Vehicle Turning Behavior Control Apparatus

This invention relates to an apparatus for controlling the behavior of a multi-wheeled automotive vehicle turning either to the left or to the right and, more particularly, to such a vehicle turning behavior control apparatus for providing different braking forces to the offside and nearside wheels of the vehicle.

For example, Japanese Utility Model Kokai No. 59-155264 discloses an anti-skid braking apparatus arranged to improve the initial vehicle heading performance by delaying the time at which brakes are applied to the offside wheels of the vehicle so as to produce a yaw moment in a direction correcting an understeer characteristic when the vehicle steering angle exceeds a predetermined value. However, such an anti-skid braking apparatus fails to provide a good vehicle turning behavior control meeting to the driver's demand during a transient condition where the steering wheel is turning. For example, the conventional anti-skid braking apparatus operates at a slow response rate and the yaw rate will rise somewhat behind an actual steering angle change when the driver turns the steering wheel at a high speed.

Therefore, it is a main object of the invention to provide a vehicle turning behavior control apparatus which can operate at a rapid response rate with respect to steering angle changes so as to provide an improved vehicle turning behavior control during transient conditions.

There is provided, in accordance with the invention, an apparatus for controlling turning behavior of a multi-wheeled automotive vehicle supported on a plurality of pairs of wheels. The apparatus comprises braking means for applying braking forces to the respective wheels, first sensor means sensitive to a vehicle steering condition for producing a first signal indicative of a sensed vehicle steering condition, second sensor means sensitive to a vehicle steering speed for producing a second signal indicative of a sensed vehicle steering speed, and a control unit coupled to the first and second sensor means. The control unit includes means for providing a difference between the braking forces applied to nearside and offside wheels of at least one of the pairs of wheels based upon the sensed vehicle steering condition to produce a yaw moment in a direction to which the vehicle turns, and means for modifying the difference based upon the sensed vehicle steering speed to increase the difference as the vehicle steering speed increases.

In another aspect of the invention, the vehicle turning behavior control apparatus comprises braking means for applying braking forces to the respective wheels, first sensor means sensitive to a vehicle steering angle for producing a first signal indicative of a sensed vehicle steering angle, second sensor means sensitive to a vehicle steering speed for producing a second signal indicative of a sensed vehicle steering speed, and a control unit coupled to the first and second sensor means. The control unit includes means for calculating a basic value .DELTA.P1 as a function of the sensed vehicle steering angle to increase the basic value as the sensed vehicle steering angle increases when the sensed vehicle steering angle exceeds a predetermined value, means for calculating a first correction factor K1 as a function of the sensed vehicle steering speed to increase the first correction factor as the sensed vehicle steering speed increases, means for calculating a difference .DELTA.P as .DELTA.P=.DELTA.P1.times.K1, and means for setting the braking means to provide the calculated difference .DELTA.P between the braking forces applied to nearside and offside wheels of at least one of the pairs of wheels so that the braking force applied to the offside wheel is smaller than the braking force applied to the nearside wheel.

This invention will be described in greater detail by reference to the following description taken in connection with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a vehicle turning behavior control apparatus embodying the invention;

FIG. 2 is a flow diagram showing the programming of the digital computer used in the vehicle turning behavior control apparatus;

FIG. 3 is a graph of vehicle steering angle .theta. versus fluid pressure difference .DELTA.P1;

FIG. 4 is a graph of vehicle steering speed .theta.' versus first correction factor K1;

FIG. 5 is a graph of vehicle speed V versus second correction factor K2; and

FIG. 6 is a flow diagram showing a modified form of the programming of the digital computer used in the vehicle turning behavior control apparatus.

With reference to the drawings, and in particular to FIG. 1, there is shown a schematic diagram of a vehicle turning behavior control apparatus embodying the invention. The invention will be described in connection with an automotive vehicle supported on a pair of front wheels 1L and 1R spaced laterally apart at equal distances from a vehicle longitudinal axis and a pair of rear wheels 2L and 2R spaced laterally apart at equal distances from the vehicle longitudinal axis. The numeral 3 designates a brake pedal which actuates the pistons in a tandem master cylinder 4 and forces fluid through a first hydraulic system to wheel cylinders 5L and 5R situated in front wheel brakes for application of brakes to the respective front wheels 1L and 1R and also through a second hydraulic system to wheel cylinders 6L and 6R situated in rear wheel brakes for application of brakes to the respective rear wheels 2L and 2R.

The first hydraulic system includes a pressure responsive change-over valve 8F which has an inlet connected through a conduit 7F to the master cylinder 4 and an outlet connected through a pilot cylinder 9F to a conduit 10F. The change-over valve 8F normally occupies in a first position, illustrated in FIG. 1, to provide communication between the master cylinder 4 and the pilot cylinder 9F. The change-over valve 8F responds to a fluid pressure by changing to a second position permitting fluid flow from the master cylinder 4 to the pilot cylinder 9F but not vice versa. The pilot cylinder 9F includes a piston 9c provided for reciprocating movement within the pilot cylinder valve bore to define output and input chambers 9a and 9b on the opposite sides of the piston 9c. The piston 9c is urged toward the illustrated first position by means of a compression spring 9d placed within the pilot cylinder valve bore. When the input chamber 9b receives a fluid pressure, the piston 9c moves against the resilient force of the compression spring 9d to force fluid from the output chamber 9a to the conduit 10F. The conduit 10F is divided into two conduits 11F and 12F, the first conduit 11F being connected through a pressure control valve 13F to the wheel cylinder 5L provided for the left front wheel 1L. The second conduit 12F is connected through another pressure control valve 14F to the wheel cylinder 5R provided for the right front wheel 1R. A pump 20F is provided to supply fluid from a reservoir 19F to the conduit 10F when it is running and an accumulator 21F is charged up the fluid flow through the conduit 10F. The pressure control valve 13F operates on a current signal i1 fed thereto to occupy one of three positions. The first position, illustrated in FIG. 1, is occupied when the current signal i1 has a level of zero amperes and the pressure control valve 13F provides communication of the conduit 11F with the wheel cylinder 5L provided for the left front wheel 1L so as to increase the fluid pressure introduced to the wheel cylinder 5L. The second position is encountered when the current signal i1 has a level of two amperes and the pressure control valve 13F interrupts the communication between the conduit 11F and the wheel cylinder 5L so as to retain the fluid pressure in the wheel cylinder 5L. The third position is encountered when the current signal i1 has a level of five amperes and the pressure control valve 13F provides communication between the wheel cylinder 5L and the reservoir 19F so as to reduce the fluid pressure in the wheel cylinder 5L. The pressure control valve 14F operates on a current signal i2 fed thereto to occupy one of three positions. The first position, illustrated in FIG. 1, is occupied when the current signal i2 has a level of zero amperes and the pressure control valve 14F provides communication of the conduit 11F with the wheel cylinder 5R provided for the right front wheel 1R so as to increase the fluid pressure introduced to the wheel cylinder 5R. The second position is encountered when the current signal i2 has a level of two amperes and the pressure control valve 14F interrupts the communication between the conduit 11F and the wheel cylinder 5R so as to retain the fluid pressure in the wheel cylinder 5R. The third position is encountered when the current signal i2 has a level of five amperes and the pressure control valve 14F provides communication between the wheel cylinder 5R and the reservoir 19F so as to reduce the fluid pressure in the wheel cylinder 5R. The pump 20F runs when the pressure control valves 13F and 14F are in the second or third position.

Similarly, the second hydraulic system includes a pressure responsive change-over valve 8R which has an inlet connected through a conduit 7R to the master cylinder 4 and an outlet connected through a pilot cylinder 9R to a conduit 10R. The change-over valve 8R normally occupies in a first position, illustrated in FIG. 1, to provide communication between the master cylinder 4 and the pilot cylinder 9R. The change-over valve 8R responds to a fluid pressure by changing to a second position permitting fluid flow from the master cylinder 4 to the pilot cylinder 9R but not vice versa. The pilot cylinder 9R includes a piston 9c provided for reciprocating movement within the pilot cylinder valve bore to define output and input chambers 9a and 9b on the opposite sides of the piston 9c. The piston 9c is urged toward the illustrated first position by means of a compression spring 9d placed within the pilot cylinder valve bore. When the input chamber 9b receives a fluid pressure, the piston 9c moves against the resilient force of the compression spring 9d to force fluid from the output chamber 9a to the conduit 10R. The conduit 10R is divided into two conduits 11R and 12R, the first conduit 11R being connected through a pressure control valve 13R to the wheel cylinder 6L provided for the left rear wheel 2L. The second conduit 12R is connected through another pressure control valve 14R to the wheel cylinder 6R provided for the right rear wheel 2R. A pump 20R is provided to supply fluid from a reservoir 19R to the conduit 10R when it is running and a accumulator 21R is charged up the fluid flow through the conduit 10R. The pressure control valve 13R operates on a current signal i3 fed thereto to occupy one of three positions. The first position, illustrated in FIG. 1, is occupied when the current signal i3 has a level of zero amperes and the pressure control valve 13R provides communication of the conduit 11R with the wheel cylinder 6L provided for the left rear wheel 2L so as to increase the fluid pressure introduced to the wheel cylinder 6L. The second position is encountered when the current signal i3 has a level of two amperes and the pressure control valve 13R interrupts the communication between the conduit 11R and the wheel cylinder 6L so as to retain the fluid pressure in the wheel cylinder 6L. The third position is encountered when the current signal i3 has a level of five amperes and the pressure control valve 13R provides communication between the wheel cylinder 6L and the reservoir 19R so as to reduce the fluid pressure in the wheel cylinder 6L. The pressure control valve 14R operates on a current signal i4 fed thereto to occupy one of three positions. The first position, illustrated in FIG. 1, is occupied when the current signal i4 has a level of zero amperes and the pressure control valve 14R provides communication of the conduit 11R with the wheel cylinder 6R provided for the right rear wheel 2R so as to increase the fluid pressure introduced to the wheel cylinder 6R. The second position is encountered when the current signal i4 has a level of two amperes and the pressure control valve 14R interrupts the communication between the conduit 11R and the wheel cylinder 6R so as to retain the fluid pressure in the wheel cylinder 6R. The third position is encountered when the current signal i4 has a level of five amperes and the pressure control valve 14R provides communication between the wheel cylinder 6R and the reservoir 19R so as to reduce the fluid pressure in the wheel cylinder 6R. The pump 20R runs when the pressure control valves 13R and 14R are in the second or third position.

