U.S. patent application number 09/780034 was filed with the patent office on 2002-04-25 for magnet configuration for a linear motor.
Invention is credited to Chitayat, Anwar.
Application Number | 20020047315 09/780034 |
Document ID | / |
Family ID | 46254048 |
Filed Date | 2002-04-25 |
United States Patent
Application |
20020047315 |
Kind Code |
A1 |
Chitayat, Anwar |
April 25, 2002 |
Magnet configuration for a linear motor
Abstract
A linear motor path forms a closed figure. Contactless power
control and position feedback permit a linear motor stage to
traverse the closed figure without interference from trailing
wires. One embodiment of the closed figure includes a racetrack
pattern with straight runs joined by curved ends. Axes of armature
windings are skewed to lie across the path in the curved ends to
maintain force on the linear motor stage. Other embodiments of the
invention includes multilevel paths wherein one portion of the path
crosses over another portion of the path. Disclosure is made of a
linear motor stage supported below the path by magnetic attraction
between permanent magnets on the stage and magnetic material in the
path.
Inventors: |
Chitayat, Anwar; (Fort
Salanga, NY) |
Correspondence
Address: |
SUSAN M. DONAHUN 704-
ROCKWELL AUTOMATION TECHNOLOGIES
1201 SOUTH SECOND STREET
MILWAUKEE WISCONSIN 53204
CLEVELAND
OH
44114
US
|
Family ID: |
46254048 |
Appl. No.: |
09/780034 |
Filed: |
February 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09780034 |
Feb 9, 2001 |
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09415166 |
Oct 8, 1999 |
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09415166 |
Oct 8, 1999 |
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09069324 |
Apr 29, 1998 |
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09069324 |
Apr 29, 1998 |
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09031009 |
Feb 26, 1998 |
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09069324 |
Apr 29, 1998 |
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09031287 |
Feb 26, 1998 |
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09069324 |
Apr 29, 1998 |
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09040132 |
Mar 17, 1998 |
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09069324 |
Apr 29, 1998 |
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09055573 |
Apr 6, 1998 |
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Current U.S.
Class: |
310/12.19 |
Current CPC
Class: |
H02K 11/00 20130101;
G01D 5/145 20130101; H02P 25/06 20130101; H02K 11/22 20160101; H02K
41/02 20130101; H02K 41/03 20130101 |
Class at
Publication: |
310/12 |
International
Class: |
H02K 041/00 |
Claims
What is claimed is:
1. A linear motor comprising: a path; said path including a
plurality of armature windings therein; a plurality of switches,
each one of said switches controlling application of power to one
of said armature windings; a linear motor stage movable on said
path; a plurality of motor permanent magnets on said stage facing
said path for interaction with said armature windings; means for
controlling actuation only of those of said switches controlling
application of power to those of said plurality of armature
windings within a magnetic influence of said motor permanent
magnets; said path describing a closed path; and means for
permitting said linear motor stage to traverse said closed
path.
2. A linear motor according to claim 1, wherein said means for
permitting includes a contactless system for controlling the
application of power to said armature windings.
3. A linear motor according to claim 2, wherein said means for
permitting further includes contactless means for producing at
least one of position and motion feedback from said movable
stage.
4. A linear motor according to claim 1, wherein: said closed path
includes at least one curved portion; each of said armature
windings includes an axis; at least some of said axes of said
armature windings in said at least one curved portion are skewed
with respect to axes of neighboring armature windings across said
path; and a skew of said axes being across a local width of said
path.
5. A path for a linear motor comprising: a plurality of armature
windings in said path; means for controlling application of power
to said plurality of armature windings; and said path including at
least first and second levels.
6. A path according to claim 5 wherein said first level includes an
upper portion which crosses over a lower level in said second
level.
7. A path according to claim 6, wherein said first and second
levels are joined to form a closed pattern.
8. A linear motor comprising: a path; said path including a
magnetic material therein; a movable stage movable on said path;
said movable stage including permanent magnets thereon facing said
magnetic material; at least a portion of said path placing said
movable stage below said path; and said permanent magnets having
sufficient magnetic attraction to said magnetic material to retain
said movable stage on said path, including said at least a portion.
Description
[0001] The present application is a continuation in part of U.S.
patent application Ser. No. 09/031,009 entitled "LINEAR MOTOR
HAVING AUTOMATIC ARMATURE WINDING SWITCHING AT MINIMUM CURRENT
POINTS" filed Feb. 26, 1998; U.S. patent application Ser. No.
09/031,287 entitled "ENCODER" filed Feb. 26, 1998; U.S. patent
application Ser. No. 09/040,132 entitled "MODULAR WIRELESS LINEAR
MOTOR" filed Mar. 17, 1998; and U.S. patent application Ser. No.
09/055,573 entitled "WIRELESS PERMANENT MAGNET LINEAR MOTOR WITH
MAGNETICALLY CONTROLLED ARMATURE SWITCHING AND MAGNETIC ENCODER"
filed Apr. 6, 1998.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a linear motor and, more
particularly, to a linear motor which is capable of following any
path, including a closed path, where continuous or discontinuous
motion in one direction is enabled.
[0003] Linear motors having stationary armatures containing coils
and movable stages containing magnets are well known in the art
Also known are linear motors having stationary magnets and moving
coils.
[0004] One type of such linear motors is disclosed in U.S. Pat. No.
4,749,921; The linear motor of the referenced disclosure has a
series of armature windings mounted to a base plate, and a stage
having a series of magnets that is free to move on the base plate.
The stage is urged in the desired direction by applying AC or DC
excitation to the coils. When such a linear motor is used in a
positioning system, the relationship between the location of the
stage and locations of the coils must be accounted for.
[0005] In one linear motor, commutator contacts are pendant from
the stage. The contacts contact one or more power rails, and one or
more coil contacts. As the stage moves along the armature, the
location of the stage, relative to the armature is automatically
accounted for by applying power to the stationary armature windings
through the commutator contacts.
[0006] In other linear motors, it is conventional to employ a
service loop of wires between the moving stage and the stationary
elements. The location of the stage is updated using a magnetic or
optical position encoder on the stage which senses markings on an
encoder tape stationary alongside the path of the stage. The
location is connected on the service loop to a stationary motor
controller.
