JP2007118785A - Steering assisting device of vehicle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent an insufficient steering assisting force in a steering assisting device of a vehicle for controlling a weak magnetic field of an electric motor. <P>SOLUTION: An electronic control unit 30 controls driving of rotation of an electric motor 15 by a vector control described in a 2-phase rotary magnetic flux coordinate system comprising a q-axis and a d-axis. A q-axis target current calculating unit 34 calculates a basic assisting force using a steering torque T and a vehicle speed V. A d-axis target current calculation unit 36 calculates a d-axis target current Id* for controlling the weak magnetic field using an angular speed ω, a q-axis command voltage Vq, and a q-axis actual current Iq of the electric motor 15. A q-axis target current correction calculating unit 37 corrects the q-axis target current Iq* on a larger side as the d-axis target current Id* or the d-axis actual current Id becomes larger. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a vehicle steering assist device including an electric motor for assisting a steering operation of a steering wheel by a driver.

  Conventionally, a steering torque applied to a steering wheel is detected, a field current is caused to flow through the electric motor in accordance with the detected steering torque, and an assist torque corresponding to the steering torque is generated in the electric motor. Steering assist devices are well known. On the other hand, the relationship between the output torque and the rotational speed of this type of electric motor is uniquely determined as shown by a solid line or a broken line in FIG. The solid line in FIG. 9 indicates the characteristics of the electric motor (torque priority characteristics) that emphasize output torque, and the broken line in FIG. 9 indicates the characteristics of the electric motor (rotation speed priority characteristics) that emphasize rotation speed.

  When the steering wheel is operated when the vehicle is stopped or when traveling at a low speed, as shown by a point A1 in FIG. 9, a large rotation speed is not required, but a large output torque (assist torque) is often required. However, when the electric motor having the characteristics shown by the broken line in FIG. 9 is used, the output torque necessary for the operation of the steering wheel when the vehicle is stopped or when traveling at low speed may not be obtained. Further, when the steering wheel is operated at a medium or high speed, the vehicle does not require a large output torque (assist torque) as shown by a point A2 in FIG. 9, but may require a large rotational speed. Many. However, when an electric motor having the characteristics shown by the solid line in FIG. 9 is used, there may be a case where a large rotational speed necessary for operation of the steering wheel during medium speed or high speed traveling of the vehicle cannot be obtained.

In order to satisfy these requirements, it is necessary to increase the size of the electric motor. To avoid the increase in size of the electric motor, for example, as shown in Patent Documents 1 and 2 below, the electric motor It has been conventionally known that the rotation of an electric motor is controlled by vector control described in a two-phase rotating magnetic flux coordinate system in which the rotation direction is q-axis and the direction orthogonal to the rotation direction is d-axis. . In this vector control, by increasing the d-axis current as the rotational speed of the electric motor increases, the coil current is caused to flow in a direction that weakens the magnetic field of the permanent magnet of the electric motor. Is changed to the characteristics of the electric motor with priority on the rotational speed, so-called field weakening control is performed. According to this field weakening control, the characteristic of the electric motor with priority on torque shown by the solid line in FIG. 10 can be changed to the characteristic of the electric motor with priority on rotational speed in the region where the rotational speed is large as shown by the broken line. This makes it possible to satisfy the aforementioned requirements for assist torque and rotational speed in various vehicle states without increasing the size of the electric motor.
JP 2000-279000 A JP 2004-40883 A

  However, when the application range of the weak magnetic field control is expanded to increase the efficiency of the electric motor and reduce the size and increase the output as in the prior art described above, the frequency of the output torque of the electric motor decreasing in the normal range increases. There is a problem that the steering feeling by the motor is insufficient and the steering feeling deteriorates.

  The present invention has been made to address the above-described problems, and an object thereof is to prevent a steering assist force from being insufficient in a vehicle steering assist device that performs field-weakening control of an electric motor.

