JP2003267235A - Electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric power steering device capable of applying a constant amount of output voltage to a power assist motor, and downsizing a boosting circuit by appropriately boosting judging a situation of generate heat generation of elements. <P>SOLUTION: This electric power steering device is characterized by that it calculates a target boost voltage Vt based on a specified motor torque command value Ts, calculates a reference duty value based on the calculated target boost voltage Vt and input voltage from a battery. Then, it calculates a q-shaft duty value based a q-shaft assist current Iqf. The smaller one between the q-shaft duty value and the reference duty value is regarded as a maximum duty value Tmax. It outputs command signals of a boost duty value Tc, of which the upper limit is set to the maximum duty value Tmax, to a boosting/ deboosting circuit. It drives the boosting/deboosting circuit based on the output signals and boosts input voltage Vin to the target boost voltage Vt. <P>COPYRIGHT: (C)2003,JPO

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 device mounted on an automobile vehicle.

[0002]

2. Description of the Related Art An electric power steering apparatus generates a proper steering assist force by driving a power assist electric motor according to a steering torque applied to a steering shaft through a steering wheel and a vehicle speed. And, this power assisted electric motor is installed in an in-vehicle computer (hereinafter referred to as "EC
U ”). Specifically, it is performed by controlling the assist current supplied to the power assist electric motor based on the steering torque signal and the vehicle speed signal.

Power is supplied to the power assist electric motor from a battery or a charging generator via the ECU. When power is supplied, an input voltage input from a battery or a charging generator is boosted by a booster circuit in the ECU and applied to the power assist electric motor. This is due to various reasons such as responsiveness at the time of sudden turning while traveling at high speed. Further, Japanese Unexamined Patent Publication No. 8-127351 discloses that the power assist motor can be downsized with a small current as a reason for using the booster circuit.

However, when the input voltage is boosted, there is a risk that the inductor, switching element, etc. used in the booster circuit may generate heat, resulting in circuit failure or the like.

To solve this problem, Japanese Patent Laid-Open No. 8-127351
The publication describes that the temperature of an element used in a booster circuit is monitored, and when the temperature exceeds a predetermined value, the output voltage to be boosted is reduced to prevent a circuit failure or the like. .

[0006]

However, if the output voltage is lowered when the temperature of the element used exceeds a predetermined value, the power temporarily supplied to the power assist motor becomes very small. This has a great influence on the steering assist force. Therefore, it is desired that the output voltage can always be applied to some extent without the output voltage being rapidly lowered. That is, it is desired to use a method for coping with the possibility of heat generation of the element in advance, rather than a method for coping ex post facto after the element has generated heat.

Further, when the power assisted electric motor requires a larger torque, a large current flows through the booster circuit, so that the booster circuit becomes large. Specifically, because a large current flows, the heat generation of the element etc. increases,
There is no choice but to use an inductor or a switching element having a size that can endure the heat generation.

Further, when the power assisted electric motor generates electric power, the generated voltage is applied to the capacitor used for smoothing the boosted voltage. Therefore, it is necessary to increase the withstand voltage of the capacitor to the voltage at which power is generated, and it is necessary to increase the size of the capacitor.

The present invention has been made in view of such circumstances, and the power assist is always performed to some extent by judging the situation in which the element may generate heat in advance and appropriately boosting the voltage. It is an object of the present invention to provide an electric power steering device that can be applied to an electric motor and can further reduce the size of a booster circuit.

[0010]

Therefore, the present inventor has diligently studied to solve this problem, and as a result of repeated trial and error,
The target boost voltage, which is the target value of the output voltage boosted according to the steering state, for example, the torque command value to the power assist motor or the q-axis assist current, is set, and the inductor voltage is increased by boosting the input voltage to the target boost voltage. He came up with the idea that the heat generation of an element or the like can be suppressed, and completed the present invention.

That is, the electric motor control means in the electric power steering apparatus of the present invention has a booster circuit, a target booster setting means, and a booster circuit control means, and the target boosted voltage is equal to or lower than a voltage at which heat generation of the booster circuit can be suppressed. It is characterized by setting to.

Here, the booster circuit is a circuit for converting an input voltage input from the power supply source into a boosted output voltage and applying the boosted output voltage to the power assist motor. The power supply source is a battery mounted on the vehicle or a charging generator. The target boosted voltage setting means is a setting means that sets a target boosted voltage that is a target value of the output voltage boosted according to the steering state. Further, the booster circuit control means is a control means for controlling the booster circuit so as to boost the input voltage to the target boosted voltage.

With this, the target boosted voltage can be set according to the steering state, so that the heat generated by boosting, such as the inductor and the element in the booster circuit, can be suppressed to a predetermined value or less. At that time, since the output voltage is not drastically lowered, a certain amount of steering assist force can be obtained and operability can be maintained. In addition, since an inductor having an appropriate size that can withstand heat generation below a predetermined value can be used, the size of the inductor can be reduced.

This target boosted voltage may be set based on the motor torque command value. Here, the electric motor torque command value is a command value of the torque required by the power assist electric motor based on the steering torque signal and the vehicle speed signal. When this motor torque command value is large, a large current may flow through the inductor of the booster circuit. Therefore, when the motor torque command value is large, the target boosted voltage can be set low, so that heat generation of the inductor can be suppressed. At that time, since the output voltage is not drastically lowered, a certain amount of steering assist force can be obtained and operability can be maintained.

The target boost voltage may be set based on the q-axis assist current. In this case, the electric motor control means needs to have an assist current detection means and a q-axis assist current calculation means.

Here, the assist current detecting means is a detecting means for detecting the assist current supplied to the power assist electric motor. The q-axis assist current calculating means is a calculating means for calculating the q-axis assist current based on the assist current. The q-axis assist current is one of the equivalent currents when the three-phase assist current is converted into two-phase (q-axis and d-axis). When the q-axis assist current is large, a large current is flowing in the inductor of the booster circuit.

Therefore, when the q-axis assist current is large, the target boosted voltage is set low and the excessive current flowing through the inductor is suppressed to prevent heat generation in the inductor. At that time, since the output voltage is not drastically lowered, a certain amount of steering assist force can be obtained and operability can be maintained.

The target boost voltage may be set based on both the motor torque command value and the q-axis assist current. Specifically, the smaller one of the target boost voltage based on the q-axis assist current and the target boost voltage based on the motor torque command value is set as the target boost voltage. In this case,
The electric motor control means needs to have an assist current detection means and a q-axis assist current calculation means. The assist current detecting means and the q-axis assist current calculating means are the same as those described above.

