Method for controlling a motor vehicle at slow speeds by means of a drive differential torque on the rear axle

Abstract

A method can be used to control a steer-by-wire steering system for a motor vehicle that has two axles each with two wheels. Two front wheels can be steered by front-wheel steering and two rear wheels can be steered by rear-wheel steering. The motor vehicle includes a single wheel drive that is assigned to one of the two axles and drives the two wheels of the corresponding axle via a differential. The motor vehicle comprises an inboard braking system. The method involves checking the motor vehicle speed and activating rear-axle steering when a motor vehicle speed should be slower than 40 km/hr. With rear-axle steering active, the following steps are performed: deactivating front-wheel steering and rear-wheel steering, determining a reference position of a first steering rod via a reference wheel steering angle, determining a differential drive torque between the rear wheels to reach the reference position via a control unit.

Claims

1. A method for controlling a steer-by-wire steering system for a motor vehicle, wherein the motor vehicle comprises two axles with two wheels each, wherein two front wheels are steerable by way of front-wheel steering and are connected to one another via a second steering rod of a steering system of the front-wheel steering, wherein two rear wheels are steerable by way of rear-wheel steering and are connected to one another via a first steering rod of a steering system of the rear-wheel steering, the motor vehicle comprising a single wheel drive that is assigned to one of the two axles and that drives the two wheels of the corresponding axle via a differential, wherein the motor vehicle comprises an inboard braking system, the method comprising: checking the motor vehicle speed; activating rear-axle steering when the motor vehicle speed should be less than a defined motor vehicle speed; and performing the following steps with the rear-axle steering active: deactivating rear-wheel steering, determining a reference position of the first steering rod by way of a reference wheel steering angle of the rear wheels, and determining a differential drive torque between the two rear wheels to reach the reference position by way of a control unit.

2. The method of claim 1 wherein the single wheel drive is disposed on a front axle of the two axles, the method comprising: generating the differential drive torque between the two rear wheels by braking one of the rear wheels; and increasing a torque provided by the single wheel drive to compensate for a loss of speed by the motor vehicle caused by the braking of the one of the rear wheels.

3. The method of claim 1 wherein the single wheel drive is a rear-wheel drive with a single actuator and an open differential, the method comprising: generating the differential drive torque between the two rear wheels by braking one of the rear wheels; and increasing a torque provided by the single wheel drive to compensate for a loss of speed by the motor vehicle caused by the braking of the one of the rear wheels.

4. The method of claim 3 comprising applying the following through a left-hand bend: T.sub.RL±T.sub.RR=2*T.sub.RR±T.sub.ped,br, wherein T.sub.RL and T.sub.RR are torques of the left and right rear wheels and T.sub.ped,br is a braking torque introduced to the left rear wheel, which is the one of the rear wheels that is being braked.

5. The method of claim 1 wherein the single wheel drive is a rear-wheel drive with two actuators, wherein each actuator drives a rear wheel, the method comprising generating the differential drive torque between the two rear wheels by introducing different torques by the two actuators.

6. The method of claim 5 comprising applying the following through a left-hand bend: a tractive force for the left rear wheel is xN−ΔT/2 and a tractive force for the right rear wheel is xN+ΔT/2.

7. The method of claim 1 comprising locking a steering gear of the rear-axle steering in a straight-line position when the motor vehicle speed is greater than the defined motor vehicle speed.

8. The method of claim 1 comprising determining the reference wheel steering angle with a steering torque that is introduced into a steering means by a driver.

9. The method of claim 1 comprising specifying the reference wheel steering angle by an autonomous or semi-autonomous driving mode.

10. A motor vehicle that is capable of and configured to perform the method of claim 1.

Description

(1) Preferred embodiments of the invention are explained in greater detail below with the help of the drawing. Identical, or functionally identical, components in this case are provided with the same reference signs shared between all the figures. In the drawings:

(2) FIG. 1: shows a schematic representation of a motor vehicle with rear wheel drive-based torque vectoring,

(3) FIG. 2: shows a schematic depiction of a motor vehicle with front-wheel drive comprising a drive motor and rear-wheel brake-based torque vectoring,

(4) FIG. 3: shows a schematic representation of a motor vehicle with rear-wheel drive comprising a drive motor and rear wheel-based torque vectoring, and

(5) FIG. 4: shows a block diagram of a control system of the motor vehicle with rear axle-based torque vectoring.

