Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1999-03-10
2001-09-04
Lee, John R. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C250S231160, C250S23700G, C356S618000
Reexamination Certificate
active
06285023
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to an optical linear scale for measuring a momentum of relative movement between two objects and more particularly to an apparatus for generating an origin signal of an optical linear scale by which a momentum of relative movement of, for example, a machine tool can be obtained as an absolute value by outputting a reference position as the origin signal in such the scale.
DESCRIPTION OF THE PRIOR ART
It is important in a precision work in a machine tool and so forth to measure accurately a momentum of relative movement of the tool toward an article to be worked.
As one of them, an optical scale using Moire fringe obtained by superimposing two sheets of optical grating has been conventionally known. This type of optical scale has, as illustrated in
FIG. 9
, a main scale
101
prepared by forming a grating (a cut line) on one face of a transparent glass scale
100
so that a transparent portion and a non-transparent portion are arranged at prescribed pitch, and an index scale
103
prepared by forming a grating (a cut line) on one face of a transparent glass scale
102
so that a transparent portion and a non-transparent portion are arranged at prescribed pitch, and ,as illustrated in
FIG. 9
, the index scale
103
is opposed to the main scale
101
at a minute interval and, simultaneously, as illustrated in
FIG. 9
, the grating of the index scale
103
is arranged so as to incline at a minute angle relative to the grating of the main scale
101
.
The gratings provided on the main scale
101
and the index scale
103
are formed by the gratings having equal pitches which are prepared by evaporating in vacuo chromium onto the glass scale
100
,
102
and etching.
According to such an arrangement as described above, the Moire fringe shown in
FIG. 10
is generated. An interval of this Moire fringe is W, and a dark portion and a light portion are formed at every interval W. The dark portion or light portion moves from the upper part to the lower part or from the lower part to the upper part according to a direction in which the index scale
103
moves relatively from right to left toward the main scale
101
. In this case, when a pitch between the main scale
101
and the index scale
103
is P and a tilt angle between them is &thgr; [rad], the interval W of the Moire fringe is shown by the following equation;
W=P/&thgr;, and the interval W of the Moire fringe is one that the interval of the grating P is enlarged optically 1/&thgr; times. Therefore, when the grating moves by one pitch P, the Moire fringe is displaced by W, and therefore, the momentum of the movement within the pitch P can be measured accurately by reading a change of W in upper and lower directions.
Then, as illustrated in
FIG. 11
, a photoelectric conversion element
110
for detecting optically a change of the Moire fringe is set to the index scale, and a light source is set to opposite side of the main scale, and a change in an electric current flowing through the photoelectric conversion element
110
is read while moving the index scale
103
relatively to the main scale
101
.
That is to say, when the index scale
103
is in a state of A relative to the main scale
101
, a quantity of light with which the photoelectric conversion element
110
is irradiated is the most largest and the electric current flowing through the photoelectric conversion element
110
comes to a maximum value I
1
. Next, being in a state of B after a relative movement, the quantity of light with which the photoelectric conversion element
110
is irradiated decreases somewhat and its electric current is I
2
, and then, being in a state of C after further movement, the photoelectric conversion element
110
is irradiated with the most smallest quantity of light and its electric current is also the most smallest I
3
. And, being in a state of D after further movement, the quantity of light with which the photoelectric conversion element
110
is irradiated increases somewhat and its electric current is I
2
, and when moving to a position in a state of E, the position is one where the quantity of light is the largest and its electric current is a maximum value I
1
.
As described above, the electric current flowing through the photoelectric conversion element
110
changes in a state of sinusoidal wave form, and simultaneously, at the time when the change has moved by one period, the main scale
101
and the index scale
103
move relatively by a grating interval P.
While only one photoelectric conversion element
110
is mounted in
FIG. 11
, in the event that two photoelectric conversion elements
110
,
111
are mounted to shift one period (interval W) plus 90 degrees respectively as illustrated in
FIG. 11
, the electric current flowing through the photoelectric conversion element
112
at phase B is shifted 90 degrees relative to the electric current flowing through the photoelectric conversion element
111
at phase A as illustrated in FIG.
13
. That is to say, in the event that the electric current flowing through the photoelectric conversion element
111
at phase A is sine wave, the electric current flowing through the photoelectric conversion element
112
at phase B is co-sine wave.
In this case, since the phase of the electric current flowing through the photoelectric conversion element
112
at phase B relative to the electric current flowing through the photoelectric conversion element
111
at phase A is 90 degrees advance phase or 90 degrees delay phase owing to the direction of relative movement of the main scale
101
and the index scale
103
, in the event that two photoelectric conversion elements are set to shift 90 degrees, the direction of the relative movement can be detected by detecting the phase between both. An outline of a perspective view of an optical scale using aforementioned principles is shown in FIG.
14
.
In
FIG. 14
, a grating having equal pitches which is prepared by vacuum-evaporated chromium are formed on a face of the elongate main scale
101
, and the index scale
103
is fixed on a face of an U-shaped holder
104
holding the main scale
101
. A grating having equal pitches which is prepared by vacuum-evaporated chromium similarly to the main scale
101
is formed on a face of the index scale
103
opposite to the main scale, and the photoelectric conversion element
111
is mounted to the reverse side of the index scale
103
.
Further, as illustrated in
FIG. 15
, a light source
105
is placed to the face of the U-shaped holder
104
which is located oppositely to the main scale
101
to detect light transmitted through the main scale
101
and the index scale
103
by means of the photoelectric conversion element
111
.
And, the main scale
101
and the index scale
103
can be moved each other.
As described previously, the grating (cut line) of the index scale
103
is opposed to the grating (cut line) of the main scale
101
at a minute interval and, simultaneously, the former can be tilted to the latter at a minute angle.
From a cross-sectional view
FIG. 15
showing a principle structure of the optical scale constituted in such a manner as described above, the light generated from the light source
105
passes through the glass-made main scale
101
and then the glass-made index scale
103
, and thereafter, is received as Moire fringe by means of the photoelectric conversion element
113
.
Signals of phase A and phase B having a phase difference of 90 degrees respectively shown in the aforementioned
FIG. 13
are generated from the photoelectric conversion element
113
, and the direction of movement and distance of movement can be measured by these two signals as described above.
While the photoelectric conversion element
113
is provided with three photoelectric conversion elements, two of them generate the above mentioned signals of phase A and phase B, and the remainder generates a signal of reference level. And, a detecting signal of more higher accuracy can be obtained by setting the quantity of light
Futaba Denshi Kogyo Kabushiki Kaisha
Lee John R.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Pyo Kevin
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