JP3718399B2 - Element resistance detector for oxygen concentration sensor - Google Patents

Description

なお、上記式（１）において、｛Ｖｏ（ｔ１）−Ｖｏ（ｔ５）｝が印加電圧の変化分ΔＶであり、それ以外の部分が素子電流Ｉの変化分ΔＩである。また、式（１）における｛Ｖｏ（ｔ１）−Ｖｏ（ｔ５）｝は、既知のΔＶａに置き換えることも可能である。
【００５２】
その後、時刻ｔ３から予め定められた所定時間Ｔ２（本実施の形態では、２００μｓ）が経過した時刻ｔ６では、トランジスタＴn1をオフさせるとともに、トランジスタＴp1をオンさせ、抵抗Ｒ５，Ｒ６間の電圧Ｖｐを通常電圧（＝３．３Ｖ）よりもΔＶｂ（＝０．２Ｖ）だけ高い電圧（＝３．５Ｖ）に変化させる。
【００５３】
時刻ｔ６から予め定められた所定時間Ｔ３（本実施の形態では、２００μｓ）が経過した時刻ｔ７では、トランジスタＴn1，Ｔp1を共にオフさせて電圧Ｖｐを通常電圧（＝３．３Ｖ）に戻す。
【００５４】
なお、素子抵抗ＺａｃＨの演算が終了した時刻ｔ６において、電圧Ｖｐを通常電圧よりも正側（最初の変化方向とは逆側）に変化させているのは、素子電流Ｉの収束を早めるためである。すなわち、Ａ／Ｆセンサ１０への印加電圧を再び通常電圧に復帰させる際に、その電圧を直接、元の通常電圧に切り換えると、Ａ／Ｆセンサ１０のセンサ素子部１１が有する容量成分に蓄えられた電荷に影響により、素子電流Ｉは電圧の復帰直後においてピーク電流を発生し、結果として元の電流値に収束するまでの時間が長くなる。そこで、本実施の形態では、印加電圧を元の通常電圧に復帰させる際に、先の電圧変化時とは逆方向の電圧を短時間印加して、素子の容量成分における電荷の放電を短時間で終了させ、センサ電流収束のための所要時間を短縮化するようにしている。この場合、Ａ／Ｆセンサ１０の素子内を移動する電荷量が正負いずれの変化時にも略同一量となるように電圧波形を設定すれば、より効果的である。
【００５５】
また、スイッチＳＷ１をオフ状態とし、印加電圧の時定数を大きくした場合（低周波域）における素子抵抗ＺａｃＬの検出時には、前記所定時間Ｔ１を１．５ｍｓ、所定時間Ｔ２，Ｔ３を２ｍｓとして、上記と同様の方法で実施する。
【００５６】
次に、マイコン２０により実行される素子抵抗ＺａｃＨ、ＺａｃＬの検出手順について、図５のフローチャートに従い説明する。なお、図５の処理は、例えば１２８ｍｓ周期で実行される。
【００５７】
さて、マイコン２０が図５の処理を開始すると、先ずステップ１００にて、時定数小のタイミングか否か、即ち高周波域の素子抵抗ＺａｃＨを検出するタイミングか否かを判定する。ここでは、例えば、１２８ｍｓ周期毎にカウントアップするカウンタを用い、そのカウント値が偶数であれば、時定数小のタイミングと判定し、カウント値が奇数であれば、時定数大のタイミングと判定する。
【００５８】
同ステップ１００にて肯定判別されると、ステップ１１０に移行して、マイコン２０は、スイッチＳＷ１をオンに切り替えるとともにステップ１１５にて所定時間Ｔ１に１３５μｓ、所定時間Ｔ２，Ｔ３に２００μｓを設定した後、ステップ１３０に移行する。一方、ステップ１００にて否定判別されると、ステップ１２０に移行して、マイコン２０は、スイッチＳＷ１をオフに切り替えるとともにステップ１２５にて所定時間Ｔ１に１．５ｍｓ、所定時間Ｔ２，Ｔ３に２．０ｍｓを設定した後、ステップ１３０に移行する。
【００５９】
そして、マイコン２０は、ステップ１３０でオペアンプＯＰ３と抵抗Ｒ１２間の電圧Ｖｉを検出し、この電圧Ｖｉが所定の基準値よりも大きいか否かを判定する。基準値は、オペアンプＯＰ３の出力能力の中心電圧に相当する。本実施の形態ではオペアンプＯＰ３の出力能力が１．５〜４．５Ｖであるとして「基準値＝３．０Ｖ」としている。
【００６０】
Ｖｉ＞基準値であれば、マイコン２０はステップ１４０に進み、以降の電圧切換の処理において、抵抗Ｒ５，Ｒ６間の電圧Ｖｐを通常電圧（空燃比検出用電圧の３．３Ｖ）に対して負側→正側の順に電圧を変化させる旨を決定する。また、Ｖｉ≦基準値であれば、マイコン２０はステップ１５０に進み、以降の電圧切換の処理において、電圧Ｖｐを通常電圧（空燃比検出用電圧の３．３Ｖ）に対して正側→負側の順に電圧を変化させる旨を決定する。
【００６１】
要するに、例えばＶｉ＝２Ｖの時（ステップ１３０がＮＯの時）、電圧Ｖｐを負側に掃引すると、電圧ＶｏがすぐにオペアンプＯＰ３の出力能力の下限値（１．５Ｖ）で飽和してしまい、完全に掃引できずに素子抵抗値が正確に求められなくなる。また逆に、例えばＶｉ＝４Ｖの時（ステップ１３０がＹＥＳの時）、電圧Ｖｐを正側に掃引すると、電圧ＶｏがすぐにオペアンプＯＰ３の出力能力の上限値（４．５Ｖ）で飽和してしまい、やはり素子抵抗値が正確に求められなくなる。これに対し、上記の通り電圧Ｖｉと基準値との比較に応じて電圧Ｖｐの掃引の方向を決定することで、電圧Ｖｉの上限値又は下限値へのはりつきが防止できる。
【００６２】
その後、マイコン２０は、ステップ１６０で電圧Ｖｏを検出し、この検出した電圧Ｖｏを前述のＶｏ（ｔ１）として記憶する。また、マイコン２０は、電圧Ｖｉを検出し、この検出した電圧Ｖｉを前述のＶｉ（ｔ２）として記憶する。
【００６３】
続いて、マイコン２０は、ステップ１７０にて、抵抗Ｒ５，Ｒ６間の電圧Ｖｐを正側或いは負側に掃引させる。このとき、前記ステップ１３０が肯定判別されていれば、トランジスタＴp1，Ｔn1のうちトランジスタＴn1をオンして、電圧Ｖｐを負側に掃引させる。また、前記ステップ１３０が否定判別されていれば、トランジスタＴp1，Ｔn1のうちトランジスタＴp1をオンして、電圧Ｖｐを正側に掃引させる。
【００６４】
そして、マイコン２０は、続くステップ１８０において、上記ステップ１７０の処理を行ってから、前記ステップ１１５又はステップ１２５で設定した所定時間Ｔ１が経過したか否かを判定し、その時間Ｔ１が経過するまで待機する。つまり、ステップ１００にて肯定判別されていれば１３５μｓが経過するまで待機し、ステップ１００にて否定判別されていれば１．５ｍｓが経過するまで待機する。
【００６５】
所定時間Ｔ１の経過後、マイコン２０は、ステップ１９０で電圧Ｖｉを検出し、この検出した電圧Ｖｉを前述のＶｉ（ｔ４）として記憶する。また、マイコン２０は電圧Ｖｏを検出し、この検出した電圧Ｖｏを前述のＶｏ（ｔ５）として記憶する。
【００６６】
そして、マイコン２０は、ステップ２００にて、上述した数式（１）を用い、電圧変化量ΔＶと電流変化量ΔＩとから素子抵抗を算出する。なお同ステップ２００では、ステップ１００にて肯定判定されていれば、高周波域の素子抵抗ＺａｃＨが算出され、ステップ１００にて否定判定されていれば、低周波域の素子抵抗ＺａｃＬが算出される。
【００６７】
続くステップ２１０において、マイコン２０は、上記ステップ１７０の処理を行ってから、前記ステップ１１５又はステップ１２５で設定した所定時間Ｔ２が経過したか否かを判定し、その時間Ｔ２が経過するまで待機する。つまり、ステップ１００にて肯定判別されていれば２００μｓが経過するまで待機し、ステップ１００にて否定判別されていれば２．０ｍｓが経過するまで待機する。
【００６８】
所定時間Ｔ２の経過後、マイコン２０は、ステップ２２０で電圧Ｖｐを前記掃引した方向とは逆側に変化させる。このとき、前記ステップ１３０が肯定判別されていれば、トランジスタＴp1をオンするとともにトランジスタＴn1をオフして電圧Ｖｐを正側に変化させ、前記ステップ１３０が否定判別されていれば、トランジスタＴp1をオフするとともにトランジスタＴn1をオンして電圧Ｖｐを負側に変化させる。そして、ステップ２３０において、マイコン２０は、上記ステップ２２０の処理を行ってから、前記ステップ１１５又はステップ１２５で設定した所定時間Ｔ３が経過したか否かを判定し、その時間Ｔ３が経過するまで待機する。つまり、ステップ１００にて肯定判別されていれば２００μｓが経過するまで待機し、ステップ１００にて否定判別されていれば２．０ｍｓが経過するまで待機する。
【００６９】
そして、マイコン２０は、ステップ２４０に進み、トランジスタＴp1，Ｔn1を共にオフ状態とすることで、電圧Ｖｐを元の通常電圧（空燃比検出用電圧＝３．３Ｖ）に戻してその後本ルーチンを一旦終了する。なお、本実施の形態では、図５のステップ１１０及び１２０の処理が時定数切替手段に相当する。
【００７０】
図６は、図５の処理にて検出される高周波域（時定数小）における素子抵抗ＺａｃＨ及び低周波域（時定数大）における素子抵抗ＺａｃＬの温度特性を示す。図６において、実線は、初期状態のＡ／Ｆセンサ１０の特性を示し、点線は、長期の使用によって劣化が進行したＡ／Ｆセンサ１０の特性を示している。図６に示すように、検出される素子抵抗ＺａｃＨ，ＺａｃＬは、素子温度が高くなるほど小さくなる。また、劣化が進行すると、所定温度において検出される素子抵抗値は大きくなる。但し、劣化により抵抗値が増大する変化割合は、高周波域（時定数小）の素子抵抗ＺａｃＨよりも、低周波域（時定数小）の素子抵抗ＺａｃＬの方が大きい。
【００７１】
従って、本実施の形態では、高周波域（時定数小）の素子抵抗ＺａｃＨにより、ヒータ１２の通電制御を実施して、低周波域（時定数小）の素子抵抗ＺａｃＬにより、Ａ／Ｆセンサ１０の劣化を判定するようにしている。