With this, the target boost voltage can be set from two directions of the motor torque command value and the q-axis assist current, so that the heat generation of the inductor can be suppressed more effectively.

Further, the electric motor control means in the electric power steering apparatus of the present invention further comprises reference duty value calculation means, assist current detection means, q-axis assist current calculation means, q-axis duty value calculation means, and duty. It has a value comparison / determination means and a command duty value setting means, and the booster circuit boosts voltage based on the command duty value.

Here, the reference duty value calculating means is
It is a calculation unit that calculates the reference duty value based on the input voltage and the target boosted voltage. The assist current detecting means and the q-axis assist current calculating means are the same as those described above. The q-axis duty value calculating means is a calculating means for calculating the q-axis duty value based on the q-axis assist current. The duty value comparison / determination means is a determination means for comparing the reference duty value and the q-axis duty value and determining which one is smaller. The command duty value setting means is a setting means that sets the duty value determined by the duty value comparison and determination means to the maximum duty value and limits the command duty value which is the command signal of the booster circuit control means based on the maximum duty value. .

As a result, when the duty value obtained from the q-axis assist current is smaller than the duty value obtained from the target boost voltage based on the motor torque command value, etc., the duty value obtained from the q-axis assist current is calculated. Used for the maximum duty value. Since the booster circuit is not controlled with a value larger than this maximum duty value, heat generation of the inductor can be suppressed from the two directions of the motor torque command value and the q-axis assist current.

Further, the reference duty value calculating means may calculate the reference duty value by using a correspondence map showing a correspondence relationship between the input voltage and the target boosted voltage. This makes it possible to shorten the calculation time of the duty value.

The frequency of the command signal of the booster circuit control means may be set to a value other than the audible frequency. This can prevent noise.

Further, the electric motor control means in the electric power steering apparatus of the present invention is characterized by further including a change rate calculation means and a boost change means.

Here, the change rate calculating means is a calculating means for calculating the change rate when the target boosted voltage changes. The boost change means is a means for slowing down the responsiveness when boosting when the rate of change exceeds a predetermined value, or for gradually changing the change in boost.

As a result, when the target boosted voltage suddenly changes during sudden steering or the like, the steering assist force is not suddenly changed, so that steering can be performed comfortably and comfortably.

Further, the electric motor control means in the electric power steering apparatus of the present invention is characterized by including a step-up / step-down circuit, power generation detection means, and step-up / step-down circuit control means.

Here, the step-up / down circuit applies an output voltage obtained by stepping up an input voltage input from a power supply source such as a battery or a charging generator to the power assist electric motor, and
This circuit reduces the back electromotive force (power generation state) output from the power assist motor and regenerates it to the power supply source. What is the power generation detection means? It is a detection means for detecting that the power assist electric motor has generated power. The step-up / step-down circuit control means reduces the voltage by the step-up / step-down circuit when the power generation detecting means detects the power generation of the power assist motor, and raises the voltage by the step-up / down circuit when the power generation detecting means does not detect the power generation of the power assist motor. It is a control means for controlling.

As a result, the voltage can be stepped down when the electric motor generates electric power, so that the withstand voltage of the capacitor for smoothing the voltage supplied to the electric motor is not increased more than necessary. As a result, the capacitor can be downsized.

[0031]

BEST MODE FOR CARRYING OUT THE INVENTION Next, the electric power steering apparatus of the present invention will be described in more detail with reference to embodiments.

(Overall Structure) As shown in FIG. 1, the electric power steering apparatus of the present invention includes an ECU 16, a power assist electric motor 14, a position detecting device 15, a steering torque detecting device 17, and a vehicle speed detecting device 11. , Battery 12
And a charging generator 13. In this embodiment, a three-phase brushless motor is used as the power assist electric motor 14.

The ECU 16 includes a microcomputer 6, a step-up / down circuit 1, a step-up / down drive circuit 2, an input voltage detection circuit 4, an output voltage detection circuit 3, an electric motor drive circuit 5, an assist current detection circuit 7, It is composed of a steering torque detection circuit 9.

First, the steering torque detecting device 17 detects the steering torque applied to the steering shaft. A steering torque signal is generated by the steering torque detection circuit 9 based on the detected steering torque. The steering torque detecting device 7 and the steering torque detecting circuit 9 are steering torque detecting means. A vehicle speed signal is generated by detecting the speed of the vehicle by the vehicle speed detecting device 11 which is a vehicle speed detecting means. Then, based on the steering torque signal and the vehicle speed signal, the three-phase brushless motor 1
4 commanded torque value (hereinafter "motor torque command value")
Is calculated by the motor torque command value calculation means of the microcomputer 6. Further, an output signal corresponding to the current supplied to the three-phase brushless motor 14 is generated by the motor control means of the microcomputer 6 based on the calculated motor torque command value.

On the other hand, the input voltage input from the battery 12 which is the power supply source or the charging generator 13 is boosted by the step-up / down circuit 1. The buck-boost circuit 1 is controlled by the buck-boost drive circuit 2 based on a command signal from the microcomputer 6. Then, based on the boosted output voltage and the output signal corresponding to the supply current, the electric motor drive circuit 5 supplies the phase current to the three-phase brushless motor 14. Then, the three-phase brushless motor 14 is driven to generate a steering assist force.

The input voltage detection circuit 4 is the step-up / down circuit 1
The voltage input to the position detecting device 1 and the output voltage detecting circuit 3 detects the voltage output from the step-up / down circuit 1.
5 detects the position signal of the three-phase brushless motor 14.

(Process of Generating Output Signal to Motor Driving Circuit 5 by Motor Control Unit of Microcomputer 6) Next, a process of generating an output signal to the motor driving circuit 5 by the microcomputer 6 will be described with reference to FIG. .

The generated motor torque command value Ts is sent to the torque current converter 601 and the torque current command value Riq is sent.
* Is output. The torque current command value Riq * is compared with a q-axis assist current Iqf, which will be described later, in the comparison unit 610. As a result, the q-axis current difference ΔIq (= | Riq * −
If there is Iqf |), the PI control unit 602 receives proportional-integral control to correct it, and the q-axis assist voltage Vq * is output.

