Abstract:
A differential control apparatus for a self-balancing scooter includes platform interfaces, such as clamping mechanisms, configured to be rigidly connected to respective foot platforms of the self-balancing scooter, and a differential drive coupler mechanically joined to the platform interfaces to impart differential rotational motion thereto in response to a mechanical control input. A rider interface control mechanism is connected to the differential drive coupler to supply the mechanical control input in response to user control actions to control movement of the self-balancing scooter in operation.

Description:
BACKGROUND 
       [0001]    The present invention generally relates to the field of self-balancing scooters and more specifically to the field of control mechanisms for self-balancing scooters. 
         [0002]    A self-balancing scooter (SBS) is a recreational transportation vehicle or device designed to carry a single passenger. Typically, an SBS has two wheels mounted symmetrically on either side of a center line of the scooter and having a common axis of rotation. Each wheel is independently driven, and steering is accomplished by differential speed control of the two wheels. 
         [0003]    One type of SBS has a unitary body with a footrest on which the passenger/rider stands. An example is shown in U.S. Pat. No. 6,302,230 and available as the “Segway®” from SEGWAY, INC., 14 Technology Drive, Bedford, NH 03110. This form of SBS is speed-controlled by tilting the body forward or backward, and it is steered by differentially increasing the speed of an “outside” wheel relative to an “inside” wheel. In the Segway “PT” device this differential speed control is accomplished by moving a “LeanSteer Frame”, essentially a handle bar support shaft, to the left or right. 
         [0004]    Another type of SBS employs a split body with mirror-image left and right halves, with each half having a drive wheel located on a common axis outboard of the rider&#39;s feet. An example of such an SBS is disclosed in U.S. Pat. No. 8,738,278 and is known colloquially as a “hoverboard”. The left and right halves are rotationally coupled to each other, allowing relative rotation of approximately plus or minus 5° in the forward/backward direction. Each half of the hover board has its own motorized and servo controlled drive wheel for propulsion. Each wheel is driven by a brushless DC motor, controlled by rotational inputs that the rider makes to each half of the hoverboard by tilting his or her feet forward or backwards. When the feet are tilted in unison, the hoverboard moves forward or backwards in a straight line; when the feet are tilted at different angles, the hoverboard turns in the direction of the less-forward-tilted foot. If the feet are tilted by the same angle but in opposing directions, the hoverboard rotates about a vertical axis. 
       SUMMARY 
       [0005]    Disclosed is a differential control mechanism that can be used with a hoverboard-type of SBS (HSBS). The disclosed control mechanism may be configured as an after-market add-on or as a component integrated into an SBS at the time of manufacture. As an add-on, the control mechanism leaves existing design and engineering of an HSBS substantially intact. If incorporated into an HSBS by a manufacturer, it can be implemented with substantially no functional impact on existing designs. 
         [0006]    In one variation the disclosed apparatus can be designed as an accessory to be added-on to existing HSBS by, for example, the end user. In another variation the disclosed apparatus can be designed to be integral to an HSBS. In either the add-on or integral variation the disclosed apparatus can be configured to allow rapid removal and replacement of the user interface component, permitting the HSBS to quickly switch between “safe” and “sports” modes of operation. 
         [0007]    In one variation the differential control mechanism is operated by a single, centrally positioned shaft. In one instance the shaft is pushed forward/pulled back for straight line travel forward or backward respectively and rotated/twisted clockwise or counterclockwise to turn right (viz., clockwise) or left (viz., counterclockwise) respectively. In such an embodiment, the inputs to the centrally positioned shaft may be substantially identical, in both direction and magnitude, to the inputs a rider makes on a child&#39;s scooter or a bicycle. 
