Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
2001-08-27
2003-06-17
Le, N. (Department: 2858)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S231130, C356S619000, C702S145000, C702S151000, C702S163000, C702S161000
Reexamination Certificate
active
06580066
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a measurement signal generating circuit for a linear scale which is adapted to measure relative movement between two objects, and more particularly to a signal processing circuit which is useful to increase an S/N ratio of a Lissajous signal outputted from a photoelectric conversion means.
In a machine tool or the like, accurate measurement of relative movement between a tool and a workpiece is highly important for carrying out precise processing of the workpiece. For this purpose, a variety of measuring devices have been manufactured as commercial products.
One of such measuring devices is practiced in the form of an optical scale utilizing Moire fringes obtained by superposing two optical gratings on each other. The optical scale is generally constructed in such a manner as shown in FIGS.
5
(
a
) and
5
(
b
). More particularly, the optical scale includes a main scale
101
including a transparent glass scale
100
which is formed thereon with gratings (cut lines) so that light-permeable sections and light-impermeable sections are arranged at predetermined pitches. The optical scale also includes an index scale
103
including a transparent glass scale
102
formed thereon with gratings so that light-permeable sections and light-impermeable sections are arranged at predetermined pitches. The main scale
101
and index scale
103
, as shown in FIG.
5
(
a
), are arranged opposite to each other at a microinterval. Also, the main scale
101
and index scale
103
, as shown in FIG.
5
(
b
), are so arranged that the gratings of the index scale
103
are inclined at a microangle with respect to the gratings of the main scale
101
.
The gratings provided on the main scale
101
and index scale
103
are formed at the same pitches by forming chromium on the glass scales
100
and
102
by vacuum deposition and then subjecting it to etching.
Such arrangement of the gratings permits generation of Moire fringes shown in FIG.
6
. The Moire fringes are formed at intervals W, so that dark portions or bright portions are obtained at the intervals W. The dark portions or bright portions are downwardly or upwardly moved depending on a direction in which the index scale
103
is laterally moved relative to the main scale
101
. In this instance, supposing that pitches of gratings of the main scale
101
and index scale
103
are indicated by P and an inclination angle between both scales
101
and
103
is indicated by &thgr; (rad), the intervals W of the Moire fringes are represented by the following expression:
W=P/&thgr;
Thus, the intervals W of the Moire fringes are optically defined to be 1/&thgr; times as large as the grating intervals P. Therefore, when the grating is moved by one pitch P, the Moire fringe is displaced by W, so that movement within the pitch P may be precisely measured by reading a variation in intervals W in a vertical direction.
For example, as shown in
FIG. 7
, a photoelectric conversion element
110
is provided on the index scale
103
and a light source is provided on a side of the main scale
101
opposite to the photoelectric conversion element
110
, so that a variation in current flowing to the photoelectric conversion element
110
may be read while moving the index scale
103
relative to the main scale
101
.
More particularly, when a pattern of the Moire fringes is at a state indicated by A in
FIG. 7
, the amount of light irradiated to the photoelectric conversion element
110
is maximized, so that a current flowing to the photoelectric conversion element
110
reaches a maximum level I
1
. Then, when the pattern is at a state indicated by B in
FIG. 7
due to relative movement between the main scale
101
and the index scale
103
, the amount of light irradiated to the photoelectric conversion element
110
is somewhat reduced, so that the current is reduced to a level I
2
. When the relative movement is further carried out to cause the pattern to be at a state indicated at C, the amount of light irradiated to the photoelectric conversion element
110
is minimized, so that the current is reduced to a minimum level I
3
. Then, when the index scale
103
is further moved relative to the main scale
101
to cause the pattern to be at a state D in
FIG. 7
, light irradiated to the photoelectric conversion element
110
is somewhat increased, resulting in the current being increased to a level I
2
. Moreover, the relative movement is further carried out to cause the pattern to be at a state E in
FIG. 7
, light irradiated to the photoelectric conversion element
110
is increased to the maximum level again, so that the current may be increased to the maximum level I
1
.
Thus, the current flowing to the photoelectric conversion element
110
is varied in a manner like a sinusoidal wave, and when the variation elapses by one period, relative movement between the main scale
101
and the index scale
103
is carried out by the grating interval P.
In
FIG. 7
, only one such photoelectric conversion element
110
is arranged. Alternatively, as shown in
FIG. 8
, an A phase photoelectric conversion element
111
and a B phase photoelectric conversion element
112
may be arranged while being deviated from each other by one period (interval W) and 90°. Such arrangement permits a current flowing to the B phase photoelectric conversion element
112
to be deviated by 90° with respect to a current flowing to the A phase photoelectric conversion element
111
, as shown in FIG.
9
. Thus, supposing that a current flowing to the A phase photoelectric conversion element
111
is in the form of a sinusoidal wave, that flowing to the B phase photoelectric conversion element
112
is in the form of cosine wave.
In this instance, a phase of the current flowing to the B phase photoelectric conversion element
112
is advanced or delayed by 90° relative to that of the current flowing to the A phase photoelectric conversion element
111
depending on a direction of relative movement between the main scale
101
and the index scale
103
. Thus, when the two photoelectric conversion elements
111
and
112
are arranged while being deviated by 90° relative to each other, a phase therebetween may be detected, resulting in a direction of the relative movement being detected.
Actually, the conventional optical scale is so constructed that any additional photoelectric conversion element is arranged at a predetermined position, to thereby concurrently output an A phase signal and a B phase signal, as well as inverted A phase and B phase signals respectively obtained by inverting the A phase and B phase signals by 180°. Such construction permits a DC component to be removed from the signal detected and ensures reliability of the signal and follow-up characteristics at a high speed.
FIG.
4
(
a
) indicates the above-described A phase signal and inverted A phase signal and the above-described B phase signal and inverted B phase signal obtained by arranging four photo-detectors at predetermined positions on the index scale.
Also, FIG.
4
(
b
) shows a circuit for forming a synthesized A-phase signal based on waveforms of the two A phase signals described above, wherein AP and −AP each indicate a photo detector for detecting Moire fringes formed by light permeating between the cut lines of the scale to convert them into an electric signal.
In a sinusoidal current of an inverted phase outputted from each of the photo detectors, one of signals thereof is inverted at a phase thereof through an inversion amplifier A
1
and synthesized in an addition circuit ADD constituted by an operational amplifier OP.
Synthesis of the B phase signal is likewise carried out in such a circuit as described above.
Such a circuit structure permits synthesis of a measurement signal from which a DC signal is removed through the single operational amplifier OP, to thereby accomplish a reduction in manufacturing cost. However, it renders adjustment in offset before the synthesis difficult, leading to a deterioration in balance between the A pha
Kuga Toshihiko
Uehira Takahisa
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