DE102018211792B4 - Method for controlling a vehicle and vehicle - Google Patents

Abstract

Method for controlling a vehicle (2), - the vehicle (2) having at least one electric machine (4) which is designed to generate a drive torque (M) which is positive for driving the vehicle (2) and which for braking the vehicle (2) is negative, - in the case of a braking request (B) to generate a deceleration, two braking strategies (Bdis, Breg) for the vehicle (2) can be selected and implemented, namely a first, dissipative braking strategy (Bdis) , In which a dissipative braking function of the vehicle (2) is activated, and a second, regenerative braking strategy (Breg), in which only a regenerative braking function of the vehicle (2) is used, - wherein no braking function is activated in the regenerative braking strategy (Breg). is as long as the drive torque (M) is positive, but with a load change of the E-machine (4) is awaited and the regenerative braking function is only activated when the load change required olgt is, - in the event that a braking request (B) occurs while the drive torque (M) is positive, for the regenerative braking strategy (Breg) is determined whether in its implementation a comfort criterion (K) can be met, - where the regenerative braking strategy (Breg) is selected and implemented if the comfort criterion (K) can be met and the dissipative braking strategy (Bdis) is otherwise selected and implemented.

Description

The invention relates to a method for controlling a vehicle and a vehicle.

A vehicle that has an electric motor for driving can also operate it as a generator in order to generate energy. In this way, a regenerative braking function can also be implemented, which slows down the vehicle by converting kinetic energy into electrical energy via the electric motor. However, due to the principle of operation, the regenerative braking function is not available as long as the e-machine is generating a positive drive torque. In this case, it is rather necessary to resort to a dissipative braking function, for example in the form of a friction brake.

In the WO 2014/090799 A1 and the U.S. 2015/0307099 A1 a target distance to a vehicle driving ahead is deliberately chosen to be greater than a critical distance, so that an additional distance is created which enables braking by means of a regenerative brake.

In the DE 10 2010 052 964 A1 a method for operating a vehicle is described, with an overrun and/or braking phase being automatically adapted to a parameter which characterizes an energy store of the vehicle.

Against this background, it is an object of the invention to operate a vehicle as energy-efficiently as possible. The vehicle should be braked regeneratively as often as possible and dissipatively as rarely as possible. In particular, a method suitable for this purpose and a corresponding vehicle are to be specified.

The object is achieved according to the invention by a method with the features according to claim 1 and by a vehicle with the features according to claim 12. Advantageous refinements, developments and variants are the subject matter of the dependent claims. The explanations in connection with the method also apply to the vehicle and vice versa.

The method is used to control a vehicle. The vehicle is in particular a motor vehicle. The vehicle has at least one electric machine, i.e. an electric machine, which is also referred to as an electric motor and generally as a drive machine. The vehicle therefore has one or more electric machines. The following explanations also apply analogously to vehicles with several electric machines. The vehicle is, for example, an electric vehicle or a hybrid vehicle. The electric machine is designed to generate a drive torque which is positive for driving the vehicle and which is negative for braking the vehicle. Correspondingly, the electric machine is connected to a chassis of the vehicle, in particular via a drive train of the vehicle. Although the e-machine is referred to as an electric motor, strictly speaking this designation only applies if the drive torque is positive. If, on the other hand, the drive torque is negative, the e-machine acts as a generator. The drive torque is also referred to simply as torque for short and is in particular a torque of the electric machine. The vehicle expediently has a battery as the energy source for the electric machine.

In the event of a braking request, i.e. if the vehicle is to be slowed down, two braking strategies can be selected and implemented for the vehicle in order to generate a deceleration. “Deceleration” is understood to mean, in particular, negative acceleration, i.e. slowing down of the vehicle. A first of the two braking strategies is a dissipative braking strategy in which a dissipative braking function of the vehicle is activated. The dissipative braking function is preferably implemented by a friction brake, which converts kinetic energy into thermal energy. A second of the two braking strategies, on the other hand, is a regenerative braking strategy in which only a regenerative braking function of the vehicle is used. The vehicle is then decelerated by the fact that the e-machine generates a negative drive torque, so that the vehicle's kinetic energy is then converted into electrical energy. This electrical energy is used to charge the vehicle's battery in particular, so that energy is recovered, i.e. recuperated.

With the regenerative braking strategy, no braking function is activated as long as the drive torque is positive, but a load change of the electric machine is awaited and the regenerative braking function is only activated when the load change has taken place. This is based on the consideration that for the regenerative braking function, i.e. if regenerative braking is to take place, the e-machine must be operated as a generator and it must therefore generate a negative drive torque. When the vehicle is being driven, however, a positive drive torque is regularly requested from the electric machine in order to overcome a driving resistance and ultimately to move the vehicle. If regenerative braking is now required as a result of a braking request, the e-machine cannot immediately meet the request for a negative drive torque, but must first carry out a load change, in which the positive drive torque is first reduced to zero in order to then generate a negative drive torque. This load change, which is accompanied at least by a reduction in the drive torque to zero and preferably even with a change in sign of the drive torque, is initially awaited for regenerative braking. Finally, the regenerative braking function is only activated when the load change has taken place and the drive torque is zero and can therefore become negative starting from this, or when the drive torque is already negative. In particular, the vehicle is not actively braked while waiting for the load change.

