Method for controlling a road vehicle with a steer-by-wire steering system in case of road wheel actuator failure

Abstract

The invention relates to a method for controlling a road vehicle (1) with a steer-by-wire steering system (2) in an alternative steering functionality mode (12) after total failure of a road wheel actuator (3), wherein the road vehicle (1) comprises two axles each with two wheels (RL, RR, FL, FR), wherein the front two wheels (FL, FR) can be steered by means of a front axle steering system and are connected to one another via a rack (10) of the steering system (2) of the front axle steering system, and wherein the road vehicle (1) has a brake system which acts on the four wheels (RL, RR, FL, FR), wherein the method includes the following steps: a) Checking the steer-by-wire steering system (2) for the presence of a road wheel actuator (3) failure and for the state of performance availability, b) Carrying out the alternative steering functionality mode (12) in the event that a failure of the road wheel actuator (3) has been detected, wherein the alternative steering functionality mode (12) includes longitudinal control (13), lateral control (14), and road wheel actuator control (15), wherein the road wheel actuator control (15) includes an on and off turning of a phase-short damping of the road wheel actuator (3).

Description

[0001] Method for controlling a road vehicle with a steer-by-wire steering system in case of road wheel actuator failure

[0002] The present invention relates to a method for controlling a road vehicle with a steer-by-wire steering system according to the preamble of claim 1 and a road vehicle designed to carry out the method.

[0003] In a Steer-by-wire steering system of a road vehicle, it is crucial to have a redundant steering system available in the event of a total failure of the functionality of the road wheel actuator.

[0004] As soon as the road wheel actuator has failed, the system must ensure that the vehicle is brought to a state depending on the type of failure in which it can safely come to a standstill in the lane, Limp Aside (turning off the main road) or Limp Home (e.g. driving to the nearest garage).

[0005] This can be achieved by an "automatic" braking function that slows the vehicle to a safe speed or to a standstill, taking into account the lateral state of the vehicle and the lateral requests from the driver.

[0006] Patent application KR. 10-2023-0137510 discloses a control method for controlling the steering of a vehicle by controlling the pressure of the rear wheel brake and the drive of the front wheels based on the yaw moment when it is determined that a failure of the electric steering system has occurred. It is an object of the present invention to provide a method for controlling a road vehicle with a steer-by-wire steering system which allows to safely stop the vehicle after failure of the road wheel actuator.

[0007] This object is achieved by a method for controlling a road vehicle with a steer-by-wire steering system having the features of claim 1 and a road vehicle designed to carry out the method.

[0008] Accordingly, a method for controlling a road vehicle with a steer-by-wire steering system in an alternative steering functionality mode after total failure of a road wheel actuator is provided, wherein the road vehicle comprises two axles each with two wheels, wherein (only) the front two wheels can be steered by means of a front axle steering system and are connected to one another via a rack of the steering system of the front axle steering system, and wherein the road vehicle has a brake system which acts on the four wheels, wherein the method includes the following steps:

[0009] a) Checking the steer-by-wire steering system for the presence of a road wheel actuator failure and for the state of performance availability of the road wheel actuator (in other words: the available power of the road wheel actuator),

[0010] b) Carrying out the alternative steering functionality mode in the event that a failure of the road wheel actuator has been detected, wherein the alternative steering functionality mode includes longitudinal control, lateral control, and road wheel actuator control, wherein the road wheel actuator control includes an on and off turning of a phase-short damping of the road wheel actuator.

[0011] Turning on and off of the phase-short damping helps to improve the lateral control in the event of a total failure of the road wheel actuator. The lateral vehicle motion is controlled by the lateral control and the road wheel actuator control. With this method, the vehicle can be safely controlled to a standstill.

[0012] In the alternative steering functionality mode it is preferred that the phaseshort damping is switched on / off based on a driver or autonomous steering unit steering request change. This ensures that the damping does not hinder steering through targeted braking.

