Abstract:
A modular actuator has a combination cam member and scissors mechanism in a compact unit that is capable of lifting heavy loads. A pull member is capable of controlling and combining two different mechanisms that travel at different rates through two ranges of motion. The cam member has a profile that is dimensioned and configured to maintain a constant range of motion.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/621,439, filed on Oct. 22, 2004. 
     
    
     BACKGROUND OF INVENTION  
       [0002]     This invention relates in general to personal mobility vehicles and more particularly, to seating systems for personal mobility vehicles. Most particularly, this invention relates to the mechanisms for lifting and/or tilting seat assemblies for wheelchairs and like vehicles.  
         [0003]     Mechanisms for lifting and/or tilting wheelchair seat assemblies are well known. Such mechanisms typically include an actuator (i.e., a lead screw, nut, motor, and gearbox). By convention, parts of the actuator are integrated in the base and parts of the actuator are integrated in the seat assembly. Such actuators encounter a high load from an initial or lowered position. This high load typically causes premature wear of the actuator. A lifting and/or tilting mechanism is needed that is in the form of a compact unit that is capable of lifting heavy loads.  
       SUMMARY OF INVENTION  
       [0004]     The present invention is directed towards a modular actuator that meets the foregoing needs. According to one embodiment of the invention, the modular actuator has a cam member in combination with a scissors mechanism in a compact unit that is capable of lifting heavy loads. Moreover, the invention is directed towards an actuator having component parts that may be integrated with the base or the seat assembly in such a manner that the parts are not integrated with both the base and the seat assembly. The invention is also directed towards a pull member that is capable of controlling and combining two different mechanisms that travel at different rates through two ranges of motion. The invention is still further directed towards a lifting and/or tilting assembly that includes two different actuator mechanisms that function to apply lifting and/or tilting force throughout different periods of operation, whereby transition from one mechanism to the other mechanism is substantially undetected by one supported by the assembly. The invention is further directed towards a cam member having a profile that is dimensioned and configured to maintain a constant range of motion and keep a constant load on the actuator.  
         [0005]     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  
       [0006]      FIG. 1  is a rear perspective view of a wheelchair and a modular actuator according to the present invention.  
         [0007]      FIGS. 2A and 2B  are enlarged front perspective views of lift modules according to alternative embodiments of the present invention.  
         [0008]      FIG. 3  is an enlarged front perspective view of a nut assembly of the lift modules shown in  FIG. 2B .  
         [0009]      FIGS. 4A and 4B  are cross-sectional views in elevation of the lift modules shown in  FIGS. 2A and 2B  in lowered positions.  
         [0010]      FIGS. 5A and 5B  are cross-sectional views in elevation of the lift modules shown in  FIGS. 2A and 2B  in partially raised positions.  
         [0011]      FIGS. 6A and 6B  are cross-sectional views in elevation of the lift modules shown in  FIGS. 2A and 2B  in fully raised positions.  
         [0012]      FIG. 7  is an enlarged partial front perspective view of the lift module shown in  FIG. 2B  in a lowered position.  
         [0013]      FIG. 8  is a graph of the load profile of the lift modules.  
         [0014]      FIG. 9  is a front perspective view of a tilt module according to one embodiment of the present invention.  
         [0015]      FIG. 10  is a rear perspective view of a tilt module according to another embodiment of the present invention.  
         [0016]      FIG. 11 a  rear perspective view of the tilt module shown in  FIG. 10  with portions removed to permit viewing of a secondary over-tilt lock and a one-way clutch and further showing enlarged detail views of the secondary over-tilt lock and the one-way clutch.  
         [0017]      FIG. 12 a  front perspective view of the tilt module shown in  FIG. 10  with portions removed to permit viewing of an over-tilt lock and a position sensor and further showing enlarged detail views of the over-tilt lock and the position sensor.  
         [0018]      FIG. 13  is front perspective view of the lift and tilt modules stacked upon one another. 
