JP3862223B2 - Electric power steering control device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electric power steering control device that reduces a driver's steering force in a vehicle in which electric motor power is directly applied to a steering system.
[0002]
[Prior art]
The electric power steering apparatus includes an electric motor in a steering system, and controls the power supplied from the electric motor (hereinafter referred to as auxiliary steering force) using a control device, thereby reducing the driver's steering force. When the driver operates the steering wheel, the front wheel is swung through the steering shaft to change the direction of the vehicle. As a configuration for reducing the steering force of the driver, an electric motor that supplies auxiliary steering force is disposed coaxially with, for example, the rack shaft, and a ball screw mechanism is provided so that the rotational power of the electric motor is reciprocated with the rack shaft. Are connected through. The control device controls the output power of the motor by outputting a motor drive signal using the steering torque as a main input (the motor drive signal is a voltage difference applied between the motor terminals). The control device is provided with an H bridge, and forward / reverse control of the electric motor is performed by switching the energization direction of the H bridge. The motor drive signal is controlled to a desired value by performing pulse width modulation control (hereinafter referred to as PWM control) for the H-bridge switch. In this way, the control device drives and controls the output power of the electric motor to assist and supply the thrust of the rack shaft of the steering gear box (see, for example, Patent Document 1).
[0003]
[Patent Document 1]
Japanese Patent Laid-Open No. 8-67266
[Problems to be solved by the invention]
However, in the electric power steering apparatus, when the direction of the steering torque is different from the direction of the motor rotation (hereinafter referred to as return control), there is a problem that a regenerative current is generated in the H bridge due to the counter electromotive force of the motor. With respect to this problem, in the conventional control, the off duty (the time ratio at which the pulse is turned off by PWM control) is increased as the motor rotation speed increases. However, the effect of the regenerative current on the motor current is large when the motor current is small (if the motor current is sufficiently large compared to the regenerative current, the effect of the regenerative current is small and adjustment may be less). . Therefore, it is necessary to reduce the off duty in accordance with the increase in the motor current. However, when the conventional control that does not consider this is performed, the off duty is output when the motor current is large, and the motor current is fed back. On duty (time ratio at which the pulse is turned on by PWM control) is output contrary to the control, and hunting (control contradiction) occurs.
[0005]
Also, the output port of the PWM signal and the gate of each FET (field effect transistor) of the H-bridge are not directly connected, and a protective resistor or the like is interposed in the current path. As a result, the rectangular wave output from the port changes its shape with a first order delay and is input to the gate. The FET has a characteristic that turns on when the gate-source voltage exceeds a specified voltage for a specified time. The larger the PWM output of the port is (the higher the on-time ratio is: higher duty), the less the influence of the first order delayed waveform rounding, so that the FET can be stably turned off. However, when the PWM output is small (off time ratio is high: low duty), it is easily affected and becomes unstable. That is, in the extremely low duty region, it becomes difficult to satisfactorily set the FET off as a result of the switching waveform becoming distorted. As a result, the regenerative current of the H-bridge during return control cannot be controlled, and a large amount of current flows through the motor. Specifically, in FIG. 4, b is originally desired to be a, and as a result, the current flowing through the electric motor increases.
[0006]
Accordingly, the present invention is, by the small to Turkey the effect of the regenerative current to the motor current, a main object to provide a means for improving the motor current control characteristic.
[0007]
[Means for Solving the Problems]
Of the present invention that solves the above-mentioned problems, the invention according to claim 1 is directed to an electric motor that supplies an auxiliary steering force to reduce a steering force of a driver, a steering torque detection means that detects a steering torque, and at least the steering and the target current determination means for determining a target current to be supplied to said motor in accordance with the steering torque by the torque detecting means detects, PWM controlled by P WM drive control means, the downstream-side switch of the upstream-side switch and the ground side of the power supply side a motor driving means for driving the electric motor with an H-bridge circuit to have a, a motor current detecting means for detecting a motor current flowing through the electric motor, and a direction of the motor rotation speed and the motor rotation to the electric motor rotates detected Motor rotation speed detecting means for performing PWM control on at least the upstream switch according to a deviation between the target current and the motor current And a feedback control means for controlling the motor current, wherein a steering torque direction detected by the steering torque detection means is different from a motor rotation direction detected by the motor rotation speed detection means. sometimes, before SL because the downstream switch as the motor current is reduced have a structure to increase the time ratio to be off, it is possible to reduce the influence of the regenerative current to the motor current. In an embodiment to be described later, this corresponds to increasing the off duty as the motor current is smaller by the PWM2 output calculation means and outputting a rectangular wave having a higher off time ratio.
