Abstract:
A cooperative traction control system that integrates throttle control and torque distribution. The system also uses dual slip controllers and methods that involve controlling the distribution of torque between wheels in the front and rear axles of a vehicle and a relatively small or no adjustment of the engine throttle (or, more generally, engine torque output) to reduce wheel slip. The control is cooperative in the sense that two controllers—a front axle torque controller and a rear axle torque controller—work together (or are controlled together) to reduce wheel slip and thereby achieve improved straight-line movement of a vehicle from a standstill.

Description:
RELATED APPLICATIONS 
     The present application claims priority to U.S. Provisional Patent Application Ser. No. 61/221,707 of the same title, filed on Jun. 30, 2009, the entire contents of which is herein incorporated by reference. 
    
    
     BACKGROUND 
     Modern four-wheel or all-wheel drive vehicles have been developed to provide greater vehicle traction over varied terrain and road surfaces. Roads may be dry, wet, icy, snow-covered, or some combination of these conditions and four-wheel or all-wheel drive vehicles offer advantages over vehicles in which just two wheels are driven (for example, either the front wheels or the rear wheels). Often, all-wheel drive vehicles use an electronically controlled system to affect the way in which the vehicle responds to certain road conditions. For example, electronic sensing units are used to monitor vehicle conditions such as wheel speed. Such sensing units provide signals to a control unit, which can alter how torque is distributed to the wheels. For example, in many current traction control systems, if wheel slip is detected, throttle control is implemented such that the torque output of the engine is reduced and, as a consequence, the torque to the driven wheels is reduced. 
     SUMMARY 
     Although traction control systems are known, they are not fully satisfactory. For example, many systems do not manipulate torque to drive wheels on the front and rear axles. In addition, many do not integrate throttle control and torque distribution. 
     One embodiment of the invention provides what the inventors refer to as “cooperative” traction control, which involves control of the distribution of torque between wheels in the front and rear axles of a vehicle and a relatively small or no adjustment of the engine throttle (or, more generally, engine torque output) to reduce wheel slip. The control is cooperative in the sense that two controllers—a front axle torque controller and a rear axle torque controller—work together (or are controlled together) to reduce wheel slip and thereby achieve improved straight-line movement of a vehicle from a standstill. 
     One embodiment of the invention provides a traction control module that includes first and second comparators. The first comparator receives a left front wheel slip value and a right front wheel slip value. The second comparator receives a left rear wheel slip value and a right rear wheel slip value. Each comparator outputs the larger of the two wheel slip values received. Each of these values is a “front axle slip value” and a “rear axle slip value,” respectively. The traction control module also includes first and second summing nodes, one to process the front axle slip value from the first comparator and one to process the rear axle slip value from the second comparator. The output of each comparator is provided to a summing node. The summing node for the front axle also receives a target slip value for the front axle. The summing node for the rear axle receives a target slip value for the rear axle. The outputs of the summing nodes represent slip errors and these values are provided, respectively, to front and rear axle controllers. The front and rear axle controller generates torque command signals based on the error signals. 
     The module also includes a third comparator. The output of the front axle controller is inverted and sent to the third comparator. The third comparator determines the lesser of the front axle command signal and actual torque provided to the front axle. The lesser of these values is provided to a third summing node, which also receives the output of the rear axle controller. The difference between these values is provided to a fourth summing node which also receives an engine target torque value from an engine controller. The output of the fourth summing node is provided to the engine to control its overall torque output. The command signal from the rear axle controller is provided to a transfer case (or similar controllable, torque-distribution device). The two command signals have the overall effect of mildly reducing the torque produced by the engine and distributing more torque to the rear axle (than the front axle) in a situation where the wheel slip of the front wheels is greater than the wheel slip of the rear wheels. 
