Abstract:
A resilient wheel structure having a rim and a hub interconnected by spokes, wherein the rim is sufficiently stiff to distribute a load to all of the spokes, and wherein the wheel is able to absorb loads in both the radial and axial directions. The rim comprises an inner and outer rim portion supported by ribs, that are located co-radially with curved spokes that allow tension and compression. The wheel structure being optimally used on a mobile robot.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Serial No. 60/244,180, filed Oct. 31, 2000. 
    
    
     STATEMENT AS TO FEDERALLY SPONSORED RESERACH 
     This invention was made in part with Government support under contract MDA972-00-C-0011 awarded by the Defense Advanced Research Projects Agency. The Government may have certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a resilient wheel structure, able to withstand significant loading. In certain embodiments, the wheels of the present invention can be mounted to a vehicle to allow the vehicle to withstand significant impacts. Such a vehicle is disclosed in U.S. Pat. No. 6,263,989, which disclosure is incorporated herein by reference. 
     A number of resilient wheel designs exist in the prior art. For example, Great Britain Pat. No 1 292 928, published Oct. 18, 1972, discloses a spring-type wheel which allows resilient radial movement of the rim relative to the hub. Also, U.S. Pat. No. 4,553,577, issued Nov. 19, 1985, discloses a wheel structure that allows each spoke to come into contact with an adjacent spoke, which limits the distortion of the spokes under load. Each of these wheels is designed to absorb radial energy only and do not provide complete load distribution. 
     The wheel structures that have been used in the prior art exhibit various shortcomings, many of which are addressed by the present invention. 
     The present invention provides a modified and improved form of such a wheel particularly suited for use with delicate electronics mounted onto a mobile platform. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention a resilient wheel structure comprises a continuous, annular rim, a hub and a plurality of spokes connecting the hub to the rim, wherein the particular curvature of the spokes provide absorption of energy caused by sudden loads in any direction, and wherein the rim is sufficiently stiff to transmit the load to all of the spokes. 
     Accordingly, the present invention has several objects and advantages. 
     It is an object of the invention to provide a wheel with a rim sufficiently stiff to effectively distribute load to all of the spokes of the wheel. 
     It is an object of the invention to have a wheel such that loading is evenly distributed. It is a further object of the invention to provide a wheel able to absorb loads from impacts or shock in any direction, not merely radial loads. 
     It is an additional object of the invention to provide a resilient wheel that can be conventionally machined. 
     It is an object of the invention to provide a wheel with maximum energy absorption capacity while minimizing the weight. 
     It is an object of the invention to provide a wheel suitable for use in a mobile robot that can be deployed by being thrown into a building or dropped from a helicopter, wherein the wheels and/or wheels with tracks protect the body of the robot during high impact deployment. 
     Other features and advantages of the invention will be apparent from the following detailed description, including the associated drawings, and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an embodiment of the wheel according to the invention; 
     FIG. 2 shows an isometric view of a quarter section along line A—A of FIG. 1; 
     FIG. 3 shows a side view of the wheel shown in FIG. 1; 
     FIG. 4 an alternative embodiment of the wheel of the invention; 
     FIGS. 5A &amp; 5B show the preferred embodiment of the present invention mounted on a mobile robot. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, the wheel  10  possess a continuous rim  40 , a hub  30  and spokes  20  all integrally molded in, or machined from, a suitable high resiliency material such as impact-resistant polymers. In other embodiments, alternative materials can be used including resilient composites and certain high strength metals such as spring steel. It should be noted that in embodiments using metals, the aspect ratios of the spokes should be altered appropriately (e.g. made significantly thinner). 
     Lightweight materials like polymers are preferred in part because the mass of the wheel contributes to the energy that must be absorbed during an impact event. In addition to strength/weight considerations, cost and ease of machinability may also be considered in choosing a suitable material. 
     In certain embodiments, the rim  40  can be constructed out of one material and the spokes  20  constructed out of a different material. For example, a metal/polymer composite can be used with the stiffer metal as the rim  40  and composite for the spokes  20 . However, such a design would require a means for bonding the two materials, for example a polyurethane cast onto the metal rim. The preferred embodiment is constructed from a homogeneous material. 
     The rim  40  is designed for maximum rigidity, which allows the force from any impact or load to be distributed to the maximum number of spokes  20 . In the preferred embodiment, the distributive effects of the rim  40  are accomplished by incorporating an inner rim  44 , and outer rim  46 , and one or more ribs  52  connecting the inner rim  44  and the outer rim  46 . The ribs  52  and the relationship between the inner rim  44  and the outer rim  46  is discussed in more detail below and with reference to FIG.  2 . 
     The dimensions of the preferred embodiment were chosen based on particular criteria dictated by its use in the mobile robot shown in FIG.  5 . One of skill in the art, knowing the design criteria for the amount of energy to absorb and the specific material or materials chosen, will be able to determine the volume of material that must actively absorb the energy. Spokes that are not used actively to absorb the energy, merely add weight to the vehicle and further increase the demands on the active spokes. 
