Steering system

Abstract

The present invention provides a steering system capable of suppressing an increase in the behavior of a vehicle caused by resonance between a steering device that steers wheels or vehicle motion and the steering of the wheels. The present invention includes a steering control unit (54) that, when a determination unit (53) determines that a steering-related frequency is a value within a resonance band, performs a restriction control that controls a front-wheel steering device (3) or a rear-wheel steering device (4) in such a manner that (i) an actual steering angle (i), which is an actual steering angle, is constantly a specified value, (ii) an absolute value of the actual steering angle gradually decreases from the specified value, (iii) a frequency of the actual steering angle becomes outside the resonance band, or (iv) an absolute value of the actual steering angle is less than an absolute value of a latest value of a target extreme value that is a target steering angle.

Description

Technical Field

[0001] The present invention relates to a steering system. Background Art

[0002] In a steering system of a vehicle, a technique has been developed to suppress noise transmitted to an actuator that steers the front wheels, thereby suppressing interference between an operation of a steering wheel by a driver and steering performed by the actuator. For example, in the steering control device described in Japanese Patent Application Laid-Open No. 2002-302058, a target yaw rate calculated from image information based on a camera is separated into a low-frequency component and a high-frequency component. The front-wheel steering unit is controlled according to the low-frequency component, and the rear-wheel steering unit is controlled according to the high-frequency component. Thereby, the above-described interference is suppressed.

[0003] Patent Document 1: Japanese Patent Application Laid-Open No. 2002-302058

[0004] In the behavior of a vehicle, it is considered that resonance occurs between a steering device that steers wheels or vehicle movement and the steering of the wheels. If resonance occurs, for example, in terms of an increase in vehicle behavior such as the vehicle turning more greatly than expected by the driver, it is possible to affect the vehicle behavior. In the above-described steering control device, resonance is not considered, and there is room for improvement in terms of suppressing an increase in behavior.

[0005] Summary of the Invention

[0006] An object of the present invention is to provide a steering system capable of suppressing an increase in vehicle behavior caused by resonance between a steering device that steers wheels or vehicle movement and the steering of the wheels.

[0007] The steering system of the present invention includes at least one of a front-wheel steering device that steers the front wheels and a rear-wheel steering device that steers the rear wheels, and a control device that controls the front-wheel steering device or the rear-wheel steering device according to a steering request. The control device includes: a target calculation unit that calculates a target steering angle based on the steering request; a frequency calculation unit that calculates a steering-related frequency that is the frequency of a steering variable related to steering; a determination unit that determines whether the steering-related frequency is a value within a specified resonance band; and a steering control unit that, when the determination unit determines that the steering-related frequency is a value outside the resonance band, performs normal control to control the front-wheel steering device or the rear-wheel steering device based on the target steering angle, and when the determination unit determines that the steering-related frequency is a value within the resonance band, performs limit control, where the limit control is a control for controlling the front-wheel steering device or the rear-wheel steering device with respect to an actual steering angle that is the actual steering angle in the following manner: (i) the actual steering angle is constantly a specified value, (ii) the absolute value of the actual steering angle gradually decreases from the specified value, (iii) the frequency of the actual steering angle becomes outside the resonance band, or (iv) the absolute value of the actual steering angle is less than the absolute value of the latest value of a target extreme value that is the extreme value of the target steering angle.

[0008] According to the present invention, when the frequency of a steering variable related to steering (for example, the operating angle of an operating member, an indication value in autonomous driving, or the actual steering angle of a wheel, etc.) that is preset is a value within a specified resonance band, limit control is performed. According to limit control (i), since the control is performed in such a way that the actual steering angle is made constant without considering the target steering angle, the control value (actual steering angle) does not fluctuate (amplitude), suppressing the generation of resonance. According to limit control (ii), since the control is performed in such a way that the absolute value of the actual steering angle gradually decreases without considering the target steering angle, the control value does not fluctuate (amplitude), suppressing the generation of resonance.

[0009] According to limit control (iii), since the control is performed in such a way that the frequency of the actual steering angle becomes outside the resonance band without considering the target steering angle, the generation of resonance is suppressed. According to limit control (iv), since the control is performed in such a way that the absolute value of the actual steering angle is less than the nearest extreme value of the target steering angle, even if resonance occurs, the pitching of the vehicle can be suppressed. Thus, according to the present invention, an increase in the behavior of the vehicle caused by resonance between the steering device that steers the wheels or vehicle movement and the steering of the wheels can be suppressed. Description of the Drawings

[0010] Figure 1 It is a structural diagram of a vehicle equipped with the steering system of the present embodiment.

[0011] Figure 2 is a timing chart for explaining the limit control of the present embodiment.

[0012] Figure 3 is a chart showing the experimental results of the present embodiment.

[0013] Figure 4 is a flowchart showing an example of the control process of the present embodiment.

[0014] Figure 5 is a timing chart for explaining another example of the limit control.