     In another embodiment, the invention provides a method of providing traction control in a vehicle having a front axle, a rear axle, and an engine that produces torque. The method includes determining a left front wheel slip value and a right front wheel slip value; comparing the left and right front wheel slip values; and generating a front axle slip value that is indicative of the greater of the two. A left rear wheel slip value and a right rear wheel slip value are determined and compared to generate a rear axle slip value that is indicative of the greater of the two. Once wheel slip has been evaluated on an axle-by-axle basis, a first slip error is determined based on the front axle slip value and a target slip value for the front axle. A second slip error based on the rear axle slip value and a target slip value for the rear axle is also determined. A first torque command output is generated by or with a front axle controller based on the first slip error. A second torque command output is generated by or with a rear axle controller based on the second slip error. 
     The torque commands are used to control the torque of the front and rear axles. However, the first torque command is modified in a manner that accounts for 1) the difference in the target torque for the front wheels and the actual torque and 2) the amount of torque that can be shifted to the rear wheels, in circumstances where the traction available to the rear wheels is greater than the traction available to the front wheels. In one implementation, this adjustment is achieved by comparing the first torque command output and an actual front axle torque value and generating an excess torque output that is indicative of the lesser of the two. A difference output based on the difference between the excess torque amount and the second torque command is determined. An engine target torque value is generated with an engine controller and an engine torque command is determined based on the difference output and the engine target torque value. The engine torque command is provided to an engine controller to control a torque output of the engine. The command signal from the rear axle controller is provided to a controllable torque distribution device (such as a transfer case) to control the amount of torque provided to the rear axle. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  illustrates throttle control used in prior traction control systems. 
         FIG. 1B  illustrates drive torque distribution used in prior traction control systems. 
         FIG. 2A  illustrates throttle control in one embodiment of cooperative traction control. 
         FIG. 2B  illustrates drive torque distribution in one embodiment of cooperative traction control. 
         FIG. 3  illustrates an embodiment of a cooperative traction control module designed for an all-wheel drive vehicle that is primarily front-wheel drive. 
         FIG. 4  illustrates an embodiment of a cooperative traction control module designed for an all-wheel drive vehicle that is primarily wheel-rear drive. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. For example, embodiments described below relate to vehicles in which the front wheels provide the primary motive force and motive torque is provided to the rear wheels only when certain conditions exist. However, the techniques described could be readily applied to other vehicles, including vehicles that are primarily rear-wheel drive and in which motive torque is provided to the front wheels under certain circumstances. Thus, in a more general sense, embodiments of the invention are applicable to vehicles with “first” and “second” axles or groups of axles and torque may be controlled based on whether wheel slip is greater at one of the two axles or groups of axles. 
       FIG. 1A  illustrates the operation of a prior vehicle traction control system that relies upon throttle control. Traditionally, throttle control involves mechanically manipulating a throttle that controls the flow of an air-fuel mixture into an internal combustion engine. In many vehicles, an accelerator or “gas pedal” operated by a vehicle driver is connected to the throttle to control the amount of air-fuel mixture into the engine and, as a consequence, the output or torque of the engine. It should also be understood that until relatively recently torque distribution was uniform and non-selective in that drive trains were, at least in general, designed to provide an equal amount of torque to drive wheels and the ability to control the amount of torque was often limited to either applying all available engine torque to the driven wheels or none. 
     In the context of the current invention, “throttle control” is not strictly limited to control of a throttle, as modern vehicles may include a variety of mechanisms that control the delivery of air and fuel to an engine in addition to or in place of a throttle. Also, modern vehicles may include electric or other motors whose output is not controlled by a throttle controlling an air-fuel mixture, but, for example, the amount of current provided to the motor. Accordingly, throttle control is used more broadly to indicate controlling the output of a vehicle engine. A vehicle engine may be an internal combustion engine, an electric motor, a hybrid drive train, a hydraulic motor, or other source of torque. 
