U.S. patent application number 09/741765 was filed with the patent office on 2001-08-30 for position measuring system.
Invention is credited to Benner, Ullrich, Holzapfel, Wolfgang.
Application Number | 20010017349 09/741765 |
Document ID | / |
Family ID | 7933988 |
Filed Date | 2001-08-30 |
United States Patent
Application |
20010017349 |
Kind Code |
A1 |
Holzapfel, Wolfgang ; et
al. |
August 30, 2001 |
Position measuring system
Abstract
A position measuring system that includes a scale having an
incremental track, which extends in a measuring direction and has
an incremental graduation with two different graduation periods. A
scanning unit moves in relation to the scale in a measuring
direction, the scanning unit has an incremental signal detection
arrangement, wherein the incremental signal detection arrangement
generates incremental signals with a first coarse signal period and
generates incremental signals with a second finer signal
period.
Inventors: |
Holzapfel, Wolfgang; (Obing,
DE) ; Benner, Ullrich; (Trostberg, DE) |
Correspondence
Address: |
BRINKS HOFER GILSON & LIONE
P.O. BOX 10395
CHICAGO
IL
60610
US
|
Family ID: |
7933988 |
Appl. No.: |
09/741765 |
Filed: |
December 19, 2000 |
Current U.S.
Class: |
250/231.13 |
Current CPC
Class: |
G01D 5/2457 20130101;
H03M 1/143 20130101; H03M 1/282 20130101; H03M 1/30 20130101 |
Class at
Publication: |
250/231.13 |
International
Class: |
G01D 005/347 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 23, 1999 |
DE |
199 62 278.7 |
Claims
We claim:
1. A position measuring system comprising: a scale comprising an
incremental track, which extends in a measuring direction and
comprises an incremental graduation with two different graduation
periods; a scanning unit which moves in relation to the scale in a
measuring direction, the scanning unit comprises an incremental
signal detection arrangement; wherein the incremental signal
detection arrangement generates incremental signals with a first
coarse signal period and generates incremental signals with a
second finer signal period.
2. The position measuring system in accordance with claim 1,
wherein the scale further comprises an absolute track arranged
parallel with the incremental track, and the absolute track
comprises an absolute coding, and the scanning unit further
comprises an absolute signal detection arrangement that generates
an absolute position signal.
3. The position measuring system in accordance with claim 2,
wherein the incremental track comprises a first incremental
graduation with a coarse graduation period and a second incremental
graduation with a fine graduation period, and wherein the
graduation periods have a whole-number relationship with respect to
each other.
4. The position measuring system in accordance with claim 3,
wherein the incremental track comprises periodically successive
first and second blocks in the measuring direction, which are
arranged in the grid of the coarse graduation period.
5. The position measuring system in accordance with claim 4,
wherein the second blocks each has periodically succeeding partial
areas in the measuring direction with different optical properties,
which are arranged in the grid of the fine graduation period.
6. The position measuring system in accordance with claim 5,
wherein the first blocks are optically high-reflecting or
low-reflecting, and the partial areas in the second blocks are
embodied to be alternatingly optically high-reflecting or
low-reflecting.
7. The position measuring system in accordance with claim 5,
wherein the partial areas in the second blocks with different
optical properties are arranged in such a way, that in connection
with the first and second blocks along the measuring direction it
is always assured, that partial areas with different optical
properties are alternatingly arranged.
8. The position measuring system in accordance with claim 3,
wherein the absolute track is designed as a serial code structure
having a bit width corresponding to the coarse graduation period of
the first incremental graduation.
9. The position measuring system in accordance with claim 2,
wherein the absolute position signal detector arrangement comprises
a plurality of detector elements, which adjoin each other in the
measuring direction and are each identically designed, and wherein
a circuit element is assigned to respectively two or more detector
elements assigned to one bit width, by which a selective read-out
of respectively one detector element is possible.
10. The position measuring system in accordance with claim 2,
wherein the scanning unit furthermore comprises a light source, as
well as a scanning plate with one or several scanning graduations
and a transparent window area.
11. The position measuring system in accordance with claim 3,
wherein the scanning unit comprises a first and a second scanning
graduation with different scanning graduation periods, which are
matched to the coarse and the fine graduation periods of the first
and second incremental graduations, respectively, so that
respective Vernier fringe patterns result in a detection plane.
12. The position measuring system in accordance with claim 10,
wherein the scanning plate comprises a first and a second scanning
graduation with different scanning graduation periods, which are
matched to the coarse and the fine graduation periods of the first
and second incremental graduations, respectively, so that
respective Vernier fringe patterns result in a detection plane.
13. The position measuring system in accordance with claim 11,
wherein the first scanning graduation is matched to the coarse
graduation period and is embodied purely as an amplitude structure,
and the second scanning graduation is matched to the fine
graduation period and is embodied as a combined phase-amplitude
structure.
14. The position measuring system in accordance with claim 13,
wherein the first scanning graduation is embodied so that a
filtering of undesired signal portions results, in particular of
those signal portions with the finer signal period.
15. The position measuring system in accordance with claim 11,
wherein the incremental signal detector arrangement comprises a
first and a second incremental signal detector arrangement, which
are arranged spatially separated from each other, and the first and
second incremental signal detector arrangements are each embodied
as structured detector arrangements comprising a plurality of
individual detector elements, which are arranged adjoining each
other in the measuring direction, and wherein those detector
elements in each incremental signal detector arrangement, which
provide output signals of identical phase, are connected with each
other.
16. The position measuring system in accordance with claim 15,
wherein every fifth one of the adjoining plurality of individual
detector elements are connected with each other, so that four
partial incremental signals per incremental signal detector
arrangement result, which respectively have a phase shift of 90
with respect to each other.
17. The position measuring system in accordance with claim 15,
wherein the width b.sub.DET of at least one of the plurality of
individual detector elements is selected in the measuring direction
in accordance with: b.sub.DET=n*TP.sub.1 , with n=1, 2, 3, . . . ,
and TP.sub.1=the coarse graduation period of the first incremental
graduation.
18. The position measuring system in accordance with claim 11,
wherein the Vernier fringe pattern has a Vernier period (.LAMBDA.),
which is selected in accordance with: .LAMBDA.=(2*m+1)*TP.sub.1
,with m=1, 2, 3, . . . , and TP.sub.1=the coarse graduation period
of the first incremental graduation.
