Patent Publication Number: US-7896358-B2

Title: Magneto-rheological inertial damping system for lift trucks

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Not applicable. 
     STATEMENT CONCERNING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     FIELD OF THE INVENTION 
     This invention relates to material handling apparatus, and more particularly, to improved arrangements for inertially damping the motion of the unpowered, suspended rear wheel commonly used on lift trucks. 
     BACKGROUND OF THE INVENTION 
     One class of narrow-aisle lift trucks employs a pair of unpowered non-steerable front wheels, or load wheels, a steerable powered drive wheel assembly rigidly mounted near one rear corner of the truck, and an unpowered vertically-sprung idler wheel assembly near the other rear corner of the truck. With all four wheels mounted on the same base frame, one wheel must be vertically sprung, or floor irregularities could result in loss of traction by the drive wheel. In some applications the vertically-sprung idler wheel assembly uses a free-wheeling, non-steered caster wheel which is self-steering. One early form of truck of that type is shown in U.S. Pat. No. 2,564,002. In various other applications the sprung idler wheel is not castered, but instead steered via a linkage. A truck of this latter type is shown in U.S. Pat. No. 3,392,797. 
     The suspended wheel is suspended from the frame of the truck by coil springs, a torsion bar or leaf springs as shown and described in U.S. Pat. No. 4,813,512, which is hereby incorporated by reference for its description of such devices. Lift trucks achieve significant economies when vehicle frames of a uniform type are used with either a castered idler wheel or a linkage-steered idler wheel. Provision of an idler wheel mounting arrangement which will readily accommodate either type of steering is disclosed in U.S. Pat. No. 3,392,797. In the idler wheel mounting arrangements disclosed in that patent, the pivot steering axis of the idler wheel is located somewhat inwardly from a lateral extremity of the truck to allow space for a castered wheel to swing. The springs used to oppose weight on the idler wheel must be aligned with the pivot or steering axis, so that they do not impose moments which would cause undue bearing wear, and hence the springs also must be located undesirably inwardly from the lateral extremity of the truck, where they tend to interfere with provisions of an unobstructed operator compartment and waste space. 
     One problem with prior art lift trucks is that they sway when the truck stops abruptly or abruptly changes direction or both. While such motion will not tip the truck, it can be disconcerting to an operator. Normally an operator will slow down and allow the tilt to naturally dissipate before resuming travel. Accordingly, such unwanted tilting or swaying reduces the efficiency of the operator and the overall productivity of lift truck operations. 
     U.S. Pat. No. 5,685,555 describes one method for providing a suspended idler wheel mounting arrangement wherein the suspension means has its motion dampened in order to limit the tilt of a lift truck following an abrupt stop or an abrupt change in direction. Here, a mechanical inertial damper is coupled between the suspended wheel and the frame. The inertial damper includes a pair of parallel outer plates, with a slider plate disposed between the plates. A pair of friction pads is provided between an outer plate and the slider plate, and frictionally engages the slider plate when the frame moves relative to the wheel to slow the relative motion between the frame and the wheel. An adjustable means, such as a belville washer or spring, is provided for adjusting pressure of the outer plates on the slider plate. 
     While this prior art system is effective in providing stability to the vehicle, this system can provide only a single level of damping during use, and thus cannot dynamically adjust for variations that occur in the height of the mast or the weight of the load. The present invention addresses these issues. 
     SUMMARY OF THE INVENTION 
     The present invention provides a shock absorbing system that minimizes truck dynamics, particularly in vehicles having tall masts, for use on uneven floors, and in vehicles that provide right angle stacking. The shock absorbing dampers of the present invention provide smoother ride characteristics and facilitate precision load handling by providing a stable ride for the operator. 
