JP2003154952A - Electric power steering device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric power steering device capable of restricting overheating at a drive circuit (a power transistor, especially) in an electric motor to protect the circuit. SOLUTION: When a microcomputer 10 determines that the motor drive circuit 20 is in an overheated state based on a target current value It, it executes a drive frequency lowering process to set a lowering control flag F1 (step S204), decrease a dividing number N to be given to a PWM signal generating circuit by a lowering constant K1 to lower a switching frequency of FET (S212), and reset the lowering control flag F1 (S220) when the lowest limit is reached in a drive frequency lowering process. When the microcomputer 10 determines that the overheated state is eliminated, it executes a drive frequency restoration process to set a restoration control flag F2 (S208), gradually increase the dividing number N by a restoration constant K2 to gradually raise the switching frequency of FET (S216), and reset the restoration control flag F2 (S220) when an initial value is reached.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric power steering apparatus that applies a steering assist force to a steering mechanism of a vehicle by driving an electric motor according to a steering torque applied to an operating means for steering the vehicle. ,
More specifically, the present invention relates to measures against heat generation in a circuit that drives an electric motor in such an electric power steering device.

[0002]

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

In such an electric power steering system, when the steering assist force is large, as in the case where the steering wheel is stationary (the steering wheel is turned while the vehicle is stopped) for a long time,
A large current may flow through the electric motor to generate heat, which may eventually damage it. Therefore, conventionally, the current flowing through the electric motor is monitored, and when the current continues to exceed the reference value for a certain period of time or more, the normal command value calculated based on the detected steering torque or the like is compared. There is known an electric power steering apparatus in which a value obtained by multiplying a predetermined coefficient smaller than 1 is used as a command value, and the feedback control is performed based on the command value to reduce the current flowing through the electric motor. Such a conventional configuration has a state (hereinafter referred to as an “overheat state”) in which a high temperature that causes a risk of thermal destruction of the elements of the motor drive circuit continues in association with the action of overload protection for the electric motor. It also has a preventive action.

[0004]

However, the above structure for limiting the current to be supplied to the electric motor is a motor drive circuit (in particular, the power transistor which constitutes the circuit).
It is not always optimal to prevent overheating of the. In particular, when a brushless motor is used as the electric motor, heat generation due to the switching operation of the drive circuit becomes larger than that in the case of a brushed motor, so an optimal configuration is desired as a measure for preventing overheating of the drive circuit.

Therefore, an object of the present invention is to provide an electric power having an optimum configuration for suppressing heat generation of a drive circuit (especially a power transistor) of an electric motor to suppress the heat generation when the drive circuit is overheated. A steering device is provided.

[0006]

SUMMARY OF THE INVENTION A first aspect of the present invention is directed to a steering mechanism of a vehicle by driving an electric motor according to a steering torque applied to an operating means for steering the vehicle. An electric power steering apparatus that provides a current detection means for detecting a current flowing through an electric motor and outputting a detection value of the current, and a target set as a value of a current to be supplied to the electric motor based on a steering torque. Control means for calculating a command value for feedback control of the electric motor based on the deviation between the current value and the detected value of the current flowing through the electric motor, and a PWM whose duty ratio changes based on the command value calculated by the control means.
PWM signal generating means for generating a signal, motor driving means for driving the electric motor based on the PWM signal generated by the PWM signal generating means, and overheat determining means for determining whether or not the motor driving means is in an overheated state. When the overheat determination means determines that the PWM signal is in the overheated state, the drive frequency lowering means changes the frequency of the PWM signal to a frequency lower than a preset initial value.

According to the first aspect of the invention, since the switching frequency by the switching element of the motor drive circuit is changed, the FET is maintained while maintaining the normal steering assist force.
Can be cooled effectively to protect it.

In a second aspect based on the first aspect, the control means includes current limiting means for limiting the current to be supplied to the electric motor when the overheat determining means determines that the electric motor is in an overheated state. .

