Note: Descriptions are shown in the official language in which they were submitted.
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APPARATUS POR MULTI-AXIS ROTATION AND TRANSLATION
FIELD OP THE INVENTION
The present invention relates to an apparatus for multi-axis rotation
and translation of a spherical body.
Manipulators capable of motion in three linear and three angular
directions singularly or in any combination are often referred to as ~~six
degrees of freedom" (6DOF) manipulators. They have many applications
such as motion simulator platforms, sensor calibration tables, precision
aiming devices, machining operations, and material handling. These
manipulators have different architectures and can be categorized into serial
and parallel configurations.
Serial 6DOF manipulators, such as a six-axis wrist-partitioned serial
manipulator, have a relatively simple kinematic structure and do not have
any closed kinematic loops. Typically each joint has its own actuator which
provides a relatively large range of motion and relatively simple control but
have generally poor positioning accuracy and very limited load-carrying
capacity (as the majority of the load capacity is taken up by actuators
themselves).
Conversely, parallel 6DOF manipulators are architecturally more
complex because they are formed with several closed kinematic loops,
typically two or more kinematic chains that connect a moving platform to a
base, where one joint in the chain is actuated and the other joints are
passive. Parallel 6DOF manipulators can support larger loads, position these
loads with greater accuracy, are typically lighter, less costly to operate
(energy savings) and require less maintenance than serial manipulators.
Their limitations are that they are highly coupled, more difficult to control
and
have very limited ranges of motion.
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The most commonly known parallel manipulator is the Stewart (or
Stewart-Gough) platform which consists of a movable platform attached to a
fixed base with six "legs" which can be characterized as universal-prismatic-
spherical kinematic chains. There are two types of Stewart platform, the
first being a 3-3 platform (i.e. 3 connecting points on the base, 3 connecting
points of the movable platform and two legs intersecting each connecting
point via a universal or ball-and-socket joint). The second type of Stewart
platform is a 6-3 platform (i.e. six connecting points on the base and three
connecting points on the movable platform which join the endpoints of two
legs). More recently, 6-6 platforms have been introduced. They are
sometimes referred to as modified Stewart platforms although they are
geometrically much more complicated.
One such parallel 6DOF manipulator is disclosed in US patent No.
5,179,525 (Griffis et al.). This manipulator comprises a movable platform
supported about a base platform by a plurality of parallel support legs and is
based upon the 3-3, 6-3 and 6-6 configurations described previously. While
these platforms have excellent structural stiffness, they have the inherent
drawback that the degrees of freedom are highly coupled. Thus, when the
platform nears its limit of motion in one direction (or degree of freedom), it
loses its ability to move in other directions (or other degrees of freedom).
A further disadvantage of the Stewart-type platform is that they often
rely upon hydraulic actuators, especially in large scale platforms where the
actuators must be able to generate large forces to support gravitational
loads.
There are other types of 6DOF manipulators that combine translation
and rotation. Newport Instruments, for example, custom makes such
platforms which typically consist of a turntable mounted to a universal joint.
The universal joint's axes are used to orient the turntable. If the turntable
has no angle limits, then the platform offers unlimited rotation about an
arbitrary axis. Unfortunately, this axis must be within the angular limits of
the universal joint, typically X30°.
Another type of 6DOF manipulator is disclosed in US patent No.
4,908,558 (Lordo et al.) for use as a flight motion simulator. This platform
is
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capable of motion in three linear and three angular directions singularly or
in
any combination and comprises a spherical rotor element which is moved
using magnetic bearings and an induction motor which generates magnetic
flux in a stator assembly. While this platform can move with six degrees of
freedom, it is very complex, requires a lot of power and is only capable of
unlimited roll. Its range of pitch and yaw are limited, as are the ranges of
its
X, Y and Z translations.
The present invention seeks to overcome, or at least mitigate, the
limitations of the above-described prior art and/or provide an alternative.
According to a first aspect of an embodiment of the invention, there is
provided an apparatus for rnulti-axis rotation and translation comprising a
spherical body having an outer surtace and a geometric center, a plurality of
roller assemblies each engaging the outer surface, a plurality of actuators
for
driving said roller assemblies, a frame for supporting the plurality of roller
assemblies and the plurality of actuators and translation means for
translating the frame along each of three orthogonal axes. The actuators are
selectively operated to drive the roller assemblies thereby imparting
unlimited angular displacement to the spherical body and rotating the
spherical body about any axis passing through the geometric center and the
translation means is operated to translate the spherical body along at least
one of the three orthogonal axes.
Preferably, each of the plurality of roller assemblies comprises active
traction means and passive slip means. The roller assemblies may be omni-
wheels each having a main wheel hub for providing traction in a direction
perpendicular to a rotation axis passing through a center of the hub and a
plurality of peripheral rollers for providing slip in a plurality of
directions
perpendicular to respective rotation axes of the plurality of peripheral
rollers.