Further, the magnetizing current command value Id * is compared with a d-axis assist current Idf, which will be described later, in the comparing section 609. As a result, the d-axis current difference ΔId (= | Id * −Idf
If there is a |, the PI control unit 603 receives proportional-integral control for correction, and the d-axis assist voltage Vd * is output. Here, the q-axis assist voltage Vq * and the d-axis assist voltage Vd * are voltages on the orthogonal coordinate system as command values to the three-phase brushless motor 14.

Then, the q-axis assist voltage Vq * and d
The shaft assist voltage Vd * is calculated by the three-phase conversion unit (dq inverse conversion unit).
At 604, it is converted into three-phase voltage Vu, Vv, Vw,
It is output to the pulse width modulation unit (PWM) 605. Based on the pulse current output by the pulse width modulator 605,
The electric motor drive circuit 5 (shown in FIG. 1) is activated to drive the three-phase brushless motor 14 through the U-phase, V-phase, and W-phase conductors.
To the power supply state.

During this period, the position detection signal detected by the position detection device (rotation angle sensor) 15 (shown in FIG. 1) connected to the three-phase brushless motor 14 is output to the electrical angle calculation circuit 612. Then, the electrical angle calculation circuit 612 determines the three-phase brushless motor 14 based on the position detection signal.
Is calculated and output as an electric signal to the two-phase conversion unit (dq conversion unit) 608. Here, the angle in parentheses indicates that there is a correlation between the electrical angle of the three-phase brushless motor 14 and the actual angle (or rotation angle), and the electrical angle and the angle are the three-phase brushless motor 14 The electrical angle is used as a specific example of the comprehensive word angle because it can be converted from each other, although it depends on the configuration and the like.

Phase currents Iu, Iv, Iw (hereinafter referred to as "assist current") supplied from the motor drive circuit 5 to the three-phase brushless motor 14 by the assist current detection circuit 7 (shown in FIG. 1) serving as assist current detection means. ") Is detected and an assist current signal is generated. Based on the assist current signal and the electrical signal output from the electrical angle calculation circuit 612, a two-phase conversion unit (dq conversion unit) 60 that is a q-axis assist current calculation unit and a d-axis assist current calculation unit.
At 8, it is converted into a two-phase current. The converted current is q
The axis assist current Iqf and the d-axis assist current Idf. The q-axis assist current Iqf and the d-axis assist current Idf are output to the comparison units 609 and 610 as described above, and are fed back to control the three-phase brushless motor 14.

(Basic Operation of Buck-Boost Circuit 1) Next, the operation of the buck-boost circuit 1 will be described with reference to FIG.

Energy is accumulated in the inductor 101 by the ON operation of the boosting switch 102, and energy is released by the OFF operation. By repeating this, a high voltage at the time of discharging is generated on the cathode side of the diode 104, which enables boosting.

That is, when the boosting switch 102 is turned on, a short-circuit current flows through the inductor 101, and when the boosting switch 102 is turned off, the current flowing through the inductor 101 is cut off. When the current flowing through the inductor 101 is cut off, a high voltage is repeatedly generated on the cathode side of the diode 104 so as to prevent a change in magnetic flux due to the cutoff of the current. The generated voltage is smoothed by the capacitor 105, and the output voltage Vo is output to the motor drive circuit 5. This is called a boost chopper method and is widely known. Since the step-down switch 103 in this case is normally in the OFF operation state, the current passes through the diode 104. In the present embodiment, the diode 104 is described in the figure for the sake of convenience, but it is possible to use a MOS-FET for the step-up switching element 102 so that a parasitic diode configured as a MOS-FET can be used. The description of being the same is omitted.

The ON / OFF operation of the step-up switch 102 and the step-down switch 103 is performed by the step-up / down circuit drive circuit 2 based on a command signal from the step-up / step-down circuit control means of the microcomputer 6. The command signal from the microcomputer 6 is a pulse wave and is a signal indicating a duty value of a predetermined frequency. Generally, when the duty ratio of the boosting switch 102 is large, the boosting ratio is large, and when the duty value is small, the boosting ratio is small.

When the step-down switch 103 is turned on, a current flows from the higher input voltage Vin to the lower output voltage Vo. That is, the input voltage Vi
When n is large, the current flows from the inductor 101 side to the capacitor 105 side through the step-down switch 103 without passing through the diode. On the other hand, when the input voltage Vin is small, it will flow backward. Normally, the input voltage V
Since in is small, it will flow backwards.

Next, FIG. 4 shows changes in the current IL flowing through the inductor 101 when the input voltage Vin is boosted, that is, when the boosting switch 102 is turned on / off. ON operation of the boost switch 102 (Ton)
At time, that is, while energy is being accumulated, the current flowing through the inductor 101 increases. On the other hand, during the OFF operation (Toff) of the boost switch 102, that is, while the energy is being released, the current flowing through the inductor 101 decreases.

Next, the method of controlling the step-up / down circuit 1 will be specifically described.

(Control by Setting Target Boost Voltage) A control method of the step-up / down circuit 1 which is a characteristic part of the present invention will be described. First, the target boost voltage setting means of the microcomputer 6 sets the target boost voltage Vt, which is the target value of the output voltage Vo, according to the steering state. The target boost voltage Vt is set based on, for example, the electric motor torque command value Ts. Target boost voltage Vt set based on electric motor torque command value Ts
An example is shown in FIG. In the electric power steering apparatus, when the steering wheel is steered, the rotational force (following ability) of the three-phase brushless motor 14 must be extracted. Therefore, the voltage applied to the three-phase brushless motor 14 needs to be higher. The electric motor torque command value Ts is relatively small when the steering wheel is steered. Further, when the steering wheel is stationary, the torque of the three-phase brushless motor 14 is required, and a large current is passed, but at this time, since the rotational force of the motor 14 is not required, the target boosted voltage Vt is set small.

Next, the target boosted voltage V to which the input voltage Vin input from the battery 12 or the charging generator 13 is set is set.
A command signal of a duty value of the boosting switch 102 (hereinafter referred to as “boosting duty value”) capable of boosting to t is generated. Here, it is well known that the input voltage Vin varies depending on the state of the vehicle (variation of electric load, etc.). Therefore, the microcomputer 6 controls the fluctuating input voltage Vi.
The command signal of the boost duty value is determined based on the target boost voltage Vt set as n. The step-up duty value means a ratio of ON operation time of the step-up switch 102 in one cycle.

