Abstract:
A drive system base or platform, such as for a mobile robot, is disclosed having multiple caster wheels, each wheel having its own first motor for independent driving and its own second motor for independent steering. Each wheel is rotably and pivotably mounted in a separate wheel module, which includes both the driving and steering motors associated with the wheel. All of the wheel modules on the base are identical and interchangeable. The two motors of each module are mounted side by side in a vertical arrangement for compactness. Each wheel module includes a suspension for allowing each wheel to move vertically and independently relative to the base. The hub and tread of each wheel are each cast concentrically around a bevel drive gear in an offset manner to provide a wheel and bevel gear that turn more smoothly and precisely.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to drive systems, and more particularly to powered caster wheel modules used to create drive systems such as for a mobile robot platform, automated guided vehicle (AGV), forklift, or omnidirectional powered roller conveyor. 
     2. Discussion of the Prior Art 
     Mobile robots have been developed in a myriad array of configurations. In general, a mobile robot will typically have a base or platform for supporting drive systems, controllers, sensors, manipulators, and whatever equipment is needed to allow the robot to perform its desired tasks. A mobile robot base can be driven by wheels, tracks, “legs” or a variety of other means. 
     It is useful for a mobile robot to be very maneuverable. The more agile the robot, the better it can deal with changing goals, obstacles, environments, and tasks. This is even more important when a manipulator is integrated with a mobile robot. 
     The mobility of a drive system increases with the ability to control an increasing number of independent degrees of freedom. For a vehicle that moves along the ground, there are three degrees of freedom available, most commonly described as two translations and one rotation. The ability to independently control all of the degrees of freedom available in the environment means that the system is omnidirectional. The ability to independently control the acceleration of all of the degrees of freedom available in the environment means that the system is holonomic. 
     To help achieve greater mobility, one or more caster mounted wheels can be utilized that pivot about a vertical axis as well as rotate about a horizontal axis. This arrangement makes the drive system omnidirectional. Preferably, the vertical steering axis does not intersect the horizontal drive axis. This offset arrangement allows the caster wheels to drive the robot and accelerate it in any direction, making it holonomic. In other words, the drive system can always create planer omnidirectional accelerations, velocities, and displacements of the robot, rather than requiring the robot wheels in some orientations to skid or to drive forward before turning to the side. 
     A mobile robot base has been previously developed which uses four caster wheels with intersecting horizontal and vertical axes for driving and steering the base. (Although the axes intersect, the “contact patch” of each wheel is offset from the vertical steering axis, making the system non-holonomic.) The driving axes of all of the wheels are linked together by a drive belt, and are driven by a single motor. Similarly, all of the steering axes of the wheels are linked together by a drive linkage, and are actuated by a second motor. However, this type of system, known as “synchro-drive,” has several drawbacks. Because the wheels must all drive in the same direction and at same speed at any given moment, certain complex maneuvering cannot be performed. The motion-transmitting belts and linkages also add complexity and backlash to the drive train. 
     Prior art mobile robot caster wheels are typically driven by a bevel gear mounted on one side the wheel, outwardly facing and concentric with the axis of rotation. Both the bevel gear and the wheel often are standard “off the shelf” components. Mounting holes are provided through a flange on the bevel gear for receiving fasteners to secure the gear to the wheel. The bevel gear is aligned with the center of the wheel before being secured. However, inaccuracies in aligning the gear and accumulation of tolerances between the gear and the wheel bearings prevent the gear from being located in a truly concentric fashion, and from being precisely perpendicular to the rotation axis. Inaccuracies in the manufacture of the wheel and over, under, or uneven tightening of the fasteners can prevent the bevel gear from being precisely located laterally with respect to the mating pinion. Such misalignments of the bevel gear cause the gear to turn inconsistently and wear prematurely. Also, lack of concentricity precision between the wheel tread or outer wheel circumference and the axis of rotation causes the mobile robot base to run unevenly. These problems can prevent the robot from accurately maintaining its desired trajectory. 
     What is needed and is not provided by the prior art is an omnidirectional or holonomic drive system that exhibits a high degree of mobility and accuracy, yet is simple, compact and reliable. 
