WO2018056077A1

WO2018056077A1 - Electric power steering apparatus

Info

Publication number: WO2018056077A1
Application number: PCT/JP2017/032425
Authority: WO
Inventors: 久純石川; 智哉安原; 輝之菊池
Original assignee: Ｋｙｂ株式会社
Priority date: 2016-09-23
Filing date: 2017-09-08
Publication date: 2018-03-29
Also published as: JP2018047815A; JP6875813B2

Abstract

This electric power steering apparatus (100) comprises an automatic steering control unit (60) which, during automatic steering, controls an electric motor (10) according to the deviation between a signal based on the steering angle target value set on the basis of the movement target position of a vehicle and a signal based on the steering angle detected by a motor rotational angle sensor (10a) and a steering angle sensor (15). The automatic steering control unit (60) has a deviation restriction unit (80) which restricts integrating control based on an integrator (67b) when the steering wheel (1) is operated by a driver during automatic steering and the value of torque detected by a torque sensor (12) reaches a first threshold value α.

Description

Electric power steering device

The present invention relates to an electric power steering apparatus.

JP6-336170A describes a steering device that automatically steers a running vehicle along a white line of a road without operating a steering wheel by controlling a motor based on image data, vehicle speed, and actual steering angle signals in front of the vehicle. Has been.

In the steering device described in JP6-336170A, when the steering operation by the driver is determined in the automatic steering mode, the steering device is switched to the manual steering mode. Specifically, when a predetermined steering torque continues for a predetermined time, it is determined that the driver is operating the steering wheel, and the automatic steering mode is switched to the manual operation mode.

In the steering device described in JP6-336170A, during the automatic steering mode, the automatic steering control is continued while the determination is made even if the driver performs manual steering. For this reason, even if the driver starts manual steering, a control signal is applied to the motor so that the steering angle of the steering matches the steering target value by automatic steering control until manual operation by the driver is determined. . As a result, the steering torque felt by the driver increases, and the driver feels uncomfortable.

The present invention has been made in view of the above problems, and an object thereof is to reduce a driver's uncomfortable feeling when a manual operation is performed during automatic steering.

According to an aspect of the present invention, an electric power steering apparatus includes a steering shaft that is coupled to a steering member that is operated by a driver and that rotates in response to the operation of the steering member by the driver, and a part of the steering shaft. A torsion bar that detects torque acting on the torsion bar, an electric motor that applies rotational torque to the steering shaft, steering assistance that controls the electric motor based on the torque detected by the torque sensor, and the outside of the vehicle A control unit that selectively performs automatic steering for controlling the electric motor based on the information of the steering wheel, and a steering angle detection unit that detects the steering angle of the steering member. The control unit moves the vehicle during automatic steering. A signal based on the rudder angle target value set based on the target position and the rudder angle detector detects it. The automatic steering control unit controls the electric motor according to the deviation from the signal based on the steering angle, and the automatic steering control unit operates during integration and an integrator for performing integral control based on the deviation. When the steering member is operated by a person, when the value of the torque detected by the torque sensor reaches the first threshold value, a limiting unit that limits the integration control based on the integrator is provided.

FIG. 1 is a configuration diagram of an electric power steering apparatus according to a first embodiment of the present invention. FIG. 2 is a block diagram of the electric power steering apparatus according to the first embodiment of the present invention. FIG. 3 is a block diagram of the automatic steering control unit of the electric power steering apparatus according to the first embodiment of the present invention. FIG. 4 is a graph showing the operation of the limiting unit according to the first embodiment of the present invention. FIG. 5 is a graph showing the operation of a modified example of the limiting unit according to the first embodiment of the present invention. FIG. 6 is a block diagram of an automatic steering control unit of an electric power steering apparatus according to a modification of the first embodiment of the present invention. FIG. 7 is a block diagram of an automatic steering control unit of an electric power steering apparatus according to a modification of the first embodiment of the present invention. FIG. 8 is a block diagram of an automatic steering control unit of the electric power steering apparatus according to the second embodiment of the present invention.

<First Embodiment>
Hereinafter, an electric power steering apparatus 100 according to a first embodiment of the present invention will be described with reference to FIGS.

As shown in FIG. 1, an electric power steering apparatus 100 is connected to a steering wheel 1 as a steering member operated by a driver and is operated by the driver (hereinafter referred to as “steering operation”). The input shaft 2 which rotates with it, the output shaft 3 linked with the rack shaft 5 which steers the wheel 6, and the torsion bar 4 which connects the input shaft 2 and the output shaft 3 are provided. A steering shaft 7 is configured by the input shaft 2, the output shaft 3, and the torsion bar 4.

A pinion gear 3a that meshes with a rack gear 5a formed on the rack shaft 5 is formed below the output shaft 3. When the steering wheel 1 is steered, the steering shaft 7 rotates, and the rotation is converted into a linear motion of the rack shaft 5 by the pinion gear 3a and the rack gear 5a, and the wheel 6 is steered via the knuckle arm 14.

The electric power steering apparatus 100 includes an electric motor 10 that is a power source for assisting the driver in steering the steering wheel 1, a speed reducer 11 that decelerates and transmits the rotation of the electric motor 10 to the output shaft 3, and driving. A torque sensor 12 that detects torque acting on the torsion bar 4 by relative rotation between the input shaft 2 and the output shaft 3 in response to a steering operation by a person, a controller 30 as a control unit that controls the electric motor 10, and the electric motor 10 And a current sensor 10b (FIG. 2) for detecting a current value flowing through

The electric motor 10 is provided with a motor rotation angle sensor 10 a that acquires the rotation angle of the electric motor 10. The motor rotation angle sensor 10a is configured by a resolver.

The reducer 11 includes a worm shaft 11a connected to the output shaft of the electric motor 10 and a worm wheel 11b connected to the output shaft 3 and meshing with the worm shaft 11a. The rotational torque output from the electric motor 10 is transmitted from the worm shaft 11 a to the worm wheel 11 b and applied to the output shaft 3.

The steering torque applied to the input shaft 2 in accordance with the steering operation by the driver is detected by the torque sensor 12, and the torque sensor 12 outputs a voltage signal corresponding to the steering torque to the controller 30. The controller 30 calculates the torque output from the electric motor 10 based on the voltage signal from the torque sensor 12, and controls the driving of the electric motor 10 so that the torque is generated. As described above, the electric power steering apparatus 100 detects the steering torque applied to the input shaft 2 by the torque sensor 12, and controls the driving of the electric motor 10 by the controller 30 based on the detection result, thereby steering the driver. Assist the operation.