  In order to achieve the above object, the structural features of the present invention include an electric motor for assisting the steering operation of the steering wheel by the driver, the rotation direction of the electric motor as the q axis, and orthogonal to the rotation direction. In a steering assist device for a vehicle having an electric control circuit for controlling the rotation of an electric motor by vector control described in a two-phase rotating magnetic flux coordinate system having a direction as a d-axis, the electric control circuit is provided to the steering handle. Steering torque detecting means for detecting a steering torque, q-axis target current determining means for determining a q-axis target current according to the detected steering torque, and a rotational control state of the electric motor for field weakening control of the electric motor D-axis target current determining means for determining a d-axis target current that changes in accordance with the d-axis current-related state quantity related to the d-axis current of the electric motor. Q-axis current correcting means for correcting the q-axis target current to calculate a corrected q-axis target current, and field control means for field-controlling the electric motor according to the calculated corrected q-axis target current and the determined d-axis target current This is because of the above.

  In this case, specifically, the q-axis current correcting means corrects the q-axis target current so that the corrected q-axis target current increases as the d-axis current of the electric motor increases. Further, the d-axis current related state quantity may be, for example, the d-axis target current determined by the d-axis target current determining means or the actual d-axis current of the electric motor.

  In the present invention configured as described above, the q-axis current correction means corrects the q-axis target current to correct the electric motor even if the field-weakening control of the electric motor is performed to increase the efficiency of the electric motor and increase the size and output. Since the output torque of the motor is ensured, a situation where the steering assist force is insufficient can be avoided, and deterioration of the driver's steering feeling can be avoided.

  Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram showing an entire vehicle steering apparatus including a steering assist apparatus according to the present invention.

  The vehicle steering apparatus includes a steering shaft 12 connected to a steering handle 11 so that an upper end thereof is integrally rotated. A pinion gear 13 is connected to a lower end of the shaft 12 so as to be integrally rotated. The pinion gear 13 meshes with rack teeth formed on the rack bar 14 to constitute a rack and pinion mechanism. Left and right front wheels FW1, FW2 are steerably connected to both ends of the rack bar 14 via tie rods and knuckle arms (not shown). The left and right front wheels FW1 and FW2 are steered left and right in accordance with the axial displacement of the rack bar 14 accompanying the rotation of the steering shaft 12 around the axis.

  An electric motor 15 for steering assist is assembled to the rack bar 14. The electric motor 15 is configured by an AC motor that is a three-phase synchronous permanent magnet motor (brushless motor). The rotating shaft of the electric motor 15 is connected to the rack bar 14 via the ball screw mechanism 16 so that power can be transmitted, and assists the steering of the left and right front wheels FW1, FW2 by the rotation. The ball screw mechanism 16 functions as a speed reducer and a rotation-linear converter. The ball screw mechanism 16 reduces the rotation of the electric motor 15 and converts it into a linear motion and transmits it to the rack bar 14. Further, instead of assembling the electric motor 15 to the rack bar 14, the electric motor 15 is assembled to the steering shaft 12, and the rotation of the electric motor 15 is transmitted to the steering shaft 12 via the speed reducer so that the shaft 12 is axially connected. You may comprise so that it may drive around.

  Next, an electric control device that controls the operation of the electric motor 15 will be described. The electric control device includes a steering torque sensor 21, a vehicle speed sensor 22, and a rotation angle sensor 23. The steering torque sensor 21 is assembled to the steering shaft 12 and detects the steering torque T acting on the steering shaft 12 by the turning operation of the steering handle 11. The steering torque T represents the magnitude of the steering torque when the steering handle 11 is steered in the right direction and the left direction by positive and negative values, respectively. Further, instead of assembling the steering torque sensor 21 to the steering shaft 12, the steering torque T may be detected from the amount of distortion in the axial direction of the rack bar 14 by being assembled to the rack bar 14. The vehicle speed sensor 22 detects the vehicle speed V and outputs a detection signal representing the vehicle speed V.

  The rotation angle sensor 23 is composed of an encoder incorporated in the electric motor 15, and is a two-phase pulse train signal having a phase different by π / 2 according to the rotation of the rotor of the electric motor 15 and a zero representing the reference rotation position. Outputs a phase pulse train signal. The detection signal from the rotation angle sensor 23 is used to calculate the rotation angle θ and the angular velocity ω of the electric motor 15. On the other hand, since the rotation angle θ of the electric motor 15 is proportional to the steering angle of the steering handle 11, the rotation angle θ is commonly used as the steering angle of the steering handle 11 in this specification. In addition, since the angular velocity ω of the electric motor 15 is proportional to the steering angular velocity of the steering handle 11, the angular velocity ω is also commonly used as the steering angular velocity of the steering handle 11 in this specification. Instead of using the detection output of the rotation angle sensor 23, a sensor for detecting the rotation angle of the steering shaft 12 or the position of the rack bar 14 in the axial direction is prepared, and the rotation angle and displacement detected by the sensor are prepared. The amount may be used as the steering angle of the steering handle 11 and the differential value thereof may be used as the steering angular velocity of the steering handle 11. The steering angle θ and the steering angular velocity ω thus detected also represent the right and left steering angles and steering angular velocities of the steering handle 11 based on positive and negative values.

