Abstract:
This method for controlling the behaviour of an automotive vehicle during cornering consisting in measuring the rotational wheel speed of the front and rear wheels of the vehicle, establishing the wheel speed difference between the front and rear wheels and modifying the brake pressure applied to the rear wheel brakes of the vehicle according to a vehicle behaviour to be obtained, comprises the steps of elaborating a function of the absolute value of the wheel speed difference between the inner front wheel and the outer rear wheel, considering the cornering direction and using said function to determine the brake pressure variations to be applied to the rear wheel brakes.

Description:
TECHNICAL FIELD 
     This invention relates to a method for controlling the behaviour of an automotive vehicle during cornering and to a braking system for implementing such a method. 
     More particularly, the present invention concerns a method wherein, using the wheel speed of the vehicle, the hydraulic brake pressure available to the wheel brakes is modified according to intended road-holding qualities. 
     BACKGROUND OF THE INVENTION 
     Some widely known types of vehicles are provided with an Anti-Lock Braking System (ABS), and with a Dynamic Rear Proportioning System (DRP) integrated within the ABS to perform the function of brake pressure proportioning between the front and rear brakes in order to provide the optimum brake force balance throughout the brake manoeuvre to enhance the performance of the braking system and to optimise the vehicle stability. 
     This type of braking system is quite efficient while the vehicle is moving in a substantial linear pathway, but is prone to be less accurate during cornering, due to the fact that the regulation of the braking balance is carried out using the front and rear wheel speed differential value which may not be high enough, during cornering, to generate a consistent data. 
     SUMMARY OF THE INVENTION 
     In view of the above problem, it is an object of the invention to provide a method for controlling the behaviour of a vehicle during cornering, capable, on the one hand, to enhance the braking performance, and on the other hand, to optimise the vehicle stability. 
     Another object of the invention is to provide a braking system carrying out the above method. 
     For this purpose, the present invention provide a method for controlling the behaviour of an automotive vehicle during cornering consisting in measuring the rotational wheel speed of the front and rear wheels of the vehicle, establishing the wheel speed difference between the front and rear wheels and modifying the brake pressure applied to the rear wheel brakes of the vehicle according to a vehicle behaviour to be obtained, comprising the steps of: 
     elaborating a function of the absolute value of the wheel speed difference between the inner front wheel and the outer rear wheel, considering the cornering direction, and 
     using said function to determine the brake pressure variations to be applied to the rear wheel brakes. 
     Further, the invention provides a braking system intended to implement the above method, said system including Dynamic Rear Proportioning means and further comprises: 
     measuring means for measuring the inner front wheel speed and the outer rear wheel speed, considering the cornering direction, 
     means for establishing the absolute value of the difference between the inner front wheel speed and the outer rear wheel speed, 
     control means for controlling the brake pressure applied to the vehicle rear wheel brakes, and 
     computing means for calculating a function of the absolute value of the difference between the inner front wheel speed and the outer rear wheel speed, said computing means being connected to said measuring means and said control means in order to modify the brake pressure applied to the rear wheel brakes using said function, according to the vehicle behaviour to be obtained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a general schematic diagram depicting a braking system according to the present invention; 
     FIG. 2 is a flow chart of the control method routine implemented in the braking system shown in FIG. 1, for enhancing the vehicle stability; 
     FIG. 3 is a flow chart of the control method routine implemented in the braking system shown in FIG. 1, for enhancing the performance of the braking system; and 
     FIGS. 4 a - 4   h  show graphs illustrating the effects of the control method according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The vehicle braking system illustrated on FIG. 1, denoted by the general reference numeral  10 , is mainly constituted by an Anti-lock Braking system (ABS)  12 , a Dynamic Rear Proportioning system (DRP) including DRP processing means and an associated memory (not shown), and an hydraulic system associated with conventional brake means. 
     The hydraulic system comprises a conventional master cylinder  16  associated with a known per-se booster  18 . 
     The master cylinder  16  may be operated by the vehicle driver through a brake pedal  20  associated with a brake switch sensor  21  for applying fluid pressure, with power assist, to front wheel brakes  22  and  24  and rear wheel brakes  26  and  28 . 
     Fluid pressure is applied to the front wheel brakes  22 ,  24  through a fluid line  30  and fluid pressure is applied to rear wheel brakes  26 ,  28  via a fluid line  32 . 
     In a conventional manner, ABS modulator  34  of the ABS system  12  is included in the fluid lines  30  and  32  and controls, under control of an ABS controller  36 , the fluid pressure applied to the front and rear brakes  22 - 28  in order to reduce the pressure to the brakes upon detection of wheel instability, for example when wheel deceleration exceeds a predetermined limit. 
