Abstract:
A system and method that maintains the dynamic stability of a material handling vehicle having a vertical lift. The method allows static vehicle properties, such as vehicle weight, wheelbase length, and wheel configuration, and dynamic operating parameters, such as vehicle velocity, floor grade, lift position, and load weight, to be accounted for when maintaining the dynamic stability of a moving material handling vehicle. The method may include calculating and predicting center-of-gravity parameters, wheel loads, and projected force vectors multiple times a second and adjusting vehicle operating parameters in response thereto to maintain vehicle stability.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    N/A 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to the field of industrial trucks and, in particular, to a dynamic stability control system for a material handling vehicle having a lifting fork. 
         [0003]    One method for improving material handling vehicle stability includes performing a static center-of-gravity (CG) analysis while the vehicle is at rest and limiting vehicle operating parameters (for example, maximum speed and steering angle) accordingly. However, this static calibration does not dynamically account for vehicle motion, changing lift heights, or environmental factors such as the grade of a driving surface. 
         [0004]    Other methods for improving vehicle stability common in consumer automobiles include calculating vehicle CG during vehicle movement and employing an anti-lock braking system (ABS) to modify the cornering ability of the vehicle. These prior art methods only consider two-dimensional vehicle movement (forward-reverse and turning) and do not, for example, account for three-dimensional CG changes due to load weights being lifted and lowered while a vehicle is in motion. 
         [0005]    It would therefore be desirable to have a method for dynamically maintaining the stability of a material handling vehicle that accounts for vehicle motion and complex CG changes imposed by a load weight. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention overcomes the drawbacks of previous methods by providing a system and method for improving the dynamic stability of a material handling vehicle that is able to dynamically assess vehicle stability and adjust vehicle operation in response. The method includes analyzing dynamic vehicle properties such as velocity, travel direction, acceleration, floor grade, load weight, lift position and predicting wheel loads and three-dimensional center-of-gravity positions. 
         [0007]    The present invention provides a method of maintaining the dynamic stability of a material handling vehicle having a vertical lift. The method includes continuously calculating dynamic center-of-gravity parameters for the vehicle over a time interval during which the vehicle is moving, wherein a vertical position of the dynamic center-of-gravity is strongly dependent on a position of the vertical lift. The method further includes continuously calculating wheel loads based on the calculated dynamic center-of-gravity parameters and adjusting vehicle operating parameters based on calculated and predicted wheel loads and center-of-gravity parameters to maintain vehicle dynamic stability. 
         [0008]    The present invention also provides a material handling vehicle including a motorized vertical lift, traction motor, steerable wheel, steering control mechanism, and brake. The material handling vehicle further includes a stability control system having a plurality of sensors configured to measure dynamic vehicle properties, a sensor input processing circuit, a vehicle memory configured to store static vehicle properties. The control system further includes a stability computer, vehicle control computer, and a plurality of vehicle function controllers configured to maintain vehicle dynamic stability in accordance with the above-mentioned method. 
         [0009]    Various other features of the present invention will be made apparent from the following detailed description and the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a perspective view of a lift truck employing a stability control system in accordance with the present invention; 
           [0011]      FIG. 2  is a schematic view of a control system for maintaining the dynamic stability of a material handling vehicle in accordance with the present invention; 
           [0012]      FIG. 3  is a flowchart setting forth the steps for assessing and maintaining the dynamic stability of a material handling vehicle in accordance with the present invention; 
           [0013]      FIGS. 4A-4C  are alternate views of a free-body diagram for a three-wheeled material handling vehicle that may be employed to calculate vehicle center-of-gravity and wheel loads in accordance with the present invention; and 
           [0014]      FIG. 5  is a schematic showing vehicle stability in relation to center-of-gravity position in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The present invention provides a system and method for maintaining the dynamic stability of a material handling vehicle having a vertical lift. Generally, the vehicle&#39;s wheel loads and dynamic CG parameters are calculated over a time period during which the vehicle is moving and the vehicles operating parameters are adjusted based on the calculated wheel loads and CG parameters, as well as predicted wheel load and CG parameters. 
