Abstract:
An electric drive attachment for a wheelchair comprising a drive housing containing an electric motor, a drive wheel which touches the ground and drives the wheelchair through frictional contact with the ground and speed reducing mechanism between the motor and drive wheel, attachment structure to attach the wheelchair drive unit to the wheelchair as a pair of clamps one on each side of the frame. Also included are torque dampening systems to reduce the shock of starting the motor, a clutch which enables manual forward movement of a wheelchair without engagement of the motor and without dragging of the drive wheel, an under run wheel and positive pressurization of the gear works and area surrounding the drive wheel to keep the housing free of debris.

Description:
FIELD OF THE INVENTION 
   This invention relates to an improved electric drive attachment for a conventional wheelchair. 
   BACKGROUND OF THE INVENTION 
   Wheelchair drive units are well known accessories and fall into two distinct categories. The first category is mounted on the wheelchair and drives the tire of one or more wheelchair wheels. The second category, into which the present invention falls, is attached to the wheelchair and has one or more independent drive wheels which rest on and drive against the ground. 
   It is the action of the drive wheels on the ground which can cause the user problems when they need to negotiate kets or pot holes. While it is known to raise the drive wheel off the ground surface by the use of a cable and lever or screw jack arrangement, these structures and methods are difficult to use or result in minimal ground clearance when actuated. 
   SUMMARY OF THE INVENTION 
   According to the present invention there is provided an electric drive attachment for a wheelchair comprising a drive housing containing an electric motor, a drive wheel which touches the ground and drives the wheelchair through frictional contact with the ground and speed reducing mechanism between the motor and drive wheel, attachment structure to attach the wheelchair drive unit to the wheelchair, said attachment structure preferably being a pair of clamps one on each side of the frame, each clamp containing a pivoting hinge which can be withdrawn from the clamp by aligning a tang on the pivot with a groove in the clamp, the axis of the hinge being parallel to the axis of the wheelchair rear wheels, two connecting struts of adjustable length fitted between the pivoting hinges and the drive housing, said connecting structure being in the shape of a ‘V’ so that the drive housing sits centrally between the two clamps and is free to pivot at the clamps on the aforementioned axis and each end of the struts able to pivot in the plane of the ‘V’ so that the ‘V’ can open and close and with the ends of the struts at the drive housing incorporating meshing gear teeth so that the struts mesh with each other and therefore open equally about a centerline between them, and such that an angle between the drive wheel ground contact point and the center of the axis of the pivoting hinges, and a vertical line through the center of the pivoting hinges is about 35 degrees, a visible mark on the drive housing that, when vertical, indicates the aforementioned angle is correct. A flexible length adjustable connector that ties the two struts to each other near the clamps thereby limits the extent to which the struts can open and limiting the forces the struts can impose on the frame. A lifting strap of adjustable length is provided which attaches to the drive unit and has a loop at the other end that slips over and is secured by a wheelchair handle. The lifting strap incorporates a shortening device that effectively shortens the length of the lifting strap when activated to thereby raise the drive housing and drive wheel off the ground. The lifting strap also affording a way to quickly raise the drive housing and drive wheel substantially clear of the ground by simply pulling upwards. The lifting strap also acts to stop the drive unit and drive wheel from under running the wheelchair by restricting the degree to which the drive unit can pivot about the pivoting hinges. An optional anti under run roller can also be utilized to further limit the angle which the drive unit can achieve with respect to the ground surface. 
   Also included are torque dampening systems to reduce the shock of starting the motor, a clutch which enables manual forward movement of a wheelchair without engagement of the motor and without dragging of the drive wheel, an anti under run roller that limits the degree to which the wheelchair drive unit can under run the wheelchair and positive pressurization of the gear works and area surrounding the drive wheel to keep the housing free of debris. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings. Further details of its configuration, construction, and operation will be best further described in the following detailed description, taken in conjunction with the accompanying drawings in which: 
       FIG. 1  shows in perspective the wheelchair drive unit fitted to an unfolded wheelchair and in a position ready for use and illustrating a horizontal strap extending between the strut pivot connection positions; 
       FIG. 2  shows a perspective a cut away view of the drive unit seen in  FIG. 1  from a bottom perspective, with a bottom housing of the drive unit removed to expose the internals; 
       FIG. 3  shows a perspective detail view of the wheelchair drive unit fitted to a folded wheelchair to illustrate how the wheelchair drive unit folds with the wheelchair; 
       FIG. 4  shows a detail view of the clamp and pivot hinge for connecting the wheelchair drive unit to the wheelchair in the locked position; 
       FIG. 5  shows a detail view of the clamp and pivot hinge seen in  FIG. 4 , but shown in the unlocked position; 
       FIG. 6  shows a simplified side view of a wheelchair taken along line  6 — 6  of FIG.  1  and illustrating the angle formed by the pivot hinge, drive unit, its drive wheel and the ground surface; 
       FIG. 7  shows a perspective view of a lifting strap with the lifter in its normal position; 
       FIG. 8  shows the lifting strap with the lifter positioned in the drive wheel raised position; 
       FIG. 9  shows a rear perspective of a switch with pivotable lockout for fitting adjacent a wheelchair handle; 
       FIG. 10  is a perspective view of a flexible battery support with an attachment strap; 
       FIG. 11  is a rear perspective view of a wheelchair and illustrating the use of the soft battery case and suspension support; 
       FIG. 12  is an exploded perspective view of the clutch which uses pivoting engagement members to enable the drive wheel to be moved forward powered by the motor as well as moved forward passively when the motor is not operating; 
       FIG. 13  is a side schematic view showing the pivoting pawls in an unengaged position; 
       FIG. 14  is a side schematic view showing the pivoting pawls in an engaged position; 
       FIG. 15  is an exposed upper perspective view of the gear box of the drive unit and illustrating the use of a radial fan for pressurizing the internals of the gear box; 
       FIG. 16  is an exploded view of the components of a radial shock reduction system to cushion initial starts of the motor against the drive wheel; 
       FIG. 17  is a second embodiment a modified version of the radial shock reduction system to cushion initial starts of the motor against the drive wheel which depends upon the construction of the drive wheel; 
       FIG. 17B  is a third embodiment of a shock reduction system to cushion initial starts of the motor against the drive wheel and which depends upon the construction of the drive wheel; 
       FIG. 18  is a view of the electrical and control schematic illustrating the rate of rise and rate of fall circuit employed in conjunction with the drive system; 
       FIG. 19  illustrates an analog realization of the block diagram of  FIG. 18  as a circuit which automatically provides protective features for the motor. 
