GB2321684A - A vehicle brake system with electric-motor driven brake actuators - Google Patents

Abstract

A vehicle brake system includes wheel brakes 100, 102, 104, 106 having electric motor-driven actuators 116, 118, 120, 122, and two batteries 140, 148 each associated with a respective pair of diagonally-opposed wheel brakes. In alternative embodiments, the two batteries are each associated with the wheel brakes of a respective axle or on a respective side of the vehicle, and in a further embodiment the brake system has only one battery. To reduce the loading of the vehicle electrical system in at least one braking situation e.g. emergency, or panic, braking, maximum current is supplied to the front axle brake actuator motors before maximum current is supplied to the rear axle brake actuator motors. If the brake pedal is released very quickly the rear brakes are released before the front brakes. The braking level may be dependent on factors such as axle load, vehicle speed, and brake lining wear as well as on brake pedal operation.

Description

Wheel brake system for vehicles with electric motor-driven brake actuators Prior Art The invention concerns a wheel brake system for a vehicle, with electric motor-driven brake actuators.

Electric motor-driven wheel brakes for vehicles are known. For example, a wheel brake is described in WO A 94/24453, in which the brake application force is generated by an electric motor. If the braking system of a vehicle is constructed with such types of electric motor-driven wheel brakes, special attention should be paid to the reliable operation of such a braking system, particularly the reliability of the power supply provided by batteries.

It is the object of the invention to develop a braking system having electric motordriven wheel brakes in such a way that the load on the batteries is as small as possible while maintaining the braking effect. This is achieved by the features of claim 1.

Advantages of the invention The invention may be used to provide a wheel-brake system in which the loading of batteries supplying the motors with current can be appreciably reduced with negligible loss of braking effect.

The braking system may be hereby reliably controlled in a particularly advantageous manner, even in situations in which the driver applies heavy braking, panic braking or full braking.

The brake system of the invention enables reliable control even when the coefficient of friction of the roadway falls suddenly from a high to a very low value and the brake is released very rapidly.

The preferred feature of claim 2 ensures that reliable control of the brakes is guaranteed even when at least one of the batteries is exhausted.

Further advantages are disclosed in the following description of exemplary embodiments or in the dependent claims.

Attention is directed to copending application 9620235.3 (publication No. 2305988) from which the present application is divided.

Drawing The invention is explained in further detail below with the aid of the embodiments illustrated in the drawing. Here Figure 1 shows a block diagram of a preferred development of a braking system with an electric motor drive. Figure 2 shows a flowchart which outlines the implementation of the braking control as a computer program. Finally, in Figure 3 the mode of operation of the braking control is explained with the aid of timing diagrams.

Description of exemplary embodiments Figure 1 shows a preferred development of an electric motor-driven braking system for vehicles. In Figure 1 are shown four wheel brakes which are assigned to the four wheels of the vehicle. Here the wheel brake 100 is allocated to the right-hand rear wheel, wheel brake 102 to the left-hand front wheel, wheel brake 104 to the left-hand rear wheel and wheel brake 106 to the right-hand front wheel. The wheel brakes are connected via mechanical links 108, 110, 112 and 114 to electric motor-driven actuators 116, 118, 120 and 122, which are assigned to each wheel brake. The electric motor-driven actuators are activated by an electronic control unit 124 via output lines 126, 128, 130 and 132. For reasons of safety and reliability, the vehicle electrical system which supplies the actuators with voltage is constructed with built-in redundancy. At the same time, each of the two brakes belonging to diagonallyopposed wheels is connected to one of the two batteries, the corresponding actuators being supplied from the associated battery. In this sense, the actuators of the righthand rear wheel 116 and of the left-hand front wheel 118 via supply lines 134 and 136, respectively, are connected to the positive pole 138 of a first battery 140.

Correspondingly, the actuators of the left-hand rear wheel 120 and of the right-hand front wheel 122 are linked via supply lines 142 and 144 to the positive pole 146 of a second battery 148.

At least one parameter for the degree of operation of the brake pedal is fed from a pedal valuator device 152 via an input line 150 to the electronic control unit 124.

Furthermore, input lines 154 to 156 are led to the electronic control unit 124 from measuring devices 158 to 160, which measure the performance variables of the braking system and/or of the vehicle. Such performance variables are for example wheel loads, a measure of the actual-value of the braking effect at each wheel brake (e.g. current, braking torque, braking force, slip, rotational speed of wheel), the wheel speeds, the vehicle speed, brake lining wear, etc. Moreover, the electronic control unit 124 is supplied with current from at least one of the batteries 140 and 148. For the sake of clarity, the corresponding connecting lead is not shown in Figure 1.

