Steering control device and steering control method

Abstract

This steering control device is configured to execute offset amount calculation processing, offset compensation processing, and offset reduction processing. The offset amount calculation processing is processing for calculating an offset amount which is a difference equivalent amount between a target turning equivalent angle and an actual turning equivalent angle. The offset compensation processing is processing for correcting the target turning equivalent angle by an offset correction amount corresponding to the offset amount. The offset reduction processing is for reducing the magnitude of the offset correction amount, and is processing for, when the magnitude of the offset correction amount is reduced to a predetermined value or less, changing a reduction rate according to the magnitude of the offset correction amount, under a condition in which the reduction rate of the magnitude of the offset amount when the magnitude of the offset correction amount is large is equal to or greater than the reduction rate when the magnitude of the offset correction amount is small.

Description

Steering control device and steering control method

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

[0002] For example, Patent Document 1 below describes a steering control device for a system in which the transmission of power between the steering wheel and the steered wheels is interrupted. This device corrects a target value of the steered angle of the steered wheels according to the difference between the target value and the actual steered angle under predetermined conditions, and then gradually changes the correction amount to zero.

[0003] Japanese Patent Application Laid-Open No. 2022-37765

[0004] However, at the point when the correction amount becomes zero, the rate of change of the target value changes suddenly, which may cause a sudden change in the amount of control operation for steering the steered wheels.

[0005] One aspect of the present disclosure provides a steering control device. The steering control device is configured to execute a target steering equivalent angle setting process, an operation process, an offset amount calculation process, an offset compensation process, and an offset reduction process in a state in which the steered wheels and the steering shaft are mechanically disconnected. The target steering equivalent angle setting process is a process for setting a target steering equivalent angle. The target steering equivalent angle is a target value of the steering equivalent angle, which is a variable indicating the steering angle of the steered wheels. The operation process is a process for operating a motor that steers the steered wheels in accordance with a differential operation amount. The differential operation amount is a control operation amount in which the rate of change of the target steering equivalent angle is a target value of a control amount. The offset amount calculation process is a process for calculating an offset amount that is an amount equivalent to the difference between the target steering equivalent angle and the actual steering equivalent angle, triggered by the satisfaction of a predetermined condition. The offset compensation process is a process for correcting the target steering equivalent angle by an offset correction amount that is in accordance with the offset amount. The offset reduction process is a process for reducing the magnitude of the offset correction amount and includes a reduction process. The reduction process is a process that changes the rate of decrease in the magnitude of the offset correction amount in accordance with the magnitude of the offset correction amount, under the condition that when the magnitude of the offset correction amount decreases to a predetermined value or less, the rate of decrease in the magnitude of the offset correction amount when the magnitude of the offset correction amount is large is equal to or greater than the rate of decrease when the magnitude of the offset correction amount is small.

[0006] Another aspect of the present disclosure provides a steering control method. The steering control method includes, in a state in which the steered wheels and the steering shaft are mechanically disconnected, executing a target steering equivalent angle setting process, executing an operation process, executing an offset amount calculation process, executing an offset compensation process, and executing an offset reduction process. The target steering equivalent angle setting process is a process for setting a target steering equivalent angle. The target steering equivalent angle is a target value of a steering equivalent angle, which is a variable indicating the steering angle of the steered wheels. The operation process is a process for operating a motor that steers the steered wheels in accordance with a differential operation amount. The differential operation amount is a control operation amount in which the rate of change of the target steering equivalent angle is a target value of a control amount. The offset amount calculation process is a process for calculating an offset amount that is an amount equivalent to the difference between the target steering equivalent angle and the actual steering equivalent angle, triggered by the satisfaction of a predetermined condition. The offset compensation process is a process for correcting the target steering equivalent angle by an offset correction amount corresponding to the offset amount. The offset reduction process is a process for reducing the magnitude of the offset correction amount and includes a reduction process in which, when the magnitude of the offset correction amount is reduced to a predetermined value or less, the reduction process changes the reduction rate in accordance with the magnitude of the offset correction amount on the condition that the reduction rate when the magnitude of the offset correction amount is large is equal to or greater than the reduction rate when the magnitude of the offset correction amount is small.

[0007] It is a diagram showing the configuration of a vehicle according to one embodiment. It is a block diagram showing part of the processing executed by the steering control device shown in Figure 1. It is a flowchart showing the procedure of the processing executed by the steering control device shown in Figure 1. It is a time chart showing the effect of the processing shown in Figure 3.

[0008] An embodiment will be described below with reference to the drawings. "Configuration of steering control system" As shown in Figure 1, a vehicle steering device 10 is a steer-by-wire type steering device. The steering device 10 includes a reaction force actuator Ar and a turning actuator At. The steering device 10 of this embodiment has a structure in which the power transmission path between the steering wheel 12 and the steered wheels 44 is mechanically disconnected.

