JP2004198351A

JP2004198351A - Gas concentration detection apparatus

Info

Publication number: JP2004198351A
Application number: JP2002369925A
Authority: JP
Inventors: Tomoo Kawase; 友生川瀬; Hidekazu Kurokawa; 英一黒川
Original assignee: Denso Corp
Current assignee: Denso Corp
Priority date: 2002-12-20
Filing date: 2002-12-20
Publication date: 2004-07-15

Abstract

<P>PROBLEM TO BE SOLVED: To speedily acquire accurate gas concentration output by speedily optimizing the inside state of a chamber for detecting gas concentration at the start of operation of a gas concentration sensor etc. <P>SOLUTION: The gas concentration sensor 100 is provided with a pump cell 110; a monitor cell 120; and a sensor cell 130. An electrode 131 opposed to a second chamber 146 among a pair of electrodes of the sensor cell 130 is an NOx active electrode. Immediately after the start of sensor operation, a control circuit 200 impresses a voltage higher than a normal one between the electrodes of the sensor cell 130 and changes the current detection resistance of the sensor cell 130 to low resistance. By this, oxygen adsorbed to the NOx active electrode of the sensor cell 130 is forcefully desorbed, and oxygen inside the second chamber 146 in which the sensor cell is installed is forcefully discharged. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

