Method for operating a motor vehicle and corresponding motor vehicle

Abstract

A method for operating a motor vehicle (1), wherein the motor vehicle (1) has a drive device (2) provided and designed for accelerating and regeneratively decelerating the motor vehicle (1), and a braking system (9) with at least one friction brake (10) provided and designed for decelerating the motor vehicle (1), and wherein, for decelerating the motor vehicle (1), a target braking force is set at least temporarily on the at least one friction brake (10), wherein a target deceleration is determined from the target braking force, and a state value describing a state of the at least one friction brake (10) is calculated from a deviation of an actual deceleration of the motor vehicle (1) from the target deceleration, characterized in that the target braking force set on the braking system (9) is a first target braking force, which is determined from a total target braking force together with a second target braking force set on the drive device (2).wherein the first target braking force is set to a first preset braking force and the second target braking force is set to a second preset braking force, which together correspond to the total target braking force, the actual deceleration is determined and stored as the target deceleration, then the first target braking force is increased based on the first preset braking force and the second target braking force is reduced based on the second preset braking force, and the state value is determined from the deviation between the actual deceleration and the stored target deceleration, and that if the state value falls below a threshold value, the target braking force set on the at least one friction brake (10) is increased based on a preset braking force to clean the at least one friction brake (10).

Description

[0001] The invention relates to a method for operating a motor vehicle, wherein the motor vehicle has a drive device provided and configured for accelerating and regeneratively decelerating the motor vehicle, as well as a braking system with at least one friction brake provided and configured for decelerating the motor vehicle. A target braking force is set at least temporarily on the at least one friction brake to decelerate the motor vehicle. The invention further relates to a motor vehicle.

[0002] For example, the prior art document DE 10 2017 205 810 A1 is known. This document describes a method for cleaning a friction brake system of a vehicle, in which a corrosion characteristic value is determined for each friction brake of the friction brake system. If a corrosion characteristic value of at least one friction brake exceeds a predetermined threshold, a brake force distribution system of the vehicle is configured to use the at least one friction brake for which the corrosion characteristic value was determined to exceed the predetermined threshold more frequently for braking during future braking operations in which the brake force distribution system is used, compared to a standard configuration of the brake force distribution system.

[0003] Furthermore, the document DE 10 2016 217 681 A1 discloses a method for operating a vehicle with a traction battery and a braking system, wherein the braking system comprises at least one friction brake, the friction surfaces of which can be brought into frictional contact with corresponding brake pads either by a driver-initiated braking operation or by a driver-independent braking operation, wherein the driver-independent braking operation takes place with a defined pressure over a defined period of time when at least one cleaning criterion is present, and an electrical machine which can be operated as a generator and which charges the traction battery in a recuperation mode, wherein the cleaning criterion is present when the charge state of the traction battery is above a threshold value.

[0004] The object of the invention is to propose a method for operating a motor vehicle which has advantages over known methods, in particular reliably ensures that the motor vehicle is ready to brake by cleaning the friction brake from time to time.

[0005] This is achieved according to the invention with a method for operating a motor vehicle with the features of claim 1. It is provided that a target deceleration is determined from the target braking force and a state value describing a state of the at least one friction brake is calculated from a deviation of an actual deceleration of the motor vehicle from the target deceleration, wherein if the state value falls below a threshold value, the target braking force set on the at least one friction brake is increased starting from a preset braking force in order to clean the at least one friction brake.

[0006] The described method serves to operate the motor vehicle. The motor vehicle has the drive device and the braking system. The drive device serves, on the one hand, to temporarily accelerate and, on the other hand, to temporarily decelerate the motor vehicle. Acceleration is understood to mean an increase in the driving speed of the motor vehicle, and deceleration is understood to mean a reduction in the driving speed. In more general terms, the drive device is therefore intended to drive the motor vehicle and, to that extent, to provide a drive torque directed towards driving the motor vehicle. The drive torque can be directed either towards accelerating or decelerating the motor vehicle, or can be directed partly towards accelerating and partly towards decelerating.

[0007] To provide the drive torque, the drive device has at least one drive unit, which is in the form of an electric machine or an electric traction machine. The drive unit can be used for regenerative deceleration of the motor vehicle, i.e., deceleration of the motor vehicle by converting the motor vehicle's kinetic energy into electrical energy. Preferably, the drive unit is electrically connected to an energy storage device for temporarily storing electrical energy, so that the electrical energy generated during regenerative deceleration of the motor vehicle is or can be temporarily stored in the energy storage device.