[0007] Generally, the important location information is the phase
of the stage relative to the phase of the armature. For example, in
a three-phase armature, the windings are disposed in repeating sets
of three for phases A, B and C. If the center of the A phase
winding is arbitrarily defined as 0 degrees, then the centers of
the B and C windings are defined as 120 and 240. There may be two,
three or more sets of windings as required for the travel distance
of the stage. Normally, all A phase windings are connected in
parallel. The same is true of all B and C phase windings. Thus,
when the location of the stage requires a certain voltage
configuration on the particular windings within the influence of
the magnets on the stage, besides powering these windings, all of
the other windings in the armature are also powered. The maximum
force obtainable from a linear motor is limited by the allowable
temperature rise in the armature windings. When all windings are
powered, whether they contribute to motor force or not, more
armature heating occurs than is strictly necessary for performing
the motor functions.
[0008] Some linear motors in the prior art have responded to this
heating problem using switches that are closed only to the armature
windings actually within the influence of the magnets.
[0009] The need for a cable loop connecting moving and stationary
elements is inconvenient, and limits the flexibility with which a
system can be designed. The wiring harness requires additional
clearance from the linear motor to prevent entanglement between the
motor and any equipment or items that may be adjacent to the linear
motor path. In addition, the wiring harness adds additional weight
to the moving element of the linear motor. Furthermore,
manufacturing of a linear motor employing a wiring harness incurs
additional cost of material and assembly labor. Therefore, it would
be desirable to eliminate the use of a wiring harness in a linear
motor to decrease the cost of assembly, decrease the overall weight
of the moving element, and to eliminate the clearance restrictions
on the linear motors utility.
[0010] Most linear motors are manufactured to follow a straight
path and to be of a predetermined fixed length. This establishes
the length of the armature, and consequently the number of armature
windings. In such linear motors, all armature windings lie parallel
to each other, with axes thereof generally 90 degrees to the travel
direction of the linear motor. In order to make a new linear motor
of any specific length, a new assembly must be tooled. Each
assembly has a set number of armature windings, a set number of
moveable magnets, and, a fixed length wiring harness associated
with the moveable element of the linear motor. The cost of
producing a linear motor is increased because each assembly must be
custom designed to a users needs, with new tooling required for
each such design. Therefore, it is particularly desirable to
produce a linear motor of a modular design.
[0011] A modular designed motor would allow easy customization for
any desired length armature winding assembly. The cost of
manufacturing a particular linear motor would be decreased since
the cost of tooling would be minimal. A data base of assembly and
outline drawings will be common to all assemblies within a family
of linear motors, easing assembly and manufacturing. A stocking of
common parts would allow quick assembly of any special length motor
assembly, from now readily available parts. The stocking of common
parts also decreases overall cost of manufacturing since materials
will be bought in bulk from common suppliers. The assembly of any
desired length armature winding assembly will enjoy a decreased
lead time. As such, a modular designed linear motor provides for a
decrease in manufacturing cost, decrease in lead time to assemble,
and increases overall utility.
[0012] Linear motors using a series of stationary armature windings
and moving magnets require a means to dissipate heat from the
coils. Linear motors having cold plates mounted on one edge of an
armature winding are known in the art. Alternatively, armature
windings having cooling coils or channels are also well known in
the art. Examples of such armatures are disclosed in U.S. Pat. No.
4,839,545. These armatures use stacked laminated magnetic
material.
[0013] Linear motors having non-magnetic armatures are also known,
an example of which is disclosed in U.S. Pat. No. 4,749,921. The
linear motor of the referenced disclosure has a non-magnetic
armature which includes a coil support structure composed of an
aluminum frame or a serpentine cooling coil. In the embodiment
having an aluminum frame, heat is carried away from the coils of
the armature via the aluminum frame and a side plate which
functions as a heat sink. Alternatively, a serpentine coil may be
employed to effect more uniform cooling within the armature. The
serpentine coils support the overlapping coils while the coils and
the armature are cast in a block of settable resin. However, the
incorporation of such a coil has the disadvantage of increasing
costs because of the complexity of assembly and material expenses.
Furthermore, while the use of the settable resin prevents the
occurrence of eddy currents, the thermal conductivity of the
settable resin is significantly less than that of metals which it
replaces and thus reduces the power dissipation capacity of the
linear motor.
[0014] Linear motors are increasingly being employed in
manufacturing equipment. In such equipment, nominal increases in
the speed of operation translate into significant savings in the
cost of production. Therefore, it is particularly desirable to
produce as much force and acceleration as possible in a given
linear motor. An increase in force generated requires either an
increase in magnetic field intensity or an increase in current
applied to coils of the armature. In a permanent magnet linear
motor, the available magnetic field intensity is limited by the
field strength of available motor magnets. Power dissipated in the
coils increases at a rate equal the square of the current.
Attendant heat generation limits the force that may be achieved
without exceeding the maximum armature temperature. Therefore,
improvements in the power dissipation capacity of linear motors
provide for increases in their utility.
OBJECTS AND SUMMARY OF THE INVENTION
[0015] Accordingly, it is an object of the invention to provide a
linear motor which overcomes the drawbacks of the prior art.
[0016] It is a further object of the invention to provide a linear
motor that is capable of traversing any desired path, including a
closed path.
[0017] It is a still further object of the invention to provide a
wireless linear motor that eliminates the need for a wiring
harness, thereby permitting the linear motor to operate over a path
which is closed on itself.
[0018] Briefly stated the present invention provides a linear motor
path that forms a closed figure. Contactless power control and
position feedback permit a linear motor stage to traverse the
closed figure without interference from trailing wires. One
embodiment of the closed figure includes a racetrack pattern with
straight runs joined by curved ends. Axes of armature windings are
skewed to lie across the path in the curved ends to maintain force
on the linear motor stage. Other embodiments of the invention
includes multilevel paths wherein one portion of the path crosses
over another portion of the path. Disclosure is made of a linear
motor stage supported below the path by magnetic attraction between
permanent magnets on the stage and magnetic material in the
path.
