JP4123580B2 - Method for detecting element resistance of oxygen concentration sensor - Google Patents

Description

となる。
上記（１）式において、「Ｔ１」はＨＰＦの時定数（図９の点Ｂに相当）であり、「Ｔ２」はＬＰＦの時定数（図９の点Ａに相当）である。ここで前述した通り、安定した抵抗周波数特性の領域を用いるには、点Ａを点Ｂよりも高周波数側にすることが必須要件となる。すなわち二つの時定数Ｔ１，Ｔ２の大小関係は必ず、
Ｔ１＞Ｔ２
となる。よって、前記（１）式から、前記図４の範囲Ａの立ち上がり時間よりも範囲Ｂの立ち下り時間が遅くなることが分かる。
【００３１】
以上の理由より、前記図４の出力電圧Ｖｂがほぼ一定となる範囲Ｂにおいて、センサ等価回路のＨＰＦの時定数の１桁下のオーダーである２．５ｍｓ程度の時間内ではセンサ電流Ｉｐがほとんど変化しなくなる。つまり、図４の範囲Ｂではどのタイミングでピーク電流ΔＩを検出してもほとんど同じ値が得られるようになる。
【００３２】
従って、図１１に示すように、正弦波形Ｌａ（図の二点鎖線の波形）の如くセンサ電流が変化する場合には検出タイミングが「ｔ」から「ｔ’」にずれることでΔＩが誤検出されるのに対し、本実施の形態では波形Ｌｂ（図の実線の波形）の如くセンサ電流が変化するため、「ｔ’」の検出タイミングでもΔＩが正確に検出できる。なお因みに、所定の時定数を持たせて切り換えた電圧の印加時間の合計「Ａ＋Ｂ」は、ピーク電流ΔＩが検出できる時間よりも長くすればよいから、数１０μｓ以上であればよい。
【００３３】
次に、バイアス制御回路４０の構成を図１２の電気回路図を用いて説明する。図１２において、バイアス制御回路４０は大別して、基準電圧回路４４と、第１の電圧供給回路４５と、第２の電圧供給回路４７と、電流検出回路５０とを有する。基準電圧回路４４は、定電圧Ｖｃｃを分圧抵抗４４ａ，４４ｂにより分圧して一定の基準電圧Ｖａを生成する。
【００３４】
第１の電圧供給回路４５は電圧フォロア回路にて構成され、基準電圧回路４４の基準電圧Ｖａと同じ電圧ＶａをＡ／Ｆセンサ３０の一方の端子（前記図２の大気側電極層３７に接続される端子）４２に供給する。より具体的には、第１の電圧供給回路４５は、正側入力端子が各分圧抵抗４４ａ，４４ｂの分圧点に接続されると共に負側入力端子がＡ／Ｆセンサ３０の一方の端子４２に接続された演算増幅器４５ａと、演算増幅器４５ａの出力端子に一端が接続された抵抗４５ｂと、この抵抗４５ｂの他端にそれぞれベースが接続されたＮＰＮトランジスタ４５ｃ及びＰＮＰトランジスタ４５ｄとを有する。ＮＰＮトランジスタ４５ｃのコレクタは定電圧Ｖｃｃに接続され、エミッタは電流検出回路５０を構成する電流検出抵抗５０ａを介してＡ／Ｆセンサ３０の一方の端子４２に接続されている。また、ＰＮＰトランジスタ４５ｄのエミッタはＮＰＮトランジスタ４５ｃのエミッタに接続され、コレクタはアースされている。
【００３５】
第２の電圧供給回路４７も同様に電圧フォロア回路にて構成され、前記ＬＰＦ２２の出力電圧Ｖｃと同じ電圧ＶｃをＡ／Ｆセンサ３０の他方の端子（前記図２の排ガス側電極層３６に接続される端子）４１に供給する。より具体的には、第２の電圧供給回路４７は、正側入力端子が前記ＬＰＦ２２の出力に接続されると共に負側入力端子がＡ／Ｆセンサ３０の他方の端子４１に接続された演算増幅器４７ａと、演算増幅器４７ａの出力端子に一端が接続された抵抗４７ｂと、この抵抗４７ｂの他端にそれぞれベースが接続されたＮＰＮトランジスタ４７ｃ及びＰＮＰトランジスタ４７ｄとを有する。ＮＰＮトランジスタ４７ｃのコレクタは定電圧Ｖｃｃに接続され、エミッタは抵抗４７ｅを介してＡ／Ｆセンサ３０の他方の端子４１に接続されている。また、ＰＮＰトランジスタ４７ｄのエミッタはＮＰＮトランジスタ４７ｃのエミッタに接続され、コレクタはアースされている。
【００３６】
上記構成により、Ａ／Ｆセンサ３０の一方の端子４２には常時一定電圧Ｖａが供給される。そして、ＬＰＦ２２を経由してＡ／Ｆセンサ３０の他方の端子４１に一定電圧Ｖａよりも低い電圧Ｖｃが供給されると、当該Ａ／Ｆセンサ３０が正バイアスされることになる。また、ＬＰＦ２２を経由してＡ／Ｆセンサ３０の他方の端子４１に一定電圧Ｖａよりも高い電圧Ｖｃが供給されると、当該Ａ／Ｆセンサ３０が負バイアスされることになる。
【００３７】
次に、上記の如く構成される空燃比検出装置の作用を説明する。
図１３は、本実施の形態における制御プログラムの概要を示すメインルーチンのフローチャートであり、そのルーチンはマイコン２０への電源投入に伴い起動される。
【００３８】
図１３において、マイコン２０は、先ずステップ１００で前回のＡ／Ｆ検出時から所定時間Ｔａが経過したか否かを判別する。ここで、所定時間Ｔａは、Ａ／Ｆの検出周期に相当する時間であって、例えば、Ｔａ＝２〜４ｍｓ程度に設定されるのが適当である。そして、前回のＡ／Ｆ検出時から所定時間Ｔａが経過していれば、マイコン２０はステップ１００を肯定判別してステップ１１０に進む。マイコン２０は、ステップ１１０で電流検出回路５０にて検出されたセンサ電流Ｉｐ（限界電流値）を読み込むと共に、予めマイコン２０内のＲＯＭに記憶されている特性マップを用いてその時のセンサ電流Ｉｐに対応するＡ／Ｆ値を検出する。Ａ／Ｆ値の検出後、マイコン２０は、図３の特性線Ｌ１を用いてその時々のＡ／Ｆ検出結果（Ｉｐ）に応じた印加電圧ＶｐをＡ／Ｆセンサ３０に印加しておく。
【００３９】
また、マイコン２０は、続くステップ１２０で前回の素子抵抗検出時から所定時間Ｔｂが経過したか否かを判別する。ここで、所定時間Ｔｂは、素子抵抗の検出周期に相当する時間であって、例えばエンジン運転状態に応じて選択的に設定される。本実施の形態では、Ａ／Ｆの変化が比較的小さい通常時（エンジンの定常運転時）にはＴｂ＝２ｓ（秒）に、Ａ／Ｆの急変時（エンジンの過渡運転時）にはＴｂ＝１２８ｍｓ（ミリ秒）に、というように所定時間Ｔｂが可変に設定されるようになっている。
【００４０】
ステップ１２０が否定判別されると、マイコン２０は、上記の如く所定時間Ｔａの経過毎にＡ／Ｆ値を検出する。また、ステップ１２０が肯定判別されると、マイコン２０は、ステップ１３０で素子抵抗検出の処理を実施する。以下、素子抵抗検出の処理を図１４のサブルーチンを用いて説明する。
【００４１】
