JPS6328763A - Steering device for vehicle - Google Patents

Abstract

PURPOSE:To control the steering response speed of a wheel to an optimum value, by a method wherein, even when turbulence and the like is not exerted on the wheel, control is made so that an actual steering angle is prevented from the occurrence of overshoot from a target steering angle. CONSTITUTION:A target steering angle computing means 610 calculates a target steering angle thetac from signals from state detecting groups 1a-1c, and an actual steering angle computing means 611 and a steering angle computing means 612 calculate an actual steering angle thetar and a steering angle velocity X2 from a signal from a rear wheel steering angle sensor 2. An error computing means 613 calculates the error of each of the target steering angle thetac and the steering angle thetar, and a switching function correcting means 618 determines a factor C according to the state detecting groups 1a-1c. A gain switching function computing means 614 determines switching functions L1 and L2 from an error X1, a steering angle velocity X2, and the factor C, based on the L1 and L2 a final command value computing means 617 is actuated through a fundamental computing means 615, a correction item computing means to output a given signal to a drive circuit 180. The drive circuit 180 outputs an average current I to a servo motor 5 to steer a steering mechanism 3.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention is a device for electrically steering front or rear wheels of a vehicle.

[Conventional technology]

Various types of steering devices for vehicles have been devised and developed, including those that control servo pulp using conventional hydraulic mechanisms, and those that utilize DC servo motors and step motors. In devices that control the servo pulp using a hydraulic mechanism or devices that use a servo motor, the servo pulp or servo motor is often driven with a current proportional to the error between the target steering angle and the actual steering angle, so-called P I D 1tllJl,
Alternatively, state feedback control or the like may be used.

[Problems to be solved by the invention] However, in PID control, when trying to increase the response, overshoot occurs with respect to the target, and for a moving vehicle, overshoot with respect to the target steering angle is a problem due to the dynamic characteristics of the vehicle. It becomes very dangerous. Furthermore, if the control is performed to eliminate overshoot, the steering of the wheels may become meaningless due to a delay in response, or the motion characteristics may worsen.

This kind of problem can be countered with PID control by feeding back various signals in a minor loop, but depending on the signal that needs to be fed back, adding a sensor or
The addition of circuits and the like makes the system and control circuit configuration extremely complicated, and the number of parameters required to match desired characteristics increases dramatically.

In addition, state feed control also requires accurate system identification of the target system, and as with PID control, problems such as overshoot and response delay occur due to system parameter fluctuations. Here too, a countermeasure can be taken by sequentially changing control gains and the like through real-time system identification using an adaptive control method, but as with PID control, there is a problem in that the system configuration and control circuit become extremely complicated.

Also, even if a step motor is used, what about the motor drive circuit? ! ! It becomes complicated, and if you try to improve responsiveness, the system configuration and control circuit will become complicated to prevent synchronization.

In view of the above points, the present invention uses a simple system configuration and control circuit to arbitrarily change the steering response speed of the wheels according to the driving condition of the vehicle in order to improve the running stability and steering performance of the vehicle. And in any case, with respect to the target steering angle,
The object of the present invention is to provide a vehicle steering system that has a control system that is robust in terms of overshoot.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a steering mechanism equipped with a servo mechanism that adjusts the steering angle of the wheels in response to an electrical command value, as shown in FIG. (M9), a state detection means (Ml) for detecting the running state of the vehicle, and a steering angle detection means (M2) for detecting the actual steering angle of the steering mechanism and outputting an actual steering angle (Or) signal; The target steering angle (
a target steering angle calculation means (M4) for calculating the target steering angle (θc); an error calculation procedure (M6) for calculating the error (Xl) from the target steering angle (θc) and the actual steering angle (Or); and a steering angular velocity of the wheels (M6). a steering angular velocity detecting means (M3) for determining X2); and determining a switching function that satisfies a condition for both to converge to zero from the error (X1) and the steering angular velocity (X2), and determining the switching function based on the switching function. a control means (M7) that outputs an electrical command value to converge the error (X1) and the steering angular velocity (X2) to zero while being constrained by the switching line determined by the state detector; The switching function correction means (~15) corrects the switching function accordingly, and the driving means (M8) drives the servo mechanism according to the electrical command value from the control means. Vehicle steering device.

