Abstract:
A clutch-driven limited force actuator is disclosed having a motor, a planetary gear mechanism in operative communication with the motor and a clutch plate. The clutch plate includes a contact surface selectively in frictional contact with the planetary gear mechanism to drive an actuator arm. In operation, the frictional force between the planetary gear mechanism and the clutch plate causes the actuator arm to actuate. The actuator arm can be connected to any element that requires actuation. The actuator of the present invention is preferably used in a wheelchair lift to actuate any mechanism that must have a safe limit on the amount of force applied to obstacles and that must have provision for direct manual operation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application No. 60/386,338 filed on Jun. 6, 2002, the contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to friction-force actuators, and particularly to friction-force actuators that can be adapted for use in operating secondary mechanisms of a wheelchair lift where safety is a concern. 
     BACKGROUND OF THE INVENTION 
     Wheelchair lifts are provided for facilitating the loading of wheelchairs and wheelchair occupants on and off vehicles. When the lift is used to load a wheelchair onto the vehicle, the lift is positioned at the ground level and is configured to allow the wheelchair and its occupant to roll onto a platform. Once the wheelchair has been loaded onto the platform, a barrier or roll stop is raised at the end of the platform assembly to prevent the wheelchair from rolling off of the platform while the lift is in motion. Barriers may be provided on the front end of the platform, the back end of the platform, as well as both sides of the platform to ensure the safety of the wheelchair occupant. When the wheelchair lift is raised to the vehicle entry level, the vehicle-side barriers drop, allowing the wheelchair to exit the platform onto the vehicle. 
     Similarly, when the wheelchair is unloaded from the vehicle, the lift is positioned at the entry level of the vehicle, with the vehicle-side barriers down, to allow the wheelchair access to the platform. When the wheelchair is securely positioned on the platform, the barriers are raised to prevent the wheelchair from rolling off of the platform during transport. The platform is then lowered from the entry level position to the ground level position. Upon arrival at the ground level, the barriers opposite the vehicle are released and lowered to allow the wheelchair to exit from the platform onto the ground. 
     It is desirable to provide a device for automating the raising and release of the platform barriers to avoid the need to manually engage the barriers during each use. Accordingly, it is desirable to provide an actuator that automates the operation of the barriers. 
     Although automatic operation of the barriers is desirable, from time to time, there may be a need to manually operate the barriers. Typically, the manual operation of automated devices requires the disengagement of the actuator and the movement of the barrier by hand. The disadvantage of these known devices is that manual operation of the barriers often causes the device to become mechanically “lost,” i.e., after manual operation, the device is left out of sequence. As a result of being out of sequence, when the device is reactivated, it often gets jammed or otherwise malfunctions. 
     Accordingly, there is a need for an actuator that automates the operation of the barriers while still allowing manual operation thereof as needed, without requiring disengagement of the actuator from the barrier during manual operation, and without resulting in the mechanical mis-sequencing of the device. 
     The automation of the barriers raises certain safety issues. Although desirable to automatically move the barriers up and down during each use, there may be some situations in which the motion of the barriers should be limited. For example if someone&#39;s foot is positioned underneath the barrier, for safety purposes, the actuator should be limited in the amount of force it applies to the barrier. Accordingly, it is desirable to provide an actuator that limits the amount of force it applies to the barrier upon contact with an intervening obstacle. 
     SUMMARY OF THE INVENTION 
     A clutch-driven limited force actuator is disclosed having a motor, a planetary gear mechanism in operative communication with the motor and a clutch plate. The clutch plate includes a contact surface in frictional contact with the planetary gear mechanism and applying a selective amount of force to drive an actuator arm. In operation, the frictional force between the planetary gear mechanism and the clutch plate causes the actuator arm to actuate. The actuator arm can be connected to any element that requires actuation. The actuator of the present invention is preferably used in a wheelchair lift to actuate the barriers of the lift. 
