JP2006027512A - Electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric power steering device capable of smoothly reducing the assist force while effectively providing it even during the fail-safe shift as an abnormality occurs in the system. <P>SOLUTION: A microcomputer 11 comprises a system abnormality detection unit 31 to detect occurrence of a system abnormality, and a DUTY limit value operation unit 32 to operate the DUTY limit value α to limit the DUTY indicated value Dx when the system abnormality occurs. The DUTY limit value operation unit 32 reduces the DUTY limit value α as the time is elapsed from the occurrence of the system abnormality. A DUTY indicated value operation unit 23 limits the DUTY indicated value Dx so as to be within a limit range to be regulated by the DUTY limit value α with the value of the DUTY ratio corresponding to the grounding electric potential as a reference. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electric power steering apparatus.

In recent years, an electric power steering device (EPS) using a motor as a drive source has been widely adopted as a vehicle power steering device.
In general, such an EPS is provided with a plurality of sensors such as a torque sensor and a rotation angle sensor, and a motor as a drive source is controlled based on a plurality of vehicle state quantities detected by these various sensors. . As a result, an optimum assist force according to the situation is applied to the steering system, and a system abnormality (system abnormality) is detected based on output signals from various sensors. When the system abnormality occurs, the motor is stopped. It is designed to be fail safe.

  However, at the time of such fail-safe transition, if the operation of the motor is stopped immediately with the occurrence of a system abnormality, the steering feeling may be seriously impaired such as suddenly feeling heavy steering.

For this reason, conventionally, when a system abnormality is detected, there is one that gradually reduces the voltage applied to the motor by reducing the duty ratio of the motor control signal with the passage of time. As a result, when a system abnormality occurs, the assist force can be gently reduced to stop the motor without causing the driver to feel uncomfortable, and as a result, the steering feeling at the time of fail-safe transition can be improved (patent) Reference 1).
Japanese Examined Patent Publication No. 7-94227

  However, in the above prior art, since the reduction is performed at a predetermined reduction ratio that decreases with the passage of time regardless of the duty ratio, even if the assist application is stopped, the assist force is relatively small and the sense of discomfort is small. However, the rationality that the reduction is performed at the same reduction ratio as when the assist force is large occurs. In particular, when the motor is rotating at a high speed, there is a problem that the counter electromotive voltage cannot be canceled due to insufficient applied voltage, and the steering feeling is deteriorated due to a decrease in the rotational speed.

  The present invention has been made in order to solve the above-mentioned problems, and its purpose is to gently reduce it while effectively providing assist force even at the time of fail-safe transition accompanying system abnormality occurrence. An object of the present invention is to provide an electric power steering apparatus capable of

  In order to solve the above problems, the invention described in claim 1 is characterized in that PWM output means for outputting a motor control signal by comparing a DUTY instruction value and a carrier wave, and ON / OFF of a switching element controlled by the motor control signal. An electric power steering apparatus comprising: a drive circuit that outputs a voltage corresponding to the DUTY instruction value when turned off; and a motor to which the voltage is applied, and applying assist force to a steering system using the motor as a drive source, Detection means for detecting an abnormality of the system, calculation means for calculating a limit value for limiting the DUTY instruction value, and the DUTY within a limit range defined by the limit value with reference to a value corresponding to a ground potential Limiting means for limiting the indicated value, and when the abnormality is detected, the calculating means reduces the limit value over time, and the limiting means When the DUTY instruction value is not within the limited range, to correct the DUTY instruction value within the limited range, and the gist.

  According to the above configuration, the limit value is reduced with time and the limit range is gradually narrowed, so that the range that the DUTY instruction value can take is limited to a value close to the value corresponding to the ground potential. Therefore, the output voltage of the drive circuit can be brought close to the ground potential over time, and as a result, the assist torque generated by the motor, that is, the assist force applied to the steering system can be gently reduced.

  Further, since the DUTY instruction value is not corrected unless the limit range is exceeded, it becomes possible to apply a voltage necessary for generating the target assist torque within the limit range to the motor. Therefore, the assist force can be effectively applied even at the time of fail-safe transition accompanying the occurrence of system abnormality.

  According to a second aspect of the present invention, the motor is a brushless motor, and the driving circuit converts a DC power source into three-phase driving power and outputs the driving power to the motor. The gist of the invention is that a correction means for correcting the DUTY instruction value corresponding to each phase is provided so that the voltage applied to the phase is in an equilibrium state.