     
    
     DETAILED DESCRIPTION  
       [0019]     Referring now to the drawings, there is illustrated in  FIG. 1 a  wheelchair  10  and a modular actuator according to the present invention. The wheelchair  10  is a power wheelchair and more particularly, a mid-wheel drive wheelchair, although the invention can be practiced with other suitable drive configurations, such as front and rear wheel drive configurations. The wheelchair  10  comprises a base  12 , which includes a frame supported on a supporting surface by a plurality of wheels, such as drive wheels  14 , front wheels  16 , and rear wheels  18 , as shown in the drawing. The drive wheels  14  can be powered by any suitable power source and can be controlled by any suitable input device, including but not limited to a joystick (not shown). A seat assembly  20  is supported by the base  12 . The seat assembly  20  comprises a seat  22  and a seat back  24 , which may articulate (i.e., recline) relative to the seat  22 . According to the present invention, the seat assembly  20  is adapted to be supported by one or more modular actuators  26 ,  76  that may be controlled to tilt and/or lift the seat assembly  20 .  
         [0020]     Modular actuators in the form of lift modules  26  are shown in  FIGS. 2A and 2B . The lift modules  26  comprise a first member  28 , which is adapted to be secured to the base  12 , and the second member  30 , to which the seat assembly  20  is adapted to be secured, and which is adapted to move relative to the first member  28 . In the illustrated embodiment of the invention, the first member  28  comprises a bottom plate  32  and a plurality of side walls, including longitudinal side walls  34   a ,  34   b  and end walls  36   a ,  36   b . Similarly, the second member  30  comprises a top plate  38  and a plurality of side walls, including longitudinal side walls  40   a ,  40   b  and end walls  42 . The first and second members  28  and  30  are configured to form housings for enclosing or otherwise supporting other elements of the lift module  26 . It should be appreciated that the particular configuration of the lift module  26  shown is provided for illustrative purposes and that the invention may be practiced with other configurations.  
         [0021]     The first and second members  28  and  30  are connected together by a lift mechanism, such as the scissors mechanism  44  shown. The scissors mechanism  44  comprise members  46 ,  48  that cross and are pivotally connected to one another. In the illustrated embodiment of the invention, forward ends of the cross members  46 ,  48  are pivotally connected proximate to forward regions of the longitudinal side walls  34   a ,  34   b ,  40   a ,  40   b  of the first and second members  28  and  30 . Rearward ends of the cross members  46 ,  48  are slidably and pivotally connected proximate to rear regions of the longitudinal side walls  34   a ,  34   b ,  40   a ,  40   b  of the first and second members  28  and  30 . The latter connection may be provided by opposing tracks  50   a ,  50   b ,  52   a ,  52   b  (shown in  FIG. 2B ) proximate the rear regions of the longitudinal side walls  34   a ,  34   b ,  40   a ,  40   b  of the first and second members  28  and  30 .  
         [0022]     The scissors mechanism  44  is preferably driven by a lead screw  54 , which in turn can be driven by a motor  56 , or other suitable prime mover. The motor  56  may indirectly drive the lead screw  54  through a gearbox  58 , as shown in the drawings. The lead screw  54  may carry for longitudinal movement a nut assembly  60 , as shown more clearly in  FIG. 3 . In the illustrated embodiment of the invention, the nut assembly  60  includes an internal thread  62  for engagement with the lead screw  54 . It should be noted that the nut assembly  60  has a nut  60   a  therein. Webbing  60   b  in a semi-cylindrical portion  60   c  of the nut assembly  60  aids in trapping the nut  60   a  within the nut assembly  60 . It should be appreciated that the illustrated nut assembly is provided for illustrative purposes and that the invention may be practiced or carried out with other assemblies or configurations.  
         [0023]     As shown in  FIG. 3 , the nut assembly  60  has means for supporting pull members. Although the supporting means may take on any form, the particular means shown includes support members and more particularly, laterally extending support members  64 . The particular support members  64  shown may be shaped to provide clearance for other elements of the lift module  26 , including but not limited to, for example, the motor  56 . The support members  64  may also be structured to withstand longitudinal forces. The support members  64  cooperate with pull members, such as the pull members  66  shown in  FIGS. 2A and 2B . The pull members  66 , as shown in the drawings, may include but are not limited to pull rods, although other pull members may be suitable for practicing the invention.  
