Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
2000-08-02
2002-04-16
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S207240
Reexamination Certificate
active
06373239
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a position detecting apparatus for detecting the distance and position for and to which a movable component of a machine tool or industrial robot has moved.
A position detecting apparatus is known, which detects the distance and position for and to which a movable component of a machine tool or industrial robot has moved.
A position detecting apparatus of this type has been proposed in Japanese Patent Application No. 9-81016 filed by the present applicant.
FIG. 1
shows the position detecting apparatus proposed in Japanese Patent Application No. 9-81016.
As shown in
FIG. 1
, the position detecting apparatus
100
comprises a scale
101
and a magnetic sensor
102
. The scale
101
is a magnetic member. The magnetic sensor
102
detects the magnetic field generated by the scale
101
.
The scale
101
is a magnet that is made of, for example, ferrite-based plastic. It is shaped like a plate having a major surface
103
. The major surface
103
is a length L
11
and a width W
10
. The scale
101
has magnetized surfaces
103
a
and
103
b
. These surfaces
103
a
and
103
b
have been magnetized in opposite polarities, in a direction perpendicular to the major surface
103
. As shown in
FIG. 2
, the magnetized surfaces
103
a
and
103
b
are separated by a boundary line m, which intersects with the longitudinal center line of the major surface
103
, the center o of the major surface
103
. (Hereinafter, the line m shall be referred to as “center line n.” The center line n and the boundary line m intersect with each other at an angle &thgr;.
The magnetic sensor
102
comprises an annular core
104
and coils
105
and
106
. The core
104
is shaped like a rectangular frame, defining a closed magnetic path. The coils
104
and
105
are wound around the opposing longer sides of the core
104
, respectively. The core
104
is made of high-permeability material such as permalloy or amorphous metal. The coils
105
and
106
have
50
turns each. Each coil is formed by winding a Cu wire having a diameter of 0.06 mm around one longer side of the core
104
. A high-frequency pulse current is made to flow in the coils
105
and
106
, which generate magnetic fields in the opposite directions.
The scale
101
and the magnetic sensor
102
, thus constructed, are spaced apart by a predetermined distance. They are arranged such that the coils
105
and
106
have their axes perpendicular to the major surface
103
of the scale
101
and set aside in the widthwise direction of the major surface
103
and that the midpoint between the axes of the coils
105
and
106
is located exactly above the center line n of the major surface
102
.
The scale
101
and the magnetic sensor
102
, thus arranged, are secured to a fixed component and movable component of a machine tool, respectively. The scale
101
and the sensor
102
may move along the center line n of the major surface
103
of the scale
101
. At any position in the lengthwise direction of the scale
101
, the magnetic sensor
102
detects a magnetic field emanating from the scale
101
.
The operating principle of the position detecting apparatus
100
will be described, on the assumption that the lengthwise direction and widthwise direction of the major surface
103
are X direction and Y direction, respectively, and that the direction perpendicular to the major surface
103
is Z direction.
FIG. 3
shows a cross section of the scale
101
, taken along a line passing the intersection o of the centerline n of the scale
101
and the boundary line in, or the center o of the major surface
103
. As shown in
FIG. 4
, the magnetic field extending in the Z direction linearly changes in Y direction, from value −W
10
/4 to value W
10
/4. It follows that the magnetic field extending in the Z direction represents the position in the Y direction of the scale
101
.
The scale
101
is designed so that its detection-effective length L
12
, measured lengthwise (in the X direction), is shorter than the length L
11
(that is, L
12
<L
11
) and the angle &thgr;, at which the lines n and m intersect with each other, is tan
−1
(d/L
12
) (d is equal to or less than W
10
/2). The magnetic sensor
102
is moved along the center line n in the X direction, within the range of the detection-effective length L
12
. The two coils
105
and
106
respectively detect the magnetic fields that emanate from the surfaces
103
a and
103
b
magnetized in the opposite polarities and which extends in the Z direction. The difference between the outputs of the coils
105
and
106
linearly changes according to their positions along the longitudinal direction of the scale
101
, in the same way as the magnetic field extending in the Z direction changes in the Y direction in the cross section of the scale
101
, which is taken along a line passing the intersection o of the lines n and m. That is, the vertical magnetic field, which the magnetic sensor
102
detects, changes in accordance with the distance between the scale
101
and the magnetic sensor
102
. This means that the positional relation between the scale
101
and the sensor
102
can be detected from the intensity of the vertical magnetic field the magnetic sensor
102
has detected.
The magnetic field generated by the scale
101
linearly changes, but for an extremely narrow range. The detection precision of the position detecting apparatus
100
described above is greatly influenced by a displacement, if any, of the scale
101
or the magnetic sensor
102
. It is difficult for the apparatus
100
to detect the positions of the scale
101
and sensor
102
with a high accuracy. It is also difficult to position the scale
101
and the sensor
102
in the process of securing them to the components of a machine tool. Inevitably, the outputs of the coils
103
a
and
103
b
differ from the desired values. The detection precision of the apparatus
100
greatly decreases when the magnetic sensor
102
is positioned outside the region in which a magnetic field changes linearly.
The magnetic sensor
102
is a coil sensor. Its dimension is the largest in the coil-winding direction, i.e., magnetism-detecting direction. In the position detecting apparatus
100
, the magnetic sensor
102
detects the magnetic field that extends in the direction perpendicular to the major surface
103
of the scale
101
. The magnetic sensor
102
must therefore be arranged so that the magnetism-detecting direction may be perpendicular to the major surface
103
of the scale
101
. Consequently, the apparatus
100
is large and massive.
BRIEF SUMMARY OF THE INVENTION
The present invention has been made in consideration of the foregoing. The object of the invention is to provide a position detecting apparatus that is small and exhibits output characteristic little influenced by a positioning error, vibration, or displacement.
To achieve the object, a position detecting apparatus according to this invention comprises: magnetic field generating means for generating a magnetic field; and magnetic field detecting means capable of moving relative to the magnetic field detecting means, for detecting the magnetic field generated by the magnetic field detecting means. The magnetic field extends at right angles to a direction in which the magnetic field detecting means moves relative to the magnetic field generating means and extends to a magnetism-sensing direction in which the magnetic field detecting means is spaced from the magnetic field generating means. The magnetic field generating means applies to the magnetic field detecting means a magnetic field whose intensity linearly changes over a prescribed distance (L
2
) in the direction in which the magnetic field detecting means moves relative to the magnetic field generating means. The positional relation between the magnetic field generating means and the magnetic field detecting means is detected from the magnetic field detected by the magnetic field detecting means.
In the position detecting apparatus, the magn
Maioli Jay H.
Patidar Jay
Sony Precision Technology Inc.
Zaveri Subhash A
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