Steering control device and steering control method

Abstract

To provide a steering control device and a steering control method that prevent a steering wheel from causing an unintended action.SOLUTION: A steering control device that controls a steer-by-wire steering device executes processes relating to: control of a current supply to a reaction force motor; and control of a current supply to a turning motor. A reaction control unit includes: a hysteresis control quantity calculating unit that calculates a hysteresis control quantity to apply hysteresis characteristics to steering reaction force, and a compensation axial force calculating unit that applies compensation axial force to the steering reaction force when it can be determined that the current supplied to the turning motor is in a high-load state. A turning control unit includes a reduction processing unit that reduces the current when it can be determined that the current supplied to the turning motor is in a high-load state. The hysteresis control quantity calculating unit includes a stopping hysteresis component calculating unit that fixes the hysteresis control quantity while it can be determined that the current supplied to the turning motor is in the high-load state.SELECTED DRAWING: Figure 9

Description

[Technical Field]

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

[0002] Steer-by-wire steering devices have been conventionally installed in vehicles. In such steer-by-wire steering devices, the drive current supplied to a steering motor is controlled by executing steering angle feedback control that causes the steering angle to follow a target steering angle. Patent Document 1, for example, describes a steering control device as a technology for controlling the drive current supplied to the steering motor.

[0003] In the above-mentioned Patent Document 1, in a situation where a current continues to be supplied to the steering motor in accordance with a steady-state deviation between the target steering angle and the steering angle, a reduction process is executed to reduce the current supplied to the steering motor. The reduction process corrects the target steering angle so that the deviation between the target steering angle and the steering angle becomes smaller. This makes it possible to reduce the current supplied to the steering motor. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-49971 Summary of the Invention [Problem to be solved by the invention]

[0005] In the above-mentioned Patent Document 1, a steering reaction force is applied to the steering wheel, allowing the driver to grasp the state of the steered wheels. The steering reaction force applied to the steering wheel reflects multiple axial forces, including a current axial force calculated based on the current supplied to the steering motor. For example, if a reduction process is executed to reduce the current supplied to the steering motor in a state where a deviation between the target steering angle and the steering angle steadily occurs, the current axial force changes as the current is reduced. This disrupts the relationship between the multiple axial forces, which can result in unintended movement of the steering wheel. [Means for solving the problem]

[0006] The steering control device that can solve the above problem controls a steer-by-wire type steering device having a structure in which the power transmission paths between a steering unit that is steered by the steering wheel of the vehicle and a steering unit that operates to move the steering shaft to which the steered wheels of the vehicle are connected are separated. The steering unit has a steering-side motor that generates a steering reaction force on the steering wheel, and the steering unit has a steering-side motor that generates a steering force that moves the steering axis, and the steering control device is configured to execute processing related to control of supply of current to the steering-side motor and processing related to control of supply of current to the steering-side motor, the processing related to control of supply of current to the steering-side motor includes a reaction force control amount calculation processing that calculates a reaction force control amount that is a control amount for controlling supply of current to the steering-side motor, and the processing related to control of supply of current to the steering-side motor includes a turning control amount calculation processing that calculates a turning control amount that is a control amount for controlling supply of current to the turning-side motor, and the reaction force control amount calculation processing The control system includes a hysteresis control amount calculation process that calculates a hysteresis control amount for adding a hysteresis characteristic obtained by changing a reference to the steering reaction force, and a limiting reaction force control amount calculation process that calculates a limiting reaction force control amount for adding a limiting reaction force to the steering reaction force to limit the steering of the steering wheel when it can be determined that the current is in a high load state due to the steered wheels not being turned while current is being supplied to the turning side motor, and the turning control amount calculation process includes a reduction process that reduces the current supplied to the turning side motor when it can be determined that the high load state exists, and the hysteresis control amount calculation process includes a change suppression process that suppresses change in the hysteresis control amount regardless of whether the steering direction of the steering wheel changes while it can be determined that the high load state exists.

[0007] According to the above configuration, when it is possible to determine that the current supplied to the steered-side motor is in a high-load state, changes in the hysteresis control variable are suppressed. Therefore, while it is possible to determine that the current supplied to the steered-side motor is in a high-load state, the steering reaction force is affected by fluctuations in magnitude due to the limited reaction force depending on the situation, but is less affected by the hysteresis control variable. This makes it possible to control the steering reaction force while it is possible to determine that the current supplied to the steered-side motor is in a high-load state. Therefore, unintended movement of the steering wheel is less likely to occur.

[0008] In the above steering control device, the reduction process is preferably a process of reducing the current supplied to the steering side motor when it is possible to determine that the vehicle is in the high load state, as well as that the vehicle is at an extremely low speed including a stopped state, and further that the steering wheel is in a state of not changing, and the change suppression process is preferably a process of suppressing changes in the hysteresis control amount regardless of whether the steering direction of the steering wheel changes or not, when it is at least possible to determine that the vehicle is in the high load state.

[0009] According to the above configuration, the change suppression process can be performed at least when it is possible to determine that the current supplied to the steered-side motor is in a high load state, which is effective for controlling the steering reaction force while it is possible to determine that the current supplied to the steered-side motor is in a high load state.

[0010] In the above steering control device, it is preferable that the hysteresis control amount calculation process includes a reference update process that updates the reference when it becomes impossible to determine whether the vehicle is in the high load state or the vehicle is traveling at an extremely low speed during execution of the change suppression process.

[0011] According to the above configuration, for example, when it becomes impossible to determine that the current supplied to the steering-side motor is in a high load state during execution of the change suppression process, it becomes unnecessary to suppress the change in the hysteresis control variable. This also applies when it becomes impossible to determine that the vehicle is traveling at an extremely low speed. Therefore, even if the change in the hysteresis control variable is suppressed, it is possible to prevent the driver from feeling uncomfortable.

[0012] In the above steering control device, it is preferable that the hysteresis control amount calculation process includes a process of calculating a value corresponding to the reference in the hysteresis characteristic when calculating the hysteresis control amount that can fix the value of the hysteresis control amount at the timing of updating the reference when the reference update process updates the reference.

[0013] According to the above configuration, when it is no longer necessary to suppress the change in the hysteresis control amount, the occurrence of an event such as a sudden change in the hysteresis control amount is suppressed, which is effective in suppressing the driver's sense of discomfort.

[0014] In the above steering control device, it is preferable that the change suppression process is a process of suppressing a change in the hysteresis control amount by fixing the hysteresis control amount. According to the above configuration, the change suppression process can be realized by a simple process.

[0015] In the above steering control device, it is preferable that the steering control device includes a judgment process for judging that the current is in a high load state due to the steered wheels not being turned while current is being supplied to the steered side motor, and that the judgment process includes a condition based on the result of a comparison of the current supplied to the steered side motor with a current threshold value.

[0016] According to the above configuration, in order to determine whether the current supplied to the steering-side motor is in a high-load state, conditions related to the current actually supplied to the steering-side motor are set, so that the situation for determining the high-load state can be optimized.

[0017] A steering control method that can solve the above problems is a method that is applied to a steer-by-wire steering device having a structure in which the power transmission paths between a steering unit that is steered by the steering wheel of a vehicle and a steering unit that operates to move a steering shaft to which the steered wheels of the vehicle are connected are separated. The steering control method includes executing a process related to control of supply of current to a steering side motor included in the steering unit, which generates a steering reaction force on the steering wheel, and executing a process related to control of supply of current to the turning side motor included in the steering unit, which generates a steering force to move the turning axis, the process related to control of supply of current to the steering side motor includes executing a reaction force control amount calculation process to calculate a reaction force control amount which is a control amount for controlling supply of current to the steering side motor, and the process related to control of supply of current to the turning side motor includes executing a turning control amount calculation process to calculate a steering control amount which is a control amount for controlling supply of current to the turning side motor, and the reaction force control amount calculation process uses hysteresis obtained by changing a reference each time the steering direction of the steering wheel changes. and, when it can be determined that the current is in a high load state due to the steered wheels not turning while current is being supplied to the turning-side motor, executing a limiting reaction force control amount calculation process to calculate a limiting reaction force control amount for adding a limiting reaction force to the steering reaction force so as to limit the steering of the steering wheel, wherein the turning control amount calculation process, when it can be determined that the high load state exists, includes executing a reduction process to reduce the current supplied to the turning-side motor, and the hysteresis control amount calculation process, while it can be determined that the high load state exists, includes executing a change suppression process to suppress a change in the hysteresis control amount regardless of whether the steering direction of the steering wheel changes. [Effects of the Invention]

[0018] According to the present invention, unintended movement of the steering wheel is less likely to occur. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a diagram showing a configuration of a vehicle steering system according to a first embodiment. [Figure 2] FIG. 2 is a block diagram showing the functions of the steering control device of FIG. 1. [Figure 3] FIG. 3 is a block diagram showing the functions of a reduction processing unit in FIG. 2; [Figure 4] FIG. 3 is a block diagram showing the functions of a reaction force control amount calculation unit in FIG. 2. [Figure 5] FIG. 3 is a block diagram showing the functions of an axial force calculation unit in FIG. 2. [Figure 6] FIG. 6 is a block diagram showing the functions of a distribution axial force calculation unit in FIG. 5. [Figure 7] FIG. 5 is a block diagram showing the function of a traveling hysteresis compensation amount calculation unit in FIG. 4. [Figure 8] 1A is a graph showing the relationship between the steering angle and the hysteresis component during turning steering, and FIG. 1B is a graph showing the relationship between the steering angle and the hysteresis component during returning steering. [Figure 9] FIG. 7 is a block diagram showing the functions of an angle axial force calculation unit in FIG. 6. [Figure 10] 10(a) to 10(d) are diagrams illustrating phenomena that occur in a comparative example. [Figure 11] FIG. 10 is a diagram illustrating the state of an axial force component. DETAILED DESCRIPTION OF THE INVENTION

[0020] First Embodiment A steering control device according to a first embodiment will be described. As shown in FIG. 1, vehicle steering system 2 includes a steering control device 1. Vehicle steering system 2 includes a steering unit 4 and a steering unit 6. Steering unit 4 is steered by a driver via a steering wheel 3 of the vehicle. Steering unit 6 steers left and right steered wheels 5 of the vehicle in accordance with the steering input to steering unit 4 by the driver. Vehicle steering system 2 has a structure in which, for example, power transmission paths between steering unit 4 and steering unit 6 are always separated. Power transmission paths between steering actuator 12 (described later) and steering actuator 31 (described later) are always separated. In other words, vehicle steering system 2 includes a steer-by-wire type steering device.

