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United States Patent |
5,777,322
|
Holzapfel
|
July 7, 1998
|
Photo-electric position measuring system having a scanning grating with
transverse graduations
Abstract
A scanning grating is provided in a photo-electric position measuring
system and the scanning grating includes several grating areas. Each one
of these grating areas has a transverse grating with a graduation period.
The transverse gratings of the grating areas are disposed offset by TT/4
perpendicularly to the measuring direction, i.e., in the Y direction, with
respect to each other. The partial light beams diffracted at the scanning
grating in the +1st and -1st order lead to an intensity modulation at a
defined scanning distance Z1. An amplitude grating is provided at the
location of this intensity modulation, so that by means of downstream
disposed photo-detectors a scanning signal is generated by the +1st
diffraction order and a scanning signal, which is phase-shifted by
180.degree. with respect to the first, is generated by the -1st
diffraction order.
Inventors:
|
Holzapfel; Wolfgang (Obing, DE)
|
Assignee:
|
Dr. Johannes Heidenhain GmbH (Traunreut, DE)
|
Appl. No.:
|
616184 |
Filed:
|
March 15, 1996 |
Foreign Application Priority Data
| Mar 25, 1995[DE] | 195 11 068.4 |
Intern'l Class: |
G01B 011/00 |
Field of Search: |
250/237 G
33/707
356/356
|
References Cited
U.S. Patent Documents
4519709 | May., 1985 | Nelle | 356/373.
|
4602436 | Jul., 1986 | Ernst | 33/125.
|
4677293 | Jun., 1987 | Michel | 250/237.
|
4778273 | Oct., 1988 | Michel | 356/374.
|
5264915 | Nov., 1993 | Huber et al. | 356/356.
|
5428445 | Jun., 1995 | Holzapfel | 356/356.
|
5497226 | Mar., 1996 | Sullivan | 356/4.
|
5519492 | May., 1996 | Holzapfel et al. | 356/356.
|
Foreign Patent Documents |
0 163 362 | Jun., 1988 | EP.
| |
0 220 757 | Mar., 1990 | EP.
| |
Other References
Theory of Fresnel Images I. Plane Periodic Objects in Monochromatic Light,
"Journal of he Optical Society of America," vol. 33, No. 1, p. 373 (1963).
Talbot Array Illuminator with Multilevel Phase Gratings, "Applied Optics,"
vol. 32, No. 7, pp. 1109-1110 (1993).
|
Primary Examiner: Westin; Edward P.
Assistant Examiner: Pyo; Kevin
Attorney, Agent or Firm: Brinks Hofer Gilson & Lione
Claims
What is claimed is:
1. A photo-electric position measuring system comprising:
a source of light;
a first grating located downstream of the source of light;
a second grating displaceable with respect to the first grating in a
measuring direction, the second grating located downstream of the first
grating; and
a plurality of photo-detectors located downstream of the second grating;
wherein at least one of said first or second grating has at least a first
and a second transverse grating areas disposed adjacent to one another in
the measuring direction, these transverse grating areas having
substantially the same transverse graduation period, the first and second
transverse grating areas are phase shifted in a direction transverse to
the measuring direction with respect to each other by a phase shift which
deviates from 180.degree., and light from the light source is diffracted
at the first and second transverse grating areas so that a diffraction
order from the first transverse grating areas and a diffraction order from
the second transverse grating areas impinge on at least a common
photo-detector.
2. A photo-electric position measuring system according to claim 1 wherein
the first and second transverse grating areas have markings that are
parallel grating lines which form an angle of 0.degree. with respect to
the measuring direction.
3. A photo-electric position measuring system according to claim 1 wherein
the first and second transverse grating areas are embodied in the form of
either one of an amplitude grating or a phase grating.
4. A photo-electric position measuring system according to claim 3 wherein
the first and second transverse grating areas form a phase grating whose
parameters are selected in such a way that a zero-th transverse
diffraction order is suppressed.
5. A photo-electric position measuring system according to claim 1 wherein
respective partial light beams in a +1st and -1st diffraction order are
generated at the first and second transverse grating areas which are
disposed transversely offset, and a photo-detector for detecting the +1st
diffraction order and a further photo-detector for detecting the -1st
diffraction order are provided, wherein the two photo-detectors generate
signals that are phase-shifted with respect to each other.
6. A photo-electric position measuring system according to claim 1 wherein
the first and second transverse grating areas are provided on the first
grating, the first and second transverse grating areas having transverse
markings of the same graduation period wherein the first and second
transverse grating areas are offset from one another by 1/4 of the
graduation period in a direction perpendicular to the measuring direction.
7. A photo-electric position measuring system according to claim 1 wherein
the first and second transverse grating areas are provided on the first
grating, the first and second transverse grating areas having transverse
markings of the same graduation period the first and second transverse
grating areas are offset with respect to each other by approximately 2/3
of the graduation period in a direction perpendicular to the measuring
direction.
8. A photo-electric position measuring system according to claim 1 wherein
the first and second transverse grating areas have equal widths in the
measuring direction.
9. A photo-electric position measuring system according to claim 1 wherein
the first and second transverse grating areas have different widths in the
measuring direction and the ratio of the widths of the grating areas is
2:1.
10. A photo-electric position measuring system in according to claim 9
wherein the first and second transverse grating areas have the same
transverse graduation period and are offset from each other in a direction
perpendicular to the measuring direction by approximately 2/3 of the
transverse graduation period.
11. A photo-electric position measuring system according to claim 1 wherein
the first and second transverse grating areas have markings that have a
phase shift in a direction perpendicular to the measuring direction that
extends sinusoidally.
12. A photo-electric position measuring system according to claim 1 wherein
the first and second transverse grating areas have markings that have a
width in a direction perpendicular to the measuring direction and whose
width varies as a function of a distance in the measuring direction.
13. A photo-electric position measuring system according to claim 1 wherein
the transverse graduation period of the first and second transverse
grating areas varies continuously as a function of a path perpendicular to
the measurement direction.
14. A photo-electric position measuring system according to claim 1 wherein
a common photo-detector detects partial beams of a predetermined order of
magnitude which are variously sharply deflected at local division periods
of the first and second transverse grating areas.
15. A photo-electric position measuring system according to claim 1
comprising several transverse grating areas of different transverse
graduation periods wherein a plurality of first transverse grating areas
have the same first graduation period and form a first group, and a
plurality of second transverse grating areas have the same second
graduation period which is different from the first graduation period and
the plurality of second transverse grating areas form a second group
wherein the grating areas of the first group are disposed phase-shifted in
the measuring direction with respect to the second group by a fraction or
by a fraction plus a multiple of a longitudinal graduation period.
16. A photo-electric position measuring system according to claim 1
comprising, several transverse grating areas in the form of echelette
gratings with different Blaze angles, wherein the transverse grating areas
with the same Blaze angle respectively form a first group, and that the
transverse grating areas of the first group are phase-shifted in the
measuring direction with respect to transverse grating areas not having
the same Blaze angle by a fraction or by a fraction plus a multiple of a
longitudinal graduation period.
