transducer apparatus according to tical radiation source means adapted to emit any one of the preceding claims including dis radiation within a first band of wavelengths, play means arranged to provide a display optical detector means arranged to receive ra- representation indicative of failure of the diation from said source means, filter means source means when the output of the detector having a transmission characteristic that is 100 means in respect of the wavelength which substantially constant over the area of the fil- normally produces the higher output falls be ter means and that varies substantially within low a predetermined value.

said first band of wavelengths, means for ef- 10. Optical transducer apparatus according fecting displacement of the filter means rela- to any one of the preceding claims, wherein tive to the path of radiation between the 105 the said optical radiation source means is pro source means and the detector means such vided by a single source.

that the proportion of radiation incident on the 11. Optical transducer apparatus according detector means via the filter means to that to Claim 10, wherein the radiation source is a incident directly on the detector means varies light-emitting diode.

according to the position of the filter means, 110 12. Optical transducer apparatus according wherein said detector means is arranged to to Claim 11, wherein the light- emitting diode provide outputs in respect of the quantity of is driven at reduced power.

radiation at two different wavelengths within 13. Optical transducer apparatus substan said first band such that by comparison of the tially as hereinbefore described with reference outputs at the two different wavelengths the 115 to Figures 1 to 4 of the accompanying draw position of the filter means relative to the radi- ings.

ation path can be determined. 14. Optical transducer apparatus substan

2. Optical transducer apparatus according to tially as hereinbefore described with reference Claim 1, wherein the wavelengths over which to Figures 1 to 4 as modified by Figure 5 of the transmission characteristic of the filter 120 the accompanying drawings.

means varies substantially are located substan- 15. Optical transducer apparatus substan tially midway along said first band, and tially as hereinbefore described with reference wherein the said two different wavelengths to Figures 1 to 4 as modified by Figure 6 of are located on opposite sides of said first the accompanying drawings.

band. 125 16. Any novel feature or combination of

3. Optical transducer apparatus according to features as hereinbefore described.

Claim 1 or 2, wherein the filter means is pro- Printed for Her Majesty's Stationery Office vided by a part only of the area of a neutrally by Burgess & Son (Abingdon) Ltd, Did 8991685, 1987.

transparent substrate, the substrate being dis- Published at The Patent Office, 25 Southampton Buildings, placeable such that the boundary between the London, WC2A 1 AY, from which copies may be obtained.