[0021] The steering unit 4 includes a steering shaft 11 and a steering actuator 12. The steering shaft 11 is connected to the steering wheel 3. The steering actuator 12 includes a reaction motor 13 and a steering-side reduction gear mechanism 14. The reaction motor 13 is a steering-side motor that applies a steering reaction force, which is a force that resists steering, to the steering wheel 3 via the steering shaft 11. The reaction motor 13 is connected to the steering shaft 11 via the steering-side reduction gear mechanism 14, which is made up of, for example, a worm and wheel. The reaction motor 13 is, for example, a three-phase brushless motor.

[0022] The steering unit 6 includes a pinion shaft 21, a rack shaft 22 as a steering shaft, and a rack housing 23. The pinion shaft 21 and the rack shaft 22 are connected with each other at a predetermined cross angle. A rack-and-pinion mechanism 24 is formed by meshing pinion teeth 21a formed on the pinion shaft 21 with rack teeth 22a formed on the rack shaft 22. The pinion shaft 21 corresponds to a rotation axis that can be converted into a steering angle θi, which is the steering position of the steered wheels 5. The rack housing 23 accommodates the rack-and-pinion mechanism 24.

[0023] One end of the pinion shaft 21 opposite to the end connected to the rack shaft 22 protrudes from a rack housing 23. Both ends of the rack shaft 22 protrude from both axial ends of the rack housing 23. Tie rods 26 are connected to both ends of the rack shaft 22 via rack ends 25 made up of ball joints. The ends of the tie rods 26 are connected to knuckles (not shown) to which the left and right steered wheels 5 are respectively attached.

[0024] The steering unit 6 includes a steering actuator 31. The steering actuator 31 includes a steering motor 32, a transmission mechanism 33, and a conversion mechanism 34. The steering motor 32 is a steering-side motor that applies a steering force to the rack shaft 22 to turn the steerable wheels 5 via the transmission mechanism 33 and the conversion mechanism 34. The steering motor 32 transmits rotation to the conversion mechanism 34 via the transmission mechanism 33, which is, for example, a belt transmission mechanism. The transmission mechanism 33 converts the rotation of the steering motor 32 into reciprocating motion of the rack shaft 22 via the conversion mechanism 34, which is, for example, a ball screw mechanism. The steering motor 32 may be, for example, a three-phase brushless motor.

[0025] In vehicle steering system 2, steering actuator 31 applies motor torque as a steering force to rack shaft 22 in response to steering by the driver, thereby changing the steering angle θi of steered wheels 5. At this time, steering actuator 12 applies a steering reaction force that resists the steering by the driver to steering wheel 3. As a result, in vehicle steering system 2, the steering reaction force, which is the motor torque applied from steering actuator 12, changes the steering torque Th required to steer steering wheel 3.

[0026] The reason for providing the pinion shaft 21 is to support the rack shaft 22 together with the pinion shaft 21 inside the rack housing 23. The rack shaft 22 is supported movably along its axial direction and is pressed toward the pinion shaft 21 by a support mechanism (not shown) provided in the vehicle steering system 2. In this way, the rack shaft 22 is supported inside the rack housing 23. However, another support mechanism may be provided to support the rack shaft 22 in the rack housing 23 without using the pinion shaft 21.

[0027] <Electrical configuration of the steering system> 1, the reaction force motor 13 and the steering motor 32 are connected to the steering control device 1. The steering control device 1 controls the operation of each of the motors 13, 32.

[0028] Detection results of various sensors are input to the steering control device 1. The various sensors include, for example, a torque sensor 41, a steering side rotation angle sensor 42, a turning side rotation angle sensor 43, and a vehicle speed sensor 44.

[0029] Torque sensor 41 is provided on steering shaft 11 between steering wheel 3 and steering-side reduction mechanism 14. Torque sensor 41 detects steering torque Th, which is a value indicating the torque applied to steering shaft 11 by the driver's steering. Steering torque Th is detected in relation to the torsion of a torsion bar 41a provided on steering shaft 11 between steering wheel 3 and steering-side reduction mechanism 14, midway along steering shaft 11. Steering-side rotation angle sensor 42 is provided on reaction motor 13. Steering-side rotation angle sensor 42 detects rotation angle θa, which is the angle of the rotation shaft of reaction motor 13, within a range of 360 degrees. Turning-side rotation angle sensor 43 is provided on steering motor 32. Steering-side rotation angle sensor 43 detects rotation angle θb, which is the angle of the rotation shaft of steering motor 32, within a range of 360 degrees. Vehicle speed sensor 44 detects vehicle speed V, which is the traveling speed of the vehicle.

[0030] <Functions of the steering control device> As shown in FIG. 2, steering control device 1 has reaction force control unit 50 and steering control unit 60. Reaction force control unit 50 controls steering wheel 3, which is the object to be controlled. Reaction force control unit 50 controls the drive of steering actuator 12, more specifically the supply of current to reaction force motor 13, in order to control the steering reaction force, which is the control variable of the object to be controlled. Steering control unit 60 controls rack shaft 22, which is the object to be controlled. Steering control unit 60 controls the drive of steering actuator 31, more specifically the supply of current to steering motor 32, in order to control the steering force, which is the control variable of the object to be controlled. Reaction force control unit 50 and steering control unit 60 transmit and receive information to and from each other via a local network such as serial communication, for example. Reaction force control unit 50 is combined with steering unit 4 to form a reaction force system RS. Steering control unit 60 is combined with steering unit 6 to form a steering system TS.

[0031] The reaction force control unit 50 is equipped with a central processing unit (hereinafter referred to as "CPU") and a memory. The reaction force control unit 50 performs various processes by having the CPU execute programs stored in the memory at predetermined calculation cycles. The turning control unit 60 is equipped with a central processing unit (hereinafter referred to as "CPU") and a memory. The turning control unit 60 performs various processes by having the CPU execute programs stored in the memory at predetermined calculation cycles. The CPU and memory constitute a microcomputer, which is a processing circuit. The memory includes computer-readable media such as RAM (Random Access Memory) and ROM (Read Only Memory). However, it is one example that various processes are realized by software. The processing circuits of the reaction force control unit 50 and the turning control unit 60 may be configured to realize at least a part of their processes by hardware circuits such as logic circuits.

[0032] Fig. 2 shows part of the processing executed by reaction force control unit 50 and steering control unit 60. The processing shown in Fig. 2 is part of the processing realized by a CPU executing a program stored in memory, and is described for each type of processing realized.

[0033] The reaction force control unit 50 has a current sensor 54. The current sensor 54 detects an actual current value Ia obtained from the current value of each phase of the reaction force motor 13 flowing through a connecting wire between the reaction force control unit 50 and the motor coil of each phase of the reaction force motor 13. The current sensor 54 acquires, as a current, the voltage drop across a shunt resistor connected to the source side of each switching element in an inverter (not shown) provided corresponding to the reaction force motor 13. For ease of explanation, FIG. 2 shows the connecting wires and current sensors for each phase collectively.

[0034] The steering control unit 60 has a current sensor 65. The current sensor 65 detects an actual current value Ib obtained from the value of a current for each phase of the steering motor 32 flowing through a connecting wire between the steering control unit 60 and the motor coil for each phase of the steering motor 32. The current sensor 65 obtains, as a current, a voltage drop across a shunt resistor connected to the source side of each switching element in an inverter (not shown) provided corresponding to the steering motor 32. For ease of explanation, in FIG. 2, the connecting wires for each phase and the current sensors for each phase are shown as one unit.

[0035] <About the reaction force control unit> As shown in Fig. 2, reaction force control unit 50 receives as input steering torque Th, vehicle speed V, rotation angle θa, pinion angle θp, and actual current value Ib. Pinion angle θp is a component calculated in steering control unit 60. Actual current value Ib is a component obtained from steering control unit 60. Reaction force control unit 50 controls the power supply to reaction force motor 13 based on steering torque Th, vehicle speed V, rotation angle θa, pinion angle θp, and actual current value Ib.

[0036] The reaction force control unit 50 includes a steering angle calculation unit 51 , a reaction force control amount calculation unit 52 , and an electric current control unit 53 . Steering angle calculation unit 51 calculates steering angle θs by inputting rotation angle θa. Steering angle θs corresponds to the rotation position of steering wheel 3. Steering angle calculation unit 51 converts rotation angle θa into an integrated angle that includes a range exceeding 360°, for example, by counting the number of rotations of reaction force motor 13 from a steering neutral position, which is the rotation position of steering wheel 3 when the vehicle is traveling straight. Steering angle calculation unit 51 calculates steering angle θs by multiplying the integrated angle obtained by conversion by a conversion coefficient based on the rotational speed ratio of steering-side reduction gear mechanism 14. The steering angle θs obtained in this way is output to steering control unit 60.

[0037] The reaction force control amount calculation unit 52 receives the steering torque Th, the vehicle speed V, the pinion angle θp, the actual current value Ib, and the steering angle θs as input. The reaction force control amount calculation unit 52 executes a reaction force control amount calculation process to calculate a reaction force torque command value Ts* based on the steering torque Th, the vehicle speed V, the pinion angle θp, the actual current value Ib, and the steering angle θs. The reaction force torque command value Ts* is a reaction force control amount that is a target for the steering reaction force of the steering wheel 3 to be generated by the reaction force motor 13.

