OPTICAL POTENTIOMETER

Abstract: An optical potentiometer comprises a pair of mutually spaced electrically resistive tracks (40, 42) defining a gap (44) in which a photoconductive amorphous silicon layer (46) is exposed. The optical potentiometer has a light source (36) and a reflector arrangement defined by a reflector body (18) mounted on the lower end of a rotary shaft (16). The reflector body (18) defines a reflective surface (24) lying on an ellipse and a planar mirror (22). The mirror (22) and surface (24) are arranged so that one focal point (F1) of the ellipse lies in the gap (44) between the tracks (40, 42), whilst a virtual focus (Fv) of the other focal point (F2) of the ellipse lies on the axis of rotation (R) of the shaft (16). The light source (36) also lies on the axis of rotation (R) so that light emanating therefrom is concentrated by the reflector arrangement into the gap (44).

Description: OPTICAL POTENTIOMETER This invention relates to an optical potentiometer. Optical potentiometers have the advantage over mechanical potentiometers that there is no wear of the electrically resistive track because there is no moving element in mechanical contact with the track. In a previously proposed optical potentiometer, cadmium sulphide or cadmium selenide is screen printed onto an insulating substrate to act as a photoconductive layer, and resistive metal (e.g. chromium) contacts are provided over the photoconductive layer and etched to define a pair of mutually parallel, electrically resistive tracks separated by a gap typically having a width of about 20//m. In such potentiometer, light from a light source is shone through a system of lenses and mirrors mounted on a control element which is movable relative to the tracks. The resultant light beam is focused on the gap between the tracks so that the photoconductive layer become electrically conducting at the location of light incidence and electrically bridges the tracks. One disadvantage of such an arrangement is that the screen printed photosensitive layer does not have consistent photoconductive properties and so it can be difficult to achieve a smooth and accurately proportional change in electrical output as relative movement between the control element and the tracks takes place. Another disadvantage is that cadmium and selenium are both poisonous. It is an object of a first aspect of the present invention to obviate or mitigate these disadvantages. According to said first aspect of the present invention, there is provided an optical potentiometer comprising a pair of mutually spaced electrically resistive tracks defining a gap, a photoconductive layer exposed in the gap between the tracks, a light source, and means for directing light from the light source to the gap between the tracks, the tracks and the directing means being relatively movable to vary the location at which the gap between the tracks is illuminated by the light source in use, characterised in that the photoconductive layer is a photosensitive amorphous silicon layer. The photosensitive amorphous silicon layer can be applied by chemical vapour deposition with very consistent photoconductive properties. The tracks will normally be formed of metal. However, the use of metal tracks may cause non-linearities in the output signal as a result of photovoltaic effects due to the formation of an electronic junction between the tracks and the photoconductive layer. It is therefore a object of a second aspect of the present invention to obviate or mitigate this disadvantage. According to said second aspect of the present invention, there is provided an optical potentiometer comprising a pair of mutually spaced electrically resistive tracks defining a gap, a photoconductive layer exposed in the gap between the tracks, a light source, and means for directing light from the light source to the gap between the tracks, the tracks and the directing means being relatively movable to vary the location at which the gap between the tracks is illuminated by the light source in use, characterised in that there is provided a respective ohmic contact layer separating the photoconductive layer from each track. Most preferably, the photoconductive layer is a photosensitive amorphous silicon layer in accordance with said first aspect of the present invention. The tracks are preferably electrically resistive metal tracks. Typically, the ohmic contact layer is defined by an n + interface having a thickness of 20 to 200A. Another disadvantage of the previously proposed optical potentiometer described above is that the system of lenses and mirror is quite bulky and expensive. It is an object of a third aspect of the present invention to obviate or mitigate the above disadvantage. According to said third aspect of the present invention, there is provided an optical potentiometer comprising a pair of mutually spaced electrically resistive tracks defining a gap, a photoconductive layer exposed in the gap between the tracks, a light source, and means for directing light from the light source into the gap between the tracks, characterised in that the directing means includes a reflector arrangement which is shaped and disposed so as to concentrate light from the