[0008]
The invention according to claim 2 has a configuration in which the time ratio at which the downstream switch is turned off is corrected in accordance with the response characteristic of the circuit constituting the motor driving means in the PWM control. According to this configuration, since the OFF duty is corrected in accordance with the response delay of the H bridge in the PWM control for the downstream switch of the H bridge, the hardware delay of the H bridge can be eliminated. In an embodiment to be described later, this corresponds to adding the PWM2 output value by the PWM2 output calculation means and the correction value output by the dead zone correction value calculation means by the PWM2 output correction means.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
[0010]
<< Overall configuration of electric power steering system >>
First, the overall configuration of the electric power steering apparatus will be described with reference to FIG. As shown in the overall configuration diagram of the electric power steering apparatus in FIG. 2, the manual steering force generating means 6 is connected to a steering shaft 2 provided integrally with the handle 1 via a connecting shaft 3 having universal joints 3a and 3b. The rack and pinion mechanism 5 provided in the steering gear box is connected to the pinion 5a. The rack shaft 7 having rack teeth 7a meshing with the pinion 5a and reciprocatingly converted in the axial direction by these meshing is connected to left and right front wheels 9 as rolling shafts via tie rods 8 at both ends thereof. ing. When the handle 1 is operated, the front wheel 9 is swung via the steering shaft 2 to change the direction of the vehicle.
[0011]
In order to reduce the steering force generated by the manual steering force generating means 6, an electric motor 10 for supplying auxiliary steering force is disposed coaxially with the rack shaft 7, and a ball screw mechanism 11 provided substantially parallel to the rack shaft 7 is provided. It is converted into thrust through the rack shaft 7. A helical gear 10 a is integrally provided on the rotor of the electric motor 10, and the helical gear 10 a is meshed with a helical gear 11 b that is integrally provided on the shaft end of the screw shaft 11 a of the ball screw mechanism 11. A nut of the ball screw mechanism 11 is connected to the rack shaft 7.
[0012]
A steering torque detector 12 for detecting a manual steering torque T acting on the pinion 5a is provided in the steering gear box, and the steering torque detection signal 12a is sent to an electric power steering control device (hereinafter simply referred to as “control device”) 14. You are typing. The control device 14 controls the output power of the electric motor 10 by outputting the electric motor drive signal 15 using the steering torque detection signal 12a as a main input signal. The motor drive signal 15 is a voltage difference applied between the motor terminals.
[0013]
<< Configuration of Electric Power Steering Control Device >>
The configuration of the electric power steering control device will be described in detail with reference to FIGS. 1 and 3. As shown in the block diagram of the electric power steering control device in FIG. 1, the control device 14 includes a target current determining means 16 for generating a target current IT that is a target value of a current signal of the electric motor 10, and at least the target current IT. PWM drive control means 17 that performs PWM control based on this, motor drive means 18 that drives the motor by receiving PWM control from this PWM drive control means 17, motor current detection means 19 that detects the motor current IM flowing through the motor 10, motor The motor rotation speed detection means 36 for detecting the motor rotation speed MV at which the motor 10 rotates, and the deviation between the target current IT output by the target current determination means 16 and the motor current IM output by the motor current detection means 19 are output. It comprises feedback control means 20. The PWM drive control means 17 includes a PWM1 output calculation means 31 for calculating an output value for turning on / off the upstream switches (FET 43 and FET 44 in FIG. 3) of the motor drive means 18, and a downstream switch (FIG. 3). PWM 41 output calculating means 32 for calculating an output value for turning on / off the FET 41 and FET 42), a dead band correction value calculating means 33 for calculating a dead band correction value from the output of the PWM 2 output calculating means 32, and an output of the PWM 2 output calculating means 32 Is constituted by a PWM2 output correction means 34 that corrects the signal with the output of the dead zone correction value calculation means 33, and a PWM signal generation means 35 that outputs the PWM signal 25 based on the output of each calculation means.
