Abstract:
A drive mechanism under the floor of a minivan is coupled to the folding ramp by a rotating arm linkage, also located beneath the floor. The extension of the folding ramp is partially controlled by dynamic braking, i.e. using the electric motor as a generator and coupling the motor to an electrical load.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to a wheelchair ramp for a minivan and, in particular, to a battery powered, folding ramp that is powered by a motor under the floor of the minivan. 
   It is known in the art to provide a ramp for enabling a person in a wheelchair to enter or leave a vehicle. Trucks, buses, and large vans have a high ground clearance and are typically provided with a lifting mechanism to augment a ramp, which would be too long or too steep otherwise. The ramp in such mechanisms merely provides a gradual transition from the ground to the height of a platform that is raised or lowered. A type of light truck known as a minivan has a lower ground clearance than larger vehicles. Thus, a ramp can be used without a lift and the ramp typically folds when stored. 
   In the prior art, minivans are typically provided with a battery powered, folding ramp having a motor located on one side of the sliding door adjacent the ramp. The motor is coupled to the ramp by gears or chain; e.g. see U.S. Pat. No. 5,871,329 (Tidrick et al.). The location for the motor reduces precious space within the cabin of the minivan and reduces the width of the ramp. 
   The Tidrick et al. patent also describes a mechanism for counterbalancing a folding ramp. Although the mechanism described works satisfactorily, the mechanism is mechanically complex and relatively expensive. Competing devices power the ramp down to the expected ground level, which may drive the ramp into the ground or stop the ramp just above ground level. On the one hand, such is necessary because the ground may be uneven or there may be a high curb, for example. On the other hand, an uncontrolled descent of a heavy metal ramp is undesirable. 
   In the prior art, it is generally assumed that a power failure occurs with a person in the minivan and that exiting the vehicle is all-important. Typically, a release mechanism is provided for extending the ramp after the door is opened. The ramp free-falls into an open position, creating a dangerous situation for someone standing near the open door. A folding metal ramp is heavy and cannot be handled by a person sitting in a wheelchair. Even for someone able to move about freely, a folding ramp may be difficult to handle and it is preferred that two people control the ramp. 
   In the event of a power failure, it is not simply a matter of extending the ramp manually. A manual release mechanism in the mechanical drive for the ramp must be re-set. This usually requires some re-assembly of the mechanical drive. Thus, for example, a power failure due to a blown fuse has the effect of stranding a wheelchair bound person or at least greatly delaying his travel while the system is restored to working order. 
   While there are many obvious solutions to the problem, such as a back-up power supply, one must realize that a ramp and a drive mechanism must fit an existing vehicle, i.e. a ramp is designed for the vehicle, not the other way around. A minivan is, by definition, a small van. Thus, most solutions to the problem are either too big, too expensive, or simply impractical. One wants to modify a minivan in a way that changes the finished appearance of the minivan as little as possible. Finding a location where a second battery, and the associated switching and charging apparatus, is both hidden and accessible is not easy in a minivan and is more difficult in a minivan that has already been heavily modified to accommodate a powered ramp. Similarly, any other solution to the problem must fit within commercially available vehicles without significantly affecting the space available for passengers or cargo. 
   In view of the foregoing, it is therefore an object of the invention to provide an electrically powered, folding ramp that has a controlled descent even when there is no electricity available. 
   Another object of the invention is to provide a compact drive mechanism for a folding ramp for a minivan, wherein the drive mechanism for the ramp is invisible from within the minivan. 
   A further object of the invention is to prevent a powered folding ramp from free-falling open in the event of a power failure. 
   Another object is to provide a controlled operation of a folding, powered ramp when power is interrupted. 
   SUMMARY OF THE INVENTION 
   The foregoing objects are achieved in this invention in which a drive mechanism under the floor of a minivan is coupled to the folding ramp by a rotating arm linkage, also located beneath the floor. The extension of the folding ramp is partially controlled by dynamic braking, i.e. using the electric motor as a generator and coupling the motor to an electrical load. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a perspective view of a folding ramp and the drive mechanism under the floor of a minivan; 
       FIG. 2  is a side view of a drive mechanism constructed in accordance with a preferred embodiment of the invention; 
       FIG. 3  is an end view of a drive mechanism constructed in accordance with a preferred embodiment of the invention; 
       FIG. 4  is a perspective view showing the floor level of a minivan; and 
       FIG. 5  is a schematic of a dynamic braking circuit constructed in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a perspective view looking out the sliding door of a minivan having folding ramp  10  attached to the minivan and extending through the opening for the sliding door. Drive mechanism  14  is positioned under the floor and is mechanically coupled to ramp  10  for raising and lowering the ramp. A battery (not shown) provides electrical power for drive mechanism  14 . 
