JP5134042B2 - Electric power steering device - Google Patents

Description

  The present invention relates to an electric power steering apparatus having a temperature estimation function for heat generation of a motor or a motor drive circuit.

  An electric power steering device (electric power steering) that energizes an auxiliary load for a steering device of an automobile or a vehicle by a rotational force of a motor, a motor that applies a steering assist force to a steering system in response to a driver's steering operation, and a motor A control unit (Electronic Control Unit, hereinafter referred to as ECU). The main part of the ECU is composed of a CPU (Central Processing Unit), and includes a calculation unit that calculates a target current of the electric motor and a motor drive unit that drives the motor according to the result.

  A schematic configuration of such an electric power steering apparatus is shown in FIG. Next, an outline of the operation will be described. A steering handle shaft 102 of the steering handle 101 is coupled to a wheel tie rod 107 via a reduction gear 103, a universal joint (handle side) 104, a universal joint (pinion rack side) 105, and a pinion rack mechanism 106. The shaft 102 is provided with a torque sensor 110 that detects the steering torque of the steering handle 101, and a motor 120 that assists the steering force of the steering handle 101 is connected to the steering handle shaft 102 via the reduction gear 103. . The motor control of the electric power steering apparatus inputs the torque value detected by the torque sensor 110, the vehicle speed signal transmitted from the vehicle speed sensor by CAN (Controller Area Network), the engine signal 140 also transmitted from the engine ECU by CAN. The motor 120 is controlled by the ECU 130 as a value. ECU 130 is mainly composed of a CPU, and motor control is executed by a program. The ECU 130 is supplied with power from a battery 170 via an ignition switch (hereinafter referred to as IGSW) 150 and a power relay 160.

  In such an electric power steering device, an FET (Field Effect Transistor) used in a motor 120 or a motor driving circuit for driving the motor 120 generates heat when a current is applied, and burns out. May lead to. For this reason, the current to be energized is limited or the power supply is shut off to protect the motor and FET from burning out.

  As a conventional technique for estimating and protecting the temperature of a motor, an FET, etc., for example, in a control device for an electric power steering device disclosed in Patent Document 1, an ambient temperature detected by a temperature sensor is used as an initial value, and an electric current is applied. By adding the temperature rise of the motor or FET due to the generated heat to the initial value (ambient temperature), the absolute temperature of the motor winding or FET junction that cannot be detected by the temperature sensor is estimated and energized using them. A method of protecting from burning or the like by limiting the current to be disclosed is disclosed.

However, in general, the temperature sensor is often installed at a location away from the winding of the motor that estimates the temperature. In the case of an FET, the temperature sensor is installed on the same substrate on which the FET is installed. In many cases, a cooling fin or the like is attached to the FET, so that a large temperature difference occurs between the temperature detected by the temperature sensor and the estimated temperature if the cooling time is not sufficiently set after energization. Care must be taken as there are cases. In other words, at the start of energization after a sufficient cooling time, the temperature of the temperature sensor and the estimated temperature of the motor or FET are the same, so the temperature detected by the temperature sensor is used as the initial value for temperature estimation. It doesn't matter. That is, with such a temperature estimation method, correct temperature estimation can be expected when temperature estimation is continued, but it is expected that correct temperature estimation is possible in the following cases where temperature estimation cannot be continued. Can not.

  For example, if the steering wheel is suddenly turned back repeatedly for a long time when entering the garage, a large current is energized for a considerable time, so that the motor and FET generate heat violently and the temperature rises significantly. When the garage entry is completed, the IGSW is turned off, and as a result, the temperature estimation is temporarily suspended. Then, after a short time, when the IGSW is turned on again and the steering wheel switching operation is resumed, the temperature of the motor windings and FET junctions is not yet sufficiently cooled. Temperature estimation is started with the ambient temperature detected by the sensor as an initial value. The temperature estimation is basically performed by adding the temperature rise due to the energization current to the initial value, so the ambient temperature, which is the temperature detected by the temperature sensor, is estimated as the initial value for estimating the temperature of the motor or FET. The temperature may be estimated to be lower than the actual motor or FET temperature. As a result, there is a possibility that the motor and the FET may be burned out by limiting the current to be energized using the temperature estimation different from the actual one.

