JP4240013B2 - Steering support device - Google Patents

Description

  The present invention relates to a steering assist device for assisting steering of a vehicle.

  There has been proposed a steering assist device that assists in traveling of a vehicle along a travel lane (see, for example, Patent Document 1). This steering assist device first acquires an image of a lane in which the host vehicle travels using a CCD camera or the like. Next, road lane information (white line) that divides the travel lane is detected from the acquired image by image recognition processing, thereby acquiring travel lane information that the host vehicle should travel. Then, based on the acquired travel lane information, a steering torque necessary for steering is obtained, and an appropriate assist torque is applied to assist the driver in steering.

In the technique of Patent Document 1, the curvature of the traveling lane, the lane offset that is the lateral deviation amount between the vehicle center line and the traveling path center line, and the differential value of the deflection angle that is the angle formed by the traveling path center line and the vehicle center line are used. Thus, an appropriate assist torque is calculated by calculating the steering torque.
JP 2001-10518 A

  By the way, the steering mechanism of the vehicle is connected by a gear box, a suspension joint or the like, and there is resistance (steering friction) due to the frictional force thereof. When the steering amount is to be increased, the steering friction is insufficient because the steering friction acts as a resistance to the output of the motor that applies the assist torque. On the other hand, when the steering amount is to be reduced, the steering friction acts as a resistance of a force for the tire to return in the straight direction, and thus works in the same way as when the output of the motor acts excessively. For this reason, the target assist torque and the assist torque that is actually applied do not match, and the controllability may be reduced.

  Therefore, it is conceivable to set different assist torques for each steering state, that is, a state in which the steering amount increases and a state in which the steering amount decreases in consideration of steering friction. In this case, it is necessary to determine the steering state and switch the assist torque setting value when the steering state changes. Here, as described above, when the assist torque is set based on the curvature of the traveling lane or the like, the turning circuit having a large curvature is more greatly affected by noise included in the detected value of the curvature than the straight road. Therefore, if the assist torque is switched on the turning circuit under the same condition as that of the straight road, the steering torque and the lane followability are changed due to the frequent change of the assist torque that is applied to the steering mechanism. May get worse.

  The present invention has been made to solve the above-described problems, and provides a steering assist device capable of applying an appropriate assist torque according to a steering state without deteriorating steering feeling and lane following ability. The purpose is to provide.

A steering assist device according to the present invention is a steering assist device that applies an assist torque to a steering mechanism of a vehicle so as to travel a predetermined position on a travel path, and calculates a target lateral acceleration of the vehicle based on a curvature of the travel path. A lateral acceleration calculating means; and an assist torque setting means for setting an assist torque based on the target lateral acceleration calculated by the target lateral acceleration calculating means. When the assist torque setting means is in a state in which the target lateral acceleration is increasing, When setting the assist torque to a larger value than when the target lateral acceleration is in a decreasing state, and determining whether or not the target lateral acceleration increase / decrease state has been switched for control , When the curvature is large, it is harder to determine that the increase / decrease state has been switched than when the curvature is small.

  When the absolute value of the target lateral acceleration is increased, that is, when the steering amount is increased, the assist torque is increased compared to when the absolute value of the target lateral acceleration is decreased, that is, when the absolute value of the steering amount is decreased. By increasing the value, it is possible to apply assist torque that matches the target lateral acceleration. The magnitude of the assist torque to be applied may be, for example, considering the steering friction. In the case of increasing the cut, the assist torque that is larger by the amount of steering friction is applied compared to the case where the steering friction is not considered. It is preferable to apply an assist torque that is smaller by the amount of steering friction than when steering friction is not taken into consideration. Further, when determining whether the target lateral acceleration has been switched from the increased state to the decreased state or from the decreased state to the increased state, when the curvature of the traveling road is large and the influence of noise or the like included in the detected value is more strongly affected, By making it difficult to determine that the increase / decrease state has been switched compared to when the curvature is small, it is possible to suppress fluctuations in the assist torque due to frequent switching. As a result, it is possible to suppress deterioration of steering feeling and lane following performance.

  The assist torque setting means switches the increase / decrease state when the target lateral acceleration changes from the increasing state to the decreasing state or from the decreasing state to the increasing state, and the amount of change in the target lateral acceleration exceeds the threshold value. It is preferable to increase the threshold value when the curvature of the traveling road is large than when the curvature is small.