The load change thus describes the process in which, starting from a positive drive torque, the request for a positive drive torque on the e-machine is reduced or switched off completely, so that the actual drive torque is finally reduced to zero and, based on this, enters the negative range and further reduced to a negative drive torque. The reduction in the area around zero, ie in a zero area, takes place in particular much more slowly than the previous reduction from the positive drive torque in the direction of zero. In vehicles, the zero range typically extends from -100 Nm to +100 Nm. This deceleration of the further reduction of the drive torque in the zero range, ie in particular when crossing zero, results, among other things, from a gear wheel edge change on the electric motor, in particular in a transmission which is connected to the electric motor. The deceleration described also applies in particular in the other direction, ie when the drive torque increases in the zero range. The negative drive torque during load changes does not result from a braking function or a braking measure, but due to the behavior of the electric machine itself, which automatically strives for idling operation without requiring a special drive torque, in which a negative drive torque is provided as a matter of principle. Appropriately, one waits until the end of the zero range is reached and a faster build-up of drive torque is possible again, so that a strong negative drive torque can and is requested immediately and a correspondingly high deceleration is built up. In vehicles, this negative drive torque at the end of the load change is typically on the order of 100 Nm, more precisely -100 Nm taking into account the sign. In contrast, a positive drive torque for moving the vehicle is typically on the order of 1000 Nm.

The invention is now based in particular on the observation that the two aforementioned braking strategies may lead to braking distances of different lengths due to the different braking functions, or that one braking function is stronger for the same braking distance. In particular, due to the waiting time required to carry out the load change, the regenerative braking strategy leads to a longer braking distance or, given the same braking distance, to stronger braking than the dissipative braking strategy, which can be carried out regardless of the sign of the drive torque. Because during the waiting time, the vehicle continues to move unbraked. However, this results in a conflict of objectives such that the regenerative braking strategy is indeed more energy-efficient, but may entail a safety risk since, depending on the situation, there may not be enough time available to wait for the necessary load change.

In the present case, the above-mentioned conflict of objectives is resolved in particular by the fact that, in the event that a braking request occurs while the drive torque is positive, it is determined specifically for the regenerative braking strategy whether a comfort criterion can be met when it is carried out. The specific design of the comfort criterion is initially not relevant here; it is more important that the comfort criterion is a boundary condition that should be adhered to when braking as far as possible and which is then used as a basis for the decision as to which of the two braking strategies is selected and implemented. The regenerative braking strategy is then actually selected and implemented if the comfort criterion can be met, otherwise the dissipative braking strategy is selected and implemented. It is therefore decided which braking strategy is used by checking whether a comfort criterion can be met. If the comfort criterion cannot be met when the regenerative braking strategy is carried out, this braking strategy is not selected, but the alternative, dissipative braking strategy is selected. In other words: if braking is required, it is checked individually whether regenerative braking is possible or not, and if possible, such regenerative braking is preferably carried out. This exploits the fact that in certain situations the ambient conditions permit a braking strategy that is less strict with regard to the deceleration generated, and the more energy-efficient braking strategy, ie the regenerative braking strategy, is then preferably executed in such situations. A predictive control of the vehicle is thus advantageously carried out in that, for braking, it is determined in advance whether the regenerative braking strategy can be implemented while complying with the comfort criterion. If so the regenerative braking strategy is also carried out, otherwise the dissipative braking strategy is carried out.

A particular advantage results for a vehicle that is controlled by means of an automatic driving system, specifically a distance control. Correspondingly, in a suitable configuration, the vehicle has an automatic driving system, by means of which the vehicle is controlled. An automatic driving system controls a vehicle in that the automatic driving system intervenes in the driving operation and in the process changes at least the speed of the vehicle. The automatic driving system preferably has distance control, speed control or both. The automatic driving system is suitably designed in such a way that, depending on the ambient conditions, it can also generate a braking request in order to slow down the vehicle, i.e. to reduce its speed. The environmental conditions are determined, for example, by means of suitable sensors, i.e. by means of one or more sensors. While a person needs a certain reaction time to move their foot from an accelerator pedal to a brake pedal and thereby trigger a braking request, the automatic driving system is designed to be significantly faster and in particular instantaneous, i.e. directly and at least faster than a human being, from an acceleration request to a change braking request. The automatic driving system is regularly faster than the load change of the e-machine, so that the automatic driving system regularly generates a braking request when the e-machine is still generating a positive drive torque, so that a regenerative braking strategy is not possible. When a vehicle is operated with an automatic driving system, a dissipative braking strategy is typically selected by the latter, since such a strategy is often only available at the moment when the braking request is made. A combination of a dissipative braking function until the load change has taken place and a subsequent switchover or switching on of a regenerative braking function is not easily possible or at least not possible in an efficient manner, e.g. because so-called blending effects appear during the transition, i.e. disadvantageous transition effects while the driving torque of the friction brake is taken over by the e-machine as a generator. This manifests itself in particular in disadvantageous acoustics or disadvantageous vibrations or a combination thereof. Blending also results in several efficiency disadvantages, namely on the one hand part of the energy is lost as heat due to the dissipative braking function and on the other hand there is still a residual braking torque even after the transition. For design, comfort and acoustic reasons, a dissipative braking function that has been activated cannot usually be canceled immediately, so that it still has an effect in the area of the negative drive torque and generates unnecessary energy losses there, e.g. because the friction partners of a friction brake are not yet completely separated from each other.