     The left-hand side of  FIG. 1A  includes a graphic representation  10  of throttle control where a certain throttle input  14  is provided to the vehicle engine to cause a vehicle to move from a standstill. As shown in  FIG. 1B , in a vehicle with four-wheel or all-wheel drive an amount of torque (represented by upwardly pointing arrows  15 - 18 ) is provided to each of the vehicle wheels. In  FIG. 1B , a situation in which the front wheels experience more slip or slippage as compared to the rear wheels is shown (as indicated by the two graphical indicators (crescents in the drawings) behind each front wheel and one graphical indicator (again, a crescent) behind each rear wheel). Such a situation might arise, for example, when the front wheels of a vehicle are on an icy spot and the rear wheels are located on ice-free pavement. In the system shown in  FIGS. 1A and 1B , once wheel slip is detected the throttle input from the driver is overridden by the traction control system (“TCS”). To reduce the slip of the front wheels, the TCS reduces the amount of torque provided to the wheels, by reducing the throttle input from the input  14  to a throttle input  22 . But as is illustrated in  FIGS. 1A and 1B , the torque is reduced is a non-selective and drastic manner. To reduce the slippage of the front wheels, the system employs a relatively large reduction in throttle input from the input  14  to the input  22 . This results in a reduction of the total drive torque (as shown by the arrows  24 - 27 , which are shorter than the arrows  15 - 18 ) and a first acceleration of the vehicle, A 1  (represented by arrow  30 ), and movement of the vehicle from position P 1  to P 2 . 
       FIGS. 2A and 2B  illustrate vehicle traction control in which the reduction of the throttle input is less than that used in the example illustrated in  FIGS. 1A and 1B . In the example illustrated in  FIGS. 2A and 2B , throttle input is reduced from the input  14  to an input  36 , which is greater than the input  22 . In addition to this smaller reduction in throttle input, torque is shifted from the front wheels to the rear wheels as is shown by arrows  37 - 40 . Arrows  39  and  40  are longer than arrows  37  and  38 , indicating that a greater amount of torque has been applied to the rear wheels. This results in increased acceleration of the vehicle, A 2  (represented by the arrow  45 ), and movement of the vehicle from position P 1  to position P 3 . P 3  is farther away from P 1  than position P 2 . Thus, improved vehicle launch (from a standstill) is achieved with the system illustrated in  FIGS. 2A and 2B . 
       FIG. 3  illustrates a cooperative traction control module  48  and the flow of information between the module  48  and other control modules in a vehicle  50 . The vehicle includes an engine  51 , a transmission  53 , and transfer case  55  (all shown schematically). As noted above, the engine may be an internal combustion engine, electric motor, or other source of torque. Also, multiple engines could be used. For example, an electric motor could be used at each wheel of the vehicle. The transfer case  55  is, in general terms, a controllable, torque-distribution device in the sense that it, in response to a command or signal, distributes torque from a source (such as an internal combustion engine) to axles connected to the wheels of the vehicle. In an embodiment with multiple engines or motors, the need for a torque distribution device is lessened as the distribution of torque may be accomplished through, for example, individually controlling each engine. 
     In the embodiment shown, the module  48  is illustrated as if it and some other components in the drawings are separate from and outside of the vehicle  50  (shown schematically). However, in most implementations, the module  48 , the vehicle controller area network (“CAN”) bus (discussed below), and other components are all located within the vehicle  50 . Sensors  56  that are part of an electronic stability control (“ESC”) system (and thus, actually located within the vehicle  50 ) collect information about the vehicle such as the rotational speed of each of the wheels of the vehicle. The wheel speed information from the ESC system sensors  56  can be processed using known techniques (as is shown by processing block  57 ) to generate four wheel slip values:  58 ,  59 ,  60 , and  61 . The value  58  is the wheel slip for the left front wheel. Value  59  is the wheel slip for the right front wheel. Values  60  and  61  correspond to the wheel slip for the left rear wheel and right rear wheel, respectively. 
     The two front wheel slip values  58  and  59  are fed to a first comparator  63 . The comparator  63  determines the larger of the two slip values  58  and  59  and outputs a front axle slip value  64 , which represents the largest amount of slip experienced by the front wheels. In a similar manner, the two rear wheel slip values  60  and  61  are fed to a second comparator  65 . The comparator  65  determines the larger of the two slip values  60  and  61  and outputs a rear axle slip value  68 , which represents the largest amount of slip experienced by the rear wheels. 
