JP7722178B2 - Steering control device - Google Patents

Description

The present invention relates to a steering control device.

Traditionally, steering control devices have been known for vehicles equipped with lane keeping assist systems that assist the driver's steering by using assist torque output by a motor.

For example, the vehicle steering device disclosed in Patent Document 1 superimposes the vibration torque of a lane departure warning on the assist torque, and changes the magnitude of the vibration torque depending on the driver's steering state.

In the motor control device disclosed in Patent Document 2, the target tracking control calculation unit calculates a steering angle steering torque command through tracking control, i.e., automatic steering, to make the steering angle follow the target steering angle commanded by the lane keeping assist system. The motor control device drives the motor according to the sum of the assist torque command, which reduces the steering load, and the steering angle steering torque command for automatic steering, and reduces the responsiveness of the automatic steering when driver intervention is detected.

JP 2017-65587 A JP 2015-33942 A

When steering angle control is in operation through automatic steering, the steering angle based on the motor angle generated when the lane departure warning vibration torque is applied to drive the motor is corrected to match the target steering angle. Because the vibrations from the steering angle control cancel out the vibrations from the lane departure warning, the vibrations that appear in the steering wheel when the lane departure warning is activated are smaller than with normal assistance, making it harder for the driver to notice the warning.

The present invention was created in light of these issues, and its purpose is to provide a steering control device that prevents a decrease in the recognizability of lane departure warnings while steering angle control is being performed by lane keeping assist control.

The present invention relates to a steering control device that assists the driver's steering with an assist torque output by a motor (80) in a vehicle equipped with a lane keeping assist system (16) and a lane departure warning system (17). This steering control device includes an assist control unit (20), a steering angle control unit (30), a vibration application control unit (50), and a motor drive control unit (65).

The assist control unit calculates an assist torque command (Ta ^* ) based on the steering torque (Ts). The steering angle control unit calculates a steering angle control torque command (Tθ*) by a servo controller (34) so that the steering angle (θ) determined according to the output of the motor follows the target steering angle (θ ^{* )} commanded by the lane keeping assist device.

When a request for activation of the lane departure warning is notified from the lane departure warning device, the vibration application control unit calculates a lane departure warning control torque command (Tv ^* ) so as to apply vibration to the assist torque.

The motor drive control unit controls the drive of the motor based on the sum (Tm ^* ) of the assist torque command, the steering angle control torque command, and the lane departure warning control torque command.

The servo controller is configured to receive a steering angle deviation (Δθ) between a target steering angle and an actual steering angle, and to adjust the ratio of the output to the input by a steering angle control gain. The steering angle control unit has a servo response correction unit (38) that corrects the steering angle control gain so as to reduce the responsiveness of the servo controller when the lane departure warning is activated compared to when it is not activated, except in cases where a predetermined exemption requirement is met.

In this invention, the responsiveness of the steering angle control torque command is reduced when steering angle control by lane keeping assist control is in operation, making it more difficult to cancel out the vibration of the lane departure warning. This allows the driver to perceive the warning even when lane keeping assist control is in operation.

FIG. 1 is a schematic diagram of an electric power steering system. FIG. 1 is a schematic diagram of an EPS-ECU (steering control device) according to a first embodiment. 5 is a flowchart of a servo response correction process according to the first embodiment. Steering angle - steering torque vibration amplitude characteristic diagram. 10 is a time chart illustrating the influence of steering angle control on LDW amplitude in a comparative example. 4 is a time chart showing a servo response correction process according to the first embodiment. FIG. 10 is a schematic configuration diagram of an EPS-ECU according to a second embodiment. 10 is a flowchart of a servo response correction process according to a second embodiment. 5 is a time chart showing the servo response correction process according to the first embodiment when the target steering angle changes. 10 is a time chart showing the servo response correction process according to the second embodiment when the target steering angle changes.