[0019] According to an embodiment of the invention, there is
provided a linear motor comprising: a path, the path including a
plurality of armature windings therein, a plurality of switches,
each one of the switches controlling application of power to one of
the armature windings, a linear motor stage movable on the path, a
plurality of motor permanent magnets on the stage facing the path
for interaction with the armature windings, means for controlling
actuation only of those of the switches controlling application of
power to those of the plurality of armature windings within a
magnetic influence of the motor permanent magnets, the path
describing a closed path, and means for permitting the linear motor
stage to traverse the closed path.
[0020] According to a feature of the invention, there is provided a
path for a linear motor comprising: a plurality of armature
windings in the path, means for controlling application of power to
the plurality of armature windings, and the path including at least
first and second levels.
[0021] According to a further feature of the invention, there is
provided a linear motor comprising: a path, the path including a
magnetic material therein, a movable stage movable on the path, the
movable stage including permanent magnets thereon facing the
magnetic material, at least a portion of the path placing the
movable stage below the path, and the permanent magnets having
sufficient magnetic attraction to the magnetic material to retain
the movable stage on the path, including the at least a
portion.
[0022] The above, and other objects, features and advantages of the
present invention will become apparent from the following
description read in conjunction with the accompanying drawings, in
which like reference numerals designate the same elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1A is a simplified schematic diagram linear motor
system according to an embodiment of the invention.
[0024] FIG. 1B is a transverse cross section taken along II-II in
FIG. 1.
[0025] FIG. 2 is a cross section taken along A-A in FIG. 1B,
showing the switching magnet and switching sensors which control
application of drive power to armature windings.
[0026] FIG. 3 is a cross section taken along C-C in FIG. 1B,
showing the relationship between the switching magnet and motor
magnets.
[0027] FIG. 3A is a cross section taken along C-C in FIG. 1B,
showing, the positional relationship between the switching magnets
and the motor magnets.
[0028] FIG. 3B is a cross section taken along C-C as in FIG. 3A,
where the movable stage has moved to the right from its position in
FIG. 3A.
[0029] FIG. 4 is cross section taken along B-B in FIG. 1B showing
the relationship between magnetic zones in the encoder magnet and
the encoder sensors.
[0030] FIG. 4A shows a shape of a beveled magnetic zone about one
of the encoder sensors from FIG. 4.
[0031] FIG. 4B shows the relationship between the output of the
encoder, sensors located at the left and right ends of the encoder
magnets in FIG. 4, and the beveled magnet zone in FIG. 4A.
[0032] FIG. 4C shows another shape of a beveled magnetic zone about
one of the encoder sensors from FIG. 4.
[0033] FIG. 5 is a schematic diagram showing an embodiment of a
wireless linear motor employing active communications elements on
the movable stage.
[0034] FIG. 6 is a schematic diagram showing an embodiment a
wireless linear motor employing an active command-response position
feedback system.
[0035] FIG. 7 is a cross section similar to FIG. 1B, except that
provision is made in the path for controlling a second movable
stage along the same path.
[0036] FIG. 8 is a cross section similar to FIG. 1B, except that
provision is made in the path for controlling any desired number of
stages along the same path.
[0037] FIG. 9 is a cross section similar to FIG. 1B, except that
provision is made in the path for controlling two or more stages
along the same path.
[0038] FIG. 10 is a cross section similar to FIG. 1B, except that
provision is made in the path for controlling three or more stages
along the same path.
[0039] FIG. 11 is a schematic diagram of a wireless linear motor
employing an active command-response system with memory on-board
the movable stage.
[0040] FIG. 12 is a diagram showing a path adapted for open-loop
control of a movable stage over one section and closed-loop control
over another section.
[0041] FIG. 13 is a diagram showing several path modules connected
together to form a path.
[0042] FIG. 14 is a diagram showing a preferred embodiment of a
path module having three encoder sensor groups spaced along the
path of the module.
[0043] FIG. 15 is a diagram showing an embodiment of two path
modules coupled together, one module having a sensor, and another
module without a sensor.
[0044] FIG. 16 is a diagram showing an alternative embodiment of a
path module having a single sensor.
[0045] FIG. 17 is a diagram of a linear motor with a path in a
racetrack shape.
[0046] FIG. 18 is an enlarged view of a portion of a curved section
of the path of FIG. 17.
[0047] FIG. 19 is a diagram of a linear motor having path with
multiple levels and wherein one portion of the path crosses over or
under another portion of the path.
[0048] FIG. 20 is a diagram of a linear motor path consisting of
two connected spirals, including multiple crossovers.
[0049] FIG. 21 is a diagram of a linear motor path in the shape of
a Moebius band.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0050] Referring to FIG. 1A, there is shown, generally at 10, a
linear motor according to the invention. A movable stage 12 is
supported and guided in any convenient manner along a path 14. Path
14 includes therein repeating sets of three armature windings 16A,
16B and 16C for receiving, respectively, phases A, B and C of
three-phase drive power produced by a motor controller 18. Phase A
of the drive power from motor controller 18 is connected on a
phase-A conductor 20A to terminals of normally-open phase-A
switches 22A. Each phase-A switch is connected to its associated
phase-A armature winding 16A. Similarly, phase-B and phase-C drive
power are connected on phase-B and phase-C conductors 20B and 20C
to terminals of phase-B and phase-C switches 22B and 22C, all
respectively. Armature windings 16A, 16B and 16C of each set are
non-interleaved. That is, they lie side by side, not overlapping as
is the case in some prior art linear motors.
[0051] All switches 22A, 22B and 22C remain open, except the
switches associated with the particular armature windings 16A, 16B
and 16C that are within the influence of motor magnets on movable
stage 12. The closed switches 22A, 22B and 22C that are closed in
this manner are indicated as 22A', 22B' and 22C', thereby apply
power to corresponding armature windings 16A', 16B' and 16C'. As
moveable stage 12 moves along path 14, those of switches 22A, 22B
and 22C which newly come under the influence of the magnets on
movable stage 12 close, and those moving out of the influence of
the magnets are opened. Thus, at any time, only the armature
windings 16A', 16B' and 16C' which can contribute to generating a
force on movable stage 12 are powered. The remainder of armature
windings 16A, 16B and 16C, not being useful for contributing to the
generation of force, remain in a quiescent, unpowered, condition.