[0008] In addition to the drive system, there is a braking system, which serves to decelerate the motor vehicle and is designed accordingly. The braking system has at least one friction brake, by means of which the motor vehicle can be decelerated. The friction brake has, for example, a first brake element, in particular a brake disc, and a second brake element, which in particular has brake shoes. To decelerate the motor vehicle, the two brake elements are brought into frictional contact with one another; for example, the brake shoes are displaced toward one another in such a way that they frictionally accommodate the brake disc between them.

[0009] Particularly preferably, the braking system naturally has a plurality of friction brakes, wherein preferably at least one such friction brake is assigned to each of the wheels of the motor vehicle, in particular to each of the wheels of the motor vehicle, preferably precisely one such friction brake. Where reference is made to the friction brake or the at least one friction brake within the scope of this description, the explanations are fundamentally analogous. Furthermore, they are preferably applicable to each of the plurality of friction brakes, if present. Conversely, the explanations for a plurality of friction brakes are of course also applicable to the friction brake or the at least one friction brake.

[0010] To decelerate the motor vehicle, the target braking force is set at least temporarily on the at least one friction brake. The friction brake is then controlled to generate the target braking force. For example, based on the braking force set on the friction brake, a braking pressure is determined, which is then applied to the braking elements, in particular the brake shoes. In this case, the friction brake is a hydraulic friction brake. Alternatively, the friction brake can of course also be an electric friction brake, a pneumatic friction brake, or the like.

[0011] Generally, however, regenerative deceleration using the drive system is preferred when decelerating a motor vehicle because the electrical energy generated during this deceleration can be used at a later time, for example, to accelerate the vehicle. For this reason, the braking system, or at least one friction brake, is used less frequently than in motor vehicles that do not allow regenerative deceleration. This results in a reduced self-cleaning effect of the friction brake. The self-cleaning effect is achieved through the frictional action of the braking elements against each other, for example, due to the frictional action of the brake shoes or corresponding brake pads on the brake disc.

[0012] For this reason, it is intended to apply the friction brake preferentially from time to time to achieve a certain cleaning effect. For example, it is intended to apply the friction brake preferentially at certain time intervals. However, this can result in increased wear on the friction brake, necessitating its early replacement. In addition, it can also have a negative impact on the comfort and noise levels of the vehicle.

[0013] For this reason, the friction brake should be cleaned as needed. For this purpose, the target braking force set on the friction brake is used to determine a target deceleration. The target deceleration is the deceleration to which the vehicle would be subjected if the friction brake were completely cleaned. The target deceleration is therefore determined from the target braking force under the assumption of a completely intact and thoroughly cleaned friction brake.

[0014] In addition, the actual deceleration of the motor vehicle is determined, for example measured, and the deviation between the target deceleration and the actual deceleration is calculated. The actual deceleration is understood to mean the deceleration of the motor vehicle caused by the friction brake, in particular only this. If the deceleration of the motor vehicle is caused solely by the friction brake, the actual deceleration corresponds to an actual deceleration of the motor vehicle, which is in particular measured. If, on the other hand, the deceleration of the motor vehicle is caused both by the friction brake and by the drive device, the actual deceleration is preferably corrected accordingly; in particular, it corresponds to the actual deceleration less the deceleration caused by the drive device.

[0015] Alternatively, it can also be provided that the actual deceleration always corresponds to the actual deceleration of the motor vehicle. In this case, a deceleration that takes into account both the deceleration caused by the friction brake and the deceleration caused by the drive device – if present – is preferably used as the target deceleration. The target deceleration is therefore the sum of the deceleration (theoretically) caused by the friction brake and the deceleration (theoretically) caused by the drive device.

[0016] The condition value that describes the condition of at least one friction brake is calculated from the deviation. The condition of the friction brake is understood here to be, for example, a corrosion condition. The further the actual deceleration deviates from the target deceleration, in particular downwards, the greater the assumed corrosion of the friction brake. The greater the deviation of the actual deceleration from the target deceleration, the smaller the condition value calculated from it. The condition value is preferably calculated in a value range from 0 to 1 or from 0% to 100%, with smaller values representing a poorer condition of the friction brake, i.e. in particular, greater corrosion of the friction brake.

