Abstract:
A wheel chair lift device includes a lift car selectively elevated above a base, along with protective skirting covering the region below the lift car. Exposed portions of the protective skirting subject to lateral inward deformation are provided with skirt deformation sensors for detecting abnormal inward deformation of such skirt portions. The skirt deformation sensors include a spring or other elongated deformable member that extends generally parallel and proximate to the portion of the skirt being sensed. A sensor detects that the elongated member has been laterally displaced from its usual longitudinal axis and generates an electrical signal. In response to such electrical signal, the lift device stops further movement of the lift car.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates generally to lifting devices, and more particularly, to a wheelchair lift device including a lift car, and having a protective skirt that restricts access below the lift car. 
         [0003]    2. Description of the Background Art 
         [0004]    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. 
         [0005]    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 portable 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. 
         [0006]    Another portable lifting device adapted for wheelchairs is disclosed within pending U.S. patent application Ser. No. 11/026,863, filed on Dec. 30, 2004, and published as U.S. Publ. No. 20060182570 (Zuercher, et al.) on Aug. 17, 2006, also assigned to the assignee of the present application. This application discloses a portable wheelchair lift device that includes a lift car that can be raised and lowered, and which provides protective skirting around the front, back, and sides of the lift device to restrict access below the lift car to help prevent injury. 
         [0007]    Applicable governmental regulations require that wheelchair lift devices include a safety skirt surrounding the base of the lift to help keep legs, arms and other body parts from being inserted under the lift car. While such safety skirting is helpful in preventing accidents, the safety skirts are often made from rather flexible, yielding material, such as rubber or plastic. If sufficient force is applied laterally inward upon such safety skirts, they readily give way and deform. Accordingly, were a lift attendant, or even a bystander, to fall against the lift device during operation, such person&#39;s legs, arms, head, or other body parts could press sufficiently hard against the safety skirting to cause it to deform. If the lift car is being lowered at such time, there is a possibility that such person&#39;s leg, arm, head, etc., could become pinched between the bottom of the lift car and the base of the lift device, posing a significant danger. In view of such dangers, applicable governmental regulations now require that such wheel chair lift devices be able to avoid injury to such persons. 
         [0008]    In view of the foregoing, it is an object of the present invention to provide a wheelchair 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. 
         [0009]    Another object of the present invention is to provide such a lift device having a safety skirt, and which is able to detect instances when the safety skirt is inwardly deformed to the extent of posing a possible danger. 
         [0010]    A further object of the present invention is to provide such a lift device which is capable of halting upward or downward movement of the lift car upon detecting that the safety skirt has been inwardly deformed to the extent of posing such danger. 
         [0011]    Yet another object of the present invention is to provide such a lift device achieving the aforementioned objectives without significantly increasing the cost or complexity of the lift device. 
         [0012]    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 
       [0013]    Briefly described, and in accordance with a preferred embodiment thereof, the present invention relates to a lift device for raising and lowering wheelchairs and the like, and including a base for resting upon the ground, a lift car that can be raised and lowered for supporting a user of a wheelchair or the like, and a lift mechanism coupled to the base and to the lift car for selectively raising, or lowering, the lift car relative to the base. A collapsible curtain panel, protective skirt, or safety skirt, has a lower end secured to the base and an upper end secured to the lift car for elevational movement therewith; this safety skirt helps to restrict access to an area located below the lift car when the lift car is raised. 
         [0014]    A deformable elongated member has a first end supported generally proximate to the base, and a second end generally supported proximate to the lift car for movement therewith. The deformable member extends lengthwise along a longitudinal axis that is proximate to the protective skirt. When a lateral, inwardly-directed force is applied to the protective skirt, the deformable elongated member is also displaced laterally from its usual longitudinal axis. 
         [0015]    A sensor detects lateral displacement of the deformable elongated member relative to its usual longitudinal axis, and generates an electrical signal that indicates such occurrence. The lift device includes a control mechanism responsive to the aforementioned electrical signal generated by the sensor for stopping further movement of the lift car until the problem is resolved. 
         [0016]    Preferably, the deformable elongated member is elastic and flexible, allowing lengthwise deformation (extension and retraction) as well as lateral deformation. A preferred example of such deformable elongated member is a tension spring. 
         [0017]    The preferred form of sensor for detecting lateral displacement of the deformable elongated member is a microswitch for opening or closing an electrical circuit when a trigger lever of the microswitch is contacted by the deformable elongated member. However, other types of sensors (optical, magnetic, ultrasonic, etc.) may also be used to detect the relative position of the deformable elongated member. 
         [0018]    The lift mechanism used to elevate the lift car relative to the base preferably includes a piston rod that is extendable from a hydraulic cylinder. The deformable elongated member preferably extends along a longitudinal axis that is generally parallel to the hydraulic cylinder; preferably, the longitudinal axis of the deformable elongated member also extends generally proximate to the hydraulic cylinder. At least a portion of the protective skirt extends generally proximate to the longitudinal axis of the deformable elongated member. 
