METHOD FOR PREVENTING MOTOR VEHICLE OVERLOADS USING TORQUE VECTORING, CORRESPONDENT STEERING SYSTEM

Description

[0001] The present invention relates to a method for preventing rollovers of a motor vehicle during transverse load changes, having the features of the preamble of claim 1, and to a steer-by-wire steering system of a motor vehicle, having the features of the preamble of claim 8.

[0002] In steer-by-wire steering systems, the position of the steered wheels is not directly linked to the steering input device, such as a steering wheel. There is a connection between the steering wheel and the steered wheels via electrical signals. The driver's steering input is detected by a steering angle sensor, and the position of the steered wheels is controlled via a steering actuator depending on the driver's steering input.

[0003] To avoid unstable driving situations such as oversteer or understeer, a process known as "torque vectoring" is used. This technique involves distributing drive torque between a left and right vehicle wheel using a specially designed differential gear to counteract understeer or oversteer. In an unstable driving situation, one wheel is braked, while the drive power is directed to the wheel with the greatest reduction in yaw moment. In other words, if understeer occurs, the control system applies more drive power to the outside wheel via a separate clutch, so that the vehicle is "pushed" into the curve. A sharp change in lateral load can result in the inside wheel being completely unloaded and the outside wheel being completely loaded. The loaded wheel can transmit a lateral force sufficient to tip the vehicle over.

[0004] A rollover stabilization system (ARS) is known from the prior art. In critical situations or when there is a risk of rollover, the outside front wheel is braked, thereby reducing the lateral force on this wheel. At the same time, the braking of this wheel creates a yaw moment about the vehicle's vertical axis, which counteracts the vehicle's steering. By reducing the lateral force and the creation of the yaw moment, lateral acceleration is reduced, thereby lowering the risk of rollover. A disadvantage of this solution is that the vehicle understeers and slides out of the curve over the front wheel, and the braking also reduces the vehicle speed. Such systems are known, for example, from DE 10 2004 046 985 A1, DE 10 2010 017 647 A1, or WO 2006 / 026259 A1.

[0005] From DE 10 2009 045 234 A1, a method for operating a drive device of a vehicle is also known, which has at least two wheel hub motors that communicate with each other by means of a master-slave communication.

[0006] Finally, DE 10 2005 046 776 A1 discloses a roll stability control system using a four-wheel drive, in which active differential gears are used alone or in addition to braking in response to a potential rollover condition to prevent the vehicle from rolling over.

[0007] It is an object of the present invention to provide an improved method and a steer-by-wire steering system for preventing rollovers of a motor vehicle during strong lateral load changes.

[0008] This object is achieved by a method for preventing rollovers of a motor vehicle during transverse load changes having the features of claim 1 and a steer-by-wire steering system of a motor vehicle having the features of claim 8.

[0009] Accordingly, a method is provided for preventing rollovers of a motor vehicle during a transverse load change, wherein the motor vehicle has an independent wheel drive which is designed to drive a wheel loaded by the transverse load change independently of at least one other wheel of the motor vehicle, wherein the following method steps are provided: Detecting a critical condition of the motor vehicle during a lateral load change, applying a drive torque by an individual wheel drive to the wheel of the motor vehicle loaded by the lateral load change in such a way that the wheel loaded by the lateral load change is caused to slip, and steering the wheel of the motor vehicle loaded by the lateral load change in the direction of travel in such a way that a rollover of the motor vehicle can be prevented.

[0010] This process causes the motor vehicle to slide, but it does not tip over. The drive torque is preferably the maximum torque of a drive motor. It is preferable if the drive torque drives the wheel loaded by the lateral load change in the direction of travel.

[0011] In a preferred embodiment, the application of the drive torque and the steering of the wheel loaded by the transverse load change take place for a limited time interval which is at least as long as a critical condition lasts.

[0012] In particular, the limited time interval is in a range between 0.1s and 0.3s.

[0013] In a preferred embodiment, the application of the drive torque and the steering of the wheel loaded by the transverse load change take place in an automatic steering state of the motor vehicle steering system.

[0014] When cornering, the wheel loaded by the lateral load change is the outer wheel.

[0015] Furthermore, a steer-by-wire steering system of a motor vehicle with a steerable front wheel axle having two steerable wheels is provided, wherein the front wheel axle has an individual wheel drive which drives wheel drives assigned to the steerable wheels individually by means of a drive control, and wherein an electric steering actuator is provided which controls the position of the steerable wheels, wherein the drive control and the steering actuator are designed to carry out the method described above.

