Patent Publication Number: US-2007102615-A1

Title: Seat tilt apparatus for a wheelchair

Description:
TECHNICAL FIELD OF THE INVENTION  
      The present invention relates generally to wheelchairs and particularly to an apparatus for tilting the seat of a wheelchair in relation to the chassis of the wheelchair. The invention also relates to a wheelchair provided with such an apparatus.  
     BACKGROUND OF THE INVENTION  
      Wheelchairs, especially electrically driven wheelchairs often have means for adjusting the position of the seat in relation to the chassis. These means may comprise a seat lift for vertically adjusting the height of the seat and a backrest tilt arrangement for tilting the backrest of the seat in relation to the bottom of the seat, i.e. for changing the angle between the backrest and the bottom of the seat.  
      Modern wheelchairs may additionally be provided with an electrically driven seat tilt apparatus, by which the entire seat, including the bottom and backrest may be tilted in relation to the chassis of the wheelchair. Such seat tilt apparatuses are provided for facilitating changing the body weight of the wheelchair occupant, to thereby avoid injuries, numbness and other inconveniences otherwise caused by static positioning of the occupant.  
      At one known type of seat tilt apparatuses the entire seat is tilted around a transverse horizontal axis, which axis is stationary in relation to the chassis. While allowing easy change of the occupant&#39;s body weight, this known type of seat tilt causes the center of gravity of the seat and occupant to move backwards during tilting. Such backward movement of the center of gravity is disadvantageous since it decreases the stability of the wheelchair and may even cause the wheelchair to fall over.  
      In order to solve this problem different so-called constant center of gravity seat tilt devices have been suggested At these types of seat tilt arrangements the axis around which the seat is tilted moves forwardly during the tilting operation. U.S. Pat. No. 6,032,976 discloses one such apparatus, comprising a seat frame which, at a rear end, is pivotally connected to a transverse support member. The support member is guided in opposing horizontal linear slots arranged in a base frame of the wheelchair. The seat frame is also connected to the base frame by means of a tilt linkage, which is formed of two bars that are pivotally connected to opposing sides of the seat frame, forward of its rear end, and to the base frame. An electrical drive member is at a forward end connected to the base frame and at a rear end connected to the transverse support member. By actuating the drive member, the support member and the rear part of the seat frame is moved forwardly while being guided by the linear slots in the base member. During this movement, the two bars force the seat frame to tilt upwards around the pivot axis, which is defined by the opposing pivotal connections between the rear end of the seat frame and the transverse support member.  
      The arrangement according to U.S. Pat. No. 6,032,976 thus enables tilting the seat while keeping the center of gravity of the seat and occupant essentially unchanged. However, this known arrangement also entails some disadvantages. It is often desired to keep the overall dimensions of a wheelchair as small as possible. Among other things such small dimensions facilitates access to narrow places, maneuvering and handling such as storage of the wheelchair. For achieving such small overall dimensions it is of great importance to keep the constructional height of the seat tilt apparatus as small as possible. However, the geometrical requirements of linkage systems, Such as the one described in U.S. Pat. No. 6,032,976 often constitute a limit of how much the constructional height of the linkage system may be reduced. In the above mentioned known apparatus the tilting movement is achieved by two linkages, each being articulated at three points. For the above mentioned constructional reasons it would be desirable to keep the vertical distance of these three articulated points as low as possible, when the seat is in its horizontal position. However, if all three points where to bee aligned on a horizontal line, the linkage would interlock and the drive member would not be able to initiate the tilting movement from the horizontal position. Therefore, at least one of the articulate points needs to be arranged at a distance from a straight line connecting the other two articulate points. In the above mentioned known apparatus the articulate point between the base frame and the pivotal bar is arranged below a straight line connecting the other two articulate points. At this arrangement any reduction of the distance between said articulate point and a line aligning the other two points needs to be compensated by a correspondingly increased force applied by the drive member to overcome the gravitational force acting on the seat frame in front of the articulate point between the seat frame and the pivotal bar. Hence, with the described known arrangement a reduction of the constructional height of the tilt apparatus will require a correspondingly more powerful drive member in order for it to be able to initiate tilting from the horizontal position. Such powerful drive members are not only expensive but also comparatively large, whereby the drive member per se adds to the total constructional height of the tilt apparatus.  
     SUMMARY OF THE INVENTION  
      It is therefore an object of the present invention to provide a seat tilt apparatus for a wheelchair, which seat tilt apparatus may be constructed with a comparatively small constructional height.  
