Optical encoder and optical rotary encoder

Details Optical encoder and optical rotary encoder Optical encoder and optical rotary encoder

2001-05-07

2004-03-30

Porta, David (Department: 2878)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C250S231180, C250S23700G, C356S615000, C356S616000

Reexamination Certificate

active

06713756

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2000-136088, filed May 9, 2000; and No. 2000-233351, filed Aug. 1, 2000; and No. 2000-337966, filed Nov. 6, 2000, the entire contents of all of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to an optical encoder, particularly to an optical encoder as an optical displacement sensor which uses optical means for detecting a displacement amount of a precision mechanism.
Moreover, the present invention relates to an optical rotary encoder which uses optical means to detect a rotary angle.
As described in the beginning, as a prior art concerning a constitution of an optical encoder, first Jpn. Pat. Appln. KOKAI Publication No. 2000-205819 by the present inventor et al. will be described as a first prior example.
FIGS. 59A
,
59
B show constitution diagrams.
A scale
2
in which a first optical modulation region formed of a predetermined-period optical pattern generating a diffraction pattern (a transmission or reflective diffraction lattice scale in
FIGS. 59A
,
59
B) is formed is irradiated with a laser mean emanating from a semiconductor laser
1
(or a surface emitting laser
10
) as a coherence light source.
Moreover, the encoder is constituted such that a specific portion of the generated diffraction pattern is detected by either a photodetector
3
or a photodetector
3
′.
Additionally, the coherence light source will also be referred to simply as a light source.
Additionally, when the coherence light source and photodetector
3
are disposed on the same side with respect to the scale
2
, as shown in
FIG. 59A
, main axes
41
,
42
of the light beam emitted from the semiconductor laser
1
(or the surface emitting laser
10
) are inclined/arranged by angle &phgr; with respect to a perpendicular line of a scale surface.
Operation of the sensor will next be described.
As shown in
FIG. 59A
, various constituting parameters will be defined as follows:
z1: length obtained by measuring a distance between the light source and the surface of the scale with a first light modulation region formed thereon on the light beam main axis;
z2: length obtained by measuring a distance between the surface of the scale with the first light modulation region formed thereon and a light receiving surface of the photodetector on the light beam main axis;
p1: pitch of the optical pattern in the first light modulation region on the scale;
p2: pitch of the diffraction pattern on the light receiving surface of the photodetector;
&thgr;x: spread angle of the light beam of the light source with respect to a pitch direction of a diffraction lattice on the scale; and
&thgr;y: spread angle of the light beam of the light source in a vertical direction with respect to the above &thgr;x.
Additionally, the light beam spread angle means an angle formed by a pair of boundary lines
9
each having a direction in which a light beam intensity becomes ½ of a peak intensity.
Moreover, “the pitch of the optical pattern in the light modulation region on the scale” means a space period of the pattern formed on the scale and having optical properties modulated.
Furthermore, “the pitch of the diffraction pattern on the light receiving surface of the photodetector” means a space period of an intensity distribution of the diffraction pattern generated on the light receiving surface.
According to a light diffraction theory, when z1 and z2 are in a specific relation satisfying the following equation (1), an intensity pattern substantially analogous to the scale diffraction lattice pattern is generated on the light receiving surface of the photodetector.
(1/
z
1)+(1/
z
2)=&lgr;/{
k
(
p
1)
2
}  (1)
In the equation, &lgr; denotes a light wavelength from the light source, and k denotes an integer.
In this case, the pitch p2 of the diffraction pattern on the light receiving surface can be represented using other constituting parameters as follows.
p
2
=p
1(
z
1+
z
2)/
z
1   (2)
When the scale
2
is displaced in the pitch direction of the diffraction lattice with respect to the light source, the diffraction pattern intensity distribution moves in a displacement direction of the scale
2
with the same space period being kept.
Therefore, a space period p21 of a light receiving area of the photodetector is set to the same value as that of p2. Every time the scale
2
moves by p1 in the pitch direction, a periodic strength signal is obtained from the photodetector. Therefore, the displacement amount of the scale
2
in the pitch direction can be detected.
Additionally, the above has been described on an assumption that the light beam extending to the scale from the light source has a constant spread angle (hereinafter referred to as “a case of a spread beam”).
Therefore, when the emanating beam from the light source is collimated to a parallel light by a lens, and the scale is irradiated with the light (hereinafter referred to as “a case of a parallel beam”), in the above equations (1) and (2), z1→∞ is assumed.
In this case, the equation (2) results in the following.
p2=p1   (2)′
Here, it is unnecessary to consider “the case of the parallel beam”, but in “the case of the spread beam”, as shown in
FIGS. 59A
,
59
B, the light source and photodetector are disposed on the same side with respect to the scale
2
(hereinafter referred to as “a reflective arrangement”) such that z1=z2. In this case, even when a space gap between the scale
2
and the light source fluctuates, the pitch of the diffraction pattern on the light receiving surface does not change from the equation (2).
Moreover, in
FIGS. 59A
,
59
B, the surface with the first light modulation region formed thereon is disposed substantially in parallel with the light receiving surface of the first photodetector. Additionally, the main axis of the first light beam is inclined/arranged with respect to the surface with the first light modulation region formed thereon only in a plane vertical to a predetermined period direction of the first light modulation region.
The diffraction pattern is regarded as a so-called shadow picture pattern. Therefore, even when the space gap between the scale
2
and the light source fluctuates because of the arrangement limiting a light beam inclined surface, a diffraction pattern intensity distribution curve is obtained as shown by curves
103
,
104
. Moreover, since the distribution does not move in a scale pitch direction, a displacement sensing is possible without being substantially influenced by the aforementioned gap fluctuation.
As described above, in both “the case of the parallel beam” and “the case of spread beam” it is an important point for the displacement sensing hardly influenced by the gap fluctuation to “dispose the light beam main axis such that the axis is inclined with respect to the surface with the light modulation region formed thereon only in the plane vertical to the predetermined period direction of the light modulation region”.
Furthermore, in practical use, a light receiving group of the photodetector with a space period p20 is displaced at an interval of p21/4, four alternately arranged light receiving groups are formed, outputs Va, Vb, Va′, Vb′ are obtained from light receiving elements of each group, and Va-Va′, Vb-Vb are utilized as so-called A phase (sine wave) and B phase (cosine wave) outputs of the encoder, respectively.
Moreover, in the Jpn. Pat. Appln. KOKAI Publication No. 2000-205819, a laser beam intensity can be monitored by obtaining an operation sum of the respective outputs Va, Vb, Va′, Vb′. Therefore, it is possible to correct an influence of a laser beam intensity change by an environment change and change with time to some degrees by feeding back a laser beam intensity change by the environment change and change with time so as to set the change to be con

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