JP4053392B2 - Capacity type electromagnetic flow meter - Google Patents

Description

の関係が成立するので、抵抗Ｒ１、Ｒ４を１００ＫΩ、抵抗Ｒ２、Ｒ３を１ＫΩとし、信号ｖ_Ｎとして、１Ｖ、５００Ｈｚの信号を抵抗Ｒ１の一方の端から印加すると、面電極４Ａとガード電極５Ａの間に空検出用微小信号ｖ_ＮＥ（ｅ_Ａ−ｖ_Ｇ）として１０ｍＶの信号が印加される。
【００２８】
この様に設定された容量式電磁流量計の動作について、図１でその概要を説明する。励磁コイル３Ａ、３Ｂに印加する励磁電流ｉ_Fで定常値を持つ矩形波の正負信号を印加すると、ほぼ同様の波形を持つ磁場が印加される。
【００２９】
ここで、被測定流体２が流れると、被測定流体２の流速に比例した起電圧が発生し、被測定流体２と面電極４Ａ、４Ｂとの間の静電容量を介して、前置増幅器６Ａ、６Ｂで増幅され、更に差動増幅器７で増幅されて、検出信号ｅとして後続の信号処理部１１に送られる。
【００３０】
信号処理部１１のＡＤＣ回路１１１では、前述した様に、励磁周波数(ｆ０)の１０倍以上の周波数でサンプリングされ、デジタル値として、流量測定処理部１１２と空判定処理部１１３に送り、ここに内蔵されたプログラム処理機能で処理されて、流量測定信号及び空判定信号を出力する。
【００３１】
次に、流量測定処理部１１２と空判定処理部１１３における処理について図４と図５で説明する。
【００３２】
図４の（ａ）は励磁電流ｉ_Ｆ、（ｂ）はＡＤＣ回路１１１のサンプリングパルスｓ_Ａ、（ｃ）はこの時の検出信号ｅの関係を示す。検出信号ｅには、前述した微分ノイズ（図４（ｃ）における波形のＤ部）が励磁電流の立ち上がりと立下り部分に重畳した波形となる。
【００３３】
この検出信号ｅは、励磁電流の図４（ｂ）に示すＡＤＣ回路１１１のサンプリングパルス信号ｓ_Ａのタイミングでサンプリングされ、デジタル値として流量測定処理部１１２で処理される。
【００３４】
以下この処理動作を説明する。流量測定処理部１１２における処理は図４（ｂ）に示す様に、微分ノイズ成分を含まない時間、Ｓ４、Ｓ５、Ｓ９、Ｓ１０のタイミングでサンプリングされた各々の値から、一周期毎の値として
（Ｓ４＋Ｓ５）−（Ｓ９＋Ｓ１０）を求める演算を行い、この値を所定の測定周期の平均値として流量信号に変換処理して出力される。応答を早くする場合は、この励磁周波数(ｆ０)の周期毎に出力する。
【００３５】
次に、空判定処理部１１３の動作について説明する。図４（ｅ）、（ｆ）は検出信号ｅを一定時間のフーリエ変換処理した結果の出力で、周波数スペクトルＥ_Ｓ（ω）を示し、各々の横軸は周波数（Ｈｚ）、縦軸はその検出信号ｅのパワーを示す。
【００３６】
この周波数スペクトルＥ_Ｓ(ω)には、検出信号ｅに含まれる信号成分、雑音成分が重畳している。信号成分としては、流速に比例する起電圧に含まれる矩形波の励磁電流に含まれる周波数成分である２００Ｈｚの基本周波数（ｆ０）成分、第３高調波（この周波数を以後ｆ３と言う）成分，第５高調波（この周波数を以後ｆ５と言う）成分がある。
【００３７】
また、雑音成分としては、空検出のために印加する５００Ｈｚ（この周波数を以後ｆ１と言う）の空検出用微小信号ｖ_ＮＥ成分と、全周波数領域において一様なパワー強度を有する前置増幅器６Ａ、６Ｂからの白色雑音ｖ_ＮＷ成分、及び前述した２００Ｈｚ以下に分布する前述した流体ノイズ成分が含まれる。
【００３８】
以下、図５を参照しながら、この検出信号ｅに重畳させた空検用微小信号ｖ_ＮＥによる空判定処理部１１３での処理手順について説明する。
【００３９】
（ステップ１）
まず、測定管１に被測定流体２の満水状態と空状態を設定し、検出信号ｅのフーリエ変換処理を行い、この周波数スペクトルＥ_Ｓ（ω）から満水時の空検出周波数(ｆ１)成分Ｅ_Ｓ（ω）_ｆ１Ｏを求める。満水状態の設定は、通常、起電圧の発生がない被測定流体２が静止した状態とするのが好ましいが、流れた状態で設定することも可能である。
【００４０】
また、フーリエ変換処理（積分）時間は、所定の時間（励磁電流の 1 周期毎）とし、励磁周波数の１０倍のサンプリング周波数２ｋＨｚにおいて、検出信号ｅを２０００点サンプルできる設定とし、空判定処理は 1 秒程度に設定する。
【００４１】