         [0019]    The hydraulic cylinder has a first end from which a piston rod is extended to raise the lift, as well as an opposing second end. In one instance, the piston rod that extends from the first end of the hydraulic cylinder is secured to the base of the lift device, and the second end of the hydraulic cylinder is secured to the lift car. In an alternate case, the piston rod is secured to the lift car, and the second end of the hydraulic cylinder is secured to the base of the lift device. In either case, the first end of the deformable elongated member can be supported generally proximate to the base, and the second end of the deformable elongated member is supported generally proximate to the lift car. For example, the second end of the deformable elongated member could be supported from the uppermost end of the hydraulic cylinder. Alternatively, the deformable elongated member can simply extend between the first and second ends of the hydraulic cylinder, such that its length remains relatively fixed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  shows a user entering the lift car from the ground. 
           [0021]      FIG. 2  shows a user being lifted in the lift car. 
           [0022]      FIG. 3  shows a user entering the lift car from the stage through the stage gate. 
           [0023]      FIG. 4  is a perspective, skeletal view of the lift device base, intermediate support rails, and lift car in an elevated position. 
           [0024]      FIG. 5  is a cut-away side view of the lift device showing the position of an electric motor, hydraulic pump, hand-operated manual pump, and one of the hydraulic cylinders used to raise the lift car. 
           [0025]      FIG. 6  is another perspective, skeletal view of the lift device, similar to  FIG. 4 , but adding the hydraulic lift cylinders, lift car gates, and front gate scissors interlock. 
           [0026]      FIG. 7  is a schematic drawing of the hydraulic lifting mechanism, including an electric motor, hydraulic gear pump, supplemental hand pump, control valves, and hydraulic cylinders. 
           [0027]      FIG. 8  is an electrical circuit schematic illustrating the switches and control circuitry for controlling the operation of the motor and solenoid valve that power the hydraulic lifting mechanism. 
           [0028]      FIG. 9  is a perspective view of a height adjustment rail, viewed from above, used to set the predetermined height to which the lift device is elevated. 
           [0029]      FIG. 10  is a perspective view of the height adjustment rail shown in  FIG. 9  viewed from below. 
           [0030]      FIG. 11  is an enlarged view of the second end of the height adjustment rail. 
           [0031]      FIG. 12  is an enlarged view of the actuator that slides within the height adjustment rail. 
           [0032]      FIG. 13  is an enlarged view of the “two-inch” electrical switch. 
           [0033]      FIG. 14  is an enlarged view of the maximum height, upper-stop switch. 
           [0034]      FIG. 15  is a side cut-away view of the height adjustment rail mounted within a side panel of the lift car. 
           [0035]      FIG. 16  is a top, cross-sectional view of the structure shown in  FIG. 15 . 
           [0036]      FIG. 17  is a perspective view of the lift device illustrating protective skirting installed thereon. 
           [0037]      FIG. 18  is a perspective view of the protective skirt associated with the front gate of the lift car prior to assembly. 
           [0038]      FIG. 19  is a perspective view of the protective skirt assembly that surrounds the sides and rear portion of the lift device. 
           [0039]      FIG. 20  is a side view of the lower portion of the lift device showing a pair of skirt sensors. 
           [0040]      FIG. 21  is a perspective cut-away view of skirt sensor components shown in  FIG. 20 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0041]    In  FIG. 1 , a lift device includes a movable lift car  162 , as well as a lifting mechanism (not shown) that selectively elevates lift car  162  relative to the ground from a lowered position to an elevated position. In  FIG. 1 , lift car  162  is shown completely lowered to the floor, and a front (lower landing) gate  164  has been opened for allowing user  166  to roll his wheelchair  168  onto the floor  170  of lift car  162  from ground level. Lift car  162  includes opposing side panels  165  and  167 . Lower landing gate (or front entry gate)  164  preferably includes an electro-mechanical interlock that prevents front entry gate  164  from being opened whenever lift car  162  is more than two inches above the fully lowered position. In addition, a safety skirt  181  completely encloses and protects the area under lift car  162 . 
         [0042]    In  FIG. 2 , user  166  is being elevated in lift car  162  toward stage height. Front gate  164 , and rear (stage) gate  172 , are both closed and secured during elevation. For safety reasons, both the lower entry gate  164  and upper stage gate  172  are preferably self-closing. 
         [0043]      FIG. 3  shows another user  166 ′, already supported on stage floor  174 , entering into lift car  162 . Rear stage gate  172  is opened, and a hinged stage docking plate  176  is lowered to allow wheelchair  168 ′ to roll smoothly onto lift car floor  170 . As stage gate  172  opens, hinged dock plate  176  is automatically lowered into position by a tether (not shown), thereby spanning any small gap between lift car floor  170  and stage  174 . Dock plate  176  rests on stage  174  and provides a smooth transition between lift car floor  170  and stage  174 . When stage gate  172  is closed, dock plate  176  is simultaneously retracted by the aforementioned tether. 
