JP2005009893A - Ultrasonic fluid measuring device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To precisely measure a propagation time t1' (t2') used for measurement of the concentration (density) and flow rate (flow velocity) of a fluid to be measured. <P>SOLUTION: Receipt of a receiving wave S3 of a predetermined period from the receiving start of the receiving head wave S1 of ultrasonic waves transmitted from one of a pair of echo sound transducers is detected. The zero cross frequency Sc of receiving waves W" on and after it is counted, and when it reaches a predetermined frequency (Nz=5→0), a receiving timing td is determined. A predetermined time trl (tr2) from the receiving start of the head wave S1 to the receiving timing determination td is subtracted from a transmitting and receiving time t1 (t2) from the ultrasonic wave transmitting start t0 to the receiving timing determination td to determine the propagation time t1' (t2'). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００１６】
【数２】

ここで、（１），（２）式の右辺第１項ｔ１’，ｔ２’は、図１０に示すように、超音波送受波器２ａ，２ｂ間の純粋な超音波伝播時間である。Ｌｍは超音波送受波器２ａ，２ｂ間の距離、Ｃｇは被測定流体の音速、Ｖｇは被測定流体の流速、θは被測定流体の流れ方向と超音波の伝播方向との成す角度である。
【００１７】
そして、流速算出の式では、伝播時間ｔ１，ｔ２から遅れ時間ｔｒ１，ｔｒ２及び検出誤差時間ｔｅを差し引いて超音波送受波器２ａ，２ｂ間の純粋な超音波伝播時間ｔ１’（＝ｔ１−ｔｒ１−ｔｅ），ｔ２’（＝ｔ２−ｔｒ２−ｔｅ）から流速Ｖｇを求める必要がある。
【００１８】
【数３】

また流量Ｑｖは、次式に示す通り、被測定流体の流れる測定管（空間）の断面積Ａ、配管内の流速分布補正係数Ｋ、および（３）式で算出した被測定流体の流速Ｖｇを積算することにより算出する。
【００１９】
【数４】

このように流速Ｖｇ、流量Ｑｖの算出においては、（３）式おいて被測定流体の順方向及び逆方向の伝播時間の差分をとっているため、空気など音速が遅い測定流体の場合や、遅れ時間ｔｒ１，ｔｒ２、検出誤差時間ｔｅに対し、伝播時間ｔ１’，ｔ２’が十分に大きい場合に限れば、実用上無視可能であるが、水素ガス等、音速が早い場合や、小型の流量計のように超音波伝播距離Ｌｍが短い場合には流速および流量計測精度に及ぼす影響は無視できないことが理解できる。
【００２０】
また、被測定流体の密度（濃度）ρを求める場合には、まず次式の通りにして音速Ｃｇを算出する。
【００２１】
【数５】

そして、次式により密度ρを算出する。
【００２２】
【数６】

ここで、γは比熱比、Ｒはガス定数、Ｔはガス温度（Ｋ）である。このように密度ρを求める（６）式では、音速Ｃｇの２乗Ｃｇ^２を代入することから遅れ時間ｔｒ１，ｔｒ２の影響および検出誤差時間ｔｅの影響は更に大きくなる関係にある。
【００２３】
以上より、特に音速Ｃｇが早い流体が測定対象である場合は、受信タイミングの正確な検知が、流速及び流量計測精度を確保するために大変重要な処理と言える。
【００２４】
本発明は上記問題点に鑑みてなされたもので、被測定流体の密度変化のみならず、電磁的あるいは音響的ノイズが重畳するようなこともあっても、正確な受信開始タイミングを検出して、被測定流体の濃度（密度）および／または流量（流速）の計測に用いる伝播時間を、精度良く測定することを目的とする。
【００２５】
【課題を解決するための手段】
そのため本発明では、超音波受信波の先頭から予め定められた周期目で受信され始めたことを検知し、この検知以降の受信波のゼロクロス回数を計数して、予め定めた回数に達した時点で受信タイミングを決定し、超音波送信開始時点から受信タイミング決定までの送受信間時間から、受信先頭波の受信開始時点から受信タイミングまでの時間を差し引いて超音波送受波間距離の伝播時間を求めるようにした。
【００２６】
【発明の効果】
本発明によれば、電磁ノイズや音響ノイズが重畳した超音波受信波に対し、受信開始を検知した後、受信波の振幅レベル（Ｓ）とノイズの振幅レベル（Ｎ）との比（Ｓ／Ｎ比）が最大値を迎える所定時間領域までゼロクロス回数を計数して待ち、所定のゼロクロス回数に達した時点を受信タイミングとして送受信間時間を求めることにより、ノイズ重畳によるゼロクロスタイミングの揺らぎが小さく、正確で純粋な超音波伝播時間を計測することができる。これにより精度の良い伝播時間が計測でき、この結果として精度の良い濃度（密度）および／または流量（流速）の計測が可能となる。
【００２７】
【発明の実施の形態】
以下、図面に基づき、本発明の実施形態について説明する。
図１は、超音波式流体計測装置の全体構成図である。なお、図中のＬｍは超音波送受波器（超音波振動子）２ａ，２ｂ間の距離、Ｃｇは被測定流体の音速、Ｖｇは被測定流体の流速、θは被測定流体の流れ方向と超音波の伝播方向との成す角度である。
【００２８】
被測定流体が流れる測定管（空間）１ｃを挟んで、一対の超音波送受波器２ａ，２ｂを音響的に対向する位置関係に所定の距離を隔てて配設している。具体的には図示の通り、測定管１ｃに設けられた超音波送受波器２ａ，２ｂが互いに向かい合うように斜めに配置されている。この一方の超音波送受波器２ａから超音波を送信して、被測定流体中で伝播した超音波を他方の超音波送受波器２ｂによって受信する（超音波透過型）。
【００２９】
そして、超音波伝播時間測定回路３により超音波送受波器２ａ，２ｂ間の超音波伝播時間ｔ１，ｔ２を流れの順方向（２ａ→２ｂ）と逆方向（２ｂ→２ａ）とでそれぞれ求める。なお図示しないが、一対の超音波送受波器２ａ，２ｂを被測定流体の流れ方向に所定の間隔を隔てて設け、一方の超音波送受波器２ａから送信した超音波を被測定流体中で伝播させ、測定管１ｃに反射させて他方の超音波送受波器２ｂにより受信するようにしてもよい（超音波反射型）。
【００３０】
図２は、超音波式流体計測装置の構成図であり、特に超音波伝播時間測定回路３の構成を示す図である。図３は、超音波伝播時間ｔ１，ｔ２の計測の説明図であり、横軸は時間、縦軸は各信号であり、Ｗｒ”は受信波（受信信号）、Ｓｒは受信検知信号、Ｇｒは感度調整電圧、Ｓｚは受信ゼロクロス信号、Ｓｃはゼロクロス計数信号、Ｓｄは受信タイミング決定信号を示している。なお、流量計においては、被測定流体の順方向および逆方向の２方向の伝播時間を計測するため、通常は時分割で処理するが、伝播時間計測の処理手順は全く同じであるため、ここでは時分割にかかるところの説明は省略する。
【００３１】
始めに送受信方向切替スイッチ３ｂによって、例えば一方の超音波送受波器２ａを送信側に選択した場合には、送信駆動回路３ａの生成する送信駆動信号Ｗｔが超音波送受波器２ａに伝達され、電気−音響変換されて被測定流体中に超音波が送信される。そして、送信駆動回路３ａは、送信駆動信号Ｗｔを生成すると同時に、超音波が送受波器２ａ，２ｂ間の距離Ｌｍを伝播する時間ｔ１，ｔ２の計数開始信号（ｓｔａｒｔ信号）を送受信間時間計数回路（タイムカウンタ）３ｆに対して出力する（図３のｔ０時点）。
