Method for checking the braking force in a vehicle

Abstract

A method is used for checking the braking force in a vehicle, which has a hydraulic vehicle brake having a brake booster and an electromechanical brake device having a brake motor. In order to build up a braking force, first of all the brake booster is activated, and subsequently an electromechanical braking force is generated by application of the brake motor. If a pressure drop occurring in the brake fluid is outside a permissible value range, a fault signal is generated.

Claims

1. A method for checking the braking force in a vehicle with a hydraulic vehicle brake, in which hydraulic braking force generated with a brake booster, and an electromechanical braking force generated by an electromechanical braking apparatus, act on a wheel-brake piston of a wheel-brake device, the method comprising: building up a braking force, including firstly, the brake booster setting a hydraulic brake fluid pressure in the wheel-brake device, and subsequently, generating an electromechanical braking force by applying torque of the electric brake motor to the wheel-brake piston thereby reducing the hydraulic brake fluid pressure in the wheel-brake device; and generating a fault signal when the reduction in the hydraulic brake fluid pressure lies outside a permissible range of values.

2. The method as claimed in claim 1, wherein: as the brake booster is setting the hydraulic brake fluid pressure, and before the brake fluid pressure is set, the electric brake motor is actuated until a brake contact-point is reached whereat the wheel-brake piston bears against a brake disk without clearance.

3. The method as claimed in claim 1, wherein after the brake booster firstly sets the hydraulic brake fluid pressure, the brake booster is not actuated further.

4. The method as claimed in claim 1, wherein the hydraulic brake fluid pressure that is firstly set by the brake booster is higher than a brake fluid pressure required for attaining a target braking force.

5. The method as claimed in claim 1, further comprising: inferring a type of fault from the reduction in the hydraulic brake fluid pressure.

6. The method as claimed in claim 5, wherein: when the reduction in the hydraulic brake fluid pressure lies below a threshold value when the fault signal is generated, an insufficient electromechanical braking force is inferred; and when the reduction in the hydraulic brake fluid pressure is not below the threshold value when the fault signal is generated a fault in the hydraulic vehicle brake is inferred.

7. The method as claimed in claim 1, wherein the method is implemented in wheel-brake devices on a rear axle of the vehicle.

8. The method as claimed in claim 1, wherein the method is implemented in a course of generating a parking-brake force.

9. The method of claim 1, further comprising: implementing the method with a closed-loop or open-loop control unit.

10. A braking system in a vehicle, comprising: a hydraulic vehicle brake; an electromechanical braking apparatus with an electric brake motor; and a closed-loop or open-loop control unit configured to trigger adjustable components of the braking system, control building up a braking force by firstly controlling the brake booster to set a hydraulic brake fluid pressure in the hydraulic vehicle brake, and subsequently controlling the electric brake motor to generate electromechanical braking force by applying torque of the electric brake motor to a wheel-brake piston thereby reducing the hydraulic brake fluid pressure; and generate a fault signal if the reduction in the hydraulic brake fluid pressure lies outside a permissible range of values.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further advantages and expedient embodiments can be gathered from the description of the figures, and from the drawings. Shown are:

(2) FIG. 1 a schematic representation of a with a hydraulic vehicle brake with a brake booster constituting an actuator, wherein the wheel-brake devices of the vehicle brake on the rear axle of the vehicle have additionally been realized as an electromechanical braking apparatus with an electric brake motor,

(3) FIG. 2 a section through an electromechanical braking apparatus with an electric brake motor,

(4) FIG. 3 a graph showing the progression of the brake pressure (solid), of the motor current of the brake motor (dashed), and of the total braking force (dash-dotted), plotted in each instance for an orderly braking process,

(5) FIG. 4 a graph corresponding to FIG. 3, plotted for a braking process with a blocked hydraulic brake line,

(6) FIG. 5 a further graph, plotted for a braking process with a hydraulic brake line with reduced cross-section,

(7) FIG. 6 a further graph, plotted for a braking process with a leakage in the hydraulic vehicle brake,

(8) FIG. 7 a further graph, plotted for a braking process with a defect in the electromechanical braking apparatus.

DETAILED DESCRIPTION

(9) In the figures, identical components have been provided with identical reference symbols.