[0015] Explanation of reference numerals

[0016] 1... Steering system; 3... Front-wheel steering device; 35... Steering motor; 4... Rear-wheel steering device; 45... Steering motor; 5... Steering ECU (control device); 51... Target operation unit; 52... Frequency operation unit; 53... Judgment unit; 54... Steering control unit. Detailed implementation mode

[0017] Hereinafter, as a mode for implementing the present invention, the steering system 1 as an embodiment of the present invention will be described in detail with reference to the drawings. In addition, the present invention can be implemented in various modes in which various changes and improvements have been made based on the knowledge of those skilled in the art, in addition to the following embodiments.

[0018] As Figure 1 shown, the steering system 1 of the embodiment includes three devices that are mechanically independent of each other, namely, an operation device 2, a front-wheel steering device 3, and a rear-wheel steering device 4. The front-wheel steering device 3 is a device for steering a pair of front wheels 10F. The rear-wheel steering device 4 is a device for steering a pair of rear wheels 10R. In addition, the steering system 1 further includes a steering ECU (equivalent to the "control device") 5 that mainly controls the front-wheel steering device 3 and the rear-wheel steering device 4. The steering system 1 is a steer-by-wire type steering system. In addition, hereinafter, there are cases where the front wheels 10F and the rear wheels 10R are collectively referred to as wheels 10. In addition, the drive wheels are, for example, the rear wheels 10R.

[0019] (Operation device)

[0020] The operation device 2 includes a steering wheel 21 as an operation member, a steering shaft 22, a steering column 23, a reaction force imparting mechanism 24, and an operation angle sensor 25. The steering wheel 21 is an operation member that is steered (steering operation) by the driver. The steering shaft 22 is a shaft member having the steering wheel 21 mounted at its front end. The steering column 23 is a member that holds the steering shaft 22 so as to be rotatable and supports it on the instrument panel reinforcement (not shown).

[0021] The reaction force imparting mechanism 24 is a mechanism that uses the reaction force motor 26, which is an electric motor supported by the steering column 23, as a power source and imparts a reaction force relative to the steering operation (hereinafter also referred to as "operation reaction force") to the steering wheel 21 via the steering shaft 22. The reaction force imparting mechanism 24 is a mechanism with a general structure including a speed reducer and the like. A rotation angle sensor 26a is provided in the reaction force motor 26. The operation angle sensor 25 is a sensor that detects the operation angle of the steering wheel 21 as the steering operation amount.

[0022] In addition, in the steering system 1, similar to a general so-called power steering system, a torsion bar 27 is assembled to the steering shaft 22. The operation device 2 has an operation torque sensor 28 for detecting an operation torque, which is an operation force applied by the driver to the steering wheel 21, based on the amount of twist of the torsion bar 27.

[0023] (Steering device)

[0024] The wheels 10 are respectively steerably supported on the vehicle body via a knuckle 90, which is a component of a suspension device. The front-wheel steering device 3 integrally steers a pair of front wheels 10F by rotating the knuckle 90. The front-wheel steering device 3 has a steering actuator 30 as a main component.

[0025] The steering actuator 30 includes a steering rod 31, a housing 32, and a rod movement mechanism 33. The steering rod 31 (also referred to as "rack") is a component whose both ends are respectively connected to the left and right knuckles 90 via linkages 34. The housing 32 is a component that supports the steering rod 31 so as to be movable left and right and is fixedly held on the vehicle body.

[0026] The rod movement mechanism 33 is a mechanism for moving the steering rod 31 left and right using the steering motor 35, which is an electric motor, as a drive source. The rod movement mechanism 33 is mainly a ball screw mechanism, that is, the main body is composed of a ball groove threaded on the steering rod 31 and a nut that is threadedly engaged with the ball groove via bearing balls and rotates by the steering motor 35. Since it is a mechanism with a general structure, the detailed description of the rod movement mechanism 33 is omitted.

[0027] A rotation angle sensor 35a and further a current sensor 35b for detecting the current supplied to itself are provided in the steering motor 35. In addition, the front-wheel steering device 3 has a steering angle sensor 36 for detecting the amount of movement of the steering rod 31 from the neutral position to the left and right in order to detect the steering angle (steering amount) of the front wheels 10F. In this way, the front-wheel steering device 3 constitutes a steer-by-wire type steering device that steers the front wheels 10F mechanically independently of the operation force of the steering wheel 21 by the force of the steering motor 35.

[0028] The rear-wheel steering device 4 integrally steers a pair of rear wheels 10R by rotating a knuckle 90. The rear-wheel steering device 4 has the same structure as the front-wheel steering device 3. That is, the rear-wheel steering device 4 includes a steering actuator 40 corresponding to the steering actuator 30. The steering actuator 40 includes a steering rod 41 corresponding to the steering rod 31, a housing 42 corresponding to the housing 32, a rod moving mechanism 43 corresponding to the rod moving mechanism 33, and a steering motor 45 corresponding to the steering motor 35. In addition, the rear-wheel steering device 4 includes a link 44 corresponding to the link 34. A rotation angle sensor 45a corresponding to the rotation angle sensor 35a and a current sensor 45b corresponding to the current sensor 35b are provided in the steering motor 45. In addition, the rear-wheel steering device 4 has a steering angle sensor 46 that detects the amount of movement of the steering rod 41 of the rear wheel 10R to the left and right from the neutral position in order to detect the steering angle (steering amount) of the rear wheel 10R.