Besides the diagonal distribution of the braking circuits as shown, a braking circuit distribution is specified in another advantageous exemplary embodiment, which combines the wheel brakes of one side of the vehicle or of one axle in a braking circuit, so that a power source is assigned to actuators which belong to different pairs of wheels.

The basic mode of operation of the controller of such a braking system is as follows.

From the degree of actuation of the brake pedal and additional performance variables such as wheel loads, brake lining wear, tyre size, battery voltage, etc., the electronic control unit 124 generates for each wheel brake a setpoint value which represents a value for the braking effect to be applied to the wheel brake (e.g. braking torque, braking force, current through the electric motor, angle of rotation of the electric motor, wheel slip, wheel speed, etc.). Control units (not illustrated) which, according to a predetermined control strategy (e.g. PID), generate output signals representing approximations of the actual-values to the setpoints, control the braking effect to the specified setpoints. In this case the actual-values are either measured at the wheel brakes or calculated from variables measured at those points. For example, the contact pressure of the brake linings or the drive torque of the electric motor, which is a direct indication of the braking effect applied to the corresponding wheel brake, can be deduced from the current through the electric motor. Furthermore, the applied braking force can measured by strain gauges or other suitable sensors and the applied braking torque determined from the brake design. The angle of rotation can be determined by suitable angular sensors on the wheel brakes.

The electronic control unit 124 converts the control unit output signals for each wheel brake into drive signals for the corresponding actuators. Depending on the design of the motors driving the electrical actuators, whether stepping motors or commutator motors or motors with electronic commutation, the drive signal represents a number of steps to be output, a mark-space ratio, a voltage value, a current value, etc. In the preferred exemplary embodiment, commutator motors through which a pulse-shaped drive signal with variable mark-space ratio can pass in both directions, are used to drive the actuators. The mark-space ratio is generated by at least one microcomputer of the control unit 124 in accordance with the control unit output signals to rotate the commutator motor against restoring forces to a predetermined position.

In some braking situations, for example panic braking, during which the driver operates the brake pedal very quickly and more or less to its full extent, in the above control system all electric motors draw maximum current in order to ensure very rapid application (e.g. in 150 milliseconds) of the braking devices, even on roadway surfaces with high friction. High peak currents are then demanded of the batteries which supply the electric motors With weakened batteries there is then the danger that the battery voltage collapses and the brakes cannot be operated as required.

To reduce the maximum current load of the batteries in these braking situations (panic braking, full braking, emergency braking with run-down batteries, etc.), in a preferred exemplary embodiment, maximum current is fed to the electric motors for the front wheel brakes. Te electric motors for the rear wheel brakes are initially operated with limited current so as to overcome the brake clearance and contact the brake linings. After the front wheel brakes have achieved their maximum effect, the current reduction of the rear brake wheels is removed and maximum current is likewise applied to these.

In addition to supplying maximum current to the electric motors in the case of panic braking, in another advantageous exemplary embodiment in the intended control system the front wheels are controlled in this or the other above-mentioned braking situations in relation to a setpoint and an actual-value, while the rear wheel brakes are only actuated with a delay in accordance with the procedure stated above.

Accordingly, during very rapid release of the brake, which becomes necessary in the case of a negative change in the coefficient of friction if the coefficient of friction of the roadway suddenly falls to a very low value (e.g. from a roadway with good grip to black ice) during braking, especially full braking, the rear wheel brakes are released first and then the front wheel brakes after a certain delay.

These measures result in a reduction in the maximum load on the vehicle electric system without appreciable lengthening of the braking distance, since the major part of the total braking force - usually about 2/3 - is applied by the front wheels.

The control of the braking system as described on a preferred exemplary embodiment is illustrated as a flowchart in Figure 2. There the electronic control unit 124 is fitted with at least one microcomputer to which the above-mentioned performance variables are fed and which generates the said output signals.

The program section illustrated in Figure 2 is started at predetermined time intervals.

