Patent Description:
An electric power steering device (EPS) as a vehicle steering device applies assist force (steering supplementary force) to a steering system of the vehicle through rotational force of a motor. The EPS applies, as the assist force, drive power of the motor, which is controlled by electrical power supplied from an inverter, to a steering shaft or a rack shaft through a transmission mechanism including a deceleration mechanism. For example, a configuration in which a first control signal generated based on a steering torque and a vehicle speed, and a second control signal generated to reduce the deviation between the steering torque and a reference steering torque generated based on a steering angle are switched in accordance with behavior of the vehicle and the motor is driven is disclosed (for example, Patent Literature <NUM>).

In a configuration in which control is performed based on a vehicle speed, when a vehicle speed signal is not normally output, control is performed by using a predetermined alternative vehicle speed in some cases. When the alternative vehicle speed is a high speed such as <NUM> [km/h], assist force is excessive in a low speed range and provides discomfort to a wheel operation by a driver in some cases.

The present invention is made in view of the above-described problem and is intended to provide a vehicle steering device capable of preventing generation of excessive steering torque in a low speed range.

In order to achieve the above object, a vehicle steering device according to one aspect of the present invention configured to assist and control a steering system of a vehicle by driving and controlling a motor configured to assist steering force, wherein when the vehicle speed of the vehicle is a predetermined alternative vehicle speed, target steering torque is reduced in accordance with the absolute value of the difference between a physical quantity generated through turning motion of the vehicle and an estimated value of the physical quantity at the alternative vehicle speed.

With the above-described configuration, it is possible to prevent generation of excessive steering torque in a low speed range.

As a desirable aspect of the vehicle steering device, it preferably comprising: a vehicle motion estimation unit configured to estimate the estimated value of the physical quantity in accordance with a steering angle; and a torque gain setting unit configured to set a torque gain for the target steering torque in accordance with the absolute value of the difference between the physical quantity and the estimated value of the physical quantity.

Accordingly, it is possible to estimate the estimated value of the physical quantity at the alternative vehicle speed in accordance with the steering angle. In addition, it is possible to set the target steering torque based on the torque gain in accordance with the absolute value of the difference between the physical quantity and the estimated value of the physical quantity.

As a desirable aspect of the vehicle steering device, it is preferable that the torque gain setting unit reduces the torque gain when the vehicle speed is the alternative vehicle speed and the absolute value of the difference between the physical quantity and the estimated value of the physical quantity is equal to or larger than a predetermined threshold value.

Accordingly, when the vehicle speed is the alternative vehicle speed, it is possible to prevent setting to a value far from an ideal target steering torque at the actual vehicle speed.

As a desirable aspect of the vehicle steering device, it is preferable that the torque gain setting unit sets the torque gain to be one when the vehicle speed is not the alternative vehicle speed and the absolute value of the difference between the physical quantity and the estimated value of the physical quantity is smaller than a predetermined threshold value, and sets the torque gain to be a value smaller than one when the vehicle speed is the alternative vehicle speed and the absolute value of the difference between the physical quantity and the estimated value of the physical quantity is equal to or larger than the threshold value.

Accordingly, it is possible to set a target steering torque to be smaller when the vehicle speed is the alternative vehicle speed and the physical quantity along with turning motion of the vehicle is far from the estimated value than when the vehicle speed is not the alternative vehicle speed or when the vehicle speed is the alternative vehicle speed but the physical quantity along with turning motion of the vehicle is not far from the estimated value. Accordingly, when the vehicle speed is the alternative vehicle speed, it is possible to prevent setting to a value far from the ideal target steering torque at the actual vehicle speed.

As a desirable aspect of the vehicle steering device, it is preferable that the torque gain setting unit gradually reduces the torque gain to the set value when the vehicle speed is the alternative vehicle speed and the absolute value of the difference between the physical quantity and the estimated value of the physical quantity is equal to or larger than the threshold value.

Accordingly, it is possible to reduce discomfort due to abrupt change of the assist force.

As a desirable aspect of the vehicle steering device, it is preferable that the physical quantity is a yaw rate, and the vehicle motion estimation unit estimates an estimated yaw rate in accordance with the steering angle.

Accordingly, it is possible to perform control by using, as a parameter, the yaw rate that is the physical quantity generated through turning motion of the vehicle.

As a desirable aspect of the vehicle steering device, it is preferable that the physical quantity is lateral acceleration, and the vehicle motion estimation unit estimates an estimated lateral acceleration in accordance with the steering angle.

Accordingly, it is possible to perform control by using, as a parameter, the lateral acceleration that is the physical quantity generated through turning motion of the vehicle.

As a desirable aspect of the vehicle steering device, it is preferable that the physical quantity is self-aligning torque, and the vehicle motion estimation unit estimates estimated self-aligning torque in accordance with the steering angle.

Accordingly, it is possible to perform control by using, as a parameter, the self-aligning torque that is the physical quantity generated through turning motion of the vehicle.

According to the present invention, it is possible to provide a vehicle steering device capable of preventing generation of excessive steering torque in a low speed range.

Modes for carrying out the invention (hereinafter referred to as embodiments) will be described below in detail with reference to the accompanying drawings. Note that, the present invention is not limited by the following embodiments. In addition, components in the embodiments described below include their equivalents such as those that could be easily thought of by the skilled person in the art and those identical in effect. Moreover, components disclosed in the embodiments described below may be combined as appropriate.

<FIG> is a diagram illustrating a typical configuration of an electric power steering device. The electric power steering device (EPS) as a vehicle steering device is coupled with steering wheels <NUM> and 8R through a column shaft (steering shaft or wheel shaft) <NUM> of a wheel <NUM>, a deceleration mechanism <NUM>, universal joints 4a and 4b, a pinion rack mechanism <NUM>, and tie rods 6a and 6b and further through hub units 7a and 7b in an order in which force provided by a steering person transfers. In addition, a torque sensor <NUM> configured to detect steering torque Ts of the wheel <NUM> and a rudder angle sensor <NUM> configured to detect a steering angle θh are provided to the column shaft <NUM> including a torsion bar, and a motor <NUM> configured to assist steering force of the wheel <NUM> is coupled with the column shaft <NUM> through the deceleration mechanism <NUM>. Electrical power is supplied from a battery <NUM> to a control unit (ECU) <NUM> configured to control the electric power steering device, and an ignition key signal is input to the control unit <NUM> through an ignition key <NUM>. The control unit <NUM> performs calculation of a current command value of an assist (steering auxiliary) command based on the steering torque Ts detected by the torque sensor <NUM> and vehicle speed Vs detected by a vehicle speed sensor <NUM>, and controls current supplied to the motor <NUM> through a voltage control command value Vref obtained by providing compensation or the like to the current command value.

The control unit <NUM> is connected with an on-board network such as a controller area network (CAN) <NUM> through which various kinds of information of a vehicle are transmitted and received. In addition, the control unit <NUM> is connectable with a non-CAN <NUM> configured to transmit and receive communication other than the CAN <NUM>, analog and digital signals, radio wave, and the like.

The control unit <NUM> is mainly configured as a CPU (including an MCU and an MPU). <FIG> is a schematic diagram illustrating a hardware configuration of the control unit configured to control the electric power steering device.

A control computer <NUM> configured as the control unit <NUM> includes a central processing unit (CPU) <NUM>, a read only memory (ROM) <NUM>, a random access memory (RAM) <NUM>, an electrically erasable programmable rom (EEPROM) <NUM>, an interface (I/F) <NUM>, an analog/digital (A/D) converter <NUM>, and a pulse width modulation (PWM) controller <NUM>, and these components are connected with a bus.

The CPU <NUM> is a processing device configured to execute a computer program for control (hereinafter referred to as a control program) of the electric power steering device and control the electric power steering device.

The ROM <NUM> stores a control program for controlling the electric power steering device. In addition, the RAM <NUM> is used as a work memory for operating the control program. The EEPROM <NUM> stores, for example, control data input to and output from the control program. The control data is used on the control program loaded onto the RAM <NUM> after the control unit <NUM> is powered on, and is overwritten to the EEPROM <NUM> at a predetermined timing.

The ROM <NUM>, the RAM <NUM>, the EEPROM <NUM>, and the like are storage devices configured to store information and are storage devices (primary storage devices) directly accessible from the CPU <NUM>.

The A/D converter <NUM> receives, for example, signals of the steering torque Ts, a detected current value Im of the motor <NUM>, and the steering angle θh and converts the signals into digital signals.

The interface <NUM> is connected with the CAN <NUM>. The interface <NUM> receives a signal (vehicle speed pulse) of a vehicle speed V from the vehicle speed sensor <NUM>.

The PWM controller <NUM> outputs a PWM control signal of each UVW phase based on a current command value to the motor <NUM>.

<FIG> is a diagram illustrating an exemplary internal block configuration of a control unit in an electric power steering device according to a comparative example. The steering torque Ts and the vehicle speed Vs are input to a current command value calculation unit <NUM>. The current command value calculation unit <NUM> refers to, based on the steering torque Ts and the vehicle speed Vs, a look-up table (such as an assist map) stored in advance and calculates a current command value Iref1 that is a control target value of current supplied to the motor <NUM>.