This contributes to a reduction in power consumption, and a
corresponding reduction in heating compared to prior-art devices in
which all armature windings are powered, regardless of whether they
are position to contribute to force.
[0052] In an application where "open-loop" drive of movable stage
12 is satisfactory, motor controller 18 produces the required
sequence of phases to drive stage 12 in the desired direction.
However, one desirable application is a "closed-loop" drive system
in which motor controller 18 receives feedback information from
movable stage 12 indicating either its position along path 14, or
increments of motion along path 14. A closed-loop system permits
accurate control of position, velocity and acceleration of movable
stage 12.
[0053] The prior art satisfies the requirement for position
feedback using wiring between movable stage 12 and motor controller
18. This is inconvenient in some applications, and impractical in
others. Impractical applications including travel of movable stage
12 along a path 14 which is closed upon itself. An example of such
a path is an oval or "race-track" pattern of value in a robotic
assembly operation, to be described in greater detail later in this
specification. That is, movable stage 12 continues in a forward
direction repeatedly traveling in the same direction on path 14.
Wiring between the movable and stationary elements for such an
application is either difficult or impossible to accomplish.
[0054] The embodiment of the invention in FIG. 1A includes a
communications device 24 which wirelessly informs motor controller
18 about the position and/or incremental motion of movable stage
12. Communications device 24 is preferably a linear encoder which
does not require connecting cables between stationary and movable
elements, as will be explained.
[0055] In the preferred embodiment, at least some of the position
or motion information is developed at stationary locations off
movable stage 12, without requiring the transmission of position
information.
[0056] It can be seen from the simplified drawing of FIG. 1A, and
the description above, that linear motor 10 requires the following
actions:
[0057] 1) control of switches 22A, 22B, 22C
[0058] 2) feedback of position or motion data
[0059] 3) drive power generation related to position
[0060] (or motion-derived position).
[0061] Referring to FIG. 1B, a cross section through path 14,
looking at the end of movable stage 12 reveals a plurality of motor
magnets 160, 162 below a plate 26. Lower surfaces of motor magnets
160, 162 are maintained closely parallel to an upper surface of
armature windings 16A, 16B and 16C. Although it does not form a
part of the present invention, armature windings 16A, B, C, may be
wound on stacked laminations of magnetic metal. In this case, the
lower surface of motor magnets 160, 162 are maintained closely
parallel to an upper surface of the stacked laminations. Some
applications may benefit from the reduction in static load on
movable stage 12 provided when armature windings 16A, 16B and 16C
contain no magnetic material. For purposes of later description,
motor magnets 160, 162 are referred to as motor magnets. Armature
windings 16A, B and C are energized as necessary to interact with
motor magnets 160, 162 whereby a translational force is generated
on movable stage 12.
[0062] A pendant arm 28 extends downward from plate 26. Pendant arm
28 has attached thereto a switching magnet 30 and an encoder magnet
32, both movable with movable stage 12. A rail 34, affixed to path
14, rises generally parallel to pendant arm 28. Rail 34 has affixed
thereto a plurality of longitudinally spaced-apart switching
sensors 36 facing switching magnet 30, and a plurality of
longitudinally spaced-apart encoder sensors 38 facing encoder
magnet 32.
[0063] Referring now to FIG. 2, switching sensors 36 are evenly
spaced along rail 34. Each switching sensor 36 is preferably
positioned on rail 34 aligned with its respective armature winding
16. In the embodiment shown, switching sensors 36 are Hall-effect
devices. Switching magnet 30 has a length in the direction of
travel roughly equal to the length of travel influenced by the
magnetic fields of motor magnets 160, 162. This length is variable
in dependence on the number of motor magnets used. In the
illustrated embodiment, the length of switching magnet 30 is
sufficient to influence nine switching sensors 36. That is, nine
armature windings 16 (three sets of phases A, B and C) are
connected at any time to their respective power conductors 20 for
magnetic interaction with motor magnets 160, 162.
[0064] Switching sensors 36 control the open and closed condition
of respective switches, as previously explained. Any convenient
type of switch may be used. In the preferred embodiment, the
switches are conventional semiconductor switches such as
thyristors. Since semiconductor switches, and the technique for
controlling their open/closed condition are well known to those
skilled in the art, a detailed description thereof is omitted.
[0065] Referring now to FIG. 3, the underside of plate 26 includes
nine motor magnets 160 equally spaced therealong. In addition, an
additional motor magnet 162 is disposed at each end of the array of
nine motor magnets 160. Motor magnets 160, 162 are tilted as shown
in a conventional fashion to reduce cogging. It will be noted that
the length of switching magnet 30 is approximately equal to the
center-to-center spacing of the end ones of the set of nine full
motor magnets 160. This length of switching magnet 30 defines the
span S of the active portion of linear motor 10. That is, only
those of armature windings 16 that lie within the span S receive
power. As armature windings 16 enter the span S, they receive
power, as they exit the span S, power is cut off.
[0066] Additional motor magnets 162, being outside the span, do not
contribute to the generation of force because armature windings 16
below them are unpowered. However, additional motor magnets 162
perform an important function. It is important to the function of
linear motor 10 that the magnetic field strength along plate 26 be
generally sinusoidal. In the absence of additional motor magnets
162, the magnetic fields produced by the two motor magnets 160 at
the ends of span S depart substantially from sinusoidal due to
fringing effects. This produces ripple in the force output. The
presence of additional motor magnets 162, by maintaining
substantially sinusoidal magnetic field variations along motor
magnets 160, avoids this source of ripple.
[0067] Additional motor magnets 162 are shown with widths that are
less than that of motor magnets 160. It has been found that a
narrower width in additional motor magnets 162 produces
satisfactory results. However, it has also been found that a wider
additional motor magnet 162 does not interfere with the function.
From the standpoint of manufacturing economy, it may be desirable
to employ only a single size magnet for both motor magnets 160 and
additional motor magnets 162, thereby reducing stocking costs, and
assembly costs.
