Abstract:
A system and method for improving the traction of a vehicle uses weight distribution control to improve wheel to driving surface friction. A driven axle and a non-driven axle support a vehicle load through adjustable suspension members. The suspension members are, for example, air bags. Actuators and controllers adjust the suspension members in a selectable predetermined manner. The selection can be based on operator preference, vehicle operating mode, or a sensed loss of traction. The driven axle carries a drive axle maximum rated load in order to maximize drive wheel traction at all times.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    Commonly assigned, copending application Ser. No. 09/107,620, filed Jun. 30, 1998 and Ser. No. 09/285,172, filed Mar. 22, 1999, relate to an improved traction and suspension control assembly of the type that can use the proportional control valve of the subject invention. The disclosure of that application is hereby incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of Invention  
           [0003]    This invention pertains to a proportional control valve for a suspension assembly associated with an air braking system for a tractor/trailer vehicle. The invention is particularly applicable to vehicles such as trucks/tractor systems known as a 6×2 vehicle employing a traction control system with air suspension control transfer. It will be appreciated, however, that the invention may have broader applications and may be advantageously employed in related environments or applications.  
           [0004]    2. Description of Related Art  
           [0005]    By way of brief background, a 6×4 truck and tractor system employs a pair of drive axles. As will be appreciated, the vehicle cost associated with a pair of drive axles is substantially greater than a 6×2 arrangement, i.e., a system in which only one of the two rear axles is a drive axle, because of the additional drive components. Since a 6×4 system has increased weight, operating costs, complexity, maintenance costs, friction, and fuel consumption associated therewith, a 6×2 assembly is highly desirable. On the other hand, a 6×2 system has decreased traction capabilities relative to a 6×4 configuration. Accordingly, a 6×2 system using an air suspension control to transfer load to the drive axle has been proposed as a preferred arrangement that achieves enhanced traction control.  
           [0006]    Generally, traction control systems employ similar principles to antilock braking systems on wet or slippery surfaces, curves, split surfaces, ice, and the like. Traction control assemblies sense when the wheels of a vehicle spin upon acceleration. This is representative of a loss of traction between the road surface and the tire. In response, drive torque is transferred to the non-spinning tires or wheels and a braking force is gently applied to the spinning wheel. This transfers the torque through the differential to the non-spinning, or more slowly spinning wheel. If both wheels spin, then the engine RPM is electronically controlled and reduced to an appropriate level.  
           [0007]    As briefly indicated above, it has been proposed to use a shift or transfer the vehicle load in a 6×2 arrangement. An air bag suspension assembly associated therewith reduces the suspension bag pressure in the non-drive axle so that more weight is transferred to the drive axle. In this manner, an increased portion of the load is transferred to the drive axle to enhance traction.  
           [0008]    Prior systems allow for load transfer manually, i.e., via driver input, or automatically with wheel slip and the onset of an automatic traction event. Manual systems that allow for load transfer are cumbersome and always present the potential for mis-use by an operator. On the other hand, if load transfer occurs automatically only after a loss of traction, then some adhesion with the road surface is already lost. This loss of adhesion can make the vehicle more difficult to move.  
           [0009]    Different driving conditions and different operator preferences require different traction control strategies. Thus, a need exists for a traction control system that can be operated in a plurality of operating modes based on vehicle operation, operator preference, or operating conditions.  
         SUMMARY OF THE INVENTION  
         [0010]    To those ends, a system and method operative to enhance the traction of a vehicle have been developed. The system comprises a drive axle, a non-drive axle, a suspension assembly operative to support a vehicle load, and an actuator assembly operatively associated with the suspension assembly for selectively transferring a predetermined ratio of vehicle load from one axle to the other axle. The method comprises the steps of providing a suspension force to a suspension control assembly associated with the drive axle, providing a suspension force to a suspension control assembly associated with the non-drive axle and regulating the forces provided to the suspension control assemblies in response to a control signal so as to control an amount of weight born by the drive axle in a plurality of predetermined manners.  
           [0011]    According to the present invention, a proportional load transfer valve shifts weight from a non-drive or tag axle to a drive axle in response to a signal from the traction control assembly during a traction event.  
           [0012]    A principal advantage of the invention is the ability to control the load transfer of the weight from the tag axle to the drive axle for enhanced traction control.  
           [0013]    Another advantage of the invention resides in the automated operation of the transfer that is responsive to a traction event.  