A compensation signal generation unit <NUM> generates a compensation signal CM. The compensation signal generation unit <NUM> includes a convergence estimation unit <NUM>, an inertia estimation unit <NUM>, and a self-aligning torque (SAT) estimation unit <NUM>. The convergence estimation unit <NUM> estimates the yaw rate of the vehicle based on the angular velocity of the motor <NUM>, and estimates a compensation value with which shake operation of the wheel <NUM> is reduced to improve convergence of the yaw of the vehicle. The inertia estimation unit <NUM> estimates the inertial force of the motor <NUM> based on the angular acceleration of the motor <NUM>, and estimates a compensation value with which the inertial force of the motor <NUM> is compensated to improve response. The SAT estimation unit <NUM> estimates self-aligning torque based on the steering torque Ts, assist torque, and the angular velocity and angular acceleration of the motor <NUM>, and estimates a compensation value with which the assist torque is compensated with the self-aligning torque as reaction force. The compensation signal generation unit <NUM> may include an estimation unit configured to estimate another compensation value in addition to the convergence estimation unit <NUM>, the inertia estimation unit <NUM>, and the SAT estimation unit <NUM>. The compensation signal CM is a sum obtained by adding, at an addition unit <NUM>, the compensation value of the convergence estimation unit <NUM> and a sum obtained by adding the compensation value of the inertia estimation unit <NUM> and the compensation value of the SAT estimation unit <NUM> at an addition unit <NUM>.

At an addition unit 32A, the compensation signal CM from the compensation signal generation unit <NUM> is added to the current command value Iref1, and characteristic compensation of a steering system is provided to the current command value Iref1 through the addition of the compensation signal CM to improve convergence, an inertia characteristic, and the like. Then, the current command value Iref1 becomes a current command value Iref2 provided with characteristic compensation through the addition unit 32A, and the current command value Iref2 is input to a current restriction unit <NUM>. At the current restriction unit <NUM>, largest current of the current command value Iref2 is restricted, and a current command value Irefm is generated. The current command value Irefm is input to a subtraction unit 32B, and a deviation I (Irefm - Im) from the detected current value Im fed back from the motor <NUM> side is calculated at the subtraction unit 32B. The deviation I is input to a PI control unit <NUM> for characteristic improvement of steering operation. Accordingly, the voltage control command value Vref, characteristics of which are improved at the PI control unit <NUM> is input to a PWM control unit <NUM>, and in addition, the motor <NUM> is PWM-driven through an inverter circuit <NUM> as a motor drive unit. The detected current value Im of the motor <NUM> is detected by a current detector <NUM> and fed back to the subtraction unit 32B. In addition, the inverter circuit <NUM> includes a field effect transistor (hereinafter referred to as a FET) as a drive element and is configured as a bridge circuit of the FET.

In assist control by the conventional electric power steering device, steering torque manually applied by a driver is detected by a torque sensor as twist torque of the torsion bar, and motor current is controlled as assist current mainly in accordance with the torque. However, when control is performed by this method, the steering torque changes depending on the steering angle because of difference in the state (for example, tilt) of a road surface in some cases. The steering torque is also affected by variation of a motor output characteristic due to long-term use in some cases.

<FIG> is a structural diagram illustrating exemplary installation of the rudder angle sensor.

The column shaft <NUM> includes a torsion bar 2A. Road surface reaction force Rr and road surface information µ act on the steering wheels <NUM> and 8R. An upper angle sensor is provided on the wheel side of the column shaft <NUM> with respect to the torsion bar 2A. A lower angle sensor is provided on the steering wheel side of the column shaft <NUM> with respect to the torsion bar 2A. The upper angle sensor detects a wheel angle θ<NUM>, and the lower angle sensor detects a column angle θ<NUM>. The steering angle θh is detected by a rudder angle sensor provided at an upper part of the column shaft <NUM>. The twist angle Δθ of the torsion bar is expressed in Expression (<NUM>) below based on the deviation between the wheel angle θ<NUM> and the column angle θ<NUM>. In addition, torsion bar torque Tt is expressed in Expression (<NUM>) below by using the twist angle Δθ of the torsion bar expressed in Expression (<NUM>). Note that, Kt represents the spring constant of the torsion bar 2A. <MAT><MAT>.

The torsion bar torque Tt may be detected by using a torque sensor. In the present embodiment, the torsion bar torque Tt is treated as the steering torque Ts.

<FIG> is a diagram illustrating an exemplary internal block configuration of the control unit according to a first embodiment.

The control unit <NUM> includes, as internal block components, a target steering torque generation unit <NUM>, a twist angle control unit <NUM>, a steering direction determination unit <NUM>, and a conversion unit <NUM>.

In the present embodiment, wheel steering by the driver is assisted and controlled by the motor <NUM> of an EPS steering system/vehicle system <NUM>. The EPS steering system/vehicle system <NUM> includes an angle sensor and an angular velocity calculation unit in addition to the motor <NUM>.

The target steering torque generation unit <NUM> generates a target steering torque Tref that is a target value of the steering torque when the steering system of the vehicle is assisted and controlled in the present disclosure. The conversion unit <NUM> converts the target steering torque Tref into a target twist angle Δθref. The twist angle control unit <NUM> generates a motor current command value Iref that is a control target value of current supplied to the motor <NUM>.

The twist angle control unit <NUM> calculates the motor current command value Iref with which the twist angle Δθ is equal to the target twist angle Δθref. The motor <NUM> is driven by the motor current command value Iref.

The steering direction determination unit <NUM> determines whether the steering direction is right or left based on a motor angular velocity ωm output from the EPS steering system/vehicle system <NUM>, and outputs a result of the determination as a steering state signal STs. <FIG> is an explanatory diagram of the steering direction.

A steering state indicating whether the steering direction is right or left can be obtained as, for example, the relation between the steering angle θh and the motor angular velocity ωm as illustrated in <FIG>. Specifically, the steering direction is determined to be "right" when the motor angular velocity ωm is a positive value, or the steering direction is determined to be "left" when the motor angular velocity ωm is a negative value. Note that, an angular velocity calculated by performing speed calculation on the steering angle θh, the wheel angle θ<NUM>, or the column angle θ<NUM> may be used in place of the motor angular velocity ωm.

The conversion unit <NUM> converts the target steering torque Tref generated at the target steering torque generation unit <NUM> into the target twist angle Δθref by using the relation of Expression (<NUM>) above.

Subsequently, exemplary basic operation at the control unit of the first embodiment will be described below. <FIG> is a flowchart illustrating exemplary operation of the control unit according to the first embodiment.

The steering direction determination unit <NUM> determines whether the steering direction is right or left based on the sign of the motor angular velocity ωm output from the EPS steering system/vehicle system <NUM>, and outputs a result of the determination as the steering state signal STs to the target steering torque generation unit <NUM> (step S10).

The target steering torque generation unit <NUM> generates the target steering torque Tref based on the vehicle speed Vs, a vehicle speed determination signal Vfail, the steering state signal STs, the steering angle θh, and a real yaw rate γre (step S20).

The conversion unit <NUM> converts the target steering torque Tref generated at the target steering torque generation unit <NUM> into the target twist angle Δθref (step S20). The target twist angle Δθref is output to the twist angle control unit <NUM>.

The twist angle control unit <NUM> calculates the motor current command value Iref based on the target twist angle Δθref, the steering angle θh, the twist angle Δθ, and the motor angular velocity ωm (step S30).

Then, current control is performed to drive the motor <NUM> based on the motor current command value Iref output from the twist angle control unit <NUM> (step S40).

<FIG> is a block diagram illustrating an exemplary configuration of the target steering torque generation unit of the first embodiment. As illustrated in <FIG>, the target steering torque generation unit <NUM> includes a basic map unit <NUM>, a multiplication unit <NUM>, a differential unit <NUM>, a damper gain map unit <NUM>, a hysteresis correction unit <NUM>, a SAT information correction unit <NUM>, a multiplication unit <NUM>, addition units <NUM>, <NUM>, and <NUM>, and a vehicle speed failure processing unit <NUM>. <FIG> is a diagram illustrating exemplary characteristics of a basic map held by the basic map unit. <FIG> is a diagram illustrating exemplary characteristics of a damper gain map held by the damper gain map unit.

The steering angle θh and the vehicle speed Vs are input to the basic map unit <NUM>. The basic map unit <NUM> outputs a torque signal Tref_a0 having the vehicle speed Vs as a parameter by using the basic map illustrated in <FIG>. Specifically, the basic map unit <NUM> outputs the torque signal Tref_a0 in accordance with the vehicle speed Vs.

As illustrated in <FIG>, the torque signal Tref_a0 has such a characteristic that the torque signal Tref_a0 increases as the magnitude (absolute value) |θh| of the steering angle θh increases. In addition, a torque signal Tref_a has such a characteristic that the torque signal Tref_a increases as the vehicle speed Vs increases. Note that, the map is configured with the magnitude |θh| of the steering angle θh in <FIG> but may be configured in accordance with the positive and negative values of the steering angle θh, and in this case, different change aspects may be applied depending on whether the steering angle θh is positive or negative.

The steering angle θh is input to the differential unit <NUM>. The differential unit <NUM> calculates a rudder angular velocity ωh that is angular velocity information by differentiating the steering angle θh. The differential unit <NUM> outputs the calculated rudder angular velocity ωh to the multiplication unit <NUM>.

The vehicle speed Vs is input to the damper gain map unit <NUM>. The damper gain map unit <NUM> outputs a damper gain DG in accordance with the vehicle speed Vs by using a vehicle speed sensitive damper gain map illustrated in <FIG>.

As illustrated in <FIG>, the damper gain DG has such a characteristic that the damper gain DG gradually increases as the vehicle speed Vs increases. The damper gain DG may be variable in accordance with the steering angle θh.

