BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a spherical motor and,
more particularly, to a spherical motor that includes a plurality of two-axis magnetic
elements positioned on one of a field sphere or an armature sphere and a plurality
of three-axis magnetic elements positioned on the other of the field sphere or the
armature sphere, where the two-axis magnetic elements generate oscillating magnetic
fields and the three-axis magnetic elements detect the oscillating magnetic fields
and then provide an actuating magnetic torque to position the armature sphere.
2. Discussion of the Related Art
There is a need to accurately point various devices, such
as antennas, sensors, detectors, etc., in a particular direction within a desired
field of view. Currently, these devices are typically mounted on a two or three
axis gimbal assembly where each axis includes a separate gimbal that is controlled
by a separate motor to point the device in the desired direction. Such gimbal assemblies
typically employ complex wrist and elbow joints that result in a relatively large
and complex system sometimes unsuitable for certain applications.
Spherical motors are known in the art that require less
space and can rotate and direct a device in three degrees of freedom. However, current
spherical motor designs typically use extremely complex algorithms and modeling
techniques that make their implementation difficult, impractical and cost prohibitive.
U.S. Patent No. 5,410,232 issued to Smith illustrates this
problem. The '232 patent discloses a spherical motor 10 including a spherical stator
12 surrounding a spherical rotor 18. Suitable bearings are provided so that the
rotor 18 can rotate within the stator 12. A motor shaft 24 is mounted to the spherical
rotor 18 and extends through a stator opening 26. The motor 10 provides three-axis
positioning of the shaft 24 within the opening 26. The spherical rotor 18 includes
a plurality of rotor magnets or poles 22 disposed on its outer surface, and the
spherical stator 12 includes a plurality of stator poles 14 disposed on its inner
surface. The stater poles 14 are controllable electric coils and the rotor poles
22 are permanent magnets defined by a magnetic core. The magnetic fields of the
poles 14 and 22 interact to provide a torque on the rotor 18 to position the shaft
The motor 10 includes an orientation sensing system 40
having a spherical grid pattern 42 provided on the outer surface of the rotor 18.
The grid pattern 42 includes a set of symmetrically spaced radial lines continuously
converging to a point P, where the motor shaft 24 is situated, and a set of parallel
lines that are orthogonal to the radial lines. The system 40 uses a mathematical
algorithm to determine the position of the rotor 18 relative to the grid pattern
42, and control the magnetic fields to position the shaft 24. Particularly, the
system 40 uses the grid pattern 42 to determine the position of the rotor 18 and
uses the magnetic field supplied to the rotor poles 22 to provide the desired torque.
The magnetic fields generated by the fixed magnet poles
22 are extremely complicated. Further, every time the rotor 18 moves, the magnetic
field that the rotor 18 sees is different. Therefore, it is necessary to accurately
know the position of the rotor 18 relative to the fixed poles 22. The sensing system
40 computes the magnetic field as seen by the rotor 18 each time the rotor 18 move.
The rotor poles 22 are turned on and off to move the rotor 18 in the desirable direction.
This operation requires a very elaborate position knowledge scheme for the rotor
18 employing complex algorithms. It would be desirable to provide a spherical motor
that was much less complex to control.
Further prior art documents :
SUMMARY OF THE INVENTION
"An Approach to Basic Design of the PM-type Spherical Motor" (Ebihara et al,
Proceedings of the 2001 IEEE International Conference on Robotics & Automation,
Seaoul, Korea, May 21-26, 2001) discloses a spherical actuator that has permanent
magnets on the inner armature sphere distributed in a triangular lattice to eliminate
detent force. The actuator magnetic elements on the outer field sphere are pole
pieces each having a single coil.
WO 02 31945 A (CLARITY LLC ;ERTEN GAMZE (US)) 18 April 2002 (2002-04-18)
shows a single permanent magnet affixed to the armature sphere of a spherical actuator.
The permanent magnet can be moved within a field generated by multiple field coils
all fixed to an outer sphere surface. Each field coil may also be used to sense
the position of the permanent magnet.
GB-A-2 330 457 (UNIV SHEFFIELD) 21 April 1999 (1999-04-21) discloses a spherical
motor which comprises an armatures sphere entirely made of a composite magnet and
has four magnetic poles. Each pole is a quarter sphere of the rotor. The outer field
sphere comprises four sets of coils. Separate hall-sensors are used for detecting
the rotor position.
