Steering control device and steering control method

Abstract

A steering control device according to the present invention is configured to execute an operation process, an offset amount calculation process, an offset compensation process, an offset reduction process, and a reduction process. The operation process is a process for operating a motor for turning a turning wheel in accordance with a differential operation amount. The offset amount calculation process is a process for calculating an offset amount, which is an amount equivalent to the difference between a target turning equivalent angle and the actual turning equivalent angle. The offset compensation process is a process for correcting the target turning equivalent angle by using an offset correction amount corresponding to the offset amount. The offset reduction process is a process for gradually reducing the magnitude of the offset correction amount. The reduction process is a process for reducing changes in a change speed of the target turning equivalent angle for calculating the operation amount when the offset compensation process is carried out.

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. When the ratio of the steered angle of the steered wheels to the steering angle suddenly changes, this device corrects the target value of the steered wheel turning angle in accordance with the difference between the previous and current target values, and then gradually changes the correction amount to zero.

[0003] Japanese Patent Application Laid-Open No. 2023-062515

[0004] However, when the process of correcting the target value based on the difference between the previous and current target values ​​is started, the rate of change of the target value may suddenly change, which may cause problems such as abnormal noise due to a sudden change in the torque of the motor that steers 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, an offset reduction process, and a 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 corresponding to the offset amount. The offset reduction process is a process for gradually decreasing the magnitude of the offset correction amount. The reduction process is a process for reducing a change in the rate of change of the target steering equivalent angle used for calculating the differential operation amount when the offset compensation process is performed.

[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, an operation process, an offset amount calculation process, an offset compensation process, an offset reduction process, and a 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 gradually reducing the magnitude of the offset correction amount. The reduction process is a process for reducing a change in the rate of change of the target steering equivalent angle used for calculating the differential operation amount when the offset compensation process is performed.

[0007] 1 is a diagram showing the configuration of a vehicle according to a first embodiment; FIG. 2 is a block diagram showing part of the processing executed by the steering control device shown in FIG. 1; FIG. 3 is a flowchart showing the procedure of the processing executed by the steering control device shown in FIG. 1; FIG. 4 is a block diagram showing details of the steering feedback processing shown in FIG. 2; FIG. 5 is a flowchart showing the procedure of the processing executed by the steering control device shown in FIG. 1; FIG. 6 is a time chart showing the effect of the first embodiment; and FIG. 7 is a flowchart showing the procedure of the processing executed by the steering control device according to a second embodiment.

[0008] <First embodiment> A first embodiment will be described below with reference to the drawings. "Configuration of steering control system" As shown in Fig. 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 greater 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 "when A is a first value," and "when A is small" corresponds to "when A is a second value smaller than the first value." This 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 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 of the target steering equivalent angle θp* 0. 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] The steering feedback process M24 is a process for calculating the steering torque command value Tt*, which is the command value of the torque of the steering motor 60, in accordance with the operation amount of the feedback control in which the steering equivalent angle θp is the control amount and the target steering equivalent angle θp* is the target value of the control amount.

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

[0027] Specifically, the predetermined 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, discontinuous changes in vehicle speed V cause discontinuous changes in target steering equivalent angle θp*0. Also, for example, the predetermined condition may be a condition that operation intervention by steering control device 70 is started. When steering intervention is started, target steering equivalent angle θp*0 is set independently of steering angle θh and vehicle speed V, and therefore target steering equivalent angle θp*0 changes discontinuously.

[0028] When the PU 72 determines that the 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). Specifically, 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 predetermined condition is met is considered to be approximately equal to the target steering equivalent angle θp*0. Therefore, when the predetermined 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 a specified 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 the amount equivalent to the difference between the target steering equivalent angle θp* and the actual steering equivalent angle θp.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] The PU 72 determines whether the offset amount Δθp0 is positive (S28). If the PU 72 determines that the offset amount Δθp0 is positive (S28: 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 (S30). On the other hand, if the PU 72 determines that the offset amount Δθp0 is not positive (S28: 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 (S32).

[0034] 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 (S36). If the determination in the process of S14 is negative, the PU 72 assigns zero to the offset correction amount Δθp (S34).

[0035] When the PU 72 completes the processes of S20 and S30 to S36, it temporarily ends the series of processes shown in Fig. 3. "Steering Feedback Process" Fig. 4 shows the details of the steering feedback process M24.

[0036] The deviation calculation process M30 calculates a value obtained by subtracting the steering equivalent angle θp from the target steering equivalent angle θp*. The proportional element M32 multiplies the output value of the deviation calculation process M30 by a proportional gain Kp and outputs a value. The integral element M34 multiplies the output of the deviation calculation process M30 by an integral gain Ki and outputs an integrated value.

