Method for controlling a vehicle's braking system

Abstract

The invention relates to a method (100) for controlling a braking system (10) of a vehicle (1) with at least one electrically driven axle (2), wherein the braking system (10) comprises at least one electric motor (11) and at least one mechanical wheel brake (12), wherein the method (100) comprises the following steps: - Detection (101) of a brake signal to initiate a braking process to decelerate the vehicle (1), - Determining (102) a recuperation torque achievable by the electric motor (11), - Determining (103) a torque that can be transferred from the vehicle to the road surface and that ensures the vehicle's driving stability (1), - Comparing (104) the desired braking torque based on the brake signal with the maximum available torque and the maximum achievable recuperation torque of the electric motor (11) and the vehicle's driving stability, - Determining (105) a target slip value, - Adjusting (106) the recuperation torque generated by the electric motor (11) to set the target slip value.

Description

[0001] The invention relates to a method for controlling a braking system of a vehicle, a computer program product and a vehicle.

[0002] Modern electric and hybrid vehicles increasingly use electric motors to assist braking, particularly through regenerative braking, which feeds energy back into the battery. This form of deceleration offers significant advantages, including increased range and reduced mechanical wear on the friction brakes. While regenerative braking is already being used effectively, challenges remain when the vehicle enters a critical slip control situation. Conventional anti-lock braking systems (ABS) intervene mechanically in such situations, individually regulating the braking torque at each wheel to ensure vehicle stability.

[0003] In vehicles without individual wheel drives, where one axle is driven by a single electric motor via a differential, the integration of precise slip control via the electric motor is particularly important, since wheel-specific control via the electric motor is not possible.

[0004] The prior art according to DE 10 2020 007 248 A1 relates to a method for controlling a vehicle's braking system, which includes both an electric motor and a mechanical wheel brake. In this method, both the brake slip by the wheel brake and by the electric motor are controlled to improve vehicle stability. In particular, the torque of the electric motor is controlled in such a way that, in combination with the wheel brake, it enables control of the vehicle's behavior. This control is achieved by combining the torques of the wheel brake and the electric motor.

[0005] A disadvantage of the known state of the art is that, in addition to the mechanical wheel brake, each wheel of the vehicle requires a separate electric motor to control the brake slip.

[0006] It is therefore an object of the present invention to overcome at least one of the disadvantages described above, at least partially. In particular, it is an object of the invention to provide a method for controlling a vehicle's braking system that functions reliably, especially even without individual wheel drive.

[0007] The foregoing problem is solved by a method according to a first aspect of the present invention, by a computer program product according to a second aspect of the present invention, and by a vehicle according to a third aspect of the present invention. Further features and details of the invention will become apparent from the dependent claims, the description, and the drawings. Features and details described in connection with the method according to the invention naturally also apply in connection with the computer program product and / or the vehicle according to the invention, and vice versa, so that the disclosure relating to the individual aspects of the invention always includes, or allows for, reciprocal reference.

[0008] According to a first aspect, the present invention relates to a method for controlling a braking system of a vehicle with at least one electrically driven axle, wherein the braking system comprises at least one electric motor and at least one mechanical wheel brake, and wherein the method comprises the following steps: - Detecting a brake signal to initiate a braking process to decelerate the vehicle, - Determining the recuperation torque achievable by the electric motor and the vehicle's driving stability, Determining the torque that can be transferred from the vehicle to a roadway, - Comparing the desired braking torque based on the brake signal with the maximum available torque, the maximum achievable recuperation torque of the electric motor, and / or the vehicle's driving stability. - Determining a target slip value, - Adjusting the recuperation torque generated by the electric motor to set the target slip value.

[0009] The process steps can be carried out at least partially simultaneously or sequentially, and the sequence of the process steps is not limited to the order defined by the numbering, so that individual steps can be carried out in different sequences. According to the invention, the process steps are primarily executed by a control unit.

[0010] The inventive method for controlling a vehicle's braking system with at least one electrically driven axle comprises several successive and / or parallel steps aimed at optimizing the use of the electric motor's braking power to decelerate the vehicle in a controlled manner. This braking system includes at least one electric motor, which can function as a generator to recover energy through recuperation, and a mechanical wheel brake, which can provide additional deceleration through friction. The inventive method makes it possible to perform slip control primarily and preferably (depending on the situation) exclusively via the electric motor. The mechanical wheel or friction brake is only activated if the electric motor is unable to fully meet the braking requirements.An inventive method enables slip control close to the actuator via the electric motor, allowing for precise and dynamic adjustment of the braking torque. It is preferred to deactivate the conventional ABS, which is implemented by the mechanical wheel brake, provided that the braking requirements can be met solely by the electric motor.

