STEER-BY-WIRE STEERING SYSTEM WITH CONTROL OF THE CURRENT LIMITATION OF A ROAD WHEEL ACTUATOR

Description

[0001] The present invention relates to a method to control a steer-by-wire steering system of a road vehicle according to the preamble of claim 1 and to a steer-by-wire steering system for a road vehicle according to the preamble of claim 8.

[0002] In steer-by-wire steering systems there is no mechanical connection between the steering wheel and the steering rack and the steerable wheels and steering movement is achieved by a position controlled road wheel actuator, whose reference position is based on the steering wheel angle.

[0003] The position controller always acts with motor torque in a way, that the reference position signal is followed by the road wheel actuator. Due to physical limitations of the road wheel actuator motor (such as maximum available motor torque at a certain motor speed), and limitations of the position controller (bandwidth), the reference road wheel angle shows a transient response.

[0004] In case of high dynamic maneuvers the transient response can cause a post steering effect. For instance, if a very fast step steer maneuver is performed on the steering wheel, the road wheels will turn during the transient, even if the maneuver on the steering wheel side has already ended. This can be felt and heard by the driver, and is very disturbing.

[0005] DE 10 2018 101 181 A1, on which the preamble of claim 1 is based, discloses a method for controlling a steer-by-wire-system of a motor vehicle in which, when a target power exceeds the maximum power of the electric motor of the road wheel actuator, the electric motor is operated at the maximum power and a counter torque applied to the steering wheel by a feedback actuator is increased.

[0006] JP 2007-230471 A discloses a method in which, when a first threshold of motor current is reached, the steering command is reduced. A feedback loop is used for further corrections of the steering command.

[0007] US 2019 / 084614 A1 discloses a diagnostic apparatus that computes a reference tracking error based on a difference between the reference position and the rack position, and can distinguish between an out-running situation (driver steering quicker than a capability of the road wheel actuator to steer the road wheels) and a faulty condition.

[0008] It is an object of the present invention to provide a method to control a steer-by-wire steering system of a road vehicle and a steer-by-wire steering system itself, which reduces the post steering effect.

[0009] This object is achieved by a method to control a steer-by-wire system of a motor vehicle having the features of claim 1 and a steer-by-wire steering system for a motor vehicle having the features of claim 8.

[0010] Accordingly, a method to control a steer-by-wire steering system for a road vehicle is provided, the steer-by-wire steering system comprising steering means (preferably a steering wheel), an road wheel actuator to actuate road wheels and a control unit that calculates a motor torque request for the road wheel actuator based on an actual or an estimated position and a position request, which is based on the position of the steering means, wherein the following steps are provided: Detection of a road wheel actuator power limit situation; Modifying the position request, so that it can be followed by the road wheel actuator within power limits; and Using the modified position request for calculating the motor torque request for the road wheel actuator.

[0011] Modifying the request to be within power limits of the road wheel actuator prevents post steering effect.

[0012] Preferably, for the detection of a road wheel actuator power limit situation the actual power state of the road wheel actuator's electric motor is compared to predefined power characteristics. The predefined power characteristics can be for example an upper power limit or a torque limit curve.

[0013] In another embodiment position-tracking behavior of the system is monitored for the detection of a road wheel actuator power limit situation. If the road wheel actuator cannot follow the position request with a tolerance, a power limit situation is detected.

[0014] It is preferred that the modification of the position request is based on a limitation of the speed of road wheel angle change with a rate limiter.

[0015] The modification of the position request can also be based on the position deviation between the actual or estimated position and the modified position request. The modification of position request can be tuned depending on vehicle speed. The purpose is to optimize for vehicle characteristic for each vehicle speed.

[0016] The position can be the position of the rack (in case a rack gear or a rack and pinion steering gear is present) or the road wheel angle.

[0017] Further a steer-by-wire steering system for a road vehicle comprising steering means, a road wheel actuator to actuate road wheels and a control unit that calculates a motor torque request for the road wheel actuator based on an actual or an estimated position and a position request, which is based on the position of the steering means, is provided, wherein the control unit is designed to carry out the above explained method. The road wheels can be actuated for example via a rack gear or a rack and pinion steering gear.

[0018] A preferred embodiment of the present invention will be described with reference to the drawings. Figure 1:is a schematic illustration of a steer-by-wire steering system of a motor vehicle; and Figure 2:shows a diagram of a step steer maneuver with modified position request.

[0019] Figure 1 is a schematic drawing of a steer-by-wire system 1 with a steering shaft 2 connected to steering means 3. There is no mechanical connection between the steering means 3 and road wheels 4. A road wheel actuator 5 operates a gear rack 6 via a rack-and-pinion gear 7, which is part of a front wheel axle 8. The front wheel axle 8 has two tie rods 9 for road wheels 4, of which only one road wheel 4 is sketched.

