JP2004271207A - Ultrasonic propagation time measuring device and fluid sensor equipped therewith - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To measure accurately an ultrasonic propagation time even if the density of a fluid to be measured is changed dynamically. <P>SOLUTION: A rise detection circuit 33 detects the increase speed of the amplitude of an ultrasonic reception signal as an amplitude rising rate. A reception sensitivity adjustment circuit 31 adjusts the reception sensitivity so that the ultrasonic reception signal has a prescribed amplitude rising rate, and a reception timing detection circuit 35 adjusts the reception sensitivity, and then detects the time when the ultrasonic reception signal becomes a reception determination value set by a reception determination value setting circuit 34, as the arrival time of a reception wave, A reception timing correction circuit 36 calculates a delay time from reception start until arrival of the ultrasonic reception signal to the reception determination value, and calculates the ultrasonic propagation time by subtracting the delay time from the arrival time. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

とする必要がある。そして、体積流量Ｑｖは上記式３を用いて、
Ｑｖ＝Ｖｇ＊Ａ＊Ｋ （Ｋは、測定管内の流速分布補正係数）… 式４
となる。
【００１３】
上記式３に示すように、流速Ｖｇ、流量Ｑｖの計測においては、被測定流体の順方向、逆方向の伝播時間の差分をとっているため、被測定流体が空気等の音速の遅いものである場合や遅れ時間ｔｒ１、ｔｒ２に対して実伝播時間ｔ１、ｔ２が十分に大きい場合には、遅れ時間ｔｒ１、ｔｒ２の影響を実用上無視できる。
【００１４】
しかし、被測定流体が燃料電池自動車の燃料ガスである水素ガス等の音速の速いものである場合や小型の流量計のように実伝播時間ｔ１、ｔ２が小さい場合には、遅れ時間ｔｒ１、ｔｒ２が流速Ｖｇ、流量Ｑｖの計測精度に及ぼす影響を無視できなくなる。
【００１５】
また、被測定流体の密度ρを求める場合には、まず、音速Ｃｇが、
Ｃｇ＝Ｌｍ・｛１／（ｔ１−ｔｒ１）＋１／（ｔ２−ｔｒ２）｝／２ … 式５
となるから、密度ρは、この音速Ｃｇを用いて
ρ＝γ・Ｒ・Ｔ＊２２．４／Ｃｇ^２ … 式６
となる。但し、γ：比熱比、Ｒ：ガス定数、Ｔ：ガス温度、である。
【００１６】
音速Ｃｇを求めるには、上記式５に示すように順方向、逆方向の伝播時間が加算されており、遅れ時間ｔｒ１、ｔｒ２の影響が流速Ｖｇ，流量Ｑｖを求める場合（式３）よりも更に大きくなる。そして、密度ρを求めるには、上記式６に示すように音速Ｃｇの２乗を用いることから、遅れ時間ｔｒ１、ｔｒ２の影響が更に大きくなる。
【００１７】
このように、特に音速が速い流体を対象として、その流速、流量、密度を計測する場合には、遅れ時間の影響が大きくなるため、単に、上記従来のように測定した超音波伝播時間を用いたのでは正確な測定が行えない。
【００１８】
従って、上記遅れ時間分を正確に補正する等して超音波伝播時間を正確に測定することは、実用的な計測精度を確保するために非常に重要な処理であると言え、この点において改良の余地があった。
【００１９】
本発明は、このような従来の問題を解決するためになされたものであり、被測定流体の密度がダイナミックに変化しても、的確に受信開始タイミングを検知すると共に、被測定流体の濃度（密度）、流速、流量の計測に用いる超音波伝播時間を精度よく測定できる超音波伝播時間測定装置及びこれを備えた流体センサを提供することを目的とする。
【００２０】
【課題を解決するための手段】
このため、本発明に係る超音波伝播時間測定装置は、超音波受信信号の振幅の増加速度を振幅立ち上がり率として検出し、前記超音波受信信号が所定の振幅立ち上がり率となるように受信感度を調整し、受信感度を調整した後、前記超音波受信信号が所定の受信判定値に到達したときを受信波の到達時間として検出し、受信開始から前記超音波受信信号が前記受信判定値に到達するまでの遅れ時間を算出し、前記到達時間から前記遅れ時間を減算して超音波伝播時間を算出するようにした。
【００２１】
また、本発明に係る流体センサは、上記のようにして算出した被測定流体中の超音波伝播時間を用いて、被測定流体の流速、流量又は密度のいずれかを計測するようにした。
【００２２】
【発明の効果】
本発明に係る超音波伝播時間測定装置によると、超音波受信信号の振幅増加速度を振幅立ち上がり率として算出し、超音波信号が所定の振幅立ち上がり率となるように受信感度が調整される。この受信感度調整後に超音波受信号が所定の受信判定値に到達したときを受信波の到達時間として検出する。これにより、一旦、本来の受信開始タイミングよりも遅れたタイミングで、常にＳＮ比よく安定して到達時間を検出できる。また、超音波受信信号は、先頭の数周期（例えば５〜８周期）までは、ほぼ直線的に振幅増加するので、例えば、受信判定値を振幅立ち上がり率で除算することにより、受信開始から超音波受信信号が受信判定値に到達するまでの遅れ時間を算出（推定）できる。
【００２３】
そして、安定して検出できる到達時間から遅れ時間（遅れ時間）を減算することにより、送信した超音波を受信するまでの伝播時間（超音波伝播時間）を精度よく求めることができる。
【００２４】
また、本発明に係る流体センサによると、上記のようにして求めた被測定流体中の超音波伝播時間に基づいて、被測定流体の流速、流量又は密度を計測するので、被測定流体が燃料電池自動車の燃料ガス等の音速の速いものである場合や流体センサを小型化した場合であっても、高精度な流速、流量又は密度の計測が可能となる。
【００２５】
【発明の実施の形態】
以下、本発明の実施形態を図に基づいて説明する。
図１は、本発明に係る超音波伝播時間測定装置を適用した流体センサのセンサジオメトリを示したものである。
【００２６】
この流体センサは、被測定流体が流れる測定管１と、音響的に対向するよう設けられた一対の超音波送受信器２ａ、２ｂと、超音波伝播時間測定回路３と、演算回路４と、を備えて構成され、一方の超音波送受信器から超音波を送信し、測定管１内で伝播させて他方の超音波送受信器で受信する。
【００２７】
超音波伝播時間測定回路３は、被測定流体の流れに対して順方向（超音波送受信器２ａ→２ｂ）と逆方向（２ｂ→２ａ）の超音波伝播時間ｔ１ 、ｔ２を測定する。この超音波伝播時間ｔ１、ｔ２は演算回路４に出力され、演算回路４は、測定した超音波伝播時間ｔ１、ｔ２を用いて被測定流体の流速Ｖｇ、流量Ｑｖ、あるいは、密度ρを計測する。
【００２８】
図２は、超音波伝播時間測定回路３の構成を示したものであり、図３は、超音波伝播時間測定処理のタイムチャートである。
図２において、受信感度調整回路（本発明に係る受信感度調整手段に相当する）３１には、超音波送受信器２ａ（又は２ｂ）で電気信号に変換された超音波受信信号（以下、単に「受信信号」という）が入力され、また、後述する立ち上がり率検出回路（本発明に係る振幅立ち上がり率検出手段に相当する）３３からの制御信号ＳＲ及び受信タイミング修正回路（本発明に係る遅れ時間算出手段及び超音波伝播時間算出手段に相当する）３６からの制御信号ｔｓが入力されるようになっている。なお、これらの制御信号ＳＲ、ｔｓは、伝播時間測定処理で得られた測定値（計測値）を参照して、受信信号を所定のレベルに調整するための制御量を示す信号である。