Here, the input voltage Vin and the target boosted voltage V
The relationship between t and the boost duty value will be described.

The change rate Ia of the current flowing from the inductor 101 when the boosting switch 102 is turned on is shown in Equation 1.
L is the inductance value of the inductor 101.

[0054]

## EQU1 ## Ia = di / dt = Vin / L Further, the rate of change Ib of the current flowing from the inductor 101 when the boosting switch 102 is turned off is shown in Equation 2.

[0055]

## EQU00002 ## Ib = di / dt = (Vo-Vin) / L Then, when the ON operation time Ton and the OFF operation time Toff of the boosting switch 102 are equal, Ia = Ib
Becomes That is, it becomes as shown in Formula 3.

[0056]

## EQU3 ## Ia * Ton = Ib * Toff Vin * Ton = (Vo-Vin) * Toff Here, when the boost duty value when the input voltage Vin is 10 V and the target boost voltage Vt is 25 V is calculated,
become that way. Note that Ton + Toff = 1. Further, the voltage drop due to the diode 104 and the loss due to the wiring resistance are not considered.

[0057]

## EQU00004 ## Ton = {(25-10) / 10} * (1-Ton) Ton = 0.6 = 60% Therefore, in this case, the command signal with the boost duty value of 60% is generated. That is, when the driving frequency of the boosting switch 102 and the value of the inductance L are fixed, the boosting duty value is uniquely determined by the input voltage Vin and the target boosting voltage Vt. FIG. 7 shows a map table of this relationship.

Next, the command signal generated by the microcomputer 6 is output to the step-up / down drive circuit 2. The step-up / down drive circuit 2 drives the step-up switch 102 and the step-down switch 103 based on the output signal.

By controlling the step-up / down circuit 1 in this way, the following effects are obtained.

When the electric motor torque command value Ts is large, a large current may flow in the step-up / down circuit 1. In such a case, the target boosted voltage setting means of the microcomputer 6 can set the target boosted voltage Vt small. Therefore, a large current does not flow in the buck-boost circuit 1 and the inductor 10
1, it is possible to suppress heat generation of the boost switch 102, the step-down switch 103, and the like.

Although the target boost voltage Vt is set based on the electric motor torque command value Ts in the present embodiment, it may be set based on the q-axis assist current Iqf. The target boosted voltage Vt at that time is shown in FIG. The larger the q-axis assist current Iqf, the smaller the target boosted voltage Vt is set. When the q-axis assist current Iqf is large, a large current may flow through the step-up / down circuit 1. In such a case, the target boosted voltage Vt can be set small. Therefore, a large current does not flow in the step-up / down circuit 1, and the inductor 101, the step-up switch 102, and the step-down switch 10
It is possible to suppress the heat generation such as 3.

Further, the microcomputer 6 selects the target boosted voltage Vt obtained based on the motor torque command value Ts and the target boosted voltage Vt obtained based on the q-axis assist current Iqf.
The smaller one may be set as the target boosted voltage Vt.

The correspondence of the target boosted voltage Vt based on the electric motor torque command value Ts or the q-axis assist current Iqf shown in FIGS. 5 and 6 can be further subdivided or the maps can be interpolated. As a result, more comfortable steerability can be realized.

Further, the input voltage Vin and the target boosted voltage Vt
It is not always necessary to preset the map table as shown in FIG. 7 showing the relationship between the boost duty value and the boost duty value, but it is possible to shorten the calculation time of the boost duty value by the microcomputer 6 by using the map table. The interpolation between maps and the number of maps can be subdivided.

When the target boosted voltage Vt is changed and the rate of change becomes equal to or higher than a predetermined value, the responsiveness at the time of boosting is slowed down by software, or the change in boosting is gradually changed by hardware. You should let me. By doing so, since the steering assist force is not suddenly changed, it is possible to comfortably and steer the steering wheel.

Noise can be prevented by setting the frequency of the command signal to a value other than the audible frequency.

(Control by Setting Boosting Duty Value)
A method of controlling the step-up / down circuit 1 which is a characteristic part of the present invention will be described.

First, the microcomputer 6 sets the target boost voltage Vt, which is the target value of the output voltage Vo, to the motor torque command value T.
It is set based on s. Next, the reference duty value calculation means of the microcomputer 6 calculates a boost duty value capable of boosting the input voltage Vin input from the battery 12 or the charging generator 13 to the set target boost voltage Vt. FIG. 7 shows the relationship between the input voltage Vin, the target boost voltage Vt, and the boost duty value. This boost duty value is used as a reference boost duty value.

On the other hand, the q-axis duty value calculating means of the microcomputer 6 calculates the q-axis boost duty value according to the q-axis assist current Iqf. This relationship is shown in FIG.

Then, the smaller of the q-axis boost duty value and the reference boost duty value is determined by the duty value comparison / determination means of the microcomputer 6. The determined boost duty value is set as the maximum duty value Tmax. Further, it is generated by the command duty value setting means using the duty value for controlling the target boosted voltage Vt as a command signal. The generated command signal is output to the buck-boost drive circuit 2. Based on the output signal, the step-up / down drive circuit 2 causes the step-up switch 102 and the step-down switch 103 to operate.
Is driven.

By thus limiting the maximum value of the boosting duty value, it is possible to suppress heat generation without causing an excessive current to flow through the inductor 101. Note that the interpolation between maps and the number of maps shown in FIGS. 5, 7, and 8 can be subdivided.

Here, a typical microcomputer 6 of the present embodiment
The process will be described with reference to the flowchart of FIG.

The input voltage Vin detected by the input voltage detection circuit 4 is read (step S1). On the other hand, based on the steering torque signal and the vehicle speed signal, the electric motor torque command value T
s is calculated (step S2). Then, the q-axis assist current Iqf, which is one of the currents converted into the two-phase current by the two-phase conversion unit (dq conversion unit) 608 based on the output signal of the assist current detection circuit 7 and the electric signal of the electric angle calculation circuit 612. Is calculated (step S3). Then, based on the calculated electric motor torque command value Ts, the target boosted voltage Vt
Is set (step S4). A reference boost duty value is calculated based on the input voltage Vin and the target boost voltage Vt. Further, the boost duty value is calculated based on the q-axis assist current Iqf. Then, the smaller one is set as the maximum duty value Tmax (step S
5). Then, the output voltage Vo detected by the output voltage detection circuit 3 is read (step S6).