         [0024]     A portion of the nut assembly  60 , a medial portion  68  in the illustrated embodiment shown in  FIG. 3 , may support one or more low friction elements. One or more low friction elements, such as the surface rollers  70  shown, may be provided for engagement with and movement relative to the first member  28  of the lift module  26 . One or more other low friction elements, such as the cam rollers  72   a ,  72   b  shown, may be provided for engagement with and movement relative to a cam member  74   a ,  74   b , which will be described in greater detail below.  
         [0025]     One or more cam members  74   a ,  74   b  are adapted to engage the low friction elements  72   a ,  72   b . As shown in  FIGS. 4A and 4B , when the lift module  26  is in a lowered or initial position, the cam rollers  72   a ,  72   b  engage the cam members  74   a ,  74   b . As the lead screw  54  is driven by the motor  56  (shown in  FIGS. 2A and 2B ) and gearbox  58 , the lead screw  54  threads the nut assembly (i.e., from left to right when viewing the drawings). The cam rollers  72   a ,  72   b  push up on the cam members  74   a ,  74   b , and the cross member  46  of the scissors mechanism  44  pushes up on the second member  30  of the lift module  26 . The cam rollers  72   a ,  72   b  and cam members  74   a ,  74   b  are beneficial to raise the second member  30  when the scissors mechanism  44  has insufficient mechanical advantage. It should be noted that the cam members  74   a ,  74   b  have slots or grooves  74   c  that receive nubs or pins  60   d  on opposing sides of the nut assembly  60 , as shown in  FIG. 3 . The grooves  74   c  and pins  60   d  function as guides and stops for the cam members  74   a ,  74   b . In addition, the pins  60   d  and grooves  74   c  function to prevent an external force from inadvertently lifting or tilting the scissors mechanism  44  prior to the nut assembly  60  transferring the load from the cam rollers  72   a ,  72   b  to the pull members  66 , as will be described in greater detail in the description that follows.  
         [0026]     A transition is made from the cam members  74   a ,  74   b  to the scissors mechanism  44  when the cam rollers  72   a ,  72   b  reach the end of the profile of the cam members  74   a ,  74   b , as shown in  FIGS. 5A and 5B . It should be noted that throughout the operation of the cam members  74   a ,  74   b , the scissors mechanism  44 , particularly the cross members  46 , is pulled by the pull members  66  (shown in  FIGS. 2A and 2B ) via the nut assembly  60 . Although the pull members  66  pull the scissors mechanism  44 , the pull members  66  are not in tension. The cross members  46  act to lift the second member  30  of the lift module  26  by virtue of the connection between the cam members  74   a ,  74   b  and the cross members  46 . At the transition between the cam members  74   a ,  74   b  and the scissors mechanism  44 , the nut assembly  60  continues to pull the pull members  66 , which are placed in tension, and which in turn pull the cross members  46  of the scissors mechanism  44 . At this transition point, the scissors mechanism  44  has a sufficient mechanical advantage to lift the second member  30  of the lift module  26 . The transition between the cam members  74   a ,  74   b  and the scissors mechanism  44  should be smooth because the cam rollers  72   a ,  72   b  reach the end of the profile of the cam members  74   a ,  74   b  when the mechanical advantage of the scissors mechanism  44  is sufficient to raise the second member  30  under load to a fully raised position, as shown in  FIGS. 6A and 6B . The pull members  66  contribute to combine two ranges of motion (i.e., via the cam members  74   a ,  74   b  and the scissors mechanism  44 ) and two rates (i.e., different rates) of motion into one continuous motion. This is graphically illustrated in  FIG. 8 , which simulates load versus time of the lift module  26 . In the graph, the vertical axis represents motor current draw or motor load and the horizontal axis represents travel of the nut assembly  60 . The relative flat curve is provided to maximize the performance of the motor  56  and gearbox  58  and reduce or eliminate any perceived transfer of function from cam members  74   a ,  74   b  to pull members  66 . Although the graph in  FIG. 8  represent an ideal operational characteristic of the invention, actual data may not support a line that is as flat, as represented.  
