JP2005225355A - Steering device for vehicle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable quick steering reaction force to be generated and to prevent deterioration of durability and reliability of a steering system when a high load is inputted to a rack shaft in low speed traveling such as a curbstone collision of a tire in a steering device for a vehicle by a steer-by-wire system. <P>SOLUTION: The steering device for the vehicle is equipped with a controller 10 calculating a target turning torque of a turning motor 7 and a target steering reaction torque of a reaction force motor 3 based on the deviation between a target steering angle and an actual steering angle, a turning motor driver 11 outputting a control command obtaining the target turning torque for the turning motor 7, and a reaction force motor driver 12 outputting the control command obtaining a target steering reaction force torque against a reaction force actuator. The steering controller 10 sets a steady-state gain C (0) of a feedback controller C(s) to be a finite value when a vehicle speed v is not more than a predetermined value. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention belongs to a technical field of a vehicle steering apparatus using a so-called steer-by-wire system in which a handle and a steering mechanism are mechanically separated.

As a conventional steer-by-wire vehicle steering apparatus, there is known a vehicle steering apparatus that controls a steering reaction force according to an estimated turning load for the purpose of giving a feeling of response to the driver according to the turning load ( For example, see Patent Document 1).
Japanese Patent Laid-Open No. 10-310074

  In general, in feedback control in which the actual turning angle follows the target turning angle, the feedback gain is set to a large value in order to improve responsiveness. Therefore, since the motor current of the actuator includes a high-frequency vibration component that is uncomfortable for the driver's steering, a low-pass filter or the like is required to reduce this.

  As a result, by removing high-frequency vibration components using a low-pass filter, direct steering reaction force cannot be generated for high load input to the steering rack during low-speed driving such as tire curb contact, etc. There was a risk that the durability and reliability of the steel would decrease.

  The present invention has been made paying attention to the above problems, and its purpose is to generate a rapid steering reaction force when a high load is applied to the rack shaft during low-speed traveling, such as tire curb contact. An object of the present invention is to provide a vehicle steering device that can prevent a decrease in durability reliability of a steering system.

In order to achieve the above-mentioned object, in the present invention, in a steering apparatus for a vehicle by a steer-by-wire system including a steering actuator and a reaction force actuator,
A target turning angle calculating means for calculating a target turning angle based on the steering wheel angle;
A turning angle servo means for calculating a target turning torque of the turning actuator based on the target turning angle and the actual turning angle;
A steering driver that outputs a control command for obtaining a target steering torque to the steering actuator;
A target steering reaction force calculating means for calculating a target steering reaction force torque based on a deviation between the target turning angle and the actual turning angle;
A reaction force driver that outputs a control command to obtain a target steering reaction force torque to the reaction force actuator;
Provided,
The turning angle servo means sets a steady gain of feedback to a finite value when the vehicle speed is equal to or lower than a predetermined value.

  In the present invention, since the steady gain of the feedback at a low speed is set to a finite value, a deviation occurs between the target turning angle and the actual turning angle. When input, it is possible to quickly reflect the steering reaction force. Therefore, the driver feels the reaction force, stops or weakens steering, or the steering force is weakened by the reaction force itself. As a result, it is possible to prevent a decrease in durability reliability of the steering system due to continued steering.

  Hereinafter, the best mode for carrying out the vehicle steering system of the present invention will be described based on the first embodiment.

  FIG. 1 is an overall configuration diagram of a steer-by-wire vehicle to which the vehicle steering apparatus according to the first embodiment is applied.

  A reaction force motor (reaction force actuator) 3 that applies a reaction force to the handle 1 is connected to the handle 1. Further, the angle (handle angle) of the handle 1 is detected by the handle angle sensor 2. Tie rods 5 and 5 are provided between the steering rack 4 and the front wheel tires 6 and 6.

  A steering motor (steering actuator) 7 that generates torque for steering the tires 6 and 6 is connected to the steering rack 4 via a transmission 8. The turning angle sensor 9 detects the rotation angle of the turning motor 7 (≈the actual turning angle of the front wheels 6 and 6).

  The steering controller 10 calculates a reaction force command torque and a steering command torque based on the outputs of the steering wheel angle sensor 2, the vehicle speed sensor 13, and the steering angle sensor 9, and provides the steering motor driver (steering driver) 11 with the reaction force command torque. The steering command torque is output, and the reaction force command torque is output to the reaction force motor driver (reaction force driver) 12.

  The steered motor driver 11 controls the current of the steered motor 7 based on the steered command torque. The reaction force motor driver 12 controls the current of the reaction force motor 3 based on the reaction force command torque.

Next, the operation will be described.
[Steering control processing]
FIG. 2 is a flowchart showing the flow of the steering control process executed by the steering controller 10, and each step will be described below.