The pressure responsive change-over valves 8F and 8R and the pilot cylinders 9F and 9R are controlled by an automatic braking system including a solenoid change-over valve 18. The solenoid change-over valve 18 has three ports. The first port connected to the pressure responsive change-over valves 8F and 8R and also to the input chambers 9b of the respective pilot cylinders 9F and 9R. The second port is connected to a reservoir 16 from which a pump 15 supplies fluid to the third port of the solenoid change-over valve 18. An accumulator 17 is charged up the fluid flow to the third port of the solenoid change-over valve 18. The solenoid change-over valve 18 operates on a current signal i5 fed thereto to occupies one of two positions. The first position, illustrated in FIG. 1, is occupied when the current signal i5 has a level of zero amperes for the purpose of operating the braking system in a foot braking mode and the solenoid change-over valve 18 provides communication between its first and second ports. In the first position, thus, no fluid pressure is applied to the change-over valves 8F and 8R and the pilot cylinders 9F and 9R. Consequently, the fluid pressure discharged from the pilot cylinders 9F and 9R to the respective conduits 10F and 10R depends upon the degree of depression of the brake pedal 3. The second position is encountered when the current signal i5 has a level of two amperes for the purpose of operating the braking system in an automatic braking mode and the solenoid change-over valve 18 provides communication between its first and third ports. In the second position, thus, a constant fluid pressure is applied to change the change-over valves 8F and 8R to the second positions preventing back-flows from the pilot cylinders 9F and 9R to the master cylinder 4. The constant fluid pressure is also introduced into the input chambers 9b of the pilot cylinders 9F and 9R to force fluid from the output chambers 9a to the respective conduits 10F and 10R. Consequently, the fluid pressure discharged from the pilot cylinders 9F and 9R does not depend upon the degree of depression of the brake pedal 3 and it depends upon the fluid pressure from the accumulator 17.

The levels of the respective current signals i1, i2, i3, i4 and i5 are repetitively determined from calculations performed by an control unit 22, these calculations being made based upon various conditions of the automotive vehicle that are sensed during its operation. These sensed conditions include steering angle .theta., brake pedal depression, wheel speeds Vw1, Vw2, Vw3 and Vw4, vehicle lateral acceleration g, and steering speed .theta.'. Thus, a steering angle sensor 23, a brake switch 24, wheel speed sensors 25, 26, 27 and 28, and a lateral acceleration sensor 29 are connected to the control unit 22.

The steering angle sensor 23 is provided to sense the degree .theta. of rotation of the steering wheel with respect to its neutral position and it produces an electric signal indicative of the sensed steering angle .theta.. The steering angle indication signal has a sign indicating the direction to which the vehicle steering handle turns. The brake switch 24 is associated with the brake pedal 3 to close to supply current from the engine battery to the control unit 22 in response to the application of foot braking to the vehicle (when the brake pedal 3 is depressed). The wheel speed sensors 25, 26, 27 and 28 are provided to sense the peripheral speeds Vw1, Vw2, Vw3 and Vw4 of rotation of the respective wheels 1L, 1R, 2L and 2R and they produces electrical signals indicative of the sensed wheel speeds Vw1, Vw2, Vw3 and Vw4. The lateral acceleration sensor (lateral G sensor) 29 senses the vehicle lateral acceleration g and produces an electric signal indicative of the sensed lateral acceleration g. An appropriate vehicle steering speed sensor may be provided for producing an electric signal indicative of a sensed vehicle steering speed .theta.'. In the illustrated embodiment, the vehicle steering speed .theta.' is calculated based upon the sensed vehicle steering angle. This calculation will be described later in greater detail.

The numeral 31L, 31R, 32L and 32R designates fluid pressure sensors provided to sense the fluid pressures P1, P2, P3 and P4 introduced into the respective wheel cylinders 5L, 5R, 6L and 6R. The fluid pressure sensors produces electric signals indicative of the sensed fluid pressures P1, P2, P3 and P4 to the control unit 22. The control unit 22 employs these fluid pressure indication signals for providing feedback controls of the fluid pressures introduced into the respective wheel cylinders 5L, 5R, 6L and 6R.