     As indicated in FIG. 1, a set of wheel speed sensors  38 ,  40 ,  42  and  44 , facing each wheel  46 ,  48 ,  50  and  52  respectively, provides the ABS controller  36  with data concerning the rotational speed of the vehicle wheels. 
     The system illustrated in FIG. 1 is completed by power supply means (not shown) associated with a switch for turning the ABS and DRP systems on and off. 
     It should be noted that the DRP system can be considered as a sub-system of the ABS system. Information shared by ABS and DRP as well as information specifically calculated for the DRP function is used for control of rear brakes  26 ,  28 . 
     The DRP processing means (which is preferably incorporated in the controller  36 ) controls a variation of the fluid pressure partition between the rear and front brakes  26 ,  28 ,  22 ,  24  according to the intended road-holding qualities, by means of a DRP algorithm stored in the memory of the DRP system and implemented in the DRP processing means. 
     The control method of variation of the brake fluid pressure according to the vehicle behaviour during cornering to be obtained will now be described with reference to FIGS. 2 and 3. 
     Referring first to FIG. 2, in order to control the vehicle behaviour during cornering, the DRP processing means first detects at step  54  that the vehicle is actually cornering, using a conventional technique, based for example on wheel speed measurements. 
     During the following step  56 , the DRP processing means detects the corner direction, namely as to whether it is a left or a right corner. 
     If the corner is a left one, DRP processing means calculates the absolute value of the difference between the left front wheel speed LFWS and the right rear wheel speed RRWS (step  58 ). 
     If the corner is a right one, DRP processing means calculates the absolute value of the difference between the right front wheel speed RFWS and the left rear wheel speed LRWS (step  60 ). 
     This calculated variable will be called hereinafter DIAG variable. 
     During the following step  62 , the DRP processing means elaborates a function which will be called below ADJ_DIAG function and which will be used to determine the brake pressure variations to be applied to the rear brakes  26 ,  28 . 
     For that purpose, DIAG variable is adjusted according to the intended road-holding qualities. Thus at step  62 , DIAG variable is multiplied by a predetermined multiplier factor stored in the memory of the system and obtained by previous machine learning, said factor constituting a gain intended to compensate for the calculated variable value according to the vehicle behaviour to be obtained and the vehicle features, for example the length thereof. 
     The multiplier factor can modify oversteering or understeering behaviour of the vehicle. 
     Next, at step  64 , the value of the function ADJ_DIAG obtained at the end of step  62  is compared with a threshold value THRESH  1 , and if said function value is above this threshold value, the value of this latter is allocated to the value of ADJ_DIAG function (step  66 ). 
     The above computed function ADJ_DIAG is then input into a low-pass filter, for example a first order filter, in order to eliminate noise (step  68 ). 
     The filtered function value is then compared with a brake release threshold THRESH  2  (step  70 ). If said function value is above this second threshold value, it is decided at step  72  that the brake fluid pressure applied to the left and right rear brakes  26  and  28  is to be released. 
     If it is not the case, during the following step  74 , the value of the filtered function ADJ_DIAG is compared with a brake holding threshold value THRESH  3  above which the pressure applied to the rear brakes is held constant. 
     Thus, if the filtered function value is above such a third threshold value, the pressure applied to the rear brakes  26 ,  28  is kept constant regardless of the pressure applied under the control of brake pedal  20  to the front brakes  22 ,  24  (step  76 ), and if it is not the case, i.e. if the filtered function value is less than the brake holding threshold value, the pressure applied to the rear brakes is kept unmodified, namely equal to the pressure in the fluid line  32  (step  78 ). 
     It is thus believed that the above mentioned routine is able, from the calculation of the absolute value of the difference between the rotational speed of the inner front wheel and the outer rear wheel, and according to desired road-holding qualities, to release, hold constant or let the rear braking pressure follow the pressure from the master cylinder  16 . 
     The DRP processing means is also able to increase the brake pressure applied to the rear brakes  26 ,  28  when the vehicle is stable enough in order to enhance the braking efficiency, as explained below with reference to FIG.  3 . 
     The routine implemented for that purpose begins with a first step  80  during which the DRP system checks whether the vehicle is actually cornering. 
     If it is the case, during the next step  82 , the adjusted function value ADJ_DIAG obtained at the end of above mentioned steps  64  and  66 , is compared with a fourth threshold value THRESH  4  which corresponds to a minimal stability condition above which the rear brake pressure increasing is allowed. 
     It is to be noted that during this step, rear brake pressure increasing is allowed only if this rear brake pressure has previously been held or released, that is, only if the corner is severe. 
     Next, at step  84 , it is checked whether the vehicle is stable enough for increasing the rear brake pressure, by comparing the value of the function ADJ_DIAG with a fifth threshold value THRESH  5  which corresponds to a maximal stability condition above which the brake pressure increasing is not allowed. 
