DE102020207728A1 - Method for setting a steering angle of a motor vehicle and steering system - Google Patents

Abstract

The invention relates to a method for adjusting a steering angle of a motor vehicle and a steering system (1) of a motor vehicle, the steering system (1) having at least one control unit (2) and at least one actuator (3) for generating a steering torque (ML) in order to Set the target steering angle, the control unit (2) being designed in such a way that it receives a signal (SB) about the availability of a braking system, the control unit (2) sending the actuating signal (sSL) for the actuator (3 ) to superimpose an alternating control signal (sa) to set the target steering angle in order to achieve a braking effect.

Description

The invention relates to a method for setting a steering angle of a motor vehicle and a steering system.

Various types of steering systems are known for motor vehicles. For example, these are designed as electromechanical steering systems or as steer-by-wire steering systems. An actuator generates a steering torque for setting a target steering angle. In steer-by-wire systems, the actuator generates the full steering torque, whereas in electromechanical steering systems, part of the steering torque is applied by the driver, which is applied to a rack via a steering column, for example.

Just like the steering system, a braking system of a motor vehicle also represents a safety-relevant system which either has to have a fallback level or has to be designed redundantly.

The invention is based on the technical problem of providing a method for setting a steering angle of a motor vehicle which also provides braking functionality. Another technical problem is the creation of a suitable control system.

The solution to the technical problem results from a method with the features of claim 1 and a steering system with the features of claim 6. Further advantageous embodiments of the invention emerge from the subclaims.

The method for setting a steering angle of a motor vehicle takes place by means of at least one control unit and an actuator for generating a steering torque in order to set a target steering angle. A signal about the availability of a brake system is also fed to the control unit, with the control unit superimposing an alternating control signal on the control signal for the actuator to determine the target steering angle when it is available below a threshold value in order to achieve a braking effect. This is based on the following knowledge: When braking, the tire builds up slip in relation to the roadway and thus builds up a force in the longitudinal direction of the vehicle between these two friction partners, which slows the vehicle down. When steering, there is also slippage between the road and the tire, which dissipates energy that has to be compensated for by the drive machine when cornering at constant speed. This effect is now used specifically as a fallback level for braking. An alternating steering movement of sufficiently high frequency causes slip on the tire without the vehicle having the opportunity to follow the steering specification due to its inertia, i.e. the motor vehicle continues to follow its target trajectory with the target steering angle. With the method according to the invention, it is estimated that 20% -50% of the braking power of the braking system can be applied via the steering system. For example, the threshold value can be selected such that it corresponds to 80% of the maximum braking power, so that this drop in braking power can be fully compensated for by the additional braking power of the steering. In addition, a corresponding warning message is preferably generated that the brake system must be checked. It can also be provided that the additional braking power is only made available once and a new engine start is prevented as long as the braking system has not been checked.

The motor vehicle can be a fully or partially automated motor vehicle, but also a vehicle that is predominantly controlled by the vehicle driver.

The method can be implemented particularly advantageously in a steer-by-wire steering system, since a reaction of the alternating actuating signal on the steering handle can very easily be excluded there. In the case of electromechanical steering systems, on the other hand, there is a reaction that either has to be compensated for or is used specifically as a haptic warning.

In one embodiment, the frequency of the alternating control signal is between 3 Hz and 20 Hz, more preferably between 8 Hz and 12 Hz. This represents a good compromise in order to dissipate sufficient energy and ensure inertia. Any deviations from the target steering angle that may nevertheless occur are to be compensated for by the actuating signal for the target steering angle.

In a further embodiment, the alternating control signal is rectangular, sinusoidal or sawtooth-shaped, since these can be implemented very easily. The rectangular configuration of the actuating signal is particularly advantageous, since the torques are set suddenly.

In a further embodiment, the alternating control signal is symmetrical. However, embodiments are also possible in which the alternating actuating signal is asymmetrical in order to generate maximum braking force. Corresponding deviations from the target steering angle must then be compensated for in some other way.

In a further embodiment, the maximum steering torque of the actuator is set by the alternating control signal in at least one steering direction, so that the braking effect is maximized. With a target steering angle of 0, this can be achieved very easily for both steering directions by means of a symmetrical, alternating control signal. If, on the other hand, the target steering angle is not equal to 0, the control signal for the target steering angle already generates a steering torque in one direction. The alternating control signal in this steering direction can now be correspondingly smaller in order to achieve the maximum steering torque in this direction, the corresponding counter-torque increasing in the other direction in order to achieve the full steering torque. This can be compensated for by an asymmetrical control signal so that the maximum steering torque is built up in both steering directions. Alternatively, the symmetrical, alternating control signal is set in such a way that the maximum steering torque is only achieved in one steering direction.

With regard to the actual design of the steering system, reference can be made in full to the preceding explanations.

• 1 ein schematisches Blockschaltbild eines Lenkungssystems,
• 2a einen beispielhaften Verlauf des Stellsignals für den Soll-Lenkwinkel bei Geradeausfahrt,
• 2b einen beispielhaften Verlauf des alternierenden Stellsignals bei Geradeausfahrt bei einer Bremsung zum Zeitpunkt t₀,
• 2c einen beispielhaften Verlauf des Lenkmoments für die Geradeausfahrt bei einer Bremsung zum Zeitpunkt to,
• 3a einen beispielhaften Verlauf des Stellsignals für den Soll-Lenkwinkel bei einer konstanten Kurvenfahrt,
• 3b einen beispielhaften Verlauf des alternierenden Stellsignals bei Kurvenfahrt bei einer Bremsung zum Zeitpunkt t₀ und
• 3c einen beispielhaften Verlauf des Lenkmoments für die konstante Kurvenfahrt bei einer Bremsung zum Zeitpunkt t₀.

