Method, control unit and device for stabilizing a vehicle in extreme driving dynamics situations

Abstract

Method for stabilizing a vehicle (1a-1c) in extreme driving dynamics situations, in particular during understeer in a curve, wherein several wheels (4-7) are automatically braked to reduce the driving speed (v) of the vehicle (1a-1c), wherein a higher braking torque (M) is applied to the rear wheels (6,7). h ) is exercised as on the front wheels (4, 5), - where an indicator size (k U ) is calculated, which indicates a tendency to understeer, - wherein, if the indicator value exceeds a predetermined threshold value, which depends on the lateral acceleration of the vehicle, target braking torques for the rear wheels are output to the corresponding actuators after the trigger threshold has been exceeded and are regulated by means of a slip control system, wherein the target braking torques result as a function of a target deceleration, wherein the target deceleration is calculated as a function of the understeer tendency and an accelerator pedal position, - where an algorithm monitors, based on wheel slip, whether the specified target braking torques can actually be implemented and - if the wheel slip (λ) on a rear wheel exceeds a predetermined threshold, the unusable braking torque is applied to the corresponding front wheel, whereby no braking torque (M) is applied to the outer front wheel (5) in the curve. b ) is exercised.

Description

[0001] The invention relates to a method for stabilizing a vehicle in extreme driving dynamics situations, in particular during understeer while cornering, according to the preamble of claim 1, and to a corresponding device according to the preamble of claim 7.

[0002] Vehicle dynamics control systems, such as ESP or DSC, serve to improve the controllability of motor vehicles in critical driving situations, e.g., during oversteer or understeer in corners, and to stabilize the vehicle. Common systems comprise sensors that record measurements of the vehicle's current driving state, a control unit with a control algorithm for controlling the sideslip angle and / or yaw rate, and at least one actuator to influence the vehicle's handling. Based on driver input, particularly the steering wheel position and the accelerator or brake pedal position, various target values ​​are calculated and compared with the actual values. If the deviation from the target values ​​exceeds a predefined threshold, the vehicle dynamics control intervenes and generates a compensating yaw moment that counteracts the vehicle's yaw motion and aligns the vehicle according to the target values.For this purpose, known vehicle dynamics control systems typically use the vehicle brakes and / or the engine control as actuators or act on the steering via a steering actuator.

[0003] Today's vehicle dynamics control systems are mostly designed to brake the inside rear wheel when the vehicle understeers. This generates a yaw moment towards the inside of the curve, which turns the vehicle more sharply. The vehicle can thus follow tighter curve radii. However, at high speeds with correspondingly high lateral accelerations, the problem arises that the inside wheels are significantly unloaded and can therefore only absorb minimal longitudinal forces. This makes it impossible to implement effective braking intervention at the inside rear wheel.

[0004] To improve vehicle stability, an additional control function (Bosch designation: EUC) was introduced, which triggers an automatic braking process with the aim of decelerating the vehicle overall and reducing lateral forces (lateral acceleration). As part of this function, the brake pressure at the wheel brakes of all four wheels is increased equally, thus slowing the vehicle. However, braking the wheels initially leads to a decrease in lateral grip. This results in a greater tendency for the front wheels to lean and consequently more pronounced understeer. Only once the vehicle speed has decreased sufficiently can the vehicle once again respond to the steering input.

[0005] German patent application DE 197 22 716 A1 discloses a method for stabilizing a vehicle in which automatic braking intervention is used to influence yaw behavior. In this method, if understeer is detected, a braking force is applied to one of the rear wheels to generate a yaw moment, while if oversteer is detected, a braking force is applied to one of the front wheels to suppress a yaw moment.

[0006] German patent application DE 196 24 198 A1 discloses a method for stabilizing a vehicle in which a single wheel is selected for braking based on a yaw rate difference in order to correct the vehicle's behavior. If understeer is detected, the rear inside wheel is braked, while if oversteer is detected, the front outside wheel is braked to adjust the actual yaw rate to a target yaw rate.

[0007] German patent application DE 199 62 549 C1 discloses a method for stabilizing a vehicle traveling at excessive speed while cornering, whereby an automatic braking process is initiated to reduce the vehicle's speed. For this purpose, a target longitudinal acceleration is determined such that the maximum lateral distance of the vehicle to the center of the lane is minimized, and this target longitudinal acceleration is then regulated without, however, providing for a specific distribution of braking torques between the rear and front wheels or a torque transfer from the rear to the front axle upon reaching a slip limit.

[0008] It is therefore the object of the present invention to provide a stabilization method and a corresponding device with which the vehicle speed can be reduced while maintaining a maximum possible lateral force.

[0009] This problem is solved according to the invention by the features specified in claim 1 and claim 6. Further embodiments of the invention are the subject of dependent claims.