19. The position measuring system in accordance with claim 3,
wherein the scanning unit comprises a single scanning graduation,
which is matched to the fine graduation period, so that a Vernier
fringe pattern with the Vernier period .LAMBDA.' results in a
detection plane.
20. The position measuring system in accordance with claim 10,
wherein the scanning plate comprises a single scanning graduation,
which is matched to the fine graduation period, so that a Vernier
fringe pattern with the Vernier period .LAMBDA.' results in a
detection plane.
21. The position measuring system in accordance with claim 19,
wherein the single scanning graduation is embodied as a combined
phase-amplitude structure.
22. The position measuring system in accordance with claim 19,
wherein the incremental signal detection arrangement comprises a
single incremental signal detection arrangement, which is embodied
as a structured detection arrangement comprising a plurality of
individual detector elements, which are arranged adjoining each
other in the measuring direction.
23. The position measuring system in accordance with claim 22,
wherein each of the plurality of individual detector elements has
an inner structure and comprises several partial detector elements,
whose geometric shape has been selected to be such that a defined
filter effect results therefrom.
24. The position measuring system in accordance with claim 19,
wherein the width b.sub.DET of at least one of the plurality of
individual detector element is selected in the measuring direction
in accordance with: b.sub.DET=n* TP.sub.1 ,with n=1, 2, 3, . . . ,
and TP.sub.1=the coarse graduation period of the first incremental
graduation.
25. The position measuring system in accordance with claim 18,
wherein a Vernier period .LAMBDA.' is selected in accordance with
.LAMBDA.'=(2*m+1)*TP.sub.1 ,with m=1, 2, 3, . . . , and
TP.sub.1=the coarse graduation period of the first incremental
graduation.
26. The position measuring system in accordance with claim 23,
wherein every third one of the several partial detector elements of
a detector element are connected via a summing element for
generating the fine incremental signals.
27. The position measuring system in accordance with claim 23,
wherein three partial detector elements of different detector
elements are connected via a summing element for generating the
incremental signals with a first coarse signal period, and the
selection of the respective partial detector elements takes place
in such a way that the resulting incremental signals have a
constant signal amplitude.
Description
[0001] Applicants claim, under 35 U.S.C. .sctn. 119, the benefit of
priority of the filing date of Dec. 23, 1999 of a German patent
application, copy attached, Ser. No 199 62 278.7, filed on the
aforementioned date, the entire contents of which is incorporated
herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a position measuring system, having
a scale and a scanning unit, which can be moved in relation to the
scale in the measuring direction.
[0004] 2. Discussion of Related Art
[0005] A first category of known devices for absolute position
determination includes an absolute track on the scale side for
coarse position determination, as well as several incremental
tracks for the incremental position determination at higher
resolution. In connection with such measuring systems for absolute
position determination, reference is made, for example, to FIG. 6
in U.S. Pat. No. 5,252,825. Known serial pseudo-random codes can be
employed for absolute position determination, for example. The
further incremental tracks customarily include incremental
graduations with graduation periods which are stepped with respect
to each other, starting with a coarse incremental graduation period
and up to a fine incremental graduation period. The finest
incremental graduation period finally determines the maximum
resolution of such a measuring system. Appropriate processing of
the scanning signals of the different tracks is required during
measuring operations in order to have the desired absolute position
available at the output side. The several tracks extend on the
scale parallel with each other in the measuring direction. Rotary,
as well as linear measuring arrangements on the basis of this
principle are known in this connection. Accordingly, different
widths of the scales are therefore required, depending on the
number of incremental tracks. In the same way, several detector
elements per track are required for the scanning unit, which is
movable with respect to the scale. With a correspondingly large
number of incremental tracks, a voluminous structure results as a
whole. For example, in case of a rotary system, a correspondingly
large diameter of the graduation disk is required. Thus, problems
sometimes arise under restricted installation conditions.
[0006] Furthermore, difficulties can arise if a metal strip is used
as the scale carrier in the described systems. Erroneous
measurements thus result if such a metal strip is not aligned
exactly parallel with the measuring direction, and the various
graduation tracks are twisted around an axis which is located
perpendicular to the graduation plane. In this case processing the
signals from the various tracks, and the absolute position
determination from these signals can sometimes become
problematical.
[0007] Furthermore, a multitude of incremental tracks require a
large illuminated area on both sides of the scale, i.e. a
relatively low illumination strength per unit of area exists on the
scanned scale. The results of this are scanning signals with small
signal amplitude, which are therefore susceptible to failure.
[0008] However, a second category of known devices for absolute
position determination includes merely a single incremental track
arranged parallel adjacent to an absolute track with a serial
absolute coding. In this connection, reference is made to DE 195 05
176 A1, for example. Although the above-mentioned problems in
connection with the structural size are avoided in such a
variation, a clearly reduced resolution in the course of the
position determination results in comparison with the above
discussed systems. If, on the other hand, the resolutions of the
incremental track and the absolute track are selected to be too
different, problems result in particular in the synchronization of
the signals from the different tracks.
[0009] Moreover, with such systems the use of metal strips for the
scale is not without problems, since again wrong measurements can
occur with the synchronization of the signals from the incremental
track and the neighboring absolute track in case of metal strips
which are not oriented parallel with the measuring direction.
Basically the always necessary synchronization of the signals from
the track with the highest resolution with the signals from the
track with the next highest resolution is most critical here.
Errors in this synchronization result in particular in cases, were
twisting of the scale occurs around an axis which is oriented
perpendicularly in relation to the scale surface. Such twisting as
a rule occurs along thin metal strips, since they cannot be cut
straight enough because of their lack of stiffness.
[0010] But the above discussed problems do no only occur in the
described categories of absolute position measuring with a serially
coded absolute track. Similar problems also exist in connection
with rotary position measuring systems which, besides a high
resolution incremental track, furthermore have an incremental track
which provides an output signal with a single signal period in the
course of one revolution.
OBJECT AND SUMMARY OF THE INVENTION
[0011] It is therefore an object of the present invention to
indicate a position measuring system of compact structure, which
will provide several incremental signals of different resolution,
if possible, from scanning an incremental track.
