Method for controlling an electromechanical steering system in a semi-autonomous driving mode of a steering system with position

Abstract

The invention relates to a method for controlling an electromechanical steering system (1) during semi-autonomous steering, comprising the following steps: determining a required motor torque (MotReqTrq) for controlling an electric motor (9) of a servo unit (10) of the electromechanical steering system (1), in which a force application adjustment (14) is carried out; the boost adjustment determines a required motor torque (MotReqTrq) as a function of a manual torque (TsuTrq) applied to the steering wheel and a vehicle speed (VhlSpd) and a steering column offset torque (TsuTrqoffset), wherein the steering column offset torque (TsuTrqoffset) reflects a position adjustment of an electric motor (9), a steering pinion or a rack.

Description

Technical Field

[0001] The invention relates to a method for controlling a steering system in a semi-autonomous or semi-automatic driving mode of an electromechanical steering system and to an electromechanical steering system of a motor vehicle. Background Art

[0002] Driver assistance systems are used in motor vehicles to assist the driver in performing various driving maneuvers. Semi-autonomous or semi-automatic driver assistance systems are known in the prior art. The vehicle's movements are controlled by the driver assistance system, but the driver can intervene and take over control. The driver assistance system includes a position controller for determining the additional steering torque. In contrast, the torque controller is responsible for driver control. If the driver intervenes in the steering while the driver assistance system is active, both controllers are used in combination to calculate the control variable for the electric motor. This can cause the two controllers to interfere with each other, resulting in undesirable vibrations that negatively impact the steering feel.

[0003] EP 284 2833 B1 implements a position controller in the form of a PID controller, the output of which is the requested motor torque. This requested motor torque is added to the required motor torque determined in a boost controller to determine the control variable for the electric motor. Summary of the Invention

[0004] It is an object of the present invention to provide a method for controlling an electromechanical steering system in a semi-autonomous or semi-automatic driving mode of the steering system, with an improved determination of the required motor torque.

[0005] This object is achieved by a method having the features of claim 1 and an electromechanical steering system having the features of claim 12 .

[0006] Therefore, a method for controlling an electromechanical steering system during a semi-autonomous or semi-automatic steering process is provided, comprising the following steps:

[0007] - determining the required motor torque for controlling the electric motor of the servo unit of the electromechanical steering system, wherein boost control (also known as Gain adjustment), which determines the required motor torque based on the manual torque applied to the steering wheel and the vehicle speed and the steering column offset torque, and wherein the steering column offset torque includes position adjustment of the electric motor, steering pinion or rack.

[0008] By performing a combination of position and torque regulation at the force regulation input, stability issues can be prevented.

[0009] Preferably, the method comprises an LQ regulator before the boost regulation, which determines the steering column offset torque either based on a reference position of the electric motor and a measured position of the electric motor, or based on a reference pinion position of a steering pinion of the steering system and a measured position of the steering pinion, or based on a reference rack position and a measured rack position.

[0010] Furthermore, the method advantageously includes a steering model for the boost control, which includes a boost control and a downstream steering model, wherein a gain curve of the boost control has different operating points and is linearized at these operating points, and a steering model exists for each of these operating points. It is preferably provided that the downstream steering model further transmits the measured electric motor position of the steering system or the measured pinion position of the steering pinion to the input of the LQ controller.

[0011] Preferably, the method comprises a state estimator which determines the state of the steering system (preferably comprising load and driver torque) and transmits this to the LQ regulator.The state estimator may comprise a Kalman filter.

[0012] In one embodiment, the LQ regulator defines a loss function that includes the square of the hysteresis error (the difference between the reference motor position or the reference pinion position or the reference rack position and the measured motor position or the measured pinion position or the measured rack position), the square of the integral of the hysteresis error, and the square of the steering column offset torque.

[0013] Furthermore, it can be provided that the method includes an integration unit and a damping unit, wherein the integration unit provides the integral of the hysteresis error and transmits it to the LQ controller, and the damping unit determines a damping torque that depends on the vehicle speed, other measured values ​​and a gain (English: boost gain) preferably derived from the boost control, wherein the gain is the tangent of the gain curve in the case of a fixed (TSU) torque.

[0014] The damping torque is preferably provided to reduce the influence of the integration of the hysteresis error when determining the steering column deflection torque in the LQ controller.

[0015] Furthermore, the method can include a steering algorithm that uses a reference motor torque and other measured values ​​to determine a required motor torque, which is added to the required motor torque determined using the boost control. The resulting value is preferably limited to a predetermined value range in a motor torque limiter and fed into the steering model.