         [0044]      FIG. 4  shows the base, intermediate lift support rails, and lift car skeleton used to fabricate the lift device. The hydraulic lifting cylinders, motor, hydraulic pump, and protective skirt, are omitted from  FIG. 4  for purposes of clarity. Base  180  includes a pair of opposing, parallel elongated metallic members  501  and  502  that are coupled to each other by cross-braces  503 ,  504  and  505 . Brackets  501  and  502  each include apertured brackets  506  and  507 , respectively, for receiving piston rods of the hydraulic lifting cylinders. A pair of U-shaped rails  508  and  509  project upwardly from metallic members  506  and  507 , respectively. Angled braces  510  and  511  are welded to rails  508  and  509 , respectively, and to the opposing ends of metallic members  501  and  502 , respectively. Cross brace  512  extends between, and couples, the upper ends of rails  508  and  509 . Partially visible in  FIG. 4  is a roller  514  which pivots upon axle  516  near the upper end of rail  508 . A similar roller (not shown) is installed at the upper end of rail  509 . 
         [0045]    Still referring to  FIG. 4 , a pair of intermediate lift support rails  518  and  520  are slidingly supported by rails  508  and  509 , respectively, for vertical movement. The aforementioned sliding support of rail  518  is provided by roller  514 , and by a lower roller (not visible) secured by an axle to the lower end of intermediate rail  518 ; this lower roller engages the inner U-shaped walls of rail  508 . Lift car  162  is, in turn, slidingly supported by intermediate lift support rails  518  and  520 . Lift car  162  includes floor  170  extending between opposing side panels  165  and  167 . Again, for purposes of clarity, the front and rear gate entry doors ( 164  and  172  in  FIGS. 1-3 ) have been removed for clarity. The upper ends of intermediate lift support rails  518  and  520  are slidingly received within side panels  167  and  165 , respectively. While not visible within  FIG. 4 , rollers secured to the upper ends of intermediate lift support rails  518  and  520 , and rollers secured within side panels  167  and  165 , allow the upper ends of intermediate lift support rails  518  and  520  to telescope within, or extend from, the bottoms of side panels  167  and  165 . 
         [0046]      FIG. 5  is a side view of the lift device in its lowered position, with the protective skirt and a portion of the side panel cut away for clarity. In addition, the springs and sensors used to detect deformation of the protective skirt have also been omitted from  FIG. 5  for clarity. Hydraulic lifting cylinder  50  has its upper end secured to bracket  522  of lift car side panel  165  for selectively raising lift car  162 . The piston rod extending from the lower end of hydraulic lifting cylinder  50  is connected by pin  524  to apertured bracket  507  of base  180 . Also visible within  FIG. 5  are electric motor  56 , rotary pump  58 , manual pump  80  (used in the event of an electrical power failure), hydraulic fluid reservoir  526  and hydraulic solenoid valve  68 . With the exception of hydraulic cylinder  50 , all of the aforementioned components fit within side panel  165  of lift car  162 . Lines  528  and  54  pass below base  180  to the opposite side of the lift device for powering the second hydraulic lift cylinder. 
         [0047]      FIG. 6  is a perspective view similar to that shown in  FIG. 4 , but rotated 180 degrees, and now including the hydraulic lift cylinders  50  and  52 , front gate  164 , and rear gate  172 . Once again, the protective skirt, skirt tension springs, and skirt sensors are omitted from this view for purposes of clarity. Front lift gate  164  includes a stabilizing scissors brace  530  that expands and contracts as lift car  162  is raised and lowered. Scissors brace  530  helps to stabilize lift car  162  when elevated. The lowermost links of scissors brace  530  are coupled to a lower support bar  532 , which is allowed to swivels outward, along with entry gate  164 , when lift car  162  is fully-lowered. Piston rods  51  and  53  are shown fully extended in  FIG. 6 . Switch assemblies  534  and  536  are also shown for operating the lift device from outside, or inside, lift car  162 , respectively. The lift car  162 , base support frame  180 , and the hydraulic lifting cylinders  50 / 52  are all preferably formed from ASTM A36, AISI 1018, or AISI 1020 Steel. All transparent windows incorporated within lift car side panels  165  and  167 , and within the front and rear gates  164  and  172  are preferably fabricated from ¼″ thick high impact strength clear thermoplastic material. 