[0011] First, a control unit detects a braking signal initiated by the driver or the vehicle assistance system to initiate braking and reduce the vehicle's speed. This braking signal represents the driver's desired deceleration torque, which serves as the basis for calculating and controlling the braking forces within the vehicle. The brake system's control unit interprets this braking signal and determines the corresponding braking force request. The control unit thus derives the desired braking torque value, which defines the target demand on the brake system.

[0012] Furthermore, the maximum recuperation torque achievable by the electric motor, also known as recuperation potential, is determined. The recuperation torque describes the braking force that the electric motor can generate by converting kinetic energy into electrical energy. This recuperation power varies depending on the operating conditions of the electric motor (e.g., temperature) and the state of charge of the vehicle battery, and therefore may not cover the driver's entire braking request under certain circumstances. The system thus calculates the maximum possible braking torque that the electric motor can generate through recuperation without overloading the battery or the motor.

[0013] In parallel, the vehicle's deliverable torque is determined, which describes the maximum braking force that can be transmitted to the road surface under the given road and adhesion conditions. This value is calculated by the control unit and is based on factors such as the road surface's coefficient of friction, the axle load, and the tires' mechanical properties. The deliverable torque thus defines the physical limit of braking force transmission without exceeding the tires' traction limit. In parallel or alternatively, the vehicle's driving stability is checked. Driving stability describes the state in which the vehicle can be moved in a controlled and stable manner without exceeding the tires' adhesion limit. Stable driving dynamics are essential for safely controlling the vehicle under all conditions, especially during braking.

[0014] In a further step, the control unit compares the desired braking torque, derived from the brake signal, with the maximum available torque and the maximum achievable recuperation torque of the electric motor, as well as the vehicle's driving stability. This test serves to determine whether the electric motor alone is capable of generating the required braking torque, i.e., whether the braking requirements can be met solely by the electric motor. If so, the mechanical wheel brakes (conventional ABS) can remain deactivated, and the electric motor takes over complete slip control. If the maximum achievable recuperation torque of the electric motor is insufficient and / or driving stability would be compromised by using the electric motor alone, the mechanical wheel brakes will preferably be used to provide support and achieve the desired deceleration.According to the invention, however, the mechanical wheel brake is not used as long as the electric motor makes it possible to brake the vehicle by means of recuperation.

[0015] Based on this, the control unit then sets a target slip value. This target slip value represents the optimal value for brake slip, i.e., the ratio between the actual wheel speed and the vehicle speed. Controlled brake slip is necessary to utilize the tire's traction limit and thus transfer maximum braking force to the road surface. The target slip value is determined dynamically so that the electric motor can be used optimally to maximize deceleration without compromising the vehicle's steerability and stability. Simultaneously, the use of the mechanical wheel brakes can be avoided as long as driving stability allows. The target slip value is therefore selected to optimally utilize the tire's traction limit, thereby achieving maximum braking force transfer to the road surface.

[0016] Finally, in a further step, the recuperation torque generated by the electric motor is adjusted by the control unit to achieve and maintain the specified target slip value. This is preferably done by controlling the electric current in the motor. A control loop continuously compares the actual slip, or a derived value calculated, for example, from the wheel speed sensors and the vehicle speed, with the target slip. If there are deviations, the recuperation torque is increased or decreased accordingly. By adjusting the recuperation torque, the electric motor is controlled so that the braking process takes place within the stable limits of tire grip, ensuring controlled deceleration.This dynamic control system allows for precise slip regulation, regulating the recuperation torque to maintain optimal brake slip. This enables high braking performance while simultaneously maximizing energy recovery. In this way, efficient and controlled deceleration of the vehicle is achieved, making optimal use of recuperation and ensuring vehicle safety on the road. If no critical driving condition is identified, brake slip regulation can be handled solely by the electric motor. The mechanical wheel brakes (conventional ABS) remain deactivated as long as the electric motor can meet the braking demands on its own.

[0017] According to the invention, the interaction between the electric motor and the mechanical wheel brake is designed such that the braking forces are optimally distributed depending on the available braking or recuperation potential of the motor. The decision as to whether the brake slip control is primarily and preferably solely via the electric motor or via the mechanical wheel brake is preferably based not only on the maximum deceleration capability but also on the thermal load of the motor. By prioritizing the use of the electric motor for slip control, the energy efficiency of the vehicle is increased, since a large portion of the kinetic energy is converted into electrical energy instead of being lost as heat through the mechanical brake. Furthermore, the wear of the mechanical brake components is reduced, which extends the service life of the braking system.The precise control of the recuperation torque enables a coordinated brake force distribution, which ensures the stability and safety of the vehicle in every driving situation.

[0018] Applied to a specific scenario, the method according to the invention can be described by way of example as follows. If the driver requests a braking torque that is greater than the available torque, the system first checks whether the electric motor has sufficient recuperation potential. If so, the braking torque is controlled solely by the electric motor, and the mechanical wheel brake remains deactivated. The electric motor varies its recuperation torque to maintain the target slip value and achieve the desired deceleration. This dynamic and efficient brake control illustrates the advantages of the method, both in terms of energy recovery and the safety and stability of the vehicle.