      Another object is to provide such a seat tilt apparatus, which allows for use of a comparatively weak and small drive member.  
      It is a further object to provide such a seat tilt apparatus by which the fore required for initiating tilting of a seat from a horizontal position is reduced.  
      A still further object is to provide such a seat tilt apparatus which is strong and reliable in use.  
      One aspect of the invention is therefore a seat tilt apparatus for a wheelchair, which wheelchair comprises a chassis and a seat. The seat tilt apparatus comprises a base member, a seat supporting member, a pivotal arm and a linear drive member. The base member is fixable to the chassis and provided with a guide slot. The seat supporting member is arranged to supportably receive the seat, movable in relation to the base member and guided by the guide slot. The pivotal arm is pivotally connected to the base member at a first point and to the seat supporting member. The linear drive member is pivotally connected to the base member and to the seat supporting member, for moving the seat supporting member in relation to the base member. The guide slot provided in the base member comprises a first portion which is proximal to the first point and which has a first general direction of extension. A second portion of the guide slot, which is distal to the first point, has a second general direction of extension, which second general direction of extension is different from the first general direction of extension.  
      By this arrangement of the guide slot, with two different portions being arranged at an angle to each other it is achieved that the drive member will act on the seat supporting member under two different gear ratios, when the seat member is guided to move along the two different portions of the guide slot movement of the seat supporting member along the second distal portion, corresponds to the initial phase of the tilting action, from the horizontal position, where the required power is at its greatest. This portion of the guide slot may thus be arranged with a lower gear ratio, such that a comparatively weak drive member may still be able to initiate the tilting action from the horizontal position. When the tilting action has been initiated and the seat supporting member moves along the second portion, the required power for continuing the tilting is lowered. This portion of the guide slot may thus be arranged with a higher gear ratio, whereby the comparatively weak drive member may continue the tilting action at a comparatively higher and satisfying speed of the tilting motion.  
      Hence, with the seat tilt apparatus according to the invention it is possible to keep the three articulate points at a comparatively small vertical distance from each other, to thereby reduce the overall constructional height of the seat tilt apparatus and the entire wheelchair. According to the invention this may further be achieved while utilizing a comparatively weak drive member, which still is capable of initiating the tilting motion and completing the motion at a desirable speed.  
      According to another aspect of the invention the first proximal portion of the guide slot is essentially horizontal and the second distal portion of the guide slot is arranged at a downward angle in relation to the first portion. By this arrangement the rear part of the seat supporting member is somewhat lifted during the initial phase of the tilting, when the seat supporting member is guided by the angled portion Since the rear part of the seat supporting member then moves vertically at the same time as the forward portion of the seat supporting member, the rotational motion of the seat supporting movement around the connection between the seat supporting member and the pivotal arm is reduced. Thereby, the load on the drive member caused by the gravitational force acting on the forward portion of the seat supporting member, in front of the articulate connection between the seat supporting member and the pivotal arm is reduced.  
      The two portions of the guide slot may preferably be connected by an intermediate curved portion. By this means, the acceleration of the tilting motion when the seat supporting member moves from the first to the second portion of the guide slot is reduced to thereby avoid any unpleasant abrupt change of speed.  
      Preferably, the seat tilt apparatus comprises two sets of seat supporting members and pivotal arms arranged in parallel to each other and being connected to a base member with two guide slots each having a first and a second portion as described above. By this means a well-balanced seat tilt apparatus is achieved which may withstand and manage considerable loads.  
      In order to further minimize the constructional height of the seat tilt apparatus and the entire wheelchair, especially when the seat tilt apparatus is fixed to a centrally positioned seat lift of the wheelchair chassis, the drive member may be arranged asymmetrical at one side of the base member.  
      At such an asymmetrical positioning of the drive member there might be a tendency that one of two seat supporting members arranged in parallel moves slightly before the other. I order to compensate for any such unbalanced motion the two pivotal arms may be fixed to the second axle at slightly different rotational angles.  
      A further aspect of the invention is a wheelchair comprising a seat tilt apparatus as described. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic front perspective view of a wheelchair provided with a seat tilt apparatus according to the invention.  