[0016] Furthermore, an electromechanical steering system for a motor vehicle is provided, wherein the steering system comprises:

[0017] a pinion connected to the lower steering shaft, said pinion meshing with a rack for steering the wheels, said rack being movably supported in the housing along the longitudinal axis,

[0018] - at least one electric motor for power steering, and

[0019] An electronic control unit for calculating the steering assistance, which is designed to carry out the method according to one of the preceding claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Below, the embodiment of the present invention is described in detail with reference to the drawings. Here, identical or functionally identical components are provided with the same reference numerals in the drawings.

[0021] The accompanying drawings show:

[0022] Figure 1 : Schematic diagram of the electromechanical steering system, and

[0023] Figure 2 : Block diagram of a method for determining the required motor torque of an electric motor of an electromechanical steering system. DETAILED DESCRIPTION

[0024] exist Figure 1 The figure schematically illustrates an electromechanical power steering system 1 for a motor vehicle having a steering wheel 2, which is rotationally fixedly coupled to an upper steering shaft 3. The driver inputs a corresponding torque as a steering command into the steering shaft 3 via the steering wheel 2. This torque is then transmitted via the upper steering shaft 3 and the lower steering shaft 4 to the steering pinion 5. The pinion 5 meshes with the toothed segments of a rack 6 in a known manner. The rack 6 is mounted displaceably in the direction of its longitudinal axis in the steering housing. The rack 6 is connected at its free end to a tie rod 7 via a ball joint (not shown). The tie rod 7 is in turn connected to each steered wheel 8 of the vehicle via a steering knuckle in a known manner. Rotation of the steering wheel 2, via the connection between the steering shaft 3 and the pinion 5, causes a longitudinal displacement of the rack 6 and, consequently, a pivoting of the steered wheels 8. The steered wheels 8 are subjected to a reaction force from the roadway 800, which acts in the opposite direction to the steering movement. Therefore, a force is required to pivot the wheels 8, which in turn requires a corresponding torque on the steering wheel 2. The electric motor 9 of the actuating unit 10 is provided to assist the driver during this steering movement.

[0025] The upper steering shaft 3 and the lower steering shaft 4 are connected to each other in a rotationally fixed manner via a universal joint. The torque sensor unit 11 detects the torsion of the upper steering shaft 3 relative to the pinion 5 as the amount of torque manually applied to the steering shaft 3 or the steering wheel 2. The servo unit 10 provides steering assistance to the driver based on the torque measured by the torque sensor unit 11. The servo unit 10 can be connected to the steering shaft 3, the steering pinion 5 or the rack 6 as a power assistance device 100, 101, 10. The corresponding power assistance device 100, 101, 10 introduces the power assistance torque into the steering shaft 3, the steering pinion 5 and / or the rack 6, thereby assisting the driver in steering work. Figure 1 The three different power assist devices 10, 100, 101 shown in FIG. 1 show alternative positions for their arrangement. Typically, only one of the positions shown is occupied by a power assist device. The servo unit 10 has an electronic control unit 12 for calculating the steering assist.

[0026] In the electronic control unit 12 , the required motor torque for the electric motor of the servo unit is determined in order to reflect the desired steering feel to the driver.

[0027] When the steering system is in semi-autonomous or semi-automatic driving mode and both the position controller and the torque controller calculate the required motor torque, coordinating the combination of the two controllers is difficult because slight vibrations can occur. It has been found that better power and stability levels can be achieved if the combination of the two controllers is implemented at the input of the force boost control.

[0028] The boost control is a feed forward control. The motor torque required by the electric motor is calculated as MotReqTrq = f(TsuTrq, VhlSpd), where f is a static lookup table, TsuTrq is the manual torque at the steering wheel measured using a torque sensor (TSU), and VhlSpd is the vehicle speed. This means that there are no fault signals in this control loop that are minimized by the regulator. The values ​​in the lookup table f determine the steering feel and the stability of the control loop. The function f is called a gain map. At a predetermined vehicle speed, the gain map becomes a curve and is called a gain curve (English: boost curve):

[0029] MotReqTrq=f(TsuTrq).

[0030] The reason for the better performance and stability when combining the two regulators before the force boost control is that the force boost control can generate high motor torque variations when the measured steering column torque varies and the gain (tangent to the gain curve) is high (at high steering column torques). This means that the force boost control can react very sensitively to the measured steering column torque and, when combining the two regulators after the force boost control is performed and the combination is also controlled by the measured steering column torque, this can cause undesirable oscillations in the control loop.