[0009] A steering shaft 14 is connected to the steering wheel 12. The reaction force actuator Ar is an actuator for applying a steering reaction force to the steering wheel 12. The steering reaction force is a force that acts in the opposite direction to the direction of operation of the steering wheel 12 by the driver. By applying the steering reaction force to the steering wheel 12, it is possible to give the driver an appropriate sense of responsiveness. The reaction force actuator Ar includes a reduction mechanism 16, a reaction force motor 20, and a reaction force inverter 22.

[0010] Reaction motor 20 is, for example, a three-phase brushless motor. The rotation shaft of reaction motor 20 is connected to steering shaft 14 via speed reducer 16. Meanwhile, steered shaft 40 extends along the vehicle width direction, which is the left-right direction in FIG. 1 . Left and right steered wheels 44 are connected to both ends of steered shaft 40 via tie rods 42, respectively. The steered angle of steered wheels 44 is changed by linear movement of steered shaft 40.

[0011] The steering actuator At includes a reduction gear mechanism 56, a steering motor 60, and a steering inverter 62. As an example, the steering motor 60 is a three-phase surface permanent magnet synchronous motor. The rotating shaft of the steering motor 60 is connected to a pinion shaft 52 via the reduction gear mechanism 56. The pinion teeth of the pinion shaft 52 mesh with rack teeth 54 of the steering shaft 40. The pinion shaft 52 and the steering shaft 40 provided with the rack teeth 54 form a rack-and-pinion mechanism 50. The torque of the steering motor 60 is applied as a steering force to the steering shaft 40 via the pinion shaft 52. In response to the rotation of the steering motor 60, the steering shaft 40 moves in the vehicle width direction, which is the left-right direction in FIG. 1 .

[0012] The steering control device 70 includes a PU 72 and a storage device 74. The PU 72 is a software processing device such as a CPU or a GPU. The storage device 74 may be an electrically non-rewritable non-volatile memory. Alternatively, the storage device 74 may be an electrically rewritable non-volatile memory or a storage medium such as a disk medium. The steering control device 70 controls the control amount by the PU 72 executing a program stored in the storage device 74.

[0013] The control object of steering control device 70 is steering wheel 12. Steering control device 70 operates reaction force actuator Ar to control the steering reaction force as a control variable of the control object. Fig. 1 shows an operation signal MSs to reaction force inverter 22. Also, the control object of steering control device 70 is steered wheels 44. Steering control device 70 operates steering actuator At to control the steering angle of steered wheels 44 as a control variable of the control object. Fig. 1 shows an operation signal MSt to steering inverter 62.

[0014] In order to control the control variable, steering control device 70 refers to steering torque Th, which is the input torque to steering shaft 14, detected by torque sensor 80. Steering control device 70 also refers to rotation angle θa of the rotation shaft of reaction force motor 20, detected by rotation angle sensor 82. Steering control device 70 also refers to currents iu1, iv1, iw1 flowing through reaction force motor 20. Currents iu1, iv1, iw1 are quantified as voltage drops across shunt resistors provided in each leg of reaction force inverter 22. Steering control device 70 also refers to rotation angle θb of the rotation shaft of turning motor 60, detected by rotation angle sensor 84. Steering control device 70 also refers to currents iu2, iv2, iw2 flowing through turning motor 60. Currents iu2, iv2, iw2 are quantified as voltage drops across shunt resistors provided in each leg of turning inverter 62. The steering control device 70 also refers to the vehicle speed V detected by the vehicle speed sensor 92 .

[0015] "Control" FIG. 2 shows part of the processing executed by the steering control device 70. The steering angle calculation process M10 is a process for calculating the steering angle θh, which is the rotation angle of the steering wheel 12, based on the rotation angle θa as an input variable. The steering angle calculation process M10 includes a process for converting the rotation angle θa into an integrated angle including a range exceeding 360° by counting the number of rotations of the reaction force motor 20 from a steering neutral position, which is the position of the steering wheel 12 when the vehicle is traveling straight. The steering angle calculation process M10 includes a process for multiplying the converted integrated angle by a conversion coefficient based on the rotational speed ratio of the reduction mechanism 16 to calculate the steering angle θh. Note that the steering angle θh may be set to a positive value when the steering angle is to the right of the steering neutral position, and a negative value when the steering angle is to the left of the steering neutral position, for example.