The control unit 22 employs the signals from the wheel speed sensors 25, 26, 27 and 28 to provide anti-skid and traction controls. For a traction control, the control unit 22 outputs a control signal to an engine output controller.

When the brake pedal 3 is depressed, the control unit 22 receives a signal indicative of this condition from the brake switch 24 and sets the current signal i5 at zero amperes to hold the solenoid change-over valve 18 in the first (off) position, illustrated in FIG. 1. As a result, the change-over valves 8F and 8R and the pilot cylinders 9F and 9R are held in the illustrated positions. The control unit 23 sets the current signals i1 to i4 at zero amperes to hold the pressure control valves 13F, 14F, 13R and 14R in the positions, illustrated in FIG. 1, unless the wheels are locked by the brakes. Consequently, the fluid pressures supplied into the respective wheel cylinders 5L, 5R, 6L and 6R are substantially proportional to the force applied by the driver to the brake pedal 3.

The control unit 22 provides an anti-skid control. For this purpose, the control unit 22 employs the wheel speeds Vw1, Vw2, Vw3 and Vw4 to calculate a pseudo vehicle speed in a well-known manner. The control unit 22 employs the wheel speeds Vw1, Vw2, Vw3 and Vw4 to calculate braking slip factors for use in determining the brake lock conditions for the respective wheels. When there is a tendency toward a brake lock condition for one of the wheels, the control unit 22 sets the corresponding one of the current signals i1, i2, i3 and i4 at two amperes to change the corresponding pressure control valve to the second position retaining the fluid pressure in the corresponding wheel cylinder. If a brake lock condition occurs for the wheel, the control unit 22 sets the current signal at five amperes to change the corresponding control valve to the third position decreasing the fluid pressure in the corresponding wheel cylinder.

The control unit 22 provides a vehicle turning behavior control. For this purpose, the control unit 22 calculates a basic value .DELTA.P1 as a function of the vehicle steering angle .theta. to increase the basic value .DELTA.P1 as the vehicle steering angle .theta. increases, a correction factor K1 as a function of the vehicle steering speed .theta.' to increase the correction factor K1 as the vehicle steering speed .theta.' increases, and a difference .DELTA.P as .DELTA.P=.DELTA.P1.times.K1. The control unit 22 sets the pressure control valves 13F, 14F, 13R and 14R to provide the calculated difference .DELTA.P between the braking forces applied to the nearside and offside wheels of the vehicle. The offside wheel is subject to a smaller braking force than the nearside wheel so as to produce a yaw moment in a direction to which the vehicle turns.

The control unit 22 employs a digital computer including a central processing unit (CPU), a random access memory (RAM), a read only memory (ROM), and an input/output control unit (I/O). The central processing unit communicates with the rest of the computer via data bus. The read only memory contains the program for operating the central processing unit and further contains appropriate data in look-up tables used in calculating appropriate values for the drive current signals i1 to i5.

FIG. 2 is a flow diagram illustrating the programming of the digital computer. The computer program is entered at the point 102 at uniform intervals of time. At the point 104 in the program, the sensed vehicle steering angle .theta., and the sensed fluid pressures P1, P2, P3 and P4 are read into the random access memory. At the point 106 in the program, a vehicle steering speed .theta.' is calculated as .theta.'=.theta.-.theta.old where .theta. is the vehicle steering speed value read in the present cycle of execution of the program and .theta.old is the vehicle steering speed value read in the last cycle of execution of the program. Since this program is entered at uniform intervals of time, the difference (.theta.-.theta.') represents a rate of change of the vehicle steering angle. At the point 108 in the program, a determination is made as to whether or not the brake switch 24 is turned on. This determination is made based upon the current signal fed from the brake switch 24. If the answer to this question is "yes", then it means that the driver depresses the brake pedal 3 and the program proceeds to the point 110 for providing a vehicle turning behavior control.

