Abstract:
Enhanced stability of a wheelchair having a lift mechanism is provided, through utilization of a support apparatus including a unitized chassis for operatively connecting a seat and wheels of the wheelchair. A tilt sensor and controller preclude operation of the wheelchair with the seat in raised position while the wheelchair is resting on or traversing an uneven or sloping surface.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to powered and non-powered wheeled chairs for use by handicapped and disabled persons.  
       BACKGROUND OF THE INVENTION  
       [0002]     Wheelchairs, both powered and unpowered, have long been used to provide handicapped and disabled persons with independent mobility to assist them in leading more normal lives.  
         [0003]     Prior powered wheelchairs, sometimes also referred to as power chairs, have utilized a frame-like chassis fabricated from a number of individual frame elements which are welded and/or bolted together to form the frame-like chassis. A non-structural decorative cover is sometimes provided, for attachment to the chassis, for esthetic or safety considerations. Examples of such frame-like chassis in powered wheelchairs are disclosed in U.S. Pat. No. 6,935,448, to Goertzen et al.; U.S. Pat. No. 6,773,032, to Redman et al.; U.S. Pat. No. 6,640,916, to Schaffner et al.; and U.S. Pat. No. 6,357,776, to Goertzen et al.  
         [0004]     The frame-like construction of the chassis in prior powered wheelchairs is undesirable for several reasons. Because such frame-like chassis require complex joining operations for a substantial number of components, the cost for constructing such frame-like chassis is relatively high. Frame-like chassis are also typically structurally inefficient, thereby causing the weight and size of the powered wheelchair to be larger than is desirable. Having a powered wheelchair weigh more, or be larger than it otherwise could be, makes a powered wheelchair having a frame-like chassis more difficult to transport, less capable of being operated in confined areas, and reduces operating time between battery charges, due to the increased power consumption by drive units in moving the extra unnecessary weight of the wheelchair. The frame-like chassis of prior powered wheelchairs also typically flex to a larger degree than is desirable, Such flexing can result in instability, or at least a perception of instability for a person operating the wheelchair.  
         [0005]     Some prior wheelchairs, both powered and unpowered, have also incorporated a lifting device, for raising the seat of the wheelchair in a manner allowing better access to countertops, wall mounted cabinets, and to facilitate use of bathroom fixtures or appliances in buildings not constructed specifically for handicapped or disabled persons. The addition of such a lifting device can also facilitate other activities, such as entry into, or exit from, a vehicle, and can allow a person seated in the wheelchair to raise themselves to a position where they can maintain normal eye-level contact with persons who are standing, during social activities. Examples of wheelchairs incorporating such lifting devices are provided by U.S. Pat. No. 6,793,232, to Wing; U.S. Pat. No. 6,431,650, to Visone; U.S. Pat. No. 5,601,302, to Beard et al.; U.S. Pat. No. 4,993,736, to Garman et al.; and U.S. Pat. No. 4,613,151, to Kielczewiski.  
         [0006]     Prior wheelchairs having lift mechanisms, regardless of the particular type of lifting apparatus utilized, have proven to be less than satisfactory and/or impractical for a variety of reasons. Some have simply been too large and cumbersome to operate for practical usage. In general, none of the prior approaches to providing a lifting apparatus in a wheelchair has provided sufficient stability, when the lifting device is fully extended to raise the seat to a maximum height above the surface upon which the wheelchair is resting, or over which the wheelchair is traveling, while the wheelchair was traversing an uneven surface, or moving up or down an access ramp.  
         [0007]     The inherent instability of prior wheelchairs having a lifting device has sometimes been the result of unavoidable flexing in the frame-like chassis to which the lifting device was attached. Additional flexing in the lifting device itself, and relative movement between components of the lifting device in prior wheelchairs, has also contributed to the instability observed or perceived at the seat of prior wheelchairs. A small movement of only one hundredth of an inch, or so, for example, due to flexing of a frame-like chassis, may be greatly magnified to become 1½ to 2 inches of movement at the seat of the wheelchair, when the lifting device is fully extended. Additional flexing or movement within the lifting device is also substantially magnified as the seat of the wheelchair is lifted further away from the chassis by the lifting device.  
         [0008]     Prior wheelchairs having lifting devices have also not heretofore incorporated any sort of sensing and control apparatus for precluding raising of the seat, or for providing automated and safe lowering of the seat, where an attempt is being made to operate the wheelchair with the seat in a raised position while the wheelchair is tilted at an angle which could result in the wheelchair tipping over.  