[Operation]

According to the above configuration, the present invention reduces the error (Xl) and steering angular velocity (x) between the target steering angle (θc) and the actual steering angle (Or).
2), control '4:[0 means M7, this error (X
1) and the steering angular velocity (X2), and based on this, find the switching line (while being constrained by the switching line, calculate the error (X1) and the steering angular velocity). An electrical command value (I f) is output in order to converge the angular velocity (X2) to zero, and in response to this, the driving means (M8) drives the servo mechanism of the steering mechanism M9 to adjust the steering angle of the wheels. Here, the switching function can be corrected by a switching function correction means M in accordance with a state signal from a state detector, such as vehicle speed.

〔Effect of the invention〕

As described above, the present invention deals with the error (X1) between the target steering angle (θc) and the actual steering angle (Or), and the steering angular velocity (X2).
converges toward zero while being constrained by the switching line, the actual steering angle can be controlled without overshooting with respect to the target steering angle (θc) even if a disturbance or the like acts on the wheels. can. Moreover, since the switching function can be corrected according to the driving condition of the vehicle, for example, the vehicle speed, etc., there is an effect that the steering angular velocity, that is, the steering response speed of the wheels can be optimally controlled according to the vehicle speed, etc. .

(Example) An example of the present invention will be described based on the drawings.Here,
An example of a four-wheel steering vehicle to which the present invention is applied to rear wheel steering is shown.

In Fig. 2, a DC servo motor 5 serving as a servo mechanism is shown.
rotates in forward and reverse directions in response to an electrical command value signal from the electrical control device 6, and is connected to the input shaft (not shown) of the rank-and-binion mechanism 3 with hydraulic power assist, that is, the input shaft of the steering mechanism (not shown), through the '$i speed gear 4 bar). A pinion gear 3b is attached to the other end of the torsion bar, and meshes with a rank 3C formed at one end of the power piston 3a. That is, one end of the torsion bar is rotated by the motor 5, the torsion bar is twisted, the throttle area of the hydraulic valve 4a is changed, and hydraulic pressure is supplied in a direction to correct the twist of the torsion bar to move the power piston 3a. ing. Both ends of the power piston 3a are each connected to a knuckle arm 3C via tie rods 3b. Rear wheel 8 is knuckle arm 3
It is supported by C so as to be swingable in the left and right direction. Therefore, by moving the power piston 3a in the direction of the arrow in the figure, the rear wheels 8 are steered left and right. When the torsion bar is no longer twisted, the throttle area of the hydraulic valve 4a becomes O, and the power piston does not move and the oil pressure becomes zero.

Here, the rear wheel steering angle can be determined from the relationship between the position of the power piston 3a and the rear wheel steering angle by detecting the position of the power piston 3a with the rear wheel steering angle sensor 2. Further, the actual rear wheel steering angular velocity can also be calculated from the rate of change in the position of the power piston 3a.

Note that 7a is connected to the power piston 3a via the hydraulic valve 4a.
7b indicates an oil tank.

1a to IC are sensors serving as state detection means for detecting the driving and traveling states of the vehicle, and output detection signals to the electrical control device 6. 1a is a front wheel steering angle sensor that detects the rotation of the steering wheel 10 and outputs a front wheel steering angle signal according to the steering angle θ of the front wheels 9; 1b is a front wheel steering angle sensor that detects the rotational speed of an axle or a wheel and outputs a front wheel steering angle signal according to the vehicle speed ■ The vehicle speed sensor 1c that outputs a signal is a yaw rate sensor that is composed of a gyro or the like and outputs a yaw rate signal corresponding to the rotational angular velocity (yaw rate θy) of the vehicle centered on the center of gravity of the vehicle.

The control device 6 will be explained based on the block diagram of FIG.

The control device 6 includes a waveform shaping circuit 61 for waveform-shaping the vehicle speed signal from the vehicle speed sensor 1b and inputting it into the microcomputer 60, and each signal from the rear wheel steering angle sensor 2, front wheel steering angle sensor las, and yaw rate sensor lc. It is composed of an analog buffer 63 for importing, an A/D converter 64 for performing analog-to-digital conversion, and a drive circuit 180 for driving the DC servo motor 5 in accordance with a command value signal from the microcomputer 60.

