Turning control device, turning control method, turning control program, and turning device

Abstract

The present invention pertains to a turning control device, a turning control method, a turning control program, and a turning device. In one aspect thereof, in a vehicle comprising a steering input member that receives a steering operation of the driver of the vehicle, and a motor that imparts a turning force to a turning wheel of the vehicle that is mechanically separated from the steering input member, when determining a steering gear ratio on the basis of a control vehicle speed corresponding to a physical quantity related to a vehicle speed, in a case where a slip state of the wheel is detected, a response delay of the control vehicle speed with respect to the physical quantity related to the vehicle speed is increased. Thus, deterioration of controllability when the wheel slips can be suppressed.

Description

Steering control device, steering control method, steering control program, and steering device

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

[0002] The steering control device of Patent Document 1 controls a steering device in which a transmission ratio between the steering angle of a steering wheel connected to a steering mechanism and the steering angle of steered wheels connected to a steering shaft of the steering mechanism is varied based on vehicle speed by operation of a motor serving as a drive source.The steering control device of Patent Document 1 calculates a target steering angle by adjusting the transmission ratio to be larger based on an increase in the absolute value of longitudinal acceleration, and also calculates a phase compensation component based on the longitudinal acceleration so that the phase leads more when the transmission ratio is small than when the transmission ratio is large, and calculates a final target steering angle by adding the target steering angle and the phase compensation component together.

[0003] Patent No. 7133393

[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 the lower the vehicle speed, in other words, the lower the vehicle speed, the quicker the steering characteristics are, thereby improving maneuverability. However, when accelerating or decelerating on a slippery road surface such as a snowy road, if the wheels slip, the detected vehicle speed value used to set the steering gear ratio changes suddenly, which can cause a sudden change in the steering gear ratio and result in a deterioration in vehicle maneuverability.

[0005] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a steering control device, a steering control method, a steering control program, and a steering device that can prevent deterioration of maneuverability when wheels slip.

[0006] In one aspect, the steering control device, steering control method, steering control program, and steering device according to the present invention, when determining the steering gear ratio based on a control vehicle speed corresponding to a physical quantity related to vehicle speed, if a wheel slip state is detected, the response delay of the control vehicle speed with respect to the physical quantity related to vehicle speed is increased.

[0007] According to the present invention, deterioration of maneuverability when wheels slip can be suppressed.

[0008] It is a system diagram showing a vehicle equipped with a steer-by-wire steering device. It is a functional block diagram showing a control process of a steering angle. It is a diagram showing the characteristics of a steering gear ratio. It is a diagram showing the correlation between the cutoff frequency of a low-pass filter and a slip state. It is a time chart explaining the effect of a process of changing the cutoff frequency.

[0009] Hereinafter, embodiments of a steering control device, a steering control method, a steering control program, and a steering device according to the present invention will be described with reference to the drawings. Figure 1 is a system diagram showing one aspect of a vehicle 100 equipped 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 equipped with steering input member 310 that receives steering operation from the driver of vehicle 100, steering operation device 400 equipped with steering motor 410 that applies steering force to front wheels 101, 102 that are steered wheels of vehicle 100, and steering control device 500 which is an electronic control device that controls steering input device 300 and steering operation device 400. Here, steering input member 310 and front wheels 101, 102 are mechanically separated. In other words, steering device 200 is 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 front wheels 101, 102 (in other words, tire angle).

[0011] Steering input device 300 includes a steering input member 310, a steering reaction force application device 320, and a steering operation amount sensor 330. Steering input member 310 is an operator that accepts steering operations by the driver, and in addition to a steering wheel, a dial-type, a stick-type, or the like may be used. The steering control device, steering control method, steering control program, and steering device according to the present invention are more suitable for vehicles that employ new types of steering input member 310, such as a dial-type that changes the steering angle with less steering operation by the driver.

[0012] The steering reaction force imparting device 320 is a device that biases the steering input member 310 toward the neutral position, and applies a steering reaction force to the steering input member 310 using 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 operation device 400 includes steering mechanism 420. Steering mechanism 420 is a mechanism that, for example, 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 of front wheels 101, 102 connected to rack shaft 421. Steering operation 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 that includes an MCU (Micro Controller Unit) 510, which is a control section that executes a steering control method. The MCU 510 executes a steering control program stored in a storage section such as a ROM provided in the steering control device 500, thereby controlling the steering reaction force imparting device 320 and the steering motor 410 and controlling the operation of the steering device 200.

