Abstract:
A wheel assembly for a small-scale machine includes a pair of opposing, individually-driven wheels, associated motors and possibly one or more gear assemblies. The motors receive separate drive signals from a control electronics subsystem of the wheel assembly. The motors can be connected to the wheels via the gear assemblies. Because the wheels are separately driven, driving the two wheel assemblies at similar or different speeds drives the wheel assembly over straight or curved paths. An orientation assembly allows the orientation of the wheels to the machine to be controllably altered. A height adjusting assembly allows the distance between the wheels and a point on the machine where the wheel assembly is attached to be adjusted. The rotational axis that the wheel assembly rotates about when re-oriented need not pass through either wheel of the wheel assembly. The wheels can be driven by control signals generated by on-board control electronics.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention is directed to drive assemblies for self-propelled wheeled devices.  
         [0003]     2. Related Art  
         [0004]     Devices or machines having self-propelled wheels typically occupy two distinct points along the size spectrum. At one end of the side spectrum lie the “mega machines” or “monster machines”, as they are commonly referred to in the media. These large-scale machines are typically used to move extremely heavy and/or large loads, and thus typically have extremely large or oversized wheels and/or tires that are typically in the six to twelve foot diameter range. Such large-scale machines are disclosed in, for example, U.S. Pat. Nos. 3,197,229; 3,280,931; 3,645,406; 4,003,447; 4,599,030; 5,165,838; 5,618,151 and U.S. Pat. No. 6,328,123 and published U.S. patent application Ser. No. 2002/0175009.  
         [0005]     Because the wheels for such large-scale machines are so large, it is possible to place electromotive, hydraulic, pneumatic or other large-scale drive mechanisms directly within the hub of such wheels. Typically, at least some, if not all, of the wheels of such large-scale machines will have independent, direct-drive motors. Due to the size of such wheels, such motors may incorporate any necessary reduction gearing in the assembly provided within the hub of the wheels. Typically, such large-scale self-propelled wheels provided only limited steering ranges, although some of the more complicated designs, such as that shown in the &#39;030 patent, will allow for 360° rotation of the wheels.  
         [0006]     However, due to the complicated nature of such drive systems, it is effectively impossible to scale such drive systems down for use with small-scale self-propelled wheels. Additionally, because the power/weight ratios of such large-scale drive systems do not adequately scale down, even if it were possible to provide a scaled-down version of the drive system, the amount of power that could be obtained from such scaled-down large-scale drive systems would be inadequate for small-scale wheels.  
         [0007]     At the other end of the size spectrum lie small-scale devices or machines having self-propelled wheels, such as wheelchairs, self-propelled material pallets and platforms, mobile robots, robotic carts and the like. Such small-scale devices are disclosed, for example, in U.S. Pat Nos. 4,573,548; 4,657,104; 5,609,216 and U.S. Pat. No. 6,853,877 and in “Development of a holonomic mobile robot for mobile manipulation tasks”, by R. Holmberg et al., FSR&#39;99 International Conference on Field and Service Robots, Pittsburgh, Pa., August 1999, pp 1-6. Each of the &#39;877 patent and Holmberg is incorporated herein by reference in its entirety.  
         [0008]     Such small-scale machines typically use a simple two-wheel drive system, where each wheel has its own motor and the machine is balanced on one or more caster wheels in addition to the two driven wheels. This simple design has significant limitations, such as poor or imprecise position, velocity and/or acceleration control, and the inability to move freely in any direction. For example, typical wheelchairs cannot move sideways. Such small-scale machines are also subject to rocking about the caster wheels, which typically results in loss of traction and vehicle sway. Additionally, the pivot point of the driven wheel on the supporting surface is often collinear with the steering axis and the pivot or contact point of the wheel is often quite small. Because such small-scale self-propelled machines typically support significant loads relative to the size of the contact area, rotating the wheels about the steering axis/pivot point typically causes the wheels to slide, rather than roll on the support surface, such as a floor, thus scuffing the support surface. Scuffing not only can leave marks or other discolorations on the support surface, but can actively damage the support surface if the sliding friction of the wheel is sufficiently high.  
         [0009]     Some small-scale self-propelled machines use a synchronous drive train, which is also know in this art as a “synchro-drive”. Small-scale self-propelled machines that use a synchro-drive system typically have a minimum of three driven wheel assemblies and all of the wheel assemblies are steered and driven in unison, i.e., synchronously. That is, each of the driven wheel assemblies receive the same drive signal indicating forward or backward motion and speed and the same steering signal which indicates direction relative to a reference point on the small-scale self-propelled machine. These signals can be either electronically or mechanically transmitted to the wheel assemblies. It should be appreciated that at least three wheels or wheel assemblies are used because three points are necessary to define a plane and thus the minimum number of wheels to define a meta-stable small-scale machine.  
         [0010]     When such machine or devices that incorporate synchro-drive systems operate, the machine or device does not rotate as the wheels pivot for steering. Accordingly, such synchro-drive machines remain in the same orientation regardless of their direction of movement. Straight-line motion is improved, because steering drift is minimized by the locked geometry of the gearing. However, locked gearing also prevents many precision movements. Since the wheels cannot move independently, such synchro-drive machines cannot pivot, spin on their axis, or perform a simulated skid as a maneuver. Additionally, large volumes of space are taken up by the centralized motors, gears, belts, chains, shafts and other mechanical devices used to transmit power from the central motors synchronously to the driven wheel assemblies.  
         [0011]     In contrast to synchro-drive machines, small-scale self-propelled holonomic machines use two or more self-propelled wheel assemblies, where the overall holonomic drive system has at least one motor for each degree of freedom. Typically, such holonomic systems use independent steering and drive motors for each wheel assembly. The drive assemblies for such holonomic systems typically have three degrees of freedom for motion within a plane. As discussed in the incorporated &#39;877 patent and Holmberg, on very small machines, small, caster-type wheels are often used, while more standard or traditional sized holonomic machines use standard round wheels. In either case, the weight distribution of the supported load on each wheel assembly is an issue, as all the weight for each wheel is concentrated onto the small surface area contact patch of the wheel. Again, rotation around the steering axis, whether in synchro-drive machines or holonomic machines, typically causes scuffing and floor damage.  
         [0012]     It should also be appreciated that such small-scale self-propelled machines, whether synchro-drive machines or holonomic machines, typically omit any suspension elements and are unable to adjust the height of the support platform relative to the wheel assembly.  
       SUMMARY OF THE DISCLOSED EMBODIMENTS  
       [0013]     Typical large-scale self-propelled vehicles provide at least one drive motor for each wheel assembly. However, due to the size of the wheel assemblies, the types of drive motors used with such large-scale machines are inappropriate for use with small-scale machines. The drive systems conventionally used with small-scale self-propelled machines do not provide sufficient mobility or maneuverability for the small-scale self-propelled machines, do not provide sufficient degrees of freedom and/or are unable to avoid scuffing and/or otherwise avoiding damage to the surface on which the small-scale self-propelled machine moves.  
         [0014]     This invention provides a dual-wheel self-propelled wheel assembly.  
         [0015]     This invention further provides dual-wheel self-propelled wheel assemblies for self-propelled machines.  
         [0016]     This invention separately provides a self-propelled wheel assembly for a self-propelled machine having improved load weight distribution,  
         [0017]     This invention separately provides a self-propelled wheel assembly that is capable of rollingly pivoting about an axis point.  
         [0018]     This invention further provides a dual-wheel self-propelled wheel assembly that is able to rollingly pivot about an axis point.  
         [0019]     This invention additionally provides a dual-wheel self-propelled wheel assembly having two drive motors that allow the drive assembly to rollingly pivot about an axis point.  
         [0020]     This invention additionally provides a dual-wheel self-propelled wheel assembly having an orientation motor and two drive motors that allows the two wheels of the wheel assembly to be individually driven as dual-wheel self-propelled wheel assembly is rollingly pivoted about an axis point by the orientation motor.  
         [0021]     This invention separately provides a self-propelled wheel assembly for a small-scale self-propelled machine having a suspension system.  