[0038] The energization control unit 53 receives the reaction torque command value Ts*, the actual current value Ia, and the rotation angle θa. The energization control unit 53 calculates a steering current command value based on the reaction torque command value Ts*. The steering current command value is a target value of the torque to be generated by the reaction motor 13. The energization control unit 53 controls the power supply to the reaction motor 13 by performing feedback control of the current value based on the input of the actual current value Ia and the rotation angle θa. This causes the reaction motor 13 to generate torque according to the reaction torque command value Ts*. In other words, it is possible to provide the driver with an appropriate sense of control according to the road reaction force.

[0039] <About the steering control unit> As shown in Fig. 2, steering control unit 60 receives as input vehicle speed V, rotation angle θb, steering torque Th, and steering angle θs. Steering control unit 60 controls the power supply to steering motor 32 based on vehicle speed V, rotation angle θb, steering torque Th, and steering angle θs. Steering angle θs is a component calculated in reaction force control unit 50.

[0040] The steering control unit 60 has a pinion angle calculation unit 61, a target pinion angle calculation unit 62, a pinion angle feedback control unit ("pinion angle F / B control unit" in the drawing) 63, and an energization control unit 64.

[0041] Pinion angle calculation unit 61 calculates pinion angle θp by inputting rotation angle θb. Pinion angle θp corresponds to the movement position of rack shaft 22, i.e., the steered position of steerable wheels 5. Pinion angle calculation unit 61 converts rotation angle θb into an integrated angle that includes a range exceeding 360°, for example, by counting the number of rotations of steering motor 32 from a rack neutral position, which is the position of rack shaft 22 when the vehicle is traveling straight. Pinion angle calculation unit 61 calculates pinion angle θp, which is the actual rotation angle of pinion shaft 21, by multiplying the integrated angle obtained by conversion by a conversion coefficient based on the rotational speed ratio of transmission mechanism 33, the lead of conversion mechanism 34, and the rotational speed ratio of rack and pinion mechanism 24. Steering motor 32 and pinion shaft 21 are linked. Therefore, there is a one-to-one correspondence between the integrated value of rotation angle θb of steering motor 32 and pinion angle θp. Using this correspondence relationship, pinion angle θp can be found from rotation angle θb of steering motor 32. Furthermore, pinion shaft 21 is meshed with rack shaft 22. Therefore, there is also a one-to-one correspondence relationship between pinion angle θp and the amount of movement of rack shaft 22. And there is also a one-to-one correspondence relationship between pinion angle θp and steering angle θi of steered wheels 5. Pinion angle θp is an example of information that can be acquired by steering actuator 31, and is also an example of a steering converted angle.

[0042] Target pinion angle calculation unit 62 receives vehicle speed V, steering torque Th, steering angle θs, and actual current value Ib as input. Target pinion angle calculation unit 62 calculates target pinion angle θp* based on vehicle speed V, steering torque Th, steering angle θs, and actual current value Ib. Target pinion angle θp* is a target steering angle that serves as a target for pinion angle θp obtained as a result of steering steered wheels 5.

[0043] More specifically, as shown in FIG. 2, the target pinion angle calculation unit 62 has a steering angle ratio variable control unit 66 and a reduction processing unit 67. The steering angle ratio variable control unit 66 receives the vehicle speed V and the steering angle θs as input. The steering angle ratio variable control unit 66 calculates a converted angle θvg by adding an adjustment amount to the steering angle θs. The converted angle θvg is a component that forms the basis of the target pinion angle θp*, which is the target for the pinion angle θp obtained as a result of steering the steered wheels 5. The adjustment amount is a component for changing the steering angle ratio of the target pinion angle θp* to the steering angle θs. The adjustment amount is obtained by reflecting the gear ratio VG. The gear ratio VG is a steering angle variable value obtained, for example, by subtracting "1" from a value that indicates the relationship between the steering angle θs as the denominator and the converted angle θvg as the numerator. The value that indicates the relationship between the steering angle θs as the denominator and the converted angle θvg as the numerator is an index that indicates the ratio of the change in the converted angle θvg to the change in the steering angle θs. Such an index is called, for example, a transmission ratio or a steering angle ratio, and defines the relationship between the steering angle θs and the pinion angle θp, that is, the relationship between the steering angle θs and the steered angle θi. In other words, the gear ratio VG is a value that indicates the difference from the case where the steering angle θs and the target pinion angle θp* correspond 1:1.

[0044] The reduction processing unit 67 receives the vehicle speed V, the steering torque Th, the actual current value Ib, and the converted angle θvg. The reduction processing unit 67 calculates the target pinion angle θp* based on the vehicle speed V, the steering torque Th, the actual current value Ib, and the converted angle θvg.

[0045] More specifically, as shown in FIG. 3, the reduction processing unit 67 includes a reduction processing flag generating unit 71, an offset angle calculating unit 72, and an angle reflection processing unit 73. Reduction process flag generation unit 71 receives vehicle speed V and steering torque Th. Reduction process flag generation unit 71 generates reduction process flag FLG1 based on vehicle speed V and steering torque Th. Reduction process flag FLG1 indicates that reduction process is to be performed to reduce actual current value Ib, which is the current supplied to steering motor 32. Reduction process flag generation unit 71 generates reduction process flag FLG1 when vehicle speed V is extremely low and steering wheel 3 is not in a hands-off state. On the other hand, reduction process flag generation unit 71 does not generate reduction process flag FLG1 when vehicle speed V is not extremely low or when steering wheel 3 is not in a hands-off state. Extremely low speed includes a vehicle stopped state such as less than 2 kilometers per hour. Low, medium, and high speed includes a vehicle traveling state such as 2 kilometers per hour or more.

[0046] The offset angle calculation unit 72 receives the actual current value Ib and the target pinion angle θp*. The offset angle calculation unit 72 calculates the offset angle θoft based on the actual current value Ib and the target pinion angle θp*. For example, the offset angle calculation unit 72 calculates the offset angle θoft using an offset angle map that defines the relationship between the target pinion angle θp* and the offset angle θoft. The offset angle calculation unit 72 calculates an offset angle θoft with a larger absolute value as the absolute value of the target pinion angle θp* increases. Furthermore, the offset angle calculation unit 72 sets the sign of the offset angle θoft based on the actual current value Ib in a direction in which the actual current value Ib decreases, i.e., in a direction in which the absolute value decreases. For example, the offset angle calculation unit 72 sets the sign of the offset angle θoft to negative when the actual current value Ib is positive, and sets the sign of the offset angle θoft to positive when the actual current value Ib is negative. When the actual current value Ib is "0", the offset angle calculation unit 72 calculates an offset angle θoft that is "0".

[0047] The angle reflection processing unit 73 receives the vehicle speed V, steering torque Th, converted angle θvg, actual current value Ib, reduction processing flag FLG1, and offset angle θoft. For example, the angle reflection processing unit 73 determines whether the actual current value Ib indicates a high-load state based on a comparison between the actual current value Ib and a current threshold value Ith. The current threshold value Ith is, for example, a value within a range determined such that, if the actual current value Ib exceeds this value, it can be determined that the actual current value Ib indicates a high-load state. If the actual current value Ib is greater than the current threshold value Ith, the angle reflection processing unit 73 determines that the actual current value Ib indicates a high-load state, i.e., that reduction processing needs to be performed. Furthermore, if the actual current value Ib is equal to or less than the current threshold value Ith, the angle reflection processing unit 73 determines that the actual current value Ib does not indicate a high-load state, i.e., that reduction processing does not need to be performed.

[0048] On the condition that reduction process flag FLG1 is input, when actual current value Ib is greater than current threshold value Ith, angle reflection processing unit 73 calculates target pinion angle θp* obtained by adding post-conversion angle θvg and offset angle θoft. That is, when actual current value Ib is greater than current threshold value Ith, angle reflection processing unit 73 calculates target pinion angle θp* having an absolute value smaller than that of post-conversion angle θvg. As a result, target pinion angle θp* becomes a value closer to pinion angle θp than post-conversion angle θvg so that the amount of change in the current pinion angle θp becomes smaller. This makes the absolute value of a feedback deviation Δθp, which will be described later, smaller than the value corresponding to the post-conversion angle θvg. The absolute value of a steering current command value, which will be described later and which is obtained based on such feedback deviation Δθp, becomes smaller than the value obtained based on the post-conversion angle θvg. The absolute value of actual current value Ib obtained based on such steering current command value becomes smaller than the value obtained based on the post-conversion angle θvg. In this way, when the reduction processing unit 67 determines that the actual current value Ib indicates a high load state while receiving the reduction processing flag FLG1, it executes processing to reduce the absolute value of the actual current value Ib.

[0049] Furthermore, angle reflection processing unit 73 calculates target pinion angle θp*, which is the converted angle θvg, when actual current value Ib is equal to or less than current threshold value Ith, on the condition that reduction processing flag FLG1 is input. As a result, target pinion angle θp* becomes a value corresponding to the converted angle θvg, which is the amount of change in the current pinion angle θp. This means that the absolute value of feedback deviation Δθp, which will be described later, becomes a value corresponding to the converted angle θvg. The absolute value of a turning current command value, which will be described later, obtained based on such feedback deviation Δθp becomes a value obtained based on the converted angle θvg. The absolute value of actual current value Ib, which is obtained based on such a turning current command value, becomes a value obtained based on the converted angle θvg. In this way, reduction processing unit 67 inputs reduction processing flag FLG1, but when it can determine that actual current value Ib is not in a high-load state, executes processing to set the absolute value of actual current value Ib to a value obtained based on the converted angle θvg.