     In a preferred embodiment of the invention, the planetary gear mechanism includes a plurality of planetary gears orbiting the spindle of the motor and a ring gear engaged with and driven by the planetary gears. The clutch plate is biased toward the ring gear by a wave washer and is selectively in frictional contact with the ring gear. In operation, the clutch plate rotates with the ring gear when the plate is in frictional contact with the ring gear. When the frictional force is overcome, the clutch plate will no longer rotate, even if the motor continues to operate. 
     Other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description. It is to be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not limitation. Many changes and modifications within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be more readily understood by referring to the accompanying drawings in which: 
         FIG. 1  is a perspective view of a preferred embodiment of the clutch-driven limited force actuator of the present invention; 
         FIG. 2  is an exploded view of a preferred embodiment of the clutch-driven limited force actuator of the present invention; 
         FIG. 3  is a top plan view of a preferred embodiment of the clutch-driven limited force actuator of the present invention; 
         FIG. 4  is a cross-sectional view of the clutch-driven limited force actuator of  FIG. 3 , taken along line  4 — 4 ; and  FIG. 5  depicts a preferred embodiment of the clutch-driven limited force actuator of the present invention as installed on a wheelchair lift. 
     
    
    
     Like numerals refer to like parts throughout the several views of the drawings. 
     DESCRIPTION OF THE INVENTION 
       FIGS. 1 through 4  illustrate a preferred embodiment of the clutch-driven limited force actuator  10  of the present invention. The actuator  10  includes a motor  12 , a planetary gear mechanism  20 , a pressure plate or clutch plate  30  and an actuation stop plate  40 . The motor  12  is preferably a 12 volt direct current motor having a clockwise rotation. It is envisioned that various types of motors can be used in the actuator of the present invention without departing from the spirit or scope of the present invention. 
     The planetary gear mechanism  20  preferably includes a sun gear  22 , ring gear  24  and a plurality of planetary gears  26 . As best shown in  FIGS. 1 and 4 , in a preferred embodiment of the present invention, the sun gear  22  is attached to the spindle of the motor  12 . When the motor  12  operates, the sun gear  22  spins. The planetary gears  26  surround the sun gear  22  and have teeth  27  to facilitate the rotation of the planetary gears  26  with respect to the sun gear  22 . The sun gear  22  includes teeth corresponding to the teeth  27  on the planetary gears  26  to facilitate the rotation of the planetary gears  26  about the sun gear  22 . 
     To preserve the positioning of the planetary gears with respect to each other, in a preferred embodiment of the invention, the planetary gear mechanism  20  includes a planetary gear carrier plate  28 . The planetary gear carrier plate  28  preferably includes a plurality of carrier posts  29 , each post corresponding to a planetary gear  28 . Each planetary gear  28  is mounted on a carrier post  29  and rotates thereon. The carrier post  29  provides the pivot axis for the rotation of the planetary gear  28 . 
     When the sun gear  22  rotates, it causes the rotation of the planetary gears  26  with respect to the sun gear  22 . Because the teeth  27  of the planetary gears  26  are engaged with the teeth  25  of the ring gear  24 , the rotation of the planetary gears  26  drives the ring gear  24 . 
     The number of planetary gears  26 , and the number of teeth on each of the planetary gears  26 , ring gear  24  and sun gear  22  can be varied without departing from the inventive concept of the present invention. In a preferred embodiment of the invention, the planetary gear mechanism  20  includes three planetary gears  26 , each having six teeth  27  thereon, a ring gear  24  with twenty-one teeth  25 , and a sun gear  22 . 
     The planetary gear mechanism  20 , and more preferably the ring gear  24 , is in frictional contact with the clutch plate  30 . The frictional force between the planetary gear mechanism  20  and the clutch plate  30  is controlled by a biasing device  50 . In a preferred embodiment of the invention, the biasing device  50  is a wave washer. The wave washer  50  is positioned such that it exerts force on the clutch plate  30 , causing the clutch plate  30  into frictional contact with the planetary gear mechanism  20 . The underside  32  of the clutch plate  30  preferably contacts the ring gear  24 . When the ring gear  24  rotates, the frictional force on the clutch plate  30  causes the clutch plate to rotate together with the ring gear  24 . As a result, the actuator arm  34  also rotates. 