The two braking strategies influence the vehicle and possibly also its occupants in different ways. In any case, certain forces act on the vehicle and the occupants when braking, which are dependent on the strength of the braking, ie on what value is required for the deceleration and, in particular, is required by the automatic driving system. Certain braking strategies can lead to heavy braking, which leads to correspondingly high forces on the occupants that are perceived as unpleasant. These high forces can also lead to injuries under certain circumstances. The braking forces can also lead to unfavorable vibration or pitching behavior of the vehicle, which the occupants can also find uncomfortable or can also be harmful to individual components of the vehicle. The aforementioned inconveniences and disadvantages are now taken into account by the comfort criterion. A core idea of the invention is then in particular that the regenerative braking strategy is always selected and implemented when this is possible without violating the comfort criterion.

In a preferred embodiment, the comfort criterion is defined by a maximum deceleration and can be met if the deceleration that is generated when a particular braking strategy is implemented does not exceed the maximum deceleration. In particular, the maximum deceleration determines what maximum forces are still acceptable on the vehicle and the occupants. At least for the regenerative braking strategy, expediently for both braking strategies, it is determined prior to a selection and implementation what effects result when the corresponding braking strategy is implemented, specifically how the deceleration develops during implementation and whether the deceleration exceeds the maximum deceleration. This is done in particular taking into account a maximum braking distance of the vehicle as an additional boundary condition, ie a maximum braking distance is specified, which should be adhered to without violating the comfort criterion. The maximum braking distance must be observed in any case, ie it is now checked whether this is possible for the regenerative braking strategy while also complying with the comfort criterion or not. Since the waiting time for the load change represents an additional and, in particular, unbraked distance for the vehicle in the regenerative braking strategy, the regenerative braking strategy may be based on a greater deceleration in order to be able to comply with the maximum braking distance. Depending on the specific situation, it may not be possible to meet the comfort criterion, so that the dissipative braking strategy is then selected and implemented. This is regularly the case with particularly short maximum braking distances, so that braking is carried out particularly early in these cases. However, if the comfort criterion can be met despite the waiting time, then the regenerative braking strategy is selected because it is more energy-efficient.

In a preferred embodiment, the vehicle is at an actual distance from an object ahead and the comfort criterion is defined by a minimum distance of the vehicle from an object ahead, in particular the object ahead, and can be met if the actual distance is when a respective braking strategy is implemented -Distance does not fall below the minimum distance. As an alternative or in addition to the maximum deceleration mentioned above, the selection of the braking strategy carried out also takes into account the actual distance, or simply distance, to an object ahead by using a minimum distance as a boundary condition. It is then checked whether the comfort criterion can be met without falling below the minimum distance. As described above, it is also checked here whether the additional distance covered by the regenerative braking strategy compared to the dissipative braking strategy makes it impossible to fulfill the comfort criterion or whether it is possible to carry out the regenerative braking strategy without violating the comfort criterion. In the former case, the dissipative braking strategy is selected in order to keep the minimum distance as well as possible, in the latter case the regenerative braking strategy is selected.

In a particularly expedient embodiment, the two aforementioned embodiments are combined with one another so that the minimum distance then corresponds to the maximum braking distance, i.e. in particular that the maximum braking distance is the distance that the vehicle travels or can travel until the minimum distance is reached. First of all, the actual distance to the object in front is determined and then, at least for the regenerative braking strategy, expediently for both braking strategies, it is determined how the actual distance develops over time and whether it is possible to comply with both the maximum braking distance and the comfort criterion .

The object lying ahead is regularly another vehicle that is driving in front of the vehicle. However, other situations are also conceivable, in which the object ahead is, for example, an oncoming vehicle or a vehicle crossing from the side, or a stop line, a barrier, a lane boundary, a building or the like. Logically, the same considerations also apply when reversing and an object then lying behind the vehicle.

In a preferred embodiment, the comfort criterion can be set, i.e. in particular that a threshold value to be maintained, which defines the comfort criterion, can be set. As a result, it is advantageously possible to adapt the comfort criterion as required and in particular to adapt it to different acceptance or tolerance values of a vehicle or an occupant. The vehicle expediently has an operating element, by means of which the comfort criterion can be set. For example, the maximum deceleration or the maximum braking distance can be set. Alternatively or additionally, the comfort criterion can be set automatically and is then set automatically, in particular, by a control unit of the vehicle. As a result, the comfort criterion is advantageously set appropriately in each case depending on the situation. In this case, a current situation is suitably determined by means of a sensor or by means of a plurality of sensors and then the comfort criterion is set as a function of the current situation. In a suitable embodiment, it is detected whether an occupant is holding something in their hand or is not wearing their seat belt or the like and then the comfort criterion is set accordingly, e.g. the maximum deceleration is reduced in order to allow only significantly weaker braking specifically for the cases mentioned. The situation is e.g. "occupant is not buckled up" or "occupant is holding something in his hand".