[0050] Furthermore, when reduction process flag FLG1 is not input, angle reflection processing unit 73 calculates target pinion angle θp*, which is the converted angle θvg, on the condition that offset angle θoft is "0." As a result, target pinion angle θp* becomes a value corresponding to the converted angle θvg, which is the amount of change in the current pinion angle θp. This means that the absolute value of a feedback deviation Δθp, which will be described later, becomes a value corresponding to the converted angle θvg. The absolute value of a turning current command value, which will be described later and which is obtained based on such feedback deviation Δθp, becomes a value obtained according to the converted angle θvg.

[0051] Furthermore, when the reduction processing flag FLG1 is not input, the angle reflection processing unit 73 executes offset angle release processing to reduce the offset angle θoft on the condition that the offset angle θoft is not 0. The offset angle release processing includes processing to reduce the offset angle θoft based on the vehicle speed V and the converted angle θvg while calculating the target pinion angle θp* by adding the converted angle θvg and the offset angle θoft.

[0052] More specifically, the offset angle release process sets the degree of decrease of the offset angle θoft based on the vehicle speed V and the converted angular velocity ωvg, which is a time derivative of the converted angle θvg. The degree of decrease includes, for example, the amount of decrease and the gradient of decrease of the offset angle θoft. For example, the offset angle release process sets the degree of decrease so that the offset angle θoft decreases more significantly as the vehicle speed V increases. Furthermore, the offset angle release process sets the degree of decrease so that the offset angle θoft decreases more significantly as the absolute value of the converted angular velocity ωvg increases. Furthermore, for example, the offset angle release process sets the offset angle θoft not to decrease when the vehicle speed V or the converted angular velocity ωvg is "0".

[0053] Pinion angle feedback control unit 63 receives as input target pinion angle θp* and pinion angle θp. Pinion angle feedback control unit 63 executes steering control amount calculation processing to calculate steering torque command value Tp* based on target pinion angle θp* and pinion angle θp. Turning torque command value Tp* is a steering control amount that serves as a target for the turning force of steered wheels 5 to be generated via turning motor 32. Pinion angle feedback control unit 63 calculates turning torque command value Tp* by executing feedback control of pinion angle θp so as to make pinion angle θp follow target pinion angle θp*. Turning torque command value Tp* is a control amount for reducing and eliminating feedback deviation Δθp obtained by subtracting pinion angle θp from target pinion angle θp*.

[0054] Energization control unit 64 receives as input steering torque command value Tp*, actual current value Ib, and rotation angle θb. Energization control unit 64 calculates a steering current command value based on steering torque command value Tp*. The steering current command value is a target value for torque to be generated in steering motor 32. Energization control unit 64 calculates a converted steering current value based on actual current value Ib and rotation angle θb. The converted steering current value is a converted value obtained by converting actual current value Ib into a current value on the dq coordinate. Energization control unit 64 controls the power supply to steering motor 32 by executing feedback control of the converted steering current value so that the converted steering current value follows the turning current command value. This causes steering motor 32 to generate torque according to steering torque command value Tp*. In other words, it is possible to rotate steered wheels 5 by an angle according to the steering force.

[0055] <Details of the reaction force control amount calculation unit> As shown in FIG. 2, the reaction force control amount calculation unit 52 has a target steering reaction force calculation unit 55, an axial force calculation unit 56, and a reflection processing unit 57.

[0056] The target steering reaction force calculation unit 55 receives the vehicle speed V, the steering torque Th, and the steering angle θs as input. The target steering reaction force calculation unit 55 calculates a steering reaction force component Tb* based on the vehicle speed V, the steering torque Th, and the steering angle θs. The steering reaction force component Tb* corresponds to the motor torque for rotating the steering wheel 3 in the steering direction of the driver, i.e., the assist force for assisting the steering of the steering wheel 3 by the driver.

[0057] The axial force calculation unit 56 receives the vehicle speed V, steering angle θs, actual current value Ib, and pinion angle θp as input. The axial force calculation unit 56 calculates the axial force component F based on the vehicle speed V, steering angle θs, actual current value Ib, and pinion angle θp. The axial force component F corresponds to a calculated axial force that is an estimate of the axial force acting on the rack shaft 22 through the steered wheels 5.

[0058] The reflection processing unit 57 receives the steering reaction force component Tb* and the axial force component F. The reflection processing unit 57 calculates a reaction torque command value Ts* obtained by subtracting the axial force component F from the steering reaction force component Tb*.

[0059] <About the target steering reaction force calculation unit> As shown in FIG. 4, the target steering reaction force calculation unit 55 has a basic control amount calculation unit 81, a traveling hysteresis compensation amount calculation unit 82, and an addition processing unit 83.

[0060] The basic control amount calculation unit 81 receives the vehicle speed V and the steering torque Th. The basic control amount calculation unit 81 calculates a basic control amount I1* based on the vehicle speed V and the steering torque Th. The basic control amount I1* is a control amount calculated in relation to the steering of the steering wheel 3. The basic control amount I1* is a basic component of the steering reaction force component Tb* and is set so that the steering of the steering wheel 3 exhibits desired characteristics. The basic control amount calculation unit 81 calculates the basic control amount I1* using, for example, an assist map that defines the relationship between the steering torque Th, the vehicle speed V, and the basic control amount I1*. The basic control amount I1* is set so that the larger the absolute value of the steering torque Th, the larger its absolute value becomes, and so that the smaller the vehicle speed V, the larger its absolute value becomes.

[0061] The driving hysteresis compensation amount calculation unit 82 receives the vehicle speed V and the steering angle θs as input. The driving hysteresis compensation amount calculation unit 82 calculates the driving hysteresis compensation amount I2* based on the vehicle speed V and the steering angle θs. The driving hysteresis compensation amount I2* is a compensation component that optimizes the hysteresis characteristics due to friction when the steering wheel 3 is in operation. The hysteresis characteristics due to friction when the steering wheel 3 is in operation are related to the mechanical friction components of the vehicle in which the vehicle steering system 2 is installed. The optimization of the hysteresis characteristics due to the mechanical friction components of the vehicle is compensated for by the driving hysteresis compensation amount I2*. The driving hysteresis compensation amount I2* compensates to optimize the hysteresis characteristics due to friction when the steering wheel 3 is in operation, for example, when the vehicle is in a driving state. The driving hysteresis compensation amount I2* has a hysteresis characteristic with respect to changes in the steering angle θs.

[0062] The addition processing unit 83 receives the basic control amount I1* and the traveling hysteresis compensation amount I2* as input, and calculates the steering reaction force component Tb* by adding the basic control amount I1* and the traveling hysteresis compensation amount I2*.

[0063] <About the axial force calculation unit> As shown in FIG. 5, the axial force calculation unit 56 has a distribution axial force calculation unit 91, a compensating axial force calculation unit 92, and an axial force summation processing unit 93.

[0064] The vehicle speed V, steering angle θs, and actual current value Ib are input to the distribution axial force calculation unit 91. The distribution axial force calculation unit 91 calculates the distribution axial force Fd based on the vehicle speed V, steering angle θs, and actual current value Ib.

[0065] More specifically, as shown in FIG. 6, the distribution axial force calculation unit 91 has an angle axial force calculation unit 101, a current axial force calculation unit 102, a distribution ratio calculation unit 103, and a distribution axial force summing unit 104.

[0066] The angular axial force calculation unit 101 receives the vehicle speed V and the steering angle θs. The angular axial force calculation unit 101 calculates the angular axial force Fr based on the vehicle speed V and the steering angle θs. The angular axial force Fr is an ideal value of the axial force defined by an arbitrarily set vehicle model. The angular axial force Fr is a value in the dimension of torque (N·m). The angular axial force Fr is calculated as an axial force that does not reflect road surface information such as minute irregularities that do not affect the lateral behavior of the vehicle or steps that affect the lateral behavior of the vehicle. The angular axial force calculation unit 101 calculates the angular axial force Fr using, for example, an angular axial force map that defines the relationship between the vehicle speed V, the steering angle θs, and the angular axial force Fr. The angular axial force Fr is set so that its absolute value increases as the absolute value of the steering angle θs increases. The angular axial force Fr is also set so that its absolute value increases as the vehicle speed V increases.

[0067] Current axial force calculation unit 102 receives actual current value Ib. Based on actual current value Ib, current axial force calculation unit 102 calculates current axial force Fi. Current axial force Fi is an axial force that actually acts on rack shaft 22, which operates to steer steered wheels 5, i.e., an estimated value of the axial force actually transmitted to rack shaft 22. Current axial force Fi is a value in the dimension of torque (N m). Current axial force Fi is calculated as a road surface reaction force, which is an axial force that reflects the road surface information. Current axial force calculation unit 102 calculates current axial force Fi using, for example, a current axial force map that defines the relationship between actual current value Ib and current axial force Fi. The current axial force Fi is set so that its absolute value increases as the absolute value of actual current value Ib increases, assuming that the torque applied to rack shaft 22 by steering motor 32 is balanced with the torque corresponding to the force applied to rack shaft 22 via steered wheels 5.

[0068] The distribution ratio calculation unit 103 receives the vehicle speed V as an input. The distribution ratio calculation unit 103 calculates the distribution ratio Di based on the vehicle speed V. The distribution ratio Di is a distribution ratio of the current axial force Fi when the axial force component F is obtained by distributing the angular axial force Fr and the current axial force Fi. In other words, the distribution ratio Di indicates a reflection state of the current axial force Fi to the axial force component F, and indicates a reflection state of the current axial force Fi to the steering reaction force component Tb*, i.e., the reaction torque command value Ts*. The distribution ratio calculation unit 103 calculates the distribution ratio Di using, for example, a distribution ratio map that defines the relationship between the vehicle speed V and the distribution ratio Di. When the vehicle speed V is relatively high, the distribution ratio Di becomes "1 (100%)." In other words, when the vehicle speed V is relatively high, this indicates that only the current axial force Fi is distributed to the axial force component F, i.e., the angular axial force Fr is not distributed. The distribution ratio includes the concept of "0 (zero)" in which only either the angular axial force Fr or the current axial force Fi is distributed to the axial force component F, for example.