図１４において、マイコン２０は、先ずステップ１３１でバイアス指令信号Ｖｒを操作し、それまでの印加電圧Ｖｐ（Ａ／Ｆ検出用電圧）に対して電圧を正側に変化させる。このとき、素子抵抗検出用電圧の印加時間（前記図４のｔ１〜ｔ２の時間）を、電圧切り換え後にピーク電流が出る時間よりも長くする。ここで、当該印加時間は先に規定した２．５ｍｓ以下であればよいが、次のＡ／Ｆを検出するまでの時間Ｔａ＝２〜４ｍｓ以内に電流が限界電流値に収束しなければならない。このため、素子電流検出用電圧の印加時間をＡ／Ｆ検出間隔よりも短くする。具体的には、電圧印加時間を数１０〜１００μｓ程度とする。
【００４２】
その後、マイコン２０は、ステップ１３２でその時の電圧変化量ΔＶと電流検出回路５０により検出されたセンサ電流の変化量ΔＩとを読み取る。かかる場合、前記図４の範囲Ｂ内でΔＩが検出されることになる。また、マイコン２０は、続くステップ１３３でΔＶ，ΔＩから素子抵抗Ｒを算出し（Ｒ＝ΔＶ／ΔＩ）、その後元のメインルーチンに戻る。
【００４３】
一方、上記の如く求められる素子抵抗Ｒは、素子温に対して図１５に示す関係を有する。すなわち、素子温が低くなるほど、素子抵抗Ｒが飛躍的に大きくなる。同図において、素子抵抗Ｒ＝９０ΩはＡ／Ｆセンサ３０の半活性温度（６００℃）に相応し、素子抵抗Ｒ＝３０ΩはＡ／Ｆセンサ３０の活性温度（７００℃）に相応する。そして、Ａ／Ｆセンサ３０のヒータ制御に際しては、前記算出した素子抵抗ＲとＡ／Ｆセンサ３０が十分に活性化していると思われる目標抵抗値（例えば３０Ω）との偏差をなくすべくヒータ３３に要求される通電量が求められ、その通電がデューティ制御される。すなわち、素子温フィードバック制御が実施され、これによりセンサ素子温が所定の活性温度に保持される。
【００４４】
以上詳述した本実施の形態によれば、以下に示す効果が得られる。
（ａ）本実施の形態では、Ａ／Ｆセンサ３０の印加電圧をステップ的に変化させてその時の電圧及び電流の変化量ΔＶ，ΔＩから素子抵抗Ｒを検出する際、所定の時定数を持たせて電圧値を切り換えると共に、前記所定の時定数を持たせて切り換えた電圧の印加時間を、電圧印加後にピーク電流が出る時間よりも長くした。この場合、電圧変化に伴ってピーク電流が出た後に電流変化が緩慢になる期間を設けることができる（前記図４の範囲Ｂ）。つまり、電圧変化に伴う電流変化量を正確に検出するための期間が拡張される。その結果、電流変化量の検出タイミングや時定数などの各種要因にバラツキが生じたとしても、センサの素子抵抗が精度良く検出できるようになる。
【００４５】
（ｂ）所定の時定数を持たせて切り換えた電圧の印加時間を、センサ特有のカットオフ周波数の逆数のオーダー以下とした。具体的には、センサ特有のカットオフ周波数を４０Ｈｚ程度とした場合、そのカットオフ周波数に対応する時定数の１桁下のオーダーである２．５ｍｓ程度で電圧印加時間を規定した。この場合、電圧印加を必要以上に長引かせることもなく、電圧印加時間を好適に設定して素子抵抗が検出できる。
【００４６】
（ｃ）上記のように素子抵抗を精度良く検出することが可能となれば、その検出結果を用いたＡ／Ｆセンサ３０の活性化制御（ヒータ３３の通電制御）が精度良く実現できる。また、素子抵抗の検出結果は、センサ３０の劣化判定にも有効に適用できることになる。
【００４７】
なお、本発明の実施の形態は、上記以外にも次のように具体化できる。
所定の時定数を持たせた電圧を印加してピーク電流が出た後、その電圧値を直前の値で一定に保つようにする。かかる実施の形態は、前記図１４のステップ１３１にて、素子抵抗検出用電圧の印加時間を制御することにより実現できる。この場合、所定の電圧値で一定に保つ時間を、センサ特有のカットオフ周波数の逆数のオーダー以下とするとよい。本構成でも、上述した実施の形態と同様に、電圧変化に伴ってピーク電流が出た後、電流変化が緩慢になる期間を設けることができ、電圧変化に伴う電流変化量を正確に検出するための期間が拡張される。その結果、電流変化量の検出タイミングや時定数などの各種要因にバラツキが生じたとしても、センサの素子抵抗が精度良く検出できるようになる。また、素子抵抗検出時の電圧印加を必要以上に長引かせることもなく、適度な電圧印加時間が設定できる。
【００４８】
素子抵抗検出電圧への切り換え後、当該電圧の印加時間を可変に設定するようにしてもよい。具体的には、
・過渡判定結果や排ガス温度などのエンジン運転状態、
・Ａ／Ｆセンサの活性度合、
・エンジン始動時からの経過時間、
などに応じて電圧印加時間を可変に設定する。この場合、電圧印加時間がより一層好ましく設定できる。
【００４９】
酸素濃度センサとして既述のコップ型限界電流式Ａ／Ｆセンサに代えて、積層型センサを用いて本発明を具体化してもよい。かかる場合にも、既述した通りの作用及び効果が得られる。
【００５０】
上記実施の形態では、車載エンジンの排ガス中の酸素濃度（Ａ／Ｆ）を検出するＡ／Ｆセンサとして本発明を適用したが、本発明の適用範囲は自動車用Ａ／Ｆセンサに限定されるものではなく、これ以外にも適用範囲を拡大することも可能である。例えば可燃性ガス（メタンガス、エタンガス等）中の酸素濃度を検出する酸素濃度センサとして具体化することも可能である。
【００５１】
上記実施の形態では、マイコン２０から出力される矩形状の信号に対してＬＰＦにて所定の時定数を持たせるようにしたが、マイコン２０にて所定の時定数を持たせた信号を生成し、その信号を用いて素子抵抗を検出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空燃比検出装置の概要を示す構成図。
【図２】Ａ／Ｆセンサの詳細な構成を示す断面図。
【図３】Ａ／Ｆセンサの電圧−電流特性を示すグラフ。
【図４】Ｄ／Ａ変換器の出力電圧Ｖｂ、ＬＰＦの出力電圧Ｖｃ及びセンサ電流Ｉｐを示す波形図。
【図５】Ａ／Ｆセンサの等価電気回路図。
【図６】Ａ／Ｆ検出用電圧をＡ／Ｆセンサに印加した状態で交流入力電圧の周波数に対するインピーダンスの軌跡を示すグラフ。
【図７】交流入力電圧の周波数と交流インピーダンスとの関係を示すグラフ。
【図８】Ａ／Ｆセンサの等価回路図と、その等価回路に対応するブロック図。
【図９】図８の等価回路の抵抗周波数特性を示すグラフ。
【図１０】Ｄ／Ａ変換器の出力Ｖｂからセンサ電流Ｉｐまでの伝達ブロックと、Ｖｂ，Ｉｐ波形とを示す図。
【図１１】電圧変化に伴う電流変化の様子を示す波形図。
【図１２】バイアス制御回路の構成を示す電気回路図。
【図１３】メインルーチンを示すフローチャート。
【図１４】素子抵抗検出サブルーチンを示すフローチャート。
【図１５】素子温と素子抵抗との関係を示すグラフ。
【図１６】従来技術において、素子抵抗検出の際の電圧変化と電流変化を示す波形図。
【符号の説明】
