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Description  |
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BACKGROUND OF THE INVENTION
The present invention relates to a novel driving device consisting of a
stator and an armature in which the armature is moved along a plane by an
electric current, and a two-dimensional precise positioning device making
use of the driving device for use in a semiconductor manufacturing
apparatus.
In the past, it has been common to use a precise positioning device in a
semiconductor manufacturing apparatus which comprises a finely displacable
loading table. Prior art methods for driving the loading table involve the
use of a step motor or a rotary type D.C. servo motor the rotational
motion being converted into a linear motion by means of a feed screw or
the like. However, in view of the mechanical structure of these devices it
is difficult to give a microfine movement of 1 micron or less to a loading
table, and if it is intended to obtain a high resolution, generally the
moving speed is slow due to the fact that the pitch of the screw becomes
fine. Consequently, in most examples the loading table was formed with a
double structure in which a microfine displacement was separately
provided, and so the device was mechanically complex and expensive. On the
other hand, recent development of D.C. linear motors has advanced and
excellent results have been obtained achieving high resolution of 1 micron
or less and high speed movement. However, both the method depending upon
feeding of a screw and the method employing a linear motor can provide
only a linear motion to the loading table. In general there are many cases
where the loading table must achieve a two-dimensional motion along a
plane. Thus, it has been common practice to construct an X-Y orthogonal
coordinate system by making use of linear driving sources which are
orthogonal to each other and providing guide rails for restraining
movements in the directions other than the driving directions.
SUMMARY OF THE INVENTION
It is one object of the present invention to provide a novel
two-dimensional current-motion converter serving as a driving device that
is movable in two-dimensional directions.
Another object of the present invention is to provide a two-dimensional
positioning device which can achieve positioning in two-dimensional
directions precisely and at a high speed without employing a double
structure or guide rails by making use of the aforementioned
two-dimensional current-motion converter.
According to one feature of the present invention, a two-dimensional
current-motion converter is provided comprising a two-dimensional array of
magnetic fields disposed perpendicular to a plane and having alternately
arranged field orientations. Each magnetic field of the array is
positioned at equal intervals or periods along said plane to form a
magnetic field group. At least four coils are being integrally fixed to
each other either directly or indirectly to form a coil group, and said
coil group is disposed within said array of magnetic fields so as to be
freely movable along said plane. The four coils are provided with a fixed
dimensional relationship, each coil being provided with an outer dimension
approximately equal to 3/2 of the period of the array and a winding width
of approximately 1/2 the period of the array. The coils are provided
spaced from one another along the plane of the array by odd number
multiples of approximately 1/2 the period of the array.
BRIEF DESCRIPTION OF THE DRAWINGS
The above-mentioned and other features and objects of the present invention
will become more apparent by reference to the following description of
preferred embodiments of the invention taken in conjunction with the
accompanying drawings, wherein:
FIG. 1 is a perspective view of a section forming a magnetic field group in
a two-dimensional current-motion converter according to the present
invention;
FIG. 2 is a plan view of the same section;
FIG. 3 is a plan view of a coil to be used in the two-dimensional
current-motion converter in FIGS. 1 and 2;
FIG. 4 is a plan view of a coil group formed by fixing four coils as shown
in FIG. 3 to each other;
FIG. 5 is a schematic view showing a positional relationship along a plane
of movement between the magnetic field group and the coil group in the
two-dimensional current-motion converter according to the present
invention;
FIG. 6 is a side view showing the same positional relationship;
FIG. 7 shows another preferred embodiment of the present invention in which
a side view of a two-dimensional current-motion converter having a
different construction of magnetic paths from those shown in FIG. 6 is
illustrated;
FIG. 8 shows two curves of a force exerted in one dimensional direction
versus a position of two coils in the coil group in the two-dimensional
current-motion converter according to the present invention;
FIG. 9 is a schematic view showing forces exerted upon one coil as
energized in the same coil group at various points within the magnetic
field group formed by the stator when the coil is moved through the
magnetic field group;
FIG. 10 is a block diagram showing a control system in the two-dimensional
precise positioning device according to the present invention; and
FIG. 11 is a block diagram showing the entire construction of the same
two-dimensional precise positioning device.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In order to facilitate understanding of the present invention, description
will start from explanation of the entire construction of the
two-dimensional precise positioning device according to the present
invention illustrated in FIG. 11.