The input shaft 2 is provided with a steering angle sensor 15 as a steering angle detector that detects a steering angle that is a rotation angle of the steering wheel 1. Since the rotation angle of the input shaft 2 and the steering angle of the steering wheel 1 are equal, the steering angle of the steering wheel 1 can be obtained by detecting the rotation angle of the input shaft 2 by the steering angle sensor 15. The detection result of the steering angle sensor 15 is output to the controller 30.

Although not shown, the rudder angle sensor 15 includes, for example, a center gear that rotates integrally with the input shaft 2 and two outer gears that mesh with the center gear, and is based on changes in magnetic flux accompanying rotation of the two outer gears. Thus, the rotation angle of the center gear, that is, the rotation angle of the input shaft 2 is calculated.

The controller 30 is detected by a CPU that controls the operation of the electric motor 10, a ROM that stores control programs and setting values necessary for the processing operation of the CPU, and various sensors such as the torque sensor 12 and the steering angle sensor 15. And a RAM for temporarily storing information.

As described above, the electric power steering device 100 automatically assists the steering operation by the driver based on the detection result of the torque sensor 12, and automatically based on information outside the vehicle without depending on the steering operation of the driver. Can be steered. The controller 30 selectively performs steering assistance for controlling the electric motor 10 based on the steering torque detected by the torque sensor 12 and automatic steering for controlling the electric motor 10 based on information outside the vehicle.

Switching from manual steering to automatic steering by the driver is performed by selecting the automatic driving mode by the driver's operation. On the other hand, switching from automatic steering to manual steering is automatically performed when an intervention determination unit 33 (see FIG. 2) of the controller 30 described later determines that the steering operation by the driver is being performed, or by the driver's operation. This is executed by releasing the operation mode.

As shown in FIG. 2, the controller 30 is set based on an assist control unit 50 that performs assist control by the electric motor 10 based on detection values from various sensors during a steering operation by the driver, and a movement target position of the vehicle. And an automatic steering control unit 60 that controls the electric motor 10 in accordance with a deviation between the signal based on the steering angle target value to be performed and the signal based on the steering angle detected by the steering angle sensor 15.

The assist control unit 50 is based on the torque detected by the torque sensor 12, the vehicle speed detected by the vehicle speed sensor 16 provided in the vehicle, and the steering angle of the steering wheel 1 detected by the steering angle sensor 15. Calculate the assist force target value.

Automatic steering is performed based on a command signal (steering angle target value) from the automatic driving system 40 of the vehicle. Specifically, the automatic driving system 40 detects a boundary line (white line) of a running lane as information outside the vehicle, and calculates a steering angle target value necessary for the host vehicle to keep running in the lane. Then, the steering angle target value is output to the automatic steering control unit 60. In other words, the steering angle target value is set based on the movement target position of the vehicle. The automatic steering control unit 60 calculates an automatic steering target value based on the steering angle of the steering wheel 1 detected by the steering angle sensor 15 and the steering angle target value input from the automatic driving system 40. Details will be described later.

The controller 30 includes a current control unit 31 that controls a current applied to the electric motor 10 based on the assist force target value and the automatic steering target value, a drive circuit 32 that controls driving of the electric motor 10, and the torque sensor 12. An intervention determination unit 33 that determines a steering intervention operation by the driver based on the torque detected by the driver. In the electric power steering device 100, the current value of the electric motor 10 detected by the current sensor 10b is fed back to the current control unit 31 to perform feedback control.

The intervention determination unit 33 determines whether a steering operation by the driver is performed based on the torque detected by the torque sensor 12 during automatic steering (hereinafter, the steering operation by the driver during automatic steering is referred to as “driving”). This is also referred to as “steering intervention by the user”.) Specifically, the intervention determination unit 33 determines that a steering intervention operation by the driver has been performed when the torque detected by the torque sensor 12 exceeds a predetermined value for a predetermined time. The intervention determination unit 33 controls the electric motor 10 only by assist control by invalidating or stopping the control by the automatic steering control unit 60 based on the determination result.

Next, the automatic steering control unit 60 will be described with reference to FIG. FIG. 3 is a block diagram of the automatic steering control unit 60.

The automatic steering control unit 60 controls the electric motor 10 based on the steering angle target value input from the automatic driving system 40 and the steering angle of the steering wheel 1 detected by the steering angle sensor 15. An angular velocity target value calculation unit 61 is provided.

The angular velocity target value calculation unit 61 converts the steering wheel 1 to the steering angle target value based on the difference between the steering angle target value output from the automatic driving system 40 and the steering angle of the steering wheel 1 detected by the steering angle sensor 15. A position control unit 62 that calculates a position control target value for controlling the electric motor 10 so that the steering angles coincide with each other, and a target based on a change per unit time of the steering angle target value output from the automatic driving system 40. A target change compensator 63 for calculating a change compensation signal.

When calculating the position control target value by the position control unit 62, the steering angle of the steering wheel 1 detected by the steering angle sensor 15 is used. However, the detection period of the steering angle sensor 15 is relatively long, and when the steering wheel 1 is quickly steered, the steering angle of the steering wheel 1 cannot be detected with high accuracy only by the steering angle sensor 15.

Therefore, in order to detect the steering angle of the steering wheel 1 with higher accuracy, the position control unit 62 uses the steering angle detected by the steering angle sensor 15 as the motor rotation angle detected by the motor rotation angle sensor 10a. A high precision rudder angle is calculated by using and correcting. More specifically, the motor rotation angle sensor 10 a detects the motor rotation angle that is the rotation angle change amount of the electric motor 10 at a cycle shorter than the detection cycle of the rudder angle sensor 15. The amount of change in the rotation angle of the output shaft 3 can be obtained from the motor rotation angle detected by the motor rotation angle sensor 10a and the reduction ratio of the reduction gear 11 (the reduction ratio of the output shaft 3 and the electric motor 10). Therefore, by adding the rotation angle change amount of the output shaft 3 obtained from the motor rotation angle detected by the motor rotation angle sensor 10a and the reduction ratio of the speed reducer 11 to the steering angle detected by the steering angle sensor 15, The high-precision steering angle of the steering wheel 1 can be calculated with a short cycle. Thereby, even when the steering wheel 1 is quickly steered, a highly accurate steering angle can be obtained. In the present embodiment, the motor rotation angle sensor 10a functions as a part of the steering angle detector.