         [0008]    Generally in implementations, the differential control mechanism replaces the “tilt the feet forward/backward” movement of a conventional HSBS, which may be unnatural and/or awkward for users, with an alternative “steer left/steer right” and “go forward/go backward” control scheme, which may be more intuitive and natural. In addition, the mechanism generally provides a rider with an additional support point to assist with balance. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0009]    The foregoing and other objects, features and advantages will be apparent from the following description in conjunction with the accompanying drawings, in which reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles involved. Of the drawings: 
           [0010]      FIG. 1  illustrates an example of current hoverboard-type self-balancing scooters; 
           [0011]      FIG. 2  illustrates a differential control apparatus installed on a hoverboard-type self-balancing scooter; 
           [0012]      FIG. 3A  is a perspective drawing of an embodiment of the differential control apparatus in a right-steering position; 
           [0013]      FIG. 3B  is a perspective drawing of an embodiment of the differential control apparatus in the neutral steering position; 
           [0014]      FIG. 3C  is a perspective drawing of an embodiment of the differential control apparatus in a left-steering position; 
           [0015]      FIG. 4  illustrates a truncated view of a differential control apparatus installed on a hoverboard-type self-balancing scooter; 
           [0016]      FIG. 5  is a truncated, exploded view of an embodiment of the differential control apparatus; 
           [0017]      FIG. 6  is a cross-section view of an embodiment of an upper clamp portion; 
           [0018]      FIGS. 7A-7C  are sectional views illustrating a differential control apparatus in a right-, neutral-, and left-steering position respectively; 
           [0019]      FIG. 8A-8B  are perspective illustrations of a second embodiment of a differential control apparatus; 
           [0020]      FIG. 9  is an exploded view of a differential drive coupler having removable drive appendages; 
           [0021]      FIG. 10  is a perspective view of a third embodiment of the differential control apparatus; 
           [0022]      FIG. 11  is s a partially exploded view of a coupler stabilization embodiment; 
           [0023]      FIG. 12  is an exploded view of a coupler stabilization variation combined with a differential drive coupler; 
           [0024]      FIG. 13  is a sectional view of a coupler stabilization embodiment integrated within the differential drive coupler. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 1  is an illustration of a hoverboard-type self-balancing scooter (HSBS) such as is available from Swagway, of 3431 William Richardson Dr., Suite F, South Bend, Ind. 46628. An HSBS has two mirror-image left and right halves with each half having a respective foot platform  20 , the platforms being joined at their inboard edges  22  with a rotary joint and each having a drive wheel  50  located on a common axis at their outboard edges. The left and right halves are rotationally coupled to each other to allow for their foot platforms to have a relative rotation of approximately plus or minus 5° in the forward/backwards direction. Each half of the hover board has its own drive-wheel servo feedback system. The servo system(s) are controlled by rotational inputs that the rider makes to each foot platform by tilting his or her feet forward or backwards as indicated by angle A. When the feet are tilted in unison, the HSBS moves forward or backwards in a straight line; when the feet are tilted at different angles, the HSBS turns in the direction of the less-forward-tilted foot. If the feet are tilted by the same angle but in opposing directions, the HSBS will rotate about a vertical axis. The angular velocity of the wheels is proportional to the platform tilt angle. 
         [0026]    As shown in  FIG. 1 , a rider stands on an HSBS with one foot on each of the two foot platforms  20 . Controlling the HSBS requires the ability to maintain one&#39;s feet at a desired angle while standing on two independently, rotationally unconstrained foot platforms. A differential control apparatus  10  illustrated in  FIG. 2  provides a more natural and easier means for controlling the tilt angles of foot platforms  20 . Control apparatus  10 , in the illustrated variation, includes a rider interface control mechanism  100 , which connects the rider&#39;s hands to a differential drive coupler  200 , and the differential drive coupler  200 , which is disposed at the common interface location of the platforms&#39; inboard edges  22 . Control apparatus  10  further includes two platform interfaces  300 , which convey the motions of differential drive coupler  200  to each foot platform. 
         [0027]    A truncated, perspective view of one variation of control apparatus  10  is shown in  FIG. 3 .  FIG. 3B  illustrates the control apparatus in the neutral steering position. While  FIGS. 3A and 3  C show it in the left and right steering positons respectively. A detail view of this variation of control apparatus  10  as installed in the vicinity of the platforms&#39; inner edges  22  is shown in  FIG. 4  and an exploded view of this variation is illustrated in  Fig. 5 . 