     In one aspect, the present invention provides a lift truck adapted to provide stability during use of the vehicle. The lift truck comprises a frame, with a motor and wheels mounted on the frame. At least one wheel is driven by the motor and another wheel is suspended from the frame by a spring. A movable lift mast is mounted on the frame for vertically extending and retracting. The lift mast includes a mass sufficient to tilt the frame of the truck such that a portion of the frame adjacent the suspended wheel changes its relative position with respect to ground when the truck stops abruptly or changes direction abruptly. A fork is adapted to move along the mast. A sensor is provided for producing a feedback signal indicating at least one of a height of the mast, a weight of a load on the fork, and a speed of the lift truck. A magneto-rheological damper is coupled between the suspended wheel and the frame. A vehicle control system is adapted to monitor the feedback signal and to drive the magneto-rheological damper to alter a damping force based on the feedback for speed, height or weight. 
     In another aspect of the invention, the vehicle control system is further adapted to drive the damper to a maximum damping force when the feedback signal exceeds a respective one of a speed, height or weight maximum damping value. The vehicle control system can also be adapted to drive the damper to a selected damping force value between the minimum damping force and the maximum damping force as a function of the feedback signal. The selected damping force can be also selected as a function of the ratio of the feedback to a maximum rated value for the lift truck. 
     In another aspect of the invention, the lift truck further comprises a second sensor for producing a second feedback signal indicative of another of the height of the mast, a weight of a load on the fork, and a speed of the lift truck. The lift truck can also include a third sensor for sensing the remaining height, weight, or speed parameter. 
     In yet another aspect of the invention the minimum damping value and the maximum damping value are calculated as a function of the rated maximum value of the parameters associated with each of the respective height of the mast, weight of a load on the fork, and speed of the lift truck. 
     The foregoing and other objects and advantages of the invention will appear in the detailed description which follows. In the description, reference is made to the accompanying drawings which illustrate a preferred embodiment of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an elevation view of a lift truck with its mast extended and supporting a load. 
         FIG. 2  is a rear elevation view of one form of lift truck incorporating a preferred form of the invention, with certain parts cut away and certain parts omitted for sake of clarity. 
         FIG. 3  is a downward section view taken at lines  3 - 3  in  FIG. 2 . 
         FIG. 4  is a partial perspective and partial cut away view of a lift truck showing an inertial damper on the suspended wheel. 
         FIG. 5  is a front elevation view of the damper mounted between two coil springs. 
         FIG. 6  is a back elevation view of the damper mounted between two coil springs. 
         FIG. 7  is a block diagram of a control system for the lift truck of  FIG. 1 . 
         FIG. 8  is a graph illustrating the current applied for percentages of maximum rated height, weight and speed levels. 
         FIG. 9  is a graph illustrating the current applied for percentages of maximum rated height, weight and speed levels for a specific vehicle. 
         FIG. 10  is a partial view of a lift truck constructed in accordance with a second embodiment of the invention. 
         FIG. 11  is a perspective view of a suspension system provided in the lift truck of  FIG. 10 . 
         FIG. 12  is a second perspective view of a suspension system provided in the lift truck of  FIG. 10 . 
         FIG. 13  is another partial view of the lift truck of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIGS. 1 and 2  illustrate a lift truck  100  constructed in accordance with one embodiment of the present invention. Referring first to  FIG. 1 , the truck  100  comprises a mast  110  including a fork  112  that is moveable along the mast  110  to raise and lower a load  114 . The mast  110  and a housing  113  are coupled to a base frame  116  of the truck  100 , and a steerable powered drive wheel assembly  20  and a vertically sprung idler wheel assembly  32  support the truck  100  below the base frame  116 . As described below, the idler wheel assembly  32  includes a magneto-rheological damper for stabilizing the truck  100  during operation and preferably also includes a spring assembly. 