[0013] It is advantageous to use a strategy handler that detects and memorizes the desired rack position at the moment the road wheel actuator fails. The desired rack position is calculated from the steering wheel angle sensor. Based on the steering wheel position, the logic recognizes whether the driver

[0014] • keeps the steering wheel angle constant,

[0015] • steers-out (steering wheel angle increased), or

[0016] • steers-in (steering wheel angle reduced).

[0017] Based on the detected steering state, the phase-short damping of the road wheel actuator is switched off / on.

[0018] A preferred method is described below:

[0019] If the driver keeps the steering wheel angle constant (e.g. straight driving, steady cornering), the phase-short damping of the road wheel actuator remains switched on even after failure in order to limit any road disturbances acting on the steering rack or self-aligning tire forces moving the rack to zero position. In certain applications, the brake pressure error (difference between request brake pressure and actual brake pressure) is also observed and if this error is too large, the damping can be switched on or remain switched on.

[0020] If the driver steers-out from center, the logic detects that the driver is changing from straight driving to a cornering condition. To allow for unhindered rack movement via the brakes, the phase-short damping of the road wheel actuator is switched off.

[0021] If the driver steers-out from an offset steering position, the logic observes a new rack position error, the current vehicle lateral acceleration and the brake pressure error (request-actual) and decides if the phase-short damping of the road wheel actuator shall be switched off. For example, if the logic observes a minimal brake pressure error and a low measured lateral acceleration, the phase-short damping may be switched off. On the contrary, if a large brake pressure error and / or a high lateral acceleration are observed, the phase-short damping remains on.

[0022] If the driver steers-in, the logic observes a new rack position error, and if the error is larger than a certain threshold, it switches off the phase-short damping of the road wheel actuator. In this way, the self-aligning forces of the tires help the rack steer-in closer to the rack center as desired by the driver.

[0023] The strategy handler can be implemented in a simpler form, in which the phase-short damping of the road wheel actuator is strictly switched on during straight driving / steer-out and turned off for steer-in maneuvers. This simpler strategy does not allow continuous switching of the phase-short damping. This simpler strategy shall be preferably implemented when:

[0024] The system cannot estimate self-aligning tire forces due to lack of reliable sensor data and / or the vehicle suspension is not optimized to aid the translation effect of the brake forces to the steering rack. In these cases, the rack cannot be moved sufficiently and the vehicle is steered mainly through the additional yaw moment from the differential braking, but not by the steering of the front wheels.

[0025] Preferably, the lateral control includes a feedforward and feedback control. The feedforward control can determine a brake yaw request based on the current lateral driver request or the current lateral autonomous driving unit request. In a preferred embodiment, the feedback control determines a brake torque request based on the deviation between the requested yaw rate from the driver or autonomous driving unit and the measured yaw rate. It is also possible that the brake torque request is determined based on the deviation between the requested rack position from the driver or autonomous driving unit and the measured rack position.

[0026] Preferably, the lateral control requests either overall vehicle yaw torque, front / rear yaw torque or individual brake pressures depending on the vehicle configuration. The longitudinal control can monitor the vehicle state and can decide on the deviation between a desired and measured longitudinal deceleration a required braking pressure to the corresponding road wheels. In a preferred embodiment the calculated braking pressure is longitudinally distributed to the road wheels in the same way as in normal operation. For example, it can be 60 / 40 (longitudinal brake pressure distribution (front / rear)) or 80 / 20.

[0027] Additionally, a road vehicle is provided that can carry out the procedure described above.

[0028] A preferred embodiment of the present invention will be described with reference to the drawings.

[0029] Figure 1: is a schematic illustration of a road vehicle with a steer-by-wire steering system; and

[0030] Figure 2: shows a block diagram of a method for controlling the road vehicle after failure of the road wheel actuator of the steer-by- wire steering system.