       FIG. 20  shows in perspective view the wheelchair drive unit fitted to the horizontal frame of an unfolded wheelchair of a type having a large wheel pivoting higher up on the wheelchair; 
       FIG. 21  shows in plan view the tie strap used to limit the opening of the struts; 
       FIG. 22  shows in side elevation the anti under run roller in its normal position; and 
       FIG. 23  shows in side elevation the anti under run roller in its operational position, acting to limit the forward movement of the drive unit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , a wheelchair  1  is shown as having an attached wheelchair drive unit  2  having an outer drive housing  3 , attachment clamps  4 , and adjustable struts  5  which position the drive unit  2  at the center of the wheelchair  1 . Wheelchair  1  is of the small wheel type where the rear wheel has a smaller diameter and an axis of pivot mounted lower with respect to the chair occupant. 
   Adjustable struts  5  are each assembled from a clamp half strut  6  and a drive housing half strut  7 . The length of each strut  5  can be locked off via wing nut  8  and a bolt (not shown). Drive wheel  9  is seen at the bottom of drive unit  2 . Also note, as is seen in a numbering sequenced for  FIG. 10 , a tie strap  170  is seen as being connected to each clamp half strut  6  near the clamp  4  via connecting shackles  171 . An adjuster  175  allows the tie strap  170  to be shortened or lengthened to suit different wheelchair  1  sizes and may limit the extent to which struts  5  can open. 
   Wheelchair  1  is seen as having near vertical frame members  10  which will hereinafter be referred to as frame  10  as it is the aspect of the frame to which the wheelchair drive unit  2  attaches as seen in FIG.  1 . The attachment may differ where a different wheelchair  1  is utilized. A lifting strap  11 , adjustable in length by strap adjuster  13 , can be used with an adjuster (discussed more fully below) and is shown as extending from the wheelchair drive unit  2  outer drive housing  3  to a wheelchair handle  12 . Wheelchair handle  12  can be any structure which can secure lifting strap  11 , and the wheelchair handle  12  was chosen in the view of  FIG. 1  due to its prominence and engage ability. 
   Lifting strap  11  is shown as having two stable positions. The configuration of lifting strap  11  and lifter actuator  13  is to provide a mechanically advantaged method of disengaging the drive unit  2  from the ground so that such disengaging lifting can be lifted by the wheelchair occupant easily. 
   The wheelchair unit  2  is also made for quick and easy complete disengagement from the wheelchair  1 . Clamp half strut  6  is connected to a removable pivot  14 . The removable pivot has a cylindrical portion extending into attachment clamp  4 . Withdrawal of the removable pivot  14  from the attachment clamp  4 , along with simple disconnection of the lifting strap  11  from the wheelchair handle  12  will complete the mechanical detachment from the wheelchair  1 . 
   Generally, with reference to  FIG. 1 , the attachment clamps  4  which house removable pivots  14  are each fastened to the wheelchair frame  10  using standard fasteners (not shown). The pivots  14  are free to rotate within the clamps  4 . The clamp half strut  6  is also able to pivot within the pivot  14 . 
   Referring to  FIG. 2 , an exposed bottom side view of the drive unit  2  is seen with a bottom portion of a housing case (not seen) removed. Prominently seen is a motor  15  with a speed reducing mechanism  16  which may include belts and gears as shown. Meshing gears  17  are seen connecting the drive housing half struts  7  so that they angularly expand about the drive unit  2  at its midpoint. As such that attachments of the struts  5  to any object will automatically center the drive unit  2  at the center of objects, and in this case the wheelchair  1  when the struts  5  are adjusted to have equal length. For wheelchair  1 , centering will cause the wheelchair  1  to be driven from the center and should drive the wheelchair  1  straight unless some other force or factor is at play. 
   Also seen is a inner drive housing  18  which provides the physical support for the motor  15 , and speed reducing mechanism  16 . A set of pivot supports not shown anchor the meshing gears  17  to the outer drive housing  3 . For clarity, electrical details have been omitted. 
   Generally, with regard to  FIG. 2 , the drive housing half strut  7  is able to pivot within the drive housing  2  and operate the meshing gears  17  at the end of each drive housing half strut  7 . This ensures that both halves pivot at the same angular rate with respect to the drive housing  2 .  FIG. 2  also shows the general compact relationship of drive wheel  9 , motor  15  and speed reducing mechanism  16  within the cut away inner drive housing  18 . 
   The effect of pivots  14  is to allow the drive unit  2  and struts  5  to pivot up and down relative to the road surface. The effect of the pivots at the end of the clamp half struts  6  the pivots at the end of the drive housing half struts  7  and the meshing gears  17  is to allow the two wheelchair frames  10  to be brought together in order to fold the wheelchair whilst maintaining the drive unit  2  centrally between the two attachment clamps  4 . 
   Referring to  FIG. 3 , a closeup view of the drive unit  2  is seen looking toward the rear side of the wheelchair  1  of  FIG. 1 , which is shown in  FIG. 2  in folded condition. Most wheelchairs  1  are set to fold in a manner which brings the vertical frame members together in a parallel approach relationship. This is shown in  FIG. 3  as the vertical frame members  10  are shown in close proximity to each other. 
