Method and apparatus for detection of position with correction of errors caused by errors in scale pitch

A method for detecting a position wherein a signal outputted from a position detecting scale is detected by at least two detectors, an output value of a preceding detector of these two detectors is stored anew at the time when an output value of a following detector of the two detectors comes near a scale position on which the output value of the preceding detector is once stored and becomes equal to the output value of the preceding detector, and a position detecting signal is outputted, these steps being repeated to detect the position.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for detecting a position of an 
object by detecting a signal from a scale serving as a reference for a 
linear encoder, a rotary encoder, etc., and more particularly to a method 
for detecting a position so as to receive a position detecting signal 
which is not affected by pitch error of a scale. 
2. Prior Art 
According to known methods for detecting a position, photoelectric elements 
read an occulting light or a moire pattern for each pitch of an optical 
scale or a moire scale, each such pitch serving as a reference of a linear 
encoder, a rotary encoder etc., Alternatively one pitch of an electric or 
magnetic scale is detected by inducing voltage in order to detect a moving 
direction of a measuring object, finally detecting a position thereof. In 
other words, a pitch scale of various scales is detected by an optical 
sensor, an electrical sensor, a magnetic sensor, etc., and a position is 
detected based on signals detected by these sensors. 
In this way, according to the known methods, since the scale to be used as 
a reference is read by photoelectric or magnetic means and the read 
information is detected and operated to output a position signal, it is 
required to minimize error of the scale itself in order to improve a 
position detecting accuracy. However, even if the error of scale itself is 
minimized, in case of a cylindrical drum rotary type scanner, for example, 
another erroneous factor due to rotation of a drum may be a problem since 
a signal of an encoder coupled coaxially with the drum is used to obtain a 
main scanning position signal. In other words, since the drum is a 
rotational body, a little radial swing cannot be avoided, and some error 
in a recording position on the drum surface is inevitably caused by this 
radial swing. Moreover, since the drum is connected with the encoder by 
coaxial coupling in this kind of scanner, an accurate centering is 
required. 
In order to meet the foregoing erroneous factor of the drum, an attempt has 
been proposed wherein the scales for position signal are formed on the 
periphery of a drum to be detected. 
In this way, even if error of the scale to be detected is minimized, 
another error in the mechanical system for driving the measuring object is 
further involved in. Accordingly, in the present state of art, it is 
generally understood that when reading information from each pitch on the 
scale by some means to obtain a position detecting signal based on the 
information, an error between two adjacent pitches as well as an error 
accumulated through the process of reading is unavoidable to a certain 
extent. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a method 
for detecting a position in which a position detecting signal can be 
obtained on the basis of a certain distance without being affected by 
external factors by detecting the foregoing error of the scale to be 
corrected so that an accurate position detection can be performed based on 
an output signal which does not include any pitch error of the scale, thus 
solving the above-discussed problems of the prior arts. 
In order to accomplish the foregoing object, a method for detecting a 
position in accordance with the present invention comprises a step of 
detecting a signal pitch by pitch, i.e., detecting each pitch by at least 
two detectors, said signal being outputted from a position detecting scale 
set to a measuring object moving relative to a measurement system to be 
read by photoelectric or magnetic means, a step of storing an output value 
of a part of the scale delivered from a first detector which precedingly 
detects a signal from the position detecting scale moving relative to the 
measurement system, a step of comparing the stored output value with an 
output value from a second detector which subsequently detects a signal 
from an area the same as said part, a step of outputting a position 
detecting signal when said two output values are coincident and a step of 
simultaneously storing an output value of the preceding first detector, 
said steps being repeated to obtain further position detecting signals to 
detect the position of the measuring object. 
By arranging the foregoing method for detecting a position, a position 
detecting signal is outputted at the time when a signal of the part on 
which the preceding first detector has detected a signal from the scale 
has been detected by the second detector, the output value from the first 
detector is restored at the same time, and a position detecting signal is 
outputted again when the second detector has detected the same value as 
the restored value, and thereafter the position detecting signal is 
outputted every time when the output value of the first detector becomes 
equal to the output value of the second detector, in other words, the 
position detecting signal is outputted on the basis of a distance fixed 
between the first detector and the second detector, thereby signals of 
uniform pitch being outputted. 
