JP3622478B2 - Air-fuel ratio detection device - Google Patents

Description

図１５に示す特性の検出部１０１では、空燃比が３０のときのセンサ電流Ｉｐが１９ｍＡになる。従って、空燃比が３０のとき、マイクロコンピュータ８５は当該空燃比を算出することができない。
【００２６】
このように、従来の駆動回路８１では高い領域の空燃比を検出することができないため、高い領域の空燃比におけるリーンバーン制御を行うことは不可能である。
ところで、抵抗Ｒ１を１００Ωより小さくし、例えば、５６Ωにすれば、式（４）より電圧Ｖ１を４．７６９ＶにすることができるためＡ／Ｄ変換器８４にてＡ／Ｄ変換することが可能になり、式（７）よりＩｐ（ｍａｘ）を２５．２４ｍＡにすることができるため図１５に示す特性の検出部１０１でも３０，４０といった高い領域の空燃比まで検出することが可能になる。しかし、抵抗Ｒ１の抵抗値を小さくすると、各電圧Ｖ１，Ｖ２の電位差が小さくなるため、Ａ／Ｄ変換器８４の変換精度が低下し、マイクロコンピュータ８５の算出する空燃比の精度も低下することになる。従って、抵抗Ｒ１の抵抗値を小さくすると、高い領域の空燃比におけるリーンバーン制御を精密に制御することができない。
【００２７】
本発明は上記問題点を解決するためになされたものであって、その目的は、リッチ領域，理論空燃比，リーン領域の各状態における空燃比を高精度に検出可能な空燃比検出装置を提供することにある。
【００２８】
【課題を解決するための手段】
かかる目的を達成するためになされた請求項１に記載の発明は、ジルコニア固体電解質と、該ジルコニア固体電解質の大気雰囲気側に形成された大気側電極と、該ジルコニア固体電解質の排気雰囲気側に形成された排気側電極と、排気雰囲気から該排気側電極へ拡散で流入するガスを抑制する拡散抵抗体とを備えた検出部を設けている。そして、該検出部の大気側電極および排気側電極に対してそれぞれ電圧を印加する駆動回路部を備える。また、前記検出部の排気側電極から流れ出すか又は流れ込む電流を検出し、その電流値に基づいて空燃比を検出する検出回路部を備える。
【００２９】
従って、本発明によれば、検出部の大気側電極ではなく、排気側電極から流れ出すか又は流れ込む電流を検出し、その電流値に基づいて空燃比を検出するため、高い領域の空燃比まで高精度に検出することが可能になり、リッチ領域および理論空燃比からリーン領域へ空燃比の検出範囲を拡大することができる。
【００３０】
具体的には、前記駆動回路部は、前記検出部の大気側電極に印加される大気側印加電圧を生成する大気側電圧生成手段と、前記検出部の排気側電極に印加される排気側印加電圧を生成する排気側電圧生成手段と、前記検出部の排気側電極と前記排気側電圧生成手段との間に接続された電流検出用抵抗とを備える。また、前記検出回路部は、前記駆動回路部の電流検出用抵抗の両端の電圧のうち少なくとも前記排気側電圧生成手段側の電圧を検出する電圧検出手段と、該電圧検出手段によって検出された電圧から前記電流検出用抵抗に流れる電流を検出する電流検出手段と、該電流検出手段によって検出された電流に基づいて空燃比を算出する空燃比算出手段とを備える。
【００３１】
従って、本発明によれば、電流検出用抵抗の抵抗値を小さくしなくても高い領域の空燃比を検出することが可能になるため、電圧検出手段をＡ／Ｄ変換器にて具体化した場合にそのＡ／Ｄ変換精度が低下するおそれがなくなり、空燃比算出手段にて算出される空燃比の精度を高めることができる。
【００３２】
次に、請求項２に記載の発明は、請求項１に記載の空燃比検出装置において、前記大気側電圧生成手段の生成する大気側印加電圧と、前記排気側電圧生成手段の生成する排気側印加電圧とが共に固定されている。
従って、請求項２に記載の発明によれば、ストイキ制御に必要な範囲の空燃比を高精度に検出することができる。
【００３３】
ところで、請求項２に記載の空燃比検出装置における前記駆動回路部は、請求項３に記載の発明のように、電圧源とグランドとの間に直列接続された複数の抵抗から成る抵抗分圧回路を備える。そして、前記大気側電圧生成手段は、前記抵抗分圧回路によって抵抗分圧された電圧により大気側印加電圧を生成する。また、前記排気側電圧生成手段は、前記抵抗分圧回路によって抵抗分圧された電圧により排気側印加電圧を生成する。
【００３４】
従って、請求項３に記載の発明によれば、抵抗のみから成る簡単な抵抗分圧回路により得られる電圧にて、大気側印加電圧および排気側印加電圧を共に生成することができる。
次に、請求項４に記載の発明は、請求項１に記載の空燃比検出装置において、前記駆動回路部は、前記検出部の大気側電極と排気側電極との間に印加される電圧に基づいて、前記大気側電圧生成手段の生成する大気側印加電圧を制御することにより、前記検出部の排気側電極から流れ出すか又は流れ込む電流を飽和させる。そして、前記空燃比算出手段は、前記飽和された電流値に基づいて空燃比を検出する。
【００３５】
従って、本発明によれば、空燃比に応じて検出部の大気側電極と排気側電極との間に印加される電圧を変化させることが可能になり、リーン領域の広範囲の空燃比を検出することができる。
ところで、請求項４に記載の空燃比検出装置における前記駆動回路部は、請求項５に記載の発明のように、電圧源とグランドとの間に直列接続された複数の抵抗から成る抵抗分圧回路と、前記検出部の大気側電極と排気側電極との間に印加される電圧に基づいた可変電圧を生成する可変電圧生成回路とを備える。そして、前記大気側電圧生成手段は、前記可変電圧生成回路の生成する可変電圧により大気側印加電圧を生成する。また、前記排気側電圧生成手段は、前記抵抗分圧回路によって抵抗分圧された電圧により排気側印加電圧を生成する。
【００３６】
従って、請求項５に記載の発明によれば、排気側印加電圧を固定して大気側印加電圧を可変することにより、空燃比に応じて検出部の大気側電極と排気側電極との間に印加される電圧を変化させることが可能になり、リーン領域の広範囲の空燃比を検出することができる。
【００３７】
また、請求項５に記載の空燃比検出装置は、請求項６に記載の発明のように、前記大気側電圧生成手段の生成する大気側印加電圧を一定値以下に抑えることにより、前記検出部の大気側電極と排気側電極との間に印加される電圧を抑制して前記検出部を保護する保護手段を備える。
【００３８】
従って、請求項６に記載の発明によれば、検出部の電極間に異常な電圧が印加されるのを回避して、検出部の劣化や破壊を防止することができる。
次に、請求項７に記載の発明は、請求項１に記載の空燃比検出装置において、前記駆動回路部は、前記検出部の大気側電極と排気側電極との間に印加される電圧に基づいて、前記大気側電圧生成手段の生成する大気側印加電圧と、前記排気側電圧生成手段の生成する排気側印加電圧とを共に可変することにより、前記検出部の排気側電極から流れ出すか又は流れ込む電流を飽和させる。そして、前記空燃比算出手段は、前記飽和された電流値に基づいて空燃比を検出する。
【００３９】
従って、本発明によれば、排気側印加電圧と大気側印加電圧とを共に可変することにより、空燃比に応じて検出部の大気側電極と排気側電極との間に印加される電圧を変化させることが可能になり、リーン領域の広範囲の空燃比を検出することができる。
【００４０】
次に、請求項８に記載の発明は、請求項１〜７のいずれか１項に記載の空燃比検出装置において、前記検出部の大気側電極と排気側電極との間の交流的な内部抵抗を検出し、その内部抵抗が一定になるように前記検出部のジルコニア固体電解質の温度を一定に制御する。
【００４１】
従って、本発明によれば、ジルコニア固体電解質の温度を常に一定に保ち、検出部の各電極間に印加される電圧に対して排気側電極から流れ出すか又は流れ込む電流の特性変動を防止することが可能になり、空燃比の検出精度をさらに高めることができる。
【００４２】
ところで、請求項８に記載の空燃比検出装置は、請求項９に記載の発明のように、前記検出部のジルコニア固体電解質を加熱するヒータを備え、前記内部抵抗が一定になるように該ヒータの作動を制御する。
また、請求項９に記載の空燃比検出装置は、請求項１０に記載の発明のように、前記排気側電圧生成手段の生成する排気側印加電圧を変化させ、その電圧の変化量と、その電圧の変化によって生じた前記検出部の排気側電極から流れ出すか又は流れ込む電流の変化量とに基づいて前記内部抵抗を検出する。
【００４３】
また、請求項９に記載の空燃比検出装置は、請求項１１に記載の発明のように、前記大気側電圧生成手段の生成する大気側印加電圧を変化させ、その電圧の変化量と、その電圧の変化によって生じた前記検出部の排気側電極から流れ出すか又は流れ込む電流の変化量とに基づいて前記内部抵抗を検出する。
【００４４】
尚、以下に述べる発明の実施の形態において、特許請求の範囲または課題を解決するための手段に記載の「検出回路部」はＡ／Ｄ変換器８４およびマイクロコンピュータ８５から構成され、同じく「大気側電圧生成手段」はオペアンプ８２から構成され、同じく「排気側電圧生成手段」はオペアンプ８３から構成され、同じく「電圧検出手段」はＡ／Ｄ変換器８４から構成され、同じく「電流検出手段」はマイクロコンピュータ８５におけるＳ１０２の処理に相当し、同じく「空燃比算出手段」はマイクロコンピュータ８５におけるＳ１０３の処理に相当し、同じく「電圧源」は電源ＶＣに相当し、同じく「可変電圧生成回路」はＤ／Ａ変換器３２から構成され、同じく「保護手段」はダイオードＤ１から構成される。
【００４５】
【発明の実施の形態】
以下、本発明を具体化した各実施形態を図面と共に説明する。尚、各実施形態において、図１３〜図１５に示した従来の形態と同じ構成部材については符号を等しくしてその詳細な説明を省略する。
【００４６】
（第１実施形態）
以下、本発明を具体化した第１実施形態を図面と共に説明する。
図２に、本実施形態の空燃比検出装置を用いた自動車用内燃機関の制御装置の全体構成を示す。
【００４７】
制御装置１１は、入力バッファ１２、出力バッファ１３、駆動回路１４、Ａ／Ｄ変換器８４、マイクロコンピュータ８５から構成されている。
内燃機関の回転数を検出する回転センサからの回転数検出信号ＮＥ，自動車の車速を検出する車速センサからの車速検出信号ＳＰＤ，スタータスイッチが操作されたか否かに対応するスタータスイッチ操作検出信号ＳＴＡなどのディジタル信号は、入力バッファ１２にてノイズ除去および波形整形が施されて０Ｖまたは５Ｖの信号に変換され、マイクロコンピュータ８５に入力される。
【００４８】
また、内燃機関の吸入空気量を検出する空気量センサからの吸入空気量検出信号ＶＧ，スロットル開度を検出するスロットル開度センサからのスロットル開度検出信号ＶＴＡ，内燃機関の冷却水温を検出する冷却水温センサからの水温検出信号ＴＨＷなどのアナログ信号は、Ａ／Ｄ変換器８４にてＡ／Ｄ変換され、マイクロコンピュータ８５に入力される。
【００４９】
駆動回路１４は検出部１０１を駆動し、駆動回路１４から出力される各電圧Ｖ１，Ｖ２はＡ／Ｄ変換器８４にてＡ／Ｄ変換され、マイクロコンピュータ８５に入力される。
マイクロコンピュータ８５は、入力バッファ１２およびＡ／Ｄ変換器８４から入力された各種信号に基づいて、インジェクタから内燃機関へ供給される燃料噴射量の制御信号ＩＮＪ，点火時期の制御信号ＩＧＴ，アイドル回転制御用バルブの制御信号ＩＳＣ，検出部１０１のヒータ１０３を制御するための制御信号ＨＴなどを生成する。マイクロコンピュータ８５の生成したこれらの制御信号は、出力バッファ１３を介して制御装置１１の外部へ出力される。
【００５０】
図１に、駆動回路１４の内部構成を示す。
本実施形態の駆動回路１４において、従来の駆動回路８１と異なるのは以下の点である。
（１−１）オペアンプ８２の出力端子は、抵抗Ｒ２を介して検出部１０１の電極１０４（大気雰囲気側）に接続されると共に、各抵抗Ｒ２，Ｒ３を介してオペアンプ８２の反転入力端子に接続されている。
【００５１】
（１−２）オペアンプ８３の出力端子は抵抗Ｒ１を介して検出部１０１の電極１０５（排気ガス雰囲気側）に接続されると共に、各抵抗Ｒ１，Ｒ４を介してオペアンプ８３の反転入力端子に接続されている。
（１−３）Ａ／Ｄ変換器８４は、抵抗Ｒ１におけるオペアンプ８３側の電圧Ｖ１と検出部１０１側の電圧Ｖ２とをそれぞれＡ／Ｄ変換し、そのＡ／Ｄ変換値をマイクロコンピュータ８５に出力する。
【００５２】
次に、本実施形態の駆動回路１４の作用について説明する。
前記したように、リーン領域においては、電極１０４から電極１０５へセンサ電流Ｉｐが流れる。このとき、オペアンプ８３の出力可能な最低電圧をＶＣＥとする。また、各オペアンプ８２，８３の非反転入力端子に印加される電圧ＶＡＦ＋，ＶＡＦ−は、両方共に可変可能であるとする。すると、電圧Ｖ１は式（８）により表される。
【００５３】
Ｖ１≧ＶＣＥ ………（式８）
オペアンプ８３にオフセット電圧が存在しないとすると電圧Ｖ２は電圧ＶＡＦ−と等しくなり、電圧Ｖ２は式（９）により表される。
Ｖ２＝ＶＡＦ−＝Ｖ１＋Ｉｐ・Ｒ１ ………（式９）
検出部１０１の各電極１０４，１０５間に印加される電圧ＶＲは各電圧ＶＡＦ＋，ＶＡＦ−の差電圧であるため、電圧ＶＡＦ＋は式（１０）により表される。
【００５４】
ＶＡＦ＋＝ＶＡＦ−＋ＶＲ ………（式１０）
オペアンプ８２の出力電圧ＶＯＰは式（１１）により表される。
ＶＯＰ＝ＶＡＦ＋＋Ｉｐ・Ｒ２ ………（式１１）
前記した駆動回路８１の場合と同様に、Ｉｐ＝２３ｍＡ、ＶＣＥ＝１．５Ｖ、ＶＲ＝０．９Ｖ、Ｒ１＝１００Ω、Ｒ２＝４７Ωとして式（８）〜（１１）に代入すると、各電圧Ｖ２，Ｖ１，ＶＡＦ＋，ＶＯＰの値は式（１２）〜（１５）に示すようになる。
【００５５】

