DE102011109842A1 - Method for operating a driver assistance device and driver assistance device - Google Patents

Abstract

The invention relates to a method for operating a driver assistance device of a motor vehicle, wherein the driver assistance device performs an environmental monitoring of the motor vehicle and initiates a braking process with a given initial braking force in the event of a collision hazard with an obstacle. It is provided that after reaching the initial braking force determines a deceleration braking force and the braking operation is continued in a deceleration phase with this, wherein the deceleration braking force is smaller than the initial braking force. The invention further relates to a driver assistance device of a motor vehicle.

Description

The invention relates to a method for operating a driver assistance device of a motor vehicle, wherein the driver assistance device performs an environmental monitoring of the motor vehicle and initiates a braking process with a given initial braking force in the event of a collision hazard with an obstacle. The invention further relates to a driver assistance device.

State of the art

Methods of the type mentioned are known from the prior art. They serve to operate the driver assistance device, which is designed for the at least partially at least partially automatic, in particular fully automatic, implementation of a driving operation of the motor vehicle.

In particular, the driver assistance device is provided for collision avoidance or at least for reducing a collision severity. For this purpose, the driver assistance device or a system affiliated with it carries out the environmental monitoring of the motor vehicle. For this purpose, at least one sensor arrangement with at least one sensor is provided in particular. In this case, the sensor is, for example, a distance sensor, by means of which it is possible to detect the presence of an obstacle in the surroundings of the motor vehicle, and preferably additionally to determine the distance of the motor vehicle from the obstacle. The sensor is preferably arranged to detect the obstacle in the direction of travel or main direction of travel of the motor vehicle. The main direction of travel is to be understood as meaning a direction which lies in the direction of a vehicle longitudinal axis of the motor vehicle. If an obstacle or a number of obstacles are detected in the environmental monitoring, the risk of collision is determined for the obstacle or each of the obstacles. For this purpose, an evaluation is carried out, for example, in which the probability of a collision is determined in particular on account of the relative speed between the motor vehicle and the respective obstacle as well as the distance between the motor vehicle and the obstacle. If there is a risk of collision with the obstacle, ie in particular if the probability is above a certain limit value, then the braking process with the specific initial braking force is initiated by the driver assistance device in order to avoid the collision. The braking force is proportional to a braking deceleration of the motor vehicle, ie to a negative acceleration. In the following, the terms braking force and braking deceleration are used synonymously. However, the braking force can also describe the set on a brake system of the motor vehicle for deceleration of the motor vehicle force or the brake pressure.

The initial braking force may correspond, for example, to a maximum braking force of the braking system of the motor vehicle or at least a fraction of it. However, it can also be a substantially arbitrary braking force, preferably determined on the basis of an operating variable of the motor vehicle. In this case, the operating variable is understood as meaning, for example, the speed of the motor vehicle, the distance of the motor vehicle from the obstacle or the relative speed of the motor vehicle relative to the obstacle. From the DE 10 2008 056 204 A1 For example, a driver assistance system for avoiding a collision of a vehicle is known, which is designed to carry out an autonomous deceleration of the speed of the motor vehicle with a first negative acceleration in a critical driving situation of the motor vehicle, which can lead to a collision, if a first condition is met is.

Subsequently, an autonomous deceleration of the speed of the motor vehicle with a second, larger negative acceleration is provided when a second condition is met and before the last time to avoid a collision has elapsed. This procedure is justified by the fact that the first negative acceleration should initially be limited, in order to avoid that a false estimate of the current situation by an environmental sensor system and electronics of the motor vehicle leads to a critical emergency stop. Only if the driver does not respond to the autonomous deceleration with the first acceleration, then the second, larger negative acceleration is used to avert the impending collision.

This means in total that the driver assistance device in critical situations in which a driver of the motor vehicle makes a mistake in the management of the motor vehicle (for example, because he is inattentive or too late or wrongly responds) briefly at least partially takes over the leadership of the motor vehicle and corrective intervention. Usually, the driver assistance device is designed such that it engages very late in the leadership of the motor vehicle to correct the error of the driver. The braking process is so far introduced with a relatively large initial braking force in order to avoid the collision with the obstacle can. Due to the large initial braking force, the duration of the braking process is usually low. At the same time strong inertial forces act on the driver. Likewise, in particular at the time at which the standstill is reached, or subsequently, a nod of the motor vehicle and additionally act on the driver. Immediately after performing the braking operation or after the completion of the braking process, however, the driver must take over the leadership of the motor vehicle as soon as possible again. In particular, if the driver is distracted or inattentive at the time of performing the braking or has a long reaction time, he is due to the braking initially possibly at least for a short time disoriented and therefore can not react properly, so even the leadership of the motor vehicle not yet again high reliability itself.