US-A-3 854 341 (QUERMANN T) 17 December 1974 (1974-12-17) is related to a
gyroscope. Two axis-field magnetic elements are used as sector motors to provide
a secondary drive which rotates the inertial element at slow rate. Additional pick
off-torque coils are used to detect the displacement of the angular momentum of
the inertial element and may also act to generate torque.
In accordance with the teachings of the present invention,
a spherical motor is disclosed that simultaneously provides motive torque in three
degrees of freedom. The spherical motor includes an outer sphere and an inner sphere
positioned therein, where one of the spheres is a stationary field sphere and the
other sphere is a rotatable armature sphere. A first set of magnetic elements is
formed on the outer sphere and a second set of magnetic elements is formed on the
inner sphere. One set of the magnetic elements are field magnetic elements that
include at least two coils providing magnetic fields in two axes. The other set
of the magnetic elements are sensor/actuator magnetic elements that include three
coils providing magnetic fields in three axes.
The field magnetic elements generate a regularly varying
magnetic field. Each sensor magnetic element senses its localized magnetic field
variations generated by the field magnetic elements and generates a torque relative
thereto to rotate the armature sphere. Over one complete field variation of the
field magnetic elements, each sensor magnetic element can produce torque about all
three axes. Because each sensor magnetic element generates the required torque vector,
no coordination is necessary between the two sets of magnetic elements.
Additional advantages and features of the present invention
will become apparent from the following description and appended claims, taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is perspective view of a spherical motor, according
to an embodiment of the present invention;
Figure 2 is a perspective view of half of an outer field
sphere removed from the spherical motor shown in figure 1 and including a plurality
of field magnetic elements;
Figures 3(a)-3(c) are perspective views of one of the field
magnetic elements shown in figure 2;
Figure 4 is a perspective view of half of an armature sphere
removed from the spherical motor shown in figure 1 and including a plurality of
sensor/actuator magnetic elements; and
Figure 5 is a perspective view of one of the sensor/actuator
magnetic elements shown in figure 4.
DETAILED DESCRIPTION OF THE EMBODIMENTS
The following discussion of the embodiments of the invention
directed to a spherical motor is merely exemplary in nature, and is in no way intended
to limit the invention or its applications or uses.
The present invention includes a spherical motor that provides
three-degrees of freedom of rotation, but does not suffer the complexities of the
spherical motors known in the art. As will be discussed in detail below, the spherical
motor of the invention employs a plurality of field magnets that create oscillating
magnetic fields that generate a voltage that is proportional to the derivative of
the magnetic fields. Sensor/actuator magnets sense these oscillating magnetic fields
over one complete oscillating magnetic field cycle. The sensor/actuator magnets
then generate actuating magnetic fields that provide torque on the moving sphere
of the motor to position it at the desired location. Thus, the motor does not need
to employ a complex vision system that determines the position of the moving sphere
of the motor.
Figure 1 is a perspective view of a spherical motor 10,
according to an embodiment of the present invention. The spherical motor 10 includes
an outer field sphere 12 and an inner armature sphere 14. As will become apparent
from the discussion below, the term "sphere" as used herein also includes a portion
of a complete sphere, and possibly less than half of a sphere. A gap is defined
between the spheres 12 and 14 so that the armature sphere 14 is free to rotate within
the field sphere 12 by any suitable mechanism, such as a forced air pocket, ball
bearings, electrostatic repulsion, fluid bearings, etc. The field sphere 12 includes
a plurality of symmetrically disposed magnetic regions 20 formed in an outer shell
22 of the sphere 12, and the armature sphere 14 includes a plurality of symmetrically
dispose magnetic regions 24 formed in an outer shell 32 of the sphere 14. The diameter
of the spheres 12 and 14 and the thickness of the shells 22 and 32 would be application
specific, and can be any dimension suitable for the purposes described herein.
The armature sphere 14 includes a motor shaft 16 mounted
to the shell 32 that extends through an opening 18 in the field sphere 12. A device
(not shown) can be mounted to the shaft 16, so that it can be positioned in a particular
direction by rotation of the sphere 14 in three-degrees of freedom. The device can
be any device that requires pointing, such as a communications antenna, sensor,
optical device, etc. Alternately, the device can be completely mounted within the
armature sphere 14. Depending on the application and the configuration of the spheres
12 and 14, the device can be directed in any direction within a 360° field-of-view
defined by the spheres 12 and 14. In a practical application, the pointing of the
device would probably be limited to a field-of-view within 180°. As will be
discussed in detail below, the spherical motor 10 employs a technique of detecting
changing or oscillating magnetic fields to provide torque on the armature sphere
14 to position the device.