(10) The hydraulic vehicle brake 1, represented in FIG. 1, for a vehicle includes a front-axle brake circuit 2 and a rear-axle brake circuit 3 for supplying and triggering wheel-brake devices 9 on each wheel of the vehicle with a brake fluid under hydraulic pressure. The two brake circuits 2, 3 are linked to a common master brake cylinder 4 which is supplied with brake fluid via a brake-fluid reservoir 5. The piston of the master brake cylinder within the master brake cylinder 4 is actuated by the driver via the brake pedal 6; the pedal travel executed by the driver is measured via a pedal-travel sensor 7. Located between the brake pedal 6 and the master brake cylinder 4 is a brake booster 10 which, for instance, includes an electric motor which preferably actuates (iBooster) the master brake cylinder 4 via a transmission. The brake booster 10 constitutes an electrically controllable actuator for influencing the brake pressure.

(11) The regulating motion of the brake pedal 6 measured by the pedal-travel sensor 7 is communicated as a sensor signal to a closed-loop or open-loop control unit 11 in which regulating signals for triggering the brake booster 10 are generated. The supply of the wheel-brake devices 9 with brake fluid takes place in each brake circuit 2, 3 via various switching valves which, together with further assemblies, are part of brake hydraulics 8. A hydraulic pump which is an integral part of an electronic stability program (ESP) pertains furthermore to the brake hydraulics 8.

(12) In FIG. 2, the wheel-brake device 9, which is arranged on a wheel on the rear axle of the vehicle, is represented in detail. The wheel-brake device 9 is part of the hydraulic vehicle brake 1 and is supplied with brake fluid 22 from the rear-axle brake circuit. The wheel-brake device 9 exhibits, in addition, an electromechanical braking apparatus which is preferably employed for immobilizing a vehicle at a standstill but may also be employed in the course of a movement of the vehicle for the purpose of decelerating the vehicle, in particular at relatively low vehicle speeds below a speed limit.

(13) The electromechanical braking apparatus includes a brake caliper 12 with a pincer 19 which overlaps a brake disk 20. By way of regulating element, the braking apparatus exhibits a DC electric motor as brake motor 13, the rotor shaft of which drives in rotation a spindle 14 on which a spindle nut 15 is rotatably mounted. In the course of a rotation of the spindle 14, the spindle nut 15 is shifted axially. The spindle nut 15 moves within a brake piston 16 which is carrier of a brake pad 17 which is pressed against the brake disk 20 by the brake piston 16.

(14) Located on the opposite side of the brake disk 20 is a further brake pad 18 which is held stationary against the pincer 19. On its outside the brake piston 16 is sealed in flow-tight manner in relation to the receiving housing via an encompassing sealing ring 23.

(15) Within the brake piston 16 the spindle nut 15 is able to move, in the course of a rotational motion of the spindle 14, axially forward in the direction toward the brake disk 20 or, in the course of a contrary rotational motion of the spindle 14, axially rearward until reaching a stop 21. For the purpose of generating a clamping force, the spindle nut 15 acts on the inner front side of the brake piston 16, as a result of which the brake piston 16 which is mounted in the braking apparatus so as to be axially displaceable is pressed with the brake pad 17 against the facing front face of the brake disk 20.

(16) For the hydraulic braking force, the hydraulic pressure of the brake fluid 22 from the hydraulic vehicle brake 1 acts on the brake piston 16. The hydraulic pressure can also be effective in assisting manner at a standstill of the vehicle upon actuation of the electromechanical braking apparatus, so that the total braking force is composed of the portion provided by the electric motor and of the hydraulic portion. During the running of the vehicle, either only the hydraulic vehicle brake is active or both the hydraulic vehicle brake and the electromechanical braking apparatus are active or only the electromechanical braking apparatus is active in order to generate braking force. The regulating signals for triggering both the adjustable components of the hydraulic vehicle brake 1 and the electromechanical wheel-brake device 9 are generated in the closed-loop or open-loop control unit 11.

(17) In each of FIGS. 3 to 7 a graph is shown with the progression of the hydraulic brake pressure p, represented by a solid line, of the motor current I, represented by a dashed line, of the brake motor, and of the total braking force F.sub.br, represented by a dash-dotted line. Between two times t4 and t5 the electromotive braking force F.sub.e runs congruently with the motor current I.

(18) FIG. 3 shows the graph for an orderly braking process with functioning hydraulic vehicle brake and functioning electromechanical braking apparatus. On the other hand, in FIGS. 4 to 6 the hydraulic vehicle brake is defective; in FIG. 7 the electromechanical braking apparatus is defective.