この満水状態での周波数スペクトルＥ_Ｓ（ω）を図４(ｅ)に示す。図４（ｅ）において、２００Ｈｚ、６００Ｈｚ、１０００Ｈｚの個所に見られるピーク値は、流速に比例する起電圧に含まれる周波数スペクトル成分で、夫々励磁周波数（ｆ０）の基本波成分、第３高調波（ｆ３）成分，第５高調波（ｆ５）を、また５００Ｈｚ個所に見られるピーク値は、空検出周波数（ｆ１）成分が現れる。
【００４２】
また、その他の周波数のスペクトル成分は，前置増幅器６Ａ、６Ｂからの白色雑音ｖ_ＮＷ成分を示し、２００Ｈｚ以下の周波数のスペクトルには、前述した流体ノイズ成分が含まれている。
【００４３】
（ステップ２）
次に、被測定流体２を流出させ面電極４Ａ、４Ｂ間に空状態を設定し、空時のＥ_Ｓ（ω）_ｆ１Ｅを求める。
【００４４】
この空状態の時の周波数スペクトルＥ_Ｓ（ω）を図４（ｆ）に破線で示す。この時の周波数スペクトルＥ_Ｓ（ω）は、図４（ｅ）に比べて大きくなる。
【００４５】
この理由について説明する。図３にした回路構成は、図７に示す様な等価回路となるので、前置増幅器６Ａの入力電圧、即ち、一方の面電極４Ａからの検出信号ｅ_Ａと空検出用微小信号ｖ_ＮＥ、及び前置増幅器６Ａの白色雑音電圧ｖ_Ｎｗの関係は、次の(１)式の様になる。
【００４６】
ｅ_Ａ＝（１＋ｊωＣ１・Ｒｓ）Ｃ２／Ｃ１×（ｖ_ＮＥ＋ｖ_Ｎｗ）・・・（１）
ここで、Ｃ１は面電極４Ａと被測定流体２の間の静電容量値、Ｒｓは被測定流体２の抵抗値、Ｃ２は面電極４Ａとガード電極５Ａ間の静電容量値を示す。
【００４７】
被測定流体２に空状態が発生すると被測定流体２の抵抗値Ｒｓはさらに高抵抗値となり、また測定管１の内壁に異物が付着すると被測定流体２内の誘電率が水の場合に比べて小さいので、Ｃ１の値はより小さな値となる。その結果、満水時に比べて空時の検出信号ｅ_Ａは大きく増幅される。
【００４８】
従って、この時の周波数スペクトルＥ_Ｓ（ω）は、満水時に図４（ｅ）であったものが、空時には、図４（ｆ）（破線）に示す様に一様に大きなパワー値を示す。また、空時の空検出周波数ｆ１でのスペクトル成分（Ｅ_Ｓ（ω）_ｆ１Ｅ＝）Ｈ_Ｅも、同様に、満水時のスペクトル成分（Ｅ_Ｓ（ω）_ｆ１Ｏ＝）Ｈ_Ｏに比べて大きな値を示すことになる。
【００４９】
（ステップ３）
次に、図５ステップ３では、ステップ１とステップ２で求めた空検出周波数（ｆ１）のスペクトルのパワーの値から、空判定値Ｎ_Ｒ（ω）_ｆ１を下記の範囲で所定の値に設定する。
【００５０】
Ｈ_Ｅ＞Ｎ_Ｒ（ω）_ｆ１＞Ｈ_Ｏ・・・（２）
(ステップ４)
この空判定値Ｎ_Ｒ（ω）_ｆ１を所定の値に設定した後、通常の流量計測を前述した演算処理で行う。
【００５１】
(ステップ５)
次に、フーリエ積分時間１秒が経過する毎に、ステップ３で設定した空判定設定値Ｎ_Ｒ（ω）_ｆ１と周波数スペクトルＥ_Ｓ（ω）の空検出周波数成分Ｅ_Ｓ（ω）_ｆ１とを比較し、空判定設定値Ｎ_Ｒ（ω）_ｆ１以下であれば流量測定を継続する。
【００５２】
(ステップ６)
ステップ５の結果、周波数スペクトルＥ_Ｓ（ω）の空検出周波数成分Ｅ_Ｓ（ω）_ｆ１が、Ｎ_Ｒ（ω）_ｆ１以上になれば空状態として判定する。
【００５３】
尚、この空検出用微小信号ｖ_ＮＥは、流速測定処理部１１２に影響を与えない様に、励磁の休止期間等のみ印加する様に空検出信号発生回路９を構成してもよい。
【００５４】
また、空検出用微小信号ｖ_ＮＥが流速測定処理部１１２に影響与えない他の方法としては、検出信号ｅにフィルタを設けてＳ／Ｎを改善しておく方法がある。
【００５５】
この方法は、図８に示す様に、ＡＤＣ回路１１１Ａ、１１１Ｂを２台設け、更に、流量測定系統には、空検出信号発生回路９からの空検出用微小信号ｖ_ＮＥ成分をカットするフィルタ１１４を介した後にＡＤＣ回路１１１Ｂを介して流速測定処理部に導く様に構成すれば、各々の並列処理が可能で、流量測定処理部１１２に空検出信号発生回路９からの信号が重畳しない高精度な流量測定が行える。
【００５６】
また、面電極４Ａとガード電極５Ａ間の静電容量値Ｃ２を予め計測しておき、空検出信号発生器９から印加する空検出用微小信号ｖ_ＮＥの周波数（ｆ１）を時分割で２種以上印可して、前述した(１)式の関係を利用して、満水時と空時の検出信号ｅを測定することによって、面電極４Ａと被測定流体２の間の静電容量値Ｃ１、被測定流体２の抵抗値Ｒｓを測定する事が出来る。
