JP2004069370A - Oxygen concentration detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an oxygen concentration detector, which satisfies both highly accurate detection within a specific detection range and broad range detection in detecting the concentration of oxygen in a gas to be detected. <P>SOLUTION: An air/fuel ratio detector 51 comprises: a current detecting resistor 7, one end of which is connected to one terminal AFp of an air/fuel ratio sensor 1 (oxygen concentration sensor) and through which a sensor current flows; a differential amplifier circuit 21, into which a voltage Vi at the end part opposite to the sensor 1 of the current detecting resistor 7 and a reference voltage Vr are inputted; an A/D converter 5 for digitalizing an output voltage Vo of the differential amplifier circuit 21; and a microcomputer 3 for detecting an air/fuel ratio based on an A/D conversion value of Vo obtained by the A/D converter 5. The air/fuel ratio detector 51 has a D/A converter 53, which enables inputting of a voltage according to an instruction from the microcomputer 3 into the differential amplifier circuit 21 as reference voltage Vr. The microcomputer 3 modifies the reference voltage Vr from the D/A converter 53 so that the output voltage Vo of the differential amplifier circuit 21 is within a convertible range of the A/D converter 5. <P>COPYRIGHT: (C)2004,JPO

Description

そして、電流検出抵抗７の抵抗値をＲ７とし、センサ電流Ｉの正の方向をリッチ時の電流方向とすると、下記の式２が成立する。
【００１２】
Ｖｉ＝Ｖ１−Ｉ×Ｒ７…式２
また、この空燃比検出装置１０１においては、下記の式３が成立する。
Ｖｒ＝Ｖ１…式３
よって、上記式１〜式３から、差動増幅回路２１の出力電圧Ｖｏは、下記の式４で表される。
【００１３】
Ｖｏ＝Ｖ１＋（Ｒ４２／Ｒ４１）×（Ｉ×Ｒ７）…式４
そこで、この空燃比検出装置１０１では、Ａ／Ｄ変換器５が、差動増幅回路２１の出力電圧Ｖｏと、センサ１のプラス側端子ＡＦｐの実際の電圧Ｖ１とをＡ／Ｄ変換し、マイコン３が、そのＡ／Ｄ変換器５による各Ａ／Ｄ変換値から、上記式４における「（Ｒ４２／Ｒ４１）×（Ｉ×Ｒ７）」の値を求め、その値から排気ガス中の酸素濃度を空燃比という形で検出している。
【００１４】
尚、式４におけるＲ７，Ｒ４１，Ｒ４２は既知であるため、マイコン３は、結局、センサ電流Ｉの値を検出していることとなり、そのセンサ電流Ｉの値から酸素濃度に相当する空燃比が検出される訳であるが、一般には、予め記憶されたデータマップから「（Ｒ４２／Ｒ４１）×（Ｉ×Ｒ７）」の値に対応する空燃比を算出する、といった手法が採られる。また、センサ１のプラス側端子ＡＦｐに実際に印加されている電圧Ｖ１は検出せずに、差動増幅回路２１の出力電圧ＶｏのＡ／Ｄ変換値のみから空燃比を検出することも考えられる。
【００１５】
そして更に、図６の空燃比検出装置１０１において、Ｒ４１とＲ４２とで決まる差動増幅回路２１の増幅率は、空燃比が該空燃比の全検出対象範囲に渡って変化した場合（換言すれば、排気ガス中の酸素濃度が該酸素濃度の全検出対象範囲に渡って変化した場合）に、差動増幅回路２１の出力電圧ＶｏがＡ／Ｄ変換器５のＡ／Ｄ変換可能範囲内で出来るだけ広範囲に変化するように設定されている。
【００１６】
例えば、空燃比の検出対象範囲がＡ／Ｆ＝１３〜１８であり、Ａ／Ｄ変換器５のＡ／Ｄ変換可能範囲が０．２Ｖ〜４．８Ｖであるとすると、差動増幅回路２１の増幅率は、Ａ／Ｆ＝１８の時（最もリーンの時）に差動増幅回路２１の出力電圧ＶｏがＡ／Ｄ変換可能範囲の下限値である０．２Ｖ或いはそれよりも若干高い電圧となり、Ａ／Ｆ＝１３の時（最もリッチの時）に差動増幅回路２１の出力電圧ＶｏがＡ／Ｄ変換可能範囲の上限値である４．８Ｖ或いはそれよりも若干低い電圧となるように設定される。
【００１７】
そして、このような増幅率の設定により、空燃比を全検出対象範囲で高精度に検出できるようにしている。つまり、差動増幅回路２１の出力電圧ＶｏをＡ／Ｄ変換器５の入力フルスケールでＡ／Ｄ変換することができ、そのＡ／Ｄ変換精度が高くなるからである。
【００１８】
【発明が解決しようとする課題】
ところで、近年、エンジンの気筒内に燃料を直接噴射する燃料噴射方式（いわゆる直噴）による更なる燃費向上や、各種法規制に基づくエミッション低減を実現するために、空燃比の検出対象範囲を、図７に例示する如く、従来の範囲（Ａ／Ｆ＝１３〜１８）よりも広い範囲（Ａ／Ｆ＝１０〜２３程度）に拡大すると共に、従来の検出対象範囲（Ａ／Ｆ＝１３〜１８）については、従来と同様の高い精度で空燃比を検出したい、という要求が生じている。
【００１９】
ここで、前述した従来の空燃比検出装置１０１において、空燃比の検出対象範囲を広くするためには、差動増幅回路２１の増幅率を下げる必要がある。つまり、増幅率を小さく設定しないと、差動増幅回路２１の出力電圧ＶｏがＡ／Ｄ変換器５のＡ／Ｄ変換可能範囲を越えてしまうからである。
【００２０】
しかしながら、差動増幅回路２１の増幅率を下げると、センサ電流の検出精度（延いては、酸素濃度，空燃比の検出精度）が悪化する。このため、高い検出精度が要求される特定の検出範囲（上記図７の例では、Ａ／Ｆ＝１３〜１８）について、必要な検出精度を確保できないという問題が生じてしまう。
【００２１】
尚、Ａ／Ｄ変換器５として、高分解能の（分解能が小さい）ものを使用することも考えられるが、そのような高分解能のＡ／Ｄ変換器は概して非常に高価であるため、装置全体のコストアップを招いてしまう。
本発明は、こうした問題に鑑みなされたものであり、被検出ガス中の酸素濃度の広範囲検出と特定の検出範囲での高精度検出とを両立させることができる酸素濃度検出装置の提供を目的としている。
【００２２】
【課題を解決するための手段及び発明の効果】
上記目的を達成するためになされた請求項１に記載の酸素濃度検出装置では、従来の装置と同様に、電圧の印加に伴い被検出ガス中の酸素濃度に応じた電流が流れる酸素濃度センサの一対の端子のうちの一方の端子に、電流検出抵抗の一端が接続されている。そして、電圧印加手段が、酸素濃度センサの前記一方の端子の電圧が第１の電圧となるように、電流検出抵抗の酸素濃度センサ側とは反対側の端部に印加する電圧を調整すると共に、酸素濃度センサの他方の端子に第１の電圧とは異なる第２の電圧を印加することにより、酸素濃度センサの両端子間に第１の電圧と第２の電圧との差分である酸素濃度検出用電圧を印加する。
【００２３】
そして更に、この酸素濃度検出装置は、電流検出抵抗の酸素濃度センサ側とは反対側の端部の電圧（以下、発生電圧という）が第１入力端子に入力されると共に、所定の基準電圧が第２入力端子に入力されて、前記基準電圧と前記発生電圧との差に応じた電圧を出力する差動増幅回路と、その差動増幅回路の出力電圧をデジタル値に変換するＡ／Ｄ変換器と、そのＡ／Ｄ変換器によるＡ／Ｄ変換値に基づいて被検出ガス中の酸素濃度を検出する検出手段とを備えている。
【００２４】
ここで特に、請求項１の酸素濃度検出装置には、検出手段からの指令に応じた電圧を出力して、その電圧を差動増幅回路の第２入力端子へ前記基準電圧として入力させる電圧可変手段が備えられている。そして、この酸素濃度検出装置において、検出手段は、差動増幅回路の出力電圧がＡ／Ｄ変換器のＡ／Ｄ変換可能範囲（即ち、Ａ／Ｄ変換器がＡ／Ｄ変換可能な電圧の範囲）内に収まるように、電圧可変手段から差動増幅回路の第２入力端子に入力される基準電圧を変更する。
【００２５】
よって、このような請求項１の酸素濃度検出装置によれば、高分解能のＡ／Ｄ変換器を用いなくても、被検出ガス中の酸素濃度の広範囲検出と、特定の検出範囲での高精度検出とを、両立させることができる。
つまり、差動増幅回路の増幅率が一定であっても、その差動増幅回路の出力電圧がＡ／Ｄ変換器のＡ／Ｄ変換可能範囲から外れないように該差動増幅回路の第２入力端子に入力される基準電圧を変えることで、酸素濃度の広範囲検出ができ、しかも、予め、差動増幅回路の増幅率を、酸素濃度の全検出対象範囲のうちで他の範囲よりも高い検出精度が要求される特定の範囲（以下、高精度検出必要範囲という）について、酸素濃度を十分な精度で検出できるように設定しておくことで、その高精度検出必要範囲での高精度検出を実現することができる。
【００２６】
具体例として、例えば、まず、差動増幅回路の増幅率を、その差動増幅回路の第２入力端子に入力される基準電圧が所定の電圧Ｖａであり、且つ、被検出ガス中の酸素濃度が高精度検出必要範囲に渡って変化した場合に、差動増幅回路の出力電圧がＡ／Ｄ変換器のＡ／Ｄ変換可能範囲内であって高精度検出必要範囲についての検出精度を実現可能な範囲に渡って変化するように設定しておく。そして、検出手段は、高精度検出必要範囲内の酸素濃度を検出する場合には、電圧可変手段から差動増幅回路の第２入力端子に入力される基準電圧を上記Ｖａに設定し、高精度検出必要範囲外の酸素濃度を検出する場合には、差動増幅回路の出力電圧がＡ／Ｄ変換器のＡ／Ｄ変換可能範囲内に収まるように、電圧可変手段から差動増幅回路の第２入力端子に入力される基準電圧を上記Ｖａとは異なる電圧に変更すれば良い。