  In this way, switching between the assist torque characteristic in the increasing direction and the assist torque characteristic in the decreasing direction is not performed instantaneously when the increase or decrease in the target lateral acceleration is switched, but after the increase or decrease is switched, the increase state or the decrease state is changed. Executes when it continues and the amount of change exceeds the threshold. By making this threshold value larger when the curvature of the traveling road is larger than when the curvature is small, it is possible to make switching difficult when the curvature of the traveling road is large than when the curvature is small.

  According to the present invention, when the target lateral acceleration is in the increasing state, the assist torque is set to be larger than when the target lateral acceleration is in the decreasing state, and when determining whether the target lateral acceleration increase / decrease state has been switched, When the curvature of the road is large, it is harder to determine that the increase / decrease state has been switched than when the curvature is small, so appropriate assistance according to the steering state without deteriorating the steering feeling and lane followability Torque can be applied.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments of the invention will be described in detail with reference to the accompanying drawings. In order to facilitate the understanding of the description, the same reference numerals are given to the same components in the drawings as much as possible, and duplicate descriptions are omitted.

  An embodiment of a steering assist device according to the present invention will be described below. FIG. 1 shows a configuration diagram of a vehicle 1 provided with the steering assist device of the present embodiment. The vehicle 1 includes an electronic control unit (hereinafter referred to as “ECU”) 2, and steering assist control (lane keeping control) is executed by the ECU 2. As shown in FIG. 1, the vehicle 1 includes a steering wheel 3. The steering wheel 3 is disposed in the vehicle interior of the vehicle 1 and steers steered wheels (here, left and right front wheels FR, FL) when operated by a driver. The steering wheel 3 is fixed to one end of the steering shaft 4. The steering shaft 4 rotates as the steering wheel 3 rotates.

  A rack bar 6 is connected to the other end of the steering shaft 4 via a steering gear box 5. The steering gear box 5 has a function of converting the rotational movement of the steering shaft 4 into the linear movement of the rack bar 6 in the axial direction. Both ends of the rack bar 6 are connected to the hub carriers of the wheels FL and FR via the knuckle arm 7. Because of this configuration, the wheels FL and FR are steered via the steering shaft 4 and the steering gear box 5 (rack bar 6) when the steering wheel 3 is rotated.

  Further, a CCD camera 8 for imaging the front is built in the rearview mirror (see FIG. 2). The CCD camera 8 photographs the surrounding situation in a predetermined area ahead through the front window 30 of the vehicle 1. Specifically, a moving image around the traveling lane 51 where the vehicle 1 on the road 50 is traveling is photographed. An image processing unit 9 is connected to the CCD camera 8. The image data of the surrounding situation photographed by the CCD camera 8 is supplied to the image processing unit 9. The image processing unit 9 performs image processing on image data from the CCD camera 8 and detects a travel lane (lane) based on a road marking line (hereinafter referred to as a white line) drawn on the road on which the vehicle 1 travels. To do. In the captured image or video, the brightness difference between the road surface and the white line drawn on it is large, so the white line that divides the driving lane is relatively easy to detect by edge detection etc. and detects the lane ahead of the vehicle Convenient for

  The image processing unit 9 is connected to the ECU 2 described above. Based on the detected lane, the image processing unit 9 calculates the curve curvature (χ = 1 / R) of the forward travel route, the offset D of the vehicle 1 with respect to the lane (the center in the longitudinal direction of the vehicle), as shown in FIG. This corresponds to the amount of lateral deviation between the axis 1a and the tangent line 51a at the vehicle centroid position of the center line of the travel lane 51.) and the yaw angle θ Corresponding to an angle formed with the tangent line 51a) is detected by calculation, and the result is sent to the ECU 2. The curve curvature χ, the offset D, and the yaw angle θ may take either positive or negative values, and the sign indicates the direction and direction. In the present embodiment, the right direction is “+” and the left direction is “−”. As a method of detecting various information amounts (curve curvature χ, vehicle offset D, yaw angle θ) of the forward travel route based on the image, a known method can be used.

