Electric power steering control apparatus and electric power steering control method

A main controller includes: a d-axis current limiter configured to calculate a d-axis current limiting value with priority, based on a dq-converted current limiting value, and calculate a limited d-axis current command value by clipping the d-axis current command value to be the d-axis current limiting value or less; a q-axis current limiter configured to calculate a q-axis current limiting value, based on the dq-converted current limiting value and the limited d-axis current command value, and calculate a limited q-axis current command value by clipping the q-axis current command value to be the q-axis current limiting value or less; and an inverter controller configured to control motor current, based on the limited d-axis current command value and the limited q-axis current command value.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/JP2015/063061 filed May 1, 2015, the contents of all of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to an electric power steering control apparatus and an electric power steering control method, and more particularly to computing a current command value for controlling the driving of an AC motor which assists in the steering of a steering wheel.

BACKGROUND ART

In a conventional electric power steering control apparatus, a command value of an assist torque, which is output to an AC motor, is determined by a current value on the q-axis (hereafter called q-axis current command value), and a current value on the d-axis (hereafter called d-axis current command value) based on the weak field control of the AC motor, in order to improve the follow-up performance of the steering wheel during high-speed steering.

When predetermined conditions are established and the q-axis current command value must be limited, the conventional electric power steering control apparatus limits not only the q-axis current command value, but also limits the d-axis current command value at the same rate as the case of limiting the q-axis current command value in order to improve the sensation of steering (e.g. see PTL 1).

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

Prior art, however, has the following problems.

FIG. 5shows graphs depicting the relationship of the d-axis current command value with respect to the motor rotation speed, and the relationship of the maximum value of the assist torque with respect to the motor rotation speed in the conventional electric power steering control apparatus. When a predetermined motor rotation speed is N1 or greater, at which the counter electromotive force of the motor exceeds the power supply voltage, the maximum value of the assist torque drops. In concrete terms, the conventional electric power steering control apparatus outputs the d-axis current command value based on the motor rotation speed, so as to suppress a drop in the maximum value of the assist torque.

As depicted inFIG. 5, if the required d-axis current command value becomes insufficient when the rotation speed is N2 or more, due to a certain limitation issue, this causes a drop in the maximum value of the assist torque at the rotation speed N2 or more.

Further, in the case of the conventional electric power steering control apparatus, if a predetermined condition is established and the q-axis current command value is limited, the d-axis current command value is also limited at the same rate as this limitation. Therefore if the steering wheel is moving rapidly, the d-axis current command value may become insufficient and the followability may drop.

With the foregoing in view, it is an object of the present invention to provide an electric power steering control apparatus and an electric power steering control method, which can improve the followability of the prior art when current limit to the AC motor, to assist the steering of the steering wheel, is limited in a high-speed steering range.

Solution to Problem

An electric power steering control apparatus according to the present invention is an electric power steering control apparatus having a main controller configured to control motor current for driving an AC motor assisting steering of a steering wheel, based on a d-axis current command value and a q-axis current command value, wherein the main controller includes, when a dq-converted current command value, which is a root-sum-square of the d-axis current command value and the q-axis current command value, is limited: a d-axis current limiter configured to calculate a d-axis current limiting value with priority, based on a dq-converted current limiting value, which is a limiting value of the dq-converted current command value, and calculate a limited d-axis current command value by clipping the d-axis current command value to be the d-axis current limiting value or less; a q-axis current limiter configured to calculate a q-axis current limiting value, based on the dq-converted current limiting value and the limited d-axis current command value calculated by the d-axis current limiter, and calculate a limited q-axis current command value by clipping the q-axis current command value to be the q-axis current limiting value or less; and an inverter controller configured to control the motor current, based on the limited d-axis current command value and the limited q-axis current command value.

An electric power steering control method according to this invention is an electric power steering method executed by a main controller controlling motor current for driving an AC motor assisting steering of a steering wheel, based on a d-axis current command value and a q-axis current command value, the method including: with the use of the main controller, when a dq-converted current command value, which is a root-sum-square of the d-axis current command value and the q-axis current command value, is limited: a first step of acquiring a dq-converted current limiting value, which is a limiting value of the dq-converted current command value; a second step of calculating a d-axis current limiting value with priority, based on the dq-converted current limiting value acquired in the first step; a third step of calculating a limited d-axis current command value by clipping the d-axis current command value to be not more than the d-axis current limiting value, which has been calculated in the second step; a fourth step of calculating a q-axis current limiting value, based on the dq-converted current limiting value acquired in the first step and the limited d-axis current command value calculated in the third step; a fifth step of calculating a limited q-axis current command value by clipping the q-axis current command value to be not more than the q-axis current limiting value, which has been calculated in the fourth step; and a sixth step of controlling the motor current, based on the limited d-axis current command value, which has been calculated in the third step, and the limited q-axis current command value, which has been calculated in the fifth step.