Next, the processing procedures of the control device 6 and the microcomputer 60 will be explained as each means based on FIG. The microcomputer 60 includes a target steering angle calculation means 610 that calculates a target steering angle θc of the rear wheels according to the status signals from the status detector group 1 (front wheel steering angle sensor 1a, vehicle speed sensor 1b, yaw rate sensor IC); Actual steering angle calculation means 611 calculates the actual rear wheel steering angle θr based on the signal from the rear wheel steering angle sensor 2, and the steering angular velocity is calculated from the rate of change of the signal from the rear wheel steering angle sensor 2, that is, the temporal differential value. A steering angular velocity calculation means 612 that calculates x2, and an error calculation means 61 that calculates an error x1 from the target steering angle θc and the actual steering angle θr.
3, and according to the vehicle speed signal etc. from the vehicle speed sensor 1b,
It is comprised of a switching failure function correction means 618 that calculates and corrects a coefficient C of a switching function in a control means 650, which will be described later.

The control means 650 controls the error Xi and the steering angular velocity X2.
From this, find the switching functions Ll and L2 that satisfy the conditions for both to converge to zero, and converge the error X1 and the steering angular velocity x2 to zero while being constrained by the switching line Z found based on this switching function. An electrical command value If is output to the drive circuit 180 in order to do so.

Hereinafter, each of the above-mentioned means will be explained in more detail. The target steering angle calculating means 610 calculates the target stage steering angle θc according to the vehicle speed (2) as shown in FIG. Note that when the target steering angle θc in the figure is positive, it indicates that the front and rear wheels are steered in the same direction (same phase), and when it is negative, it indicates that the wheels are in opposite phases. Incidentally, FIG. 4 shows the relationship when the front wheel steering angle θf and the yaw rate signal have a certain value. This target steering angle θc is calculated according to the vehicle speed ■ and the front wheel steering angle θf so that the ratio of the rear wheel target steering angle θc to the front wheel steering angle θf (target steering ratio: θc/θf) becomes a predetermined ratio. , and at a predetermined vehicle speed ■o or higher, the yaw rate value θy from the yaw rate sensor IC is corrected to obtain the desired yaw rate gain (ratio of the yaw rate value θy to the front wheel steering angle θf). The actual steering angle calculation means 611
The actual steering angle θr is calculated from the position of the power piston 3a based on a data map that has been previously measured and stored as a correspondence between the position of a and the rear wheel steering angle, or on the basis of a calculation formula showing this correspondence.

The switching function correction means 618 calculates a coefficient C (positive constant) according to the vehicle speed ■, for example, as shown in FIG. 6 ta+. In this way, the coefficient C is almost constant in the low speed range where the vehicle speed ■ is up to the predetermined speed ■1, and in the medium speed range from ■1 to ■2 the vehicle speed ■
It increases as the speed increases, and decreases in the high speed range above the predetermined speed (2). Note that the coefficient C is set below a predetermined upper limit value C for stably controlling the rear wheel steering. In addition to the characteristics shown in FIG. 6(a), the coefficient C may be freely set so as to obtain desired vehicle motion characteristics. For example, in order to perform more precise control 1111, the coefficient C may be obtained from a two-dimensional map stored in advance based on the vehicle speed 2 and the front wheel steering angular velocity θf as shown in FIG.

At this time, the front wheel steering angular velocity θr is calculated by the front wheel steering angular velocity calculation means 619 from the signal from the front wheel steering angle sensor 1a, as shown in FIG.

The control means 650 includes gain switching function calculation means 614
, basic type calculation means 615, correction term calculation means 616, and final command value calculation means 617. The gain switching function calculating means 614 calculates the following equation +11. from the error x1 between the target steering angle θc and the actual steering angle θr, the steering angular velocity X2, and the coefficient C. gain switching function Ll shown in f2)
, L2 is calculated.

(1) ... L1=C-Xi-X2(2) ・
...L2=L1.XI Here, the coefficient C is a coefficient calculated by the switching function correction means 618 described above.

The basic type calculation means 615 calculates the basic term parameters shown in the following equation (3) and (41) based on the positive and negative values of the gain switching function L2.
Calculate the housing Ib.

+31 ・When L2≧0 1b=Ga・Xl(4)
... When L2<0 1b=Gb·X2 Here, the constants Ga and Gb are constants set to stably control the rear wheels.

The correction term calculating means 616 calculates a correction term 1m expressed by the following equations (51, (6)) based on the positive or negative of the gain switching function L1.
Calculate.

