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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a linear encoder for performing positional
detection of relative motion that is used in mechanisms that perform said
relative motion such as, for example, machine tools and industrial robots,
and to a guide unit formed by adding a guiding device, having a track rail
and so forth, that guides said relative motion, to said linear encoder.
2. Description of the Prior Art
An example of an apparatus of the prior art containing this type of guide
unit is the direct drive unit indicated in FIG. 1. In contrast to a guide
unit comprising the above-mentioned linear encoder and guiding device,
this direct drive unit is provided with, for example, a linear motor for
the driving device.
As indicated in FIG. 1, said direct drive unit has two track rails 2 that
are mounted in parallel on base 1 and in which tracks are formed in the
lengthwise direction in each to serve as a guiding device, and a slider 4
that is straddled across both said track rails 2 and guided by said
tracks. In addition, a linear motor has a primary side (not shown) and a
secondary side 6. Said primary side is composed of mutually connected
permanent magnets and electromagnets and attached to the bottom surface of
said slider 4. The secondary side 6 is composed of rectangular members
wherein a plurality of inductor teeth (reference numeral not shown),
having high magnetic permeability, are formed so as to be arranged in a
row in the direction of the tracks. These inductor teeth and the magnetic
poles of the above-mentioned electromagnets are arranged relative to each
other.
On the other hand, a linear encoder is arranged on base 1 along one of
track rails 2. Said linear encoder has a detected element in the form of a
multipolar magnetized, long permanent magnet 8, wherein magnetic poles (N
and S) are arranged in alternating fashion in a direction parallel to the
tracks of said track rails 2, and an electromagnetic conversion element
(not shown) in the form of a detecting element mounted on the bottom
surface of the side of slider 4 so as to oppose said permanent magnet 8.
The current position of slider 4 can be detected from the output of said
electromagnetic conversion element that moves together with said slider 4
so as to travel longitudinally along permanent magnet 8.
Furthermore, reference number 9 in FIG. 1 indicates a cable guide housing a
connection cable (not shown) for supplying a power source to the
above-mentioned primary side of the linear motor, as well as for obtaining
the output generated by the above-mentioned electromagnetic conversion
element. As indicated in this figure, this cable guide 9 is composed of a
plurality of links coupled in a row so as to be able to mutually pivot
freely. Together with being formed so that said connection cable maintains
a prescribed curvature even during movement of slider 4, it also serves to
protect said connection cable from damage.
In the linear encoder equipped on the above-mentioned direct drive unit,
since an electromagnetic conversion element is arranged on slider 4, the
connection cable for obtaining the output signals from said
electromagnetic conversion element must be arranged so as to be pulled
around the entire moving range of slider 4. Moreover, cable guide 9 and so
forth are also required which together with making the constitution
complex, has the shortcoming of hindering smooth operation on the moving
side, including slider 4, by the above-mentioned connection cable and
cable guide 9.
In addition, since permanent magnet 8 equipped on the above-mentioned
linear encoder is of a long shape, there is the additional shortcoming of
the entire apparatus in which said linear encoder is incorporated being
large in size.
SUMMARY OF THE INVENTION
Thus, in consideration of the above-mentioned shortcomings of the prior
art, a first object of the present invention is to provide a linear
encoder that does not require the connection cable to be pulled around and
has a simple constitution, while also contributing to smooth operation of
the moving side as well as reduced size of the apparatus. In addition, a
second object of the present invention is to provide a guide unit equipped
with said linear encoder in addition to a guiding device composed of a
track rail and so forth.
The present invention is a linear encoder that performs positional
detection of relative motion of two objects, comprising a plurality of
detecting elements arranged in a row at prescribed intervals along the
direction of relative motion with respect to the stationary side of each
of the objects performing relative motion, and a detected element arranged
on the moving side corresponding to the above-mentioned stationary side so
as to be able to oppose the above-mentioned detecting elements.
In addition, the present invention is a guide unit equipped with a track
rail in which tracks are formed in the lengthwise direction, a slider
guided by the above-mentioned tracks, and a linear encoder for detecting
the position of the above-mentioned slider with respect to the
above-mentioned track rail, wherein, the above-mentioned linear encoder is
composed of a plurality of detecting elements arranged in a row at
prescribed intervals along the above-mentioned tracks on the
above-mentioned track rail, and a detected element arranged on the
above-mentioned slider so as to be able to oppose the above-mentioned
detecting elements.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of the essential components of a direct drive
unit containing a linear encoder and guide unit of the prior art.
FIG. 2 is a perspective view, including a partial cross-section, of the
essential components of the direct drive unit containing a linear encoder
and guide unit pertaining to the present invention.
FIG. 3 is a side view, including a partial cross-section, indicating the
direct drive unit indicated in FIG. 2 provided on a base.
FIG. 4 is a cross-sectional view taken along line A--A relating to FIG. 3.
FIG. 5 is a perspective view of a field magnet equipped on the direct drive
unit indicated in FIGS. 2 through 4.
