Photoelectric displacement detecting apparatus

A photoelectric displacement detecting apparatus for measuring a length from a repeated number of bright and dark portions of light formed by the repetitions of overlappings of slits in respective slit rows during a relative movement between a main scale and an index scale, the apparatus comprising reference marks provided on the main scale at suitable intervals, a mark detecting device provided on the index scale, identification marks provided in spaces between the reference marks for identifying these spaces, and an identification mark reading device actuated by the mark detecting device. An identification mark ascertaining device is provided for comparing the identification mark read by the identification mark reading device with the identification mark preset by a mark selecting-setting device and ascertaining whether both identification marks coincide with each other or not. The apparatus further comprises an origin setting device for making the position of the detected reference mark to be the length measuring origin on condition of a coincidence signal from the identification mark ascertaining device and the mark detection signal from the mark detecting device immediately after the input of the coincidence signal.

DESCRIPTION 
1. Technical Field 
This invention relates to a photoelectric displacement detecting apparatus, 
and more particularly to improvements in a photoelectric displacement 
detecting apparatus comprising a main scale with a length-measuring slit 
row having regular pitches, an index scale provided thereon with a 
length-measuring gate slit row being equal in pitch to the 
length-measuring slit row of the main scale and disposed in a manner to be 
in parallel to the direction of the slit row and movable relative to the 
main scale, and a length-measuring photo-electric transducer for receiving 
light transmitted through or reflected by the main scale and the index 
scale producing a length-measuring signal from the number of repeated 
bright and dark portions of the light formed by the repetitions of 
overlappings of respective slits during a relative movement between the 
main scale and the index scale. 
2. Background Art 
The photoelectric displacement detecting apparatus of the type described 
are adopted in many fields because displacements can be detected in 
non-contact with an object to be detected and with high accuracy. However, 
since with such a functional curve that the detection signals repeat the 
increases or decreases against the length-measuring direction is drawn, 
even if a detection signal is processed to be turned into a digital 
signal, for example, and indicated digitally, the physical absolute origin 
is unclear only from this signal. In consequence, there has been needed 
another means to specify the absolute origin. 
As measures for specifying the absolute origin in the photoelectric 
displacement detecting apparatus of the type described, there have 
heretofore been types shown below. 
Firstly, there is a method for forcibly zero-setting an indicated value of 
digital indicators, including a counter and the like, when the main scale 
and the index scale are in a desirable positional relationship to each 
other. 
This method is easy and simple, however there are some cases where it is 
difficult to determine the absolute origin with the accuracy of .mu.m 
order. 
Furthermore, when the power is cut off during measurement, a repeated work 
of zero-clear is necessitated, and further, such a disadvantage has been 
presented that, even with the renewed zero-clear, it is difficult to 
zero-set an origin in coincidence with the preceding origin. 
Next, there is such a method that, on the main scale, there are provided 
reference marks in parallel to the length-measuring slit row separately of 
the length-measuring slit row, and, on the index scale, there are provided 
marks associated therewith for sensing the reference marks separately of 
the length-measuring gate slit row, whereby the reference mark is detected 
by this associated mark and the position thus detected is made to be the 
absolute origin. 
The absolute origin is accurately obtainable by this method. However, a 
multiplicity of reference marks are arranged along the length-measuring 
slit row, and at the time of actual measurement, an associated mark of the 
index scale is superposed on one of the plurality of reference marks, 
whereby the reference mark should be read to set an absolute origin. In 
consequence, it becomes necessary to conduct operations to specify one 
reference mark from the plurality of reference marks, thus presenting the 
disadvantage that the absolute origin cannot be easily and quickly set. 
Furthermore, as another measure, there is a method described in Japanese 
Patent Publication No. 40684/1983, wherein a multiplicity of reference 
marks are provided on the main scale in parallel to the length-measuring 
slit row. Reference mark selecting switches displaceable along the main 
scale are provided having one of the reference mark selecting switches 
previously fixed to a position associated with a desirable reference mark, 
and when the associated mark on the index scale detects a reference mark 
and simultaneously the reference mark selecting switch is actuated, the 
reference mark is set as the absolute origin. 