(5)... When L1≧0 1 m=M[6) =
When −L 1 < O, Im=−M where constant M is M≧
A constant of 0, which is used to suppress disturbances received by the rear wheels, and is set to a value larger than the predicted disturbance value that is expected to be applied to the rear wheel steering control system such as the rear wheel steering mechanism and servo mechanism. Disturbances are suppressed. The expected disturbance value corresponds to the current value that should be input to the servo motor 5 in order to suppress the disturbance when the disturbance received by the wheels is converted into the rotation of the motor shaft of the servo motor 5.

The final command value calculation means 617 calculates the final command value If expressed by the following equation (7) based on the basic model If and the correction term 1m.

(71-1f = l b + l mThis command value If
is output to the drive circuit 180 as a current command value, that is, a duty ratio.

Next, the overall operation of the control means 650 will be explained based on FIG. 7 ta+. FIG. 7(a) is a diagram showing a phase plane trajectory with error X1 on the X axis and steering angular velocity x2 on the Y axis, where the origin O is the error Xi and the steering angular velocity x2.
When both are zero, that is, the target steering angle θc is indicated, and the solid line Z indicates the switching line obtained as the gain switching function L1=O. Therefore, the shaded area in the figure is L2=L1・X1
Indicates an area where ≧0. When the state determined by the error
5 is the basic type! b is calculated as Ib=Ga-Xi, and when it comes to other areas, basic type 1b is calculated as Ib-Gb-X
Calculate as I. Further, the above state is Ll(=C-XI-
When it is in the region of X2)≧O, that is, the above switching line Z
When it is in the lower region, the correction term calculating means 616 calculates the correction term 1m as Im=M, and when it is in the region above the switching vAZ, the correction term 1m is calculated as Im=−M.

Here, for example, if the command value or the target steering angle θc changes and the error x 1 between the actual steering angle θr and the target steering angle θc becomes XIO [this state is the state of FIG.
IO, O)], the drive circuit 180 supplies a positive current (If>0) to the servo motor 5 according to the final command value If (=Ga・X1+M) obtained based on the above calculation.
Power the iJ1 and steer the steering groove. Then, the steering angular velocity X2 of the rear wheels rapidly increases (accelerates) from zero, while the error X1 decreases. Then, when reaching the state point P2 beyond the switching cotton Z, the basic type 1b and the correction term 1m are each Gb・
×1 and -M, new command value If (=Gb-X
The current changes to a reverse current (If<O) according to I-M), and reaches state point P3. When the switching line Z is crossed and the state point P3 is reached, a positive current based on 1f=Ga-Xi0M is applied to the servo motor 5 again, and the state point P4 is reached. In this way, the state is controlled so that it converges to the origin (0,0), that is, to the target steering angle θc, without overshoot while drawing a trajectory constrained by the switching line Z. Note that even if the parameters vary due to other causes and deviate from the state points PI and P2, convergence control to the target steering angle θc will be performed without overshoot while being restrained by the switching line Z.

In addition, the case where there is an overshoot in FIG. 7 (al) is indicated by point vAR.
If the control described in 2 is not performed, the error X1 goes from positive to zero at one end, then repeats negative, zero, and positive again, and then converges to the origin. In other words, it can be seen that after exceeding the target steering angle and overshooting, the vehicle overshoots again in the opposite direction.

Next, based on FIG. 7(b), gain switching function L1
The case where the coefficient C of is corrected by the switching function correction means 618 will be explained. The coefficient C is set to a larger coefficient C'(C'
>C>0), the switching line Z becomes a switching eAz' with a large slope.

Then, the state point P1 exceeds the above-mentioned state point P2 and reaches the state point Q3, which is constrained by the switching line Z', and then the state point Q
4. It reaches the origin while being restrained by Q5 and switching line Z'. In other words, the state point Q is a state where the steering angular velocity X2 is larger and the error X1 is smaller than the state point P2.
The control time from Pl to the origin (target steering angle) becomes shorter,
Improves responsiveness. In this way, by correcting the coefficient C to be larger or smaller depending on the running/driving condition of the vehicle, an appropriate response speed corresponding to the condition can be obtained.