In the first step 200 the performance variables required for control, e.g. axle load, vehicle speed, brake lining wear, etc., the degree of operation of the brake pedal, as well as the evaluated control actual-value for the braking effect (e.g. braking torque, braking force, braking current, etc.) are read in. In the following step 202 the setpoints for the braking effect of the individual wheel brakes are determined in relation to the degree of operation of the brake pedal, and where applicable, the performance variables according to predetermined characteristics, tables and/or calculation steps. The following interrogation step 204 checks if there is an operational situation which could cause the vehicle electrical system to be overloaded. In the preferred exemplary embodiment, this is the so-called panic braking in which the driver operates the brake pedal very quickly and more or less to its full extent in order to obtain the maximum braking effect. Other operating situations are full braking where the brake pedal is fully operated, or braking processes where at least one battery is run down (emergency operation). If none of these operating situations applies, i.e. under normal operating conditions, according to step 206 the drive signal variable, in the preferred exemplary embodiment the mark-space ratio T, is determined in the control loop for each wheel brake according to setpoints and actual-values and is output. The program section is ended after step 206 and is restarted at the next predetermined time interval.

In the preferred exemplary embodiment, step 204 checks whether a panic braking situation exists. This is effected by taking into account the operating speed of the brake pedal and, if applicable, the absolute value of the degree of operation. If the speed at which the brake pedal is operated exceeds a predetermined value and the degree of operation of the brake pedal is above a specified value, a panic braking situation is recognized. If the degree of operation of the brake pedal exceeds a predetermined threshold, panic braking is assumed to have ended. In the first case the "yes" result is established in step 204 and in the latter case the result is "no".

If panic braking exists, then the driver wants maximum braking effect, because of a hazardous situation, for example. Therefore in step 208 the drive signal variables for the front wheel brakes rVL and TVR are set to the maximum specified value. On the other hand, the drive signal variables for the rear wheel brakes TH1 and THR are set to a predetermined value TO which applies the brake pads and overcomes the brake clearance. This achieves maximum braking effect at the front wheel brakes and in this way meets the driver's wishes and at the same time reducing the load on the vehicle electrical system since the rear wheel brakes draw scarcely any current.

Interrogation step 210 checks if the front wheel brakes develop their maximum braking effect. This is preferably effected with the help of the control actual-value which is compared to a predetermined value indicating the maximum braking effect, with the aid of the vehicle deceleration or when an anti-locking system controller is activated. If this is the case, according to step 212 the drive signal for the rear axle brakes is also set to the specified maximum value, which is possibly less than that of the front wheel brakes. An anti-lock system control is of course superimposed on the braking controller illustrated in Figure 2, which ensures that the wheels do not lock in the braking situation described. According to a known procedure, the anti-locking system controller acts on the controlled variable of the brake of the wheel tending to lock up; in the present case it reduces the braking current or the drive signal, or reverses it direction. It is still more advantageous if it reduces the current in the front wheel brakes immediately to zero and actively releases the front wheel brakes, i.e. by reversing the current, after actively releasing the rear wheel brakes.

After steps 206, 212, or in the case of a "no" response after step 210, according to an advantageous addition, step 214 checks if the driver is releasing the brake very quickly. Like in step 204, the speed at which the brake pedal is being operated is utilized and compared to the specified setpoint. If the brake pedal is not released quickly, the program section is ended and the predetermined time period restarted.

If rapid release of the brakes is detected, then according to a preferred exemplary embodiment of the invention, in step 216 the drive signal parameters for the rear wheel brakes tHL, THR are determined from the current setpoints and actual-values (maximum current in the opposite direction). This results in immediate release of the rear axle brakes as desired by the driver. Initially, the drive signal parameters for the front wheel brakes remain unchanged. After the expiry of a predetermined time delay in step 218, according to step 220 the drive signal parameters for the front wheel brakes tVR and TVL are determined from the current setpoints and actual-values and the program section is ended. The maximum time delay amounts to a few 100 milliseconds and is designed to ensure complete release of the rear axle brakes within the delay interval.

As mentioned above, the procedure is preferably is implemented in an advantageous way not only during so-called panic braking, but also in other braking situations. In the case of full braking, a corresponding action take place in accordance with steps 208 to 220. If the brake system according to the invention is used as an emergency braking operation when the battery is run down, maximum braking effect is not attempted in steps 208, 212 and 210, but the drive signal variables are determined according to the setpoints and actual-values. The question asked in step 210 is then whether the front wheel brakes have achieved the intended braking effect (setpoint).