[0029] The rear-wheel steering device 4 steers the rear wheels 10R independently of the front wheels 10F under the control of the steering ECU 5. The rear-wheel steering device 4, which is the control object of the steering ECU 5, is the same as the front-wheel steering device 3 and is mechanically independent of the operating force of the steering wheel 21, and constitutes a steer-by-wire type steering device that steers the rear wheels 10R by the force of the steering motor 45. Since the detailed description of the rear-wheel steering device 4 can be referred to the description of the front-wheel steering device 3, it is omitted.

[0030] (Control device)

[0031] The steering ECU 5 is an electronic control unit including a CPU, a memory, etc. The illustration of the communication line is omitted for the steering ECU 5, but it is communicably connected to each device and each sensor. The communication within the vehicle uses CAN (car area network or controllable area network).

[0032] The steering ECU 5 performs steering control for steering the wheels 10 according to a steering request, that is, the operating angle of the steering wheel 21 in the case of manual driving. The steering ECU 5 obtains the operating angle of the steering wheel 21 based on the rotation angle of the reaction force motor 26 detected by the rotation angle sensor 26a. The steering ECU 5 determines a target front-wheel steering angle that is the target of the steering angle of the front wheels 10F based on the operating angle.

[0033] The steering ECU 5 determines a target rotation angle, which is the target of the rotation angle of the steering motor 35, based on the target front-wheel steering angle. The steering ECU 5 detects the actual rotation angle (hereinafter referred to as "actual rotation angle") of the steering motor 35 by means of the rotation angle sensor 35a, and determines a rotation angle deviation, which is the deviation of the actual rotation angle from the target rotation angle. The torque generated by the steering motor 35 is called steering torque, and the steering ECU 5 determines the steering torque to be generated according to a feedback control rule based on the rotation angle deviation.

[0034] The current supplied to the steering motor 35 is called steering current, and the steering torque is generally in a proportional relationship with the steering current. According to this relationship, the steering ECU 5 determines the steering current to be supplied to the steering motor 35 based on the determined steering torque, and supplies this steering current to the steering motor 35.

[0035] In addition, the steering ECU 5 determines a target rear-wheel steering angle, which is the target of the steering angle of the rear wheels 10R, based on the operation angle and vehicle speed information. For example, the vehicle speed is calculated based on the detection results of the wheel speed sensors 82 provided for each wheel 10. The steering ECU 5 controls the steering actuator 40 based on the target rear-wheel steering angle in the same way as the steering control of the front wheels 10F. The steering ECU 5 can control the front wheels 10F and the rear wheels 10R to be in the same phase or in the opposite phase according to driving conditions such as the vehicle speed. The steering ECU 5 controls the steering angle of the rear wheels 10R as needed. The steering ECU 5 can also control only the steering angle of the front wheels 10F according to the steering requirement and driving conditions.

[0036] In addition, the steering ECU 5 performs a reaction force control for giving a sense of operation to the driver with respect to the steering operation. The steering ECU 5 determines an operation reaction force based on a steering load component FS and an operation force reduction component FA, which are two components. The steering load component FS is a component related to the steering force (steering torque of the steering motor 35) required to steer the front wheels 10F, and is determined based on the steering current supplied to the steering motor 35. Although detailed description is omitted, it is recognized that the larger the steering current, the greater the steering load of the front wheels 10F, and the steering load component FS is determined to be a larger value.

[0037] On the other hand, the operation force reduction component FA can be considered as a component for giving a sense of operation in a so-called power steering system to the driver. In a power steering system, generally, an auxiliary torque corresponding to the operation torque is given to the steering shaft 22. The steering ECU 5 detects the operation torque by means of the operation torque sensor 28. The steering ECU 5 determines a reaction force current, which is the current supplied to the reaction force motor 26, based on the operation reaction force, and supplies the determined reaction force current to the reaction force motor 26.

[0038] In this way, it can be said that the steering ECU 5 has a steering control unit that executes steering control and an operation control unit that controls the operation device 2, such as executing reaction force control. In addition, the steering ECU 5 may be composed of multiple ECUs. For example, the steering ECU 5 may be configured to include an operation ECU of the operation device 2, a front-wheel steering ECU that controls the front-wheel steering device 3, and a rear-wheel steering ECU that controls the rear-wheel steering device 4, which are communicably connected to each other. In addition, the devices and ECUs in the steer-by-wire steering system 1 are redundantly configured respectively.

[0039] (Normal control and limit control)

[0040] The steering ECU 5 includes a target calculation unit 51, a frequency calculation unit 52, a determination unit 53, and a steering control unit 54. The target calculation unit 51 calculates a target steering angle based on a steering request. More specifically, the target calculation unit 51 calculates a target front-wheel steering angle, which is the target value of the steering angle of the front wheels 10F, and a target rear-wheel steering angle, which is the target value of the steering angle of the rear wheels 10R, based on the steering request. The steering request is, for example, the operation angle of the steering wheel 21 received by the steering ECU 5 (the detection value of the operation angle sensor 25) or the indication value (indication angle) from other ECUs in autonomous driving or the like. The target calculation unit 51 calculates the target steering angle based on, for example, the detected operation angle and operation torque, using calculation formulas, maps, etc.