Other objects and features of the present invention will become apparent in 
the course of the following description in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 1, 2 and 3 showing an embodiment of the present 
invention, a position detecting scale (2) in the rotating direction is 
formed on a periphery of a drum by corroding a surface of the drum through 
etching or the like so that a light is irregularly reflected on the etched 
portion while being regularly reflected on the non-etched portion. In this 
embodiment, the scale thus prepared is adapted to be optically read. A 
position detecting scale (14) in the subscanning direction is Similarly 
prepared and is also eptically read. 
Prior to the description of the principle with reference to FIG. 1, an 
arrangement of one optical system of this embodiment is described 
referring to FIGS. 2 and 3. 
In FIG. 2, a rotary drum (1) is driven by a drive motor not illustrated. In 
this connection, the drum is supposed to be rotated in the main scanning 
direction. A recording head (3) moves in the subscanning direction by a 
feed screw driven by a drive motor (5), i.e., in the direction parallel to 
the axis of the rotary drum (1). Any scale of a position detecting scale 
(2) is detected by a pickup head (6). This pickup head (6) emits light 
from a light source (11) on the scale (2) through an illuminating 
condenser lens (10), a half mirror (9), a 1/4.lambda. plate (8) and an 
objective lens (7). An image on the scale (2) is focused on a line sensor 
(12), wherein four photoelectric elements are arranged, through the 
objective lens (7) and the half mirror (9). The 1/4.lambda. plate (8) 
serves for preventing the light reflected from the scale from returning to 
the light source causing interference thereby. A pickup head (13) of a 
position detecting scale (14) disposed in the subscanning direction along 
the rotary drum (1) is also arranged in the same manner to read the scale 
(14), but in this case the scale (14) is fixed or stationary and the 
pickup head (13) is fixed to a recording head (3) to move together. 
FIG. 4 shows a state wherein scales of the scale (2) are focused on the 
line sensor (12). A scale pitch on the formed image is made equal to a 
pitch of each photoelectric element of the line sensor (12) by adjusting 
lateral magnification of the image formed on the line sensor (12), i.e., 
by adjusting a ratio of a distance between the objective lens (7) and the 
surface of the scale (2) to that between the objective lens (7) and the 
line sensor (12). Oblique line portions of the formed image in FIG. 4 
shows irregular reflection to which etching is applied and blank portions 
regular reflection. 
FIG. 5 shows photoelectric outputs (A.sub.0), (B.sub.0), (C.sub.0), 
(D.sub.0) of the respective photoelectric elements (A), (B), (C), (D). 
They are outputted in the form of triangle waves since a black-and-white 
pattern moves on the rectangular sensor. These outputs are usually used as 
position detecting signals, but the scale pitch on the formed image is not 
perfectly coincident with the pitch of the sensor by the interference with 
pitch error of the scale as described above. 
Referring now to FIG. 1, a principle on which a position detecting signal 
is outputted is described hereinafter. FIG. 1 shows how to process a 
signal from the line sensor (12) for the purpose of obtaining a position 
detecting signal not interfered with the pitch error of the scale. Each 
photoelectric element (A) (B) (C) (D) are divided into two groups in order 
of detection, i.e., one element group (D) (C) and the other (B) (A). When 
establishing outputs of these groups as (D.sub.0) (C.sub.0) and (B.sub.0) 
(A.sub.0) respectively and taking processing outputs (C.sub.0 
-D.sub.0)/(C.sub.0 +D.sub.0), (A.sub.0 -.sub.0)/(A.sub.0 +B.sub.0), 
triangular waves are drawn when moving the black-and-white pattern (2) in 
the direction of the arrow as shown in FIG. 1 (a) (b). 
