Turning control device, turning control method, and turning device

Abstract

According to a turning control device, a turning control method, and a turning device of the present invention, in one aspect thereof: a first physical quantity related to a steering operation amount of a steering input member and a second physical quantity generated when a vehicle turns are acquired; a ratio of the steering operation amount to a turning angle of a turning wheel is varied in accordance with a traveling state of the vehicle; and, in relation to a delay in a change in the turning angle relative to a change in the steering operation amount, when the ratio is small, a target turning angle that increases the degree of the delay in the change in the turning angle with respect to a change in the steering operation amount is acquired on the basis of the second physical amount, and a turning motor is controlled such that the turning angle becomes the target turning angle. This makes it possible to prevent the driver from feeling that the steering response is too sensitive while ensuring good maneuverability.

Description

Steering control device, steering control method, and steering device

[0001] The present invention relates to a steering control device, a steering control method, and a steering device.

[0002] The steering system of Patent Document 1 is a steering system configured so that in a first steering angle region, the ratio of the amount of change in steering angle to the amount of change in steering angle is greater than in a second steering angle region, which is a region where the steering angle is smaller than in the first steering angle region; regardless of whether the steering is increasing or decreasing, the degree of delay in the change in steering angle relative to the change in steering angle is greater in the first steering angle region than in the second steering angle region; when the steering angle is in the second steering angle region, the ratio of the amount of change in steering angle to the amount of change in steering angle is smaller when the vehicle speed is high than when the vehicle speed is low, and the degree of delay in the change in steering angle relative to the change in steering angle is smaller; and when the steering angle is in the first steering angle region, the ratio of the amount of change in steering angle to the amount of change in steering angle is greater when the vehicle speed is high than when the vehicle speed is low, and the degree of delay in the change in steering angle relative to the change in steering angle is greater.

[0003] Patent No. 7091995

[0004] In steer-by-wire steering devices, the steering gear ratio, which is the ratio of the steering operation amount of the steering input member to the steering angle of the steered wheels, is made smaller (quick ratio) as the vehicle speed decreases, thereby improving maneuverability. However, when making a U-turn, for example, a small amount of steering operation can cause large vehicle behavior (yaw rate, lateral acceleration, etc.), which can make the driver feel that the steering response is too sensitive.

[0005] The present invention has been made in consideration of the conventional situation, and an object of the present invention is to provide a steering control device, a steering control method, and a steering device that can prevent the driver from feeling that the steering response is too sensitive while ensuring good maneuverability.

[0006] In one aspect, the steering control device, steering control method, and steering device according to the present invention acquire a first physical quantity related to a steering operation amount of a steering input member that accepts a steering operation by a driver, and a second physical quantity that is generated when the vehicle turns, vary a ratio of the steering operation amount to the steering angle of the steered wheels in accordance with a running state of the vehicle, and when the ratio of the steering operation amount to the steering angle is small, acquire a target steering angle based on the second physical quantity for increasing the degree of delay in change of the steering angle relative to a change in the steering operation amount, and control a motor that applies a steering force to the steered wheels so that the steering angle becomes the acquired target steering angle.

[0007] According to the present invention, it is possible to prevent the driver from feeling that the steering response is too sensitive while ensuring good maneuverability.

[0008] 1 is a schematic diagram showing a vehicle equipped with a steer-by-wire steering device. FIG. 2 is a block diagram showing the steering control function of the steering control device. FIG. 3 is a diagram showing the correlation between vehicle speed and steering gear ratio. FIG. 4 is a diagram illustrating an example of a variable characteristic of the cutoff frequency in a turning-back operation. FIG. 5 is a diagram illustrating an example of a characteristic of gradually decreasing the cutoff frequency in response to a gradual increase in yaw rate. FIG. 6 is a diagram showing changes in steering operation amount and vehicle behavior when there is no delay in change in steering angle relative to a change in steering operation amount. FIG. 7 is a diagram showing changes in steering operation amount, steering angle, and vehicle behavior when the degree of delay is variable in accordance with the steering operation amount. FIG. 8 is a diagram illustrating changes in steering operation amount, steering angle, and vehicle behavior when the degree of delay is variable in accordance with the yaw rate and vehicle speed.

