JP5233215B2 - Electric power steering device - Google Patents

Description

  The present invention relates to an electric power steering apparatus that executes feedback control based on a deviation between a target assist force set according to a steering torque applied to a steering member and an actual assist force of a steering assist motor.

  The electric power steering device detects a steering torque applied to a steering member such as a steering wheel by a torque sensor, and drives a steering assist motor that assists the operation of the steering mechanism according to the detected steering torque. Thus, the steering operation force by the driver is reduced.

  In the conventional electric power steering apparatus, for example, a detected value of the steering torque and a detected value of the vehicle speed sensor are stored in a map storing correspondence data between the steering torque and the target assist force (specifically, the target current of the motor) using the vehicle speed as a parameter. Is used to find the target current value of the motor, the actual assist force of the steering assist motor is detected by the actual motor current, and the motor current is controlled so that the deviation between this actual current value and the target current value is small Feedback control is performed. The map is set so that the target assist force is reduced at high speeds to prevent steering wobbling while the target assist force is increased at low speeds to increase the steering effort reduction effect.

The motor current is controlled by switching the motor drive circuit with a high-frequency pulse signal and changing the duty ratio of the high-frequency pulse signal according to the deviation between the actual current value of the motor and the target current value. ing. The motor current is detected by measuring the voltage across a shunt resistor provided in the motor drive circuit (see, for example, Patent Documents 1 and 2). In these electric power steering devices, when the motor current detection circuit fails, the feedback control is switched to the open loop control.
Japanese Patent Laid-Open No. 10-167086 JP 2002-87304 A

  In the conventional electric power steering apparatus, since the switching noise of the high-frequency pulse signal of the motor drive circuit affects the detected value of the motor current, the detected signal of the motor current is averaged by passing it through a filter with a large time constant. The detected current value is used for feedback control. For this reason, there is a problem that the response of the steering assist force is delayed with respect to the change of the steering torque, and the steering feeling is poor. As a countermeasure against this problem, it is effective to increase the increase rate of the target current with respect to the steering torque in the map or to increase the gain of the feedback control loop in order to improve the steering response. If the increase rate of the target current or the gain of the feedback control loop is increased too much, vibration may occur and the steering feeling may be deteriorated.

  Open loop control is effective as another means for improving responsiveness, but in this case, characteristic variations (individual differences) such as the coil resistance and rotor magnetic flux of the steering assist motor appear directly in the actual assist force. Since it is difficult to obtain a desired steering assist force with high accuracy, it is necessary to use a high-quality motor with little individual difference. However, even if such a high-quality motor is used, the motor characteristics may change due to external conditions such as temperature changes during operation, and the steering feeling may be reduced.

  The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide an electric power steering apparatus that can improve the response of steering assistance while suppressing the occurrence of vibration and obtain a good steering feeling. .

According to a first aspect of the present invention, there is provided an electric power steering apparatus comprising: a detected value of a torque sensor for detecting a steering torque applied to a steering member; a detected value of a vehicle speed sensor for detecting a vehicle speed; In an electric power steering apparatus that applies target value to a formula or map set as a parameter to obtain a target assist force and performs feedback control based on a deviation between the obtained target assist force and the actual assist force of the steering assist motor The steering torque and the detection value of the vehicle speed sensor are set such that the increase rate of the target assist force with respect to the steering torque exceeds the set value in the equation or map, determining means for determining whether or not the condition of the vehicle, when it is determined that the condition is satisfied by the determination means, Characterized in that the serial feedback control and a control switching means switches to an open-loop control based on the target assist force.

The electric power steering apparatus according to a second aspect of the present invention is the electric power steering apparatus according to the first aspect, wherein the equation or the map indicates that when the vehicle speed is lower than a set vehicle speed, the increase rate of the target assist force with respect to the steering torque is smaller than the steering torque. It is characterized in that it is changed in a stepwise manner so as to become larger on the larger side, and the increasing rate on the larger side of the steering torque is set to exceed the set value .

  According to a third aspect of the present invention, there is provided an electric power steering device according to the first or second aspect, wherein the target auxiliary force after switching from the feedback control to the open loop control is the target auxiliary force in the feedback control before switching or the target auxiliary force. Target assisting force setting means for setting a value close to.

  According to a fourth aspect of the present invention, there is provided an electric power steering apparatus according to any one of the first to third aspects of the invention, wherein the target auxiliary force is the driving voltage, rotational speed, temperature, and driving circuit of the steering auxiliary motor. A target auxiliary force correcting means for correcting based on at least one of the temperatures is provided.