[0012] This object is attained by a position measuring system that
includes a scale having an incremental track, which extends in a
measuring direction and has an incremental graduation with two
different graduation periods. A scanning unit moves in relation to
the scale in a measuring direction, the scanning unit has an
incremental signal detection arrangement, wherein the incremental
signal detection arrangement generates incremental signals with a
first coarse signal period and generates incremental signals with a
second finer signal period.
[0013] The position measuring system in accordance with the present
invention thus permits the generation of at least two incremental
signals of different resolution while scanning only one incremental
track.
[0014] A variation of the position measuring system of the
invention, which in particular is suitable for absolute position
determination, only includes two separate tracks. One of the two
tracks is embodied as an absolute track and in the course of
scanning provides coarse absolute position information. However,
the second one of the two tracks, i.e. the incremental track,
comprises an incremental graduation with at least two different
graduation periods, and in the course of scanning provides at least
two separate incremental signals with different signal periods. It
is therefore possible to derive at least two incremental signals of
different resolution from a single incremental track without the
structural size of the total system becoming too large.
[0015] Moreover, the position measuring system in accordance with
the present invention has been shown to be relatively indifferent
to possible tilting of the scanning unit with respect to the
scanned scale. With the above discussed categories of absolute
position measuring systems in particular, the above mentioned
synchronization of the signals from the track with the highest
resolution with the signals of the track with the next highest
resolution is now less critical, since these signals are obtained
from a single, or common, track. This means that it is also
possible to employ metal strips as the scale carrier. Because of
this indifference to tilting or twisting of the scanning unit with
respect to the scale, extreme demands are no longer made on
mechanical components, such as machine guideways etc., which are
employed in connection with the system in accordance with the
invention.
[0016] It should be furthermore pointed out that in the case of
optical scanning, a smaller illuminated field is sufficient for
signal generation as a whole because of the reduced lateral
dimensions of the two tracks. This in turn results in a greater
illumination strength per unit of surface, and therefore a greater
signal strength, or reduced susceptibility to failure.
[0017] Although embodiments of the position measuring system of the
present invention will be described in what follows, each of which
is based on optical, or photoelectric, scanning, it should be
pointed out here that the present invention can of course also be
carried out in connection with other physical scanning principles.
It should also be mentioned that the system of the invention can be
designed as a linear, as well as a rotary position measuring
system.
[0018] It is of course also possible to provide, besides the
exemplary embodiments of absolute position measuring systems
explained in detail in what follows, alternative position measuring
systems. For example, a rotary system with only a single
incremental track is conceivable, whose scanning provides a first
coarse incremental signal with one signal period per revolution,
and a second, high-resolution incremental signal, etc.
[0019] Further advantages of the present invention ensue from the
subsequent description of exemplary embodiments by means of the
attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1a is a schematic representation of the path of the
scanning beams in a first exemplary embodiment of the position
measuring system according to the present invention;
[0021] FIG. 1b is a top plan view of the embodiment of the scale
shown in FIG. 1a;
[0022] FIG. 1c is an enlarged section from FIG. 1b;
[0023] FIG. 1d is a top plan view of the embodiment of the scanning
plate of the scanning unit in the position measuring system shown
in FIG. 1a;
[0024] FIG. 1e is a plan view of the detection plane of the
embodiment of the scanning unit shown in FIG. 1a;
[0025] FIG. 1f shows a portion of an embodiment of a detector
arrangement shown in FIG. 1e, as well as the Vernier fringe pattern
scanned by the detector arrangement;
[0026] FIG. 1g represents the scanning signals detected by an
incremental signal detector arrangement according to the present
invention;
[0027] FIG. 2a is a schematic representation of the path of the
scanning beams in a second exemplary embodiment of the position
measuring system according to the present invention;
[0028] FIG. 2b is a top plan view of the embodiment of the scale
shown in FIG. 2a;
[0029] FIG. 2c is a plan view of the detection plane of the
embodiment of the scanning unit shown in FIG. 2a;
[0030] FIG. 2d is a schematic representation of the detection plane
of the embodiment of the scanning unit in the position measuring
system shown in FIG. 2a;
[0031] FIG. 2e is a plan view of a portion of the detection plane
of the embodiment of the scanning unit of the position measuring
system shown in FIG. 2a, as well as the fringe pattern scanned by
the scanning unit.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] A first exemplary embodiment of the position measuring
system will be explained in what follows by means of FIGS. 1a to
1g. This is a device for absolute position determination.
[0033] Here, FIG. 1a shows the principal path of the scanning beams
of the position measuring system of the present invention, which is
designed as a linear incident light measuring system. The position
measuring system includes a scale 10 extending in the measuring
direction x, as well as a scanning unit 20, which is movable with
respect to the scale 10 in the measuring direction x. In the
representation shown in FIG. 1a, the measuring direction x is
oriented perpendicularly with respect to the drawing plane. The
scale 10 and the scanning unit 20 are connected with machine
elements, which are movable with respect to each other, for
example, and whose positions in relation to each other are to be
determined. These can be, for example, a tool and a workpiece in a
numerically controlled machine tool. In this case, the signals
generated by the position measuring system in accordance with the
present invention are further processed in a machine tool control
device, not represented.
[0034] In this embodiment, the scale of the position measuring
system of the present invention includes two tracks 12, 13, which
are arranged on a scale carrier 11 and are scanned by the scanning
unit 20 for the absolute position detection. The two tracks 12, 13
extend in the measuring direction x and, in the represented
incident light variation of the position measuring system, each
includes a sequence of partial areas of different reflection
properties, namely of partial areas of high reflectivity and
partial areas of low reflectivity.
[0035] The shape of the two tracks 12, 13 of the first exemplary
embodiment can be seen in the top plan view in FIG. 1b. A first
track 13, hereinafter called absolute track 13, has an absolute
coding in the form of a pseudo-random code. The pseudo-random code
includes an aperiodic sequence of high-reflecting and
low-reflecting partial areas 13.1, 13.2, each of which has the same
width in the measuring direction x. A coarse absolute position
signal ABS can be generated in a known manner from scanning the
absolute track 13, but its resolution is not yet sufficient for the
desired highly precise position measurement. It is of course also
possible to alternatively provide serial codings in the absolute
track 13, such as block codes, Manchester codes, etc.