         [0048]      FIG. 7  is a schematic diagram of a hydraulic control system that may be used to control the wheel chair lift device in one preferred embodiment. A pair of hydraulic lifting cylinders, including left side cylinder  50  and right side cylinder  52 , are provided to raise and lower the wheel chair lift. In this preferred embodiment, hydraulic cylinders  50  and  52  are of the type generally available from Ram Industries Inc., a Canadian company having a U.S. distribution facility in Minot, N. Dak. Left side cylinder  50  is preferably of the type available from Ram Industries Inc. as Model No. R4505901 (3000 psi operating pressure, 2.5″ bore, 40.5″ stroke, 1.125″ rod), while right side cylinder  52  is preferably a Model No. R4505902 (3000 psi operating pressure, 2.75″ bore, 40.5″ stroke, 1.125″ rod). Cylinders  50  and  52  each include an expansion chamber and a retraction chamber. The expansion chamber of cylinder  50  is coupled by tube  54  to the retraction chamber of cylinder  52 . When the lift is being raised, pressurized hydraulic fluid is forced into the expansion chamber of cylinder  52 , extending piston rod  53 , compressing fluid in the retraction chamber of cylinder  52 , and forcing the compressed fluid into the expansion chamber of cylinder  50  for extending piston rod  51 . Alternatively, when the lift is being lowered, pressurized hydraulic fluid is forced into the retraction chamber of cylinder  50 , retracting piston rod  51 , compressing fluid in the expansion chamber of cylinder  50 , and forcing the compressed fluid through tube  54  into the retraction chamber of cylinder  52  for retracting piston rod  53 . 
         [0049]    Still referring to  FIG. 7 , electric motor  56  rotates in a fixed direction to rotate the input drive shaft of hydraulic fluid pump  58 . In the preferred embodiment, motor  56  is a one-half horsepower, 120 V AC electric pump motor of the type commercially available from Leeson Electric Corporation of Grafton, Wis. Pump  58  is preferably a close-coupled, hydraulic gear pump of the type commercially available from JS Barnes Corp./Haldex Hydraulics Corporation of Rockford, Ill. under Part No. G 1112H1A109NPG, having a cubic displacement of 0.194 cubic inches. Pump  58  draws hydraulic fluid from inlet  60  via fluid return line  61  and pumps hydraulic fluid out under pressure through check valve  62 . Relief valve  64  is provided as part of pump  58  and can be adjusted to permit a selected amount of pressurized hydraulic fluid to be directed back to fluid return line  61 . 
         [0050]    Still referring to  FIG. 7 , hydraulic fluid pressurized by pump  58  is supplied via high pressure conduit  66  to the high pressure inlet of a solenoid valve  68 . Solenoid valve  68  also includes a low pressure outlet coupled to return conduit  72  for coupling to fluid return line  61 . Solenoid valve  68  is normally biased (by a spring) to a position for raising cylinders  50  and  52 . In this case, solenoid valve  68  assumes the default crossed-over position shown in  FIG. 7 , wherein high pressure inlet line  66  is coupled to line  74 , and low pressure outlet  72  is coupled to line  76 . Preferably, solenoid valve  68  is a 12 VDC solenoid valve with manual override of the type commercially available from Hydac Technology Corporation, Hydraulics Division, of Glendale Heights, Ill., under Part Number WK08Y-01-M-C-N, with electrical coil Part Number 12 DS-40-1836. 
         [0051]    In the event of a power failure, motor  56  that powers hydraulic pump  58  will no longer operate. For this reason, hydraulic hand pump  80  is provided in an emergency to raise and lower the lift car without electrical power. Still referring to  FIG. 7 , hand-operated fluid pump  80  includes a fluid inlet coupled through a check valve  82  to low pressure return line  72  for receiving unpressurized hydraulic fluid. Pump  80  also includes a high-pressure outlet port for supplying pressurized hydraulic fluid through check valve  84  to high pressure line  66 . A lever can be reciprocated by an operator to raise or lower the lift using such hand-operated pump  80  if motor  56  is suddenly lacking any electrical power. Pump  80  is preferably of the type available from HydraForce, Inc. of Lincolnshire, Ill. under part number HP 10-21B-0-N-B. 
         [0052]    As shown in  FIG. 7 , pilot-operated check valve  88  couples line  76  to the retraction chamber of hydraulic cylinder  50 . Valve  88  is preferably of the type commercially available from Hydac Technology Corporation, Hydraulics Division, of Glendale Heights, Ill., under Part Number RP08A-01C-NS-15-4. Line  74  is coupled by an over-center, counter-balance, spring-biased valve  90  to the expansion chamber of cylinder  52 . Valve  90  is preferably of the type commercially available from Hydac Technology Corporation, Hydraulics Division, of Glendale Heights, Ill., under Part Number RS08-01-C-N-4-500V. Valve  90  is adjustable to help ensure that cylinders  50  and  52  expand and retract at the same rate. 
         [0053]    The electrical schematic of  FIG. 8  includes pump motor  56  electrically coupled across 110 Volt power lines  100  and  102 , protected by fuses  101  and  103 , respectively. The housing of motor  56  is coupled by ground line  104  to ground conductor  106 . Element  108  is coupled in series between motor  56  and “hot” power line  100  and represents the contacts of motor relay  110  (also shown in  FIG. 8 ) that selectively applies power to motor  56 . The 110 Volt service lines  100  and  102 , and ground conductor  106 , are also coupled to a regulated 12 Volt D.C. power supply  111 . Power supply  111  provides a source of a regulated 12 volt D.C. voltage on line  112  relative to low-power ground line  114 . 