【０００１】
【発明の属する技術分野】
本発明は、被検出ガス中の特定成分のガス濃度を検出するガス濃度検出装置に関するものである。
【０００２】
【従来の技術】
この種のガス濃度検出装置として、限界電流式のガス濃度センサを用い、例えば車両用エンジンから排出される排ガス中のＮＯｘ（窒素酸化物）を検出するものがある。ガス濃度センサは、例えばポンプセル、センサセル及びモニタセルからなる３セル構造を有し、ポンプセルでは、第１チャンバに導入した排ガス中の酸素の排出又は汲み込みが行われ同時に排ガス中の酸素濃度検出が行われる。また、センサセルでは、ポンプセルを通過した後の第２チャンバ内のガスからＮＯｘ濃度（特定成分のガス濃度）が検出され、モニタセルでは、同第２チャンバ内の残留酸素濃度が検出される（例えば、特許文献１参照）。
【０００３】
【特許文献１】
特許第２８８５３３６号公報
【０００４】
【発明が解決しようとする課題】
上記ガス濃度センサでは、当該センサを放置しておくと第１及び第２のチャンバ内に大気が導入され、比較的高濃度（約２０．９％）の酸素が充満する。すると、センサ作動開始直後には、第１及び第２のチャンバ内における高濃度の酸素が原因で正確なセンサ出力が得られなくなるという事態が生ずる。実機でのデータを解析すると、ガス濃度センサの作動開始直後には、センサセル電流が過剰に大きくなり、その後、所定のＮＯｘ濃度検出範囲の安定した電流値に収束するのに長い時間を要することが確認された。従って、センサセル電流、すなわちＮＯｘ濃度出力の正常化に要する時間が長引くことが問題となる。
【０００５】
一方、ＮＯｘ濃度に応じて流れるセンサセル電流はｎＡ（ナノアンペア）オーダの電流値であり、この微弱電流を精度良く検出するには電流検出抵抗の抵抗値を数百ｋΩ以上にまで大きくする必要がある。すると、チャンバ内に充満する高濃度酸素の排出に長い時間を要し、やはり上述の如くセンサセル電流、すなわちＮＯｘ濃度出力の正常化に要する時間が長引くという問題を生ずる。
【０００６】
本発明は上記事情に鑑みてなされたものであり、その目的とするところは、ガス濃度センサの作動開始時等においてチャンバ（ガス室）内の状態を早期にガス濃度検出に最適な状態とし、正確なガス濃度出力をいち早く得ることができるガス濃度検出装置を提供することである。
【０００７】
【課題を解決するための手段】
請求項１記載のガス濃度検出装置は、次の構成のガス濃度センサに適用される。すなわち、ガス濃度センサは、各々に固体電解質と該固体電解質に配置された一対の電極とよりなる第１セル及び第２セルを有し、第１セルではガス室に導入した被検出ガス中の酸素量が所定の低濃度レベルに調整され、第２セルでは第１セルでの酸素量調整後のガスから特定成分のガス濃度が検出される。また、前記第２セルの一対の電極のうちガス室に対面する電極は特定成分に対して活性な特定成分活性電極となっている。そして、請求項１に記載の発明ではその特徴として、酸素離脱手段により、前記第２セルの特定成分活性電極に吸着した酸素が強制的に離脱される。なお、特定成分活性電極とは、ＮＯｘ等の特定成分を分解・還元し得る能力を有する電極を言う。
【０００８】
要するに、上記構成のガス濃度センサでは、センサ放置状態等において特定成分活性電極に酸素が吸着すると、第２セルの出力が過剰に大きくなり、センサ作動開始直後において特定成分活性電極の吸着酸素が排除されるまで正確なガス濃度検出ができない。例えばＮＯｘセンサにあってＮＯｘ活性電極としてロジウムを用いた場合には当該電極に多くの酸素が吸着し、その酸素排除に長い時間を要することから上記問題が顕著になる。これに対し本発明では、第２セルの特定成分活性電極に吸着した酸素を強制的に離脱する構成としたため、吸着酸素が早期に排除できる。それ故、センサ作動開始時等においてガス室内の状態が早期にガス濃度検出に最適な状態となり、正確なガス濃度出力をいち早く得ることができるようになる。
【０００９】
前記酸素離脱手段として具体的には、請求項２に記載したように、所定期間内で通常よりも高い電圧を前記第２セルの電極間に印加すると良い。この場合、第２セルへの印加電圧を高くすることにより、第２セルの特定成分活性電極に多くのエネルギが付与され、当該活性電極において吸着酸素の分解が促進される。故に、吸着酸素が早期に排除できる。
【００１０】
一般にジルコニア等よりなる固体電解質では０〜０．９Ｖの範囲で電圧が印加され、通常時は０．４５Ｖ程度の電圧が印加される。かかる場合において、請求項３に記載したように、通常時の印加電圧に対して０．２Ｖ以上高い電圧を前記第２セルの電極間に印加すると良い。つまりこれは、第２セルに０．６５Ｖ以上の電圧を印加することに相当し、実際には０．６５Ｖ〜０．９Ｖの範囲で、より望ましくは０．７Ｖ前後で当該印加電圧を設定すると良い。
【００１１】
請求項４に記載の発明では、前記酸素離脱手段により、ガス濃度センサの作動開始直後において酸素離脱の処理が実施される。つまり、第２セルの特定成分活性電極における酸素吸着の問題は、ガス濃度センサを放置状態から作動開始した直後に起こるものであり、かかる作動開始直後に酸素離脱の処理を実施するのが望ましい。
【００１２】
ガス濃度センサとして、ガス室が、被検出ガスが導入される第１ガス室と前記第２セルが設けられる第２ガス室とよりなり且つ、第１セルが、第１ガス室側及び第２ガス室側に各々分離して設けられた第１の酸素ポンプセル及び第２の酸素ポンプセルを有するものがある。このガス濃度センサを用いたガス濃度検出装置では、請求項５に記載したように、前記酸素離脱手段により、ガス濃度センサの作動開始直後において前記第２の酸素ポンプセルの出力が所定値を超えることを条件に酸素離脱の処理が実施されると良い。なお、第１の酸素ポンプセル及び第２の酸素ポンプセルは、それぞれにポンプセル及びモニタセルとも称される。
【００１３】
つまり、上記構成のガス濃度センサおいて、センサ放置後であれば作動開始直後には、第２ガス室内に充満する比較的高濃度の酸素により第２の酸素ポンプセル（モニタセル）の出力も過剰に大きくなるが、ガス濃度センサの作動停止直後の再作動時であれば、こうした事態は生じない。故に、第２の酸素ポンプセルの出力レベルに応じて酸素離脱の処理を実施すれば、当該酸素離脱の処理を必要時にのみ実施することができるようになる。
【００１４】
上記請求項５の発明では請求項６に記載したように、前記酸素離脱手段は、前記第２の酸素ポンプセルの出力が所定値を超えた後、同所定値以下に収束したことを条件に酸素離脱の処理を終了すると良い。つまり、第２セルに比べて第２の酸素ポンプセルは出力の収束が早く、その出力レベルによれば第２ガス室内の余剰酸素の排出が進んだこと、ひいては第２セル電極の酸素離脱が進んだことが判断できる。故に、適正な期間での酸素離脱の処理が実施できる。
【００１５】
また、請求項７に記載の発明では、前記第２セルの特定成分活性電極における酸素吸着の状態を推測し、該推測した酸素吸着の状態に応じて、前記酸素離脱手段により酸素離脱を実施するための所定期間の長さを可変に設定する。この場合、実際の酸素吸着の程度に合わせて酸素離脱の処理が実施でき、当該処理の適正化が可能となる。例えば、ガス濃度センサの放置時間に応じて酸素吸着の状態を推測することとし、放置時間が長いほど、酸素離脱を実施する所定時間を長くする。
【００１６】
また、請求項８，９に記載の発明では、ガス成分排出手段により、前記ガス室内に存在する酸素又は特定成分が強制的に早期排出される。センサ放置状態等においてガス室内に大気が導入されると、当該ガス室内が酸素過多の状態となり、センサ作動開始直後において特定成分のガス濃度検出に支障が出る。また、ガス室内で特定成分過多となる場合にも、当該特定成分のガス濃度検出に支障が出る。これに対し本発明によれば、ガス室内で余剰となる酸素や特定成分が早期に排出できることから、センサ作動開始時等においてガス室内の状態が早期にガス濃度検出に最適な状態となり、正確なガス濃度出力をいち早く得ることができるようになる。特に、上記請求項１乃至７の発明とガス成分排出手段とを組み合わせることで相乗的な効果が期待でき、より一層望ましいガス濃度検出装置が実現できる。
【００１７】
請求項１０に記載の発明では、ガス濃度センサの作動開始直後における所定期間でガス成分排出の処理が実施される。つまり、ガス室（特に後述する第２ガス室）内での酸素過多の問題は、ガス濃度センサを放置状態から作動開始した直後に起こるものであり、かかる作動開始直後に酸素排出の処理を実施するのが望ましい。
【００１８】
ガス室が第１ガス室及び第２ガス室に区画され、第２セルが第１ガス室よりも下流の第２ガス室側に設けられるセンサ構成にあっては、第２ガス室には第１セルによる酸素ポンプ作用が及ばない。故に、請求項１１に記載したように、前記ガス成分排出手段により、第２ガス室内の余剰酸素又は余剰特定成分が排出される構成とすると良い。
【００１９】
前記第１セルが、第１ガス室側及び第２ガス室側に各々分離して設けられた第１の酸素ポンプセル及び第２の酸素ポンプセルを有するガス濃度センサでは、請求項１２に記載したように、ガス濃度センサの作動開始直後において前記第２の酸素ポンプセルの出力が所定値を超えることを条件にガス成分排出の処理が実施されると良い。センサ放置後であれば作動開始直後には、第２ガス室内に充満する比較的高濃度の酸素により第２の酸素ポンプセル（モニタセル）の出力も過剰に大きくなるが、ガス濃度センサの作動停止直後の再作動時であれば、こうした影響は小さくなる。故に、第２の酸素ポンプセルの出力レベルに応じてガス成分排出の処理を実施すれば、当該処理を必要時にのみ実施することができるようになる。
【００２０】
上記請求項１２の発明では請求項１３に記載したように、前記ガス成分排出手段は、前記第２の酸素ポンプセルの出力が所定値を超えた後、同所定値以下に収束したことを条件にガス成分排出の処理を終了すると良い。つまり、第２セルに比べて第２の酸素ポンプセルは出力の収束が早く、その出力レベルによれば第２ガス室内の余剰酸素の排出が進んだことが判断できる。故に、適正な期間でのガス濃度排出の処理が実施できる。
【００２１】
また、請求項１４に記載の発明では、前記第２ガス室内の余剰酸素の量又は余剰特定成分の量を推測し、該推測した余剰酸素の量又は余剰特定成分の量に応じて、前記ガス成分排出手段によりガス成分排出を実施するための所定期間の長さを可変に設定する。この場合、実際の酸素吸着の程度に合わせてガス成分排出の処理が実施でき、当該処理の適正化が可能となる。例えば、ガス濃度センサの放置時間に応じて余剰酸素の量又は余剰特定成分の量を推測することとし、放置時間が長いほど、ガス濃度排出を実施する所定時間を長くする。
【００２２】
また、請求項１５に記載の発明では、ガス室内が特定成分の高濃度雰囲気に移行した際において、当該ガス室内に存在する特定成分が強制的に早期排出される。仮に被検出ガス中における特定成分のガス濃度が一時的に増加すると、それに対応してガス室内が特定成分過多の状態となるが、当該特定成分の排出が早期に実施できる。
【００２３】
車両用エンジンでは、例えば高負荷運転時や高回転運転時等においてＮＯｘの排出量が増える傾向にあり、ＮＯｘ過多の状態となればその余剰分を排出できるまで正確なＮＯｘ濃度出力が得られなくなる。そこで、請求項１６に記載したように、エンジンがＮＯｘ排出増加傾向となる運転状態に移行した際、ガス室内に存在するＮＯｘが強制的に早期排出されると良い。
【００２４】
また、請求項１７に記載の発明では、前記ガス成分排出手段は電流操作手段を備え、該電流操作手段により、前記第２セルに流れる電流が所定期間にて強制的に増大される。つまり、第２セルでは、特定成分を還元又は分解しその際発生する酸素イオン量に応じて電流を流すが、その電流は微弱電流であり、検出精度向上のため電流検出抵抗は高抵抗のものが使用される。故に、第２セルによる酸素等の排出能力は元々低レベルに制限されるが、本発明により、酸素等の排出能力が高められる（後述する第２の酸素ポンプセルを用いる場合も同様）。
【００２５】
より具体的な構成として、請求項１８に記載したように、前記第２セルの電流検出のための電流検出抵抗を、ガス濃度検出に適した抵抗値とそれよりも小さな抵抗値とで切り替え可能とし、前記電流操作手段は、電流検出抵抗の抵抗値を小さいものに切り替えることにより第２セル電流を強制的に大きくすると良い。
【００２６】
かかる場合、請求項１９に記載したように、前記電流検出抵抗は並列関係にある少なくとも２つの抵抗を有し、これら抵抗を選択的に接続又は遮断する構成であると良い。
【００２７】
また、素子抵抗値（例えば素子インピーダンス）を検出する構成を有するガス濃度検出装置では、第２セルの印加電圧又は電流を交流的に変化させ、その時の電流及び電圧の変化量を電流検出抵抗により計測して素子抵抗値が検出される。このとき、ガス濃度検出時とガス濃度センサの素子抵抗検出時とでは第２セルに流れる電流レベルが大小相違するため、電流検出抵抗の抵抗値を切り替えることが考えられる。かかる構成において、請求項２０に記載したように、前記電流操作手段は、素子抵抗検出時に適用する抵抗を用いて第２セル電流を強制的に大きくすると良い。本構成では、新たに構成を付加せずとも上記の電流操作が実現でき、コスト面でのメリットも得られるようになる。
【００２８】
ガス濃度センサとして、ガス室が、被検出ガスが導入される第１ガス室と前記第２セルが設けられる第２ガス室とよりなり且つ、第１セルが、第１ガス室側及び第２ガス室側に各々分離して設けられた第１の酸素ポンプセル及び第２の酸素ポンプセルを有するものを用いる場合、請求項２１に記載したように、前記電流操作手段は、前記第２セルに代えて又はこれに加えて、第２の酸素ポンプセル（モニタセル）に流れる電流を所定期間にて強制的に大きくし前記第２ガス室内に存在する酸素を強制的に排出させると良い。本構成では、第２の酸素ポンプセルによる酸素排出能力が高められる。故に、既述の発明と同様に、所望の効果が達成できる。
【００２９】
上記請求項２１の発明でも、大小異なる抵抗値を切り替える構成や、素子抵抗検出用の抵抗を流用する構成等が採用できる（上記請求項１８〜２０と同様）。すなわち、
・請求項２２に記載の発明では、第２の酸素ポンプセルの電流検出のための電流検出抵抗を、酸素濃度検出に適した抵抗値とそれよりも小さな抵抗値とで切り替え可能とし、前記電流操作手段は、電流検出抵抗の抵抗値を小さいものに切り替えることにより第２の酸素ポンプセル電流を強制的に大きくする。
・請求項２３に記載の発明では、前記電流検出抵抗は並列関係にある少なくとも２つの抵抗を有し、これら抵抗を選択的に接続又は遮断する構成とした。
・請求項２４に記載の発明では、前記電流操作手段は、素子抵抗検出時に適用する抵抗を用いて第２の酸素ポンプセル電流を強制的に大きくする。
【００３０】
一方、請求項２５に記載の発明では、ガス濃度センサは、固体電解質と該固体電解質に配置された一対の電極とよりなる検出セルを有し、前記一対の電極のうちガス室に対面する電極を特定成分に対して活性な特定成分活性電極としている。そして、酸素離脱手段により、前記検出セルの特定成分活性電極に吸着した酸素が強制的に離脱される。本発明によれば、検出セルの特定成分活性電極に吸着した酸素が早期に排除できるため、センサ作動開始時等においてガス室内の状態が早期にガス濃度検出に最適な状態となり、正確なガス濃度出力をいち早く得ることができるようになる。
【００３１】
請求項２６に記載の発明では、前記酸素離脱手段は、所定時間内で通常よりも高い電圧を前記検出セルの電極間に印加する。従って、検出セルの特定成分活性電極に多くのエネルギが付与され、当該活性電極において吸着酸素の分解が促進される。故に、吸着酸素が早期に排除できる。
【００３２】
【発明の実施の形態】
以下、この発明を具体化した一実施の形態を図面に従って説明する。本実施の形態におけるガス濃度検出装置は、例えば自動車用エンジンに適用されるものであって、限界電流式ガス濃度センサを用い、被検出ガスである排ガスから酸素濃度を検出すると共に特定成分のガス濃度としてのＮＯｘ濃度を検出する。
【００３３】
先ずはじめに、ガス濃度センサの構成を図２を用いて説明する。図２のガス濃度センサは、ポンプセル、モニタセル及びセンサセルからなる３セル構造を有し、排ガス中の酸素濃度とＮＯｘ濃度とを同時に検出可能な、いわゆる複合型ガスセンサとして具体化されている。なお、ポンプセル及びモニタセルが「第１セル」に相当し、これら各セルにより、チャンバ（ガス室）に導入した排ガス中の酸素量が所定の低濃度レベルに調整される。また、センサセルが「第２セル」に相当し、同センサセルにより、酸素量調整後のガスからＮＯｘ濃度が検出される。モニタセルは、ポンプセル同様、ガス中の酸素ポンプの機能を具備しており、ポンプセルが「第１の酸素ポンプセル」に相当し、モニタセルが「第２の酸素ポンプセル」に相当する。図２（ａ）は、センサ素子の先端部構造を示す断面図であり、図２（ｂ）は、図２（ａ）のＡ−Ａ線断面図である。
【００３４】
ガス濃度センサ１００において、ジルコニア等の酸素イオン伝導性材料からなる固体電解質（固体電解質素子）１４１，１４２はシート状をなし、アルミナ等の絶縁材料からなるスペーサ１４３を介して図の上下に所定間隔を隔てて積層されている。このうち、図の上側の固体電解質１４１にはピンホール１４１ａが形成されており、このピンホール１４１ａを介して当該センサ周囲の排ガスが第１チャンバ１４４内に導入される。第１チャンバ１４４は、絞り部１４５を介して第２チャンバ１４６に連通している。第１チャンバ１４４が「第１ガス室」に相当し、第２チャンバ１４６が「第２ガス室」に相当する。符号１４７は多孔質拡散層である。
【００３５】
図の下側の固体電解質１４２には、第１チャンバ１４４に対面するようにしてポンプセル１１０が設けられており、ポンプセル１１０は、第１チャンバ１４４内に導入した排ガス中の酸素を排出又は汲み込む働きをすると共に酸素排出又は汲み込みの際に排ガス中の酸素濃度を検出する。ここで、ポンプセル１１０は、固体電解質１４２を挟んで上下一対の電極１１１，１１２を有し、そのうち特に第１チャンバ１４４側の電極１１１はＮＯｘ不活性電極（ＮＯｘガスを分解し難い電極）である。ポンプセル１１０は、第１チャンバ１４４内に存在する酸素を分解して電極１１２より大気通路１５０側に排出する。
【００３６】
また、図の上側の固体電解質１４１には、第２チャンバ１４６に対面するようにしてモニタセル１２０及びセンサセル１３０が設けられている。モニタセル１２０は、第２チャンバ１４６内の残留酸素濃度に応じて起電力、又は電圧印加に伴い電流出力を発生する。また、センサセル１３０は、ポンプセル１１０を通過した後のガスからＮＯｘ濃度を検出する。
【００３７】
特に本実施の形態では、図２（ｂ）に示すように、排ガスの流れ方向に対して同等位置になるよう、モニタセル１２０及びセンサセル１３０が並列に配置されると共に、これら各セル１２０，１３０の大気通路１４８側の電極が共通電極１２２となっている。すなわち、モニタセル１２０は、固体電解質１４１とそれを挟んで対向配置された電極１２１及び共通電極１２２とにより構成され、センサセル１３０は、同じく固体電解質１４１とそれを挟んで対向配置された電極１３１及び共通電極１２２とにより構成されている。モニタセル１２０の電極１２１（第２チャンバ１４６側の電極）はＮＯｘガスに不活性なＡｕ−Ｐｔ等の貴金属からなるのに対し、センサセル１３０の電極１３１（第２チャンバ１４６側の電極）はＮＯｘガスに活性なロジウムＲｈ、白金Ｐｔ等の貴金属からなり、この電極１３１が「特定成分活性電極」に相当する。
【００３８】
図３（ａ）は、モニタセル１２０及びセンサセル１３０の電極配置を第２チャンバ１４６側から見た平断面図であり、図３（ｂ）は、これら各セルの電極配置を大気通路１４８側から見た平断面図である。本構成によれば、モニタセル１２０及びセンサセル１３０では排ガス導入距離が同じになる。その結果、ポンプセル１１０通過後の残留酸素に対するモニタセル１２０とセンサセル１３０との感度が同等になり、精度の高いガス濃度検出が可能になる。但し、モニタセル１２０及びセンサセル１３０の電極は、図３（ａ）のように、排ガスの流れ方向に沿って並列に配置すること以外に、排ガスの流れ方向に前後（すなわち、図の左右）に配置することも可能である。例えば、モニタセル１２０を上流側（図の左側）に、センサセル１３０を下流側（図の右側）に配置する。また、各セルにおいて共通電極１２２を用いることも必須ではなく、各セル個別の電極を用いることも可能である。
【００３９】
固体電解質１４２の図の下面にはアルミナ等よりなる絶縁層１４９が設けられ、この絶縁層１４９により大気通路１５０が形成されている。また、絶縁層１４９には、センサ全体を加熱するためのヒータ１５１が埋設されている。ヒータ１５１はポンプセル１１０、モニタセル１２０及びセンサセル１３０を含めたセンサ全体を活性状態にすべく、外部からの給電により熱エネルギを発生させる。
【００４０】
上記構成のガス濃度センサ１００において、排ガスは多孔質拡散層１４７及びピンホール１４１ａを通って第１チャンバ１４４に導入される。そして、この排ガスがポンプセル１１０近傍を通過する際、ポンプセル電極１１１，１１２間に電圧Ｖｐを印加することで分解反応が起こり、第１チャンバ１４４内の酸素濃度に応じてポンプセル１１０を介して酸素が出し入れされる。なおこのとき、第１チャンバ１４４側のポンプセル電極１１１がＮＯｘ不活性電極であるので、ポンプセル１１０では排ガス中のＮＯｘは分解されず、酸素のみが分解されて大気通路１５０に排出される。そして、ポンプセル１１０に流れた電流（ポンプセル電流Ｉｐ）により、排ガス中に含まれる酸素濃度が検出される。
【００４１】
その後、ポンプセル１１０近傍を通過した排ガスは第２チャンバ１４６に流れ込み、モニタセル１２０では、ガス中の残留酸素濃度に応じた出力が発生する。モニタセル１２０の出力は、モニタセル電極１２１，１２２間に所定の電圧Ｖｍを印加することでモニタセル電流Ｉｍとして検出される。また、センサセル電極１３１，１２２間に所定の電圧Ｖｓを印加することでガス中のＮＯｘが還元分解され、その際発生する酸素が大気通路１４８に排出される。その際、センサセル１３０に流れた電流（センサセル電流Ｉｓ）により、排ガス中に含まれるＮＯｘ濃度が検出される。
【００４２】
因みに、ポンプセル１１０では、その都度の排ガス中の酸素濃度（すなわちポンプセル電流Ｉｐ）に応じて印加電圧Ｖｐが可変に制御されるようになっており、一例として、当該ポンプセル１１０の限界電流特性に基づき作成された印加電圧マップを用い、その都度のポンプセル電流Ｉｐに応じて印加電圧Ｖｐが制御される。これにより、排ガス中の酸素濃度が高くなるほど印加電圧が高電圧側にシフトするようにして印加電圧制御が実施される。それ以外に、第２チャンバ１４６内の残留酸素濃度が一定になるように、言い換えればモニタセル電流Ｉｍが一定となるように印加電圧Ｖｐがフィードバック制御される構成であっても良い。以上の制御により、第１チャンバ１４４に導入された排ガス中の酸素をいち早く排出し、酸素排出後における残留酸素濃度を所望の低濃度レベルに保持することが可能となる。
【００４３】
ところで、上述した通りガス濃度センサ１００では、ポンプセル１１０で排ガス中の余剰酸素が排出された後、低濃度レベルの残留酸素を含むガスがモニタセル１２０及びセンサセル１３０に給送される。そして、モニタセル１２０においてはガス中の残留酸素濃度に応じてモニタセル電流Ｉｍが計測され、センサセル１３０においてはガス中のＮＯｘ濃度に応じてセンサセル電流Ｉｓが計測される。但しこのとき、センサセル１３０では、本来ガス中のＮＯｘだけが還元分解されてそれに伴う電流値が計測されるのが望ましいが、実際にはガス中の残留酸素分（微量酸素量）に応じた電流成分も併せて計測される。つまり、計測されるセンサセル電流ＩｓにはＮＯｘ反応分と残留酸素反応分とが含まれ、このうち残留酸素反応分がオフセット誤差となる。そこで本実施の形態では、センサセル電流Ｉｓからオフセット誤差分を排除すべく、計測されたセンサセル電流Ｉｓからモニタセル電流Ｉｍを減算し、その差分値（Ｉｓ−Ｉｍ）に基づいてＮＯｘ濃度出力を得るようにしている。
【００４４】
次に、ガス濃度検出装置の電気的な構成を図１を用いて説明する。なお図１には、前述のガス濃度センサ１００を用いたガス濃度検出装置を示すが、モニタセル１２０及びセンサセル１３０の電極配置については、便宜上、横並びの状態で示す。
【００４５】
図１において、制御回路２００は、ＣＰＵ、Ａ／Ｄ変換器、Ｄ／Ａ変換器、Ｉ／Ｏポート等を備える周知のマイクロコンピュータで構成されており、各セル１１０〜１３０の印加電圧指令値をＤ／Ａ変換器（Ｄ／Ａ０〜Ｄ／Ａ２）より適宜出力する。また、制御回路２００は、各セル１１０〜１３０に流れる電流の計測結果を取り込むべく、各端子Ｖｃ、Ｖｅ、Ｖｄ、Ｖｂ、Ｖｇ、Ｖｈの電圧をＡ／Ｄ変換器（Ａ／Ｄ０〜Ａ／Ｄ５）より各々入力する。制御回路２００は、ポンプセル１１０やセンサセル１３０で計測された電流値に基づいて排ガス中の酸素濃度（Ａ／Ｆ）やＮＯｘ濃度を演算し、その演算結果をＤ／Ａ変換器（Ｄ／Ａ４，Ｄ／Ａ３）や通信回路を介して外部のエンジンＥＣＵ等に出力する。
【００４６】
回路構成について詳しくは、ポンプセル１１０において一方の電極１１２には、基準電源２０１及びオペアンプ２０２により基準電圧Ｖａが印加され、他方の電極１１１には、オペアンプ２０３及び電流検出抵抗２０４を介して制御回路２００の指令電圧Ｖｂが印加される。指令電圧Ｖｂの印加に際し、排ガス中の酸素濃度に応じてポンプセル１１０に電流が流れると、その電流が電流検出抵抗２０４により検出される。つまり、電流検出抵抗２０４の両端子電圧Ｖｂ，Ｖｄが制御回路２００に取り込まれ、その電圧Ｖｂ，Ｖｄによりポンプセル電流Ｉｐが算出される。
【００４７】
なお、エンジン運転状態の変化等によりポンプセル電流Ｉｐが変化する際、その変化量が大きいと電流の取込値にテーリング等が発生し、電流計測の精度が落ちてしまう。故に、そのテーリング防止のために毎回の電流変化幅を制限し、更になまし処理を施してポンプセル電流の計測値を確定することとしている。
【００４８】
また、モニタセル１２０及びセンサセル１３０の共通電極１２２には、基準電源２０５及びオペアンプ２０６により基準電圧Ｖｆが印加され、共通電極１２２とは異なる方のセンサセル電極１３１には、オペアンプ２０７及び電流検出抵抗２０８を介して制御回路２００の指令電圧Ｖｇが印加される。指令電圧Ｖｇの印加に際し、ガス中のＮＯｘ濃度に応じてセンサセル１３０に電流が流れると、その電流が電流検出抵抗２０８により検出される。つまり、電流検出抵抗２０８の両端子電圧Ｖｇ，Ｖｈが制御回路２００に取り込まれ、その電圧Ｖｇ，Ｖｈによりセンサセル電流Ｉｓが算出される。
【００４９】
また、共通電極１２２とは異なる方のモニタセル電極１２１には、オペアンプ２０９及び電流検出抵抗２１０を介して制御回路２００の指令電圧Ｖｃが印加される。指令電圧Ｖｃの印加に際し、ガス中の残留酸素濃度に応じてモニタセル１２０に電流が流れると、その電流が電流検出抵抗２１０により検出される。つまり、電流検出抵抗２１０の両端子電圧Ｖｃ，Ｖｅが制御回路２００に取り込まれ、その電圧Ｖｃ，Ｖｅによりモニタセル電流Ｉｍが算出される。
【００５０】
ヒータ駆動系の構成は図示を省略するが、制御回路２００内のＣＰＵは、ヒータ駆動用のＭＯＳＦＥＴ等に対してデューティ制御信号を出力し、電源（例えばバッテリ電源）からヒータ１５１への供給電力をＰＷＭ制御する。このとき、モニタセル１２０及びセンサセル１３０での検出電流はヒータ電流に比べて微弱な電流であるため、ヒータ通電に伴い発生するノイズが上記各セル１２０，１３０による検出電流に悪影響を及ぼすおそれがある。そのノイズ対策として本実施の形態では、センサセル１３０及びモニタセル１２０用の各電流検出抵抗２０８，２１０に並列にそれぞれコンデンサ２１１，２１２を設けてフィルタを構成している。これにより、ヒータ通電に伴うノイズの問題が効果的に抑制できることとなる。また、上述した通りセンサセル電流Ｉｓとモニタセル電流Ｉｍとの差分値（Ｉｓ−Ｉｍ）に基づいてＮＯｘ濃度出力を得るようにしている。そのため、センサセル１３０及びモニタセル１２０において電流検出抵抗２０８，２１０とフィルタ用コンデンサ２１１，２１２の定数を合わせて時定数などを一致させると良い。
【００５１】
なお、センサセル電流Ｉｓとモニタセル電流Ｉｍとの差分値（Ｉｓ−Ｉｍ）を求める場合において、センサセル電流信号とモニタセル電流信号とを差動増幅回路に入力し、該差動増幅回路の出力（すなわち、Ｉｓ−Ｉｍ）を制御回路２００にてＡ／Ｄ変換する構成とするのが望ましい。これにより、差動増幅回路のゲインを大きくすることができ、ＮＯｘ濃度信号（Ｉｓ−Ｉｍ）のダイナミックレンジが広がる。故に、ＮＯｘ濃度の検出精度が向上する。
【００５２】
また本実施の形態では、いわゆる掃引法を用い、センサセル１３０を対象に「素子抵抗値」としての素子インピーダンスが検出される。つまり、センサセル１３０のインピーダンス検出時において、制御回路２００により、センサセル印加電圧（指令電圧Ｖｇ）が正側又は負側の少なくとも何れかに瞬間的に（例えば数１０〜１００μｓｅｃ程度の時間で）切り替えられる。この交流的に変化する電圧は、図示しないローパスフィルタ（ＬＰＦ）により正弦波的になまされつつセンサセル１３０に印加される。交流電圧の周波数は１０ｋＨｚ以上が望ましく、ＬＰＦの時定数は５μｓｅｃ程度で設定される。そして、その時の電圧変化量と電流変化量とからセンサセル１３０の素子インピーダンスが算出される（インピーダンス＝電圧変化量／電流変化量）。センサセル１３０の印加電圧を交流的に変化させる構成の他、センサセル１３０に流れる電流を交流的に変化させる構成であっても良い。
【００５３】
ガス濃度センサ１００のヒータ制御に際し、素子インピーダンスを所定の目標値に制御する素子抵抗フィードバック制御が実施される。これにより、各セル１１０〜１３０が所望の活性状態に保持されるようになっている。
【００５４】
なお、素子抵抗値として、素子インピーダンスに代えてその逆数である素子アドミタンスを検出する構成であっても良い。また、センサセル１３０のインピーダンス（又はアドミタンス）を検出することに代えて、モニタセル１２０のインピーダンス（又はアドミタンス）を検出する構成や、ポンプセル１１０のインピーダンス（又はアドミタンス）を検出する構成としても良い。
【００５５】
因みに、モニタセル１２０及びセンサセル１３０では、一方の電極を共通電極１２２としたため、基準電圧側のドライブ回路が削減できるというメリットや、ガス濃度センサ１００からのリード線の取り出し本数が削減できるというメリットが得られる。また、モニタセル１２０とセンサセル１３０とは同じ固体電解質１４１で隣り合って形成されるため、掃引時には隣の電極に電流が流れ、素子インピーダンスの検出精度が悪化することが懸念されるが、共通電極１２２を設けることで一方の電極が同じ電位となり、この影響が低減できる。
【００５６】
また、センサセル１３０ではＮＯｘにより流れる電流はμＡオーダであり、検出したいＮＯｘ濃度範囲ではｎＡ（ナノアンペア）オーダという非常に微弱な電流値となる。そのため、検出精度を確保しようとすると、電流検出抵抗２０８の抵抗値はＭΩオーダとなる。これに対し、インピーダンス検出のための掃引時にはｍＡオーダの電流が流れる。このオーダの異なる電流を同じ検出抵抗で検出すると、オーバーレンジしたり、検出精度が悪くなったりする。そこで本実施の形態では、センサセル１３０によるＮＯｘ濃度検出時とインピーダンス検出時とで電流検出抵抗を切り替えることとしている。具体的には、電流検出抵抗２０８に並列に、別の電流検出抵抗２１５とスイッチ回路２１６（例えば、半導体スイッチ）とを設けている。そして、制御回路２００のＩ／Ｏポートからの出力により、スイッチ回路２１６をＯＮ／ＯＦＦさせるよう構成している。この場合、通常のガス濃度検出時には、スイッチ回路２１６がＯＦＦ（開放）され、電流検出抵抗２０８によるＭΩオーダの抵抗でセンサセル電流Ｉｓが検出される。これに対し、インピーダンス検出時には、スイッチ回路２１６がＯＮ（閉鎖）され、電流検出抵抗２０８及び２１５による数百Ω程度の抵抗でセンサセル電流Ｉｓが検出される。
【００５７】
ところで、上記構成のガス濃度センサ１００では、センサ作動開始前（エンジン始動前）の放置状態においてセンサセル１３０のＮＯｘ活性電極に多くの酸素が吸着されることや、センサ放置状態では第１及び第２チャンバ１４４，１４６内に大気レベルの比較的高濃度（約２０．９％）の酸素が充満されることから、センサ作動開始後において正確なＮＯｘ濃度出力が得られるまでに長い時間を要するという事態が生ずる。特に、ＮＯｘ活性電極にロジウムを添加した本構成では、ロジウムの酸素吸着（酸素ストレージ）により上記不都合を招く。そこで、本実施の形態のガス濃度検出装置では、上記不都合を解消すべく、次の対策を講ずることとしている。
（１）センサ作動開始直後において、センサセル電極に通常時よりも高い電圧を印加し、センサセル電極（ＮＯｘ活性電極）の吸着酸素を強制的に離脱させる（酸素離脱手段）。
（２）センサ作動開始直後において、センサセル１３０用の電流検出抵抗を低抵抗に切り替えてセンサセル電流を大きくし、第２チャンバ１４６内の酸素を強制的に早期排出させる（ガス成分排出手段、電流操作手段）。
【００５８】
ここで、上記（２）についてその概要を説明する。センサセル１３０に接続された電流検出抵抗２０８はＭΩオーダの抵抗値を有し、仮に抵抗値が１ＭΩであればセンサセル１３０に流せる電流は最大６μＡ程度となる。モニタセル１２０も同様であることから、第２チャンバ側のセンサセル１３０及びモニタセル１２０で流せる電流、すなわちセンサセル電流とモニタセル電流との合計は最大１２μＡ程度となる。これが第２チャンバ側で流せる最大電流になり、同第２チャンバ側での酸素排出能力にも相当する。この場合、センサセル１３０及びモニタセル１２０で流せる電流が上記の如く制限されるため、第２チャンバ１４６での余剰酸素排出が遅れ、ひいてはＮＯｘ濃度出力の正常化が遅れるという事態を招く。エンジンの通常運転状態で第１チャンバ１４４内に導入されるガス中の酸素はポンプセル１１０にて排出されるが、エンジン始動前に第１及び第２のチャンバ１４４，１４６に満たされた大気による酸素はポンプセル１１０による排出に加え、センサセル１３０又はモニタセル１２０にて早期に排出してやる必要がある。
