Patent Publication Number: US-8973713-B2

Title: Height adjustment system for wheelchair lift

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is related to a co-ending application Ser. No. 13/288,927, filed concurrently herewith, and entitled “Low Profile Wheelchair Lift With Direct-Acting Hydraulic Cylinders”, assigned to the assignee of the present application. 
     The present application is related to a co-pending application Ser. No. 13/288,936, filed concurrently herewith, and entitled “Wheelchair Lift Device with Pinned Floor Struts”, assigned to the assignee of the present application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to lifting devices, and more particularly, to a wheelchair lift device to provide access to stages, platforms, risers and other elevated structures for individuals with disabilities. 
     2. Description of the Background Art 
     Under the Americans With Disabilities Act of 1990 (the “ADA”), the U.S. government required that public buildings be accessible to the disabled. For persons requiring a wheelchair for mobility, abrupt changes in floor elevation have to be modified to enable access by wheelchair. The ADA permits vertical lifting devices to be used instead of a ramp. 
     Lifting devices for the disabled are known in the prior art. For example, U.S. Pat. No. 5,105,915 (Gary) describes a lifting device having a car including fixed sides and short, one-piece ramps at each end. The car is raised and lowered by a pantograph jack including a hydraulic pump driven by an electric motor controlled by switches. The patent also describes several lifting devices of the prior art. Another wheelchair lifting device is disclosed in U.S. Pat. No. 6,182,798 to Brady, et al., and assigned to AGM Container Controls, Inc., the assignee of the present invention. The &#39;798 patent discloses a lift device with gates at both ends of the lift car, transparent walls, a loading ramp, a dock plate, a stage height sensor, and numerous safety features. In addition, U.S. Pat. No. 7,926,618, also assigned to the assignee of the present invention, discloses a lift device suitable for elevating wheel chair-bound individuals to stages or platforms. 
     Wheel chair lift devices are often used repeatedly in conjunction with the same stage or platform, whereby the lift car is elevated numerous times to the very same height. It is therefore desirable to provide a control mechanism by which the maximum elevational height of the lift can be set in advance, or programmed, thereby automatically stopping the lift at the stage height repeatedly and consistently. The wheel chair lift device disclosed in assignee&#39;s prior U.S. Pat. No. 7,926,618 discloses a height adjustment mechanism accessible through a panel of the lift car for varying the elevational height of the lift. A rotatable arm is used to set the elevational height, and a knob secured to the end of such rotatable arm slides within a circular slot. The knob can be loosened to move the knob within the circular slot, thereby repositioning the rotatable arm. Once the knob is set to the desired elevational height, the knob is re-tightened, and the access panel is closed. 
     An alternate height adjustment mechanism is disclosed in assignee&#39;s U.S. Pat. No. 7,721,850 for use with a fixed-installation lift, wherein a cable attached to an actuator moves the actuator as the lift car moves, the actuator eventually engaging a microswitch when the lift reaches the desired maximum height. Adjustment of the maximum desired height requires an installer to adjust the relative position of the microswitch along a rail traversed by the actuator. 
     Portable wheelchair lifting devices generally require that the height to which the lift car is elevated be readily adjustable. Such lift devices are frequently moved from one stage or platform to another, and the elevations of two or more stages or platforms often differ from one another. On the other hand, once a portable lift is transported to a particular location, and the maximum height has been re-adjusted to suit the particular platform or stage at the new location, further height adjustments are neither required nor recommended. 
     Therefore, it is important to be able to quickly and easily adjust the maximum height to which the lift is elevated each time the lift is moved to a different platform or stage. Once the maximum height is set for the new stage or platform, it is also important that the lift should be able to raise the platform of the lift device repeatedly, and reliably, to the pre-set maximum height. Clearly, it would be advantageous to be able to verify that the mechanism used to signal that the maximum height has been reached is, in fact, operational before permitting the lift car to elevate; if the maximum height detection system is not working properly, and the lift is permitted to be elevated, the lift will not automatically stop when it reaches the desired maximum height. 
     In view of the foregoing, it is an object of the present invention to provide a wheel chair lift device suitable for lifting wheelchair-bound users up to the height of stages, platforms, risers and the like in a safe and reliable manner, and comporting with all applicable ADA requirements. 
     Another object of the present invention is to provide such a lift device that is relatively inexpensive, easy to construct and use, and simple to maintain. 
     A further object of the present invention is to provide such a lift device wherein the maximum height to which the lift car is raised can be quickly and easily adjusted for allowing the lift device to be repeatedly raised to the height of the platform with which the lift device is currently being used. 
     A still further object of the present invention is to confirm that the control system used to halt further elevation of the lift car, upon reaching the selected maximum height, is operational before the lift car is significantly elevated. 
     These and other objects of the present invention will become more apparent to those skilled in the art as the description of the present invention proceeds. 
     SUMMARY OF THE INVENTION 
     Briefly described, and in accordance with one aspect thereof, the present invention relates to a lift device used to provide access to a stage, platform, or the like for individuals with disabilities, including persons who rely upon wheelchairs or crutches to move about. The lift device includes a base for resting on the ground, and first and second guide members attached to, and extending generally vertically upward from, opposing sides of the base. A lift car is provided to support and elevate an occupant of a wheelchair. This lift car includes a structural frame, as well as a floor panel supported between the lower portions of first and second opposing sides of the structural frame. 
     A lifting mechanism, e.g., hydraulic cylinders, is provided to raise and lower the lift car. This lifting typically includes a motor for powering the lift mechanism. While a motor is used in the preferred embodiment to rotate a hydraulic pump, other types of wheel chair lift devices might use the motor to rotate a threaded rod, a worm gear, drive gear, or other mechanism for selectively causing the lift car to raise or lower. Irrespective of the specific lift mechanism used, a height adjust system is provided for stopping the operation of the motor powering the lift mechanism, and for stopping further raising of the lift car, when the lift car reaches a desired maximum height. The height adjust system permits the adjustment of the maximum height to which the lift car may be repeatedly lifted, e.g., the height of a platform or stage with which the lift is currently being used. 
     In the preferred embodiment, the height adjustment system includes a light-sending element having a magnetic backing for being releasably secured along one of the fixed vertical guide members at a selected height for sending light. The associated vertical guide member is metallic for allowing the light sending element to be magnetically attracted thereto. An optical sensor is secured to the lift car facing the vertical guide member for sensing light sent from an area lying proximate to the guide member. When the optical sensor receives light from the light-sending element, the optical sensor generates an electrical signal that prevents further operation of the motor in the direction that would further elevate the lift car. 
     In the preferred embodiment, the light-sending element is a passive element, i.e., a reflector or mirror, although the light-sending element could alternatively be an actual source of light. Preferably, a source of light is provided on the lift car, for example, proximate the optical sensor, for directing a beam of light toward the fixed vertical guide member. When the lift car has reached the desired maximum height, the reflector intercepts and reflects the beam of light back to the optical sensor. 