[0019] Within the scope of the invention, it can be advantageous that the vehicle's driving stability is continuously monitored, and the electric motor takes over slip control by adjusting the recuperation torque as long as driving stability is maintained. Thus, the vehicle's driving stability is continuously monitored, allowing the electric motor to take over slip control by adjusting the recuperation torque and thereby continuously maintaining the optimal braking slip value. This continuous monitoring of driving stability ensures that the vehicle remains stable even during braking by the electric motor, particularly under varying road conditions or dynamic driving situations.By integrating this continuous monitoring, the system can react quickly and precisely to changes in road conditions or vehicle dynamics and use the electric motor for slip control, as long as driving safety is ensured.

[0020] Vehicle dynamics control describes the interplay of various vehicle movements, such as acceleration, braking, and steering, to keep the vehicle within safe physical limits. Through a detailed analysis of the vehicle dynamics, the system detects, for example, changes in yaw rate, lateral acceleration, or wheel slip rate that could indicate potential instabilities. This continuous feedback enables precise control of the braking torque by the electric motor, ensuring that the vehicle remains on the desired path and can be safely controlled.

[0021] Another advantage of this method is the reduction of mechanical stress on the braking system. Since the electric motor is used for traction control as long as stability allows, the mechanical wheel brakes are used less frequently. This load-based distribution of the braking process between the electric motor and mechanical brakes extends the service life of the braking system, reduces maintenance requirements, and thus lowers operating costs. Furthermore, the continuous use of the electric motor for brake force control improves the vehicle's energy efficiency. Because the electric motor acts as a generator, some of the vehicle's kinetic energy is converted into electrical energy and fed back into the battery. This energy recovery through recuperation increases the vehicle's range, as some of the energy required for propulsion is covered by the recovered energy.

[0022] The combination of slip control and continuous monitoring of driving stability also increases the system's responsiveness. Should the vehicle approach its stability limit, the system can take early countermeasures to prevent instability. However, if no critical driving condition is identified, brake slip control can be managed solely by the electric motor. As long as driving stability can be maintained, the electric motor takes over slip control by selectively adjusting the recuperation torque. This reduces the intervention of mechanical braking systems, which not only minimizes wear on the brake components but also enables a smoother and more comfortable driving experience. A further advantage of this advanced system lies in the precise control of the electric motor.Thanks to the high control speed of the electric motor, adjustments can be made almost without delay, which significantly increases the effectiveness of the slip control.

[0023] Within the scope of the invention, it is conceivable that driving stability is determined by detecting the difference in wheel speeds between the left and right wheels of an axle, particularly to determine whether individual wheel ABS control is necessary. Accordingly, driving stability is monitored by detecting the difference in wheel speeds between the left and right wheels of the electrically driven axle. This continuous monitoring of the speed differences, also referred to as wheel speed differential, provides a reliable method for detecting stability deviations. The differential speed describes the deviation in the speed of the two wheels and is an indicator of driving behavior, especially for changing, varying road conditions (e.g., one wheel already on black ice, the other on dry asphalt). The method determines whether the differential speed exceeds a defined threshold.When this threshold is reached, it indicates that the vehicle's stability may be compromised because the wheels are subjected to uneven loads. In such cases, the braking force is adjusted accordingly to prevent over- or under-braking of the wheels. A greater difference therefore suggests that one wheel may be braking too hard, indicating the onset of wheel slip and potential instability.

[0024] Within the scope of the invention, it is also conceivable that driving stability is determined by detecting the difference in wheel speeds between the front and rear axles, particularly to determine whether individual wheel ABS control is necessary. Accordingly, driving stability is monitored by detecting the difference in wheel speeds between the front and rear wheels of an axle. This continuous monitoring of the speed differences, also referred to as axle differential, provides a reliable method for detecting stability deviations. The differential speed describes the deviation in the speed of the two axles and is an indicator of driving behavior, especially for changing, different road conditions (e.g., one axle already on black ice, the other on dry asphalt). The method determines whether the axle differential speed exceeds a defined threshold.When this threshold is reached, it indicates that the vehicle's stability may be compromised because the wheels on each axle are subjected to different loads. In such cases, the braking force is adjusted accordingly to prevent over- or under-braking of the axles. A larger difference therefore suggests that one axle may be braking too hard, indicating the onset of wheel slip and potential instability.

[0025] By detecting these speed differences, the braking system can optionally assess whether individual wheel ABS control is necessary, addressing the specific requirements of each wheel. Individual wheel control describes ABS control at the wheel level, where each wheel is individually monitored to ensure optimal grip and stability. This type of control is implemented by the ABS (Anti-lock Braking System), which prevents individual wheels from locking up, thus ensuring improved steering and stability. Integrating wheel-level ABS control into the system allows for adjustments to the braking force, ensuring that the vehicle remains stable and all wheels operate within their optimal slip range.