  In order to solve this problem, in the control device for the electric power steering apparatus disclosed in Patent Document 2, even if the power supply is temporarily shut down, the estimated temperature is cooled down to a predetermined temperature or lower, or the estimated temperature and the temperature sensor. The temperature estimation method is used in which the temperature estimation is continued until the temperature difference from the detected temperature becomes a predetermined value or less. In the case of garage entry as described above, even if an IGSW operation is performed in which the power is temporarily shut off and the power is turned on again, temperature estimation is continued until the motor and FET are sufficiently cooled. The temperature can be estimated.

  Further, in the electric power steering device for a vehicle shown in Patent Document 3, since the battery power is consumed when the temperature estimation is continued after the power is shut off, the power is supplied at the estimated temperature immediately before the power is shut off. After the data is stored in a non-volatile memory that can hold data even if it is not, the power supply is shut off to interrupt temperature estimation. And when the power is turned on again, the method of reading the estimated temperature at the time of power shut-off from the nonvolatile memory as the initial value of the temperature estimation and restarting the temperature estimation is adopted. The temperature is estimated not to burn out.

  Furthermore, in the electric power steering device disclosed in Patent Document 4, the engine speed is detected, the engine stall (hereinafter referred to as “engine stall”), and the estimated temperature is stored in the nonvolatile memory when the engine stops. When steering is resumed, the motor and FET burn out by continuing temperature estimation with the higher one of the ambient temperature detected by the temperature sensor and the temperature stored in the nonvolatile memory as the initial value. It is supposed that the temperature is not.

JP-A-11-286278 JP 2002-362393 A JP 2001-138828 A JP 2005-263010 A

As described above, in the conventional temperature estimation method, when it is known in advance that the power supply is shut off, the temperature estimation is continued using the power shutdown signal or the estimated temperature is stored in the nonvolatile memory. Or let me. However, when the amount of power stored in the battery is small when cranking after occurrence of the engine stall, a large current flows when the cell motor is started, so that the voltage of the instantaneous battery power supply is greatly reduced and the CPU It may be reset (RESET) due to a drop. The battery voltage is restored immediately after cranking, but the estimated temperature that resumes after the restoration is reset to the detected temperature (initial value) detected by the temperature sensor. If the temperature of the FET of the motor or motor drive circuit is rising due to the steering operation before the engine stall etc., the low detection temperature detected by the temperature sensor is estimated even though it is not cooled sufficiently during the cranking time. Since it is misjudged that there is room in temperature by using it for temperature calculation, current limit is started from the initial value Ovh-Iref1 as shown in the overheat protection current of FIG. However, there is a problem that there is a possibility of burning the FET of the motor or the motor driving circuit.

  In addition, when the engine speed is detected and the engine is stopped when the engine is stopped as the timing to be stored in the nonvolatile memory, the estimated temperature is stored in the nonvolatile memory. Compared to the above, since the number of times of writing limit is small, there is a problem that the cost increases depending on the number of data to be stored.

  Also, as a characteristic of volatile memory, the retention voltage of stored data tends to be low at extremely low temperatures, and even when the power supply voltage of the battery is less than the guaranteed storage data voltage of the volatile memory of the CPU at low temperatures, the stored data Even when you want to use the detected temperature (initial value) detected by the temperature sensor (for example, when you stop the engine and start the engine the next day) The temperature data of the volatile memory stored in a state where the temperature of the vehicle is high is used, and the steering feels heavy. Therefore, there has been a problem that it is desirable not to use data stored in the volatile memory at low temperatures.

  The present invention has been made to solve the above-described problems. Even when the CPU is reset and restarted due to a voltage drop of the battery power generated by cranking or the like, the accurate temperature is maintained. An object of the present invention is to provide an electric power steering device that can perform estimation and protect a motor and an FET from burning due to heat generation.

In order to solve the above-described problems, an electric power steering apparatus according to the present invention includes a motor that applies a steering assist force to a steering system of a vehicle, a control unit that includes a CPU, a volatile memory, and a motor drive circuit that drives the motor. A temperature sensor that detects an ambient temperature of the control unit, and limits a motor current value that can be energized from the ambient temperature detected by the temperature sensor according to an estimated temperature inside the control unit and a current status of the motor current. And the estimated temperature and the overheat protection limit current value are stored in the volatile memory, and the motor drive is performed based on the estimated temperature and the overheat protection control current value. current supplied to the motor is controlled by a circuit, the predetermined temperature of the ambient temperature, a preset The motor drive circuit based on the estimated temperature and the overheat protection control current value using data of the estimated temperature and the overheat protection limit current value stored in the volatile memory. When the current supplied to the motor is controlled, and the power supply voltage supplied to the CPU and the volatile memory is equal to or lower than the reset voltage for resetting the CPU and equal to or higher than the data storage guarantee voltage of the volatile memory. The current supplied to the motor is controlled by the motor drive circuit based on the estimated temperature and the overheat protection limit current value stored in the volatile memory before the CPU is reset. It is characterized by.