The multiplication unit <NUM> multiplies the rudder angular velocity ωh output from the differential unit <NUM> by the damper gain DG output from the damper gain map unit <NUM>, and outputs a result of the multiplication as a torque signal Tref_b to the addition unit <NUM>.

The steering direction determination unit <NUM> performs determination as illustrated in, for example, <FIG>. The steering angle θh, the vehicle speed Vs, and the steering state signal STs, which is a result of the determination illustrated in <FIG>, are input to the hysteresis correction unit <NUM>. The hysteresis correction unit <NUM> calculates a torque signal Tref_c based on the steering angle θh and the steering state signal STs by using Expressions (<NUM>) and (<NUM>) below. Note that, in Expressions (<NUM>) and (<NUM>) below, x represents the steering angle θh, and yR = Tref_c and yL = Tref_c represent the torque signal (fourth torque signal) Tref_c. In addition, a coefficient "a" is a value larger than one, and a coefficient "c" is a value larger than zero. A coefficient Ahys indicates the output width of a hysteresis characteristic, and the coefficient "c" indicates the roundness of the hysteresis characteristic. <MAT><MAT>.

In a case of right steering, the torque signal (fourth torque signal) Tref_c (yR) is calculated by using Expression (<NUM>) above. In a case of left steering, the torque signal (fourth torque signal) Tref_c (yL) is calculated by using Expression (<NUM>) above. Note that, when switching is made from right steering to left steering or when switching is made from left steering to right steering, a coefficient "b" or "b'" indicated in Expression (<NUM>) or (<NUM>) below is substituted into Expressions (<NUM>) and (<NUM>) above after steering switching based on the values of final coordinates (x<NUM>, y<NUM>) that are the previous values of the steering angle θh and the torque signal Tref_c. Accordingly, continuity through steering switching is maintained. <MAT><MAT>.

Expressions (<NUM>) and (<NUM>) above can be derived by substituting x<NUM> into x and substituting y<NUM> into yR and yL in Expressions (<NUM>) and (<NUM>) above.

For example, when Napierian logarithm e is used as the coefficient "a", Expressions (<NUM>), (<NUM>), (<NUM>), and (<NUM>) above can be expressed as Expressions (<NUM>), (<NUM>), (<NUM>), and (<NUM>) below, respectively. <MAT><MAT><MAT><MAT>.

<FIG> is a diagram illustrating exemplary characteristics of the hysteresis correction unit. The example illustrated in <FIG> indicates an exemplary characteristic of the torque signal Tref_c subjected to hysteresis correction when Ahys = <NUM> [Nm] and c = <NUM> are set in Expressions (<NUM>) and (<NUM>) above and steering is performed from <NUM> [deg] to +<NUM> [deg] or -<NUM> [deg]. As illustrated in <FIG>, the torque signal Tref_c output from the hysteresis correction unit <NUM> has a hysteresis characteristic such as the origin at zero → L1 (thin line) → L2 (dashed line) → L3 (bold line).

Note that, the coefficient Ahys, which indicates the output width of the hysteresis characteristic, and the coefficient "c", which indicates the roundness thereof may be variable in accordance with one or both of the vehicle speed Vs and the steering angle θh.

In addition, the rudder angular velocity ωh is obtained through the differential calculation on the steering angle θh but is provided with low-pass filter (LPF) processing as appropriate to reduce influence of noise in a higher range. In addition, the differential calculation and the LPF processing may be performed with a high-pass filter (HPF) and a gain. Moreover the rudder angular velocity ωh may be calculated by performing the differential calculation and the LPF processing not on the steering angle θh but on a wheel angle θ1 detected by the upper angle sensor or a column angle θ2 detected by the lower angle sensor. The motor angular velocity ωm may be used as the angular velocity information in place of the rudder angular velocity ωh, and in this case, the differential unit <NUM> is not needed.

As illustrated in <FIG>, the steering angle θh, the vehicle speed determination signal Vfail, and the real yaw rate γre detected by a yaw rate sensor <NUM> (refer to <FIG>) provided to the own-vehicle are input to the vehicle speed failure processing unit <NUM>.

The vehicle speed sensor <NUM> (refer to <FIG>) outputs, as a vehicle speed signal, a pulse signal in accordance with the vehicle speed. When the vehicle speed sensor <NUM> fails and the vehicle speed signal (pulse signal in accordance with the vehicle speed) is not normally output, control based on the vehicle speed Vs cannot be performed. Thus, when the vehicle speed signal is not normally output, control using a predetermined alternative vehicle speed is performed.

The vehicle speed determination signal Vfail is a signal indicating whether the vehicle speed signal is normally output from the vehicle speed sensor <NUM>. When the vehicle speed signal is not normally output, the predetermined alternative vehicle speed is input as the vehicle speed Vs to the vehicle speed failure processing unit <NUM>. In other words, the vehicle speed determination signal Vfail is a signal indicating whether the vehicle speed Vs is the alternative vehicle speed. In the present embodiment, the alternative vehicle speed is set to be, for example, <NUM> [km/h]. Note that, a component configured to output the vehicle speed determination signal Vfail and the alternative vehicle speed may be configured as, for example, a circuit outside the control unit <NUM>.

The present embodiment describes an example in which the real yaw rate γre detected by the yaw rate sensor <NUM> is input as a physical quantity generated through turning motion of the vehicle. Real lateral acceleration detected by a lateral acceleration sensor <NUM> (refer to <FIG>) provided to the own-vehicle may be input as the physical quantity generated through turning motion of the vehicle in place of the real yaw rate γre.

<FIG> is a block diagram illustrating an exemplary configuration of the vehicle speed failure processing unit of the first embodiment. The vehicle speed failure processing unit <NUM> of the first embodiment includes a vehicle motion estimation unit <NUM> and a torque gain setting unit <NUM>.

The steering angle θh is input to the vehicle motion estimation unit <NUM>. The vehicle motion estimation unit <NUM> holds an estimated yaw rate map representing the relation between the steering angle θh and a yaw rate γ at an alternative speed (for example, <NUM> [km/h]). <FIG> is a diagram illustrating exemplary characteristics of the estimated yaw rate map held by a vehicle motion estimation unit of the first embodiment. Note that, the relation between the steering angle θh and the yaw rate γ may be expressed by using an expression based on, for example, a vehicle model called a single-track Model.

The vehicle motion estimation unit <NUM> outputs an estimated yaw rate γest in accordance with the steering angle θh by using the estimated yaw rate map (the expression indicating the relation between the steering angle θh and the yaw rate γ at the alternative speed) illustrated in <FIG>.

The estimated yaw rate γest output from the vehicle motion estimation unit <NUM>, the vehicle speed determination signal Vfail, and the real yaw rate γre are input to the torque gain setting unit <NUM>. The torque gain setting unit <NUM> generates a torque gain AG based on the estimated yaw rate γest, the vehicle speed determination signal Vfail, and the real yaw rate γre.

Specifically, the torque gain setting unit <NUM> determines whether the vehicle speed Vs is normally detected, in other words, whether the vehicle speed Vs is the alternative vehicle speed based on the vehicle speed determination signal Vfail. When the vehicle speed Vs is the alternative vehicle speed, the torque gain setting unit <NUM> generates the torque gain AG in accordance with the absolute value |γest - γre| of the difference between the estimated yaw rate γest and the real yaw rate γre. In the present embodiment, the torque gain setting unit <NUM> holds a predetermined threshold value B for the absolute value |γest - γre| of the difference between the estimated yaw rate γest and the real yaw rate γre.

<FIG> is an explanatory diagram of specific operation at the torque gain setting unit of the first embodiment. In the example illustrated in <FIG>, a solid line represents the absolute value |γest| of the estimated yaw rate γest. In addition, in the example illustrated in <FIG>, a dashed line represents a value smaller than the absolute value |γest| of the estimated yaw rate γest by the predetermined threshold value B.

The torque gain setting unit <NUM> reduces the torque gain AG when the vehicle speed Vs is the alternative vehicle speed and the absolute value |γest - γre| of the difference between the estimated yaw rate γest and the real yaw rate γre is equal to or larger than the threshold value B.

The example illustrated in <FIG> indicates a point Ex where the absolute value of the steering angle θh is |θh1| and the absolute value of the real yaw rate γre is |γre1|. <FIG> illustrates an example in which the absolute value |γest - γre| of the difference between the estimated yaw rate γest and the real yaw rate γre is equal to or larger than the threshold value B (|γest - γre| ≥ B).

The torque gain AG of the first embodiment is expressed in Expression (<NUM>) below. In Expression (<NUM>) below, a coefficient "A" is a real number equal to or larger than one.

When the vehicle speed Vs is the alternative vehicle speed and |γest - γre| ≥ B is satisfied, the torque gain setting unit <NUM> sets the torque gain AG to be smaller than one. In other words, the coefficient "A" indicated in Expression (<NUM>) above is set to be a value larger than one.

Note that, when the vehicle speed Vs is normally detected, in other words, when the vehicle speed determination signal Vfail indicates that the vehicle speed Vs is normal, the torque gain setting unit <NUM> sets the torque gain AG to be one. The torque gain setting unit <NUM> sets the torque gain AG to be one also when the absolute value |γest - γre| of the difference between the estimated yaw rate γest and the real yaw rate γre when the vehicle speed Vs is the alternative vehicle speed is smaller than the threshold value B (|γest - γre|<B). In other words, the coefficient "A" indicated in Expression (<NUM>) above is set to be one.

<FIG> is a flowchart illustrating exemplary processing at the vehicle speed failure processing unit of the first embodiment.

The torque gain setting unit <NUM> determines whether the vehicle speed Vs is the alternative vehicle speed based on the vehicle speed determination signal Vfail (step S101).