According to the second aspect of the invention, in addition to the cooling effect due to the reduction of the switching frequency, the cooling effect due to the limitation of the current to be supplied to the motor is utilized to make the switching included in the motor drive circuit. The element can be cooled quickly.

In a third aspect based on the first aspect, the overheat determining means determines whether or not the motor drive means is in an overheated state based on the current to be supplied from the motor drive means to the electric motor. Characterize.

According to the third aspect of the invention, by monitoring only the current to be supplied to the electric motor, the switching element included in the motor drive circuit can be easily operated without requiring a special temperature sensor. An overheat condition can be detected.

In a fourth aspect based on the first aspect, the overheat determining means includes temperature detecting means for detecting the temperature of the motor driving means, and the temperature detected by the temperature detecting means exceeds a predetermined threshold value. In this case, the motor driving means is determined to be in an overheated state.

According to the fourth aspect, the overheated state of the switching element included in the motor drive circuit can be accurately detected based on the temperature detected by the temperature sensor regardless of the current to be supplied to the electric motor. You can

A fifth aspect of the present invention further comprises drive frequency restoring means for restoring the frequency of the PWM signal changed by the drive frequency lowering means to an initial value, wherein the overheat determining means is the motor driving means. The drive frequency restoring means includes a cancellation determining means for determining whether or not the overheating state has been resolved, and the drive frequency restoring means determines the heat dissipation time of the motor driving means when the cancellation determining means determines that the overheating state of the motor driving means has been resolved. It is characterized in that the frequency of the PWM signal is returned to the initial value in a corresponding manner.

According to the fifth aspect of the invention, when it is determined that the overheated state has disappeared after the driving frequency lowering process,
By slowly returning the switching frequency to the initial value in a manner according to the heat radiation time of the motor drive circuit, it is possible to take a sufficient time for cooling the switching elements included in the motor drive circuit.

[0016]

DETAILED DESCRIPTION OF THE INVENTION Embodiments of the present invention will be described below with reference to the accompanying drawings. <1. Overall Configuration> FIG. 1 is a schematic diagram showing the configuration of an electric power steering apparatus according to an embodiment of the present invention together with a vehicle configuration related thereto. This electric power steering apparatus includes a steering shaft 102 having one end fixed to a steering wheel (steering wheel) 100 as an operating means for steering, and a rack and pinion mechanism 10 connected to the other end of the steering shaft 102.
4, a torque sensor 3 that detects a steering torque applied to the steering shaft 102 by operating the steering wheel 100, and an electric motor 6 that generates a steering assist force for reducing a driver's load due to steering operation (steering operation). , A reduction gear 7 for transmitting the steering assist force generated by the motor 6 to the steering shaft 102, and a power supply from a vehicle-mounted battery 8 via an ignition switch 9 to obtain sensor signals from the torque sensor 3 and the vehicle speed sensor 4. An electronic control unit (ECU) 5 that controls the drive of the motor 6 based on the above.

When a driver operates the steering wheel 100 in a vehicle equipped with such an electric power steering device, the steering torque due to the operation is detected by the torque sensor 3, and the detected value Ts of the steering torque and the vehicle speed sensor 4 are detected. The ECU 5 drives the motor 6 based on the determined vehicle speed. As a result, the motor 6 generates a steering assist force, and the steering assist force is applied to the steering shaft 102 via the reduction gear 7, whereby the load on the driver due to the steering operation is reduced. That is, the sum of the steering torque Ts applied by the steering wheel operation and the torque (hereinafter referred to as “steering assist torque”) Ta generated by the steering assist force generated by the motor 6 is output torque Tb via the steering shaft 102 as the rack pinion. Provided to the mechanism 104. As a result, when the pinion shaft rotates, the rotation is converted into a reciprocating motion of the rack shaft by the rack and pinion mechanism 104. Both ends of the rack shaft are connecting members 10 each including a tie rod and a knuckle arm.
It is connected to the wheel 108 via 6, and the direction of the wheel 108 changes according to the reciprocating motion of the rack shaft.