[0069] The allocation ratio conversion processing unit 107 receives the allocation ratio Di. The allocation ratio conversion processing unit 107 calculates the allocation ratio Dr by subtracting the allocation ratio Di from a predetermined reference value "1". The allocation ratio Dr is the allocation ratio of the angular axial force Fr when the axial force component F is obtained by allocating the angular axial force Fr and the current axial force Fi. In other words, the allocation ratio Dr indicates the reflection state of the angular axial force Fr to the axial force component F, and indicates the reflection state of the angular axial force Fr to the steering reaction force component Tb*, i.e., the reaction torque command value Ts*. The sum of the allocation ratio Dr and the allocation ratio Di is set to, for example, the reference value "1", i.e., the combined allocation ratio is set to "100%".

[0070] Each distribution ratio Di, Dr is reflected in the corresponding axial force Fi, Fr. More specifically, the distribution ratio reflection processing unit 105 calculates the final angular axial force Frm by multiplying the angular axial force Fr by the distribution ratio Dr. The distribution ratio reflection processing unit 106 calculates the final current axial force Fim by multiplying the current axial force Fi by the distribution ratio Di.

[0071] The angular axial force Frm and the current axial force Fim are input to the distributed axial force summing processor 104. The distributed axial force summing processor 104 calculates an axial force component F obtained by adding the angular axial force Frm and the current axial force Fim.

[0072] The compensating axial force calculation unit 92 receives the vehicle speed V and the actual current value Ib. The compensating axial force calculation unit 92 executes a limiting reaction force control amount calculation process to calculate the compensating axial force Fc based on the vehicle speed V and the actual current value Ib. The compensating axial force Fc is a limiting reaction force control amount for adding a limiting reaction force to the steering reaction force to limit further steering of the steering wheel 3. The compensating axial force calculation unit 92 determines that a curb-hitting state has occurred when the vehicle speed V is extremely low and the actual current value Ib is greater than the current threshold value Ith. The curb-hitting state is, for example, a state in which the steered wheels 5 have hit an obstacle such as a curb and the rack shaft 22 cannot move. The current threshold value Ith is, for example, a value within a range defined as a value above which it can be determined that a curb-hitting state has occurred in which the steered wheels 5 have hit an obstacle such as a curb and the rack shaft 22 cannot move.

[0073] For example, in the case of hitting a curb or the like, even though the steerable wheels 5 cannot be steered in one direction toward the obstacle, there is a possibility that the steering wheel 3 will be steered in that direction beyond the stop position corresponding to the stop position of the steerable wheels 5. In this case, with regard to the control for steering the steerable wheels 5, it is not possible to match the pinion angle θp with the target pinion angle θp*, and the feedback deviation Δθp does not become small, so it can be determined that the actual current value Ib is in a high load state. In other words, there is a high possibility that a situation will occur in which current will continue to be supplied to the steering motor 32.

[0074] The axial force summing processor 93 receives the distributed axial force Fd and the compensating axial force Fc as input, and calculates the axial force component F obtained by adding the distributed axial force Fd and the compensating axial force Fc. <About the driving hysteresis compensation amount calculation unit> As shown in FIG. 7, the running hysteresis compensation amount calculation unit 84 has a running zero point calculation unit 121, a running hysteresis component calculation unit 122, and a running vehicle speed gain calculation unit 123.

[0075] The driving zero point calculation unit 121 inputs the steering angle θs. The driving zero point calculation unit 121 calculates a driving zero point Pr based on the steering angle θs. The driving zero point Pr is a calculation origin that serves as a reference when calculating the driving hysteresis component Fhy1. The driving hysteresis component Fhy1 is a component that forms the basis of the driving hysteresis compensation amount I2*. When determining the start of forward steering or the start of return steering, the driving zero point calculation unit 121 calculates the driving zero point Pr, which is the steering angle θs at the start position. The driving zero point calculation unit 121 determines the start of forward steering or the start of return steering based on an increase or decrease in the steering angle θs, i.e., the sign of the steering angular velocity ωs, which is the time derivative of the steering angle θs.

[0076] For example, forward steering refers to continuous steering in one direction with the same steering direction. Return steering refers to steering for a steering angle θs within a small predetermined range after the steering direction has changed. In other words, the driving zero point calculation unit 121 determines that the start of forward steering is the time when the steering angular velocity ωs starts to change from "0" to either a positive or negative sign. The driving zero point calculation unit 121 determines that the start of return steering is the time when the sign of the steering angular velocity ωs starts to change between positive and negative during forward steering. The driving zero point calculation unit 121 then determines that the start of forward steering is the time when the steering angle θs has completed changing by an amount corresponding to the predetermined range after determining that return steering has started.

[0077] The steering angle θs and the driving zero point Pr are input to the driving hysteresis component calculation unit 122. The driving hysteresis component calculation unit 122 calculates the driving hysteresis component Fhy1 based on the steering angle θs and the driving zero point Pr.

[0078] 8(a) and 8(b), the driving hysteresis component calculation unit 122 includes hysteresis maps M1 and M2 that define the relationship between the steering angle θs and the driving hysteresis component Fhy1. In the hysteresis maps M1 and M2, "θs" indicates the amount of change in the steering angle θs when the driving zero point Pr is set as the origin, that is, the angle deviation θhs.

[0079] In the hysteresis map M1, the absolute value of the driving hysteresis component Fhy1 is set to increase as the absolute value of the angular deviation θhs increases. Furthermore, in the hysteresis map M1, the absolute value of the driving hysteresis component Fhy1, which is the rate of change of the angular deviation θhs at the time of rising, decreases as the absolute value of the angular deviation θhs increases. In the hysteresis map M1, the absolute value of the driving hysteresis component Fhy1 is set to saturate when the angular deviation θhs is equal to or greater than a predetermined range. In the hysteresis map M2, the driving hysteresis component Fhy1 is calculated to be proportional to the angular deviation θhs. Furthermore, in the hysteresis maps M1 and M2, the absolute value of the driving hysteresis component Fhy1 and the absolute value of the hysteresis gradient increase as the vehicle speed V decreases. For example, the hysteresis map M1 exhibits a characteristic MA1 when the vehicle speed V is low, medium, or high, and exhibits a characteristic MB1 when the vehicle speed V is extremely low. The hysteresis map M2 exhibits a characteristic MA2 when the vehicle speed V is low, medium, or high, and exhibits a characteristic MB2 when the vehicle speed V is extremely low.

[0080] During turning steering, the driving hysteresis component calculation unit 122 uses the hysteresis map M1 to calculate the driving hysteresis component Fhy1. During turning steering to the right, the driving hysteresis component calculation unit 122 uses the value shown in the first quadrant with the driving zero point Pr at the start position of the turning steering as the origin of the hysteresis map M1. When turning steering to the left, the driving hysteresis component calculation unit 122 uses the value shown in the third quadrant with the driving zero point Pr at the start position of the turning steering as the origin of the hysteresis map M1.

[0081] During return steering, the driving hysteresis component calculation unit 122 uses the hysteresis map M2 to calculate the driving hysteresis component Fhy1. When steering back to the right, the driving hysteresis component calculation unit 122 uses the value shown in the first quadrant with the driving zero point Pr at the start position of the return steering as the origin of the hysteresis map M2. When steering back to the left, the driving hysteresis component calculation unit 122 uses the value shown in the third quadrant with the driving zero point Pr at the start position of the return steering as the origin of the hysteresis map M2.

[0082] The traveling vehicle speed gain calculation unit 123 receives the vehicle speed V. The traveling vehicle speed gain calculation unit 123 calculates the traveling vehicle speed gain Dv1 based on the vehicle speed V. For example, the traveling vehicle speed gain calculation unit 123 calculates the traveling vehicle speed gain Dv1 using a traveling vehicle speed gain map that defines the relationship between the vehicle speed V and the traveling vehicle speed gain Dv1. When the vehicle speed V is extremely low, the traveling vehicle speed gain calculation unit 123 calculates the traveling vehicle speed gain Dv1 to be "0." Furthermore, when the vehicle speed V is low, medium, or high, the traveling vehicle speed gain calculation unit 123 calculates the traveling vehicle speed gain Dv1 to be "1."

[0083] The running hysteresis component Fhy1 and the running vehicle speed gain Dv1 are input to the multiplication unit 124. The multiplication unit 124 calculates the running hysteresis compensation amount I2* by multiplying the running hysteresis component Fhy1 by the running vehicle speed gain Dv1.

[0084] <About the angle and axial force calculation section> As shown in FIG. 9, the angle axial force calculation section 101 has an axial force basic component calculation section 131, a hysteresis control amount calculation section 132, and a hysteresis reflection processing section 133.

[0085] The axial force basic component calculation unit 131 receives the steering angle θs. The axial force basic component calculation unit 131 calculates the axial force basic component Frb based on the steering angle θs. The axial force basic component Frb is the basic component of the angular axial force Fr. The axial force basic component Frb is set so that its absolute value increases as the absolute value of the steering angle θs increases. The axial force basic component Frb is a value in the dimension of torque (N m).

[0086] The hysteresis control amount calculation unit 132 receives the vehicle speed V, the steering angle θs, and the actual current value Ib as input. The hysteresis control amount calculation unit 132 executes a hysteresis control amount calculation process to calculate a hysteresis component Fhyr for the angular axial force based on the vehicle speed V, the steering angle θs, and the actual current value Ib. The hysteresis component Fhyr for the angular axial force is a hysteresis component to be added to the angular axial force Fr.

[0087] More specifically, the hysteresis control amount calculation unit 132 has a high load flag generation unit 134, a stopping zero point calculation unit 135, a stopping hysteresis component calculation unit 136, a zero point inverse calculation unit 137, and a stopping vehicle speed gain calculation unit 138.