また、センサ電流Ｉｐの検出可能な最大値Ｉｐ（ｍａｘ）は、式（８）〜（１１）から求められる式（１６）により表される。
【００５６】
Ｉｐ（ｍａｘ）＝（ＶＣ−ＶＲ−ＶＣＥ）／Ｒ１＝２６ｍＡ ………（式１６）
従って、式（１３）（１４）を満足するように各電圧ＶＡＦ＋，ＶＡＦ−を設定することにより、各電圧Ｖ１，Ｖ２を共に５Ｖ以下にすることができる。Ａ／Ｄ変換器８４はプラス側電源ＶＣおよびグランドに接続されて電源供給がなされているため、電源電圧ＶＣを５Ｖとすると、Ａ／Ｄ変換値の最大値は５Ｖとなる。よって、式（１２）（１３）より、各電圧Ｖ１，Ｖ２はＡ／Ｄ変換器８４にてＡ／Ｄ変換可能になる。
【００５７】
図１５に示す特性の検出部１０１では、空燃比が４０のときのセンサ電流Ｉｐが２３ｍＡになる。従って、式（１６）より、空燃比が４０のときも、マイクロコンピュータ８５は当該空燃比を算出することができる。
尚、式（１５）より、オペアンプ８２の出力電圧ＶＯＰは５Ｖを越えるが、各オペアンプ８２，８３の電源ＶＢを車載バッテリから供給するようにすれば、車載バッテリの電圧は１４Ｖであることから、オペアンプ８２の出力電圧ＶＯＰの最大値は１４Ｖになり何ら問題とはならない。
【００５８】
次に、本実施形態においてマイクロコンピュータ８５が実行する処理の詳細を、図３に示すフローチャートを用いて説明する。
イグニッションスイッチがオンされて車載バッテリから電源が供給されることによりマイクロコンピュータ８５が起動すると、内蔵ＲＯＭに記録されている制御プログラムに従い、コンピュータによる各種演算処理によって、以下の各ステップの処理を実行する。
【００５９】
まず、ステップ（以下、Ｓという）１０１において、抵抗Ｒ１の両端の電圧Ｖ１，Ｖ２のＡ／Ｄ変換をＡ／Ｄ変換器８４に実行させ、そのＡ／Ｄ変換値を受け取る。
次に、Ｓ１０２において、前記式（１）に示すように、駆動回路１４から出力される各電圧Ｖ１，Ｖ２と抵抗Ｒ１の抵抗値とに基づいて、検出部１０１の各電極１０４，１０５間に流れるセンサ電流Ｉｐを算出する。
【００６０】
次に、Ｓ１０３において、予め内蔵ＲＯＭに記録されているデータテーブルを用いたテーブル補間により、センサ電流Ｉｐに対応した空燃比（Ａ／Ｆ）を算出する。ここで、センサ電流Ｉｐと空燃比とは完全にリニアな直線関係にはならないため、予め何点かのセンサ電流Ｉｐに対応する空燃比を求めてデータテーブルを作成しておく必要がある。
【００６１】
次に、Ｓ１０１へ戻るが、本実施形態ではこのＳ１０１からＳ１０３のルーチンを８ｍｓのタイミングで繰り返し行う。
そして、マイクロコンピュータ８５は、算出した空燃比に応じて燃料噴射量をフィードバック制御して燃料噴射量制御信号ＩＮＪを生成することにより、内燃機関に供給される混合気の空燃比を目標値に制御する。
【００６２】
このように、本実施形態の駆動回路１４によれば、高い領域の空燃比を検出することが可能になるため、内燃機関の制御装置１１は高い領域の空燃比におけるリーンバーン制御を行うことができる。そして、抵抗Ｒ１の抵抗値を小さくする必要がないため、高い領域の空燃比を検出する際にも、Ａ／Ｄ変換器８４の変換精度が低下するおそれはなく、マイクロコンピュータ８５の算出する空燃比の精度を高めることができる。従って、検出精度を低下させることなく、リッチ領域および理論空燃比からリーン領域へ空燃比の検出領域を拡大することができる。
【００６３】
ここで、オペアンプ８３の非反転入力端子に印加される電圧ＶＡＦ−が固定の場合、電圧Ｖ２は電圧ＶＡＦ−と等しくなるため、マイクロコンピュータ８５は電圧Ｖ２の値を予め内蔵ＲＯＭに記憶しておくことにより認知することが可能であり、Ａ／Ｄ変換器８４において電圧Ｖ２をＡ／Ｄ変換する必要はない。しかし、オペアンプ８３にオフセット電圧が存在する場合など、電圧Ｖ２が電圧ＶＡＦ−と等しくならない場合にもセンサ電流Ｉｐを正確に検出するため、上記のようにＡ／Ｄ変換器８４において電圧Ｖ２をＡ／Ｄ変換することが望ましい。但し、電圧Ｖ２が電圧ＶＡＦ−と等しい条件が満足されるならば、Ａ／Ｄ変換器８４における電圧Ｖ２のＡ／Ｄ変換を省くことも可能である。
【００６４】
（第２実施形態）
以下、第１実施形態をより具体化した第２実施形態を図面と共に説明する。尚、本実施形態において、第１実施形態と同じ構成部材については符号を等しくしてその詳細な説明を省略する。
【００６５】
図４に、本実施形態の駆動回路２１の内部構成を示す。
駆動回路２１において、図１に示す第１実施形態の駆動回路１４と異なるのは、電源ＶＣとグランドとの間に直列接続された各抵抗Ｒ５〜Ｒ７による抵抗分圧によって各電圧ＶＡＦ＋，ＶＡＦ−を生成している点である。
【００６６】
次に、本実施形態の駆動回路２１の作用について説明する。
本実施形態は、リーンバーン制御を行わず、ストイキ制御のみを行う場合に適用したものである。ストイキ制御時には、例えば、空燃比（Ａ／Ｆ）が１３から１８の間でほぼ制御されるため、図１５に示す特性の検出部１０１では、センサ電流Ｉｐを−１０ｍＡ以上１０ｍＡ以下の範囲（−１０ｍＡ≦Ｉｐ≦１０ｍＡ）で検出できれば十分である。また、図１５の点線（ａ）に示すように、印加電圧ＶＲも一定値（＝０．４Ｖ）に設定すればよい。尚、リーン領域においてセンサ電流Ｉｐが電極１０４から電極１０５へ流れる場合にセンサ電流Ｉｐは正の値をとり、リッチ領域においてセンサ電流Ｉｐが電極１０５から電極１０４へ流れる場合にセンサ電流Ｉｐは負の値をとる。
【００６７】
センサ電流Ｉｐのとる範囲が−１０ｍＡ以上１０ｍＡ以下であるため、リッチ領域におけるセンサ電流の絶対値ＩｐＲとリーン領域におけるセンサ電流の絶対値ＩｐＬとが等しくなるように電圧ＶＡＦ−を設定する。ここで、電圧Ｖ１の最大値をＶ１（ｍａｘ）とすると、センサ電流ＩｐＲの最大値ＩｐＲ（ｍａｘ）は式（１７）により表される。また、センサ電流ＩｐＬの最大値ＩｐＬ（ｍａｘ）は式（１８）により表される。
【００６８】

ここで、電源電圧ＶＣを５Ｖとすると最大値Ｖ１（ｍａｘ）は５Ｖになる。そこで、ＩｐＲ（ｍａｘ）＝ＩｐＬ（ｍａｘ）、Ｖ１（ｍａｘ）＝５Ｖ、ＶＣＥ＝１．５Ｖ、Ｒ１＝１００Ωとして式（１７）（１８）に代入すると、各電圧ＶＡＦ−，ＶＡＦ＋は式（１９）（２０）に示すようになる。そのため、ＩｐＲ（ｍａｘ）およびＩｐＬ（ｍａｘ）は式（２１）に示すようになる。
【００６９】

従って、式（１９）（２０）を満足するように各抵抗Ｒ５〜Ｒ７の抵抗値を設定することにより、図１５に示す特性の検出部１０１を用いてストイキ制御を行うことができる。
【００７０】
尚、本実施形態においてマイクロコンピュータ８５が実行する処理は、第１実施形態と同じであるため説明を省略する。
このように、本実施形態の駆動回路２１によれば、各抵抗Ｒ５〜Ｒ７による簡単な抵抗分圧回路を用いて各電圧ＶＡＦ＋，ＶＡＦ−を生成することにより、ストイキ制御に必要な範囲の空燃比を高精度に検出することができる。
【００７１】
また、第１実施形態の駆動回路１４に各抵抗Ｒ５〜Ｒ７による抵抗分圧回路を接続するだけで本実施形態の駆動回路２１を作成することができる。そのため、駆動回路１４においてオペアンプ８２，８３および抵抗Ｒ１〜Ｒ４が実装されるプリント基板を駆動回路２１に流用することが可能になり、各駆動回路１４，２１にてプリント基板を共通化することができる。そして、本実施形態においてマイクロコンピュータ８５が実行する処理は第１実施形態と同じであるため、マイクロコンピュータ８５の制御プログラムについても、本実施形態と第１実施形態とで共通化することができる。従って、本実施形態によれば、第１実施形態とプリント基板および制御プログラム（ソフトウェア）を共通化することが可能になるため、コストダウンを図ることができる。
【００７２】
（第３実施形態）
以下、第１実施形態をより具体化した第３実施形態を図面と共に説明する。尚、本実施形態において、第１実施形態と同じ構成部材については符号を等しくしてその詳細な説明を省略する。
【００７３】
図５に、本実施形態の駆動回路３１の内部構成を示す。
駆動回路３１において、図１に示す第１実施形態の駆動回路１４と異なるのは以下の点である。
（３−１）駆動回路３１は、オペアンプ８２，８３および抵抗Ｒ１〜Ｒ４に加えて、抵抗Ｒ８〜Ｒ１０、ダイオードＤ１、Ｄ／Ａ変換器３２から構成されている。
【００７４】
（３−２）Ｄ／Ａ変換器３２は、マイクロコンピュータ８５の制御に従い、Ｄ／Ａ変換値としての電圧ＶＡＦ＋を生成する。ここで、Ｄ／Ａ変換器３２はプラス側電源ＶＣおよびグランドに接続されて電源供給がなされている。
（３−３）オペアンプ８２の非反転入力端子には、Ｄ／Ａ変換器３２から抵抗Ｒ１０を介して電圧ＶＡＦ＋が印加されている。尚、抵抗Ｒ１０は静電気からオペアンプ８２の非反転入力端子を保護するために設けられている。
【００７５】
（３−４）オペアンプ８３の非反転入力端子には、電源ＶＣとグランドとの間に直列接続された各抵抗Ｒ８，Ｒ９による抵抗分圧によって生成された電圧ＶＡＦ−が印加されている。また、オペアンプ８３の非反転入力端子はダイオードＤ１のカソードに接続され、ダイオードＤ１のアノードはオペアンプ８２の非反転入力端子に接続されている。
【００７６】
次に、本実施形態の駆動回路３１の作用について説明する。
図１５に示す特性の検出部１０１を用いた場合、１２から４０までの広範囲の空燃比（Ａ／Ｆ）に対応するセンサ電流Ｉｐを飽和電流値にて検出しようとすると、図１５の点線（ｂ）に示すように、空燃比に応じて印加電圧ＶＲを変化させなければならない。そこで、本実施形態においては、各抵抗Ｒ８，Ｒ９による抵抗分圧によって電圧ＶＡＦ−を固定すると共に、Ｄ／Ａ変換器３２によって電圧ＶＡＦ＋を可変することにより、空燃比に応じて印加電圧ＶＲを変化させるようにしている。
【００７７】
図１５に示す特性の検出部１０１では、１２から４０の範囲の空燃比に対応するセンサ電流Ｉｐを飽和電流値にて検出する場合、センサ電流Ｉｐを−１０ｍＡ以上２５ｍＡ以下の範囲（−１０ｍＡ≦Ｉｐ≦２５ｍＡ）で検出する必要がある。
【００７８】
前記式（１７）（１８）より、式（２２）が求められる。
ＩｐＲ（ｍａｘ）＋ＩｐＬ（ｍａｘ）＝（Ｖ１（ｍａｘ）−ＶＣＥ）／Ｒ１ ………（２２）
ここで、ＩｐＲ（ｍａｘ）＝１０ｍＡ、ＩｐＬ（ｍａｘ）＝２５ｍＡ、Ｖ１（ｍａｘ）＝５Ｖ、ＶＣＥ＝１．５Ｖとして式（２２）に代入すると、抵抗Ｒ１は式（２３）に示すようになる。
【００７９】