Thus, in the electric power steering apparatus 100, the steering angle of the steering wheel 1 is determined by the motor rotation angle of the electric motor 10 detected by the motor rotation angle sensor 10a and the rotation of the steering wheel 1 detected by the steering angle sensor 15. Calculation is performed based on the angle. Thereby, a highly accurate rudder angle can be obtained. For example, the detection cycle of the rudder angle sensor 15 is about 10 msec and the detection cycle of the motor rotation angle sensor 10a is 1 msec or less, but if the detection cycle of the rudder angle sensor 15 is short, it is detected by the rudder angle sensor 15. Only the rudder angle may be used.

The position control unit 62 calculates the difference between the steering angle target value and the steering angle of the steering wheel 1, performs PD control on this difference, and multiplies a predetermined gain to obtain the target angular velocity of the steering wheel 1. The corresponding position control target value is calculated. Specifically, the position control unit 62 determines the steering wheel 1 based on the difference between the steering angle target value output from the automatic driving system 40 and the actual steering angle of the steering wheel 1 detected by the steering angle sensor 15. The position control target value for making the rudder angle equal to the rudder angle target value is calculated. That is, the position control unit 62 performs position feedback control based on the difference between the steering angle target value and the actual steering angle of the steering wheel 1. The gain corresponds to, for example, the reciprocal of unit time.

The target change compensation unit 63 divides the difference between the previous steering angle target value output from the automatic driving system 40 and the current steering angle target value by a unit time (interval at which the steering angle target value is output), and PID. By executing the control, the target change compensation signal is calculated.

The angular velocity target value calculation unit 61 adds the target change compensation signal calculated by the target change compensation unit 63 to the position control target value calculated by the position control unit 62. This is intended to improve the followability to changes in the steering angle target value. This will be described in detail below.

If the steering angle target value changes abruptly, the control by the position control unit 62 may not be able to follow the control of the electric motor 10 with respect to the change in the steering angle target value (a delay occurs). A case will be described as an example where the steering angle target value changes suddenly from a state where it is larger than the actual steering angle to a state where it is smaller than the actual steering angle.

Since the steering angle target value is larger than the actual steering angle when the steering angle target value starts to change, the position control unit 62 outputs a position control target value that increases the steering angle of the steering wheel 1. . For this reason, the electric motor 10 is driven so that the steering angle of the steering wheel 1 is increased. Even if the steering angle target value is decreased, the position control unit 62 is configured to steer the steering wheel 1 so that the actual steering angle matches the steering angle target value if the actual steering angle is smaller than the steering angle target value. A position control target value that increases the angle is output. When the rudder angle target value further decreases and becomes smaller than the actual rudder angle, the position control unit 62 starts the position where the rudder angle of the steering wheel 1 becomes smaller from this point in order to follow the rudder angle target value. Outputs the control target value. As described above, in the control by the position control unit 62, the steering angle of the steering wheel 1 is increased until the actual steering angle matches the steering angle target value even if the steering angle target value suddenly decreases. Since the output of such a position control target value is continued, a delay is likely to occur.

Therefore, the angular velocity target value calculator 61 calculates a target change compensation signal corresponding to the change speed of the steering angle target value, and the target change compensator 63 calculates the position control target value calculated by the position controller 62. The target change compensation signal is added to generate an angular velocity target value. Thereby, even if the steering angle target value changes rapidly, the change in the steering angle target value is compensated by the target change compensation signal calculated by the target change compensation unit 63. Therefore, by controlling with the angular velocity target value generated by adding the target change compensation signal to the position control target value, the steering angle is controlled rather than controlling only with the position control target value calculated by the position control unit 62. The followability to changes in the target value is improved.

The automatic steering control unit 60 calculates the twist angle of the torsion bar 4 based on the torque detected by the torque sensor 12, and controls the electric motor 10 in a direction to suppress the twist of the torsion bar 4 based on the twist angle. The vibration compensation unit 70 for calculating the vibration compensation signal is further provided.

The vibration compensation unit 70 includes a torsion angle calculation unit 71 that calculates the torsion angle of the torsion bar 4 from the torque detected by the torque sensor 12. The vibration compensation unit 70 performs PD control on the twist angle calculated by the twist angle calculation unit 71 and calculates a vibration compensation signal.

The vibration compensation signal is a signal for suppressing the vibration of the steering wheel 1 generated due to the inertia of the steering wheel 1 and the spring property of the torsion bar 4. Here, the vibration of the steering wheel 1 caused by the inertia of the steering wheel 1 and the spring property of the torsion bar 4 will be specifically described.

When the electric motor 10 is activated during automatic steering, the rotation of the electric motor 10 is transmitted to the output shaft 3 via the speed reducer 11. When the output shaft 3 rotates, the steering wheel 1 rotates via the torsion bar 4 and the input shaft 2. At this time, the torsion bar 4 has a spring property, and the inertia that tries to stay in place acts on the steering wheel 1, so that the torsion bar 4 is twisted when the output shaft 3 starts to rotate. Then, the steering wheel 1 starts rotating with a delay from the output shaft 3 by the amount of twisting of the torsion bar 4. Thereafter, even if the electric motor 10 stops, the steering wheel 1 continues to rotate due to the spring force of the torsion bar 4 and the inertia of the steering wheel, and the torsion bar 4 is moved in the reverse direction beyond the position where the twist of the torsion bar 4 has been eliminated. Twist into. Thus, when the torsion bar 4 is twisted in the opposite direction, a difference is generated between the steering angle detected by the steering angle sensor 15 and the steering angle target value. As a result, the electric motor 10 operates to rotate the output shaft 3 in the reverse direction in order to eliminate this difference. At the same time, the torsion bar 4 rotates in the opposite direction by the spring force so as to eliminate the twist of the torsion bar 4 itself. The electric motor 10 stops when the steering angle target value is reached, but the steering wheel 1 continues to rotate due to its inertia even when the twist of the torsion bar 4 is eliminated, and the torsion bar 4 is further reversed. It will twist. By repeating such an operation, the steering wheel 1 vibrates.

