JP2004061450A

JP2004061450A - 2-wire electromagnetic flowmeter

Info

Publication number: JP2004061450A
Application number: JP2002223839A
Authority: JP
Inventors: Ichiro Mitsutake; 光武　一郎
Original assignee: Azbil Corp
Current assignee: Azbil Corp
Priority date: 2002-07-31
Filing date: 2002-07-31
Publication date: 2004-02-26
Anticipated expiration: 2022-07-31
Also published as: JP3996464B2

Abstract

【課題】計測値に応じて多段階に設定される励磁電流の限界値をさらに大きくする。
【解決手段】励磁回路３と流量測定出力回路６との接続ラインＬＡ中にスイッチＳＷ１を設ける。接続ラインＬＢ中にスイッチＳＷ２を設ける。接続ラインＬＣ中にスイッチＳＷ３を設ける。ＣＰＵ６−４での計測値が２０％を上回る前まではスイッチＳＷ１をオン（スイッチＳＷ２，ＳＷ３はオフ）とし、直列励磁回路方式とする。計測値が２０％を上回った場合には、スイッチＳＷ２，ＳＷ３をオン（スイッチＳＷ１はオフ）とし、並列励磁回路方式とする。並列励磁回路方式では、励磁電圧回路３−１での励磁電圧Ｖｅｘを８．５Ｖから１４Ｖに切り替える。
【選択図】　　　　図１An object of the present invention is to further increase a limit value of an exciting current set in multiple stages according to a measured value.
A switch SW1 is provided in a connection line LA between an excitation circuit 3 and a flow measurement output circuit 6. A switch SW2 is provided in the connection line LB. A switch SW3 is provided in the connection line LC. Until the value measured by the CPU 6-4 exceeds 20%, the switch SW1 is turned on (the switches SW2 and SW3 are turned off), and the series excitation circuit system is used. When the measured value exceeds 20%, the switches SW2 and SW3 are turned on (the switch SW1 is turned off), and the parallel excitation circuit system is used. In the parallel excitation circuit system, the excitation voltage Vex in the excitation voltage circuit 3-1 is switched from 8.5V to 14V.
[Selection diagram] Fig. 1

Description

【０００１】
【発明の属する技術分野】
この発明は、各種プロセス系において導電性を有する流体の流量を測定する電磁流量計に関し、特に直流電源に接続された一対の信号線より外部電圧が供給され、この外部電圧を供給する一対の信号線に流れる出力電流を調整することによって計測値を出力する２線式電磁流量計に関するものである。
【０００２】
【従来の技術】
従来より、この種の２線式電磁流量計においては、測定管内を流れる流体の流れ方向に対してその磁界の発生方向を垂直として配置された励磁コイルへ所定周波数で矩形波状の励磁電流Ｉｅｘを供給し、励磁コイルの発生磁界と直交して測定管内に配置された電極間に得られる信号起電力（流量に比例した信号）を検出し、この検出した信号起電力に基づいてＣＰＵでの演算処理により計測値を０〜１００％値として求め、この２線式電磁流量計に外部電圧を供給する一対の信号線に流れる電流（出力電流）を、上記求めた計測値に応じて４〜２０ｍＡの電流範囲で調整するようにしている。
【０００３】
図６に従来の２線式電磁流量計の概略を示す。同図において、１００は２線式電磁流量計、２００は直流電源であり、２線式電磁流量計１００には一対の信号線Ｌ１，Ｌ２を介して直流電源２００からの外部電圧Ｖｓが供給される。信号線Ｌ２には外部負荷ＲＬとして例えば２５０Ωの抵抗が接続されている。また、直流電源２００は、ＤＣ２４Ｖとされている。この場合、２線式電磁流量計１００に供給される外部電圧Ｖｓは、直流電源２００の電源電圧（ＤＣ２４Ｖ）から外部負荷ＲＬにける電圧降下を差し引いた値とされる。
【０００４】
２線式電磁流量計１００において、１は測定管、２は測定管１内を流れる流体の流れ方向に対してその磁界の発生方向を垂直として配置された励磁コイル、３は励磁コイル２へ矩形波状の励磁電流Ｉｅｘを周期的に供給する励磁回路、４ａ，４ｂは励磁コイル２の発生磁界と直交して測定管１内に配置された検出電極、５は接地電極、６は検出電極４ａ，４ｂ間に得られる信号起電力を検出し、この検出される信号起電力に基づいて計測値を求め、この求めた計測値に応じて直流電源２００に戻される出力電流Ｉｏｕｔを４〜２０ｍＡの電流範囲で調整する流量測定出力回路である。
【０００５】
励磁回路３は励磁電圧回路（定電圧回路）３−１と電流調整回路（ＣＣＳ）３−２とを有している。励磁電圧回路３−１は励磁電圧Ｖｅｘとして８．５Ｖの定電圧を生成する。電流調整回路３−２は、励磁電圧回路３−１からの励磁電圧Ｖｅｘを励磁コイル２に印加するとともに、その極性が交互に切り替わる矩形波状の励磁電流Ｉｅｘを生成する。励磁電流Ｉｅｘの値（波高値）は、流量測定出力回路６における計測値に応じ、図７に示すように多段階に切り替えられる。
【０００６】
流量測定出力回路６は、信号起電力検出回路６−１と、サンプルホールド回路６−２と、Ａ／Ｄ変換回路６−３と、ＣＰＵ６−４と、Ｄ／Ａ変換回路６−５と、電流調整回路（ＣＣＳ）６−６と、これらの回路に電源電圧を供給する定電圧回路６−７とを有している。
【０００７】