         [0054]    The heart of the control system for controlling the lift is an IDEC Smart Relay module  116  commercially available from IDEC Izumi Corporation of Sunnyvale, Calif. under part number FL1C. This module is a compact, expandable, fully programmable, CPU that can replace multiple timers, counters, and relays. As indicated in  FIG. 8 , module  116  is coupled to 12 volt D.C. power lines  112  and  114 . Module  116  includes a series of input terminals coupled to conductors designated by reference numerals  118 ,  120 ,  122 ,  124 ,  126 ,  128 ,  130  and  132 . Module  116  also includes output terminals  134  and  136 . 
         [0055]    Input terminal  118  is the “UP” terminal; when a “high” voltage is applied to input  118 , module  116  is signaled to raise the lift. Input terminal  120  is the “DOWN” terminal; when a high voltage is applied to input  120 , module  116  is signaled to lower the lift. As will be described in greater detail below, there are three toggle switches (grouped together in  FIG. 8  within dashed box  138 ) positioned about lift car  162  for selecting upward or downward movement of the lift car. 
         [0056]    Input terminal  122  is coupled in series with two right-side skirt sensor switches  142  and  144 , described in greater detail below. Switches  142  and  144  detect deflection of the protective skirt on the right side of the lift device. Switches  142  and  144  are normally closed to apply a “high level” on conductor  122 . If either switch  142  or switch  142  is opened due to deflection of the protective skirt, then movement of lift car  162  (upward or downward) ceases. 
         [0057]    Similarly, input terminal  128  is coupled in series with two left-side skirt sensor switches  156  and  140 , described in greater detail below. Switches  156  and  140  detect deflection of the protective skirt on the left side of the lift device. Switches  156  and  140  are normally closed to apply a “high level” on conductor  128 . If either switch  156  or switch  140  is opened due to deflection of the protective skirt, then movement of lift car  162  (upward or downward) ceases. 
         [0058]    Input terminal  124  is the “2 Inch Switch” terminal and is coupled to “2 Inch Switch” 146. When lift car  162  is being raised from the ground, the electrical contacts of switch  146  are closed as the floor of the lift car reaches approximately two inches above the ground. The 2 Inch Switch  146  signals, via input terminal  124 , that the floor of the lift car has raised to approximately two inches above the ground. One of the safety features provided in the preferred embodiment relates to ensuring that the front gate ( 164  in  FIG. 6 ) of the lift car is securely locked closed once the floor of the lift car has raised two inches off of the ground. If the floor of the lift car has raised more than two inches off of the ground, but a front gate safety interlock bolt has not engaged, then further elevation of the lift car is prevented. 
         [0059]    Input terminal  126  is the “Lockbolt” terminal and is used to signal that the front gate safety interlock bolt, briefly described in the preceding paragraph, is engaged. The electrical contacts of lockbolt switch  148  are closed when the interlock bolt is engaged, but such electrical contacts open if the interlock bolt is not engaged. As mentioned above, safe operation of the lift is ensured by confirming that the front gate safety interlock bolt has engaged, and hence, that the front gate (or lower landing gate) is securely locked, before allowing the lift car to elevate more than a few inches off of the ground. 
         [0060]    Input terminal  130  is the “Landing Gate” terminal and is used to detect whether the front landing gate (i.e., front gate  164  in  FIG. 6 ) and rear landing gate (i.e., rear gate  172  in  FIG. 6 , the gate providing access to an elevated stage) are closed. The electrical contacts of upper landing gate switch  150  open if the rear gate is open, and close when the rear gate is closed. Likewise, the electrical contacts of lower landing gate switch  152  open if the front gate is open, and close when the front gate is closed. When all gates are closed, switches  150  and  152  are closed, and a “high level” signal is conveyed to conductor  130 , allowing lift car  162  to continue movement; if not, movement of the lift ceases. 
         [0061]    Finally, input terminal  132  is the “Height” terminal and is used to signal whether or not the lift car has reached a pre-selected height. An electrical height switch  154  can be adjusted, in a manner to be described in greater detail below, to cause its electrical contacts to be open if the lift car is below a desired height, but to close such electrical contacts when the lift car reaches the pre-selected height, thereby signaling relay module  116  to prevent further elevation of lift car  162 . 
         [0062]    Still referring to  FIG. 8 , output terminal  134  is coupled to one side of solenoid valve  68 , the other side of which is coupled to ground line  114 . Module  116  provides a “low” voltage when it is desired to raise the lift, and provides a “high” (+12 V DC) voltage when it is desired to lower the lift. Referring briefly to  FIG. 7 , it can be seen that, depending upon the position of solenoid-controlled valve  68 , the direction in which pressurized hydraulic fluid is directed into hydraulic cylinders  50  and  52  can be reversed by actuating solenoid valve  68 . 
         [0063]    As shown in  FIG. 8 , output terminal  136  of module  116  is coupled to one side of motor relay coil  110 , the other side of which is coupled to ground line  114 . When module  116  causes output terminal  136  to assume a “high” (+12 V DC) output state, motor relay coil  110  is energized, and the electrical contacts of motor relay  108  are closed to energize pump motor  56 . As is also shown in  FIG. 8 , a normally-closed emergency stop button  160  may be positioned inside lift car  162  to shut down the operation of the lift during an emergency. 
