Method for Operating an Automated Parking Brake

Abstract

A method for operating an automated parking brake in a motor vehicle with a hydraulic actuator for generating a hydraulic force component and an electromechanical actuator for generating an electromechanical force component, includes overlaying the hydraulic force component and the electromechanical force component to achieve a total clamping force for a parking brake process. The method further includes setting, on occurrence of a first condition, a first hydraulic pressure level, and setting, on occurrence of a second condition, a second hydraulic pressure level. The method also includes holding substantially constant the set first hydraulic pressure level with the hydraulic actuator until the occurrence of the second condition.

Claims

1. A method for operating an automated parking brake in a motor vehicle having a hydraulic actuator for generating a hydraulic force component and an electromechanical actuator for generating an electromechanical force component, the method comprising: overlaying the hydraulic force component and the electromechanical force component to achieve a total clamping force for a parking brake process, setting a first hydraulic pressure level on occurrence of a first condition; setting a second hydraulic pressure level on occurrence of a second condition; and holding substantially constant the set first hydraulic pressure level with the hydraulic actuator until the occurrence of the second condition.

2. The method according to claim 1, further comprising: defining the first hydraulic pressure level as a pressure for stopping the vehicle; and defining a target pressure of the automated parking brake as a pressure for parking the vehicle, wherein the second hydraulic pressure level corresponds to the defined target pressure.

3. The method according to claim 2, further comprising: taking into account a slope of the roadway in the defining the first hydraulic pressure level and/or defining the second hydraulic pressure level to enable stopping and/or parking the vehicle on a momentary roadway incline.

4. The method according claim 1, further comprising: holding substantially constant the set first hydraulic pressure level until the occurrence of the second condition by control of the hydraulic actuator and/or self-inhibition in a system of the hydraulic actuator.

5. The method according to claim 1, further comprising: activating the hydraulic actuator such that the setting of the first hydraulic pressure level, the holding substantially constant of the set first hydraulic pressure level, and the setting of the second hydraulic pressure level take place as part of a single activation.

6. The method according to claim 1, further comprising: identifying a parking brake request; and closing inlet valves on a front axle of the vehicle when the parking brake request is identified, or the first hydraulic pressure level is reached.

7. The method according to claim 1, wherein on reaching the second hydraulic pressure level until a total clamping force is achieved, the method further comprises: holding constant the second hydraulic pressure level by further adjustment of the hydraulic actuator; and/or closing switchover valves on the rear axle of the vehicle.

8. The method according to claim 1, further comprising: checking whether the second hydraulic pressure level is correctly set by taking into account a value of the electromechanical actuator representing a force curve and/or a current gradient.

9. The method according to claim 1, further comprising: checking whether the second hydraulic pressure level is correctly set by taking into account a first value of the electromechanical actuator representing a force curve and/or a current gradient during a first time interval, and a second value of the electromechanical actuator representing a force curve and/or a current gradient during a second time interval, and comparing the first value and the second value.

10. The method according to claim 9, further comprising: identifying a correct setting of the second hydraulic pressure level if the first value is less than the second value.

11. The method according to claim 8, further comprising: taking into account values of several electromechanical actuators in the check; and comparing the values of several electromechanical actuators, wherein the values of several electromechanical actuators represent a force curve of a respective one of the several electromechanical actuators and/or a current gradient.

12. The method according to claim 1, further comprising: determining the first and the second hydraulic pressure levels with a pressure sensor, a travel sensor, and/or algorithms for estimating pressure and/or travel and/or position of a component, wherein the component is a spindle nut.

13. A control unit for operating an automated parking brake for a motor vehicle having a hydraulic actuator for generating a hydraulic force component and an electromechanical actuator for generating an electromechanical force component, the control unit comprising: a non-transitory computer readable medium having program instructions configured to cause the control unit to overlay the hydraulic force component and the electromechanical force component to achieve a total clamping force for a parking brake process, to set a first hydraulic pressure level on occurrence of a first condition, to set a second hydraulic pressure level on occurrence of a second condition, and to hold substantially constant the set first hydraulic pressure level with the hydraulic actuator until the occurrence of the second condition.