【００２７】
ところで、電圧可変手段としては、例えば、抵抗分圧によって発生させた複数通りの電圧のうちの１つを選択スイッチ等によって択一的に出力する、といった回路を用いても良いが、請求項２に記載の如く、検出手段からのデジタル値に応じた電圧を出力するＤ／Ａ変換器を用いれば、任意の電圧を出力することができるという点で有利である。
【００２８】
また、請求項３に記載のように、電圧可変手段として、検出手段から出力される２値振幅のオン／オフ信号を平滑化して出力する積分回路を用い、検出手段が、上記オン／オフ信号のデューティ比により、電圧可変手段としての積分回路の出力電圧を変更するように構成すれば、差動増幅回路への基準電圧を非常に安価な構成で任意に変更することができる。つまり、積分回路は、一般にコンデンサと抵抗から構成することができ、Ｄ／Ａ変換器よりも非常に安価であるからである。
【００２９】
次に、請求項４に記載の酸素濃度検出装置では、差動増幅回路の増幅率が、その差動増幅回路の第２入力端子に入力される基準電圧が前記第１の電圧であり、且つ、被検出ガス中の酸素濃度が全検出対象範囲のうちの高精度検出必要範囲（即ち、他の範囲よりも高い検出精度が要求される特定の範囲）に渡って変化した場合に、差動増幅回路の出力電圧がＡ／Ｄ変換器のＡ／Ｄ変換可能範囲内であって高精度検出必要範囲についての検出精度を実現可能な範囲に渡って変化するように設定されている。そして、検出手段は、高精度検出必要範囲内の酸素濃度を検出する場合には、電圧可変手段から差動増幅回路の第２入力端子に入力される基準電圧を前記第１の電圧に設定し、高精度検出必要範囲外の酸素濃度を検出する場合には、差動増幅回路の出力電圧がＡ／Ｄ変換器のＡ／Ｄ変換可能範囲内に収まるように、電圧可変手段から差動増幅回路の第２入力端子に入力される基準電圧を変更する。
【００３０】
つまり、この請求項４に記載の酸素濃度検出装置は、上記具体例での所定の電圧Ｖａを、電流検出抵抗の一端が接続される方の酸素濃度センサの端子に印加される第１の電圧（図６の端子ＡＦｐに印加される電圧Ｖ１に相当）にしたものである。
【００３１】
そして、このような請求項４の酸素濃度検出装置によれば、高精度検出必要範囲について、従来と同じ検出ロジックをそのまま使うことができるという利点がある。即ち、差動増幅回路への基準電圧が変われば、その差動増幅回路の出力電圧とセンサ電流（酸素濃度センサ及び電流検出抵抗に流れる電流）との関係も変わるため、検出手段は、Ａ／Ｄ変換器による差動増幅回路の出力電圧のＡ／Ｄ変換値と、自分が電圧可変手段から差動増幅回路に供給させている基準電圧の値とから、酸素濃度を検出すれば良いが、高精度検出必要範囲の酸素濃度を検出する場合に、差動増幅回路への基準電圧を第１の電圧とすれば、その差動増幅回路へは、図６の従来装置と同様に、電流検出抵抗の両端の電圧が２つの入力信号として入力されることとなり、従来装置と等価な回路構成になるからである。
【００３２】
次に、請求項５に記載の酸素濃度検出装置には、請求項４の酸素濃度検出装置に対して、差動増幅回路の第２入力端子を酸素濃度センサの前記一方の端子（即ち、電流検出抵抗の一端が接続される端子）と電圧可変手段の電圧出力端子との何れかに接続させる切替手段が、追加して設けられている。
【００３３】
そして、検出手段は、高精度検出必要範囲内の酸素濃度を検出する場合には、電圧可変手段の出力電圧を差動増幅回路の第２入力端子に入力させることに代えて、切替手段により、差動増幅回路の第２入力端子を酸素濃度センサの前記一方の端子に接続させ、高精度検出必要範囲外の酸素濃度を検出する場合には、切替手段により、差動増幅回路の第２入力端子を電圧可変手段の電圧出力端子に接続させると共に、差動増幅回路の出力電圧がＡ／Ｄ変換器のＡ／Ｄ変換可能範囲内に収まるように、電圧可変手段から差動増幅回路の第２入力端子に入力される基準電圧を変更する。
【００３４】
つまり、請求項５の酸素濃度検出装置では、高精度検出必要範囲内の酸素濃度を検出する場合に、電流検出抵抗の両端が差動増幅回路の２つの入力端子に接続されるようにしている。そして、この酸素濃度検出装置によれば、高精度検出必要範囲内の酸素濃度を検出する場合に、電圧可変手段の電圧出力誤差による影響が排除されるため、酸素濃度をより正確に検出することができるようになる。
【００３５】
ところで、検出手段は、差動増幅回路の出力電圧がＡ／Ｄ変換可能範囲を超えそうになったこと（換言すれば、差動増幅回路への基準電圧を変更すべきタイミングが到来したこと）を、差動増幅回路の出力電圧のＡ／Ｄ変換器によるＡ／Ｄ変換値から判断するように構成すれば良い。
【００３６】
但し、差動増幅回路の出力電圧が急激に変化する状況が起こり得ると共に、その出力電圧が急激に変化した場合に、基準電圧の変更タイミングをＡ／Ｄ変換値から判断していたのでは間に合わない（出力電圧がＡ／Ｄ変換可能範囲を超えてしまう）虞があるならば、請求項６に記載のように、差動増幅回路の出力電圧と所定のしきい値電圧とを大小比較する比較器を設け、検出手段は、その比較器の比較結果に基づいて、差動増幅回路への基準電圧の変更を実施するように構成すれば良い。尚、この場合、比較器のしきい値電圧は、Ａ／Ｄ変換可能範囲を超える少し手前の電圧に設定しておけば良く、検出手段は、比較器の出力反転を検知した時に、差動増幅回路への基準電圧を変更すれば良い。
【００３７】
そして、このような請求項６の酸素濃度検出装置によれば、差動増幅回路の出力電圧が急激に変化しても、その差動増幅回路への基準電圧を速やかに変更して、該差動増幅回路の出力電圧を確実にＡ／Ｄ変換可能範囲内に納めることができるようになる。
【００３８】
【発明の実施の形態】
以下、本発明が適用された実施形態の空燃比検出装置（酸素濃度検出装置）について、図面を用いて説明する。
まず図１は、第１実施形態の空燃比検出装置５１の構成を表す回路図である。尚、図１において、前述した図６の空燃比検出装置１０１と同じ構成要素については、同一の符号を付しているため、詳細な説明は省略する。
【００３９】
図１に示すように、本第１実施形態の空燃比検出装置５１は、図６に示した従来の空燃比検出装置１０１と比較すると、ハードウェア面では、マイコン３からの指令であるデジタル値に応じた電圧を出力するＤ／Ａ変換器（ＤＡＣ）５３を備えており、そのＤ／Ａ変換器５３の電圧出力端子が、差動増幅回路２１における抵抗４４のオペアンプ４３側とは反対側の端部（差動増幅回路の第２入力端子に相当し、以下、第２入力端子という）に接続されている。つまり、Ｄ／Ａ変換器５３の出力電圧が、差動増幅回路２１の第２入力端子に基準電圧Ｖｒとして入力されるようになっている。
【００４０】
尚、本実施形態では、Ｄ／Ａ変換器５３が電圧可変手段に相当し、マイコン３が検出手段に相当し、電圧印加回路９，１１が電圧印加手段に相当している。また、差動増幅回路２１における抵抗４１のオペアンプ４３及び抵抗４２側とは反対側の端部が、差動増幅回路の第１入力端子に相当している。
【００４１】
そして、本第１実施形態の空燃比検出装置５１において、マイコン３は、差動増幅回路２１の出力電圧ＶｏがＡ／Ｄ変換器５のＡ／Ｄ変換可能範囲（本実施形態では０．２Ｖ〜４．８Ｖ）内に収まるように、Ｄ／Ａ変換器５３から差動増幅回路２１の第２入力端子に入力される基準電圧Ｖｒを変更する。
【００４２】
具体的に説明すると、まず、本第１実施形態の空燃比検出装置５１では、空燃比の検出対象範囲が、従来装置１０１の検出対象範囲（Ａ／Ｆ＝１３〜１８）よりも広いＡ／Ｆ＝１０〜２３となっている（図７参照）。そして更に、その検出対象範囲（Ａ／Ｆ＝１０〜２３）のうちで、従来装置１０１の検出対象範囲と同じ範囲（Ａ／Ｆ＝１３〜１８）は、従来装置１０１と同様の高い精度で空燃比を検出すべき高精度検出必要範囲となっている。
【００４３】
そこで、まず、本第１実施形態の空燃比検出装置５１において、差動増幅回路２１の増幅率は、差動増幅回路２１の第２入力端子に入力される基準電圧Ｖｒが空燃比センサ１のプラス側端子ＡＦｐに印加される第１の電圧Ｖ１（＝３．３Ｖ）であり、且つ、空燃比が高精度検出必要範囲（Ａ／Ｆ＝１３〜１８）に渡って変化した場合に、図２の中央部分に示す如く、差動増幅回路２１の出力電圧ＶｏがＡ／Ｄ変換器５のＡ／Ｄ変換可能範囲内であって高精度検出必要範囲についての検出精度を実現可能な出来るだけ広い範囲（本実施形態では例えば０．３Ｖ〜４．７Ｖ）に渡って変化するように設定されている。
【００４４】
そして、マイコン３は、図２に示すように、高精度検出必要範囲内の空燃比を検出する場合には、Ｄ／Ａ変換器５３から差動増幅回路２１の第２入力端子に供給する基準電圧Ｖｒを第１の電圧Ｖ１（＝３．３Ｖ）に設定し、高精度検出必要範囲外（Ａ／Ｆ＝１０〜１３，１８〜２３）の空燃比を検出する場合には、差動増幅回路２１の出力電圧ＶｏがＡ／Ｄ変換器５のＡ／Ｄ変換可能範囲内に収まるように、Ｄ／Ａ変換器５３から差動増幅回路２１への基準電圧Ｖｒを変更するようになっている。尚、マイコン３は、差動増幅回路２１の出力電圧ＶｏのＡ／Ｄ変換器５によるＡ／Ｄ変換値から、その差動増幅回路２１の出力電圧ＶｏがＡ／Ｄ変換器５のＡ／Ｄ変換可能範囲から外れそうになったことを検知すると、Ｄ／Ａ変換器５３へのデジタル値を変えて、差動増幅回路２１への基準電圧Ｖｒを変更する。
【００４５】
また、マイコン３は、高精度検出必要範囲内の空燃比を検出する場合には、前述した従来装置１０１と同じ検出ロジックで空燃比を検出する。つまり、マイコン３は、Ａ／Ｄ変換器５によるＡ／Ｄ変換値から、差動増幅回路２１の出力電圧Ｖｏと、空燃比センサ１のプラス側端子ＡＦｐの実際の電圧（以下、プラス側端子電圧という）Ｖ１とを検出して、前述した式４における「（Ｒ４２／Ｒ４１）×（Ｉ×Ｒ７）」の値を求め、その値（換言すれば、センサ電流Ｉの検出値）から、所定の計算式や予め記憶されたデータマップに基づき空燃比を検出する。
【００４６】
これに対して、高精度検出必要範囲以外（Ａ／Ｆ＝１０〜１３，１８〜２３）の空燃比を検出する場合、マイコン３は、Ａ／Ｄ変換器５によるＡ／Ｄ変換値から、差動増幅回路２１の出力電圧Ｖｏと、空燃比センサ１のプラス側端子電圧Ｖ１とを検出すると共に、その両検出値Ｖｏ，Ｖ１と、自分がＤ／Ａ変換器５３から差動増幅回路２１に供給させている基準電圧Ｖｒの値とから、空燃比を検出する。
【００４７】