   As can be seen, the bringing together of the vertical frame members  10  causes the removable pivots  14  to be brought together and the struts  5  brought to a generally parallel orientation. A lower portion of the lifting strap  11  is seen, with the remaining upper portion of the lifting strap removed for clarity and not shown in FIG.  3 . The dimensionality shown has the vertical frame members  10  brought close but still far enough to accommodate the width of the struts  5  and the attachment clamps  4 . Where a wheelchair  1  has a closer fold, differently dimensioned attachment clamps  4  may be provided to give a greater clearance. The electrical connections into the drive unit  2  are also shown as cut, truncated, or removed as both a switch connection and a power connection and will be discussed in further details. To illustrate quick removability, removable pivots  14  and a lifting strap  11 , as well as the not yet mentioned on/off switch, battery pack and cables are not shown.  FIG. 3  emphasizes the folded wheelchair  1  in which the struts  5  lie parallel to each other with the drive unit  2  maintained centrally between the attachment clamps  14 . 
   Referring to  FIG. 4 , a closeup view of a portion of the vertical frame member  10  at the point of connection of the attachment clamp  4  is shown. The removable pivot  14  is shown from an end on view as having a tang  20  not aligned with a key or recess  21  in the clamp  4 . The removable pivot  14  cannot therefore be withdrawn while the tang  20  is out of alignment with the key or recess  21 . The tang  20  cannot be brought to a position of disengagement unless the drive unit  2  is swung extremely below the wheelchair  1 , as by tilting what would have to be an empty wheelchair forward to enable the drive unit  2  to achieve a low angle. As a result, the drive unit  2  is in stable position so long as the wheelchair is occupied. Further, because of the positioning of the optional anti under run roller  53  shown later in  FIGS. 21 and 22 , if the wheel  9  begins to under run the wheelchair  1  as it pivots on pivots  14 , the anti under run roller  53  comes into contact with the road surface and reduces the contact force between the wheel  9  and the ground leading to the wheel  9  slipping against the ground instead of gripping against the ground. 
   Referring to  FIG. 5 , a closeup view of a portion of the vertical frame member  10  at the point of connection of the attachment clamp  4  is shown while the clamp half strut  6  is in a position nearly parallel with the vertical frame member  10 . This has enabled the tang  20  to align with the keyhole recess  21  such that the removable pivot  14  is enabled to disengage the attachment clamp  4 . As can be seen from the angle, and in respect of  FIG. 1 , the drive unit  2  would have to be brought so far below as to be in alignment, or nearly in alignment with the vertical frame members  10 . Where attachment to frame members which are other than vertical are desired, clamps  4  which have a differently located key or recess  21 . 
   Generally with regard to  FIG. 5 , the pivot  14  is emphasized from an end on view with its tang  20  aligned with the recess  21  in the attachment clamp  4 . The two pivots  14  can therefore be withdrawn simultaneously in order to more quickly and easily remove the wheelchair drive unit  2  from the wheelchair  1 . 
   Referring to  FIG. 6 , a profile side view of the wheelchair  1  is seen with the drive unit in place with its drive wheel  9  shown to the rear of the ground contact made with a large rear wheel  22 . An angled line is drawn from the removable pivots  14  generally along side the struts  5  and to the point of contact of the drive wheel  9 . An angle alpha is shown as taken between the angled line through the contact point of the drive wheel  9  on the ground and a vertical line taken from the center of the pivot  14  through to the ground. 
   It is clear that the attachment clamp  4  can attach to the vertical frame member  10  at various points along its vertical extent, such that a low attachment would result in a larger angle alpha and a higher attachment along vertical frame member  10  would result in a smaller angle alpha. This angle alpha is very important as the vertical force component Fg that pushes the drive wheel  9  down onto the ground is equal to F multiplied by cosine (alpha) where F is the force produced by the wheelchair drive unit  2  along the struts  5 . 
   By having an angle alpha of about 35 degrees, the wheelchair drive unit  2  does not rely solely upon gravity to maintain sufficient grip to prevent slippage between the drive wheel  9  and the ground. The minimum angle alpha can be defined as the arctangent of the coefficient of friction between the rubber drive wheel tire  9  and the ground, any angle less than this will ensure that there is sufficient grip although an angle approaching zero would create a larger force Fg than is necessary. To ensure the angle alpha is optimized a mark  27  on the wheelchair drive unit indicates that the angle is correct when the mark  27  is visually positioned by the installer to assume a vertical condition. Again, the installer who is initially positioning the attachment clamps  4  up and down the vertical frame members  10  and adjusting the length of struts  5  can manually adjust the attachment clamps  4  as the mark  27  is observed to obtain the optimum position. 
   Referring to  FIG. 7  the top portion of the lifting strap  11  is shown with the lifter actuator  13  seen as a rigid member connected to one side of two generally vertical portions of the lifting strap  11 , and is shown in the normal stable first position in which the drive wheel  9  is seen resting on the ground. A loop  22  is used to attach the lifting straps to another structure such as the wheelchair handle  12  which was shown in FIG.  1 . 
   Four components of the lifting strap  11  include upper components  23  and  25  as well as lower components  28  and  29 . Components  23  and  28  are generally delineated by the point  26  at which the end of the lifter actuator  13  is attached. Components  25  and  29  are generally delineated by the presence of a stop  31  which is a doubling over of the strap material as shown in  FIG. 7 , or some other blocking structure which will prevent further downward movement of the upper curved end  33  of the lifter actuator  13 . 
   The use of the lifter actuator  13  enables a relatively weak person to apply a lifting force to the lifting strap  11  by applying a much lesser downward force to the lifter actuator  13 . Referring to  FIG. 8  the application of this downward force is illustrated. The top of the lifter strap  11  is shown with the lifter actuator  13  in the horizontal second stable position to place the lifter strap  11  in an overall raised position to lift the drive unit  2  upwardly. Upward movement of the drive unit  2  while the wheelchair is on a flat ground surface will disengage the drive unit  2  from the ground. This, in turn, raises the drive wheel  9  off the ground. 