信号起電力検出回路６−１は、接地電極５の電位を基準として、電極４ａ，４ｂ間に得られる信号起電力を検出する。サンプルホールド回路６−２は、信号起電力検出回路６−１が検出する信号起電力を入力とし、この信号起電力の値を極性が切り替わる直前でサンプルホールドする。Ａ／Ｄ変換回路６−３は、サンプルホールド回路６−２によってサンプルホールドされた信号起電力（アナログ値）をデジタル値に変換し、ＣＰＵ６−４へ送る。
【０００８】
ＣＰＵ６−４は、Ａ／Ｄ変換回路６−３からの信号起電力に基づいて計測値（０〜１００％）を求め、Ｄ／Ａ変換回路６−５へ出力する。Ｄ／Ａ変換回路６−５は、ＣＰＵ６−４からの計測値（デジタル値）をアナログ値に変換し、電流調整回路６−６へ送る。電流調整回路６−６は、コンパレータＣＰ１とトランジスタＱ１と抵抗Ｒ１とを有し、コンパレータＣＰ１によってトランジスタＱ１のベース電流を調整することによって、トランジスタＱ１のコレクタ−エミッタ間を流れる電流ＩｃｃｓをＤ／Ａ変換回路６−５からの計測値に応じて調整する。また、ＣＰＵ６−４は、Ａ／Ｄ変換回路６−３からの信号起電力に基づいて求めた計測値に応じ、図７に示した関係で励磁電流Ｉｅｘが励磁コイル２へ供給されるように、励磁回路３の電流調整回路３−２へ指令を与える。
【０００９】
〔直流励磁回路方式〕
この２線式電磁流量計１００では、励磁回路３と流量測定出力回路６とが信号線Ｌ１とＬ２との間に直列に接続されており、励磁回路３を流れる電流が流量測定出力回路６に流れ込んで、直流電源２００に戻される出力電流Ｉｏｕｔとなる。このような方式を直列励磁回路方式と呼んでいる。
【００１０】
例えば、ＣＰＵ６−４での計測値が０％である場合、励磁回路３での励磁電流Ｉｅｘの値は３．５ｍＡとされる。励磁回路３では、励磁電圧回路３−１での励磁電圧Ｖｅｘの生成に０．５ｍＡの電流が必要であり、励磁電圧回路３−１側を流れる電流をＩａとすると、励磁回路３を流れる電流Ｉ１は、Ｉ１＝Ｉａ＋Ｉｅｘ＝０．５ｍＡ＋３．５ｍＡ＝４ｍＡとなる。
【００１１】
この４ｍＡの電流が流量測定出力回路６へ流れ込む。流量測定出力回路６では、定電圧回路６−７側に流れる電流をＩｂとした場合、この電流ＩｂはＣＰＵ６−４などを駆動するために３ｍＡ必要であり、トランジスタＱ１側を流れる電流Ｉｃｃｓが１ｍＡに調整されるとすると、流量測定出力回路６を流れる電流Ｉ２（Ｉ２＝Ｉｃｃｓ＋Ｉｂ）は４ｍＡとなり、励磁回路３を流れる電流Ｉ１と流量測定出力回路６を流れる電流Ｉ２とが等しくなり、直流電源２００に戻される出力電流Ｉｏｕｔが計測値０％を示す４ｍＡとなる。
【００１２】
ＣＰＵ６−４での計測値が例えば１０％となれば、ＣＰＵ６−４は出力電流ＩｏｕｔをＩｏｕｔ＝４ｍＡ＋１．６ｍＡ＝５．６ｍＡとするために、トランジスタＱ１側を流れる電流Ｉｃｃｓを２．６ｍＡに調整する。この場合、励磁回路３では、励磁電流Ｉｅｘ＝３．５ｍＡとされているために、励磁電圧回路３−１側を流れる電流ＩａがＩａ＝２．１ｍＡとなる。
【００１３】
〔励磁電流Ｉｅｘの値を計測値に応じて多段階に変える理由〕
励磁電流Ｉｅｘの値は、図７に示した関係に従い、ＣＰＵ６−４での計測値に応じて多段階に切り替えられる。このような励磁電流Ｉｅｘの値を多段階に切り替える方式を多点励磁切替方式と呼んでいる。多点励磁切替方式としないとすると、すなわち励磁電流Ｉｅｘの値を３．５ｍＡに固定すると、流体を貫く磁束密度が小さいため、信号起電力検出回路６−１で得られる信号起電力が小さく、流速に応じて電極４ａ，４ｂに重畳してくるノイズの影響を受けて出力が大きくふらつく。すなわち、高流量になるにつれて信号起電力に含まれるノイズの割合が高くなって、Ｓ／Ｎ比が低下し、安定した流量計測ができなくなる。
【００１４】
信号起電力検出回路６−１で得られる信号起電力をｅとすると、信号起電力ｅは、ｅ＝ｋ・Ｂ・ｖ・Ｄとして表される。なお、この式において、ｋは定数、Ｄは測定管１の口径、ｖは平均流速、Ｂは発生磁束密度である。ここで、Ｂは励磁電流Ｉｅｘに比例し、励磁電流Ｉｅｘを大きくすれば同じ流速でも信号起電力ｅが大きくなる。そこで、図６に示した従来の２線式電磁流量計１００では、計測値に応じて、すなわち計測値に応ずる出力電流（４〜２０ｍＡ）が大きくなれば、その大きくなった分を利用して、励磁電流Ｉｅｘを大きな値に切り替えるようにしている。
【００１５】
例えば、計測値が２０％となると、励磁電流Ｉｅｘの値が６．７ｍＡに切り替えられる。すなわち、計測値２０％に応ずる出力電流Ｉｏｕｔは７．２ｍＡであり、励磁回路３ではＩａ＝０．５ｍＡ必要なので、励磁電流Ｉｅｘとして６．７ｍＡまで流すことができる。計測値が４０％となると、励磁電流Ｉｅｘの値が９．９ｍＡに切り替えられる。すなわち、計測値４０％に応ずる出力電流Ｉｏｕｔは１０．４ｍＡであり、励磁回路３ではＩａ＝０．５ｍＡ必要なので、励磁電流Ｉｅｘとして９．９ｍＡまで流すことができる。このように、計測値に応じて励磁電流Ｉｅｘを大きな値に切り替えることにより、信号起電力ｅを大きくして、Ｓ／Ｎ比を向上させ、安定した流量計測が可能となる。
【００１６】
【発明が解決しようとする課題】
しかしながら、上述した従来の２線式電磁流量計１００では、直流励磁回路方式としているために、２線式電磁流量計１００に供給される外部電圧Ｖｓ、すなわち直流電源２００の電源電圧ＤＣ２４Ｖから外部負荷ＲＬにおける電圧降下Ｉｏｕｔ×ＲＬを差し引いた電圧Ｖｓが励磁回路３と流量測定出力回路６とに分圧される。このため、励磁電圧回路３−１が生成する励磁電圧Ｖｅｘは８．５Ｖと小さく、励磁コイル２へ供給される矩形波状の励磁電流Ｉｅｘは、その値が大きくなるほど所要の値までの立ち上がり時間が長くなる。
【００１７】
図８は励磁電流Ｉｅｘの値をＩｅｘ＝３．５ｍＡ、６．７ｍＡ、９．９ｍＡ、１２ｍＡと切り替えた場合の立ち上がり波形であり、励磁電流Ｉｅｘの値が３．５ｍＡと小さい場合には、同図に示す波形Ｉのように即座に立ち上がる。しかし、励磁電圧回路３−１が生成する励磁で電圧は変わらないため、励磁電流Ｉｅｘの値が６．７ｍＡ、９．９ｍＡ、１２ｍＡと大きくなると、同図に示す波形ＩＩ，ＩＩＩ　，ＩＶのようにように立ち上がり時間が長くなり、極性が切り替わる直前の定常域（Ｉｅｘが所要値に達した後の平坦な波形部分）ｔａが短くなる。
【００１８】
サンプルホールド回路６−２では信号起電力ｅの値を極性が切り替わる直前でサンプルホールドする。例えば、信号起電力ｅの極性が切り替わる直前の５ｍｓの間の信号起電力ｅをサンプリングし、その平均値を保持する。励磁電流Ｉｅｘの値が１２ｍＡの場合、励磁電流Ｉｅｘの極性が切り替わる直前の定常域ｔａは５ｍｓ程度であり、サンプリングされる信号起電力ｅはかろうじて安定した励磁電流Ｉｅｘにより得られる値となる。
【００１９】