<2. Configuration and operation of control device> Fig. 2
FIG. 3 is a block diagram showing a detailed configuration of an ECU 5 that is a control device of the electric power steering device. This E
The CU 5 includes the microcomputer 10 and the PWM signal generation circuit 17
And a current detector 19 and a motor drive circuit 20.

The torque sensor 3 detects a steering torque applied by operating the steering wheel 100. That is,
In the steering shaft 102, a torsion bar is interposed between a portion on the handlebar side and a portion to which the steering assist torque Ta is applied via the reduction gear 7.
The torque sensor 3 detects the steering torque by detecting the twist of the torsion bar. The steering torque detection value Ts thus detected is output from the torque sensor 3 as a steering torque detection signal, and the microcomputer 10
Entered in.

The vehicle speed sensor 4 detects the vehicle speed of the vehicle equipped with this electric power steering device and outputs a signal indicating the detected value Ss as a vehicle speed signal. This vehicle speed signal is also input to the microcomputer 10.

The current detector 19 detects a current actually supplied to the motor 6, that is, a current flowing through the motor 6, and outputs a current detection value Is indicating the current. This detected current value Is is also input to the microcomputer 10.

The PWM signal generating circuit 17 is a pulse signal having a duty ratio D corresponding to a command value V generated by a microcomputer 10 described later, that is, a pulse width modulation signal (a pulse width modulation signal whose pulse width changes according to the command value V described later). Below "PWM
Signal)). The frequency of the PWM signal for driving the motor drive circuit 20 (hereinafter referred to as “drive frequency”) is a frequency division number N given from the microcomputer 10.
Determined by Hereinafter, the detailed configuration of the PWM signal generation circuit will be described with reference to FIG.

The PWM signal generating circuit 17 includes a reference clock generating unit 171, a frequency dividing circuit 172 composed of, for example, a counter, and a PWM signal generating unit 173. The reference clock generator 171 generates the reference clock Sb having a predetermined frequency. The reference clock generator 1
71 may be omitted and the clock of the ECU 5 or the microcomputer 10 may be used as the reference clock Sb. The frequency dividing circuit 172 generates a frequency division signal Sf by dividing the reference clock Sb input from the reference clock generation unit 171 by a frequency division number N smaller than 1 input from the microcomputer 10. The PWM signal generator 173 generates a PWM signal having a duty ratio D based on the frequency-divided signal Sf and supplies the PWM signal to the motor drive circuit 20. The frequency division number N, the frequency f0 of the reference clock Sb, and the frequency f1 of the frequency-divided signal Sf have the relationship of the following expression (1). f1 = f0 × N (where N <1) (1) Further, the function of the PWM signal generation circuit 17 as described above is
It may be realized by the microcomputer 10.

Next, the microcomputer 10 generates the appropriate steering assist force based on the input steering torque detection signal Ts, vehicle speed signal Ss, and current detection value Is, and the PWM signal generation circuit 17 To the duty ratio D. Further, the microcomputer 10 changes the frequency division number N so as to suppress heat generation when the motor drive circuit 20 is overheated. The operation of the microcomputer 10 will be described below with reference to the flowchart shown in FIG.

First, the microcomputer 10 receives the detected steering torque value Ts, the detected vehicle speed value Ss, and the detected current value Is, respectively (step S12). Next, microcomputer 1
In the case of 0, phase compensation is applied to the received steering torque detection value Ts, and the value after the phase compensation and the vehicle speed detection value S
Based on s, a process of calculating the value (target current value) It of the target current to be supplied to the motor 6 (hereinafter referred to as "target current setting process") is performed (step S14). In particular,
The microcomputer 10 refers to a table (referred to as an “assist table”) showing the relationship between the target current value It to be supplied to the motor 6 and the steering torque in order to generate an appropriate steering assist force, using a vehicle speed as a parameter. Target current value It
To set.

