Abstract:
A traction control system includes a master cylinder containing brake fluid, braking devices configured to apply braking force to associated wheels of the vehicle, a brake pedal operable by a driver of the vehicle to generate braking force by pressurizing the brake fluid, and means for storing pressurized brake fluid to apply and temporarily hold a braking force at a slipping driven wheel. A method of providing traction control for a vehicle includes manually switching the vehicle from a normal operating mode to a traction control mode, sensing the slippage of a driven wheel, applying a braking force to the slipping driven wheel in response to a driver of the vehicle pressing a brake pedal, maintaining the braking force on the slipping driven wheel after the brake pedal is released, and gradually releasing the braking force as the slipping driven wheel gains traction.

Description:
BACKGROUND 
       [0001]    The present invention relates to braking systems for vehicles. More particularly, the invention relates to traction control systems for vehicles that incorporate features and functions of the braking system to help the driver maintain the vehicle&#39;s wheels in traction with the road surface. 
       SUMMARY 
       [0002]    In one embodiment, the invention provides a vehicle traction control system including a master cylinder containing a quantity of brake fluid, a plurality of braking devices configured to apply a braking force to an associated one of a plurality of wheels of the vehicle, a brake pedal operable in response to a driver of the vehicle to generate braking force by pressurizing the brake fluid, and means for storing brake fluid pressurized by application of the brake pedal to apply and temporarily hold a braking force at a slipping driven wheel. 
         [0003]    In another embodiment the invention provides a method of operating a braking system of a vehicle to provide both a braking function and a traction control function. A master cylinder is provided coupled with a brake pedal of the vehicle and configured to generate hydraulic pressure transferable to a plurality of braking devices at a plurality of wheels of the vehicle to apply braking force to the plurality of wheels of the vehicle when a driver of the vehicle presses on the brake pedal. Hydraulic pressure is generated and a corresponding braking force is applied to each of the plurality of wheels of the vehicle in response to the driver of the vehicle pressing on the brake pedal. The braking force on each of the plurality of wheels is released in response to the brake pedal being released. Slipping of a driven one of the plurality of wheels is sensed. Hydraulic pressure is generated to selectively apply a braking force to only the driven wheel that was sensed to be slipping, the hydraulic pressure being generated without operating any pumps in the brake fluid circuit between the slipping driven wheel and the master cylinder. 
         [0004]    In yet another embodiment, the invention provides a vehicle traction control system including a brake pedal operable by a driver of the vehicle, wherein the brake pedal is coupled to a plurality of braking devices, each of which is configured to apply a braking force to reduce the rotational speed of an associated wheel in response to the brake pedal being operated by the driver in a normal operating mode of the vehicle. The braking force at each of the plurality of braking devices is configured to terminate upon release of the brake pedal when the vehicle is in the normal operating mode. A driver-actuable switch is configured to change an operational mode of the vehicle from the normal mode to a traction control mode while the vehicle is stopped. Means are provided for identifying slipping of a driven wheel of the vehicle and for retaining a braking force supplied by the brake pedal on the slipping driven wheel after the brake pedal is released by the driver when the vehicle is in the traction control mode. Further means are provided for gradually releasing the braking force on the slipping driven wheel as the slipping driven wheel gains traction. 
         [0005]    In yet another embodiment, the invention provides a method of providing a traction control feature in a vehicle. The vehicle is manually switched from a normal operating mode to a traction control mode. The slippage of a driven wheel is sensed. A braking force is applied to the slipping driven wheel in response to a driver of the vehicle pressing a brake pedal. The braking force is maintained on the slipping driven wheel after the brake pedal is released. The braking force is gradually released as the slipping driven wheel gains traction. 
         [0006]    Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic diagram illustrating a conventional braking system with anti-lock and traction control features. The braking system is shown in an at-rest state. 
           [0008]      FIG. 2  is a schematic diagram of the conventional braking system of  FIG. 1  shown during normal braking pressure build. 
           [0009]      FIG. 3  is a schematic diagram of the conventional braking system of  FIG. 1  shown during normal braking pressure release. 
           [0010]      FIG. 4  is a schematic diagram of the conventional braking system of  FIG. 1  shown during traction control pressure build. 
