JP2008068666A - Electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric power steering device which suppresses a torque ripple in a high-speed area of a motor even in a rapid turn-back steering of a steering wheel by restricting a current command value based on a desired output characteristic, an angular speed, a motor, and a transmission direction of a steering auxiliary force so as to set a small d-axis current as much as possible in a range satisfying a required specification during the d-axis weak field control of vector control, where the vibration is not generated in the steering wheel, and there is no sense of incongruity in a steering wheel operation. <P>SOLUTION: The electric power steering device controls the motor, drives the motor by a current command value calculated based on a steering torque or the like, and gives a steering auxiliary force to a steering system of a vehicle. The electric power steering device is provided with a restriction function part for restricting a current command value based on the desired output characteristic, the angular speed, the motor, and the transmission direction of the steering auxiliary force. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electric power steering device that applies a steering assist force to a steering system by PWM control using a permanent magnet type brushed DC motor and a brushless DC motor, and particularly to a left and right control system of a motor and a steering assist force transmission mechanism. The present invention relates to an electric power steering apparatus in which PWM duty does not saturate even when there is a difference in efficiency, steering sound does not deteriorate, and the driver does not feel uncomfortable.

  2. Description of the Related Art An electric power steering device that uses a rotational force of a motor to assist an auxiliary force so that a vehicle handle can be operated lightly is widely used. In this electric power steering apparatus, an auxiliary force is applied to the steering shaft or the rack shaft by a transmission mechanism such as a gear or a belt via a reduction gear.

  A general configuration of such an electric power steering apparatus is shown in FIG. The column shaft 302 of the steering handle 301 is connected to a tie rod 306 of the steering wheel via a reduction gear 303, universal joints 304a and 304b, and a pinion rack mechanism 305. The column shaft 302 is provided with a torque sensor 307 that detects the steering torque of the steering handle 301, and a motor 308 that assists the steering force of the steering handle 301 is connected to the column shaft 302 via the reduction gear 303. ing.

  In the electric power steering apparatus having such a configuration, the steering torque transmitted from the steering handle 301 by the driver's steering operation is detected by the torque sensor 307, and the current command value calculated based on the torque signal and the vehicle speed is detected. The motor 308 is driven and controlled by the PWM control, and this driving becomes an assisting force for the driver's steering operation, and the driver can perform the steering operation with a light force. In other words, whether the steering wheel steering feel is good or bad depends on what current command value is calculated from the steering torque output by the steering wheel operation and how the motor 308 is controlled based on the current command value. The performance of the power steering device is greatly affected.

  Therefore, before describing the problem for realizing desirable motor control in the electric power steering apparatus from such a viewpoint, general motor characteristics of the motor used in the electric power steering apparatus and torque control of the motor will be described.

First, motor characteristics will be described. The normal operation region of the motor can be defined by a torque-speed characteristic (Tn characteristic) derived from the motor output equation. Therefore, a motor output equation relating to a three-phase brushless DC motor (BLDC motor) can be expressed as Equation 1.
(Equation 1)
v = EMF + R · i + L · di / dt
Here, v is the phase voltage of the motor, i is the phase current of the motor, EMF is the phase back electromotive force, R is the resistance value per phase of the motor, and L is the inductance value per phase.

Here, if the saturation state (PWM duty is 100%), the battery voltage Vbat for driving the motor is applied to the two windings of the motor. Can do.
(Equation 2)
Vbat = EMF _LL + 2R · I
Here, EMF _LL is a counter electromotive voltage measured between two phases, and I is a motor current.

Next, when Formula 2 is calculated using Formula 3 which is an equation of the back electromotive force EMF and Formula 4 which is a torque equation, Formula 5 can be derived.
(Equation 3)
EMF _LL = Ke · ω
Here, Ke is a counter electromotive voltage constant, and ω is an angular velocity (rotational speed).
(Equation 4)
T = Kt · I
Here, Kt is a torque constant.
(Equation 5)
ω = ω ₀ (1- (I / I ₀ )) = ω ₀ (1- (T / T ₀ )) [rad / s]
Here, ω ₀ = Vbat / Ke is an angular velocity at no load (torque is zero), and I ₀
= Vbat / 2R is a restraint current (angular velocity 0), and T ₀ = Kt is a restraint torque (stall torque).

When the above equation 5 is rewritten with the unit being rpm, the equation 6 is obtained.
(Equation 6)
n = n ₀ (1- (I / I ₀ )) = n (1- (T / T ₀ )) [rpm]

The above formula 6 represents a linear torque-speed characteristic (Tn characteristic).

Here, the torque-speed characteristic (Tn characteristic) of an actual brushless DC motor is slightly different from Equation 6 and can be expressed as Equation 7 below.
(Equation 7)
n = n ₀ − (n ₀ −n _rated ) · T / T _rated
Here, n ₀ is a no-load rotational speed, n _rated is a rated rotational speed, and T _rated is a rated torque.

Expressions 6 and 7 are shown in FIG. In FIG. 16, point A is a point indicating the rating, and point B is a point indicating no load. The broken line indicated by Equation 6 is an ideal straight line, whereas the actual characteristic (solid line) indicated by Equation 7 is slightly different from the ideal straight line. This is due to the influence of the inductance value L of the motor. As the larger current is applied, the actual characteristics become more distant from the ideal straight line.

  Here, the meaning indicated by the Tn characteristic in FIG. 16 indicates the limit of the motor. In the lower region of the Tn characteristic of FIG. 16, the motor can operate from the stop state to the maximum angular velocity and output the maximum torque without exceeding the thermal and electrical limits.

  In FIG. 17, characteristic 1 represents a Tn characteristic of a motor with a small output, and characteristic 3 represents a motor characteristic with a large output. Further, if the characteristic 2 represents the motor load characteristic of the electric power steering apparatus, the load characteristic of the characteristic 2 can be covered in the whole area if the high output motor represented by the characteristic 3 can be used. There is a problem that the outer shape becomes large. Therefore, if an attempt is made to cover the load characteristic 2 with a motor having a small output of the characteristic 1, it cannot be covered in the high speed rotation region. Therefore, as a method of covering the load characteristic 2 of the characteristic 2 with the motor of characteristic 1, a method of changing the Tn characteristic of the motor of characteristic 1 to the Tn characteristic of characteristic 4 using field weakening control in the motor vector control. Can be considered.

