JP7578076B2 - Steering control device - Google Patents

Description

The present invention relates to a steering control device.

Conventionally, in a steering device that controls the assist torque output by a motor, a technique is known in which a servo controller calculates the motor output command so that the steering torque follows a target steering torque.

For example, in the steering control device disclosed in Patent Document 1, a servo controller (called an assist controller in Patent Document 1) generates a base assist command using PID control so that the steering torque follows the target steering torque.

Patent No. 6252027

When the absolute value of the steering torque saturates due to sudden steering or hitting the rack end, the differential component of the PID control causes a large change in the assist torque that pushes back the steering wheel. As a result, when saturated by sudden steering, it becomes heavy for a moment, which may lead to a behavior that is pushed back when hitting the end.

The present invention was created in light of these points, and its purpose is to provide a steering control device that prevents the steering wheel from being pushed back when the absolute value of the steering torque becomes saturated.

The steering control device of the present invention includes a servo controller (400) and a steering torque saturation determination unit (38). The servo controller performs PID control and calculates an output command (Tb ^* ) for a motor (80) so that the steering torque follows a target steering torque (Ts ^* ). The steering torque saturation determination unit determines that the absolute value of the steering torque is saturated.

When the steering torque saturation determination unit determines that the absolute value of the steering torque is saturated, the servo controller resets at least the input to the low-pass filter (54) in the calculation of the differential component of the PID control and the past values stored in the low-pass filter, and resets the calculation of the differential component of the PID control, which is an element related to the assist torque whose phase is advanced relative to the steering torque.

Preferably, the servo controller is configured such that, in the differential component calculation of the PID control, a value obtained by subtracting the previous value of the steering torque deviation differential or zero from the current value of the steering torque deviation differential, which is the output of the low-pass filter, in a subtractor (56) is input to a differential gain multiplier (57). When the steering torque saturation determination unit determines that the absolute value of the steering torque is saturated, the value subtracted from the current value of the steering torque deviation differential in the subtractor becomes zero.

Specific configurations for resetting the calculations include performing the calculations using 0 instead of the original input value, or performing the calculations using the results of clearing stored past values to 0.

In this invention, when the absolute value of the steering torque is saturated, the "element related to the assist torque whose phase is advanced relative to the steering torque (e.g., the differential component of PID control)" becomes 0, so that no torque in the direction of pushing back the steering wheel is generated, and steering becomes possible without being pushed back. This eliminates the momentary weighting that occurs even when the steering wheel is saturated due to sudden steering. In addition, the steering wheel stops at the end without being pushed back.

FIG. 1 is a schematic diagram of an electric power steering system. 1 is a schematic configuration diagram of an ECU (steering control device) according to an embodiment; FIG. 2 is a block diagram of a servo controller according to an embodiment. FIG. 4 is a detailed block diagram of the differential component calculation unit in FIG. 3 . 6A and 6B are diagrams showing the operation of servo control calculation according to a comparative example and an embodiment of the present invention, respectively. 4 is a time chart showing the behavior of an actual vehicle in a comparative example; 4 is a time chart showing the behavior of an actual vehicle according to the present embodiment;

Below, an embodiment of the steering control device of the present invention will be described with reference to the drawings. The ECU as the "steering control device" is applied to the electric power steering system or steer-by-wire system of a vehicle, and calculates the motor output command. In the following embodiment, an example in which it is mainly applied to an electric power steering system is shown. In the electric power steering system, the steering control device outputs an assist torque command to a steering assist motor.

[Electric power steering system configuration]
The configuration of the electric power steering system will be described with reference to Fig. 1. Note that the symbols of the assist torque Ta and the commands Ta ^* and Tb ^* are shown in Fig. 2. The electric power steering system 1 is a system that assists the operation of a steering wheel 91 by a driver using the driving torque of a motor 80. The steering wheel 91 is fixed to one end of a steering shaft 92, and an intermediate shaft 93 is provided on the other end of the steering shaft 92. The steering shaft 92 and the intermediate shaft 93 are connected by a torsion bar of a torque sensor 94, and these constitute a steering shaft 95. The torque sensor 94 detects the steering torque Ts based on the torsion angle of the torsion bar.