         [0009]     It is desirable, therefore, to provide an improved wheelchair, and/or apparatuses for use in a wheelchair, which overcome one or more of the problems and observed in prior wheelchairs discussed above.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     The invention provides an improved wheelchair through use of a unitized chassis for operatively connecting a seat and wheels of the wheelchair. The unitized chassis utilizes integrally joined plates for bearing structural loads more efficiently and effectively than prior frame-like wheelchair chassis, to provide a wheelchair chassis that is substantially stiffer and lighter weight than prior wheel chair chassis. The unitized chassis also functions as a protective and decorative cover, thereby eliminating the need for a separate non-structural cover of the type typically used in prior wheelchairs having frame-like chassis.  
         [0011]     As used herein, with respect to various embodiments of the invention, the term unitized structure refers to a structure having individual components which are integrally joined to one another by a process such as welding or brazing, or to structures having individual elements which are formed, molded, cast, or cured, in such a manner that the individual elements are permanently and integrally connected to one another, rather than being separably joined by bolted connections or other types of removable fasteners. A unitized chassis, according to the invention, may be constructed from a variety of metallic materials, such as aluminum, from non-metallic materials, such as plastics or composite materials, or from a combination of metallic and non-metallic materials.  
         [0012]     In one form of the invention, a unitized chassis is provided for operatively connecting a seat and wheels of a wheelchair, wherein the chassis includes a substantially planar base plate, and a peripheral side plate. The planar base plate defines a periphery of the base plate, a longitudinal axis of the chassis, and a transverse axis of the chassis extending substantially perpendicular to the longitudinal axis. The peripheral side plate has an edge thereof joined as a unitized structure to the periphery of the base plate, and has a width thereof extending substantially orthogonally to the base plate. The base plate and peripheral side plate of the chassis may be formed from a single piece of material. The chassis may be constructed such that, when the chassis is oriented for operative attachment of the seat and wheels of the wheelchair, the base plate and peripheral side plate of the unitized chassis form an open-bottomed box-shape shell, with the base plate forming a top surface of the chassis, and the peripheral side plate depending substantially downward from the base plate.  
         [0013]     A unitized chassis, according to the invention, may further include at least one longitudinal rib, integrally joined to the base plate, and disposed inboard of the peripheral side plate. The longitudinal rib may be integrally joined at one or both longitudinal ends thereof to the peripheral side plate. The unitized chassis may further include at least one secondary plate, which is offset from the base plate, and joined as a unitized structure to both the peripheral side plate and the longitudinal rib. The secondary plate may be further joined as a unitized structure to the base plate. The unitized chassis may also include one or more transverse ribs integrally joined to the base plate. Transverse ribs may also be integrally joined to other parts of the chassis, such as longitudinal ribs or the peripheral side wall.  
         [0014]     The invention may also take the form of a wheelchair apparatus including a seat, wheels, and a unitized chassis, according to the invention, operatively connecting the seat and the wheels. The wheelchair apparatus may further include a motor for driving a driven wheel of the wheelchair by an operative connection between the motor and the driven wheel. A wheelchair, according to the invention, may also include a lift mechanism operatively connecting the seat to the chassis. The lift mechanism may be a scissors lift mechanism.  
         [0015]     A wheelchair, according to the invention, may further include a controller having a tilt sensor for sensing an angle of tilt of the wheelchair, and precluding extension of the lift mechanism if the sensed angle of tilt exceeds a predetermined safe angle of tilt. The controller, in a wheelchair according to the invention, may further retract the lift mechanism to a fully lowered position, at a controlled rate of retraction, if the lift mechanism is extended when the tilt sensor detects that the angle of tilt of the wheelchair exceeds the predetermined safe angle of tilt. The predetermined safe angle of tilt may be a function of the extension of the lift mechanism from the fully lowered position and/or the direction of the angle of tilt.  
         [0016]     In some forms of the invention, the lift mechanism may be a scissors lift mechanism, having an upper and a lower inner frame, and upper and lower pairs of outer links. The upper and lower inner frames may each have left and right side links thereof joined into a unitized structure by a non-protruding cross member, with each of the left and right side links of the upper and lower inner frames having a respective upper and lower attachment point at opposite ends thereof and a respective intermediate attachment point disposed between the upper and lower attachment points. Each of the outer links in the upper and lower pairs of outer links have upper and lower attachment points at opposite ends thereof and intermediate attachment point disposed between the upper and lower attachment points of the outer links.  
         [0017]     The pair of lower outer links includes a right and left outer link, rotatably joined at respective intermediate attachment points thereof to the right and left side links of the lower inner frame respectively, at the intermediate attachment points of the right and left side links of the lower inner frame. The pair of upper outer links includes a right and a left upper outer link rotatably joined at the respective intermediate attachment points thereof to the right and left links of the upper inner frame respectively, at the intermediate attachment points of the right and left side links of the upper inner frame. The lower ends of the right and left upper outer links are rotatably joined respectively to the attachment points at the upper ends of the right and left side links of the lower inner frame. The upper ends of the right and left lower outer links are rotatably joined respectively to the attachment points at the lower ends of the right and left side links of the upper inner frame.  