  It is well known in the art to control a motor of an electric power steering device by vector control that also takes field weakening control into consideration. For example, in Japanese Patent Laid-Open No. 2001-18822 (Patent Document 1), the motor of the electric power steering apparatus is controlled using vector control.

  FIG. 18 shows a basic configuration of an electric power steering apparatus using the vector control of Patent Document 1.

  Based on the torque command value Tref, the command current determining means 324 calculates the d-axis and q-axis current command values idref and iqref. On the other hand, the motor currents ia, ib, and ic of the motor 308 are detected by the current detection means 341, 342, and 343, respectively, and the detected currents ia, ib, and ic are detected on the dq2 axis by the three-phase / two-phase conversion means 345 It is converted into currents id and iq. Subtracting units 325 and 326 calculate deviation currents between the d-axis and q-axis current command values idref and iqref and the fed back currents id and iq. The deviation current is input to the PI control means 328, and voltage command values vd and vq are calculated so that the deviation current is zero. The motor 308 is a three-phase motor, and the voltage command values vd and vq are converted into three-phase voltage command values va, vb and vc by the two-phase / three-phase conversion means 336.

  The PWM control means 337 generates a PWM-controlled gate signal based on the three-phase voltage command values va, vb, vc. The inverter 338 is driven by the gate signal generated by the PWM control means 337, and the motor 308 is supplied with such a current that the deviation current becomes zero. The resolver 316 detects the angle (rotational position) θ of the motor 308, and the angular velocity conversion means 348 calculates the angular velocity (rotational speed) ω from the angle θ, which is used for vector control.

  In such vector control, field-weakening control is used in the high-speed rotation region of the motor.

Here, the motor 308 is vector-controlled based on the steering assist current command value Iref calculated based on the steering torque (or vehicle speed, etc.) detected by the torque sensor 307. When this vector control is expressed by a mathematical expression, the following Expression 8 or Expression 9 is obtained. Equation 8 shows the case where there is no field weakening control (Id = 0), and Equation 9 shows the case where field weakening control is being executed (Id ≠ 0).
(Equation 8)
Iq = Iref
Id = 0
(Equation 9)
Iq = Iref
Id ≠ 0

On the other hand, when the motor current Is is expressed by the d-axis current command value Id and the q-axis current command value Iq, the following formula 10 is obtained.
(Equation 10)
Is = √ (Iq ² + Id ² )

In the vector control of the motor under such a current relationship, if the handle is turned back rapidly, the motor cannot output the required torque, and the field weakening control is executed. That is, in the high speed rotation region, the motor current Is may fall into a saturated state (duty = 100%).

  When the motor current is saturated, the current waveform is distorted and the torque ripple of the motor is increased. As a result, vibration is generated in the handle or abnormal noise is generated from the motor.

  Thus, if control is performed beyond the limit of the constant torque output, the motor current is saturated and the torque ripple becomes large, and the driver feels vibration and discomfort in the steering operation.

  Therefore, as a control method corresponding to such a problem, a control method as disclosed in JP-A-8-142886 (Patent Document 2) is known. That is, a steering assist force target value is calculated based on a steering torque detector that detects steering torque of the steering system, a motor that generates a steering assist force for the steering system, and at least a torque detection value of the steering torque detector. The steering assist force command value is calculated from the steering assist force target value by the calculation of the feedback control including the integral calculation, the control means for outputting the steering assist force command value, and the drive current corresponding to the output of the control means. In a control device for an electric power steering apparatus provided with drive means for supplying to a motor, the steering assist force command value of the control means is limited to a predetermined range corresponding to the upper and lower limit values of the drive current of the motor and the drive means Limiting means for outputting, and correcting means for correcting the integration amount in the integral calculation with a correction value based on the difference between the output value of the limiting means and the steering assist force command value are provided.

Thus, in the apparatus shown in Patent Document 2, since the motor current is forcibly limited, saturation of the motor current can be prevented, so that generation of vibration can be prevented.
JP 2001-18822 A JP-A-8-142886

  However, in the device described in Patent Document 2, since the output value to the PWM circuit is limited by the limiter, it is not possible to remove the uncomfortable feeling of steering operation for the driver.

  Here, there are three concepts for field weakening control of a permanent magnet type brushless DC motor. That is, constant voltage constant power control, constant current constant power control, and constant current constant voltage control. Although the first two concepts are relatively simple, constant power conditions are not maintained over a wide range of speeds. The third concept is more accurate because limit conditions are taken into account. However, motor resistance is often ignored, giving significant calculation errors, especially for small motors, and motor resistance is relatively large with respect to inductance. That is, it is necessary to consider the influence of inductance as well as motor resistance.

  In the electric power steering system, the steering assist torque is a function of the steering torque. When the steering wheel is turned back very quickly, the required steering assist torque is large, and the motor cannot follow immediately, so that accurate assist torque cannot be applied. This is because the current command value (reference current) that is directly proportional to the required torque cannot be flowed well in the above-described system (limited motor power, duty saturation) that limits the motor current. In this state, torque ripple is generated, which causes a sense of incongruity in steering operation and noise.

  In addition, in the electric power steering system, there is a problem that the steering force varies depending on the steering direction when there is a difference between the left rotation and the right rotation of the reduction gear efficiency or a difference in the axial load applied to the front shaft. .

  The present invention has been made under the circumstances described above, and an object of the present invention is to set a d-axis current as small as possible within a range that satisfies the required specifications during vector-controlled d-axis field weakening control. By limiting the current command value based on the output characteristics and angular velocity, and the transmission direction of the motor and steering assist force, the torque ripple in the high-speed region of the motor is suppressed even during the rapid turn steering of the steering wheel, and vibration occurs in the steering wheel. It is another object of the present invention to provide an electric power steering device that does not give a sense of incongruity to steering operation. This is because if field-weakening control is used near the limit of motor performance, there is a problem that noise increases due to torque ripple or the like.