[0020] When a driver operates the steering means 3, steering shaft 2 is rotated, which is detected by a shaft sensor, which is not shown in the drawings. When the vehicle is switched on, a control unit 10 calculates an operation signal for the road wheel actuator 5 from the signal detected by the shaft sensor. By operating gear rack 6 with the operation signal, the front wheel axle 8 is moved sideways and the road wheels 4 are turned. At the same time, forces introduced in the wheel axle 8 from the road wheels 4 are recognized by another sensor not shown in the drawings, and a feedback signal is calculated, which is applied to the steering shaft 2 by a feedback actuator 11, so that the operator can recognize the feedback in the steering means 3.

[0021] The control unit 10 includes a position controller, which calculates a motor torque request based on the actual or an estimated position of the rack 6 and a requested rack position. The road wheel actuator 5 thus follows a reference position. The requested rack position is dependent on the steering wheel angle. The road wheel actuator has power limits. They constrain the output torque and thus prevent damage of the electric motor. In order to avoid post steering the following method steps are carried out. At first a power limit situation is detected. Then as a result, the reference position is artificially modified, so that it can be followed by the road wheel actuator within the power limits. The road wheel actuator can operate at power limit. By the time the steering wheel movement slows down, the modification leads to a deviation between the modified and actual requested road wheel angle position and the unmodified steering wheel angle based road wheel angle request. But post-steering will be significantly shorter, because the road wheel actuator can follow the reference position with transient response. If the road wheel actuator actuates below power limits, the torque is preferably not requested, because it would lead to slowing down of the system, meaning the rack would not move as fast as it would be possible.

[0022] Figure 2 shows a diagram of a step steer maneuver between 0 and 1. The normalized positions are plotted against time. Shown are the resulting position, the modified, actual position request and the unmodified steering wheel angle based position request. The unmodified steering wheel angle based position request increases from 0 to 1 over time with a very high gradient. Just before reaching a value of 1 the curve shows a rapid but constant decrease of the gradient. The steering maneuver leads to a motor torque request which exceeds the upper power limits of the road wheel actuator. The request is thus adjusted to a modified, actual request, based on which a motor torque is calculated, which can be provided by the electric motor of the road wheel actuator within its limits. This modified, actual request is almost equal to the unmodified steering wheel angle based position request in the beginning of the steering maneuver. It flattens down to the end of the steering maneuver to a constant value of around 0.8. As can be seen from the curve of the resulting position, this modified, actual request can be followed by the road wheel actuator, limited only by the transient response. Post-steering does not occur.

[0023] For detection of a power limit situation, predefined power characteristics of the system can be used, for example an upper power limit or a torque limit curve, that corresponds with a maximum amount of torque that could be produced by the electric motor of the road wheel actuator at any given motor speed. In order to detect a power limit situation, the actual power state of the electric motor is compared to the predefined power characteristics. A power limit situation is detected, if the limits are exceeded.

[0024] In another embodiment the position tracking behavior of the system is monitored to detect a power limit situation. If the actuator cannot follow the reference position with a tolerance, the system detects a power limit situation.

[0025] The modification of the request can be based on a limitation of the speed of road wheel angle change with a rate limiter, which results in a request below the power limit of the system. In another embodiment the modification can be tracking behavior based. In other words, the speed of road wheel angle change is slowed down by a rate, which is based on the position deviation between the actual position and the modified, actual request.

Claims

1. Method to control a steer-by-wire steering system (1) for a road vehicle, the steer-by-wire steering system (1) comprising steering means (3), an road wheel actuator (5) to actuate road wheels (4) and a control unit (10) that calculates a motor torque request for the road wheel actuator (5) based on an actual or an estimated position and a position request, which is based on the position of the steering means (3), wherein the following steps are provided: • Detection of a road wheel actuator power limit situation; • Modifying the position request, so that it can be followed by the road wheel actuator (3) within power limits; and • Using the modified position request for calculating the motor torque request for the road wheel actuator (5); characterized in that the step of modifying the position request is based on the position deviation between the actual or estimated position and the modified position request.

2. Method according to claim 1, characterized in that for the detection of a road wheel actuator power limit situation the actual power state of the road wheel actuator's electric motor is compared to predefined power characteristics.

3. Method according to claim 2, characterized in that the predefined power characteristics are an upper power limit or a torque limit curve.

4. Method according to claim 1, characterized in that for the detection of a road wheel actuator power limit situation position tracking behavior of the system is monitored and if the road wheel actuator (5) cannot follow the position request with a tolerance, a power limit situation is detected.

5. Method according to one of the preceding claims, characterized in that the modification of the position request is based on a limitation of the speed of road wheel angle change with a rate limiter.

6. Method according to one of the preceding claims, characterized in that the position is the position of the rack or the road wheel angle.

7. A steer-by-wire steering system (1) for a road vehicle comprising steering means (3), a road wheel actuator (5) to actuate road wheels (4) and a control unit (10) that calculates a motor torque request for the road wheel actuator (5) based on an actual or an estimated position and a position request, which is based on the position of the steering means (3), wherein the control unit (10) is designed to carry out the method according to one of the preceding claims.