Below, several embodiments of the steering control device will be described with reference to the drawings. Substantially identical components in the various embodiments will be assigned the same reference numerals and will not be described again. The first and second embodiments will be collectively referred to as "the present embodiment." The steering control device of this embodiment is applied to an electric power steering system of a vehicle equipped with a lane keeping assist device and a lane departure warning device. In each embodiment, the EPS-ECU corresponds to the "steering control device." Furthermore, the LKA (Lane Keep Assist)-ECU corresponds to the "lane keeping assist device," and the LDW (Lane Departure Warning)-ECU corresponds to the "lane departure warning device." LKA stands for lane keeping assist, and LDW stands for lane departure warning.

The LKA-ECU commands a target steering angle to maintain the vehicle in its lane during automatic steering. The LDW-ECU issues a warning to the driver if the vehicle is about to depart from its lane during driver steering or automatic steering. In this embodiment, instead of or in addition to an audible warning or display, the LDW-ECU works with the EPS-ECU to vibrate the steering wheel. The EPS-ECU assists the driver's steering with the assist torque output by the motor.

[Electric power steering system configuration]
As shown in Figure 1, the electric power steering system 1 is basically a system that assists the driver in operating a steering wheel 91 by using the driving torque of a motor 80. The steering wheel 91 is fixed to one end of a steering shaft 92, and an intermediate shaft 93 is provided on the other end of the steering shaft 92. The steering shaft 92 and the intermediate shaft 93 are connected by a torsion bar of a torque sensor 94, and these constitute a steering axis 95. The torque sensor 94 detects the steering torque Ts based on the torsion angle of the torsion bar.

A gearbox 96 including a pinion gear 961 and a rack 962 is provided at the end of the intermediate shaft 93 opposite the torque sensor 94. When the driver turns the steering wheel 91, the pinion gear 961 rotates together with the intermediate shaft 93, and as the pinion gear 961 rotates, the rack 962 moves left and right. Tie rods 97 are provided at both ends of the rack 962 and are connected to the tire 99 via knuckle arms 98. The tie rod 97 moves back and forth left and right, pulling and pushing the knuckle arms 98, changing the orientation of the tire 99.

The motor 80 is, for example, a three-phase AC brushless motor, and outputs drive torque according to the drive voltage Vd output from the EPS-ECU 15. In the case of a three-phase AC motor, the drive voltage Vd refers to the voltage of each of the U, V, and W phases. The rotation of the motor 80 is transmitted to the intermediate shaft 93 via a reduction mechanism 85 composed of a worm gear 86, a worm wheel 87, etc. The rotation of the intermediate shaft 93 caused by steering the steering wheel 91 or reaction force from the road surface is also transmitted to the motor 80 via the reduction mechanism 85.

Note that the electric power steering system 1 shown in FIG. 1 is a column-assist type in which rotation of the motor 80 is transmitted to the steering shaft 95, but the EPS-ECU 15 of this embodiment can also be applied to a rack-assist type electric power steering system or a steer-by-wire system in which the steering wheel and steered wheels are mechanically separated. In other embodiments, a multi-phase AC motor other than a three-phase motor or a brushed DC motor may be used as the motor.

Here, the entire mechanism that transmits the steering force of the steering wheel 91 from the steering wheel 91 to the tires 99 is referred to as the "steering system mechanism 100." The EPS-ECU 15 controls the driving torque output by the motor 80 to the steering system mechanism 100, thereby controlling the steering torque Ts generated by the steering system mechanism 100. The EPS-ECU 15 obtains the steering torque Ts and the steering angle θ from the steering system mechanism 100. The EPS-ECU 15 also obtains the vehicle speed V detected by a vehicle speed sensor 11 installed at a predetermined location on the vehicle.

Furthermore, the EPS-ECU 15 acquires the target steering angle θ ^* and the steering angle control request flag F1 from the LKA-ECU 16, and acquires the LDW operation request flag F2 from the LDW-ECU 17. During automatic steering, the LKA-ECU 16 outputs the steering angle control request flag F1 to the EPS-ECU 15. As referred to in Patent Document 2 (JP 2015-33942 A) and the like, the LKA-ECU 16 sets a target course based on the driving lane and the position of the host vehicle detected from video footage from an on-board camera, and outputs the target steering angle θ ^* for traveling along the target course to the EPS-ECU 15.