         [0016]    Referring now to the Figures, and more particularly to  FIG. 1 , one embodiment of a material handling vehicle or lift truck  10  which incorporates the present invention is shown. The material handling vehicle  10  includes an operator compartment  12  comprising a body  14  with an opening  16  for entry and exit of the operator. The compartment  12  includes a control handle  18  mounted to the body  14  at the front of the operator compartment  12  proximate the vertical lift  19  and forks  20  carrying a load  21 . The lift truck  10  further includes a floor switch  22  positioned on the floor  24  of the compartment  12 . A steering wheel  26  is also provided in the compartment  12  disposed above the turning wheel  28  it controls. The lift truck  10  includes two load wheels  30  proximate to the fork  20  and vertical lift  21 . Although the material handling vehicle  10  as shown by way of example as a standing, fore-aft stance operator configuration lift truck, it will be apparent to those of skill in the art that the present invention is not limited to vehicles of this type, and can also be provided in various other types of material handling and lift vehicle configurations. For brevity and simplicity, material handling vehicles are hereinafter referred to simply as “vehicles” and “loaded vehicles” when carrying a load weight. 
         [0017]    Referring now to  FIG. 2 , one embodiment of a control system  34  configured to maintain vehicle dynamic stability in accordance with the present invention is shown. The control system  34  includes an array of sensors  36  linked to a sensor input processing circuit  38 , which are together configured to acquire and process signals describing dynamic vehicle properties such as speed, direction, steering angle, floor grade, tilt, load weight, lift position, and sideshift. For example, the sensor array  36  may employ a motor controller, tachometer, or encoder to measure vehicle speed; a potentiometer or feedback from a steering control circuit to measure steering angle; a load cell, hydraulic pressure transducer, or strain gauge to measure load weight; an encoder to measure lift height; or three-axis accelerometers to measure tilt, sideshift, reach, and floor grade. The sensor input processing circuit  38  is linked to a vehicle computer system  40  that includes a stability CPU  42 , vehicle memory  44 , and vehicle control computer  46 , which together analyze static vehicle properties and dynamic vehicle properties to assess vehicle stability. Changes to vehicle operating parameters based on the assessed vehicle stability are communicated from the vehicle control computer  46  to function controllers  48 , which adjust the operation of vehicle actuators, motors, and display systems  50  to maintain vehicle stability. For example, adjusted vehicle operating parameters may be received by a lift function controller  52  that activates a motor  54  to change lift position; a travel function controller  56  to relay maximum speed limitations to a vehicle motor  58 ; a display controller  60  and display  62  to communicate present or pending changes in vehicle operating parameters to a driver; and a steering function controller  68  that directs a steering motor  70  to limit steering angle. The vehicle control computer may also include a braking function controller  64  and brake  66  to adjust vehicle speed. 
         [0018]    Referring to  FIG. 3 , the above lift truck  10  and control system  34  may be employed to maintain vehicle dynamic stability. A method for maintaining dynamic vehicle stability starts at process block  100  with the input of vehicle data to the vehicle computer system  40 . Vehicle data, which is retrieved from the vehicle memory  44 , may include static vehicle properties such as unloaded vehicle weight and CG, wheelbase length, and wheel width and configuration. At process blocks  102  and  104  respectively load weight and carriage height are input from the sensor array  36  and sensor input processing circuit  38  to the computer system  40 . A residual capacity is then calculated at process block  106  to determine if vehicle capacity, for example, vehicle position and load weight, is within acceptable bounds. If, at decision block  108 , it is decided that vehicle capacity is exceeded, then the driver is notified at process block  110  and vehicle operation may be limited at process block  111 . If vehicle capacity is within the acceptable bounds, then carriage position and vehicle incline angle are input at process blocks  112  and  114  respectively. 
         [0019]    Referring now to  FIGS. 3 and 4 , loaded vehicle CG is calculated at process block  116  by the stability CPU  42  based on static vehicle properties input at process block  100  and the dynamic vehicle properties such as those input at process blocks  102 ,  104 ,  112 , and  114 . For example, the free-body diagram (FBD) shown in  FIG. 4  shows the position of the CG, indicated by X CG , Y CG , and Z CG , in relation to the turning wheel and load wheels of a three-wheel material handling vehicle and the loaded weight W at the CG. It should be noted that Y CG  is strongly dependent on load weight and lift position and that heavy load weights at increasing lift heights elevate the CG and reduce vehicle stability. If, at decision block  118 , the vehicle is deemed stable, then vehicle speed is input at process block  120  and vehicle movement is assessed at decision block  122 . If the vehicle is moving, then the steering angle is input at process block  124  and operator commands are input at process block  126 . 
         [0020]    At process block  128 , the effects of vehicle movement on wheel loading are calculated. For example, wheel loads for a three-wheeled vehicle can be calculated by again considering the FBD of  FIG. 4 , which describes the distance A from the vehicle centerline C L  to the turning wheel  28 , the distance B from the C L  to the load wheels  30 , and the distance L between the turning wheel  28  and the axis-of-rotation of the load wheels  30 . From these distances and the steering angle θ input at process block  124 , a heading angle α and turning radius r are calculated using the following equations: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       α 
                       = 
                       
