Method for controlling a parking brake in a vehicle

Abstract

In a method for controlling a parking brake in a vehicle, in order to set a clearance between the brake pad and the brake disk, a supply voltage is first applied to an electric motor, after which the supply voltage is switched off before the brake pad comes into contact with the brake disk.

Claims

1. A method for controlling a parking brake in a vehicle, the parking brake including an electromechanical braking device having an electric brake motor configured to displace a brake pad on a brake piston in a direction of a brake disk, the method comprising: applying a supply voltage to the electric brake motor to set the brake pad in motion in the direction of the brake disk from a starting position; and after applying the supply voltage, switching off the supply voltage to the electric brake motor in response to the supply voltage having been applied to the electric brake motor for a predefined time period and before the brake pad comes into contact with the brake disk, the brake pad being allowed to continue to move in the direction of the brake disk after switching off the supply voltage.

2. The method according to claim 1 further comprising: determining a distance traveled by the brake pad in the direction of the brake disk from the starting position based on the motor constant of the electric brake motor.

3. The method according to claim 1 further comprising: after switching off the supply voltage, detecting whether there is a clamping force between the brake pad and the brake disk; and slowing a rotation of the electric brake motor in response to the clamping force being detected and the clamping force exceeding a threshold value.

4. The method according to claim 3, the slowing the rotation of the electric brake motor further comprising: slowing the rotation of the electric brake motor by controlling the electric brake motor to rotate in a direction opposite a current rotating direction of the electric brake motor.

5. The method according to claim 3, the slowing the rotation of the electric brake motor further comprising: slowing the rotation of the electric brake motor by short-circuiting a power output stage of the electric brake motor.

6. The method according to claim 1, wherein the applying the supply voltage and the switching off the supply voltage are performed one of (i) before an automated parking procedure and (ii) during the automated parking procedure.

7. The method according to claim 1, wherein the applying the supply voltage and the switching off the supply voltage are performed one of (i) before an operation of the vehicle on a roller dynamometer and (ii) during the operation of the vehicle on a roller dynamometer.

8. A control unit for controlling a parking brake in a vehicle, the parking brake including an electromechanical braking device having an electric brake motor configured to displace a brake pad on a brake piston in a direction of a brake disk, the control unit being one of closed-loop and open-loop, the control unit comprising: a processor configured to: apply a supply voltage to the electric brake motor to set the brake pad in motion in the direction of the brake disk from a starting position; determine a motor constant of the electric brake motor while the brake pad moves in the direction of the brake disk; and after applying the supply voltage, switch off the supply voltage to the electric brake motor after the motor constant of the brake motor is ascertained and before the brake pad comes into contact with the brake disk, the brake pad being allowed to continue to move in the direction of the brake disk after switching off the supply voltage.

9. A parking brake in a vehicle, the parking brake comprising: an electromechanical braking device having an electric brake motor configured to displace a brake pad on a brake piston in a direction of a brake disk; and control unit, the control unit being one of closed-loop and open-loop, the control unit being configured to: apply a supply voltage to the electric brake motor to set the brake pad in motion in the direction of the brake disk from a starting position; determine a switch-off time based on the supply voltage, a measured motor current of the electric brake motor, and a desired distance to be set between the brake pad and the brake; and after applying the supply voltage, switch off the supply voltage to the electric brake motor at the switch off time before the brake pad comes into contact with the brake disk, the brake pad being allowed to continue to move in the direction of the brake disk after switching off the supply voltage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Exemplary embodiments of the disclosure are presented in the drawings an are explained in more detail in the description below.

(2) In the drawings:

(3) FIG. 1 shows a section through an electromechanical parking brake for a vehicle, in which the clamping force can be generated via an electric brake motor,

(4) FIG. 2 shows a graph of the time-dependent progression of the supply voltage for the brake motor, the motor current, the motor speed, and the clamping force during the engagement process of the parking brake,

(5) FIG. 3 shows a graph of the shapes of the curves during the switch-off of the supply voltage,

(6) FIG. 4 shows a block diagram having process steps for setting the clearance between the brake pad and the brake disk, depicted for a switch-off of the supply voltage after a fixedly predefined time period,

(7) FIG. 5 shows a block diagram for setting the clearance in an alternative embodiment, in which the supply voltage is switched off only after the motor constant of the brake motor has been ascertained,

(8) FIG. 6 shows a further block diagram for setting the clearance, in which the switch-off time of the supply voltage is computationally determined on the basis of the idle travel of the brake motor.

