An optical electronic scale reading apparatus comprising a scale read by a read head to determine the relative displacement therebetween. The read head comprises reflective surfaces so arranged that light from a source is reflected therefrom and interacts with the scale at first, second and third positions. At the third position, a fringe field is developed which has the same pitch as the scale. A detector is provided to receive light modulations during relative movement of the scale and read head as the fringe field interacts with the scale at the third position. These modulations are counted to determine the relative displacement between the scale and read head.

BACKGROUND OF THE INVENTION 
This invention relates to apparatus for measuring displacement between a 
scale having a periodic structure and a read head. 
A known type of such apparatus comprises a plurality of gratings, one of 
which is defined by the periodic structure of the scale. An illumination 
means interacts with one or more of the gratings to produce an interaction 
product including a spatially periodic fringe field. Another of the 
gratings forms an analyser grating situated in a position co-planar with 
the fringe field and having a periodicity which is substantially the same 
as the periodicity of the fringe field, whereby the fringe field and the 
analyser grating co-operate to produce light modulations when, in 
operation, the scale and the read head are displaced one relative to the 
other. Means are provided for detecting the light modulations in order to 
measure the displacement. Examples of such apparatus are shown in U.S. 
Pat. No. 3,812,352 (MacGovern) and in Diffraction Gratings, M. C. Hutley, 
Academic Press, 1982, pages 293-304. 
For example, the known apparatus may include a source of light arranged for 
illumination of the periodic structure of the scale and interacting 
therewith to produce the interaction product including the fringe field, 
the analyser grating being provided in the read head. Alternatively, the 
apparatus may comprise a source of light arranged for illumination of the 
periodic structure of the scale and interacting therewith to produce a 
first interaction product, an index grating positioned in the light path 
of the first interaction product and interacting therewith to produce a 
second interaction product including said fringe field, both the index 
grating and said analyser grating being provided in the read head. 
It is expensive to provide gratings in the read head, and so it would be 
advantageous to avoid the use of a grating in the read head or to reduce 
the number of such gratings. 
SUMMARY OF THE INVENTION 
The present invention is characterised in that at least two of said 
gratings are constituted by said periodic structure of the scale; and in 
that the read head includes reflector means positioned in the light path 
of at least one of the interaction products to reflect said interaction 
product back onto the scale.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIGS. 1 to 3, a reflective scale 10 is defined by periodic 
light and dark marks 11 provided on an elongate scale member 12 orientated 
with reference to directions X,Y,Z. The marks 11 lie in the X-Y plane and 
effectively form a diffraction grating. A readhead 13 comprises a light 
source 14 for projecting a beam L of light on to the scale, a lens 15 for 
collimating the light, a prism 16 having flat reflective surfaces 17,18 
for reflecting the light between them and the scale, and a lens 19 for 
focussing the light finally reflected from the scale on to an 
opto-electronic transducer 20. The angle between the surfaces 17,18 is 
90.degree.. The readhead is so positioned that the surfaces 17,18 each lie 
at 45.degree. to the X-Y plane and each lie at 90.degree. to the X-Z 
plane. The surfaces 17,18 are therefore concave in a longitudinal plane 
normal to the scale. The scale 10 and the readhead 13 are movable one 
relative to the other in the direction X and, as will be explained, the 
optical interaction between the scale and the prism produces modulations 
of the light beam L, readable by the transducer 20. A count of the 
corresponding transducer output by a counter 21 constitutes a measure of 
said movement. Although the counter 21 is shown as part of the read head 
13, it may in practice be provided separately. 
More specifically, rays R1 produced by the lens 15 have a first interaction 
with the marks 11 of the scale at a location A whereby the marks become 
periodic light sources producing rays R2. The rays R2 are reflected by the 
surfaces 17,18 at regions B,C back on to the scale at a region D for a 
second interaction with the marks 11 thereby producing diffracted rays R3 
projected back into the prism. The surfaces 17,18 reflect the rays R3 at 
regions E,F back on to the scale for a third interaction with the marks 11 
at a location G where the diffracted rays form a fringe field FF 
interacting with the marks to produce rays R4. The grating formed by the 
scale is not blazed to favour any particular diffraction order, and so the 
fringe field FF has a pitch which is the same as the marks 11. During said 
relative movement the fringe field FF interacts with the marks 11 to 
modulate the rays R4 such that each modulation represents one cyclic 
displacement between the fringe field and one pitch of the marks. 