When the vehicle speed Vs is not the alternative vehicle speed (No at step S101), in other words, when the vehicle speed Vs is normally detected, the torque gain setting unit <NUM> sets the coefficient "A" in the torque gain AG = <NUM>/A to be one (step S103), and ends the processing.

When the vehicle speed Vs is the alternative vehicle speed (Yes at step S101), the vehicle motion estimation unit <NUM> outputs the estimated yaw rate γest in accordance with the steering angle θh by using the estimated yaw rate map illustrated in, for example, <FIG> (step S102).

The torque gain setting unit <NUM> calculates the absolute value |γest - γre| of the difference between the estimated yaw rate γest and the real yaw rate γre (step S104).

Subsequently, the torque gain setting unit <NUM> determines whether the absolute value |γest - γre| of the difference between the estimated yaw rate γest and the real yaw rate γre is equal to or larger than the predetermined threshold value B (|γest - γre| ≥ B) (step S105).

When the absolute value |γest - γre| of the difference between the estimated yaw rate γest and the real yaw rate γre is smaller than the threshold value B (|γest - γre|<B) (No at step S105), the torque gain setting unit <NUM> sets the coefficient "A" in the torque gain AG = <NUM>/A to be one (step S103), and ends the processing.

When the absolute value |γest - γre| of the difference between the estimated yaw rate γest and the real yaw rate γre is equal to or larger than the threshold value B (|γest - γre| ≥ B) (Yes at step S105), the torque gain setting unit <NUM> sets the coefficient "A" in the torque gain AG = <NUM>/A to be a predetermined value larger than one (step S106), and ends the processing.

Referring back to <FIG>, the multiplication unit <NUM> multiplies the torque signal Tref_a0 output from the basic map unit <NUM> by the torque gain AG output from the vehicle speed failure processing unit <NUM>, and outputs a result of the multiplication as the torque signal Tref_a to the addition unit <NUM>.

<FIG> is a diagram illustrating an exemplary effect of the torque gain AG output from the vehicle speed failure processing unit. When the vehicle speed Vs is the alternative vehicle speed, the predetermined alternative vehicle speed (for example, <NUM> [km/h]) is input as the vehicle speed Vs to the basic map unit <NUM>. In this case, the value of the torque signal Tref_a0 output from the basic map unit <NUM> is a value in accordance with the alternative speed (in this example, <NUM> [km/h]).

With a configuration in which the vehicle speed failure processing unit <NUM> of the first embodiment is not employed, the torque signal Tref_a0 output from the basic map unit <NUM> is output as the torque signal Tref_a.

The torque signals Tref_a, Tref_b, and Tref_c obtained as described above are added together at the addition units <NUM> and <NUM> and output as the target steering torque Tref.

With the configuration in which the vehicle speed failure processing unit <NUM> of the first embodiment is not employed, the target steering torque Tref becomes a large value in accordance with the alternative vehicle speed, for example, when the driver largely operates the wheel <NUM> before stopping the vehicle while the vehicle speed sensor <NUM> fails and the alternative vehicle speed (for example, <NUM> [km/h]) is output as the vehicle speed Vs, and then the vehicle stops with the steering angle θh at, for example, <NUM> [deg]. When the driver takes hands off the wheel <NUM> in this state, the steering angle θh is controlled to decrease by assist control. Thus, for example, when the driver operates the wheel <NUM> and stops the wheel <NUM> in a right or left state to turn right or left at an intersection, the driver needs to hold the wheel <NUM>. Thus, anomalous behavior called self-steering, which is not intended by the driver, occurs.

The above-described anomalous behavior can be prevented by employing the vehicle speed failure processing unit <NUM> of the first embodiment. In the example illustrated in <FIG>, the torque gain AG output from the torque gain setting unit <NUM> is <NUM> (in other words, the coefficient "A" in the torque gain AG = <NUM>/A is <NUM>). Accordingly, the value of the torque signal Tref_a obtained by multiplying a value C of the torque signal Tref_a0 output from the basic map unit <NUM> by the torque gain AG (=<NUM>) at the multiplication unit <NUM> is <NUM>/<NUM> of the value of the torque signal Tref_a0, in other words, C/<NUM>. Thus, it is possible to prevent generation of excessive steering torque that causes self-steering during stopping due to assist control in a state in which the vehicle speed sensor <NUM> fails and the alternative vehicle speed (for example, <NUM> [km/h]) is output as the vehicle speed Vs.

Note that, the position at which the multiplication unit <NUM> is provided is not limited to a later stage of the basic map unit <NUM> as illustrated in <FIG>, but may be, for example, a later stage of the addition units <NUM> and <NUM>.

When the vehicle speed Vs is the alternative vehicle speed and the absolute value |γest - γre| of the difference between the estimated yaw rate γest and the real yaw rate γre is equal to or larger than the threshold value B, the torque gain setting unit <NUM> may gradually reduce the value of the torque gain AG at stages from one, or may change the torque gain AG in accordance with the magnitude of the absolute value |γest - γre| of the difference between the estimated yaw rate γest and the real yaw rate γre. Accordingly, it is possible to reduce discomfort due to abrupt change of assist force.

The yaw rate sensor <NUM> configured to detect the real yaw rate γre only needs to output a detected value, for example, when the steering angle θh changes by several [deg], and does not need to be particularly highly accurate. Thus, it is possible to use the yaw rate sensor <NUM> that is relatively inexpensive.

The detected value of the yaw rate sensor <NUM> is desirably directly input to the control unit <NUM>, not through the CAN <NUM>. Accordingly, it is possible to prevent the above-described anomalous behavior when the alternative vehicle speed is input as the vehicle speed Vs due to failure of the CAN <NUM>.

The yaw rate sensor <NUM> desirably has a self-diagnosis function. This can prevent assist function failure and, for example, makes it possible to notify the driver of anomaly through a provided warning lamp.

The twist angle control unit <NUM> of the first embodiment (refer to <FIG>) will be described below with reference to <FIG>.

<FIG> is a block diagram illustrating an exemplary configuration of the twist angle control unit of the first embodiment. The twist angle control unit <NUM> calculates the motor current command value Iref based on the target twist angle Δθref, the twist angle Δθ, the steering angle θh, and the motor angular velocity ωm. The twist angle control unit <NUM> includes a twist angle feedback (FB) compensation unit <NUM>, a speed control unit <NUM>, a stabilization compensation unit <NUM>, an output restriction unit <NUM>, a rudder angle disturbance compensation unit <NUM>, a subtraction unit <NUM>, an addition unit <NUM>, and a speed reduction ratio unit <NUM>.

The target twist angle Δθref output from the conversion unit <NUM> is input to the subtraction unit <NUM> through addition. The twist angle Δθ is input to the subtraction unit <NUM> through subtraction. The steering angle θh is input to the rudder angle disturbance compensation unit <NUM>. The motor angular velocity ωm is input to the stabilization compensation unit <NUM>.

The twist angle FB compensation unit <NUM> multiplies a deviation Δθ0 between the target twist angle Δθref and the twist angle Δθ, which is calculated at the subtraction unit <NUM>, by a compensation value CFB (transfer function) and outputs a target column angular velocity ωref1 with which the twist angle Δθ follows the target twist angle Δθref. The target column angular velocity ωref1 is output to the addition unit <NUM> through addition. The compensation value CFB may be a simple gain Kpp, or a typically used compensation value such as a PI control compensation value.

The rudder angle disturbance compensation unit <NUM> multiplies the steering angle θh by a compensation value Ch (transfer function) and outputs a target column angular velocity wref2. The target column angular velocity wref2 is output to the addition unit <NUM> through addition.

The addition unit <NUM> adds the target column angular velocity ωref1 and the target column angular velocity ωref2, and outputs a result of the addition as a target column angular velocity wref to the speed control unit <NUM>. Accordingly, it is possible to reduce influence on the torsion bar twist angle Δθ due to change of the steering angle θh input by the driver, thereby improving the capability of the twist angle Δθ to follow the target twist angle Δθref in response to abrupt steering.

When the steering angle θh changes in response to steering by the driver, the change of the steering angle θh affects the twist angle Δθ as disturbance, and error occurs to the target twist angle Δθref. In particular, upon abrupt steering, significant error occurs to the target twist angle Δθref due to change of the steering angle θh. A basic purpose of the rudder angle disturbance compensation unit <NUM> is to reduce influence of the steering angle θh as disturbance.

The speed control unit <NUM> calculates, through I-P control (proportional preceding PI control), a motor current command value Is with which a column angular velocity ωc follows the target column angular velocity ωref. The column angular velocity ωc may be a value obtained by multiplying the motor angular velocity ωm by a speed reduction ratio <NUM>/N of the speed reduction ratio unit <NUM> as a deceleration mechanism as illustrated in <FIG>.

A subtraction unit <NUM> calculates the difference between (ωref - ωc) the target column angular velocity ωref and the column angular velocity ωc. An integral unit <NUM> integrates the difference between (ωref - ωc) the target column angular velocity ωref and the column angular velocity ωc and inputs a result of the integration to a subtraction unit <NUM> through addition.

A twist angular velocity ωt is also output to a proportional unit <NUM>. The proportional unit <NUM> performs proportional processing with a gain Kvp on the column angular velocity ωc and inputs a result of the proportional processing to the subtraction unit <NUM> through subtraction. A result of the subtraction at the subtraction unit <NUM> is output as the motor current command value Is. Note that, the speed control unit <NUM> may calculate the motor current command value Is not by I-P control but by a typically used control method such as PI control, P (proportional) control, PID (proportional-integral-differential) control, PI-D control (differential preceding PID control), model matching control, or model reference control.