  These steering torque sensor 21, vehicle speed sensor 22 and rotation angle sensor 23 are connected to the electronic control unit 30. The electronic control unit 30 includes a microcomputer including a CPU, ROM, RAM, and the like as main components, and also includes a drive control circuit for the electric motor 15. Next, the electronic control unit 30 will be described in detail. FIG. 2 is an overall block diagram of the electronic control unit 30 including functional blocks representing the functions of the microcomputer realized by executing the program. The electronic control unit 30 controls the rotation of the electric motor 15 by vector control described in a two-phase rotating magnetic flux coordinate system in which the rotation direction of the electric motor 15 is the q axis and the direction orthogonal to the rotation direction is the d axis. . In addition, regarding the q-axis and the d-axis, when the expression method is changed, the d-axis is a field direction by the permanent magnet of the electric motor 15 and the q-axis is a direction orthogonal thereto.

  The electronic control unit 30 includes a basic assist force calculation unit 31 and a compensation value calculation unit 32. The basic assist force calculation unit 31 has a basic assist force table that stores a basic assist force Tas that changes as shown in the characteristic graph of FIG. 3 according to the steering torque T and the vehicle speed V. The basic assist force calculation unit 31 inputs the steering torque T from the steering torque sensor 21 and the vehicle speed V from the vehicle speed sensor 22, and calculates the basic assist force Tas by referring to the basic assist force table. In this case, the basic assist force Tas increases as the steering torque T increases and decreases as the vehicle speed V increases. The characteristic graph of FIG. 3 shows only the relationship between the positive region, that is, the steering torque T and the basic assist force Tas in the right direction, but the negative region, that is, the left direction steering torque T and the basic assist force Tas, The characteristic graph of FIG. 3 is moved to a point-symmetrical position around the origin. In this embodiment, the basic assist force Tas is calculated using the basic assist force table, but instead of the basic assist force table, a basic assist force Tas that changes according to the steering torque T and the vehicle speed V is defined. Alternatively, the basic assist force Tas may be calculated using the same function.

  The compensation value calculation unit 32, together with the vehicle speed V, a rotation angle θ of an electric motor 15 described later (corresponding to a steering angle θ of the steering handle 11) and an angular velocity ω of the electric motor 15 (corresponding to a steering angular velocity ω of the steering handle 11). To calculate a compensation value Trt for the basic assist force Tas. That is, the compensation value calculation unit 32 basically responds to the return force to the basic position of the steering shaft 12 that increases in proportion to the steering angle θ and the rotation of the steering shaft 12 that increases in proportion to the steering angular velocity ω. The sum of the return torque corresponding to the resistance force is calculated as the compensation value Trt. The compensation value Trt increases as the vehicle speed V increases. The compensation value Trt may be calculated by adding signals from other various sensors.

  These calculated basic assist force Tas and compensation value Trt are input to the calculation unit 33. The calculation unit 33 adds the basic assist force Tas and the compensation value Trt, and supplies the addition result to the q-axis target current calculation unit 34 as the target command torque T *. The q-axis target current calculation unit 34 calculates a q-axis target current Iq * proportional to the target command torque T *. The q-axis target current Iq * is a q-axis component current in vector control described by the two-phase rotating magnetic flux coordinate system, and controls the magnitude of the rotational torque generated by the electric motor 15.