         [0028]    In this variation, control apparatus  10  is operated by the rider using rider interface control mechanism  100 . Rider interface control mechanism  100  in the illustrated variation includes an extension shaft  110  and, optionally, a cross-bar handle  120 . Extension shaft  110  extends upwardly from a rider interface section  210  of differential drive coupler  200  to a convenient height for the rider. Extension shaft  110  is pushed forward to go forward, pulled backwards to go backwards, and rotated about its axis Z to steer left or right. The axis Z is also referred to herein as the “Z axis”. Cross-bar handle  120  increases the rider&#39;s torque for applying rotation about axis Z. In other variations of differential control apparatus  10 , different mechanisms may be used to allow the rider to manipulate differential drive coupler  200  without grasping coupler  200  directly. It will be noted that extension shaft  110  can be a separate element, as in the illustrated variation, or its function could be incorporated into drive coupler  200  by extending the length of rider interface section  210 . A separate extension shaft allows the designer to select different materials for the drive coupler and the extension shaft and permits the extension shaft to be removed and re-installed to allow the HSBS to be controlled alternatively by the usual foot tilting method or by use of the differential control apparatus. A separate extension shaft also allows the differential drive augmented HSBS to be stored or transported more compactly. 
         [0029]    Differential drive coupler  200  further includes a drive coupler section  220  which is the section of drive coupler  200  that transfers the motions of the coupler to the two platform interfaces  300 , where the left-side platform interface  300 L and the right-side platform interface  300 R are, in most variations, mirror images of each other matching the typical mirror image designs of the foot platforms. In the illustrated variation, drive coupler section  220  includes a cylindrical body  218  disposed coaxially with extension shaft  110  and between the left platform interface  300 L and the right platform interface  300 R. Drive coupler section  220  further includes upper drive appendages  230 U and lower drive appendages  230 L that extend radially outwardly from cylindrical body  218 . Drive appendages  230  transfer the motions of differential drive coupler  200  to the foot platforms  20  through the platform interfaces  300 . 
         [0030]    As illustrated, differential drive coupler  200  is disposed between the left and right platform interfaces and includes at least one drive appendage  230  directed toward the left and right foot platform respectively. Each platform interface  300  has one or more drive appendage seats  330  designed to engage the one or more drive appendages  230  such that a rotary motion of differential drive coupler  200  about an axis parallel to a nominal Z axis is converted into foot platform rotation about a Y axis that is parallel to the axis of rotation of the foot platforms. 
         [0031]    As shown in  FIGS. 3, 4, and 5 , in one variation the differential control apparatus  10  is designed as an aftermarket accessory that may be attached to an HSBS such as manufactured by multiple manufacturers. In one variation, platform interfaces  300  include a clamping mechanism that allows the right and left platform interfaces to be firmly attached to the appropriate foot platforms. For example, as illustrated in the exploded view of  FIG. 5 , a differential control apparatus  10  designed for an HSBS having foot platforms  20  that are cylindrical at their inboard edges  22  has platform interfaces  300  each having a clamp with two substantially semi-circular portions. An upper clamp portion  310  incorporates the drive appendage seats  330 U,  330 L and is a generally arcuate half-ring with a curved inner surface  312  matched to the radius of the foot platform at its inboard edge  22 . Similarly, a lower clamp portion  315  is an arcuate half-ring with the same inner radius surface  317 . The two clamp portions are attached to each other by a hinge  318 . The two clamp portions, when open, may be fitted around the inner edge  22  of foot platform  20 , after which they are closed around the foot platform and locked together by a fastening mechanism The locking mechanism may be, for example, a machine bolt  320  as shown in  FIG. 4 . In some embodiments the inner surfaces  312 ,  317  of the clamp portions are lined with a padding material  319  which may both protect the surface of the HSBS from being marred and may increase the friction between the clamp portions and the HSBS, reducing the compressional force required to keep the platform interfaces from slipping inside the clamp portions. 