According to the electric power steering apparatus of the first aspect of the invention, the torque sensor detection value and the vehicle speed sensor detection value are applied to an expression or map in which the relationship of the target auxiliary force to the steering torque is set with the vehicle speed as a parameter. The assist force is obtained, and the detected value of the torque sensor and the detected value of the vehicle speed sensor are set so as to correspond to a region in which the increase rate of the target assist force with respect to the steering torque exceeds a set value in the equation or the map. steering torque and determines whether or not the condition of the vehicle, if the detected value and the detection value of the vehicle speed sensor of the torque sensor does not satisfy the condition of the steering torque and the vehicle speed, the target assist force said determined run the feedback control based on the deviation of the actual assisting force of the motor for steering assist, whereas, the detection value and the detection value of the vehicle speed sensor of the torque sensor and said steering torque及 If the condition is satisfied for the vehicle speed, it executes the open loop control based on the target assist force. That is, under conditions where the increase rate of the target assist force with respect to the steering torque is not large, a stable steering assist force is generated by feedback control regardless of variations in the characteristics of the steering assist motor, and the increase rate of the target assist force with respect to the steering torque is increased. Under large conditions, the steering assist force is generated with good response by open loop control.
Therefore, when steering assist control is executed using the above formula or map in a region where the increase rate of the target assist force with respect to the steering torque is large, the response of the steering assist is improved while suppressing the occurrence of vibration. An electric power steering apparatus that can provide a steering feeling can be realized.

According to the electric power steering apparatus of the second invention, whether or not the detected value of the vehicle speed sensor is lower than the set vehicle speed, and the increase rate of the target assist force with respect to the steering torque is larger than the smaller side of the steering torque. The detected value of the torque sensor is changed in a stepwise manner so as to increase in the region on the side, and the detected value of the torque sensor with respect to the equation or map set so that the increase rate on the large side of the steering torque exceeds the set value. determining whether the larger side of the torque, when the detection value of and the torque sensor at a lower speed than the set vehicle speed detection value of the vehicle speed sensor does not satisfy the condition that the larger side of the steering torque, the formula or The feedback control based on the deviation between the target assist force obtained by applying the torque sensor detection value and the vehicle speed sensor detection value to the map and the actual assist force of the steering assist motor is executed, while the vehicle speed sensor detection is performed. value And a low speed than the constant speed, when the detected value of the torque sensor is larger side of the steering torque, to perform open loop control based on the target assist force. That is, when the detected value of the torque sensor becomes large when the steering operation is performed at a large steering angle when the vehicle speed is low, the steering assist force is generated with good responsiveness by the open loop control .

  According to the electric power steering apparatus of the third aspect of the invention, the target auxiliary force after switching from the feedback control to the open loop control is set to the target auxiliary force in the feedback control before switching or a value close to the target auxiliary force. Therefore, it is possible to realize an electric power steering apparatus that does not cause a sense of incongruity due to the discontinuity of the steering assist force when switching from the feedback control to the open loop control, and provides a smooth steering feeling.

According to the electric power steering apparatus of the fourth aspect of the invention, the target auxiliary force in the open loop control is corrected based on at least one of the driving voltage, the rotational speed, the temperature, and the driving circuit temperature of the steering auxiliary motor. . Examples of specific correction contents are shown below.
When the motor driving voltage is high, the target assisting force is reduced, and when the motor driving voltage is low, the target assisting force is increased. Thereby, the fluctuation | variation of a steering assist force can be suppressed irrespective of the fluctuation | variation of a motor drive voltage.
When the rotational speed of the motor is fast, the target assisting force is increased, and when the rotational speed is slow, the target assisting force is decreased. Thereby, the fluctuation | variation of a steering assist force can be suppressed irrespective of the fluctuation | variation of a motor rotational speed.
When the motor temperature is high, the target assist force is increased, and when the motor temperature is low, the target assist force is decreased. Thereby, the fluctuation | variation of a steering assist force can be suppressed irrespective of the fluctuation | variation of motor temperature.
When the motor drive circuit temperature is high, the target assist force is increased, and when the motor drive circuit temperature is low, the target assist force is decreased. Thereby, the fluctuation | variation of a steering assist force can be suppressed irrespective of the fluctuation | variation of the drive circuit temperature of a motor.
By correcting the target assist force as described above, it is possible to execute open loop control while dealing with fluctuations in external conditions such as motor drive voltage, rotation speed, temperature, and drive circuit temperature. An electric power steering device with steering characteristics can be realized.