[0010] A key aspect of the invention is to avoid braking, or only minimally brake, the front wheels, which are critical for cornering behavior, in an understeering vehicle, while braking the two rear wheels much more strongly. According to a preferred embodiment, the vehicle is braked only at the two rear wheels; the front wheels, on the other hand, remain unbraked or are significantly underbraked. This has the significant advantage that the lateral grip at the front wheels is maintained, allowing the vehicle to negotiate the smallest possible turning radius.

[0011] According to the invention, the strength of the braking intervention depends on the understeer tendency of the vehicle. This can be determined, for example, from a measured yaw rate and a calculated yaw rate.

[0012] Either the same or different braking torque can be applied to the rear wheels. Preferably, a greater braking torque is applied to the inside rear wheel than to the outside rear wheel in order to generate a counter-yaw moment towards the inside of the curve in the case of an understeering vehicle.

[0013] With the front wheels, a greater braking torque can be applied to the inside front wheel than to the outside front wheel in a turn, in order to avoid reducing the lateral grip potential of the outside wheel. The outside front wheel has the greatest lateral grip potential and is therefore particularly critical when cornering. According to the invention, it is not braked at all during the entire control process.

[0014] To detect understeering behavior, the control algorithm according to the invention calculates an indicator value that shows the understeer tendency. This indicator value is preferably a function of the yaw rate, such as a quotient of the yaw rate desired by the driver and a measured yaw rate. The yaw rate desired by the driver results from various parameters, in particular the vehicle geometry, the speed, and the steering angle, and is preferably calculated in a control unit. The measured yaw rate can, for example, be measured using a yaw rate sensor.

[0015] The braking function according to the invention is triggered when the indicator value exceeds a predetermined threshold. According to the invention, this threshold depends on the lateral acceleration of the vehicle.

[0016] According to the invention, once the trigger threshold is exceeded, target torques for the rear wheels are output to the corresponding actuators and regulated using a slip control system. The algorithm monitors, based on wheel slip, whether the specified target braking torques can actually be applied. If the wheel slip λ at a rear wheel exceeds a predetermined threshold, the unapplicable braking torque is applied to the corresponding front wheel. According to the invention, only the inside front wheel is braked.

[0017] A vehicle dynamics control system according to the invention comprises sensors for detecting various driving condition variables, a control unit that processes the sensor signals and, upon exceeding a trigger threshold, initiates automatic braking of the vehicle by means of the wheel brakes or controls the wheel brakes in such a way that they initiate automatic braking of the vehicle. The control algorithm stored in the control unit is designed such that, in the event of understeer of the vehicle, a higher braking torque is exerted on the rear wheels (either individually or collectively) than on the front wheels, whereby, according to the invention, the outer front wheel is not braked.

[0018] The invention is explained in more detail below with reference to the accompanying drawings. These show: Fig. 1. The cornering behavior of a vehicle with different vehicle dynamics control systems; Fig. 2. the course of different yaw rates during cornering with an understeering vehicle; Fig. 3 the essential procedural steps of a method for braking a vehicle while cornering; and Fig. 4 a block diagram of a vehicle dynamics control system with extended braking function.

[0019] Fig. Figure 1 shows several vehicles (1a-1c) equipped with different vehicle dynamics controllers in an understeer situation. The vehicle dynamics controllers use different control strategies to keep the vehicles in their lane (3).

[0020] The vehicle's dynamic control system (1a) on the far right brakes the inside rear wheel in a known manner to generate a counter-yaw moment towards the inside of the curve. The longitudinal force acting on the left rear wheel (6) is represented by an arrow F.

[0021] The vehicle dynamics control system of the middle vehicle 1b additionally utilizes a known function (EUC) that increases the wheel brake pressure equally at all four wheels 4-7. The braking forces acting on wheels 4-7 are represented by arrows F1, F2, and F3. The braking force F3 at wheels 4 and 5 is symmetrical and, due to the generally higher braking coefficient (CP), is higher than at the rear axle. The braking force at the inside rear wheel 6 is higher than at the outside rear wheel 7 in order to generate a counter-yawing moment, as mentioned above.

[0022] The left vehicle 1c comprises a vehicle dynamics controller according to the invention, which, in an understeer situation, primarily brakes the wheels 6, 7 of the rear axle. The front wheels 4, 5, which are most important for the vehicle's cornering behavior and must absorb the highest possible lateral forces, are only braked slightly or not at all. This preserves the lateral grip potential of the front wheels 4, 5, while still braking the vehicle 1c so that it can follow the smallest possible turning radius.

[0023] In the illustrated embodiment, all four wheels 4-7 are braked to varying degrees. The inner rear wheel 6 is braked most strongly, followed by the outer rear wheel 7. The two front wheels 4, 5 are braked only very weakly, with the outer front wheel 5 not being braked at all according to the invention. With this brake distribution, the left vehicle 1c can follow a significantly smaller curve radius than the other two vehicles 1a, 1b with known vehicle dynamics controllers.