１０…エンジン、２０…マイコン（マイクロコンピュータ）、２２…ＬＰＦ（ローパスフィルタ）、３０…酸素濃度センサとしての限界電流式Ａ／Ｆセンサ、４０…バイアス制御回路。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an oxygen concentration sensor for detecting, for example, an oxygen concentration in exhaust gas from an in-vehicle engine, and relates to a method for detecting element resistance using a frequency characteristic of a voltage current unique to the sensor.
[0002]
[Prior art]
In recent air-fuel ratio control of in-vehicle engines, for example, there is a demand for higher control accuracy and a demand for lean burn, and in order to meet these demands, the air-fuel ratio of the air-fuel mixture sucked into the engine (in the exhaust gas) A linear air-fuel ratio sensor (oxygen concentration sensor) that linearly detects (oxygen concentration) over a wide area is embodied. In such an air-fuel ratio sensor, in order to know, for example, its active state or deteriorated state, it is necessary to accurately detect the internal resistance (element resistance) of the sensor element.
[0003]
The inventors of the present application have proposed a method of detecting an element resistance in Japanese Patent Application No. 8-207410 (Japanese Patent Laid-Open No. 9-292364). This is because, as shown in FIG. 16, a voltage having a predetermined time constant is applied to the air-fuel ratio sensor on a one-off basis to detect a peak current ΔI (current change amount) after elapse of time t, and the voltage at that time The element resistance was detected from the change amount ΔV and the peak current ΔI (element resistance = ΔV / ΔI). That is, when detecting the element resistance, the generation of excessive peak current is suppressed by changing the applied voltage of the sensor with a waveform having a predetermined time constant. As a result, the sensor current value can be accurately measured, and the element resistance of the sensor can be accurately detected.
[0004]
[Problems to be solved by the invention]
However, in the above-described conventional technology, if the detection time t varies or the time constant varies, the peak current ΔI may not be detected accurately. For example, as shown in FIG. 16, when the timing for detecting ΔI is delayed and the detection time becomes “t ′”, ΔI is detected as a value smaller than the actual value, and the element resistance is erroneously detected to be larger. This problem occurs because the period during which ΔI can be accurately detected is narrow.
[0005]
The present invention has been made paying attention to the above problems, and the object of the present invention is to detect the resistance of the sensor element regardless of variations in various factors such as the detection timing and time constant of the current change amount accompanying the voltage change. An object of the present invention is to provide an element resistance detection method for an oxygen concentration sensor that can accurately detect the oxygen concentration sensor.