However, even with this method, it becomes necessary to conduct operations 
of judging as to which reference mark to be selected for a position where 
the reference mark selecting switch should be located, or which reference 
mark selecting switch to be used out of a plurality of associated mark 
selecting switches. This method is disadvantageous in that the absolute 
origin cannot be automatically and quickly set. 
Furthermore, the above-described provision of the reference mark selecting 
switches presents such disadvantages that the displacement detecting 
apparatus itself becomes long and large-sized, and moreover, the 
manufacturing cost increases. 
The present invention has been developed to obviate the above-described 
disadvantages of the prior art and has as its object the provision of a 
photoelectric displacement detecting apparatus wherein an absolute origin 
can be easily, quickly and automatically set. 
Furthermore, another object of the present invention is to provide a 
photoelectric displacement detecting apparatus wherein an absolute origin 
can be reliably set and detected with no increase in weight and length of 
the apparatus and no increase in cost. 
DISCLOSURE OF THE INVENTION 
The present invention contemplates a photoelectric displacement detecting 
apparatus comprising a main scale with a length-measuring slit row having 
regular pitches, an index scale with a length-measuring gate slit row 
being equal in pitch to the length-measuring slit row of the main scale 
and disposed in a manner to be parallel to the direction of the slit row 
and movable relative to the main scale, and a length-measuring 
photo-electric transducer for receiving light transmitted through or 
reflected by the main scale and the index scale producing a 
length-measuring signal from the number of repeated bright and dark 
portions of the light formed by the repetitions of overlappings of 
respective slits during a relative movement between the main scale and the 
index scale. The apparatus further comprises reference marks provided on 
the main scale in the direction of the length-measuring slit row located 
at suitable intervals, a mark detecting device provided on the index scale 
for detecting the reference marks, and identification marks arranged in 
spaces between the reference marks for identifying the spaces. The 
identification mark reading device is actuated in response to a detection 
signal from the mark detecting device to read the identification mark. The 
apparatus further comprises an identification mark ascertaining device for 
comparing the identification mark read by the identification mark reading 
device with an identification mark preset by a mark selecting-setting 
device to ascertain whether both identification marks coincide with each 
other or not, and for outputting a coincidence signal, an origin setting 
device is provided for making the position of the detected reference mark 
to be the length-measuring origin on condition of the coincidence signal 
from the identification mark ascertaining device and the mark detection 
signal from the mark detecting device immediately after the input of the 
aforesaid coincidence signal. 
To the above end, the present invention contemplates that the reference 
marks are arranged in parallel to the length-measuring slit row and the 
identification marks are arranged in patterns different from one another 
between the reference marks. 
To the above end, the present invention contemplates that the 
identification marks are constituted by identification slit rows different 
in number of slits within the respective spaces and adjacent the reference 
marks, and the identification reading device comprises reading slits 
provided on the index scale as opposed to slits of the identification slit 
rows and a reading photo-electric transducer for outputting a mark 
identification signal on the basis of the number of repetitions of the 
bright and dark portions of the received light formed by the repetitions 
of the overlappings between the reading slits and the identification slit 
rows. 
To the above end, the present invention contemplates that the numbers of 
slits of the identification slit rows between the respective reference 
marks are determined such that the numbers of the length-measuring slit 
groups in positions symmetrical to each other are made equal to each other 
in directions to opposite ends of the main scale from the substantially 
central position of the main scale. 
To the above end, the present invention contemplates that the 
identification marks are provided in the respective spaces adjacent the 
reference marks on one side only. 
To the above end, the present invention contemplates that the origin 
setting device sets the origin such that a counter for counting signals 
outputted from the length-measuring photo-electric transducer is set to 
zero. 