Next, the output voltage Vo and the target boosted voltage Vt
Are compared, and if they match, the process ends, and the current boosted state is maintained (step S7: Y). However, if they are different, that is, if the target boosted voltage is changed, the processing is different depending on whether the changed target boosted voltage Vt is larger or smaller than the output voltage Vo, and the comparison is performed (step 8). When the changed target boost voltage Vt is higher than the output voltage Vo (step S8: Y), the boost duty value Tc currently commanded is decreased by a predetermined value (step S10). On the other hand, when the changed target boost voltage Vt is smaller than the output voltage Vo (step S8: N), the boost duty value Tc is increased by a predetermined value (step S).
9). Then, the changed boosting duty value Tc is compared with the calculated maximum duty value Tmax, and if they match, the process ends, and the boosting duty value Tc is used for boosting (step S11). If they do not match,
The output voltage Vo and the target boosted voltage Vt are compared to see if they match, and if they match, the process ends in this case as well, and boosting is performed based on the target boosted voltage Vt (step S12).
However, if they do not match, the process returns to step S8 and the process of changing the boost duty value is performed.

(Control of Step-Down Processing During Power Generation of Three-Phase Brushless Motor 14) A method of controlling the step-up / step-down circuit 1 which is a characteristic part of the present invention will be described.

The three-phase brushless motor 14 may be in a state of generating a counter electromotive force (power generation state) depending on the vehicle and the steering state. Therefore, in the case of the power generation state, by turning on the step-down switch 103 shown in FIG. 3, it can be returned to the battery 12 as regenerative energy. The power generation state is determined by comparing the target boosted voltage Vt and the output voltage Vo by the power generation detection means. That is, when the output voltage Vo exceeds the target boosted voltage Vt by a predetermined value, the power generation state is determined.

When the power assisted motor generates power, the generated voltage is applied to the capacitor 105 (shown in FIG. 3) used to smooth the boosted voltage. In such a case, by turning on the step-down switch 103, the voltage generated in the capacitor 105 is not applied, so that the withstand voltage of the capacitor 105 cannot be increased more than necessary. Therefore, the capacitor 104 can be downsized. However, the step-down switch 10
The ON operation of No. 3 needs to be performed when the boosting switch 102 is in the OFF operation. When the step-up switch 102 and the step-down switch 103 are simultaneously turned on, the capacitor 105
The voltage applied to the three-phase brushless motor 14 stored in the short-circuited state becomes a short-circuited state, which causes an uncomfortable feeling in steering the steering wheel. In the case of power generation, the boost switch 10
This is because an excessive current will flow in 2 and cause a failure of the step-up / down circuit 1. This operation is performed by the microcomputer 6
Therefore, it is possible to prohibit the simultaneous ON operation. It is also possible to prohibit simultaneous ON operation by hardware, and it is possible to prevent a failure even when the microcomputer 6 malfunctions.

(Control by Setting Frequency) A control method of the step-up / down circuit 1 which is a characteristic part of the present invention will be described.

First, the frequency signal setting means of the microcomputer 6 sets the frequency f for driving the boosting switch 102 based on the electric motor torque command value Ts. The frequency f set based on the electric motor torque command value Ts is shown in FIG. This frequency f is the sum (P) of the ON operation time (Ton) and the OFF operation time (Toff) of the boost switch 102.
WM cycle). Next, a boosting duty value capable of boosting the input voltage Vin to a predetermined voltage is calculated. Then, the command signal is generated based on the set frequency f and the calculated boost duty value. The generated command signal is output to the buck-boost drive circuit 2 by the buck-boost circuit control means. Based on the output signal, the step-up / down drive circuit 2 drives the step-up switch 102 and the step-down switch 103 to step up the input voltage Vin.

Therefore, when the electric motor torque command value Ts is large, the frequency f can be set high. Therefore, a large current does not flow in the buck-boost circuit 1 and the inductor 1
The heat generation of 01 can be suppressed. At that time, since the output voltage Vo is not lowered, an appropriate steering assist force can be obtained and operability can be maintained. Moreover, since the heat generation of the inductor 101 can be suppressed, the size of the inductor 101 can be reduced.

Here, the reason why the heat generation of the inductor 101 can be suppressed by changing the frequency f will be described with reference to FIGS. 10 and 11.

FIG. 10 shows a typical example of the DC superimposition characteristic of the inductor 101, which is the relationship between the inductance L and the current IL flowing through the inductor 101. As shown in FIG. 10, there is a region where the inductance L decreases when the current value exceeds a certain value. This region is called a characteristic saturation region.

Then, referring to FIG. 11, for the current IL flowing through the inductor 101, the boost switch 102 is turned on.
The frequency f of OFF operation is 20 kHz (cycle = 50 μs)
In case of, the solid line a indicates 40 kHz (cycle = 25 μs)
The case is shown by a broken line b. Here, the frequency is 20 kHz
The ON operation of the boost switch 102 in the case of z is T1on.
And the OFF operation is set to T1off, and the frequency f is 40 kHz.
In the case of z, the ON operation of the boost switch 102 is set to T2on.
Then, the OFF operation is set to T2off.

First, when the frequency f is 20 kHz,
When the boost switch 102 is turned on (T1on), the current flowing through the inductor 101 increases and reaches the characteristic saturation region, so that the inductance L decreases. As a result, the current flowing through the inductor 101 further increases, causing the inductor 101 to generate heat. When the frequency f is 40 kHz, the current flowing through the inductor 101 does not reach the characteristic saturation region, so that heat generation of the inductor 101 can be suppressed. Thus, by raising the frequency f, the heat generation of the inductor 101 can be suppressed.

Although the frequency f is set based on the electric motor torque command value Ts in this embodiment, it may be set based on the q-axis assist current Iqf. The frequency f at that time is shown in FIG. The larger the q-axis assist current Iqf, the higher the frequency f is set. In this case, a large current may flow through the step-up / down circuit 1, causing the inductor 101 to generate heat. Therefore, by increasing the frequency f, the inductor 101 is prevented from decreasing the output voltage Vo.
Can suppress the heat generation.