The upper and lower limit values of the motor current command value Is are set in advance at the output restriction unit <NUM>. The motor current command value Iref is output with restriction on the upper and lower limit values of the motor current command value Is.

Note that, the configuration of the twist angle control unit <NUM> in the present embodiment is exemplary and may be different from the configuration illustrated in <FIG>. For example, the twist angle control unit <NUM> may not include the rudder angle disturbance compensation unit <NUM>, the addition unit <NUM>, nor the speed reduction ratio unit <NUM>.

<FIG> is a diagram illustrating an exemplary internal block configuration of a control unit according to a second embodiment. Note that, a component same as that in the configuration described above in the first embodiment is denoted by the same reference sign and duplicate description thereof is omitted. A control unit (ECU) 30a according to the second embodiment is different from that of the first embodiment in the configurations of a target steering torque generation unit 200a and a twist angle control unit 300a.

The steering torque Ts and a motor angle θm in addition to the steering angle θh, the vehicle speed Vs, and the vehicle speed determination signal Vfail are input to the target steering torque generation unit 200a.

The twist angle control unit 300a calculates a motor current command value Imc with which the twist angle Δθ is equal to the target twist angle Δθref. The motor <NUM> is driven by the motor current command value Imc.

<FIG> is a block diagram illustrating an exemplary configuration of the target steering torque generation unit of the second embodiment. As illustrated in <FIG>, the target steering torque generation unit 200a of the second embodiment includes the SAT information correction unit <NUM> and an addition unit <NUM> in addition to the configuration described in the first embodiment. In addition, the target steering torque generation unit 200a is different from that of the first embodiment in the configuration of a vehicle speed failure processing unit 280a.

The steering angle θh, the vehicle speed Vs, the steering torque Ts, the motor angle θm, and the motor current command value Imc are input to the SAT information correction unit <NUM>. The SAT information correction unit <NUM> calculates self-aligning torque (SAT) based on the steering torque Ts, the motor angle θm, and the motor current command value Imc and further provides filter processing, gain multiplication, and restriction processing to calculate a torque signal (first torque signal) Tref_d.

<FIG> is a block diagram illustrating an exemplary configuration of the SAT information correction unit. The SAT information correction unit <NUM> includes a SAT calculation unit <NUM>, a filter unit <NUM>, a steering torque sensitive gain unit <NUM>, a vehicle speed sensitive gain unit <NUM>, a rudder angle sensitive gain unit <NUM>, and a restriction unit <NUM>.

The status of torque generated between a road surface and steering will be described below with reference to <FIG> is a schematic diagram illustrating the status of torque generated between the road surface and steering.

The steering torque Ts is generated as the driver steers the wheel, and the motor <NUM> generates assist torque (motor torque) Tm in accordance with the steering torque Ts. As a result, the wheel is rotated, self-aligning torque TSAT is generated as reaction force. In this case, torque as resistance against wheel steering is generated by column-shaft conversion inertia (inertia that acts on the column shaft by the motor <NUM> (rotor thereof), the deceleration mechanism, and the like) J and friction (static friction) Fr. In addition, physical torque (viscosity torque) expressed as a damper term (damper coefficient DM) is generated by the rotational speed of the motor <NUM>. The equation of motion in Expression (<NUM>) below is obtained from balancing among these forces.

In Expression (<NUM>) above, ωM is a motor angular velocity subjected to column-shaft conversion (conversion into a value for the column shaft), and αM is a motor angular acceleration subjected to column-shaft conversion. When Expression (<NUM>) above is solved for TSAT, Expression (<NUM>) below is obtained.

As understood from Expression (<NUM>) above, when the column-shaft conversion inertia J, the static friction Fr, and the damper coefficient DM are determined as constants in advance, the self-aligning torque TSAT can be calculated from the motor angular velocity ωM, the motor angular acceleration αM, the assist torque Tm, and the steering torque Ts. Note that, for simplification, the column-shaft conversion inertia J may be a value converted for the column shaft by using a relational expression of motor inertia and a speed reduction ratio.

The steering torque Ts, the motor angle θm, and the motor current command value Imc are input to the SAT calculation unit <NUM>. The SAT calculation unit <NUM> calculates the self-aligning torque TSAT by using Expression (<NUM>) above. The SAT calculation unit <NUM> includes a conversion unit 251A, an angular velocity calculation unit 251B, an angular acceleration calculation unit 251C, a block 251D, a block 251E, a block 251F, a block <NUM>, and adders <NUM>, 251I, and 251J.

The motor current command value Imc is input to the conversion unit 251A. The conversion unit 251A calculates the assist torque Tm subjected to column-shaft conversion through multiplication by a predetermined gear ratio and a predetermined torque constant.

The motor angle θm is input to the angular velocity calculation unit 251B. The angular velocity calculation unit 251B calculates the motor angular velocity ωM subjected to column-shaft conversion through differential processing and gear ratio multiplication.

The motor angular velocity ωM is input to the angular acceleration calculation unit 251C. The angular acceleration calculation unit 251C calculates the motor angular acceleration αM subjected to column-shaft conversion by differentiating the motor angular velocity ωM.

Then, the self-aligning torque TSAT is calculated with a configuration as illustrated in <FIG> based on Math. <NUM> by the block 251D, the block 251E, the block 251F, the block <NUM>, and the adders <NUM>, 251I, and 251J by using the input steering torque Ts and the assist torque Tm, the motor angular velocity ωM, and the motor angular acceleration αM thus calculated.

The motor angular velocity ωM output from the angular velocity calculation unit 251B is input to the block 251D. The block 251D functions as a sign function and outputs the sign of the input data.

The motor angular velocity ωM output from the angular velocity calculation unit 251B is input to the block 251E. The block 251E multiplies the input data by the damper coefficient DM and outputs a result of the multiplication.

The block 251F multiplies the input data from the block 251D by the static friction Fr and outputs a result of the multiplication.

The motor angular acceleration αM output from the angular acceleration calculation unit 251C is input to the block <NUM>. The block <NUM> multiplies the input data by the column-shaft conversion inertia J and outputs a result of the multiplication.

The adder <NUM> adds the steering torque Ts and the assist torque Tm output from the conversion unit 251A.

The adder 251I subtracts the output from the block <NUM> from the output from the adder <NUM>.

The adder 251J adds the output from the block 251E and the output from the block 251F and subtracts the output from the adder 251I.

With the above-described configuration, Expression (<NUM>) above can be achieved. Specifically, the self-aligning torque TSAT is calculated by the configuration of the SAT calculation unit <NUM> illustrated in <FIG>.

Note that, when the column angle can be directly detected, the column angle may be used as angle information in place of the motor angle θm. In this case, column-shaft conversion is unnecessary. In addition, a signal obtained by subjected the motor angular velocity ωm from the EPS steering system/vehicle system <NUM> to column-shaft conversion may be input as the motor angular velocity ωM in place of the motor angle θm, and the differential processing on the motor angle θm may be omitted. Moreover, the self-aligning torque TSAT may be calculated by a method other than that described above or may be a measured value, not a calculated value.

To utilize the self-aligning torque TSAT calculated at the SAT calculation unit <NUM> and appropriately convey the self-aligning torque TSAT to the driver as a steering feeling, information desired to be conveyed is extracted from the self-aligning torque TSAT by the filter unit <NUM>, the amount of conveyance is adjusted by the steering torque sensitive gain unit <NUM>, the vehicle speed sensitive gain unit <NUM>, and the rudder angle sensitive gain unit <NUM>, and the upper and lower limit values thereof are further adjusted by the restriction unit <NUM>.

The self-aligning torque TSAT from the SAT calculation unit <NUM> is input to the filter unit <NUM>. The filter unit <NUM> performs filter processing on the self-aligning torque TSAT through, for example, a bandpass filter and outputs SAT information TST<NUM>.

The SAT information TST<NUM> output from the filter unit <NUM> and the steering torque Ts are input to the steering torque sensitive gain unit <NUM>. The steering torque sensitive gain unit <NUM> sets a steering torque sensitive gain.

<FIG> is a diagram illustrating exemplary characteristics of the steering torque sensitive gain. As illustrated in <FIG>, the steering torque sensitive gain unit <NUM> sets the steering torque sensitive gain so that sensitivity is high at on-center vicinity corresponding to a straight traveling state. The steering torque sensitive gain unit <NUM> multiplies the SAT information TST<NUM> by the steering torque sensitive gain set in accordance with the steering torque Ts and outputs SAT information TST<NUM>.

<FIG> illustrates an example in which the steering torque sensitive gain is fixed at <NUM> when the steering torque Ts is equal to or smaller than Ts1 (for example, <NUM>), fixed at a value smaller than <NUM> when the steering torque Ts is equal to or larger than Ts2 (> Ts1) (for example, <NUM>), or set to decrease at a constant ratio when the steering torque Ts is between Ts1 and Ts2.

The SAT information TST<NUM> output from the steering torque sensitive gain unit <NUM> and the vehicle speed Vs are input to the vehicle speed sensitive gain unit <NUM>. The vehicle speed sensitive gain unit <NUM> sets a vehicle speed sensitive gain.

<FIG> is a diagram illustrating exemplary characteristics of the vehicle speed sensitive gain. As illustrated in <FIG>, the vehicle speed sensitive gain unit <NUM> sets the vehicle speed sensitive gain so that sensitivity at fast travel is high. The vehicle speed sensitive gain unit <NUM> multiplies the SAT information TST<NUM> by the vehicle speed sensitive gain set in accordance with the vehicle speed Vs, and outputs SAT information TST<NUM>.