[0026] Detecting and evaluating wheel speed differences and / or axle speed differences enables a rapid and targeted response to specific driving situations. This not only improves safety but also increases the efficiency of the braking system, as individual wheel ABS control is only activated when needed. This selective control ensures stable overall braking performance while individually adjusting the wheel units to optimally support driving dynamics.

[0027] Within the scope of the invention, it can be provided that the control unit switches to the wheel-individual ABS control of the mechanical wheel brake when the difference in wheel speeds exceeds a predetermined threshold, in particular over a defined period or a defined integral value. In this further embodiment of the method, an automatic switch to the wheel-individual ABS control of the mechanical wheel brake occurs as soon as the difference in wheel speeds between the wheels of one axle or the wheels of two axles exceeds a predetermined threshold, in particular over a defined period or a defined integral value.This threshold can therefore preferably be determined over a defined period or based on a defined integral value and serves as a limit indicating when individual wheel braking control becomes necessary to ensure vehicle stability. In this context, the term threshold refers to a defined threshold for the difference in wheel speeds, which is determined according to vehicle type and driving conditions and acts as the trigger for switching to individual wheel control. If this threshold is reached or exceeded, it indicates that one of the wheels is rotating significantly faster or slower than the other, typically indicating increased wheel slip. The definition of a threshold can advantageously be based on different criteria. A time-based threshold specifies how long a wheel speed difference may exceed a certain limit before the switchover occurs.An integral value can take into account the cumulative deviation of wheel speeds over time, thereby identifying particularly long-lasting and significant differences.

[0028] The differential rotational speed describes the difference in speed between the two wheels on an axle and is an indicator of driving behavior, particularly under changing road conditions (e.g., one wheel on ice, the other on dry asphalt). The procedure determines whether the differential rotational speed exceeds a defined threshold, especially over a defined period, or a defined integral value. Reaching this threshold indicates that the vehicle's stability may be compromised due to unequal loads on the wheels. In such cases, the braking force can be adjusted accordingly, and / or the target brake slip can be adjusted, and / or the system can switch to individual wheel ABS control of the mechanical wheel brakes to prevent over- or under-braking of the wheels.

[0029] The axle differential speed describes the difference in speed between the wheels of two axles and is an indicator of driving behavior, particularly under varying road conditions (e.g., one axle on ice, the other on dry asphalt). The procedure determines whether the axle differential speed exceeds a defined threshold, especially over a defined period, or a defined integral value. Reaching this threshold indicates that the vehicle's stability may be compromised due to unequal loads on the axles. In such cases, the braking force can be adjusted accordingly, and / or the target brake slip can be adjusted, and / or the vehicle can switch to individual wheel ABS control of the mechanical wheel brakes to prevent over- or under-braking of the wheels.

[0030] One advantage of this method is the ability to adaptively adjust the vehicle's braking performance to changing driving conditions. Since the switch to individual wheel ABS control only occurs when a threshold is exceeded, regular traction control is maintained via the electric motor for as long as possible. This adaptive switching contributes to the efficiency of the braking system, as the mechanical brakes are only subjected to greater stress when precise control of individual wheels is required.

[0031] It is also conceivable that vehicle stability is determined by the control unit detecting the vehicle's yaw rate, and that a deviation between the target and actual yaw rate, particularly over a defined period, is used as a criterion for switching to individual wheel ABS control. Thus, the vehicle's stability is monitored by detecting the yaw rate, i.e., the vehicle's rotational movement around its vertical axis. The yaw rate provides information about the vehicle's current rotational speed around this axis and is therefore a key measure of cornering behavior. Controlling the yaw rate is particularly important for the vehicle's stability and steerability, as it describes how quickly the vehicle rotates into or out of a curve. Preferably, the actual yaw rate is measured using a yaw rate sensor that detects this rotational speed in real time.The target yaw rate is calculated by a control unit and is based on input values ​​such as steering angle, vehicle speed, and lateral acceleration. The deviation between the target and actual yaw rate provides information about whether the vehicle is stable or if a critical driving situation exists. The system continuously compares the measured actual yaw rate with a target yaw rate that is defined for the current speed, steering angle, and dynamic requirements of the vehicle. The target yaw rate thus represents the desired or ideal value for the rotational movement at which the vehicle remains stable and driving stability is ensured.

[0032] A defined deviation, particularly over a specific period, is used as a criterion to switch to individual wheel ABS control. This time-based assessment ensures that, for example, short-term fluctuations or measurement errors do not trigger unnecessary intervention, but rather that only significant and persistent stability problems are taken into account. As soon as the threshold is reached, the ABS takes over control of the braking forces at each individual wheel to restore stability and ensure vehicle controllability. The individual wheel ABS control intervenes selectively to adjust the braking forces at each wheel in such a way that the difference between the target and actual yaw rate is minimized and driving stability is restored.