Further, the microcomputer 10 uses the target current value It and the detected current value Is output from the current detector 19.
The deviation It-Is from and is calculated. And this deviation It
A calculation (hereinafter referred to as "feedback control calculation") for generating a command value (hereinafter simply referred to as "command value") V for feedback control is performed by a proportional-plus-integral control calculation based on -Is (step S16). This command value V
So that the current of the target current value It flows through the motor 6,
It is a command value for performing feedback control based on the deviation It-Is, and is given by the following equation (1). V = Kp · (It−Is) + Ki · ∫ (It−Is) dt (1) where Kp is the gain of proportional control in the proportional integral control calculation, and Ki is the gain of integral control in the proportional integral control calculation. is there. Then, the microcomputer 10 calculates the duty ratio D to be given to the PWM signal generation circuit 17 based on the command value V.

The microcomputer 10 gives the duty ratio D calculated as described above to the PWM signal generating circuit 17 (step S18).

Finally, the microcomputer 10 determines whether or not the motor drive circuit 20 is overheated, and if necessary,
In order to change the switching frequency of the power transistor included in the motor drive circuit 20, which will be described later, processing for changing the frequency division number N to be given to the PWM signal generation circuit 17 (hereinafter referred to as "switching frequency change processing") is performed (subroutine step). S20). Details of this switching frequency changing process will be described later. In this way, the microcomputer 10 repeats the processing of steps S12 to S20 shown in FIG. Note that the above-mentioned change processing is performed in step S
It is not necessary to be executed after step 18, and the above step S12
It may be performed at any time between S18 and S18. Further, the change processing may be configured as an independent process (task) that is executed in parallel with the loop processing.

Next, the motor drive circuit 20 will be described in detail. The motor drive circuit 20 is connected to the battery 8 via the ignition switch 9 and applies a voltage according to the duty ratio D of the PWM signal to the motor 6. FIG. 5 is a circuit diagram showing a configuration example of the motor drive circuit 20. In this example, a bridge circuit is configured by four field effect transistors (hereinafter referred to as “FET”) 21 to 24 used as power transistors, and the bridge circuit is provided between a power line and a ground line. It is connected. Then, a torque (hereinafter referred to as “rightward torque”) in a direction for assisting the rightward steering is supplied to the motor 6
If it is to be generated, the PWM signal is input from the PWM signal generation circuit 17 to the gates of the FETs 21 and 24, and a predetermined signal for turning them off is input to the gates of the FETs 22 and 23. As a result, the FETs 21, 24 are turned on only during a period (ON period) corresponding to the pulse width of the PWM signal.
Is turned on, and a voltage corresponding to the command value V is applied to the motor 6
Is applied to the motor 6, the motor 6 generates a rightward torque having a magnitude corresponding to the current flowing by the voltage application. on the other hand,
When the motor 6 is required to generate a torque in the direction of assisting the leftward steering (hereinafter referred to as “leftward torque”), the PW
The PWM signal is input to the gates of the FETs 22 and 23 from the M signal generation circuit 17, and a predetermined signal for turning them off is input to the gates of the FETs 21 and 24. As a result, a period corresponding to the pulse width of the PWM signal (ON period)
Only the FETs 22 and 23 are turned on, a voltage having a magnitude corresponding to the command value V is applied to the motor 6, and the motor 6 generates a leftward torque having a magnitude corresponding to the current flowing by the voltage application.

Here, the FET generates Joule heat during its ON period, and also generates heat due to switching loss generated during switching operation that is turned ON / OFF corresponding to the ON period and OFF period of the PWM signal. .
Therefore, the higher the switching frequency of the FET, the larger the amount of heat generation. Therefore, the motor drive circuit 2
When 0 is overheated, the PWM signal generation circuit 17
The frequency division number N to be given to is reduced. That way, PW
The FET is turned on / off in response to the ON period and the OFF period of the PWM signal when the driving frequency of the M signal becomes low.
The switching frequency will decrease.

