Abstract:
An improved control method that utilizes active brake controls to provide vehicle control during conditions of carriage motion tendency about a longitudinal vehicle axis. Accordingly, the control may be integrated with known active brake controls that regulate yaw rate and/or slip angle or traction controls that regulate front wheel slippage, thereby significantly improving the functionality of the respective control for little or no additional cost. A measure of vehicle carriage motion tendency is determined by computing a control parameter based on the roll angle of the vehicle and the rate of change of roll angle. If the computed control parameter exceeds a first calibrated threshold, a condition of carriage motion tendency is detected and the active brake control actuators are activated to reduce the velocity of the front wheels of the vehicle by a delta velocity command determined as a function of the roll angle and roll angle rate. When the computed control parameter falls below a second calibrated threshold, the delta velocity command is gradually decayed to exit the control.

Description:
TECHNICAL FIELD 
     This invention relates to a motor vehicle active brake control (ABC) method. 
     BACKGROUND OF THE INVENTION 
     Active brake controls (ABC) have been used to improve vehicle handling under conditions of non-linear wheel operation (i.e., lateral wheel movement relative to a road surface) by applying differential braking forces to left and right wheels of the vehicle with the objective of bringing the vehicle yaw rate and/or slip angle into conformance with desired values provided by a linear reference model. See, for example, the U.S. Pat. No. 5,720,533 to Pastor et al., issued on Feb. 24, 1998, assigned to the assignee of the present invention, and incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an improved control method that utilizes active braking of the front wheels of a vehicle to provide vehicle balance control during conditions of carriage motion tendency relative to the longitudinal vehicle axis. Accordingly, the control may be integrated with known active brake controls that regulate yaw rate and/or slip angle, or traction controls that regulate front wheel slippage, thereby significantly improving the functionality of the respective control for little or no additional cost. 
     According to the invention, a measure of carriage motion tendency of a revolving type relative to the longitudinal axis (hereinafter “carriage motion tendency”) is determined by computing a control parameter based on the roll angle of the vehicle and the rate of change of roll angle. If the computed control parameter exceeds an enable threshold, a condition of carriage motion tendency is detected and the brake control actuators are activated to reduce the velocity of the front wheels of the vehicle by a delta velocity command determined as a function of the roll angle and roll angle rate, so as to saturate the lateral adhesion capability of the front tires. When the computed control parameter falls below a disable threshold, the delta velocity command is gradually decayed to exit the control. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of a vehicle including an electronic controller and associated input and output devices constituting a control system for carrying out an active brake control according to this invention. 
     FIG. 2 is a main loop flow diagram representative of computer program instructions executed by the electronic controller of FIG. 1 in carrying out the control of this invention. 
     FIG. 3 is a flow diagram detailing a flow diagram block of FIG. 2 concerning the processing of carriage motion tendency information. 
     FIG. 4 is a flow diagram of an example implementation for block  130  of FIG. 2 concerning the determination of a delta velocity command. 
     FIG. 5 is a flow diagram of an example implementation of block  140  of FIG. 2 concerning entry and exit conditions for the balance control. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 depicts a mechanization of an active brake control (ABC) on a front wheel drive vehicle  10 . The vehicle  10  includes a brake system including a brake pedal  64  mechanically coupled to a master cylinder  66  for producing hydraulic pressure in proportion to the force applied to pedal  64 . The master cylinder  66 , which may include a pneumatic booster (not shown), proportions the hydraulic pressure among the front and rear brake supply lines  48  and  50  in a conventional manner. Front supply lines  48  are coupled to the left front service brake  20  via ABC actuator  52 , and to the right front service brake  22  via ABC actuator  54 . Rear supply lines  50  are coupled directly to the left and right rear wheel brakes  24  and  26 . 