[0088] The high load flag generation unit 134 receives the actual current value Ib. The high load flag generation unit 134 generates a high load flag FLG2 based on the actual current value Ib. The high load flag FLG2 indicates that the actual current value Ib is in a high load state. The high load flag generation unit 134 performs a determination process to determine whether the actual current value Ib is in a high load state, for example, based on the result of a comparison between the actual current value Ib and a current threshold value Ith. The current threshold value Ith is the same value as that used to determine whether the steered wheels 5 have hit an obstacle such as a curb and the rack shaft 22 is unable to move. The high load flag generation unit 134 generates the high load flag FLG2 when the actual current value Ib is greater than the current threshold value Ith. On the other hand, the high load flag generation unit 134 does not generate the high load flag FLG2 when the actual current value Ib is equal to or less than the current threshold value Ith.

[0089] The vehicle speed V, steering angle θs, high load flag FLG2, and corrected zero point Pc are input to the vehicle stopping zero point calculation unit 135. The corrected zero point Pc is a component calculated in the zero point inverse calculation unit 137. The vehicle stopping zero point calculation unit 135 calculates the vehicle stopping zero point Ps based on the vehicle speed V, steering angle θs, high load flag FLG2, and corrected zero point Pc. The vehicle stopping zero point Ps is a calculation origin that serves as a reference when calculating the vehicle stopping hysteresis component Fhy2. The vehicle stopping hysteresis component Fhy2 is a component that is added to the axial force basic component Frb so that the angular axial force Fr has hysteresis characteristics. When determining the start of turning steering or the start of returning steering, the stopping zero point calculation unit 135 calculates the stopping zero point Ps, which is the steering angle θs at the start position, in the same way that the driving zero point calculation unit 121 calculates the driving zero point Pr.

[0090] While the high load flag FLG2 is not input, the vehicle-stopping zero point calculation unit 135 calculates and updates the vehicle-stopping zero point Ps every time it determines the start of turning steering or the start of returning steering.

[0091] Furthermore, while the high load flag FLG2 is being input, the parking zero point calculation unit 135 fixes and holds the parking zero point Ps regardless of whether it determines the start of turning steering or the start of returning steering.

[0092] Furthermore, in a state in which the high load flag FLG2 is input, if the vehicle speed V is not extremely slow or the high load flag FLG2 is no longer input, the stopping zero point calculation unit 135 executes a reference update process to calculate and update the stopping zero point Ps, which is the corrected zero point Pc. The stopping zero point calculation unit 135 updates the stopping zero point Ps regardless of whether it determines the start of turning steering or the start of returning steering.

[0093] The vehicle speed V, the steering angle θs, the high load flag FLG2, and the vehicle stopping zero point Ps are input to the vehicle stopping hysteresis component calculation unit 136. The vehicle stopping hysteresis component calculation unit 136 calculates the vehicle stopping hysteresis component Fhy2 based on the vehicle speed V, the steering angle θs, the high load flag FLG2, and the vehicle stopping zero point Ps.

[0094] When the high load flag FLG2 is not input, the vehicle-stopping hysteresis component calculation unit 136 calculates the vehicle-stopping hysteresis component Fhy2 in the same manner as the running hysteresis component calculation unit 122 calculates the running hysteresis component Fhy1. That is, the vehicle-stopping hysteresis component calculation unit 136 calculates the vehicle-stopping hysteresis component Fhy2 using the hysteresis maps M1 and M2.

[0095] Furthermore, when the high load flag FLG2 is input, the vehicle-stopping hysteresis component calculation unit 136 executes a change suppression process to fix and maintain the vehicle-stopping hysteresis component Fhy2, regardless of whether it determines that turning-in steering or turning-back steering has started. The vehicle-stopping hysteresis component Fhy2 is fixed to its immediately preceding value. Maintaining the vehicle-stopping hysteresis component Fhy2 fixed suppresses changes in the vehicle-stopping hysteresis component Fhy2.

[0096] The zero point inverse calculation unit 137 receives the steering angle θs and the parking hysteresis component Fhy2. The zero point inverse calculation unit 137 calculates a corrected zero point Pc based on the steering angle θs and the parking hysteresis component Fhy2. The corrected zero point Pc is a component for correcting the parking zero point Ps so that, when updating the parking zero point Ps, the value of the parking hysteresis component Fhy2 at the timing of the update can be fixed. The zero point inverse calculation unit 137 calculates the corrected zero point Pc so as to offset the parking zero point Ps to either the positive or negative side of the angle deviation θhs.

[0097] The process of calculating the correction zero point Pc includes considering a coordinate system in which the horizontal axis represents the angular deviation θhs and the vertical axis represents the parking hysteresis component Fhy2 for the hysteresis maps M1 and M2. The process of calculating the correction zero point Pc includes specifying target coordinates representing the input parking hysteresis component Fhy2 and the corresponding angular deviation θhs for the hysteresis maps M1 and M2 corresponding to the current situation. The process of calculating the correction zero point Pc includes offsetting the characteristics of the hysteresis maps M1 and M2 corresponding to the current situation so as to shift the characteristics entirely to either the positive or negative side of the horizontal axis until they overlap with the specified target coordinates. The process of calculating the correction zero point Pc includes specifying an intersection angle, which is the point of intersection with the horizontal axis obtained from the offset characteristics for the hysteresis maps M1 and M2 corresponding to the current situation. The process of calculating the correction zero point Pc includes subtracting the angle deviation θhs, which is the identified intersection angle, from the steering angle θs corresponding to the stopping zero point Ps at that time, and setting the value obtained as the correction zero point Pc.

[0098] The stopping vehicle speed gain calculation unit 138 receives the vehicle speed V. The stopping vehicle speed gain calculation unit 138 calculates the stopping vehicle speed gain Dv2 based on the vehicle speed V. For example, the stopping vehicle speed gain calculation unit 138 calculates the stopping vehicle speed gain Dv2 using a stopping vehicle speed gain map that defines the relationship between the vehicle speed V and the stopping vehicle speed gain Dv2. When the vehicle speed V is in a low, medium, or high state, the stopping vehicle speed gain calculation unit 138 calculates the stopping vehicle speed gain Dv2 to be "0." Furthermore, when the vehicle speed V is extremely low, the stopping vehicle speed gain calculation unit 138 calculates the stopping vehicle speed gain Dv2 so that it linearly increases toward "1" as the vehicle speed V becomes smaller.

[0099] The multiplication processing unit 139 receives the vehicle stopping hysteresis component Fhy2 and the vehicle stopping vehicle speed gain Dv2. The multiplication processing unit 139 calculates the angular axial force hysteresis component Fhyr by multiplying the vehicle stopping hysteresis component Fhy2 by the vehicle stopping vehicle speed gain Dv2.

[0100] The axial force basic component Frb and the angular axial force hysteresis component Fhyr are input to the hysteresis reflection processing unit 133. The hysteresis reflection processing unit 133 calculates the angular axial force Fr obtained by adding the axial force basic component Frb and the angular axial force hysteresis component Fhyr.

[0101] <Operation of this embodiment> Below, a comparative example will be described in which the vehicle stopping hysteresis component calculation unit 136 does not fix and hold the vehicle stopping hysteresis component Fhy2 even when it can determine that the actual current value Ib is in a high load state.

[0102] For example, as shown in FIG. 10(a), when the vehicle is stopped, steering to the left from the rack neutral position indicated by "N" in the figure can cause the steered wheels 5 to hit an obstacle such as a curb, preventing the rack shaft 22 from moving. In this curb-hitting state, the pinion angle θp cannot be made to match the target pinion angle θp*, and the feedback deviation Δθp does not decrease. The compensating axial force calculation unit 92 determines an increase in the absolute value of the actual current value Ib, and adds a compensating axial force Fc to the steering reaction force. The compensating axial force Fc acts to the right in the figure ("S1" in the figure) to limit further steering of the steering wheel 3.

[0103] Subsequently, when the steering wheel 3 is released, the compensating axial force Fc acts, causing the steering wheel 3 to move back toward the rack neutral position. In the distributed axial force calculation unit 91, the compensating axial force Fc acts and determines a change in the direction of movement of the steering wheel 3, and the distributed axial force Fd including the stopping hysteresis component Fhy2 is added to the steering reaction force. The stopping hysteresis component Fhy2 acts to the left in the figure, which is the side opposite to the direction in which the compensating axial force Fc acts ("S2" in the figure).

[0104] Next, because the steering wheel 3 is in a hands-off state, the movement of the steering wheel 3 stops at a position where the compensating axial force Fc and the distributed axial force Fd including the parking hysteresis component Fhy2 are balanced. The position where the compensating axial force Fc and the distributed axial force Fd including the parking hysteresis component Fhy2 are balanced may be shifted from the position where the compensating axial force Fc starts to be added to the steering reaction force. At a position corresponding to the steering angle θs, the target pinion angle θp* is fixed and held at a value corresponding to the position where the movement of the steering wheel 3 stops ("S3" in the figure).

[0105] Furthermore, as shown in FIG. 10(b), since the pinion angle θp is in a state of hitting a curb or the like, the pinion angle θp cannot be made to coincide with the target pinion angle θp*, and the feedback deviation Δθp remains steadily. The reduction process flag generation unit 71 generates a reduction process flag FLG1. The angle reflection processing unit 73 determines that the actual current value Ib is in a high load state. The angle reflection processing unit 73 calculates the target pinion angle θp* as a value closer to the pinion angle θp than the position corresponding to the steering angle θs ("S4" in the figure). This reduces the feedback deviation Δθp.

[0106] 10(c), in the state where the wheel is hitting a curb or the like, the feedback deviation Δθp decreases, causing the actual current value Ib to decrease ("S5" in the figure). When the compensating axial force calculation unit 92 no longer determines that the absolute value of the actual current value Ib is increasing, the compensating axial force Fc is no longer taken into account in the steering reaction force ("S6" in the figure).

[0107] 10(d), when the compensating axial force Fc is no longer added to the steering reaction force, the balance between the compensating axial force Fc and the distributed axial force Fd including the parking hysteresis component Fhy2 is lost. As a result, the steering wheel 3 moves to the left in the figure, which is the end position indicated by "E" in the figure ("S7" in the figure).

