GB2331338A

GB2331338A - Brake system for a motor vehicle

Info

Publication number: GB2331338A
Application number: GB9816545A
Authority: GB
Inventors: Michael Doericht
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1997-07-30
Filing date: 1998-07-29
Publication date: 1999-05-19
Anticipated expiration: 2018-07-29
Also published as: FR2766779A1; GB9816545D0; FR2766779B1; GB2331338B

Abstract

A brake system 1 for a motor vehicle has electrically controlled wheel brake actuators 12, 26, operated, for example, by actuating the brake pedal. The brake actuators are included in first and second braking circuits 10, 28 which are supplied separately by electric power stores (batteries) 8, 24. A common generator unit 6 charges battery 8 directly and battery 24 via a charging device 22. In an alternative embodiment (Fig.2), the batteries are charged by individual generator units (6,31). The temperatures, terminal voltages and charging and discharging currents of the batteries are monitored by a unit 18, which also includes a buffer battery (44, Fig.4) to give warning of battery failure. If the generator unit(s) fail(s), and unit 18 detects a critical discharge of the batteries, the brakes are applied.

Description

2331338 2-1-e system for a motor vehicle is The invention relates to an

electrically actuated brake system for a motor vehicle.

New demands on the brake systems of motor vehicles, such as antiskid systems, driving stability control systems, traction control systems or traction controls, so-called intelligent cruise controllers, brake assistants etc., together with the demand for a reduction of the assembly and maintenance costs, which are very considerable with the presently generally common hydraulic brake systems, have led to the development of new, purely electrical brake systems (also known by the name brakeby-wire) (DE 195 48 392 A1). With such electrical brake systems the driver is not directly connected to the brake by a linkage. The braking torque request coming from the driver is no longer transmitted directly as force by way of a hydraulic system, but only as a signal by way of an electrical connection. With the aid of this signal an electrical braking actuator is controlled which, with the aid of an electrical power supply, generates a force on the individual brakes which generate the desired braking torque using a friction element. With a disc brake the braking torque is transmitted as so called brake application force at a calliper to the brake disc.

Because the brakes of a vehicle which is allowed to be in public traffic represent an important safety device which is subject to legal requirements, and because in the case of an electrically operated brake the driver no longer has the possibility of braking the vehicle directly, with muscle power, special measures for guaranteeing the operating electrical power are necessary. The operational reliability and availability of the vehicle and particularly the brakes must be guaranteed at all times.

The invention aims to create an electrical brake system for motor vehicles, wherein the supply of electrical power to the brake system is made much more reliable. In this respect, both the operating devices and the control devices of the brake system should be reliably supplied at all times with electrical power.

According to a f irst aspect of the invention there is provided a brake system for a motor vehicle including: a plurality of wheel brakes each activated independently by a corresponding electrically controlled wheel brake actuator, depending on actuations of a brake pedal, a first braking circuit supplying at least one of the brake actuators, a second braking circuit supplying at least one other of the brake actuators, a first circuit having a first power store, the first braking circuit and additional electrical consumers, a second circuit having a second power store and the second braking circuit, and a generator which is connected to the first circuit and to a charging device for the second circuit.

According to a second aspect of the invention there is provided a brake system for a motor vehicle including: an electrically controlled wheel brake actuator, by means of which the individual wheel brakes are activated independently of each other and depending on actuations of the brake pedal, a first braking circuit supplying at least one of the brake actuators, a second braking circuit supplying at least one other of the brake actuators, a first circuit which has a first power store and the first braking circuit and additional electrical consumers, a second circuit which has a second power store and the second braking circuit and a first generator which is connected to the first circuit and a second generator connected to the second circuit for charging it.

The advantages of the invention lie in particular in that the necessary operational reliability and availability of the brake system is achieved with little expenditure on components and costs. The supply of power to the brake system which has several braking circuits is arranged in such a way that a failure or an impairment of the supply of power to one braking circuit does not impair the operability of the remaining braking circuit or circuits. In this way, adequate braking ability of the motor vehicle is always maintained.

Embodiments provide a system for monitoring the electrical power stores of the individual braking circuits which makes it possible to detect in good time an expected failure of a power store, and to take appropriate measures, for example initiating a warning signal to the driver, before there is a total failure of the braking circuit concerned.