         [0064]    Referring now to  FIGS. 9 and 10 , the preferred embodiment of the height adjustment mechanism, used to adjust the maximum height to which lift car  162  can be elevated, will now be described. A generally U-shaped, elongated rail  540  extends between first and second opposing ends  542  and  544 . Rail  540  is preferably made of metal, and the lower edges of side walls  546  and  548  preferably turn back inwardly inside rail  540  to form two inwardly directed flanges  550  and  552 , as best illustrated in the enlarged end view shown in  FIG. 11 . Mounting pins  543  and  545  extend transversely through the first and second ends  542  and  544 , respectively, of rail  540 . 
         [0065]    An actuator  554  is slidingly received within rail  540 , and a transverse tab  556  extends from actuator  554  below rail  540 . The features of actuator  554  are best observed in the enlarged view of  FIG. 12 . Actuator  554  is preferably formed of plastic, and is ideally machined from Nylon material. As shown in  FIG. 12 , the side walls of actuator  554  have opposing slots  558  and  560  formed therein; these slots are slidingly engaged by inwardly directed flanges  550  and  552  of rail  540  for allowing actuator  554  to slide along rail  540  between the first end  542  and the second end  544  thereof, while being captured therein. Mounting pins  543  and  545  prevent actuator  554  from exiting from either end of rail  540 . Transverse tab  556  is secured to the underside of plastic actuator body  554  by a pair of screws  557  and  559 . 
         [0066]    Still referring to  FIGS. 9 and 10 , a first proximity sensor, in the form of an electrical microswitch  562 , is mounted on rail  540  generally closer to second end  544  of rail  540  than to first end  542 . Switch  562  is preferably similar to those sold under Part No. BZ-2RW82-A2 by Honeywell Microswitch. Switch  562  corresponds to the upper stop switch  154  in the electrical schematic of  FIG. 8 . As shown in  FIG. 14 , switch  562  includes a lever arm  564  having a cam roller  566  at its distal end. Switch  562  is secured by a pair of screws  568  and  570  to a vertical wall of angle bracket  572 . The upper horizontal wall of angle bracket  572  is adapted to engage the upper, horizontal central wall of rail  540 . 
         [0067]    As indicated in  FIG. 9 , a series of slots, including slot  574 , are formed along the upper, horizontal central wall of rail  540 . Alternatively, one long continuous slot could be formed in the upper, horizontal central wall of rail  540 , if desired. Similarly, a slot  576  is formed in upper horizontal wall of angle bracket  572 . As will be explained below, maximum elevation height of the lift car is adjusted by moving, and re-tightening, angle bracket  572  relative to rail  540 . Referring to  FIG. 14 , a screw  578  extends through a lockwasher  580  from the underside of angle bracket  572 , through slot  576 . Turning to  FIG. 9 , the threaded tip of screw  578  is received within a mating lockwasher and nut (collectively designated by reference numeral  582 ). The length of slot  576 , along with the lengths and spacings of slots  574 , permit virtually infinite adjustment of the position of switch  562  along rail  540 . During installation of the lift device, the installer adjusts the position of switch  562  along rail  540  to make the lift car stop so that the floor  170  of the lift car is even with the stage  174 . 
         [0068]    Referring jointly to  FIGS. 10 ,  11  and  12 , a constant force spring  584  is wrapped about a plastic drum  585  for rotation about mounting pin  543 . Constant force spring  584  is similar to the constant force springs often found within tape measures for causing the elongated tape to retract. The free end  586  of constant force spring  584  is coupled with actuator  554 . Constant force spring  584  thereby serves as a biasing member for biasing actuator  554  toward first end  542  of rail  540 , and away from second end  544  of rail  540 . While this biasing force is preferably created by a constant force spring, the biasing force could alternatively be created using the force of gravity, as by attaching a weight, via a cable and pulley, to actuator  554 , or by simply mounting rail  540  at an angle to the horizontal (with first end  542  being the lowermost point) and attaching a weight directly to actuator  554 . 
         [0069]    Actuator  554  is disposed generally proximate to first end  542  of rail  540  when lift car  162  is in its lowered position on the ground. A first end of a flexible cable  590  extends into rail  540  from second end  544  and is attached to actuator  554  by anchor  592 . Cable  590  is preferably formed of braided wire of the type known as aircraft cable. As will be described in more detail below, as lift car  162  is elevated, cable  590  pulls on actuator  554  against the biasing force of spring  584 , causing actuator  554  to slide toward second end  544  of rail  540 , and toward switch  562 . As actuator  554  nears switch  562 , tab  556  engages cam roller  566  of lever arm  564 , closing microswitch  562 . The closing of switch  562  corresponds to the generation of an electrical signal that indicates that actuator  554  is proximate to switch  562 , and that the maximum height of the lift car has been achieved. Relay module  116  (see  FIG. 8 ) is responsive to this electrical signal for halting any further elevation of the lift car. 