At the point 110 in the program, a fluid pressure difference value .DELTA.P is calculated as a function of vehicle steering angle .theta. and vehicle steering speed .theta.'. For this purpose, the central processing unit calculates a basic fluid pressure difference value .DELTA.P1 from a relationship programmed into the computer. This relationship specifies the basic fluid pressure difference value .DELTA.P1 as a function of steering angle .theta.. One example of such a relationship is shown in FIG. 3 where the basic fluid pressure difference value .DELTA.P1 is zero when the steering angle .theta. is equal to or less than a predetermined value .theta.1 and it increases as the steering angle .theta. increases when the steering angle .theta. is greater than the predetermined value .theta.1. A correction factor K1 is used to modify the calculated basic fluid pressure difference value .DELTA.P1 so as to obtain the fluid pressure difference value .DELTA.P as .DELTA.P=K1.times..DELTA.P1. The correction factor K1 is calculated from a relationship programmed into the computer. This relationship defines the correction factor K1 as a function of vehicle steering speed .theta. '. One example of such a relationship is shown in FIG. 4 where the correction factor K1 increases as the vehicle steering speed .theta.' increases toward a predetermined maximum value (for example, 1.0). The calculated fluid pressure difference value .DELTA.P corresponds to a difference between the fluid pressures to be introduced into the wheel cylinders 5L and 5R provided for the respective front wheels 1L and 1R and also to the difference between the fluid pressures to be introduced into the wheel cylinders 6L and 6R provided for the respective rear wheels 2L and 2R.

At the point 112 in the program, target values P1L, P1R, P2L and P2R for the fluid pressures to be introduced into the wheel cylinders 5L, 5R, 6L and 6R provided for the respective wheels 1L, 1R, 2L and 2R are calculated. These target values are calculated in such a manner that a smaller braking force is applied to an outer or offside one of the front wheels positioned on the outside of a circle in which the vehicle moves than is applied to the other inner or nearside front wheel and a smaller braking force is applied to an outer or offside one of the rear wheels positioned on the outside of the circle than is applied to the other inner or nearside rear wheel. It is to be understood that the direction to which the cornering path is curved is determined based upon the sign of the signal from the steering angle sensor 23.

For example, when the vehicle is turning to the left, the right front wheel (offside wheel) 1R is subject to a smaller braking force than the left front wheel (nearside wheel) 1L and the right rear wheel (offside wheel) 2R is subject to a smaller braking force than the left rear wheel (nearside wheel) 2L. For this purpose, the central processing unit calculates a target value P1L for the fluid pressure to be introduced into the wheel cylinder 5L provided for the left front wheel 1L as P1L=P1 where P1 is the sensed fluid pressure introduced into the wheel cylinder 5L and a target value P1R for the fluid pressure to be introduced into the wheel cylinder 5R provided for the right front wheel 1R as P1R=P1-.DELTA.P. The central processing unit further calculates a target value P2L for the fluid pressure to be introduced into the wheel cylinder 6L provided for the left rear wheel 2L as P2L=P3 where P3 is the sensed fluid pressure introduced into the wheel cylinder 6L and a target value P2R for the fluid pressure to be introduced into the wheel cylinder 6R provided for the right rear wheel 2R as P2R=P3-.DELTA.P.

When the vehicle is turning to the right, the left front wheel 1L (offside wheel) is subject to a smaller braking force than the right front wheel (nearside wheel) 1R and the left rear wheel (offside wheel) 2L is subject to a smaller braking force than the right rear wheel (nearside wheel) 2R. For this purpose, the central processing unit calculates a target value P1R for the fluid pressure to be introduced into the wheel cylinder 5R as P1R=P2 where P2 is the sensed fluid pressure introduced into the wheel cylinder 5R and a target value P1L for the fluid pressure to be introduced into the wheel cylinder 5L provided for the left front wheel 1L as P1L=P2-.DELTA.P. The central processing unit further calculates a target value P2R for the fluid pressure to be introduced into the wheel cylinder 6R as P2R=P4 where P4 is the sensed fluid pressure introduced into the wheel cylinder 6R and a target value P2L for the fluid pressure to be introduced into the wheel cylinder 6L provided for the left rear wheel 2L as P2L=P4-.DELTA.P.

At the point 116 in the program, the central processing unit calculates target values for the current signals i1, i2, i3 and i4 applied to the respective pressure control valves 13F, 14F, 13R and 14R. These calculations are made based upon the calculated target values P1L, P1R, P2L and P2R. At the point 116 in the program, the calculate target current signal values are transferred to the input/output control unit. The input/output control unit sets the current signals i1, i2, i3 and i4 to cause the pressure control valves 13F, 14F, 13R and 14R to control the fluid pressures to the wheel cylinders 5L, 5R, 6L and 6R to the calculated target values P1L, P1R, P2L and P2R, respectively. In the illustrated case, the input/output control unit maintains the current signals i1 and i3 as they stand and takes the current signals i2 and i4 in the form of on-off signals to reduce the fluid pressures P2 and P4 when the vehicle moves along a left-handed cornering path and it maintains the current signals i2 and i4 as they stand and takes the current signals i1 and i3 in the form of on-off signals to reduce the fluid pressures P1 and P3 when the vehicle moves along a right-handed cornering path. The input/output control unit employs the fluid pressure sensors to provide feedback controls of the fluid pressures P1, P2, P3 and P4. After the calculated values are transferred to the input/output control unit, the program proceeds to the end point 120.