     In a preferred embodiment of the invention, the clutch plate  30  and ring gear  24  are made of different materials to prevent galling. The clutch plate  30  is preferably made of a bearing material such as brass or bronze. The ring gear  24  is preferably made of steel. Given the force applied by the wave washer  50  and the contact surface area between the clutch plate  30  and ring gear  24 , both of which are designed properties of the assembly, those skilled in the art will be able to calculate the friction force between the clutch plate  30  and the ring gear  24 . The friction force directly equates to the amount of force the actuator arm  34  can exert. As long as the clutch plate  30  is in frictional contact with the ring gear  24 , the actuator  10  applies a constant force on the actuator arm  34 . If additional force is required, the strength and the compression of the biasing device or wave washer  50  can be increased, causing it to apply additional biasing force on the clutch plate  30 . 
     To limit the rotation of the clutch plate  30 , the actuator  10  of the present invention preferably includes an actuation stop plate  40 . In a preferred embodiment of the invention, the actuation stop plate  40  includes a plurality of apertures  42  that correspond to apertures  14  on the motor  12 . Spacers  16  are placed between the stop plate  40  and the motor  12  to attach the actuation stop plate  40  spaced apart from the motor  12 . Upon alignment of the spacers  16  and the apertures  14 ,  42 , the actuation stop plate  40  is fastened to the motor  12 . 
     When attached to the motor  12 , the actuation stop plate  40  provides a first limiting member  44  and a second limiting member  46 . The rotation of the actuator arm  34  is limited by the first and second limiting members  44 ,  46 . The distance between the first and second limiting members  44 ,  46  represents the range of motion of the device actuated by the actuator arm  34 . 
     The actuator  10  of the present invention is never thrown out of sequence. There are no surfaces that separate and re-engage each other. Upon operating the motor  12 , the actuator arm  34  will travel until it reaches a limiting member  44 ,  46 . If the motor  12  continues to run after the actuator arm  34  has reached a limiting member, the force of the limiting member on the actuator arm  34  overcomes the friction force between the clutch plate  30  and the planetary gear mechanism  20  and the clutch plate  30  will no longer rotate with the ring gear  24 . Accordingly, the actuator will not jam or otherwise become mechanically “lost.” 
     spacers  52 , spacer plates  54 , wear plates  56  and washers  58  can be used as known by those in the art to ensure smooth operation of the various components of the actuator. 
     It is envisioned that the clutch-driven limited force actuator  10  can be used in a variety of applications. In a preferred embodiment of the invention, as best shown in  FIG. 5 , the actuator  10  is used in a wheelchair lift  100  to facilitate the movement of the barriers  110 . Actuator arm  34  is operatively connected to a barrier  110  by an extension arm  112 . When actuated, the actuator arm  34  actuates the barrier  110  via the extension arm  112 . The frictional force between the clutch plate  30  and the planetary gear mechanism  20  is adjusted to be sufficient to overcome the force of the barriers  110  and to move the barriers to the position desired. 
     If during deployment of the barriers  110 , an obstacle is encountered, or in the event that the barriers must be manually operated, the actuator will limit the application of force with minimal interruption and effect on the operation of the actuator. For example, if an obstacle is placed in the path of the barriers, the force of the obstacle will cause the clutch to slip. While the motor is operating, the ring gear  24  will continue to turn, sliding on the clutch plate  30  and thereby applying force to the actuator arm  34 . However, the force applied by the actuator arm  34  is limited. Upon removal of the obstacle, the actuator arm  34  continues rotating and applying force on the barriers. The actuator is never taken out of sequence and it is not necessary to disengage the actuator from the barrier to manually operate the barrier. 
     The embodiments described above are exemplary embodiments of a clutch-driven limited force actuator of the present invention. Those skilled in the art may now make numerous uses of, and departures from, the above-described embodiments without departing from the inventive concepts disclosed herein. Accordingly, the present invention is to be defined solely by the scope of the following claims.