     Referring now also to  FIG. 2 , a cutaway view of the housing  113  of the lift truck  100  is shown. The drive wheel assembly  20  includes a traction motor  49  which drives a drive wheel  11 , and a steering motor  47  that is fixedly mounted relative to the base frame  116  of the truck  100  and is operated by a conventional steering control  16  ( FIG. 4 ), which is controlled by the operator to select a direction of motion for the drive wheel  11  and truck  100 . The drive wheel assembly  20  can be constructed, for example, as described in U.S. Pat. No. 5,685,555, which is incorporated by reference for its description of this assembly and the associated steering linkages. Various other methods of constructing a drive wheel assembly will be apparent to those of skill in the art. 
     Referring still to  FIG. 2 , the idler wheel assembly  32  is coupled to the housing  113  on an opposing side of the housing  113  from the drive wheel  11 . Referring now also to  FIG. 5 , the idler wheel assembly  32  is shown journalled by means of a roller thrust bearing  40  near the outer end of a rigid A-frame arm, or lever member  34 , which is shown pivotally mounted on the base frame  116  of the truck  100 , near the lateral center of the truck  100 , by trunnion bearings  35  so that A-frame lever member  34  may rotate limited amounts about a horizontal longitudinally-extending axis x-x ( FIG. 3 ). A pair of compression springs  42 ,  43  are shown interposed between the outer end of the A-frame lever member and a plate affixed to the base frame  116  of the truck  100 . Hence springs  42 ,  43  compress in accordance with the vertical weight imposed on the idler wheel  16 , and as the truck  100  travels over irregular floor surfaces the idler wheel  16  may move upwardly and downwardly relative to the frame  116  of the truck  100  to insure that adequate weight to provide traction is always imposed on the powered drive wheel  11  of drive unit  20 . As shown in  FIG. 1 , when truck  100  stops abruptly or abruptly changes direction, the springs  42 ,  43  are compressed and oscillate, thereby causing the mast  110  to oscillate, for example, in the direction of arrow  103 . Such oscillation is enhanced by a load  114  carried on fork  112  that is extended to the top of the mast  110 . Although a specific direction of oscillation is shown here, the induced oscillation can be in a lateral direction, in a longitudinal direction, or both. 
     As floor surface irregularities cause the A-frame lever member  34  to rotate about axis x-x, the steering axis of the idler wheel assembly departs slightly from the vertical, and because the idler wheel steering shaft is journalled in lever member  34  for rotation about a fixed axis, the slight rotation of the lever member causes floor contact of the idler wheel  16  to vary between the inside and outside edges of the idler wheel tire. Appreciable rotation of lever member  34  occurs when floor irregularities are encountered, when there is a rapid change in motion, or when the brakes are applied quickly. 
     Referring still to  FIG. 2 , idler wheel assembly  32  includes an idler wheel  16  (shown partially cutaway in  FIG. 2 ), and a vertical pivot or steering shaft  52  ( FIG. 3 ). Referring now also to  FIGS. 4 ,  5 , and  6 , the idler wheel assembly  32  comprises a plate  44  that is coupled to an inside wall of the housing  113 , and springs  42  and  43  are coupled between the plate  44  and the lever member  34 , substantially in parallel with a magneto-rheological damper  150 . The damper  150  includes a housing  149  that contains a magneto-rheological fluid, and an extendable arm  151  that extends and retracts from the housing  149 . A ring connector  153  is provided at the end of the arm  151 , and a ring connector  155  is provided at the opposing end of the housing. When a magnetic field is applied to the fluid, by applying a voltage and current to the fluid in the housing  149 , the fluid changes from a liquid to a near solid, increasing the damping force of the damper  150 . Although a number of commercial devices are available for providing this function, one example of a magneto-rheological device suitable in the present application is the RD-1005-3 MR Damper from Lord Corporation of Cary N.C. 
     Referring still to  FIGS. 5 and 6 , a mounting member  161  is coupled to the plate  44 , and a mounting member  163  is coupled to the lever arm  34 . Each of the mounting members  161  and  163  include two legs, which are positioned on opposing sides of the ring connectors  153  and  155 , respectively, at opposing ends of the damper  150 , and include bores that axially align with bores in the legs (not shown). Fasteners,  157  and  159 , are connected to the mounting members  161  and  163  through the ring connector  153  and  155 , respectively, coupling the opposing ends of the damper  150  to the plate  44  and the lever arm  34 . 