[0031] Figure 1 schematically shows a road vehicle 1. The steer-by-wire steering system 2 is a front axle steering with a central road wheel actuator 3 via which both front wheels FL,FR can be steered in a coupled manner. The road wheel actuator 3 acts on the front wheels FL,FR via the rack-and-pinion steering gear 4. The rack-and-pinion steering gear can be replaced by a rack only system. Each wheel FL,FR,RL,RR is individually assigned a brake 5FL, 5FR, 5RL, 5RR. The steering unit 7 including the steering controller and the feedback actuator is connected to and in communication with the road wheel actuator 3. The four brake actuators 5FL, 5FR, 5RL, 5RR of the wheels are connected to a separate brake control unit 6. The steering controller communicates with the brake control unit 6 (not shown). The feedback actuator introduces a feedback signal into the steering shaft 8, which is coupled to the steering wheel 9, so that the driver can recognize a feedback in the steering wheel 9 from forces introduced in the rack 10 from the wheels FR, FL as if there were a mechanical

[0032] connection. In the event that the road wheel actuator 3 of the steer-by-wire steering system 2 fails, i.e. the front axle steering is no longer available, the vehicle switches to an alternative steering functionality (ASF) mode to slow down the vehicle to a safe state while fulfilling the drivers' lateral requests. The brakes of the vehicle are used to generate a steering motion (yaw rate). Additionally, electric motor phase modulation within the road wheel actuator is used to enhance steering capabilities by counteracting any inherent self-aligning torque from the suspension. In the following the method for controlling the steer-by-wire steering system is explained in detail.

[0033] As shown in Figure 2, the vehicle status is continuously monitored by evaluating the vehicle sensor signals 11 via the vehicle bus. The ASF mode 12 comprises longitudinal control 13, lateral control 14 and road wheel actuator control 15.

[0034] The longitudinal control 13 is responsible for the overall deceleration of the vehicle. The longitudinal control 13 monitors the vehicle state and decides on the desired deceleration depending on the current lateral acceleration and the vehicle speed. Depending on the deviation between the desired and measured longitudinal deceleration, the longitudinal control sends a required braking pressure to the electronic control unit 16 of the brake system (as shown in Figure 1 with reference number 6) and requests breaking of the corresponding breaks 5FL, 5FR, 5RL, 5RR. The longitudinal control may request overall brake pressure to all four brakes or just a target deceleration (e.g. to the ESP). In detail, the brake control unit can process the deceleration request and decide for itself how it is distributed to the individual road wheels. The alternative is to "bypass" the distribution logic of the brake control unit and request directly pressure to the individual road wheels.

[0035] The lateral control 14 realizes the steering request of the driver or the autonomous driving unit (AD) through a combination of forward and feedback control. The lateral control 14 observes the vehicle speed, the rack position request, the measured yaw rate and further calculates the yaw rate request. The control is split in three parts: 1) Feedforward control of a brake yaw torque request based on the current lateral driver or AD request,

[0036] 2) Feedback control of a brake torque request based on the deviation between the requested yaw rate from the driver or AD and the measured yaw rate or instead analogous rack position feedback control, and

[0037] 3) Road wheel actuator control 15 via electric motor phase modulation depending on vehicle state.

[0038] The lateral control can request either overall vehicle yaw torque, front / rear yaw torque or individual brake pressures via the electronic control unit 16 of the brake system depending on the vehicle configuration.

[0039] Part 3 will be described in more detail in the following. In case of a road wheel actuator failure in a steer-by-wire steering system, the steerable front wheels would have a chance of being moved by external inputs (e.g. cornering forces on the front tires). One way to mitigate this is by shorting the road wheel actuator motor phases after a failure occurs. This will result in a damping behavior which limits the rate at which the rack is moved from the lateral tire forces. The phase-short damping generates torque as a function of the road wheel actuator motor rotor speed and can counteract those tire forces.

[0040] The road wheel actuator control of the ASF mode 12 implemented in the electronic control unit of the steer-by-wire steering system is able to turn on and off the phase-short damping of the road wheel actuator 3. The phaseshort damping is a very important part of the ASF mode as in the moment of road wheel actuator failure it can prevent losing the trajectory before enough brake forces are generated to keep the vehicle on the desired path. However, if the driver wants to change direction after failure, phase-short damping has to be switched off as otherwise it limits any wanted rack movement realized from the front brakes.