The above end, the present invention contemplates a photoelectric 
displacement detecting apparatus comprising a main scale with a 
length-measuring slit row having regular pitches, an index scale with a 
length-measuring gate slit row being equal in pitch to the 
length-measuring slit row of the main scale and disposed in a manner to be 
in parallel to the direction of the slit row and movable relative to the 
main scale, and a length-measuring photo-electric transducer for receiving 
light transmitted through or reflected by the main scale and the index 
scale and producing a length-measuring signal from the number of repeated 
bright and dark portions of the light formed by the repetitions of 
overlappings of respective slits during a relative movement between the 
main scale and the index scale. A plurality of reference position 
detecting sections are provided on several intermediate portions of the 
length-measuring slit row at irregular intervals longer than the 
length-measuring gate slit row, each of the sections having a length 
shorter than the length-measuring gate slit row. The length of each of the 
sections is within such a range of length that a variation in the quantity 
of light to the length-measuring photo-electric transducer does not affect 
the measuring accuracy, and the length-measuring slits are lacking from 
the positions, where the sections are provided, whereby the 
length-measuring slit row is broken off and divided into length-measuring 
slit groups different from one another in slit number. 
The apparatus comprises reference marks formed on the reference position 
detecting sections, a mark detecting device provided on the index scale 
for detecting the reference marks, and a slit group reading devices 
actuated in response to a detection signal of the mark detecting means to 
read the number of slits of said length-measuring groups. A 
length-measuring slit group ascertaining device is provided for comparing 
the number of slits read by the slit group reading means with the number 
of slits preset by a slit group selecting-setting device to ascertain 
whether both slit numbers coincide with each other or not and outputting a 
coincidence signal. The apparatus further comprises origin setting means 
for making the position of the detected reference mark to be the 
length-measuring origin on condition of the coincidence signal from the 
length-measuring slit group ascertaining means and the mark detection 
signal immediately after the input of the aforesaid coincidence signal. 
To the above end, the present invention contemplates that the mark 
detecting means is constituted by the detection marks associated in form 
with the reference marks and a photo-electric transducer for sensing an 
overlapping of the detection mark and the reference mark. 
To the above end, the present invention contemplates that the reference 
marks are formed into slits of a random pattern, and the detection marks 
of the mark detecting means are formed into reference gate slits formed of 
slits of a random pattern, which are associated with the reference marks. 
To the above end, the present invention contemplates that the reference 
marks are formed of non-slit portions provided at least at opposite sides 
of the length-measuring slit groups. 
To the above end, the present invention contemplates that the slit group 
reading means is a length-measuring counter for counting length-measuring 
signals outputted from the length-measuring photo-electric transducer. 
To the above end, the present invention contemplates a photoelectric 
displacement detecting apparatus comprising a main scale with a 
length-measuring slit row having regular pitches, an index scale with a 
length-measuring gate slit row being equal in pitch to the 
length-measuring slit row of the main scale and disposed in a manner to be 
in paralled to the direction of the slit row and movable relative to the 
main scale, and a length-measuring photo-electric transducer for receiving 
light transmitted through or reflected by the main scale and the index 
scale anrd producing a length-measuring signal from the number of repeated 
bright and dark portions of the light formed by the repetitions of 
overlappings of respective slits during a relative movement between the 
main scale and the index scale. The apparatus further comprises a 
plurality of reference marks arranged on the main scale in parallel to the 
length-measuring slit row and in irregular pitches, a mark detecting 
device provided on the index scale for detecting the reference marks, a 
slit counting device actuated in response to a detection signal from the 
mark detecting device to count the number of slits of the length-measuring 
slit groups between the reference mark and the succeeding reference mark 
when the length-measuring slits between the reference marks are formed 
into the length-measuring slit groups, an identification mark ascertaining 
device for comparing the number of slits counted by the slit counting 
device with the number of slits preset by a reference mark 
selecting-setting device to ascertain whether both slit numbers coincide 
with each other and outputting a coincidence signal, and an origin setting 
means for making the position of the detected reference mark to be the 
length-measuring origin on condition of the coincidence signal from the 
identification mark ascertaining device and the mark detection signal form 
the mark detecting device immediately after the input of the aforesaid 
coincidence signal. 