According to a second aspect of an embodiment of the invention, there is
provided an apparatus for multi-axis rotation comprising a spherical body
having an outer surface and a geometric center, a plurality of roller
assemblies each engaging the outer surface, a plurality of actuators for
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driving the roller assemblies and a frame for supporting the plurality of
roller
assemblies and the plurality of actuators. The actuators are selectively
operated to drive the roller assemblies thereby imparting unlimited angular
displacement to the spherical body and rotating the spherical body about any
axis passing through the geometric center.
An embodiment of the invention will now be described by way of
example with reference to the accompanying drawings, in which;
Figure 1 is a perspective view of an embodiment of the invention;
Figure 2 is a perspective view of part of an embodiment of the
invention;
Figure 3A is a side view of part of an embodiment of the invention;
Figure 3B is a perspective view of part of an embodiment of the
invention;
Figure 4 is a side view of part of an embodiment of the invention;
Figure 5 is a detailed sectional view of part of an embodiment of the
invention.
DETAILED DESCRIPTION OE THE INVENTION
An embodiment of the invention will be described in reference to X, Y
and Z axes as indicated in Figures 1 and 2. The term 'roll" refers to rotation
about the X-axis, the term "pitch" refers to rotation about the Y-axis and the
term "yaw" refers to rotation about the Z-axis (vertical).
Referring to Figures 1 and 2, there is illustrated an apparatus 10 for
multi-axis rotation and translation comprising a spherical body 12 supported
by a frame 14, a plurality of roller assemblies 16, a plurality of actuators
20
for driving the roller assemblies 16, respectively, and translation means 24.
The Z-axis passes through the geometric center 26 of the spherical body 12.
The actuators 20 may be of any suitable configuration but as shown are three
variable speed DC motors 22A, 22B and 22C.
In the embodiment shown in Figures 1 and 2, the roller assemblies 16
comprise three omni-wheels 18A, 18B and 18C. It will be understood by
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those skilled in the art that there are other configurations of roller
assemblies
which would meet the design criteria of the invention (as described below).
For example, there could be any number of omni-wheels contacting the outer
surface of the spherical body 12.
As shown in Figures 3A and 3B, the omni-wheels 18A, 18B and 18C
(sometimes referred to as "omni-directional" wheels) each comprise a split
wheel hub 30 that supports a plurality of passive peripheral rollers 38A, 388,
38C, 38D, 40A, 40B, 40C and 40D (38D not shown). The split wheel hub 30
has first and second integral hub halves 34 and 36, respectively, each
supporting four peripheral rollers 38A, 38B, 38C, 38D and 40A, 40B, 40C,
40D, respectively. Each of the four peripheral rollers 38A, 38B, 38C and 38D
of the first hub half 34 is spaced circumferentially between an adjacent pair
of the rollers 40A, 408, 40C and 40D in the second hub half 36. Each of the
peripheral rollers is positioned at approximately 90° to the periphery
of the
wheel hub 30 to allow for near friction-free movement perpendicular to the
axis of rotation 42 of the wheel hub. In this way, each of the omni-wheels
18A, 18B and 18C provides traction in a direction perpendicular to the axis of
rotation 42 of the wheel hub while permitting slip in a plurality of
directions
perpendicular to the respective rotation axes 44A, 44B, 44C, 44D, 46A, 46B,
46C and 46D of the rollers 38A, 38B, 38C, 38D, 40A, 40B, 40C and 40D,
respectively.
It should be noted that any suitable roller assemblies or other devices
that provide the necessary traction and slip may be used. Preferably, each of
the roller assemblies will have a substantially circular circumferential
profile
and will not induce significant vibrations in the spherical body 12.
The three omni-wheels 18A, 18B and 18C contact the spherical body
12 at three points 48A, 48B and 48C, respectively, distributed substantially
symmetrically about the Z axis below the reference equator 60 of the
spherical body 12 (the reference equator 60 divides the spherical body 12
into two equal parts). The contact points 48A, 48B and 48C of the omni-
wheels 18A, 18B and 18C, respectively, are angularly spaced in the XY plane
by 120° and form the vertices of an equilateral triangle. This geometry
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creates equal distribution of static weight of the spherical body 12 on each
of
the omni-wheels 18A, 18B and 18C.
It should also be noted that the contact points 48A, 48B and 48C of
the omni-wheels 18A, 188 and 18C, respectively, do not need to be angularly
spaced in the XY plane by 120°. Any suitable angular spacing may be
used.
