Abstract:
A system and method is provided for braking a vehicle having multiple wheels. The method includes applying friction braking and powertrain braking to the vehicle. The method includes reducing the powertrain braking by a predetermined amount. The method also includes compensating the friction braking in an amount that is substantially the same as the predetermined amount. The method further includes redistributing the friction braking to the multiple wheels and modulating the friction braking being applied to the multiple wheels, thereby reducing the occurrence of a wheel lockup by the multiple wheels.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of U.S. provisional application Ser. No. 60/598,008, filed Aug. 2, 2004, entitled “IMPROVED ABS PERFORMANCE WITH BRAKE-BY-WIRE ON LOW MU,” the entire contents of which is hereby incorporated by reference. 

   TECHNICAL FIELD 
   The present invention relates to a system and method for braking a vehicle. 
   BACKGROUND 
   Powertrain braking (also referred to as compression or engine braking) occurs when a powertrain of a vehicle delivers a braking torque (or force) that causes the vehicle to decelerate. Powertrain braking is typically produced as a result of the engine&#39;s compression losses during high engine revolutions per minute (RPM) and a low throttle position. It is well known that the amount of powertrain braking produced can cause the wheels of the vehicle to lockup on low friction surfaces, which is undesirable for a vehicle operator. Additionally, the conventional powertrain and friction braking systems lack efficient control and distribution of the powertrain braking produced to significantly reduce the occurrence of wheel lockup events. 
   The present invention was conceived in view of these and other disadvantages of conventional powertrain and friction braking systems. 
   SUMMARY 
   The present invention provides a system and method for improved braking of a vehicle. A method of braking a vehicle is disclosed wherein the vehicle has multiple wheels and the vehicle is configured to apply friction braking and powertrain braking to the vehicle. The method includes reducing the powertrain braking by a predetermined amount. The method also includes compensating the friction braking in an amount that is substantially the same as the predetermined amount. In one aspect, compensation of the friction braking occurs by adding braking torque to the friction braking. The method further includes redistributing the compensated friction braking to the wheels and modulating the friction braking being applied to the wheels thereby reducing the occurrence of a wheel lockup. In one embodiment, the friction braking is redistributed to a vehicle having four wheels. In such an embodiment, redistributing the braking force to four wheels enables improved removal of one or more of the vehicle wheels from a lockup state. 
   The braking system includes a powertrain configured to apply powertrain braking to the vehicle and a friction braking system for applying friction braking to the vehicle. The braking system also includes a controller operable with the powertrain for reducing the powertrain braking by a predetermined amount. The braking system further includes a braking module integrated with the friction braking system and operable with the controller for compensating the friction braking in an amount that is substantially the same as the predetermined amount and for redistributing the friction braking among the vehicle wheels and modulating the friction braking being applied to the wheels, thereby reducing the occurrence of a wheel lockup. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the invention will be apparent from the following detailed description and the appended claims, taken in conjunction with the accompanying drawings, in which: 
       FIGS. 1A and 1B  illustrate system diagrams of a vehicle that is configured to apply friction braking and powertrain braking to a vehicle in accordance with an embodiment of the present invention; and 
       FIG. 2  illustrates a flow diagram of a method for compensating a friction braking system for improved steerability and vehicle stability in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   By way of example, a system and methodology for implementing the present invention is described below. The system and methodology may be adapted, modified or rearranged to best-fit a particular implementation without departing from the scope of the present invention. 
   Referring to  FIG. 1 , a vehicle  12  is illustrated that enables monitoring and compensation of powertrain braking and friction braking for improved vehicle steerability and stability. Vehicle  12  includes a powertrain having an engine  14 , a transmission  16  and a drive shaft  18 . As recognized by one of ordinary skill in the art, engine  14  responds to a vehicle operator request to decelerate or accelerate vehicle  12  through the use of an accelerator pedal  15 . Accordingly, vehicle operator request includes both an acceleration and a deceleration request. The acceleration request occurs when a vehicle operator applies force to accelerator pedal  15  to accelerate. The deceleration request occurs when the vehicle operator does not apply force to accelerator pedal  15  or applies an amount of force (i.e., typically a minimal amount of force while the vehicle is in motion) to accelerator pedal  15  so as to cause vehicle  12  to decelerate. 
   Drive shaft  18  mechanically couples transmission  16  to a differential  20 . Differential  20  is mechanically coupled to wheels  22  thereby enabling movement of vehicle  12  in response to motive force from engine  14 . As shown, vehicle  12  further includes friction brakes  24 . Brakes  24  include a brake disc  25 , a caliper  26 , and a speed sensor  28  that communicates with an anti-lock braking system (ABS) module  34 . Caliper  26  is operable with brake disc  25  for slowing and/or stopping vehicle  12 . ABS module  34  is operable with a pressure adjustment unit  32 . In response to a brake request from a brake pedal  30 , pressure adjustment unit  32  is configured to enable proper distribution of braking fluid to brakes  24  through the use of liquid pressure passages  36 . Although the embodiment shown in  FIG. 1  illustrates a braking system that utilizes hydraulics, it is recognized that the friction braking system of  FIG. 1  may be a pure brake-by-wire (BBW) system, an electromechanical braking system or a hydro-mechanical braking system without departing from the scope of the present invention. In either embodiment, controller  38  is operable with ABS module  34  and engine  14  for monitoring and controlling the performance of engine  14 . 