【００７２】
ここで、ヒータ通電制御を説明する。ヒータ１２の通電開始後において、素子温度が比較的低い領域では、素子温度の上昇を促進して早期に活性化させるために、ヒータデューティが１００％に設定され、ヒータ１２の１００％通電制御が実施される。その後、センサ素子部１１がほぼ半活性化状態になった時の素子温度（素子抵抗値）に到達した時点で、１００％通電制御から素子抵抗フィードバック制御に移行し、同フィードバック制御において、素子抵抗値ＺａｃＨを目標素子抵抗値（素子温度を活性温度に維持するための素子抵抗値）に一致させるようにヒータ１２への通電量が制御される。
【００７３】
初期状態のＡ／Ｆセンサ１０では、図６に示すように、目標温度（例えば、７００℃）に対応した目標素子抵抗値（初期目標値）ＺａｃＨｔ0となるようにヒータ１２への通電量が制御される。また、本実施の形態では、センサ素子部１１の劣化状態に応じて、目標素子抵抗値が補正されて、補正後の目標素子抵抗値により、ヒータ１２の通電制御が実施されるようになっている。
【００７４】
次に、素子抵抗フィードバック制御時におけるヒータ駆動処理を、図７のフローチャートに従い説明する。なお、図７の処理は、マイコン２０により例えば２５６ｍｓ周期で実行される。
【００７５】
マイコン２０は、ステップ３００において、図５の処理にて検出された高周波域の素子抵抗ＺａｃＨが所定範囲内であるか否かを判定し、同ステップ３００にて肯定判別された場合、ステップ３１０に移行して、劣化を判定するための処理を実行する。一方、ステップ３００にて否定判別された場合、劣化を判定するための処理を迂回してステップ３６０に移行する。つまり、本実施の形態では、素子抵抗ＺａｃＨがほぼ目標素子抵抗値に一致しているとき（素子温度が活性温度域に維持されているとき）に劣化判定処理が実施される。
【００７６】
ステップ３１０において、マイコン２０は、低周波域の素子抵抗値ＺａｃＬが所定値（図６の劣化判定値ＺａｃＬｆ）以下か否かを判定し、所定値ＺａｃＬｆを超えていれば、Ａ／Ｆセンサ１０が劣化していると判断してステップ３２０に移行する。そして、マイコン２０は、劣化異常フラグＸＦＡＩＬを、センサの劣化異常を意味する「１」とした後、ステップ３３０にてヒータ１２の通電を禁止して本処理を終了する。一方、素子抵抗値ＺａｃＬが所定値（劣化判定値ＺａｃＬｆ）以下であれば、マイコン２０はＡ／Ｆセンサ１０が正常に動作していると判定してステップ３４０に移行し、劣化異常フラグＸＦＡＩＬを、センサの正常を意味する「０」に維持する。その後、ステップ３５０にて、マイコン２０は、素子抵抗ＺａｃＬの値に基づいて目標素子抵抗値を補正する。ここでは、初期状態における目標温度での抵抗値ＺａｃＬ0（図６参照）を基準とし、検出された素子抵抗値ＺａｃＬの変化量を求め、その変化量に応じて目標素子抵抗値を補正する。具体的には、例えば、素子抵抗値ＺａｃＬの変化量の１／５を初期目標値ＺａｃＨｔ0に加え、その抵抗値を新たな目標素子抵抗値ＺａｃＨｔとする。なお、ステップ３００にて、否定判別された場合は、ステップ３５０の処理が実施されないので、目標素子抵抗値ＺａｃＨｔは前回の処理時の値が保持される。
【００７７】
そして、マイコン２０は、ステップ３６０にて、図５の処理にて検出した素子抵抗値ＺａｃＨを目標素子抵抗値ＺａｃＨｔに一致させるように、ヒータデューティ比ＤＵＹＴを算出する。本実施の形態では、その一例としてＰＩＤ制御手順を用いることとしている。つまり、次の式（２）〜（４）により比例項ＧＰ，積分項ＧＩ，微分項ＧＤを算出する。
【００７８】
ＧＰ＝ＫＰ・（ＺａｃＨ−ＺａｃＨｔ） …（２）
ＧＩ＝ＧＩi-1 ＋ＫＩ・（ＺａｃＨ−ＺａｃＨｔ） …（３）
ＧＤ＝ＫＤ・（ＺａｃＨ−ＺａｃＨi-1 ） …（４）
但し、上式において、「ＫＰ」は比例定数、「ＫＩ」は積分定数、「ＫＤ」は微分定数を表し、添字「ｉ−１」は前回処理時の値を表す。
【００７９】
また、上記比例項ＧＰ，積分項ＧＩ，微分項ＧＤを加算してデューティ比ＤＵＴＹを算出する（ＤＵＴＹ＝ＧＰ＋ＧＩ＋ＧＤ）。なお、こうしたヒータ制御のためのデューティ比ＤＵＴＹの算出は、上記のＰＩＤ制御に限定されるものではなく、ＰＩ制御やＰ制御を実施するようにしてもよい。
【００８０】
その後、マイコン２０はステップ３７０に進み、上記ステップ３６０で求めたヒータデューティ比ＤＵＴＹによりヒータ１２を通電し、その後本処理を終了する。
【００８１】
なお、ステップ３２０において、劣化異常フラグに「１」がセットされた場合は、マイコン２０は、空燃比フィードバック制御を禁止するととに、警告ランプ（図示せず）を点灯または点滅させ、運転者に警告する。
【００８２】
本実施の形態では、図７のステップ３１０の処理が劣化判定手段に相当し、ステップ３５０の処理が補正手段に相当する。また、ステップ３６０及び３７０の処理がヒータ制御手段に相当する。
【００８３】
以上詳述した本実施の形態によれば、以下に示す効果が得られる。
（１）印加電圧の時定数を切り替えることにより、異なる周波数域での素子抵抗ＺａｃＨ、ＺａｃＬが検出される。この場合、時定数固定の従来技術とは異なり、劣化による変化が小さい素子抵抗ＺａｃＨと劣化による変化が大きい素子抵抗ＺａｃＬとを検出できる。つまり、用途に応じて異なる周波数帯域の素子抵抗が検出できる。
【００８４】
（２）時定数が切り替えられると、印加電圧が素子抵抗検出用の電圧に変化した後、素子電流Ｉが収束するタイミングが変化するが、本実施の形態では、切り替えられた時定数に応じて、素子電流Ｉを検出するタイミングが変更される。この場合、適正なるタイミングで素子電流Ｉを検出でき、時定数切り替え時にも素子抵抗ＺａｃＨ，ＺａｃＬを正しく検出できる。
【００８５】
（３）劣化による変化が小さい素子抵抗ＺａｃＨに基づき目標素子抵抗値ＺａｃＨｔとなるようにヒータ１２への通電量（デューティ比ＤＵＹＴ）が制御されるので、素子温度が活性温度域に維持され、排ガス中の酸素濃度の検出精度を良好に保つことができる。また、劣化による変化が大きい素子抵抗ＺａｃＬに基づきＡ／Ｆセンサ１０劣化が判定されるので、Ａ／Ｆセンサ１０の劣化も精度よく検出できる。
【００８６】
（４）本実施の形態では、劣化による変化が大きい素子抵抗ＺａｃＬに基づいて、目標素子抵抗値ＺａｃＨｔを補正するようにした。このようにすれば、素子温度に対する素子抵抗値ＺａｃＨとの関係は、劣化が進行すると図６の実線から点線へと変化するが、その関係が劣化度合に応じて補正されることとなり、素子温度を活性温度域に保つことができる。つまり、補正せずに初期の目標素子抵抗値ＺａｃＨｔ0となるように、ヒータ１２の通電制御を実施した場合、劣化に応じて実際の素子温度は、目標温度から高温側にズレてしまう。このため、センサ素子部１１が加熱により破壊されてしまうおそれがある。これに対し、本実施の形態によれば、目標素子抵抗値ＺａｃＨｔを補正することによって、劣化が進行したとしても素子温度を活性温度域に維持することができ、センサ破壊を防止できる。
【００８７】
なお本発明は、上記以外に次の形態にて具体化できる。
上記実施の形態では、劣化による変化が大きい素子抵抗ＺａｃＬに基づいてＡ／Ｆセンサ１０の劣化を判定するものであったが、劣化による変化が大きい素子抵抗ＺａｃＬと劣化による変化が小さい素子抵抗ＺａｃＨとの差（ＺａｃＬ−ＺａｃＨ）から劣化を判定してもよい。また、その差（ＺａｃＬ−ＺａｃＨ）から目標素子抵抗値ＺａｃＨｔの補正を行うようにしてもよい。さらには、検出した素子抵抗値ＺａｃＬと初期の素子抵抗値ＺａｃＬ0との差（ＺａｃＬ−ＺａｃＬ0）から劣化を判定してもよい。
【００８８】
上記実施の形態では、コップ型のＡ／Ｆセンサ１０を適用したが、例えば、積層型のＡ／Ｆセンサを適用してもよい。
また、上記実施の形態では、劣化を判定するために、時定数を切り替えたが、センサの個体差や用いるセンサの種類（例えば、積層型又はコップ型）により時定数を切り替えてもよい。この場合、センサの個体差やセンサの種類に応じて、最適な周波数帯域での素子抵抗が検出できる。従って、ヒータ通電の制御性が悪化したり、望み通りに素子温度が制御できなくなるといった不都合を回避できる。
【００８９】
上記実施の形態では、２つの時定数を切り替えるものであったがこれに限定するものではなく、３つ以上の時定数を切り替える構成としてもよい。
【図面の簡単な説明】
【図１】発明の実施の形態における車載ＥＣＵの概要を示す電気的構成図。
【図２】Ａ／Ｆセンサの構成を示す断面図。
【図３】交流入力電圧の周波数とインピーダンスとの関係を示す図。
【図４】素子抵抗検出時のＶｏ電圧波形と、Ｖｉ電圧波形とを示すタイミングチャート。
【図５】素子抵抗検出処理を示すフローチャート。
【図６】素子温度と素子抵抗値との関係を示す図。
【図７】ヒータ通電処理を示すフローチャート。
【図８】Ａ／Ｆセンサの概略構成を示す図。
【図９】Ａ／Ｆセンサの等価電気回路図。
【図１０】Ａ／Ｆセンサの複素インピーダンス特性を示す図。
【符号の説明】