[0016] A preferred embodiment of the invention is explained in more detail below with reference to the drawings. Similar or similarly functioning components are designated by the same reference numerals in the figures. They show: Fig. 1: a schematic representation of a steer-by-wire steering system with two separate wheel drives on the front axle, Fig. 2: a schematic representation of a steer-by-wire steering system in plan view, Fig. 3: a diagram of the temporal course of torque and wheel steering angle for controlling the outer wheel in the curve, Fig. 4: a schematic representation of the forces acting on the motor vehicle, and Fig. 5: a diagram of the course of the tire lateral force as a function of the drive torque.

[0017] In the Figure 1A steer-by-wire steering system 1 is shown. A steering angle sensor (not shown) is attached to a steering shaft 2. This sensor detects the driver's steering angle applied by turning a steering input means 3, which in the example is designed as a steering wheel. However, a steering torque can also be detected. Furthermore, a feedback actuator 4 is attached to the steering shaft 2. This feedback actuator 4 serves to simulate the reactions from the road surface on the steering wheel 3 and thus provides the driver with feedback about the steering and handling behavior of the vehicle. The driver's steering input is transmitted via the angle of rotation of the steering shaft 2 measured by the angle of rotation sensor via signal lines to a steering control unit 5. The steering control unit 5, depending on further input variables, controls an electric steering actuator 6, which controls the position of the steered wheels 70, 71.The steering actuator 6 acts indirectly on the steered wheels 70, 71 via a steering rack and pinion steering gear 8, such as a rack-and-pinion steering gear, as well as tie rods 9 and other components. Drive motors 10 are assigned to the steerable wheels 70, 71, which drive the wheels 70, 71 separately in the form of an individual wheel drive. A drive controller 11 determines the drive torques for the steerable wheels 70, 71 based on the angle of rotation of the steering shaft 2 measured by the angle of rotation sensor and other signals and controls the respective drive motor 10 accordingly.

[0018] In Figure 2The motor vehicle is shown schematically with the two axles, wherein the drive of the steerable wheels 70, 71 is arranged on the front axle 12. The front axle 12 comprises, with respect to a direction of travel, a first steerable wheel 71 and a second steerable wheel 70, which are connected to one another via the rack 13 of the rack and pinion steering gear. When the rack 13 is displaced transversely to the direction of travel x to the right or left, the wheels are pivoted about a respective pivot point 140, 141. A first drive motor 10 is arranged on the left in the direction of travel and a second drive motor 10 is arranged on the right in the direction of travel. The wheel drive motors 10 are each connected to the steerable wheels 70, 71 via drive shafts. The wheel drive motors 10 are preferably electric motors. The drive control 11 controls the first drive motor 10 via a first signal line S1 and the second drive motor 10 via a second signal line S2.The drive control 11 also receives information about the status of the rear axle via a signal line S3. The vehicle is moving in direction x at a speed v.

[0019] A critical load change that could lead to a rollover of the vehicle is detected based on the measured lateral acceleration of the vehicle and the known parameters of the vehicle's mass, track width, and center of gravity. Upon detection of the critical condition, the system switches from a manual steering condition to an automatic steering condition. In this case, "manual steering condition" means that the driver causes the wheels to turn by operating the steering wheel. Assistance systems can influence the steering of the wheels. In the automatic steering condition, however, the steer-by-wire steering system takes over steering the wheels independently of the steering input. The automatic steering condition is maintained for a limited time interval Δt, namely for as long as the critical condition lasts, preferably in a range between 0.1 s and 0.3 s.

[0020] During the automatic steering state, a torque T drive and a wheel steering angle ϕ LW are applied to the loaded outer wheel. Figure 3shows the temporal progression of the torque T drive and the wheel steering angle ϕ LW . The drive torque T drive rotates the loaded wheel in the direction of travel x. Preferably, the applied drive torque T drive is the maximum torque available from the drive motor 10, which can briefly exceed the continuous torque by far. This causes the loaded wheel to slip. The term "slip" refers to the condition when, when the vehicle wheel accelerates, the surface speed of the wheel differs from the vehicle speed v. The drive slip limits the lateral force that can be transferred from the wheel to the road. The vehicle begins to slide. All wheels are in contact with the road surface. This prevents the vehicle from tipping over.During the drive torque peak, the driven wheel is steered slightly back toward the direction of travel to prevent the lateral forces acting on the vehicle from increasing further. This is because a smaller wheel steering angle ϕ LW generates less lateral acceleration and thus also less lateral force FQ . The wheel is steered back only far enough to prevent the vehicle from tipping over, i.e., the direction of travel is not necessarily achieved when steering back.