17. A photo-electric position measuring system according to claim 15
wherein the first and second groups are spaced apart in the measuring
direction by (m+1/4)TA, wherein m=0, 1, 2 . . .
18. A photo-electric position measuring system according to claim 15 or 16
wherein light from the light source is diffracted into first diffracted
light beams at the first transverse grating areas, which are transversely
offset with respect to each other and are of the same graduation period or
have the same Blaze angle, are directed to several photo-detectors, and
first scanning signals are generated which are phase-shifted with respect
to each other, and that further transverse grating areas, which are
transversely offset with respect to each other, are provided, whose
graduation periods and/or Blaze angle differ from that of the first
transverse grating areas, wherein by these further transverse grating
areas second diffracted light beams are directed to further
photo-detectors and at least a further scanning signal, which is
phase-shifted with respect to the first scanning signals, is generated.
19. A photo-electric position measuring system according to claim 1
comprising a scanning plate and a scale grid wherein light from the light
source, collimated by a lens impinges on the scanning plate having a
scanning grating, and then upon the scale grid located downstream of the
scanning plate to create partial light beams which are focused by a
further lens onto several photo-detectors.
20. A photo-electric position measuring system according to claim 1
comprising a scanning plate and a reflective scale grid wherein light from
the light source, collimated by a lens impinges on the scanning plate
having a scanning grating, and then upon the scale grid located downstream
of the scanning plate to create partial light beams which are again
diffracted through the scanning grating and are focused on several
photo-detectors by a lens.
21. A photo-electric position measuring system according to claim 19
wherein the scale grating and the scanning grating have the same
graduation period in the measuring direction.
22. A photo-electric position measuring system according to claim 21
wherein the scale grating is either one of an amplitude or a phase
grating.
23. A photo-electric position measuring system comprising:
a source of light;
a first grating located downstream of the source of light;
a second grating displaceable with respect to the first grating in a
measuring direction, the second grating located downstream of the first
grating; and
a plurality of photo-detectors located downstream of the second grating for
generating position-dependent signals that are phase shifted with respect
to each other;
wherein at least one of said first or second grating has a plurality of
transverse grating areas disposed adjacent to one another in the measuring
direction, the plurality of transverse grating areas having a graduation
period substantially the same, wherein the plurality of transverse areas
are phase shifted in a direction transverse to the measuring direction
with respect to each other by a phase shift which deviates from
180.degree.; and light from the light source is diffracted at the
plurality of transverse grating areas so that a plurality of diffraction
orders result wherein a first diffraction order creates a first
interference strip system and a second diffraction order creates a second
interference strip system, wherein both the first and second interference
strip systems are phase shifted with respect to each other.
24. A photo-electric position measuring system according to claim 23
wherein the first and second transverse grating areas have markings that
are parallel grating lines which form an angle of 0.degree. with respect
to the measuring direction.
25. A photo-electric position measuring system according to claim 23
wherein the first and second transverse grating areas are embodied in the
form of either one of an amplitude grating or a phase grating.
26. A photo-electric position measuring system according to claim 25
wherein the first and second transverse grating areas form a phase grating
whose parameters are selected in such a way that a zero-th transverse
diffraction order is suppressed.
27. A photo-electric position measuring system according to claim 23
wherein respective partial light beams in a +1st and -1st diffraction
order are generated at the first and second transverse grating areas which
are disposed transversely offset, and a photo-detector for detecting the
+1st diffraction order and a further photo-detector for detecting the -1st
diffraction order are provided, wherein the two photo-detectors generate
signals that are phase-shifted with respect to each other.
28. A photo-electric position measuring system according to claim 23
wherein the first and second transverse grating areas are provided on the
first grating, the first and second transverse grating areas having
transverse markings of the same graduation period wherein the first and
second transverse grating areas are offset from one another by 1/4 of the
graduation period in a direction perpendicular to the measuring direction.
29. A photo-electric position measuring system according to claim 23
wherein the first and second transverse grating areas are provided on the
first grating, the first and second transverse grating areas having
transverse markings of the same graduation period the first and second
transverse grating areas are offset with respect to each other by
approximately 2/3 of the graduation period in a direction perpendicular to
the measuring direction.
30. A photo-electric position measuring system according to claim 23
wherein the first and second transverse grating areas have equal widths in
the measuring direction.
31. A photo-electric position measuring system according to claim 23
wherein the first and second transverse grating areas have different
widths in the measuring direction and the ratio of the widths of the
grating areas is 2:1.
32. A photo-electric position measuring system in according to claim 31
wherein the first and second transverse grating areas have the same
transverse graduation period and are offset from each other in a direction
perpendicular to the measuring direction by approximately 2/3 of the
transverse graduation period.
33. A photo-electric position measuring system according to claim 23
wherein the first and second transverse grating areas have markings that
have a phase shift in a direction perpendicular to the measuring direction
that extends sinusoidally.
34. A photo-electric position measuring system according to claim 23
wherein the first and second transverse grating areas have markings that
have a width in a direction perpendicular to the measuring direction and
whose width varies as a function of a distance in the measuring direction.
35. A photo-electric position measuring system according to claim 23
wherein the transverse graduation period of the first and second
transverse grating areas varies continuously as a function of a path
perpendicular to the measurement direction.
36. A photo-electric position measuring system according to claim 35
wherein a common photo-detector detects partial beams of a predetermined
order of magnitude which are variously sharply deflected at local division
periods of the first and second transverse grating areas.
37. A photo-electric position measuring system according to claim 23
comprising several transverse grating areas of different transverse
graduation periods wherein a plurality of first transverse grating areas
have the same first graduation period and form a first group, and a
plurality of second transverse grating areas have the same second
graduation period which is different from the first graduation period and
the plurality of second transverse grating areas form a second group
wherein the grating areas of the first group are disposed phase-shifted in
the measuring direction with respect to the second group by a fraction or
by a fraction plus a multiple of a longitudinal graduation period.
38. A photo-electric position measuring system according to claim 23
comprising, several transverse grating areas in the form of echelette
gratings with different Blaze angles, wherein the transverse grating areas
with the same Blaze angle respectively form a first group, and that the
transverse grating areas of the first group are phase-shifted in the
measuring direction with respect to transverse grating areas not having
the same Blaze angle by a fraction or by a fraction plus a multiple of a
longitudinal graduation period.
39. A photo-electric position-measuring system according to claim 37
wherein the first and second groups are spaced apart in the measuring
direction by (m+1/4)TA, wherein m=0, 1, 2, . . .