A gear box 96 including a pinion gear 961 and a rack 962 is provided at the end of the intermediate shaft 93 opposite the torque sensor 94. When the driver turns the steering wheel 91, the pinion gear 961 rotates together with the intermediate shaft 93, and as the pinion gear 961 rotates, the rack 962 moves left and right. Tie rods 97 provided at both ends of the rack 962 are connected to the tires 99 via knuckle arms 98. The tie rods 97 reciprocate left and right, pulling and pushing the knuckle arms 98, changing the orientation of the tires 99.

The motor 80 is, for example, a three-phase AC brushless motor, and outputs an assist torque Ta that assists the steering force of the steering wheel 91 according to the drive voltage Vd output from the ECU 10. In the case of a three-phase AC motor, the drive voltage Vd means the phase voltages of the U phase, V phase, and W phase. The rotation of the motor 80 is transmitted to the intermediate shaft 93 via a reduction mechanism 85 composed of a worm gear 86, a worm wheel 87, etc. In addition, the rotation of the intermediate shaft 93 caused by the steering of the steering wheel 91 or the reaction force from the road surface is transmitted to the motor 80 via the reduction mechanism 85.

The electric power steering system 1 shown in FIG. 1 is a column-assist type in which the rotation of the motor 80 is transmitted to the steering shaft 95, but the ECU 10 of this embodiment can also be applied to a rack-assist type electric power steering system. In other embodiments, a multi-phase AC motor other than a three-phase motor or a brushed DC motor may be used as the steering assist motor.

Here, the entire mechanism through which the steering force of the steering wheel 91 is transmitted from the steering wheel 91 to the tires 99 is referred to as the "steering system mechanism 100." The ECU 10 controls the assist torque Ta output by the motor 80 connected to the steering system mechanism 100, thereby controlling the steering torque Ts generated by the steering system mechanism 100. The ECU 10 also acquires the vehicle speed V detected by a vehicle speed sensor 11 provided at a predetermined location on the vehicle.

The ECU 10 operates using power from an on-board battery (not shown) and calculates an assist torque command Ta ^{* based on the steering torque Ts detected by the torque sensor 94 and the vehicle speed V detected by the vehicle speed sensor 11. The ECU 10 then applies a drive voltage Vd calculated based on the assist torque command Ta* to} the motor 80, thereby causing the steering system mechanism 100 to generate a steering torque Ts.

The various arithmetic processes in the ECU 10 may be software processes in which a CPU executes a program prestored in a physical memory device such as a ROM, or may be hardware processes in which a dedicated electronic circuit is used.

[Configuration of ECU]
(One embodiment)
The configuration of the ECU 10 according to the embodiment will be described with reference to Fig. 2. The ECU 10 includes an estimated load torque calculation unit 20, a target steering torque calculation unit 30, a steering torque saturation determination unit 38, a servo controller 400, and a current feedback unit ("FB" in the figure) 70.

The estimated load torque calculation unit 20 calculates the estimated load torque Tx based on the target steering torque Ts ^* and the base assist command Tb ^* . The estimated load torque Tx is a load torque that acts on the steering shaft 95 of the steering system mechanism 100 and changes according to steering. The positive and negative of the estimated load torque Tx and the steering torque Ts are defined according to the rotation direction of the steering shaft 95 such that the torque in one rotation direction is positive and the torque in the opposite direction is negative.

The estimated load torque calculation unit 20 includes an adder 21 and a low-pass filter ("LPF" in the figure) 22. The adder 21 adds the base assist command Tb ^* fed back from the servo controller 400 and the target steering torque Ts ^* fed back from the target steering torque calculation unit 30. The low-pass filter 22 extracts components of a predetermined frequency, for example, a band of 10 Hz or less, from the added torque. The estimated load torque calculation unit 20 outputs the frequency components extracted by the low-pass filter 22 as the estimated load torque Tx.

The target steering torque calculation unit 30 calculates the target steering torque Ts ^* using a map 33 that defines the relationship between the estimated load torque Tx and the target steering torque Ts ^* . The target steering torque calculation unit 30 includes a sign determination unit ("sgn" in the figure) 31, an absolute value determination unit ("|u|" in the figure) 32, a map 33, and a multiplier 34. The sign determination unit 31 determines whether the estimated load torque Tx is positive or negative, i.e., the sign according to the rotation direction of the steering shaft 95. The absolute value determination unit 32 calculates the absolute value of the input u, i.e., the estimated load torque Tx.

