DISPLACEMENT DETECTION

Abstract: Apparatus for determining displacement of an object comprises a light source (1) for propagating polychromatic light along a path to a detector (7). Radiation modulation means such as a filter (5) is provided in predetermined fixed location with respect to the source (1) and the path of the light is adjusted in response to a displacement of the object such as to vary the distributed spectral content of the light reaching the detector (7). The detector comprises first and second photoresponsive elements (10, 11), of differing responsivity with respect to wavelength, and a microprocessor (14) receiving signals from the photoresponsive elements. The microprocessor calculates the colour of the light incident on the detector (7) as represented by two or more parameters on the chromaticity (CIE) diagram, and interprets the colour of the light incident on the detector (7) in terms of the displacement of the object.

Description: This invention relates to the detection of the displacement of an object. Parameters such as position, velocity, acceleration, pressure and temperature can all be translated into a displacment and determined by the detection of that displacement. Displacement measuring systems are known in which a spectral splitting device, such as a prism or grating, splits a light beam into its wavelength components. Displacement of the spectral splitting device is measured by the detection of the wavelength received at a given detection point. By way of example of this type of measuring system, reference is directed to French Patent Specification FR-A-2569001 which describes an apparatus and method for determining displacement of an object, comprising a light source for propagating polychromatic light along a path; a detector having at least first and second photo-responsive elements, the responsivity with respect to wavelength of the first element being different from that of the second; radiation modulation means disposed in predetermined fixed location with respect to the detector; means for adjusting the path of the polychromatic light such as to vary the distributed spectral content of the light reaching the detector, the path adjustment means being responsive to the displacment of the object; and an analysis means. Attention is also directed to EP-A-0 246 005, falling under Article 54 (3), EPC, disclosing a method and arrangement for displacement detection similar to that of the present invention. It is an object of the present invention to provide an improvement to this type of displacment measuring system. In accordance with a first aspect of the present invention, the analysis means calculates, from the signals from the photo-responsive elements, the colour of the radiation incident on the detector as represented by two or more parameters on the chomaticity (CIE) diagram, the analysis means being adapted to interpret the colour of the radiation reaching the detector in terms of the displacement of the object. In one convenient arrangement, two different photoresponsive elements are employed, each with its own wavelength responsivity characteristic. Alternatively, one or both of the photoresponsive elements includes a coloured filter to impart a colour response characteristic, thereby allowing two identical photoresponsive elements to be employed, if desired. Preferably, the responsivities with respect to wavelength of the first and second photoresponsive elements are such that their respective wavelength/intensity curves overlap for at least part of the wavelength spectrum. A change in colour is therefore determined by assessing the change in the whole of a selected part of the spectrum (colour modulation) as opposed to merely detecting the change at one or more selected wavelengths (wavelength modulation). Thus a change from colour A (represented by a wavelength/intensity curve A) to colour B (represented by wavelength/intensity curve B) will be calculated from the area between the two curves thereby giving a more complete analysis of 'true' colour. Wavelength modulation is limited in that it is a calculation based on the distance between the two curves at one or more selected wavelengths. By the term 'polychromatic light' there is herein meant any multi-wavelength radiation, and is specifically meant to include both visible light and infra-red radiation. The term 'colour' whilst used herein for ease of understanding, should in no way imply that only visible light may be employed. Where the apparatus employs radiation outside the visible spectrum, the term 'colour' will refer to the spectral distribution of the radiation. Preferably there is provided a focusing element adapted to focus the polychromatic light on to the radiation modulation means. Conveniently the path adjustment means comprises means for moving the focusing element in response to a displacement of the object. Movement of the focusing element varies the proportion of the light beam which passes through the radiation modulation means, thereby varying the colour of the beam reaching the detector. Additionally or alternatively there is provided a reflective surface positioned such as to reflect the focused polychromatic light on to the radiation modulation means, the path adjustment means comprising means for moving one or both of the focusing element and reflective surface so as to vary the distance therebetween. A change in the distance between the focusing element and the reflective surface will again vary the proportion of the light beam which passes through the radiation modulation means. Preferably the reflective surface is movable with respect to the focusing element in response to displacement of the - 4 object. Conceivably, the reflective surface is provided by the object the displacement of which is to be determined, typically in the form of a reflective diaphragm in a pressure sensor. In one convenient arrangement the radiation modulation means is integrally formed within the focusing element. The focusing element is conveniently in the form of a sphere, or alternatively a cylinder, preferably of optically transparent glass. The radiation modulation means preferably comprises a filter which attenuates the intensity of transmitted wavelengths to different degrees. The filter conveniently comprises an elongate element the