  According to the above configuration, even when the DUTY instruction value is limited, the balanced state of each phase voltage can be maintained and the occurrence of torque ripple can be suppressed, whereby a suitable steering feeling can be maintained. .

  ADVANTAGE OF THE INVENTION According to this invention, the electric power steering apparatus which can be reduced gently can be provided, providing an assist force effectively also at the time of the fail safe transfer accompanying system abnormality generation | occurrence | production.

Hereinafter, an embodiment in which the present invention is embodied in an electric power steering apparatus (EPS) will be described with reference to the drawings.
As shown in FIG. 1, the EPS 1 includes a motor 2 as a drive source that applies assist force to the steering system of the vehicle, and an ECU 3 that controls the motor 2.

  A steering wheel (steering) 4 is connected to a rack 6 via a steering shaft 5, and the rotation of the steering shaft 5 accompanying a steering operation is caused by a reciprocating linear motion of the rack 6 by a rack and pinion mechanism (not shown). It is converted and transmitted to the steering wheel 8. The EPS 1 of the present embodiment is a so-called rack type EPS in which the motor 2 is arranged coaxially with the rack 6, and the assist torque generated by the motor 2 is transmitted to the rack 6 via a ball feed mechanism (not shown). . And ECU3 controls the assist force given to a steering system by controlling the assist torque which this motor 2 generate | occur | produces.

  As shown in FIG. 2, the ECU 3 includes a microcomputer 11 that outputs a motor control signal and a PWM inverter 12 as a drive circuit that supplies drive power to the motor 2 based on the motor control signal. The motor 2 of this embodiment is a brushless motor, and the PWM inverter 12 supplies three-phase (U, V, W) driving power based on the motor control signal.

  A torque sensor 14, a vehicle speed sensor 15 (see FIG. 1), current sensors 16u, 16v, 16w, and a rotation angle sensor 19 are connected to the microcomputer 11, and the microcomputer 11 is based on output signals of these sensors. Thus, the steering torque τ, the vehicle speed V, the phase current values Iu, Iv, Iw of the motor 2 and their rotation angles (electrical angles) θ are detected. The microcomputer 11 determines the assist torque to be applied to the steering system based on the detected steering torque τ, the vehicle speed V, the phase current values Iu, Iv, Iw of the motor 2 and the rotation angle θ thereof. A motor control signal is output to cause the motor 2 to generate the assist torque.

  More specifically, the microcomputer 11 calculates a current command value calculation unit 21 that calculates a current command value Iq * that is a control target amount of assist torque generated by the motor 2, and each of the motors 2 based on the current command value Iq *. And a phase voltage command value calculation unit 22 for calculating a command value of a voltage applied to the phase, that is, a phase voltage command value Vu *, Vv *, Vw *.

  In this embodiment, the steering torque τ detected by the torque sensor 14 and the vehicle speed V detected by the vehicle speed sensor are input to the current command value calculation unit 21. Calculates the current command value Iq * based on the steering torque τ and the vehicle speed V. Then, the current command value calculation unit 21 outputs the current command value Iq * calculated by the calculation to the phase voltage command value calculation unit 22.

  The phase voltage command value calculator 22 includes the current command value Iq *, the phase current values Iu, Iv, Iw detected by the current sensors 16u, 16v, 16w, and the rotation detected by the rotation angle sensor 19. The angle θ is input. The phase voltage command value calculator 22 calculates the phase voltage command values Vu *, Vv *, Vw * based on the phase current values Iu, Iv, Iw, the rotation angle θ, and the current command value Iq *. Do.

  In the present embodiment, the phase voltage command value calculation unit 22 calculates the phase voltage command values Vu *, Vv *, and Vw * by current control in the dq coordinate system. That is, the current command value Iq * is input to the phase voltage command value calculation unit 22 as a q-axis current command value, and each phase current value Iu, Iv, Iw is d / q converted based on the rotation angle θ. Then, the phase voltage command value calculation unit 22 calculates the d and q axis voltage command values based on the q axis current command value and the d and q axis current values calculated by the d / q conversion. Each phase voltage command value Vu *, Vv *, Vw * is calculated by inversely converting the shaft voltage command value by d / q.