【００５９】
上述した図１の構成では、電流検出抵抗２０８に並列に別の電流検出抵抗２１５とスイッチ回路２１６とが接続され、スイッチ回路２１６のＯＮ／ＯＦＦによりセンサセル電流検出のための抵抗値が大小切り替えられるようになっている。従って、電流検出抵抗２０８，２１５の抵抗値をそれぞれ１ＭΩ、５００Ωとした場合、スイッチ回路２１６をＯＦＦすることで、センサセル１３０を流れる電流が電流検出抵抗２０８の抵抗値１ＭΩにより検出されるのに対し、スイッチ回路２１６をＯＮすることで、同じくセンサセル１３０を流れる電流が電流検出抵抗２０８及び２１５の合成抵抗値４９９．８Ωにより検出される。故に、センサセル１３０に流せる電流レベルが操作できるようになり（本実施の形態では、最大２０ｍＡ程度の電流）、第２チャンバ１４６内における余剰酸素の早期排出を強制的に行わせることができるようになる。
【００６０】
素子インピーダンスの値は数十Ω程度であるため、インピーダンス検出のための電流検出抵抗は数百Ωオーダの抵抗値となる。この場合、インピーダンス検出のための電流検出抵抗の抵抗値はＮＯｘ濃度検出のための抵抗値より格段に小さいため、このインピーダンス検出用の抵抗を利用することができる。これにより、新たな構成を付加せずともセンサセル電流の上記操作を行うことができ、コスト面でのメリットも得られるようになる。
【００６１】
以下、上記対策の具体的な実施態様を説明する。図４は、センサセル印加電圧の制御手順を示すフローチャートであり、この処理は制御回路２００内のＣＰＵにより所定時間周期で実施される。
【００６２】
図４において、先ずステップＳ１０１では、モニタセル電流Ｉｍが所定の判定値Ｋａよりも大きいか否かを判別する。判定値Ｋａは、第２チャンバ１４６内の酸素が高濃度になっていることを判定するための判定レベルであり、例えばＫａ＝５００ｎＡとする。この場合、第２チャンバ１４６内の酸素が高濃度となり、モニタセル電流Ｉｍが過剰に大きくなると、センサセル電極（ＮＯｘ活性電極）に多くの酸素が吸着されていると推測でき、センサセル印加電圧の高電圧化や抵抗切り替え（センサセル電流操作）の処理を要する旨判断される。
【００６３】
また、ステップＳ１０２では、ガス濃度センサ１００の作動開始後、所定時間が経過したか否かを判別する。この所定時間は、センサセル印加電圧の高電圧化や抵抗切り替えの処理を実施するための実施期間に相当し、固定の時間としても良いし、可変設定される時間としても良い。所定時間を固定とする場合、例えば５０秒とする。また、所定時間を可変とする場合、センサセル電極の酸素吸着状態に応じて当該所定時間を設定する。例えば、素子インピーダンスの値、ポンプセル電流（すなわち第１チャンバ１４４内の酸素濃度）、センサ放置時間（すなわちエンジン停止時間）などに応じてセンサセル電極の酸素吸着状態を推測し、その推測結果に応じて所定時間を設定することが考えられる。
【００６４】
そして、Ｉｍ≦Ｋａであるか、又は所定時間経過後であれば、センサセル印加電圧の高電圧化や抵抗切り替えの処理を実施することなく、通常の印加電圧制御を実施する。すなわち、ステップＳ１０３ではセンサセル印加電圧を例えば０．４５Ｖとして設定し、続くステップＳ１０４ではスイッチ回路２１６をＯＦＦする。その後、ステップＳ１０７では、上記設定したセンサセル印加電圧を出力する。これにより、センサセル印加電圧＝０．４５Ｖ、電流検出抵抗＝１ＭΩといった条件でセンサセル１３０の初期制御が実施される。
【００６５】
これに対し、Ｉｍ＞Ｋａであり且つ所定時間の経過前であれば、センサセル印加電圧の高電圧化や抵抗切り替えの処理を実施する。すなわち、ステップＳ１０５ではセンサセル印加電圧を通常時よりも高い０．７Ｖとし、続くステップＳ１０６ではスイッチ回路２１６をＯＮする。「０．７Ｖ」の印加電圧は一例であり、通常時の印加電圧に対して０．２Ｖ以上高い電圧、実際には０．６５Ｖ〜０．９Ｖの範囲内の電圧でセンサセル印加電圧が設定されると良い。その後、ステップＳ１０７では、上記設定したセンサセル印加電圧を出力する。これにより、センサセル印加電圧＝０．７Ｖ、電流検出抵抗＝約５００Ωといった条件でセンサセル１３０の初期制御（ここでは、センサセル印加電圧の高電圧化及び抵抗切り替えの処理）が実施される。
【００６６】
なお、上記ステップＳ１０１におけるモニタセル電流Ｉｍの条件は、センサセル印加電圧の高電圧化や抵抗切り替えの処理の開始条件としてのみ採用しても良く、或いは、当該モニタセル電流条件をなくすことも可能である。またこの他に、上記ステップＳ１０２における時間条件をなくすことも可能である。
【００６７】
次に、センサセル印加電圧の高電圧化や抵抗切り替えの処理による具体的な作用効果を説明する。但し便宜上、先ずはじめにセンサセル印加電圧の高電圧化についてその作用効果を図５を従い説明する。図５は、高電圧化処理に際してのセンサセル電流Ｉｓ、モニタセル電流Ｉｍ等の推移を示すタイムチャートである。なお、図中の一点鎖線は、センサ作動開始当初からセンサセル印加電圧固定（Ｖｓ＝０．４５Ｖ固定）とした場合の電流推移を示す。以下の説明では、センサ作動開始時には、センサセル電極に多くの酸素が吸着され、且つ第２チャンバ１４６内がほぼ大気状態になっていることとしている。
【００６８】
センサ作動開始に伴いセンサセル電流Ｉｓ及びモニタセル電流Ｉｍは共に急上昇し、計測可能な上限値（例えば２５００ｎＡ）に達する。モニタセル電流Ｉｍは判定値Ｋａを超える。また、素子アドミタンスＭＡｄｍ（素子インピーダンスの逆数）は、活性化の進行に伴い図示の如く変化する。このとき、センサセル印加電圧Ｖｓとして通常時よりも高電圧の０．７Ｖが設定され、センサセル１３０に印加される。その後、第２チャンバ１４６内の酸素の低濃度化に伴いモニタセル電流Ｉｍがセンサセル電流Ｉｓよりも先に低下し始め、該モニタセル電流Ｉｍが判定値Ｋａ以下になると、センサセル印加電圧Ｖｓが通常時の０．４５Ｖに変更される（図のタイミングｔ１）。但し、センサセル印加電圧の高電圧化の開始条件とするモニタセル電流Ｉｍの判定値と終了条件とする同判定値とを異なるレベルに設定することも可能である。
【００６９】
センサセル電流Ｉｓはモニタセル電流Ｉｍよりも遅れて低下し、図の実線で示すように推移する。このとき、センサセル印加電圧固定とした場合の電流推移（図の一点鎖線）と比較すると、本実施の形態ではセンサセル電流Ｉｓが早期に低下することが分かる。実線及び一点鎖線で示すセンサセル電流Ｉｓの推移の違いは、センサ作動開始当初においてセンサセル印加電圧Ｖｓを高くしたことによるものであり、センサセル印加電圧の高電圧化によりセンサセル電極（ＮＯｘ活性電極）に高エネルギが付与される。従って、当該センサセル電極において吸着酸素の分解が促進され、結果としてセンサセル電流が早期に低下する。
【００７０】
実際には、所定レベル（例えば１０ｐｐｍ）のＮＯｘ濃度出力が得られるまでの時間を比較すると、センサセル印加電圧の高電圧化を行わない場合よりも約１００秒程度の時間短縮が可能となる。つまり、ＮＯｘ濃度出力の正常化が早まり、これはセンサセル１３０としての活性化が早まることであるとも言える。
【００７１】
また、図６は、センサセル印加電圧の高電圧化に加え、抵抗切り替え（チャンバ内酸素の強制排出）の処理を実施した際のセンサセル電流Ｉｓ、モニタセル電流Ｉｍ等の推移を示すタイムチャートである。なお、図中の一点鎖線は、電流検出抵抗の切り替えを行わない場合の電流推移を示す。
【００７２】
センサ作動開始直後、スイッチ回路２１６がＯＮされると、電流検出抵抗２０８，２１５の合成抵抗を通じてセンサセル１３０に電流が流れる。このとき、前出の事例で言えば合成抵抗が約５００Ωとなり、センサセル電流Ｉｓは最大で０．０４ｍＡ（４０ｎＡ）程度流れる。故に、センサセル１３０による酸素排出能力が高められ、第２チャンバ１４６内の酸素排出が促進される。その後、モニタセル電流Ｉｍが判定値Ｋａ以下になると、スイッチ回路２１６がＯＦＦされる（図のタイミングｔ１１）。以降、電流検出抵抗２０８の抵抗１ＭΩにより通常通りセンサセル電流Ｉｓの計測（すなわち、ＮＯｘ濃度検出）が実施される。
【００７３】
センサセル電流Ｉｓはモニタセル電流Ｉｍよりも遅れて低下し、図の実線で示すように推移する。このとき、電流検出抵抗の切り替えを行わない場合の電流推移（図の一点鎖線）と比較すると、本実施の形態ではセンサセル電流Ｉｓ及びモニタセルＩｍが早期に低下することが分かる。従って、第２チャンバ１４６の余剰酸素の早期排出により、センサセル電流Ｉｓが早期に収束し、ひいてはＮＯｘ濃度出力がいち早く早期正できるようになる。
【００７４】
実際には、所定レベル（例えば１０ｐｐｍ）のＮＯｘ濃度出力が得られるまでの時間を比較すると、抵抗切り替えを行わない場合よりも約１００秒程度の時間短縮が可能となる。
【００７５】
以上詳述した本実施の形態によれば、以下の優れた効果が得られる。
【００７６】
センサ作動開始直後において、センサセル１３０のＮＯｘ活性電極に吸着した酸素が強制的に離脱されること、及び、第２チャンバ１４６内に存在する余剰酸素が強制的に早期排出されることにより、センサ作動開始時において第２チャンバ１４６内の状態が早期にＮＯｘ濃度検出に最適な状態となり、正確なＮＯｘ濃度出力をいち早く得ることができるようになる。
【００７７】
なお、本発明は上記実施の形態の記載内容に限定されず、例えば次のように実施してもよい。
【００７８】
上記実施の形態では、（１）センサセル印加電圧を高電圧化してセンサセル電極の吸着酸素を強制離脱させること、及び、（２）電流検出抵抗を大小切り替えてチャンバ内酸素を強制排出させることを共に実施する構成であったが、その実施態様を変更する。つまり、上記（１）のみを実施する構成、又は上記（２）のみを実施する構成としたり、上記（１），（２）をその都度のセンサ状態に応じて選択的に実施する構成としたりしても良い。
【００７９】
上記実施の形態では、第２チャンバ１４６内における酸素の強制排出を実施するにあたりセンサセル１３０側の電流検出抵抗を切り替える構成としたが、これに代えてモニタセル１２０側の電流検出抵抗を切り替える構成とすることも可能である。又は、センサセル１３０及びモニタセル１２０の両方で電流検出抵抗を切り替える構成とすることも可能である。モニタセル１２０側で抵抗切り替えを行う場合にも、上記同様、素子インピーダンス検出用の抵抗を流用すると良い。本構成であっても、モニタセル１２０による酸素排出能力が高められ、第２チャンバ１４６内に存在する酸素が強制的に排出される。故に、既述の実施の形態と同様に、所望の効果が達成できる。
【００８０】
上記図６のタイムチャートからも分かるように、センサセル１３０（又はモニタセル１２０）の電流検出抵抗を大小切り替えると、通常は計測できない電流レベルまで電流計測が可能となる。本実施の形態の場合、センサセル１３０（又はモニタセル１２０）に最大２０ｍＡ程度まで電流を流すことが可能となり、ｍＡオーダまでの電流計測が可能となる。このとき、センサ作動開始直後にセンサセル電流（又はモニタセル電流）がどの程度の電流レベルに達するかを計測することが可能となり、その電流レベルに応じてセンサセル電極への吸着酸素量を推測したり、第２チャンバ内の余剰酸素量を推測したりすることが可能となる。従って、センサセル印加電圧の高電圧化や抵抗切り替えの処理を実施するための所定時間を、その電流レベルに応じて設定するといった構成を採用することも可能となる。
【００８１】
上記実施の形態では、センサセル印加電圧の高電圧化を行う際、所定期間内でセンサセル印加電圧を一定値（０．７Ｖ）としたが、その電圧値を所定期間内で変化させる構成であっても良い。例えば、高電圧の印加後、徐々に電圧を降下させて通常値に移行させる構成とする。
【００８２】
車両用エンジンでは、例えば高負荷運転時や高回転運転時等においてＮＯｘの排出量が増える傾向にあり、第２チャンバ１４６内でＮＯｘ過多の状態となればその余剰分が排出されるまで正確なＮＯｘ濃度出力が得られなくなる。そこで、エンジンがＮＯｘ排出増加傾向となる運転状態に移行した際において、センサセル１３０に流れる電流を増大し、結果として第２チャンバ１４６内に存在するＮＯｘを強制的に早期排出するよう構成すると良い。具体的には、スロットル開度信号や燃料噴射量等に応じてその都度のエンジン負荷状態を判定し、又はエンジン回転数信号に応じてその都度のエンジン回転状態を判定し、高負荷状態又は高回転状態であれば、電流検出抵抗を低抵抗側に切り替えれば良い。
【００８３】
上記実施の形態では、図２の構造のガス濃度センサ１００について適用例を説明したが、勿論他のガス濃度センサについて適用しても良く、適用可能なガス濃度センサの具体例を以下に説明する。
【００８４】
図７に示すガス濃度センサ３００では、符号３０１，３０２，３０３，３０４で示すシート状の部材が全て固体電解質（例えばジルコニア）で構成されており、これら固体電解質３０１〜３０４が図の上下に積層されている。固体電解質３０１，３０３の間には、多孔質アルミナ等の絶縁材料からなる律速層３０５ａ，３０５ｂを隔てて第１チャンバ３０６及び第２チャンバ３０７が区画形成されている。固体電解質３０４により大気通路３０８が形成され、同固体電解質３０４にはヒータ３０９が埋設されている。
【００８５】
また、ガス濃度センサ３００には、第１ポンプセル３１０、第２ポンプセル３２０、センサセル３３０及びモニタセル３４０が設けられており、本センサ３００の特徴としてこれら各セルが固体電解質同士により結合されている。かかる構成において、第１ポンプセル３１０では、一対の電極３１１，３１２間に電圧Ｖｐ１が印加され、その時流れる第１ポンプセル電流Ｉｐ１が検出される。第２ポンプセル３２０では、電極３１３，３１４間に電圧Ｖｐ２が印加され、その時流れる第２ポンプセル電流Ｉｐ２が検出される。センサセル３３０では、電極３１４，３１５間に電圧Ｖｓが印加され、その時流れるセンサセル電流Ｉｓが検出される。更に、モニタセル３４０では、電極３１４，３１６間で起電力信号Ｖｍが検出される。なお、第１チャンバ３０６及び第２チャンバ３０７に対面する各セルの電極３１２，３１３，３１５，３１６のうち、センサセル３３０の電極３１５はＮＯｘに対して活性なロジウム、白金等よりなるＮＯｘ活性電極（特定成分活性電極）であり、他の電極はＮＯｘに対して不活性なＮＯｘ不活性電極である。
【００８６】
排ガスは、センサ先端部の律速層３０５ａを介して第１チャンバ３０６に導入される。排ガス中の大部分の酸素は、モニタセル３４０の起電力信号Ｖｍにより検出されると共に、この信号Ｖｍに応じて第１ポンプセル３１０の印加電圧Ｖｐ１が制御されることで電極３１１から外部へ排出される。残ったガスは律速層３０５ｂを通過して第２チャンバ３０７に導入され、当該ガス中の残留酸素は第２ポンプセル３２０にて電圧Ｖｐ２の印加により分解されて大気通路３０８に排出される。また、ガス中のＮＯｘは、センサセル３３０にて電圧Ｖｓの印加により分解されて大気通路３０８に排出され、その際流れたセンサセル電流Ｉｓに基づいてＮＯｘ濃度値が算出される。
【００８７】
なお図７のガス濃度センサ３００では、第１ポンプセル３１０及び第２ポンプセル３２０が「第１セル」に、センサセル３３０が「第２セル」にそれぞれ相当する。また、第１チャンバ３０６が「第１ガス室」に、第２チャンバ３０７が「第２ガス室」にそれぞれ相当する。
【００８８】
また、センサセル３３０を対象に素子インピーダンスを検出する構成とした場合において、センサセル３３０用の電流検出回路３５０には、センサセル３３０によるＮＯｘ濃度検出時とインピーダンス検出時とで電流検出抵抗を切り替える構成が付加されている。電流検出回路３５０の詳細についての図示は省略するが、図１に示すセンサセル用回路構成と同様に、高抵抗（ＭΩオーダ）の電流検出抵抗、それに並列接続された低抵抗（数百Ωオーダ）の電流検出抵抗、抵抗値を大小切り替えるためのスイッチ回路などが設けられている。この場合、通常のガス濃度検出時には、高抵抗の電流検出抵抗でセンサセル電流Ｉｓが検出され、インピーダンス検出時には、低抵抗の電流検出抵抗でセンサセル電流Ｉｓが検出される。
【００８９】
そして更に、上記構成において、（１）センサセル印加電圧を高電圧化してセンサセルＮＯｘ活性電極の吸着酸素を強制離脱させること、及び、（２）電流検出抵抗を大小切り替えてチャンバ内酸素を強制排出させることとしている。これにより、センサ作動開始時において第２チャンバ３０７内の状態が早期にＮＯｘ濃度検出に最適な状態となり、正確なＮＯｘ濃度出力をいち早く得ることができるようになる。但し、上記（１）のみを実施する構成、上記（２）のみを実施する構成、上記（１），（２）をその都度のセンサ状態に応じて選択的に実施する構成が適宜採用できる。また、センサセル３３０に代えて第２ポンプセル３２０側の電流検出抵抗を切り替える構成、又は、センサセル３３０及び第２ポンプセル３２０の両方で電流検出抵抗を切り替える構成とすることも可能である。
【００９０】
ＮＯｘ濃度を検出可能なガス濃度センサの他に、特定成分のガス濃度としてＨＣ濃度やＣＯ濃度を検出可能なガス濃度センサにも適用できる。この場合、ポンプセルにて被検出ガス中の余剰酸素を排出し、センサセルにて余剰酸素排出後のガスからＨＣやＣＯを分解してＨＣ濃度やＣＯ濃度を検出する。更に、自動車用以外のガス濃度検出装置に用いること、排ガス以外のガスを被検出ガスとすることも可能である。
【図面の簡単な説明】
【図１】発明の実施の形態におけるガス濃度検出装置の概要を示す構成図である。
【図２】ガス濃度センサの構成を示す断面図である。
【図３】モニタセル及びセンサセルの電極配置を示す平断面図である。
【図４】センサセルの印加電圧制御処理を示すフローチャートである。
【図５】センサ作動開始時におけるセンサセル電流やモニタセル電流等の推移を示すタイムチャートである。
【図６】センサ作動開始時におけるセンサセル電流やモニタセル電流等の推移を示すタイムチャートである。
【図７】別のガス濃度センサの構成を示す断面図である。
【符号の説明】
１００…ガス濃度センサ、１１０…ポンプセル、１１１，１１２…電極、１２０…モニタセル、１２１，１２２…電極、１３０…センサセル、１３１…電極、１４１，１４２…固体電解質、１４４…第１チャンバ、１４６…第２チャンバ、２００…制御回路、２０８，２１０，２１５…電流検出抵抗、２１６…スイッチ回路、３００…ガス濃度センサ、３０１，３０３…固体電解質、３０６…第１チャンバ、３０７…第２チャンバ、３１０…第１ポンプセル、３１１〜３１６…電極、３２０…第２ポンプセル、３３０…センサセル、３４０…モニタセル。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a gas concentration detection device that detects a gas concentration of a specific component in a gas to be detected.
[0002]
[Prior art]
As this type of gas concentration detecting device, there is a device which uses a limiting current type gas concentration sensor and detects, for example, NOx (nitrogen oxide) in exhaust gas discharged from a vehicle engine. The gas concentration sensor has, for example, a three-cell structure including a pump cell, a sensor cell, and a monitor cell. In the pump cell, oxygen in exhaust gas introduced into the first chamber is discharged or pumped, and at the same time, oxygen concentration in exhaust gas is detected. Is Further, the sensor cell detects the NOx concentration (gas concentration of a specific component) from the gas in the second chamber after passing through the pump cell, and the monitor cell detects the residual oxygen concentration in the second chamber (for example, Patent Document 1).
[0003]
[Patent Document 1]
Japanese Patent No. 2885336
[0004]
[Problems to be solved by the invention]
In the gas concentration sensor, when the sensor is left unattended, the atmosphere is introduced into the first and second chambers, and is filled with a relatively high concentration (about 20.9%) of oxygen. Then, immediately after the start of the sensor operation, a situation arises in which accurate sensor output cannot be obtained due to the high concentration of oxygen in the first and second chambers. Analyzing the data from the actual machine, it is found that immediately after the operation of the gas concentration sensor, the sensor cell current becomes excessively large, and then it takes a long time to converge to a stable current value in a predetermined NOx concentration detection range. confirmed. Therefore, there is a problem that the time required for normalizing the sensor cell current, that is, the NOx concentration output is prolonged.
[0005]
On the other hand, the sensor cell current flowing according to the NOx concentration is a current value on the order of nA (nano-ampere), and it is necessary to increase the resistance value of the current detection resistor to several hundred kΩ or more in order to accurately detect this weak current. is there. Then, it takes a long time to discharge the high-concentration oxygen filled in the chamber, which also causes a problem that the time required for normalizing the sensor cell current, that is, the NOx concentration output is prolonged.
[0006]
The present invention has been made in view of the above circumstances, and a purpose thereof is to make a state in a chamber (gas chamber) an optimal state for gas concentration detection at an early stage at the start of operation of a gas concentration sensor, and the like. An object of the present invention is to provide a gas concentration detecting device capable of promptly obtaining an accurate gas concentration output.
[0007]
[Means for Solving the Problems]
The gas concentration detecting device according to claim 1 is applied to a gas concentration sensor having the following configuration. That is, the gas concentration sensor has a first cell and a second cell each including a solid electrolyte and a pair of electrodes disposed on the solid electrolyte. In the first cell, the gas contained in the gas to be detected introduced into the gas chamber is used. The oxygen amount is adjusted to a predetermined low concentration level, and the second cell detects the gas concentration of the specific component from the gas after the oxygen amount adjustment in the first cell. The electrode facing the gas chamber of the pair of electrodes of the second cell is a specific component active electrode that is active for a specific component. The first aspect of the invention is characterized in that oxygen adsorbed on the specific component active electrode of the second cell is forcibly released by the oxygen releasing means. The specific component active electrode refers to an electrode having an ability to decompose and reduce a specific component such as NOx.
[0008]
In short, in the gas concentration sensor having the above-described configuration, when oxygen is adsorbed on the specific component active electrode while the sensor is left unattended, the output of the second cell becomes excessively large, and the adsorbed oxygen of the specific component active electrode is eliminated immediately after the sensor operation starts. Gas concentration cannot be detected accurately until it is detected. For example, when rhodium is used as a NOx active electrode in a NOx sensor, a large amount of oxygen is adsorbed on the electrode, and it takes a long time to remove the oxygen. On the other hand, in the present invention, since the oxygen adsorbed on the specific component active electrode of the second cell is forcibly released, the adsorbed oxygen can be eliminated early. Therefore, at the start of the operation of the sensor or the like, the state in the gas chamber becomes an optimal state for detecting the gas concentration at an early stage, and an accurate gas concentration output can be obtained promptly.
[0009]
Specifically, as the oxygen desorbing means, it is preferable to apply a higher voltage than usual within a predetermined period between the electrodes of the second cell, as described in claim 2. In this case, by increasing the voltage applied to the second cell, a large amount of energy is applied to the specific component active electrode of the second cell, and the decomposition of adsorbed oxygen is promoted at the active electrode. Therefore, the adsorbed oxygen can be eliminated early.
[0010]
In general, a voltage in the range of 0 to 0.9 V is applied to a solid electrolyte made of zirconia or the like, and a voltage of about 0.45 V is normally applied. In such a case, as described in claim 3, it is preferable to apply a voltage higher than the normal applied voltage by 0.2 V or more between the electrodes of the second cell. That is, this corresponds to applying a voltage of 0.65 V or more to the second cell, and in practice, setting the applied voltage in the range of 0.65 V to 0.9 V, more preferably around 0.7 V good.
[0011]
According to the fourth aspect of the present invention, the oxygen desorbing means performs the process of desorbing oxygen immediately after the start of the operation of the gas concentration sensor. In other words, the problem of oxygen adsorption on the specific component active electrode of the second cell occurs immediately after the gas concentration sensor starts operating from a standing state, and it is desirable to perform the oxygen desorption process immediately after the operation starts.
[0012]