In step S1, a target tire turning angle δ ^* is calculated based on the steering wheel angle sensor 2 and other signals (corresponding to a target turning angle calculation means), and the process proceeds to step S2. As an example of a method for calculating the target tire turning angle δ ^* , the following equation (1) can be given.
δ * ＝ G (v) ・ θ… (1)
Here, θ corresponds to the steering wheel angle, and G (v) corresponds to the steering gear ratio. v represents the vehicle speed, and G (v) indicates that the vehicle speed is variable. Many other methods have been proposed, but since they are out of the scope of this patent, details are omitted.

  In step S2, the characteristic of the turning angle servo is determined based on the signal from the vehicle speed sensor 13 (corresponding to the turning servo means), and the process proceeds to step S3. Details of this will be described later.

In step S3, the target torque T _δ ^* of the steered motor 7 is calculated from the actual tire turning angle δ and the target tire turning angle δ ^* , which are signals from the turning angle sensor 9, and the process proceeds to step S4.

In step S4, the target torque _Tθ ^* of the reaction force motor 3 is calculated (corresponding to target steering reaction force calculation means), and this control is finished. Details of this will be described later.

  The contents of the steered motor driver 11 and the reaction force motor driver 12 are current servos. The design of the current servo uses the control system design theory. This is a known technique, and details are omitted here.

Next, details of step S2 and step S4 of the first embodiment will be described.
[Determination of turning angle servo characteristics]
First, the method for determining the turning angle servo characteristic in step S2 will be described. FIG. 3 shows a turning angle servo control block by a two-degree-of-freedom control system. In FIG. 3, P (s) represents a controlled object model and C (s) represents a feedback controller. F (s) represents the target value response of the actual turning angle δ with respect to the target turning angle δ ^* . The output of F (s) will be expressed as a target turning angle correction value δ ^** .

First, an example of P (s) derivation method is shown. The model around the turning axis is approximated by the following equation (2).
T _δ = I _δ・ s ² δ + C _δ・ sδ + ξN _f (s) / D _f (s) ・ δ… (2)
Here, s is the Laplace operator, and T _δ is the converted value around the turning torque of the turning motor torque, and I _δ is the turning value of the turning device including the motor, transmission, steering rack, tie rod, tire, etc. moment of inertia around the steering axis, the C _[delta] is a viscosity coefficient of around turning shaft of the turning device. Further, ξ is a caster rail, and N _f (s) / D _f (s) is a transfer function from the turning angle δ to the front wheel lateral force (total of left and right wheels).

When this is expressed as a transfer function, P (s) shown in the following equation (3) is obtained.
δ = [D _f (s) / {I _δ · s ² + C _δ · s + ξN _f (s)}] · T _δ = P (s) · T _δ … (3)
For example, F (s) has a first-order lag characteristic. When it is written as a transfer function, the following equation (4) is obtained.
F (s) = 1 / (T _a · s + 1) (4)
Here, Ta represents _a delay time. FIG. 4 shows the response of the target turning angle correction value δ ^** when the target turning angle δ ^* changes stepwise at time zero.

The feedback controller C (s) is designed so that the system composed of P (s) and C (s) is stable. However, the C (s) setting is changed according to the vehicle speed v. The input / output relationship of the feedback controller C (s) is shown in the following equation (5).
T _{δ_fb} ^* = −C (s) ・ (δ−δ ^** ) = − {N _{_fb} (s) / D _{_fb} (s)} (δ−δ ^** )… (5)
Here, s represents a Laplace operator. T _{δ_fb} ^* represents a feedback amount of the steered motor command torque T _δ ^* . The feedforward component is F (s) / P (s) · δ ^* in FIG.

At low speed, C (s) is set so that steady feedback with respect to the deviation δ-δ ^** becomes finite. That is, C (0) is set to be a finite value. An example of realizing C (s) by PD control (proportional + differential control) is shown in the following formula (6).
C (s) ＝ K _pδ + K _dδ・ s (6)
Here, K _pδ and K _dδ represent (proportional / differential gain). C (0) is called the steady gain of the feedback controller C (s).

The steady gain in Equation (6) is K _pδ . K _dδ is _set according to K _pδ so that the feedback system becomes stable.

The steady gain C (0) is increased as the speed increases. As in equation (7), C (0) may be infinite by including s in the denominator.
C (s) ＝ − N _{_fb} (s) / {s ・ D _{_fb} (s)}… (7)

FIG. 5 shows an example of the vehicle speed dependence of the steady gain.
As a result, at low speed, there is a possibility that a deviation occurs between the target tire turning angle correction value δ ^** and the actual tire turning angle δ. Therefore, if the vehicle speed range with a low steady gain is set wide, the range of rack axial force estimation described later is widened, but the range where the deviation δ-δ ^** is also widened.