           [0014]    Yet another advantage of the invention is found in the ability to selectively transfer weight to the drive axle under normal operating conditions in order to prevent a traction event.  
           [0015]    A further advantage of the present invention is the ability to change traction control strategy based on vehicle operating mode, for example using a first strategy or mode when in a low gear and a second strategy or mode when in a high gear.  
           [0016]    Still another advantage of the invention is found in the ability to limit the proportional load transfer.  
           [0017]    Still other advantages and benefits of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed description.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The invention may take physical form in certain parts and arrangements of parts, preferred embodiments of which will be described in detail in the specification and illustrated in the accompanying drawings. The drawings include:  
         [0019]    [0019]FIG. 1 which is a schematic representation of a 6×2 traction control system with an air suspension assembly that transfers load between the drive and tag axles;  
         [0020]    [0020]FIG. 2 is a longitudinal cross-section of a preferred proportional load transfer valve;  
         [0021]    [0021]FIG. 3 is a graphical representation of the supply and delivery pressures provided by the valve of FIG. 2 under various designs and operating modes;  
         [0022]    [0022]FIG. 4 is a second preferred arrangement of a proportional load transfer valve; and,  
         [0023]    [0023]FIG. 5 is a graphical representation of the supply pressure versus the delivery pressure provided by the valve of FIG. 4. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    Referring now to the drawings which illustrate preferred embodiments of the invention only and are not intended to limit the invention, the FIGURES show a traction and suspension control assembly A used in a tractor/trailer vehicle. More particularly, FIG. 1 schematically illustrates a tractor cab having a first or drive axle  10  and a second or non-drive axle  12 . Mounted on opposite ends of the axles are rear wheels  14 ,  16 , respectively, that support the weight imposed on the tractor when it pulls a loaded trailer, or in an unloaded condition known as a bobtail mode. In this 6×2 system, only one of the axles is driven and the second or rear axle, known as a tag axle, aids in supporting the weight from the trailer.  
         [0025]    The illustrated tractor shown in FIG. 1 is equipped with a conventional antilock/traction control assembly commercially available, for example, from the assignee of the present application. The antilock/traction control system includes a source of compressed air represented by cylinder  20 . As is well-known in the art, a compressor periodically charges the cylinder so that a sufficient air supply or reservoir is provided for braking and suspension needs. The pressurized air is supplied to an antilock/traction controller  22  which includes an antilock electronic control unit  24  and a traction solenoid valve  26  and relay valve  28 . The controller regulates air to modulator valves  30 ,  32  which supply air brake actuators  34  associated with the rear wheels  14 ,  16 .  
         [0026]    Sensors  40  are associated with the wheels  14  on the drive axle  10  to monitor the rotation of the wheels. The sensors provide representative signals of wheel spinning conditions to the control unit. If a wheel is spinning, i.e., a differential traction control event is sensed, braking is gently applied to the spinning wheel. Likewise, if all of the drive wheels are slipping, the RPM of the engine can be reduced and braking gently applied to the wheels. This transfers torque to the wheel(s) in a manner well-known with conventional traction control assemblies. Lines  42 ,  44  extend from the modulators  30 ,  32 , respectively, to supply the brake actuators associated with the drive axle through first branch passages  42   a ,  44   a . Likewise, second branch passages  42   b ,  44   b  communicate air to the brake actuators associated with the non-drive wheels  16 .  
         [0027]    An air bag suspension assembly  50  is also schematically represented in FIG. 1. It includes air bags  52  disposed in pairs adjacent the wheels of the first and second axles. By selectively increasing or decreasing pressure in the air bags, the vehicle load or weight can be shifted between the drive and non-drive axles. As will be appreciated, the air bags associated with one of the axles work in tandem to shift the vehicle load as desired for enhanced traction.  
         [0028]    First and second control pressure lines  60   a ,  60   b  extend from the traction solenoid  26  and a transmission low range selector (not shown) respectively. The first and second control lines  60   a ,  60   b  provide control pressure to a double check valve  61 . The double check valve  61  connects one of the control lines  60   a ,  60   b  to the air suspension control valves  54 . The control line selected is the one supplying the highest pressure. In addition, proportioning valves  62  associated with the brake actuators of the non-drive tag axle  12  are provided control pressure the double check valve  61 . Typically, the traction solenoid  26  supplies 110 to 120 psi air pressure to the first control line  60   a  during a traction event and 0 psi otherwise. The transmission low range selector supplies air pressure at 60 psi when the transmission is in low gear. Otherwise the transmission low range selector supplies 0 psi to the double check valve  61  via the second control line  60   b . If a traction control event occurs, air pressure from the antilock/traction controller is directed to the suspension control valves  54  and the proportioning valves  62  to isolate the braking to the non-drive axle from the drive axle. This initiates a load transfer and limits brake drag on the non-drive axle during a traction event.  