The invention is explained in more detail below on the basis of a preferred embodiment. The figures show:

• 1 a schematic block diagram of a steering system,
• 2a an exemplary course of the control signal for the target steering angle when driving straight ahead,
• 2 B an exemplary course of the alternating actuating signal when driving straight ahead with braking at time t ₀ ,
• 2c an exemplary curve of the steering torque for straight-ahead travel when braking at time to,
• 3a an exemplary course of the control signal for the target steering angle during constant cornering,
• 3b an exemplary course of the alternating actuating signal when cornering during braking at time t ₀ and
• 3c an exemplary curve of the steering torque for constant cornering during braking at time t ₀ .

In the 1 Figure 3 is a schematic block diagram of a steering system 1 shown. The steering system 1 has a control unit 2 as well as an actuator 3 to generate a steering torque. The actuator 3 has power electronics for this purpose 4th as well as an electric motor 5 on, which has a gear, not shown, with a rack 6th is connected, which sets the steering angle on the wheels, not shown. The steering system continues 1 a steering handle 7th as well as a sensor 8th on. The sensor 8th can be a rotation angle or torque sensor that sends the driver's request to the steering handle 7th detected and sent to the control unit 2 transmitted. The steering system shown 1 is a steer-by-wire steering system, so there is no mechanical connection between the steering handle 7th and rack 6th exists. The control unit 2 also receives signals FA from driver assistance systems such as a lane keeping assistance system. The control unit continues to receive 2 a signal S _B from a control unit 9 of a braking system on the availability of the braking system. If the availability falls below a threshold value, the control unit generates 2 an alternating control signal that is superimposed on the control signal for a target steering angle. In this context, superposition is understood to mean that the two control signals are added and the resulting control signal is sent to the actuator 3 is supplied, the alternating control signal generating a braking force on the wheels without influencing the set target steering angle. Finally is the control unit 2 with a display unit 10 connected to inform the vehicle driver when the steering system 1 works as a supporting braking system.

The mode of operation will now be explained briefly as an example for straight travel and constant cornering.

In the 2a is the control signal s _SL for setting a target steering angle over time for driving straight ahead. Since the target steering angle is 0 °, the control signal is s _SL = 0. At time t ₀ , the steering system should 1 (please refer 1 ) provide a braking force, including the control unit 2 an alternating control signal s _a for the actuator 3 (please refer 1 ) generates what is in 2 B is shown, so that a steering torque M _L sets that alternates between the maximum steering torques ± M _{L, max} with the frequency f. This is in 2c shown. The resulting path signal of the rack 6th is then a sinusoidal signal.

In the 3a is the control signal s _SL for setting a target steering angle over time for constant cornering, wherein s _SL = const ≠ 0. If a braking force is then to be generated again at time t _{0, the control signal} s _SL an alternating control signal for the target steering angle s _a superimposed (see 3b) . The alternating control signal s _a is rectangular and symmetrical. The steering torque alternates accordingly symmetrically M _L . The level of the amplitude of the alternating control signal s _a is selected in such a way that the maximum steering torque M _{L, max is} achieved in one steering direction. This is in 3c shown, assuming that the control signal s _SL a steering torque M _L = ½ M _{L, max} generated. The steering torque then oscillates between M _{L, max} and 0.

List of reference symbols

1

Steering system

2

Control unit

3

Actuator

4th

Power electronics

5

Electric motor

6th

Rack

7th

Steering handle

8th

sensor

9

Control unit

10

Display unit

sSL

Control signal for target steering angle

sa

alternating control signal

ML

Steering torque

SB

Signal about the availability of a braking system

Claims (10)

Method for setting a steering angle of a motor vehicle by means of at least one control unit (2) and an actuator (3) for generating a steering torque (M _L ) in order to set a target steering angle, characterized in that the control unit (2) sends a signal (S _B ) is supplied via the availability of a brake system, the control unit (2) superimposing _{an alternating control signal (s a ) on the control signal (s SL} ) for the actuator (3) for setting the target steering angle when the availability is below a threshold value in order to to achieve a braking effect. Procedure according to Claim 1 , characterized in that the frequency (f) of the alternating control signal (s _a ) is between 3 Hz and 20 Hz. Procedure according to Claim 1 or 2 , characterized in that the alternating control signal (s _a ) is square, sinusoidal or sawtooth-shaped. Method according to one of the preceding claims, characterized in that the alternating control signal (s _a ) is symmetrical. Method according to one of the preceding claims, characterized in that the maximum steering torque (± M _{L, max} ) of the actuator (3) is set by the alternating control signal (s _{a) in at least one steering direction.} Steering system (1) of a motor vehicle, the steering system (1) having at least one control unit (2) and at least one actuator (3) for generating a steering torque (M _L ) in order to set a target steering angle, characterized in that the control unit ( 2) is designed _{to receive a signal (S B} ) about the availability of a brake system, the control unit (2) if the availability is below a threshold value, the control signal (s _SL ) for the actuator (3) for setting the target steering angle to superimpose an alternating control signal (s _a ) in order to achieve a braking effect. Steering system according to Claim 6 , characterized in that the frequency (f) of the alternating control signal (s _a ) is between 3 Hz and 20 Hz. Steering system according to Claim 6 , characterized in that the frequency (f) of the alternating control signal (s _a ) is square, sinusoidal or sawtooth-shaped. Steering system according to one of the Claims 6 until 8th , characterized in that the alternating control signal (s _a ) is symmetrical. Steering system according to one of the Claims 6 until 9 , characterized in that the control unit (2) is designed such that the maximum steering torque (± M _{L, max} ) of the actuator (3) is set _{by the alternating control signal (s a) at least in one steering direction.}

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