The switching frequency changing process by the microcomputer 10 is basically performed by a process similar to the conventional process for limiting the electric current supplied to the electric motor to protect the electric motor from overload. it can. The detailed contents of the switching frequency changing process will be described below with reference to the flowchart shown in FIG.

First, the microcomputer 10 calculates a maximum value (hereinafter, referred to as "target current maximum value") MCS of the target current value It for a fixed time (for example, 1 second), and the target current maximum value MCS is predetermined. The number of times the allowable upper limit value MC01 is exceeded within a predetermined time (for example, 30 seconds) is counted (step S202). If the number of counts is equal to or greater than the predetermined threshold number, the microcomputer 10 causes the FE
It is determined that T is in the overheated state, and the lowering control flag F1 is set (step S204). If the FET is not determined to be overheated, the microcomputer 10
Counts how many times the target current maximum value MCS falls below a predetermined allowable lower limit value MC02 within a predetermined time (step S206). If the number of counts is equal to or greater than the predetermined threshold number, the microcomputer 10 causes the FE
A cancellation determination process is performed to determine that the overheated state of T has been resolved, the drop control flag F1 is reset, and the return control flag F2 is set (step S208).

The above FET overheat determination processing may be performed based on the integrated value of the target current value It or the detected current value Is. Further, in the above process, it is determined that the FET is no longer in the overheated state after a certain period of time has passed since the decrease control flag F1 was set or after the period of time required for heat dissipation calculated by the microcomputer 10 has elapsed. The return control flag F2 may be set accordingly.

Next, the microcomputer 10 causes the drop control flag F
It is determined whether 1 is set (step S21
0), if set, the frequency division number N to be given to the PWM signal generation circuit 17 is reduced by a predetermined reduction constant K1 (step S212). If not set, the microcomputer 10 determines whether or not the return control flag F2 is set (step S214), and if it is set, the frequency division number N to be given to the PWM signal generation circuit 17 is set. It is increased by a predetermined restoration constant K2 (step S216). Here, it is assumed that the frequency division number N is given an initial value in advance, and for example, the frequency division number N of the initial value is PWM.
The drive frequency when applied to the signal generation circuit 17 is 2
The driving frequency is 0 KHz, and the driving frequency when the reduction constant K1 is subtracted is about 17 to 19 KHz.

The return constant K2 is preferably a value smaller than the decrease constant K1 (that is, K1> K2), and is a constant value determined so that the drive frequency gradually returns to the initial value. Alternatively, it may be a numerical value calculated by the microcomputer 10 so that a time interval necessary for heat dissipation can be provided. Thus, the return constant K2 is F
In consideration of the heat dissipation mode of ET, a value is selected such that the frequency division number N gradually returns to the initial value after a predetermined time has elapsed.

Next, the microcomputer 10 determines whether or not the frequency division number N has decreased to a predetermined lower limit value or has returned to the initial value (step S218), and if the condition is satisfied, it decreases. The control flag F1 and the return control flag F2 are reset (step S220). The lower limit value of the frequency division number N is, for example, 17 KHz when the switching frequency is
It is preferably selected so as not to drop below (that is, near the upper limit of the audible frequency).

As described above, when the microcomputer 10 determines that the motor drive circuit 20 is in an overheated state, the microcomputer 10 reduces the frequency division number N to the above lower limit value so that the drive frequency of the FET included in the circuit is lowered. A drive frequency lowering process for reducing the constant K1 is performed. After that, when the microcomputer 10 determines that the overheated state is eliminated, the FET 10 radiates heat for a necessary time, so typically, the frequency division number N is increased by a restoration constant K2 so as to gradually return to the initial value. Drive frequency recovery processing is performed.

