Abstract:
A method for automatically decelerating a vehicle having a first side with a first front wheel brake and a first rear wheel brake and second side with a second front wheel brake and a second rear wheel brake, and in which the vehicle further has a braking system with a pump operable to deliver a flow of pressurized hydraulic fluid to the first and second front wheel brakes and to the first and second rear wheel brakes, includes receiving a signal associated with an emergency braking event. The method further includes increasing the hydraulic pressure at the first and second front wheel brakes at a greater rate than at the first and second rear wheel brakes. The method also includes directing hydraulic fluid from the first front wheel brake to the second rear wheel brake upon achieving a targeted level of deceleration or wheel slip.

Description:
BACKGROUND 
     The present invention relates to a system and method for controlling the hydraulic pressure within an emergency braking system. 
     SUMMARY 
     The invention provides an improved hydraulic emergency braking system control strategy that includes modifying and controlling the brake pressure build profile for the front and rear axles of a vehicle, leading to greater vehicle stability and more efficient use of system hydraulic energy in view of the limited electrical/hydraulic power available from the brake modulation system. 
     In one embodiment of a method for automatically decelerating a vehicle having a first side with a first front wheel brake and a first rear wheel brake and second side with a second front wheel brake and a second rear wheel brake, and in which the vehicle further has a braking system with a pump operable to deliver a flow of pressurized hydraulic fluid to the first and second front wheel brakes and to the first and second rear wheel brakes, the method includes receiving a signal associated with an emergency braking event. The method further includes increasing the hydraulic pressure at the first and second front wheel brakes at a greater rate than at the first and second rear wheel brakes. The method also includes directing hydraulic fluid from the first front wheel brake to the second rear wheel brake upon achieving a targeted level of deceleration or wheel slip. 
     In one embodiment of a method for automatically decelerating a vehicle having a first side with a first front wheel brake and a first rear wheel brake and second side with a second front wheel brake and a second rear wheel brake, and in which the vehicle further has a braking system with a pump operable to deliver a flow of pressurized hydraulic fluid to the first and second front wheel brakes and to the first and second rear wheel brakes, the method includes receiving a signal associated with an emergency braking event. The method further includes increasing the hydraulic pressure at the first and second front wheel brakes at a greater rate than at the first and second rear wheel brakes. The method also includes increasing the hydraulic pressure at the first and second rear wheel brakes while maintaining hydraulic pressure at the first and second front wheel brakes upon achieving a targeted level of deceleration or wheel slip. 
     In one embodiment of a braking system for autonomously decelerating a vehicle without vehicle driver input, in which the vehicle includes a first front wheel brake and a first rear wheel brake proximate a vehicle first side and a second front wheel brake and a second rear wheel brake proximate a vehicle second side, the system includes a first hydraulic circuit operably associated with the first front wheel brake and the second rear wheel brake and a second hydraulic circuit operatively associated with the second front wheel brake and the first rear wheel brake. A controller is configured to receive a signal associated with an emergency braking event, increase the hydraulic pressure at the first and second front wheel brakes at a greater rate than at the first and second rear wheel brakes, and, upon achieving a targeted level of deceleration or wheel slip, direct hydraulic fluid from the first front wheel brake to the second rear wheel brake. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of a diagonally split hydraulic brake system circuit. 
         FIG. 2  is a schematic of a parallel hydraulic brake system circuit. 
         FIG. 3  is a brake pressure versus time graph of a diagonally split hydraulic brake system in accordance with the invention. 
         FIG. 4  is a flow chart of a method of braking a diagonally split hydraulic brake system in accordance with the invention. 
         FIG. 5  is a brake pressure versus time graph of a parallel hydraulic brake system in accordance with the invention. 
         FIG. 6  is a flow chart of a method of braking a parallel hydraulic brake system in accordance with the invention. 
     
    
    
     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. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. And as used herein and in the appended claims, the terms “upper”, “lower”, “top”, “bottom”, “front”, “back”, and other directional terms are not intended to require any particular orientation, but are instead used for purposes of description only. 
