METHOD FOR OPERATING A STEERING SYSTEM

Description

[0001] The invention relates to a method for operating a steering system of a vehicle, comprising a force feedback actuator which acts on a steering handle to map reaction forces of at least one steered wheel of the steering system as a function of at least one road surface condition, a vehicle speed or a current steering angle of the wheel, wherein the actuator comprises an electric machine drivable by at least one first phase system or second phase system, according to the preamble of patent claim 1.

[0002] Electric power steering (EPS) systems are state of the art in today's passenger cars or are increasingly replacing the familiar hydraulic power steering systems. In such electric power steering systems, a steering handle, designed as a steering wheel, is mechanically connected to a steering gear via a steering shaft, which in turn is connected to the vehicle's steered wheels via tie rods and wheel steering arms. Planned future vehicle generations will enable at least partially automated driving, whereby a driver will no longer be constantly responsible for steering and controlling the vehicle. In such steering systems, it is theoretically possible to dispense with mechanical control from the steering handle via the steering gear to the steered wheels, and to implement a steer-by-wire steering system.

[0003] With such steer-by-wire steering systems, the driver receives no direct physical feedback of wheel reaction forces on their steering input due to the lack of mechanical coupling between the steering handle and the steered wheels. This lack of physical feedback makes it difficult for the driver to reliably assess current driving situations and limits their ability to initiate situation-specific and appropriate steering maneuvers or even acceleration and deceleration of the vehicle. This can compromise driving safety.

[0004] In order to give the vehicle driver a realistic driving experience at the steering handle, it is known to record parameters such as steering angle, driving speed, or wheel reaction forces, etc., from an actual, current driving situation or to calculate them in a simulation. From these parameters, a feedback signal is generated, which is then fed to a control and / or regulating device for a force feedback actuator. The force feedback actuator represents a manual torque or steering wheel actuator and, depending on the feedback signal, can apply a restoring torque to the steering handle corresponding to the actual wheel reaction torques or the reaction forces of the steered wheels. Such steering devices thus give the driver the impression of a real driving situation, similar to a mechanical or power steering system, which facilitates the driver's intuitive response to changing driving situations.

[0005] DE 10 2008 036 730 A1 describes a steer-by-wire steering system with a steering handle that has a force feedback actuator driven by an electric motor. The electric motor can be controlled by a control and / or regulating device that regulates the motor current via a converter depending on measured values ​​describing the respective current driving situation. The motor shaft of the electric motor is operatively connected to the steering handle, whereby the current motor torque can be identical to a manual torque applied to the steering handle. During operation of the steering system, the force feedback actuator generates a steering torque that must be overcome by the driver.

[0006] Basically, such force feedback actuators are formed from the electric machine and a reduction gear, which makes it possible to generate corresponding manual torques even with small connection values ​​of the electric machine.

[0007] DE 10 2006 056 042 A1 describes a system for controlling an electromechanical steering system, comprising a first control unit with an electronic immobilizer and a steering control unit for controlling the electromechanical steering system. The steering control unit inhibits the electromechanical steering system in a first state and releases the electromechanical steering system in a second state to enable steering. When the system is started, the steering control unit initially assumes the first state and switches to the second state after the electronic immobilizer is released. By integrating the steering control unit into the electronic immobilizer system, separate steering locks can be completely eliminated.

[0008] US 2009 / 0026998 A1 describes a pulse-width-controlled three-phase DC motor with two three-phase systems, or a plurality of three-phase systems, which, when operated alternately, enable the motor to drive a force-feedback actuator, for example, to actuate a pedal or steering handle of a vehicle. The motor can be used either as a steering actuator to provide auxiliary torque or as a force-feedback actuator.

[0009] DE 10 2019 208 201 B3 describes a method for operating a steering system of a vehicle, wherein an actuator is provided for generating a torque in a steering handle of the vehicle and at least the method steps Generating the torque up to a maximum torque when the maximum torque is enabled and limiting the maximum torque to a limit torque, detecting a vehicle situation in the vehicle, releasing the maximum torque when the vehicle situation entry and / or exit of the driver is detected in order to stiffen the steering handle, limiting the maximum torque when the vehicle situation is detected as a driving operation of the vehicle, includes.

