Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Reexamination Certificate
1999-11-23
2002-06-18
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
C360S077030, C250S231160
Reexamination Certificate
active
06407378
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to an encoder device, and more particularly, to an optical encoder device that is thinner and at the same time provides highly accurate encoder output.
2. Description of the Related Art
In a magnetic disk drive, for example, a head carriage having a magnetic head is moved in a radial direction of a disk and the magnetic head is made to trace a selected track as the position of the head carriage is detected by the encoder device. Efforts are underway to make encoder devices of this type thinner and more compact while at the same time providing highly accurate encoder output.
The conventional encoder device has two light-receiving elements such as photodiodes placed 90 degrees apart, with two signals phase A and phase B having the same periods being output from light-receiving elements that receive the light from the light-emitting elements. From the two signals phase A and phase B the direction and distance that the head carriage has traveled is obtained.
More recently, in an effort to obtain more accurate encoder device output, four light-receiving elements have come to be used and four signals phase A, phase B, inverted phase A and inverted phase B extracted and the phase A and inverted phase A, as well as the phase and the inverted phase B, are differentially amplified.
FIG. 1
is a schematic structural diagram of a conventional encoder device. As indicated in the diagram, in the conventional encoder device
1
the light-receiving element
2
A and the light-receiving element
2
B are disposed so as to have phases 90 degrees different from each other, the light-receiving element
2
a
of the inverted phase A and the light-receiving element
2
A are disposed so as to have phases 180 degrees different from each other and the light-receiving element
2
b
of the inverted phase B and the light-receiving element
2
B are disposed so as to have phases 180 degrees different from each other.
Additionally, in the conventional encoder device
1
a light-emitting element
3
is disposed at a location opposite the light-receiving elements
2
A,
2
B,
2
a
and
2
b
, the light-receiving elements
2
A,
2
B,
2
a
and
2
b
symmetrically disposed with respect to a center line of the light-emitting element
3
. A main scale
5
made of a single piece of plastic is provided between a lens
3
a
of the light-emitting element
3
and the light-receiving elements
2
A,
2
B,
2
a
and
2
b
. The main scale
5
has slits
4
spaced at regular intervals, the slits
4
being shown in
FIG. 1
as blank openings in the main scale
5
.
Light emitted from the light-emitting element
3
is diffused at predetermined angles by the lens
3
a
so as to reach the light-receiving elements
2
A,
2
B,
2
a
and
2
b
. When the main scale
5
, which is movable, moves in a direction D with respect to the light-emitting element
3
, the light emitted from the light-emitting element
3
passes through the slits
4
in the main scale
5
and strikes the light-receiving elements
2
A,
2
B,
2
a
and
2
b
. The intensity of the light received at each of the light-receiving elements
2
A,
2
B,
2
a
and
2
b
varies as the main scale
5
moves and its position changes with respect to the light-receiving elements
2
A,
2
B,
2
a
and
2
b.
As a result, a waveform signal is obtained from each of the light-receiving elements
2
A,
2
B,
2
a
and
2
b
which corresponds to variations in the level of light received at the light-receiving elements
2
A,
2
B,
2
a
and
2
b
as the main scale
5
changes position with respect to the light-receiving elements
2
A,
2
B,
2
a
and
2
b
. Signals from the light-receiving elements
2
A,
2
B,
2
a
and
2
b
are input into a circuit not shown in the diagram, so that a phase A signal output from the light-receiving element
2
A and a phase a signal output from the light-receiving element
2
a
are differentially amplified to obtain an A′ phase signal (=A−a) and, similarly, a phase B signal output from the light-receiving element
2
B and a phase b signal output from the light-receiving element
2
b
are differentially amplified to obtain a B′ phase signal (=B−b). The A′ phase signal and the B′ phase signal have phases 90 degrees different from each other.
The arrangement of the light-receiving elements
2
A,
2
B,
2
a
and
2
b
is not important so long as phase A signals and phase B signals having phases 90 degrees different from each other and having the same period are output from the encoder device.
FIG. 2
is a diagram showing the conventional arrangement of the light-receiving elements
2
A,
2
B,
2
a
and
2
b
. As shown in the diagram, light-receiving element
2
B is positioned to one side of light-receiving element
2
A so as to have a phase 90 degrees different from that of light-receiving element
2
A, and light-receiving element
2
a
is positioned to one side of light-receiving element
2
b
so as to have a phase 90 degrees different from that of light-receiving element
2
b.
By positioning light-receiving elements
2
A,
2
B,
2
a
and
2
b
as described above, the light-receiving elements
2
A,
2
B,
2
a
and
2
b
are spaced an equal distance apart, that is, are spaced so as have a phase difference of 90 degrees. With such an arrangement of the light-receiving elements
2
A,
2
B,
2
a
and
2
b
, interference between the light-receiving elements
2
A,
2
B,
2
a
and
2
b
can be reduced and the sensitivity of the light-receiving elements
2
A,
2
B,
2
a
and
2
b
can be improved.
However, in the conventional encoder device
1
having the structure described above, when a given slit
4
of the main scale
5
passes a position opposite a central portion of the lens
3
a
of the light-emitting element
3
, the light emitted from the light-emitting element
3
via the lens
3
a
is not in the form of parallel beams of light but is dispersed at predetermined angles and, at the same time, diffracted by the edges of the slits
4
, and thus light leaks from the slits
4
. As a result, the light-receiving elements
2
A and
2
b
, which are positioned near the central portion of the lens
3
a
, are affected by the above-described leaked light and the detectional accuracy of the light-receiving elements
2
A and
2
b
is degraded.
Moreover, although it is desirable to make the encoder device slimmer, the effect of the above-described leaked light only increases as the light-receiving elements
2
A,
2
B,
2
a
and
2
b
are positioned closer to the lens
3
a
in an effort to make the encoder device slimmer.
It should be noted that although in
FIG. 1
the leaked light appears to penetrate the main scale
5
, in actuality the leaked light is cut off by the main scale
5
(the slanted line sections shown in
FIG. 1
) once a given slit
4
has passed the position opposite the central portion of the lens
3
a
, and hence does not strike the light-receiving elements
2
A and
2
b.
Further, the volume of light is particularly heavy around a central axis and surrounding area of the lens
3
a
, and as a result the effect of leaked light tends to be more pronounced thereabout. Thus light-receiving elements
2
A and
2
b
are particularly susceptible to the effects of leaked light because they are positioned closer to the central portion of the lens
3
a
than light-receiving elements
2
B and
2
a.
As a result, the accuracy and reliability of the phase A signal and the phase b signal output from the light-receiving elements
2
A and
2
b
declines.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an improved and useful encoder device in which the above-mentioned disadvantages are eliminated.
The above-described object of the present invention is achieved by an encoder device comprising:
a main scale with slit-like openings at regular intervals;
light-emitting means for emitting and directing light toward the main scale;
light-receiving means including four light receiving members for
Anderson Kill & Olick
Le Que T.
Lieberstein Eugene
Luu Thanh X.
Meller Michael N.
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