As a gas concentration sensor, the gas chamber includes a first gas chamber into which a gas to be detected is introduced, and a second gas chamber in which the second cell is provided, and the first cell is connected to the first gas chamber and the second gas chamber. Some have a first oxygen pump cell and a second oxygen pump cell which are separately provided on the gas chamber side. In the gas concentration detecting device using the gas concentration sensor, as described in claim 5, the output of the second oxygen pump cell exceeds a predetermined value immediately after the start of the operation of the gas concentration sensor by the oxygen desorbing means. The condition of oxygen desorption is preferably carried out under the following conditions. Note that the first oxygen pump cell and the second oxygen pump cell are also referred to as a pump cell and a monitor cell, respectively.
[0013]
That is, in the gas concentration sensor having the above configuration, if the sensor is left unattended, immediately after the operation starts, the output of the second oxygen pump cell (monitor cell) is excessively increased due to the relatively high concentration of oxygen filled in the second gas chamber. However, such a situation does not occur if the gas concentration sensor is restarted immediately after the operation is stopped. Therefore, if the process of desorbing oxygen is performed according to the output level of the second oxygen pump cell, the process of desorbing oxygen can be performed only when necessary.
[0014]
In the fifth aspect of the present invention, as described in the sixth aspect, after the output of the second oxygen pump cell exceeds a predetermined value, the oxygen desorbing means converges on the condition that the output converges to the predetermined value or less. It is good to end the withdrawal process. That is, the output of the second oxygen pump cell converges faster than that of the second cell, and according to the output level, the excess oxygen in the second gas chamber is discharged, and thus the oxygen desorption of the second cell electrode is advanced. Can be determined. Therefore, the process of oxygen desorption can be performed for an appropriate period.
[0015]
In the invention according to claim 7, the state of oxygen adsorption in the specific component active electrode of the second cell is estimated, and oxygen is released by the oxygen desorbing means according to the estimated state of oxygen adsorption. Variably set the length of the predetermined period for the operation. In this case, the process of desorbing oxygen can be performed according to the actual degree of oxygen adsorption, and the process can be optimized. For example, the state of oxygen adsorption is estimated according to the standing time of the gas concentration sensor, and the longer the leaving time, the longer the predetermined time for performing oxygen desorption.
[0016]
According to the eighth and ninth aspects of the present invention, oxygen or a specific component present in the gas chamber is forcibly discharged early by the gas component discharging means. If the atmosphere is introduced into the gas chamber while the sensor is left unattended, the gas chamber becomes excessively oxygen, which hinders the detection of the gas concentration of the specific component immediately after the operation of the sensor is started. In addition, when the specific component is excessive in the gas chamber, the detection of the gas concentration of the specific component is hindered. On the other hand, according to the present invention, since the excess oxygen and specific components in the gas chamber can be discharged early, the state in the gas chamber becomes an optimal state for detecting the gas concentration at the beginning of operation of the sensor, etc. Gas concentration output can be obtained quickly. In particular, a synergistic effect can be expected by combining the inventions of claims 1 to 7 with the gas component discharging means, and a more desirable gas concentration detecting device can be realized.
[0017]
According to the tenth aspect, the process of discharging the gas component is performed in a predetermined period immediately after the start of the operation of the gas concentration sensor. That is, the problem of excessive oxygen in the gas chamber (particularly, a second gas chamber described later) occurs immediately after the gas concentration sensor is started to operate from a state of being left unattended. It is desirable to do.
[0018]
In a sensor configuration in which the gas chamber is divided into a first gas chamber and a second gas chamber, and the second cell is provided on the second gas chamber side downstream of the first gas chamber, the second gas chamber has a second gas chamber. The oxygen pump action by one cell does not reach. Therefore, as described in claim 11, it is preferable that the gas component discharging means is configured to discharge the excess oxygen or the excess specific component in the second gas chamber.
[0019]
The gas concentration sensor according to claim 12, wherein the first cell has a first oxygen pump cell and a second oxygen pump cell which are separately provided on a first gas chamber side and a second gas chamber side, respectively. Preferably, immediately after the start of the operation of the gas concentration sensor, the gas component discharging process is performed on condition that the output of the second oxygen pump cell exceeds a predetermined value. If the sensor is left unattended, the output of the second oxygen pump cell (monitor cell) becomes excessively large immediately after the start of operation due to the relatively high concentration of oxygen filling the second gas chamber. When the is restarted, these effects are reduced. Therefore, if the gas component discharge processing is performed according to the output level of the second oxygen pump cell, the processing can be performed only when necessary.
[0020]
According to the twelfth aspect of the present invention, as described in the thirteenth aspect, the gas component discharging means is provided on condition that the output of the second oxygen pump cell converges to a predetermined value or less after the output exceeds a predetermined value. It is preferable to end the gas component discharging process. In other words, the output of the second oxygen pump cell converges faster than that of the second cell, and it can be determined from the output level that the discharge of excess oxygen in the second gas chamber has progressed. Therefore, the process of discharging the gas concentration in an appropriate period can be performed.
[0021]
In the invention according to claim 14, the amount of surplus oxygen or the amount of surplus specific component in the second gas chamber is estimated, and the gas is supplied in accordance with the estimated amount of surplus oxygen or the amount of surplus specific component. The length of the predetermined period for performing the gas component discharge by the component discharge means is variably set. In this case, the gas component discharge processing can be performed according to the actual degree of oxygen adsorption, and the processing can be optimized. For example, the amount of the excess oxygen or the amount of the excess specific component is estimated according to the leaving time of the gas concentration sensor, and the longer the leaving time is, the longer the predetermined time for performing the gas concentration discharge is.
[0022]
In the invention according to claim 15, when the inside of the gas chamber shifts to a high concentration atmosphere of a specific component, the specific component present in the gas chamber is forcibly discharged early. If the gas concentration of the specific component in the gas to be detected temporarily increases, the gas chamber becomes correspondingly excessive in the specific component, but the specific component can be discharged early.
[0023]
In a vehicle engine, for example, the amount of NOx emission tends to increase during high-load operation or high-speed operation. If the amount of NOx becomes excessive, an accurate NOx concentration output cannot be obtained until the excess can be exhausted. . Therefore, as described in claim 16, when the engine shifts to an operating state in which NOx emission tends to increase, NOx present in the gas chamber may be forcibly exhausted early.
[0024]
In the invention according to claim 17, the gas component discharging means includes a current operating means, and the current operating means forcibly increases the current flowing through the second cell in a predetermined period. In other words, in the second cell, a specific component is reduced or decomposed, and a current flows according to the amount of oxygen ions generated at that time. The current is a weak current, and the current detection resistor has a high resistance to improve detection accuracy. Is used. Therefore, the discharge capacity of oxygen and the like by the second cell is originally limited to a low level, but the discharge capacity of oxygen and the like is enhanced by the present invention (the same applies to the case where a second oxygen pump cell described later is used).
[0025]
As a more specific configuration, as described in claim 18, the current detection resistor for current detection of the second cell can be switched between a resistance value suitable for gas concentration detection and a smaller resistance value. The current operation means may forcibly increase the second cell current by switching the resistance value of the current detection resistor to a smaller value.
[0026]
In such a case, it is preferable that the current detection resistor has at least two resistors in a parallel relationship and selectively connects or disconnects these resistors.
[0027]
In a gas concentration detection device having a configuration for detecting an element resistance value (for example, an element impedance), the applied voltage or current of the second cell is changed in an alternating manner, and the current and the amount of change in the voltage at that time are detected by a current detection resistor. The element resistance is detected by measurement. At this time, the level of the current flowing through the second cell differs between when the gas concentration is detected and when the element resistance of the gas concentration sensor is detected. Therefore, it is possible to switch the resistance value of the current detection resistor. In this configuration, it is preferable that the current operation means forcibly increases the second cell current by using a resistor applied at the time of detecting the element resistance. In the present configuration, the above-described current operation can be realized without adding a new configuration, and a merit in cost can be obtained.
[0028]
As a gas concentration sensor, the gas chamber includes a first gas chamber into which a gas to be detected is introduced, and a second gas chamber in which the second cell is provided, and the first cell is connected to the first gas chamber and the second gas chamber. In the case of using one having a first oxygen pump cell and a second oxygen pump cell which are separately provided on the gas chamber side, as described in claim 21, the current operating means is replaced with the second cell. In addition to or in addition to this, the current flowing through the second oxygen pump cell (monitor cell) may be forcibly increased for a predetermined period to forcibly discharge oxygen present in the second gas chamber. With this configuration, the oxygen discharge capacity of the second oxygen pump cell is enhanced. Therefore, a desired effect can be achieved similarly to the above-described invention.
[0029]
Also in the invention of the twenty-first aspect, a configuration in which resistance values different in magnitude are switched, a configuration in which a resistance for detecting an element resistance is diverted, and the like can be adopted (similar to the above-described claims 18 to 20). That is,
In the invention according to claim 22, the current detection resistor for current detection of the second oxygen pump cell can be switched between a resistance value suitable for oxygen concentration detection and a resistance value smaller than that, and the current operation The means forcibly increases the second oxygen pump cell current by switching the resistance value of the current detection resistor to a smaller value.
In the twenty-third aspect of the present invention, the current detection resistor has at least two resistors in a parallel relationship, and these resistors are selectively connected or disconnected.
In the invention described in claim 24, the current operating means forcibly increases the second oxygen pump cell current by using a resistor applied when detecting the element resistance.
[0030]
On the other hand, in the invention according to claim 25, the gas concentration sensor has a detection cell including a solid electrolyte and a pair of electrodes disposed on the solid electrolyte, and an electrode facing the gas chamber among the pair of electrodes. Is a specific component active electrode active for the specific component. Then, the oxygen adsorbed on the specific component active electrode of the detection cell is forcibly released by the oxygen releasing means. According to the present invention, since the oxygen adsorbed on the specific component active electrode of the detection cell can be removed early, the state in the gas chamber becomes optimal for gas concentration detection at an early stage such as at the start of operation of the sensor. Output can be obtained quickly.
[0031]
In the invention according to claim 26, the oxygen desorbing means applies a voltage higher than usual between the electrodes of the detection cell within a predetermined time. Accordingly, a large amount of energy is applied to the specific component active electrode of the detection cell, and the decomposition of adsorbed oxygen is promoted at the active electrode. Therefore, the adsorbed oxygen can be eliminated early.
[0032]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described below with reference to the drawings. The gas concentration detection device according to the present embodiment is applied to, for example, an automobile engine, and uses a limiting current type gas concentration sensor to detect the oxygen concentration from the exhaust gas to be detected and to detect a gas having a specific component. The NOx concentration as the concentration is detected.
[0033]