         [0027]     Another modular actuator according to the present invention is illustrated in  FIGS. 9 and 10 . This modular actuator is in the form of a tilt module  76 . The tilt module  76  is similar to the lift module  26  described above in that the tilt module  76  comprises a first member  28 , which is adapted to be secured to a wheelchair base, and the second member  30 , to which a wheelchair seat is adapted to be secured, and which moves relative to the first member  28 . In the illustrated embodiment of the invention, the first member  28  comprises a bottom plate  32  and a plurality of side walls, including longitudinal side walls  34   a ,  34   b  and end walls  36   a ,  36   b . Similarly, the second member  30  comprises a top plate  38  and a plurality of side walls, including longitudinal side walls  40   a ,  40   b  and an end wall  42 . The first and second members  28  and  30  are configured to form housings for enclosing or otherwise supporting other elements of the tilt module  76 . It should be appreciated that the particular configuration of the tilt module  76  shown is provided for illustrative purposes and that the invention may be practiced with other configurations.  
         [0028]     The first and second members  28  and  30  are connected together by a modified scissors mechanism, such as the scissors mechanism  78  shown. The scissors mechanism  78  comprises one or more tilt members  80  and struts  82  that are pivotally connected to the tilt members  80 . In the illustrated embodiment of the invention, lower ends of the tilt members  80  are slidably and pivotally connected proximate to rearward regions of the longitudinal side walls  34   a ,  34   b  of the first member  28 . Lower ends of the struts  82  are pivotally connected proximate to forward regions of the longitudinal side walls  34   a ,  34   b  of the first members  28 . The former connection may be provided by opposing tracks  84   a ,  84   b  (shown in  FIG. 9 ) proximate the rear regions of the longitudinal side walls  34   a ,  34   b  of the first members  28 . The latter connection may be provided by opposing pivot bosses  86   b  (only one shown in  FIG. 9 ) proximate the forward regions of the longitudinal side walls  34   a ,  34   b  of the first members  28 .  
         [0029]     The tilt module  76 , like the lift module  26  described above, is preferably driven by a lead screw  54  (shown in  FIG. 9 ), which in turn can be driven by a motor  56  (shown in  FIG. 9 ), or other suitable prime mover. The motor  56  may indirectly drive the lead screw  54  through a gearbox  58  (shown in  FIG. 9 ). The lead screw  54  may carry for longitudinal movement a nut assembly  60 , like that shown in  FIG. 3 . In the preferred embodiment of the invention, the nut assembly  60  includes an internal thread  62  for engagement with the lead screw  54 . It should be noted that the nut assembly  60  has a nut  60   a  therein. Webbing  60   b  in a semi-cylindrical portion  60   c  of the nut assembly  60  aids in trapping the nut  60   a  within the nut assembly  60 . It should be appreciated that the illustrated nut assembly is provided for illustrative purposes and that the invention may be practiced or carried out with other assemblies or configurations.  
         [0030]     As described above with reference to the lift module  26 , the nut assembly  60  has means for supporting pull members. Although the supporting means may take on any form, the particular means shown includes support members and more particularly, laterally extending support members  64 . The particular support members  64  shown may be shaped to provide clearance for other elements of the tilt module  76 , including but not limited to, for example, the motor  56 . The support members  64  may also be structured to withstand longitudinal forces. The support members  64  cooperate with pull members  66 , such as, for example, the pull rods shown in  FIG. 10 , although other pull members may be suitable for practicing the invention.  
         [0031]     A portion of the nut assembly  60 , a medial portion  68  in the illustrated embodiment, supports one or more low friction elements. One or more low friction elements, such as the surface rollers  70  shown, may be provided for engagement with and movement relative to the second member  30  of the tilt module  76 . One or more other low friction elements, such as the cam rollers  72   a ,  72   b  shown, may be provided for engagement with and movement relative to a cam member  74   a ,  74   b.    
         [0032]     One or more cam members  74   a ,  74   b  are adapted to engage the low friction elements  72   a ,  72   b . When the tilt module  76  is in a lowered or initial position, the cam rollers  72   a ,  72   b  engage the cam members  74   a ,  74   b . As the lead screw  54  is driven by the motor  56  and gearbox  58 , the lead screw  54  threads the nut assembly  60 . The cam rollers  72   a ,  72   b  push up on the cam members  74   a ,  74   b , which push up on the second member  30  of the tilt module  76 . The cam rollers  72   a ,  72   b  and cam members  74   a ,  74   b  are beneficial to push up the second member  28  when the tilt mechanism  44  has insufficient mechanical advantage.  
