Abstract:
A wheelchair comprises a caster, a drive wheel, a sensor, and a controller. The sensor is provided for sensing operational characteristics of the caster. The controller is provided for controlling at least one of the speed, acceleration and deceleration of the drive wheel when the sensor senses a certain operational characteristic of the caster.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/565,608, filed on Apr. 27, 2004. 
     
    
     BACKGROUND OF INVENTION  
       [0002]     This invention relates in general to land vehicles and, more particularly, to a control system for land vehicles. Most particularly, the invention relates to a control system for power wheelchairs.  
         [0003]     A conventional wheel configuration for a powered wheelchair typically includes two drive wheels (i.e., motor driven wheels) and two or four idler wheels or casters. Steering of the wheelchair is achieved by differentially driving the drive wheels (i.e., causing the drive wheels to rotate at different speeds and/or directions). A typical wheelchair caster includes a wheel, a fork, and a stem. The stem is adapted to rotate about a vertically oriented axis. Caster stem bearings or bushings allow the stem to rotate smoothly, with low friction. The fork extends from the stem and supports the wheel for rotational movement about an axle. The axle is offset with respect to the stem. The offset distance of the axle with respect to the stem is known as the caster “trail”. As the caster is displaced during wheelchair motion, the fork will rotate about the stem such that the caster points in the direction of travel of the wheelchair. This trailing action is passive and bears no impact on the steering control of the wheelchair other than to follow in the direction dictated by the drive wheels. That is to say, the casters provide support and stability for the wheelchair but play no active role in steering the wheelchair or keeping the wheelchair on track.  
         [0004]     During the operation of the wheelchair, the caster, though a passive component, may, for various reasons, not be pointed in the direction of travel of the wheelchair. In this condition, it may be undesirable for the wheelchair occupant to have unrestricted control of the speed, acceleration and deceleration of the wheelchair.  
         [0005]     In addition, the caster may flutter (i.e., be subject to oscillating movement) during the wheelchair operation. This is undesirable because such movement affects the smoothness of the wheelchair operation and produces an annoying fluttering sound.  
         [0006]     What is needed is a wheelchair with improved control system that controls the operation of the drive wheels in response to the operational characteristics of the caster.  
       SUMMARY OF INVENTION  
       [0007]     The present invention is directed toward a wheelchair that meets the foregoing needs. The wheelchair comprises a caster, a drive wheel, a sensor for sensing operational characteristics of the caster, and a controller for controlling at least one of the speed, acceleration or deceleration of the drive wheel when the sensor senses a certain operational characteristics of the caster.  
         [0008]     Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0009]      FIG. 1  is a diagrammatic representation of a control system for controlling the drive wheels of a wheelchair in response to certain operational characteristics of a wheelchair caster. 
     
    
     DETAILED DESCRIPTION  
       [0010]     Referring now to the drawings, there is illustrated in  FIG. 1 a  power wheelchair  10  having casters  12  and feedback sensors  16 , such as potentiometers or encoders, which are controlled by a controller  18 , such as a closed loop servo system controller. A steering control device  20 , such as a joystick, may serve as an input to direct the wheelchair  10  in a direction of travel. A control algorithm or look-up table may be used to coordinate the operational characteristics of the casters  12  and drive wheels  24 , which are differentially driven to steer the wheelchair  10  in a desired direction of travel. Feedback sensors  26  may also measure the speed, acceleration and deceleration of the drive wheels  24 . The wheelchair  10  may be a front, mid, rear or all-wheel drive configuration.  
         [0011]     The orientation of the casters  12 , as measured by the feedback sensors  16 , is provided in the form of a signal to the controller  18 . The controller  18 , via the control algorithm or look-up table, may be compare the orientation of the casters  12  to a desired direction of travel, as input through the steering control device  20 , to determine whether the casters  12  are pointed in the desired direction of travel. If the casters  12  are not pointed in the desired direction of travel, the controller  18  may control the speed, acceleration and deceleration of the drive wheels  24  to minimize the speed, acceleration and deceleration of the drive wheels  24 . The orientation of the casters  12 , the desired direction of travel of the wheelchair  10 , and the speed, acceleration and deceleration of the drive wheels  24  is monitored until the casters  12  are pointed in the desired direction of travel of the wheelchair  10 , at which point, the control of the speed, acceleration and deceleration of the drive wheels  24  returns to a default control mode (e.g., a standard or conventional control mode as prescribed for a particular wheelchair occupant and wheelchair). The orientation of the casters  12 , the desired direction of travel of the wheelchair  10 , and the speed, acceleration and deceleration of the drive wheels  24  may be continually monitored to determine if a direction of travel changes (i.e., via the steering control device  20 ), which differs from the direction that the casters  12  are pointing, and which requires further control of the speed, acceleration and deceleration of the drive wheels  24 . Alternatively, the orientation of the casters  12  and the desired direction of travel of the wheelchair  10  may be monitored when the wheelchair  10  is idle to determine if a desired direction of travel differs from the direction that the casters  12  are pointing. In such case, the casters  12  can be oriented the the desired direction of travel prior to permitting the drive wheels  24  to be driven.  
         [0012]     In addition to sensing orientation, the feedback sensor  16  may sense caster flutter, or oscillating movement of the caster stem, and provide a signal to the controller  18  when such flutter is present. If caster flutter is present, the controller  18  controls the speed and acceleration of the drive wheels  24  to minimize the speed and acceleration of the drive wheels  24  until the flutter ceases. Once the flutter ceases, control of the speed, acceleration and deceleration of the drive wheels  24  returns to a default control mode. The feedback sensors  16  may be continually monitored to determine if the caster flutter exists and if further control of the speed and acceleration of the drive wheels  24  is required.  
         [0013]     The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.