Embodiments of an electric power steering apparatus according to the present invention will be described below with reference to the drawings.
FIG. 1 is a schematic diagram showing an overall configuration of an electric power steering apparatus according to the present invention.
In FIG. 1, reference numeral 1 denotes a rack and pinion type steering mechanism. The steering mechanism 1 includes a rack shaft 5 supported in an axially movable manner in a rack housing 4 extending in the left-right direction of the vehicle body, a rack A pinion shaft 2 rotatably supported inside a pinion housing 8 that intersects the middle of the housing 4 is provided. Both ends of the rack shaft 5 are projected to the outside of both sides of the rack housing 4, and these projecting ends are connected to the left and right steering wheels 7, 7 via separate tie rods 6, 6. A pinion (not shown) is integrally formed at the lower part of the pinion shaft 2 extending inside the pinion housing 8, and this pinion is a rack provided on the outer surface of the middle portion of the rack shaft 5 at the intersection with the rack housing 4. Is engaged.

  An upper end of the pinion shaft 2 protruding outside the pinion housing 8 is connected to a steering wheel 9 as a steering member via a steering shaft 3. The steering shaft 3 is rotatably supported inside a cylindrical housing 31, and the housing 31 is attached to the interior of a vehicle compartment (not shown) while maintaining an inclined posture with the front facing down. The pinion shaft 2 is connected to the protruding end of the steering shaft 3 below the housing 31, and the steering wheel 9 is also connected to the protruding end upward.

  With the above configuration, when the steering wheel 9 is rotated for steering, this rotation is transmitted to the pinion shaft 2 via the steering shaft 3, and the rotation of the pinion shaft 2 is performed at the meshing portion of the pinion and the rack. It is converted into the movement of the rack shaft 5 in the axial length direction. The movement of the rack shaft 5 thus generated is transmitted to the left and right steered wheels 7 and 7 via the respective tie rods 6 and 6, and the steered wheels 7 and 7 are turned and steered.

  A torque sensor 10 that detects a steering torque applied to the steering wheel 9 is provided in the middle of the housing 31, and a steering assist motor 18 is attached at a position below the torque sensor 10.

  The torque sensor 10 divides the steering shaft 3 into two upper and lower axes coaxially connected by a torsion bar having a known torsion characteristic, and detects a relative angular displacement caused by the torsion bar torsion between these two axes. It has a known configuration. The torque sensor 10 outputs a steering torque signal corresponding to the relative angular displacement associated with torsion of the torsion bar. The steering assisting motor 18 is attached to the outside of the housing 31 with its shaft center being substantially orthogonal to the steering shaft 3, and the rotation of the output shaft of the motor 18 extending inside the housing 31 is transmitted to the inside of the housing 31. The speed is reduced by a provided worm gear reducer and transmitted to the steering shaft 3.

  The steering assist motor 18 provided in this way is driven by a drive signal output from the control unit 30. A steering torque signal detected by the torque sensor 10 is input to the control unit 30, and a vehicle speed signal detected by the vehicle speed sensor 13 disposed at an appropriate position of the vehicle is input.

FIG. 2 is a block diagram showing the control unit 30 shown in FIG.
The steering torque signal is given to the phase compensator 11, and the steering torque signal phase-compensated by the phase compensator 11 is given to the control unit 30 comprising a microprocessor. A vehicle speed signal is also given to the control unit 30.

  The storage means 14 stores a map 14A representing the relationship between the steering torque value and the motor current target value with the vehicle speed as a parameter. The control unit 30 applies the detected value of the vehicle speed and the detected value of the steering torque to the map 14 </ b> A in the storage unit 14 to obtain the target value of the motor current. The target value of the motor current is a value indicating the target value in the steering assist control of the motor 18 and corresponds to the target assist force. The control unit 30 determines the steering assist direction based on the sign of the steering torque signal.