           [0011]      FIG. 5  is a schematic diagram of the conventional braking system of  FIG. 1  shown during traction control pressure release. 
           [0012]      FIG. 6  is a schematic diagram illustrating a braking system of the invention with anti-lock and traction control features. The braking system is shown during traction control pressure build. 
           [0013]      FIG. 7  is a schematic diagram of the braking system of  FIG. 6  shown during traction control pressure release. 
           [0014]      FIG. 8  is a schematic diagram illustrating the parameters controlling and controlled by the braking system of  FIGS. 6 and 7 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    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. 
         [0016]      FIGS. 1-5  are schematic diagrams of an existing braking system  10  having anti-lock braking capability and traction control capability. The invention, as shown in the schematic diagrams of  FIGS. 6 and 7  and described with reference to those figures below, is an improvement of the system shown in  FIGS. 1-5 . The existing braking system  10  is described first so that the improvements and advantages of the invention are better understood when introduced. 
         [0017]    The braking system  10  includes a brake pedal  14  that is actuable by a driver of the vehicle (such as any typical automobile—not shown). The brake pedal  14  is actuated by pressure applied from the driver&#39;s foot when it is desired to slow or stop the vehicle, or to keep the vehicle in a stopped state once stopped. The brake pedal  14  is coupled to a piston shaft  18  that actuates two pistons within a master cylinder assembly  22 . The master cylinder assembly  22  includes a reservoir  26  for containing a volume of hydraulic fluid (“brake fluid”) and further includes a body  30  in which two separate chambers are formed, each containing one of the pistons that are mounted on the piston shaft  18 . The master cylinder body  30  includes two outlets  34 ,  36 —one from each of the chambers so that two independent brake fluid circuits  40 ,  42  are established for redundancy to maintain some braking ability in the event that one of the brake fluid circuits becomes inoperable. 
         [0018]    The braking system  10  further includes a plurality of braking devices  48  for slowing and stopping the vehicle wheels from rotating. In the most typical construction, disc-type braking devices are utilized. Each disc-type braking device  48  includes a rotor  52  coupled for rotation with a wheel of the vehicle and a caliper  56  that selectively applies a squeezing pressure to the rotor  52  to slow the rotor  52  (and vehicle wheel) by friction. In a four-wheeled vehicle, the left-rear and right-front braking devices  48  are operated on the first brake circuit  40 , and the left-front and right-rear braking devices  48  are operated on the second brake circuit  42 . 
         [0019]    Although different braking devices  48  are actuated by the brake circuits  40 ,  42 , the layout and function of the brake circuits  40 ,  42  are identical. The types of braking devices  48  may vary from one braking system to another or within the braking system  10  (i.e., larger disc brakes for front wheels or disc-type front brakes with drum-type rear brakes). The size/type of the components within either brake circuit  40 ,  42  may also vary, but it will be understood that the second brake circuit  42  includes the same basic features and functions as the first brake circuit  40 , which is described in detail. The reference numbers of all parts of the first brake circuit  40  are shared with the corresponding parts of the second brake circuit  42 . 
         [0020]    During normal braking operation, the brake fluid in the brake circuit  40  (and also in the brake circuit  42 ) must be compressed to hydraulically actuate the braking devices  48  as shown in  FIG. 2 . This is accomplished by movement of the pistons within the chambers of the master cylinder  22 . Pressurized brake fluid in the first circuit  40  is in communication with each of the braking devices  48  through a single normally-open pilot valve  60  and separate normally-open inlet valves  64  that are in parallel with each other in a position “downstream” of the pilot valve  60  (i.e., closer to the braking devices  48 ). In parallel with each inlet valve  64  is a one-way check valve  68 . 
         [0021]    A normally-closed outlet valve  72  is provided in communication with each of the braking devices  48 . When the system is operating normally and the brake pedal  14  is pressed by the driver, high pressure brake fluid is in communication with the braking devices  48  through the valves  60 ,  64 . The normally-closed outlet valves  72  keep the high pressure brake fluid in communication with the braking devices  48 , isolating the brake fluid supply path from the separate brake fluid return path (discussed later). 
