JP7624148B2 - Steering gear - Google Patents

Description

This invention relates to a steering device.

Patent Document 1 discloses the following technology. That is, the tire slip angle at which the tire cornering force generated in the wheel is at its maximum is stored as the target slip angle. When it is determined that the vehicle may come into contact with an obstacle ahead, a steering angle at which the tire slip angle is equal to the target slip angle is calculated as the target steering angle. Then, the steering actuator is driven and controlled so that the actual steering angle becomes the target steering angle. As a result, the steering angle of the wheel is controlled so that the steering angle of the wheel is equal to the steering angle at which the tire cornering force is at its maximum, improving the ability to avoid contact with the obstacle ahead.

JP 2007-168698 A

The objective of one embodiment of the present invention is to provide a steering device that can prevent the tire slip angle of the steered wheels from exceeding the tire slip angle at which the tire cornering force is maximized, and can prevent the vehicle from becoming unstable when the vehicle moves from an area with a low road friction coefficient to an area with a high road friction coefficient.

One embodiment of the present invention provides a steering device including a road surface friction coefficient estimation unit that estimates a road surface friction coefficient, which is the friction coefficient of the road surface on which the vehicle is traveling; a filter unit that smooths the change over time in the road surface friction coefficient estimated by the road surface friction coefficient estimation unit; a limit tire slip angle estimation unit that uses the road surface friction coefficient after filtering by the filter unit to estimate a limit tire slip angle, which is the tire slip angle at which the tire cornering force of the steered wheel is maximized; and a tire slip angle limiting unit that limits the tire slip angle of the steered wheel so as to prevent the tire slip angle of the steered wheel from exceeding the limit tire slip angle.

This configuration can prevent the tire slip angle of the steered wheels from exceeding the tire slip angle at which the tire cornering force is maximized. This configuration can also prevent the vehicle from becoming unstable when the vehicle enters an area with a high road friction coefficient from an area with a low road friction coefficient.

FIG. 1 is a schematic diagram showing a schematic configuration of an electric power steering system to which a steering device according to an embodiment of the present invention is applied. FIG. 2 is a block diagram showing the electrical configuration of the motor control ECU, and is used to explain the electrical configuration related to the operation in the automatic steering mode. FIG. 3 is a block diagram showing the configuration of the first steering envelope control unit. FIG. 4 is a block diagram showing the configuration of the road friction estimating unit. FIG. 5 is a block diagram showing the configuration of the angle control unit. FIG. 6 is a block diagram showing the configuration of the torque control unit. FIG. 7A is a graph for explaining the operation of the comparative example when the vehicle enters from the high μ region to the low μ region. FIG. 7B is a graph for explaining the operation of the comparative example when the vehicle enters from the low μ region to the high μ region. FIG. 8A is a schematic diagram showing that in the comparative example, when the vehicle enters from the low μ region to the high μ region, the vehicle state tends to become unstable. FIG. 8B is a schematic diagram showing that in this embodiment, the vehicle state is less likely to become unstable when the vehicle enters from the low μ region into the high μ region. FIG. 9A is a time chart showing the change in the road surface friction coefficient μ input to the limit tire slip angle estimating section when the vehicle enters from a low μ region to a high μ region. FIG. 9B is a time chart showing the change over time in the steering angle limit value θ _sl calculated by the steering angle limit value calculation unit when the vehicle enters from the low μ region into the high μ region. FIG. 9C is a time chart showing the change over time in the target steering angle θ _SEC after the limiting process calculated by the limiting section when the vehicle enters from the low μ region into the high μ region. FIG. 10 is a block diagram showing the electrical configuration of the motor control ECU, and is used to explain the electrical configuration related to the operation in the manual steering mode. FIG. 11 is a block diagram showing the configuration of the second steering envelope control unit. FIG. 12 is a graph showing an example of setting the target assist torque T _at relative to the steering torque T _d . FIG. 13 is a flowchart for explaining a modified example of the filter section.

[Description of the embodiments of the present invention]
One embodiment of the present invention provides a steering device including a road surface friction coefficient estimation unit that estimates a road surface friction coefficient, which is the friction coefficient of the road surface on which a vehicle is traveling; a filter unit that smooths out changes over time in the road surface friction coefficient estimated by the road surface friction coefficient estimation unit; a limit tire slip angle estimation unit that uses the road surface friction coefficient after filtering by the filter unit to estimate a limit tire slip angle, which is the tire slip angle at which the tire cornering force of the steered wheel is maximized; and a tire slip angle limiting unit that limits the tire slip angle of the steered wheel so as to prevent the tire slip angle of the steered wheel from exceeding the limit tire slip angle.

This configuration can prevent the tire slip angle of the steered wheels from exceeding the tire slip angle at which the tire cornering force is maximized, and also prevents the vehicle from becoming unstable when the vehicle enters an area with a high road friction coefficient from an area with a low road friction coefficient.
In one embodiment of the present invention, the filter unit is configured to smooth out the change over time in the road surface friction coefficient estimated by the road surface friction coefficient estimation unit only when the road surface friction coefficient estimated by the road surface friction coefficient estimation unit changes from a small road surface friction coefficient to a large road surface friction coefficient.

In one embodiment of the present invention, the tire slip angle limiting section is configured to limit the turning angle of the steered wheels so that the tire slip angle of the steered wheels does not exceed the limit tire slip angle.
In one embodiment of the present invention, the tire slip angle limiting unit includes a tire slip angle calculation unit that calculates a tire slip angle of the steered wheel, a steering angle limit value calculation unit that calculates a steering angle limit value for limiting the steering angle using the tire slip angle and the limit tire slip angle, and a limiting unit that limits a target steering angle for automatic steering using the steering angle limit value.

In one embodiment of the present invention, the tire slip angle limiting unit includes an electric motor for generating assist torque, a target assist torque setting unit for setting a target assist torque, and a motor control unit for controlling the electric motor, and the tire slip angle limiting unit includes a tire slip angle calculation unit for calculating a tire slip angle of the steered wheel, a steering angle limit value calculation unit for calculating a steering angle limit value for limiting the steering angle using the tire slip angle and the limit tire slip angle, and a steering angle limiting torque calculation unit for calculating a steering angle limiting torque for limiting the steering angle using the steering angle limit value and the steering angle, and the motor control unit is configured to control the torque of the electric motor using the target assist torque and the steering angle limiting torque.
Detailed Description of the Embodiments of the Invention
Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings.
[1] Schematic Configuration of Electric Power Steering System FIG. 1 is a schematic diagram showing the schematic configuration of an electric power steering system to which a steering device according to one embodiment of the present invention is applied.