[0036] The second track 12, which hereinafter will be called the
incremental track 12, is arranged directly adjoining and parallel
to the absolute track 13. The same as the absolute track 13, the
incremental track 12 also extends in the measuring direction x. The
incremental track 12 is used for generating periodic incremental
signals INC1, INC2 of higher resolution, which are employed in a
known manner for determining the relative positions of the scale 10
and the scanning unit 20. In this variation, the incremental
signals INC1, INC2 are combined in a suitable manner with the
absolute position signals ABS from the absolute track 13 in order
to determine in this way the absolute position of the parts which
can be moved with respect to each other. It is possible here that
the determination of the respective absolute position from the
different scanning signals can already be performed at the position
of the measuring system, or only later in a downstream connected
evaluation unit, for example a numerical machine tool control.
[0037] In accordance with the present invention, the incremental
track 12 in the exemplary embodiment permits the generation of
incremental signals INC1, INC2 with two different signal periods
SP1, SP2. The scanned incremental track 12 has two different
graduation periods TP.sub.1, TP.sub.2 for this purpose. The
incremental track 12 in this exemplary embodiment includes a
periodic sequence of first and second blocks B1, B2. The graduation
period TP.sub.1, which represents the coarser one of the two
graduation periods TP.sub.1, TP.sub.2 in the incremental track 12,
is defined by the sum of the width of two successive blocks B1, B2.
In this embodiment the widths of the two blocks B1, B2 have not
been selected to be identical. However, the coarse graduation
period TP.sub.1, defined by the sum of the widths, is mainly
important for scanning. Depending on the type of scanning of the
incremental track 12, it can also be advantageous to embody the
blocks B1, B2 to be of identical width.
[0038] In the variation represented, the first blocks B1 are
embodied to be low-reflecting, however, the second blocks B2
include a periodic sequence of further partial areas 12.1, 12.2
with low-reflecting and high-reflecting optical properties. An
enlarged partial representation of a second block B2 is shown in
FIG. 1c. As can be seen in FIG. 1c, the partial areas 12.1, 12.2 in
the second blocks B2 are designed as narrow rectangular-shaped
areas, whose longitudinal axes extend in the y-direction in the
scale plane, i.e. are oriented perpendicularly in relation to the
measuring direction x. Each one of the different partial areas
12.1, 12.2 in the second blocks B2 has the identical dimensions.
The fine graduation period TP.sub.2 of the incremental track 12 is
defined by the sum of two successive partial areas 12.1, 12.2 in
the first blocks B1, which is also illustrated in FIG. 1c.
[0039] In the embodiment of the position measuring system
represented, the fine graduation period TP.sub.2 of the incremental
track 12 has been selected to be smaller by a factor of 10 than the
coarser graduation period TP.sub.1 of the incremental track 12,
i.e. TP.sub.1=200 .mu.m and TP.sub.2=20 .mu.m.
[0040] In principle, the coarser graduation period TP.sub.1 of the
incremental track 12 should be selected as a whole number multiple
of the finer graduation period TP.sub.2. Only in this way is it
assured that successive blocks B2 represent portions of a
continuous incremental graduation with the finer graduation period
TP.sub.2. Accordingly, it must always be assured that
low-reflecting partial areas 12.1 and high-reflecting partial areas
12.2 are alternatingly arranged over the entire measurement
distance. This applies in particular if the low-reflecting blocks
B1 interrupt the incremental graduation with the finer graduation
period TP.sub.2. Thus, a high-reflecting partial area 12.2 in the
block B2 immediately adjoining in the measuring direction follows a
low-reflecting partial area 12.1 at the edge of a block B2.
Incidentally, the representation of the incremental track 12 in
FIG. 1b does not show this in detail.
[0041] In this exemplary embodiment, the bit width of the
pseudo-random code of the absolute track 13 is furthermore matched
to the coarser graduation period TP.sub.1 of the incremental
graduation 12. This means that the width in the measuring direction
x of the partial areas 13.1, 13.2 in the absolute track 13 has been
selected to be identical with the graduation period TP.sub.1 of the
incremental track 12.
[0042] Alternatively with the explained embodiment of the scale 10
it is of course also possible to design the various partial areas
of the two tracks 12, 13 on the scale carrier 11 to have other
optical properties, i.e. to possibly interchange the high- and
low-reflecting partial areas. In the same way the explained
embodiment of a scale can also be transferred to a transmitted
light system. In this case it would then be necessary to design the
respective partial areas in the two tracks to be transparent and
non-transparent. Moreover, it can principally also be provided to
design the incremental tracks in such a way that more than two
incremental signals with different signal periods can be obtained
from them.
[0043] The scanning unit 20, which is also schematically
represented in FIG. 1a, includes a light source 21, for example an
LED, an optical collimation device 22, a scanning plate 23, as well
as a detector unit 24 for detecting the various scanning signals.
The paths of the scanning beams for generating the incremental
signals INC1, INC2, as well as the absolute position signal ABS,
will be separately explained in what follows.
[0044] In the first embodiment represented, following collimation
the light beams emitted by the light source 21, which aid in the
generation of the various incremental signals, reach the two
scanning graduations 23.1, 23.2 in the scanning plate 23 via the
optical collimation device 22. A top plan view of the scanning
plate 23 is represented in FIG. 1d. The two scanning graduations
23.1, 23.2 have different scanning graduation periods ATP.sub.1,
ATP.sub.2, which are matched to the two graduation periods
TP.sub.1, TP.sub.2 of the incremental track 12. Here, the first
scanning graduation 23.1 is matched to the coarser graduation
period TP.sub.1 and is used to generate the incremental signals
INC1 with the coarser signal period SP1. In a possible embodiment,
the corresponding scanning graduation period ATP.sub.1 is selected
to be slightly different from the coarser graduation period
TP.sub.1=200 .mu.m. But the scanning graduation period ATP.sub.2 of
the second scanning graduation 23.2 in the scanning plate 23 is
matched to the finer graduation period TP.sub.2 in the incremental
track 12 and is used for generating the incremental signals INC2 of
the finer signal period SP2. The scanning graduation period
ATP.sub.2 of the second scanning graduation 23.2 is selected to be
slightly different from the finer graduation period TP.sub.2=20
.mu.m.
[0045] The first scanning graduation 23.1 with the coarser scanning
graduation period ATP.sub.1 is embodied purely as an amplitude
grating. The second scanning graduation 23.2 with the finer
scanning graduation period ATP.sub.2, however, is designed as a
combined phase/amplitude grating, such as described in German
Patent Application 199 41 318.5. Such a design of the second
scanning graduation 23.2 assures that a defined invariability of
the measuring system against possible fluctuations of the scanning
distance is provided.
