DE102011114481A1 - Method for controlling brake system of motor vehicle by wheel of vehicle, involves reducing recuperation moment during reduction phase in magnitude which is greater than the friction braking torque and/or the recuperation moment - Google Patents

Abstract

A slip detection unit (24) is arranged for detecting the occurrence of slip (lambda) through a friction brake (42). Upon detection of an unacceptably high slip of wheel (40) during braking, a friction braking torque (T-BRK) and a recuperation moment (T-EM) are applied for reducing the braking torque reduction phase. The recuperation moment during braking torque reduction phase is temporarily reduced in magnitude which is greater than that of the friction braking torque and/or the recuperation moment. Independent claims are included for the following: (1) a control device for controlling brake system of motor vehicle; and (2) a method for determining control parameters of friction brake control unit of friction assembly.

Description

The present invention relates to a method for controlling a brake system of a motor vehicle, by which at least one wheel of the motor vehicle, which is monitored by a slip detection unit with respect to the occurrence of a slip, can be braked by means of a friction brake and an electric machine which can be operated as a generator.

In modern motor vehicles, a continuous slip control on the wheels of the vehicle takes place in order to ensure the stability of the vehicle, especially during braking. If, in the case of a braking operation, one or more of the wheels become locked, the vehicle may lose control. It is therefore attempted to keep the wheel slip caused by a braking of the wheels below a critical slip value. In conventionally powered vehicles, such a limitation is performed by means of an ABS system as a subordinate control of an ESP system. These systems use wheel-specific interventions in the brake system, in which the friction brake of the affected wheel is controlled accordingly. In other words, the brake pressure actually requested by a driver of the vehicle by means of a corresponding brake pedal actuation is reduced by the opening of suitable valves in order to avoid wheel blockages. Various braking strategies are used, which are adapted to the current driving situation. For example, in some situations a smooth braking strategy may be used while in other situations an aggressive braking strategy is indicated to quickly reduce vehicle speed.

In vehicles with at least partially electric drive, the kinetic energy of the vehicle during braking can be used to generate electrical energy and to charge an energy store (so-called "recuperation"). During braking, first of all a braking torque (recuperation torque) to be applied by an electric machine and / or a braking torque (friction braking torque) to be applied by the friction brake are determined on the basis of a desired total braking torque. For example, if the vehicle is on a slippery surface during the braking process (often referred to as a "μ-low ground"), then the requested braking torque may cause overbraking of one or more wheels. A critical wheel slip is exceeded and the wheel or wheels can even block. In conventional systems, the ESP system takes over the wheel slip control in order to solve the mentioned blockade (s) by reducing the total braking torque or the corresponding wheel-specific braking torques or to avoid them in advance. The ESP system ensures that the braking torque applied by the friction brake is reduced. At the same time, the recuperation is ended or a reduced quasi-static REF recovery torque is set.

The above-described mode of operation means that energy recovery during the braking process is at least temporarily suspended and / or permanently reduced until the critical driving situation ends. The ESP intervention also has a negative impact on ride comfort during braking.

It is therefore an object of the present invention to provide a method of the type mentioned, which allows in particular in the situation described above by way of example an optimized energy recovery with simultaneous comfort increase. Safety aspects should also be taken into account by increasing the dynamics of the braking process.

The solution of this object is achieved by a method having the features of claim 1.

According to the invention, a friction braking torque to be applied by the friction brake and a recuperation torque to be applied by the electric machine are reduced during a brake torque reduction phase during the detection of an inadmissibly high slip of a wheel until a corrected total braking torque to be applied by the friction brake and the electric machine is achieved. The Rekuperationsmoment is temporarily reduced during the braking torque reduction phase in amount more than the friction brake torque to account for a different temporal response of the friction brake and the electric machine. Alternatively or additionally, it is provided that the recuperation torque undergoes a local minimum during the brake torque reduction phase.

The method according to the invention makes use of the problem underlying the invention that the electric machine can respond faster to changes in it Braking torque can react as the friction brake. The recuperation torque is therefore initially - absolute or relative - reduced more than the friction brake torque to implement as quickly as possible the changed specification for the applied total braking torque. The dynamics of the brake system is therefore improved and is no longer limited by the brake torque reduction characteristic - ie the response to a brake torque reduction request - the friction brake.

Basically, regardless of or in addition to the above-described measure, the recuperation torque can only be lowered temporarily during the brake torque reduction phase. This reduction means that the reduction of the total braking torque can be implemented faster than by the friction brake alone. After reaching a magnitude minimum value of the Rekuperationsmoments this is raised again during the brake torque reduction phase to convert as much of the degraded during the braking process kinetic energy of the vehicle into electrical energy. Ie. if it allows the brake torque reduction characteristic of the friction brake - so if, for example, the braking force generated by the friction brake has been degraded sufficiently strong - the Rekuperationsmoment can be increased again to recover as much as possible of the braking energy.