The unfolded view of FIG. 3 shows that the movement of the scale relative 
to the light beam L at the regions A,D,G occurs in directions X1, X2 and 
X3 respectively. It will be seen that the direction X2 is opposite to the 
directions X1, X3, because of the reversal produced by the arrangement of 
the reflective surfaces 17,18. Thus for every passage of a pitch of the 
marks relative to the beam L at the region A, four fringes of the field FF 
pass relative to each pitch of the marks 11 at the region G. Therefore the 
counter 21 produces four counts for every one pitch movement of the scale 
and in this way a four-fold interpolation of the scale movement is 
achieved. This is a significant advantage over prior art devices. 
It should be noted that the device described is relatively insensitive to 
the stand-off distance SO between the scale and the reflector. In such a 
triple interaction device, the condition for producing a fringe field with 
the required pitch is that the path length from the first interaction to 
the second should equal that from the second interaction to the third. 
Since both these path lengths involve a reflection by the reflector, they 
depend to the same extent on the stand-off distance, and the condition is 
satisfied automatically if the stand-off varies. 
Changing the stand-off distance, SO, between the prism 16 and the scale 10 
changes the phase of the transducer output, and two or more (preferably 
three) prisms at appropriately different distances SO may be used to 
establish phase quadrature signals for determining direction of scale 
reading and, if desired, for further interpolation. The outputs of a 
respective transducer 20 for each prism are taken to a circuit such as 
described in International Patent Publication No. WO87/07943 for this 
purpose. A similar effect is obtained by positioning a simple prism 16 so 
as to produce moire fringes at the region G, e.g. by a small tilt of the 
prism about an axis in the X,Y or the Z direction. A plurality of 
transducers 20 is arranged laterally across the scale so as to view 
different phases of the moire fringes, as explained in the book by Hutley 
cited above. 
In a second example (FIG. 4) the scale is in the form of a phase grating 
310 and diffraction takes place at each of the regions A,D,G. This 
generates quadrature modulation of the diffracted orders in the part of 
the beam, 3L, emanating from the region G, in the sense that the -1, 0 and 
+1 order have different phases of the light modulation. A lens 319 
focusses these diffracted orders on to three respective transducers 420, 
whose outputs are processed as described in WO87/07943. 
In a third example (FIG. 5) the readhead, 413, has a single reflective 
surface 417 being a concave two-dimensional mirror, preferably of 
elliptical shape. In this case the beam, 4L, interacts only twice with the 
scale 10, at the foci 4A, 4B of the ellipse. Thus, the mirror 417 focusses 
an optical image of the scale region 4A onto the region 4B. This optical 
image then acts as a fringe field as in the previous examples, to give 
light modulations as this fringe field moves in the opposite direction to 
the grating at region 4B. Desirably, the exact shape of the surface 417 is 
optimised by computer design techniques to adjust the elliptical form to 
give good focussing qualities over regions around 4A, 4B, rather than just 
at the exact focal points. Of course, the surface 417 need not be 
elliptical; it may be a general conic section or an aspheric surface to 
give foci at desired regions. Alternatively, it can be replaced by a lens 
in front of a plane mirror, or other catadioptric imaging system, suitably 
optimised to focus region 4A onto region 4B. 
In a fourth example (FIG. 6,7), there is provided a readhead 613 having 
three reflective surfaces in the form of a corner cube retroreflector 616. 
This arrangement is insensitive to yaw error, that is, error in the 
relative position of the readhead about an axis in the Z direction. 
Otherwise, however, it works in a similar manner to the example of FIGS. 1 
and 2. Because of the yaw insensitivity, it is also possible to arrange 
such a read head laterally across the scale, instead of longitudinally 
along it. A fifth example employing a read head 213 having a 
retroreflector 216 arranged in this manner is shown in FIG. 8. In this 
case, operation is as in the first example, but triangulation errors due 
to variations in the stand-off distance are substantially eliminated. 
