Position measuring system

A position measuring system for increasing operational dependability, wherein the absolute position is generated by scanning a chain code and erroneously scanned code words are detected and excluded from further processing. A plurality of codes are scanned at areas of the code track which are distanced from each other and are supplied to an error check device.

FIELD OF THE INVENTION 
The invention relates to a position measuring system for determining the 
absolute position of a scanning unit which can be displaced relative to a 
measurement representation in the measuring direction. 
BACKGROUND OF THE INVENTION 
A known position measuring system has been described in Japanese Patent 
Publication JP 4-1522 A. In this publication, the position measuring 
system is represented in FIG. 2. The measurement representation consists 
of a code track with code elements arranged one behind the other in the 
measuring direction. FIG. 2 of the Japanese Patent Publication JP 4-1522A 
illustrates the position measuring system. The code elements are provided 
in a pseudo-random distribution, so that a defined number of successive 
code elements form a code word and all code elements form a continuous 
sequence of different code words. Such a code is also called a chain code. 
The measurement representation is scanned by several groups of detector 
elements in order to simultaneously scan several complete code words which 
do not overlap each other. These code words are then supplied to an error 
check device for detecting reading errors. In the error check device, the 
distances between the actual positions of the code words are compared with 
the distances between the detector groups. If the two distances do not 
match, a warning signal is triggered. A disadvantage of this position 
measuring system is that it is only possible to detect whether code words 
have been erroneously read. The system cannot detect which particular code 
words have been erroneously read. 
SUMMARY OF THE PRESENT INVENTION 
The present invention provides a position measuring system with a high 
degree of operational dependability. In particular, the device of the 
present invention detects errors in the scanned section of the code track, 
and removes erroneous code words. The probability that at least one code 
word is correctly read in the course of scanning several code words which 
are completely independent of each other, is very great, thereby 
increasing the operational dependability of the position measuring system. 
In the presently preferred embodiment, the position measuring system is 
used to determine the absolute position of a scanning unit which can be 
displaced relative to a measurement representation in a measuring 
direction. The system includes a measurement representation having a code 
track with code elements following each other in the measuring direction 
and forming a sequence of different code words; a scanning unit having a 
plurality of detector elements following each other in the measuring 
direction for simultaneously scanning several complete, non-overlapping 
code words; and an error check device receiving the code words or decoded 
code words and checking the code words or decoded code words for errors, 
whereby a correct code word is recognized from several simultaneously 
detected code words and is used to determine the instantaneous absolute 
position, and whereby code words recognized as being erroneous are 
excluded from further processing.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
A preferred embodiment of a portion of a measurement representation 10 of 
the present invention is shown in FIG. 1. The measurement representation 
10 includes a code track 12 having code elements C. Some code elements 
such as C35, C37, C46, C66 and C86 are shown by hatched lines and are 
preferably opaque. Other code elements such as code element C36 is 
transparent. The code elements preferably have a width P1 and sequentially 
follow each other in the measurement direction X. The code elements C form 
a chain code, i.e., they are pseudo-randomly distributed in the measuring 
direction X and form a continuous sequence of different code words. A 
particularly simple and space-saving construction of an absolute position 
measuring system is possible using chain codes. 
In the method of the present invention, the measurement representation 10 
is binary scanned with a binary value 0 assigned to each opaque code 
element C and a binary value 1 assigned to each transparent code element 
C. However, the code elements C can also be designed so they can be 
scanned magnetically or capacitively. Such chain codes are extensively 
described in U.S. Pat. No. 5,068,529, which is hereby specifically 
incorporated by reference. 