In a preferred embodiment, the vehicle has an automatic driving system, in particular as described above, and the automatic driving system regulates an actual distance of the vehicle from an object ahead to a target distance. In a cutting-in situation, the automatic driving system generates a brake request if the actual distance is less than the target distance due to the cutting-in situation because another, second object appears between the vehicle and the first object originally lying ahead. A cutting-in situation is particularly characterized in that the actual distance corresponds to the target distance within a tolerance range and is abruptly reduced by another object pushing into the space between the vehicle and the object in front. This is regularly the case when a third vehicle cuts in from an adjacent lane or from a driveway, driveway, exit or entrance. The other The object is now the object in front and relative to the vehicle, due to the principle, is not at the target distance, but much closer. The automatic driving system therefore recognizes a reduced actual distance that falls below the target distance and then triggers a braking request, in particular in order to restore the target distance as far as possible, with the corresponding consequence that it is then also determined whether the regenerative braking strategy is being carried out for this purpose can be used or whether the dissipative braking strategy must be selected.

A cut-in situation generally represents a first scenario for an automatic driving system, which is characterized in that the target distance from an object in front is initially actually maintained, i.e. the actual distance corresponds to the target distance. Based on this, a situation now arises in which the actual distance decreases and thus falls below the target distance, so that a delay is necessary. In this case, it is expediently determined for both braking strategies what deceleration is necessary in order to avoid a collision with the further object, ie the new object ahead, and in particular in order to restore the setpoint distance. It is determined whether the comfort criterion can be met when using the regenerative braking strategy. If the comfort criterion can be met, the regenerative braking strategy is selected and implemented accordingly, otherwise the dissipative braking strategy is selected and implemented.

In a preferred embodiment, the vehicle has an automatic driving system, in particular as described above. The automatic driving system regulates an actual distance of the vehicle from an object ahead to a target distance and generates a braking request when approaching, namely if the actual distance decreases and is greater than the target distance. In order to trigger the braking request, it is not absolutely necessary for the actual distance to actually fall below the target distance; rather, it is sufficient if this is calculated in advance for a future point in time. An approach situation is characterized in particular by the fact that the actual distance from the vehicle ahead decreases over time, since the vehicle, viewed relatively, has a higher speed than the object ahead. In other words: the vehicle catches up with the object ahead. Since there is a risk that the vehicle will collide with the object in front when the vehicle is approaching, the automatic driver reacts accordingly to avoid this. The automatic driving system therefore measures the actual distance and recognizes an approach situation when the actual distance decreases. The automatic driving system then triggers a braking request, with the corresponding consequence that it is then also determined whether the regenerative braking strategy can be carried out for this purpose or whether the dissipative braking strategy must be selected.

An approach situation generally represents a second scenario for automatic driving, which is characterized in that the actual distance is greater than a target distance, i.e. there is still room for a further reduction in the actual distance without a delay being necessary at first is. As already mentioned, this is the case when colliding with or approaching a vehicle in front. In this case, a difference between the actual distance and the setpoint distance is expediently taken into account when selecting the braking strategy. In particular, this difference is available as an additional distance and waiting time to wait for the load change of the electric machine, so that the comfort criterion may be easier to meet. In one possible embodiment, the distance to the object ahead is briefly undershot in the regenerative braking strategy.

In a preferred embodiment, it is anticipated that the braking request will be made in the future, and it is then determined for the regenerative braking strategy whether the comfort criterion can be met during its implementation, even before the braking request is actually made, so that there is a difference between the actual distance and the target -Distance is used to wait for the load change. In other words: the braking request is triggered earlier than intended, so to speak, namely before the actual distance corresponds to the target distance, since the ability to meet the comfort criterion is checked while the actual distance is still greater than the target distance. Since, as mentioned, there is still room for a further reduction in the actual distance when approaching, the difference between the actual distance and the target distance is used to advantage in order to select one of the two braking strategies early, ie before the target distance is reached if necessary also to be carried out. A braking request therefore occurs in the future and it is correspondingly anticipated or also predicted that the braking request will be triggered in the future and then, before this braking request is actually triggered, it is determined for the regenerative braking strategy whether the comfort criterion can be met when it is implemented. A significant advantage here is in particular that the additional difference is now available for braking and as a result only a smaller deceleration needs to be requested, so that the comfort criterion can be met more easily. In a suitable embodiment, the Dif The difference between the actual distance and the target distance means that the two braking strategies do not differ, or differ only slightly, with regard to their respective effects, in particular with regard to the comfort criterion. The difference is therefore used as a waiting time in order to then carry out the regenerative braking strategy with the same or a similar deceleration, in particular deceleration characteristic curve, as the dissipative braking strategy, which is otherwise carried out when the setpoint distance is reached. If the difference is sufficient to carry out the regenerative braking strategy in such a way that it fulfills the comfort criterion and preferably fulfills it at least as well as the dissipative braking strategy, then the regenerative braking strategy is also selected and carried out.

In a preferred embodiment, the actual distance is measured using a distance sensor or, alternatively or additionally, determined using Car2Car or Car2X communication. In particular, the distance sensor is part of the automatic driving system. The distance sensor is, for example, a radar or a laser range finder or the like. Car2Car or, more generally, Car2X communication is characterized in that a communication connection is set up between the vehicle and another vehicle or object, via which the vehicle exchanges data with the other vehicle or object. In one embodiment, the other vehicle or object is then the object ahead, in another embodiment the vehicle is another vehicle in the vicinity or the object is a camera at the edge of the road. The data is specifically data that the automatic driving system uses to determine the actual distance, e.g. distance data or GPS data or the actual distance itself.