In the examples of FIGS. 6-8, the corollary of the yaw insensitivity is 
that moire fringes cannot be produced by misaligning the read head. FIGS. 
9-12 therefore show modifications which enable phase information to be 
obtained for determining the direction of travel and for interpolation. 
Referring to FIGS. 9 to 11, the device shown is generally similar to the 
examples above, in that it has a scale 10 having periodic marks, over 
which a readhead 13 is movable in a longitudinal direction. The readhead 
13 includes a light source 14 (such as a light emitting diode), a corner 
cube retro-reflector 516, and three laterally spaced detectors 120, 220, 
320. The light from the source 14 is collected by a lens 15, which however 
does not produce a parallel collimated beam--the beam is still slightly 
divergent so that it can be considered as producing several slightly 
divergent rays as described below. In particular three such rays are 
described below, being the rays which end up at the detectors 120,220,320. 
FIG. 9 shows that the path of one such ray has a first leg L1 to an 
interaction with the scale 10 at a point A. It is then reflected along a 
leg L2, and retro-reflected by the retro-reflector 516 along a leg L3 to 
have a second interaction with the scale at a point D. The resulting 
fringes pass along a leg L4 and are further retro-reflected by the 
retro-reflector along a leg L5 to have a third interaction with the scale 
at a point G. The resulting modulated light passes along a leg L6 of the 
ray to the detector. In FIG. 11, the legs L1-L6 of the ray are shown by 
thicker lines, while internal reflections within the retro-reflector are 
shown by thinner lines. 
As described in the previous examples, the result of the reflection from 
point A to point D is to produce a fringe field at point G, having the 
same periodicity as the scale. The fringes of this field then interact 
further with the scale at point G (by a kind of "shuttering" effect) to 
produce light modulations which can be detected by the detectors 120, 220, 
320. 
Unlike the systems of the previous examples, the present embodiments of the 
invention provide some means for introducing different phase shifts into 
the rays which reach the detectors. More particularly, the different rays 
have interactions with the scale at different phase angles of the scale 
pitch. This alters the phasing of the resulting detected fringes. They can 
then be analysed to produce direction information, in a similar manner to 
the moire fringes produced by misaligning a prism or mirror reflector. 
In FIG. 9, this phase shift means comprises a simple parallel-sided glass 
plate mounted within the readhead 13. As can be seen from FIGS. 10 & 11, 
the glass plate 30 only extends partially across the width of the scale 
10. In particular, considering the ray 1 in FIG. 11, which is a 
`straight-through` ray from the source 14 to the centre detector 120, none 
of the legs L1 to L6 of this ray pass through the plate 30. Consequently, 
the plate introduces no phase shift at all to this ray. In contrast to 
this, ray 2 (which goes to the detector 220) passes through the plate 30 
on leg L2 (between the points A and D) and on leg L5 (between the points D 
and G). Transmission through the glass plate at an oblique angle produces 
an offset in the path of the ray, by refraction so that it interacts with 
the scale 10 slightly further along the scale than ray 1, at a different 
phase angle of the scale pitch. It also travels a greater distance than 
would otherwise be the case, but it will be seen that the increase in the 
path length is the same in legs L2 and L5, so that there is still equality 
of path length between the points of interaction A, D and the points D, G. 
This equality of path length is important for producing the desired 
fringes, as discussed above. 
A simple readhead could just make use of the two rays, ray 1 and ray 2 just 
described. The thickness of the plate 30 is chosen so that the detectors 
120, 220 produce quadrature sine and cosine outputs. These are fed to a 
counter and/or interpolator, in a well known manner. Because there are now 
the two separate quadrature outputs from the readhead, it is possible to 
derive the necessary direction information (as well as the count of the 
distance moved) from the single readhead, whereas our previous examples 
required two or more readheads to provide such information. The direction 
is given by the relative phases of the quadrature outputs, as is well 
known. 