[0010] This realizes a comfort function related to an entry aid. For this purpose, the force feedback actuator is provided with redundant windings, preferably at least six or exactly six windings. All windings are used to generate the maximum torque of the actuator, and only a portion of the windings, preferably half, are used or energized to generate the feedback torque.

[0011] WO 2021 085 228 A1 discloses a steering system with a force feedback actuator, in which the force feedback actuator is controlled by two redundant systems consisting of control units and motor drive units. The steering system is designed to switch from one system to the other in the event of a malfunction.

[0012] US 2020 / 0247462 A1 discloses a similar system in which a force feedback actuator motor with dual windings is controlled by two frequency converters. US 2020 / 03315221 discloses a force feedback actuator with two independent, communicating actuators, of which only one is operated in the normal state. In the event of abnormal events, the system can switch to a mode in which both actuators are operated independently of each other.

[0013] DE 11 2019 002 534 T5 discloses a steering system with a force feedback actuator, in which the force feedback actuator is controlled by two redundant systems consisting of control units and motor drive units. Both systems normally operate simultaneously with a limited electrical power. In the event of a fault in one system, the power limit of the other system is increased.

[0014] US 2003 / 0230448 A1 discloses a steering system with a force feedback actuator, in which the force feedback actuator provides a specific haptic feedback in the event of a malfunction if one or both motors of the force feedback actuator fail.

[0015] The known steering systems with force feedback actuators exhibit torque jumps or fluctuations, particularly when switching the power supply from one phase system to another phase system, which are disruptive to proper control of the vehicle while the vehicle is moving.

[0016] Based on this prior art, the invention is therefore based on the object of specifying a method for operating a steering system of a vehicle with a force feedback actuator, the driver of which remains unaffected by system-related torque or reaction force changes on the steering handle while driving.

[0017] The problem is solved by a method for operating a steering system having the entirety of the features of patent claim 1.

[0018] The invention relates to a method for operating a steering system of a vehicle having a force feedback actuator that acts on a steering handle to map reaction forces of at least one steered wheel of the steering system as a function of at least one road surface condition, a vehicle speed, or a current steering angle of the steered wheel. The force feedback actuator comprises an electric machine that can be driven by at least a first phase system and a second phase system, and switching of the at least first phase system and / or at least second phase system is carried out by means of a control and / or regulating device of the steering system. Switching of a phase system is understood here to mean using or activating it.

[0019] Alternatively, switching from one phase system to another can occur when a substantially stationary steering handle is detected and / or when the vehicle is traveling straight ahead. This has the advantage of preventing a torque or force surge from the force feedback actuator and its impact on the steering handle. This prevents the driver from experiencing irritating haptic feedback on the hand-held steering handle while driving. Another advantage is that using different phase systems at different times reduces the load on each individual phase system, thus increasing their service life. Furthermore, if one phase system fails, another phase system can replace the failed phase system. This also increases operational reliability.By using several phase systems at the same time, a higher torque can be advantageously generated.

[0020] Furthermore, switching between the at least first phase system and the at least second phase system occurs depending on a sensor evaluation and / or a state of the vehicle. A sensor evaluation describes, for example, the sensory detection of rotational speeds and / or angular positions of the steering handle and / or the wheels, the vehicle speed, people sitting in the vehicle, in particular the driver, the open, closed or locked door state, the steering wheel position and / or the status and / or status change of the ignition. The advantage here is that switching can take place unnoticed by the driver, for example. This can contribute to increased safety and a better driving experience by avoiding irritation to the driver.

[0021] Preferably, the phase systems are switched based on a steering wheel characteristic curve of the steering system. Particularly preferably, the steering wheel characteristic curve is a speed-torque characteristic curve. For this purpose, the steering wheel characteristic curve for the steering handle generally has a steering range for operating the steering system while driving and a holding range for providing entry and exit assistance, with the steering handle being mechanically fixed relative to the vehicle by the force feedback actuator. A change between the holding range and steering range can occur, for example, at essentially 360° / s. The advantage here is that a steering wheel characteristic curve allows the phase systems to be switched without the driver noticing.