[0017] If the condition value falls below the threshold, i.e., if the condition value is smaller than the threshold, the friction brake should be cleaned. To do this, the target braking force set on the friction brake is increased, starting from the specified braking force. The specified braking force is the braking force specified by the vehicle for the friction brake. The target braking force is the braking force set on the friction brake. The target braking force preferably deviates from the specified braking force only for cleaning the friction brake. Apart from cleaning, the target braking force always corresponds to the specified braking force.

[0018] It may be provided that the target braking force is increased only to clean the friction brake if the motor vehicle is to be decelerated, i.e., to decelerate the motor vehicle. However, it may also be provided that the target braking force is increased starting from a default braking force of zero. In this case, the drive device is preferably adjusted such that the speed of the motor vehicle remains constant despite the increased target braking force being different from zero.

[0019] In the manner described, the friction brake can be cleaned as needed. In particular, the friction brake is only cleaned when, based on a comparison of the actual deceleration with the target deceleration, it is determined that the friction brake is not achieving the desired braking performance, but is falling short. This reliably prevents premature replacement of the friction brake, as well as the aforementioned reduction in comfort and noise.

[0020] A further development of the invention provides that the target deceleration is determined as the output variable of a relationship in the form of a table, a characteristic map, and / or a mathematical function, which has the target braking force as its input variable. Overall, the relationship therefore has the target braking force as its input variable and the target deceleration as its output variable. To determine the target deceleration, the target braking force is fed into the relationship, whereupon the target deceleration is available as the output variable. The relationship is in the form of a table, a characteristic map, or a mathematical relationship. This enables quick and easy calculation of the target deceleration, for which even low computing power is sufficient.

[0021] A further development of the invention provides that one of the following variables is used as an additional input variable to determine the target deceleration using the relationship: speed of the motor vehicle, mass of the motor vehicle, ambient temperature, brake temperature, age of the friction brake, and driving resistance. The target braking force is therefore not the only input variable of the relationship. Rather, at least one of the aforementioned variables can be used as an additional input variable. Particularly preferably, several of the aforementioned variables serve as additional input variables, in particular all of the aforementioned variables.

[0022] The speed of the motor vehicle is understood to be the current driving speed of the motor vehicle. The mass of the motor vehicle describes the current mass, which can be fixed or estimated, for example. The ambient temperature is the temperature in the environment of the motor vehicle, whereas the brake temperature is the current temperature of the friction brake, in particular the brake disc. The age of the friction brake can also be taken into account in the calculation of the target deceleration, whereby the age is given, for example, in the form of operating hours of the friction brake. As the age of the friction brake increases, the deceleration resulting from the target braking force can be reduced, so it is sensible to also use the age when determining the target deceleration.

[0023] Driving resistance is ultimately the sum of all resistances that the motor vehicle must overcome during operation. Driving resistance includes at least one of the following resistances: air resistance, rolling resistance, gradient resistance, and acceleration resistance. At least one of these resistances can therefore be used to determine the target deceleration; however, this is preferably done for several of the resistances mentioned, or even all of them. In the manner described, an extremely precise determination of the target deceleration is possible, which can be achieved using the friction brake.

[0024] A further development of the invention provides that after a certain period of time has elapsed since the target braking force was increased and / or when the limit value is reached or exceeded by the state value, the target braking force is reset to the specified braking force. Cleaning of the friction brake should, of course, only be carried out for as long as necessary to prevent excessive stress on the friction brake and thus premature wear. Accordingly, the target braking force is reset to the specified braking force after a certain period of time. It should be noted that the specified braking force can, of course, change over time, so that the specified braking force to which the target braking force is reset has a different value than the specified braking force at the start of cleaning the friction brake.

[0025] The target braking force is reset to the default braking force, for example, after a certain period of time has elapsed since the target braking force was increased. The time period is selected such that complete cleaning of the friction brake can be assumed. For example, the time period is selected to be constant. However, it is preferably calculated based on the state value.

[0026] Particularly preferred is the longer the time period, the smaller the condition value is at the beginning of cleaning.