   Engine  14  is adapted to apply a powertrain braking to vehicle  12 . Additionally, powertrain braking may result from any device located on vehicle  12  that is capable of delivering an acceleration or deceleration torque (or force) as commanded by the accelerator pedal. Referring to  FIG. 1B , a hybrid-electric vehicle (HEV) embodiment of vehicle  12  is shown having a motor/generator  27 . Motor/generator  27  is configured to deliver an acceleration or deceleration torque to vehicle  12 . A planetary gear set  17  and a second gear set  19  coupled to motor/generator  27  further enable the delivery of the acceleration or deceleration torque. Additionally, a controller  21  is configured to generate control signals for motor/generator  27 . A battery  23  is configured to provide power to vehicle  12  and receive an electrical charge from motor/generator  27 . Accordingly, in a HEV embodiment of vehicle  12  as shown in  FIG. 1B , powertrain braking may be delivered by HEV motor/generator  27  or engine  14 . In the case of a fuel-cell electric vehicle (FCEV) embodiment of vehicle  12 , powertrain braking may be delivered by a FCEV motor. 
   Nevertheless, in one aspect, the powertrain braking may be applied in response to a deceleration request from the vehicle operator. It is recognized that the powertrain braking applied to vehicle  12  may be of a magnitude that causes wheels  22  to experience a lockup. It is also recognized that powertrain braking may occur as a result of the vehicle operator placing transmission  16  into a lower gear ratio. In conventional vehicles, the powertrain braking is applied to those wheels that are directly coupled to the powertrain of the vehicle (e.g., the set of wheels mechanically coupled to the engine via the transmission). According to one aspect of the present invention, friction brakes  24 , controller  38  and ABS module  34  enable redistribution of the powertrain braking to all four wheels of vehicle  12 . Additionally, ABS module  34  enables modulation of the friction braking applied to wheels  22 . 
   Now referring to  FIG. 2 , a flow diagram illustrates a method for braking a vehicle having powertrain and friction braking functionality, in the event of a wheel lockup. Accordingly, block  50  is an entry point to the method. At block  52 , the method determines whether a wheel lockup event has or is occurring. As such, ABS module  34  and speed sensors  28  enable determination of the wheel lockup event. If there is no wheel lockup event, the method proceeds to block  60  wherein the method returns to block  52 . When the method determines that there is a wheel lockup event, the powertrain braking applied by engine  14  is reduced by a predetermined amount. 
   Controller  38  is configured to monitor the amount of powertrain braking being applied by engine  14 . Controller  38  is further configured to determine, based on various parameters such as vehicle speed and engine RPM, the amount of powertrain braking that causes a wheel lockup event to occur. Accordingly, the reduction in powertrain braking is of a sufficient magnitude to reduce the occurrence of a wheel lockup event. Thus, based on the various vehicle parameters, controller  38  generates signals for engine  14  to reduce the amount of powertrain braking created. Alternatively, the powertrain braking may be reduced by mechanically decoupling the powertrain from wheels  22 . In the case of an HEV or FCEV vehicle, powertrain braking may be reduced in response to controller  38  generating signals for the HEV or FCEV electric motor that cause the electric motor to reduce the amount of powertrain braking force being applied to the vehicle powertrain. As described above, powertrain braking may be produced by any powertrain device such as engine  14  and alternatively, an electric motor, in the case of HEV or FCEV vehicles. In one aspect of the invention, the HEV and FCEV vehicle typically include a controller configured to generate signals that cause the electric motor and/or the internal combustion engine to apply a powertrain braking force to the vehicle. It is recognized that the powertrain braking force may be applied independent of inputs from the vehicle operator. 
   As described above, controller  38  communicates with ABS module  34  to compensate the friction braking being applied to wheels  22 . In one embodiment, compensation of friction braking occurs by adding braking torque to the friction braking in an amount that is substantially the same as the predetermined amount of powertrain braking reduced in block  54 . In the case of an HEV or FCEV vehicle, braking torque may be added by controller  38  generating signals via ABS module  34  for the friction braking system to increase the amount of braking force (or torque) being applied to the wheels  22 . As such, in one aspect of the invention, the vehicle operator generates a braking request by depressing brake pedal  30 . Accordingly, the friction braking applied by brakes  24 , in response to the braking request, is compensated to improve vehicle braking and reduce occurrence of the lockup by wheels  22 , as illustrated by block  56 . The compensated friction braking is then redistributed to wheels  22  via the friction braking system as shown in block  57 . Additionally, the friction braking applied to wheels  22  may be modulated through the use of ABS module  34  as illustrated in block  58 . Following block  58 , block  60  occurs where the method returns to block  52 . 
   As described in the foregoing, the powertrain braking is reduced by a predetermined amount and the friction braking is compensated and redistributed to the wheels of vehicle  12  through the use of a friction braking system having anti-lock braking functionality. It is recognized that vehicle  12  may be any vehicle configured to monitor and distribute powertrain braking and friction braking. As such, vehicle  12  may be a hybrid electric vehicle (HEV) or a fuel cell electric vehicle (FCEV). It is further recognized that vehicle  12  is configured to apply a braking force (e.g., friction braking and powertrain braking) that is independent of a braking request from brake pedal  30  or an acceleration/deceleration request from accelerator pedal  15 , through the use of controller  38 , an adaptive speed control system integrated with engine  14 , and ABS module  34 . Additionally, although vehicle  12  illustrates a braking system that utilizes hydraulics, the braking system of vehicle  12  may be a pure BBW system, an electromechanical braking system or a hydro-mechanical braking system. 
   Furthermore, although the embodiments herein, in some instances, have been described in the context of operating within the torque domain, it is recognized that the present invention may be utilized within an acceleration, power or force domain without departing from the scope of the present invention. Accordingly, while embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.