[0022] It is therefore the steering range of the characteristic curve in question that is designed here to exhibit no torque jump. This also includes reaching the end stop positions (electrical or mechanical) of the steered wheel(s) of the vehicle. The force feedback actuator with its associated control and / or regulating device and corresponding sensors, such as the steering angle sensor, steering torque sensor, and the like, are subjected to less stress by the switching from one phase system to the other according to the invention. They are subjected to less thermal stress, and their availability and service life are improved or extended. If the steering system is functioning properly, the vehicle driver will not even notice when switching from one phase system to the other.

[0023] Since steering systems can have different designs, especially different steering trapezes, and different self-resetting properties when exiting a corner, switching from one phase system to another can occur at different maximum steering angles depending on the respective steering system. For example, a steering system with a more indifferent design or characteristics allows switching between phase systems even at larger steering angles, as is possible with a steering system with greater self-resetting properties.

[0024] In an alternative embodiment of the method, a steering torque of no more than 0.1 Nm is detected in addition to the aforementioned steering parameters before switching from one phase system to the other, and switching is only performed when a very low steering torque or low wheel reaction forces are present. This also improves safety during vehicle operation.

[0025] Preferably, switching from one phase system to the other occurs independently of the vehicle speed while the vehicle is moving, allowing switching, for example, even when accelerating straight ahead on a highway or motorway. This allows even very long journeys to be carried out without any particular stress, particularly thermal stress, on components of the force feedback actuator and its control and / or regulation device, including converters.

[0026] While the vehicle is moving, preferably only one phase system of the electric machine of the force feedback actuator is energized and it can be switched from one phase system to the other phase system in a predeterminable time interval, but also adaptively controlled and depending on an individual load situation.

[0027] Furthermore, it is preferred that the switching from one phase system to the other phase system is carried out depending on further parameters, such as an operating time and / or a stress situation of the phase systems and the force feedback actuator.

[0028] In order to make the switching to another phase system unnoticed by the driver of the vehicle, a ramp function, in particular a torque ramp for the electric machine of the force feedback actuator, is provided in a preferred embodiment, which can be stored, for example, in a data storage device of the control and / or regulating device.

[0029] Switching a respective phase system with converters on or off enables a self-diagnosis function to be provided to check the electrical and electronic components, such as the windings and semiconductor elements of the converters, as well as all components that include the torque-generating or flux-generating current components of the electrical machine's control system. As an example, the application of a d-current while analyzing the respective phase current and / or the speed and / or torque of a rotor of the electrical machine is mentioned here as part of the self-diagnosis.

[0030] Regarding the term d-current, it should be noted that three-phase electrical machines are known to have one or more three-phase systems, with such a three-phase system being represented in a spatial coordinate system with the U, V, and W axes. Using a so-called d / q transformation, such a three-phase system can be converted into a two-phase coordinate system with the d and q axes. In contrast to the UVW coordinate system, the d / q coordinate system is not fixed, but rotates with the rotor of the three-phase machine, with the q-axis being perpendicular to the magnetic excitation in the rotor and the d-axis (d-current) being aligned parallel to the magnetic excitation in the rotor. The d-current therefore only leads to the amplification or attenuation of the magnetic field of the three-phase machine and has no influence on the torque developed by the three-phase machine or its rotor speed.

[0031] In a cost-effective, robust embodiment of the steering system, the electric machine of the force feedback actuator is designed as a three-phase machine with two phase systems of three phases each, with at least one converter, preferably with two B6 half-bridges, being provided for each phase system. PWM or PAM modulation methods can preferably be used to operate the electric machine of the force feedback actuator.

[0032] The converters are assumed to have ideal proportional behavior, whereby multi-variable current controllers can also be used for dynamic decoupling of the torque-generating and flux-generating current variables in order to eliminate torque ripple.

[0033] In order to implement a locking function for the steering handle, the method according to the invention provides for one, preferably two phase systems, each with three phases (a, b, c) and with a total of preferably six phases, to be energized, with at least one converter being provided per phase system and one or two phase systems implementing a holding function for the steering handle as soon as the opening of the driver's door of the vehicle is detected. After detection of an actuation of an ignition lock or a seat occupancy of the driver's seat, for example, it is possible to switch to just one phase system. Energization of at least two phase systems is also provided when electrical or mechanical end stop positions of the steered wheels are or have been reached. It is advantageous here that the driver can hold on to the steering handle when getting into and / or out of the vehicle.

