GB2090656A

GB2090656A - Optical Position Sensing

Info

Publication number: GB2090656A
Application number: GB8200319A
Authority: GB
Original assignee: General Electric Co PLC
Current assignee: General Electric Co PLC
Priority date: 1981-01-07
Filing date: 1982-01-06
Publication date: 1982-07-14
Also published as: GB2090656B

Abstract

The light emitted by a combination of at least two luminescent materials coated 6, on a body 7 varies with position on the body surface, enabling the position and/or orientation of the body to be detected photoelectrically, e.g. by sensing at a fixed detection point either the mean luminescent lifetime or the relative luminescent intensities. The mixing ratio of a pair of materials may vary continuously in one direction along the body, or a uniform mixture may be overlaid by a non-uniform layer selectively absorbing light from one of the materials. A second pair of materials may be used likewise for sensing in a second direction. For discrete position sensing, the body bears uniform areas of different materials. In a further variant, the luminescent strips are overlaid by a binary coded apertured mask and the luminescent light observed via a fixed triple aperture mask is analysed. As shown light from xenon arc lamp 1 is directed via chopper 3, filter 4, dichroic mirrors 8, 9 and optical fibre 5 to the coating 6, and luminescent light is returned through fibre 5, mirror 9 and filter 10 to a detector 11. The mixing ratio of two materials is determined at 12 by phase comparison of the detector output and a signal from chopper 3. <IMAGE>

Description

SPECIFICATION Optical Sensing Device This invention relates to optical sensing devices and analogous devices using electromagnetic radiation of other wavelengths, particularly but not exclusively for use in remote sensing of the positions of objects.

Hitherto, such devices have been affected by variations in the intensity of the radiation source and also by optical noise. These disadvantages are eliminated in the present invention, which also has advantages over electrical position sensors in that it is not susceptible to electrical interference and does not require a remote power supply. The present invention is therefore particu;arly suitable for use in extreme environments.

According to the present invention, apparatus for sensing the position and/or orientation of a body comprises a surface provided with a coating incorporating a plurality of luminescent materials, each having a different luminescent lifetime and/or a different luminescent wavelength, the composition of the said coating being nonuniform, so that on illumination of the said coating, different parts of the said coated surface are characterised by different relative intensities of components of the resulting luminescent emission, each of which components is characterised by a different luminescent wavelength and/or a different luminescent lifetime; and, separate from said body, means for illuminating at least one region of the said surface with exciting radiation, thereby to excite luminescent emission in the said materials within the said region, the region or regions illuminated being determined by the position and/or the orientation of the said body; means for separating the exciting radiation from the luminescent emission; detection means for detecting the luminescent emission; and means for determining the relative intensities of the detected components of luminescent emission.

It will be seen that this will give an indication of the position and/or orientation of the mobile body with respect to the illuminating radiation. The non-uniform composition of the coating may be achieved either by varying the ratio of the surface concentrations of the luminescent materials over the surface or by doping the coating nonuniformly with a selective absorber, so that, in use, luminescent emission from each material is absorbed to a different extent, the difference depending on the concentration of the absorber.

The former method is preferable. Suitable materials include calcium chlorophosphate activated with antimony and manganese, magnesium fluorogermanate activated with manganese, and many lanthanide oxide mixtures.

The luminescent materials may be luminescent with respect to either one photon or multi-photon excitation.

In the case where the luminescent materials are characterised by different luminescent lifetimes, a restricted region of the coated area is illuminated by a modulated beam of exciting radiation and the resulting luminescence detected and monitored. The exciting radiation and the luminescent emission may both be transmitted through a single optic fibre. Preferably the coated area contains two luminescent materials. Typical series of exciting and emitted pulses are shown in Figure 1. The height of the luminescent pulses will vary with the source intensity but the phase will be independent of the source intensity and will be a function of the rate of build-up and decay of the luminescent pulses.The phase will therefore depend on the relative amounts of the luminescent materials illuminated by the beam of exciting radiation, and will thus provide a measure of the displacement of the object in any direction perpendicular to the beam of exciting radiation in which the relative surface concentrations of the luminescent materials vary. The phase of the luminescent pulses may be measured by a lock-in amplifier. The decay of the luminescence may also be analysed in other ways, however; for example by using sample-hold techniques or a storage oscilloscope. The measured rate of decay will be a function of the relative amounts of the materials illuminated by the beam, and will thus provide an indication of position.

The invention will now be further described by way of an example with reference to the accompanying drawings.

Figure 2 shows an apparatus comprising Xenon arc lamp 1 powered by a power supply 2.

A rotating perforated disc 3 serves to chop the light into pulses which are then allowed to pass through a filter 4 which removes light of the frequencies of emission of the luminescent materials. Two dichroic mirrors 8, 9 are used to further remove light of these frequencies from the beam which is then focussed onto the end of an optic fibre 5 leading to the luminescent material 6 which is attached to an object 7, movement of the position of which is to be detected. The luminescent material 6 comprises a mixture of calcium chlorophosphate activated with antimony and manganese and of magnesium fluorogermanate activated with manganese. The proportions of these luminescent materials vary monotonically along the path that the incident light takes across the surface of the body as the object moves. The two luminescent materials emit light at 580 nm and 620 nm respectively.