                         A 
                          
                         
                             
                         
                          
                         
                           tan 
                           ( 
                           
                             
                               L 
                               - 
                               
                                 X 
                                 CG 
                               
                             
                             
                               
                                 L 
                                 
                                   tan 
                                    
                                   
                                       
                                   
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                               - 
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                               + 
                               A 
                             
                           
                           ) 
                         
                       
                     
                     ; 
                   
                    
                   
                     
 
                   
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                   and 
                 
               
               
                 
                   Eqn 
                   . 
                   
                       
                   
                    
                   1 
                 
               
             
             
               
                 
                   r 
                   = 
                   
                     
                       
                         L 
                         - 
                         
                           X 
                           CG 
                         
                       
                       
                         sin 
                          
                         
                             
                         
                          
                         α 
                       
                     
                     . 
                   
                 
               
               
                 
                   Eqn 
                   . 
                   
                       
                   
                    
                   2 
                 
               
             
           
         
       
     
         [0021]    Normal and tangential accelerations, a t  and a n  respectively, are then calculated using the following equations: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         a 
                         i 
                       
                       = 
                       
                         
                           v 
                           - 
                           
                             v 
                             o 
                           
                         
                         t 
                       
                     
                     ; 
                   
                    
                   
                     
 
                   
                    
                   and 
                 
               
               
                 
                   Eqn 
                   . 
                   
                       
                   
                    
                   3 
                 
               
             
             
               
                 
                   
                     
                       a 
                       n 
                     
                     = 
                     
                       
                         v 
                         2 
                       
                       r 
                     
                   
                   ; 
                 
               
               
                 
                   Eqn 
                   . 
                   
                       
                   
                    
                   4 
                 
               
             
           
         
       
     
         [0022]    where v is current vehicle velocity, v o  is the last measured vehicle velocity, t is the time between velocity measurements. It is then possible, using these values and by analyzing the FBD of  FIG. 3 , to produce the following equations describing wheel load: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       N 
                       D 
                     
                     = 
                     
                       
                         
                           
                             
                               
                                 W 
                                  
                                 
                                   ( 
                                   
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                                       CG 
                                     
                                   
                                   ) 
                                 
                                  
                                 
                                   cos 
                                    
                                   
                                     ( 
                                     
                                       γ 
                                       F 
                                     
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                               - 
                               
                                 
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                                  
                                 
                                   ( 
                                   
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                                     F 
                                   
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                                   CG 
                                 
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                                 ( 
                                 
                                   
                                     
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                   5 
                 
               
             
             
               
                 
                   
                     
                       
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                       = 
                       
                         
                           
                             
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                                 ( 
                                 
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                                     CG 
                                   
                                 
                                 ) 
                               
                             
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                                
                               
                                 ( 
                                 
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                           2 
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                           B 
                         
                       
                     
                     ; 
                   
                    
                   
                     
 
                   
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                   and 
                 
               
               
                 
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                   . 
                   
                       
                   
                    
                   6 
                 
               
             
             
               
                 
                   
                     
                       N 
                       
                         L 
                          
                         
                             
                         
                          
                         2 
                       
                     
                     = 
                     
                       
                         W 
                          
                         
                             
                         
                          
                         
                           cos 
                            
                           
                             ( 
                             
                               α 
                               L 
                             
                             ) 
                           
                         
                          
                         
                           cos 
                            
                           
                             ( 
                             
                               α 
                               F 
                             
                             ) 
                           
                         
                       
                       - 
                       
                         N 
                         D 
                       
                       - 
                       
                         N 
                         
                           L 
                            
                           
                               
                           
                            
                           1 
                         
                       
                     
                   
                   ; 
                 
               
               
                 
                   Eqn 
                   . 
                   
