Reference mark identification system for measuring instrument

A measuring system is described which includes a scale which defines a measuring graduation and an array of reference marks. The reference marks are positioned at predetermined absolute positions with respect to the graduation and are used to generate reproduceable electric reference control pulses. In order to identify each of the reference marks a serially allocated code mark is provided which is made up of at least one nonzero code mark segment. Each nonzero code mark segment agrees in spatial pattern with the nonzero associated reference mark in the measuring direction. In order to distinguish nonzero code mark segments from reference marks, the width of the nonzero code mark segments perpendicular to the measuring direction is made less than the width of the reference marks.

BACKGROUND OF THE INVENTION 
The present invention relates to a reference mark identification system for 
use with a measuring system of the type which includes a measuring scale 
extending along a measuring direction, a measuring graduation defined by 
the scale, a plurality of reference marks, all of which define a single, 
preselected pattern along the measuring direction and each of which is 
positioned at a predetermined absolute position with respect to the 
graduation, and means for scanning the reference marks to generate 
reference pulses in response thereto. 
In such a measuring system, reference control pulses generated at the 
reference marks can be used in various ways. For example, such reference 
pulses can be used to set the counter of the measuring system to zero in 
order to define a zero position of the measuring system. Alternately, such 
reference control pulses can be used to load a predetermined position 
value into the counter and to start the measuring process. Furthermore, 
such reference pulses can be used to control interference pulses as well 
as to act on a control arrangement coupled to the counter. 
German Patent DE-PS 24 16 212 discloses an incremental length or angle 
measuring system in which a scale defines an incremental graduation and a 
plurality of reference marks on a separate track alongside the incremental 
graduation. The absolute values of these reference marks are determined 
from the different spacings between the individual reference marks. The 
spacings between the reference marks are determined by scanning the 
incremental graduation. Therefore, if the absolute position of any single 
reference mark is to be determined, two reference marks must be scanned. 
This process is relatively complicated and time consuming if for example 
two such reference marks lie far apart. Furthermore, in the event of a 
faulty or erroneous counting of the increments between two reference 
marks, the separation between the two reference marks can be measured 
inaccurately, and this can lead to false identification of the reference 
marks. 
In German Patent DE-PS 29 52 106 there is described an incremental length 
or angle measuring system which includes a scale that defines both a 
measuring graduation and a plurality of reference marks situated alongside 
the graduation. In this system each of the reference marks is 
characterized by a unique line group distribution, different from all the 
other reference marks. The individual reference marks are scanned by 
scanning fields in a scanning unit and each reference mark has a 
particular scanning field allocated to it which defines the same line 
group distribution as the associated reference mark. This arrangement is 
relatively expensive, since the line group distributions of the individual 
reference marks should be distinguished from one another as strongly as 
possible in order to make possible an unambiguous identification of the 
individual reference marks. Furthermore, the scanning unit must include an 
identical scanning field for each of the reference marks to be identified. 
German DE-OS No. 30 39 483 describes an incremental length or angle 
measuring system which defines a graduation track and a reference mark 
track arranged alongside the graduation track. A code mark track is 
arranged parallel to the graduation track and it includes code marks which 
identify respective ones of the reference marks. For the scanning of the 
reference marks and of the associated code marks, separate scanning fields 
on a scanning plate of a scanning unit are provided. Thus, the segments of 
the code marks are scanned by scanning fields which are provided 
particularly for these code mark segments and are different from the 
scanning fields used to scan the reference marks. 
BACKGROUND OF THE INVENTION 
The present invention is directed to an improved measuring system of the 
general type described initially above, in which the scanning of reference 
marks and associated code mark segments is substantially simplified. 
According to this invention, a measuring system of the type described 
initially above is provided with a plurality of code marks, each 
positioned between two adjacent ones of the reference marks and each 
serially associated with a respective one of the reference marks. Each of 
the code marks comprises at least one nonzero code mark segment which 
defines a spatial pattern along the measuring direction which is identical 
to that of the reference marks. 
The important advantages of this invention come from the fact that only a 
single scanning field is required on the scanning plate of the scanning 
unit to scan both the reference marks and the associated code mark 
segments. This is because both the reference marks and the nonzero code 
mark segments have the same spatial pattern, at least in the measuring 
direction. Because of the structural agreement between the nonzero code 
mark segments and the reference marks, the manufacture of both the 
measuring scale and the scanning plate are considerably simplified. For 
this reason a particularly economical measuring system results. Further 
advantageous features of the invention are set forth in the attached 
dependent claims. 
The 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.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
Turning now to the drawings, FIG. 1 shows a schematic view of a 
photoelectric incremental length measuring system that includes a 
measuring scale M and a scanning unit A. The scale M and the scanning unit 
A are connected in each case (in a manner not shown) with objects, the 
position of which is to be measured. These objects may comprise for 
example the slide piece and bed of a processing machine. The scale M 
serves as a carrier for an incremental graduation T which takes the form 
of a line grid as shown in FIG. 2. This incremental graduation T is 
scanned in direct lumination, photoelectrically without physical contact 
by the scanning unit A. A series of reference marks R.sub.n (n=1, 2, 3, . 