The vibration compensation unit 70 outputs a vibration compensation signal to the electric motor 10 in order to suppress such vibration. Specifically, when the output shaft 3 rotates to the left and the torsion bar 4 is twisted, the vibration compensator 70 twists the torsion bar 4 according to the torsion angle of the torsion bar 4. A vibration compensation signal that suppresses the rotation direction, that is, the leftward rotation of the output shaft 3, is output to the electric motor 10. Thereby, since the torque to the left rotation direction of the electric motor 10 is suppressed, the twist of the torsion bar 4 is suppressed. Therefore, vibration of the steering wheel 1 caused by the inertia of the steering wheel 1 and the torsion of the torsion bar 4 is suppressed. Similarly, when the output shaft 3 rotates in the right direction and the torsion bar 4 is twisted, the vibration compensation unit 70 suppresses the twist of the torsion bar 4 according to the twist angle of the torsion bar 4, that is, Then, a vibration compensation signal that suppresses the rightward rotation of the output shaft 3 is output to the electric motor 10. Thereby, since the torque to the right rotation direction of the electric motor 10 is suppressed, the twist of the torsion bar 4 is suppressed. Therefore, vibration of the steering wheel 1 caused by the inertia of the steering wheel 1 and the torsion of the torsion bar 4 is suppressed. If the torsion angle calculated by the torsion angle calculation unit 71 is large, the vibration compensation signal is calculated so as to suppress the rotation (torque) of the electric motor 10 in the direction in which the torsion bar 4 is twisted. Is done. That is, the vibration compensation signal is calculated to a value corresponding to the magnitude of the torsion angle, and acts on the electric motor 10 in a direction to cancel the torsion, so that the torsion bar 4 can be accurately prevented from being twisted.

The automatic steering control unit 60 is a corrected angular velocity target value calculation unit that calculates a corrected angular velocity target value based on the angular velocity target value calculated by the angular velocity target value calculation unit 61 and the vibration compensation signal calculated by the vibration compensation unit 70. An adder 64, an angular velocity calculation unit 65 for calculating the angular velocity of the steering wheel 1 from the motor rotation angle detected by the motor rotation angle sensor 10a, and a corrected angular velocity target value calculated by the adder 64 and the angular velocity calculation unit 65. A subtractor 66 serving as a deviation calculating unit that calculates a deviation d (difference) from the calculated angular velocity of the steering wheel 1, and automatic steering for controlling the electric motor 10 based on the deviation d calculated by the subtractor 66. And an angular velocity control unit 67 that generates a target value.

The adder 64 adds a vibration compensation signal to the angular velocity target value and calculates a corrected angular velocity target value.

The angular velocity calculation unit 65 calculates the angular velocity of the rotating shaft of the electric motor 10 from the motor rotation number per unit time detected by the motor rotation angle sensor 10a. Furthermore, the angular velocity calculation unit 65 calculates the angular velocity of the steering wheel 1 by dividing the reduction ratio of the reduction gear 11 by the angular velocity of the rotating shaft of the electric motor 10. Thus, the angular velocity of the steering wheel 1 calculated by the angular velocity calculating unit 65 corresponds to the actual angular velocity of the steering wheel 1. Note that the angular velocity of the steering wheel 1 may be calculated based on the steering angle detected by the steering angle sensor 15 as long as the detection cycle of the steering angle sensor 15 is short.

The angular velocity control unit 67 includes a proportional unit 67a for performing proportional control based on the deviation d, an integrator 67b for performing integral control based on the deviation d, and a differential for performing differential control based on the deviation d. And a device 67c. The angular velocity control unit 67 performs PID control based on the deviation d by the proportional unit 67a, the integrator 67b, and the differentiator 67c to generate an automatic steering target value. Specifically, the angular velocity control unit 67 generates an automatic steering target value for controlling the electric motor 10 based on the deviation d so that the actual angular velocity of the steering wheel 1 matches the corrected angular velocity target value. That is, the angular velocity control unit 67 performs speed feedback control based on a deviation d (difference) between the target value of the angular velocity of the steering wheel 1 and the actual angular velocity of the steering wheel 1.

As described above, in the automatic steering controller 60, the adder 64 adds the vibration compensation signal calculated by the vibration compensator 70 to the angular velocity target value to calculate the corrected angular velocity target value. Since the deviation d between the corrected angular velocity target value calculated by the adder 64 and the angular velocity fed back by the angular velocity calculator 65 is a difference between the signals to which the vibration compensation signal is added, the deviation d is a relatively small value. Become. Thereby, the control in the angular velocity control part 67 can be stabilized.

When the steering wheel 1 is operated by the driver during automatic steering, the automatic steering control unit 60 is calculated by the subtractor 66 when the torque value detected by the torque sensor 12 reaches the first threshold value α. A deviation limiting unit 80 is further provided as a limiting unit that limits the input of the deviation d to the angular velocity control unit 67.

As shown in FIG. 4, when the torque acting on the torsion bar 4 reaches the first threshold value α, the deviation limiting unit 80 determines the deviation between the signal based on the steering angle target value and the signal based on the steering angle. Specifically, the deviation d between the angular velocity calculated by the subtractor 66 and the corrected angular velocity target value is output as zero. The deviation limiting unit 80 outputs the deviation d as it is when the torque acting on the torsion bar 4 decreases to a second threshold value β that is smaller than the first threshold value α. As described above, since the dead zone is provided between the first threshold value α and the second threshold value β in the deviation limiting unit 80, for example, the torque changes in small increments across the first threshold value α. Occasional hunting can be prevented.

Referring to FIG. 3, the automatic steering by the automatic steering control unit 60 configured as described above will be described.

When the steering angle target value is input from the automatic driving system 40 to the angular velocity target value calculation unit 61, the angular velocity target value calculation unit 61 makes the steering angle detected by the steering angle sensor 15 coincide with the steering angle target value. An angular velocity target value for controlling the electric motor 10 is calculated. Further, the corrected angular velocity target value is calculated by adding the vibration compensation signal calculated by the vibration compensation unit 70 to the target angular velocity value. In parallel with this, the angular velocity calculator 65 calculates the actual angular velocity of the steering wheel 1 from the rotation angle of the motor detected by the motor rotation angle sensor 10a.

Based on the difference (deviation d) between the corrected angular velocity target value calculated by the adder 64 and the angular velocity of the steering wheel 1 calculated by the angular velocity calculator 65, the angular velocity control unit 67 converts the steering wheel 1 into the corrected angular velocity target value. The automatic steering target value for controlling the electric motor 10 is calculated so that the actual angular velocities match.