つまり、本実施形態の空燃比検出装置５１において、差動増幅回路２１の出力電圧Ｖｏは、前述した式１と式２から、下記の式５のように表すことができる。
Ｖｏ＝Ｖｒ＋（Ｒ４２／Ｒ４１）×（Ｖｒ−Ｖ１）＋（Ｒ４２／Ｒ４１）×（Ｉ×Ｒ７）…式５
よって、ＶｏとＶ１との検出値（実測値）と、Ｄ／Ａ変換器５３から差動増幅回路２１に供給している基準電圧Ｖｒとから、上記式５における「（Ｒ４２／Ｒ４１）×（Ｉ×Ｒ７）」の値を求め、その値から空燃比を検出すれば良い。
【００４８】
尚、Ｄ／Ａ変換器５３から出力する基準電圧Ｖｒを第１の電圧Ｖ１（＝３．３Ｖ）に設定した場合、差動増幅回路２１の出力電圧Ｖｏは、前述した式４となるが、その式４のＶｏと式５のＶｏとの差（即ち、Ｖｒ＝Ｖ１にした場合のＶｏと、Ｖｒ≠Ｖ１にした場合のＶｏとの差）Ｄは、下記式６のようになる。
【００４９】
Ｄ＝（１＋Ｒ４２／Ｒ４１）×（Ｖｒ−Ｖ１）…式６
よって、例えば、Ｖｒ＝Ｖ１であると仮定した場合の差動増幅回路２１の出力電圧Ｖｏと空燃比との関係を示したデータマップから、Ｖｏの検出値に対応した空燃比を求める構成であれば、高精度検出必要範囲以外（Ａ／Ｆ＝１０〜１３，１８〜２３）の空燃比を検出する場合には、Ｖｏの検出値から上記式６の差Ｄを引いた値を、データマップに当てはめて空燃比を求めても良い。
【００５０】
また、Ｖｏの検出値から空燃比を求めるためのデータマップを、「（Ｖｒ−Ｖ１）」の値に応じて切り換えるようにしても良い。
以上のような本第１実施形態の空燃比検出装置５１によれば、Ａ／Ｄ変換器５として、高分解能の高価なＡ／Ｄ変換器を用いなくても、空燃比の広範囲検出と、特定の範囲（高精度検出必要範囲）での高精度検出とを、両立させることができる。つまり、差動増幅回路２１の増幅率が高精度検出必要範囲を狙った値に設定されているため、その高精度検出必要範囲での高精度検出が可能であり、また、高精度検出必要範囲以外の拡大された範囲については、差動増幅回路２１の出力電圧ＶｏがＡ／Ｄ変換器５の入力フルスケール（０．２Ｖ〜４．８Ｖ）に収まるように、Ｄ／Ａ変換器５３から差動増幅回路２１への基準電圧Ｖｒが調整されるからである。
【００５１】
次に、第２実施形態の空燃比検出装置について、図３を用い説明する。尚、図３において、前述した図１及び図６と同じ構成要素については、同一の符号を付しているため、詳細な説明は省略する。
図３に示すように、本第２実施形態の空燃比検出装置５５は、図１に示した第１実施形態の空燃比検出装置５１と比較すると、下記の（１）〜（３）の点が異なっている。
【００５２】
（１）：Ｄ／Ａ変換器５３の電圧出力端子は、差動増幅回路２１の第２入力端子に直接接続されていない。
（２）：その代わりに、差動増幅回路２１の第２入力端子を空燃比センサ１のプラス側端子ＡＦｐとＤ／Ａ変換器５３の電圧出力端子との何れかに接続させる切替回路５７（切替手段に相当）が追加されている。
【００５３】
そして、この切替回路５７は、２つのアナログスイッチ５８，５９を有しており、マイコン３からの切替信号Ｓｃがローレベルならば、アナログスイッチ５８だけがオンして差動増幅回路２１の第２入力端子をＤ／Ａ変換器５３の電圧出力端子に接続させ、逆に、マイコン３からの切替信号Ｓｃがハイレベルならば、アナログスイッチ５９だけがオンして差動増幅回路２１の第２入力端子を空燃比センサ１のプラス側端子ＡＦｐに接続させる。
【００５４】
（３）：マイコン３は、高精度検出必要範囲（Ａ／Ｆ＝１３〜１８）内の空燃比を検出する場合には、切替回路５７により、差動増幅回路２１の第２入力端子を空燃比センサ１のプラス側端子ＡＦｐに接続させ、高精度検出必要範囲外の空燃比を検出する場合には、切替回路５７により、差動増幅回路２１の第２入力端子をＤ／Ａ変換器５３の電圧出力端子に接続させた上で、第１実施形態と同様に、差動増幅回路２１の出力電圧ＶｏがＡ／Ｄ変換器５のＡ／Ｄ変換可能範囲内に収まるように、Ｄ／Ａ変換器５３から差動増幅回路２１への基準電圧Ｖｒを変更する。
【００５５】
つまり、第２実施形態の空燃比検出装置５５では、高精度検出必要範囲内の空燃比を検出する場合に、電流検出抵抗７の両端が差動増幅回路２１の２つの入力端子に接続されるようにしている。
そして、このような空燃比検出装置５５によれば、高精度検出必要範囲内の空燃比を検出する場合に、Ｄ／Ａ変換器５３の電圧出力誤差による影響が排除されるため、空燃比をより正確に検出することができるようになる。
【００５６】
次に、第３実施形態の空燃比検出装置について、図４を用い説明する。尚、図４において、前述した図１及び図６と同じ構成要素については、同一の符号を付しているため、詳細な説明は省略する。
図４に示すように、本第３実施形態の空燃比検出装置６１は、図１に示した第１実施形態の空燃比検出装置５１と比較すると、差動増幅回路２１への基準電圧Ｖｒを可変にするための電圧可変手段として、Ｄ／Ａ変換器５３ではなく、マイコン３出力される２値振幅のオン／オフ信号（本実施形態では、ハイレベルが５Ｖであり、ローレベルが０Ｖである）を平滑化して差動増幅回路２１の第２入力端子に出力する積分回路６２を用いている点が異なっている。
【００５７】
尚、積分回路６２は、マイコン３における上記オン／オフ信号の出力端子と差動増幅回路２１の第２入力端子（抵抗４４のオペアンプ４３側とは反対側の端部）との間に直列に挿入された抵抗６３と、その抵抗６３の差動増幅回路２１側の端部に一端が接続され、他端が接地電位に接続されたコンデンサ６５と、から構成されている。
【００５８】
そして、本第３実施形態の空燃比検出装置６１において、マイコン３は、積分回路６２へ出力するオン／オフ信号のデューティ比により、その積分回路６２から差動増幅回路２１の第２入力端子に出力される基準電圧Ｖｒを、第１実施形態と同様に変更する。つまり、積分回路６２の出力電圧はマイコン３からのオン／オフ信号のデューティ比に応じて変化するからである。
【００５９】
このような空燃比検出装置６１によれば、差動増幅回路２１への基準電圧Ｖｒを非常に安価な構成で任意に変更することができ有利である。
次に、第４実施形態の空燃比検出装置について、図５を用い説明する。尚、図５において、前述した図１及び図６と同じ構成要素については、同一の符号を付しているため、詳細な説明は省略する。
【００６０】
図５に示すように、本第４実施形態の空燃比検出装置６７は、図１に示した第１実施形態の空燃比検出装置５１と比較すると、下記の（Ａ）及び（Ｂ）の点が異なっている。
（Ａ）：電源電圧ＶＳ（＝５Ｖ）を分圧することにより所定のしきい値電圧Ｖｔｈを発生する２つの分圧抵抗７１，７３と、その分圧抵抗７１，７３の接続点に生じるしきい値電圧Ｖｔｈと差動増幅回路２１の出力電圧Ｖｏとを大小比較する比較器６９とが追加されている。そして、比較器６９は、Ｖｏ＜Ｖｔｈの場合には、マイコン３への出力信号をハイレベルにし、Ｖｏ≧Ｖｔｈになると、マイコン３への出力信号をハイレベルからローレベルに反転させる。
【００６１】
また、しきい値電圧Ｖｔｈは、Ａ／Ｄ変換可能範囲の上限値（＝４．８Ｖ）よりも若干低く、且つ、高精度検出必要範囲の空燃比を検出する場合における差動増幅回路２１の出力電圧Ｖｏの最大値（＝４．７Ｖ）と同じかそれよりも若干高い電圧（本実施形態では４．７５Ｖ）に設定されている。
【００６２】
（Ｂ）：マイコン３は、高精度検出必要範囲内の空燃比を検出している場合に、差動増幅回路２１の出力電圧Ｖｏがしきい値電圧Ｖｔｈ以上になって比較器６９の出力信号がハイレベルからローレベルに反転すると、Ｄ／Ａ変換器５３から差動増幅回路２１の第２入力端子に供給する基準電圧Ｖｒを、第１の電圧Ｖ１（＝３．３Ｖ）から、それよりも低い所定電圧Ｖｂに変更して、差動増幅回路２１の出力電圧ＶｏがＡ／Ｄ変換器５のＡ／Ｄ変換可能範囲内に収まるようにし、その状態で、高精度検出必要範囲よりもリッチ側の空燃比を検出する。
【００６３】
また、マイコン３は、差動増幅回路２１への基準電圧Ｖｒを上記所定電圧Ｖｂにしている状態で、差動増幅回路２１の出力電圧ＶｏがＡ／Ｆ＝１３（高精度検出必要範囲の最リッチ値）に該当する値にまで低下したことを、Ａ／Ｄ変換器５によるＶｏのＡ／Ｄ変換値に基づき検知したならば、差動増幅回路２１への基準電圧Ｖｒを第１の電圧Ｖ１に戻して、高精度検出必要範囲内の空燃比を検出する状態に戻る。
【００６４】
このような第４実施形態の空燃比検出装置６７によれば、例えば、燃料噴射制御で空燃比を急激にリーンからリッチにするような制御が行われて、差動増幅回路２１の出力電圧Ｖｏが急に大きくなったしたとしても、マイコン３は、そのことを比較器６９の出力によって速やかに検知することができ、延いては、差動増幅回路２１への基準電圧Ｖｒを速やかに変更して、該差動増幅回路２１の出力電圧Ｖｏを確実にＡ／Ｄ変換可能範囲内に納めることができる。
【００６５】
特に、差動増幅回路２１の出力電圧Ｖｏが急激に変化した場合に、基準電圧Ｖｒを変更すべきことをＡ／Ｄ変換器５によるＶｏのＡ／Ｄ変換値から判断していたのでは間に合わない（ＶｏがＡ／Ｄ変換可能範囲を超えてしまう）虞がある場合に有効である。
【００６６】
尚、この第４実施形態では、差動増幅回路２１への基準電圧Ｖｒを、高精度検出必要範囲の空燃比を検出するための第１の電圧Ｖ１から、その高精度検出必要範囲よりも１段階リッチ側の範囲の空燃比を検出するための所定電圧Ｖｂへと切り替えるタイミングを、マイコン３に知らせるために、１つの比較器６９を設けたが、基準電圧Ｖｒを速やかに切り替えるべきタイミングが複数存在するのであれば、その切り替えタイミング毎（換言すれば、Ｖｏと大小比較すべきしきい値毎）に比較器を設ければ良い。
【００６７】
以上、本発明の一実施形態について説明したが、本発明は、種々の形態を採り得ることは言うまでもない。
例えば、電圧可変手段として積分回路６２を用いる図４の構成は、第２実施形態や第４実施形態の空燃比検出装置５５，６７に対しても、同様に適用することができる。
【図面の簡単な説明】
【図１】第１実施形態の空燃比検出装置の構成を表す回路図である。
【図２】第１実施形態の空燃比検出装置の作用を表す図である。
【図３】第２実施形態の空燃比検出装置の構成を表す回路図である。
【図４】第３実施形態の空燃比検出装置の構成を表す回路図である。
【図５】第４実施形態の空燃比検出装置の構成を表す回路図である。
【図６】従来の空燃比検出装置の構成を表す回路図である。
【図７】空燃比センサの電圧−電流特性の一例を示すグラフである。
【符号の説明】