【００２９】
そして、図３に示すように、受信感度調整回路３１は、送信波Ｗｔが送信されてから制御信号ｔｓが示す時間（ｔ１（又はｔ２）の期間）までは、ノイズ信号Ｗｎの最大値（最大電圧）Ｖｎを所定の値に抑えるように受信感度を調整する。一方、制御信号ｔｓが示す時間に到達すると、前回の伝播時間測定における受信信号の振幅の立ち上がり率（以下、単に「立ち上がり率」という）の検出結果に応じて、今回の受信感度を調整する。例えば、前回の立ち上がり率が不足していた場合には感度を上昇させる方向に調整する一方、前回の立ち上がり率が過大であった場合には感度を低下させる方向に調整する。
【００３０】
かかる受信感度調整回路３１の具体的な構成としては、より簡単には図４に示すようなものがある。受信感度調整回路３１の第１の構成を示す図４において、まず、制御量調整回路３１１は、前回の立ち上がり率を信号処理上都合のよい規定値と比較して過不足を求めて、その過不足を補うための制御量を決定して出力する。なお、かかる制御量調整回路３１１としては、例えば、ＰＩＤ（比例・積分・微分）制御回路を用いることができる。
【００３１】
出力された制御量情報は、次回の受信処理に備えて制御電圧記憶回路３１２に記憶される。なお、かかる制御電圧記憶回路３１２としては、例えば、メモリＩＣやサンプルホールド回路が該当する。
【００３２】
そして、受信処理時には、制御電圧記憶回路３１２に記憶された制御量が電圧制御増幅器３１３へと伝達され、制御量に応じた増幅度で超音波受信信号が増幅されて出力される。なお、かかる電圧制御増幅器３１３としては、例えば、アナログ乗算器を用いることができる。
【００３３】
図５は、受信感度調整回路３１の他の構成を示したものである。この受信感度調整回路３１は、上記したように、受信前のノイズレベルを抑えつつ受信信号の立ち上がり率を調整するものである。
【００３４】
図５において、Ｖｎ制御量調整回路３１４は、受信前のノイズの最大振幅値があらかじめ定めた規定値となるような制御量Ｖｇｎを決定して出力する。また、ｔａｎθ制御量調整回路３１５は、受信後における受信信号の立ち上がり率が、あらかじめ定めた規定値になるような制御量Ｖｇａを決定して出力する。なお、かかるＶｎ制御量調整回路３１４及びｔａｎθ制御量調整回路３１５としては、上記制御量調整回路３１１と同様に、ＰＩＤ制御回路を用いることができる。
【００３５】
制御電圧発生回路３１６は、内部に時間カウンタを備えており、超音波の送信開始タイミングｔ０（図３参照）の信号を受けて時間計数を開始すると共に、受信感度調整電圧としてＶｎ制御量調整回路３１４から出力された制御量Ｖｇｎを電圧制御増幅器３１３に出力する。そして、時間カウンタの計数値が受信開始タイミングｔｓ（図３参照）に到達したらｔａｎθ制御量調整回路３１５から出力された制御量Ｖｇａに切り替えて電圧制御増幅器３１３に出力する。従って、制御電圧発生回路３１６の出力する受信感度制御電圧は、図３においてＶｇで示すタイミングの信号となる。これにより、後述する立ち上がり率の検出や受信判定において、受信開始前のノイズによる誤動作（誤検出、誤判定）を防止するようにしている。また、被測定流体の密度が時々刻々と変化して受信信号の立ち上がり率が変化しても、受信感度を調整することで、立ち上がり率の検出や受信判定を一定の条件で実行できるようにしている。
【００３６】
図２に戻って、上記受信感度調整回路３１にて感度調整された後、受信信号は包絡線検出回路３２に入力される。入力された受信信号は、この包絡線検出回路３２において、次段の立ち上がり率検出回路３３で処理しやすいように、交番信号から直流信号である包絡線信号Ｗｅに変換される。なお、かかる包絡線検出回路３２としては、例えば、一般的なダイオードとコンデンサとを用いた検波回路を用いることができる。
【００３７】
包絡線検出回路３２から出力された包絡線信号Ｗｅは、立ち上がり率検出回路３３に入力される。この立ち上がり率検出回路３３では、入力された包絡線信号Ｗｅが、立ち上がり率検出開始電圧（第１レベル）Ｖａ及び立ち上がり率検出終了電圧（第２レベル）Ｖｂと比較判定され（図３のＡ点、Ｂ点）、立ち上がり開始タイミングｔａと立ち上がり終了タイミングｔｂとを検出する。そして、次式７を実行することにより、立ち上がり率ｔａｎθを検出する。なお、上記立ち上がり率検出開始電圧Ｖａは、ノイズ信号Ｗｎの最大値Ｖｎに対して所定の比率を乗算することにより設定し（Ｖａ＝Ｖｎ＊１．１）、上記立ち上がり率検出終了電圧Ｖｂは、受信信号の飽和電圧Ｖｓに対して所定の比率を乗算して設定する（Ｖｂ＝Ｖｓ＊０．９）。
【００３８】
ｔａｎθ＝（Ｖｂ−Ｖａ）／（ｔｂ−ｔａ）＝ΔＶｅ／Δｔｓ … 式７
かかる立ち上がり率検出回路３３の具体的な構成としては、図６に示すようなものがある。
【００３９】
図６において、立ち上がり開始タイミング検出回路３３１は、受信信号（包絡線信号）Ｗｅが上記立ち上がり率検出開始電圧Ｖａに到達したときにパルス信号を出力し、立ち上がり終了タイミング検出回路３３２は、上記立ち上がり率検出終了電圧Ｖｂに到達したときにパルス信号を出力するようになっている。なお、かかる立ち上がり開始タイミング検出回路３３１及び立ち上がり終了タイミング検出回路３３２としては、例えば電圧コンパレータＩＣを用いることができる。
【００４０】
時間計数カウンタ３３３では、立ち上がり開始タイミング検出回路３３１からの出力パルスで時間計数を開始し、立ち上がり終了タイミング検出回路３３２からの出力パルスで時間計数を停止することで、立ち上がり率検出時間Δｔｓを求める。そして、除算器３３４において、上記式７を実行して立ち上がり率（ｔａｎθ）を示す制御信号ＳＲを出力する。
【００４１】
次に、受信判定値設定回路３４について説明する。受信判定値Ｖｔｈは、より簡易には、図３に示すタイムチャートにあるように、上記立ち上がり率検出終了電圧Ｖｂと兼用することもできる（この場合、受信タイミングｔｄ＝立ち上がり率検出終了タイミングｔｂとなる）。
【００４２】
しかしながら、受信判定値Ｖｔｈは伝播時間測定値ｔ１（又はｔ２）の基準となる受信タイミングｔｄを検出するための重要な値であり、受信タイミングｔｄにゆらぎが生じないように、包絡線信号Ｗｅの振幅変化率が大きいところに設定することが望ましい。
【００４３】
そこで、本実施形態では、図７に示す超音波伝播時間のタイムチャートにあるように、受信判定値Ｖｔｈを上記立ち上がり率検出終了電圧Ｖｂとは独立した値として設定することで、設定自由度の増加を図りつつ、安定した伝播時間測定を行えるようにしている。
【００４４】
この独立した受信判定値Ｖｔｈは、受信判定値設定回路３４において、次式８を実行することにより設定される。
Ｖｔｈ＝Ｖｂ−｛ｎ＊ΔＶｅ／ＩＮＴ（２＊ｆｕ＊Δｔｓ）｝ … 式８
但し、ｎ：奇数、ｆｕ：超音波周波数、である。
【００４５】
すなわち、立ち上がり率検出の開始から終了までの時間（立ち上がり率検出時間）Δｔｓに超音波周波数ｆｕの２倍を乗算した値の整数値を求め、この整数値で立ち上がり率検出終了電圧Ｖｂと開始電圧Ｖａとの差（立ち上がり振幅差）ΔＶｅを除算した値を奇数倍し、これを立ち上がり検出終了電圧Ｖｂから減算することで受信判定値Ｖｔｈを設定している。なお、ここでは、立ち上がり検出終了電圧Ｖｂから減算しているが、立ち上がり検出開始電圧Ｖａに加算して受信判定値Ｖｔｈを設定するようにしてもよい。これにより、受信判定値Ｖｔｈを常に包絡線信号Ｗｅの変化が大きいところに設定することが可能となる（図７のＣ点参照）。
【００４６】
受信タイミング検出回路（本発明に係る到達時間検出手段に相当する）３５では、包絡線信号Ｗｅと受信判定値Ｖｔｈとを比較して、包絡線信号Ｗｅが受信判定値Ｖｔｈに到達したときに受信タイミングｔｄを検出する。そして、受信タイミングｔｄを検出したら、送信タイミングｔ０からの時間計測を停止することで受信タイミングｔｄまでの時間を受信波の到達時間Ｔ｛＝ｔ１（又はｔ２）＋ｔｒ｝として検出する。
【００４７】
次に、検出した到達時間Ｔは、受信タイミング修正回路３６に入力される。この受信タイミング修正回路３６では、立ち上がり率（ｔａｎθ）及び受信判定値Ｖｔｈから次式９で求めた修正量（すなわち、受信開始タイミングから超音波受信信号が受信判定値Ｖｔｈに到達するまでの遅れ時間）ｔｒを、検出した到達時間Ｔから減算して伝播時間ｔ１（又はｔ２）を求めて出力する。