[0031] However, if the combination is performed before the force control, the high sensitivity of the gain curve has less influence on the combination, allowing for a more stable control loop. To achieve the combined position and torque control before the force control, the steering column deflection torque is determined to be equivalent to the position or angle control of the servo unit's electric motor. To this end, an LQG (linear-quadratic-Gaussian controller) controller is implemented, which, together with the force control and motor controller, performs the position control function. These three controllers are connected in series.

[0032] Figure 2 An embodiment of the software architecture of the LQG regulator 13 for the boost control 14 is shown. The goal of the boost control is to generate an assist torque that assists the driver when steering the vehicle. The boost control is based on a gain curve or a gain map. The gain curve defines the static relationship between the input torque applied by the driver and the assist torque generated by the electric motor. In simple systems, where there is a linear relationship between the input torque and the output torque, the gain curve can have the form of a straight line. In practical applications, a simple "straight line" gain curve does not produce satisfactory results. Therefore, the gain curve is a nonlinear curve that must be linearized at multiple operating points.

[0033] A steering model 15 for force control has been developed, which includes a conventional steering model 16 (steering mechanics, motor torque dynamics of the electromechanical steering system) and force control 14 with a linearized gain curve. A steering model is provided for each operating point that needs to be linearized. An LQ regulator 17 (English: linear-quadratic controller) is used, which can perform position or angle control of the electric motor using the corresponding steering model 15 in a closed control loop. The LQ regulator 17 is a regulator with state feedback. Therefore, it is necessary to determine the state of the steering system. This task is undertaken by a state estimator 18, in particular a Kalman filter. Therefore, the proposed regulator is an LQG (LQ+Kalman) regulator 13.

[0034] The state estimator 18 calculates a reference motor torque RefMotTrq based on the required motor torque (MotReqTrq_Lim). The reference motor torque (RefMotTrq) is considered as a delayed required motor torque. To this end, the returned required motor torque (MotReqTrq_Lim) is transmitted to the state estimator 18 with a time delay by means of a delay unit 19.

[0035] As input signals, the LQ regulator 17 receives the state value (EstStates) estimated by the state estimator 18, the reference position (angle) of the electric motor, and the measured position (angle) of the electric motor, or equivalently, the rack position / steering column position / pinion position (RefPinAngRad, PinAngRad), as shown in the figure. The LQ regulator 17 determines the steering column offset torque (TsuTrq_offset), which is additionally input to the gain regulator 14 in addition to the measured value.

[0036] The LQG controller 13 is defined by a loss function that includes the square of the hysteresis error (the difference between the reference position (RefPinAngRad or reference motor angle or reference rack position) and the measured position (PinAngRad or motor angle or rack position)), the square of the integral of the hysteresis error, and the square of the steering column offset torque (TsuTrq_offset). An integration unit 20 is provided that determines the integral of the hysteresis error 21 (IntegralError) and passes it on to the LQG controller.

[0037] Integration can cause significant overshoot, so the integrated component can be reduced by a suitable factor. Overshoot can also be reduced by setting an additive damping torque. For this purpose, a damping unit 22 is provided, in which the damping is determined and fed into the LQ regulator 17. When the measured position is close to the reference position, the speed difference between the reference position and the measured position is large, and the measured position is inclined toward the reference position, the amount of integration is artificially reduced.

[0038] In such a situation, when the vehicle speed is low (below <10 km / h), the amount of integration is also reduced, for example by reducing the integral gain in the LQ regulator.

[0039] The additional damping torque determined by means of the damping unit 22 is dependent on the vehicle speed (VhlSpd), other measured values ​​and a boost gain derived from the boost control 14. At lower vehicle speeds, higher damping is required.

[0040] The boost control 14 calculates the required motor torque (MotReqTrq_boost) of the electric motor using the steering column offset torque (TsuTrq_offset). The boost control 14 has a stabilization filter (PDT1 link) to improve the stability margin of the control loop.

[0041] Other steering characteristics, such as steering wheel centering and damping, are implemented by a steering algorithm (SA) 23, the output of which is also a required motor torque (MotReqTrq_SA). The required motor torque determined by the steering algorithm 23 is added to the required motor torque (MotReqTrq_boost) determined by the gain adjustment 14 to calculate the required net motor torque (MotReqTrq). The required net motor torque is limited to a predetermined range by a motor torque limiter (MTL) 24 to ensure a desired safety level.

[0042] The required motor torque is achieved by the electromechanical steering system, which causes the rack position to change as a function of the required motor torque, the driver's manual torque (T_drv) and the load torque (T_load).