[0027] Additionally or alternatively, the target braking force is reset if the status value reaches or exceeds the limit value, i.e., is equal to or greater than it. The status value continues to be determined during cleaning, for example, at specific intervals or continuously. This allows for extremely needs-based cleaning of the friction brake. Particularly preferably, the target braking force is reset if one of the two aforementioned conditions is met. It is therefore not necessary for both conditions to be met, although this can also be assumed.

[0028] A further development of the invention provides that a difference between the target braking force and the specified braking force is set depending on the condition value. In particular, the smaller the condition value, the greater the difference is selected. In other words, the smaller the condition value, i.e., the worse the condition of the friction brake, the more intensive the cleaning of the friction brake is. This achieves needs-based cleaning of the friction brake, while simultaneously preventing excessive wear on the friction brake.

[0029] A further development of the invention provides that the target braking force set on the braking system is a first target braking force, which, together with a second target braking force set on the drive device, is determined from a total target braking force. It has already been pointed out above that the motor vehicle can be decelerated not only by means of the braking system, but also by means of the drive device. Accordingly, the total target braking force is first determined and specified. From this, the first target braking force and the second target braking force are then calculated, with the first target braking force being set on the braking system and the second target braking force being set on the drive device.

[0030] The total target braking force is divided between the first target braking force and the second target braking force, for example, according to a predetermined ratio. This ratio can be constant, but can also depend on at least one of the following variables: the charge level of the energy storage device, the total target braking force, the speed of the vehicle, and the mass of the vehicle.

[0031] It can be provided that, in a first operating mode, the deceleration of the motor vehicle occurs solely by means of the drive system. In a second operating mode, the deceleration of the motor vehicle occurs jointly with the help of the drive system and the braking system. In a third operating mode, deceleration occurs, for example, solely with the help of the braking system, i.e., without the drive system.

[0032] In the first operating mode, the first target braking force is zero, whereas the second target braking force equals the total target braking force. In the second operating mode, the sum of the first target braking force and the second target braking force equals the total target braking force. In the third operating mode, the first target braking force equals the total target braking force and the second target braking force equals zero.

[0033] To clean the friction brake, either the second operating mode or the third operating mode is used. It should be noted that if multiple friction brakes are present and form part of the braking system, multiple first target braking forces are present, namely a first target braking force for each of the multiple friction brakes. Whenever the first target braking force is mentioned in this description, it should always be noted that multiple first target braking forces can be present and, in this respect, the total target braking force is composed of the multiple target braking forces and the second target braking force. In the manner described, efficient and safe deceleration of the motor vehicle can always be achieved.

[0034] As already indicated above, the actual deceleration is preferably only the deceleration caused by the friction brake. The actual deceleration is determined from the actual deceleration of the motor vehicle in such a way that it is independent of the deceleration of the motor vehicle caused by the drive device. This approach is used particularly in the second operating mode. In the third operating mode, the actual deceleration corresponds to the actual deceleration, since the motor vehicle is braked solely by the friction brake.

[0035] A further development of the invention provides that when the first target braking force is increased for cleaning the at least one friction brake, the second target braking force is reduced. It is therefore provided that the total target braking force immediately before cleaning and immediately at the start of cleaning is the same. While the first target braking force is increased by a first amount based on a first specified braking force, the second target braking force is reduced by a second amount based on a second specified braking force, wherein the first amount and the second amount are identical. Cleaning the friction brake therefore has no influence on the driving operation of the motor vehicle.

[0036] A further development of the invention provides that the first target braking force is set to a first specified braking force and the second target braking force is set to a second specified braking force, which in sum correspond to the total target braking force, the actual deceleration is determined and stored as the target deceleration, then the first target braking force is increased starting from the first specified braking force and the second target braking force is reduced starting from the second specified braking force, and the state value is determined from the deviation between the actual deceleration and the stored target deceleration.

[0037] The two target braking forces are then adjusted to the corresponding preset braking forces. The first target braking force and the second target braking force add up to the total target braking force, so that the first target braking force and the second target braking force add up to the total target braking force. At the same time or subsequently, the actual deceleration is determined, for example, measured, which is present at the total target braking force. This actual deceleration is stored as the target deceleration. In this case, the actual deceleration can correspond to the actual deceleration of the vehicle. However, here too, the actual deceleration is preferably the deceleration achieved solely by the friction brake.