FIG. 11 also shows a third ray, ray 3, which passes through the glass plate 
30 on legs L3 and L4, but not on legs L2 and L5. This ray ends up at the 
detector 320, to produce a third phase shifted output which can be 
combined with the outputs of detectors 120, 220. These three signals can 
be arranged to be in tripliture, and combined by circuits of the type 
disclosed in FIGS. 16 & 17 of our earlier International Patent 
Specification No. WO87/07943, in order to give quadrature outputs. 
Although FIGS. 9 to 11 show the length of the plate 30 to be sufficiently 
long that the ray 2 also passes through it on legs L1 and L6 of its path, 
as well as legs L2 and L5, this has no effect in practice. The place in 
which it is particularly desired to introduce an offset along the scale, 
so as to give a relative phase shift (of equal amounts) between the 
resulting fringe patterns, is between the points A, D and the points D, G. 
The system is relatively tolerant to misalignments of the plate 30 such 
that it is not parallel to the scale 10. The use of a retro-reflector 
means that the system is also relatively tolerant to yaw misalignments of 
the retro-reflector, since such a device has the property of always 
returning an incident ray in a parallel direction, whatever its alignment. 
It follows that the system described is very insensitive to misalignment 
of the readhead as a whole relative to the scale. 
In theory, the three outputs of the detectors 120, 220, or 320 shown in 
FIG. 11 cannot be combined in the manner disclosed in FIGS. 16 and 17 of 
specification No. WO87/07943, because the phase shifts of the detectors 
320 and 220 relative to the detector 120 are identical (because both ray 2 
and ray 3 have passed through the glass plate 30 on two legs of their 
paths). However, in practice we have found that useful results are 
nevertheless obtained. We presently attribute this to misalignments in the 
setup of the apparatus, such that the phase shifts of ray 2 and ray 3 are 
not exactly equal. 
FIG. 12 shows an improvement over FIG. 11 intended to give different phase 
shifts to each of the three rays, ray 1, ray 2, ray 3. Here, a glass plate 
is used which has a thickness t for most of its length, except for a 
central region 30A which has a double thickness of 2t. Such a double 
thickness can easily be produced by bonding together two pieces of glass 
of thickness t. The result is that ray 2, on its legs L2 and L5, passes 
through the plate at a region where it has the single thickness t; whereas 
ray 3 passes through the glass on its legs L3 and L4 where there is a 
double thickness 2t. Consequently, ray 3 has twice the offset and its 
fringes have twice the phase shift (relative to ray 1) of ray 2. Put 
another way, ray 1 will lead ray 2 by a phase shift which is equal to the 
phase shift by which ray 3 lags ray 2. Such a phase relationship of the 
three outputs of the detectors is ideal for feeding to the tripliture 
combination circuit described in FIGS. 16 and 17 of International 
Specification No. WO87/07943, because of the symmetry of the phase shifts. 
Of course, it is likely that the phase difference between ray 1 and ray 2 
will not be exactly equal to that between ray 2 and ray 3, but the 
arrangements shown in WO87/07943 are specifically designed to produce 
outputs which are accurately in quadrature (sine and cosine) from such 
imperfect inputs so as to allow the derivation of direction information 
and to allow interpolation of the signals. 
The same effect as shown in FIG. 12 can be produced in other ways. For 
example, the central region 30A of the plate 30 could have a different 
refractive index from the rest of the plate, instead of a different 
thickness, though this might be difficult to put into practice. 
The actual phase shift introduced is also affected by the angle of the 
plate 30 relative to the scale (if not parallel) and by the angle of 
incidence of the light ray onto the plate (which is governed by the 
relative orientations of the source and the detectors). Thus, to produce 
different phase shifts for ray 2 and ray 3, one could use a complex plate 
which was tilted at a different angle in the region of legs L3 and L4 than 
in the regions of legs L2 and L5. Alternatively, to alter the relative 
orientation of the source and detectors as between ray 2 and ray 3, one 
could arrange the detectors in a diagonal line, spaced apart 
longitudinally as well as laterally, as shown at 120, 220', 320' in FIG. 
10. A similar effect can be produced by orienting the readhead relative to 
the scale in the manner shown in FIG. 8. 