【００４８】
ｔｒ＝Ｖｔｈ／ｔａｎθ … 式９
なお、上記したように、常に包絡線信号Ｗｅの変化が大きいところで受信タイミングを検出するので、立ち上がり率検出終了電圧Ｖｂを受信判定値Ｖｔｈとして兼用しない場合であっても、立ち上がり率検出終了電圧Ｖｂを立ち上がり率（ｔａｎθ）で除算して修正量ｔｒを算出しても、すなわち、ｔｒ＝Ｖｂ／ｔａｎθとしても十分に精度を確保できる（誤差は無視できる程度のものとなる）。
【００４９】
また、より高精度な伝播時間の測定を行う場合には、図７に示すｔｚを受信タイミングとする。すなわち、包絡線信号Ｗｅを受信判定値Ｖｔｈと比較して検出した受信タイミングｔｄ以降の受信信号Ｗｒの立ち上がりゼロクロス点（Ｄ点）を受信タイミングｔｚとする。この場合の修正量ｔｒ’は、受信開始タイミングｔｓから受信タイミングｔｚまでの時間であると共に、受信信号Ｗｒの周期Ｔｕの整数倍となる。
【００５０】
そこで、かかる関係を利用し、上記受信タイミング修正回路３６において次式１０を実行することで修正量ｔｒ’を求め、検出した到達時間Ｔから差し引いて本来の伝播時間ｔ１（又はｔ２）を求めて出力する。
【００５１】
ｔｒ’＝｛Ｋｃ＋ＩＮＴ（ｔｒ＊ｆｕ）｝＊Ｔｕ … 式１０
但し、Ｋｃ：補正係数であり、ここでは、１を用いる。
なお、ここでは、立ち上がりセロクロス点を受信タイミングとしたが、同様に、立下りゼロクロス点としても有効であり、この場合には、補正係数Ｋｃ＝０．５とする。
【００５２】
本実施形態における流体センサでは、以上のようにして超音波伝播時間ｔ１、ｔ２の測定が行われることになるが、これを図示しない切り替え回路によって被測定流体の流れに対して順方向と逆方向とで交互に超音波の送受信を行うようになっている。
【００５３】
そして、このようにして測定した超音波伝播時間は演算回路４に入力され、被測定流体の流速Ｖｇ、流量Ｑｖ、密度ρのいずれかが計測される。なお、この場合には、到達時間Ｔから遅れ時間に相当する修正量ｔｒが減算され、精度よく伝播時間ｔ１、ｔ２が測定されているので、上記式３及び式５から遅れ時間ｔｒ１、ｔｒ２を取り除いたものが実行されることになる。
【００５４】
次に、上記一連の信号処理の手順を図８に示す。
図８において、電源が投入されると（ステップ１、図中Ｓ１と記す。以下同じ）、各種制御量や受信判定値等の初期値が設定される（ステップ２）。そして、超音波送受信方向の切り替え（２ａ→２ｂ／２ｂ→２ａ）が行われ（ステップ３）、送信に先駆けて受信前ノイズレベルを抑えるための受信前感度Ｖｇｎを設定する（ステップ４）。この受信前感度Ｖｇｎは、上記したように、ノイズ信号の最大値（振幅）を所定値に抑えるように（規定値となるように）設定されるものであり、電源投入直後の初回は初期値が適用されるが、以降は前回の処理で検出されたノイズレベルの規定値に対する過不足量が適用されることになる。
【００５５】
続いて、ステップ２において送信側に切り替えられた超音波送受信器から超音波を送信し（ステップ５）、受信開始タイミングｔｓに到達するまで待つ（ステップ６）。この受信開始タイミングｔｓとしては、前回の処理で測定された超音波伝播時間が適用される。
【００５６】
受信開始タイミングｔｓに到達したら、今回の測定におけるノイズレベルの最大値が規定値にあるか否か（所定のノイズレベルであるか否か）を判定する（ステップ７）。そして、ノイズレベルの最大値が規定値にない場合には、その過不足量に応じた感度補正を実行する共に、この感度補正量を記憶、更新して次回の受信前感度調整に備える（ステップ８）。一方、ノイズレベルが規定値にあるときは、感度補正量の更新は行われない。
【００５７】
続いて、受信感度を受信後感度Ｖｇａに設定する（ステップ９）。この受信後感度Ｖｇａは、上記したように、受信信号の立ち上がり率（ｔａｎθ）が規定値となるように（所定のレベルとなるように）設定されるものであり、電源投入直後の初回は初期値が適用されるが、以降は前回の処理で検出された立ち上がり率（ｔａｎθ）の規定値に対する過不足量が適用されることになる。
【００５８】
そして、受信信号の振幅が受信判定値Ｖｔｈに達した時点で受信タイミングｔｄを検出する（ステップ１０）。なお、ここで用いる受信判定値Ｖｔｈは、電源投入直後の初回は初期値が適用されるが、以降は前回の処理において上記式８により算出された受信判定値が適用される。
【００５９】
次に、今回の受信信号の立ち上がり率（ｔａｎθ）を検出し（ステップ１１）、検出した立ち上がり率（ｔａｎθ）が規定値にあるか否かを判定する（ステップ１２）。立ち上がり率（ｔａｎθ）が規定値にない場合には、その過不足量に応じた感度補正を実行すると共に、この感度補正量を記憶、更新して次回の受信後感度調整に備える（ステップ１３）。また、同時に、受信判定値Ｖｔｈも今回の結果（ノイズ最大値Ｖｎ、立ち上がり時間Δｔｓ等）に応じた値に変更し、次回の受信タイミングの検出に備える（ステップ１４）。一方、立ち上がり率（ｔａｎθ）が規定値にある場合には、感度補正量の更新は行われない。
【００６０】
続いて、検出した立ち上がり率（ｔａｎθ）と立ち上がり率検出終了電圧Ｖｂとから上記式９により修正量ｔｒを算出し（ステップ１５）、この修正量ｔｒを到達時間Ｔから減算して伝播時間をｔ１（又はｔ２）を求め、これを出力する（ステップ１６）。なお、受信タイミングを、立ち上がり率検出終了電圧Ｖｂを超えた後のゼロレベル（Ｄ点）とした場合には、上記式１０により修正量ｔｒ’が算出される。以降、送受信方向の切り替え処理処理（ステップ２）に戻り、一連の処理を繰り返し実行する。
【００６１】
以上説明した実施形態では、前回測定した超音波伝播時間（今回の受信開始タイミングの推測値）の前後で受信感度を切り替えるようにし、前回測定した超音波伝播時間の経過前はノイズレベルが受信判定値Ｖｔｈを超えない所定レベルになるように受信感度を調整する一方、前回測定した超音波伝播時間の経過後は受信信号が所定の立ち上がり率となるように受信感度を調整するので、被測定流体の密度が変化して受信信号強度が変化しても、その変化に追従して、常に良好なＳＮ比を確保した超音波伝播時間の測定が可能となる。なお、前回の超音波伝播時間の測定におけるノイズレベル又は立ち上がり率に基づいて、今回の受信感度を調整するよう構成すれば、比較的容易に上記効果を得ることができる。
【００６２】
また、受信信号が、前回の最大ノイズＶｎに対して所定量大きい立ち上がり率検出開始電圧Ｖａに到達（立ち上がり率検出の開始座標Ａ点）から、前回の受信信号の飽和電圧Ｖｓに対して所定量小さい立ち上がり率検出終了電圧Ｖｂ（立ち上がり率検出の終了座標Ｂ点）に到達するまでの振幅増加速度（両座標間の傾き）を立ち上がり率として検出するので、被測定流体の密度が変化して受信開始タイミングが変化したり、受信信号強度や立ち上がり率が変化したりしても、その変化に追従して、常に的確な立ち上がり率を検出できる。
【００６３】
また、受信判定値Ｖｔｈを上記式８により算出・設定するので、被測定流体の密度が変化して、受信信号強度や立ち上がり率が変化しても、その変化に追従しつつ、包絡線信号Ｗｅの変化が大きいところに受信判定値を設定でき、変動の少ない安定した超音波伝播時間の測定が可能となる。なお、受信判定値Ｖｔｈを立ち上がり率検出終了電圧Ｖｂとすることにより、より簡易な構成として演算処理等の軽減を図ることも可能である。なお、これらの場合には、受信判定値Ｖｔｈ又は立ち上がり率検出終了電圧Ｖｂを立ち上がり率（ｔａｎθ）で除算することで遅れ時間ｔｒを高精度に推定できる。
【００６４】
また、超音波受信信号が立ち上がり率検出終了電圧Ｖｂに到達した後のゼロレベルを受信判定値とすることで、上記修正量（遅れ時間）ｔｒ’は受信信号の周期の整数倍となるので、かかる関係を利用して修正量ｔｒ’を上記式１０により算出することで、より精度のよい超音波伝播時間の測定が可能となる。また、受信信号のゼロクロス点を受信タイミングとする高精度タイプの伝播時間測定に対して容易に適用できる。
【００６５】
尚、以上の実施形態は本発明の基本的概念とその代表例を示したものであり、これらに限定されるものではなく、種々の変形実施が可能であることは言うまでもない。
【図面の簡単な説明】
【図１】本発明に係る超音波伝播時間測定装置を適用した流体センサの概略構成図である。
【図２】本発明に係る超音波伝播時間測定装置の基本構成を示す図である。
【図３】超音波伝播時間の測定を示すタイムチャートである。