         [0022]     This invention separately provides a self-propelled wheel assembly for a small-scale self-propelled machine having a suspension system having a compliant load sensorber.  
         [0023]     This invention separately provides a self-propelled wheel assembly for a small-scale self-propelled machine having a height adjusting motor that adjust a distance between a reference point on the machine and the bottom surface of the wheel of the drive assembly.  
         [0024]     This invention separately provides a small-scale self-propelled machine having a plurality of self-propelled wheel assemblies, where at least one of the self-propelled wheel assemblies can adjust a height of a reference surface of the machine relative to the ground that supports at least that drive assembly, to allow the small-scale self-propelled machine to lean or tip.  
         [0025]     In various exemplary embodiments of devices and methods according to this invention, a wheel assembly for a small-scale machine, device or load includes a pair of opposing, individually-driven wheels, associated motors and associated gear assemblies. The motors receive separate drive signals from a control electronics subsystem of the wheel assembly. The motors are connected to the wheels via the gear assemblies. Because the wheels are separately driven, the wheel assembly can be driven over a curved path by driving the two wheel assemblies at different speeds or rates.  
         [0026]     In various exemplary embodiments of devices and methods according to this invention, the wheel assembly includes some or all of an orientation drive motor, an orientation gear, an orientation drive assembly and an orientation gear assembly. The drive assembly includes a gear that is meshingly engaged with the orientation gear. When the orientation motor drives the drive assembly gear, such as through the gear assembly, the drive assembly gear walks around the circumference of the orientation gear. In various exemplary embodiments, the orientation gear is maintained at a fixed position relative to the machine, device or load supported by the wheel assembly. Accordingly, as the drive assembly gear walks around the circumference of the orientation gear, the orientation of the pair of opposing, individually-driven wheels relative to the machine, device or load supported by the wheel assembly changes.  
         [0027]     In various exemplary embodiments, the rotational axis that the wheel assembly rotates about when the drive assembly gear walks around the circumference of the orientation gear does not pass through either of the wheels of the pair of opposing, individually-driven wheels. Thus, when the wheel assembly is re-oriented by driving the drive assembly gear, the wheels can be allowed to rotate freely, or can be controllably rotated at the appropriate rate, so the wheels rotate and move along a curved path that is centered on the rotational axis that the wheel assembly. In various other exemplary embodiments, the wheel assembly can be re-oriented by driving the drive assembly gear at the same time that the opposing, individually-driven wheels are driven to move the machine, device or load supported by the wheel assembly relative to the surface they are resting on. In this situation, the machine, device or load moves along a complicated curved path resulting from the simultaneous lateral movement provided by the opposing, individually-driven wheels and the change in orientation of those wheels provided by the orientation drive motor.  
         [0028]     In various exemplary embodiments of devices and methods according to this invention, the wheel assembly includes a height adjusting motor, an associated gear, a vertical lift screw and associated vertical lift nut, a guide shaft, a thrust bearing and a bearing housing containing a centering bearing and a load bearing that are all positioned around the guide shaft, and a mounting plate that is attached to the bearing housing. The guide shaft is connected to a frame that carries the pair of opposing, individually-driven wheels, the associated motors and the associated gear assemblies, and the orientation drive motor, the orientation gear, the orientation drive assembly and the orientation gear assembly. The load of the machine, device or load that is supported by the wheel assembly is transferred from the mounting plate through the load bearings and the compliant load sensorber to the vertical lift nut, then to side plates of the frame, and then to the wheels.  
         [0029]     The centering bearing and the load bearing contained within the bearing housing allow the mounting plate to rotate relative to the guide shaft. In various exemplary embodiments, the orientation gear is at least indirectly attached to the mounting plate. In this situation, when the orientation motor drives the orientation drive gear to walk around the circumference of the orientation gear, the guide shaft and the rest of the elements attached to the frame rotate relative to the mounting plate via the load and centering bearings.  
         [0030]     In various exemplary embodiments, the relative position between the mounting plate and the frame can be adjusted by appropriately energizing the height adjusting motor. Depending on the direction the height adjusting motor is driven, the vertical lift screw is rotated relative to the vertical lift nut to move the frame closer to or farther away from the mounting plate. Because the mounting plate is attached to the machine, device or load that is supported by the wheel assembly, and the wheels on the support surface support the frame, this alters the distance between the machine, device or load and the wheels. If this distance is reduced, and the machine, device or load that is otherwise fully supported, such as by the other wheel assemblies, this will cause the wheels to be lifted from the support surface Otherwise, this will cause the machine, device or load that is supported by the wheel assembly to dip or tilt toward the support surface. If this distance is increased, and the machine, device or load is supported by this wheel assembly, the machine, device or load will be lifted or tilted away from the support surface. Otherwise, the wheels will be lowered toward the support surface.  
         [0031]     Depending on the direction(s) the two wheels are driven at, and the individual rotational rate(s)s the two wheels are driven at, the pair of opposing, individually-driven wheels can move along a straight or curved path in either forward or backward directions, and/or can rotate about a rotational axis. The location of the rotational axis can be positioned at any point between the two wheels or at any point lying along a line that is parallel to the rotational axes of the wheels. When the wheels are driven in opposite directions, the rotational axis will lie between the wheels. The position of the rotational axis will depend on the relative rotational rates of the two wheels. If the two rotational rates are equal, the rotational axis will lie at the midpoint between the two wheels. If one wheel rotates more slowly that the other wheel, the rotational axis will lie closer to that slower-rotating wheel. When the wheels are driven in the same direction, the wheel assembly will move in a straight line when the rotational rates are equal. If one wheel rotates more slowly that the other wheel, the rotational axis will lie on that side of the wheel assembly. The distance from the wheel assembly to the rotational axis will depend on the relative rotational rates and the distance between the two wheels.  
         [0032]     These and other features and advantages of various exemplary embodiments of systems and methods according to this invention are described in, or are apparent from, the following detailed descriptions of various exemplary embodiments of various devices, structures and/or methods according to this invention. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0033]     Various exemplary embodiments of the systems and methods according to this invention will be described in detail, with reference to the following FIGS., wherein:  
         [0034]      FIG. 1  is a perspective view of one exemplary embodiment of a dual-wheel self-propelled wheel assembly according to this invention;  
         [0035]      FIG. 2  is a front plan view of the dual-wheel self-propelled wheel assembly shown in  FIG. 1 ;  
         [0036]      FIG. 3  is a rear view of the dual-wheel self-propelled wheel assembly shown in  FIG. 1 ;  
         [0037]      FIG. 4  is a side plan view of the dual-wheel self-propelled wheel assembly shown in  FIG. 1 ;  
         [0038]      FIG. 5  is a cross sectional view of one wheel gear assembly taken along the line of  5  shown in  FIG. 2 ;  
         [0039]      FIG. 6  is a cross sectional view of the wheel motor, wheel gear assembly and wheel taken along the line  6  shown in  FIG. 4 ;  
         [0040]      FIG. 7  is a cross sectional view of the wheel motor, wheel gear assembly and wheel taken along the line  7  shown in  FIG. 4 ;  
         [0041]      FIG. 8  is a cross sectional view of the orientation wheel motor, orientation drive gear assembly and orientation drive/steering assembly taken along the line  8  shown in  FIG. 4 ;  
         [0042]      FIG. 9  is a side plan view of one exemplary embodiment of the height adjusting assembly and adjacent components of the dual-wheel self-propelled wheel assembly shown in  FIG. 1 ;  
         [0043]      FIG. 10  illustrates one exemplary embodiment of a self-propelled multi-wheel pallet or robotic cart using the dual-wheel self-propelled wheel assembly shown in  FIG. 1 ;  
         [0044]      FIG. 11  is a perspective view of one exemplary embodiment of a wheelchair that incorporates a pair of dual-wheel self-propelled wheel assembly shown in  FIG. 1 ;  
         [0045]      FIG. 12  is a perspective view of one exemplary embodiment of a pallet jack that incorporates the dual-wheel self-propelled wheel assembly shown in  FIG. 1  and;  
         [0046]      FIG. 13  is a perspective view of one exemplary embodiment of a 3-wheeled mobile robotic platform or robotic cart that incorporates the dual-wheel self-propelled wheel assembly shown in  FIG. 1 . 
