Method and assembly for boosting the brake force of an electrohydraulic motor vehicle brake system

Abstract

The invention relates to a technique for boosting the brake force of an electrohydraulic motor vehicle brake system in a mode in which, as a result of a mechanical push-through, an actuating force onto a brake pedal acts upon a master cylinder of the brake system. According to an aspect of this technique, the method comprises the steps of: determining a value of a first variable indicating a current deceleration of the vehicle; determining, based on the first variable, a value of a second variable indicating the actuating force; determining, based on the second variable, a required brake boost; and controlling an electromechanical actuator acting upon the master brake cylinder to obtain the required brake boost.

Claims

1. Method for boosting a brake force of an electrohydraulic motor vehicle brake system in a mode in which an actuating force on a brake pedal acts upon a brake master cylinder in which brake pressures are generated by pressurizing a hydraulic fluid of the brake system by a mechanical push-through, comprising the steps: determining a value of a first variable that is indicative of a current vehicle deceleration; on the basis of the value of the first variable, determining a value of a second variable that is indicative of the actuating force; on the basis of the value of the second variable, determining a required brake force boost; and controlling an electromechanical actuator, which acts upon the master cylinder, to achieve the required brake force boost.

2. Method according to claim 1, the current vehicle deceleration resulting from the actuating force on the brake pedal and a current brake force boost from the electromechanical actuator.

3. Method according to claim 2, the value of the second variable being determined from the value of the first variable and the current brake force boost.

4. Method according to claim 3, the current brake force boost being determined from an electric current consumption of the electromechanical actuator.

5. Method according to claim 3, the determination of the value of the second variable being based on the assumption of a known relationship between the current brake force boost and the actuating force.

6. Method according to claim 1, the determination of the required brake force boost being based on the assumption of a known relationship between the required brake force boost and the second variable.

7. Method according to claim 1, the first variable being the vehicle deceleration itself, a distance traveled by a piston in the brake master cylinder, a hydraulic pressure in the brake system or a total brake force.

8. Method according to claim 1, the second variable being the actuating force itself, a hydraulic pressure component in the brake system that results from the actuating force, or a brake force component that results from the actuating force.

9. Method according to claim 1, the steps being performed repeatedly during a braking operation.

10. Method according to claim 1, the mode being activated during an ongoing braking operation.

11. Method according to claim 1, the mode being a fallback mode and, in a regular operating mode, the brake pedal being decoupled from the brake master cylinder and a brake force being generated solely by the electromechanical actuator.

12. Method according to claim 1, the method being performed as a response to a failure of a sensor for detection of a braking request.

13. Method according to claim 12, the method being performed as a response to the failure of a pedal travel sensor.

14. Method according to claim 1, wherein each step is performed by a program code means of a computer program, the method including the step of executing the program code means of the computer program on a control unit (ECU).

15. Method according to claim 1, wherein the actuating force relates to the force by means of which a driver operates the brake pedal.

16. Control unit (ECU) of a motor vehicle, comprising a computer program product having a program code that instructs the control unit to generate an output signal to an electromechanical actuator to boost a brake force of an electrohydraulic motor vehicle brake system in a mode in which an actuating force on a brake pedal acts upon a brake master cylinder of the brake system by a mechanical push-through, the ECU receiving a signal from a sensor and the program code determining a value of a first variable based on the sensor signal that is indicative of a current vehicle deceleration; on the basis of the value of the first variable, the program code determines a value of a second variable that is indicative of the actuating force; on the basis of the value of the second variable, the program code determines a required brake force boost; and the ECU generates the output signal to control the electromechanical actuator to act upon the master cylinder, to achieve the required brake force boost.

17. Method according to claim 16, wherein the actuating force relates to the force by means of which a driver operates the brake pedal.