[0024] Fig. Figure 2 shows the progression of different yaw rates during a cornering maneuver in which vehicle 1c understeers. Curve 8 shows the progression of a target yaw rate, representing the driver's intention. As can be seen, the driver increasingly turns the steering wheel until reaching maximum steering lock. Curve 9 shows the maximum achievable yaw rate under the given driving conditions (vehicle, tires, road surface, speed, etc.). This line should be achieved with the aid of optimal vehicle dynamics control. Curve 10 shows an actual yaw rate measured during cornering. As can be seen, the measured yaw rate is sometimes significantly lower than the maximum possible yaw rate 9. In this range, vehicle 1 drifts more than necessary towards the outside of the curve.

[0025] Fig. Figure 3 shows the essential procedural steps of a method for braking a vehicle in an understeer situation. In step 15, the algorithm first calculates an understeer indicator k. U which indicates an understeer tendency of the vehicle 1c. For the understeer indicator k U In simplified terms: kU=ψsetψmeasure ψsoll=f(vehicle geometry, Lw, vFzg)

[0026] In this case, Ψ soll a target yaw rate, which is derived from the vehicle geometry, the vehicle speed v Fzg and the steering wheel angle Lw, representing the driver's intent. ψ mess is a measured yaw rate.

[0027] If this quotient exceeds a predetermined lateral acceleration-dependent threshold SW (step 16), the braking function according to the invention is automatically activated. In step 17, a target deceleration a is first set. xsoll calculated by the understeer indicator kU and depends on the accelerator pedal position. Therefore: axsoll=f(kU, accelerator pedal position)

[0028] This results in wheel braking torques of 4.5 for the rear wheels and, if applicable, 6.7 for the front wheels. For the rear wheels, the following applies initially, until the slip limit is reached on one wheel: Mh=mFzg⋅axsoll⋅rRad2

[0029] The torques calculated in step 18 are output as target torques for the rear wheels 6 and 7 and regulated using a slip control system. Based on the wheel slip λ in step 19, the algorithm monitors whether the specified target braking torques M are being achieved. hcan also be implemented. If the wheel slip λ exceeds a predefined threshold SW (J), the unusable braking torque is applied to the corresponding front wheel. Preferably, only the inside front wheel 4 (step 20) is braked. Otherwise (N), the function ends. Basically, the individual wheel torques are distributed so that there is either no yaw moment overall or a yaw moment in the direction of the inside of the curve.

[0030] Fig. Figure 4 shows a system for performing an automatic braking maneuver in an understeer situation. The system essentially comprises a control unit 23 in which the braking function is stored as an algorithm, a sensor system 24 for recording various driving condition variables, and the actuators 29 of the vehicle braking system, which are controlled by the control unit 23.

[0031] The algorithm stored in control unit 23 includes a unit 25 for calculating the understeer indicator k. U , a unit 26 for calculating a target deceleration and a unit 27 which calculates the wheel braking torques M depending on the target deceleration FL ,M FR ,M RL ,M RR The braking torques are calculated for each individual wheel. They are output to the individual brake actuators 29 via an interface 28.

[0032] The device works, for example, as described above, using the following: Fig. 3 was explained.

Claims

[1] Method for stabilizing a vehicle (1a-1c) in extreme driving dynamics situations, in particular during understeer in a curve, wherein several wheels (4-7) are automatically braked to reduce the driving speed (v) of the vehicle (1a-1c), wherein a higher braking torque (M) is applied to the rear wheels (6,7). h ) is exercised as on the front wheels (4, 5), - where an indicator size (k U ) is calculated, which indicates a tendency to understeer, - wherein, if the indicator value exceeds a predetermined threshold value, which depends on the lateral acceleration of the vehicle, target braking torques for the rear wheels are output to the corresponding actuators after the trigger threshold has been exceeded and are regulated by means of a slip control system, wherein the target braking torques result as a function of a target deceleration, wherein the target deceleration is calculated as a function of the understeer tendency and an accelerator pedal position, - where an algorithm monitors, based on wheel slip, whether the specified target braking torques can actually be implemented and - if the wheel slip (λ) on a rear wheel exceeds a predetermined threshold, the unusable braking torque is applied to the corresponding front wheel, whereby no braking torque (M) is applied to the outer front wheel (5) in the curve. b ) is exercised. [2] Method according to claim 1, characterized by, that a braking torque (M) is applied to both rear wheels (4, 5). h ) is exercised. [3] Method according to any one of the preceding claims, characterized by , that a greater braking torque (M) is present at the inside front wheel (4) when cornering. h ) is exerted on the outer front wheel (5). [4] Method according to claim 1, characterized by , that the indicator size is a function of the yaw rate (ψ) desired by the driver d ) and a measured yaw rate (ψ m ) is. [5] Control unit, wherein the control unit is configured to perform all steps of the method according to any one of claims 1 to 4. [6] Device for stabilizing a vehicle (1a-1c) in extreme driving dynamic situations, in particular during understeer in a curve, comprising a sensor system (24) for detecting various driving condition variables (ψ, ay, v) and a control unit (23) according to the preceding claim.

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