It will be seen that the noted disadvantages of FIGS. 6-8 are essentially 
overcome by having at least two detectors in the readhead, with the paths 
of rays to the respective detectors being so arranged as to introduce a 
phase shift of one resulting fringe pattern relative to the other. 
Naturally, the use of a glass plate such as the plate 30 is only one 
possible means for producing this end and many other such means of 
introducing a phase shift can be envisioned. 
FIG. 13 shows an example having only two interactions with the scale, 
instead of three. A light source 714 projects a divergent beam 7L through 
a transmissive scale 710, at 7A. Reflective surfaces 717,718 reflect the 
beam back onto the scale, to produce a fringe field FF at region 7G. 
This interacts again with the grating of the scale, to produce light 
modulations which can be detected by a detector 720. An imaging system 719 
images the plane containing the fringe field and the scale onto the 
detector. To ensure that the fringes have the same periodicity as the 
scale, the distance from the source 714 to the first interaction at 7A 
should equal the distance from 7A to 7G. This system produces three counts 
at the detector for every one pitch movement of the scale. 
Alternatively, in a modification, the device of FIG. 13 may include an 
optional lens 750, to collimate the beam 7L. In this case, the resulting 
fringe field is produced by a Fresnel-type mechanism (rather than the 
Fraunhofer-type mechanism as previously). Such a fringe field, however, is 
only formed in discrete Talbot planes, and so the scale 710 must be 
positioned co-planar with one of these Talbot planes. This system produces 
only two counts at the detector for each one pitch movement of the scale. 
FIG. 14 shows an example similar to FIG. 13, except that the scale 810 is 
reflective rather than transmissive. Here, the surfaces 817 and 818 are 
polarising beam splitters, in a glass block 816. Incident light from a 
source 814 passes through the beam splitter 817 and through a quarter wave 
plate 860, to have a first interaction with the scale 810 at a region 8A. 
The interaction product passes back through the quarter wave plate 860 to 
the surface 817. The two passes through the plate 860 cause a 90 degree 
rotation of the plane of polarisation, so that now the interaction product 
is reflected by the surface 817, and by the surface 818, to the scale 
region 8G. Here, a fringe field is developed, under the same conditions as 
discussed for FIG. 13, and is modulated by the scale. The resulting 
modulated light passes back through the surface 818. Again, the light has 
passed twice through a quarter wave plate 862, and so now it is of the 
correct polarisation to be transmitted to a detector 820. Of course, in 
practice the two quarter wave plates 860,862 may be amalgamated into a 
single plate on the underside of the glass block 816. As in FIG. 13, there 
may be an optional collimating lens 850. 
The devices of FIGS. 13 and 14 require a coherent source of light. This 
could be a point source (a pin-hole) or a line source. More practically, 
however, the source should be a laser diode. 
A reason for preferring a triple interaction over a double interaction, 
therefore, is that the light source can be diffuse and broad-band 
(non-coherent) thus saving the expense of a laser diode. Moreover, the 
double interaction only produces two or three counts at the transducer for 
every single pitch movement of the scale, instead of the four counts of 
the example shown in FIG. 1. 
It is also possible to have a simple version of a system such as in FIG. 9, 
with point or line source and detectors with only two interactions (at 
points A and D). However, as before it is preferred to have three 
interactions because this produces a significant multiplication of the 
effect detected and consequently a greater resolution. It also enables the 
use of extended source and detectors rather than point or line source and 
detectors. 
Double interaction systems such as FIGS. 13 and 14 can, if desired, have a 
single grating added to the readhead to make them into triple interaction 
systems. For example, the readhead may have a grating in the place of the 
optional lens 750 in FIG. 13, or between the position 7G and the detector 
system. The light then has two interactions with gratings formed by the 
scale, and one with the grating on the readhead. The interaction positions 
should be spaced apart to ensure that the fringe field has a pitch 
matching that of the final grating, in each case. Such a system only 
requires one grating on the readhead, compared with two in prior art 
triple interaction systems, but it will normally be preferred to use one 
of the other triple interaction systems described herein, which do not 
require any gratings on the readhead.