  A steering angle sensor 10 and a vehicle speed sensor 11 are also connected to the ECU 2. The steering angle sensor 10 outputs a signal corresponding to the steering angle of the steering wheel 3. The vehicle speed sensor 11 is a wheel speed sensor attached to each wheel, and generates a pulse signal at a cycle corresponding to the speed of the vehicle 1. The output signal of the steering angle sensor 10 and the output signal of the vehicle speed sensor 11 are respectively supplied to the ECU 2. The ECU 2 detects the steering angle based on the output signal of the steering angle sensor 10 and detects the vehicle speed based on the output signal of the vehicle speed sensor 11.

  In addition, a yaw rate sensor 12 and a navigation system 13 are also connected to the ECU 2. The yaw rate sensor 12 is disposed near the center of gravity of the vehicle 1, detects the yaw rate around the center of gravity vertical axis, and sends the detection result to the ECU 2. The navigation system 13 is a device for detecting the position of the vehicle 1 using GPS or the like. The navigation system 13 also has a function of detecting a situation such as a curve curvature (χ) and a gradient in front of the vehicle 1. The ECU 2 uses the navigation system 13 to grasp the position of the vehicle 1 and the road situation expected to travel.

  Further, a motor driver 14 is also connected to the ECU 2. The motor driver 14 is connected to a motor (actuator) 15 disposed in the steering gear box 5 described above. Although not shown, a ball screw groove is formed on a part of the outer peripheral surface of the rack bar 6, and a ball nut having a ball screw groove corresponding to the ball screw groove on the inner peripheral surface of the rotor of the motor 15. Is fixed. A plurality of bearing balls are accommodated between the pair of ball screw grooves, and when the motor 15 is driven, the rotor rotates to assist the axial movement of the rack bar 6, that is, the steering.

  The motor driver 14 supplies a drive current to the motor 15 in accordance with a command signal from the ECU 2. The motor 15 applies an assist torque corresponding to the drive current supplied from the motor driver 14 to the rack bar 6. The ECU 2 supplies a command signal to the motor driver 14 according to the logic described later and drives the motor 15 to displace the rack bar 6 and steer the wheels FL and FR.

  Next, the steering assist control according to the present embodiment will be specifically described. FIG. 4 is a block diagram showing the operation of the steering assist control.

  First, the front situation of the vehicle 1 is imaged by the CCD camera 8, and the situation (curve curvature χ) of the travel lane 51, the offset D and the yaw angle θ of the host vehicle 1 are obtained by the image processing unit 9 based on the captured image. And are calculated. The curve curvature χ is obtained by geometrically obtaining the curvature R of the forward curve from the captured image and taking the reciprocal thereof. As a geometrical calculation method, it may be performed with reference to the lateral displacement amount of the white line in front of the host vehicle 1 at a predetermined distance and the inclination of the tangent line of the white line in front of the host vehicle 1 at a predetermined distance.

Further, the target offset and yaw angle with respect to the travel route are determined in advance as the target offset D ₀ and the target yaw angle θ ₀ .

In calculating the control amount to the motor driver 14, it is necessary to calculate the yaw rate ω as the control amount. The yaw rate omega is obtained as the sum of the yaw rate omega _theta compensating the yaw rate omega _d and the yaw angle theta for compensating the offset D in the yaw rate omega _r based on the curve curvature chi.

First, based on the vehicle 1 ahead of the curve curvature χ (= 1 / R), obtains the yaw rate omega _r necessary for running along the vehicle 1 on the curve. The curve curvature χ (= 1 / R) is input to a feedforward controller (F / F controller) 21 that constitutes the ECU 2, and a yaw rate ω _r related to the curve curvature χ is calculated according to a predetermined characteristic.

The yaw rate ω _d required to compensate the offset D (converge to the target value) is calculated by multiplying the deviation (D ₀ −D) between the offset D and the target offset D ₀ by the coefficient Kd.

The yaw rate ω _θ necessary for compensating the yaw angle θ (converging to the target) is calculated by multiplying the deviation (θ ₀ −θ) between the yaw angle θ and the target yaw angle θ ₀ by a coefficient K _θ. .

  The target yaw rate ω is calculated by adding the three yaw rates calculated in this way. This target yaw rate ω is converted into the target lateral acceleration G using the vehicle speed Vn detected by the vehicle speed sensor 11, and the torque calculator 22 constituting the ECU 2 is required to generate the target lateral acceleration G. The steering amount, that is, the steering torque T of the motor 15 is calculated. Thus, the ECU 2 configured to include the feedforward controller 21 and the torque calculator 22 functions as a target lateral acceleration calculation unit and an assist torque setting unit.