  The present invention relates to an electric power steering apparatus that controls a motor and drives the motor by a current command value calculated based on a steering torque or the like to apply a steering assist force to a steering system of a vehicle. The above-described object of the present invention is achieved by providing a limiting function unit that limits the current command value based on desired output characteristics and angular velocity, and the transmission direction of the motor and steering assist force.

  The above-described object of the present invention is to configure the limiting function unit with a CW lookup table and a CCW lookup table, and a switch for switching to the CW lookup table or the CCW lookup table according to the transmission direction. Or the limit function unit is further set in consideration of the front axle load, or the limit function unit is further set in consideration of a change in battery voltage, or the motor is vectorized. This is achieved more effectively by being driven by control, or when the motor is a permanent magnet brushless DC motor.

  According to the present invention, it is possible to reduce operating noise and torque ripple in a high speed region by reducing current error due to excessive reference current, and it is possible to reduce duty by calculating an accurate current command value limit value. As a result, the torque ripple caused by the distortion of the motor current waveform can be suppressed, so that the abnormal noise is not generated from the motor even when the steering wheel is turned quickly, and the steering wheel is uncomfortable. The electric power steering device can be provided, and even if there is a difference between the left and right rotations of the reduction gear efficiency, and there is a difference in the axial load applied to the front shaft, the limiting characteristics can be adjusted to the left of the reducer. The effect of the same steering force regardless of the steering direction is achieved by taking into account the difference in the efficiency of right rotation and the difference in axle load applied to the left and right wheels of the front axle. .

  When used near the limit of motor performance, there is a problem that noise is increased due to saturation of PWM duty. Therefore, the present applicant has proposed a method of limiting the current command value so that the duty is not saturated (Japanese Patent Application No. 2005-76537). Further, during d-axis current control, the current command value is limited so as to set the d-axis current as small as possible within a range that satisfies the required specifications. As a result, torque ripple in the high speed region of the motor is suppressed even when the steering wheel is turned quickly, and as a result, vibration does not occur in the steering wheel, and the steering wheel does not feel strange.

  First, the calculation principle of the limit value Ireflim of the current command value that limits the steering assist current command value Iref, which is a premise of the present invention, will be described. The calculation of the current limit value when the maximum output characteristic is used as the rated output characteristic will be described below.

The rated output Pn of the motor can be calculated as shown in Equation 11 as a motor display value.
(Equation 11)
Pn = Tn · ω _n
However, Tn is a rated torque and ω _n is a rated angular velocity.

The maximum output Pmax of the motor means the maximum output that can always be operated, and is a characteristic given by the manufacturer of the motor. In general, the maximum output Pmax and the rated output Pn have the following relationship:
(Equation 12)
Pmax> Pn

Equation 12 means that the point of the maximum output Pmax of the motor is different from the rated operating point of the motor, and this depends on the circuit configuration of the motor drive. In the motor of the electric power steering apparatus, the maximum output Pmax is generated in the region of high-speed rotation than the rated angular speed omega _n.

When the characteristic 2 of the rated output Pn and the characteristic 4 of the maximum output Pmax are displayed on the same drawing, it is as shown in FIG. From FIG. 1, the characteristic 4 indicating the maximum output Pmax has a relationship obtained by shifting the characteristic 2 indicating the rated output Pn by n-offset (denoted as “ _nos ”) in the rotational axis (vertical axis) direction. I understand that. When the angular velocity ω (rad / s) corresponding to the display of the rotational speed n (rpm) is displayed, the offset no _os can be displayed as the angular velocity “ω _os ”. That is, the angular velocity omega _os relates rotational axis direction, is shifted by the offset _{n os.} Here, the maximum output angular velocity is defined as (ω _n + ω _os ), and this relationship is expressed by the following equation (13).
(Equation 13)
Pmax = Tn · (ω _n + ω _os )

This is based on the maximum output Pmax which is the rated output characteristic. That is, the condition is that the motor must never exceed the maximum output Pmax in the steady state. If this is expressed in an equation, the following equation 14 is obtained.
(Equation 14)
P <Pmax

Further, since the power P is a product of the torque T and the angular velocity ω (P = T · ω), the limit Tlim of the torque T can be expressed as shown in Expression 15 when expressed again using Expression 14.
(Equation 15)
Tlim <Pmax / ω _m
Here, ω _m is the mechanical angular velocity of the motor.

Then, by substituting Equation 13 into Equation 15, Equation 16 is obtained.
(Equation 16)
Tlim <Tn · (ω _n + ω _os ) / ω _m

The limit value Ireflim of the current command value is determined from Expression 16, and can be expressed as Expression 17.
(Equation 17)
Ireflim = Tlim / Kt <(Tn · (ω _n + ω _os ) / ω _m ) / Kt

The meaning of Equation 17 is that the limit value Ireflim of the current command value is obtained from the rotation speed (angular velocity ω _m ) of the motor based on the maximum output characteristic that is the rated output characteristic of the motor, and the limit of the obtained current command value is obtained. The motor driving current is limited based on the value Ireflim.

Then, from the equations 4 and 17, the following equation 18 is obtained.
(Equation 18)
Ireflim <In · (ω _n + ω _os ) / ω _m
Here, In is a rated current value of the motor.

If Expression 18 is used, the torque constant Kt of the motor is not necessary. When the current command value is limited from the angular velocity ω, the above equation 17 or 18 is the basic formula.

Next, PVC control as an example of vector control to which the present invention can be applied will be described with reference to FIG.