Disclosure of the invention

In contrast, the method for operating a driver assistance device with the features of claim 1 has the advantage that the driver despite the implementation of the braking process with a relatively high initial braking force can easily take over again the leadership of the motor vehicle itself. For this purpose, the invention provides that after reaching the initial braking force determines a deceleration braking force and the braking process is continued in a deceleration phase with this, wherein the deceleration braking force is smaller than the initial braking force. Thus, as is known from the prior art, the braking process is initiated with the determined initial braking force and carried out during an initial braking phase. After the initial braking phase has elapsed, however, the braking process should not be terminated, but should be continued in a deceleration phase with the lower deceleration braking force. The initial braking phase is followed by the deceleration phase.

In order to minimize the effects of the autonomous braking process on the driver, the deceleration braking force should be less than the initial braking force. It is therefore intended to vary the braking force during the implementation of the braking operation over time and preferably to reduce it steadily. In this way, it is possible for the driver of the motor vehicle to orient himself and to adjust to the driving situation changed by the braking process. He can thus better or more reliable take over the further guidance of the motor vehicle after completion of the braking process. With the procedure described, the motor vehicle can be safely decelerated or decelerated, but with much less severity than known from the prior art. With the thus achieved "Soff stop" effect takes place a reduction of the vehicle preload and the braking process is comfortable for the driver or other occupants of the motor vehicle. In addition, the pitch of the motor vehicle, which is excited by the braking process, significantly weaker and has a lower amplitude. The driver is so far less affected than known from the prior art method. Preferably, the braking operation is initiated only if the instantaneous speed of the motor vehicle - which is present before the braking operation - is less than or equal to a certain maximum speed, for example 20 km / h to 50 km / h.

A development of the invention provides that the deceleration phase is initiated immediately after reaching the initial braking force or after an initial braking phase with a certain initial braking time, wherein preferably the deceleration phase is continued until the motor vehicle is at a standstill. In the initial braking phase, the actually present braking force is usually adjusted over a certain period of time in the direction of the initial braking force. It may, as known from the prior art, be provided initially to use a smaller braking force and set only after failure of a reaction of the driver within a certain reaction period, the braking force to the initial braking force. As soon as the initial braking force is reached in the initial braking phase, the initial braking phase can be ended and the deceleration phase can be initiated. The delay phase follows in this respect directly to the initial braking phase. Alternatively, the deceleration phase can also be initiated only after the initial braking phase has been carried out over the specific initial braking time duration. During the deceleration phase, the braking operation is continued with the deceleration braking force. This is preferably provided to a standstill of the motor vehicle. After carrying out the initial braking phase and the deceleration phase, the motor vehicle is therefore at a standstill. Alternatively, however, it may also be provided to carry out the deceleration phase only until a specific speed difference has been reached at a speed present before the braking operation. For example, during the initial braking phase and the deceleration phase, the speed should be reduced by the specific speed difference, which may in particular be 10 km / h to 40 km / h, and the braking process subsequently terminated.

A further development of the invention provides that the initial braking time duration, the initial braking force, the deceleration braking time duration and / or the deceleration braking force as a function of at least one operating variable of the motor vehicle, in particular the speed, the distance to the obstacle and / or the relative speed of the motor vehicle to the obstacle, be determined. The initial braking time and / or the Initial braking force are preferably determined such that until the initiation of the subsequent to the initial braking phase deceleration phase, the risk of collision is eliminated or at least significantly reduced. Accordingly, within the initial braking time period, the braking operation is performed with the initial braking force such that the motor vehicle is decelerated to a speed at which the risk of collision with the obstacle no longer exists or at least is no longer probable. For this purpose, the initial braking time duration and / or the initial braking force are determined as a function of the at least one operating variable of the motor vehicle. Accordingly, in addition or alternatively, the deceleration braking force and the deceleration braking time can be set in dependence on the at least one operating variable of the motor vehicle to give the driver of the motor vehicle the opportunity to reorient himself so that he can carry out the driving operation of the motor vehicle himself after completion of the braking process. In this way, the comfort for the driver is to be increased even during the autonomous execution of the braking process.