  Here, all of the steering torque of the motor 15 is used to steer the wheels FL and FR due to friction between the interlocking mechanisms in the steering gear box 5 constituting the steering system, the rack bar 6, the knuckle arm 7, and the like. I don't mean. Therefore, in practice, it is necessary to apply a driving force that balances the steering friction with the force required for turning the wheels FL and FR. This steering friction acts as a reaction force in the direction of turning, so when increasing the rudder angle, a steering torque that matches (force required for turning + steering friction) is required. When the angle is decreased, a steering torque matching (force required for steering-steering friction) is sufficient.

  Therefore, the torque calculator 22 calculates a characteristic curve when the target lateral acceleration G increases and a characteristic curve when the target lateral acceleration G decreases when calculating the amount of the steering torque T applied to the wheels FL and FR. Reference is made to a characteristic curve in which a hysteresis width is provided. FIG. 5 is a graph that is referred to when the steering assist device applies the steering torque T to the vehicle 1 based on the target lateral acceleration G. A diagram A indicated by a solid line in FIG. 5 corresponds to an increased state of right steering. A diagram B indicated by a broken line in FIG. 5 corresponds to a right steering switching back state. Similarly, a diagram C indicated by a solid line corresponds to a left steering addition state, and a diagram D indicated by a broken line corresponds to a left steering return state.

Here, the diagrams A to D shown in FIG. 5 are set as follows, for example.

  Here, the coefficients a, b, G1, G2, and w are preset according to vehicle characteristics. This can be obtained from the characteristics by measuring the lateral acceleration with respect to the required torque in the actual vehicle. The coefficient a is an increase amount of the required torque with respect to the increase amount of the lateral acceleration, and corresponds to the inclination a in FIG. The coefficient b is a value at which the lateral acceleration is first generated when the absolute value of the required torque is increased. In FIG. 5, it corresponds to the intercept b. G1 and G2 are constants for giving stability to the control when the vehicle 1 is traveling on a straight road. The coefficient w is expressed as a hysteresis width in FIG. 5, which is obtained as a difference between the required torque actually obtained when the lateral acceleration is in the increasing state and when the lateral acceleration is in the decreasing state.

  Next, with reference to FIG. 6, the characteristic curve (diagrams A to D) selection process corresponding to the steering state will be described. FIG. 6 is a flowchart showing the processing procedure of the characteristic curve selection processing.

  In step S100, the target lateral acceleration G is subjected to low-pass filter processing to remove high frequency components. In the subsequent step S102, a determination is made as to whether or not the vehicle 1 is making a curve turn based on the detected curvature of the travel path. Here, when it is determined that the vehicle 1 is not turning, that is, when it is determined that the vehicle 1 is traveling on a straight road, the process proceeds to step S104. On the other hand, if it is determined that the vehicle 1 is turning, the process proceeds to step S106.

In step S104, the target lateral acceleration fluctuation threshold value ΔG during straight traveling is calculated. This target lateral acceleration fluctuation threshold ΔG is calculated by the following equation (1).
ΔG = β (1)
Here, β is a target lateral acceleration fluctuation threshold during straight running, and is stored in the ECU 2 in advance. Thereafter, the process proceeds to step 108.

On the other hand, in step S106, the target lateral acceleration fluctuation threshold value ΔG during curve turning is calculated. This target lateral acceleration fluctuation threshold ΔG is calculated by the following equation (2).
ΔG = β + α × | G _ffw | (2)
Here, α is a coefficient that considers noise included in the detected curvature value. G _ffw is a feed-forward lateral acceleration at the time of turning of the curve, and is a value calculated by the formula G _ffw = (curvature χ × velocity V _n ² × Gain) /9.81. As shown in Expression (2), the target lateral acceleration fluctuation threshold value ΔG during curve turning is larger than the target lateral acceleration fluctuation threshold value ΔG during straight traveling.