[0009] Hereinafter, embodiments of a steering control device, a steering control method, and a steering device according to the present invention will be described with reference to the drawings. Figure 1 is a schematic diagram showing one aspect of a vehicle 100 fitted with a steer-by-wire type steering device 200. Vehicle 100 is a four-wheeled automobile equipped with a pair of left and right front wheels 101, 102 and a pair of left and right rear wheels 103, 104.

[0010] Steering device 200 has steering input device 300 to which the steering operation of the driver of vehicle 100 is input via steering input member 310, steering device 400 equipped with steering motor 410 (road wheel actuator) for applying steering force to the steered wheels (front wheels 101, 102) of vehicle 100, and steering control device 500 which is an electronic control device that controls steering input device 300 and steering device 400. Here, steering input member 310 and front wheels 101, 102 which are steered wheels are mechanically separated, and steering device 200 is equipped with a steer-by-wire steering system in which steering motor 410 is controlled based on a signal of the driver's steering operation, thereby changing the steering angle of the steered wheels (in other words, tire angle).

[0011] The steering input device 300 includes a steering input member 310, a steering reaction force imparting device 320, a steering operation amount sensor 330, etc. The steering input member 310 is an operator that accepts steering operations by the driver, and in addition to a steering wheel, a dial-type, stick-type, or other steering input member is used. As will be described in detail later, the present invention has the effect of ensuring good maneuverability while preventing the driver from feeling that the steering response is too sensitive. Therefore, the present invention is more suitable for vehicles that employ new types of steering input member 310, such as a dial-type, that change the steering angle with a small steering operation by the driver.

[0012] The steering reaction force imparting device 320 is a means for biasing the steering input member 310 toward the neutral position (the straight-ahead position of the vehicle 100), and applies a steering reaction force to the steering input member 310 by means of a motor or the like. The steering operation amount sensor 330 is a sensor that detects the steering operation amount, which is the amount of operation of the steering input member 310. For example, if the steering input member 310 is a steering wheel or a dial-type steering input member, the steering operation amount sensor 330 detects the steering operation angle θ, which is the rotation angle of the steering shaft or dial, as the steering operation amount.

[0013] Steering device 400 includes steering mechanism 420. Steering mechanism 420 is a mechanism that converts the rotational motion of steering motor 410 into linear motion of rack shaft 421 by a rack-and-pinion system, thereby changing the steering angle (tire angle) of front wheels 101, 102 connected to rack shaft 421. Steering device 400 also includes rack stroke sensor 430 that detects rack stroke RS [mm] (in other words, actual rack position), which is the stroke amount of rack shaft 421 that correlates with the steering angle of front wheels 101, 102, and motor rotation angle sensor 440 that detects rotation angle θm [deg] of steering motor 410.

[0014] The steering control device 500 is an electronic control device equipped with an MCU (Micro Controller Unit) 510 which is a control section that executes a steering control method. The steering control device 500 controls the operation of the steering device 200 by controlling the steering reaction force imparting device 320 and the steering motor 410 that are provided in the steering device 200. In detail, the MCU 510 calculates and processes various signals acquired from the outside to determine control signals for the steering reaction force imparting device 320 and the steering motor 410, and outputs the determined control signals. The MCU 510 can also be referred to as a microcomputer, processor, processing device, arithmetic device, etc.

[0015] Vehicle 100 also includes wheel speed sensors 621-624 that detect wheel speeds WS1-WS4, which are the rotational speeds of wheels 101-104, respectively, an acceleration sensor 630 that detects the acceleration in the longitudinal direction and the acceleration in the lateral direction of vehicle 100, and a yaw rate sensor 640 that detects the yaw rate generated in vehicle 100. MCU 510 of steering control device 500 acquires output signals from steering operation amount sensor 330, rack stroke sensor 430, motor rotation angle sensor 440, wheel speed sensors 621-624, acceleration sensor 630 and yaw rate sensor 640.