     The micro-processor based controller  68  controls the operation of the ABC actuators  52 ,  54  via lines  70 ,  72  for braking the front wheels  12 ,  14  independent of master cylinder  66  as required to enhance the lateral carriage (i.e., body) position control of the vehicle. The controller  68  receives various inputs, including wheel speed signals on lines  36 ,  38 ,  40 ,  42  from respective wheel speed sensors  28 ,  30 ,  32 ,  34 ; an optional brake pedal travel signal on line  84  from pedal travel sensor  82 ; a steering wheel angle signal on line  62  from angle sensor  61 , and relative position signals on lines  86 ,  88 ,  90 ,  92  from wheel position sensors  94 ,  96 ,  98 ,  99 . The sensors  28 ,  30 ,  32 ,  34 ,  61 ,  82   94 ,  96 ,  98 ,  99  may be implemented with conventional devices in a manner known to those skilled in the art. In particular, the relative position sensors  94 ,  96 ,  98 ,  99  sense the relative positions of the sprung and un-sprung frame elements (not shown) at each corner of the vehicle  10 , and are commonly used in active suspension control systems, such as the Real Time Damping System (RTD) featured in certain vehicles produced by General Motors Corporation. Other sensors such as yaw rate and lateral acceleration sensors may also be used. 
     In performing active brake control, the controller  68  modifies the normal braking of one or more of the wheels  12 ,  14  via the respective actuators  52 ,  54  in order to produce a corrective yaw moment. Exemplary actuators are shown and described in detail in the U.S. Pat. No. 5,366,291, assigned to the assignee of the present invention. In general, however, the actuators  52 ,  54  operate during active brake control to isolate the respective brakes  20 ,  22  from the master cylinder  66 , and then increase or decrease the respective brake pressures to cause the individual wheel speeds to conform to respective target speeds chosen to produce the desired handling characteristic. See, for example, the U.S. Pat. No. 5,015,040 to Lin, issued on May 14, 1991, assigned to the assignee of the present invention, and incorporated herein by reference. In yaw rate and/or slip angle control, the target speeds are selected to produce a left-to-right wheel speed differential corresponding to a desired corrective yaw moment. In the control of the present invention, the target speeds are selected to reduce the velocity of the front wheels of the vehicle by a delta velocity command determined as a function of the roll angle and roll angle rate of the body or carriage. Other controls, such as anti-lock brake control, may also be integrated into the active brake system. 
     A main flow diagram for an active brake control incorporating the balance control of this invention is set forth in FIG.  2 . Referring to FIG. 2, the reference numeral  100  designates a series of initialization instructions executed at the initiation of vehicle operation for properly initializing certain variables and flags to initial values. Thereafter, the block  110  is executed to read the various sensor inputs, including the wheel speed signals on lines  36 - 42  and the relative position signals on lines  86 - 92 . Then the block  120  is executed process the relative position information, determining filtered and unfiltered values of the vehicle roll angle RA and roll rate RR; see following description of FIG.  3 . Then block  130  is executed to determine a ΔVelocity command or velocity reduction command for the front wheels  12 ,  14  based on the filtered roll angle FRA and the filtered roll rate FRR; see following description of FIG.  4 . The entry/exit conditions for balance control are determined at block  140 , described below in detail in reference to the flow diagram of FIG.  5 . Finally, the block  150  is executed to control the brake actuators  52  and  54  to regulate the front wheel speeds at the target values when balance control is active. 
     Referring to FIG. 3, the main flow diagram block  120  comprises the individual steps of computing current values of roll angle RA and roll rate RR based on the relative position signals lf, rf, lr and rr, and filtered versions of such values. The relative position signals lf, rf, lr and rr correspond to the position measurements at the left-front, right-front, left-rear and right-rear wheels  12 ,  14 ,  16 ,  18 . In block  160 , the term CAL 1  represents a calibration constant that converts the left-to-right relative position difference (lf−rf+lr−rr) to roll angle; for small angles, CAL 1  has a value of 360/π/T, in degrees/mm, where T is the vehicle track. In block  162 , the roll rate RR is determined according to the difference between the current and last roll angles the time period between the two samples. The filtered roll angle FRA and filtered roll rate FRR are determined at blocks  164  and  166  with simple first order filters having respective gain factors CAL 3  and CAL 4 . Sample values of the gain factors are CAL 3 =0.4 and CAL 4 =0.05. 
     Referring to FIG. 4, the main flow diagram block  130  comprises the individual steps of determining proportional and derivative ΔVelocity terms ΔVELp, ΔVELd and an overall ΔVelocity term ΔVEL. The proportional term ΔVELp is determined as a function of the filtered roll angle FRA as indicated at block  170 , and the derivative term ΔVELd is determined as a function of the filtered roll rate FRR as indicated at block  172 . The proportional and derivative terms may be computed or determined by table look-up. As indicated at block  174 , the overall ΔVelocity term ΔVEL is computed as the sum of ΔVELp and ΔVELd. 