To the above end, the present invention contemplates that the slit counting 
device is a length-measuring counter for counting length-measuring signals 
outputted from the length-measuring transducer. 
To the above end, the present invention contemplates that the slit counting 
means is an auxiliary counter formed separately of the length-measuring 
counter for counting the length-measuring signals outputted from the 
length-measuring photo-electric transducer.

BEST MODE FOR CARRYING OUT THE INVENTION 
Description will hereunder be given of the present invention with reference 
to the drawings. 
According to this embodiment, a photoelectric displacement detecting 
apparatus is provided comprising a main scale 10 provided thereon with a 
length-measuring slit row 12 having regular pitches, an index scale 16 
provided thereon with a length-measuring gate slit row 14 being equal in 
pitch to the length-measuring slit row 12 of the main scale 10 and 
disposed in a manner to be in parallel to the direction of the slit row 
and movable relative to the main scale 10, and a length-measuring 
photo-electric transducer 18 for receiving light transmitted through or 
reflected by the main scale 10 and the index scale 16 and producing a 
length-measuring signal from the number of repeated bright and dark 
portions of the light formed by the repetitions of overlappings of 
respective slits during a relative movement between the main scale 10 and 
the index scale 16. The apparatus further comprises a plurality of 
reference marks 20 (20A, 20B, 20C . . . ) provided on the main scale 10 in 
the direction of the length-measuring slit row 12 at suitable intervals, a 
mark detecting means 22 provided on the index scale 16, for detecting the 
reference marks 20, identification marks 26 arranged in spaces 24 (24A, 
24B, 24C . . . ) between the reference marks 20, for identifying the 
spaces 24 identification mark reading means 28 actuated in response to a 
detection signal from the mark detecting device 22 to read the 
identification mark 26 identification mark ascertaining means 32 for 
comparing the identification mark 26 read by the identification mark 
reading device 28 with an identification mark preset by a mark 
selecting-setting device 30 to ascertain whether both identification marks 
coincide with each other or not and for outputting a coincidence signal, 
and origin setting means 34 for making the position of the detected 
reference mark to be the length-measuring origin on condition of the 
coincidence signal from the identification mark ascertaining device 32 and 
the mark detection signal from the mark detecting device 22 immediately 
after the input of the aforesaid coincidence signal. 
The reference marks 20 are arranged in parallel to the length-measuring 
slit row 12, and the identification marks 26 are disposed in patterns 
different from one another between these reference marks 20. More 
specifically, the identification marks 26 are constituted by 
identification slit rows 26A, 26B and 26C . . . different in slit number 
from one another, disposed adjacent the reference marks 20A, 20B and 20C 
and in the respective spaces 24A,24B and 24C . . . , and the 
identification mark reading device 28 includes a reading slit 36 
associated with the identification slit rows 26A, 26B and 26C . . . and a 
reading photo-electric transducer 38 for outputting a mark identification 
signal on the basis of the number of repetitions of the bright and dark 
portions of the light received which are formed by the repetitions of 
overlappings of the reading slit 36 with the identification slits 26A, 26B 
and 26C . . . . 
As shown in FIG. 1, the reference marks 20 are formed upwardly of the 
length-measuring slit row 12 of the main scale 10, in parallel to the 
length-measuring slit row 12 and at regular intervals. 
Furthermore, the identification slit rows 26A, 26B and 26C . . . are about 
one half the length of the reference marks 20. Referring to FIG. 1, in the 
space 24A between the reference mark 20A at the left end and the reference 
mark 20B disposed rightwardly of the reference mark 20A, one pitch of the 
identification slit row 26A is formed contiguously to the right side of 
the reference mark 20A and also two pitches of the identification slit row 
26A are formed contiguously to the left side of the reference mark 20B, 
thus totally forming three pitches. 