It may be provided that the aforementioned variables are determined once, continuously or at intervals during the braking process. During the initial braking phase, the initial braking time duration and / or the initial braking force or, during the deceleration phase, the deceleration braking time duration and / or the deceleration braking force are thus determined accordingly. The one-time determination means that the respective variable is determined at the beginning of the respective phase. In the continuous determination, a continuous tracking of the respective size is provided. If the determination is provided at intervals, then there is a certain period of time between two points in time in which the respective variable is to be determined, so that no continuous determination takes place.

A development of the invention provides that after reaching the standstill of the motor vehicle, the braking operation is continued in a holding phase with a holding brake force over a holding brake period. The holding phase is preferably initiated immediately after reaching the standstill of the motor vehicle. The holding phase follows in this respect directly to the delay phase. In order to end the deceleration phase or to initiate the holding phase, the driver assistance device has means for detecting the stoppage of the motor vehicle. The holding brake force is constant in particular over time and preferably corresponds to the deceleration braking force at the conclusion of the deceleration phase or when the holding phase is initiated. The holding brake period may be preset, but also, for example, depending on the severity of the braking process, so the Initialbremszeitdauer, the Initialbremskraft, the delay brake duration and / or the deceleration braking force determined.

A development of the invention provides that after reaching the standstill of the motor vehicle and / or after the holding phase, the braking operation is continued in a startup phase with a Anrollbremskraft over a Anrollbremszeitdauer, the Anrollbremskraft reduced over the Anrollbremszeitdauer to zero and over time as a function of a state variable of the motor vehicle, in particular an inclination of the motor vehicle, a loading state and / or a direction of movement of the motor vehicle, is determined. Accordingly, the start-up phase or the deceleration phase should be followed by the start-up phase. In this, the braking operation is continued with the Anrollbremskraft over the Anrollbremszeitdauer. The Anrollbremskraft is reduced to zero. During the Anrollbremsphase the driver of the motor vehicle is thus given the opportunity to control the driving operation of the motor vehicle to resume. The Anrollbremskraft is preferably determined in dependence on the at least one state variable of the motor vehicle. The state variable is in particular the inclination of the motor vehicle or the background of the motor vehicle, the loading state and / or the direction of movement. In this way, when the motor vehicle is on an incline, the driver is given the opportunity to set a desired torque of a drive system of the motor vehicle to a value which allows the motor vehicle to start on the incline. This is especially true when the direction of movement of the motor vehicle is directed against the main direction of travel. In this case, the motor vehicle moves - usually due to the influence of gravity and against a driver's request - backwards. Because the movement is due to the influence of gravity and (at least predominantly) is not effected by the drive system of the motor vehicle, the direction of movement can also be referred to as rolling direction. With the determination of the Anrollbremskraft over time, the course of Anrollbremskraft over time is determined.

A development of the invention provides that at least during a present at least during the deceleration phase torque reduction phase, a target torque of a drive system of the motor vehicle is smaller than a default torque of a driver of the motor vehicle, in particular equal to a minimum torque of a drive system selected. The torque reduction phase is thus in particular at the beginning of the Delay phase or even already initiated at the beginning of the initial braking phase or the braking process. The torque reduction phase runs parallel to the initial braking phase, the deceleration phase, the holding phase and / or the startup phase. The torque reduction phase is preferably present during the entire initial braking phase and the entire deceleration phase, that is to say in particular until the motor vehicle is at a standstill. During the torque reduction phase, filtering of the driver's default torque is performed.

The default torque of the driver is the moment, which he pretends for example by means of an accelerator pedal, so the desired by the driver torque. Usually, during the driving operation of the motor vehicle, the target torque of the drive system, ie the torque which is set on the drive system, is substantially equal to the default torque or is readjusted with only a slight time delay. During the torque reduction phase, however, the target torque of the drive system should be selected smaller than the default torque, so that the drive system does not work against the brake of the motor vehicle during the braking process. Because no moment is needed to accelerate the motor vehicle during the initial braking phase or the deceleration phase, it is advantageous if the nominal torque is completely decoupled from the default torque during the torque reduction phase and equal to the minimum torque of the drive system is selected, ie the moment, which is just an operation of the drive system.