         [0022]    Under normal circumstances, when the driver releases the brake pedal  14 , the pressure in the brake fluid is relieved by expanding back “upstream” into the master cylinder  22  through the same path that the brake pressure was originally supplied to the braking devices  48  (through the normally-open pilot valve  60  and the normally-open inlet valves  64 —see  FIG. 3 ). Thus, during normal braking, no valves of the brake circuit  40  need to be actuated whatsoever. 
         [0023]    As well-known in the art, anti-lock braking is effected by sensing impending wheel lock (skidding on road surface) with a sensor and relieving a predetermined amount of brake pressure from the locked wheel(s) by opening the normally-closed outlet valve  72  associated with the locked wheel(s). This allows for better control of the vehicle during hard braking. Brake fluid released from the supply side to the return side through either of the outlet valves  72  is accumulated at an accumulator chamber  76  and can be later returned to the reservoir  26  of the master cylinder  22  by a self-priming pump  80  that is driven by a motor  82 . 
         [0024]    Next, the traction control function of the existing braking system  10  is described. The basic function of traction control is to apply selective braking force at a slipping wheel to maintain greater control of the vehicle through greater traction with the road surface. This is usually accomplished automatically or “on-the-fly” without input from the driver by using a sensor to monitor for wheel slip and then activating the braking system  10  as described below. Although the terms “slip” and “slipping wheel” are used herein, it should be understood that the various components (e.g., sensors, controller, controlled valves) available today may operate fast enough to virtually eliminate noticeable slipping or spinning. 
         [0025]      FIG. 4  illustrates the braking system  10  building brake pressure for traction control functionality. Notice that the brake pedal  14  is not being pressed by the driver. The pilot valve  60  and the inlet valves  64  are all switched from their normally-open state to a closed state which prevents the transfer of brake fluid and the pressurization of brake fluid across the valves  60 ,  64 . A prime valve  86  in parallel with the pilot valve  60  is switched from a normally-closed state to an open state. This allows the motor-driven pump  80  to be operated to draw brake fluid from the master cylinder  22  through the prime valve  86 . Operation of the pump  80  supplies pressurized brake fluid to each of the inlet valves  64 , which are closed until the vehicle&#39;s control unit senses wheel slip and demands braking force at one or more of the braking devices  48 . When traction control braking is needed, the inlet valve(s)  64  are opened in a controlled manner to limit the slippage of the slipping wheel on the road surface. To release the pressure in the brake fluid that was pressurized during traction control type pressure build, the pilot valve  60  and the prime valve  86  are returned to their normal or at-rest states so that the pump  80  sends the brake fluid back to the master cylinder  22  through the pilot valve  60  (see  FIG. 5 ). 
         [0026]    In view of the above description, it should be clear that the pilot valve  60 , the inlet valves  64 , the outlet valves  72 , the prime valve  86 , and the pump  80  are all required in order to provide the braking system  10  with the desired anti-lock braking functionality and the “on-the-fly” traction control functionality. Thus, the braking system  10  is irreducibly complex for the functions required of it. 
         [0027]      FIGS. 6 and 7  are schematic diagrams illustrating a braking system  100  of the invention that provides anti-lock braking functionality and limited driver-induced traction control functionality without either of the pilot valve  60  or the prime valve  86  of the existing braking system  10 . Thus, the braking system  100  of the invention provides most of the capability of the existing system  10  with much less cost and complexity. The braking system  100  and its various functions are discussed in greater detail below. Some details of the components and functionality of the brake system  100  is common with the braking system  10  and may not be repeated. Reference is made to the above description. Furthermore, elements of the brake system  100  that are common with elements of the brake system  10  of  FIGS. 1-5  are given the same reference number, incremented by  100  in  FIGS. 6 and 7 . 