         [0018]     A scissors mechanism, according to the invention, may further include a lower guide apparatus having a translating element operatively joined to a guide element for translating movement of the translating element relative to the guide element. The attachment points at the lower ends of either the lower outer links, or the left and right side links of the lower inner frame, are rotatably connected to the translating element of the lower guide apparatus. The lower guide apparatus may include a pair of guide rods spaced from, and extending parallel to one another.  
         [0019]     In some forms of the invention, the attachment points at the lower ends of the lower outer links of a scissors lift mechanism may be rotatably attached to the unitized chassis, and the attachment points at the lower ends of the right and left side links of the lower inner frame of the scissors mechanism may be attached to the translating element of the lower guide apparatus, with the guide element of the lower guide apparatus being attached to the chassis in a manner constraining the lower ends of the lower inner frame to translate along the longitudinal axis of the chassis, as the scissors mechanism moves between a fully raised and a fully lowered position of the scissors mechanism with respect to the chassis.  
         [0020]     A scissors lift mechanism, according to the invention, may further include a linear actuator operatively attached between the chassis and the translating element of the lower guide apparatus for moving the translating element in a longitudinal direction along the guide element. The linear actuator may have first end thereof rotatably attached to the chassis and a second end thereof rotatably attached to the lower inner frame, with the actuator providing relative linear movement between the first and second ends of the linear actuator. In some forms of the invention, the linear actuator may be powered. A powered linear actuator, according to the invention, may include a hydraulic cylinder for moving the first and second ends of the linear actuator relative to one another.  
         [0021]     A scissors mechanism, according to the invention, may further include an upper guide apparatus having a translating element operatively joined to a guide element for translating movement of the translating element of the upper guide apparatus relative to the guide element of the upper guide apparatus. The upper ends of either the upper outer links, or the left and right side links of the upper inner frame, may be rotatably connected to the translating element of the upper guide apparatus. The guide element of the upper guide apparatus may include a pair of guide rods spaced from, and extending parallel to one another.  
         [0022]     The scissors mechanism, according to the invention, may further include a seat mounting plate, with the upper ends of the right and left side links of the upper inner frame being attached to the seat mounting plate, and the upper ends of the upper outer links being rotatably attached to the translating element of the upper guide apparatus, with the guide element of the upper guide apparatus being attached to the seat mounting plate in a manner constraining the upper ends of the upper inner frame to translate along the longitudinal axis of the chassis, as the scissors mechanism moves between a fully raised and a fully lowered position of the seat mounting plate with respect to the chassis.  
         [0023]     A seat mounting plate, according to the invention, may be a unitized structure having a substantially planar mounting base defining a periphery of the mounting plate, a longitudinal axis of the mounting base, and a transverse axis of the mounting plate extending substantially perpendicular to the longitudinal axis of the mounting plate. The seat mounting plate may also include a peripheral mounting plate side wall, having an edge thereof joined as a unitized structure to the periphery of the mounting base, and having a width thereof extending substantially orthogonally to the mounting plate. The peripheral mounting plate side wall may also include a stiffening flange.  
         [0024]     Other aspects, objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:  
         [0026]      FIG. 1  is a perspective top view of an exemplary embodiment of a powered wheelchair, according to the invention, with a seat of the wheelchair in a raised position.  
         [0027]      FIG. 2  is a bottom perspective view of the exemplary embodiment of the wheelchair of  FIG. 1 , with the seat in a raised position.  
         [0028]      FIGS. 3 and 4  are left side and front orthographic views, respectively, of the exemplary embodiment of the wheelchair shown in  FIGS. 1 and 2 , with the seat in a raised position.  
         [0029]      FIGS. 5 and 6  are top and bottom perspective views, respectively of the exemplary embodiment of the wheelchair shown in  FIG. 1 , with the seat in a fully lowered position.  
         [0030]      FIGS. 7 and 8  are front and right side orthographic views, respectively of the exemplary embodiment of the wheelchair shown in  FIG. 1 , with the seat in a fully lowered position.  
         [0031]      FIG. 9  is an orthographic bottom view of the exemplary embodiment of the wheelchair shown in  FIG. 1 , with various components removed to illustrate various construction and connection details of the exemplary embodiment  
         [0032]      FIG. 10  is a schematic illustration of the connections of a tilt sensor and controller within the exemplary embodiment of the wheelchair shown in  FIG. 1 .  