  The electronic control unit 30 includes a field-weakening control parameter calculation unit 35 related to field-weakening control for increasing the efficiency and reducing the size and output of the electric motor 15. The field-weakening control parameter calculation unit 35 inputs, in detail, an angular velocity ω of the electric motor 15, which will be described later, a q-axis command voltage Vq * ′ for the electric motor 15, and a q-axis actual current Iq of the electric motor 15, and the first to third parameters. Referring to the table, first to third parameters Cw, Cq, Ci corresponding to the angular velocity ω, q-axis command voltage Vq * ′, and q-axis actual current Iq are calculated. These first to third parameters Cw, Cq, and Ci are supplied to the d-axis target current calculation unit 36. The d-axis target current calculator 36 multiplies the first to third parameters Cw, Cq, and Ci by a positive coefficient k to calculate the d-axis target current Id * (= k · Cw · Cq · Ci). The d-axis target current Id * is a d-axis component current in vector control described in the two-phase rotating magnetic flux coordinate system, and is used to weaken the field of the electric motor 15.

  Next, the first to third parameters Cw, Cq, and Ci will be described. As shown in the characteristic graph of FIG. 4, the first parameter table indicates “0” when the angular velocity ω of the electric motor 15 is small, and indicates a substantially constant positive value when the angular velocity ω is large. Is remembered. In other words, the first parameter Cw indicating a value that increases as the angular velocity ω increases is stored. Therefore, the first parameter Cw determined according to this characteristic means that the field-weakening current is increased in a region where the rotational speed of the electric motor 15 is large. The characteristic is changed (see FIG. 10). Further, the first parameter Cw prevents useless field weakening current from flowing when the rotation speed of the electric motor 15 is low, that is, when the rotation speed of the steering handle 11 is low.

  As shown in the characteristic graph of FIG. 5, the second parameter table indicates “0” when the q-axis command voltage Vq * ′ of the electric motor 15 is small, and is substantially constant when the q-axis command voltage Vq * ′ is large. The second parameter Cq indicating a positive value of is stored. In other words, the second parameter Cq indicating a value that increases as the q-axis command voltage Vq * ′ increases is stored. The large q-axis command voltage Vq means that the q-axis command current ΔIq, which will be described in detail later, is large, that is, the q-axis target current Iq * (corrected q-axis target current Iq * ′) and the actual q-axis current of the electric motor 15. This means that the deviation from Iq is large, and the field weakening current of the electric motor 15 increases as the deviation increases. As a result, the second parameter Cq increases the rotational speed of the electric motor 15 by performing field-weakening control when the steering handle 11 is slowly and smallly rotated while the vehicle is running, and the deviation is large. Prevents the field-weakening current from flowing when it is small.

  As shown in the characteristic graph of FIG. 6, the third parameter table shows a substantially constant positive value in a portion where the q-axis actual current Iq is small, and “0” in a portion where the q-axis actual current Iq is large. The parameter Ci is stored. In other words, the first parameter Ci indicating a value that decreases as the q-axis actual current Iq increases is stored. The third parameter Ci is the steering torque of the steering handle 11 when the steering assist force by the electric motor 15 is controlled to decrease when the steering handle 11 is rotated faster while the angular velocity ω of the electric motor 15 is large. To avoid increasing. In the present embodiment, the first to third parameters Cw, Cq, and Ci are calculated using the first to third parameter tables. However, instead of these tables, the angular velocity ω, q-axis command is calculated. Functions that define first to third parameters Cw, Cq, and Ci that change in accordance with the voltage Vq * ′ and the q-axis actual current Iq are prepared, and the first to third parameters Cw, Cq and Ci may be calculated.

  The calculated q-axis target current Iq * and d-axis target current Id * are supplied to the q-axis target current correction calculation unit 37. The q-axis target current correction calculation unit 37 corrects the q-axis target current Iq * using the d-axis target current Id * by executing the q-axis current correction calculation program of FIG. That is, the q-axis target current correction calculation unit 37 inputs the q-axis target current Iq * from the q-axis target current calculation unit 34 (step S11), and inputs the d-axis target current Id * from the d-axis target current calculation unit 36. (Step S12). Then, the q-axis target current correction calculation unit 37 calculates the correction coefficient α corresponding to the d-axis target current Id * with reference to the correction coefficient table (step S13). The correction coefficient table is built in the q-axis target current correction calculation unit 37, and stores a positive correction coefficient α that decreases as the d-axis target current Id * increases as shown in the characteristic graph of FIG. . In the present embodiment, the correction coefficient α is calculated using the correction coefficient table. However, instead of the correction coefficient table, a function that defines the correction coefficient α that changes according to the d-axis target current Id * is prepared. In addition, the correction coefficient α may be calculated using the same function.