Advantageous Effects of Invention

According to this invention, when current is limited while using the steering wheel, the d-axis current command value can be output, regardless the limiting rate of the q-axis current value, if the d-axis current command value is the d-axis current limiting value or less, and the d-axis current command value is calculated with priority over the q-axis current command value. By including this configuration, the limiting of the d-axis current command value can be more relaxed than prior art in a high rotation speed range of the AC motor. As a result, an electric power steering control apparatus and an electric power steering control method, which can improve the followability more than prior art, can be provided.

DESCRIPTION OF EMBODIMENTS

Embodiments of an electric power steering control apparatus and an electric power steering control method of this invention will now be described with reference to the drawings.

FIG. 1is a general block diagram depicting an electric power steering control apparatus according to Embodiment 1 of this invention. In concrete terms, the electric power steering control apparatus depicted inFIG. 1is constituted by: a main controller10(hereafter ECU10) which is installed in a vehicle for controlling electric power steering; a motor1which assists the steering of the steering wheel; a torque sensor2which is installed near the steering wheel and detects the steering torque; a vehicle speed sensor3which detects the vehicle speed; a resolver sensor4which detects the rotation angle of the motor1; and a temperature sensor5which detects the ambient temperature of the ECU10.

The ECU10, which includes a CPU, is constituted by a q-axis current computing unit11, a motor rotation speed computing unit12, a d-axis current computing unit13, a dq-converted current limiting value computing unit14, a d-axis current limiter15, a q-axis current limiter16, and an inverter controller17.

The q-axis current computing unit11receives the inputs of a steering torque signal Trq detected by the torque sensor2, and a vehicle speed signal Vs detected by the vehicle speed sensor, and sets a q-axis current command value Iq. The motor rotation speed computing unit12computes the motor rotation speed N per unit time from a rotation angle θ of the motor1detected by the resolver sensor4.

The d-axis current computing unit13sets a d-axis current command value Id based on the q-axis current command value Iq which was set by the q-axis current computing unit11, and the motor rotation speed N per unit time which was computed by the motor rotation speed computing unit12.

The dq-converted current limiting value computing unit14determines a dq-converted current limiting value Idq_LIM, which is a motor current limiting value, based on the ambient temperature T detected by the temperature sensor5.

The d-axis current limiter15limits the d-axis current command value Id, which was set by the d-axis current computing unit13, using the dq-converted current limiting value Idq_LIM determined by the dq-converted current limiting value computing unit14. The q-axis current limiter16limits the q-axis current command value Iq, which was set by the q-axis current computing unit11, using a d-axis current command value Id′ limited by the d-axis current limiter15(hereafter called limited d-axis current command value Id′), and the dq-converted current limiting value Idq_LIM determined by the dq-converted current limiting value computing unit14.

The inverter controller17outputs the three-phase current Iu, Iv and Iw based on a q-axis current command value Iq′ limited by the q-axis current limiter16(hereafter called limited q-axis current command value Iq′), the limited d-axis current command value Id′ limited by the d-axis current limiter15, and the rotation angle θ of the motor1detected by the resolver sensor4, and drives the motor1.

Here a case when the maximum value of the motor current is a dq-converted current rated value Idq-MAX and the maximum value of the d-axis current is a d-axis current rated value Id_MAX will be considered. In this case, the d-axis current limiter15calculates the d-axis current limiting value Id_LIM according to the following Expression (1), using the dq-converted current limiting value Idq_LIM. Further, the d-axis current limiter15calculates the limited d-axis current command value Id′ by limiting the d-axis current command value Id to the d-axis current limiting value Id_LIM or less according to the following Expression (2).

Then the q-axis current limiter16calculates the q-axis current limiting value (Iq_LIM) according to the following Expression (3), using the limited d-axis current command value Id′ and the dq-converted current limiting value Idq_LIM. Further, the q-axis current limiter16calculates the limited q-axis current limiting value Iq′ by limiting the q-axis current command value Iq′ to the q-axis current limiting value Iq_LIM or less according to the following Expression (4).
Iq_LIM=√{square root over (Idq_LIM2−Id′2)}  (3)
Iq′=MIN(Iq,Iq_LIM)  (4)

In other words, the ECU10of Embodiment 1 computes the current command values according to the following procedure.