[0033] Using yaw rate as a measure of stability and comparing the target and actual yaw rate offers the advantage of allowing the system to react very quickly to dynamic driving situations. Since the yaw rate provides direct feedback on cornering, the braking system can stabilize the vehicle even before the driver perceives a loss of control. This continuous adjustment and monitoring of the yaw rate optimizes the vehicle's cornering stability and increases safety, especially at high speeds or when driving on slippery or uneven road surfaces. The targeted, wheel-specific adjustment of braking force results in balanced steering dynamics and precise slip control, making the vehicle safer and more stable.

[0034] This method of using yaw rate as a criterion for driving stability also offers adaptive adjustment of the braking system to changing driving conditions, as the vehicle can switch quickly and effectively between different braking modes. Through individual wheel control, the braking force is precisely tailored to the needs of each individual wheel.

[0035] It is also conceivable that when switching to individual wheel ABS control of the mechanical wheel brakes, the target values ​​for the recuperation torque and the mechanical braking force (i.e., the braking torque of the mechanical wheel brakes) are adjusted to ensure vehicle stability. Thus, when switching to individual wheel ABS control of the mechanical wheel brakes, the calculation of the target values ​​for the recuperation torque and the mechanical braking force is dynamically adjusted to guarantee vehicle stability. These target values ​​represent the desired values ​​for the recuperation torque generated by the electric motor and the braking force applied by the mechanical wheel brakes to achieve the desired deceleration effect.Particularly during the switch to wheel-individual ABS control, these target values ​​are continuously adjusted so that the braking system responds to the current driving conditions and the specific requirements of each individual wheel.

[0036] By adjusting the recuperation torque, the electric motor can continue to contribute a certain degree of deceleration and recuperate energy through braking, thus increasing the vehicle's energy efficiency. At the same time, the mechanical braking force is adjusted to provide the necessary braking pressure individually for each wheel, especially for wheels closer to the traction limit, which therefore require increased slip control.

[0037] Another advantage of this approach is the load distribution between the electric motor and the mechanical wheel brake, which makes the braking performance more efficient and distributes the load on the entire braking system more evenly.

[0038] Within the scope of the invention, it may be provided that the following step is included: - Rules of a, in particular constant, base braking torque of the mechanical wheel brake, depending on the applicable torque and the maximum achievable recuperation torque of the electric motor.

[0039] The base braking torque is deliberately kept below the applied torque at the traction limit to prevent wheel lock-up and simultaneously enable precise and efficient brake control. The recuperation torque of the electric motor is used to make the braking effect as energy-efficient as possible, by utilizing the electric motor's generator function for a large portion of the deceleration, thereby feeding energy back into the traction battery.

[0040] This process step allows the combined recuperation torque of the electric motor and the mechanical base braking torque to exceed the traction potential. This ensures sufficient braking force is provided to bring the wheels to their traction limit. The electric motor is controlled to handle the majority of the braking effect, enabling efficient slip control due to its high control speed and precision. The mechanical brake provides only the base braking torque to ensure basic stability and / or serve as a reserve in case of sudden changes in traction conditions. A further advantage lies in the provision of sufficient control reserve for the recuperation torque.This reserve allows the electric motor to quickly counteract abrupt changes in traction potential, such as those caused by changing road conditions, until the mechanical brakes provide the necessary braking force. Slip control is primarily achieved via the electric motor, as its high dynamics allow for precise and rapid interventions.

[0041] To further illustrate this, consider the following scenario. The driver requests a braking force greater than the available torque – the maximum torque that can be transferred to the road surface without loss of traction. This means that the physical limits of vehicle stability have been reached, and braking control must be activated to prevent exceeding the traction limit.

[0042] In this situation, the procedure is executed such that the recuperation torque of the electric motor initially handles the slip control. Due to its fast control speed and high precision, the electric motor can control the wheel speeds and thus, for example, prevent the wheels from locking up. However, in this scenario, the recuperation torque is insufficient to bring the wheels to their traction limit and fully provide the required braking force. Therefore, the base braking torque of the mechanical brake is additionally controlled to compensate for the difference between the available braking torque and the recuperation torque.

[0043] The base braking torque is precisely adjusted to provide the necessary additional braking force without exceeding the traction limit. This distribution of braking force means the mechanical brake only serves to provide the missing braking power, while the electric motor continues to handle primary slip control. This distribution has the advantage of relieving the mechanical brake, while simultaneously allowing the electric motor to precisely control traction to ensure maximum stability and safety.