Specific embodiments of the invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:

Figure 1 shows a brake system in accordance with the invention, Figure 2 shows another embodiment of a brake system in accordance with the invention, Figure 3 shows another embodiment of a brake system in accordance with the invention, Figure 4 shows a system for monitoring the state of charge of a brake system in accordance with the invention, shown as a block diagram, and Figures 5 and 6 show diagrams which clarify the characteristic of the terminal voltage of a battery in dependence upon operating variables with two different states of charge.

For reasons of clarity, the embodiments of the invention which are described in the following relate is to two-circuit brake systems in four-wheel motor vehicles, where in each case two wheel brake devices belong to one braking circuit. The following designs can also be applied to brake systems with more than two circuits, with each braking circuit in this respect preferably being allocated an electrical power store and/or corresponding charging and disconnecting devices.

A brake system 1 (Figure 1) in accordance with the invention contains a generator 2 (alternator) which is driven by the motor, not shown here, of a motor vehicle and which generates a direct voltage U,;, the magnitude of which is determined by a generator controller 3. An overvoltage protector 4 is provided, in order to avoid a destruction of connected electronic devices by way of transient voltage peaks, in the case of a sudden great reduction in load. Generator, controller and overvoltage protector are preferably combined to form a generator unit 6 which is known per se.

The generator unit 6 supplies current into a first circuit 7, called primary circuit in the following, which contains an electrical power store 8, also called primary battery in the following. The primary circuit also contains the usual consumers of electrical power (electrical consumers) in a motor vehicle (starter, ignition, lighting, display instruments, fan motors, rear-window heating etc.) as well as several, in this case two, wheel brake actuators or wheel brake devices 12 belonging to a primary braking circuit 10.

The charging current and discharge current of the primary battery 8 is measured by a contactless current sensor 13. Because of the relatively high discharge currents which occur in motor vehicles, and because of endeavours to achieve a high level of security against failure, it is an advantage to have as few cable breaks as possible in lines which are used for power transmission, so the simpler and more economical alternative current measurement with shunt resistors is therefore not used in this specific embodiment.

Each of the usual consumers of electrical power, denoted here as a whole by the reference number 14, of the primary circuit 10, and the two wheel brake actuators 12, are protected against a short circuit by way of suitable protection means 16, for example fusible links or electronic fuses. An electronic protection means 16 of a type which can be activated and deactivated by a control signal from an electronic circuit arrangement is preferably used. The control signals can be generated, for example, by the system for monitoring charge 18. With such electronic protection means it is possible to protect the primary battery from an unwanted discharge when the vehicle is switched off, for example as a result of the driver failing to switch off the headlight. Optionally, the circuit to each consumer is interrupted by activating an electronic protection means 16. This function can also be used, in the event of an inadequate state of charge of the primary battery, to deliberately disconnect less important consumers, for example the heated rear window or the passenger heating, and to use the remaining residual charge of the primary battery exclusively for operating the wheel brake devices.

The temperature T, of the primary battery 8 is measured by a temperature sensor 17. The measurement is delivered, together with the measured values of the charging current and discharge current, to a circuit arrangement for monitoring and controlling the state of charge, which will be referred to as the system 18 for monitoring the charge. In this respect, the temperature sensor 17 is advantageously accommodated directly on the battery 8 in order to achieve a good thermal coupling between the battery and the sensor.

-6 The voltage U, is picked up by the system for monitoring the charge 18 directly at the battery 8. On the basis of the received and evaluated measured data, the system for monitoring the charge 18 controls the generator controller 3, in particular by outputting a target value (desired value) to the generator controller. The generator controller 3 then controls the output of the generator unit 6 to follow this target value. The function of the system for monitoring the charge is described in more detail below.

The generator voltage U,, also supplies a charging device 22, provided with a return protection system 20, which is constructed, for example, as a semiconductor rectifier, and with a charging controller 21, for an electrical power store 24 of a second circuit 25, called secondary battery and secondary circuit in the following. The secondary circuit only contains wheel brake actuators 26 of a second braking circuit 28, called secondary braking circuit in the following. The output voltage of the charging device 22 is U,.

The voltage U, of the secondary battery, its charging and discharge currents and its temperature are measured and transferred to the device for monitoring the state of charge 18. The latter determines from these measurements the target values for controlling the charging device 22. The charging controller 21 controls the output of the charging device 22 so that the output follows the target value. The charging device 22 with return protection system 20 is constructed, for example, as a pulse-widthmodulated clocked inductive voltage transformer. The mode of operation of such clocked inductive voltage transformers (step-up converter, stepup/step-down converter, isolating transformer etc.) is known. The use of an isolating transformer makes it possible, by way of its isolation in the through path, to dispense is with a separate return protection system.