         [0033]     A transition is made from the cam members  74   a ,  74   b  to the tilt members  80  and struts  82  when the cam rollers  72   a ,  72   b  reach the end of the profile of the cam members  74   a ,  74   b . At this transition, the tilt members  80  are pulled by the pull members  66  via the nut assembly  60 . This transition should be smooth because the cam rollers  72   a ,  72   b  reach the end of the profile of the cam members  74   a ,  74   b  when the mechanical advantage of the tilt members  80  and struts  82  is sufficient to raise the first member  28  under load to a fully raised position, as shown in the drawings. The pull members  66  contribute to combine two ranges of motion (i.e., via the cam members  74   a ,  74   b  and the tilt members  80  and struts  82 ) and two rates of travel into one continuous motion.  
         [0034]     According to a preferred embodiment of the invention, an over-tilt lock, as shown in  FIGS. 9 and 10 , may be provided. The over-tilt lock locks out the pull members  66  between a minimum and a maximum tilt angle (e.g., between about 0 degrees and about 50 degrees) just prior to when the user&#39;s weight shifts and goes from a positive force in a clockwise direction (when viewing  FIG. 9 ) to a positive force in a counter clockwise direction (when viewing  FIG. 9 ) about the tilt pivot point. This prevents the seat assembly  20  from inadvertently over tilting. Obviously, the affects of shifts in the user&#39;s weight are dependent on, for example, the user&#39;s weight, the center of gravity of the seat assembly  20  and the user combined, and the relationship between the center of gravity and the tilt axis. Positioning of the module  76  and/or the motor  56  and/or the gearbox  58  could effectively position the center of gravity over the module  76  and the wheelchair base  12  as desired.  
         [0035]     It should be noted that a spring  106  may be provided to function as an over-tilt mechanism to be used in conjunction with or in lieu of the over-tilt lock. As stated above, a load reversal may occur as the tilt module  76  tilts back beyond a certain threshold, that is, when the user&#39;s weight shifts and goes from a positive force in one direction to a positive force in another clockwise direction about the tilt pivot point, or to a negative force. The spring  106  may be provided for positively biasing the load in as this shift occurs to a negative force.  
         [0036]     In addition, the spring functions a load compensating mechanism may be employed to balance the load applied against the motor  56  during operation of the modules  26 ,  76 . For example, in the illustrated embodiments of the invention, one or more helical coil springs  106  cooperate with the scissors mechanisms  44 ,  78 . A spring  106  may be carried by a rigid rod, which is within the spring  106 , and which may be present on opposing sides of the modules  26 ,  76 . The rod may be free to move in a longitudinal direction through operation of the modules  26 ,  76  and prevent the spring  106  from moving radially. In the illustrated embodiment of the invention, a first end of the rod is connected to a movable block  108  while a second end of the rod, opposite the first end, is a free end (i.e., not physical fixed). It should be noted that the spring  106  may be at least partially encased in a sleeve  110 , tube, or the like, which further prevents the spring  106  from moving radially. In this way, the spring  106  is maintained along a substantially linear axis coincident with the longitudinal axes of the rod and the sleeve  110 . Throughout the operation of the scissors mechanisms  44 ,  78 , the mechanical advantage may not be constant. As a consequence, the load on the motor  56  may be greater during certain periods of operation. During periods when the load on the motor  56  is reduced, the motor  56  may operate at a greater speed. This may be undesirable. That is to say, it may be desirable to operate the motor  56  at a substantially constant rate of motion. In accordance with the invention, the spring  106  may be compressed by the scissors mechanisms  44 ,  78  during operation of the modules  26 ,  76 . As a consequence, the load applied by the spring  106  increases as the scissors mechanisms  44 ,  78  gain a greater mechanical advantage. Since the spring  106  is compressed throughout the operation of the modules  26 ,  76 , the spring  106  could offer little to no resistance initially when the modules  26 ,  76 , and more particularly, the scissors mechanisms  44 ,  78  are first operated (i.e., when the load on the motor  56  is greatest), and offer greater resistance as the load on the motor  56  decreases due to the mechanical advantage of the scissors mechanisms  44 ,  78 . In this way, the motor  56  may be operated at a substantially constant rate of motion.  