[0068] Referring now to FIG. 3A, the positional relationships of
switching magnet 30 and motor magnets 160, 162 are shown, using a
reduced set of 5 motor magnets interacting with 4 armature
windings, for purposes of explanation. As movable stage 12 moves,
switching magnet and motor magnets 160, 162 move together with it,
maintaining the same relative positions. As movable stage 12 moves
along, those switching sensors 36 adjacent switching magnet 30 turn
on their respective switches. Switching sensors 36 that are not
adjacent switching magnet 30 maintain their respective switches
turned off. In the condition shown, switching sensors 36 centered
on armature windings 16-2, 16-3, and 164 are adjacent switching
magnet 30, and these armature windings are connected to drive
power. The switching sensors 36 centered on armature windings 16-1.
16-5 and 16-6 are not adjacent switching magnet 30, and therefore,
these switching sensors 36 maintain armature windings 16-1, 16-5
and 16-6 cut off from drive power. The centers of all motor magnets
160 shown are offset from the centers of the armature windings 16
most closely adjacent. Therefore all turned-on armature windings 16
produce force by the interaction of their magnetic fields with the
magnetic fields of the three nearest motor magnets 160.
[0069] Referring now to FIG. 3B, movable stage 12 has moved to the
right from its position in FIG. 3A until the center of the
right-hand motor magnet 160 is centered over the center of armature
winding 16-5. In this relationship, the end of switching magnet 30
just reaches a position adjacent switching sensor 36. This is a
minimum-current position. Thus, at this instant, switching sensor
36 closes its switch to connect armature winding 16-S to its power
source. In this center-overlapped condition, armature winding 16-5
is incapable of generating a force. Thus, the current in armature
winding 16-5 is at a minimum, and the switching takes place at
minimum current to armature winding 16-5. Similarly, at about this
same instant, the left-hand end of switching magnet 30 passes off
the switching sensor 36 aligned with armature winding 16-2, thereby
cutting off power to armature winding 16-2. The center of left-hand
motor magnet 160 is aligned with the center of armature winding
16-2 at this time. Thus, the current to armature winding 16-2 is
minimum at this time. The above switching at minimum current
reduces electrical switching noise which would be generated if
switching were to take place at times when an energized armature
winding 16 is generating force, or a deenergized armature winding
16 would generate a force immediately upon energization.
[0070] For a three-phase drive system, a minimum of five motor
magnets is required to interact at any time with a minimum of four
armature windings, or vice versa. If additional force is desired,
magnets can be added in increments of four. That is, the number of
magnets=5+4L where L is an integer, including zero. The number of
armature windings in span S=(number of motor magnets in span S)-1.
The embodiment in FIGS. 2 and 3 employ 5+(4.times.1)=9 magnets. The
positioning of the magnets is such that the center-to-center
spacing of the extreme ends of the 9 magnets is equal to the
center-to-center spacing of 8 armature windings.
[0071] Referring now to FIG. 4, encoder magnet 32 includes
alternating magnetic zones alternating with north and south
polarities facing encoder sensors 38. Accordingly, each encoder
sensor 38 is exposed to alternating positive and negative magnetic
fields as encoder magnet 32 passes it. The zones at the extreme
ends of encoder magnet 32 are beveled magnetic zones 42. Beveled
magnetic zones 42 produce an increasing or decreasing magnetic
field as it moves onto or off an encoder sensor 38. Beveled
magnetic zones 42 are illustrated as linear ramps. Motors using
such linear ramps have been built and tested successfully. However,
a shape other than a linear ramp may give improved results. It is
known that the magnetic field of a motor magnet decreases as the
square of the distance from the magnet. Thus, to have an increase
in magnetic field at one beveled zone that is substantially equal
to the decrease in the magnetic field at the opposite magnetic
zone, the bevel shape may be described by a squared law.
[0072] Referring momentarily to FIG. 4A, a shape of beveled
magnetic zone which satisfies the rule that, for equal increments
of motion of beveled magnetic zone 42', there are equal changes in
magnetic field at encoder sensor 3 8 is represented by the
equation:
y=a+bx.sup.2
[0073] where:
[0074] y is the distance from the surface of the magnet to encoder
sensor 38,
[0075] x is the position along beveled magnetic zone 42', and
[0076] a and b are constants.
[0077] Experience dictates that other factors besides the square
law above affects the relationship between magnetic field and
distance. The shape of beveled magnetic zones 42' may require
modification from the square law to account for such other
factors.
[0078] Referring now to FIG. 4B, when the ideal shape of beveled
magnetic zones 42' is attained, the outputs of the encoder sensors
at the left and right ends of encoder magnet 32 should approximate
the figure. That is, the sum of the signal from the left beveled
magnetic zone 42', and the signal from the right beveled magnetic
zone 42' should remain about constant.
[0079] Returning now to FIG. 4, each encoder sensor 38 is
preferably a Hall-effect device. A Hall-effect device produces a
current when exposed to one magnetic polarity (north or south) but
is insensitive to the opposite magnetic polarity. Encoder sensors
38 are disposed into encoder sensor groups 40 consisting of four
encoder sensors 38 spaced in the direction of travel. Each encoder
sensor group 40 is spaced from its neighboring encoder sensor group
by a distance D. Distance D is seen to be equal to the
center-to-center distance between the beveled magnetic zones 42 at
the ends of encoder magnet 32. The four encoder sensors 38 in each
encoder sensor group 40 are spaced in the direction of travel of
movable stage 12 in relation to the center-to-center distance
between magnetic zones in encoder magnet 32. For purposes of
description, the center-to-center distance between magnetic zones
of like polarity is considered to be 360.degree.. Thus, the
center-to-center distance between adjacent magnetic zones is
considered to be 180.degree., and the distance between the center
of a zone and its edge is considered to be 90.degree..