FIG. 3 is a diagram showing an example of a map 14A representing the relationship between the steering torque value with the vehicle speed as a parameter and the target value of the motor current. FIG. 3A shows the steering torque value and the motor current when the vehicle speed is high. A broken line representing the relationship with the target value is shown, and (b) shows a broken line representing the relationship between the steering torque value and the target value of the motor current when the vehicle speed is low.
The broken line shown in (a) indicates that when the steering torque value exceeds the predetermined dead zone torque value T1, the target value of the motor current increases proportionally as the steering torque value increases, and the steering torque value exceeds the predetermined value T3. Then, the target value of the motor current is determined to be saturated. In the broken line shown in (b), the increase rate of the target value of the motor current with respect to the steering torque value is small between the torque values T1 and T2 where the steering torque value exceeds the dead zone, and between the torque values T2 and T3 (where T1 < It is set with a two-stage increase rate so as to increase with T2 <T3). Here, the increase rate of the target value of the motor current between the torque values T2 and T3 of the broken line shown in (b) is the target value of the motor current between the torque values T1 and T3 of the broken line shown in (a). It is set larger than the rate of increase. As described above, the relationship between the steering torque value and the target value of the motor current is such that the target value of the motor current with respect to the steering torque value decreases and the saturation value of the target value decreases as the vehicle speed value increases. It is prescribed as follows. Actually, in order to represent the relationship between the steering torque value and the target value of the motor current for each vehicle speed value at fine intervals, not just the broken lines shown in FIGS. A large number of broken lines are stored and selected according to the vehicle speed value.

  The motor current of the steering assist motor 18 is detected by the motor current detection circuit 19, and the detection signal of the motor current detection circuit 19 is input to the control unit 30. The motor current detection circuit 19 detects the magnitude of the motor current. The detected value of the motor current detected by the motor current detection circuit 19 corresponds to the actual auxiliary force of the motor 18.

  The steering assist motor 18 includes a resolver 21 that outputs a signal for detecting the rotor position. The output signal of the resolver 21 is processed by the motor position detection circuit 22 and output from the motor position detection circuit 22. A rotor position signal is input to the motor drive circuit 20 and the control unit 30.

  Although not shown, the motor drive circuit 20 is configured by a circuit in which drive elements (for example, FETs) are arranged in a bridge, and a motor rotation direction signal indicating a steering assist direction input from the control unit 30 and motor drive. Based on the PWM (Pulse Width Modulation) duty value indicating the magnitude of the current, the motor 18 is driven so as to have the magnitude of the drive current designated in the designated rotation direction.

  A motor temperature detector 25 that detects the temperature of the motor 18, a motor drive circuit temperature detector 26 that detects the temperature of the motor drive circuit 20, and a power supply voltage that detects the output voltage of the battery 24 corresponding to the drive voltage of the motor 18. A detection circuit 27 is provided, and each detection value thereof is input to the control unit 30.

  Next, the steering assist control operation of the electric power steering apparatus configured as described above will be described with reference to the flowcharts of FIGS. 4 and 5 are flowcharts showing the steering assist control operation of the control unit 30 in the electric power steering apparatus according to the present invention.

  First, the control unit 30 reads the detected value of the steering torque and the detected value of the vehicle speed (steps S1 and S2), and calculates the target value of the motor current using the map 14A based on both the detected values of the steering torque and the vehicle speed. (Step S3). Further, the control unit 30 determines the rotation direction of the motor 18 for assisting steering based on the detected value of the steering torque (step S4).

  Next, the control unit 30 determines whether or not the detected value of the steering torque is larger than the steering torque value T2 and the detected value of the vehicle speed is slower than the speed Vα (step S5). Specifically, as shown in the map 14A in FIG. 3, when the steering torque value is between T2 and T3, the vehicle speed when the rate of increase of the target assist force with respect to the steering torque becomes the set value α is Vα. The region hatched above the broken line corresponding to the vehicle speed Vα is the region of the detected value of the steering torque and the detected value of the vehicle speed that satisfies the determination condition in step S5. The slope of the broken line corresponding to the vehicle speed Vα is set to a value between the slope of the broken line (b) when the vehicle speed is low and the slope of the broken line (a) when the vehicle speed is high. The hatched area that satisfies the determination condition corresponds to a condition that the steering operation is performed at a large steering angle when the vehicle speed is low.

  Corresponding to the determination whether or not the determination in step S5 satisfies the specific condition, if the condition is not satisfied (S5; NO), the feedback control in step S10 and after is executed, and if the condition is satisfied (S5; YES) Then, the open loop control from step S20 is executed.