[0014]
In the control device 14, the target current determining means 16 determines the target current IT based on the steering torque detection signal 12 a, the PWM drive control means 17 performs PWM control based on at least the target current IT, and the motor driving means 18 Under the PWM control, the output power of the electric motor 10 is driven and controlled. Thus, the steering force of the driver is detected by the steering torque detecting means 12 of the manual steering force generating means 6 (see FIG. 2), and the output power of the electric motor 10 is driven and controlled by the control device 14 to control the steering gear box. The thrust of the rack shaft 7 (see FIG. 2) is supplementarily supplied.
[0015]
FIG. 3 is a circuit diagram of an H bridge constituting the motor driving means 18. The H bridge includes FETs (field effect transistors) 41 to 44, the electric motor 10, a power source, a resistor, and a ground. Each FET is on / off controlled by the PWM signal 25 output from the PWM signal generating means 35 (see FIG. 1). The current flowing through the H bridge is determined by the ON state and the OFF state of each FET, and the direction of the current flowing into the electric motor 10 is controlled.
[0016]
<< PWM drive control means >>
Next, the PWM drive control means will be described in detail with reference to FIG. 1 and FIG. First, motor drive control in the case of forward control will be described. The PWM signal generation means 35 compares the direction of the manual steering torque T (actual input is the steering torque detection signal 12a) detected by the steering torque detection means 12 with the direction of the motor rotation detected by the motor rotation speed detection means 36. . If these two directions are the same, forward control is performed. At that time, the on / off control of each FET switch by the PWM signal generation means 35 at the time of supplying the right direction auxiliary steering force (hereinafter referred to as right assist) is as follows. That is, the FET 41 and FET 44 are off, the FET 42 is on, and the FET 43 is a PWM1 output value by the PWM1 output calculation means 31. As a result, the current flow becomes “FET43 → motor M → FET42 → ground”. Further, on / off control of each FET switch at the time of left-side auxiliary steering force supply (hereinafter referred to as left assist) is as follows. That is, the FET 41 is on, the FET 42 and the FET 43 are off, and the FET 44 is the PWM1 output value. The current at this time flows as “FET44 → motor M → FET41 → ground”.
[0017]
In either case, the PWM1 output value by the PWM1 output calculation means 31 can be expressed by, for example, Equation 1. Here, the PWM1 output calculation means 31 inputs a deviation (IT-IM) between the target current IT output from the feedback control means 20 and the motor current IM, and performs proportional calculation and integration calculation on the deviation, respectively. The product multiplied by individual coefficients is added. The proportional coefficient KP and the integral coefficient KI are stored as constant data in a ROM (not shown) in the control device 14 and can be referred to at any time. The PWM1 output of Equation 1 indicates the ratio of time that the ON state of the signal continues with respect to the period of the rectangular wave, that is, ON Duty (see Equation 2 and FIG. 4). Therefore, according to Equation 1, as the deviation (IT-IM) is larger, a rectangular wave having a higher on-time ratio is output, and the motor current IM tends to increase.
[0018]
PWM1 output = (IT−IM) × KP + Σ (IT−IM) × KI Equation 1
(IT: target current, IM: motor current, KP: proportional coefficient, KI: integral coefficient)
[0019]
On Duty = Ton / (Ton + Toff) Equation 2
[0020]
Next, motor drive control in the case of return control will be described. Similar to the forward control, the PWM signal generating means 35 compares the direction of the manual steering torque T and the direction of the motor rotation, and performs return control if the two directions are different. The on / off control of each FET switch by the PWM signal generation means 35 at the time of the right assist is as follows. That is, the FET 41 and FET 44 are off, the FET 42 is PWM2 output, and the FET 43 is PWM1 output. The current flow at this time is as follows: “FET 43 → motor M → FET 42 → ground”. The on / off control of each FET switch during left assist is as follows. That is, the FET 41 outputs PWM2, the FET 42 and FET 43 are off, and the FET 44 outputs PWM1. The current at this time flows as “FET44 → motor M → FET41 → ground”.
