KR20210003570A - Steering control method and control system of motor driven power steering system - Google Patents

Abstract

The present invention is to provide an electric power steering control method to which a rack force estimator reflecting friction torque is applied, and a control system thereof. According to the present invention, provide is an electric power steering control method comprising the steps of: estimating a friction torque acting between the ground and a wheel rotated by steering; estimating a rack force acting on a rack gear based on a rack force estimation model using a yaw rate, a lateral acceleration, and the estimated friction torque as input variables; and calculating a target steering torque based on a virtual steering system model using the estimated rack force.

Description

Electric power steering control method and control system {STEERING CONTROL METHOD AND CONTROL SYSTEM OF MOTOR DRIVEN POWER STEERING SYSTEM}

The present invention relates to an electric power steering control system and a control method in which a rack force estimator reflecting friction torque to rack force acting on a rack gear is applied.

The existing open-loop based MDPS (Mortor Driven Power Steering) control has a disadvantage in that its performance may vary depending on the distribution of hardware, and a lot of repetitive tuning is required to secure the desired target steering performance.

However, such open-loop-based control can be overcome through closed-loop-based feedback control.

That is, the feedback control has the advantage of improving the robustness of control and tuning efficiency compared to the open loop method by generating a target steering torque to be controlled in a look-up table method and controlling the feedback.

However, in the case of such feedback control, it is difficult to predict the performance of the control logic in the early stage of designing the feedback controller, and thus a steering system system using a virtual steering system model was used to improve development efficiency.

However, the rack force estimator according to the prior art estimates the lag force based on the yaw rate and the lateral acceleration, so that the target if the yaw rate and lateral acceleration are measured very small in a stationary state or a low-speed driving condition. As the steering torque is reduced and the steering wheel feels heavy, discomfort is caused, and there is a limit that the freedom to generate the target steering torque is also reduced.

The matters described as the background art are only for enhancing an understanding of the background of the present invention, and should not be taken as acknowledging that they correspond to the prior art already known to those of ordinary skill in the art.

KR 10-2017-0019669 A

The present invention has been proposed to solve this problem, and is to provide an electric power steering control method and control system to which a rack force estimator reflecting friction torque is applied.

An electric power steering control method according to the present invention for achieving the above object comprises: estimating a friction torque acting between a wheel rotated by steering and a ground; Estimating a rack force acting on the rack gear based on a rack force estimation model using yaw rate, lateral acceleration, and estimated friction torque as input variables; And calculating a target steering torque based on a virtual steering system model using the estimated rack force.

In the step of estimating the friction torque, the friction torque acting in a direction preventing the rotation of the wheel rotated by the movement of the rack gear may be estimated based on the steering angular velocity of the steering wheel.

In the step of estimating the friction torque, a friction gain that decreases in size as the driving speed of the vehicle increases may be reflected in the estimated friction torque.

In the step of estimating the lag force, the yaw rate and the lateral acceleration may be measured values measured by a sensor.

In the step of estimating the rack force, the rack force estimation model is a feedback control model in which the rack force, friction torque, and lateral acceleration are input and the yaw rate is output. The yaw rate, friction torque, and lateral acceleration are determined by the Youla transfer function. It can be converted into a feed forward control model in which input and lag force is output.

In the step of estimating lag force, the lag force may be estimated using the following transfer function.

,

,

이고,

는 요 레이트,

는 횡가속도,

는 차량의 관성모멘트,

는 바퀴의 트레일, m은 차량의 질량, a 및 b는 각각 차량의 무게중심과 앞바퀴 및 뒷바퀴의 회전축 사이의 거리, l는 랙기어와 앞바퀴의 조향축 사이의 이격거리(effective arm), τ는 시상수이다.here,

,

,

ego,

The yo rate,

Is the lateral acceleration,

Is the moment of inertia of the vehicle,

Is the trail of the wheel, m is the mass of the vehicle, a and b are the distances between the vehicle's center of gravity and the rotational axes of the front and rear wheels, respectively, l is the effective arm between the rack gear and the steering axis of the front wheel, τ is It is a time constant.

In the step of calculating the target steering torque, the virtual steering system model may calculate the target steering torque using a state equation using the steering angular velocity and the rack force acting on the rack gear as input variables.