The electric power steering system 1 includes a steering wheel (handle) 2 as a steering member for steering the vehicle, a steering mechanism 4 that steers the steered wheels 3 in conjunction with the rotation of the steering wheel 2, and a steering assist mechanism 5 that assists the driver in steering. The steering wheel 2 and the steering mechanism 4 are mechanically connected via a steering shaft 6 and an intermediate shaft 7.

The steering shaft 6 includes an input shaft 8 connected to the steering wheel 2, and an output shaft 9 connected to the intermediate shaft 7. The input shaft 8 and the output shaft 9 are connected via a torsion bar 10 so as to be capable of relative rotation.
A torque sensor 12 is disposed near the torsion bar 10. The torque sensor 12 detects a steering torque (torsion bar torque) _Td applied to the steering wheel 2 based on the relative rotational displacement amount between the input shaft 8 and the output shaft 9. In this embodiment, the steering torque Td detected by the torque sensor 12 is detected, for example, as a positive value for torque for steering leftward and a negative value for torque for steering rightward, and the magnitude of the _steering torque _Td increases as the absolute value increases.

The steering mechanism 4 is a rack-and-pinion mechanism including a pinion shaft 13 and a rack shaft 14 as a steering shaft. The steered wheels 3 are connected to each end of the rack shaft 14 via tie rods 15 and knuckle arms (not shown). The pinion shaft 13 is connected to the intermediate shaft 7. The pinion shaft 13 rotates in conjunction with the steering of the steering wheel 2. A pinion 16 is connected to the tip of the pinion shaft 13.

The rack shaft 14 extends linearly in the left-right direction of the vehicle. A rack 17 that meshes with the pinion 16 is formed in the middle of the rack shaft 14 in the axial direction. The pinion 16 and rack 17 convert the rotation of the pinion shaft 13 into axial movement of the rack shaft 14. The steered wheels 3 can be steered by moving the rack shaft 14 in the axial direction.

When the steering wheel 2 is steered (rotated), this rotation is transmitted to the pinion shaft 13 via the steering shaft 6 and the intermediate shaft 7. The rotation of the pinion shaft 13 is then converted into axial movement of the rack shaft 14 by the pinion 16 and the rack 17. As a result, the steered wheels 3 are steered.
The steering assist mechanism 5 includes an electric motor 18 for generating a steering assist force (assist torque), and a reducer 19 for amplifying the output torque of the electric motor 18 and transmitting it to the steering mechanism 4. The reducer 19 is made up of a worm gear mechanism including a worm gear 20 and a worm wheel 21 meshing with the worm gear 20. The reducer 19 is housed in a gear housing 22 serving as a transmission mechanism housing. Hereinafter, the reduction ratio (gear ratio) of the reducer 19 may be represented as i _dec . The reduction ratio i _dec is defined as the ratio θ _wg /θ _wo of the rotation angle θ _wg of the worm gear 20 to the rotation angle θ _ww of the worm wheel 21.

The worm gear 20 is rotationally driven by the electric motor 18. In addition, the worm wheel 21 is connected to the output shaft 9 so as to be integrally rotatable therewith.
When the worm gear 20 is rotationally driven by the electric motor 18, the worm wheel 21 is rotationally driven, and motor torque is applied to the steering shaft 6, causing the steering shaft 6 (output shaft 9) to rotate. The rotation of the steering shaft 6 is then transmitted to the pinion shaft 13 via the intermediate shaft 7. The rotation of the pinion shaft 13 is converted into axial movement of the rack shaft 14. This causes the steered wheels 3 to be turned. In other words, by rotating the worm gear 20 with the electric motor 18, steering assistance by the electric motor 18 and steering of the steered wheels 3 become possible. The electric motor 18 is provided with a rotation angle sensor 23 for detecting the rotation angle of the rotor of the electric motor 18.

The vehicle is equipped with a CCD (Charge Coupled Device) camera 25 that photographs the road ahead in the direction of travel of the vehicle, a GPS (Global Positioning System) 26 for detecting the vehicle's position, a radar 27 for detecting road shapes and obstacles, and a map information memory 28 that stores map information.
The vehicle is further equipped with a vehicle speed sensor 29, a yaw rate sensor 30, a lateral acceleration sensor 31, a first mode switch 33, and a second mode switch 34. The vehicle speed sensor 29 is a sensor for detecting a vehicle speed V. The yaw rate sensor 30 is a sensor for detecting a yaw rate γ. The yaw rate γ is a rotational angular velocity around a vertical axis passing through the center of gravity of the vehicle.

The lateral acceleration sensor 31 is a sensor for detecting the lateral acceleration _Ay . The lateral acceleration _Ay is an acceleration acting in the y-axis direction, which is the width direction of the vehicle. The first mode switch 33 is a switch for setting the steering mode to the manual steering mode. The second mode switch 34 is a switch for setting the steering mode to the automatic steering mode.
The CCD camera 25, GPS 26, radar 27, map information memory 28, vehicle speed sensor 29, yaw rate sensor 30, lateral acceleration sensor 31, first mode switch 33 and second mode switch 34 are connected to a host ECU (Electronic Control Unit) 201 for performing driving assistance control and automatic driving control. The host ECU 201 recognizes the surrounding environment, estimates the vehicle's position, plans a route, etc., based on the information obtained by the CCD camera 25, GPS 26 and radar 27 and the map information, and determines control target values for steering and drive actuators.

In this embodiment, the host ECU 201 sets a target steering angle θ _auto for automatic steering. In this embodiment, the automatic steering control is, for example, control for driving a vehicle along a target trajectory. The target steering angle θ _auto is a target value of the steering angle for automatically driving the vehicle along the target trajectory. The process for setting such a target steering angle θ _auto is well known, and therefore a detailed description thereof will be omitted here.

In addition, the host ECU 201 generates a mode signal S _mode indicating the manual steering mode or the automatic steering mode based on the output signals of the first mode switch 33 and the second mode switch 34 .
The target steering angle θ _auto set by the host ECU 201, the mode signal S _mode , the vehicle speed V detected by the vehicle speed sensor 29, the yaw rate γ detected by the yaw rate sensor 30, and the lateral acceleration _Ay detected by the lateral acceleration sensor 31 are provided to the motor control ECU 202 via the in-vehicle network. The steering torque T _d detected by the torque sensor 12 and the output signal of the rotation angle sensor 23 are input to the motor control ECU 202. The motor control ECU 202 controls the electric motor 18 based on these input signals and information provided by the host ECU 201.