そこで、ＩｐＲ（ｍａｘ）＝１０ｍＡ、ＩｐＬ（ｍａｘ）＝２５ｍＡ、Ｖ１（ｍａｘ）＝５Ｖ、ＶＣＥ＝１．５Ｖ、Ｒ１＝１００Ωとして式（１７）（１８）に代入すると、電圧ＶＡＦ−は式（２４）に示すようになる。
【００８０】
ＶＡＦ−＝４．０Ｖ ………（２４）
但し、式（２４）にて求めた電圧ＶＡＦ−により必要な印加電圧ＶＲが得られるか否かを確認する必要がある。Ｄ／Ａ変換器３２はプラス側電源ＶＣおよびグランドに接続されて電源供給がなされているため、電源電圧ＶＣを５Ｖとすると、Ｄ／Ａ変換値の最大値は５Ｖとなる。ここで、図１５の点線（ｂ）より印加電圧ＶＲの最大値ＶＲ（ｍａｘ）を０．９Ｖとすると、式（２４）より電圧ＶＡＦ−は４．０Ｖであるため、前記式（１０）より電圧ＶＡＦ＋の最大値は４．９Ｖとなり、Ｄ／Ａ変換器３２の出力の最大値である５Ｖより小さいため何ら問題とはならない。
【００８１】
しかし、式（２４）にて求めた電圧ＶＡＦ−により必要な印加電圧ＶＲが得られない場合には、抵抗Ｒ１および電圧ＶＡＦ−を再設定する。すなわち、前記式（１６）より求められる式（２５）に各値ＶＣ，ＶＲ（ｍａｘ），ＶＣＥ，ＩｐＬ（ｍａｘ）を代入して抵抗Ｒ１を求める。
【００８２】
Ｒ１＝（ＶＣ−ＶＲ（ｍａｘ）−ＶＣＥ）／ＩｐＬ（ｍａｘ） ………（２５）
また、電源電圧ＶＣを５ＶとするとＤ／Ａ変換器３２の出力の最大値（すなわち、電圧ＶＡＦ＋の最大値）は５Ｖとなるため、ＶＡＦ＋＝５Ｖと必要な印加電圧ＶＲの値とを前記式（１０）に代入して電圧ＶＡＦ−を求める。
【００８３】
従って、求めた電圧ＶＡＦ−が得られるように各抵抗Ｒ８，Ｒ９の抵抗値を設定することにより、図１５に示す特性の検出部１０１を用いて、１２から４０の範囲の空燃比に対応するセンサ電流Ｉｐを飽和電流値にて検出することができる。次に、本実施形態においてマイクロコンピュータ８５が実行する処理の詳細を、図６に示すフローチャートを用いて説明する。
【００８４】
図６に示すＳ１０１〜Ｓ１０３の各ステップについては、図３に示すそれと同じである。
そして、Ｓ１０３に続くＳ１０４において、予め内蔵ＲＯＭに記録されている検出部１０１における印加電圧ＶＲとセンサ電流Ｉｐの特性データ（図１５の点線（ｂ））に基づき、Ｓ１０２にて算出したセンサ電流Ｉｐに対応する印加電圧ＶＲを算出する。
【００８５】
次に、Ｓ１０５において、算出した印加電圧ＶＲと電圧ＶＡＦ−（＝Ｖ２）とを前記式（１０）に代入して電圧ＶＡＦ＋を算出し、その電圧ＶＡＦ＋をＤ／Ａ変換器３２から生成させる。
次に、Ｓ１０１へ戻るが、本実施形態ではこのＳ１０１からＳ１０５のルーチンを８ｍｓのタイミングで繰り返し行う。
【００８６】
このように、本実施形態の駆動回路３１によれば、各抵抗Ｒ８，Ｒ９による簡単な抵抗分圧回路を用いて固定した電圧ＶＡＦ−を生成すると共に、Ｄ／Ａ変換器３２によって電圧ＶＡＦ＋を可変することにより、空燃比に応じて印加電圧ＶＲを変化させることが可能になり、リーン領域の広範囲の空燃比を検出することができる。
【００８７】
尚、電圧ＶＡＦ−を生成するＤ／Ａ変換器を加え、各電圧ＶＡＦ＋，ＶＡＦ−を共に可変することにより印加電圧ＶＲを可変するようにしてもよいが、その場合はＤ／Ａ変換器が２つ必要となりコストが増大することになる。
ところで、ダイオードＤ１は検出部１０１を保護するために設けられている。すなわち、センサ電流Ｉｐが飽和電流値にならない領域で検出部１０１を使用すると、検出部１０１が劣化したり破壊されたりする。そのため、図１５に示す特性の検出部１０１を用いた場合には、印加電圧ＶＲを±１Ｖ以内にする必要がある。しかし、Ｄ／Ａ変換器３２またはマイクロコンピュータ８５が異常な動作を起こした場合や、Ｄ／Ａ変換器３２とマイクロコンピュータ８５とを接続する配線に異常が生じた場合などには、異常に高いか又は低い電圧ＶＡＦ＋が生成されることがあり、印加電圧ＶＲが±１Ｖを越えることがある。
【００８８】
例えば、生成された電圧ＶＡＦ＋が電圧ＶＡＦ−よりも低い場合（ＶＡＦ＋＜ＶＡＦ−）、電圧ＶＡＦ＋は式（２５）により表される。
ＶＡＦ＋＝ＶＡＦ−−ＶＦ ………（式２５）
そのため、印加電圧ＶＲ（＝ＶＡＦ＋−ＶＡＦ−）はしきい値電圧ＶＦと等しくなり、通常、しきい値電圧ＶＦは１Ｖ以下であるため、印加電圧ＶＲを±１Ｖ以内におさめることができる。従って、ダイオードＤ１によって各オペアンプ８２，８３の非反転入力端子間をクランプすることにより、電圧ＶＡＦ＋が電圧ＶＡＦ−よりも低い場合に印加電圧ＶＲが異常に高くなるか又は低くなるかして検出部１０１に劣化や破壊が起こるのを回避することができる。
【００８９】
また、生成された電圧ＶＡＦ＋が異常に高い場合、検出部１０１に劣化や破壊が起こらない印加電圧ＶＲの最大値ＶＲ（ｍａｘ）を１Ｖとし、電源電圧ＶＣを５ＶとしてＤ／Ａ変換器３２の出力の最大値（すなわち、電圧ＶＡＦ＋の最大値）を５Ｖとすると、前記式（１０）より電圧ＶＡＦ−を４Ｖ以上に設定しておけば、前記最大値ＶＲ（ｍａｘ）を１Ｖ以下におさめることができる。ここで、前記式（２４）より電圧ＶＡＦ−は４．０Ｖであるため、実用上、最大値ＶＲ（ｍａｘ）を１Ｖ以下にすることができる。従って、前記最大値ＶＲ（ｍａｘ）および電圧ＶＡＦ＋の最大値に応じて電圧ＶＡＦ−を最適に設定することにより、電圧ＶＡＦ＋が異常に高い場合に印加電圧ＶＲが異常に高くなるか又は低くなるかして検出部１０１に劣化や破壊が起こるのを回避することができる。
【００９０】
（第４実施形態）
以下、第３実施形態を一部変更した第４実施形態を図面と共に説明する。尚、本実施形態において、第３実施形態と同じ構成部材については符号を等しくしてその詳細な説明を省略する。
【００９１】
図７に、本実施形態の駆動回路４１の内部構成を示す。
駆動回路４１において、図５に示す第３実施形態の駆動回路３１と異なるのは以下の点である。
（４−１）駆動回路４１は、オペアンプ８２，８３、抵抗Ｒ１〜Ｒ４，Ｒ８〜Ｒ１０、ダイオードＤ１、Ｄ／Ａ変換器３２に加えて、抵抗Ｒ１１〜Ｒ１４、コンデンサＣ１、トランジスタＴｒ１，Ｔｒ２から構成されている。
【００９２】
（４−２）オペアンプ８３の非反転入力端子とグランドとの間に、コンデンサＣ１が接続されている。
（４−３）直列に接続されたＰＮＰトランジスタＴｒ１および抵抗Ｒ１１が抵抗Ｒ８と並列に接続され、直列に接続されたＮＰＮトランジスタＴｒ２および抵抗Ｒ１２が抵抗Ｒ９と並列に接続されている。各トランジスタＴｒ１，Ｔｒ２はそれぞれ各抵抗Ｒ１３，Ｒ１４を介してマイクロコンピュータ８５に接続され、各トランジスタＴｒ１，Ｔｒ２のオン・オフはマイクロコンピュータ８５によって制御される。尚、各抵抗Ｒ１３，Ｒ１４は静電気から各トランジスタＴｒ１，Ｔｒ２を保護するために設けられている。
【００９３】
次に、本実施形態の駆動回路４１の作用について説明する。
検出部１０１において、ジルコニア固体電解質１０２の温度が変化すると、印加電圧ＶＲに対するセンサ電流Ｉｐの特性が変動するため、空燃比の検出精度が低下するという問題がある。ジルコニア固体電解質１０２の温度を一定に保つには、検出部１０１の交流的な内部抵抗を検出し、当該内部抵抗が一定値になるようにヒータ１０３をオン・オフ制御すればよい。
【００９４】
駆動回路４１において、空燃比を検出するためにセンサ電流Ｉｐを検出する際には各トランジスタＴｒ１，Ｔｒ２を共にオフさせる。すると、駆動回路４１は第３実施形態の駆動回路３１と同じ構成となり、オペアンプ８３の非反転入力端子には各抵抗Ｒ８，Ｒ９による抵抗分圧によって生成された電圧ＶＡＦ−が印加され、オペアンプ８２の非反転入力端子にはＤ／Ａ変換器３２から抵抗Ｒ１０を介して電圧ＶＡＦ＋が印加される。従って、駆動回路４１においても、駆動回路３１と同様に、各抵抗Ｒ８，Ｒ９による簡単な抵抗分圧回路を用いて固定した電圧ＶＡＦ−を生成すると共に、Ｄ／Ａ変換器３２によって電圧ＶＡＦ＋を可変することにより、空燃比に応じて印加電圧ＶＲを変化させることが可能になり、リーン領域の広範囲の空燃比を検出することができる。
【００９５】
尚、本実施形態において、空燃比を検出する際にマイクロコンピュータ８５が実行する処理は、第３実施形態と同じであるため説明を省略する。
次に、本実施形態において、検出部１０１の交流的な内部抵抗を検出する際にマイクロコンピュータ８５が実行する処理の詳細を、図８に示すフローチャートを用いて説明する。
【００９６】
図９に、検出部１０１の交流的な内部抵抗を検出する際における、各トランジスタＴｒ１，Ｔｒ２のオン・オフ状態および各電圧ＶＡＦ−（＝Ｖ２），Ｖ１の時間変位を示す。尚、オペアンプ８３にオフセット電圧が存在しないとしているため、電圧ＶＡＦ−は電圧Ｖ２と等しくなる。
【００９７】
図８に示すように、まず、Ｓ２０１において、抵抗Ｒ１の両端の電圧Ｖ１，Ｖ２のＡ／Ｄ変換をＡ／Ｄ変換器８４に実行させ、そのＡ／Ｄ変換値を受け取る。このとき、各トランジスタＴｒ１，Ｔｒ２は共にオフさせておく。図９に示すように、ここで検出した電圧Ｖ１の電圧値を電圧Ｖ１０と表記し、電圧Ｖ２の電圧値を電圧Ｖ２０と表記する。また、このときに流れるセンサ電流Ｉｐ（以下、「Ｉ０」と表記する）は、式（２６）により表される。
【００９８】
Ｉｐ＝Ｉ０＝（Ｖ２０−Ｖ１０）／Ｒ１ ………（式２６）
次に、Ｓ２０２において、トランジスタＴｒ２をオンさせると共に、マイクロコンピュータ８５の内蔵カウンタのカウンタ値をクリアする。
次に、Ｓ２０３において、Ｓ２０２にて内蔵カウンタのカウンタ値をクリアしてから１３５μｓ経過したか否かを判定し、１３５μｓ経過した時点でＳ２０４へ移行する。
【００９９】
Ｓ２０４において、抵抗Ｒ１の両端の電圧Ｖ１，Ｖ２のＡ／Ｄ変換をＡ／Ｄ変換器８４に実行させ、そのＡ／Ｄ変換値を受け取る。
このとき、トランジスタＴｒ１はオフ、トランジスタＴｒ２はオンされているため、各抵抗Ｒ９，Ｒ１２が並列に接続される。その結果、電圧ＶＡＦ−は各抵抗Ｒ８，Ｒ９，Ｒ１２の直並列回路により算出される電圧値となり、その電圧値は、図９に示すように、前記電圧Ｖ２０より低い電圧Ｖ２１になる。また、電圧ＶＡＦ−の低下に伴い、電圧Ｖ１の電圧値も低下して前記電圧Ｖ１０より低い電圧Ｖ１１になる。従って、リッチ領域の場合と同様に、電極１０５から電極１０４へセンサ電流Ｉｐが流れる。このときに流れるセンサ電流Ｉｐ（以下、「Ｉ１」と表記する）は、式（２７）により表される。
【０１００】
Ｉｐ＝Ｉ１＝（Ｖ２１−Ｖ１１）／Ｒ１ ………（式２７）
式（２６）（２７）より、電圧ＶＡＦ−の変化に伴うセンサ電流Ｉｐの変化量ΔＩは式（２８）により表される。また、電圧ＶＡＦ−の変化量ΔＶは式（２９）により表される。
【０１０１】