  The phase voltage command values Vu *, Vv *, Vw * calculated by the phase voltage command value calculation unit 22 are output to the DUTY instruction value calculation unit 23. Then, the DUTY instruction value calculator 23 calculates the DUTY instruction values Du, Dv, Dw of each phase based on the phase voltage command values Vu *, Vv *, Vw *.

  As shown in FIG. 3, the DUTY instruction value Dx (X = U, V, W, and so on) is a higher-order FET (described later) corresponding to the X phase of the PWM inverter 12 within one cycle of the triangular wave δ that is a carrier wave. ) Is turned on, that is, a value indicating a DUTY ratio (on-duty ratio), and the value of the X-phase output voltage is defined by this DUTY ratio. Specifically, when the DUTY ratio is greater than “50”, the X-phase output voltage is “positive”, and when it is less than “50”, the X-phase output voltage is “negative”. When the DUTY ratio is “50”, the X-phase output voltage becomes the ground potential.

  As shown in FIG. 2, the DUTY instruction value calculating unit 23 outputs the DUTY instruction value Dx to the PWM output unit 24, and the PWM output unit 24 is based on the comparison between the input DUTY instruction value Dx and the triangular wave δ. A motor control signal is generated (see FIG. 3). Then, the PWM output unit 24 outputs the motor control signal to the PWM inverter 12.

  On the other hand, the PWM inverter 12 is composed of a plurality of (2 × 3) power MOSFETs (hereinafter simply referred to as FETs) corresponding to the number of phases of the motor 2, specifically, FETs 26 a and 26 d in series. A circuit, a series circuit of FETs 26b and 26e, and a series circuit of FETs 26c and 26f are connected in parallel. The connection point 27u of the FETs 26a and 26d is connected to the U-phase coil of the motor 2, the connection point 27v of the FETs 26b and 26e is connected to the V-phase coil of the motor 2, and the connection point 27w of the FETs 26c and 26f is the W of the motor 2. Connected to the phase coil.

  The motor control signal output from the microcomputer 11 is applied to the gate terminals of the FETs 26a to 26f. The FETs 26a to 26f are turned on / off in response to the motor control signal, whereby the DC voltage supplied from the DC power supply 30 is converted into three-phase (U, V, W) driving power, and the motor 2 To be supplied.

(When a system error occurs)
Next, a control mode when a system abnormality occurs in the electric power steering apparatus according to the present embodiment will be described.

  As shown in FIG. 2, the microcomputer 11 of this embodiment includes a system abnormality detection unit 31 that detects the occurrence of a system abnormality, and a DUTY that calculates a DUTY limit value α for restricting the DUTY instruction value Dx when the system abnormality occurs. The DUTY limit value calculation unit 32 reduces the DUTY limit value α as time elapses from the occurrence of the system abnormality. Then, as shown in FIG. 3, the DUTY instruction value calculation unit 23 is within the limit range R defined by the DUTY limit value α with reference to the value of the DUTY ratio corresponding to the ground potential, that is, “50”. The DUTY instruction value Dx is limited (DUTY limitation).

  More specifically, the system abnormality detection unit 31 receives an output signal from each sensor and an abnormality signal from a host ECU (not shown), and the system abnormality detection unit 31 receives the signals from these signals. Based on this, the occurrence of system abnormality is detected. When a system abnormality is detected, a system abnormality detection signal indicating that a system abnormality has occurred is output to the DUTY limit value calculation unit 32.

  Incidentally, the “system abnormality” here does not lead to the complete loss of the EPS function, such as an abnormality in the output signals of various sensors, but it is desirable to stop the operation of the motor 2 to make it fail-safe. It refers to the state.

  The DUTY limit value calculation unit 32 calculates the DUTY limit value α using the input of the system abnormality detection signal from the system abnormality detection unit 31 as a trigger, and outputs the DUTY limit value α to the DUTY instruction value calculation unit 23.

  In the present embodiment, the DUTY limit value calculation unit 32 has a map 33 in which the elapsed time T from the occurrence of the system abnormality and the DUTY limit value α are related (see FIG. 4). The DUTY limit value α is set so as to gradually decrease from the maximum value “50” as the elapsed time T increases. Then, the DUTY limit value calculating unit 32 calculates the DUTY limit value α by referring to the map 33. Accordingly, the DUTY limit value α output to the DUTY instruction value calculation unit 23 is reduced with the passage of time from the occurrence of the system abnormality.