The use of the voltage transformer allows adaptation to different operating states. For example, it is possible to select the nominal voltage U, of the secondary battery 24 to be greater than the nominal voltage U, of the primary battery. An advantage in particular with regard to the greater braking powers on the front axle is if the secondary battery supplies the two front axle braking actuators 12 because, as a result of a higher voltage, there is an improved dynamic response of these braking actuators.

Even in the case of identical nominal voltages of the two batteries it is an advantage to use a controlled voltage transformer with a return protection system for charging the secondary battery and not to select the simpler option of non-separated parallel connection of the two batteries. The latter option has the following disadvantage, even if a return protection system and protection means against short circuit allow a certain circuit isolation. If, due to the batteries ageing at different rates, for example brought about by them being used to different degrees, one of the batteries has a smaller actual capacity than the other battery which is still in a better state, then, due to the charging conditions derived therefrom, the "poor" battery compared with the "good" battery is supplied with less charge, that is to say that its actual capacity decreases even more in this way, and a battery failure within a relatively short time is unavoidable. With a separate charging device also for the secondary battery it is possible to charge both batteries according to their actual capacity independently of each other and therefore to attain an extended battery life.

The charging device 22 exchanges data indicating the state of the batteries with a central brake control device 30, namely by way of the system 18 for monitoring the charge. The central brake control device 30 can then take suitable measures if necessary, for example stopping the vehicle if there is insufficient residual charge in both batteries.

In the second embodiment of a brake system 1b in accordance with the invention, which can be seen in Figure 2, an isolation of the two circuits takes place by using two generator units 6 and 31. These generator units as separate structural units can be designed with a respective driving belt per generator 2, 31a or as a double generator with a common driving belt. The assembly of such double generators is known. For a design of such double generators which is as economical as possible, the two generator systems, the rotors of which are disposed on a common shaft, are designed with almost the same nominal power.

In this respect, it is recommended to use primary and secondary batteries 8 and 24 with the same nominal capacity and to divide the load into approximately equal portions on the primary and secondary circuit and 32. For this purpose there are also further electrical consumers in the secondary circuit 32 in addition to the braking actuators 26, which consumers are protected against short circuit by suitable protection means 16. Electronic protection means are also preferably used here in order to protect the battery from complete discharge, as described in the first embodiment.

7 The states of charge of the two batteries 8 and 24 are detected by the system 18 for monitoring the charge according to the method described in the first exemplary embodiment, the system for monitoring the state of charge assuming control of the generator controllers 3 and 33 depending on the measured variables (battery voltage, charging and discharge current. battery temperature). The generator unit 31 also contains an overvoltage protector 34.

In the third embodiment of a brake system le in accordance with the invention, which can be seen in Figure 3, low power consumers 35 are combined in their own circuit, called an auxiliary circuit 36 in the following, with a low supply voltage, for example 12V, whereas consumers 37 of high power, including the braking actuators 12 and 26, are divided into two circuits, called main circuits 38 and 39 in the following, with higher operating voltage, for example 36V. The assembly and function of the two main circuits correspond to the circuits of Figure 2.

The consumers 35 in the auxiliary circuit are supplied by way of a stepdown converter 40 from one of the two main circuits 38 or 39. For a circuit isolation between the main circuit and the auxiliary circuit the step-down converter 40 is designed as an isolating transformer. The supply of a greater voltage than the 12V hitherto usual in automobiles to consumers of higher power brings advantages with respect to efficiency because the line losses decrease due to the smaller currents. With constant power of the braking actuators the operating current decreases, for example, by the factor by which the new operating voltage is greater than the hitherto normal 12V. In exactly the same way there results a cost reduction with respect to the semiconductors in the output stages of the braking actuators because of the smaller operating currents, because the power loss decreases, even for a given conducting resistance, quadratically with the current reduction. Cheaper semiconductors of smaller maximum power loss can therefore be used and the expenditure for cooling measures also decreases.