  Next, the q-axis target current correction calculation unit 37 divides the q-axis target current Iq * by the calculated correction coefficient α to obtain a corrected q-axis target current Iq * ′ obtained by correcting the q-axis target current Iq *. The calculated q-axis target current Iq * ′ is output to the calculation unit 38 (step S15). As a result, the corrected q-axis target current Iq * ′ indicates a value obtained by correcting the q-axis target current Iq * to be larger as the d-axis target current Id * becomes larger.

  Since the d-axis target current Id * is substantially the same as the d-axis actual current Id described later in detail, the q-axis target current correction calculation unit 37 performs the d-axis target current Id * as shown by a broken line in FIG. Instead, the corrected q-axis target current Iq * ′ obtained by correcting the q-axis target current Iq * may be calculated using the d-axis actual current Id. In this case, the correction coefficient table stores a correction coefficient α that changes in the same manner as in the case of the d-axis target current Id * according to the d-axis actual current Id as shown in parentheses in FIG. Then, the q-axis target current correction calculation unit 37 inputs the d-axis actual current Id as shown in parentheses in step S12 of FIG. 7, and corrects using the changed correction coefficient table in step S13. The coefficient α is calculated. The remaining processing is the same as in the case of the d-axis target current Id *. In this case, the correction coefficient α may be calculated using a function instead of the correction coefficient table, as in the case where the d-axis target current Id * is used.

  The calculation unit 38 subtracts the q-axis actual current Iq from the corrected q-axis target current Iq * ′, and supplies the subtraction result to the proportional-integral control unit (PI control unit) 41 as the q-axis command current ΔIq. The calculation unit 39 subtracts the d-axis actual current Id from the d-axis target current Id * and supplies the subtraction result to the proportional-integral control unit (PI control unit) 42 as the d-axis command current ΔId. The proportional-integral control units 41 and 42 calculate the q-axis and d-axis command currents Iq *, I-d by the proportional-integral calculation based on the q-axis command current ΔIq and the d-axis command current ΔId. The q-axis and d-axis command voltages Vq * and Vd * are calculated so as to follow Id *.

  These q-axis and d-axis command voltages Vq * and Vd * are corrected by the non-interference correction value calculation unit 43 and the calculation units 44 and 45 to be 2 as q-axis and d-axis correction command voltages Vq * ′ and Vd * ′. This is supplied to the phase / 3-phase coordinate conversion unit 46. The non-interference correction value calculator 43 calculates the non-interference correction value −ω for the q-axis and d-axis command voltages Vq * and Vd * based on the q-axis and d-axis actual currents Iq and Id and the angular velocity ω of the rotor. Calculate (φa + La · Id), ω · La · Iq. The inductance La and the magnetic flux φa are predetermined constants. The calculation units 44 and 45 subtract the non-interference correction values −ω · (φa + La · Id) and ω · La · Iq from the q-axis and d-axis command voltages Vq * and Vd *, respectively, to correct the q-axis and d-axis. The command voltage Vq * ′ = Vq * + ω · (φa + La · Id) and Vd * ′ = Vd * −ω · La · Iq are calculated.

  The two-phase / three-phase coordinate conversion unit 46 converts the q-axis and d-axis correction command voltages Vq * ′ and Vd * ′ into the three-phase command voltages Vu *, Vv *, and Vw *, and the converted three-phase command. The voltages Vu *, Vv *, and Vw * are supplied to the PWM voltage generator 47. The PWM voltage generator 47 outputs PWM control voltage signals UU, VU, WU corresponding to the three-phase command voltages Vu *, Vv *, Vw * to the inverter circuit 48. The inverter circuit 48 generates three-phase excitation voltage signals Vu, Vv, Vw corresponding to the PWM control voltage signals UU, VU, WU, and outputs the excitation voltage signals Vu, Vv, Vw to a three-phase excitation current path. Are supplied to the electric motors 15 respectively. Current sensors 51 and 52 are provided in two of the three-phase excitation current paths, and each of the current sensors 51 and 52 excites two of the three-phase excitation currents Iu, Iv, and Iw for the electric motor 15. The currents Iu and Iw are detected and output to the three-phase / two-phase coordinate conversion unit 53. The three-phase / two-phase coordinate conversion unit 53 is also supplied with the excitation current Iv calculated by the calculation unit 54 based on the actual currents Iu and Iw. The three-phase / two-phase coordinate conversion unit 53 converts these three-phase actual currents Iu, Iv, and Iw into two-phase actual currents Id and Iq.