40. A photo-electric position measuring system according to claim 38 and 39
wherein light from the light source is diffracted into first diffracted
light beams at the first transverse grating areas, which are transversely
offset with respect to each other and are of the same graduation period or
have the same Blaze angle, are directed to several photo-detectors, and
first scanning signals are generated which are phase-shifted with respect
to each other, and that further transverse grating areas, which are
transversely offset with respect to each other, are provided, whose
graduation periods and/or Blaze angle differ from that of the first
transverse grating areas, wherein by these further transverse grating
areas second diffracted light beams are directed to further
photo-detectors and at least a further scanning signal, which is
phase-shifted with respect to the first scanning signals, is generated.
41. A photo-electric position measuring system according to claim 23
comprising a scanning plate and a scale grid wherein light from the light
source, collimated by a lens impinges on the scanning plate having a
scanning grating, and then upon the scale grid located downstream of the
scanning plate to create partial light beams which are focused by a
further lens onto several photo-detectors.
42. A photo-electric position measuring system according to claim 23
comprising a scanning plate and a reflective scale grid wherein light from
the light source, collimated by a lens impinges on the scanning plate
having a scanning grating, and then upon the scale grid located downstream
of the scanning plate to create partial light beams which are again
diffracted through the scanning grating and are focused on several
photo-detectors by a lens.
43. A photo-electric position measuring system according to claim 41
wherein the scale grating and the scanning grating have the same
graduation period in the measuring direction.
44. A photo-electric position measuring system according to claim 43
wherein the scale grating is either one of an amplitude or a phase
grating.
45. A photo-electric position measuring system comprising:
a source of light;
a first grating located downstream of the source of light;
a second grating displaceable with respect to the first grating in a
measuring direction, the second grating located downstream of the first
grating;
wherein at least one of said first or second grating has at least a first
and second transverse grating areas disposed adjacent to one another in
the measuring direction, the first and second transverse grating areas
having a transverse graduation period substantially the same, the first
and second grating areas are phase shifted in a direction transverse to
the measuring direction with respect to each other by a phase shift which
deviates from 180.degree., and light from the light source is diffracted
at the first and second transverse grating areas so that a transverse
diffraction order of the first transverse grating areas and a transverse
diffraction order of the second transverse grating area impinge on a first
common photo-detector and wherein the grating has at least a further third
and fourth transverse grating areas disposed adjacent to one another in
the measuring direction, the third and fourth transverse grating areas
having substantially the same transverse graduation period, and are phase
shifted in a direction transverse to the measuring direction with respect
to each other by a phase shift which deviates from 180.degree., and light
from the light source is diffracted at the third and fourth transverse
grating areas so that a transverse diffraction order of the third
transverse grating areas and a transverse diffraction order of the fourth
transverse grating areas impinge on a further second common
photo-detector, thus the first and second common photo-detectors generate
two position-dependent signals that are phase shifted with respect to each
other.
46. A photo-electric position measuring system according to claim 1 wherein
a first diffraction order from the first transverse grating areas and a
first diffraction order from the second transverse grating areas creates a
first interference strip system and impinge on at least a common first
photo-detector and wherein a second diffraction order from the first
transverse grating areas and a second diffraction order from the second
transverse grating areas creates a second interference strip system and
impinge on at least a common second photo-detector, so that the first and
second photo-detectors generate two position-dependent signals that are
phase shifted with respect to each other.
47. A photo-electric position measuring system according to claim 45
wherein the transverse grating period of the first and second transverse
grating areas varies from the transverse grating period of the third and
fourth transverse grating areas.
Description
FIELD OF THE INVENTION
The present invention relates to a photo-electric position measuring system
wherein the light from a light source is modulated as a function of the
position of several gratings which are displaceable with respect to each
other, in that light beams diffracted at the gratings interfere with each
other, and several photo-detectors are provided to form electrical signals
which are phase-shifted with respect to each other.
European Patent Publication No. EP 0 163 362 B1 discloses a position
measuring system of this type. A reflecting scale grating is displaceable
with respect to a scanning grating. The scanning grating is a phase
grating with a defined relationship between the strip widths and the
groove widths in order to generate three electrical signals which are
phase-shifted by 120.degree. with respect to each other. A group of
diffracted rays of the same direction is focused on each of the three
detectors. In connection with these groups of diffracting rays of the same
direction, reference is also made to so-called resultant diffraction
orders. The diffraction rays of the n-th resultant diffraction order are
the group of rays which exit the total system of the two gratings
directionally in a way as if, aside from the reflection at the scale, they
had been deflected at only one grating in the n-th diffraction order.
U.S. Pat. No. 4,677,293 discloses a further position measuring system of
this type. The area of a reference marker of a scale consists of a
transverse graduation which is scanned by means of a diaphragm structure,
i.e., amplitude graduation of the scanning plate. This transverse
graduation consists of several strip-shaped diffracting elements which are
disposed next to each other in the measuring direction. The diffracting
elements are transverse gratings whose grating strips extend parallel with
the measuring direction. The individual diffracting elements differ with
respect to their transverse graduation periods and therefore deflect an
impinging light beam in different directions. If this transverse
graduation is illuminated through the gaps of the scanning plate,
deflected light beams are generated whose angle of deflection is a
function of the transverse graduation period and thus of the illuminated
transverse grating area, from which the scale position is derived.
Differently deflected light beams are focused by a lens on different
photo-detectors in the focal plane of the lens.
European Patent Publication No. EP 0 220 757 B1 discloses a position
measuring system in which the scale also has a transverse graduation. The
graduation consists of reflecting areas and areas with a transverse
graduation disposed alternatingly in the measuring direction. This
transverse graduation is a phase grating whose grating parameter has been
selected such that the 0-th diffraction order being generated is cancelled
and the further diffraction orders do not impinge on the photo-detector.
Thus the areas with the transverse graduations are considered by the
photo-detectors to be non-reflecting areas.
Cross gratings and chessboard gratings are furthermore known as scales for
two measuring directions. The grating lines of the position measuring
system in accordance with U.S. Pat. No. 5,264,915, for example, extend
diagonally with respect to the two measuring directions so that impinging
light is diffracted in two directions. These gratings are not designed for
generating scanning signals which are phase-shifted with respect to each
other since the transverse grating areas which are located next to each
other have a transverse phase shift of 0.degree. or of 180.degree..
SUMMARY OF THE INVENTION
It is an object of the present invention to create a photo-electric
position measuring system which is simply designed and can be
cost-effectively produced.
An advantage of the position measuring system in accordance with the
present invention lies in that position-dependent scanning signals with
high levels and limited frequencies can be generated by means of (quasi)
single field scanning, which are indifferent to soiling and/or graduation
errors of the scale. In addition, relatively large production tolerances
of the phase graduation are permissible, because of which cost-effective
production is made possible. A further advantage is that scanning signals
which are phase-shifted with respect to each other can be generated in a
simple manner.
The present invention itself, together with further objects and attendant
advantages, will best be understood by reference to the following detailed
description, taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a front view of a scanning grating in accordance with a preferred
embodiment of the present invention.