(Procedure 1) When the d-axis current command value Id and the q-axis current command value Iq are limited by the dq-converted current limiting value Idq_LIM for any reason, such as overheating protection, the ECU10calculates the limited d-axis current command value Id′ according to the above Expressions (1) and (2).

(Procedure 2) Then within the dq-converted current limiting value Idq_LIM, the ECU10calculates the limited q-axis current command value Iq′ according to the above Expressions (3) and (4).

In other words, a technical feature of the electric power steering control apparatus of Embodiment 1 is that the optimum limited q-axis current command value Iq′ is calculated after calculating the optimum limited d-axis current command value Id′ with priority. As a result, an electric power steering control apparatus, which can implement high followability, can be obtained.

Now the effect obtained by the electric power steering control apparatus of Embodiment 1 will be described in detail, by comparing it with the conventional current limiting method according to PTL 1.FIG. 2is a diagram depicting the current limiting method by the electric power steering control apparatus according to Embodiment 1 of this invention.FIG. 3, on the other hand, is a diagram depicting the current limiting method by the electric power steering control apparatus according to PTL 1.

FIG. 2, which depicts the current control method according to Embodiment 1, is a vector diagram of motor current, where the abscissa indicates the q-axis current command value, and the ordinate indicates the d-axis current command value, and each reference sign denotes the following.Semicircle21: A semicircle when the dq-converted current limiting value Idg_LIM is the rated value Idq_MAX of the dq-converted current command value.Semicircle22: A semicircle when the dq-converted current limiting value Idq_LIM is limited to a value smaller than that of the semicircle21due to current limitation.Line23: A limiting value when the d-axis current limiting value Id_LIM is the rated value Id_MAX of the d-axis current command value.Line24: A limiting value when the d-axis current limiting value Id_LIM is a value lower than the line23due to current limitation.Combined vector31: A dq-converted current command value Idq* generated by combining the q-axis current command value and the d-axis current command value, that is, a combined vector when the magnitude of the vector is the same as the dq-converted current limiting value indicated by the semicircle21.d-axis vector32: A vector which indicates the limited d-axis current command value Id′ as a d-axis component corresponding to the combined vector31.q-axis vector33: A vector which indicates the limited q-axis current command value Iq′ as a q-axis component corresponding to the combined vector31.Combined vector41: A dq-converted current command value Idq* generated by combining the q-axis current command value and the d-axis current command value, that is, a combined vector when the magnitude of the vector is the same as the dq-converted current limiting value indicated by the semicircle22.d-axis vector42: A vector which indicates the limited d-axis current command value Id′ as a d-axis component corresponding to the combined vector41.q-axis vector43: A vector which indicates the limited q-axis current command value Iq′ as a q-axis component corresponding to the combined vector41.

Now the current control method according to Embodiment 1, in the case when the current limitation is activated and the dq-converted current limiting value Idq_LIM drops from the state of the semicircle21to the state of the semicircle22, will be described in details with reference toFIG. 2.

When the dq-converted current limiting value Idq_LIM decreases, the d-axis current limiting value Id_LIM drops from line23to the line24according to the above Expression (1). As a result, the limited d-axis current command value Id′ is calculated according to the above Expression (2), and drops from the vector32to the vector42. In other words, the limited d-axis current command value Id′ is calculated with priority.

Then the q-axis current limiting value Iq_LIM is determined according to the above Expression (3), and as a result, the limited q-axis current command value Iq′ is calculated according to the above Expression (4), and drops from the vector33to the vector43. Then, finally the current that flows to the motor1becomes the combined vector41of the vector42and the vector43.

FIG. 3, which depicts the current control method according to PTL 1, on the other hand, is a vector diagram of motor current, where the abscissa indicates the q-axis current command value, and the ordinate indicates the d-axis current command value just likeFIG. 2described above, and each reference sign denotes the following. The reference signs that are the same asFIG. 2denote the same meanings, for which description is omitted.Combined vector51: A dq-converted current command value Idq* generated by combining the q-axis current command value and the d-axis current command value, that is, a combined vector when the magnitude of the vector is the same as the dq-converted current limiting value indicated by the semicircle22.d-axis vector52: A vector which indicates the limited d-axis current command value Id′ as a d-axis component corresponding to the combined vector51.q-axis vector53: A vector which indicates the limited q-axis current command value Iq′ as a q-axis component corresponding to the combined vector51.