     In the preferred embodiment of the wheel  10  shown in FIG. 1, there are eighteen identical spokes  20  equispaced around the wheel  10 . Each spoke  20  extends from a rim origin point  42  at the inner rim  44  to a hub origin point  32 . 
     In the preferred embodiment, each spoke  20  can be described to have three portions, each curving about a unique point. The first spoke portion  22  of the spoke  20  is connected to the hub  30  at the hub origin point  32 . In the preferred embodiment, the first spoke portion  22  curves in a generally clockwise direction extending from the hub  30  towards the rim  40 . A second spoke portion  24  is connected to the rim  40  at the rim origin point  42 , and curves in a counterclockwise direction from the rim  40  toward the hub  30 . The third spoke portion  26  connects the first spoke portion  22  to the second spoke portion  24 , and curves in a counterclockwise direction from the hub  30  toward the rim  40 . 
     In the embodiment shown in FIG. 1, the first and second spoke portions  22  &amp;  24 , respectively, are considerably shorter than the third spoke portion  26 . In the embodiment shown, the length of the third spoke portion  26  can be greater than the hub-to-rim radial distance of the wheel  10 . 
     During a loading or impact event, in order to minimize localized stress on the wheel  10  in general and at the hub original point  32  in particular, the spoke portions  22 ,  24  and  26  are each arcs of a circle and their meeting locations are tangent to one another. 
     The dimensions of the preferred embodiment were chosen specifically for use on the mobile robot shown in FIG. 5, and one of skill in the art will be able to adjust the dimensions for use on any number of vehicles with varying load requirements. The preferred embodiment was designed to have an outer rim  46  diameter of approximately 15.5 cm and an inner hub diameter of 3.8 cm. Moreover, the wheels, in the configuration shown in FIG. 5, are required to absorb approximately 55 kg-meters of energy. In the preferred embodiment, the hub  30  thickness of approximately 6.4 mm provides adequate load transfer, leaving approximately 5.3 cm radially for both the spokes  20  and the rim  40 . In the preferred embodiment, the rim  40  is approximately 16 mm thick, and the spokes  20  are approximately 3 mm thick and approximately 37 mm in length. While a thicker rim  40  can be used for effective transmitting of a load to the spokes  20 , one of skill in the art will recognize that the spokes must have adequate length to accommodate the sufficient curvature to allow the spokes to expand and contract enough to accommodate the deformation transferred by the stiffer rim. 
     In the preferred embodiment, the radii of spoke portions  22  &amp;  24  are sufficiently large to reduce stress concentrations endemic in sharp corner interactions between the spoke portions and the rim  40  or hub  30 . One of skill in the art will recognize that, for a given diameter wheel  10 , as the radii of the first and second spoke portions  22  &amp;  24  is increased, the length of the third spoke portion  26  must be decreased (for a given curvature), resulting in less spoke length for absorbing energy. In the preferred embodiment, the first and second spoke portions  22  &amp;  24  should each be between 5-20%, preferably 15%, of the total length of spoke  20 , with the third spoke portion  26  constituting the remainder of the spoke  20 . 
     One of skill in the art will recognize that the general shape of the third spoke portion  26  is well known in the art as a structure than can deform in three axes and have no inflection points, thus providing zero stress transitions from tension to compression along the surface of the spoke. In the preferred embodiment, three axes deformations are necessary not only to absorb energy from radial shocks, but also from axial loads and side impacts. 
     In the preferred embodiment, each spoke  20  is identical; that is, the length and curvature of each first spoke portion  22  is identical, the length and curvature of each second spoke portion  24  is identical and the length and curvature of each third spoke portion  26  is identical. In other embodiments, each spoke  20  need not be identical. 
     The spokes of the preferred embodiment are of uniform thickness and have a substantially rectangular cross section. In other embodiments, the spokes  20  can be thicker along their median line than they are at the face of the wheel. Such a design may be necessary in a molded wheel in order to accommodate draft angles in the mold. 
     Although the benefits inherent in the present invention are not limited to machined wheels, in the preferred embodiment, the number, spacing and orientation of the spokes  20  have been chosen to allow for ease of machinability. For example, in the preferred embodiment shown in FIG. 1, the relatively large radii of first and spoke portions  22  and  24  allow the design to be easily machined with conventional methods such as conventional milling machines; this is advantageous in that some high-resiliency materials, such as Nyloil, an oil-impregnated cast Nylon-6, that cannot be readily molded or cast into such intricate shapes. When using such materials, conventional machining is the preferred fabrication alternative. 