【図４】受信感度調整手段の構成を示す図である。
【図５】受信感度調整手段の他の構成を示す図である。
【図６】振幅立ち上がり率検出手段の構成を示す図である。
【図７】他の超音波伝播時間の測定を示すタイムチャートである。
【図８】超音波伝播時間の測定（信号処理）を示すフローチャートである。
【図９】従来の超音波伝播測定を説明するための図である。
【符号の説明】
１…測定管、２ａ，２ｂ…超音波送受信器、３…超音波伝播時間測定回路、４…演算回路、３１…受信感度調整回路、３２…包絡線検出開度、３３…立ち上がり率検出回路、３４…受信判定値設定回路、３５…受信タイミング検出回路、３６…受信タイミング修正回路、３１１…制御量調整回路、３１２…制御電圧記憶回路（メモリ）、３１３…電圧制御増幅器、３１４…Ｖｎ制御量調整回路（受信前の感度制御量調整回路）、３１５…ｔａｎθ制御量調整回路（受信後の感度調整回路）、３１６制御電圧発生回路、３３１…立ち上がり開始タイミング検出回路、３３２…立ち上がり終了タイミング検出回路、３３３…時間計測カウンタ、Ｗｔ…送信波、Ｗｒ…受信波、Ｗｅ…受信波の包絡線、ｔｄ１…流れ順方向の受信タイミング、ｔｄ２…流れ逆方向の受信タイミング、Ｖｔｈ…受信判定値（閾値）、Ｖｓ…受信信号飽和電圧、Ｖａ…立ち上がり率検出の開始電圧、Ｖｂ…立ち上がり率検出の終了電圧、Ｖｎ…ノイズ信号最大電圧、Ｖｇ…感度調整電圧、ΔＶｅ…立ち上がり率検出電圧、Δｔｓ…立ち上がり率検出時間、ｔｓ…受信開始タイミング、ｔａ…立ち上がり率検出開始タイミング、ｔｂ…立ち上がり率検出終了タイミング、ｔｄ…受信タイミング、ｔｚ…ゼロクロス受信判定タイプの受信タイミング、ｔｒ…（到達時間）修正量、ｔｒ’…ゼロクロス受信判定タイプの（到達時間）修正量[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an ultrasonic propagation time measuring apparatus and a fluid sensor that measures the flow velocity, flow rate, concentration, etc. of a fluid to be measured using the ultrasonic propagation time measured by the ultrasonic propagation time measuring apparatus, and in particular, measures the ultrasonic wave propagation time measurement accuracy. Technology to improve.
[0002]
[Prior art]
Conventionally, a propagation time (ultrasonic propagation time) from transmission of an ultrasonic wave (transmission wave) into a fluid to be measured by an ultrasonic element until reception of the transmission wave is measured, and the measured ultrasonic propagation time is measured. A device (fluid sensor) for measuring a gas concentration (density), a flow velocity or a flow rate of a fluid to be measured by using the same is known. In the measurement of the ultrasonic propagation time in these devices, how to detect the reception timing is important. There are methods described in Patent Literature 1 and Patent Literature 2 as the detection method.
[0003]
In Patent Literature 1, a reception determination value is set by multiplying the amplitude integral value of the previous reception wave by a predetermined ratio of 1 or less, and the reception timing when the amplitude integration value of the current reception wave reaches the reception determination value is set as reception timing. I have.
[0004]
Further, in Patent Document 2, a reception determination value is set by multiplying the maximum amplitude value of the previous reception wave by a predetermined ratio, and when the amplitude of the current reception wave exceeds the reception determination value and becomes zero level ( (Zero cross point) is set as the reception timing.
[0005]
[Patent Document 1]
JP 2001-124745 A
[Patent Document 2]
JP-A-2002-323361
[0006]
[Problems to be solved by the invention]
According to such a conventional technique, when the change in the density of the fluid to be measured is relatively small, even if the received signal strength fluctuates due to factors such as a pressure change or deterioration of the ultrasonic transceiver, reception is performed in accordance with the fluctuation. Since the determination value is also automatically adjusted, accurate and stable measurement of the ultrasonic propagation time is possible.
[0007]
However, in the case of a fluid to be measured whose gas density dynamically changes in accordance with the operation state, such as a fuel gas of a fuel cell vehicle, which has been actively developed for practical use in recent years, the above-described conventional technology does not provide accurate measurement. Thus, it was not possible to detect a proper reception timing, and thus it was not possible to measure the ultrasonic propagation time with the accuracy required for measuring the concentration (density), flow velocity, and flow rate.