(19) On the basis of the properly functioning progression in FIG. 3, the mode of operation for a braking process at a standstill of the vehicle for the purpose of generating a clamping force immobilizing the vehicle will be elucidated. Firstly, in a first step, the brake booster in the hydraulic vehicle brake is triggered, and a defined hydraulic brake pressure p2 is set in the hydraulic vehicle brake. The beginning of the rise in the hydraulic brake pressure p occurs at time t1. At time t3 the target brake pressure has been attained, which is retained in the further progression up until time t4; in the time-interval between t3 and t4 the brake booster is not actuated further, so the brake pressure p remains constant. The rise occurs linearly between times t2 and t3, in which connection a progression deviating therefrom, for instance an S-shaped progression of the rise, may occur where appropriate.

(20) At time t2, which follows shortly after the start-time t1, the electric brake motor of the electromechanical braking apparatus is actuated, and torque is applied in the direction of the build-up of a braking force. The triggering of the electric brake motor is characterized by a brief peak value in the motor current I, which in the further progression in the period between times t3 and t4 drops to a low value slightly above the zero line, which is characterized by the idling of the electromechanical braking apparatus. At time t4 the clearance between the brake pad on the brake piston, which is shifted by the electric brake motor, and the brake disk is overcome, whereupon in the time-interval between t4 and t5 an electromechanical braking force F.sub.e is built up which runs parallel to or congruently with the motor current I.

(21) With the build-up of electromechanical braking force F.sub.e the piston is displaced by the brake motor, as a result of which the volume available for the brake fluid increases, and the brake pressure p drops from the target brake pressure p2 to a reduced brake pressure p1 which is approximately one half of the target brake pressure p2. The total braking force F.sub.br, which is composed of the electromechanical component F.sub.e and the hydraulic braking-force component, rises progressively up until time t5, at which the linearly rising electromechanical braking force F.sub.e has reached its maximum and the hydraulic brake pressure p drops to zero in ramp-like manner.

(22) FIG. 4 shows a case of a fault in the braking system, in which a hydraulic brake line of the hydraulic vehicle brake is blocked, for instance by being pinched, so that no brake fluid is able to flow through this brake line. Although the target pressure p2 is attained upon actuation of the brake booster, the drop in pressure between times t4 and t5 is less than the set drop in pressure according to FIG. 3. The drop in pressure corresponds to approximately one half of the set drop in pressure, and this can be sensed via pressure sensors, whereupon a corresponding fault signal can be generated. From the extent of the actual drop in pressure, the type of fault, namely a pinched brake line in the hydraulic vehicle brake, can be inferred.

(23) FIG. 5 represents the case of a fault of a hydraulic brake line with reduced cross-section, which may happen, for instance, as a result of pinching of the brake line. The reduced cross-section of the brake line leads to a throttling behavior, this being expressed, between times t3 and t4, in a drop in pressure in comparison with the set pressure p2. After attaining the set pressure p2 at time t3, the hydraulic brake pressure drops to a lower brake-pressure level which is retained up until time t4, at which electromechanical braking force is generated.

(24) The drop in pressure in the time-interval between t4 and t5 is greater than in the case of a blocked brake-pressure line (FIG. 4) but less than the set drop in pressure (FIG. 3). From the extent of the drop in pressure at time t5, the type of fault can likewise be inferred.

(25) FIG. 6 shows the graph with a premature drop in pressure by reason of a leakage in the hydraulic vehicle brake. With the attaining of the set pressure p2 at time t3, the hydraulic brake pressure p already drops continuously during the idling phase between t3 and t4. With the build-up of electromechanical braking force between times t4 and t5, the brake-pressure level p diminishes further and drops below pressure level p1 which in the case of an intact braking system (FIG. 3) is attained at time t5. Also from this drop in pressure, the type of fault can be inferred.

(26) In FIG. 7 the braking situation with properly functioning hydraulic vehicle brake but with a fault in the electromechanical braking apparatus is represented. By reason of this fault, the brake piston is displaced less far than in the case of orderly functioning, so that a smaller additional volume for the brake fluid arises and correspondingly the drop in pressure in the progression of the brake pressure p turns out to be less than in the case of orderly functioning according to FIG. 3. The drop in pressure is also less than in the case of a blocked hydraulic brake line according to FIG. 4, so the type of fault can likewise be inferred.