[0041] The frequency calculation unit 52 calculates a steering-related frequency, which is the frequency of a steering variable related to steering. The steering variable is, for example, the operation angle of the steering wheel 21, the indication value from other ECUs in autonomous driving or the like, or the actual steering angle (the detection values of the steering angle sensors 36 and 46) as the actual steering angle. The frequency calculation unit 52 calculates the variable extreme value, which is the extreme value of the steering variable, and calculates the steering-related frequency based on the time Ta from the generation of the variable extreme value to the generation of the next variable extreme value (the time between consecutive variable extreme values).

[0042] The extreme value is the value at which the slope of the value change changes from positive to negative or from negative to positive, and can also be said to be the value at which the slope of the tangent line (differential value) becomes 0. The value at which the slope changes from positive to negative is the maximum value, and the value at which the slope changes from negative to positive is the minimum value. The frequency calculation unit 52 calculates (detects) the variable extreme value based on, for example, the slope of the change of the steering variable.

[0043] In the frequency operation unit 52, as a determination criterion for whether the detected extreme value is a variable extreme value, in addition to the change in the positive or negative of the slope, it is also set that the absolute value of the steering variable is above a threshold value, and the difference between the determined extreme value and the next determined extreme value is above a difference threshold value. Thereby, the false detection of variable extreme values caused by noise can be suppressed. The frequency f is the reciprocal of the period T. The time Ta between consecutive variable extreme values can be inferred to be half of the period of the steering variable, and thus the steering-related frequency can be calculated based on the time Ta.

[0044] The determination unit 53 determines whether the steering-related frequency is a value within a specified resonance band. In the determination unit 53, as the specified resonance band, the resonance bands of the actuators 30 and 40 that are the objects of the subsequent limit control and the resonance band of the vehicle motion system are set. In the present embodiment, since the object of the limit control is the rear-wheel steering device 4, at least the resonance band of the actuator 40 of the rear-wheel steering device 4 and the resonance band of the vehicle motion system are stored in the determination unit 53.

[0045] The resonance band of the vehicle motion system is the resonance band in a moving vehicle and changes, for example, according to the vehicle speed. The resonance frequency of the vehicle motion system can be said to be, for example, the frequency (horizontal axis) at which the gain (vertical axis) in the Bode diagram of the second-order delay system increases. The gain corresponds to the value obtained by dividing the yaw rate (or lateral acceleration) by the operation angle (yaw rate / operation angle). For example, if the steering wheel 21 is continuously operated left and right and the vehicle turns left and right, when the frequency of the operation angle becomes a certain frequency, there is a problem that the turning (yawing) of the vehicle becomes larger. At this time, it can be said that the frequency of the operation angle has entered a value within the resonance band of the vehicle motion system. The resonance band of the vehicle motion system is set for each vehicle speed.

[0046] The resonance band of the actuator 40 is, for example, the resonance band that takes the feedback control of the steering motor 45 as an important factor. In this case, the resonance frequency of the actuator 40 can be said to be, for example, the frequency (horizontal axis) at which the gain (vertical axis) in the Bode diagram of the second-order delay system increases. In the feedback control, based on the difference between the target steering angle and the actual steering angle, the steering current (control current) of the steering motor 45 is determined so that the difference becomes smaller. In the operation in the steering ECU 5, for example, a specified coefficient (parameter) is multiplied by the difference to calculate the steering current.

[0047] In the feedback control of the motor, generally speaking, the higher the frequency of the input becomes, the greater the delay in the output. Thus, if an input of a certain frequency continues, the difference becomes larger and the steering current increases, resulting in resonance. For example, by changing the coefficient (parameter) in the operation of the feedback control, the resonance frequency band of the actuator 40 can be changed. The method of considering the resonance frequency band of the actuator 30 is the same as that of the actuator 40. The resonance frequency band of the vehicle motion system and the resonance frequency bands of the actuators 30 and 40 can be obtained in advance through experiments and simulations.

[0048] For the determination unit 53 of the present embodiment, as the steering-related frequency, in the case of manual driving, the frequency of the operation angle of the steering wheel 21 is used, and in the case of autonomous driving, the frequency of the indicated value is used. The determination unit 53 determines, based on the detection value of the operation angle sensor 25 during manual driving, whether the frequency of the operation angle is a value within the resonance frequency band of the actuator 40 and whether the frequency of the operation angle is a value within the resonance frequency band of the vehicle motion system.

[0049] In the case where the determination unit 53 determines that the steering-related frequency is a value outside the resonance frequency band, the steering control unit 54 performs normal control for controlling the rear-wheel steering device 4 based on the target rear-wheel steering angle. The steering control unit 54 performs normal control for the front-wheel steering device 3 as control based on the target front-wheel steering angle without considering the steering-related frequency.