The motor control ECU 202 controls the electric motor 18 by performing an operation in the automatic steering mode when the mode signal S _mode is a signal representing the automatic steering mode, and controls the electric motor 18 by performing an operation in the manual steering mode when the mode signal S _mode is a signal representing the manual steering mode.
[2] Electrical configuration of motor control ECU 202 in automatic steering mode [2.1] Overall configuration FIG. 2 is a block diagram showing the electrical configuration of motor control ECU 202, and is used to explain the electrical configuration related to operation in automatic steering mode.

The motor control ECU 202 includes a microcomputer 50, a drive circuit (inverter circuit) 41 controlled by the microcomputer 50 and supplying power to the electric motor 18, and a current detection circuit 42 for detecting the current flowing through the electric motor 18 (hereinafter referred to as "motor current I _m ").
The microcomputer 50 includes a CPU and memory (ROM, RAM, non-volatile memory, etc.), and functions as a plurality of functional processing sections by executing a predetermined program. The plurality of functional processing sections include a rotation angle calculation section 51, a rack axial force estimation section 52, a first steering envelope control section 53, an angle control section 54, a torque control section (current control section) 55, and a tire angle calculation section 59, which are related to the automatic steering mode.
[2.2] Rotation angle calculation unit 51 and tire angle calculation unit 59
The rotation angle calculation unit 51 calculates the rotor rotation angle _θm of the electric motor 18 based on the output signal of the rotation angle sensor 23 .

The tire angle calculation unit 59 calculates the turning angle of the steered wheels 3 (hereinafter referred to as "tire angle _θt ") by dividing the rotor rotation angle _θm calculated by the rotation angle calculation unit 51 by the reduction gear ratio _idec and dividing the result by the overall gear ratio _ioagr . The overall gear ratio _ioagr is the ratio of the steering angle of the steering wheel 2 (rotation angle _θs of the output shaft 9) to the tire angle _θt of the steered wheels 3, and is set in advance.
[2.3] Rack axial force estimation unit 52
The rack axial force estimating unit 52 estimates the rack axial force F rack, which is the axial force acting on the rack 17, based on the motor current I _m detected by the current detection circuit 42, the rotor rotation angle θ _m calculated by the rotation angle calculating unit 51, and the steering torque T _d . The estimated value of _{the rack axial force F rack is} represented as ^F _rack . As the rack axial force estimating unit 52, for example, the rack axial force estimating unit disclosed in JP 2021-949 A may be used.
[2.4] First steering envelope control unit 53
The first steering envelope control unit 53 calculates a target steering angle θ _SEC after a limiting process for preventing the tire slip angle of the steered wheels 3 from exceeding a tire slip angle α _sl of a tire grip limit. The tire slip angle α _sl of a tire grip limit is a tire slip angle at which the cornering force of the steered wheels 3 is maximized.

FIG. 3 is a block diagram showing the configuration of the first steering envelope control unit 53.
The first steering envelope control unit 53 includes a road surface friction estimating unit 61, a filter unit 62, a limit tire slip angle estimating unit 63, a tire slip angle estimating unit 64, a steering angle limit value calculating unit 65, and a limiting unit 66. The steering angle limit value calculating unit 65 is an example of the "steering angle limit value calculating unit" in the present invention.

FIG. 4 is a block diagram showing the configuration of the road surface friction estimating unit 61. As shown in FIG.
The road surface friction estimating unit 61 estimates the road surface friction coefficient μ, the vehicle body slip angle β, and the yaw rate γ based on the rack axial force estimated value ^ _Frack , yaw rate γ, lateral acceleration _Ay , tire angle _θt , and vehicle speed V calculated by the rack axial force estimating unit 52. The estimated values of the road surface friction coefficient μ, the vehicle body slip angle β, and the yaw rate γ are represented by ^μ, ^β, and ^γ, respectively. Note that the rotor rotation angle _θm calculated by the rotation angle calculating unit 51 may be used instead of the tire angle _θt .

The road surface friction estimating unit 61 includes an observed value calculating unit 71 , an estimated value calculating unit 72 , a subtraction unit 73 , and a gain calculating unit 74 .
The observation value calculation unit 71 calculates an observation quantity y necessary for calculating a state quantity estimated value ^x including the road surface friction coefficient estimated value ^μ as a state variable. The estimate value calculation unit 72 calculates the state quantity estimated value ^x and also calculates an observation quantity estimated value ^y which is the estimate value ^y of the observation quantity y. The subtraction unit 73 calculates an observation deviation Δy which is the deviation between the observation quantity y and the observation quantity estimated value ^y. The gain calculation unit 74 calculates a Kalman gain K for correcting the state quantity estimated value ^x based on the observation deviation Δy.

Specifically, the lateral acceleration Ay, the yaw rate γ, and the rack axis estimated value ^ _Frack are input to the observed value calculation unit 71. Based on the rack axis estimated value ^ _Frack , the observed value calculation unit 71 calculates the kingpin torque Tkpa, which is a force about the kingpin axis of the steered wheels 3, as a tire force acting on the steered wheels 3. Specifically, the observed value calculation unit 71 calculates the kingpin torque _Tkpa as the observed value by multiplying the rack axis estimated value ^ _Frack , which is approximated to a tie rod force acting in the axial direction of the tie rod ₁₅ , and the knuckle arm length determined by the vehicle specifications.

It is known that the kingpin torque _Tkpa as an observation value can be approximated by adding the tire alignment torque to the product of the tire cornering force and the caster trail. The observation value calculation unit 71 then calculates a third-order observation value y[ _Ay , γ, _Tkpa ] (3 x 1 matrix) having the lateral acceleration _Ay , the yaw rate γ, and the kingpin torque _Tkpa as observation values. The observation value y obtained in this manner is provided to the subtraction unit 73.

The tire angle _θt , the vehicle speed V, and the Kalman gain K are input to the estimate value calculation unit 72. The estimate value calculation unit 72 uses the tire angle _θt and the vehicle speed V as input quantities u, and calculates a state quantity estimate value ^x and an observation quantity estimate value ^y based on, for example, a two-wheel, two-degree-of-freedom planar vehicle model as a dynamics model of the road surface friction coefficient.
Specifically, the estimate calculation unit 72 calculates a third-order state quantity estimate ^x[^β, ^γ, ^μ] (3×1 matrix) including an estimate ^β of the vehicle body slip angle β, an estimate ^γ of the yaw rate γ, and an estimate ^μ of the road surface friction coefficient μ. The vehicle body slip angle β is an angle about a vertical axis passing through the center of gravity of the vehicle between the front-rear direction of the vehicle body and the vehicle traveling direction.