According to the invention, the design-related, different temporal response characteristics of the electric machine and the friction brake are thus taken into account in optimizing the time course of the recuperation torque and the friction braking torque during the braking force reduction phase. As a rule, it should be achieved by the optimization that the sum of the two braking torques corresponds as well as possible to a target specification of the total braking torque. But the braking strategy selected in the specific driving situation can also be taken into account in the optimization.

Under an impermissibly high slip is to be understood as a slip, which adversely affects the driving dynamics of the vehicle and is therefore undesirable. It can be defined by a threshold. If a currently determined slip exceeds this threshold, then an impermissible state is detected, which triggers the measures described above. In particular, the threshold is chosen so that the loss of control over the vehicle is reliably avoided.

Both measures described above have in common that the blocking of the wheel is reliably prevented by the coordinated interaction of the electric machine and the friction brake. By utilizing the rapid response of the electric machine, the dynamics of the braking process can be improved compared to conventional braking systems. An inventively controlled braking system thus not only ensures optimized energy recovery, but also the dynamics of the braking process and the associated ride comfort are improved. In addition, a situationally adapted reduction of the recuperation torque enables a simple realization of different braking strategies.

In the context of the present disclosure, information on a reduction or increase in a braking torque or on comparisons of different braking torques is always to be understood as an amount. This applies analogously to the slip values or slip parameters used.

According to one embodiment of the method, the corrected total torque after the brake torque reduction phase in terms of amount by a correction braking torque is less than a total braking torque before the detection of an impermissibly high wheel slip.

It is also possible that the correction braking torque is variable in time and, in particular during the braking torque reduction phase, temporarily assumes values which lead to an increase in the total braking torque. In fact, it has been recognized that a brief increase in the total braking torque can, in certain cases, have a supporting effect in order to convert an unstable slip situation into a stable slip situation. In other words, this initially counterintuitive approach stabilizes and ultimately accelerates the suppression of inadmissibly high levels of slippage.

The correction braking torque is chosen in particular such that the detected wheel slip below a threshold -. B. a Referenzradschlupf - is lowered. Ie. It should be ensured that the slippage of the monitored wheel - usually all wheels of the vehicle monitored - remains below a critical threshold, so that on the one hand, the braking effect provided is maximum and on the other hand, a loss of control of the vehicle is avoided. It can be provided that the correction braking torque is determined by the slip detection unit.

The recuperation torque can be increased again as soon as the friction brake torque has reached a value reduced by a threshold value. For example, the threshold may be the correction braking torque itself.

It may be provided that the recuperation torque after the brake-torque reduction phase has an amount which is less reduced than that of the friction-braking torque in comparison to the situation before the brake-torque reduction phase in order to ensure the most efficient energy recovery possible. In particular, the recuperation torque after the brake torque reduction phase essentially corresponds to the level before said phase. In other words, the Rekuperationsmoment is only temporarily lowered in this embodiment of the method in order to achieve a reduction of the total braking torque as quickly as possible. The actual reduction of the total braking torque is provided by the friction brake after completion of the brake torque reduction phase.

• – Bestimmung eines durch die Reibbremse aufzubringenden Reibbremsmoments und eines durch die elektrische Maschine aufzubringenden Rekuperationsmoments auf Basis eines angeforderten Bremsmoments;
• – Bestimmung eines Korrekturbremsmoments, wenn durch die Schlupferkennungseinheit ein Schlupf des Rads detektiert wird;
• – Bestimmung eines Reibbremsreferenzmoments auf Basis des Korrekturbremsmoments und des Reibbremsmoments;
• – Bestimmung eines Rekuperationsreferenzmoments auf Basis des Korrekturbremsmoments, des Rekuperationsmoments und eines das unterschiedliche zeitliche Ansprechverhalten der Reibbremse und der elektrischen Maschine berücksichtigenden Modifikationsterms.

According to one embodiment, the method comprises the following steps:

• Determining a friction braking torque to be applied by the friction brake and a recuperation torque to be applied by the electric machine on the basis of a requested braking torque;
• - Determining a correction braking torque when a slip of the wheel is detected by the slip detection unit;
• Determining a friction brake reference torque based on the correction brake torque and the friction brake torque;
• - Determining a Rekuperationsreferenzmoments based on the correction braking torque, the Rekuperationsmoments and the different temporal response of the friction brake and the electric machine into account modifying term.

The modification term can be variable in time. It may even in certain cases during the braking torque reduction phase at least temporarily adopt a value which results in a corrected recuperation torque resulting in an acceleration of the wheel. This approach contributes to a more rapid stabilization of the slip state of the wheel, as a result of which the slip suppression is more controlled and often even faster.

The modification term depends, in particular, on a time characteristic of a friction-braking torque change-that is, the response of the friction brake with respect to a reduction of the brake-torque. In order to optimize the energy recovery while maintaining high control dynamics, the modification term can be selected to be complementary to the characteristic of the friction brake torque change of the friction brake, so that the modification term minimizes a difference between a total braking torque specification and an ACTUAL state actually produced by a reduction of the friction braking torque.