From the block diagram of Figure 4 the basic assembly of the system 18 for monitoring the charge can is be seen. The battery voltages U, and U2 of both circuits are respectively led by way of an overvoltage protector 42 to a decoupling circuit arrangement 43, constructed as a diode circuit arrangement in the embodiment, which ensures that, in the event of a failure of a circuit, the circuit arrangement is supplied from the other circuit and in this way the system 18 for monitoring the state of charge continues to remain ready for operation. A buffer battery 44 is provided in order to guarantee, in the event of a double circuit failure, the operational reliability of the circuit arrangement for a limited period. The central brake control device 30 is likewise provided with a buffer battery, not shown here, and is supplied from both circuits, wherein each individual circuit can also be used for the supply here. A voltage controller 46 is connected to the decoupling circuit arrangement 43, the voltage controller making the internal supply voltage available for the modules of the system for monitoring the charge 18.

The terminal voltage, the temperature and the charging and discharging currents of each of the two batteries are measured and supplied to a signal processing (circuit) 47 which prepares the measured data for a state of charge determination (circuit) 48. This circuit 48 is preferably realized by a microcomputer which can also contain the signal processing circuit 47, the voltage controller 46 and additional circuit components. Data about the state of charge of the batteries is exchanged by way of a communications interface 49 with the central brake control device. The operability of the internal modules of the system for monitoring the state of charge 18 is monitored by a self-diagnosis module 50.

The battery state is evaluated in accordance with the following method.

is A measured terminal voltage U or its ratio q=U/U, to the no-load voltage depends, for a given battery, on its temperature T, the discharge current I or its ratio p=I/I, to low-temperature test current I,, the age or the present capacity and the state of charge. The nominal capacity in operation is a constant, it being determined by the vehicle manufacturer and not changing during the operation of the vehicle. The ageing of the battery is determined by the loads to which the battery is subjected during operation, and influences its present capacity. Because of the many and diverse influences it can scarcely be detected by measurement. For a certain battery age and a certain state of charge the following relationship for the measured terminal voltage results:

q=U/U,=f (T, p) In this respect, the function f(T,p) depends, as said, on the age and on the state of charge, and in the same way also on the parameters battery type and nominal capacity. It can be calculated algorithmically from battery models or determined by measuring performance characteristics.

The diagram which can be seen in Figure 5 illustrates qualitatively the function f,(T,p) for a new battery with 100% state of charge, where the entire nominal capacity is therefore available.

The diagram which can be seen in Figure 6 illustrates qualitatively the function f,(T,p) for the same battery with a present state of charge which now only corresponds to about 25% of the nominal capacity. The greater reduction of the terminal voltage in the event of loading the battery with high discharge currents is characteristic. The reduction of the terminal voltage in the event of loading, at a -12 specified temperature, depends on the state of charge of the battery: a battery with a poor state of charge has a correspondingly high internal resistance. In relation to the nominal capacity of a new battery, the internal resistance of the battery in the course of its life is influenced by two variables. Firstly, by the battery age which reduces the capacity of the battery, i.e. even a fully charged old battery has a smaller charge stored than a fully charged new battery. Generally, the internal resistance increases with increasing age. Secondly, however, the internal resistance also depends on the present state of charge, namely it becomes greater, the closer the battery is to the state of discharge.

To ascertain whether a battery is still suitable for use as a power store for an electrical brake system or not, two variables must therefore be determined: the present capacity and the present state of charge. The state of charge is calculated by a charge balance with the measured charging and discharge currents:

2 Q'=E (77,0I,At) + Q, In this equation, I, is the current measured at the sampling instant t,+ kAt, Q, the value of the charge balance at the instant t,.

However, the charge introduced into the battery is not completely stored, but only in part. The charging current I, is therefore weighted with a factor -qk - In this respect, the weighting factor n, depends both on the present capacity and on the present state of charge. In the overcharging range, where the battery starts to gas, 77,=0. In the gassing range the electrical power supplied into the battery is no longer stored, but is used for the electrolytic decomposition of the water and for the thermal heating of the is battery. However, this state, which has a very disadvantageous effect on the life expectancy of the battery, can be avoided by the following strategy.

The state of charge determination arrangement 48 controls the generator 2 or the charging device 22 in such a way that the charging voltage for the respective battery 8, 24 only lies insignificantly above the end of charge, this being the no-load voltage of the fully charged battery. When the gassing range is approached, first of all the charging current decreases and after that the temperature of the battery begins to rise and announces the transition into the gassing range. This is recognized by the state of charge determination arrangement 48 which thereupon reduces the charging voltage accordingly.