If the drive system has an electric machine, the minimum torque can be zero or correspond to a torque required by a secondary system. For an internal combustion engine, the minimum torque must be at least equal to a loss torque, which is present for example due to friction losses. After standstill of the motor vehicle, so usually after the deceleration phase, the target torque is usually greater than the minimum torque can be - if desired by the driver - to simplify a start of the motor vehicle on a slope. Alternatively, however, the filtering of the default torque during the entire braking process, ie including the holding phase and the start-up phase, be performed.

Furthermore, it may be provided to carry out the torque reduction phase over a minimum period of time. This means that, for example, after completion of the deceleration phase, the torque reduction phase continues until the minimum time duration has been reached or exceeded. The minimum period of time is chosen such that it gives the driver of the motor vehicle ample opportunity to adjust to the changed driving situation and to react accordingly. Accordingly, a (permanent) accelerator operation by the driver is irrelevant during the minimum period of time. Only when carrying out a demolition action described below, the torque reduction phase can be terminated before the minimum period expires.

A further development of the invention provides that the torque reduction phase is followed by a torque buildup phase during which the desired torque is adjusted to the default torque continuously or stepwise, wherein in particular the torque buildup period of the torque buildup phase and / or the desired torque over time as a function of the state variable, preferably the inclination of the motor vehicle, the loading state and / or the direction of movement of the motor vehicle, are determined. Like the torque reduction phase, the torque buildup phase can run parallel to the initial brake phase, the deceleration phase, the hold phase and / or the startup phase. The torque buildup phase immediately follows the torque reduction phase. During the torque build-up phase, the desired torque which is kept constant during the torque reduction phase is to be increased again, in particular to be adjusted to the driver's default torque. This is provided either steadily or stepwise. In the case of continuous matching, in contrast to the stepwise adjustment, no jumps are provided in the course of the setpoint torque over time.

The torque buildup time may depend on the state quantity. This also applies to the desired torque or to the course of the desired torque over time during the torque buildup phase. The matching is achieved by filtering. The matching can be stopped when falling below a minimum difference between the setpoint torque and the setpoint torque so as not to delay the achievement of the setpoint torque by the setpoint torque. After falling below the minimum difference, therefore, the setpoint torque is set equal to the default torque.

In particular, it is provided to set the torque buildup time period and / or the setpoint torque over time in such a way that starting up of the motor vehicle is also possible without problems on an incline. So if there is a direction of movement of the motor vehicle, which is opposite to a direction of movement, upon reaching the default torque is directed, it is advantageous that target torque to adapt faster to the default torque to those desired by the driver or to achieve predetermined direction of movement.

A development of the invention provides that the torque reduction phase and / or the torque buildup phase are aborted when the driver, in particular by means of at least one accelerator pedal, performs a cancellation action and / or a maximum period of time is exceeded. Situations are conceivable in which the driver wants to accelerate the motor vehicle again immediately after the braking process. If, however, the nominal torque is subjected to filtering during the torque reduction phase or the torque build-up phase so that it is smaller than the default torque, then such acceleration is not possible. For this purpose, the driver should be able to signal to the driver assistance device that, despite the braking process, an acceleration is desired which is stronger than that which can be achieved by means of the momentarily present desired torque. For this purpose, the demolition action is provided, which can be carried out for example by means of the at least one accelerator pedal. The abort action is in particular a pedal change, for example, from a brake pedal to an accelerator pedal, an accelerator pedal zero crossing, ie a complete unloading and subsequent re-actuation of the accelerator pedal, and / or a pumping operation of the accelerator pedal. The latter means a quick, multiple actuation of the accelerator pedal. When the cancellation action is carried out, the torque reduction phase and / or the torque buildup phase are also preferably terminated even if the minimum duration of the torque reduction phase has not yet been reached.

Alternatively or additionally, it can be provided that the torque reduction phase and / or the torque buildup phase are aborted when a maximum time duration is reached or exceeded. It is therefore intended to reduce the torque only over a limited period of time, namely the maximum period of time, because it is assumed that the driver can take over the leadership of the motor vehicle again after the maximum time has expired. Therefore, upon occurrence of this condition, the torque reduction phase and / or the torque buildup phase should be aborted and then the desired torque should be equalized or set to the default torque.