         [0033]      FIG. 11  is a top perspective view of a unitized chassis of the exemplary embodiment of the wheelchair shown in  FIG. 1 .  
         [0034]      FIG. 12  is an orthographic bottom view of the unitized chassis of  FIG. 11 .  
         [0035]      FIG. 13  is a bottom perspective view of the unitized chassis of  FIG. 11 .  
         [0036]      FIG. 14  is a partial cut away bottom perspective view of the unitized chassis of  FIG. 11 .  
         [0037]      FIG. 15  is a partially cut away top perspective view of a seat mounting plate of a scissors lift mechanism, of the exemplary embodiment of the wheelchair shown in  FIG. 1 .  
         [0038]      FIG. 16  is a top orthographic view of a guide apparatus of a scissors lift mechanism of the exemplary embodiment of the wheelchair of  FIG. 1 .  
         [0039]      FIG. 17  is a sectional orthographic view, taken along line  17 - 17  of  FIG. 16 , illustrating various internal features of the components of the guide apparatus of  FIG. 16 . 
     
    
       [0040]     While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0041]      FIGS. 1-8  show an exemplary embodiment of a powered wheelchair  100 , according to the invention, having a seat  102  operatively joined to a pair of rear-mounted drive wheels  104  and a pair of front-mounted casters  106 , by a wheelchair support apparatus  108  which includes a unitized chassis  110  and a scissors lift mechanism  112 .  FIGS. 1-4  illustrate the wheelchair  100 , from various angles, with the scissors lift mechanism  112  in a fully raised position.  FIGS. 5-8  show the powered wheelchair  100 , from various angles, with the scissors lift mechanism  112  in a fully lowered position.  
         [0042]     As shown by dashed lines in  FIG. 3 , and solid lines in  FIGS. 5 and 6 , the exemplary embodiment of the powered wheelchair  100  also includes a bellows-like protective shroud  114  around the scissors mechanism  112 . A rear shroud  116  is attached at the top rear of the chassis  110  as a partial protective cover for an actuator, to be described below, and to provide mounting for tail and directional lights, as best seen in  FIG. 8 . Protective/decorative covers  120  are also provided at the two front corners of the chassis  110 .  
         [0043]     As shown in  FIG. 9 , each of the rear wheels  104  is connected to be driven directly by a drive motor assembly  122 , through an operative connection  124 . The drive motor assemblies  122  are fastened directly to the chassis  110  by a series of bolts passing through holes  124  in secondary plates  126  of the chassis  110 . For clarity of illustration, only the left drive motor assembly  122  and operative connection  124  are shown in  FIG. 9 , but it will be understood, by those having skill in the art, that each of the drive wheels  104  is similarly connected by an operative connection  124  to a right drive motor assembly  122  bolted to a secondary base  126  on the right side of the chassis  110  by bolts passing through holes  124  in the secondary base  126 .  
         [0044]     As shown in  FIGS. 2, 6  and  9 , the powered wheelchair  100  of the exemplary embodiment also includes a battery  128  mounted centrally within a battery compartment  129  of the unitized chassis  110 , and a battery cover  130  for closing the battery compartment  129 . As shown in  FIG. 6 , the exemplary embodiment of the powered wheelchair  100  further includes a battery charger/power supply  132 , mounted on an underside of the unitized chassis  110  and operatively connected between the battery  128  and the drive motor assemblies  122 .  
         [0045]     As illustrated schematically in  FIG. 10 , the exemplary embodiment of the powered wheelchair  100  also includes a controller  134  having a tilt sensor  136  for sensing an angle of tilt, in one or more directions, of the wheelchair  100 . The tilt sensor  136  and controller  134  are operatively connected between the seat  102 , the unitized chassis  110  and a linear actuator  138 , to be described in more detail below of the scissors lift mechanism  112  for precluding extension of the lift mechanism if the sensed angle of tilt exceeds a predetermined safe angle of tilt. The controller  134 , of the exemplary embodiment, also retracts the scissors lift mechanism to the fully lowered position, at a controlled rate of retraction, if the lift mechanism is extended when the tilt sensor  136  detects that the angle of tilt of the wheelchair  100  exceeds the predetermined safe angle of tilt. The predetermined safe angle of tilt, in the controller  134  of the exemplary embodiment, is a function of the degree of extension of the lift mechanism  112  from the fully lowered position, such that the predetermined safe angle of tilt that is allowable will be greater when the seat  102  is closer to the chassis  110  then when the seat  102  is extended further away from the chassis  110  toward the fully raised position of the seat  102 . In various embodiments of the invention, the tilt sensor  136  and controller  134  may be mounted together, or separately, at various locations on the various components of the wheelchair  100 . The controller  134  may also be programmed to allow different predetermined safe angles of tilt in various directions, i.e., the safe angle of tilt allowed in a fore and aft direction may be different from a safe angle of tilt allowed in a sized-to side direction, or in various directions therebetween.  