     Referring now to  FIG. 7 , a block diagram of a control system for one embodiment of a lift truck  100  constructed in accordance with the present invention is shown. The lift truck  100  comprises a vehicle control system  12  which receives operator input signals from the operator control handle  14 , the steering wheel  17 , a key switch  18 , and the floor switch  19  and, based on the received signals, provides command signals to each of a lift motor control  23  and a drive system  25  including both a traction motor control  27  and a steer motor control  29 . The drive system  25  provides a motive force for driving the truck  100  in a selected direction, while the lift motor control  23  drives forks  112  along the mast  110  to raise or lower a load  114 . The lift truck  100  and vehicle control system  12  are powered by one or more battery  37 , coupled to the vehicle control system  12 , drive system  25 , steer motor control  29 , and lift motor control  23  through a bank of fuses or circuit breakers  39 . 
     As noted above, the operator inputs include a key switch  18 , floor switch  19 , steering wheel  17 , and an operator control handle  14 . The key switch  18  is activated to apply power to the vehicle control system  12 , thereby enabling the lift truck  100 . The floor switch  19  provides a signal to the vehicle control system  12  for operating the brake  22  to provide a deadman braking device, disabling motion of the vehicle unless the floor switch  19  is activated by the operator. 
     The operator control handle  14  provides a travel request signal to the vehicle control system  12 . Typically, the handle  14  is rotated in a vertical plane to provide a travel direction and speed command of motion for the lift truck  10 , and includes a switch  15  located on the top of the handle  14  that can provide a tilt up/down function when activated in the forward and reverse directions and a sideshift right and left function when activated to the right and left directions. A plurality of control actuators  41  located on the handle  14  provide a number of additional functions, and can include, for example, a reach push button, a retract push button, and a horn push button as well as a potentiometer providing a lift function. A number of other functions could also be provided, depending on the construction and intended use of the lift truck  10 . 
     The traction motor control  27  drives the traction motor  49  which is connected to wheel  11  to provide motive force to the lift truck. The speed and direction of the traction motor  49  and associated wheel  11  is selected by the operator from the operator control handle  14 , and is typically monitored and controlled through feedback provided by a speed sensor  45  which can be an encoder or other feedback device coupled to the traction motor  49 . The wheel  11  is also connected to friction brake  22  through the traction motor  49 , to provide both a service and parking brake function for the lift truck  10 . The friction brake  22  can be a spring-activated brake that defaults to a “brake on” position, such that the switch  20  and associated brake  22  therefore provide the deadman braking function. The operator must provide a signal indicating that the deadman brake is to be released to drive the truck, here provided by the floor switch  19 , as described above. The traction motor  49  is typically an electric motor, and the associated friction brakes  22  can be either electrically operated or hydraulically operated devices. Although one friction brake  22 , motor  49 , and wheel  11  are shown, the lift truck  100  can include one or more of these elements. Various other types of braking systems could also be used. 
     The steer motor control  29  is connected to drive a steer motor  47  and associated steerable wheel  11  in a direction selected by the operator by rotating the steering wheel  17 , described above. The direction of rotation of the steerable wheel  11  determines the direction of motion of the lift truck  10 . 