【００５７】
このようにすれば、抵抗値の変化か、容量値の変化かが判別できるので、空状態が被測定流体２の導電率の変化によるものであるか、或いは異物等の付着によるものかを予測判定することが可能となる。
【００５８】
以上述べた様に、本発明の前記実施の形態によれば、検出信号ｅとして、既知の固定された空検出用微小信号ｖ_ＮＥを外部から印加しているので、検出信号ｅの微分波形の歪みに依存しない空判定が安定して行える。
【００５９】
また、空判定の処理はフーリエ変換処理による統計処理によって周波数分離され、Ｓ／Ｎ比を高く出来るので、被測定流体２が満水と空の間の半満水状態の判定も可能となる。
【００６０】
更に、励磁周波数を流体ノイズの周波数成分が小さくなる周波数以上に設定したので、流体ノイズの変動に影響されない安定した流量測定と空判定が可能となる。
【００６１】
（第２の実施の形態）
次に、図１、図４、及び図６を参照して、本発明における第２の実施の形態の作用について説明する。
【００６２】
第１の実施の形態では、空検出信号発生回路９によって新たな空検出用微小信号ｖ_ＮＥを印加して、この信号の周波数（ｆ１）の周波数スペクトル成分Ｅ_Ｓ（ω）_ｆ１の変化から空状態の変化を検出したが、本第２の発明の実施の形態では、検出信号eに含まれる前置増幅器６Ａ、６Ｂからの白色雑音ｖ_ＮＷ成分のスペクトルパワーの積算値Ｅ_ＳＮ（ω）_Σの変化から、空判定を行うようにしている。
【００６３】
尚、この積算する周波数範囲は、励磁周波数（ｆ０、ｆ３、ｆ５）成分を除く２００Ｈｚから１０００Ｈｚまでの範囲とする。
【００６４】
従って、第２の実施の形態と第１の実施の形態との構成の差異は、検出部１０に空検出信号発生回路９がなく、ガード電極５Ａからの接続は、ガード電極５Ｂからの接続と同様に、前置増幅器６Ａの出力と−入力端とを接続している。その他の構成は第１の実施の形態と同じである。
【００６５】
検出信号ｅのフーリエ変換出力の周波数スペクトルＥ_Ｓ（ω）は、図４（ｇ）に示す様に、満水時に実線で示すスペクトルパワーであったものが、空時には破線の様に大きくパワーが変化する。この理由は、前述した空状態の時の方が流体の内部容量値（Ｃ１）が小さくなり、大きく増幅されることによる。
【００６６】
以下図４（ｇ）の周波数スペクトルＥ_Ｓ（ω）のデータについて図６で、空判定処理部１１３でのこの処理手順を説明する。
【００６７】
(ステップ１、ステップ２)
まず、第１の実施の形態と同様に、測定管１の被測定流体２の満水時と空時の状態を設定し、フーリエ変換処理後の周波数スペクトルＥ_Ｓ（ω）から下記式によって、白色雑音ｖ_ＮＷの積算値、周波数スペクトルＥ_ＳＮ(ω)_Σを求める。
【００６８】
E_SN(ω) _Σ =E_S(ω) _Σ -E_S(ω)_{0 〜 f0}-E_S(ω)_f3+f5 ・・・（３）（３）式の積算範囲を図示すると、図４（ｇ）の斜線の範囲となる。
【００６９】
即ちＥ_ＳＮ(ω)_Σは、周波数が０からｆ５の全周波数範囲のスペクトル成分Ｅｓ（ω）_Σから、周波数が０からｆ０（２００Ｈｚ）の範囲の成分Ｅｓ（ω）_０〜ｆ０と、励磁周波数のｆ３、ｆ５の高調波周波数成分Ｅｓ（ω）_{ｆ３＋ｆ５}を除いたものである。
【００７０】
上演算式において、満水時の値、Ｅ_ＳＮ（ω）_ΣＯ（ステップ１）、空時の値、Ｅ_ＳＮ（ω）_ΣＥ（ステップ２）を求める。
【００７１】
(ステップ３)
次に、空判定値Ｎ_Ｒ（ω）_Σとして所定の値を下記の範囲内で設定する。
【００７２】
Ｅ_ＳＮ（ω）_ΣＥ＞Ｎ_Ｒ（ω）_Σ＞Ｅ_ＳＮ（ω）_ΣＯ・・・（４）
（ステップ４）
上記の空判定値Ｎ_Ｒ（ω）_Σを設定した後、通常の流量計測を前述した演算処理で行う。
【００７３】
（ステップ５）
次に、フーリエ積分時間１秒が経過する毎に、ステップ３で設定した空判定設定値Ｎ_Ｒ（ω）_Σと（３）式で求めた白色雑音ｖ_ＮＷのスペクトル積分値Ｅ_ＳＮ(ω)_Σを比較し、空判定値Ｎ_Ｒ（ω）_Σ以下であれば流量測定を継続する。
【００７４】
(ステップ６)
(ステップ５)で空判定値Ｎ_Ｒ（ω）_Σ以上になれば空状態として判定する。
【００７５】
