Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
2000-10-27
2003-06-10
Turner, Samuel A. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
Reexamination Certificate
active
06577401
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a displacement measuring apparatus and, more particularly, to an apparatus for converting two light beams that have been diffracted by a diffraction grating into electric signals by a photoelectric conversion element and for measuring a displacement of the diffraction grating with high accuracy.
2. Description of Related Art
As an apparatus for measuring a displacement with high accuracy, there has conventionally been known a displacement measuring apparatus called a linear gauge. Generally, a photoelectric transmission type linear encoder has been employed for this displacement measuring apparatus. The photoelectric transmission type linear encoder consists of a light source (a light-emitting diode) and a light-receiving element (a photodiode) facing each other, a fixed index scale having a scale of pitches in the order of &mgr;m and provided in a space between the light source and the light-receiving element, and a main scale that moves linked with the move of a measured object and having the same pitches as those of the fixed index scale. According to this displacement measuring apparatus, when the main scale moves linked with the move of the measured object, a light that is transmitted through the window of the index scale is interrupted intermittently. Based on the brightness and darkness formed by this intermittent interference of the light, two sinusoidal wave signals that have a phase difference of &pgr;/2 (rad) are output in the same cycle. Therefore, the distance of a movement of the main scale has been electrically detected from these signals.
In contrast to this photoelectric transmission type displacement measuring apparatus, in recent years, a displacement measuring apparatus has been put into practical use that has a diffraction grating with a fine pitch and a scale that moves with the movement of a measured object. According to this displacement measuring apparatus, a coherent light beam is irradiated onto the diffraction grating on the scale, and the apparatus measures a change in the intensity of the interference light between beams diffracted by the diffraction grating. The displacement of the scale is measured based on this effect. According to the displacement measuring apparatus that uses the diffraction grating, a beam splitter splits a light beam emitted from the coherent light source into two light beams. Each light beam is incident to the diffraction grating on the scale, Another beam splitter combines the light beams diffracted by the diffraction grating. Then, a photoelectric conversion element measures the intensity of the interference light. In order to measure the displacement with high resolution, it is necessary to provide a diffraction grating having a very fine pitch with a magnitude the same as the wavelength of the light source. For this purpose, a diffraction grating is generally used that is holographically recorded with interference fringes based on the interference of two light beams from a laser. The optical system of the displacement measuring apparatus is broadly classified into a transmission type apparatus that detects interference between diffracted light beams that are transmitted through the diffraction grating, and a reflection type apparatus that detects interference between diffracted light beams that are reflected by mirrors.
A conventional transmission type displacement measuring apparatus
50
will be explained in detail with reference to FIG.
1
. The transmission type displacement measuring apparatus
50
has a scale
14
that moves with a member of which the displacement is to be measured. On this scale
14
, there is provided a diffraction grating
15
marked with lines in a direction perpendicular to a move direction of the scale. On one of the two sides of this scale
14
, there are a polarized-beam splitter
51
and a coherent light source
11
as represented by a laser diode. A collimating lens
12
is provided between the light source
11
and the polarizing beam splitter
51
. On the other of the two sides of this scale
14
, a beam splitter
52
and photoelectric conversion elements
18
A and
18
B are disposed. A quarter-wave plate
17
and a polarizer
53
A are disposed between the beam splitter
52
and the photoelectric conversion elements
18
A. A polarizer
53
B is disposed between the beam splitter
52
and the photoelectric conversion element
18
B.
In the displacement measuring apparatus
50
having the above-described structure, a light beam that has been emitted from the light source
11
is adjusted into a collimated beam by the collimating lens
12
, and the beam is split into two diffracted lights by the polarizing beam splitter
51
. The light beams split into two are then incident to the scale
14
respectively, and reach the diffraction grating
15
. These two light beams are diffracted by the diffraction grating
15
. First-order diffracted lights having± signs diffracted by the diffraction grating
15
(it is possible to utilize high-order diffracted lights, but generally, ± first-order diffracted lights having high diffraction efficiency are used) are combined together by the beam splitter
52
. The combined diffracted light beams are incident to the polarizers
53
A and
53
B respectively. The beams incident to the polarizers
53
A and
53
B are converted into linearly polarized beams respectively. The polarized beams are incident to the photoelectric conversion elements
18
A and
18
B as interference light beams of components of the same polarization directions respectively. These interference lights are converted into electric signals, which are then used for detecting the displacement distance.
The intensity of the interference light becomes a sinusoidal wave signal having a displacement of d/2 as one cycle when the pitch of the diffraction grating
15
is expressed as d. One of the two combined light beams is transmitted through the quarter-wave plate
17
to obtain a signal of which the phase is deviated by &pgr;/2 (rad) from the phase of the other interference signal, This signal is used to detect the proceeding direction of the scale
14
and to detect the phase of the output signal. For example, when the pitch of the diffraction grating is 0.5 &mgr;m, the output signal becomes a sinusoidal wave signal having 0.25 go wave length as one cycle. It is possible to obtain a high resolution of 0.01 &mgr;m when the wavelength of the output signal is divided into 25 and interpolation is used.
However, according to the conventional displacement measuring apparatus
50
, in order to measure a phase modulation volume based on a shift of the diffraction grating
15
as an interference signal, it is necessary to provide the optical elements having a function of branching a light beam and a function of combining light beams, in addition to the diffraction grating
15
. In other words, as main.parts of the optical system, it is necessary to provide the polarizing beam splitter.
51
for splitting the light beam emitted from the light source
11
into two light beams based on the polarization components and for making the split light beams incident to the diffraction grating
15
respectively, and the beam splitter
52
for combining the two split light beams together. Further, in order to obtain two phase signals it is necessary to use the quarter-wave plate
17
to give a phase difference of &pgr;/2 between the orthogonal polarized components, and it is necessary to insert the polarizers
53
A and
53
B before the light sources
18
A and
18
B respectively. The orthogonal polarized components need to be extracted in this way. Further, as the phase precision of the wave plate
17
influences the measurement precision, it is necessary to provide the wave plate
17
of high precision in order to achieve the measurement of a displacement with high precision.
The polarizing beam splitter
51
and the high-precision wave plate
17
are expensive. In addition, it is also necessary to provide the beam spli
Citizen Watch Co. Ltd.
Lyons Michael A.
Turner Samuel A.
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