   In  FIG. 8 , the lifting actuator  13  is shown in a position after the curved end  33  has been pushed until it rests against the stop  31  while the connected end of the lifter actuator  13  pivots about its connection to the junction of portions  23  and  28 , about a loop  26 . The four components  23 ,  25 ,  28  and  29  now form a diamond shape, the net effect of which is to shorten the overall length of the lifting strap  11 . 
   In the installation the wheelchair drive unit  2  is fitted to the wheelchair  1  using the clamps  4 . The struts  5  are then adjusted until the drive unit  2  is centrally located between the clamps  4  and the indicator mark  27  is generally as vertical as possible while the wheelchair is on flat ground. Adjustment of the struts  5  with horizontal wheelchair members may be accomplished with the clamps  4  or with other types of clamps, and where other members are used, care should be taken to insure that proper mounting can be effectuated. With the lifting actuator  13  in the normal, non-lifted position shown in  FIG. 7 , the loop  22  is affixed to the handle  12  and the lifting strap  11  is then adjusted until there is no significant slack in it. 
   Once the above adjustment has been made, to raise the drive wheel  9  and drive unit  2  off the ground the lifting actuator  13  is moved to the horizontal, raised position as is shown in FIG.  8 . To drop the drive wheel  9  and wheelchair drive unit  2  back onto the ground the lifting actuator  13  is moved back to the normal position seen in FIG.  7 . The lifter actuator  13  can be manufactured in a variety of lengths and widths to affect lifting over a variety of height differentials and with a variety of different widths and thicknesses of lifting strap  11 . 
   While operating the wheelchair under powered movement and in forward motion, to clear a curb stone or other obstruction the drive wheel  9  and wheelchair drive unit  2  can be raised instantly and substantially by pulling up the lower strap  24  directly by hand, if necessary. Raising the drive unit  2  by hand will minimize the shock and impact to the drive unit  2 . However, for small step downs or step ups, the forward momentum of the wheelchair  1  and the flexibility of the drive unit  2  should minimize any disruption. 
   To quickly remove the wheelchair drive unit  2  from the wheelchair  1  the lifting strap  11  is disconnected by loosening the lifting strap  11  and removing loop  22  from the wheelchair handle  12 . The wheelchair drive unit  2  can then be dropped down until angle alpha is approximately zero typically by up tilting the wheelchair as by raising it or tilting it forward to provide clearance to achieve such a zero angle. Once the zero angle is achieved, the tangs  20  on removable pivots  14  then align with recesses  21  and the removable pivots  14  can then be easily withdrawn from the attachment clamps  4 . The wheelchair drive unit  2  can then be withdrawn from the wheelchair  1  typically after disconnecting a located on/off switch and battery to be shown below. 
   Referring to  FIG. 9 , a closeup of handle  12  illustrates a plunger switch assembly  41  which includes a base housing  43  connected by an electrical cord  45 . Base housing  43  has a plunger  47  which includes a vertical member  49  and an angled thumb support  51 . To activate the switch assembly  41  to an “on” position the thumb support  51  is depressed gently driving the vertical member  49  into the base housing  43 . 
   The position of the thumb support  51  is generally overlying the handle  12 . In its full downward travel, the thumb support  51  has a lower surface which rests on either the handle  12  or a portion of the base housing  43  which overlies the handle  12 . In this configuration there is good support underneath the thumb support  51  to guard against any damage from undue or inadvertent thumb pressure. As can be seen by the arrow, the plunger  47  is rotatable to bring the thumb support  51  from its position over the handle  12  and generally to a position parallel with the handle  12 , either forward or rearwardly in direction. In this position the plunger  47  is locked out of an ability to close the switch to activate the wheelchair drive unit  2 . This feature will prevent accidental triggering of the plunger  47  and inadvertent turning on of the wheelchair drive unit  2 . 
   Referring to  FIG. 10 , a view of an open cloth bag  61  surrounds a battery  63 . The cloth bag  61  has a long strap  65  two or more short straps  66  and a pair of flaps  67  and  69  fitted with areas of hook and loop members  68  to ensure that the battery  63  is secured. A connector  71  is provided to fit closely onto the battery  63  and has an overbite portion to fit onto the battery terminals and to form a locking relationship with the battery  63  and battery cable  62 . The close fit of the connector enables the flaps  67  and  69  to be brought tightly over the top of the battery  63  to both securing the battery  63  within the cloth bag  61  and thus further securing the connector  71  and the short length of the battery cable  62  onto the battery  63 . 
   Referring to  FIG. 11 , a rear view of the wheelchair  1  shows the cloth bag  61  containing the battery  63  in place beneath the seat area of wheelchair  1 . The short straps  66  are used to suspend the battery  63  and cloth bag  61  from any convenient horizontal support  75  on wheelchair  1 . The long strap  65  can then be used to pull the battery  63  and cloth bag  61  away from any tire  70  that may impinge on it by looping over any further convenient horizontal support  75  on wheelchair  1  and pulling it taught enough to take up the slack. The cloth bag  61  and straps  65 ,  66  extend generally between horizontal supports  75  on the wheelchair  1 . However, the straps  66  provide a configuration such that the battery  63  and cloth bag  61  can be attached or suspended from the wheelchair  1  in any manner which will give it secure support. 
   Referring to  FIG. 12 , an exploded view of a drive assembly  81  includes an ultimate drive gear  83  having drive teeth  85  and a space  87 . Within the space  87  are a series of raised circumferentially inwardly directed engagement structures  89 . Each of the engagement structures  89  has a circumferentially inwardly predominant axial cam surface  90  with respect to the overall axis of depth of the space  87 . The axial depth enables a wider profile on the engagement structure  89  for engagement by an associated one of a series of pawls  91 . The pawls  91  are individually pivotally attached via pivot pin  95  pivoting in pawl hole  96  in clutch plate  111 . Each pawl  91  has a cam follower half  101  and a load bearing half  97  that includes a cam following face  98 . The pawls  91  are individually pivotally attached via pivot pin  95  pivoting in pawl hole  96  in clutch plate  111 . Each pawl  91  has a cam follower half  101  and a load bearing half  97  that includes a cam following face  98 . Each of the pawls  91  may preferably be made from a polymer material and have a distant curved end  103  which interfits with a curved space  105  in one side of the engagement structure  89 . The cam follower half  101  of pawl  91  and the cam following face  98  of the load bearing half  97  of the pawl  91  both contact the cam face  90  of the engagement structure  89  with causes the pawl  91  to be moved between two positions. Bearing  93  fits into ultimate drive gear  83  to locate it onto a common axle not shown. Bearing  107  fits into clutch plate  111  to locate it onto a common axle not shown. 