<FIG> illustrates an example in which the vehicle speed sensitive gain is fixed at <NUM> when the vehicle speed Vs is equal to or higher than Vs2 (for example, <NUM>/h), fixed at a value smaller than <NUM> when the vehicle speed Vs is equal to or smaller than Vs1 (<Vs2) (for example, <NUM>/h), or set to increase at a constant ratio when the vehicle speed Vs is between Vs1 and Vs2.

The SAT information TST<NUM> output from the vehicle speed sensitive gain unit <NUM> and the steering angle θh are input to the rudder angle sensitive gain unit <NUM>. The rudder angle sensitive gain unit <NUM> sets a rudder angle sensitive gain.

<FIG> is a diagram illustrating exemplary characteristics of the rudder angle sensitive gain. As illustrated in <FIG>, the rudder angle sensitive gain unit <NUM> sets the rudder angle sensitive gain to start acting at a predetermined steering angle and have high sensitivity when the steering angle is large. The rudder angle sensitive gain unit <NUM> multiplies the SAT information TST<NUM> by the rudder angle sensitive gain set in accordance with the steering angle θh, and outputs a torque signal Tref_d0.

<FIG> illustrates an example in which the rudder angle sensitive gain is a predetermined gain value Gα when the steering angle θh is equal to or smaller than θh1 (for example, <NUM> deg), fixed at <NUM> when the steering angle θh is equal to or larger than θh2 (for example, <NUM> deg), or set to increase at a constant ratio when the steering angle θh is between θh1 and θh2. To have high sensitivity when the steering angle θh is large, Gα may be set to be in the range of <NUM> ≤ Gα < <NUM>. To have high sensitivity when the steering angle θh is small, Gα may be set to be in the range of <NUM> < Gα although not illustrated. To avoid sensitivity change due to the steering angle θh, Gα may be set to be one.

The torque signal Tref_d0 output from the rudder angle sensitive gain unit <NUM> is input to the restriction unit <NUM>. The upper and lower limit values of the torque signal Tref_d0 are set to the restriction unit <NUM>.

<FIG> is a diagram illustrating exemplary setting of the upper and lower limit values of the torque signal at the restriction unit. As illustrated in <FIG>, the upper and lower limit values of the torque signal Tref_d0 are set to the restriction unit <NUM> in advance, and the restriction unit <NUM> outputs, as a torque signal Tref_d, the upper limit value when the torque signal Tref_d0 that is input is equal to or larger than the upper limit value, the lower limit value when the torque signal Tref_d0 that is input is equal to or smaller than the lower limit value, or the torque signal Tref_d0 otherwise.

Note that, the steering torque sensitive gain, the vehicle speed sensitive gain, and the rudder angle sensitive gain may have curved characteristics in place of linear characteristics as illustrated in <FIG>, and <FIG>. In addition, settings of the steering torque sensitive gain, the vehicle speed sensitive gain, and the rudder angle sensitive gain may be adjusted as appropriate in accordance with a steering feeling. In addition, the restriction unit <NUM> may be omitted, for example, when the magnitude of a torque signal is not likely to increase or is prevented by another means. The steering torque sensitive gain unit <NUM>, the vehicle speed sensitive gain unit <NUM>, and the rudder angle sensitive gain unit <NUM> may also be omitted as appropriate. In addition, installation positions of the steering torque sensitive gain, the vehicle speed sensitive gain, and the rudder angle sensitive gain may be interchanged. In addition, for example, the steering torque sensitive gain, the vehicle speed sensitive gain, and the rudder angle sensitive gain may be determined in parallel and used to multiply the SAT information TST<NUM> at one component.

Thus, the configuration of the SAT information correction unit <NUM> in the present embodiment is exemplary and may be different from the configuration illustrated in <FIG>.

<FIG> is a block diagram illustrating an exemplary configuration of the vehicle speed failure processing unit of the second embodiment. The vehicle speed failure processing unit 280a of the second embodiment includes a vehicle motion estimation unit 281a and a torque gain setting unit 282a.

The present embodiment describes an example in which the self-aligning torque TSAT calculated by the SAT calculation unit <NUM> described above is input as the physical quantity generated through turning motion of the vehicle.

The steering angle θh is input to the vehicle motion estimation unit 281a. The vehicle motion estimation unit 281a holds an estimated self-aligning torque map representing the relation between the steering angle θh and the self-aligning torque TSAT at the alternative speed (for example, <NUM> [km/h]). <FIG> is a diagram illustrating exemplary characteristics of the estimated self-aligning torque map held by the vehicle motion estimation unit of the second embodiment. Note that, instead of the estimated self-aligning torque map illustrated in <FIG>, for example, an expression representing the relation between the steering angle θh and the self-aligning torque TSAT at the alternative speed may be used for the relation between the steering angle θh and the self-aligning torque TSAT.

The vehicle motion estimation unit 281a outputs estimated self-aligning torque Test in accordance with the steering angle θh by using the estimated self-aligning torque map (or the expression representing the relation between the steering angle θh and the self-aligning torque TSAT at the alternative speed).

The estimated self-aligning torque Test output from the vehicle motion estimation unit 281a, the vehicle speed determination signal Vfail, and the self-aligning torque TSAT are input to the torque gain setting unit 282a. The torque gain setting unit <NUM> generates the torque gain AG based on the estimated self-aligning torque Test, the vehicle speed determination signal Vfail, and the self-aligning torque TSAT.

Specifically, the torque gain setting unit 282a determines whether the vehicle speed Vs is normally detected, in other words, whether the vehicle speed Vs is the alternative vehicle speed based on the vehicle speed determination signal Vfail. When the vehicle speed Vs is the alternative vehicle speed, the torque gain setting unit 282a generates the torque gain AG in accordance with the absolute value |Test - TSAT| of the difference between the estimated self-aligning torque Test and the self-aligning torque TSAT. In the present embodiment, the torque gain setting unit 282a holds a predetermined threshold value E for the absolute value |Test - TSAT| of the difference between the estimated self-aligning torque Test and the self-aligning torque TSAT.

<FIG> is an explanatory diagram of specific operation at the torque gain setting unit of the second embodiment. In the example illustrated in <FIG>, a solid line represents the absolute value |Test| of the estimated self-aligning torque Test. In addition, in the example illustrated in <FIG>, a dashed line represents a value smaller than the absolute value |Test| of the estimated self-aligning torque Test by the predetermined threshold value E.

The torque gain setting unit 282a reduces the torque gain AG when the vehicle speed Vs is the alternative vehicle speed and the absolute value |Test - TSAT| of the difference between the estimated self-aligning torque Test and the self-aligning torque TSAT is equal to or larger than the threshold value E.

In the example illustrated in <FIG>, the absolute value of the steering angle θh is |θh1| and the absolute value of the self-aligning torque TSAT is |TSAT<NUM>|. <FIG> illustrates an example in which the absolute value |Test - TSAT| of the difference between the estimated self-aligning torque Test and the self-aligning torque TSAT is equal to or larger than the threshold value E (|γest - γre| ≥ E).

The torque gain AG of the second embodiment is expressed in Expression (<NUM>) below. In Expression (<NUM>) below, a coefficient "D" is a real number equal to or larger than one.

When the vehicle speed Vs is the alternative vehicle speed and |Test - TSAT| ≥ E is satisfied, the torque gain setting unit 282a sets the torque gain AG to be smaller than one. In other words, the coefficient "D" indicated in Expression (<NUM>) above is set to be a value larger than one.

Note that, when the vehicle speed Vs is normally detected, the torque gain setting unit 282a sets the torque gain AG to be one. The torque gain setting unit 282a sets the torque gain AG to be one also when the vehicle speed Vs is the alternative vehicle speed and the absolute value |Test - TSAT| of the difference between the estimated self-aligning torque Test and the self-aligning torque TSAT is smaller than the threshold value E (|Test - TSAT| < E). In other words, the coefficient "D" indicated in Expression (<NUM>) above is set to be one.

<FIG> is a diagram illustrating exemplary processing at the vehicle speed failure processing unit of the second embodiment.

The torque gain setting unit 282a determines whether the vehicle speed Vs is the alternative vehicle speed based on the vehicle speed determination signal Vfail (step S201).

When the vehicle speed Vs is not the alternative vehicle speed (No at step S201), in other words, when the vehicle speed Vs is normally detected, the torque gain setting unit 282a sets the coefficient "D" in the torque gain AG = <NUM>/D to be one (step S203), and ends the processing.

When the vehicle speed Vs is the alternative vehicle speed (Yes at step S202), the vehicle motion estimation unit 281a outputs the estimated self-aligning torque Test in accordance with the steering angle θh by using the estimated self-aligning torque map illustrated in, for example, <FIG> (step S202).

The torque gain setting unit 282a calculates the absolute value |Test - TSAT| of the difference between the estimated self-aligning torque Test and the self-aligning torque TSAT (step S204).

Subsequently, the torque gain setting unit 282a determines whether the absolute value |Test - TSAT| of the difference between the estimated self-aligning torque Test and the self-aligning torque TSAT is equal to or larger than the predetermined threshold value E (|Test - TSAT| ≥ E) (step S205).

When the absolute value |Test - TSAT| of the difference between the estimated self-aligning torque Test and the self-aligning torque TSAT is smaller than the threshold value E (|Test - TSAT| < E) (No at step S205), the torque gain setting unit 282a sets the coefficient "D" in the torque gain AG = <NUM>/D to be one (step S203), and ends the processing.