<3. Modified Example> In the above embodiment, it is determined that the motor drive circuit 20 is in the overheated state based on the target current value It (or the detected current value Is), but it is determined whether the temperature of the motor drive circuit 20 exceeds the threshold value. You may judge. Therefore, in the modified example of the present embodiment, a temperature sensor is provided inside the motor drive circuit 20 (typically in the vicinity of the FETs 21 to 24). FIG. 7 is a block diagram showing a configuration of a control device (ECU) in the electric power steering device further including a temperature sensor. The temperature sensor includes an element such as a thermistor for detecting the temperature, and supplies the detected temperature Js to the microcomputer 10. Also,
The microcomputer 10 determines whether or not the state in which the detected temperature Js exceeds the upper limit temperature indicating the overheat state lasts for a time that causes a risk of causing thermal destruction, instead of the process shown in step S202 of FIG. . Similarly, the microcomputer 10 operates as shown in FIG.
Instead of the process shown in step S206 of
It is determined whether or not the temperature has fallen below a predetermined return temperature indicating the elimination of the overheat condition for a certain period of time. With such a configuration, when the microcomputer 10 directly measures the temperature of the motor drive circuit 20 and determines that the motor drive circuit 20 is in the overheated state, the microcomputer 10 performs the drive frequency lowering process, and then the temperature is lowered and the overheated state is eliminated. If it is determined, the drive frequency restoration process may be performed. In the above-described embodiment, the switching frequency changing process is performed with the overheated state of the motor drive circuit 20 as a determination target. However, the overheated state of the motor 6 is changed by the same method as that of the above-described embodiment (for example, a temperature sensor is provided near the motor 6). The switching frequency changing process may be performed as a determination target depending on the mounting method).

Further, in the above-described embodiment, only the process of changing the switching frequency is performed as a countermeasure against the overheating of the FET. In addition, the upper limit value (or the target current) of the target current value It to be supplied to the motor can also be taken. The value It itself) may be set or changed to limit the current supplied to the motor. That is, the microcomputer 10
In addition to the process shown in step S212 of FIG. 6, a current limiting process of lowering the upper limit value of the target current value It from the initial value or setting a new lower upper limit value is further performed, and the process shown in step S216 is performed. In addition, current limit restoration processing for returning the upper limit value of the target current value It to the initial value may be further performed. With the configuration in which the switching frequency lowering process and the process for limiting the current to be supplied to the motor are combined, the overheated state of the motor drive circuit 20 is quickly eliminated.

Although the motor drive circuit 20 is a circuit for driving the brushed motor 6 as shown in FIG. 5, even if it is a circuit for driving a brushless motor, an overheated state due to the same or more switching loss may occur. Since this occurs, the configuration of the above embodiment can be applied in the same manner.

<4. Action and Effect> According to the above embodiment, the microcomputer 10 controls the FET 2 of the motor drive circuit 20.
When it is determined that 1 to 24 are overheated, these FE
In order to reduce the switching frequency of T, the frequency division number N to be given to the PWM signal generation circuit 17 is reduced. After that, when the microcomputer 10 determines that the overheated state has disappeared, it typically performs a process of returning the switching frequency so as to gradually return to the initial value for heat dissipation. Thus, according to the above-described embodiment, since only the switching frequency is changed without changing the current to be supplied to the motor 6, the FET is effectively cooled while maintaining the normal steering assist force, This can be protected.

Further, according to the above embodiment, the microcomputer 1
0 determines whether or not the FETs 21 to 24 of the motor drive circuit 20 are overheated based on the target current value It to be supplied to the motor 6. As described above, according to the above-described embodiment, by monitoring only the current to be supplied to the motor 6, it is possible to detect the overheated state of the FET with a simple configuration without requiring a special temperature sensor. .

On the other hand, according to a modification of the above-described embodiment, the microcomputer 10 uses the FET of the motor drive circuit 20.
A temperature sensor determines whether 21 to 24 are in an overheated state. Thus, according to the modification of the above-described embodiment, the overheated state of the FET can be detected reliably and accurately regardless of the current value to be supplied to the motor 6.

Further, according to another modification of the above-described embodiment, the microcomputer 10 performs the process of changing the switching frequency and the upper limit value (or the target current value It itself) of the target current value It to be supplied to the motor. ),
The target current value It is limited. As described above, according to the modified example of the above-described embodiment, in addition to the cooling effect by the reduction of the switching frequency, the cooling effect by limiting the current to be supplied to the motor is utilized to rapidly cool the FET. be able to.