   The cam face  90  is ramped so that force engagement cannot be had from rotation in the other direction. This enables, as will be shown, the wheelchair drive unit  2  to “over run” or put another way, enables the drive wheel  9  to turn in the forward direction when the motor  15  is off. This is useful when the wheelchair is being pushed forward. Otherwise, the lifter actuator  13  would have to be lifted each time that non-driven forward movement was desired. Otherwise, a positive connection with the drive wheel  9  would cause it to drag if the wheelchair  1  is pushed forward when the motor  15  isn&#39;t running. If the ground surface is rough, a dragging drive wheel  9  could cause flat spots and loss of continuous drive ability. 
   The clutch plate  111  supports wheel drive transfer pegs  113  which fit through matching apertures of the drive wheel  9  (not shown in FIG.  12 ). A central hub  115  is a support about which the drive wheel  9  fits. 
   Referring to  FIG. 13 , a semi-sectional view taken from the vantage point of line  13 — 13  of  FIG. 12  illustrates a schematic orientational view of the pawls  91  prior to engagement into its respective curved space  105 . It can be seen that if the pawls  91  move counterclockwise with respect to drive gear  83  and taken in respect of the views of  FIGS. 13 and 14 , the pawls  91  will circle the space  87  traveling in a counterclockwise direction and bump against, and pass over the engagement structures  89 . This will cause the cam follower half  101  will come into contact with the cam face  90  which will move the pawl  91  to a position in which cam follower face  98  will contact the cam face  90  if the drive wheel  9  continues to rotate causing the pawl to move back to its original position relative to clutch plate  111  not shown. 
   However, should the direction of travel of the pawls  91  be reversed, the distant curved ends  103  of the pawls  91  will slide along the cylindrical inner wall of the space  87  and into engagement with their next closest respective curved space  105 . This is shown in FIG.  14 .  FIG. 14  illustrates the position of the pawls  91  during mechanical driving. If the distant ends  103  of pawls  91  were not in the correct position to engage with the next respective curved space  105 , the action of the cam follower half  101  and cam following face  98  against the cam face  90  will move the pawl  91  into a suitable position as the pawl  91  rotates due to the driving motion of motor  15  via speed reducing mechanism  16 . 
   Referring to  FIG. 15  a view of an upper quarter quadrant of the outer drive housing  3  shown removed to illustrate structure utilized in producing a pressurized gearbox effect. A radial fan  121  is seen along an upper extent of the inner drive housing  18 . Radial fan  121  has with curved, pressure inducing blades. The radial fan  121  is mounted directly on the motor  15  along with the Pinion. The motor  15  is also shown connected to a drive sprocket  123  by a belt  125 . Thus, when the wheelchair drive unit  2  is powered and moving forward, the radial fan  121  will operate. The action of the fan is to pressurize the gearbox portion of the inner drive housing  18  to about 2 millibar of pressure. This pressurization action reduces the amount of dirt that can enter the gearbox and is achieved by having the radial fan  121  draw air into the gearbox space through a vent  129  consisting of a series of slots in the outer housing  3  and a hole in the inner drive housing  18 . The exit point for the air is the space immediately between the drive wheel  8  and the inner drive housing  18  and through bleed holes  92  in the ultimate drive gear  83  (seen in FIG.  12 ), thereby blowing out dirt that may otherwise find work itself into the inner drive housing  18  and drive assembly  81 . Holes (not shown) in the inner drive housing  18  also allow air to stream over the hot motor  15  and hot electronic components not shown. 
   In the closeup drawing of  FIG. 15 , also seen are a few other features, including a charging jack  139  (shown partially covered) which enables easy access for the recharging of the attached battery  63  seen in FIG.  10 . 
   Referring to  FIG. 16 , a perspective view of the hub  161  of drive wheel  9  and illustrating the use of a shock absorbing hub  151  with polymeric or rubber inserts  153  in order to limit the transmission of starting and stopping type rotational shocks from the drive wheel  9 , via the hub  151 , is shown. Each of the rubber inserts  153  includes a large opening  155  superimposed at one side of an oval cross sectional shape. 
   The rubber inserts  153  are also shown as having a smaller bore  159  which will help control the deformation of the rubber insert  153  in the direction which it will be compressed between the wheel drive transfer pegs  113  and the forward side of the associated oval opening  157  of hub  151 . As can be seen, there are five rubber inserts  153  matching five oval openings  157  in hub  151 . The direction of force turning is counterclockwise from the perspective of FIG.  16 . Initial turning tends to compress slightly the five rubber inserts  153  to create a much gentler beginning motion. Once the rubber inserts  153  are initially compressed under the instant starting force, and once forward motion starts, the rubber inserts  153  will decompress slightly as forward motion starts. 
   Referring to  FIG. 17 , a second possible embodiment includes a drive wheel  9  material made of a shock absorbing material which removes the need for individual rubber inserts  153 . A drive wheel  161  is made of softer material, especially an intermediate layer  163  which connects a hub  165  and an outer layer  167 . In this case, the hub  165  includes circular drive transfer bores  169  for directly accepting the drive transfer pegs  113 . 