Furthermore, referring to FIG. 1, the identification slit row 26B provided 
in the space 24B between the second reference mark 20B from left and the 
third reference mark 20C rightwardly adjacent the reference mark 20B 
includes two pitches disposed contiguously to the right side of the 
reference mark 20B, as shown, and two pitches disposed contiguously to the 
left side of the reference mark 20C, thus totally forming four pitches. 
Thus, the identification slit row 26C in the space 24C totals up to five 
pitches, and the identification slit row 26D in the space 24D totals up to 
six pitches. 
The mark detecting means 22 is constituted by a reference gate slit 23 
having a height and a pitch being equal to the reference marks 20 and a 
reading photo-electric transducer 38 in the identification mark reading 
means 28. Furthermore, a reading slit 36 in the identification mark 
reading device 28 is constituted by slits disposed contiguously to an 
opposite side of a standard gate slit 23 and each having a height being 
equal to the slit of the identification slit row in the identification 
mark 26. 
Furthermore, the length-measuring gate slit row 14 is constituted by a pair 
of a first and a second gate slit rows 14A and 14B, which are shifted by 
one-half pitch from each other. 
These first and second length-measuring gate slit rows 14A and 14B are 
disposed separately of each other in the direction of the relative 
movement between the index scale 16 and the main scale 10, and one of the 
length-measuring gate slit rows is shifted by one half pitch from the 
other. 
Furthermore, the length-measuring photo-electric transducer 18 is 
constituted by first and second length-measuring photo-electric 
transducers 18A and 18B as opposed to the first and the second 
length-measuring photo-electric transducers 14A and 14B. 
In FIG. 2, designated at 40 is a signal processing section, which converts 
an analogue input signal from the first length-measuring photo-electric 
transducer 18A into a digital pulse signal and outputs the same to a 
length-measuring counter 42, discriminates whether the index scale 16 
moves to the right or to the left in the drawing from a phase difference 
between input signals from the first and the second length-measuring 
photo-electric transducers 18A and 18B, and outputs a plus or a minus 
signal to the length-measuring counter 42 depending on the direction of 
the movement. 
Furthermore, in FIG. 2, denoted at 44 is a signal processing section of the 
reference mark detecting device 22 and 46 and identification mark counter 
of the identification mark reading means 28, respectively. When the 
reference mark detecting device 22 detects the reference mark 20, the 
signal processing section 44 is adapted to output a counter clear signal 
to an identification mark counter 46 and a mark detection signal to the 
origin setting device 34, respectively. 
Furthermore, the identification mark counter 46 is adapted to count the 
slit number of the identification slit row read by the identification mark 
reading device 28 and outputs the same to the identification mark 
ascertaining device 32. 
In the drawing, indicated at 48 is an indicating section, which is adapted 
to indicate or record a measuring dimension corresponding to outputs from 
the length-measuring counter 42, i.e. pulse count number. 
Description will now be given of action of the above embodiment with 
reference to FIGS. 3 and 4. 
Firstly, in FIG. 1, when the reference mark 20D, which is fourth from the 
left end, is made to be an absolute origin, the slit number or the pitch 
number of the identification slit row 26C is set by the mark 
selecting-setting device 30. 
As shown in FIG. 3, in step 101, the identification ascertaining device 32 
reads a value set by the mark selecting-setting device 30, i.e. a signal 
for identifying the identification slit row. 
In this state, when the index scale 16 is moved to the right in FIG. 1 
relative to the main scale 10, the reading slits 36 in the identification 
mark reading device 28 provided on the index scale 16 successively scans 
the identification slit rows 26B, 26C and 26D. 
At the same time, the reference gate slit 23 interposed between the reading 
slits 36 scans the reference marks 20B, subsequently, the reference mark 
20C, and further, the reference mark 20D, successively. 
In this case, on the basis of the number of the bright and dark portions of 
the light received by the reading photo-electric transducer 38, the 
identification mark counter 46 counts the slit numbers of the 
identification slit rows 26B, 26C and 26D, successively (Refer to Step 
102). More specifically, as shown in FIG. 4, there are counted the number 
8 of intersections of the signal waveforms and the reference mark level 
between the reference marks 20C and 20D and the number 9 between the 
reference marks 20D and 20E, respectively. 