FIG. 6 is an enlarged explanatory drawing of the essential components of
the linear encoder pertaining to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The following provides an explanation of the direct drive unit, including a
linear encoder and guide unit, in the form of embodiments of the present
invention with reference to the drawings.
First, the following provides an explanation of the constitution of the
guide unit pertaining to the present invention.
As indicated in FIGS. 2 through 4, this guide unit has track rail 11,
formed so that the shape of the cross-section perpendicular to its
lengthwise direction is roughly that of the letter "U" opened upward,
rolling element circulating path 12 (reference numeral indicated in FIGS.
3 and 4, to be described in detail to follow), and a slider able to
perform relative motion with respect to said track rail 11 in the form of
sliding unit 13. One each of tracks having a roughly semi-circular
cross-section, in the form of track grooves 11a, are formed in the
lengthwise direction on the right and left outer sides of track rail 11.
However, the number of these track grooves 11a is not necessarily limited
to two. Two of the above-mentioned rolling element circulating paths 12
are provided to correspond to each of these track grooves 11a. A plurality
of balls 14 are arranged and contained within said rolling element
circulating paths 12 to bear the load between track rail 11 and sliding
unit 13. The balls 14 circulate while rolling over the track grooves 11a
with movement of the sliding unit 13.
Sliding unit 13 has casing 16 straddled across track rail 11, a pair of end
caps 17a and 17b connected to both ends of said casing 16, and two seals
18a and 18b attached to each of the outer surfaces of said end caps 17a
and 17b. Furthermore, grease nipple 19 for supplying grease to the
above-mentioned balls 14 is attached to end cap 17a. As indicated in FIGS.
3 and 4, each rolling element circulating path 12 is composed of load
bearing track groove 12a and return path 12b formed linearly and mutually
in parallel on both the left and right ends of casing 16, and a pair of
roughly semi-circular direction changing paths 12c and 12d that are formed
in both end caps 17a and 17b and that connect said load bearing track
groove 12a and return path 12b at both ends. Furthermore, the
above-mentioned load bearing track groove 12a is opposed to track groove
11a of track rail 11.
The guide unit having the above-mentioned constitution is arranged on, for
example, a frame indicated in FIGS. 3 and 4 (the entire frame is not
shown), or in other words, an object of the stationary side in the form of
a flat base 22. Track rail 11 is fastened to said base 22 by a plurality
of fastening members in the form of bolts (with hexagon sockets) 23.
Therefore, track rail 11 has a flat mounting surface 11b on its bottom for
mounting to base 22. Furthermore, as indicated in FIGS. 3 and 4,
countersunk portions 11c, having a diameter larger than the heads of said
bolts 23, and holes 11d, having a diameter slightly larger than the
threaded portions of bolts 23, are arranged mutually concentrically and in
a row in the lengthwise direction of said track rail 11. Bolts 23 are
screwed into base 22 by being inserted into said countersunk portions 11c
and holes 11d so that they are completely embedded. In addition, as
indicated in FIG. 2, a plurality of threaded holes 16a are formed in the
upper surface of casing 16 of sliding unit 13 to allow fastening of a
workpiece and so forth to said casing 16 by screwing bolts (with hexagon
sockets, not shown) into these threaded holes 16a.
Next, the following provides a detailed description of the linear motor
provided in the form of a driving device.
As indicated in FIGS. 2 and 4, the primary side of said linear motor has
rectangular plate-shaped coil yoke 25, provided so as to extend over
roughly the entire length of track rail 11 on said track rail 11, and a
plurality of armature coils 26 arranged in a row on said coil yoke 25.
Furthermore, coil yoke 25 is arranged near the bottom of track rail 11,
which is formed so that the shape of its cross-section is in the shape of
the letter "U" opened upward. More specifically, one each of support ledge
11e and support projection 11f is formed mutually in parallel near the
bottom of track rail 11 in said track rail 11. Coil yoke 25 is supported
by said support ledge 11e and support projection 11f, and mounted to track
rail 11 with adhesive or small screws and so forth. In addition, as
indicated in the drawings, each armature coil 26 is respectively wound in
the form of, for example, a roughly rectangular loop (and including those
shaped in the form of a diamond or parallelogram).
As indicated in FIG. 4, electronic component group 28, consisting of an IC,
transistors and so forth, is provided on the lower surface of coil yoke
25. Wiring for electrically connecting these electronic components is
provided by etching and so forth on the upper surface of coil yoke 25.
In addition, as indicated in FIGS. 2 and 4, thin boards 29, made of plastic
and so forth, are attached to the upper surface of each armature coil 26
for mounting each of said armature coils.
On the other hand, the secondary side is composed in the manner described
below.
As indicated in FIG. 4, said secondary side has magnet yoke 32 attached to
the lower surface of casing 16, a constituent member of sliding unit 13,
and field magnet 33 composed of a permanent magnet attached to the bottom
of said magnet yoke 32 in opposition to each of the above-mentioned
armature coils 26 of the primary side. As is clear from FIG. 5, field
magnet 33 is formed overall roughly into the shape of a rectangular plate
(including that formed into the shape of a diamond or parallelogram), and
is magnetized so that a plurality of, and in this case 5, N and S magnetic
poles are alternately arranged along the lengthwise direction of track
rail 11, or in other words, direction L in which there is relative
movement of the primary and secondary sides.