[Brief description of drawings]

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

FIG. 2 is a block diagram showing a configuration of a control device (ECU) in the electric power steering device according to the above embodiment.

FIG. 3 is a block diagram showing a configuration of a PWM signal generation circuit in the above embodiment.

FIG. 4 is a flowchart showing the operation of the microcomputer in the above embodiment.

FIG. 5 is a circuit diagram showing a configuration of a motor drive circuit in the above embodiment.

FIG. 6 is a flowchart showing the detailed contents of a switching frequency changing process of the microcomputer in the above embodiment.

FIG. 7 is a block diagram showing a configuration of a control device (ECU) in an electric power steering device according to a modified example of the above embodiment.

[Explanation of symbols]

3 ... Torque sensor 4 ... Vehicle speed sensor 5 ... Electronic control unit (ECU) 6 ... Motor 8 ... Battery 10 ... Microcomputer 17 ... PWM signal generation circuit 19 ... Current detector 20 ... Motor drive circuit 30 ... Temperature sensor 171 ... Reference clock generation unit 172 ... Divider circuit 173 ... PWM signal generator It ... Target current value Is ... Current detection value (motor current) Ts ... Detected value of steering torque Ss ... Detected value of vehicle speed Sb ... Reference clock Sf ... Divided signal V ... Command value N ... Division number D: Duty ratio

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) H02P 5/17 H02P 5/17 H // B62D 107: 00 B62D 107: 00 119: 00 119: 00 F term (Reference) 2F051 AA01 AB06 AC01 AC07 BA03 3D032 DA15 DA64 DA67 EC23 3D033 CA16 CA20 5H571 AA03 BB10 CC04 EE02 GG04 HA09 HD02 KK05 LL01 LL22 MM06

Claims (5)

[Claims] 1. An electric power steering apparatus for applying a steering assist force to a steering mechanism of a vehicle by driving the electric motor according to a steering torque applied to an operating means for steering the vehicle, the electric motor comprising: A current detection unit that detects a flowing current and outputs a detection value of the current, a target current value set based on the steering torque as a value of a current to be supplied to the electric motor, and the current flowing through the electric motor. Control means for calculating a command value for feedback control of the electric motor based on a deviation from a detected value, and a PWM signal for generating a PWM signal with a varying duty ratio based on the command value calculated by the control means Generating means, and a motor for driving the electric motor based on the PWM signal generated by the PWM signal generating means. A drive unit, an overheat determination unit that determines whether the motor drive unit is in an overheated state, and if the overheated determination unit determines that the motor is in an overheated state, the frequency of the PWM signal is set to a preset initial value. An electric power steering apparatus comprising: a drive frequency lowering unit that changes the frequency to a value lower than a value. 2. The control means includes a current limiting means for limiting a current to be supplied to the electric motor when the overheat determining means determines that the electric motor is in an overheated state. Electric power steering device. 3. The overheat determination means determines whether or not the motor drive means is in an overheated state based on a current to be supplied from the motor drive means to the electric motor. The electric power steering device according to 1. 4. The overheat determination means includes a temperature detection means for detecting the temperature of the motor drive means, and when the temperature detected by the temperature detection means exceeds a predetermined threshold value, the motor drive means. The electric power steering apparatus according to claim 1, wherein the electric power steering apparatus is determined to be in an overheated state. 5. The driving frequency returning means for returning the frequency of the PWM signal changed by the driving frequency lowering means to the initial value is further provided, and the overheat judging means eliminates an overheated state of the motor driving means. In the case where it is determined by the elimination determining means that the overheated state of the motor driving means has been eliminated, the drive frequency restoring means determines whether or not the motor driving means has been eliminated. The electric power steering apparatus according to claim 1, wherein the frequency of the PWM signal is returned to the initial value in this manner.