【００３２】
また、送受信方向切替スイッチ３ｂによって受信側に選択された他方の超音波送受波器２ｂでは、超音波送受波器２ａから送信され被測定流体中を伝播した超音波を受信し、音響−電気変換された受信波Ｗｒ”が受信増幅回路３ｃへ入力される。
【００３３】
この受信増幅回路３ｃでは、受信波Ｗｒ”が、超音波信号以外の不要なノイズ信号を除去するためのフィルター回路（図示しない）を介して、次に述べる受信検知回路３ｄに必要な振幅レベルまで増幅し出力される。なお、この時の増幅率は、受信検知回路３ｄから入力される感度調整電圧Ｇｒで調整されるように構成してある。
【００３４】
受信検知回路３ｄでは、初期増幅電圧（低感度電圧）Ｇ０を出力して、超音波送受波器２ｂが受信波Ｗｒ”の先頭から予め定められた周期目（図３では３周期目（Ｓ３））で受信され始めたことを検知して、受信検知信号Ｓｒを出力する。なお、所定周期目は、受信波Ｗｒ”がしきい値Ｖｔｈに達した場合の周期目に設定する。
【００３５】
また、これと同時に感度調整電圧Ｇｒを受信増幅回路３ｃの増幅率を増大する電圧（高感度電圧）Ｇ１に切り替える（第３図−（４））。これにより受信検知（Ｓ３）以降の受信波Ｗｒ”は、受信信号飽和振幅レベルＶｓである振幅飽和領域まで増幅されて略矩形波形となる（第３図−（２））。これは、振幅が飽和振幅レベル（所定振幅レベル）Ｖｓを越えた時の受信信号Ｗｒ”を飽和振幅レベルＶｓにホールドするためである。
【００３６】
更に受信波Ｗｒ”は、次段の受信タイミング決定回路３ｅのカウンタ入力仕様に合わせて、例えば０−５Ｖのロジック信号へ変換される（第３図−（５）受信ゼロクロス信号Ｓｚ）。
【００３７】
ここで、受信検知回路３ｄおよび受信タイミング決定回路３ｅの具体例を図５において説明する。
受信検知回路３ｄでは、受信波Ｗｒ”は電圧コンパレータ３ｄ１を用いることで、ゼロレベルＬｚと比較されてロジック信号に変換される。これにより受信ゼロクロス信号Ｓｚを出力する。一方、受信検知信号Ｓｒも電圧コンパレータ３ｄ２によって、しきい値Ｖｔｈと比較されてロジック信号へ変換される。これにより受信検知信号Ｓｒを出力する。
【００３８】
また、受信タイミング決定回路３ｅでは、受信ゼロクロス信号Ｓｚはゲート３ｅ２（またはアナログスイッチ素子）にて信号伝達が制御されている。このゲート３ｅ２は、フリップフロップ素子３ｅ１で生成される信号で制御され、その制御信号は送信駆動信号Ｗｔでゲートを閉（Ｒｅｓｅｔ）に、そして受信検知信号Ｓｒでゲートを開（Ｓｅｔ）にするように設計されている。そして、受信検知以降のゼロクロス信号がダウンカウンタ３ｅ３に入力され、設定したゼロカウント回数計数して受信タイミング信号Ｓｄを出力する。
【００３９】
これにより、受信タイミング決定回路３ｅでは入力された受信ゼロクロス信号Ｓｚに対して、受信検知された以降、つまり受信検知信号Ｓｒ以降のゼロクロス信号Ｓｃを計数し、例えば受信検知以降のゼロクロス回数（カウント値）Ｎｚ＝５と予め設定されている場合は、第３図（６）に示すように、ゼロクロス計数信号Ｓｃが入力される毎にカウント値Ｎｚを−１（ダウンカウント）し、カウント値Ｎｚがゼロに達した時点（Ｎｚ＝０）で受信タイミング決定信号Ｓｄを出力する。この処理により受信検知（Ｓ３）以降の所定ゼロクロス回数（５回）で受信タイミングｔｄを決定する。
【００４０】
なお、ここではゼロクロス回数Ｎｚ＝５としたが、これは先頭波から数えると８周期目（第３図−Ｓ８）に相当する。一般的に圧電素子を振動板に貼り付けた形式の超音波送受波器では、送信駆動信号Ｗｔが印加されてから、徐々に振動振幅が増し、概略５周期目以降では最大振幅で振動する特性にある。従って、ゼロクロス回数Ｎｚは５以上を設定することで良好な結果を得ることができる。
【００４１】
ここで、再度図２を参照して説明する。
前述の受信タイミング決定回路３ｅからの受信タイミング信号Ｓｄは、送受信経過時間計数回路３ｆに入力される。この送受信経過時間計数回路３ｆには、送信駆動回路３ａの送信駆動信号Ｗｔも入力される。
【００４２】
そして、送受信経過時間計数回路３ｆは、受信タイミング決定回路３ｅからの受信タイミング信号Ｓｄを受けて、送信駆動回路３ａの送信駆動信号Ｗｔの出力と共に開始していた時間計数を停止させる。そして、この時間計数結果ｔ１（ｔ２）は、超音波伝播時間算出回路３ｇに入力される。
【００４３】
超音波伝播時間算出回路３ｇは、時間計数結果ｔ１（ｔ２）から、受信感度調整電圧Ｇｒ及びしきい値Ｖｔｈで定められた受信検知時間ｔａとゼロクロス計数時間ｔｂ（図３参照）とを差し引いて、本来求めたい超音波伝播時間ｔ１’（＝ｔ１−ｔｒ１−ｔｅ），ｔ２’（＝ｔ２−ｔｒ２−ｔｅ）を出力する。なお、検出誤差時間ｔｅは、本発明の効果にて省略することができる。
【００４４】
ここで前述の（３）式の遅れ時間ｔｒ１（ｔｒ２）は、次式の通り、受信検知時間ｔａと、ゼロクロス計数時間ｔｂとの和により表される。
【００４５】
ｔｒ１（ｔｒ２）＝ｔａ＋ｔｂ ・・・（７）
ここで、受信検知時間ｔａは、受信波Ｗｒ”の先頭波（Ｓ１）の受信時点から受信検知時点（Ｓ３）までの時間である。ゼロクロス計数時間ｔｂは、受信検知時点（Ｓ３）から受信タイミング決定時点（ｔｄ）までの時間である。このゼロクロス計数時間ｔｂは、次式の通り、ゼロクロス回数（所定値）Ｎｚを超音波周波数ｆｕ（Ｈｚ）で割ることで求める。
【００４６】
ｔｂ＝Ｎｚ／ｆｕ （ｓｅｃ） ・・・（８）
以上により超音波伝播時間ｔ１’（ｔ２’）の計測が実行されることになる。すなわち、図５に示す通り、超音波送信開始時点ｔ０から受信タイミングｔｄまでの送受信間時間ｔ１（ｔ２）から、受信先頭波（Ｓ１）の受信開始時点から受信タイミング決定ｔｄまでの時間ｔｒ１（ｔｒ２）を差し引いて（ｔ１−ｔｒ１，ｔ２−ｔｒ２）、超音波送受波間距離Ｌｍの伝播時間ｔ１’（ｔ２’）を求める。
【００４７】
そして、流量／濃度演算回路４において、（３）式及び（４）式を用いて（（３）式及び（５）式においてｔ１−ｔｒ１−ｔｅ＝ｔ１’、ｔ２−ｔｒ２−ｔｅ＝ｔ２’とする。但し、ｔｅ＝０）被測定流体の流量値Ｑｖを、（５）式及び（６）式を用いて密度（濃度）値をそれぞれ算出して出力する。なお、（３）式と（５）式とでは、ｔ１（ｔ２）とｔｒ１（ｔｒ２）とを分離して表しているが、超音波伝播時間算出回路３ｇで得た超音波伝播時間ｔ１’（ｔ２’）は既に遅れ時間ｔｒ１（ｔｒ２）を差し引いているので、そのまま括弧内第１項および第２項の分母にｔ１’，ｔ２’を代入して流速Ｖｇ、音速Ｃｇを求めることになる。
【００４８】
次に、この一連の信号処理手順をコンピュータシステムで実施する場合の実施形態について、図８のフローチャートを用いて説明する。なお、後述する別の実施形態については、点線で囲んでおり、ここでの説明は省略する。
【００４９】
ステップ１００（図には「Ｓ１００」と示す。以下同様）では、電源を投入し、ステップ１０１では、ゼロクロス回数Ｎｚやしきい値Ｖｔｈ等の初期値が設定される。
【００５０】
ステップ１０２では、超音波送受波器２ａ，２ｂの送受信方向を切り替え、ステップ１０３では、送信駆動回路３ａから超音波の送信駆動信号Ｗｔを出力すると共に送受信経過時間計数回路３ｆの送受信間時間（ｔ０からの経過時間）の計数を開始する。
【００５１】
ステップ１０４では、超音波送受波器２ｂが受信する受信波Ｗｒ”が、先頭波から所定周期目の受信波（Ｓ３）を検知したか否か、すなわち受信検知タイミングを判断する。受信検知した場合には、ステップ１０５へ進む。一方、受信検知されない場合には、受信検知されるまで待つ。
【００５２】
ステップ１０５では、受信増幅回路３ｃの増幅率を上げて（Ｇ０→Ｇ１）、得られた受信波Ｗｒ”をデジタル変換して読み込み、ステップ１０６では、ゼロクロス回数Ｎｚをカウントして、ステップ１１０へ進む。
【００５３】
