ANTILOCK BRAKE CONTROL DEVICE

Abstract: An antilock brake control device includes wheel speed sensors (S1-S4) for detecting the wheel speeds of the wheels (FL,FR,RL,RR) of an automobile, an electronic control circuit (10) for calculating the wheel speeds and slip rates of the wheels (FL,FR,RL,RR), the deceleration of the vehicle and the estimated vehicle speed based upon the signals from the sensors (S1-S4) to produce control signals based upon the results of calculation, and a fluid pressure control unit for controlling the braking forces on the rear wheels (RR,RL) independently of each other. The electronic control circuit (10) is adapted to calculate the sum of slip rates of both rear wheels (RR,RL) to control both rear wheels (RR,RL) based upon the information supplied from the sensor (S2,S4) for one of the rear wheels (RR,RL) rotating at a lower speed, if the sum of slip rates is larger than a predetermined value and to control all the wheels (FR,FL,RR,RL) independently of one another based upon the information from the sensors (S1-S4) for the respective wheels (FR,FL,RR,RL), if the sum of slip rates is smaller than the predetermined value.

Description: The present invention relates to an antilock brake control device capable of braking the wheels of an automobile, especially its rear wheels, with higher efficiency. In order to brake the wheels of an automobile with maximum efficiency, the braking pressure has to be controlled so as to follow changes in the coefficient of friction between the road surface and the wheels. For this purpose the brake assembly should be provided with an antilock device which is adapted to increase, hold and reduce the braking pressure so as to repeatedly and alternately release and apply the brake at very short time intervals even while the brake pedal is being trodden. Such an antilock control device comprises wheel speed sensors for detecting the wheel speeds, an electronic control circuit for calculating the wheel speeds, estimated vehicle speed (reference wheel speed) and slip rates based upon the wheel speed signals to produce control signals such as pressure signals to produce control signals such as pressure reduction, pressure hold and pressure increase signals, and a fluid pressure control unit which controls the braking pressure generated in the master cylinder in response to the control signals and supplies it to the wheel cylinder. More specifically, the electronic control circuit performs various arithmetic operations based upon the wheel speed signals to give the slip rate and deceleration, judges from the fluctuation of the slip rates and deceleration whether each wheel is falling into a locked state or recovering from the locked state, and gives control signals such as pressure reduction, pressure hold and pressure increase signals on the basis of the above judgement. A typical prior art fluid pressure control unit (e.g. FR-A-2.626.221) comprises solenoid valves (optionally provided with cutoff valves or flow control valves), check valves, hydraulic pumps with motors, accumulators and reservoir tanks. The valves of one of the abovementioned types are provided in the brake pressure lines between the master cylinder and the respective wheel cylinders to control the flow of braking pressure or pump pressure by opening and closing the valves. As a method for controlling the flow of braking pressure to the wheel cylinders, it is known to provide one set of the control unit for each vehicle wheel to control the wheels independently of one another (four-channel mode), to provide one set of the control unit for each of the right and left front wheels and one for both rear wheels (three-channel mode) and to provide one set for each of the right and left front wheels and adapt so that the hydraulic pressure acting on each of the rear wheels will follow the hydraulic pressure acting on one of the front wheels (2-channel mode). As a method of giving fluid pressure control signals to the above-described hydraulic circuit, it is known to control the two wheels at each side as one unit and reduce the braking pressure on all the wheels if the wheels at one side which are controlled with a lower fluid pressure (which means that the coefficient of friction with the road surface is smaller) show a tendency to lock (such a control method is hereinafter referred to as the select-low mode). Another known method is to reduce the braking pressure on all the wheels if the wheels at one side which are controlled with a higher fluid pressure show a tendency to lock (select-high mode). It is also known to control the braking pressure on the wheels independently of one another according to the road condition (independent mode). The select-low mode is effective in increasing the resistance of the wheels to lateral forces and thus in improving the directional stability and the drivability of the vehicle, but has a problem that the braking distance tends to be long because of an insufficient braking force. On the other hand, if the wheels are controlled on the select-high mode, the vehicle can be braked with a sufficient braking force, but its directional stability will be insufficient. With the independent mode, the wheels can be controlled individually so as to precisely conform to the changing road condition. In view of the advantages and disadvantages of each control mode, it is desirable to control the front