[Reaction torque motor calculation]
Next, details of step S4 will be described. The reaction motor command torque is expressed by the following equation (8).
T _θ ^* = K _pθ・ (δ−δ ^** ) + K _dθ・ s ・ (δ−δ ^** )… (8)
The sign of the physical quantity related to rotation is assumed to be left rotation positive. That is, θ and δ are counterclockwise positive, and when _Tθ ^* is positive, the handle generates a reaction force of the handle in the direction in which the counterclockwise rotation occurs.

Consider a situation where the deviation δ-δ ^** is positive, that is, the actual turning angle is turned to the left of the target value. At this time, since the current is generated so that the reaction force is also cut to the left, the sign of K _pθ is set to be positive. In addition, since the current is generated so that the deviation δ-δ ^** changes in the positive direction, i.e., when the actual turning angle starts to turn to the left with respect to the target value, the sign of K _dθ Set to positive.

[Problems of conventional technology]
In the steer-by-wire vehicle steering apparatus, since there is no mechanical connection between the handle and the steering gear, a reaction force corresponding to the steering is generated on the handle using a reaction force actuator. The method of generating the reaction force of the steering wheel using the signals of the yaw rate sensor, acceleration sensor, and steering wheel angle sensor is also being studied, but these signals are used to inform the driver that the tire has touched the curb as a reaction force. Is not enough.

  In other words, since yaw rate and acceleration are physical quantities that represent vehicle behavior, there is a delay in the change that occurs after the tire contacts the curb, so the reaction force generated based on it also has a delay. This is because a feeling of reaction force cannot be obtained. Further, because of the steer-by-wire mechanism, the handle angle does not change even when an external force such as a curb is applied to the steering rack. Therefore, the external force cannot be generated by the reaction force control based on the handle angle.

  Therefore, as a means for generating the reaction force of the curb contact, it can be considered that the reaction force is generated by the rack axial force. For example, a constant multiple of the rack axial force is applied to the reaction force. Although it is conceivable to directly measure the rack axial force, an axial force sensor is generally difficult in terms of durability, reliability, and other mountability. Therefore, rack axial force estimation disclosed in “Steering control device” of JP-A-10-310074 is being studied.

  As described above, in steer-by-wire, the target tire turning angle is calculated based on the steering wheel angle and other information, and the actual tire turning angle is servo-controlled so as to coincide with the target tire turning angle. A tire steering motor is connected to the steering gear, and it is an outline of the above prior art to estimate the rack axial force based on the current flowing through the motor.

  Generally, the feedback gain is set to a large value so that there is no error between the actual tire turning angle and the target tire turning angle. As a result, the motor current includes a high frequency component as shown in FIG. If this constant multiple is applied to the reaction force, an unpleasant high-frequency vibration component is added to the reaction force, so it is necessary to reduce high-frequency noise using a low-pass filter or the like.

  As a result, a direct reaction force at the time of curb contact is generated in situations where the steering wheel is steered suddenly at low speed and the tire hits the curb at a low speed, or when a large rack axial force is applied to the steering gear by climbing on the curb at low speed Since the driver continues to steer, the durability reliability of the steering system including the steering gear may be lowered in some cases.

[Operation of steady gain at low speed]
In contrast, in the vehicle steering system of the first embodiment, the steady state gain C (0) of the feedback controller C (s) is set to a finite value at low speeds, and the rack axial force is quickly reflected in the steering reaction force. As a result, the steering wheel and the tire are cut back, and the rack axial force is reduced, so that it is possible to prevent a decrease in durability reliability of the front wheel steering system in the above situation. The reason will be described below.

At low speed, the steady gain of the steering angle servo control is a finite value, so that when the rack axial force is applied, a deviation occurs between the target tire turning angle correction value δ ^** and the actual tire turning angle δ. Become. The rack axial force _Fδ and deviation δ-δ ^** when using PD control have the following relationship.
F _δ = {− K _pδ・ (δ−δ ^** ) −K _dδ・ s ・ (δ−δ ^** )} ・ c _δ (9)

In the equation (9), c _δ is a coefficient for converting the steered motor torque into the rack axial force. As shown in equation (9), it can be seen that the deviation increases as the rack axial force increases. Equation (8), which is the reaction torque, is shown again.
T _θ ^* = K _pθ・ (δ−δ ^** ) + K _dθ・ s ・ (δ−δ ^** )… (8)

Deviation δ-δ ^** occurs when the tire is struck hard by curbing the steering wheel or when the curb is climbing up, and the reaction force is quickly reflected on the steering wheel by equation (8).

Here, the meaning of “quick” is supplemented. As in the PD control represented by the equation (9), since the deviation is allowed at a low speed, the deviation Δ−δ ^** does not switch between positive and negative. That is, since no high frequency noise is generated, even if the reaction force is given by the equation (8), no high frequency reaction force is generated. In the conventional estimation using current, a low-pass filter is required, and a quick reaction force cannot be generated. However, since a reaction force is applied without a low-pass filter as shown in Equation (8), a quick reaction force can be generated.