ステップ１１０では、ゼロクロス回数に達するまで計数を継続する。そして、ゼロクロス回数に達した（Ｎｚ＝０）時は、ステップ１１１で送受信経過時間計数回路３ｆの送受信間時間計数を停止して、この時の計数値ｔ１（ｔ２）を読み取る。
【００５４】
次に、ステップ１１２では、送受信間時間ｔ１（ｔ２）から、先頭波（Ｓ１）の受信時点から受信検知（Ｓ３）時点までの遅れ時間ｔａと、受信検知（Ｓ３）時点からゼロクロス回数に要した時間ｔｂとの和、すなわち遅れ時間ｔｒ１（ｔｒ２）を差し引いて（ｔ１−ｔｒ１，ｔ２−ｔｒ２）、超音波伝播時間ｔ１’（ｔ２’）を算出する。
【００５５】
ステップ１１３では、求めた超音波伝播時間ｔ１’（ｔ２’）から前述の（３）式〜（６）式を用いて流量Ｑｖおよび濃度（密度：ρ）を算出し出力する。以降、送受信方向の切り替え処理まで戻り、一連の処理を繰り返し実行する。
【００５６】
本実施形態によれば、被測定流体が流れる空間（測定管１ｃ）を挟んで一対の超音波送受波器２ａ，２ｂを音響的に対向する位置関係に所定の距離を隔てて設け、この超音波送受波器の一方２ａから送信した超音波が被測定流体中で伝播して他方の超音波送受波器２ｂによって受信されるまでの超音波伝播時間を流れの順方向と逆方向とでそれぞれ求めて、これらの超音波伝播時間に基づいて被測定流体の流量Ｑｖあるいは濃度（密度：ρ）を求める超音波式流体計測装置において、超音波受信波の先頭（Ｓ１）から予め定められた周期目（Ｓ３）で受信され始めたことを検知する受信検知手段（ステップ１０４，受信検知回路３ｄ）と、受信検知（Ｓ３）以降の受信波のゼロクロス回数Ｓｃを計数して、予め定めた回数（Ｎｚ＝５）に達した時点（Ｓ８）で受信タイミングｔｄを決定する受信タイミング決定手段（ステップ１１０，受信タイミング決定回路３ｅ）と、超音波送信開始時点ｔ０から受信タイミングｔｄまでの送受信間時間ｔ１（ｔ２）から、受信先頭波（Ｓ１）の受信開始時点から前記受信タイミング決定ｔｄまでの時間ｔｒ１（ｔｒ２）を差し引いて超音波送受波間距離Ｌｍの伝播時間ｔ１’（ｔ２’）を求めるようにした超音波伝播時間算出手段（ステップ１１２，超音波伝播時間算出回路３ｇ）と、を備えた。このため、電磁ノイズや音響ノイズが重畳した超音波受信波に対し、受信開始を検知した後、受信波の振幅レベル（Ｓ）とノイズの振幅レベル（Ｎ）との比（Ｓ／Ｎ比）が最大値を迎える所定時間領域までゼロクロス回数を計数して待ち、所定のゼロクロス回数に達した時点を受信タイミングとして送受信間時間を求めることにより、ノイズ重畳によるゼロクロスタイミングの揺らぎが小さく、正確で純粋な超音波伝播時間を計測することができる。これにより精度の良い伝播時間が計測でき、この結果として精度の良い濃度（密度）および／または流量（流速）の計測が可能となる。
【００５７】
また本実施形態によれば、少なくとも２つ以上の振幅増幅率（Ｇ０，Ｇ１）が設定可能な受信波増幅手段（ステップ１０５，受信増幅回路３ｃ）と、この受信波増幅手段が受信検知（Ｓ３）以降の振幅増幅率Ｇｒを増加させ、振幅が所定振幅レベルＶｓを超えた時の受信波Ｗｒ”を所定振幅レベルＶｓにホールドする振幅レベルホールド手段と、を備える。このため、受信検知（Ｓ３）以降の振幅情報が不要となり、この替わりに正確な時間情報が要求される時間領域の受信波Ｗｒ”を高倍率（Ｇ１）に増幅し（矩形波化）、ゼロクロス前後の振幅変化率を高めることで、ノイズ重畳した場合の時間軸のずれを少なくすることができる。そして、精度の良い伝播時間が計測でき、この結果として精度の良い濃度（密度）や流量（流速）の計測が可能となる。
【００５８】
次に、干渉波が発生した場合における超音波伝播時間の誤計測防止の実施形態について図２，４，６〜８を用いて説明する。
まず受信波Ｗｒ”に多重反射等により干渉波が発生すると、図４の受信波Ｗｒ”のＳ７に示すように、振幅が一時的にしきい値Ｖｔｈより低下する。また干渉波発生以降においては、位相変化の影響により、先頭波との時間的関連性（ゼロクロス時刻は周期の整数倍）にずれが生じるため、干渉波の発生以降の受信タイミング決定ｔｄには計測誤差が発生するおそれがある。
【００５９】
そこで本実施形態では、ゼロクロス計数を実行しながら同時に受信波Ｗｒ”の包絡線Ｗｅの低下を検知する。そして、図４の受信波Ｗｒ”のＳ７に示すように包絡線Ｗｅの低下を検知した時には、その時のゼロクロス計数値（図４ではＮｚ＝１）を記憶しておき、次回のゼロクロス回数を次式で求めた回数へと変更する。
【００６０】
次回のゼロクロス回数＝現在のゼロクロス回数−（包絡線低下時のゼロクロス計数値＋１） ・・・（９）
例えば図４では、現在のゼロクロス回数は５であり（Ｎｚ＝５）、包絡線低下時のゼロクロス計数値は１であるため、（９）式から、次回のゼロクロス回数は３となる。そして、次回のゼロクロス計数は、受信検知以降のゼロクロス計数信号Ｓｃに示すように、回数が５から３へ変更される（Ｎｚ＝５→（３））ため、受信タイミングＳｄは干渉波の前のＳ６周期（ｔｄ’）で決定されることになり、計測誤差を回避できる。
【００６１】
続いて、本実施形態で追加した（図２の点線で囲んだ部分）干渉波検知回路３ｈおよびゼロクロス回数変更回路３ｉについて、図６及び図７を用いて説明する。
【００６２】
干渉波検知回路３ｈでは、図４の受信波Ｗｒ”の包絡線Ｗｅを生成するために、図６に示す通り、ピークホールド回路３ｈ２を用いて、受信波Ｗｒ”の周期毎の最大振幅値をホールドしている。そしてホールド値は、干渉による急峻な振幅低下に追従するために、周期毎の受信ゼロクロス信号Ｓｚでピーク値をサンプルするようにしている。
【００６３】
このサンプル信号は、単安定マルチバイブレータ３ｈ１により受信ゼロクロス信号Ｓｚを、ピークホールド時間に相当するパルス幅Ｐｗの信号として生成する。なお、この時のパルス幅Ｐｗの設定例は、次式の通り、１．２５を超音波周波数ｆｕ（Ｈｚ）で割って求める。
【００６４】
Ｐｗ＝１．２５／ｆｕ （ｓｅｃ） ・・・（１０）
この（１０）式の分子は次回のサンプルタイミングが、次回の振幅ピーク近辺のタイミングに整合させるための定数であり、１．２５は丁度次回の振幅ピークタイミングに相当する。このようにして生成した包絡線信号Ｗｅは、電圧コンパレータ３ｈ３に入力される。
【００６５】
この電圧コンパレータ３ｈ３は、包絡線信号Ｗｅと干渉波検知判定電圧（所定値）Ｖｈとを比較して、干渉波によるレベル低下があった（Ｗｅ＜Ｖｈ）場合、正パルスの干渉波検知信号Ｓｈを出力する。
【００６６】
また図７に示す通り、ゼロクロス回数変更回路３ｉは、ダウンカウンタ（ゼロクロス計数カウンタ）３ｅ３のカウント値（Ｄａｔａ）がデータレジスタ３ｉ１に入力され、干渉波検知信号Ｓｈが正パルスになった（干渉を検知）時のゼロクロスカウント値を記憶する。
【００６７】
続いてデジタルコンパレータ３ｉ２を用いて、記憶したゼロクロスカウント値がゼロに達しているか否かを判定し、もし達していない場合は、前述の（９）式を実行するためにデータレジスタ（メモリ）３ｉ１の記憶値に１を加え（加算器３ｉ３）、その結果をゼロクロス回数初期値Ｎｚｏから差し引く（減算器３ｉ４）。この結果はデジタルコンパレータ３ｉ２の判定結果で制御される切替スイッチ（セレクタ）３ｉ５で選択されて、次回のゼロクロス回数Ｎｚとして、ゼロクロス計数カウンタ３ｅ３へ入力される。
【００６８】
一方、干渉波が検出されずに正常にゼロクロス計数値Ｓｃがゼロに達した場合は、切替スイッチ３ｉ５にてゼロクロス回数初期値Ｎｚｏが次回のゼロクロス回数Ｎｚとなる。これにより干渉波が無くなった場合は、予め定めたゼロクロス回数に自動復帰させるようになっている。
【００６９】
以上により片方向の超音波伝播時間計測が実行されることになるが、これを被測定流体の流れの順逆双方向について交互に行い、双方向一対の超音波伝播時間ｔ１’（＝ｔ１−ｔｒ１−ｔｅ。但し、ｔｅ＝０），ｔ２’（＝ｔ２−ｔｒ２−ｔｅ。但し、ｔｅ＝０）を得る（図２参照）。そして、流量／濃度演算回路４において、（３）式及び（４）式を用いて（但し、各式においてｔ１＝ｔ１’、ｔ２＝ｔ２’として）被測定流体の流量値Ｑｖを、（５）式及び（６）式を用いて密度（濃度）値をそれぞれ算出して出力する。なお、（３）式と（５）式とでは、ｔ１（ｔ２）とｔｒ１（ｔｒ２）とを分離して表しているが、超音波伝播時間算出回路３ｇで得た超音波伝播時間ｔ１’，ｔ２’は既に遅れ時間ｔｒ１（ｔｒ２）を差し引いているので、そのまま括弧内第１項および第２項の分母にｔ１’，ｔ２’を代入して流速Ｖｇ、音速Ｃｇを求めることになる。