FIG. 2 is a cross-sectional view of the scanning grating shown in FIG. 1
taken along line II--II and illustrates the resultant beam path generated.
FIG. 3 is a cross-sectional schematic of a position measuring system with
the scanning grating shown in FIGS. 1 and 2.
FIG. 4 is front view of a scanning grating according to a second preferred
embodiment of the present invention.
FIG. 5 is a front view of a scanning grating according to a third preferred
embodiment of the present invention.
FIG. 6 is a front view of a scanning grating according to a fourth
embodiment of the present invention.
FIG. 7 is a front view of a scanning grating according to a fifth
embodiment of the present invention for filtering harmonic waves.
FIG. 8 is a front view of a scanning grating according to a sixth
embodiment of the present invention for generating four scanning signals.
FIG. 9 is a cross-sectional schematic of a position measuring system with
the scanning grating shown in FIG. 8.
FIG. 10 illustrates a position measuring system with the scanning grating
shown in FIG. 8 in a spatial representation.
FIG. 11 is a front view of a scanning grating according to a seventh
embodiment of the present invention.
FIG. 12 is a front view of a scanning grating according to an eighth
preferred embodiment of the present invention.
FIG. 13 is a front view of a scanning grating according to a ninth
preferred embodiment of the present invention for the generation of three
signals which are phase-shifted by 120.degree. with respect to each other.
FIG. 14 illustrates an arrangement of the photo-detectors that can be used
in a position measuring system with the scanning grating shown in FIG. 13.
FIG. 15 illustrates a position measuring system with the scanning grating
shown in FIG. 13 in a spatial representation.
FIG. 16 illustrates a position measuring system according to another
preferred embodiment of the present invention.
FIG. 17 is a front view of a scanning grating shown in the position
measuring system of FIG. 16 according to a tenth preferred embodiment.
FIG. 18 is a cross-sectional view of the scanning grating shown in FIG. 17
taken along line I--I.
FIG. 19 is a cross-sectional view of the scanning grating shown in FIG. 17
taken along line II--II.
FIG. 20 is a cross-sectional view of a position measuring system according
to the prior art.
DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS
FIG. 20 is a cross-sectional view of a position measuring system according
to the prior art. The functional principle of the position measuring
system according to the present invention will be more easily understood
if the functional principle of a known position measuring system such as
that shown in FIG. 20 is first examined and briefly described. The
position measuring system includes a light source 100, a collimating lens
101, phase grating 102, amplitude grating 103, and photodetector 105. The
light from light source 100 is collimated by collimating lens 101 and
impinges on phase grating 102. Phase grating 102 has a graduation period
TA. The phase grating 102 has approximately equal strip and groove widths
and a phase deviation of 90.degree.. In this case the phase deviation of
90.degree. means that the strip height, taking the diffraction index into
consideration, is such that immediately after passing through the phase
grating 102, i.e. in the near field, a collimated light beam has a wave
front with a local phase shift of 90.degree. (.lambda./4) of the strip
areas with respect to the gap areas. By means of the effect of this phase
grating 102, interference strips 104 of the period TA are generated at
distances Z1 where Z1=(n+1/2)TA.sup.2 /.lambda. where n=0, 1, 2 . . . ;
.lambda.=light wave length, TA.sup.2 /.lambda.=talbot distance. A further
grating 103 with an amplitude graduation of the graduation period TM where
TM=TA is located at one of these distances Z1. Depending on the position
of the two gratings 102, 103 with respect to each other in the measuring
direction X, and of the grating 103 with respect to the reference strips
104, the transmitted light output is different and is detected by a
photo-detector 105. With a displacement of the two gratings 102 and 103
with respect to each other in the measuring direction X, the
photo-detector 105 provides a periodic scanning signal.
In the present invention, the first grating 102 is embodied as a scanning
grating with a special transverse graduation to generate several
position-dependent scanning signals which are phase-shifted with respect
to each other. FIG. 1 is a front view of a scanning grating 1 according to
a first preferred embodiment of the present invention. The scanning
grating 1 consists of a periodic joining in the measuring direction X of
transversely structured first and second grating areas 2 and 3
respectively. Viewed in the measuring direction X, these grating areas 2
and 3 are at least of even width. The combined width of a first and a
second grating area 2, 3 forms the graduation period TA and is identical
with the graduation period TM of the scale grating 4 which is shown in
FIG. 3 and will be described hereinafter.
The first and second grating areas 2, 3 consist of transverse gratings with
the same transverse graduation period TT, viewed in the Y direction. These
grating strips or markings of the transverse grating areas 2, 3 are
displaced with respect to each other in the Y direction by a quarter of
the transverse graduation period TT. Both grating areas 2, 3 are
furthermore embodied as phase gratings whose strips and gaps are of such
size that preferably the even-numbered transverse diffraction orders
(0-th, .+-.2, .+-.4, . . . ) are suppressed. For this purpose the
transverse strip and gap elements have approximately the same width (in
the Y direction) and the strips have a phase height of 180.degree.
(.lambda./2). If a collimated light beam impinges on this scanning grating
1, each transversely structured grating area 2, 3 essentially splits the
incident light beam into a +1st and a -1st transverse diffraction order.
Because of the uniform transverse graduation period TT of both grating
areas 2, 3, like transverse diffraction orders of both grating areas 2, 3
have the same deflection angle in the Y direction. Two partial light beams
which are phase-shifted with respect to each other in the near field, i.e.
directly at the scanning grating 1, are created in each diffraction order
by the offset of the grating strips of both grating areas 2, 3 in the Y
direction. This phase shift of the partial light beams is shown in FIG. 2.
FIG. 2 is a cross-sectional view of the scanning grating shown in FIG. 1
taken along line II--II and illustrates the resultant beam path generated.
Grating 2 is shown in solid line and grating 3 is shown is dashed line. A
light beam impinging on the first grating area 2 is split into the +1st
and -1st diffraction order, the partial light beams being generated are
shown in solid lines. A light beam impinging on the second grating area 3
is also split in the +1st and -1st diffraction orders, the partial light
beams are shown in dashed lines. The phase shift of the partial light
beams of both areas 2, 3 in the +1st transverse diffraction order is
+90.degree. (.lambda./4), and in the -1st transverse diffraction order it
is -90.degree. (-.lambda./4). These local phase shifts should be compared
with the local phase shift of the prior art position measuring system
shown in FIG. 20. The local effect of the scanning grating 1 according to
the present invention is therefore similar to that of a conventional phase
scanning graduation, if only one of the two respective transverse
diffraction orders (+1st or 31 1st) is considered individually and the
transverse deflection of the partial light beams is disregarded. With a
phase shift of +90.degree. of the area 2 with respect to the area 3, the
area 3 furthermore corresponds to the strip of a conventional phase
scanning graduation. In contrast thereto, with a phase shift of
-90.degree. the area 2 corresponds to a gap of a conventional phase
scanning graduation. Thus the effect of the graduation according to the
present invention in +1st and -1st transverse diffraction orders are those
of a conventional scanning graduation, but displaced by half a graduation
period (180.degree.).