式（２８）（２９）より、検出部１０１の交流的な内部抵抗Ｚは式（３０）により表される。
【０１０２】
Ｚ＝ΔＶ／ΔＩ ………（式３０）
従って、内部抵抗Ｚが一定値になるように、検出部１０１のヒータ１０３をオン・オフ制御するための制御信号ＨＴを生成する。
次に、Ｓ２０５において、Ｓ２０２にて内蔵カウンタのカウンタ値をクリアしてから２００μｓ経過したか否かを判定し、２００μｓ経過した時点でＳ２０６へ移行する。
【０１０３】
次に、Ｓ２０６において、トランジスタＴｒ１をオン、トランジスタＴｒ２をオフさせると共に、内蔵カウンタのカウンタ値をクリアする。このとき、各抵抗Ｒ８，Ｒ１１が並列に接続され、電圧ＶＡＦ−は各抵抗Ｒ８，Ｒ９，Ｒ１１の直並列回路により算出される電圧値となり、その電圧値は、図９に示すように、前記電圧Ｖ２０より高い電圧Ｖ２２になる。また、電圧ＶＡＦ−の上昇に伴い、電圧Ｖ１の電圧値も上昇して前記電圧Ｖ１０より高い電圧Ｖ１２になる。
【０１０４】
次に、Ｓ２０７において、Ｓ２０２にて内蔵カウンタのカウンタ値をクリアしてから２００μｓ経過したか否かを判定し、２００μｓ経過した時点でＳ２０６へ移行する。
次に、Ｓ２０８において、トランジスタＴｒ１をオフさせる。
【０１０５】
このとき、Ｓ２０６にて各電圧ＶＡＦ−，Ｖ１はそれぞれ各電圧Ｖ２２，Ｖ１２と高い電圧値にされているため、センサ電流Ｉｐは速やかに元の電流値Ｉ０に復帰する。すなわち、Ｓ２０４にて各電圧ＶＡＦ−，Ｖ１はそれぞれ各電圧Ｖ２１，Ｖ１１と低い電圧値にされているため、Ｓ２０６にて各トランジスタＴｒ１，Ｔｒ２を共にオフさせたとしても、センサ電流Ｉｐは元の電流値Ｉ０にすぐには復帰しない。そこで、Ｓ２０４にて一旦低下させた各電圧ＶＡＦ−，Ｖ１をＳ２０６にて上昇させて逆方向に振ることにより、センサ電流Ｉｐを速やかに元の電流値Ｉ０に復帰させることができる。
【０１０６】
次に、Ｓ２０１へ戻るが、本実施形態ではこのＳ２０１からＳ２０８のルーチンを１２８ｍｓのタイミングで繰り返し行う。
尚、Ｓ２０１からのルーチンを開始するタイミングは、図６に示すＳ１０１からのルーチンを開始するタイミングの中間のタイミングに設定する。すなわち、図６に示すＳ１０１からのルーチンは８ｍｓのタイミングで繰り返されるため、Ｓ２０１からのルーチンを開始するタイミングは、Ｓ１０１からのルーチンを開始してから４ｍｓ後に設定する。これは、Ｓ２０１からのルーチンにより各電圧ＶＡＦ−，Ｖ１が変化してから安定化するまでにある程度の時間を要するためであり、各電圧ＶＡＦ−，Ｖ１が安定化した後にＳ１０１からのルーチンを行うことでセンサ電流Ｉｐを正確に検出することができる。
【０１０７】
ところで、コンデンサＣ１は、図９に示すように、各電圧ＶＡＦ−，Ｖ１の波形の立ち下がりを鈍らせるために設けられている。すなわち、コンデンサＣ１を省いた場合には、各電圧ＶＡＦ−，Ｖ１の波形にオーバーシュートやリンギングが生じ、そのオーバーシュートやリンギングにより各電圧Ｖ２１，Ｖ１１の電圧値が変動するおそれがある。そこで、コンデンサＣ１を設けることにより、各抵抗Ｒ８，Ｒ９，Ｒ１２およびコンデンサＣ１により規定される時定数によって各電圧ＶＡＦ−，Ｖ１の波形の立ち下がりを鈍らせ、オーバーシュートやリンギングの発生を防止して、各電圧Ｖ２１，Ｖ１１を安定化することができる。
【０１０８】
このように、本実施形態の駆動回路４１によれば、検出部１０１の交流的な内部抵抗Ｚを検出し、当該内部抵抗Ｚが一定値になるようにヒータ１０３をオン・オフ制御することにより、ジルコニア固体電解質１０２の温度を常に一定に保ち、印加電圧ＶＲに対するセンサ電流Ｉｐの特性変動を防止することが可能になる。そのため、第３実施形態の駆動回路３１の効果に加えて、マイクロコンピュータ８５の算出する空燃比の精度をさらに高めることができる。
【０１０９】
尚、本実施形態においては、トランジスタＴｒ１をオフ、トランジスタＴｒ２をオンさせて各電圧ＶＡＦ−，Ｖ１を低下させ、リッチ領域の場合と同様に電極１０５から電極１０４へセンサ電流Ｉｐを流すことにより、前記変化量ΔＩ，ΔＶを求めている。しかし、本実施形態とは逆に、トランジスタＴｒ１をオン、トランジスタＴｒ２をオフさせて各電圧ＶＡＦ−，Ｖ１を上昇させ、リーン領域の場合と同様に電極１０４から電極１０５へセンサ電流Ｉｐを流すことにより、前記変化量ΔＩ，ΔＶを求めるようにしてもよい。
【０１１０】
但し、前記のように、センサ電流Ｉｐの検出範囲には制限がある。そのため、リーン領域に対応する空燃比を検出する際に、リーン領域の場合と同様に電極１０５から電極１０４へセンサ電流Ｉｐを流すと、センサ電流Ｉｐが検出範囲を越えるおそれがある。また、リッチ領域に対応する空燃比を検出する際に、リッチ領域の場合と同様に電極１０４から電極１０５へセンサ電流Ｉｐを流すと、センサ電流Ｉｐが検出範囲を越えるおそれがある。
【０１１１】
従って、リーン領域に対応する空燃比を検出する際には、本実施形態のように、リッチ領域の場合と同じ方向にセンサ電流Ｉｐを流すことにより、前記変化量ΔＩ，ΔＶを求めるのが望ましい。また、リッチ領域に対応する空燃比を検出する際には、リーン領域の場合と同じ方向にセンサ電流Ｉｐを流すことにより、前記変化量ΔＩ，ΔＶを求めるのが望ましい。
【０１１２】
ところで、第３実施形態の駆動回路３１において、Ｄ／Ａ変換器３２により電圧ＶＡＦ＋を可変させたときの抵抗Ｒ１の両端の電圧と抵抗Ｒ２の検出部１０１側の電圧を検出することにより、前記変化量ΔＩ，ΔＶを求めるようにしてもよい。
【０１１３】
但し、この場合には、抵抗Ｒ２の検出部１０１側の電圧を検出するＡ／Ｄ変換器を追加する必要があり、コストが増大することになる。
また、この場合、マイクロコンピュータ８５は、Ｄ／Ａ変換器３２により電圧ＶＡＦ＋を可変させたときの抵抗Ｒ２の検出部１０１側の電圧値の変化量ΔＶを予め内蔵ＲＯＭに記憶しておくことにより認知することも可能であり、このようにすれば、抵抗Ｒ２の検出部１０１側の電圧を検出するＡ／Ｄ変換器を追加する必要はない。しかし、Ｄ／Ａ変換器３２の出力に誤差が存在する場合や、オペアンプ８２にオフセット電圧が存在する場合などを考慮すると、前記変化量ΔＩ，ΔＶを正確に求めるには、抵抗Ｒ２の検出部１０１側の電圧をＡ／Ｄ変換器にて検出するのが望ましい。
【０１１４】
尚、検出部１０１の交流的な内部抵抗を検出し、当該内部抵抗が一定値になるようにヒータ１０３を制御する技術については、特開昭５９−２１４７５６号公報に開示されている。しかし、同公報に記載の技術は、酸素センサ（本実施形態の検出部１０１に相当する）の出力側に所定周波数のバイアス電圧を抵抗を介して印加し、酸素センサの合成出力電圧の振幅レベルを検出し、当該振幅レベルに基づいてヒータの作動を制御するというものであり、本実施形態とは全く異なるものである。
【０１１５】
（第５実施形態）
以下、第２実施形態を一部変更した第５実施形態を図面と共に説明する。尚、本実施形態において、第２，第４実施形態と同じ構成部材については符号を等しくしてその詳細な説明を省略する。
【０１１６】
図１０に、本実施形態の駆動回路５１の内部構成を示す。
駆動回路５１においては、第２実施形態の駆動回路２１と同様に、各抵抗Ｒ５〜Ｒ７による抵抗分圧回路を用いて電圧ＶＡＦ＋を生成すると共に、第４実施形態の駆動回路４１と同様に、抵抗Ｒ８，Ｒ９，Ｒ１１〜Ｒ１４、コンデンサＣ１、トランジスタＴｒ１，Ｔｒ２を備えており、空燃比を検出するためにセンサ電流Ｉｐを検出する際には、各トランジスタＴｒ１，Ｔｒ２を共にオフさせ、各抵抗Ｒ８，Ｒ９による抵抗分圧回路を用いて電圧ＶＡＦ−を生成する。そして、駆動回路５１において、検出部１０１の交流的な内部抵抗を検出する際には、第４実施形態と同様に、図８および図９にて説明したように各トランジスタＴｒ１，Ｔｒ２をオン・オフ制御する。
【０１１７】
従って、本実施形態の駆動回路５１によれば、第２実施形態の駆動回路２１の効果に加えて、第４実施形態の駆動回路４１と同様に、ヒータ１０３をオン・オフ制御してジルコニア固体電解質１０２の温度を常に一定に保つことにより、マイクロコンピュータ８５の算出する空燃比の精度をさらに高めることができる。
【０１１８】
（第６実施形態）
以下、第５実施形態を一部変更した第６実施形態を図面と共に説明する。尚、本実施形態において、第５実施形態と同じ構成部材については符号を等しくしてその詳細な説明を省略する。
【０１１９】
図１１に、本実施形態の駆動回路６１の内部構成を示す。
駆動回路６１においては、各抵抗Ｒ５〜Ｒ７による抵抗分圧回路を用いて各電圧ＶＡＦ＋，ＶＡＦ−を生成し、電圧ＶＡＦ−については、オペアンプ６２から構成されるボルテージホロワから抵抗Ｒ１５を介してオペアンプ８３の非反転入力端子に印加している。また、駆動回路６１は、第５実施形態の駆動回路５１と同様に、Ｒ１１〜Ｒ１４、コンデンサＣ１、トランジスタＴｒ１，Ｔｒ２を備えており、検出部１０１の交流的な内部抵抗を検出する際には、図８および図９にて説明したように各トランジスタＴｒ１，Ｔｒ２をオン・オフ制御する。
【０１２０】
ところで、第５実施形態の駆動回路５１において、電圧ＶＡＦ＋については各抵抗Ｒ５〜Ｒ７による抵抗分圧回路を用いてを生成し、電圧ＶＡＦ−については各抵抗Ｒ８，Ｒ９による抵抗分圧回路を用いて生成している。そのため、各電圧ＶＡＦ＋，ＶＡＦ−を正確に設定するには各抵抗Ｒ５〜Ｒ８の抵抗値を正確に設定する必要があり、各抵抗Ｒ５〜Ｒ８の抵抗値に誤差が存在する場合には、各電圧ＶＡＦ＋，ＶＡＦ−を正確に設定することができずセンサ電流Ｉｐの検出精度が低下するという欠点がある。
【０１２１】
それに対して、本実施形態の駆動回路６１においては、各抵抗Ｒ５〜Ｒ７による抵抗分圧回路を用いて各電圧ＶＡＦ＋，ＶＡＦ−を生成しているため、各電圧ＶＡＦ＋，ＶＡＦ−を正確に設定することが可能になり、センサ電流Ｉｐの検出精度を向上させることができる。但し、各トランジスタＴｒ１，Ｔｒ２のオン・オフ制御時に各抵抗Ｒ５〜Ｒ７が影響を及ぼすのを防止するために、オペアンプ６２から構成されるボルテージホロワを設ける必要があり、第５実施形態の駆動回路５１に比べてコストが増大することになる。尚、抵抗Ｒ１５は静電気からオペアンプ８３の非反転入力端子を保護するために設けられている。
【０１２２】
ところで、本発明は上記各実施形態に限定されるものではなく、以下のように変更してもよく、その場合でも、上記各実施形態と同様の作用および効果を得ることができる。
［１］センサ電流Ｉｐに対して各オペアンプ８２，８３の出力電流の供給能力が不足する場合には、各オペアンプ８２，８３の出力端子に、プッシュプル構成のトランジスタによる電流増幅回路を接続し、その電流増幅回路にて各オペアンプ８２，８３の出力電流を必要なレベルまで増幅すればよい。
【０１２３】
［２］検出部１０１のジルコニア固体電解質１０２の形状については、袋管状のものだけでなく、電極１０４に大気を導入できる形状であればどのような形状でもよく、例えば、図１２に示すように平板状にしてもよい。尚、図１２に示す検出部１０１において、図１３に示すものと同じ構成部材については符号を等しくしてある。
【０１２４】
図１２において、平板状のジルコニア固体電解質１０２には、通路１１１および拡散室１１２が設けられている。通路１１１の内壁におけるジルコニア固体電解質１０２には電極１０４が形成され、拡散室１１２の内壁におけるジルコニア固体電解質１０２には電極１０５が形成されている。また、拡散室１１２には外部と連通する１個の孔が設けられており、この孔により拡散抵抗体１０６が構成されている。そして、大気は通路１１１を介して電極１０４へ導入され、排気ガス中の残存酸素や未燃ガスは拡散抵抗体１０６から拡散室１１２を介して電極１０５へ拡散で流入する。また、ジルコニア固体電解質１０２にはアルミナから成る絶縁層１１３が固着され、その絶縁層１１３内には線状のヒータ１０３が配置されている。そして、ヒータ１０３は、絶縁層１１３を介してジルコニア固体電解質１０２を加熱して酸素イオンの導電性を向上させる。
【図面の簡単な説明】
【図１】本発明を具体化した第１実施形態の駆動回路を示す回路図。
【図２】本発明を具体化した第１〜第６実施形態の空燃比検出装置を用いた自動車用内燃機関の制御装置を示すブロック図。
【図３】第１，第２、第５，第６実施形態におけるマイクロコンピュータの処理内容を説明するためのフローチャート。
【図４】第２実施形態の駆動回路を示す回路図。
【図５】第３実施形態の駆動回路を示す回路図。
【図６】第３，第４実施形態におけるマイクロコンピュータの処理内容を説明するためのフローチャート。
【図７】第４実施形態の駆動回路を示す回路図。
【図８】第４，第５，第６実施形態におけるマイクロコンピュータの処理内容を説明するためのフローチャート。
【図９】第４，第５，第６実施形態の動作を説明するためのタイミングチャート。
【図１０】第５実施形態の駆動回路を示す回路図。
【図１１】第６実施形態の駆動回路を示す回路図。
【図１２】検出部の概略構造を示す一部断面図。
【図１３】検出部の概略構造を示す一部断面図。
【図１４】従来の駆動回路を示す回路図。
【図１５】検出部の特性図。
【符号の説明】
１４，２１，３１，４１，５１，６１…駆動回路 ３２…Ｄ／Ａ変換器
８２，８３…オペアンプ ８４…Ａ／Ｄ変換器
８５…マイクロコンピュータ １０１…検出部
１０２…ジルコニア固体電解質 １０３…ヒータ １０４…大気側電極
１０５…排気側電極 １０６…拡散抵抗体 Ｄ１…ダイオード
Ｒ１〜Ｒ１５…抵抗 Ｔｒ１，Ｔｒ２…トランジスタ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an air-fuel ratio detection apparatus.
[0002]
[Prior art]
Conventionally, in order to control the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine to a target value, a technique of providing an air-fuel ratio detection device in an exhaust pipe of the internal combustion engine and performing feedback control of the fuel supply amount according to the detected air-fuel ratio Is widely used.
[0003]
Such an air-fuel ratio detection apparatus includes an air-fuel ratio detection unit (air-fuel ratio sensor), a drive circuit that drives the detection unit, and an A / F for obtaining the air-fuel ratio by processing an output signal of the drive circuit. It is composed of a D converter and a microcomputer, and generally uses a zirconia solid electrolyte as a detection part.
[0004]
FIG. 13 is a partial cross-sectional view illustrating a schematic structure of an example of a detection unit (air-fuel ratio sensor).
The detection unit 101 includes a zirconia solid electrolyte 102, a heater 103, electrodes 104 and 105, and a diffusion resistor 106.
[0005]
The cup-type detection unit 101 is disposed in an exhaust pipe (not shown) of the internal combustion engine.
Air is introduced into the bag-shaped zirconia solid electrolyte 102. The rod-shaped heater 103 disposed inside the zirconia solid electrolyte 102 heats the zirconia solid electrolyte 102 to at least 600 ° C. or more to improve the conductivity of oxygen ions.
[0006]
An air side electrode 104 is formed on the air atmosphere side of the zirconia solid electrolyte 102, and an exhaust side electrode 105 is formed on the exhaust gas atmosphere side. Each of the electrodes 104 and 105 is made of a porous platinum material having a film thickness of several μm to several tens of μm.
The diffusion resistor 106 formed on the surface of the electrode 105 suppresses the inflow of oxygen or carbon monoxide which is an untwisted gas from the exhaust gas atmosphere into the electrode 105 by diffusion. The diffusion resistor 106 is formed to be porous by plasma spraying spinel or the like, and the film thickness is set to several hundred μm in order to increase the diffusion resistivity.
[0007]
FIG. 14 shows a conventional drive circuit 81 for driving the detection unit 101.
The drive circuit 81 includes operational amplifiers 82 and 83 and resistors R1 to R4.
The output terminal of the operational amplifier 82 is connected to the electrode 104 of the detection unit 101 via the resistor R1, and is connected to the inverting input terminal of the operational amplifier 82 via the resistors R1 and R3. The output terminal of the operational amplifier 83 is connected to the electrode 105 of the detection unit 101 via the resistor R2, and is connected to the inverting input terminal of the operational amplifier 83 via the resistors R2 and R4. The atmospheric-side applied voltage VAF + is applied to the non-inverting input terminal of the operational amplifier 82, and the exhaust-side applied voltage VAF− is applied to the non-inverting input terminal of the operational amplifier 83. Here, each of the operational amplifiers 82 and 83 is connected to the plus-side power supply VB and the ground to be supplied with power and perform a single power supply operation. Hereinafter, the resistance values of the resistors R1 to R4 are denoted as “R1” to “R4”, respectively.
[0008]
Since the voltage VAF + is applied from the operational amplifier 82 to the electrode 104 of the detection unit 101 and the voltage VAF− is applied from the operational amplifier 83 to the electrode 105, the voltage difference between the voltages VAF + and VAF− is applied between the electrodes 104 and 105. VR (= VAF + −VAF−) is applied.
[0009]
In the lean region (λ> 1) where the excess air ratio λ is greater than 1, the voltage of the electrode 105 is lower than the voltage of the electrode 104 by VR, so that the residual oxygen in the exhaust gas atmosphere is diffused by this excitation voltage VR. The oxygen ions are converted into oxygen ions at the electrode 105 through 106, and the oxygen ions are transferred to the electrode 104 side by the oxygen pump action through the zirconia solid electrolyte 102, and are oxidized again at the electrode 104 to become oxygen gas and released into the atmosphere. Is done. At this time, a sensor current (pump current) Ip flows from the electrode 104 to the electrode 105 in the direction opposite to the flow of oxygen ions. This sensor current Ip corresponds to the amount of oxygen flowing into the electrode 105 from the exhaust gas atmosphere via the diffusion resistor 106 by diffusion.
[0010]
At the stoichiometric air-fuel ratio (λ = 1) where the excess air ratio λ is 1, the amount of residual oxygen in the exhaust gas flowing into the electrode 105 through the diffusion resistor 106 by diffusion and the residual unburned gas such as carbon monoxide. Is the chemical equivalent ratio, and both are completely burned by the catalytic action of the electrode 105. Therefore, there is no oxygen near the electrode 105, and no oxygen ions are transferred through the zirconia solid electrolyte 102 even when a voltage is excited between the electrodes 104 and 105. Therefore, the sensor current Ip flowing between the electrodes 104 and 105 becomes zero (Ip = 0).
[0011]
In the rich region where the excess air ratio λ is less than 1 (λ <1), contrary to the lean region, oxygen ions flow from the electrode 104 to the electrode 105, and this oxygen ion flow is caused by the oxygen concentration in the vicinity of the electrode 105. And is oxidized again at the electrode 105 to become oxygen gas. This oxygen gas is diffused into the electrode 105 from the exhaust gas atmosphere via the diffusion resistor 106 and unburned, such as carbon monoxide. Burn the gas. Therefore, the amount of oxygen ions transferred from the electrode 104 side to the electrode 105 side in the zirconia solid electrolyte 102 becomes a value corresponding to the amount of unburned gas flowing into the electrode 105 by diffusion. At this time, the sensor current Ip flows from the electrode 105 to the electrode 104 in the direction opposite to the flow of oxygen ions.
[0012]
Both ends of the resistor R1 are connected to the A / D converter 84, respectively. The A / D converter 84 performs A / D conversion on the voltage V1 on the operational amplifier 82 side and the voltage V2 on the detection unit 101 side in the resistor R1 under the control of the microcomputer 85, and converts the A / D conversion value into the microcomputer. Output to 85. Here, the A / D converter 84 is connected to the plus-side power source VC and the ground and is supplied with power. Hereinafter, the voltages of the power supplies VB and VC are denoted as “VB” and “VC”, respectively.
[0013]
The microcomputer 85 has a known configuration including a CPU, a ROM, a RAM, and an I / O circuit. As shown in the equation (1), the microcomputer 85 has voltages V1 and V2 across the resistor R1 and a resistance value R1 of the resistor R1. Based on this, the sensor current Ip flowing between the electrodes 104 and 105 of the detection unit 101 is calculated.
[0014]
Ip = (V1-V2) / R1 (Equation 1)
The microcomputer 85 calculates the air-fuel ratio based on the sensor current Ip, and feedback-controls the fuel supply amount to the internal combustion engine in accordance with the calculated air-fuel ratio, so that the air-fuel mixture supplied to the internal combustion engine is empty. Control the fuel ratio to the target value.
[0015]
By the way, in the driving circuit 81, when the non-inverting input terminal of the operational amplifier 83 is short-circuited with the power source VB, the resistor R2 causes the excessive current to flow into the non-inverting input terminal of the operational amplifier 83 or flow out from the output terminal. It is provided to prevent destruction and protect the output terminal of the operational amplifier 83 from static electricity, and its resistance value is set to about 47Ω.
[0016]
Since the operational amplifier 82 is provided with the resistor R1, by setting the resistance value to 47Ω or more, if the non-inverting input terminal of the operational amplifier 82 is short-circuited with the power supply VB, an excessive current is not applied to the operational amplifier 82. It is possible to prevent the operational amplifier 82 from being broken by flowing into the inverting input terminal or from the output terminal, and to protect the output terminal of the operational amplifier 82 from static electricity.
[0017]
Further, the resistors R3 and R4 are provided to protect the output terminals of the operational amplifiers 82 and 83 from static electricity, and the resistance value is set to about 1 kΩ.
Here, when the voltage VAF + applied to the non-inverting input terminal of the operational amplifier 82 is fixed, the voltage V2 becomes equal to the voltage VAF +. Therefore, the microcomputer 85 stores the value of the voltage V2 in the built-in ROM in advance. The A / D converter 84 does not need to A / D convert the voltage V2. However, in order to accurately detect the sensor current Ip even when the voltage V2 is not equal to the voltage VAF +, such as when an offset voltage exists in the operational amplifier 82, the A / D converter 84 converts the voltage V2 to A / D as described above. It is desirable to perform D conversion.
[0018]
By the way, Japanese Patent Laid-Open No. 61-180131 discloses a circuit similar to the drive circuit 81 shown in FIG. 3 is different from the drive circuit 81 in that the resistors R2 to R4 are omitted and short-circuited, and the output terminal of the operational amplifier 83 and the non-inverting input terminal of the operational amplifier 82 are different. The only difference is that a voltage source for setting the excitation voltage VR is connected between them. The operation of each of the resistors R2 to R4 is as described above, and even when the resistors R2 to R4 are omitted and shorted, the basic operation of the drive circuit 81 is not affected. In the driving circuit 81, the voltages VAF + and VAF− are applied to the non-inverting input terminals of the operational amplifiers 82 and 83, and the difference voltage VR between the voltages VAF + and VAF− becomes an excitation voltage. It functions in the same way as the voltage source. Therefore, the circuit described in the publication performs basically the same operation as the drive circuit 81.
[0019]
[Problems to be solved by the invention]
In recent years, in order to protect the environment, reduction of carbon dioxide in exhaust gas is required, and further reduction in fuel consumption is required. Therefore, in addition to the stoichiometric control for controlling the operation of the internal combustion engine at the stoichiometric air-fuel ratio (λ = 1), it is essential to perform the lean burn control for controlling the operation in the lean region (λ> 1). That is, it is required to perform precise control by switching between stoichiometric control and lean burn control according to the operating state of the internal combustion engine. However, the conventional drive circuit 81 shown in FIG. 14 has a problem that the detection accuracy is lowered when detecting a high range air-fuel ratio (for example, 25 or more) in order to perform lean burn control.
[0020]
As described above, the sensor current Ip flows from the electrode 104 to the electrode 105 in the lean region. At this time, the lowest voltage that can be output from the operational amplifier 83 is VCE. Further, it is assumed that the voltages VAF + and VAF− applied to the non-inverting input terminals of the operational amplifiers 82 and 83 are both variable. Then, the voltage VAF− is expressed by Expression (2).
[0021]
VAF− ≧ VCE + Ip · R2 (Equation 2)
If there is no offset voltage in the operational amplifier 82, the voltage V2 is equal to the voltage VAF +, and the voltage VR applied between the electrodes 104 and 105 of the detection unit 101 is a difference voltage between the voltages VAF + and VAF−. V2 is represented by Formula (3).
[0022]
V2 = VAF + = VAF− + VR (Equation 3)
Moreover, the voltage V1 is represented by Formula (4).
V1 = V2 + Ip · R1 (Equation 4)
FIG. 15 shows a characteristic example of the applied voltage VR and the sensor current Ip in the detection unit 101 when the zirconia solid electrolyte 102 is heated to 700 ° C. by the heater 103.
[0023]
In this characteristic example, the sensor current Ip when the air-fuel ratio (A / F) is 40 is 23 mA.
Here, when Ip = 23 mA, VCE = 1.5 V, VR = 0.9 V, R1 = 100Ω, and R2 = 47Ω are substituted into the equations (2) to (4), the values of the voltages V2 and V1 are expressed by the equation (5). ) As shown in (6).
[0024]
V2 ≧ 3.481V (Equation 5)
V1 ≧ 5.781V (Equation 6)
Since the A / D converter 84 is connected to the plus-side power source VC and the ground and is supplied with power, if the power source voltage VC is 5V, the maximum value of the A / D conversion value is 5V. Therefore, the voltage V2 can be A / D converted by the A / D converter 84, but the voltage V1 cannot be A / D converted by the A / D converter 84. Therefore, when the air-fuel ratio is 40, the microcomputer 85 cannot calculate the air-fuel ratio.
[0025]
By the way, the maximum detectable value Ip (max) of the sensor current Ip is expressed by Expression (7) obtained from Expressions (1) to (3).

In the characteristic detection unit 101 shown in FIG. 15, the sensor current Ip when the air-fuel ratio is 30 is 19 mA. Therefore, when the air-fuel ratio is 30, the microcomputer 85 cannot calculate the air-fuel ratio.
[0026]
Thus, since the conventional drive circuit 81 cannot detect the air-fuel ratio in the high region, it is impossible to perform the lean burn control in the air-fuel ratio in the high region.
By the way, if the resistance R1 is made smaller than 100Ω, for example, 56Ω, the voltage V1 can be set to 4.769V from the equation (4), so that the A / D converter 84 can perform A / D conversion. Thus, from equation (7), Ip (max) can be set to 25.24 mA. Therefore, even the detection unit 101 having the characteristics shown in FIG. However, if the resistance value of the resistor R1 is reduced, the potential difference between the voltages V1 and V2 is reduced, so that the conversion accuracy of the A / D converter 84 is reduced and the accuracy of the air-fuel ratio calculated by the microcomputer 85 is also reduced. become. Therefore, if the resistance value of the resistor R1 is reduced, the lean burn control at a high air-fuel ratio cannot be accurately controlled.
[0027]
The present invention has been made to solve the above problems, and an object of the present invention is to provide an air-fuel ratio detection device capable of accurately detecting the air-fuel ratio in each state of the rich region, the theoretical air-fuel ratio, and the lean region. There is to do.
[0028]
[Means for Solving the Problems]
The invention according to claim 1, which has been made to achieve the above object, is formed on the zirconia solid electrolyte, the atmosphere-side electrode formed on the atmosphere side of the zirconia solid electrolyte, and the exhaust atmosphere side of the zirconia solid electrolyte. A detection unit is provided that includes the exhaust-side electrode and a diffusion resistor that suppresses the gas flowing from the exhaust atmosphere to the exhaust-side electrode by diffusion. And the drive circuit part which applies a voltage with respect to the atmosphere side electrode and exhaust side electrode of this detection part is provided, respectively. Further, a detection circuit unit is provided for detecting a current flowing in or flowing from the exhaust side electrode of the detection unit and detecting an air-fuel ratio based on the current value.
[0029]
Therefore, according to the present invention, the current flowing out from or flowing into the exhaust side electrode, not the atmosphere side electrode of the detection unit, is detected, and the air-fuel ratio is detected based on the current value. It becomes possible to detect accurately, and the detection range of the air-fuel ratio can be expanded from the rich region and the theoretical air-fuel ratio to the lean region.
[0030]
In particular, The drive circuit unit is ,Previous An atmospheric-side voltage generating means for generating an atmospheric-side applied voltage applied to the atmospheric-side electrode of the detector; an exhaust-side voltage generating means for generating an exhaust-side applied voltage applied to the exhaust-side electrode of the detecting section; A current detection resistor connected between the exhaust side electrode of the detection unit and the exhaust side voltage generating means; In addition, the detection circuit unit includes a voltage detection unit that detects at least the voltage on the exhaust side voltage generation unit side of the voltage across the current detection resistor of the drive circuit unit, and a voltage detected by the voltage detection unit Current detection means for detecting a current flowing from the current detection resistor to the current detection resistor; Flow inspection Air-fuel ratio calculating means for calculating the air-fuel ratio based on the current detected by the output means.
[0031]
Therefore, Book According to the invention, since it becomes possible to detect the air-fuel ratio in a high region without reducing the resistance value of the current detection resistor, when the voltage detection means is embodied by an A / D converter, There is no possibility that the A / D conversion accuracy is lowered, and the accuracy of the air-fuel ratio calculated by the air-fuel ratio calculating means can be increased.
[0032]
Next, the claim 2 The invention described in claim 1 In the air-fuel ratio detection device described in 1), the atmospheric-side applied voltage generated by the atmospheric-side voltage generating means and the exhaust-side applied voltage generated by the exhaust-side voltage generating means are both fixed.
Therefore, the claims 2 According to the invention described in (1), the air-fuel ratio in the range necessary for stoichiometric control can be detected with high accuracy.
[0033]
By the way, claims 2 The drive circuit unit in the air-fuel ratio detection device according to claim 1, 3 As in the invention described in (1), a resistance voltage dividing circuit comprising a plurality of resistors connected in series between a voltage source and a ground is provided. And the said atmospheric | air side voltage generation means produces | generates the atmospheric side applied voltage with the voltage by which resistance division was carried out by the said resistance voltage dividing circuit. The exhaust-side voltage generation means generates an exhaust-side applied voltage based on a voltage that is resistance-divided by the resistance voltage dividing circuit.
[0034]
Therefore, the claims 3 According to the invention described in (1), it is possible to generate both the atmospheric-side applied voltage and the exhaust-side applied voltage with a voltage obtained by a simple resistance voltage dividing circuit composed of only a resistor.
Next, the claim 4 The invention described in claim 1 The air-fuel ratio detection device according to claim 1, wherein the drive circuit unit is configured to generate an atmospheric-side application generated by the atmospheric-side voltage generation unit based on a voltage applied between an atmospheric-side electrode and an exhaust-side electrode of the detection unit. By controlling the voltage, the current flowing out from or flowing into the exhaust side electrode of the detection unit is saturated. And the air-fuel ratio calculation means Detects an air-fuel ratio based on the saturated current value.
[0035]
Therefore, according to the present invention, it is possible to change the voltage applied between the atmosphere-side electrode and the exhaust-side electrode of the detection unit according to the air-fuel ratio, and detect a wide range of air-fuel ratio in the lean region. be able to.
By the way, claims 4 The drive circuit unit in the air-fuel ratio detection device according to claim 1, 5 The voltage applied between the resistance voltage dividing circuit composed of a plurality of resistors connected in series between the voltage source and the ground, and the atmosphere side electrode and the exhaust side electrode of the detection unit And a variable voltage generation circuit for generating a variable voltage based on the above. And the said atmospheric | air side voltage generation means produces | generates an atmospheric | air side applied voltage with the variable voltage which the said variable voltage generation circuit produces | generates. The exhaust-side voltage generation means generates an exhaust-side applied voltage based on a voltage that is resistance-divided by the resistance voltage dividing circuit.
[0036]
Therefore, the claims 5 According to the invention described in the above, the voltage applied between the atmosphere side electrode and the exhaust side electrode of the detection unit according to the air-fuel ratio is changed by fixing the exhaust side applied voltage and varying the atmosphere side applied voltage. It is possible to change the air-fuel ratio in a wide range in the lean region.
[0037]
Claims 5 The air-fuel ratio detection device according to claim 1 6 As described in the invention described above, the voltage applied between the atmosphere-side electrode and the exhaust-side electrode of the detection unit can be reduced by suppressing the atmosphere-side applied voltage generated by the atmosphere-side voltage generating means to a certain value or less. Protecting means for suppressing and protecting the detection unit is provided.
[0038]
Therefore, the claims 6 According to the invention described in (1), it is possible to prevent an abnormal voltage from being applied between the electrodes of the detection unit, thereby preventing the detection unit from being deteriorated or broken.
Next, the claim 7 The invention described in claim 1 The air-fuel ratio detection device according to claim 1, wherein the drive circuit unit is configured to generate an atmospheric-side application generated by the atmospheric-side voltage generation unit based on a voltage applied between an atmospheric-side electrode and an exhaust-side electrode of the detection unit. By varying both the voltage and the exhaust-side applied voltage generated by the exhaust-side voltage generating means, the current flowing out from or flowing into the exhaust-side electrode of the detection unit is saturated. And the air-fuel ratio calculation means Detects an air-fuel ratio based on the saturated current value.
[0039]
Therefore, according to the present invention, by changing both the exhaust side applied voltage and the atmospheric side applied voltage, the voltage applied between the atmosphere side electrode and the exhaust side electrode of the detector is changed according to the air-fuel ratio. This makes it possible to detect a wide range of air-fuel ratios in the lean region.
[0040]
Next, the claim 8 The invention described in claim 1 7 In the air-fuel ratio detection device according to any one of the above, the detection unit detects an AC internal resistance between the atmosphere-side electrode and the exhaust-side electrode of the detection unit and makes the internal resistance constant. The temperature of the zirconia solid electrolyte is controlled to be constant.
[0041]
Therefore, according to the present invention, it is possible to keep the temperature of the zirconia solid electrolyte constant, and to prevent fluctuations in the characteristics of the current that flows out or flows in from the exhaust side electrode with respect to the voltage applied between the electrodes of the detection unit. As a result, the air-fuel ratio detection accuracy can be further improved.
[0042]
By the way, claims 8 The air-fuel ratio detection device according to claim 1 9 As described in the invention, a heater for heating the zirconia solid electrolyte of the detection unit is provided, and the operation of the heater is controlled so that the internal resistance becomes constant.
Claims 9 The air-fuel ratio detecting device according to claim 1 0 The exhaust side applied voltage generated by the exhaust side voltage generating means is changed, and the amount of change in the voltage and the exhaust side electrode of the detection unit caused by the change in voltage are The internal resistance is detected based on the amount of change in the flowing current.
[0043]
Claims 9 The air-fuel ratio detecting device according to claim 1 1 The atmospheric side applied voltage generated by the atmospheric voltage generating means is changed, and the amount of change in the voltage flows out from the exhaust side electrode of the detection unit caused by the change in voltage or The internal resistance is detected based on the amount of change in the flowing current.
[0044]
In the embodiments of the invention described below, the “detection circuit unit” described in the claims or means for solving the problems is composed of an A / D converter 84 and a microcomputer 85, The “side voltage generating means” is composed of the operational amplifier 82, the “exhaust side voltage generating means” is composed of the operational amplifier 83, the “voltage detecting means” is composed of the A / D converter 84, and the “current detecting means” is also the same. Corresponds to the process of S102 in the microcomputer 85, the “air-fuel ratio calculating means” corresponds to the process of S103 in the microcomputer 85, the “voltage source” corresponds to the power source VC, and the “variable voltage generation circuit”. Is composed of a D / A converter 32, and the “protection means” is composed of a diode D1.
[0045]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments embodying the present invention will be described with reference to the drawings. In each embodiment, the same components as those of the conventional embodiment shown in FIGS. 13 to 15 are denoted by the same reference numerals, and detailed description thereof is omitted.
[0046]
(First embodiment)
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings.
FIG. 2 shows an overall configuration of a control device for an internal combustion engine for automobiles using the air-fuel ratio detection device of the present embodiment.
[0047]
The control device 11 includes an input buffer 12, an output buffer 13, a drive circuit 14, an A / D converter 84, and a microcomputer 85.
A rotation speed detection signal NE from a rotation sensor for detecting the rotation speed of the internal combustion engine, a vehicle speed detection signal SPD from a vehicle speed sensor for detecting the vehicle speed of the automobile, a starter switch operation detection signal STA corresponding to whether or not the starter switch has been operated. The digital signal is subjected to noise removal and waveform shaping in the input buffer 12, converted into a 0V or 5V signal, and input to the microcomputer 85.
[0048]
Also, an intake air amount detection signal VG from an air amount sensor for detecting the intake air amount of the internal combustion engine, a throttle opening detection signal VTA from a throttle opening sensor for detecting the throttle opening, and a cooling water temperature of the internal combustion engine are detected. Analog signals such as a water temperature detection signal THW from the cooling water temperature sensor are A / D converted by the A / D converter 84 and input to the microcomputer 85.
[0049]
The drive circuit 14 drives the detection unit 101, and the voltages V 1 and V 2 output from the drive circuit 14 are A / D converted by the A / D converter 84 and input to the microcomputer 85.
The microcomputer 85 controls the fuel injection amount control signal INJ supplied from the injector to the internal combustion engine, the ignition timing control signal IGT, idle rotation based on various signals input from the input buffer 12 and the A / D converter 84. A control signal ISC for the control valve, a control signal HT for controlling the heater 103 of the detection unit 101, and the like are generated. These control signals generated by the microcomputer 85 are output to the outside of the control device 11 via the output buffer 13.
[0050]
FIG. 1 shows the internal configuration of the drive circuit 14.
The drive circuit 14 of the present embodiment differs from the conventional drive circuit 81 in the following points.
(1-1) The output terminal of the operational amplifier 82 is connected to the electrode 104 (atmosphere side) of the detection unit 101 through the resistor R2, and is connected to the inverting input terminal of the operational amplifier 82 through the resistors R2 and R3. Has been.
[0051]
(1-2) The output terminal of the operational amplifier 83 is connected to the electrode 105 (exhaust gas atmosphere side) of the detection unit 101 through the resistor R1, and is connected to the inverting input terminal of the operational amplifier 83 through the resistors R1 and R4. Has been.
(1-3) The A / D converter 84 performs A / D conversion on the voltage V1 on the operational amplifier 83 side and the voltage V2 on the detection unit 101 side in the resistor R1, and sends the A / D conversion value to the microcomputer 85. Output.
[0052]
Next, the operation of the drive circuit 14 of this embodiment will be described.
As described above, the sensor current Ip flows from the electrode 104 to the electrode 105 in the lean region. At this time, the lowest voltage that can be output from the operational amplifier 83 is VCE. Further, it is assumed that the voltages VAF + and VAF− applied to the non-inverting input terminals of the operational amplifiers 82 and 83 are both variable. Then, the voltage V1 is expressed by Expression (8).
[0053]
V1 ≧ VCE (Equation 8)
If there is no offset voltage in the operational amplifier 83, the voltage V2 is equal to the voltage VAF−, and the voltage V2 is expressed by Expression (9).
V2 = VAF− = V1 + Ip · R1 (Equation 9)
Since the voltage VR applied between the electrodes 104 and 105 of the detection unit 101 is a difference voltage between the voltages VAF + and VAF−, the voltage VAF + is expressed by Expression (10).
[0054]
VAF + = VAF− + VR (Equation 10)
The output voltage VOP of the operational amplifier 82 is expressed by Expression (11).
VOP = VAF ++ Ip · R2 (Equation 11)
As in the case of the drive circuit 81 described above, when Ip = 23 mA, VCE = 1.5 V, VR = 0.9 V, R1 = 100Ω, R2 = 47Ω and substituting into the equations (8) to (11), each voltage V2 , V1, VAF +, and VOP are as shown in equations (12) to (15).
[0055]

Further, the maximum detectable value Ip (max) of the sensor current Ip is expressed by Expression (16) obtained from Expressions (8) to (11).
[0056]
Ip (max) = (VC−VR−VCE) / R1 = 26 mA (Equation 16)
Accordingly, by setting the voltages VAF + and VAF− so as to satisfy the expressions (13) and (14), both the voltages V1 and V2 can be set to 5 V or less. Since the A / D converter 84 is connected to the plus-side power source VC and the ground and is supplied with power, if the power source voltage VC is 5V, the maximum value of the A / D conversion value is 5V. Therefore, from the expressions (12) and (13), each of the voltages V1 and V2 can be A / D converted by the A / D converter 84.
[0057]
In the characteristic detection unit 101 shown in FIG. 15, the sensor current Ip when the air-fuel ratio is 40 is 23 mA. Therefore, from the equation (16), even when the air-fuel ratio is 40, the microcomputer 85 can calculate the air-fuel ratio.
From the equation (15), although the output voltage VOP of the operational amplifier 82 exceeds 5V, if the power source VB of each of the operational amplifiers 82 and 83 is supplied from the vehicle battery, the voltage of the vehicle battery is 14V. The maximum value of the output voltage VOP of the operational amplifier 82 is 14V, which is not a problem.
[0058]
Next, details of processing executed by the microcomputer 85 in the present embodiment will be described with reference to the flowchart shown in FIG.
When the microcomputer 85 starts when the ignition switch is turned on and power is supplied from the vehicle-mounted battery, the following steps are executed by various calculation processes by the computer according to the control program recorded in the built-in ROM. .
[0059]
First, in step (hereinafter referred to as S) 101, the A / D converter 84 is caused to execute A / D conversion of the voltages V1 and V2 across the resistor R1, and the A / D conversion value is received.
Next, in S102, based on the voltages V1 and V2 output from the drive circuit 14 and the resistance value of the resistor R1, as shown in the equation (1), between the electrodes 104 and 105 of the detection unit 101, as shown in FIG. The flowing sensor current Ip is calculated.
[0060]
Next, in S103, the air-fuel ratio (A / F) corresponding to the sensor current Ip is calculated by table interpolation using a data table recorded in advance in the built-in ROM. Here, since the sensor current Ip and the air-fuel ratio do not have a completely linear linear relationship, it is necessary to determine the air-fuel ratio corresponding to some sensor current Ip in advance and create a data table.
[0061]
Next, the process returns to S101, but in this embodiment, the routine from S101 to S103 is repeated at a timing of 8 ms.
The microcomputer 85 controls the air-fuel ratio of the air-fuel mixture supplied to the internal combustion engine to the target value by feedback-controlling the fuel injection amount according to the calculated air-fuel ratio and generating the fuel injection amount control signal INJ. To do.
[0062]
Thus, according to the drive circuit 14 of the present embodiment, it becomes possible to detect the air-fuel ratio in the high region, and therefore the control device 11 of the internal combustion engine can perform lean burn control in the air-fuel ratio in the high region. it can. Since it is not necessary to reduce the resistance value of the resistor R1, there is no possibility that the conversion accuracy of the A / D converter 84 will decrease even when detecting an air-fuel ratio in a high region, and the sky calculated by the microcomputer 85 will be reduced. The accuracy of the fuel ratio can be increased. Therefore, the air-fuel ratio detection region can be expanded from the rich region and the theoretical air-fuel ratio to the lean region without reducing the detection accuracy.
[0063]
Here, when the voltage VAF− applied to the non-inverting input terminal of the operational amplifier 83 is fixed, the voltage V2 becomes equal to the voltage VAF−. Therefore, the microcomputer 85 stores the value of the voltage V2 in the built-in ROM in advance. It is not necessary to A / D convert the voltage V2 in the A / D converter 84. However, in order to accurately detect the sensor current Ip even when the voltage V2 is not equal to the voltage VAF−, such as when an offset voltage is present in the operational amplifier 83, the A / D converter 84 converts the voltage V2 to A as described above. / D conversion is desirable. However, if the condition that the voltage V2 is equal to the voltage VAF− is satisfied, the A / D conversion of the voltage V2 in the A / D converter 84 can be omitted.
[0064]
(Second Embodiment)
Hereinafter, a second embodiment that is a more specific embodiment of the first embodiment will be described with reference to the drawings. In the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.
[0065]
FIG. 4 shows the internal configuration of the drive circuit 21 of the present embodiment.
The drive circuit 21 differs from the drive circuit 14 of the first embodiment shown in FIG. 1 in that the voltages VAF + and VAF− are generated by resistance voltage division by the resistors R5 to R7 connected in series between the power supply VC and the ground. This is the point that generates
[0066]
Next, the operation of the drive circuit 21 of this embodiment will be described.
This embodiment is applied when only lean control is performed without performing lean burn control. During stoichiometric control, for example, the air-fuel ratio (A / F) is substantially controlled between 13 and 18, so that the detection unit 101 having the characteristics shown in FIG. 15 has a sensor current Ip within a range of −10 mA to 10 mA (− 10 mA ≦ Ip ≦ 10 mA) is sufficient. Further, as indicated by a dotted line (a) in FIG. 15, the applied voltage VR may be set to a constant value (= 0.4V). The sensor current Ip takes a positive value when the sensor current Ip flows from the electrode 104 to the electrode 105 in the lean region, and the sensor current Ip becomes negative when the sensor current Ip flows from the electrode 105 to the electrode 104 in the rich region. Takes a value.
[0067]
Since the range of the sensor current Ip is -10 mA or more and 10 mA or less, the voltage VAF− is set so that the absolute value IpR of the sensor current in the rich region is equal to the absolute value IpL of the sensor current in the lean region. Here, assuming that the maximum value of the voltage V1 is V1 (max), the maximum value IpR (max) of the sensor current IpR is expressed by Expression (17). Further, the maximum value IpL (max) of the sensor current IpL is expressed by Expression (18).
[0068]

Here, when the power supply voltage VC is 5V, the maximum value V1 (max) is 5V. Therefore, when IpR (max) = IpL (max), V1 (max) = 5V, VCE = 1.5V, and R1 = 100Ω are substituted into the equations (17) and (18), the voltages VAF− and VAF + are expressed by the equation (19). ) As shown in (20). Therefore, IpR (max) and IpL (max) are as shown in Expression (21).
[0069]

Accordingly, by setting the resistance values of the resistors R5 to R7 so as to satisfy the expressions (19) and (20), the stoichiometric control can be performed using the detection unit 101 having the characteristics shown in FIG.
[0070]
In addition, since the process which the microcomputer 85 performs in this embodiment is the same as 1st Embodiment, description is abbreviate | omitted.
As described above, according to the drive circuit 21 of the present embodiment, the voltages VAF + and VAF− are generated using a simple resistance voltage dividing circuit including the resistors R5 to R7, so that an empty space in the range necessary for the stoichiometric control is obtained. The fuel ratio can be detected with high accuracy.
[0071]
Further, the drive circuit 21 of the present embodiment can be created simply by connecting the resistance voltage dividing circuit including the resistors R5 to R7 to the drive circuit 14 of the first embodiment. Therefore, the printed circuit board on which the operational amplifiers 82 and 83 and the resistors R1 to R4 are mounted in the drive circuit 14 can be used for the drive circuit 21, and the drive circuit 14 and 21 can share the printed circuit board. it can. Since the processing executed by the microcomputer 85 in the present embodiment is the same as that in the first embodiment, the control program for the microcomputer 85 can be shared between the present embodiment and the first embodiment. Therefore, according to the present embodiment, since the printed circuit board and the control program (software) can be shared with the first embodiment, the cost can be reduced.
[0072]
(Third embodiment)
Hereinafter, a third embodiment that is a more specific embodiment of the first embodiment will be described with reference to the drawings. In the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.
[0073]
FIG. 5 shows an internal configuration of the drive circuit 31 of the present embodiment.
The drive circuit 31 differs from the drive circuit 14 of the first embodiment shown in FIG. 1 in the following points.
(3-1) The driving circuit 31 includes resistors R8 to R10, a diode D1, and a D / A converter 32 in addition to the operational amplifiers 82 and 83 and the resistors R1 to R4.
[0074]
(3-2) The D / A converter 32 generates a voltage VAF + as a D / A conversion value under the control of the microcomputer 85. Here, the D / A converter 32 is connected to the plus-side power source VC and the ground and is supplied with power.
(3-3) The voltage VAF + is applied to the non-inverting input terminal of the operational amplifier 82 from the D / A converter 32 via the resistor R10. The resistor R10 is provided to protect the non-inverting input terminal of the operational amplifier 82 from static electricity.
[0075]
(3-4) The non-inverting input terminal of the operational amplifier 83 is applied with a voltage VAF− generated by resistance division by the resistors R8 and R9 connected in series between the power supply VC and the ground. The non-inverting input terminal of the operational amplifier 83 is connected to the cathode of the diode D 1, and the anode of the diode D 1 is connected to the non-inverting input terminal of the operational amplifier 82.
[0076]
Next, the operation of the drive circuit 31 of this embodiment will be described.
When the detection unit 101 having the characteristics shown in FIG. 15 is used, if the sensor current Ip corresponding to a wide range of air-fuel ratios (A / F) from 12 to 40 is detected by the saturation current value, the dotted line ( As shown in b), the applied voltage VR must be changed according to the air-fuel ratio. Therefore, in the present embodiment, the voltage VAF− is fixed by resistance division by the resistors R8 and R9, and the voltage VAF + is varied by the D / A converter 32, whereby the applied voltage VR is changed according to the air-fuel ratio. I try to change it.
[0077]
In the detection unit 101 having the characteristics shown in FIG. 15, when the sensor current Ip corresponding to the air-fuel ratio in the range of 12 to 40 is detected with the saturation current value, the sensor current Ip is in the range of −10 mA to 25 mA (−10 mA ≦ It is necessary to detect at Ip ≦ 25 mA).
[0078]
Equation (22) is obtained from Equations (17) and (18).
IpR (max) + IpL (max) = (V1 (max) −VCE) / R1 (22)
Here, when IpR (max) = 10 mA, IpL (max) = 25 mA, V1 (max) = 5V, and VCE = 1.5V are substituted into the equation (22), the resistor R1 becomes as shown in the equation (23). .
[0079]

Thus, when IpR (max) = 10 mA, IpL (max) = 25 mA, V1 (max) = 5V, VCE = 1.5V, R1 = 100Ω and substitution into equations (17) and (18), the voltage VAF− 24).
[0080]
VAF− = 4.0V (24)
However, it is necessary to confirm whether or not the necessary applied voltage VR can be obtained by the voltage VAF− obtained by the equation (24). Since the D / A converter 32 is connected to the plus-side power source VC and the ground and is supplied with power, if the power source voltage VC is 5V, the maximum value of the D / A conversion value is 5V. Here, when the maximum value VR (max) of the applied voltage VR is 0.9V from the dotted line (b) in FIG. 15, the voltage VAF− is 4.0V from the equation (24). The maximum value of the voltage VAF + is 4.9 V, which is less than 5 V, which is the maximum value of the output of the D / A converter 32, and there is no problem.
[0081]
However, when the necessary applied voltage VR cannot be obtained by the voltage VAF− obtained by the equation (24), the resistor R1 and the voltage VAF− are reset. That is, the resistance R1 is obtained by substituting the values VC, VR (max), VCE, IpL (max) into the equation (25) obtained from the equation (16).
[0082]
R1 = (VC−VR (max) −VCE) / IpL (max) (25)
Further, when the power supply voltage VC is 5V, the maximum value of the output of the D / A converter 32 (that is, the maximum value of the voltage VAF +) is 5V. Therefore, VAF + = 5V and the value of the required applied voltage VR are expressed by the above equation. Substituting into (10), the voltage VAF− is obtained.
[0083]
Accordingly, by setting the resistance values of the resistors R8 and R9 so that the obtained voltage VAF− is obtained, the air-fuel ratio in the range of 12 to 40 is accommodated by using the detection unit 101 having the characteristics shown in FIG. The sensor current Ip can be detected by the saturation current value. Next, details of processing executed by the microcomputer 85 in the present embodiment will be described with reference to a flowchart shown in FIG.
[0084]
Each step of S101 to S103 shown in FIG. 6 is the same as that shown in FIG.
In S104 following S103, the sensor current Ip calculated in S102 based on the characteristic data (dotted line (b) in FIG. 15) of the applied voltage VR and the sensor current Ip in the detection unit 101 recorded in advance in the built-in ROM. The applied voltage VR corresponding to is calculated.
[0085]
Next, in S105, the calculated applied voltage VR and the voltage VAF − (= V2) are substituted into the equation (10) to calculate the voltage VAF +, and the voltage VAF + is generated from the D / A converter 32.
Next, the process returns to S101. In this embodiment, the routine from S101 to S105 is repeated at a timing of 8 ms.
[0086]
As described above, according to the drive circuit 31 of the present embodiment, the fixed voltage VAF− is generated using a simple resistance voltage dividing circuit including the resistors R8 and R9, and the voltage VAF + is generated by the D / A converter 32. By making it variable, the applied voltage VR can be changed according to the air-fuel ratio, and a wide range of air-fuel ratio in the lean region can be detected.
[0087]
Note that a D / A converter that generates the voltage VAF− may be added, and the applied voltage VR may be varied by varying both the voltages VAF + and VAF−. In this case, the D / A converter may be Two are required, which increases the cost.
Incidentally, the diode D <b> 1 is provided to protect the detection unit 101. That is, if the detection unit 101 is used in a region where the sensor current Ip does not reach the saturation current value, the detection unit 101 is deteriorated or destroyed. Therefore, when the detection unit 101 having the characteristics shown in FIG. 15 is used, the applied voltage VR needs to be within ± 1V. However, when the D / A converter 32 or the microcomputer 85 performs an abnormal operation or when an abnormality occurs in the wiring connecting the D / A converter 32 and the microcomputer 85, it is abnormally high. Or a low voltage VAF + may be generated, and the applied voltage VR may exceed ± 1V.
[0088]
For example, when the generated voltage VAF + is lower than the voltage VAF− (VAF + <VAF−), the voltage VAF + is expressed by Expression (25).
VAF + = VAF−−VF (Equation 25)
Therefore, the applied voltage VR (= VAF + −VAF−) is equal to the threshold voltage VF, and normally the threshold voltage VF is 1 V or less, so that the applied voltage VR can be kept within ± 1 V. Therefore, by clamping between the non-inverting input terminals of the operational amplifiers 82 and 83 by the diode D1, when the voltage VAF + is lower than the voltage VAF−, the applied voltage VR becomes abnormally high or low, thereby detecting the detection unit. It is possible to avoid the deterioration and destruction of the 101.
[0089]
When the generated voltage VAF + is abnormally high, the maximum value VR (max) of the applied voltage VR that does not cause deterioration or destruction of the detection unit 101 is set to 1V, the power supply voltage VC is set to 5V, and the D / A converter 32 Assuming that the maximum output value (that is, the maximum value of the voltage VAF +) is 5V, the maximum value VR (max) can be reduced to 1V or less if the voltage VAF− is set to 4V or more from the above equation (10). Can do. Here, since the voltage VAF− is 4.0 V from the equation (24), the maximum value VR (max) can be practically set to 1 V or less. Therefore, by optimally setting the voltage VAF− according to the maximum value VR (max) and the maximum value of the voltage VAF +, whether the applied voltage VR becomes abnormally high or low when the voltage VAF + is abnormally high Thus, it is possible to avoid the deterioration or destruction of the detection unit 101.
[0090]
(Fourth embodiment)
Hereinafter, a fourth embodiment in which the third embodiment is partially changed will be described with reference to the drawings. In the present embodiment, the same components as those in the third embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.
[0091]
FIG. 7 shows an internal configuration of the drive circuit 41 of the present embodiment.
The drive circuit 41 differs from the drive circuit 31 of the third embodiment shown in FIG. 5 in the following points.
(4-1) The drive circuit 41 includes operational amplifiers 82 and 83, resistors R1 to R4, R8 to R10, a diode D1, and a D / A converter 32, as well as resistors R11 to R14, a capacitor C1, and transistors Tr1 and Tr2. It is configured.
[0092]
(4-2) A capacitor C1 is connected between the non-inverting input terminal of the operational amplifier 83 and the ground.
(4-3) The PNP transistor Tr1 and the resistor R11 connected in series are connected in parallel with the resistor R8, and the NPN transistor Tr2 and the resistor R12 connected in series are connected in parallel with the resistor R9. The transistors Tr1 and Tr2 are connected to the microcomputer 85 via the resistors R13 and R14, respectively, and the on / off of the transistors Tr1 and Tr2 is controlled by the microcomputer 85. The resistors R13 and R14 are provided to protect the transistors Tr1 and Tr2 from static electricity.
[0093]
Next, the operation of the drive circuit 41 of this embodiment will be described.
When the temperature of the zirconia solid electrolyte 102 changes in the detection unit 101, the characteristic of the sensor current Ip with respect to the applied voltage VR changes, which causes a problem that the air-fuel ratio detection accuracy decreases. In order to keep the temperature of the zirconia solid electrolyte 102 constant, the AC internal resistance of the detection unit 101 may be detected, and the heater 103 may be turned on / off so that the internal resistance becomes a constant value.
[0094]
In the drive circuit 41, when detecting the sensor current Ip to detect the air-fuel ratio, both the transistors Tr1 and Tr2 are turned off. Then, the drive circuit 41 has the same configuration as the drive circuit 31 of the third embodiment, and the voltage VAF− generated by the resistance voltage division by the resistors R8 and R9 is applied to the non-inverting input terminal of the operational amplifier 83, and the operational amplifier 82 The voltage VAF + is applied from the D / A converter 32 to the non-inverting input terminal via the resistor R10. Accordingly, in the drive circuit 41 as well as the drive circuit 31, a fixed voltage VAF− is generated using a simple resistance voltage dividing circuit including the resistors R 8 and R 9, and the voltage VAF + is generated by the D / A converter 32. By making it variable, the applied voltage VR can be changed according to the air-fuel ratio, and a wide range of air-fuel ratio in the lean region can be detected.
[0095]
In the present embodiment, the processing executed by the microcomputer 85 when detecting the air-fuel ratio is the same as that in the third embodiment, and a description thereof will be omitted.
Next, in the present embodiment, details of processing executed by the microcomputer 85 when detecting the AC internal resistance of the detection unit 101 will be described with reference to the flowchart shown in FIG.
[0096]
FIG. 9 shows the on / off states of the transistors Tr1 and Tr2 and the time displacements of the voltages VAF − (= V2) and V1 when detecting the internal internal resistance of the detection unit 101. Note that since the offset voltage does not exist in the operational amplifier 83, the voltage VAF− is equal to the voltage V2.
[0097]
As shown in FIG. 8, first, in S201, the A / D converter 84 is caused to execute A / D conversion of the voltages V1 and V2 across the resistor R1, and the A / D conversion value is received. At this time, the transistors Tr1 and Tr2 are both turned off. As shown in FIG. 9, the voltage value of the voltage V1 detected here is expressed as a voltage V10, and the voltage value of the voltage V2 is expressed as a voltage V20. Further, the sensor current Ip flowing at this time (hereinafter referred to as “I0”) is expressed by Expression (26).
[0098]
Ip = I0 = (V20−V10) / R1 (Equation 26)
Next, in S202, the transistor Tr2 is turned on and the counter value of the built-in counter of the microcomputer 85 is cleared.
Next, in S203, it is determined whether or not 135 μs has elapsed since the counter value of the built-in counter was cleared in S202, and the process proceeds to S204 when 135 μs has elapsed.
[0099]
In S204, the A / D converter 84 is caused to execute A / D conversion of the voltages V1 and V2 across the resistor R1, and the A / D conversion value is received.
At this time, since the transistor Tr1 is off and the transistor Tr2 is on, the resistors R9 and R12 are connected in parallel. As a result, the voltage VAF− becomes a voltage value calculated by a series-parallel circuit of the resistors R8, R9, R12, and the voltage value becomes a voltage V21 lower than the voltage V20 as shown in FIG. Further, as the voltage VAF− decreases, the voltage value of the voltage V1 also decreases and becomes a voltage V11 lower than the voltage V10. Accordingly, the sensor current Ip flows from the electrode 105 to the electrode 104 as in the rich region. The sensor current Ip flowing at this time (hereinafter referred to as “I1”) is expressed by Expression (27).
[0100]
Ip = I1 = (V21−V11) / R1 (Equation 27)
From Expressions (26) and (27), the change amount ΔI of the sensor current Ip accompanying the change of the voltage VAF− is expressed by Expression (28). Further, the change amount ΔV of the voltage VAF− is expressed by Expression (29).
[0101]