A position at the time the processing output (A.sub.0 -B.sub.0)/(A.sub.0 
+B.sub.0)=0 is a position of gravity center since the black-and-white 
pattern pitch is made equal to the sensor pitch. If there is no pitch 
error of the scale, both of the processing output (A.sub.0 
-B.sub.0)/(A.sub.0 +B.sub.0) of the sensor group (A) (B) and the 
processing output (C.sub.0 -D.sub.0)/(C.sub.0 +D.sub.0) of the sensor 
group (C) (D) adjacent to the group (A) (B) at a scale pitch will be zero 
at the position of the center of gravity. However, the processing outputs 
include actually some error since the pitch error and accumulation thereof 
are caused in the scale pattern itself. 
An extent of deviation of a scale pitch of a detecting object with respect 
to a distance between the center lines of the sensors (A) (B) and the 
sensors (C) (D) can be successfully determined by reading a value of the 
processing output (C.sub.0 -D.sub.0)/(C.sub.0 +D.sub.0) at the time when 
the processing output (A.sub.0 -B.sub.0)/(A.sub.0 +B.sub.0) is zero in the 
relative movement of either the scale (2) or the line sensor (12). 
In other word, an output value (.alpha.) of the (C.sub.0 -D.sub.0)/(C.sub.0 
+D.sub.0) [i.e., a value at the time of t.sub.1 in FIG. 1 (b)] read out at 
the moment the processing output (A.sub.0 -B.sub.0)/(A.sub.0 +B.sub.0)=0 
[i.e., at the time of t.sub.1 in FIG. 1 (a)] is the pitch error of the 
scale. 
Thus according to the present invention, the processing output (A.sub.0 
-B.sub.0)/(A.sub.0 +B.sub.0) serves as a trigger while the output (C.sub.0 
-D.sub.0)/(C.sub.0 +D.sub.0) as a correction signal. Accordingly, in FIG. 
1, a position detecting signal is outputted at the time t.sub.1 when the 
processing output (A.sub.0 -B.sub.0)/(A.sub.0 +B.sub.0) of the sensors (A) 
(B) showing a center of gravity of a part n.sub.1 of the scale (2) is 
zero, and at the same time the value (.alpha.) of the processing output 
(C.sub.0 -D.sub.0)/(C.sub.0 +C.sub.0) of a part n.sub.2 of the scale is 
fetched. Then the scale (2) moves in the direction of the arrow and the 
position detecting signal is outputted at the time t.sub.2 when the 
processing output (A.sub.0 -B.sub.0)/(A.sub.0 +B.sub.0) detected with 
respect to the part n.sub.2 by the sensors (A) (B) is zero. And at the 
time t.sub.2, a value (.beta.) of the processing output (C.sub.0 
-D.sub.0)/(C.sub.0 +D.sub.0) of a part n.sub.3 of the preceding sensors 
(C) (D) is fetched along with the movement of the scale (2). The position 
detecting signal is outputted when moving the scale to detect the part 
n.sub.3 by using the sensors (A) (B) and the processing output (A.sub.0 
-B.sub.0)/(A.sub.0 +B.sub.0) thereof is (.beta.). At the same time, a 
value (.gamma.) of the processing output (C.sub.0 -D.sub.0)/(C.sub.0 
+D.sub.0) is fetched, and thereafter the position detecting signal is 
outputted in the same manner. 
If there is some pitch error in the scale, the distance 1.sub.1 between the 
centers of gravity of two scales (i.e., the scale pitch) advances to 
1.sub.1 ', 1.sub.1 " producing different deviations as shown in FIG. 1 
(a). Accordingly, the position detecting signal is outputted at the moment 
when the gravity center position of the following sensors pass through a 
point where the scale has been precedingly detected so that the output is 
obtained by every distance 1.sub.2 between the gravity center positions of 
the sensors. Since the distance 1.sub.2 between the sensors is always 
fixed and constant, any signal outputted on the basis of this constant 
distance is a signal of equal and uniform pitch available for the method 
of this invention. 