【００７０】
次に、干渉波が影響する場合における一連の信号処理手順をコンピュータシステムで実施する場合の実施形態について、図８のフローチャートを用いて説明する。なお、前述の実施形態におけるフローチャートと同じものについては、説明を省略する。
【００７１】
本実施形態では、干渉波が影響する場合においても伝播時間を正確に測定するために、干渉波の有無を判断し、ゼロクロス回数Ｎｚを変更する。
ステップ１００〜ステップ１０６では、前述の実施形態と同じである。ステップ１０６でゼロクロス回数Ｎｚをカウントした後に、ステップ１０７へ進む。
【００７２】
ステップ１０７では、干渉波検知回路３ｈが干渉波を検知したか否かを判断する。干渉波が検知された場合には、計測エラー処理を行うためステップ１０８へ進む。一方、干渉波が検知されなかった場合には、ステップ１１０へ進む。
【００７３】
ステップ１０８では、今回の計測データは確からしさが無いため破棄し、代わりに前回の計測データに置き換える等の処理を行う。
これに合わせて、次回の計測でも干渉波により計測エラーを生じる可能性があるため、ステップ１０９で、ゼロクロス変更回路３ｉにより、次回のゼロクロス回数を、今回干渉波を検出したゼロクロス設定回数以下に変更する。そして、ステップ１１１へ進み、前述の処理を行う。
【００７４】
一方、ステップ１０７で干渉波が検出されない場合は、ステップ１１０でゼロクロス回数に達するまで計数を継続する。そして、ゼロクロス回数に達した（Ｎｚ＝０）時は、ステップ１１１で送受信経過時間計数回路３ｆの送受信間時間計数を停止して、この時の計数値ｔ１を読み取る。
【００７５】
本実施形態によれば、受信タイミング決定手段（ステップ１１０，受信タイミング決定回路３ｅ）に設定するゼロクロス回数Ｎｚにおいて、受信検知（Ｓ３）以降の受信波Ｗｒ”の包絡線Ｗｅが干渉波によって所定振幅レベルＶｔｈより低下したタイミングを検知する干渉波検知手段（ステップ１０７，コンパレータ３ｉ２）と、この干渉波検知タイミングに基づき前記ゼロクロス回数Ｎｚを変更するゼロクロス回数変更手段（ステップ１０９，加算器３ｉ３，減算器３ｉ４）と、を備え、このゼロクロス回数変更手段は、現在のゼロクロス回数（Ｎｚ＝５）に達する以前に干渉波検出手段にて所定振幅レベルＶｓの低下が検出された場合（Ｓ７）、現在のゼロクロス回数（Ｎｚ＝５）から所定振幅レベル低下時（Ｓ７）のゼロクロス回数に１を加えた回数を差し引いて（（９）式）、次回計測時のゼロクロス回数に変更する（ステップ１０９）。このため、超音波送受波器間や被測定流体の測定管１ｃの内壁等に反射した反射波と、受信波との干渉によりゼロクロスのタイミングずれや消失が生じた場合、そのゼロクロスを計数する前に受信タイミングを決定するように変更することから、誤った伝播時間の計測を防止することができ、この結果として信頼性の高い濃度（密度：ρ）や流量Ｑｖ（流速Ｖｇ）計測が可能となる。
【図面の簡単な説明】
【図１】代表的な超音波流体計測装置の全体構成を説明する図
【図２】本発明の基本的構成の実施形態を説明する図
【図３】本発明の超音波伝播時間計測を説明する図
【図４】本発明の他の実施形態の超音波伝播時間計測を説明する図
【図５】本発明の受信検知回路および受信タイミング決定回路を示す図
【図６】本発明の干渉波検知回路を示す図
【図７】本発明のゼロクロス回数変更回路を示す図
【図８】本発明の信号処理のフローチャート
【図９】従来の超音波伝播時間計測を説明する図
【図１０】従来の超音波伝播時間計測の問題点を説明する図
【符号の説明】
１ｃ 測定管
２ａ、２ｂ 超音波送受波器
３ 超音波伝播時間測定回路
３ａ 送信駆動回路
３ｂ 送受信方向切替スイッチ
３ｃ 受信増幅回路
３ｄ 受信検知回路
３ｄ１、３ｄ２ 電圧コンパレータ
３ｅ 受信タイミング決定回路
３ｅ１ フリップフロップ素子
３ｅ２ ゲート
３ｅ３ ダウンカウンタ
３ｆ 送受信経過時間計数回路
３ｇ 超音波伝播時間算出回路
３ｈ 干渉波検知回路
３ｈ１ 単安定マルチバイブレータ
３ｈ２ ピークホールド回路
３ｈ３ 電圧コンパレータ
３ｉ ゼロクロス回数変更回路
３ｉ１ データレジスタ
３ｉ２ コンパレータ
３ｉ３ 加算器
３ｉ４ 減算器
３ｉ５ 切替スイッチ
４ 流量／濃度演算回路[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a fluid measuring device (flow meter, densitometer) using ultrasonic propagation time in a fluid to be measured.
[0002]
[Prior art]
Conventionally, as disclosed in Patent Document 1 or Patent Document 2, it is known to detect the propagation time of ultrasonic waves and measure the flow rate (flow velocity) and gas concentration (density) of a fluid to be measured. Hereinafter, a description will be given with reference to FIG.
[0003]
FIG. 9 is an explanatory diagram of conventional ultrasonic propagation time measurement, where the horizontal axis indicates time t, the vertical axis indicates a transmission drive wave (signal) Wt, a flow forward reception wave Wr1, and a flow reverse reception wave. Wr2 is shown. In the figure, t1 and t2 are propagation times between the ultrasonic transducers 2a and 2b (see FIG. 1), tr1 and tr2 are delay times from the reception time of the head wave of the ultrasonic reception wave to the reception timing, and td1. , Td2 are reception timings, Wth is a reception determination threshold value, and Vp is a maximum amplitude voltage.
[0004]
A pair of ultrasonic transducers 2a and 2b are provided in a positional relationship facing each other across a space in which the fluid to be measured flows, with a predetermined distance Lm therebetween, and an ultrasonic wave is transmitted from one ultrasonic transducer 2a. The ultrasonic wave transmitted and propagated in the fluid to be measured is received by the other ultrasonic transducer 2b (see FIG. 1).