[0050] In the case where the determination unit 53 determines that the steering-related frequency is a value within the resonance frequency band, the steering control unit 54 performs limit control. The limit control is control for the rear-wheel steering device 4 in a manner that (i) the actual steering angle is constantly a specified value, (ii) the absolute value of the actual steering angle gradually decreases from the specified value, (iii) the frequency of the actual steering angle becomes outside the resonance frequency band, or (iv) the absolute value of the actual steering angle becomes smaller than the absolute value of the latest value of the target extreme value that is the extreme value of the target rear-wheel steering angle. The limit control is released (stopped) by the steering-related frequency becoming a value outside the resonance frequency band. The limit control of this example controls the rear-wheel steering device 4 in a manner that the actual steering angle is constantly a specified value. The specified value of this example is set to 0.

[0051] After the steering-related frequency becomes a value within the specified resonance frequency band, in the case where it is determined that the number of times of the variable extreme value has become equal to or more than the extreme value determination threshold (here, 3 times), the steering control unit 54 makes the target rear-wheel steering angle constant at 0 on the condition that the absolute value of the target rear-wheel steering angle becomes below the switching threshold. The switching threshold is set to the specified value (here, 0) or a value near the specified value. The extreme value determination threshold is set such that limit control is not performed for an emergency avoidance operation of the steering wheel 21.

[0052] In the stage where the detected consecutive variable extreme values are two, it is possible that the calculated steering-related frequency is caused by an emergency avoidance operation. Even if the steering-related frequency is a value within the resonance band, the limit control is not executed, and thus the actual steering angle of the rear-wheel steering device 4 is not restricted. Further, in the case where the steering-related frequency continuously remains a value within the resonance band and the third variable extreme value is detected (determined), the steering ECU 5 turns on the flag for permitting the execution of the limit control (hereinafter, referred to as the limit flag), and changes to the state where the execution of the limit control is permitted. In other words, the limit flag is turned on when the steering-related frequency calculated from the first variable extreme value and the second variable extreme value is within the resonance band, and the steering-related frequency calculated from the second variable extreme value and the third variable extreme value is within the resonance band. The extreme value determination threshold is set to a value of 3 or more to prevent the limit control from interfering with the emergency avoidance.

[0053] Further, the steering ECU 5 may be configured to turn on the limit flag on the condition that the third variable extreme value is detected within a specified time from the detection of the second variable extreme value when the steering-related frequency calculated from the first variable extreme value and the second variable extreme value is within the resonance band. In this case, even if the steering-related frequency calculated from the second variable extreme value and the third variable extreme value is outside the resonance band, the limit flag is turned on by the detection of the third variable extreme value.

[0054] If the absolute value of the target steering angle becomes equal to or less than the switching threshold (here, 0) in the state where the limit flag is turned on, the control mode is changed from the normal control to the limit control (i.e., the limit control is executed), and the target rear-wheel steering angle becomes 0. In the limit control, the target rear-wheel steering angle is maintained at 0. By setting the switching threshold to a value at or near the specified value in the limit control, a sharp and large change in the target rear-wheel steering angle due to the start of the limit control is suppressed. The limit control is continued until the steering-related frequency becomes a value outside the resonance band.

[0055] As an example, as Figure 2 shown, for the input of the operation angle, the steering ECU 5 detects the variable extreme value at time t1 and the next variable extreme value at time t2. The steering ECU 5 calculates the time (t2 - t1) between the variable extreme value of the input operation angle and the next variable extreme value. The steering ECU 5 calculates the frequency of the operation angle based on the calculation result. The steering ECU 5 determines whether the frequency of the operation angle is a value within the specified resonance band. Here, as an example for explanation, it is determined whether it is a value within the resonance band of the actuator 40. Actually, the same resonance band as that of the actuator 40 is used, and the resonance band of the vehicle motion system is also determined, but the description thereof is omitted.

[0056] When the frequency of the operation angle is within the resonance band of the actuator 40, the steering ECU 5 measures the number of variable extrema that are the basis for the operation as the frequency in this determination. When the number of consecutive variable extrema reaches the extremum determination threshold, it changes to a state where limit control can be executed. In this example, the extremum determination threshold is 3 times. Therefore, when it is determined twice in a row that the frequency of the operation angle is within the resonance band of the same object (here, the actuator 40), the steering ECU 5 turns on the limit flag. That is, when it is determined at time t2 that the frequency of the operation angle is within the resonance band of the actuator 40, at the time t3 when a variable extremum is detected, the time (t3 - t2) from the variable extremum at time t2 to the variable extremum at time t3 is calculated. When it is determined that the frequency of the operation angle based on this time (t3 - t2) is within the resonance band of the same object (actuator 40) as the previous determination, the limit flag is turned on. That is, in this example, the limit flag is turned on at time t3. In addition, the target extremum (the extremum of the target rear wheel steering angle) at time t3 is the latest value (the most recent target extremum) of the target extremum before the execution of the limit control.

[0057] If the absolute value of the target rear wheel steering angle becomes the switching threshold (here, 0) in the state where the limit flag is on, limit control is executed (time t4). Through the limit control, until it is determined that the frequency of the operation angle is outside the resonance band of the actuator 40 (that is, from time t4 to time t7), the target rear wheel steering angle remains 0. In the limit control, the determination of whether the frequency of the operation angle is within the resonance band is the same as above, and is executed based on the extremum determination threshold (3 consecutive extrema). That is, when the frequency based on times t5 and t6 is outside the resonance band and the frequency based on times t6 and t7 is outside the resonance band, it is determined that the frequency of the operation angle is outside the resonance band. In addition, the extremum determination threshold in the limit control can be a value different from the extremum determination threshold in the normal control. For example, it can also be twice.