wheels on the independent or select-high mode to assure a sufficient braking force and to control the rear wheels on the select-low mode to keep a high directional stability. However, with the abovementioned prior art antilock brake control devices, either the whole wheels were controlled always independently of one another or the rear wheels were controlled always on the select-low mode irrespective of the road condition. Suppose now that the rear wheels are always controlled on the select-low mode. Since it is practically impossible to eliminate the manufacturing errors for the fluid pressure control unit, the braking pressures on right and left wheels can increase and decrease in different manners from each other, thus bringing about differece in braking pressure between right and left wheels, in spite of the fact that the same control signals are given to them. Even if the difference in pressure is small at first, it will widen gradually because the rear wheels are always controlled on the select-low mode. It will be appreciated from Fig. 7 that the distance between the curves b and a and the distance between the curve b′ and a′ increase with time (wherein the curves b and b′ indicate the wheel speed and the braking pressure, respectively, if the pressures on both wheels are to increase and decrease in exact conformity with each other, whereas the curves a and a′ indicate the actual wheel speed and the braking pressure, respectively. This means that the braking force on one of the rear wheels at the low pressure side is not effectively utilized. With the independent control mode where all the wheels are controlled independently of one another, if the difference in the coefficient of friction of the road surface between right and left sides of the vehicle is large and if the sum of slip rates of both rear wheels exceeds a predetermined value, the difference between the braking forces applied to both rear wheels will widen excessively. This will bring about a lag in the brake timing between both rear wheels and a difference in the pressure increase and pressure reduction intervals between both rear wheels as shown by curves c , c′ , d and d′ in Fig. 8, thus hampering the directional stability of the vehicle (the vehicle tends to lose its stability in the areas A and B). In such a case, the rear wheels should rather be controlled on the select-low mode in order to improve the directional stability . But, it is not desirable to control the rear wheels always on the select-low mode. If the sum of slip rates of both rear wheels falls below a predetermined value, the difference between the braking forces applied to both rear wheels is so small as not to hamper the directional stability of the vehicle. Controlling the rear wheels on the select-low mode in such a situation would unnecessarily reduce the braking force on the rear wheels, thus increasing the braking distance. It is an object of the present invention to provide an improved antilock brake control device which obviates the abovesaid shortcomings and which is capable of controlling the braking forces on the rear brakes of an automobile in an improved manner. In accordance with the present invention, while the sum of slip rates of the rear wheels is above a predetermined value, the select-low mode is applied to the rear wheels. When it decreases below the predetermined value, the mode is switched from the select-low to the independent. In other words, while the sum is larger than the predetermined value, both of the rear wheels are controlled simultaneously on the basis of the wheel speed information about one of the rear wheels which is rotating at a lower speed, and while it is smaller than the predetermined value, all the wheels are controlled independently of one another on the basis of the wheel speed information about the respective wheels. By treading the brake pedal, the antilock control will start. When the wheel speeds fall owing to an increase in the braking pressure, the electronic control circuit will calculate the slip rate and the deceleration for each wheel, and the sum of slip rates of both rear wheels on the basis of the wheel speeds detected and judge from the results of calculation whether or not the sum of slip rates is larger than a predetermined value. If judged so, the wheel speeds of both rear wheels are compared with each other and the higher wheel speed is replaced with the slower wheel speed. The deceleration and the slip rate for each wheel are again calculated based upon the renewed wheel speeds to judge whether each wheel is showing a tendency to lock or recovering from the locked state. It is then determined based upon the above judgement which one of the pressure increase, pressure hold and pressure reduction signals is to be given. In other words, the rear wheels are controlled on the select-low mode. If the sum of slip rates is smaller than the predetermined value, the wheels are controlled not on the select-low mode but independently of one another. Namely the input signals for each wheel are used to control the wheels independently. Other features and objects of the present invention will become apparent from the following description taken with reference to the accompanying drawings, in which: Fig. 1 is a block diagram of the electronic control circuit in the antilock brake control device embodying the present invention; Fig. 2 is a block