18. Electrohydraulic actuating assembly for a motor vehicle brake system, comprising a brake master cylinder; an electromechanical actuator at least for boosting brake force, the electromechanical actuator acting upon the brake master cylinder; a component that, by means of a mechanical push-through, enables an actuating force on a brake pedal to be transmitted to the brake master cylinder; and a control unit (ECU), which is designed to determine a value of a first variable that is indicative of a current vehicle deceleration; to determine, on the basis of the value of the first variable, a value of a second variable that is indicative of the actuating force; to determine, on the basis of the value of the second variable, a required brake force boost; and to control the electromechanical actuator to achieve the required brake force boost.

19. Actuating assembly according to claim 18, the component being configured for operating in a fallback mode, and the actuating assembly being configured, in a regular operating mode, to decouple the brake pedal from the brake master cylinder, and to generate a brake force solely by the electromechanical actuator.

20. Actuating assembly according to claim 18, the actuating assembly being realized without a sensor for detection of a braking request.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1: an embodiment of a motor vehicle brake system having an electrohydraulic actuating assembly;

(2) FIG. 2 a schematic representation of the determination of the required brake force boost for the purpose of controlling an electromechanical actuator of the actuating assembly according to FIG. 1;

(3) FIG. 3 an embodiment of a method for boosting the brake force of the motor vehicle brake system according to FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(4) The electrohydraulic motor vehicle brake system 10 represented in FIG. 1 comprises an electrohydraulic actuating assembly 12, which operates by means of a hydraulic fluid. Some of the hydraulic fluid is stored in an unpressurized container 14. The container 14 is connected to a brake master cylinder 16, in which brake pressures are generated by pressurizing the hydraulic fluid. For this purpose, two movable pistons 18 and 20, which delimit two hydraulic chambers 22 and 24 that are separate from each other, are accommodated, as a tandem arrangement, in the brake master cylinder 16. Two brake circuits I. and II. are connected to outputs of the chambers 22, 24, each brake circuit acting upon two of a total of four wheel brakes FL (front left), FR (front right), and RL (rear left), RR (rear right).

(5) Depending on which wheel brake is actuated via which brake circuit, there is a front/rear axle division, which means that the one brake circuit actuates the wheel brakes of the front axle, and the other brake circuit actuates those of the rear axle, or there is a diagonal division, which means that each brake circuit actuates the wheel brake of a front wheel and that of the diagonally opposite rear wheel. A front/rear axle division is shown exemplarily in FIG. 1.

(6) Actuation of the brake master cylinder 16 may be effected by means of an electromechanical actuator 26 (as an electrical brake pressure generator) and a mechanical final control element 28 (as a mechanical brake pressure generator), jointly or separately from each other. For this purpose, both the electromechanical actuator 26 and the mechanical final control element 28 act, on the input side, upon the end face of the piston 20 that is opposite the hydraulic chamber 24, in order to cause the pistons 18 and 20 (because of their tandem arrangement) to be moved longitudinally.

(7) As an alternative to this, the electromechanical actuator 26 may act in combination with a cylinder-piston means that is fluidically coupled to the brake master cylinder 16, in order to actuate the pistons 18, 20 electrohydraulically (not represented in FIG. 1). Specifically, for example, the cylinder-piston means acting in combination with the electromechanical actuator 26 may be fluidically coupled, on the outlet side, to the piston 20 of the brake master cylinder 16, in such a manner that a hydraulic pressure in the cylinder-piston means that is generated upon actuation of the actuator 26 acts directly upon the end face of the piston 20 that is opposite the hydraulic chamber 24. The piston 20 is then made to move longitudinally, because of the hydraulic pressure acting upon the piston 20. The hydraulic pressure generated in the cylinder-piston means by the electromechanical actuator 26 may be used solely to actuate the piston 20 in the brake master cylinder 16 or, in the course of a brake force boost, to assist the piston actuation effected by means of the mechanical final control element 28.

(8) The electromechanical actuator 26 comprises an electric motor 30 that, via a transmission 32, 34, acts upon the piston 20 on the input side. The electric motor 30 and the transmission 32, 34 are disposed concentrically in relation to each other, the transmission 32, 34 being realized, for example, as a nut/spindle arrangement, which preferably has a recirculating ball means. The nut 32 of the transmission is rotatably mounted, and the transmission spindle 34 acting upon the piston 20 is mounted in a rotationally fixed manner, in order to convert rotary motions of the electric motor 30 into longitudinal motions of the spindle 34, and therefore of the pistons 18 and 20. Generally, the transmission 32, 34 may be designed to convert rotary motions of the electric motor 30 into a longitudinal motion acting upon the piston 20.