From the equations (28) and (29), the AC internal resistance Z of the detection unit 101 is expressed by the equation (30).
[0102]
Z = ΔV / ΔI (Equation 30)
Therefore, the control signal HT for on / off control of the heater 103 of the detection unit 101 is generated so that the internal resistance Z becomes a constant value.
Next, in S205, it is determined whether or not 200 μs has elapsed since the counter value of the built-in counter was cleared in S202, and the process proceeds to S206 when 200 μs has elapsed.
[0103]
In step S206, the transistor Tr1 is turned on, the transistor Tr2 is turned off, and the counter value of the built-in counter is cleared. At this time, the resistors R8 and R11 are connected in parallel, and the voltage VAF− becomes a voltage value calculated by a series-parallel circuit of the resistors R8, R9, and R11, and the voltage value is as shown in FIG. The voltage V22 is higher than the voltage V20. As the voltage VAF− increases, the voltage value of the voltage V1 also increases and becomes a voltage V12 higher than the voltage V10.
[0104]
Next, in S207, it is determined whether or not 200 μs has elapsed since the counter value of the built-in counter was cleared in S202, and the process proceeds to S206 when 200 μs has elapsed.
Next, in S208, the transistor Tr1 is turned off.
[0105]
At this time, since the voltages VAF− and V1 are set to high voltage values of the voltages V22 and V12 in S206, the sensor current Ip quickly returns to the original current value I0. In other words, since the voltages VAF− and V1 are set to low voltage values of the voltages V21 and V11 in S204, respectively, even if the transistors Tr1 and Tr2 are both turned off in S206, the sensor current Ip is not changed. It does not immediately return to the current value I0. Therefore, the sensor current Ip can be quickly returned to the original current value I0 by increasing each voltage VAF−, V1 once decreased in S204 and swinging it in the reverse direction in S206.
[0106]
Next, the process returns to S201, but in this embodiment, the routine from S201 to S208 is repeatedly performed at a timing of 128 ms.
Note that the timing for starting the routine from S201 is set to an intermediate timing from the timing for starting the routine from S101 shown in FIG. That is, since the routine from S101 shown in FIG. 6 is repeated at a timing of 8 ms, the timing for starting the routine from S201 is set 4 ms after the routine from S101 is started. This is because a certain amount of time is required until the voltages VAF− and V1 are stabilized after the routines from S201 are changed. After the voltages VAF− and V1 are stabilized, the routine from S101 is performed. Thus, the sensor current Ip can be accurately detected.
[0107]
Incidentally, as shown in FIG. 9, the capacitor C1 is provided in order to blunt the falling of the waveform of each voltage VAF−, V1. That is, when the capacitor C1 is omitted, overshoot and ringing occur in the waveforms of the voltages VAF− and V1, and the voltage values of the voltages V21 and V11 may vary due to the overshoot and ringing. Therefore, by providing the capacitor C1, the falling of the waveform of each voltage VAF−, V1 is blunted by the time constants defined by the resistors R8, R9, R12 and the capacitor C1, thereby preventing the occurrence of overshoot and ringing. Thus, the voltages V21 and V11 can be stabilized.
[0108]
Thus, according to the drive circuit 41 of the present embodiment, the AC internal resistance Z of the detection unit 101 is detected, and the heater 103 is turned on / off so that the internal resistance Z becomes a constant value. In addition, it is possible to keep the temperature of the zirconia solid electrolyte 102 constant and to prevent the fluctuation of the sensor current Ip with respect to the applied voltage VR. Therefore, in addition to the effect of the drive circuit 31 of the third embodiment, the accuracy of the air-fuel ratio calculated by the microcomputer 85 can be further improved.
[0109]
In this embodiment, the transistor Tr1 is turned off and the transistor Tr2 is turned on to reduce the voltages VAF− and V1, and the sensor current Ip is caused to flow from the electrode 105 to the electrode 104 in the same manner as in the rich region. The change amounts ΔI and ΔV are obtained. However, contrary to the present embodiment, the transistor Tr1 is turned on and the transistor Tr2 is turned off to increase the voltages VAF− and V1, and the sensor current Ip is caused to flow from the electrode 104 to the electrode 105 as in the lean region. Thus, the change amounts ΔI and ΔV may be obtained.
[0110]
However, as described above, the detection range of the sensor current Ip is limited. Therefore, when detecting the air-fuel ratio corresponding to the lean region, if the sensor current Ip is passed from the electrode 105 to the electrode 104 as in the lean region, the sensor current Ip may exceed the detection range. Further, when the air-fuel ratio corresponding to the rich region is detected, if the sensor current Ip is passed from the electrode 104 to the electrode 105 as in the rich region, the sensor current Ip may exceed the detection range.
[0111]
Therefore, when the air-fuel ratio corresponding to the lean region is detected, it is desirable to obtain the change amounts ΔI and ΔV by causing the sensor current Ip to flow in the same direction as in the rich region as in this embodiment. . Further, when detecting the air-fuel ratio corresponding to the rich region, it is desirable to obtain the change amounts ΔI and ΔV by causing the sensor current Ip to flow in the same direction as in the lean region.
[0112]
By the way, in the drive circuit 31 of the third embodiment, the voltage at both ends of the resistor R1 and the voltage on the detection unit 101 side of the resistor R2 when the voltage VAF + is varied by the D / A converter 32 are detected. The change amounts ΔI and ΔV may be obtained.
[0113]
However, in this case, it is necessary to add an A / D converter that detects the voltage on the detection unit 101 side of the resistor R2, and the cost increases.
Further, in this case, the microcomputer 85 stores in the built-in ROM in advance a change amount ΔV of the voltage value on the detection unit 101 side of the resistor R2 when the voltage VAF + is varied by the D / A converter 32. It is also possible to recognize, and in this way, it is not necessary to add an A / D converter that detects the voltage on the detection unit 101 side of the resistor R2. However, considering the case where there is an error in the output of the D / A converter 32 or the case where the offset voltage is present in the operational amplifier 82, in order to accurately determine the change amounts ΔI and ΔV, the detection unit of the resistor R2 It is desirable to detect the voltage on the 101 side with an A / D converter.
[0114]
A technique for detecting the AC internal resistance of the detection unit 101 and controlling the heater 103 so that the internal resistance becomes a constant value is disclosed in Japanese Patent Laid-Open No. 59-214756. However, the technique described in this publication applies a bias voltage of a predetermined frequency to the output side of the oxygen sensor (corresponding to the detection unit 101 of the present embodiment) via a resistor, and the amplitude level of the combined output voltage of the oxygen sensor. Is detected, and the operation of the heater is controlled based on the amplitude level, which is completely different from the present embodiment.
[0115]
(Fifth embodiment)
Hereinafter, a fifth embodiment in which the second embodiment is partially modified will be described with reference to the drawings. In the present embodiment, the same components as those in the second and fourth embodiments are denoted by the same reference numerals, and detailed description thereof is omitted.
[0116]
FIG. 10 shows the internal configuration of the drive circuit 51 of this embodiment.
In the drive circuit 51, similarly to the drive circuit 21 of the second embodiment, the voltage VAF + is generated by using a resistance voltage dividing circuit including the resistors R5 to R7, and similarly to the drive circuit 41 of the fourth embodiment, The resistors R8, R9, R11 to R14, the capacitor C1, and the transistors Tr1 and Tr2 are provided. When detecting the sensor current Ip in order to detect the air-fuel ratio, both the transistors Tr1 and Tr2 are turned off, and the resistors A voltage VAF− is generated using a resistance voltage dividing circuit composed of R8 and R9. When the drive circuit 51 detects the AC internal resistance of the detection unit 101, the transistors Tr1 and Tr2 are turned on / off as described in FIGS. Control off.
[0117]
Therefore, according to the drive circuit 51 of the present embodiment, in addition to the effect of the drive circuit 21 of the second embodiment, the heater 103 is controlled to be turned on / off in the same manner as the drive circuit 41 of the fourth embodiment, so that the zirconia solid state. By always keeping the temperature of the electrolyte 102 constant, the accuracy of the air-fuel ratio calculated by the microcomputer 85 can be further increased.
[0118]
(Sixth embodiment)
The sixth embodiment, which is a partial modification of the fifth embodiment, will be described below with reference to the drawings. In the present embodiment, the same components as those in the fifth embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.
[0119]
FIG. 11 shows the internal configuration of the drive circuit 61 of this embodiment.
In the drive circuit 61, each voltage VAF +, VAF− is generated by using a resistance voltage dividing circuit by each resistor R5 to R7, and the voltage VAF− is supplied from the voltage follower constituted by the operational amplifier 62 through the resistor R15. This is applied to the non-inverting input terminal of the operational amplifier 83. Similarly to the drive circuit 51 of the fifth embodiment, the drive circuit 61 includes R11 to R14, a capacitor C1, and transistors Tr1 and Tr2. When the AC internal resistance of the detection unit 101 is detected, As described with reference to FIGS. 8 and 9, the transistors Tr1 and Tr2 are turned on / off.
[0120]
By the way, in the drive circuit 51 of the fifth embodiment, the voltage VAF + is generated using a resistance voltage dividing circuit using resistors R5 to R7, and the voltage VAF− is generated using a resistance voltage dividing circuit including resistors R8 and R9. Is generated. Therefore, in order to accurately set the voltages VAF + and VAF−, it is necessary to accurately set the resistance values of the resistors R5 to R8. When there is an error in the resistance values of the resistors R5 to R8, There is a drawback that the voltages VAF + and VAF− cannot be set accurately and the detection accuracy of the sensor current Ip is lowered.
[0121]
On the other hand, in the drive circuit 61 of the present embodiment, the voltages VAF + and VAF− are generated using the resistance voltage dividing circuit including the resistors R5 to R7, and therefore the voltages VAF + and VAF− are accurately set. Thus, the detection accuracy of the sensor current Ip can be improved. However, in order to prevent the resistors R5 to R7 from affecting the transistors Tr1 and Tr2 during on / off control, it is necessary to provide a voltage follower composed of the operational amplifier 62. The driving of the fifth embodiment Compared with the circuit 51, the cost increases. The resistor R15 is provided to protect the non-inverting input terminal of the operational amplifier 83 from static electricity.
[0122]
By the way, this invention is not limited to said each embodiment, You may change as follows, Even in that case, the effect | action and effect similar to said each embodiment can be acquired.
[1] When the output current supply capability of each of the operational amplifiers 82 and 83 is insufficient with respect to the sensor current Ip, a current amplification circuit using a push-pull configuration transistor is connected to the output terminal of each of the operational amplifiers 82 and 83. What is necessary is just to amplify the output current of each operational amplifier 82,83 to a required level in the current amplifier circuit.
[0123]
[2] The shape of the zirconia solid electrolyte 102 of the detection unit 101 is not limited to a tubular shape, but may be any shape as long as it can introduce air into the electrode 104. For example, as shown in FIG. It may be flat. In addition, in the detection part 101 shown in FIG. 12, the code | symbol is made equal about the same structural member as shown in FIG.
[0124]
In FIG. 12, a plate-like zirconia solid electrolyte 102 is provided with a passage 111 and a diffusion chamber 112. An electrode 104 is formed on the zirconia solid electrolyte 102 on the inner wall of the passage 111, and an electrode 105 is formed on the zirconia solid electrolyte 102 on the inner wall of the diffusion chamber 112. The diffusion chamber 112 is provided with one hole communicating with the outside, and the diffusion resistor 106 is configured by this hole. The atmosphere is introduced to the electrode 104 through the passage 111, and residual oxygen and unburned gas in the exhaust gas flows from the diffusion resistor 106 into the electrode 105 through the diffusion chamber 112 by diffusion. Further, an insulating layer 113 made of alumina is fixed to the zirconia solid electrolyte 102, and a linear heater 103 is disposed in the insulating layer 113. The heater 103 heats the zirconia solid electrolyte 102 through the insulating layer 113 to improve the conductivity of oxygen ions.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a drive circuit according to a first embodiment embodying the present invention.
FIG. 2 is a block diagram showing a control apparatus for an automobile internal combustion engine using air-fuel ratio detection apparatuses according to first to sixth embodiments embodying the present invention.
FIG. 3 is a flowchart for explaining processing contents of the microcomputer in the first, second, fifth and sixth embodiments;
FIG. 4 is a circuit diagram showing a drive circuit according to a second embodiment.
FIG. 5 is a circuit diagram showing a drive circuit according to a third embodiment.
FIG. 6 is a flowchart for explaining processing contents of a microcomputer in the third and fourth embodiments.
FIG. 7 is a circuit diagram showing a drive circuit according to a fourth embodiment.
FIG. 8 is a flowchart for explaining processing contents of a microcomputer according to fourth, fifth, and sixth embodiments;
FIG. 9 is a timing chart for explaining the operation of the fourth, fifth and sixth embodiments.
FIG. 10 is a circuit diagram showing a drive circuit according to a fifth embodiment.
FIG. 11 is a circuit diagram showing a drive circuit according to a sixth embodiment.
FIG. 12 is a partial cross-sectional view showing a schematic structure of a detection unit.
FIG. 13 is a partial cross-sectional view showing a schematic structure of a detection unit.
FIG. 14 is a circuit diagram showing a conventional driving circuit.
FIG. 15 is a characteristic diagram of a detection unit.
[Explanation of symbols]
14, 21, 31, 41, 51, 61 ... drive circuit 32 ... D / A converter
82, 83 ... operational amplifier 84 ... A / D converter
85 ... Microcomputer 101 ... Detector
102 ... Solid electrolyte of zirconia 103 ... Heater 104 ... Air side electrode
105 ... Exhaust side electrode 106 ... Diffusion resistor D1 ... Diode
R1 to R15 ... resistors Tr1, Tr2 ... transistors

Claims (11)

A zirconia solid electrolyte, an air electrode formed on the air atmosphere side of the zirconia solid electrolyte, an exhaust gas electrode formed on the exhaust atmosphere side of the zirconia solid electrolyte, and diffused into the exhaust gas electrode from the exhaust atmosphere A detection unit comprising a diffusion resistor for suppressing gas
A drive circuit section for applying a voltage to each of the atmosphere side electrode and the exhaust side electrode of the detection section;
An air- fuel ratio detection apparatus comprising: a detection circuit unit that detects a current flowing out or flowing in from an exhaust-side electrode of the detection unit and detects an air-fuel ratio based on the current value ;
Before Symbol driver circuit portion,
Atmospheric-side voltage generating means for generating an atmospheric-side applied voltage applied to the atmospheric-side electrode of the detection unit;
Exhaust-side voltage generating means for generating an exhaust-side applied voltage applied to the exhaust-side electrode of the detection unit;
A current detection resistor connected between the exhaust side electrode of the detection unit and the exhaust side voltage generation means;
The detection circuit unit includes:
Voltage detection means for detecting at least the voltage on the exhaust side voltage generation means side among the voltages at both ends of the current detection resistor of the drive circuit section;
Current detection means for detecting a current flowing through the current detection resistor from the voltage detected by the voltage detection means;
Air-fuel ratio detecting device characterized by comprising an air-fuel ratio calculating means for calculating an air-fuel ratio on the basis of the current detected by the electric Nagareken detecting means. The air-fuel ratio detection apparatus according to claim 1 ,
An air-fuel ratio detection apparatus, wherein an air-side applied voltage generated by the air-side voltage generating means and an exhaust-side applied voltage generated by the exhaust-side voltage generating means are both fixed. The air-fuel ratio detection apparatus according to claim 2 ,
The drive circuit unit includes a resistance voltage dividing circuit including a plurality of resistors connected in series between a voltage source and a ground.
The atmosphere-side voltage generating means generates an atmosphere-side applied voltage from a voltage that is resistance-divided by the resistance voltage dividing circuit,
The air-fuel ratio detection apparatus according to claim 1, wherein the exhaust-side voltage generation means generates an exhaust-side applied voltage based on a voltage that is resistance-divided by the resistance voltage-dividing circuit. The air-fuel ratio detection apparatus according to claim 1 ,
The drive circuit unit controls the detection of the atmospheric side applied voltage generated by the atmospheric side voltage generation unit based on a voltage applied between the atmospheric side electrode and the exhaust side electrode of the detection unit. Saturate the current flowing in or flowing out of the exhaust side electrode of the part,
The air-fuel ratio calculating device detects the air-fuel ratio based on the saturated current value. In the air-fuel ratio detection apparatus according to claim 4 ,
The drive circuit unit includes a resistance voltage dividing circuit including a plurality of resistors connected in series between a voltage source and a ground;
A variable voltage generation circuit that generates a variable voltage based on a voltage applied between the atmosphere side electrode and the exhaust side electrode of the detection unit;
The atmosphere side voltage generation means generates an atmosphere side applied voltage by a variable voltage generated by the variable voltage generation circuit,
The air-fuel ratio detection apparatus according to claim 1, wherein the exhaust-side voltage generation means generates an exhaust-side applied voltage based on a voltage that is resistance-divided by the resistance voltage-dividing circuit. In the air-fuel ratio detection device according to claim 5 ,
By suppressing the atmospheric-side applied voltage generated by the atmospheric-side voltage generating means to a certain value or less, the voltage applied between the atmospheric-side electrode and the exhaust-side electrode of the detection unit is suppressed and the detection unit is protected. An air-fuel ratio detection apparatus comprising a protective means for The air-fuel ratio detection apparatus according to claim 1 ,
The drive circuit unit includes an atmospheric-side applied voltage generated by the atmospheric-side voltage generation unit based on a voltage applied between the atmospheric-side electrode and the exhaust-side electrode of the detection unit, and the exhaust-side voltage generation unit. And saturating the current flowing in or flowing out from the exhaust side electrode of the detection unit by varying both the exhaust side applied voltage generated by
The air-fuel ratio calculating device detects the air-fuel ratio based on the saturated current value. In the air-fuel ratio detection device according to any one of claims 1 to 7 ,
An AC internal resistance between the atmosphere side electrode and the exhaust side electrode of the detection unit is detected, and the temperature of the zirconia solid electrolyte of the detection unit is controlled to be constant so that the internal resistance becomes constant. An air-fuel ratio detection device. The air-fuel ratio detection apparatus according to claim 8 ,
An air-fuel ratio detection apparatus comprising a heater for heating the zirconia solid electrolyte of the detection unit, and controlling the operation of the heater so that the internal resistance becomes constant. In the air-fuel ratio detection device according to claim 9 ,
The exhaust side applied voltage generated by the exhaust side voltage generating means is changed, and the amount of change in the voltage and the amount of change in the current flowing in or flowing from the exhaust side electrode of the detection unit caused by the change in the voltage. And detecting the internal resistance. In the air-fuel ratio detection device according to claim 9 ,
Based on the amount of change in the voltage applied to the atmosphere side voltage generated by the atmosphere side voltage generation means and the amount of change in the current flowing in or flowing out from the exhaust side electrode of the detection unit caused by the change in the voltage. And detecting the internal resistance.

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