[0015] In detail, MCU 510 performs arithmetic processing on various signals acquired from outside in accordance with a steering control program to determine a control signal for steering reaction force application device 320 and a control signal for steering motor 410, and outputs the determined control signals. Note that MCU 510 can also be referred to as a microcomputer, processor, processing device, arithmetic device, etc.

[0016] 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. Vehicle 100 also includes an acceleration sensor 630 that detects the longitudinal acceleration and lateral acceleration 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.

[0017] Here, we will outline the control process of steering motor 410 by MCU 510 of steering control device 500. MCU 510 acquires a signal of the steering operation angle, which is a physical quantity related to the operation amount of steering input member 310, from steering operation amount sensor 330, and also acquires wheel speed signals, which are physical quantities related to the vehicle speed, from wheel speed sensors 621-624. MCU 510 then variably sets the steering gear ratio based on vehicle speed information determined from the wheel speed signal, and determines a target rack stroke (in other words, a target rack position) corresponding to the target steering angle of front wheels 101, 102 from the steering operation angle, which is the operation amount of steering input member 310, and the steering gear ratio.

[0018] In the present application, the above steering gear ratio is the ratio of the amount of operation of steering input member 310 to the steering angle of the steered wheels, and is defined as "steering gear ratio = amount of operation / steering angle." After determining the target rack stroke, MCU 510 acquires information about the actual rack stroke detected by rack stroke sensor 430, and controls steering motor 410 so that the actual rack stroke approaches the target rack stroke. In other words, MCU 510 controls the steering angle of front wheels 101, 102 to the target value by controlling steering motor 410 based on the target rack stroke.

[0019] MCU 510 can determine the actual rack stroke, in other words, the actual steering angle, 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 the pinion shaft of the rack and pinion.

[0020] Furthermore, the MCU 510 can acquire a detected value of the rotation angle of the motor constituting the steering reaction force imparting device 320 as a physical quantity related to the operation amount of the steering input member 310. Furthermore, if the steering input device 300 includes a mechanism that converts the rotational motion of the steering input member 310 into linear motion, the MCU 510 can acquire a detected value of the stroke amount of a movable member that moves linearly in response to the rotation of the steering input member 310 as a physical quantity related to the operation amount of the steering input member 310.

[0021] Here, the MCU 510 is configured to perform a process of calculating a control vehicle speed used for variable setting of the steering gear ratio from the vehicle speed calculated from the wheel speeds, and to variably set the steering gear ratio based on the control vehicle speed. Furthermore, when the MCU 510 detects a slip state of the wheels 101-104 of the vehicle 100, it increases the response delay of the control vehicle speed relative to the vehicle speed calculated from the wheel speeds compared to when there is no slip state. Note that the slip state of the wheels 101-104 includes a wheel spin state and a wheel lock state.

[0022] When accelerating or decelerating on a slippery road surface such as a snowy road, if the wheels 101-104 slip, the detected value of the vehicle speed based on the wheel speed changes suddenly. Therefore, if the MCU 510 variably sets the steering gear ratio based on the detected value of the vehicle speed based on the wheel speed, the steering gear ratio will change suddenly as slip occurs, deteriorating the maneuverability of the vehicle 100. Therefore, when the wheels 101-104 slip, the MCU 510 increases the response delay of the control vehicle speed used to set the steering gear ratio relative to the detected value of the vehicle speed based on the wheel speed, thereby preventing a sudden change in the steering gear ratio and reducing the impact on the maneuverability of the vehicle 100.

[0023] The steering angle control process, including the process for setting the control vehicle speed, executed by MCU 510 will be described in detail below. Fig. 2 is a functional block diagram showing the steering angle control process. To set the target steering angle, MCU 510 acquires detection signals from steering operation amount sensor 330, wheel speed sensors 621-624, and acceleration sensor 630. MCU 510 then outputs a control signal to steering motor 410.