DETAILED DESCRIPTION

(9) FIG. 1 shows an electromechanical parking brake 1 for fixing a vehicle at a standstill. The parking brake 1 comprises a brake caliper 2 having a gripping device 9, which overlaps a brake disk 10. The parking brake 1 comprises, as the actuator, a direct-current electric motor as the brake motor 3, the rotor shaft of which drives a spindle 4 in a rotating manner, on which spindle a spindle nut 5 is rotatably mounted. During a rotation of the spindle 4, the spindle nut 5 is axially displaced. The spindle nut 5 moves within a brake piston 6, which is the carrier of a brake pad 7, which brake pad is pressed against the brake disk 10 by the brake piston 6. Another brake pad 8, which is fixedly held on the gripping device 9, is located on the opposite side of the brake disk 10.

(10) Within the brake piston 6, during a rotary motion of the spindle 4 axially forward, the spindle nut 5 can move in the direction toward the brake disk 10 and, during an opposing rotary motion of the spindle 4 axially backward, said spindle nut can move until it reaches a stop 11. In order to generate a clamping force, the spindle nut 5 acts upon the inner end face of the brake piston 6, whereby the brake piston 6which is mounted in the parking brake 1 so as to be axially displaceablehaving the brake pad 7 is pressed against the facing end face of the brake disk 10.

(11) The brake motor 3 is controlled by a closed-loop or open-loop control unit 12, which is part of the parking brake system comprising the parking brake 1. The closed-loop or open-loop control unit 12 delivers, as the output, a supply voltage U.sub.S0, which is applied to the electric brake motor 3.

(12) The parking brake can be supported by a hydraulic vehicle brake, if necessary, and so the clamping force is composed of an electric-motor portion and a hydraulic portion. In the case of the hydraulic support, the back side of the brake piston 6 facing the brake motor is acted upon with pressurized hydraulic fluid.

(13) FIG. 2 shows a graph of the current progression I.sub.A, the supply voltage U.sub.S0, and the progression of the motor angular speed w of the electric brake motor as a function of time for an engagement process. FIG. 2 also shows a graph of the electromechanical clamping force F.sub.KI, which is generated by the electric brake motor, and the distance s covered by the brake motor or by an actuator acted upon by the brake motor, during the engagement process.

(14) The engagement process starts at the time t1 by way of an electrical voltage being applied and the brake motor being energized when the circuit is closed. The start phase (phase I) lasts from the time t1 to the time t2. At the time t2, the supply voltage U.sub.S0 and the motor angular speed w have reached their maximum. The phase between t2 and t3 is the idle phase (phase II), in which the current I.sub.A moves at a maximum level. This is followed, at the time t3, by the force build-up phase (phase III) up to the time t4, in which the brake pads rest against the brake disk and are pressed against the brake disk with an increasing clamping force F.sub.KI. At the time t4, the electric brake motor is switched off by opening the electric circuit, and so, as the progression continues, the angular speed w of the brake motor drops to zero.

(15) The force-increase point coincides with the phase of the force build-up at the time t3. The force build-up or the progression of the clamping force F.sub.KI can be determined, for example, on the basis of the progression of the current I.sub.A of the brake motor, which has the same progression, in principle, as the electromechanical clamping force F.sub.KI. Proceeding from the low level during the idle phase between t2 and t3, the current progression I.sub.A increases steeply at the beginning of the time t3. This increase in the current can be detected and utilized for determining the force-increase point. In principle, the progression of the force build-up can also be determined from the progression of the voltage or the speed, or from any combination of the signals for the current, voltage, and speed.

(16) The graph according to FIG. 3 also shows the progression over time of the supply voltage U.sub.S0, the distance s covered, the motor angular speed to, the motor current I.sub.A, and the clamping force F.sub.KI, although the supply voltage U.sub.S0 is maintained only up to a time t.sub.Ansteuer, after which the supply voltage U.sub.S0 is switched off. At this time, the supply voltage U.sub.S0 has reached its nominal level, the motor speed is at an idle level. The brake pad on the brake piston is still spaced from the brake disk, and therefore no clamping force F.sub.KI has been built up yet, either.

(17) When the supply voltage U.sub.S0 is switched off, said supply voltage returns to the value zero, while the rotor shaft of the electric brake motor continues to rotate, due to inertia, and therefore the distance s covered continues to increase while, simultaneously, the angular speed to decreases. When contact between the brake pad and the brake disk is achieved, the clamping force F.sub.KI increases to a value greater than zero.

(18) This procedure is intended for setting the clearance, i.e., the spacing between the brake pad and the brake disk, to a reduced value before the actual engagement process of the parking brake, wherein the clearance can be reduced to zero, if necessary. In the latter case, however, the resultant clamping force F.sub.KI should be below a clamping force threshold value.