As seen in Patent Document 1 (JP 2017-65587 A) and elsewhere, the LDW-ECU 17 determines whether there is a risk of lane departure or collision with an obstacle based on the steering angle, vehicle speed, video from an onboard camera, yaw rate, lateral acceleration, etc. If it determines that there is a risk of lane departure or collision with an obstacle, the LDW-ECU 17 outputs an LDW operation request flag F2 to the EPS-ECU 15.

The EPS-ECU 15 operates using power from an on-board battery (not shown) and calculates various torque commands (described below) based on the acquired information. The EPS-ECU 15 drives the motor 80 by applying a drive voltage Vd to the motor 80 in accordance with the sum of the calculated torque commands. Note that the various calculation processes in the EPS-ECU 15 may be software processes in which a CPU executes a program pre-stored in a physical memory device such as a ROM, or may be hardware processes using dedicated electronic circuits.

[Configuration of EPS-ECU]
Next, the configuration of the EPS-ECU 15 of each embodiment will be described in order. The reference numeral of the EPS-ECU of each embodiment has the embodiment number added as the third digit following "15".

(First embodiment)
The first embodiment will be described with reference to Figures 2 to 6. As shown in Figure 2, the EPS-ECU 151 of the first embodiment includes an assist control unit 20, a vibration control unit 50, a steering angle control unit 30, and a motor drive control unit 65. For convenience of explanation, the vibration control unit 50 will be described before the steering angle control unit 30.

The assist control unit 20 calculates the assist torque command Ta ^* in accordance with the driver's steering torque Ts. Specifically, the assist control unit 20 calculates the assist torque command Ta ^* based on the steering torque Ts and the vehicle speed V so as to achieve a transmission feeling in accordance with the road reaction force (or road load) and a feel in accordance with the steering state. When the LDW-ECU 17 notifies the LDW operation request by using flag F2, the vibration application control unit 50 calculates the LDW control torque command Tv ^* so as to apply vibration to the assist torque to make the driver aware of it.

The steering angle control unit 30 has a steering angle deviation calculator 33, a servo controller 34, and a servo response correction unit 38. The steering angle deviation calculator 33 calculates the steering angle deviation Δθ (= θ ^{* -} θ) between the steering angle θ and the target steering angle θ * commanded by the LKA-ECU 16. The servo controller 34 calculates the steering angle control torque command Tθ ^* by PID control so that the steering angle θ follows the target steering angle θ ^* , i.e., so that the steering angle deviation Δθ approaches 0.

When the LDW-ECU 17 notifies the servo response correction unit 38 of a request to operate the LDW using flag F2, i.e., when the LDW is in operation, the servo response correction unit 38 corrects the steering angle control gain so that, in principle, the responsiveness of the servo controller 34 is reduced compared to when the LDW is not in operation. Here, "in principle" means that cases where certain exemption requirements are met are excluded as exceptions. Cases where certain exemption requirements are met will be described later in the second embodiment. This processing by the servo response correction unit 38 is referred to as "servo response correction processing."

The positive and negative signs of the torque commands Ta ^* , Tθ ^* , Tv ^* , and Tm ^* are defined according to the rotation direction in which the torque is applied. For example, torque applied in the counterclockwise direction is defined as positive, and torque applied in the clockwise direction is defined as negative. The positive and negative signs of the steering angle θ are defined, for example, as an angle to the left of the neutral position being positive, and an angle to the right of the neutral position being negative.