Figure 2 is a perspective view of half of the field sphere
12 separated from the motor 10. A two-axis field magnetic element 26 is symmetrically
positioned within each magnetic region 20. Figures 3(a)-3(c) show a perspective
view of one of the magnetic elements 26 separated from the field sphere 12. Each
magnetic element 26 includes a first coil 28 wrapped around a ferrite core 34, and
a second coil 30 wrapped around a ferrite core 36. where the cores 34 and 36 are
orthogonal to each other. However, as will be appreciated by those skilled in the
art from the discussion herein, the coils 28 and 30 do not need to be orthogonal
to each other for the spherical motor 10 to operate within the scope of the present
In this embodiment, the field sphere 12 includes fourteen
magnetic regions 20 each including a single magnetic element 26. However, this is
by way of a non-limiting example in that a practical field sphere for a spherical
motor probably would include many more magnetic regions 20 and magnetic elements
26. The symmetrical positioning of the magnetic regions 20 on the shell 22 is also
by way of a non-limiting example in that the regions 20 and the elements 26 can
be disposed on the shell 22 in any suitable configuration. The magnetic regions
20 generally define a confined area of the magnetic fields for each particular magnetic
element 26, but the magnetic fields of the elements 26 can overlap without affecting
the operation of the system 10. Further, a common voltage source can be employed
to operate all of the magnetic elements 26.
The direction of the combined magnetic field from the coils
28 and 30 is determined by the direction of the current traveling through the coils
28 and 30 when a positive or negative voltage potential is applied thereto. In figure
3(a), the coil 30 is receiving a positive potential and the coil 28 is off, so that
the direction of the current flow through the coil 30 creates a magnetic field 38
along the axis of the core 36. In figure 3(b), the coils 28 and 30 are both receiving
a positive potential so that the direction of the current flow through the coils
28 and 30 creates the combined magnetic field 38 at a 45° angle relative to
the axes of the cores 34 and 36 in the direction indicated. In figure 3(c), the
coil 28 is receiving a positive potential and the coil 30 is off, so that the direction
of the current flow through the coil 28 creates the magnetic field 38 along the
axis of the core 34. If the coils 28 and 30 are wound in the opposite direction,
then the magnetic field would be in the opposite direction for the same voltage
As is apparent, if a negative voltage is also applied to
the coils 28 and 30 in the manner as described herein the direction of the magnetic
field 38 will rotate 360° in the plane of the cores 34 and 36. Thus, a two-axis
field generator can be created by discreetly changing the potential applied to the
coils 28 and 30 in the sequence (+, off), (+, +), (off, +), (-, +), (-, off), (-,
-), (off, -), (+, -), where the magnetic field rotates in discreet 45° steps.
The magnetic field 38 can also be caused to rotate continuously by applying a sinusoidal
voltage potential to the coils 28 and 30 that are 90° apart in phase.
According to the invention, the relative orientation of
the magnetic elements 26 and the rotating magnetic fields that they generate is
not important. It is only necessary that the magnetic fields move so that they can
be detected. Also, if the magnetic fields did not move, a torque could never be
generated parallel to the magnetic fields due to the nature of the cross product
magnetic torque law. By providing a moving magnetic field, an average torque can
be generated in any direction. Further, the magnetic field 38 does not need to rotate
360° for the spherical motor 10 to operate according to the invention. Also,
additional coils, including coils providing three-axis magnetic fields, can be employed
in each magnetic element 26 to provide the moving magnetic field within the scope
of the present invention.
Figure 4 is a perspective view of a portion of the armature
sphere 14 separated from the motor 10. Each of the plurality of magnetic regions
24 includes a three-axis sensor/actuator magnetic armature element 42 disposed therein.
A perspective view of one of the magnetic elements 42 removed from the armature
sphere 14 is shown in figure 5. Each sensor/actuator magnetic element 42 includes
a first coil 44 wrapped around a core extending along a first axis, and a second
coil 46 wrapped around a core extending in a second axis perpendicular to the first
axis, as shown. Four separate coils 48, 50, 52 and 54 are positioned in each quadrant
defined by the axes of the coils 44 and 46, where each coil 48-54 is wrapped around
a core extending along an axis perpendicular to both the first and second axes to
provide the three axes. Therefore, the magnetic element 42 senses or provides a
magnetic field in any direction.