     SUMMARY OF THE INVENTION 
     Broadly stated, a drive system constructed according to the present invention can provide a robot, vehicle, or other device with a high degree of mobility and accuracy, yet is simple, compact and reliable. 
     In accordance with one aspect of the present invention, a drive system is provided with multiple caster wheels, each wheel having its own separate motor for driving the wheel and its own separate motor for steering. This allows each wheel to be driven and steered independently. The motion of multiple wheels can be coordinated for increased mobility. Complex linkages interconnecting the wheels are also eliminated. 
     In accordance with another aspect of the invention, each wheel is mounted in a separate wheel module, which includes both the driving and steering motors associated with the wheel. This modular arrangement of powered caster wheels allows a drive system for a mobile robot base or other device to be designed and built much easier than before. Since there are no mechanical, motion-transmitting linkages between modules, each can be built and tested independently. The easily removable modules can be extracted for maintenance. Using a common module in several places in a drive system reduces the cost of the system because of the increased number of each part. Common modules are easily replaced when damaged. Modules can be fastened to different sizes and configurations of robot bases to produce different vehicles without redesigning the drive system. The design is easily scaled to produce a powered caster wheel module of any size. 
     In accordance with still another aspect of the invention, the drive and steering motors are arranged in compact orientation such that their armatures are both vertically aligned and their outer housings are close together. This arrangement provides an efficient use of space and a compact footprint. Because the wheel module takes up less space on the mobile robot base or device, space is made available for more wheel modules or other components. 
     In accordance with yet another aspect of the invention, each wheel module is individually mounted on a resilient suspension to give a robot base a suspension. This ensures that all of the wheels maintain contact with the ground for precise motion and position tracking of the robot, and allows for smooth driving of the vehicle on uneven terrain. 
     In accordance with yet another aspect of the invention, a bevel gear is provided on one side of each wheel for driving the wheel, and bearings for rotatably mounting the wheel are located within a precision bore through the wheel. A rigid hub and resilient tire tread are cast in place around the bevel gear to form a wheel having a bevel gear and outer tire circumference that are highly concentric with the axis of rotation of the wheel. This arrangement provides for smooth rotation of each wheel and precise alignment between each bevel gear and its associated drive pinion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view showing a mobile robot base and four powered caster wheel modules constructed according to the present invention. 
     FIG. 2A is a perspective view showing the exterior components of a powered caster wheel module. 
     FIG. 2B is a perspective view showing the interior components of a powered caster wheel module. 
     FIG. 3A is a perspective view showing the components of the caster steering gear train. 
     FIG. 3B is another perspective view similar to FIG. 3A showing the components from a different angle. 
     FIG. 4A is a perspective view showing the components of the caster translation gear train. 
     FIG. 4B is another perspective view similar to FIG. 4A showing the components from a different angle. 
     FIG. 5A is a perspective view showing a mobile robot base suspended from a powered caster wheel module (with the lower housing and base shown in phantom for clarity.) 
     FIG. 5B is a perspective view showing the suspension components of a powered caster wheel module (with the lower housing shown in phantom for clarity.) 
     FIG. 5C is a perspective view similar to FIG. 5B showing just the suspension components. 
     FIG. 6 is an exploded perspective view showing a powered caster wheel module. 
     FIG. 7A is a broken-away side elevation view showing a wheel. 
     FIG. 7B is a cross-sectional view taken along line  7 B— 7 B in FIG. 7A, and shows portions of a hub mold and a tread mold in phantom. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a mobile robot base  12  is shown configured for mounting four powered caster wheel modules  14 . The four modules  14  are essentially identical, and serve to movably support robot base  12 . Base  12  provides a platform for carrying the rest of a robot (not shown), such as a main enclosure, power source, controllers, sensors, manipulators, and other such components. 