In a preferred embodiment, in the dissipative braking strategy, the vehicle is braked dissipatively immediately after the braking request. “Immediately” is understood to mean, in particular, that the braking request is not awaited but that the dissipative braking function is activated immediately. In the case of a friction brake with brake linings for the wheels of the vehicle, the brake linings are brought into contact with braking surfaces on the wheels as soon as the brake request is generated. This happens in particular while the e-machine is still generating a positive drive torque. The load change is therefore not waited for.

In a preferred embodiment, in the regenerative braking strategy, the regenerative braking function is only activated after a waiting time that corresponds at least to a time that the electric machine needs to switch from a positive drive torque to a drive torque of zero or to a negative drive torque. A reduction to zero or a change in sign of the drive torque is therefore awaited. In order to determine whether the comfort criterion can be met despite the waiting time, the waiting time is calculated using the current positive drive torque and one or more gradients, which describe the reduction in the drive torque. It is therefore predicted how the drive torque will develop over the subsequent period and when the load change will then take place. The gradient is determined in particular by the fact that the drive torque required by the electric machine is reduced when the braking request is triggered.

The waiting time is preferably at least 0.1 s and at most 1 s, particularly preferably at most 0.5 s. The aforementioned values result in particular from the observation that the load change in typical vehicles and in typical driving situations lasts a maximum of about 0.5 s .

The dissipative braking strategy is expediently selected and implemented generally and specifically when the comfort criterion cannot be met with the regenerative braking strategy, even if the comfort criterion cannot be met with it. In a preferred embodiment, the regenerative braking scenario is then used as standard and the dissipative braking scenario is used in an emergency situation in which the comfort criterion cannot be met for either of the two braking strategies. An emergency situation is therefore characterized in particular by the fact that the comfort criterion cannot be met in any case. At least the dissipative braking function is activated here, since with this an immediate braking effect can be achieved and the vehicle is advantageously maximally, although possibly not sufficiently, decelerated. This is based on the consideration that in an emergency situation, regardless of the comfort criterion, the greatest possible deceleration should be achieved as quickly as possible in order to avoid or at least reduce an accident and the damage and injuries associated therewith. An emergency situation typically requires immediate emergency braking. Since the dissipative braking function, in contrast to the regenerative braking function, can already be activated with a positive drive torque, the dissipative braking strategy is preferably used in an emergency situation. In one variant, the regenerative braking function is also activated after the load change has taken place in order to generate additional deceleration.

If the drive torque is already negative when the braking request is generated, purpose moderately carried out the regenerative braking strategy immediately. In this case, there is no need to wait for a load change, so that the regenerative braking strategy initially has no disadvantages compared to the dissipative braking strategy and the conflict of objectives mentioned at the beginning does not arise in the selection. In situations in which the maximum deceleration that can be achieved with the regenerative braking function is not sufficient to meet the braking request, the dissipative braking function is suitably also activated.

A vehicle according to the invention is designed to be controlled using a method according to one of the preceding claims. For this purpose, the vehicle has in particular a control unit. As described above, the vehicle preferably has an automatic driving system, which is in particular a part of the control unit.

The object is also achieved in particular by a computer program product (file or data medium) containing an executable program which automatically executes the method described above when installed on a computer. In this case, the computer is in particular a control unit of a vehicle as described above.

• 1 ein Fahrzeug,
• 2 ein Blockdiagramm eines Verfahrens zur Steuerung des Fahrzeugs,
• 3 die zeitliche Entwicklung eines Antriebsmoments für verschiedene Bremsstrategien,
• 4 das Fahrzeug in einer Einschersituation,
• 5 ein Ist-Abstand zu einem vorausliegenden Objekt während der Einschersituation,
• 6 das Fahrzeug in einer Annäherungssituation,
• 7 ein Ist-Abstand zu einem vorausliegenden Objekt während der Annäherungssituation.

An exemplary embodiment of the invention is explained in more detail below with reference to a drawing. They each show schematically:

• 1 a vehicle,
• 2 a block diagram of a method for controlling the vehicle,
• 3 the temporal development of a drive torque for different braking strategies,
• 4 the vehicle in a cut-in situation,
• 5 an actual distance to an object ahead during the cut-in situation,
• 6 the vehicle in an approach situation,
• 7 an actual distance to an object ahead during the approach situation.

In 1 a vehicle 2 is shown, which has an electric machine 4 and which is, for example, an electric vehicle. The electric machine 4 is designed to generate a drive torque M, which is positive for driving the vehicle 2 and which is negative for braking the vehicle 2 . Correspondingly, the electric machine 4 is connected to a chassis 6 of the vehicle 2 via a drive train of the vehicle 2 that is not shown in detail. If the drive torque M of the electric machine 4 is negative, it acts as a generator and can be used to slow down the vehicle 2 . The vehicle 2 has a battery (not shown in detail) as the energy source for the electric machine 4 .

The vehicle 2 is designed using a special method as in 2 shown to be controlled. The vehicle 2 has a control unit 8 for this purpose. In the present case, the vehicle 2 also has an automatic driving system 10 which is part of the control unit 8 in the exemplary embodiment shown.