If the answer to the question inputted at the point 108 is "no", then it means that the brake pedal 3 is released and the program proceeds to the point 118 where the target fluid pressure value .DELTA.P is set at zero. Following this, the program proceeds to the point 112. In this case, the control unit 23 provide no vehicle turning behavior control when the brake pedal 3 is released.

Although the fluid pressure difference value .DELTA.P has been described as calculated as a function of steering angle .theta. and steering speed .theta.', it is to be understood that the fluid pressure difference value .DELTA.P may be calculated as a function of steering angle .theta., steering speed .theta.' and vehicle speed V. In this case, the fluid pressure difference value .DELTA.P is calculated as .DELTA.P=K1.times.K2.times..DELTA.P1 where K2 is a correction factor calculated from a relationship which specifies the correction factor K2 as a function of vehicle speed V. One example of such a relationship is shown in FIG. 5 where the correction factor K2 decreases from 1.0 as the vehicle speed V increases. The vehicle speed V may be calculated based upon the speeds of the driven wheels of the vehicle.

When the brake pedal 3 is depressed to apply a braking force to each of the wheels of the vehicle turning either to the right or to the left, the braking forces applied to the offside wheels are smaller than those corresponding to the brake pedal force. As a result, the vehicle is subject to a yaw moment in the direction to which the vehicle is turning so as to promote the tendency of the vehicle to turn in the direction. If the braking force difference is determined as a function of steering angle, the vehicle turning behavior control has a slow response rate with respect to a steering angle change when the steering wheel is turned at a high speed. According to this embodiment, the difference between the braking forces applied to the offside and inside wheels is corrected for the vehicle steering speed .theta.' in such a manner that the braking force difference increases as the vehicle steering speed .theta.' increases. As a result, the yaw moment increases as the rate .theta.' of change of the vehicle steering angle .theta. increases.

Although the vehicle turning condition is detected based upon the vehicle steering angle .theta., it is to be understood that the yaw rate or the lateral acceleration g sensed by the lateral acceleration sensor 29 may be used singly or in combination with the vehicle steering angle .theta..

FIG. 6 is a flow diagram illustrating a modified form of the programming of the digital computer. The computer program is entered at the point 202 at uniform intervals of time. At the point 204 in the program, the sensed steering angle .theta. and the sensed fluid pressures P1, P2, P3 and P4 are read into the random access memory. At the point 206 in the program, a vehicle steering speed .theta.' is calculated as .theta.'=.theta.-.theta.old where .theta. is the vehicle steering speed value read in the present cycle of execution of the program and .theta.old is the vehicle steering speed value read in the last cycle of execution of the program. Since this program is entered at uniform intervals of time, the difference (.theta.-.theta.old) represents a rate of change of the vehicle steering angle. At the point 208 in the program, a determination is made as to whether or not the brake switch 24 is turned on. This determination is made based upon the current signal fed from the brake switch 24. If the answer to this question is "yes", then it means that the driver depresses the brake pedal 3 and the program proceeds to the point 210 for providing a vehicle turning behavior control.

At the point 210 in the program, a fluid pressure difference value .DELTA.P is calculated as a function of steering angle .theta. and steering speed .theta.'. For this purpose, the central processing unit calculates a basic fluid pressure difference value .DELTA.P1 from a relationship programmed into the computer. This relationship specifies the basic fluid pressure difference value .DELTA.P1 as a function of steering angle .theta.. One example of such a relationship is shown in FIG. 3 where the basic fluid pressure difference value .DELTA.P1 is zero when the steering angle .theta. is equal to or less than a predetermined value .theta.1 and it increases as the steering angle .theta. increases when the steering angle .theta. is greater than the predetermined value .theta.1. A correction factor K1 is used to modify the calculated basic fluid pressure difference value .DELTA.P1 so as to obtain the fluid pressure difference value .DELTA.P as .DELTA.P=K1.times..DELTA.P1. The correction factor K1 is calculated from a relationship programmed into the computer. This relationship defines the correction factor K1 as a function of steering speed .theta.'. One example of such a relationship is shown in FIG. 4 where the correction factor K1 increases as the steering speed .theta.' increases. The calculated fluid pressure difference value .DELTA.P corresponds to a difference between the fluid pressures to be introduced into the wheel cylinders 5L and 5R provided for the respective front wheels 1L and 1R and also to the difference between the fluid pressures to be introduced into the wheel cylinders 6L and 6R provided for the respective rear wheels 2L and 2R.

At the point 214 in the program, target values P1L, P1R, P2L and P2R for the fluid pressures to be introduced into the wheel cylinders 5L, 5R, 6L and 6R provided for the respective wheels 1L, 1R, 2L and 2R are calculated. These target values are calculated in such a manner that a smaller braking force is applied to an outer or offside one of the front wheels positioned on the outside of a circle in which the vehicle moves than is applied to the other inner or nearside front wheel and a smaller braking force is applied to an outer or offside one of the rear wheels positioned on the outside of the circle than is applied to the other inner or nearside rear wheel. It is to be understood that the direction to which the cornering path is curved is determined based upon the sign of the signal from the steering angle sensor 23.