Further, the temperature TL of the inductor 101 may be detected by the inductor temperature monitoring means and the frequency f may be set based on the temperature. Here, the step-up / down circuit 1 used in this case is shown in FIG. The same parts as those in the step-up / down circuit 1 shown in FIG. The buck-boost circuit 1 is
Further, it has a temperature sensor 107 such as a thermistor. The temperature sensor 107 is installed near the inductor 101. Then, the detected temperature is input to the microcomputer 6,
Temperature calculation processing is performed. A frequency f suitable for the temperature TL of the inductor 101 is shown in FIG. The higher the temperature TL of the inductor 101, the higher the frequency f is set.

Now, the processing of the microcomputer 6 in this case will be described with reference to the flowchart of FIG.

First, the initial value of the frequency f is set (step S31). For example, it is set to 20 kHz. next,
The temperature TL of the inductor 101 detected by the temperature sensor 107 is read (step S32). Its temperature TL
Determines whether or not the conditional expression “−40 ≦ TL ≦ 85” is satisfied (step S33). When the conditional expression is satisfied, the frequency f is maintained at the initial value. That is, frequency 2
Drive at 0 kHz. When the conditional expression is not satisfied, the frequency f1 is calculated based on the table of FIG. 14 according to the temperature TL of the inductor 101 (step S34). The frequency f1 is reset as the frequency f (step S35).

Therefore, by determining the excessive temperature rise state of the inductor 101 by the microcomputer 6, it is possible to suppress the heat generation of the inductor 101 without lowering the output voltage Vo.

The temperature Tc of the step-up switch 102 or the temperature Td of the step-down switch 103 may be detected by the element temperature monitoring means, and the frequency f may be set based on the temperature. The step-up switch 102 and the step-down switch are
If the ON / OFF operation is intense, heat will be generated.
That is, when the frequency f is high, heat is generated. Here, the step-up / down circuit 1 used in this case is shown in FIG. The step-up / down circuit 1 further includes temperature sensors 106 and 107.
The temperature sensor 106 is installed near the boost switch 102. The temperature sensor 108 is a step-down switch 103.
Will be installed near the. Then, the detected temperature is input to the microcomputer 6 and subjected to temperature calculation processing. A frequency f suitable for the temperature Tc of the boost switch 102 is shown in FIG. Further, the frequency f suitable for the temperature Td of the step-down switch 103 is
Is shown in FIG. The higher the switch temperature, the lower the frequency f is set. Therefore, switching loss can be reduced.

Here, the temperature Tc of the boosting switch 102
The processing of the microcomputer 6 when the frequency f is set based on the above will be described with reference to the flowchart of FIG.

First, the initial value of the frequency f is set (step S41). For example, it is set to 20 kHz. next,
Boosting switch 10 detected by temperature sensor 106
The temperature Tc of 2 is read (step S42). It is determined whether the temperature Tc satisfies the conditional expression “−40 ≦ Tc ≦ 120” (step S43). When the condition is satisfied, the frequency f is maintained at the initial value. That is, it is driven at a frequency of 20 kHz. When the conditional expression is not satisfied, the temperature of the boost switch 102 is detected according to the detected temperature Tc of FIG.
The frequency f1 is calculated based on the table of step 5 (step S4).
4). When the calculated frequency f1 is less than or equal to the initial value f,
The frequency f is maintained at the initial value (step S4).
5). Otherwise, the frequency f1 is reset as the frequency f (step S46).

Therefore, the microcomputer 6 determines the excessive temperature rise state of the step-up switch 102 and the step-down switch 103, so that the inductor 1 can be operated without lowering the output voltage Vo.
The heat generation of 01 can be suppressed.

Noise can be prevented by setting the frequency f to a value other than the audible frequency.

Note that if the frequency f is set too high, control becomes difficult. Therefore, it is advisable to set an upper limit value of the frequency f and change it within a range below that. In that case, heat generation may be suppressed by setting the target boost voltage Vt, setting the boost duty value, and the like.

(Synchronous Control of Operation of Step-Up Switch 102 and Step-Down Switch 103) A method of controlling the step-up / step-down circuit 1 which is a characteristic part of the present invention will be described.

The control method of this embodiment is characterized in that the step-down switch 103 is positively turned on. That is, when the q-axis assist current Iqf is larger than the predetermined value, the operation of the step-up switch 102 and the step-down switch 103 are synchronously controlled. This synchronous control means the step-down switch 103 when the step-up switch 102 is turned on.
Is turned off, or OF of the boost switch 102 is turned off.
The control is such that the step-down switch 103 is turned on during the F operation. When the q-axis assist current Iqf is smaller than the predetermined value, the boost switch 102 is turned on.
The step-down switch 103 is turned off regardless of the OFF operation
Control to operate. Then, the microcomputer 6 generates a command signal for operating the step-up switch 102 and the step-down switch 103 of the step-up / step-down circuit 1 in this way. The generated command signal is the buck-boost drive circuit 2
Is output to. A step-up / down switch 102 and a step-down switch 1 are driven by a step-up / down drive circuit 2 based on the output signal.
03 is driven, and the input voltage Vin is boosted.

However, when the step-down switch 103 is turned on, current normally flows backward. Therefore, it is necessary to determine the state in which the current does not flow backward and perform the synchronous control.

Now, a state in which the current is not reversed even when the synchronous control is performed will be described with reference to FIGS. 20 and 21.

FIG. 20 shows a change in the current IL flowing through the inductor 101 when the steering assist force is small, that is, when the q-axis assist current Iqf is small. During the ON operation (Ts_on) of the boost switch 102, that is, while energy is being accumulated, the current flowing through the inductor 101 increases. On the other hand, the OF of the boost switch 102
During the F operation (Ts_off), that is, while releasing energy, the current flowing through the inductor 101 decreases. The ON / OFF operation of the boost switch 102 is repeated at a predetermined cycle (PWM cycle). If synchronous control is performed in this case, the OF
During the F operation (Ts_off), the step-down switch 103 is O
Since the N operation is performed, the current flows backward during the time (Tr) when the current does not flow in the inductor 101. That is, the output voltage Vo drops.

From this, it is understood that the time (Tr) during which no current flows in the inductor 101 can be set to 0 in order to perform the synchronous control without causing the current to flow backward. That is, as shown in FIG. 21, when the current always flows through the inductor 101, the current does not flow backward even if the synchronous control is performed. 21 shows the q-axis assist current I.
The change in the current IL flowing through the inductor 101 when qf becomes large is shown. Further, Td_on represents an ON operation of the step-down switch 103, and Td_off represents an OFF operation of the step-down switch 103.