In FIG. 11, reference numeral 150 designates a loading table which is, in
the illustrated example, movable only in the directions indicated by
arrows 151,152 and 153. In the following, with respect to FIG. 11, the
direction of the arrow 151 is called "X-direction", the direction of the
arrow 152 is called "Y-direction" and the direction of the arrow 153 is
called ".theta.-direction". The loading table 150 can be moved in the X-,
Y- and .theta.-directions with an extremely small force, for example, by
disposing flat plate type static pressure pneumatic bearings as shown at
157.
Reference numerals 154, 155 and 156 are components of a two-dimensional
current-motion converter according to the present invention, numeral 154
designates a stator forming a magnetic field, and numerals 155 and 156,
respectively, designate coil groups serving as armatures. The two
armatures 155 and 156 are mounted on the bottom of the loading table
separated in the X-direction, they use the magnetic field of the stator
154 in common, and their plane of movement is in parallel to the plane of
movement of the loading table.
The respective coil groups have a capability of simultaneously generating a
force in the X-direction and a force in the Y-direcion, and by using the
two coil groups together it is possible to obtain a torque in the
.theta.-direction. While the two coil groups are kept apart in the
X-direction in the illustrated example, they could be kept apart in any
arbitrary direction within the X-Y plane, and while the magnetic field
serves as a stator and the coil group serves as an armature in the
illustrated example, it is possible to reverse this relation. In addition,
in order that the area of the magnetic field forming member is reduced and
yet a wide motion area can be obtained, it is also possible that a number
of additional coil groups would be added and these coil groups would be
switched depending upon the position of the armature.
Reference numerals 158, 159, 160 and 161 designate component elements of a
position detector for knowing a present position of the loading table 150.
While the plane mirrors and the laser ranging devices well-known in the
prior art are used in the illustrated example, any kind of position
detector could be used so long as it can detect displacements in the X-,
Y- and .theta.-directions of the loading table 150. Reference numeral 158
designates plane mirrors having a high precision, and reference numerals
159, 160 and 161 designate detectors for the respective axes in the laser
ranging device. A displacement in the X-direction is obtained by the laser
ranging device 159, displacements in the Y-direction at two positions on
the loading table are obtained by the laser ranging devices 160 and 161,
respectively, and a displacement in the .theta.-direction is obtained from
a difference between detected values of the laser ranging devices 160 and
161. The laser ranging device used in this example achieves a high
resolution of 0.079 microns or less by employing a plane mirror.
Reference numerals 162, 163 and 164 designate a first group of counters for
holding positional relationships between the magnetic field and the coil
groups with respect to the position in the X-direction of the loading
table and with respect to the two positions in the Y-direction of the
loading table, and these counters are always renewed by the outputs from
the position detectors 159, 160 and 161.
Reference numerals 165, 166 and 167 designate a second group of counters
for always holding the present position of the loading table according to
the signals sent from the position detectors 159, 160 and 161. The counter
165 indicates the present position in the X-direction of the loading
table, and the counters 166 and 167 indicate the present positions in the
Y-direction of two points on the loading table. The outputs of the second
group of counters are respectively applied to digital subtractors 170, 171
and 172. On the other hand, reference numerals 168a and 169a designate
digital signal devices which issue digital signals representing desired
stop positions in the X-direction and in the Y-direction, respectively. A
signal 168 issued from the device 168a is applied to the digital
subtractor 170, and a signal 169 issued from the device 169a is applied to
the digital subtractors 171 and 172. Accordingly, the output of the
digital subtractor 170 represents a difference between the desired stop
position in the X-direction and the present position, that is, it becomes
a position error digital signal, and likewise the outputs of the digital
subtractors 171 and 172 become position error digital signals in the
Y-direction of two different points on the loading table.
