Abstract:
An apparatus for multi-axis rotation and translation comprises a spherical body, a plurality of roller assemblies each engaging the outer surface of the spherical body, 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 its geometric center. The translation means may be operated to translate said spherical body along at least one of said three orthogonal axes. The apparatus is particularly applicable to use as a manipulator with six degrees of freedom (unlimited rotational displacement and translational displacement limited only by the boundaries of the workspace).

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to an apparatus for multi-axis rotation and translation of a spherical body.  
       BACKGROUND OF THE INVENTION  
       [0002]     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.  
         [0003]     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).  
         [0004]     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.  
         [0005]     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.  
         [0006]     One such parallel 6DOF manipulator is disclosed in U.S. Pat. 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).  
         [0007]     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.  
         [0008]     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&#39;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 ±30°.  
         [0009]     Another type of 6DOF manipulator is disclosed in U.S. Pat. No. 4,908,558 (Lordo et al.) for use as a flight motion simulator. This platform is 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.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention seeks to overcome, or at least mitigate, the limitations of the above-described prior art and/or provide an alternative.  
         [0011]     According to a first aspect of an embodiment of the invention, there is provided an apparatus for multi-axis rotation and translation 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 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.  
         [0012]     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.  
         [0013]     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 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. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     An embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which:  
         [0015]      FIG. 1  is a perspective view of an embodiment of the invention;  
         [0016]      FIG. 2  is a perspective view of part of an embodiment of the invention;  
         [0017]      FIG. 3A  is a side view of part of an embodiment of the invention;  
         [0018]      FIG. 3B  is a perspective view of part of an embodiment of the invention;  
         [0019]      FIG. 4  is a side view of part of an embodiment of the invention;  
         [0020]      FIG. 5  is a detailed sectional view of part of an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     An embodiment of the invention will be described in reference to X, Y and Z axes as indicated in  FIGS. 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).  
         [0022]     Referring to  FIGS. 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  22 A,  22 B and  22 C.  
         [0023]     In the embodiment shown in  FIGS. 1 and 2 , the roller assemblies  16  comprise three omni-wheels  18 A,  18 B and  18 C. It will be understood by 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 .  
         [0024]     As shown in  FIGS. 3A and 3B , the omni-wheels  18 A,  18 B and  18 C (sometimes referred to as “omni-directional” wheels) each comprise a split wheel hub  30  that supports a plurality of passive peripheral rollers  38 A,  38 B,  38 C,  38 D,  40 A,  40 B,  40 C and  40 D ( 38 D not shown). The split wheel hub  30  has first and second integral hub halves  34  and  36 , respectively, each supporting four peripheral rollers  38 A,  38 B,  38 C,  38 D and  40 A,  40 B,  40 C,  40 D, respectively. Each of the four peripheral rollers  38 A,  38 B,  38 C and  38 D of the first hub half  34  is spaced circumferentially between an adjacent pair of the rollers  40 A,  40 B,  40 C and  40 D 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  18 A,  18 B and  18 C 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  44 A,  44 B,  44 C,  44 D,  46 A,  46 B,  46 C and  46 D of the rollers  38 A,  38 B,  38 C,  38 D,  40 A,  40 B,  40 C and  40 D, respectively.  
         [0025]     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 .  
         [0026]     The three omni-wheels  18 A,  18 B and  18 C contact the spherical body  12  at three points  48 A,  48 B and  48 C, 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  48 A,  48 B and  48 C of the omni-wheels  18 A,  18 B and  18 C, respectively, are angularly spaced in the XY plane by 120° and form the vertices of an equilateral triangle. This geometry creates equal distribution of static weight of the spherical body  12  on each of the omni-wheels  18 A,  18 B and  18 C.  
         [0027]     It should also be noted that the contact points  48 A,  48 B and  48 C of the omni-wheels  18 A,  18 B and  18 C, respectively, do not need to be angularly spaced in the XY plane by 120°. Any suitable angular spacing may be used.  
         [0028]     Likewise, while in the above description, the omni-wheels  18 A,  18 B and  18 C engage the spherical body  12  below its reference equator  60 , any number of configurations may be used. For example, the omni-wheels  18 A,  18 B and  18 C may be distributed so that the angular spacing of their respective contact points  48 A,  48 B and  48 C 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 ).  
         [0029]     Referring also to  FIG. 4 , the three variable speed DC motors  22 A,  22 B and  22 C are independently operable so as to rotate at different speeds, or the same speed if desired. Each of the motors  22 A,  22 B and  22 C are coupled to a corresponding one of the omni-wheels  18 A,  18 B and  18 C by corresponding one of three elongate drive pins  62 A,  62 B and  62 C. The motors  22 A,  22 B and  22 C and the drive pins  62 A,  62 B and  62 C are all coupled to the frame  14 , as will be explained in more detail below.  