[0038] The actual deceleration is used below as the target deceleration. Therefore, the target deceleration is not determined using this relationship, but rather calculated from the current deceleration of the vehicle. After the target deceleration is stored, the first target braking force is increased and the second target braking force is decreased. This occurs in such a way that the total target braking force before increasing the first target braking force or before decreasing the second target braking force is equal to the total target braking force after increasing and decreasing. In other words, the total target braking force should be kept constant.

[0039] The actual deceleration is then determined again, and the deviation between the actual deceleration and the stored target deceleration is calculated. From this deviation, the state value is then calculated again, and the procedure described above is continued. This approach eliminates the need for the relationship to determine the target deceleration and calculates the state value solely from the vehicle's actual deceleration. This results in greater accuracy.

[0040] A further development of the invention provides that the at least one friction brake is a component of several friction brakes, and that the determination of the target deceleration and the calculation of the state value for the several friction brakes are performed individually or in groups. Thus, there is not just one friction brake, but rather the braking system has several friction brakes, each of which includes the at least one friction brake. Preferably, at least one of these several friction brakes is assigned to each wheel of the motor vehicle.

[0041] It can be provided that the target deceleration, the deviation, and the state value are determined for each of the friction brakes. However, it can also be provided that the friction brakes are grouped together and that the target deceleration, the deviation, and the state value are determined for each of the groups. For example, a first of the groups comprises the friction brakes of a first wheel axle of the motor vehicle, and a second of the groups comprises the friction brakes of a second wheel axle of the motor vehicle. Thus, axle-specific determination of the state value is provided. By determining the state value individually or for each axle, a particularly high level of accuracy of the state value is achieved, thus ensuring a high level of operational reliability of the motor vehicle.

[0042] The invention further relates to a motor vehicle, in particular for carrying out the method according to the statements in the context of this description, wherein the motor vehicle has a drive device provided and designed for accelerating and for regeneratively decelerating the motor vehicle and a braking system with at least one friction brake provided and designed for decelerating the motor vehicle, and wherein the motor vehicle is provided and designed to at least temporarily set a target braking force on the at least one friction brake for decelerating the motor vehicle.

[0043] The motor vehicle is further provided and designed to determine a target deceleration from the target braking force and to calculate a state value describing a state of the at least one friction brake from a deviation of an actual deceleration of the motor vehicle from the target deceleration, wherein if the state value falls below a threshold value, the target braking force set on the at least one friction brake is increased starting from a preset braking force in order to clean the at least one friction brake.

[0044] The advantages of such a design of the motor vehicle or such a procedure have already been pointed out.

[0045] Both the motor vehicle and the method for its operation can be further developed in accordance with the explanations in this description, so that reference is made to these in this respect.

[0046] The invention will be explained in more detail below with reference to the exemplary embodiments shown in the drawings, without limiting the invention. Figure a schematic representation of the motor vehicle with a drive device and a braking system.

[0047] The figure shows a schematic representation of a motor vehicle 1, which has a drive device 2 for at least temporarily driving wheels 3 of the motor vehicle 1. Two of the wheels 3 are assigned to a first wheel axle 4 and two wheels 3 to a second wheel axle 5. The first wheel axle 4 is, for example, the front wheel axle and the second wheel axle 5 is the rear wheel axle. The drive device 2 is drive-connected to the wheels 3 of the respective wheel axles 4 and 5 via an axle differential 6 or 7, respectively. A clutch 8 can also be present between the drive device 2 and the axle differential 7 of the second wheel axle 5.

[0048] The drive device 2 has at least one drive unit by means of which the motor vehicle 1 can be accelerated and decelerated regeneratively. To ensure reliable deceleration of the motor vehicle 1 at all times, it also has a braking system 9 that has at least one friction brake 10, or in the exemplary embodiment shown here, several friction brakes 10. Preferably, one of the friction brakes 10 is drive-relatedly assigned to each of the wheels 3, so that the respective wheel 3 can be braked or decelerated with the corresponding friction brake 10.