１…空燃比センサ（酸素濃度センサ）、ＡＦｍ…マイナス側端子、ＡＦｐ…プラス側端子、３…マイコン、５…Ａ／Ｄ変換器、７…電流検出抵抗、９，１１…電圧印加回路、１３〜１９，２３，２９，３１，３３，３９，４１，４２，４４，４５，６３，７１，７３…抵抗、２１…差動増幅回路、２５，３５，４３…オペアンプ、２７，３７…ＮＰＮトランジスタ、５１，５５，６１，６７…空燃比検出装置、５３…Ｄ／Ａ変換器、５７…切替回路、５８，５９…アナログスイッチ、６２…積分回路、６５…コンデンサ、６９…比較器[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an oxygen concentration detection device that detects, for example, the oxygen concentration in exhaust gas of a vehicle engine.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, in the air-fuel ratio control of a vehicle-mounted engine, an oxygen concentration sensor (a fuel-air ratio of an air-fuel mixture sucked into the engine) that can linearly detect an oxygen concentration in an exhaust gas as a gas to be detected. Hereinafter, an air-fuel ratio sensor or simply a sensor is used.
[0003]
More specifically, this type of air-fuel ratio sensor, as shown in FIG. 7 showing an example of its voltage-current characteristics, changes the oxygen concentration (and hence the air-fuel ratio) in the exhaust gas with the application of a voltage. It is configured such that a corresponding current flows. In FIG. 7, the horizontal axis represents the value of the voltage applied to the air-fuel ratio sensor, and the vertical axis represents the value of the sensor current flowing through the air-fuel ratio sensor. In FIG. 7, the linear portion of the sensor current parallel to the horizontal axis indicates the limit current of the sensor, and the value of the limit current corresponds to the value of the air-fuel ratio (A / F).
[0004]
Therefore, in an air-fuel ratio detection device (oxygen concentration detection device) using such an air-fuel ratio sensor, a predetermined oxygen concentration detection voltage is applied between a pair of terminals of the air-fuel ratio sensor and the air-fuel ratio sensor flows through the air-fuel ratio sensor. The air-fuel ratio is calculated by detecting the limit current.
Next, FIG. 6 shows a configuration example of a conventional air-fuel ratio detection device 101. Note that this type of air-fuel ratio detection device is generally incorporated in a control device that controls an engine.
[0005]
As shown in FIG. 6, an air-fuel ratio sensor 1 as an oxygen concentration sensor is connected to the air-fuel ratio detection device 101, and the air-fuel ratio sensor 1 is attached to an exhaust pipe of an engine (not shown).
The air-fuel ratio detection device 101 has one end connected to one of the microcomputer 3, the A / D converter (ADC) 5, and one of the pair of terminals AFp and AFm of the sensor 1 (the plus side terminal AFp in this example). And the opposite side of the current detection resistor 7 from the sensor 1 side such that the voltage of the plus side terminal AFp of the sensor 1 becomes the first voltage V1 (3.3 V in this example). A voltage application circuit 9 for adjusting the voltage applied to the end of the sensor 1; and a voltage for oxygen concentration detection to be applied to the negative terminal AFm of the sensor 1 between the two terminals AFp and AFm of the sensor 1 more than the first voltage V1. The voltage application circuit 11 that applies a second voltage V2 (in this example, 2.9 V) that is lower by only (in this example, 0.4 V) and the constant power supply voltage VS (in this example, 5 V) to divide the voltage. 1 voltage V1 (= 3.3V) Generated voltage dividing resistors 13 and 15, two voltage dividing resistors 17 and 19 that generate the second voltage V <b> 2 (= 2.9 V) by dividing the power supply voltage VS, and a current detecting resistor 7. A differential amplifier circuit 21 to which the voltage at both ends is input.
[0006]
Here, in the voltage application circuit 9, the first voltage V 1 (= 3.3 V) generated at the connection point between the voltage dividing resistors 13 and 15 is input to the non-inverting input terminal, and the inverting input terminal passes through the input protection resistor 23. The operational amplifier 25 connected to the plus side terminal AFp of the sensor 1, the base is connected to the output terminal of the operational amplifier 25, the collector is connected to the power supply voltage VS, and the emitter is connected to the sensor 1 side of the current detection resistor 7. An NPN transistor 27 for voltage output is connected to the opposite end, and a pull-down resistor 29 is connected between the emitter of the transistor 27 and the ground potential. In the voltage application circuit 9, the emitter voltage of the transistor 27 (that is, the current detection resistor 7 on the side opposite to the sensor 1 side) is set so that the voltage of the plus side terminal AFp of the sensor 1 always becomes the first voltage V1. ) Is adjusted.