The estimate calculation unit 72 calculates a third-order observation quantity estimate ^y[^Ay, ^γ, ^ _Tkpa ] (3×1 matrix) including an estimate ^ _Ay of the lateral acceleration _Ay , an estimate ^ _γ of the yaw rate γ, and an estimate ^Tkpa of the kingpin torque _Tkpa . The observation quantity estimate ^ _y obtained in this manner is provided to the subtraction unit 73.
The subtractor 73 subtracts the observation quantity estimate ^y from the observation quantity y to calculate an observation deviation Δy. This observation deviation Δy is provided to a gain calculator 74. The gain calculator 74 calculates a Kalman gain K for correcting the state quantity estimate ^x based on the observation deviation Δy so as to reduce the observation deviation Δy. The estimate calculator 72 corrects the state quantity estimate ^x based on the Kalman gain K.

In this embodiment, the observed value calculator 71, the estimated value calculator 72, and the gain calculator 74 configure an extended Kalman filter EKF, which is a type of nonlinear Kalman filter. The road surface friction estimator 61 configures an estimation observer using the extended Kalman filter EKF.
For ease of explanation, hereinafter, the estimated value ^μ of the road surface friction coefficient μ calculated by the road surface friction estimating unit 61 will be referred to as the "road surface friction coefficient μ," and the estimated value ^β of the vehicle body slip angle β calculated by the road surface friction estimating unit 61 will be referred to as the "vehicle body slip angle β." The "road surface friction coefficient μ" estimated by the road surface friction estimating unit 61 is provided to a filter unit 62. The "vehicle body slip angle β" estimated by the road surface friction estimating unit 61 is provided to a tire slip angle estimating unit 64.

Returning to FIG. 3, the filter unit 62 includes a filter 62A for smoothing the change over time in the road surface friction coefficient μ estimated by the road surface friction estimation unit 61. The filter 62A may be a low-pass filter. In this embodiment, the filter unit 62 performs filtering processing on the road surface friction coefficient μ provided by the road surface friction estimation unit 61 using the filter 62A. The filtered road surface friction coefficient μ output from the filter unit 62 is provided to the limit tire slip angle estimation unit 63.

The tire slip angle estimating section 64 estimates a tire slip angle _αf of the steered wheels 3 (hereinafter simply referred to as "tire slip angle α") based on the vehicle body slip angle _β provided by the road surface friction estimating section 61, the vehicle speed V, the yaw rate γ, the front wheel base lf which is the distance in the front-to-rear direction of the vehicle from the center of gravity of the vehicle to the axle of the front wheels, and the tire angle _θt . Specifically, the tire slip angle estimating section 64 calculates the tire slip angle α based on the following equation (1).

The limit tire slip angle estimating section 63 calculates the tire slip angle at the tire grip limit (hereinafter referred to as the "limit tire slip angle αsl") based on the sign of the tire slip angle _α estimated by the tire slip angle estimating section 64, the road surface friction coefficient μ output from the filter section 62, the wheel load _Fz of the steered wheels 3, and the cornering stiffness Cα. The cornering stiffness _Cα is a rise gradient of the tire cornering force Fy of the steered wheels 3 with respect to the tire slip angle _{α in a range where the tire slip angle α is very small, and is set in advance. The tire cornering force Fy of the} steered wheels 3 is a force acting from the road surface on the tires of the steered wheels 3 in a direction perpendicular to the tire traveling direction.

Specifically, the limit tire slip angle estimating section 63 calculates the limit tire slip angle α _sl based on the following equation (2).

The steering angle limit value calculation unit 65 calculates the steering angle at the tire grip limit as the steering angle limit value θsl based on the overall gear ratio i _oagr , the tire slip angle α estimated by the tire slip angle estimation unit 64, the vehicle speed _V , the yaw rate γ, the front wheel base l _f , the road surface friction coefficient μ output from the filter unit 62, the wheel load _Fz of the steered wheels 3, and the cornering stiffness C _α . The steering angle limit value calculation unit 65 is an example of the "steering angle limit value calculation unit" in the present invention.

Specifically, the steering angle limit value calculation unit 65 calculates the steering angle limit value θ _sl based on the following equation (3).

The limiting section 66 calculates a target steering angle θ _SEC after the limiting process based on the target steering angle θ _auto and the steering angle limit value θ _sl calculated by the steering angle limit value calculation section 65 .
Specifically, the limiting section 66 calculates the target steering angle θ _SEC after the limiting process based on the following expressions (4a) and (4b).

In the formulas (4a) and (4b), G _α is an adjustment gain, and is set to a value within the range of 0<G _α ≦1. In this embodiment, G _α is set to G _α =1.
If G _α =1, when the absolute value |α| of the tire slip angle α is smaller than the absolute value |α _sl | of the limit tire slip angle α _sl , the target steering angle θ _auto is calculated as the target steering angle θ _SEC after the limiting process according to the formula (4a). Therefore, in this case, the target steering angle θ _auto is not limited.

On the other hand, when the absolute value |α| of the tire slip angle α is equal to or greater than the absolute value | _α | of the limit tire slip angle _αsl , the steering angle limit value _θsl is calculated as the target steering angle _θsec after the limiting process according to the formula (4b). Therefore, in this case, the target steering angle _θauto is replaced with the steering angle limit value _θsl .
The tire slip angle estimating section 64, the steering angle limit value calculating section 65, and the limiting section 66 in FIG. 3 are an example of the "tire slip angle limiting section" of the present invention.
[2.5] Configuration of Angle Control Unit 54 The angle control unit 54 calculates a target motor torque _Tmt, which is a target value of the motor torque of the electric motor 18, based on the target steering angle _θSEC after the limiting process.

FIG. 5 is a block diagram showing the configuration of the angle control unit 54.
The angle control unit 54 includes a feedback control unit 101 , a feedforward control unit 102 , a torque addition unit 103 , a first reduction ratio division unit 104 , and a second reduction ratio division unit 105 .
The first reduction ratio division unit 104 converts the rotor rotation angle _θm into a rotation angle (actual steering angle) _θs of the output shaft 9 (steering shaft 6) by dividing the rotor rotation angle _θm calculated by the rotation angle calculation unit 51 (see Figure 2) by the reduction ratio i _dec .

