Rotary encoder

A rotary encoder for producing a pair of pulse trains with a desired phase difference therebetween is provided. The encoder comprises a rotary plate provided with a plurality of slits arranged in the form of a ring and a stationary slit plate provided with two sets of slits spaced apart at a particular distance. In accordance with the present invention, a single light source and a pair of light-receiving elements are provided, and the rotary and stationary slit plates are provided such that their distance may be appropriately adjusted. With such a structure, a phase difference between the two signals obtained by the light-receiving elements may be precisely adjusted to an intended value.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a rotary encoder and in particular to a 
rotary encoder for producing two pulse trains with a desired phase 
difference therebetween. In applications, a rotary encoder of the present 
invention may be used in a driving system for an image sensor head of a 
facsimile machine or a recording head of a printer as coupled to a motor, 
pulley or wire guide in the driving system, thereby detecting the scanning 
position, scanning speed, scanning direction, etc. of such a head. 
2. Description of the Prior Art 
A rotary encoder for producing two pulse trains to control the operation of 
a reader or recorder head in a facsimile or printing machine is well known 
in the art. A typical example of such a prior art rotary encoder is 
schematically shown in FIGS. 1 through 3. Such a rotary encoder usually 
comprises a rotary plate and a stationary slit plate. FIG. 1 shows a 
rotary plate 1 which is circular is shape and which is provided with a 
plurality of slits 2 equally spaced apart from each other. The slits 2 are 
provided to the plate 1 to allow the passage of light therethrough and 
arranged in the form of a ring. Thus, the outer edges of the slits 2 are 
located at a radius a from the center 0 of rotation of the plate 1 and 
their inner edges are all located at a radius b from the center 0. 
FIG. 2 shows a stationary slit plate 10 which is to be used in combination 
with the rotary plate 1. The stationary slit plate 10 is also provided 
with a first set 11 of slits and a second set 12 of slits, each set having 
the same number of slits which are structurally the same as the slits 2 of 
the rotary plate 1. It is to be noted, however, that the relative 
positional relationship between the first and second slit sets 11 and 12 
is such that when the slits of the first set 11 are precisely aligned with 
the slits 2 of the rotary plate 1, the slits of the second set 12 are 
misaligned with the slits 2 by a quarter of the slit pitch. 
As shown in FIG. 3, the stationary slit plate 10 is disposed above the 
rotary plate 1 such that the slits of the first and second sets 11 and 12 
may be brought into alignment with some of the slits 2 alternately. 
Disposed above the stationary slit plate 10 are first and second light 
emitting elements 31 and 32 such as L.E.D's.; whereas, disposed below the 
rotary plate 1 are first and second light receiving elements 21 and 22 
such as photodiodes. Accordingly, the first light receiving element 21 may 
receive light emitted from the first light emitting element 31 through the 
slits of the first set 11 and the slits 2. Likewise, the second light 
receiving element 22 may receive light emitted from the second light 
emitting element 32 through the slits of the second set 12 and the slits 
2. It should be noted that the rotary plate 1 may be rotated clockwise or 
counter-clockwise with respect to the stationary slit plate 10 as 
indicated by the solid line and dotted line arrows. 
Under the circumstances, when the rotary plate 1 is rotated to cause 
relative movement between the slits 2 and the slits of the first and 
second sets 11 and 12, the first and second light receiving elements 21 
and 22 supply output signals shown by waveforms E and F in FIG. 4. These 
waveforms E and F may be converted into pulse trains e and f by a 
well-known method. It is to be noted that there is a phase difference of 
90.degree. between the pulse trains e and f. In FIG. 4, "T" indicates 
time, and "N" and "R" indicate rotational direction of the rotary plate 1 
with "N" indicating normal rotation and "R" indicating reversed rotation. 
Thus, when the rotary plate 1 is in normal rotation, the pulse train e is 
90.degree. ahead of the pulse train f; whereas, when the rotary plate 1 is 
in reversed rotation, the pulse train f is 90.degree. ahead of the pulse 
train e. This leads to the fact that, by detecting the phase difference 
between the pulse trains e and f, the rotational direction of the rotary 
plate 1 may be determined. Moreover, the rotational speed and the amount 
of rotation of the rotary plate 1 may be determined by using at least 
either one of the pulse trains e and f. 