light source into the gap. The optical potentiometer according to said third aspect of the present invention is most preferably a rotary potentiometer where at least part of the reflector arrangement is mounted for rotary movement on a shaft, the tracks and gap being arcuate and centred upon the axis of rotation of the shaft. Preferably, said reflector arrangement includes a reflective surface lying on the surface of at least one conic section. Preferably, the conic section is an ellipse. Concentration of the light may be effected by locating the reflecting surface so that the light source and the gap lie at respective focal points of the ellipse. This principle is illustrated in accompanying Fig 1 where, in a two-dimensional sense, light emanating from one focus (F 2 ) of the ellipse and reflecting off any point on the inner surface of the ellipse will always pass through the other focus (F,). However, a more compact arrangement can be achieved by positioning at least one mirror (e.g. a planar or conic section mirror) relative to the elliptical reflecting surface and to the light source in such a way that at least one of the light source and the gap is disposed at a, or a respective, virtual focal point of the ellipse, e.g. so that light from the light source which has reflected off the mirror appears to have emanated from the actual focal point. This principle is illustrated in Figs 2a and 2b where light source L is disposed at the virtual focal point F v corresponding to focus F 2 . Preferably, and as shown in Fig 2a, the cross-sectional shape (i.e. the shape ir, any plane perpendicular to the elliptical section illustrated in Fig 2b) of the reflective surface is arcuate . It is to be appreciated that the mirror (mirror M) forms part of the reflector arrangement. Instead of basing the reflective surface on an elliptical section, it is also within the scope of the present invention to base it on a section which is made up of a multiplicity of arcs approximating to an elliptical section, or a faceted design (e.g. an array of reflecting surface regions approximating to an ellipse or any of the other forms specified herein). Conveniently, at least one, and preferably both, of the form of the reflective surface and the form of the mirror are defined by a body which is preferably formed of plastics material and which has an appropriately shaped surface or surfaces rendered suitably reflective, e.g. by deposition of a reflective film, such as by a vacuum evaporation process. In a convenient embodiment, the body has a hole therein which receives the rotor shaft and which has an inclined surface defining the mirror. It may not be necessary for a reflective film to be provided on such inclined surface since it may be inherently reflective due to total internal reflection. The body may be solid and in which case it should be transparent and rendered suitably reflective as necessary by provision of a suitably reflective film. In such a case, a surface of the body through which light passes in use is shaped so as to permit light reflected off at least one internal reflective surface in the body to be concentrated in the gap between the tracks. Such surface through which light passes may be planar or non-planar. As an alternative to being solid, the body may be of shell-like construction with an internally dished form defining at least one of the shape of the reflective surface and the shape of the mirror. In this "shell" configuration, the material of construction may be opaque (e.g. metallic) provided that the surface is highly reflective or has a highly reflective coating thereon. The gap between the tracks is typically about 20μm. However, the light beam which is incident upon the gap typically has a diameter of about 0.5 to 1mm. This may not enable particularly effective use of the light beam. Accordingly, it is an object of a fourth aspect of the present invention to obviate or mitigate this disadvantage. According to said fourth aspect of the present invention, there is provided an optical potentiometer wherein the gap is of serpentine shape, and the arrangement is such that, in use, the beam of light from the light source is scanned along the direction of longitudinal extent of the serpentine gap. Most preferably, the serpentine gap is defined between interdigitated fingers on the respective tracks. Embodiments of the present invention will now be described in further detail, by way of example, with reference to Figs 3 to 6 of the accompanying drawings, in which:- Fig 3 is an axial section through an optical potentiometer according to the present invention, Fig 4 is a plan view of a substrate disk carrying the potentiometer tracks, Fig 5 is a scrap section on the line A-A of Fig 4, and Fig 6 is a much enlarged plan view showing a small part of a preferred design of tracks. Referring now to Fig 3 of the drawings, the optical potentiometer comprises a housing 10 with lid 12 carrying bearings 14 in which an operating shaft 16 is rotatably mounted. The lower end of the shaft 16 carries a shaped reflector body 18 which, in this embodiment, is formed of a solid transparent plastics (eg a polycarbonate plastic material) moulded with an accurately chamfered recess 20 therein