   Referring to  FIG. 17B , a third possible embodiment includes a drive wheel  9  incorporating a hub  151  that has oval openings  157  within it. The rubber material of the drive wheel  9  extends to fill the oval openings  157  thereby eliminating the need for separate inserts. A smaller bore  159  will help control the deformation of the rubber in oval opening  157  of hub  151  in the direction which it will be compressed between the wheel drive transfer pegs  113  and the forward side of the associated oval opening  157  of hub  151 . 
   Referring to  FIG. 18 , an overall schematic illustrating a an over current, over temperature, under voltage, soft start and rate of rise/rate of fall circuit shown within an overall schematic  201 . Where items are named which were seen in earlier figures, the original numbering will be used. The overall block schematic of  FIG. 18 , as well as the circuitry diagram of  FIG. 19  give a control system which can both detect and react to a set reaction conditions, individually or collectively, and which include one or more of an over current condition, a rate of fall condition, a rate of rise condition, a high temperature condition and an under voltage condition 
   BATTERY  63  is connected to motor  15  via MOTOR CURRENT SENSOR block  205  and controlled via Motor Switch block  203 . When a power switch  204  (seen physically as the plunger switch assembly  41 ) is switched on, an UNDER VOLTAGE DETECTOR block  209  begins to monitor the voltage of Battery  63  and switches the motor  15  off if battery voltage drops below a predetermined level, or if a MOTOR TEMPERATURE SENSOR block  216  begins to monitor the temperature of motor  15  and shuts the motor  15  down for a minimum predetermined time via a TEMPERATURE SWITCH TIMER block  215  if the temperature rises above a predetermined limit and the combined action of a RAMP GENERATOR block  221  and a pulse width modulator PWM GENERATOR block  219  begin to progressively switch on MOTOR SWITCH block  203  which causes MOTOR CURRENT SENSOR block  205  to send a signal to an OVER CURRENT PROTECTOR block  207  which momentarily switches off the motor via RAMP GENERATOR RESTART block  220  if the current rises above a predetermined limit, RATE OF RISE (ROR) block  217  which monitors the current utilized and if the current utilized rises above a predetermined rate (amps per second) the circuit momentarily shuts down the motor  15  via RAMP GENERATOR RESTART block  220  and Rate of Fall (ROF) block  214  which monitors the current utilized and if the current utilized drops above a predetermined rate (amps per second) shuts off power momentarily via RAMP GENERATOR RESTART block  220 . 
   An OPTIONAL ROR/ROF PROHIBITOR block  222  delays the output signal from the MOTOR CURRENT SENSOR block  205  to the ROR block  217  and ROF block  214  until the RAMP GENERATOR block  221  has completed its ramp generation and the MOTOR SWITCH block  203  is fully on. 
   The circuit  201  has several parts, including the rate of rise (ROR) and rate of fall (ROF) components. The ROR portion monitors the current utilized and if the current utilized rises above a predetermined rate (amps per second) the circuit shuts down the motor  15 . The physical condition which would cause an ROR trip condition is one in which the wheelchair suddenly slows quickly under positive force conditions, for example, as the wheelchair  1  bumps into a curb or wall. This results in a rapid slowing of the motor leading to a surge in current and torque. The ROR circuit reacts more quickly than a simple over current circuit. A simple over current circuit would allow current to rise all the way up to a preset limit before cutting power, however the ROR circuitry of the present invention will cut power as soon as the current begins to rise rapidly towards the preset limit. 
   The ROF circuit also monitors the current and shuts off power if the current drops above a predetermined rate (amps per second). The condition which would cause a ROF trip is one in which the wheelchair drive unit suddenly and unexpectedly speeds up. This would occur if the drive wheel was turning slower than its top speed and then suddenly sped up, for example if it slipped on loose gravel, ice, oil or simply bounced up and lost traction with the pavement. The motor would normally suddenly speed up to its no load speed with a corresponding reduction in current. 
   Referring to  FIG. 19 , an analog realization of the circuit  201  provides an operating system for both operation and protection of the motor  15 . At the upper right of  FIG. 19  is a flag entitled +12V which represents the positive 12 volt connection to the battery. The flag entitled MOTOR+ indicates the positive lead for the motor  15 . The flag GND is the generalized ground flag. The plunger switch assembly  41  is shown as connecting the positive 12 volt connection to the battery to the positive lead for the motor  15 . 
   The motor  15  power input terminals are paralleled by a capacitor C 10 . A series combination of capacitors C 16  and C 17  are connected in parallel to capacitor C 10 . The center connection of capacitors C 16  and C 17  are connected into the motor  15  at a center of the stator windings. The motor positive lead is also connected to ground through a parallel combination of capacitor C 2  and capacitor C 1 . 
   The motor positive lead is connected into the negative input of an operational amplifier U 1 : 1  through a resistor R 2 . The negative input of an operational amplifier U 1 : 1  is connected to ground through a parallel combination of resistors R 3  and capacitor C 4 . A positive input of an operational amplifier U 1 : 1  is connected to its output through a resistor R 4 . The operational amplifier U 1 : 1  rails include the 12 volt supply voltage and ground. 
   The output of input operational amplifier U 1 : 1  is connected to a negative input of an operational amplifier U 1 : 2  through a resistor R 6 . A capacitor C 3  connects the negative input operational amplifier U 1 : 2  to its output. The output of operational amplifier U 1 : 2  is connected back through the positive input of operational amplifier U 1 : 1  through a resistor R 5 . 
   At the lower left side of  FIG. 19 , a circuit begins with a connection labeled VCC which represents the 12 volt supply power, written at the left side to simplify the schematic. The voltage potential is connected to ground through a first series combination of a resistor R 29 , thermistor R 9  and resistor R 32 . The voltage potential is connected to ground through a second series combination of a resistor R 30 , and resistor R 31 . 
   A connection between the thermistor R 9  and R 29  is supplied to the negative input of an operational amplified U 2 : 1 . A connection between resistors R 30  and R 31  is supplied to the positive input of an operational amplified U 2 : 1 . The output of operational amplified U 2 : 1  is supplied to the positive input of an operational amplifier U 2 : 2  through a diode D 10 . The positive input of operational amplified U 2 : 2  is also connected to ground through a parallel combination of capacitor C 8  and a resistor R 26 . 