14. An automated parking brake for a motor vehicle, comprising: a hydraulic actuator configured to generate a hydraulic force component; an electromechanical actuator configured to generate an electromechanical force component; and a control unit configured to overlay the hydraulic force component and the electromechanical force component to achieve a total clamping force for a parking brake process, to set a first hydraulic pressure level on occurrence of a first condition, to set a second hydraulic pressure level on occurrence of a second condition, and to hold substantially constant the set first hydraulic pressure level with the hydraulic actuator until the occurrence of the second condition.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0043] In the figures:

[0044] FIG. 1 shows a diagrammatic section view of a brake device with an automated parking brake in motor on caliper construction, and

[0045] FIG. 2 shows a diagrammatic depiction of the force curve during and after the brake application phase of a parking brake process according to the application, and

[0046] FIG. 3 shows a diagrammatic depiction of a pV curve for the rear axle and front axle, and

[0047] FIG. 4 shows a diagram with the time-dependent curve of the motor current of the electromechanical actuator and the hydraulic actuator, the hydraulic brake pressure and the brake pressure in the master cylinder, and the total brake force.

DETAILED DESCRIPTION

[0048] FIG. 1 shows a diagrammatic section view of a brake device 1 for a vehicle. The brake device 1 has an automated parking brake 13 (also known as automatic parking brake or APB for short) which, by means of an electromechanical actuator 2 (electric motor) can exert a clamping force for parking the vehicle. For this, the electromechanical actuator 2 of the parking brake 13 shown drives a spindle 13 mounted in an axial direction, in particular a threaded spindle 13. At its end remote from the actuator 2, the spindle 3 is provided with a spindle nut 4 which, when the automated parking brake 13 is applied, lies against the brake piston 5. The parking brake 13 thus transmits a force to the brake pads 8, 8 or brake disc 7. Here, the spindle nut 4 lies on an inner end face of the brake piston 5 (also called the back of the brake piston floor or inner piston floor). On a rotary movement of the actuator 2 and a resulting rotary movement of the spindle 3, the spindle nut 4 is moved in the axial direction. The spindle nut 4 and the brake piston 5 are mounted in a brake caliper 6 which grips a brake disc 7 in the manner of pincers.

[0049] A brake pad 8, 8 is arranged on both sides of the brake disc 7. When the brake device 1 is applied by means of the automated parking brake 13, the electric motor (actuator 2) turns, whereupon the spindle nut 4 and the brake piston 5 are moved in the axial direction towards the brake disc 7 in order to generate a predefined clamping force between the brake pads 8, 8 and the brake disc 7. Because of the spindle drive and the associated self-inhibition, a force generated at the parking brake 13 by means of activation of the electric motor is maintained even when the activation is terminated.

[0050] The automated parking brake 13 is configured e.g. as a motor on caliper system and combined with the service brake 14. The parking brake 13 could also be regarded as integrated in the system of the service brake 14. Both the automated parking brake 13 and the service brake 14 act on the same brake piston 5 and the same brake caliper 6 to build up a braking force of the brake disc 7. The service brake 14 however has a separate hydraulic actuator 10, e.g. a foot brake pedal with a brake force amplifier. The service brake 14 is configured in FIG. 1 as a hydraulic system, wherein the hydraulic actuator 10 is supported by the ESP pump or an electromechanical brake force amplifier (e.g. Bosch iBooster) or can be implemented thereby. Further embodiments of the actuator 10 are conceivable, e.g. in the form of a so-called IPB (integrated power brake) which in principle constitutes a brake-by-wire system in which a plunger is used to build up hydraulic pressure. On service braking, a predefined clamping force is built up hydraulically between the brake pads 8, 8 and the brake disc 7. To build up a brake force by means of the hydraulic service brake 14, a medium 11, in particular a substantially incompressible brake fluid 11, is pressed into a fluid chamber delimited by the brake piston 5 and the brake caliper 6. The brake piston 5 is sealed against the environment by means of a piston sealing ring 12.

[0051] The brake actuators 2 and 10 are activated by means of one or more end stages, i.e. by means of a control unit 9 which e.g. may be a control unit of a driving dynamics systems such as ESP (electronic stability program) or another control unit.

[0052] When the automated parking brake 13 is activated, first the idle travel or play must be overcome before a braking force can be built up. The idle travel is e.g. the distance which the spindle nut 4 must overcome by rotation of the spindle 3 in order to come into contact with the brake piston 5.

[0053] The play refers to the distance between the brake pads 8, 8 and the brake disc 7 in disc brake systems of motor vehicles. This process usually takes a relatively long time in relation to the total activation, in particular for the automated parking brake 13. At the end of such a preparation phase, the brake pads 8, 8 are laid against the brake disc 7 and the force build-up begins on further activation. FIG. 1 shows the state in which the idle travel and play are already overcome. Here, the brake pads 8, 8 are placed against the brake disc 7 and all brakes, i.e. the parking brake 13 and the service brake 14, may on subsequent activation immediately build up a braking force at the corresponding wheel. The descriptions in relation to the play apply accordingly to the service brake 14, but because of the higher pressure build-up dynamic, overcoming an idle travel takes less time than for the parking brake 13.