These emissions are conducted through the fibre 5, filtered to remove any residual exciting radiation and are focussed onto a silicon detector 11. The phase shift of the emitted radiation incident on the detector is determined by a lock-in amplifier 12 with reference to a signal taken from the light chopper 3. The previously described optical system may be convenientiy replaced by an entirely fibre optic system incorporating fibre couplers. The output of the amplifier 12 is a measure of the position of the object 7.

In the case where the luminescent materials are characterised by different luminescent wavelengths, the beam of exciting radiation may be continuous. The luminescence emitted from the illuminated area is collected and split up into appropriate components of different wavelength by means such as fiiters and prisms, dichroic mirrors or diffraction gratings, according to the wavelengths being used, such that each of said components corresponds to a respective illuminated luminescent material in the coated area. The intensities of said components are monitored, and the relative amounts of the luminescent materials illuminated by the beam of exciting radiation found from the relative intensities of the respective components of different wavelength. The position of the object is thus determined.

In both the embodiments described above, the relative surface concentrations of the luminescent materials may vary either discretely or continuously over the coated area, leading to discrete and analogue position sensing respectively.

Figure 3 illustrates three possible distributions of luminescent materials (a), (b) and (c) which are suitable for discrete sensing of, respectively, the one-dimensional position, two-dimensional position, and angular displacement of the object.

The detectable movements of the object are illustrated by arrows in each case. Each typical section S of the coated area may consist of either one luminescent material or a uniform mixture of luminescent materials. Different sections of uniform composition may therefore be characterised either by different luminescent lifetimes or different mean luminescent lifetimes, or by different luminescent wavelengths of sets of luminescent wavelengths. The illuminated region is preferably just large enough to completely cover any section of uniform composition within the coated area, as illustrated by the shaded regions in Figure 3. A slight movement of the object will then be detectable irrespective of the position of said illuminated region.

Analogue position-sensing may be carried out by using a coated area containing two luminescent materials such that their relative surface concentrations vary in a direction parallel to the direction of movement of the object.

An alternative method of utilising luminescent materials for position sensing will now be described by way of example with reference to Figure 4. A stationary mask c), pierced with a row of holes as shown, is superimposed in front of a plate a) which is provided with a plurality of adjacent coatings 1, 2 and 3 of luminescent materials. The coatings of luminescent materials lie perpendicular to the row of holes in the mask c). A mask b) is attached to the object whose position is to be determined, and moves freely with the object between the plate a) and the mask c) in the direction indicated by the arrows. A beam of exciting radiation is directed onto the front of the mask c) and passes through the holes onto the moving mask b).Depending on the position of the object, radiation passes through one of the sets of holes A, B, C, D, E, F or G in the moving mask and falls on strips 1, 2, (1, and 2), 3, (1 and 3), (2 and 3), or (1, 2 and 3) respectively.

Some of the resulting luminescence passes back through the overlapping holes in the masks b) and c) and is collected and identified by its mean lifetime and/or distribution of wavelengths according to any of the methods disclosed in the examples herein. Knowing the characteristics (i.e.

the lifetime or the wavelength) of the luminescence emitted from each coating, the position of the object may be found. Thus overlap of the sets of holes A, B, C, D, E, F and G with the row of holes in the mask c) corresponds to the binarydigital positions 1,10,11, 100,101, 110 and 111 respectively. Provided that the mask c) is aligned with the plate so that radiation passing through any given hole in the mask c) falls on only one strip of luminescent material on the plate a), the measured displacement of the object will be unaffected by slight inaccuracies in the positioning of the luminescent strips.Since the holes in the mask b) may be accurately located with respect to one another, the method is capable of high accuracy, and high precision may be achieved even with a relatively small number of strips of luminescent material, since n strips enable 2n - 1 positions to be distinguished.

A further embodiment of the invention which is particularly suitable for two-dimensional position sensing will now be described by way of example with reference to Figure 5.

Pulsed light from a source 1 passes through a filter 2 and a dichroic mirror 3 and is directed by an optic fibre 4 onto a coated area 5 of the object whose position is to be determined.

The coated area is provided with four luminescent materials; A, B, C and D such that the ratio of the surface concentrations of B and A increases in the x direction and is constant in the y direction, while the corresponding ratio for D and C increases in the y direction but remains constant in the x direction. The filter 2 blocks out components of the radiation from the source 1 having wavelengths similar to any of the emissive luminescent wavelengths of any of the materials A, B, C or D. The emissive luminescent wavelengths A', B', C', D' of respectively, A, B C and D increase in the orderA' < B' < C' < D'.

Preferably (D'-C') < 50 nm, (C'-B') > 100 nm, (B'-A') < 50 nm.