しかし、励磁電流Ｉｅｘの値が大凡ではあるが１２ｍＡを越えると、励磁電流Ｉｅｘが変化している間の信号起電力ｅがサンプリングされるようになり、電極４ａ，４ｂに生じる渦電流などによって、流量の計測値に誤差が含まれるものとなる。このため、従来の多点励磁切替方式の２線式電磁流量計１００では、計測値に応じて多段階に設定される励磁電流Ｉｅｘの限界値が１２ｍＡ程度とされており、これ以上励磁電流Ｉｅｘの値を大きくすることができなかった。
【００２０】
本発明はこのような課題を解決するためになされたもので、その目的とするところは、計測値に応じて多段階に設定される励磁電流の限界値をさらに大きくすることのできる２線式電磁流量計を提供することにある。
【００２１】
【課題を解決するための手段】
このような目的を達成するために本発明は、上述した２線式電磁流量計において、流量測定出力回路における計測値が所定値を越えた場合、一対の信号線の間に直列に接続されている励磁回路と流量測定出力回路との接続を並列に切り替える切替手段を設けたものである。
この発明によれば、流量測定出力回路における計測値が例えば２０％を越えると、それまで一対の信号線の間に直列に接続されていた励磁回路と流量測定出力回路とが並列に接続される。すなわち、それまで外部電圧が励磁回路と流量測定出力回路とに分圧されていたものが、励磁回路と流量測定出力回路とのそれぞれに印加されるものとなる。このため、励磁回路において生成される励磁電圧をそれまでの励磁電圧よりも大きくすることが可能となり、励磁電流の立ち上がり時間を早くし、極性が切り替わる直前の定常域を長くすることができるようになる。これにより、励磁電流を例えば１２ｍＡ以上としても、極性が切り替わる直前の定常域として５ｍｓ以上を確保することが可能となり、計測値に応じて多段階に設定される励磁電流の限界値を１２ｍＡ以上とすることが可能となる。
【００２２】
【発明の実施の形態】
以下、本発明を図面に基づいて詳細に説明する。図１はこの発明に係る２線式電磁流量計の一実施の形態の概略を示す図である。同図において、図６と同一符号は図６を参照して説明した構成要素と同一或いは同等構成要素を示し、その説明は省略する。
【００２３】
この２線式電磁流量計１００Ａにおいては、励磁回路３と流量測定出力回路６との接続ラインＬＡ中に第１のスイッチＳＷ１を設け、励磁電圧回路３−１とスイッチＳＷ１との接続ライン上の点Ｐ１と抵抗Ｒ１と外部負荷ＲＬとの間の接続ライン上の点Ｐ２との間（接続ラインＬＢ中）に第２のスイッチＳＷ２を設け、信号線Ｌ１と励磁電圧回路３−１との接続ライン上の点Ｐ３とスイッチＳＷ１と流量測定出力回路６との接続ライン上の点Ｐ４との間（接続ラインＬＣ中）に第３のスイッチＳＷ３を設けている。
【００２４】
また、流量測定出力回路６におけるＣＰＵ６−４にスイッチＳＷ１〜ＳＷ３のオン・オフを制御する機能を付加し、Ａ／Ｄ変換回路６−３から出力されたＣＰＵ６−４での計測値が０〜２０％の範囲ではスイッチＳＷ１をオン（スイッチＳＷ２，ＳＷ３はオフ）とし、ＣＰＵ６−４での計測値が２０％を上回った場合には、スイッチＳＷ２，ＳＷ３をオン（スイッチＳＷ１はオフ）とするようにしている。
【００２５】
また、Ａ／Ｄ変換回路６−３からの信号起電力に基づいて求めた計測値に応じ、図７に示した関係ではなく、図２に示された関係で定まる値の電流を励磁電流とするように励磁回路３の電流調整回路３−２へＣＰＵ６−４より指令を与えるようにしている。また、ＣＰＵ６−４での計測値が２０％を上回った場合、ＣＰＵ６−４より励磁電圧回路３−１へ指令を与え、スイッチＳＷ１〜ＳＷ３の切り替えに合わせて励磁電圧Ｖｅｘの値を８．５Ｖから１４Ｖの定電圧へ切り替えるようにしている。
【００２６】
〔直列励磁回路方式：０％〜２０％〕
ＣＰＵ６−４は、Ａ／Ｄ変換回路６−３からの信号起電力に基づいて計測値を求め、この計測値が０〜２０％の範囲ではスイッチＳＷ１をオンとし、スイッチＳＷ２，ＳＷ３をオフとする。これにより、励磁回路３と流量測定出力回路６とが信号線Ｌ１とＬ２との間に直列に接続され、２線式電磁流量計１００Ａは従来の２線式電磁流量計１００と同様の動作を行う。
【００２７】
図３にこの場合の２線式電磁流量計１００Ａにおける回路接続を単純化して示す。この場合、励磁回路３における励磁電圧回路３−１が生成する励磁電圧Ｖｅｘは８．５Ｖとされ、電流調整回路３−２が励磁コイル２へ供給する励磁電流Ｉｅｘの値は３．５ｍＡとされ、励磁電圧回路３−１側を流れる電流Ｉａは計測値０％〜２０％に対応して０．５〜３．７ｍＡとされる。また、流量測定出力回路６では、定電圧回路６−７側への電流Ｉｂが３ｍＡとされ、トランジスタＱ１側への電流Ｉｃｃｓが計測値０％〜２０％に対応して１〜４．２ｍＡとされる。
【００２８】
〔並列励磁回路方式：２０％〜１００％〕
ＣＰＵ６−４は、Ａ／Ｄ変換回路６−３からの信号起電力に基づいて求めた計測値が２０％を上回ると、すなわち直流電源２００に戻される出力電流Ｉｏｕｔが７．２ｍＡを超えると、スイッチＳＷ１をオフとし、スイッチＳＷ２，ＳＷ３をオンとする。これにより、それまで信号線Ｌ１とＬ２との間に直列に接続されていた励磁回路３と流量測定出力回路６とが、信号線Ｌ１とＬ２との間に並列に接続されるようになる。
【００２９】
図４にこの場合の２線式電磁流量計１００Ａにおける回路接続を単純化して示す。この場合、直流電源２００からの入力電流Ｉｉｎ＝７．２ｍＡは、励磁回路３への電流Ｉ１（Ｉ１＝Ｉｅｘ＋Ｉａ）と流量測定出力回路６への電流Ｉ２（Ｉ２＝Ｉｂ＋Ｉｃｃｓ）とに分流される。この例では、励磁回路３における励磁コイル２への励磁電流Ｉｅｘの値は３．５ｍＡとされ、励磁電圧回路３−１での励磁電圧Ｖｅｘの生成にＩａ＝０．５ｍＡの電流が必要とされ、流量測定出力回路６では定電圧回路６−７側への電流Ｉｂとして３ｍＡを必要とする。これらの電流ＩｅｘとＩｃとＩｂとを足し合わせると、３．５＋０．５＋３＝７ｍＡとなり、直流電源２００からの入力電流Ｉｉｎ＝７．２ｍＡとほゞ一致する。なお、７．２ｍＡと７ｍＡとの差０．２ｍＡについては、流量測定出力回路６におけるトランジスタＱ１側を流れる電流Ｉｃｃｓとする。
【００３０】
また、ＣＰＵ６−４は、計測値が２０％を上回ると、励磁電圧回路３−１へ指令を与え、励磁電圧Ｖｅｘの値を８．５Ｖから１４Ｖへと切り替える。この励磁電圧Ｖｅｘの８．５Ｖから１４Ｖへの切り替えは、スイッチＳＷ１〜ＳＷ３の切り替えによって励磁回路３と流量測定出力回路６とが信号線Ｌ１とＬ２との間に並列に接続されることによって実現可能となるものである。
【００３１】
すなわち、計測値が２０％を上回る前は、励磁回路３と流量測定出力回路６とは信号線Ｌ１とＬ２との間に直列に接続されている。この場合、２線式電磁流量計１００Ａの外部電圧Ｖｓは励磁回路３と流量測定出力回路６とに分圧され、励磁回路３への供給電圧は小さい。このため、励磁電圧Ｖｅｘの値は、８．５Ｖ程度としかできない。これに対し、計測値が２０％を上回ると、励磁回路３と流量測定出力回路６とは信号線Ｌ１とＬ２との間に並列に接続される。この場合、２線式電磁流量計１００Ａの外部電圧Ｖｓは励磁回路３と流量測定出力回路６とのぞれぞれに印加されるものとなり、励磁回路３への供給電圧が大きくなる。これにより、励磁電圧回路３−１が生成する励磁電圧Ｖｅｘを、１４Ｖ程度まで大きくすることが可能となる。