[0046] Following passage through the two scanning graduations 23.1,
23.2, the respective beams impinge on the incremental track 12 on
the scale 10 and are there reflected back in the direction toward
the scanning plate 23. The reflected beams pass through a
transparent window area in the scanning plate 23 before impinging
on the respective incremental signal detector arrangements 24.1,
24.2 in the detector unit 24. Only one of the two detector
arrangements 24.1, 24.1 of this exemplary embodiment can be seen in
FIG. 1a, so that reference is made to the schematic top plan view
on the detection plane in FIG. 1e with respect to the actual
construction of the detector unit 24. As can be seen there, a first
incremental signal detector arrangement 24.1 is provided for the
detector unit 24 for generating the incremental signals INC1 with
the coarse signal period SP1, as well as a second incremental
signal detector arrangement 24.2 for generating the incremental
signals INC2 with the fine signal period SP2. Respectively, one
pair of incremental signals INC1, INC2 with the coarse and fine
signal periods SP1, SP2 is of course generated on the output side,
wherein these are respectively phase-shifted by 90. Regarding the
detailed design of the detector unit 24, reference is made to the
still to follow detailed description of FIG. 1 e.
[0047] The beams, which in this example are used for generating the
coarse absolute position signals ABS pass, after collimation by the
optical collimation device 22, through the transparent window area
23.3 of the scanning plate 23 and reach the absolute track 13 on
the scale 10. A reflection back in the direction of the scanning
plate 23 takes place from there, where the reflected beams again
pass through the transparent window area before impinging on the
absolute position signal detector arrangement 24.3 in the detector
unit 24. Thus, in a known manner only an image or shadow projection
of the respectively just detected section of the pseudo-random code
on the absolute position signal detector arrangement 24.3 is
provided by this path of the beams.
[0048] A plan view of the detection plane of the detector unit 24
is represented in a schematic form in FIG. 1e. Here, the absolute
position signal detector arrangement 24.3, which includes a
multitude of individual opto-electronic detector elements 24.3A to
24.4K, which are arranged following each other in the measuring
direction x, can be seen in the upper part. In the exemplary
embodiment represented, respectively two adjoining detector
elements are used for scanning a bit of the pseudo-random code in
the absolute track 13, i.e. there are two groups of detector
elements 24.3A to 24.4K, which are used for scanning the bits of
the pseudo-random code. However, only respectively one of the two
detector elements 24.3A to 24.3K per bit is read out during
measuring operations, which is intended to be illustrated in FIG.
1e by the schematically indicated circuit elements SA to SN above
the detector elements 24.3A to 24.3K. In the present exemplary
embodiment, one such circuit element SA to SN is therefore assigned
to two detector elements 24.3A to 24.3K, which are assigned to one
bit width. In this case the selective readout of the respective
detector elements takes place as a function of which of the two
groups of detector elements 24.3A to 24.3K is located closer to the
center of the respective bit fields of the pseudo-random code.
During measuring operations, the incremental signal INC1 with the
coarser signal period SP1 is used to decide this.
[0049] The scanning signals, which are generated in the detector
elements 24.3A to 24.3K of the absolute position signal detector
arrangement 24.3, are respectively fed to trigger elements, not
represented, which provide signals with logical HIGH and LOW levels
on their output side, which are further processed for the absolute
position determination. The trigger elements are also preferably
arranged in integrated form on the carrier substrate of the
detector unit 24. For generating the output signals, a reference
signal is moreover present at the trigger elements, which either
has a constant signal level, or is varied in a known manner as a
function of the signal level of the scanning signals. The output
signals of the trigger elements are conducted to a signal
processing unit, also not represented, which further processes
them, as well as the generated incremental signals, and generates
an output signal ABS, which delivers the desired coarse absolute
position.
[0050] The two incremental signal detector arrangements 24.1, 24.2
of the detector unit 24 can be seen in the lower portion of the
plan view in FIG. 1e which, in this exemplary embodiment, are used
for scanning the incremental track and for generating the
incremental signals INC1, INC2 with the signal periods SP1, SP2.
The incremental signal detector arrangement 24.1, shown on the left
in FIG. 1e, is used for generating the coarser incremental signal
INC1, while the incremental signal detector arrangement 24.2 shown
on the right is employed for generating the finer incremental
signal INC2.
[0051] The two incremental signal detector arrangements 24.1, 24.2
basically have the same structure, and are each designed as
so-called structured detector arrangements, or detector arrangement
arrays. In a known manner, these include a plurality of individual
opto-electronic elements 24.1A, 24.1B . . . , 24.2A, 24.2B . . . ,
all of which are identically embodied in the incremental signal
detector arrangement 24.1, 24.2 and are arranged, adjoining each
other, in the measuring direction x. Those detector elements 24.1A,
24.1B 24.2A, 24 . . . of a detector arrangement 24.1, 24.2, which
provide scanning signals of identical phase, are respectively wired
together at the output side. In the represented embodiment, the
fifth detector elements in the two detector arrangements 24.1, 24.2
are respectively wired together, so that a total of four partial
increment signals INC1.sub.A, INC1.sub.B, INC1.sub.C, INC1.sub.D,
INC2.sub.A, INC2.sub.B, INC2.sub.C, INC2.sub.D, which are
respectively phase-shifted by 90, are provided per detector
arrangement 24.1, 24.2 on their output sides. For generating the
two incremental signals INC1.sub.0, INC1.sub.90, or INC2.sub.0,
INC2.sub.90, which are phase-shifted by 90, these are then
differentially connected in a known manner, which is not
represented in FIG. 1e, so that respectively one pair of
incremental signals, phase-shifted by 90, results per detector
arrangement 24.1, 24.2. For the sake of simplicity, only one
respective incremental signal INC1, INC2 was mentioned above, this
will also be continued in the further course of the
description.
[0052] Periodic fringe patterns respectively appear in the
detection plane in the area of the two incremental signal detector
arrangements 24.1, 24.2, which are the result of the interaction of
the beams emitted by the light source 21 with the incremental
tracks 12, as well as with the respective scanning graduations
23.1, 23.2 in the scanning plate 23. Because of the slightly
different graduation periods TP.sub.1 and ATP.sub.1, or TP.sub.2
and ATP.sub.2, the periods of the respectively associated fringe
patterns are clearly larger than TP.sub.1 and TP.sub.2.
