JP2002370662A - Electric power steering device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric power steering device for obtaining a superior steering feeling in the whole area of steering torque by restraining overs and shorts of steering assisting force. SOLUTION: A feedback control command value Vfb is generated by a proportional-plug-integral control operation on the basis of a deviation between a target current value It set on the basis of the steering torque T and a detecting value Is of an electric current flowing to a motor for generating the steering assisting force. A feedforward control command value Vff is generated by a differential control operation on the basis of the steering torque T. At this time, FF control gain G(T) having a value changing according to the steering torque T, that is, FF control gain G(T) having a negative correlation with the steering torque T when the steering torque T is a prescribed value or more, is generated by an FF control gain operation part 33. The FF control command value Vff being a multiplication value of this FF control gain G(T) and a differential value dT/dt of the steering torque is added to the FB control command value Vfb, and the motor 6 is driven on the basis of the whole command value V being the additive result.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric power steering apparatus which applies a steering assisting force to a steering mechanism of a vehicle by driving an electric motor in accordance with a steering torque applied to operation means for steering the vehicle. .

[0002]

2. Description of the Related Art Conventionally, there has been used an electric power steering device which applies a steering assist force to a steering mechanism by driving an electric motor in accordance with a steering torque applied to a steering wheel (steering wheel) by a driver.
In this electric power steering device, a torque sensor for detecting a steering torque applied to a steering wheel, which is an operating means for steering, is provided. Based on the steering torque detected by the torque sensor, a current to be supplied to the electric motor is determined. The target value is set. Then, a command value to be given to the drive means of the electric motor is generated based on the deviation between the target value and the value of the current actually flowing through the electric motor. The driving means of the electric motor includes, for example, a PWM signal generation circuit that generates a pulse width modulation signal (PWM signal) having a duty ratio according to the command value, and a power transistor that turns on / off according to the duty ratio of the PWM signal. And a voltage according to the duty ratio, that is, a voltage according to the command value, is applied to the electric motor. The current flowing through the electric motor by the application of the voltage is detected by a current detector, and the difference between the detected value and the target value is used as a deviation for generating the command value. In this way, in the electric power steering device, feedback control is performed so that a current of a target value set based on the steering torque flows to the electric motor.

In the above-described conventional electric power steering apparatus, when starting to turn from the neutral position of the steering wheel or turning the steering wheel at the neutral position, the target value which is the current value to be supplied to the electric motor is changed with respect to the target value. There is a characteristic that the supply of the actual current to the motor is delayed. As described above, when the electric motor starts up, the followability of the current actually flowing through the electric motor to the target value, that is, the control responsiveness is not good. There was a problem that was not good.

On the other hand, there has been proposed an electric power steering apparatus in which control responsiveness is improved by performing feedforward control in addition to the feedback control. For example, a command value for feedforward control is obtained by multiplying a differential value of the steering torque detected by the torque sensor by a constant feedforward control gain, and this command value is used as a command value for the feedback control. , And the result of the addition is used as the final command value for the driving means of the electric motor. According to such an electric power steering device, when the steering torque rises, the ability to follow the target value of the current actually flowing to the electric motor is improved, and as a result, the steering starts from the neutral position and starts at the neutral position. Also, a good steering feeling can be obtained in steering at the time of turning back.

[0005]

However, in the conventional electric power steering apparatus to which the above-mentioned feedforward control is applied, the steering torque is increased or decreased when a certain degree or more of the steering torque is applied. When the steering operation is performed, the generated steering assist force becomes excessive or insufficient. For example, when the steering wheel is turned (when the steering wheel is located at a position other than near neutral and a certain large steering torque is applied), the driver does not particularly feel the unevenness or delay of the steering torque. In such a case, if the driver further turns the steering wheel at a certain speed or more, an extra steering assist force is generated, and as a result, the steering feeling is deteriorated.

In view of the above, the present invention provides an electric power steering apparatus capable of suppressing the occurrence of excessive or insufficient steering assisting force as described above and obtaining a good steering feeling in the entire steering torque range. With the goal.