[0005]
Then, ultrasonic propagation times t1 and t2 between the ultrasonic transducers 2a and 2b are respectively obtained in the forward direction and the reverse direction of the flow, and based on the ultrasonic propagation times t1 and t2, the flow rate of the fluid to be measured or Calculate the concentration. In the measurement of the ultrasonic propagation times t1 and t2, it is important how accurately and precisely the reception timings td1 and td2 are detected.
[0006]
As a reception timing detection method, when the amplitude values of the reception waves Wr1 and Wr2 shown in FIG. 9 exceed a predetermined reception determination threshold value Vth determined with reference to the maximum amplitude value Vp of the previous reception wave and the like. Detection of reception of ultrasonic waves, detection of zero-crossing times td1 and td2 of a plurality of received waves immediately after or before and reception of propagation times t1 and t2 from the ultrasonic transmission start time t0 to zero-crossing times td1 and td2 of the received waves There is a method of measuring the timings td1 and td2.
[0007]
According to this method, when the received signal intensity varies in a similar manner due to a change in temperature or pressure of the fluid to be measured or deterioration of the ultrasonic transducer (indicated by the broken line of the received wave Wr1 in FIG. 9), it depends on the degree of variation. This is an effective propagation time measurement method because it automatically adjusts the reception determination threshold value Vth and detects a zero-crossing point at which the temporal positional relationship does not change even if the reception amplitude changes.
[0008]
[Patent Document 1]
JP 2002-323361 A
[Patent Document 2]
JP 2002-162269 A
[0009]
[Problems to be solved by the invention]
However, when electromagnetic noise, flow sound of the fluid to be measured, or acoustic noise from a sound generator other than the ultrasonic transducer is superimposed on the received signal, the received wave is shown as a received wave Wr1 (Wr2) in FIG. Thus, the received waveform Wr ′ is obtained by adding the noise waveform Wn and the ultrasonic reception waveform. For this reason, it is desired to detect the reception timing at the point P1, which is the zero-cross point of the original received wave, but the reception timing td is detected at the zero-cross point P2 where noise is superimposed, and a detection error time te occurs.
[0010]
Further, the detection error time te changes not only with the noise amplitude but also with the noise frequency and phase. For example, if the phase of the noise waveform Wn in FIG. 10 is inverted by 180 deg, the undulation of the received waveform Wr ′ occurs. Inverted, and this time, the zero-crossing time point P2 is detected earlier than the zero-crossing time point P1 of the original received wave, so that the obtained propagation time t1 (t2) is shortened. Thus, the amount and polarity of the detection error time te are not necessarily uniform.
[0011]
Therefore, there is a problem that the accuracy of the propagation time cannot be maintained / improved only by measuring the ultrasonic propagation time as described in Patent Document 1 and Patent Document 2.
[0012]
Further, as means for solving such a problem, generally, before detecting the reception timing, a method of removing noise through a band pass filter set so that only the reception wave passes, or conversely, the reception waveform Wr ′. A method of extracting a noise component from a filter and using it as a detection criterion for detecting a zero cross level, or a method of canceling the noise component by adding an anti-phase waveform of the extracted noise to the received waveform Wr ′ can be considered.
[0013]
However, in these methods, not only the circuit is complicated, but in the case of a band-pass filter, for example, the frequency close to the frequency of the received wave cannot be removed, whereas in the method of canceling with the antiphase waveform of noise, Amplitude and time errors occur during extraction and conversion to the opposite phase, so it is not easy to completely cancel the noise component. As described above, any method leaves a problem.
[0014]
Here, the relationship between the propagation time, concentration (density), flow velocity, and flow rate calculation formula will be described. The symbols used here are the same as those in FIG.
The time between transmission and reception in the forward direction of the measurement fluid (ultrasonic transducer 2a → 2b) detected by the ultrasonic propagation time measuring circuit 3 (see FIG. 1) and the reverse direction of the flow (ultrasonic transducer 2b → 2a) ) And the transmission / reception time have the following relationship.
[0015]
[Expression 1]

[0016]
[Expression 2]

Here, the first terms t1 ′ and t2 ′ on the right side of the equations (1) and (2) are pure ultrasonic propagation times between the ultrasonic transducers 2a and 2b, as shown in FIG. Lm is the distance between the ultrasonic transducers 2a and 2b, Cg is the sound velocity of the fluid to be measured, Vg is the flow velocity of the fluid to be measured, and θ is the angle formed by the flow direction of the fluid to be measured and the propagation direction of the ultrasonic waves. .
[0017]
In the flow velocity calculation formula, the pure ultrasonic propagation time t1 ′ (= t1−tr1) between the ultrasonic transducers 2a and 2b is obtained by subtracting the delay times tr1 and tr2 and the detection error time te from the propagation times t1 and t2. -Te), t2 '(= t2-tr2-te), it is necessary to obtain the flow velocity Vg.
[0018]
[Equation 3]

Further, as shown in the following equation, the flow rate Qv is the cross-sectional area A of the measurement pipe (space) through which the fluid to be measured flows, the flow velocity distribution correction coefficient K in the piping, and the flow velocity Vg of the fluid to be measured calculated by the equation (3). Calculate by integrating.
[0019]
[Expression 4]

Thus, in the calculation of the flow velocity Vg and the flow rate Qv, the difference between the forward and reverse propagation times of the fluid to be measured in equation (3) is taken, so in the case of a measurement fluid with a slow sound velocity such as air, It is practically negligible as long as the propagation times t1 ′ and t2 ′ are sufficiently large with respect to the delay times tr1 and tr2 and the detection error time te. It can be understood that when the ultrasonic propagation distance Lm is short, such as a meter, the influence on the flow velocity and flow rate measurement accuracy cannot be ignored.
[0020]
Further, when obtaining the density (concentration) ρ of the fluid to be measured, first, the sound velocity Cg is calculated according to the following equation.
[0021]
[Equation 5]

Then, the density ρ is calculated by the following equation.
[0022]
[Formula 6]

Here, γ is a specific heat ratio, R is a gas constant, and T is a gas temperature (K). Thus, in the equation (6) for obtaining the density ρ, the square Cg of the sound velocity Cg ² Therefore, the influence of the delay times tr1 and tr2 and the influence of the detection error time te are further increased.
[0023]
From the above, it can be said that the accurate detection of the reception timing is a very important process for ensuring the flow velocity and the flow rate measurement accuracy, particularly when the fluid having a high sound velocity Cg is the measurement object.
[0024]
The present invention has been made in view of the above problems, and detects not only the density change of the fluid to be measured but also the accurate reception start timing even when electromagnetic or acoustic noise may be superimposed. An object of the present invention is to accurately measure the propagation time used for measuring the concentration (density) and / or flow rate (flow velocity) of the fluid to be measured.
[0025]
[Means for Solving the Problems]
Therefore, in the present invention, it is detected that reception has started at a predetermined cycle from the beginning of the ultrasonic reception wave, and the number of zero crosses of the reception wave after this detection is counted, and when the predetermined number of times is reached To determine the reception timing, and subtract the time from the reception start time of the reception head wave to the reception timing from the transmission / reception time from the start of ultrasonic transmission until the reception timing is determined to obtain the propagation time of the distance between the ultrasonic transmission and reception waves I made it.
[0026]
【The invention's effect】
According to the present invention, after detecting the start of reception with respect to an ultrasonic reception wave on which electromagnetic noise or acoustic noise is superimposed, the ratio of the amplitude level (S) of the reception wave to the amplitude level (N) of the noise (S / (N ratio) counts and waits for the number of zero crosses until a predetermined time region where the maximum value reaches the maximum value, and obtains the time between transmission and reception as the reception timing when the predetermined number of zero crosses is reached. Accurate and pure ultrasonic propagation time can be measured. Thereby, it is possible to measure the propagation time with high accuracy, and as a result, it is possible to measure the concentration (density) and / or the flow rate (flow velocity) with high accuracy.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is an overall configuration diagram of an ultrasonic fluid measuring apparatus. In the figure, Lm is the distance between the ultrasonic transducers (ultrasound transducers) 2a and 2b, Cg is the sound velocity of the fluid to be measured, Vg is the flow velocity of the fluid to be measured, and θ is the flow direction of the fluid to be measured. This is the angle formed with the propagation direction of the ultrasonic wave.
[0028]
A pair of ultrasonic transducers 2a and 2b are disposed at a predetermined distance in an acoustically opposed relationship with a measurement tube (space) 1c through which a fluid to be measured flows. Specifically, as shown in the figure, the ultrasonic transducers 2a and 2b provided in the measurement tube 1c are arranged obliquely so as to face each other. An ultrasonic wave is transmitted from the one ultrasonic transducer 2a, and the ultrasonic wave propagated in the fluid to be measured is received by the other ultrasonic transducer 2b (ultrasonic transmission type).