【００３２】
励磁電圧回路３−１が生成する励磁電圧Ｖｅｘが１４Ｖとされると、それまでの励磁電圧Ｖｅｘ＝８．５Ｖに対し、励磁電流Ｉｅｘの立ち上がり時間が早くなり、極性が切り替わる直前の定常域も長くなる。励磁電流Ｉｅｘの値が３．５ｍＡ程度では顕著な差はないが、流量信号（入力電圧）の増大に伴って励磁電流Ｉｅｘの値が大きくなるほどその差が大きくなり、例えば並列励磁回路方式で励磁電流Ｉｅｘを最大限流すことの可能な１６．５ｍＡとした場合、図５に示すように定常域ｔａとして６ｍｓ程度を確保することができる。すなわち、従来は励磁電流Ｉｅｘの限界値を１２ｍＡ程度としかできなかったが、計測値２０％を境として直列励磁回路方式から並列励磁回路方式に切り替えることによって、励磁電流Ｉｅｘの限界値を最大限流すことの可能な１６．５ｍＡまで大きくすることができる。
【００３３】
ＣＰＵ６−４は、計測値が４０％を上回ると、励磁電流Ｉｅｘの値を６．９ｍＡに切り替える。すなわち、計測値４０％に応ずる出力電流Ｉｏｕｔは１０．４ｍＡであり、励磁回路３ではＩａ＝０．５ｍＡ必要であり、流量測定出力回路６では３ｍＡ必要であるので、流量測定出力回路６でのトランジスタＱ１側への電流Ｉｃｃｓを０ｍＡとすれば、励磁電流Ｉｅｘとして６．９ｍＡ流すことができる。
【００３４】
ＣＰＵ６−４は、計測値が６０％を上回るまでは、励磁電流Ｉｅｘの値を６．９ｍＡとする。この間の余剰な電流はトランジスタＱ１側への電流Ｉｃｃｓとして流す。そして、計測値が６０％を上回ると、励磁電流Ｉｅｘの値を１０．１ｍＡに切り替える。すなわち、計測値６０％に応ずる出力電流Ｉｏｕｔは１３．６ｍＡであり、励磁回路３ではＩａ＝０．５ｍＡ必要であり、流量測定出力回路６では３ｍＡ必要であるので、流量測定出力回路６でのトランジスタＱ１側への電流Ｉｃｃｓを０ｍＡとすれば、励磁電流Ｉｅｘとして１０．１ｍＡ流すことができる。
【００３５】
以下、同様にして、ＣＰＵ６−４は、計測値が８０％を上回ると励磁電流Ｉｅｘの値を１３．３ｍＡとし、計測値が１００％となると励磁電流Ｉｅｘの値を１６．５ｍＡとする。励磁電流Ｉｅｘが１２ｍＡを超えても、本実施の形態では並列励磁回路方式として励磁電圧Ｖｅｘが１４Ｖと高くされていることから、励磁電流Ｉｅｘの立ち上がり時間が早く、極性が切り替わる直前の定常域も長くなることから、安定した励磁電流Ｉｅｘにより得られる信号起電力ｅをサンプリングして、正確な計測値を求めることができるようになる。
【００３６】
なお、上述した実施の形態では、計測値が２０％を上回った場合に直列励磁回路方式から並列励磁回路方式に切り替えるようしたが、必ずしも２０％を直列励磁回路方式から並列励磁回路方式への切り替えポイントとしなくてもよい。例えば、計測値５０％を直列励磁回路方式から並列励磁回路方式への切り替えポイントするなどとしてもよい。また、図２に示した計測値と励磁電流Ｉｅｘとの関係はあくまでも一例として示したものであり、励磁電流Ｉｅｘの切り替えポイントを変えたり、励磁電流Ｉｅｘの値を変えるなど、種々の変形が自在である。
【００３７】
【発明の効果】
以上説明したことから明らかなように本発明によれば、流量測定出力回路における計測値が所定値を越えた場合、一対の信号線の間に直列に接続されている励磁回路と流量測定出力回路との接続を並列に切り替える切替手段を設けたので、計測値が所定値を越えるまでは外部電圧が励磁回路と流量測定出力回路とに分圧されていたものが、計測値が所定値を超えると励磁回路と流量測定出力回路とのそれぞれに印加されるものとなり、これにより励磁回路において生成される励磁電圧をそれまでの励磁電圧よりも大きくすることが可能となり、励磁電流の立ち上がり時間を早くし、極性が切り替わる直前の定常域を長くし、計測値に応じて多段階に設定される励磁電流の限界値をさらに大きくすることができるようになる。
【図面の簡単な説明】
【図１】本発明に係る２線式電磁流量計の一実施の形態の概略を示す図である。
【図２】この２線式電磁流量計における計測値と励磁電流Ｉｅｘの指示値との関係を示す図である。
【図３】この２線式電磁流量計における計測値が２０％を上回る前までの回路接続（直列励磁回路方式）を単純化して示した図である。
【図４】この２線式電磁流量計における計測値が２０％を上回った時の回路接続（並列励磁回路方式）を単純化して示した図である。
【図５】並列励磁回路方式に切り替えられた場合の励磁電流Ｉｅｘの値を１６．５ｍＡとしたときの立ち上がり波形を例示する図である。
【図６】従来の２線式電磁流量計の概略を示す図である。
【図７】従来の２線式電磁流量計における計測値と励磁電流Ｉｅｘの指示値との関係を示す図である。
【図８】励磁電流Ｉｅｘの値をＩｅｘ＝３．５ｍＡ、６．７ｍＡ、９．９ｍＡ、１２ｍＡと切り替えた場合の立ち上がり波形を例示する図である。
【符号の説明】
１…測定管、２…励磁コイル、３…励磁回路、３−１…励磁電圧回路、３−２…電流調整回路、４ａ，４ｂ…電極、５…接地電極、６…流量測定出力回路、６−１…信号起電力検出回路、６−２…サンプルホールド回路、６−３…Ａ／Ｄ変換回路、６−４…ＣＰＵ、６−５…Ｄ／Ａ変換回路、６−６…電流調整回路、ＣＰ１…コンパレータ、Ｑ１…トランジスタ、Ｒ１…抵抗、６−７…定電圧回路、ＳＷ１〜ＳＷ３…スイッチ、ＬＡ，ＬＢ，ＬＣ…接続ライン、Ｌ１，Ｌ２…信号線、ＲＬ…外部負荷、１００Ａ…２線式電磁流量計、２００…直流電源。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an electromagnetic flowmeter for measuring the flow rate of a fluid having conductivity in various process systems, and in particular, an external voltage is supplied from a pair of signal lines connected to a DC power supply, and a pair of signals for supplying the external voltage The present invention relates to a two-wire electromagnetic flowmeter that outputs a measured value by adjusting an output current flowing through a wire.