[0007]
In the voltage application circuit 11, the second voltage V2 (= 2.9 V) generated at the connection point between the voltage dividing resistors 17 and 19 is input to the non-inverting input terminal, and the inverting input terminal passes through the input protection resistor 33. The operational amplifier 35 connected to the negative terminal AFm of the sensor 1, the base is connected to the output terminal of the operational amplifier 35, the collector is connected to the power supply voltage VS, and the emitter is connected via the resistor 31 to the negative terminal of the sensor 1. It includes an NPN transistor 37 for voltage output connected to AFm, and a pull-down resistor 39 connected between the emitter of the transistor 37 and the ground potential. In the voltage application circuit 11, similarly to the voltage application circuit 9, the emitter voltage of the transistor 37 is adjusted so that the voltage of the negative terminal AFm of the sensor 1 always becomes the second voltage V2. As a result, the voltage applied to the negative terminal AFm of the sensor 1 is always maintained at the second voltage V2.
[0008]
On the other hand, the differential amplifier circuit 21 includes a resistor 41 having one end connected to the end of the current detection resistor 7 on the side opposite to the sensor 1 side, a resistor 42 having one end connected to the other end of the resistor 41, An output terminal is connected to the other end of the resistor 42, an operational amplifier 43 having an inverting input terminal connected to a connection point between the resistors 41 and 42, one end is connected to a non-inverting input terminal of the operational amplifier, and the other end is connected. And a resistor 44 connected to the plus side terminal AFp of the sensor 1. The output voltage Vo of the differential amplifier circuit 21 (in other words, the output voltage of the operational amplifier 43) is input to the A / D converter 5.
[0009]
Further, the voltage actually applied to the positive terminal AFp of the sensor 1 is also input to the A / D converter 5 via the input protection resistor 45.
In the air-fuel ratio detection device 101 having the above configuration, a voltage of 0.4 V is applied between the two terminals AFp and AFm of the air-fuel ratio sensor 1 by the two voltage application circuits 9 and 11. Then, in this state, a current (sensor current) flowing through the air-fuel ratio sensor 1 flows through the current detection resistor 7, and the sensor current is supplied with a voltage when lean (A / F> 14). The current flows from the circuit 9 side to the plus side terminal AFp of the sensor 1 (to the left in FIG. 6), and when rich (A / F <14), from the plus side terminal AFp of the sensor 1 to the voltage application circuit 9 side. (Right direction in FIG. 6).
[0010]
Further, a voltage input to an end of the differential amplifier circuit 21 opposite to the operational amplifier 43 and the resistor 42 side of the resistor 41 is defined as Vi, and a voltage of the resistor 44 in the differential amplifier circuit 21 opposite to the operational amplifier 43 side is opposite to the voltage. Assuming that the voltage (reference voltage) input to the end is Vr, the resistance value of the resistor 41 is R41, and the resistance value of the resistor 42 is R42, the output voltage Vo of the differential amplifier circuit 21 is expressed by the following equation 1. expressed.
[0011]

If the resistance value of the current detection resistor 7 is R7 and the positive direction of the sensor current I is the current direction at the time of rich, the following equation 2 is established.
[0012]
Vi = V1−I × R7 Equation 2
Further, in the air-fuel ratio detecting device 101, the following Expression 3 is satisfied.
Vr = V1 Equation 3
Therefore, from the above equations 1 to 3, the output voltage Vo of the differential amplifier circuit 21 is expressed by the following equation 4.
[0013]
Vo = V1 + (R42 / R41) × (I × R7) Equation 4
Therefore, in the air-fuel ratio detection device 101, the A / D converter 5 A / D converts the output voltage Vo of the differential amplifier circuit 21 and the actual voltage V1 of the plus terminal AFp of the sensor 1 into a microcomputer. 3 obtains the value of “(R42 / R41) × (I × R7)” in the above equation 4 from each A / D conversion value obtained by the A / D converter 5, and calculates the oxygen concentration in the exhaust gas from the value. Is detected in the form of an air-fuel ratio.
[0014]
Since R7, R41, and R42 in Equation 4 are known, the microcomputer 3 eventually detects the value of the sensor current I, and the air-fuel ratio corresponding to the oxygen concentration is determined from the value of the sensor current I. Although it is detected, a method of calculating the air-fuel ratio corresponding to the value of “(R42 / R41) × (I × R7)” from a data map stored in advance is generally adopted. It is also conceivable to detect the air-fuel ratio only from the A / D conversion value of the output voltage Vo of the differential amplifier circuit 21 without detecting the voltage V1 actually applied to the plus terminal AFp of the sensor 1. .
[0015]
Further, in the air-fuel ratio detection device 101 of FIG. 6, the amplification factor of the differential amplifier circuit 21 determined by R41 and R42 is determined when the air-fuel ratio changes over the entire detection target range of the air-fuel ratio (in other words, When the oxygen concentration in the exhaust gas changes over the entire detection range of the oxygen concentration), the output voltage Vo of the differential amplifier circuit 21 falls within the A / D conversion range of the A / D converter 5. It is set to vary as widely as possible.
[0016]
For example, assuming that the detection range of the air-fuel ratio is A / F = 13 to 18 and the A / D conversion possible range of the A / D converter 5 is 0.2 V to 4.8 V, the differential amplifier 21 When the A / F = 18 (leanest), the output voltage Vo of the differential amplifier circuit 21 is 0.2V which is the lower limit of the A / D convertible range or a voltage slightly higher than 0.2V. When A / F = 13 (at the time of the richest state), the output voltage Vo of the differential amplifier circuit 21 is set to the upper limit value of the A / D conversion possible range of 4.8 V or a voltage slightly lower than the upper limit value. Is set to
[0017]
By setting such an amplification factor, the air-fuel ratio can be detected with high accuracy in the entire detection target range. That is, the output voltage Vo of the differential amplifier circuit 21 can be A / D-converted at the input full scale of the A / D converter 5, and the A / D conversion accuracy is improved.
[0018]
[Problems to be solved by the invention]
By the way, in recent years, in order to further improve fuel efficiency by a fuel injection method of directly injecting fuel into the cylinder of an engine (so-called direct injection), and to reduce emissions based on various laws and regulations, the detection range of the air-fuel ratio has been increased. As illustrated in FIG. 7, the detection range is expanded to a wider range (A / F = about 10 to 23) than the conventional range (A / F = 13 to 18), and the conventional detection target range (A / F = 13 to 18). Regarding 18), there has been a demand for detecting the air-fuel ratio with the same high accuracy as in the past.
[0019]
Here, in the above-described conventional air-fuel ratio detection device 101, it is necessary to reduce the amplification factor of the differential amplifier circuit 21 in order to widen the detection range of the air-fuel ratio. That is, unless the amplification factor is set to a small value, the output voltage Vo of the differential amplifier circuit 21 exceeds the A / D conversion possible range of the A / D converter 5.
[0020]
However, when the amplification factor of the differential amplifier circuit 21 is reduced, the detection accuracy of the sensor current (and the detection accuracy of the oxygen concentration and the air-fuel ratio) deteriorates. For this reason, a problem arises in that a necessary detection accuracy cannot be secured in a specific detection range (A / F = 13 to 18 in the example of FIG. 7) where high detection accuracy is required.
[0021]
A high-resolution (low-resolution) A / D converter may be used as the A / D converter 5, but such a high-resolution A / D converter is generally very expensive, so that Cost increase.
The present invention has been made in view of such a problem, and an object of the present invention is to provide an oxygen concentration detection device that can achieve both wide-range detection of oxygen concentration in a gas to be detected and high-precision detection in a specific detection range. I have.
[0022]
Means for Solving the Problems and Effects of the Invention
In order to achieve the above object, in the oxygen concentration detecting device according to claim 1, as in the conventional device, an oxygen concentration sensor in which a current according to the oxygen concentration in the gas to be detected flows with application of a voltage. One end of the current detection resistor is connected to one of the pair of terminals. Then, the voltage applying means adjusts the voltage applied to the end of the current detection resistor opposite to the oxygen concentration sensor side so that the voltage of the one terminal of the oxygen concentration sensor becomes the first voltage, and By applying a second voltage different from the first voltage to the other terminal of the oxygen concentration sensor, the oxygen concentration, which is the difference between the first voltage and the second voltage, is applied between both terminals of the oxygen concentration sensor. Apply the detection voltage.
[0023]
Further, in this oxygen concentration detection device, a voltage (hereinafter, referred to as a generated voltage) at an end of the current detection resistor opposite to the oxygen concentration sensor side is input to the first input terminal, and a predetermined reference voltage is applied. A differential amplifier circuit that is input to a second input terminal and outputs a voltage corresponding to a difference between the reference voltage and the generated voltage, and an A / D converter that converts an output voltage of the differential amplifier circuit into a digital value And a detecting means for detecting the oxygen concentration in the gas to be detected based on the A / D converted value by the A / D converter.
[0024]
Here, in particular, in the oxygen concentration detecting device according to claim 1, a voltage variable according to a command output from the detecting means is output, and the voltage is input to the second input terminal of the differential amplifier circuit as the reference voltage. Means are provided. In this oxygen concentration detecting device, the detecting means sets the output voltage of the differential amplifier circuit to the A / D convertible range of the A / D converter (that is, the voltage of the voltage at which the A / D converter can perform the A / D conversion). The reference voltage input from the voltage variable means to the second input terminal of the differential amplifier circuit is changed so as to fall within the range.
[0025]
Therefore, according to the oxygen concentration detecting device of the first aspect, even without using a high-resolution A / D converter, the oxygen concentration in the gas to be detected can be detected in a wide range and the oxygen concentration in a specific detection range can be increased. Accuracy detection can be compatible.
In other words, even if the amplification factor of the differential amplifier circuit is constant, the output voltage of the differential amplifier circuit does not deviate from the A / D convertible range of the A / D converter. By changing the reference voltage input to the input terminal, a wide range of oxygen concentration can be detected, and the amplification factor of the differential amplifier circuit is higher than the other ranges in the entire oxygen concentration detection range in advance. For a specific range where detection accuracy is required (hereinafter referred to as a high-accuracy detection required range), setting is made so that the oxygen concentration can be detected with sufficient accuracy. Can be realized.
[0026]
As a specific example, for example, first, the amplification factor of the differential amplifier circuit is determined by determining that the reference voltage input to the second input terminal of the differential amplifier circuit is a predetermined voltage Va, and that the oxygen concentration in the detected gas is Is changed over the high-accuracy detection required range, the output voltage of the differential amplifier circuit is within the A / D conversion range of the A / D converter, and the detection accuracy in the high-accuracy detection required range can be realized. It is set to change over a wide range. Then, when detecting the oxygen concentration within the high-accuracy detection required range, the detection means sets the reference voltage input from the voltage variable means to the second input terminal of the differential amplifier circuit to the above Va, When detecting the oxygen concentration outside the required detection range, the voltage varying means controls the output voltage of the differential amplifier circuit so that the output voltage falls within the A / D convertible range of the A / D converter. The reference voltage input to the two input terminals may be changed to a voltage different from Va.
[0027]
By the way, as the voltage varying means, for example, a circuit that selectively outputs one of a plurality of voltages generated by resistance voltage division by a selection switch or the like may be used. As described in above, using a D / A converter that outputs a voltage corresponding to the digital value from the detection means is advantageous in that an arbitrary voltage can be output.
[0028]
According to a third aspect of the present invention, as the voltage varying means, an integrating circuit for smoothing and outputting a binary amplitude on / off signal output from the detecting means is used, and the detecting means uses the on / off signal. If the output voltage of the integrating circuit as the voltage varying means is changed according to the duty ratio, the reference voltage to the differential amplifier circuit can be arbitrarily changed with a very inexpensive configuration. In other words, the integration circuit can be generally composed of a capacitor and a resistor, and is much cheaper than a D / A converter.
[0029]
Next, in the oxygen concentration detection device according to claim 4, the amplification factor of the differential amplifier circuit is such that the reference voltage input to the second input terminal of the differential amplifier circuit is the first voltage, and When the oxygen concentration in the gas to be detected changes over the required range of high-accuracy detection (ie, a specific range requiring higher detection accuracy than other ranges) of the entire detection target range, the differential The output voltage of the amplifier circuit is set so as to vary within a range in which the A / D converter can perform the A / D conversion and the detection accuracy in the high-accuracy detection required range can be realized. When detecting the oxygen concentration within the high-accuracy detection required range, the detection means sets the reference voltage input from the voltage variable means to the second input terminal of the differential amplifier circuit to the first voltage. When detecting the oxygen concentration outside the required range of high-precision detection, the voltage variable means controls the differential amplification so that the output voltage of the differential amplifier circuit falls within the A / D conversion-capable range of the A / D converter. The reference voltage input to the second input terminal of the circuit is changed.
[0030]
That is, in the oxygen concentration detecting apparatus according to the fourth aspect, the predetermined voltage Va in the above specific example is applied to the terminal of the oxygen concentration sensor to which one end of the current detection resistor is connected. (Corresponding to the voltage V1 applied to the terminal AFp in FIG. 6).
[0031]
According to the oxygen concentration detection device of the fourth aspect, there is an advantage that the same detection logic as that of the related art can be used as it is in the high-accuracy detection required range. That is, if the reference voltage to the differential amplifier circuit changes, the relationship between the output voltage of the differential amplifier circuit and the sensor current (current flowing through the oxygen concentration sensor and the current detection resistor) also changes. The oxygen concentration may be detected from the A / D conversion value of the output voltage of the differential amplifier circuit by the D converter and the value of the reference voltage supplied to the differential amplifier circuit from the voltage variable means. When the reference voltage to the differential amplifier circuit is set to the first voltage when detecting the oxygen concentration in the required range of the high-precision detection, the differential amplifier circuit is supplied with the current detection as in the conventional device of FIG. This is because the voltages at both ends of the resistor are input as two input signals, and the circuit configuration is equivalent to that of the conventional device.
[0032]
Next, the oxygen concentration detecting device according to claim 5 is different from the oxygen concentration detecting device according to claim 4 in that the second input terminal of the differential amplifier circuit is connected to the one terminal of the oxygen concentration sensor (that is, the current Switching means for connecting to one of the terminal to which one end of the detection resistor is connected) and the voltage output terminal of the voltage variable means is additionally provided.
[0033]
Then, when detecting the oxygen concentration within the high-accuracy detection required range, the detection means replaces the input of the output voltage of the voltage variable means to the second input terminal of the differential amplifier circuit by the switching means. When the second input terminal of the differential amplifier circuit is connected to the one terminal of the oxygen concentration sensor to detect the oxygen concentration outside the required range of the high-accuracy detection, the switching means uses the second input terminal of the differential amplifier circuit. The terminal is connected to the voltage output terminal of the voltage variable means, and the voltage variable means is connected to the second terminal of the differential amplifier circuit so that the output voltage of the differential amplifier circuit falls within the A / D converter possible range of the A / D converter. Change the reference voltage input to the two input terminals.
[0034]
In other words, in the oxygen concentration detecting device according to the fifth aspect, both ends of the current detection resistor are connected to the two input terminals of the differential amplifier circuit when detecting the oxygen concentration within the high-accuracy detection required range. . According to this oxygen concentration detection device, when detecting the oxygen concentration within the high-accuracy detection required range, the influence of the voltage output error of the voltage variable means is eliminated, so that the oxygen concentration can be detected more accurately. Will be able to
[0035]
By the way, the detecting means detects that the output voltage of the differential amplifier circuit is about to exceed the A / D convertible range (in other words, that the timing to change the reference voltage to the differential amplifier circuit has arrived). May be determined from the A / D conversion value of the output voltage of the differential amplifier circuit by the A / D converter.
[0036]
However, a situation in which the output voltage of the differential amplifier circuit changes rapidly may occur, and when the output voltage changes rapidly, it is not enough to judge the change timing of the reference voltage from the A / D conversion value. If there is a possibility that the voltage does not exist (the output voltage exceeds the A / D conversion possible range), the magnitude of the output voltage of the differential amplifier circuit is compared with a predetermined threshold voltage. A comparator may be provided, and the detection means may be configured to change the reference voltage to the differential amplifier circuit based on the comparison result of the comparator. In this case, the threshold voltage of the comparator may be set to a voltage slightly before the A / D conversion possible range, and the detection means detects the differential inversion when detecting the output inversion of the comparator. What is necessary is just to change the reference voltage to an amplifier circuit.
[0037]
According to the oxygen concentration detecting device of the sixth aspect, even if the output voltage of the differential amplifier circuit changes abruptly, the reference voltage to the differential amplifier circuit is quickly changed, and The output voltage of the dynamic amplifying circuit can be reliably kept within the A / D conversion possible range.
[0038]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an air-fuel ratio detection device (oxygen concentration detection device) according to an embodiment to which the present invention is applied will be described with reference to the drawings.
First, FIG. 1 is a circuit diagram illustrating a configuration of the air-fuel ratio detection device 51 of the first embodiment. In FIG. 1, the same components as those of the air-fuel ratio detection device 101 of FIG. 6 described above are denoted by the same reference numerals, and a detailed description thereof will be omitted.
[0039]
As shown in FIG. 1, the air-fuel ratio detection device 51 of the first embodiment is different from the conventional air-fuel ratio detection device 101 shown in FIG. And a D / A converter (DAC) 53 that outputs a voltage corresponding to the differential amplifier circuit 53. The voltage output terminal of the D / A converter 53 is connected to the opposite side of the resistor 44 of the differential amplifier circuit 21 from the operational amplifier 43 side. (Corresponding to a second input terminal of the differential amplifier circuit, hereinafter, referred to as a second input terminal). That is, the output voltage of the D / A converter 53 is input to the second input terminal of the differential amplifier circuit 21 as the reference voltage Vr.
[0040]
In the present embodiment, the D / A converter 53 corresponds to a voltage varying unit, the microcomputer 3 corresponds to a detecting unit, and the voltage applying circuits 9 and 11 correspond to a voltage applying unit. Further, an end of the resistor 41 in the differential amplifier circuit 21 opposite to the operational amplifier 43 and the resistor 42 side corresponds to a first input terminal of the differential amplifier circuit.
[0041]
Then, in the air-fuel ratio detection device 51 of the first embodiment, the microcomputer 3 determines that the output voltage Vo of the differential amplifier circuit 21 is within the A / D convertible range of the A / D converter 5 (0.2 V in the present embodiment). The reference voltage Vr input from the D / A converter 53 to the second input terminal of the differential amplifier circuit 21 is changed so as to be within 4.8 V).
[0042]
More specifically, first, in the air-fuel ratio detection device 51 of the first embodiment, the detection range of the air-fuel ratio is larger than the detection range of the conventional device 101 (A / F = 13 to 18). F = 10 to 23 (see FIG. 7). Further, in the detection target range (A / F = 10 to 23), the same range (A / F = 13 to 18) as the detection target range of the conventional device 101 is as accurate as the conventional device 101. This is a high-accuracy detection required range where the air-fuel ratio should be detected.