[0053] These fringe patterns will be called Vernier fringe patterns
in what follows. The same as the graduation structures, the
resultant Vernier fringe patterns are oriented in the incremental
track 12, or in the scanning graduations 23.1, 23.2. In case of a
relative movement between the scale 10 and the scanning unit 20,
the Vernier fringe patterns created in this way wander in the
measuring direction x over the respective incremental signal
detector arrangement 24.1, 24.2, which provide the incremental
signals INC1, INC2, which are modulated as a function of the
displacement. Because of the dimensioning of the scanning
graduations 23.1, 23.2, the Vernier fringe patterns move at
different speeds over the respective incremental signal detector
arrangement 24.1, 24.5 when the scale 10 and the scanning unit 20
move in relation to each other. Therefore periodic incremental
signals INC1, INC2 with different signal periods SP1, SP2, which in
this example differ by approximately a factor of 10, are present at
the different detector arrangements 24.1, 24.2.
[0054] Because of the dimensioning of the scanning graduations
23.1, 23.2 selected in this embodiment which, as mentioned above,
are matched to the two incremental graduation periods TP.sub.1,
TP.sub.2, Vernier fringe patterns of equal periodicity result on
the two incremental signal detector arrangements 24.1, 24.2. The
periods of the Vernier fringe patterns which are identical on the
two incremental signal detector arrangements 24.1, 24.2, will be
identified by A in what follows. The resultant Vernier fringe
pattern VS is represented in connection with a portion of the
incremental signal detector arrangement 24.1 in FIG. 1f.
[0055] In principle, it is possible by means of a suitable
dimensioning of the scanning graduations 23.1, 23.2 in the scanning
plate 23 to generate Vernier fringe patterns, which have different
Vernier periods A, on the two incremental signal detector
arrangements 24.1, 24.2. However, the generation of identical
Vernier periods A offers definite advantages. It is then for
example possible to use incremental signal detector arrangements
24.1, 24.2, which are identically designed, for the coarse and fine
incremental signals.
[0056] In the end, the Vernier fringe pattern VS represented in
FIG. 1f results from the selected design of the incremental track
12 on the scale 10 and represents the superimposition of a
resulting purely Vernier fringe pattern from the scanning of the
fine graduation period TP.sub.2 and of a cast shadow pattern from
the scanning of the coarse graduation period TP,.
[0057] In order to obtain, in accordance with the invention, the
desired incremental signals INC1, INC2 with different signal
periods SP1, SP2 from the Vernier fringe patterns VS on the two
incremental signal detector arrangements 24.1, 24.2, a number of
further measures are provided for the device of the invention.
These measures, explained in what follows, are used inter alia for
the filtering of signal portions of the periodicity SP2 out of the
incremental signal INC1, and of signal portions of the periodicity
SP1 out of the incremental signal INC2. This means that by these
measures it is assured that the individual incremental signals
INC1, INC2 are generated without being affected by the respectively
other signal. The respectively undesired signal portions are
suitably filtered out.
[0058] Thus, in this exemplary embodiment, steps are provided on
the part of the scanning graduation 23.1 which causes the filtering
of undesired signal portions from the coarse incremental signal
INC1. For example, such signal portions can result from the
simultaneous scanning of the finer graduation period TP.sub.2 and
can interfere with the incremental signal INC1. Such interference
is particularly disadvantageous if it is intended to further
divide, or interpolate, the coarse incremental signal INC1 later,
for which as a rule the best possible optimal shape, i.e. a sine-
or cosine- shape, of the scanning signals is a prerequisite. For
such filtering the scanning graduation 23.1, which is designed as
an amplitude grating, is provided with filtering properties. Known
filtering processes are being employed for this and, for example,
the transparent partial areas of the scanning graduation 23.1 are
designed with cosine-shaped openings, such as known from GB 2 116
313 A, for example. Alternatively, filtering of the harmonic wave
portion of the periodicity SP2 could also be assured in a known
manner by the suitable displacement of the strips of the scanning
graduation 23.1, i.e. different known measures are available for
filtering this signal portion.
[0059] Advantageously, further rules regarding the dimensions
should be observed for assuring the generation, which is as free as
possible of interferences, of incremental signals INC2 with the
finer signal period SP2.
[0060] Thus, the width b.sub.DET of a single detector element
24.2A, 24.2B, . . . in the detector arrangement 24.2, which is used
for generating the incremental signal INC2 with the finer signal
period SP2, is selected in accordance with the equation
b.sub.DET=n*TP.sub.1 (Equ. 1)
[0061] wherein n=1, 2, 3, . . .
[0062] This means that the width b.sub.DET of a single detector
element 24.2A, 24.2B, . . . of this detector arrangement is in the
end selected to be a whole-number multiple of the coarse graduation
period TP.sub.1 in the incremental track. In the present case, n=1
applies. It is assured by this dimensioning rule that possible
offset fluctuations of the periodicity SP1 are prevented in the
course of generating the fine incremental signal INC2 with the aid
of the incremental signal detector arrangement 24.2.
[0063] Moreover, in the present exemplary embodiment, the detector
elements 24.1 A, 24.1 B of the detector arrangement 24.1 for
generating the incremental signal INC1 with the coarse signal
period SP1 are also designed to be wide in the same way, however,
in principle this is not required.
[0064] Furthermore, in case of a relative movement between the
scale and the scanning unit, the incremental signal detector
arrangement 24.2 provides incremental signals INC2 with the fine
signal period SP2 without any further measures being required,
however, their amplitude is modulated with the coarse signal period
SP1.
[0065] A representation of such an incremental signal is shown in
FIG. 1g. However, for dependable further processing of the fine
incremental signals INC2, the most even amplitude of these signals
possible is advantageous. To assure this, the Vernier period A of
the Vernier fringe pattern VS in accordance with the equation
.LAMBDA.=(2*m+1)*TP.sub.1 (Equ. 2)
[0066] ,wherein m=1, 2, 3, . . . , is therefore preferably
selected.
[0067] In addition, in this case the width of the blocks B1 and B2
in the incremental track should be selected to be identical in the
measuring direction x, in contrast to the example in FIG. 1b.