[0080] It is conventional for encoders to produce a sine and a
cosine signal, relatively 90.degree. out of phase, for use in
detecting the direction of incremental motion of a stage. With
magnetically actuated Hall-effect devices, this conventional
technique presents a problem in that a Hall effect device responds
only to one magnetic polarity (north or south) and is insensitive
to the opposite polarity. To solve this problem, each encoder
sensor group 40 includes one encoder sensor 38s+ for producing a
sine+ output, and a second encoder sensor 38s- for producing a
sine- output Encoder sensor 38s- in encoder sensor group 40 is
spaced 180.degree. in the direction of travel from its companion
encoder sensor 38s+. When the sine+ and sine- signals are added in
motor controller 18, the desired sinusoidal sine signal is
available. A cosine+ encoder sensor 38c+ is spaced 90.degree. in
the direction of travel from sine+ encoder sensor 38s+. A cosine-
encoder sensor 38c- is spaced 180.degree. in the direction of
travel from its companion cosine+ encoder sensor 38c+. When the
cosine+ and cosine- signals are added in motor controller 18, the
desired cosine signal is generated.
[0081] The spacing D between encoder sensor groups 40 is such that,
as a particular encoder sensor 38 in one encoder sensor group 40 is
aligned with beveled magnetic zone 42 at one end of encoder magnet
32, its counterpart is aligned with beveled magnetic zone 42 at the
opposite end of encoder magnet 32. As illustrated, for example,
when sine+ encoder sensor 38s+ in the left-hand encoder sensor
group 40 is aligned with the center of the left-hand beveled
magnetic zone 42, its counterpart sine+ encoder sensor 38s+ is
aligned with the right-hand beveled magnetic zone 42 at right end
of encoder magnet 32.
[0082] All corresponding encoder sensors 38 are connected in
parallel to a line connected to motor controller 18. Four separate
lines are illustrated to carry the +/- sine/cosine signals. As
movable stage 12 moves along, the encoder sensor 38 coming into
alignment with beveled magnetic zone 42 at one end of encoder
magnet 32 produces an increasing signal while the encoder sensor 38
moving out of alignment with beveled magnetic zone 42 at that end
produces a decreasing signal. Since all corresponding encoder
sensor signals are added, the signal transition, as one encoder
sensor group 40 becomes active, and its neighbor encoder sensor
group 40 becomes inactive is smooth, without a discontinuity that
would interfere with detecting motion. One skilled in the art will
understand that the above spacing can be increased by 360.degree.
between any +/- pair of encoder sensors 38 without affecting the
resulting output signal. Also, in some applications, since the
outputs of sine encoder sensors are, in theory, 180.degree. out of
phase with each other, both sine encoder outputs could be applied
to a single conductor for connection to motor controller 18. In
other applications, four separate conductors, as illustrated, may
be desired.
[0083] In a preferred embodiment of linear motor 10, a third
encoder sensor group 40 (not shown) is disposed midway between the
illustrated encoder sensor groups 40. This has the advantage that,
during the transition of beveled magnetic zones 42 at the ends of
encoder magnet 32 from one encoder sensor group 40 to the next
encoder sensor group 40, resulting departures of the encoder signal
due to tolerances in the lengths of encoder magnet 32, and the
precise spacing of encoder sensor groups 40 is at least partially
swamped out by the signal generated by an encoder sensor group 40
located midway between the ends of encoder magnet 32.
[0084] Referring again to FIG. 1A, it will be recognized that the
functions of communications device 24 are satisfied by the
above-described wireless magnetic system for communicating the
motion of movable stage 12 to motor controller, without requiring
any active devices on movable stage 12. One limitation on such a
system is the difficulty in producing closely spaced alternating
magnetic zones in encoder magnet 32. Thus, the positional
resolution of such a system is relatively crude.
[0085] Referring now to FIG. 5, one solution to the resolution
problem includes a conventional encoder tape 44 in a fixed location
along path 14, and a conventional optical encoder sensor 46 on
movable stage 12. Encoder tape 44 is ruled with fine parallel
lines. Optical encoder sensor 46 focuses one or more spots of light
on encoder tape 44, and detects the changes in light reflected
therefrom as lines and non-lines pass in front of it. Generally,
optical encoder sensor 46 produces sine and cosine signals for
determining motion. Since the parallel lines on encoder tape 44 are
closely spaced, very fine resolution is possible. An optical
encoder system can be added to the less precise magnetic encoder
system in order to obtain enhanced position resolution.
[0086] The sine and cosine outputs of optical encoder sensor 46 are
applied to a pulse generator 48. The output of pulse generator 48
is applied to a transmitter 52. Transmitter 52 transmits the pulse
data to a data receiver 54. Although the system is shown with
antennas, implying that transmission and reception use radio
frequency, in fact, any wireless transmission system may be used.
This includes radio, optical (preferably infrared), and any other
technique capable of transmitting the information, without
requiring connecting wires, from movable stage 12 to stationary
motor controller 18.
[0087] The embodiment of the invention of FIG. 5 has the
disadvantage that transmitter 52 is active at all times. Since the
system is wireless, the illustrated apparatus on movable stage 12
is battery operated. Full-time operation of transmitter 52 reduces
battery life.
[0088] Referring now to FIG. 6, an embodiment of the invention adds
to the embodiment of FIG. 5, a command transmitter 56 in motor
controller 18, a receiver 58 and a counter 50 in movable stage 12.
In this embodiment, transmitter 52 remains off until commanded
through receiver 58 to transmit the count stored in counter 50. The
command to transmit is sent from command transmitter 56 to receiver
58. Although this embodiment requires that receiver 58 remain
active at all times, the power drain of a solid state receiver is
generally lower than that of a transmitter. As in prior
embodiments, any wireless technology may be used in receiver 58 and
command transmitter 56.
[0089] In one embodiment of the invention, the magnetic encoder
system may be omitted, and the entire encoder operation may be
accomplished using optical encoder sensor 46 facing optical encoder
tape 44, and transmitting the position or motion data from the
stage using electromagnetic means, such as described above.
[0090] Referring now to FIG. 7, an embodiment of the invention is
shown in which it is possible to drive more than one movable stage
12 along path 14. Each movable stage 12 requires independent
application of armature power from motor controller 18, independent
armature switching and independent position communication from the
movable stage back to motor controller 18. The embodiment in FIG. 7
continues to show movable stage 12, but adds a second rail 34' on
the second side of path 14 for use by a second movable stage (not
shown). The second movable stage is similar to movable stage 12,
except that a pendant arm 28' (not shown), supporting switching and
encoder magnets (not shown), if in a visible position, would be
located on the left side of the drawing. Second rail 34' includes
encoder sensors 38' and switching sensors 36', corresponding to the
encoder and switching sensors of the embodiment of FIG. 1B.