[0043]
Therefore, first, in the air-fuel ratio detection device 51 of the first embodiment, the amplification factor of the differential amplifier circuit 21 is such that the reference voltage Vr input to the second input terminal of the differential amplifier circuit 21 is equal to the reference voltage Vr of the air-fuel ratio sensor 1. When the first voltage V1 (= 3.3 V) applied to the plus side terminal AFp and the air-fuel ratio changes over the high-accuracy detection required range (A / F = 13 to 18), FIG. As shown in the center part of FIG. 2, the output voltage Vo of the differential amplifier circuit 21 is within the A / D converter possible range of the A / D converter 5 and the detection accuracy in the required range of high accuracy detection can be realized as much as possible. It is set to change over a wide range (for example, 0.3 V to 4.7 V in the present embodiment).
[0044]
Then, as shown in FIG. 2, when detecting the air-fuel ratio within the high-accuracy detection required range, the microcomputer 3 supplies the reference supplied from the D / A converter 53 to the second input terminal of the differential amplifier circuit 21. When the voltage Vr is set to the first voltage V1 (= 3.3 V) and an air-fuel ratio outside the required range of high-precision detection (A / F = 10 to 13, 18 to 23) is detected, differential amplification is performed. The reference voltage Vr from the D / A converter 53 to the differential amplifier circuit 21 is changed so that the output voltage Vo of the circuit 21 falls within the A / D converter 5 possible range. I have. Note that the microcomputer 3 calculates the output voltage Vo of the differential amplifier circuit 21 from the A / D conversion value of the output voltage Vo of the differential amplifier circuit 21 based on the A / D conversion value of the output voltage Vo of the A / D converter 5. When it is detected that it is likely to be out of the D-convertible range, the digital value to the D / A converter 53 is changed, and the reference voltage Vr to the differential amplifier circuit 21 is changed.
[0045]
When detecting the air-fuel ratio within the high-accuracy detection required range, the microcomputer 3 detects the air-fuel ratio using the same detection logic as the conventional device 101 described above. That is, the microcomputer 3 determines the output voltage Vo of the differential amplifier circuit 21 and the actual voltage of the plus terminal AFp of the air-fuel ratio sensor 1 (hereinafter, plus terminal) from the A / D converted value by the A / D converter 5. V1), the value of “(R42 / R41) × (I × R7)” in Equation 4 described above is obtained, and from the value (in other words, the detected value of the sensor current I), The air-fuel ratio is detected on the basis of the calculation formula and the data map stored in advance.
[0046]
On the other hand, when detecting an air-fuel ratio outside the required range of high-accuracy detection (A / F = 10 to 13, 18 to 23), the microcomputer 3 calculates the A / D conversion value from the A / D converter 5 The output voltage Vo of the differential amplifier circuit 21 and the positive terminal voltage V1 of the air-fuel ratio sensor 1 are detected, and both the detected values Vo and V1 are detected by the D / A converter 53 from the differential amplifier circuit 21. Is detected from the value of the reference voltage Vr supplied to the controller.
[0047]
That is, in the air-fuel ratio detection device 51 of the present embodiment, the output voltage Vo of the differential amplifier circuit 21 can be expressed by the following Expression 5 from Expressions 1 and 2 described above.
Vo = Vr + (R42 / R41) × (Vr−V1) + (R42 / R41) × (I × R7) Equation 5
Therefore, from the detection values (actually measured values) of Vo and V1 and the reference voltage Vr supplied to the differential amplifier circuit 21 from the D / A converter 53, “(R42 / R41) × ( I × R7) ”, and the air-fuel ratio may be detected from the value.
[0048]
Note that when the reference voltage Vr output from the D / A converter 53 is set to the first voltage V1 (= 3.3 V), the output voltage Vo of the differential amplifier circuit 21 becomes the above-described equation (4). The difference D between Vo in Equation 4 and Vo in Equation 5 (that is, the difference between Vo when Vr = V1 and Vo when Vr ≠ V1) is as shown in Equation 6 below.
[0049]
D = (1 + R42 / R41) × (Vr−V1) Equation 6
Therefore, for example, an air-fuel ratio corresponding to the detected value of Vo may be obtained from a data map showing the relationship between the output voltage Vo of the differential amplifier circuit 21 and the air-fuel ratio when it is assumed that Vr = V1. For example, when detecting an air-fuel ratio outside the required range of high-accuracy detection (A / F = 10 to 13, 18 to 23), a value obtained by subtracting the difference D of the above equation 6 from the detected value of Vo is represented by a data map. The air-fuel ratio may be obtained by applying
[0050]
Further, the data map for obtaining the air-fuel ratio from the detected value of Vo may be switched according to the value of “(Vr−V1)”.
According to the air-fuel ratio detection device 51 of the first embodiment as described above, even if an expensive A / D converter with high resolution is not used as the A / D converter 5, a wide range of air-fuel ratio detection can be performed. High accuracy detection in a specific range (high accuracy detection required range) can be compatible. That is, since the amplification factor of the differential amplifier circuit 21 is set to a value aiming at the high-accuracy detection required range, high-accuracy detection is possible within the high-accuracy detection required range. With respect to the expanded range other than the above, the D / A converter 53 supplies the output voltage Vo of the differential amplifier circuit 21 to the input full scale (0.2 V to 4.8 V) of the A / D converter 5. This is because the reference voltage Vr to the differential amplifier circuit 21 is adjusted.
[0051]
Next, an air-fuel ratio detection device according to a second embodiment will be described with reference to FIG. In FIG. 3, the same components as those in FIGS. 1 and 6 described above are denoted by the same reference numerals, and a detailed description thereof will be omitted.
As shown in FIG. 3, the air-fuel ratio detection device 55 of the second embodiment is different from the air-fuel ratio detection device 51 of the first embodiment shown in FIG. 1 in the following points (1) to (3). Are different.
[0052]
(1): The voltage output terminal of the D / A converter 53 is not directly connected to the second input terminal of the differential amplifier circuit 21.
(2): Instead, a switching circuit 57 that connects the second input terminal of the differential amplifier circuit 21 to one of the plus terminal AFp of the air-fuel ratio sensor 1 and the voltage output terminal of the D / A converter 53 ( (Corresponding to switching means).
[0053]
The switching circuit 57 has two analog switches 58 and 59. When the switching signal Sc from the microcomputer 3 is at a low level, only the analog switch 58 is turned on and the second switch of the differential amplifier circuit 21 is turned on. The input terminal is connected to the voltage output terminal of the D / A converter 53. Conversely, if the switching signal Sc from the microcomputer 3 is at a high level, only the analog switch 59 is turned on and the second input of the differential amplifier circuit 21 is turned on. The terminal is connected to the positive terminal AFp of the air-fuel ratio sensor 1.
[0054]
(3): When detecting the air-fuel ratio within the high-accuracy detection required range (A / F = 13 to 18), the microcomputer 3 causes the switching circuit 57 to empty the second input terminal of the differential amplifier circuit 21. When connecting to the plus side terminal AFp of the fuel ratio sensor 1 and detecting an air-fuel ratio outside the required range of high accuracy detection, the switching circuit 57 connects the second input terminal of the differential amplifier circuit 21 to the D / A converter 53. And a voltage output terminal of the differential amplifier 21 in the same manner as in the first embodiment, so that the output voltage Vo of the differential amplifier circuit 21 falls within the A / D converter possible range of the A / D converter 5. The reference voltage Vr from the A converter 53 to the differential amplifier circuit 21 is changed.
[0055]
That is, in the air-fuel ratio detection device 55 of the second embodiment, when detecting the air-fuel ratio within the high-accuracy detection required range, both ends of the current detection resistor 7 are connected to the two input terminals of the differential amplifier circuit 21. Like that.
According to such an air-fuel ratio detecting device 55, when detecting the air-fuel ratio within the high-accuracy detection required range, the influence of the voltage output error of the D / A converter 53 is eliminated. It becomes possible to detect more accurately.
[0056]
Next, an air-fuel ratio detection device according to a third embodiment will be described with reference to FIG. Note that, in FIG. 4, the same components as those in FIGS. 1 and 6 described above are denoted by the same reference numerals, and a detailed description thereof will be omitted.
As shown in FIG. 4, the air-fuel ratio detection device 61 of the third embodiment is different from the air-fuel ratio detection device 51 of the first embodiment shown in FIG. As a voltage varying means for making the voltage variable, instead of the D / A converter 53, a binary amplitude on / off signal output from the microcomputer 3 (in the present embodiment, the high level is 5V and the low level is 0V, Is different from the first embodiment in that an integrating circuit 62 for smoothing the output of the differential amplifier 21 and outputting the smoothed output to the second input terminal of the differential amplifier circuit 21 is used.
[0057]
The integrating circuit 62 is connected in series between the output terminal of the on / off signal of the microcomputer 3 and the second input terminal of the differential amplifier circuit 21 (the end of the resistor 44 on the side opposite to the operational amplifier 43). It comprises a resistor 63 inserted, and a capacitor 65 having one end connected to the differential amplifier circuit 21 side end of the resistor 63 and the other end connected to the ground potential.
[0058]
Then, in the air-fuel ratio detection device 61 of the third embodiment, the microcomputer 3 uses the duty ratio of the on / off signal output to the integration circuit 62 to send the signal from the integration circuit 62 to the second input terminal of the differential amplifier circuit 21. The output reference voltage Vr is changed in the same manner as in the first embodiment. That is, the output voltage of the integration circuit 62 changes according to the duty ratio of the on / off signal from the microcomputer 3.
[0059]
According to such an air-fuel ratio detecting device 61, the reference voltage Vr to the differential amplifier circuit 21 can be arbitrarily changed with a very inexpensive configuration, which is advantageous.
Next, an air-fuel ratio detection device according to a fourth embodiment will be described with reference to FIG. In FIG. 5, the same components as those in FIGS. 1 and 6 described above are denoted by the same reference numerals, and detailed description thereof will be omitted.