It can be provided that the recuperation torque is modified based on the modification term until the corrected friction braking torque has reached a desired value, in particular until the corrected friction braking torque has changed substantially by the amount of the correction braking torque. The modification term may depend on the corrected friction braking torque. In particular, the modification term also includes the respectively selected braking strategy.

The invention is further based on the object to provide a suitable control device with which a method for controlling a brake system of the type mentioned, in particular a method according to at least one of the embodiments described above, is feasible.

• – eine Energiesteuereinheit zur Bestimmung eines durch die Reibbremse aufzubringenden Reibbremsmoments und eines durch die elektrische Maschine aufzubringenden Rekuperationsmoments auf Basis eines angeforderten Bremsmoments;
• – eine Schlupferkennungseinheit zur Bestimmung eines Korrekturbremsmoments, wenn ein unzulässig hoher Schlupf des Rads detektiert wird;
• – eine Reibbremssteuereinheit zur Bestimmung eines Reibbremsreferenzmoments auf Basis des Korrekturbremsmoments und des Reibbremsmoments und
• – eine Maschinensteuereinheit zur Bestimmung eines Rekuperationsreferenzmoments auf Basis des Korrekturbremsmoments, des Rekuperationsmoments und eines Modifikationsterms, durch den das unterschiedliche zeitliche Ansprechverhalten der Reibbremse und der elektrischen Maschine berücksichtigbar ist.

A control device for controlling a brake system, by means of which at least one slip-monitored wheel of the vehicle can be braked by means of a friction brake and an electric brake which can be operated as a generator, comprises:

• A power control unit for determining a friction braking torque to be applied by the friction brake and a recuperation torque to be applied by the electric machine based on a requested braking torque;
• A slip detection unit for determining a correction braking torque when an excessively high slip of the wheel is detected;
• A friction brake control unit for determining a friction brake reference torque on the basis of the correction brake torque and the friction brake torque and
• - An engine control unit for determining a Rekuperationsreferenzmoments based on the correction braking torque, the Rekuperationsmoments and a modification term, by which the different temporal response of the friction brake and the electric machine is taken into account.

An input of the friction brake control unit can be connected via a summation element to an output of the energy control unit and to an output of the slip detection unit. Alternatively or additionally, an input of the engine control unit via a summation with an output of the power control unit, with an output of the slip detection unit and with an output of the friction brake control unit in connection. The above-mentioned inputs or outputs are not to be understood exclusively as analog inputs or outputs. It may also be provided that said units are provided by suitable digital components that are in electronic contact with each other.

When carrying out the method according to the invention, the selected control parameters play a decisive role. They can also be used to implement the braking strategies that are suitable for the respective braking situation. In other words, the different operating strategies are reflected in these parameters. For example, it is possible to provide a plurality of parameter sets which enable, for example, comfort-oriented braking or as dynamic a braking as possible.

The determination of the control parameters is comparatively complicated with conventional methods. It is therefore a further object of the present invention to provide a simplified method for determining control parameters of a friction brake control unit of a friction brake and an engine control unit of a generator-operated electric machine, wherein the friction brake and the electric machine form a brake system. The brake system allows the braking of at least one wheel of a motor vehicle. It can be controlled in particular by a method according to at least one of the embodiments described above.

According to the invention, the control parameters of the friction brake control unit and the control parameters of the engine control unit are each determined by a Linear Quadratic Regulator (LQR) based approach, which involves minimizing a quality function dependent on state variables of the brake system and control variables of the respective control unit. The state variables and / or the manipulated variables are weighted differently in the method according to the invention for the definition of different brake characteristics or braking strategies of the brake system.

By varying the weighting of certain state variables and / or manipulated variables, a comparatively simple calculation of the control parameters can already be undertaken. The number of parameters to be varied in the course of minimization is low by using the method according to the invention in comparison to conventional methods for determining corresponding control parameters.

To be particularly efficient, it has proven to determine the control parameters of the friction brake control unit of the friction brake weighting of a characteristic state variable of the friction brake, z. B. a hydraulic brake pressure of a hydraulically actuated friction brake - analogous applies to pneumatically or electromechanically actuated friction brakes -, and to vary a manipulated variable of the friction brake control unit. To determine the control parameters of the machine control unit, a weighting of a control error, a state variable which is proportional to a recuperation torque generated by the electric machine, and a manipulated variable of the machine control unit can be varied. The control error is in particular a difference between the actually applied recuperation torque and a recuperation reference torque (desired recuperation torque).

Further embodiments of the invention are indicated in the description, the claims and the attached drawings.

Hereinafter, the present invention will be explained purely by way of example with reference to advantageous embodiments with reference to the accompanying drawings. Show it:

1 FIG. 2 schematically a section of a conventional brake system of a motor vehicle with a friction brake and with an electrical machine, FIG.

2 a schematic representation of an embodiment of the control device according to the invention and

3 to 5 the temporal evolution of the recuperation torque and the friction brake torque according to various braking strategies.