         [0046]      FIGS. 11-14  show the unitized chassis  110  of the exemplary embodiment of the powered wheelchair  100 , in various orientations, to aid in understanding of the construction of the unitized chassis  110 . The unitized chassis  110  includes a substantially planar base plate  140  and a peripheral side plate  142 . The base plate  140  defines a periphery  144  of the base plate  140 , a longitudinal axis  146  of the chassis  110 , and a transverse axis  148  of the chassis  110  extending substantially perpendicular to the longitudinal axis  146  of the chassis. The peripheral side plate  142  has an upper edge thereof joined as a unitized structure to the periphery  144  of the base plate  140 , and a width  150  thereof extending substantially orthogonally base plate  140  of the unitized chassis  110 . The width  150  of the peripheral side plate  142  varies, in the exemplary embodiment, as the peripheral side plate  142  extends around the entire periphery  144  of the base plate  140 .  
         [0047]     In the exemplary embodiment, the base plate  140  of the chassis  110  is substantially rectangular in shape, with the periphery  144  thereof forming longitudinally spaced substantially parallel front and rear edges  152 ,  154  of the base plate  140 , and transversely spaced, substantially parallel, right and left edges  156 ,  158  of the base plate  140 . The peripheral side plate  142  of the exemplary embodiment of the unitized chassis  110  includes right and left transversely spaced portions  160   162  which extend substantially longitudinally along the right and left sides of the chassis  110 , and front and rear longitudinally spaced portions  164 ,  166  thereof which extend substantially transversely across the front and rear of the unitized chassis  110  respectively. The right and left longitudinally spaced portions  160 ,  162  of the peripheral side plate  142  are joined into a unitized structure with the base plate  140  along the front and rear edges  152 ,  154  of the base plate  140 , and the right and left transversely spaced portions  160 ,  162  of the peripheral side plate  142  are joined in a unitized structure to the base plate  140  along the right and left edges  156 ,  158 , respectively, of the base plate  140 . Adjoining corners of the transversely and longitudinally spaced portions  160 ,  162 ,  164 ,  166  of the peripheral side wall  142  of the chassis  110  are also joined together to form a unitized structure.  
         [0048]     By virtue of the above described orientation and connection of the base plate  140  and peripheral side plate  142 , when the chassis  110  is oriented for operative attachment of the seat  102  and wheels  104 ,  106  of the wheelchair  100 , the base plate  140  and peripheral side plate  142  of the chassis  110  form an open-bottom box-shaped shell, with the base plate  140  forming a top surface  167  of the chassis  110 , and the peripheral side plate  142  depending substantially downward from the base plate  140 .  
         [0049]     As shown in  FIGS. 12 and 13 , the unitized chassis  110 , of the exemplary embodiment, includes first and second longitudinal ribs  168 ,  170  disposed inboard of the right and left transversely spaced portions  160 ,  162  of the peripheral side plate  142 , and centered generally about the longitudinal axis  146  of the unitized chassis  110 . The first and second longitudinal ribs  168 ,  170  are integrally joined as a unitized structure to the base plate  140  and to the front and rear longitudinally spaced portions  164 ,  166  of the peripheral side plate  142 .  
         [0050]     As shown in  FIGS. 12-14 , the secondary base plates  126 , in the exemplary embodiment of the chassis  110  include a horizontal portion  172  thereof, which is offset from the base plate  140 , and joined as a unitized structure to both the peripheral side plate  142  and an adjacent one of the first and second longitudinal ribs  168 ,  170 . The base plates  126 , of the exemplary embodiment, are substantially L-shaped and include a vertical portion  174 , extending from the forward end of the horizontal portion  172  of the secondary base plate  126 . The vertical portion  174  of the secondary base plate is joined as a unitized structure to the base plate  140 , the peripheral side plate  142  and an adjacent one of the first and second longitudinal ribs  168 ,  170 .  
         [0051]     As best seen in  FIG. 11 , the base plate  140  of the unitized chassis  110 , of the exemplary embodiment, includes a pair of access holes  176 , to facilitate attachment of the drive motor assemblies  122  to the horizontal portions  172  of the secondary base portion  126  of the chassis  110 .  
         [0052]     As shown in  FIG. 11 , the right and left transversely spaced portions  160 ,  162  of the peripheral side plate  142  of the chassis  110  include downwardly opening slots  178  therein for passage therethrough of the operative connection  124  between the drive motor assemblies  122  and the driven wheels  104 .  