The map 33 is shown as a map in the positive region of the estimated load torque Tx, i.e., a map of absolute values. In the negative region of the estimated load torque Tx, the map is symmetrical with respect to the origin with respect to the positive region. In the multiplier 34, the absolute value of the target steering torque Ts ^* calculated using the map is multiplied by a sign corresponding to the sign of the estimated load torque Tx. The target steering torque Ts ^* output by the target steering torque calculation unit 30 is input to a deviation calculator 39 and is also fed back to the estimated load torque calculation unit 20.

The steering torque saturation determination unit 38 determines whether the absolute value |Ts| of the steering torque is saturated. Specifically, when the absolute value |Ts| of the steering torque is equal to or greater than a predetermined steering torque saturation determination threshold, the steering torque saturation determination unit 38 sets the steering torque saturation flag to ON. On the other hand, when the absolute value |Ts| of the steering torque is less than the predetermined steering torque saturation determination threshold, the steering torque saturation flag to OFF. The steering torque saturation flag is output to the servo controller 400.

The steering torque saturation judgment threshold is set to the maximum value of the absolute value of the steering torque |Ts| (e.g., 7.5 Nm). If the signal input to the servo controller 400 has been subjected to some kind of gain correction or limiting process, it is preferable to set the threshold to a value equivalent to that maximum value.

The deviation calculator 39 calculates a steering torque deviation ΔT (=Ts ^* -Ts) which is the difference between the target steering torque Ts ^* and the steering torque Ts. The steering torque deviation ΔT is input to the servo controller 400. The servo controller 400 calculates a base assist command Tb ^* so that the steering torque Ts follows the target steering torque Ts ^* . A detailed configuration of the servo controller 400 of this embodiment will be described later with reference to Figs. 3 and 4.

In the configuration example of Fig. 2, since the correction torque for the base assist command Tb ^* is not calculated, the base assist command Tb ^* is output as it is as the assist torque command Ta ^* . Note that in the configuration in which the correction torque is calculated as shown in Fig. 2 of Patent Document 1 (Japanese Patent No. 6252027), the sum of the base assist command Tb ^* and the correction torque is output as the assist torque command Ta ^* .

The current feedback unit 70 applies a drive voltage Vd to the motor 80 so that an assist torque corresponding to the base assist command Tb ^* is applied, particularly to the steering shaft 95 which is closer to the tire 99 than the torque sensor 94. The current feedback control technology is well known in the field of motor control, and therefore a detailed description thereof will be omitted.

The configuration of the servo controller 400 of this embodiment is shown in Figs. 3 and 4. Figs. 3 and 4 show a configuration in which the servo control calculation is equivalently converted into a discrete equation. The servo controller 400 includes a PID controller 410 and an accumulation processing unit 490. The PID controller 410 includes a proportional component calculation unit 430, an integral component calculation unit 440, and a differential component calculation unit 450, similar to the configuration of the assist controller disclosed in Fig. 4 of Patent Document 1. The PID controller 410 performs PID control calculation based on the steering torque deviation ΔT.

The delay element 453 of the proportional component calculation unit 430 and the delay element 454 of the integral component calculation unit 440 extract the previous value of the steering torque deviation ΔT. In the proportional component calculation unit 430, the steering torque deviation ΔT from which the previous value has been subtracted in the subtractor 463 is multiplied by the proportional gain Kp in the proportional gain multiplier 473. In the integral component calculation unit 440, the steering torque deviation ΔT from which the previous value has been added in the adder 464 is multiplied by the integral gain Ki in the integral gain multiplier 474.

The pseudo differential calculation unit 50 of the differential component calculation unit 450 calculates the steering torque deviation differential D(ΔT) by pseudo differentiation. The discrete pseudo differential "D" corresponds to the calculation function of (s/(τs+1) ² ) (where s is the Laplace operator and τ is the time constant) in terms of a transfer function of a continuous system. The delay element 55 extracts the previous value of the steering torque deviation differential D(ΔT).

In the differential component calculation unit 450, the steering torque deviation differential D (ΔT) from which the previous value has been subtracted in the subtractor 56 is multiplied by the differential gain Kd in the differential gain multiplier 57. The steering torque saturation flag is input to the pseudo differential calculation unit 50 and delay element 55 of the differential component calculation unit 450, and a "reset procedure" in the differential component calculation is executed. A detailed configuration of the reset procedure will be described later with reference to FIG. 4.