[0024] MCU 510 has, as functional sections for steering control, low-pass filter processing section 511, steering gear ratio setting section 512, target steering angle setting section 513, steering control section 514, estimated longitudinal acceleration calculation section 515, deviation amount calculation section 516, and cut-off frequency setting section 517. Low-pass filter processing section 511 acquires a vehicle speed signal determined from the wheel speeds detected by wheel speed sensors 621-624, and attenuates, of the vehicle speed signal, frequency components lower than the cut-off frequency, while gradually reducing frequency components higher than the cut-off frequency.

[0025] Then, low-pass filter processing unit 511 outputs the vehicle speed signal after low-pass filtering, that is, the vehicle speed signal in which frequency components higher than the cutoff frequency have been attenuated, as a control vehicle speed signal used to set the steering gear ratio. Note that the sensor for detecting the vehicle speed is not limited to wheel speed sensors 621-624, and may be a sensor that detects the rotation of the output shaft of the transmission or the drive shaft, etc.

[0026] Steering gear ratio setting section 512 acquires the control vehicle speed signal output by low-pass filter processing section 511 and sets the steering gear ratio based on the control vehicle speed. Figure 3 is a diagram showing how the steering gear ratio is made variable in accordance with the control vehicle speed, with the horizontal axis representing the steering operation amount and the vertical axis representing the target steering angle, and showing the correlation between the steering operation amount and the target steering angle for each control vehicle speed. Here, the steering gear ratio is set to a smaller value as the control vehicle speed is lower.

[0027] Target steering angle setting section 513 acquires a steering operation amount signal from steering operation amount sensor 330, and acquires a steering gear ratio signal from steering gear ratio setting section 512. Target steering angle setting section 513 then determines a target steering angle, which is a target value for the steering angle of front wheels 101, 102, based on the steering operation amount and the steering gear ratio. Steering control section 514 acquires the target steering angle signal from target steering angle setting section 513, and outputs a control signal to steering motor 410 so that the steering angle of front wheels 101, 102, which are steered wheels, becomes the target steering angle.

[0028] Here, the MCU 510 performs processing to change the cutoff frequency in the low-pass filter processing unit 511 in accordance with the slip state of the wheels 101-104 using an estimated longitudinal acceleration calculation unit 515, a deviation amount calculation unit 516, and a cutoff frequency setting unit 517. The estimated longitudinal acceleration calculation unit 515 obtains an estimated longitudinal acceleration, which is an estimated value of the longitudinal acceleration of the vehicle 100, by time differentiating the detection value of the vehicle speed obtained from the wheel speeds detected by the wheel speed sensors 621-624.

[0029] The deviation amount calculation unit 516 acquires the estimated longitudinal acceleration calculated by the estimated longitudinal acceleration calculation unit 515 and the actual longitudinal acceleration, which is the longitudinal acceleration of the vehicle 100 detected by the acceleration sensor 630, and compares the estimated longitudinal acceleration with the actual longitudinal acceleration. The deviation amount calculation unit 516 then subtracts the actual longitudinal acceleration from the estimated longitudinal acceleration to calculate the deviation amount of the longitudinal acceleration. Here, the deviation amount of the longitudinal acceleration represents the slip state of the wheels 101-104, and the deviation amount calculation unit 516 functions as a slip determination unit.

[0030] The cutoff frequency setting unit 517 acquires the deviation amount of the longitudinal acceleration calculated by the deviation amount calculation unit 516, and variably sets the cutoff frequency of the low-pass filter processing unit 511, in other words, the strength of the low-pass filter, according to the deviation amount of the longitudinal acceleration. Fig. 4 is a diagram illustrating an example of the correlation between the cutoff frequency of the low-pass filter processing unit 511 and the deviation amount of the longitudinal acceleration, with the horizontal axis representing the deviation amount of the longitudinal acceleration and the vertical axis representing the cutoff frequency or the strength of the low-pass filter.