The automatic steering target value calculated in this way is input to the current control unit 31, and the current control unit 31 controls the electric motor 10 via the drive circuit 32. The electric motor 10 rotates the steering shaft 7 via the speed reducer 11 so that the angular velocity of the steering wheel 1 becomes the steering angle target value.

When the electric motor 10 starts rotating, the torsion bar 4 is twisted due to the inertia of the steering wheel 1 and the spring property of the torsion bar 4 as described above. When the torsion bar 4 is twisted, torque is detected by the torque sensor 12, and this torque is input to the torsion angle calculation unit 71 of the vibration compensation unit 70. The torsion angle calculation unit 71 calculates the torsion angle of the torsion bar 4. The vibration compensation unit 70 calculates a vibration compensation signal for controlling the electric motor 10 in a direction to suppress the twist of the torsion bar 4 based on the twist angle. The vibration compensation signal calculated by the vibration compensator 70 is added to the angular velocity target value calculated by the angular velocity target value calculator 61 in the adder 64. Then, a subtractor 66 calculates a difference (deviation d) between the corrected angular velocity target value and the angular velocity of the steering wheel 1, and the deviation d is input to the angular velocity controller 67. The angular velocity control unit 67 performs PID control based on the deviation d, and calculates an automatic steering target value for controlling the electric motor 10. Thus, the electric motor 10 suppresses the twist of the torsion bar 4 according to the torsion angle of the torsion bar 4 by the automatic steering target value to which the vibration compensation signal calculated by the vibration compensator 70 is added. The angular velocity of the steering wheel 1 is controlled to be the steering angle target value while being controlled in the direction. In other words, the electric motor 10 is controlled so that the angular velocity of the steering wheel 1 becomes the steering angle target value while the torque in the rotational direction is suppressed by an amount corresponding to the magnitude of the torsion angle of the torsion bar 4. Thereby, the vibration of the steering wheel 1 generated due to the inertia of the steering wheel 1 and the spring property of the torsion bar 4 can be suppressed.

Next, control when a steering intervention operation by the driver is performed during automatic steering will be described.

When the driver operates the steering wheel 1 during automatic steering, torque acts on the torsion bar 4. Torque acting on the torsion bar 4 is detected by the torque sensor 12 and input to the deviation limiting unit 80. The deviation limiting unit 80 outputs the deviation d calculated by the subtractor 66 as 0 when the input torque becomes equal to or greater than the first threshold value α. As a result, the deviation d is input to the angular velocity control unit 67 as zero.

At the same time, the intervention determination unit 33 determines whether a steering intervention operation by the driver is being performed. Specifically, when the detected torque is equal to or greater than the first threshold value α for a certain period of time, the intervention determination unit 33 determines that the driver is performing a steering intervention operation and performs the electric drive by the automatic steering control unit 60. The control of the motor 10 is invalidated or stopped. Even if the detected torque is a value equal to or greater than the first threshold value α, the steering intervention operation by the driver is not performed if the torque does not continue the value equal to or greater than the first threshold value α for a certain period of time. Alternatively, it is assumed that the steering intervention operation is interrupted, and the control of the electric motor 10 by the automatic steering control unit 60 is continued. Note that the first threshold value α and the determination time are set to values that can prevent erroneous determination when a driver's hand or the like hits the steering wheel 1 or when vibration occurs in the steering wheel 1 as described above. The

In the electric power steering apparatus 100, when the torque detected by the torque sensor 12 becomes equal to or higher than the first threshold value α, the deviation speed limiter 80 controls the angular velocity control unit regardless of whether the determination by the intervention determination unit 33 is completed. The deviation d input to 67 is set to zero. Accordingly, even if there is a deviation d between the angular velocity and the corrected angular velocity target value, 0 is input to the angular velocity control unit 67 as the deviation d. That is, even if there is a deviation d between the angular velocity and the corrected angular velocity target value, by inputting 0 as the deviation d to the angular velocity control unit 67, a proportional device 67a, an integrator 67b of the angular velocity control unit 67, and The control by the differentiator 67c is limited. In other words, by inputting 0 as the deviation d to the angular velocity control unit 67, the control by the proportional device 67a, the integrator 67b, and the differentiator 67c of the angular velocity control unit 67 is substantially invalidated. As a result, the automatic steering target value output from the angular velocity control unit 67 is constant, and the torque generated by the electric motor 10 based on the signal from the automatic steering control unit 60 (automatic steering target value) is also constant. Therefore, an increase in torque generated by the electric motor 10 based on the automatic steering target value is suppressed.

As described above, in the electric power steering apparatus 100, when the steering wheel 1 is operated by the driver during automatic steering and the torque detected by the torque sensor 12 reaches the first threshold value α, the intervention determination unit 33. Regardless of the determination status, the deviation limiting unit 80 limits the control by the angular velocity control unit 67. Thereby, the raise of the torque which the electric motor 10 generate | occur | produces based on an automatic steering target value can be suppressed. Therefore, when the driver operates the steering wheel 1 during automatic steering, it is possible to reduce a sense of discomfort due to an increase in torque that the driver learns from the steering wheel 1.

If such control is not performed in the automatic steering control unit 60, the angular velocity control unit 67 makes the steering angle closer to the steering angle target value even while the intervention determination unit 33 determines the steering intervention operation. The automatic steering target value is generated (such that the deviation is reduced). Even if the electric motor 10 is controlled based on the automatic steering target value generated in this way, the difference from the steering angle target value cannot be resolved because the driver operates the steering wheel 1. Therefore, the angular velocity control unit 67 integrates the deviation d by the integrator 67b to eliminate the deviation d, and generates an automatic steering target value that is larger than the previous automatic steering target value. As a result, the current value for driving the electric motor 10 increases, and the torque acting in the direction in which the steering angle approaches the steering angle target value, that is, the direction opposite to the driver's steering operation, increases.

While the intervention determination unit 33 determines the manual operation, such feedback control is repeated, and the angular velocity control unit 67 sequentially integrates the deviation d by the integrator 67b, so that the automatic steering target value also increases sequentially. Become. As a result, the torque generated by the electric motor 10 based on the automatic steering target value in the direction opposite to the driver's steering operation gradually increases.