1 schematically shows a section of a conventional brake system of a motor vehicle. A wheel 40 The vehicle is equipped with a hydraulically actuated friction brake 42 and an electric machine 44 braked. The machine 44 is as a motor to drive the wheel 40 and operated as a generator. In generator mode of the machine 44 This converts the kinetic energy of the vehicle into electrical energy, ie the vehicle is braked with energy recovery.

The friction brake 42 is through a vehicle dynamics control 46 (ESP system) controlled, the inputs 14 a driver of the vehicle receives and processes. For vehicles that do not yet have an ESP system, the brake can 42 also be controlled only by an ABS system.

The inputs 14 For example, may be a steering angle of the vehicle and the respective position of an accelerator pedal and a brake pedal. The inputs 14 are also used by a vehicle control unit 48 received with the vehicle dynamics control 46 communicates and which is the electric machine 44 controls. From the machine operated in generator mode 44 generated electrical energy is - in from the unit 48 controlled way - directly or indirectly to a battery 50 directed.

The regulation 46 and the unit 48 thus essentially independent of each other control the friction brake 42 and the machine 44 , Avoidance of excessive slippage of the wheel 40 during a braking process is controlled by the scheme 46 causes by the friction brake 42 friction brake torque generated reduced, in particular by reducing a hydraulic brake pressure. The procedure described above is associated with the disadvantages already explained, which can be avoided by the method according to the invention and when using a correspondingly designed control device.

2 shows a control device 10 with an input unit 12 that from the driver input 14 a requested braking torque T _req and a Referenzradschlupf λ _ref calculated. The requested braking torque T _req is sent to a power control unit 16 to hand over. The energy control unit 16 calculated from one of the friction brake 42 applied Reibbremsmoment T _B and one of the electric machine 44 applied Rekuperationsmoment T _E.

The specific braking torques T _B , T _E are summation members 18 . 18 ' to a friction brake control unit 20 or a machine control unit 22 passed, in turn, an actuator of the friction brake 42 or the machine 44 Taxes. In principle, the actuator can also be in the friction brake control unit 20 or the machine 44 in the machine control unit 22 be integrated.

The control units 20 . 22 are thus components of an actuator system G _A , which in a braking operation on the wheel system G - comprising the at least one wheel 40 - _Actual total braking _torque T _whl generated. If, for example, the vehicle is _traveling on a slippery surface during the braking process, then the generated total braking _torque T _whl can lead to wheel slip λ occurring.

The control device 10 has a slip detection unit 24 on, which continuously determines the currently present wheel slip λ and engages when a critical threshold is exceeded, for example when the reference wheel slip λ _{ref is} exceeded. For this purpose, the slip detection unit determines 24 a correction _{braking torque} T _ref , the summation members 18 . 18 ' is passed on and thus ultimately in the determination of a Rekuperationsreferenzmoments T _{EM, ref} and a Reibbremsreferenzmoments T _{BRK, ref} received. Actually be provided by the friction brake 42 or from the machine 44 the moments T _BRK and T _EM , which are referred to as corrected friction brake torque T _BRK and as corrected recuperation torque T _EM . While the friction brake control unit 20 the Reibbremsreferenzmoment T _{BRK, ref} only on the basis of the requested Reibbremsmoments T _B and the correction braking torque T _ref determined, in the determination of T Rekuperationsreferenzmoments _{EM, ref} and the corrected Reibbremsmoment T _BRK considered.

It thus becomes the greater response dynamics of the electric machine 44 used on modified braking torque _requirements to the total braking torque T _whl for quickly lifting an oversized Radschlupfs λ quickly lower. To ensure efficient energy recovery at the same time, the response of the brake 42 considered in the calculation of the corrected Rekuperationsmoments T _EM , which symbolically by a connection of the control unit 20 with the summation element 18 ' is shown. The recuperation torque T _EM is thus calculated not only on the basis of T _E and T _ref , but there is an additional value, which ultimately depends on the properties of the friction brake 42 depends. This value can be called a modification term.

In this context, it should be noted that the summation terms 18 . 18 ' not necessarily be considered as separate components. They are to be understood as symbolic elements which stand for the combination of different signals, for the interaction of the various input and output signals of the control units 16 . 20 . 22 . 24 to clarify.

Due to the consideration of the corrected friction braking torque T _BRK , due to the compared to the electric machine 44 relatively sluggish response of the friction brake 42 declines quite slowly, the corrected Rekuperationsmoment T _{EM is} initially comparatively strongly reduced after the output of the correction braking _torque T _ref , in order to achieve the desired reduction of the total braking _torque T _whl as quickly as possible. Once that of the friction brake 42 corrected corrective braking torque T _{BRK has been} sufficiently reduced, the corrected Rekuperationsmoment T _{EM can} be raised again in order to achieve maximum efficiency in energy recovery.

In other words, that's from the electric machine 44 generated recuperation torque T _EM only so long and reduced so much until the friction brake 42 make their contribution to the reduction of the total braking _torque T _whl . For example, the torque T _EM is increased again when T _{BRK has} dropped by a certain value from T _B.