  According to the electric power steering device of the present invention of the present invention, not only when the battery power supply voltage generated at the time of cranking decreases, the power supply voltage supplied to the CPU of the ECU of the electric power steering device reaches the reset voltage of the CPU. Even if the battery voltage drops, if the battery power supply voltage when the battery power supply voltage drops is equal to or higher than the storage data guarantee voltage of the volatile memory of the CPU, the temperature of the CPU that is higher than the detected temperature is not the detected temperature from the temperature sensor. By using the estimated temperature and the overheat protection control current value stored before resetting, temperature estimation can be continued, so that it is possible to protect the motor windings, the motor drive circuit FET, and the like from burning. .

FIG. 3 is a block diagram showing a configuration of a control unit of the electric power steering apparatus in the first embodiment. 3 is a flowchart showing an overheat protection processing procedure in the first embodiment. 3 is a flowchart illustrating a procedure for reading data from a volatile memory according to the first embodiment. 3 is a flowchart illustrating a procedure for storing data in a volatile memory according to the first embodiment. It is a figure which shows the electric current waveform of the overheat protection current of the electric power steering apparatus in Embodiment 1. FIG. FIG. 10 is a diagram showing a relationship between a reset voltage of a CPU of a control unit and a storage holding voltage of a volatile memory in the second embodiment. 12 is a flowchart illustrating a procedure for reading data from a volatile memory according to the third embodiment. It is a whole lineblock diagram showing the outline of an electric power steering device. It is a figure which shows the current waveform of the overheat protection current of the conventional electric power steering apparatus.

  Hereinafter, an electric power steering apparatus according to an embodiment of the present invention will be described with reference to FIGS.

Embodiment 1 FIG.
FIG. 1 is a block diagram illustrating a configuration of a control unit of the electric power steering apparatus according to the first embodiment. FIG. 8 is an overall configuration diagram showing an outline of the electric power steering apparatus used in the first embodiment. Since the entire configuration of the electric power steering apparatus is the same as that of the conventional one, description thereof is omitted.

  As shown in FIG. 1, a control unit (ECU) 130 of the electric power steering apparatus includes a temperature sensor 904 that is mounted on a board of the ECU 130 and detects an ambient temperature Temp, and an ECU from the ambient temperature Temp detected by the temperature sensor 904. The temperature estimation processing unit 903 that estimates the internal temperature, the volatile memory 905 of the CPU 131 that stores the estimated temperature Tcont and the overheat protection limit current Ovh_Iref, and the overheat protection limit current from the estimated temperature Tcont and the motor current value Im The overheat protection processing unit 902 for deriving Ovh-Iref, the torque value Tr detected by the torque sensor 110 and the vehicle speed signal sensor 140 for detecting the steering state of the steering wheel, the current command value Iref from the vehicle speed Vcont and the overheat protection limit current Ovh-Iref. Calculate Current command value processing unit 901, motor current detection circuit 908 for detecting motor current Im from the signal of motor drive circuit 909, subtraction circuit 911 for calculating deviation ΔI between current command value Iref and motor current Im, and deviation ΔI From the voltage command value Vref, a PWM circuit 907 that generates a PWM (Pulse Width Modulation) signal from the voltage command value Vref, and a motor current is supplied to the motor 120 based on the PWM signal. And a motor drive circuit 909.

  Here, the CPU 131 includes a current command value processing unit 901, an overheat protection processing unit 902, a temperature estimation processing unit 903, and a volatile memory 905. The ECU 130 is supplied with power from the battery 170 via the power relay 160. The battery 170 is also a power source for the motor drive circuit 909. Thereby, the steering assist force required by the steering handle 101 is applied by the motor 120. As an example of the motor drive circuit 909, an inverter composed of a switching element such as an FET is used.