[0016] The steering equivalent angle calculation process M12 calculates the steering equivalent angle θp based on the rotation angle θb as an input variable. The steering equivalent angle θp is a variable that indicates the steering angle of the steered wheels 44. The steering equivalent angle calculation process M12 includes, for example, a process of counting the number of rotations of the steering motor 60 from a rack neutral position, which is the position of the steering shaft 40 when the vehicle is traveling straight, and converting the counted number of rotations into an integrated angle that includes a range exceeding 360°. The steering equivalent angle calculation process M12 includes a process of multiplying the converted integrated angle by a conversion coefficient based on the rotational speed ratio of the reduction mechanism 56 to calculate the steering equivalent angle θp. That is, in this embodiment, as an example, the steering equivalent angle θp is set to a value equal to the actual rotation angle of the pinion shaft 52. Note that the steering equivalent angle θp may be set to a positive value when the angle is to the right of the rack neutral position and a negative value when the angle is to the left of the rack neutral position.

[0017] Target reaction force calculation process M14 is a process for calculating target reaction force Tr* corresponding to the steering reaction force to be applied to steering wheel 12, based on input variables including steering torque Th, vehicle speed V, steering equivalent angle θp, and q-axis current iqt. Here, q-axis current iqt is the q-axis current flowing through steering motor 60. q-axis current iqt is a variable indicating the torque of steering motor 60. q-axis current iqt is calculated by PU 72 based on currents iu2, iv2, iw2 and rotation angle θb. More specifically, target reaction force calculation process M14 includes a process for changing target reaction force Tr* in accordance with the magnitude of the torque of steering motor 60 under the following condition. This condition is a condition that the magnitude of target reaction force Tr* when the torque of steering motor 60 is large is equal to or greater than the magnitude of target reaction force Tr* when the torque of steering motor 60 is small. The target reaction force calculation process M14 also includes a process for changing the target reaction force Tr* in accordance with the steering equivalent angle θp as an input variable under the following condition: The magnitude of the target reaction force Tr* when the steering equivalent angle θp is large is equal to or greater than the magnitude of the target reaction force Tr* when the steering equivalent angle θp is small.

[0018] The target reaction force Tr* is actually a command value for the reaction force motor 20. The steering reaction force is obtained by multiplying the target reaction force Tr* by a coefficient corresponding to the reduction ratio of the reduction mechanism 16. Note that in descriptions such as "changing B according to A under the condition that when A is large, B is equal to or larger than B when A is small," the case where A is large and the case where A is small refer to a relative magnitude relationship when comparing the two. For example, "when A is large" corresponds to "A is a first value," and "when A is small" corresponds to "A is a second value smaller than the first value." The above description also means that, depending on the settings of the first and second values, B when A is the first value may be larger than B when A is the second value. The above description also means that B is changed according to A so that A when B is large is larger than A when B is small.

[0019] The reaction force operation process M16 is a process for outputting an operation signal MSs for the reaction force inverter 22 based on input variables, such as the target reaction force Tr*, currents iu1, iv1, iw1, and rotation angle θa. The reaction force operation process M16 includes a process for calculating d-axis and q-axis current command values ​​based on the target reaction force Tr*. The reaction force operation process M16 also includes a process for calculating d-axis and q-axis currents based on the currents iu1, iv1, iw1, and rotation angle θa. The reaction force operation process M16 then includes a process for calculating an operation signal MSs for operating the reaction force inverter 22 so that the d-axis and q-axis currents approach the command values.

[0020] The target steering equivalent angle setting process M18 is a process for calculating a target steering equivalent angle θp*0 based on the input variables of steering angle θh and vehicle speed V. The target steering equivalent angle θp*0 is a target value of the steering equivalent angle θp according to the operation of the steering wheel 12 by the driver. The target steering equivalent angle setting process M18 includes a process for changing the target steering angle ratio, which is the ratio between the steering angle θh and the target steering equivalent angle θp*0, according to the vehicle speed V, etc.

[0021] The offset correction amount calculation process M20 is a process for calculating an offset correction amount Δθp for the target steering equivalent angle θp*0 based on the input variables of the steering angle θh and the vehicle speed V. The offset correction process M22 is a process for calculating the target steering equivalent angle θp* by subtracting the offset correction amount Δθp from the target steering equivalent angle θp*0.

[0022] Steering feedback process M24 is a process for calculating steering torque command value Tt*, which is a command value for the torque of steering motor 60, in accordance with the manipulated variable of feedback control in which steering equivalent angle θp is the controlled variable and target steering equivalent angle θp* is the target value of the controlled variable. In more detail, steering feedback process M24 is a process for, as an example, substituting the sum of the output value of the proportional element, the output value of the integral element, and the output value of the derivative element for steering torque command value Tt*. The proportional element is a value obtained by multiplying the difference between target steering equivalent angle θp* and steering equivalent angle θp by proportional gain Kp. The integral element is an integrated value of the value obtained by multiplying the difference by integral gain Ki. The derivative element is a value obtained by multiplying the first-order time derivative of the difference by derivative gain Kd.