前述した（３）式では、スペクトル積分値Ｅ_ＳＮ(ω)_Σとして、励磁周波数成分（ｆ０）、（ｆ３）、（ｆ５）と励磁周波数（ｆ０）以下の成分を除外したが、前置増幅器６Ａ、６Ｂからの白色雑音ｖ_ＮＷ成分が励磁周波数成分に比べて充分大きいパワーがあれば、これらの流速に比例する起電圧の周波数成分を除外することなく判定することも可能である。
【００７６】
以上述べた様に、第２の実施の形態によれば、流体ノイズや微分ノイズに影響されにくい、空判定が可能となる。又、ソフトウエアー機能の追加のみで、新たな構成部品を要しないで実現できる。
【００７７】
【発明の効果】
以上説明した様に、本発明によれば、周波数スペクトルの状態変化によって微分ノイズと流体ノイズの周波数範囲を除外した範囲で空判定処理をしたことにより、Ｓ／Ｎ比が高く、流体ノイズ、微分ノイズの影響を受けにくい空状態が検出可能な容量式電磁流量計を提供できる。
【図面の簡単な説明】
【図１】 本発明の実施の形態を示す構成図。
【図２】 低導電率流体の流体ノイズの説明図。
【図３】 空検出用信号の印加方法の実施態様説明図。
【図４】 本発明の作用を説明図。
【図５】 本発明の第１の実施の形態の説明図。
【図６】 本発明の第２の実施の形態の説明図。
【図７】 空検出原理を説明する等価回路図。
【図８】 信号処理部の他の実施事例。
【符号の説明】
１ 測定管
２ 被測定流体
３Ａ、３Ｂ 励磁コイル
４Ａ、４Ｂ 面電極
５Ａ、５Ｂ ガード電極
６Ａ、６Ｂ 前置増幅器
７ 差動増幅器
８ 励磁回路
９ 空検出信号発生回路
１０ 検出部
１１ 信号処理部
９２ 空検出信号発振器
１１１、１１１Ａ，１１１Ｂ ＡＤＣ回路
１１２ 流量測定処理部
１１３ 空判定処理部
１１４ フィルタ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an electromagnetic flow meter that measures the flow rate of a fluid to be measured flowing through a measuring tube, and more particularly to a capacitive electromagnetic flow meter that can detect an empty state of a fluid to be measured.
[0002]
[Prior art]
Generally, when a fluid is transported using a pump or the like, depending on the state of the piping, the piping may become empty when the pump stops. At this time, since an error occurs in the output of the electromagnetic flow meter, an electromagnetic flow meter having an empty detection function has been developed. In the case of a liquid contact type electromagnetic flow meter, the impedance between the electrodes for detecting the electromotive voltage proportional to the flow velocity of the fluid is greatly different between when the water is full and when it is empty, so that it is possible to detect the sky relatively easily.
[0003]
However, in the case of a capacitive electromagnetic flow meter in which the electrode does not contact the fluid to be measured, since the impedance between the electrodes is very high, it is difficult to detect the sky.