     The output  64  is sent to summing node  69 . The summing node  69  receives another input  72  that represents a predetermined or target value for allowable slip at the front axle. The target slip at the front axle  72  is an empirical value (i.e., a value determined based on observation or experimentation). The summing node  69  determines the difference of the two inputs  64  and  72  and outputs a value  74  indicating the amount of front axle slip error. 
     The rear axle slip value  68  is sent to summing node  70 . Summing node  70  receives a second input  71  that represents a predetermined or target value for allowable slip at the rear axle (which like the input  72  is an empirical value). The summing node  70  determines the difference between the two inputs  68  and  71  and outputs a value  75  indicating the amount of rear axle slip error. The rear axle slip error  75  is sent to a rear axle controller  76 . The rear axle controller  76  generates a command signal  77  that includes a target torque value for the rear axle. (In  FIG. 3 , the label MSoH_CTCS is used to identify the signal  77 ). 
     The front axle slip error  74  is provided to a front axle controller  78 . The front axle controller  78  uses the front axle slip error  74  to determine an amount of torque to apply to the wheels connected to the front axle. Note that a large (in relative terms) value for the front axle slip error  74  is indicative of a relatively large amount of wheel slip difference. In response to a front axle slip error having such a value, the front axle controller generates a command or output  81  to reduce the amount of torque provided to the front wheels. 
     When there is slippage, the command signal or output  81  of the front axle controller  78  is indicative of an excess amount of torque on the front axle. (In  FIG. 3 , the label “Excess Torque VA” is used to identify the output  81 ). The output  81  is inverted in an inverter  82  and the inverted value is delivered to a third comparator  83 . The third comparator  83  also receives an input  84  that represents the actual or measured front axle torque of the vehicle  50 . (In  FIG. 3 , the label “Measured Torque VA” is used to identify the input  84 ). The comparator  83  generates an output  86 , which is the lesser of the input  84  and the inverted output  81 . 
     The output  86  is provided to a summing node  87 . The summing node  87  also receives the output or command signal  77  of the rear axle controller  76 . The summing node  87  determines the difference between the command signal  77  (target torque) and the output  86  of the comparator  83 . The summing node  87  generates an output  88  which is the difference between the excessive torque at the front axle and the additional amount of torque that can be applied to the rear axle (without slippage at the rear axle). 
     The output  88  is sent to a fourth summing node  89 . The summing node  89  receives an engine target torque value  91  which is a signal generated by a TCS controller  93 . The TCS controller  93  generates the engine target torque value based on upon information from the ESC sensors  56 . The summing node  89  generates an output  100  that is delivered to CAN bus  102  and addressed to an engine controller  105 . The command signal  77  is also routed to the CAN bus  102  and addressed to the transfer case  55 . The control achieved in reaction to the two command signals  77  and  100  results in torque distribution as illustrated in  FIG. 2B  when the wheel slip of the front wheels is greater than wheel slip of the rear wheels. In addition, the control technique results in better integration of the control provided by the front and rear axle controllers  78  and  76 , in what can be termed a “cooperative” approach. 
       FIG. 4  illustrates an embodiment of cooperative traction control implemented in a traction control module designed for use in a vehicle that is primarily rear-wheel drive. As can be seen, in this embodiment torque is transferred from the rear wheels to the front wheels in a manner that is similar to the situation described above with respect to  FIGS. 2A ,  2 B, and  3 . Since there are similarities between the primarily front-wheel and primarily rear-wheel drive modalities, no further discussion of  FIG. 4  is provided. 
     Thus, the invention provides, among other things, a traction control module in which the transfer of torque from, for example, the front wheels to rear wheels, is controlled by two controllers each of which performs control on an axle-by-axle basis (i.e., control to both wheels connected to a front axle and control to both wheels connected to a rear axle). Various features and advantages of the invention are set forth in the following claims.