Scale for use for measurement of the displacement of an object to be examined, and displacement measuring apparatus

A scale, for measuring the displacement of an object to be examined, has a division and a mark for alignment of the division, and a displacement measuring apparatus, for measuring the displacement of a scale, has a device for reading the division of the scale and a detector for detecting the positional error of the division relative to the direction of the displacement.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a scale for use for measurement of the 
displacement of an object to be examined and to a displacement measuring 
apparatus. More particularly, the invention relates to a scale and a 
measuring apparatus which display their effects when it is desired to 
highly accurately measure the displacement of an object to be examined to 
obtain, for example, the amount of movement and the speed of movement of 
such object. 
2. Related Background Art 
Heretofore, a linear encoder has often been used for the detection of the 
position and amount of movement of an object to be measured such as an X-Y 
stage. Particularly, optical type linear encoders have been used in 
various fields because of their ability to accomplish highly accurate 
measurement of displacement. 
FIG. 1 of the accompanying drawings schematically shows the construction of 
an optical type linear encoder according to the prior art. In FIG. 1, the 
reference numeral 61 designates a scale provided with a division 61a. The 
reference numeral 68 denotes detecting means provided therein with a 
light-emitting element and a light receiving sensor for receiving the 
light from the division 61a and reading the division 61a. The reference 
numeral 69 designates an object which is a movable stage movable in a 
predetermined direction as indicated by arrow A. The scale 61 is attached 
to the movable stage 69, and is moved with the movement of the movable 
stage 69 relative to the detecting means 68 fixed to a base plate, not 
shown. 
The linear encoder shown in FIG. 1 detects the displacement of the division 
61a on the scale 61 resulting from the movement of the movable stage 69, 
by the detecting means 68, thereby detecting the amount of displacement of 
the movable stage 69. 
The division 61a comprises light-transmitting portions and 
light-intercepting portions both having a slit-like shape, and these 
portions are alternately arranged at a predetermined pitch in a direction 
orthogonal to the lengthwise direction of the slit (the widthwise 
direction). The light from the aforementioned light-emitting element 
irradiates an area including several light-transmitting portions and 
light-intercepting portions, and the aforementioned light receiving sensor 
photoelectrically converts the light passed through the light-transmitting 
portions. 
FIG. 2 of the accompanying drawings is an illustration showing a state in 
which the direction of movement A of the movable stage 69 and the 
direction of arrangement of the portions of the division 61a of the scale 
61 are not coincident with each other but are inclined, and FIG. 3 of the 
accompanying drawings is an enlarged view showing the state of the 
division 61a of the scale 61 in the state shown in FIG. 2. 
If as shown in FIGS. 2 and 3, the direction of movement A of the movable 
stage 69 and the direction of arrangement B of the portions of the 
division 61a of the scale 61 are not coincident with each other, the 
output signal from the detecting means 18 will be as follows: 
EQU .epsilon.=L2-L1 
EQU L1=L2 cos .theta. 
.epsilon.=L2(1-cos .theta.) 
where .eta. is the angle formed between the direction A and the direction 
B, L1 is the read value of the division 61a of the scale 61 when the 
movable stage 69 is displaced, L2 is the actual amount of movement of the 
movable stage 69, and .epsilon. is the difference between L1 and L2. 
Accordingly, assuming that L2=100 mm and .theta.=0.08.degree., there occurs 
an error of .epsilon.=0.1 .mu.m, and simply because the division is 
inclined by only 0.08.degree. with respect to the direction of movement of 
the object to be examined, accurate measurement in the unit of submicron 
becomes impossible. 
SUMMARY OF THE INVENTION 
The present invention has been made in view of the above-noted problem 
peculiar to the prior art and the object thereof is to provide a scale and 
an apparatus which can accomplish highly accurate measurement of 
displacement. 
To achieve this object, the scale of the present invention is for measuring 
the displacement of an object to be examined and is characterized by a 
division and a mark for alignment of said division, and the apparatus of 
the present invention is for measuring the displacement of the scale and 
is characterized by means for reading the division of the scale and means 
for detecting the positional error of said division relative to the 
direction of said displacement. 