(9) The mechanical final control element 28 has an actuating member 36, which is disposed so as to be displaceable in the longitudinal direction, concentrically in relation to the electric motor 30. The actuating member 36, which is coupled in a jointed manner to the brake pedal 38, is able, like the spindle 34 (and independently thereof), to act upon the piston 20 on the input side, in order cause the pistons 18 and 20 to move longitudinally.

(10) Whether actuation of the brake master cylinder 16 is effected by means of the electromechanical actuator 12 and/or by means of the mechanical final control element 28 can be selected by means of a coupling and decoupling means (not represented). If, according to one implementation, the mechanical final control element 28 is fully decoupled from the brake master cylinder 16, for a brake-by-wire operation (service brake mode) the brake master cylinder 12 is actuated exclusively by means of the electromechanical actuator 20. For this purpose, the actuating travel s and the associated actuating force F imitated on the brake pedal 38 by the driver are sensed by means of two sensor means 40 and 42. In addition, a pedal reaction behaviour is provided by means of a simulation means (not represented) upon actuation of the brake pedal 38. It must be pointed out that, in alternative embodiments, one or both of the sensor means 40 and 42 may be omitted.

(11) In an electronic control unit ECU, the sensed actuating travel s and the associated actuating force F are evaluated to determine the brake pressure requirement (i.e. the braking intention) of the driver. The electronic control unit ECU effects the electrical control of the electric motor 30 of the electromechanical actuator 26 in dependence on the brake pressure requirement. The brake pressure p generated upon actuation of the brake master cylinder 16 is sensed by means of a sensor means 44, and in the electronic control unit ECU is cyclically compared with the brake pressure requirement in order to control the brake pressure p by closed-loop or open-loop control. Since, owing to the tandem arrangement of the pistons 18 and 20 of the brake master cylinder 16, a (substantially) corresponding brake pressure p is generated for both brake circuits I. and II., sensing of the brake pressure p requires only one sensor means 44, which in this case senses the brake pressure p generated in brake circuit II.

(12) Should a defect of the electromechanical actuator 26 occur, for example a fault in the electrical control of the electric motor 30, or a malfunction of one of the sensor means 40 and 42 for detecting of a braking request, it becomes possible for the brake master cylinder 16 to be actuated directly, by means of the coupling and decoupling means (not represented), in dependence on an actuation of the brake pedal 38, in order to ensure an emergency braking operation (fallback mode).

(13) As described, in the fallback mode, owing to the mechanical push-through, the actuating force applied on the brake pedal 38 by the driver acts directly upon the brake master cylinder 16. In other words, the brake pedal 38 is rigidly coupled to the end face of the cylinder 20 that faces towards the brake pedal 38, via the actuating member 36. Depending on the defect or malfunction, control of the electromechanical actuator 12 is additionally effected in this case, in order to provide an additive brake force boost. This is described in greater detail below.

(14) In the hydraulic connection to the brake master cylinder 16, a valve arrangement 46, 48, 50 and 52 is assigned, respectively, to each of the wheel brakes FL, FR, RL and RR. The valve arrangements 46, 48, 50 and 52 are each designed as electromagnetically actuated 2/2-way valves, which are open in the non-actuated state (as represented). The electrical control of the valve arrangements 46, 48, 50 and 52 is likewise effected by the electronic control unit ECU.

(15) In the present embodiment, the setting of individual brake pressures in the individual wheel brakes FL, FR, RL and RR, as is required, inter alia, for an anti-lock braking system (ABS), a traction control system (TCS), a dynamic drive control system (ESP) and the like, is effected in multiplex mode.

(16) In multiplex mode, the setting of the individual wheel brake pressures is effected, for example, within multiplex cycles Tz, which succeed one another with a cycle time in an order of magnitude of typically 10 ms. In this case, a current multiplex cycle Tz(n) is divided, according to the number of wheel brakes FL, FR, RL and RR to be actuated, into time intervals (of equal duration), in which the brake pressures required for the wheel brakes FL, FR, RL and RR are set centrally in succession by the brake pressure generator 10 and, by means of the valve arrangement 46, 48, 50 and 52 assigned to the respective wheel brake FL, FR, RL and RR, are held until the succeeding multiplex cycle (Tz(n+1). Consequently, in the case of four wheel brakes FL, FR, RL and RR to be operated, there are (at least) four time intervals.

(17) As already mentioned, the fallback mode may be activated in various situations, for example because of a fault in the electrical control of the electric motor 30, or because of a malfunction of one of the sensor means 40 and 42 for detecting of a braking request. If only one of the sensor means 40 and 42 fails, or if there is a comparable defect, the electromechanical actuator 26 as such remains operational. In such cases, therefore, it is provided that the electromechanical actuator 26 is operated in addition to the mechanical push-through, in order to provide a brake force boost. The known problem of excessively long braking distances in the fallback mode can be solved in this way.

(18) The solution proposed here includes an assessment of the driver's intention (in the case of, for example, a defect of both sensor means 40 and 42) and, based thereon, a determination of the required brake force boost. The assessment may be based on the assumption of a known relationship between the required brake force boost and the (unknown) actuating force applied by the driver. Generally, the solution proposed in the present embodiment is based on the knowledge that, if the result (e.g. vehicle deceleration or brake pressure) of two actions is known and, in addition, one of the two actions (e.g. the brake force boost) is known), the second of the two actions (e.g. the actuating force applied by the driver) can be determined, and the required brake force boost can be determined therefrom.

(19) This situation is now explained with reference to the schematic flow diagram according to FIG. 2. In this connection, reference is also made to FIG. 3, which illustrates an embodiment of a method for boosting brake force in combination with the brake system according to FIG. 1.

(20) The procedure represented in FIG. 2 can be started if, for example in the context of a braking operation (i.e. in the case of an electromechanical actuator 26 controlled by the control unit ECU) the need to activate the fallback mode is identified, assuming at the same time that the electromechanical actuator 26 is able to continue to operate. Such a situation may exist, for example, in the case of failure of one or more of the sensor means 40, 42 for detection of a braking request. In such a case, upon activation of the fallback mode, the mechanical push-through also becomes activated, in which case the brake force boost (still) provided by the electromechanical actuator 26 and the actuating force resulting from the mechanical push-through can then act additively upon the piston 20 in the brake master cylinder 16, at least for a short time. The input variable for controlling the electromechanical actuator 26 is absent, however, and can be filled in as described in the following.

(21) It is assumed according to FIG. 2 that the current vehicle deceleration a.sub.Veh in the fallback mode results from two components, namely, on the one handowing to the mechanical push-throughthe (now unknown) actuating force on the brake pedal 38 F.sub.in and, on the other hand, the brake force boost (currently) provided by the electromechanical actuator 26. There is a known relationship between the brake force boost and the electric current consumption I.sub.Boost of the electromechanical actuator 26.

(22) It is additionally assumed that there is also a known relationship between the current brake force boost (i.e. the electric current consumption I.sub.Boost), on the one hand, and the actuating force F.sub.in generated by the driver, on the other hand. This known relationship may be a predefined boost factor. For example, the brake system 10 may be designed to boost the actuating force F.sub.in generally by a factor 4.

(23) The current vehicle deceleration a.sub.Veh may be determined, for example, by means of an acceleration sensor or wheel rotational-speed sensor, by means of a displacement sensor that senses the movement of one of the pistons 18, 20, by means of the pressure sensor 44 or by means of a brake force sensor (step 302 in FIG. 3). The current brake force boost may be determined, as described above, by means of the electric current consumption I.sub.Boost of the electromechanical actuator 26, or in another way. From this information, despite failure of the displacement sensor 40 and/or of the force sensor 42, the actuating force F.sub.in can then be estimatedas represented in FIG. 2(step 304 in FIG. 3).