In the step of calculating the target steering torque, the state equation can be derived by setting the torsional displacement of the column, the momentum of the rack gear, the momentum of the steering wheel, and the transfer displacement of the rack gear as state variables.

In the step of calculating the target steering torque, the target steering torque may be calculated by reflecting the assist gain set as a variable of the vehicle speed.

After the step of calculating the target steering torque, the step of feedback-controlling the steering motor to follow the calculated target steering torque; may further include.

The electric power steering control system according to the present invention for achieving the above object comprises: a friction torque estimator for estimating a friction torque acting between a wheel rotated by steering and a ground; A rack force estimator for estimating a rack force acting on a rack gear based on a rack force estimation model using the yaw rate, lateral acceleration, and friction torque estimated by the friction torque estimation unit as input variables; And a steering torque calculation unit that calculates a target steering torque based on a virtual steering system model using the estimated rack force.

It may further include a motor control unit for controlling the steering motor using the target steering torque calculated by the steering torque calculation unit.

According to the electric power steering control method and control system of the present invention, handling performance and convenience are secured by reflecting the friction torque between the wheel and the ground in a stationary or low-speed driving condition.

In addition, by calculating a target steering torque based on a virtual steering system model, it is possible to predict steering performance, thereby improving the development efficiency of steering control technology.

1 is a flow chart of an electric power steering control method according to an embodiment of the present invention.
2 is a block diagram of an electric power steering control system according to an embodiment of the present invention.
3 shows a friction torque model according to an embodiment of the present invention.
4 is a graph of a friction gain according to an embodiment of the present invention.
5 illustrates a bicycle model according to an embodiment of the present invention, and FIG. 6 is a bond graph according to the bicycle model of FIG. 5.
7A is a diagram of a feedback control model for rack force according to an embodiment of the present invention, and FIG. 7B is a diagram of a feed forward control model converted from the control model of FIG. 7A.
8 is a graph showing the handling torque of a steering wheel according to a change in friction gain according to an embodiment of the present invention, and FIG. 9 is a target of a handling torque according to the prior art and the electric power steering control method of the present invention. It is a graph comparing the actual.

Specific structural or functional descriptions of the embodiments of the present invention disclosed in this specification or application are exemplified only for the purpose of describing the embodiments according to the present invention, and the embodiments according to the present invention may be implemented in various forms. And should not be construed as limited to the embodiments described in this specification or application.

Since the embodiments according to the present invention can be modified in various ways and have various forms, specific embodiments are illustrated in the drawings and will be described in detail in the present specification or application. However, this is not intended to limit the embodiments according to the concept of the present invention to a specific form of disclosure, and it should be understood that all changes, equivalents, and substitutes included in the spirit and scope of the present invention are included.

Terms such as first and/or second may be used to describe various components, but the components should not be limited by the terms. The terms are only for the purpose of distinguishing one component from other components, for example, without departing from the scope of the rights according to the concept of the present invention, the first component may be named as the second component, and similarly The second component may also be referred to as a first component.

When a component is referred to as being "connected" or "connected" to another component, it is understood that it may be directly connected or connected to the other component, but other components may exist in the middle. Should be. On the other hand, when a component is referred to as being "directly connected" or "directly connected" to another component, it should be understood that there is no other component in the middle. Other expressions describing the relationship between components, such as "between" and "just between" or "adjacent to" and "directly adjacent to" should be interpreted as well.

The terms used in this specification are used only to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In the present specification, terms such as "comprise" or "have" are intended to designate the presence of a set feature, number, step, action, component, part, or combination thereof, but one or more other features or numbers It is to be understood that the presence or additional possibilities of, steps, actions, components, parts, or combinations thereof are not excluded in advance.

Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. Terms as defined in a commonly used dictionary should be interpreted as having a meaning consistent with the meaning in the context of the related technology, and should not be interpreted as an ideal or excessively formal meaning unless explicitly defined in the present specification. .

Hereinafter, the present invention will be described in detail by describing a preferred embodiment of the present invention with reference to the accompanying drawings. The same reference numerals in each drawing indicate the same member.

The electric power steering system applicable to the present invention is a steering system that generates steering force or assists the steering force using an electric motor, and may be a Motor Driven Power Steering (MDPS) system or a Steer by wire (SBW) system.