  In general, in vector control, when a three-phase motor is controlled, the current command value is calculated by separating the two-axis components of d-axis (excitation component) and q-axis (torque component), and the motor feedback current is also three-phase. Is divided into d-axis and q-axis components to execute PI control, and finally, a three-phase motor is controlled by performing two-phase / three-phase conversion at the stage of applying a gate signal to the inverter. In contrast to such vector control, the PVC control uses the calculation with the d-axis and q-axis components to calculate the three-phase current reference values Iavref, Ibvref, and Icvref, and feeds back the motor current. The PI control stage is characterized by three-phase control.

A current command value calculation unit (not shown) calculates a steering assist current command value Iref based on the steering torque T, the vehicle speed, etc., and the steering assist current command value Iref is converted into a conversion unit in the vector control phase command value calculation unit 100. 106, the q-axis current reference value calculation unit 103 and the d-axis current reference value calculation unit 105 are input. In place of the steering assist current command value Iref, a torque command value Tref (= Kt · Iref) converted to torque may be used. An angle θ _e of the motor 208 is detected by the resolver 209, and an angular velocity ω _e is calculated by a differentiation circuit 24 that receives the angle θ _e . The steering assist current command value Iref, the angle θ _e and the angular velocity ω _{e are} input to the vector control phase command value calculation unit 100, and the phase control reference value calculation unit 100 calculates each phase current reference value Iavref, Ibvref, Icvref, The current reference values Iavref, Ibvref, and Icvref are input to the PI control unit 21 through the subtraction units 20-1, 20-2, and 20-3, respectively.

In the vector control phase command value calculation unit 100, the d-axis current reference value calculation unit 105 calculates the d-axis current reference value Idref expressed by the following equation (19). The base angular velocity omega _b is obtained by converting the steering assist current command value Iref in terms of section 106.
(Equation 19)
Idref = − | Iref | · sin (acos (ω _b / ω _m ))

As can be seen from (acos (ω _b / ω _m ) in Equation 19, when the angular velocity ω _{m of} the motor becomes higher than the base angular velocity ω _b , the d-axis current reference value Idref appears as a value. When the angular velocity ω _m becomes higher than the base angular velocity ω _b , field weakening control is executed.

On the other hand, the conversion unit 101 inputs the angle θ _e and the angular velocity ω _e to calculate the phase back electromotive voltages ea, eb, and ec of the motor 208, and the three-phase / two-phase conversion unit 102 performs counter-electromotive force of the d axis and the q axis. The voltages ed and eq are calculated respectively. The q-axis current reference value calculation unit 103 receives the d-axis and q-axis back electromotive voltages ed and eq, the angular velocity ω _e , the d-axis current reference value Idref, and the steering assist current command value Iref, and the following equation 20 The q-axis current reference value Iqref is calculated according to
(Equation 20)
Iqref = 2/3 (Iref × ω _m −ed × Idref) / eq

Therefore, when field weakening control is not executed, Idref = 0, so that Iqref = Iref. The two-phase / three-phase converter 104 receives the d-axis current reference value Idref and the q-axis current reference value Iqref and calculates the three-phase current reference values Iavref, Ibvref, and Icvref.

  The control after the three-phase current reference values Iavref, Ibvref, and Icvref are calculated by the 2-phase / 3-phase converter 104 is exactly the same as general feedback control. That is, the phase currents Ia, Ib, and Ic of the motor 208 are detected by the current detection circuits 32-1, 32-2, and 32-3, and the three-phase currents are detected by the subtraction units 20-1, 20-2, and 20-3. Deviation currents from the reference values Iavref, Ibvref, and Icvref are calculated, and the deviation currents are input to the PI control unit 21. The PI control unit 21 calculates the voltage command values Va, Vb, and Vc so that the deviation current is zero, and executes feedback control. With the voltage command values Va, Vb, and Vc as inputs, the PWM control unit 31 calculates a PWM gate signal to the inverter 31. The inverter 31 is PWM-controlled by the gate signal, and each phase current Ia, Ib, Ic and current reference are calculated. The inverter 31 is current-controlled so that the deviations from the values Iavref, Ibvref, and Icvref are zero.

The above is the description of the PVC control. As described above, the d-axis current reference value Idref is 0 when the angular velocity ω _{e of the} motor 208 is equal to or less than the base angular velocity ω _b , that is, the field weakening control is not executed. Therefore, Iqref = Iref, and each phase current Ia, Ib, Ic does not saturate (duty = 100%), so there is no problem. However, when the angular velocity ω _{e of the} motor 208 exceeds the base angular velocity ω _b , the d-axis current reference value Idref is no longer 0, and √ (Iq ² + Id ² ) may exceed the current In at the maximum output. If an attempt is made to output the motor current equal to or greater than the current In at the maximum output, the duty becomes 100%, the torque ripple increases, and problems such as the generation of abnormal noise of the motor 208 occur. Therefore, the steering assist current command value Iref needs to be limited by the variable limit value Ireflim expressed by Equation 18 when the angular velocity ω _{e of the} motor 208 becomes high and the angular velocity is higher than the angular velocity at which the field weakening control is executed. In addition, when field-weakening control is used near the limit of motor performance, there is a problem that noise increases due to torque ripple or the like.

Therefore, the limit of the current command value at the base speed (when Id = 0) is calculated from the maximum motor conditions (Vmax, Imax) (where Vmax, Imax is the voltage and current at maximum output Pmax in Equation 12), Calculate the limit of the current command value in the high speed region (when the negative d-axis current command value Id is input to the control system). Establish the required motor torque-speed (Tn) characteristics from the performance specifications requested by the customer, input the steering assist current command value Iref, the motor angular speed ω _e, and the battery voltage Vdc, and set the maximum conditions (Vmax, Imax ) To calculate a d-axis current command value Id for field weakening control. Each of the above calculations is executed by a look-up table or the like designed in advance to speed up the calculation in the microcomputer, etc., and the change in the battery voltage Vdc is applied by calculating the motor speed change and the current resulting from the voltage change. Can be done. These change signals are also calculated by an additional lookup table or the like.

The configuration of the current command value calculation unit in that case is as shown in FIG. 3, the steering assist current command value Iref is input to the comparison units 13 and 14, and the estimated or detected angular velocity ω _e is the look-up table 11. And 12, and is input to the lookup table 15, and is further input to the detection switching unit 17 that detects whether the angular velocity ω _e is equal to or higher than the base angular velocity ω _b and switches the contacts c 1 and c 2 of the switch 16. . The limit value Ireflim of the current command value calculated by the lookup table 11 is input to the comparison unit 13, and the limit value Ireflim_base of the current command value calculated by the lookup table 12 is input to the comparison unit 14. In this example, the battery voltage Vdc is not necessary. The limit value Iref_lim of the current command value output from the comparison unit 13 is given to the contact c2 of the switch 16, and the base limit value Iref_base of the current command value output from the comparison unit 14 is given to the contact c1 of the switch 16.

  Next, calculation of the steering assist current command value Iref will be described.

When showing the expression of a-b-c-phase of the three-phase motor 5, for example, the phase voltage _{v an} for a phase, the phase current _{i a,} the phase resistance Ra, phase inductance La, phase counter-electromotive force voltage _{e a} Is expressed by the equation (21).
(Equation 21)
v _an = i _a · Ra + La · di _a / dt + e _a

For a balanced motor having the same characteristics in each phase, in one phase, the equation 21 is usually expressed as the following equation 22.
(Equation 22)
v = i * R + L * di / dt + e
Here, R = Ra = Rb = Rc is the phase resistance of the motor, and L = La = Lb = Lc
Is a motor phase inductance, and e is a phase counter electromotive voltage.

The voltages v _d and v _q on the dq axis are expressed by the following equations 23 and 24, respectively.
(Equation 23)
v _d = i _d · R + Ld · di _d / dt−ω _e · Ld · i _q
(Equation 24)
_{v q = i q · R +} Lq · di q / dt + ω e · Lq · i d + ω e · Ψ
Here, Ld is the d-axis inductance of the motor, Lq is the q-axis inductance of the motor, and Ld = Lq = L for an SPM (Surface PM) motor. Also, Ψ
Is a magnetic flux acting on the d-axis and is generally Ψ = 4/3 · Ke / (2 × p) (Ke is a counter electromotive voltage constant and p is the number of pole pairs).

For a sinusoidal motor, the d-axis current _id and the q-axis current _iq change in a DC manner, and their derivatives are zero. Therefore, Equations 23 and 24 are expressed by Equations 25 and 26 below, respectively. It is expressed in a steady state.
(Equation 25)
v _d = i _d · R−ω _e · Ld · i _q
(Equation 26)
_{v q = i q · R +} ω e · Lq · i d + ω e · Ψ

On the other hand, the q-axis current i _q in the rectangular wave motor is not a direct current, and its change and differentiation should take into account Δi _q = 0 and di _q / dt ≠ 0 in the steady state. Therefore, it can be defined as the following Expression 27.
(Equation 27)
i _q = Iref + Δi _q
Here, Iref is a current command value, and Δi _q is a q-axis current i _q (Δi _q = i _q
-Iref).

Therefore, by substituting Equation 26 into Equation 25 and Equation 26, the following Equation 28 and Equation 29 are obtained, respectively.
(Equation 28)
v _d = i _d · R−ω _e · Ld · (Iref + Δi _q )
(Equation 29)
_{v q = (Iref + Δi q} ) · R + Lq · di q / dt + ω e · Lq · i d
+ Ω _e・ Ψ
The differential Lq · di _q / dt in Equation 29 is difficult to calculate based on large variables in the function of speed and current, but if the maximum current constant is maintained, the peak of Lq · di _q / dt is It can be replaced with the following 30 speed functions.
(Equation 30)
Lq · di _q / dt = Lq · di _q / dθ · dθ / dt = Lq · di _q / dθ · ω _e
= K · ω _e
Here, k is a coefficient representing a linear relationship between the differentiation of the q-axis current i _q and the angular velocity ω _e .

Therefore, the following equations 31 and 32 hold.
(Equation 31)
v _d = i _d · R−ω _e · Ld · Iref−ω _e · Ld · Δi _q
(Expression 32)
_{v q = i ref · R +} ω e · Lq · i d + ω e · Ψ + k · ω e + Δi q · R

“Ω _e · Ld · Δi _q ” in Equation 31 and “k · ω _e + Δi _q · R” in Equation 32 are additional terms of the rectangular wave motor.

For a rectangular wave motor, the control can be realized by the PVC method described above (see FIG. 2). Since the current and other variables are not sinusoidal, the dq equation cannot be used to analyze a square wave motor. However, by introducing another change Δi _q and coefficient k, analysis on the dq axis becomes possible.

The counter electromotive voltage waveform and current waveform of a sine wave motor are sine waves, and when converted into three phases / dq axes, both the d axis component (excitation component) and the q axis component (torque component) become DC values (constant values). . On the other hand, in the rectangular wave motor, the counter electromotive voltage waveform and the current waveform are pseudo rectangular waves, and in addition to the sine wave of the primary component, the sine waves of 3, 5, 7,. When three-phase / dq-axis conversion is performed, neither the d-axis component nor the q-axis component becomes a direct current value (constant value), but is a function of the electrical angle. Specifically, the 5th and 7th order components in the 3 phase appear as the 6th order component on the dq axis, and similarly, the 11th and 13th order components in the 3 phase appear as the 12th order component on the dq axis. 3,9,... The next component disappears because it is a zero phase. If the function of the electrical angle remains as it is, the calculation for calculating the limit value is complicated, so it is necessary to simplify it and the purpose is to obtain the limit value, so the maximum value of the vibrating component is handled here. The change Δi _q and the coefficient k are parameters representing the maximum value depending on the electrical angle. By introducing these parameters that are not a function of the electrical angle, the limit value can be set even in the rectangular wave motor as in the sine wave motor. Can be calculated.

The maximum value of the q-axis current i _q is expressed by the following equation 33, and the theoretical value of the q-axis current i _q is expressed by the following equation 34.
(Expression 33)
i _q = Iref + Δi _q
(Equation 34)
i _q = Iref + (i ₃ + i ₅ ) sin6θ + (i ₁₁ + i ₁₃ ) sin12θ
+ ...