  As shown in FIG. 3, the DUTY instruction value calculation unit 23 uses the DUTY ratio value corresponding to the ground potential, that is, “50” based on the DUTY limit value α input from the DUTY limit value calculation unit 32. The restriction range R (“50 + α” to “50−α”) to be set is set. When the DUTY instruction value Dx corresponding to the phase voltage command value Vx * input from the phase voltage command value calculation unit 22 is not within the limit range R (Dx> 50 + α or Dx <50−α), Correction is performed so that the DUTY instruction value Dx is within the limit range R.

  Specifically, when the DUTY instruction value Dx is larger than the upper limit value of the limit range R (Dx> 50 + α), the DUTY instruction value Dx is set to the upper limit value (Dx = 50 + α), and the DUTY instruction value Dx is limited. When it is smaller than the lower limit value of the range R (Dx <50−α), the DUTY instruction value Dx is corrected to the lower limit value (Dx = 50−α).

  Then, as the DUTY limit value α input from the DUTY limit value calculation unit 32 decreases with the passage of time and the limit range R gradually narrows, the possible range of the DUTY instruction value Dx is limited to a value closer to “50”. The output voltage of the PWM inverter 12, that is, the phase voltage of each phase approaches the ground potential.

  Further, in the present embodiment, when the DUTY instruction value calculation unit 23 corrects the X-phase DUTY instruction value Dx, the DUTY instruction value calculation unit 23 is configured so that the X-phase phase voltage and the other-phase phase voltage are in an equilibrium state. Correct the DUTY instruction value of the phase. As a result, even when DUTY is restricted, it is possible to maintain the balanced state of each phase voltage and suppress the occurrence of torque ripple.

  Note that this phase voltage is balanced by, for example, the value of the DUTY ratio corresponding to the ground potential, that is, the correction ratio β (β = α / (Dx−50)) of the DUTY instruction value Dx based on “50”. Alternatively, β = α / (50−Dx)) is obtained, and the other-phase DUTY instruction value is corrected based on the correction ratio β.

Next, a processing procedure for DUTY restriction when a system abnormality occurs will be described.
As shown in the flowchart of FIG. 5, when the phase voltage command value Vx * is input from the phase voltage command value calculation unit 22 (step 101), the DUTY instruction value calculation unit 23 corresponds to the in-phase voltage command value Vx *. The DUTY instruction value Dx is calculated (step 102).

  Next, the DUTY instruction value calculator 23 determines whether or not the DUTY limit value α is input from the DUTY limit value calculator 32, that is, whether or not a system abnormality has occurred (step 103). When the DUTY limit value α is input (step 103: YES), the following steps 104 to 109 are executed, so that the DUTY instruction value Dx is equal to the DUTY ratio corresponding to the ground potential. The DUTY restriction is executed so as to be within the restriction range R defined by the DUTY restriction value α with reference to the value “50”.

  More specifically, when the DUTY limit value α is input in step 103 (step 103: YES), the DUTY instruction value calculation unit 23 first determines that the DUTY instruction value Dx calculated in step 102 is the limit range R. It is determined whether it is larger than the upper limit value (Dx> 50 + α, step 104). When the DUTY instruction value Dx is larger than the upper limit value of the limit range R (step 104: YES), the DUTY instruction value Dx is corrected to the upper limit value of the limit range R (Dx = 50 + α, step 105). Further, the DUTY instruction value of the other phase is corrected so that the phase voltage of each phase is in an equilibrium state (step 106).

  On the other hand, when it is determined in step 104 that the DUTY instruction value Dx is smaller than the upper limit value of the limit range R (step 104: NO), the DUTY instruction value calculation unit 23 subsequently determines that the DUTY instruction value Dx is within the limit range. It is determined whether it is smaller than the lower limit value of R (Dx <50−α, step 107). When the DUTY instruction value Dx is smaller than the lower limit value of the limit range R (step 107: YES), the DUTY instruction value Dx is corrected to the lower limit value of the limit range R (Dx = 50−α, step 108). In addition, the DUTY instruction value of the other phase is corrected so that the phase voltage of each phase is in an equilibrium state (step 109).

  When the DUTY instruction value calculation unit 23 determines in step 103 that there is no input of the DUTY limit value α (step 103: NO), or in step 107, the DUTY instruction value Dx is the lower limit of the limit range R. If it is determined that the value is greater than or equal to the value (step 107: NO), the DUTY instruction value Dx is not corrected.