In particular, the present invention relates to a method and system for setting a target steering torque (T _{q_ref} ) for controlling an electric power steering system including an electric motor, and controlling the electric power steering system accordingly.

1 is a flowchart of a method for controlling electric power steering according to an embodiment of the present invention, and FIG. 2 is a configuration diagram of an electric power steering control system according to an embodiment of the present invention.

1 to 2, a method for controlling electric power steering according to an embodiment of the present invention includes estimating a friction torque acting between a wheel rotated by steering and a ground; Estimating a rack force acting on the rack gear based on a rack force estimation model using yaw rate, lateral acceleration, and estimated friction torque as input variables; And calculating a target steering torque based on a virtual steering system model using the estimated rack force.

According to the present invention, by estimating the rack force by reflecting the friction torque between the wheel or tire and the road surface, the target steering torque is reduced in a stopped state or a low-speed driving condition that occurs in the case of the electric power steering control method according to the prior art. You can solve the problem. Accordingly, the driver's handling performance and convenience are secured.

In particular, the target steering torque is calculated to be small because the yaw rate and the lateral acceleration are very small or measured as a value of zero in a stopped state or a low-speed driving condition.

The friction torque to solve this problem is a torque according to the Scrub-friction effect, which is a torque acting on a wheel that rotates as the rack gear moves in the left and right directions.

In the step of estimating the lag force, the lag force may be estimated based on a lag force estimation model using the yaw rate, the lateral acceleration, and the estimated friction torque as input variables.

In the step of estimating the lag force according to an embodiment, the yaw rate and the lateral acceleration may be measured values measured by a sensor.

That is, the yaw rate and the lateral acceleration may be sensed using the yaw rate sensor and the lateral acceleration sensor included in the vehicle.

3 shows a friction torque model according to an embodiment of the present invention.

Referring further to FIG. 3, in the friction torque model according to an embodiment of the present invention, the front wheel of a vehicle is steered by a rack gear, and the following state equation may be established.

는 랙포스,

는 랙포스와 조향축 사이의 이격거리(effective arm),

는 바퀴의 트레일(Trail),

는 앞바퀴에 작용하는 횡력(side force),

는 마찰토크(Scrub-friction Torque)이다.here,

Is the rack force,

Is the effective arm between the rack force and the steering shaft,

Is the trail of the wheel,

Is the side force acting on the front wheel,

Is the Scrub-friction Torque.

)은 앞바퀴를 옆에서 보았을 때 킹핀 중심선이 노면을 가로지르는 점과 타이어 중심선이 노면에서 교차하는 점과의 거리를 말한다.The trail of the wheel (

) Refers to the distance between the point where the kingpin center line crosses the road surface and the point where the tire center line crosses the road surface when viewed from the side.

This is summarized in terms of rack force as follows.

Specifically, in the step of estimating the friction torque, the friction torque acting in a direction that prevents the rotation of the wheel rotated by the movement of the rack gear may be estimated based on the steering angular velocity of the steering wheel.

), 속도에 대한 Viscous를 반영한 viscous 마찰력(

)에 의해 추정할 수 있다.The friction torque can be calculated using the steering angular velocity as an input variable, for example, the Coulomb friction force (

), viscous friction force reflecting Viscous against speed (

) Can be estimated.

In addition, various friction models, such as Darl Friction Model or Lugre Friction Model, in which the friction force is expressed as a differential equation for the Stress-Strain curve can be used. In addition, the friction torque may be estimated using a nonlinear model reflecting the static friction phenomenon and the Stribeck effect caused by the stick-slip.

4 is a graph of a friction gain according to an embodiment of the present invention.

Referring further to FIG. 4, in the step of estimating the friction torque, a friction gain that decreases in size as the driving speed of the vehicle increases may be reflected in the estimated friction torque.

Since the value of the friction torque becomes very small as the driving speed of the vehicle increases, the friction gain can be drastically reduced as the driving speed increases.

Accordingly, the friction torque is largely reflected in a vehicle stop state or a low-speed condition, and the influence of the friction torque may be reduced in a high-speed condition.

5 illustrates a bicycle model according to an embodiment of the present invention, and FIG. 6 is a bond graph according to the bicycle model of FIG. 5.