         [0053]     As shown in  FIGS. 12-14 , the chassis  110  also includes an interior transverse rib extending between the first and second longitudinal ribs  168 ,  170  and joined as a unitized structure to the first and second longitudinal ribs  168 ,  170 , and the base plate  140  of the chassis  110 . An additional pair of third and fourth longitudinal ribs  182 ,  184  are disposed inboard of the first and second longitudinal ribs  168 ,  170  respectively, and are integrally joined as a unitized structure to the interior transverse rib  180 , the base plate  140 , and the rear longitudinally spaced portion  166  of the peripheral side wall  142 .  
         [0054]     A series of mounting holes  186  are provided at the forward corners of the unitized chassis  110  for a bolted attachment of the front mounted casters  106 . Comer gussets  188  are integrally joined to the inside surfaces of the peripheral side wall, at the front corners of the chassis  110 , and the rear corners of the chassis  110  are angled to provide additional stiffness to the unitized chassis  110 . The base plate  140  of the chassis  110  also includes a clearance hole  190  for passage therethrough of the linear actuator  138  of the scissors lift mechanism  112 , in a manner described in more detail below and best seen in  FIG. 9 .  
         [0055]     A unitized chassis, according to the invention, may be fabricated by a variety of methods, and from a variety of materials. In the unitized chassis  110  of the exemplary embodiment, a welded construction is used. A blank cut from a single sheet of aluminum includes the base plate  140 , the right and left portions  160 ,  162 , and front and rear portions  164 ,  166  of the peripheral side wall  142  of the chassis  110 . The right, left, front, and rear portions  160 ,  162 ,  164 ,  166  are all bent perpendicularly in the same direction from the base  140  portion of the blank, and the adjoining corners of the right, left, front, and rear portions  160 ,  162 ,  164 ,  166  of the blank are welded to one another to form the peripheral side wall  142  of the chassis  110 , resulting in the open-bottomed box-like shell of the chassis  110 . The first through fourth longitudinal ribs  168 ,  170 ,  182 ,  184 , the interior transverse rib  180 , the secondary bases  126 , and the corner gussets  188  are then positioned inside of the shell and welded to the shell, and one another, to form the completed unitized chassis  110  of the exemplary embodiment of the powered wheelchair  100 . To facilitate fixturing and manufacture, the internal components of the chassis  110  include tabs (not shown) at various points along their interface with the shell, which slide into corresponding slots (not shown) in the shell of the chassis  110  to facilitate fabrication.  
         [0056]     Those having skill in the art will recognize that a unitized chassis, according to the invention, may be fabricated in a number of different ways and from a variety of materials other than aluminum. For example, the unitized chassis  110  of the exemplary embodiment can alternatively be fabricated as a thin-wall aluminum casting, rather than as a welded assembly. A combination of investment casting and welding can also be utilized. In embodiments of the invention utilizing non-metallic materials, such as plastic or composites, a unitized chassis, according to the invention, may be fabricated by a variety of known molding and/or joining techniques. For a unitized chassis, according to the invention fabricated from either metallic or non-metallic materials, or from a combination of various types of materials, the integral joints between various components forming the unitized structure may also include the use of adhesive bonding, for example.  
         [0057]     As shown in  FIGS. 1-4 , the scissors lift mechanism  112  of the wheelchair support apparatus  108 , of the exemplary embodiment of the powered wheelchair  100 , includes a plurality of operatively connected links, and a linear actuator  138  having a first end  192  thereof operatively attached to the chassis  110  by an actuator attachment rod  194  which passes through holes in the first through fourth longitudinal ribs  168 ,  170 ,  182 ,  184 , in the manner shown in  FIGS. 2, 4  and  9 . A second end  196  of the linear actuator  138  is pivotably attached to a bracket  198  extending from one of the links, in the form of a lower inner frame  200  of the scissors lift mechanism  112 . In the exemplary embodiment, the linear actuator is a hydraulic cylinder, having an attached electric motor driven pump for supplying pressurized hydraulic fluid to the hydraulic cylinder for selectively extending and contracting the scissors lift mechanism  112  between the fully raised and fully lowered position of the seat  102  with respect to the chassis  110 .  
         [0058]     In addition to the lower inner frame  200 , the scissors lift mechanism  112  of the exemplary embodiment also includes: an upper inner frame  202 ; right and left outer upper links  204 ,  206 ; right and left lower outer links  208 ,  210 ; a lower guide apparatus  212 ; an upper guide apparatus  214 ; and a seat mounting plate  216  of unitized construction.  