[0016] Here, we will outline the control process of steering motor 410 by MCU 510 of steering control device 500. MCU 510 acquires information on the steering operation amount (steering operation angle) of steering input member 310 from steering operation amount sensor 330, and also acquires information on the vehicle speed of vehicle 100 from the output signals of wheel speed sensors 621-624. MCU 510 then variably sets the steering gear ratio from the vehicle speed information, and determines a target rack stroke (in other words, a target rack position) equivalent to the target steering angle of front wheels 101, 102 from the steering operation amount and steering gear ratio.

[0017] After determining the target rack stroke, MCU 510 acquires information about the actual rack stroke (in other words, the actual rack position or the actual steering angle) detected by rack stroke sensor 430, and controls steering motor 410 so that the actual rack stroke approaches the target rack stroke. The above-mentioned steering gear ratio is the ratio of the steering operation amount of steering input member 310 to the steering angle of the steered wheels, and is defined as "steering gear ratio = steering operation amount / steering angle".

[0018] In this embodiment, MCU 510 detects the actual rack stroke using rack stroke sensor 430, but can also determine the actual rack stroke from rotation angle θm of steering motor 410 detected by motor rotation angle sensor 440. MCU 510 can also determine the actual rack stroke using a rotation angle sensor that detects the rotation angle of a pinion shaft of the rack and pinion. MCU 510 can also use a signal of the rotation angle of a motor that constitutes steering reaction force imparting device 320 as information about the steering operation amount of steering input member 310. If steering input device 300 includes a mechanism that converts the rotational motion of steering input member 310 into linear motion, MCU 510 can acquire, as information about the steering operation amount of steering input member 310, a signal from a sensor that detects the stroke amount of a movable member that moves linearly in response to the rotation of steering input member 310.

[0019] As described above, MCU 510 has a function to variably set the steering gear ratio based on information about the vehicle speed, which is a traveling state of vehicle 100. Furthermore, as will be described in detail later, when the steering gear ratio is small (in other words, when the vehicle speed is low), MCU 510 has a function to acquire a target steering angle that increases the degree of delay in the change in the steering angle relative to a change in the steering operation amount, based on physical quantities that occur when vehicle 100 turns. Note that the physical quantities that occur when vehicle 100 turns are physical quantities related to the turning behavior of vehicle 100, such as yaw rate and lateral acceleration.

[0020] 2 is a block diagram showing the steering angle control function of MCU 510 of steering control device 500. MCU 510 acquires information on the steering operation amount of steering input member 310, in other words, a first physical quantity related to the steering operation amount, from steering operation amount sensor 330. MCU 510 also acquires information on the yaw rate generated in vehicle 100, in other words, a second physical quantity generated when vehicle 100 turns, from yaw rate sensor 640. Note that MCU 510 of this embodiment uses yaw rate information as the second physical quantity generated when vehicle 100 turns, but lateral acceleration can also be used as the second physical quantity, and further, yaw rate and lateral acceleration can be used as the second physical quantity. Furthermore, MCU 510 acquires information on the vehicle speed of vehicle 100 from output signals of wheel speed sensors 621-624.

[0021] MCU 510 has functional units including a steering gear ratio control unit 511, a low-pass filter processing unit 512, a turning control unit 513, a first cutoff frequency setting unit 514, a second cutoff frequency setting unit 515, and a turning / returning determination unit 516. Steering gear ratio control unit 511 acquires information on the steering operation amount and information on the vehicle speed. Then, steering gear ratio control unit 511 variably sets the steering gear ratio based on the vehicle speed, and converts the steering operation amount into a target rack stroke (target turning angle) based on the steering gear ratio.