     
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
       [0047]     As outlined above, current technologies for self-propelled wheel assemblies are either designed for very large scale industrial uses that are inappropriate for small scale uses or, if appropriate for small scale uses, have serious drawbacks. That is, current small scale self-propelled wheel assemblies for indoor mobile robots, wheelchairs, pallet jacks, platforms and the like are either insufficiently maneuverable, are too large and/or are powered inappropriately. The following discussion is directed to one exemplary embodiment of a wheel assembly useable with such indoor mobile and/or autonomous robots, robotic carts, wheelchairs, pallet jacks, robotic platforms or pallets and the like. This exemplary wheel assembly has been designed in view of a number of design considerations. It should be appreciated that other exemplary embodiments according to this invention may differ from the exemplary embodiment described herein, based at least in part on which of the following design considerations any such other exemplary embodiment has been designed in view of.  
         [0048]     In particular, the design considerations considered when developing the disclosed exemplary embodiment include that the wheel assembly be self contained, that the wheel assembly be easily removable from the larger overall assembly as a unit, that the wheel assembly can be quickly electrically and/or mechanically disconnected from the overall assembly as a unit, that the wheel assembly have a significant weight-bearing capability and specifically have, in various exemplary embodiments, a weight bearing capability of at least around 1000 pounds, that the wheel assembly have a continuous orientation rotation that is not limited, that the wheel assembly have a height adjustment feature, and/or that the wheel assembly meet certain speed and/or acceleration specifications. Because various exemplary embodiments of the wheel assembly according to this invention are intended for at least indoor use, and tethering the wheel assembly, and thus the device to which it is mounted, to an electrical outlet is generally undesirable, battery power is desirable.  
         [0049]     Various exemplary embodiments of devices that incorporate three or more wheel assemblies according to this invention, when operating under heavy load conditions, could consume on average up to about 5000 watts when driving the wheels, which, in household power terms, it is approximately 6.7 horsepower. Additionally, maximizing battery volume with a low center of gravity to avoid tipping over the overall device is also desirable.  
         [0050]     Because various exemplary embodiments are intended for indoor use, the overall size of the device to which the wheel assemblies are attached must be able to fit through doorways and be maneuverable within, and otherwise appropriate for, such indoor spaces. For example, soft surfaces like vinyl or linoleum require, for the wheel assembly to be surface-friendly, that the wheel assembly apply a low force per square inch to such floor surfaces. Because indoor use is desirable, and the width of doorways and other interior spaces is limited, limiting the width of the wheel assemblies is also desirable.  
         [0051]      FIGS. 1-4  show a side perspective view, and front, rear and side plan views, respectively, of one exemplary embodiment of a dual-wheel self-propelled wheel assembly  100  according to this invention. As shown in  FIGS. 1-4 , the dual-wheel self-propelled wheel assembly  100  includes a support structure  110  that supports the various motors or actuators, such as motors  120  and  150 , various gear assemblies, such as gear assemblies  130  and  160 , and an orientation drive/steering assembly  170 , a height adjusting assembly  180  and various electrical connections and electronics  190 . As shown in  FIGS. 1-4 , the support structure  110  includes a pair of nominally vertically-oriented side plates  111 , a pair of nominally horizontally-oriented base plates  112  that extend between the side plates  111  and one or more struts  113  that also extend between the side plates  111 . The support structure  110  also includes a central hollow guide shaft  114 , a bearing housing  115 , a centering bearing  116  and a load bearing  117 , which can be better seen in see  FIG. 9 , positioned between the guide shaft  114  and the bearing housing  115 , and a mounting plate  118 . The mounting plate  118  is useable to mount or otherwise connect or attach the dual-wheel self-propelled wheel assembly  100  to an appropriate device, such as a robotic pallet and/or cart that is to be self-propelled, a wheelchair, a pallet jack, a robotics platform, robotic cart or other mobile platform or the like.  
         [0052]     As shown in  FIGS. 1-4 , the dual-wheeled self-propelled wheel assembly  100  includes a pair of wheel motors  120  that are mounted to and extend through the side plates  111 . Each wheel motor  120  is drivingly connected through the wheel gear assembly  130  to a wheel  140 . Similarly, the orientation motor  150  is mounted via an orientation drive mount  168  of the orientation drive gear assembly  160  to the side plates  111 . The orientation motor  150  extends through the orientation drive mount  168  and is operationally connected to the orientation drive/steering assembly  170  via the orientation drive gear assembly  160 . The orientation motor  150  rotates the dual-wheeled self-propelled wheel assembly  100  about a vertical axis. In particular, this vertical axis is the axis of the guide shaft  114 .  
         [0053]     As also shown in  FIGS. 1-4 , the height adjusting assembly  180  is mounted to the lower base plate  112  of the dual-wheeled self-propelled wheel assembly  100 . The height adjusting assembly  180  extends through the upper base plate  112  and supports the orientation drive/steering assembly  170 . As discussed in greater detail below, operating the height adjusting assembly  180  allows the relative positioning between a vertical lift nut  185  and a vertical lift screw  184  to be altered or adjusted. This alters the overall distance between the mounting plate  118  and the surface on which the dual-wheeled self-propelled wheel assembly  100  rests. By adjusting the vertical lift screw  184  and thus the height of the dual-wheel self-propelled wheel assembly  100 , the dual-wheel self-propelled wheel assembly  100  can cause the robotic cart to which it is mounted to lean, to lift its wheels and the like.  
         [0054]     As shown in  FIGS. 1-4 , the orientation drive/steering assembly  170  includes a stationary steering gear  175  that is attached via a plurality of spacers  176  to the mounting plate  118 . It should be appreciated that the stationary steering gear  175  includes a plurality of gear teeth along its inner surface and interacts with a drive gear of the orientation drive gear assembly  170  which is driven by the orientation motor  150 , to cause the dual-wheeled self-propelled wheel assembly  100  to rotate clockwise or counterclockwise about the axis of the guide shaft  114  when the orientation motor  150  is activated. It should be appreciated that, due to the dual wheels  140 , the axis of rotation does not pass through the points of contact of the wheels  140  on the support surface.  
         [0055]     As shown in  FIGS. 1-4 , each of the wheels  140  includes, in part, a tire  148  that is mounted on a rim  146 . A gear hubcap  137  is associated with each wheel  140 . The gear hubcap  137  of a corresponding wheel gear assembly  130  transmits drive torque to the corresponding wheel  140 , as well as covers and protects that wheel gear assembly  130  from contamination and the like.  
         [0056]     Finally, as shown in  FIGS. 1-4 , a plurality of control electronics  194  are mounted on an electronics platform  192 . Control commands, data, signals and electrical power are provided to the control electronics  194 , as well as the wheel motors  120 , the orientation motor  150 , and an actuator of the height adjusting assembly  180  via rotational signal power coupler  196  from a remotely located controller or control device (not shown) and/or from another one of the wheel assemblies  100 . The control electronics  194  also outputs various control command and data signals back to the remotely located controller or control device, and/or to another one of the wheel assemblies  100 . It should be appreciated that the rotational signal power coupler  196  is typically implemented using slip rings, rotary electrical connectors or the like.  
         [0057]      FIG. 5  is a cross sectional view of the wheel gear assembly  130  and the wheel  140 , taken along the line  5 - 5  shown in  FIG. 2 . As shown in  FIGS. 5 and 6 , the wheel gear assembly  130  includes a hypocyclic cam  131  that is mounted around an output shaft  122  of the wheel motor  120 . The hypocyclic cam  131  has two lobes  131   a  and  131   b . Each cam lobe  131   a  and  131   b  is circular and is associated with a different one of two sets  136   a  and  136   b  of gear elements of the wheel gear assembly  130 . As shown in  FIGS. 5 and 6 , the first lobe  131   a  and the second lobe  131   b  of the cam  131  are each generally circular. However, the cam  131  is mounted to the output shaft  122  of the motor  120  such that the geometric centers of the first and second lobes  131   a  and  131   b  of the hypocyclic cam  131  are not aligned with the rotational axis of the output shaft  122 .  
