The present invention relates generally to a linear motor and, more
particularly, to a linear motor which has stationary armature windings and a movable
motor stage carrying magnets, preferably permanent magnets.
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.
One type of such linear motors is disclosed in U.S. patent 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.
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..
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.
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.
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.
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
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.
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.
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. Patent 4,839,545. These armatures
use stacked laminated magnetic material.
Linear motors having non-magnetic armatures are also known, an example
of which is disclosed in US Patent 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.
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
Certain tasks require that operations take place at more than one
location. Sometimes, materials or workpieces must be delivered simultaneously at
two or more locations. This is difficult or impossible to accomplish with a linear
motor which is capable of moving a single movable stage on its path. Especially
in the case of paths which are closed on themselves, the problem of interference
between service loops connecting two or more movable stages to stationary elements
is not easily solved.
According to one aspect of the present invention, there is provided
a linear motor comprising:
- a plurality of armature windings defining a path;
- said plurality of armature windings being non-interlaced;
- a stage movable on said path;
- said stage including n permanent magnets facing said plurality of armature
- said plurality of permanent magnets having alternating magnetic polarities in
faces thereof facing said plurality of armature windings;
- said n permanent magnets occupying a span S;
- m of said plurality of armature windings occupying said span S;
- m ≠ n;
- each of said plurality of armature windings including a switch for connecting
drive power thereto;
- means for switching drive power to those of said plurality of armature windings
facing said n permanent magnets; and
- automatic means for performing said switching only when an end one of said n
magnets in said span S is substantially centered on one of said armature windings,
whereby switching is performed at minimum current, with the generation of minimum
Preferably, the invention provides a wireless linear motor having
the ability to chain together a series of modular coil assemblies to accommodate
any desired length motor which is simple and cost effective to produce.
In a preferred embodiment of the invention a wireless linear motor
is provided which does not require the use of a wiring harness. This decreases
the cost of assembly, decreases the weight of such assembly, and decreases the
required clearance surrounding the motor.
In another embodiment of the invention a wireless linear motor has
Briefly stated, the present invention provides a linear motor having
permanent magnets on a movable stage which also includes non-interleaved armature
windings on a path. The magnets are arranged m along the span A of the movable
stage. The armature windings are arranged with n windings occupying a distance
equal to the span S. A switching device automatically switches power on to the
armature windings entering the span, and switches power off to the windings leaving
the span. The switching is done when the a magnet is aligned with the coil whose
power is being switched on or off in order to minimizes switching noise. Switching
is conveniently done using Hall effect devices controlling the switches, and a
switching magnet, movable with the stage, having a length equal to the span. As
the switching magnet comes within magnetic interaction distance of a Hall effect
device, the Hall effect device closes its associated switch. As the switching magnet
moves out of magnetic interaction distance with the Hall effect device, the Hall
effect device closes its associated switch. The array of magnets includes one additional
magnet outside each end of the span to maintain a sinusoidal magnetic field strength
across the entire span.
In another aspect of the invention, a linear motor comprises: a plurality
of armature windings defining a path, the plurality of armature windings being
non-interlaced, a stage movable on the path, the stage including n permanent magnets
facing the plurality of armature windings, the plurality of permanent magnets having
alternating magnetic polarities in faces thereof facing the plurality of armature
windings, the n permanent magnets occupying a span S, m of the plurality of armature
windings occupying the span S, m ≠ n, each of the plurality of armature windings
including a switch for connecting drive power thereto, means for switching drive
power to those of the plurality of armature windings facing the n permanent magnets,
and automatic means for performing the switching only when an end one of the n
magnets in the span S is substantially centered on one of the armature windings,
whereby switching is performed at minimum current, with the generation of minimum
In another aspect of the invention, there is provided a permanent
magnet array for a linear motor comprising: a plurality of permanent magnets disposed
along a span S, the span S corresponding to all of the permanent magnets effective
for producing a force in the linear motor, the plurality of permanent magnets having
alternating polarities, a first additional permanent magnet in the array outside
a first end of the span S, the first additional permanent magnet having a magnetic
polarity opposite a magnetic polarity of a first adjacent permanent magnet in the
span S, a second additional permanent magnet in the array outside a second end
of the span S, the second additional permanent magnet having a magnetic polarity
opposite a magnetic polarity of a first adjacent permanent magnet in the span S,
and the first and second additional permanent magnets maintaining substantially
sinusoidal magnetic field strengths in the span S.
The present invention also comprehends a linear motor which includes
at least two movable motor stages movable on the same path.
In another aspect of the invention a linear motor has means for independent
application of drive power to at least two movable stages, and provision for position
or motion feedback from each of the at least two movable stages to a motor controller.
Briefly stated, the present invention provides a motor controller
which produces independent first and second drive power signals for independent
application to first and second linear motor stages movable on the same path. Each
armature winding in the path is connected to a first switch receiving the first
drive power and to a second switch receiving the second drive power. A first switch
control device associated with the first linear motor stage actuates the first
switches in its vicinity, while leaving the remaining first switches unactuated.
A second switch control device associated with the second linear motor stage actuates
the second switches in its vicinity, while leaving the remaining second switches
unactuated. Each linear motor stage has an encoder associated with it to inform
the motor controller of its position and/or motion.
In another aspect of the invention, there is provided a linear motor
comprising: a motor controller, a path, the path including a plurality of armature
windings therein, at least first and second linear motor stages independently movable
on the path, a plurality of motor permanent magnets on each of the first and second
stages facing the path for interaction with the armature windings, a first encoder
associated with the first linear motor stage, the first encoder including wireless
means for communication at least one of motion and position of the first linear
motor stage to a motor controller, a second encoder associated with the second
linear motor stage, the second encoder including wireless means for communication
of at least one of motion and position of the second linear motor stage to the
motor controller, the path including first means for applying first drive power
from the motor controller to those of the plurality of armature windings within
a magnetic influence of the motor permanent magnets on the first stage, the path
including second means for applying second drive power from the motor controller
to those of the plurality of armature windings within a magnetic influence of the
motor permanent magnets on the second stage, and the first and second means being
simultaneously operable, whereby simultaneous independent drive of the first and
second linear motor stages is enabled.
