The invention relates to optical electronic scale reading apparatus, used for example in determining the extent and direction of relative motion between scale and a readhead. In our co-pending International Application No O89/05440 we disclosed the illumination of a scale for generating a set of primary diffraction orders, and consequent light modulations at an analyser grating, together with a splitting means for generating a plurality of sets of primary orders in order to provide at a single analyser grating a plurality of such light modulations in a phase shifted relationship.

The present invention provides a simplified construction of such an apparatus. According to the present invention there is provided an opto-electronic scale reading apparatus comprising a scale and a readhead, the readhead comprising: an analyser grating; an illuminating means for illuminating the scale with a single beam of light, and generating a set of primary diffraction orders, thereby to produce interference fringes in the plane of the analyser grating and a light modulation upon relative movement of the scale and the readhead; splitting means for splitting said set of primary orders of diffraction into a plurality of sets, thereby providing at a single said analyser grating a plurality of light modulations in a phase-shifted relationship; characterised by an aperature provided substantially in the plane of said splitting means for limiting the light passing from the scale to the analyser grating.

The splitting means may comprise any means suitable for this purpose e.g. a prism, but preferably the splitting

means comprise an auxiliary grating. Lines defining the auxiliary grating may extend either parallel to, or at an angle to lines defining the analyser grating.

The illuminating means may be provided by a coherent beam of light incident directly upon the scale, or alternatively a beam of non-coherent light and index grating provided upbeam of the auxiliary grating.

The facia to light modulations are typically spatially indistinct at the analyser grating. However, the aperture enables separate detection of the various light modulations without the need for a foccussing lens.

Embodiments to the present invention will now be described, by way of example, and with reference to the accompanying drawings in which;

Fig. 1 shows an apparatus according to the first embodiment of the present invention. Fig. 2 shows a section on the line II-II in Fig. 2; Fig. 3 shows a perspective view of an apparatus according to a second embodiment of the present invention; and Fig. 4 shows a section on IV-IV in Fig. 3.

The apparatus of the two illustrated embodiments is described with reference to three mutually perpendicular directions X,Y, and Z. A scale 11 has reflective marks as defined by lines 11A spaced apart in the X direction, and in an XY plane. The readhead 10 is supported for movement relative to the scale 11 in the X direction, and is spaced from the scale 11 in the Z direction.

Referring now specifically to Figs. 1 and 2, a readhead 10 comprises a non-coherent light source S which projects a beam of non-coherent light onto a scale 11 via an index grating 12, and an aperture Al. The light source S and the index grating 12 cooperate to create a periodic light

pattern, which interacts with the scale 11 to produce a set of primary diffraction orders BO, Bl and B2. The primary diffraction orders BO, Bl and B2 interfere with each other to produce a set of interference fringes at an analyser grating 13, which upon relative movement of the scale 11 and the readhead 10 cause a light modulation. In the example shown however an auxiliary grating 14 is placed in the path of the primary orders of diffraction BO, Bl and B2. The lines of grating 14 extend parallel to the lines of the analyser grating 13, and the auxiliary grating 14 interacts with the set of primary orders B0, Bl and B2 to generate three sets of such primary orders; each set of primary orders corresponding to a secondary order of diffraction DO, DI and D2 produced by the auxiliary grating 14. The gratings 13, 14 may be provided on opposite sides of a glass plate 10A which may also have the grating 12 provided thereon. Typical values for the pitch of the gratings are 0.008mm for the gratings 12, 13, and 0.004mm for the grating 14. Generally, the pitch of grating 14 is selected to give a required separation of the split beams DO, DI and D2, in the plane of the transducers Tl, T2 and T3.

By adjusting various parameters of the apparatus (e.g. the incident angle of the beam upon the scale 11, the width of the aperatures Al and A2, the pitch of the gratings, and the separation of the gratings 13 and 14) each one set of primary orders can be made to produce a light modulation having a phase shift (PI, P2, P3) relative to the other light modulations. In the diagrammatic representation shown of this embodiment in Fig. 1 the three sets of primary orders are spatially distinct at the analyser grating 13. However, this is not an essential feature of the present invention, and by displacing the transducers Tl, T2, and T3, spatially separate light modulations corresponding to each of the sets of primary orders may be detected.