     In order to accurately position the reflector upon the vertical guide member, a placement tool is preferably provided, along with a reference port formed in the lift car. The placement tool includes a first end for being held by a user and a second end for releasably supporting the reflector. The lift car reference port is preferably disposed in a side wall of the lift car proximate to the vertical guide member; the reference port is aligned with the optical sensor in the sense that a beam of light passing through the reference port will strike the optical sensor. The reference port is adapted to slidingly receive the placement tool; accordingly, a technician can set the maximum height of the lift car by simply raising the lift car to the desired height, inserting the placement tool into the reference port, securing the reflector along the vertical guide member, and thereafter withdrawing the placement tool. 
     In the preferred embodiment, the placement tool can be releasably engaged with the reflector by placing the second end of the placement tool over the reflector and rotating the placement tool in a first rotational direction (e.g., clockwise). The placement tool can be disengaged from the reflector by rotating the placement tool in the opposite rotational direction (e.g., counter-clockwise). Ideally, the placement tool can remain in engagement with the reference port as the placement tool is rotated in either the first or second direction. Thus, if the reflector is not yet attached to the vertical guide member, the reflector can be engaged with the second end of the placement tool; the placement tool can be inserted into the reference port: the placement tool can be advanced toward the vertical guide member until the reflector is magnetically attached thereto; the placement tool can then be rotated within the reference report to disengage the reflector from the placement tool; and the placement tool can thereafter be withdrawn from the reference port. On the other hand, if the reflector is already attached to the vertical guide member and needs to be moved, then the placement tool may be inserted into the reference port; the placement tool can be advanced toward the reflector until the second end of the placement tool overlies the reflector; the placement tool can then be rotated to engage the reflector while the placement tool remains within the reference port; the placement tool can be slid away from the vertical guide member to detach the reflector from the vertical guide member; and the placement tool may then be withdrawn from the reference port to remove the reflector. In the preferred embodiment, the lift car includes a storage element for supporting the placement tool when it is not in use. 
     As noted earlier, it would be advantageous to confirm that the height adjust system is functioning properly before allowing the lift to be elevated. If the height adjust system were not functioning properly, and if this fact could be detected early on, then one could prevent the lift from being elevated until such problem is resolved. Such a failsafe confirmation technique is easily incorporated into the height adjust system just described. A second reflector is preferably permanently secured to the vertical guide member at a point that is aligned with the reference port of the lift car when the lift car is fully-lowered. When the lift car is fully lowered, the optical sensor receives light reflected by the second reflector, and generates a failsafe electrical signal in response thereto. The height adjust system is programmed to permit operation of the motor in the lifting mechanism when the lift car is in its fully-lowered position if the failsafe electrical signal is present. On the other hand, the height adjust system is programmed to prevent operation of the motor in the lifting mechanism, in the direction that would elevate the lift car, if the failsafe electrical signal is not present when the lift is in its fully-lowered position. Thus, if the optical sensor is not receiving light from the permanent reflector when the lift car is fully lowered, as would indicate a problem with either the light source or the optical sensor, then the lift will “never make it off the ground”. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the wheelchair lift device of the present invention positioned adjacent an auditorium stage for lifting a wheelchair occupant up to stage level. 
         FIG. 2  is a side view of the wheelchair lift device in its lowered position, and partially cut-away to reveal the platform of the lift car. 
         FIG. 3  is a side view of the wheelchair lift device similar to that shown in  FIG. 2  and further including caster wheels installed below the lift car for transport. 
         FIG. 4A  is an end view of the wheelchair lift device and depicting the front end of the lift device through which a user enters or exits when the lift car is fully-lowered. 
         FIG. 4B  is an end view of the wheelchair lift device and depicting the rear end of the lift device through which a user enters or exits when the lift car is elevated to stage level. 
         FIG. 5  is a top view of the wheelchair lift device and illustrating, in phantom lines, how the front gate and rear gate of the lift car swing open. 
         FIG. 6A  is a side view of the wheelchair lift device in an elevated position, and with several components omitted to reveal internal features. 
         FIG. 6B  is a sectional side view, similar to that of  FIG. 6A , but wherein a tubular vertical support beam is sectioned to reveal a hydraulic piston rod extending therethrough. 
         FIG. 6C  is another sectional side view, similar to that of  FIG. 6B , but wherein the hydraulic cylinder and lift car frame are sectioned, and wherein the hydraulic pump and associated electric motor are omitted to reveal the positioning of hydraulic tubing lines. 
         FIG. 7  is a perspective view of the wheelchair lift device in an elevated position as viewed from below the lift device to reveal a framework used to support the platform of the lift car, and wherein several components have been omitted to reveal internal features. 
         FIG. 8  is a perspective view of the wheelchair lift device in an elevated position as viewed from above the lift device, and wherein several components have been omitted to reveal internal features. 
         FIG. 9  is a perspective view which schematically illustrates the configuration of hydraulic tubing lines that extend below and around the lift car. 
         FIG. 10  is a schematic drawing illustrating the hydraulic components used to elevate and lower the lift car. 
         FIG. 11  is an electrical schematic showing the principal electrical components of the wheelchair lift device for controlling the elevation and lowering of the lift car. 
         FIGS. 12A ,  12 B and  12 C are schematic figures which illustrate how loading the platform of the lift car can deform the normally vertical orientation of the lift car, and how such problem is addressed in the preferred embodiment of the present invention. 
         FIG. 13  is a partial perspective view (with decorative skins omitted) of a light source and optical sensor used to control the maximum lift height, as well as a height adjustment tool placed in its stowed position. 
         FIG. 14  is a partial perspective view of one side of the lift car (with decorative skins omitted), and illustrating a U-shaped bracket serving as a reference guide when setting the maximum height of the lift car. 
         FIG. 15A  is a partial perspective view similar to  FIG. 14  but wherein the height adjustment tool is inserted into the U-shaped bracket to accurately place an optical reflector. 
         FIG. 15B  is a partial perspective view similar to  FIG. 15A  but wherein the height adjustment tool is being withdrawn to reveal the optical reflector placed thereby. 
         FIG. 16A  is a perspective close-up front view of the optical reflector shown in  FIG. 15B . 
         FIG. 16B  is a perspective close-up rear view of the optical reflector shown in  FIG. 16A . 
         FIG. 17A  is a partial perspective close-up view of the functional end of the height adjustment tool shown in  FIGS. 15A and 15B , before engaging the optical reflector. 
         FIG. 17B  is a partial perspective close-up view of the functional end of the height adjustment tool shown in  FIGS. 15A and 15B , after engaging the optical reflector. 
         FIG. 18  is a partial perspective close-up view of one of the pivot-mounted flooring struts used to support the lift car floor from the lift car frame. 