[0108] Subsequently, as a result of the steering wheel 3 being operated, the reduction process flag FLG1 is no longer generated in the reduction process flag generation unit 71. As a result, the angle reflection processing unit 73 executes the offset angle release process, and the target pinion angle θp* is calculated as a value that moves away from the pinion angle θp ("S8" in the drawing).

[0109] Furthermore, the amount by which the steering wheel 3 moves to the left in the diagram is superimposed on the value by which the target pinion angle θp* moves away from the pinion angle θp. This increases the feedback deviation Δθp. As a result, the compensating axial force calculation unit 92 determines that the absolute value of the actual current value Ib has increased, and adds a compensating axial force Fc to the steering reaction force. The compensating axial force Fc acts to the right in the diagram ("S9" in the diagram) to limit further steering of the steering wheel 3. After that, "S1" to "S9" in the diagram are repeated.

[0110] In contrast, in this embodiment, when it is possible to determine that the actual current value Ib indicates a high load state, the vehicle-stopping hysteresis component Fhy2 is fixed. Therefore, while it is possible to determine that the actual current value Ib indicates a high load state, the steering reaction force is subject to the influence of the compensating axial force Fc, which varies in magnitude depending on the situation, but is less susceptible to the influence of the vehicle-stopping hysteresis component Fhy2.

[0111] For example, as shown in Fig. 11, the parking hysteresis component Fhy2 is fixed at a fixed value Y1 at which the compensating axial force Fc begins to be added to the steering reaction force. Even if the compensating axial force Fc is added to the steering reaction force, it is merely added to the fixed value Y1 within a range exceeding the angle X1 at which the compensating axial force Fc begins to be added to the steering reaction force. In other words, the compensating axial force Fc acts as a spring component for the fixed parking hysteresis component Fhy2.

[0112] In this embodiment, in contrast to "S2" in the figure for the comparative example, the stopping hysteresis component Fhy2 is fixed at a value at which the compensating axial force Fc starts to be added to the steering reaction force. This prevents the steering wheel 3 from returning to the rack neutral position from the position at which the compensating axial force Fc starts to be added to the steering reaction force when hitting a curb or the like.

[0113] Furthermore, in this embodiment, even if an increase in the absolute value of the actual current value Ib is no longer detected, as opposed to "S6" and "S7" in the comparative example, the stopping hysteresis component Fhy2 is fixed, so movement of the steering wheel 3 to the left in the figure is suppressed.

[0114] In this embodiment, when the steering wheel 3 is prevented from moving to the left in the figure at "S7" in the comparative example, the steering wheel 3 may move back toward the rack neutral position. Therefore, even if the offset angle release process is executed in "S8" in the comparative example, the target pinion angle θp* is prevented from being calculated to move away from the pinion angle θp. In other words, in "S9" in the comparative example, the compensating axial force calculation unit 92 determines an increase in the absolute value of the actual current value Ib, thereby preventing the compensating axial force Fc from being added to the steering reaction force.

[0115] <Effects of this embodiment> (1-1) While it is possible to determine that the actual current value Ib is in a high-load state, the compensating axial force Fc acts as a spring component for the fixed vehicle-stop hysteresis component Fhy2. This makes it possible to control the steering reaction force while it is possible to determine that the actual current value Ib is in a high-load state. Therefore, unintended movement of the steering wheel 3 is less likely to occur.

[0116] (1-2) The reduction processing unit 67 includes a process for determining whether the actual current value Ib indicates a high-load state, whether the vehicle speed V is extremely low, and whether the steering wheel 3 is in a hands-off state. The hysteresis control amount calculation unit 132 includes a vehicle-stop hysteresis component calculation unit 136 that fixes the vehicle-stop hysteresis component Fhy2 when the high-load flag FLG2 is input. This allows the vehicle-stop hysteresis component Fhy2 to be fixed when it can be determined that the actual current value Ib indicates a high-load state. This is effective in controlling the steering reaction force while it can be determined that the actual current value Ib indicates a high-load state.

[0117] (1-3) The hysteresis control amount calculation unit 132 includes a vehicle-stopping zero point calculation unit 135 that updates the vehicle-stopping zero point Ps when the vehicle speed V is low, medium, or high, or when the high-load flag FLG2 is no longer input, while the high-load flag FLG2 is being input. For example, if it becomes impossible to determine that the actual current value Ib is in a high-load state while the vehicle-stopping hysteresis component Fhy2 is being fixed, there is no need to fix the vehicle-stopping hysteresis component Fhy2. The same applies to the case where it becomes impossible to determine that the vehicle is traveling at an extremely low speed. Therefore, even if the vehicle-stopping hysteresis component Fhy2 is fixed, the driver's discomfort is reduced.

[0118] (1-4) The hysteresis control amount calculation unit 132 includes a zero point inverse calculation unit 137 that calculates the corrected zero point Pc when the parking zero point calculation unit 135 updates the parking zero point Ps. This prevents the parking hysteresis component Fhy2 from suddenly changing when there is no need to fix the parking hysteresis component Fhy2. This is effective in preventing the driver from feeling uncomfortable.

[0119] (1-5) The vehicle-stopping hysteresis component calculation unit 136 performs processing to suppress changes in the vehicle-stopping hysteresis component Fhy2 by fixing the vehicle-stopping hysteresis component Fhy2. This allows the vehicle-stopping hysteresis component calculation unit 136 to be implemented with simple processing.

[0120] (1-6) Steering control device 1 includes high-load flag generation unit 134 that determines whether actual current value Ib indicates a high-load state due to the steered wheels 5 not being steered while current is being supplied to steering motor 32. High-load flag generation unit 134 includes a condition based on the result of a comparison of the magnitude of actual current value Ib with current threshold value Ith. This sets a condition related to the current actually supplied to steering motor 32 to determine whether actual current value Ib indicates a high-load state, thereby optimizing the circumstances under which the high-load state is determined.

[0121] <Second embodiment> The second embodiment will be described below with reference to the drawings, focusing on the differences from the first embodiment. For ease of explanation, the same components as those in the first embodiment will be assigned the same reference numerals as those in the first embodiment, and the description thereof will be omitted.

[0122] As indicated by brackets in FIG. 9 , the high load flag generation unit 134 of the angle axial force calculation unit 101 of this embodiment is configured to input a compensating axial force Fc instead of inputting the actual current value Ib. The high load flag generation unit 134 generates a high load flag FLG2 based on the compensating axial force Fc. The high load flag generation unit 134 determines whether the actual current value Ib indicates a high load state, for example, based on the result of comparing the compensating axial force Fc with the axial force threshold value Fth. The axial force threshold value Fth is a value within a range determined to determine whether the compensating axial force Fc is being generated due to a curb-impact state in which the steered wheels 5 have struck an obstacle such as a curb and the rack shaft 22 cannot move. The high load flag generation unit 134 generates a high load flag FLG2 when the compensating axial force Fc is greater than the axial force threshold value Fth. On the other hand, when the compensating axial force Fc is equal to or less than the axial force threshold value Fth, the high load flag generating unit 134 does not generate the high load flag FLG2.

[0123] According to the second embodiment described above, the same actions and effects as those of the first embodiment can be obtained. <Other embodiments> The above-described embodiments may be modified as follows: In addition, the following other embodiments may be combined with each other within the scope of technical compatibility.

[0124] In the first embodiment, the current threshold value Ith in the angle reflection processing unit 73 and the current threshold value Ith in the high load flag generation unit 134 may be different values. In each embodiment, compensating axial force Fc may be a component for informing the driver of a deviation in the relationship between steering angle θs and pinion angle θp. A deviation in the relationship between steering angle θs and pinion angle θp may occur due to a state in which the pinion angle θp hits a curb or the like. Alternatively, a deviation in the relationship between steering angle θs and pinion angle θp may occur due to restrictions on the operation of steering motor 32 based on a heat generation state or voltage state. When a deviation occurs between steering angle θs and pinion angle θp, pinion angle θp cannot be made to match target pinion angle θp*, and feedback deviation Δθp does not decrease, so that it can be determined that actual current value Ib is in a high load state. The other embodiments described herein also provide functions and effects similar to those of the first embodiment. In the first embodiment, the high load flag generation unit 134 may be configured to input a component that is an index of the magnitude of deviation in the relationship between the steering angle θs and the pinion angle θp, instead of inputting the actual current value Ib.

[0125] In each embodiment, the reduction processing unit 67 may determine that the reduction processing needs to be performed if the angle reflection processing unit 73 determines that the actual current value Ib is in a high load state. In other words, the reduction processing unit 67 may not include the reduction processing flag generation unit 71.

[0126] In each embodiment, the reduction process flag generation unit 71 may be a process executed by the offset angle calculation unit 72 and the angle reflection processing unit 73. The reduction process flag generation unit 71 may also include a process for determining whether the actual current value Ib is in a high load state.

[0127] In each embodiment, high load flag generation unit 134 may include a condition based on a change in pinion angle θp or a change in feedback deviation Δθp, instead of the condition based on actual current value Ib. Also, high load flag generation unit 134 may include a condition based on a change in pinion angle θp or a change in feedback deviation Δθp, in addition to the condition based on actual current value Ib. High load flag generation unit 134 may take into account the output torque of steering motor 32, instead of actual current value Ib.

[0128] In each embodiment, the high load flag generation unit 134 may be a process executed by the steering control unit 60. Furthermore, the high load flag generation unit 134 may be a process executed by a control device other than the steering control device 1.

[0129] In each embodiment, the hysteresis control amount calculation unit 132 may not be configured to calculate the corrected zero point Pc. In other words, when the high load flag FLG2 is input, if the vehicle speed V is not extremely slow or the high load flag FLG2 is no longer input, the stopping zero point calculation unit 135 calculates and updates the stopping zero point Ps each time it determines the start of turning steering, etc. Note that in the other embodiments described herein, the zero point inverse calculation unit 137 may be eliminated.