         [0070]    It will be recalled that it is also desirable to generate a signal indicating that the lift car has been raised slightly above the ground, e.g., by two inches above the ground. This signal can easily be generated using the height adjustment rail and actuator already described above. Referring again to  FIGS. 9 and 10 , a second microswitch  594  is secured to a second angle bracket  596 . Microswitch  594  may be of the same type used for switch  562 . Second angle bracket  596  is adjustably mounted to rail  540  using a screw  598  and nut  599  in the same manner already described above for angle bracket  572 . However, second angle bracket  596  is mounted proximate to first end  542  of rail  540 , between first end  542  and switch  562 . As lift car  162  begins to rise, the tab  556  of actuator  554  engages cam roller  600  (see  FIG. 13 ) of switch  594 , closing switch  594 , and signaling that lift car  162  has left the ground. The exact position of switch  594  along rail  540  can be set, as desired, to trigger when the lift car  162  is a fixed number of inches above the ground. 
         [0071]    Turning to  FIGS. 15 and 16 , height adjustment rail  540  is shown after being mounted within side panel  167  of lift car  162 , via mounting pins  543  and  545 . As shown in  FIG. 15 , rail  540  is preferably mounted to extend substantially horizontally, and is secured to side panel  167  of the lift car; accordingly, as lift car  162  rises and falls, rail  540  rises and falls along with it. When lift car  162  is fully-lowered, actuator  554  (and its tab  556 ) are disposed all the way to the right, near the first end  542  of rail  540 , and tab  556  does not yet engage cam roller  600 . The first end of cable  590  is secured to actuator  554 , and the second end of flexible cable  590  is coupled to an anchor point below the second end  544  of rail  540 . This anchor point could be a point on base  180  of the lift. Alternatively, the anchor point can be a location on the lifting mechanism of the lift device, for example, a point on hydraulic lift cylinder  52 . In that event, the second end of cable  590  can advantageously be anchored to hydraulic cylinder  52  by a hose clamp secured about the hydraulic cylinder; the second end of cable  590  is inserted inside the hose clamp, and the hose clamp is tightened. 
         [0072]    As shown in  FIG. 15 , flexible cable  590  includes a first generally horizontal portion extending generally between actuator  554  and second end  544  of rail  540 , generally parallel to rail  540 . Flexible cable  590  also includes a second portion that extends generally between second end  544  of rail  540  and the anchor point; this second portion of flexible cable  590  extends at a substantial angle relative to rail  540 . If desired, a pulley or roller can be provided on mounting pin  545  to guide cable  590  around the bend. 
         [0073]    As lift car  162  elevates, cable  590  pulls actuator  554  from right to left (relative to  FIGS. 15 and 16 ), first tripping cam roller  600  and later tripping cam roller  566  to halt further elevation. Once again, while rail  540  is preferably mounted horizontally, as shown in  FIG. 15 , it is possible to position rail  540  at an angle to the horizontal, or even vertically, in which case, actuator  554  could be biased away from second end  544  of rail  540  by the force of gravity, as by attaching a weight to actuator  554 . 
         [0074]    While rail  540  is preferably mounted to lift car  162 , it is also possible to mount rail  540  to a fixed portion of the lift device (e.g., to a portion of base  180 ). In that event, the second end of flexible cable  590  should be attached to an anchor point above rail  540 ; this anchor point should be one that rises when lift car  162  is elevated, and that anchor point could be a point on the lift car itself. 
         [0075]      FIG. 17  shows the lift device partially elevated, and better illustrates the protective skirting that encircles the base of the lift device. As used herein, the term “collapsible curtain panel” is intended to include such protective skirting. Protective skirt  179  raises and collapses as front gate  164  of lift car  162  elevates and lowers, respectively. As shown in  FIG. 18 , protective skirt  179  consists of accordion-like flexible plastic pleated fabric; the pleats have vertically aligned holes formed near their opposing ends for slidingly receiving a pair of support rods  606  and  608 . Mounting hardware  610 ,  612 ,  614  and  616  is used to secure the upper portions of support rods  606  and  608  within the opposing side frame members of front gate  164 . The lower edge of skirt  179  is secured to lower support bar  532 , and the upper edge of skirt  179  is secured to the lower frame member of front gate  164  for elevation therewith. 
         [0076]    Referring briefly to  FIG. 6 , scissors brace  530  extends upwardly from lower support bar  532 ; scissors brace is hidden from view in  FIG. 17 , but extends just behind protective skirt  179 . Scissors brace  530  is sufficiently rigid to support protective skirt against significant inward deformation; thus, even if a bystander leaned against, or fell against, protective skirt  179 , there is little risk of injury to such person as a result of continued elevation, or continued lowering, of lift car  162 . 