     FIG. 2 shows an isometric cutaway view of slightly more than one-quarter of the preferred embodiment of the wheel  10  shown in FIG.  1 . FIG. 2, therefore, shows the detail of the preferred embodiment of rim  40 . In the preferred embodiment of the present invention, the inner rim  44  is interconnected to the outer rim  46  by a plurality of ribs  52 . In the preferred embodiment, there are an equal number of ribs  52  and spokes  20 , with the location of each rib  52  being co-radial with each spoke  20 . In other embodiments, as few as one rib  52  or as many ribs  52 , including a solid rim without ribs or approaching a solid rim, as can be accommodated may be chosen, in greater or lesser numbers than the number of spokes  20 . In addition, in other embodiments, the location of the ribs  52  need not be co-radial with the location of a spoke  20 . 
     In the preferred embodiment, the dimensions of the rim  40  were chosen based upon weight constraints, which tend to minimize the thickness of the inner rim  44  and the outer rim  46 , and the limitations of certain low-cost manufacturing techniques. In the preferred embodiment, the inner rim  44  and the outer rim  46 , and the ribs  52  are each approximately 1.6 mm thick. One of skill in the art will recognize that varying performance criteria and manufacturing techniques will allow for a great variance in the dimensions of the rim  40  and its constituent parts. 
     Using the thickness of these elements as a design constraint, an optimal distance between the inner rim. 44  and the outer rim  46  can be determined based on the desired stiffness, where rim stiffness increases by the cube of the distance. In the preferred embodiment, the distance between the inner rim  44  and the outer rim  46  —which is also a dimension of the ribs  52  —is approximately 16 mm. One of skill in the art will be able to adjust these dimensions as needed to adequately distribute the loading to all of the spokes. 
     In the preferred embodiment, the spacing and the number of ribs  52  were chosen to match the spacing and the number of spokes  20 , as the rim  40  do not provide adequate load support in other locations. Placing the ribs  52  and the spokes  20  co-radially provides the most direct load path from the surface of the wheel  10  to the spokes  20 . 
     In order to provide an even greater measure of load distribution to the plurality to spokes  20 , the preferred embodiment of the rim  40  includes a center wall  56 . As shown in FIG. 2, the center wall  56  extends from the outer rim  46  to the inner rim  44 , and is substantially perpendicular both to the inner rim  44  and the outer rim  46 . Therefore, the center wall  56  and the inner and outer rims  44  &amp;  46  from an annular I-beam, which is particularly well-suited to transmit load into each of the spokes  20 , due to its very high area moment of inertia. 
     In alternative embodiments, a center wall can be used intermittently, depending on the weight and strength requirements. In yet another embodiment, multiple center walls, preferably on each surface of the wheel, can be used. In yet other embodiments, the cross-section of the rim  30  could be a box, an “X” or other sufficiently structurally sound element. One of skill in the art will recognize that the cross-section will be chosen both for its strength-weight characteristics as well as its ease of manufacturability. 
     In the preferred embodiment, the center wall  56  is 2.5 mm thick. In other embodiments with a center wall, the center wall  56  could be as thick as the wheel  10  itself, albeit at the sacrifice of additional weight. 
     FIG. 3 shows an exterior view of the wheel  10  along a radius. In this particular embodiment, cleats  60  protrude from the outer rim  46  to engage tracks, if any. In the preferred embodiment, there are  36  cleats  60  equispaced on the outer surface of outer rim  46 . The number of cleats  60  is dependent only on the track design. Other embodiments based on an identical design can support a round wheel as easily as a track; in the case of a wheel, the tire would encompass the rim. 
     In an alternative embodiment shown in FIG. 4, a second material is used between the spokes  20  to provide additional support. Since the voids between the spokes  20  change volume as the wheel deforms, a lightweight energy absorbing material  70  can be used to fill these voids for higher overall energy absorption. Because materials can absorb three times as much energy in pure tension/compression as can be absorbed in bending, a filler material  70  provides additional support in certain embodiments. The filler material  70  should be a material more compliant than the spoke material, and preferably considerably more lightweight. Examples of such materials include, by way of example only, blow-molded polyurethane commonly used in athletic shoe soles. This design allows significant additional energy to be absorbed with little additional weight. In this alternative embodiment, the compliant filler material  70  should be affixed to the wheel material. Therefore, the filler material should also be amenable to bonding. 
     While the wheel of the present invention can be used in any number of vehicles, it is of particular utility in providing protection to a mobile robot designed to withstand significant loadings and sudden impacts. One such design is the mobile robot  80  shown in FIGS. 5A &amp; 5B, in which the electronic components  85  are housed within the track volume. The resilient wheels  10  of the present invention then provide impact resistance that allows the robot to operate even after collisions, falls, or tumbles. Furthermore, this impact resistance allows deploying the robot in a variety of ways including tossing it from a height, such as from a window or from a helicopter. 
     Although the description above contain many specificities, there should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. 
     Other embodiments of the invention are within the scope of the following claims.