Therefore, in the electric power steering apparatus 100, when the steering wheel 1 is operated by the driver during automatic steering, the value of the torque detected by the torque sensor 12 is the first value regardless of the determination by the intervention determination unit 33. When the threshold value is reached, the deviation limiting unit 80 limits the control by the angular velocity control unit 67, particularly the integration control by the integrator 67b. As a result, an increase in torque acting in the direction opposite to the driver's steering operation can be suppressed. Therefore, when the driver operates the steering wheel 1 during automatic steering, it is possible to reduce a sense of discomfort due to an increase in torque that the driver learns from the steering wheel 1. Furthermore, since the sudden decrease in the torque when the automatic steering control is invalidated or stopped by the intervention determination unit 33 is suppressed, it is possible to reduce a sense of incongruity due to a rapid change in the torque.

In the above-described embodiment, the deviation limiting unit 80 is configured to be switched by the first threshold value α or the second threshold value β. However, as shown in FIG. When the detected torque is in the range from the first threshold value α to the second threshold value β, the deviation d may be reduced as the torque increases. With this configuration, the torque change can be smoothed.

Moreover, in the said embodiment, although the threshold value of the intervention determination part 33 was made into the 1st threshold value (alpha), it is not restricted to this, The threshold value of the intervention determination part 33 is made into the 3rd threshold value (gamma), and 1st threshold value (alpha) is set to 3rd. A value smaller than the threshold value γ may be set. That is, the threshold for intervention determination by the intervention determination unit 33 and the threshold for limiting control by the angular velocity control unit 67 may be set to different torques. For example, when the speed of the steering intervention operation by the driver is slow, it takes time for the torque to reach the first threshold value α, so that the integration of the deviation d by the integrator 67b further progresses. Therefore, by setting the first threshold value α to a value smaller than the third threshold value γ, the integration control based on the integrator 67b is limited when the integration of the deviation d by the integrator 67b is small. Thereby, since the raise of the automatic steering target value produced | generated by the angular velocity control part 67 can be suppressed early, a driver | operator's uncomfortable feeling accompanying the raise of a torque can be reduced more.

According to the above 1st Embodiment, there exist the following effects.

In the electric power steering apparatus 100, the automatic steering control unit 60, when the steering wheel 1 is operated by the driver during automatic steering, when the torque value detected by the torque sensor 12 reaches the first threshold value α. The deviation limiting unit 80 limits the control by the proportional unit 67a, the integrator 67b, and the differentiator 67c. This suppresses an increase in torque generated by the electric motor 10 based on the automatic steering target value generated by the automatic steering control unit 60 when the driver operates the steering wheel 1 during automatic steering. it can. Therefore, it is possible to reduce a sense of incongruity due to an increase in torque that the driver learns from the steering wheel 1 during the steering intervention operation. Further, since the sudden decrease in torque when the automatic steering control is invalidated or stopped by the intervention determination unit 33, the uncomfortable feeling due to the rapid change in torque that the driver learns from the steering wheel 1 can be reduced.

In the electric power steering apparatus 100, the automatic steering control unit 60 sets the deviation d to 0 by the deviation limiting unit 80 when the torque acting on the torsion bar 4 is equal to or greater than the first threshold value α. The control by the proportional unit 67a, the integrator 67b and the differentiator 67c can be more reliably limited. Therefore, an increase in torque generated by the electric motor 10 can be further suppressed based on the automatic steering target value generated by the automatic steering control unit 60.

In the electric power steering apparatus 100, the automatic steering control unit 60 further includes an angular velocity calculation unit 65 that calculates the angular velocity of the steering wheel 1. Thereby, since the electric motor 10 is controlled based on the difference between the target angular velocity value and the angular velocity of the steering wheel 1 in addition to the steering angle, the control accuracy and responsiveness are improved.

In the electric power steering apparatus 100, the automatic steering control unit 60 suppresses the twist of the torsion bar 4 based on the angular velocity target value calculated by the angular velocity target value calculation unit 61 and the torsion angle calculated by the vibration compensation unit 70. The electric motor 10 is controlled based on the vibration compensation signal for controlling the electric motor 10. Thus, the electric motor 10 is controlled so that the angular velocity of the steering wheel 1 becomes the steering angle target value while the torque in the rotational direction is suppressed based on the twist angle of the torsion bar 4. Therefore, vibration of the steering wheel 1 caused by the inertia of the steering wheel 1 and the spring property of the torsion bar 4 can be suppressed.

In the electric power steering apparatus 100, the deviation d is set to 0 by the deviation limiting unit 80. However, the deviation d can be set to 0 by using the changeover switch 180 shown in the modification shown in FIG. Hereinafter, modified examples will be described.

In the modification shown in FIG. 6, the automatic steering control unit 60 replaces the deviation limiting unit 80, and when the torque acting on the torsion bar 4 reaches the first threshold value α, the angular velocity calculation unit 65 serves as the angular velocity target value. The selector switch 180 is provided as a limiting unit that outputs the angular velocity calculated by the above.

The changeover switch 180 is provided between the adder 64 and the subtractor 66. When the torque acting on the torsion bar 4 reaches the first threshold value α, the selector switch 180 is switched so as to output the angular velocity calculated by the angular velocity calculator 65 as the angular velocity target value. Thereby, the deviation calculated by the subtractor 66 can be made zero.

Further, as in the modification shown in FIG. 7, when the torque acting on the torsion bar 4 reaches the first threshold value α, switching is performed so that the angular velocity target value is output as the angular velocity calculated by the angular velocity calculator 65. For example, a changeover switch 180 may be provided. Even in this case, the deviation d calculated by the subtractor 66 can be made zero.

Furthermore, in the above embodiment, the threshold values α, β, γ may be changed according to the vehicle speed. For example, the first threshold value α may be decreased as the vehicle speed increases. When the vehicle speed is high, the driver rarely operates the steering wheel 1 abruptly. In other words, when the vehicle speed is high, a large torque is hardly generated in the torsion bar 4. For this reason, by reducing the first threshold value α, the control by the angular velocity control unit 67 is limited even if the torque generated by the steering operation is small. Thereby, the uncomfortable feeling due to the increase in torque that the driver learns from the steering wheel 1 can be appropriately reduced according to the vehicle speed.

Second Embodiment
Next, with reference to FIG. 8, an electric power steering apparatus 200 according to a second embodiment of the present invention will be described. Below, it demonstrates centering on a different point from the said 1st Embodiment, the same code | symbol is attached | subjected to the structure same as the electric power steering apparatus 100 of 1st Embodiment, and description is abbreviate | omitted.