         [0058]     That is, as shown in  FIGS. 5 and 6 , the center of the first lobe  131   a  of the hypocyclic cam  131  is offset from the center of the output shaft  122  of the wheel motor  120 . The second lobe  131   b  is likewise offset from the center of the output shaft  122  of the wheel motor  120 . That is, lobes  131   a  and  131   b  of the hypocyclic cam  131  are eccentrically mounted to the output shaft  122 . In various exemplary embodiments, the first and second lobes  131   a  and  131   b  are offset from the rotational axis of the output shafts along directions that are 180° apart. That is, the first and second lobes  131   a  and  131   b  are 180° apart.  
         [0059]     A hypocyclic cam bearing  132   a  is mounted around the hypocyclic cam  131 , and a hypocyclic inner gear  133   a  is mounted around the hypocyclic cam bearing  132   a . A pair of drive torque pins  134   a  and  134   b  extend through the hypocyclic inner gears  133   a  and  133   b . Each drive torque pin  134   a  and  134   b  is attached at one end to the gear hubcap  137 .  
         [0060]     As shown in  FIG. 5 , the hypocyclic inner gear  133   a  rotates within an outer hypocyclic gear  135   a  in a hypocyclic or epicyclic manner. The hypocyclic inner gear  133   a  has a plurality of gear teeth on its outer surface, while the hypocyclic outer gear  135   a  has a larger number of gear teeth on its inner edge facing the gear teeth of the hypocyclic inner gear  133   a . It should be appreciated that the axis of rotation of the hypocyclic outer gear  135   a  is collinear with, or aligned with, the rotational axis of the output shaft  122  of the wheel motor  120 . A load bearing  142  of the wheel  140  is mounted around the outside of the hypocyclic outer gear  135   a , while the rim  146  is mounted on the outside of the wheel load bearing  142 .  
         [0061]      FIGS. 6 and 7  show two different side cross sectional views through the side plate  111 , the wheel motor  120 , the wheel gear assembly  130  and the wheel  140  along the lines  6 - 6  and  7 - 7  shown in  FIG. 4 . As shown in  FIGS. 6 and 7 , the motor  120 , which is not shown in cross section, is mounted on the side plate  111 . In particular, the output shaft  122  of the wheel motor  120  extends through the side plate  111  and into the wheel gear assembly  130 . As shown in  FIGS. 6 and 7 , the wheel gear assembly includes the hypocyclic cam  131  and the gear hub cap  137  and inboard and outboard sets  136   a  and  136   b  respectively of the hypocyclic cam bearings  132   a  and  132   b , the hypocyclic inner gears  133   a  and  133   b , and the hypocyclic outer gears  135   a  and  135   b , as well as the plurality of drive torque pins  134   a  and  134   b.    
         [0062]     As shown in  FIGS. 6 and 7 , the hypocyclic cam  131  has an inboard portion corresponding to the inner set  136   a  of the hypocyclic cam bearing  132   a , the hypocyclic inner gear  133   a  and the hypocyclic outer gear  135   a  that is eccentrically offset from the output shaft  122  differently than is an outboard portion of the hypocyclic cam  131  and the outer set  136   b  of the hypocyclic cam bearing  132   b , the hypocyclic inner gear  133   b  and the hypocyclic outer gear  135   b . It should be appreciated that, in various exemplary embodiments, the inboard and outboard portions of the hypocyclic cam  131  are arranged such that the maximum deviation of those portions from the axis of rotation of the output shaft  122  are spaced from each other circumferentially by 180°. should also be appreciated that no offset between the inner and outer portions could be used, although this tends to reduce the usefulness of having the inner and outer sets  136   a  and  136   b.    
         [0063]     As shown in  FIGS. 6 and 7 , the load bearing  142  of the wheel  140  extends over both of the inner and outer hypocyclic outer gears  135   a  and  135   b . A load-bearing retainer  144  connects to the rim  146  and retains the load bearing  142  in the desired position relative to the inner and outer hypocyclic outer gears  135   a  and  135   b . Similarly, the gear hubcap  137  is connected by the drive torque pins  134   a  and  134   b  to the inboard and outboard hypocyclic inner gears  133   a  and  133   b . The gear hubcap  137  also contacts the outer edge of the load bearing  142  and the rim  146  to help maintain these elements in position and/or to dirt, dust and other contaminants from entering the wheel gear assembly  130 .  
         [0064]     It should be appreciated that the wheel gear assembly  130  is, in various exemplary embodiments, implemented using the hypocyclic gears, because the hypocyclic gears take up relatively little space laterally, i.e., along the axis of the output shafts  122  of the wheel motors  120 . As outlined above, based on the desired torque and rotational velocity of the wheels  140 , in view of the particular size and type of the motor  120 , a fairly high gear reduction ratio was required. However, as outlined above, because limiting the lateral width of the wheel assemblies, and thus reducing the minimum width of any device to which the dual-wheeled self-propelled wheel assemblies  100  are attached to, is desirable, many gear systems were inappropriate.  
         [0065]     In various exemplary embodiments of the dual-wheeled self-propelled wheel assembly  100 , powerful, compact, short and relatively light motors are desirable for at least the wheel motors  120 . Typical iron-core armature electrical motors have many problems, including long axial lengths, large radial sizes, poor power to weight ratios and large weights. Accordingly, in various exemplary embodiments of the dual-wheeled self-propelled wheel assembly  100 , at least the wheel motors  120  are implemented using a zero-winding copper core motor called the ServoDisc motor, which is available through Kollmorgen Motion Technologies Group of Cormack, N.Y. However, it should be appreciated that the wheel motors  120 , and the orientation motor  150  as well, can be implemented using direct drive motors, brushed or brushless DC motors, brushed or brushless AC motors, pneumatic motors, hydraulic motors, or any other known or later developed motor that has appropriate size, power, and/or power to weight ratio values.  
         [0066]     This motor is a brushed or brushless dc motor with a linear torque curve. In contrast, most motors have a torque curve where the torque is based on the rotational speed of the motor, where high torque is generated under low speed conditions and low torque is generated under higher speed conditions. In contrast, the torque curve in the ServoDisc motor is a line perpendicular to the torque axis such that the ServoDisc motor outputs the same torque at full speed as it does at just greater than zero rpm. In addition, ServoDisc motors have high power to weight and power to size ratios and have a short axial length. For example, various exemplary embodiments of the ServoDisc motor have axial lengths of three inches or less.  
         [0067]     It should be appreciated that, while the above-outlined description of various exemplary embodiments of dual-wheeled self-propelled wheel assemblies according to this invention discussed electric motors in detail, the motors usable with various exemplary embodiments of dual-wheeled self-propelled wheel assemblies according to this invention are not limited to such electric motors. Rather, any appropriate known or later-developed motor, such as, for example, mechanical, electrical, hydraulic and/or pneumatic motor systems can be used with dual-wheeled self-propelled wheel assemblies according to this invention.  
         [0068]     However, the gear ratio range defined by using the ServoDisc motor was problematic. In particular, using the ServoDisc motor suggested a fairly high gear reduction, potentially requiring a long gear train requiring significant space. However, as outlined above, it is desirable to minimize the lateral space of the dual-wheeled self-propelled wheel assembly  100 . In particular, while various pinion-based gear combinations, various compound gear combinations and various planetary gear combinations can be used, all these gear systems require significant space between the wheel motor  120  and the wheel  140 . While, when space constraints are not a consideration, such gear systems are perfectly acceptable, when trying to limit the width of the dual-wheeled self-propelled wheel assembly to improve its usability in interior spaces, such gear systems could be problematic. Likewise, harmonic drive gear systems could be used. However, in addition to causing wider wheel spacing, as in the other gear systems mentioned above, harmonic gears are relatively quite expensive and generally require using low profile, large-diameter bearings.  