[0023] The steering operation process M26 is a process for outputting an operation signal MSt for the steering inverter 62 based on the input variables of the steering torque command value Tt*, currents iu2, iv2, iw2, and rotation angle θb. The steering operation process M26 includes a process for calculating current command values ​​for the d and q axes based on the steering torque command value Tt*. The steering operation process M26 also includes a process for calculating currents for the d and q axes based on the currents iu2, iv2, iw2 and the rotation angle θb. The steering operation process M26 then includes a process for calculating an operation signal MSt for operating the steering inverter 62 so that the currents for the d and q axes approach the command values.

[0024] "Details of Offset Correction Amount Calculation Process M20" Fig. 3 shows a detailed procedure of the offset correction amount calculation process M20. The process shown in Fig. 3 is realized by the PU 72 repeatedly executing a program stored in the storage device 74, for example, at a predetermined interval. Note that, hereinafter, the step number of each process is represented by a number preceded by "S."

[0025] In the series of processes shown in Fig. 3, the PU 72 first acquires the vehicle speed V, the steering equivalent angle θp, and the target steering equivalent angle θp*0 (S10). Next, the PU 72 determines whether or not the flag F is "1" (S12). The value of the flag F is set to "1" when the absolute value of the offset correction amount Δθp is set to a value greater than zero. The value of the flag F is set to "0" when the absolute value of the offset correction amount Δθp is zero.

[0026] When PU 72 determines that flag F is "0" (S12: NO), it determines whether a predetermined condition is met under which the absolute value of offset correction amount Δθp is set to a value greater than zero (S14). The predetermined condition is a condition under which the deviation between target steering equivalent angle θp* and steering equivalent angle θp becomes large. The predetermined condition may be the following first condition or second condition. The first condition is a condition under which target steering equivalent angle θp*0 is discontinuously changed.

[0027] Specifically, the first condition may be, for example, a condition that an abnormality has occurred in vehicle speed sensor 92. In other words, if vehicle speed sensor 92 is configured with sensors that detect the speeds of a plurality of wheels, the method of calculating vehicle speed V may be changed if an abnormality occurs in any one of the wheel speed sensors. In such a case, the target steering equivalent angle θp*0 changes discontinuously due to discontinuous changes in vehicle speed V. Also, for example, the first condition may be a condition that steering intervention by steering control device 70 is started. When steering intervention is started, target steering equivalent angle θp*0 changes discontinuously because the target steering equivalent angle θp*0 is set independently of steering angle θh and vehicle speed V.

[0028] The second condition is a condition that the target equivalent steering angle θp*0 deviates from the equivalent steering angle θp even though the steered wheels 44 can be steered. The second condition may be, for example, a condition that the vehicle's start switch is switched from an off state to an on state. When the steering wheel 12 is rotated with the start switch in the off state, the steering angle θh and the equivalent steering angle θp become inconsistent. Therefore, at the time of the start switch switching, even though the steered wheels 44 can be steered, the target equivalent steering angle θp*0 deviates significantly from the equivalent steering angle θp. Alternatively, for example, the second condition may be a condition that the current limit of the steering motor 60 is released. Specifically, for example, if the steered wheels 44 hit an obstacle such as a curb while the current of the steering motor 60 is limited, it becomes difficult to steer the steered wheels 44 toward the obstacle. In this state, if the steering wheel 12 is steered toward the side that hits the obstacle, the target equivalent steering angle θp*0 changes as the steering angle θh changes. In contrast, the equivalent steering angle θp is maintained at a constant value. If the current limitation of the steering motor 60 is released in this state, the target equivalent steering angle θp*0 and the equivalent steering angle θp will differ significantly, even though the steered wheels 44 can be controlled to be steered.

[0029] When the PU 72 determines that a predetermined condition is met (S14: YES), it assigns "1" to flag F (S16). Then, the PU 72 calculates an offset amount Δθp0, which is an amount equivalent to the difference between the target steering equivalent angle θp* and the actual steering equivalent angle θp (S18). More specifically, when the first condition is met, the PU 72 assigns a value obtained by subtracting the previous value "θp*0(n-1)" of the target steering equivalent angle θp*0 from the current value "θp*0(n)" of the target steering equivalent angle θp*0 to the offset amount Δθp0. Note that the steering equivalent angle θp immediately before the first condition is met is considered to be approximately equal to the target steering equivalent angle θp*0. Therefore, when the first condition is met, the "value obtained by subtracting the previous value from the current value of the target steering equivalent angle θp*0" can be considered to be the difference between the target steering equivalent angle θp* and the actual steering equivalent angle θp. In other words, when the first condition is met, the "value obtained by subtracting the previous value from the current value of the target steering equivalent angle θp*0" can be regarded as an amount equivalent to the difference between the target steering equivalent angle θp* and the actual steering equivalent angle θp. On the other hand, when the second condition is met, the PU 72 assigns the value obtained by subtracting the steering equivalent angle θp from the target steering equivalent angle θp*0 to the offset amount Δθp0.