[0041] The road wheel actuator is preferably an electrical machine with a redundant design with two independent sets of three-phase windings. A phase short circuit can be created in particular by establishing an electrical connection between two phase lines, especially between phase lines that are assigned to different phases. Of course, it is also possible for all phase lines or selected but not all phase lines to be short-circuited together, i.e. electrically connected.

[0042] If a phase short circuit is established, the electrical machine is operated in generator mode. In generator mode, the electrical machine is not excited to move. Instead, a current dependent on the kinetic energy of moving parts of the electrical machine is induced in the motor phases. In this case, a phase short-circuit leads to a braking torque which acts in the opposite direction of rotation.

[0043] Preferably two phases of one set of windings are connected to ground, so that two phases are shorted to ground and the other four phases are not shorted. For the application described, however, it is also possible to

[0044] - shorten only one phase to ground or

[0045] - to shorten two or three phases together or

[0046] - to shorten two phases of one set of windings together and two phases of the other set of windings.

[0047] It is preferred to activate and deactivate only one phase short configuration (no control of single phases).

[0048] The brake requests of the lateral control 14 and the longitudinal control 13 are sent to an electronic control system 16 of the brake system, which calculates the resulting brake requests for each brake 5FL, 5FR, 5R.L, 5R.R. individually. In addition to the steering wheel actuator 3, the steer-by-wire steering system 2 includes the steering unit 7 with the feedback actuator, also known as the handwheel actuator. The steering request 17 initiated by the driver into the steering wheel or the steering request given by an AD is used in the ASF mode 12 for lateral control 14.

Claims

Claims1. Method for controlling a road vehicle (1) with a steer-by-wire steering system (2) in an alternative steering functionality mode (12) after total failure of a road wheel actuator (3), wherein the road vehicle (1) comprises two axles each with two wheels (RL, RR, FL, FR), wherein the front two wheels (FL, FR) can be steered by means of a front axle steering system and are connected to one another via a rack (10) of the steering system (2) of the front axle steering system, and wherein the road vehicle (1) has a brake system which acts on the four wheels (RL, RR, FL, FR), wherein the method includes the following steps:a) Checking the steer-by-wire steering system (2) for the presence of a road wheel actuator (3) failure and for the state of performance availability of the road wheel actuator (3),b) Carrying out the alternative steering functionality mode (12) in the event that a failure of the road wheel actuator (3) has been detected, characterized in that the alternative steering functionality mode (12) includes longitudinal control (13), lateral control (14), and road wheel actuator control (15), wherein the road wheel actuator control (15) includes an on and off turning of a phase-short damping of the road wheel actuator (3).

2. Method according to claim 1, characterized in that in alternative steering functionality mode (12) the phase-short damping is switched on / off based on a driver or autonomous steering unit steering request change.

3. Method according to claim 1, characterized in that the lateral control (14) includes a feedforward and feedback control.

4. Method according to claim 3, characterized in that the feedforwardcontrol determines a brake yaw request based on the current lateral driver or autonomous driving unit request.

5. Method according to claim 3 or 4, characterized in that the feedback control determines a brake torque request based on the deviation between the requested yaw rate from the driver or autonomous driving unit and the measured yaw rate or based on the deviation between the requested rack position from the driver or autonomous driving unit and the measured rack position.

6. Method according to one of the preceding claims 1 to 4, characterized in that the lateral control (14) requests either overall vehicle yaw torque, front / rear yaw torque or individual brake pressures depending on the vehicle configuration.

7. Method according to one of the preceding claims 1 to 4, characterized in that the longitudinal control (13) monitors the vehicle state and decides on the deviation between a desired and measured longitudinal deceleration a required braking pressure to the corresponding road wheels.

8. Road vehicle (1) designed to carry out the method according to one of the preceding claims 1 to 4.

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