5 to 6, the equation of motion according to the bicycle model according to an embodiment of the present invention is as follows.

및

는 각각 회전관성 및 측방에 대한 모멘텀 변화율이고,

및

는 각각 앞바퀴 및 뒷바퀴의 횡력이며,

는 요 레이트(Yaw Rate)이다.here,

And

Is the rate of change of momentum for rotational inertia and lateral, respectively,

And

Is the lateral force of the front and rear wheels, respectively,

Is the yaw rate.

)와 측방 모멘텀 변화율(

) 사이의 관계는 아래와 같다.Also, lateral acceleration (

) And lateral momentum rate of change (

The relationship between) is as follows.

Substituting this into the above formula ②, the following formula can be derived.

Again, by substituting this into the above equation ①, the following equation can be derived.

)에 관한 수식으로 정리할 수 있다.If this is summarized by substituting the following equation for the rack force, the rate of change of momentum for the rotational inertia (

) Can be summarized as an equation.

)은 요 레이트(Yaw Rate) 변화율과 아래와 같은 관계식을 갖는다.In addition, the rate of change of momentum for rotational inertia (

) Has the following relationship with the rate of change of the yaw rate.

Therefore, the above equation is summarized as a transfer function as follows.

7A is a diagram of a feedback control model for rack force according to an embodiment of the present invention, and FIG. 7B is a diagram of a feed forward control model converted from the control model of FIG. 7A.

7A to 7B, in the step of estimating the rack force, the rack force estimation model is a feedback control model in which the rack force, friction torque, and lateral acceleration are input and the yaw rate is output by the Youla transfer function. It can be converted into a feed forward control model in which yaw rate, friction torque, and lateral acceleration are input and lag force is output.

As illustrated in FIG. 7A, in order to estimate the lag force according to an embodiment of the present invention, a diagram of a control output observer (COO) scheme may be used.

Here, the transfer function according to each input variable may be as follows.

However, the lag force to be estimated here has a feedback control structure in which the yaw rate is output by being input rather than an output.

This can be converted into a feed forward control model in which yaw rate, friction torque, and lateral acceleration are input and lag force is output, as shown in FIG. 7B.

Specifically, the feedback control model may be converted into a feed forward control model by a Youla transfer function. Accordingly, it may be redefined as a transfer function for the rack force so that the rack force is output. If redefined as a transfer function for rack force in the feedback control model, it is as follows.

In particular, Gp1 is an integrator having a gain, and since the pole of s=0 is unstable, the transfer function Y can be set by Youla-Kucera Parameterization (YKP), and the transfer function related to the rack force is as follows.

Here, the target plant transfer function (Gp1) is as follows, and the closed-loop transfer function is as follows. Here, τ is a time constant and determines the bandwidth of the entire estimator system.

Accordingly, the transfer function Y can be defined as follows.

Therefore, in the step of estimating the lag force, the lag force may be estimated using the following transfer function.

,

,

이고,

는 요 레이트,

는 횡가속도,

는 차량의 관성모멘트,

는 바퀴의 트레일, m은 차량의 질량, a 및 b는 각각 차량의 무게중심과 앞바퀴 및 뒷바퀴의 회전축 사이의 거리, l는 랙기어와 앞바퀴의 조향축 사이의 이격거리(effective arm), τ는 시상수이다.here,

,

,

ego,

The yo rate,

Is the lateral acceleration,

Is the moment of inertia of the vehicle,

Is the trail of the wheel, m is the mass of the vehicle, a and b are the distances between the vehicle's center of gravity and the rotational axes of the front and rear wheels, respectively, l is the effective arm between the rack gear and the steering axis of the front wheel, τ is It is a time constant.

In the step of calculating the target steering torque, the virtual steering system model may calculate the target steering torque using a state equation using the steering angular velocity and the rack force acting on the rack gear as input variables.

In the step of calculating the target steering torque, the state equation can be derived by setting the torsional displacement of the column, the momentum of the rack gear, the momentum of the steering wheel, and the transfer displacement of the rack gear as state variables.