When the slit pitch is small and/or the rotational speed of the rotary 
plate 1 is high, the cycle of the pulse trains e and f is short so that 
the phase difference between the two pulse trains e and f is difficult to 
detect. In other words, it is now required to detect pulse edges of the 
pulse trains e and f at high speed. Such a difficulty mainly stems from 
such reasons as possible manufacturing errors in the slits; possibly 
differing photoemitting characteristics between the two light emitting 
elements 21 and 22; possibly differing photosensitivity characteristics 
between the two light receiving elements 31 and 32; and possibly differing 
conversion characteristics in converting the analog signals E and F into 
pulse trains e and f, respectively. 
Because of the above, the phase difference between the two pulse trains e 
and f cannot be precisely set at 90.degree. and it also fluctuates due to 
dimensional errors in individual slits. As the actual phase difference 
deviates more from the intended value of 90.degree., it becomes more 
difficult to determine the rotational direction of the rotary plate 1 and 
a probability of erroneous determination increases. In particular, when 
the slit pitch is small and/or the rotational speed of the rotary plate 1 
is high, the amount of deviation from the intended phase difference of 
90.degree. becomes more material in the accurate determination of the 
rotational conditions of the rotary plate 1 because the cycle of the pulse 
train e, f is short. The phase difference is also sensitive to the 
distance between the rotary plate 1 and the stationary slit plate 10 as 
well as to position and orientation of the light emitting 21, 22 and 
receiving 31, 32 elements. Accordingly, in accordance with the prior art, 
an elaborate and time-consuming step of precise positioning of elements is 
required. 
SUMMARY OF THE INVENTION 
The above-described disadvantages are overcome with the present invention 
which provides a rotary encoder capable of producing two pulse trains with 
an intended phase difference therebetween. In accordance with the present 
invention, there is provided a rotary encoder which comprises a first 
plate which is rotatably supported in the plane defined by its surface and 
which is provided with a plurality of slits spaced apart from each other 
and arranged in the form of a ring; driving means for driving to rotate 
said first plate; a second plate disposed at one side of said first plate 
and provided with a first set of slits and a second set of slits, said 
first and second slit sets having the same predetermined number of slits; 
a light source disposed at the side opposite to said one side of said 
first plate; a pair of light-receiving elements the first of which is 
disposed to receive light from said light source through the slits of said 
first plate and the first set of slits of said second plate and the second 
of which is disposed to receive light from said light source through the 
slits of said first plate and the second set of slits of said second 
plate; and adjusting means for adjusting the position of either one or 
both of said first and second plates with respect to said pair of 
light-receiving elements. 
In accordance with one embodiment of the present invention, the driving 
means includes a driving motor having a driving shaft and the first plate 
is so mounted on the driving shaft that the position of the first plate 
may be adjusted along the axis of the driving shaft. In accordance with 
another embodiment of the present invention, the adjusting means includes 
a first positioning member having a first threaded portion and fixed to 
said driving shaft, a second positioning member, to which the first plate 
is fixed, having a second threaded portion engageable with the first 
threaded portion, and fastening means for releasably fastening the second 
positioning member to the driving shaft. 
It is therefore an object of the present invention to provide a rotary 
encoder which allows to obtain an intended phase difference between a pair 
of pulse trains. 
Another object of the present invention is to provide a rotary encoder 
which has a simplified arrangement of elements. 
A further object of the present invention is to provide a rotary encoder 
which is compact in size and thus easy to manufacture. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFFERED EMBODIMENTS 
It is to be noted that the embodiment of the present invention disclosed 
herein employs the similar rotary and stationary plates disclosed in FIGS. 
1 and 2, respectively, and that like numerals indicate like elements. 
As shown in FIG. 5, in accordance with one embodiment of the present 
invention, the rotary plate 1 is releasably fixed to the driving shaft of 
a driving motor M. A support member 30 is fixedly mounted on an end plate 
EP which is fixed to the housing of the motor M. The support member 30 is 
provided with a recess into which a part of the rotary plate 1 is 
inserted. The support member 30 supports a single light emitting diode 20 
on one side of the rotary plate 1 and the stationary slit plate 10 and a 
pair of photodiodes 31 and 32 on the other side of the rotary plate 1. 