[0044] With regard to the present invention, it is conceivable that a control reserve is provided in the recuperation torque of the electric motor. In this further embodiment of the method, a control reserve is provided in the recuperation torque of the electric motor, which makes it possible to dynamically adjust the braking torque as needed. This control reserve describes a defined range within the maximum recuperation potential, which is deliberately not fully utilized in order to maintain additional control capacity in situations where the braking torque must be precisely adjusted. This control reserve allows the system to react quickly to changing road or vehicle conditions and fine-tune the recuperation torque without having to immediately engage the mechanical brakes.This is particularly advantageous when the vehicle needs to remain stable on slippery or changing road surfaces, as the control reserve enables highly dynamic slip control. Slip control is primarily achieved via the electric motor, since its high dynamics allow for precise and rapid interventions.

[0045] Effective slip management, supported by the control reserve during recuperation, allows the system to decelerate the vehicle in a controlled manner, even in critical driving situations, and to maintain stability. Because the control reserve allows for a rapid response to changes in slip, excessive wheel lock-up is prevented, ensuring optimal brake force transmission.

[0046] By integrating a control reserve during recuperation, the braking system becomes more efficient and flexible overall, as it is prepared for a wide range of driving situations. This control reserve ensures that the system provides stable and responsive braking even in dynamic or demanding driving situations.

[0047] Within the scope of the invention, it may be provided that the following step is included: - Activating the ABS control via the mechanical wheel brake in combination with engine drag torque control to regulate the braking torque and prevent overbraking of the axle.

[0048] Motor drag torque control, also known as MSR (motor drag torque control), is a system that deliberately adjusts the negative torque of the motor, also called engine braking torque, in electric and hybrid vehicles that are not intended to coast or glide freely, as soon as the driver releases the accelerator pedal. This control ensures that the engine drag torque is balanced with the braking force of the mechanical wheel brakes, thus preventing unwanted wheel lock-up, especially on slippery or icy roads.

[0049] By combining mechanical wheel brakes and MSR, the braking system can precisely adapt the braking force to the dynamic requirements of the axle, which improves the stability of the vehicle while ensuring wheel traction.

[0050] The combination of these two control mechanisms supports the vehicle's handling in demanding driving situations by ensuring smooth deceleration and reducing the risk of axle lock-up. Handling, in this context, refers to the vehicle's stability and responsiveness to braking maneuvers, as well as the driver's ability to steer and control the vehicle safely.

[0051] Within the scope of the invention, it can be advantageous for the target slip value to be transmitted to the electric motor via a speed interface, and for the braking signal to be transmitted to the electric motor as the regenerative target torque. In this embodiment of the method, the target slip value is transmitted to the electric motor via a speed interface, while the braking signal is passed on to the electric motor as the regenerative target torque. This interface serves to supply the electric motor with the necessary target values ​​so that it can precisely implement the desired slip control. The target slip value describes the optimal value of the braking slip that the electric motor should maintain in order to achieve maximum traction and optimal braking force transmission. By transmitting this value via the speed interface, the electric motor can dynamically adjust the braking torque so that the desired braking slip is maintained even under changing driving conditions.

[0052] The brake signal, representing the driver's braking request, is transmitted to the electric motor as a regenerative target torque. The term "regenerative target torque" describes the target torque that the electric motor should apply to achieve the desired deceleration while simultaneously recovering energy. This transmission as a regenerative target torque enables the electric motor to implement the braking request in such a way that the largest possible proportion of the kinetic energy is converted into electrical energy and fed back into the battery. This form of control not only precisely implements the driver's request but also optimizes the vehicle's energy efficiency by maximizing electrical recuperation. The speed interface allows for precise adjustment of the rotational speeds and thus exact control of the braking torque, which stabilizes driving behavior and increases safety.

[0053] Furthermore, this technology supports maximized recuperation efficiency. Since the braking signal is transmitted as a regenerative target torque, the electric motor can integrate the braking request into a recuperation process at any time, efficiently recovering the energy that would be lost during purely mechanical braking. Maximizing recuperation efficiency leads to increased range and optimized energy use, as the recovered energy is used directly to charge the vehicle battery.

[0054] Within the scope of the invention, it is conceivable that a speed control system, particularly one located near the actuator, is activated when the target brake slip value (defined positively here) is exceeded or a minimum speed of the electric motor or at least one wheel, particularly a mixed speed of different wheels, is undershot. Consequently, a speed control system located near the actuator is activated as soon as the target slip value is exceeded or a minimum speed of the electric motor or at least one wheel is undershot. This control system ensures that the electric motor reacts to the predefined speed limits and prevents these limits from being undershot.

[0055] The target slip value refers to the optimal braking slip at which the ratio between vehicle speed and wheel circumferential speed is set to achieve maximum traction and braking force transmission. Exceeding this target slip can lead to undesirable wheel lock-up, which would compromise vehicle stability. The activated actuator-near speed control counteracts such conditions by immediately adjusting the electric motor's torque to quickly restore the target slip value and ensure control of the vehicle's movement.