FIG. 3 is a cross-sectional schematic of a position measuring system with
the scanning grating shown in FIGS. 1 and 2. The position measuring system
includes a light source 8, a lens 9, a scanning grating 1 on a scanning
plate 19, a grating 4, a lens 5 and detectors 6 and 7. If such a scanning
grating 1 is illuminated with collimated light, two interference strip
systems are essentially created approximately at the known distances Z1
where Z1=(n+1/2)TA.sup.2 /.lambda., one starting from the +1st transverse
diffraction order and one starting from the -1st transverse diffraction
order. However, for the reasons pointed out above, both interference strip
systems are phase shifted by 180.degree. with respect to each other so
that the interference strip maxima of the one interference strip system
coincide with the minima of the other. If scale grating 4, in the form of
an amplitude graduation, is located at one of the distances Z1, the
intensity of the transmitted light of the +1st and of the -1st transverse
diffraction order is modulated in phase opposition in the course of a
relative displacement of both gratings 1 and 4. The +1st and -1st
transverse diffraction orders are guided by lens 5 to separate
photo-detectors 6, 7 so that the latter provide signals which are
correspondingly phase-shifted with respect to each other.
More particularly, the light from light source 8, preferably an LED or a
semiconductor laser diode, is collimated by first lens 9 and reaches the
scanning grating 1. The transmitted light beams impinge on the scale
grating 4 located at a distance Z1 where Z1=(n+1/2)TA.sup.2 /.lambda.,
preferably with n=0. Because a spatially extended light source 8 is
preferably used, somewhat shorter distances are preferably selected
because the contrast of the interference strip systems, and thus the
degree of modulation of the scanning signals, falls off toward greater
scanning distances because of the divergence of the light source 8 and the
collimating lens 9. With noticeable transverse deflection angles, the
optimal scanning distance is also reduced because of small transverse
graduation periods TT of the scanning grating 1.
The scale grating 4 is an amplitude grating with transparent and
non-transparent strips arranged sequentially in the measuring direction X.
The side edges of the strips extend in the Y direction (see FIG. 10 for an
exploded view of a portion of scale grating 4). The light beams
transmitted from the scale grating 4 are focused by means of lens 5 on the
photo-detectors 6, 7 which are arranged spaced apart from each other in
the Y direction, i.e. perpendicularly to the measuring direction X. The
+1st transverse diffraction orders of the two grating areas 2, 3 of the
scanning grating 1 impinge on the photo-detector 6, and the -1st
transverse diffraction orders of the two grating areas 2, 3 of the
scanning grating 1 impinge on the photo-detector 7. Thus the
photo-detectors 6, 7 provide two signals which are phase-shifted by
180.degree. with respect to each other. An image, which as a rule is very
small, of the light source 8 is generated on the photo-detectors 6, 7
through the effect of the lenses 5, 9. Thus, in spite of large scanning
areas on the scale grating 4, it is possible to utilize very small, and
therefore fast, photo-detectors 6, 7. The two scanning signals created in
this way are derived from a common scanning area of the scanning grating 1
and of the scale grating 4, so that it is possible by means of the
invention to gain the advantages of single field scanning.
In the example described so far, each graduation period TA of the scanning
grating 1 has two grating areas 2 and 3 in the measuring direction X,
which have a transverse phase shift deviating from 0.degree. and
180.degree.. However, in accordance with the invention every graduation
period TA can be divided into several grating areas with respectively
arbitrary phase shifts. It is important in this connection that for
generating the scanning signals which are phase-shifted with respect to
each other, several grating areas are involved which have transverse phase
shifts deviating from 0.degree. and 180.degree..
FIG. 4 is a front view of a scanning grating according to a second
preferred embodiment of the present invention. The scanning grating 10 has
four areas 11, 12, 13, 14 of approximately the same width per graduation
period TA. Each area 11, 12, 13, 14 has a transverse graduation with
grating strips or markings of the same graduation period TT and extending
in the measuring direction X. The grating strips of the second area 12 are
disposed phase-shifted by 90.degree. with respect to the grating strips of
the first area 11, the grating strips of the third area 13 are again
phase-shifted by 90.degree. with respect to the grating strips of the
second area 12, and the grating strips of the fourth area 14 are
equi-phased with the grating strips of the second area 12. This results in
phase positions of 0.degree., 90.degree., 180.degree. and 90.degree..
If such a scanning grating 10 is employed in a position measuring system in
accordance with FIG. 3, the optimal scanning distance is approximately Z1
where Z1=(n+1/4)TA.sup.2 /.lambda., in which two signals, phase-shifted by
180.degree. with respect to each other, of the +1st and -1st transverse
diffraction order are generated by the photo-detectors 6, 7. This reduced
scanning distance Z1 is particularly advantageous if the modulation degree
in a larger scanning distance Z1 would be reduced too much because of the
divergence of the illuminating unit of the light source 8 and lens 9.
For defined applications it may also be advantageous to obtain a larger
scanning distance Z1, for example for reducing the harmonic wave content
of the scanning signals by means of the increased effect of the divergence
of the illuminating unit 8, 9. FIG. 5 is a front view of a scanning
grating according to a third preferred embodiment of the present
invention. The scanning grating 20 is suitable for reducing the harmonic
wave content of the scanning signals. The scanning grating 20 again
consists of a grating with a graduation period TA in the measuring
direction X having four transverse grating areas 21 to 24. The markings or
grating strips of the transverse grating areas 21 to 24 are arranged
phase-shifted with respect to each other in such a way that the phase
positions of 0.degree., 90.degree., 0.degree. and -90.degree. result. The
optimal scanning distance Z1 is equal to (n+3/4)TA.sup.2 /.lambda. in this
case.
FIG. 6 is a front view of a scanning grating according to a fourth
preferred embodiment of the present invention. The scanning grating 30 has
a continuous course of the phase shift beyond a longitudinal graduation
period TA, i.e. in the measuring direction X. Defined properties of the
position measuring system can be optimized by this continuous course of
the markings of both transverse grating areas 31 and 32. The harmonic wave
content of the scanning signals can be reduced in this way, or the optimal
scanning distance can be set to a predetermined value Z1 where
Z1=(n+C)TA.sup.2 /.lambda., where C is any value. In particular, with the
sinusoidal course selected in FIG. 6 it is possible to greatly reduce the
third harmonic. In this case the amplitude a of the transverse strip
displacement is the result of solving following equation:
a/TT=X3/(4.pi.sin(3.pi..multidot.Z1.multidot..lambda./TA.sup.2)),
wherein X3=zero point of the Bessel function J3(X3)=0 and Delta
Y=a.multidot.sin(2.pi..lambda./TA) local transverse strip displacement
where Delta Y is the instantaneous amplitude as a function of the
instantaneous value X and the symbol .multidot. represents the
multiplication operation.