The command adder 60 calculates a final assist torque command Tm ^* , which is the sum of the assist torque command Ta ^* , the steering angle control torque command Tθ ^* , and the LDW control torque command Tv ^* , and outputs the calculated final assist torque command Tm ^* to the motor drive control unit 65. The motor drive control unit 65 drives the motor 80 by applying a drive voltage Vd to the motor 80 in accordance with the final assist torque command Tm*. This causes the motor 80 to output an assist torque Ta corresponding to the final assist torque command Tm ^* . The steering angle θ, which is determined according to the output of the motor, is fed back to the steering angle control unit 30.

The servo response correction process according to the first embodiment will be described with reference to the flowchart in Figure 3. In the following description of the flowchart, the symbol "S" denotes a step. In S1, it is determined whether the LDW is operating based on flag F2 from the LDW-ECU 17.

When the LDW is operating, a YES determination is made in S1, and the steering angle control gain is reduced in S3 to reduce the responsiveness of the servo control. For example, K is set to 0.3. When the LDW is not operating, a NO determination is made in S1, and the steering angle control gain is set to the reference value K = 1 in S4. This routine is executed repeatedly at a predetermined calculation interval.

Figure 4 shows the relationship between steering angle θ and steering torque vibration amplitude for each steering angle control gain. Here, 0≦θ≦25 [deg]. When the steering angle control gain is K=1 while the LKA is operating, as shown by the two-dot chain line, the steering torque vibration amplitude decreases as the steering angle θ increases from the neutral position (θ=0). When the LKA is not operating, i.e., the driver is steering, the steering angle control gain corresponds to K=0. In this case, as shown by the dashed line, the characteristic line for steering torque vibration amplitude is generally larger than the characteristic line for K=1. The difference is greater in areas where the steering angle θ is smaller.

When the steering angle control gain is corrected from K=1 to K=0.3 using servo response correction processing, the characteristic line for steering torque vibration amplitude approaches the characteristic line for K=0, as shown by the solid line. In other words, the effect of steering angle control on LDW amplitude is suppressed. Note that if the steering angle control gain is set to a value between K=0.3 and K=1 (for example, K=0.5), it is estimated that characteristics between these characteristic lines will be obtained.

The influence of steering angle control on LDW amplitude in the comparative example and the first embodiment will be described with reference to the time charts of Figures 5 and 6. From top to bottom, each figure shows the operation of LKA, target steering angle θ ^* and steering angle θ, steering angle control gain K, steering angle control torque command Tθ ^* , LDW operation, LDW control torque command Tv ^* , final assist torque command Tm ^* , and steering torque vibration amplitude. The dimension of the steering angle is angle ([deg]), and the dimension of each torque command is torque ([Nm]).

In this time chart, the LDW operates twice. Oscillations of the LDW control torque command Tv ^* are generated during the first LDW operation period W1 and the second LDW operation period W2. Because the LKA is not operating during the first LDW operation period W1, the LDW control torque command Tv ^* is not affected by the steering angle control, but becomes an oscillation component of the final assist torque command Tm ^* as it is, and is reflected in the steering torque oscillation amplitude.

After the first LDW operation period W1, the LKA begins operating at time t1. When the target steering angle θ ^* changes at time t2, the steering angle θ changes to follow the target steering angle θ ^* and reaches the target steering angle θ ^* at time t3. After that, the LDW operation period W2 occurs while the LKA is operating. At this time, steering angle control works to bring the steering angle θ, which has vibrated due to LDW vibration, closer to the target steering angle ^θ *, and a vibration component in the opposite direction to the vibration of the steering angle θ is applied to the steering angle control torque command Tθ ^* .

In the comparative example shown in Figure 5, the steering angle control gain is K = 1 regardless of the operation of the LKA. Therefore, as shown by (*Z1), the amplitude of the steering angle control torque command Tθ ^* during the second LDW operation period W2 is large. Then, as shown by (*Z2), the vibration of the steering angle control torque command Tθ ^* cancels out the vibration component of the LDW control torque command Tv ^* . Therefore, the vibration appearing in the steering wheel when the LDW is operating is smaller than during normal assist when the LKA is not operating, making it difficult for the driver to perceive the warning.