(6) In FIG. 1 a motor vehicle 1 with two axles 10,20 and four wheels FL,FR,RL,RR is schematically depicted, wherein only the rear wheels RL,RR are drivable (rear-wheel drive) and the drive 2 of the rear wheels RL,RR is arranged on a rear axle 20. The drive 2 has two separate actuators 21,22. A left wheel drive motor 202 is arranged on the left in the direction of travel and a right wheel drive motor 201 is arranged on the right in the direction of travel. The wheel drive motors 201,202 are each connected to the steerable rear wheels RL,RR via drive shafts 5. The steerable rear wheels RL,RR are connected to one another via a rack gear 3 of a rack and pinion steering gear 4. If the rack gear 3 is displaced transversely to the direction of travel to the right or left, the wheels RL,RR are pivoted about a pivot point in each case. The front wheels FL,FR are likewise steerable and connected to one another via a second rack gear 30 of a second rack and pinion steering gear 40.

(7) The actuators 21,22 are controlled by means of a reference wheel steering angle α.sub.RW,ref of the steerable rear wheels RL,RR. The rear wheels RL,RR are driven in such a manner that a differential torque AT occurs between the rear wheels FL,FR, which is proportionate to the reference wheel steering angle α.sub.RW,ref or else a driver's steering intention. In other words, the reference wheel steering angle α.sub.RW,ref f the steerable rear wheels FL,FR is incorporated in the control of the rear-wheel drive. When driving round a left-hand bend, as shown in FIG. 1, the tractive force for the left rear wheel is xN (e.g. 100 N)−ΔT/2 and the tractive force for the right rear wheel is xN+ΔT/2. The differential torque leads to a displacement of the rack gear 3 on the rear axle 20 and to a yawing moment ψ about the vertical axis of the motor vehicle.

(8) FIG. 2 shows a further exemplary embodiment of the invention. The motor vehicle 1 has front-wheel steering and rear-wheel steering, and also front-wheel drive 6. The front wheels FL,FR of the front-wheel steering are connected to one another via a second rack gear 30 of a second rack and pinion steering gear 40. The rear wheels RL,RR of the rear-wheel drive are connected to one another via a first rack gear 3 of a first rack and pinion steering gear 4. The front-wheel drive 6 has a single actuator 61, in particular an electric motor, which drives the front wheels FL,FR via a differential. At slow speeds, in order to improve the maneuverability, a steering torque is provided by braking one of the rear wheels RL,RR. The braking system on the rear-wheel axle is arranged on the drivetrain. A braking torque generated by the braking system is transmitted via the drive shaft of the rear-wheel drive to the wheel being braked. The braking system is therefore referred to as an inboard braking system. Depending on the reference wheel steering angle α.sub.RW,ref of the rear wheels RL,RR, a reference rack gear position S.sub.R,ref of the first rack gear 3 is calculated. The rack gear position is regulated by means of a control system which comprises arbitration software. This software incorporates the chassis geometry, the properties of the braking system and the algebraic sign of the reference rack gear position S.sub.R,ref, in order to determine a rear wheel to be braked RR,RL, and the brake pressure required for the braking action. So that the motor vehicle 1 does not lose speed due to the braking action, the front wheel drive provides additional torque, which corresponds to the braking torque T.sub.RRped,br and makes up for the loss of speed. A position of an accelerator pedal of the motor vehicle, preferably an accelerator pedal angle, and also a position of a brake pedal, preferably a brake pedal angle, are relayed to the control system, in order to identify an acceleration or braking of the motor vehicle and therefore calculate the additional torque required.

(9) FIG. 2 shows driving around a left-hand bend. The reference rack gear position S.sub.R,ref and the reference wheel steering angle α.sub.RW,ref are used in calculating the braking torque T.sub.RLped,br to be applied by the left rear wheel RL.

(10) Through braking of the one rear wheel, a yawing moment is produced about the vertical axis and the first rack gear 3 of the first rack and pinion steering gear 4 is displaced, as a result of which the motor vehicle 1 is deflected. The drive control of the front wheel drive 6 controls the right front wheel FR and the left front wheel FL accordingly, each of which applies a tractive force of xN (e.g. 100 N)+F.sub.RLped,br/2, wherein F.sub.RLped,br, is the force compensating for the braking torque T.sub.RLped,br. The front-axle steering can be supported by a torque generated by means of the actuator. The motor vehicle is steered by means of rear-wheel brake-based torque vectoring, without a drive on the rear axle.