. .) is provided on the scale M alongside of the graduation T. Each of the 
reference marks R.sub.n is identical to the others and each defines an 
identical line group with a predetermined line distribution or spatial 
pattern along the measuring direction. The measuring direction is 
indicated by the arrow labeled X in FIG. 1. The scanning unit operates to 
generate periodic scanning signals S.sub.1, S.sub.2 during the scanning of 
the graduation T. These scanning signals are amplified in the scanning 
unit A and transformed into square wave signals S.sub.1 ', S.sub.2 ' which 
are applied to an electronic counter Z. The counter Z operates to count 
the period of the square wave signals S.sub.1 ', S.sub.2 ' and to display 
a measurement value in digital form. The square wave signals S.sub.1 ', 
S.sub.2 ' are phase-shifted with respect to one another by a quarter of 
the grid constant (division period) of the graduation T in order to allow 
accurate discrimination of the scanning direction. The scanning unit A 
also operates to generate reference signals RS.sub.n in response to the 
scanning of the reference marks R.sub.n. The reference signals RS.sub.n 
are amplified in the scanning unit A, converted into square wave signals 
RS.sub.n ', and applied as inputs to the counter Z. 
The reference signals RS.sub.n can be used to trigger various functions in 
the counter Z. For example, in response to the reference signals RS.sub.n 
an incremental measuring system can be made into a quasiabsolute measuring 
system if to each reference mark R.sub.n there is allocated a number which 
represents its absolute position with respect to an invariable zero point. 
Furthermore, a predetermined one of the reference marks R.sub.n can be 
used to set the counter Z to the value zero upon the generation of the 
reference signal RS.sub.n obtained from that reference mark R.sub.n. Such 
functions are only possible however if the selected one of the reference 
mars R.sub.n can be clearly identified and distinguished from the other 
reference marks R.sub.n. 
In order to allow such identification of each individual reference mark 
R.sub.n, a code mark C.sub.n is serially allocated to each of the 
reference marks R.sub.n. Each of the code marks C.sub.n is made up of at 
least one code mark segment C.sub.nm (n, m=1, 2, 3, . . .). Each of the 
nonzero code mark segments C.sub.nm defines the same preselected spatial 
pattern as that of the reference marks R.sub.n, at least in the measuring 
direction. 
As shown in FIG. 2, the code mark segments C.sub.nm between the individual 
identical reference marks R.sub.n are serially allocated to the reference 
marks R.sub.n on the scale M. That is, the code mark segments are 
positioned such that the scanning system included in the scanning unit A 
for the scanning the reference marks R.sub.n also scans the code mark 
segments C.sub.nm. Of the entire series of reference marks R.sub.n only 
one reference mark R.sub.n is shown in FIG. 2. As shown in FIG. 2, four 
code mark segments C.sub.n1 -C.sub.n4 are associated with the reference 
mark R.sub.n. In order to allow the code mark segments C.sub.n1 -C.sub.n4 
to be distinguished from the reference mark R.sub.n, the width a of the 
code mark segments C.sub.n1 -C.sub.n4 measured perpendicularly to the 
measuring direction X is less than the width b of the reference mark 
R.sub.n. In the embodiment of FIG. 1 a=b/2. 
The scanning unit A included in the measuring system for scanning the scale 
M includes a scanning plate AP as shown in FIG. 2. This scanning plate AP 
defines two scanning fields AT.sub.1, AT.sub.2 which are offset with 
respect to one another by a quarter of the grid constant of the graduation 
T. Each of these scanning fields AT.sub.1, AT.sub.2 is identical with the 
graduation T, and photosensors (not shown) are aligned with the scanning 
fields AT.sub.1, AT.sub.2 to scan the graduation T and to generate in 
response thereto the scanning signals S.sub.1, S.sub.2. In addition, for 
the scanning of the reference marks R.sub.n and of the code mark segments 
C.sub.n1 -C.sub.n4 there is provided on the scanning plate AP a scanning 
field AR. A single photosensor (not shown) is aligned with the scanning 
field AR in order to generate both the reference signals RS.sub.n and the 
code signals CS.sub.n1 -CS.sub.n4. The spatial pattern of the scanning 
field AR is identical with the spatial pattern of the reference marks 
R.sub.n and of the nonzero code mark segments C.sub.nm. When the code mark 
segments C.sub.n1 -C.sub.n4 are scanned in the scanning of the scale M 
from left to right, the corresponding code signals CS.sub.n1 -CS.sub.n4 
shown in FIG. 3 are generated. These code signals CS.sub.n1 -CS.sub.n4 in 
the example of FIG. 3 define the binary signal "1010". This is because the 
nonzero code mark segments C.sub.n1, C.sub.n3 are characterized by the 
presence of a spatial pattern identical to that of the reference marks 
R.sub.n, while the zero code mark segments C.sub.n2, C.sub.n4 are 
characterized by the absence of such a pattern. This binary signal 1010 
clearly identify the associated reference mark R.sub.n. Since the width b 
of the reference mark R.sub.n is twice as great as the width a of the code 
mark segments C.sub.n1 -C.sub.n4, the code signals CS.sub.n1 -CS.sub.n4 
can be clearly distinguished from the reference signals RS.sub.n by reason 
of the different signal amplitudes. Of course, the ratio between the 
widths a and b can be chosen arbitrarily as necessary to provide the 
desired level of discrimination. 