     FIG. 5 details the entry/exit conditions of the main flow diagram block  140 . Essentially, the balance control is enabled or disabled based on a balance control sum BCSUM determined at block  180 . As indicated BCSUM is defined as the absolute value of the sum of the filtered roll angle FRA and the filtered roll rate FRR, multiplied by a calibrated scale factor CAL 5 . If balance control (BC) is inactive and BCSUM is less than or equal to a calibrated enable threshold CAL 6 , as determined at blocks  182  and  184 , balance control remains inactive. However, if BCSUM exceeds the enable threshold CAL 6  indicating possible impending carriage motion or rotation about the vehicle longitudinal axis, blocks  186 - 188  are executed to activate balance or carriage motion tendency control (BC), to set the target speeds for the front wheels  12 ,  14 , and to initialize exit condition parameters. The target speeds indicated at block  187  include a right-front target speed RFtar determined according to the difference (ω RF −ΔVEL) and a left-front target speed LFtar determined according to the difference (ω LF −ΔVEL), where ω RF  and ω LF  represent the vehicle reference speed at the right and left wheels  14 ,  12 , respectively. It will be recognized by those skilled in the art that the vehicle reference speed may be variously determined depending on the type of vehicle drive train; in a front wheel drive vehicle, for example, the vehicle reference speed may be determined as the average of the rear wheel speeds. In any case, the vehicle reference speed at a given front wheel  12 ,  14  can then be determined based on steering angle or other factors. The exit condition parameters initialized at block  188  include a balance control factor BCF (initialized to 1.0) and a balance control timer BCTMR (initialized to zero). 
     Once block  182  determines that balance control (BC) is active, and BCSUM remains at least as great as a calibrated disable threshold CAL 7 , as determined at block  190 , balance control remains active and blocks  187 - 188  are executed to update the right-front and left-front target speeds RFtar, LFtar and to re-initialize the exit condition parameters. However, if BCSUM is below the disable threshold, the blocks  192 - 204  are executed to gradually decay ΔVEL to zero. The balance control factor BCF, initialized to 1.0 at block  188 , is updated at block  192  according to the difference (1−BCTMR/CAL 8 ), where CAL 8  is a calibrated value corresponding to the decay period, and applied to ΔVEL at block  198 . The balance control timer BCTMR, initialized to zero at block  188 , is incremented at block  194 , limited to CAL 8  at block  196 , and compared to CAL 8  at block  200 . So long as BCTMR is less than CAL 8 , the block  202  is executed to update the right-front and left-front target speeds RFtar, LFtar as described above in reference to block  187 , but using the attenuated value of ΔVEL determined at block  198 . When BCTMR has been incremented up to CAL 8 , the term ΔVEL is fully decayed since factor BCF is zero, and blocks  202  and  188  are executed to deactivate balance control (BC) and to re-initialize the exit condition parameters BCF and BCTMR. By way of example, and without limitation, sample values of the calibration parameters defined in FIG. 4 are as follows: CAL 5 =0.4, CAL 6 =6.0, CAL 7 =3.0, and CAL 8  is set to a value corresponding to 1.0 sec. 
     In operation, it will thus be seen that carriage motion tendency control is enabled to reduce the front wheel speeds by a determined amount ΔVEL whenever BCSUM is greater than the enable threshold CAL 6 . When BCSUM falls below the disable threshold CAL 7 , the determined amount ΔVEL is attenuated by a gradually diminishing gain factor BCF to gradually return the vehicle to normal operation. However, if BCSUM rises above the disable threshold CAL 7  before the factor BCF has been reduced to zero, the control is reactivated at the determined amount ΔVEL and the gain factor BCF is reinitialized to 1.0. The determined amount ΔVEL is based on the filtered roll angle FRA and filtered roll rate FRR, which in turn, are determined based on the relative position signals lf, lr, rf, rr. Thus, this invention is easily implemented in an active brake control or traction control vehicle with only a small additional cost. Obviously, the control may alternatively be implemented as a stand-alone system, if desired. While disclosed in reference to the illustrated embodiment, it is expected that various modifications to the above-described control will occur to those skilled in the art, and it should be understood that controls incorporating such modifications may fall within the scope of the present invention, which is defined by the appended claims.