In the first embodiment shown in Figure 6, during the LDW operation periods W1 and W2, the servo response corrector 38 corrects the steering angle control gain from K = 1 to K = 0.3, which reduces the responsiveness of the steering angle control torque command Tθ ^* . Therefore, as indicated by (*A1), the amplitude of the steering angle control torque command Tθ ^* during the second LDW operation period W2 is smaller than that shown in Figure 5. Then, as indicated by (*A2), it becomes more difficult for the vibration of the steering angle control torque command Tθ ^* to cancel out the vibration component of the LDW control torque command Tv ^* . This allows the driver to perceive the warning even during LKA operation.

Second Embodiment
Next, a second embodiment will be described with reference to Figures 7 to 10. As shown in Figure 7, in an EPS-ECU 152 of the second embodiment, the steering angle deviation Δθ calculated by a steering angle deviation calculator 33 is output to a servo response correction unit 38 in addition to a servo controller 34. The rest of the configuration is the same as that of the EPS-ECU 151 of the first embodiment, and therefore description thereof will be omitted.

In principle, when the LDW is activated, the servo response correction unit 38 executes the servo response correction process in the same manner as in the first embodiment. However, when the absolute value of the steering angle deviation Δθ is greater than a predetermined steering angle deviation threshold Δθth, the servo response correction unit 38 does not execute the servo response correction process, assuming that the "case satisfies predetermined exemption requirements."

Figure 8 shows a flowchart of the servo response correction process according to the second embodiment. Steps that are essentially the same as those in Figure 3 of the first embodiment are assigned the same step numbers and their explanations are omitted. If the answer in S1 is YES while the LDW is operating, the absolute value of the steering angle deviation Δθ is compared with the steering angle deviation threshold value Δθth in S2.

When the absolute value of the steering angle deviation Δθ is equal to or less than the steering angle deviation threshold Δθth, a YES determination is made in S2. In this case, as in the first embodiment, the steering angle control gain is reduced in S3 to reduce the responsiveness of the servo control. For example, K is set to 0.3. When the absolute value of the steering angle deviation Δθ is greater than the steering angle deviation threshold Δθth, a NO determination is made in S2. In this case, the steering angle control gain is set to K = 1 in S4. In other words, the same steering angle control gain is used as when the LDW is not operating.

The differences between the servo response correction processes according to the first and second embodiments when the target steering angle changes will be described with reference to the time charts in Figures 9 and 10. From top to bottom, each figure shows the operation of the LKA, the target steering angle θ ^* and steering angle θ, the steering angle deviation Δθ, the steering angle control gain K, the steering angle control torque command Tθ ^* , and the operation of the LDW.

As in Figures 5 and 6, the LKA starts operating at time t1. When the target steering angle θ ^* changes at time t2, the absolute value of the steering angle deviation Δθ increases. As the steering angle θ follows the target steering angle θ ^* , the steering angle deviation Δθ gradually decreases, converging to zero at time t3.

The first embodiment shown in Figure 9 corresponds to a comparative example for the second embodiment. In the first embodiment, the steering angle control gain is always corrected to a small value while the LDW is operating. During the first LDW operation period W1 before time t1, the LKA is not operating and does not affect steering angle control.

On the other hand, during the second LDW operation period W2, the steering angle control gain is corrected to a smaller value while the LKA is operating. As explained with reference to FIG. 6, by reducing the responsiveness of the steering angle control torque command Tθ ^* as shown by (*A1), it is possible to prevent a decrease in the noticeability of the warning. However, the tradeoff of reducing the responsiveness of the steering angle control is that the tracking ability of the steering angle θ to the target steering angle θ ^* decreases as shown by (*A3). In other words, the gradient of change in the steering angle θ during the LDW operation period W2 becomes smaller, and the response time Tres from time t2 to t3 becomes longer.