         FIG. 19  is a perspective view of a permanent optical reflector used to test the functionality of the optical system before allowing the lift car to be elevated. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A wheel chair lift device constructed in accordance with a preferred embodiment of the present invention is designated generally within  FIG. 1  by reference numeral  30 . Lift device  30  is adapted to provide access to an elevated stage or platform  32  by a disabled individual, e.g., wheel chair occupant  34 . Lift device  30  is positioned adjacent wall  38  of platform  32 . As shown in  FIG. 1 , front entry gate  40  of lift device  30  is opened, and individual  34  can board lift car  42  by wheeling onto lift car floor  44 . Lift car  42  includes two opposing side walls  46  and  48 , each provided with a transparent window  50  and  52 , respectively. A rear exit gate  54  can be opened after lift car  42  is elevated sufficiently to raise lift car floor  44  to the same height as platform  32  for allowing individual  34  to wheel onto platform  32 . This procedure can be reversed when individual  34  wishes to return back to ground level. 
       FIG. 2  is a side view of lift car  42  in its lowered position. Front entry gate  40  is hinged to side wall  48  by hinges  56  and  58 . Handle  60  is provided on the exterior of front entry gate  40  to aid in opening front entry gate  40 . Up-down toggle switch  62  is provided adjacent entry gate  40  to cause lift car  42  to be raised or lowered. A grab bar, shown by dashed lines  64  through window  52 , extends across the length of lift car  42  to aid a user. At the other end of lift car  42 , rear exit gate  54  is hinged to side wall  48  by hinges  66  and  68 . A hinged dock plate  70  is provided at the lower end of rear exit gate  54 ; hinged dock plate  70  pivots downwardly to meet with platform  32  as rear exit gate  54  is opened. Handle  72  is provided on the exterior of rear exit gate  54 , and another up-down toggle switch  74  is provided adjacent rear exit gate  54  to cause lift car  42  to be raised or lowered. Panel  76  is secured to the exterior of side wall  48 , and in the case of a power loss, panel  76  may be removed to permit access to a hand-operated hydraulic pump for safely lowering lift car  42  back to the ground. 
       FIG. 3  is a side view of lift car  42  as viewed from the opposite side as that shown in  FIG. 2 . In  FIG. 3 , removable access panel  78  permits access to a storage area wherein four casters are stored for use when transporting lift device  30 . Indeed, in  FIG. 3 , such casters, including rigid caster  80  and swivel caster  82 , are installed on the bottom of lift car to facilitate transport of lift device  30 . Also visible within  FIG. 3  is an electrical power cord  84 , including a ground fault circuit interrupter (GFCI)  86 , used to supply electrical power for operating lift device  30 . 
       FIGS. 4A and 4B  are end views of lift car  42 , and show the front entry gate  40  and rear exit gate  54 , respectively. Front entry gate  40  preferably includes a transparent window  88  of high-impact thermoplastic; likewise, rear exit gate  54  includes a transparent window  90  formed of high-impact thermoplastic. 
       FIG. 5  is a top view of lift car  42 . Front entry gate  40  is shown in solid lines in its closed position, and in dashed lines in an opened position. Rear exit gate  54  is likewise shown in solid lines in its closed position, and in dashed lines in an opened position. 
     Turning now to  FIGS. 6A. 6B  and  6 C, lift device  30  is shown with lift car  42  in an elevated position, and wherein the decorative/protective skins that usually cover side walls  46  and  48  removed. It may now be seen that lift device  30  includes a base  92 , including base side member  94 , for resting upon a floor when the wheelchair lift is in use. It will be noted briefly that base  92  is actually lifted off of the floor when, as shown in  FIG. 3 , the caster wheels are installed for transporting lift device  30 . Referring briefly to  FIGS. 7 and 8 , it will be seen that base  92  also includes an opposing base side member  96  opposite base side member  94 , and that base side members  94  and  96  are interconnected by base cross members  98 ,  100  and  102 . As shown in  FIGS. 7 and 8 , these cross members are preferably formed as telescoping members for allowing the length of such cross members to be adjusted. Fastening screws, such as screw  104 , can be loosened to set the length of such cross members, and then tightened to maintain the desired length. Construction of cross members  98 ,  100  and  102  in this manner helps to allow lift device  30  to be collapsed to a narrower width when being transported through narrow passageways. 
     Referring jointly to  FIGS. 6A through 8 , a first guide member  106  extends generally vertically upward from base side member  94 . First guide member  106  includes lower end  108  and upper end  110 . Lower end  108  of first guide member  106  is fixedly coupled to base side member  94 . Similarly, a second guide member  112  is secured at its lower end to base side member  96 , and extends generally vertically upward therefrom. In the preferred embodiment, the tubular members forming base  92  and guide members  106  and  112  are all formed of ASTM A36 steel. Unless otherwise described, the joints attaching such members to each other are formed by welding. In the preferred embodiment, guide members  106  and  112  are of rectangular cross-section and each include a hollow internal channel. 
     Still referring to  FIGS. 6A-6C ,  7  and  8 , lift car  42  includes a structural frame that has two opposing sides  114  and  116 . First side  114  is a generally rectangular shape including outer vertical members  118  and  120 , inner vertical members  122  and  124 , upper horizontal members  126 ,  128 , and  130 , and lower horizontal members  132 ,  134  and  136 . First side  114  extends generally vertically from lower horizontal members  132 ,  134 , and  136  to upper horizontal members  126 ,  128  and  130 . Second side  116  is essentially a mirror image of first side  114 . The manner in which first and second sides  114  and  116  are interconnected below lift car floor panel  44  will be described later. 
     In the preferred embodiment, lift car  42  is raised and lowered by a first hydraulic cylinder  138  and a second hydraulic cylinder  140 . First hydraulic cylinder  138  has a closed upper end, or butt end,  144 , and an opposing lower open end  146 . First hydraulic cylinder  138  has a piston rod  142  extendable from lower open end  146  (see  FIGS. 6B and 6C ). Piston rod  142  has a free end  148  extendable away from first hydraulic cylinder  138 , and an opposing captive end which remains within first hydraulic cylinder  138  at all times. Butt end  144  of first hydraulic cylinder  138  includes a tubular mounting bracket  145  (see  FIG. 9 ) for receiving bolt  150  which secures butt end  144  to upper structural frame member  128 ; thus, first hydraulic cylinder  138  moves up and down along with lift car  42 . Free end  148  of piston rod  142  is secured by bolt  149  to lower end  108  of vertical guide member  106 , and hence, to base  92 ; in this sense, free end  148  of piston rod  142  is fixedly coupled to a first side of base  92 . Also visible within  FIG. 6A  is an adhesive-backed plastic strip  139  secured vertically along cylinder  138  facing away from the center of lift car  42 . If desired, two more plastic strips may be similarly secured along cylinder  138 , facing inward (i.e., toward the center of lift car  42 ), and facing forward (i.e., toward vertical frame member  118 ), respectively. Plastic bumpers (not shown) may also be secured on the corresponding inner walls of guide member  106  near its upper end  110 , i.e., on the forwardmost inner wall of guide member  106 , and on the two inner walls perpendicular thereto). While contact between cylinder  138  and guide member  106  is preferably avoided altogether, the presence of such plastic strips and corresponding plastic bumpers ensures that any sliding contact which does result will avoid metal-to-metal scraping. To some extent, such plastic-to-plastic engagement may help further stabilize the lift when elevated. 