[0031] Here, when the absolute value of the deviation amount of the longitudinal acceleration is equal to or less than a predetermined value, the cutoff frequency is set to the highest setting frequency and the strength of the low-pass filter is set to the lowest setting. In other words, when the absolute value of the deviation amount of the longitudinal acceleration is equal to or less than a predetermined value, the estimated longitudinal acceleration and the actual longitudinal acceleration are substantially equal, and the wheels 101-104 are substantially not slipping. In a state where the wheels 101-104 are substantially not slipping, the cutoff frequency is set relatively high, so that the control vehicle speed follows the detected vehicle speed without a significant delay, and the steering gear ratio is changed to follow the actual vehicle speed.

[0032] On the other hand, in a region where the absolute value of the deviation in longitudinal acceleration exceeds a predetermined value, the cutoff frequency is changed to a lower frequency in proportion to the increase in the absolute value of the deviation, and the strength of the low-pass filter increases as the absolute value of the deviation in longitudinal acceleration increases. In other words, in a region where the absolute value of the deviation in longitudinal acceleration exceeds a predetermined value, the larger the deviation, the greater the response delay of the control vehicle speed used to set the steering gear ratio relative to the detected value of the vehicle speed based on the wheel speed.

[0033] When the absolute value of the deviation amount of longitudinal acceleration exceeds a predetermined value, the wheels 101-104 are slipping, and the detected value of the vehicle speed based on the wheel speed changes suddenly due to the slip of the wheels 101-104, causing a deviation from the body speed of the vehicle 100. If the steering gear ratio is changed in such a slip state in accordance with the detected value of the vehicle speed based on the wheel speed, the steering gear ratio changes suddenly regardless of the body speed, and the maneuverability of the vehicle 100 deteriorates.

[0034] Therefore, when the wheels 101-104 are slipping, the MCU 510 lowers the cutoff frequency of the low-pass filter processing unit 511 to increase the response delay of the control vehicle speed relative to the detected value of the vehicle speed based on the wheel speed compared to when the wheels 101-104 are not slipping, thereby preventing the steering gear ratio from unnecessarily following a sudden change in the detected value of the vehicle speed based on the wheel speed. This prevents a sudden change in the steering gear ratio when the wheels 101-104 slip, and thus prevents a deterioration in the maneuverability of the vehicle 100.

[0035] FIG. 5 is a time chart for explaining the effect of performing processing to lower the cutoff frequency of the low-pass filter processing unit 511 when the wheels 101-104 are slipping. When the wheels 101-104 slip, the vehicle speed calculated from the wheel speed changes suddenly. However, at this time, the deviation in the longitudinal acceleration increases, so the cutoff frequency of the low-pass filter processing unit 511 is changed to a lower frequency than when there is no slip. As a result, the response change of the control vehicle speed is delayed relative to the vehicle speed calculated from the wheel speed, the change in the control vehicle speed becomes gradual, and the change in the steering gear ratio set based on the control vehicle speed also becomes gradual. Therefore, even if the wheels 101-104 slip, a sudden change in the steering gear ratio is prevented, allowing the driver to steer the vehicle 100 without any discomfort.

[0036] 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.

[0037] For example, the determination of slippage of the wheels 101-104 is not limited to a determination based on the magnitude of the difference between the estimated longitudinal acceleration and the actual longitudinal acceleration. The MCU 510 can determine the slippage state based on, for example, the rotational speed of each wheel 101-104 detected by the wheel speed sensors 621-624, the rate of change in the rotational speed of each wheel 101-104, etc.

[0038] Furthermore, the MCU 510 can determine slippage based on the grip force (friction force) between the tires and the road surface. Furthermore, if the vehicle 100 is equipped with a GPS (Global Positioning System), the MCU 510 can determine slippage based on the difference between the vehicle speed (vehicle speed) calculated from changes in the measurement position of the vehicle 100 and the vehicle speed calculated from the outputs of the wheel speed sensors 621-624. Furthermore, the MCU 510 can determine slippage when the rate of change in the vehicle speed calculated from the outputs of the wheel speed sensors 621-624 exceeds a normal rate of change.