[0006]
[Means for Solving the Problems]
In the present invention, as a premise thereof, the present invention is applied to an oxygen concentration sensor that outputs a current signal corresponding to the oxygen concentration in a gas to be detected with the application of a voltage, and the voltage applied to the sensor for oxygen concentration detection is set to a predetermined value. By switching to a voltage for detecting the element resistance of the sensor with a time constant, the element resistance of the sensor is detected from the voltage change at that time and the current change accompanying the voltage change. According to such a method, generation of an excessive peak current that becomes a problem when detecting element resistance is suppressed.
[0007]
The invention described in claim 1 is characterized in that the application time of the voltage switched with the predetermined time constant is made longer than the time when the peak current appears after the voltage switching. According to the present invention, at the time of detecting the element resistance, it is possible to provide a period in which the current change becomes slow after the peak current is generated with the voltage change. That is, the period for accurately detecting the amount of current change accompanying the voltage change is extended. As a result, even if various factors such as the detection timing of the current change amount and the time constant vary, the element resistance of the sensor can be detected with high accuracy.
[0008]
According to a second aspect of the present invention, the voltage application time switched with the predetermined time constant is set.
Sensor-specific cutoff frequency Frequency Reciprocal of Cycle time represented by It is as follows. This regulates the length of voltage application time, and in a limiting current type oxygen concentration sensor using a solid electrolyte such as zirconia, if the application time is prescribed based on the cut-off frequency peculiar to the sensor, the voltage The element resistance can be detected by suitably setting the voltage application time without prolonging the application more than necessary. For example, when the cut-off frequency peculiar to the sensor is set to about 40 Hz, the voltage application time may be defined by about 2.5 ms which is an order one digit lower than the time constant.
[0009]
In the invention according to claim 3, after a voltage having the predetermined time constant is applied and a peak current is generated, the voltage value is calculated. Voltage at the time of peak current generation It is characterized by keeping the value constant. According to the present invention, as in the first aspect of the present invention, at the time of detecting the element resistance, it is possible to provide a period in which the current change becomes slow after the peak current is generated along with the voltage change. The period for accurately detecting the amount of current change is extended. As a result, even if various factors such as the detection timing of the current change amount and the time constant vary, the element resistance of the sensor can be detected with high accuracy.
[0010]
In a fourth aspect of the present invention, the When peak current occurs The time to keep constant at the voltage value of Frequency Reciprocal of Cycle time represented by It is as follows. According to the present invention, as in the second aspect of the present invention, an appropriate voltage application time can be set without prolonging the voltage application at the time of detecting the element resistance more than necessary.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment in which the present invention is embodied in an air-fuel ratio detection apparatus will be described with reference to the drawings. The air-fuel ratio detection apparatus in the present embodiment is applied to an electronically controlled gasoline injection engine mounted on an automobile. In the air-fuel ratio control system of the engine, the engine is based on the detection result by the air-fuel ratio detection apparatus. The fuel injection amount to the desired air-fuel ratio is controlled. In the following description, an air-fuel ratio (A / F) detection procedure using an air-fuel ratio sensor and an element resistance detection procedure using the frequency characteristics of the sensor will be described in detail.
[0012]
FIG. 1 is a configuration diagram showing an outline of an air-fuel ratio detection apparatus according to the present embodiment. In FIG. 1, the air-fuel ratio detection device includes a limit current type air-fuel ratio sensor (hereinafter referred to as an A / F sensor) 30 as an oxygen concentration sensor, and the A / F sensor 30 extends from an engine body 11 of the engine 10. Attached to the tube 12. The A / F sensor 30 outputs a linear air-fuel ratio detection signal proportional to the oxygen concentration in the exhaust gas in accordance with application of a voltage commanded from a microcomputer (hereinafter referred to as a microcomputer) 20. The microcomputer 20 is composed of a well-known CPU, ROM, RAM, and the like for executing various arithmetic processes, and controls a bias control circuit 40 and a heater control circuit 25 described later according to a predetermined control program.
[0013]
FIG. 2 is a cross-sectional view showing an outline of the A / F sensor 30. In FIG. 2, the A / F sensor 30 protrudes toward the inside of the exhaust pipe 12, and the sensor 30 is roughly divided into a cover 31, a sensor body 32, and a heater 33. The cover 31 has a U-shaped cross section, and a plurality of small holes 31a communicating with the inside and outside of the cover are formed on the peripheral wall. The sensor body 32 generates a limit current corresponding to the oxygen concentration in the air-fuel ratio lean region or the unburned gas (CO, HC, H2 etc.) concentration in the air-fuel ratio rich region.
[0014]
The configuration of the sensor body 32 will be described in detail. In the sensor body 32, an exhaust gas side electrode layer 36 is fixed to the outer surface of the solid electrolyte layer 34 formed in a cup shape in cross section, and an atmosphere side electrode layer 37 is fixed to the inner surface. A diffusion resistance layer 35 is formed outside the exhaust gas side electrode layer 36 by a plasma spraying method or the like. The solid electrolyte layer 34 is made of an oxygen ion conductive oxide sintered body in which CaO, MgO, Y2 O3, Yb2 O3 or the like is dissolved as a stabilizer in ZrO2, HfO2, ThO2, Bi2 O3 or the like as a stabilizer. Consists of heat-resistant inorganic materials such as alumina, magnesia, siliceous, spinel, mullite. Both the exhaust gas side electrode layer 36 and the atmosphere side electrode layer 37 are made of a noble metal having high catalytic activity such as platinum, and the surface thereof is subjected to porous chemical plating or the like. The area and thickness of the exhaust gas side electrode layer 36 are about 10 to 100 mm ^ 2 (square millimeter) and about 0.5 to 2.0 μm, while the area and thickness of the atmosphere side electrode layer 37 are about It is 10 mm ^ 2 (square millimeter) or more and about 0.5 to 2.0 μm.
[0015]
The heater 33 is accommodated in the atmosphere-side electrode layer 37, and heats the sensor body 32 (the atmosphere-side electrode layer 37, the solid electrode layer 34, the exhaust gas-side electrode layer 36, and the diffusion resistance layer 35) with the generated heat energy. The heater 33 has a heat generation capacity sufficient to activate the sensor body 32.