[0002]
[Prior art]
Conventionally, in this type of two-wire electromagnetic flowmeter, a rectangular wave-shaped exciting current Iex at a predetermined frequency is applied to an exciting coil arranged with the direction of generation of the magnetic field perpendicular to the flow direction of the fluid flowing in the measuring tube. The sensor detects a signal electromotive force (a signal proportional to the flow rate) obtained between the electrodes arranged in the measurement tube at right angles to the magnetic field generated by the excitation coil, and calculates the CPU based on the detected signal electromotive force. The measured value is determined as a value of 0 to 100% by the processing, and the current (output current) flowing through a pair of signal lines for supplying an external voltage to the two-wire electromagnetic flow meter is 4 to 20 mA according to the measured value. The current range is adjusted.
[0003]
FIG. 6 schematically shows a conventional two-wire electromagnetic flowmeter. In the figure, 100 is a two-wire electromagnetic flow meter, 200 is a DC power supply, and an external voltage Vs from the DC power supply 200 is supplied to the two-wire electromagnetic flow meter 100 via a pair of signal lines L1 and L2. You. A resistance of, for example, 250Ω is connected to the signal line L2 as an external load RL. The DC power supply 200 is set to DC 24V. In this case, the external voltage Vs supplied to the two-wire electromagnetic flowmeter 100 is a value obtained by subtracting the voltage drop at the external load RL from the power supply voltage (DC 24 V) of the DC power supply 200.
[0004]
In the two-wire electromagnetic flow meter 100, 1 is a measurement tube, 2 is an excitation coil arranged so that the direction of generation of the magnetic field is perpendicular to the flow direction of the fluid flowing in the measurement tube 1, and 3 is rectangular to the excitation coil 2. An excitation circuit 4a, 4b for periodically supplying a wave-like excitation current Iex is a detection electrode arranged in the measuring tube 1 at right angles to the magnetic field generated by the excitation coil 2, 5 is a ground electrode, 6 is a detection electrode 4a, 4b is detected, a measured value is obtained based on the detected signal electromotive force, and the output current Iout returned to the DC power supply 200 according to the obtained measured value is set to a current of 4 to 20 mA. It is a flow measurement output circuit that adjusts in a range.
[0005]
The excitation circuit 3 has an excitation voltage circuit (constant voltage circuit) 3-1 and a current adjustment circuit (CCS) 3-2. The excitation voltage circuit 3-1 generates a constant voltage of 8.5 V as the excitation voltage Vex. The current adjustment circuit 3-2 applies the excitation voltage Vex from the excitation voltage circuit 3-1 to the excitation coil 2, and generates a rectangular wave excitation current Iex whose polarity is alternately switched. The value (peak value) of the exciting current Iex is switched in multiple stages according to the measurement value in the flow rate measurement output circuit 6, as shown in FIG.
[0006]
The flow rate measurement output circuit 6 includes a signal electromotive force detection circuit 6-1, a sample hold circuit 6-2, an A / D conversion circuit 6-3, a CPU 6-4, a D / A conversion circuit 6-5, It has a current adjustment circuit (CCS) 6-6 and a constant voltage circuit 6-7 for supplying a power supply voltage to these circuits.
[0007]
The signal electromotive force detection circuit 6-1 detects a signal electromotive force obtained between the electrodes 4a and 4b with reference to the potential of the ground electrode 5. The sample and hold circuit 6-2 receives the signal electromotive force detected by the signal electromotive force detection circuit 6-1 as an input, and samples and holds the value of the signal electromotive force immediately before the polarity is switched. The A / D conversion circuit 6-3 converts the signal electromotive force (analog value) sampled and held by the sample and hold circuit 6-2 into a digital value, and sends the digital value to the CPU 6-4.
[0008]
The CPU 6-4 obtains a measured value (0 to 100%) based on the signal electromotive force from the A / D conversion circuit 6-3 and outputs it to the D / A conversion circuit 6-5. The D / A conversion circuit 6-5 converts the measurement value (digital value) from the CPU 6-4 into an analog value and sends the analog value to the current adjustment circuit 6-6. The current adjustment circuit 6-6 has a comparator CP1, a transistor Q1, and a resistor R1, and adjusts the base current of the transistor Q1 by the comparator CP1 to thereby convert the current Iccs flowing between the collector and the emitter of the transistor Q1 into D / A. The adjustment is performed according to the measurement value from the conversion circuit 6-5. The CPU 6-4 supplies the exciting current Iex to the exciting coil 2 according to the measurement value obtained based on the signal electromotive force from the A / D conversion circuit 6-3 in the relationship shown in FIG. , And gives a command to the current adjustment circuit 3-2 of the excitation circuit 3.
[0009]
(DC excitation circuit method)
In the two-wire electromagnetic flow meter 100, the excitation circuit 3 and the flow measurement output circuit 6 are connected in series between the signal lines L1 and L2, and the current flowing through the excitation circuit 3 is supplied to the flow measurement output circuit 6 The output current Iout flows in and returns to the DC power supply 200. Such a system is called a series excitation circuit system.