Also shown in FIG. 1 is a portion of a scanning unit 14 for scanning the 
code track 12 of the measurement representation 10. The scanning unit 14 
can be displaced relative to the measurement representation 10 and 
includes a plurality of detector elements D arranged one behind the other 
in the measuring direction X. Preferably, these detector elements D are 
light-sensitive regions of a photoelectric diode array, such as a 
charge-coupled device (CCD) array, that sense the transparent and opaque 
regions of the code track 12. As shown in FIG. 1, if an opaque region of 
code track 12, such as C46, is scanned by a detector element D, such as 
detector element D1, no light is sensed by the detector element D which 
results in a negligible output voltage and a binary value 0 assigned to 
the region of the detector element D. If a transparent region of code 
track 12, such as C47, is scanned by a detector element D, such as 
detector element D2, light is sensed by the detector element D which 
results in a large output voltage and a binary value 1 assigned to the 
region of the detector element D. The distance P2 between the detector 
elements D is equal to the width P1 of a code element C. However, the 
invention can also be employed if the width P1 of a code element C 
deviates from P2, in particular if P1 is a multiple of P2. 
The scanning unit 14 has at least a sufficient number of detector elements 
D to read three complete, non-overlapping code words CW. In the presently 
preferred embodiment, a code word CW consists of 16 bits, and the distance 
between two code words is 4 bits. The scanning area is 56 bits in length 
(i.e., 48 bits of code word and 8 bits of spacing between code words). 
FIG. 2 shows a schematic representation of a first preferred embodiment of 
the device of the present invention. The device 16 includes a scanning 
unit 14, a decoding device 18, an error check device 20, an electronic 
follow up device 22 and a selector circuit 24. Scanning unit 14 generates 
three code words CW46, CW66 and CW86 at a common scanning time at 
locations on the code track 12 as shown, for example, in FIG. 1. The code 
words are supplied to a decoding device 18 for determining the absolute 
positions AW46, AW66 and AW86 of each code word CW46, CW66 and CW86, 
respectively. Decoding device 18 can be a memory in the form of an EPROM 
table, whose addresses are the code words CW46, CW66 and CW86. In 
accordance with the invention, the absolute positions AW46, AW66 and AW86 
derived from code words CW46, CW66 and CW86 are supplied to the error 
check device 20. Alternatively, the code words CW46, CW66 and CW86 can be 
supplied to the error check device 20 as shown by dotted lines in FIG. 2. 
Preferably, error check device 20 is a microprocessor. Error check device 
20 calculates all possible differences, i.e., the detected actual 
distances IA1, IA2 and IA3 of the code words CW46, CW66, CW86, and 
compares them with the set known distances SA1, SA2 and SA3 (FIG. 1). The 
set distances SA1, SA2, SA3 result from the arrangement of the detector 
elements D (FIG. 1). The set distance SA1 between the code word CW46 and 
the code word CW66 and the set distance SA2 between the code word CW66 and 
the code word CW86 is preferably 20 bits, corresponding to the length of a 
code word plus the spacing of 4 bits. The set distance SA3 between the 
code word CW46 and the code word CW86 is preferably 40 bits, corresponding 
to the length of two code words plus two spacings of 4 bits. If 
corresponding distances IA and SA do not match, the error check device 20 
will issue a warning signal W indicating that an erroneous code was 
detected. 
In the presently preferred embodiment as shown in FIG. 2, the result of the 
comparison indicates that code words CW66 and CW86 have been erroneously 
read. These code words are blocked from further processing by selector 
circuit 24. Code word CW46, on the other hand, was correctly read and 
selector circuit 24 releases it to the electronic follow-up device 22 for 
further processing. Preferably, selector circuit 24 is a component of a 
processor that selects which code words to pass to the electronic 
follow-up device 22 for further processing. In FIG. 2, the selector 
circuit is shown as a switching mechanism to illustrate the selection 
function. Electronic follow-up device 22 can be, for example, a display 
unit that displays the measured position or a drive controller, such as a 
numerically controlled controller, that moves a machine part. 