[0034] Both phase systems, together with the control and / or regulating device, are suitable for implementing electronic limit stop functions of the steering system. The converters are constructed in a conventional manner with semiconductor elements in half-bridge or full-bridge technology. The electric motor is preferably a three-phase motor, but can also have other suitable designs, such as DC or AC motors.

[0035] Furthermore, it is preferred that the electrical and / or mechanical end stops of the wheel be indicated to the driver via at least two phase systems. This also allows the driver to visualize when the end stops of the steered wheels have been reached, thereby improving vehicle handling.

[0036] The invention is explained in more detail below using an exemplary embodiment. The figures show: Fig. 1 is a schematic view of a steer-by-wire steering system with which the method according to the invention can be carried out, Fig. 2 is a schematic view of a circuit of an electric machine for a force feedback actuator which is applied to the steering handle of the steering system in Fig. 1 and Fig. 3 shows an example of a steering wheel characteristic curve of the steer-by-wire steering system in Fig. 1 .

[0037] Fig. 1shows a schematic view of a steering system 1, which is designed as a steer-by-wire steering system and serves to steer steered wheels 4 of a passenger car (not shown in its entirety). A steering handle 3, designed as a steering wheel, is operatively connected to a shaft of a force feedback actuator 2 by means of a steering shaft. Furthermore, the steering handle 3 is operatively connected to a steering wheel angle sensor (not shown in detail). The force feedback actuator 2 and the steering wheel angle sensor are connected to a control and / or regulating device 8 via a control line 11, which transmits both sensor signals from the steering wheel angle sensor and control commands for the force feedback actuator 2.

[0038] The control and / or regulating device 8 is connected to further sensors, not shown, for measuring wheel steering angles, the driving speed or to sensors for measuring the seat occupancy of the vehicle and to further sensors which describe the respective current driving state and driving parameters of the vehicle.

[0039] A steering actuator 12, for example in the form of an electric machine that drives a recirculating ball nut and axially displaces a recirculating ball screw, serves to adjust the wheel steering angles of the steered wheels 4 and is connected to them in an articulated manner via a tie rod 13, 14 and wheel steering levers 15, 16.

[0040] The force feedback actuator 2 serves to haptically feed back the restoring forces of the steered wheels 4 to the driver of the vehicle (not shown) and enables authentic feedback about the current driving situation of the vehicle despite the lack of a mechanical connection between the wheels 4 and the steering handle 3.

[0041] The restoring torque of the force feedback actuator 2 is determined by the control and / or regulating device 8 from the determined driving speed, the current steering angle at the steering handle 3 and a reaction torque of the steered wheels 4 and a corresponding signal is impressed on the force feedback actuator 2.

[0042] Three-phase DC motors have proven particularly suitable for driving the force feedback actuator 2, as electrical machines 7. Their three phases a, b, and c are controlled via a respective converter 9, 10 using power semiconductors such as transistors or thyristors. This is done in combination with pulse width modulation.

[0043] Fig. 2 schematically illustrates a connection of an electrical machine 7 with such a converter circuit, wherein two converters 9, 10 are provided for two phase systems 5, 6, which can redundantly and alternately take over the operation of the machine 7.

[0044] How Fig. 2 shows, two converter half-bridges (Q 1 +Q 2 ), (Q 3 +Q 4 ), (Q 5 +Q 6 ) are provided for controlling the first phase system 5 and two converter half-bridges (Q 7 +Q 8 ), (Q 9 +Q 10 ), (Q 11 +Q 12 ) are provided for controlling the second phase system 6.

[0045] It may also be advantageous to use only one B6 bridge and two oppositely arranged transistors per phase a, b, c for each converter 9, 10 in order to save costs.

[0046] Since the operation of a three-phase system while the vehicle is moving offers haptic and acoustic advantages over the operation of a six-phase system, which may have more torque ripple and is more complex to represent in terms of structure, it is intended to operate the steering system 1 alternately with the first phase system 5 and the second phase system 6.

[0047] A switching process between the phase systems 5 and 6 carried out by the control and / or regulating device 8 takes place when a low steering speed, in particular straight-ahead driving, is detected, or when a small steering angle is present.

[0048] Thanks to this design measure, the driver ideally does not notice the switching, since this always occurs even with a very low steering torque, e.g., 0.1 Nm or less, applied to the steering handle 3. In principle, the current driving speed is irrelevant for the switching and can advantageously occur at specific time intervals.

