GB1581943A - Anti-lock ontrol systems for braking systems for vehicles - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO ANTI-LOCK CONTROL SYSTEMS FOR BRAKING SYSTEMS FOR VEHICLES (71) We, ROBERT BOSCH GMBH, a German Company, of Postfach 50, 7 Stuttgart 1, Federal Republic of Germany, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to anti-lock control systems for braking systems for motor vehicles.

Efforts are in progress to simplify anti-lock control systems for braking systems for vehicles by combining separate control functions; however, such control systems have substantial operational disadvantages according to road surface and loading conditions. Thus it is known, for example, to use a common pressure control device for two separate wheels. In such a case, optimal braking of only one of the two wheels is possible, of course. In particular, in the case of an axlewise wheel combination, extreme differences in the frictional coefficients of the road surface, which affect the vehicle laterally, for example slush at the edge of the road and dry concrete in the middle of the road, result in substantial underbraking or overbraking, according to the method of control, of the individual wheels.

Basically, the control system of a motor vehicle may be arranged according to three different criteria, whose effects are clearly different, particularly when road conditions differ on both sides of the vehicle.

1. "Select low" control In this case, brake pressure is adjusted with reference to the wheel which has the greatest amount of slip (the lowest wheel speed); that is, only the low value of frictional efficiency is used. The result is an unacceptably long vehicle stopping distance.

2. "Select high" control In a "select high" control system, the brake pressure is adjusted with reference to the wheel which has the least amount of slip (the highest wheel speed); that is, the high value of frictional efficiency is used, so that substantially improved braking of the vehicle is achievable.

The resultant tractional stability of a locking wheel, or, should there be other antilocking axles for which the same selection criteria are used, the locking wheels, is, however, substantially reduced when frictional efficiency is low. In addition, owing to no reference speed being provided, axle control purely according to the "select high" method is not absolutely proof against locking when the control is operating in borderline regions.

3. "Select high/select low" control A control system which uses a "select high/select low" combination for the respective axle does, in fact, as compared with purely "select low" control, enable improved vehicle braking to be achieved; however, owing to the constant alternation of the "select high/select low" combination, an intermittent yawing moment is unavoidable.

Reduced comfort, and ultimately substantial steering corrections, are the negative results.

According to the present invention an anti-lock control system for a braking system for a motor vehicle having at least two wheels on at least two axles comprises control means individual to each axle for regulating the brake pressure applied to brakes of the wheels of that axle and selectively responsive to a "select high" control signal or a "select low" control signal, control circuits individual to each axle for deriving from the rotational speeds of the wheels of that axle a first signal that can be used as the "select high" control signal and a second signal that can be used as the "select low" control signal, and a combinational control circuit responsive to the first and second signals of each axle for selecting when the control means individual to which axle is to be responsive to "select high" control signals and, simultaneously with switching of the control means individual to the selected axle to be responsive to "select high" control, switching the control means individual to the other axle to "select low" control and vice versa.

Preferably the axle loading of a motor vehicle axle is detectable by means of a pressure switch, by means of which a signal can be supplied to the combinational control circuit when a threshold value is exceeded.

Different criteria for determining the highest wheel or axle loading at any time are conceivable: If, in the vehicle to be fitted with an anti-lock control system, there is installed a device for determining the loading, such as is necessary, for example, for regulating the length of the headlamp beam or for load-dependent control of brake pressure (General Delivery and Performance Regulations), only a simple switch is necessary, which, by means of a switch contact (electrical or mechanical), or alternatively by means of a predetermined pressure threshold or stroke threshold, makes it possible to determine the loading condition in which the wheel loading is greater in the case of this axle.

The present invention is further described hereinafter, by way of example, with reference to the accompanying drawings, in which: Fig. 1 shows curves of percentage slip against coefficient of friction, such as occur in asymmetrical road surface conditions, with corresponding frictional efficiency, Fig. 2 shows an anti-lock control system according to a first embodiment of the invention, Fig. 3 shows part of a control circuit for the system of Fig. 2, Fig. 4 shows an anti-lock control system according to a second embodiment of the invention, and Fig. 5 shows part of a control circuit for the system of Fig. 4.

Fig. 1 shows a graph, in which the percentage wheel slip is shown on the abscissa, and the coefficient of friction u. is shown on the ordinate. A slip curve 1 for high frictional efficiency ("select high"), and a -slip curve 2 for low frictional efficiency ("select low"), are shown on the graph. The "select high/ select low" control range lies between the highest point of the slip curve 1 and the lowest point of the slip curve 2, and is denoted by the reference numeral 3. This is the control range to be covered by the herein proposed control system.

An anti-lock control system in accordance a first embodiment of the invention is illustrated in Fig. 2 and enables this object to be achieved.

A front axle 4, having two wheels 5 and 6, has a sensor 7, 8 and a brake cylinder 9, 10 for each respective wheel. A pressure control valve 12, by means of which the brake pressure in the brake cylinders 9 and 10 can be regulated in order to prevent locking, is provided in a brake line 11, which connects with the brake cylinders 9 and 10.