to receive the correspondingly chamfered lower end of shaft 16. In the axial section illustrated in Fig. 3, an upper surface 24 of the body 18 is elliptically curved and has focal points F, and F 2 . In cross-section, i.e. perpendicular to the section illustrated in Fig. 3, the upper surface 24 is arcuately curved, with the arcs being centred on the focal axis of the ellipse. The chamfered surface 22 is planar and acts as a mirror. The surface 24 may be provided with a vacuum evaporated film of aluminium thereon to render it internally reflective. The same film may be provided within the recess 18 over the chamfered surface 22. However, it is not essential to provide such a film on the surface 22 if the latter can act as a reflector by total internal reflection. The surface 22 is so inclined and positioned relative to surface 24 and the axis of rotation R that light rays passing along the axis of rotation R and incident upon the surface 22 are reflected towards the surface 24 to be re-reflected so as to pass through the focal point F,. In other words, point F v on axis R is the virtual focus of F 2 . The lower surface of the body 18 is planar and is disposed a short distance above a fixed, circular glass plate 26 defining a transparent substrate. The plate 26 is disposed perpendicularly to the axis of rotation R and is retained against an annular mounting flange 28 by means of a retaining plate 30 and intervening annular spacer 32. The retaining plate 30 has a central holder 34 which holds a light- emitting diode 36 in alignment with the axis of rotation R. The light emitting diode has a relatively narrow cone of emission (about 20°) which is directed along the axis of rotation R towards the mirror 22. Provided on the undersurface of the plate 26 are a pair of almost totally annular tracks 40 and 42 which are separated by an almost totally annular gap 44. The tracks 40 and 42 and the gap 44 are centred on the axis of rotation R. The focal point F, lies substantially in the gap 44. Referring now to Fig 5, the tracks 40 and 42 are formed on an amorphous silicon layer 46 provided on undersurface of the plate 26, there being an n + doped layer 48 between the amorphous silicon layer 46 and the tracks 40 and 42. Thus, the amorphous silicon layer 46 is exposed in the gap 44 between the tracks 40 and 42. In order to produce the assembly illustrated in Fig 5, the amorphous silicon layer 46 is applied to the glass plate 26 by chemical vapour deposition in a per se known manner to a depth which is typically about 1μm. Towards the end of the chemical vapour deposition process, an n + dopant (e.g. phosphinc) is introduced to produce the layer 40 which typically has a thickness of 20 to 200A. The resultant layers are then masked and etched to produce an annular shape where the centre of the plate 26 is left clear for light transmission therethrough. Following this, a resistive metal (e.g. chromium) film is deposited on the n + doped layer 48 and then masked and etched to define the tracks 40 and 42 with intervening gap 44, the latter extending through the layer 48 so that the amorphous silicon layer 46 is exposed within the gap 44. Alternatively, the etching of the n + layer and the resistive metal track can both be carried out after the whole deposition process is complete. Leads 50 (see Fig 4) are then attached to adjacent ends of the tracks 40 and 42. The tracks 40 and 42, which are delicate items, can then be protected against ingress of moisture and other contamination by potting them in a suitable resin together with the LED 36 and amplifier electronics (not shown) provided in the cavity defined within the spacer 32. In use, when the light emitting diode 36 is illuminated, light therefrom passes through the transparent plate 26 to be incident upon mirror 22 and reflected towards the reflector 24 where it is re-reflected towards focal point F,. The above-described shape of the reflector 24 causes the conical I y expanding light beam emanating from the light emitting diode 36 to be concentrated by being brought partly to a focus within the gap 44. Typically, at this stage, the diameter of the beam incident upon the gap 44 is about 1 mm, whilst the gap 44 has a width of 20/ m. Thus, that region of the amorphous silicon layer 46 which is exposed in the illuminated region of the gap 44 is rendered photoconductive and electrically bridges the tracks 40 and 42 at such region. Because the tracks 40 and 42 are resistive metal tracks, it will be appreciated that the resistance across the leads 50 will depend upon the rotary position of the shaft 16 relative to the tracks 40 and 42. In order to increase the effectiveness of the light beam on the photoconductive layer 46, it is preferred to use a track and gap arrangement as illustrated on a very much larger scale in Fig 6 where it will be seen that the tracks 40 and 42 are provided with respective narrow fingers 40a and 42a which are interdigitated so that the gap 44 is of locally serpentine form extending transversely of the general arcuate direction of extent of the gap 44. Such an arrangement reduces the resistance of the conductive path at that region of the photoconductive layer 46 which, at any instant, is being illuminated.