[0049] This will Fig. 3 The steering range shown (dashed line) is enabled. This also prevents thermal overload of each three-phase system 5, 6 during operation of the steering system 1 and overall improves the availability and service life of the motor control of the electric machine 7.

[0050] The switching between phase systems 5 and 6 can also be carried out depending on the operating time of the steering system 1 during the journey or an overall operating time.

[0051] In order to ensure a further increase in comfort during operation of the steering system 1, the switching takes place taking into account a ramp function and in particular taking into account a torque ramp function for the shaft of the electric machine 7 of the force feedback actuator 2.

[0052] The currently deactivated phase system 5 or 6 can be subjected to self-diagnosis, thus further improving the operational reliability of the steering system 1. If one phase system 5 or 6 fails, operation can be safely maintained with the remaining phase system. Operating only one phase system 5 or 6 also ensures that no synchronization problems can occur between the converters 9, 10, thus avoiding haptic losses, for example due to torque ripple in the actuating force of the force feedback actuator 2.

[0053] How Fig. 3As shown, the method can also be used to implement a locking or holding function for the steering handle 3, which is illustrated by the solid line, the holding area. By energizing both phase systems 5, 6, a maximum value of approximately twice that achieved in steering mode—away from the end stops of the steered wheels—can be achieved. This allows the driver to support themselves on the steering wheel before and after the journey in order to get in or out of the vehicle. This also allows the driver to visualize when the end stops of the steered wheels have been reached. List of reference symbols

[0054] (1)Steering system (2)Force feedback actuator (3)Steering handle (4)Steered wheel (5)First phase system (6)Second phase system (7)Electrical machine (8)Control and / or regulating device (9)Converter (10)Converter (11)Control line (12)Steering actuator (13)Tie rod (14)Tie rod (15)Wheel steering arm (16)Wheel steering arm (Q 1- Q 12 )Converter half-bridge (a)Phase (b)Phase (c)Phase

Claims

1. Method for operating a steering system (1) of a vehicle, comprising a force feedback actuator (2) acting on a steering handle (3) to map reaction forces of at least one steered wheel (4) of the steering system (1) depending on at least one road surface condition, a vehicle speed, or a current steering angle of the steered wheel (4), the force feedback actuator (2) comprising an electric machine (7) drivable by at least one first phase system (5) and a second phase system (6), switching from the at least first phase system (5) and / or at least second phase system (6) taking place by means of an open-loop and / or closed-loop control device (8) of the steering system (1), switching between the at least first phase system (5) and the at least second phase system (6) taking place depending on a sensor evaluation and / or a state of the vehicle, characterized in that a steering torque is detected before switching, the steering torque not being greater than 0.1 Nm, and / or in that switching takes place when a substantially stationary steering handle is detected and / or when the vehicle is traveling straight ahead.

2. Method according to claim 1, characterized in that switching of the phase systems (5, 6) takes place based on a steering wheel characteristic curve of the steering system.

3. Method according to either of the preceding claims, characterized in that switching between the phase systems (5, 6) takes place while driving and independently of the particular driving speed of the vehicle.

4. Method according to any of the preceding claims, characterized in that while the vehicle is moving, switching is carried out from one phase system (5) to the other phase system (6) and vice versa at a predeterminable time interval.

5. Method according to any of the preceding claims, characterized in that switching from one phase system (5) to the other phase system (6) is carried out depending on further parameters, such as an operating time and / or a load situation of the phase systems (5, 6) and the force feedback actuator (2).

6. Method according to any of the preceding claims, characterized in that at predeterminable time intervals, the particular inactive phase system (5, 6) is subjected to a self-diagnosis, the self-diagnosis taking place by applying a d-current to amplify or weaken the magnetic field by analyzing the particular phase current and / or the speed and / or the torque of a rotor of the electric machine (7).

7. Method according to any of the preceding claims, characterized in that the electric machine (7) has at least two phase systems (5, 6) each having three phases (a, b, c), at least one converter (9, 10) being provided per phase system (5, 6), one or two phase systems (5, 6) implementing a holding function for the steering handle (3).

8. Method according to any of the preceding claims, characterized in that electrical and / or mechanical end stops of the wheel (4) are indicated to the driver via at least two phase systems (5, 6).