Next, the drive circuit 180 for the servo motor 5 will be explained based on FIG. 8. When the command value from the final command value calculating means 617 is If>O and the motor 5 is to rotate forward, the first terminal a is at a high level, the second terminal is at a low level, and the third terminal C is at a command value. A current command value of an ON-OFF pulse with a duty ratio proportional to the absolute value 11fl of If is supplied. Also, when rotating the motor 5 in reverse at If〈0, the first. A low level is supplied to both the second terminals a and b, and the above-mentioned current command value is supplied to the third terminal C.

The operation of drive circuit 180 will now be described. For example, If > 0
Since terminal a is at a high level when , the inverter 181
The output of becomes low level, and transistor 182 is turned off.
, transistor 183 also remains OFF. On the other hand, when the transistor 184 is turned on, the transistor 1
85 is also maintained ON. Also, since the terminal is at a low level, transistors 186 and 187 are turned off.
Transistor 188 remains OFF. Therefore, the battery voltage V is output to the output terminal m only through the transistor 185. Here, when a duty ratio pulse is applied to terminal c, as the duty ratio pulse turns ON (high level) to OFF (low level), the transistors 189, 190° 191 also switch ON and OFF.
When the transistor 191 is turned on, a positive current flows through the motor through the output terminal m, the motor 5, the output terminal n, and the transistor 191. Also, transistor 1
・Even when 91 turns from ON to OFF, the current (energy) remaining in the motor coil flows through diode 1.
92, transistor 185, output terminal m, motor 5, and output terminal n, the positive current continues to flow for a predetermined period of time corresponding to the inductance of the motor coil. As a result, the average type fLI that actually flows through the motor 5 is the duty ratio (O
N-duty ratio). Also, t
When r<o, each transistor mentioned above is turned on or off.
It is reversed, and the battery voltage ■8 is generated at the output terminal n.

And the duty ratio pulse applied to the terminal is 0N-
Finally, the transistor 188 turns OFF.
A negative current proportional to the duty ratio (ON-duty ratio) flows through the FFL and motor 5. (Note that when the duty ratio of transistor 188 is OFF, diode -1
"193, the residual current in the motor coil flows through the transistor 183). That is, this drive circuit 180
By using the current command value If as shown in FIG.
An average current I proportional to is supplied to the motor 5 as a motor current.

The operation of the control device will be explained based on the means shown in FIG. 4 described above. First, the target steering angle calculation means 610 calculates the target (rear wheel) steering angle θc from the signal of the state detector group 1, and calculates the actual steering angle? The A calculation means 611 and the steering angular velocity calculation means 612 calculate the actual steering angle θr and the steering angular velocity X2 from the signals from the rear wheel steering angle sensor 2, respectively, and the error calculation means 613 calculates the target steering angle θc and the actual steering angle θr. Calculate the error. Further, the switching function correction means 618 determines the coefficient C according to the state detector group 1. Next, the gain switching function calculation means 614 of the control means 650 calculates the error X1, the steering angular velocity X2, and the coefficient C.
The switching functions LL and L2 are obtained from the fundamental term calculation means 6.
15 calculates the basic term 1b based on the function L2, and the correction term calculation means 616 calculates the correction term 1m based on the function L1, and finally the final command value calculation means 617
calculates the command value If from the basic term 1b and correction term 1m and outputs it to the drive circuit 180, and the drive circuit 180 outputs the average current I to the servo motor 5! The three-rudder mechanism 3 will be steered.

Having said the above, by correcting the coefficient C according to the vehicle speed ■, or the vehicle speed ■ and the front wheel steering angular velocity bf as shown in FIG. 6 ta+ or FIG.
As shown in Figure 2, the control response speed can be changed appropriately. Therefore, the control can be changed arbitrarily, such as low response in the low speed range, high response in the medium speed range, and low response in the high speed range for safety reasons.
Notching can be done easily and arbitrarily. In addition, as mentioned above, the shape G (Xl, X2) is controlled to converge to the target steering angle θc while being constrained by the switching line, so
There is no overshoot, and it is possible to control the steering mechanism with high precision, which is robust against disturbances and parameter changes.