Then, in step S108, the target lateral acceleration G is changed to decreasing state from increasing state, and state change determination reference value becomes a reference lateral acceleration G _lim target lateral acceleration variation threshold ΔG or more from the target lateral acceleration G A determination is made as to whether or not. This is to determine whether or not the steering state has changed from the right steering increase state to the return state. Specifically, the determination is made based on whether or not the following equation (3) is satisfied. .
Target lateral acceleration G <reference lateral acceleration G _lim −ΔG (3)

  Here, when the above equation (3) is established, in step S110, a flag LEFT indicating that the assist torque is to be applied in the left direction is set in the steering direction flag F_GD indicating the direction in which the assist torque is to be applied. The characteristic curve selected when calculating the torque T is switched from the diagram A to the diagram B. Thereafter, the process proceeds to step S116.

  Here, switching of the characteristic curve will be described with reference to an example of a change in the target lateral acceleration G during straight running shown in FIG. 7 and an example of a change in the target lateral acceleration G during curve turning shown in FIG. 7 and 8, the vertical axis represents the target lateral acceleration G, and the horizontal axis represents time. As shown in FIG. 7, when the target lateral acceleration G changes from an increasing state to a decreasing state at time t0 during straight running, and then the target lateral acceleration G continues to decrease, the amount of decrease travels straight at time t1. The characteristic curve selected when calculating the steering torque T that exceeds the target lateral acceleration fluctuation threshold ΔG1 at the time is switched from the diagram A to the diagram B.

  On the other hand, as shown in FIG. 8, when the target lateral acceleration G changes from the increasing state to the decreasing state at time t0 and the target lateral acceleration G continues to decrease thereafter at the time t0, as shown in FIG. The characteristic curve selected when calculating the steering torque T that exceeds the target lateral acceleration fluctuation threshold ΔG2 at the time of the curve turning is switched from the diagram A to the diagram B. Here, since ΔG1 <ΔG2, the characteristic curve is not switched until the decrease amount of the target lateral acceleration G becomes larger during the curve turning than during the straight running.

Returning to FIG. When the above equation (3) is not established, the process proceeds to step S112. In step S112, the target lateral acceleration G is changed from the decreasing state to the increasing state, and the target lateral acceleration G is increased from the reference lateral acceleration _Glim, which is a reference value for determining the state change, by the target lateral acceleration fluctuation threshold ΔG or more. A determination is made whether or not. This is to determine whether or not the steering state has changed from the increased left steering state to the return state. Specifically, the determination is made based on whether or not the following equation (4) is satisfied. .
Target lateral acceleration G> reference lateral acceleration G _lim + ΔG (4)

  Here, when the above equation (4) is satisfied, in step S114, the steering direction flag F_GD indicating the direction in which the assist torque is applied is set with the flag RIGHT indicating that the assist torque is applied in the right direction. The characteristic curve selected when calculating the torque T is switched from the diagram C to the diagram D. Thereafter, the process proceeds to step S116. On the other hand, when the above equation (4) is not satisfied, the process is temporarily exited.

At step S116, the target lateral acceleration G is substituted for the reference lateral acceleration _{G lim,} the reference lateral acceleration _{G lim} is updated. Thereafter, the process is temporarily exited.

  Returning to FIG. 4 and continuing the description, the ECU 2 instructs the motor driver 14 to drive the motor 15 according to the steering torque T thus obtained. As a result, the left and right front wheels FR and FL are steered, and the vehicle 1 is turned to maintain the lane. When the vehicle 1 turns, the CCD camera 8 captures the front situation again, and the above is repeated.

  Here, FIG. 9 shows an example of a change in steering torque calculated by the steering assist device according to the present embodiment. The vertical axis in FIG. 9 is the steering torque T, and the horizontal axis is the time. Moreover, in FIG. 9, the steering torque change calculated by the steering assistance apparatus which concerns on this embodiment is shown as a continuous line, and the steering torque change calculated by the conventional steering assistance apparatus is shown with a broken line.

  According to the conventional steering assist device, the characteristic curve is switched between times t1 and t2 and between times t3 and t4 due to the influence of noise or the like included in the detected curvature value, and a torque jump occurs. Since the steering torque T fluctuates every time a torque jump occurs, the conventional steering assist device has poor lane following ability as shown by the broken line in FIG. On the other hand, according to the steering assist device according to the present embodiment, an unnecessary torque jump does not occur, so that the lane followability of the vehicle is improved as shown by a solid line in FIG.