Thus, the processing output serving as a reference for the following 
sensors is fetched as a trigger, while the processing output of the 
preceding sensors as a correction signal, the processing output of the 
following sensors is determined based on the correction signal to replace 
the scale pitch with the sensor pitch, each position detecting signal is 
outputted by every sensor pitch, and as a result such interference as 
scale pitch error, irregular moving speed of the scale or the sensors, 
etc. is perfectly and successfully shut off. 
FIG. 6 is a block diagram showing a circuit system arranged based on the 
foregoing principle, and FIG. 7 shows wave forms of outputs of various 
parts. Output of each photoelectric element (A) (B) (C) (D) of the line 
sensor (12) is converted to a voltage signal by conversion circuits (20) 
(21) (22) (23) to enter in differential amplifiers (24) (25) and adding 
amplifiers (38) (39) and further delivered to dividing amplifiers (40) 
(41). Output (a) of the dividing circuit (40) enters in a Schmitt circuit 
(26) and a comparison circuit (31). On the other hand, output (b) of the 
dividing circuit (41) enters in either a sample holding circuit (29) or 
(30) by way of an analog switch (35). Output of the sample holding circuit 
(29) or (30) enters in the comparison circuit (31) by way of an analog 
switch (36). Output (c) of the Schmitt circuit (26) enters in an inverter 
(27) and a flip-flop (28). Output (d) of the flip-flop (28) makes on-off 
control of the analog switches (35) (36) (37). Output (e) of the inverter 
(27) and output (f) of the comparison circuit (31) enter in an AND circuit 
(32). Output (g) of the AND circuit (32) enters in monostable 
multivibrators (33) (34). Output pulse (i) for timing of the monostable 
multivibrator (34) enters either in the sample holding circuit (29) or 
(30) through the analog switch (37). 
In the foregoing arrangement, when the processing output (a) of the sensors 
(A) (B) is zero, initial value of the sample holding circuit (29) is zero, 
and the comparison circuit (30) judges that both of them are zero. The "L" 
signal (c) of the Schmitt circuit (26) is inverted by the inverter (27) to 
be a "H" signal, which opens the AND circuit (32) to increase the 
processing output (a) of the sensors (A) (B). When this output (a) gets 
over zero, the output signal (f) is outputted as first transition signal 
from the comparison circuit (31) and the first transition signal (g) is 
outputted by way of the AND circuit (32). The monostable multivibrator 
(33) is driven by the first transition output to output the position 
detecting signal (h). The monostable multivibrator (34) is driven by the 
first transition output of the AND circuit (32), and the output pulse 
thereof is applied to the sample holding circuit (30) as a timing for 
sample holding by way of the analog switch (37). The processing output 
(".alpha." in FIG. 1) of the sensors (C) (D) at this moment is held in the 
sample holding circuit (30) through the analog switch (35). At the time of 
reducing the processing output (a) of the sensors (A) (B), the output of 
the flip-flop (28) is inverted by the first transition output of the 
Schmitt circuit (26) to switch the analog switches (35) (36) (37). The 
foregoing operation is now performed when the processing output (a) of the 
sensors (A) (B) is coincident with the value ".alpha." held in the sample 
holding circuit (30) to output the position detecting signal and to hold a 
value ".beta." of the processing output (b) of the sensors (C) (D). The 
foregoing operation is repeated thereafter in the same manner. 
In the embodiment described above, two pairs of sensors adjacent to each 
other are disposed corresponding to the scale pitches. This is because 
when obtaining the processing output of the sensors, a detection accuracy 
is improved by the detection of the gravity center positions of the 
scales. 