The feedback control unit 101 is provided to bring the actual steering angle θ _s closer to the target steering angle θ _SEC after the limiting process. The feedback control unit 101 includes an angle deviation calculation unit 101A and a PD control unit 101B. The angle deviation calculation unit 101A calculates the deviation Δθ (=θ _SEC -θ _s ) between the target steering angle θ _SEC and the actual steering angle θ _s .
The PD control unit 101B performs a PD calculation (proportional differential calculation) on the angle deviation Δθ calculated by the angle deviation calculation unit 101A to calculate a feedback control torque _Tfb . The feedback control torque _Tfb is provided to a torque addition unit 103.

The feedforward control unit 102 is provided to improve the control response by compensating for a delay in response due to the inertia of the electric power steering system 1. The feedforward control unit 102 includes an angular acceleration calculation unit 102A and an inertia multiplication unit 102B. The angular acceleration calculation unit 102A calculates a target angular acceleration d ² θ _SEC /dt ² by second-order differentiation of the target steering angle θ _SEC after the limiting process.

The inertia multiplication unit 102B calculates a feedforward control torque T _ff (=J·d ² θ _SEC /dt ² ) by multiplying the target angular acceleration d ² θ _SEC /dt ² calculated by the angular acceleration calculation unit 102A by the inertia J of the electric power steering system 1. The inertia J is obtained, for example, from a physical model (not shown) of the electric power steering system 1. The feedforward control torque T _ff is provided to the torque addition unit 103 as an inertia compensation value.

The torque addition unit 103 calculates a target steering torque _Tst (= _Tfb + _Tff ), which is a target torque for the output shaft 9, by adding the feedforward control torque _Tff to the feedback control torque _Tfb .
The target steering torque _Tst is provided to a second reduction ratio division section 105. The second reduction ratio division section 105 calculates a target motor torque _Tmt by dividing the target steering torque _Tst by the reduction ratio _idec . This target motor torque _Tmt is provided to the torque control section 55 (see FIG. 2).
[2.6] Configuration of Torque Control Unit 55 The torque control unit 55 controls the drive circuit 41 so that the motor torque of the electric motor 18 approaches the target motor torque _Tmt . That is, the torque control unit 55 performs torque control (torque feedback control) of the electric motor 18.

FIG. 6 is a block diagram showing the configuration of the torque control unit 55.
The torque control unit 55 includes a target motor current calculation unit 111 , a current deviation calculation unit 112 , a PI control unit 113 , and a PWM (Pulse Width Modulation) control unit 114 .
The target motor current calculation unit 111 calculates the target motor current I _mt by dividing the target motor torque T _mt calculated by the angle control unit 54 (see FIGS. 2 and 5) by the torque constant K _t of the electric motor 18 .

The current deviation calculation section 112 calculates the deviation ΔI (=I _mt −I _m ) between the target motor current I _mt obtained by the target motor current calculation section 111 and the motor current I _m detected by the current detection circuit 42 .
The PI control unit 113 performs a PI calculation (proportional integral calculation) on the current deviation ΔI calculated by the current deviation calculation unit 112 to generate a drive command value for guiding the motor current I _m flowing through the electric motor 18 to a target motor current I _mt . The PWM control unit 114 generates a PWM control signal with a duty ratio corresponding to the drive command value, and supplies the PWM control signal to the drive circuit 41. As a result, power corresponding to the drive command value is supplied to the electric motor 18.
[2.7] Effects of the embodiment in automatic steering mode In the above-described embodiment, for example, if G _α =1, when the absolute value |α| of the tire slip angle α is equal to or greater than the absolute value |α _sl | of the limit tire slip angle α _sl , the steering angle limit value θ _sl is calculated as the target steering angle θ _SEC after the limit process (see formula (4b)). This prevents the steering angle θ from becoming larger than the steering angle limit value θ _sl (the steering angle at the tire grip limit). This prevents the tire slip angle α of the steered wheels 3 from exceeding the limit slip angle α _sl (the tire slip angle at which the tire cornering force _Fy is at its maximum), and therefore prevents the vehicle state from becoming unstable.

In addition, when G _α is set to a value less than 1, the steering angle limit value θ _sl is calculated as the target steering angle θ _SEC after the limiting process before |α| reaches |α _sl |, so that the steering angle θ _s can be gradually limited before |α| reaches |α _sl |.
In addition, in the above-described embodiment, the first steering envelope control unit 53 includes a filter unit 62 for smoothing the change over time in the road surface friction coefficient μ, thereby preventing the vehicle from becoming unstable when the vehicle enters a region where the road surface friction coefficient is high (hereinafter referred to as the "high μ region") from a region where the road surface friction coefficient is low (hereinafter referred to as the "low μ region").

In the following, a configuration that is substantially the same as the above-described embodiment, but in which the first steering envelope control unit 53 does not include the filter unit 62, will be referred to as a comparative example.
Using Figures 7A and 7B, it will be explained that when a vehicle enters from a low μ region to a high μ region, the vehicle state is more likely to become unstable than when the vehicle enters from a high μ region to a low μ region.

7A and 7B, curve S1 is a graph showing the relationship of tire cornering force (tire grip force) _Fy to tire slip angle α when the vehicle is traveling in a low μ region. Curve S2 is a graph showing the relationship of tire cornering force _Fy to tire slip angle α when the vehicle is traveling in a high μ region. Curve S3 shows the target tire slip angle α _tar according to the target steering angle θ _auto .

FIG. 7A is a graph for explaining the operation of the comparative example when the vehicle enters from the high μ region to the low μ region.
When the vehicle is traveling in a high μ region, if the target tire slip angle α _tar increases, the tire slip angle α increases as shown by the arrow a1. In this case, the tire slip angle α is limited by a relatively large limit tire slip angle α _slH calculated using a relatively large road surface friction coefficient μ (curve S2).

When the vehicle enters from the high μ region to the low μ region while the target tire slip angle α _tar is maintained, the tire cornering force F _y decreases, as indicated by the arrow a2. Also, the relatively large limit tire slip angle α _slH changes to a relatively small limit tire slip angle α _slL calculated using a relatively small road surface friction coefficient μ (curve S1), as indicated by the arrow b1. As a result, the tire slip angle α is limited by the relatively small limit tire slip angle α _slL . As a result, the steering angle limit value θ _sl is set to a relatively small value, and as indicated by the arrow a3, the tire slip angle α decreases and the tire cornering force F _y remains almost unchanged, so that the vehicle state is less likely to become unstable.