[0029]
Then, the ultrasonic propagation time measuring circuit 3 determines the ultrasonic propagation times t1 and t2 between the ultrasonic transducers 2a and 2b in the forward direction (2a → 2b) and the reverse direction (2b → 2a), respectively. Although not shown, a pair of ultrasonic transducers 2a and 2b is provided at a predetermined interval in the flow direction of the fluid to be measured, and the ultrasonic waves transmitted from one ultrasonic transducer 2a are contained in the fluid to be measured. It may be propagated, reflected by the measurement tube 1c and received by the other ultrasonic transducer 2b (ultrasonic reflection type).
[0030]
FIG. 2 is a configuration diagram of the ultrasonic fluid measuring apparatus, and more particularly, a configuration of the ultrasonic propagation time measuring circuit 3. FIG. 3 is an explanatory diagram of the measurement of the ultrasonic propagation times t1 and t2, where the horizontal axis is time, the vertical axis is each signal, Wr ″ is a received wave (received signal), Sr is a received detection signal, and Gr is Sensitivity adjustment voltage, Sz is a reception zero-cross signal, Sc is a zero-cross count signal, Sd is a reception timing determination signal, and in the flowmeter, the propagation time of the fluid to be measured in the forward direction and the reverse direction is determined. In order to make a measurement, the process is usually performed in a time division manner. However, since the processing procedure of the propagation time measurement is exactly the same, the description relating to the time division is omitted here.
[0031]
First, when one ultrasonic transducer 2a is selected on the transmission side by the transmission / reception direction changeover switch 3b, for example, the transmission drive signal Wt generated by the transmission drive circuit 3a is transmitted to the ultrasonic transducer 2a, The electro-acoustic conversion is performed and ultrasonic waves are transmitted into the fluid to be measured. The transmission drive circuit 3a generates the transmission drive signal Wt, and at the same time, counts the count start signal (start signal) at the times t1 and t2 when the ultrasonic wave propagates the distance Lm between the transducers 2a and 2b. Output to the circuit (time counter) 3f (at time t0 in FIG. 3).
[0032]
The other ultrasonic transducer 2b selected on the receiving side by the transmission / reception direction change-over switch 3b receives the ultrasonic wave transmitted from the ultrasonic transducer 2a and propagated in the fluid to be measured, and is subjected to acoustic-electric conversion. The received wave Wr ″ is input to the reception amplifier circuit 3c.
[0033]
In the reception amplification circuit 3c, the reception wave Wr ″ is passed through a filter circuit (not shown) for removing unnecessary noise signals other than the ultrasonic signal up to an amplitude level necessary for the reception detection circuit 3d described below. The amplification factor at this time is configured to be adjusted by the sensitivity adjustment voltage Gr input from the reception detection circuit 3d.
[0034]
In the reception detection circuit 3d, an initial amplified voltage (low sensitivity voltage) G0 is output, and the ultrasonic transducer 2b receives a predetermined period from the head of the reception wave Wr ″ (the third period (S3 in FIG. 3)). ) And the reception detection signal Sr is output.The predetermined period is set to the period when the reception wave Wr ″ reaches the threshold value Vth.
[0035]
At the same time, the sensitivity adjustment voltage Gr is switched to a voltage (high sensitivity voltage) G1 that increases the amplification factor of the reception amplifier circuit 3c (FIG. 3 (4)). As a result, the reception wave Wr ″ after the reception detection (S3) is amplified to an amplitude saturation region that is the reception signal saturation amplitude level Vs to become a substantially rectangular waveform (FIG. 3 (2)). This is because the received signal Wr ″ when the saturation amplitude level (predetermined amplitude level) Vs is exceeded is held at the saturation amplitude level Vs.
[0036]
Further, the received wave Wr ″ is converted into, for example, a 0-5V logic signal in accordance with the counter input specification of the reception timing determination circuit 3e at the next stage (FIG. 3 (5) received zero-cross signal Sz).
[0037]
Here, specific examples of the reception detection circuit 3d and the reception timing determination circuit 3e will be described with reference to FIG.
In the reception detection circuit 3d, the reception wave Wr ″ is compared with the zero level Lz by using the voltage comparator 3d1 and converted into a logic signal. As a result, the reception zero cross signal Sz is output. The voltage comparator 3d2 compares it with the threshold value Vth and converts it to a logic signal, thereby outputting the reception detection signal Sr.
[0038]
In the reception timing determination circuit 3e, the signal transmission of the reception zero cross signal Sz is controlled by the gate 3e2 (or analog switch element). The gate 3e2 is controlled by a signal generated by the flip-flop element 3e1, and the control signal causes the transmission drive signal Wt to close the gate (Reset) and the reception detection signal Sr to open the gate (Set). Designed to. Then, the zero cross signal after the detection of the reception is input to the down counter 3e3, and the reception timing signal Sd is output by counting the set number of zero counts.
[0039]
As a result, the reception timing determination circuit 3e counts the zero-cross signal Sc after the reception detection is performed on the input reception zero-cross signal Sz, that is, after the reception detection signal Sr. ) When Nz = 5 is set in advance, as shown in FIG. 3 (6), the count value Nz is decremented by -1 (down-counting) every time the zero-crossing count signal Sc is input. The reception timing determination signal Sd is output at the time when it reaches zero (Nz = 0). With this process, the reception timing td is determined by the predetermined number of zero crossings (5 times) after the reception detection (S3).
[0040]
Here, the number of zero crossings Nz = 5, but this corresponds to the eighth period (FIG. 3-S8) when counted from the leading wave. In general, in an ultrasonic transducer of a type in which a piezoelectric element is attached to a diaphragm, the vibration amplitude gradually increases after the transmission drive signal Wt is applied, and the characteristic of oscillating at the maximum amplitude after approximately the fifth cycle. It is in. Therefore, a good result can be obtained by setting the number of zero crossings Nz to 5 or more.
[0041]
Here, description will be given with reference to FIG. 2 again.
The reception timing signal Sd from the reception timing determination circuit 3e is input to the transmission / reception elapsed time counting circuit 3f. The transmission drive signal Wt of the transmission drive circuit 3a is also input to the transmission / reception elapsed time counting circuit 3f.
[0042]
Then, the transmission / reception elapsed time counting circuit 3f receives the reception timing signal Sd from the reception timing determination circuit 3e, and stops the time counting started together with the output of the transmission drive signal Wt of the transmission drive circuit 3a. The time count result t1 (t2) is input to the ultrasonic propagation time calculation circuit 3g.
[0043]
The ultrasonic propagation time calculation circuit 3g subtracts the reception detection time ta and the zero-cross count time tb (see FIG. 3) determined by the reception sensitivity adjustment voltage Gr and the threshold value Vth from the time count result t1 (t2). The ultrasonic propagation times t1 ′ (= t1−tr1−te) and t2 ′ (= t2−tr2−te) to be originally obtained are output. The detection error time te can be omitted due to the effect of the present invention.
[0044]
Here, the delay time tr1 (tr2) of the above-described equation (3) is represented by the sum of the reception detection time ta and the zero-crossing counting time tb as the following equation.
[0045]
tr1 (tr2) = ta + tb (7)
Here, the reception detection time ta is the time from the reception time of the leading wave (S1) of the reception wave Wr ″ to the reception detection time (S3). The zero cross count time tb is the reception timing from the reception detection time (S3). This zero cross count time tb is obtained by dividing the zero cross count (predetermined value) Nz by the ultrasonic frequency fu (Hz) as shown in the following equation.
[0046]
tb = Nz / fu (sec) (8)
The ultrasonic propagation time t1 ′ (t2 ′) is measured as described above. That is, as shown in FIG. 5, the time tr1 (tr2) from the transmission / reception time t1 (t2) from the ultrasonic transmission start time t0 to the reception timing td to the reception timing determination td from the reception start time of the reception head wave (S1). ) Is subtracted (t1-tr1, t2-tr2) to determine the propagation time t1 ′ (t2 ′) of the ultrasonic transmission / reception distance Lm.