         [0037]     The over-tilt lock function of the tilt module  76  is embodied in two separate mechanisms. When the nut assembly  60  is in contact with the cam members  74   a ,  74   b , the over tilt function may be carried out by the pins  60   d  on the nut assembly  60  engaging the grooves  74   c  on the cam members  74   a ,  74   b , as shown in  FIG. 11 . When the cam rollers  72   a ,  72   b  disengage from the cam members  74   a ,  74   b , a secondary over tilt lock is engaged. As shown in  FIG. 11 , there are two pull members  66  per side of the tilting module  76 . The lower set of pull members  66  each have a single timing pin  98  and a single locking pin (not shown) extending in a radial direction from the long axis of the pull member  66 . The timing pin  98  engages an “S” shaped timing slot  102  in the base  32  of the tilt module  76 . The locking pin is housed in a matching slot  104  in the lateral support members  64  of the nut assembly  60  (shown in  FIG. 3 ). As the cam members  74   a ,  74   b  transition the length of the bottom plate  32 , the nut assembly  60  pulls all the pull members  66 . The lower set of pull rods  66  with the timing pin  98  is fully extended between the nut assembly  60  and the pivot connections at the rearward ends of the scissors mechanism  78 . The lower pull rod timing pin  98  is moved through the timing slot  102  in the base  32  while this pull rod  66  is pulled into a tensile load condition. While this transition to a tensile load condition is accomplished, the locking pin in the lower pull rod  66  comes out of the mating slot in the lateral support members  64  of the nut assembly  60  and, through the rotation of the pull rod  66  via the timing pin  98 , is now prevented from retreating back into the mating slot  104 . With this pull rod  66  now prevented from retreating back into the support arm member  64  in over tilt condition, this pull rod  66  becomes a compression member instead of a tensile member, holding the parts stationary and preventing inadvertent over tilt.  
         [0038]     The present invention may further comprise a position sensor  90 , as shown in  FIG. 12 . The sensor  90 , for example, may be in the form of an optical encoder or reader that counts the number of turns or revolutions of the lead screw  54  to determine the position of the actuator. This sensor may be used in conjunction with or in lieu of position switches, such as end switches and the inhibit switches  96   c  described below, to reduce available tilt angle, adjust the angle at which reduced speed of the wheelchair base will be implemented, or act as a fine tuning adjustment for what is considered a bottom angle or “home” position for the tilt.  
         [0039]     The present invention may further comprise a one-way anti-back drive clutch  92 , as shown in  FIG. 12 , that prevents the weight of the user from closing the actuator. For example, the motor  56  may be free to turn unencumbered by frictional resistance in one direction to raise the user. However, resistance prevents the actuator from creeping down due to the load imposed on the actuator by the weight of the user. This is particularly useful when lead screw  54  is not a self locking thread design and the nut assembly  60  is as low friction as possible to allow a smaller motor per weight. Under conditions where the motor  56  is neither being driven up or down, the weight of the user can be enough to cause the lead screw  54  to be back driven in a downward command direction. The one-way clutch  92  may freewheel when the motor  56  rotates in a direction to increase the tilt angle of the tilt module  76 , or to increase the lift height of the lift module  26 . When electrical power to the motor  56  is removed or the motor  56  is driven in a direction to reduce the tilt angle, or the lift height, the one way clutch engages and impedes the tendency of the motor  56  to reduce the tilt angle. The motor  56  can overpower the clutch on demand, but the weight of the user is not sufficient to reduce tilt angle, or the lift height. This function can also be accomplished by a conventional electrical brake (not shown) on the motor armature shaft. The clutch is preferred over that of an electrical brake as release of an electrical brake can allow over rapid decrease in tilt angle, or lift height.  