[0039] 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 operation device, 410...steering motor, 500...steering control device, 510...MCU, 511...low-pass filter processing unit, 512...steering gear ratio setting unit, 513...target steering angle setting unit, 514...steering control unit, 515...estimated longitudinal acceleration calculation unit, 516...deviation amount calculation unit, 517...cut-off frequency setting unit, 621-624...wheel speed sensors, 630...acceleration sensor

Claims

1. A steering control device provided on a vehicle having a steering input member that receives steering operation from a vehicle driver, and a motor that applies a steering force to steered wheels of the vehicle that are mechanically separated from the steering input member, the steering control device obtaining a physical quantity related to the amount of operation of the steering input member and a physical quantity related to the vehicle speed, and when determining a steering gear ratio, which is the ratio of the amount of operation to the steering angle of the steered wheels, based on a control vehicle speed that is in accordance with the physical quantity related to the vehicle speed, increasing a response delay of the control vehicle speed with respect to the physical quantity related to the vehicle speed if a slip state of wheels of the vehicle is detected, calculating a target value of the steering angle based on the steering gear ratio determined based on the control vehicle speed and the physical quantity related to the amount of operation of the steering input member, and controlling the motor so that the steering angle becomes the target value.

2. A steering control device as claimed in claim 1, wherein an actual acceleration representing the actual acceleration of the vehicle is acquired, and the detection of the slip state is carried out based on a comparison between an estimated acceleration, which is an acceleration estimated from a physical quantity related to the vehicle speed, and the actual acceleration.

3. A steering control device according to claim 2, wherein the greater the difference between the estimated acceleration and the actual acceleration, the greater the response delay of the control vehicle speed relative to the physical quantity related to the vehicle speed.

4. A steering control device as claimed in claim 1, wherein, when a slip state of the vehicle wheels is detected, the control vehicle speed is determined by low-pass filtering the physical quantity related to the vehicle speed, and the response delay of the control vehicle speed relative to the physical quantity related to the vehicle speed is increased.

5. A steering control method executed by a steering control device provided on a vehicle having a steering input member that accepts steering operation by a vehicle driver, and a motor that applies a steering force to steered wheels of the vehicle that are mechanically separated from the steering input member, the steering control method comprising: obtaining a physical quantity related to the operation amount of the steering input member and a physical quantity related to the vehicle speed of the vehicle; when determining a steering gear ratio, which is the ratio of the operation amount to the steered angle of the steered wheels, based on a control vehicle speed that is in accordance with the physical quantity related to the vehicle speed, increasing a response delay of the control vehicle speed with respect to the physical quantity related to the vehicle speed if a slip state of wheels of the vehicle is detected; determining a target value of the steering angle based on the steering gear ratio determined based on the control vehicle speed and the physical quantity related to the operation amount of the steering input member; and controlling the motor so that the steering angle becomes the target value.

6. A steering control program executed by a steering control device provided on a vehicle having a steering input member that accepts steering operation by a vehicle driver, and a motor that applies a steering force to steered wheels of the vehicle that are mechanically separated from the steering input member, the steering control program acquiring a physical quantity related to the operation amount of the steering input member and a physical quantity related to the vehicle speed of the vehicle, and when determining a steering gear ratio, which is the ratio of the operation amount to the steered angle of the steered wheels, based on a control vehicle speed that is in accordance with the physical quantity related to the vehicle speed, increasing a response delay of the control vehicle speed with respect to the physical quantity related to the vehicle speed if a slip state of wheels of the vehicle is detected, calculating a target value of the steering angle based on the steering gear ratio determined based on the control vehicle speed and the physical quantity related to the operation amount of the steering input member, and controlling the motor so that the steering angle becomes the target value.

7. A steering device comprising: a steering input member that accepts steering operation by a vehicle driver; a motor that applies a steering force to steered wheels of the vehicle that are mechanically separated from the steering input member; and a steering control device that acquires a physical quantity related to the operation amount of the steering input member and a physical quantity related to the vehicle speed of the vehicle, and when determining a steering gear ratio, which is the ratio of the operation amount to the steered angle of the steered wheels, based on a control vehicle speed that corresponds to the physical quantity related to the vehicle speed, increases a response delay of the control vehicle speed with respect to the physical quantity related to the vehicle speed if a slip state of wheels of the vehicle is detected, calculates a target value of the steering angle based on the steering gear ratio determined based on the control vehicle speed and the physical quantity related to the operation amount of the steering input member, and controls the motor so that the steering angle becomes the target value.

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