[0004]
In order to detect this sky, the differential noise superimposed on the detection signal at the rise and fall of the excitation current is detected, and the difference in differential noise fluctuation caused by the change in inter-electrode impedance at the time of sky or foreign matter adhesion is obtained. An empty state is detected (hereinafter referred to as empty detection, which is defined as including foreign matter adhesion). (For example, refer to Patent Document 1).
[0005]
[Patent Document 1]
JP-A-8-261808 [0006]
[Problems to be solved by the invention]
However, since this method is a method of detecting the peak value of the noise component (hereinafter referred to as differential noise) induced in the detection signal when the excitation current changes, the differential noise is detected by the noise component superimposed on the detection signal. The peak value of changes.
[0007]
For example, the fluctuation of the exciting magnetic field waveform is caused by the fluctuation of the eddy current that is a demagnetizing field of the exciting magnetic field applied by the exciting coil, so that the waveform of the differential noise is distorted.
[0008]
In addition, when the fluid to be measured is a low conductor, a low-frequency potential variation called fluid noise occurs in the fluid to be measured due to the fluid to be measured flowing. This noise has a characteristic that fluctuates in proportion to the flow velocity. However, since this noise component is also superimposed on the detection signal, the waveform of the differential noise is distorted.
[0009]
As described above, in the method of capturing the peak value of the differential noise, it is easily affected by the distortion of the waveform of the differential noise, and it is difficult to stably detect the sky against the fluctuation of the fluid noise.