[0007]

Means for Solving the Problems and Effects of the Invention According to a first aspect of the present invention, a steering assist force is applied to a steering mechanism of a vehicle by driving an electric motor in accordance with a steering torque applied to operation means for steering the vehicle. An electric power steering device that provides a target value for setting a target value of a current to be supplied to the electric motor based on the steering torque detected by the torque detection unit. Setting means, current detection means for detecting a current flowing in the electric motor and outputting a detected value of the current, motor driving means for driving the electric motor in accordance with a given command value, and motor driving means A command value for feedback control to be given is generated by a proportional integral control operation based on a deviation between the target value and the current detection value. Proportional-integral control means, feed-forward control means for generating a command value for feed-forward control to be given to the motor drive means by differential control operation based on the detected steering torque, and feed-forward control means for feedback control. An addition means for calculating an added value of the command value and the command value for the feedforward control, and providing the added value to the motor driving means, wherein the feedforward control means adjusts a gain of the differential control operation. It is characterized in that it is changed according to the detected steering torque.

According to the first aspect of the present invention, the gain of the differential control operation for obtaining the command value for the feedforward control is changed in accordance with the steering torque, so that the feedforward control has an excessive function. The occurrence of excessive or insufficient steering assist force can be suppressed.

[0009] In a second aspect based on the first aspect, the feedforward control means includes a differential means for obtaining a differential value of the detected steering torque, and a differential control means for determining when the detected steering torque is larger than a predetermined value. Gain changing means for changing the gain of the differential control calculation so that the gain of the differential control calculation has a negative correlation with the detected steering torque; and the gain changing means changing the differential value obtained by the differentiating means. And a multiplication means for generating a command value for the feedback control by multiplying the gain obtained by the above.

According to the second aspect, when the steering torque is larger than the predetermined value, the gain of the differential control calculation has a negative correlation with the steering torque. When the steering torque equal to or more than the predetermined value is applied, the operation of the feedforward control is suppressed. This suppresses the occurrence of excessive or insufficient steering assist force due to excessive operation of the feedforward control.

[0011]

Embodiments of the present invention will be described below with reference to the accompanying drawings. <1. Overall Configuration> FIG. 1 is a schematic diagram showing a configuration of an electric power steering apparatus according to an embodiment of the present invention, together with a vehicle configuration related thereto. This electric power steering apparatus includes a steering shaft 102 having one end fixed to a steering wheel (steering wheel) 100 as an operating means for steering, and a rack and pinion mechanism 104 connected to the other end of the steering shaft 102.
A torque sensor 3 for detecting a steering torque applied to the steering shaft 102 by operating the steering wheel 100; and an electric motor 6 for generating a steering assist force for reducing a driver's load due to the steering operation (steering operation).
And a reduction gear 7 for transmitting the steering assist force generated by the motor 6 to the steering shaft 102, and receiving a power supply from the vehicle-mounted battery 8 and controlling the motor 6 based on sensor signals from the torque sensor 3 and the vehicle speed sensor 4. An electronic control unit (ECU) 5 for controlling driving. When the driver operates the steering wheel 100 in a vehicle equipped with such an electric power steering device, the steering torque due to the operation is detected by the torque sensor 3, and the detected value Ts of the steering torque and the vehicle speed detected by the vehicle speed sensor are displayed. , The motor 6 is driven by the ECU 5. As a result, the motor 6 generates a steering assist force, and the steering assist force is applied to the steering shaft 102 via the reduction gear 7, whereby the load on the driver due to the steering operation is reduced. That is, the steering torque Ts applied by operating the steering wheel and the torque (hereinafter referred to as “steering assist torque”) T due to the steering assist force generated by the motor 6.
The sum with a is given to the rack and pinion mechanism 104 via the steering shaft 102 as the output torque Tb. As a result, when the pinion shaft rotates, the rotation is converted by the rack and pinion mechanism 104 into a reciprocating motion of the rack shaft. Both ends of the rack shaft are connected to wheels 108 via a connecting member 106 comprising a tie rod and a knuckle arm.
And the wheels 1 according to the reciprocating motion of the rack shaft.
08 changes direction.