         [0028]    The braking system  100  of  FIGS. 6 and 7  (and the vehicle in which the braking system  100  is implemented) operates in a normal mode, an anti-lock braking mode, and a traction control mode. In normal mode, the driver presses on the brake pedal  114  and hydraulic pressure in the brake fluid is generated at the master cylinder  122  and conveyed along individual brake fluid supply lines  115  through the normally-open inlet valves  164  associated with each braking device  148  so that the braking device  148  is actuated to slow/stop the corresponding vehicle wheel. In one construction, this can occur by a hydraulically actuated brake caliper  156  that is actuated by the pressurized brake fluid to squeeze or clamp onto the brake disc or rotor  152  that rotates with the wheel. Alternate braking devices may operate in different ways while still falling within the scope of the invention. In normal mode, when the driver releases the brake pedal  114 , the pressure in the brake fluid subsides by expanding back “upstream” into the master cylinder  122  through the same path that the brake pressure was originally supplied to the braking devices  148  (through the normally-open inlet valves  164  along the brake fluid supply lines  115 ). Thus, like with the braking system  10  of  FIGS. 1-5 , no valves are actuated whatsoever during operation in normal mode. 
         [0029]    Anti-lock braking is also carried out in much the same way as with the braking system  10  of  FIGS. 1-5 . When braking-induced wheel lock is sensed (or sensed to be imminent), brake pressure is released in a controlled manner from the affected braking device(s)  148  via the associated outlet valve(s)  172 . Excess brake fluid that is bled through the outlet valve(s)  172  during anti-lock braking operation is fed through brake fluid return lines  125  to the accumulator chamber  176  and is later returned to the master cylinder  122  by the motor-driven self-priming pump  180 . 
         [0030]    During traction control mode, the braking system  100  provides the same basic function as the existing braking system  10  in that brake pressure is applied to a slipping wheel and the brake pressure is gradually reduced as the slipping wheel regains traction with the road surface. However, because the braking system  100  does not include the pilot valve  60  or the prime valve  86  of the existing braking system  10 , the braking system  100  does not build and retain fluid pressure in the braking circuit(s)  140 ,  142  for later gradual application to the braking device(s)  148  while the vehicle is in motion. Thus, the traction control mode of the braking system  100  does not operate “on-the-fly” to automatically intervene during normal driving, and is instead manually selected and the pressure in the hydraulic brake fluid is manually generated. 
         [0031]    The driver may manually put the braking system  100  into traction control mode when the vehicle becomes stuck on a slick or loose road surface such as ice, mud, etc. or when the vehicle becomes stuck due to being off of the road surface. Thus, the traction control mode of the braking system  100  serves to enable the driver to maneuver the vehicle out of a stuck condition once the driver realizes that the vehicle has become stuck. For this purpose and referring now to  FIG. 8 , the brake system  100  is provided with an input such as a push-button-actuated switch  150  that is marked “TCS On/Off”, “Vehicle Stuck”, “Manual Traction Control” etc. and that is actuable by the driver of the vehicle to put the braking system  100  into traction control mode. The switch  150  is coupled to a controller  159 , such as the vehicle&#39;s main control module, that is configured to control the valves  164 ,  172  of the braking system  100 . The controller  159  may be the same controller that controls the outlet valves  172  during anti-lock braking action. As shown in  FIG. 8 , the controller  159  may be in communication with various sensors such as wheel speed sensors (WSS)  161  at each wheel so that information regarding each wheel&#39;s speed is conveyed to the controller  159  for enacting the proper actuation of the valves  164 ,  172  during anti-lock braking action and traction control action. 
         [0032]    For example, if the controller  159  receives signals from the wheel speed sensors  161  indicating that a slip threshold has been exceeded for one or more wheels (i.e., rotating significantly slower than one or more of the other wheels during braking), the controller  159  is configured to identify the wheel that has inadequate traction and requires anti-lock operation. The controller  159  signals to the outlet valve  172  associated with the appropriate wheel(s) to relieve brake pressure at the corresponding brake device(s)  148  until the wheel speeds are within the slip threshold. The monitoring of the wheel speeds and activation of the outlet valve(s)  172  may take place many times per second so that maximum braking performance is achieved. 