Luminescence from the materials lying within the illuminated area 6 of the coated area 5 is collected and transmitted down the optic fibre 4 and reflected by the dichroic mirror 3 onto the dichroic mirror 7. The latter separates wavelengths A' and B' from C' and D', reflecting the former pair through the filter 8 to a detector 10, and transmitting the latter pair through the filter 9 to a detector 1 The signals from detectors 10 and 11 can then be separately processed by phase-sensitive detection electronics to measure the overall delay in the phase of the luminescence from the exciting light.

The x and y coordinates are determined from the measured luminescent lifetimes which themselves are known weighted functions of the relative amounts of the luminescent materials present in the restricted illuminated area.

Provided that the luminescent excitation wavelength of A is similar to that of B, and that of C is similar to that of D, the measurements of weighted mean lifetime of the above pairs of materials will not be affected by changes in the emission spectrum of the light source 11, in the attenuation spectrum of the optic fibre 4, or in the relative sensitivity to different wavelengths of the detectors 10 and 1 The above method of position sensing is therefore exceptionally reliable.

Furthermore, the method of position-sensing using luminescent lifetimes is fail-safe in that any breakdown in a detector or blockage of an optical path will be immediately detected and will not result in a false indication of position.

Claims

1. Apparatus for sensing the position and/or orientation of a mobile body, comprising on said body, a surface provided with a coating incorporating a plurality of luminescent materials, each having a different luminesecent lifetime and/or a different luminescent wavelength, the composition of the said coating being nonuniform, so that on illumination of the said coating, different parts of the said coated surface are characterised by different relative intensities of components of the resulting luminescent emission, each of which components is dharacterised by a different luminescent wavelength and/or a different luminescent lifetime; and, separate from said body, means for illuminating at least one region of the said surface with exciting radiation, thereby to excite luminescent emission in the said materials within the said region, the region or regions illuminated being determined by the position and/or the orientation of the said body; means for separating the exciting radiation from the luminescent emission; detection means for detecting the luminescent emission; and means for determining the relative intensities of the detected components of luminescent emission.

2. Apparatus according to claim 1 , wherein the ratio of surface concentrations of the luminescent materials varies discontinuously over the coated part of the said surface.

3. Apparatus according to claim 1, wherein the ratio of the surface concentrations of the luminescent materials varies continuously over the coated part of the said surface.

4. Apparatus according to claim 2, in which the said surface is provided with a plurality of coatings of luminescent materials, each of uniform composition, lying side by side and running approximately parallel to the locus of the mobile body, a mask pierced with sets of holes is carried by the mobile body so as to be superimposed on, but spaced from, the said surface, each hole being superimposed on a single coating, and a beam of exciting radiation is directed onto the front of the mask so as to pass through one of the said sets of holes and thereby excite said luminescent emission in those strips exposed to the said beam by holes in the said set.

5. Apparatus according to claim 4 for detecting the linear position of the mobile body.

6. Apparatus according to any of claims 2 to 5 in which the said components of luminescent emission are characterised by different luminescent wavelength.

7. Apparatus accordingto any of claims 2 to 5 in which the said components of luminescent emission are characterised by different luminescent lifetime and means is provided for modulating the intensity of said exciting radiation.

8. Apparatus according to claim 7 in which the said exciting radiation is pulsed.

9. Apparatus according to claim 7 or claim 8 incorporating means for measuring the phase of the said luminescent emission.

10. Apparatus according to claim 9 in which the said means is a lock-in amplifier.

11. Apparatus according to any preceding claim in which the exciting radiation and luminescent emission are both transmitted through a single optic fibre.

1 2. Apparatus according to any of claims 2 to 11 for sensing two-dimensional position or orientation in which the said coating incorporates at least two pairs of luminescent materials such that the ratio of the surface concentrations of the materials of one said pair increases along one axis but is substantially constant along the perpendicular axis while the ratio of surface concentrations of the materials of the other said pair increases along the said perpendicular axis but is substantially constant along the said one axis, prism means is provided for separating the luminescent emission produced by one said pair of luminescent materials from the luminescent emission produced by the other said pair of luminescent materials and means is provided for measuring the mean luminescent lifetime of the luminescent emission produced by each said pair of materials.

1 3. Apparatus according to claim 12 in which the emissive luminescent wavelengths of the materials within each said pair differ from each other by less than 50 nm and differ from the emissive luminescent wavelengths of the other said pair by more than 100 nm.

14. Apparatus according to any preceding claim in which one or more of the materials is susceptible to multi-photon excitation.

1 5. A method of sensing the position and/or orientation of a mobile body, comprising applying to an area of a surface of the body a coating of two or more luminescent materials, each distributed non-uniformly over the area, the materials having different luminescent lifetimes o emitting luminescent radiation at different wavelengths, illuminating a restricted region of the coated area by a stationary modulated beam of exciting raaiation, and detecting the resulting luminescence of the coating at said region.

1 6. Apparatus substantially as described hereinabove with reference to Figure 2 of the accompanying drawings.

1 7. Apparatus substantially as described hereinabove with reference to Figure 4 of the accompanying drawings.

1 8. Apparatus substantially as described hereinabove with reference to Figure 5 of the accompanying drawings.