The preferred exemplary embodiment illustrated in Figure 2 is explained in Figure 3 with the aid of timing diagrams. Here Figure 3a, for example, shows the characteristic of the degree of operation PW of the brake pedal in a panic braking situation. Figure 3b shows the current waveform resulting from the solution according to the invention at a front wheel and a rear wheel, while Figure 3c illustrates the waveform of the applied braking torque at a selected front wheel and a selected rear wheel.

The driver operates the brake pedal very quickly and almost completely. The rate of actuation and the degree of operation itself exceed the prescribed setpoints, so that a panic braking situation is detected. The result of this is that at the time TO, or shortly thereafter, as shown in Figure 3b, the current in the front wheel brakes is controlled to its maximum value and in the rear wheel brakes to a value 10.

Correspondingly, the applied braking torque for the front wheel brakes rises at the time TO from zero to its maximum value, while at the rear wheel brakes no appreciable braking effect has yet occurred. At time T 1, the system detects that the maximum braking effect is being applied at the front wheel brakes. As shown in Figures 3b and 3c, this causes at time T1 an increased current in the rear wheel brakes, with a corresponding increase in braking effect up to the intended maximum value. During panic braking, the predetermined values (possibly corrected by an antilocking system control) are maintained. At time T3 the driver releases the brake pedal very quickly. The rate of actuation exceeds a predetermined limiting value, so that rapid release of the brake is necessary. According to the action described above, from time T3 the current in the rear wheel brakes is reduced to zero in accordance with the requirement and then increased in the opposite direction if necessary. The braking torque falls accordingly. After a specified time delay the corresponding action is applied to the front wheel brakes from time T4.

If at time T3 the driver releases the pedal slowly and the degree of operation falls below the prescribed threshold, the end of the panic braking situation is detected and front wheel and rear wheel brakes are then simultaneously set to the then existing setpoints and actual-values (normal operation).

Besides the use of the braking controller described above in braking systems having a redundant vehicle electrical system, this is also used in braking systems which have only one power source (battery).

Claims (12)

Claims:

1. A vehicle wheel brake system with electric motor-driven brake actuators, having an electronic control unit which at least in relation to the driver's braking request, applies drive signals for the electric motor-driven actuators of the brakes assigned to the wheels, the actuators being supplied with current from at least one power source, wherein in at least one braking situation the wheel brakes of the front axle and those of the rear axle are operated one after the other.

2. Vehicle wheel brake system according to claim 1, wherein one power source is assigned to one front wheel brake and one rear wheel brake and another power source is assigned to the other front wheel brake and the other rear wheel brake.

3. Vehicle wheel brake system according to claim 1, wherein at least one of the prescribed braking situations is a panic braking situation, a full braking situation or an emergency braking situation with a run-down battery.

4. Vehicle wheel brake system according to any preceding claim, wherein in at least one braking situation the front wheel brakes of the vehicle are supplied with maximum current and on reaching a predetermined braking effect the rear wheel brakes of the vehicle are also supplied with maximum current.

5. Vehicle wheel brake system according to any preceding claim, wherein in panic braking situations the front wheel brakes are first supplied with maximum current and on reaching the maximum braking effect at the front wheels the rear axle brakes are supplied with maximum current.

6. Vehicle wheel brake system according to claim 5, wherein the maximum braking effect is detected with the aid of the measured actual torque, the measured braking force, the measured current or when an anti-locking system control is activated.

7. Vehicle wheel brake system according to any preceding claim, wherein the brakes of the rear wheels are first supplied with a current value that ensures that brake linings of the brakes make contact and a brake clearance is overcome.

8. Vehicle wheel brake system according to any preceding claim, wherein drive signals for the actuators are set according to a setpoint specified by the driver and a measured actual value of braking effect.

9. Vehicle wheel brake system according to claim 8, wherein in the emergency braking mode with a run down battery, the front wheel brakes are set according to the setpoint and after reaching the desired braking effect the rear axle brakes are set according to the setpoint.

10. Vehicle wheel brake system according to any preceding claim, wherein the rear axles brakes are released first and the front wheel brakes are released after a predetermined time delay.

11. Vehicle wheel brake system according to any preceding claim, wherein an anti-locking system is provided which, if there is a tendency for at least one of the wheels to lock, the current in the front wheel brakes is immediately reduced to zero, the rear axle brakes are released and after the rear axle brakes are released the front axle brakes are released by reversing the current.

12. Vehicle wheel brake system for a vehicle, substantially as herein described with reference to the accompanying drawings.