When the absolute value |Test - TSAT| of the difference between the estimated self-aligning torque Test and the self-aligning torque TSAT is equal to or larger than the threshold value E (|Test - TSAT| ≥ E) (Yes at step S205), the torque gain setting unit 282a sets the coefficient "D" in the torque gain AG = <NUM>/D to be a predetermined value larger than one (step S206), and ends the processing.

The multiplication unit <NUM> multiplies the torque signal Tref_a0 output from the basic map unit <NUM> by the torque gain AG output from the vehicle speed failure processing unit 280a, and outputs a result of the multiplication as the torque signal Tref_a to the addition unit <NUM>.

The torque signals Tref_a, Tref_b, Tref_c, and Tref_d obtained as described above are added at the addition units <NUM>, <NUM>, and <NUM> and output as the target steering torque Tref.

As described above, effects same as those of the first embodiment can be obtained with a configuration in which the self-aligning torque is employed as the physical quantity generated through turning motion of the vehicle, in place of the yaw rate described in the first embodiment. Specifically, when the vehicle speed failure processing unit 280a of the second embodiment is employed, it is possible to prevent generation of excessive steering torque that causes self-steering during stopping due to assist control in a state in which the vehicle speed sensor <NUM> fails and the alternative vehicle speed (for example, <NUM> [km/h]) is output as the vehicle speed Vs.

Note that, the vehicle speed failure processing unit <NUM> of the first embodiment may be employed in place of the vehicle speed failure processing unit 280a of the second embodiment. In this case, the yaw rate or the lateral acceleration may be employed as the physical quantity generated through turning motion of the vehicle, in place of the self-aligning torque.

The twist angle control unit 300a of the second embodiment will be described below with reference to <FIG>.

<FIG> is a block diagram illustrating an exemplary configuration of the twist angle control unit of the second embodiment. The twist angle control unit 300a calculates the motor current command value Imc based on the target twist angle Δθref, the twist angle Δθ, and the motor angular velocity ωm. The twist angle control unit 300a includes the twist angle feedback (FB) compensation unit <NUM>, a twist angular velocity calculation unit <NUM>, the speed control unit <NUM>, the stabilization compensation unit <NUM>, the output restriction unit <NUM>, the subtraction unit <NUM>, and an addition unit <NUM>.

The target twist angle Δθref output from the conversion unit <NUM> is input to the subtraction unit <NUM> through addition. The twist angle Δθ is input to the subtraction unit <NUM> through subtraction and input to the twist angular velocity calculation unit <NUM>. The motor angular velocity ωm is input to the stabilization compensation unit <NUM>.

The twist angle FB compensation unit <NUM> multiplies the deviation Δθ0 between the target twist angle Δθref and the twist angle Δθ, which is calculated at the subtraction unit <NUM>, by the compensation value CFB (transfer function) and outputs a target twist angular velocity ωref with which the twist angle Δθ follows the target twist angle Δθref. The compensation value CFB may be a simple gain Kpp, or a typically used compensation value such as a PI control compensation value.

The target twist angular velocity ωref is input to the speed control unit <NUM>. With the twist angle FB compensation unit <NUM> and the speed control unit <NUM>, it is possible to cause the twist angle Δθ to follow the target twist angle Δθref, thereby achieving desired steering torque.

The twist angular velocity calculation unit <NUM> calculates the twist angular velocity ωt by performing differential arithmetic processing on the twist angle Δθ. The twist angular velocity ωt is output to the speed control unit <NUM>. The twist angular velocity calculation unit <NUM> may perform, as differential calculation, pseudo differentiation with a HPF and a gain. Alternatively, the twist angular velocity calculation unit <NUM> may calculate the twist angular velocity ωt by another means or not from the twist angle Δθ and may output the calculated twist angular velocity ωt to the speed control unit <NUM>.

The speed control unit <NUM> calculates, by I-P control (proportional preceding PI control), a motor current command value Imca1 with which the twist angular velocity ωt follows the target twist angular velocity ωref.

The subtraction unit <NUM> calculates the difference (ωref - ωt) between the target twist angular velocity ωref and the twist angular velocity ωt. The integral unit <NUM> integrates the difference (ωref - ωt) between the target twist angular velocity ωref and the twist angular velocity ωt, and inputs a result of the integration to the subtraction unit <NUM> through addition.

The twist angular velocity ωt is also output to the proportional unit <NUM>. The proportional unit <NUM> performs proportional processing with the gain Kvp on the twist angular velocity ωt and inputs a result of the proportional processing to the subtraction unit <NUM> through subtraction. A result of the subtraction at the subtraction unit <NUM> is output as the motor current command value Imca1. Note that, the speed control unit <NUM> may calculate the motor current command value Imca1 not by I-P control but by typically used control method such as PI control, P (proportional) control, PID (proportional-integral-differential) control, PI-D control (differential preceding PID control), model matching control, or model reference control.

The stabilization compensation unit <NUM> has a compensation value Cs (transfer function) and calculates a motor current command value Imca2 from the motor angular velocity ωm. When gains of the twist angle FB compensation unit <NUM> and the speed control unit <NUM> are increased to improve the following capability and the disturbance characteristic, a controlled oscillation phenomenon occurs in a higher range. To avoid this, the transfer function (Cs) necessary for stabilization of the motor angular velocity ωm is set to the stabilization compensation unit <NUM>. Accordingly, stabilization of the entire EPS control system can be achieved.

The addition unit <NUM> adds the motor current command value Imca1 from the speed control unit <NUM> and the motor current command value Imca2 from the stabilization compensation unit <NUM>, and outputs a result of the addition as a motor current command value Imcb.

The upper and lower limit values of the motor current command value Imcb are set to the output restriction unit <NUM> in advance. The output restriction unit <NUM> outputs the motor current command value Imc with restriction on the upper and lower limit values of the motor current command value Imcb.

Note that, the configuration of the twist angle control unit 300a in the present embodiment is exemplary and may be different from the configuration illustrated in <FIG>. For example, the twist angle control unit 300a may not include the stabilization compensation unit <NUM>.

Although the present disclosure is applied to a column-type EPS as one vehicle steering device in the first and second embodiments, the present disclosure is not limited to an upstream-type EPS such as a column-type EPS and is applicable to a downstream-type EPS such as a rack-pinion EPS. Moreover, since feedback control is performed based on a target twist angle, the present disclosure is also applicable to, for example, a steer-by-wire (SBW) reaction force device including at least a torsion bar (with an optional spring constant) and a twist angle detection sensor. The following describes an embodiment (third embodiment) when the present disclosure is applied to a SBW reaction force device including a torsion bar.

First, the entire SBW system including a SBW reaction force device will be described below. <FIG> is a diagram illustrating an exemplary configuration of the SBW system in a manner corresponding to the typical configuration of the electric power steering device illustrated in <FIG>. Note that, a component same as that in the configuration described above in the first and second embodiments is denoted by the same reference sign and detailed description thereof is omitted.

The SBW system is a system that includes no intermediate shaft mechanically connected with the column shaft <NUM> at the universal joint 4a in <FIG> and conveys an operation of the wheel <NUM> to a rotation mechanism constituted by the steering wheels <NUM> and 8R and the like through an electric signal. As illustrated in <FIG>, the SBW system includes a reaction force device <NUM> and a drive device <NUM>, and a control unit (ECU) <NUM> controls the devices. The reaction force device <NUM> performs detection of the steering angle θh at the rudder angle sensor <NUM> and simultaneously transfers, to the driver as reaction force torque, a motion state of the vehicle conveyed from the steering wheels <NUM> and 8R. The reaction force torque is generated by a reaction force motor <NUM>. Note that, although some SBW systems include no torsion bar in the reaction force device, a SBW system to which the present disclosure is applied includes a torsion bar, and the steering torque Ts is detected at the torque sensor <NUM>. In addition, an angle sensor <NUM> detects the motor angle θm of the reaction force motor <NUM>. The drive device <NUM> drives a drive motor <NUM> in accordance with steering of the wheel <NUM> by the driver and provides drive power thereof to the pinion rack mechanism <NUM> through a gear <NUM> to rotate the steering wheels <NUM> and 8R through the tie rods 6a and 6b. An angle sensor <NUM> is disposed near the pinion rack mechanism <NUM> and detects a turning angle θt of the steering wheels <NUM> and 8R. For cooperative control of the reaction force device <NUM> and the drive device <NUM>, the ECU <NUM> generates a voltage control command value Vref1 with which the reaction force motor <NUM> is driven and controlled and a voltage control command value Vref2 with which the drive motor <NUM> is driven and controlled, based on, for example, the vehicle speed Vs from the vehicle speed sensor <NUM> in addition to information such as the steering angle θh and the turning angle θt output from the devices.

The following describes the configuration of the third embodiment in which the present disclosure is applied to such a SBW system.

<FIG> is a block diagram illustrating the configuration of the third embodiment. In the third embodiment, control (hereinafter referred to as "twist angle control") on the twist angle Δθ and control (hereinafter referred to as "turning angle control") on the turning angle θt are performed to control the reaction force device by the twist angle control and to control the drive device by the turning angle control. Note that, the drive device may be controlled by another control method.