FIG. 7B is a graph for explaining the operation of the comparative example when the vehicle enters from the low μ region to the high μ region.
When the vehicle is traveling in a low μ region, if the target tire slip angle α _tar increases, the tire slip angle α increases as shown by the arrow a1. However, in this case, the tire slip angle α is limited by a relatively small limit tire slip angle α _slL calculated using a relatively small road surface friction coefficient μ (curve S1).

When the vehicle enters the high μ region from the low μ region while maintaining the target tire slip angle α _tar , the tire cornering force _Fy increases as shown by the arrow a2, and the limit tire slip angle α _sl changes suddenly to a relatively large limit tire slip angle α _slH calculated using the relatively large road surface friction coefficient μ (curve S2) as shown by the arrow b1.
As a result, the steering angle limit value _θsl is set to a relatively large value, and the tire slip angle α increases as shown by the arrow a3, causing the tire cornering force _Fy to increase rapidly, so that the vehicle state is likely to become unstable as shown in Fig. 8A. In Fig. 8A, the region E _L indicated by hatching extending diagonally upward toward the right is the low μ region, and the region E _H indicated by hatching extending diagonally downward toward the right is the high μ region (the same applies to Fig. 8B described later).

In this embodiment, the first steering envelope control unit 53 includes a filter unit 62 for smoothing the change over time in the road surface friction coefficient μ. This makes it possible to suppress a sudden increase in the limit tire slip angle α _sl even when the vehicle enters a high μ region from a low μ region. This makes it possible to suppress a sudden increase in the tire cornering force F _y and the steering angle limit value θ _sl . This makes it possible to suppress the vehicle state from becoming unstable even when the vehicle enters a high μ region from a low μ region, as shown in FIG. 8B.

9A is a time chart showing the change in road surface friction coefficient μ input to limit tire slip angle estimator 63 when the vehicle enters from a low μ region to a high μ region. In Fig. 9A, the dashed line shows the change in road surface friction coefficient μ in the comparative example, and the solid line shows the change in road surface friction coefficient μ in this embodiment.
9B is a time chart showing the change over time of the steering angle limit value _θsl calculated by the steering angle limit value calculation unit 65 when the vehicle enters from the low μ region to the high μ region. In FIG. 9B, the dashed line shows the change in the steering angle limit value _θsl in the comparative example, and the solid line shows the change in the steering angle limit value _θsl in this embodiment.

9C is a time chart showing the change over time in the target steering angle θ _SEC after the limiting process calculated by the limiting unit 66 when the vehicle enters the high μ region from the low μ region. In FIG. 9C, the two-dot chain line shows the target steering angle θ _AUTO , the dashed line shows the change in the target steering angle θ _SEC after the limiting process in the comparative example, and the solid line shows the change in the target steering angle θ _SEC after the limiting process in the present embodiment.

In the comparative example, at time t1 when the vehicle enters the high μ region from the low μ region, the road friction coefficient μ increases abruptly (see FIG. 9A), the steering angle limit value θ _sl increases abruptly (see FIG. 9B), and the target steering angle θ _SEC increases abruptly (see FIG. 9C).
In contrast, in this embodiment, at time t1 when the vehicle enters the high μ region from the low μ region, the road friction coefficient μ increases gradually (see FIG. 9A), the steering angle limit value θ _sl increases gradually (see FIG. 9B), and the target steering angle θ _SEC increases gradually (see FIG. 9C). This makes it possible to prevent the vehicle state from becoming unstable when the vehicle enters the high μ region from the low μ region.
[3] Electrical configuration of motor control ECU 202 in manual steering mode [3.1] Overall configuration FIG. 10 is a block diagram showing the electrical configuration of motor control ECU 202, and is used to explain the electrical configuration related to operation in manual steering mode.

The motor control ECU 202 includes a microcomputer 50, a drive circuit 41 that is controlled by the microcomputer 50 and supplies power to the electric motor 18, and a current detection circuit 42 that detects the current (motor current I _m ) flowing through the electric motor 18.
The microcomputer 50 includes a CPU and a memory, and functions as a plurality of functional processing units by executing a predetermined program. The plurality of functional processing units include a rotation angle calculation unit 51, a rack axial force estimation unit 52, a second steering envelope control unit 56, a target assist torque setting unit 57, a torque addition unit 58, a torque control unit 55, and a tire angle calculation unit 59, which are related to the manual mode. The torque control unit 55 is an example of the "motor control unit" in the present invention.

The rotation angle calculation unit 51, the tire angle calculation unit 59, the rack axial force estimating unit 52, and the torque control unit 55 are the same as the rotation angle calculation unit 51, the tire angle calculation unit 59, the rack axial force estimating unit 52, and the torque control unit 55 in FIG. 2, respectively, and therefore their description will be omitted.
[3.2] Second steering envelope control section 56
The second steering envelope control unit 56 calculates an envelope control torque (steering angle limiting torque) _TSEC for preventing the tire slip angle α from exceeding the limit tire slip angle _αsl .

FIG. 11 is a block diagram showing the configuration of the second steering envelope control unit 56.
The second steering envelope control unit 56 includes a road surface friction estimator 61, a filter unit 62, a limit tire slip angle estimator 63, a tire slip angle estimator 64, a steering angle limit value calculator 65, a steering angle calculator 67, and an envelope control torque calculator 68. The steering angle limit value calculator 65 is an example of the "steering angle limit value calculator" in the present invention. The envelope control torque calculator 68 is an example of the "steering angle limiting torque calculator" in the present invention.

Road surface friction estimating unit 61, filter unit 62, limit tire slip angle estimating unit 63, tire slip angle estimating unit 64 and steering angle limit value calculating unit 65 are the same as road surface friction estimating unit 61, filter unit 62, limit tire slip angle estimating unit 63, tire slip angle estimating unit 64 and steering angle limit value calculating unit 65 in FIG. 3, respectively, and therefore their description will be omitted.
The steering angle calculation unit 67 calculates an actual steering angle (rotation angle of the output shaft 9) _θs by multiplying the tire angle θt _by the overall gear ratio i _oagr . The steering angle calculation unit 67 may calculate the actual steering angle _θs by dividing the rotor rotation angle _θm calculated by the rotation angle calculation unit 51 by the reduction ratio i _dec .