  Further, the two-phase pulse train signal and the zero-phase pulse train signal from the rotation angle sensor 23 are continuously supplied to the electrical angle converter 55 at a predetermined sampling period. The electrical angle conversion unit 55 calculates an electrical angle of the rotor of the electric motor 15 with respect to the stator based on each pulse train signal and supplies the calculated electrical angle to the angular velocity conversion unit 56. The angular velocity conversion unit 56 calculates the angular velocity of the rotor relative to the stator by differentiating the electrical angle. These electric angles and angular velocities correspond to the rotation angle (steering angle of the steering handle 11) θ and the angular velocity (steering angular velocity of the steering handle 11) ω of the electric motor 15, and these rotation angles θ and angular velocity ω are described above. The compensation value calculation unit 32, the field weakening control parameter calculation unit 35, the 2-phase / 3-phase coordinate conversion unit 46, the 3-phase / 2-phase coordinate conversion unit 53, and the like are also used.

  Next, the operation of the embodiment configured as described above will be described. When the driver turns the steering handle 11, the turning operation is transmitted to the rack bar 14 via the steering shaft 12 and the pinion gear 13, and the left and right front wheels FW1 and FW2 are moved by the axial displacement of the rack bar 14. Steered. At the same time, the steering torque sensor 21 detects the steering torque T applied to the steering shaft 12, and the electric motor 15 is servo-controlled by the electronic control unit 30 so that the rack bar 14 is driven with the assist torque corresponding to the steering torque T. Since it is driven, the left and right front wheels FW1, FW2 are steered while being assisted by the driving force of the electric motor 15.

  In the servo control by the electronic control unit 30, the basic assist force calculation unit 31, the compensation value calculation unit 32, and the calculation unit 33 perform the detected steering torque T, the vehicle speed V, the rotation angle of the electric motor 15 (the steering of the steering handle 11). The target command torque T * is calculated based on (angle) θ and the angular velocity (steering angular velocity of the steering handle 11) ω of the electric motor 15, and the q-axis target current calculation unit 34 calculates the q-axis based on the target command torque T *. Calculate the target current Iq *. Further, the d-axis target current calculation unit 36 calculates the first to third parameters Cw calculated by the field weakening control parameter calculation unit 35 based on the angular velocity ω, the q-axis command voltage Vq, and the q-axis actual current Iq of the electric motor 15. , Cq, and Ci, the d-axis target current Iq * is calculated. The calculation units 38 and 39, the proportional integration control units 41 and 42, the two-phase / three-phase coordinate conversion unit 46, the PWM voltage generation unit 47, and the inverter circuit 48 are converted into the current sensors 51 and 52, the three-phase / two-phase coordinate conversion. The electric motor 15 is controlled using the d-axis and q-axis actual currents Id and Iq fed back by the unit 53 and the calculation unit 54. In this case, the non-interference correction value calculation unit 43 and the calculation units 44 and 45 are configured so that the d-axis and q-axis command voltages from the proportional-integral control units 41 and 42 are used to cancel the speed electromotive force that interferes between the d and q axes. Correct Vd * and Vq *.

  In such servo control, the rotation of the electric motor 15 is controlled by vector control described by a two-phase rotating magnetic flux coordinate system in which the rotation direction of the electric motor 15 is the q axis and the direction orthogonal to the rotation direction is the d axis. Is done. In this vector control, the d-axis target current calculation unit 36 calculates the d-axis target current Id * based on the first to third parameters Cw, Cq, Ci calculated by the field weakening control parameter calculation unit 35. Thus, field weakening control is performed on the d-axis current of the electric motor 15 in accordance with the angular velocity ω, the q-axis command voltage Vq, and the q-axis actual current Iq. Therefore, efficiency improvement and small size and high output of the electric motor 15 are expected by the field weakening control, and the flow of useless field weakening current is also avoided.