3 shows in the right diagram by way of example the course of the corrected friction braking torque T _BRK during a brake torque reduction phase B between the times t = 0.3 and about t = 0.7 s. Initially, the total braking _torque T _whl -1000 Nm and is composed to each -500 Nm from the Rekuperationsmoment T _E and the friction braking torque T _B. Since initially - ie before the time t = 0.3 s - no unacceptably high wheel slip λ is present, the correction braking torque T _{ref is} equal to zero. Therefore: T _E = T _EM and T _B = T _BRK .

At time t = 0.3 s it is detected that the monitored wheel 40 has an inadmissibly high slip λ (λ> λ _ref ). A reaction of the slip detection unit 24 is required to stop the increase of the slip λ or to limit λ to the reference wheel slip λ _ref . It is recognized that the total braking _torque T _whl must be reduced.

The slip detection unit 24 steps in and specifies a correction braking torque T _ref = +200 Nm. The reference torque (T _B + T _ref + T _E ) therefore changes from -1000 Nm to -800 Nm. If the recuperation torque T _{E is} not changed - ie T _EM = T _E -, the friction brake reference torque T _{BRK, ref changes} from -500 Nm to -300 Nm. The modification term is temporally constant and zero in this example. Ie. the behavior of the friction brake 42 has no influence on the recuperation torque T _E.

In consequence of the instructions of the slip detection unit 24 the total braking _torque T _whl only decreases due to the reduction of the decreasing hydraulic pressure in the friction brake actuator system 42 , The total braking _torque T _whl therefore decreases comparatively slowly from -1000 Nm to -800 Nm. In order to make clear how large the difference between the course of the reference _torque (T _B + T _ref + T _E ) and the actual reduction of T _whl , the course of the total rotational _torque T _whl in the left diagram of 3 shown shifted by 500 Nm. It can be seen thereby that comparatively large differences between the actual curve (= T _{whl -T E} ) and the target curve (= T _B + T _ref ) are present during the braking torque _{reduction phase B.} Although the energy recovery at the in 3 is not affected, the dynamics of the braking system to limit wheel slip is unsatisfactory.

Therefore, the electric machine 44 during Phase B was included in the brake torque reduction, as in 4 you can see. The total braking _torque T _whl was - as in the left diagram of 3 - in the left diagram of the 4 shifted by 500 Nm compared to the 3 Improved adjustment of the actual total braking torque T _whl to the target braking torque (T _B + T _ref ) to illustrate. The right diagram of the 4 corresponds to the representation of the right diagram of the 3 ,

In contrast to the procedure according to 3 the Rekuperationsmoment is initially greatly reduced during the braking torque reduction phase B, so that the corrected Rekuperationsmoment T _EM early reaches a minimum value T _{EM, min} and then rises again until T _{EM is} again about as large as T _E before the brake torque reduction phase B, too to ensure efficient energy recovery in phase B.

If the corrected recuperation torque T _EM and the corrected friction braking torque T _{BRK are} summed at each time point, a curve of the total braking _torque T _{whl is obtained} , which _{comparatively approximates} the rectangular function of the total _target braking _torque (T _B + T _ref + T _E ). This is achieved by the appropriate correction of the recuperation torque T _EM , into which the temporal characteristic of the corrected friction braking torque T _BRK flows. In other words, the course of T _EM at the beginning of phase B depends on the one hand on the characteristic of T _BRK, on the other hand, the magnitude of the correction _{braking torque} T _ref is also included, which in turn depends inter alia on the current driving situation.

Notwithstanding T _E and T _B can also be of different sizes before the braking torque reduction phase B of the depicted situation. Depending on the existing conditions and the specific requirement profile can also be provided that T _EM after completion of the brake torque reduction phase B is smaller or even greater than T _E before the start of the brake torque reduction phase B. Incidentally, this also applies to the basis of the 3 and 5 exemplified scenarios. The decisive factor in the procedure according to 4 However, that T _{EM is} lowered at the beginning of phase B - in absolute or relative terms - stronger than the friction braking torque T _BRK .

In other words, the recuperation torque T _{EM is} initially reduced disproportionately strong and raised again in the course of phase B, as soon as the friction brake torque T _{BRK has been} reduced sufficiently. The faster response of the electric machine 44 On modified braking torque requirements is thus used to an inertia of the friction brake 42 to compensate.

5 shows a further embodiment of the method, wherein the total braking _torque T _whl in the left diagram again shown shifted by 500 Nm, so that instead of a reduction of -1000 Nm to -800 Nm, a reduction of -500 Nm to -300 Nm is shown. Unlike the scenario of 4 T _{EM is} lowered less. The in 5 reached amount of T _{em, min} is significantly larger than the in 4 which represents a smaller reduction of the recuperation torque T _EM during phase B. Also, the local extremum is significantly wider in the course of T _EM , so that the brake-torque reduction phase B extends from the time t = 0.3 s to about t = 0.9 s. Because the course of T _{BRK is} unchanged because it is essentially due to the construction of the friction brake 42 and the design of the corresponding actuator is given, results in a profile of the total braking _torque T _whl , which fits less well to the desired desired course (T _B + T _ref + T _E ) as T _whl in 4 so that a more gentle, comfort-oriented braking torque reduction takes place. In the 4 On the other hand, the dynamics shown are advantageous in cases where a high lateral tire force is required, for example during braking maneuvers in a curve or in an evasive maneuver. Since it is very important in these situations, as soon as possible to implement the desire for a reduced total braking torque, the electric machine 44 initially more demanded in the Gesamtbremsmomentreduction, as this can react very quickly.