Likewise, while in the above description, the omni-wheels 18A, 18B
and 18C engage the spherical body 12 below its reference equator 60, any
number of configurations may be used. For example, the omni-wheels 18A,
18B and 18C may be distributed so that the angular spacing of their
respective contact points 48A, 48B and 48C is substantially equal in both the
XY plane and the XZ or YZ plane (i.e. with at least one of the omni-wheels
above the reference equator 60 of the spherical body 12).
Referring also to Figure 4, the three variable speed DC motors 22A,
22B and 22C are independently operable so as to rotate at different speeds,
or the same speed if desired. Each of the motors 22A, 22B and 22C are
coupled to a corresponding one of the omni-wheels 18A, 18B and 18C by
corresponding one of three elongate drive pins 62A, 62B and 62C. The
motors 22A, 22B and 22C and the drive pins 62A, 62B and 62C are all
coupled to the frame 14, as will be explained in more detail below.
The frame 14 comprises three support members 64A, 64B and 64C, an
annular member 66, three arcuate members 68A, 68B and 68C, three angled
shelves 70A, 70B and 70C, three link arms 72A, 72B and 72C and a coupling
74. The support members 64A, 64B and 64C each have vertical portions
76A, 76B and 76C positioned slightly outwards of the outer surface of the
spherical body 12 and extending from the translation means 24 to the height
of the geometric center 26 of the spherical body 12. The support members
64A, 64B and 64C also each have horizontal foot portions 78A, 78B and 78C
each extending from the lower ends (i.e. distal to the reference equator 60)
of the vertical portions 76A, 76B and 76C towards the Z axis. The upper
ends (i.e. proximal to the reference equator 60) of the vertical portions 76A,
76B and 76C each engage the annular member 66 at respective connection
sites 80A, 80B and 80C.
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The annular member 66 has a diameter that is slightly larger than the
diameter of the spherical body 12. The three arcuate members 68A, 68B and
68C, each having a radius of curvature slightly larger than the radius of
curvature of the outer surface of the spherical body 12, extend upwardly and
towards the Z-axis from the connection sites 80A, 80B and 80C and are
coupled to the coupling 74 which lies on the Z-axis above the spherical body
12.
As best seen in Figure 5, the top of the frame 14 has a bore 90 for
slidably receiving a biasing means, shown as a compression spring 92. The
compression spring 92 engages a ball bearing 94 which in turn engages the
outer surface of the spherical body 12. The compression spring 92 is
compressed by a drive rod 96. The compression spring 92, ball bearing 94
and drive rod 96 can be used to manually or automatically apply a force to
the outer surface of said spherical body 12. Application of this force
increases the normal forces (and therefore the traction) of the omni-wheels
18A, 18B and 18C on the outer surface of the spherical body 12 thus
preventing unwanted slippage between the omni-wheels 18A, 18B and 18C
and the spherical body 12. The compression spring 92 also allows for any
vibrations of the spherical body 12. Of course, the compression spring 92,
ball bearing 94 and drive rod 96 may be dispensed with if there is enough
traction caused by the weight of the spherical body 12 for the omni-wheels
18A, 18B and 18C to rotate it.
Three hollow cylindrical members 98A, 98B and 98C extend outwardly
(away from the Z-axis) and upwardly from the vertical portions 76A, 76B and
76C of the three support members 64A, 64B and 64C, respectively, for
telescopically receiving the drive pins 62A, 62B and 62C, respectively. The
lower ends of the drive pins 62A, 62B and 62C engage respective horizontal
foot portions 78A, 78B and 78C close to the Z-axis. The outermost end
portions of each of the three cylindrical members 98A, 98B and 98C each
engage respective lower end portions of the three angled shelves 70A, 70B
and 70C, upon which the motors 22A, 22B and 22C are supported. Each of
the shelves 70A, 70B and 70C is substantially perpendicular to respective one
of the cylindrical members 98A, 98B and 98C. The respective upper end
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portions of the three angled shelves 70A, 70B and 70C are coupled to the
annular member 66 by a respective one of three link arms ?2A, 72B and 72C
extending vertically upwardly from a respective one of the three connection
sites 80A, 80B and 80C.
Those skilled in the art would appreciate that any suitable frame or
support structure may be used to support the roller assemblies and the
actuators without departing from the spirit and scope of the invention.
The horizontal foot portions 78A, 78B and 78C of each of the support
members 64A, 64B and 64C are resiliently attached to the translation means
24, which is a set of three independent orthogonal linear translation stages
104A, 104B and 104C for moving the frame in directions parallel to the X, Y
and Z axes, respectively. The translation stages 104A and 104B are linear
gantry-type translation stages each comprising a pair of parallel rails
106A;106B, a platform 108A;108B and means 110A;110B for moving said
platform 108A;108B along said pair of parallel rails 106A;106B. The third
translation stage 104C is a vertical prismatic joint actuated by a ball-screw.