[0130] In each embodiment, when the high load flag FLG2 is input, the stopping zero point calculation unit 135 may calculate and update the stopping zero point Ps each time it determines that turning steering has started, etc., if the high load flag FLG2 is no longer input.

[0131] In each embodiment, the vehicle-stopping hysteresis component calculation unit 136 may suppress the change in the vehicle-stopping hysteresis component Fhy2, and may achieve this by, for example, limiting the amount of change.

[0132] In each embodiment, the traveling hysteresis compensation amount I2* may be a component that adds hysteresis to the steering torque Th. In other words, the traveling hysteresis compensation amount calculation unit 82 may be implemented as processing that is executed immediately before the basic control amount calculation unit 81.

[0133] In each embodiment, the axial force calculation unit 56 may calculate a vehicle state quantity axial force that can be calculated based on the vehicle speed V, lateral acceleration, and yaw rate, instead of the current axial force Fi. Also, the axial force calculation unit 56 may calculate a tire axial force that is obtained in consideration of the tire force acting on the steered wheels 5, instead of the current axial force Fi. The vehicle state quantity axial force and the tire axial force can also be used by being added to the angle axial force Fr and the current axial force Fi.

[0134] In each embodiment, the angle axial force calculation unit 101 may be configured to input the pinion angle θp or the target pinion angle θp* instead of the steering angle θs. In each embodiment, the current axial force calculation unit 102 needs to use at least the actual current value Ib, and may use other elements such as the vehicle speed V in combination.

[0135] In each embodiment, the distribution ratio calculation unit 103 needs to use at least the vehicle speed V, and may use the pinion angle θp, the target pinion angle θp*, or the steering angle θs, or may use a combination of other elements.

[0136] In each embodiment, the sum of the distribution ratio Dr and the distribution ratio Di may be set to a value less than the reference value "1" or greater than the reference value "1". In each embodiment, the basic control amount calculation unit 81 may be configured to input the steering angle θs instead of the steering torque Th when calculating the basic control amount I1*. Also, the basic control amount calculation unit 81 does not have to use the vehicle speed V, and may use other elements in combination.

[0137] In each embodiment, the target steering reaction force calculation unit 55 may calculate, as the steering reaction force component Tb*, a value calculated by executing torque feedback control that causes the steering torque Th to follow the target steering torque calculated based on the steering torque Th.

[0138] In each embodiment, the steering angle calculation unit 51 may calculate the steering angle θs by taking into account the amount of torsion of the steering shaft 11 that corresponds to the steering torque Th. In each embodiment, the steering angle θs may be a value detected by a steering angle sensor that directly detects the rotation angle of the steering shaft 11. The steering angle sensor may be provided, for example, on the steering shaft 11 between the steering wheel 3 and the torque sensor 41.

[0139] In each embodiment, the steering actuator 12 does not necessarily have to include the steering-side reduction mechanism 14. In each embodiment, reaction motor 13 is not limited to a three-phase brushless motor. For example, it may be a DC motor with brushes. The other embodiments described herein can be similarly applied to steering motor 32.

[0140] In each embodiment, the pinion angle θp may be obtained by converting a detected value of the movement amount of the rack shaft 22. In this case, the control amount and the like related to the pinion angle θp are converted using the detected value of the movement amount of the rack shaft 22.

[0141] In each embodiment, steering unit 6 transmits the rotation of steering motor 32 to conversion mechanism 34 via transmission mechanism 33, but this is not limiting. For example, steering unit 6 may be configured so that the rotation of steering motor 32 is transmitted to conversion mechanism 34 via a gear mechanism. Also, steering unit 6 may be configured so that steering motor 32 directly rotates conversion mechanism 34. Furthermore, steering unit 6 may be configured to include a second rack-and-pinion mechanism, and so that the rotation of steering motor 32 is converted into reciprocating motion of rack shaft 22 by the second rack-and-pinion mechanism.

[0142] In each embodiment, the steering unit 6 is not limited to a configuration in which the right steered wheels 5 and the left steered wheels 5 are linked together. In other words, the right steered wheels 5 and the left steered wheels 5 may be independently controlled.

[0143] In each embodiment, the vehicle steering system 2 has a linkless structure in which the power transmission path between the steering unit 4 and the turning unit 6 is always separated, but this is not limited to this. For example, the vehicle steering system 2 may have a structure in which the steering unit 4 and the turning unit 6 can be mechanically separated by a clutch. [Explanation of symbols]

[0144] 1...Steering control device 2...Vehicle steering system 3...Steering wheel 4...Steering unit 5...Steering wheel 6...Steering unit 13...Reaction motor (steering side motor) 32...Steering motor (steering side motor) 50...Reaction force control unit 60...Steering control unit 67...Reduction processing section 92...Compensation axial force calculation section 132...Hysteresis control amount calculation unit 134...High load flag generation unit 135...Stopping zero point calculation unit 136...Stopping hysteresis component calculation unit 137...Zero point inverse calculation unit

Claims

1. A steering control device that controls a steer-by-wire type steering device having a structure in which a power transmission path between a steering unit steered by a steering wheel of a vehicle and a steering unit that operates to move a steering shaft to which steered wheels of the vehicle are connected is separated, the steering unit has a steering-side motor that generates a steering reaction force on the steering wheel, and the turning unit has a turning-side motor that generates a steering force that moves the turning shaft, the steering control device is configured to execute a process related to control of supply of current to the steering-side motor and a process related to control of supply of current to the steered-side motor, the processing related to the control of the supply of current to the steering-side motor includes a reaction force control amount calculation processing for calculating a reaction force control amount which is a control amount for controlling the supply of current to the steering-side motor, the processing related to control of supply of current to the turning-side motor includes a turning control amount calculation processing of calculating a turning control amount which is a control amount for controlling supply of current to the turning-side motor, The reaction force control amount calculation process includes: a hysteresis control amount calculation process for calculating a hysteresis control amount for adding a hysteresis characteristic obtained by changing a reference every time the steering direction of the steering wheel changes to the steering reaction force; a limit reaction force control amount calculation process for calculating a limit reaction force control amount for adding a limit reaction force to the steering reaction force in order to limit steering of the steering wheel when it can be determined that the current is in a high load state due to the steered wheels not being turned while current is being supplied to the turning-side motor, the steering control amount calculation process includes a reduction process of reducing a current supplied to the steering-side motor when it can be determined that the high-load state exists, The steering control device includes a change suppression process that suppresses changes in the hysteresis control amount regardless of whether the steering direction of the steering wheel changes while it can be determined that the high load state is present, in which the hysteresis control amount calculation process includes a change suppression process that suppresses changes in the hysteresis control amount regardless of whether the steering direction of the steering wheel changes.

2. the reduction process is a process of reducing the current supplied to the steered-side motor when it is possible to determine that the vehicle is in the high-load state, and also when it is possible to determine that the vehicle is in an extremely low speed state including a stopped state, and when it is possible to determine that the steering wheel is in a stationary state, 2. The steering control device according to claim 1, wherein the change suppression process is a process for suppressing a change in the hysteresis control amount regardless of whether the steering direction of the steering wheel changes or not, when it is at least possible to determine that the high load state exists.

3. 3. The steering control device according to claim 2, wherein the hysteresis control amount calculation process includes a reference update process that updates the reference when it becomes impossible to determine whether the vehicle is in the high load state or the vehicle is traveling at the extremely low speed during execution of the change suppression process.

4. 4. The steering control device according to claim 3, wherein the hysteresis control amount calculation process includes a process of calculating a value corresponding to the reference in the hysteresis characteristic when calculating the hysteresis control amount that can fix the value of the hysteresis control amount at the timing of updating the reference when the reference update process updates the reference.

5. 5. The steering control device according to claim 1, wherein the change suppression process is a process for suppressing a change in the hysteresis control amount by fixing the hysteresis control amount.

6. the steering control device includes a determination process for determining that the current is in a high load state due to the steered wheels not being turned while a current is being supplied to the steered-side motor, 5. The steering control device according to claim 1, wherein the determination process includes a condition based on a result of comparing the magnitude of the current supplied to the steered-side motor with a current threshold value.

7. A steering control method applied to a steer-by-wire steering device having a structure in which a power transmission path between a steering unit steered by a steering wheel of a vehicle and a steering unit that operates to move a steering shaft to which steered wheels of the vehicle are connected is separated, comprising: The steering control method includes: executing a process related to control of supply of current to a steering-side motor included in the steering unit, the steering-side motor generating a steering reaction force on the steering wheel; and executing a process related to control of supply of current to a steering-side motor included in the steering unit, the steering-side motor generating a steering force that moves the steering axis, the processing related to the control of the supply of current to the steering-side motor includes executing a reaction force control amount calculation processing to calculate a reaction force control amount which is a control amount for controlling the supply of current to the steering-side motor, the processing related to control of supply of current to the turning-side motor includes executing a turning control amount calculation processing to calculate a turning control amount which is a control amount for controlling supply of current to the turning-side motor, The reaction force control amount calculation process includes: executing a hysteresis control amount calculation process for calculating a hysteresis control amount for adding a hysteresis characteristic obtained by changing a reference every time the steering direction of the steering wheel changes to the steering reaction force; When it is determined that the current is in a high load state due to the steered wheels not being turned while a current is being supplied to the turning-side motor, executing a limit reaction force control amount calculation process for calculating a limit reaction force control amount for adding a limit reaction force to the steering reaction force in order to limit the steering of the steering wheel, the turning control amount calculation process includes, when it is determined that the high load state exists, executing a reduction process of reducing a current supplied to the turning-side motor, The hysteresis control amount calculation process includes executing a change suppression process that suppresses changes in the hysteresis control amount regardless of whether the steering direction of the steering wheel changes while it can be determined that the high load state is present.

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