The influence of ageing on the weighting factor n,, is taken into account by an internal resistance measurement. During a braking procedure, due to the relatively high current requirement of the actuators, current is drawn from the batteries. With the primary battery 8, current can also already be used by the normal electrical consumers, with the drawn current correspondingly increasing in the event of a braking procedure. Because of their relatively large magnitude, these current loads represent a considerable load on the batteries. In this respect, the terminal voltage will decrease to a greater or lesser degree according to the present state of charge. The ratio U/U, with a certain current and a known temperature and a known present charging balance is used to adjust the weighting factor 77,.

If the ratio U/U, falls below a lower limiting value, depending on the present state of charge (Q,,) and on the age (present capacity, internal resistance) different measures are taken: with insufficient residual capacity due to ageing, a replacement of the battery concerned is recommended. If the battery is still suitable and is only temporarily discharged, and if the generator is running so that the battery is recharged, only a battery warning is emitted. If the generator fails during the journey the vehicle remains ready for operation until the system for monitoring the charge 18 ascertains a critical discharge of the batteries. If necessary, i.e. if the vehicle has not already failed due to the motor control unit, which likewise requires a minimum operating voltage which lies above that of the braking actuators, a forced braking is introduced and the vehicle is stopped. Due to the residual escapement of the actuators or also possibly as a result of using mechanical locking devices on the actuators, the brakes are left in the applied state until proper readiness for operation is again produced by eliminating the defects in the power supply.

The control of the electrical wheel brake actuators 12, 26 for actuating the wheel brakes is generally known and is therefore not described in more detail here. It takes place on the one hand by way of a brake pedal and on the other hand by way of the modern brake control and regulating systems mentioned in the introduction, such as anti-skid system controllers and suchlike.

-is-

Claims

1. A brake system for a motor vehicle including:

a plurality of wheel brakes each activated independently by a corresponding electrically controlled wheel brake actuator, depending on actuations of a brake pedal, a first braking circuit supplying at least one of the brake actuators, a second braking circuit supplying at least one other of the brake actuators, a first circuit having a first power store, the first braking circuit and additional electrical consumers, a second circuit having a second power store and the second braking circuit, and a generator which is connected to the first circuit and to a charging device for the second circuit.

2. A brake system according to claim 1, wherein the charging device for the second circuit is provided with a return protection system between its input and the output of the generator.

3. A brake system according to claim 2, wherein the charging device is constructed as a pulse-widthmodulated clocked voltage transformer.

4. A brake system for a motor vehicle including:

a plurality of wheel brakes each activated independently by a corresponding electrically controlled wheel brake actuator, depending on actuations of a brake pedal, a first braking circuit supplying at least one of the brake actuators, a second braking circuit supplying at least one other of the brake actuators, is a first circuit which has a first power store and the f irst braking circuit and additional electrical consumers, a second circuit which has a second power store and the second braking circuit, a first generator which is connected to the first circuit, and a second generator connected to the second circuit.

5. A brake system according to claim 4, characterized in that the first and the second generators are constructed as a double generator, the rotors of which are arranged on a common shaft and which are driven by a common driving belt.

6. A brake system according to any preceding claim further including a system for monitoring the state of charge of the first power store, which system evaluates the voltage, the temperature and the discharge and charging currents of the first power store and generates therefrom a target value for a generator controller.

7. A brake system according to any preceding claim further including a system for monitoring the state of charge of the second power store, which system evaluates the voltage, the temperature and the discharge and charging currents of the second power store and generates therefrom a target value for controlling the charging device.

8. A brake system according to any preceding claim wherein a third circuit is provided including electrical consumers with a low power requirement, and the supply voltage of the first and the second circuit is higher than the supply voltage of the third circuit.

9. A brake system according to any preceding claim, characterized in that the additional electrical consumers are provided with electronic protection means which can be activated and deactivated by a control signal from an electronic circuit arrangement.

10. A brake system according to any preceding claim, further including a self-diagnosis module for diagnosing the operability of the system for monitoring the state of charge.

11. A brake system according to any preceding claim in which wheel brakes and actuators are provided for each wheel.

12. A brake system according to any preceding claim, wherein the f irst braking circuit controls some of the actuators, and the second braking circuit controls the remainder of the actuators.

13. A brake system substantially as herein described with reference to and as shown in Figure 1, Figure 2 or Figure 3 each together with Figures 4 to 6 of the accompanying drawings.