The scenarios described above by way of example can be determined in advance and suitably in the control device 10 be deposited. Depending on the detected driving situation, the appropriate braking strategy is selected. It can be used a deposited scenarios or interpolated between stored scenarios to meet the current situation. Ultimately flows into the wheel slip control thus that rapid SET changes with the help of the electric machine 44 and slower TARGET changes with the friction brake 42 can be implemented, with the focus on keeping the energy recovery always comparatively large. This is achieved in particular by the fact that the electrical machine 44 provided recuperation T _EM during the braking torque reduction phase B is initially reduced more than the friction brake torque T _BRK and / or that the recuperation torque T _{EM is} only temporarily reduced to keep the energy recovery even during a brake torque reduction phase B as large as possible.

For determining suitable control parameters for controlling the friction brake control unit 20 and the engine control unit 22 is resorting to an approach with a "linear quadratic regulator", which is a quadratic function to be minimized J (x, u) = ∫∞ / 0 (x ^T Qx + ru ² ) dt (I) where x is a system state vector, u is a manipulated variable, Q is a weighting matrix for the system state vector, and r is a weighting factor for the manipulated variable u.

Q is preferably a diagonal matrix of the shape

In principle - although not in connection with a brake system and in particular with a brake system comprising a friction brake and an electric machine - an optimization approach based on a "linear quadratic regulator" is known, so that no further explanation is needed. However, it is essential how the non-negative weighting factors q ₁ ,..., Q _{n of} the positive semidefinite matrix Q and r are chosen. r should be chosen so that x → 0 happens quickly, avoiding oscillating the system. In addition, the manipulated variable u should be minimal.

The present braking system is characterized by a first-order system (friction brake 42 ) and a second-order system (electric machine 44 ). For the hydraulic friction brake 42 the linear-quadratic-regulator (LQR) approach mentioned above is used. The hydraulic brake pressure p is used as a state variable x. By varying the weighting factors associated with the pressure p, ratio parameters k ₁ , k ₂ can finally be determined which put the hydraulic pressure p, a friction brake reference torque T _{BRK, ref} and a control input quantity u _BRK into a mathematical relationship: u _BRK = -k ₂ p + k ₁ T _{BRK, ref} (III), allowing a behavior of the friction brake 42 is shown.

Analogously, in connection with the electric machine 44 and the recuperation torque T _EM generated by it in response to a brake-torque reduction request. Again, an LQR approach is used. To determine here are the ratio parameters k ₃ and k ₄ . This is done on the basis of a state variable x, a control error e and a manipulated variable u _EM associated weighting factors q ₁ -q _n or r. As a result, the control of the machine is obtained 44 descriptive equation U _EM = -k ₄ x + k ₃ ∫ t / 0 (T _{EM, ref} (τ) - T _EM (τ)) dτ (IV) where the integrand is the control error e of the machine 44 and x is a state quantity that is proportional to that of the machine 44 generated Rekuperationsmoment T _EM is.

In other words, for the friction brake 42 and for the machine 44 ultimately each created a mathematical model based on differential equations. Such systems can be described with state space models. The time-variable quantities x are referred to as state variables. They are internal system quantities that can be converted into other quantities by means of state space transformations. The system dynamics does not change due to the mathematical transformation. Only the description of the system can be simplified by the state space transformation. The manipulated variables u _BRK , u _EM are the respective internal signals in the control unit 20 respectively. 22 ,

To determine the control parameters of the control units 20 . 22 In each case a state controller is used. Ie. the state variables x determined by measurement or by means of observer approaches are multiplied by the said ratio parameters k ₁ , k ₂ , k ₃ , k ₄ . The results are combined into a linear combination, which in turn supplies a nominal manipulated variable u _BRK , u _{EM of} the respective actuator. Decisive now is the choice of suitable parameters k ₁ , k ₂ , k ₃ , k ₄ . By using the LQR approach, for the calculation of k ₁ , k ₂ , k ₃ , k _4, weighting factors q ₁ -q _n for the state variables x and weighting factors r ₁ -r _m for the manipulated variables can be selected. The weighting factors q ₁ -q _{n of} the state variables x are found in the matrix Q again. The weighting factors of the manipulated variables u can be found in r. Are the weighting factors q ₁ -q _n or r ₁ -r _m assigned to the state variables x and the manipulated variables u, respectively chosen large, this has a great effect on the quality functional J and vice versa. In accordance with the weights, the parameters k ₁ , k ₂ , k ₃ , k _{4 result} .