[0030] Next, the PU 72 assigns the offset amount Δθp0 to the offset correction amount Δθp (S20). On the other hand, if the PU 72 determines that the flag F is "1" (S12: YES), it determines whether the offset correction amount Δθp is zero (S22). If the PU 72 determines that the offset correction amount Δθp is greater than zero (S22: NO), it calculates the target steering equivalent angular velocity ωp*0 (S24). The target steering equivalent angular velocity ωp*0 is the rate of change of the target steering equivalent angle θp*0. The target steering equivalent angular velocity ωp*0 is calculated by the PU 72 based on the target steering equivalent angle θp*0 as an input variable.

[0031] The PU 72 calculates a decrease amount Δ that determines the decrease rate of the magnitude of the offset correction amount Δθp (S26). Specifically, the PU 72 first calculates a steering angular velocity sensitive base value Δωb based on the target steering equivalent angular velocity ωp*0 as an input variable. More specifically, the PU 72 changes the steering angular velocity sensitive base value Δωb based on the target steering equivalent angular velocity ωp*0 as an input variable under the following condition: the steering angular velocity sensitive base value Δωb when the absolute value of the target steering equivalent angular velocity ωp*0 is large is equal to or greater than the steering angular velocity sensitive base value Δωb when the absolute value of the target steering equivalent angular velocity ωp*0 is small. This process may be a process of map-calculating the steering angular velocity sensitive base value Δωb using map data based on the target steering equivalent angular velocity ωp*0 as an input variable. Here, the map data is data in which the target steering equivalent angular velocity ωp*0 is an input variable and the steering angular velocity sensitive base value Δωb is an output variable.

[0032] Note that map data is a set of data consisting of discrete values ​​of input variables and values ​​of output variables corresponding to each of the input variable values. Furthermore, map calculation may be a process in which, when the value of an input variable matches one of the input variable values ​​in the map data, the value of the corresponding output variable in the map data is the calculation result. Furthermore, map calculation may be a process in which, when the value of an input variable does not match any of the input variable values ​​in the map data, the calculation result is a value obtained by interpolating the values ​​of multiple output variables included in the map data. Alternatively, map calculation may be a process in which, when the value of an input variable does not match any of the input variable values ​​in the map data, the calculation result is the value of the output variable in the map data that corresponds to the closest value of the multiple input variables included in the map data.

[0033] The PU 72 also multiplies the steering angular velocity-sensitive base value Δωb, which is an input variable, by a gain Gv and assigns the resulting value to the steering angular velocity-sensitive decrease amount Δω. The gain Gv is calculated by the PU 72 based on the vehicle speed V, which is an input variable. This process may be a map calculation using map data in which the vehicle speed V is an input variable and the gain Gv is an output variable. The PU 72 also calculates the vehicle speed-sensitive decrease amount Δv based on the vehicle speed V, which is an input variable. This process may be a map calculation using map data in which the vehicle speed V is an input variable and the vehicle speed-sensitive decrease amount Δv is an output variable. The PU 72 then assigns the smaller of the steering angular velocity-sensitive decrease amount Δω and the vehicle speed-sensitive decrease amount Δv to the decrease amount Δ.

[0034] Next, the PU 72 multiplies the decrease amount Δ by the remaining amount gain Gθ and assigns the resulting value to the decrease amount Δ (S28). The remaining amount gain Gθ is calculated by the PU 72 in accordance with the absolute value of the offset correction amount Δθp as an input variable.

[0035] When the absolute value of the offset correction amount Δθp is greater than a predetermined value Δθp1, the PU 72 assigns "1" to the remaining amount gain Gθ. When the absolute value of the offset correction amount Δθp is equal to or less than the predetermined value Δθp1, the PU 72 changes the remaining amount gain Gθ according to the absolute value under the following condition: the remaining amount gain Gθ when the absolute value of the offset correction amount Δθp is large is equal to or greater than the remaining amount gain Gθ when the absolute value is small. In addition, the PU 72 sets the minimum gain Gmin, which is the remaining amount gain Gθ when the offset correction amount Δθp is zero, to a value greater than "0."

[0036] For example, the remaining amount gain Gθ is calculated by the PU 72 using map data stored in advance in the storage device 74. Here, the map data is data in which the offset correction amount Δθp is an input variable and the remaining amount gain Gθ is an output variable.