     Thus, if the adjusted value lies within the range delimited by the above fourth and fifth threshold values, it is then checked, during the next steps  86  and  88 , whether the vehicle speed lies within a predetermined range, for example above 20 km/h (LOSPEED) and below 120 km/h (HISPEED), in order to avoid any pressure increasing if the vehicle speed is less than this lower speed value (step  86 ), which would render the rear brake pressure increasing of no interest and above the upper speed value (step  88 ), which may render the brake pressure increasing excessive. 
     Next, at step  90 , if the vehicle speed lies within said predetermined range, the brake pressure applied to the rear brakes  26 ,  28  which has been previously held or released, is increased in order to enhance the braking efficiency. 
     Preferably, the brake pressure is increased in the form of pressure steps in order to be in a position to check, after each step, the vehicle stability. 
     After each step, the number of steps is compared with a step number threshold value THRESH  6  (step  92 ). 
     If the total number of steps previously applied reaches this threshold value, the rear brake pressure is then continuously increased up to a value equal to the pressure value in the fluid line  32  (step  94 ). 
     It should be noted that the above mentioned routines described in reference to FIGS. 2 and 3 permit, by means of the calculation of the absolute value of the difference between the inner front wheel speed and the outer rear wheel speed, which provide some information about the corner radius and the vehicle speed, to hold at a constant value, or release, or increase, or let the rear brake pressure follow the pressure applied by the master cylinder  16  of a vehicle while cornering, in order, on the one hand, to enhance the vehicle stability, and, on the other hand, if the vehicle is stable enough, to enhance the braking efficiency by reducing the braking distance. 
     The results obtained through the above-mentioned routines will now be given with reference to FIGS. 4 a  to  4   h.    
     FIGS. 4 a - 4   d  depict the brake pressure, PRES_LF, PRES_RF, PRES_LR and PRES_RR as a function of time, applied to the left and right front and rear brakes, respectively, of a vehicle during cornering, at about 120 km/h. 
     FIG. 4 e  depicts the variations of a signal CORNER during an aggressive manoeuvre delivered at the end of the above-mentioned step  56  during which the cornering is detected. This signal is high as long as the vehicle is cornering and is low when the vehicle is moving along a linear pathway. 
     FIGS. 4 f  and  4   g  show the variation, as a function of time, of the filtered function ADJ_DIAG and of the speed vehicle SPEED, respectively. 
     The details of the function ADJ_DIAG shown at FIG. 4 f  have been magnified in order to improve clarity. 
     Finally, FIG. 4 h  illustrates the signal DRP_MODE corresponding to the running mode of the DRP processing means. 
     In FIG. 4 h  the signal DRP_MODE is high when, during step  70 , it has been detected that the value of ADJ_DIAG is above the corresponding release threshold value and that the rear brake pressure is to be released. 
     This signal is low when it has been decided, at step  92 , that the rear brake pressure is to be reapplied up to a value equal to that of the pressure in the fluid line  32 . 
     In addition, this signal is set at a intermediate value when, at step  74 , it has been decided that the brake pressure applied to the rear brakes  26 ,  28  is to be kept substantially constant. 
     At time t0, the vehicle driver actuates the brake pedal  20 , which tends to increase the brake pressure applied to the front and rear brakes  22 - 28 . 
     It can be seen on FIG. 4 f  that as soon as the adjusted function ADJ_DIAG is above the brake holding threshold value T1 (time t1), the DRP system is set in a hold mode, holding thus the right and left rear brake pressure substantially constant (FIGS. 4 c  and  4   d ). 
     Besides, FIG. 4 f  shows that as soon as the value of ADJ_DIAG is above the above-mentioned brake release threshold value T2 (time t2), the DRP system is set in a release mode in order to release the right and left rear brake pressure, rendering the signals shown on FIGS. 4 c  and  4   d  substantially equal to 0 until ADJ_DIAG drops below this second threshold value T2. 
     If the value of ADJ_DIAG drops below the brake holding threshold value (time t3), increasing of the rear brake pressure is allowed. 
     When the value of ADJ_DIAG drops below the above-mentioned fifth threshold value T3 (time t4), which indicates that the vehicle is in an acceptable stability condition, it can be seen on FIGS. 4 c  and  4   d , that the right and left rear brake pressure is increased up to the value of the pressure applied to the front brakes, that is to says, the rear brake pressure controlled through the brake pedal  20  is kept unmodified. 
     In this case, as mentioned above, the brake pressure is first increased in the form of steps and, when the number of steps is above a predetermined value, the pressure is reapplied in a substantially continuous way (time t5) in order to shorten the braking distance.