         [0059]     As shown in  FIG. 4 , the upper and lower inner frames  200 ,  202  each include left and right side links  218 ,  220  joined into a unitized structure by a non-protruding cross member  222 . In the exemplary embodiment, the left and right side links  218 ,  220  and the cross member  222  of the upper and lower inner frames  200 ,  202  are fabricated from square tubing joined into a unitized structure by welding the cross members  222  between the left and right side links  218 ,  220 . The cross members  222  are positioned and connected to the left and right side links  218 ,  220  in such a manner that the upper and lower facing surfaces (when the seat is in a fully lowered position) of the cross members  222  are flush with the upper and lower surfaces of the left and right side links  218 ,  220  of the upper and lower inner frames  200 ,  202 . Having the cross member  222  joined in this manner, facilitates contraction of the scissors lift mechanism  112  to a minimal height when the seat  102  is in the fully lowered position.  
         [0060]     In the exemplary embodiment, an additional cross bar  224 , of solid rectangular shaped material, is also welded to the left and right side links  218 ,  220  of the upper and lower inner frames  200 ,  202 , to provide additional rigidity and strength to the scissors lift mechanism  112 . The upper and lower inner frames  200 ,  202  of the scissors lift mechanism  112 , of the exemplary embodiment, are essentially identical to one another, to facilitate manufacturing and reduce inventory, except for the addition of the bracket  198  to the cross member  222  of the lower inner frame  202  for attachment of the second end  196  of the linear actuator  138 .  
         [0061]     The right and left upper outer links  204 ,  206  are rotatably attached at their mid-points to the mid-points of the left and right side links  218 ,  220  of the upper inner frame  202 . In similar fashion, the left and right lower outer links  208 ,  210  are rotatably attached at their respective mid-points to the left and right side links  218 ,  220  of the lower inner frame  200 . The upper ends of the left and right lower outer links  210 ,  212  are rotatably attached to the lower ends of the left and right side links  218 ,  220  of the upper inner frame  202 . The lower ends of the left and right upper outer links  206 ,  204  are rotatably attached to the upper ends of the left and right side links  218 ,  220  of the lower inner frame  200 .  
         [0062]     As shown on  FIG. 15 , the seat mounting plate  216 , of the exemplary embodiment, is a unitized structure having a substantially planar mounting base  226  defining a periphery  228  of the mounting base, a longitudinal axis  230  of the mounting plate, and a transverse axis  232  of the mounting plate  216  extending substantially perpendicular to the longitudinal axis of the mounting plate  216 . The seat mounting plate  216 , of the exemplary embodiment, also includes a peripheral mounting plate side wall  234 , having an upper edge thereof joined as a unitized structure to the periphery  228  of the mounting base  226 , and having a width  236  thereof extending substantially orthogonally to the mounting base  226 . Adjoining corners of the various portions of the peripheral mounting plate side wall  234  are integrally joined into a unitized structure, in the same manner as described above with regard to the unitized chassis  110 . The unitized seat mounting plate  216  of the exemplary embodiment also includes a stiffening flange  238  extending inward from the peripheral mounting plate side wall  234 , as shown in the partial cutaway of one corner of the seat mounting plate  216  in  FIG. 15 .  
         [0063]     The unitized seat mounting plate  216 , of the exemplary embodiment, may be fabricated in a variety of forms and from a variety of materials, in the same manner as described above with regard to fabrication of a unitized chassis, in accordance with the invention.  
         [0064]     As shown in  FIGS. 16 and 17 , the lower guide apparatus  212  includes a translating element  240 , operatively joined to a guide element in the form of two guide rods  242  which are spaced from and extend parallel to one another between front and rear support brackets  244 ,  246 , for translating movement of the translating element  240  relative to the guider rods  242 . Specifically, the translating element  240  is a substantially solid bar having threaded attachments  248  at opposite ends thereof for rotatable attachment to the lower ends of the right and left side links  220 ,  218  of the lower inner frame  200 . The translating element further includes a pair of linear bearings  250  for close-tolerance passage therethrough of the guide rods  242 . As will be understood, by those having skill in the art, when the lower guide apparatus  212  is attached to the upper surface of the unitized frame  110  of the exemplary embodiment, as illustrated in  FIG. 1 , with the guide rods  242  extending substantially parallel to the longitudinal axis  146  of the chassis  110 , the lower ends of the inner frame  200  rotatably attached to opposite ends of the translating element  240 , and the lower ends of the left and right lower outer links  210 ,  208  rotatably attached to the unitized chassis  110  by mounting brackets  252 , the lower guide apparatus  212  will constrain the lower end of the lower inner frame  200  to move fore and aft along the longitudinal axis  146  as the linear actuator  138  contracts and extends.  