Next, calculation of the current command value limit value Ireflim and the d-axis current command value Id in the current command value calculation unit will be described. First, voltage and current limiting conditions are as follows.
(1) The voltage constant is as follows.

又は
（数３６）
ｖ_ｄ ^２＋ｖ_ｑ ^２≦Ｖｍａｘ^２
ここで、Ｖｄｃはコントローラの入力で測定されるバッテリ電圧であり、ｋ_ｓはモ
デュレーション技術(modulation technique)を示す安全係数である。

（２）電流定数は下記の通りである。
（数３７）
ｉ_ｄ ^２＋ｉ_ｑ ^２≦Ｉｍａｘ^２
ここで、Ｉｍａｘはモータ又はインバータの最大電流であり、定常時は小さい。

数３１及び数３２を数３６に代入することによって、下記数３８を得る。
（数３８）
Ｖｍａｘ^２＝（ｉ_ｄ・Ｒ−ω_ｅ・Ｌ・Ｉ_{ｒｅｆｌｉｍ}−ω_ｅ・Ｌ・Δｉ_ｑ）^２
＋（Ｉ_{ｒｅｆｌｉｍ}・Ｒ＋ω_ｅ・Ｌ・Ｉ_ｄ＋ω_ｅ・Ψ＋ｋ・ω_ｅ＋Δｉ_ｑ・Ｒ）^２

そして、ｄ軸電流指令値Ｉｄは数３８から次のように導かれる。

Or (Equation 36)
v _d ² + v _q ² ≦ Vmax ²
Here, Vdc is the battery voltage measured at the input of the controller, and k _s is a safety factor indicating a modulation technique.

(2) The current constant is as follows.
(Equation 37)
i _d ² + i _q ² ≦ Imax ²
Here, Imax is the maximum current of the motor or the inverter, and is small during normal operation.

By substituting Equations 31 and 32 into Equation 36, the following Equation 38 is obtained.
(Equation 38)
_{^{Vmax 2 = (i d · R}} -ω e · L · I reflim -ω e · L · Δi q) 2
+ (I _reflim · R + ω _e · L · I _d + ω _e · Ψ + k · ω _e + Δi _q · R) ²

The d-axis current command value Id is derived from Equation 38 as follows.

ただし、Ａ１及びＢ１はそれぞれ下記数４０及び数４１である。

However, A1 and B1 are the following number 40 and number 41, respectively.

また、数３７を数３９に代入して電流指令値Ｉｒｅｆについて解くと、下記数４２となる。

Further, when Equation 37 is substituted into Equation 39 and the current command value Iref is solved, the following Equation 42 is obtained.

これが電流指令値Ｉｒｅｆの制限値であり、ｄ軸電流指令値Ｉｄが最大電流のケースでは下記数４３を使用する。

This is the limit value of the current command value Iref, and the following equation 43 is used when the d-axis current command value Id is the maximum current.

なお、正弦波モータの場合にはΔＩｑ＝０であり、ｋ＝０である。また、ΔＩｑ及びｋは数４２の計算では定数に保持される。

In the case of a sine wave motor, ΔIq = 0 and k = 0. ΔIq and k are held constant in the calculation of Equation 42.

The following can be said from the above equations. If the motor parameter and the battery voltage Vdc are constant, the current command value Iref is a function of only the speed, and is expressed by the following equation 44.
(Equation 44)
Ireflim = f (ω _e )

If the battery voltage Vdc changes, the change value is taken into account, so that
(Equation 45)
Ireflim = f (ω _e , V _dc )

In addition, the algorithm uses Equation 42 to calculate the limit value Ireflim of the current command value as follows.

(1) When Iref> Ireflim
Iref = Ireflim, Id = Idmax
(2) When Iref ≦ Ireflim
Iref = Iref, Id = Id

FIG. 4 shows a characteristic example (three regions) of the d-axis current command value Id, the current command value Iref, and the motor rotation speed. The data of the characteristic A is stored in the lookup table 11 and the data of the characteristic B is looked up. It is stored in the table 12. Further, data of characteristic C is stored in the lookup table 15. Similarly, FIG. 5 shows a characteristic example of the characteristic example (four regions) of the d-axis current command value Id and the current command value Iref and the motor rotation speed, and the data of the characteristic A is stored in the look-up table 11 and the characteristic. B data is stored in the lookup table 12, and characteristic C data is stored in the lookup table 15.

  In such a configuration, an example of the operation will be described with reference to the flowchart of FIG.

First, the steering assist current command value calculating portion based on the steering torque and the vehicle speed calculates a steering assist current command value Iref (step S1), and inputs the angular velocity omega _e of the motor 5 (step S2). This order is arbitrary. Next, based on the input angular velocity ω _e , the lookup table 11 calculates a limit value Ireflim of the current command value based on the characteristic A of FIG. 4 or FIG. 5 (step S3). 4 or based on the characteristic B of FIG. 5, the base limit value Ireflim_base of the current command value is calculated (step S4), the current command value limit value Ireflim is input to the comparison unit 13, and the base limit value Ireflim_base of the current command value is Input to the comparison unit 14. Note that the calculation order of the current command value limit value Ireflim and the current command value base limit value Ireflim_base is arbitrary.

The comparison unit 13 calculates a current command value limit value Iref_lim based on the comparison between the steering assist current command value Iref and the current command value limit value Ireflim (step S5), and the comparison unit 14 calculates the steering assist current command value Iref and the current. Based on the comparison of the command value base limit value Ireflim_base, the current command value limit value Iref_base and the d-axis current command value Id = 0 are calculated (step S6). The current command value limit value Iref_lim is input to the contact c2 of the switch 16, and the current command value base limit value Ireflim_base and the d-axis current command value Id = 0 are input to the contact c1 of the switch 16. Look-up table 15 based on the limit value Iref_lim angular velocity omega _e and the current command value, to calculate a d-axis current command value Id based on the characteristics C of FIG. 4 or FIG. 5 (step S7).