A further development of the invention provides that at least one reversible belt tensioner for tightening a safety belt is activated when the braking process is initiated. The motor vehicle therefore has the belt tensioner, which is reversible, that allows a multiple activation. Preferably, the belt tensioners of all safety belts are activated, but in particular only the belt tensioners, which seats are assigned, which are actually occupied. By activating the belt tensioner, the seat belt is tightened and the driver or another occupant of the motor vehicle is held in his seat, so that a displacement of the driver or the occupant due to inertial forces is at least partially prevented.

The invention further relates to a driver assistance device of a motor vehicle, in particular for carrying out the method described above, wherein the driver assistance device is provided for carrying out an environmental monitoring of the motor vehicle and for initiating a braking process with a specific initial braking force in the event of a risk of collision with an obstacle. In this case, the driver assistance device should be designed to determine, after reaching the initial braking force, a deceleration braking force and to continue the braking process in a deceleration phase with the deceleration braking force being smaller than the initial braking force. The driver assistance device can also be provided for carrying out further driving support measures. The method which is to be carried out by means of the driver assistance device can be developed as described above.

The invention will be explained in more detail with reference to the embodiments illustrated in the drawings, without any limitation of the invention. The only one shows

Figure is a diagram in which a braking force and a target brake pressure are plotted over time.

The figure shows a diagram in which two gradients 1 and 2 each a braking force or braking delay a and a course 3 a desired brake pressure p _desired are plotted over the time t. The history 1 shows the deceleration, which is determined by a known from the prior art driver assistance device during the course 2 represents the predetermined by a driver assistance device according to the invention braking deceleration. The desired brake pressure of the course 3 corresponds to the braking delay according to the course 2 , It should again be noted that the terms braking force and braking deceleration are used interchangeably and are interchangeable. Instead of an initial braking force can therefore be talk of the initial braking delay, without resulting in another meaning.

For t ≦ t ₁ is a motor vehicle, not shown here in a normal driving operation, with no braking operation is performed. The motor vehicle has the driver assistance device, which performs an environmental monitoring of the motor vehicle. If an obstacle in the surroundings of the motor vehicle is detected during environmental monitoring and if there is a risk of collision with it, then the driver assistance device initiates a braking process with a specific initial braking force a _I. The braking process starts at t = t ₁ .

It can be seen that for both gradients 1 and 2 the braking deceleration a increases between t ₁ <t ≦ t ₂ with a first, smaller slope (seen in absolute values). Between t ₂ <t ≦ t _{3 there} is a greater slope of the braking deceleration. Between t ₃ <t ≤ t ₄ the braking delay remains the same for both courses 1 and 2 and corresponds to a _I. The time span between t ₁ <t ≦ t ₄ is referred to as the initial brake duration Δt _{I of} an initial braking phase. Alternatively, only the time span in which the initial braking force actually exists, that is, t ₃ <t ≦ t ₄ , can represent the initial braking time duration.

The course 1 known from the prior art shows that the braking deceleration a is initially increased from a = 0 to a = a _I and then remains constant until termination of the braking process at t = t ₈ . Only after completion of the braking process, the braking deceleration drops to zero. The initial braking phase thus extends over the duration of the entire braking process.

According to the invention, ie according to history 2 , on the other hand, after reaching the initial braking force, a deceleration phase is initiated, which is carried out in the exemplary embodiment shown here between t ₄ <t ≦ t ₆ . In the deceleration phase, the braking operation is continued for a deceleration braking period Δt _V with a deceleration braking force, the deceleration braking force being smaller than the initial braking force. The deceleration phase is carried out until the motor vehicle is at a standstill, ie the braking process continues up to this point. In this case, the initial braking time duration, the initial braking force, the delay braking time duration and / or the deceleration braking force can be determined as a function of at least one operating variable of the motor vehicle. The operating variable, for example, the speed of the motor vehicle, the distance of the motor vehicle to the obstacle and / or the relative speed of the motor vehicle to understand the obstacle.

In the exemplary embodiment shown here, the motor vehicle is at standstill at the time t = t ₆ . After reaching the standstill of the motor vehicle at this time, the braking operation in a holding phase with a holding brake force a _H over a holding brake time .DELTA.t _{H should be} continued. The holding brake period here corresponds to the period between t ₆ <t ≦ t ₇ . During the holding phase, the holding brake force remains constant. The holding brake duration is, for example, constant.