[0008]
More specifically, since the ultrasonic attenuation changes according to the density change, when the gas density dynamically changes, the received signal strength (amplitude of the received wave) greatly changes, and the directional angle of the ultrasonic wave and the like. When the state of refraction and diffraction in the propagation path also changes, many phase changes occur. Then, as the received signal, the change in the signal strength (amplitude) and the change in the phase are mixed to form a complicated interference waveform. However, simply setting the reception determination value based on the amplitude integration value or the maximum amplitude value could not detect the accurate reception timing, and could not maintain high measurement accuracy of the ultrasonic propagation time. .
[0009]
Next, the effect of the delay from the start of actual reception to the measurement of the propagation time when measuring the concentration (density), flow velocity, and flow rate of the fluid to be measured by measuring the ultrasonic propagation time will be described.
[0010]
In a typical ultrasonic flowmeter, a pair of ultrasonic transceivers are arranged at positions acoustically opposed to a measurement tube through which a fluid to be measured flows, and ultrasonic waves transmitted from one ultrasonic transceiver are transmitted. Is propagated in the measuring tube and received by the other ultrasonic transceiver, and the ultrasonic propagation times t1 and t2 in the forward and reverse directions with respect to the flow of the fluid to be measured are measured, and based on the measured ultrasonic propagation time Then, the flow velocity Vg of the fluid to be measured is obtained, and the flow velocity Vg is multiplied by the cross-sectional area of the measuring tube 1 to obtain the volume flow rate of the fluid to be measured.
[0011]
In this case, the measured ultrasonic wave propagation times t1 and t2 have the relationship of the following equations 1 and 2.
t1 = {Lm / (Cg + Vg * cosθ)} + tr1 Formula 1
t2 = {Lm / (Cg−Vg * cos θ)} + tr2 Equation 2
Here, Lm: propagation distance of the ultrasonic wave, Cg: sound velocity in the measuring tube (in the stationary fluid), Vg: flow velocity of the fluid to be measured, θ: sound of the tube axis of the measuring tube 1 and the ultrasonic transceivers 2a, 2b. Angles with axes, tr1, tr2: delay times from reception start timing (leading wave) to propagation time measurement timing (see FIG. 9, hereinafter simply referred to as "delay time").
[0012]
Therefore, in the calculation of the flow velocity Vg of the fluid to be measured, the delay times tr1 and tr2 are subtracted from the measured ultrasonic propagation times t1 and t2, respectively.

It is necessary to Then, the volume flow rate Qv is calculated by using the above equation (3).
Qv = Vg * A * K (K is a flow velocity distribution correction coefficient in the measurement tube) Equation 4
It becomes.
[0013]
As shown in the above formula 3, in the measurement of the flow velocity Vg and the flow rate Qv, the difference in the forward and backward propagation times of the fluid to be measured is taken, so that the fluid to be measured has a low sound velocity such as air. In some cases or when the actual propagation times t1 and t2 are sufficiently larger than the delay times tr1 and tr2, the effects of the delay times tr1 and tr2 can be practically ignored.
[0014]
However, when the fluid to be measured is a high sound speed such as hydrogen gas as a fuel gas of a fuel cell vehicle, or when the actual propagation times t1 and t2 are small as in a small flow meter, the delay times tr1 and tr2 Influence on the measurement accuracy of the flow velocity Vg and the flow rate Qv cannot be ignored.
[0015]
When determining the density ρ of the fluid to be measured, first, the sound speed Cg is
Cg = Lm · {1 / (t1-tr1) + 1 / (t2-tr2)} / 2 Expression 5
Therefore, the density ρ is calculated using this sound speed Cg.
ρ = γ · RT · 22.4 / Cg ² … Equation 6
It becomes. Here, γ: specific heat ratio, R: gas constant, T: gas temperature.
[0016]
To calculate the sound velocity Cg, the forward and backward propagation times are added as shown in the above equation 5, and the influence of the delay times tr1 and tr2 is greater than when the flow velocity Vg and the flow rate Qv are obtained (equation 3). It becomes even larger. Then, to obtain the density ρ, the square of the sound speed Cg is used as shown in the above equation 6, so that the effects of the delay times tr1 and tr2 are further increased.
[0017]
As described above, when measuring the flow velocity, flow rate, and density of a fluid having a particularly high sound velocity, the influence of the delay time becomes large. If it is, accurate measurement cannot be performed.
[0018]
Therefore, it can be said that accurately measuring the ultrasonic propagation time by accurately correcting the delay time or the like is a very important process for ensuring practical measurement accuracy. There was room for
[0019]
The present invention has been made to solve such a conventional problem. Even if the density of the fluid to be measured dynamically changes, the reception start timing can be accurately detected and the concentration of the fluid to be measured ( It is an object of the present invention to provide an ultrasonic propagation time measuring device capable of accurately measuring an ultrasonic propagation time used for measuring density, flow velocity, and flow rate, and a fluid sensor including the same.
[0020]
[Means for Solving the Problems]
For this reason, the ultrasonic propagation time measuring device according to the present invention detects the rate of increase in the amplitude of the ultrasonic reception signal as an amplitude rise rate, and adjusts the reception sensitivity so that the ultrasonic reception signal has a predetermined amplitude rise rate. After adjusting the reception sensitivity, the time when the ultrasonic reception signal reaches a predetermined reception determination value is detected as the arrival time of the reception wave, and the ultrasonic reception signal reaches the reception determination value from the start of reception. Then, the delay time is calculated, and the ultrasonic wave propagation time is calculated by subtracting the delay time from the arrival time.
[0021]
In addition, the fluid sensor according to the present invention measures any one of the flow velocity, the flow rate, and the density of the measured fluid using the ultrasonic wave propagation time in the measured fluid calculated as described above.
[0022]
【The invention's effect】
According to the ultrasonic propagation time measuring device of the present invention, the rate of increase in the amplitude of the ultrasonic reception signal is calculated as the amplitude rise rate, and the reception sensitivity is adjusted so that the ultrasonic signal has a predetermined amplitude rise rate. After the reception sensitivity is adjusted, the time when the ultrasonic reception signal reaches a predetermined reception determination value is detected as the arrival time of the reception wave. As a result, the arrival time can always be detected stably with a good SN ratio at a timing that is later than the original reception start timing. Also, the amplitude of the ultrasonic reception signal increases almost linearly up to the first several cycles (for example, 5 to 8 cycles). The delay time until the sound wave reception signal reaches the reception determination value can be calculated (estimated).
[0023]
Then, by subtracting the delay time (delay time) from the arrival time that can be detected stably, the propagation time (ultrasonic propagation time) until the transmitted ultrasonic wave is received can be accurately obtained.
[0024]
Further, according to the fluid sensor according to the present invention, the flow rate, the flow rate, or the density of the measured fluid is measured based on the ultrasonic wave propagation time in the measured fluid determined as described above. Even when the speed of sound such as fuel gas of a battery-powered vehicle is high or when the fluid sensor is downsized, it is possible to measure the flow velocity, flow rate or density with high accuracy.
[0025]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a sensor geometry of a fluid sensor to which the ultrasonic propagation time measuring device according to the present invention is applied.
[0026]
This fluid sensor includes a measuring tube 1 through which a fluid to be measured flows, a pair of ultrasonic transceivers 2a and 2b provided so as to be acoustically opposed, an ultrasonic propagation time measuring circuit 3, and an arithmetic circuit 4. Ultrasonic waves are transmitted from one ultrasonic transceiver, propagated in the measuring tube 1, and received by the other ultrasonic transceiver.