First, the configuration of the gas concentration sensor will be described with reference to FIG. The gas concentration sensor of FIG. 2 has a three-cell structure including a pump cell, a monitor cell, and a sensor cell, and is embodied as a so-called composite gas sensor that can simultaneously detect the oxygen concentration and the NOx concentration in exhaust gas. Note that the pump cell and the monitor cell correspond to the “first cell”, and these cells adjust the amount of oxygen in the exhaust gas introduced into the chamber (gas chamber) to a predetermined low concentration level. The sensor cell corresponds to the “second cell”, and the sensor cell detects the NOx concentration from the gas after the oxygen amount adjustment. The monitor cell has a function of an oxygen pump in gas, similarly to the pump cell. The pump cell corresponds to a “first oxygen pump cell”, and the monitor cell corresponds to a “second oxygen pump cell”. FIG. 2A is a cross-sectional view illustrating the structure of the distal end portion of the sensor element, and FIG. 2B is a cross-sectional view taken along line AA of FIG. 2A.
[0034]
In the gas concentration sensor 100, solid electrolytes (solid electrolyte elements) 141 and 142 made of an oxygen ion conductive material such as zirconia are formed in a sheet shape, and are provided at predetermined intervals above and below the figure via spacers 143 made of an insulating material such as alumina. Are stacked. Among them, a pinhole 141a is formed in the solid electrolyte 141 on the upper side of the figure, and exhaust gas around the sensor is introduced into the first chamber 144 via the pinhole 141a. The first chamber 144 communicates with the second chamber 146 via the throttle 145. The first chamber 144 corresponds to a “first gas chamber”, and the second chamber 146 corresponds to a “second gas chamber”. Reference numeral 147 is a porous diffusion layer.
[0035]
A pump cell 110 is provided in the lower solid electrolyte 142 so as to face the first chamber 144, and the pump cell 110 discharges or pumps oxygen in exhaust gas introduced into the first chamber 144. It works and detects the oxygen concentration in the exhaust gas when discharging or pumping oxygen. Here, the pump cell 110 has a pair of upper and lower electrodes 111 and 112 with the solid electrolyte 142 interposed therebetween. Among them, the electrode 111 on the side of the first chamber 144 is a NOx inert electrode (an electrode that hardly decomposes NOx gas). . The pump cell 110 decomposes oxygen existing in the first chamber 144 and discharges the oxygen from the electrode 112 to the atmosphere passage 150 side.
[0036]
Further, a monitor cell 120 and a sensor cell 130 are provided on the solid electrolyte 141 on the upper side of the figure so as to face the second chamber 146. The monitor cell 120 generates an electromotive force according to the residual oxygen concentration in the second chamber 146, or generates a current output according to the application of a voltage. The sensor cell 130 detects the NOx concentration from the gas after passing through the pump cell 110.
[0037]
In particular, in the present embodiment, as shown in FIG. 2B, the monitor cell 120 and the sensor cell 130 are arranged in parallel so as to be at the same position with respect to the flow direction of the exhaust gas. The electrode on the side of the atmosphere passage 148 is the common electrode 122. That is, the monitor cell 120 is composed of the solid electrolyte 141 and the electrode 121 and the common electrode 122 disposed opposite each other with the solid electrolyte 141 interposed therebetween, and the sensor cell 130 is similarly configured with the solid electrolyte 141 and the electrode 131 disposed opposite thereto with the electrode 131 interposed therebetween. And an electrode 122. The electrode 121 of the monitor cell 120 (electrode on the second chamber 146 side) is made of a noble metal such as Au-Pt which is inert to NOx gas, whereas the electrode 131 of the sensor cell 130 (electrode on the second chamber 146 side) is NOx gas. The electrode 131 corresponds to a "specific component active electrode".
[0038]
FIG. 3A is a plan sectional view of the electrode arrangement of the monitor cell 120 and the sensor cell 130 viewed from the second chamber 146 side, and FIG. 3B is a view of the electrode arrangement of each of these cells viewed from the atmosphere passage 148 side. FIG. According to this configuration, the exhaust gas introduction distance is the same in the monitor cell 120 and the sensor cell 130. As a result, the sensitivity of the monitor cell 120 and the sensor cell 130 to residual oxygen after passing through the pump cell 110 becomes equal, and highly accurate gas concentration detection becomes possible. However, the electrodes of the monitor cell 120 and the sensor cell 130 are arranged in parallel in the flow direction of the exhaust gas as shown in FIG. It is also possible. For example, the monitor cell 120 is arranged on the upstream side (left side in the figure), and the sensor cell 130 is arranged on the downstream side (right side in the figure). Further, it is not essential to use the common electrode 122 in each cell, and it is also possible to use an individual electrode for each cell.
[0039]
An insulating layer 149 made of alumina or the like is provided on the lower surface of the solid electrolyte 142 in the figure, and the insulating layer 149 forms an air passage 150. Further, a heater 151 for heating the entire sensor is embedded in the insulating layer 149. The heater 151 generates heat energy by external power supply in order to activate the entire sensor including the pump cell 110, the monitor cell 120, and the sensor cell 130.
[0040]
In the gas concentration sensor 100 having the above configuration, the exhaust gas is introduced into the first chamber 144 through the porous diffusion layer 147 and the pinhole 141a. When this exhaust gas passes near the pump cell 110, a decomposition reaction occurs by applying a voltage Vp between the pump cell electrodes 111 and 112, and oxygen is passed through the pump cell 110 according to the oxygen concentration in the first chamber 144. Get in and out. At this time, since the pump cell electrode 111 on the first chamber 144 side is a NOx inactive electrode, NOx in the exhaust gas is not decomposed in the pump cell 110, and only oxygen is decomposed and discharged to the atmosphere passage 150. Then, the concentration of oxygen contained in the exhaust gas is detected based on the current flowing through the pump cell 110 (pump cell current Ip).
[0041]
Thereafter, the exhaust gas that has passed near the pump cell 110 flows into the second chamber 146, and the monitor cell 120 generates an output according to the residual oxygen concentration in the gas. The output of the monitor cell 120 is detected as a monitor cell current Im by applying a predetermined voltage Vm between the monitor cell electrodes 121 and 122. In addition, when a predetermined voltage Vs is applied between the sensor cell electrodes 131 and 122, NOx in the gas is reduced and decomposed, and oxygen generated at that time is discharged to the atmosphere passage 148. At this time, the concentration of NOx contained in the exhaust gas is detected based on the current flowing through the sensor cell 130 (sensor cell current Is).
[0042]
Incidentally, in the pump cell 110, the applied voltage Vp is variably controlled according to the oxygen concentration in the exhaust gas (that is, the pump cell current Ip) in each case. For example, based on the limit current characteristic of the pump cell 110, Using the created applied voltage map, the applied voltage Vp is controlled according to the pump cell current Ip in each case. Thereby, the applied voltage control is performed such that the applied voltage shifts to the higher voltage side as the oxygen concentration in the exhaust gas increases. Alternatively, the applied voltage Vp may be feedback-controlled so that the concentration of residual oxygen in the second chamber 146 is constant, in other words, the monitor cell current Im is constant. With the above control, oxygen in the exhaust gas introduced into the first chamber 144 can be quickly discharged, and the residual oxygen concentration after the oxygen discharge can be maintained at a desired low concentration level.
[0043]
By the way, as described above, in the gas concentration sensor 100, after excess oxygen in exhaust gas is exhausted by the pump cell 110, gas containing low-concentration level residual oxygen is supplied to the monitor cell 120 and the sensor cell 130. The monitor cell 120 measures the monitor cell current Im according to the residual oxygen concentration in the gas, and the sensor cell 130 measures the sensor cell current Is according to the NOx concentration in the gas. However, at this time, in the sensor cell 130, it is desirable that only the NOx in the gas is originally reduced and decomposed, and the current value accompanying the measurement is actually measured. However, in actuality, the current corresponding to the residual oxygen content (a trace amount of oxygen) in the gas is measured. The components are also measured together. That is, the measured sensor cell current Is includes the NOx reaction component and the residual oxygen reaction component, of which the residual oxygen reaction component becomes the offset error. Therefore, in the present embodiment, the monitor cell current Im is subtracted from the measured sensor cell current Is in order to eliminate the offset error from the sensor cell current Is, and the NOx concentration output is obtained based on the difference value (Is-Im). I have to.
[0044]
Next, the electrical configuration of the gas concentration detection device will be described with reference to FIG. Although FIG. 1 shows a gas concentration detection device using the above-described gas concentration sensor 100, the electrode arrangement of the monitor cell 120 and the sensor cell 130 is shown side by side for convenience.
[0045]
In FIG. 1, a control circuit 200 includes a well-known microcomputer including a CPU, an A / D converter, a D / A converter, an I / O port, and the like. Are appropriately output from the D / A converters (D / A0 to D / A2). Further, the control circuit 200 converts the voltages of the terminals Vc, Ve, Vd, Vb, Vg, and Vh into A / D converters (A / D0 to A / D) in order to take in the measurement results of the current flowing through the cells 110 to 130. Input each from D5). The control circuit 200 calculates the oxygen concentration (A / F) and NOx concentration in the exhaust gas based on the current values measured by the pump cell 110 and the sensor cell 130, and calculates the D / A converter (D / A4, D / A3) or an external engine ECU via a communication circuit.
[0046]
More specifically, in the pump cell 110, a reference voltage Va is applied to one electrode 112 of the pump cell 110 by a reference power supply 201 and an operational amplifier 202, and a control circuit 200 is applied to the other electrode 111 via an operational amplifier 203 and a current detection resistor 204. Is applied. When a current flows through the pump cell 110 according to the oxygen concentration in the exhaust gas when the command voltage Vb is applied, the current is detected by the current detection resistor 204. That is, both terminal voltages Vb and Vd of the current detection resistor 204 are taken into the control circuit 200, and the pump cell current Ip is calculated from the voltages Vb and Vd.
[0047]
When the pump cell current Ip changes due to a change in the engine operating state or the like, if the amount of the change is large, tailing or the like occurs in the current intake value, and the accuracy of the current measurement decreases. Therefore, in order to prevent the tailing, the current variation width is limited each time, and a smoothing process is further performed to determine the measured value of the pump cell current.
[0048]
The reference voltage Vf is applied to the common electrode 122 of the monitor cell 120 and the sensor cell 130 by the reference power supply 205 and the operational amplifier 206, and the operational amplifier 207 and the current detection resistor 208 are applied to the sensor cell electrode 131 different from the common electrode 122. The command voltage Vg of the control circuit 200 is applied via the control circuit 200. When a current flows through the sensor cell 130 in accordance with the NOx concentration in the gas when the command voltage Vg is applied, the current is detected by the current detection resistor 208. That is, both terminal voltages Vg and Vh of the current detection resistor 208 are taken into the control circuit 200, and the sensor cell current Is is calculated based on the voltages Vg and Vh.
[0049]
The command voltage Vc of the control circuit 200 is applied to the monitor cell electrode 121 different from the common electrode 122 via the operational amplifier 209 and the current detection resistor 210. When a current flows through the monitor cell 120 according to the residual oxygen concentration in the gas when the command voltage Vc is applied, the current is detected by the current detection resistor 210. That is, both terminal voltages Vc and Ve of the current detection resistor 210 are taken into the control circuit 200, and the monitor cell current Im is calculated based on the voltages Vc and Ve.
[0050]
Although the configuration of the heater driving system is not shown, the CPU in the control circuit 200 outputs a duty control signal to the heater driving MOSFET and the like to reduce the power supplied from the power supply (for example, a battery power supply) to the heater 151. PWM control is performed. At this time, since the detection currents in the monitor cell 120 and the sensor cell 130 are weaker than the heater current, noise generated due to heater energization may adversely affect the detection currents in the cells 120 and 130. In the present embodiment, as a countermeasure against the noise, capacitors 211 and 212 are provided in parallel with the current detection resistors 208 and 210 for the sensor cell 130 and the monitor cell 120, respectively, to form a filter. As a result, the problem of noise due to heater energization can be effectively suppressed. Further, as described above, the NOx concentration output is obtained based on the difference value (Is-Im) between the sensor cell current Is and the monitor cell current Im. Therefore, in the sensor cell 130 and the monitor cell 120, it is preferable that the constants of the current detection resistors 208 and 210 and the filter capacitors 211 and 212 are matched to match the time constant.
[0051]
When a difference value (Is−Im) between the sensor cell current Is and the monitor cell current Im is obtained, the sensor cell current signal and the monitor cell current signal are input to a differential amplifier circuit, and the output of the differential amplifier circuit (ie, It is preferable that the control circuit 200 performs A / D conversion of Is-Im. Thereby, the gain of the differential amplifier circuit can be increased, and the dynamic range of the NOx concentration signal (Is-Im) is widened. Therefore, the detection accuracy of the NOx concentration is improved.
