Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2001-01-02
2002-12-10
Kim, Robert H. (Department: 2882)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S23700G, C250S23700G, C250S559190, C250S559290, C250S559380, C356S614000, C356S615000, C356S616000
Reexamination Certificate
active
06492637
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a dimension measuring apparatus for optically detecting the movement of an optical scale (grid) or, in particular, to a configuration of an optical system and a signal processing system of a dimension measuring apparatus.
BACKGROUND ART
In a production line for precision members, demand is strong for in-line non-contact type measurement of the distance traveled by a machining head or the size of a workpiece. As a simple measuring instrument often used for this purpose, an optical scale formed with a multiplicity of fine grids having a binary light transmission distribution of black and white at a predetermined pitch is mounted on a probe, and the moving distance of the probe is optically read.
FIG. 21A
shows an example of a configuration of a conventional dimension measuring apparatus
700
. The light emitted from a light source
701
such as a white lamp or a light-emitting diode (LED) is radiated on an optical scale
710
through a collimating lens
702
. The optical scale
710
is configured with three grids including a moving grid
703
and stationary grids
704
A,
704
B. The moving grid
703
is mounted on a probe (not shown), and is adapted to move in the directions A, B indicated by arrows with the movement of the probe. The stationary grids
704
A,
704
B are fixed at a specific position behind the moving grid
703
.
The moving grid
703
and the stationary grids
704
A,
704
B each have a rectangular pattern of binary (black and white) light transmission distribution formed on a glass substrate, and have the same shape and the same pitch.
FIG. 21B
shows an example
715
of a light transmission distribution of a grid. A black pattern
716
transmitting a small amount of light and a white pattern
718
transmitting a large amount of light are both rectangular in shape and formed alternately. One pitch of the grid has a length of a, and the width of the black pattern and the white pattern are both a/2. Normally, a grid of a=10 &mgr;m is used.
The stationary grid
704
A is arranged so that it is shifted by one fourth of a pitch of the grid length with respect to the stationary grid
704
B, and two photodetectors
705
A,
705
B are arranged behind the stationary grids
704
A,
704
B, respectively. The photodetector
705
A outputs an A-phase signal
720
, and the photodetector
705
B outputs a B-phase signal
725
. The A-phase signal
720
and the B-phase signal
725
(hereinafter collectively referred to as the two-phase signals) both have the transmitted light intensity changed sinusoidally with the movement of the moving grid
703
. Since the grid positions of the stationary grids
704
A and
704
B are shifted by one fourth of a pitch, the phases of the A-phase signal
720
and the B-phase signal
725
are shifted by &pgr;n/2 from each other.
In the case where the grids having the configuration described above are irradiated with parallel light rays, the distance L between the moving grid
703
and the stationary grids
704
A,
704
B is required to be set to a specific value (Fourier image distance) determined by the relation between the wavelength of the light source
701
and the length a of one pitch of the grid. The Fourier image is defined as a light transmission distribution substantially equivalent to the geometric shape of the moving grid
703
irradiated with parallel light rays from the light source
701
. In the case where the wavelength &lgr; of the light source is 0.7 &mgr;m and the grid pitch length a is 10 &mgr;m, L is about 140 &mgr;m. The light transmission distribution at a position different from the Fourier image surface has a deteriorated contrast, and therefore the two-phase signals are required to be measured with high contrast by disposing the stationary grids
704
A,
704
B at the Fourier image distance of the moving grid
703
.
The signal processing will be explained with reference to the waveform examples
720
and
725
shown in FIG.
21
C. The A-phase signal
720
and the B-phase signal
725
are both a sinusoidal wave which moves one cycle when the moving grid
703
moves one pitch of its grid length. In
FIG. 21C
, the phase of the A-phase signal
720
is &pgr;/2 ahead of the phase of the B-phase signal
725
. An example is the case in which the probe begins to move at a position
741
and stops at a position
742
. The moving distance from position
741
to position
742
is the dimension to be measured, and for the measurement, the moving distance equal to an integral multiple of one pitch of the grids and the moving distance not longer than one pitch of the grids are both required to be detected.
A sinusoidal wave number counter
731
in
FIG. 21A
counts one signal for each cycle of the sinusoidal wave and thereby counts the number (integer) of pitches of the moving grid
703
. For example, the number is counted for each period of the sinusoidal wave assuming the mesial intensity position (
743
,
745
,
746
, etc.) of the amplitude of the A-phase signal
720
as a trigger point. For detecting the number, it is necessary to determine the direction in which the moving grid
703
moves, and the direction of movement is determined from the lead and lag of the phase between the two-phase signals. In the case where the phase of the A-phase signal
720
is advanced, for example, the number is counted upward, while in the case where the B-phase signal
725
is advanced, the number is counted downward.
The moving distance not more than one pitch of the grid is the distance La between the movement start position
741
and the reference position
743
, and the distance Lb between the reference position
746
and the stop position
742
. The resolution and accuracy of the dimension measurement are determined by the accuracy of detection of the distances La, Lb. Therefore, it is important how finely one cycle of the sinusoidal wave is segmented for detecting the grid stop position. In view of this, the phase of the sinusoidal wave corresponding to the positions
741
and
742
is detected from the intensity of the sinusoidal wave. For attaining the resolution of 0.1 &mgr;m in the case where the length a of one pitch of the grid is 10 &mgr;m, for example, the phase is required to be detected by segmenting one pitch of the grid into 100 parts.
A phase quadrant determination unit
732
determines the phase quadrant (1 to 4) of the sinusoidal wave from the relation of the intensity of the two-phase signals between the grid stop positions
741
and
742
. It is determined that the phase of the A-phase signal
720
is in the second quadrant (&pgr;n/2 to &pgr;) and the phase of the B-phase signal
725
is in the third quadrant (&pgr; to 3/2&pgr;) at the position
741
. The phase detector
733
standardizes the amplitude of both the A-phase signal
720
and the B-phase signal
725
as the magnitude of ±1 and, based on the standardized intensity (Va, Vb), detects the phase of the grid stop positions
741
,
742
from the arc tangent (Va/Vb) equation, for example. In the process, a trigonometric function table
734
for storing the tangent values of the trigonometric function is prepared, and the phase is determined by referring to the values in the table.
In the case where the phase detected at the grid stop position
742
is &phgr;, for example, the distance not more than one grid pitch is Lb=a&phgr;/(2&pgr;). The same applies to the distance La. In the case where the grid pitch a is segmented into 100 parts for detection, the phase is required to be detected with an error of not more than 3 degrees. The dimension calculation unit
735
calculates the dimension from the sum of the moving distance equal to an integer multiple of one grid pitch detected by the sinusoidal wave signal number counter
731
and the moving distance not more than one grid pitch detected by the phase detector
733
. In this way, the conventional dimension measuring apparatus is based on the measurement of the intensity of the two-phase sinusoidal wave signals by generating the same signals.
As described abov
Citizen Watch Co. Ltd.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Ho Allen C.
Kim Robert H.
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