[0037] The PU 72 determines whether the offset amount Δθp0 is positive (S30). If the PU 72 determines that the offset amount Δθp0 is positive (S30: YES), the PU 72 assigns the larger of either zero or a value obtained by subtracting the decrease amount Δ from the offset correction amount Δθp to the offset correction amount Δθp (S32). On the other hand, if the PU 72 determines that the offset amount Δθp0 is not positive (S30: NO), the PU 72 assigns the larger of either zero or a value obtained by adding the decrease amount Δ to the offset correction amount Δθp to the offset correction amount Δθp (S34).

[0038] As is clear from the processes of S32 and S34, the decrease amount Δ is a decrease variable that defines the decrease rate of the magnitude of the offset correction amount Δθp. Incidentally, the decrease rate of the magnitude of the offset correction amount Δθp is expressed as "Δ / Tc" using the cycle Tc of the series of processes shown in FIG.

[0039] On the other hand, if the PU 72 determines that the offset correction amount Δθp is zero (S22: YES), it assigns "0" to flag F (S38). If the determination in the process of S14 is negative, the PU 72 assigns zero to the offset correction amount Δθp (S36).

[0040] When the PU 72 completes the processes of S20 and S32 to S38, it temporarily ends the series of processes shown in Fig. 3. <Functions and Effects of the Present Embodiment> Fig. 4 shows the transition of the absolute value of the offset correction amount Δθp, its first derivative, and its second derivative.

[0041] As shown in FIG. 4 , when the absolute value of the offset correction amount Δθp decreases to or below a predetermined value Δθp1 due to the offset correction amount Δθp being decreased in accordance with the decrease amount Δ, the remaining amount gain Gθ is set to a value smaller than "1." Therefore, the decrease amount Δ decreases compared to when the absolute value of the offset correction amount Δθp is greater than the predetermined value Δθp1. Therefore, the decrease rate of the absolute value of the offset correction amount Δθp decreases. A decrease in the decrease rate of the absolute value of the offset correction amount Δθp results in an increase in the value of the second-order derivative of the absolute value of the offset correction amount Δθp, but the magnitude of this increase is limited.

[0042] In contrast to this, the dashed dotted line in Figure 4 shows a case where the rate of decrease of the absolute value of the offset correction amount Δθp is constant without using the remaining amount gain Gθ. In this case, at time t2 when the offset correction amount Δθp becomes zero, the value of the second-order derivative of the absolute value of the offset correction amount Δθp becomes excessively large. As a result, there is a risk that the absolute value of the output value of the derivative element in the steering feedback process M24 will become excessively large.

[0043] According to the present embodiment described above, the following further actions and effects can be obtained. (1) When the absolute value of the offset correction amount Δθp is greater than a predetermined value Δθp1, the PU 72 sets the remaining gain Gθ to "1." This allows the absolute value of the offset correction amount Δθp to be reduced by the decrease amount Δ calculated by the processing of S26, which is a value adapted based on the steering feel, etc. Therefore, the driver can be provided with the desired steering feel, etc. Then, the PU 72 reduces the decrease amount Δ calculated by the processing of S26 only when the absolute value of the offset correction amount Δθp is equal to or less than the predetermined value Δθp1. This prevents a sudden change in the absolute value of the output value of the derivative element without interfering as much as possible with the desired effect of the decrease amount Δ calculated by the processing of S26.

[0044] (2) The PU 72 sets the minimum gain Gmin, which is the remaining gain Gθ when the offset correction amount Δθp is zero, to a value greater than "0." This allows the rate of decrease of the absolute value of the offset correction amount Δθp to be set to a constant or higher value, even when the absolute value of the offset correction amount Δθp becomes small. Therefore, the absolute value of the offset correction amount Δθp can be reliably decreased to zero.

[0045] (3) The PU 72 corrects the decrease amount Δ using the remaining amount gain Gθ. This makes it easier to reduce the number of steps required for adaptation compared to a case where the decrease amount Δ is calculated using a map based on the offset correction amount Δθp in addition to the target steering equivalent angular velocity ωp*0 and the vehicle speed V.

[0046] (4) The PU 72 sets the decrease amount Δ based on the steering angular velocity sensitive base value Δωb. As a result, the decrease amount Δ becomes larger the more rapid the steering of the steered wheels 44 is desired, and therefore the actual target steering equivalent angle θp* can be made to quickly approach the target steering equivalent angle θp*0.

[0047] <Other Embodiments> This embodiment can be modified as follows: This embodiment and the following modifications can be combined and implemented within the scope of technical compatibility.

[0048] "Regarding operation processing" The operation amount of the closed-loop control does not necessarily have to be the sum of the output value of the proportional element, the output value of the derivative element, and the output value of the integral element. For example, the operation amount of the closed-loop control may be the sum of the output value of the proportional element and the output value of the derivative element. Also, for example, the operation amount of the closed-loop control may be the sum of the output value of the derivative element and the output value of the integral element.