Furthermore, when the reference gate slit 23 in the mark detecting means 22 
coincides with the reference mark 20, since the reference gate slit 23 and 
the reference mark 20 has a slit length as long as about two times the 
slit length of the identification slit rows 26B, 26C and 26D, the output 
from the reading photo-electric transducer 38 becomes larger than those in 
the identification slit rows as shown in FIG. 4, so that the reference 
marks 20 can be detected as distinguished from the identification slit 
rows. When the reference mark is detected, a reference gate signal is 
outputted to the identification mark counter 46, the identification mark 
ascertaining device 32 and the origin setting device 34 (Refer to Step 
103). 
The identification mark ascertaining device 32 reads a counted number of 
the identification mark counter 46 in response to a reference gate signal 
inputted from the signal processing section 44 (Refer to Step 104). When 
the counted number thus read is equal to a value set by the mark 
selecting-setting device 30, a coincidence signal is outputted to the 
origin setting means 34 (Refer to Step 105). 
When inputted thereto with the coincidence signal from the identification 
mark ascertaining device 32 and the reference gate signal outputted from 
the signal processing section 44 at the time of detecting the succeeding 
reference mark 26D, this origin setting device 34 causes the 
length-measuring counter 42 to clear and makes it to be an absolute origin 
(Refer to Sep 106). 
Furthermore, while outputting the coincidence signal between the set value 
and the counted value to the origin setting device 34, the identification 
mark ascertaining device 32 also outputs the coincidence signal to the 
identification mark counter 46 to cause it to clear (Refer to Step 107). 
Furthermore, the identification mark ascertaining device 32 clears the 
counted value reading, when the counted value differs from the value set 
by the mark selecting-setting device 30 (Refer to Step 108), and also 
outputs a clear signal to the identification mark counter 46 to clear it 
(Refer to Step 109). 
In consequence, the indication section 48 indicates or records a distance 
from the absolute origin set as above. 
In the above case, as shown in FIG. 4, the pulse number obtained from 
signals corresponding to the identification slit row 26C of the space 24C 
after the reference mark 20C is detected and counted, whereby the 
reference mark 20D is identified. When the index scale 16 is moved from 
the right to the left in FIG. 1 relative to the main scale 10, the 
reference mark 20D is identified on the basis of the pulse number of the 
identification slit row 26D. 
When the reference mark 20 at another position is made to be the absolute 
origin, a pertinent identification slit row is selected by the mark 
selecting-setting device 30, and the selected one is read by the 
identification mark ascertaining device 32, so that the reference mark 20 
adjacent the pertinent identification slit row can be made to be the 
absolute origin. 
If such an arrangement is adopted that, in the mark selecting-setting 
device 30, an absolute origin can be successively changed and selected in 
accordance with a predetermined program, then, the mark selecting-setting 
device 30 is interlocked with a control device of a numerically controlled 
machine tool or the like, for example, whereby the absolute origin is 
successively changeable according to the order of machining works, so that 
the optimum measuring can be automatically conducted. 
Furthermore, the absolute origin can be quickly and easily changed in 
accordance with the shape and size of a material to be worked on through 
machining. 
Here, the identification slit rows 26A, 26C and 26C . . . are formed such 
that the pitch number is successively increased from the side of 26A. 
However, the pitch number may be increased or decreased at ransom, and the 
increased pitch number may be one pitch at the minimum. 
Namely, when widths of the bright and the dark portions of the 
length-measuring slit row 12 in the length-measuring slit row 12 and of 
the length-measuring slit in the length-measuring gate slit row 14 are 
made to be 4 .mu.m for example, respectively, as described above, and an 
analogue signal obtained from the length-measuring photo-electric 
transducer 18 is divided into eight portions, one of the pulse signals 
introduced from the signal processing section 38 comes to be 1 .mu.m. 