Next, the following provides an explanation of the linear encoder for
positional detection of sliding unit 13 with respect to track rail 11.
As is indicated in FIGS. 2 and 4, board 29 provided on track rail 11
extends farther to the outside than the edge of armature coil 26, and a
detecting element in the form of electromagnetic conversion element 37 is
mounted on the top (or bottom) of this extending portion. Said
electromagnetic conversion element 37 detects changes in magnetic field in
the form of changes in the value of electrical resistance, and a plurality
of said electromagnetic conversion elements 37 are, for example, arranged
in a row along the track at prescribed intervals at locations
corresponding to each armature coil 26 as indicated in FIG. 2. Although
not indicated in the drawings, an output signal acquisition device in the
form of a connection cable or flexible board is provided on the stationary
side for obtaining output signals from these electromagnetic conversion
elements 37.
In addition, a detected element in the form of permanent magnet 38 is
mounted by means of magnet yoke 39 on the bottom surface of sliding unit
13 so as to be in opposition to these electromagnetic conversion elements
37. Changes in the magnetic field accompanying movement of sliding unit 13
are then detected by said electromagnetic conversion elements 37.
The following provides a detailed explanation of the above-mentioned linear
encoder based on FIG. 6.
FIG. 6 indicates an enlarged view of the position and length of permanent
magnet 38 with respect to the direction of movement of sliding unit 13, or
in other words, the direction of orientation of electromagnetic conversion
elements 37. As is clear from said drawing, permanent magnet 38 is
magnetized with a plurality of, for example, 8 S and N magnetic poles
arranged in alternating fashion in the direction of the track.
In said constitution, when permanent magnet 38 moves along the track
together with sliding unit 13, a change occurs in the resistance value
corresponding to the change in the magnetic field in electromagnetic
conversion elements 37 in opposition to said permanent magnet 38. After
permanent magnet 38 passes over a specified electromagnetic conversion
element 37, changes in the resistance value occur successively in each
electromagnetic conversion element 37 as a result of a similar change in
the magnetic field being applied to the adjacent electromagnetic
conversion element 37. The change in the output of each electromagnetic
conversion element 37 based on the change in this resistance value is
detected. The position of sliding unit 13 can then be determined by signal
processing and computation. For example, after setting a reference
position in advance, it should then be determined from what
electromagnetic conversion element 37 an output was produced with respect
to said reference position.
Furthermore, as described above, since permanent magnet 38 is multipolar
magnetized so that the S and N magnetic poles are alternating, an output
signal can be obtained that corresponds to the number of each magnetic
pole with each electromagnetic conversion element 37. As a result, the
resolution of position detection can be improved. However, in cases when
such a degree of high resolution is not required, permanent magnet 38 may
be composed of a single magnetic pole instead of the multiple number of
magnetic poles as described above.
In addition, electrical division and processing of the output waveforms of
electromagnetic conversion elements 37 allows resolution to be increased
further.
In the present embodiment, the following constitution is employed in order
to obtain the output from each electromagnetic conversion element 37 in
the most efficient manner.
More specifically, as indicated in FIG. 6, when the total length of
permanent magnet 38 in the track direction is taken to be M, and the
arrangement pitch of each magnetic pole (N,S) of said permanent magnet 38
and each electromagnetic conversion element 37 is taken to be t and p,
respectively, a constitution is formed so that M=p-t.
Together with this eliminating any dead areas in position detection as a
result of any magnetic pole always acting only on a specific
electromagnetic conversion element 37, it also results in the obtaining of
only one detection output at all times. Thus, signal processing,
computation and so forth following detection signal output can be
performed efficiently.
Furthermore, although a linear motor is used for the driving device in the
above-mentioned embodiment, the present invention is not limited to a type
that uses this particular driving device.
In addition, although a guiding device in the form of rolling elements in
the form of balls 14 circulating within sliding unit 13 is used in the
above-mentioned embodiment, a guiding device of a different constitution
may also be applied. In addition, although balls are used for the rolling
elements in the above-mentioned embodiment, a constitution may also be
employed in which rollers are used.
Moreover, although the detection device that detects the position of
sliding unit 13 with respect to track rail 11 is composed of
electromagnetic conversion elements 37 and permanent magnet 38, the
present invention is not limited to said constitution, but rather various
other combinations can also be applied, such as, for example, a
constitution wherein a light reflecting plate is used for the detected
element and this light reflecting plate is detected by optical detecting
elements.
According to the present invention as explained above, since a detecting
device, to which a connection cable and so forth is connected for the
obtaining of output signals, is provided on the stationary side, the
pulling around of said connection cable is not required, which together
with simplifying the constitution, offers a first advantage of
facilitating smooth operation of the moving side due to said connection
cable not hindering the operation of the moving side.
In addition, according to the present invention, since a long permanent
magnet 8 like that of the apparatus of the prior art is not required, the
apparatus can be reduced in size, thus offering a second advantage of the
present invention.
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