         [0069]     Hypocyclic gearings operate similarly to harmonic drive gears but at a fraction of the cost. Hypocyclic gears operate under a spyrographic effect by having an inner gear having an axis of rotation that is offset from the axis of rotation of a drive element for that internal gear. The hypocyclic inner gear has gear teeth on its exterior surface, while the hypocyclic outer gear has gear teeth on its inner surface. The hypocyclic outer gear also has its axis of rotation collinear with the axis of rotation of the drive element.  
         [0070]     In the exemplary embodiment shown in  FIGS. 5 and 6 , the hypocyclic cam  131  has two lobes  131   a  and  131   b , such that the inside hypocyclic cam bearings  132   a  and  132   b  and the hypocyclic inner gears  133   a  and  133   b  use cam offsets that are spaced 180° apart around the circumference of the hypocyclic cam  131  i.e., are 180° out of phase. This tends to balance the forces applied to the hypocyclic cam  131  and the output shaft  122 . In the exemplary embodiment shown in  FIG. 5 , each lobe  131   a  and  131   b  of the hypocyclic cam  131  has an axis of rotation that is collinear with the axis of rotation of the corresponding hypocyclic inner gear  132   a  and  132   b , respectively, but which, like the hypocyclic inner gears  132   a  and  132   b , is offset from the axis of rotation of the output shaft  122  and the hypocyclic outer gears  135   a  and  135   b.    
         [0071]     An analysis of the gear teeth loading on a single one of the hypocyclic gear sets  136   a  and  136   b  of the exemplary embodiment shown in  FIGS. 5-7  indicated that the gear loading was within tolerances, but did not provide a large safety margin. At the same time, to keep the wheel gear assembly  130  and the wheel  140  relatively narrow, the load bearing  142  between the wheel gear assembly  130  and the wheel  140  needed to be almost as large as the rim  146 , suggesting the load bearing  142  could be used as part of the rim  146  of the wheel  140 . Accordingly, in the exemplary embodiment shown in  FIG. 5 , the outer hypocyclic ring gears  135   a  and  135   b  were modified to allow the large wheel bearing  142  to fit over them, so they act as the hub for the load bearing  142 . However, the appropriate commercially available bearing was significantly wider than each of the hypocyclic gear sets  136   a  or  136   b  alone. Accordingly, providing two hypocyclic gear sets  136   a  and  136   b  in this particular exemplary embodiment does not consume significant additional space, while reducing the gear teeth loading, resulting in a large safety margin and long average life for the gear sets  136   a  and  136   b.    
         [0072]     To transfer torque from the hypocyclic inner gears  133   a  and  133   b  to the wheel  140 , passages are provided in the hypocyclic inner gears  133   a  and  133   b . The drive torque pins  134   a  and  134   b  are mounted on the gear hub cap  137  and inserted through the passages in the hypocyclic inner gears  133   a  and  133   b  to transfer the drive torque from the hypocyclic inner gears  133   a  and  133   b  to the wheel  140 . The gear hub cap  137 , as outlined above, also encapsulates the hypocyclic gear sets  136   a  and  136   b  to protect against oil leakage and to protect against contamination by dirt and other environmental contaminants.  
         [0073]     It should be appreciated that, while the above-outlined description of exemplary embodiments of dual-wheeled self-propelled wheel assemblies according to this invention discussed hypocyclic gears in detail, dual-wheeled self-propelled wheel assemblies according to this invention are not limited to using hypocyclic gears. Rather, it should be appreciated that any known or later-developed gear structures that allow any space, weight and/or size constraints to be met can be used in place of the hypocyclic gears described above. For example, so-called “2-stage cycloidal” gears, available from Nabtesco Precision U.S.A., Novi, Mich., could be used in place of the hypocyclic gears described above.  
         [0074]      FIG. 8  is a cross-sectional view, taken along the line  8  in  FIG. 4 , of one exemplary embodiment of the orientation motor  150  and the orientation drive gear assembly  160 . It should be appreciated that the orientation motor  150  is connected via its output shaft  152  to a hypocyclic cam  161  of the orientation drive gear assembly  160 . Like the hypocyclic cam  131 , the hypocyclic cam  161  has two lobes  161   a  and  161   b  that are 180° out of phase. Similarly, like the wheel gear assembly  130 , the orientation drive gear assembly  160  has two hypocyclic gear sets  167   a  and  167   b . A pair of hypocyclic cam bearings  162   a  and  162   b  are provided between the hypocyclic cam  161  and a pair of hypocyclic inner gears  163   a  and  163   b . A pair of hypocyclic outer gears  165   a  and  165   b  are provided around, and mesh with, the hypocyclic inner gears  163   a  and  163   b , respectively. An orientation drive mount  168  is used to connect the orientation drive gear assembly  160  to the support structure  110 , as well as to connect the orientation motor  150  to the hypocyclic outer gears  165   a  and  165   b.    
         [0075]     It should be appreciated that the orientation drive gear assembly  160  operates essentially identically to the wheel gear assembly  130 . In particular, the hypocyclic drive hub  171  is connected via one or more drive torque pins (not shown) to the hypocyclic inner gears  163   a  and  163   b . The hypocyclic drive hub  171  is supported within an orientation bearing retainer  173  by a pair of orientation bearings  172 . The orientation bearing retainer  173  is also attached to the hypocyclic outer gears  165   a  and  165   b . An orientation pinion gear  174  is mounted to the outer end of the hypocyclic drive hub  171 .  
         [0076]     In operation, the orientation pinion gearing  174  meshingly engages with the gear teeth on the inner surface of the stationary steering gear  175 . When the orientation motor  150  is energized, rotation of the output shaft  152  causes the hypocyclic inner gears  163   a  and  163   b , and thus hypocyclic drive hub  171 , to rotate. Rotation of the hypocyclic drive hub  171  is transferred to the orientation pinion gear  174 . The pinion gear  174 , when rotating, causes the support structure  110  to rotate about the rotational axis established by the guide shaft  114  as the pinion gear  174  “walks” around the inside of the stationary steering gear  175 .  
         [0077]     As a result, when the orientation motor  150  is energized, the dual-wheel self-propelled wheel assembly  100  rotates about a rotational axis established by the drive shaft  114  which is spaced away from the contact points of the tires  148  on the support surface on which the dual-wheel self-propelled wheel assembly  100  rests. As a consequence, when the orientation motor  150 , along with the two wheel motors  120 , is actuated, the two wheels  140  of the dual-wheel self-propelled wheel assembly  100  rollingly rotate in opposite directions about the axis of the guide shaft  114 . Conventional wheel assemblies have their rotational axes passing through the contact points on the support surface, and thus scuff, mar, or otherwise damage the support surface due to the sliding movement of their tires on the support surface when they are re-oriented. In contrast, re-orienting the dual-wheel self-propelled wheel assembly  100  according to this invention does not mar, scuff or otherwise damage the surface on which it is resting, as the tires  148  roll as the dual-wheel, self-propelled wheel assembly  100  is re-oriented.  
         [0078]     It should also be appreciated that the orientation motor  150  can be operated in either direction and can be operated indefinitely. That is, the orientation pinion gear  174  can complete multiple circles around the interior circumference of the stationary searing gear  175  without running into any limits, stops or the like which limit the overall rotational freedom of the support structure  100  to rotate about the axis of the guide shaft  114 .  
         [0079]      FIG. 9  outlines one exemplary embodiment of the height adjusting assembly  180  and the electrical connections and electronics  190 . As shown in  FIG. 9 , in various exemplary embodiments, the guide shaft  114  is mounted to the lower base plate  112 . A combination vertical position motor and gear box  181  has a vertical lift pinion gear  182  attached to its output shaft. The vertical lift pinion gear  182  has gear teeth on its outer surface that meshingly engage with the gear teeth formed in the outer surface of a vertical lift drive gear  183 . The vertical lift drive gear  183  is threadingly engaged with a vertical lift screw  184 . In various exemplary embodiments, the vertical lift screw  184  sits around, but is able to freely rotate around, the guide shaft  114 .  