[0016]
In the A / F sensor 30 configured as described above, the sensor main body 32 generates a limit current corresponding to the oxygen concentration in the lean region from the theoretical air-fuel ratio point. In this case, the limiting current corresponding to the oxygen concentration is determined by the area of the exhaust gas side electrode layer 36, the thickness of the diffusion resistance layer 35, the porosity, and the average pore diameter. The sensor main body 32 can detect the oxygen concentration with a linear characteristic, and a high temperature of about 600 ° C. or higher is required to activate the sensor main body 32. Since the active temperature range is narrow, the active region cannot be controlled by heating only with the exhaust gas of the engine 10. Therefore, in the present embodiment, the sensor body 32 is heated to the activation temperature range by the heating control of the heater 33. Note that in the region on the richer side than the stoichiometric air-fuel ratio, the concentration of unburned gas such as carbon monoxide (CO) changes almost linearly with respect to the air-fuel ratio, and the sensor body 32 corresponds to the concentration of CO or the like. Generate limit current.
[0017]
The voltage-current characteristics of the sensor body 32 will be described with reference to FIG. According to FIG. 3, the inflow current to the solid electrolyte layer 34 of the sensor main body 32 proportional to the detection A / F of the A / F sensor 30 and the applied voltage to the solid electrolyte layer 34 have linear characteristics. I understand. In such a case, the straight line portion parallel to the voltage axis V specifies the limit current of the sensor body 32, and the increase / decrease in the limit current (sensor current) is the increase / decrease in A / F (ie, the degree of lean / rich). It corresponds to. That is, the limit current increases as A / F becomes leaner, and the limit current decreases as A / F becomes richer.
[0018]
In this voltage-current characteristic, a voltage region smaller than the straight line portion parallel to the voltage axis V is a resistance dominant region, and the slope of the primary straight line portion in the resistance dominant region is the solid electrolyte layer 34 in the sensor body 32. Specified internal resistance (this is referred to as element resistance). Since the element resistance changes with a change in temperature, when the temperature of the sensor main body 32 decreases, the inclination decreases due to the increase in element resistance.
[0019]
On the other hand, in FIG. 1, a bias command signal (digital signal) Vr for applying a voltage to the A / F sensor 30 is input from the microcomputer 20 to the D / A converter 21, and the D / A converter 21 performs analog processing. After being converted to the signal Vb, it is input to an LPF (low-pass filter) 22. The output voltage Vc from which the high-frequency component of the analog signal Vb has been removed by the LPF 22 is input to a bias control circuit 40 for applying a voltage for A / F detection or element resistance detection to the A / F sensor 30. The At the time of A / F detection, the applied voltage Vp corresponding to the A / F at that time is set using the characteristic line L1 of FIG. 3, whereas at the time of element resistance detection, it is a single and predetermined frequency signal consisting of a predetermined frequency signal. A voltage having a time constant of is applied.
[0020]
The current detection circuit 50 in the bias control circuit 40 detects a current value that flows as a voltage is applied to the A / F sensor 30. An analog signal having a current value detected by the current detection circuit 50 is input to the microcomputer 20 via the A / D converter 23. The detailed configuration of the bias control circuit 40 will be described later. The operation of the heater 33 attached to the A / F sensor 30 is controlled by the heater control circuit 25. That is, the heater control circuit 25 performs duty control on the power supplied from the battery power source (not shown) to the heater 33 according to the element temperature of the A / F sensor 30 and the heater temperature, and controls the heating of the heater 33.
[0021]
Here, the details of the command voltage applied to the A / F sensor 30 when the element resistance is detected will be described. That is, the microcomputer 20 outputs a bias command signal Vr as a digital signal, and this bias command signal Vr has a single time and a predetermined time constant when passing through the D / A converter 21 and the LPF 22. Converted to voltage (analog signal). FIG. 4 shows an example of signal waveforms of the output voltage Vb of the D / A converter 21, the output voltage Vc of the LPF 22, and the sensor current Ip when detecting the element resistance. In this case, the output voltage Vb of the D / A converter 21 is switched to a voltage value higher by “ΔV” than the immediately preceding applied voltage Vp (A / F detection voltage) at time t1, and the output voltage Vb is restored at time t2. To the applied voltage Vp.
[0022]
The portion of the range A in the output voltage (Vc waveform) of the LPF 22 is an annealed signal from which the high frequency component has been removed by giving a predetermined time constant, and the portion of the range B in the Vc waveform is the LPF 22 The effect of annealing is eliminated and the voltage is constant. The sensor current Ip rises with the same time constant as the voltage Vc in the range A where the voltage Vc rises with the time constant of the LPF 22, and rises with a time constant slower than the rise time constant of the range A in the range B where the voltage Vc is almost constant. It is supposed to go down. In this range B, since the time constant is slow, the current Ip hardly changes in practice. That is, no matter what timing within the range B is detected, the peak current ΔI (current change amount) has almost no detection error of ΔI. The reason will be described later.
[0023]
The voltage having a predetermined time constant referred to in the present embodiment is a signal including a single frequency component, and its frequency is determined as follows.
FIG. 5 is an equivalent circuit of the A / F sensor 30. In this equivalent circuit, Rg is the particle resistance of the solid electrolyte to oxygen ions, Ri and Ci are the particle resistance and grain boundary capacitance at the solid electrolyte particle interface, and Rf and Cf are the electrode interface resistance and electrode interface capacitance, respectively.
[0024]
FIG. 6 shows the complex impedance characteristic of the A / F sensor 30 expressed as shown in FIG. In the figure, “Zreal” on the horizontal axis represents the real part of the complex impedance, and “Zimaginary” on the vertical axis represents the imaginary part. At this time, the impedance ZAC is
ZAC = Zreal + j ・ Zimaginary
Represented as: The point A in FIG. 6 shows the impedance characteristic at a frequency of 1 kHz. When the frequency is lower than that, the characteristic is on the right side of the point A, and when the frequency is high, the characteristic is on the left side of the point A. That is, in the vicinity of 1 kHz, the total value of Rg and Ri is detected as impedance.
[0025]
FIG. 7 is obtained by converting the horizontal axis into the frequency f and the vertical axis into the impedance ZAC in FIG. According to FIG. 7, the impedance ZAC converges to a predetermined value (Rg + Ri) at a frequency of 1 kHz to 10 MHz. Further, the impedance ZAC decreases on the higher frequency side than near 10 MHz, and converges to a predetermined value (Rg) smaller than (Rg + Ri). From this, it can be seen that, in order to detect the impedance ZAC in a stable state with high accuracy, it is desirable that the impedance ZAC is set to a constant value of about 1 kHz to 10 MHz regardless of the frequency f. In the present embodiment, the AC frequency at the time of voltage switching is set to 1 kHz, and a time constant of about 159 μs is set by the LPF 22 in order to obtain the rising edge of the waveform (range A in the Vc waveform in FIG. 4). Incidentally, the lower limit (the upper limit of the frequency) of the time constant at the time of voltage change is limited by the processing capability of the D / A converter 21 and the A / D converter 23, and the time constant can be obtained by using a high-speed circuit. It is also possible to extend the lower limit of.
[0026]
For this reason, when switching the voltage applied to the A / F sensor 30, the microcomputer 20 outputs a digital signal containing a frequency component of about 1 kHz, and the digital signal is predetermined as it passes through the D / A converter 21 and the LPF 22. Is converted to a signal having a time constant (approximately 159 μs). Since the command signal output from the microcomputer 20 is a rectangular signal, the signal generation can be easily realized.
[0027]
Here, the reason why the time constant at which the sensor current Ip falls in the range B in FIG. 4 is slower than the time constant in the range A will be described. First, in order to simplify the explanation, the sensor equivalent circuit (FIG. 5) is simplified as shown in FIG. This is because the rising time constant of the voltage Vc actually applied to the A / F sensor 30 is about several kHz to several tens of kHz, so that the current almost passes through the Rg-Ri-Cf path in FIG. Because. Therefore, the equivalent circuit of FIG. 8A can be expressed by a simple HPF model as shown in FIG.
[0028]
FIG. 9 shows the resistance frequency characteristics of the equivalent circuit of FIG. It can be seen that the characteristics of FIG. 9 are substantially the same as the frequency characteristics in the vicinity of the point A in FIG. Point B in the figure corresponds to the cutoff frequency of the HPF in FIG.
[0029]
FIG. 10A shows a transmission block from the output Vb of the D / A converter 21 of FIG. 1 to the current Ip flowing through the A / F sensor 30. FIG. The front-stage LPF corresponds to the LPF 22 in FIG. 1, and the rear-stage HPF corresponds to the HPF in FIG. According to this block diagram, the voltage waveform Vb and the current waveform Ip when the step voltage is applied to the A / F sensor 30 are shown in FIGS. 10B and 10C, and the equation of the time function of the sensor current Ip is
[0030]
[Expression 1]

It becomes.
In the above equation (1), “T1” is the HPF time constant (corresponding to point B in FIG. 9), and “T2” is the LPF time constant (corresponding to point A in FIG. 9). As described above, in order to use a region having a stable resistance frequency characteristic, it is essential to set the point A to be higher in frequency than the point B. That is, the magnitude relationship between the two time constants T1 and T2 is always
T1> T2
It becomes. Therefore, from the equation (1), it can be seen that the fall time of the range B is later than the rise time of the range A in FIG.
[0031]
For the above reason, in the range B in which the output voltage Vb of FIG. 4 is almost constant, the sensor current Ip is almost within a time of about 2.5 ms, which is an order one digit lower than the HPF time constant of the sensor equivalent circuit. It will not change. That is, in the range B in FIG. 4, almost the same value can be obtained no matter what timing the peak current ΔI is detected.
[0032]
Therefore, as shown in FIG. 11, when the sensor current changes like a sine waveform La (the waveform of the two-dot chain line in the figure), ΔI is erroneously detected by shifting the detection timing from “t” to “t ′”. On the other hand, in the present embodiment, since the sensor current changes like a waveform Lb (solid line waveform in the figure), ΔI can be accurately detected even at the detection timing of “t ′”. Incidentally, the total applied time “A + B” of the voltages switched with a predetermined time constant may be longer than the time during which the peak current ΔI can be detected, and may be several tens μs or more.
[0033]
Next, the configuration of the bias control circuit 40 will be described with reference to the electric circuit diagram of FIG. In FIG. 12, the bias control circuit 40 is roughly divided into a reference voltage circuit 44, a first voltage supply circuit 45, a second voltage supply circuit 47, and a current detection circuit 50. The reference voltage circuit 44 divides the constant voltage Vcc by the voltage dividing resistors 44a and 44b to generate a constant reference voltage Va.
[0034]
The first voltage supply circuit 45 is configured by a voltage follower circuit, and the same voltage Va as the reference voltage Va of the reference voltage circuit 44 is connected to one terminal of the A / F sensor 30 (atmospheric side electrode layer 37 in FIG. 2). Terminal 42). More specifically, in the first voltage supply circuit 45, the positive input terminal is connected to the voltage dividing point of each of the voltage dividing resistors 44a and 44b, and the negative input terminal is one terminal of the A / F sensor 30. 42, an operational amplifier 45a connected to 42, a resistor 45b having one end connected to the output terminal of the operational amplifier 45a, and an NPN transistor 45c and a PNP transistor 45d each having a base connected to the other end of the resistor 45b. The collector of the NPN transistor 45 c is connected to the constant voltage Vcc, and the emitter is connected to one terminal 42 of the A / F sensor 30 through the current detection resistor 50 a constituting the current detection circuit 50. The emitter of the PNP transistor 45d is connected to the emitter of the NPN transistor 45c, and the collector is grounded.