[0043] The required net motor torque (MotReqTrq_Lim) of the electric motor enters the downstream steering model 15. Furthermore, the driver's manual torque (T_drv) and the load torque (T_load) are incorporated into the steering model 15. The steering model 15 determines the pinion position (PinAngRad), which is fed as an input variable to the LQ controller 17. Furthermore, the motor angular velocity (MotAsp) and the measured motor torque (motor current, T_mot) as well as the steering column torque applied by the driver (T_TSU or TSDU torque) are measured and transmitted as measured values ​​25 to the corresponding inputs of the components in the control loop.

[0044] The following values ​​are preferably measured:

[0045] - steering column torque applied by the driver (T_TSU or TSDU torque),

[0046] -Motor torque (T_mot),

[0047] -Motor angular speed of the electric motor (MotAspRps) and

[0048] -Motor position, or equivalently steering column / pinion position (PinAngRad).

Claims

1. A method for controlling an electromechanical steering system (1) during a semi-autonomous or semi-automatic steering process, comprising the following steps: - determining a required motor torque (MotReqTrq) of an electric motor (9) for controlling a servo unit (10) of an electromechanical steering system (1), wherein an acceleration control (14) is performed, said acceleration control determining the required motor torque (MotReqTrq) as a function of a manual torque (TsuTrq) applied to the steering wheel and the vehicle speed (VhlSpd) as well as a steering column offset torque (TsuTrq_offset), wherein the steering column offset torque (TsuTrq_offset) reflects a position control of the electric motor (9), the steering pinion or the steering rack.

2. The method according to claim 1, characterized in that The method comprises an LQ regulator (17) which is connected upstream of the boost regulation (14) and which determines the steering column offset torque (TsuTrq_offset) based on a reference position of the electric motor and a measured position of the electric motor, or based on a reference pinion position (RefPinAngRad) of a steering pinion of the steering system and a measured position (PinAngRad) of the steering pinion, or based on a reference rack position and a measured rack position.

3. The method according to claim 1 or 2, characterized in that The method comprises a steering model (15) for a force adjustment (14), the steering model comprising a force adjustment (14) and a steering model (16) connected downstream, wherein a gain curve of the gain adjustment (14) has different operating points and is linearized at these operating points, and a steering model exists for each of these operating points.

4. The method according to claims 2 and 3, characterized in that The downstream steering model (16) transmits the measured position of the electric motor or the measured pinion position (PinAngRad) of the steering pinion of the steering system to the input of the LQ controller (17).

5. The method according to any one of the preceding claims, when dependent on claim 2, characterized in that The method comprises a state estimator (18) which determines a state of the steering system and transmits the state of the steering system to the LQ controller (17).

6. The method according to claim 5, characterized in that The state estimator (18) includes a Kalman filter.

7. The method according to any one of the preceding claims, when dependent on claim 2, characterized in that The LQ regulator (17) defines a loss function including the square of the hysteresis error, the square of the integral of the hysteresis error, and the square of the steering column offset torque (TsuTrq_offset).

8. The method according to claim 7, characterized in that The method comprises an integration unit (20) and a damping unit (22), wherein the integration unit (20) provides the integral of the hysteresis error and transmits it to the LQ regulator (17), and the damping unit (22) determines a damping torque that depends on the vehicle speed (VhlSpd), other measured values ​​and a gain derived from the boost control (14).

9. The method according to claim 8, characterized in that The damping torque is provided for reducing the influence of the integration of the hysteresis error when determining the steering column offset torque (TsuTrq_offset) in the LQ controller (17).

10. The method according to any one of the preceding claims, characterized in that The method comprises a steering algorithm (23) which determines a required motor torque (MotReqTrq_SA) by means of a reference motor torque and other measured values ​​(25), said required motor torque (MotReqTrq_SA) being added to a required motor torque (MotReqTrq_boost) determined by means of the boost control (14).

11. The method according to claim 10, characterized in that The value resulting from the addition is limited to a predetermined value range in a motor torque limiter (24), and the value resulting therefrom (MotReqTrq_Lim) is introduced into the steering model (16).

12. An electromechanical steering system (1) for a motor vehicle, comprising a pinion (5) connected to the lower steering shaft (4), said pinion meshing with a rack (6) for steering the wheels (8), said rack (6) being supported in the housing so as to be displaceable along the longitudinal axis, - at least one electric motor (9) for power steering, and An electronic control unit (12) for calculating the steering assistance, which is designed to carry out the method according to one of the preceding claims.

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