[0052]
In the present embodiment, an element impedance as an “element resistance value” is detected for the sensor cell 130 using a so-called sweep method. That is, when the impedance of the sensor cell 130 is detected, the control circuit 200 switches the sensor cell applied voltage (command voltage Vg) to at least one of the positive side and the negative side instantaneously (for example, in a time of about several tens to 100 μsec). . This alternating voltage is applied to the sensor cell 130 while being sinusoidally smoothed by a low-pass filter (LPF) (not shown). The frequency of the AC voltage is preferably 10 kHz or more, and the time constant of the LPF is set at about 5 μsec. Then, the element impedance of the sensor cell 130 is calculated from the voltage change amount and the current change amount at that time (impedance = voltage change amount / current change amount). In addition to the configuration in which the applied voltage of the sensor cell 130 is changed in an alternating manner, the configuration in which the current flowing in the sensor cell 130 is changed in an alternating manner may be used.
[0053]
In controlling the heater of the gas concentration sensor 100, element resistance feedback control for controlling the element impedance to a predetermined target value is performed. Thus, each cell 110 to 130 is maintained in a desired active state.
[0054]
Note that, instead of the element impedance, the element admittance, which is the reciprocal of the element impedance, may be detected as the element resistance value. Instead of detecting the impedance (or admittance) of the sensor cell 130, a configuration for detecting the impedance (or admittance) of the monitor cell 120 or a configuration for detecting the impedance (or admittance) of the pump cell 110 may be employed.
[0055]
Incidentally, in the monitor cell 120 and the sensor cell 130, one of the electrodes is used as the common electrode 122, so that the drive circuit on the reference voltage side can be reduced, and the number of lead wires from the gas concentration sensor 100 can be reduced. Can be In addition, since the monitor cell 120 and the sensor cell 130 are formed adjacent to each other with the same solid electrolyte 141, a current flows to the adjacent electrode during the sweep, and the detection accuracy of the element impedance may be deteriorated. Is provided, one electrode has the same potential, and this effect can be reduced.
[0056]
In the sensor cell 130, the current flowing by NOx is on the order of μA, and has a very weak current value on the order of nA (nanoampere) in the NOx concentration range to be detected. Therefore, in order to ensure detection accuracy, the resistance value of the current detection resistor 208 is on the order of MΩ. On the other hand, a current on the order of mA flows during sweeping for impedance detection. If currents of different orders are detected by the same detection resistor, over-range may occur or detection accuracy may deteriorate. Therefore, in the present embodiment, the current detection resistance is switched between when the sensor cell 130 detects the NOx concentration and when the impedance is detected. Specifically, another current detection resistor 215 and a switch circuit 216 (for example, a semiconductor switch) are provided in parallel with the current detection resistor 208. The switch circuit 216 is configured to be turned on / off by an output from an I / O port of the control circuit 200. In this case, at the time of normal gas concentration detection, the switch circuit 216 is turned off (open), and the sensor cell current Is is detected by a resistance of the order of MΩ by the current detection resistance 208. On the other hand, at the time of impedance detection, the switch circuit 216 is turned on (closed), and the sensor cell current Is is detected by a resistance of about several hundred Ω by the current detection resistors 208 and 215.
[0057]
By the way, in the gas concentration sensor 100 having the above-described configuration, a large amount of oxygen is adsorbed on the NOx active electrode of the sensor cell 130 in the idle state before the sensor operation is started (before the engine is started). Since the chambers 144 and 146 are filled with a relatively high concentration (about 20.9%) of oxygen at the atmospheric level, it takes a long time to obtain an accurate NOx concentration output after the sensor operation is started. Occurs. In particular, in the present configuration in which rhodium is added to the NOx active electrode, the above-described disadvantage is caused by oxygen adsorption (oxygen storage) of rhodium. Therefore, in the gas concentration detection device of the present embodiment, the following measures are taken to solve the above-mentioned inconvenience.
(1) Immediately after the start of the sensor operation, a voltage higher than usual is applied to the sensor cell electrode to forcibly release the adsorbed oxygen from the sensor cell electrode (NOx active electrode) (oxygen desorbing means).
(2) Immediately after the start of operation of the sensor, the current detection resistor for the sensor cell 130 is switched to a low resistance to increase the sensor cell current and forcibly exhaust oxygen in the second chamber 146 at an early stage (gas component discharging means, current operation means).
[0058]
Here, the outline of the above (2) will be described. The current detection resistor 208 connected to the sensor cell 130 has a resistance value on the order of MΩ, and if the resistance value is 1 MΩ, the maximum current that can flow through the sensor cell 130 is about 6 μA. Since the same applies to the monitor cell 120, the current that can flow in the sensor cell 130 and the monitor cell 120 on the second chamber side, that is, the total of the sensor cell current and the monitor cell current is about 12 μA at the maximum. This is the maximum current that can flow on the second chamber side, and also corresponds to the oxygen discharge capacity on the second chamber side. In this case, since the current that can be passed through the sensor cell 130 and the monitor cell 120 is limited as described above, the discharge of excess oxygen in the second chamber 146 is delayed, and the normalization of the NOx concentration output is delayed. Oxygen in the gas introduced into the first chamber 144 in the normal operation state of the engine is exhausted by the pump cell 110, but before the engine starts, oxygen in the atmosphere filled in the first and second chambers 144 and 146 is released. Needs to be discharged early by the sensor cell 130 or the monitor cell 120 in addition to the discharge by the pump cell 110.
[0059]
In the configuration of FIG. 1 described above, another current detection resistor 215 and a switch circuit 216 are connected in parallel to the current detection resistor 208, and the resistance value for sensor cell current detection is switched by ON / OFF of the switch circuit 216. It has become. Therefore, when the resistance values of the current detection resistors 208 and 215 are 1 MΩ and 500 Ω, respectively, the current flowing through the sensor cell 130 is detected by the resistance value 1 MΩ of the current detection resistor 208 by turning off the switch circuit 216. When the switch circuit 216 is turned ON, the current flowing through the sensor cell 130 is detected by the combined resistance value of the current detection resistors 208 and 215 of 499.8Ω. Therefore, the current level that can be passed through the sensor cell 130 can be controlled (in this embodiment, a current of about 20 mA at the maximum), and the early discharge of excess oxygen in the second chamber 146 can be forcibly performed. Become.
[0060]
Since the value of the element impedance is about several tens of ohms, the current detection resistor for detecting the impedance has a resistance value on the order of several hundred ohms. In this case, since the resistance value of the current detection resistor for detecting the impedance is much smaller than the resistance value for detecting the NOx concentration, the resistance for detecting the impedance can be used. As a result, the above-described operation of the sensor cell current can be performed without adding a new configuration, and a merit in cost can be obtained.
[0061]
Hereinafter, specific embodiments of the above measures will be described. FIG. 4 is a flowchart showing a control procedure of the sensor cell applied voltage. This processing is executed by the CPU in the control circuit 200 at a predetermined time period.
[0062]
In FIG. 4, first, in step S101, it is determined whether or not the monitor cell current Im is larger than a predetermined determination value Ka. The determination value Ka is a determination level for determining that oxygen in the second chamber 146 has a high concentration, and is set to, for example, Ka = 500 nA. In this case, when the oxygen concentration in the second chamber 146 becomes high and the monitor cell current Im becomes excessively large, it can be estimated that a large amount of oxygen is adsorbed on the sensor cell electrode (NOx active electrode), and the high voltage of the sensor cell applied voltage is obtained. It is determined that it is necessary to perform the process of changing the resistance or switching the resistance (operating the sensor cell current).
[0063]
In step S102, it is determined whether or not a predetermined time has elapsed after the start of the operation of the gas concentration sensor 100. The predetermined time corresponds to an execution period for performing a process of increasing the voltage applied to the sensor cell or switching the resistance, and may be a fixed time or a variably set time. When the predetermined time is fixed, for example, it is set to 50 seconds. When the predetermined time is variable, the predetermined time is set according to the oxygen adsorption state of the sensor cell electrode. For example, the oxygen adsorption state of the sensor cell electrode is estimated according to the element impedance value, the pump cell current (that is, the oxygen concentration in the first chamber 144), the sensor leaving time (that is, the engine stop time), and the like. It is conceivable to set a predetermined time.
[0064]
Then, if Im ≦ Ka or after a lapse of a predetermined time, normal applied voltage control is performed without performing the process of increasing the applied voltage of the sensor cell or switching the resistance. That is, in step S103, the sensor cell applied voltage is set to, for example, 0.45 V, and in the subsequent step S104, the switch circuit 216 is turned off. Thereafter, in step S107, the set sensor cell applied voltage is output. As a result, the initial control of the sensor cell 130 is performed under the condition that the applied voltage of the sensor cell is 0.45 V and the current detection resistance is 1 MΩ.
[0065]
On the other hand, if Im> Ka and before the lapse of the predetermined time, the process of increasing the voltage applied to the sensor cell and switching the resistance is performed. That is, in step S105, the voltage applied to the sensor cell is set to 0.7 V, which is higher than normal, and in the subsequent step S106, the switch circuit 216 is turned on. The applied voltage of “0.7 V” is an example, and the applied voltage of the sensor cell is set at a voltage 0.2 V or more higher than the applied voltage in a normal state, and in practice, a voltage in the range of 0.65 V to 0.9 V. Good to be. Thereafter, in step S107, the set sensor cell applied voltage is output. Thereby, the initial control of the sensor cell 130 (here, the process of increasing the applied voltage of the sensor cell and switching the resistance) is performed under the condition that the applied voltage of the sensor cell is 0.7 V and the current detection resistance is about 500Ω.
[0066]
Note that the condition of the monitor cell current Im in step S101 may be employed only as a condition for starting the process of increasing the applied voltage of the sensor cell or switching the resistance, or the monitor cell current condition may be eliminated. In addition, it is also possible to eliminate the time condition in step S102.
[0067]
Next, a specific operation and effect of the process of increasing the voltage applied to the sensor cell and switching the resistance will be described. However, for the sake of convenience, the effect of increasing the voltage applied to the sensor cell will first be described with reference to FIG. FIG. 5 is a time chart showing changes in the sensor cell current Is, the monitor cell current Im, and the like during the high voltage processing. The dashed line in the figure shows the current transition when the sensor cell applied voltage is fixed (Vs = 0.45 V fixed) from the beginning of the sensor operation. In the following description, it is assumed that a large amount of oxygen is adsorbed on the sensor cell electrode at the start of the sensor operation, and that the inside of the second chamber 146 is almost in the atmospheric state.
[0068]
With the start of the sensor operation, the sensor cell current Is and the monitor cell current Im both rise sharply and reach a measurable upper limit (for example, 2500 nA). The monitor cell current Im exceeds the determination value Ka. The element admittance MAdm (reciprocal of the element impedance) changes as the activation proceeds, as shown in the figure. At this time, 0.7 V, which is a higher voltage than usual, is set as the sensor cell applied voltage Vs and applied to the sensor cell 130. Thereafter, the monitor cell current Im starts to decrease earlier than the sensor cell current Is due to the low concentration of oxygen in the second chamber 146, and when the monitor cell current Im becomes equal to or less than the determination value Ka, the sensor cell applied voltage Vs becomes lower than the normal value. It is changed to 0.45 V (timing t1 in the figure). However, it is also possible to set different levels for the determination value of the monitor cell current Im as a start condition for increasing the applied voltage of the sensor cell and the determination value for the end condition.
[0069]
The sensor cell current Is decreases later than the monitor cell current Im and changes as shown by the solid line in the figure. At this time, in comparison with the current transition when the applied voltage of the sensor cell is fixed (the dashed line in the figure), it is understood that the sensor cell current Is decreases early in the present embodiment. The difference between the changes in the sensor cell current Is indicated by the solid line and the dashed line is due to the increase in the sensor cell applied voltage Vs at the beginning of the sensor operation, and the sensor cell electrode (NOx active electrode) becomes high due to the increase in the sensor cell applied voltage. Energy is applied. Therefore, the decomposition of the adsorbed oxygen is promoted in the sensor cell electrode, and as a result, the sensor cell current decreases early.