  That is, when there is no abnormality in the system or when the DUTY instruction value Dx is within the limit range R, the DUTY instruction value calculation unit 23 does not correct the DUTY instruction value Dx and calculates the DUTY instruction value calculated in step 102 above. The value Dx, that is, the DUTY instruction value Dx corresponding to the phase voltage command value Vx * is output to the PWM output unit 24 as it is (step 110).

  As described above, while the DUTY instruction value calculation unit 23 repeats the processing of steps 101 to 110, the DUTY limit value calculation unit 32 reduces the DUTY limit value α output to the DUTY instruction value calculation unit 23 over time. To do. The limit range R is gradually narrowed by the reduction of the DUTY limit value α, and the possible range of the DUTY instruction value Dx is limited to a value closer to “50”, so that the output voltage of the PWM inverter 12, that is, each phase Thus, the assist torque generated by the motor 2, that is, the assist force applied to the steering system is gently reduced.

  In this embodiment, the PWM output unit 24 is a PWM output unit, the system abnormality detection unit 31 is a detection unit, the DUTY limit value calculation unit 32 is a calculation unit, and the DUTY instruction value calculation unit 23 is a limit unit and correction. Configure the means.

As described above, according to the present embodiment, the following features can be obtained.
(1) The microcomputer 11 includes a system abnormality detection unit 31 that detects the occurrence of a system abnormality, and a DUTY limit value calculation unit 32 that calculates a DUTY limit value α for limiting the DUTY instruction value Dx when the system abnormality occurs. The DUTY limit value calculation unit 32 reduces the DUTY limit value α as time elapses from the occurrence of the system abnormality. Then, the DUTY instruction value calculation unit 23 limits the DUTY instruction value Dx to be within the limit range R defined by the DUTY limit value α with reference to the value of the DUTY ratio corresponding to the ground potential, that is, “50”. .

  With such a configuration, the DUTY limit value α decreases with time and the limit range R gradually narrows, so that the possible range of the DUTY instruction value Dx is limited to a value closer to “50”. . Accordingly, the output voltage of the PWM inverter 12, that is, the phase voltage of each phase, can be brought close to the ground potential over time, and as a result, the assist torque generated by the motor 2, that is, the assist force applied to the steering system is gently reduced. Can be reduced.

  (2) If the DUTY instruction value Dx is larger than the upper limit value of the limit range R (Dx> 50 + α), the DUTY instruction value calculation unit 23 sets the DUTY instruction value Dx to the upper limit value (Dx = 50 + α). When the instruction value Dx is smaller than the lower limit value of the limit range R (Dx <50−α), the DUTY instruction value Dx is corrected to the lower limit value (Dx = 50−α). When the DUTY instruction value Dx is within the limit range R, the DUTY instruction value Dx is not corrected, and the DUTY instruction value Dx corresponding to the phase voltage command value Vx * is output to the PWM output unit 24 as it is.

  With such a configuration, the DUTY instruction value Dx is not corrected unless the limit range R is exceeded. Therefore, within the limit range R, a voltage corresponding to the phase voltage command value Vx * based on the current command value Iq *, which is the control target amount of the assist torque, can be applied to each phase of the motor 2, thereby causing a system abnormality. The assist force can be effectively applied even at the time of fail-safe transition accompanying the generation.

  (3) When the DUTY instruction value calculation unit 23 corrects the X-phase DUTY instruction value Dx, the other-phase DUTY instruction is set so that the X-phase phase voltage and the other-phase phase voltage are in an equilibrium state. Correct the value. As a result, even when DUTY is restricted, the balanced state of each phase voltage can be maintained and the occurrence of torque ripple can be suppressed, and a suitable steering feeling can be maintained.

In addition, you may change each said embodiment as follows.
In the present embodiment, the present invention is embodied in the electric power steering device, but may be embodied in other motor control devices used in addition to the electric power steering device.

  In the EPS 1 of the present embodiment, the motor 2 as a drive source is a brushless motor, and U, V, and W three-phase drive power is supplied to the motor 2, but the present invention uses a DC motor as the drive source. It may be embodied in EPS.

  In the present embodiment, the DUTY instruction value calculation unit 23 constitutes the restriction unit and the correction unit. However, the restriction unit and the correction unit may be provided in addition to the DUTY instruction value calculation unit 23.