         [0032]    The cutaway side view shown in  FIG. 6  illustrates one variation of upper clamp portion  310 . A function of upper clamp portion  310  is to properly locate the one or more drive appendage seats  330  to engage drive appendages  230  to transfer the motions of the differential drive coupler to the foot platform. The illustrated variation, as discussed above, includes two drive appendage seats; an upper appendage seat  330 U and a lower appendage seat  330 L. It will be noted that one seat, in this example lower appendage seat  330 L, is circular in cross-section while the other, in this case upper appendage seat  330 U, is generally oval in cross-section. This oval shape accommodates the greater circumferential motion of seat  330 U (relative to seat  330 L) when upper clamp portion  310  rotates about a Y axis shown as extending into the plane of  FIG. 6 . Alternatively, upper seat  330 U may be circular and lower seat  330 L may be oval. As will be discussed below, this oval shape may produce a “dead band” in speed control. 
         [0033]    One motion transferred from drive coupler  200  to the foot platforms through the platform interfaces  300  is the Z-axis rotation of the drive coupler that is converted to the differential forward-backwards pitch motion (viz., motion about the Y axis) of the foot platforms. This pitch motion is used to steer the HSBS. As was noted above, it is the difference in foot platform pitch that steers the HSBS while the average pitch determines its speed, forward or backward. Sectional views in  FIGS. 7A-7C  illustrate for one embodiment how Z-axis rotation of drive coupler  200  is converted into pure differential pitch of foot platform interfaces  300 R,  300 L, where “pure differential pitch” means interface  300 R and  300 L pitch by an equal amount but in opposing senses.  FIG. 7B  shows the apparatus in the neutral or non-turning condition while  FIGS. 7A and 7C  show the apparatus in the right-turning and left-turning conditions respectively. 
         [0034]    As shown in  FIG. 7B , in the non-turning condition lower drive appendages  230 L are aligned parallel to the Y axis, forcing platform interfaces  300 R,  300 L to be similarly aligned, that is, being at equal pitch angles. In  FIG. 7A  drive coupler section  220  has been rotated about the Z axis to dispose the right side drive appendage behind (that is, at a negative X position) the neutral position of  FIG. 7B  and to dispose the left side drive appendage ahead (positive X positon) of the neutral position of  FIG. 7B . Because the platform interfaces are coupled to their respective appendages,  FIG. 7A  shows the corresponding features of interfaces  300 R and  300 L displaced accordingly. 
         [0035]    The embodiment illustrated in  FIG. 7  may further include an integral rotation limit. Drive coupler section  220  is designed to fit in a pocket formed by a pair of open regions in the platform interfaces. In the sectional views in  FIG. 7  the edges of this pocket are visible as a leading edge  340  and a trailing edge  342 . When the HSBS is in the non-turning condition, as shown in  FIG. 7B , neither leading edge  340  nor trailing edge  342  is in contact with drive coupler section  220 . As the rider initiates a turn, the two halves of the pocket become displaced from each other such that the leading edge of one half of the pocket and the trailing edge of the other half of the pocket begin to move toward drive coupler section  220 . The tightest turning radius allowed by the design is achieved when the two edges of the pocket reach interface section  220 . As illustrated in  FIG. 7C  for a left hand turn, the minimum turning radius is achieved when lead edge  340  of the pocket in left platform interface  300 L touches drive coupler section  220 . By symmetry, the trailing edge of the pocket in right platform interface  300 R touches drive coupler section  220  at the same time. 