A scanning distance Z1 where Z1=(n+1/2)TA.sup.2 /.lambda. is particularly
advantageous in this case since the fundamental wave assumes a large
value. On the other hand a maximal modulation degree of the scanning
signals is obtained at a given scanning distance Z1 for
a/TT=X1max/(4.pi.sin(n.multidot.Z1.multidot..lambda./TA.sup.2)) with the
maximum X1max of the Bessel function J1. In this case the expansion of the
light source must be taken into consideration and results in somewhat
smaller scanning distances Z1.
FIG. 7 is a front view of a scanning grating according to a fifth preferred
embodiment of the present invention for filtering harmonic waves. The
scanning grating shown in FIG. 7 has transverse markings, or grating
strips of different widths, b, along the measuring direction X. Thus the
diffraction efficiency in the individual diffraction orders .+-.1, .+-.2,
. . . of transverse diffraction order depends on the location X along the
measuring direction X. These transverse diffraction orders considered in
the near field correspond to the effect of a combination of conventional
phase and amplitude graduations, since both the phase and the amplitude of
the partial light beams are modulated in the near field. The width b in
the Y direction of the individual markings varies and if it is selected,
for example in accordance with the equation:
##EQU1##
the scanning signals are free of harmonics at approximately the following
distance Z1 where Z1=(n+1/2)TA.sup.2 /.lambda..
This scanning grating 40, consisting of two grating areas 41, 42
phase-shifted by TT/4 with respect to each other, wherein the width b(x)
of the markings, strip or gap, varies in accordance with the above
equation.
FIG. 8 is a front view of a scanning grating according to a sixth preferred
embodiment of the present invention for generating four scanning signals.
The scanning plate 59 has two groups of scanning gratings 51, 52, which
are arranged interleaved with each other in the measuring direction X.
Each group consists of three scanning gratings of the same kind. The
scanning gratings 51 or 52 of one group are arranged offset from each
other by a multiple of the longitudinal graduation period TA. Each
scanning grating 51, 52 in turn consists of three longitudinal graduation
periods TA. The scanning gratings 51, 52 of the two groups differ by the
transverse graduation periods TT1 and TT2, which have values preferably of
about 5 .mu.m and about 7 .mu.m respectively, for example. The scanning
gratings 52 of the second group are disposed spaced apart in the measuring
direction X by (m+1/4)TA with respect to the scanning gratings 51 of the
first group, wherein m=1, 2 . . . By means of this geometric offset, the
intensity modulations of the .+-.1st transverse diffraction order of the
scanning gratings 52 of the second group are phase-shifted by respectively
90.degree. with respect to those of the first group. In this way four
scanning signals phase-shifted by respectively 90.degree. with respect to
each other are obtained by separate detection of respectively the +1st and
-1st diffraction orders of both groups. It is of course possible for each
group to consist of more than three scanning gratings 51, 52, and each
scanning grating 51, 52 of more than three graduation periods TA.
FIG. 9 is a cross-sectional schematic of a position measuring system with
the scanning grating shown in FIG. 8. The position measuring system
includes a light source 8, a lens 9, a scanning plate 59 with scanning
gratings 51 and 52 disposed thereon, a scale plate 4, a lens 5 and
detectors 6, 7, 61, and 71. The light from light source 8 is collimated by
the lens 9 and reaches the scanning plate 59. The partial light beams of
the .+-.1st diffraction order of the scanning gratings 51, 52 of both
groups impinge on the scale grating 4 at the known scanning distance Z1,
and the transmitted partial light beams are focused on four
photo-detectors 6, 7 and 61, 71 by the further lens 5. The four
photo-detectors 6, 7, 61, 71 are disposed above each other in the Y
direction. The light beams are deflected by the scanning grating 51 in a
different way from that of the scanning grating 52 by means of the
different transverse graduation periods TT1 and TT2 and are in this way
focused by the second lens on different photo-detectors 6, 7; 61, 71. The
light beams which are deflected by the scanning gratings 51 into the +1st
diffraction order impinge on the photo-detector 61, those deflected by the
scanning gratings 51 into the -1st diffraction order impinge on the
photo-detector 71. The light beams which are deflected by the scanning
gratings 52 into the +1st diffraction order impinge on the photo-detector
6, those deflected by the scanning gratings 52 into the -1st diffraction
order impinge on the photo-detector 7. Each of the scanning gratings 51,
52 can be embodied in accordance with the already described embodiments of
FIGS. 1 to 7 or in a combination of these.
FIG. 10 illustrates a position measuring system with the scanning grating
shown in FIG. 8 in a spatial representation. So far the two position
measuring systems described operate in accordance with the transmitted
light method. However, the present invention can also be realized with the
so-called incident light method as shown in FIG. 10. The result is a
particularly simple structure with only one lens 9 which also takes over
the function of lens 5. The scanning plate 59 is embodied in accordance
with FIG. 8. Since the scale grating 4 is made to be reflecting, light
passes twice through the scanning plate 59 so that the partial light beams
deflected in the +1st and -1st transverse diffraction order during the
first passage are again transversely deflected in the +1st and -1st
diffraction order in the second passage and exit in the resultant +2nd and
-2nd transverse diffraction order (relative to the direction). The four
photo-detectors 6, 7, 61, 71 are disposed in or near the focal plane of
the lens 9 so that they detect these .+-.2nd resultant transverse
diffraction orders of the two groups of scanning gratings 51, 52.
During the first passage through the scanning gratings 51, 52 the partial
light beams are deflected in the Y direction and therefore again reach the
scanning gratings 51, 52 offset in the Y direction following reflection at
the scale grating 4. The field limits of the individual scanning gratings
51, 52 trim the partial light beam during the second passage. This shading
is different for both scanning gratings 51, 52 at a given scanning
distance Z1 because of the different transverse graduation periods TT1,
TT2. This shading effect can be compensated by appropriately different
dimensions of the scanning gratings 51, 52 in the Y direction.
The partial light beams exiting during the second passage through the
scanning plate 59 in the .+-.2nd resultant transverse diffraction order
are superimpositions of different light beams which are differently
transversely deflected in the two passages. Thus, a partial light beam
deflected in the first passage in +3rd and in the second passage in -1st
transverse diffraction order exits in +2nd resultant transverse
diffraction order and reaches the photo-detector 6 or 61. Since this
partial light beam travels a path which is different in comparison to the
partial light beam deflected twice in +1st transverse diffraction order,
it is possible with a coherent superimposition of both partial light beams
to generate a scanning signal which is strongly dependent upon the
scanning distance Z1. Coherent superimposition should therefore be
avoided, for example by using a chronologically and/or spatially
incoherent light source, such as an LED or a longitudinal or transverse
multi-mode laser, in particular a semiconductor laser diode.