[0058] If it is determined that the frequency of the operating angle is a value outside the resonance band of the actuator 40 (time t7), the steering ECU 5 performs a switching process that gradually brings the restricted target steering angle, which is the target value in the restricted control, closer to the unrestricted target rear-wheel steering angle. When the restricted target steering angle and the target rear-wheel steering angle match (time t8), normal control is started. In this example, the control mode switches from restricted control to normal control via the switching process. Normal control is executed from time t8, and the comparison between the frequency of the operating angle and the resonance band continues. The above control is the same in the comparison between the steering-related frequency and the resonance band of the vehicle motion system. In addition, when the difference between the restricted target steering angle and the target steering angle at time t7 is equal to or less than a specified restoration determination threshold, the steering ECU 5 may also switch the control mode from restricted control to normal control without performing the switching process.

[0059] (Effect of this Embodiment)

[0060] According to this embodiment, when the frequency of the steering variable related to steering is a value within a specified resonance band, restricted control is executed. According to the restricted control of this embodiment, the actual steering angle of the rear wheel 10R is controlled to be constant regardless of the target rear-wheel steering angle. Therefore, the control value (actual steering angle) does not fluctuate (amplitude), suppressing the generation of resonance. That is, according to this embodiment, it is possible to suppress an increase in the behavior of the vehicle caused by resonance between the steering device that steers the wheels or vehicle motion and the steering of the wheels.

[0061] In addition, in the restricted control, the rear wheel 10R is controlled so that the actual steering angle is constantly 0. Thereby, it is suppressed that the rear wheel 10R obstructs the behavior of the vehicle caused by the steering of the front wheel 10F. That is, by controlling the actual steering angle of the front wheel 10F according to the target front-wheel steering angle and keeping the actual steering angle of the rear wheel 10R constantly 0, it is possible to achieve the target steering angle with the front wheel 10F and suppress resonance in the rear wheel 10R. In this way, the steering control unit 54 is configured to perform normal control on the front-wheel steering device 3 without considering the steering-related frequency, and selectively perform normal control and restricted control on the rear-wheel steering device 4 based on the steering-related frequency and the resonance band. According to this configuration, it is possible to balance the responsiveness and the stability of the behavior of the vehicle.

[0062] In Figure 3 the graph, the measured values of the first vehicle that performs restricted control on the rear-wheel steering device 4 in a vehicle equipped with the front-wheel steering device 3 and the rear-wheel steering device 4 (diamond mark: with rear-wheel steering and countermeasure), the measured values of the second vehicle that does not perform restricted control on the rear-wheel steering device 4 (square mark: with rear-wheel steering and no countermeasure), and the measured values of the third vehicle equipped only with the front-wheel steering device 3 (triangle mark: without rear-wheel steering) are depicted. In Figure 3In this case, the horizontal axis is the frequency of the operating angle as the steering-related frequency, and the vertical axis is the lateral acceleration / operating angle. It can be said that the larger the value of the vertical axis, the greater the shimmy (behavior) of the vehicle. The limit control here is the limit control of the present embodiment described above.

[0063] According to Figure 3 the measurement results, when comparing between the first vehicle and the second vehicle, in the region where the steering-related frequency is greater than a specific frequency, the measured value of the first vehicle (with limit control) is less than the measured value of the second vehicle (without limit control) and the same as that of the third vehicle. When the frequency of the operating angle enters the resonance band in the second vehicle, resonance occurs, and the lateral acceleration / operating angle becomes larger at frequencies higher than that. In the first vehicle, since the limit control is executed, even when the frequency of the operating angle enters the resonance band, resonance is suppressed, and the lateral acceleration / operating angle is as small as that of the third vehicle at frequencies higher than that. Thus, by executing the limit control, an increase in the behavior of the vehicle can be suppressed.

[0064] In addition, since the specified value is 0, in the limit control, the absolute value of the actual steering angle is less than the absolute value of the latest value of the target extreme value, which is the extreme value of the target rear-wheel steering angle. Thus, even if resonance occurs, the absolute value of the control is suppressed to be smaller than the target value, so an increase in the behavior of the vehicle is suppressed. In addition, in the present embodiment, the steering-related frequency is calculated based on the time from the generation of a variable extreme value to the generation of the next variable extreme value. Thus, a frequency corresponding to the actual input value can be calculated.

[0065] (An example of the control process)

[0066] Refer to Figure 4 An example of the control process will be described. The steering ECU 5 determines whether the current situation is in normal control (not in limit control) as condition A (S101). When condition A is satisfied, that is, when the current situation is in normal control (S101: Yes), the steering ECU 5 determines whether the steering-related frequency is a value within the resonance band as condition B (lower limit value of resonance frequency determination ≤ steering-related frequency ≤ upper limit value of resonance frequency determination) (S102).