The basic configuration of the electric power steering apparatus 200 is the same as that of the electric power steering apparatus 100 according to the first embodiment. In the electric power steering apparatus 100 according to the first embodiment, the deviation d input to the proportional unit 67a, the integrator 67b, and the differentiator 67c is limited, whereas in the electric power steering apparatus 200, the integrator 67b is used. The difference is that only the input deviation d is limited.

In the electric power steering apparatus 200, the automatic steering control unit 60 is provided between the subtractor 66 and the integrator 67b, and when the torque acting on the torsion bar 4 reaches the first threshold value α, the integrator 67b. An integration limiting unit 280 is provided as a limiting unit that limits only the integration control based on.

The integration limiting unit 280 sets the deviation input to the integrator 67b to 0 when the torque acting on the torsion bar 4 reaches the first threshold value α. That is, in the electric power steering apparatus 200, when the torque acting on the torsion bar 4 reaches the first threshold value α, only the integration control based on the integrator 67b is limited.

Even when the integration control based on the integrator 67b is restricted, the angular velocity control unit 67 performs the proportional control by the proportional device 67a and the differential control by the differentiator 67c. However, in the proportional control and the differential control, even if the deviation d exists, the deviation d is not integrated unlike the integral control. For this reason, the automatic steering target value does not gradually increase as in the case where the integral control is performed. Therefore, an increase in torque generated by the electric motor 10 based on the automatic steering target value can be suppressed when the driver operates the steering wheel 1 during automatic steering. Therefore, also in the electric power steering apparatus 200, it is possible to reduce a sense of incongruity due to an increase in torque that the driver learns from the steering wheel 1.

The integration limiting unit 280 may be provided on the output side of the integrator 67b. In this case, the integration limiting unit 280 can be configured by, for example, an ON / OFF switch.

The electric power steering apparatus 200 of the second embodiment described above can achieve the same effects as those of the first embodiment.

The configuration, operation, and effect of the embodiment of the present invention configured as described above will be described together.

The electric

power steering devices

100 and 200 are connected to a steering member (steering wheel 1) operated by a driver, and rotate according to the operation of the steering member (steering wheel 1) by the driver, and the steering shaft. 7, a torque sensor 12 that detects torque acting on the torsion bar 4, an electric motor 10 that applies rotational torque to the steering shaft 7, and torque detected by the torque sensor 12. A control unit (controller 30) that selectively performs steering assistance for controlling the electric motor 10 based on the above and automatic steering for controlling the electric motor 10 based on information outside the vehicle, and a steering angle of the steering member (steering wheel 1). Rudder angle detector A rotation angle sensor 10a and a rudder angle sensor 15), and a control unit (controller 30) includes a signal and a rudder angle detection unit based on a rudder angle target value set based on a moving target position of the vehicle during automatic steering. An automatic steering control unit 60 that controls the electric motor 10 in accordance with a deviation (deviation d) from a signal based on the steering angle detected by the (motor rotation angle sensor 10a and the steering angle sensor 15) includes an automatic steering control unit. 60 is detected by the torque sensor 12 when the steering member (steering wheel 1) is operated by the driver during the automatic steering, and the integrator 67b for performing integral control based on the deviation (deviation d). When the torque value reaches the first threshold value α, a limiting unit (deviation limiting unit 80, changeover switch 180, integration limiting unit 280) that limits integration control based on the integrator 67b is provided. To do.

According to this configuration, when the steering member (steering wheel 1) is operated by the driver during automatic steering, the electric motor 10 is generated based on a signal (automatic steering target value) from the automatic steering control unit 60. An increase in torque can be suppressed. Therefore, when the driver operates the steering member (steering wheel 1) during automatic steering, it is possible to reduce a sense of discomfort due to an increase in torque that the driver learns from the steering member (steering wheel 1).

Further, in the electric power steering apparatus 100, the limiting unit (deviation limiting unit 80) is based on the signal based on the steering angle target value and the steering angle when the torque acting on the torsion bar 4 reaches the first threshold value α. The integration control based on the integrator 67b is limited by outputting the deviation (deviation d) from the signal as zero.

In the electric power steering apparatus 100, the limiter (changeover switch 180) outputs a signal based on the steering angle as a signal based on the steering angle target value when the torque acting on the torsion bar 4 reaches the first threshold value α. By doing so, the integration control based on the integrator 67b is limited.

In the electric power steering apparatus 100, when the torque acting on the torsion bar 4 reaches or exceeds the first threshold value α, the limiting unit (changeover switch 180) outputs a signal based on the steering angle target value as a signal based on the steering angle. By outputting, the integration control based on the integrator 67b is limited.

According to these configurations, when the torque acting on the torsion bar 4 is equal to or greater than the first threshold value α, the deviation d input to the angular velocity control unit 67 can be set to zero. Thereby, since the deviation d is not integrated by the integrator 67b and a signal (automatic steering target value) is not generated, it is ensured that the torque acting in the direction opposite to the steering operation of the driver increases. Can be suppressed.

In the electric power steering apparatus 200, the automatic steering control unit 60 further includes a proportional unit 67a for performing proportional control based on the deviation d and a differentiator 67c for performing differential control based on the deviation d. The limiting unit (integration limiting unit 280) limits only the integration control based on the integrator 67b when the torque acting on the torsion bar 4 reaches the first threshold value α.

According to this configuration, since the integration control based on the integrator 67b is limited, the deviation d is not integrated and the signal (automatic steering target value) generated by the automatic steering control unit 60 does not increase. Therefore, when the driver operates the steering member (steering wheel 1) during automatic steering, an increase in torque generated by the electric motor 10 is suppressed based on a signal (automatic steering target value) from the automatic steering control unit 60. can do.

Further, in the electric

power steering devices

100 and 200, the limiting unit (deviation limiting unit 80) is configured such that when the torque acting on the torsion bar 4 reaches the first threshold value α, the signal and the steering angle based on the steering angle target value. When the torque acting on the torsion bar 4 decreases to a second threshold value β smaller than the first threshold value α, a signal based on the steering angle target value is output. And the deviation (deviation d) from the signal based on the steering angle are output as they are.

According to this configuration, the control is not switched even if the torque acting on the torsion bar 4 changes in small increments across the first threshold value α, so that the control is stable.

Further, in the electric

power steering devices

100 and 200, the limiting unit (deviation limiting unit 80) is configured such that the torque acting on the torsion bar 4 is within the range from the first threshold value α to the second threshold value β. As the torque acting on 4 increases, the deviation (deviation d) between the signal based on the steering angle target value and the signal based on the steering angle is reduced.