１…車載ＥＣＵ、１０…酸素濃度センサとしてのＡ／Ｆセンサ、１１…センサ素子部、１２…ヒータ、２０…マイコン、Ｔp1，Ｔn1…電圧変更手段としてのトランジスタ、Ｒ１０，Ｒ１１…抵抗、Ｃ１…コンデンサ、ＳＷ１…スイッチ。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to, for example, an oxygen concentration sensor for detecting the oxygen concentration in exhaust gas from a vehicle-mounted engine, and relates to an element resistance detection device for an oxygen concentration sensor that detects the element resistance of the oxygen concentration sensor.
[0002]
[Prior art]
As this type of oxygen concentration sensor, a limiting current type air-fuel ratio sensor is known that detects the oxygen concentration in exhaust gas using a solid electrolyte. In this sensor, since the temperature dependence of the output signal is large, it is necessary to keep a sensor element made of a solid electrolyte or the like in an active state in order to maintain good oxygen concentration detection accuracy. For this reason, the sensor element is heated and maintained in an active state by controlling energization of a heater attached to the sensor.
[0003]
In the energization control of the heater, using the fact that the resistance value (element resistance value) of the sensor element has a predetermined correspondence with the temperature (element temperature) of the sensor element, the detected element resistance value is set to the target resistance value. The amount of current supplied to the heater is controlled so as to match the above.
[0004]
As a method of detecting the element resistance value of the oxygen concentration sensor, there is a method in which the applied voltage is changed stepwise with a predetermined time constant, and the element resistance value of the oxygen concentration sensor is detected by the voltage change and the current change. Known (for example, JP-A-11-201935).
[0005]
As shown in FIG. 8, this type of oxygen concentration sensor includes a solid electrolyte layer, a pair of electrodes on both sides of the solid electrolyte layer, and a diffusion resistance layer. The oxygen concentration sensor can be represented by the equivalent circuit of FIG. In this equivalent circuit, Ra1 and Ra4 are electrode interface resistances, and Ca1 and Ca4 are electrode interface capacitances. Ra2 is the particle resistance of the solid electrolyte with respect to oxygen ions, and Ra3 and Ca3 are the grain boundary resistance and the grain boundary capacity at the interface of the solid electrolyte particles, respectively. VEP is an electromotive force generated according to the oxygen concentration in the exhaust gas.
[0006]
FIG. 10 shows the complex impedance characteristic of the oxygen concentration sensor expressed as shown in FIG. In FIG. 10, the horizontal axis represents the real part of the complex impedance, and the vertical axis represents the imaginary part. In other words, the impedance Ri of the oxygen concentration sensor is the sum of Ra1, Ra2, Ra3, and Ra4 (= Ra1 + Ra2 + Ra3 + Ra4) at a low frequency of about 0 Hz, and the impedance of the oxygen concentration sensor decreases as the frequency increases. In the vicinity of point A in FIG. 10 (frequency = 1 kHz to 5 kHz), there is little impedance change with respect to the frequency, and the total value of Ra2 and Ra3 is detected as AC impedance.
[0007]
Therefore, the time constant of the applied voltage is determined in order to detect the AC impedance (element resistance) composed of Ra2 + Ra3.
[0008]
[Problems to be solved by the invention]
However, in the conventional element resistance detection device, the element resistance of the oxygen concentration sensor is detected with a fixed time constant, and the following problems occur.
[0009]
For example, conventionally, a technique has been proposed in which an element resistance detected for performing heater control is used and a sensor abnormality is determined based on a change in the element resistance. However, as described above, when detecting the element resistance composed of Ra2 + Ra3, there is little change in the element resistance due to deterioration. For this reason, there is a possibility that deterioration detection cannot be performed with high accuracy.