[0067] When condition B is satisfied, that is, when the steering-related frequency is a value within the resonance band (S102: Yes), the steering ECU 5 determines as condition C whether the number of consecutive variable extreme values that satisfy condition B reaches the extreme value determination threshold. Here, it is determined whether condition B is satisfied for three consecutive variable extreme values (S103). When the counting flag described later is turned on, the steering ECU 5 determines that condition C is satisfied in step S103.

[0068] When condition C is satisfied, that is, when condition B is satisfied and the number of consecutive variable extreme values reaches the extreme value determination threshold (S103: Yes), the steering ECU 5 determines whether the target rear wheel steering angle reaches a specified value (here, 0) as condition D (S104). If the target rear wheel steering angle becomes 0 and condition D is satisfied (S104: Yes), the steering ECU 5 starts the limit control and closes the counting flag (S105).

[0069] On the other hand, if condition C is not satisfied (S103: No), the steering ECU 5 turns on the counting flag (S106). In addition, if condition B is not satisfied in step S102 (S102: No), the steering ECU 5 turns off the counting flag (S107). Further, in the case where the steering ECU 5 uses the counting function for measuring the number of consecutive variable extreme values that satisfy condition B, for example, step S106 includes a process of incrementing the count number, and steps S105 and S107 include a process of resetting the count.

[0070] When condition A is not satisfied, that is, when the current situation is in the limit control (S101: No), the steering ECU 5 determines as condition E whether the steering-related frequency is a value outside the resonance band (the steering-related frequency < the lower determination limit value of the resonance frequency, or the steering-related frequency > the upper determination limit value of the resonance frequency) (S108). When condition E is satisfied, that is, when the steering-related frequency is a value outside the resonance band (S108: Yes), the steering ECU 5 determines as condition F whether the difference between the limit target steering angle and the target rear wheel steering angle is below the restoration determination threshold (S109).

[0071] When condition F is satisfied, that is, when the difference between the limit target steering angle and the target rear wheel steering angle is below the restoration determination threshold (S109: Yes), the steering ECU 5 completes the limit control and starts the normal control (S110). That is, if condition F is satisfied, the steering ECU 5 switches the control mode from the limit control to the normal control. On the other hand, when condition F is not satisfied, that is, when the difference between the limit target steering angle and the target rear wheel steering angle is greater than the restoration determination threshold (S109: No), the steering ECU 5 performs a switching process to change the limit target steering angle in such a way that the limit target steering angle slowly approaches the target rear wheel steering angle, and starts the normal control at the moment when the limit target steering angle and the target rear wheel steering angle coincide (S111). Thereby, a sharp change in the target value is suppressed. The steering ECU 5 repeatedly executes such a process. In addition, "resonance frequency" is typically described in the above process, but this can be replaced with the resonance frequency of the actuator or the resonance frequency of the vehicle motion system.

[0072] (Other)

[0073] In the above-mentioned limit control (i), the rear-wheel steering device 4 is controlled in such a way that the actual steering angle is constantly the specified value. And, the absolute value of the specified value is set to be smaller than the absolute value of the latest value of the target extreme value. As in this embodiment, as an example, the specified value is set to 0.

[0074] The limit control is not limited to the above case. Other examples of the limit control will be described below. The steering ECU 5 can also be configured to: in the limit control (ii), as Figure 5 shown, control the rear-wheel steering device 4 in such a way that the absolute value of the actual steering angle gradually decreases from the specified value. Thereby, the control value does not generate fluctuations (amplitude), suppressing the generation of resonance. In addition, in this example, since the specified value is set to be smaller than the absolute value of the latest value of the target extreme value, the increase in the vehicle's behavior can be further suppressed. If as Figure 5 shown, in the limit control, when the actual steering angle becomes 0 by gradually decreasing, it is also possible to control in such a way that the actual steering angle is constantly 0 until the limit release flag. That is, it is also possible to execute the limit control (i) after the execution of the limit control (ii). In this way, the steering ECU 5 can also change the method (type) of the limit control in the limit control.

[0075] In addition, the steering ECU 5 can also control the rear-wheel steering device 4 in the limit control (iii) in such a way that the frequency of the actual steering angle becomes outside the resonance band. Thereby, the frequency of the control value also becomes outside the resonance band, suppressing the generation of resonance. However, in the limit control (iii), the possibility of deviation between the operation angle and the actual steering angle is relatively high. Therefore, from the viewpoint of the driver's operation feeling, other limit controls are preferred.

[0076] In addition, the steering ECU 5 can also control the rear-wheel steering device 4 in the limit control (iv) in such a way that the absolute value of the actual steering angle is smaller than the absolute value of the latest value of the target extreme value. Thereby, even if resonance occurs without changing the steering-related frequency, the magnitude of the vehicle's shimmy (behavior) becomes smaller. In this way, according to this structure, the increase in the vehicle's behavior caused by the resonance between the rear-wheel steering device 4 or the vehicle movement and the wheel steering can be suppressed.