     Similarly, second hydraulic cylinder  140  has its butt end secured to the upper portion of second side  116  of the lift car structural frame by bolt  152  (see  FIGS. 7 and 8 ); thus, second hydraulic cylinder  140  likewise moves up and down along with lift car  42 . A piston rod likewise is extendable downwardly from the lower end of second hydraulic cylinder  140 , and the free end  154  (see  FIG. 7 ) of this piston rod is secured by bolt  156  to the lower end of vertical guide member  112 , and hence, to base  92 ; in this sense, free end  148  of piston rod  142  is fixedly coupled to a second side of base  92 . As will be clear to those skilled in the art, pressurized hydraulic fluid can be selectively applied to fittings on hydraulic cylinders  138  and  140  to either extend or retract their respective piston rods. Since the free ends of such piston rods are fixedly attached to base  92 , extension of such piston rods forces hydraulic cylinders  138  and  140 , and hence lift car  42 , upwardly. In contrast, retraction of such piston rods within hydraulic cylinders  138  and  140  lowers lift car  42  back toward the ground. 
     It will be noted that both of the hydraulic cylinders  138  and  140  are oriented vertically, and such hydraulic cylinders directly drive lift car  42 . If the piston rods of such cylinders are extended by one additional inch, then lift car  42  raises by one additional inch. Moreover, it should be noted that hydraulic cylinders  138  and  140  are effectively mounted “upside-down” compared to typical uses of such hydraulic cylinders. In a typical lift device, the butt ends of the hydraulic cylinders are secured to a fixed structure, and the free ends of the movable piston rods are secured to the car or platform that elevates. However, in the preferred embodiment of the present invention, the typical configuration is reversed. Unexpected benefits of reversing the typical configuration are discussed below. 
     Still referring jointly to  FIGS. 6A-6C ,  7  and  8 , the upper end  110  of first guide member  106  is received within first side  114  of the lift car structural frame. More specifically, upper end  110  of guide member  106  extends just inside lower horizontal frame member  132 , and between vertical frame members  122  and  124 . As lift car  42  is lowered further toward base  92 , guide member  106  continues to be received within first side  114  of the lift car structural frame until, when lift car  42  is fully-lowered, upper end  140  of guide member  106  lies closely proximate to upper frame member  128 . Likewise, second guide member  112  is received with second side  116  of the lift car structural frame. 
     It will be recalled that one of the objects of the present invention is to provide a wheel chair lift wherein the lift car is highly stable, particularly when the lift is elevated. In this regard, rollers are provided at the lower ends of the first and second sides  114  and  116  of the lift car structural frame to engage vertical guide members  106  and  112  for allowing vertical movement of lift car  42 , while maintaining the lower portion of lift car  42  in close alignment with guide members  106  and  112 . First guide member  106  includes a vertical planar face  158 , shown best in  FIGS. 7 and 8 . A similar vertical planar face  160  is provided on the opposite wall of guide member  106 . Lower roller  162 , and upper roller  164 , are pivotally coupled to the lower end of vertical frame member  122  for rollingly engaging vertical face  158  of guide member  106 . A second set of rollers  166  and  168  are likewise provided on the lower end of vertical frame member  124  for rollingly engaging opposing vertical face  160  of guide member  106 . Preferably, the distance between the first set of rollers  162  and  164  and the second set of rollers  166  and  168  can be adjusted to closely match the distance between opposing vertical faces  158  and  160 . Thus, as lift car  42  rises, lowers, or stays at any given height, all of such rollers are in close engagement with guide member  106  to maintain lift car  42  directly above base  92  at all times. While not shown in detail, it should be understood that identical rollers are provided proximate the lower end of second side  116  of the lift car structural frame to rollingly engage opposing faces of second guide member  112 . While not shown in the drawings, rollers may also be provided, if desired, to engage one or both of the exterior faces of guide members  106  and  112  that lie perpendicular to vertical faces  158  and  160 . 
     It will also be recalled that one of the objectives of the present invention is to provide a wheel chair lift device wherein no moving parts of the lift mechanism are exposed, apart from the lift car itself. In this regard,  FIGS. 6A-6C  illustrate that the lower, open end  146  of first hydraulic cylinder  138  extends into the hollow internal channel of first guide member  106  and moves therethrough as the lift car  42  moves up and down. Any extended portion of piston rod  142  is always enclosed within guide member  106 . Likewise, the lower, open end of second hydraulic cylinder  140  extends within the hollow internal channel of second guide member  112  and moves therethrough as lift car  42  moves up and down; any extended portion of the piston rod associated with cylinder  140  is always enclosed within guide member  112 . Thus, all moving parts of the lift mechanism are enclosed within either guide members  106 / 112  or within side walls  114 / 116  of lift car structural frame. Accordingly, apart from movement of lift car  42  itself, there are no other exposed moving parts that could injure a passerby or which could become intertwined with foreign objects. 
     Vertical guide members  106  and  112  are illustrated in the drawings as having a rectangular cross-section, surrounding a hollow, rectangular internal channel. Those skilled in the art will appreciate however, that the tubular stock from which vertical guide members  106  and  112  are made could be square tubing, circular tubing, or even C-shaped stock defining a C-shaped internal channel having one open face; in the latter instance, the open face preferably is directed toward the center of the lift, i.e., the two open faces of the two guide members are directed toward one another. 