   Drive mechanism  14  includes gear motor  15  and drive shaft  17 . The drive shaft is preferably journaled through plates at the gear box and at the free end to support the considerable off-axis forces applied to the drive shaft. The plates, in turn, are fastened to the frame of the minivan. The details of drive shaft  17  are best seen in  FIGS. 2 and 3 . Drive shaft  17  includes arm  21  having one end welded to drive shaft  17  and the other end drilled to receive a pin for attaching clevis  22 . Arm  23  has one end welded to drive shaft  17  and the other end drilled to receive a pin for attaching clevis  24 . Arms  21  and  23  are substantially parallel. Shaft  28  includes clevis  22  at one end and clevis  31  ( FIG. 3 ) at the other end. Shaft  29  includes clevis  24  at one end and clevis  32  ( FIG. 3 ) at the other end. Clevis  31  and clevis  32  are attached to brackets (not shown) on ramp  10  for rotating the ramp about hinge  35  ( FIG. 1 ). 
   In operation, drive shaft  17  is supported at each end by bearings in bulkheads  41  and  42  ( FIG. 3 ), which are adapted to be attached to the frame of a minivan. Because the throw of arms  21  and  23  is relatively short, and the throw on the brackets attached to the ramp is similarly short, substantial torque is applied to drive shaft  17  to operate the ramp. As drive shaft  17  rotates, the motion is converted to linear motion and back to rotary motion by the arms and brackets. 
   In one embodiment of the invention, drive mechanism  14  is about four inches in diameter and fits entirely under the floor of a minivan, as illustrated in  FIG. 4 . Floor plate  50  is completely flat and include only small slots  52  and  53  to accommodate a portion of ramp  10 . None of the interior space of the minivan is taken up by the drive mechanism. This not only provides a much cleaner installation but also gives the user more room to maneuver a wheelchair within the small confines of a minivan. 
   In accordance with another aspect of the invention, dynamic braking is used to provide a controlled descent of the ramp. In  FIG. 2 , switch  60  is attached to drive shaft  17  and opens when the drive shaft turns to a certain position. When switch  60  opens, power is cut to gear motor  15  ( FIG. 1 ) and ramp  10  turns the motor through the integral gearbox. Because the gearbox is now operating in a step-up gear ratio, there is significant mechanical drag on the descent of ramp  10 . This drag is further enhanced by coupling a resistive load (not shown) to gear motor  15  to dissipate as heat the electrical energy being generated by the motor. 
     FIG. 5  is a partial schematic of the motor control circuitry showing the dynamic braking circuit for controlling the descent of the ramp. The battery in the minivan, or an auxiliary battery, is coupled to gear motor  15  through terminals  63  and  64 . The polarity of the connection is reversed, depending upon whether the ramp (not shown in  FIG. 5 ) is to be raised or lowered. When the transmission of the minivan is in drive or reverse, the motor leads are shorted, essentially locking motor  15 . The shorting mechanism is not shown. 
   During power operation (with the transmission of the minivan in park or neutral), current is sent through motor  15  and relay  71 , closing contacts  73 . The ramp is lowered until switch  60  signals that the ramp is approximately ten inches above expected ground level. At this point, current is removed, switch  72  is closed, and motor  15  generates current that is dissipated through resistor  75 , dynamically braking the descent of the ramp. To raise the ramp, switch  72  is opened, and the opposite polarity current is applied to terminals  63  and  64 . Relay  71  closes contacts  73  and the ramp is raised. 
   During manual operation (with the transmission of the minivan in park or neutral), no current Is sent to the motor and switch  72  is closed. A person pushes or pulls the ramp to lower the ramp. Motor  15  generates current that is dissipated through resistor  75 , dynamically braking the descent of the ramp. As a person lifts the ramp, motor  15  freewheels because diode  76  blocks current in the opposite direction from resistor  75 . The gearbox presents nominal resistance but the leverage provided by the ramp easily overcomes this resistance. 
   The invention thus provides a compact drive mechanism for a folding ramp for a minivan, wherein the drive mechanism for the ramp is invisible from within the minivan. The drive mechanism is made more compact by providing dynamic braking, rather than a complex support structure, for the ramp. The invention provides an electrically powered, folding ramp that has a controlled descent even when there is no electricity available. 
   Having thus described the invention, it will be apparent to those of skill in the art that various modifications can be made within the scope of the invention. For example, the dynamic braking can be variable rather than constant, as described above. For example, plural loads can be switched into the circuit as the ramp descends to increase braking effect. Alternatively, the loads can be non-linear, e.g. an incandescent lamp. Further, one can rely on mechanical drag alone prior to applying dynamic braking when the ramp is more nearly vertical. Programmable logic, e.g. a microcontroller, can be used instead of fixed logic for controlling the operation of the ramp. Various position sensors can be used instead of switch  60  for opening the circuit to gear motor  15 . Limit switches and various other safety items are not shown but would typically be included in the control mechanism for the ramp. A separate motor and gear box can be used instead of gearmotor  15  and a hydraulic pump and hydraulic motor can be used instead of an electric motor.