[0021]
In any of the above cases, the PWM1 output can be expressed by, for example, Expression 1 as in the forward control. In either case, the PWM2 output by the PWM2 output calculation means 32 and the deadband correction value by the deadband correction value calculation means 33 can be expressed by Expression 3 and Expression 4 as an example. Here, the PWM2 output calculation means 32 inputs the motor rotation speed MV and the motor current IM, performs a predetermined calculation to eliminate the influence of the regenerative current due to the back electromotive force of the motor, and outputs it. In addition, the dead zone correction value calculation means 33 is 3% of the output value of the PWM2 output calculation means 32 as a “dead zone correction value” that is added to the target output value in advance in order to avoid a response delay of the hardware constituting the H bridge. The minute is output. Then, the two output values are added by the PWM2 output correcting means 34 and outputted to the PWM signal generating means 35. The constants K1, K2, and K3 are stored in a ROM (not shown) and can always be referred to. Further, the PWM2 output of Expression 3 indicates the ratio of the time during which the signal OFF state continues with respect to the period of the rectangular wave, that is, the off duty (see Expression 5 and FIG. 4). Therefore, according to Equation 3, as the motor current IM is smaller, a rectangular wave having a higher off-time ratio is output, and the motor current IM is suppressed.
[0022]
PWM2 output = MV × K1 × [1- (IM / 100A-K2) × K3]
... Formula 3
Dead band correction value = 0.03 x PWM2 output (4)
(MV: motor rotation speed, IM: motor current, K1, K2, K3: constant, A: ampere unit)
[0023]
OFF Duty = Toff / (Ton + Toff) Equation 5
[0024]
In the above embodiment, the dead zone correction value calculation means 33 receives the output value of the PWM2 output calculation means 32 and outputs 3% of the output value. However, it is not necessarily 3%, and the motor drive means 18 is configured as H. It can be changed to an appropriate value depending on the response characteristics of the bridge and the characteristics of the constituent elements. Further, other signals may be input in accordance with the characteristics and a result of performing an appropriate calculation for correction may be output. Various changes can be made without departing from the spirit of the present invention.
[0025]
【The invention's effect】
According to the first aspect of the invention, since the influence of the regenerative current can be reduced, the return control in the state where the electric motor is rotating in the electric motor control can be stabilized.
[0026]
According to the second aspect of the present invention, even when there is a hardware response delay, the current falling point for the off duty can be made uniform.
[0027]
In addition, due to the effect described above, the electric power steering control is smoothly performed, so that the steering feeling of the driver can be improved.
[Brief description of the drawings]
FIG. 1 is a block diagram of an electric power steering control device according to the present invention.
FIG. 2 is an overall configuration diagram of an electric power steering apparatus according to the present invention.
FIG. 3 is a circuit diagram of an H bridge in the electric power steering control device according to the present invention.
FIG. 4 is a diagram showing circuit characteristics of an H bridge in the electric power steering control device according to the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Handle 2 ... Steering shaft 3 ... Connection shaft 5 ... Rack & pinion mechanism 6 ... Manual steering force generation means 10 ... Electric motor 11 ... Ball screw mechanism 12 ... Steering torque detection means 14 ... Electric power steering control device 16 ... Target current determination Means 17 ... PWM drive control means 18 ... Motor drive means 41, 42, 43, 44 ... FET (field effect transistor)

Claims (2)

An electric motor for supplying auxiliary steering force to reduce the steering force of the driver;
Steering torque detection means for detecting steering torque;
Target current determining means for determining a target current to be supplied to the electric motor according to at least the steering torque detected by the steering torque detecting means ;
Is PWM controlled by the PWM drive control means, a motor driving means for driving the electric motor with an H-bridge circuit to have a downstream switch upstream switch and the ground side of the power supply side,
Motor current detecting means for detecting a motor current flowing in the motor;
Electric motor rotation speed detecting means for detecting an electric motor rotation speed at which the electric motor rotates and a direction of motor rotation;
Feedback control means for controlling the motor current by PWM controlling at least the upstream switch according to a deviation between the target current and the motor current;
In an electric power steering apparatus comprising:
Wherein the steering torque detection means and the direction of the steering torque detected by the time in which the direction of motor rotation by the electric motor rotational speed detecting means has detected is different, the downstream switch according to the previous SL motor current becomes smaller the time ratio turns off Having a configuration to enlarge,
An electric power steering control device. In the PWM control, having a configuration for correcting a time ratio at which the downstream side switch is turned off in accordance with response characteristics of a circuit constituting the electric motor driving unit,
The electric power steering control device according to claim 1.

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