[0035]
Similarly, the second voltage supply circuit 47 is configured by a voltage follower circuit, and the same voltage Vc as the output voltage Vc of the LPF 22 is connected to the other terminal of the A / F sensor 30 (the exhaust gas side electrode layer 36 in FIG. 2). Terminal 41). More specifically, the second voltage supply circuit 47 includes an operational amplifier having a positive input terminal connected to the output of the LPF 22 and a negative input terminal connected to the other terminal 41 of the A / F sensor 30. 47a, a resistor 47b having one end connected to the output terminal of the operational amplifier 47a, and an NPN transistor 47c and a PNP transistor 47d each having a base connected to the other end of the resistor 47b. The collector of the NPN transistor 47c is connected to the constant voltage Vcc, and the emitter is connected to the other terminal 41 of the A / F sensor 30 via the resistor 47e. The emitter of the PNP transistor 47d is connected to the emitter of the NPN transistor 47c, and the collector is grounded.
[0036]
With the above configuration, the constant voltage Va is always supplied to one terminal 42 of the A / F sensor 30. When the voltage Vc lower than the constant voltage Va is supplied to the other terminal 41 of the A / F sensor 30 via the LPF 22, the A / F sensor 30 is positively biased. Further, when the voltage Vc higher than the constant voltage Va is supplied to the other terminal 41 of the A / F sensor 30 via the LPF 22, the A / F sensor 30 is negatively biased.
[0037]
Next, the operation of the air-fuel ratio detection apparatus configured as described above will be described.
FIG. 13 is a flowchart of a main routine showing an outline of the control program in the present embodiment, and this routine is started when the power to the microcomputer 20 is turned on.
[0038]
In FIG. 13, the microcomputer 20 first determines in step 100 whether or not a predetermined time Ta has elapsed since the previous A / F detection. Here, the predetermined time Ta is a time corresponding to the detection cycle of A / F, and is suitably set to about Ta = 2 to 4 ms, for example. If the predetermined time Ta has elapsed since the last A / F detection, the microcomputer 20 makes a positive determination in step 100 and proceeds to step 110. The microcomputer 20 reads the sensor current Ip (limit current value) detected by the current detection circuit 50 in step 110, and uses the characteristic map stored in advance in the ROM in the microcomputer 20 as the sensor current Ip at that time. The corresponding A / F value is detected. After detecting the A / F value, the microcomputer 20 applies an applied voltage Vp corresponding to the A / F detection result (Ip) at that time to the A / F sensor 30 using the characteristic line L1 of FIG.
[0039]
In step 120, the microcomputer 20 determines whether or not a predetermined time Tb has elapsed since the previous element resistance detection. Here, the predetermined time Tb is a time corresponding to the detection cycle of the element resistance, and is selectively set according to, for example, the engine operating state. In the present embodiment, Tb = 2 s (seconds) at normal times (when the engine is in steady operation) where the change in A / F is relatively small, and Tb when there is a sudden change in A / F (when the engine is in transient operation). The predetermined time Tb is variably set to = 128 ms (milliseconds).
[0040]
If the determination at step 120 is negative, the microcomputer 20 detects the A / F value every time the predetermined time Ta elapses as described above. If step 120 is positively determined, the microcomputer 20 performs element resistance detection processing at step 130. The element resistance detection process will be described below using the subroutine of FIG.
[0041]
In FIG. 14, the microcomputer 20 first operates the bias command signal Vr in step 131 to change the voltage to the positive side with respect to the applied voltage Vp (A / F detection voltage) so far. At this time, the application time of the element resistance detection voltage (time t1 to t2 in FIG. 4) is set longer than the time when the peak current is generated after the voltage is switched. Here, the application time may be 2.5 ms or less as defined above, but the current must converge to the limit current value within a time Ta = 2 to 4 ms until the next A / F is detected. . For this reason, the application time of the element current detection voltage is made shorter than the A / F detection interval. Specifically, the voltage application time is set to about several tens to 100 μs.
[0042]
Thereafter, the microcomputer 20 reads the voltage change amount ΔV at that time and the sensor current change amount ΔI detected by the current detection circuit 50 in step 132. In such a case, ΔI is detected within the range B in FIG. In step 133, the microcomputer 20 calculates the element resistance R from ΔV and ΔI (R = ΔV / ΔI), and then returns to the original main routine.
[0043]
On the other hand, the element resistance R obtained as described above has the relationship shown in FIG. 15 with respect to the element temperature. That is, the element resistance R increases dramatically as the element temperature decreases. In the figure, the element resistance R = 90Ω corresponds to the half-active temperature (600 ° C.) of the A / F sensor 30, and the element resistance R = 30Ω corresponds to the activation temperature (700 ° C.) of the A / F sensor 30. In controlling the heater of the A / F sensor 30, the heater 33 is used to eliminate a deviation between the calculated element resistance R and a target resistance value (for example, 30Ω) that the A / F sensor 30 is considered to be sufficiently activated. The energization amount required for the current is obtained, and the energization is duty controlled. That is, element temperature feedback control is performed, whereby the sensor element temperature is maintained at a predetermined activation temperature.
[0044]
According to the embodiment described in detail above, the following effects can be obtained.
(A) In the present embodiment, when the applied voltage of the A / F sensor 30 is changed stepwise and the element resistance R is detected from the change amounts ΔV and ΔI of the voltage and current at that time, a predetermined time constant is provided. In addition to switching the voltage value, the application time of the voltage having the predetermined time constant is set longer than the time when the peak current is generated after the voltage application. In this case, it is possible to provide a period in which the current change becomes slow after the peak current is generated with the voltage change (range B in FIG. 4). That is, the period for accurately detecting the amount of current change accompanying the voltage change is extended. As a result, even if various factors such as the detection timing of the current change amount and the time constant vary, the element resistance of the sensor can be detected with high accuracy.