  The q-axis target current Iq * calculated by the q-axis target current calculation unit 34 is corrected by the q-axis target current correction calculation unit 37 according to the d-axis target current Id *. In the correction of the q-axis target current Iq *, the d-axis target current Id * or the d-axis actual current Id is increased, so that the q-axis target current Iq * is corrected. Therefore, according to the above-described embodiment, the output assist torque of the electric motor 15 is ensured even if the field weakening control of the electric motor is performed to increase the efficiency of the electric motor 15 and to increase the size and output, so that the steering assist force is insufficient. It is possible to avoid such a situation, and the deterioration of the steering feeling of the driver can be avoided.

  Furthermore, the present invention is not limited to the above-described embodiment and its modifications, and various modifications can be employed within the scope of the present invention.

  For example, in the embodiment described above, the first to third parameters Cw and Cq that change according to the angular velocity ω, the q-axis command voltage Vq, and the q-axis actual current Iq of the electric motor 15 for field-weakening control of the electric motor 15. , Ci is used to determine the d-axis target current Id *. However, the d-axis target current Id * is determined using only one or two of the first to third parameters Cw, Cq, and Ci, in particular, only the first parameter Cw that changes according to the angular velocity ω. You may do it. Further, the first to third parameters Cw, Cq, Ci and d-axis are used by using variables representing the rotation control state of the electric motor 15 such as the angular speed ω, the q-axis command voltage Vq and the q-axis actual current Iq of the electric motor 15. Although the target current Id * is determined, the target current Id * may be determined using another variable representing the rotation control state of the electric motor 15.

1 is a schematic diagram of a vehicle steering device including a steering assist device according to an embodiment of the present invention. It is a block diagram which shows the detail of the electronic control unit of FIG. It is a characteristic graph which shows the relationship between steering torque and basic assist force. It is a characteristic graph which shows the relationship between the angular velocity of an electric motor, and the 1st parameter Cw in a field-weakening control parameter. It is a characteristic graph which shows the relationship between the q-axis command voltage of an electric motor, and the 2nd parameter Cq in a field-weakening control parameter. It is a characteristic graph which shows the relationship between the q-axis real current of an electric motor, and the 3rd parameter Ci in the field-weakening control parameter. It is a flowchart which shows a q-axis current correction calculation program. It is a characteristic graph which shows the relationship between d-axis target current Id * and d-axis actual current Id, and a correction coefficient. It is a characteristic graph which shows the relationship between the rotational speed of an electric motor, and output torque. It is a characteristic graph which shows the relationship between the rotational speed of an electric motor and output torque for demonstrating field weakening control.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 ... Steering handle, 15 ... Electric motor, 21 ... Steering torque sensor, 22 ... Vehicle speed sensor, 23 ... Rotation angle sensor, 30 ... Electronic control unit, 31 ... Basic assist force calculating part, 34 ... q-axis target current calculating part, 35... Field weakening control parameter calculator, 36... D-axis target current calculator, 37.

Claims (4)

An electric motor for assisting the steering operation of the steering wheel by the driver, and a two-phase rotating magnetic flux coordinate system in which the rotation direction of the electric motor is the q axis and the direction orthogonal to the rotation direction is the d axis. A steering assist device for a vehicle comprising: an electric control circuit that controls rotation of the electric motor by vector control;
Steering torque detection means for detecting steering torque applied to the steering wheel;
Q-axis target current determining means for determining a q-axis target current according to the detected steering torque;
D-axis target current determining means for determining a d-axis target current that changes according to a rotation control state of the electric motor for field-weakening control of the electric motor;
Q-axis current correction means for correcting the determined q-axis target current according to a d-axis current-related state quantity related to the d-axis current of the electric motor and calculating a corrected q-axis target current;
A vehicle steering assist device comprising field control means for field-controlling the electric motor according to the calculated corrected q-axis target current and the determined d-axis target current.   2. The vehicle according to claim 1, wherein the q-axis current correction unit corrects the q-axis target current so that the correction q-axis target current increases as the d-axis current of the electric motor increases. Steering assist device.   The vehicle steering assist device according to claim 1, wherein the d-axis current related state quantity is a d-axis target current determined by the d-axis target current determining unit. The vehicle steering assist device according to claim 1, wherein the d-axis current related state quantity is an actual d-axis current of the electric motor.

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