For example, when the vehicle is turning to the left, the right front wheel (offside wheel) 1R is subject to a smaller braking force than the left front wheel (nearside wheel) 1L and the right rear wheel (offside wheel) 2R is subject to a smaller braking force than the left rear wheel (nearside wheel) 2L. For this purpose, the central processing unit calculates a target value P1L for the fluid pressure to be introduced into the wheel cylinder 5L provided for the left front wheel 1L as P1L=P1 where P1 is the sensed fluid pressure introduced into the wheel cylinder 5L and a target value P1R for the fluid pressure to be introduced into the wheel cylinder 5R provided for the right front wheel 1R as P1R=P1-.DELTA.P. The central processing unit further calculates a target value P2L for the fluid pressure to be introduced into the wheel cylinder 6L provided for the left rear wheel 2L as P2L=P3 where P3 is the sensed fluid pressure introduced into the wheel cylinder 6L and a target value P2R for the fluid pressure to be introduced into the wheel cylinder 6R provided for the right rear wheel 2R as P2R=P3-.DELTA.P.

When the vehicle is turning to the right, the left front wheel 1L (offside wheel) is subject to a smaller braking force than the right front wheel (nearside wheel) 1R and the left rear wheel (offside wheel) 2L is subject to a smaller braking force than the right rear wheel (nearside wheel) 2R. For this purpose, the central processing unit calculates a target value P1R for the fluid pressure to be introduced into the wheel cylinder 5R as P1R=P2 where P2 is the sensed fluid pressure introduced into the wheel cylinder 5R and a target value P1L for the fluid pressure to be introduced into the wheel cylinder 5L provided for the left front wheel 1L as P1L=P2-.DELTA.P. The central processing unit further calculates a target value P2R for the fluid pressure to be introduced into the wheel cylinder 6R as P2R=P4 where P4 is the sensed fluid pressure introduced into the wheel cylinder 6R and a target value P2L for the fluid pressure to be introduced into the wheel cylinder 6L provided for the left rear wheel 2L as P2L=P4-.DELTA.P.

At the point 216 in the program, the central processing unit calculates target values for the current signals i1, i2, i3 and i4 applied to the respective pressure control valves 13F, 14F, 13R and 14R. These calculations are made based upon the calculated target values P1L, P1R, P2L and P2R. The central processing unit also a target value for the current signal i5 applied to the solenoid change-over valve 18. At the point 214 in the program, the calculate target current signal values are transferred to the input/output control unit. The input/output control unit sets the current signals i1, i2, i3 and i4 to cause the pressure control valves 13F, 14F, 13R and 14R to control the fluid pressures to the wheel cylinders 5L, 5R, 6L and 6R to the calculated target values P1L, P1R, P2L and P2R, respectively. In the illustrated case, the input/output control unit maintains the current signals i1 and i3 as they stand and takes the current signals i2 and i4 in the form of on-off signals to reduce the fluid pressures P2 and P4 when the vehicle moves along a left-handed cornering path and it maintains the current signals i2 and i4 as they stand and takes the current signals i1 and i3 in the form of on-off signals to reduce the fluid pressures P1 and P3 when the vehicle moves along a right-handed cornering path. The input/output control unit employs the fluid pressure sensors to provide feedback controls of the fluid pressures P1, P2, P3 and P4. The input/output control unit produces a current signal i5 having a set level to the solenoid change-over valve 18. After the calculated values are transferred to the input/output control unit, the program proceeds to the end point 228.

If the answer to the question inputted at the point 208 is "no", then it means that the brake pedal 3 is released and the program proceeds to another determination step at the point 218. This determination is as to whether or not it is a time at which the automatic braking mode is required. If the answer to this question is "yes", then it means that the vehicle is in a predetermined turning condition and the program proceeds to the point 220 where the current signal i5 is set at two amperes in order to energize the solenoid change-over valve 18 so as to place the braking system in an automatic braking mode.

At the point 222 in the program, a fluid pressure difference value .DELTA.P is calculated as a function of steering angle .theta. and steering speed .theta.'. For this purpose, the central processing unit calculates a basic fluid pressure difference value .DELTA.P1 from a relationship programmed into the computer. This relationship specifies the basic fluid pressure difference value .DELTA.P1 as a function of steering angle .theta.. One example of such a relationship is shown in FIG. 3 where the basic fluid pressure difference value .DELTA.P1 is zero when the steering angle .theta. is equal to or less than a predetermined value .theta.1 and it increases as the steering angle .theta. increases when the steering angle .theta. is greater than the predetermined value .theta.1. A correction factor K1 is used to modify the calculated basic fluid pressure difference value .DELTA.P1 so as to obtain the fluid pressure difference value .DELTA.P as .DELTA.P=K1.times..DELTA.P1. The correction factor K1 is calculated from a relationship programmed into the computer. This relationship defines the correction factor K1 as a function of steering speed .theta.'. One example of such a relationship is shown in FIG. 4 where the correction factor K1 increases as the steering speed .theta.' increases. The calculated fluid pressure difference value .DELTA.P corresponds to a difference between the fluid pressures to be introduced into the wheel cylinders 5L and 5R provided for the respective front wheels 1L and 1R and also to the difference between the fluid pressures to be introduced into the wheel cylinders 6L and 6R provided for the respective rear wheels 2L and 2R.