The PID component adder 48 outputs the target torque TM obtained by adding the proportional component, the integral component, and the derivative component for each control cycle. The accumulation processing unit 490 accumulates the target torque TM and calculates the current value Tb ^* _n of the base assist command. The accumulation processing is synonymous with the integral processing, but the term "accumulation" is used here to distinguish it from the integral control of the PID.

The accumulation processing unit 490 includes an adder 491, a delay element 492, and a limiting calculator 494. The adder 491 adds the previous value Tb ^* _n-1 of the base assist command input via the delay element 492 to the current value of the torque TM to be processed. The limiting calculator 494 limits the sum of the adder 491 to a limiting value that can be output as an assist torque. This addresses the wind-up problem, that is, the phenomenon in which, when a deviation continues to occur, a value larger than the allowable output is taken by integration, and then, when the sign of the deviation becomes reversed, the reduction in output is delayed.

The servo control formula is as follows: The steering torque deviation ΔT is expressed by formula (1).
ΔT=Ts ^* -Ts...(1)

The base assist command Tb ^* is expressed by the following equation (2): In the configuration of Fig. 3, the proportional gain Kp, the integral gain Ki, and the derivative gain Kd are all set to negative values.

The proportional component, integral component, and differential component added by the PID component adder 48 pass through the accumulation processing unit 490, and are reflected in the amounts equivalent to each element of the PID control shown in equation (2) (proportional control torque, integral control torque, differential control torque), thereby calculating the base assist command Tb ^* .

In order to discretize equation (2), the bilinear transformation equation expressed by equation (3) is substituted into equation (2) and rearranged to obtain equation (4). In equation (3), ts indicates the calculation period. In addition, in Figure 3, (ts/2)Ki is collectively written as "Ki".

Here, attention is paid to the phase relationship between the steering torque Ts and the amount equivalent to the differential control torque of the PID control obtained by the accumulation processing of the accumulation processing unit 490. In FIG. 3, the differential component itself output from the differential gain multiplier 57 is equivalent to a signal obtained by performing a second-order differential control calculation of the steering torque deviation ΔT through the calculation of the delay element 55 and the subtractor 56. This differential component passes through the accumulation processing unit 490 to become the differential control torque of the PID control. The accumulated differential control torque corresponds to an "assist torque whose phase is advanced relative to the steering torque". Therefore, the differential component, which is an element related to the differential control torque, corresponds to an "element related to the assist torque whose phase is advanced relative to the steering torque".

The proportional component shown in FIG. 3 is a signal obtained by subtracting the previous value through calculations by the delay element 453 and the subtractor 463, and therefore has a phase lead relative to the steering torque. However, the amount equivalent to the proportional control torque of the PID control obtained by the accumulation processing of the accumulation processing unit 490 has the same phase as the steering torque, and does not correspond to an "assist torque whose phase leads relative to the steering torque." Therefore, the proportional component, which is an element related to the proportional control torque, does not correspond to an "element related to the assist torque whose phase leads relative to the steering torque."

The detailed configuration of the differential component calculation unit 450 will be described with reference to FIG. 4. First, the configuration related to normal differential component calculation excluding the reset process will be described. The pseudo-differential calculation unit 50 has a delay element 51 and a subtractor 52 for the input of the steering torque deviation ΔT, and a multiplier 53 and a low-pass filter 54 provided on the output side of the subtractor 52. The multiplier 53 multiplies the difference between the current value and the previous value of the steering torque deviation ΔT by the reciprocal of the calculation period ts (1/ts) to output the steering torque deviation difference differential.

The low-pass filter 54 removes high-frequency components from the input steering torque deviation difference differential and outputs the steering torque deviation differential D(ΔT). The RAM inside the low-pass filter 54 stores multiple previous values or multiple past values going back from the previous value depending on the calculation configuration, and outputs a value that has been filtered based on these past values.

The delay element 55, subtractor 56, and gain multiplier 57 for the steering torque deviation differential D(ΔT) are as described above with reference to FIG. 3. The output of the subtractor 56, i.e., the difference between the current value and the previous value of the steering torque deviation differential D(ΔT), is called the steering torque deviation differential difference. In the example configuration of the PID controller shown in FIG. 3 and FIG. 4, the steering torque deviation differential difference is multiplied by the differential gain Kd. Note that the calculation of the fourth term on the right side of equation (4) may be realized by using another configuration of the PID controller.