[0047]
Then, in the flow rate / concentration calculation circuit 4, using the formulas (3) and (4) (in the formulas (3) and (5), t1-tr1-te = t1 ′, t2-tr2-te = t2 ′ However, te = 0) Calculates and outputs the flow rate value Qv of the fluid to be measured by calculating the density (concentration) value using the equations (5) and (6). In the equations (3) and (5), t1 (t2) and tr1 (tr2) are shown separately, but the ultrasonic wave propagation time t1 ′ (obtained by the ultrasonic wave propagation time calculation circuit 3g). Since t2 ′) has already subtracted the delay time tr1 (tr2), t1 ′ and t2 ′ are substituted for the denominators of the first and second terms in parentheses to obtain the flow velocity Vg and the sonic velocity Cg.
[0048]
Next, an embodiment in which this series of signal processing procedures is performed by a computer system will be described using the flowchart of FIG. In addition, about another embodiment mentioned later, it encloses with the dotted line and abbreviate | omits description here.
[0049]
In step 100 (shown as “S100” in the figure, the same applies hereinafter), power is turned on, and in step 101, initial values such as the number of zero crossings Nz and the threshold value Vth are set.
[0050]
In step 102, the transmission / reception directions of the ultrasonic transducers 2a and 2b are switched. In step 103, an ultrasonic transmission drive signal Wt is output from the transmission drive circuit 3a and the transmission / reception time (t0) of the transmission / reception elapsed time counting circuit 3f is output. (Elapsed time from) is started.
[0051]
In step 104, it is determined whether or not the received wave Wr ″ received by the ultrasonic transducer 2b has detected a received wave (S3) having a predetermined period from the top wave, that is, the reception detection timing. In step 105, on the other hand, if reception is not detected, the process waits until reception is detected.
[0052]
In step 105, the amplification factor of the reception amplification circuit 3c is increased (G0 → G1), and the obtained reception wave Wr ″ is digitally converted and read. In step 106, the number of zero crossings Nz is counted and the process proceeds to step 110. .
[0053]
In step 110, counting is continued until the number of zero crossings is reached. When the number of times of zero crossing is reached (Nz = 0), the transmission / reception elapsed time counting circuit 3f stops the transmission / reception time counting in step 111, and the count value t1 (t2) at this time is read.
[0054]
Next, in step 112, a delay time ta from the reception time of the leading wave (S1) to the reception detection (S3) time from the transmission / reception time t1 (t2) and the number of zero crossings from the reception detection (S3) time are required. The ultrasonic propagation time t1 ′ (t2 ′) is calculated by subtracting the sum with the time tb, that is, the delay time tr1 (tr2) (t1-tr1, t2-tr2).
[0055]
In step 113, the flow rate Qv and the concentration (density: ρ) are calculated and output from the obtained ultrasonic propagation time t1 ′ (t2 ′) using the above-described equations (3) to (6). Thereafter, the process returns to the transmission / reception direction switching process, and a series of processes are repeatedly executed.
[0056]
According to the present embodiment, the pair of ultrasonic transducers 2a and 2b is provided at a predetermined distance apart from each other across the space (measuring tube 1c) through which the fluid to be measured flows. The ultrasonic propagation time until the ultrasonic wave transmitted from one of the ultrasonic transducers 2a propagates in the fluid to be measured and is received by the other ultrasonic transducer 2b is shown in the forward and reverse directions of the flow, respectively. In an ultrasonic fluid measuring device that determines the flow rate Qv or concentration (density: ρ) of the fluid to be measured based on these ultrasonic propagation times, a predetermined cycle from the head (S1) of the ultrasonic wave reception A reception detection means (step 104, reception detection circuit 3d) for detecting that reception has started at the eye (S3), and the number of zero crosses Sc of the received wave after reception detection (S3) are counted, and a predetermined number of times ( Nz = 5) From the reception timing determination means (step 110, reception timing determination circuit 3e) for determining the reception timing td at the time (S8) and the transmission / reception time t1 (t2) from the ultrasonic transmission start time t0 to the reception timing td, the reception head Ultrasonic propagation time calculation means for subtracting the time tr1 (tr2) from the reception start time of the wave (S1) to the reception timing determination td to obtain the propagation time t1 ′ (t2 ′) of the ultrasonic transmission / reception distance Lm. (Step 112, ultrasonic propagation time calculation circuit 3g). For this reason, the ratio of the amplitude level (S) of the received wave to the amplitude level (N) of the received wave (S / N ratio) after detecting the start of reception of the ultrasonic wave with electromagnetic noise or acoustic noise superimposed on it. Counts and waits for the number of zero crossings until the predetermined time range reaches the maximum value, and obtains the time between transmission and reception using the time when the predetermined number of zero crossings is reached as the reception timing. Ultrasonic propagation time can be measured. Thereby, it is possible to measure the propagation time with high accuracy, and as a result, it is possible to measure the concentration (density) and / or the flow rate (flow velocity) with high accuracy.
[0057]
Further, according to the present embodiment, the reception wave amplification means (step 105, reception amplification circuit 3c) in which at least two amplitude amplification factors (G0, G1) can be set, and the reception wave amplification means are configured to detect reception (S3). ) Amplitude level holding means for increasing the amplitude amplification factor Gr thereafter and holding the received wave Wr ″ when the amplitude exceeds the predetermined amplitude level Vs at the predetermined amplitude level Vs. For this reason, reception detection (S3) is provided. ) Subsequent amplitude information becomes unnecessary, and instead of this, the received wave Wr ″ in the time domain for which accurate time information is required is amplified to a high magnification (G1) (rectangular wave), and the amplitude change rate before and after the zero crossing is increased. As a result, it is possible to reduce the shift of the time axis when noise is superimposed. And it is possible to measure the propagation time with high accuracy, and as a result, it is possible to measure the concentration (density) and flow rate (flow velocity) with high accuracy.
[0058]
Next, an embodiment for preventing erroneous measurement of ultrasonic propagation time when an interference wave occurs will be described with reference to FIGS.
First, when an interference wave is generated in the received wave Wr ″ due to multiple reflection or the like, the amplitude temporarily falls below the threshold value Vth as indicated by S7 of the received wave Wr ″ in FIG. In addition, after the generation of the interference wave, due to the effect of the phase change, the temporal relationship with the leading wave (zero crossing time is an integer multiple of the period) is shifted, so that the reception timing determination td after the generation of the interference wave is measured. An error may occur.
[0059]
Therefore, in the present embodiment, a decrease in the envelope We of the received wave Wr ″ is detected at the same time as performing zero-crossing. Then, a decrease in the envelope We is detected as indicated by S7 in the received wave Wr ″ in FIG. Sometimes, the zero cross count value at that time (Nz = 1 in FIG. 4) is stored, and the next zero cross number is changed to the number obtained by the following equation.
[0060]
Next number of zero crossings = current number of zero crossings− (zero cross count value when envelope decreases) +1 (9)
For example, in FIG. 4, the current number of zero crossings is 5 (Nz = 5), and the zero cross count value when the envelope is lowered is 1, so the next zero cross number is 3 from equation (9). In the next zero cross count, the number of times is changed from 5 to 3 (Nz = 5 → (3)) as indicated by the zero cross count signal Sc after detection of reception (Nz = 5 → (3)). This is determined in the S6 cycle (td ′), and measurement errors can be avoided.
[0061]
Next, the interference wave detection circuit 3h and the zero-crossing frequency changing circuit 3i added in this embodiment (portion surrounded by a dotted line in FIG. 2) will be described with reference to FIGS.
[0062]
In the interference wave detection circuit 3h, in order to generate the envelope We of the reception wave Wr ″ of FIG. 4, the peak hold circuit 3h2 is used to generate the maximum amplitude value for each period of the reception wave Wr ″ as shown in FIG. Hold it. The hold value samples the peak value with the received zero-cross signal Sz for each period in order to follow a sharp amplitude drop due to interference.
[0063]
This sample signal is generated by the monostable multivibrator 3h1 as a signal having a pulse width Pw corresponding to the peak hold time. An example of setting the pulse width Pw at this time is obtained by dividing 1.25 by the ultrasonic frequency fu (Hz) as shown in the following equation.
[0064]
Pw = 1.25 / fu (sec) (10)
The numerator of the equation (10) is a constant for matching the next sample timing with the timing around the next amplitude peak, and 1.25 corresponds to the next amplitude peak timing. The envelope signal We generated in this way is input to the voltage comparator 3h3.
[0065]
The voltage comparator 3h3 compares the envelope signal We and the interference wave detection determination voltage (predetermined value) Vh, and when there is a level drop due to the interference wave (We <Vh), the positive pulse interference wave detection signal Sh Is output.
