Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2000-05-30
2003-10-07
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06631005
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon claims the benefit of priority from the prior Japanese Patent Application No. 11-153743 filed Jun. 1, 1999 the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to an optical displacement sensor, and in particular, to an optical displacement sensor for detecting displacement of a precision mechanism.
First, as a conventional technique for optical displacement sensors of the above kind, the optical displacement sensor disclosed in Jpn. Pat. Application No. 11-6411 and comprising a vertical cavity surface emitting laser as a light source will be described.
The configuration and operation of the optical displacement sensor according to this conventional technique will be explained with reference to
FIGS. 9A and 9B
.
The optical displacement sensor according to this conventional technique is configured so that a transmissive scale
2
having a periodic pattern formed thereon is irradiated with laser beams emitted from a semiconductor laser that is a coherent light source
1
so that a particular portion of the diffraction interference pattern generated by the irradiation is detected by a photodetector
3
.
The sensor operation of this type will be described below.
First, as shown in
FIGS. 9A and 9B
, each configuration parameter is defined as:
z
1
: distance between the light source and a surface of the scale on which a diffraction grating is formed,
z
2
: distance between the of the scale with the diffraction grating formed thereon and a light receiving surface of the photodetector,
p
1
: pitch of the diffraction grating on the scale,
p
2
: pitch of the diffraction interference pattern on the light receiving surface of the photodetector,
&thgr;x: spread angle of light beams from the light source with respect to a pitch direction of the diffraction grating on the scale, and
&thgr;y: spread angle of light beams emitted from the light source, in a direction perpendicular to the &thgr;x (the spread angle of light beams refers to the angle between a pair of boundary lines
6
at each of which the light beam intensity is half of a peak value).
The “pitch of the diffraction grating on the scale” means the spatial cycle of the periodic pattern formed on the scale
2
and having its optical characteristics modulated.
In addition, the “pitch of the diffraction interference pattern on the light receiving surface of the photodetector” means the spatial cycle of the intensity distribution of the diffraction interference pattern generated on the light receiving surface of the photodetector
3
.
According to the light diffraction theory, an intensity pattern similar to the grating pattern on the scale
2
is generated on the light receiving surface of the photodetector
3
when the z
1
and z
2
defined above have such a particular relationship as meets the relationship shown in the following Equation (1):
(
1
/
z
1
)+(
1
/
z
2
)=&lgr;/
kp
1
2
(1)
where &lgr; denotes the wavelength of light beams emitted from the light source and k is an integer.
In this case, other configuration parameters can be used to express the pitch p
2
of the grating pattern on the light receiving surface as shown in the following Equation (2):
p
2
=
p
1
(
z
1
+
z
2
)/
z
1
(2)
When the scale
2
is displaced in the pitch direction of the diffraction grating with respect to the light source
1
, the intensity distribution of the grating pattern moves in the displacement direction of the scale
2
with the same spatial cycle maintained.
Thus, by setting the spatial cycle p
20
of light receiving areas
4
of the photodetector
3
to be equal to the value p
2
, a periodical intensity signal is obtained from the photodetector each time the scale
2
moves in the pitch direction by the pi, thereby allowing detection of the displacement of the scale
2
in the pitch direction.
The operation of conventional displacement sensors will be explained below.
Laser beams emitted from the vertical cavity surface emitting laser that is the coherent light source
1
are formed by the diffraction grating on the scale into a diffraction interference pattern on the light receiving surface of the photodetector
3
, the pattern having a constant cycle pi (z
1
+z
2
)/z
1
.
Since the light receiving areas
4
on the photodetector
3
, constituting light intensity-detecting means, are formed in the pitch direction of the diffraction grating at distances of np
1
(z
1
+z
2
)/z
1
, these light-receiving areas detect only the same particular phase portion of the diffraction interference pattern on the light receiving surface.
When the scale
2
is displaced in the pitch direction of the diffraction grating by x
1
, the diffraction interference pattern on the light receiving surface is displaced in the same direction by x
2
=x
1
(z
1
+z
2
)/z
1
. Consequently, each time the scale
2
is displaced in the pitch direction of the diffraction grating by one pitch, the light intensity-detecting means provides output signals with a periodically varying intensity.
A primary axis of light beams emitted from this surface emitting laser light source is shown at reference numeral
5
, and beam boundaries at which the light beam intensity is half that of the primary axis are shown at reference numeral
6
.
Additionally, a remote tangent to each of the beam boundary curves
6
is shown at reference numeral
6
′, and the angle between the tangents
6
′, which are opposed to each other with respect to the primary axis of the light beams, is defined as &thgr;x and &thgr;y in directions x and y, respectively, and the &thgr;x and &thgr;y are referred to as the spread angle of the light beams.
In the surface emitting laser, by freely setting the dimensions of an emission window in an element to vary beam diameters &ohgr;
ox
, &ohgr;
oy
on an emission surface, the &thgr;x and &thgr;y can be set at ones of a broad range of values due to diffraction of the light beams.
Further, when an inclined base
11
is provided, the grating surface of the scale
2
and the light receiving surface of the photodetector
3
are inclined with respect to the primary axis of the light beams emitted from the laser light source. Accordingly, when light emitted from the laser light source
1
is reflected from a surface of the scale
2
or the photodetector
3
, it can be prevented from returning to the laser light source
1
to restrain optical reflection noise in the laser light from being superposed on output signals from the sensor.
Thus, the optical displacement sensor using the vertical cavity surface emitting laser light source shown in
FIGS. 9A and 9B
can sense the displacement of the scale more accurately and reliably.
On the other hand, the principle on which the displacement sensor is based when the diffraction grating is irradiated with parallel beams will be described with reference to FIG.
10
.
A measuring movable grating
103
is a rectangular-wave grating comprising transparent and opaque portions alternated at equal distances and mounted on a measured object in its moving direction.
On the other hand, a small fixed grating
104
having the same shape as the movable grating
103
is located opposite and close to the movable grating
103
, and in this arrangement, a light source
101
applies parallel beams via a lens
102
so that transmitted beams are detected by a photodetector
106
via a lens
105
.
When fringe directions of the two gratings
103
,
104
are maintained exactly in parallel, the scale distance between the gratings is defined as z, the scale pitch is defined as p
1
, the oscillation wavelength of a laser is defined as &lgr;, and k denotes an integer. Then, when these values are adjusted to meet the following equation:
z=kp
1
2
/&lgr; (3)
a luminous flux entering the photodetector
106
repeatedly increases or decreases each time the measuring movable grating
103
moves by one pitch.
In this case, if the two gra
Komazaki Iwao
Yamamoto Eiji
Frishauf Holtz Goodman & Chick P.C.
Olympus Optical Co,. Ltd.
Turner Samuel A.
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