       FIG. 2  is a front perspective view of the seat tilt apparatus schematically illustrated in  FIG. 1   
       FIG. 3  is a front perspective view of a base member forming part of the seat tilt apparatus shown in  FIG. 2   
       FIGS. 3   a - c  are side views showing the seat tilt apparatus in different positions, corresponding to the seat being horizontal, partially tilted and fully tilted. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 1  shows an electrical wheelchair comprising a chassis  1 , a seat  2  and a seat tilt apparatus  3  according to the invention. The chassis supports two front wheels  4 , which are driven by electrically powered motors and a pivotal rear wheel  5 . The chassis also comprise an electrical seat lift device (not shown) for vertical movement of the seat  2 . In the figure, the seat lift is in its lowest position.  
      The seat  2  comprises a seat bottom  7 , a backrest  8  and arm rests  9 , which armrests are connected to the backrest  8 . In the shown embodiment the backrest  8  is fixedly connected to the seat bottom  7 . The backrest  8  may however also be pivotal in relation to the seat bottom by means of a backrest tilt arrangement, which is known per se to the skilled person. The seat bottom  7  is further fixed to a seat frame  6 , which also supports a leg rest  10 .  
      As will be described more in detail below, the seat frame  6  is fixed to the seat tilt apparatus  3  and the seat tilt apparatus is in turn fixed to a vertically movable part of the seat lift device.  
      With reference to  FIGS. 2 and 3 , the seat tilt apparatus comprises a base member  11 , which is shown separately in  FIG. 3 . The base member  11  comprises a central mounting plate  12  with a plurality of mounting holes  13  for being secured by screws or the like to the vertically movable part of the seat lift or to a stationary part of the chassis. Hence, the seat lift may be omitted if desired. The base member  11  also comprises two side members  14  extending downwards from the mounting plate  12  in parallel to each other and perpendicular to the mounting plate  12 . In the shown embodiment the side members  14  are separate parts fixedly secured to the mounting plate  12 . The mounting plate  12  and side members may however also form an integral part. A seating  15  for a bearing is formed in a forward lower portion of each side member  14 . A guide slot  16  is formed in each side member  14 . Each guide slot comprises a first forward portion  16   a  which is proximal to the seating  15  and a second rear portion  16   b  which is distal to the seating  15 . The first  16   a  and second  16   b  portions are connected by an intermediate third portion  16   c.  The first portion  16   a  is generally horizontal and essentially longer than the combined length of the second  16   b  and third  16   c  portion.-The second portion  16   b  is angled downward to the right as seen in the figure at an angle of approximately 45°.  
      One of the side members  14  further comprises a shaft journal  17 , which protrudes sideways, perpendicularly from an upper forward portion of the side member.  
      As best seen in  FIG. 2  the seat tilt apparatus also comprises a seat supporting member  20 . The seat supporting member comprises two support arms  21 . The support arms  21  are provided with mounting holes  23  for fixation of the seat frame  6 . Both support arms  21  are fixed to a first axle  22 , which extends in the transverse direction through both guide slots  16 . The first axle  22  is displaceable in the longitudinal direction of the guide slots  16  and guided in the vertical direction by the edges of the guide slots  16 . The first axle  22  is also pivotal relative to the base member  11 , around its own axis for allowing the support arms  21  to pivot around the axis of the first axle  22 . Both support arms  21  are fixed from rotation in relation to the first axle  22 , whereby the rotational angle between the two support arms  21 , in relation to the first axle  22  is fixed. In the shown embodiment the two support arms are parallel to each other, i.e. the rotational angle them between, in relation to the first axle is zero.  
      The seat tilt apparatus  3  further comprises two pivotal arms  24 . Both pivotal arms  24  are, at a first proximal end, connected to a common second axle  25 , which is journalled in bearings received in the seatings  15  (se  FIG. 3 ). The second distal end of each pivotal arm  24  is pivotally connected to an upper portion of one of the supporting arms  21 , by means of a screw and nut combination  26  extending through corresponding through holes in the respective supporting arm  21  and pivotal arm  24 .  
      A linear drive member  27  is at a first forward end  27   a  pivotally connected to the shaft journal  17  of the base member  11 . A second rear end  27   b  of the drive member  27  is pivotally connected to a portion of the first axle  22 , which portion extends sideways from the guide slot, in the same direction as the shaft journal. This portion of the first axle  22  is received by a cylindrical bore arranged at the rear end of the linear drive member. This portion of the first axle  22 , being in engagement with the cylindrical bore, is slightly cambered. By this means a slight angular adjustment of the first axle  22  is allowed without causing a corresponding angular adjustment of the drive member  27  when the drive member acts on the first axle  22  under load Such an angular adjustment of the drive member could otherwise cause damage to the drive member. The linear drive member  27  further comprises an electrical motor  28  and a transmission unit  29  for retracting and expelling the second end  27   b  of the linear drive member in relation to the first end  27   a.    