         [0065]     Construction and mounting of the upper guide apparatus  214  to the seat mounting plate  216  is essentially identical to the construction and mounting described above for the lower guide apparatus  212 , with the exception that the front support bracket  244  of the lower guide apparatus  212  is eliminated in the upper guide apparatus  216 , with the forward ends of the guide rods  242  of the upper guide apparatus being fastened directly to the peripheral mounting plate side wall  234  through a pair of holes  254  extending through the front of the peripheral mounting plate side wall  234 .  
         [0066]     The upper ends of the right and left upper outer links  204 ,  206  are attached to opposite ends of the translating element of the upper guide apparatus, and the upper ends of the upper inner frame  202  are rotatably attached to brackets  256  extending from the bottom surface of the seat mounting plate  216 , in such a manner that the translating element  244  of the upper guide apparatus  214  is constrained to move fore and aft the longitudinal axis  146 ,  230  of the chassis  110  and seat mounting plate  216  as the linear actuator  138  contracts and extends.  
         [0067]     In practicing the invention, all of the rotatable connections between the various elements of the scissors lift mechanism  112  are preferably closely toleranced and aligned to minimize clearance and any movement other than rotation in the joints. Those having skill in the art will readily recognize that, by virtue of the construction and connection of the lower and upper inner frames  200 ,  202 , and other elements of the scissors lift mechanism, such as the upper and lower guide apparatuses  212 ,  214 , in conjunction with the structurally non-flexing unitized chassis  110  and seat mounting plate  216 , the exemplary embodiment, of the powered wheelchair  100  may be safely operated with the seat  102  in the fully raised position with substantially less movement and instability than occurred in prior wheelchairs having a lifting mechanism. It will be further recognized, that, by virtue of the construction and relative location of the various components of the exemplary embodiment of the powered wheelchair  100 , the center of gravity of the powered wheelchair  100  is very low, with reference to the surface supporting the wheelchair  100 , to thereby also further enhance stability of the wheelchair  100 . It will be yet further noted, by those having skill in the art, that the relative location of attachment points of the scissors lift mechanism  112  with respect to the drive wheels  104  and casters  106 , and the manner in which the scissors lift mechanism  112  is attached to the seat mounting plate  216  also enhance stability of the exemplary embodiment of the powered wheelchair  100 .  
         [0068]     Attachment of the linear actuator  138 , in the manner described above, and as shown in the drawings, with the first end  192  of the linear actuator  138  located below and behind the drive wheels  104  allows a relatively long stroke in the linear actuator  138 , and, provides an advantageous alignment of the actuator  138  with the remainder of the scissors lift mechanism  112  which reduces the force that is required to be produced the linear actuator in moving the seat from the fully lowered to the fully raised position.  
         [0069]     By virtue of the various aspects of the invention described above, the inventor has constructed a wheelchair, essentially identical to the exemplary embodiment of the powered wheelchair  100  described herein, which is substantially lighter in weight than prior powered wheelchairs performing similar functions. Specifically, the wheelchair constructed by the inventor weighs approximately 150 to 170 pounds, depending on the particular seat configuration and accessories selected, which is capable of carrying and lifting an individual weighing 325 pounds. Prior powered wheelchairs offering similar functionally weigh 225 to 400 pounds.  
         [0070]     Those having skill in the art will recognize that, although the invention has been described herein with regard to the exemplary embodiment of the powered wheelchair  100 , various aspects and features of the invention may be provided in a variety of other forms in alternate embodiments of the invention. For example, other embodiments of the invention may utilize other types of linear actuators, such as ball-screw-type actuators, for example. The linear actuator may also be connected in a different fashion, from that described above, such as between the chassis and the translating element of either upper or lower guide apparatus of a scissors lift mechanism, according to the invention. Specifically, a linear actuator, such as a ball-screw or hydraulic or pneumatic cylinder, may be operatively attached between the translating element and one of the mounting brackets of the upper and/or lower guide apparatuses of a scissors lift mechanism, according to the invention. The guide rods of a guide apparatus, according to the invention may also take the form of a rotatable screw engaging a nut mounted in a translating element of a scissors lift mechanism, according to the invention.  
         [0071]     Those having skill in the art will also recognize that the invention may be practiced with lift mechanisms of a type other than the scissors lift mechanism described herein with regard to the exemplary embodiment and certain aspects of the invention.  
         [0072]     Those skilled in the art will yet further recognize that the invention may also be practiced in non-powered wheelchairs, and in both powered and non-powered wheelchairs without a lift mechanism. A scissors lift mechanism, according to the invention may also incorporate a manually operable linear actuator rather than a powered linear actuator of the type described above.  
         [0073]     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.  
         [0074]     Preferred embodiments of this invention are described herein, including the best mode known to the inventor for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventor expects skilled artisans to employ such variations as appropriate, and the inventor intends for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.