The code signals CS.sub.n1 -CS.sub.n4 are applied to an evaluating circuit 
included in the counter Z to permit absolute identification of the 
associated reference mark R.sub.n. If the scanning unit A scans the scale 
M in a positive measuring direction from left to right, then the next code 
mark segments C.sub.n1, m follow after the reference mark R.sub.n and 
thereafter follows the associated reference mark R.sub.n+1, and so forth. 
In order in this scanning direction to be able to recognize the first code 
mark segment C.sub.n+1, 1, the code mark segment C.sub.n+1, 1 should have 
for example the binary value "1". As a general matter, the code mark 
segments C.sub.nm can include beginning of code mark and end of code mark 
information items. With the recognition of the beginning of code mark 
information, the evaluation circuit in the counter Z can be prepared that 
thereupon code information is to be read out following the beginning of 
code mark information. The end of code mark information serves to assure 
that the entire code information has been read out. In this way, a 
possible reversal of the measuring direction in the middle of the scanning 
of code information can be recognized. 
When the scanning unit A is moved in a positive measuring direction from 
left to right the code information of the code mark segments C.sub.nm is 
scanned before the associated reference mark R.sub.n is scanned. In the 
scanning movement in a negative measuring direction from right to left, by 
means of the direction-dependent evaluation of the code information can be 
recognized that after the code mark segments C.sub.n+1, m there must 
follow the reference mark R.sub.n. 
The code mark segments C.sub.nm and the associated reference marks R.sub.n 
can follow upon one another without gaps therebetween, or alternately a 
predetermined spacing can be provided between them. The gapless 
arrangement is preferred if there is only limited space available between 
individual reference marks R.sub.n. Preferably, the code mark segments 
C.sub.nm and the reference marks R.sub.n are applied as a whole-numbered 
fraction or a whole numbered multiple of the graduation (division) period 
of the incremental graduation T on the scale M, so that the scanning of 
the code mark segments C.sub.nm and the reference marks R.sub.n occurs in 
the rhythm of the scanning of the incremental graduation T. 
FIG. 4 shows an enlarged fragmentary view of portions of the scale M of 
FIG. 2. As shown in FIG. 4, a reference mark R.sub.n as well as the 
associated code mark segments C'.sub.n1 -C'.sub.n4 and the first code mark 
segment C'.sub.n+1, 1 of the reference mark R.sub.n+1 (not shown) are 
arranged serially in sequence along the length of the scale M. The 
reference mark R.sub.n+1 (not shown) would follow the reference mark 
R.sub.n in the positive measuring direction X. Each of the nonzero code 
mark segments C'.sub.n1 -C'.sub.n4 and C'.sub.n+1, 1 -C'.sub.n+1, 4 are 
completely identical to the reference mark R.sub.n and are scanned by the 
scanning field AR of the scanning plate AP as shown in FIG. 2. 
In order to distinguish the reference mark R.sub.n from the associated code 
mark segments C'.sub.n1 -C'.sub.n4, the spacing r between the reference 
mark R.sub.n and the adjacent code mark segment C'.sub.n4 differs from the 
spacing c between adjacent individual ones of the code mark segments 
C'.sub.n1 -C'.sub.n4. In order to distinguish the reference mark R.sub.n 
from the non-associated adjacent code mark segment C'.sub.n+1, 1, the 
spacing between the reference mark R.sub.n and the code mark segment 
C'.sub.n+1, 1 is equal to the value u, which is different from the 
spacings c and r. In the example of FIG. 4, the code signals CS'.sub.n1 
-CS'.sub.n4 yield the binary signal "1101". In order to distinguish the 
reference marks R.sub.n from the code mark segments C'.sub.n1 -C'.sub.n4 
and C'.sub.n+1, 1 the corresponding spacings c, r, u' between the centers 
of gravity or centers of the reference marks R.sub.n and code mark 
segments C'.sub.nm can be used instead of the spacings c, r, u. The 
reference marks R.sub.n and code mark segments c'.sub.nm as well as the 
spacings c, r, u, c', r', u' in the measuring direction X again are 
preferably formed as whole-number parts and/or whole number multiples of 
the division or graduation period of the incremental graduation T on the 
scale M. The code signals CS.sub.nm as well as the spacings c, r, u, c', 
r', u' are evaluated in the evaluating arrangement of the counter Z. This 
evaluating arrangement can include a selected circuit which operates to 
select certain ones of the reference marks R.sub.n to be brought into 
operation from the total series of reference marks R.sub.n. 
Of course, it should be understood that a wide range of changes and 
modifications can be made to the preferred embodiments described above. 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.