[0022] Here, the steering gear ratio control unit 511 sets the steering gear ratio to a smaller value as the vehicle speed decreases, that is, a steering gear ratio that is controlled to a larger steering angle for the same steering operation amount. Note that in this application, the magnitude of the steering operation amount and the steering angle indicates the absolute value of the amount of displacement from the neutral position (the straight-ahead position of the vehicle 100).

[0023] Fig. 3 is a diagram showing the correlation between vehicle speed and steering gear ratio. Fig. 3 is a graph with the horizontal axis representing the steering operation amount, the vertical axis representing the turning angle (tire angle), and the origin representing the neutral position, and illustrates the characteristics of the steering gear ratio for representative vehicle speeds of 20 km / h, 60 km / h, and 120 km / h. Here, the turning angle increases in proportion to the increase in steering operation amount, but the larger the slope, which is the ratio of the increase in steering angle to the increase in steering operation amount, the smaller the steering gear ratio.

[0024] In Figure 3, the slope is maximum at the minimum speed of 20 km / h, minimum at the maximum speed of 120 km / h, and set to an intermediate value at the intermediate speed of 60 km / h. In other words, Figure 3 shows the characteristic (quick ratio) in which the steering gear ratio is changed to a smaller value as the vehicle speed decreases, and the steering angle is controlled to a larger value as the vehicle speed decreases for the same steering operation amount.

[0025] Steering gear ratio control unit 511 outputs a signal of a target rack stroke (target steering angle) calculated from the steering operation amount and the steering gear ratio to low-pass filter processing unit 512. Low-pass filter processing unit 512 applies low-pass filter processing to the signal of the target rack stroke (target steering angle) calculated based on the steering operation amount, thereby causing a phase delay in the change in the target steering angle relative to a change in the steering operation amount. In other words, low-pass filter processing unit 512 attenuates frequency components of the target rack stroke signal that are higher than the cutoff frequency, thereby causing a delay in the change in the steering angle relative to a change in the steering operation amount, and ultimately in the turning behavior of vehicle 100.

[0026] In this way, MCU 510 determines the degree of delay in the change in the steering angle relative to a change in the steering operation amount using a low-pass filter. As will be described later, MCU 510 has a function of varying the cutoff frequency in low-pass filter processing unit 512 based on the yaw rate, which is a physical quantity generated when vehicle 100 turns, and the vehicle speed, which indicates the traveling state of vehicle 100.

[0027] Steering control unit 513 acquires the target rack stroke signal that has been subjected to low pass filtering in low pass filter processing unit 512, and also acquires the actual rack stroke signal detected by rack stroke sensor 430 or the like. Steering control unit 513 then controls the current of steering motor 410 so that the actual rack stroke approaches the target rack stroke, that is, so that the actual steering angle approaches the target steering angle.

[0028] On the other hand, first cutoff frequency setting unit 514, second cutoff frequency setting unit 515, and steering increase / return determination unit 516 are functional units for varying the cutoff frequency in low-pass filter processing unit 512, in other words, the degree of delay in change of steering angle relative to change in steering operation amount. Here, first cutoff frequency setting unit 514 varies the cutoff frequency in low-pass filter processing unit 512 when the driver's steering operation is an operation to increase the steering angle. On the other hand, second cutoff frequency setting unit 515 varies the cutoff frequency in low-pass filter processing unit 512 when the driver's steering operation is an operation to return the steering angle.

[0029] Further, steering increase / return determination unit 516 determines whether the driver's steering operation is an operation to increase the steering angle or an operation to return the steering angle, based on the output of steering operation amount sensor 330 or the output of yaw rate sensor 640. An operation to increase the steering angle is an operation to increase the absolute value of the steering angle when the steering angle at the neutral position (straight ahead position) is set to zero, and a steering return operation is an operation to decrease the absolute value of the steering angle.