The apparatus of the present invention has positional deviation detecting 
means and can therefore be adjusted so as to correct the detected 
positional error. This adjustment includes adjusting the position of the 
scale, or correcting the signal from the reading means so that said 
positional error may not affect the result of measurement. Also, by using 
the scale of the present invention as a scale, positional deviation can be 
detected accurately. 
Further features and specific forms of the present invention will appear in 
detail in the following description of some embodiment thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 4 which is a schematic view showing an embodiment of the 
present invention, the reference numeral 1 designates the scale of the 
present invention which has a division and a mark for alignment of the 
division formed on a transparent base plate. The scale 1 is attached to a 
movable stage as shown in FIG. 1. Denoted by 1a is a division similar to 
the division 61a of FIG. 1, and the division 1a comprises 
light-transmitting portions and light-intercepting portions arranged along 
a predetermined direction. The reference numeral 40 designates a straight 
mark for alignment, and this mark is provided so that the lengthwise edge 
thereof is parallel to the direction of arrangement of the portions of the 
division 1a (said predetermined direction). The mark 40 is formed on the 
surface of the scale 1 with a reflecting film or a light-intercepting film 
provided thereon, and is designed such that a light irradiating the mark 
does not pass through the mark. The reference numeral 2 denotes a 
light-emitting element comprising an LED, and the reference numeral 3 
designates a condensing lens for condensing the light beam from the 
lightemitting element 2 and forming a light spot on the scale 1 so as to 
illuminate the mark 40 by part by the light beam. The reference numeral 4 
denotes a light receiving element for receiving the part of the light beam 
from the light-emitting element 2 which has passed through the upper or 
lower transparent portion of the mark 40. 
The light-emitting element 2, the condensing lens 3 and the light receiving 
element 4 function as detecting means for detecting any positional 
deviation of the division, and these are integrated and contained in a 
case, not shown. The division 1a is read by the detecting means 68 
described with reference to FIG. 1. 
In the present embodiment, the light beam from the light-emitting element 2 
is caused to enter the scale 1 by the condensing lens 3. The light beam 
passed through the marginal transparent portion of the mark 40 is received 
by the light receiving element 4. The shape of the mark 40 provided on the 
scale 1 and the shape of the light spot formed on the scale 1 are 
determined so that when the movable stage which is an object to be 
examined is moved and thereby displaced from end to end, at least the 
quantities of light entering the light receiving element 4 at the opposite 
ends of the mark 40 become equal when the direction of movement A of the 
movable stage and the direction of arrangement of the portions of the 
division la of the scale 1 (the direction B in FIG. 3) coincide with each 
other. 
Specifically, design is made such that when the light spot 45 illuminates 
the vicinity of the end portions of the mark 40, one half of the light 
spot (substantially circular) is intercepted by the mark 40 and the other 
half of the light spot passes through the scale 1 and is received by the 
light receiving element 4, and the lengthwise edges of the mark 40 are 
parallel to each other and the width of the mark 40 does not vary from end 
to end and therefore, if the mark 40 is not inclined with respect to the 
direction of movement A of the scale 1, the quantity of light received by 
the light receiving element 4 is constant even if the scale 1 is displaced 
in the direction A as previously described. 
On the other hand, if the direction of arrangement of the portions of the 
division 1a is inclined with respect to the direction of movement A of the 
scale 1 (the two directions are not coincident) and the edges of the mark 
40 are inclined with respect to the direction of movement A, the quantity 
of light received by the light receiving element 4 varies gradually in 
conformity with the displacement of the scale 1 in the direction A. 
Accordingly, the level of the output signal from the light receiving 
element 4 varies gradually. 
Thus, in the present embodiment, before the displacement of the scale 1 
(the movable stage) is detected, the position of the scale 1 is adjusted 
while the output signal from the light receiving element 4 is monitored by 
the use of the detecting means (2, 3, 4) and the mark 40 so that the 
quantity of light passed through the vicinity of the mark 40 on the scale 
and entering the light receiving element 4 may not vary with the movement 
of the scale 1, whereby the direction of movement of the scale 1 (the 
movable stage) and the direction of arrangement of the portions of the 
division 1a of the scale 1 are made coincident with each other. 