[0049] The manipulated variable of the closed-loop control does not necessarily have to consist of only the manipulated variable of the classical control. For example, the manipulated variable of the closed-loop control may include a disturbance estimated by a disturbance observer.

[0050] The operation of the motor according to the manipulated variable of the closed-loop control is not limited to the operation of the motor according to only the manipulated variable of the closed-loop control. For example, the operation of the motor according to the sum of the manipulated variable of the closed-loop control and the manipulated variable of the open-loop control may be used.

[0051] The motor operation according to the differential manipulated variable is not limited to the motor operation according to the manipulated variable of closed-loop control in which the target steering equivalent angle is the target value of the controlled variable. For example, the controlled variable does not have to include the steering equivalent angle.

[0052] Regarding the Decrease Variable Setting Process: The steering angular velocity variable, which is an input variable in the process of calculating the steering angular velocity sensitive base value Δωb, is not limited to the target steering equivalent angular velocity ωp*0. The steering angular velocity variable may be, for example, the rate of change of the steering angle θh.

[0053] The amount of decrease Δ is not limited to the value obtained by multiplying the steering angular velocity sensitive base value Δωb by the gain Gv that depends on the vehicle speed V. For example, the gain Gv may be a fixed value. With map data stored in the storage device 74, the amount of decrease Δ may be calculated by the PU 72 using a map in accordance with the rate of change of the target steering equivalent angle θp*0 and the vehicle speed V. Here, the map data is data in which the rate of change of the target steering equivalent angle θp*0 and the vehicle speed V are input variables and the amount of decrease Δ is an output variable.

[0054] The decrease amount Δ may be the sum of the steering angular velocity-sensitive decrease amount Δω and the vehicle speed-sensitive decrease amount Δv. Here, the PU 72 may calculate the steering angular velocity-sensitive decrease amount Δω in accordance with the target steering equivalent angular velocity ωp*0 under the following condition. This condition is a condition in which the magnitude of the steering angular velocity-sensitive decrease amount Δω when the absolute value of the target steering equivalent angular velocity ωp*0 is large is equal to or greater than the magnitude of the steering angular velocity-sensitive decrease amount Δω when the absolute value of the rate of change of the target steering equivalent angular velocity ωp*0 is small. Furthermore, the PU 72 may calculate the vehicle speed-sensitive decrease amount Δv in accordance with the vehicle speed V under the following condition. This condition is a condition in which the vehicle speed-sensitive decrease amount Δv when the vehicle speed V is large is equal to or greater than the vehicle speed-sensitive decrease amount Δv when the vehicle speed V is small.

[0055] Regarding the Reduction Process: It is not essential that the remaining amount gain Gθ be greater than “0” when the offset correction amount Δθp is zero.

[0056] It is not essential that the value of the remaining amount gain Gθ be "1" when the absolute value of the offset correction amount Δθp is larger than the predetermined value Δθp1. For example, if a predetermined condition is met when the absolute value of the offset correction amount Δθp is larger than the predetermined value Δθp1, the remaining amount gain Gθ may be gradually increased from a value smaller than "1" to "1" as the absolute value of the offset correction amount Δθp decreases. This makes it possible to limit the magnitude of the rate of change of the target steering equivalent angle θp* as the offset correction amount Δθp starts to decrease.

[0057] Furthermore, for example, the value of the remaining amount gain Gθ when the offset correction amount Δθp is larger than the predetermined value Δθp1 may be 0.9. In short, it is sufficient that the value of the remaining amount gain Gθ when the offset correction amount Δθp is larger than the predetermined value Δθp1 is a constant value and is larger than when the offset correction amount Δθp is equal to or smaller than the predetermined value Δθp1.

[0058] Regarding the offset reduction process, it is not essential that the reduction process be a process of multiplying the reduction amount Δ, which is the value of the reduction variable set by the reduction variable setting process, by the remaining amount gain Gθ. For example, the offset reduction process may be a process of map-calculating the reduction amount Δ using map data in which the vehicle speed V, the target steering equivalent angular velocity ωp*0, and the offset correction amount Δθp are input variables and the reduction amount Δ is an output variable.

[0059] "Regarding the steering control device" The steering control device is not limited to one that executes various processes using a PU. For example, it may be equipped with a dedicated hardware circuit, such as an ASIC, that executes at least part of the processes executed in the above embodiments. That is, the steering control device may be equipped with any of the following processing circuits (a) to (c). (a) A processing circuit that includes a processing device that executes all of the above processes in accordance with a program, and a program storage device such as a memory device that stores the program. (b) A processing circuit that includes a processing device and program storage device that executes part of the above processes in accordance with a program, and a dedicated hardware circuit that executes the remaining processes. (c) A processing circuit that includes a dedicated hardware circuit that executes all of the above processes. Here, there may be multiple software execution devices that include a processing device and a program storage device, and multiple dedicated hardware circuits.