<2. Configuration of Control Apparatus> FIG. 2 is a block diagram showing a configuration of the electric power steering apparatus from a control viewpoint. The ECU 5 which is a control device of the electric power steering device includes a phase compensation filter 10.
, A target current calculation unit 12, a subtractor 14, a PI control unit 15, an adder 16, a PWM signal generation circuit 17, a motor drive circuit 20, a current detector 19, a feed forward control unit 30, And the phase compensation filter 1
The detected value Ts of the steering torque detected by the torque sensor 3 is input to 0, and the detected value of the vehicle speed detected by the vehicle speed sensor 4 is input to the target current calculator 12. The phase compensation filter 10, the target current calculation unit 12, the subtractor 14, the PI control unit 15, the adder 16, and the feedforward control unit 30 among the components of the ECU 5 require the microcomputer to execute a predetermined program. Can be realized as software, but may be realized as hardware.

The torque sensor 3 detects a difference Tb-Ta between the output torque Tb and the steering assist torque Ta as a steering torque given by operating the steering wheel 100. That is, the torsion bar is interposed between the steering wheel side portion of the steering shaft 102 and the portion to which the steering assist torque Ta is applied via the reduction gear 7, and the torque sensor 3 detects the torsion of the torsion bar. Thus, the steering torque is detected. The detected value of the steering torque detected in this manner is output from the torque sensor 3 as a detection signal Ts, and is output from the phase compensation filter 1.
Input to 0. The phase compensation filter 10 performs phase compensation on the detection signal Ts, and outputs a signal after the phase compensation as a steering torque signal T. The steering torque signal T is input to the target current calculator 12 and the feedforward controller 30.

A vehicle speed sensor 4 detects a vehicle speed of a vehicle on which the electric power steering device is mounted, and outputs a signal indicating the detected value as a vehicle speed signal. This vehicle speed signal is also input to the target current calculation unit 12.

The target current calculation section 12 calculates a target current value (target current value) It to be supplied to the motor 6 based on the steering torque signal T and the vehicle speed signal. Specifically, in order to generate an appropriate steering assist force, the value It of the target current to be supplied to the motor 6 and the steering torque T (code “T”)
The table also indicates the steering torque itself), and a table indicating the vehicle speed as a parameter, that is, a table as shown in FIG. 3 (called an “assist table”) is stored in the target current calculation unit 12 in advance. The target current calculator 12 sets the target current value It with reference to the assist table.

The current detector 19 detects a current actually supplied to the motor 6, that is, a current flowing through the motor 6, and outputs a current detection value Is indicating the current.

The subtractor 14 calculates a deviation It-Is between the target current value It output from the target current calculator 12 and the current detection value Is output from the current detector 19.

The PI control unit performs a proportional-integral control operation based on the difference It-Is to calculate the PWM signal
(Hereinafter, referred to as “FB control command value”) Vfb for feedback control to be given to the controller. This FB
The control command value Vfb is a command value for performing feedback control based on the difference It-Is so that a current having the target current value It flows through the motor 6, and is given by the following equation. Vfb = Kp · (It−Is) + Ki · ∫ (It−Is) dt (1) where Kp is the gain of the proportional control in the PI control unit 15, and Ki is the gain of the integral control in the PI control unit 15. is there.

The feed-forward control section 30 performs a differential control operation, which will be described later, based on the steering torque signal T input from the phase compensation filter 10 to generate a PWM signal.
A command value (hereinafter referred to as “FF control command value”) Vff for feedforward control to be given to 7 is generated. The FF control command value Vff is added to the FB control command value Vfb by the adder 16 in order to improve the followability of the current actually flowing through the motor 6 to the target current value It, that is, the control responsiveness.

In this way, the FB control command value Vfb and F
The added value V = Vfb + Vff with the F control command value Vff is the adder 1
The added value (hereinafter referred to as “all command values”) V is supplied to the PWM signal generation circuit 17.

The PWM signal generation circuit 17 outputs a pulse signal having a duty ratio according to the total command value V, that is, a pulse width modulation signal (hereinafter, referred to as a “PWM signal”) whose pulse width changes according to the total command value V. Generate.