       FIGS. 1 and 2  illustrate a brake-pressure modulation device in a brake system of a motor vehicle. The system includes a brake master cylinder  102  with a reservoir  104  actuated by means of a pedal  106  coupled to a brake booster  110 . A hydraulic unit  114  operatively coupled to the master cylinder  102  is comprised of first and second circuits  120 ,  124 .  FIG. 1  illustrates a diagonally split system such that the first and second circuits  120 ,  124  separately control 1) the front-right (FR) and rear-left (RL) wheels and 2) the rear-right (RR) and front-left (FL) wheels.  FIG. 2  illustrates a parallel system such that the first and second circuits  120 ,  124  separately control the front wheels (FR, FL) and the rear wheels (RR, RL). Each circuit  120 ,  124  includes an outlet  128  from the master cylinder  102 , and an inlet solenoid valve  130  and an outlet solenoid valve  134  for carrying out braking operations on each wheel brake. The brake-pressure modulation device also includes, for each circuit  120 ,  124 , a low-pressure accumulator  140  and a return pump  144 . The return pump  144  is driven by an electric motor  150 . 
     For traction control, each circuit  120 ,  124  further includes a changeover valve  160  and a shut-off valve  164  mounted between the master cylinder  102  and the return pump  144 . The valves  130 ,  134 ,  160 ,  164  are electromagnetic solenoid valves. Various other functional components are illustrated in  FIGS. 1 and 2 , to include relief valves  170  and filters  174 , not all of which are numbered to provide greater clarity. 
     A control unit  190  controls the operation of the vehicle braking associated with the circuits  120 ,  124 , to include anti-lock braking and traction control, and the emergency braking sequences to be further described below. 
     A hydraulic brake system for a road vehicle, such as shown in  FIGS. 1 and 2 , has a certain brake effectiveness, generally defined as brake torque that can be generated by a given amount of hydraulic pressure, and a certain brake stiffness, generally defined as the brake pressure that can be generated by a given flow volume of hydraulic fluid. The brake stiffness and the brake effectiveness are typically not equal between the front axle (wheels FR and FL) and the rear axle (wheels RR and RL). 
     As known by those of ordinary skill in the art, during braking, deceleration forces at the center of gravity of the vehicle, which is above the level at which tire forces are transmitted to the ground, increase the vertical force loading on the front axle and decrease the vertical force loading on the rear axle. These vertical loading forces are further influenced by the design, tuning, and state of the vehicle&#39;s suspension, which can significantly affect the loading profile over time at each axle. These effects can be most pronounced for rapidly changing vehicle deceleration. Modern braking systems use closed loop feedback control using various sensors and a brake pressure modulation system to modify (lessen) the increase of brake pressure to the rear axle relative to the pressure being supplied to the front axle above some predetermined amount. 
     Vehicle sensing systems make use of cameras, radar, or other “smart” technology to detect a potential emergency braking event and provide an external signal to initiate a slowing of the vehicle without driver brake pedal input. Although automatic or autonomous braking offers quicker reaction and faster deceleration than possible through driver-initiated braking, such systems can, by commanding the brake modulation system to build hydraulic pressure to automatically rapidly decelerate the vehicle, cause an uncontrolled pressure build-up at both the front and rear axles. This rate of pressure build at both axles is a function of the flow rate to the brakes and the stiffness of the brakes receiving the flow. 
     The control strategy of the present invention modifies the build-up of brake pressure between the front and rear axles over the course of an autonomous braking event, i.e., a braking event automatically initiated and controlled by a subsystem not influenced or directed by the vehicle driver. 