[0030] In other words, further steering / returning determination unit 516 determines whether the driver's steering operation is an operation to further turn steering input member 310 from the neutral position, or an operation to return steering from the neutral position. Therefore, further steering / returning determination unit 516 can determine whether the driver's steering operation is an operation to further turn the steering angle or an operation to return the steering angle, based on whether the steering operation amount is changing in the direction away from the neutral position or in the direction approaching the neutral position.

[0031] Furthermore, the yaw rate increases as the steering angle is turned further, and conversely, the yaw rate decreases as the steering angle is turned back. Therefore, steering further / returning determination unit 516 can determine whether the driver's steering operation is an operation to turn the steering angle further or an operation to turn back the steering angle, based on the increase or decrease in yaw rate.

[0032] Then, based on the determination signal output by further steering / returning determination section 516, if the steering operation is further, first cutoff frequency setting section 514 performs variable processing of the cutoff frequency, and if the steering operation is returning, second cutoff frequency setting section 515 performs variable processing of the cutoff frequency. In other words, MCU 510 has a function of varying the cutoff frequency in the low-pass filter processing of the target rack stroke (target steering angle) depending on whether the steering operation by the driver is an operation to further turn the steering angle or an operation to return the steering angle.

[0033] Fig. 4 is a cutoff frequency map showing the variable characteristics of the cutoff frequency in the first cutoff frequency setting unit 514. Fig. 5 is a cutoff frequency map showing the variable characteristics of the cutoff frequency in the second cutoff frequency setting unit 515. The cutoff frequency maps in the first cutoff frequency setting unit 514 and the second cutoff frequency setting unit 515 have vehicle speed on the horizontal axis and yaw rate on the vertical axis, with cutoff frequency data defined at each intersection. In the cutoff frequency maps shown in Figs. 4 and 5, the cutoff frequency is set to one of four types: very low, low, medium, and high. However, the cutoff frequency may be switched more finely in the maps.

[0034] The first cutoff frequency setting unit 514 and the second cutoff frequency setting unit 515 acquire a vehicle speed signal and a yaw rate signal, and extract cutoff frequency data corresponding to the combination of the acquired vehicle speed signal and yaw rate signal from a cutoff frequency map. Then, the first cutoff frequency setting unit 514 and the second cutoff frequency setting unit 515 set the cutoff frequency extracted from the cutoff frequency map as the cutoff frequency in the low-pass filter processing unit 512.

[0035] The cutoff frequency maps in first cutoff frequency setting unit 514 and second cutoff frequency setting unit 515 are set so that the cutoff frequency is lower when the vehicle speed is low (low vehicle speed range) than when the vehicle speed is high (medium to high vehicle speed range), that is, so that the attenuation range is expanded toward the low frequency side. In other words, the cutoff frequency map is set so that the degree of delay in the change in the steering angle in response to a change in the steering operation amount is greater when the vehicle speed is low than when the vehicle speed is high.

[0036] Furthermore, the cutoff frequency maps in first cutoff frequency setting section 514 and second cutoff frequency setting section 515 are set so that in the low vehicle speed range, the higher the yaw rate (second physical quantity), the lower the cutoff frequency becomes, and the greater the degree of delay in the change in the steering angle relative to a change in the steering operation amount. In other words, first cutoff frequency setting section 514 and second cutoff frequency setting section 515 vary the cutoff frequency so that the degree of delay in the change in the steering angle relative to a change in the steering operation amount becomes greater the lower the vehicle speed and the greater the yaw rate.

[0037] By setting the degree of delay in the change in the steering angle relative to the change in the steering operation amount as described above, MCU 510 can prevent large vehicle behavior (such as yaw rate or lateral acceleration) from occurring with a small steering operation amount in a situation where the yaw rate is large at low speed, such as when making a U-turn with vehicle 100. This makes it possible to prevent the driver from feeling that the steering response is too sensitive in a situation where the yaw rate is large at low speed, such as when making a U-turn with vehicle 100.