(24) Considered for the purpose of estimating the actuating force F.sub.in is the balance of forces ensuing at the piston 20 that delimits the hydraulic chamber 24, the sensor means 44 being connected to the output of the latter for the purpose of sensing the brake pressure p. This is obtained on the basis of the hydraulic force F.sub.p generated in the hydraulic chamber 24, the actuating force F.sub.in and the boost force F.sub.Boost provided by the electromechanical actuator 26:
F.sub.p=F.sub.in+F.sub.Boost

(25) This results in the actuating force F.sub.in:
F.sub.in =F.sub.pF.sub.Boost

(26) The hydraulic force F.sub.p is obtained as the product of the operating pressure p, sensed by means of the sensor means 44, and the effective working area A.sub.20 of the piston 20, as constants:
F.sub.p=A.sub.20*p

(27) The boost force F.sub.Boost is obtained as the product of the electric current consumption I.sub.Boost of the electric motor 30 and the constant K.sub.30 defined by the characteristic values of the electric motor 30 and of the transmission 32, 34:
F.sub.Boost=K.sub.30*I.sub.Boost

(28) The input force F.sub.in can thus be estimated, or calculated, according to the following formula:
F.sub.in=A.sub.20*pK.sub.30*I.sub.Boost

(29) From the estimated actuating force F.sub.in, in turn, since the boost factor is known, it is possible to deduce the brake force boost required for the next cycle, and the therewith associated electric current consumption I.sub.Boost of the electromechanical actuator 26. This procedure corresponds to step 306 in FIG. 3. In a further step, 308, the electromechanical actuator 26 is then controlled by means of the control unit ECU to achieve the required brake force boost.

(30) In the meantime, the driver will have changed the actuating force on the brake pedal 38 (in an unknown manner), such that a new actuating force F.sub.in is present. Accordingly, a new vehicle deceleration a.sub.Veh ensues. Then, in a next cycleas indicated by broken lines in FIG. 2the new actuating force F.sub.in can be estimated on the basis of the new vehicle deceleration a.sub.Veh and the newly set brake force boost.

(31) The steps represented in FIG. 2 are repeated cyclically until an end of the braking request has been detected (i.e. the estimated actuating force is zero or close to zero).

(32) The procedure outlined in FIG. 2 may also be started if there is no sensor means 40, 42 at all for detection of a braking request or, alternatively, if the corresponding sensor means 40, 42 has (have) already failed in the lead-up to a braking operation. In this case, it is simply assumed at the start that the current brake force boost and the associated energizing of the electromechanical actuator 26 (i.e. I.sub.Boost) are equal to zero, and the current vehicle deceleration therefore results exclusively from the mechanical push-through.

(33) It is understood that the implementation of the procedure outlined here is equally suited to boosting the brake force in the case of a brake system 10 in which a mechanical push-through is always present. In the case of such brake systems, it would therefore also be possible to dispense with one or both of the sensor means 40 and 42 for detection of a braking request.

(34) As is evident from the embodiments described, the procedure proposed here makes it possible to maintain a brake force boost even in such cases in which, conventionally, an unboosted push-through operation would be effected in the fallback mode. Unnecessary or precautionary switch-off of the electromechanical actuator 26 can therefore be precluded. Overall, therefore, there are very few remaining cases of fault in which there is no brake force boost available and the driver himself/herself has to apply the entire brake force. The everyday usefulness of brake-by-wire systems and systems for electrohydraulic boosting of brake force is thereby increased.

(35) When the procedure proposed here is implemented, the known and accustomed pedal feel may change; in particular, the pedal travel may lengthen by a reasonable extent. This is advantageous, however, since this also provides the driver with a haptic indication that the brake system possibly has to be checked for a fault.

(36) Finally, it must also be mentioned that a practical embodiment has been explained exemplarily on the basis of FIGS. 1 to 3. It is therefore at the discretion of persons skilled in the art to effect modifications and combinations within the scope of the claims and the description.

(37) In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit of scope.