The envelope control torque calculation unit 68 calculates the envelope control torque (torque for limiting steering angle) T SEC based on the actual steering angle θ _s calculated by the steering angle calculation unit 67 and the limit tire slip angle α _sl estimated by the limit tire slip angle estimation unit 63. Specifically, the envelope control torque calculation unit 68 calculates _{the envelope control torque T SEC based} on the following equations (5a) and (5b).

In (5a) and (5b), _Gα is an adjustment gain and is set to a value within the range of 0< _Gα ≦1. In this embodiment, _Gα is set to _Gα =1. In formula (5b), k is a virtual spring constant and is preset. Also, c is a virtual viscous damping coefficient and is preset.
The tire slip angle estimating section 64, the steering angle limit value calculating section 65 and the envelope control torque calculating section 68 in FIG. 11 are an example of the "tire slip angle limiting section" of the present invention.
[3.3] Target assist torque setting unit 57
Returning to Fig. 10, the target assist torque setting unit 57 sets a target assist torque _T at which is a target value of the assist torque. The target assist torque setting unit 57 sets the target assist torque T _at based on the steering torque T _d detected by the torque sensor 12. An example of setting the target assist torque T _at with respect to the steering torque T _d is shown in Fig. 12.

The target assist torque T _at is set to a positive value when a steering assist force for steering leftward is to be generated from the electric motor 18, and is set to a negative value when a steering assist force for steering rightward is to be generated from the electric motor 18. The target assist torque T _at is positive for a positive value of the steering torque T _d , and is negative for a negative value of the steering torque T _d . The target assist torque T _at is set so that its absolute value increases as the absolute value of the steering torque T _d increases.

The target assist torque setting unit 57 may calculate the target assist torque T _at by multiplying the steering torque T _d by a preset constant.
[3.4] Torque addition unit 58
The torque addition section 58 calculates the target motor torque _Tmt by adding the envelope control torque _TSEC calculated by the envelope control torque calculation section 68 to the target assist torque _Tat calculated by the target assist torque setting section 57.

The target motor torque _Tmt is given to the torque control section 55. The torque control section 55 controls the drive circuit 41 so that the motor torque _Tm of the electric motor 18 approaches the target motor torque _Tmt .
[3.5] Effects of the embodiment in manual steering mode In the above-described embodiment, if G _α =1, for example, and the absolute value |α| of the tire slip angle _α is less than the absolute value |α _sl | of the limit tire slip angle α sl , the envelope control torque T _SEC becomes zero, and the target assist torque T _at is calculated as the target motor torque T _mt (see equation (5a)). Therefore, in this case, normal manual steering is performed.

When |α| is equal to or greater than | _αsl |, the envelope control torque _TSEC calculated by equation (5b) is added to the target assist torque _Tat to calculate the target motor torque _Tmt . In this case, since the absolute value | _θs | of the steering angle _θs is greater than the absolute value | _θsl | of the steering angle limit value _θsl , the envelope control torque _TSEC becomes a negative value, and the target motor torque _Tmt becomes smaller than the target assist torque _Tat .

The target motor torque _Tmt becomes smaller as the absolute value | _θs | of the steering angle _θs becomes larger than the absolute value | _θsl | of the steering angle limit value _θsl . This makes the driver feel that the steering wheel is heavy compared to normal manual steering. This makes it difficult for the driver to steer, so the steering angle θ is prevented from becoming larger than the steering angle limit value _θsl (the steering angle at the tire grip limit). This makes it possible to prevent the tire slip angle α of the steered wheels 3 from exceeding the limit slip angle _αsl (the tire slip angle at which the tire cornering force _Fy is maximum), and therefore makes it possible to prevent the vehicle state from becoming unstable.

When _Gα is set to a value less than 1, the envelope control torque _TSEC is calculated based on equation (5b) before |α| reaches | _αsl |, so that the steering angle _θs can be gradually limited before |α| reaches | _αsl |.
In addition, in the above-described embodiment, the first steering envelope control unit 53 includes a filter unit 62 for smoothing the change over time in the road surface friction coefficient μ, thereby preventing the vehicle state from becoming unstable when the vehicle enters a high μ region from a low μ region.
[3.6] Modifications of Envelope Control Torque Calculation Unit 68 The envelope control torque calculation unit 68 may calculate the envelope control torque T _SEC based on the following equations (6a) and (6b).

G _α is an adjustment gain and is set to a value within the range of 0<G _α ≦1.
For example, if G _α =1, when the absolute value |α| of the tire slip angle _α is less than the absolute value |α _sl | of the limit tire slip angle α sl , the envelope control torque T _SEC becomes zero, and the target assist torque T _at is calculated as the target motor torque T _mt (see equation (6a)). Therefore, in this case, normal manual steering is performed.

When |α| is equal to or greater than | _αsl |, the envelope control torque _TSEC calculated by equation (6b) is added to the target assist torque _Tat to calculate the target motor torque _Tmt . In this case, since the absolute value | _θs | of the steering angle _θs is greater than the absolute value | _θsl | of the steering angle limit value _θsl , the envelope control torque _TSEC is a positive value, and the target motor torque _Tmt is greater than the target assist torque _Tat .

The target motor torque _Tmt increases as the absolute value | _θs | of the steering angle _θs increases compared to the absolute value | _θsl | of the steering angle limit value _θsl . This makes the driver feel that the steering wheel is lighter than during normal manual steering. This gives the driver a feeling of slipping, and he or she tries to return the steering wheel. This prevents the steering angle θ from becoming larger than the steering angle limit value _θsl (the steering angle at the tire grip limit). This prevents the tire slip angle α of the steered wheels 3 from exceeding the limit slip angle _αsl (the tire slip angle at which the tire cornering force _Fy is at its maximum), and thus prevents the vehicle state from becoming unstable.

When _Gα is set to a value less than 1, the envelope control torque _TSEC is calculated based on equation (6b) before |α| reaches | _αsl |, so that the steering angle _θs can be gradually limited before |α| reaches | _αsl |.
[4] Other Modifications Although the embodiment of the present invention has been described above, the present invention can be embodied in other forms. For example, in the above-described embodiment, the filter unit 62 makes the time change of the road surface friction coefficient μ gentle regardless of the change in the road surface friction coefficient μ provided by the road surface friction estimation unit 61. However, the time change of the road surface friction coefficient μ may be made gentle only when the road surface friction coefficient μ provided by the road surface friction estimation unit 61 changes from a small value to a large value. For example, the filter unit 62 may perform an operation as shown in FIG. 13.