Specifically, in the virtual steering system model, the steering angular velocity (ω _sw ) and the rack force (F _rack ) are applied as input variables, and the inertia of the steering wheel (J _sw ) and the stiffness of the column used as the reaction force mechanism (K _t ), column damper (B _t ), column friction torque (T _{fric_c} ), pinion radius (R _p ), and rack gear weight (M _r ) are applied as system characteristic parameters, input variables and system characteristics Target steering torque (T _{q_ref} ) can be applied as an output variable according to the relationship of parameters.

The state equation can be derived using a bond graph for a virtual steering system model, and it can be expressed as follows by exemplifying a bond graph.

In addition, in the step of calculating the target steering torque, the state equation can be derived by setting the torsional displacement of the column, the momentum of the rack gear, the momentum of the steering wheel and the transfer displacement of the rack gear as state variables.

Specifically, the state equation is derived using the bond graph, which is the torsional displacement of the column (q ₅ ), the momentum of the rack gear (P ₁₀ ), the momentum of the steering wheel (P ₂ ) and the rack gear. It can be derived by setting the transfer displacement of (q ₁₃ ) as a state variable, and for example, it can be calculated as follows.

The present invention calculates the target steering torque (T _{q_ref} ) acting on the column through the numerical integration of the state equation, and in particular, the target steering torque (T _{q_ref} ) can be calculated by the following equation (1).

.........(1)

.........(One)

T _{q_ref} : Target steering torque

K _t : column stiffness

q ₅ : Torsional displacement of column

B _t : Damper of the column

: 컬럼의 비틀림변위 변화율

: Column torsional displacement rate of change

By calculating the target steering torque (T _{q_ref} ) based on the virtual steering system model, the steering performance can be predicted to increase the development efficiency of the steering control technology, and the characteristics of the steering system system can be freely changed. By creating a sense of steering, tuning efficiency is improved.

When applied to the SBW system in which the mechanical connection structure between the steering wheel and the steering gearbox is disconnected, it is possible to generate a steering reaction force and steering feeling similar to that in which a mechanical steering system is installed.

In the step of calculating the target steering torque, the target steering torque may be calculated by reflecting the assist gain Ka _a set as a variable of the vehicle speed Vs. The assist gain (K _a ) may be determined by the gain determining unit 60 by receiving the driving speed Vs of the vehicle.

Specifically, the assist gain to the target steering torque (T _{q_ref)} which is calculated by the formula (K _a) for multiplying the assist gain (K _a) the change of the target steering torque (T _{q_ref)} can be configured to be in accordance with the have. This can be summarized as the following equation.

Here, 0 <assist gain (Ka) ≤ 1.

That is, when the value of the target steering torque (T _{q_ref)} being excessively high output, by applying the assist gain (K _a) it is possible to decrease the target steering torque (T _{q_ref).}

The assist gain Ka may be a value that varies depending on the steering angle and the driving speed of the vehicle. In one embodiment, the assist gain Ka may be pre-mapped to decrease as the steering angle increases, and may be pre-mapped to decrease as the driving speed of the vehicle increases.

After the step of calculating the target steering torque, the step of feedback-controlling the steering motor 40 to follow the calculated target steering torque; may further include.

That is, the motor controller 50 may feedback control the control amount of the steering motor 40 so that the actual steering torque input to the motor coincides with the target steering torque T _{q_ref} .

Referring back to Fig. 2, the electric power steering system according to an embodiment of the present invention includes a friction torque estimator 10 for estimating a friction torque acting between a wheel rotated by steering and the ground; A rack force estimator 20 for estimating a rack force acting on the rack gear based on a rack force estimation model using the yaw rate, lateral acceleration, and friction torque estimated by the friction torque estimation unit as input variables; And a steering torque calculation unit 30 that calculates a target steering torque based on a virtual steering system model using the estimated rack force.

It may further include a motor controller 50 for controlling the steering motor 40 using the target steering torque calculated by the steering torque calculation unit 30;

Here, the friction torque estimator 10, the rack force estimator 20, the steering torque calculation unit 30, and the motor controller 50 according to an exemplary embodiment of the present invention are configured to control the operation of various components of the vehicle. Through a nonvolatile memory (not shown) configured to store data related to an algorithm or software instructions for reproducing the algorithm, and a processor (not shown) configured to perform an operation described below using data stored in the memory. Can be implemented. Here, the memory and the processor may be implemented as separate chips. Alternatively, the memory and processor may be implemented as a single chip integrated with each other. A processor can take the form of one or more processors.