[0027]
The ultrasonic propagation time measuring circuit 3 measures the ultrasonic propagation times t1 and t2 in the forward direction (ultrasonic transceivers 2a → 2b) and the reverse direction (2b → 2a) with respect to the flow of the fluid to be measured. The ultrasonic propagation times t1 and t2 are output to the arithmetic circuit 4, and the arithmetic circuit 4 measures the flow velocity Vg, the flow rate Qv, or the density ρ of the fluid to be measured using the measured ultrasonic propagation times t1 and t2. .
[0028]
FIG. 2 shows the configuration of the ultrasonic wave propagation time measuring circuit 3, and FIG. 3 is a time chart of the ultrasonic wave propagation time measuring process.
In FIG. 2, a reception sensitivity adjustment circuit (corresponding to a reception sensitivity adjustment means according to the present invention) 31 includes an ultrasonic reception signal (hereinafter simply referred to as “the reception signal”) converted into an electric signal by the ultrasonic transceiver 2a (or 2b). A reception signal ”), a control signal SR from a rise rate detection circuit 33 (corresponding to an amplitude rise rate detection means according to the present invention) described later, and a reception timing correction circuit (delay time calculation according to the present invention). (Corresponding to the means and the ultrasonic wave propagation time calculating means) 36. Note that these control signals SR and ts are signals indicating a control amount for adjusting a received signal to a predetermined level with reference to a measured value (measured value) obtained in the propagation time measuring process.
[0029]
Then, as shown in FIG. 3, the reception sensitivity adjustment circuit 31 outputs the maximum value (maximum value) of the noise signal Wn from the time when the transmission wave Wt is transmitted until the time (t1 (or t2)) indicated by the control signal ts. The reception sensitivity is adjusted so that the voltage) Vn is suppressed to a predetermined value. On the other hand, when the time indicated by the control signal ts is reached, the current reception sensitivity is adjusted according to the detection result of the rise rate of the amplitude of the received signal (hereinafter simply referred to as “rise rate”) in the previous propagation time measurement. For example, if the previous rising rate is insufficient, the sensitivity is adjusted to increase the sensitivity, while if the previous rising rate is excessive, the sensitivity is adjusted to decrease the sensitivity.
[0030]
As a specific configuration of the reception sensitivity adjustment circuit 31, there is a more simplified configuration as shown in FIG. In FIG. 4 showing the first configuration of the reception sensitivity adjustment circuit 31, first, the control amount adjustment circuit 311 compares the previous rising rate with a prescribed value convenient for signal processing to determine excess or deficiency. A control amount for compensating for the shortage is determined and output. As the control amount adjusting circuit 311, for example, a PID (proportional / integral / differential) control circuit can be used.
[0031]
The output control amount information is stored in the control voltage storage circuit 312 in preparation for the next reception process. The control voltage storage circuit 312 corresponds to, for example, a memory IC or a sample and hold circuit.
[0032]
Then, at the time of reception processing, the control amount stored in the control voltage storage circuit 312 is transmitted to the voltage control amplifier 313, and the ultrasonic reception signal is amplified and output at an amplification degree according to the control amount. Note that, as the voltage control amplifier 313, for example, an analog multiplier can be used.
[0033]
FIG. 5 shows another configuration of the reception sensitivity adjustment circuit 31. As described above, the reception sensitivity adjustment circuit 31 adjusts the rise rate of the reception signal while suppressing the noise level before reception.
[0034]
In FIG. 5, a Vn control amount adjustment circuit 314 determines and outputs a control amount Vgn such that the maximum amplitude value of noise before reception becomes a predetermined specified value. Further, the tan θ control amount adjustment circuit 315 determines and outputs the control amount Vga such that the rising rate of the received signal after reception becomes a predetermined value. Note that a PID control circuit can be used as the Vn control amount adjustment circuit 314 and the tan θ control amount adjustment circuit 315, similarly to the control amount adjustment circuit 311.
[0035]
The control voltage generation circuit 316 includes a time counter therein, starts counting the time in response to the signal of the ultrasonic wave transmission start timing t0 (see FIG. 3), and sets the Vn control amount adjustment circuit as the reception sensitivity adjustment voltage. The control amount Vgn output from 314 is output to the voltage control amplifier 313. When the count value of the time counter reaches the reception start timing ts (see FIG. 3), the control value is switched to the control amount Vga output from the tan θ control amount adjustment circuit 315 and output to the voltage control amplifier 313. Therefore, the reception sensitivity control voltage output from the control voltage generation circuit 316 is a signal at the timing indicated by Vg in FIG. This prevents malfunctions (erroneous detection and erroneous determination) due to noise before the start of reception in detection of a rising rate and reception determination described later. In addition, even if the density of the fluid to be measured changes every moment and the rising rate of the received signal changes, by adjusting the receiving sensitivity, the detection of the rising rate and the reception judgment can be performed under certain conditions. I have.
[0036]
Returning to FIG. 2, after the sensitivity is adjusted by the reception sensitivity adjustment circuit 31, the received signal is input to the envelope detection circuit 32. The input received signal is converted from an alternating signal into an envelope signal We, which is a DC signal, in the envelope detection circuit 32 so that it can be easily processed by the rising rate detection circuit 33 in the next stage. As the envelope detection circuit 32, for example, a detection circuit using a general diode and a capacitor can be used.
[0037]
The envelope signal We output from the envelope detection circuit 32 is input to the rise rate detection circuit 33. In the rise rate detection circuit 33, the input envelope signal We is compared with a rise rate detection start voltage (first level) Va and a rise rate detection end voltage (second level) Vb and determined (point A in FIG. 3). , B point), the rising start timing ta and the rising end timing tb are detected. Then, by executing the following equation 7, the rise rate tanθ is detected. The rise rate detection start voltage Va is set by multiplying the maximum value Vn of the noise signal Wn by a predetermined ratio (Va = Vn * 1.1), and the rise rate detection end voltage Vb is The saturation voltage Vs of the received signal is set by multiplying it by a predetermined ratio (Vb = Vs * 0.9).
[0038]
tan θ = (Vb−Va) / (tb−ta) = ΔVe / Δts Equation 7
A specific configuration of the rise rate detection circuit 33 is as shown in FIG.
[0039]
6, a rising start timing detection circuit 331 outputs a pulse signal when a reception signal (envelope signal) We reaches the rising rate detection start voltage Va, and a rising end timing detection circuit 332 outputs the pulse rate. A pulse signal is output when the voltage reaches the detection end voltage Vb. As the rising start timing detection circuit 331 and the rising end timing detection circuit 332, for example, a voltage comparator IC can be used.
[0040]
The time counting counter 333 starts the time counting with the output pulse from the rising start timing detection circuit 331 and stops the time counting with the output pulse from the rising end timing detection circuit 332, thereby obtaining the rising rate detection time Δts. Then, the divider 334 outputs the control signal SR indicating the rising rate (tan θ) by executing the above equation (7).
[0041]
Next, the reception determination value setting circuit 34 will be described. More simply, the reception determination value Vth can also be used as the rising rate detection end voltage Vb as shown in the time chart of FIG. 3 (in this case, the reception timing td = the rising rate detection end timing tb. Become).
[0042]
However, the reception determination value Vth is an important value for detecting the reception timing td, which is a reference for the measured propagation time t1 (or t2), and the reception signal Vd is determined so that the reception timing td does not fluctuate. It is desirable to set it at a place where the amplitude change rate is large.
[0043]
Therefore, in the present embodiment, as shown in the time chart of the ultrasonic propagation time shown in FIG. 7, the reception determination value Vth is set as a value independent of the rise rate detection end voltage Vb, so that the degree of freedom of the setting can be improved. It is possible to perform stable propagation time measurement while increasing the number.
[0044]
The independent reception determination value Vth is set by executing the following equation 8 in the reception determination value setting circuit 34.