[0038] If the cutoff frequency extracted from the cutoff frequency map is applied as is to low-pass filter processing unit 512 and the cutoff frequency in low-pass filter processing unit 512 changes in a stepwise manner, this will result in a sudden change in response to the steering angle, which may deteriorate the driver's steering ability and ride comfort. Therefore, when applying the cutoff frequency extracted from the cutoff frequency map to low-pass filter processing unit 512, first cutoff frequency setting unit 514 and second cutoff frequency setting unit 515 gradually decrease the cutoff frequency set in low-pass filter processing unit 512 in response to a gradual increase in yaw rate.

[0039] Fig. 6 is a diagram illustrating an example of a gradual decrease in the cutoff frequency with respect to a gradual increase in the yaw rate. Fig. 6 shows a case in which the cutoff frequency is linearly decreased with respect to a gradual increase in the yaw rate, and a case in which the cutoff frequency is gradually decreased with respect to a gradual increase in the yaw rate so that the slope of the cutoff frequency increases in a quadratic curve. When the cutoff frequency extracted from the map switches, for example, from medium to low to extremely low with a gradual increase in the yaw rate, the MCU 510 can linearly decrease the cutoff frequency set in the low-pass filter processing unit 512 with respect to a gradual increase in the yaw rate, as shown by the solid line in Fig. 6.

[0040] Furthermore, when the cutoff frequency extracted from the map switches, for example, from medium to low to very low as the yaw rate gradually increases, the MCU 510 can gradually decrease the cutoff frequency set in the low-pass filter processing unit 512 so that the slope increases like a quadratic curve with respect to the gradual increase in yaw rate, as shown by the dotted line in Figure 6. If the cutoff frequency is gradually decreased so that the slope increases like a quadratic curve with respect to the gradual increase in yaw rate, the driver is less likely to notice the change in responsiveness than if the cutoff frequency is gradually decreased linearly. In this way, if the cutoff frequency set in the low-pass filter processing unit 512 is gradually decreased as the yaw rate gradually increases, the driver is less likely to notice the change in steering response, and the driver's discomfort with the maneuverability of the vehicle 100 can be suppressed.

[0041] Next, the difference between the cutoff frequency during further steering and the cutoff frequency during back steering will be described. The cutoff frequency set by the second cutoff frequency setting unit 515 during back steering is set to be lower than the cutoff frequency set by the first cutoff frequency setting unit 514 during further steering.

[0042] In other words, the cutoff frequency set in the returning operation state is set lower than the cutoff frequency set in the further steering operation state so that the degree of delay in the change in the steering angle relative to a change in the steering operation amount is greater during a returning operation than during a further steering operation. Drivers generally tend to perceive an overly sensitive steering response during a returning operation. Therefore, MCU 510 increases the degree of delay in the change in the steering angle relative to a change in the steering operation amount during a returning operation compared to during a further steering operation, thereby ensuring good maneuverability and preventing the driver from perceiving an overly sensitive steering response.

[0043] Note that if the cutoff frequency changes suddenly when switching between further steering and returning steering, the steering operation may become jerky. Therefore, the MCU 510 sets the cutoff frequency (degree of delay) when the yaw rate is zero to the same value for both returning steering and further steering, regardless of the vehicle speed. In one aspect shown in Figures 4 and 5, the cutoff frequency set by the first cutoff frequency setting unit 514 during further steering is "low" at a vehicle speed of 0 km / h, "medium" at a vehicle speed of 20 km / h, and "high" at a vehicle speed of 40 km / h or higher. Similarly, the cutoff frequency set by the second cutoff frequency setting unit 515 during returning steering is "low" at a vehicle speed of 0 km / h, "medium" at a vehicle speed of 20 km / h, and "high" at a vehicle speed of 40 km / h or higher.

[0044] In other words, when switching between further steering and returning steering, the yaw rate temporarily becomes zero. Therefore, if the cutoff frequency when the yaw rate is zero is the same during both returning steering and further steering, the cutoff frequency (degree of delay) does not change when switching between further steering and returning steering, preventing jerky steering operation.