Next, as an additional configuration for the reset procedure, a switch 535 is provided on the input side to the low-pass filter 54 of the pseudo differential calculation unit 50. Also, although not shown, a switch for switching the target input for filter processing and a means for clearing RAM values to 0 are provided inside the low-pass filter 54. Furthermore, a switch 555 is provided on the previous value input side of the steering torque deviation differential subtractor 56. The steering torque saturation flag is input to these switch 535, low-pass filter 54, and switch 555.

The steering torque saturation determination unit 38 determines whether the absolute value |Ts| of the steering torque is saturated before performing the differential component calculation of the fourth term on the right side of equation (4) or as the first step when performing the calculation of the entire PID control. When the steering torque saturation determination unit 38 determines that the absolute value |Ts| of the steering torque is saturated and the steering torque saturation flag is ON, the differential component calculation unit 450 executes a reset process as follows. The corresponding numbers <1> to <3> are written in the input fields of the steering torque saturation flag in FIG. 4. Hereinafter, "0" in the reset process is interpreted as including not only strict 0 but also "a value close to 0" that does not affect the calculation result of the differential component.

<1> The switch 535 of the pseudo differential calculation unit 50 is switched to the "0" side, and 0 is input to the low-pass filter 54 instead of the original steering torque deviation difference differential. As a result, the current input to the low-pass filter 54 becomes 0.

<2> Instead of the multiple past values, 0 is used as the processing target input of the low-pass filter 54. That is, the filter process is performed using 0. Alternatively, all of the multiple past values stored in the RAM are cleared to 0. As a result, the current value D(ΔT) _n of the steering torque deviation differential output by the low-pass filter 54 becomes 0.

<3> The switch 555 on the previous value input side of the steering torque deviation differential subtractor 56 is switched to the "0" side. As a result, 0 is input to the subtractor 56 instead of the original previous value D(ΔT) _n-1 of the steering torque deviation differential output by the delay element 55. As a result, the steering torque deviation differential difference input to the differential gain multiplier 57 becomes 0. Therefore, the differential component output by the differential component calculation unit 450 becomes 0, and the reset process is completed.

By performing steps <1> to <3>, when the absolute value |Ts| of the steering torque reaches saturation, the differential component becomes zero, and at the time of the next calculation, a signal that continues from that state is obtained. Of <1> to <3>, steps <1> and <2>, i.e., the input to the low-pass filter 54 in the differential component calculation and the processing to reset the past values stored in the low-pass filter 54, are essential in all cases. On the other hand, the reset processing in <3> may be omitted depending on the configuration of the PID control.

Next, the technical significance of this embodiment will be described with reference to Figs. 5 to 7. Fig. 5(a) shows the operation of the servo control calculation of a comparative example, and Fig. 5(b) shows the operation of the servo control calculation of this embodiment. Figs. 6 and 7 show actual vehicle data of the comparative example and this embodiment, respectively. The calculation of the comparative example corresponds to the normal servo control calculation according to the conventional technology of Patent Document 1 and the like. The servo output is the base assist command Tb ^* .

Figures 5(a) and (b) show the waveforms of the steering torque Ts, steering torque deviation differential D (ΔT), and the differential component. The value of the steering torque Ts is positive, so the "absolute value" is omitted. The steering torque Ts increases, reaches saturation, and then decreases. The waveforms of the steering torque Ts and steering torque deviation differential D (ΔT) are the same in the comparative example and this embodiment, but the waveforms of the differential component are different.

The differential component is added to the proportional component, integral component, and previous servo output, and is limited to become the new servo output. In the comparative example, when steering suddenly toward the end, the servo output increases or reaches the upper output limit until the time before the hatched portion (see actual vehicle data in Figure 6). After that, when the steering torque Ts saturates due to hitting the end, etc., a large differential component is generated on the negative side, which acts to reduce the servo output.

In this embodiment, when the steering torque Ts becomes saturated, such as when hitting the end, the differential component becomes 0 by resetting the internal state of the low-pass filter 54 in the differential component calculation. At this time, the servo output is not affected by the differential component, and a decrease in the servo output is avoided (see actual vehicle data in Figure 7).

6 and 7 show, from top to bottom, the steering angle, steering torque Ts, differential component, servo output, steering angular acceleration, and steering angular velocity. The end contact stop section is a period of approximately 0.03 to 0.05 [sec], and the steering torque Ts reaches saturation in the latter half of the end contact stop section. Here, the "absolute value" is omitted because the value of the steering torque Ts is positive in both Figs. 6 and 7. Also, the target steering torque Ts ^* is assumed to be constant.

In the comparative example shown in Figure 6, as indicated by (*1), a sudden differential component occurs in the direction of pushing back the steering wheel when the steering torque Ts saturates. As a result, as indicated by (*2), the servo output is reduced by the differential component. For example, it suddenly decreases from approximately 100 [Nm] to approximately 50 [Nm].

Also, as shown in (*3), after the end contact stop section has passed, the steering angular velocity goes from positive to negative (exceeding 0) (approximately -100 [deg/s]). In other words, the steering wheel is being bounced back.

In the present embodiment shown in FIG. 7, a reset procedure is performed on the differential component calculation when the steering torque Ts is saturated, so that the output of the differential component becomes almost 0, as shown in (*4). Therefore, as shown in (*5), the servo output is constant when the end is hit and does not decrease. Also, as shown in (*6), after the end hit stop section has passed, the steering angular velocity is maintained at almost 0, and the behavior of the steering wheel being bounced back is eliminated.

The change in the differential component and servo output at around 0.08 [sec] is due to the driver's intention to return the control, and is not a problematic phenomenon.

In this manner, in this embodiment, when the steering torque Ts becomes saturated, the servo controller 400 resets the differential component calculation of the PID control, so that no torque is generated in the direction of pushing back the steering wheel, and steering becomes possible without being pushed back. This eliminates the momentary weighting that occurs when the steering wheel becomes saturated due to sudden steering. In addition, when the steering wheel hits the end, it stops at that position without being pushed back.

Other Embodiments
(a) The servo controller 400 is not limited to one that performs PID control, and may calculate the base assist command Tb ^* using a transfer function so that the steering torque Ts follows the target steering torque Ts ^* . In this configuration, the servo controller 400 resets the calculation of the "element related to the assist torque whose phase is advanced relative to the steering torque" instead of the calculation of the differential component of the PID control. This makes it possible to prevent the steering wheel from being pushed back when the absolute value of the steering torque is saturated, as in the above embodiment.

(b) The estimated load torque calculation unit 20 may calculate the estimated load torque Tx based on the steering torque Ts instead of the target steering torque Ts ^* and based on the assist torque Ta instead of the base assist command Tb ^* .

(c) The steering control device of the present invention is not limited to electric power steering systems, and may be applied to a reaction force control device of a steer-by-wire system in which the steering wheel and the steered wheels are mechanically separated, as disclosed in Fig. 11 of Patent Document 1. In this case, the servo controller controls the steering torque Ts detected by the reaction force detection device so as to follow the target steering torque Ts ^* calculated in accordance with the load and steering angle calculated by the tire steering device.

The present invention is not limited to these embodiments, and can be implemented in various forms without departing from the spirit of the invention.

10...ECU (steering control device),
38 . . . Steering torque saturation determination unit,
400...Servo controller,
80...Motor.

Claims (2)

a servo controller (400) that performs PID control and calculates an output command (Tb ^* ) of a motor (80) so that the steering torque (Ts) follows a target steering torque (Ts ^* );
A steering torque saturation determination unit (38) that determines whether an absolute value of the steering torque is saturated;
Equipped with
When the steering torque saturation determination unit determines that the absolute value of the steering torque is saturated, the servo controller resets at least the input to a low-pass filter (54) in the calculation of the differential component of the PID control and the past values stored in the low-pass filter, and resets the calculation of the differential component of the PID control, which is an element related to the assist torque whose phase is advanced relative to the steering torque. the servo controller is configured such that, in a differential component calculation of PID control, a value obtained by subtracting a previous value of the steering torque deviation differential or 0 from a current value of the steering torque deviation differential, which is an output of the low-pass filter, in a subtractor (56) is input to a differential gain multiplier (57);
2. The steering control device according to claim 1, wherein when the steering torque saturation determination unit determines that the absolute value of the steering torque is saturated, the value subtracted by the subtractor from the current value of the steering torque deviation differential becomes zero .

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