  Next, the operation of the electric power steering apparatus according to Embodiment 1 will be described with reference to FIGS. First, basic control of the electric power steering apparatus will be described first, and then a temperature estimation method and an overheat protection method that are main parts of the first embodiment will be described.

  The winding of the motor 120, the core magnetic body, or the FET of the motor drive circuit 909 may change in characteristics or burn out due to overheating, so it is necessary to estimate the temperature inside the ECU and protect it from overheating. In the present embodiment, even when the power supply voltage Ev supplied to the CPU 131 drops to a voltage at which the CPU 131 is reset, the temperature estimation process is performed using the data stored in the volatile memory 905 of the CPU 131. An example of protecting the FET of the motor 120 and the motor drive circuit 909 from overheating will be described.

  First, the ambient temperature Temp detected by the temperature sensor 904 is input to the temperature estimation processing unit 903. Estimated temperature TCont estimated by the temperature estimation processing unit 903 in the volatile memory 905 that can retain the memory even when the power supply voltage Ev of the battery 170 supplied to the CPU 131 decreases to a voltage at which the CPU 131 is reset. The overheat protection limiting current Ovh-Iref calculated by the overheat protection processing unit 902 is written. The written estimated temperature TCont and overheat protection limit current Ovh-Iref are read from the volatile memory 905 to the temperature estimation processing unit 903 and the overheat protection processing unit 902.

  FIG. 2 shows a basic flowchart for estimating the temperature inside the ECU from the ambient temperature Temp detected by the temperature sensor 904 and performing overheat protection processing of the motor 120 and the motor drive circuit 909. First, when the IGSW 150 is turned on, an ignition on signal is read (step S51), and the CPU 131 is initialized (step S52). Next, the estimated temperature TCont is initialized (step S53). Subsequently, a data read process for reading data of the estimated temperature Tcont and the overheat protection limit current Ovh-Iref stored from the volatile memory 905 is executed (step S54). The details of the data reading process (step S54) will be described later with reference to a flowchart showing a data reading procedure from the volatile memory of FIG.

  Further, a temperature estimation process for calculating the estimated temperature TCont by the temperature estimation processing unit 903 using the read data is executed (step S55). Further, overheat protection processing is executed by the overheat protection processing unit 902 based on the calculated estimated temperature TCont, and the overheat protection limited current Ovh-Iref is calculated according to the overheat protection temperature (step S56). Data storage processing (step S57) for storing the calculated estimated temperature Tcont and overheat protection limit current Ovh-Iref in the volatile memory 905 is executed. Details of this data storage process (step S57) will be described later with reference to a flowchart showing a data storage procedure in the volatile memory of FIG.

Next, the current command value processing unit 901 refers to the torque value Tr and the vehicle speed Vcont obtained from the torque sensor 110 and the vehicle speed signal sensor 140, and the overheat protection limit current Ovh− output from the overheat protection processing unit 902. A current command value Iref is calculated using Iref (step S58). The subtraction circuit 911 calculates a deviation ΔI from the current command value Iref and the motor current Im. The proportional integration circuit 906 obtains the voltage command value Vref from the calculated deviation ΔI and outputs it to the PWM circuit 907. . In the PWM circuit 907, a PWM signal is generated from the voltage command value Vref. The motor current Im supplied to the motor 120 is controlled by the motor drive circuit 909 based on this PWM signal.

  By limiting the motor current Im flowing through the motor 120, the winding temperature of the motor 120 and the FET of the motor drive circuit 909 are protected from overheating. The motor current detection circuit 908 detects the motor current Im from the motor drive circuit 909, and the motor current Im is sent to the subtraction circuit 911 to be used for calculating the deviation ΔI.

  After the series of processes, it is determined whether the IGSW 150 is on or off (step S59). If IGSW 150 is on, the process returns to the data reading process (step S54), and steps S54 to S58 are repeated again. If the IGSW 150 is off, the overheat protection process is terminated.