         [0036]    In the illustrated variation, another motion transferred from drive coupler  200  to the foot platforms through the platform interfaces  300  is rotation of the drive coupler  200  around the Y axis to effect velocity (that is, speed and direction) control. Rotation of drive coupler  200  around the Y axis is typically accomplished by displacing the top of rider interface control mechanism  100 , that is, the top of extension shaft  110 , forward or backward from its neutral position. That motion is converted to a simultaneous forward or backward pitch motion (viz., motion about the Y axis) of both foot platforms, driving the HSBS forward or backward. The successful transfer of the forward/backward extension shaft  110  motion to the platform interfaces  300  requires a substantially rigid connection between shaft  110  and interfaces  300  in the X-Z plane. In the embodiment presented above, that substantially rigid connection is provided by incorporating a second drive appendage (in this variation upper drive appendage  230 U) and upper drive appendage seat  330 U into each side of the apparatus. It will be clear to one of ordinary skill in the mechanical arts that drive coupler  200  is constrained in the X-Z plane by a pair of appendages/appendage seats linearly displaced from one another in that plane. 
         [0037]    As was noted above, as shown in  FIG. 6 , upper drive appendage seat  330 U is oval, not round like lower drive appendage seat  330 L. In this variation the upper seat is elongated into a generally oval shape to account for the greater circumferential distance travelled by upper seat  330 U, compared to lower seat  330 L, when upper clamp portion  310  rotates about the Y axis. It will be noted that, in this variation, drive coupler  200 , and by extension rider interface control mechanism  100 , when in the neutral steering position, may be rocked forward or rearwards by several degrees without imparting any change of tilt to the two foot platforms. 
         [0038]    The differential control apparatus  10  discussed above is configured to be an aftermarket accessory for an HSBS that can be installed by the end-consumer. For example, the consumer can clamp, for example, the left platform interface  300  L onto the left foot platform  20 , attach the differential drive coupler  200  by inserting either set of drive appendages  230  into the appendage seats in the left platform interface, and then lock the drive coupler  200  in place by installing the right platform interface  300 R over the protruding second set of drive appendages  230  and tightening the clamping mechanism around the right foot platform  20 R. Installation is completed by inserting control extension shaft  110  into rider interface section  210  and tightening one or more locking screws  212 . Removal of the apparatus  10  merely involves reversing these steps, leaving the HSBS in its initial condition. 
         [0039]    In another variation the differential control apparatus  10  can be configured to be an integral component of the HSBS by the manufacturer. As illustrated in  FIG. 8A  and exploded view  FIG. 8B , the integral version of differential control apparatus  10  includes platform interfaces  300  that are built directly into foot platforms  20 . The differential drive coupler  200  used in an integral differential control apparatus  10  is in all respects equivalent to the differential drive coupler discussed above. For example, in the variation shown in  FIG. 8B  the drive coupler  200  includes cylindrical body  218  having attached thereto two sets of drive appendages  230  and rider interface section  210  into which extension shaft  110  is inserted. Platform interfaces  300 , in lieu of being clamped in place by upper clamp portion  310  and lower clamp portion  315 , are instead molded directly into foot platforms  20 . The functionality of the upper clamp portion is provided by an upward directed molded projection having the requisite drive appendage seats  330  to match drive appendages  230 . 
         [0040]    In one variation of drive coupler  200  the drive appendages are cast in place or otherwise permanently attached to cylindrical body  218 . This variation provides a strong apparatus well suited to withstanding the forces required to tilt foot platforms  20  when they are carrying the weight of a rider. However, a drive coupler with permanently in place drive appendages cannot be removed easily from between the two platform interfaces  300  without either separating the platform interfaces or modifying (and thereby complicating) the platform interfaces by including some movable or removable sections to allow the drive appendages to escape the drive appendage seats, typically in an upward (Z-axis) direction. 
         [0041]    Another variation of drive coupler  200  provides an alternative means to allow the drive coupler to be removed from between the platform interfaces. As illustrated in close-up  FIG. 9 , cylindrical body  218  may include threaded attachment points  231  designed to accept threaded drive appendages  230 . The spherical ends of the threaded drive appendages are adapted to allow the drive appendages to be screwed in or out of attachment points  231  by, for example, incorporating a slot for a chisel blade screwdriver or a hexagonal hole matching a standard Allen wrench size. 