As the description of the braking system and on the wheels 40 the vehicle acting forces is subject to inaccuracies, the currently present wheel slip λ can not be controlled completely accurately to the reference slip λ _ref in the rule. There may be a permanent error in certain cases. To compensate for this, is for the electric machine 44 adds a state quantity x that is proportional to T _EM . The recuperation torque-proportional state variable x and the control error e thus form the relevant state variables for the manipulated variable u _EM , while the brake pressure p and the Reibbremsreferenzmoment T _{BRK, ref is used} to define the manipulated variable u _BRK .

In advance of the control parameter determination, the characteristics of a specific braking strategy are thus determined and the above-described weighting factors q ₁ -q _n , r ₁ -r _{m selected} accordingly. The weighting factors q ₁ -q _n , r ₁ -r _m again form the basis for a calculation of the parameters k ₁ -k ₄ . For each braking strategy or braking scenario, individual parameters k ₁ -k _{4 are} determined. It is understood that the scenarios of 3 to 5 have only exemplary character.

LIST OF REFERENCE NUMBERS

10

control device

12

input unit

14

driver input

16

Power control unit

18, 18 '

Summation member

20

Reibbremssteuereinheit

22

Engine control unit

24

Slip detection unit

40

wheel

42

friction brake

44

electric machine

46

electronic stability program

48

Vehicle control unit

50

battery

λ _ref

Referenzradschlupf

λ

wheel slip

G _A

actuator system

G

wheel system

B

Braking torque reduction phase

T _B

Reibbremsmoment

T _E

recuperation

T _{BRK, ref}

Reibbremsreferenzmoment

T _{EM, ref}

Rekuperationsreferenzmoment

T _BRK

corrected friction braking torque

T _EM

corrected recuperation torque

TEM _{, min}

minimal recuperation moment

T _req

requested braking torque

T _whl

Total braking torque

T _ref

Correction braking torque

t

time

x

state variable

u, u _BRK , u _EM

manipulated variable

k ₁ , k ₂ , ...

ratio parameter

J

quality function

Q

weighting matrix

q ₁ , q ₂ , ... r ₁ , r ₂ , ...

weighting factor

p

hydraulic pressure

Claims (21)