In the above embodiments, the present invention was applied to steering the rear wheels, but it goes without saying that it may also be applied to steering the front wheels. Furthermore, the above-mentioned steering angular velocity x2 is calculated by the rear wheel steering angle sensor 2.
Although the calculation was performed based on the signal from the hydraulic power assist rack-and-binion mechanism 3, it may also be calculated from the rotation speed of the pinion gear 3b of the rack-and-binion mechanism 3 with hydraulic power assist. As for the steering mechanism, the input shaft of a power-assisted rack-and-binion mechanism was driven by a servo motor 5, but a servo valve was used instead of the hydraulic valve 4a of the power-assisted rank-and-binion mechanism. It may also be controlled directly.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the configuration of the present invention, Fig. 2 is a schematic block diagram showing an embodiment of the present invention, Fig. 3 is an electric circuit block diagram showing an electrical control device (6), and Fig. 4 is a block diagram showing the configuration of the present invention. 5 is a block diagram showing the procedure of the microcomputer (60), FIG. 5 is a characteristic diagram showing the target steering angle (θc) with respect to vehicle speed (V), and FIG. A characteristic diagram showing the coefficient C of the switching function with respect to the steering angular velocity θr, Fig. 7 ta1.
A diagram showing a phase plane trajectory with the steering angular velocity (X2) on the axis,
FIG. 8 is a circuit diagram showing details of the drive circuit (180), FIG.
The figure is a characteristic diagram showing the motor current with respect to the current command value If. 1a...Front wheel steering angle sensor, lb...Vehicle speed sensor. IC... Yaw rate sensor, 2... Rear wheel steering angle sensor. 4...Low speed gear, 5...DC servo motor, 6...
・Electrical control device, 8... Rear wheel, 9... Front wheel, Ml
...State detection means, M2...Steering angle detection means, M3
...Steering angular velocity detection means, M4...Target l-mulberry steering angle calculation means, M5...Switching function correction means, M6...
Error calculation means 1M7...control means, M8...driving means, M9...steering mechanism. Representative Patent Attorney Takashi Okabe ■0 Small - Missing Figure 5 (a) ↑ I no. k-f;

Claims (5)

[Claims] (1) A steering mechanism equipped with a servo mechanism that adjusts the steering angle of the wheels in response to an electrical command value, a state detection means that detects the running state of the vehicle, and a state detection means that detects the actual steering angle of the steering mechanism. a steering angle detection means for outputting an actual steering angle (θr) signal; a target steering angle calculation means for calculating a target steering angle (θc) of the steering mechanism according to a state signal from the state detector; and the target steering angle. (θc) and the actual steering angle (θr); an error calculation procedure for determining the error (X1) from the actual steering angle (θr); a steering angular velocity detection means for determining the steering angular velocity (X2) of the wheels; and the error (X1) and the steering angular velocity (X2). ), find a switching function that satisfies the conditions for both to converge to zero, and converge the error (X1) and the steering angular velocity (X2) to zero while being constrained by the switching line found based on the switching function. a control means for outputting an electrical command value to control the switching function; a switching function correction means for correcting the switching function according to a state signal from the state detector; and a switching function correction means for correcting the switching function according to the electrical command value from the control means A vehicle steering device comprising: a drive means for driving a mechanism; (2) The vehicle steering system according to claim 1, wherein the switching function correction means corrects the switching function based on a vehicle speed signal from a state detection means for detecting vehicle speed. (3) The steering mechanism is a steering mechanism that steers the rear wheels of the vehicle, and the switching function correction means receives a vehicle speed signal and a front wheel steering angular velocity signal from a state detection means that respectively detects the vehicle speed and the front wheel steering angular velocity. 3. The vehicle steering system according to claim 1, wherein the switching function is corrected based on . (4) The switching functions L1 and L2 are calculated by the following formula L1=C・
X1-X2, L2=L1・X1 (however, C is a coefficient where C>0), and
3. The vehicle steering system according to claim 2, wherein the coefficient C is calculated based on the vehicle speed signal. (5) The control means includes a switching function calculation means for calculating the switching functions L1 and L2, and a basic term (Ib) of an electrical command value to the servo mechanism from the error (X1) and the switching function L2. A fundamental term calculating means for calculating from the following formula (a) when L2≧0, Ib=Ga・X1 (b) when L2<0, Ib=Gb・X1 (however, Ga and Gb are constants); and the switching function. The correction term (Im) of the electrical command value from L1 to the servo mechanism is expressed by the following formula (a) When L1≧0, Im=M (b) When L1<0, Im=-M (However, M is M≧ a final command value calculation unit that calculates a final command value If (If=Ib+Im) of the electrical command value from the basic term (Ib) and the correction term (Im); 5. The vehicle steering system according to claim 4, further comprising means for controlling the vehicle.