In another aspect of the invention, there is provided a linear motor
comprising: a motor controller, the motor controller producing first and second
motor drive power, a path, the path including a plurality of armature windings
therein, a plurality of first switches, each of the plurality of first switches
controlling the application of the first motor drive power to a respective one
of the plurality of armature windings: a plurality of second switches, each of
the plurality of second switches controlling the application of the second motor
drive power to a respective one of the plurality of armature windings, first and
second linear motor stages movable on the path, the first and second linear motor
stages each including a plurality of permanent magnets thereon facing the plurality
of armature windings, for magnetic interaction with the armature windings to produce
a force in a direction of the path, a first switch controller, the first switch
controller including means for controlling the application of the first motor drive
power only to those of the plurality of armature windings within a magnetic influence
of the permanent magnets on the first linear motor stage, a second switch controller,
and the second switch controller including means for controlling the application
of the second motor drive power only to those of the plurality of armature windings
within a magnetic influence of the permanent magnets on the second linear motor
stage, whereby the first linear motor stage is driven only by the first motor drive
power and the second linear motor stage is driven by the second motor drive power
independently of the first linear motor stage, and the first and second linear
motor stages can be driven to any point on the path.
The present invention also comprehends a linear motor comprising:
a path, a plurality of armature windings in the path, first and second linear motor
stages movable on the path, a first plurality of switches for applying a first
motor drive power only to those of the plurality of armature windings in the vicinity
of the first linear motor stage, and a second plurality of switches for applying
a second motor drive power only to those of the plurality of armature windings
in the vicinity of the second linear motor stage, whereby the first and second
linear motor stages are independently driveable.
Various embodiments of the present invention will now be more particularly
described, by way of example, with reference to the accompanying drawings, in which:
- Fig. 1A is a simplified schematic diagram of a linear motor system according
to an embodiment of the invention.
- Fig. 1B is a transverse cross section taken along II-II in Fig. 1.
- 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
- Fig. 3 is a cross section taken along CC in Fig. 1B, showing the relationship
between the switching magnet and motor magnets.
- Fig. 3A is a cross section taken along CC in Fig. 1B, showing, the positional
relationship between the switching magnets and the motor magnets.
- Fig. 3B is a cross section taken along CC as in Fig. 3A, where the movable stage
has moved to the right from its position in Fig. 3A.
- Fig. 4 is a cross section taken along B-B in Fig. 1B showing the relationship
between magnetic zones in the encoder magnet and the encoder sensors.
- Fig. 4A shows a shape of a beveled magnetic zone about one of the encoder sensors
from Fig. 4.
- Fig. 4B shows the relationship between the output of the encoder sensors located
at the left and right ends of the encoder magnets in Figure 4, and the beveled
magnet zone in Fig. 4A.
- Fig. 4C shows another shape of a beveled magnetic zone about one of the encoder
sensors from Fig. 4.
- Fig. 5 is a schematic diagram showing an embodiment of a wireless linear motor
employing active communications elements on the movable stage.
- Fig. 6 is a schematic diagram showing an embodiment of a wireless linear motor
employing an active command-response position feedback system.
- 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.
- 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.
- 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.
- 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.
- Fig. 11 is a schematic diagram of a wireless linear motor employing an active
command-response system with memory on-board the movable stage.
- 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.
- Fig. 13 is a diagram showing several path modules connected together to form
- 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.
- 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.
- Fig. 16 is a diagram showing an alternative embodiment of a path module having
a single sensor.
- Fig. 17 is a diagram of a linear motor with a path in a racetrack shape.
- Fig. 18 is an enlarged view of a portion of a curved section of the path of
- 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.
- Fig. 20 is a diagram of a linear motor path consisting of two connected spirals,
including multiple crossovers.
- Fig. 21 is a diagram of a linear motor path in the shape of a Moebius band.
Referring now to the drawings, and particularly 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
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.
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.
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.
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.
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.
It can be seen from the simplified drawing of Fig. 1A, and the description
above, that linear motor 10 requires the following actions:
- 1) control of switches 22A, 22B, 22C
- 2) feedback of position or motion data
- 3) drive power generation related to position (or motion-derived position).
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.
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.
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,
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.
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.
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.
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.
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 16-4 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.
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 16-5 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.
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 x 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.
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 square law.
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 38 is represented
by the equation:
y = a + bx2
- y is the distance from the surface of the magnet to encoder sensor 38,
- x is the position along beveled magnetic zone 42', and
- a and b are constants.
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.
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.
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°. Thus, the center-to-center distance between
adjacent magnetic zones is considered to be 180°, and the distance between the
center of a zone and its edge is considered to be 90°.
It is conventional for encoders to produce a sine and a cosine signal,
relatively 90° 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° 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° in the direction
of travel from sine+ encoder sensor 38s+. A cosine- encoder sensor 38c- is spaced
180° 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.
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.
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° 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° 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.
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.
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.
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.
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 antennae, 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.
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.
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.
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.
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.
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'.
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'... 28n to each movable stage 12.
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.
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
It is also possible to provide path switching, similar to the switching
used on railroads, to direct movable stage 12 flexibly along different paths.
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.
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).
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.
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.
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.
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
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
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.
Dashed box 82 in Fig. 17 is expanded in Fig. 18 to enable description.
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'.
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 x 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.
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
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.
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.
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.
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.
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.