At the point 224 in the program, target values P1L, P1R, P2L and P2R for the fluid pressures to be introduced into the wheel cylinders 5L, 5R, 6L and 6R provided for the respective wheels 1L, 1R, 2L and 2R are calculated. These target values are calculated in such a manner that the fluid pressures (Pout) to be introduced into the wheel cylinders provided for the outer or offset wheels positioned on the outside of a circle in which the vehicle moves are set at zero and the fluid pressures (Pin) to be introduced into the wheel cylinders provided for the inner or nearside wheels positioned on the inside of the circle are set at the calculated fluid pressure difference .DELTA.P.

For example, when the vehicle is turning to the left, the central processing unit sets zero for a target value P1R for the fluid pressure to be introduced into the wheel cylinder 5R provided for the right front wheel 1R and .DELTA.P for a target value P1L for the fluid pressure to be introduced into the wheel cylinder 5L provided for the left front wheel 1L. The central processing unit further sets zero for a target value P2R for the fluid pressure to be introduced into the wheel cylinder 6R provided for the right rear wheel 2R and .DELTA.P for a target value P2L for the fluid pressure to be introduced into the wheel cylinder 6L provided for the left rear wheel 2L.

When the vehicle is turning to the right, the central processing unit sets zero for a target value P1L for the fluid pressure to be introduced into the wheel cylinder 5L provided for the left front wheel 1L and .DELTA.P for a target value P1R for the fluid pressure to be introduced into the wheel cylinder 5R provided for the right front wheel 1R. The central processing unit further sets zero for a target value P2L for the fluid pressure to be introduced into the wheel cylinder 6L provided for the left rear wheel 2L and .DELTA.P for a target value P2R for the fluid pressure to be introduced into the wheel cylinder 6R provided for the right rear wheel 2R. Following this, the program proceeds to the point 214.

If the answer to the question inputted at the point 218 is "no", then the program proceeds to the point 226 where the fluid pressure difference .DELTA.P is set at zero. Following this, the program proceeds to the point 224. In this case, neither vehicle turning behavior control nor no automatic braking operation is performed.

Although the fluid pressure difference value .DELTA.P has been described as calculated as a function of steering angle .theta. and steering speed .theta.', it is to be understood that the fluid pressure difference value .DELTA.P may be calculated as a function of steering angle .theta., steering speed .theta.' and vehicle speed V. In this case, the fluid pressure difference value .DELTA.P is calculated as .DELTA.P=K1.times.K2 X .DELTA.P1 where K2 is a correction factor calculated from a relationship which specifies the correction factor K2 as a function of vehicle speed V. One example of such a relationship is shown in FIG. 5 where the correction factor K2 decreases from 1.0 as the vehicle speed V increases. The vehicle speed V may be calculated based upon the speeds of the driven wheels of the vehicle.

In this modification, the vehicle turning behavior control is made during an automatic braking mode when the vehicle is turning either to the left or to the right. The difference between the braking forces applied to the offside and inside wheels is corrected for the steering speed .theta.' in a direction to increase the yaw moment as the steering speed .theta.' increases.

Although the vehicle turning condition is detected based upon the steering angle .theta., it is to be understood that the yaw rate or the lateral acceleration g sensed by the lateral acceleration sensor 29 may be used singly or in combination with the steering angle .theta..

Although the invention has been described in connection with a vehicle turning behavior control apparatus arranged to provide a difference between the braking forces applied to the respective front wheels and between the braking forces applied to the respective rear wheels, it is to be understood that the vehicle turning behavior control apparatus may be arranged to provide a difference between the braking forces applied to the respective front wheels or between the braking forces applied to the rear wheels.

In addition, the fluid pressure difference .DELTA.P may be obtained as a function of time. In this case, the fluid pressure difference .DELTA.P gradually decreases to zero as time progresses after the fluid pressure difference .DELTA.P is calculated once.

Although the fluid pressure sensors 31L, 31R, 32L and 32R are used to provide feedback control of the fluid pressures to the respective wheel cylinders 5L, 5R, 6L and 6R, it is to be understood that the vehicle turning behavior control apparatus may be arranged to provide an open loop control for these fluid pressures. In this case, the fluid pressure sensors may be removed.