[0054] FIG. 2 shows a diagrammatic depiction of the force curve F during and after a brake application process according to the disclosure, over a temporal perspective t. The method starts at the beginning of phase 1. First a defined hydraulic pressure value is built up. For this, for example, an actuator of the service brake system is activated. This could for example be the iBooster. In phase P1, the idle travel and play of the service brake are overcome. In a phase P2, the electrohydraulic force component F.sub.hydr is generated. For this, a defined pressure value is generated. As soon as the pressure value has been generated, this need merely be maintained in the further course. In the present example, the actuator of the parking brake system is activated at the same time as activation of the actuator of the service brake system. The idle travel of the parking brake is overcome in phase P3. After overcoming the idle travel of the parking brake, i.e. when the spindle nut lies against the brake piston, on a further deflection of the spindle nut, a steep force rise occurs since the brake system is already pretensioned by means of the hydraulic service brake. The actual superposition of the parking brake and service brake takes place in this phase P4. By activating the parking brake, the electromechanical force component F.sub.mech is generated. This is overlaid over the present electrohydraulic force component F.sub.hydr and increases the achieved total clamping force F.sub.ges. The actuator of the parking brake is activated until the required total clamping force F.sub.ges has been reached. This activation of the parking brake, due to the displacement of the brake piston, leads to an increase in the fluid volume between the brake caliper and the brake piston. Because of this increase in fluid volume, the hydraulic pressure may need to be adjusted by means of the service brake. This may take place in a targeted fashion by means of an iBooster system which is equipped with corresponding force sensors and means for pressure monitoring. When the total clamping force F.sub.ges required has been achieved, the activation is terminated, i.e. the electromechanical and electrohydraulic actuators are disengaged at the transition between phases 4 and 6. This prevents a further build-up of force. The disengagement of the actuators also leads to a reduction in the electromechanical force component F.sub.mech and the electrohydraulic force component F.sub.hydr. The total clamping force F.sub.ges built up is maintained however, even after termination of the brake application process, since the exemplary parking brake as described is provided with a self-inhibition, as depicted in phase P5. Only an active operation of the parking brake in the reverse direction leads to a release of the parking brake, which is not however depicted in FIG. 2.

[0055] FIG. 3 shows a diagrammatic depiction of an exemplary pV curve (pressure-volume curve) for the rear axle (HA) and the front axle (VA) of in vehicle. Using the pV curve of the corresponding vehicle, the necessary volumes to be displaced can be determined from the desired target pressure. In the method described, the hydraulic force proportion is adjusted via the travel of the hydraulic actuator. The displaced volume can be calculated from this, taking into account the master cylinder piston area.

[0056] An electric brake force amplifier is assumed below as an example, which already for component reasons contains a pedal travel sensor. An alternative would be a system with a plunger and travel sensor. If for example 20 bar are required to stop the vehicle, the travel by which the electric brake force amplifier must move the push-rod must be calculated as follows: from the pV curves, a volume capacity at each brake caliper on the front axle of 0.68 cm.sup.3 and at the rear axle of 0.31 cm.sup.3 can be read. Therefore the following total volume must be displaced: V.sub.1=2*V.sub.VA+2*V.sub.HAI=2*0.69 cm.sup.3+2*0.31 cm.sup.3=1.98 cm.sup.3. With a master brake cylinder diameter of d.sub.HZ=23.4 mm, the area A.sub.HZ can be calculated as follows: A.sub.HZ=(d.sub.HZ).sup.2*pi/4=((23.4 mm).sup.2*3.14)/4=430 mm.sup.2. This then gives a push-rod travel of S.sub.push=V.sub.1/A.sub.HZ=1.98 cm.sup.3/4.30 cm.sup.2=0.46 cm.

[0057] At low pressures, there is no linear correlation between pressure and displaced volume, so the pressure build-up in t1 to t3 is shown accordingly (see FIG. 4). During the idle phase, the push-rod is held in position in order to provide a constant hydraulic pressure. The inlet valves on the front axle are also closed.