A target steering torque generation unit 200b generates the target steering torque Tref based on the vehicle speed Vs, the vehicle speed determination signal Vfail, the steering angle θh, and the real yaw rate γre. The conversion unit <NUM> converts the target steering torque Tref generated at the target steering torque generation unit 200b into the target twist angle Δθref. The target twist angle Δθref is output to the twist angle control unit <NUM>. In the twist angle control, such control that the twist angle Δθ follows the target twist angle Δθref calculated through the target steering torque generation unit 200b and the conversion unit <NUM> by using the steering angle θh and the like is performed with configurations and operations same as those of the second embodiment. The motor angle θm is detected at the angle sensor <NUM>, and the motor angular velocity ωm is calculated by differentiating the motor angle θm at an angular velocity calculation unit <NUM>. The turning angle θt is detected at the angle sensor <NUM>. In addition, although detailed description is not performed as processing in the EPS steering system/vehicle system <NUM> in the first embodiment, a current control unit <NUM> performs current control by driving the reaction force motor <NUM> based on the motor current command value Imc output from the twist angle control unit 300a and a current value Imr of the reaction force motor <NUM> detected at a motor current detector <NUM> with configurations and operations same as those of the subtraction unit 32B, the PI control unit <NUM>, the PWM control unit <NUM>, and the inverter <NUM> illustrated in <FIG>.

In the turning angle control, a target turning angle θtref is generated based on the steering angle θh at a target turning angle generation unit <NUM>, the target turning angle θtref together with the turning angle θt is input to a turning angle control unit <NUM>, and a motor current command value Imct with which the turning angle θt is equal to the target turning angle θtref is calculated at the turning angle control unit <NUM>. Then, a current control unit <NUM> performs current control by driving the drive motor <NUM> based on the motor current command value Imct and a current value Imd of the drive motor <NUM> detected at a motor current detector <NUM> with configurations and operations same as those of the current control unit <NUM>.

<FIG> is a diagram illustrating an exemplary configuration of the target turning angle generation unit. The target turning angle generation unit <NUM> includes a restriction unit <NUM>, a rate restriction unit <NUM>, and a correction unit <NUM>.

The restriction unit <NUM> outputs a steering angle θh1 with restriction on the upper and lower limit values of the steering angle θh. Similarly to the output restriction unit <NUM> in the twist angle control unit 300a illustrated in <FIG>, the upper and lower limit values of the steering angle θh are set in advance and restricted.

To avoid abrupt change of the steering angle, the rate restriction unit <NUM> provides restriction by setting a restriction value for the change amount of the steering angle θh1, and outputs the steering angle θh2. For example, the change amount is set to be the difference from the steering angle θh1 at the previous sample. When the absolute value of the change amount is larger than a predetermined value (restriction value), the steering angle θh1 is increased or decreased so that the absolute value of the change amount becomes equal to the restriction value, and the increased or decreased steering angle θh1 is outputs as the steering angle θh2. When the absolute value of the change amount is equal to or smaller than the restriction value, the steering angle θh1 is directly output as the steering angle θh2. Note that, restriction may be provided by setting the upper and lower limit values of the change amount instead of setting the restriction value for the absolute value of the change amount, or restriction may be provided on a change rate or a difference rate in place of the change amount.

The correction unit <NUM> corrects the steering angle θh2 and outputs the target turning angle θtref. For example, as in a case of the basic map unit <NUM> in the target steering torque generation unit 200b, the target turning angle θtref is calculated from the steering angle θh2 by using a map that defines a characteristic of the target turning angle θtref for the magnitude |θh2| of the steering angle θh2. Alternatively, the target turning angle θtref may be calculated by simply multiplying the steering angle θh2 by a predetermined gain.

<FIG> is a diagram illustrating an exemplary configuration of the turning angle control unit. The configuration of the turning angle control unit <NUM> is same as the exemplary configuration of the twist angle control unit 300a illustrated in <FIG> from which the stabilization compensation unit <NUM> and the addition unit <NUM> are removed, the target turning angle θtref and the turning angle θt are input in place of the target twist angle Δθref and the twist angle Δθ, and the configurations and operations of a turning angle feedback (FB) compensation unit <NUM>, a turning angular velocity calculation unit <NUM>, a speed control unit <NUM>, an output restriction unit <NUM>, and a subtraction unit <NUM> are same as those of the twist angle FB compensation unit <NUM>, the twist angular velocity calculation unit <NUM>, the speed control unit <NUM>, the output restriction unit <NUM>, and the subtraction unit <NUM>, respectively.

Exemplary operation of the third embodiment in such a configuration will be described below with reference to a flowchart in <FIG> is a flowchart illustrating the exemplary operation of the third embodiment.

Once operation is started, the angle sensor <NUM> detects the turning angle θt and the angle sensor <NUM> detects the motor angle θm (step S110), and the turning angle θt and the motor angle θm are input to the turning angle control unit <NUM> and the angular velocity calculation unit <NUM>, respectively.

The angular velocity calculation unit <NUM> calculates the motor angular velocity ωm by differentiating the motor angle θm and outputs the calculated motor angular velocity ωm to the twist angle control unit 300a (step S120).

Thereafter, the target steering torque generation unit 200b executes operation same as that at steps S10 to S40 illustrated in <FIG> to perform current control by driving the reaction force motor <NUM> (steps S130 to S160).

In the turning angle control, the target turning angle generation unit <NUM> receives the steering angle θh, and the steering angle θh is input to the restriction unit <NUM>. The restriction unit <NUM> restricts the upper and lower limit values of the steering angle θh to upper and lower limit values set in advance (step S170) and outputs the steering angle θh as the steering angle θh1 to the rate restriction unit <NUM>. The rate restriction unit <NUM> restricts the change amount of the steering angle θh1 based on a restriction value set in advance (step S180) and outputs the steering angle θh1 as the steering angle θh2 to the correction unit <NUM>. The correction unit <NUM> obtains the target turning angle θtref by correcting the steering angle θh2 (step S190) and outputs the target turning angle θtref to the turning angle control unit <NUM>.

Having received the turning angle θt and the target turning angle θtref, the turning angle control unit <NUM> calculates a deviation Δθt0 by subtracting the turning angle θt from the target turning angle θtref at the subtraction unit <NUM> (step S200). The deviation Δθt0 is input to the turning angle FB compensation unit <NUM>, and the turning angle FB compensation unit <NUM> compensates the deviation Δθt0 by multiplying the deviation Δθt0 by a compensation value (step S210) and outputs a target turning angular velocity ωtref to the speed control unit <NUM>. The turning angular velocity calculation unit <NUM> receives the turning angle θt, calculates a turning angular velocity wtt through differential calculation on the turning angle θt (step S220) and outputs the turning angular velocity wtt to the speed control unit <NUM>. Similarly to the speed control unit <NUM>, the speed control unit <NUM> calculates a motor current command value Imcta by I-P control (step S230) and outputs the motor current command value Imcta to the output restriction unit <NUM>. The output restriction unit <NUM> restricts the upper and lower limit values of the motor current command value Imcta to upper and lower limit values set in advance (step S240) and outputs the motor current command value Imcta as the motor current command value Imct (step S250).

The motor current command value Imct is input to the current control unit <NUM>, and the current control unit <NUM> performs current control by driving the drive motor <NUM> based on the motor current command value Imct and the current value Imd of the drive motor <NUM> detected by the motor current detector <NUM> (step S260).

Note that, the order of data input, calculation, and the like in <FIG> may be changed as appropriate. Similarly to the speed control unit <NUM> in the twist angle control unit 300a, the speed control unit <NUM> in the turning angle control unit <NUM> may perform PI control, P control, PID control, PI-D control, or the like in place of I-P control and only needs to perform any of P control, I control, and D control, and following control at the turning angle control unit <NUM> and the twist angle control unit 300a may be performed in a typically used control structure. The turning angle control unit <NUM> is not limited to a control configuration used for a vehicle device but may have any control configuration with which a real angle (in this example, the turning angle θt) follows a target angle (in this example, the target turning angle θtref), and for example, may have a control configuration used for an industrial positioning device, an industrial robot, or the like.

In the third embodiment, one ECU <NUM> controls the reaction force device <NUM> and the drive device <NUM> as illustrated in <FIG>, but an ECU for the reaction force device <NUM> and an ECU for the drive device <NUM> may be provided. In this case, the ECUs perform data transmission and reception through communication. In addition, although the SBW system illustrated in <FIG> has no mechanical connection between the reaction force device <NUM> and the drive device <NUM>, the present disclosure is also applicable to a SBW system including a mechanical torque transmission mechanism configured to mechanically connect the column shaft <NUM> and the rotation mechanism through a clutch or the like when anomaly has occurred to the system. In such a SBW system, when the system is normal, the clutch is turned off to set mechanical torque transfer to an open state, or when the system is anomalous, the clutch is turned on to set mechanical torque transfer to an enabled state.

The twist angle control units <NUM> and 300a in the above-described first to third embodiments directly calculate the motor current command value Imc and an assist current command value Iac, but before calculating the motor current command value and the assist current command value, may first calculate motor torque (target torque) to be output. In this case, a typically used relation between motor current and motor torque is used to calculate the motor current command value and the assist current command value from the motor torque.

Claim 1:
A vehicle steering device configured to assist and control a steering system (<NUM>) of a vehicle by driving and controlling a motor configured to assist steering force, comprising:
a vehicle speed sensor that outputs, as a vehicle speed signal, a pulse signal in accordance with a vehicle speed of the vehicle, wherein
when the vehicle speed sensor fails and the vehicle speed signal is not normally output, control using a predetermined alternative vehicle speed is performed instead of the vehicle speed signal, and target steering torque is reduced in accordance with the absolute value of the difference between a physical quantity generated through turning motion of the vehicle and an estimated value of the physical quantity at the alternative vehicle speed.