[0066]
Further, as shown in FIG. 7, in the zero crossing number changing circuit 3i, the count value (Data) of the down counter (zero crossing counter) 3e3 is inputted to the data register 3i1, and the interference wave detection signal Sh becomes a positive pulse (interference is detected). The zero cross count value at the time of detection) is stored.
[0067]
Subsequently, the digital comparator 3i2 is used to determine whether or not the stored zero cross count value has reached zero. If not, the data register (memory) 3i1 is used to execute the above-described equation (9). 1 is added to the stored value (adder 3i3), and the result is subtracted from the zero cross count initial value Nzo (subtractor 3i4). This result is selected by the changeover switch (selector) 3i5 controlled by the determination result of the digital comparator 3i2, and is input to the zero cross count counter 3e3 as the next zero cross number Nz.
[0068]
On the other hand, when the zero-cross count value Sc normally reaches zero without detecting an interference wave, the zero-cross count initial value Nzo becomes the next zero-cross count Nz at the changeover switch 3i5. As a result, when the interference wave disappears, it is automatically returned to a predetermined number of zero crossings.
[0069]
The ultrasonic propagation time measurement in one direction is executed as described above. This is alternately performed in both the forward and reverse directions of the flow of the fluid to be measured, and a pair of bidirectional ultrasonic propagation times t1 ′ (= t1−tr1). -Te, where te = 0) and t2 '(= t2-tr2-te, where te = 0) are obtained (see FIG. 2). Then, in the flow rate / concentration calculation circuit 4, the flow rate value Qv of the fluid to be measured is expressed as (5) using the formulas (3) and (4) (where t1 = t1 ′ and t2 = t2 ′ in each formula). ) And (6) are used to calculate and output density (concentration) values. In the equations (3) and (5), t1 (t2) and tr1 (tr2) are shown separately, but the ultrasonic propagation time t1 ′ obtained by the ultrasonic propagation time calculation circuit 3g, Since t2 ′ has already subtracted the delay time tr1 (tr2), t1 ′ and t2 ′ are substituted into the denominators of the first and second terms in parentheses to obtain the flow velocity Vg and the sound velocity Cg.
[0070]
Next, an embodiment in which a series of signal processing procedures when an interference wave affects the computer system will be described with reference to the flowchart of FIG. In addition, description is abbreviate | omitted about the same thing as the flowchart in above-mentioned embodiment.
[0071]
In the present embodiment, in order to accurately measure the propagation time even when an interference wave affects, the presence or absence of the interference wave is determined, and the number of zero crossings Nz is changed.
Steps 100 to 106 are the same as those in the above-described embodiment. After counting the number of zero crossings Nz in step 106, the process proceeds to step 107.
[0072]
In step 107, it is determined whether or not the interference wave detection circuit 3h has detected an interference wave. If an interference wave is detected, the process proceeds to step 108 to perform measurement error processing. On the other hand, if no interference wave is detected, the process proceeds to step 110.
[0073]
In step 108, since the current measurement data is not certain, it is discarded and processing such as replacement with the previous measurement data is performed instead.
In accordance with this, since a measurement error may occur due to the interference wave in the next measurement, the next zero-cross number is changed to the number of zero-cross setting times or less in which the next interference wave is detected by the zero-cross change circuit 3i in step 109. To do. Then, the process proceeds to step 111 and the above-described processing is performed.
[0074]
On the other hand, if no interference wave is detected in step 107, counting is continued until the number of zero crossings is reached in step 110. When the number of zero crossings has been reached (Nz = 0), the transmission / reception elapsed time counting circuit 3f stops counting the transmission / reception time in step 111, and the count value t1 at this time is read.
[0075]
According to the present embodiment, the envelope We of the reception wave Wr ″ after reception detection (S3) has a predetermined amplitude due to the interference wave at the zero-cross count Nz set in the reception timing determination means (step 110, reception timing determination circuit 3e). Interference wave detecting means (step 107, comparator 3i2) for detecting timing lower than level Vth, and zero cross number changing means (step 109, adder 3i3, subtractor) for changing the zero cross number Nz based on the interference wave detection timing. 3i4), and this zero-crossing number changing means, when a decrease in the predetermined amplitude level Vs is detected by the interference wave detecting means before reaching the current zero-crossing number (Nz = 5) (S7) From the number of zero crossings (Nz = 5) to the number of zero crossings when the predetermined amplitude level drops (S7) Is subtracted (equation (9)) to change to the number of zero crosses at the next measurement (step 109), so that it is reflected between the ultrasonic transducers and the inner wall of the measurement pipe 1c of the fluid to be measured. If there is a zero-cross timing shift or disappearance due to interference between the reflected wave and the received wave, it is changed so that the reception timing is determined before counting the zero cross, thus preventing erroneous propagation time measurement. As a result, highly reliable concentration (density: ρ) and flow rate Qv (flow velocity Vg) can be measured.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining the overall configuration of a typical ultrasonic fluid measuring apparatus.
FIG. 2 is a diagram illustrating an embodiment of a basic configuration of the present invention
FIG. 3 is a diagram for explaining ultrasonic propagation time measurement according to the present invention.
FIG. 4 is a diagram for explaining ultrasonic propagation time measurement according to another embodiment of the present invention.
FIG. 5 is a diagram showing a reception detection circuit and a reception timing determination circuit according to the present invention.
FIG. 6 is a diagram showing an interference wave detection circuit according to the present invention.
FIG. 7 is a diagram showing a zero crossing number changing circuit of the present invention.
FIG. 8 is a flowchart of signal processing according to the present invention.
FIG. 9 is a diagram for explaining conventional ultrasonic propagation time measurement;
FIG. 10 is a diagram for explaining the problems of conventional ultrasonic propagation time measurement.
[Explanation of symbols]
1c Measuring tube
2a, 2b Ultrasonic transducer
3 Ultrasonic propagation time measurement circuit
3a Transmission drive circuit
3b Transmission / reception direction selector switch
3c Receive amplifier circuit
3d reception detection circuit
3d1, 3d2 voltage comparator
3e Reception timing decision circuit
3e1 flip-flop element
3e2 gate
3e3 down counter
3f Transmission / reception elapsed time counting circuit
3g Ultrasonic propagation time calculation circuit
3h Interference wave detection circuit
3h1 monostable multivibrator
3h2 peak hold circuit
3h3 voltage comparator
3i Zero cross frequency change circuit
3i1 data register
3i2 comparator
3i3 adder
3i4 subtractor
3i5 selector switch
4 Flow rate / concentration calculation circuit

Claims (3)

A pair of ultrasonic transducers is provided at a predetermined distance in an acoustically opposed positional relationship across a space in which the fluid to be measured flows, and the ultrasonic waves transmitted from one of the ultrasonic transducers are received by the fluid to be measured The ultrasonic propagation time until it is propagated in and received by the other ultrasonic transducer is determined in the forward direction and the reverse direction of the flow, and the flow rate of the fluid to be measured is determined based on these ultrasonic propagation times. Alternatively, in an ultrasonic fluid measuring device for determining the concentration,
A reception detection means for detecting that reception has started at a predetermined cycle from the head of the ultrasonic reception wave;
A reception timing determination means for counting the number of zero crossings of the received wave after the detection of detection and determining a reception timing when the predetermined number of times is reached;
Ultrasound in which the propagation time of the distance between ultrasonic transmission and reception waves is obtained by subtracting the time from the reception start time of the reception head wave to the reception timing determination from the transmission / reception time from the ultrasonic transmission start time to the reception timing determination. Propagation time calculation means;
An ultrasonic fluid measuring device comprising: Received wave amplification means capable of setting at least two amplitude amplification factors;
An amplitude level holding means for holding the received wave at a predetermined amplitude level when the received wave amplifying means increases the amplitude amplification factor after detection of reception and the amplitude exceeds a predetermined amplitude level;
The ultrasonic fluid measuring device according to claim 1, further comprising: An interference wave detecting means for detecting a timing at which the envelope of the received wave after the detection of reception is lowered from a predetermined amplitude level by the interference wave at the number of zero crosses set in the reception timing determining means;
Zero cross number changing means for changing the number of zero crosses based on the interference wave detection timing,
The zero crossing number changing means is a number obtained by adding 1 to the zero crossing number when the predetermined amplitude level is lowered from the current zero crossing number when the decrease of the amplitude level is detected by the interference wave detecting means before reaching the current zero crossing number. The ultrasonic fluid measurement device according to claim 1, wherein the number of times of zero crossing is subtracted and changed to the number of zero crossings at the next measurement.

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