     The lift motor control  33  provides command signals to control a lift motor  51  which is connected to a hydraulic circuit  53  for driving the forks  112  along the mast  110 , thereby moving the load  114  up or down, depending on the direction selected at the control handle  14 . In some applications, the mast  110  can be a telescoping mast, as shown here. Here, additional hydraulic circuitry is provided to raise or lower the mast  110  as well as the forks  112 . Sensors  117  and  115  can be provided for monitoring the height of the mast  110  and the weight of the load  114 , respectively. The sensor  117  can be, for example, an encoder driven by a belt or cable. The sensor  115  can be a transducer that measures pressure, which is then converted to a weight by the vehicle control system  12  as a function of the pressure of the hydraulic fluid. Based on the height of the mast  110 , the weight of the load  114 , and the speed of the truck  100 , the vehicle control system  12  drives the magneto-rheological damper  150  to stabilize the lift truck  100 , as described more fully below. Although specific sensors are discussed above, various other sensing methods can be used. For example, weight can be measured using fork scales, and height by using ultrasonic, radar, laser, or infrared measuring devices. Other types of measuring devices will be apparent to those of skill in the art. 
     Referring again to  FIG. 1 , in operation, as the truck  100  moves backward and abruptly stops, the mast  110  can begin to tilt in the direction indicated by arrow  103  and pivot about a line between the drive wheel contact with the floor and the right front load wheel contact with the floor so that the base  116  of the truck  100  compresses the springs  42 ,  43 . Without the damper  150  the truck  100  would oscillate aided by springs  42 ,  43 . Once oscillation begins in typical prior art vehicles, the truck continues to oscillate until the oscillation is dissipated through friction inherent in the suspension members. However, with the magneto-rheological damper  150 , the vehicle control system  12  can activate the magneto-rheological damper  150  to retard the motion of the frame  116 . 
     Referring still to  FIG. 7  and now also to  FIG. 8 , a graph illustrating the application of the damper  150  is shown. As shown in the graph of  FIG. 8 , current can be applied to the damper  150  by the vehicle control system  12  to adjust the damping force of damper  150  under varying height, weight, and speed conditions as shown. During operation, the vehicle control system  12  receives speed feedback from sensor  45 , height feedback from the height sensor  117  and weight feedback from the weight sensor  115 . Based on these feedback signals, the vehicle control system  12  adjusts the current applied to the damper  150 , thereby adjusting the damping force applied by the damper  150 . 
     Referring now specifically to  FIG. 8 , when no load is on the mast  110  and the mast  110  is in a lowered state, the vehicle control system  12  retains the damping force of the damper  150  at a minimum value. When any of the speed, weight, and height parameters reaches a predetermined minimum damping value, the vehicle controller  12  begins applying current to the damper  150 , such that the damper  150  begins applying a damping force at a selected value. The applied current is ramped up at a steady rate, shown here as linear, until any of the speed of the vehicle, the height of the mast, or the weight of the load reaches a maximum damping value. At this level, the vehicle control system  12  drives the damper  150  to a maximum damping force level, and the vehicle controller  12  continues to apply the maximum current until the mast height, load weight, and speed all fall below the maximum value. By adjusting the damper as described, additional stability is provided when lifting or transporting a heavy load, when driving the truck with the mast  110  in an extended position, and when driving the lift truck  100  at a relatively high rate of speed or abruptly changing the direction of travel. When the damper  150  is activated, the truck  100  receives additional stabilizing support, thereby limiting instability, and truck sway or oscillation. When the damper  150  is not active, as, for example, during unloaded operation, the suspension of the truck is relatively soft, limiting operator fatigue. 
     Referring still to  FIG. 8 , it has been shown experimentally that applying a damping force when the speed of the lift truck, weight of the load, or height of the mast exceeds 25% of the maximum rated value provides stability to the vehicle, while maintaining a soft ride when damping is not required. To maintain stability, the amount of damping can be increased linearly as the speed, height or weight increase between 25% and 50% of the maximum rated value. After any of the speed, height, or weight values exceeds 50% of the maximum rated value, the maximum damping value is applied until all of these values falls below 50%. Although no example is shown here, it will be apparent that these factors can be varied, while generally increasing damping as the height, weight, and/or speed of the vehicle increases and decreasing the damping as these parameters decrease. 