Thereafter, the detection switch 17 the angular velocity omega _e is equal to or a base angular velocity omega _b above (step S10), and the contact of the switch 16 for the angular velocity omega _e field weakening control if the base angular velocity omega _b above Is switched to c1 (step S11), and the base limit value Iref_base of the current command value from the comparison unit 14 and the d-axis current command value Id = 0 are output as the limit value Iref_lim (step S13). If the angular velocity ω _e is smaller than the base angular velocity ω _b , the contact of the switch 16 is switched to c2 (step S12), and the current command value limit value Ireflim_lim from the comparison unit 13 is output as it is as the current command value limit value Iref_lim. (Step S13).

Next, the response to the battery voltage Vdc change will be described.

If required to hold the q-axis current command value Iq and d-axis current command value Id constant, variation ΔVdc battery voltage Vdc is a change in the angular velocity omega _e. That is, it is possible to the d-axis current limit function only of the angular velocity omega _e. If the battery voltage Vdc and the angular velocity ω are functions of two variables, the capacity of the lookup table with sufficient accuracy is large, and the ROM capacity is wasted. Further, if a plurality of maps are complemented and obtained like an assist map, it takes a long calculation time. Therefore, the change in the battery voltage Vdc is expressed as a change in the angular velocity ω, and the d-axis current limit value is a function of only the angular velocity ω.

Assuming that the voltage of Equation 25 changes from V → V + ΔV and the angular velocity ω of Equation 26 changes from ω → ω + Δω, the following Equations 46 and 47 are obtained.
(Equation 46)
v _d + Δv _d = i _d · R− (ω _e + Δω _e ) Ld · i _q
(Equation 47)
_{v q + Δv q = i q} · R + (ω e + Δω e) Lq · i d + (ω e + Δω e) Ψ

Then, the following formulas 48 and 49 are obtained by subtracting the formulas 25 and 26 from the formulas 46 and 47, respectively.
(Formula 48)
Δv _d = −ω _e · Ld · i _q
(Equation 49)
_{Δv q = Δω e · Lq ·} i d + Δω e · Ψ

Equations 48 and 49 are relational expressions for changes in voltage ΔV and angular velocity Δω. A relational expression (look-up table) for changes in voltage ΔV and angular velocity Δω and a d-axis current limit value approximation formula or complementary calculation of a function of only angular velocity ω are not required.

The following formula 50 is established.
(Equation 50)
Δv _d ² + Δv _q ² = ΔV ²
However,
(Equation 51)
ΔV = (Vdc−Vdcn) / 2

である。数５３はΔωの変化がルックアップテーブルで計算され、ΔＶで乗算されることを意味している。

It is. Equation 53 means that the change in Δω is calculated by a lookup table and multiplied by ΔV.

When the above calculation is implemented as a device, the configuration shown in FIG. 7 is obtained, and the battery voltage change adaptation unit 60 and the addition unit 18 are added to the current command value calculation unit of FIG. The battery voltage change adaptation unit 60 includes a setting unit 62 for outputting the parameter Vdcn, a subtraction unit 63 for the battery voltage Vdc and the parameter Vdcn, a d-axis current command value Id and a current command value limit value Iref_lim from the current command value calculation unit. And a multiplication unit 64 that multiplies the voltage ΔV from the subtraction unit 63 by the adaptation calculation value from the lookup table 61 and outputs the angular velocity change Δω _e. It has. The d-axis current command value Id calculated by the current command value calculation unit and the limit value Iref_lim of the current command value are input to the lookup table 61, and the battery voltage Vdc is input to the subtraction unit 63 and ΔV (= Vdc). -Vdcn). The angular velocity change Δω _e obtained by the multiplier 64 is input to the adder 18, and (ω _e + Δω _e ) added to the angular velocity ω _e is input to the current command value calculator.

  In such a configuration, an example of the operation will be described with reference to the flowchart of FIG.

First, the steering assist current command value calculation unit calculates the steering assist current command value Iref based on the steering torque and the vehicle speed (step S20), and the angular velocity change _{e of the} motor 5 from the battery voltage change adaptation unit 60 _The amount of change (ω _e + Δω _e ) is calculated by inputting _e (step S21). This order is arbitrary. Next, based on the calculated change amount (ω _e + Δω _e ), the lookup table 11 calculates a limit value Ireflim of the current command value (step S22), and the lookup table 12 calculates the base limit value Ireflim_base of the current command value. (Step S23), the current command value limit value Ireflim is input to the comparison unit 13, and the current command value base limit value Ireflim_base is input to the comparison unit 14. Note that the calculation order of the current command value limit value Ireflim and the current command value base limit value Ireflim_base is arbitrary.

The comparison unit 13 calculates a current command value limit value Iref_lim based on the comparison between the steering assist current command value Iref and the current command value limit value Ireflim (step S24), and the comparison unit 14 calculates the steering assist current command value Iref and the current. Based on the comparison of the command value base limit value Ireflim_base, the current command value limit value Iref_base and the d-axis current command value Id = 0 are calculated (step S25). The current command value limit value Iref_lim is input to the contact c2 of the switch 16, and the current command value base limit value Ireflim_base and the d-axis current command value Id = 0 are input to the contact c1 of the switch 16. The lookup table 15 calculates the d-axis current command value Id based on the change amount (ω _e + Δω _e ) and the current command value limit value Iref_lim (step S26).

Thereafter, the detection switch 17 change amount (ω _e + Δω _e) it is determined whether or not the base angular velocity omega _b above (step S30), there by a change amount (ω _e + Δω _e) the base angular velocity omega _b above For weak field control, the contact of the switch 16 is switched to c1 (step S31), and the base limit value Iref_base and d-axis current command value Id = 0 of the current command value from the comparison unit 14 are output as the limit value Iref_lim (step S31). S33). If the change amount (ω _e + Δω _e ) is smaller than the base angular velocity ω _b , the contact of the switch 16 is switched to c2 (step S32), and the current command value limit value Ireflim_lim from the comparison unit 13 is directly used as the current command value. The limit value Iref_lim is output (step S33).