[0070]
Actually, when comparing the time until a NOx concentration output of a predetermined level (for example, 10 ppm) is obtained, the time can be reduced by about 100 seconds as compared with a case where the voltage applied to the sensor cell is not increased. That is, it can be said that the normalization of the NOx concentration output is accelerated, which means that the activation of the sensor cell 130 is accelerated.
[0071]
FIG. 6 is a time chart showing changes in the sensor cell current Is, the monitor cell current Im, and the like when a process of switching resistance (forcibly discharging oxygen in the chamber) is performed in addition to increasing the voltage applied to the sensor cell. Note that the alternate long and short dash line in the figure indicates a current transition when the current detection resistor is not switched.
[0072]
Immediately after the start of the sensor operation, when the switch circuit 216 is turned on, a current flows to the sensor cell 130 through the combined resistance of the current detection resistors 208 and 215. At this time, in the case described above, the combined resistance becomes about 500Ω, and the sensor cell current Is flows about 0.04 mA (40 nA) at the maximum. Therefore, the oxygen discharging capability of the sensor cell 130 is enhanced, and the oxygen discharging in the second chamber 146 is promoted. Thereafter, when the monitor cell current Im becomes equal to or smaller than the determination value Ka, the switch circuit 216 is turned off (timing t11 in the figure). Thereafter, measurement of the sensor cell current Is (that is, NOx concentration detection) is performed as usual by the resistance of 1 MΩ of the current detection resistor 208.
[0073]
The sensor cell current Is decreases later than the monitor cell current Im and changes as shown by the solid line in the figure. At this time, in comparison with the current transition when the current detection resistor is not switched (the dashed line in the figure), it can be seen that in the present embodiment, the sensor cell current Is and the monitor cell Im decrease early. Therefore, due to the early discharge of the surplus oxygen in the second chamber 146, the sensor cell current Is converges early and the NOx concentration output can be quickly corrected.
[0074]
Actually, when comparing the time until the NOx concentration output of a predetermined level (for example, 10 ppm) is obtained, the time can be reduced by about 100 seconds as compared with the case where the resistance switching is not performed.
[0075]
According to the present embodiment described in detail above, the following excellent effects can be obtained.
[0076]
Immediately after the sensor operation is started, the oxygen adsorbed on the NOx active electrode of the sensor cell 130 is forcibly desorbed, and the excess oxygen present in the second chamber 146 is forcibly discharged at an early stage. At the start, the state in the second chamber 146 becomes an optimum state for NOx concentration detection at an early stage, and an accurate NOx concentration output can be promptly obtained.
[0077]
The present invention is not limited to the description in the above embodiment, and may be implemented, for example, as follows.
[0078]
In the above embodiment, both (1) increasing the applied voltage of the sensor cell to forcibly desorb the adsorbed oxygen from the sensor cell electrode, and (2) forcibly discharging the oxygen in the chamber by switching the current detection resistance between large and small. Although the configuration is implemented, the embodiment is changed. In other words, a configuration in which only the above (1) is performed, a configuration in which only the above (2) is performed, or a configuration in which the above (1) and (2) are selectively performed according to the sensor state at each time are adopted. You may.
[0079]
In the above-described embodiment, the configuration in which the current detection resistance on the sensor cell 130 side is switched when forcibly discharging oxygen in the second chamber 146 is used, but a configuration in which the current detection resistance on the monitor cell 120 side is switched instead. It is also possible. Alternatively, a configuration in which the current detection resistance is switched in both the sensor cell 130 and the monitor cell 120 is also possible. Also in the case where the resistance switching is performed on the monitor cell 120 side, it is preferable to use the resistance for detecting the element impedance similarly to the above. Even with this configuration, the oxygen discharge capability of the monitor cell 120 is enhanced, and the oxygen present in the second chamber 146 is forcibly discharged. Therefore, a desired effect can be achieved similarly to the above-described embodiment.
[0080]
As can be seen from the time chart of FIG. 6, when the current detection resistance of the sensor cell 130 (or the monitor cell 120) is switched, the current can be measured to a current level that cannot be measured normally. In the case of the present embodiment, it is possible to supply a current of up to about 20 mA to the sensor cell 130 (or the monitor cell 120), and it is possible to measure a current up to the order of mA. At this time, it is possible to measure how much the sensor cell current (or monitor cell current) reaches immediately after the start of the sensor operation, and to estimate the amount of oxygen adsorbed on the sensor cell electrode according to the current level, It is possible to estimate the amount of surplus oxygen in the second chamber. Therefore, it is also possible to adopt a configuration in which the predetermined time for performing the process of increasing the applied voltage of the sensor cell or switching the resistance is set according to the current level.
[0081]
In the above embodiment, when the voltage applied to the sensor cell is increased, the voltage applied to the sensor cell is set to a constant value (0.7 V) within a predetermined period, but the voltage value is changed within the predetermined period. Is also good. For example, after applying a high voltage, the voltage is gradually decreased to shift to a normal value.
[0082]
In a vehicle engine, for example, during a high-load operation or a high-speed operation, the amount of NOx emission tends to increase, and if the amount of NOx in the second chamber 146 becomes excessive, accurate NOx is discharged until the surplus amount is exhausted. NOx concentration output cannot be obtained. Therefore, when the engine shifts to an operating state in which the NOx emission tends to increase, the current flowing through the sensor cell 130 may be increased, and as a result, the NOx present in the second chamber 146 may be forcibly discharged early. Specifically, the respective engine load states are determined in accordance with the throttle opening signal, the fuel injection amount, or the like, or the respective engine rotation states are determined in accordance with the engine speed signal. In the rotating state, the current detection resistor may be switched to the low resistance side.
[0083]
In the above-described embodiment, an example of application to the gas concentration sensor 100 having the structure shown in FIG. 2 has been described. However, the invention may be applied to other gas concentration sensors, and a specific example of an applicable gas concentration sensor will be described below. .
[0084]
In the gas concentration sensor 300 shown in FIG. 7, the sheet-like members denoted by reference numerals 301, 302, 303, and 304 are all formed of a solid electrolyte (for example, zirconia), and these solid electrolytes 301 to 304 are stacked at the top and bottom of the drawing. Have been. A first chamber 306 and a second chamber 307 are defined between the solid electrolytes 301 and 303 with a rate controlling layer 305a and 305b made of an insulating material such as porous alumina. An air passage 308 is formed by the solid electrolyte 304, and a heater 309 is embedded in the solid electrolyte 304.
[0085]
Further, the gas concentration sensor 300 is provided with a first pump cell 310, a second pump cell 320, a sensor cell 330, and a monitor cell 340. As a characteristic of the sensor 300, these cells are connected to each other by solid electrolytes. In such a configuration, in the first pump cell 310, the voltage Vp1 is applied between the pair of electrodes 311 and 312, and the first pump cell current Ip1 flowing at that time is detected. In the second pump cell 320, the voltage Vp2 is applied between the electrodes 313 and 314, and the second pump cell current Ip2 flowing at that time is detected. In the sensor cell 330, the voltage Vs is applied between the electrodes 314 and 315, and the sensor cell current Is flowing at that time is detected. Further, in the monitor cell 340, an electromotive force signal Vm is detected between the electrodes 314 and 316. Among the electrodes 312, 313, 315, and 316 of the cells facing the first chamber 306 and the second chamber 307, the electrode 315 of the sensor cell 330 is a NOx active electrode made of rhodium, platinum, or the like active on NOx. The other electrode is a NOx inactive electrode which is inert to NOx.
[0086]
The exhaust gas is introduced into the first chamber 306 via the rate limiting layer 305a at the sensor tip. Most of the oxygen in the exhaust gas is detected by the electromotive force signal Vm of the monitor cell 340, and is discharged from the electrode 311 to the outside by controlling the applied voltage Vp1 of the first pump cell 310 according to the signal Vm. . The remaining gas passes through the rate controlling layer 305b and is introduced into the second chamber 307, and the residual oxygen in the gas is decomposed by the application of the voltage Vp2 in the second pump cell 320 and discharged to the atmosphere passage 308. The NOx in the gas is decomposed by the application of the voltage Vs in the sensor cell 330 and discharged to the atmosphere passage 308, and the NOx concentration value is calculated based on the sensor cell current Is flowing at that time.
[0087]
In the gas concentration sensor 300 of FIG. 7, the first pump cell 310 and the second pump cell 320 correspond to “first cell”, and the sensor cell 330 corresponds to “second cell”. Further, the first chamber 306 corresponds to a “first gas chamber”, and the second chamber 307 corresponds to a “second gas chamber”.
[0088]
In the case where the element impedance is detected for the sensor cell 330, the current detection circuit 350 for the sensor cell 330 has a configuration in which the current detection resistance is switched between when the sensor cell 330 detects the NOx concentration and when the impedance is detected. Have been. Although illustration of the details of the current detection circuit 350 is omitted, similarly to the circuit configuration for the sensor cell shown in FIG. 1, a high resistance (MΩ order) current detection resistor and a low resistance connected in parallel thereto (several hundred Ω order) And a switch circuit for switching the resistance value. In this case, at the time of normal gas concentration detection, the sensor cell current Is is detected by the high-resistance current detection resistor, and at the time of impedance detection, the sensor cell current Is is detected by the low-resistance current detection resistor.
[0089]
Further, in the above configuration, (1) the applied voltage to the sensor cell is increased to forcibly release the adsorbed oxygen from the sensor cell NOx active electrode, and (2) the current detection resistor is switched between large and small to forcibly discharge oxygen in the chamber. I'm supposed to. As a result, at the start of the sensor operation, the state in the second chamber 307 becomes optimal for the NOx concentration detection at an early stage, and an accurate NOx concentration output can be obtained quickly. However, a configuration that implements only the above (1), a configuration that implements only the above (2), and a configuration that selectively implements the above (1) and (2) depending on the sensor state at each time can be appropriately adopted. In addition, a configuration in which the current detection resistance of the second pump cell 320 is switched in place of the sensor cell 330, or a configuration in which the current detection resistance is switched in both the sensor cell 330 and the second pump cell 320 is also possible.
[0090]
In addition to the gas concentration sensor capable of detecting the NOx concentration, the present invention can be applied to a gas concentration sensor capable of detecting the HC concentration and the CO concentration as the gas concentrations of the specific components. In this case, surplus oxygen in the gas to be detected is discharged by the pump cell, and HC and CO are decomposed by the sensor cell to detect HC concentration and CO concentration. Furthermore, it is also possible to use the gas concentration detecting device other than those for automobiles, and to use gas other than exhaust gas as the gas to be detected.
[Brief description of the drawings]
FIG. 1 is a configuration diagram illustrating an outline of a gas concentration detection device according to an embodiment of the present invention.
FIG. 2 is a cross-sectional view illustrating a configuration of a gas concentration sensor.
FIG. 3 is a plan sectional view showing electrode arrangement of a monitor cell and a sensor cell.
FIG. 4 is a flowchart showing an applied voltage control process of a sensor cell.
FIG. 5 is a time chart showing changes in a sensor cell current, a monitor cell current, and the like at the start of sensor operation.
FIG. 6 is a time chart showing changes in a sensor cell current, a monitor cell current, and the like at the start of sensor operation.
FIG. 7 is a cross-sectional view illustrating a configuration of another gas concentration sensor.
[Explanation of symbols]
100 gas concentration sensor, 110 pump cell, 111, 112 electrode, 120 monitor cell, 121, 122 electrode, 130 sensor cell, 131 electrode, 141, 142 solid electrolyte, 144 first chamber, 146th 2 chambers, 200 control circuit, 208, 210, 215 current detection resistor, 216 switch circuit, 300 gas concentration sensor, 301, 303 solid electrolyte, 306 first chamber, 307 second chamber, 310 1st pump cell, 311-316 ... electrode, 320 ... 2nd pump cell, 330 ... sensor cell, 340 ... monitor cell.