     Each powered caster wheel module  14  includes a translation motor  16 , a steering motor  18 , a main housing  20 , a lower forked housing  22 , and a wheel  24 . Each module  14  can be either rigidly mounted to base  12  with three standoffs  26 , or movably attached with a suspension, as will be further described below. Standoffs  26  (if used), main housings  20 , and motors  14  and  16  reside above base  12 , while wheels  24  and the lower portions of lower housings  22  protrude through holes  28  in base  12 . 
     Since each module  14  has its own motors, each wheel  24  may be independently driven either forward or reverse at any speed, and may be independently steered in any direction. 
     Referring to FIGS. 2A and 2B, the components of a powered caster wheel module are shown. Wheel  24  is vertically mounted in lower housing  22  to rotate about wheel shaft  30  and horizontal axis  32 . Two bearings  34  rotably support wheel hub  36  on shaft  30 . Lower housing  22  is mounted to main housing  20  by ring bearing  38  (and by a secondary bearing  39 , as will be described later), permitting lower housing  22  to rotate 360 degrees about vertical axis  40 . Horizontal axis  32  and vertical axis  40  are mutually orthogonal. Preferably, axes  32  and  40  do not intersect and are offset 2.0 cm from each another. As shown in FIG. 2B, main housing  20  and lower housing  22  carry the components that drive and steer wheel  24 , as will be described next. 
     Referring to FIGS. 3A and 3B, the steering gear train will now be described. Steering pinion  42  is attached to the shaft  44  of steering motor  18 , and engages steering gear  46 , which is attached to the top of lower housing  22 . Therefore, when steering motor  18  is energized (in either direction), lower housing  22  with wheel  24  rotates within ring bearing  38 . Steering encoder  48  is mounted to the top end of steering motor  18  and is connected to motor shaft  44  to electronically indicate to the robot&#39;s motion controller (not shown) the incremental steering movement of wheel  24 . A homing sensor  50  (shown in FIGS. 2A and 2B) has components connected to both main housing  20  and lower housing  22  to allow the motion controller to know the absolute steering position of wheel  24 , as is well know in the art. 
     Turning the steering axis not only changes the direction of wheel  24 , but also causes a displacement orthogonal to the direction of wheel  24 . This is what makes holonomic motion possible. 
     Referring to FIGS. 4A and 4B, the translation gear train will now be described. Translation pinion  52  is attached to the shaft  54  of translation motor  16 , and engages translation gear  56 . Translation reducing gear  58  is rigidly attached to gear  56 , and rotates therewith around translation idler shaft  60 . Idler shaft  60  is rigidly attached to lower housing  22 . Reducing gear  58  drives translation offset gear  62 . Offset gear  62  drives translation bevel gear pinion  64 , as gears  62  and  64  are rigidly attached to opposite ends of translation driveshaft  66 . Driveshaft  66  is rotably mounted within lower housing  22  by two bearings  68 . Translation bevel gear pinion  64  drives translation bevel gear  70 , which is attached to hub  36  of wheel  24 . Therefore, when translation motor  16  is energized (in either direction), wheel  24  is driven through the translation gear train to rotate about its horizontal rotation axis  32  to drive a portion of the robot base  12  forward or reverse in the direction that wheel  24  is steered in. As with the steering gear train previously described above, the incremental motion of the translation gear train is sent to the motion controller by translation encoder  72  mounted atop translation motor  16 . 
     Referring to FIG. 2B, it will be appreciated by those skilled in the relevant art that the steering and translation gear trains of this arrangement are not completely independent. Wheel  24  can be translated without affecting steering, but can not be steered without affecting translation. In particular, because translation drive shaft  66  is located on lower housing  22 , it moves with lower housing  22  and pivots about vertical axis  40  when wheel  24  is steered. This motion causes attached translation offset gear  62  to rotate as it orbits or “walks around” the centrally located reducing gear  58 . This in turn causes translation drive shaft  66  and translation bevel gear pinion to rotate and drive translation bevel gear and wheel  24 . This unwanted coupling of the steering and translation motions can easily be negated by the electronic motion control system. In other words, whenever steering motor  18  is energized, translation motor  16  can also be energized (or increased or reduced in speed if already energized) at a predetermined speed to compensate for the translation effect caused by steering motor  18 . 