[0068] In the present example, m=2 was selected from which, at
TP.sub.1=200 .mu.m, a required Vernier period .LAMBDA.=1 mm results
in the detection plane. In principle, the required Vernier period
.LAMBDA. can be definitely set by means of the selection of
different graduation periods for the scale and the scanning
graduations.
[0069] By means of this selection of the Vernier period .LAMBDA.
and the push-pull, or differential, connection of those detector
elements which have output signals with a relative phase shift of
180 the above mentioned undesired amplitude modulation of the fine
incremental signal INC2 mentioned above can be eliminated. Here,
the formation of the difference finally takes place from the
respective partial incremental signals which stem from the
inversely designed areas of the coarse graduation period TP.sub.1
of the incremental track 12. The undesired amplitude modulations of
the partial incremental signal then cancel each other out.
Incremental signals of constant amplitude are then present on the
output side.
[0070] It is moreover also possible to eliminate, or at least
minimize, undesired harmonic portions of the higher resolving
incremental signal INC2 by known filtering methods. In this case
such filtering methods must be provided either for the scanning
graduation 23.2, or for the detector arrangement 24.2.
[0071] The discussed measures now assure that it is possible to
dependably obtain the incremental signals INC1, INC2 with different
signal periods SP1, SP2 by scanning the incremental track 12. In
this embodiment of the position measuring system in accordance with
the invention it is possible in a known manner to assure the
high-resolution position determination by the combination with the
coarse absolute position signal ABS from the absolute track 13.
[0072] As already mentioned above, the different scanning signals
can already be suitably combined with each other in the device of
the invention, in order to precisely determine the respectively
actual absolute position. But it is also possible to transmit the
various scanning signals to a downstream-connected evaluation unit,
where the determination of the absolute position, etc., from these
signals takes place.
[0073] A second embodiment of the device in accordance with the
present invention will be described in what follows by means of
FIGS. 2a to 2e. In this case, only the essential differences with
respect to the first embodiment will be addressed, i.e. identical
components of the device essentially have the same functions,
unless possible differences are explained in what follows. This
embodiment of the position measuring system in accordance with the
present invention again is a device for absolute position
determination which, besides an incremental tracks being scanned in
accordance with the invention, furthermore has a serially coded
absolute track.
[0074] FIG. 2a again shows the basic scanning beam path in the
second exemplary embodiment of the position measuring system in
accordance with the present invention. But the differences with the
first variation in the way the incremental signals are generated
cannot be seen in this representation. In contrast to the first
exemplary embodiment in FIGS. 1a to If, it is now no longer
provided to detect the two incremental signals INC1, INC2 with the
different signal periods SP1, SP2 by two spatially separated
incremental signal detector arrangements, instead a modified common
incremental signal detector arrangement 240.1 of the detector unit
240 is to be used.
[0075] Regarding the scanning beam course for generating the
absolute position signals ABS and the structure of the absolute
position signal detector arrangement 240.3, there are no
differences with the first variation, therefore reference is made
only to the above explanations.
[0076] However, differences with the above explained variation
exist in connection with various measures which are of importance
in regard to the generation of the different incremental signals
INC1, INC2.
[0077] Thus, it can be seen in FIG. 2a that there is a somewhat
modified embodiment of the incremental track 120 on the scale 100,
because the blocks B1', B2' of this track now have the same width
in the measuring direction x. The coarse graduation period TP.sub.1
results from the sum of these widths which, however, as a whole has
been selected to be identical with the graduation period TP.sub.1
in the above example, i.e. TP.sub.1=200 .mu.m. The blocks B2' are
structured the same as in the first exemplary embodiment. Here,
again the fine graduation period TP.sub.1 in the blocks B2' has
been selected in accordance with TP.sub.1=20 .mu.m.
[0078] As can be seen in FIG. 2c, in contrast to the above example,
there is now only a single scanning graduation 230.2 arranged next
to the transparent window area 230.3, and it has a similar length
in the measuring direction x as the transparent window area 230.3.
The scanning graduation 230.2 is designed identically to the
scanning graduation 23.2 in the above sample, which was only used
there for generating the high-resolution incremental signals INC2.
Therefore the scanning graduation 230.2 in this example has a
scanning graduation period ATP.sub.2, which is slightly different
from the finer graduation period TP.sub.2=20 .mu.m of the
incremental track 120. The single scanning graduation 230.2 of this
variation is embodied the same as the scanning graduation 23.2 in
the first exemplary embodiment, namely as a combined
phase/amplitude structure as proposed in German Patent Application
199 41 318.5 of Applicant.
[0079] Only a single Vernier strip system VS' at the position of
the incremental signal detector arrangement 240.1 exists in the
detection plane of the detector unit 240 for generating the two
incremental signals INC1, INC2 with the different signal periods
SP1, SP2. As can be seen from the schematic plan view of the
detection plane in FIG. 2d, only a single incremental signal
detector arrangement 240.1 is therefore provided, by which the
Vernier fringe pattern is scanned in this plane for generating the
two incremental signals INC1, INC2.
[0080] For the detailed explanation of the single incremental
signal detector arrangement 240.1 provided in this example and for
the generation of the two incremental signals INC1, INC2, reference
is made to FIG. 2e, which shows a section of the incremental signal
detector arrangement 240.1, as well as the Vernier fringe pattern
VS', which has the Vernier period .LAMBDA.' and which is scanned by
it. The same as in the example already explained above, on the one
hand the scanned Vernier fringe pattern VS' has the Vernier period
.LAMBDA.', which is the end result of scanning the graduation
period TP.sub.2. Moreover, again a pattern of a periodicity
TP.sub.1 is superimposed on this Vernier fringe pattern VS',
wherein the periodicity TP.sub.1 is clearly less than the Vernier
period .LAMBDA.'.
[0081] Only a small section of the incremental signal detector
arrangement 240.1 is represented in FIG. 2e, which only extends
over one Vernier period .LAMBDA.'. A multiply repeated sequence of
the represented section, extending in the measuring direction x, is
provided in actual use.