The two front-axle wheel sensors 7 and 8 are connected to an electronic control circuit 13, to which are connected also two rear-axle wheel sensors 14 and 15. A rear axle 16 has two pairs of wheels 17 and 18, and a brake cylinder 19, 20 for each pair. The brake cylinders 19 and 20 are connected via a brake line 21 to a pressure control valve 22, which regulates the brake pressure in the brake cylinders 19 and 20 in order to prevent locking.

A device 23 for sensing the loading of the vehicle is connected by means of a line 24 to the pressure control valve 22, and a pressure switch 25, which operates as a threshold value switch and which is connected by means of its own electrical lead 26 to the electronic control circuit 13, is provided in the line 24.

Part of a control circuit suitable for the two-axle vehicle in accordance with Fig. 2 and which provides an economical method of achieving a "select high/select low" combination is illustrated in Fig. 3.

Two frequency-to-voltage converters 27, 28; 29,30 in the electronic control circuit 13 receive output signals from the respective sensors 7,8; 14,15 of each of the two axles 4 and 16 and these form part of the control circuits individual to each axle. The maximum values circuits 31, 32 and the mean values (circuits 33, 34) are compared in a combinational control circuit in the control circuit 13. Comparison (in circuits 35 or circuit 36) of the maximum and mean values provided by the output signals of the frequency-to-voltage converters of a respective axle provides a reliable criterion for control according to the "select high" control when the difference between these two values (i.e. maximum value and mean value for an axle) exceeds a relatively high predetermined value. For the actual control of the regulated brake pressures only one frequency-to-voltage converter signal is used to regulate the brake pressure for each axle: more specifically when one axle, for example the front axle, uses for example the left-hand wheel for controlling, i.e. a signal from the frequency-to-voltage converter 27 regulates the control valve 12, the other axle, i.e. the rear axle, uses the right hand wheel for controlling, i.e. the signal from the frequencyto-voltage converter 30 regulates the control valve 22, and vice versa. The brake pressures of wheels not used for controlling are regulated simultaneously with the brake pressures of the controlling wheel of the respective axle.

This ensures that on asymmetrical road surfaces one axle is always controlled according to the "select high" control, and the other is controlled simultaneously according to the "select low" control.

A selector store 39 in the electronic control circuit 13 switches over this selection of the frequency-to-voltage converter when a corresponding input signal indicates the necessity. Switching of the selector store 39 takes place when the "select high" control is indicated for the front axle (logic 1 signal from the output of the circuit 35) and the greater wheel loading is on the rear axle logic 1 signal from selector switch 40). The wheel loading as detected by the device 23 is evaluated as a need for the selector switch 40 to switch over. The inverse case also causes the desired switching, namely when "select high" control is indicated for the rear axle (logic 1 signal from the circuit 36) and the smaller wheel loading is on the rear axle (logic 0 signal from the selector switch 40).

The initial state of the selector store 39 is optional, but it may be preset for a preferred state.

Fig. 4 shows a vehicle having a different control system. The reference numerals used correspond to those used in Fig. 2. In this case no load sensing device is provided in the vehicle fitted with the anti-lock control system. However, in this case also it is possible to sense the state of loading approximately by means of the brake pressure regulated by means of the respective anti-lock brake pressure control valve 12, 22.

However, since in this case the coefficient of friction at the road surface is also a component determining the wheel locking brake pressure, in accordance with the equation p = f(, G) where p = brake pressure, = coefficient of friction, G = axle loading, additional electronic circuitry is necessary as compared with the embodiment of Fig. 3; this circuitry is shown in Fig. 5.

Since measurement of brake pressure is performed on the axle which shows the greatest variations in loading, it is first necessary to ensure that this axle switches to "select high" control at the commencement of controlled braking in order to sense the maximum transmissible brake pressure. This is achieved by the triggering of a test time circuit 37 at the commencement of a brake pressure regulation process by means of an OR gate Oi and an AND gate U1. During the test time of the test time circuit 37 (logic 1 output signal), it is established whether the "select high" control is indicated for the front axle (logic 1 output signal from the circuit switch 35) without the pressure switch 25 having responded. These conditions serve to cause switching of a selector store 38 which performs the same function as the selector store 39 of Fig. 3.The selector store 38 establishes that "select high" rear axle control is maintained. If, notwithstanding "select high" rear axle control, the pressure switch does not respond, switching to "select high" control of the front axle takes place.

The control system ensures that substantially always the more heavily loaded vehicle axle is "select high" controlled. Efficient braking can thus be achieved in every case. It is necessary only to measure the loading indirectly or directly, and all the wheels of the vehicle must be provided with sensors.

WHAT WE CLAIM IS: 1. An anti-lock control system for a braking system for a motor vehicle having at least two wheels on at least two axles, comprising control means individual to each axle for regulating the brake pressure applied to brakes of the wheels of that axle and selectively responsive to a "select high" control signal or a "select low" control signal, control circuits individual to each axle for deriving from the rotational speeds of the wheels of that axle a first signal that can be used as the "select high" control signal and a second signal that can be used as the "select low" control signal, and a combinational control circuit responsive to the first and second signals of each axle for selecting when the control means individual to which axle is to be responsive to "select high" control signals and, simultaneously with switching of the control means individual to the selected axle to be responsive to "select high" control, switching the control means individual to the other axle to "select low" control and vice versa.