When the synchronous control is performed, the heat generation of the step-up / down circuit 1 can be suppressed. The reason for this will be described.

When the step-down switch 103 is turned off, the current does not flow backward, so that the current flows in the inductor 101.
Side to the condenser 105 side. Then, when the step-down switch 103 is turned on, the current passes through the step-down switch 103 without passing through the diode 104. Here, the power consumption Pd of the diode 104 and the power consumption Pm during the ON operation of the step-down switch 103 are given by
Shown in.

[0104]

[Equation 5] Pd = (diode forward voltage) * (current) Pm = (current) ² * (ON resistance of the second switch) Generally, when the step-down switch 103 is ON, the power consumption Pd of the diode 104 is lower than the power consumption Pd. Power consumption Pm is smaller. Therefore, the heat generation of the step-down switch 103 is smaller than the heat generation of the diode 104, and the heat generation of the entire step-up / down circuit 1 can be suppressed.

Therefore, when the q-axis assist current Iqf is larger than the predetermined value, the heat generation of the step-up / down circuit 1 can be suppressed by performing the synchronous control. That is, the current does not pass through the diode 104, but passes through the step-down switch 103 whose power consumption is smaller than that of the diode 104, so that the heat generation of the diode 104 can be suppressed. Further, it is possible to surely increase the pressure without backflow.

FIG. 22 shows changes in the current IL flowing through the inductor 101 when the q-axis assist current Iqf is maximum.
Even in this state, when the synchronous control is performed, it is clear that the buck-boost circuit 1 can be prevented from generating heat without backflow.

Further, in this embodiment, the q-axis assist current I
Although the synchronous control is performed based on qf, the input voltage V
You may perform based on in. That is, the input voltage Vin
Is greater than or equal to a predetermined voltage, synchronous control is performed. Input voltage Vi
When n is large, the rate of boosting is small, so there is a risk of backflow. Therefore, it is possible to perform the synchronous control only when the backflow does not occur, to reliably boost the voltage and to suppress the heat generation of the step-up / down circuit 1.

Further, the boost switch 102 is turned ON / OFF.
ON operation (Ts_o) during F operation time (PWM cycle)
n) When the time ratio is larger than a predetermined value, synchronous control is performed. Therefore, the synchronous control can be performed in a wider range than the case based on the input voltage Vin and the q-axis assist current Iqf described above, and thus the heat generation of the step-up / down circuit 1 can be further suppressed.

Here, an example of the processing of the microcomputer 6 of this embodiment will be described with reference to the flowchart of FIG.

The input voltage Vin detected by the input voltage detection circuit 4 is read (step S41). On the other hand, the electric motor torque command value Ts is calculated based on the steering torque signal and the vehicle speed signal (step S42). The duty value of the boost switch 102 is calculated based on the calculated electric motor torque command value Ts (step S43). Then, the output signal of the assist current detection circuit 7 and the electrical angle calculation circuit 6
Two-phase converter (dq converter) 60 based on 12 electrical signals
The q-axis assist current Iqf, which is one of the currents converted into the two-phase current by 8 is calculated (step S44).

Then, it is determined whether the input voltage Vin is 16V or less (step S45). When this condition is satisfied, it is further determined whether the q-axis assist current Iqf is 35 A or more (step S46). If this condition is also satisfied, it is determined that the condition for starting the synchronous control is satisfied, and the synchronous control is started (step S48). When the input voltage Vin is not 16 V or less (step S45: N)
If the q-axis assist current Iqf is not equal to or higher than 35 A (step S46: N), it is determined whether the calculated duty value of the boost switch 102 is larger than a predetermined value (step S47). This predetermined value is the input voltage V
This corresponds to the duty value set based on the in and the q-axis assist current Iqf. This relationship is shown in FIG. For example, when the input voltage is 8 V and the q-axis assist current Iqf is 25 A or more, the predetermined value is 60.
As a result of the determination, if this condition is satisfied (step S4)
7: Y), synchronous control is started (step S4).
8). If not, the synchronous control is stopped and the process ends (step S49).

FIG. 25 shows the relationship between the input voltage Vin, the q-axis assist current Iqf, and the predetermined value (corresponding to the duty value) to be compared in step S47. The solid line shows the state where the current does not flow backward even when the synchronous control is performed. In the region a, the input voltage Vin is 16 V or less,
This is the case where the q-axis assist current Iqf is 35 A or more. That is, the area corresponds to the case where the conditions of steps S45 and S46 of FIG. 23 are satisfied.

The interpolation between the maps in the map table shown in FIG. 24 and the number of maps can be subdivided.

[0114]

According to the electric power steering apparatus of the present invention, it is possible to always apply a certain amount of output voltage to the power assist electric motor by judging in advance the situation in which the element may generate heat and appropriately boosting the voltage. Further, the booster circuit can be downsized.

[Brief description of drawings]

FIG. 1 is a diagram showing an overall configuration of an electric power steering apparatus of the present invention.

FIG. 2 is a block diagram showing a process of generating an output signal to an electric motor drive circuit by a microcomputer.

FIG. 3 is an electric circuit diagram showing a step-up / down circuit of the present invention.

FIG. 4 is a diagram showing a change in a current flowing through an inductor during ON / OFF operation of a boost switch.

FIG. 5 is a table showing target boosted voltages based on electric motor torque command values.

FIG. 6 is a table showing target boosted voltages based on q-axis assist current.

FIG. 7 is a table showing maximum duty values with respect to an input voltage and a target boost voltage.

FIG. 8 is a table showing maximum duty values based on q-axis assist current.

FIG. 9 is a flowchart showing processing of a microcomputer in a control method by setting a boosted duty value.

FIG. 10 is a diagram showing a DC superimposition characteristic of an inductor.

FIG. 11 is a diagram showing a change in a current flowing through the inductor during ON / OFF operation of the boost switch in the control method by setting the frequency.

FIG. 12 is a table showing frequencies based on electric motor torque command values.

FIG. 13 is a table showing frequencies based on q-axis assist current.

FIG. 14 is a table showing frequencies based on the temperature of the inductor.

FIG. 15 is a table showing frequencies based on the temperature of the boost switch.

FIG. 16 is a table showing frequencies based on the temperature of the step-down switch.

FIG. 17 is an electric circuit diagram showing a step-up / down circuit.

FIG. 18 is a flowchart showing the processing of the microcomputer in the control method by setting the frequency.