      In  FIG. 4   a  the seat tilt apparatus is shown when the seat supporting member is in a first horizontal position. In this position the first axle  22  is received by the downwardly angled distal portion  16   b  of the guide slots  16  in both side members  14 . When the electrical motor  28  is actuated for rotation at a constant speed in a first direction, the rear end  27   b  (covered in  FIG. 4   a ) of the drive member  27  is retracted towards the forward end  27   a.  This linear retraction is also effected under a constant linear speed. The first axle  22  is guided by the distal portion  16   b  of the guide slot  16  to move diagonally up to the left as seen in the figures. During this diagonal movement, the speed of the first axis in the horizontal forward direction is lower than the linear speed of the drive member rear end  27   b.  It is only motion of the first axle  22  in the horizontal direction which causes the seat supporting member  20  to pivot around the articulate points  26  between the seat supporting member  20  and the pivotal arms  24 . Thereby, the comparatively low speed in the horizontal direction constitutes a low gear ratio between the movement of the drive member and the horizontal movement of the first axle  22  which causes the seat member  20  to tilt. This in turn lowers the load on the drive member  27 , whereby a comparatively weak and small drive member suffices for achieving a satisfactory initiation of the tilt action, even if the seat is occupied by a heavy person.  
      The fact that the first axle  22 , during this initial phase, moves in a vertical direction further accomplishes that the rear portion of the seat supporting member  20  is lifted at the same time as the forward portion. The decreased pivotal movement of the seat supporting member around the connections  26  further lowers the load on the drive member, which load is caused by the gravitational force acting on the forward portion of the seat supporting member in front of the articulate points  26  and the lever arm constituted by this forward portion of the seat support member  20 .  
      In  FIG. 4   b  a partly tilted position is illustrated. At this position the first axle  22  has entered the intermediate curved portion  16   c  of the guide slot  16 . During its movement in this curved portion the first axle  22  gradually increases its speed in the horizontal direction and the vertical speed is gradually decreased correspondingly. This change of speed in the horizontal direction corresponds to an increase of the gear ratio between the drive member  27  and the seat supporting member  20 . At this phase of the tilting motion, the load on the drive member is lower than in the initial phase, whereby the comparatively weak drive member still is capable to continue the tilting motion in spite of the increased gear ration.  
      In  FIG. 4   c  the seat supporting member  20  has reached its fully tilted position. As seen in the figure the first axle  22  has then been displaced horizontally along the first proximal portion  16   a  of the guide slot  16 . During this displacement the first axle  22  has moved at its full horizontal speed, which horizontal speed is essentially the same as the linear speed of the drive member, due to the small angle between the horizontal portion  16   a  of the guide slot and the longitudinal extension of the drive member  27 . At this final movement the load on the drive member  27  is comparatively low and the tilting motion may be completed at swiftly at a desired speed.  
      According to an embodiment which is not shown in the figures, the two pivotal arms  21  of the supporting member  20  may be fixed to the second axle  25  with a small rotational angle them between. Especially a first pivotal arm being arranged at the opposite side of the base member in relation to the linear drive member, may be slightly displaced in the clockwise direction as shown in the figures, in relation to a second pivotal arm arranged adjacent to the drive member. When the seat supporting member is in the horizontal position, the free end of a first supporting arm connected to the first pivotal arm will thereby be at a slightly higher level than the free end of the second supporting arm connected to the second pivotal arm. When the drive member acts on the first axle under heavy load there might be a tendency of angular adjustment of the first axle. Such an angular adjustment of the first axle could then cause the supporting arm being adjacent to the drive member to move slightly before the other supporting arm. With an angular arrangement of the pivotal arms according to this embodiment, any such unbalanced movement of the two supporting arms is compensated for and the two arms will be leveled under the load from the seat.  
      According to the embodiments described above the seat tilt apparatus comprises a double linkage having two supporting arms, two pivotal arms and two guide slots. It is however also contemplated that for some applications it might suffice for the seat tilt apparatus to comprise a single linkage, whereby one supporting arm, one pivotal arm and one guide slot may be omitted. The skilled person also realizes that the seat tilt apparatus may comprise a multiple linkage with several sets of supporting arms, pivotal arms and guide slots arranged in parallel to each other.  
      Above, exemplifying embodiments of the invention have been described and illustrated. However, the invention should not be seen as limited to these embodiments but may be varied within the scope of the attached claims.