  According to the present embodiment, in consideration of steering friction, when it is increased (corresponding to diagrams A and C in FIG. 5) and when it is switched back (corresponding to diagrams B and D in FIG. 5). Thus, by making the steering torque output from the motor 15 different, the lateral acceleration of the vehicle can be matched with the target lateral acceleration G.

  However, a vibration component may be added to the target lateral acceleration G due to factors such as measurement error and noise included in the detected value of the road curvature. For this reason, when the switching determination of the increase / decrease direction is performed only from the amount of change between the target lateral acceleration G of the previous time step and the target lateral acceleration G of the current time step, the steering assist torque that is frequently applied and is applied. May change in vibration, and as a result, controllability may be reduced. In addition, if switching judgment is performed under the same conditions during straight running and curve turning, the steering assist torque that is applied due to frequent switching during curve turning that is more affected by noise etc. changes in vibration. There is a risk.

  According to this embodiment, after the increase / decrease of the target lateral acceleration G is switched, switching is performed when the increase state or the decrease state continues and the amount of change exceeds the target lateral acceleration fluctuation threshold ΔG. In addition, since the target lateral acceleration fluctuation threshold ΔG is set larger when the curvature of the traveling road is larger than when the curvature is small, switching is performed when the curvature of the traveling road is large compared to when the curvature is small. Can be difficult. As a result, variation in assist torque due to frequent switching is suppressed, and deterioration of steering feeling, lane followability, and the like can be suppressed.

  In the above description, the case where the steering state is changed from the increased state of the right steering to the returned state and the case where the steering state is changed from the increased state of the left steering to the returned state are described as examples. The present invention can be similarly applied to the case where the steering state changes from the switchback state to the increase state and the steering state changes from the switchback state to the left steering state.

  As mentioned above, although preferred embodiment of this invention was described, it cannot be overemphasized that this invention is not limited to the said embodiment. For example, the graph of the steering torque T with respect to the target lateral acceleration G shown in FIG. 5 is not limited to a straight line, and can be appropriately changed with a curve or the like according to vehicle characteristics.

It is a block diagram of the vehicle carrying the steering assistance device of this embodiment. It is a figure explaining the acquisition condition of the traveling lane by a CCD camera. It is a figure explaining a road parameter. It is a block diagram which shows operation | movement of steering assistance control. It is a diagram showing the steering torque with respect to a target lateral acceleration. It is a flowchart which shows the process sequence of the selection process of a characteristic curve. It is a graph which shows an example of the target lateral acceleration change at the time of straight running. It is a graph which shows an example of the target lateral acceleration change at the time of curve turning. It is a figure which shows an example of a steering torque change. It is a figure which shows the lane followability of the vehicle carrying the steering assistance device of this embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Vehicle, 2 ... ECU, 3 ... Steering wheel, 4 ... Steering shaft, 5 ... Steering gear box, 6 ... Rack bar, 7 ... Knuckle arm, 8 ... Camera, 9 ... Image processing part, 10 ... Steering angle sensor, DESCRIPTION OF SYMBOLS 11 ... Vehicle speed sensor, 12 ... Yaw rate sensor, 13 ... Navigation system, 14 ... Motor driver, 15 ... Motor, 21 ... Feed forward controller, 22 ... Torque calculator, 50 ... Road, 51 ... Driving lane.

Claims (2)

In a steering assist device that applies assist torque to a steering mechanism of a vehicle so as to travel a predetermined position on a travel path,
Target lateral acceleration calculating means for calculating a target lateral acceleration of the vehicle based on the curvature of the travel path;
Assist torque setting means for setting an assist torque based on the target lateral acceleration calculated by the target lateral acceleration calculation means,
The assist torque setting means increases the assist torque set value when the target lateral acceleration is in an increasing state than when the target lateral acceleration is in a decreasing state. When determining whether or not switching has been performed, it is more difficult to determine that the increase / decrease state has been switched when the curvature of the travel path is large than when the curvature is small. The assist torque setting means switches the increase / decrease state when the target lateral acceleration changes from the increasing state to the decreasing state or from the decreasing state to the increasing state and then the change amount of the target lateral acceleration exceeds the threshold value. 2. The steering assist device according to claim 1, wherein when the curvature of the traveling road is large, the threshold value is made larger than when the curvature is small.

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