(19) The switch-off time t.sub.Ansteuer of the supply voltage U.sub.S0 can be defined in different ways. According to a first variant embodiment, the switch-off time t.sub.Ansteuer is a fixedly predefined time. This time is advantageously selected in such a way that, first, the supply voltage U.sub.S0 and the motor angular velocity co have reached their stationary values, but the clearance is largely or completely reduced when the supply voltage U.sub.S0 is switched off and the brake motor runs out or slows down. Depending on the clearance desired, the brake motor can be reactivated after completion of the run-out or in the end phase of the run-out and can be acted upon with supply voltage, specifically either in the direction of the brake disk or in the opposite direction.

(20) FIG. 4 shows a block diagram for setting a desired clearance in the parking brake using a fixedly predefined switch-off time t.sub.Ansteuer. In a first block 20, the engagement process in the parking brake is started first by way of the supply voltage U.sub.S0 being applied to the electric brake motor. The idle speed has been reached in the following method step 21. In the next method step 22, the supply voltage U.sub.S0 is switched off after the switch-off time t.sub.Ansteuer has been reached, after which the brake motor enters the run-out mode. In the method step 23, the run-out of the brake motor has ended, and the spindle nut having the brake piston and brake pad has come to a standstill.

(21) This is followed by a monitoring block 24 having different queries related to a desired clamping force F.sub.KI in the standstill-state of the brake motor. In the query block 24, an initial query is carried out in step 25 to determine whether a clamping force F.sub.KI was built up while the brake motor was at a standstill, which can be detected, for example, on the basis of the motor current. If this is not the case, the brake pad is spaced from the brake disk, and there is clearance. In this case, the no branch (N) is followed to the next method step 26, in which a query is carried out as to whether a clamping force build-up is desired. If so, the yes branch (Y) is followed to the method step 27, according to which the brake motor is briefly controlled in the engagement direction in order to overcome the clearance and bring the brake pad into contact with the brake disk, and so an at least small clamping force is built up. In the subsequent step 28, the method for setting the clearance is now ended.

(22) However, if the result of the query in step 26 is that a build-up of clamping force is not desired, the no branch is followed directly to step 28, to end the setting of the clearance.

(23) If the result of the query in the method step 25 as to whether a clamping force has actually been built-up, the yes branch is followed to the step 29, where a query is carried out as to whether the build-up of clamping force is also desired. If so, the yes branch is followed to the step 28, to end the method. Otherwise, the no branch is followed to step 30, in which the brake motor is briefly controlled in the opposite direction for disengagement, and so the brake pad is moved away from the brake disk and the clamping force is reduced back to zero. Next, the method is ended according to step 28.

(24) In one variant embodiment, the switch-off time t.sub.Ansteuer is not fixedly predefined, but rather the supply voltage is switched off only after the motor constant k.sub.M of the brake motor in the ongoing method has been estimated. With consideration for the motor constant k.sub.M, the distance x(t) covered by the spindle nut or the brake piston having the brake padwhich nut is driven by the slowing-down rotor shaft of the brake motorcan calculate in the run-out mode, with the supply voltage U.sub.S0 switched off:

(25) $w\left(t\right)=\frac{1}{k_M}.Math.u_{\mathrm{EMK}}\left(t\right)$$x\left(t\right)=\frac{s_{\mathrm{SP}}}{k_M*2**i_{\mathrm{Getr}}}*u_{\mathrm{EMK}}\left(t\right)dt$

(26) In this case, U.sub.EMK stands for the voltage induced in the run-out of the brake motor, s.sub.SP is the thread pitch required to convert the rotary motion of a spindle into a translatory motion of the spindle nut and, therefore, of the brake piston and the brake pad, wherein the spindle is driven by the rotor shaft of the brake motor. The parameter i.sub.Getr represents all the gear stages that bring about a gear ratio between the rotor shaft of the brake motor and the spindle. The induced voltage U.sub.EMK can be measured.

(27) If the distance covered in the run-out mode is known, it is possible to actively intervene, if necessary, by accelerating or braking the brake motor, for example, by correspondingly switching a control logic.

(28) The associated block diagram is shown in FIG. 5. The only difference from the block diagram according to FIG. 4 is in the method steps 40 to 44, which are adjoined by the monitoring block 24, which is identical to that from FIG. 4.

(29) According to the first method step 40, first the engagement process of the parking brake is started by applying the supply voltage U.sub.S0 until the idle speed is reached in the next step 41. Subsequently, a query is carried out in the method step 42 as to whether the motor constant k.sub.M has already been calculated in the on-going method. If not, the no branch is followed back to the beginning of the method 42 and the query is carried out again at cyclic intervals.