[0077] In addition, the above-described restrictive controls (i) to (iv) including this embodiment can be executed on the front-wheel steering device 3. For example, the steering ECU 5 can also execute the restrictive control on the front-wheel steering device 3 and execute the normal control on the rear-wheel steering device 4. The steering ECU 5 can also be configured to execute the restrictive control on one of the front-wheel steering device 3 and the rear-wheel steering device 4 and execute the normal control on the other of the front-wheel steering device 3 and the rear-wheel steering device 4 if the start condition of the restrictive control is satisfied. In addition, when the vehicle is equipped with only the front-wheel steering device 3, even if the steering ECU 5 executes the restrictive control on the front-wheel steering device 3, depending on the type of the restrictive control, a deviation may occur between the operation angle and the actual steering angle, but the behavior of the vehicle can be suppressed.

[0078] In addition, in the above embodiment, in the restrictive control, the actual steering angle is controlled based on the restricted target steering angle obtained by correcting (restricting) the target steering angle. The control of the actual steering angle in the restrictive control is not limited to this. For example, the steering control unit 54 can also change the arithmetic parameters of the steering motor 45 in the restrictive control. For example, by changing the coefficient (parameter) multiplied by the difference value in the arithmetic operation of converting the difference between the target steering angle and the actual steering angle into the steering current supplied to the steering motor 45, the control of the actual steering angle in the restrictive control can also be achieved.

[0079] In addition, according to the present invention, since the actual steering angle is controlled to be in a specified state in the restrictive control, the steering system can be applied not only to a by-wire type but also, for example, to a system in which the operation of an operation member is mechanically linked to a wheel such as a power steering type. For example, in a power steering type steering system, in the restrictive control, the auxiliary force with respect to the operation angle is reduced as compared with the normal control, and thus an increase in the behavior caused by resonance can be suppressed. In addition, for example, it can also be configured such that one of the front-wheel steering device and the rear-wheel steering device is of the power steering type and the other of the front-wheel steering device and the rear-wheel steering device is of the by-wire type. For example, it can also be configured such that the front-wheel steering device is of the power steering type and the rear-wheel steering device is of the by-wire type, and the object of the restrictive control is the rear-wheel steering device. By making the object of the restrictive control a by-wire type steering system, the actual steering angle can be restricted more freely without considering the influence on the operation of the driver. The present invention can be applied, for example, to a steering system (for example, variable gear ratio steering: VGRS) in which the operation of an operation unit (steering wheel) is independent of the control of the steering angle of the wheel.

Claims

1. A steering system, comprising: At least one of a front-wheel steering device for steering the front wheels and a rear-wheel steering device for steering the rear wheels; and A control device for controlling the front-wheel steering device or the rear-wheel steering device according to a steering requirement, Wherein, The control device includes: A target calculation unit that calculates a target steering angle based on the steering requirement; A frequency calculation unit that calculates a steering-related frequency, which is a frequency of a steering variable related to steering; A determination unit that determines whether the steering-related frequency is a value within a specified resonance band; And A steering control unit that, when the determination unit determines that the steering-related frequency is a value outside the resonance band, performs normal control for controlling the front-wheel steering device or the rear-wheel steering device based on the target steering angle, and when the determination unit determines that the steering-related frequency is a value within the resonance band, performs limit control, The limit control is a control for controlling the front-wheel steering device or the rear-wheel steering device with respect to an actual steering angle as an actual steering angle in the following manner: (i) A manner in which the actual steering angle is constantly a specified value, (ii) A manner in which the absolute value of the actual steering angle gradually decreases from the specified value, (iii) A manner in which the frequency of the actual steering angle becomes outside the resonance band, or (iv) A manner in which the absolute value of the actual steering angle is less than the absolute value of the latest value of a target extreme value that is an extreme value of the target steering angle, The frequency calculation unit calculates a variable extreme value that is an extreme value of the steering variable, and calculates the steering-related frequency based on the time from the generation of the variable extreme value to the generation of the next variable extreme value.

2. The steering system according to claim 1, wherein, The limit control is a control for controlling the front-wheel steering device or the rear-wheel steering device in a manner in which the actual steering angle is constantly the specified value, The absolute value of the specified value is less than the absolute value of the latest value of a target extreme value that is an extreme value of the target steering angle.

3. The steering system according to claim 2, wherein, The specified value is 0.

4. The steering system according to claim 1, wherein, The limit control is a control for controlling the front-wheel steering device or the rear-wheel steering device in a manner in which the absolute value of the actual steering angle gradually decreases from the specified value, The absolute value of the specified value is less than the absolute value of the latest value of a target extreme value that is an extreme value of the target steering angle.

5. The steering system according to claim 1, wherein, The front-wheel steering device or the rear-wheel steering device, which is a control object of the control device, constitutes a steer-by-wire steering device that steers the wheels mechanically independently of the operating force of an operating member and by the force of a steering motor.

6. The steering system according to claim 1, wherein, The steering control unit controls the front-wheel steering device or the rear-wheel steering device based on a limited target steering angle obtained by correcting the target steering angle during the limit control.

7. The steering system according to claim 5, wherein, The steering control unit changes the operational parameters of the steering motor in the restriction control.

8. The steering system according to any one of claims 1 to 7, wherein the steering control unit performs the normal control on the front-wheel steering device without considering the steering-related frequency, and selectively performs the normal control and the restriction control on the rear-wheel steering device based on the steering-related frequency and the resonance frequency band.

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