  In the feedback control after step S10, the control unit 30 reads the detected value of the motor current (step S10), and subtracts the detected value of the motor current from the target value of the motor current read in step 3. The deviation between the target value and the detected value of the motor current is calculated (step S11). Next, the control unit 30 calculates a duty value D in the PWM signal based on the obtained deviation (step S12), and uses the motor rotation direction determined in the duty value D and step 4 as a motor drive signal. To drive the motor 18 (step S13).

  As described above, the control unit 30 uses the detected value of the torque sensor 10 and the detected value of the vehicle speed sensor 13 as a map 14A in which the relationship between the target assist force (target value of the motor current) with respect to the steering torque is set using the vehicle speed as a parameter. The target assist force (target value of the motor current) is obtained by applying the feedback control based on the deviation between the determined target assist force and the actual assist force (detected value of the motor current) of the steering assist motor 18. Run.

  When shifting to the open loop control after step S20, the control unit 30 first determines whether or not within a predetermined time after switching from the feedback control (step S20), and within a predetermined time after switching from the feedback control. If so (S20; YES), the target value of the motor current in the feedback control before switching is set as the target value of the motor current in the open loop control after control switching (step S21). Next, the duty value D1 in the PWM signal is calculated and determined based on the set target value of the motor current after the control switching (step S22), and the motor rotation direction determined in the duty value D1 and step 4 is driven by the motor. A signal is output to the motor drive circuit 20 to drive the motor 18 (step S23).

  In the setting process of the target value of the motor current in the open loop control in step S21, as a specific example of the target value of the motor current in the feedback control before the control switching or a value close to the target value of the motor current, The target value of the motor current at the sampling time or the average value of the target values of the motor current at a plurality of sampling times within a predetermined time before switching is set as the target value of the motor current in the open loop control.

  On the other hand, if the control unit 30 determines that it is not within the predetermined time after switching from the feedback control (S20; NO), the PWM in the open loop control is performed based on the target value of the motor current obtained in step S3. The duty value D2 in the signal is calculated and determined (step S24). Next, the control unit 30 reads the motor drive voltage detection value output from the power supply voltage detection circuit 27 (step S25), and calculates the motor rotation speed based on the change in the rotor position signal output from the motor position detection circuit 22 (step S25). S26), the detected values of the motor temperature output from the motor temperature detector 25 and the motor drive circuit temperature output from the motor drive circuit temperature detector 26 are read (steps S27 to S28), and these detected values and calculated values are read. (Step S29), the PWM correction coefficient is used to correct the duty value D2 in the PWM signal (step S30), the corrected duty value D2 ′ and the motor rotation determined in step 4 A direction signal is output to the motor drive circuit 20 to drive the motor 18 (step S31).

An example of the correction contents made by the target auxiliary force correction means will be described. When the motor drive voltage is high, the drive force increases even with the same motor drive current. Therefore, the target value of the motor current is corrected to be small. When the motor drive voltage is low, the drive force decreases even with the same motor drive current. Therefore, the motor current target value is corrected to be increased.
When the rotational speed is high, the influence of the back electromotive force of the motor is large. Therefore, to compensate for the counter electromotive force, the target value of the motor current is corrected to be large, and when the rotational speed is slow, the back electromotive force of the motor is Since the influence is reduced, correction is made so that the target value of the motor current is reduced.
Since the motor resistance (impedance) increases when the motor temperature is high, the motor current (impedance) is corrected to increase, and when the motor temperature is low, the motor resistance (impedance) decreases. Correct as follows.
When the temperature of the motor drive circuit 20 is high, the on-resistance of the drive element (for example, FET) becomes high. Therefore, in order to compensate for the increase in resistance, correction is made to increase the target value of the current. On the other hand, when the value is low, the on-resistance of the drive element (FET) becomes low, so correction is made so as to reduce the target value of the motor current.

  Then, even after switching to the open loop control, the control unit 30 determines that the detected value of the steering torque becomes smaller than the steering torque value T2 or the detected value of the vehicle speed becomes faster than the speed Vα (step S5). Reference), and returns to the feedback control after step S10.

  As described above, with the electric power steering apparatus of the present invention, the increase rate of the target assist force with respect to the steering torque is smaller than the set value α and the conditions of the steering torque and the vehicle speed (for example, the vehicle speed shown in FIG. 3), feedback control is executed to realize stable steering without wobbling, and the conditions of the steering torque and vehicle speed at which the increase rate of the target assist force with respect to the steering torque exceeds the set value α (for example, (b) of FIG. When the specific condition is satisfied when the steering torque shown in (2) is the region between the torque values T2 and T3), the vibration is generated by switching to the open loop control based on the target auxiliary force. A good steering feeling with good responsiveness can be realized while suppressing. That is, when a large steering angle operation is performed in a low speed state, a large steering assist force can be generated with good responsiveness without vibration with respect to a change in steering torque by open loop control.