Out of 2 it becomes clear that the automatic driving system 10 has a distance control system in the present case, in which it is checked repeatedly or continuously whether an actual distance I of the vehicle 2 from an object 12 lying ahead is less than a target distance S, in which case the automatic driving system 10 has a Braking request B triggers. In the case of a braking request B, ie if the vehicle 2 is to be slowed down, two braking strategies Bdis, Breg can be selected and implemented for the vehicle 2 in order to generate a deceleration. A first braking strategy Bdis is a dissipative braking strategy Bdis, in which a dissipative braking function of vehicle 2 is activated. The dissipative braking function is implemented here by a friction brake 14 . A second braking strategy Breg, on the other hand, is a regenerative braking strategy Breg, in which only a regenerative braking function of vehicle 2 is used. The vehicle 2 is then decelerated in that the electric machine 4 generates a negative drive torque M, so that the kinetic energy of the vehicle 2 is then converted into electrical energy. The battery of the vehicle 2 is then charged with this electrical energy, so that energy is thus recovered, ie recuperated.

The two Bremstatrategies Bdis, Breg are below based on 3 explained in more detail, which shows the progression of the drive torque M over time t, ie the progression over time, in each case for an exemplary case of implementation of the two braking strategies Bdis, Breg. First of all, it is assumed, merely by way of example, that the drive torque M is constant up to a point in time t0. A braking request B is then generated at time t0 and vehicle 2 is to be slowed down. Out of 3 it can now be seen that with the regenerative braking strategy Breg initially no braking function is activated as long as the drive torque M is positive. Rather, a load change of the electric machine 4 is awaited and the regenerative braking function Breg is only activated when the load change has taken place. The point in time t1 marks the point in time at which the sign of the drive torque M changes, ie the difference between the points in time t0, t1 corresponds to a waiting time tw, which is awaited in order to complete the load change draw. This is based on the consideration that for the regenerative braking function, the electric machine 4 must be operated as a generator and it must therefore generate a negative drive torque M. When the vehicle 2 is being driven, however, a positive drive torque M is requested by the electric machine 4 in order to overcome a driving resistance, as is evident from the time period up to the point in time t0. In the event of a braking request B, the e-machine 4 cannot immediately meet the request for a negative drive torque M for a regenerative braking strategy Breg, but must first carry out a load change in which the positive drive torque M is first reduced to zero and then a negative drive torque M to create. This load change is initially awaited for regenerative braking, and the regenerative braking function is only activated when the load change has taken place and drive torque M is negative, namely after time t1. Vehicle 2 is therefore not actively braked while waiting for the load change. In the case of the dissipative braking strategy Bdis, on the other hand, the vehicle 2 is braked dissipatively immediately following the braking request B. The load change is therefore not waited for.

Due to the waiting time tw, which is necessary to carry out the load change, the regenerative braking strategy Breg regularly leads to a longer braking distance or, given the same braking distance, to stronger braking than the dissipative braking strategy Bdis, which can be carried out regardless of the sign of the drive torque M. Because during the waiting time tw, the vehicle 2 continues to move unbraked. However, this results in a conflict of objectives such that although the regenerative braking strategy Breg is more energy-efficient, it may entail a safety risk since, depending on the situation, there may not be enough time available to wait for the necessary load change. This conflict of objectives is now resolved in that, in the event that a braking request B occurs while the drive torque M is positive, specifically for the regenerative braking strategy Breg as in 2 shown it is determined whether a comfort criterion K can be fulfilled when it is carried out. The specific configuration of the comfort criterion K is initially not relevant; it is more important that the comfort criterion K is a boundary condition which should be adhered to when braking as far as possible and on which the decision between the two braking strategies Bdis, Breg is based. The regenerative braking strategy Breg is then actually selected and implemented if the comfort criterion K can be fulfilled, otherwise the dissipative braking strategy Bdis is selected and implemented. It is therefore decided which braking strategy Bdis, Breg is used by checking whether a comfort criterion K can be met. If the comfort criterion K cannot be met when performing the regenerative braking strategy Breg, this braking strategy Breg is also not selected, but the alternative, dissipative braking strategy Bdis is selected. Thus, if braking is required, it is checked individually whether regenerative braking is possible or not, and if possible, such regenerative braking is preferably carried out.

In the exemplary embodiment shown, the automatic driving system 10 is designed in such a way that it can generate a braking request B, depending on the ambient conditions, in order to slow down the vehicle 2 . The environmental conditions are determined, for example, by means of suitable sensors that are not shown in detail. While a person needs a certain reaction time to move his foot from an accelerator pedal to a brake pedal and thereby trigger a braking request B, the automatic driving system 10 is designed to switch from an acceleration request to a braking request B much more quickly. The automatic driving system 10 is regularly faster than the load change of the electric machine 4, so that a braking request B is regularly generated when the electric machine 4 is still generating a positive drive torque M, so that a regenerative braking strategy Breg is not possible, as shown 3 becomes clear. When operating with an automatic driving system 10, it would in principle be possible to always choose a dissipative braking strategy Bdis and to do without a regenerative braking strategy Breg entirely, with the corresponding disadvantages. In the present case, in contrast to this, the load change is simply awaited as a function of the comfort criterion K, in order then to use the regenerative braking strategy Breg at a later point in time t1.