Another possibility for avoiding coherent superimposition consists in
embodying the grating areas 81, 82 (see FIG. 11) of a scanning grating 80
in the form of a so-called chirped grating, whose transverse graduation
period TT continuously changes by a small amount as a function of the path
in the Y direction. The impinging light beams are differently deflected in
the Y direction because of this continuous variation of the local
transverse graduation period TT(Y). These differently strongly deflected
partial light beams are directed on a common photo-detector 6 or 7. The
path length differences between the superimposed partial light beams which
impinge on one of the photo-detectors 6, 7 are therefore
position-dependent in the Y direction and are averaged, so that the
coherent superimposition is destroyed. This embodiment is particularly
advantageous when using a laser light source.
A particular advantage of the position measuring system represented in FIG.
10 is the position of the so-called neutral point of rotation. The neutral
point of rotation is defined as the point around which the scanning device
6 to 9, 61, 71 or the scale 4 can be tilted without the scanning signals
being phase-shifted with respect to their set position, so that the
position measuring value determined by an electronic evaluation device
remains constant. The neutral point of rotation of the position measuring
system in accordance with FIG. 10 is located in the plane of the scale
grating 4. Waviness of the scale surface, in particular in the measuring
direction X, which corresponds to local tilting of the scale grating 4
around an axis of rotation located in the plane of the scale grating 4,
therefore does not affect the determined position measuring value, which
leads to considerable increases in the accuracy of the measuring system.
The reason for this is that during the second passage through the scanning
grating 59 the light beams are deflected in the Y direction independently
of the position in the X direction. The intensity of the passing partial
light beams is independent of the place where they impinge in the
measuring direction X. For this purpose the photo-detectors 6, 7, 61, 71
should detect all longitudinal diffraction orders, at least all 0th and
.+-.1st, and possibly also the .+-.2nd diffraction orders in addition.
The scanning plate 59 in accordance with FIG. 8 can be modified in such a
way that a purely single field scanning is made possible. Such a scanning
plate 57 is represented in FIG. 12. In contrast to FIG. 8, the grating
areas 53, 54, offset from each other by 1/4 of the transverse graduation
periods TT1 of the scanning grating 51 of the transverse graduation period
TT1 are not disposed directly next to each other in the measuring
direction X, instead one respective grating area 55, 56 of the further
scanning grid 52 with the transverse graduation period TT2 is disposed
between them. Each graduation period TA of the scanning grating 50 created
in this manner consists of four equally wide grating areas 53 to 56. The
respectively first and third grating areas 53, 54 have the same transverse
graduation period TT1 of, for example about 5 .mu.m, wherein the markings
(grating strips) of the grating areas 53, 54 are phase-shifted by TT1/4,
corresponding to 90.degree. with respect to each other. They generate two
first scanning signals, phase-shifted by 180.degree. with respect to each
other, in the associated first transverse diffraction orders. The second
and fourth grating areas 55, 56 located between them also have a common
transverse graduation period TT2 of, for example, about 7 .mu.m which,
however, is different from TT1 wherein the transverse grating strips are
again phase-shifted in the Y direction by TT2/4, corresponding to
90.degree. with respect to each other. They also generate in the
associated first transverse diffraction directions two scanning signals
phase-shifted by 180.degree. with respect to each other. However, because
of the geometric offset (in the measuring direction X) of 90.degree. of
the grating areas 53, 54 with respect to grating areas 55, 56, these two
scanning signals are phase-shifted by 90.degree. with respect to the first
two scanning signals so that four scanning signals 0.degree., 90.degree.,
180.degree., 270.degree., phase-shifted by 90.degree. with respect to each
other, are derived from a common area of the scanning plate 90 and thus
also from the scale grating 4.
By means of the present invention its is basically possible to replace any
prior art phase grating wherein the phase deviation is realized by means
of strips and gaps of different height or differing refractive index
alternatingly disposed in the measuring direction X, by a transverse
grating with markings in several grating areas which are transversely
geometrically phase-shifted with respect to each other. In this way it is
also possible to realize in a particularly simple manner the scanning
grating (reference grating) used in European Patent Publication No. EP 0
163 362 B1. To generate three signals, phase-shifted by 120.degree. with
respect to each other, two transverse grating areas 91, 92 with markings
of the same graduation period TT, which are transversely phase-shifted by
2TT/3 (corresponding to 120.degree.) with respect to each other, are
provided in this further scanning grating 90 in accordance with the
present invention, which is represented in FIG. 13. The grating areas 91,
92 can again be embodied as amplitude or as phase gratings. It is also
particularly advantageous here to embody the transverse grating areas 91,
92 as phase gratings whose parameters are selected such that the 0th,
.+-.2nd, .+-.4th, . . . transverse diffraction orders are suppressed.
The width of the two grating areas 91, 92 constitutes the graduation period
TA. One of the grating areas 91 has a width of 2TA/3, and the other area a
width of TA/3 (viewed in the measuring direction).
A position measuring system using the scanning grating 90 shown in FIG. 13
is represented in FIG. 15. The light source 8 illuminates, via a
collimating lens 9, a scanning plate 99 with the scanning grating 90. The
transmitted light beams impinge on the reflecting scale grating 4 and are
again directed to the scanning grating 90. Partial light beams, diffracted
in the measuring direction X as well as transversely, impinge on the
photo-detectors 6, 62, 63. By means of double transverse diffraction in
the .+-.1st diffraction order, the partial beams of the .+-.2nd
diffraction order impinge on the photo-detectors 6, 62, 63.
A possible arrangement of photo-detectors 6, 62, 63 and 7, 72, 73 is
represented in FIG. 14. But since the photo-detectors 7, 72, 73 generate
the same signals (0.degree., 120.degree., -120.degree.) as the
photo-detectors 6, 62, 63, they are not used in the position measuring
system in FIG. 15.
The employment of a transversely deflecting scanning grating 90 has the
advantage that with simultaneous scanning of a reference marker 93, 94 a
separation of the partial light beams can be realized by the selection of
the grating parameters of the transverse grating areas 91, 92 and of the
reference marking 93. No additional deflection elements in the form of
prisms are required to prevent interference between the partial light
beams from the reference marking 93, 94 as well as the scale and scanning
gratings 4, 90. It is particularly advantageous to embody the reference
markings 93, 94 in a known manner in the measuring direction X in the form
of a chirped grating, in particular in accordance with FIG. 3 of the
not-yet-published European Patent Application No. 95102328.2. The chirped
grating strips of the reference marking 93 have a transverse graduation so
that two signals, phase-shifted by 180.degree. with respect to each other,
can be generated by means of the photo-detectors 95, 96.