Since this displacement detecting apparatus has a resolving-power of 1 
.mu.m, even if a difference between the length-measuring slit groups is 
one pitch of the length-measuring slit, it can be discriminated. 
Additionally, in the above embodiment, the identification slit row is 
specified by the mark selecting-setting device 30 on the basis of the 
pulse number corresponding to the slit row. However, this specification 
may be made by some other specifying means, e.g., the specification may be 
made by use of the slit number of the identification slit row from the end 
portion of the main scale 10. Or, when the absolute origin is successively 
changed in the numerically controlled machine tool or the like as 
described above, the length-measuring slit group may be specified by use 
of a control signal of a control device 50 of the machine tool or the like 
as indicated by two-dot chain lines in FIG. 2. 
Furthermore, in the above embodiment, description has been given to the 
case where the index scale 16 moves relative to the main scale 10 from the 
left to the right in FIG. 1. However, as the means for specifying the 
reference mark 20 in relation to the identification slit row, the 
identification slit row disposed rightwardly of the reference mark 20 in 
the drawing or the identification slit rows on opposite sides of the 
reference mark 20 may be utilized to specify the absolute origin. 
Furthermore, in the above embodiment, the slit numbers of the 
identification slit rows have been successively increased from the left to 
the right in the drawing as described above. However, the slit numbers may 
be successively decreased, or, each of pairs of identification slit rows 
arranged at symmetrical positions to the right and left from the central 
portion of the main scale 10 may be equal in slit number to each other for 
example. 
In this case, there is no need of discriminating the movement to the right 
or left of the index scale. 
Further, the identification slit rows may be arranged such that the slit 
numbers are circulated in a successive increase or decrease. 
Furthermore, the identification slit rows in the identification mark 26 are 
arranged on the right and left sides of the reference mark 20. However, in 
one and the same space 24, the identification slit rows in the space 24 
may be arranged on the right or left side of the reference mark 20 in one 
way or the other. 
When the identification slit rows of each of the identification marks are 
arranged on the right or left side of the reference mark 20 as described 
above, there is no need of discriminating the movement to the right or 
left of the index scale 16 relative to the main scale 10. 
Furthermore, the reference marks 20 have been formed of slits. However, the 
present invention need not necessarily be limited to this, and, for 
example, the reference marks 22 may be formed of marks completely 
different from the slits. In these cases, the reference gate slits 34 
should necessarily be marks for detecting patterns associated with the 
reference marks 22. 
Furthermore, identification slit rows 25 have had slits shorter than the 
slits of the reference marks 20, and further, as associated therewith, the 
reference gate slits 23 have been formed to be long and the reading slits 
36 short. However, such an arrangement may be adopted that, as shown in 
FIG. 5(A) for example, the reference marks 20 are formed of slits of 
random patterns similar to those in the embodiment shown in FIG. 1, the 
identification slit row 26 has slits of very small patterns for the 
transmission or the reflection, and further, the reference gate slit 23 
and the reading slits 36 have a slit or slits of random patterns 
associated with the reference marks 20. 
In this case, signals having levels of two types different from each other 
as shown in FIG. 5(B) can be obtained from outputs of a single light 
receiver, so that the reference marks can be detected. 
Furthermore, the identification slit row 26 has been provided as being 
associated with one of the reference marks 20, however, such an 
arrangement may be adopted that two or more, e.g., ten reference marks 20 
are formed into a group, an identification slit row 26 is provided for 
identifying this group, this identification slit row 26 is detected, and 
thereafter, a predetermined absolute origin can be found from the number 
of the reference marks 20 detected, i.e., the number of the reference gate 
signals. 
In this case, a counter for counting the reference gate signals should be 
provided separately. 
In this embodiment, even when a large scale of five meter, for example, in 
which absolute origins are arranged at regular intervals of 50 mm, is 
used, one identification slit row should be provided for every ten 
absolute origins. After all, the identification of ten identification slit 
rows 26 should be performed. 