Table 1 illustrates how the selector circuit 24 blocks (code word invalid) 
or releases (code word valid) defined code words CW46 to CW86 for further 
processing as a function of the comparison results: 
TABLE 1 
______________________________________ 
Result IA1 = IA2 = IA3 = 
Possibility 
SA1 SA2 SA3 CW46 CW66 CW86 
______________________________________ 
1 Yes No No valid valid invalid 
2 No Yes No invalid 
valid valid 
3 No No Yes valid invalid 
valid 
4 Yes Yes No invalid 
invalid 
invalid 
5 No Yes Yes invalid 
invalid 
invalid 
6 Yes No Yes invalid 
invalid 
invalid 
7 Yes Yes Yes valid valid valid 
8 No No No invalid 
invalid 
invalid 
______________________________________ 
It can be seen from Table 1 that of the three code words CW46, CW66 and 
CW86, any one can be detected as invalid, and at least one of the 
remaining two code words can still be supplied to the electronic follow-up 
device 22 as a correct code word for further processing without the 
measuring operation being interrupted. The higher the number of 
non-overlapping code words CW that are scanned, the higher the number of 
erroneous code words that can occur while the position measuring system 
maintains the ability to identify valid code words and supply them for 
further processing. Further, the more code words CW that are 
simultaneously scanned, the more likely it is that at least one of the 
code words CW is recognized as being correct, thereby increasing the 
operational dependability of the system. It has been shown that by 
scanning four code words it is possible to considerably increase the 
operational dependability of the system while maintaining a reasonable 
expense, since it is already possible to let two code words be erroneous. 
FIG. 3 shows a schematic representation of a second preferred embodiment of 
the device of the present invention. Similar to device shown in FIG. 2, 
the device 32 includes a scanning unit 28, an error check device 30, an 
electronic follow up device 32 and a selector circuit 34. At least two 
non-overlapping code words CW100 and CW150 are read off the code track 12 
(FIG. 1) by means of the scanning unit 28. The code of the code track 12 
(FIG. 1) contains control bits, for example, so that erroneously read out 
code word CW100 is detected in accordance with known rules. The two code 
words CW100 and CW150 are supplied to an error check device 30 in which 
each code word CW100 and CW150 is independently checked in accordance with 
known rules. Code word CW100, recognized as being erroneous, is removed 
from further processing by the selector circuit 34, while one of the 
correct code word CW150 is supplied to the electronic follow up device 32. 
In the simplest case, the error check of a code word CW100 or CW150 is 
performed by a known parity check or by scanning of redundant bits in 
accordance with German Patent Publication Nos. DE 12 87 630 C or DE 80 25 
487 U1. Rules for checking a code word CW100 or CW150 are known. Such 
rules are disclosed, for example, in the book by W. Wesley Peterson 
entitled "Prhfbare und korrigierbare Codes" Codes Which can Be Checked 
and Corrected!, Oldenbourg Verlag, publishers, 1967, particularly on pages 
184 to 215. In a manner not shown, it is also possible to supply decoded 
code words to the error check device 30 in place of the code words CW100 
and CW150. 
The code words or absolute positions which have been supplied as being 
correct to the electronic follow-up device 22,32 are corrected as a 
function of the set position of the detector elements D (FIG. 1). 
Referring to FIG. 2, if, for example, the correct absolute position is 
obtained from the first ten detector elements D, and these detector 
elements D are used as the reference position, the code word CW46 can be 
supplied directly or decoded to the electronic follow-up device 22 (FIG. 
2). However, if in this case the code word CW86 is recognized as being 
correct, it is necessary to correct the code word CW86 by the distance 
SA3. This correction can be performed, for example, by subtracting SA1 
from CW66 or by subtracting SA3 from CW86. A further option for taking the 
distances SA1 and SA3 into consideration with respect to the selected 
reference position consists of using the decoding device 18 in such a way 
that in an error-free case AW46=AW66=AW86. This is achieved, for example, 
in the decoding table for the code word CW66 by reading out CW66 displaced 
by twenty places with respect to the decoding table of the code word CW46, 
and for the code word CW86 by reading out CW86 displaced by forty places 
with respect to the decoding table of the code word CW46. 
The invention can be used in a particularly advantageous manner when 
scanning a single-track chain code where each displacement of the scanning 
unit 14 by a single code element C results in different code words. The 
invention can also be used in connection with absolute position measuring 
devices in which a single-track block code is present. Further, the 
invention can be employed for longitudinal and angular measuring systems. 
For fine resolution, an incremental track can be disposed next to the code 
track. It is therefore intended that the foregoing detailed description be 
regarded as illustrative rather than limiting and that it be understood 
that it is the following claims, including all equivalents, which are 
intended to define the scope of this invention.