[0049] To decelerate the motor vehicle 1, a target braking force is set at least temporarily on the at least one friction brake 10. It is now provided that a target deceleration is determined from the target braking force. If an actual deceleration of the motor vehicle 1 deviates from the target deceleration, a state value is calculated from the deviation, which describes the current state of the at least one friction brake 10. If the state value falls below a threshold value, cleaning of the corresponding friction brake 10 is initiated. For this purpose, the target braking force set on the friction brake 10 is increased starting from a specified braking force in order to remove corrosion from the friction brake 10. Because the friction brake 10 is only cleaned when actually necessary, a particularly long service life of the friction brakes 10 is achieved. LIST OF REFERENCE SYMBOLS: 1 motor vehicle 2 drive device 3 wheel 4 1. Wheel axle 5 2nd wheel axle 6 axle differential 7 axle differential 8 clutch 9 Braking system 10 Friction brake

Claims

[1] Method for operating a motor vehicle (1), wherein the motor vehicle (1) has a drive device (2) provided and designed for accelerating and for regenerative deceleration of the motor vehicle (1) and a braking system (9) with at least one friction brake (10) provided and designed for decelerating the motor vehicle (1), and wherein, for decelerating the motor vehicle (1), a target braking force is set at least temporarily on the at least one friction brake (10), wherein a target deceleration is determined from the target braking force and a state value describing a state of the at least one friction brake (10) is calculated from a deviation of an actual deceleration of the motor vehicle (1) from the target deceleration, characterized bythat the target braking force set on the braking system (9) is a first target braking force which is determined from a total target braking force together with a second target braking force set on the drive device (2), wherein the first target braking force is set to a first specified braking force and the second target braking force is set to a second specified braking force, which in sum correspond to the total target braking force, the actual deceleration is determined and stored as the target deceleration, then the first target braking force is increased based on the first specified braking force and the second target braking force is reduced based on the second specified braking force, and the state value is determined from the deviation between the actual deceleration and the stored target deceleration,and that if the state value falls below a threshold value, the target braking force set on the at least one friction brake (10) is increased for cleaning the at least one friction brake (10) starting from a preset braking force. [2] Method according to claim 1, characterized by that the target deceleration is determined as the output variable of a relationship in the form of a table, a characteristic map and / or a mathematical function, which has the target braking force as the input variable. [3] Method according to one of the preceding claims, characterized by that to determine the target deceleration by means of the relationship, one of the following variables is used as a further input variable: speed of the motor vehicle (1), mass of the motor vehicle (1), ambient temperature, brake temperature, age of the friction brake (10) and driving resistance. [4] Method according to one of the preceding claims, characterized bythat after a certain period of time has elapsed since the target braking force was increased and / or when the limit value is reached or exceeded by the state value, the target braking force is reset to the specified braking force. [5] Method according to one of the preceding claims, characterized by that a difference between the target braking force and the specified braking force is set depending on the state value. [6] Method according to one of the preceding claims, characterized by that when the first target braking force is increased to clean the at least one friction brake (10), the second target braking force is reduced. [7] Method according to one of the preceding claims, characterized by that the at least one friction brake (10) is a component of several friction brakes (10) and the determination of the target deceleration and the calculation of the state value for the several friction brakes (10) are carried out individually or in groups. [8] Motor vehicle (1), in particular for carrying out the method according to one or more of the preceding claims, wherein the motor vehicle (1) has a drive device (2) provided and designed for accelerating and for regenerative deceleration of the motor vehicle (1), and a braking system (9) with at least one friction brake (10) provided and designed for decelerating the motor vehicle (1), and wherein the motor vehicle (1) is provided and designed to at least temporarily set a target braking force on the at least one friction brake (10) for decelerating the motor vehicle (1), wherein the motor vehicle (1) is further provided and designed to determine a target deceleration from the target braking force and to calculate a state value describing a state of the at least one friction brake (10) from a deviation of an actual deceleration of the motor vehicle (1) from the target deceleration, characterized bythat the target braking force set on the braking system (9) is a first target braking force which is determined from a total target braking force together with a second target braking force set on the drive device (2), wherein the first target braking force is set to a first specified braking force and the second target braking force is set to a second specified braking force, which in sum correspond to the total target braking force, the actual deceleration is determined and stored as the target deceleration, then the first target braking force is increased based on the first specified braking force and the second target braking force is reduced based on the second specified braking force, and the state value is determined from the deviation between the actual deceleration and the stored target deceleration,and that if the state value falls below a threshold value, the target braking force set on the at least one friction brake (10) is increased for cleaning the at least one friction brake (10) starting from a preset braking force.

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