[0010]
Further, by determining the frequency at point A shown in FIG. 10, the element resistance composed of Ra2 + Ra3 can be detected. However, depending on the individual difference of the sensor and the type of oxygen concentration sensor such as cup type or stacked type, point A May change. If the point A changes, the correct element resistance as an AC impedance cannot be detected. In this case, the controllability of heater energization deteriorates and the element temperature cannot be controlled as desired.
[0011]
The present invention has been made paying attention to the above-mentioned problems, and an object of the present invention is to provide an element resistance detection device for an oxygen concentration sensor that can detect element resistances in different frequency bands depending on applications. Is to provide.
[0012]
[Means for Solving the Problems]
As described above, the oxygen concentration sensor using a solid electrolyte is represented by the equivalent circuit of FIG. 9, and its element resistance (AC impedance) changes according to the frequency of the AC input voltage. In addition, by changing the applied voltage with a predetermined time constant, a single signal including a frequency component is applied to the oxygen concentration sensor, and the element is detected from the voltage change at that time and the current change associated therewith. The resistance value is an AC impedance at a predetermined frequency determined by a time constant. That is, as in the first aspect of the invention, the element resistance values in different frequency bands are detected by adopting a configuration in which the time constant of the applied voltage is switched by the time constant switching means.
[0013]
  Therefore, unlike the prior art with a fixed time constant, for example, an element resistance value in a frequency band suitable for heater energization control and an element resistance value in a frequency band suitable for deterioration determination can be detected. In addition, it is possible to detect the element resistance value in the optimum frequency band according to the individual difference of the sensor and the type of the oxygen concentration sensor such as the cup type or the laminated type.
  In addition, the degree of change in the element resistance value due to sensor deterioration is smaller in the high frequency range and larger in the lower frequency range. That is, the degree of change due to the progress of deterioration increases as the element resistance value is detected when the time constant is increased.
  Therefore, according to the first aspect of the present invention, the time constant switching means switches to the first time constant having a small time constant, and the element resistance value with little change due to deterioration is detected. Based on the element resistance value, the heater control means controls the energization amount to the heater so that the target element resistance value is obtained. On the other hand, the time constant switching means switches to a second time constant having a large time constant, and an element resistance value having a large change due to deterioration is detected. Then, deterioration of the oxygen concentration sensor is determined by the deterioration determining means based on the element resistance value. In this case, the element temperature is maintained in the activation temperature range, the detection accuracy of the oxygen concentration in the gas to be detected can be kept good, and the deterioration of the oxygen concentration sensor can also be accurately determined.
As described above, the element resistance value composed of the particle resistance Ra2 and the grain boundary resistance Ra3 is detected at the frequency near the point A in FIG. 10, and when the frequency becomes lower than the frequency near the point A, the electrode interface resistance Ra1. , Ra4 is detected. The particle resistance Ra2 and the grain boundary resistance Ra3 of the solid electrolyte do not change even when deterioration progresses due to long-term use, but the electrode interface resistances Ra1 and Ra4 increase as the deterioration progresses. Therefore, the first time constant is set so as to detect the element resistance value composed of the particle resistance Ra2 and the grain boundary resistance Ra3, and the second time constant detects the element resistance value including the electrode interface resistances Ra1 and Ra4. It is set as follows.
[0014]
  When the time constant is switched, the timing at which the current flowing through the sensor element portion converges after the applied voltage changes is changed. However, according to the invention of claim 2, the time constant is switched.When,ThatWhenDepending on the time constant ofBy timing change meansSince the timing for detecting the current of the sensor element portion after the voltage change is changed, the current of the sensor element portion is detected at an appropriate timing. Therefore, the element resistance can be correctly detected even when the time constant is switched.
[0019]
  Since the element resistance value detected when switching to the second time constant is largely changed due to deterioration, the degree of deterioration of the sensor can be determined according to the change. In this case, the claim3As described above, the target element resistance value for controlling the energization of the heater is corrected by the correction means based on the element resistance detected when the second time constant is switched. As a result, even if the relationship between the element resistance value and the element temperature changes due to deterioration, the relationship is corrected, and the element temperature can be maintained in the active temperature range.
[0020]
  Claim4According to the invention described in the above, the voltage to be applied to the sensor element unit is switched stepwise by the voltage changing means, and the output voltage of the voltage changing means is input to the RC circuit composed of the resistor and the capacitor. . As a result, the voltage applied to the sensor element section is changed with a predetermined time constant, and this time constant is switched by changing the resistance component of the RC circuit using the switch means.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings. In the present embodiment, the present invention is applied to an air-fuel ratio control system for a vehicle engine, and is embodied as an air-fuel ratio detection device for the system. An electronic control unit (hereinafter referred to as an in-vehicle ECU) mounted on a vehicle uses an air-fuel ratio feedback (F / B) based on a detection result by a limit current type air-fuel ratio sensor (A / F sensor) provided in an engine exhaust pipe. ) Control is performed, and heater heating control is performed to constantly keep the sensor in an active state while monitoring the element temperature of the A / F sensor. The details will be described below.
[0022]
FIG. 1 is an electrical configuration diagram showing an outline of an in-vehicle ECU 1 and an A / F sensor 10 driven by the ECU 1 in the present embodiment. As shown in FIG. 1, an A / F sensor 10 as an oxygen concentration sensor has a sensor element unit 11 and a heater 12 as its main components, and is connected to an in-vehicle ECU 1.
[0023]
The A / F sensor 10 is attached to an exhaust pipe 14 (see FIG. 2) extending from the engine body (not shown). The A / F sensor 10 has an oxygen concentration in the exhaust gas as the ECU 1 applies a voltage. A current proportional to the current flows. The in-vehicle ECU 1 includes a microcomputer 20 (hereinafter referred to as a microcomputer). The microcomputer 20 detects a current flowing through the A / F sensor 10 and performs air-fuel ratio F / B control based on the detection result. Further, the microcomputer 20 detects the element resistance value of the sensor element unit 11 and controls the energization amount to the heater 12 based on the detection result, so that the A / F sensor 10 reaches a predetermined activation temperature (for example, 700 ° C.). Retained.
[0024]
Here, the configuration of the A / F sensor 10 will be described with reference to FIG. In FIG. 2, the A / F sensor 10 protrudes toward the inside of the exhaust pipe 14, and the sensor 10 is roughly divided into a cover 15, a sensor element unit 11, and a heater 12. The cover 15 has a U-shaped cross section, and a plurality of small holes 15a communicating with the inside and outside of the cover are formed on the peripheral wall. The sensor element unit 11 generates a limit current corresponding to the oxygen concentration in the air-fuel ratio lean region or the unburned gas (CO, HC, H2, etc.) concentration in the air-fuel ratio rich region.
[0025]
The configuration of the sensor element unit 11 will be described in detail. In the sensor element unit 11, the exhaust gas side electrode layer 17 is fixed to the outer surface of the solid electrolyte layer 16 formed in a cup shape in cross section, and the atmosphere side electrode layer 18 is fixed to the inner surface. A diffusion resistance layer 19 is formed outside the exhaust gas side electrode layer 17 by a plasma spraying method or the like. The solid electrolyte layer 16 is made of an oxygen ion conductive oxide sintered body in which CaO, MgO, Y2 O3, Yb2 O3 or the like is dissolved in ZrO2, HfO2, ThO2, Bi2 O3 or the like as a stabilizer. Consists of heat-resistant inorganic materials such as alumina, magnesia, siliceous, spinel, mullite. Both the exhaust gas side electrode layer 17 and the atmosphere side electrode layer 18 are made of a noble metal with high catalytic activity such as platinum, and the surface thereof is subjected to porous chemical plating or the like. The area and thickness of the exhaust gas side electrode layer 17 are about 10 to 100 mm 2 (square millimeter) and about 0.5 to 2.0 μm, while the area and thickness of the atmosphere side electrode layer 18 are about It is 10 mm ^ 2 (square millimeter) or more and about 0.5 to 2.0 μm.
[0026]
The heater 12 is accommodated in the atmosphere-side electrode layer 18 and heats the sensor element unit 11 (the atmosphere-side electrode layer 18, the solid electrolyte layer 16, the exhaust gas-side electrode layer 17, and the diffusion resistance layer 19) by the heat generation energy. The heater 12 has a heat generation capacity sufficient to activate the sensor element unit 11.
[0027]
In the A / F sensor 10 having the above configuration, the sensor element unit 11 generates a limit current corresponding to the oxygen concentration in the lean region from the theoretical air-fuel ratio point. In this case, the limit current corresponding to the oxygen concentration is determined by the area of the exhaust gas side electrode layer 17, the thickness of the diffusion resistance layer 19, the porosity, and the average pore diameter. The sensor element unit 11 can detect the oxygen concentration with a linear characteristic. A high temperature of about 600 ° C. or higher is required to activate the sensor element unit 11, and the sensor element 11 Since the active temperature range of the part 11 is narrow, the sensor element part 11 cannot be maintained in the active region by heating only the exhaust gas of the engine. Therefore, the sensor element unit 11 is heated to the active temperature range by the heating control of the heater 12. Note that in a region on the rich side of the stoichiometric air-fuel ratio, the concentration of carbon monoxide (CO) or the like in the unburned gas changes substantially linearly with respect to the air-fuel ratio, and the sensor element unit 11 corresponds to the concentration of CO or the like. Generates a limiting current.
[0028]
As shown in FIG. 1, the heater 12 of the A / F sensor 10 has one end connected to the battery power source + B and the other end connected to a switching element in the ECU 1, for example, the drain of the MOSFET 21. The MOSFET 21 has a source connected to the ground and a gate connected to the output port of the microcomputer 20. The microcomputer 20 controls on / off of the MOSFET 21 to control the energization amount (heater duty) of the heater 12.
[0029]
In the in-vehicle ECU 1, three resistors R1, R2, and R3 are connected in series between the power supply voltage Vc (= 5V) and the ground (= 0V). By dividing the power supply voltage Vc by these resistors, the first voltage V1 is generated at the connection portion between the resistors R1 and R2, and the second voltage V2 is generated at the connection portion between the resistors R2 and R3. Is done. In the present embodiment, the resistance values of the resistors R1, R2, and R3 are set to 3.9 kΩ, 390Ω, and 2.21 kΩ, respectively. Accordingly, the first voltage V1 generated at the connection point between the resistors R1 and R2 is 3.0V, and the second voltage V2 generated at the connection point between the resistors R2 and R3 is 3.3V.
[0030]
A connection portion between the resistor R1 and the resistor R2 is connected to a non-inverting input terminal of the operational amplifier OP1, and an output terminal of the operational amplifier OP1 is connected to a minus side terminal AF− of the sensor element portion 11. The operational amplifier OP1 has an inverting input terminal and an output terminal connected to each other, and operates as a voltage follower. As a result, the first voltage V1 (= 3.0 V) input to the non-inverting input terminal of the operational amplifier OP1 becomes equal to the output voltage of the operational amplifier OP1, and the first voltage V1 is the negative terminal AF− of the sensor element unit 11. Is input.
[0031]
On the other hand, the connection portion between the resistor R2 and the resistor R3 is connected to the non-inverting input terminal of the operational amplifier OP2, and the inverting input terminal and the output terminal are connected in the operational amplifier OP2. Since the operational amplifier OP2 also operates as a voltage follower, the second voltage V2 (= 3.3 V) input to the non-inverting input terminal of the operational amplifier OP2 becomes equal to the output voltage of the operational amplifier OP2. The output terminal of the operational amplifier OP2 is connected between the resistor R5 and the resistor R6 via the resistor R4. The resistor R5 is connected to the power supply voltage Vc via a PNP transistor Tp1, and the resistor R6 is connected to the ground via an NPN transistor Tn1. The bases of the transistors Tp1 and Tn1 are connected to the output port of the microcomputer 20 via resistors R7 and R8, respectively, and the microcomputer 20 switches the transistors Tp1 and Tn1 on / off. By this switching, the voltage Vp between the resistors R5 and R6 changes.
[0032]
Specifically, if the two transistors Tp1 and Tn1 are both off, the voltage Vp between the resistors R5 and R6 is the second voltage V2 output from the operational amplifier OP2. If only the transistor Tn1 of the two transistors Tp1 and Tn1 is in the on state, the voltage Vp is the difference between the second voltage V2 and the ground potential (that is, the second voltage V2) between the resistor R4 and the resistor R6. The divided voltage (V2-ΔVa) is obtained. On the contrary, if only the transistor Tp1 of the two transistors Tp1 and Tn1 is in the on state, the voltage Vp is obtained by dividing the differential voltage (VC−V2) between the power supply voltage VC and the second voltage V2 by the resistors R4 and R5. The pressed voltage (V2 + ΔVb).
[0033]
In the present embodiment, the resistance values of the resistors R4, R5, and R6 are set to 200Ω, 1.5 kΩ, and 3.09 kΩ, respectively. Therefore, the divided voltage (V2−ΔVa) is 3.1V, and the divided voltage (V2 + ΔVb) is 3.5V. That is, both ΔVa and ΔVb are 0.2V.
[0034]
A connection portion between the resistor R5 and the resistor R6 is connected to the non-inverting input terminal of the operational amplifier OP3 through the resistor R10, and is connected to the non-inverting input terminal of the operational amplifier OP3 through the switch SW1 and the resistor R11. The switch SW1 is connected to the output port of the microcomputer 20, and can be switched on or off by the microcomputer 20. A capacitor C1 is connected between the non-inverting input terminal of the operational amplifier OP3 and the ground.
[0035]
In the present embodiment, a circuit constituted by the resistors R10 and R11 and the capacitor C1 corresponds to an RC circuit. The switch SW1 corresponds to the switch means, and the transistors Tp1 and Tn1 correspond to the voltage changing means.
[0036]
When the vehicle-mounted ECU 1 is configured as described above, when the voltage Vp between the resistors R5 and R6 is changed by switching the transistors Tp1 and Tn1, it is determined by the resistor R10 and the capacitor C1 or the parallel circuit of the resistors R10 and R11 and the capacitor C1. The voltage Vp is changed in accordance with the time constant to be input and input to the non-inverting input terminal of the operational amplifier OP3. Here, the time constant when the switch SW1 is OFF is mainly determined by the resistor R10 and the capacitor C1, and the time constant when the switch SW1 is ON is mainly the parallel circuit of the resistors R10 and R11 and the capacitor C1. Determined by. Accordingly, when the switch SW1 is turned from on to off, the time constant is switched from small to large.
[0037]
The output terminal of the operational amplifier OP3 is connected to the plus side terminal AF + of the sensor element unit 11 via the resistor R12, and the inverting input terminal of the operational amplifier OP3 is also connected to the plus side terminal AF + of the sensor element unit 11. As a result, the voltage input to the non-inverting input terminal of the operational amplifier OP3 is applied to the plus side terminal AF + of the sensor element unit 11, and the sensor element unit 11 is connected to the resistor R12 connected to the plus side terminal AF +. A flowing element current (limit current flowing according to the oxygen concentration in the exhaust gas) I flows.
[0038]
Both ends of the resistor R12 are connected to the A / D converter 22, and both-end voltages Vi and Vo of the resistor R12 are taken into the microcomputer 20 via the A / D converter 22. Thereby, the oxygen concentration in the exhaust gas and the resistance value (element resistance value) of the sensor element unit 11 are detected.
[0039]
Specifically, at the time of detecting the oxygen concentration, the two transistors Tp1 and Tn1 are both kept off by the microcomputer 20. In this state, since the second voltage V2 (= 3.3 V) output from the operational amplifier OP2 is output as it is to the non-inverting input terminal of the operational amplifier OP3, the second voltage V2 of the sensor element unit 11 is applied to the positive side terminal AF +. V2 is applied. On the other hand, the first voltage V <b> 1 (= 3.0 V) is applied to the negative terminal AF− of the sensor element unit 11. That is, the difference (V2−V1 = 0.3V) between the first voltage V1 and the second voltage V2 generated by dividing by the resistors R1, R2, and R3 is the two terminals AF + and AF− of the A / F sensor 10. Applied between. And with the application of the voltage, the limiting current according to the oxygen concentration in the exhaust gas at that time flows through the A / F sensor 10. At this time, since the same current as the element current I flowing through the sensor element unit 11 flows through the resistor R12, the difference between the potentials at both ends of the resistor R12 becomes a value proportional to the element current I.
[0040]
Accordingly, the microcomputer 20 divides the difference (Vi−Vo) between the voltages Vi and Vo across the resistor R12 input via the A / D converter 22 by the resistance value of the resistor R12, thereby The flowing element current I is obtained. The microcomputer 20 calculates the air-fuel ratio (oxygen concentration of exhaust gas) of the air-fuel mixture from this element current I every 4 ms, for example.