The outputs of the subtractors 170, 171 and 172 are converted into analog
signals 176, 177 and 178 by means of digital-analog converters shown at
173, 174 and 175, respectively. Accordingly, the signal 176 is an
X-direction position error analog signal, and the signals 177 and 178 are
Y-direction position error analog signals at two different points on the
loading table. The signals 176, 177 and 178 are applied to adders 185, 186
and 187, respectively, also they are differentiated by differentiators
179, 180 and 181, respectively, to produce the respective velocity signals
and then applied to the adders 185, 186 and 187, respectively, with their
algebraic sign reversed, and further for the purpose of improvements in
response to minute displacements they are integrated by integrators 182,
183 and 184 and then also applied to the adders 185, 186 and 187,
respectively, to produce drive demand signals 188, 189 and 190,
respectively.
Reference numerals 194 and 195 designate driving force distribution control
circuits forming a two-dimensional driving device according to the present
invention as will be explained later, which are the circuits for obtaining
driving forces proportional to the magnitude and polarity of the drive
demand signals by distributing the drive demand signals in the X- and
Y-directions over the integrally formed four coils as currents according
to the positional relationship between the coils and the magnetic field.
To the control circuit 194 are input the signal 188 as an X-direction
drive demand signal, the signal 189 as a Y-direction drive demand signal,
further a signal 191 as a positional relation signal between the magnetic
field and the coils in the X-direction and a signal 192 as a positional
relation signal between the magnetic fields and the coils in the
Y-direction, and a signal group 196 for controlling the coil group 155 is
output from the control circuit 194. Likewise to the control circuit 195
are input the aforementioned signals 188 and 190 as X-direction and
Y-direction drive demand signals, respectively, and the aforementioned
signals 191 and 193 as positional relation signals between the magnets and
the coils in the X- and Y-directions, respectively, and a signal group 197
for controlling the coil group 156 is output from the control circuit 195.
Accordingly, the coil groups 155 and 156 generate forces proportional to
the X- and Y-direction drive demand signals 188 and 189 and the X- and
Y-direction drive demand signals 188 and 190, respectively and thereby the
position and attitude of the loading table can be corrected.
If this system including the first counter group 162, 163 and 164, the
drive distribution control circuits 194 and 195, the stator 154 and the
armatures 155 and 156 is deemed jointly as a D.C. motor having driving
forces in the X-, Y- and .theta.-directions, then this is equivalent to
the heretofore known D.C. servo system. Accordingly, the loading table 150
moves towards the position of the X-Y coordinate given by the signals 168
and 169, and it stops at the position where given coordinates become equal
to the coordinate obtained from the above-described position detector.
Furthermore, since feedback is provided such that the coordinates obtained
from the Y-direction position detectors, placed at two points having
different X-coordinates, may be held equal to the signal 169, the loading
table 150 does not require a guide rail, and yet rotation of the loading
table will not occur.
Thus, to the present invention, the basically well-known D.C. servo system,
which was a driving device consisting of a rotary type D.C. motor or a
linear D.C. motor in the prior art, is replaced by a special novel
two-dimensional current-motion converter as indicated by reference
numerals 154, 155 and 156, and jointly with a first counter group 162, 163
and 164 forming a control circuit and a drive distribution control circuit
194 and 195 constructs a special two-dimensional driving device. In the
following, description will be made on this two-dimensional driving
device, that is, a two-dimensional current-motion converter and a drive
circuit therefor in combination.
The two-dimensional current-force converter according to the present
invention makes use of a force generated when an electric current is
passed through a coil placed in a magnetic field, and as a matter of
course, though either the magnetic field side or the coil side could be
used as a stator. However, in the following description, for convenience
the magnetic field side is handled as a stator and the coil side is
handled as an armature.
At first, one example of a magnetic field is shown in FIG. 1. In FIG. 1,
reference numeral 11 designates a member forming a part of a magnetic path
and holding permanent magnets, numeral 12 designates permanent magnets
which are magnetized in the direction of magnetic axes represented by
arrows 13 and 14, and they are arrayed in the direction of arrows 15 and
16 at equal intervals with their N and S poles aligned alternately.
Accordingly, the directions 13 and 14 of the magnetic fields are arrayed
alternately.
FIG. 2 is a plan view of the magnet array in FIG. 1, reference numeral 21
corresponds to numeral 11 in FIG. 1 and reference numeral 22 corresponds
to numeral 12 in FIG. 1. Reference numerals 24 and 26 designate periods of
magnetic fields in the two orthogonal directions, and reference numerals
23 and 25 designate periods of arrays of the permanent magnets. In the
illustrated example, permanent magnets are disposed on a plate of soft
iron. However, so long as the illustrated magnetic fields can be obtained,
as a matter of course, the material and structure of the magnetic field
forming means is of no matter. In other words, the permanent magnet could
be replaced by electromagnets and every such modification is included
within the scope of the present invention.