[0056] According to a second aspect, the present invention further relates to a computer program product comprising instructions which, when executed by a control unit, cause the control unit to execute a method according to one of the preceding claims. This results in the same advantages with respect to a computer program product according to the invention as have already been described with respect to a method according to the invention.

[0057] According to a third aspect, the present invention further relates to a vehicle comprising a braking system for decelerating a vehicle with an electric motor and a mechanical wheel brake, and a control unit for carrying out a method according to the invention.

[0058] This results in the same advantages with regard to a vehicle according to the invention as have already been described with regard to a method according to the invention and / or a computer program product according to the invention.

[0059] Further advantages, features, and details of the invention will become apparent from the following description, in which a single embodiment of the invention is described in detail with reference to the drawing. The features mentioned in the claims and in the description can each be essential to the invention individually or in any combination. Fig. 1 schematically a vehicle according to the invention.

[0060] The Fig.Figure 1 schematically shows a possible embodiment of the vehicle 1 with a braking system 10 comprising both an electric motor 11 and mechanical wheel brakes 12. The driven axle 2 with wheels 3 and 4 is driven by the electric motor 11. The vehicle 1 is additionally equipped with a brake pedal 5 and a driver assistance system 6, each of which can generate a braking signal that is transmitted to the control unit 20. This braking system 10 is designed to enable traction control and driving stability through precise control of the braking forces and the integration of recuperation.

[0061] The process begins with the detection of a brake signal 101, which is generated either by the driver via the brake pedal 5 or automatically by the driver assistance system 6. This signal represents the desired braking action and serves as the basis for the control unit 20 to initiate the braking process. Furthermore, the maximum recuperation torque of the electric motor 11 is determined 102, which describes the maximum braking force that the electric motor 11 can generate in generator mode. This torque depends, for example, on the power electronics and the traction battery of the vehicle 1 and represents a parameter for controlling the braking force.

[0062] The control unit 20 further determines the transmissible torque of the vehicle 1, which represents the maximum braking force that can be transferred to the road surface without loss of traction. This value is influenced, for example, by the current friction conditions between the tires and the road surface, the speed of the vehicle 1, and the axle load. At the same time, the control unit 20 continuously monitors the driving stability of the vehicle 1, using two parameters in particular: the difference in wheel speeds between wheels 3 and 4 of axle 2 and / or the difference in wheel speed between the wheels of the other axle and / or the difference in wheel speed between axle 2 and the other axle and / or the yaw rate of the vehicle 1.

[0063] The difference in wheel speeds is used to detect traction differences between the wheels. The difference in axle speeds is used to detect traction differences between the axles. This is intended to determine early on whether individual wheel control of the braking forces is required. Additionally, the yaw rate of the vehicle 1 is detected by a yaw rate sensor. A deviation between the target yaw rate, calculated from inputs such as steering angle and speed, and the actual yaw rate is analyzed by the control unit 20. If this deviation exceeds a defined threshold value over a specific period, the control unit 20 interprets this as a potential loss of driving stability and can initiate the switchover 107 to individual wheel ABS control of the wheel brakes 12.

[0064] As long as driving stability can be maintained, the control unit 20 performs the slip control exclusively via the electric motor 11. By adjusting 106 the recuperation torque, the target slip value, previously determined in step 105, is precisely set. This allows efficient use of recuperation for brake force provision and reduces wear on the mechanical brakes 12. The brake signal is preferably transmitted directly to the electric motor 11 as the regenerative target torque, while the target slip value is preferably transmitted via a speed interface. Thus, the conventional ABS control of the friction brake can be deactivated, as sufficient recuperation potential is available to use the electric motor as an actuator, which will perform the slip control to prevent the wheels 3 and 4 from locking up.

[0065] If the recuperation torque of the electric motor 11 is insufficient to bring the wheels 3 and 4 to their traction limit, a base braking torque of the mechanical wheel brakes 12 is controlled 108. This base braking torque is preferably defined such that it always remains smaller than the applicable torque to prevent the wheels 3 and 4 from locking up, thereby supplementing the recuperation torque. Taking into account the applicable torque and the recuperation potential, minus an adjustable controller reserve, a constant base torque can be calculated. This results in the sum of the base torque and the required regenerative target torque being greater than the applicable torque, with the proportion of the regenerative target torque being chosen to be as large as possible, considering the controller reserve. The base torque can be continuously controlled or adjusted depending on the magnitude of the actual recuperation torque.

[0066] Furthermore, a control reserve is to be maintained during the recuperation phase of the electric motor 11, so that it can react immediately to sudden changes in traction conditions before the mechanical brakes 12 actively engage. To prevent overbraking of axle 2, a motor drag torque control can also be activated, which adjusts the braking torque of the electric motor 11.