The sensor/actuator magnetic elements 42 are used to sense
local magnetic field properties and produce torque about all three axes. Because
the magnetic fields generated by the magnetic elements 26 are moving, the magnetic
elements 42 can sense the direction of a localized magnetic field around it. The
elements 26 can then use their magnetic fields to generate a torque relative to
the moving magnetic fields to move the armature sphere 14 relative to the field
sphere 12. In one embodiment, each magnetic element 42 will sense its localized
magnetic field through one complete cycle of the moving magnetic field.
When the elements 42 are sensing, a control system (not
shown) records the voltages on the coils 44-54 as the localized magnetic field for
that element 42 moves through its cycle. If a magnetic armature element 42 is sensing
the magnetic field of one of the magnetic field elements 26, it will determine the
direction of the magnetic field 38 as it moves relative to the coils 28 and 30.
The control system then calculates the magnetic field as it appears locally to that
particular armature element 42, and assumes it will be the same for the next cycle.
In other words, the control system knows where the magnetic field is because the
localized magnetic fields are rotating at a particular rate.
After measuring for one complete cycle, the armature magnetic
elements 42 will then generate an actuating magnetic field in a particular direction
that interacts with the field magnetic element fields, so that the magnetic fields
cause a torque on the armature sphere 14. In other words, the control system will
apply a voltage potential to the coils 44-54 in each of the magnetic elements 42
so that for the next cycle of the moving magnetic field, which will now be known
by the system, a desired torque can be applied to the armature sphere 14 to position
the shaft 16. Thus, any torque in any direction can be generated by the system.
In this embodiment, the magnetic elements 42 are sensing
the magnetic fields for one cycle of the magnetic fields, and then actuating the
armature sphere 14 for the next cycle of the magnetic fields in an alternating sequence.
The magnetic elements 42 can all be sensing and then all be actuating together.
Alternatively, some of the magnetic elements 42 can be sensing while other of the
magnetic elements 42 are actuating.
During the sensing phase, each magnetic element 42 is sensing
the magnetic field around it, which may be provided by one or more of the field
elements 26. Therefore, the orientation of the field elements 26 on the shell 22
is not important. Because each element 42 generates the desired torque vector, no
coordination is necessary between the various armature elements 42. Once the global
torque requests to the motor 10 is transformed into a local actuator set reference
frame, a simple local controller can manipulate the coils 44-54 in the armature
elements 42 to generate a torque vector parallel to the torque requested to position
the shaft 16.
As discussed above, when a magnetic element 42 acts as
sensing elements and then as an actuating element, the motor 10 has a 50% duty cycle
in that the magnetic elements 42 will be sensing half the time and actuating the
other half of the time. The duty cycle can be varied by changing the sensing and
actuating times. In an alternate embodiment, the magnetic elements 42 can be providing
actuation continuously. In this embodiment, the armature elements 42 do not sense
the oscillating magnetic fields generated by the field elements 26, but sense the
back electromotive force (EMF) generated by the field elements 26. It is still necessary
that magnetic fields are moving. Thus, by sensing the back EMF of the magnetic fields
generated by the field elements 26, the armature elements 42 are sensing and actuating
at the same time.
Variations of the embodiments discussed above can be made
within the scope of the present invention. For example, the outer sphere can be
the armature sphere that moves relative to the inner, field sphere. The three-axis
magnetic elements 42 and the two-axis magnetic elements 26 can be positioned on
either the field sphere or the armature sphere, regardless which of the inner and
outer sphere is the field sphere 12 and the armature sphere 14. Further, the field
sphere 12 and the armature sphere 14 do not need to have the identical distribution
of magnetic elements thereon. The number of field magnetic elements 26 and armature
magnetic elements 42 will be determined for different applications. If the requested
torque is going to be calculated in the base frame, it may make more sense to have
the outer sphere be the armature sphere. However, if the requested torque is going
to be calculated in the reference frame, it may make more sense for the inner sphere
to be the armature sphere.
The foregoing discussion discloses and describes merely
exemplary embodiments of the present invention. One skilled in the art will readily
recognize from such discussion and from the accompanying drawings and claims that
various changes, modifications and variations can be made therein without departing
from the scope of the invention as defined in the following claims.