         [0040]     The present invention may also have a switch rail  94 , as shown in  FIG. 12 , or may otherwise support one or more position switches. For example, the switch rail  94  shown may support two end switches (not shown) and an inhibit switch  96   c  between the end switches. The end switches are threshold switches that prevent further operation of the actuator when the actuator reaches upper and lower operating limits. For example, the end switches may be normally open switched. When the nut assembly  60  reaches one end of the switch rail  94 , a support member  64  may engage an end switch to close the end switch at that end of the switch rail  94 . The closed switch may produce a signal representing that the nut assembly  60  has been driven a threshold distance in a first direction. A controller could recognize this as a threshold condition and, for example, prevent the motor  56  from further operating and thus prevent the tilt module  76  from tilting further back. However, the motor  56  remains operable to turn in an opposite direction to return the module  76  forward toward a non-tilted orientation until, for example, the support member  64  of the nut assembly  60  closes the other end switch at the other end of the switch rail  94 . It should be appreciated that the end switches need not be normally open switches. The type of end switch used depending on the design of the electronics. Regardless of the switch orientation, the switches may operate as described above. The inhibit switch  96   c  may prevent operation of the wheelchair  10 , or limit performance of the wheelchair  10  under a certain condition. The inhibit switch  96   c  may signal the wheelchair base  10  when a certain height of lift, or a certain degree of tilt has been attained. Above a certain amount of lift or degree of tilt, the top operating speed of the wheelchair can or will be reduced to reduce or eliminate the occurrence of accidents resulting from a compromised driving position or compromised stability standpoint. For example, the inhibit switch  96   c  may be a mid-travel rocker switch, which may, for example, be a single pole switch, double throw switch that is thrown in a first direction to decrease the motor speed beyond a certain tilt angle, either when raising or lowering the top plate  38 , and that is thrown in a second direction to permit the motor  56  to operate at an increased motor speed below that tilt angle. That is to say the mid-travel rocker switch may function to reduce the available drive motor speed above a certain angle of tilt, via activation by the nut assembly  60 , and restore full drive motor speed below that angle as the nut assembly  60  directs the switch throw back to a full motor speed setting.  
         [0041]     The modular actuators (i.e., the lift and tilt modules  26 ,  76 ) according to the present invention are compact units that have high load carrying abilities. The actuators are adapted for use in new wheelchairs, or for use in converting existing wheelchairs, to wheelchairs having lift and tilt capabilities. The actuators could integrate with existing wheelchairs in a relatively short time with minimal efforts. The likeness of the lift and tilt modules  26 ,  76  would minimize component parts. That is to say, but for the scissors mechanisms  44 ,  78 , the component part of the lift and tilt modules  26 ,  76  may be substantially identical. Since the lift and tilt modules  26 ,  76  use common components to achieve two distinctly different modes of operations (i.e., lifting and tilting operations), economy in inventory and production is achieved. Moreover, the lift and tilt modules  26 ,  76  may be removable, reversible, and adjustable. The lift and tilt modules  26 ,  76  may function as structural elements of the wheelchair, and have low profiles that are particularly useful for wheelchair seat lifting and center of gravity seat tilting. This also allows lift and tilt wheelchairs to be offered with a low seat to floor height. High load bearing characteristics are achieved through the entire cycle of operation by two working mechanisms. During angles of low mechanical advantage for the scissors mechanism  44 , or the tilt members  80  and struts  82 , the cam members  74   a ,  74   b  function to push the second member  30  upward. The scissors mechanism  44 , or the tilt members  80  and struts  82 , takes over when enough mechanical advantage is available. The cam members  74   a ,  74   b  lifts loads in a low profile. These two mechanisms (i.e., cam members  74   a ,  74   b  and scissors mechanism  44 , or the tilt members  80  and struts  82 ) may be tuned (i.e., dimensioned and configured) to keep the load on the motor  56  and gearbox  58  constant to maximize the life of the drive train (i.e., the motor  56  and gearbox  58 ).  
         [0042]     It should also be appreciated that the pull members  66  marry the cam members  74   a ,  74   b  and the scissors mechanism  80 ,  82  as well as the scissors mechanism  44  so that the difference in the rate of motion of the two components (i.e., cam members  74   a ,  74   b  and the scissors mechanism  44 ) is not noticeable.  
         [0043]     As shown in  FIG. 13 , the lift and tilt modules  26  and  76  are adapted to be stacked upon one another. This permits the wheelchair seat assembly  20  to be lifted, tilted, or lifted and tilted, as shown.  
         [0044]     In a preferred embodiment of the invention, each module (i.e., lift and tilt module) is relatively thin and is most preferably about two inches in height to insure that the minimum seat height is maintained.  
         [0045]     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.