Conductors 20'A, B and C carry motor drive power, separately
generated in motor controller 18, to the switches feeding power to
the armature windings 16A, B and C, along paths separate from
conductors 20A, B and C. In this manner, the second stage is
separately controlled, and its motion is separately fed back to
motor controller 18.
[0091] Referring now to FIG. 8, there is shown an embodiment of the
invention adapted to controlling and driving two movable stages 12
(and 12', not shown). In this embodiment, rail 34', besides
supporting encoder sensor 38 and switching sensor 36, also
supports, spaced below, a second encoder sensor 38' and a second
switching sensor 36'. It will be understood power to armature
windings 16A, B and C is independently controlled by separate
switches that feed motor power from conductors 20A, B and C, when
influenced by switching magnet 30, and from conductors 20'A, B and
C when influenced by switching magnet 30'.
[0092] Referring to FIG. 9, a second movable stage 12' is shown,
for use with rail 34' of FIG. 8. Second movable stage 12' includes
a pendant arm 28', on the same side of movable stage 12 of FIG. 8,
but extending further downward to accommodate an encoder magnet 32'
and switching magnet 30' aligned with second encoder sensors 38'
and second switching sensors 36', respectively. It would be clear
to one skilled in the art that more than two movable stages could
be controlled by adding additional elements to rail 34', and by
installing suitably long pendant arms 28, 28' . . . 28.sup.n to
each movable stage 12.
[0093] The present invention is not limited to two movable stages
on a single path. Any number of movable stages may be controlled
independently along the same path 14. Referring to FIG. 10, for
example, three rails 34, 34' and 34" are spaced parallel to each
other outward from path 14. Each of rails 34, 34' and 34" includes
thereon encoder sensors 38, 38' and 38", and switching sensors 36,
36' and 36". Each movable stage 12, 12' and 12" (only movable stage
12 is shown) includes a pendant arm 28, 28' and 28" (only pendant
arm 28 is shown) adjacent to the sensors on its respective rail 34,
etc. Encoder magnets 32, 32' and 32" (only encoder magnet 32 is
shown), and switching magnets 30, 30' and 30" (only switching
magnet 30 is shown) are installed on their respective pendant arms.
With the interleaving of pendant arms 28, etc. between rails 34,
etc., as many stages 12, etc. as desired may be accommodated,
driven and controlled on a single path 14.
[0094] In some applications, it may be desirable to have
closed-loop control in some regions of the path for precise
positioning, but where open-loop control may be desirable over
other regions of the path. Referring to FIG. 12, a region of
closed-loop control 60, along path 14 receives drive power from
motor controller 18 on a first set of conductors 20A, B, and C
through magnetically actuated switches 22A, B and C, as previously
described. Position or motion feedback in region 60, as previously
described, permits motor controller 18 to accurately control the
position and velocity of movable stage 12. A region of open-loop
control 62, along path 14 receives drive power from motor
controller 18 on a second set of conductors 20'A, B and C. When
movable stage 12 is in region 62, motion feedback is either not
generated, or is not responded to by motor controller 18. Instead,
motor controller 18 generates a programmed phase sequence for
open-loop control of movable stage 12. This drives movable stage at
a predetermined speed. Once a region of closed-loop control is
attained, movable stage 12 resumes operation under control of motor
controller 18.
[0095] It is also possible to provide path switching, similar to
the switching used on railroads, to direct movable stage 12
flexibly along different paths.
[0096] Referring now to FIG. 11, an embodiment, similar to that of
FIG. 6,. adds a memory 64 for receiving commanded motion
information. Once commanded motion information is stored, it is
continuously compared with the content of counter 50 until a
commanded condition is attained. During the interval between
storage of the information, and the accomplishment of the commanded
condition, transmitter 52 may remain quiescent. In some
applications, receiver 58 may also remain quiescent during such
interval, thereby consuming a minimum amount of battery power.
[0097] Referring now to FIG. 13, the power consumption of the
above-described system is independent of the length of path 14,
since only active armature windings 16 are energized. Consequently,
it is convenient to be able to construct a path 14 of any length by
simply adding path modules 66 end to end. Each path module 66
includes at least one armature winding 16, an associated portion of
rail 34 and conductors 20A, B and C. Conductors 20A, B and C on
adjacent path modules are connected together by connectors 68. Path
modules 66 are illustrated to contain three armature windings 16A,
B and C. It will be understood that switching sensors, together
with their semiconductor switches, for the contained armature
windings are mounted on the portion of rail 34 associated with that
path module 66. In addition, position feedback, if magnetic encoder
sensing is used, is also included on suitable path modules 66. As
noted above, encoder sensors are spaced relatively widely apart. In
a preferred embodiment, each path module should be long enough to
contain at least one encoder sensor group. One system of this sort
has been long enough to contain 9 armature windings (3 repetitions
of phases A, B and C armatures).
[0098] Referring now to FIG. 14, a preferred embodiment of a path
module 70 includes armature windings, as described above, plus
three encoder sensor groups 40 spaced D/2 apart (D is the
center-to-center spacing of beveled magnetic zones 42 at the ends
of encoder magnet 32). Path module 70 extends a distance D/4 beyond
the outer encoder sensor groups 40. In this way, when the next path
module 70 is connected end to end, the distance between the nearest
encoder sensor groups 40 on the mated path modules 70 is D/2 as is
desired. Path modules 70 are connected together to form a path 14'
of any desired length or shape.