Furthermore, in the above embodiment, the identification mark ascertaining 
device 32 is adapted to compare the pulse number specified by the mark 
selecting-setting device 30 with the pulse number inputted from the 
identification mark reading device 32, and output a coincidence signal 
when both pulse numbers coincide with each other. However, if the 
coincidence signal is outputted only when both pulse numbers accurately 
coincide with each other, an accurate outputting operation may become 
difficult to perform in timing. In consequence, a predetermined tolerance 
is provided above and below a preset value, and the coincidence signal 
should be outputted when both pulse numbers coincide with each other 
within this tolerance. However, in this case, each of the differences in 
slit number between the respective identification slit rows should be made 
larger in value than the tolerance. 
In the above embodiment, the reference marks 20 are arranged in parallel to 
the length-measuring slit row 12 and the identification marks 26 are 
arranged between the respective reference marks 20 in patterns different 
from one another. However, such an arrangement may be adopted that, for 
example, as shown in FIGS. 6 and 7, the reference marks 20 (20A, 20B and 
20C . . . ) are formed at a plurality of portions where the 
length-measuring slits are removed at irregular intervals on the 
length-measuring slit row 12, the identification marks are constituted by 
the length-measuring slit groups 12A, 12B and 12C . . . and the 
identification mark reading means is formed of a length-measuring counter 
42 for counting a length-measuring signal outputted from the 
length-measuring photo-electric transducer 18. 
In this embodiment, the reference gate slit 23 is disposed in series with 
the length-measuring gate slit row 14, and the reference marks 20 and the 
reference gate slit 23 are formed of slits of a random pattern discernible 
from the length-measuring slit row 12 or a slit row identical with the 
length-measuring slit row 12 and having blank portions on opposite sides 
thereof to be distinguished from the length-measuring slit row 12. 
The arrangements other than the above are similar to those in the first 
embodiment shown in FIG. 2, whereby same reference numerals in FIG. 2 are 
used to designate same or similar parts corresponding to ones as shown in 
FIG. 2, so that the detailed description will be omitted. 
In this embodiment, the reference marks 20 are provided at several 
intermediate positions of the length-measuring slit row 12, so that the 
width of the main scale 10 can be decreased. 
Furthermore, as far as the respective length-measuring slit groups 12A, 12B 
and 12C . . . are equal in their pitches, the lengths of the blank 
portions of the length-measuring slit row 12, where the reference mark 20 
is provided, may not be constant, so that the main scale 10 can be easily 
produced. 
Description will hereunder be given of the third embodiment of the present 
invention with reference to FIGS. 8 and 9. 
In this third embodiment, the reference marks 20 (20A, 20B and 20C . . . ) 
are arranged along the length-measuring slit row 12, in parallel thereto 
and at irregular intervals, the reference marks are constituted by the 
length-measuring slit row 12 disposed between the reference marks 20, and 
the identification mark reading device is formed of the length-measuring 
counter 42 for counting the length-measuring signal outputted from the 
length-measuring photo-elecric transducer 18. 
More specifically, in this embodiment, the length-measuring slit row 12 
disposed between the respective reference marks 20A, 20B and 20C . . . are 
made to be the respective length-measuring slit groups 12A, 12B and 12C . 
. . and the length-measuring slit groups are different in slit number from 
one another, to thereby constitute the identification marks. 
In consequence, in this embodiment, the slit numbers of the 
length-measuring slit groups 12A, 12B and 12C . . . which are associated 
with the reference marks 20, respectively, are counted, thus enabling to 
specify the reference marks 20. 
The arrangements other than the above are similar to those in the first 
embodiment, whereby same reference numerals are used to designate same or 
similar parts corresponding to one as shown in the first embodiment, so 
that the detailed description will be omitted. 
INDUSTRIAL APPLICABILITY 
As has been described hereinabove, the present invention is so useful that, 
in the photoelectric displacement detecting apparatus, the absolute origin 
can be easily, reliably and automatically set with a simplified 
arrangement and without the apparatus being large-sized, increased in 
weight and increased in cost to a considerable extent.