         [0042]    This variation of drive coupler  200  may be removed from between fixed platform interfaces such as are illustrated in  FIG. 8  if the platform interfaces include appendage seats that provide clearance for the spherical ends to pass. For example, as shown in  FIG. 7B , drive appendage  230 U is screwed into cylindrical body  218  while its spherical head is disposed in appendage seat  330 U. As is illustrated, appendage  230 U may be unscrewed from body  218  and will pass unimpeded through seat  330 U. 
         [0043]      FIG. 10  illustrates an alternative embodiment for the differential drive coupler and foot platform interface(s) which uses matching beveled gears to achieve differential function. That is, rotating coupler  200  clockwise causes the left foot platform to rotate downwards and the right foot platform to rotate upwards, steering the HSBS to the “right”, or clockwise, as would be expected and pushing coupler  200  fore and aft causes both foot platforms to rotate downwards or upwards appropriately. As illustrated, this embodiment includes a bevel gear  250  mounted at the end of drive coupler section  220  and two beveled gears  350  affixed to the left and right foot platforms  20 , where they function as at least one a part of the platform interfaces  300 . Note that this embodiment requires a means of coupler vertical stabilization to hold the coupler (and extension shaft  110 ) from rotating unconstrained in the X-Z plane, a rotation that is used to control the velocity of the HSBS. 
         [0044]    In some variations of a beveled-gear differential drive coupler, vertical stabilization is provided by adding a pair of drive appendages  230 U to coupler  200 , displaced along the Z axis from beveled gear  250 , along with a matching set of appendage seats  330 U on an extended section of a platform interface  300 , where the “U” or “Upper” designation has been retained to indicate that the appendages and seats are displaced in the positive Z-direction from the bevel gears  350 . 
         [0045]    As was discussed above in relationship to  FIGS. 5 and 6  and noted with regard to the variation in  FIG. 10 , drive coupler  200  vertical stabilization can be accomplished using a set of drive appendages and appendage seats displaced in the Z-direction from the primary differential drive connection between drive coupler  200  and platform interfaces  300 . As was noted, the variation disclosed in that discussion uses a substantially oval appendage seat to avoid over constraining the motion of the second drive appendage. The oval appendage seat does create a small control “dead band” wherein extension shaft  110  can move fore and aft without affecting the velocity of the HSBS. In some situations this dead band may be undesirable, in which case alternative stabilization variations may be utilized. For example, as shown in  FIG. 11 , a stabilization bushing  245  may be installed on cylindrical body  218  to constrain drive coupler  200  relative to platform interfaces  300  in the fore-aft direction only. 
         [0046]    Stabilization bushing  245  includes an annulus that surrounds cylindrical body  218  and a pair of stabilization appendages  240  which may be substantially identical to drive appendages  230  or may be simple rods, as illustrated, extending radially from bushing  245 . Stabilization appendages  240  are inserted into matching appendage seats  331  U whose design will follow from their usage. 
         [0047]    For example, as shown in  FIG. 11 , when appendages  240  are substantially rods, appendage seats  331 U are frustrated conical holes wherein the holes taper outwardly between the inward face of upper clamp portion  310  and the exterior face of upper clamp portion  310 . As shown in  FIG. 11  bushing  245  is unconstrained along the Z axis. In some variations it is desirable to use this bushing to limit the Z-axis motion of the rider interface control mechanism  100 , that is, to hold the control mechanism in place. In such variations a retaining snap ring may be installed around cylindrical body  218 , directly below the nominal position of bushing  245 , thereby limiting drive coupler  200  from pulling up and away from the platform interfaces. 
         [0048]    It should be noted that cylindrical body  218  is truncated in  FIG. 11 ; the extended portion of cylindrical body  218  could have, for example bevel gear  250  or another pair of drive appendages  230  attached. Similarly, upper clamp portions  310  are illustrated with just one set of appendage seats to focus on the stabilization function being illustrated. 