         [0080]     A vertical lift nut  185  is threaded around the vertical lift screw  184  and is connected to a height adjusting plate  119 . In various exemplary embodiments, the height adjusting plate  119  is a nut retaining plate to which the vertical lift nut  185  is mounted. In such exemplary embodiments, the vertical position of the vertical lift nut  185  is fixed. Thus, rotation of the vertical lift screw  184  inside the vertical lift nut  185  causes the vertical lift screw  184  to move up or down along the guide shaft  114  relative to the vertical lift nut  185  and the height adjusting plate  119 , depending on the direction of the rotation and the direction of the threads on the vertical lift screw and nut  184  and  185 . It should be appreciated that, in various exemplary embodiments, the vertical lift screw and nut  184  and  185  use “acme” type threads. Such acme or square threads better support the loads placed on the vertical lift screw and nut  184  and  185 . It should also be appreciated that, while acme threads are more useful in this exemplary embodiment, they are not required.  
         [0081]     As shown in  FIG. 9 , a plurality of thrust bearing washers  186  are interspersed with a thrust bearing  187 , a compliant load sensorber  188  and a precision washer  189  to form a compliant stack. The load bearing  117  and the centering bearing  116 , which are contained within the bearing housing  115  shown in  FIGS. 1-4 , are also provided around the guide shaft  114 . The guide shaft  114  slides inside the centering bearing  116  and the load bearing  117  as the vertical lift screw  184  moves up and down. It should be appreciated that, in various exemplary embodiments, the guide shaft  114 , and/or contact surfaces of the vertical lift screw  184 , the load bearing  117  and/or the centering bearing  116  are coated with a low-friction or slippery coating, such as Teflon. The guide shaft  114  and/or contact surfaces of the vertical lift screw  184 , the load bearing  117  and/or the centering bearing  116  can also be lubricated to provide for mechanical slip on their surfaces.  
         [0082]     When the dual-wheel self-propelled wheel assembly  100  is attached to a particular device, such as a pallet, a wheelchair, a pallet jack, a robotics platform or the like, the mounting plate  118  is attached to the attachment surface of the particular device. The device load supported by the dual-wheel, self-propelied wheel assembly  100 , which includes the device&#39;s mass and the mass of any materials supported by the device, are transferred through the mounting plate  118  and the bearing housing  115  to the load bearing  117  that is mounted around the guide shaft  114 . Thus, the load supported by the dual-wheel self-propelled wheel assembly  100  is transferred through the mounting plate  118 , the load bearing  117 , and the stack comprising the plurality of thrust bearing washers  186 , the thrust bearing  187 , the compliant load sensorber  188  and the precision washer  189 , to the vertical lift screw  184 . The load is then transferred from the vertical lift screw  184  to the vertical lift nut  185  and the height adjusting plate  119 .  
         [0083]     That is, the stack comprising the plurality of thrust bearing washers  186 , the thrust bearing  187 , the compliant load sensorber  188  and the precision washer  189  are located above the vertical lift screw  184  and are supported by the upper surface of the vertical lift screw  184 . The stack comprising the plurality of thrust bearing washers  186 , the thrust bearing  187 , the compliant load sensorber  188  and the precision washer  189  are located below and support the load bearing  117 . Accordingly, as the vertical lift screw  184  moves up and down inside the vertical lift nut  185 , the stack comprising the plurality of thrust bearing washers  186 , the thrust bearing  187 , the compliant load sensorber  188  and the precision washer  189  moves up and down with the vertical lift screw  184 . The load bearing  117  moves up and down with the compliant stack. Accordingly, moving the vertical lift screw  184  up and down changes the distance between the vertical lift nut  185  and the load bearing  117 .  
         [0084]     It should be appreciated that, in various exemplary embodiments, the precision washer  189  is located below the tapered load bearing  117 . The precision washer  189  fits loosely over the guide shaft  114  and provides a flat surface between the tapered load bearing  117  and the compliant load sensorber  188 . In various exemplary embodiments, the compliant load sensorber  188  is a soft non-metallic commercial O-ring or washer. Typically, O-rings have several advantages over metal springs, such as a compact size, high load ratings, low cost and wide availability. O-rings can be purchased with a wide variety of diameters, thicknesses and materials to provide a compliant interface between load elements.  
         [0085]     It should be appreciated that, in various exemplary embodiments, the compliant load sensorber or washer  188  is sized to provide the desired support for the intended load range. The compliant load sensorber  188  typically also has a sufficiently large diameter so that it does not contact the guide shaft  114  when under full compression. It should also be appreciated that, in various exemplary embodiments, the compliant load sensorber or washer  188  provides a cushioning effect for momentary load spikes. In such exemplary embodiments, the compliant load sensorber  188  acts as a spring in a suspension system, with a shock absorbing response from the friction of the guide shaft  114  as it slides inside the centering bearing  116  and the load bearing  117 .  
         [0086]     The spring rate for the compliant load sensorber  188  can be calibrated to indicate a load on the wheel assembly. This load can be determined by measuring the deflection of the compliant load sensorber  188 . As the compliant load sensorber  188  deforms over time, the control electronics  194  can be programmed to correct for this deformation, as well as to generate a signal when it is necessary or desirable to replace the compliant load sensorber  188 . A mismatched compliant load sensorber  188  can also be denoted in the control electronics  194 .  
         [0087]     It should be appreciated that, in various exemplary embodiments, the loose-fit precision thrust bearing washer  186  provides a lower contact surface below the compliant load sensorber  188 . This loose-fit precision thrust bearing washer  186  is in contact with the thrust bearing  187 , which is also sized and positioned loosely on the guide shaft  114 . The thrust bearing  187  allows any rotational forces to be decoupled away from the compliant load sensorber  188 , as allowing rotation on the compliant load sensorber  188  could lead to inaccurate measurements and wear. It should be appreciated that, in various exemplary embodiments, another optional loose fit precision thrust bearing washer  186  is located below the thrust bearing  187 . This optional loose fit precision thrust bearing washer  186  provides the thrust bearing  187  with a smooth surface to operate from.  
         [0088]     The compliant load sensorber  188  and the related thrust bearing  187  transmit the load of the supported device from the tapered load bearing  117  without imparting any direct load forces over the guide shaft  114 . These forces are transferred to the vertical lift screw  184  that slides on the outside of the guide shaft  114 . The vertical lift nut  185  transfers the supported load through the suspension elements to the tapered load bearing  117 . The vertical lift screw  184  is an active load transfer element, in that the load is directly transferred between the vertical lift nut  185  and the threads of the vertical lift screw  184 . It should be appreciated that, in various exemplary embodiments, the combination of the suspension elements and the ability to measure the load on the sensorber  188 , coupled to the vertical lift system, provides a way to control the load and height as related to an active suspension system. The suspension allows for smother vehicle ride, as bumps and vibrations are reduced. The suspension spring element that forms a load cell can provide feedback signals to the local computer and/or drive the vertical lift screw to react to the changes in the load or to needed height adjustments. Thus, the suspension spring element that forms a load cell, which form an active suspension mode.  
         [0089]     Thus, operating the height adjusting assembly  180  allows the relative position between the vertical lift nut  185  and the vertical lift screw  184  to be altered. This alters the overall distance between the vertical lift nut  185  and the tapered load bearing  117 , and thus the distance between the mounting plate  118  and the surface on which the dual-wheeled self-propelled wheel assembly  100  rests. By adjusting the relative position of the vertical lift screw  184  to the vertical lift nut  185 , and thus the height of the dual-wheel self-propelled wheel assembly  100 , the dual-wheel self-propelled wheel assembly  100  can cause the device to which it is mounted to lean, to lift the wheels  140  of the dual-wheel self-propelled wheel assembly  100  and the like. It should be appreciated that, in various exemplary embodiments, the position of the vertical lift nut  185  relative to the height adjusting plate  119  is fixed. In such exemplary embodiments, as the vertical lift screw  184  rotates, the vertical lift screw  184  moves up and down along the guide shaft  114 , pushing the stack comprising the plurality of thrust bearing washers  186 , the thrust bearing  187 , the compliant load sensorber  188  and the precision washer  189 , and the tapered load bearing  117  up and down. For this reason, in such exemplary embodiments, the vertical lift pinion gear  182  is not fully meshed with the vertical lift drive gear  183 .  