[0045] Next, the effect of the control of the cutoff frequency (degree of delay) by MCU 510 will be described with reference to Figures 7 to 9. Figure 7 illustrates an example of a change in vehicle behavior relative to the steering operation angle (= steering angle) on the assumption that there is no delay in the change in the steering angle relative to a change in the steering operation angle (operation amount). Note that the vehicle behavior indicates a change in a second physical quantity, such as yaw rate or lateral acceleration, that occurs when vehicle 100 turns. Here, even if there is no delay in the change in the steering angle relative to a change in the steering operation angle, the vehicle behavior occurs with a delay relative to the movement of the steered wheels.

[0046] 8 illustrates changes in the steering angle, the turning angle, and the vehicle behavior when the degree of delay in the change in the steering angle relative to a change in the steering angle is increased in a region where the steering angle is large compared to a region where the steering angle is small. In the case of FIG. 8, even if the yaw rate is still large when steering back, the degree of delay in the change in the steering angle relative to a change in the steering angle is reduced by reducing the steering angle, so the driver feels that the steering response is too sensitive. In other words, if the degree of delay is made variable according to the steering angle regardless of the vehicle behavior (yaw rate, lateral acceleration, etc.), the driver will feel too sensitive in a situation where the yaw rate is still large even when the steering angle is small, because the degree of delay is changed to a small value.

[0047] 9 illustrates changes in the steering operation angle, turning angle, and vehicle behavior when MCU 510 varies the cutoff frequency (degree of delay) according to the yaw rate and vehicle speed. MCU 510 lowers the cutoff frequency (increases the degree of delay) when the yaw rate is large compared to when the yaw rate is small. Therefore, in a situation where the yaw rate is large even with a small steering operation angle, the cutoff frequency is maintained low, thereby preventing overly sensitive vehicle behavior. Furthermore, MCU 510 increases the cutoff frequency (decreases the degree of delay) in the medium-to-high vehicle speed range compared to the low vehicle speed range, resulting in a higher steering operation response in the medium-to-high vehicle speed range compared to the low vehicle speed range, thereby ensuring, for example, the emergency avoidance capability of vehicle 100.

[0048] The technical ideas described in the above embodiments can be used in appropriate combinations as long as no contradictions arise. Furthermore, although the contents of the present invention have been specifically described with reference to preferred embodiments, it is obvious that a person skilled in the art can adopt various modified embodiments based on the basic technical ideas and teachings of the present invention.

[0049] For example, instead of applying low-pass filtering to the target rack stroke (target steering angle), MCU 510 can apply low-pass filtering to the signal of the steering operation amount detected by steering operation amount sensor 330 and set the target rack stroke (target steering angle) from the steering operation amount signal after low-pass filtering, thereby controlling the degree of delay in the change in the steering angle relative to a change in the steering operation amount. Also, instead of searching a map for the cut-off frequency in the low-pass filtering, MCU 510 can set the cut-off frequency based on a function with vehicle speed and yaw rate as variables.

[0050] Furthermore, steering control device 500 can be equipped separately with a first microcomputer that determines the target rack stroke (target steering angle), and a second microcomputer that controls steering motor 410 based on the target rack stroke (target steering angle). Furthermore, steering control device 500 can vary the cutoff frequency in low pass filter processing based on the yaw rate and lateral acceleration.