The two braking strategies Bdis, Breg influence vehicle 2 and possibly also its occupants in different ways. Certain forces act on the vehicle 2 and the occupants during braking, which are dependent on the strength of the braking, that is to say on which value is required by the automatic driving system 10 for the deceleration. Certain braking strategies can lead to heavy braking with corresponding consequences for the vehicle 2 and the occupants. This is now taken into account by the comfort criterion K. As 2 shows, the regenerative braking strategy Breg is then always selected and implemented if this is possible without violating the comfort criterion K.

In the exemplary embodiment shown, the comfort criterion K is defined by a maximum deceleration and can be fulfilled if, when a respective braking strategy Bdis, Breg the delay which is thereby generated does not exceed the maximum delay. The maximum deceleration determines the maximum forces that are still acceptable on the vehicle and the occupants. Before one of the two braking strategies Bdis, Breg is selected and implemented, it is then determined what effects result when the corresponding braking strategy Bdis, Breg is implemented, specifically how the deceleration develops during implementation and whether the deceleration exceeds the maximum deceleration. In one variant, this takes place taking into account a maximum braking distance of vehicle 2 as an additional boundary condition.

Since in the regenerative braking strategy Breg the waiting time tw for the load change represents an additional and, in particular, unbraked distance for the vehicle 2, the regenerative braking strategy Breg may need to be based on a greater deceleration in order to be able to comply with the maximum braking distance. Depending on the specific situation, it may not be possible to comply with the comfort criterion K, so that the dissipative braking strategy Bdis is then selected and implemented. This is regularly the case with particularly short maximum braking distances, so that braking is carried out particularly early in these cases. However, if the comfort criterion K can be met despite the waiting time tw, then the regenerative braking strategy Breg is selected because it is more energy-efficient.

In one variant, the vehicle 2 is, alternatively or additionally, at an actual distance I from an object 12 ahead, and the comfort criterion K is defined by a minimum distance between the vehicle 2 and the object 12 ahead, and can be met if, when carrying out a respective Braking strategy Bdis, Breg the actual distance I does not fall below the minimum distance. As an alternative or in addition to the maximum deceleration mentioned above, the actual distance I is also taken into account when selecting the braking strategy Bdis, Breg that is carried out, in that a minimum distance is used as a boundary condition. It is then checked whether the comfort criterion K can be met without going below the minimum distance, and one of the two braking strategies Bdis, Breg is then selected accordingly.

The object 12 lying ahead is regularly another vehicle which is driving in front of the vehicle 2 . In the exemplary embodiment shown, the automatic driving system 10 adjusts an actual distance I of the vehicle 2 to the object 12 ahead to a target distance S, so that a distance control is implemented. As explained in more detail below, are in the 4 and 6 Two special scenarios are now shown, in which the actual distance I changes with respect to the object 12 ahead, which causes the automatic driving system 10 to react. the 5 and 7 then show the development of the actual distance I over time as a result of the two braking strategies Bdis, Breg for the two scenarios. The times t0, t1 in the 5 and 7 the times t0, t1 in 3 , so that the change in the actual distance I is comparable to the development of the drive torque M.

In 4 a cut-in situation is shown in a view from above as a first scenario, which is characterized in that the actual distance I initially corresponds to the target distance S and is suddenly reduced in that another object 16, here a third vehicle, into the space between the vehicle 2 and the object 12 ahead. The individual movements are in 4 indicated by arrows. The further object 16 is now the object 12 ahead - in 4 represented by a rectangle with a dashed border - and now much closer. The automatic driving system 10 thus recognizes a reduced actual distance I, which falls below the setpoint distance S, and then triggers a braking request B. This is particularly evident in 5 . The actual distance I initially corresponds constantly to the setpoint distance S until the third vehicle cuts in in front of vehicle 2 at time t0. As a result, a braking request B is triggered and it is checked how the situation is developing for the two braking strategies Bdis, Breg. In the 5 The shown development of the actual distance I over time t for the two braking strategies Bdis, Breg can vary in detail depending on the specific situation, but it is clearly recognizable that the dissipative braking strategy Bdis immediately leads to an increase in the actual distance I, while initially no change occurs in the regenerative braking strategy Breg. With the regenerative braking strategy Breg, the actual distance I is only increased again after the waiting time tw. Depending on the behavior of the third vehicle, for example if it is slower than vehicle 2, a correspondingly stronger braking may be necessary, which may no longer meet comfort criterion K. In this case, the dissipative braking strategy Bdis is selected. If, on the other hand, the comfort criterion K can be met, the regenerative braking strategy Breg is selected.

In 6 is shown as a second scenario, an approach situation, which is characterized in that the actual distance I to the object 12 ahead decreases over time, since the vehicle 2, viewed relatively, has a higher speed - in 6 illustrated by corresponding arrows - has as the object 12 lying ahead, so that the vehicle 2 ahead of the object 12 overtakes. Like before everything 7 shows, in the case shown, the actual distance I is initially greater than the target distance S and the vehicle 2 approaches the object 12. In this approach situation, there is a risk that the vehicle 2 will drive onto the object 12 ahead, so that the Driving automation 10 responds accordingly to avoid this. The automatic driving system 10 thus measures the actual distance I and recognizes the approach situation in that the actual distance I decreases. The automatic driving system 10 then triggers a braking request B at time t0. In the present case, the braking request B is already triggered before the actual distance I falls below the setpoint distance S in order to avoid this situation in advance. In one variant, on the other hand, the braking request B is only triggered when the actual distance I corresponds to the target distance S or falls below it.