FIG. 19 is a flowchart showing the processing of another microcomputer in the control method by setting the frequency.

FIG. 20 is a diagram showing changes in the current flowing through the inductor when the boost switch is turned ON / OFF.

FIG. 21 is a diagram showing changes in the current flowing through the inductor when the boost switch is turned on and off.

FIG. 22 is a diagram showing changes in the current flowing through the inductor when the boost switch is turned on and off.

FIG. 23 is a flowchart showing the processing of the microcomputer in the synchronous control method.

FIG. 24 is a diagram showing a predetermined value based on an input voltage and a q-axis assist current.

FIG. 25 is a diagram showing a relationship among an input voltage, a q-axis assist current, and a predetermined value.

[Explanation of symbols]

1 ・ ・ ・ Buck-boost circuit 2 ... Buck-boost drive circuit 5 ... Motor drive circuit 6 ... Microcomputer 7 Assist current detection circuit 9 Steering torque detection circuit 12 ... Battery 13 ・ ・ ・ Charging generator 14 ・ ・ ・ Three-phase brushless motor 16 ... ECU 101 ... Inductor 102 ... Step-up switch 103 ・ ・ ・ Step-down switch 104 ... Diode 105 ・ ・ ・ Capacitor 106, 107, 108 ... Temperature sensor

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) B62D 119: 00 H02P 6/02 371A F term (reference) 3D032 CC48 DA15 DA23 DA63 DA64 DA65 DD10 EB04 EC23 EC24 GG01 3D033 CA03 CA13 CA16 CA20 CA21 5H560 AA10 BB04 BB12 DA00 DC03 DC05 DC12 DC13 DC20 EB01 SS02 TT10 TT12 XA02 XA12 XA13 5H576 AA15 BB03 BB04 BB06 CC02 DD08 EE01 EE11 GG01 GG04 HB02 JJ03 JJ17 JJ23 JJ24 KK06 LL22 LL24 LL38 LL41 LL42 LL43 MM02 MM03 MM06

Claims (9)

[Claims] 1. A power assist electric motor for assisting steering force, steering torque detecting means for detecting a steering torque applied to a steering shaft, and a steering torque signal, and a vehicle speed for detecting a vehicle speed to generate a vehicle speed signal. Detecting means, electric motor torque command value calculating means for calculating a command value of torque required by the power assist electric motor based on the steering torque signal and the vehicle speed signal, and an electric motor controlling the power assist electric motor based on the electric motor torque command value. In an electric power steering apparatus having a control means and an electric power supply source for supplying drive power to the power assist electric motor via the electric motor control means, the electric motor control means controls the input voltage input from the electric power supply source. A booster circuit that converts the boosted output voltage and applies the boosted output voltage to the power assist motor; Target boosted voltage setting means for setting a target boosted voltage which is a target value of the output voltage according to the steering state, and a booster circuit for controlling the booster circuit to boost the input voltage to the target boosted voltage. An electric power steering apparatus comprising: a control unit. 2. The electric power steering apparatus according to claim 1, wherein the target boost voltage is set based on the electric motor torque command value. 3. The electric motor control means further includes assist current detection means for detecting an assist current supplied to the power assist electric motor, and a q-axis assist current which is an equivalent current obtained by converting the assist current into two phases. The electric power steering apparatus according to claim 1, further comprising: a q-axis assist current calculating unit that calculates the target boost voltage, the target boost voltage being set based on the q-axis assist current. 4. The electric motor control means further includes assist current detection means for detecting an assist current supplied to the power assist electric motor, and a q-axis assist current which is an equivalent current obtained by converting the assist current into two phases. Q-axis assist current calculating means for calculating the target boost voltage setting means, and the target boost voltage setting means sets the smaller one of the target boost voltage based on the q-axis assist current and the target boost voltage based on the motor torque command value to the target boost voltage. The electric power steering apparatus according to claim 1, wherein the electric power steering apparatus is set to a voltage. 5. The electric motor control means further detects a reference duty value calculation means for calculating a reference duty value based on the input voltage and the target boosted voltage, and an assist current supplied to the power assist electric motor. Assist current detecting means, q-axis assist current calculating means for calculating a q-axis assist current that is an equivalent current obtained by converting the assist current into two phases, and q based on the q-axis assist current.
Q-axis duty value calculation means for calculating the axis duty value, duty value comparison determination means for comparing the reference duty value and the q-axis duty value to determine which is smaller, and the duty value comparison determination means. And a command duty value setting means for limiting the command duty value which is a command signal of the booster circuit control means based on the maximum duty value, and the booster circuit has the command duty value. 3. The electric power steering device according to claim 1, wherein the voltage is boosted based on the value. 6. The reference duty value calculating means calculates the reference duty value by using a correspondence map showing a correspondence relationship between the input voltage and the target boosted voltage. Electric power steering device. 7. The electric power steering apparatus according to claim 1, wherein the frequency of the command signal of the booster circuit control means is set to a value other than the audible frequency. 8. The electric motor control means further includes a change rate calculation means for calculating a change rate when the target boosted voltage changes, and a responsiveness for boosting when the change rate exceeds a predetermined value. 8. The electric power steering apparatus according to claim 1, further comprising a boost change means for slowing down or gradually changing a change in boost. 9. A power assist motor for assisting steering force, steering torque detecting means for detecting a steering torque applied to a steering shaft to generate a steering torque signal, and vehicle speed for detecting a vehicle speed to generate a vehicle speed signal. Detecting means, electric motor torque command value calculating means for calculating a command value of torque required by the power assist electric motor based on the steering torque signal and the vehicle speed signal, and an electric motor controlling the power assist electric motor based on the electric motor torque command value. In an electric power steering apparatus having a control means and an electric power supply source for supplying drive power to the power assist electric motor via the electric motor control means, the electric motor control means controls the input voltage input from the electric power supply source. The power assist electric motor is converted into a boosted output voltage and applied to the power assist electric motor. A step-up / down circuit that steps down the counter electromotive voltage output from the machine to regenerate the power supply source, a power generation detection unit that detects that the power assisted motor has generated power, and the power generation detection unit is the power assisted motor. A step-up / step-down circuit control means for controlling the step-down voltage to be stepped down by the step-up / step-down circuit when power generation is detected, and stepping up by the step-up / step-down circuit when the power generation detecting means does not detect the power generation of the power assist motor. An electric power steering device characterized by: