JP4178260B2 - Electric power steering device - Google Patents

Description

ただし、Ｌはモータのインダクタンス、ｓはラプラス演算子、Ｒはモータの内部抵抗、Ｒ’は配線や駆動回路等の外部抵抗である。
【００２５】
次に、外部抵抗Ｒ’が電流制御系に与える影響を全くないものと仮定した場合、電流制御系の閉ループ伝達関数Ｇ₄(ｓ)は、Ｇ₃(ｓ)においてＲ’＝０を代入した次式のように示される。
【数２】

分母についてＧ₃(ｓ)とＧ₄(ｓ)とを比較すると、ｓの１次の係数については、Ｇ₃(ｓ)の方がＧ₄(ｓ)よりもＲ’／Ｌ大きくなっている。１次遅れ系の伝達関数の周波数応答においては、１次の係数が大きくなるにしたがい低周波数帯域での位相の遅れが大きくなる。すなわちＲ’＝０とみなすことができない場合、ｓの１次の係数が大きくなり、外部抵抗Ｒ’が電流制御系に与える影響が位相の遅れとなって現れる。
【００２６】
前述のとおり、従来構成における電流制御系の電流周波数特性は、モータ・駆動回路系の折点周波数である５７Ｈｚよりも低い周波数帯域で位相が遅れているので、上記外部抵抗Ｒ’が電流制御系に与える影響があるものと推測される。
【００２７】
そこで、電流制御系に影響を与える外部抵抗Ｒ’をも考慮に入れてＰＩゲインを設定した場合、電流制御系の閉ループ伝達関数Ｇ₅(ｓ)は次式のように示される。
【数３】

上記Ｇ₅(ｓ)において、２πｆ_c ＝(Ｒ＋Ｒ’)／Ｌと設定した場合の伝達関数Ｇ₆(ｓ)は次式で表される。
Ｇ₆(ｓ)＝（Ｒ＋Ｒ’）／（Ｌｓ＋Ｒ＋Ｒ’） ・・・（７）
ここで、モータ・駆動回路系の伝達関数Ｇ₇(ｓ)は次式で表される。
Ｇ₇(ｓ)＝１／（Ｌｓ＋Ｒ＋Ｒ’） ・・・（８）
Ｇ₆(ｓ)とＧ₇(ｓ)とを比較すると、両者の時定数は共にＬ／（Ｒ＋Ｒ’）であって同一となっている。すなわち、外部抵抗Ｒ’を含めてＰＩ制御のパラメータ（比例ゲインおよび積分ゲインまたは積分時間）を設定した場合、数式上においては、電流制御系の特性としてモータ・駆動回路系と同様の特性が得られることになる。
【００２８】
次に、外部抵抗Ｒ’を含めてＰＩゲインを設定した場合の実験データによる検討を行った。図５は、外部抵抗Ｒ’を含めてＰＩゲインを設定したＰＩ制御をおこなった場合の電流制御系のゲイン特性を示す電流周波数特性図である。図６は、外部抵抗Ｒ’を含めてＰＩゲインを設定したＰＩ制御をおこなった場合の電流制御系の位相特性を示す電流周波数特性図である。一方、従来構成における電流制御系の電流周波数特性は、前述のとおり図２および図３に示している。図２と図５とを比較すると、図５ではゲイン特性が改善されていることがわかり、図３と図６とを比較すると、図６では位相特性が改善されていることがわかる。すなわち、外部抵抗Ｒ’を含めてＰＩゲインを設定したＰＩ制御をおこなうと、電流制御系の電流周波数特性がモータ・駆動回路系の電流周波数特性に近づき、応答性が高くなることがわかる。
【００２９】
以上のように、数式および実験データに基づく検討の結果、外部抵抗Ｒ’が電流制御系に影響を与えており、外部抵抗Ｒ’を考慮に入れて電流制御系を設計することにより、電流制御系の電流周波数特性がモータ・駆動回路系本来の特性に近づくことがわかった。
【００３０】
そこで、上記の基礎検討の結果に基づき、実際に外部抵抗Ｒ’を考慮に入れて電動パワーステアリング装置の電流制御系の設計を行い、その設計により得られる電動パワーステアリング装置を本発明の一実施形態として以下に説明する。
【００３１】
＜２．実施形態＞
以下、添付図面を参照しつつ、本発明の一実施形態について説明する。
図７は、本発明の一実施形態に係る電動パワーステアリング装置の構成を、それに関連する車両構成と共に示す概略図である。この電動パワーステアリング装置は、操舵のための操作手段としてのハンドル（ステアリングホイール）１００に一端が固着されるステアリングシャフト１０２と、そのステアリングシャフト１０２の他端に連結されたラックピニオン機構１０４と、ハンドル１００の操作によってステアリングシャフト１０２に加えられる操舵トルクを検出するトルクセンサ３と、ハンドル操作（操舵操作）による運転者の負荷を軽減するための操舵補助力を発生させる電動モータ６と、そのモータ６の発生する操舵補助力をステアリングシャフト１０２に伝達する減速ギヤ７と、車載バッテリ８から電源の供給を受けて、トルクセンサ３や車速センサ４からのセンサ信号に基づきモータ６の駆動を制御する電子制御ユニット（ＥＣＵ）５とを備えている。
【００３２】
図８は、上記電動パワーステアリング装置を制御的観点から見た構成を示すブロック図である。上記電動パワーステアリング装置の制御装置であるＥＣＵ５は、位相補償フィルタ１０と、目標電流設定部１２と、減算器１４と、ＰＩ制御部１５と、モータ駆動回路２０と、電流検出器１９とを備えている。位相補償フィルタ１０には、トルクセンサ３によって検出される操舵トルクの検出値Ｔｓが入力される。位相補償フィルタ１０は、検出値Ｔｓに基づき位相補償を施し、その位相補償後の信号を操舵トルク信号Ｔとして出力する。目標電流設定部１２は、この操舵トルク信号Ｔと、車速センサ４によって検出される車両速度の検出値とを受け取り、モータに供給すべき目標電流値Ｉｔを算出する。減算器１４は、目標電流設定部１２から出力される目標電流値Ｉｔと電流検出器１９から出力される電流検出値Ｉｓとの偏差Ｉｔ−Ｉｓを算出する。ＰＩ制御部１５は、この偏差Ｉｔ−Ｉｓに基づき比例積分制御演算によって、電圧指令値Ｖを出力する。モータ駆動回路２０は、ＰＩ制御部１５が出力する電圧指令値Ｖに基づいて、モータに電圧を印加する。この電圧印加によりモータ６に電流が流れる。
【００３３】
図９は、伝達関数を用いて示した、本実施形態に係る電流制御系のブロック線図である。ここでは、図８における、ＰＩ制御部１５と、電流検出器１９と、モータ駆動回路およびモータをひとつの伝達要素とみなしたモータ・駆動回路系２００とを伝達関数を用いて示している。図９において、電流検出器１９の伝達関数はＧ₂’(ｓ)に示したとおり、「１」とみなすことができる。また、モータ・駆動回路系２００の伝達関数Ｇ₈(ｓ)は次式で表される。
Ｇ₈(ｓ)＝１／（Ｌｓ＋Ｒ＋Ｒ’） ・・・（９）
ただし、Ｌはモータのインダクタンス、ｓはラプラス演算子、Ｒはモータの内部抵抗、Ｒ’は配線や駆動回路等の外部抵抗である。
【００３４】
ＰＩ制御部１５は、前述のとおり、目標電流値と実電流値との偏差に基づいて、モータ・駆動回路系２００に与える電圧指令値Ｖを出力する。ここで、本実施形態では伝達関数Ｇ₉(ｓ)が次式となるようにＰＩ制御部１５を設計する。すなわち、ＰＩ制御部１５の伝達関数Ｇ₉(ｓ)をω_c（τ_c・ｓ＋１）／ｓとおいた場合における当該ＰＩ制御部１５の第１の制御パラメータであるτ_cを、上記モータ・駆動回路系２００の時定数Ｌ／（Ｒ＋Ｒ’）に等しくなるように設定する。
Ｇ₉(ｓ)＝２πｆ_c (Ｌｓ+Ｒ＋Ｒ’)／ｓ ・・・（１０）
すなわち、τ_c＝Ｌ／(Ｒ＋Ｒ’)、ω_c＝２πｆ_c(Ｒ＋Ｒ’)
ただし、ｆ_c はＰＩゲイン、Ｌはモータのインダクタンス、ｓはラプラス演算子、Ｒはモータの内部抵抗、Ｒ’は配線や駆動回路等の外部抵抗である。なお、測定データの一例を示すと、Ｌ＝０．０９２ｍＨ、Ｒ＝０．０２６Ω、Ｒ’＝０．０３６Ωであり、外部抵抗Ｒ’は内部抵抗Ｒに比べて無視できない値を有していることがわかる。この外部抵抗Ｒ’には、モータ駆動回路２０で使用される電界効果型トランジスタ（ＦＥＴ）のオン抵抗が数ｍΩ、配線抵抗が数ｍΩ〜数十ｍΩ、コネクタ抵抗が数ｍΩ〜数十ｍΩ含まれている。
【００３５】
以上のように設計すると、この電流制御系の閉ループ伝達関数Ｇ₁₀(ｓ)は次式で表される。
【数４】

このようにして電流制御系は１次遅れ要素（１次遅れ系）となり、その時定数は、１／（２πｆ_c）であるので、ＰＩゲインｆ_cが電流制御系の折点周波数となる。ところで、式（９）よりモータ・駆動回路系２００の時定数はＬ／(Ｒ＋Ｒ’)である。そこで本実施形態では、電流制御系の時定数１／（２πｆ_c）がモータ・駆動回路系２００の時定数Ｌ／(Ｒ＋Ｒ’)に等しくなるように、ＰＩゲインｆ_cを
ｆ_c＝(Ｒ＋Ｒ’)／（２πＬ）
と設定する。これは、電流制御系の時定数１／（２πｆ_c）がモータ・駆動回路系２００の時定数Ｌ／(Ｒ＋Ｒ’)に等しくなるように、ＰＩ制御部１５の比例ゲイン２πｆ_cＬをＲ＋Ｒ’に設定したこと（すなわちＰＩ制御部１５の第２の制御パラメータであるω_cを(Ｒ＋Ｒ’)²／Ｌに設定したこと）と等価である。
【００３６】
ＰＩ制御部１５のＰＩゲインｆ_cまたは比例ゲイン２πｆ_cＬまたは第２の制御パラメータω_cをこのように設定することにより、電流制御系の閉ループ伝達関数Ｇ₁₀(ｓ)とモータ・駆動回路系２００の伝達関数Ｇ₈(ｓ)とは、下記式に示す関係となる。

上記より、電流制御系とモータ・駆動回路系２００とは、同一の折点周波数ｆ_c＝（Ｒ＋Ｒ’)／（２πＬ）を有する１次遅れ系であって、同一の周波数特性（ゲイン特性および位相特性）を有することになる。すなわち、両者の伝達関数は同一の周波数特性を持つ。
【００３７】
以上説明したように、前述した基礎検討の結果にしたがい外部抵抗Ｒ’を考慮に入れて電流制御系を設計することにより、電流制御系の伝達関数とモータ・駆動回路系２００の伝達関数とが同じ特性を持つ伝達関数となった。
【００３８】
したがって、本実施形態に係る電流制御系の電流周波数特性は、モータ・駆動回路系が本来持つ電流周波数特性を引き出すこととなり、十分な応答性を確保することができる。これにより、所望のトルク周波数特性が得られ、例えば１０Ｈｚ以上の折点周波数（１次遅れ系では４５度位相遅れ、２次遅れ系では９０度位相遅れとなる周波数）にも対応可能となる。また、外乱の多い路面の走行時や不感帯とアシスト域の境界付近の操舵時におけるハンドルのふらつきも防止することができる。さらに、モータ単体のトルクリップルの補償を十分に行うことができるようになる。さらにまた、電流制御系全体でモータ・駆動回路系２００自体と同等の電流周波数特性を有するようになるので、慣性補償やダンピング補償等のダイナミック補償のための制御手段の機能がより十分に発揮されるようになる。
【図面の簡単な説明】
【図１】本発明の一実施形態に係るモータ・駆動回路系に信号を入力した場合の入力に対する出力のゲインと位相とを示すボード線図である。
【図２】従来構成における電流制御系のゲイン特性を示す電流周波数特性図である。
【図３】従来構成における電流制御系の位相特性を示す電流周波数特性図である。
【図４】従来構成における制御装置において、伝達関数を用いて示した電流制御系のブロック線図である。
【図５】本実施形態に係る電流制御系のゲイン特性を示す電流周波数特性図である。
【図６】本実施形態に係る電流制御系の位相特性を示す電流周波数特性図である。
【図７】本実施形態に係る電動パワーステアリング装置の構成をそれに関連する車両構成と共に示す概略図である。
【図８】本実施形態に係る電動パワーステアリング装置を制御的観点から見た構成を示すブロック図である。
【図９】本実施形態に係る制御装置において、伝達関数を用いて示した電流制御系のブロック線図である。
【符号の説明】
３ …トルクセンサ
４ …車速センサ
５ …電子制御ユニット（ＥＵＣ）
６ …モータ
７ …減速ギヤ
８ …バッテリ
１０ …位相補償フィルタ
１２ …目標電流設定部
１４ …減算部
１５ …ＰＩ制御部
１９ …電流検出器
２０ …モータ駆動回路
１００…ハンドル（ステアリングホイール）
１０２…ステアリングシャフト
１０４…ラックピニオン機構
２００…モータ・駆動回路系
Ｉｔ …目標電流値
Ｉｓ …電流検出値
Ｔ …操舵トルク信号
Ｔｓ …操舵トルクの検出値
Ｖ …電圧指令値
ｆ_c …ＰＩゲイン
Ｌ …モータのインダクタンス
Ｒ …モータの内部抵抗
Ｒ’ …外部抵抗[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electric power steering apparatus that controls a current flowing through a motor by proportional-integral control (PI control).
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in an electric power steering apparatus, current control for flowing a current corresponding to the magnitude of steering torque to a motor has been performed. This electric power steering apparatus is provided with a torque sensor for detecting a steering torque applied to a steering wheel as an operation means for steering, and a current to be supplied to the motor based on the steering torque detected by the torque sensor is provided. A target value (hereinafter referred to as “target current value”) is set. Further, a PI control unit that generates a command value to be given to the motor drive means by proportional integral calculation based on a deviation between the target current value and the amount of current actually flowing to the motor (hereinafter referred to as “actual current value”). Is provided.
[0003]
As the current control, the proportional integral control (hereinafter referred to as “PI control”) that calculates the deviation between the target current value and the actual current value and performs the proportional integral calculation based on the deviation as described above is common. . By the proportional-integral control, a command value for the current flowing through the motor is given to the motor drive circuit, and the motor drive circuit applies a voltage to the motor based on the command value (hereinafter, the motor drive circuit and the motor are connected to each other). What is regarded as one transfer element is called "motor / drive circuit system").
[0004]
Incidentally, in order to improve the responsiveness in the current control system, it is generally performed to increase the gain of the PI control unit (hereinafter referred to as “PI gain”). However, the stability of the entire system decreases as the PI gain is increased. Therefore, for the frequency band where the PI gain is lowered to a range where practical stability can be obtained and the responsiveness decreases, the degree of responsiveness reduction is reduced by providing a phase compensator and inertia compensator. Yes.
[0005]
Japanese Patent Laid-Open No. 2001-61292 provides model error correction means for multiplying a deviation between a target current value and an actual current value by a correction gain, and realizes a constant current response by sufficiently increasing the correction gain. A technique has been proposed (see Patent Document 1).
[0006]
[Patent Document 1]
Japanese Patent Laid-Open No. 2001-61292
[Problems to be solved by the invention]
In order to investigate the characteristics of the current control system, it is possible to measure changes in gain and phase due to frequency differences when the input to the current control system is the target current value and the output from the current control system is the actual current value. In general, the change in gain and phase due to the difference in frequency when the input to the current control system is the target current value and the output from the current control system is the actual current value "). When the current control system is designed by the conventional method as described above, the current frequency characteristic inherent in the motor / drive circuit system is not obtained, and the responsiveness is not sufficient in a practical frequency band. Therefore, for example, there are the following problems.
[0008]
A change due to a difference in frequency of the magnitude of the torque obtained with respect to the steering torque is called a torque frequency characteristic, but a desired torque frequency characteristic corresponding to the magnitude of the steering torque cannot be obtained. Even when the desired torque frequency characteristics are obtained, the actual current value does not follow the target current value that changes frequently, for example, when driving on a road surface with a lot of disturbance or when steering near the boundary between the dead zone and the assist zone. The handle fluctuates. In addition, when a compensation value for compensating torque ripple of the motor alone is added to the target current value in a practical frequency band, the actual current value does not follow the target current value. Can not do enough.
[0009]
Therefore, the present invention provides an electric power steering device in which the current frequency characteristic of the current control system is brought close to the original current frequency characteristic of the motor / driving circuit system in a practical frequency band by reviewing the design of the current control system. For the purpose.
[0010]
[Means for Solving the Problems and Effects of the Invention]
A first invention is an electric power steering device that applies a steering assist force to a steering mechanism of a vehicle by driving an electric motor in accordance with an operation for steering the vehicle,
Target current setting means for setting a target value of current to be supplied to the electric motor in accordance with the operation;
Proportional-integral control means for generating a command value for feedback control on the electric motor by proportional-integral control calculation based on a deviation between the current target value and the value of the current actually flowing to the electric motor;
A motor drive circuit for driving the electric motor according to the command value,
Of the proportional integral control means comprising a first control parameter is a tau _c and the second control parameter a is omega _c in the case where the transfer function of the proportional-integral control means put the _{ω c (τ c · s +} 1) / s The control parameter is set based on an inductance of the electric motor, an internal resistance of the electric motor, and an external resistance including a wiring resistance of the electric motor and the motor drive circuit.
[0011]
According to the first aspect, the control parameter of the proportional integral control means is set in consideration of the external resistance, so that the current frequency characteristic of the current control system is changed to the original current frequency characteristic of the motor / drive circuit system. Close enough to ensure sufficient responsiveness.
[0012]
According to a second invention, in the first invention,
The first control parameter τ _c of the proportional-integral control means is set to a value corresponding to a time constant of a motor / drive circuit system which is a first-order lag element including the electric motor and the motor drive circuit. It is characterized by being.
[0013]
According to the second aspect of the invention, the first control parameter τ _c of the proportional-integral control means is equal to the time constant of the motor / drive circuit system which is a first-order lag system, so that the current control system (closed loop transfer function) Is a first order lag system. As a result, the current frequency characteristic of the current control system approaches the original current frequency characteristic of the motor / drive circuit system, and sufficient response can be ensured.
[0014]
According to a third invention, in the second invention,
The second control parameter ω _c of the proportional-integral control means is a first-order lag element corresponding to a closed-loop transfer function of a current control system including the proportional-integral control means, the motor drive circuit, and the electric motor. The constant is set to be equal to the time constant of the motor / drive circuit system.
[0015]
According to the third aspect of the invention, the time constant of the first-order lag element corresponding to the closed-loop transfer function of the current control system becomes equal to the time constant of the motor / drive circuit system. It has the same characteristic as the current frequency characteristic of the circuit system.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
<1. Basic study>
In order to solve the problems of the current control system according to the prior art, first, a basic study was conducted in order to investigate the cause of the problem in the prior art. Hereinafter, this basic study will be described with reference to the accompanying drawings before describing embodiments of the present invention.
[0017]
First, the characteristics of the motor / drive circuit system will be described. FIG. 1 is a Bode diagram showing a gain and a phase of an output with respect to an input when a predetermined signal is input to a motor / drive circuit system. As shown in FIG. 1, the break frequency of the motor / drive circuit system is about 57 Hz, and the phase delay at the break frequency is 45 degrees. It can also be seen that the phase delay with respect to the frequency is as follows. For example, when the frequency is 10 Hz, the phase delay is about 10 degrees, when the frequency is 20 Hz, the phase delay is about 20 degrees, and when the frequency is 100 Hz, the phase delay is about 60 degrees.
[0018]
Next, the characteristics of the current control system will be described. Conventionally, the PI control unit is designed so that the transfer function G ₁ (s) is expressed by the following equation (1).
_{G 1 (s) = 2πf c} (Ls + R) / s ··· (1)
Where f _c is a PI gain, L is a motor inductance, s is a variable based on Laplace transform and is an operator corresponding to a differential operator (hereinafter abbreviated as “Laplace operator”), and R is an internal resistance of the motor. It is. Incidentally, if the transfer function of the PI controller are set as shown by the formula (1), the overall current control system is the primary delay element to the break point frequency the PI gain f _c (however, later external Ignore resistor R ').
[0019]
FIG. 2 is a current frequency characteristic diagram showing the gain characteristic of the current control system in the conventional configuration designed so that the transfer function of the PI control unit is G ₁ (s). FIG. 3 is a current frequency characteristic diagram showing phase characteristics of the current control system in the conventional configuration. As shown in FIG. 2, toward at high PI gain f _c than at low PI gain f _c is, it can be seen that the gain of the current control system is high. As also shown in FIG. 3, who at high PI gain f _c than at low PI gain f _c is, it can be seen that the phase delay of the current control system is small.
[0020]
Looking at the phase characteristics when f _c = 100 Hz in FIG. 3, when the frequency is 10 Hz, the phase delay is about 10 degrees, when the frequency is 20 Hz, the phase delay is about 20 degrees, and the frequency is about 60 Hz. The phase delay is 45 degrees. Here, the phase characteristic of f _c = 100 Hz shows a characteristic close to the phase characteristic of the motor / drive circuit system shown in FIG. 1. When f _c is smaller than 100 Hz, it is more than when f _c is 100 Hz. It can be seen that the phase delay increases. As described above, comparing FIG. 1, FIG. 2 and FIG. 3, it can be said that the current frequency characteristic of the current control system approaches the current frequency characteristic of the motor / drive circuit system as the PI gain is increased. However, it can be seen that as the PI gain is increased from 25 Hz to 100 Hz, the improvement of the gain characteristic and the phase characteristic with respect to the change in the frequency is not so much seen. This is because even if the PI gain is increased, the performance of the motor does not follow. Further, in the current control system in the conventional configuration, when f _c = 100 Hz, a characteristic close to the phase characteristic of the motor / drive circuit system is obtained, and when f _c becomes 100 Hz or less, the phase delay becomes large. It can be seen that the phase is delayed in a frequency band lower than 57 Hz, which is the corner frequency of the drive circuit system.
[0021]
As described above, there is a difference between the current frequency characteristics of the current control system and the current frequency characteristics of the motor / drive circuit system. In order to investigate the cause, we decided to review the design of the current control system. In the current control system, the PI control unit gives a voltage command value to the motor / drive circuit system based on the deviation between the target current value and the actual current value. Here, the motor / drive circuit system includes a circuit for driving the motor, and there is a resistance other than the motor. However, conventionally, when designing a current control system, only the internal resistance of the motor is considered, and the resistance other than the motor is not considered.
[0022]
In view of this, external resistances such as wiring and driving circuits that have not been considered in the design of current control systems in the past (hereinafter, external resistances such as wiring and driving circuits are indicated by “R ′”) are not limited. The effect of the external resistance R ′ on the current control system was examined.
[0023]
FIG. 4 is a block diagram of a current control system in a conventional configuration, shown using a transfer function. The current control system includes a PI control unit 15, a motor / drive circuit system 200, and a current detector 19. The current detector 19 detects and outputs the actual current value flowing through the motor. The PI control unit 15 receives a deviation between the target current value and the actual current value. Further, the current detector 19 is provided with a low-pass filter so that a signal having a specific frequency or less passes. The transfer function G ₂ (s) of the current detector 19 is expressed by the following equation (2).
G ₂ (s) = 1 / (τs + 1) (2)
Here, τ is a time constant of the low-pass filter, and s is a Laplace operator.
Here, while the corner frequency of the low-pass filter is several KHz, the practical frequency band of the current control system according to the present invention is a low frequency band of 50 Hz or less. The effect on the closed-loop transfer function of the system need not be considered. That is, the transfer function G ₂ (s) of the current detector 19 can be regarded as a function equal to the transfer function G ₂ ′ (s) expressed by the following equation.
G ₂ '(s) = 1 (3)
[0024]
From the above, the closed loop transfer function G ₃ (s) of the current control system in the configuration of FIG. 4 is expressed as the following equation (4).
[Expression 1]

However, L is an inductance of the motor, s is a Laplace operator, R is an internal resistance of the motor, and R ′ is an external resistance such as a wiring or a drive circuit.
[0025]
Next, assuming that the external resistance R ′ has no influence on the current control system, the closed loop transfer function G ₄ (s) of the current control system substitutes R ′ = 0 in G ₃ (s). It is shown as the following formula.
[Expression 2]

G ₃ (s) and is compared with the G ₄ (s) for the denominator, for first-order coefficient of the s, towards G ₃ (s) is larger R '/ L than G ₄ (s) . In the frequency response of the transfer function of the first-order lag system, the phase delay in the low frequency band increases as the first-order coefficient increases. That is, when R ′ = 0 cannot be considered, the first-order coefficient of s increases, and the influence of the external resistance R ′ on the current control system appears as a phase delay.
[0026]
As described above, the current frequency characteristic of the current control system in the conventional configuration is delayed in phase in a frequency band lower than 57 Hz, which is the break frequency of the motor / drive circuit system. It is presumed to have an effect on
[0027]
Therefore, when the PI gain is set in consideration of the external resistance R ′ that affects the current control system, the closed loop transfer function G ₅ (s) of the current control system is expressed by the following equation.
[Equation 3]

In the above G ₅ (s), the transfer function G ₆ (s) when 2πf _c = (R + R ′) / L is expressed by the following equation.
G ₆ (s) = (R + R ′) / (Ls + R + R ′) (7)
Here, the transfer function G ₇ (s) of the motor / drive circuit system is expressed by the following equation.
G ₇ (s) = 1 / (Ls + R + R ′) (8)
Comparing G ₆ (s) and G ₇ (s), both time constants are L / (R + R ′) and are the same. That is, when the PI control parameters (proportional gain and integral gain or integration time) including the external resistance R ′ are set, the same characteristics as those of the motor / drive circuit system are obtained as the characteristics of the current control system. Will be.
[0028]
Next, examination was performed based on experimental data when the PI gain including the external resistance R ′ was set. FIG. 5 is a current frequency characteristic diagram showing the gain characteristics of the current control system when PI control including PI gain including the external resistance R ′ is performed. FIG. 6 is a current frequency characteristic diagram showing the phase characteristics of the current control system when PI control including PI resistance including the external resistance R ′ is performed. On the other hand, the current frequency characteristics of the current control system in the conventional configuration are shown in FIGS. 2 and 3 as described above. When FIG. 2 is compared with FIG. 5, it can be seen that gain characteristics are improved in FIG. 5, and when FIG. 3 is compared with FIG. 6, phase characteristics are improved in FIG. That is, it is understood that when PI control including PI resistance including the external resistance R ′ is performed, the current frequency characteristic of the current control system approaches the current frequency characteristic of the motor / drive circuit system, and the responsiveness increases.
[0029]
As described above, as a result of the examination based on the mathematical formula and the experimental data, the external resistance R ′ has an influence on the current control system, and the current control system is designed by taking the external resistance R ′ into consideration. It was found that the current frequency characteristics of the system approached the original characteristics of the motor / drive circuit system.
[0030]
Therefore, based on the result of the above basic study, the current control system of the electric power steering apparatus is actually designed in consideration of the external resistance R ′, and the electric power steering apparatus obtained by the design is implemented in one embodiment of the present invention. The form will be described below.
[0031]
<2. Embodiment>
Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.
FIG. 7 is a schematic diagram showing the configuration of the electric power steering apparatus according to one embodiment of the present invention, together with the vehicle configuration related thereto. This electric power steering apparatus includes a steering shaft 102 having one end fixed to a handle (steering wheel) 100 as an operation means for steering, a rack and pinion mechanism 104 connected to the other end of the steering shaft 102, a handle A torque sensor 3 for detecting a steering torque applied to the steering shaft 102 by an operation of 100, an electric motor 6 for generating a steering assist force for reducing a driver's load due to a steering operation (steering operation), and the motor 6 Is an electronic gear that controls the driving of the motor 6 based on sensor signals from the torque sensor 3 and the vehicle speed sensor 4 upon receiving power from the vehicle-mounted battery 8 and the reduction gear 7 that transmits the steering assist force generated by the vehicle to the steering shaft 102. And a control unit (ECU) 5.
[0032]
FIG. 8 is a block diagram showing a configuration of the electric power steering apparatus as viewed from a control viewpoint. The ECU 5 that is a control device for the electric power steering apparatus includes a phase compensation filter 10, a target current setting unit 12, a subtractor 14, a PI control unit 15, a motor drive circuit 20, and a current detector 19. ing. A detected value Ts of the steering torque detected by the torque sensor 3 is input to the phase compensation filter 10. The phase compensation filter 10 performs phase compensation based on the detection value Ts, and outputs a signal after the phase compensation as a steering torque signal T. The target current setting unit 12 receives the steering torque signal T and the detected value of the vehicle speed detected by the vehicle speed sensor 4, and calculates a target current value It to be supplied to the motor. The subtractor 14 calculates a deviation It−Is between the target current value It output from the target current setting unit 12 and the current detection value Is output from the current detector 19. The PI control unit 15 outputs a voltage command value V by proportional-integral control calculation based on the deviation It-Is. The motor drive circuit 20 applies a voltage to the motor based on the voltage command value V output from the PI control unit 15. By applying this voltage, a current flows through the motor 6.
[0033]
FIG. 9 is a block diagram of the current control system according to the present embodiment, shown using a transfer function. Here, the PI control unit 15, the current detector 19, and the motor / drive circuit system 200 in which the motor drive circuit and the motor are regarded as one transfer element in FIG. 8 are shown using transfer functions. In FIG. 9, the transfer function of the current detector 19 can be regarded as “1” as indicated by G ₂ ′ (s). The transfer function G ₈ (s) of the motor / drive circuit system 200 is expressed by the following equation.
G ₈ (s) = 1 / (Ls + R + R ′) (9)
However, L is an inductance of the motor, s is a Laplace operator, R is an internal resistance of the motor, and R ′ is an external resistance such as a wiring or a drive circuit.
[0034]
As described above, the PI control unit 15 outputs the voltage command value V to be given to the motor / drive circuit system 200 based on the deviation between the target current value and the actual current value. Here, in this embodiment, the PI control unit 15 is designed so that the transfer function G ₉ (s) is expressed by the following equation. That is, when the transfer function G ₉ (s) of the PI control unit 15 is set to ω _c (τ _c · s + 1) / s, τ _c which is the first control parameter of the PI control unit 15 is set as the motor / drive. It is set to be equal to the time constant L / (R + R ′) of the circuit system 200.
_{G 9 (s) = 2πf c} (Ls + R + R ') / s ··· (10)
_{That, τ c = L / (R} + R '), ω c = 2πf c (R + R')
Here, f _c is a PI gain, L is an inductance of the motor, s is a Laplace operator, R is an internal resistance of the motor, and R ′ is an external resistance such as a wiring or a drive circuit. An example of measurement data is L = 0.092 mH, R = 0.026Ω, and R ′ = 0.036Ω, and the external resistance R ′ has a value that cannot be ignored compared to the internal resistance R. I understand that. The external resistance R ′ includes a field effect transistor (FET) used in the motor drive circuit 20 having an on-resistance of several mΩ, a wiring resistance of several mΩ to several tens of mΩ, and a connector resistance of several mΩ to several tens of mΩ. It is.
[0035]
When designed as described above, the closed-loop transfer function G ₁₀ (s) of this current control system is expressed by the following equation.
[Expression 4]

In this way, the current control system is a primary delay element (first-order system), and the time constant are the 1 / (2 [pi] f _c), PI gain f _c is the corner frequency of the current control system. By the way, the time constant of the motor / drive circuit system 200 is L / (R + R ′) from the equation (9). Therefore, in the present embodiment, the PI gain f _{c is set} to f _c = (R + R) so that the time constant 1 / (2πf _c ) of the current control system is equal to the time constant L / (R + R ′) of the motor / drive circuit system 200. ') / (2πL)
And set. This is because the proportional gain 2πf _c L of the PI control unit 15 is set to R + R ′ so that the time constant 1 / (2πf _c ) of the current control system is equal to the time constant L / (R + R ′) of the motor / drive circuit system 200. (That is, the second control parameter ω _c of the PI controller 15 is set to (R + R ′) ² / L).
[0036]
The PI gain f _c or proportional gain 2 [pi] f _c L or the second control parameter omega _c of the PI controller 15 by setting like this, the motor-driving circuit system with closed-loop transfer function G ₁₀ of the current control system (s) The transfer function G ₈ (s) of 200 is represented by the following equation.

From the above, the current control system and the motor / drive circuit system 200 are first-order lag systems having the same corner frequency f _c = (R + R ′) / (2πL), and have the same frequency characteristics (gain characteristics and Phase characteristics). That is, both transfer functions have the same frequency characteristic.
[0037]
As described above, the current control system and the transfer function of the motor / drive circuit system 200 can be obtained by designing the current control system in consideration of the external resistance R ′ in accordance with the result of the basic examination described above. The transfer function has the same characteristics.
[0038]
Therefore, the current frequency characteristic of the current control system according to the present embodiment draws out the current frequency characteristic inherent in the motor / drive circuit system, and sufficient responsiveness can be ensured. As a result, a desired torque frequency characteristic can be obtained, and for example, it is possible to cope with a break frequency of 10 Hz or more (a frequency that is 45 degrees phase lag in the first order lag system and 90 degrees phase lag in the second order lag system). Further, it is possible to prevent the steering wheel from wobbling when traveling on a road surface with a lot of disturbances or during steering near the boundary between the dead zone and the assist zone. Furthermore, it becomes possible to sufficiently compensate for torque ripple of the motor alone. Furthermore, since the current control system as a whole has the same current frequency characteristics as the motor / drive circuit system 200 itself, the function of the control means for dynamic compensation such as inertia compensation and damping compensation is more fully exhibited. Become so.
[Brief description of the drawings]
FIG. 1 is a Bode diagram showing a gain and a phase of an output with respect to an input when a signal is input to a motor / drive circuit system according to an embodiment of the present invention.
FIG. 2 is a current frequency characteristic diagram showing gain characteristics of a current control system in a conventional configuration.
FIG. 3 is a current frequency characteristic diagram showing phase characteristics of a current control system in a conventional configuration.
FIG. 4 is a block diagram of a current control system shown using a transfer function in a control device in a conventional configuration.
FIG. 5 is a current frequency characteristic diagram showing gain characteristics of the current control system according to the present embodiment.
FIG. 6 is a current frequency characteristic diagram showing phase characteristics of the current control system according to the present embodiment.
FIG. 7 is a schematic view showing the configuration of the electric power steering apparatus according to the present embodiment together with the vehicle configuration related thereto.
FIG. 8 is a block diagram showing a configuration of the electric power steering apparatus according to the present embodiment as viewed from a control viewpoint.
FIG. 9 is a block diagram of a current control system shown by using a transfer function in the control device according to the present embodiment.
[Explanation of symbols]
3 ... Torque sensor 4 ... Vehicle speed sensor 5 ... Electronic control unit (EUC)
DESCRIPTION OF SYMBOLS 6 ... Motor 7 ... Reduction gear 8 ... Battery 10 ... Phase compensation filter 12 ... Target current setting part 14 ... Subtraction part 15 ... PI control part 19 ... Current detector 20 ... Motor drive circuit 100 ... Steering wheel (steering wheel)
102 ... steering shaft 104 ... rack-pinion mechanism 200 ... motor-driving circuit system It ... target electric current detection value V ... voltage command value of the value Is ... current detection value T ... steering torque signal Ts ... steering torque f _c ... PI gain L ... Motor inductance R ... Motor internal resistance R '... External resistance

Claims (3)

An electric power steering device that applies a steering assist force to a steering mechanism of a vehicle by driving an electric motor according to an operation for steering the vehicle,
Target current setting means for setting a target value of current to be supplied to the electric motor in accordance with the operation;
Proportional-integral control means for generating a command value for feedback control on the electric motor by proportional-integral control calculation based on a deviation between the current target value and the value of the current actually flowing to the electric motor;
A motor drive circuit for driving the electric motor according to the command value,
Of the proportional integral control means comprising a first control parameter is a tau _c and the second control parameter a is omega _c in the case where the transfer function of the proportional-integral control means put the _{ω c (τ c · s +} 1) / s The control parameter is set based on an inductance of the electric motor, an internal resistance of the electric motor, and an external resistance including a wiring resistance of the electric motor and the motor drive circuit. . The first control parameter τ _c of the proportional-integral control means is set to a value corresponding to a time constant of a motor / drive circuit system which is a first-order lag element including the electric motor and the motor drive circuit. The electric power steering apparatus according to claim 1, wherein: The second control parameter ω _c of the proportional-integral control means is a first-order lag element corresponding to a closed-loop transfer function of a current control system including the proportional-integral control means, the motor drive circuit, and the electric motor. 3. The electric power steering apparatus according to claim 2, wherein a constant is set to be equal to a time constant of the motor / drive circuit system.

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