The battery voltage change adaptation unit 60 inputs the battery voltage Vdc (step S34), calculates a difference voltage ΔV (= Vdc−Vdcn) from a preset parameter Vdcn, and inputs the difference voltage ΔV (= Vdc−Vdcn) to the multiplication unit 64 (step S35). . On the other hand, the d-axis current command value Id and the current command value limit value Iref_lim are input from the current command value calculation unit 10 to the lookup table 61 (step S36), and the lookup table 61 stores the d-axis current command value Id and the current. An adaptation calculation value is calculated based on the limit value Iref_lim of the command value (step S37), and the adaptation calculation value is input to the multiplication unit 64. The multiplication unit 64 calculates an angular velocity change Δω _e based on the adaptive calculation value and the difference voltage ΔV, and inputs it to the addition unit 18 (step S38).

  Here, the steering device has a problem that the steering force varies depending on the steering direction due to the difference between the left and right rotations of the reduction gear efficiency and the difference in the axial load applied to the front shaft. However, the above-described limiting characteristic has a problem that the driver feels uncomfortable because it is not considered that the steering force varies depending on the steering direction with efficiency such as a reduction gear.

  For this reason, the present invention sets the limiting characteristics in consideration of the difference between the left rotation (CCW) and the right rotation (CW) of a mechanical system such as a speed reducer, and the axle load applied to the left and right wheels of the front shaft. It solves the problem that the steering force varies depending on the direction, so that the driver does not feel uncomfortable.

  The present invention will be described below.

  For example, when the mechanical friction is CW> CCW, for the current limit value Ireflim, the data of the CW lookup table is increased as shown in FIG. 9 and the current limit value Ireflim_base is increased. As shown in FIG. 10, the CW lookup table data is increased. Therefore, in the present invention, since the lookup tables 11 and 12 are set so that the current limit values Ireflim and Ireflim_base in the CW direction become large at the same rotation speed, the same characteristics can be obtained in the CW direction and the CCW direction. it can. If the data for CW and CCW is not set in this way, the steering force is required as compared with CCW at CW, and the driver feels the steering wheel heavy and feels as a difference between right and left.

As described above, in the present invention, as shown in FIGS. 11 and 12, the rotation direction RD indicating CW or CCW is input to the look-up tables 11 and 12 that calculate the limit values Ireflim and Ireflim_base of the current command value and output. I am trying to switch. The look-up tables 11 and 12 are configured as shown in FIGS. 13 and 14. The look-up table 11 includes a CW look-up table 11-1, a CCW look-up table 11-2, and a rotation direction. The switch 11-3 switches the look-up table output by the signal RD. The look-up table 12 includes a CW look-up table 12-1, a CCW look-up table 12-2, and a switch 12-3 that switches the look-up table output according to the rotation direction signal RD. .
The current command value is limited based on the desired output characteristics and angular velocity, and the transmission direction of the motor and steering assist force. Therefore, the torque ripple in the high-speed region of the motor is suppressed and the handle vibrates even during the rapid turning of the steering wheel. Does not occur, and the steering operation in the CW direction and the CCW direction does not cause a sense of incongruity.

It is a figure for demonstrating the principle used as the premise of this invention. It is a block diagram which shows the structural example of the pseudo vector control which can apply this invention. It is a block diagram which shows the structural example of an electric current command value calculation part. It is a figure which shows the example of a characteristic (3 area | region) of d-axis current command value and current command value, and a motor rotational speed. It is a figure which shows the example of a characteristic (4 area | region) of d-axis current command value and current command value, and a motor rotational speed. It is a flowchart which shows the operation example of the premise technique of this invention. It is a block block diagram which shows the other example of the premise technique of this invention. It is a flowchart which shows the operation example. It is a figure which shows the example of a characteristic by a steering direction. It is a figure which shows the example of a characteristic by a steering direction. It is a block block diagram which shows the structural example of this invention. It is a block block diagram which shows the other structural example of this invention. It is a block block diagram which shows the structural example of the lookup table of this invention. It is a block block diagram which shows the structural example of the lookup table of this invention. It is a block diagram of a general electric power steering device. It is a figure which shows the Tn characteristic of a motor. It is a figure which shows a motor characteristic and a motor load characteristic in piles. It is a block diagram which shows the structural example of the conventional vector control apparatus.

Explanation of symbols

11, 12, 15 Look-up table 13, 14 Comparison unit 16 Switch 17 Detection switching unit 21 PI control unit 24 Differentiation circuit 30 PWM control unit 31 Inverter 60 Battery voltage change adaptation unit 61 Look-up table 64 Multiplication unit 100 Vector control phase command Value calculation unit 101, 106 Conversion unit 102 3-phase / 2-phase conversion unit 103 q-axis current reference value calculation unit 104 2-phase / 3-phase conversion unit 105 d-axis current reference value calculation unit 208, 308 Motor 209 Resolver 301 Steering handle 303 Reduction gear 307 Torque sensor 324 Command current determination means

Claims (6)

In an electric power steering apparatus configured to control a motor and drive the motor with a current command value calculated based on a steering torque or the like to apply a steering assist force to a steering system of a vehicle. An electric power steering apparatus comprising: a limiting function unit configured to limit the current command value based on the angular velocity and the transmission direction of the motor and the steering assist force. The said limitation function part is comprised by the switch for switching to the said CW look-up table or the CCW look-up table according to the said transmission direction, and the CW look-up table and CCW look-up table. Electric power steering device. The electric power steering apparatus according to claim 1 or 2, wherein the restriction function unit is further set in consideration of a front axle weight. The electric power steering apparatus according to claim 3, wherein the limiting function unit is further set in consideration of a change in battery voltage. The electric power steering apparatus according to any one of claims 1 to 4, wherein the motor is driven by vector control. The electric power steering apparatus according to claim 5, wherein the motor is a permanent magnet type brushless DC motor.

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