Now the current control method according to PTL1, in the case when the current limitation is activated and the dq-converted current limiting value Idq_LIM is dropped from the state of the semicircle21to the state of the semicircle22, just like the above mentioned case ofFIG. 2, will be described in details with reference toFIG. 3.

In PTL1, the d-axis current command value and the q-axis current command value are decreased at a same rate so that the vector51, which corresponds to the limited dq-converted current command value, becomes the dq-converted current limiting value which corresponds to the value indicated by the semicircle22, whereby the vector52and the vector53are calculated.

Therefore the limited d-axis current command value and the limited q-axis current command value drop as indicated by the vector52and the vector53respectively, and the combined current of the limited d-axis current command value and the limited q-axis current command value becomes as indicated by the vector51.

In comparing the results inFIG. 2andFIG. 3, both the vector41and the vector51, which respectively correspond to the dq-converted current command value, that is, the combined value of the limited d-axis current command value and the limited q-axis current command value, are limited to the value indicated by the semicircle22.

On the other hand, in comparing the vector42and the vector52, which respectively indicate the limited d-axis current command value Id′, the vector42, when current was limited according to Embodiment 1, is larger, indicating more d-axis current can be supplied, than the vector52when current was limited according to PTL1. As a result, the current limiting method according to Embodiment 1 can improve followability more than the current limiting method according to PTL 1.

FIG. 4is a flow chart depicting a series of processing of the electric power steering control method executed by the ECU10according to Embodiment 1 of this invention. First in step S401, the q-axis current command value Iq is calculated by the q-axis current computing unit11, and the d-axis current command value Id is calculated by the d-axis current computing unit13, just like prior art.

Then in step S402, the dq-converted current limiting value computing unit14calculates the dq-converted current limiting value Idq_LIM. In the above description on the example based onFIG. 1, the dq-converted current limiting value computing unit14determines the dq-converted current limiting value Idq_LIM, which is the limiting value of the motor current, based on the ambient temperature T detected by the temperature sensor5.

However, the dq-converted current limiting value Idq_LIM need not always be calculated based on the temperature value detected by the temperature sensor5. The value to limit current of the AC motor to assist steering of the steering wheel in the high-speed steering range may be acquired by the ECU10from an outside source, or may be calculated based on other detected values, for which prior art can be applied.

Then in step S403, the d-axis current limiter15calculates the d-axis current limiting value Id_LIM using the above Expression (1), based on the dq-converted current limiting value Idq_LIM calculated in step S402. In other words, the d-axis current limiting value Id_LIM is calculated with priority over the q-axis current limiting value Iq_LIM.

Then in step S404, the d-axis current limiter15clips the d-axis current command value Id calculated in step S401using the above Expression (2) to be the d-axis current limiting value Id_LIM, which was calculated in step S403, or less, so as to calculate the limited d-axis current command value Id′.

Then in step S405, the q-axis current limiter16calculates the q-axis current limiting value Iq_LIM using the above Expression (3) based on the dq-converted current limiting value Idq_LIM calculated in step S402and the limited d-axis current command value Id′ calculated in step S404.

Further in step S406, the q-axis current limiter16clips the q-axis current command value Iq calculated in step S401using the above Expression (4) to be the q-axis current limiting value Iq_LIM, which was calculated in step S405, or less, so as to calculate the limited q-axis current command value Iq′.

Then finally in step S407, the inverter controller17controls the three-phase current Iu, Iv and Iw for driving the motor1, using the limited d-axis current command value Id′ calculated in step S404and the limited q-axis current command value Iq′ calculated in step S406.

In this way, a technical feature of the electrical power steering control method according to Embodiment 1 is that the limited q-axis current command value Iq′ is determined after determining the limited d-axis current command value Id′ with priority in step S403to step S406. As a result, followability can be improved more than prior art when current to the AC motor to assist steering of the steering wheel is limited in the high-speed steering region.

As described above, according to Embodiment 1, when current is limited when steering the steering wheel, the d-axis current command value can be output if the d-axis current command value is the d-axis current limiting value or less, regardless the limiting rate of the q-axis current command value, and the d-axis current command value can be calculated with priority over the q-axis current command value. As a result, compared with the prior art, the d-axis current command value is not limited in the high rotation region of the AC motor, and the followability can be improved to the maximum.