[0041]
On the other hand, when detecting the element resistance value of the A / F sensor 10, either one of the transistors Tp1 and Tn1 is turned on by the microcomputer 20, and a voltage applied to the sensor element unit 11 for detecting the concentration is set to a predetermined value. Change with a time constant. At this time, the resistance value (element resistance value) of the sensor element unit 11 is detected from the current change (ΔI) accompanying the voltage change (ΔV).
[0042]
The voltage having a predetermined time constant in the present embodiment is a single signal including a frequency component. That is, the resistance value of the sensor element unit 11 detected by sweeping the voltage applied to the A / F sensor 10 corresponds to an AC impedance in a frequency band corresponding to a predetermined time constant.
[0043]
FIG. 3 shows the frequency characteristics of the A / F sensor 10 when the element temperature is 700 ° C. As indicated by a solid line in the figure, the impedance of the A / F sensor 10 gradually decreases as the frequency increases. Here, as described above, the equivalent circuit of the A / F sensor 10 is shown in FIG. 9, and the AC impedance converges to the total value (≈30Ω) of the particle resistance Ra2 and the grain boundary resistance Ra3 in the high frequency range. Also, the dotted line in FIG. 3 shows the characteristics of the A / F sensor 10 that has deteriorated due to long-term use. That is, the impedance of the A / F sensor 10 hardly changes even if it deteriorates in the high frequency range, but increases as the deterioration progresses in the low frequency range. This is because, in the A / F sensor 10 having a solid electrolyte, the values of Ra2 of the particle resistance and Ra3 of the grain boundary resistance of the solid electrolyte do not change even if they deteriorate, but the resistances of the electrode interface resistances Ra1 and Ra4. This is because the value increases as the deterioration progresses.
[0044]
Therefore, in the present embodiment, the time constant of the applied voltage is changed by switching the switch SW1, and the impedance in the high frequency range (element resistance ZacH) with little change due to degradation and the impedance in the low frequency range with great variation due to degradation. (Element resistance ZacL) is detected. Here, the time constant when the switch is on corresponds to the first time constant, and the time constant when the switch is off corresponds to the second time constant. The first time constant when the switch is turned on is set so as to detect an element resistance value composed of the particle resistance Ra2 and the grain boundary resistance Ra3 in a high frequency range (for example, 4.5 kHz). The second time constant when the switch is turned off is set so as to detect element resistance values including the electrode interface resistances Ra1 and Ra4 in a low frequency range (for example, 45 Hz).
[0045]
Specifically, when the element resistance value of the A / F sensor 10 is detected, the voltage applied to the A / F sensor 10 to detect the air-fuel ratio changes every predetermined time (for example, 128 ms) as shown in FIG. Let In the present embodiment, the switch SW1 is turned on / off every predetermined time (for example, 128 ms), and the element resistance ZacH in the high frequency range (small time constant) and the low frequency range (time constant). The element resistance ZacL in (large) is alternately detected.
[0046]
Here, a method of detecting the element resistance ZacH when the switch SW1 is turned on and the time constant of the applied voltage is reduced (high frequency region) will be described with reference to the timing chart of FIG. FIG. 4B shows an enlarged view of a portion surrounded by an ellipse in FIG.
[0047]
At time t1 in FIG. 4B, the voltage Vo between the resistor R12 and the plus terminal AF + is detected. The voltage detected at this time t1 is referred to as “Vo (t1)”.
At time t2 immediately after time t1, the voltage Vi between the operational amplifier OP3 and the resistor R12 is detected. The voltage detected at this time t2 is referred to as “Vi (t2)”.
[0048]
At time t3 immediately after time t2, the transistor Tn1 is turned on among the transistors Tp1 and Tn1, and the voltage Vp between the resistors R5 and R6 is set to ΔVa (= 0.0 V) than the normal voltage for air-fuel ratio detection (= 3.3 V). 2V) to a lower voltage (= 3.1V).
[0049]
At a time t4 when a predetermined time T1 (135 μs in the present embodiment) elapses from the time t3, the voltage Vi between the operational amplifier OP3 and the resistor R12 is detected. The voltage detected at time t4 is referred to as “Vi (t4)”. The time T1 is set to a time when the change ΔI of the element current I is expected to reach a peak from time t3.
[0050]
At time t5 immediately after time t4, the voltage Vo between the resistor R12 and the positive terminal AF + is detected. The voltage detected at time t5 is referred to as “Vo (t5)”. And element resistance ZacH is calculated by the following numerical formula (1). However, the resistance value of the resistor R12 is Rs.
[0051]
[Expression 1]

In the above formula (1), {Vo (t1) −Vo (t5)} is the change ΔV in the applied voltage, and the other part is the change ΔI in the element current I. Also, {Vo (t1) −Vo (t5)} in the equation (1) can be replaced with a known ΔVa.
[0052]
Thereafter, at time t6 when a predetermined time T2 (200 μs in the present embodiment) elapses from time t3, the transistor Tn1 is turned off and the transistor Tp1 is turned on, and the voltage Vp between the resistors R5 and R6 is set. The voltage is changed to a voltage (= 3.5 V) higher by ΔVb (= 0.2 V) than the normal voltage (= 3.3 V).
[0053]
At a time t7 when a predetermined time T3 (200 μs in the present embodiment) elapses from the time t6, both the transistors Tn1 and Tp1 are turned off to return the voltage Vp to the normal voltage (= 3.3V).
[0054]
At time t6 when the calculation of the element resistance ZacH is completed, the voltage Vp is changed to the positive side (opposite to the first change direction) from the normal voltage because the convergence of the element current I is accelerated. is there. That is, when the voltage applied to the A / F sensor 10 is restored to the normal voltage again, if the voltage is directly switched to the original normal voltage, the voltage is stored in the capacitance component of the sensor element unit 11 of the A / F sensor 10. Due to the influence of the generated electric charge, the element current I generates a peak current immediately after the voltage is restored, and as a result, it takes a long time to converge to the original current value. Therefore, in this embodiment, when the applied voltage is restored to the original normal voltage, a voltage in the direction opposite to that at the time of the previous voltage change is applied for a short time to discharge the charge in the capacitance component of the element for a short time. The time required for converging the sensor current is shortened. In this case, it is more effective if the voltage waveform is set so that the amount of charge moving in the element of the A / F sensor 10 becomes substantially the same even when the change is positive or negative.
[0055]
When the element resistance ZacL is detected when the switch SW1 is turned off and the time constant of the applied voltage is increased (low frequency range), the predetermined time T1 is set to 1.5 ms, the predetermined times T2 and T3 are set to 2 ms, The same method is used.
[0056]
Next, the detection procedure of the element resistances ZacH and ZacL executed by the microcomputer 20 will be described with reference to the flowchart of FIG. Note that the processing of FIG. 5 is executed, for example, at a cycle of 128 ms.
[0057]
When the microcomputer 20 starts the processing of FIG. 5, first, at step 100, it is determined whether or not it is a timing with a small time constant, that is, whether or not it is a timing for detecting the element resistance ZacH in the high frequency region. Here, for example, a counter that counts up every 128 ms cycle is used. If the count value is an even number, the timing is determined to be a small time constant, and if the count value is an odd number, the timing is determined to be a large time constant. .
[0058]
If an affirmative determination is made in step 100, the process proceeds to step 110, where the microcomputer 20 switches on the switch SW1 and sets 135 μs for a predetermined time T1 and 200 μs for a predetermined time T2 and T3 in step 115. The process proceeds to step 130. On the other hand, if a negative determination is made in step 100, the process proceeds to step 120, where the microcomputer 20 switches the switch SW1 to OFF, and in step 125, 1.5 ms at a predetermined time T1, and 2. at a predetermined time T2, T3. After setting 0 ms, the process proceeds to step 130.
[0059]
In step 130, the microcomputer 20 detects the voltage Vi between the operational amplifier OP3 and the resistor R12, and determines whether or not the voltage Vi is larger than a predetermined reference value. The reference value corresponds to the center voltage of the output capability of the operational amplifier OP3. In this embodiment, it is assumed that the output capability of the operational amplifier OP3 is 1.5 to 4.5V, and “reference value = 3.0V”.
[0060]
If Vi> reference value, the microcomputer 20 proceeds to step 140, and in the subsequent voltage switching process, the voltage Vp between the resistors R5 and R6 is negative with respect to the normal voltage (air-fuel ratio detection voltage 3.3V). It is determined that the voltage is changed in the order of the side to the positive side. If Vi ≦ reference value, the microcomputer 20 proceeds to step 150, and in the subsequent voltage switching process, the voltage Vp is changed from the positive side to the negative side with respect to the normal voltage (3.3 V of the air-fuel ratio detection voltage). It is determined that the voltage is changed in the order of.
[0061]
In short, for example, when Vi = 2V (when step 130 is NO), when the voltage Vp is swept to the negative side, the voltage Vo immediately saturates at the lower limit (1.5V) of the output capability of the operational amplifier OP3. The element resistance value cannot be obtained accurately because sweeping is impossible. Conversely, for example, when Vi = 4V (when step 130 is YES), when the voltage Vp is swept to the positive side, the voltage Vo immediately saturates at the upper limit value (4.5V) of the operational amplifier OP3. As a result, the element resistance value cannot be obtained accurately. On the other hand, sticking to the upper limit value or lower limit value of the voltage Vi can be prevented by determining the sweep direction of the voltage Vp in accordance with the comparison between the voltage Vi and the reference value as described above.
[0062]
Thereafter, the microcomputer 20 detects the voltage Vo in step 160, and stores the detected voltage Vo as the aforementioned Vo (t1). Further, the microcomputer 20 detects the voltage Vi and stores the detected voltage Vi as the aforementioned Vi (t2).
[0063]
Subsequently, in step 170, the microcomputer 20 sweeps the voltage Vp between the resistors R5 and R6 to the positive side or the negative side. At this time, if the determination at step 130 is affirmative, the transistor Tn1 of the transistors Tp1 and Tn1 is turned on to sweep the voltage Vp to the negative side. If the determination at step 130 is negative, the transistor Tp1 of the transistors Tp1 and Tn1 is turned on to sweep the voltage Vp to the positive side.
[0064]
Then, in the following step 180, the microcomputer 20 determines whether or not the predetermined time T1 set in the step 115 or step 125 has elapsed after performing the processing of the step 170, and until the time T1 has elapsed. stand by. That is, if an affirmative determination is made in step 100, the process waits until 135 μs elapses, and if a negative determination is made in step 100, the process waits until 1.5 ms elapses.
[0065]
After elapse of the predetermined time T1, the microcomputer 20 detects the voltage Vi in step 190, and stores the detected voltage Vi as the aforementioned Vi (t4). Further, the microcomputer 20 detects the voltage Vo, and stores the detected voltage Vo as the aforementioned Vo (t5).
[0066]
Then, in step 200, the microcomputer 20 calculates the element resistance from the voltage change amount ΔV and the current change amount ΔI using the above-described equation (1). In step 200, if an affirmative determination is made in step 100, a high-frequency element resistance ZacH is calculated, and if a negative determination is made in step 100, a low-frequency element resistance ZacL is calculated.
[0067]
In the following step 210, the microcomputer 20 determines whether or not the predetermined time T2 set in the step 115 or step 125 has elapsed after performing the processing of step 170, and waits until the time T2 elapses. . That is, if an affirmative determination is made in step 100, the process waits until 200 μs has elapsed, and if a negative determination is made in step 100, the process waits until 2.0 ms elapses.
[0068]
After the elapse of the predetermined time T2, the microcomputer 20 changes the voltage Vp to the opposite side to the swept direction in step 220. At this time, if step 130 is positively determined, the transistor Tp1 is turned on and the transistor Tn1 is turned off to change the voltage Vp to the positive side. If step 130 is negatively determined, the transistor Tp1 is turned off. At the same time, the transistor Tn1 is turned on to change the voltage Vp to the negative side. In step 230, the microcomputer 20 determines whether or not the predetermined time T3 set in step 115 or 125 has elapsed after performing the processing in step 220, and waits until the time T3 elapses. To do. That is, if an affirmative determination is made in step 100, the process waits until 200 μs has elapsed, and if a negative determination is made in step 100, the process waits until 2.0 ms elapses.
[0069]
Then, the microcomputer 20 proceeds to step 240 to turn off both of the transistors Tp1 and Tn1, thereby returning the voltage Vp to the original normal voltage (air-fuel ratio detection voltage = 3.3 V), and then this routine is temporarily executed. finish. In the present embodiment, the processing of steps 110 and 120 in FIG. 5 corresponds to time constant switching means.
[0070]
FIG. 6 shows temperature characteristics of the element resistance ZacH in the high frequency range (small time constant) and the element resistance ZacL in the low frequency range (large time constant) detected by the processing of FIG. In FIG. 6, the solid line indicates the characteristic of the A / F sensor 10 in the initial state, and the dotted line indicates the characteristic of the A / F sensor 10 that has deteriorated due to long-term use. As shown in FIG. 6, the detected element resistances ZacH and ZacL decrease as the element temperature increases. As the deterioration progresses, the element resistance value detected at a predetermined temperature increases. However, the rate of change in which the resistance value increases due to deterioration is greater in the low frequency region (small time constant) element resistance ZacL than in the high frequency region (low time constant) element resistance ZacH.
[0071]
Therefore, in the present embodiment, the energization control of the heater 12 is performed by the element resistance ZacH in the high frequency range (small time constant), and the A / F sensor 10 is controlled by the element resistance ZacL in the low frequency range (small time constant). The deterioration is judged.
[0072]
Here, heater energization control will be described. In the region where the element temperature is relatively low after the energization of the heater 12 is started, the heater duty is set to 100% in order to accelerate the activation of the element temperature and activate it at an early stage. To be implemented. Thereafter, when the sensor temperature reaches the element temperature (element resistance value) when the sensor element portion 11 is almost in the semi-activated state, the element resistance feedback control is shifted from the 100% energization control. The energization amount to the heater 12 is controlled so that the value ZacH matches the target element resistance value (element resistance value for maintaining the element temperature at the activation temperature).
[0073]
In the A / F sensor 10 in the initial state, as shown in FIG. 6, the energization amount to the heater 12 is controlled so that the target element resistance value (initial target value) ZacHt0 corresponding to the target temperature (for example, 700 ° C.) is obtained. Is done. Further, in the present embodiment, the target element resistance value is corrected according to the deterioration state of the sensor element unit 11, and the energization control of the heater 12 is performed based on the corrected target element resistance value. Yes.
[0074]
Next, heater drive processing during element resistance feedback control will be described with reference to the flowchart of FIG. 7 is executed by the microcomputer 20 at a cycle of, for example, 256 ms.
[0075]
In step 300, the microcomputer 20 determines whether or not the high-frequency element resistance ZacH detected in the process of FIG. 5 is within the predetermined range. If the determination in step 300 is affirmative, the microcomputer 20 proceeds to step 310. The process proceeds to execute a process for determining deterioration. On the other hand, if a negative determination is made in step 300, the process proceeds to step 360, bypassing the process for determining deterioration. That is, in the present embodiment, the deterioration determination process is performed when the element resistance ZacH substantially matches the target element resistance value (when the element temperature is maintained in the active temperature range).
[0076]
In step 310, the microcomputer 20 determines whether or not the element resistance value ZacL in the low frequency range is equal to or less than a predetermined value (degradation determination value ZacLf in FIG. 6). The process proceeds to step 320. Then, the microcomputer 20 sets the deterioration abnormality flag XFAIL to “1” which means the deterioration abnormality of the sensor, and then prohibits the energization of the heater 12 in step 330 and ends this processing. On the other hand, if the element resistance value ZacL is equal to or smaller than the predetermined value (degradation determination value ZacLf), the microcomputer 20 determines that the A / F sensor 10 is operating normally and proceeds to step 340 to set the deterioration abnormality flag XFAIL. , “0”, which means that the sensor is normal, is maintained. Thereafter, in step 350, the microcomputer 20 corrects the target element resistance value based on the value of the element resistance ZacL. Here, based on the resistance value ZacL0 (see FIG. 6) at the target temperature in the initial state as a reference, the detected change amount of the element resistance value ZacL is obtained, and the target element resistance value is corrected according to the change amount. Specifically, for example, 1/5 of the change amount of the element resistance value ZacL is added to the initial target value ZacHt0, and the resistance value is set as a new target element resistance value ZacHt. If a negative determination is made in step 300, the process of step 350 is not performed, and the target element resistance value ZacHt is held at the value of the previous process.