  As a result, the steering wheel generates a torque to be turned back, and the driver feels the reaction force to stop or weaken the steering, or the steering force is weakened by the reaction force itself. As a result, it is possible to effectively prevent a decrease in durability reliability of the front wheel steering system due to continued steering.

  In conclusion, by setting the steady gain of the feedback controller C (s) to be finite at low speeds and increasing it at higher speeds, quick reaction force control at low speeds can be achieved, and high-speed driving operation can be accurately performed. Both can be balanced. Further, by setting the steady gain of the turning angle servo at a low speed at a relatively low value, the rigidity of the turning is lowered, and as a result, it is possible to suppress the decrease in the durability of the front wheel turning mechanism due to the curb striking.

As described in the previous discussion, the steering amount tends to increase at lower speeds, and the accuracy thereof tends not to be required. Therefore, the allowable range of the deviation δ-δ ^** increases. That is, the steady gain C (0) of the turning angle servo can be set smaller as the speed decreases. Therefore, the deviation becomes large even with the same rack axial force, so that the reaction force feedback coefficients K _pθ and K _dθ can be set small in order to generate the same reaction force. As a result, the influence of the deviation quantum error on the reaction force is reduced, and smoother reaction force control can be realized.

Next, the effect will be described.
With the vehicle steering apparatus according to the first embodiment, the following effects can be obtained.

(1) The steering controller 10 sets the steady gain C (0) of the feedback controller C (s) to a finite value when the vehicle speed v is less than or equal to a predetermined value. Therefore, when the rack axial force is applied, the target tire is set. There is a deviation between the turning angle correction value δ ^** and the actual tire turning angle δ, and the reaction force is quickly reflected in the steering wheel 1, and as a result, the steering is continued by the driver and the durability reliability of the steering system is reduced. Can be prevented.

(2) Since the steering controller 10 sets the steady gain C (0) of the feedback controller C (s) to a larger value as the vehicle speed v increases, the target tire turning angle correction value δ ^* increases as the vehicle speed v increases ^. Deviation between ^* and actual tire turning angle δ can be reduced. Therefore, it is possible to achieve both the quick reaction force control at the low speed and the accuracy of the driving operation in the high-speed traveling region.

(3) The steering controller 10 includes the reaction force motor command torque T _θ ^* including a constant multiple of the deviation between the target tire turning angle correction value δ ^** and the actual tire turning angle δ, and a differential value of the deviation (formula (Refer to (8)), the reaction force can be generated more quickly than in the prior art in which the reaction force is applied through the low-pass filter.

1 is an overall configuration diagram of a steer-by-wire vehicle to which a vehicle steering apparatus according to a first embodiment is applied. 4 is a flowchart showing a flow of steering control processing executed by the steering controller 10. It is a turning angle servo control block by a two-degree-of-freedom control system. It is a figure which shows the response of target turning angle correction value (delta) ^** . It is a figure which shows the vehicle speed dependence of a steady gain. It is a figure which shows the frequency characteristic of the motor current when a feedback gain is set large.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Handle 2 Handle angle sensor 3 Reaction force motor 4 Steering rack 5 Tie rod 6 Front wheel tire 7 Steering motor 8 Transmission 9 Steering angle sensor 10 Steering controller 11 Steering motor driver 12 Reaction force motor driver 13 Vehicle speed sensor

Claims (3)

A steering gear for steering the front wheels mechanically separated from the steering wheel that receives the steering input;
A steering actuator that is coupled to the front-wheel steering steering gear and generates torque for steering the front wheels;
A reaction force actuator that generates a torque that applies a steering reaction force to the handle;
In a vehicle steering apparatus comprising:
A target turning angle calculating means for calculating a target turning angle based on the steering wheel angle;
A turning angle servo means for calculating a target turning torque of the turning actuator based on the target turning angle and the actual turning angle;
A steering driver that outputs a control command for obtaining a target steering torque to the steering actuator;
A target steering reaction force calculating means for calculating a target steering reaction force torque based on a deviation between the target turning angle and the actual turning angle;
A reaction force driver that outputs a control command to obtain a target steering reaction force torque to the reaction force actuator;
Provided,
The steering angle servo means sets a steady gain of feedback to a finite value when the vehicle speed is a predetermined value or less. The vehicle steering apparatus according to claim 1,
The steering angle servo means sets a steady gain of feedback to a larger value as the vehicle speed increases. The vehicle steering apparatus according to claim 1 or 2,
The target steering reaction force calculation means sets the target steering reaction force torque in a form including a constant multiple of the deviation between the target turning angle and the actual turning angle and a differential value of the deviation. apparatus.

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