Therefore, the photodiodes 31 and 32 disposed at the bottom of the recess 
may receive light emitted from the light emitting diode 20 through the 
slits 2 of the rotary plate 1 and through the first set slit 11 and 
through the second set of slits 12 respectively. The support member 30 
together with L.E.D. 20, slit plate 10 and photodiodes 31, 32 constitutes 
a photosensor unit. 
As shown in FIG. 6, fixedly mounted on the driving shaft of the motor M is 
a first positioning member 41 which has a male-threaded portion. The 
rotary plate 1 is provided with a central through-hole to which a second 
positioning member 42 is fixed. The second positioning member 42 has a 
female-threaded portion into which the male-threaded portion of the first 
positioning member 41 may be screwed. The second positioning member 42 is 
also provided with a screw 43 and, therefore, the second positioning 
member 42 may be temporarily fixed to the driving shaft of the motor M by 
tightening the screw 43. 
By untightening the screw 43, the second positioning member 42 may be 
rotated clock-wise or counter-clockwise against the first positioning 
member 41 fixed to the driving shaft of the motor M so that the rotary 
plate 1 may change its position along the axis of the driving shaft of the 
motor M. Such a change in axial position of the rotary plate 1 is 
indicated by the reference character G in FIG. 7. By changing the axial 
position of the rotary plate 1 as described above, the relative distance 1 
between the rotary plate 1 and the stationary slit plate 10 may be easily 
adjusted, thereby allowing to obtain an intended phase defference between 
a pair of pulse trains produced. Upon completion of adjustment, the screw 
43 may be tightened to fix the second positioning member 42 to the driving 
shaft of the motor M. 
FIG. 7 schematically shows the principle of the present invention. As 
shown, there is provided a single light source 20 which is located 
approximately at a center between the two sets 11, 12 of slits provided in 
the stationary slit plate 10. At least one of the rotary plate 1 and the 
stationary plate 10 is provided to be adjustable in position toward or 
away from the other in order to precisely set a phase difference between a 
pair of pulse trains at a desired value. 
In the structure shown in FIG. 7, both of the photodiodes 31 and 32 receive 
light from the same light source 20, and the phase difference between a 
pair of pulse trains formed by signals from the photodiodes 31 and 32 is 
dependent upon three relative positional relationships: distance l1 
between the rotary plate 1 and the stationary slit plate 10, distance l2 
between the light source 20 and the photodiode mounting surface, and 
distance l3 between the stationary slit plate 10 and the photodiode 
mounting surface. Besides, in an actual light emitting diode, there is a 
light intensity distribution in the direction normal to an optical axis PC 
and such distribution also varies along the axis PC, which also 
constitutes another factor causing to change a phase difference. 
Suppose that the rotary plate 1 is in rotation in the direction indicated 
by the arrow H. Under the condition, when the rotary plate 2 is moved away 
from the stationary slit plate 10 along the axis PC, a phase for the 
amount of light received by the photodiode 31 advances; whereas, a phase 
for the amount of light received by the photodiode 32 delays. On the other 
hand, if the rotary plate 2 is moved closer to the stationary slit plate 
10, the photodiode 31 receives light in delayed phase and the photodiode 
32 receives light in advanced phase. It is to be noted that similar 
adjustments in phase difference may be carried out by moving the 
stationary slit plate 10 along the axis PC with holding the rotary plate 1 
stationary along the axis PC. In this manner, in accordance with the 
present invention, it is made possible to obtain a phase difference having 
an intended value by using the single light source and providing the 
rotary plate and the stationary slit plate at least one of which may be 
adjustably positioned closer to or away from the other. 
It should be appreciated that the number of elements required is decreased 
as compared with the prior art. And the conventional rotary and stationary 
slit plates may be used without changes. Moreover, the manufacturing error 
in the relative distance between the first and second sets of slits are 
not critical because such an error is effectively corrected by the 
adjusting mechanism of the present invnetion. It should also be noted that 
the required adjustment in phase difference may be carried out even after 
assemblage to a driving system. 
While the above provides a full and complete disclosure of the preferred 
embodiments of the present invention, various modifications, alternate 
constructions and equivalents may be employed without departing from the 
true spirit and scope of the invention. Therefore, the above description 
and illustration should not be construed as limiting the scope of the 
invention, which is defined by the appended claims.