The motor drive circuit 20 applies a voltage corresponding to the pulse width (duty ratio) of the PWM signal to the motor 6. FIG. 4 is a circuit diagram showing a configuration example of the motor drive circuit 20. In this example, four power field effect transistors (hereinafter referred to as “FETs”) 21 to 24 are used.
Constitutes a bridge circuit, which is connected between the power supply line of the battery 8 and the ground line. When the motor 6 is to generate a torque in a direction that assists rightward steering (hereinafter referred to as “rightward torque”), the PWM signal generation circuit 17 outputs
The PWM signal is input to gates 21 and 24, and FE
A predetermined signal for turning them off is input to the gates of T22 and T23. As a result, the FETs 21 and 24 are turned on only for a period corresponding to the pulse width of the PWM signal, and a voltage having a magnitude corresponding to the total command value V is applied to the motor 6. Generates a rightward torque of the corresponding magnitude. On the other hand, when the motor 6 is to generate torque in a direction that assists left steering (hereinafter referred to as “left torque”), the PWM signal generation circuit 17
, The PWM signal is input to the gates of the FETs 22 and 23, and a predetermined signal for turning them off is input to the gates of the FETs 21 and 24. As a result, the FETs 22 and 23 are turned on only for a period corresponding to the pulse width of the PWM signal, and a voltage having a magnitude corresponding to the total command value V is applied to the motor 6. Generates a leftward torque of a corresponding magnitude.

As described above, the PWM signal generation circuit 17
Current is supplied to the motor 6 in accordance with the total command value V given to the motor driving means comprising the motor driving circuit 20 and the motor 6 generates a torque having a magnitude corresponding to the current. The current detector 19 detects a current supplied to the motor 6 at this time, that is, a current flowing through the motor 6, and outputs a current detection value Is. This current detection value Is is input to the subtractor 14, and is used to calculate the deviation It-Is as described above.

The torque Tm (leftward torque and rightward torque) generated by the motor 6 driven as described above is transmitted to the steering shaft 102 via the reduction gear 7 as the steering assist torque Ta.

<3. Configuration of Feedforward Control Unit>
As described above, the feedforward control unit 30 is provided to improve the ability of the current supplied to the motor 6 to follow the target current value It, and performs a differential control operation based on the steering torque signal T. To generate the FF control command value Vff. FIG. 5A is a block diagram illustrating a configuration of the feedforward control unit 30. As shown in the figure, the feedforward control unit 30 in the present embodiment includes a first low-pass filter 31, a differentiation operation unit 32, a multiplication unit 34, and a second low-pass filter 35.
And an FF control gain calculator 33.
Steering torque signal T output from phase compensation filter 10
Is input to the feedforward control unit 30. After passing through the first low-pass filter 31, the steering torque signal T is input to the differential operation unit 32, where the differential value dT / dt of the steering torque signal T is calculated.

The steering torque signal T is also input to the FF gain calculation unit 33, and the FF control gain calculation unit 33
As a gain of the differential control calculation for generating the control command value Vff, an FF control gain G (T) whose value changes according to the steering torque T is output. That is, the FF control gain calculation unit 33 previously holds a table as shown in FIG. 5B that gives a relationship between the steering torque T and the FF control gain G, and based on the input steering torque signal T, The FF control gain G (T) to be output is obtained with reference to the table. According to the table shown in FIG. 5B, the FF control gain G (T) is 0.0005 (constant value) in the range of 0 ≦ T ≦ 2, and the steering torque is 2 ≦ T ≦
The steering torque decreases monotonously in the range of 4, and becomes 0 in the range of T ≧ 4. The relationship between the FF control gain G and the steering torque T is not limited to the relationship shown in FIG. 5B, but when the steering torque T is small (in a region near zero), the FF control gain G becomes smaller. When the steering torque T becomes a relatively large value and the steering torque T increases to some extent, the FF control gain monotonically decreases with respect to the steering torque T, or the steering torque T and the FF control gain G have a negative correlation. It is preferable to set. Instead of holding such a table, a function or an arithmetic processing routine for providing a relationship between the steering torque T and the FF control gain G is prepared in advance, and the function or the arithmetic processing routine is calculated based on the input steering torque signal T. F to be output by the processing routine
The F control gain G (T) may be calculated.