One example of a coil is illustrated in FIG. 3, and one example of an array
of coils in one coil group is illustrated in FIG. 4. Reference numerals 31
and 34 designate an outer dimension of the coil, and in the illustrated
example, it is nearly equal 2/3 of the periods 23 and 25, respectively, of
the magnetic fields. Reference numerals 32 and 35 designate winding widths
of the coil, and in the illustrated example it is equal to nearly 1/2 of
the periods 23 and 25, respectively, of the periods of the magnetic field.
Accordingly at the center of the coil is provided a hole having dimensions
33 and 36, respectively, that is nearly 1/2 of the periods 23 and 25,
respectively, of the magnetic fields. Reference numeral 39 designates a
center line of the coil, and edges 37 and 38 of the square of the center
line have nearly equal dimensions to the periods 23 and 25 of the magnetic
fields. According to the method of controlling this two dimensional
current-motion converter as will be described later, preferably the coil
should have a square or rectangular shape as shown in FIG. 3. However, the
present invention can be practiced even in the case where the coil is
circular or nearly circular.
Now description will be made of an array of coils comprising a coil group.
Each coil group of the two-dimensional current-motion converter of the
present invention requires at least four coils, and these coils in
combination generate a driving force. While it is possible to use more
than four coils and to use these coils either simultaneously or switched
from one to another, this in principle is no different from the case of
using four coils, and the following description will be made with respect
to an example employing four coils.
Reference numerals 43, 44, 45 and 46 each represent the coil shown in FIG.
3. In this example, the coil 44 is displaced from the coil 43 in the
direction of an arrow 41 by nearly 2/3 of the period of the magnetic field
in this direction. The coil 45 is displaced from the coil 43 in the
direction of an arrow 42 by nearly 2/3 of the period of the magnetic field
in this direction, and the coil 46 is placed it is displaced in the
directions of arrows 41 and 42 by nearly 2/3 of the period of the magnetic
field in these directions. While the coils have all been displaced by 2/3
of the period of the array of the magnetic field in the illustrated
example, in principle, the dimension of displacement could be any odd
number multiple of 1/2 of the period of the magnetic field.
FIG. 5 is a plan view of the relation between the magnetic field
distribution shown in FIG. 2 and the coil group shown in FIG. 4. In the
state shown in FIG. 5, even if an electric current is passed through the
coil 53, forces exerted upon the coil by the magnetic field directed in
the same direction placed on a diagonal would off set each other, and so
no force is generated. On the other hand, if an electric current is passed
through the coil 54, then a driving force depending upon the direction of
the electric current is generated along an axis represented by arrows 58.
Likewise, if an electric current is passed through the coil 55, then a
driving force depending upon the direction of the electric current is
generated along an axis represented by arrows 57. The coil 56 is placed
outside of the magnetic field, and hence, even if an electric current is
passed through this coil, a driving force would not be generated. The
driving capability of the respective coils will periodically change as the
position of the coils changes. At first, the change of the driving
capability will be explained with respect to a one-dimensional direction,
as represented by arrows 57 or arrows 58.
FIGS. 6 and 7 are examples of a cross section taken along arrows 57 or
arrows 58. The difference between FIGS. 6 and 7 exists in the method of
forming a return path of magnetic flux. In FIG. 6, reference numerals 61
and 62 designate a stator forming a magnetic field, and numerals 67 and 68
designate another stator serving as a return path of the magnetic flux,
and the magnetic flux is efficiently closed between a set of adjacent
permanent magnets as shown by an arrow 65. Reference numerals 63 and 64
designate coils which have a movable plane along the direction of an arrow
66. On the other hand, in FIG. 7, reference numerals 71 and 73 designate a
stator forming a magnetic field, and the magnetic flux closes as shown by
an arrow 76 passing through a magnetic conductor 72 which moves jointly
with coils 74 and 75 of the armature. Reference numeral 77 designates a
movable plane of an armature. In either case, the variation of a driving
capability of a coil is as shown in FIG. 8. In FIG. 8, an arrow 81
designates a direction of movement of a coil, and an arrow 82 indicates
the magnitude and direction of a driving capability. Numerals 85 and 84
designate driving capability curves of the coils 63 and 64 or the coils 74
and 75, which generally depend upon the shape of the magnetic path, and
are close to a triangular waveform, a trapezoidal waveform or a sinusoidal
waveform having a period equal to that of the magnetic field. The driving
capability curves of the two coils have 90.degree. out-of phase
relationship, hence even when one of the curves is zero the other curve
has a driving capability. Furthermore, since the driving capability curves
in FIG. 8 take similar curves independently for both directions indicated
by arrows 57 and 58, respectively, the respective axial components of the
driving capability at any arbitrary point on the movable plane is the sum
of the respective curves.
A vector sum of the respective axial components represents a resultant
driving capability at any arbitrary point on the movable plane. This is
shown in FIG. 9.
FIG. 9 is an illustration of a force exerted upon a coil at various
positions of the coil when one coil is moved within a magnetic field
formed by a stator while a fixed D.C. current is being passed through the
coil. In this figure, a direction and a length of an arrow represent the
direction and magnitude of the force exerted upon the coil at that
position. In this figure, reference numerals 91 and 92 designate the
period of the magnetic field in the respective directions.
As described previously, since the positional relationship of a group of
four coils is such that each coil is placed at a period equal to an odd
multiple of 1/2 of the period of the magnetic field in the respective
axial directions, the four coils are moving in FIG. 9, for instance, while
maintaining a mutual positional relationship as reprsented by 93, 94, 95
and 96. Accordingly, upon the respective coils are exerted forces having
different directions and magnitude, respectively. Hence, by regulating
distribution of currents fed to these four coils, a driving force having
any direction and any magnitude on the plane of movement can be generated
in the coil group.
However, generally this distribution ratio would become a complex function
of the position of the coil group that is different depending upon a
distribution of a magnetic flux and a shape of coils.
FIG. 10 is a block diagram of one example of a system for carrying out this
current distribution. Reference numerals 100 and 101 designates
X-direction and Y-direction drive signal inputs from an external system,
and in the illustrated example, they are analog signals having a magnitude
and an algebraic sign. Reference numerals 102, 103 104 and 105 designate
respective coils, and the positional relationship between this coil group
and the magnetic field is measured by position detectors 106 and 107, and
they are always maintained in counters 108 and 109, respectively.
Accordingly, reference numerals 110 and 111 designate digital signals
indicating the positional relationship between the X-direction and
Y-direction magnetic fields and the coil. Reference numerals 112 and 113
are current distribution signal generator circuits for feeding four
digital values, respectively, which serve as distribution factors for
distributing the X-direction and Y-direction drive signals to the four
coils depending upon the coil position signals 110 and 111, and in the
illustrated example by making use of a read-only memory it is made
possible to approximate a distribution ratio represented by a complex
functions as fine as possible. Two sets of digital outputs issued from the
current distribution signal generator circuits 112 and 113 are applied to
corresponding ones of two sets of variable gain amplifier groups, each
group consisting of four variable gain amplifiers, and multiplication is
effected by analog inputs 100 and 101, respectively. In the illustrated
example, the variable gain amplifiers 114 and 115 are multiplier type
digital-analog converters including multiplication of algebraic signs.
The drive demand signals distributed for the respective coils by the
respective amplifier groups 114 and 115 are added by adders 116 for the
respective coils, then respectively passed through voltage-current
converters 117 and converted into a drive signal group 118 to drive the
corresponding coil.
In this way, in response to the drive demand signals indicated by 100 and
101, the driving force components in the respective directions 119 and 120
can be obtained in proportion to the direction and magnitude. The
above-mentioned is the operation principle of the two-dimensional driving
device constructed according to the present invention, this is equivalent
to the nature of the D.C. motor in the prior art, and accordingly, it is
possible to assemble the two-dimensional driving device in the existing
D.C. servo system.
While the description of operation of one example of a two-dimensional
precise positioning device according to the present invention has been
provided above, it is a matter of course that the shape of the loading
table, the shapes of the permanent magnet and the coil and their
arrangement could be modified without departing from the spirit of the
present invention.
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