Method for controlling a brake system of a motor vehicle, by which at least one wheel ( 40 ) of the vehicle, which is controlled by a slip detection unit ( 24 ) is monitored for the occurrence of a slip (λ) by means of a friction brake ( 42 ) and an electric machine that can be operated as a generator ( 44 ) is brakable, wherein upon detection of an impermissibly high slip (λ) of the wheel ( 40 ) during a braking operation through the friction brake ( 42 ) applied Reibbremsmoment (T _BRK ) and a through the electric machine ( 44 ) to be applied during a brake torque reduction phase (B) to be applied by a friction brake (T _EM ) 42 ) and the electric machine ( 44 corrected total braking torque (T _whl ) is to be applied, the recuperation torque (T _EM ) during the brake torque reduction phase (B) is temporarily reduced in magnitude more than the friction brake torque (T _BRK ) and / or the recuperation (T _EM ) during the braking torque reduction phase (B ) undergoes a local minimum (T _{EM, min} ) in order to achieve a different temporal response of the friction brake ( 42 ) and the electric machine ( 44 ). A method according to claim 1, characterized in that the corrected total braking torque (T _whl ) after the brake torque _{reduction phase} (B) amount by a correction braking torque (T _ref ) is less than a total braking torque (T _whl ) before the detection of an impermissibly high wheel slip (λ). A method according to claim 2, characterized in that the correction braking torque (T _ref ) is variable in time and, in particular during the braking torque _{reduction phase} (B) temporarily assumes values which lead to an increase in the total braking torque (T _whl ). A method according to claim 2 or 3, characterized in that the correction braking torque (T _ref ) is selected so that the detected wheel slip (λ) is lowered below or to a threshold value (λ _ref ). Method according to at least one of claims 2 to 4, characterized in that the correction braking torque (T _ref ) by the slip detection unit ( 24 ) is determined. Method according to at least one of the preceding claims, characterized in that the recuperation torque (T _EM ) is increased again as soon as the friction braking torque (T _BRK ) has reached a value reduced by a threshold value. Method according to at least one of the preceding claims, characterized in that the recuperation torque (T _EM ) after the brake torque reduction phase (B) has an amount which is less reduced compared to the situation before the brake torque reduction phase (B) than that of the friction brake torque (T _BRK ), wherein the recuperation torque (T _EM ) after the brake torque reduction phase (B) in particular substantially the same amount as before the brake torque reduction phase (B). Method according to at least one of the preceding claims, characterized in that the friction brake torque (T _BRK ) is reduced continuously or stepwise during the brake torque reduction phase (B). Method according to at least one of the preceding claims, comprising the steps of: determining by the friction brake ( 42 ) applied Reibbremsmoments (T _B ) and one by the electrical machine ( 44 ) to be applied on the basis of a requested braking torque (T _req ) _{Rekuperationsmoments} (T _E ); Determining a correction braking torque (T _ref ), if by the slip detection unit ( 24 ) an impermissibly high slip (λ) of the wheel ( 40 ) is detected; Determining a Reibbremsreferenzmoments (T _{BRK, ref} ) based on the correction braking torque (T _ref ) and the friction brake torque (T _B ); Determining a recuperation reference torque (T _{EM, ref} ) on the basis of the correction braking torque (T _ref ), the recuperation torque (T _E ) and the different temporal response of the friction brake ( 42 ) and the electric machine ( 44 ) modification term. A method according to claim 9, characterized in that the modification term is variable in time. A method according to claim 10, characterized in that the modification term during the braking torque reduction phase (B) at least temporarily assumes a value, by which a corrected Rekuperationsmoment (T _EM ) results, which leads to an acceleration of the wheel ( 40 ) leads. Method according to at least one of claims 9 to 11, characterized in that the modification term of a temporal characteristic of a Reibbremsmomentänderung the friction brake ( 42 ), in particular complementary to the characteristic of the Reibbremsmomentänderung the friction brake ( 42 ). Method according to at least one of Claims 9 to 12, characterized in that the recuperation torque (T _EM ) is modified based on the modification term until the corrected friction braking torque (T _BRK ) has reached a desired value, in particular until the corrected friction braking torque (T _EM ) T _BRK ) has changed substantially by the amount of the correction braking torque (T _ref ). Method according to at least one of Claims 9 to 13, characterized in that the modification term is dependent on the corrected friction braking torque (T _BRK ). Control device for controlling a brake system of a motor vehicle, in particular according to a method according to at least one of claims 1 to 12, by which at least one slip-monitored wheel ( 40 ) of the vehicle by means of a friction brake ( 42 ) and an electric machine that can be operated as a generator ( 44 ), comprising: a power control unit ( 16 ) for determining a friction brake ( 42 ) applied Reibbremsmoments (T _B , T _BRK ) and one by the electric machine ( 44 ) to be applied recuperation (T _E , T _EM ) based on a requested braking torque (T _req ); a slip detection unit ( 24 ) for determining a correction braking torque (T _ref ), when an impermissibly high slip (λ) of the wheel ( 40 ) is detected; a friction brake control unit ( 20 ) for determining a friction brake reference torque (T _{BRK, ref} ) on the basis of the correction brake torque (T _ref ) and the friction brake torque (T _B ) and an engine control unit ( 22 ) for determining a recuperation reference torque (T _{EM, ref} ) on the basis of the correction braking torque (T _ref ), the recuperation torque (T _E ) and a modification term, by which the different temporal response of the friction brake ( 42 ) and the electric machine ( 44 ) is considered. Control device according to claim 15, characterized in that an input of the friction brake control unit ( 20 ) via a summation element ( 18 ) with an output of the energy control unit ( 16 ) and with an output of the slip detection unit ( 24 ). Control device according to claim 15 or 16, characterized in that an input of the machine control unit ( 22 ) via a summation element ( 18 ' ) with an output of the energy control unit ( 16 ), with an output of the slip detection unit ( 24 ) and with an output of the friction brake control unit ( 20 ). Method for determining control parameters (k ₁ , k ₂ , k ₃ , k ₄ ) of a friction brake control unit ( 20 ) a friction brake ( 42 ) and a machine control unit ( 22 ) of an electric machine that can be operated as a generator ( 44 ), wherein the friction brake ( 42 ) and the electric machine ( 44 ) form a braking system through which at least one wheel ( 40 ) of a motor vehicle is brakable, wherein the brake system is controllable in particular by a method according to claims 1 to 12, and wherein the control parameters (k ₁ , k ₂ , k ₃ , k ₄ ) of the Reibbremssteuereinheit ( 20 ) and the machine control unit ( 22 ) are each determined by a linear quadratic regulator-based approach, which minimizes one of state variables (x, p) of the brake system and manipulated variables (u _EM , u _BRK ) of the respective control unit ( 22 respectively. 20 ), wherein the state variables (x, p) and / or the manipulated variables (u _EM , u _BRK ) are weighted differently in order to define different braking characteristics of the brake system. A method according to claim 18, characterized in that for determining the control parameters (k ₁ , k ₂ ) of the friction brake control unit ( 20 ) of the friction brake ( 42 ) a weighting of a characteristic state variable (p) of the friction brake ( 42 ) and a manipulated variable (u _BRK ) of the friction brake control unit ( 20 ) can be varied. Method according to claim 18 or 19, characterized in that for determining the control parameters (k ₃ , k ₄ ) of the machine control unit ( 22 ) a weighting of a control error (e), a state variable (x) which is proportional to one by the electric machine ( 44 Is) regeneration torque generated (T _EM), and (a manipulated variable (u _EM) of the engine control unit 22 ) can be varied. A method according to claim 20, characterized in that the control error (e) a difference between an actually applied recovery torque (T _EM) and a Rekuperationsreferenzmoment _(EM T _ref,) is.

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