(11) A motor vehicle 1 with rear-wheel drive 2 and front-wheel and rear-wheel steering are depicted in FIG. 3. The front wheels FL,FR of the front-wheel steering are connected to one another via a second rack gear 30 of a second rack and pinion steering gear 40. The rear wheels RL,RR of the rear-wheel steering are connected to one another via a second rack gear 3 of a second rack and pinion steering gear 4. The rear wheel drive has a single actuator 2, in particular an electric motor, which drives the rear wheels RL,RR via an open differential (without a locking mechanism) or a partially open differential.

(12) In order to increase maneuverability at slow speeds, a steering torque and a yawing moment about the vertical axis of the motor vehicle are provided by braking one of the rear wheels RL.

(13) Depending on the reference wheel steering angle α.sub.RW,ref, a reference rack gear position S.sub.R,ref of the first rack gear 3 is calculated. The rack gear position is regulated by means of a control system which comprises arbitration software. This software uses the chassis geometry, the properties of the braking system and the algebraic symbol of the reference rack gear position S.sub.R,ref to determine a rear wheel which is to be braked and the brake pressure. So that the motor vehicle 1 does not lose speed on account of the braking action, the actuator 21 of the rear wheel drive 2 provides additional torque which compensates for the loss of speed. A position of an accelerator pedal of the motor vehicle, preferably an accelerator pedal angle α, and also a position of a brake pedal, preferably a brake pedal angle, are relayed to the control system, so as to identify acceleration or braking of the motor vehicle, in order to calculate the additional torque.

(14) The following equation applies to the left-hand bend depicted in FIG. 3:

(15) T.sub.RL+T.sub.RR=2*T.sub.RR+T.sub.RLped,br, where T.sub.RL and T.sub.RR are the torque of the left and right rear wheel and T.sub.RLped,br is the braking torque applied to the rear wheel RL to be braked. F.sub.RR is therefore equal to 2*xN+F.sub.RLped,br.

(16) The braking system on the rear wheel axle is arranged on the drivetrain. A braking torque generated by the braking system is transmitted via the drive shaft of the rear wheel drive 2 to the wheel to be braked RL. The braking system is therefore referred to as an “inboard braking system”. The forces acting on the wheels are therefore mainly generated from the torque transmitted via the drive shaft and the frictional force of the point of contact of the wheel with the road.

(17) The resulting force acts on the centre of the wheel, as a result of which a stronger steering torque can be achieved with a smaller torque than is traditionally the case with a larger king pin inclination.

(18) In this case, the rear-wheel brakes-based torque vectoring represents particularly favourable and simple rear-axle steering which allows for agile control of the motor vehicle at a low speed.

(19) At higher speeds, in particular at speeds of over 40 km/hr, it is preferable for the steering gear of the rear-axle steering to be locked in the straight-line position (mid-position), in order to achieve steady handling. The locking is preferably achieved by means of a locking mechanism for the rear axle, which releases said rear axle or locks it, depending on the defined operating states of the vehicle. In particular, the steering should be locked when a predefined speed of the vehicle is exceeded.

(20) A drive torque difference between the left and right rear wheel for controlling the motor vehicle is used when a predefined vehicle speed is exceeded and/or a maximum lateral acceleration is exceeded.

(21) In order to achieve a maximum effect through the torque difference, the suspension of the wheels and the steering geometry resulting from this must be selected accordingly. For this purpose, the caster and the scrub radius, in particular, must be defined, in order to maximize the yawing moment resulting from the differential torque.

(22) FIG. 4 shows a block diagram of a control system of the motor vehicle with rear axle-based torque vectoring for slow vehicle speeds. In a first unit 7, the steering torque T.sub.SW introduced into a steering wheel by the driver or a reference torque T.sub.req required by an autonomous driving mode are converted into a reference wheel steering angle α.sub.RW,ref of the rear wheels RL,RR. A second unit 8 determines from the reference wheel steering angle α.sub.RW,ref a reference rack gear position S.sub.R,ref. The reference rack gear position S.sub.R,ref is compared with a measured actual rack gear position S.sub.R. The rack gear force F.sub.Rack is determined in a third unit 9 from the reference rack gear position S.sub.R,ref and the braking torque T.sub.ped,br and/or the acceleration torque T.sub.ped,acc of the rear wheels. In a fourth unit 10, the way in which the motor vehicle and the vehicle wheels behave with a constant speed and cornering, acceleration and cornering and when braking while cornering is then determined from this and subsequently implemented for the respective vehicle state.

(23) As disclosed in the previously described embodiments, the drive may be arranged on the front axle or the rear axle. The drive comprises at least one actuator.

(24) A steering action can be initiated both by a driver by turning a steering wheel or by moving another kind of steering means, and also by a control system of an autonomous or semi-autonomous motor vehicle.