[0045]
(B) The application time of the voltage switched with a predetermined time constant was set to be less than the order of the reciprocal of the cut-off frequency specific to the sensor. Specifically, when the cut-off frequency peculiar to the sensor is about 40 Hz, the voltage application time is defined at about 2.5 ms, which is an order one digit lower than the time constant corresponding to the cut-off frequency. In this case, the device resistance can be detected by suitably setting the voltage application time without prolonging the voltage application more than necessary.
[0046]
(C) If the element resistance can be detected with high accuracy as described above, activation control of the A / F sensor 30 (energization control of the heater 33) using the detection result can be realized with high accuracy. Further, the detection result of the element resistance can be effectively applied to the deterioration determination of the sensor 30.
[0047]
In addition to the above, the embodiment of the present invention can be embodied as follows.
After applying a voltage having a predetermined time constant and generating a peak current, the voltage value is kept constant at the immediately preceding value. Such an embodiment can be realized by controlling the application time of the element resistance detection voltage in step 131 of FIG. In this case, it is preferable that the time during which the voltage is kept constant at a predetermined voltage value is less than or equal to the order of the reciprocal of the cutoff frequency specific to the sensor. Even in this configuration, as in the above-described embodiment, after the peak current is generated with the voltage change, a period in which the current change becomes slow can be provided, and the current change amount with the voltage change can be accurately detected. The period for is extended. As a result, even if various factors such as the detection timing of the current change amount and the time constant vary, the element resistance of the sensor can be detected with high accuracy. Further, an appropriate voltage application time can be set without prolonging the voltage application at the time of detecting the element resistance more than necessary.
[0048]
After switching to the element resistance detection voltage, the voltage application time may be set variably. In particular,
-Engine operating conditions such as transient judgment results and exhaust gas temperature,
・ Activity of A / F sensor,
・ Elapsed time since engine startup,
The voltage application time is variably set according to the above. In this case, the voltage application time can be set even more preferably.
[0049]
Instead of the above-described cup-type limiting current type A / F sensor as the oxygen concentration sensor, the present invention may be embodied by using a laminated sensor. Even in such a case, the actions and effects as described above can be obtained.
[0050]
In the above embodiment, the present invention is applied as an A / F sensor for detecting the oxygen concentration (A / F) in the exhaust gas of the vehicle-mounted engine. However, the scope of the present invention is limited to an A / F sensor for automobiles. In addition to this, the scope of application can be expanded. For example, it can be embodied as an oxygen concentration sensor that detects the oxygen concentration in a combustible gas (methane gas, ethane gas, etc.).
[0051]
In the above embodiment, the rectangular signal output from the microcomputer 20 has a predetermined time constant by the LPF, but the microcomputer 20 generates a signal having the predetermined time constant. It is also possible to detect the element resistance using the signal.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing an outline of an air-fuel ratio detection apparatus in an embodiment of the invention.
FIG. 2 is a cross-sectional view showing a detailed configuration of an A / F sensor.
FIG. 3 is a graph showing voltage-current characteristics of an A / F sensor.
FIG. 4 is a waveform diagram showing an output voltage Vb of the D / A converter, an output voltage Vc of the LPF, and a sensor current Ip.
FIG. 5 is an equivalent electric circuit diagram of the A / F sensor.
FIG. 6 is a graph showing a locus of impedance with respect to the frequency of an AC input voltage in a state where an A / F detection voltage is applied to the A / F sensor.
FIG. 7 is a graph showing the relationship between the frequency of AC input voltage and AC impedance.
FIG. 8 is an equivalent circuit diagram of an A / F sensor and a block diagram corresponding to the equivalent circuit.
9 is a graph showing resistance frequency characteristics of the equivalent circuit of FIG.
FIG. 10 is a diagram showing a transmission block from an output Vb of a D / A converter to a sensor current Ip, and Vb and Ip waveforms.
FIG. 11 is a waveform diagram showing a state of a current change accompanying a voltage change.
FIG. 12 is an electric circuit diagram showing a configuration of a bias control circuit.
FIG. 13 is a flowchart showing a main routine.
FIG. 14 is a flowchart showing an element resistance detection subroutine.
FIG. 15 is a graph showing the relationship between element temperature and element resistance.
FIG. 16 is a waveform diagram showing voltage change and current change when detecting element resistance in the prior art.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Engine, 20 ... Microcomputer (microcomputer), 22 ... LPF (low-pass filter), 30 ... Limit current type A / F sensor as an oxygen concentration sensor, 40 ... Bias control circuit.

Claims (4)

Applied to an oxygen concentration sensor that outputs a current signal corresponding to the oxygen concentration in the gas to be detected with the application of voltage,
The voltage applied to the sensor for oxygen concentration detection is switched to a voltage for detecting the element resistance of the sensor with a predetermined time constant, and the voltage change at that time and the current change accompanying the voltage change In the element resistance detection method of the oxygen concentration sensor for detecting the element resistance of the sensor,
A method for detecting an element resistance of an oxygen concentration sensor, characterized in that an application time of a voltage switched with the predetermined time constant is made longer than a time during which a peak current is generated after voltage switching. 2. The element resistance detection of the oxygen concentration sensor according to claim 1, wherein an application time of the voltage switched with the predetermined time constant is set to a cycle time represented by a reciprocal of a frequency that is a sensor-specific cutoff frequency. Method. Applied to an oxygen concentration sensor that outputs a current signal corresponding to the oxygen concentration in the gas to be detected with the application of voltage,
The voltage applied to the sensor for oxygen concentration detection is switched to a voltage for detecting the element resistance of the sensor with a predetermined time constant, and the voltage change at that time and the current change accompanying the voltage change In the element resistance detection method of the oxygen concentration sensor for detecting the element resistance of the sensor,
A method for detecting an element resistance of an oxygen concentration sensor, wherein after applying a voltage having the predetermined time constant and generating a peak current, the voltage value is kept constant at the voltage value at the time of the generation of the peak current. . 4. The element resistance detection method for an oxygen concentration sensor according to claim 3, wherein the time during which the voltage value at the time of peak current generation is kept constant is equal to or shorter than the cycle time represented by the reciprocal of the frequency that is a sensor-specific cutoff frequency.

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