[0021] During the intervention period Δt, the vehicle should be moved away from a state in which the vehicle is in danger of tipping over.

[0022] The following relationship applies, as in the Figure 4 shown: FQ *h=F m *b / 2, where according to the force equation: In transverse direction: FQ =FR

[0023] In the vertical direction: FN = F m = m*g, where m is the vehicle mass, g is the acceleration due to gravity, b is the vehicle width from the vehicle center to the wheel center, and h is the vehicle height from the road surface to the vehicle center. In this state, the vehicle does not yet tip over. However, it is in the tipping limit state. Therefore, the tire lateral force FR between the vehicle wheels and the road must be reduced. The connection to this is that drive slip reduces the transferable tire lateral force FQ. As a result, the spinning vehicle wheels on the loaded side transfer less lateral force, so the vehicle slides laterally along a larger curve radius.

[0024] To reduce the tire lateral force FR, the drive torque T drive must be increased. The relationship between tire force and drive torque is shown in Figure 5 shown.

[0025] To increase the curve radius, the vehicle lateral force FQ must be reduced. F Q = m * a y , where the lateral acceleration ay = v 2 l × φlw 1 + v vch 2 <mprescripts / > <none / > ∧ where l = axle distance between a wheel center of a front wheel and a wheel center of a rear wheel center along the same side of the vehicle; m is the vehicle mass, v is the vehicle speed, and v ch is the characteristic speed.

[0026] To reduce the lateral tire force, the wheel steering angle ϕ LW must be reduced because steering the tires into a straight position increases the curve radius.

[0027] Due to the combination of torque T drive and wheel steering angle ϕ LW, the vehicle skids but does not tip over, enabling a faster transition to the manual mode. After the limited time interval Δt, the driver resumes steering, and the torque T drive and wheel steering angle ϕ LW are reduced or no longer applied. If it is detected that the critical condition no longer exists, the system can switch from the automatic to the manual mode even before the intervention time Δt has elapsed.

Claims

1. A method for preventing roll-over of a motor vehicle in the event of a transverse load change, wherein a critical state of the motor vehicle in the event of a transverse load change is identified, characterized in that the motor vehicle has an individual-wheel drive which is designed to drive a wheel (70, 71) that is loaded by the transverse load change independently of the at least one other wheel (70, 71) of the motor vehicle, and in that the following method steps are provided: • Applying a drive torque (TDrive) by an individual-wheel drive (150, 151) to the motor vehicle wheel (70, 71) that is loaded by the transverse load change in such a manner that the wheel (70, 71) that is loaded by the transverse load change is caused to slip, such that the loaded wheel (70, 71) spins, wherein the drive slip reduces the transmissible transverse tyre force (FQ) and the motor vehicle slips in the transverse direction along a greater bend radius, and • Steering the motor vehicle wheel (70, 71) that is loaded by the transverse load change in the direction of a direction of travel (x) in such a manner that a roll-over of the motor vehicle can be prevented.

2. The method as claimed in claim 1, characterized in that the drive torque (TDrive) is the maximum torque of a drive motor (10).

3. The method as claimed in claim 1 or 2, characterized in that the drive torque (TDrive) drives the wheel (70, 71) that is loaded by the transverse load change in the direction of travel (x).

4. The method as claimed in one of the preceding claims, characterized in that the application of the drive torque (TDrive) and the steering of the wheel that is loaded by the transverse load change take place for a limited period of time (Δt) which is at least as long as a critical state lasts.

5. The method as claimed in claim 4, characterized in that the limited period of time (Δt) lies within a range of between 0.1 s and 0.3 s.

6. The method as claimed in one of the preceding claims, characterized in that the application of the drive torque (TDrive) and the steering of the wheel (70, 71) that is loaded by the transverse load change take place in an automatic steering state of the motor vehicle steering system.

7. The method as claimed in one of the preceding claims, characterized in that the wheel (70, 71) that is loaded by the transverse load change is the wheel on the outside of the bend.

8. A steer-by-wire steering system of a motor vehicle having a steerable front wheel axle (12) having two steerable wheels (70, 71), wherein the front wheel axle (12) has an individual-wheel drive which uses a drive controller (11) to individually drive wheel drives (150, 151) assigned to the steerable wheels (70, 71), and wherein an electric steering actuator (6) is provided which controls the position of the steerable wheels (70, 71), characterized in that the drive controller (11) and the steering actuator (6) are designed as to carry out the method as claimed in one of claims 1 to 7.