In the second embodiment shown in Figure 10, when the steering angle θ changes and the absolute value of the steering angle deviation Δθ becomes greater than the steering angle deviation threshold Δθth, priority is given to steering angle control tracking over preventing a decrease in warning noticeability. In other words, the exemption requirements are met, and processing to reduce the responsiveness of the servo controller 34 is not executed. During the LDW operation period W2, because the absolute value of the steering angle deviation Δθ is greater than the steering angle deviation threshold Δθth, the servo controller 34 uses the same steering angle control gain (i.e., K = 1) as when the LDW is not operating.

As a result, as shown by (*B1), the tracking ability of the steering angle θ relative to the target steering angle θ ^* does not decrease even during the LDW operation period W2. When the absolute value of the steering angle deviation Δθ becomes smaller than the steering angle deviation threshold Δθth, the exemption requirement is lifted. When the LDW is operated after time t3, if the absolute value of the steering angle deviation Δθ is equal to or smaller than the steering angle deviation threshold Δθth, the servo response correction unit 38 reduces the responsiveness of the servo controller 34 as a rule. In this way, in the second embodiment, the priority between warning noticeability and steering angle control tracking can be switched according to changes in the steering angle θ.

(Other embodiments)
The exemption requirement for the servo response correction process may be set as a parameter such as the "acceleration of steering angle change" which corresponds to the second derivative of the steering angle θ, in addition to the steering angle deviation Δθ, i.e., the first derivative of the steering angle θ.

The present invention is not limited to these embodiments, and can be implemented in various forms without departing from its spirit.

The control unit and method described in the present disclosure may be implemented by a special-purpose computer provided by configuring a processor and memory programmed to perform one or more functions embodied in a computer program. Alternatively, the control unit and method described in the present disclosure may be implemented by a special-purpose computer provided by configuring a processor with one or more dedicated hardware logic circuits. Alternatively, the control unit and method described in the present disclosure may be implemented by one or more special-purpose computers configured by combining a processor and memory programmed to perform one or more functions with a processor configured with one or more hardware logic circuits. Furthermore, the computer program may be stored as instructions executed by a computer on a computer-readable non-transitory tangible recording medium.

15...EPS-ECU (steering control unit),
16...LKA-ECU (lane keeping assist device),
17...LDW-ECU (Lane Departure Warning System),
20... Assist control unit,
34...Servo controller,
38...Servo response correction unit,
50...Vibration applying control unit,
65...Motor drive control unit,
80...Motor.

Claims (2)

A steering control device for a vehicle equipped with a lane keeping assist device (16) and a lane departure warning device (17), which assists a driver's steering with an assist torque output by a motor (80),
an assist control unit (20) that calculates an assist torque command (Ta ^* ) based on the steering torque (Ts);
a steering angle control unit (30) that calculates a steering angle control torque command (Tθ ^* ) by a servo controller (34) so that the steering angle (θ) determined according to the output of the motor follows a target steering angle (θ ^* ) commanded by the lane keeping assist device;
a vibration application control unit (50) that calculates a lane departure warning control torque command (Tv ^* ) so as to apply vibration to the assist torque when a request for activation of the lane departure warning is notified from the lane departure warning device;
a motor drive control unit (65) that controls the drive of the motor based on a sum (Tm ^* ) of the assist torque command, the steering angle control torque command, and the lane departure warning control torque command;
Equipped with
the servo controller is configured to receive a steering angle deviation (Δθ) between the target steering angle and the actual steering angle, and to adjust the ratio of the output to the input by a steering angle control gain;
The steering control device has a servo response correction unit (38 ) that corrects the steering angle control gain so as to reduce the responsiveness of the servo controller when the lane departure warning is activated compared to when the lane departure warning is not activated, except in cases where specified exemption requirements are met. When the absolute value of the steering angle deviation (Δθ), which is the difference between the steering angle and the target steering angle, is greater than a predetermined steering angle deviation threshold, the following applies:
2. The steering control device according to claim 1, wherein the servo controller uses the same steering angle control gain when the lane departure warning is activated as when the lane departure warning is deactivated.