By the direction of arrangement of the portions of the division 1a of the 
scale 1 and the direction of movement of the scale 1 being thus made 
coincident with each other, the signal obtained when the division 1a is 
read by detecting means, not shown, exactly corresponds to the amount of 
movement of the scale 1. Consequently, the displacement of the scale 1, 
i.e., the object to be examined, can be measured very accurately. 
In the present embodiment, the detecting means (2, 3, 4) is designed such 
that the light transmitted through the scale 1 is received by the light 
receiving element 4, but alternatively, the light receiving element 4 may 
be installed on the same side as the light-emitting element 2 with respect 
to the scale 1 and the reflected light from the mark 40 may be received by 
the light receiving element 4. 
Also, the division la of the scale 1 in the present embodiment, like the 
division 61a shown in FIG. 1, is comprised of slit-like light-transmitting 
portions and light-intercepting portions alternately arranged, but 
alternatively, may be comprised of slit-like light-transmitting portions 
and reflecting portions alternately arranged. To increase the resolving 
power of the scale and of the apparatus, a diffraction grating is used as 
the division 1a. The available forms of this diffraction grating include 
various forms such as a construction in which the above-described slitlike 
portions are arranged at a very small pitch, a hologram and a relief type 
grating in which grooves are periodically arranged on a transparent 
substrate. Such diffraction grating is illuminated by light and several 
diffracted lights created by the diffraction grating are caused to 
interfere with one another to thereby form an interference fringe, which 
is photoelectrically converted to thereby obtain a signal conforming to 
the displacement of the scale. Various detecting means for reading this 
diffraction grating have heretofore been proposed and are well known to 
those skilled in the art and therefore need not be described in detail 
herein, but it should be noted that the present invention is suitable for 
a displacement measuring apparatus having a high resolving power of this 
type. 
FIG. 5 shows a modification of the mark 40 provided on the scale 1 of the 
embodiment shown in FIG. 4. In FIG. 5, the reference numeral 41 designates 
marks for alignment. Instead of the mark 40 being provided so as to cover 
the whole range of the division 1a of the scale 1 as in the embodiment 
shown in FIG. 4, the marks 41 are provided at locations adjacent to the 
opposite ends of the division la of the scale 1. The marks 41 of the 
present embodiment each comprise a rectangular reflecting film, and the 
lengthwise direction of the marks 41 coincides with the direction of 
arrangement of the portions (the light-transmitting portions and the 
light-intercepting portions) of the division 1a. That is, the marks 41 are 
formed so that the center lines (the phantom lines) of the marks 41 
widthwisely bisecting the marks 41 which correspond to the lengthwise 
direction of the marks 41 coincide with each other. As in the embodiment 
shown in FIG. 4, the position of the scale 1 is adjusted so that the 
quantities of light passed through the transparent portions near the marks 
41 at the opposite ends and entering the light receiving element 4 become 
equal, whereby the direction of movement A of the scale 1 (the movable 
stage) and the direction of arrangement B of the portions of the division 
la are made coincident with each other. 
FIG. 6 is a schematic view showing another embodiment of the present 
invention. In FIG. 6, the reference numeral 60 designates marks each 
comprising a narrow slit. The marks 60 are provided near the opposite ends 
of the same division 1a as those in the previously described embodiments, 
and are formed of a reflecting film which reflects light. Like the marks 
41 of FIG. 5, the two marks 60 are such that the segment liking (the 
centers of) these marks is parallel to the direction of arrangement of the 
portions of the division 1a of the scale 1. The reference numeral 5 
denotes a light receiving portion comprising two divided sensors 5a and 
5b. The sensors 5a and 5b output signals conforming to the quantities of 
light independently entering them. The two sensors 5a and 5b are arranged 
in a direction orthogonal to the direction of arrangement of the portions 
of the division 1a of the scale 1. By a condensing lens 3, the mark 60 on 
the scale 1 is imaged on the two sensors 5a and 5b so that the center line 
of the image of the mark 60 coincides with the border line of the sensors 
5a and 5b. The reference numeral 2 designates a light-emitting element, 
and the reference numeral 6 denotes a half-mirror. 
The light-emitting element 2, the light receiving portion 5, the 
half-mirror 6 and the condensing lens 3 together constitute positional 
deviation detecting means, and these are integrated and contained in a 
case, not shown. This case is secured on a base plate, not shown. 