[0060]
As shown in FIG. 5, the air-fuel ratio detecting device 67 of the fourth embodiment is different from the air-fuel ratio detecting device 51 of the first embodiment shown in FIG. 1 in the following points (A) and (B). Are different.
(A): Two voltage dividing resistors 71 and 73 that generate a predetermined threshold voltage Vth by dividing the power supply voltage VS (= 5 V), and a threshold generated at a connection point between the voltage dividing resistors 71 and 73. A comparator 69 for comparing the value voltage Vth with the output voltage Vo of the differential amplifier circuit 21 is added. The comparator 69 sets the output signal to the microcomputer 3 to high level when Vo <Vth, and inverts the output signal to the microcomputer 3 from high level to low level when Vo ≧ Vth.
[0061]
Further, the threshold voltage Vth is slightly lower than the upper limit value (= 4.8 V) of the A / D conversion possible range, and the threshold voltage Vth of the differential amplifier circuit 21 when detecting the air-fuel ratio in the high accuracy detection required range. The voltage is set to be equal to or slightly higher than the maximum value (= 4.7 V) of the output voltage Vo (4.75 V in the present embodiment).
[0062]
(B): When the microcomputer 3 detects the air-fuel ratio within the high-accuracy detection required range, the output voltage Vo of the differential amplifier circuit 21 becomes equal to or higher than the threshold voltage Vth and the output signal of the comparator 69 Is inverted from the high level to the low level, the reference voltage Vr supplied from the D / A converter 53 to the second input terminal of the differential amplifier circuit 21 is increased from the first voltage V1 (= 3.3 V). Is also changed to a lower predetermined voltage Vb so that the output voltage Vo of the differential amplifier circuit 21 falls within the A / D conversion possible range of the A / D converter 5, and in that state, the output voltage Vo becomes higher than the high-accuracy detection required range. The rich side air-fuel ratio is detected.
[0063]
Also, the microcomputer 3 sets the reference voltage Vr to the differential amplifier circuit 21 to the predetermined voltage Vb and sets the output voltage Vo of the differential amplifier circuit 21 to A / F = 13 (the maximum value of the high-accuracy detection required range). If it is detected based on the A / D conversion value of Vo by the A / D converter 5 that the voltage has dropped to the value corresponding to the rich value), the reference voltage Vr to the differential amplifier circuit 21 is changed to the first voltage. It returns to V1 and returns to the state of detecting the air-fuel ratio within the high-accuracy detection required range.
[0064]
According to the air-fuel ratio detection device 67 of the fourth embodiment, for example, control is performed such that the air-fuel ratio is suddenly changed from lean to rich by fuel injection control, and the output voltage Vo of the differential amplifier circuit 21 is controlled. Even if the value suddenly becomes large, the microcomputer 3 can quickly detect the fact by the output of the comparator 69, and, consequently, quickly change the reference voltage Vr to the differential amplifier circuit 21. As a result, the output voltage Vo of the differential amplifier circuit 21 can be reliably kept within the A / D conversion possible range.
[0065]
In particular, when the output voltage Vo of the differential amplifier circuit 21 suddenly changes, it is not enough to determine that the reference voltage Vr should be changed from the A / D conversion value of Vo by the A / D converter 5. This is effective when there is no possibility (Vo exceeds the A / D conversion possible range).
[0066]
In the fourth embodiment, the reference voltage Vr to the differential amplifier circuit 21 is changed from the first voltage V1 for detecting the air-fuel ratio in the high-accuracy detection required range to one more than the high-accuracy detection required range. One comparator 69 is provided to notify the microcomputer 3 of the timing of switching to the predetermined voltage Vb for detecting the air-fuel ratio in the step rich side range. If there is, a comparator may be provided for each switching timing (in other words, for each threshold value to be compared with Vo in magnitude).
[0067]
As mentioned above, although one Embodiment of this invention was described, it cannot be overemphasized that this invention can take various forms.
For example, the configuration of FIG. 4 using the integration circuit 62 as the voltage variable means can be similarly applied to the air-fuel ratio detection devices 55 and 67 of the second and fourth embodiments.
[Brief description of the drawings]
FIG. 1 is a circuit diagram illustrating a configuration of an air-fuel ratio detection device according to a first embodiment.
FIG. 2 is a diagram illustrating an operation of the air-fuel ratio detection device according to the first embodiment.
FIG. 3 is a circuit diagram illustrating a configuration of an air-fuel ratio detection device according to a second embodiment.
FIG. 4 is a circuit diagram illustrating a configuration of an air-fuel ratio detection device according to a third embodiment.
FIG. 5 is a circuit diagram illustrating a configuration of an air-fuel ratio detection device according to a fourth embodiment.
FIG. 6 is a circuit diagram illustrating a configuration of a conventional air-fuel ratio detection device.
FIG. 7 is a graph showing an example of a voltage-current characteristic of the air-fuel ratio sensor.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Air-fuel ratio sensor (oxygen concentration sensor), AFm ... Minus side terminal, AFp ... Plus side terminal, 3 ... Microcomputer, 5 ... A / D converter, 7 ... Current detection resistor, 9, 11 ... Voltage application circuit, 13 19, 23, 29, 31, 33, 39, 41, 42, 44, 45, 63, 71, 73... Resistors, 21... Differential amplifier circuits, 25, 35, 43. , 51, 55, 61, 67 ... air-fuel ratio detection device, 53 ... D / A converter, 57 ... switching circuit, 58, 59 ... analog switch, 62 ... integration circuit, 65 ... capacitor, 69 ... comparator

Claims (6)

Among a pair of terminals of the oxygen concentration sensor through which a current according to the oxygen concentration in the gas to be detected flows with the application of the voltage, a current detection resistor having one end connected to one terminal,
The voltage applied to the end of the current detection resistor opposite to the oxygen concentration sensor side is adjusted so that the voltage of the one terminal of the oxygen concentration sensor becomes the first voltage, and the oxygen concentration By applying a second voltage different from the first voltage to the other terminal of the sensor, oxygen which is a difference between the first voltage and the second voltage is applied between both terminals of the oxygen concentration sensor. A voltage application means for applying a voltage for concentration detection, and a voltage (hereinafter referred to as a generated voltage) at an end of the current detection resistor opposite to the oxygen concentration sensor side is input to a first input terminal and a predetermined A differential amplifier circuit that receives the reference voltage of the second input terminal and outputs a voltage corresponding to a difference between the reference voltage and the generated voltage
An A / D converter for converting an output voltage of the differential amplifier circuit into a digital value;
Detecting means for detecting the oxygen concentration in the gas to be detected based on the A / D converted value by the A / D converter;
An oxygen concentration detection device comprising:
A voltage varying unit that outputs a voltage according to a command from the detection unit and causes the voltage to be input to the second input terminal of the differential amplifier circuit as the reference voltage,
The detection unit is configured to output the voltage from the voltage variable unit so that an output voltage of the differential amplifier circuit falls within an A / D conversion range, which is a range of a voltage that can be A / D converted by the A / D converter. Changing a reference voltage input to a second input terminal of the differential amplifier circuit;
An oxygen concentration detection device characterized by the above-mentioned. The oxygen concentration detecting device according to claim 1,
The voltage varying unit is a D / A converter that outputs a voltage according to a digital value from the detection unit;
An oxygen concentration detection device characterized by the above-mentioned. The oxygen concentration detecting device according to claim 1,
The voltage variable means includes an integration circuit for smoothing and outputting a binary amplitude on / off signal output from the detection means,
The detecting unit changes an output voltage of the voltage varying unit according to a duty ratio of the on / off signal;
An oxygen concentration detection device characterized by the above-mentioned. The oxygen concentration detecting device according to any one of claims 1 to 3,
The amplification factor of the differential amplifier circuit is
The reference voltage input to the second input terminal of the differential amplifier circuit is the first voltage, and the oxygen concentration in the gas to be detected is different from the entire detection range of the oxygen concentration. If the output voltage of the differential amplifier circuit is within the range where the A / D conversion is possible and the detection accuracy can be realized when the output voltage changes over a specific range where a detection accuracy higher than the range is required. Is set to change across
The detecting means,
When detecting the oxygen concentration within the specific range, a reference voltage input from the voltage variable means to the second input terminal of the differential amplifier circuit is set to the first voltage, When detecting the outside oxygen concentration, the input from the voltage variable means to the second input terminal of the differential amplifier circuit so that the output voltage of the differential amplifier circuit falls within the A / D convertible range. Changing the reference voltage,
An oxygen concentration detection device characterized by the above-mentioned. The oxygen concentration detecting device according to claim 4,
Switching means for connecting a second input terminal of the differential amplifier circuit to one of the one terminal of the oxygen concentration sensor and a voltage output terminal of the voltage variable means,
The detecting means,
When detecting the oxygen concentration within the specific range, instead of inputting the output voltage of the voltage variable unit to the second input terminal of the differential amplifier circuit, the switching unit performs the differential amplification. Connecting a second input terminal of the circuit to the one terminal of the oxygen concentration sensor;
When detecting the oxygen concentration outside the specific range, the switching means connects the second input terminal of the differential amplifier circuit to the voltage output terminal of the voltage variable means, Changing a reference voltage input from the voltage variable means to a second input terminal of the differential amplifier circuit so that an output voltage falls within the A / D conversion possible range;
An oxygen concentration detection device characterized by the above-mentioned. The oxygen concentration detection device according to any one of claims 1 to 5,
A comparator for comparing the output voltage of the differential amplifier circuit with a predetermined threshold voltage,
The detecting unit changes the reference voltage based on a comparison result of the comparator;
An oxygen concentration detection device characterized by the above-mentioned.

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