         [0029]    Particular structural details of a preferred proportional load transfer valve are shown in FIG. 2. A valve housing  80  includes a first or supply port  82  that communicates with the air bags suspension assembly associated with the drive axle. In the valve position shown in FIG. 2, the supply port  82  is in communication with a second or delivery port  84  that communicates with the tag axle suspension airbags. In addition, communication with a third or exhaust port  86  is precluded as a result of the biased inlet/exhaust valve  88 . The inlet/exhaust valve is shown in a closed position with exhaust seat  90 , although it will be understood that the valve  88  is selectively moved from the seat in response to pressure changes. A fourth or control port  92  receives a pneumatic signal from the double check valve  61 . Therefore, for example, during a traction event the double check valve  61  delivers control pressure from the traction control solenoid to the control port  92 . The pneumatic pressure introduced at the control port pressurizes the underside of piston  94 . Pressure on underside of piston  94  overcomes biasing forces of a spring  96  and pressure at the supply port  82 . If the force provided by the control port pressure is greater that the sum of forces provided by the spring  96  (if equipped) and the supply port pressure, the piston is moved upwardly from the position shown in FIG. 2. This movement allows the inlet/exhaust valve  88  to open relative to the exhaust seat  92 . Pressure at the delivery port  84  pressurizes the underside of a second piston  95  and therefore tends to open the inlet/exhaust valve. Pressure at the supply port  82  pressurizes the topside of the second piston  95  and therefore tends to close the inlet/exhaust valve. The exposed surface areas of the underside and topside of the second piston are different. For example, the topside surface area of the second piston  95  is half that of the underside. Therefore, the inlet/exhaust port toggles from open to closed when the pressure at the delivery port  84  falls below half that of the supply port  82 . Of course other significant ratios may be used without departing from the scope and intent of the present invention.  
         [0030]    The inlet/exhaust port  86  is selectively opened and closed when the delivery port  84  pressure is more than half the supply port pressure. When the inlet/exhaust valve is open, air from the tag axle air bags flows through delivery port  84  and through the exhaust port  86  to atmosphere. When enough air escapes the tag axle air bags to reduce the pressure to half that of the drive axle air bags, the second piston  95  is forced downwardly and the inlet/exhaust valve is closed. In this manner, the air pressure in the tag axle air bags is reduced to half that of the drive axle air bags during a traction control event. Again, these ratios can be changed as a result of different surface area ratios between the topside and underside of the second piston  95 .  
         [0031]    As a result of the reduction in tag axle air bag pressure more of the burden of the trailer load shifts to the drive axle air bags. A level sensor (not shown) takes note of this shift and sends a control signal to a height control valve  99 . The height control valve  95  reacts to return the trailer to its original position by increasing the pressure in the drive axle air bags through fluid delivery lines  97 . If the trailer is carrying a heavy load, the drive axle air bag pressure and therefore the pressure at the supply port  82 , reaches a level whereby the control port  92  pressure lifting piston  96  is overcome and the piston  96  moves downward. Piston  96  comes in contact with the second piston  95  thereby increasing the effective surface area of the topside of the second piston  95 . The increase in topside surface area effectively changes the pressure ratios between the drive axle and tag axle air bags. In this way, heavy loads automatically shared more evenly than light loads during a traction event and the weight on the drive axle is kept at an appropriate level.  
         [0032]    The arrangement illustrated in FIG. 1 and FIG. 2 is intended to the distribute the weight of a load on the drive and tag axles in a predetermined and controlled manner. For example, in a first instance the weight of the load is normally controlled to be distributed evenly over the two axles. Only during a traction control event is more weight carried by the drive axle. In a second mode, even under normal conditions, most of the weight of a load, up to for example the weight limit of the axle, is carried by the drive axle. Only that portion of the load that is above the weight limit of the drive axle is carried by the tag axle. In this way maximum traction is provided all the time. Should a traction event occur in the second mode, additional weight, for example, even above the rating of the drive axle, can be transferred to the drive axle from the tag axle for a brief or abbreviated period of time.  