diagram of the hydraulic circuit of the same; Fig. 3 is a flow chart of the program stored in the electronic control circuit; Fig. 4 is a flow chart of a portion of an ordinary antilock control program; Fig. 5 is a flow chart of the main program; Fig. 6 is a flow chart of another example of the program of Fig. 3; and Fig. 7 and 8 are graphs showing how a prior art antilock control device operates. As shown in Fig. 1, wheel speed signals detected by wheel speed sensors S1 to S4 are fed into an A/D converter 11 so as to be converted into pulse signals, which are then processed in a pulse processing circuit 12 and fed into a microcomputer 13. The microcomputer 13 calculates the deceleration, estimated vehicle speed, slip rates, etc. on the basis of the pulse signals representative of the wheel speeds of the respective wheels and feeds control signals as a result of the above calculations to a solenoid actuating circuit 14 and a motor actuating circuit 15. The control signals from the microcomputer 13 are also fed to a failsafe relay actuating circuit 16 and a warning lamp actuating circuit 17. W.L represents a warning lamp. The solenoid actuating circuit 14 feeds four separate control signals to four pairs of solenoid valves 22 and 22′ (two-position changeover valves) to turn ON or OFF the respective pairs of valves independently of one another and thus to increase, maintain or reduce the braking pressure on the brake cylinder for each vehicle wheel. As shown in Fig. 2, the hydraulic circuit in this embodiment includes a master cylinder 21, the solenoid valves 22 and 22′ (the former being provided in the braking pressure circuit and the latter in the return line), hydraulic pumps 23, a motor 24, accumulators 25, reservoir tanks 26 and check valves 27. The braking pressure generated in the master cylinder 21 is transmitted through two lines to the right and left front wheels and then to the left and right rear wheels through lines crossing each other in an X shape. One pair of solenoid valves 22 and 22′ and one check valve 27 are provided for each wheel. The hydraulic pumps 23, accumulators 25 and reservoir tanks 26 are provided each in the respective brake lines at both sides of the hydraulic circuit. The hydraulic pumps 23 are driven by a single dual-shaft type drive motor 24. This type of hydraulic circuit is called a circulating type circuit. Figs. 3 to 8 show how the antilock brake control device according to the present invention operates. When the vehicle starts, the signals representative of the individual wheel speeds are fed into the electronic control unit 10, and the microcomputer 13 calculates the wheel speeds V FLH , V FRH , V RLH and V RRH of the respective wheels. The antilock control will be activated upon treading the brake pedal. As shown in Fig. 3, upon application of the brake, the ECU will calculate the estimated vehicle speed VREF and the sum of slip rates of the rear wheels SR = (2V REF - V RLH -V RRH ). Next, judgement is made as to whether or not the SR value is larger than a threshold value THR, which is such a value that if the SR value is larger than the threshold value THR, the stability of the vehicle is supposed to be influenced. In other words, if the SR value is smaller than the threshold value THR, the vehicle is judged to be travelling rather stably. Thus the wheels can be controlled independently of one another so as to follow the road condition, and the wheel speeds V(1) - V(4) detected are used as they are. If the SR value is larger than the threshold value THR, the rear wheels are controlled on the select-low principle. Namely, the wheel speeds of both rear wheels are compared with each other and if the wheel speed V RRH of the right rear wheel turns out to be smaller than the wheel speed V RLH of the left rear wheel, the wheel speed V RRH is assigned to the wheel speed V(3) of the left rear wheel. If the right rear wheel is rotating at a higher speed, the wheel speed V RLH is assigned to the wheel speed V(4) of the right rear wheel. In other words, the rear wheels are controlled based upon the wheel speed information from the wheel rotating at a lower speed. According to the surface condition of the road, one of the abovementioned three options is selected and the V(i) values (i = 1 - 4) are determined accordingly. The deceleration Dv, slip rates S and the like are calculated on the basis of the thus renewed V(i) values to evaluate the V(i) values. The results of evaluation are given by ϑ(i). It is determined on the basis of the ϑ(i) values which of the pressure increase, pressure hold and pressure reduction signals is to be issued. This decision of the control signal may be made in accordance with the flow chart for ordinary antilock control shown in Fig. 4. Before entering the flow chart in Fig. 4, initialization is executed in the main program (which is to be described later) and a control phase, which is a combination of pressure increase, pressure hold and pressure reduction commands, is set to Phase I. The control phase includes three phases, i.e. Phase I wherein the pressure increase command is given continuously, Phase II wherein the pressure reduction and pressure hold commands are alternately given and Phase III wherein the pressure increase and pressure hold commands are alternately given. With the control phase set to Phase I, the routine shown by the flow chart of Fig. 4 is executed. If