colour absorption characteristics of which vary along its length. In one convenient arrangement the filter comprises an elongate element having a coloured portion and a transparent portion and wherein the transition between said coloured and transparent portions occurs at a sharply defined edge. Conveniently the sharply defined edge extends obliquely relative to the longitudinal axis of the elongate element. Where the radiation modulation means is integrally formed within the focusing element, it may conveniently comprise a coloured core, spherical or cylindrical in shape depending on the shape of the focusing element. The invention further resides in a method of determining displacmeent of an object employing apparatus as herein described. In particular, a method of detecting displacmeent of an object comprises the steps of propagating polychromatic light along a path to a detector which has at least first and second photoresponsive elements, the responsivity with respect to wavelength of the first element being different from that of the second, supporting radiation modulation means in predetermined fixed relation with respect to the detector, and adjusting the path of the polychromatic light in response to displacement of the object such as to vary the distributed spectral content of the light reaching the detector, characterised by calculating, from the signals from the photo-responsive elements, the colour of the radiation incident on the detector as represented by two or more parameters on the chromaticity (CIE) diagram, and interpreting the colour of the radiation incident on the detector in terms of the displacement of the object. The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which Figure 1 is a schematic diagram of one embodiment of apparatus according to the present invention; Figures 2 to 4 are schematic diagrams of further embodiments of apparatus according to the invention; Figure 5 is a schematic diagram of an embodiment of a modulating element for use in the apparatus of Figures. 1 to 3; and Figure 6 is a sectional diagram of one practical embodiment of apparatus according to the invention. Referring to Figures 1 and 2, displacement sensing apparatus comprises a light source 1, adjacent which is an optical fibre 2 which transmits light from the source 1 to a gap 3 in which is provided a transparent sphere 4 and a chromatic modulator 5. Light traversing the gap 3 is transmitted by means of a further optical fibre 6 to a detector shown generally at 7. The sphere 4 is mounted so as to be movable in response to the displacment to be measured, either in a longitudinal direction as shown in Figure 1, or in a lateral direction as shown in Figure 2. The chromatic modulator 5 comprises a filter element, including a colourless portion 8 and a coloured portion 9, with a sharp division therebetween. The detector 7 comprises two photoresponsive elements 10 and 11, the responsivity of which with respect to wavelength differs one from the other. Signals from the elements 10 and 11 are fed via lines 12 and 13 respectively to a microprocessor 14. The output of the microprocessor activates a display unit 15, signals being passed thereto via line 16. As the sphere 4 moves as a result of the displacement, its focusing effect upon the light emerging from the optical fibre 2 is altered, causing a different proportion of the light to be passed through the coloured portion 9 of the chromatic modulator 5. This causes a change in the colour of the light reaching the detector 7. The light impinging on the photoresponsive elements 10 and 11 produces two different output signals, which signals are analysed by the microprocessor 14. The microprocessor 14 calculates the colour of the light in terms of two parameters on the Chromaticity (CIE) Diagram from the signals from the photoresponsive elements 10 and 11. The parameters are compared with those in a look-up table stored in a read only memory of the microprocessor, and the corresponding value of the displacement is obtained. The displacement is then displayed by means of the display unit 15, which is typically an LED display. Figure 3 shows a version of the apparatus in which the light emerging from the optical fibre 2 passes through the sphere 4 and is reflected by a reflective diaphragm 17. The reflected light passes back through the sphere 4 before passing through the chromatic modulator 5 and reaching the detector 7 as before. In this version of the apparatus the diaphragm 17 and not the sphere 4 moves in response to the displacement to be measured, the diaphragm moving longitudinally as shown in figure 3. This causes a change in the path of the light passing back through the sphere and hence the proportion of light passing through the coloured portion 9 of the chromatic modulator 5. The colour of the light reaching the detector is analyised as before to give an indication of the displacement of the diaphragm. Figure 4 shows a version of the apparatus of figure 3 in which the sphere and chromatic modulator are merged into a single unit. The sphere 4 comprises a transparent outer portion 18 with a coloured core 19. As before the movement of the diaphragm 17 changes the path of the light through the sphere and hence the proportion of the light passing through the coloured core 19. This produces the change of colour at the detector 7 required to indicate a displacement of the diaphragm 17. Figure 5 showns an alternative chromatic modulator 20, which can be used instead of the chromatic modulator 5 in the displacement detecting apparatus of figures 1 to 3. The modulator 20 comprises a colourless portion 8 and a coloured portion 9 as before, but in the modulator 20 the division therebetween is at an oblique angle. This type of modulator provides a more gradual change in colour along its length than the modulator 5. Figure 6 shows a compact form of sensing head such as would be used in an industrial sensor. The optical fibres 2 and 6 are located within a housing 26 and are connected to larger fibres 21 and 22 encased in resin shown generally at 23. The chromatic modulator 5 is in the form of a glass filter and the sphere 4 is mounted by means of supports 24, 25. A casing 27, removably connected to the housing 26, includes the diaphragm 17. This arrangement is suitable for use as a pressure sensor.