It should be noted that the apparatus shown in the drawings could be
mounted to or rest on any suitable surtace or structure.
Linear combinations of angular displacement and speed of each of the
three omni-wheels 18A, 18B and 18C are executed, either manually or
automatically (as will be discussed below) to impart the desired angular
displacement and speed of the spherical body 12. The motors 22A, 22B and
22C drive each of the omni-wheels 18A, 18B and 18C to execute the desired
angular displacement by varying the velocity/force contribution of each omni-
wheel so that the rotation axis can be varied to any linear combination of the
principal axes. For example, if solely yaw motion is desired, all three omni-
wheels are driven in the same direction at the same speed. For solely pitch
motion, two of the omni-wheels are driven in opposite directions with equal
speed and the third omni-wheel is not actuated, but provides the necessary
slip on its passive axis. For solely roil motion, two of the omni-wheels must
be driven in the same direction at the same speed, and the third omni-wheel
must be driven in the opposite direction at twice the speed of the other two
omni-wheels. The overall rotational velocity of the spherical body 12 will
also
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depend upon the weight of the spherical body 12 itself, the relative
contributions of each of the omni-wheels 18A, 18B and 18C and their
respective contact surfaces.
Simultaneously, the spherical body 12 may be moved parallel to the
three translation axes by the translation stages 104A, 104B and 104C. Thus,
the rotation and translation are independent of each other, that is to say the
rotational and translational actuation are completely decoupled. This means
that the spherical body 12 can thus be positioned anywhere within the
reachable workspace of the translation stage with any orientation about any
axis through the geometric center 26 of the spherical body 12.
It should be noted that those skilled in the art would recognize that
any suitable translation means rnay be used in place of the above-described
translation stages 104A, 104B and 104C. In addition, if no translation is
desired, i.e. purely rotational displacement, the translation means may be
dispensed with altogether.
The spherical body and frame may be made of a rigid material or a
non-rigid material.
The motors 22A, 22B and 22C and/or the translation stages 104A,
104B and 104C may be controlled using manual control means or automatic
control means. Automatic control means may comprise a computer and
motor interface. The computer could calculate the appropriate combination
of rotation and translation for a desired movement and send the appropriate
signals to the three motors 22A, 22B and 22C and/or the translation stages
104A, 1048 and 104C.
While in the above-described embodiment of the invention, no
feedback is used (i.e. the apparatus is manually controlled or controlled
using
open-loop control), feedback may be implemented (i.e. closed-loop control)
to adjust the relative contributions of the omni-wheels 18A, 18B and 18C to
compensate for deviations from the desired angular displacement of the
spherical body 12 and/or discrepancies between the desired angular velocity
of the spherical body and the angular velocity of the omni-wheels 18A. 18B
and 18C (this effect is sometimes referred to as scrub). For example, optical
feedback may be used to determine the angular displacement. Likewise,
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velocity detection at the omni-wheels 18A, 18B and 18C may be used to
determine the angular velocity of the spherical body.
Embodiments of the present invention effectively combine the benefits
of both serial and parallel manipulators resulting in a parallel architecture
capable of accurate positioning and large load capacity with unlimited range
of angular displacement and translational displacement limited only by the
translation range of the translation stage(s). Due to the decoupling of the
rotational and translational actuation, embodiments of the present invention
can be controlled with a high degree of accuracy, and where a computer is
used as control means, with relative ease of computation. Effectively,
embodiments of the present invention which use a computer as control
means provide the computational simplicity of a six-axis wrist partitioned
serial manipulator, but have the structural stiffness of a six-legged Stewart-
Gough type platform. Due to the unlimited orienting workspace,
embodiments of the present invention have an even broader range of
applications that the Stewart-Gough type platforms.
In addition, embodiments of the present invention have the additional
advantage that the actuators 20 (e.g. standard DC motors 22A, 22B and
22C) do not require as much power as the actuators used in the prior art,
namely hydraulic actuators, magnetic bearings and large induction motors.
This lower power requirement is also a consequence of having multiple wheel
assemblies acting together to actuate the rotational displacement.
Embodiments of the invention are scalable so it is conceivable that the
apparatus of the present invention could be applied to large-scale vehicle
simulator platforms. Likewise, it is conceivable that the apparatus of the
present invention could be scalable to micro scale platforms or smaller.
Embodiments of the invention may also be applied to satellite motion
control because of the need for apparatus that operates reliably in the
weightlessness of space. In particular, embodiments of the invention can be
used to emulate conditions of weightlessness for satellite sensor and control
system development, calibration and testing.
While the invention has been described in detail in the foregoing
specification, it will be understood by those skilled in the art that
variations
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may be made without departing from the spirit and scope of the invention,
being limited only by the appended claims.