Vth = Vb− {n * ΔVe / INT (2 * fu * Δts)} Equation 8
Here, n: odd number, fu: ultrasonic frequency.
[0045]
That is, the integer from the start to the end of the rise rate detection (rise rate detection time) Δts multiplied by twice the ultrasonic frequency fu is obtained, and the rise rate detection end voltage Vb and the start voltage are calculated using the integer values. The reception determination value Vth is set by multiplying the value obtained by dividing the difference from Va (rising amplitude difference) ΔVe by an odd number and subtracting this from the rising detection end voltage Vb. In this case, the value is subtracted from the rising detection end voltage Vb, but may be added to the rising detection start voltage Va to set the reception determination value Vth. This makes it possible to always set the reception determination value Vth to a place where the change of the envelope signal We is large (see the point C in FIG. 7).
[0046]
The reception timing detection circuit (corresponding to the arrival time detection means according to the present invention) 35 compares the envelope signal We with the reception determination value Vth, and performs reception when the envelope signal We reaches the reception determination value Vth. The timing td is detected. When the reception timing td is detected, the time measurement from the transmission timing t0 is stopped, and the time until the reception timing td is detected as the arrival time of the received wave T 波 = t1 (or t2) + tr｝.
[0047]
Next, the detected arrival time T is input to the reception timing correction circuit 36. In the reception timing correction circuit 36, the correction amount calculated from the rising rate (tan θ) and the reception determination value Vth by the following equation 9 (that is, the delay time from the reception start timing until the ultrasonic reception signal reaches the reception determination value Vth) ) Tr is subtracted from the detected arrival time T to obtain and output the propagation time t1 (or t2).
[0048]
tr = Vth / tan θ Expression 9
As described above, since the reception timing is always detected where the change of the envelope signal We is large, even when the rising rate detection end voltage Vb is not used as the reception determination value Vth, the rising rate detection end voltage Vb is used. Is divided by the rise rate (tan θ) to calculate the correction amount tr, that is, even if tr = Vb / tan θ, sufficient accuracy can be ensured (the error is negligible).
[0049]
When measuring the propagation time with higher accuracy, tz shown in FIG. 7 is set as the reception timing. That is, the rising zero-cross point (point D) of the reception signal Wr after the reception timing td detected by comparing the envelope signal We with the reception determination value Vth is set as the reception timing tz. The correction amount tr 'in this case is a time from the reception start timing ts to the reception timing tz and is an integral multiple of the period Tu of the reception signal Wr.
[0050]
Therefore, using this relationship, the correction amount tr 'is obtained by executing the following equation 10 in the reception timing correction circuit 36, and is subtracted from the detected arrival time T to obtain the original propagation time t1 (or t2). Output.
[0051]
tr ′ = {Kc + INT (tr * fu)} * Tu Expression 10
Here, Kc is a correction coefficient, and 1 is used here.
Here, the rising zero crossing point is set as the reception timing, but it is also effective as the falling zero crossing point. In this case, the correction coefficient Kc is set to 0.5.
[0052]
In the fluid sensor according to the present embodiment, the measurement of the ultrasonic wave propagation times t1 and t2 is performed as described above. However, the measurement is performed by a switching circuit (not shown) in the forward and reverse directions with respect to the flow of the fluid to be measured. The transmission and reception of the ultrasonic wave are alternately performed with the above.
[0053]
Then, the ultrasonic propagation time measured in this way is input to the arithmetic circuit 4, and one of the flow velocity Vg, the flow rate Qv, and the density ρ of the fluid to be measured is measured. In this case, since the correction amount tr corresponding to the delay time is subtracted from the arrival time T and the propagation times t1 and t2 are measured with high accuracy, the delay times tr1 and tr2 are calculated from the above equations 3 and 5. The removed one will be executed.
[0054]
Next, FIG. 8 shows a procedure of the above series of signal processing.
In FIG. 8, when the power is turned on (Step 1, described as S1 in the figure, the same applies hereinafter), initial values such as various control amounts and reception determination values are set (Step 2). Then, the ultrasonic transmission / reception direction is switched (2a → 2b / 2b → 2a) (step 3), and a pre-reception sensitivity Vgn for suppressing the pre-reception noise level is set prior to transmission (step 4). As described above, the pre-reception sensitivity Vgn is set so that the maximum value (amplitude) of the noise signal is suppressed to a predetermined value (to be a specified value). Is applied, but thereafter, the excess or deficiency with respect to the specified value of the noise level detected in the previous processing is applied.
[0055]
Subsequently, an ultrasonic wave is transmitted from the ultrasonic transceiver switched to the transmitting side in step 2 (step 5), and waits until the reception start timing ts is reached (step 6). As the reception start timing ts, the ultrasonic propagation time measured in the previous processing is applied.
[0056]
When the reception start timing ts has been reached, it is determined whether or not the maximum value of the noise level in the current measurement is a specified value (whether or not the noise level is a predetermined noise level) (step 7). If the maximum value of the noise level is not at the specified value, the sensitivity is corrected in accordance with the excess or deficiency, and the sensitivity correction is stored and updated to prepare for the next pre-reception sensitivity adjustment. 8). On the other hand, when the noise level is at the specified value, the sensitivity correction amount is not updated.
[0057]
Subsequently, the receiving sensitivity is set to the post-receiving sensitivity Vga (step 9). As described above, the post-reception sensitivity Vga is set so that the rising rate (tan θ) of the reception signal becomes a specified value (a predetermined level). The value is applied, and thereafter, the excess or deficiency amount with respect to the prescribed value of the rise rate (tan θ) detected in the previous processing is applied.
[0058]
Then, when the amplitude of the reception signal reaches the reception determination value Vth, the reception timing td is detected (step 10). As the reception determination value Vth used here, an initial value is applied for the first time immediately after the power is turned on, but thereafter, the reception determination value calculated by the above equation 8 in the previous processing is applied.
[0059]
Next, the rising rate (tan θ) of the current reception signal is detected (step 11), and it is determined whether the detected rising rate (tan θ) is a specified value (step 12). If the rise rate (tan θ) is not at the specified value, the sensitivity is corrected in accordance with the excess or deficiency, and the sensitivity correction is stored and updated to prepare for the next sensitivity adjustment after reception (step 13). . At the same time, the reception determination value Vth is also changed to a value corresponding to the current result (the maximum noise value Vn, the rise time Δts, and the like) to prepare for the detection of the next reception timing (step 14). On the other hand, when the rise rate (tan θ) is at the specified value, the sensitivity correction amount is not updated.
[0060]
Subsequently, the correction amount tr is calculated from the detected rise rate (tan θ) and the rise rate detection end voltage Vb by the above equation 9 (step 15), and the correction amount tr is subtracted from the arrival time T to calculate the propagation time t1. (Or t2) is obtained and output (step 16). When the reception timing is set to the zero level (point D) after exceeding the rising rate detection end voltage Vb, the correction amount tr 'is calculated by the above equation (10). Thereafter, the process returns to the transmission / reception direction switching process (step 2), and a series of processes is repeatedly executed.
[0061]
In the embodiment described above, the reception sensitivity is switched before and after the previously measured ultrasonic wave propagation time (estimated value of the current reception start timing), and the noise level is determined to be not received before the previous measured ultrasonic wave propagation time elapses. Since the reception sensitivity is adjusted so as to be a predetermined level not exceeding the value Vth, and after the lapse of the previously measured ultrasonic wave propagation time, the reception sensitivity is adjusted so that the reception signal has a predetermined rise rate. Even if the received signal strength changes due to the change in the density of the received signal, it is possible to follow the change to measure the ultrasonic propagation time while always ensuring a good SN ratio. In addition, if the present reception sensitivity is adjusted based on the noise level or the rise rate in the previous measurement of the ultrasonic propagation time, the above effect can be obtained relatively easily.