8 is a graph showing the handling torque of a steering wheel according to a change in friction gain according to an embodiment of the present invention, and FIG. 9 is a target of a handling torque according to the prior art and the electric power steering control method of the present invention. It is a graph comparing the actual.

In particular, FIGS. 8 to 9 are simulation test results through sine wave input when the vehicle is actually stopped.

8 to 9, according to the electric power steering control method according to an embodiment of the present invention, the more the friction gain is increased, the greater the reflection effect of the friction torque of the present invention occurs. Accordingly, the steering wheel handling You can see the phenomenon that the torque decreases.

That is, as the friction torque is reflected, the amount of the handling torque for the driver to rotate the steering wheel in the vehicle stopping condition is reduced, thereby demonstrating the effect of increasing steering convenience.

Although illustrated and described in connection with specific embodiments of the present invention, it is understood in the art that the present invention can be variously improved and changed within the scope of the technical spirit of the present invention provided by the following claims. It will be obvious to a person of ordinary knowledge.

10: friction torque estimator 20: rack force estimator
30: steering torque calculation unit 40: steering motor
50: motor controller

Claims (12)

Estimating a friction torque acting between the ground and the wheel rotated by steering;
Estimating a rack force acting on the rack gear based on a rack force estimation model using yaw rate, lateral acceleration, and estimated friction torque as input variables; And
An electric power steering control method comprising; calculating a target steering torque based on a virtual steering system model using the estimated rack force. The method according to claim 1,
In the step of estimating the friction torque, the electric power steering control method, characterized in that, based on the steering angular velocity of the steering wheel, the friction torque acting in a direction preventing rotation of the wheel rotated by the movement of the rack gear is estimated. The method according to claim 1,
In the step of estimating the friction torque, an electric power steering control method, characterized in that the estimated friction torque reflects a friction gain that decreases in size as the driving speed of the vehicle increases. The method according to claim 1,
In the step of estimating the rack force, the yaw rate and the lateral acceleration are measured values measured by a sensor. The method according to claim 1,
In the step of estimating the rack force, the rack force estimation model is a feedback control model in which the rack force, friction torque, and lateral acceleration are input and the yaw rate is output. The yaw rate, friction torque, and lateral acceleration are determined by the Youla transfer function. An electric power steering control method, characterized in that converted to a feed forward control model in which input and rack force is output. The method according to claim 1,
In the step of estimating the rack force, the electric power steering control method, characterized in that the rack force is estimated using the following transfer function.

here,

,

,

ego,

The yo rate,

Is the lateral acceleration,

Is the moment of inertia of the vehicle,

Is the trail of the wheel, m is the mass of the vehicle, a and b are the distances between the vehicle's center of gravity and the rotational axes of the front and rear wheels, respectively, l is the effective arm between the rack gear and the steering axis of the front wheel, τ is It is a time constant. The method according to claim 1,
In the step of calculating the target steering torque, the virtual steering system model calculates the target steering torque using a state equation that uses the steering angular velocity and the rack force acting on the rack gear as input variables. The method of claim 7,
In the step of calculating the target steering torque, the state equation is derived by setting the torsional displacement of the column, the momentum of the rack gear, the momentum of the steering wheel and the transfer displacement of the rack gear as state variables. Way. The method according to claim 1,
In the step of calculating the target steering torque, the target steering torque is calculated by reflecting the assist gain set as a variable of the vehicle speed. The method according to claim 1,
After the step of calculating the target steering torque, the step of feedback-controlling the steering motor to follow the calculated target steering torque; electric power steering control method further comprising a. A friction torque estimator for estimating a friction torque acting between a wheel rotated by steering and the ground;
A rack force estimator for estimating a rack force acting on a rack gear based on a rack force estimation model using the yaw rate, lateral acceleration, and friction torque estimated by the friction torque estimation unit as input variables; And
An electric power steering control system comprising a; steering torque calculation unit for calculating a target steering torque based on a virtual steering system model using the estimated rack force. The method of claim 11,
A motor controller for controlling the steering motor by using the target steering torque calculated by the steering torque calculator. The electric power steering control system further comprising.

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