  According to the electric power steering apparatus of the present invention, the target auxiliary force after switching from the feedback control to the open loop control is set to a target auxiliary force in the feedback control before switching or a value close to the target auxiliary force. In addition, there is no fear of discomfort due to discontinuity of the steering assist force when switching from feedback control to open loop control, and a smooth and responsive steering feeling can be realized.

  Further, according to the electric power steering apparatus of the present invention, the target assist force (target value of the motor current) in the open loop control is used as the drive voltage, the rotational speed, the motor temperature, and the motor drive circuit 20 of the steering assist motor 18. Therefore, it is not necessary to use a high-quality motor with little characteristic variation, and it is possible to realize good steering characteristics with good responsiveness while responding to external conditions.

  In the above embodiment, the control unit 30 stores the detected value of the steering torque and the detected value of the vehicle speed in the map 14A in which the relationship between the steering torque value and the target auxiliary force (target value of the motor current) is stored using the vehicle speed as a parameter. However, in addition to this, the relationship between the steering torque value for each vehicle speed and the target auxiliary force is stored as an equation and selected according to the detected value of the vehicle speed. It is also possible to apply the detected value of the steering torque to the above formula and obtain the target auxiliary force by calculation.

  In the above embodiment, the target auxiliary force correcting means (control unit 30) applies the target auxiliary force in the open loop control to all of the drive voltage, rotational speed, temperature, and drive circuit temperature of the steering assist motor 18. The correction is made based on this, but the correction may be made based on at least one of these pieces of information.

  In the above embodiment, an example in which the electric power steering apparatus according to the present invention is applied to a vehicle including the rack and pinion type steering mechanism 1 is shown, but the present invention is applicable regardless of the type of the steering mechanism. Is possible.

1 is a schematic diagram showing an overall configuration of an electric power steering apparatus according to the present invention. It is a block diagram which shows the control part and surroundings of the electric power steering apparatus which concern on this invention. It is a figure which shows an example of the map showing the relationship between the steering torque value which made the vehicle speed the parameter, and the target value of motor current. 3 is a flowchart showing a steering assist control operation of the electric power steering apparatus according to the present invention. 3 is a flowchart showing a steering assist control operation of the electric power steering apparatus according to the present invention.

Explanation of symbols

  9 steering wheel (steering member), 10 torque sensor, 13 vehicle speed sensor, 14A map, 18 motor, 30 control unit (determination means, control switching means, target auxiliary force setting means, target auxiliary force correction means)

Claims (4)

The detection value of the torque sensor for detecting the steering torque applied to the steering member and the detection value of the vehicle speed sensor for detecting the vehicle speed are applied to an expression or map in which the relationship of the target assist force to the steering torque is set using the vehicle speed as a parameter. In the electric power steering device that calculates the target assist force and executes feedback control based on the deviation between the determined target assist force and the actual assist force of the steering assist motor,
The detected value of the torque sensor and the detected value of the vehicle speed sensor are set so that the increase rate of the target assist force with respect to the steering torque exceeds a set value in the equation or the map, and the steering torque and the A determination unit that determines whether or not a vehicle speed condition is satisfied; and a control switching unit that switches from the feedback control to an open loop control based on the target auxiliary force when the determination unit determines that the condition is satisfied. an electric power steering apparatus characterized in that it comprises. The equation or map changes stepwise so that when the vehicle speed is lower than the set vehicle speed, the increase rate of the target assist force with respect to the steering torque is larger on the larger side than the smaller side of the steering torque, 2. The electric power steering apparatus according to claim 1, wherein an increase rate on the large side of the steering torque is set to exceed the set value .   The target auxiliary force setting means for setting the target auxiliary force after switching from the feedback control to the open loop control to a target auxiliary force in the feedback control before switching or a value close to the target auxiliary force is provided. The electric power steering apparatus as described.   The target auxiliary force correction means for correcting the target auxiliary force in the open loop control based on at least one of a driving voltage, a rotation speed, a temperature, and a driving circuit temperature of the steering assist motor is provided. The electric power steering device according to claim 3.

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