   The supply voltage VCC is connected to the negative input of operational amplified U 2 : 2  through a series combination of resistor R 1  and resistor R 27 . The connection between resistors resistor R 1  and resistor R 27  is also connected to ground through a parallel combination of zener diode D 12  and a capacitor C 3 . The negative input of operational amplifier U 2 : 2  is also connected to ground through a capacitor R 28 . The output of operational amplifier U 2 : 2  is connected through diode D 11  to an output labeled CON. 
   At the lower left side of the  FIG. 19 , the connection CON is also connected through a series connected diode D 20  and capacitor C 23  to ground. The connection between the diode D 20  and capacitor C 23  is connected to a connection CON-C and to the collector of a transistor Q 9 , the emitter being grounded. The base of transistor Q 9  is connected to ground through a series connected resistor R 46  and a capacitor C 19 . The connection between the resistor R 46  and a capacitor C 19  is connected to the collector of a transistor Q 8  through a resistor R 45 . The collector of transistor Q 8  is connected to the voltage source VCC through a resistor R 42  and to a connection CON-B through a resistor R 48 . The emitter of transistor Q 8  is grounded and the base of the emitter is connected to ground through a resistor R 47 . The emitter of transistor Q 8  is also connected through a series combination of a resistor R 43  and a diode D 19  to the positive input of an operational amplifier U 1 : 3 . The negative input of operational amplifier U 1 : 3  is connected to the output of operational amplifier U 1 : 2 . 
   The output of operational amplifier U 1 : 2  is connected to the supply voltage VCC through a resistor R 10 , and to the gate of a transistor Q 5  through a parallel combination of capacitor C 9  and resistor R 11 . The drain of transistor Q 5  is connected to the negative motor  15  terminal and to ground through a capacitor C 21 . The source of transistor Q 5  is connected to ground through a resistor R 14 , and is also connected to ground through reverse biased diode D 17 . 
   Positive input terminal to operational amplifier U 1 : 3  is also connected through a resistor R 28  to an emitter of a transistor Q 2 . The voltage source VCC is connected through a resistor R 7  to the collector of transistor Q 2 , and to the base of transistor Q 2  through a pair of series connected diodes D 8  and D 9 . The base of transistor Q 2  is connected to ground through a resistor R 8 . The emitter of transistor Q 2  is also connected to ground through a capacitor C 6  and to the collector of a transistor Q 1 , the emitter of transistor Q 1  connected to ground. The base of transistor Q 1  is connected to ground through reverse biased diode D 6 , resistor R 12 , a base of a transistor Q 6 . The base of transistor Q 1  is connected to the supply voltage VCC through a series connection of capacitor C 7  and a resistor R 13 . The connection between capacitor C 7  and resistor R 13  is connected to ground through the parallel combination of a resistor R 33  and a reverse biased diode D 18 . 
   The supply voltage VCC is connected to the collector of transistor Q 3  through a resistor R 24  and to the base of transistor Q 3  through a series combination of resistor R 22  and diode D 3 . The base of transistor Q 3  is connected to ground through resistor R 25  while the emitter of transistor Q 3  is grounded. The collector of transistor Q 3  is connected to ground through a capacitor C 11  and to the collector of transistor Q 6  through a series combination of resistor R 23  and diode D 4 . The collector of transistor Q 6  is connected to ground through capacitor C 12  and to the source of transistor Q 5  through a resistor R 15  and to the positive input of an operational amplifier U 1 : 4   
   The negative input of operational amplifier U 1 : 4  is connected to ground through the parallel combination of resistor R 18  and capacitor C 13 , and is connected tho the supply voltage VCC through a series combination of resistors R 16  and R 19 . The connection between resistors R 16  and R 19  is connected to ground through a resistor R 17 . 
   The output of operational amplifier U 1 : 4  is connected back though its positive input through a diode D 5  and to the base of a transistor Q 4  through a series combination of diode D 2  and a resistor R 20 . The connection between diode D 2  and a resistor R 20  forms a connection labeled CON. The emitter of transistor Q 4  is grounded and the collector is connected to the supply voltage VCC through a resistor R 21 . The collector of transistor Q 4  is also connected to the positive input of operational amplifier U 1 : 3  through a diode D 7 . 
   The source of transistor Q 5  is connected to ground through a series combination of resistor R 37  and capacitor C 14 . The connection between resistor R 37  and capacitor C 14 . Is connected to an operational amplifier U 2 : 3 . The supply voltage VCC is connected to the negative input of operational amplifier U 2 : 3  through a series combination of resistors R 36  and R 35 . The connection between resistors R 36  and R 35  is connected to the source of transistor Q 5  through a diode D 14 , the negative input of operational amplifier U 2 : 3  is also connected to the source of transistor Q 5  through a resistor R 34 . The output of operational amplifier U 2 : 3  is connected through a diode D 15  to form the connection labeled CON-C. 
   The source of transistor Q 5  is connected to a positive input of an operational amplifier U 2 : 4  through a resistor R 44 . Positive input of operational amplifier U 2 : 4  is connected to ground through a capacitor C 20 . The supply voltage VCC is connected to the source of transistor Q 5  through a parallel combination of a reverse biased diode D 16  in parallel with a series combination of resistors R 39  and R 40 . The connection between resistors R 39  and R 40  are connected to ground through a capacitor C 15  and into the negative input of operational amplifier U 2 : 4 . The output of operational amplifier U 2 : 4  is connected to the connection CON-A through a diode D 13 , and into the collector of a transistor Q 7 . The emitter of transistor Q 7  is grounded and the base of transistor Q 7  is connected to connection CON-B. 