  Details of the data read process (step S54) will be described with reference to a flowchart showing a data read procedure from the volatile memory of FIG. Since the stored estimated temperature TCont and the overheat protection limit current Ovh-Iref are read from the volatile memory 905 only at the first time when the CPU 131 of the ECU 130 is activated, it is first determined whether or not the first time reading is completed. Performed (step S61). If the first reading is completed, reading from the volatile memory 905 is not executed. However, if the initial read is not completed, a read determination is performed using a checksum or the like to check whether there is an abnormality in the read data (step S62). If the read determination is NG, the data of the estimated temperature Tcont and the overheat protection limit current Ovh-Iref read from the volatile memory 905 are not used, and the detected temperature (initial value) at the time of starting the CPU is used. If the read determination is OK, the estimated temperature Tcont and the overheat protection limit current Ovh-Iref stored in the volatile memory 905 are read and used for the calculation of the temperature estimation process (step S55) and the overheat protection process (step S56). (Step S63). Next, in order to indicate that the initial read is completed, it is clearly indicated that the initial read is completed using a flag or the like, and the initial read completion process is executed (step S64). This completes the data reading process.

  The details of the data storage process (step S57) will be described with reference to the flowchart showing the data storage procedure in the volatile memory of FIG. Here, the estimated temperature Tcont and the overheat protection limit current Ovh-Iref processed in the estimated temperature process and the overheat protection process are written in the volatile memory 905 (step S81). Thereafter, the checksum for reading data determination used for the reading determination is updated (step S82). This completes the data storage process.

  When the motor 120 winding and the motor drive circuit 909 FET are restarted in a state where the temperature is not sufficiently lowered, the current waveform of the overheat protection current shown in FIG. In addition, when the CPU 131 is reset after Tsec from the stop, the overheat protection current Ovh-Iref1 due to the low detection temperature detected by the temperature sensor is used as an initial value, and thus an excessive current may flow to the motor 120. There is a possibility that the winding of the motor 120 and the FET of the motor drive circuit 909 may be burned out.

In contrast, in the first embodiment, this overheat protection current Ovh-Iref1 is not used, and the estimated temperature Tcont stored in the volatile memory 905 before the CPU 131 is reset.
When the CPU 131 is reset after Tsec from the stop as shown in the current waveform of the overheat protection current in FIG. Since the protection current Ovh-Iref2 is resumed, the possibility that the winding of the motor 120 and the FET of the motor drive circuit 909 are burned out is reduced.

  As described above, in the electric power steering apparatus according to the first embodiment, the stored estimated temperature TCont and the overheat protection limit current Ovh-Iref are read from the volatile memory only for the first time, and writing is performed each time the temperature estimation process is completed. By performing each time, when the CPU recovers from the reset, the estimated temperature and the overheat protection current immediately before the reset are used. Therefore, the temperature estimation process and the overheat are performed based on the data in the high temperature state before the CPU is reset. The protection process can be resumed, and the effect of preventing the motor and FET from being burned out is exhibited.

Embodiment 2. FIG.
FIG. 6 shows the relationship between the reset voltage of the CPU of the control unit and the storage holding voltage of the volatile memory in the electric power steering apparatus of the second embodiment. Here, the configuration of the control unit of the electric power steering apparatus and the basic flowchart for performing the overheat protection process are the same as those in FIG. 1 and FIG.

  Depending on the CPU 131 used for the ECU 130, there may be no difference between the voltage at which the CPU 131 is reset and the storage data guarantee voltage in the volatile memory 905 of the CPU 131. Therefore, in the second embodiment, the CPU of the control unit in FIG. As shown by the relationship between the reset voltage and the storage holding voltage of the volatile memory, the voltage Vth1 at which the CPU 131 is reset is set to be higher than the storage data guarantee voltage Vth2 of the volatile memory of the CPU 131. Thereby, there is an effect that it is possible to clarify the conditions that can prevent burnout of the winding of the motor 120 and the FET of the motor drive circuit 909 more reliably.

Embodiment 3 FIG.
FIG. 7 shows a data read processing procedure in the overheat protection processing flow of the electric power steering apparatus in the third embodiment. Here, the configuration of the control unit of the electric power steering apparatus and the basic flowchart for performing the overheat protection process are the same as those in FIG. 1 and FIG.

  In the first embodiment and the second embodiment, the data reading process (step S54) in FIG. 2 is performed every time of startup. However, as a characteristic of the volatile memory 905, the retention voltage of stored data is low at low temperatures. Even if the power supply voltage Ev of the battery 170 is lower than the storage data guarantee voltage of the volatile memory 905 of the CPU 131, there is a possibility that the storage data is retained, and the detection that the temperature sensor 904 originally detects is possible. Even when you want to use the temperature (initial value) (stop the engine, start the engine the next day, etc.), the data in the volatile memory 905 stored at a high temperature will be used. You may feel stress.