         [0090]     It should be appreciated that the gear forces of the orientation pinion gear  174 , as it interacts with the stationary steering gear  175 , are able to move the entire dual-wheel self-propelled wheel assembly  100  with sufficient torque to counteract any mis-directed wheel torques, such as that occur when one wheel slips, or when a wheel rolls over a small obstacle such as a pencil, a stone, dirt or the like. The wheel motors  120 , the orientation motor  150  and the vertical position combined motor/gear box  181  are all mounted on the rotating portion of the dual-wheel self-propelled wheel assembly  100 . This rotating portion is the portion that is able to rotate continuously around the stationary steering gear  175 . Therefore, some device or structure useable to connect signals and power from the stationary portion of the dual-wheel self-propelled wheel assembly  100  to the rotating portion is desirable. In the exemplary embodiment shown in  FIGS. 1-9 , since this exemplary embodiment uses only electrical devices, an electrical slip ring is used to connect the signal lines and power lines between these two portions of the dual-wheel self-propelled wheel assembly  100 . While optical, fiber optic, radio frequency, and electromagnetic coupling could be used, the low cost and reliability of slip rings makes them a more economical and higher reliability solution.  
         [0091]     It should be appreciated that the rotational signal power coupler  196 , which is in this exemplary embodiment preferably a slip ring, is positioned at the very top of the dual-wheel self-propelled wheel assembly  100 . In the exemplary embodiment shown in  FIGS. 1-9 , the electronics platform  192  and the control electronics  194  mounted on the electronics platform  192  are positioned directly below the rotational signal power coupler  196 . However, it should be appreciated that this is a matter of design choice only. The actual location of the control electronics  194  within the dual-wheel self-propelled wheel assembly  100  is not critical.  
         [0092]      FIG. 10  shows a robotic pallet or cart  200  that can use a number of the dual-wheel self-propelled wheel assemblies  100  according to this invention. As shown in  FIG. 10 , the robotic pallet or cart  200  includes a pallet surface  220  having a plurality of slots  210  formed in it. As shown in  FIG. 10 , one dual-wheel self-propelled wheel assembly  100  can be inserted into each of the slots  210  such that the pallet surface  220  rests on the mounting plate  118 . By bolting or otherwise attaching the pallet surface  220  to the mounting plate  118 , the various dual-wheel self-propelled wheel assemblies  100  can be securely attached to the pallet  200 .  
         [0093]     It should be appreciated that, by attaching six of the dual-wheel self-propelled wheel assemblies  100  to the pallet  200 , that the pallet  200  can be made extremely maneuverable, both laterally and vertically. In particular, by appropriately driving the orientation motor  150 , each of the six dual-wheel self-propelled wheel assemblies  100  can individually orient its wheels  140 . Thus, if it is desirable to move the pallet  200  forward along a direction A, all of the wheels  140  may be rotated so they are parallel to the direction A before the wheel motors  120  are actuated for forward motion. Alternatively, all of the orientation motors  150  can be actuated so that the wheels  140  are parallel to a direction B. Then, when the wheel motors  120  are actuated for forward motion, the pallet  200 , without changing the orientation of the pallet surface  220  in space, can be moved sideways along the direction B.  
         [0094]     However, if the orientation of the pallet surface  220  needs to be altered, such as by rotating the pallet  200  about an axis C, various ones of the dual-wheel self-propelled wheel assemblies  100  can be oriented in one direction, while other ones are oriented in other directions to allow the pallet  200  to be rotated or spun about the vertical axis C. That is, each wheel assembly  100  can be oriented at a different angle relative to some reference direction, so that the wheel assemblies  100  are all at appropriate angles, so that, as the pallet  200  pivots around the axis C, all of the wheel assemblies  100  roll around the axis C, and non of the wheel assemblies  100  need to be dragged around the axis C. Of course, any combinations of motions along the direction A, the direction B and the rotational axis C can be implemented using appropriate orientations for the wheels  140  and by dynamically changing the orientation of the wheels  140  as the various six dual-wheel self-propelled wheel assemblies  100  are driven forward or backward by actuating the wheel motors  120 .  
         [0095]      FIG. 11  illustrates one exemplary embodiment of a self-propelled wheelchair  300  that incorporates a pair of one exemplary embodiment of the dual-wheel, self-propelled wheel assemblies  100  according to this invention. As shown in  FIG. 11 , the wheelchair  300  includes a body  310 , a pair of wheel assembly controllers  320  connected to a pair of the dual-wheel self-propelled wheel assemblies  100  and a pair of casters  330 . The wheelchair body  310  includes a headrest  311 , a back  312 , a pair of armrests  313 , a seat  314 , a pair of wheel assembly mounting structures  315  and a foot rest  316 . In the exemplary embodiment shown in  FIG. 11 , the pair of wheel assembly controllers  320  are mounted on the arm rests  313 , while the dual-wheel self-propelled wheel assemblies  100  are mounted to the wheelchair  300  by securely attaching the mounting plate  118  to the mounting structures  315 . By appropriately controlling the orientation of the dual-wheel self-propelled wheel assembly  100 , and appropriately controlling the forward or backward rotation of each of the wheels  140  of each dual-wheel self-propelled wheel assemblies  100 , the wheelchair  300  can be made to move forward or backwards, sideways, or to spin about an axis extending through the seat  314  and/or the seat back  312 . In face, the wheelchair  300  can actually use any axis to rotate about, such as an axis extending through one of the wheel assemblies or even one of the front casters, such that the wheelchair  300  pivots on one of the wheel assemblies, or even on one of the front casters.  
         [0096]      FIG. 12  shows one exemplary embodiment of a pallet jack  400  that incorporates one exemplary embodiment of a dual-wheel self-propelled wheel assembly  100  according to this invention. As shown in  FIG. 12 , the pallet jack  400  includes a pallet jack body  410  and a wheel assembly controller  420 . The pallet jack body  410  includes a base  412 , a back  413 , a pair of forks  414 , a pair of braces  416  and a pair of batteries  415 , although only one battery  415  is shown. By appropriately controlling the dual-wheel self-propelled wheel assembly  100  using the wheel assembly controller  420 , the pallet jack  400  can be maneuvered forward and backward, sideways and/or rotated about a desired rotational axis. It should be appreciated that, because the rotational direction and speed of each of the wheel motors  120  can be individually controlled, a rotational axis of the pallet jack  400  can be located anywhere forward of the support member  416  and specifically does not need to be located at the rotational axis of the front fork wheels  422 .  
         [0097]      FIG. 13  illustrates one exemplary embodiment of a self-propelled robotic cart or platform  500  that can be used to support an mobile robot, one or more robotic arms or any other desired load. As shown in  FIG. 13 , the self-propelled robotic cart or platform  500  includes three of the dual-wheel self-propelled wheel assemblies  100 . As shown in  FIG. 13 , the robotic cart or platform  500  includes a platform body  510  including a base  512 , a wheel skirt  514  and a floor base  516 . Typically, the mounting plates  118  of the various dual-wheel self-propelled wheel assemblies  100  would be securely attached to the upper base  512 . However, in  FIG. 13 , the dual-wheel self-propelled wheel assemblies  100  are shown spaced away from the upper base  512  to better illustrate them. It should be appreciated that, in the exemplary embodiment shown in  FIG. 13 , the electronics platform  192  and the control electronics  194  are not located at the top of the guide shaft  114 , as in the previously described exemplary embodiments.  
         [0098]     As outlined above, the height adjusting assembly  180  incorporated into each of the dual-wheel self-propelled wheel assemblies  100  allows the distance between the mounting plate  118  and the vertical lift nut  185  to be adjusted. Because the mounting plate  118  is ultimately supported by the vertical lift nut  185 , this also causes the distance between the bottom of the tires  148  and the top if the mounting plate  118  to be adjusted. Because the mounting plate  118  is typically mounted to the device incorporating the dual-wheel self-propelled wheel assembly  100 , the height of that surface of that device, such as the robotic pallet or cart  200 , the wheelchair  300 , the pallet jack  400 , or the robotic cart or platform  500 , can be adjusted. It should be appreciated that each of the robotic pallet or cart  200 , the wheelchair  300 , the pallet jack  400 , and the robotic cart or platform  500  illustrate different uses and advantages of the height adjusting assembly  180 . In the following detailed description, reducing the height of the dual-wheel self-propelled wheel assembly  100  refers to reducing the distance between the vertical lift nut  185  and the mounting plate  118 , while increasing the height of the dual-wheel self-propelled wheel assembly  100  refers to increasing this distance.  