[0037] The differential calculation M36 is a process for outputting a first-order time differential value of the target turning equivalent angle θp*. As an example, the differential calculation M36 may be a process for calculating a first-order time differential value using a backward difference of the target turning equivalent angle θp*.

[0038] The low-pass filter M38 is a process in which the output value of the differential operation M36 is an input variable and the target steering equivalent angular velocity ωp*L is an output variable. The low-pass filter M38 is, for example, a first-order lag filter expressed as "K / (1+T·s)".

[0039] The differential calculation M40 is a process for outputting a steering equivalent angular velocity ωp, which is a first-order time differential value of the steering equivalent angle θp. As an example, the differential calculation M40 may be a process for calculating a first-order time differential value using a backward difference of the steering equivalent angle θp.

[0040] The deviation calculation process M42 is a process for outputting a value obtained by subtracting the turning-equivalent angular velocity ωp from the target turning-equivalent angular velocity ωp*L. The differential gain multiplication process M44 is a process for multiplying the output value of the deviation calculation process M42 by a differential gain Kd.

[0041] The addition process M46 is a process of substituting the sum of the output value of the proportional element M32, the output value of the integral element M34, and the output value of the differential gain multiplication process M44 into the turning torque command value Tt*. The filter constant change process M48 is a process of changing the filter constant of the low-pass filter M38 to change the cutoff frequency of the low-pass filter M38.

[0042] "Filter Constant Changing Process M48" Fig. 5 shows the detailed procedure of the filter constant changing process M48. The series of processes shown in Fig. 5 is realized by the PU 72 repeatedly executing a program stored in the storage device 74, for example, at a predetermined interval.

[0043] 5, the PU 72 first determines whether the change flag FL is "1" (S40). The change flag FL is set to "1" when the value of the filter constant of the low-pass filter M38 has been changed from its normal value. The change flag FL is set to "0" when the value of the filter constant of the low-pass filter M38 is set to its normal value.

[0044] When the PU 72 determines that the change flag FL is "0" (S40: NO), it determines whether or not the value of flag F has changed from "0" to "1" (S42). This process is a process for determining whether or not a difference in offset amount Δθp0 has occurred between the target steering equivalent angle θp* and the target steering equivalent angle θp*0. When the PU 72 determines that the value of flag F has changed from "0" to "1" (S42: YES), it assigns "1" to the change flag FL (S44). Furthermore, the PU 72 assigns the previous target steering equivalent angular velocity ωp*(n-1) to the target steering equivalent angular velocity ωp*(n), which is the current value of the input variable of the low-pass filter M38 (S46). This process is a process for holding the input variable of the low-pass filter M38 at the previous value.

[0045] The PU 72 also assigns the sudden change suppression value T2 to the filter constant T of the low-pass filter M38 (S48). The sudden change suppression value T2 is set to a value greater than the normal value T1.

[0046] On the other hand, when the PU 72 determines that the change flag FL is “1” (S40: YES), the PU 72 increments the counter C (S50). The counter C measures the duration of the state in which the filter constant T is changed to the sudden change suppression value T2.

[0047] The PU 72 determines whether or not a condition A is established, that is, whether the counter C is equal to or greater than the threshold value Cth (S52). This process is a process for determining whether or not the duration of the state in which the filter constant T is changed to the sudden change suppression value T2 has reached a predetermined time. The predetermined time is set to a time during which the magnitude of change in the target steering equivalent angular velocity ωp* is expected to be equal to or less than a predetermined value.

[0048] If the PU 72 determines that the condition A is met (YES at S52), the PU 72 assigns the normal value T1 to the filter constant T (S54). Then, the PU 72 assigns "0" to the change flag FL (S56).

[0049] On the other hand, if the PU 72 determines that the condition A is not satisfied (S52: NO), the PU 72 proceeds to the process of S48. Note that the PU 72 temporarily ends the series of processes shown in FIG. 5 when it completes the processes of S48 and S56 or when it makes a negative determination in the process of S42.

[0050] <Operations and Effects of the Present Embodiment> FIG. 6 shows a situation in which a predetermined condition is met when the target steering equivalent angle θp* changes in accordance with a change in the steering angle θh.

[0051] The left side of Fig. 6 shows a case where the filter constant change process M48 shown in Fig. 5 is not executed. In this case, the process of S20 is executed when a predetermined condition is met at time t1, so that the target steering equivalent angle θp* does not change during one control cycle. As a result, the target steering equivalent angular velocity ωp* decreases to zero. This sudden change in the target steering equivalent angular velocity ωp* leads to a sudden change in the differential element shown as the "D term" in the figure.

[0052] In contrast, the right side of Fig. 6 shows a case where the filter constant change process M48 shown in Fig. 5 is executed. In this case, the target steering equivalent angular velocity ωp* as an input variable of the low-pass filter M38 is held over one control cycle from time t1. This prevents the differential element from suddenly changing immediately after a predetermined condition is met.

[0053] Incidentally, when a predetermined condition is met, there is a tendency for the gear ratio, which is the ratio of the target steering equivalent angle θp*0 to the steering angle θh, to be changed. Therefore, the rate of change of the target steering equivalent angle θp*0 changes before and after the predetermined condition is met. This change leads to a change in the differential element. In response to this, when the predetermined condition is met, the PU 72 changes the filter constant T to the sudden change suppression value T2. This lowers the cutoff frequency of the low-pass filter M38, thereby sufficiently suppressing changes in the target steering equivalent angular velocity ωp*L.

[0054] According to the present embodiment described above, the following further actions and effects can be obtained. (1-1) The decrease amount Δ that defines the decrease rate of the offset correction amount Δθp is set in accordance with the steering angular velocity-sensitive decrease amount Δω. Here, the steering angular velocity-sensitive decrease amount Δω has a positive correlation with the magnitude of the rate of change of the target steering equivalent angle θp*0. Therefore, when the rate of change of the steering angle θh is large, the magnitude of the steering angular velocity-sensitive decrease amount Δω becomes particularly large. Therefore, if the filter constant change process M48 is not executed when the rate of change of the steering angle θh is large, a sudden change in the differential element becomes particularly noticeable. Therefore, the filter constant change process M48 is particularly useful.

[0055] Second Embodiment A second embodiment will be described below with reference to the drawings, focusing on differences from the first embodiment.

[0056] Fig. 7 shows the detailed procedure of the filter constant change process M48. The series of processes shown in Fig. 7 are realized by the PU 72 repeatedly executing a program stored in the storage device 74, for example, at a predetermined interval. For convenience, processes in Fig. 7 that correspond to those shown in Fig. 5 are denoted by the same reference numerals.

[0057] In the series of processes shown in FIG. 7 , when the PU 72 determines that the change flag FL is "1" (S40: YES), it determines whether or not condition B is met (S52a). Condition B is a condition that the absolute value of the difference between the target steering-equivalent angular velocity ωp*L and the target steering-equivalent angular velocity ωp* is equal to or less than a threshold value Δωth. This process is a process for determining whether or not the change in the target steering-equivalent angular velocity ωp* has become small. When the PU 72 determines that condition B is not met (S52a: NO), the PU 72 proceeds to the process of S48. On the other hand, when the PU 72 determines that condition B is met (S52a: YES), the PU 72 proceeds to the process of S54.

[0058] <Functions and Effects of Second Embodiment> When it is determined that the absolute value of the difference is equal to or less than the threshold value Δωth, the PU 72 returns the filter constant T to the normal value T1. As a result, when it is confirmed that the change in the target steering equivalent angular velocity ωp* has become small, the filter constant T can be returned to the normal value T1.

[0059] <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.

[0060] "Regarding the reduction process" The low-pass filter for reducing the change in the difference between the rate of change of the target steering equivalent angular velocity ωp* and the rate of change of the steering equivalent angle θp is not limited to a filter for which the target steering equivalent angular velocity ωp* is an input variable. For example, the low-pass filter for reducing the change in the difference may be a filter for which the difference between the target steering equivalent angular velocity ωp* and the steering equivalent angular velocity ωp is an input variable. In that case, the reduction process may be a process for changing a filter constant. Furthermore, the reduction process may be a process for setting the value of the differential term to a value obtained by multiplying the output value of a low-pass filter for which the difference between the target steering equivalent angular velocity ωp* and the steering equivalent angular velocity ωp is an input variable by a derivative gain Kd, only during the period in which the change flag FL is set to "1".

[0061] The low-pass filter for reducing the change in the difference between the rate of change of the target turning equivalent angle θp* and the rate of change of the turning equivalent angle θp does not necessarily have to be a first-order lag filter. The condition for returning the filter constant T from the sudden change suppression value T2 to the normal value T1 is not limited to condition A or condition B. For example, the condition for returning the filter constant T from the sudden change suppression value T2 to the normal value T1 may be a condition in which the logical sum of condition A and condition B is true.

[0062] The method for returning the filter constant T to the normal value T1 is not limited to the method of returning it stepwise from the sudden change suppression value T2 to the normal value T1. For example, the method for returning the filter constant T to the normal value T1 may be a method of gradually transitioning from the sudden change suppression value T2 to the normal value T1.

[0063] The process of reflecting the amount equivalent to the rate of change of the target steering equivalent angle before the predetermined condition is met in the input variable of low-pass filter M38 at the time when offset compensation processing is started is not limited to the process of S46. For example, the process of reflecting the amount equivalent to the rate of change of the target steering equivalent angular velocity ωp*(n) at the time when offset compensation processing is started may be a process of setting "ωp*(n-1)±Δ / Tc", where Tc is the update period of the target steering equivalent angle θp*. Furthermore, when the process of S30 is executed, "Δ / Tc" is added to the previous target steering equivalent angular velocity ωp*(n-1), while when the process of S32 is executed, "Δ / Tc" is subtracted from the previous target steering equivalent angular velocity ωp*(n-1).

[0064] The reduction process does not necessarily include both the processes of S46 and S48. "Regarding the operation process" 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] Regarding the difference equivalent amount: The difference equivalent amount that determines the offset amount Δθp0 is not limited to the 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. For example, the difference equivalent amount that determines the offset amount Δθp0 may be the value obtained by subtracting the steering equivalent angle θp from the target steering equivalent angle θp*0.

[0069] "Regarding the Predetermined Condition" The predetermined condition is not limited to the condition exemplified in the above embodiment. For example, it may be 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. This 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 misaligned. 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. Also, for example, this condition may be a condition that the current limit on the steering motor 60 is released. Specifically, for example, if the steered wheels 44 hit an obstacle such as a curb while the current limit on the steering motor 60 is in effect, 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.

[0070] Regarding the Offset Reduction 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.

[0071] 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.

[0072] 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 change the steering angular velocity-sensitive decrease amount Δω in accordance with the target steering equivalent angular velocity ωp*0 under the following condition. The condition is that 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 change the vehicle speed-sensitive decrease amount Δv in accordance with the vehicle speed V under the following condition. The condition is that 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.

[0073] "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.

[0074] 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 a 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 the 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 according to the offset amount, A steering control device, wherein the reduction process is a process for reducing a change in a rate of change of the target steering equivalent angle used for calculating the differential operation amount when the offset compensation process is performed.

2. A steering control device according to claim 1, wherein the reduction process includes a process for reducing the change in the rate of change of the target steering equivalent angle by changing a filter constant of a low-pass filter process when the offset compensation process is performed.

3. A steering control device according to claim 2, wherein an input variable of the low-pass filter processing is the rate of change of the target equivalent steering angle, an output variable of the low-pass filter processing is an input variable of a process for calculating the difference between the rate of change of the target equivalent steering angle and the rate of change of the equivalent steering angle, and the difference is used to calculate the differential operation amount.

4. A steering control device according to claim 3, wherein the reduction process includes a process of restoring the value of the filter constant to the value before change when the absolute value of the difference between the input variable of the low-pass filter process and the output variable of the low-pass filter process is equal to or less than a predetermined value.

5. A steering control device according to claim 3, wherein the reduction process includes a process of restoring the value of the filter constant to the value before the change when a predetermined time has elapsed since the start of the offset compensation process.

6. A steering control device as described in claim 1, wherein the reduction process includes a process of reflecting an amount equivalent to the rate of change of the target steering equivalent angle before the predetermined condition is met in the rate of change of the target steering equivalent angle used in calculating the differential operation amount at the time when the offset compensation process is started.

7. A steering control device as claimed in claim 6, wherein the offset compensation process includes a process of setting an initial value of the offset correction amount to the offset amount, and the reflecting process includes a process of holding the rate of change of the target steering equivalent angle used in calculating the differential operation amount.

8. A steering control device as described in claim 1, configured to execute an offset reduction process, wherein the offset reduction process is a process of gradually reducing the magnitude of the offset correction amount, the offset reduction process includes angular velocity sensitive process, and the angular velocity sensitive process is a process of changing the reduction rate in accordance with the value of the steering angular velocity variable on the condition that the reduction rate of the offset correction amount when the magnitude of the value of the steering angular velocity variable is large is equal to or greater than the reduction rate when the magnitude of 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.

9. 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.

10. A steering control method for controlling a steering device in a state in which steered wheels and a 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 a 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 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 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. The steering control method, wherein the reduction process is a process of reducing a change in a rate of change of the target steering equivalent angle used in calculating the differential operation amount when the offset compensation process is performed.

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