According to this configuration, as the torque acting on the torsion bar 4 increases, the deviation (deviation d) between the signal based on the steering angle target value and the signal based on the steering angle is reduced, so that the change in torque is smoothed. Can do. Therefore, it is possible to reduce a sense of incongruity due to a change in torque that the driver learns from the steering member (steering wheel 1).

In the electric

power steering devices

100 and 200, the automatic steering control unit 60 includes an angular velocity calculation unit 65 that calculates an angular velocity of the steering member (steering wheel 1) based on the steering angle, a steering angle target value, and a steering angle. An angular velocity target value calculation unit 61 for calculating an angular velocity target value for controlling the electric motor 10 based on the difference, and the deviation (deviation d) between the signal based on the steering angle target value and the signal based on the steering angle is an angular velocity. And the angular velocity target value d.

According to this configuration, since the electric motor 10 is controlled based on the angular velocity, the control accuracy and responsiveness are improved as compared with the control based only on the steering angle.

The electric

power steering devices

100 and 200 further include an intervention determination unit 33 that stops the control by the automatic steering control unit 60 when the torque acting on the torsion bar 4 is equal to or greater than the third threshold value γ for a certain period of time. The first threshold value α is set to a value smaller than the third threshold value γ.

According to this configuration, by setting the first threshold value α to a value smaller than the third threshold value γ, integration control based on the integrator 67b can be limited when the integration of the deviation d by the integrator 67b is small. . As a result, the increase in the signal (automatic steering target value) generated by the angular velocity control unit 67 can be suppressed early, so that the driver's uncomfortable feeling associated with the increase in torque can be further reduced when the steering operation speed is slow.

Further, in the electric

power steering devices

100 and 200, the automatic steering control unit 60 decreases the first threshold value α as the vehicle speed increases.

According to this configuration, the uncomfortable feeling due to the increase in torque that the driver learns from the steering member (steering wheel 1) can be appropriately reduced according to the vehicle speed.

The embodiment of the present invention has been described above. However, the above embodiment only shows a part of application examples of the present invention, and the technical scope of the present invention is limited to the specific configuration of the above embodiment. Absent.

In the electric

power steering devices

100 and 200, the vibration compensation unit 70 and the intervention determination unit 33 may not be provided. The automatic steering target value is generated by the angular velocity target value and the angular velocity, but the signal based on the steering angle target value, the signal based on the steering angle detected by the steering angle sensor 15 and the motor rotation angle sensor 10a, Anything may be used as long as it is based on the deviation.

This application claims priority based on Japanese Patent Application No. 2016-185149 filed with the Japan Patent Office on September 23, 2016, the entire contents of which are incorporated herein by reference.

Claims

An electric power steering device,
A steering shaft coupled to a steering member operated by a driver and rotating in accordance with the operation of the steering member by the driver;
A torsion bar constituting a part of the steering shaft;
A torque sensor for detecting torque acting on the torsion bar;
An electric motor for applying rotational torque to the steering shaft;
A control unit that selectively performs steering assistance for controlling the electric motor based on torque detected by the torque sensor and automatic steering for controlling the electric motor based on information outside the vehicle;
A steering angle detector that detects a steering angle of the steering member,
In response to a deviation between a signal based on a steering angle target value set based on a moving target position of a vehicle and a signal based on the steering angle detected by the steering angle detection unit during the automatic steering. An automatic steering control unit for controlling the electric motor;
The automatic steering control unit
An integrator for performing integral control based on the deviation;
When the steering member is operated by the driver during the automatic steering, if the value of the torque detected by the torque sensor reaches a first threshold value, a limiting unit that limits integration control based on the integrator; ,
An electric power steering apparatus.
The electric power steering apparatus according to claim 1,
The restriction unit is
When the torque acting on the torsion bar reaches the first threshold, based on the integrator, the deviation between the signal based on the steering angle target value and the signal based on the steering angle is output as zero. Electric power steering device that limits integral control.
The electric power steering apparatus according to claim 1,
The restriction unit is
When the torque acting on the torsion bar reaches the first threshold, the electric control is performed to limit the integration control based on the integrator by outputting the signal based on the steering angle as a signal based on the steering angle target value. Power steering device.
The electric power steering apparatus according to claim 1,
The restriction unit is
When the torque acting on the torsion bar reaches the first threshold value or more, the integration control based on the integrator is limited by outputting a signal based on the steering angle target value as a signal based on the steering angle. Electric power steering device.
The electric power steering apparatus according to claim 1,
The automatic steering control unit
A proportional device for performing proportional control based on the deviation;
A differentiator for performing differential control based on the deviation,
The restriction unit is
An electric power steering device that limits only integral control based on the integrator when a torque acting on the torsion bar reaches the first threshold value.
The electric power steering apparatus according to claim 1,
The restriction unit is
When the torque acting on the torsion bar reaches the first threshold value, the deviation between the signal based on the steering angle target value and the signal based on the steering angle is output as 0 and acts on the torsion bar. An electric power steering apparatus that outputs the deviation between the signal based on the steering angle target value and the signal based on the steering angle as it is when the torque decreases to a second threshold smaller than the first threshold.
The electric power steering apparatus according to claim 6,
The restriction unit is
When the torque acting on the torsion bar is within the range from the first threshold value to the second threshold value, the signal based on the steering angle target value and the steering angle are increased as the torque acting on the torsion bar increases. An electric power steering device for reducing the deviation from the signal based on the above.
The electric power steering apparatus according to claim 1,
The automatic steering control unit
An angular velocity calculator that calculates an angular velocity of the steering member based on the rudder angle;
An angular velocity target value calculation unit that calculates an angular velocity target value for controlling the electric motor based on the steering angle target value and the steering angle;
The electric power steering apparatus, wherein the deviation between the signal based on the steering angle target value and the signal based on the steering angle is a deviation between the angular velocity and the angular velocity target value.
The electric power steering apparatus according to claim 1,
When the torque acting on the torsion bar is equal to or greater than a third threshold for a certain time, further comprising an intervention determination unit that stops control by the automatic steering control unit,
The electric power steering apparatus, wherein the first threshold is set to a value smaller than the third threshold.
The electric power steering apparatus according to claim 1,
The automatic steering control unit is an electric power steering device that reduces the first threshold as the vehicle speed increases.