[0082] In this embodiment, the periodically arranged detector
elements 240.1A, . . . 240.1D provided in the incremental signal
detector arrangement 240.1 now have such an interior structure that
each detector element 240.1A, . . . 240.1D in turn includes a total
of three individual partial detector elements 240.1A1, 240.1A2,
240.1A3, as represented in an enlargement in FIG. 2f for the
detector element 240.1A. The geometrical shape of the individual
partial detector elements 240.1A1, 240.1A2, 240.1A3 has been
selected here in such a way that a defined filtering effect results
in the course of scanning the coarse graduation period TP.sub.1.
For this purpose, on the one hand, the width of each detector
element 240.1A, . . . 240.1D is selected to be equal to the coarse
graduation period TP.sub.1. On the other hand, the inner partial
detector element 240.1A2 and the combination of the respectively
outer partial detector elements 240.1A1, 240.1A3 have cosine-shaped
boundary lines. The period of the respective cosine function has
been selected to be identical with the coarse graduation period
TP.sub.1.
[0083] The detector element 240.1B adjoining the detector element
240.1A also includes three partial detector elements, wherein the
arrangement of the three partial detector elements in this
exemplary embodiment is provided to be turned by 180. The next
detector element 240.1C is again designed identical to the first
detector element 240.1A, while the fourth detector element 240.1D
again is designed the same as the second detector element
240.1B.
[0084] By the selected shaping of the partial detector elements
240.1A1, 240.1A2, 240.1A3 it is possible to assure in a known
manner a filtering of undesired signal portions at the detector.
This is necessary in this exemplary embodiment since, in contrast
to the previous variation, only a single scanning graduation is
provided, which does not have an implicit filter effect. As has
been explained above, filtering of the resulting scanning signal is
required for generating the coarse incremental signal INC1 in order
to eliminate, inter alia, signal portions which stem from the finer
graduation period TP.sub.2 in the incremental track. While in the
above example this took place by appropriate filtering measures at
the scanning graduation 23.1, in this example the same filtering
effect is now caused by the geometric layout of the partial
detector elements 240.1A1, 240.1A2, 240.1A3.
[0085] It is of course possible to cause this desired filtering
effect at the detector by means of alternative geometries of the
partial detector elements 240.1A1, 240.1A2, 240.1A3. In this case a
multiple sequential arrangement of such partial detector elements
in the y-direction, etc. is possible.
[0086] For generating the fine partial incremental signals
INC2.sub.A, . . . INC2.sub.D, the various partial detector elements
240.1A1, 240.1A2, 240.1A3 are connected with each other in the way
represented in the lower portion of FIG. 2e. This means that the
respective scanning signals of three partial detector elements
240.1Aa, 240.1A2, 240.1A3 of a detector element 240.1A are added
together, which takes place with the aid of suitable summing
elements 240.S1A, . . . 240.S1D. The fine partial incremental
signals INC2.sub.A, . . . INC2D present at the outputs of the
summing elements 240.S1A, . . . 240.S1D which have the identical
phase relationships, are also combined and added in such a way that
in the end the four fine partial incremental signals INC2A, . . .
INC2D, respectively phase-shifted by 90, are present at the output
and are further processed in a known manner.
[0087] For generating the coarse partial incremental signals
INC1.sub.A, . . . INC1.sub.D, the wiring together of the different
partial detector elements 240.1A1, 240.1A2, 240.1A3 of the
individual detector elements 240.1A, . . . 240.1D takes place in
the way represented in the upper part of FIG. 2e. A constant
amplitude of the resulting incremental signals INC1.sub.A, . . .
INC1.sub.D in particular is again assured here because of the
selected wiring. For this purpose, respectively three specifically
selected partial detector elements are connected together, or added
together via the summing elements 240.S2A, 240.S2D, for generating
an incremental signal INC1.sub.A, . . . INC1D at the output. For
example, for generating the incremental signal INC1.sub.A, the
center partial detector element 240.1A2 of the detector element
240.1A and the two outer partial detector elements 240.1C1, 240.1C3
of the detector element 240.1C are wired together, etc. In this
case, the selection of the partial detector elements to be wired
together takes place with the point of view that the incremental
signal INC1 resulting from this should have a constant signal
amplitude. For this purpose, respectively three partial detector
elements are therefore wired, or summed, together, which register
identical areas of the cast shadow pattern with the period TP.sub.1
in the Vernier fringe pattern VS' which, however, occur at locally
different positions in the Vernier fringe pattern VS', in
particular at complementary first and second positions of the
Vernier fringe pattern. For example, the partial detector element
240.1A2 essentially registers a dark area of the cast shadow
pattern at a first position of the Vernier fringe pattern VS'. The
two partial detector elements 240.1C1, 240.1C3 also register dark
areas of the cast shadow pattern, but at a second position of the
Vernier fringe pattern, which is offset by half a Vernier strip
period and at which a Vernier fringe pattern complementary to the
first position is present. The wiring of the remaining detector
elements takes place analogously with these considerations.
Finally, the desired amplitude constancy of the coarse incremental
signals INC1.sub.A, . . . INC1.sub.D at the output results by
adding the different signals together.
[0088] As already indicated several times, the present invention is
of course not limited to the two explicitly explained exemplary
embodiments, i.e. there are also alternative execution variations
within the framework of the present invention. For example, it is
possible to provide different whole-number relationships between
the graduation periods TP.sub.1, TP.sub.2 in the incremental track.
In the same way other relationships can of course also be provided
between the graduation periods TP.sub.1, TP.sub.2 of the
incremental track and the respective bit width in the absolute
track.
[0089] Moreover, there is the possibility of arranging or
integrating a variety of components for signal processing, or
signal editing, on the detector side directly on the appropriate
carrier substrate. These can be switches, trigger stages,
current/voltage converters, etc., for example.
[0090] It is also possible to provide, alternatively to the Vernier
fringe pattern, for the generation of a Moir fringe pattern in a
known manner and to scan it.
[0091] Finally, it is also possible within the framework of the
present invention to operate not only with a strictly collimated
illumination, but also with a divergent illumination. In this case
it is necessary, in addition to the above explained considerations,
to observe appropriate enlargement factors when dimensioning the
respective detector arrangement.
[0092] Moreover, alternative variations can also be produced, which
only include a single incremental track at the scale, for example,
which provides two incremental signals of different periodicity in
the course of scanning, etc.
[0093] The foregoing description is provided to illustrate the
invention, and is not to be construed as a limitation. Numerous
additions, substitutions and other changes can be made to the
invention without departing from its scope as set forth in the
appended claims.
* * * * *