2. A system as claimed in claim 1, wherein a device is used for sensing, directly or indirectly, the loading of a said axle, and the axle having the greatest axle loading is controllable by means of the combinational control circuit.

3. A system as claimed in claim 2, wherein said axle having the greatest axle loading is controllable in accordance with the "select high" control method.

4. A system as claimed in claim 2 or 3 wherein the loading of an axle is detectable by means of a said device in the form of a pressure switch, by means of which a signal may be supplied to the combinational control circuit when a threshold value is exceeded.

5. Anti-lock control systems for braking systems for vehicles, constructed and arranged and adapted to operate substantially as hereinbefore particularly described with reference to Figs. 1, 2 and 3 or Figs. 1, 4 and 5 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. surfaces one axle is always controlled according to the "select high" control, and the other is controlled simultaneously according to the "select low" control. A selector store 39 in the electronic control circuit 13 switches over this selection of the frequency-to-voltage converter when a corresponding input signal indicates the necessity. Switching of the selector store 39 takes place when the "select high" control is indicated for the front axle (logic 1 signal from the output of the circuit 35) and the greater wheel loading is on the rear axle logic 1 signal from selector switch 40). The wheel loading as detected by the device 23 is evaluated as a need for the selector switch 40 to switch over. The inverse case also causes the desired switching, namely when "select high" control is indicated for the rear axle (logic 1 signal from the circuit 36) and the smaller wheel loading is on the rear axle (logic 0 signal from the selector switch 40). The initial state of the selector store 39 is optional, but it may be preset for a preferred state. Fig. 4 shows a vehicle having a different control system. The reference numerals used correspond to those used in Fig. 2. In this case no load sensing device is provided in the vehicle fitted with the anti-lock control system. However, in this case also it is possible to sense the state of loading approximately by means of the brake pressure regulated by means of the respective anti-lock brake pressure control valve 12, 22. However, since in this case the coefficient of friction at the road surface is also a component determining the wheel locking brake pressure, in accordance with the equation p = f(, G) where p = brake pressure, = coefficient of friction, G = axle loading, additional electronic circuitry is necessary as compared with the embodiment of Fig. 3; this circuitry is shown in Fig. 5. Since measurement of brake pressure is performed on the axle which shows the greatest variations in loading, it is first necessary to ensure that this axle switches to "select high" control at the commencement of controlled braking in order to sense the maximum transmissible brake pressure. This is achieved by the triggering of a test time circuit 37 at the commencement of a brake pressure regulation process by means of an OR gate Oi and an AND gate U1. During the test time of the test time circuit 37 (logic 1 output signal), it is established whether the "select high" control is indicated for the front axle (logic 1 output signal from the circuit switch 35) without the pressure switch 25 having responded. These conditions serve to cause switching of a selector store 38 which performs the same function as the selector store 39 of Fig. 3.The selector store 38 establishes that "select high" rear axle control is maintained. If, notwithstanding "select high" rear axle control, the pressure switch does not respond, switching to "select high" control of the front axle takes place. The control system ensures that substantially always the more heavily loaded vehicle axle is "select high" controlled. Efficient braking can thus be achieved in every case. It is necessary only to measure the loading indirectly or directly, and all the wheels of the vehicle must be provided with sensors. WHAT WE CLAIM IS:

1. An anti-lock control system for a braking system for a motor vehicle having at least two wheels on at least two axles, comprising control means individual to each axle for regulating the brake pressure applied to brakes of the wheels of that axle and selectively responsive to a "select high" control signal or a "select low" control signal, control circuits individual to each axle for deriving from the rotational speeds of the wheels of that axle a first signal that can be used as the "select high" control signal and a second signal that can be used as the "select low" control signal, and a combinational control circuit responsive to the first and second signals of each axle for selecting when the control means individual to which axle is to be responsive to "select high" control signals and, simultaneously with switching of the control means individual to the selected axle to be responsive to "select high" control, switching the control means individual to the other axle to "select low" control and vice versa.

2. A system as claimed in claim 1, wherein a device is used for sensing, directly or indirectly, the loading of a said axle, and the axle having the greatest axle loading is controllable by means of the combinational control circuit.

3. A system as claimed in claim 2, wherein said axle having the greatest axle loading is controllable in accordance with the "select high" control method.

4. A system as claimed in claim 2 or 3 wherein the loading of an axle is detectable by means of a said device in the form of a pressure switch, by means of which a signal may be supplied to the combinational control circuit when a threshold value is exceeded.

5. Anti-lock control systems for braking systems for vehicles, constructed and arranged and adapted to operate substantially as hereinbefore particularly described with reference to Figs. 1, 2 and 3 or Figs. 1, 4 and 5 of the accompanying drawings.