-The method of making the phase voltage of each phase into an equilibrium state is not limited to the method of this embodiment, and any method may be used.
In the present embodiment, the map 33 is used to reduce the DUTY limit value α as time elapses from the occurrence of a system abnormality. However, the DUTY limit value α is a predetermined value that decreases the DUTY limit value α as time elapses. It is good also as calculating using a function.

Next, technical ideas other than the claims that can be understood from the above embodiments will be described.
(A) In the electric power steering apparatus according to claim 1, the motor is a brushless motor, and the drive circuit converts a DC power supply into three-phase drive power and outputs the three-phase drive power to the motor. The limiting means sets a value obtained by adding the limit value to a value corresponding to a ground potential as an upper limit value of the limit range, and when the DUTY instruction value is larger than the upper limit value, the DUTY instruction value is set to the upper limit value. An electric power steering device characterized by

  (B) In the electric power steering apparatus according to claim 1, the motor is a brushless motor, and the driving circuit converts a DC power source into three-phase driving power and outputs the driving power to the motor, The limiting means sets the value obtained by subtracting the limit value from the value corresponding to the ground potential as the lower limit of the limit range, and when the DUTY instruction value is smaller than the lower limit value, sets the DUTY instruction value to the lower limit value. An electric power steering device characterized by correcting.

  (C) PWM output means for outputting a motor control signal by comparing the DUTY instruction value with a carrier wave, and a drive circuit for outputting a voltage corresponding to the DUTY instruction value by turning on / off a switching element controlled by the motor control signal And a motor control device for controlling the motor by applying the output voltage, a detection means for detecting a system abnormality, and a calculation means for calculating a limit value for limiting the DUTY instruction value And a limiting means for limiting the DUTY instruction value within a limit range defined by the limit value with reference to a value corresponding to a ground potential, and the computing means passes the limit value over time when the abnormality is detected. The limiting means corrects the DUTY instruction value within the limit range when the DUTY instruction value is not within the limit range. It, motor control device according to claim.

1 is a schematic configuration diagram of an electric power steering device (EPS) of the present embodiment. The control block diagram of EPS of this embodiment. The wave form diagram which shows the aspect of the DUTY restriction | limiting at the time of system abnormality occurrence. The schematic diagram of the map with which elapsed time and the DUTY limit value were related. The flowchart which shows the process sequence of DUTY restriction | limiting at the time of system abnormality generation | occurrence | production.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Electric power steering apparatus (EPS), 2 ... Motor, 3 ... ECU, 4 ... Steering wheel, 8 ... Steering wheel, 11 ... Microcomputer, 12 ... PWM inverter, 14 ... Torque sensor, 15 ... Vehicle speed sensor, 16u, 16v , 16w ... current sensor, 19 ... rotation angle sensor, 21 ... current command value calculation unit, 22 ... phase voltage command value calculation unit, 23 ... DUTY instruction value calculation unit, 24 ... PWM output unit, 26a to 26f ... FET, 30 ... DC power supply, 31 ... System abnormality detection unit, 32 ... DUTY limit value calculation unit, Iq * ... Current command value, Vx * (Vu *, Vv *, Vw *) ... Phase voltage command value, Dx (Du, Dv, Dw) ... DUTY instruction value, δ ... triangular wave, α ... DUTY limit value, R ... limit range, T ... elapsed time.

Claims (2)

PWM output means for outputting a motor control signal by comparing a DUTY instruction value and a carrier wave; a drive circuit for outputting a voltage corresponding to the DUTY instruction value by turning on / off a switching element controlled by the motor control signal; A motor to which a voltage is applied, and an electric power steering device that applies assist force to a steering system using the motor as a drive source,
Detection means for detecting an abnormality of the system;
Computing means for computing a limit value for limiting the DUTY instruction value;
Limiting means for limiting the DUTY instruction value within a limiting range defined by the limiting value with reference to a value corresponding to a ground potential;
The calculation means reduces the limit value with time when the abnormality is detected,
The electric power steering apparatus according to claim 1, wherein when the DUTY instruction value is not within the restriction range, the restriction means corrects the DUTY instruction value within the restriction range. The electric power steering apparatus according to claim 1, wherein
The motor is a brushless motor, and the driving circuit converts a DC power source into three-phase driving power and outputs the driving power to the motor,
Correction means for correcting the DUTY instruction value corresponding to each phase so that the voltage applied to each phase of the motor at equilibrium is in an equilibrium state;
An electric power steering device.

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