         [0033]    In the event that the vehicle becomes stuck or one or more driven wheels are sensed to be spinning, the controller  159  receives signals from the wheel speed sensors  161  so that the spinning wheel(s) can be identified. As used herein a “spinning” wheel is a driven wheel that is or has been sensed to be rotating faster than a rate corresponding to the vehicle&#39;s rate of travel, although the “spinning wheel” may not be perceptibly spinning or may have stopped spinning. The driver actuates the switch  150  to turn the traction control system on. The controller  159  recalls which driven wheel(s) were spinning. The controller  159  closes all of the inlet valves  164  for the wheel(s) that were not sensed to be spinning (including all non-driven wheels and any driven wheel that was not sensed to be spinning). This action allows braking force to be targeted to only the spinning wheel(s).  FIGS. 6 and 7  illustrate an example in which the left-front wheel is sensed to be spinning while the other three wheels are not. Accordingly, in this example, the inlet valves  164  associated with the left-rear, right-front, and right-rear wheels are actuated to move to the closed state. 
         [0034]    When the traction control system is on, the braking system  100  enters a dedicated mode for getting the vehicle out of a stuck condition. The traction control mode is not suitable for regular driving of the vehicle, and in some constructions is only able to be actuated when the vehicle is stopped. With the braking system  100  in the traction control mode, the driver presses on the brake pedal  114  to generate pressure in the braking circuits  140 ,  142 . However, hydraulic pressure is only transmitted to the braking device(s)  148  that are not blocked by closed inlet valves  164 . Thus, in the example of  FIGS. 6 and 7 , braking force is only applied at the braking device  148  associated with the slipping left-front wheel. The driver may be prompted (e.g., by the controller  159 ) to press on the brake pedal  114  by a message on a vehicle display after the traction control mode is activated. Because the hydraulic pressure is generated manually by the driver pressing on the brake pedal  114 , no pumps (e.g., the pump  180  in the corresponding brake fluid circuit  142  between the slipping wheel and the master cylinder  122 ) are operated to build hydraulic brake fluid pressure for traction control. 
         [0035]    After the braking force is generated at the braking device(s)  148  of the slipping wheel(s), the brake pedal  114  is released. In some constructions, the vehicle&#39;s display may prompt the driver that the brake pedal  114  can be released. After the braking force has been established, and before the driver releases pressure from the brake pedal  114 , any inlet valve(s)  164  that were open during the building of hydraulic brake pressure are closed. As shown in  FIG. 7 , the inlet valve  164  for the left-front braking device  148  has moved from the open state to the closed state. Thus, the braking device  148  for the left-front wheel is activated to apply braking force to the wheel, and the brake pressure in the circuit  142  is trapped between the inlet valve  164  and the braking device  148  for the left-front wheel so that the braking force initially generated by the driver&#39;s actuation of the brake pedal  114  is temporarily held without further interaction from the driver. 
         [0036]    After releasing the brake pedal  114 , the driver may press on the accelerator pedal (not shown) in order to move the vehicle. The braking force at the slipping wheel(s) (the left-front wheel in the example of  FIGS. 6 and 7 ) is gradually released to allow an increasing amount of driving power to reach the slipping wheel as the vehicle begins to move successfully out of the stuck condition. The gradual release of braking force is accomplished by controlling gradual or pulsed opening of the outlet valve  172  to gradually release brake fluid that has been trapped between the inlet valve  164  and the braking device  148 , which relieves hydraulic pressure in the trapped brake fluid and lessens the amount of the braking force. 
         [0037]    If the first attempt to move the vehicle out of the stuck condition is unsuccessful or the vehicle becomes stuck again, the manual traction control mode is re-activated by the driver. The driver may be required to actuate the switch  150  again and will re-pressurize the brake fluid by pressing on the brake pedal  114  again. This process can be repeated as necessary to release the vehicle from the stuck condition. Once the vehicle regains normal traction, as sensed by the wheel speed sensors  161 , the manual traction control mode is automatically shut off and the vehicle returns to the normal mode in which braking only occurs while pressure on the brake pedal  114  exists (i.e., brake pressure is not stored). 
         [0038]    Optionally, in some constructions, the structure of the brake system  100  is further simplified while retaining traction control functionality by removing the anti-lock braking feature. This allows the elimination of the pumps  180 , the motor  182 , and the accumulator chambers  176  from the illustrated braking system  100 . The manual traction control functionality remains in-tact as described above. Thus, a braking system can be provided with traction control functionality with a minimum of hardware. 
         [0039]    Various features and advantages of the invention are set forth in the following claims.