the deceleration and slip rates calculated from the V(i) values (i = 1 to 4) do not indicate any locking tendency of the wheels, the control signal for Phase I is applied to the solenoids for the respective wheels. While this routine is repeated with the control phase maintained at Phase I, i.e. with the braking pressure on the wheels continuously increasing, the deceleration and the slip rates will change until the wheels are falling into a locked state. If the locking tendency of the wheels is detected, the control phase will switch from Phase I to Phase II in which the pressure reduction and pressure hold commands are alternately given to reduce the braking pressure and allow the wheels to recover from the locked state. When the wheels are found out to be recovering from the locked state, the control phase will switch from Phase II to Phase III. As far as the surface condition of the road remains substantially unchanged, the control phase will switch from Phase I to Phase II and then to Phase III because in this way the vehicle can be braked with the highest efficiency and can be slowed down or stopped quickly. However, if the coefficient of friction between the road surface and the wheels should change drastically, the control phase is adapted to switch from Phase III to Phase II and then back to Phase III. Fig. 5 shows an example of the main program in the microcomputer 13. The sub-programs shown by the flow charts in Figs. 3 and 4 are adapted to interrupt the main program at regular time intervals. In the main program, after initial check and initialization steps have been executed, the control phase is set to Phase I and an interrupt permission is given. The following non-emergent judgement steps are repeated indefinitely as far as no abnormal state is detected and thus constitute an infinite loop. The subprograms are executed at uniform time intervals while temporarily interrupting the main program. A program shown in Fig. 6 is substantially the same as the sub-program shown in Fig. 3 with the exception that the V(i) values (i = 1 to 4) are evaluated at an earlier stage. If the wheels are to be controlled on the select-low principle, the results of evaluation ϑ(i) (i = 1 to 4) including the deceleration and the slip rates are all replaced with the results of evaluation for the low-speed side.

Claims: An antilock brake control device comprising wheel speed sensors (S1-S4) for detecting the wheel speeds of the wheels (FL,FR,RL,RR) of a motor vehicle, an electronic control circuit (10) for calculating the wheel speeds and slip rates of the wheels, the deceleration of the vehicle and the estimated vehicle speeds on the basis of the wheel speed signals from said wheel speed sensors (S1-S4) to produce fluid pressure control signals on the basis of the results of calculation, and a fluid pressure control unit (14,22) for controlling at least the braking forces for the rear wheels (RL,RR) independently of each other on the basis of said fluid pressure control signals, characterized in that said electronic control circuit (10) having means for evaluating the sum of said slip rates of the rear wheels (RL,RR) so as to control both of the rear wheels (RL,RR) simultaneously on the basis of the wheel speed information about one of the rear wheels (RL,RR) which is rotating at a lower speed, if said sum is larger than a predetermined value and so as to control all the wheels (FR,FL,RR,RL) independently of one another on the basis of the wheel speed information about the respective wheels, if said sum is smaller than said predetermined value. An antilock brake control device as claimed in claim 1, wherein said electronic control circuit (10) is adapted to compare the wheel speeds of both rear wheels (RL,RR) with each other to replace the value of wheel speed of one of the rear wheels which is rotating at a higher speed with the value of wheel speed of the other rear wheel, calculate the deceleration and the slip rates on the basis of said renewed values of wheel speeds and supply control signals to said fluid pressure control unit (14,22) to control the rear wheels (RL,RR) on the basis of the results of calculation, if said sum of slip rates is larger than said predetermined value, said electronic control unit being adapted to supply control signals to said fluid pressure control unit to control all the wheels (FL,FR,RL,RR) independently of one another, if said sum of slip rates is smaller than said predetermined value. An antilock brake control device as claimed in claim 1, wherein said electronic control circuit (10) is adapted to compare the wheel speeds of both rear wheels (RL,RR) with each other to replace the results of calculation of the wheel speeds, estimated vehicle speed, deceleration and slip rates for one of the rear wheels rotating at a higher speed with those for the other rear wheel to feed control signals given as a result of said replacement to said fluid pressure control unit (14,22), if said sum of slip rates is larger than said predetermined value, said electronic control circuit (10) being adapted to feed control signals produced as the results of said calculation to said fluid pressure control unit (14,22), if said sum of slip rates is smaller than said predetermined value.

Inventors: YOSHINO, MASATO C/O ITAMI WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1989
Main classification: B60T8
IPC codes: B60T8