[0077]
In step 360, the microcomputer 20 calculates the heater duty ratio DUYT so that the element resistance value ZacH detected in the process of FIG. 5 matches the target element resistance value ZacHt. In this embodiment, a PID control procedure is used as an example. That is, the proportional term GP, the integral term GI, and the differential term GD are calculated by the following equations (2) to (4).
[0078]
GP = KP · (ZacH−ZacHt) (2)
GI = GIi-1 + KI. (ZacH-ZacHt) (3)
GD = KD. (ZacH-ZacHi-1) (4)
In the above equation, “KP” represents a proportional constant, “KI” represents an integral constant, “KD” represents a differential constant, and the subscript “i−1” represents a value at the time of the previous process.
[0079]
Further, the duty ratio DUTY is calculated by adding the proportional term GP, the integral term GI, and the differential term GD (DUTY = GP + GI + GD). The calculation of the duty ratio DUTY for heater control is not limited to the PID control described above, and PI control or P control may be performed.
[0080]
Thereafter, the microcomputer 20 proceeds to step 370, energizes the heater 12 with the heater duty ratio DUTY obtained in step 360, and then ends this process.
[0081]
If "1" is set in the deterioration abnormality flag in step 320, the microcomputer 20 prohibits the air-fuel ratio feedback control and lights or blinks a warning lamp (not shown) to prompt the driver. Warning.
[0082]
In the present embodiment, the process of step 310 in FIG. 7 corresponds to the deterioration determination unit, and the process of step 350 corresponds to the correction unit. Further, the processing in steps 360 and 370 corresponds to the heater control means.
[0083]
According to the embodiment described in detail above, the following effects can be obtained.
(1) By switching the time constant of the applied voltage, element resistances ZacH and ZacL in different frequency ranges are detected. In this case, unlike the prior art in which the time constant is fixed, the element resistance ZacH having a small change due to deterioration and the element resistance ZacL having a large change due to deterioration can be detected. That is, the element resistance in a different frequency band can be detected depending on the application.
[0084]
(2) When the time constant is switched, the timing at which the element current I converges changes after the applied voltage has changed to the element resistance detection voltage. In the present embodiment, the time constant is changed according to the switched time constant. The timing for detecting the element current I is changed. In this case, the element current I can be detected at an appropriate timing, and the element resistances ZacH and ZacL can be correctly detected even when the time constant is switched.
[0085]
(3) Since the energization amount (duty ratio DUYT) to the heater 12 is controlled so that the target resistance value ZacHt becomes the target element resistance value ZacHt based on the element resistance ZacH whose change due to deterioration is small, the element temperature is maintained in the active temperature range and the exhaust gas The detection accuracy of the oxygen concentration inside can be kept good. Further, since the A / F sensor 10 deterioration is determined based on the element resistance ZacL, which has a large change due to deterioration, the deterioration of the A / F sensor 10 can be detected with high accuracy.
[0086]
(4) In the present embodiment, the target element resistance value ZacHt is corrected based on the element resistance ZacL that is largely changed due to deterioration. In this manner, the relationship between the element temperature and the element resistance value ZacH changes from the solid line to the dotted line in FIG. 6 as the deterioration progresses, but this relationship is corrected according to the degree of deterioration, and the element temperature Can be maintained in the active temperature range. That is, when the energization control of the heater 12 is performed so that the initial target element resistance value ZacHt0 is obtained without correction, the actual element temperature is shifted from the target temperature to the high temperature side according to the deterioration. For this reason, there exists a possibility that the sensor element part 11 may be destroyed by heating. On the other hand, according to the present embodiment, by correcting the target element resistance value ZacHt, the element temperature can be maintained in the active temperature range even if the deterioration proceeds, and sensor breakdown can be prevented.
[0087]
In addition to the above, the present invention can be embodied in the following forms.
In the above embodiment, the deterioration of the A / F sensor 10 is determined based on the element resistance ZacL having a large change due to deterioration, but the element resistance ZacL having a large change due to deterioration and the element resistance ZacH having a small change due to deterioration. Deterioration may be determined from the difference between (ZacL−ZacH). Further, the target element resistance value ZacHt may be corrected from the difference (ZacL−ZacH). Furthermore, the deterioration may be determined from the difference (ZacL−ZacL0) between the detected element resistance value ZacL and the initial element resistance value ZacL0.
[0088]
In the above embodiment, the cup-type A / F sensor 10 is applied. However, for example, a stacked-type A / F sensor may be applied.
In the above embodiment, the time constant is switched in order to determine the deterioration. However, the time constant may be switched depending on the individual difference of the sensor and the type of sensor used (for example, stacked type or cup type). In this case, the element resistance in the optimum frequency band can be detected according to the individual difference of the sensor and the type of sensor. Accordingly, it is possible to avoid the disadvantage that the controllability of the heater energization is deteriorated or the element temperature cannot be controlled as desired.
[0089]
In the above embodiment, two time constants are switched. However, the present invention is not limited to this, and a configuration in which three or more time constants are switched may be employed.
[Brief description of the drawings]
FIG. 1 is an electrical configuration diagram showing an outline of an in-vehicle ECU in an embodiment of the invention.
FIG. 2 is a cross-sectional view showing a configuration of an A / F sensor.
FIG. 3 is a diagram showing the relationship between the frequency and impedance of an AC input voltage.
FIG. 4 is a timing chart showing a Vo voltage waveform and a Vi voltage waveform when element resistance is detected.
FIG. 5 is a flowchart showing element resistance detection processing;
FIG. 6 is a diagram showing a relationship between element temperature and element resistance value.
FIG. 7 is a flowchart showing heater energization processing.
FIG. 8 is a diagram showing a schematic configuration of an A / F sensor.
FIG. 9 is an equivalent electric circuit diagram of the A / F sensor.
FIG. 10 is a diagram showing complex impedance characteristics of an A / F sensor.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Vehicle-mounted ECU, 10 ... A / F sensor as an oxygen concentration sensor, 11 ... Sensor element part, 12 ... Heater, 20 ... Microcomputer, Tp1, Tn1 ... Transistor as voltage change means, R10, R11 ... Resistance, C1 ... Capacitor, SW1 ... switch.

Claims (4)

An applied voltage applied to the sensor element unit, which is applied to an oxygen concentration sensor that has a sensor element unit using a solid electrolyte and a heater for heating the sensor element unit and detects an oxygen concentration in a gas to be detected. In an element resistance detection device of an oxygen concentration sensor that detects a resistance value of the sensor element section from a voltage change at that time and a current change accompanying the change with a predetermined time constant,
A time constant switching means for switching the time constant of the applied voltage to either the first time constant or the second time constant larger than the first time constant;
The time constant switching means is
Heater control means for controlling an energization amount to the heater based on an element resistance value detected when the first time constant is switched;
Deterioration determining means for determining deterioration of the oxygen concentration sensor based on an element resistance value detected when the second time constant is switched to , wherein the first time constant is the sensor An element resistance value including a particle resistance and a grain boundary resistance in the element part is set to be detected, and the second time constant is set to detect an element resistance value including an electrode interface resistance in the sensor element part. An element resistance detection device for an oxygen concentration sensor. When changing the applied voltage to detect the element resistance, the current flowing through the sensor element unit before the voltage change is detected, and the current flowing through the sensor element unit after a predetermined time has elapsed after the voltage change Current detection means for detecting
In the current detection means, element resistance calculation means for calculating an element resistance based on a difference between currents detected when a predetermined time has elapsed before and after the voltage change; and
When the time constant is switched by the time constant switching means, it further comprises timing changing means for changing the timing for detecting the current of the sensor element section after voltage change according to the time constant at that time. The device resistance detection device for an oxygen concentration sensor according to claim 1. 2. A correction means for correcting a target element resistance value for controlling energization of the heater based on an element resistance detected when the second time constant is switched to the second time constant. 3. An element resistance detection device for an oxygen concentration sensor according to 2. Voltage changing means for switching the voltage to be applied to the sensor element section in a stepwise manner;
An RC circuit comprising a resistor and a capacitor and changing the output voltage of the voltage changing means with a predetermined time constant;
The oxygen concentration sensor according to any one of claims 1 to 3 , wherein the time constant switching means changes a resistance component of the RC circuit using a switch means. Element resistance detection device .

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