  Therefore, in the third embodiment, when the engine is started at a low temperature, the stored data stored in the volatile memory 905 of the CPU 131 is not used, but the detected temperature (initial value) at the time of starting the CPU is used. It is. The data read process is performed according to the data read process procedure in the overheat protection process flow shown in FIG.

The basic flowchart for performing the overheat protection process in FIG. 2 is the same as that in the first embodiment and the second embodiment. However, since there is a difference from the data read processing procedure in FIG. Is performed using the procedure for reading data from the volatile memory shown in FIG.

  Next, a procedure for reading data from the volatile memory according to the third embodiment will be described. Since the stored estimated temperature TCont and the overheat protection limit current Ovh-Iref are read from the volatile memory 905 only at the first time when the CPU 131 of the ECU 130 is activated, it is first determined whether or not the first time reading is completed. It is executed (step S71). If the first reading is completed, reading from the volatile memory 905 is not executed. However, if the initial reading is not completed, it is determined whether or not the detected ambient temperature Temp is lower than the low temperature Tth (step S72). If the detected ambient temperature Temp is lower than the low temperature Tth, reading from the volatile memory 905 is not executed, and the detected temperature (initial value) detected when the CPU is activated is used. If the detected ambient temperature Temp is equal to or higher than the low temperature Tth, a checksum or the like is executed to check whether there is an abnormality in the read data, and read quality is determined (step S73). If the read determination is NG, the data of the estimated temperature Tcont and the overheat protection limit current Ovh-Iref read from the volatile memory 905 are not used, and the detected temperature (initial value) at the time of starting the CPU is used. If the read determination is OK, the estimated temperature Tcont and the overheat protection limit current Ovh-Iref stored in the volatile memory 905 are read and used for the calculation of the temperature estimation process (step S55) and the overheat protection process (step S56). (Step S74). Next, in order to indicate that the initial read is completed, the initial read is clearly indicated using a flag or the like, and the initial read completion process is performed (step S75). This completes the data reading process. Here, the low temperature Tth is a very low temperature assuming a cold region.

  As described above, in the electric power steering apparatus according to Embodiment 3, if the detected ambient temperature Temp is lower than the low temperature Tth, reading from the volatile memory is not performed, and the temperature is estimated from the detected temperature at the time of starting the CPU. By performing the process and the overheat protection process, there is an effect that the control of the electric power steering can be realized without giving the stress that the steering is heavy due to the limitation of the overheat protection limit current at a low temperature.

  Although the volatile memory 905 has been described as a cache memory built in the CPU 131 in the embodiment, it may be a DRAM of an external memory.

  Moreover, in the figure, the same code | symbol shows the same or an equivalent part.

120 motor 130 control unit (ECU)
131 CPU
170 Battery 901 Current command value processing unit 902 Overheat protection processing unit 903 Temperature estimation processing unit 904 Temperature sensor 905 Volatile memory 909 Motor drive circuit

Claims (2)

A motor for applying a steering assist force to the vehicle steering system;
A control unit having a CPU, a volatile memory, and a motor drive circuit for driving the motor;
A temperature sensor for detecting the ambient temperature of the control unit,
From the ambient temperature detected by the temperature sensor, an estimated temperature inside the control unit and an overheat protection limit current value that limits a motor current value that can be energized according to the energization status of the motor current are calculated, and the estimated temperature and The overheat protection limit current value is stored in the volatile memory, and the current supplied to the motor by the motor drive circuit is controlled based on the estimated temperature and the overheat protection control current value,
When the ambient temperature is equal to or higher than a predetermined temperature set in advance, the estimated temperature and the overheat protection control current using the data of the estimated temperature and the overheat protection limit current value stored in the volatile memory. A current supplied to the motor is controlled by the motor drive circuit based on the value;
If the power supply voltage supplied to the CPU and the volatile memory is equal to or lower than the reset voltage at which the CPU is reset and equal to or higher than the data storage guarantee voltage of the volatile memory, before the CPU is reset An electric power steering apparatus, wherein a current supplied to the motor is controlled by the motor drive circuit based on the estimated temperature and the overheat protection limit current value stored in the volatile memory.   2. The electric power steering apparatus according to claim 1, wherein a reset voltage of the CPU is set higher than a data storage guarantee voltage of the volatile memory.

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