   A set of values for the circuit of  FIG. 19  is as follows: Resistors: R 1 , 680 ohms; R 2 , 10 k ohms; R 3 , 4.7 k ohms; R 4 , 100 k ohms; R 5 , 20 k ohms; R 6 , 100 k ohms; R 7 , 2.7 k ohms; R 8 , 1.2 k ohms; R 9 , Thermistor; R 10 , 10 k ohms; R 11 , 510 ohms; R 12 , 1 k ohms; R 13 , 390 ohms; R 14 , 0.0042 megohms; R 15 , 100 k ohms; R 16 , 12 k ohms, R 17 , 100 ohms; R 18 , 620 ohms; R 19 , 390 ohms; R 20  10 k ohms; R 21 , 10 k ohms; R 22 , 4.7 k ohms; R 23 , 100 k ohms; R 24 , 2 megaohms; R 25 , 300 ohms; R 26 , 82 k ohms; R 27 , 3 k ohms; R 28 , 1.8 k ohms; R 29 , 3.3 k ohms; R 30 , 3.3 k ohms; R 31 , 5.6 k ohms; R 32 , 4.3 k ohms; R 33 , 1.0 k ohms; R 34 , 100 ohms; R 35 , 10 k ohms; R 36 , 1.5 k ohms; R 37 , 47 k ohms; R 38 , 10 k ohms; R 39 , 1.0 megaohms; R 40 , 10 k ohms; R 41 , 1.5 k ohms; R 42 , 1.0 k ohms; R 43 , 470 ohms; R 44 , 1.0 k ohms; R 45 , 10 k ohms; R 46 , 10 k ohms; R 47 , 680 k ohms; and R 48 , 10 k ohms. 
   Capacitor values are C 1 , 470 microfarads; C 2 , 0.1 microfarads; C 3 , 102 microfarads; C 4 , 102 microfarads; C 5 , 0.1 microfarads; C 6 , 220 microfarads; C 7 , 334 microfarads; C 8 , 220 microfarads; C 9 , 1500 picofarads; C 10 , 0.1 microfarads; C 11 , 3.3 microfarads; C 12 , 104 microfarads; C 13 , 473 microfarads; C 14 , 10 microfarads; C 15 , 10 microfarads; C 16 , 330 picofarads; C 17 , 330 picofarads; C 19 , 100 microfarads; C 20 , 473 microfarads; C 21 , 0.1 microfarads; and C 23 , 0.47 microfarads. 
   Preferably the U 1  operational amplifiers are part number LM324A, while the U 2  operational amplifiers are part number LM324B. Transistors are preferably Q 1 , 9014C; Q 2 , 9012; Q 3 , 9014C; Q 4 , S8050C; Q 5 , 1RL3202; Q 6 , 9014C; Q 7 , S8050C; Q 8 , 9014C; and Q 9 , 9014C. 
   Diodes may preferably be D 1 , 1N5404; D 2 , 1N4148; D 3 , 1N4148; D 4 , 1N4148; D 5 , 1N4148; D 6 , D 7 , 1N4148; D 8 , 1N4148; D 9 , 1N4148; D 10 , 1N4148; D 12 , 4.7V, 0.5W D 13 , 1N4148; D 14 , 2.4V, 0.5W; D 15  1N4148; D 17 , 1N4148; D 18  7.5V, 0.5W; D 19  7.5V, 0.5W; and D 20 , 1N4148; 
   In general the association between the circuitry and its function is as follows. The OVER CURRENT PROTECTOR block  207  is associated with components U 1 : 4  and Q 4 . UNDER VOLTAGE DETECTOR block  209  is associated with Q 3 . The RATE OF FALL block  214  is associated with U 2 : 3 . The RAMP GENERATOR RESTART block  220  is associated with Q 1  and Q 2 . TEMPERATURE SWITCH TIMER block  215  is associated with U 2 : 1  and U 2 : 2 . PWM GENERATOR block  219  is associated with U 1 : 1 , U 1 : 2  and U 1 : 3 . The ROR/ROF PROHIBITOR block  223  is associated with Q 8  and Q 9 . 
   Referring to  FIG. 20  a detail perspective view of a different style wheelchair  251  shows the clamps  4  of wheelchair drive unit  2  fitted to horizontal frame members  253  as example of a different mounting. 
   Referring to  FIG. 21  a plan view shows the tie strap  170  used to limit the extent to which struts  5  can open and comprising shackles  171 , an adjuster  175  connected to one shackle  171  by a first piece of low stretch material  173  and a second piece of low stretch material  177  that passes through the adjuster  175  and which may be adjusted to reduce the length of the second piece of low stretch material  177  between second shackle  171  and adjuster  175 . Shackles  171  are connected to each clamp half strut  6  not shown near the attachment clamps  4 . 
   Referring to  FIG. 22  a side elevation of wheelchair drive unit  2  in normal use shows the position of optional anti under run roller  53  which is mounted on anti under run roller axle  55  and in normal use is substantially clear of pavement  57 . The normal use position will involve an angle beta between the struts  5  and the ground  57 . 
   Referring to  FIG. 23  a side elevation of wheelchair drive unit  2  in an under run condition in which the wheelchair drive unit  2  has moved to a position in which aforementioned angle beta has increased to angle delta. At the angle delta, the optional anti under run roller  53  then comes into contact with pavement  57  and the contact force between tire  9  and the pavement  57  is reduced. Tire  9  can no longer drive the wheelchair drive unit  2  forward to increase the angle beyond the angle delta This reduction in contact force causes tire  9  to slip against pavement  57  thereby stopping further reduction of angle alpha not shown. 
   While the present invention has been described in terms of an wheelchair drive unit, and more particularly to a universal applicability device which depends from a wheelchair, the particular structure and system which utilizes a physical and electrical control setup which provides both ease and a universal applicability to the users. 
   Although the invention has been derived with reference to particular illustrative embodiments thereof, many changes and modifications of the invention may become apparent to those skilled in the art without departing from the spirit and scope of the invention. Therefore, included within the patent warranted hereon are all such changes and modifications as may reasonably and properly be included within the scope of this contribution to the art.