Optical encoder

Details Optical encoder Optical encoder

2000-06-07

2004-10-12

Glick, Edward J. (Department: 2882)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C250S231130

Reexamination Certificate

active

06803560

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical encoder for detecting movement information with high accuracy.
2. Related Background Art
The conventionally known methods for detecting the position or speed of a moving object are roughly classified in methods with a magnetic encoder and methods with an optical encoder. Optical encoders are usually comprised of a light-projecting section, a light-receiving section, and a scale and the scale is normally made of a thin SUS material by precise press blanking or by etching.
In recent years, however, suggestions have been made on the optical encoders using the scale of a transparent material provided with grooves of V-shaped cross section, for example, as described in Japanese Patent Application Laid-Open No. 11-23324 etc., and they are used in printers, copying machines, and so on.
FIG. 1
is a perspective view of an optical system in a self-emitting optical encoder of a conventional example and
FIG. 2
is a cross-sectional view thereof. The optical encoder is provided with a light-irradiating device
3
comprised of a light source
1
such as an LED or a semiconductor laser for emitting coherent light, for example, of the wavelength of 632.8 nm, and a lens system
2
consisting of a spherical lens or an aspherical lens; an optical scale
4
with a grating having the phase difference detecting function and amplitude diffraction grating function; a concave mirror
5
having a curved surface matching with the Fourier transform surface of the grating and having the optical axis O
1
decentered by a center difference &Dgr; relative to the optical axis O of a central beam of incident light; and a light-receiving device
6
consisting of light-receiving elements
6
a
,
6
b
,
6
c
being three photodetectors. The output of the light-receiving device
6
is connected to a signal processing unit
7
having a pulse-counting circuit and a rotational direction determining circuit, and the light-irradiating device
3
and light-receiving device
6
are held in a fixed state in a housing
8
. The optical scale
4
is attached to part of a rotating body not illustrated and is under rotation in the direction of an arrow D about the rotational axis O
2
together with the rotating body.
FIG. 3
is a plan view of the optical scale in which the grating of the optical scale
4
is formed so that two slopes I
1
, I
2
forming a V-groove, and one flat F appear alternately at a predetermined pitch P and are formed continuously in radial directions, as illustrated in
FIGS. 4A
,
4
B. The width of the V-groove is P/2, and each of the two slopes I
1
, I
2
forming the V-groove has the width of P/4 and is inclined at an angle not less than the critical angle, for example, at the angle &thgr;=45°, relative to the flat F.
The grating has a first region
4
a
of the shape illustrated in
FIG. 4A
radially inside and a second region
4
b
of the shape illustrated in
FIG. 4B
radially outside. Each of FIG.
4
A and
FIG. 4B
includes a front view and a cross-sectional view of the corresponding region. Since the scale grooves are radially continuous, the number N
1
of V-grooves in the first region
4
a
is equal to the number N
2
of V-grooves in the second region
4
b
(N
1
=N
2
). A ratio (R
2
/R
1
) of the distance R
2
from the rotation center O
2
of the optical scale
4
to the second region
4
b
, to the distance R
1
similarly to the first region
4
a
is equal to a ratio (P
2
/P
1
) of the scale pitch P
2
of the second region
4
b
to the scale pitch P
1
of the first region
4
a
(i.e., R
2
/R
1
=P
2
/P
1
).
The light from the light source
1
being one element of the light-emitting device
3
is condensed by the lens system
2
onto the optical scale
4
. The light incident to the first region
4
a
of the optical scale
4
is diffracted by the grating and the nth-order diffracted light (0-order and ±1-order diffracted light) is condensed at or near the pupil position of the concave mirror
5
.
The concave mirror
5
reflects these three diffracted light beams thus condensed to form an interference pattern image based on these three diffracted beams in the second region
4
b
on the surface of the optical scale
4
. At this time, with movement of the optical scale
4
in the rotation direction D, the thus formed image moves in the direction opposite to the rotation direction D. Namely, the interference pattern image is displaced relative to the grating by double the movement of the optical scale
4
. This enables acquisition of rotation information in the resolution of double the grating formed in the optical scale
4
.
Beams based on the phase relation between the interference pattern image formed near the second region
4
b
of the optical scale
4
and the V-grooves of the grating are geometrically refracted by the second region
4
b
, three beams emerging from the second region
4
b
are received by the three light-receiving elements
6
a
,
6
b
,
6
c
of the light-receiving device
6
, respectively, and signals from this light-receiving device
6
are processed by the signal processing unit
7
to obtain the rotation information.
FIG. 5A
shows the convergent light incident onto the grating of the first region
4
a
of the optical scale
4
, and beams arriving at the flats F of the grating among the light travel through the flats F toward the concave mirror
5
to be focused on the surface thereof. Since the slope angle of the slopes I
1
is set over the critical angle, a beam arriving at each slope I
1
forming the V-groove is totally reflected toward the other slope I
2
together forming a V-groove and then is totally reflected again by the slope I
2
.
In this manner the beams finally arriving at the slopes I
1
of the grating are reflected back opposite to the incident direction without entering the inside of the optical scale
4
. Likewise, the beams arriving at the other slopes I
2
are also totally reflected twice back opposite to the incident direction. Therefore, the beams arriving at the two slopes I
1
, I
2
are not transmitted but reflected by the optical scale
4
, whereas only the beams arriving at the flats F travel through the optical scale
4
, in the first region
4
a.
In the first region
4
a
the V-grooved grating has the optical action similar to the transmissive amplitude diffraction grating. Namely, the light is diffracted by the grating of the first region
4
a
to generate beams of 0-order, ±1-order, ±2-order, . . . diffracted light by the action of the grating, and the beams are condensed on the surface of the concave mirror
5
. The diffracted light thus condensed is reflected by the concave mirror
5
to enter the second region
4
b
of the optical scale
4
, as illustrated in
FIG. 5B
, thereby forming an image of radial grooves on the surface of the optical scale
4
. Since the first region
4
a
and the second region
4
b
are radially different regions (which may overlap with each other in part) of the radial grating on the surface of the optical scale
4
, the grating pitches of the first region
4
a
and the second region
4
b
are different from each other, and the inside and outside pitches of the optical scale
4
are also different even in the irradiation area of the second region
4
b.
In this prior art example, therefore, the grating of the first region
4
a
is enlargingly projected onto the second region
4
b
so that a reversed image thereof may be formed at the same pitch as that of the radial grating of the optical scale
4
. For this purpose, the concave mirror
5
is designed to have a desired radius R of curvature and be decentered from the optical axis O of the incident light and the deviation &Dgr; of the concave mirror
5
from the optical axis O of incidence is set so as to make the enlargement projection magnification optimum. In this way the pitches of the radial grating are matched in part for formation of the grating image of the first region
4
a
on the surface of the second region
4
b
by the

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