     Earlier, it was noted that the mounting of the hydraulic cylinders in an upside-down configuration provides unexpected advantages. Referring again to the hydraulic component schematic of  FIG. 9 , the hydraulic circuit includes hydraulic fluid reservoir  170 , a hydraulic pump/manifold unit  172 , an emergency hand-operated pump  174  for use during electrical power outages, and an electric motor  176  coupled to hydraulic pump/manifold unit  172  for rotating the same to pressurize hydraulic fluid. In the preferred embodiment, electric motor  176  is a capacitor-start, ½ horsepower, 120 Volt AC motor, e.g., Leeson-brand Model No. A42C17NB11 available from the Leeson Electric division of Royal Beloit Corporation of Grafton, Wis. The hydraulic pump/manifold unit  172 , manual pump  174 , and fluid reservoir  170 , are available from Bucher Hydraulics of Grand Rapids, Mich. While not shown in the drawings, a short length of tubing is inserted into a socket of manual pump  174  to provide leverage during use. As shown in  FIGS. 6A-6C ,  7  and  8 , all of such hydraulic components are supported within first side  114  of the lift car structural frame, and move up and down together with lift car  42 . As further indicated in the schematic drawing of  FIG. 9 , first cylinder  138  includes a lowermost fitting  178  and an uppermost fitting  180 . Lowermost fitting  178  is coupled to the lower end of a section of rigid steel tubing  182 . Rigid tubing  182  extends upwardly along, and parallel to cylinder  138 ; the upper end of rigid tubing  182  forms an inverted U-shape and mates with a flexible hose  184  connected to hydraulic pump/manifold unit  172 . The upper fitting  180  of first cylinder  138  is coupled to rigid tube  186  which extends downwardly toward the bottom portion of the lift car structural frame, but is spaced further apart from first cylinder  138  as compared with tubing  182 . The lower end of tubing  186  connects with a rigid “elbow” tube  188 , which in turn couples to a flexible hose  190  that passes below the lift car floor to second side  116  of the lift car structural frame. 
     On second side  116 , flexible hose  190  is coupled through rigid “elbow” tube  192  to another rigid tube  194 . Rigid tube  194  extends upwardly from elbow tube  192 , forms a U-shaped bend, and extends back downwardly parallel with, and closely proximate to second cylinder  140 , finally connecting with lowermost fitting  196 . At the upper end of second cylinder  140 , rigid tubing  198  is coupled to uppermost fitting  200 , and then extends downwardly to the lower portion of lift car  42 , where it connects through a further elbow tube  202 . The other end of elbow tube  202  is coupled with a second flexible hose  204  which again passes below the lift car floor back to first side  114 . On first side  114 , flexible hose  204  is coupled through elbow tube  206  to a flexible hose  210 . Flexible hose  210  extends upwardly therefrom and connects back to hydraulic pump/manifold unit  172 . 
     It may be noted that all of the components shown in  FIG. 9 , including all of the hydraulic tubing, are supported by lift car  42  and travel up and down together with lift car  42 . Flexible hoses  190  and  204  are provided merely for allowing the width of lift car  42  to be collapsed, if desired, for transport through narrow passageways, without causing a need to disconnect any hydraulic tubing. On the other hand, if it is not necessary to collapse the width of the lift car (e.g., where lift device  30  is to be used only in conjunction with a single platform on a permanent basis), then flexible hoses  190  and  204  could instead be provided as rigid tubing. 
     As shown best in  FIGS. 6A-6C , rigid tubing  182  is maintained closely proximate and parallel to first cylinder  138  as tubing  182  passes downwardly toward lowermost fitting  178 . This ensures that, as lift car  42  is lowered, and cylinder  138  is received within the hollow internal channel of first guide member  106 , there will be no interference, or binding, between tubing  182  and the inner walls of guide member  106 . Likewise, the vertical portion of rigid tubing  194  that couples to lowermost fitting  196  on cylinder  140  (see  FIG. 9 ) is maintained closely proximate and parallel to second cylinder  140  as lift car  42  is lowered, and cylinder  140  is received within the hollow internal channel of second guide member  112 . This again ensures that there will be no interference or binding between tubing  194  and the inner walls of guide member  112 . Were it necessary to use flexible hoses in place of rigid tubing  182  and  194  to allow for relative movement between hydraulic components, such hoses could flex in a manner that would interfere with the free movement of cylinders  138  and  140  within guide members  106  and  112 , respectively. 
     It will be recalled that another object of the present invention is to support lift car  42  for elevation in a manner that will maintain side walls  46  and  48  (see  FIG. 1 ) in a vertical orientation when lift car  42  is elevated and under load. Referring to  FIGS. 12A-12C ,  FIG. 12A  shows lift car  42  in an unloaded condition; side walls  46  and  48  are vertical and parallel to each other, as desired. In  FIG. 12B , wheel chair occupant  34  is shown supported in lift car  42 , with lift car  42  in an elevated position; under load, lift car floor  44  bows downwardly, creating a twisting moment upon the base of side walls  46  and  48 . This twisting moment rotates side walls  46  and  48  away from their original vertical orientation, causing the upper portions of side walls  46  and  48  to tilt toward one another. When occupant  34  wishes to exit lift car  42  onto a stage or platform, side walls  46  and  48  tend to pinch the rear exit gate, interfering with the opening thereof. This problem would not arise if the lifting force were applied directly below lift car floor  44 . However, as explained earlier, it is advantageous to avoid the need to position the lifting mechanism directly below lift car  42  in order to allow lift car floor  44  to be lowered as close to the ground as possible, thereby avoiding the need for a separate entrance ramp. Accordingly, it is preferred to apply the lifting force to side walls  46  and  48 , and indirectly couple such lifting force to lift car floor  44 . 
     As shown in  FIG. 12C , the problem of deforming side walls  46  and  48  out of their original vertical orientation can be resolved by coupling lift car floor  44  to side walls  46  and  48  in a manner which allows the sides of lift car floor  44  to pivot relative to side walls  46  and  48 . Within the schematic drawing of  FIG. 12C , pivot links  212  and  214  pivotally couple the opposing sides of lift car floor  44  to the lower portions of side walls  46  and  48  so that deformation of floor  44  under load is not coupled to side walls  46  and  48 , thereby avoiding the problem of pinching the rear exit gate. In practice, a series of floor support struts  216 ,  218 , and  220  (see  FIGS. 6A-6C  and  FIG. 7 ) extend below car lift floor panel  44  for supporting floor panel  44 . Each of such floor support struts  216 ,  218 , and  220  has a first end pinned, i.e., pivotally connected, to a lower horizontal frame member of first side  114  of the lift car structural frame, and has a second opposing end pinned to a lower horizontal frame member of second side  116  of the lift car structural frame. For example, along side  114 , floor support struts  216  and  218  are pinned to lower frame member  134 , while floor support strut  220  is pinned to lower frame member  136 . Turning briefly to  FIG. 18 , one end of floor support strut  218  is shown in greater detail. A U-shaped yoke  222  receives a first end of floor support strut  218 . Yoke  222  is rigidly connected, as by welding, to the underside of frame member  134 . The shaft of bolt  224  passes through aligned apertures formed in the end of floor support strut  218  and yoke  222 . Yoke  222  includes two parallel flanges, and the aperture formed in the flange that is furthest from the head of bolt  224  has threads formed therein to threadedly engage the end of bolt  224 . Bolt  224  is not tightened to a point that would restrict relative movement between strut  218  and yoke  222 . Accordingly, bolt  224  forms a pivotal connection between the end of strut  218  and lower frame member  134 . 