Claims

Each has a first cell and a second cell each comprising a solid electrolyte and a pair of electrodes disposed on the solid electrolyte, and the first cell has a predetermined low concentration of oxygen in the gas to be detected introduced into the gas chamber. The second cell detects the gas concentration of a specific component from the gas after the adjustment of the oxygen amount in the first cell in the second cell, and the electrode facing the gas chamber among the pair of electrodes in the second cell. In a gas concentration detection device applied to a gas concentration sensor that is a specific component active electrode active for a specific component,
A gas concentration detection device comprising an oxygen desorbing means for forcibly desorbing oxygen adsorbed on the specific component active electrode of the second cell.

2. The gas concentration detecting device according to claim 1, wherein the oxygen desorbing means applies a voltage higher than usual within a predetermined period between the electrodes of the second cell.

3. The gas concentration detecting device according to claim 2, wherein a voltage higher than a normal applied voltage by 0.2 V or more is applied between the electrodes of the second cell.

The gas concentration detecting device according to any one of claims 1 to 3, wherein the oxygen desorbing means performs the oxygen desorption process for a predetermined period immediately after the operation of the gas concentration sensor is started.

In the gas concentration sensor, the gas chamber includes a first gas chamber into which a gas to be detected is introduced and a second gas chamber in which the second cell is provided, and the first cell is located on a first gas chamber side. And a first oxygen pump cell and a second oxygen pump cell which are separately provided on the side of the second gas chamber, respectively, wherein the oxygen desorbing means is provided with the second oxygen pump immediately after the gas concentration sensor starts operating. The gas concentration detecting device according to any one of claims 1 to 4, wherein the process of desorbing oxygen is performed on condition that an output of the pump cell exceeds a predetermined value.

6. The gas concentration detecting device according to claim 5, wherein the oxygen desorbing means terminates the oxygen desorption process on condition that the output of the second oxygen pump cell exceeds a predetermined value and converges on the predetermined value or less. Gas concentration detector.

The state of oxygen adsorption at the specific component active electrode of the second cell is estimated, and the length of a predetermined period for performing oxygen desorption by the oxygen desorption means is variably set according to the estimated state of oxygen adsorption. The gas concentration detecting device according to any one of claims 1 to 4.

The gas concentration detecting device according to any one of claims 1 to 7, further comprising a gas component discharging means for forcibly discharging oxygen or a specific component existing in the gas chamber at an early stage.

Each has a first cell and a second cell each comprising a solid electrolyte and a pair of electrodes disposed on the solid electrolyte, and the first cell has a predetermined low concentration of oxygen in the gas to be detected introduced into the gas chamber. In a gas concentration detection device applied to a gas concentration sensor that detects a gas concentration of a specific component from the gas after adjusting the oxygen amount in the first cell in the second cell while adjusting to a level,
A gas concentration detecting device, comprising: gas component discharging means for forcibly discharging oxygen or a specific component present in the gas chamber at an early stage.

The gas concentration detecting device according to claim 8, wherein the gas component discharging means performs a gas component discharging process for a predetermined period immediately after the operation of the gas concentration sensor is started.

The gas chamber includes a first gas chamber on the upstream side with respect to the flow of the gas to be detected and a second gas chamber on the downstream side, and the second cell is provided on the second gas chamber side, and the gas component discharging means includes: The gas concentration detecting device according to any one of claims 8 to 10, which discharges excess oxygen or a surplus specific component in the second gas chamber.

12. The gas concentration detection device according to claim 11, wherein the gas concentration sensor includes a first oxygen pump cell and a second oxygen pump cell, wherein the first cell is provided separately on a first gas chamber side and a second gas chamber side. Wherein the gas component discharging means performs a gas component discharging process on the condition that the output of the second oxygen pump cell exceeds a predetermined value immediately after the start of the operation of the gas concentration sensor. Concentration detection device.

13. The gas concentration detecting device according to claim 12, wherein the gas component discharging means performs the process of discharging the gas component on condition that the output of the second oxygen pump cell exceeds a predetermined value and converges to the predetermined value or less. Finish the gas concentration detector.

Estimating the amount of the excess oxygen or the amount of the excess specific component in the second gas chamber, and discharging the gas component by the gas component discharging means according to the estimated amount of the excess oxygen or the amount of the excess specific component. 14. The gas concentration detecting device according to claim 11, wherein the length of the predetermined period is variably set.

The gas concentration detection device according to any one of claims 8 to 14, wherein the gas component discharging means forcibly discharges the specific component present in the gas chamber at an early stage when the gas chamber shifts to a high concentration atmosphere of the specific component. apparatus.

A gas concentration detecting device for detecting exhaust gas of a vehicle engine, wherein the gas component discharging means forcibly removes NOx present in a gas chamber when the engine shifts to an operating state in which NOx emission tends to increase. The gas concentration detection device according to any one of claims 8 to 14, wherein the gas concentration is discharged early.

17. The gas concentration detecting device according to claim 8, wherein the gas component discharging means includes a current operating means for forcibly increasing a current flowing through the second cell for a predetermined period.

The current detection resistor for current detection of the second cell can be switched between a resistance value suitable for gas concentration detection and a smaller resistance value, and the current operating means reduces the resistance value of the current detection resistor. 18. The gas concentration detection device according to claim 17, wherein the second cell current is forcibly increased by switching to the second cell current.

19. The gas concentration detection device according to claim 18, wherein the current detection resistor has at least two resistors in a parallel relationship, and selectively connects or disconnects these resistors.

Means for alternatingly changing the applied voltage or current of the second cell, measuring the current and the amount of change in voltage at that time with a current detection resistor to detect the element resistance value. In a gas concentration detection device configured so that the resistance value of the current detection resistor can be switched between at the time of detection,
20. The gas concentration detection device according to claim 17, wherein the current operation means forcibly increases the second cell current using a resistor applied at the time of detecting an element resistance.

In the gas concentration sensor, the gas chamber includes a first gas chamber into which a gas to be detected is introduced and a second gas chamber in which the second cell is provided, and the first cell is located on the first gas chamber side. And a first oxygen pump cell and a second oxygen pump cell which are separately provided on the second gas chamber side, respectively, wherein the current operating means is provided in place of or in addition to the second cell. 21. The gas concentration detecting device according to claim 17, wherein a current flowing through the second oxygen pump cell is forcibly increased for a predetermined period to forcibly discharge oxygen present in the second gas chamber.

The current detection resistor for current detection of the second oxygen pump cell can be switched between a resistance value suitable for oxygen concentration detection and a resistance value smaller than the resistance value, and the current operating means includes a resistance value of the current detection resistor. 22. The gas concentration detection device according to claim 21, wherein the second oxygen pump cell current is forcibly increased by switching to a smaller value.

23. The gas concentration detection device according to claim 22, wherein the current detection resistor has at least two resistors in a parallel relationship, and selectively connects or disconnects these resistors.

A means for alternatingly changing the applied voltage or current of the second oxygen pump cell, measuring the current and the amount of change in voltage at that time with a current detection resistor and detecting the element resistance value, and detecting the oxygen concentration. In a gas concentration detection device configured so that the resistance value of the current detection resistor can be switched between when the element resistance is detected,
24. The gas concentration detection device according to claim 21, wherein the current operation means forcibly increases the second oxygen pump cell current using a resistance applied when detecting element resistance.

A gas concentration comprising a detection cell consisting of a solid electrolyte and a pair of electrodes disposed on the solid electrolyte, wherein the electrode facing the gas chamber of the pair of electrodes is a specific component active electrode active for a specific component. A gas concentration detection device, which is applied to a sensor, and further includes oxygen desorption means for forcibly desorbing oxygen adsorbed on a specific component active electrode of the detection cell.

26. The gas concentration detecting device according to claim 25, wherein the oxygen desorbing means applies a voltage higher than usual between the electrodes of the detection cell within a predetermined time.