FIG. 16 illustrates a position measuring system according to another
preferred embodiment of the present invention. Scanning gratings 151 and
152 are embodied on the scanning grating 159. Two groups of scanning
gratings 151, 152 are provided for scanning the scale grating 4. Each
group consists of two scanning gratings 151 or 152 of the same type. The
two scanning gratings 151 or 152 of each group are disposed offset from
each other by a multiple of the longitudinal graduation period TA. Each
scanning grating 151, 152 in turn consists of five longitudinal graduation
periods TA. In contrast to the exemplary embodiment shown in FIG. 8, the
scanning gratings 151, 152 of the two groups differ not by different
transverse graduation period, but by the Blaze angle .phi.1, as can be
seen from FIGS. 18 and 19. In addition, the scanning gratings 152 of the
second group are arranged spaced apart by (m+1/4)TA in the measuring
direction X with respect to the scanning gratings 151 of the first group,
wherein m=0, 1, 2, . . . By means of the Blaze effect, the scanning
gratings 152 essentially direct the impinging light beam only into the
+1st transverse diffraction order. Following reflection at the scale
grating 4, it is again deflected in the +1st transverse diffraction order
during the second passage through the scanning grating 152, so that it
exits in +2nd resultant transverse diffraction order and is focused by
means of the lens 9 on the photo-detectors 64, 65 which detect the various
longitudinal diffraction orders.
By means of the reversed Blaze angle .phi.1 of the scanning grating 151,
the associated light beam is guided in an analogous manner on the
photo-detectors 74, 75, which are again at a longitudinal distance from
each other.
It is possible by means of a suitable design of the scanning areas 153, 154
and 155, 156 of the scanning gratings 151, 152 to phase-shift the scanning
signals obtained from the photo-detectors 64, 65 and 74, 75 with respect
to each other by a desired value. (See FIG. 17) For example, if the
transverse grating areas 155, 156 have the same graduation period TT and
are disposed transversely offset by 2 TT/3 with respect to each other, it
is possible to achieve a phase shift of 90.degree. between the scanning
signals of the photo-detectors 64 and 65. Viewed in the measuring
direction X, respectively one grating area 155 and one grating area 156
constitute a graduation period TA, wherein the width of the grating area
156 is approximately TA/3. As indicated in FIG. 17, the same conditions
apply with respect to the grating areas 153 and 154 of the scanning
grating 151 for achieving a phase shift by 90.degree. between the scanning
signals of the photo-detectors 74, 75.
It is possible to set the phase shift of the scanning signals of the
photo-detectors 64, 65 with respect to the photo-detectors 74, 75 by a
geometric displacement of the grating area 152 with respect to the grating
area 151. If this displacement is, for example, TA/4, the scanning signals
of the photo-detectors 64, 65 are phase-shifted by 180.degree. with
respect to each other in comparison with the photo-detectors 74, 75. The
customarily required four scanning signals, phase-shifted by 90.degree.
with respect to each other, can be obtained in a simple manner in this
way, as well as the advantages of a single field scanning. The light yield
in the transverse diffraction orders used (.+-.1st) is particularly great
in this exemplary embodiment if the suitable Blaze angle .phi.1 of the
grating areas 153 to 156 is selected. The requirements for this are well
known to those of ordinary skill in the art and can be found in the
applicable technical books under "echelette grating". The employment of
the echelette gratings, shown in particular in FIGS. 18 and 19, is
especially advantageous if, as in the so-called three grating transmitters
described in European Patent Publication No. EP 0 163 362 B1, the +nth and
the -nth transverse diffraction orders are modulated in equiphase and
therefore cannot provide any additional information.
For a better distinction between the Blaze angles .phi.1, in FIG. 17 the
grating areas 153, 154 with the downward directed Blaze angle .phi.1 are
cross-hatched differently than the grating areas 155, 156 with an upward
directed Blaze angle .phi.1.
In this exemplary embodiment the reference marking 193 also consists of a
chirped scanning grating, which consists of grating areas 195 and 196
alternately disposed in the measuring direction X and whose widths
continuously decrease in the measuring direction X. The grating areas 195
and 196 have opposite Blaze angles .phi.2, which can be seen in the
sections I--I and II--II in FIGS. 18 and 19. The grating areas 195 guide
the impinging light beam to the photo-detector 95, the grating areas 196
guide the impinging light beam to the photo-detector 96. In this way the
photo-detectors 95 and 96 supply the timing or counter-timing signal of
the chirped reference marking 194.
It is easily possible by means of a different selection of the transverse
graduation periods TT, TTR and/or the Blaze angles .phi.1, .phi.2 to
separate the individual diffracted light beams in the focal plane of the
lens 9 by means of the use of transverse grating areas 153 to 156 and 195,
196 for incremental as well as reference marking scanning. In this way
elaborate light-deflecting means, such as prisms or mirrors, are no longer
required. The Blaze-angled grating represented can be made in a
particularly advantageous manner by stamping.
A particular advantage of the scanning gratings in accordance with the
present invention 1, 10, 20, 30, 40, 51, 52, 80, 50, 90 is that the local
phase shift of the light beam diffracted in the +nth and -nth (n=1, 2, 3 .
. . ) transverse diffraction order does not depend on the phase deviation
and the strip width of the transverse grating, but instead is solely
provided by the geometric displacement of the transverse grating areas 2,
3, 11 to 14, 21 to 24, 31, 32, 41, 42, 53 to 56, 81, 82, 91, 92. Because
of this, the tolerances of the phase deviation and strip width of the
transverse grating areas 2, 3, 11 to 14, 21 to 24, 31, 32, 41, 42, 53 to
56, 81, 82, 91, 92 are so great that production is cost-effective in
comparison with conventional phase graduations.
In all embodiments the phase grating can be designed as a phase structure
in the form of the surface relief represented in FIG. 2 or as a phase
structure by means of a location-dependent variable refractive index, or
also as an amplitude structure in the form of location-dependent variable
reflection, absorption or transmission. The phase structure has the
particular advantage that, as already explained, the light intensity can
be directed into defined diffraction orders. It is therefore particularly
advantageous to use the .+-.1st diffraction order, however, in accordance
with the present invention it is also possible to employ other diffraction
orders.
Since in connection with the grating in accordance with the present
invention both the position of the markings in the Y direction and their
width in the Y and X-directions can be selected locally at will, it is
possible in this way to produce any desired combination between a phase
graduation and an amplitude graduation in a simple manner. It is possible
by means of the grating in accordance with the present invention to select
the desired phase shifts of the scanning signals directly derived from
this grating and the scanning signals can be optimized in addition.
It is particularly advantageous if the scanning plate, which is relatively
small in the measuring direction X, is embodied in accordance with the
present invention. However, it is also within the scope of the present
invention to design the scale correspondingly.
In all examples the photo-detectors can be disposed at different distances,
i.e. independently of the angle of incidence of the partial light beams
impinging on the lens 5 or 9, such as extensively explained in European
Patent Publication No. EP 0 576 720 A2.
The present invention can be employed in connection with linear and angular
measuring systems.
It is to be understood that the forms of the invention described herewith
are to be taken as preferred examples and that various changes in the
shape, size and arrangement of parts may be resorted to, without departing
from the spirit of the present invention or scope of the claims.
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