It is the aim of this invention to provide such an encoder, whose life can be expected to be virtually indefi¬ nite, and without significant deterioration.

According to the present invention there is provided a non-contacting digital encoder comprising a first component for transmitting a light beam and a second component with a photo-sensitive element overlying portions of spaced conductors, the components being relatively movable so that the light beam traverses the conductors and photo-sensitive element and coded information related to the position of the light beam is derived from the conductors by virtue of their electrical connection or non-connection through the illumi¬ nated conducting portion of the photo-sensitive element.

The motion may be linear or rotary and although with a linear device the beam will usually be derived from a light source that moves, while the second component remains stationary, that arrangement could be reversed. With a rotary device, it may be convenient to have both the light source and the second component stationary, while the movement of the beam is obtained by moving a reflective

device in the path of the beam from the source.

The digital output can be absolute or incremental. An absolute bit pattern will represent the actual position of the slider at any point along its travel while an incremental output can be used to increase or decrease an external counter when there is movement of the slider.

When no light falls on the photo-sensitive material it offers a relatively high resistance electrical circuit. But when light from the light source falls on to the photo- sensitive material, the electrical characteristics of the material change and a relatively low resistance path is generated. This change in property of the material can be used effectively to connect two conductors together electri¬ cally by the beam spanning a gap between them bridged by the photo-sensitive material. The electrical signal output pattern is determined and controlled by the inter-relation¬ ship of the photo-sensitive material and the conducting material on which it is overlaid.

The moving component may be manually operated or power driven, and in the latter case its position may be governed, as in known encoders, using the coded information of the output signal.

For a better understanding of the invention, some embodiments will now be described, by way of example, with reference to the accompanying drawing, in which:

Figure 1 is a diagrammatic side view of a linear digital encoder,

Figure 2 is a diagrammatic plan view of part of the

linear encoder of Figure 1,

Figure 3 is a diagrammatic axial section of a rotary digital encoder, and

Figure 4 is a diagrammatic plan view of part of the rotary encoder shown in Figure 3.

In Figure 1, a slider 1 can move linearly from left to right (and vice versa) along guide rods 2, only one of which is visible. The slider carries an optical system which can project a beam downwards from a source 3, and in this example the beam is flattened to illuminate a thin straight line transverse to the rods when impinging on a horizontal surface.

Extending along below the rod 2 is a substrate 4 with two parallel conductors 5 and 6. As shown in the simplified form of Figure 2, the conductor 5 has projections 7 extend¬ ing towards the opposite conductor 6. The pattern may be much more complicated than the one shown.

Overlying the conductor 6 and the tips of the projec¬ tions 7 is photo-sensitive material 8. In this example, the material covers an area of simple rectangular shape. However, it would be possible to form one of its longer sides, at least, with fingers extending out to overlie the main portions of the conductors 5 and/or 6, leaving the gaps between the fingers to expose the substrate 4. This may be alternative or additional to the arrangements of Figure 2, but for the remainder of this description it will mostly assumed that a conductor provides the irregularities or discontinuities and that the photo-sensitive material has a

simple profile.

Two alternative positions of illumination by the light beam from the source 3 are indicated by the transverse lines X and Y. External electrical connections (not shown) are made to the conductors 5 and 6.

Where the light beam illuminates the surface of the photo-sensitive material 8 a relatively low resistance electric circuit path is produced. Therefore, at position X there is effectively an electrical connection made between the conductors 5 and 6 via the projection 7 traversed by the beam. With the appropriate external electrical circuit, a voltage connected to one conductor will appear on the other. However, when the beam is at position Y, where there is a gap between two projections 7, no connection is made between the conductors 5 and 6. By designing and producing the conductors 5 and 6 and/or the material 8 with an appropriate pattern, the electrical connection between the two conduc¬ tors can be made and broken as required as the light source 3 moves along the length of the conductors over the surface of the material 8.

It will be understood that, if preferred, the light source could be held stationary and the substrate 4 moved, although this would be unusual.

Any number of conductors can be interconnected by this method, limited primarily by the accuracy of the projections

7 and the light beam from the light source 3. The accuracy of the conductors 5 and 6 and the photo-sensitive material are less important. A number of different codes can be

generated, including binary and grey absolute codes and a quadrature incremental code.

The same principles apply to the rotary encoder of Figures 3 and 4, where similar references plus 10 (i.e. 13 corresponds to 3) indicate corresponding parts. The conduc¬ tors 15 and 16 are basically annular and concentric, and the light source 13 is at the centre of them and also mounted on the substrate 14. The beam from this source is projected towards an angled mirror 19 at the centre of a co-axial rotary member 20, whence it is reflected to another angled mirror 21 at the periphery of the member 20, and then back towards the conductors 15 and 16. It impinges as a radial line. Thus there are no problems with electrical connection to a member which can rotate endlessly about its axis. However, where such endless rotation need not be provided, the light source 13 could be on the rotary member and substrate 14 fixed, or vice versa.

There are several suitable materials for the photo- sensitive layer including lead sulphide, cadmium sulphide and cadmium selenide.