[0067] In situations where the target slip value is exceeded or the minimum speed of the electric motor 11 or one of the wheels 3, 4 is undershot, the control unit 20 can activate speed control. This ensures that the braking forces are dynamically adjusted to maintain the stability of the vehicle 1 and prevent wheel spin or locking. Simultaneously, when switching 107 to individual wheel ABS control, the target values ​​for the recuperation torque of the electric motor 11 and the mechanical braking torques at the wheels are precisely adjusted to guarantee the stability of the vehicle 1 in every driving situation.

[0068] Only if the recuperation potential – be it due to high thermal load, a limited adjustment range of the electric motor 11, or a higher available torque than the electric motor 11 can inherently provide – is so significantly reduced (e.g., lower than the control reserve) that insufficient adjustment range is available for slip control by the electric motor 11, is the brake slip control performed by the friction brake 12. In these cases, a constant regenerative torque could be superimposed on the friction brake torque, which is limited by a control system.

[0069] This control method combines the advantages of recuperation with the safety and stability provided by mechanical brakes. It ensures that vehicle 1 always operates within the physical limits of traction while simultaneously maximizing braking efficiency. Reference symbol list 1 vehicle 2-axis 3 right wheel 4 left wheel 5 Brake 6 Driver assistance systems 10 Braking system 11 Electric motor 12 Wheel brake 20 Control unit QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2020 007 248 A1

[0004]

Claims

Method (100) for controlling a braking system (10) of a vehicle (1) with at least one electrically driven axle (2), wherein the braking system (10) comprises at least one electric motor (11) and at least one mechanical wheel brake (12), wherein the method (100) comprises the following steps: - Receiving (101) a braking signal to initiate a braking process to decelerate the vehicle (1) by a control unit (20), - Determining (102) a recuperation torque achievable by the electric motor (11) by the control unit (20), - Determining (103) a torque that can be transferred from the vehicle (1) to a road surface by the control unit (20) to ensure the vehicle's (1) stability.- The control unit (20) compares (104) the desired braking torque based on the brake signal with the maximum available torque and the maximum achievable recuperation torque of the electric motor (11) and the vehicle's driving stability, - The control unit (20) determines (105) a target slip value, - The control unit (20) adjusts (106) the recuperation torque generated by the electric motor (11) to set the target slip value. Method (100) according to claim 1 , characterized in that the driving stability of the vehicle (1) is continuously monitored, and the electric motor (11) takes over the slip control by adjusting the recuperation torque as long as the driving stability is maintained. Method (100) according to claim 1 or 2, characterized in that the driving stability is determined by detecting a difference in wheel speeds between the left wheel (4) and the right wheel (3) of the electrically driven axle (2), in particular to determine whether individual wheel ABS control is necessary. Method (100) according to claim 3, characterized in that a switch (107) to the wheel-individual ABS control of the mechanical wheel brake (12) is carried out by the control unit (20) when the difference in wheel speeds exceeds a predetermined threshold value, in particular over a defined period of time or a defined integral value. Method (100) according to one of the preceding claims, characterized in that the driving stability is determined by detecting the yaw rate of the vehicle (1), and a deviation between target and actual yaw rate, in particular over a defined period of time, is used as a criterion for switching (107) to wheel-individual ABS control by the control unit (20). Method (100) according to one of the preceding claims, characterized in that when switching (107) to the wheel-individual ABS control of the mechanical wheel brake (12) by the control unit (20), the setpoint values ​​for the recuperation torque and a mechanical braking torque are adjusted to ensure the driving stability of the vehicle (1). Method (100) according to one of the preceding claims, characterized by the further step: - Controlling (108) a, in particular constant, base braking torque of the mechanical wheel brake (12) by the control unit (20), depending on the deployable torque and the maximum achievable recuperation torque of the electric motor (11). Method (100) according to one of the preceding claims, characterized in that a control reserve is provided in the recuperation moment of the electric motor (11). Method (100) according to one of the preceding claims, characterized in that a motor drag torque control is activated to control the braking torque and to prevent overbraking of the axle (2). Method (100) according to one of the preceding claims, characterized in that the target slip value is transmitted to the electric motor (11) via a speed interface, and the brake signal is transmitted to the electric motor (11) as a regenerative target torque. Method (100) according to one of the preceding claims, characterized in that a speed control is activated when the target slip value is exceeded, a minimum speed of the electric motor (11) or at least of a wheel (3, 4) is undershot. Computer program product comprising instructions which, when executed by a control unit (20), cause the control unit (20) to execute a method (100) according to one of the preceding claims. Vehicle (1) comprising a braking system (10) for decelerating a vehicle (1) with an electric motor (11) and a mechanical wheel brake (12), and a control unit (20) for performing a method (100) according to one of claims 1 to 11.

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