The multiplier 34 has a differential value dT / dt of the steering torque signal T output from the differential calculator 32 and an FF control gain G output from the FF control gain calculator 33.
(T) is input, and the multiplication unit 34 outputs G (T) · dT / dt, which is a value of the multiplication. This multiplied value G
(T) · dT / dt is output from the feedforward control unit 30 as the FF control command value Vff after passing through the second low-pass filter 35.

As described above, the feedforward control unit 3
At 0, the gain G (T) of the differential control operation is changed according to the steering torque T according to the relationship shown in FIG.
0 is different from the conventional feedforward control unit 30b in which the FF control gain G is fixed regardless of the steering torque T as shown in FIG. For this reason, in the present embodiment,
When the steering torque T is small (when T ≦ 2 [Nm]), such as when the steering wheel 100 starts to be turned from the neutral position, the feedforward control works satisfactorily, but when the steering torque T is a certain level or more. When (T ≧ 2 [Nm]), the function of the feedforward control decreases as the steering torque T increases, and when the steering torque T exceeds a predetermined value (T
≧ 4 [Nm]) The feedforward control does not work.

<4. Operation and Effect> According to the above-described embodiment, the ECU 5 as the control device operates as follows. Now, assuming that the driver starts turning the steering wheel 100 from the neutral position and the steering torque T gradually increases from zero, the target current calculation unit 12 should flow the motor 6 according to the steering torque T (and the vehicle speed). Target current value It
Is set. Then, the PI control unit 15 determines the FB control command value Vfb based on the difference It−Is between the target current value It and the detection value Is of the current actually flowing to the motor 6.
Is generated. With the control using only the FB control command value Vfb, the current of the motor 6 cannot quickly follow the target value It when the motor 6 starts up due to the characteristics of the motor 6 and the drive circuit 20 thereof. On the other hand, in the present embodiment, the feedforward control unit 30 adds the FF control gain G to the differential value dT / dt of the steering torque.
FF control command Vff is generated, and when the steering torque T is small, the FF control gain G (T) is set to a relatively large value (see FIG. 5B). Then, the generated FF control command value Vff is the FB control command value V
The total command value V is generated by being added to fb, and the motor 6 is driven based on the total command value V. Thereby, the control responsiveness when the motor 6 starts up is improved, and when the steering wheel 100 starts to be turned from the neutral position, uneven steering torque and a sense of delay do not occur, and a good steering feeling can be obtained.

Next, when the driver turns the steering wheel 100 and turns a certain amount of steering torque (for example, T ≧ 2 [N
Let us consider a case where the driver further turns the steering wheel 100 at a certain speed or more while m]) is being added.
In this case, the steering torque increases at a rate of change equal to or greater than a certain value.
Even with control using only the B control command value Vfb, relatively good control responsiveness can be obtained. Therefore, when the FF control gain is set to a fixed value as in the related art (see the multiplication unit 34b shown in FIG. 6), the feed is increased in a direction to increase the steering assist force according to the differential value (> 0) of the steering torque T. Forward control works more than necessary. As a result, the steering assist force becomes excessive and the steering feeling deteriorates. On the other hand, in the present embodiment, the steering torque T is a certain degree or more (for example, T ≧ 2 [N
m]), the FF control gain G (T) decreases or becomes zero as the steering torque increases, as shown in FIG.
The control command value Vff) is suppressed, and as a result, generation of an excessive steering assist force is suppressed. Therefore, when a relatively large steering torque T is applied, a good steering feeling can be obtained even when the driver further turns the steering wheel 100 at a certain speed or more.

Next, let us consider a case where the driver tries to turn back the steering wheel 100 from a state where the steering wheel 100 has been cut to some extent. In this case, the steering torque T decreases from a certain value (for example, T ≧ 2 [Nm]) at a predetermined change rate. Even with control using only the control command value Vfb, relatively good control responsiveness can be obtained. Therefore, when the FF control gain is set to a fixed value as in the related art, the feedforward control is performed more than necessary in a direction in which the steering assist force is reduced according to the differential value (<0) of the steering torque. As a result, the steering assist force is insufficient, and the steering feeling deteriorates. On the other hand, in the present embodiment, when the steering torque T is a certain degree or more (for example, T ≧ 2 [Nm]), FIG.
As shown in (b), the FF control gain G (T) decreases or becomes zero as the steering torque T increases, so that the operation of the feedforward control is suppressed, and as a result, the steering assist force is excessively reduced. The occurrence of insufficient steering assist force is suppressed. Therefore, even when the driver tries to turn the steering wheel 100 and the steering torque T decreases when a certain large steering torque T is applied, a good steering feeling can be obtained. Can be

As described above, according to the present embodiment, by changing the FF control gain according to the steering torque, it is possible to suppress the occurrence of excessive or insufficient steering assist force due to the excessive action of the feedforward control. As a result, a good steering feeling can be obtained over the entire region of the steering torque T.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a configuration of an electric power steering device according to an embodiment of the present invention, together with a vehicle configuration related thereto.

FIG. 2 is a block diagram showing a configuration of the electric power steering apparatus according to the embodiment from a control viewpoint.

FIG. 3 is a diagram showing an assist table for setting a target current to be supplied to a motor in the embodiment.

FIG. 4 is a circuit diagram showing a configuration of a motor drive circuit in the embodiment.

FIG. 5A is a block diagram illustrating a configuration of a feedforward control unit according to the embodiment, and FIG. 5B is a characteristic diagram illustrating a relationship between FF control gain and steering torque according to the embodiment.

FIG. 6 is a block diagram illustrating a configuration of a feedforward control unit in a conventional electric power steering device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 3 ... Torque sensor 5 ... Electronic control unit (ECU) 6 ... Motor 12 ... Target current calculation part 14 ... Subtraction part 15 ... PI control part 16 ... Addition part 17 ... PWM signal generation circuit 19 ... Current detector 20 ... Motor drive circuit Numeral 30: feed forward control unit 32: differential operation unit 33: FF control gain operation unit 34: multiplication unit It: target current value Is: current detection value (motor current) T: steering torque (steering torque signal) Ts: steering torque Detected value Ta: steering assist torque Tb: output torque Vfb: FB control command value Vff: FF control command value V: all command values

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) B62D 137: 00 B62D 137: 00 F term (Reference) 3D032 CC08 CC12 DA15 DA23 DA64 DC01 DC02 DC03 DC08 DC12 DC17 DC33 DC34 DC35 DD02 DD06 DD10 DD17 DD18 EA01 EB11 EC23 EC24 GG01 3D033 CA03 CA13 CA16 CA20 CA21 5H571 AA03 BB04 CC02 EE01 EE02 EE03 FF07 FF08 GG04 GG08 GG10 HA09 HB01 HC02 HD02 JJ23 JJ24 JJ26 KK22LL01

Claims (2)

[Claims] 1. An electric power steering device for applying a steering assist force to a steering mechanism of a vehicle by driving an electric motor in accordance with a steering torque applied to an operation means for steering a vehicle, wherein the steering torque is Torque detection means for detecting, target value setting means for setting a target value of a current to be supplied to the electric motor based on a steering torque detected by the torque detection means, and detecting a current flowing in the electric motor. Current detection means for outputting a detected value of the current; motor drive means for driving the electric motor in accordance with a given command value; and a target value for feedback control to be given to the motor drive means, the target value A proportional-integral control means for generating by a proportional-integral control operation based on a deviation between the motor driving value and the current detection value; A feedforward control means for generating a command value for feedforward control to be given to the stage by a differential control operation based on the detected steering torque; and a command value for the feedback control and a command value for the feedforward control. An addition means for calculating an addition value with the command value, and providing the addition value to the motor driving means, wherein the feedforward control means changes a gain of the differential control operation according to the detected steering torque. An electric power steering device, comprising: 2. The feed-forward control means includes: a differential means for obtaining a differential value of the detected steering torque; and a gain of the differential control operation when the detected steering torque is larger than a predetermined value. Gain changing means for changing the gain of the differential control operation so as to have a negative correlation with the obtained steering torque, by multiplying a differential value obtained by the differentiating means by a gain obtained by the gain changing means, The electric power steering apparatus according to claim 1, further comprising: a multiplying unit that generates a command value for the feedback control.