In general, in an approach situation, due to the large actual distance I, there is still room for a further reduction in the actual distance I without initially having to delay. The difference between the actual distance I and the comparatively smaller target distance S is available as an additional distance and waiting time tw to wait for the load change of the electric machine 4, so that the comfort criterion K is easier to meet. in the in 7 case shown, the target distance S is briefly undershot in the regenerative braking strategy. This is accepted as long as the comfort criterion does not fall below a predefined minimum distance Amin, which is therefore not necessarily equal to the target distance S. This consideration of a minimum distance Amin is also possible in the case of a cut-in situation and is also carried out in a variant that is not shown.

In the present case, the actual distance I is measured using a distance sensor 18 . In an alternative or in addition that is not shown, the actual distance I is determined by means of Car2Car or Car2X communication, for which, for example, the control unit 8 is appropriately designed.

If the drive torque M when generating the braking request B is not as in 3 shown positive, but already negative, the regenerative braking strategy Breg is carried out immediately. In this case, there is no need to wait for a load change, so that the regenerative braking strategy Breg initially has no disadvantages compared to the dissipative braking strategy Bdis.

Reference List

2

vehicle

4

electric machine

6

landing gear

8th

control unit

10

driving automation

12

object

14

friction brake

16

other/further object

18

distance sensor

B

brake request

Bdis

dissipative braking strategy

breg

regenerative braking strategy

I

actual distance

K

comfort criterion

M

drive torque

S

target distance

t

time

t0, t1

time

partly

waiting period

Claims (12)

Method for controlling a vehicle (2), - Wherein the vehicle (2) has at least one electric machine (4) which is designed to generate a drive torque (M) which is positive for driving the vehicle (2) and which is negative for braking the vehicle (2). , - Wherein the case of a braking request (B) to generate a deceleration, two braking strategies (Bdis, Breg) for the vehicle (2) can be selected and implemented, namely a first, dissipative braking strategy (Bdis), in which a dissipative braking function of the vehicle (2 ) is activated, and a second, regenerative braking strategy (Breg), in which only a regenerative braking function of the vehicle (2) is used, - With the regenerative braking strategy (Breg), no braking function is activated as long as the drive torque (M) is positive, but a load change of the electric machine (4) is awaited and the regenerative braking function is only activated when the load change has taken place , - In the event that a braking request (B) occurs while the drive torque (M) is positive, it is determined for the regenerative braking strategy (Breg) whether a comfort criterion (K) can be met when it is carried out, - Wherein the regenerative braking strategy (Breg) is selected and implemented if the comfort criterion (K) can be met and where otherwise the dissipative braking strategy (Bdis) is selected and implemented. procedure after claim 1 , wherein the comfort criterion (K) is defined by a maximum deceleration and can be met if the deceleration that is generated when a respective braking strategy (Bdis, Breg) is carried out does not exceed the maximum deceleration. Procedure according to one of Claims 1 or 2 , wherein the vehicle (2) is spaced from an object (12) ahead at an actual distance (I), the comfort criterion (K) being defined by a minimum distance (Amin) of the vehicle (2) to an object (12) ahead is and can be fulfilled if the actual distance (I) does not fall below the minimum distance (Amin) when a respective braking strategy (Bdis, Breg) is carried out. Procedure according to one of Claims 1 until 3 , whereby the comfort criterion (K) is adjustable. Procedure according to one of Claims 1 until 4 , wherein the vehicle (2) has an automatic driving mechanism (10) which adjusts an actual distance (I) of the vehicle (2) to an object (12) ahead to a target distance (S) and which, in a cutting-in situation, requests braking (B) generated, if the actual distance (I) is less than the desired distance (S), that another object (16) between the vehicle (2) and the object (12) ahead appears. Procedure according to one of Claims 1 until 5 , wherein the vehicle (2) has an automatic driving mechanism (10) which regulates an actual distance (I) of the vehicle (2) to an object (12) ahead to a target distance (S) and which requests braking in an approaching situation (B) generated, namely if the actual distance (I) decreases and is greater than the target distance (S). procedure after claim 6 , where it is anticipated that the braking request (B) will take place in the future, and it is then determined for the regenerative braking strategy (Breg) whether the comfort criterion (K) can be met when it is implemented, even before the braking request (B) actually occurs, so that a difference between the actual distance (I) and the target distance (S) is used to wait for the load change. Procedure according to one of claims 3 until 7 , wherein the actual distance (I) is measured using a distance sensor (18) or is determined using Car2Car or Car2X communication. Procedure according to one of Claims 1 until 8th , wherein in the dissipative braking strategy (Bdis), the vehicle (2) is braked dissipatively immediately following the braking request (B). Procedure according to one of Claims 1 until 9 , In the regenerative braking strategy (Breg), the regenerative braking function is activated only after a waiting time (tw), which corresponds at least to a time that the electric machine (4) needs to go from a positive drive torque to a zero or one negative drive torque (M) to switch. Procedure according to one of Claims 1 until 10 , the regenerative braking strategy (Breg) being used as standard and the dissipative braking strategy (Bdis) being used in an emergency situation in which the comfort criterion (K) cannot be met for either of the two braking strategies (Bdis, Breg). Vehicle (2), which is designed by means of a method according to one of Claims 1 until 11 to be controlled.

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