[0051] 100...vehicle, 101, 102...front wheels (steered wheels), 200...steering device, 300...steering input device, 310...steering input member, 330...steering operation amount sensor, 400...steering device, 410...steering motor, 500...steering control device, 510...MCU, 511...steering gear ratio control unit, 512...low-pass filter processing unit, 513...steering control unit, 514...first cut-off frequency setting unit, 515...second cut-off frequency setting unit, 516...steering further / returning determination unit, 621-624...wheel speed sensors, 630...acceleration sensor, 640...yaw rate sensor

Claims

1. A steering control device that is installed on a vehicle having a steering input member that accepts steering operation by a driver, and a motor that applies a steering force to steered wheels of the vehicle that are mechanically separated from the steering input member, and controls the steering angle of the steered wheels by controlling the motor based on the steering operation of the driver, the steering control device obtains a first physical quantity related to the steering operation amount of the steering input member and a second physical quantity that is generated when the vehicle turns, varies a ratio of the steering operation amount to the steering angle according to the running state of the vehicle, and when the ratio of the steering operation amount to the steering angle is small, obtains a target steering angle based on the second physical quantity that increases the degree of delay in the change of the steering angle relative to the change in the steering operation amount, and controls the motor so that the steering angle becomes the obtained target steering angle.

2. A steering control device according to claim 1, wherein the second physical quantity is a physical quantity related to a yaw rate generated in the vehicle.

3. A steering control device as claimed in claim 2, wherein the steering control device obtains a physical quantity related to the vehicle speed as the driving state, varies the ratio of the steering operation amount to the steering angle so as to be smaller as the vehicle speed is lower, and increases the degree of delay in the change in the steering angle relative to the change in the steering operation amount when the vehicle speed is low compared to when the vehicle speed is high.

4. A steering control device as described in claim 2, which determines whether the steering operation by the driver is an operation in the direction of turning the steering input member further from the neutral position, or an operation in the direction of steering back from the neutral position, and when the steering input member is an operation in the direction of steering back, increases the degree of delay in the change in the steering angle relative to a change in the steering operation amount compared to when the steering input member is an operation in the direction of turning further.

5. A steering control device as claimed in claim 4, wherein the steering control device acquires a physical quantity related to the vehicle speed of the vehicle, and when the second physical quantity is zero, the magnitude of the degree of delay in the change in the steering angle relative to the change in the steering operation amount is made the same during the further turning operation and during the return turning operation, regardless of the driving state.

6. A steering control device according to claim 1, wherein the degree of delay in the change in the steering angle relative to the change in the steering operation amount is determined using a low-pass filter.

7. A steering control device according to claim 6, wherein the cutoff frequency of the low-pass filter gradually decreases as the second physical quantity gradually increases.

8. A steering control device according to claim 7, wherein the cutoff frequency of said low-pass filter gradually decreases so that the slope of a quadratic curve increases as said second physical quantity gradually increases.

9. A steering control method executed by a steering control device provided on a vehicle equipped with a steering device having a steering input member that accepts steering operation by a driver and a motor that applies a steering force to steered wheels of the vehicle mechanically separated from the steering input member, and which changes the steering angle of the steered wheels by controlling the motor based on the steering operation of the driver, the steering control method comprising the steps of: obtaining a first physical quantity related to the steering operation amount of the steering input member and a second physical quantity that is generated when the vehicle turns; varying a ratio of the steering operation amount to the steering angle in accordance with a running state of the vehicle; when the ratio of the steering operation amount to the steering angle is small, obtaining a target steering angle that increases the degree of delay in the change of the steering angle relative to the change in the steering operation amount based on the second physical quantity; and controlling the motor so that the steering angle becomes the obtained target steering angle.

10. A steering device having a steering input member that accepts steering operation by a driver; a motor that applies a steering force to steered wheels of a vehicle mechanically separated from the steering input member; and a steering control device that controls the steering angle of the steered wheels by controlling the motor based on the steering operation of the driver, wherein the steering control device: obtains a first physical quantity related to the steering operation amount of the steering input member and a second physical quantity that is generated when the vehicle turns; varies a ratio of the steering operation amount to the steering angle in accordance with a running state of the vehicle; when the ratio of the steering operation amount to the steering angle is small, obtains a target steering angle that increases the degree of delay in the change of the steering angle relative to the change in the steering operation amount based on the second physical quantity; and controls the motor so that the steering angle becomes the obtained target steering angle.

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