         [0030]     The frame  14  comprises three support members  64 A,  64 B and  64 C, an annular member  66 , three arcuate members  68 A,  68 B and  68 C, three angled shelves  70 A,  70 B and  70 C, three link arms  72 A,  72 B and  72 C and a coupling  74 . The support members  64 A,  64 B and  64 C each have vertical portions  76 A,  76 B and  76 C 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  64 A,  64 B and  64 C also each have horizontal foot portions  78 A,  78 B and  78 C each extending from the lower ends (i.e. distal to the reference equator  60 ) of the vertical portions  76 A,  76 B and  76 C towards the Z axis. The upper ends (i.e. proximal to the reference equator  60 ) of the vertical portions  76 A,  76 B and  76 C each engage the annular member  66  at respective connection sites  80 A,  80 B and  80 C.  
         [0031]     The annular member  66  has a diameter that is slightly larger than the diameter of the spherical body  12 . The three arcuate members  68 A,  68 B and  68 C, 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  80 A,  80 B and  80 C and are coupled to the coupling  74  which lies on the Z-axis above the spherical body  12 .  
         [0032]     As best seen in  FIG. 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  18 A,  18 B and  18 C on the outer surface of the spherical body  12  thus preventing unwanted slippage between the omni-wheels  18 A,  18 B and  18 C 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  18 A,  18 B and  18 C to rotate it.  
         [0033]     Three hollow cylindrical members  98 A,  98 B and  98 C extend outwardly (away from the Z-axis) and upwardly from the vertical portions  76 A,  76 B and  76 C of the three support members  64 A,  64 B and  64 C, respectively, for telescopically receiving the drive pins  62 A,  62 B and  62 C, respectively. The lower ends of the drive pins  62 A,  62 B and  62 C engage respective horizontal foot portions  78 A,  78 B and  78 C close to the Z-axis. The outermost end portions of each of the three cylindrical members  98 A,  98 B and  98 C each engage respective lower end portions of the three angled shelves  70 A,  70 B and  70 C, upon which the motors  22 A,  22 B and  22 C are supported. Each of the shelves  70 A,  70 B and  70 C is substantially perpendicular to respective one of the cylindrical members  98 A,  98 B and  98 C. The respective upper end portions of the three angled shelves  70 A,  70 B and  70 C are coupled to the annular member  66  by a respective one of three link arms  72 A,  72 B and  72 C extending vertically upwardly from a respective one of the three connection sites  80 A,  80 B and  80 C.  
         [0034]     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.  
         [0035]     The horizontal foot portions  78 A,  78 B and  78 C of each of the support members  64 A,  64 B and  64 C are resiliently attached to the translation means  24 , which is a set of three independent orthogonal linear translation stages  104 A,  104 B and  104 C for moving the frame in directions parallel to the X, Y and Z axes, respectively. The translation stages  104 A and  104 B are linear gantry-type translation stages each comprising a pair of parallel rails  106 A; 106 B, a platform  108 A; 108 B and means  110 A; 110 B for moving said platform  108 A; 108 B along said pair of parallel rails  106 A; 106 B. The third translation stage  104 C is a vertical prismatic joint actuated by a ball-screw.  
         [0036]     It should be noted that the apparatus shown in the drawings could be mounted to or rest on any suitable surface or structure.  
         [0037]     Linear combinations of angular displacement and speed of each of the three omni-wheels  18 A,  18 B and  18 C 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  22 A,  22 B and  22 C drive each of the omni-wheels  18 A,  18 B and  18 C 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 roll 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 depend upon the weight of the spherical body  12  itself, the relative contributions of each of the omni-wheels  18 A,  18 B and  18 C and their respective contact surfaces.  
         [0038]     Simultaneously, the spherical body  12  may be moved parallel to the three translation axes by the translation stages  104 A,  104 B and  104 C. 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 .  
         [0039]     It should be noted that those skilled in the art would recognize that any suitable translation means may be used in place of the above-described translation stages  104 A,  104 B and  104 C. In addition, if no translation is desired, i.e. purely rotational displacement, the translation means may be dispensed with altogether.  
         [0040]     The spherical body and frame may be made of a rigid material or a non-rigid material.  
         [0041]     The motors  22 A,  22 B and  22 C and/or the translation stages  104 A,  104 B and  104 C 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  22 A,  22 B and  22 C and/or the translation stages  104 A,  104 B and  104 C.  
         [0042]     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  18 A,  18 B and  18 C 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  18 A.  18 B and  18 C (this effect is sometimes referred to as scrub). For example, optical feedback may be used to determine the angular displacement. Likewise, velocity detection at the omni-wheels  18 A,  18 B and  18 C may be used to determine the angular velocity of the spherical body.  
         [0043]     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.  
         [0044]     In addition, embodiments of the present invention have the additional advantage that the actuators  20  (e.g. standard DC motors  22 A,  22 B and  22 C) 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.  
         [0045]     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.  
         [0046]     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.  
         [0047]     While the invention has been described in detail in the foregoing specification, it will be understood by those skilled in the art that variations may be made without departing from the spirit and scope of the invention, being limited only by the appended claims.