[0010]
For example, when the measurement value of the CPU 6-4 is 0%, the value of the excitation current Iex in the excitation circuit 3 is set to 3.5 mA. In the excitation circuit 3, a current of 0.5 mA is required for generating the excitation voltage Vex in the excitation voltage circuit 3-1. If the current flowing in the excitation voltage circuit 3-1 is Ia, the current flowing in the excitation circuit 3 I1 is I1 = Ia + Iex = 0.5 mA + 3.5 mA = 4 mA.
[0011]
This 4 mA current flows into the flow measurement output circuit 6. In the flow rate measurement output circuit 6, when the current flowing to the constant voltage circuit 6-7 side is Ib, the current Ib needs 3 mA to drive the CPU 6-4 and the like, and the current Iccs flowing to the transistor Q1 side is 1 mA. , The current I2 flowing through the flow measurement output circuit 6 (I2 = Iccs + Ib) becomes 4 mA, the current I1 flowing through the excitation circuit 3 becomes equal to the current I2 flowing through the flow measurement output circuit 6, and the DC power supply 200 Is 4 mA indicating the measured value of 0%.
[0012]
If the measured value of the CPU 6-4 becomes, for example, 10%, the CPU 6-4 adjusts the current Iccs flowing through the transistor Q1 to 2.6 mA in order to set the output current Iout to Iout = 4 mA + 1.6 mA = 5.6 mA. I do. In this case, in the excitation circuit 3, since the excitation current Iex = 3.5 mA, the current Ia flowing through the excitation voltage circuit 3-1 is Ia = 2.1 mA.
[0013]
[Reason for changing the value of the excitation current Iex in multiple stages according to the measured value]
The value of the excitation current Iex is switched in multiple stages according to the value measured by the CPU 6-4 according to the relationship shown in FIG. Such a method of switching the value of the excitation current Iex in multiple stages is called a multipoint excitation switching method. If the multi-point excitation switching method is not used, that is, if the value of the excitation current Iex is fixed to 3.5 mA, the signal electromotive force obtained by the signal electromotive force detection circuit 6-1 is small because the magnetic flux density penetrating the fluid is small, The output fluctuates greatly under the influence of noise superimposed on the electrodes 4a and 4b according to the flow velocity. That is, as the flow rate increases, the ratio of noise included in the signal electromotive force increases, the S / N ratio decreases, and stable flow rate measurement cannot be performed.
[0014]
Assuming that the signal electromotive force obtained by the signal electromotive force detection circuit 6-1 is e, the signal electromotive force e is expressed as e = kBvD. In this equation, k is a constant, D is the diameter of the measuring tube 1, v is the average flow velocity, and B is the generated magnetic flux density. Here, B is proportional to the exciting current Iex, and if the exciting current Iex is increased, the signal electromotive force e increases even at the same flow velocity. Therefore, in the conventional two-wire electromagnetic flow meter 100 shown in FIG. 6, if the output current (4 to 20 mA) corresponding to the measured value increases, that is, if the output current (4 to 20 mA) increases, the increased amount is used. , The exciting current Iex is switched to a large value.
[0015]
For example, when the measured value becomes 20%, the value of the excitation current Iex is switched to 6.7 mA. That is, the output current Iout corresponding to the measured value of 20% is 7.2 mA, and the exciting circuit 3 requires Ia = 0.5 mA, so that the exciting current Iex can flow up to 6.7 mA. When the measured value becomes 40%, the value of the excitation current Iex is switched to 9.9 mA. That is, the output current Iout corresponding to the measured value of 40% is 10.4 mA, and Ia = 0.5 mA is required in the excitation circuit 3, so that the excitation current Iex can flow up to 9.9 mA. As described above, by switching the excitation current Iex to a large value according to the measured value, the signal electromotive force e is increased, the S / N ratio is improved, and a stable flow rate measurement can be performed.
[0016]
[Problems to be solved by the invention]
However, in the above-described conventional two-wire electromagnetic flow meter 100, since the DC excitation circuit system is used, the external voltage Vs supplied to the two-wire electromagnetic flow meter 100, that is, the external load V DC of the DC power supply 200 is reduced to the external load. The voltage Vs obtained by subtracting the voltage drop Iout × RL in the RL is divided into the excitation circuit 3 and the flow rate measurement output circuit 6. For this reason, the exciting voltage Vex generated by the exciting voltage circuit 3-1 is as small as 8.5 V, and the rectangular wave-like exciting current Iex supplied to the exciting coil 2 has a rising time to a required value as the value increases. become longer.
[0017]
FIG. 8 shows a rising waveform when the value of the excitation current Iex is switched to Iex = 3.5 mA, 6.7 mA, 9.9 mA, and 12 mA. When the value of the excitation current Iex is as small as 3.5 mA, the same waveform is obtained. It rises immediately like the waveform I shown in the figure. However, since the voltage does not change by the excitation generated by the excitation voltage circuit 3-1, when the value of the excitation current Iex increases to 6.7 mA, 9.9 mA, and 12 mA, waveforms II, III, and IV shown in FIG. As described above, the rise time becomes longer, and the steady region (a flat waveform portion after Iex reaches a required value) ta immediately before the polarity is switched becomes shorter.
[0018]
The sample hold circuit 6-2 samples and holds the value of the signal electromotive force e immediately before the polarity is switched. For example, the signal electromotive force e is sampled for 5 ms immediately before the polarity of the signal electromotive force e is switched, and the average value is held. When the value of the exciting current Iex is 12 mA, the steady-state region ta immediately before the polarity of the exciting current Iex is switched is about 5 ms, and the signal electromotive force e to be sampled is a value obtained by the barely stable exciting current Iex.
[0019]
However, when the value of the exciting current Iex generally exceeds 12 mA, the signal electromotive force e while the exciting current Iex is changing is sampled, and an eddy current generated in the electrodes 4a and 4b causes An error is included in the measured value of the flow rate. For this reason, in the conventional two-wire electromagnetic flowmeter 100 of the multi-point excitation switching method, the limit value of the excitation current Iex set in multiple stages according to the measured value is about 12 mA, and the excitation current Iex Could not be increased.
[0020]
The present invention has been made to solve such a problem, and an object of the present invention is to provide a two-wire system capable of further increasing the limit value of the excitation current set in multiple stages according to the measured value. An object of the present invention is to provide an electromagnetic flowmeter.
[0021]
[Means for Solving the Problems]
In order to achieve such an object, the present invention relates to the above-described two-wire electromagnetic flow meter, wherein when a measurement value in a flow measurement output circuit exceeds a predetermined value, the two-wire electromagnetic flow meter is connected in series between a pair of signal lines. A switching means for switching the connection between the excitation circuit and the flow rate measurement output circuit in parallel.