[0058] As soon as the APB begins to build up clamping force, because of the volume capacity of the APB, the pressure in the system falls. This means firstly that the volume capacity must be compensated by further advance of the push-rod, and secondly that the target pressure in the rear axle must be built up. In order to achieve a pressure of for example 70 bar when a 20 bar pressure is already present, the following volume is necessary: V.sub.HA2=2*0.43 cm.sup.3=0.86 cm.sup.3. This corresponds to a push-rod travel of s.sub.push=V.sub.HA2/A.sub.HZ=0.86 cm.sup.3/4.30 cm.sup.2=0.2 cm. The volume displaced by the APB can be calculated as follows: with a known brake caliper stiffness of c.sub.brake=40 kN/mm and a target clamping force for example of F.sub.APB=10 kN, and a rear axle brake piston diameter of d.sub.piston.rear=38 mm, hence a rear axle brake piston area of A.sub.piston,rear=d.sub.piston,rear.sup.2*pi/4=(38 mm).sup.2*3.14/4=1134 mm.sup.2, the piston travel on force build-up can be calculated as follows: s.sub.piston=F.sub.APB/c.sub.brake=10 kN/(40 kN/mm)=0.25 mm, and consequently the volume displaced by the APB: V.sub.APB=2*A.sub.piston,rear*S.sub.piston=567 mm.sup.30.57 cm.sup.3. In order to hold the pressure at a constant 70 bar during the force build-up by the brake force amplifier, accordingly a further advance of the push-rod by s.sub.push,APB=V.sub.APB/A.sub.HZ=0.57 cm.sup.3/430 cm.sup.20.13 cm is required.

[0059] During the simultaneous hydraulic and electromechanical force build-up (see t.sub.4 to t.sub.5 in FIG. 4), as described above, a lower current gradient is expected than in the subsequent electromechanical clamping force build-up phase with static support pressure level (see t.sub.5 to t.sub.6 in FIG. 4). By using algorithms to estimate the position of the spindle nut of an APB system, the actual spindle nut travel in the force build-up can be calculated constantly and, because of the continuous hydraulic path, supplied to the push-rod regulation of the electric brake force amplifier as a guide parameter. In this way, the method can also be used to monitor pressure in the rear axle brakes even without a pressure sensor. After reaching the target clamping force level, the electromechanical actuator is disengaged and because of its self-inhibiting design, remains in its position. Then the hydraulic pressure can be released.

[0060] FIG. 4 shows a diagram with electrical and hydraulic status parameters on a brake application process for parking the vehicle when stationary. At time t.sub.i, via an electrically controllable actuator of the hydraulic vehicle brake, a hydraulic brake pressure p is generated, for example by actuation of the ESP pump. Here, I.sub.hydr indicates the curve of the current intensity of the hydraulic actuator. This initially rises sharply on activation (starting peak). Until the first pressure level p1 is reached, the current intensity remains substantially constant at a defined height. At time t.sub.3, the hydraulic brake pressure reaches the first level p1.

[0061] At time t.sub.2, the power begins to be supplied to the electric brake motor (electromechanical actuator) with motor current I.sub.mech (i.e. current intensity of the electromechanical actuator), which after a starting pulse falls to an idle current value and maintains this over the period between t.sub.3 and t.sub.4. At time t.sub.3, the hydraulic brake pressure p reaches a pre-pressure value which is retained until time t.sub.4; the phase between t.sub.3 and t.sub.4 constitutes the idle phase of the electric brake motor. As long as the idle travel is overcome, the pressure p is held constant at pressure level p1. The current intensity I.sub.hydr of the hydraulic actuator required for this is lower than for pressure generation.

[0062] At time t.sub.4, via the electric brake motor, an electromechanical braking force is generated and accordingly the motor current I.sub.mech rises starting from the level of the idle current. Then the hydraulic actuator is activated with a higher current intensity I.sub.hydr in order to set the desired second pressure level p2. Here, the hydraulic brake pressure p rises further starting from the first level p1, so that by overlaying the hydraulic and electromechanical brake forces, a total brake force F.sub.ges is set.

[0063] At time t.sub.5, the hydraulic brake pressure reaches its maximum p2 which is retained until time t.sub.6 and then falls again to 0 by time t.sub.7. In the period between t.sub.5 and t.sub.6, the hydraulic pressure level p2 reached is held constant and adjusted by the hydraulic actuator. This takes place with a reduced current intensity I.sub.hydr. In the period between t.sub.5 and t.sub.6, the electromechanical brake force rises further in synchrony with the brake current I.sub.mech, until a maximum is reached.