       FIG. 3  illustrates a graph  200  of the hydraulic brake pressure for the front axle (line  210 ) and for the rear axle (line  220 ) of a diagonally split system over time during an autonomous braking event occurring at time t 1 . Prior to time t 1 , in response to a first signal from the vehicle sensing system, the braking system may initiate a “pre-pressurization” operation of the pump  144  to re-seat the brake pads into optimal position and thereby remove the lag time associated with the volumetric inefficiencies caused by “knockback,” a brake system phenomenon understood by those of skill in the art. Referring to  FIGS. 3 and 4 , after any pre-pressurization to eliminate the effects of knockback (step  300 ), upon receiving a signal from the controller  190  associated with an emergency braking event (step  310 ), the braking system actuates an automatic braking sequence and hydraulic pressure builds in both the front and the rear brakes (step  320 ) from time t 1  until a predetermined pressure or control point  230  is reached at time t 2  (step  330 ). This buildup of pressure may occur at an equal rate, as illustrated, or may be a different rate for the front brakes and for the rear brakes. The control point  230  is a tuneable value that is a function of the particular vehicle dynamics and the specific brake system and represents a condition at or near the Z-critical deceleration point. The point  230  is not necessarily a function of wheel slip, specific brake pressure, or deceleration but is tuned to maximize the highest level of vehicle deceleration in the shortest possible time. At time t 2  pressure to the rear axle brakes (one in each circuit  120 ,  124  of  FIG. 1 ) is modulated to a rate less than the rate for the front axle brakes (step  340 ). In some applications, the modulation of pressure to the rear axle is a generally steady pressure hold, as shown in  FIG. 3 . 
     Due to the modulation of the pressure to the rear axle, all of the hydraulic flow from the modulation system is now provided exclusively to the front brakes, i.e., both circuits  120 ,  124  are active but feed only the front axle brakes (FR, FL), as the front axle pressure build continues. The front axle brakes, which have higher vertical loading due to vehicle weight distribution and braking dynamics, can support a higher brake torque than the rear brakes and are more efficient at generating brake torque per hydraulic flow volume (a product of brake effectiveness and brake stiffness). 
     When the vehicle achieves its targeted level of deceleration or wheel slip at time t 3  (step  350 ), if the rear axle can support additional braking at time t 3  (step  360 ), the high pressure fluid at a front axle brake can be released, or vented, to the paired diagonal rear axle brake (e.g., FL to RR or FR to RL) if necessary (step  370 ). At time t 4 , if permitted by the current braking conditions (step  380 ), more traditional hydraulic brake modulation, i.e., anti-lock braking, may occur as the vehicle is at or above its target deceleration or wheel slip (step  390 ). 
     With this control strategy, hydraulic energy is not wasted by pressure dumps to a low pressure accumulator and the more efficient front axle brakes receive all of the hydraulic flow subsequent to the rear axle pressure hold. Additionally, the stability of the vehicle is enhanced because the rear axle pressure hold is initiated at an appropriately determined level to prevent hydraulic overshoot of pressure control on the rear axle. 
       FIG. 5  illustrates a graph  400  of the hydraulic brake pressure for the front axle (line  410 ) and for the rear axle (line  420 ) of a front/rear split or parallel system over time during an autonomous braking event occurring at time t 1 . As with the diagonally split system, the braking system may initiate a “pre-pressurization” operation of the pump  144  prior to time t 1  to remedy the effects of knockback. Referring to  FIGS. 5 and 6 , after any pre-pressurization to eliminate the effects of knockback (step  500 ), upon receiving a signal from the controller  190  associated with an emergency braking event (step  510 ), the braking system actuates an automatic braking sequence and hydraulic pressure builds in both the front and the rear hydraulic circuits  120 ,  124  (step  520 ) from time t 1  until a predetermined pressure or control point  430  at time t 2  (step  530 ), as previously described. Though illustrated as a different rate of pressure buildup for the front and rear brakes, the rate may be the same for each, as shown in  FIG. 3 . At time t 2  pressure to the rear axle brakes (circuit  124  of  FIG. 2 ) is modulated to a rate less than the rate for the front axle brakes (step  540 ), e.g., modulated to a steady pressure hold, as shown in  FIG. 5 . As with the diagonally split system, the hydraulic flow is now provided exclusively to the front brakes, i.e., circuit  120  is active and feeds the front axle brakes (FR, FL). 
     When the front axle brakes reach the targeted brake levels, determined in part by wheel slide and/or deceleration at time t 3  (step  550 ), the front brakes can be maintained at a pressure hold (step  560 ). If the rear axle can support additional braking at time t 3  (step  570 ) pressure can optionally be increased to the rear axle brakes (circuit  124  of  FIG. 2 ) (step  580 ) in conjunction with further hydraulic brake modulation, such as anti-lock braking (step  590 ). 
     Various features and advantages of the invention are set forth in the following claims.