         [0049]      FIG. 12  shows another embodiment of the stabilization bushing variation. In this variation drive appendages  230  are attached to an appendage cup  235  which is locked to cylindrical body  218  in rotation by a roll pin  222  and kept attached to cylindrical body  218  by screw  236 . Stabilization appendages  240  are affixed to bushing  245 , which is free-floating on body  218 . The upper and lower appendage seats  330  can now both be round (that is, fabricated identically as drilled holes) since any differential motions between the upper and lower appendage seats are accommodated by the unconstrained stabilization bushing, as is discussed below. 
         [0050]    It will be noted that one effect of the appendages travelling in their arcs about the Z axis is that they move in and out of the X-Z plane of the appendage seats; this motion is easily accommodated by making the seats simple drilled cylindrical holes. Similarly, the effect of the appendage seats travelling in their arcs about the Y axis is that the seats move up and down by a small amount in the X-Z plane. This motion can be ignored when there is only one set of appendages since that one set of appendages, along with the cylindrical body to which they are attached, can move “up and down”, that is, toward and away from the Y axis, with the appendage seat, eliminating any binding. Further, the distances up and down that the rider interface control mechanism travels during steering an HSBS can easily be calculated to be too small to be felt by a rider as he or she twists the rider interface control mechanism  100 . 
         [0051]    On the other hand, when two sets of appendages are used, as they are in the variations of  FIGS. 12-13 , the differential up and down motion between upper appendage seat  330 U and lower appendage seat  330 L can result in the appendages binding in the appendage seats unless some additional relief is provided, as described below. 
         [0052]      FIG. 13  illustrates a cross-sectional view of another variation of the use of a second set of appendages to stabilize the rider interface control mechanism. In this variation the extra manufacturing complexity of a stabilization bushing and an appendage cup as was illustrated in  FIG. 12  is eliminated. Mechanical engineering analysis shows that the tangential distances traversed by the upper and lower appendages when they are rotated about the Z axis at radii R U  R L  are proportional to the tangential distances traversed by their respective upper and lower appendage seats as they are rotated about the Z axis at radial heights C U  and C L  respectively. That is, the loci of travel in the X-Y plane of the upper and lower appendages and the upper and lower appendage seats will be substantially identical if the condition 
         [0000]    
       
         
           
             
               
                 R 
                 U 
               
               
                 R 
                 L 
               
             
             = 
             
               
                 C 
                 U 
               
               
                 C 
                 L 
               
             
           
         
       
     
         [0000]    is met. The small differences in these loci of travel that result from the appendages travelling in arcs about the Z axis while the appendage seats are travelling in arcs about the Y axis are compensated as described above for a single appendage for motions in and out of the X-Z plane. As is clear from basic geometry, the differential “up and down” motion, AZ , of the appendage seats is 
         [0000]      ( C   U   −C   L )*(1−Cos(θ))
 
         [0000]    where θ is the angle of rotation of the foot platform. This small amount of differential motion would be enough to bind the appendages if both appendage seats were machined to be a close fit to their respective appendages. To avoid this issue, as illustrated in  FIG. 13 , in one variation lower appendage seats  330 L can be matched with a close manufacturing tolerance  345  to the radius of lower appendages  230 L; the tolerance 0. 0005″ minimum to 0. 0045″ maximum radial clearance to eliminate any perceived “sloppy” feel in the up-and-down direction while the upper appendage seats  330 U have enough clearance  350  to accommodate AZ with some additional manufacturing tolerance on the order of clearance  345 . 
         [0053]    In one alternative, the platform interface could be screwed into top surface of foot platform instead of being clamped around, or the rider interface control mechanism can be at an outboard edge of foot platform and connected to central drive coupler section by parallelogram linkages running under the feet. 
         [0054]    The devices and systems described above are provided as non-limiting examples of details of construction and arrangement of components. There may be variations of devices, systems and methods that are capable of other embodiments and of being practiced or of being carried out in various ways. For example, platform interface  300  may be screwed into the top surface of foot platform  20  instead of being clamped around the platform. Further, for example, rider interface control mechanism  200  can be attached at an outboard edge of foot platform and connected to central drive coupler section  220  by parallelogram linkages running under the feet Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” “having,” “containing,” “involving,” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.