[0060] Regarding the steering actuator: As the steering actuator At, for example, one in which steering motor 60 is arranged coaxially with steering shaft 40 may be used. Alternatively, for example, one connected to steering shaft 40 via a belt-type reducer using a ball screw mechanism may be used.

Claims

1. A system is configured to execute a target steering equivalent angle setting process, an operation process, an offset amount calculation process, an offset compensation process, and an offset reduction process when the steered wheels and the steering shaft are mechanically disconnected, wherein the target steering equivalent angle setting process is a process for setting a target steering equivalent angle, and the target steering equivalent angle is a target value of the steering equivalent angle which is a variable indicating the steering angle of the steered wheels, the operation process is a process for operating a motor that steers the steered wheels in accordance with a differential operation amount, and the differential operation amount is a control operation amount whose target value is a control amount is a rate of change of the target steering equivalent angle, and the offset amount calculation process is a process for calculating an offset amount which is an amount equivalent to the difference between the target steering equivalent angle and the actual steering equivalent angle when a predetermined condition is met as a trigger, and the offset compensation process is a process for correcting the target steering equivalent angle by an offset correction amount corresponding to the offset amount, and the offset reduction process is a process for reducing the magnitude of the offset correction amount, and the offset reduction process includes a reduction process, The reduction process is a steering control device that, when the magnitude of the offset correction amount decreases to a predetermined value or less, changes the reduction rate in accordance with the magnitude of the offset correction amount under the condition that the reduction rate in the magnitude of the offset correction amount when the magnitude of the offset correction amount is large is equal to or greater than the reduction rate in the magnitude of the offset correction amount when the magnitude of the offset correction amount is small.

2. The steering control device according to claim 1, wherein the minimum value of the speed of decrease by the reduction process is greater than zero.

3. A steering control device as described in claim 1, wherein the offset reduction process comprises a reduction variable setting process, the reduction variable setting process is a process of setting the value of a reduction variable which is a variable indicating the reduction rate of the offset amount, and the reduction process is a process of multiplying the value of the reduction variable by a gain, and changing the gain according to the magnitude of the offset correction amount under the condition that the gain when the magnitude of the offset correction amount is large is equal to or greater than the gain when the magnitude of the offset correction amount is small.

4. A steering control device as described in claim 3, wherein the reduction variable setting process includes a process of changing the value of the reduction variable in accordance with the value of the steering angular velocity variable under the condition that the reduction rate indicated by the value of the reduction variable when the value of the steering angular velocity variable is large is equal to or greater than the reduction rate indicated by the value of the reduction variable when the value of the steering angular velocity variable is small, and the steering angular velocity variable is a variable indicating the rate of change of the target steering equivalent angle.

5. A steering control device as described in claim 1, wherein the operation processing is processing for operating a motor that steers the steered wheels in accordance with an operation amount of closed-loop control in which the target equivalent steering angle is a control amount, and the operation amount of the closed-loop control is calculated based on the differential operation amount, which is the output value of a differential element in which the difference between the target equivalent steering angle and the equivalent steering angle is an input variable.

6. A steering control method applied when the steered wheels and the steering shaft are mechanically disconnected, the steering control method comprising the steps of: executing a target steering equivalent angle setting process; executing an operation process; executing an offset amount calculation process; executing an offset compensation process; and executing an offset reduction process; the target steering equivalent angle setting process is a process for setting a target steering equivalent angle; the target steering equivalent angle is a target value of the steering equivalent angle which is a variable indicating the steering angle of the steered wheels; the operation process is a process for operating a motor which steers the steered wheels in accordance with a differential operation amount; the differential operation amount is a control operation amount whose target value is a control amount whose change rate of the target steering equivalent angle is a control amount; the offset amount calculation process is a process for calculating an offset amount which is an amount equivalent to the difference between the target steering equivalent angle and the actual steering equivalent angle when a predetermined condition is met as a trigger; the offset compensation process is a process for correcting the target steering equivalent angle by an offset correction amount which is in accordance with the offset amount; and the offset reduction process is a process for reducing the magnitude of the offset correction amount. The offset reduction process includes a reduction process, and the reduction process is a steering control method in which, when the magnitude of the offset correction amount is reduced to a predetermined value or less, the reduction rate of the magnitude of the offset amount when the magnitude of the offset correction amount is large is changed to be equal to or greater than the reduction rate when the magnitude of the offset correction amount is small.

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