Besides dividing operation is performed in order to correct nonuniformity 
or unevenness in the density of the scales. It is to be noted that when 
scale is formed by etching as in this embodiment, a high working accuracy 
is secured for the gravity center position than edge portions, and such 
characteristic is also utilized in this embodiment. Accordingly, the pitch 
error of the scales adjacent to each other is not so large that the 
accumulated pitches (.alpha., .beta., .gamma. . . . ) do not get out of 
the straight line area of the triangle waves drawn by the differential 
signals when pitch errors are detected to be used as correction signals 
during one revolution of the rotary drum in this embodiment. In addition, 
in the case of less unevenness in the scale density, the adding 
multipliers (38) (39) and dividers (40) (41) are not required, and the 
outputs (a) (b) of the differential multipliers (24) (25) can be used as 
signals (a) (b). Furthermore, when the accuracy of the scale is high to a 
certain extent, it is possible to dispose two sensors (A) (C) at a 
distance of one pitch of the scale as shown in FIG. 8 to perform the 
processing as above described. In such arrangement, when the sensor (A) 
outputs a value (.alpha..sub.1) for a scale part n.sub.1 at the time 
t.sub.1, output (.beta..sub.1) of the sensor (C) for a part n.sub.2 is 
fetched. The position detecting signal is outputted when the sensor (A) 
outputs the (.beta..sub.1) for the part n.sub.2 at t.sub.2, and at the 
same time output (.gamma..sub.1) of the sensor (C) for a part n.sub.3 is 
fetched to perform the same processing thereafter to obtain the position 
detecting signal. In other words, when obtaining two signal for one pitch 
of the scale, the position detecting signal can be obtained by converting 
the sensor pitch fixed on the two signals to a reference signal. 
If there is some irregularity in the moving speed of the scale, the output 
value (C.sub.0 -D.sub.0)/C.sub.0 +D.sub.0) at every time when the value 
(a) of the output (A.sub.0 -B.sub.0)/(A.sub.0 +B.sub.0) is zero shows the 
irregularity between the scales. 
Furthermore, as shown in FIG. 9, it is possible to divide each sensor (A) 
(B) (C) (D) of FIG. 1 into sensor groups (or pairs) (A.sub.1) (A.sub.2), 
(B.sub.1) (B.sub.2), (C.sub.1) (C.sub.2), (D.sub.1) (D.sub.2) forming a 
line sensor (42). By using such line sensor (42), the same processing as 
FIG. 1 can be performed by adding outputs of these sensors (A.sub.1) 
(A.sub.2), (B.sub.1) (B.sub.2), (C.sub.1) (C.sub.2), (D.sub.1) (D.sub.2) 
and obtaining difference output thereof. Furthermore, by obtaining a 
difference output of each added output of the sensors (A.sub.2) (B.sub.1) 
and (B.sub.2) (C.sub.1) as well as a difference output of each added 
output of the sensors (A.sub.1) (B.sub.1) and (B.sub.1) (B.sub.2), it is 
possible to obtain an output signal with deviation of 1/4 pitch (phase 
difference 90.degree.) so that the moving direction of the scale is 
discriminated by detecting the advance or delay of the phase. 
In the embodiment described above, the scale is read by optical means, but 
such magnetic means as hall device can be also available for the reading. 
In effect, any signal can be used so far as variation of the scale is 
detected thereby. 
In the arrangement shown in FIG. 2, the pickup head (13) is attached to the 
head (3) and the position detecting scale (14) is fixed, but it is also 
possible to attach the position detecting scale (14) to the head (3) and 
fix the pickup head (13) for the relative movement. The method according 
to this invention is further available for original picture scanning 
input. 
Thus, by the position detecting method according to the present invention, 
the signal obtained corresponding to the scale pitch is fetched as error 
detecting signal by detecting the error between the pitches adjacent to 
each other, the fetched signal serving as a correction signal is converted 
to an uniform and equal pitch signal corresponding to a distance 
established between two detectors disposed for detecting the same part of 
the scale to be outputted and the output signal is used as a position 
detecting signal, and as a result a highly accurate detection of position 
is attained without interference with the pitch error of the scales. 
Furthermore, when the moving speed of the scale is not constant, the 
output value (C.sub.0 -D.sub.0)/(C.sub.0 +D.sub.0 ) at the time the output 
(A.sub.0 -B.sub.0)/(A.sub.0 +B.sub.0) is zero always represents the 
irregularity of the scale, whereby such irregularity can be exactly 
detected. 
In addition, the method according to the present invention is applicable 
when two different types of signals from the scale are to be detected. In 
this way, the method according to the present invention can be applied to 
wide range of position detecting operation.