(30) If the motor parameter k.sub.M is already available, however, the yes branch is followed to the next step 43, according to which the supply voltage U.sub.S0 is switched off and the brake mode is transferred into the run-out mode. In the step 44, the brake motor has come to a standstill. This is followed by the monitoring block 24, in which a check is carried out to determine whether a clamping force was built up and a clamping force is desired, after which the motor is briefly controlled in the engagement direction or in the disengagement direction, if necessary.

(31) In another variant embodiment, the switch-off time t.sub.Ansteuer of the supply voltage U.sub.S0 is determined on the basis of idle travel x.sub.Leerweg, which is calculated, as follows, via addition
x.sub.Leerweg=x.sub.Anlauf+x.sub.Leerlauf+x.sub.Auslauf
of the components x.sub.Anlauf, x.sub.Leerlauf and X.sub.Anstauf. x.sub.Anlauf in this case stands for the distance that the spindle nut or the brake piston covers after the application of the supply voltage until the idle speed of the motor is reached. x.sub.Leerlauf stands for the distance that the spindle nut or the brake piston covers during the idling of the motor, and x.sub.Auslauf stands for the distance that the spindle nut or the brake piston cover after the switch-off of the supply voltage U.sub.S0 in the slow-down or run-out phase. x.sub.Anlauf, x.sub.Leerlauf and x.sub.Auslauf can be ascertained from

(32) $x_{\mathrm{Anlauf}}=\frac{s_{\mathrm{SP}}}{k_M*2**i_{\mathrm{Getr}}}*{}_0^{t_{{\mathrm{Leerlauf}}_{\mathrm{Beginn}}}}\left[u_S\left(t\right)-R_{\mathrm{ges}}*I_A\left(t\right)\right]dt$$x_{\mathrm{Leerlauf}}=\frac{s_{\mathrm{SP}}}{k_M*2**i_{\mathrm{Getr}}}*\left[u_{S0}-R_{\mathrm{ges}}*I_0\right]*t_{\mathrm{Ansteuer}}$$x_{\mathrm{Auslauf}}=\frac{s_{\mathrm{SP}}}{k_M*2**i_{\mathrm{Getr}}}*{}_{0_{{\mathrm{Auslauf}}_{\mathrm{Anfang}}}}^{t_{{\mathrm{Auslauf}}_{\mathrm{Ende}}}}{u}_{\mathrm{EMK}}\left(t\right)dt$
with consideration for the sampled motor current I.sub.A during the start-up phase, the total resistance of the motor R.sub.ges, the idle current I.sub.0 during the idle phase, and the induced voltage U.sub.EMK during the slow-down phase. On the basis thereof and with reference to the following

(33) $t_{\mathrm{Ansteuer}}=\frac{\frac{x_{\mathrm{Leerweg}}*k_M*2**i_{\mathrm{Getr}}}{s_{\mathrm{SP}}}}{U_{S0}\left(t\right)-R_{\mathrm{ges}}*I_{A0}\left(t\right)}-\frac{{}_0^{t_{{\mathrm{Leerlaf}}_{\mathrm{Beginn}}}}\left[u_S\left(t\right)-R_{\mathrm{ges}}*t_A\left(t\right)\right]dt-{}_{t_{{\mathrm{Auslauf}}_{\mathrm{Anfang}}}}^{t_{{\mathrm{Auslauf}}_{\mathrm{Ende}}}}{u}_{\mathrm{EMK}}\left(t\right)*dt}{U_{S0}\left(t\right)-R_{\mathrm{ges}}*I_{A0}\left(t\right)}$
the switch-off time t.sub.Ansteuer for a desired clearance can be precisely determined, which clearance represents the difference between the idle travel x.sub.Leerweg, which was computationally determined, of the specific situation and the actual idle travel.

(34) The associated block diagram is shown in FIG. 6. In the first method step 50, the engagement process of the parking brake is started again until the idle speed is reached in step 51. Subsequently, in step 52, the calculation of x.sub.Anlauf, x.sub.Leerlauf and x.sub.Auslauf is carried out, on the basis of which the switch-off time t.sub.Ansteuer can be ascertained. In the step 53, the supply voltage is switched off at the switch-off time t.sub.Ansteuer and in step 54 the spindle nut or the brake piston comes to a standstill.

(35) This is followed, as in FIGS. 4 and 5, by the query or monitoring block 24 with a query of the clamping force and, if necessary, a control of the brake motor in the engagement or disengagement direction.