According to the present invention, when the measured value in the flow measurement output circuit exceeds, for example, 20%, the excitation circuit and the flow measurement output circuit that have been connected in series between the pair of signal lines until then are connected in parallel. . That is, the external voltage that has been divided into the excitation circuit and the flow measurement output circuit until then is applied to each of the excitation circuit and the flow measurement output circuit. For this reason, it is possible to make the excitation voltage generated in the excitation circuit larger than the previous excitation voltage, so that the rise time of the excitation current can be shortened, and the stationary region immediately before the polarity is switched can be lengthened. Become. As a result, even if the exciting current is set to, for example, 12 mA or more, it becomes possible to secure 5 ms or more as a steady region immediately before the polarity is switched, and the limit value of the exciting current set in multiple steps according to the measured value is set to 12 mA or more. It is possible to do.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram schematically showing an embodiment of a two-wire electromagnetic flow meter according to the present invention. 6, the same reference numerals as those in FIG. 6 denote the same or equivalent components as those described with reference to FIG. 6, and a description thereof will be omitted.
[0023]
In the two-wire electromagnetic flow meter 100A, a first switch SW1 is provided in a connection line LA between the excitation circuit 3 and the flow measurement output circuit 6, and a first switch SW1 is provided on a connection line between the excitation voltage circuit 3-1 and the switch SW1. A second switch SW2 is provided between the point P1 and a point P2 on the connection line between the resistor R1 and the external load RL (in the connection line LB) to connect the signal line L1 to the excitation voltage circuit 3-1. A third switch SW3 is provided between a point P3 on the line and a point P4 on the connection line between the switch SW1 and the flow rate measurement output circuit 6 (in the connection line LC).
[0024]
Further, a function of controlling the on / off of the switches SW1 to SW3 is added to the CPU 6-4 in the flow rate measurement output circuit 6 so that the measured value of the CPU 6-4 output from the A / D conversion circuit 6-3 is 0 to 0. In the range of 20%, the switch SW1 is turned on (the switches SW2 and SW3 are turned off), and when the measured value in the CPU 6-4 exceeds 20%, the switches SW2 and SW3 are turned on (the switch SW1 is turned off). Like that.
[0025]
Further, according to the measurement value obtained based on the signal electromotive force from the A / D conversion circuit 6-3, a current having a value determined by the relationship shown in FIG. 2 instead of the relationship shown in FIG. The CPU 6-4 gives a command to the current adjustment circuit 3-2 of the excitation circuit 3 so as to perform the operation. When the measured value of the CPU 6-4 exceeds 20%, a command is given from the CPU 6-4 to the excitation voltage circuit 3-1 to change the value of the excitation voltage Vex to 8.5 V in accordance with the switching of the switches SW1 to SW3. To a constant voltage of 14V.
[0026]
[Series excitation circuit method: 0% to 20%]
The CPU 6-4 obtains a measured value based on the signal electromotive force from the A / D conversion circuit 6-3, and turns on the switch SW1 and turns off the switches SW2 and SW3 when the measured value is in the range of 0 to 20%. I do. Thus, the excitation circuit 3 and the flow measurement output circuit 6 are connected in series between the signal lines L1 and L2, and the two-wire electromagnetic flow meter 100A performs the same operation as the conventional two-wire electromagnetic flow meter 100. Do.
[0027]
FIG. 3 shows a simplified circuit connection in the two-wire electromagnetic flow meter 100A in this case. In this case, the exciting voltage Vex generated by the exciting voltage circuit 3-1 in the exciting circuit 3 is set to 8.5 V, and the value of the exciting current Iex supplied to the exciting coil 2 by the current adjusting circuit 3-2 is set to 3.5 mA. The current Ia flowing through the excitation voltage circuit 3-1 is set to 0.5 to 3.7 mA corresponding to the measured value of 0% to 20%. In the flow measurement output circuit 6, the current Ib to the constant voltage circuit 6-7 is set to 3 mA, and the current Iccs to the transistor Q1 is set to 1 to 4.2 mA corresponding to the measured value of 0% to 20%. Is done.
[0028]
[Parallel excitation circuit type: 20% to 100%]
When the measured value obtained based on the signal electromotive force from the A / D conversion circuit 6-3 exceeds 20%, that is, when the output current Iout returned to the DC power supply 200 exceeds 7.2 mA, the CPU 6-4 determines that The switch SW1 is turned off, and the switches SW2 and SW3 are turned on. As a result, the excitation circuit 3 and the flow rate measurement output circuit 6, which have been connected in series between the signal lines L1 and L2, are connected in parallel between the signal lines L1 and L2.
[0029]
FIG. 4 shows a simplified circuit connection in the two-wire electromagnetic flow meter 100A in this case. In this case, the input current Iin = 7.2 mA from the DC power supply 200 is divided into a current I1 (I1 = Iex + Ia) to the excitation circuit 3 and a current I2 (I2 = Ib + Iccs) to the flow rate measurement output circuit 6. In this example, the value of the exciting current Iex to the exciting coil 2 in the exciting circuit 3 is 3.5 mA, and a current of Ia = 0.5 mA is required to generate the exciting voltage Vex in the exciting voltage circuit 3-1. The flow measurement output circuit 6 needs 3 mA as the current Ib to the constant voltage circuit 6-7. The sum of these currents Iex, Ic and Ib is 3.5 + 0.5 + 3 = 7 mA, which is almost equal to the input current Iin from the DC power supply 200 = 7.2 mA. Note that the difference 0.2 mA between 7.2 mA and 7 mA is the current Iccs flowing on the transistor Q1 side in the flow rate measurement output circuit 6.
[0030]
When the measured value exceeds 20%, the CPU 6-4 gives a command to the excitation voltage circuit 3-1 to switch the value of the excitation voltage Vex from 8.5V to 14V. The switching of the excitation voltage Vex from 8.5 V to 14 V is realized by connecting the excitation circuit 3 and the flow rate measurement output circuit 6 in parallel between the signal lines L1 and L2 by switching the switches SW1 to SW3. It is possible.
[0031]
That is, before the measured value exceeds 20%, the excitation circuit 3 and the flow rate measurement output circuit 6 are connected in series between the signal lines L1 and L2. In this case, the external voltage Vs of the two-wire electromagnetic flow meter 100A is divided into the excitation circuit 3 and the flow measurement output circuit 6, and the supply voltage to the excitation circuit 3 is small. For this reason, the value of the excitation voltage Vex can be only about 8.5V. On the other hand, when the measured value exceeds 20%, the excitation circuit 3 and the flow rate measurement output circuit 6 are connected in parallel between the signal lines L1 and L2. In this case, the external voltage Vs of the two-wire electromagnetic flow meter 100A is applied to each of the excitation circuit 3 and the flow measurement output circuit 6, and the supply voltage to the excitation circuit 3 increases. This makes it possible to increase the excitation voltage Vex generated by the excitation voltage circuit 3-1 to about 14V.