     Floor panel  44  rests upon, and is preferably screwed to, the upper surfaces of floor support struts  216 ,  218 , and  220 , so that they alone transfer the load on lift car floor  44  to the first and second sides  114  and  116  of the lift car structural frame. In this manner, any rotational torque induced in floor panel  44 , and into floor support struts  216 ,  218  and  220 , under loading by the occupant of the wheel chair, is isolated from first and second sides  114  and  116  of the lift car structural frame. Therefore, first and second sides  114  and  116  of the lift car structural frame retain their generally vertical orientation. Screws used to secure floor panel  44  to floor support struts  216 ,  218 , and  220  should be easy to remove, since floor panel  44  needs to be removed before collapsing lift car  42  to a narrower width. Likewise, the bolts used to “pin” at least one end of floor support struts  216 ,  218 , and  220  are preferably easy to remove, again for allowing the width of the lift car structural frame to be collapsed after floor panel  44  is removed for transport through narrow passageways. 
     In order to ensure the integrity of the lift car structural frame, and to reliably couple together first and second sides  114  and  116  of the structural frame, a series of four frame struts, which includes those designated  226 ,  228 ,  230  and  231  in the drawings, are also preferably provided, as shown in  FIGS. 6A-6C  and  FIG. 7 . Each such frame strut has a first end fixedly connected, as by welding, to a lower horizontal frame member of first side  114  of the lift car structural frame, and has a second opposing end fixedly connected, as by welding, to a lower horizontal frame member of second side  116  of the lift car structural frame. For example, frame struts  226  and  228  have their first ends welded to horizontal frame member  134 , while frame strut  231  has its first end welded to horizontal frame member  136 . Each of such frame struts is spaced sufficiently below lift car floor panel  44  to avoid contact therewith, even when the lift car is under load. Accordingly, the load applied to the lift car floor is borne solely by floor support struts  216 ,  218 , and  220 . 
     In order to allow the lift car width to be collapsed for transport, each of frame struts  226 ,  228 ,  230  and  231  is preferably provided as a pair of sliding strut members that slidingly engage each other. For example, in  FIG. 7 , frame strut  226  is actually formed by sliding members  232  and  234 . At least one releasable fastener, e.g., a clamping screw, is provided where the two sliding members mate for allowing the length of each such frame strut assembly to be adjusted. This permits the spacing between first and second sides  114  and  116  of the lift car structural frame to be varied between a deployed condition for use, and a collapsed position for transport. In the preferred embodiment, each such pair of sliding strut members telescopically nest with each other. 
     It will be recalled that one of the objectives of the present invention is to be able to quickly and easily adjust the maximum height to which the lift is elevated each time the lift is moved to a different platform or stage. A related objective is to be able to raise the floor of the lift car repeatedly, and reliably, to the pre-set maximum height. Referring now to  FIGS. 13 ,  14 ,  15 A, and  15 B, an improved optical height detection and adjustment system is disclosed. Within  FIG. 13 , a lower portion of second side  116  of the lift car structural frame is shown. To place  FIG. 13  in context, lower horizontal frame members  236  and  238  extend along the lower portion of second side  116  proximate to vertical frame member  240 ; vertical frame member  240  is visible in  FIG. 8  and lies adjacent to rear exit gate  54  when such gate is closed. An L-shaped mounting bracket  242  is secured by one or more screws  244  to vertical frame member  240 . Screw  244  is inserted within a vertically-extending slot  246  formed in mounting bracket  242 , which allows for adjustment of the height of mounting bracket  242  relative to horizontal frame member  236 . A light source  248  is secured to mounting bracket  242  for emitting a focused beam of light generally parallel to horizontal frame members  236  and  238 , and toward second guide member  112 . An optical sensor  250  is also secured to mounting bracket  242 . Optical sensor  250  is preferably of the type commercially available from Banner Engineering Corp. of Minneapolis, Minn. under part number QS18VP6LV, which includes both optical sensor  250  and light source  248 . Optical sensor  250  extends past the edge of mounting bracket  242  but is shielded from the beam emitted directly by light source  248 . Optical sensor  250  is also focused toward second guide member  112 , and is responsive to light originally sourced from light source  248 , after being reflected back toward optical sensor  250  from the direction of second guide member  112 . Also visible within  FIG. 13  is a reflector placement tool  252  stowed within holder  254 . The purpose of placement tool  252  will become more apparent as the present description proceeds. 
       FIG. 14  is also a view of the lower portion of second side  116  of the lift car structural frame, and shows in particular vertical guide member  112  received within second side  116 . Within  FIG. 14 , roller  256  corresponds to one of the rollers used to rollingly engage vertical face  113  of guide member  112 . It will be noted that a bracket  258  is secured to the lower portion of second side  116 , closely proximate in which guide member  112  is received thereby. Bracket  258  has a U-shaped reference port, or saddle,  260  formed therein. Referring back to  FIG. 13  briefly, the light beam emitted by light source  248  is directed to pass through reference port  260  for striking vertical face  113  of guide member  112 . Likewise, optical sensor  250  is aligned with reference port  260  for receiving light reflected from guide member  112 , through reference port  260 , back toward optical sensor  250 . 
     Light source  248  and optical sensor  250  form part of a height adjust system for stopping the operation of electric motor  176  in the direction that would further elevate lift car  42 . This height adjust system stops motor  176  from further raising lift car  42  when it reaches a desired, predetermined maximum height. In order to set the predetermined maximum height, a reflector  262  is used, as shown in  FIGS. 15B ,  16 A, and  16 B. As shown best in  FIG. 16A , reflector  262  includes a front reflective face  264  encased in a metal housing  266 . Preferably, reflector  262  includes a magnetic backing  268  (see  FIG. 16B ). Reflector  262  is adapted to be removably secured along vertical face  113  of guide member  112 , outside the path of roller  256 , and laterally aligned with reference port  260 . Reflector  262  may be regarded as a “light-sending element” in the sense that it sends light originally emitted by light source  248  back toward optical sensor  250 . When lift car  42  is elevated to the point at which reflector  262  becomes vertically aligned with reference port  260 , reflector  262  intercepts the beam of light emitted from light source  248  and reflects it back. Light reflected by reflector  262  strikes optical sensor  250 , which then generates an electrical signal used to disable motor  176  from further elevating lift car  42 . 
     Thus, by releasably securing reflector  262  along vertical face  113  of guide member  112 , using magnetic backing  268 , reflector  262  can be used to quickly and easily set the desired maximum height. After positioning lift device  30  adjacent a stage or platform, a technician opens access panel  78  (see  FIG. 3 ) to retrieve reflector placement tool  252  from its holder  254 . The technician operates the lift by pressing “UP” and “DOWN” buttons until the lift car floor  44  is exactly even with upper platform  32  of the stage. As shown in  FIG. 13 , placement tool  252  includes a first end  253  for being held by a user, and an enlarged second end  255  for releasably engaging reflector  262 . The technician then engages reflector  262  with second end  255  of placement tool  252 . As shown in  FIG. 16A , reflector housing  266  may include a threaded perimeter  267 . Also, as shown in  FIG. 17A , the enlarged second end  255  may include a pair of detent pins  257  and  259  which threadedly engage perimeter  267  when placement tool  252  is rotated relative to reflector  262 , as shown in  FIG. 17B . Rotation of placement tool  252  about its longitudinal axis in a first direction (e.g., clockwise) engages reflector  262  in second end  255 ; rotation of placement tool  252  about its longitudinal axis in the opposite direction (e.g., counter-clockwise) disengages reflector  262  from second end  255 . 
     Once reflector  262  is engaged within second end  255  of placement tool  252 , the technician lowers the central shaft of placement tool  252  within reference port  260  until it rests upon the bottom of reference port  260 . The technician then advances second end  255  toward guide member  112  by sliding placement tool  252  horizontally until magnetic backing  268  of reflector  262  engages vertical face  113  of guide member  112 , as shown in  FIG. 15  A. The technician then rotates placement tool in the direction which allows reflector  262  to become disengaged from placement tool  252 , placement tool is returned to its holder  254  for later use, and access panel  78  is closed. The procedure for removing reflector from vertical face  113  of guide member  112  simply involves the reversal of the steps just described. 
     It will be recalled that a further object of the present invention is to provide a method of testing the functionality of the height adjust system before lift car  42  is actually elevated.  FIG. 19  shows the lower end of second guide member  112 , and its vertical face  113 , with lift car  42  in an elevated position and out of view. A second, permanent reflector  270  is secured by screws  272  and  274  near the lower end of vertical face  113 . When lift car  42  is fully-lowered, reflector  270  is aligned with reference port  260  of  FIG. 14 ; accordingly, assuming that light source  248  and optical sensor  250  (see  FIG. 13 ) are working properly, optical sensor  250  detects light reflected by permanent reflector  270 , and signals the electronic control circuit that the height adjust system is operational. Elevation of lift car  42  is then permitted above floor level. If, on the other hand, optical sensor  250  does not signal that it has detected light from reflector  270 , then the electronic control circuit does not permit lift car  42  to be elevated. 
     The operation of lift device  30  will now be described with reference to the schematic of  FIG. 10 . A pair of hydraulic lifting cylinders  138 ′ and  140 ′ (corresponding to cylinders  138  and  140  in  FIG. 9 ) raise and lower lift car  42  (not shown). Preferably, hydraulic cylinders  138 / 138 ′ and  140 / 140 ′ are of the type generally available from Ram Industries Inc., a Canadian company based in Yorkton, Saskatchewan, Canada. Cylinder  138 ′ is preferably of the type available from Ram Industries Inc. as Model No. R4506994 (3000 psi operating pressure, 2.5″ bore, 41.5″ stroke, 1.125″ piston rod diameter), while cylinder  140 ′ is preferably a Model No. R4506995 (3000 psi operating pressure, 2.75″ bore, 40.5″ stroke, 1.125″ piston rod diameter). Cylinders  138 ′ and  140 ′ each include an expansion chamber and a refraction chamber. The expansion chamber of cylinder  138 ′ is coupled by tube  300  to the retraction chamber of cylinder  140 ′. When lift car  42  is being raised, pressurized hydraulic fluid is forced into the expansion chamber of cylinder  138 ′, extending piston rod  142 ′, compressing fluid in the retraction chamber of cylinder  138 ′, and forcing the compressed fluid into the expansion chamber of cylinder  140 ′ for extending piston rod  302 . Alternatively, when the lift is being lowered, pressurized hydraulic fluid is forced into the retraction chamber of cylinder  140 ′, retracting piston rod  302 , compressing fluid in the expansion chamber of cylinder  140 ′, and forcing the compressed fluid through tube  300  into the refraction chamber of cylinder  138 ′ for retracting piston rod  142 ′. 
     Still referring to  FIG. 7 , electric motor  176 ′ rotates in a fixed direction to rotate the input drive shaft of hydraulic fluid pump  172 ′. Pump  172 ′ draws hydraulic fluid from low pressure side  170 ′, and pumps hydraulic fluid out under pressure through check valve  304 . Relief valve  306 , which may be integral with pump  172 ′, can be adjusted to permit a selected amount of pressurized hydraulic fluid to be directed back to low pressure side  170 ′. 
     Still referring to  FIG. 7 , hydraulic fluid pressurized by pump  172 ′ is supplied via high pressure conduit  308  to the high pressure inlet of a solenoid valve  310 . Solenoid valve  310  also includes a low pressure outlet coupled to return conduit for coupling to low pressure side  170 ′. Solenoid valve  310  is normally biased (by a spring) to a position for extending piston rods  142 ′ and  302 ′. In this case, solenoid valve  310  assumes the default crossed-over position shown in  FIG. 7 , wherein high pressure inlet line  308  is coupled to line  314 , and low pressure outlet  312  is coupled to line  316 . Preferably, solenoid valve  310  is a 24 VDC solenoid valve with manual override commercially available from the Deltrol Fluid Products Division of Deltrol Corporation of Bellwood, Ill. of Glendale Heights, Ill., under Part Number DSV2-4C0. 
     In the event of a power failure, motor  176 ′ that powers hydraulic pump/manifold unit  172 ′ will no longer operate. For this reason, hydraulic hand pump  174 ′ is provided in an emergency to raise and lower the lift car without electrical power. Still referring to  FIG. 7 , hand-operated fluid pump  174 ′ includes a fluid inlet coupled through a check valve  318  to low pressure return line  312  for receiving un-pressurized hydraulic fluid. Pump  174 ′ also includes a high-pressure outlet port for supplying pressurized hydraulic fluid through check valve  320  to high pressure line  308 . A lever can be reciprocated by an operator to raise or lower the lift using such hand-operated pump  174 ′ if motor  176 ′ is lacking electrical power. Pump  174 ′ may similar to the type available from the Deltrol Fluid Products Division of Deltrol Corporation of Bellwood, Ill. of Glendale Heights, Ill., under Part Number DHP-100. 
     As shown in  FIG. 7 , pilot-operated check valve  322  couples line  316  to the retraction chamber of hydraulic cylinder  140 ′. Valve  322  is preferably of the type commercially available from HydraForce, Inc. of Lincolnshire, Ill., under Part Number PC08-30. Line  314  is coupled by an over-center, counter-balance, spring-biased valve  324  to the expansion chamber of cylinder  138 ′. Valve  324  is preferably similar to the type commercially available from Bucher Hydraulics—Illinois, Inc. (formerly, “Command Controls Corp.”) of Elgin, Ill., under Part Number CBPA-08. Valve  324  is adjustable to help ensure that cylinders  138 ′ and  140 ′ expand and retract at the same rate. 
       FIG. 11  provides an electrical schematic illustrating the circuitry used to control the operation of lift device  30 . Power input lines  400  and  401  supply 120 VAC electrical power. Line  402  represents a system ground. Referring briefly to  FIG. 6B , electrical power is conveyed from the floor up to lift car  42  by guiding an electrical cable  85  from GFCI device  86  upwardly through guide member  106  to its upper end  110 . As cable  85  exits from upper end  110  of guide member  106 , cable  85  bends downwardly and enters into a cable chain  87  of the type available from Igus Inc. of East Providence, R.I. Cable chain  87  forms a movable loop  89  at its lowermost point and then passes upwardly into first side  114  of the lift car structural frame. The upper end of cable chain  87  is secured to a mounting bracket for electrical control panel, and the electrical cable secured within cable chain  87  exits from cable chain  87  just before reaching the upper end of cable chain  87 . As lift car  42  moves up and down, the height of loop  89  also moves up and down, but the electrical cable always lies safely within first side  114 . 
     Electric motor  176 , used to operate the hydraulic pump, is coupled across lines  400  and  401  under the control of a motor relay (MR)  404 . Motor relay  404  is preferably of the type available from Magnecraft, a division of Schneider Electric, of Des Plaines, Ill., under part number 781XAXM4L-24D. Power lines  400  and  401 , and system ground  402 , are also coupled to an AC to DC power converter  406 . Output lines  408  and  410  from converter  406  provide a regulated source of 24-volt DC power and ground, respectively. 
     The heart of the electronic control circuitry is a so-called “smart relay” logic controller  412 . Smart relay  412  may be of the type commercially available from IDEC Corporation of Sunnyvale, Calif., under model number FL1EB12RCE. Two of the input signals  414  and  416  supplied to smart relay  412  are the “UP” switches and “DOWN” switches provided near the front entry gate (switch  62 ), near the rear exit gate (switch  74 ), and inside lift car  42  (switch  65  in  FIG. 5 ). Each of such switches is provided in the form of a “rocker” switch wherein movement in the “UP” direction is requested by rocking the switch in one direction, and movement in the “DOWN” direction is requested by rocking the switch in the opposite direction. The three “UP” switches are coupled in parallel to input  414  to signal that the lift car should be raised, and the three “DOWN” switches are coupled in parallel to input  416  to signal that the lift car should be lowered. 
     Input  418  of smart relay  412  is coupled to a series of eight safety pan switches, all coupled in series with each other. These safety pan switches are distributed about the periphery of the lower portion of lift car  42  adjacent a “safety pan” that is suspended from the bottom of lift car  42 . In the event that the safety pan contacts a foreign object before lift car  42  is fully-lowered to the ground, the safety pan engages, and actuates, one or more of such safety pan switches, signaling that the pump motor should immediately stop to avoid injury or damage. These safety pan switches are normally closed, and the actuation (i.e., opening) of any safety pan switch, among the series-connected group of such switches, triggers the electronic control circuit to stop the lift. 
     Input  420  of smart relay  412  is coupled to a pair of gate switches coupled in series with each other, and is further coupled in series with a keyed master on/off switch. The gate switches are provided at the front entry gate  40  and rear exit gate  54 . Each such switch provides a conductive path only if its respective gate is closed. Smart relay  412  will allow operation of the pump motor only if the master on/off switch is set to “on”, and both gate switches are closed (i.e., conductive). 
     Input  422  of smart relay  412  is coupled to a lock switch; this lock switch is used to unlock the front entry gate  40 . If the lock switch is opened, indicating that the front entry gate is unlocked, then smart relay  412  will not allow lift car  42  to move. 
     Input  424  of smart relay  412  is coupled to a lower terminal stop switch. This lower terminal stop switch is located in first side  114  of the lift car structural frame near the upper end of cylinder  138  and is contacted by the upper end of guide member  106  about one inch before lift car  42  reaches the ground. In this manner, smart relay  412  can disregard the subsequent triggering of the safety pan switches which follows as the safety pan makes contact with the ground. 
     Input  426  of smart relay  412  is coupled to optical sensor  250  of the height adjust system. Input  426  receives the failsafe signal when the lift is fully-lowered to confirm that the height adjust system is functional before allowing motor  176  to elevate lift car  42 . Input  426  also receives the maximum height signal generated by optical sensor  250  when lift car  42  has been elevated to the pre-set maximum height. In this regard, smart relay  412  can distinguish between the failsafe signal (when the lift car is fully lowered) and the maximum height signal (when the lift is almost fully-raised) by noting whether or not the lower terminal stop switch is open or closed. If the lower terminal stop switch is closed, then the lift is no more than perhaps one inch above the ground, and the signal generated by optical sensor  250  is a failsafe signal. On the other hand, if the lower terminal stop switch is open, then the lift has already elevated more than one inch, and the signal generated by optical sensor  250  must be indicating that the maximum desired height has been reached. 
     Smart relay  412  generates three output signals in response to the aforementioned input signals. Output signal  427  is applied to a lock solenoid  428  which, as described above, must be energized before allowing front entry gate  40  to be opened. Output signal  429  is applied to solenoid valve  310  (see  FIG. 10 ) to control the direction (up or down) in which lift car  42  is moved when the hydraulic pump motor is operated. Finally, output signal  430  is applied, through normally closed “E-Stop” switch  432  to the controlling input terminal of motor relay  404 ; it will be recalled that the output terminals of motor relay  404  are used to control the application of 120 VAC power across pump motor  176 . If the occupant of lift car  42  depresses Emergency Stop switch  432 , motor relay  404  immediately disconnects 120 VAC power from pump motor  176 . 
     Those skilled in the art will now appreciate that an improved wheel chair lift has been described for safely and reliably lifting wheelchair-bound users up to the height of stages, platforms, risers and the like. The disclosed lift device has a low profile and avoids any significant interference with an audience&#39;s view of events taking place. The disclosed lift uses direct-drive hydraulic cylinders to minimize the size, weight and cost of the lift device without sacrificing stability. The disclosed lift device essentially limits exposed moving parts to the lift car itself, without requiring other exposed moving components around and/or below the lift device which might otherwise require a protective skirt. The disclosed lift device is relatively inexpensive, easy to construct and use, simple to maintain, and easy to collapse and/or transport. 
     Moreover, the disclosed lift device allows the lift car floor to be lowered to the ground to avoid the need for an entry ramp, while avoiding deformation of the lift car side walls away from their usual vertical orientation. The height adjust system described above allows a user to quickly and easily adjust the maximum height to which the lift car is raised, thereby allowing the lift device to be repeatedly raised to the height of the platform with which the lift device is currently being used. In addition, the above-described failsafe feature of the height adjust system verifies that the control system used to halt further elevation of the lift car after reaching the selected maximum height, is operational before permitting the lift car to be elevated significantly. 
     While the present invention has been described with respect to a preferred embodiment thereof, such description is for illustrative purposes only, and is not to be construed as limiting the scope of the invention. Various modifications and changes may be made to the described embodiments by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.