[0099] Referring now to FIG. 15, another preferred embodiment
includes two path modules 72, 74 having armature windings, as
described above. One module has an encoder sensor group 40, and
another module does not contain an encoder sensor. Path modules 72,
74 are connected together to form a path 14" such that encoder
sensor groups 40 in path modules 72 are spaced D/2 apart (D is the
center-to-center spacing of beveled magnetic zones 42 at the ends
of encoder magnet 32). Any desired path 14" can be achieved using a
combination of path modules 72 and 74. It is understood by one
skilled in the art that other arrangements of path modules 72, 74
can be used to form any desired shape or length path 14" and any
other desired spacing of encoder sensor groups 40, so long as
provision is made for spacing encoder sensor groups 40 a desired
repeating distance apart. One embodiment includes a modular path
module from which encoder sensor groups are omitted. However,
provision is made for clamping, or otherwise affixing, encoder
sensor groups 40 anywhere along the assembled modular path. When
affixing the encoder sensor groups 40, the appropriate spacing (D,
D/2, D/4, etc.) is observed to ensure that the encoding signal is
produced without distortion or dropouts during transitions from one
path module to another.
[0100] Referring now to FIG. 16, an alternative embodiment of a
path module 76 includes armature windings, as described above, and
an encoder sensor group 40. Modules 76 are connected together to
form a path 14'" such that encoder sensor groups 40 in path modules
76 are spaced D/2 apart (D is the center-to-center spacing of
beveled magnetic zones 42 at the ends of encoder magnet 32). Any
desired length or shape path 14'" can be achieved using a
combination of path modules 76.
[0101] The connection of signals and power along linear motor 10,
especially in the case of modular devices, has been described with
wires and connectors joining wires in adjacent modules. Other
techniques for carrying signals and power may be employed without
departing from the spirit and scope of the invention. For example,
instead of using wires, conductive traces on a rigid or flexible
substrate may be used.
[0102] It will be noted that path 14 is shown as containing curves.
It is a feature of the present invention that path 14 is not
restricted to a straight line, as is frequently the case with the
prior art. Instead, due to the nature of the present invention,
linear motor 10 can be arranged to follow any desired path,
including a straight path, curved path 14 as shown, or a closed
path wherein movable stage 12 can repeatedly trace a closed path,
moving in a single direction, or moving back and forth to desired
locations anywhere along the open or closed path.
[0103] Referring now to FIG. 17, a linear motor 10' includes a path
14' which is closed on itself in a racetrack pattern. That is, path
14' includes straight and parallel runs 78 joined by curved ends
80. Movable stage 12 is driven, as described to any point on path
14'. In the preferred embodiment, movable stage 12 may continue in
one direction indefinitely, or may move in one direction, then in
the other, without limitation. This freedom of movement is enabled
by the wireless control and feedback described herein.
[0104] Dashed box 82 in FIG. 17 is expanded in FIG. 18 to enable
description.
[0105] All armature windings 16A, 16B and 16C include an axis 84,
illustrated by a line in each armature winding. All axes 84 in runs
78 lie substantially parallel to each other, as shown in armature
windings 16A and 16B at the lower left of the figure. Axes 84 in
curved ends 80, however, do not lie parallel to each other.
Instead, axes 84 in curved ends 80 are tilted with respect to each
other so that they lie across the shortest transverse distance of
path 14'. In this way, repeating sets of armature windings 16A, 16B
and 16C at enabled to generate the desired force for urging movable
stage 12 along path 14'.
[0106] One skilled in the art will recognize that accommodation
must be made in the actuation times of switches 22A, 22B and 22C
for the tilting of axes 84 in curved ends 80. One possibility
includes adjusting an upstream-downstream dimension of armature
windings 16A, 16B and 16C so that center-to-center dimensions
between end ones of each set of four such windings in curved ends
80 remains the same as the center-to-center dimensions between
corresponding windings in runs 78. In this manner, the span S of
four armature windings 16 remains the same in curved ends 80 as the
span S of 5+(n.times.4) motor magnets 160 (n=0, 1, 2, . . . ) in
straight runs 78. Switching sensors 36 are located along curved
ends 80 so that their respective switches are actuated at
minimum-current times, as previously explained.
[0107] A racetrack shape, as in FIGS. 17 and 18 do not exhaust the
possible shapes of path that can be attained with the present
invention. Any shape can be accommodated.
[0108] Referring now to FIG. 19, a multilevel path 86 is equally
within the contemplation of the present invention. A lower portion
88 of path 86 passes under an upper portion 90, thereof. Movable
stage 12 may be positioned anywhere on path 86. In cases where two
or more movable stages 12 are employed on path 86, the possibility
exists that one movable stage 12 may cross on upper portion 90 at
the same time that a second movable stage 12 on lower portion 88
passes under upper portion 90.
[0109] Referring now to FIG. 20, a further illustration of a
multilevel path 86' includes a down spiral 92 aside a down and up
spiral 94. Spirals 92 and 94 are connected into a single path 86'
by crossing elements 96 and 98. Spiral paths are frequently seen in
conveyor systems to increase the residence time of objects in a
location. For example, in a bakery operation, spirals are
frequently used to permit time for newly baked goods to cool,
before being discharged to packaging or further processing.
[0110] To illustrate the complete flexibility of the present
invention, a path may be laid out as a Moebius band 100, as shown
in FIG. 21. A Moebius band is characterized as having only a single
edge and a single surface, rather than having two edges and two
surfaces, as in other examples of paths in the above description. A
toy Moebius band is constructed by making a half twist in a strip
of paper and then connecting the ends together. One proves that the
strip has only a single surface by drawing a line down the center
of the strip. Eventually, the end of the line meets the beginning
of the line without having turned the strip over. Similarly, one
can draw a line along the edge of the strip, and find the end of
the line joining the beginning of the line, without crossing over
from one edge to the other, since the strip has only a single
edge.
[0111] The views of paths in the foregoing must not be considered
to be top views. Indeed, important applications of the invention
include those in which movable stage 12 is located below its path.
Especially in the case where the path includes magnetic material,
motor magnets 160, and additional magnets 162 in movable stage 12
may be relied on to support movable stage by magnetic attraction to
the magnetic material in the path. Other types of support are
equally within the contemplation of the invention. In some cases,
some portions of the path may be below and supporting movable stage
12, and other portions of the path may be above movable stage 12,
as movable stage completes a full traverse of the path.
[0112] Having described preferred embodiments of the invention with
reference to the accompanying drawings, it is to be understood that
the invention is not limited to those precise embodiments, and that
various changes and modifications may be effected therein by one
skilled in the art without departing from the scope or spirit of
the invention as defined in the appended claims.
* * * * *