Abstract:
A vehicle lift control maintains multiple points of a lift system within the same horizontal plane during vertical movement of the lift engagement structure by synchronizing the movement thereof. A vertical trajectory is compared to actual positions to generate a raise signal. A position synchronization circuit synchronizes the vertical actuation of the moveable lift components by determining a proportional-integral error signal.

Description:
[0001]    This application hereby incorporates by reference U.S. patent application Ser. No. 10/055,800, filed Oct. 26, 2001, titled Electronically Controlled Vehicle Lift And Vehicle Service System and U.S. Provisional Application Serial No. 60/243,827, filed Oct. 27, 2000, titled Lift With Controls, both of which are commonly owned herewith. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates generally to vehicle lifts and their controls, and more particularly to a vehicle lift control adapted for maintaining multiple points of a lift system within the same horizontal plane during vertical movement of the lift superstructure by synchronizing the movement thereof. The invention is disclosed in conjunction with a hydraulic fluid control system, although equally applicable to an electrically actuated system.  
           [0003]    There are a variety of vehicle lift types which have more than one independent vertically movable superstructure. Examples of such lifts are those commonly referred to as two post and four post lifts. Other examples of such lifts include parallelogram lifts, scissors lifts and portable lifts. The movement of the superstructure may be linear or non-linear, and may have a horizontal motion component in addition to the vertical movement component. As defined by the Automotive Lift Institute ALI ALCTV-1998 standards, the types of vehicle lift superstructures include frame engaging type, axle engaging type, roll on/drive on type and fork type. As used herein, superstructure includes all vehicle lifting interfaces between the lifting apparatus and the vehicle, of any configuration now known or later developed.  
           [0004]    Such lifts include respective actuators for each independently moveable superstructure to effect the vertical movement. Although typically the actuators are hydraulic, electromechanical actuators, such as a screw type, are also used.  
           [0005]    Various factors affect the vertical movement of superstructures, such as unequal loading, wear, and inherent differences in the actuators, such as hydraulic components for hydraulically actuated lifts. Differences in the respective vertical positions of the independently superstructures can pose significant problems. Synchronizing the vertical movement of each superstructure in order to maintain them in the same horizontal plane requires precisely controlling each respective actuator relative to the others to match the vertical movements, despite the differences which exist between each respective actuator. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0006]    The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention. In the drawings:  
         [0007]    [0007]FIG. 1 is a schematic diagram of an embodiment of a control in accordance with the present invention, embodied as a hydraulic fluid control system including the controller and hydraulic circuit.  
         [0008]    [0008]FIG. 2 is a control diagram showing the complete raise control including the raise circuit and the position synchronization circuit for a pair of superstructures.  
         [0009]    [0009]FIG. 3 is a control diagram showing the complete lower control including the lowering circuit and the position synchronization circuit for a pair of vertically superstructures  
         [0010]    [0010]FIG. 4 is a control diagram showing the lift position synchronization circuit for two pairs of superstructures.  
         [0011]    [0011]FIG. 5 is a control diagram illustrating the generation of movement control signals for raising each superstructure of each of two pairs.  
         [0012]    [0012]FIG. 6 is a schematic diagram of another embodiment of a control in accordance with the present invention showing the controller and a different hydraulic circuit different from that of FIG. 1. 
     
    
       [0013]    Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    Referring now to the drawings in detail, wherein like numerals indicate the same elements throughout the views, FIG. 1 illustrates a vehicle lift, generally indicated at  2 . Lift  2  is illustrated as a two post lift, including a pair of independently moveable actuators  4  and  6  which cause the respective superstructures (not shown) to move. In the depicted embodiment, first and second actuators  4  and  6  are illustrated as respective hydraulic cylinders, although they may be any actuator suitable for the control system. First and second actuators  4  and  6  are in fluid communication with a source of hydraulic fluid  8 . Pressurized hydraulic fluid is provided by pump  10  at discharge  10   a . Each actuator  4  and  6  has a respective proportional flow control valve  12  and  14  interposed between its actuator and source of hydraulic fluid  8 .  
         [0015]    The hydraulic fluid flow is divided at  16 , with a portion of the flow going to (from, when lowered) each respective actuator  4  and  6  as controlled by first and second proportional flow control valves  12  and  14 . As illustrated, isolation check valve  18  is located in the hydraulic line of either actuator  4  or  6  (shown in FIG. 1 in hydraulic line  20  of actuator  6 ), between  16  and second flow control valve  14  to prevent potential leakage from either actuator  4  or  6  through the respective flow control valve  12  and  14  from affecting the position of the other actuator.  
         [0016]    Isolation check valve  18  can be eliminated if significant leakage through first and second flow control valves  12  and  14  does not occur. In the embodiment depicted, equalizing the hydraulic losses between  16  and actuator  4 , and  16  and actuator  6 , makes it easier to set gain factors (described below). To achieve this, an additional restriction may be included in hydraulic line  20   a  between  16  and actuator  4  to duplicate the hydraulic loss between  16  and actuator  6 , which includes isolation check valve  18 . This may be accomplished in many ways, such as through the addition of an orifice (not shown) or another isolation check valve (not shown) between  16  and actuator  4 .  
         [0017]    The hydraulic circuit includes lowering control valve  22  which is closed except when the superstructures are being lowered.  
         [0018]    Lift  2  includes position sensors  24  and  26 . Each position sensor  24  and  26  is operable to sense the vertical position of the respective superstructure. This may be done by directly sensing the moving component of the actuator, such as in the depicted embodiment a cylinder piston rod, sensing vertical position of the superstructure, or sensing any lift component whose position is related to the position of the superstructure. Recognizing that the position and movement of the superstructures may be determined without direct reference to the superstructures, as used herein, references to the position or movement of a superstructure are also references to the position or movement of any lift component whose position or movement is indicative of the position or movement of a superstructure, including for example the actuators.  
         [0019]    Position sensors  24  and  26  are illustrated as string potentiometers, which generate analog signals that are converted to digital signals for processing. Any position measuring sensor having adequate resolution may be used in the teachings of this invention, including by way of non-limiting examples, optical encoders, LVDT, displacement laser, photo sensor, sonar displacement, radar, etc. Additionally, position may be sensed by other methods, such as by integrating velocity over time. As used herein, position sensor includes any structure or algorithm capable of generating a signal indicative of position.  
         [0020]    Lift  2  includes controller  28  which includes an interface configured to receive position signals from position sensors  24  and  26 , and to generate movement control signals to control the movement of the superstructures. Movement control signals control the movement of the superstructures by controlling or directing the operation, directly or indirectly, of the lift components (in the depicted embodiment, the actuators) which effect the movement of the superstructure. Controller  28  is connected to first and second flow control valves  12  and  14 , isolation check valve  18 , lowering valve  22  and pump motor  30 , and includes the appropriate drivers on driver board  32  to actuate them. Controller  28  is illustrated as receiving input from other lift sensors (as detailed in copending application Ser. No. 10/055,800), controlling the entire lift operation. It is noted that controller  28  may be a stand alone controller (separate from the lift controller which controls the other lift functions) dedicated only to controlling the movement of the superstructures in response to a command from a lift controller.  
         [0021]    In the depicted embodiment, controller  28  includes a computer processor which is configured to execute the software implemented control algorithms every 10 milliseconds. Controller  28  generates movement control signals which control the operation of first and second flow control valves  12  and  14  to allow the required flow volume to the respective actuators  4  and  6  to synchronize the vertical actuation of the pair of superstructures.  
         [0022]    [0022]FIG. 2 is a control diagram showing the complete raise control, generally indicated at  34 , including raise circuit  36  and position synchronization circuit  38  for the pair of superstructures. When the lift is instructed to raise the superstructures, complete raise control  34  effects the controlled, synchronized movement of the superstructures based on input from position sensors  24 ,  26 . Raise circuit  36  is a feed back control loop which is configured to command the pair of superstructures to an upward vertical trajectory. Raise circuit  36  compares the desired position of the superstructures indicated by vertical trajectory signal  40  (xd) to the actual positions indicated respectively by position signals  42  and  44  (x 1  and x 2 ) generated by position sensors  24 ,  26 . The respective differences between each set of two signals, representing the error between the desired position and the actual position, is multiplied by a raise gain factor Kp, to generate first raise signal  46  for the first superstructure and second raise signal  48  for the second superstructure, respectively. Although in the depicted embodiment, Kp was the same for each superstructure, alternatively Kp could be unique for each.  
         [0023]    In the embodiment depicted, vertical trajectory signal  40  is a linear function of time, wherein the desired position xd is incremented a predetermined distance for each predetermined time interval. It is noted that the vertical trajectory may be any suitable trajectory establishing the desired position of the superstructures (directly or indirectly) based on any relevant criteria. By way of non-limiting example, it may be linear or non-linear, it may be based on prior movement or position, or the passage of time. Alternatively, first and second raise signals  46  and  48  could be fixed signals, independent of the positions of the superstructures.  
         [0024]    The vertical trajectory signal resets when the lift is stopped and restarted. Thus, if the upward motion of the lift is stopped at a time when the actual position of the lift lags behind the desired position as defined by the vertical trajectory signal  40 , upon restarting the upward motion, the vertical trajectory signal  40  starts from the actual position of the superstructures.  
         [0025]    There are various ways to establish the starting position from which the vertical trajectory signal is initiated. In the depicted embodiment, one of the posts is considered a master and the other is considered slave. When the lift is instructed to raise, the actual position of the superstructures of the master post is used as the starting position from which the vertical trajectory signal starts. Of course, there are other ways in which to establish the starting position of the vertical trajectory signal, such as the average of the actual positions of the two posts.  
         [0026]    In the embodiment depicted, vertical trajectory signal  40  is generated by controller  28 . Alternatively vertical trajectory signal  40  could be received as an input to controller  28 , being generated elsewhere.  
         [0027]    Position synchronization circuit  38 , a differential feedback control loop, is configured to synchronize the vertical actuation/movement of the pair of superstructures during raising. In the depicted embodiment, position synchronization circuit  38  is a cross coupled proportional-integral controller which generates a single proportional-integral error signal relative to the respective vertical positions of the superstructures. As shown, position synchronization circuit  38  includes proportional control  38   a  and integral control  38   b , both of which start with the error between the two positions, x 1  and x 2 , indicated by  50 . Output  52  of proportional control  38   a  is the error  50  multiplied by a raise gain factor Kpc 1 . Output  54  of integral control  38   b  is the error  50  multiplied by a raise gain factor Kic 1 , summed with the integral output  54   a  of integral control  38   b  from the preceding execution of integral control  38   b . Output  52  and output  54  are summed to generate proportional-integral error signal  56 .  
         [0028]    Controller  28 , in response to first raise signal  46  and proportional-integral error signal  56 , generates a first movement control signal  58  for the first superstructure. In the depicted embodiment, first movement control signal  58  is generated by subtracting proportional-integral error signal  56  from first raise signal  46 . First movement control signal  58  controls, in this embodiment, first flow control valve  12  so as to effect the volume of fluid flowing to and therefore the operation of first actuator  4  and, concomitantly, the first superstructure.  
         [0029]    Controller  28 , in response to second raise signal  48  and proportional-integral error signal  56 , generates a second movement control signal  60  for the second superstructure. In the depicted embodiment, second movement control signal  60  is generated by adding proportional-integral error signal  56  to second raise signal  48 . Second movement control signal  60  controls, in this embodiment, second flow control valve  14  so as to effect the volume of fluid flowing to and therefore the operation of second actuator  6  and, concomitantly, the second superstructure.  
         [0030]    [0030]FIG. 3 is a control diagram showing the complete lower control, generally indicated at  62 , including lowering circuit  64 , and position synchronization circuit  66 , a differential feedback control loop, for the pair of superstructures. When the lift is instructed to lower the superstructures, complete lower control  62  effects the controlled movement of the superstructures.  
         [0031]    Lowering circuit  64  is configured to generate first lowering signal  68  for the first superstructure and to generate second lowering signal  70  for the second superstructure. In the depicted embodiment, lowering signals are constant, not varying in dependence with the positions of the superstructures or time. Although in the depicted embodiment, lowering signals  68  and  70  are equal, they could be unique for each superstructure. Lowering signals  68  and  70  may alternatively be respectively generated in response to the positions of the superstructures, such as based on the differences between a vertical trajectory and the actual positions.  
         [0032]    Position synchronization circuit  66  is similar to position synchronization circuit  38 . Position synchronization circuit  66  is configured to synchronize the vertical actuation/movement of the pair of superstructures during lowering. In the depicted embodiment, position synchronization circuit  66  is a cross coupled proportional-integral controller which generates a single proportional-integral error signal relative to the respective vertical positions of the superstructures. As shown, position synchronization circuit  66  includes proportional control  66   a  and integral control  66   b , both of which start with the error between the two positions, x 1  and x 2 , indicated by  72 . Output  74  of proportional control  66   a  is the error  72  multiplied by a lowering gain factor Kpc 2 . Output  76  of integral control  66   b  is the error  72  multiplied by a lowering gain factor Kic 2 , summed with the integral output  76   a  of integral control  66   b  from the preceding execution of integral control  66   b . Output  74  and output  76  are summed to generate proportional-integral error signal  78 .  
         [0033]    Controller  28 , in response to first lowering signal  68  and proportional-integral error signal  78 , generates a first movement control signal  80  for the first superstructure. In the depicted embodiment, first movement control signal  80  is generated by adding proportional-integral error signal  78  to first lowering signal  68 . First movement control signal  80  controls, in this embodiment, first flow control valve  12  so as to effect the volume of fluid flowing from and therefore the operation of first actuator  4  and, concomitantly, the first superstructure.  
         [0034]    Controller  28 , in response to second lowering signal  70  and proportional-integral error signal  78 , generates a second movement control signal  82  for the second superstructure. In the depicted embodiment, second movement control signal  82  is generated by subtracting proportional-integral error signal  78  from second lowering signal  70 . Second movement control signal  82  controls, in this embodiment, second flow control valve  14  so as to effect the volume of fluid flowing from and therefore the operation of second actuator  6  and, concomitantly, the second superstructure.  
         [0035]    The present invention is also applicable to lifts having more than one pair of superstructures. For example, this invention may be used on a four post lift which has two pairs of superstructures, each pair comprising a left and right side of a respective end of the lift or each pair comprising the left side and the right side of the lift. The invention may used with an odd number of superstructures, such as by treating one of the superstructures as being a pair “locked” together. More than two pairs may be used, with one of the pairs being the control or target pair.  
         [0036]    For a four post lift, the controller includes an interface configured to receive first and second position signals of the first pair, and to receive third and fourth positions signals of the second pair. The complete up control and complete down control as described above are used for each pair (first and second superstructures; third and fourth superstructures). The respective gain factors between the pairs, or between any superstructures, may be different. Differences in the hydraulic circuits (such as due to different hydraulic hose lengths) can result in the need or use of different gain factors.  
         [0037]    The controller is further configured to synchronize the first and second pairs relative to each other through a lift position synchronization control which in the depicted embodiment reduces the difference between the average of the positions of the first pair and the mean of the positions of the second pair.  
         [0038]    [0038]FIG. 4 is a control diagram showing the lift position synchronization circuit, a differential feedback control loop, generally indicated at  84 , for synchronizing the two pairs during raising. As shown, lift position synchronization circuit  84  includes proportional control  84   a  and integral control  84   b , both of which start with the error, indicated by  86 , between the first pair and the second pair by subtracting the positions of the second pair, x 3  and x 4 , from the positions of the first pair, x 1  and x 2 . Output  88  of proportional control  84   a  is the error  86  multiplied by a raise gain factor Kpcc. Output  90  of integral control  84   b  is the error  86  multiplied by a raise gain factor Kicc, summed with the integral output  90   a  integral control  84   b  from the preceding execution of integral control  84   b . Output  88  and output  90  are summed to generate lift proportional-integral error signal  92 .  
         [0039]    [0039]FIG. 5 is a control diagram illustrating the generation of movement control signals for raising each superstructure of each of the two pairs. The controller, in response to first raise signal  94 , first pair proportional-integral error signal  96  and lift proportional-integral error signal  92 , generates a first movement control signal  98  for the first superstructure. In the depicted embodiment, first movement control signal  98  is generated by subtracting lift proportional-integral error signal  92  and first pair proportional-integral error signal  96  from first raise signal  94 . First movement control signal  98  controls, in this embodiment, first flow control valve  12  so as to effect the volume of fluid flowing to and therefore the operation of first actuator  4  and, concomitantly, the first superstructure.  
         [0040]    The controller, in response to second raise signal  100 , first pair proportional-integral error signal  96  and lift proportional-integral error signal  92 , generates a second movement control signal  102  for the second superstructure. In the depicted embodiment, second movement control signal  102  is generated by adding subtracting lift proportional-integral error signal  92  from the sum of first pair proportional-integral error signal  96  and first raise signal  100 . Second movement control signal  102  controls, in this embodiment, second flow control valve  14  so as to effect the volume of fluid flowing to and therefore the operation of second actuator  6  and, concomitantly, the second superstructure.  
         [0041]    Still referring to FIG. 5, the controller, in response to third raise signal  104 , second pair proportional-integral error signal  106  and lift proportional-integral error signal  92 , generates a third movement control signal  108  for the third superstructure. In the depicted embodiment, third movement control signal  108  is generated by subtracting second pair proportional-integral error signal  106  from the sum of lift proportional-integral error signal  92  and third raise signal  104 . Third movement control signal  108  controls, in this embodiment, third flow control valve  110  so as to effect the volume of fluid flowing to and therefore the operation of the third actuator (not shown) and, concomitantly, the third superstructure.  
         [0042]    The controller, in response to fourth raise signal  112 , second pair proportional-integral error signal  106  lift proportional-integral error signal  92 , generates a fourth movement control signal  114  for the fourth superstructure. In the depicted embodiment, fourth movement control signal  114  is generated by summing fourth raise signal  112 , second pair proportional-integral error signal  106  and lift proportional-integral error signal  92 . Fourth movement control signal  114  controls, in this embodiment, fourth flow control valve  116  so as to effect the volume of fluid flowing to and therefore the operation of the fourth actuator (not shown) and, concomitantly, the fourth superstructure.  
         [0043]    During lowering, the controller executes the lift position synchronization algorithm as shown in FIG. 4, except that the lowering gain factors are not necessarily the same as the raise gain factors. In the depicted embodiment, the lowering gain factors were different from the raise gain factors. During lowering, in the depicted embodiment, the arithmetic operations are reversed for the lift proportional-integral error signal: The lift proportional-integral error signal is added to generate the first and second movement signals (instead of subtracted as shown in FIG. 5) and subtracted to generate the third and fourth movement signals (instead of added as shown in FIG. 5).  
         [0044]    The gain factors described above may be set using any appropriate method, such as the well known Zigler-Nichols tuning methods, or empirically. In determining the gain factors empirically, the integral control was disabled and multiple cycles of different loads were raised and lowered to find the optimum gain factor for the proportional control. The integral control was then enabled and those gain factors determined through multiple cycles of different loads.  
         [0045]    The following table sets forth two examples of the gain factors and up rate:  
                                                                     Example 1   Example 2                                        Kp   1.0   6.0           Kpc1   0.5   6.0           Kic1   0.15   0.3           Kpc2   1.5   6.0           Kic2   0.25   0.25           Xdown1   65   50           Xdown2   175   175           up rate   2.0 in/sec   1.8 in/sec                      
 
         [0046]    It is noted, as seen above, that gain factors may be 1.  
         [0047]    The controller preferably includes a calibration algorithm for the position sensors. In the depicted embodiment, whenever the lift is being commanded to move when it is near either end of its range of travel and the position sensors do not indicate movement for a predetermined period of time, the calibration algorithm is executed. In such a situation, it is assumed that the lift is at the end of its range of travel. The algorithm correlates the position sensor output as corresponding to the maximum or minimum position of the lift, as appropriate. The inclusion of a calibration algorithm allows a range of position sensor locations, reducing the manufacturing cost.  
         [0048]    The present invention may be used with a variety of actuators and hydraulic circuits. FIG. 6 illustrates an alternate embodiment of the hydraulic circuit. In this vehicle lift, generally indicated at  118 , the difference in comparison to FIG. 1 lies in that control of the flow of hydraulic fluid to actuators  4  and  6  is accomplished through the use of individual motors  120  and  128  and pumps  122  and  130  for each superstructure, with each motor/pump being controlled by a respective variable frequency drive (VFD) motor controller  124  and  132  to effect raising the lift and through the use of respective proportioning flow control valves  126  and  134  to effect lowering the lift. Alternatively, individual motors  120 ,  128  could drive a screw type actuator.  
         [0049]    As illustrated, each motor/pump  120 / 122  and  128 / 130  has a respective associated source of hydraulic fluid  136  and  138 , although a single source could be associated with both motors and pumps. Each pump  122  and  130  has a respective discharge  122   a  and  130   a  which is in fluid communication with its respective actuator  4  and  6 .  
         [0050]    Controller  140  includes the appropriate drivers for the VFD motor controllers  124  and  132 , and executes the control algorithms as described above to synchronize the vertical actuation of the superstructures. By varying the speed of the respective motors  120  and  132 , the hydraulic fluid flow rate to the respective actuators  4  and  6  varies for raising.  
         [0051]    In summary, numerous benefits have been described which result from employing the concepts of the invention. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiment was chosen and described in order to best illustrate the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.