         [0099]     For example, in the robotic pallet or cart  200  shown in  FIG. 10 , reducing the height of one of the corner dual-wheel self-propelled wheel assemblies  100  will typically not cause the robotic pallet or cart  200  to tip, lean or incline. Rather, this is more likely to raise the tires  148  away from the support surface that the robotic pallet or cart  200  is moving over. This will allow the robotic pallet or cart  200  to maneuver over small barriers in its path, such as door thresholds, thrust offsets in concrete floors, and the like. In this case, each of the various dual-wheel self-propelled wheel assemblies  100  can be reduced in height in turn to “step” over the small barrier in the surface.  
         [0100]     If multiple ones of the dual-wheel self-propelled wheel assemblies  100  are progressively reduced in height, the robotic pallet or cart  200  will tilt, lean or incline. This can be useful in several instances. For example, if the small barrier in the floor cannot be “stepped” over but must be rolled over, by adjusting the wheels, the overall height of the robotic pallet or platform, and its load, can be maintained, so that the load does not contact an upper threshold of the doorway or some other structure that may be above it. Likewise, if the load on the pallet is slightly unbalanced, such that its balance can be improved by leaning, tilting or inclining the surface  220  of the robotic pallet or cart  200 , this can also be accomplished. Finally, if the compliant load sensorber  188  is used as a load sensor and/or if a tilt sensor or one or more additional load sensors or the like are incorporated into the robotic pallet or platform  200  and appropriately connected to the control system for the dual-wheel self-propelled wheel assemblies  100 , the heights of the various dual-wheel self-propelled wheel assemblies  100  can be dynamically adjusted, based on sensed tilts of the pallet surface  200  and/or the loads on the dual-wheel, self-propelled wheel assemblies  100 , to reduce, or ideally eliminate, such tilts or inclines and/or balance such loads, so that the load on the robotic pallet or cart  200  is not unbalanced and otherwise does not slide off the robotic pallet or cart  200 .  
         [0101]     Similarly, the heights of the dual-wheel self-propelled wheel assemblies  100  attached to the wheelchair  300  can also be adjusted to modify the tilt, lean or incline of the seat  314 , to allow the wheelchair  300  to crossover small barriers, such as door thresholds and the like, similarly to the robotic pallet or platform  200 . Because each user of a wheelchair  300  that incorporates the dual-wheel self-propelled wheel assemblies  100  will have a different use for the wheelchair  300 , and will come to the wheelchair  300  with a different disability, providing a particular tilt, lean or incline to the wheel chair  300  may make the user more comfortable and/or may make it easier or less awkward for the user to interact with his or her environment.  
         [0102]     As outlined above, the pallet jack  400  includes only a single one of the dual-wheel self-propelled wheel assemblies  100 . Thus, the ability of the pallet jack  400  to lean and/or tilt in any direction is significantly constrained. Nevertheless, adjusting the height of the dual-wheel self-propelled wheel assembly  100  incorporated into the pallet jack  400  allows the operator of the pallet jack to selectively engage and disengage with a pallet or other load to be engaged, moved and/or relocated using the pallet jack  400 .  
         [0103]     The self-propelled robotic cart or platform  500  includes three of the dual-wheel self-propelled wheel assemblies  100 , as three points allow a plane to be defined. Accordingly, by appropriately raising and lowering the height adjusting assembly  180  of each of the three dual-wheel self-propelled wheel assemblies  100 , the orientation of the self-propelled platform  500  can be altered so that it is inclined, slanted or tipped. Additionally, it should be appreciated that a sensorber  188  can measure the load experienced by the wheel assembly  100  of which it is a part. In response, the vertical lift motor  181  can move the vertical load nut  185  up and down along the vertical lift screw  184 , which can shift the load and/or adjust the load that each wheel assembly  100  experiences. Thus, a mobile vehicle were operating on a soft floor, such as one made of vinyl, this feature could be used to make sure that no one wheel assembly  100  placed too much force in one spot, which could dent the floor or cause other damage.  
         [0104]     It should further be appreciated that in various exemplary embodiments, the dual-wheel self-wheel assemblies  100  according to this invention have 4 degrees of freedom, corresponding to the two drive motors  120 , the orientation motor  150  and the height adjusting motor/gear box  181 . Furthermore, as outlined above, because each of the drive motors  120  can be independently operated in both forward and backward directions and at different rates of rotation, any individual dual-wheel self-propelled wheel assembly  100  can be made to rotate around any desired point in space. That is, the dual-wheel self-propelled wheel assembly  100  is not constrained to rotate around the rotational axis of the guide shaft  114  or a particular point of contact of the tires  148  on the support surface. Rather, for any desired point in space, that point in space can be made into a virtual axis of rotation by altering the orientation of the dual-wheel self-propelled wheel assembly  100 . By appropriately setting the rotational directions and speeds of the two wheels  140  based on their differing radial distances from the desired point in space, the dual-wheel self-propelled wheel assembly  100  will travel along a circular path relative to that desired point in space. Thus, for example, the pallet jack  400 , which includes a single one of the dual-wheel self-propelled wheel assembly  100 , can not be made to rotate around any virtual axis located at any point in space, as this motion is constrained by the other fixed wheels. However, it can rotate about any point using one or both of the fixed wheels as a reference.  
         [0105]     For mobile devices that include a plurality of the dual-wheel self-propelled wheel assemblies  100 , and no fixed non-steer able wheels, such as the robotic pallet or the robotic cart  200 , the wheelchair  300 , and/or the robotic platform or cart  500 , each of the various dual-wheel self-propelled wheel assemblies  100  can be individually oriented and the two wheel motors  120  and/or the orientation motor  150  of the various dual-wheel self-propelled wheel assemblies  100  are individually actuated or energized depending on the distance of each of the wheels  140  of that dual-wheel self-propelled wheel assembly  100  from the desired virtual axis of rotation. Any axis of rotation can be used for these mobile devices. The mobile device can use any axis of rotation ranging from about a single wheel of a dual-wheel assembly  100 , to a point located anywhere inside the mobile device, to a point located anywhere outside the mobile device. (In contrast, synchro drive-type robots use a gear train, belts or the like to keep all the wheels pointed the same direction. Thus, synchro drive-type robots cannot pivot about an axis and can only have a limited ability to steer to simulate rotation or pivoting about an axis.  
         [0106]     It should also be appreciated that each of the wheel motors  120 , the orientation motor  150  and the height adjusting motor and gear box  181  can be provided with incremental and/or absolute position encoders such that position information can be provided to either an onboard controller contained in the control electronics  194  and/or to a centralized controller via the rotational signal power coupler  196 . In particular, incremental position encoders would be appropriate for the wheels  140 , as it is generally only necessary to know the incremental movement of the wheels  140  since the last position reading. In contrast, the steering motor  150  could use an absolute encoder, as it is often desirable to know the absolute position of the lower portion of the dual-wheel self-propelled wheel assembly  100  relative to the upper portion of the dual-wheel self-propelled wheel assembly  100  that is attached to the particular mobile device or structure. Likewise, the height adjusting motor and gear box  181  should include an absolute encoder that indicates the absolute height of the height adjusting plate  119  above the lower base plate  112 .  
         [0107]     While this invention has been described in conjunction with the exemplary embodiments outlined above, various alternatives, modifications, variations, improvements and/or substantial equivalents, whether known or that are or may be presently foreseen, may become apparent to those having at least ordinary skill in the art. Accordingly, the exemplary embodiments of the invention, as set forth above, are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit or scope of the invention. Therefore, the invention is intended to embrace all known or earlier developed alternatives, modifications, variations, improvements and/or substantial equivalents.