         [0077]    At the opposite end of the lift device, below stage gate  172 , there is also little risk of injury to others present because the lift device is typically permanently installed so that its rear side abuts a stage or other structure. Accordingly, persons would find it difficult to position themselves adjacent to the protective skirt  603  (see  FIG. 19 ) that covers the rear side of the lift device below stage gate  172 . 
         [0078]    Referring briefly to  FIG. 19 , it will be noted that the protective skirts that shield the rear portion, and two sides, of the lift device can be fabricated as a single structure, again preferably from accordion-like flexible plastic pleated fabric. Protective skirt  604  extends below side panel  165  of lift car  162 , as shown in  FIG. 17 . Protective skirt  603  extends below the rear of lift car  162 , and protective skirt  181  extends below side panel  167  of lift car  162 , as shown in  FIGS. 1-3 . The upper end  618  of protective skirt  604  is secured to side panel  165  of lift car  162  for movement therewith, and the lower end  620  of protective skirt  604  is secured to base member  502 . 
         [0079]    Protective skirt  604  and opposing protective skirt  181  are both accessible to bystanders. While protective skirts  604  and  181  help to prevent arms and legs of bystanders from being poked under lift car  162 , such protective skirts are necessarily flexible to facilitate expansion and retraction as lift car  162  is elevated and lowered. In view of such flexibility, protective skirts  604  and  181  will yield to significant inward pressure, as when a person leans against, or falls against, one of such skirts. A person&#39;s body could subsequently become pinched between the lower portion of lift car  162  and the ground if the lift car continued down toward the ground. It is therefore advisable to halt any further movement of lift car  162  if either protective skirt  604  or protective skirt  181  is inwardly deformed. 
         [0080]    To prevent further lift car movement when either protective skirt  604  or protective skirt  181  is inwardly deformed, a series of skirt sensors are provided along the opposing sides of the lift device, as will now be described with reference to  FIGS. 20 and 21 . For clarity, protective skirt  604  is omitted from  FIGS. 20 and 21 . A first deformable elongated, elastic tension spring  630  has a first end  632  engaged with an anchor loop  634  on apertured bracket  507  near base  180 . Second end  636  of elongated spring is secured to a hook or loop  638  anchored to an upper portion of hydraulic lift cylinder  50  by circular hose clamp  640 , generally proximate lift car  162  for movement therewith. Spring  630  extends along hydraulic cylinder  50  facing, and adjacent to, protective skirt  604 . As hydraulic cylinder  50  extends its piston rod to raise lift car  162 , spring  630  stretches and elongates, but the longitudinal axis of spring  630  always extends generally across, and proximate to, protective skirt  604 . If protective skirt  604  were deformed inwardly, as by someone falling against it, and applying a lateral force thereto, the contact between protective skirt  604  and spring  630  also laterally displaces spring  630 . 
         [0081]    In  FIG. 20 , a microswitch  650  is mounted to hydraulic cylinder  50  by hose clamp  652 . Microswitch  650  is similar to those described above for use with the height adjustment mechanism; preferably skirt sensor switch  650  is a Model No. BZ-2RW8299-A2 from Honeywell Microswitch, including an adjustable pre-travel feature. Microswitch  650  corresponds to one of the skirt sensor switches  142 ,  144 ,  156 , and  140  described above in conjunction with the electrical schematic of  FIG. 8 . Switch  650  is normally “closed” to form an electrical short circuit. The cam roller on the lever arm of switch  650  is positioned just behind spring  630 ; as a result, any significant lateral deformation of tension spring  630 , away from its longitudinal axis, causes switch  650  to “open”, breaking the electrical path. 
         [0082]    For added protection, a second tension spring  660  is also secured along hydraulic cylinder  50 . Tension spring  660  has a first end secured to a hook or loop mounted to the lower end of hydraulic cylinder  50  by hose clamp  666 . The upper end  668  of spring  660  is secured to an upper portion of hydraulic cylinder  50  by hose clamp  670 . As shown in  FIG. 21 , another microswitch  672 , similar to switch  650 , and including lever arm  674  and cam roller  676 , is mounted to hydraulic cylinder  50  by hose clamp  678 . Cam roller  676  is disposed just behind spring  660  to detect any lateral deflection thereof caused by deformation of protective skirt  604 . When cam roller  676  of switch  672  is contacted by spring  660 , switch  672  opens. As explained above in conjunction with  FIG. 8 , when any of the skirt sensor switches open, relay module  116  immediately halts any further movement of lift car  162  until the problem is resolved. 
         [0083]    Those skilled in the art will now appreciate that a lift device has been described that is suitable for lifting wheelchair-bound users up to the height of stages and the like in a safe, reliable and repeatable manner, and complying with all applicable ADA requirements. The lift device includes protective skirting about the base of the lift device, while being able to detect instances when the safety skirt is inwardly deformed to the extent of posing a possible danger. Upon detecting such danger, the lift device immediately halts any further upward or downward movement of the lift car until the cause of such problem has been resolved. Moreover, the additional components used to detect lateral deformation of the skirt are relatively inexpensive and do not significantly increase the complexity of the lift device. 
         [0084]    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.