         [0033]    These various operating modes are achieved, for example, through the design and selection of second piston  95  topside and underside surface areas, as well as spring  96  force and control pressures delivered to the control port  92 . Referring to FIG. 3, a first line  310  represents a relationship between drive and tag axle bag pressures in the case where the weight of the load is evenly distributed. Operation along this curve is achieved by applying 0 psi to the control port  92 . The spring  96  urges the piston downwardly and the drive axle bags are placed in communication with the tag axle bags through a path from the supply port  82  and the delivery port  84 . The height control valve  95  varies the pressure in all the bags as necessary to keep the load level. Second and third lines illustrate operation during a traction event on a system normally operated along the first line  310 . During a traction event, a control pressure, for example 120 psi, is delivered to the control port  92 . The piston  94  is forced upwardly and air is bled from the axle bags, as described above, until the tag axle bag pressure is reduced to allow the inlet/exhaust valve to close. If the trailer is lightly loaded, then the height control valve  95  will increase the drive axle air bag pressure a small amount in order to re-level the trailer. In this case operation is along the second line  322  until the traction event has ended. If the trailer is carrying a heavy load then the height control valve will have to increase the drive axle air bag pressure further. This eventually urges the piston  96  into contact with the second piston  95  effectively changing the surface area ratio of the two sides of the second piston  95 . Therefore, operation is along the third line  324 . For example, when the drive axle air bag pressure is in this higher range the spring force of spring  96  and the force due to the pressure at the supply port  82  are enough to overcome the control port  92  pressure. The piston  96  presses down on the second piston  95  and the drive axle and tag axle air bags are placed in communication. Air flows from the drive axle bags to the tag axle bags due to the pressure differential between them. This increases the tag axle bag pressure.  
         [0034]    The newly increased tag axle bag pressure works in concert with the control pressure and eventually closes the communication path. Therefore, operation continues at a new equilibrium point somewhere on line  324 . The selection and design of appropriate components for suspension control valves  54  confines operation to be along the first, second and third lines  310 ,  322 ,  324 . Of course, other modes of operation are possible. By making different selections, for example, selecting a higher topside to underside surface area ratio for the second piston  95 , and normally providing a control pressure of, for example 60 psi, a higher percentage of trailer load can be placed on the drive axle. For example, fifth and sixth lines  332 ,  334  illustrate operation when the topside to underside surface area ratio of the second piston  95  is approximately 3.5 to 1 and a control pressure of 60 psi is applied. When there is no traction event and the transmission is in a low gear the transmission low range selector supplies fluid line  60   b  with 60 psi of air pressure and fluid line  60   a  carries 0 psi. Therefore, the double check  61  valve delivers the higher 60 psi pressure to the control ports  92  of the suspension control valves  54 . If the transmission is shifted to a higher gear range the control pressure drops to 0 psi and operation reverts to line  310 . Alternatively if there is a traction event, then line  60   a  carries 120 psi and the double check valve  61  switches and supplies 120 psi to the control ports  92 . This has the effect of shifting operation to lines  342 ,  344 . Operating on the lines  342 ,  344  shifts even more weight to the drive axle in order to improve traction.  
         [0035]    [0035]FIGS. 4 and 5 are substantially identical to the embodiment of FIGS. 2 and 3, except that a different setting for the blend-back force is used. As will be apparent, spring  96  from the embodiment of FIG. 2 is eliminated. Whereas the embodiment of FIGS. 2 and 3 has a setting of 60 psi, the proportional load transfer valve of FIG. 4 is set at 80 psi. Thus, the curve of FIG. 5 illustrates the operating characteristics of the valve that may be compared to those shown in FIG. 3. It will be appreciated, however, that still other settings can be used to achieve a desired operational curve and that effectively transfer a portion of the weight on the tag axle to the drive axle in response to a pneumatic signal from a traction control event. In order to prevent damage to, for example the drive axle  10 , the suspension control valves  54  are preferably designed to ensure that prolonged overloading of the drive axle is prevented.  
         [0036]    It will be appreciated that the valve may adopt other configurations than the particular structural configuration illustrated herein that achieves these objectives.  
         [0037]    The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon a reading and understanding of this specification. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.