[0062]
Further, from when the received signal reaches the rising rate detection start voltage Va that is larger than the previous maximum noise Vn by a predetermined amount (point A at the start coordinate of the rising rate detection), the received signal reaches a predetermined amount relative to the saturation voltage Vs of the previous received signal. Since the rate of amplitude increase (slope between the two coordinates) until reaching the small rising rate detection end voltage Vb (end point B of the rising rate detection) is detected as the rising rate, the density of the fluid to be measured changes and reception is performed. Even if the start timing changes or the received signal strength or the rise rate changes, an accurate rise rate can always be detected by following the change.
[0063]
Further, since the reception determination value Vth is calculated and set by the above equation 8, even if the density of the fluid to be measured changes and the reception signal strength or the rise rate changes, the envelope signal We is kept while following the changes. Can be set at a place where the change of the ultrasonic wave is large, and a stable measurement of the ultrasonic propagation time with little fluctuation can be performed. By setting the reception determination value Vth to the rise rate detection end voltage Vb, it is also possible to reduce arithmetic processing and the like with a simpler configuration. In these cases, the delay time tr can be estimated with high accuracy by dividing the reception determination value Vth or the rise rate detection end voltage Vb by the rise rate (tan θ).
[0064]
Also, by setting the zero level after the ultrasonic reception signal reaches the rising rate detection end voltage Vb as the reception determination value, the correction amount (delay time) tr 'becomes an integral multiple of the period of the reception signal. By calculating the correction amount tr 'by using the above equation 10 using such a relationship, it is possible to more accurately measure the ultrasonic propagation time. Further, the present invention can be easily applied to a high-accuracy type propagation time measurement using a zero-cross point of a received signal as a reception timing.
[0065]
The above embodiment shows the basic concept of the present invention and a typical example thereof, and is not limited to these. Needless to say, various modifications can be made.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a fluid sensor to which an ultrasonic propagation time measuring device according to the present invention is applied.
FIG. 2 is a diagram showing a basic configuration of an ultrasonic propagation time measuring device according to the present invention.
FIG. 3 is a time chart showing measurement of an ultrasonic propagation time.
FIG. 4 is a diagram illustrating a configuration of a reception sensitivity adjustment unit.
FIG. 5 is a diagram showing another configuration of the receiving sensitivity adjusting means.
FIG. 6 is a diagram illustrating a configuration of an amplitude rise rate detection unit.
FIG. 7 is a time chart showing another measurement of the ultrasonic propagation time.
FIG. 8 is a flowchart showing measurement (signal processing) of an ultrasonic propagation time.
FIG. 9 is a diagram for explaining conventional ultrasonic wave propagation measurement.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Measuring tube, 2a, 2b ... Ultrasonic transceiver, 3 ... Ultrasonic propagation time measuring circuit, 4 ... Operation circuit, 31 ... Reception sensitivity adjustment circuit, 32 ... Envelope detection opening degree, 33 ... Rise rate detection circuit, 34: reception determination value setting circuit, 35: reception timing detection circuit, 36: reception timing correction circuit, 311: control amount adjustment circuit, 312: control voltage storage circuit (memory), 313: voltage control amplifier, 314: Vn control amount Adjustment circuit (sensitivity control amount adjustment circuit before reception), 315 ... tan θ control amount adjustment circuit (sensitivity adjustment circuit after reception), 316 control voltage generation circuit, 331 ... Rise start timing detection circuit, 332 ... Rise end timing detection circuit 333: time measurement counter, Wt: transmission wave, Wr: reception wave, We: envelope of reception wave, td1: reception timing in the forward direction of flow, td2 Reception timing in the reverse direction of flow, Vth: reception determination value (threshold), Vs: reception signal saturation voltage, Va: start voltage of rise rate detection, Vb: end voltage of rise rate detection, Vn: noise signal maximum voltage, Vg ... Sensitivity adjustment voltage, ΔVe: rise rate detection voltage, Δts: rise rate detection time, ts: reception start timing, ta: rise rate detection start timing, tb: rise rate detection end timing, td: reception timing, tz: zero-cross reception judgment Type of reception timing, tr ... (arrival time) correction amount, tr '... Zero-cross reception determination type (arrival time) correction amount

Claims (9)

An ultrasonic wave having a pair of ultrasonic transceivers provided at acoustically opposed positions and measuring a propagation time until an ultrasonic wave transmitted from one ultrasonic transceiver is received by the other ultrasonic transceiver A transit time measuring device,
Amplitude rise rate detection means for detecting an increase rate of the amplitude of the ultrasonic reception signal as an amplitude rise rate,
Reception sensitivity adjustment means for adjusting the reception sensitivity so that the ultrasonic reception signal has a predetermined amplitude rise rate,
After adjusting the reception sensitivity, arrival time detection means for detecting when the ultrasonic reception signal reaches a predetermined reception determination value as the arrival time of the reception wave,
Delay time calculation means for calculating a delay time from the start of reception until the ultrasonic reception signal reaches the reception determination value,
Ultrasonic propagation time calculating means for calculating the ultrasonic propagation time by subtracting the delay time from the arrival time,
An ultrasonic propagation time measuring device comprising: The reception sensitivity adjusting means adjusts the reception sensitivity so that the noise level becomes a predetermined level that does not exceed the reception determination value until a predetermined time has elapsed from the transmission of the ultrasonic wave. 2. The ultrasonic propagation time measuring device according to claim 1, wherein the reception sensitivity is adjusted so that the sound wave reception signal has the predetermined amplitude rise rate. 3. The ultrasonic propagation time measuring device according to claim 2, wherein the predetermined time is an ultrasonic propagation time measured last time. 4. The ultrasonic propagation time measuring apparatus according to claim 2, wherein said reception sensitivity adjustment means adjusts the current reception sensitivity based on a noise level or an amplitude rise rate detected last time. The amplitude rise rate detection means sets a time point at which the ultrasonic reception signal reaches a first level that is larger by a predetermined amount than the previous maximum noise as a start coordinate of the amplitude rise rate detection, and saturates the ultrasonic reception signal. A time when the ultrasonic reception signal reaches a second level smaller by a predetermined amount than a level is set as an end coordinate of the amplitude rise rate detection, and a gradient between the start coordinate and the end coordinate is detected as the amplitude rise rate. The ultrasonic propagation time measuring device according to any one of claims 1 to 4, wherein: The arrival time detecting means sets the second level as the reception determination value,
The ultrasonic propagation time measuring device according to claim 5, wherein the delay time calculating means calculates the delay time by dividing the second level by the amplitude rise rate. The arrival time detecting means determines a difference between the second level and the first level as a time from when the ultrasonic reception signal reaches the first level to when the ultrasonic reception signal reaches the second level and the ultrasonic wave. Divide by twice the frequency of the value, the value of the odd multiple of the result, the value added to the first level or the value subtracted from the second level, as the predetermined reception determination value,
The ultrasonic propagation time measuring device according to claim 5, wherein the arrival time detecting means calculates the delay time by dividing the reception determination value or the second level by the amplitude rise rate. The arrival time detection means, the reception level is zero level after the ultrasonic reception signal exceeds the second level, the reception determination value,
The delay time calculating means calculates an integer value of a value obtained by multiplying the time from when the ultrasonic reception signal reaches the first level to when the ultrasonic reception signal reaches the second level by the frequency of the ultrasonic wave, The ultrasonic propagation time measuring apparatus according to claim 5, wherein the delay time is calculated by multiplying an integer value by a time corresponding to one cycle of the ultrasonic reception signal. An ultrasonic propagation time measurement device according to any one of claims 1 to 8,
A fluid sensor, wherein at least one of a flow velocity, a flow rate, and a density of the measured fluid is measured based on an ultrasonic propagation time in the measured fluid measured by the ultrasonic propagation time measuring device.

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