[0032]
When the excitation voltage Vex generated by the excitation voltage circuit 3-1 is set to 14 V, the rising time of the excitation current Iex is earlier than the excitation voltage Vex = 8.5 V up to that point, and the steady range immediately before the polarity is switched is also increased. become longer. Although there is no significant difference when the value of the excitation current Iex is about 3.5 mA, the difference increases as the value of the excitation current Iex increases with an increase in the flow signal (input voltage). When the current Iex is set to 16.5 mA at which the maximum current can flow, a steady area ta of about 6 ms can be secured as shown in FIG. That is, conventionally, the limit value of the excitation current Iex could only be about 12 mA, but the limit value of the excitation current Iex was maximized by switching from the series excitation circuit method to the parallel excitation circuit method at a measurement value of 20%. It can be increased to 16.5 mA that can flow.
[0033]
When the measured value exceeds 40%, the CPU 6-4 switches the value of the excitation current Iex to 6.9 mA. That is, the output current Iout corresponding to the measured value of 40% is 10.4 mA, the excitation circuit 3 requires Ia = 0.5 mA, and the flow measurement output circuit 6 requires 3 mA. If the current Iccs to the transistor Q1 side is 0 mA, 6.9 mA can flow as the excitation current Iex.
[0034]
The CPU 6-4 sets the value of the excitation current Iex to 6.9 mA until the measured value exceeds 60%. The surplus current during this period flows as the current Iccs to the transistor Q1. When the measured value exceeds 60%, the value of the exciting current Iex is switched to 10.1 mA. That is, the output current Iout corresponding to the measured value of 60% is 13.6 mA, the excitation circuit 3 requires Ia = 0.5 mA, and the flow measurement output circuit 6 requires 3 mA. Assuming that the current Iccs to the transistor Q1 side is 0 mA, 10.1 mA can flow as the exciting current Iex.
[0035]
Similarly, when the measured value exceeds 80%, the CPU 6-4 sets the value of the exciting current Iex to 13.3 mA, and when the measured value reaches 100%, the CPU 6-4 sets the value of the exciting current Iex to 16.5 mA. Even if the excitation current Iex exceeds 12 mA, in this embodiment, the excitation voltage Vex is increased to 14 V as a parallel excitation circuit system, so the rising time of the excitation current Iex is short, and the steady-state region immediately before the polarity is switched is also increased. Since the length becomes longer, the signal electromotive force e obtained by the stable excitation current Iex is sampled, and an accurate measurement value can be obtained.
[0036]
In the above-described embodiment, when the measured value exceeds 20%, the system is switched from the series excitation circuit system to the parallel excitation circuit system. However, it is not always necessary to switch 20% from the series excitation circuit system to the parallel excitation circuit system. You do not need to make points. For example, a measurement value of 50% may be a switching point from the series excitation circuit system to the parallel excitation circuit system. Further, the relationship between the measured value and the exciting current Iex shown in FIG. 2 is merely an example, and various modifications such as changing the switching point of the exciting current Iex and changing the value of the exciting current Iex are possible. It is.
[0037]
【The invention's effect】
As is apparent from the above description, according to the present invention, when the measured value in the flow rate measurement output circuit exceeds a predetermined value, the excitation circuit and the flow rate measurement output circuit connected in series between a pair of signal lines. The switching means for switching the connection in parallel to the external circuit is provided, so that the external voltage is divided into the excitation circuit and the flow rate measurement output circuit until the measured value exceeds the predetermined value, but the measured value exceeds the predetermined value. And the excitation circuit and the flow rate measurement output circuit are applied to each of the excitation circuits.This makes it possible to make the excitation voltage generated in the excitation circuit larger than the previous excitation voltage. However, the stationary region immediately before the polarity is switched can be lengthened, and the limit value of the exciting current set in multiple stages according to the measured value can be further increased.
[Brief description of the drawings]
FIG. 1 is a view schematically showing an embodiment of a two-wire electromagnetic flow meter according to the present invention.
FIG. 2 is a diagram showing a relationship between a measured value and an indicated value of an exciting current Iex in the two-wire electromagnetic flow meter.
FIG. 3 is a diagram showing a simplified circuit connection (series excitation circuit method) before a measured value in this two-wire electromagnetic flow meter exceeds 20%.
FIG. 4 is a simplified diagram showing circuit connection (parallel excitation circuit system) when a measured value of the two-wire electromagnetic flowmeter exceeds 20%.
FIG. 5 is a diagram illustrating a rising waveform when the value of the excitation current Iex is 16.5 mA when switching to the parallel excitation circuit method is performed.
FIG. 6 is a diagram schematically showing a conventional two-wire electromagnetic flow meter.
FIG. 7 is a diagram showing a relationship between a measured value in a conventional two-wire electromagnetic flowmeter and an indicated value of an exciting current Iex.
FIG. 8 is a diagram illustrating a rising waveform when the value of the excitation current Iex is switched to Iex = 3.5 mA, 6.7 mA, 9.9 mA, and 12 mA.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Measurement tube, 2 ... Excitation coil, 3 ... Excitation circuit, 3-1 ... Excitation voltage circuit, 3-2 ... Current adjustment circuit, 4a, 4b ... Electrode, 5 ... Ground electrode, 6 ... Flow rate measurement output circuit, 6 -1 ... signal electromotive force detection circuit, 6-2 ... sample and hold circuit, 6-3 ... A / D conversion circuit, 6-4 ... CPU, 6-5 ... D / A conversion circuit, 6-6 ... current adjustment circuit CP1, comparator, Q1 transistor, R1 resistor, 6-7 constant voltage circuit, SW1 to SW3 switch, LA, LB, LC connection line, L1, L2 signal line, RL external load, 100A 2-wire electromagnetic flow meter, 200 DC power supply.

Claims

An excitation coil arranged so that the direction of generation of the magnetic field is perpendicular to the flow direction of the fluid flowing in the measurement tube;
An excitation circuit that generates an excitation voltage to the excitation coil and supplies an excitation current whose polarity periodically changes;
A signal electromotive force obtained between electrodes arranged in the measurement tube orthogonal to the flow direction of the fluid flowing in the measurement tube and the direction of the generated magnetic field of the excitation coil is detected, and based on the detected signal electromotive force, The output current flowing through a pair of signal lines for supplying an external voltage is adjusted within a predetermined current range in accordance with the obtained measurement value, and the output current from the excitation circuit is adjusted in accordance with the obtained measurement value. A flow rate measurement output circuit that switches the value of the exciting current to the exciting coil in multiple stages,
In a two-wire electromagnetic flow meter, wherein the excitation circuit and the flow measurement output circuit are connected in series between the pair of signal lines,
When the measurement value in the flow rate measurement output circuit exceeds a predetermined value, a switching unit that switches a connection between the excitation circuit and the flow rate measurement output circuit connected in series between the pair of signal lines in parallel. A two-wire electromagnetic flowmeter, comprising:

The two-wire electromagnetic flow meter according to claim 1,
When the measurement value in the flow rate measurement output circuit exceeds a predetermined value, the switching unit switches an excitation voltage to the excitation coil in the excitation circuit to an excitation voltage higher than a previous excitation voltage. 2-wire electromagnetic flowmeter.