Claims: CLAIMS 1. An optical potentiometer comprising a pair of mutually spaced, electrically resistive tracks (40, 42) defining a gap (44), a photoconductive layer (46) exposed in the gap (44) between the tracks (40 and 42), a light source (36), and means (22, 24) for directing light from the light source (36) into the gap (44) between the tracks (40, 42), characterised in that the directing means (22, 24) includes a reflector arrangement {22, 24) which is shaped and disposed so as to concentrate light from the light source (36) into the gap (44). 2. An optical potentiometer as claimed in claim 1, which is a rotary potentiometer, wherein at least part of the reflector arrangement (22, 24) is mounted for rotary movement on a rotary shaft (16), and wherein the tracks (40, 42) and the gap (44) are arcuate and centred upon the axis of rotation of the shaft (16). 3. An optical potentiometer as claimed in claim 1 or 2, wherein said reflector arrangement (22, 24) includes a reflective surface (24) lying on the surface of at least one conic section. 4. An optical potentiometer as claimed in claim 3, wherein the conic section is an ellipse. 5. An optical potentiometer as claimed in claim 4, wherein the reflecting surface (24) is located so that the light source (36) and the gap (44) lie at respective focal points of the ellipse. 6. An optical potentiometer as claimed in claim 4, wherein said reflector arrangement (22, 24) further includes at least one mirror (22) positioned relative to said reflecting surface (24) and to the light source (36) in such a way that at least one of the light source (36) and the gap (44) is disposed at a, or a respective, virtual focal point of the ellipse. 7. An optical potentiometer as claimed in claim 6, wherein the cross-section of said reflective surface (24) is arcuate in any plane perpendicular to the focal axis of the ellipse. 8. An optical potentiometer as claimed in claim 6 or 7, wherein the mirror (22) is planar or of conic section. 9. An optical potentiometer as claimed in any one of claims 4 to 8, wherein the ellipse is replaced by a section made up of a multiplicity of arcs approximating to an elliptical section, or a faceted design comprising an array of reflecting surface regions approximating to an elliptical section. 10. An optical potentiometer as claimed in any one of claims 3 to 9, wherein at least one of the form of the reflective surface (24) and the form of the mirror (22) is defined by a body (18) having an appropriately shaped surface or surfaces rendered reflective. 11. An optical potentiometer as claimed in claim 10, wherein the body (18) has a hole (20) therein which receives the rotary shaft (16) and which has an inclined surface defining the mirror (22). 12. An optical potentiometer as claimed in claim 10 or 11, wherein the body (18) is formed of a transparent material and has a surface through which light passes in use and which is shaped so as to permit light reflected off at least one internal reflective surface (22,24) in the body (18) to be concentrated into the gap (44). 13. An optical potentiometer as claimed in claim 12, wherein said surface through which light passes is planar. 14. An optical potentiometer as claimed in claim 12, wherein said surface through which light passes is non-planar. 15. An optical potentiometer comprising a pair of mutually spaced, electrically resistive tracks (40, 42) defining a gap (44), a photoconductive layer (46) exposed in the gap (44) between the tracks (40 and 42), a light source (36), and means (22, 24) for directing light from the light source (36) into the gap (44) between the tracks (40, 42), characterised in that the gap (44) is of serpentine shape, and in that the arrangement is such that, in use, the beam of light from the light source (36) is scanned along the direction of longitudinal extent of the serpentine gap (44). 16. An optical potentiometer as claimed in claim 15, wherein the serpentine gap (44) is defined between interdigitated fingers (40a, 42a) on the respective tracks (40, 42). 17. An optical potentiometer comprising a pair of mutually spaced electrically resistive tracks (40, 42) defining a gap (44), a photoconductive layer (46) exposed in the gap (44) between the tracks (40, 42), a light source (36), and means (22, 24) for directing light from the light source (36) to the gap (44) between the tracks (40, 42), the tracks (40, 42) and the directing means (22, 24) being relatively movable to vary the location at which the gap (44) between the tracks (40, 42) is illuminated by the light source (36) in use, characterised in that the photoconductive layer (46) is a photosensitive amorphous silicon layer. 18. An optical potentiometer as claimed in claim 17, wherein the photosensitive amorphous silicon layer (46) is a chemical vapour deposited layer. 19. An optical potentiometer comprising a pair of mutually spaced, electrically resistive tracks (40, 42) defining a gap (44), a photoconductive layer (46) exposed in the gap (44) between the tracks (40, 42), a light source (36), and means (22, 24) for directing light from the light source (36) to the gap (44) between the tracks (40, 42), the tracks (40, 42) and the directing means (22, 24) being relatively movable to vary the location at which the gap (44) between the tracks (40, 42) is illuminated by the light source (36) in use, characterised in that there is provided a respective ohmic contact layer (48) separating the photoconductive layer (46) from each track (40, 42). 20. An optical potentiometer as claimed in claim 19, wherein the photoconductor layer (36) is a photosensitive amorphous silicon layer. 21. An optical potentiometer as claimed in claim 19 or 20, wherein the ohmic contact layer (48) is defined by an n + interface having a thickness of 20-200 Λ.

Inventors: ALEXANDER ALFRED JOHN, ASHRAF MUFTI MOHMED, ALEXANDER, ALFRED, JOHN, ASHRAF, MUFTI, MOHMED
Applicants: ALEXANDER ALFRED JOHN, ASHRAF MUFTI MOHMED, PENNY & GILES ELECTRONIC COMPO, ALEXANDER, ALFRED, JOHN, ASHRAF, MUFTI, MOHMED, PENNY & GILES ELECTRONIC COMPONENTS LIMITED
Application year: 1995
Main classification: H01L31
IPC codes: H01L31