                       
                   
                    
                   7 
                 
               
             
           
         
       
     
         [0023]    where γ L  is the lateral ground angle and γ F  is the fore/aft ground angle as determined at process block  114 . In this case, N D  is the load at the turning wheel, N L1  is the load at the left load wheel, and N L2  is the load at the right load wheel. 
         [0024]    Referring to  FIG. 3 , at decision block  130  it is decided if the wheel loads are acceptable. If unacceptable, for example, a wheel load approaching zero or another predetermined threshold, then the system notifies the operator at process block  110  and adjusts vehicle operation at process block  111  to maintain vehicle stability. For example, the computer system  40  may adjust vehicle operation by limiting or reducing the vehicle speed and communicate these changes to the operator via the display controller  60  and display  62 . Advantageously, the present invention further improves vehicle dynamic stability by allowing future CG parameters and wheel loads to be predicted based on trends in the measured dynamic vehicle properties and for vehicle operating parameters to be adjusted accordingly. 
         [0025]    Referring to  FIGS. 3 and 5 , at process block  102  the CG position determined at process block  84  is compared to a range of stable CG positions. It is contemplated that this may be performed by locating the CG position  200  within a stability map  202  relating a range of potential CG positions to vehicle stability. It should be noted that the stability map  202  is for a four-wheeled material handling vehicles having two turning wheels  28  and two load wheels  30 . The stability map  202  may include a preferred region  204 , limited region  206 , and undesirable region  208  whose sizes are dependent on system operating parameters. For example, applications requiring a high top speed may employ more stringent vehicle stability requirements and thus reduce the size of the preferred region  204 . At process block  134 , trends in measured dynamic vehicle properties, CG parameters, and wheel loads are analyzed to predict future vehicle stability. This may be achieved, for example, by analyzing trends in CG position  200  to determine its likelihood of entering the limited region  206  or by analyzing wheel loading trends to ensure that they remain within stable bounds. To adequately model future vehicle stability it is contemplated that the CG parameters and wheel loads are calculated approximately ten times per second. 
         [0026]    At process block  136 , vehicle operation rules are input to the computer system and, at process block  138 , parameters relating to future vehicle stability, for example, predicted wheel loads or CG position, are compared to the vehicle operation rules to determine if vehicle operating parameters should be adjusted in response. If, at decision block  140 , it is decided that vehicle operating parameters should be adjusted, then the driver is notified at process block  110  and the control system specifies an appropriate change in vehicle operating parameters to maintain vehicle stability at process block  111 . For example, if a wheel load falls below a minimum threshold specified by the vehicle operation rules, then vehicle speed may be limited to prevent further reduction in wheel load and the accompanying reduction in vehicle stability. It is contemplated that vehicle dynamic stability may also be improved in such an event by limiting steering angle, lift height, or vehicle speed. 
         [0027]    In addition to the calculated CG parameters and wheel loads, potential force vectors projected by the vehicle may also be analyzed to maintain vehicle dynamic stability. An accelerating vehicle projects a force approximately equaling the mass of the vehicle (including a load) times vehicle acceleration. This force vector, which is centered at the CG and projected in the direction of travel, is typically counteracted by the weight of the vehicle. However, if the projected force vector exceeds the vehicle weight, then the vehicle parameters may require modification. Therefore, the present invention may analyze trends in the projected force vector and adjust vehicle operation if the force vector exceeds a threshold specified by the vehicle operation rules. 
         [0028]    The present invention provides another method for maintaining vehicle dynamic stability. Possible low-stability scenarios such as a sudden change in vehicle speed or direction can be modeled and vehicle CG, wheel loads, and force vectors can be predicted in the event of such a scenario. If the modeled CG parameters, wheel loads, and force vectors fall outside a preferred range, then vehicle operation parameters may be adjusted to improve vehicle stability during the potential low-stability scenario. 
         [0029]    The present invention has been described in accordance with the embodiments shown, and one of ordinary skill in the art will readily recognize that there could be variations to the embodiments, and any variations would be within the spirit and scope of the present invention. It is contemplated that addition sensors and vehicle properties could be employed to further improve vehicle stability. Conversely, vehicle properties and the associate hardware used to measure and process them may be excluded from the present invention to reduce system costs and complexity. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.