     Referring now to  FIG. 9 , in one specific example, the speed of the vehicle varies from zero to eight miles per hour, the weight of a load that can be carried by the forks  112  of the vehicle is limited to about four thousand pounds, and the mast is extendable between zero and four hundred inches. Here, the vehicle control system  12  applies no current to the damper  150 , and the applied damping force is therefore is substantially zero, until at least one of the speed, weight, and height exceeds a minimum damping value. Here, specifically, the vehicle controller drives the controller at zero amps until the speed of the lift truck  100  exceeds two miles per hour, the weight of the load  114  carried on the fork  112  exceeds one thousand pounds, or the height of the mast exceeds one hundred inches. When any of these minimum damping values are exceeded, the vehicle controller  12  beings to apply current to the damper  150 , such that the damper  150  begins applying a damping force to the idler wheel assembly  32 . The current applied by the vehicle controller  12  is ramped up at a steady rate until any of the speed, weight, or height values exceeds a maximum damping value, specifically four miles per hour, two thousand pounds or two hundred inches, respectively. At this level, the vehicle controller  12  applies the maximum current of one amp to the damper  150 , providing a counter-force of about 1500N and continues to apply this level of damping until each of the speed, height, and weight falls below the maximum damping value. Additionally, although the damping force is shown increasing linearly, the force can be stepped up in various range levels or otherwise adjusted based on the characteristics of the vehicle. 
     Although the vehicle control system  12  is described above as receiving input from each of the speed sensor  44 , height sensor  117  and weight sensor  115 , the damper  150  can also be adjusted based on input from any one or more of these sensors. Furthermore, although specific percentages for adjusting the damping are described above, more generally speaking, the damping force should be increased as the vehicle speed increases, the height of the mast increases and the weight of the load increases. Using these guidelines, the damping of the vehicle can be adjusted for different levels. 
     Referring now to  FIGS. 10-13 , an alternative embodiment of a lift truck including a magneto-rheological damping systems is shown, wherein like numbers are used for elements described with reference to  FIGS. 1-6  above. As described above, the lift truck  100  includes a drive wheel assembly  20  including a traction motor  49 , steering motor  47 , and drive wheel  11 . An idler wheel  16  is also suspended from the frame. Here, however, the suspension system provided below the floor  182  is a walking beam suspension system  170 . 
     Referring now to  FIGS. 11 and 12 , the walking beam suspension system  170  includes a first beam assembly  172 , and a second beam assembly  180  that are pivotably coupled together at a pivot point  184 . The idler wheel  16  is coupled to the distal end of the second beam assembly  180 , and the drive wheel assembly  20  is coupled to the distal end of the first beam assembly  172 . As shown here, the distal end of the first beam assembly  172  can comprise a first and second L-shaped beams  174  and  176 . Optionally, springs  42  and  43  can be coupled to the second beam assembly  180  at one end, and to a plate  44  coupled to an inside wall of the housing  113  as described above with reference to  FIG. 4 . To stabilize the truck and limit oscillations, a magneto-rheological damper  150  can be coupled between the second beam assembly  180  and a plate  44  that is coupled to an inside wall of the housing  113 , as described above with reference to  FIGS. 5 and 6 . Alternatively, or in addition to the magneto-rheological damper  150 , a magneto-rheological damper  184  can be coupled to the drive motor assembly  20  as, for example, between a motor mounting plate  178  and a substantially vertical toe plate that forms part of the housing  113  ( FIG. 13 ). The magneto-rheological damper  184  can also be coupled anywhere between the motor mounting plate  178  or first beam assembly  172  and the housing  113 , or more generally between the suspension system and the housing. 
     A preferred embodiment of the invention has been described in considerable detail. Many modifications and variations to the preferred embodiment described will be apparent to a person of ordinary skill in the art. It should be understood, therefore, that the methods and apparatuses described above are only illustrative and do not limit the scope of the invention, and that various modifications could be made by those skilled in the art that would fall within the scope of the invention. To apprise the public of the scope of this invention, the following claims are made: