JP2005081848A - Control method for electric power steering device and electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a control method for an electric power steering device equipped with an enhanced steering feeling and provide such an electric power steering device. <P>SOLUTION: A PWM signal is impressed on a motor 8 with the output shaft fixed in such a way that the duty ratio ranges between 0-100% (or with the duty ratio until the maximum allowable current flows in the motor 8), and the duty vs. motor current characteristic is obtained. Then the axis of ordinates and the axis of abscissa of the duty vs. motor current characteristic are exchanged so that the motor current vs. duty current curve is obtained. According to the invention, the constant of the polynomial function using the target amperage Itrg as parameter is decided so that approximation is made to the motor current vs. duty curve, and the obtained constant is installed in a feedforward duty arithmetic operation part 23a. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for controlling an electric power steering apparatus and an electric power steering apparatus, and more particularly to a technique for improving steering feeling.

  In recent years, an increasing number of passenger cars and the like are equipped with an electric power steering device instead of the conventional hydraulic power steering device. In the electric power steering device, there is no direct engine drive loss because an in-vehicle battery is used as the power source of the electric motor, and since the electric motor is started only at the steering assist, a decrease in driving fuel consumption can be suppressed, There is a feature that control by an ECU (electronic control unit) can be performed very easily.

  In the electric power steering apparatus, the ECU first sets a target current value according to the steering input, the vehicle speed, etc., compares the target current value with the actual current value supplied to the electric motor, and then compares the comparison result (deviation). ) To control the drive of the electric motor. The ECU has an IC (microcomputer) that controls the electric power steering device, and the microcomputer is, in other words, the target current value corresponding to the steering input and the actual current value supplied to the electric motor match. Current feedback is performed so that the deviation between the two becomes zero. In the current feedback loop including the ECU, for example, a PI function or the like is provided in order to quickly increase the responsiveness by quickly setting the above-described deviation value to zero. Has been increased.

  However, such feedback control alone is not necessarily sufficient in response to changes in the actual current value. For this reason, in recent electric power steering apparatuses, a technique is generally employed in which feedforward control is performed in addition to feedback control to improve the response of the steering assist force (see, for example, Patent Document 1 and Patent Document 2). The feedforward control generates a feedforward control amount based on the target current value. For example, in a region where the target current value is very small, a value proportional to the target current value is generated as a drive signal for the electric motor. Output to the circuit.

Usually, an output switching element such as an H-type bridge circuit power FET (field effect transistor) or IGBT (insulated gate bipolar transistor) is generally used to supply current to the electric motor. In this type of output switching element, when the switching pattern is the upper and lower arm simultaneous switching system and control is performed by PWM (Pulse Width Modulation), the motor current Im (A) with respect to the switching duty (%) is As shown in FIG. 4, it has a non-linear output characteristic. Therefore, in a region where the change of the motor current Im with respect to the change of the duty is extremely small (a range indicated by A′-0-A in FIG. 4), the responsiveness is very poor only by the feedback control. Therefore, in the conventional electric power steering apparatus, the ECU obtains a feedforward duty Dff proportional to the target current value Itrg using an arbitrary feedforward gain Kff. Normally, the feedforward gain Kff is set to a single value, and the product (Kff · Itrg) of this and the target current Itrg is calculated as the feedforward control amount (that is, the feedforward duty Dff).
JP 2002-234457 A (page 2-3, FIG. 1) JP 2001-287658 A (page 4-5, FIG. 2)

  By the way, if the output switching element described above strictly describes the range of A′-0-A, as shown by a line X in FIG. It has a curved output characteristic. The curvature of the output characteristic is caused by a time constant determined by the inductance and resistance value of the electric motor, and the degree thereof varies depending on the magnitude of the inductance and resistance value of the electric motor. However, as described above, in the conventional feedforward control, since the feedforward gain Kff in the range of A′-0-A is set to a single value, the control amount (feedforward duty Dff) is As shown by line Y in FIG. 5, the line intersects line X at points B ′ and B. Since the feedforward duty Dff is more dominant than the feedback duty Dfb in the range of A′-0-A, the target current Itrg and the motor current Im are deviated.

  As a result, as can be seen from FIG. 9 (a diagram showing a change in the motor current Im with respect to the target current value Itrg), the motor current Im is lower than the target current value Itrg in the region below the point a due to a shortage of the feedforward duty Dff. Thus, in the region between the points a and b, the motor current Im becomes higher than the target current value Itrg due to the excessive feedforward duty Dff. Further, in the feedforward control, a limit is set for the feedforward duty Dff, and the feedforward duty Dff reaches the limit because the feedforward control is limited so that the feedforward control is not performed in a duty range equal to or higher than the sudden change point of the output characteristics. In the region between points b and c, the motor current Im becomes lower than the target current value Itrg again.

As a result, appropriate steering assist according to the steering input and the vehicle speed is not performed in a region where the steering input is small (when the steering wheel starts to be turned or when the steering wheel is turned back, etc.), and the driver lacks assistance immediately after starting the steering wheel. , Then over-assist, and again lack of assist.
The present invention has been made in view of such a background, and an object of the present invention is to provide an electric power steering apparatus control method and an electric power steering apparatus that improve the steering feeling.

  According to a first aspect of the present invention, there is provided a control method for an electric power steering apparatus, comprising: an H-type bridge circuit for supplying current to an electric motor; and an upper switch and a lower switch constituting the H-type bridge circuit, The method of controlling an electric power steering apparatus that performs PWM control of both of the above and performing feedback control according to the deviation between the target current value of the electric motor and the actual current supplied to the electric motor, and the target current value Feedforward control that increases as the target current value increases. In the feedforward control, the feedforward gain in the region where the target current value is small is made larger than the feedforward gain in the region where the target current value is large. .

  According to this configuration, the deviation of the actual current value from the target current value due to the output characteristics of the H-type bridge circuit can be suppressed, and the deviation between the target current value and the motor current can be reduced.

  According to a second aspect of the present invention, there is provided an electric power steering apparatus comprising: a steering input detecting means for detecting a steering input to the steering system; an electric motor used for steering assist for the steering system; and an actual drive of the electric motor. Motor current detection means for detecting current; target current value setting means for setting a target current value to be supplied to the electric motor based on a detection result of the steering input detection means; the target current value and the actual drive current; In the electric power steering apparatus having a feedback control means for performing feedback control according to the deviation of the power and a feedforward control means for performing feedforward control according to the target current value, the feedforward control means includes the feedforward control. In this case, a polynomial approximation method is used.

  According to this configuration, for example, even when the output switching element has an output characteristic curved in a certain region, the feedforward control corresponding to the output characteristic can be performed by appropriately determining a constant of the polynomial by an experiment or the like. Thus, the deviation between the target current value and the motor current can be reduced.

  According to such a control method of the electric power steering device and the electric power steering device, the linearity of the motor current with respect to the target current value is remarkably improved, and the assist is provided at the start of turning or turning of the steering wheel having a small steering input. Unnecessary fluctuations in force are suppressed, and the steering feeling can be improved.

  Hereinafter, an embodiment in which the present invention is applied to a rack assist type electric power steering apparatus will be described in detail with reference to the drawings. FIG. 1 is a schematic configuration diagram illustrating an entire electric power steering apparatus according to the embodiment, and FIG. 2 is a block configuration diagram of a control apparatus according to the embodiment.

First, the overall configuration of the electric power steering apparatus 1 will be described with reference to FIG.
The electric power steering apparatus 1 includes an electric motor 8, a ball screw mechanism 11, and an electric motor 8 that apply assist force to the manual steering mechanism 2 of the steering system S including the steering system S from the steering wheel 3 to the steering wheels W and W. It is comprised from the control apparatus 12 etc. which control. In the electric power steering device 1, the motor driving means 13 generates a motor voltage Vm according to the target duty Dtrg from the control device 12, and the electric motor 8 is driven by this motor voltage Vm.

  The manual steering mechanism 2 includes a steering wheel 3 that is steered by a driver, a steering shaft 4 that is integral with the steering wheel 3, and a rack and pinion mechanism 7 that is connected to the steering shaft 4 via a connecting shaft 5. It is configured. The rack and pinion mechanism 7 is built in the steering gear box 6, and the pinion 7 a rotates integrally with the connecting shaft 5. The connecting shaft 5 is provided with universal joints 5a and 5b at both ends in order to absorb the deviation of the angular phase from the steering shaft 4 and the pinion 7a. In the rack and pinion mechanism 7, rack teeth 7 b that mesh with the pinion 7 a are formed on the rack shaft 9, and the rotational movement of the pinion 7 a is caused in the lateral direction (vehicle width direction) of the rack shaft 9 by meshing of the pinion 7 a and the rack teeth 7 b. Converted to reciprocating motion. The rack shaft 9 is connected to left and right front wheels W, W as steering wheels via tie rods 10, 10 at both ends thereof.

  In the electric power steering apparatus 1, an electric motor 8 is disposed coaxially with the rack shaft 9 in order to generate an assist force (auxiliary steering force). The rotational force of the electric motor 8 is converted into a thrust by a ball screw mechanism 11 provided coaxially with the rack shaft 9, and this thrust acts on the rack shaft 9 (ball screw shaft 11a) as an assist force.

  Detection values V, T, Im of the vehicle speed sensor VS, the steering torque sensor TS, and the motor current detection means 14 are input to the control device 12. Then, the control device 12 determines the magnitude and direction of the motor current Im to be supplied to the electric motor 8 according to the detected values V, T, Im, and outputs the target duty Dtrg to the motor driving means 13.

  The vehicle speed sensor VS detects the vehicle speed as the number of pulses per unit time, and transmits an electrical signal corresponding to the detected number of pulses to the control device 12 as a vehicle speed detection value V. The vehicle speed sensor VS may be a dedicated sensor for the electric power steering apparatus 1 or may be a vehicle speed sensor for other systems such as a device for controlling brake lock and a device for controlling traction of driving wheels. May be.

  The steering torque sensor TS is disposed in the steering gear box 6 and detects the magnitude and direction of the steering torque (steering input) applied to the steering wheel 3 by the driver. Then, the steering torque sensor TS transmits an electric signal corresponding to the detected steering torque to the control device 12 as a torque detection value T. The torque detection value T includes information on the steering torque indicating the magnitude and the torque direction indicating the direction of the torque. The torque direction is represented by a plus value / minus value of the steering torque, and the plus value indicates that the steering torque direction is on the right. The negative value indicates that the steering torque direction is the left direction. The detected torque value T is processed as a digital signal in the control device 12.

  The motor current detection means 14 includes a resistor or a Hall element connected in series with the electric motor 8 and detects the magnitude and direction of the motor current Im actually supplied to the electric motor 8. The motor current detection unit 14 feeds back (negative feedback) the motor current value Imo (digital value) corresponding to the motor current Im (analog value) to the control device 12. The motor current value Imo includes information indicating the magnitude of the current and the direction of the current (direction of auxiliary assist), and the direction of the current is expressed as a plus / minus value of the motor current value Imo. The auxiliary assist direction is the right direction, and a negative value is the auxiliary assist direction is the left direction.

  The motor driving means 13 drives the electric motor 8 by applying a motor voltage Vm to the electric motor 8 according to the target duty Dtrg. For example, as shown in FIG. 3, the motor driving means 13 includes a bridge circuit and a power source (vehicle battery) 13e composed of switching elements of four power FETs (Field Effect Transistors) 13a, 13b, 13c, and 13d. Composed. When the target duty Dtrg is input to the gates G1, G2, G3, and G4 of the power FETs 13a, 13b, 13c, and 13d, the motor voltage Vm is applied to the electric motor 8 according to the target duty Dtrg. Then, a motor current Im flows through the electric motor 8, and the electric motor 8 generates an auxiliary steering torque proportional to the motor current Im.

Next, the configuration of the control device 12 will be described in detail with reference to FIG.
The control device 12 is composed of an IC (microcomputer) and peripheral circuits (not shown), and when the microcomputer reads a program written in a ROM (not shown), each module of the program (target current setting means 21, deviation calculation means 22, The feedforward duty calculation means 23 and the like are executed to control the electric power steering apparatus 1. In addition, the control device 12 controls the electric power steering device 1, and an input / output port for inputting / outputting various signals / information / commands, etc., and an AD for converting an analog signal into a digital signal and digitally processing it by a microcomputer. A converter, a DA converter that converts a digital signal processed by the microcomputer into an analog signal, and the like are provided.

  The control device 12 will be described in further detail. The control device 12 mainly includes a target current setting unit 21, a deviation calculation unit 22, a feed forward duty calculation unit 23, a proportional / integral control unit 24, a feedback duty calculation unit 25, and an addition unit 26. The control device 12 has a configuration that performs feedback and feedforward control of the electric motor 8 in order to generate an auxiliary steering torque.

  The target current setting means 21 inputs the torque detection value T from the steering torque sensor TS and the vehicle speed detection value V from the vehicle speed sensor VS, and sets the target current value Itrg corresponding to these torque detection value T and vehicle speed detection value V. Has the function to set. The target current value Itrg is a value that includes information on a current that serves as a reference for setting a target motor current to be supplied to the electric motor 8. Incidentally, the target current value Itrg is associated with the torque detection value T when the torque detection value T is near 0 (setting of the dead zone), and when the torque detection value T becomes equal to or greater than the predetermined torque detection value T, Values that increase with increasing are associated. Further, the target current value Itrg is associated with the vehicle speed detection value V with a large value at a low vehicle speed with a large road reaction force, and with a small value at a high vehicle speed to ensure stability during traveling. Yes. Since the maximum current value that can be supplied to the electric motor 8 is specified, the target current value Itrg is set to be equal to or less than the maximum current value.

  The deviation calculation means 22 receives the target current value Itrg from the target current setting means 21 and the current detection value (motor current value) Imo from the motor current detection means 14 and inputs the target current value Itgr and the motor current value Imo. It has a function of calculating the deviation ΔI.

  The feed forward duty calculation means 23 includes a feed forward duty calculation unit 23a and a feed forward duty limit unit 23b. The feedforward duty calculation unit 23a receives the target current value Itrg and calculates a feedforward duty calculation value Dffo using a polynomial approximation method. The feedforward duty limit unit 23b inputs a feedforward duty calculation value Dffo, a target duty Dtrg and a target current value Itrg as described later, and the feedforward duty calculation value Dffo is a specified limit duty (output in FIG. 4 described later). When the characteristic sudden change points A and A ′) are reached, the calculation is saturated to prevent overshoot of the motor current Im.

  The proportional / integral control means 24 is a proportional element (proportional control means) 24a that performs proportional processing on the deviation K · ΔI to calculate the proportional signal Ip, and similarly performs integration processing on the deviation K · ΔI to obtain an integrated signal. It includes an integration element (integration control means) 24b for calculating Ii, and an adder 24c that adds the proportional signal Ip and the integral signal Ii to obtain the proportional / integral signal Ipi. A differential control means for performing differential control on the deviation K · ΔI may be provided, and the proportional / integral control means 24 may be used as the PID control means.

  The feedback duty calculator 25 generates a feedback duty Dfb according to the proportional / integral signal Ipi. The adding unit 26 adds the feedforward duty Dff generated by the feedforward duty calculating unit 23 to the feedback duty Dfb generated by the feedback duty calculating unit 25 to generate a target duty Dtrg.

Next, the operation of the motor driving unit 13 will be described with reference to FIG.
The motor drive means 13 comprises a bridge circuit with four power FETs 13a, 13b, 13c and 13d, and a voltage of 12V is supplied from a power source (vehicle battery) 13e. Further, in the motor driving means 13, the electric motor 8 is connected in series between the power FET 13a and the power FET 13d and in series between the power FET 13b and the power FET 13c. The power FETs 13a and 13b are turned on when the target duty Dtrg (PWM signal or OFF signal) is input to each of the gates G1 and G2 and the PWM signal is input and the logic level is 1. The power FETs 13c and 13d are turned on when an ON signal or an OFF signal is input to each of the gates G3 and G4 and an ON signal is input. The motor drive means 13 PWM drives the electric motor 8 in the forward rotation direction (the generated auxiliary steering torque is rightward) by the power FET 13a and the power FET 13d, and reversely rotates the electric motor 8 by the power FET 13b and the power FET 13c ( The generated auxiliary steering torque is PWM-driven in the left direction). The motor voltage Vm applied to the electric motor 8 is determined by the duty ratio of the PWM signal. The motor current Im supplied to the electric motor 8 corresponds to the motor voltage Vm. For example, when the duty ratio of the PWM signal is 7 (logic level 1): 3 (logic level 0), 12V × (7/10) = 8.4V is the motor voltage Vm, and the electric motor 8 averages 8. 4V is applied.

Next, a procedure for obtaining the feedforward duty Dff in this embodiment will be described.
In developing the electric power steering apparatus 1, the person in charge of development has a duty ratio of 0 to 100% (or a duty ratio until the maximum allowable current flows in the electric motor 8) in the electric motor 8 with the output shaft fixed. A PWM signal is applied, and output characteristics (duty-motor current characteristics) as shown in FIG. 4 are first obtained. In FIG. 4, the right direction indicates a positive duty and the left direction indicates a negative duty. Here, the feedback control has a gain approximately matched between the duty range below the A ′ point and the duty range above the A point in FIG. 4, but the duty range of A′-0-A where the system gain is extremely low. Then, controllability deteriorates only by feedback control. FIG. 5 is an enlarged view of the duty range of A′-0-A in FIG.

  Next, the developer in charge replaces the vertical axis and the horizontal axis of the duty-motor current characteristic in FIG. 4 to obtain the motor current-duty curve shown in FIG. In FIG. 6, the symbol Im indicates the actual current of the electric motor 8. If this is regarded as the target current value Itrg, the duty ratio for obtaining the target current value Itrg can be obtained. Therefore, the developer in charge determines a constant of a polynomial function having the target current value Itrg as a parameter so as to approximate the motor current-duty curve of FIG. 6 in the current range of A′-0-A. It is incorporated in the feedforward duty calculator 23a.

For example, when a cubic polynomial is adopted,
Dffo = Kff1 · Itrg ³ + Kff2 · Itrg ² + Kff3 · Itrg
3 constants (Kff1, Kff2, Kff3) may be determined,
If a 4th order polynomial is used,
^{Dffo = Kff1 · Itrg 4 + Kff2} · Itrg 3 + Kff3 · Itrg 2 + Kff4 · Itrg
In this case, four constants (Kff1, Kff2, Kff3, Kff4) may be determined.
In order to improve the control accuracy, the order of the polynomial may be increased. However, the order may be set to a necessary and sufficient order in consideration of the processing capability and processing time of the microcomputer in the control device 12.

  As a result of approximation by a third-order polynomial function by the present inventors, a target current value-duty curve as shown in FIG. 7 is obtained, and this polynomial function is incorporated into the feedforward duty calculation unit 23a to perform a bench in an actual machine. As a result of the test, it was confirmed that the linearity of the motor current Im with respect to the target current value Itrg was significantly improved as shown in the graph of FIG. Furthermore, in the electric power steering apparatus 1, the occurrence of unnecessary fluctuations in the assist force can be suppressed even when the steering wheel having a small steering input is started or turned back, and the steering feeling can be improved.

  Although description of specific embodiment is finished above, the aspect of the present invention is not limited to this embodiment. For example, the embodiment described above is an application of the present invention to a rack assist type electric power steering device, but it can also be applied to a column assist type or pinion assist type electric power steering device. In the above embodiment, the polynomial approximation method is used for the feedforward control. However, the method for increasing the feedforward gain in the region where the target current value is small is not limited to this. Furthermore, the specific structure of the electric power steering device and the specific configuration of the control device can be changed as appropriate without departing from the spirit of the present invention.

It is a schematic structure figure showing the whole electric power steering device concerning an embodiment. It is a block block diagram of the control apparatus which concerns on embodiment. It is a circuit diagram which shows the structure of the motor drive means which concerns on embodiment. It is a graph which shows the duty-motor current characteristic in an embodiment. FIG. 5 is a diagram showing the duty range of A′-0-A in FIG. 4 by enlarging the vertical axis. It is a figure which shows the motor current-duty curve in embodiment. It is a figure which shows the target electric current value-duty curve in embodiment. It is a figure which shows the change of the motor electric current Im with respect to the target electric current value Itrg in embodiment. It is a figure showing the change of the motor electric current Im with respect to the target electric current value Itrg in a conventional apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electric power steering apparatus 8 Electric motor 21 Target current setting means 23 Feed forward duty calculation means (feed forward control means)
25 Feedback duty calculation means (feedback control means)
S Steering system TS Torque sensor (steering input detection means)

Claims (2)

A method for controlling an electric power steering apparatus having an H-type bridge circuit for supplying current to an electric motor and performing PWM control of both an upper switch and a lower switch constituting the H-type bridge circuit. ,
While performing feedback control according to the deviation between the target current value of the electric motor and the actual current supplied to the electric motor,
Perform feedforward control that increases as the target current value increases,
In the feedforward control, the feedforward gain in the region where the target current value is small is made larger than the feedforward gain in the region where the target current value is large. Steering input detection means for detecting steering input to the steering system;
An electric motor used for steering assist for the steering system;
Motor current detection means for detecting the actual current supplied to the electric motor;
Target current setting means for setting a target current value to be supplied to the electric motor based on the detection result of the steering input detection means;
Feedback control means for performing feedback control according to a deviation between the target current value and the actual current;
In an electric power steering apparatus having feedforward control means for performing feedforward control according to the target current value,
The electric power steering apparatus, wherein the feedforward control means uses a polynomial approximation method in the feedforward control.

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