The outputs of the transducers Tl, T2, and T3 are typically an electrical signal whose amplitude is proportional to the intensity of light incident upon the transducer. These outputs may be sent to a quadrature circuit which produces two sinusoidally varying outputs having a quadrature relationship; such a circuit is shown in our copending patent application WO87/07943.

A second embodiment of the present invention will now be described "with reference to Figs. 3 and 4. A readhead 10 comprises a non-coherent light source S, which projects light onto a scale 11 via an index grating 12, and at an angle α to the normal of the scale. As described in the previous embodiment, the scale interacts with the periodic light pattern produced by the light source S and index grating 12 o, produce a set of primary diffraction orders B0, Bl, and B2 which interfere with each other at the analyser grating 13 to produce fringes, and a light modulation consequent to movement of the readhead 10 relative to the scale 11. The present invention differs from the first embodiment in that a single aperture A3 is provided which extends in the X direction. The part of the aperture in register with the index grating 12 is completely clear, whereas an auxiliary grating 14 is provided in the part of the aperature in register with the analyser grating 13. As with the previous embodiment, the auxiliary grating 14 generates a plurality of sets of primary orders, each one corresponding to a secondary order DO, DI and D2. The lines 14A of the auxiliary grating 14 extend substantially to the X direction, but are offset therefrom by an angle θ. Consequently, each one of the secondary orders DO, DI, and D2 is offset from the other secondary orders in the X direction. Thus, each one set of primary orders B0, Bl, and B2 is offset from each other set of primary orders in the X direction, and the light modulations corresponding to each set of primary

orders will occur in a phase-shifted relationship. The offset of the three orders is indicated schematically by the points of intersection Rl, R2, and R3. Each of the light modulations is seen by a transducer Tl, T2, T3, which produces an electrical signal corresponding to the intensity of light incident thereupon. These signals are processed as discussed in the first embodiment. Referring now to Fig. 4, it can be seen that the three sets of primary orders are not spatially distinct at the analyser grating 13. However, as discussed above, the offset of the transducers Tl, T2 and T3 from the readhead 10 in the Z direction enables a satisfactory separation of these orders.

One of the consequences of the grating 14 having lines 14A which extend substantially parallel to the X direction, is that the phases corresponding to the orders DI and D2 are each offset in the same direction with respect to the phase corresponding to the order DO by some amount Δ. This offet results in the Lisajous figure corresponding to the outputs of the quadrature circuit (discussed above) becoming elliptical. Perfect quadrature signals produce a circular Lisajous figure. This eliptical Lisajous figure is undesirable since interpolation of the quadrature outputs becomes inaccurate, and the apparatus is much more susceptible to misalignment of the readhead and scale. Thus, to compensate for this, certain design parameters (for example the angle θ of the lines on the auxiliary grating 14, the thickness of the glass plate, and the angle α at which light is incident from the index grating 12 onto the scale 11) must be changed. The following parameters have been found to provide suitable quadratures outputs for an angle θ of 41 minutes: plate thickness 0.95mm; angle α = 30° plate thickness 0.61mm; angle a = 23° plate thickness 0.03mm; angle a - 11°

The width of the apertures is to be chosen to enable individual detection of each of the phase-shifted light modulations by their respective transducer. The width of the aperture is thus chosen having regard to all other parameters of the apparatus.

In a modification of the present invention, a beam of coherent light is incident directly upon the scale, thus obviating the need for the index grating 12. In this modification, smaller apertures will usually be provided than for a corresponding beam of non-coherent light.

It has been found that for the purpose of processing the transducer outputs, the orders +1,0, and -l are sufficient. Further orders, i.e. +2, -2 and beyond may be used but are unnecessary in this case. The analyser grating maybe a phase grating such that the +2, -2 orders are avoided and further orders are so weak as to be negligible.