     Referring to FIGS. 5A-5C, the suspension feature of the present invention will be described. Main housing  20  includes three vertical bores adjacent to its three apexes for receiving three guide rods  74 . Guide rods  74  are rigidly mounted to base  12  in a vertical fashion by fasteners. Bushings  76  are press fit into the bores to provide close fitting, sliding contact with guide rods  74 . This arrangement allows powered caster wheel module  14  to have a vertical travel of over an inch relative to base  12 . A resilient bumper (not shown), preferably made from urethane tubing, is located over the bottom of each guide rod  74  just above base  12  to cushion the movement of main housing  20  at the lower end of its travel. 
     Main housing  20  also includes two other vertical bores having reduced diameters at their lower ends, each for receiving and retaining a compression spring  78 . Two bolts  80  each have a shank that passes through a spring  78 , through the reduced diameter portion of one of the vertical bores, and through a hole in base  12 . A nut  82  threaded onto the bottom of each bolt  80  is tightened against base  12 , thereby captivating and compressing spring  78  between the reduced diameter of the bore and the head of bolt  80  residing in the bore. This arrangement allows base  12  to be suspended from powered caster wheel modules  14 , with each module  14  having an independent suspension. When a wheel  24  of one or more modules  14  encounters a bump or uneven ground surface, module  14  can compensate by compressing springs  78  and rising upward on guide rods  74 . 
     As previously indicated, powered caster wheel modules  14  can be mounted to base  12  in a fixed manner without a suspension. This is accomplished by mounting standoffs  26  to base  12  in place of guide rods  74  and bumpers, and removing springs  78  and bolts  80 . Module  14  is held down on standoffs  26  by shorter bolts (not shown) which replace bolts  80  in housing  20 . Everything else remains the same. 
     The above described module mounting system (either using the suspension system or standoffs  26 ) also allows each independent powered caster wheel module  14  to be quickly and easily removed and reinstalled on base  12 , such as for maintenance, repair, or replacement. Since modules  14  are not interconnected, there are no belts or linkages to remove, replace, tension, adjust, etc. To remove module  14 , electrical connections and two nuts  82  need only be removed. When module  14  is removed from base  12 , bolts  80  and springs  78  are retained in their bores by motors  16  and  18 . 
     In the preferred embodiment of powered caster wheel module  14 , main housing  20  is 8.65 inches wide, 6.56 inches deep, and 3.00 inches tall. Its generally triangular shape allows room for other components, such as rectangularly-shaped framework and batteries, to be mounted to base  12  between modules  14 . 
     Referring to FIG. 6, an exploded view of the lower housing assembly  83  is shown, providing more detail than the previous drawings. Ring bearing  38  is press fit onto turned portion  84  on lower housing  22 . Boss  85  on top of lower housing  22  serves to center steering gear  46 . Four fasteners  86  attach steering gear  46  to the top of lower housing  22 . 
     Bores  88  and  90  are provided through steering gear  46  and lower housing  22 , respectively, for receiving translation drive shaft  66 . Fastener  92  and various washers and spacers (shown but not labeled) are used to attach bevel gear pinion  64  to drive shaft  66  and to hold drive shaft  66  rotably in place. 
     Fastener  94  engages the bottom of idler shaft  60  to fixedly secure idler shaft  60  within a stepped bore through the center of lower housing  22 . Two bushings  96  and two bearings  113  are used to rotatably mount translation gear  56  and reducing gear  58  on idler shaft  60 . 
     Rubber grease boot  98  is partially and slidably received within recess  100  in wheel hub  36 , and fits between wheel  24  and one fork of lower housing  22  to cover beveled gear  70  and pinion  64 . Fins  102  and  104  formed on grease boot  98  are received within groove  106  formed on the fork of lower housing  22  to prevent grease boot  98  from rotating with wheel  24 . Fastener  108  is used to clamp down split collar  110  on the opposite fork onto split sleeve  112  to fix the preload of bearings  34  and fixedly retain wheel shaft  30 . 
     The lower housing assembly  83  is retained in main housing  20  by ring bearing  38  which is press fit into main housing  20 , and by secondary bearing  39 . Secondary bearing  39  is attached to the top of idler shaft  60  by fastener  115 , and bears against a stepped-diameter bore in main housing  20  through which idler shaft  60  passes. Ring bearing  38  carries the thrust from supporting the weight of the robot in normal operation. Secondary bearing  39  carries the thrust from supporting the weight of lower housing assembly  83  when the robot is raised such that wheel  24  leaves the ground. 
     Referring to FIGS. 7A and 7B, the preferred construction of wheel  24  will be described. Wheel  24  comprises translation bevel gear  70 , hub  36 , and tread  114 . Bevel gear  70  is preferably an “off the shelf” component with a precision bore  116 , relative to which the gear&#39;s teeth  118  have been accurately machined. Bore  116  also serves to receive wheel bearings  34  (shown in FIG. 6.) Snap ring grooves  120  are provided within bore  116  for positioning bearings  34 . Since bore  116  provides a reference both for machining teeth  118  and for locating wheel bearings  34 , teeth  118  will be positioned in a highly concentric fashion around the axis of rotation  32  and in a plane precisely perpendicular thereto. With this arrangement, bevel gear  70  will mate much more precisely with pinion  64  than will a bevel gear ring of the prior art which is bolted to the wheel hub. Also, these assembly and alignment steps are eliminated in the manufacture of wheel  24 . 
     Wheel hub  36  and tread  114  are cast in place around bevel gear  70  to provide a wheel  24  that also rotates more precisely around the axis of rotation  32 . Bore  116  of bevel gear  70  is first placed over a precision mandrel  122  in a hub mold  124  to accurately center gear  70  in mold  124 . Hub mold  124  is then filled with a hardenable resin around gear  70  and allowed to cure to form rigid hub  36  as shown. Preferably, a gravity mold process is used and the resin is a RS2920 toughened epoxy. Alternatively, a high durometer urethane or other liquid forming a rigid material can be used. 
     In a similar manner, bevel gear  70  and hardened hub  36  are placed in a tread mold  126 , and are accurately centered in mold  126  by the precise fit between gear bore  116  and mandrel  128 . Tread mold  126  is then filled with a hardenable tread material around hub  36 , preferably an 80A durometer polyurethane, and allowed to cure to form resilient tread  114 . Preferably, the outer circumference of tread mold  126  is rounded to form a crown  130  having a maximum diameter of 5.000 inches and a width of 0.630 inches. 
     To provide a more secure and dimensionally stable wheel tread  114 , perforation slots  132  are formed axially through hub  36  adjacent to its outer circumference during the hub casting process by a series of prongs  134  protruding from one half of hub mold  124 . Slots  132  are then filled by the tread material during the tread casting process. Because slots  132  are closed apertures (i.e. do not extend radially outward), the tread material formed within slots  132  helps prevent tread  114  from shifting or separating from hub  36 . Alternatively, slots  132  can be any shape of closed aperture extending axially through hub  36 . 
     The combination of mounting wheel bearings  34  directly within a precision bore  116  through bevel gear  70 , casting hub  36  directly onto gear  70 , casting tread  114  directly onto hub  36 , and providing perforation slots  132  through hub  36  to help retain tread  114 , results in a wheel assembly that runs more accurately and smoothly than powered caster wheels found on prior art mobile robots. 
     With little or no modification, the preferred embodiment of the inventive drive system described above can be used with devices other than mobile robots. For instance, the same or similar wheels and wheel modules can be used to create drive systems for automated guided vehicles (AGV&#39;s), forklifts, or omnidirectional powered roller conveyors. To create a material handling roller conveyor, the mobile robot base described above is inverted and remains stationary. Material having a generally horizontal bottom surface can then be located on the upwardly facing wheels to be rotated or translated in any horizontal direction by the wheels. 
     The above descriptions and drawings are for illustrative purposes only, and are not exhaustive of possible alternate embodiments of the invention. It is to be understood that the present invention is not limited to the sole embodiments described above and illustrated herein, but encompasses any and all variations falling within the scope of the appended claims.