Claims: Apparatus for determining displacement of an object, comprising: a light source (1) for propagating polychromatic light along a path; a detector (7) comprising at least first and second photo-responsive elements (10, 11), the responsivity with respect to wavelength of the first element being different from that of the second; radiation modulation means (5; 19; 20) disposed in predetermined fixed location with respect to the detector (7); and means for adjusting the path of the polychromatic light such as to vary the distributed spectral content of the light reaching the detector (7), the path adjustment means being responsive to the displacement of the object; characterised by analysis means (14) for calculating, from the signals from the photo-responsive elements (10, 11), the colour of the radiation incident on the detector (7) as represented by two or more parameters on the chromaticity (CIE) diagram, the analysis means being adapted to interpret the colour of the radiation reaching the detector (7) in terms of the displacmeent of the object. Apparatus according to claim 1 characterised in that there is provided a focusing element (4) adapted to focus the polychromatic light on to the radiation modulation means (5; 19; 20). Apparatus according to Claim 2 characterised in that the path adjustment means comprises means for moving the focusing element (4) in response to displacement of the object. Apparatus according to Claim 2 characterised in that there is provided a reflective surface (17) positioned such as to reflect the focused polychromatic light on to the radiation modulation means (5;19;20), the path adjustment means comprising means for moving one or both of the focusing element (4) and the reflective surface (17) so as to vary the distance therebetween. Apparatus according to Claim 4 characterised in that the reflective surface (17) is movable with respect to the focusing element (4) in response to a displacement of the object. Apparatus according to Claim 5 characterised in that the reflective surface (17) is provided by the object the displacement of which is to be determined. Apparatus according to any of Claims 4 to 6 characterised in that the radiation modulation means (19) is integrally formed within the focusing element (4). Apparatus according to any of Claims 1 to 7 characterised in that the radiation modulation means (5;20) comprises a filter which attenuates the intensity of transmitted wavelengths to different degrees. Apparatus according to Claim 8 characterised in that the filter (5;20) comprises an elongate element the colour absorption characteristics of which vary along its length. Apparatus according to Claim 9 characterised in that the filter (5;20) comprises an elongate element having a coloured portion (9) and a transparent portion (8) and wherein the transition between said coloured and transparent portions occurs at a sharply defined edge. Apparatus according to Claim 10 characterised in that the sharply defined edge extends obliquely relative to the longitudinal axis of the elongate element (20). A method of detecting displacement of an object, comprising propagating polychromatic light along a path to a detector (7) which has at least first and second photo-responsive elements (10, 11) the responsivity with respect to wavelength of the first element being different from that of the second, supporting radiation modulation means (5; 19; 20) in predetermined fixed relation with respect to the detector (7), and adjusting the path of the polychromatic light in response to displacement of the object, such as to vary the distributed spectral content of the light reaching the detector (7), characterised by calculating, from the signals from the photo-responsive elements (10, 11), the colour of the radiation incident on the detector (7) as represented by two or more parameters on the chromaticity (CIE) diagram, and interpreting the colour of the radiation incident on the detector (7) in terms of the displacement of the object.

Inventors: HENDERSON, PHILLIP JAMES
Applicants: THE UNIVERSITY OF LIVERPOOL
Application year: 1987
Main classification: G01D5
IPC codes: G01D5, G01B11