That is, the filter unit 62 first calculates the difference (μ _k - _{μ k-1} ) between the current value μ _k of the road surface friction coefficient μ provided by the road surface friction estimation unit 61 and the previous value μ _k-1 (step S1). Next, the filter unit 62 determines whether the difference (μ _k - μ _k-1 ) is equal to or greater than a predetermined threshold value A _th (where A _th > 0) (step S2).
If (μ _k −μ _k−1 )<A _th (step S2: NO), the filter unit 62 returns to step S1.

In step S2, when it is determined that (μ _k -μ _k-1 ) ≥ A _th (step S2: YES), the filter unit 62 performs a filter process on the road surface friction coefficient μ using the filter 62A (step S3), and then the process returns to step S1.
In addition, in the above embodiment, the angle control unit 54 (see FIG. 2) includes the feedforward control unit 102, but the feedforward control unit 102 may be omitted. In this case, the feedback control torque _Tfb calculated by the feedback control unit 101 becomes the target steering torque _Tst .

In the above embodiment, the steering angle _θs (tire angle _θt of the steered wheels 3) is limited so that the tire slip angle α of the steered wheels 3 does not exceed the limit tire slip angle _αsl . However, since the tire slip angle α is a function of the tire angle _θt as shown in equation (1) and also a function of the vehicle speed V, the vehicle speed V may be limited so that the tire slip angle α of the steered wheels 3 does not exceed the limit tire slip angle _αsl .

In this case, the vehicle speed V is limited by, for example, brake control. However, in the brake control, it takes time to decelerate the vehicle due to the moment of inertia of the vehicle. Therefore, the method of limiting the steering angle _θs (tire angle _θt ) has the advantage of being more responsive than the method of limiting the vehicle speed V.
In addition, in the above embodiment, the first steering envelope control unit 53 and the second steering envelope control unit 56 are provided in the motor control ECU 202, but they may be provided in the host ECU 201.

In the above embodiment, the present invention is applied to a column-type EPS, but the present invention can be applied to an EPS other than the column type. The present invention can also be applied to a steer-by-wire system.
Although the embodiments of the present invention have been described in detail, these are merely examples used to clarify the technical content of the present invention, and the present invention should not be interpreted as being limited to these examples, and the scope of the present invention is limited only by the appended claims.

1... Electric power steering system, 3... Steering wheel, 4... Steering mechanism, 18... Electric motor, 51... Rotation angle calculation unit, 52... Rack axial force estimation unit, 53... First steering envelope control unit, 54... Angle control unit, 55... Torque control unit, 56... Second steering envelope control unit, 57... Target assist torque setting unit, 58... Torque addition unit, 61... Road surface friction estimation unit, 62... Filter unit, 63... Limit tire slip angle estimation unit, 64... Tire slip angle estimation unit, 65... Steering angle limit value calculation unit, 66... Limit unit, 67... Steering angle calculation unit, 68... Envelope control torque calculation unit

Claims (5)

a road surface friction coefficient estimation unit that estimates a road surface friction coefficient, which is a friction coefficient of a road surface on which a vehicle is traveling;
a filter unit for smoothing a change over time in the road surface friction coefficient estimated by the road surface friction coefficient estimation unit;
a limit tire slip angle estimating unit that estimates a limit tire slip angle, which is a tire slip angle at which a tire cornering force of a steered wheel becomes maximum, using the road surface friction coefficient after filtering by the filter unit;
a tire slip angle limiting unit that limits the tire slip angle of the steered wheels so as to prevent the tire slip angle of the steered wheels from exceeding the limit tire slip angle ,
The filter unit is configured to smooth out the change over time in the road surface friction coefficient estimated by the road surface friction coefficient estimation unit only when the road surface friction coefficient estimated by the road surface friction coefficient estimation unit changes from a small road surface friction coefficient to a large road surface friction coefficient . 2. The steering device according to claim 1 , wherein the tire slip angle limiting unit is configured to limit the turning angle of the steered wheels so that the tire slip angle of the steered wheels does not exceed the limit tire slip angle. The tire slip angle limiting portion is
a tire slip angle calculation unit that calculates a tire slip angle of the steered wheels;
a steering angle limit value calculation unit that calculates a steering angle limit value for limiting the steering angle by using the tire slip angle and the limit tire slip angle;
The steering device according to claim 2 , further comprising: a limiting unit that limits a target steering angle for automatic steering by using the steering angle limit value. an electric motor for generating an assist torque;
A target assist torque setting unit that sets a target assist torque;
a motor control unit that controls the electric motor,
The tire slip angle limiting portion is
a tire slip angle calculation unit that calculates a tire slip angle of the steered wheels;
a steering angle limit value calculation unit that calculates a steering angle limit value for limiting the steering angle by using the tire slip angle and the limit tire slip angle;
a steering angle limiting torque calculation unit that calculates a steering angle limiting torque for limiting the steering angle by using the steering angle limit value and the steering angle,
The motor control unit uses the target assist torque and the steering angle limiting torque.
The steering apparatus according to claim 2 , wherein the electric motor is configured to be torque controlled. a road surface friction coefficient estimation unit that estimates a road surface friction coefficient, which is a friction coefficient of a road surface on which a vehicle is traveling;
a filter unit for smoothing a change over time in the road surface friction coefficient estimated by the road surface friction coefficient estimation unit;
a limit tire slip angle estimating unit that estimates a limit tire slip angle, which is a tire slip angle at which a tire cornering force of a steered wheel becomes maximum, using the road surface friction coefficient after filtering by the filter unit;
a tire slip angle limiting unit that limits a turning angle of the steered wheels so that the tire slip angle of the steered wheels does not exceed the limit tire slip angle;
an electric motor for generating an assist torque;
A target assist torque setting unit that sets a target assist torque;
a motor control unit that controls the electric motor,
The tire slip angle limiting portion is
a tire slip angle calculation unit that calculates a tire slip angle of the steered wheels;
a steering angle limit value calculation unit that calculates a steering angle limit value for limiting the steering angle by using the tire slip angle and the limit tire slip angle;
a steering angle limiting torque calculation unit that calculates a steering angle limiting torque for limiting the steering angle by using the steering angle limit value and the steering angle,
The motor control unit uses the target assist torque and the steering angle limiting torque.
A steering device configured to torque control the electric motor.

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