The holding phase is followed by a start-up phase of the motor vehicle. In this, the braking operation is continued with a Anrollbremskraft over a Anrollbremszeitdauer .DELTA.t _A. The Anrollbremskraft at the beginning of the Anrollphase corresponds to the holding brake force or the holding brake force at the end of the holding phase. The Anrollbremskraft should now be reduced over the Anrollbremszeitdauer .DELTA.t _A to zero. In this case, for example, a linear reduction over time t can be provided. Alternatively, the Anrollbremskraft can also be determined in dependence on a state variable of the motor vehicle. The state variable is in particular an inclination of the motor vehicle or of a roadway on which the motor vehicle is located and / or a loading state of the motor vehicle. After the start-up phase, the braking process is ended, so that the braking deceleration from the time t = t _{8 is} again equal to zero.

Unlike the course 1 the braking force a allows the course of the invention 2 the braking force a gentler deceleration of the motor vehicle, so that the driver after reaching standstill has sufficient time to adapt to the changed driving situation and subsequently resume the driving itself.

QUOTES INCLUDE IN THE DESCRIPTION

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Cited patent literature

• DE 102008056204 A1 [0004]

Claims (10)

Method for operating a driver assistance device of a motor vehicle, wherein the driver assistance device carries out an environmental monitoring of the motor vehicle and initiates a braking process with a given initial braking force in the event of collision with an obstacle, characterized in that after reaching the initial braking force a deceleration braking force is determined and the braking process continues in a deceleration phase is, wherein the deceleration braking force is smaller than the initial braking force. A method according to claim 1, characterized in that the deceleration phase is initiated immediately after reaching the initial braking force or after an initial braking phase with a certain initial braking time, wherein preferably the deceleration phase is continued until a standstill of the motor vehicle. Method according to one of the preceding claims, characterized in that the Initialbremszeitdauer, the initial braking force, the delay brake time and / or the deceleration braking force depending on at least one operating variable of the motor vehicle, in particular the speed, the distance to the obstacle and / or the relative speed of the motor vehicle the obstacle. Method according to one of the preceding claims, characterized in that after reaching the standstill of the motor vehicle, the braking operation is continued in a holding phase with a holding brake force over a holding brake period. Method according to one of the preceding claims, characterized in that after reaching the standstill of the motor vehicle and / or after the holding phase, the braking operation is continued in a startup phase with a Anrollbremskraft over a Anrollbremszeitdauer, wherein the Anrollbremskraft reduced over the Anrollbremszeitdauer to zero and above Time is determined in dependence on a state variable of the motor vehicle, in particular an inclination of the motor vehicle, a loading state and / or a direction of movement of the motor vehicle. Method according to one of the preceding claims, characterized in that at least during a present at least during the deceleration phase torque reduction phase, a desired torque of a drive system of the motor vehicle is smaller than a default torque of a driver of the motor vehicle, in particular equal to a minimum torque of the drive system selected. Method according to one of the preceding claims, characterized in that the torque reduction phase is followed by a torque buildup phase, during which the desired torque is adjusted to the default torque continuously or stepwise, wherein in particular the torque buildup time of the torque buildup phase and / or the desired torque over time as a function of the State variable, preferably the inclination of the motor vehicle, the loading state and / or the direction of movement of the motor vehicle, are determined. Method according to one of the preceding claims, characterized in that the torque reduction phase and / or the torque buildup phase are aborted when the driver, in particular by means of at least one accelerator pedal, performs a cancellation action and / or a maximum time period is exceeded. Method according to one of the preceding claims, characterized in that at least one reversible belt tensioner for tightening a seat belt is activated upon initiation of the braking operation. Driver assistance device of a motor vehicle, in particular for carrying out the method according to one or more of the preceding claims, wherein the driver assistance device is provided for carrying out environmental monitoring of the motor vehicle and for initiating a braking process with a specific initial braking force in the event of collision with an obstacle, characterized in that the driver assistance device thereto is formed, after reaching the initial braking force to determine a deceleration braking force and continue the braking force in a deceleration phase with this, wherein the deceleration braking force is smaller than the initial braking force.

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