In the present embodiment, part of the light beam from the light-emitting 
element 2 is reflected by the half-mirror 6 and is directed onto the mark 
60 on the scale 1 by the condensing lens 3. The reflected light beam from 
the mark 60 is condensed by the condensing lens 3 and is passed through 
the half-mirror 6, whereafter it is caused to enter the light receiving 
portion 5 and the image of the mark 60 is formed on the sensors 5a and 5b. 
Accordingly, the reflected light beams from the two marks 60 at the 
opposite ends of the scale 1 (the mark images) are successively received 
by the two sensors 5a and 5b, and during the detection of the marks, the 
inclination of the scale 1 is adjusted so that the output signals from the 
two sensors 5a and 5b become equal to each other, whereby the direction of 
arrangement of the portions of the division 1a of the scale 1 and the 
direction of movement A of the scale 1 (the movable stage) are made 
coincident with each other. 
In the embodiments described above, the marks provided on the scale 1 may 
be constructed of a transmitting portion having its periphery shielded 
from light, instead of being formed of a reflecting film. Also, the marks 
may be constructed, for example, of a member having magnetism, instead of 
an optical member, and (the positions of) the marks may be read by 
magnetic detecting means. 
In each of the embodiments described above, a mark or marks for alignment 
are provided on the scale, but without such a mark or marks being 
provided, the deviation of the direction of arrangement of the portions of 
the division relative to the direction of movement of the scale can still 
be detected. One method is to utilize, for example, the light-transmitting 
portions and light-intercepting portions constituting the division as a 
pattern replacing the mark or marks for alignment. Another method is to 
utilize the signal from detecting means for reading the division. The 
detecting means of this type produces a sine-wave-like pulse in response 
to the displacement of the scale, and the width of this pulse is 
determined in conformity with the pitch of the division in the direction 
of movement of the scale 1. Accordingly, if the division becomes inclined 
with respect to the direction of movement of the scale, this pulse width 
varies and therefore, the inclination of the scale can be detected on the 
basis of this pulse width. Consequently, the deviation of the direction of 
arrangement of the portions of the division relative to the direction of 
movement of the scale can be detected on the basis of the signal from this 
detecting means. 
Of course, the position of the scale 1 can be adjusted on the basis of the 
deviation detected by said another method to thereby improve the 
measurement accuracy, and it is also possible to improve the measurement 
accuracy without adjusting the position of the scale. FIG. 7 shows such 
embodiment. 
FIG. 7 is a schematic view showing still another embodiment of the present 
invention. In FIG. 7, the reference numeral 68 designates detecting means 
for reading the division 1a of the scale 1, the reference numeral 69 
denotes a movable stage movable in the directions of arrows A, the 
reference numeral 71 designates a processing device for receiving a signal 
from (the light receiving element of) the detecting means 68 and 
processing it, and the reference numeral 72 denotes an indicator for 
indicating the amount of movement of the scale 1 (the movable stage 69) 
measured by the processing device 71. 
The processing device 71 has the function of detecting the deviation of the 
direction of arrangement of the portions of the division 1a of the scale 1 
relative to the direction of movement A of the stage 69 (the angle .theta. 
formed between the two directions) on the basis of the signal from the 
detecting means 68 by the method as described previously. As indicated by 
a broken line, the processing device 71 may be endowed with the function 
of calculating such deviation on the basis of the signal from the light 
receiving element 4 of the detecting means (2, 3, 4; 2, 3, 4, 5, 6) shown 
in FIGS. 4-6. 
Here, if the angle .theta. between the direction of arrangement of the 
portions of the division of the scale and the direction of movement A of 
the stage is detected by the processing device 71, the amount of movement 
L1 read by the signal obtained from the detecting means 68 can be 
converted into L2 by the processing device 71 by the use of a conversion 
equation L2=L1/cos .theta., whereby the true amount of movement L2 of the 
scale 1 can be obtained easily and the correct amount of movement of the 
scale 1, i.e., the movable stage 69, can be indicated by the indicator 72 
provided at the subsequent stage. 
According to the apparatus of the present embodiment, the positional 
adjustment of the scale 1 is not effected and therefore, the setting time 
can be shortened. Also, as required, the angle .theta. may be periodically 
found to thereby automatically correct the influence thereof.