[0010]
The present invention has been made in view of the above points, and an object of the present invention is to provide a capacitive flow meter capable of detecting the sky that is not easily affected by fluid noise and differential noise.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, a capacitive electromagnetic flow meter according to claim 1 of the present invention applies a magnetic field to a fluid to be measured flowing through a measurement tube, and outputs a signal corresponding to the flow rate of the fluid to be measured. A capacitive electromagnetic flow meter that detects a capacitance formed between a fluid and a pair of surface electrodes, wherein the capacitive electromagnetic flow meter detects a signal corresponding to a flow rate; and A signal processing unit for obtaining a flow rate from a detection signal detected by the detection unit, the detection unit being disposed opposite to the outer wall of the measurement tube through which the fluid to be measured flows, and a rectangular wave having a fundamental frequency of 200 Hz or more A pair of surface electrodes and a guard electrode disposed opposite to the outer wall of the measurement tube in a direction orthogonal to the direction of the magnetic field generated by the excitation coil, and the surface electrode as a non-inverting input A girder arranged to cover the surface electrode A pair of preamplifiers in which electrodes are connected to an inverting input and an inverting input is connected to an output; an output of one of the preamplifiers; and a guard electrode corresponding to one surface electrode of the preamplifier Between the empty detection signal generating circuit for applying a sine wave signal having a frequency higher than the fundamental frequency of the excitation current at a predetermined signal level, and a differential amplifier for obtaining a difference between the outputs of the pair of preamplifiers. The signal processing unit samples an ADC circuit that performs AD conversion on the output of the differential amplifier, and the output of the ADC circuit at a predetermined timing excluding differential noise for each period of the excitation current. a flow measurement unit for determining the flow rate from the output, only the frequency components of the sine wave signal to the output of the ADC circuit, separated and extracted from the fundamental frequency component and its harmonic component of the exciting current, the The output of the frequency components of the release-extracted the sinusoidal signal is compared with a predetermined level, characterized in that a said empty determination processing unit determines an empty state of the fluid to be measured of the measuring tube.
In order to achieve the above object, a capacitive electromagnetic flow meter according to claim 4 of the present invention applies a magnetic field to a fluid under measurement flowing through a measuring tube, and outputs a signal corresponding to the flow rate of the fluid under measurement. A capacitive electromagnetic flow meter that detects a capacitance formed between a fluid and a pair of surface electrodes, wherein the capacitive electromagnetic flow meter detects a signal corresponding to a flow rate; and A signal processing unit for obtaining a flow rate from a detection signal detected by the detection unit, the detection unit being disposed opposite to the outer wall of the measurement tube through which the fluid to be measured flows, and a rectangular wave having a fundamental frequency of 200 Hz or more A pair of surface electrodes and a guard electrode disposed opposite to the outer wall of the measurement tube in a direction orthogonal to the direction of the magnetic field generated by the excitation coil, and the surface electrode as a non-inverting input A girder arranged to cover the surface electrode A pair of preamplifiers each having an electrode connected to an inverting input and further having an inverting input connected to an output; and a differential amplifier for obtaining a difference between outputs of the pair of preamplifiers, An ADC circuit for AD-converting the output of the differential amplifier, and a flow rate measurement for obtaining the flow rate from the output sampled at a predetermined timing excluding differential noise for each cycle of the excitation current with respect to the output of the ADC circuit The processing unit and the output of the ADC circuit are subjected to a Fourier transform process for each period of the excitation current, and a white noise frequency component of 200 Hz or more obtained by removing the fundamental frequency component and the harmonic component of the excitation current from the frequency spectrum. the separated extract, the separated extracted integration value of the white noise frequency component compared with a predetermined level, said air-size constant process of determining an empty status of the fluid to be measured of the measuring tube Characterized by comprising and.
[0012]
ADVANTAGE OF THE INVENTION According to this invention, the capacity | capacitance type electromagnetic flow meter which can detect the sky which is hard to receive to the influence of fluid noise and differential noise can be provided.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
Hereinafter, embodiments of the present invention will be described with reference to FIGS. FIG. 1 is a schematic view illustrating a capacitive electromagnetic flow meter according to the present invention.
[0014]
A capacitive electromagnetic flow meter according to an embodiment of the present invention includes a detection unit 10 that detects an electromotive voltage proportional to a flow velocity, and a signal process that performs flow rate measurement processing and empty determination processing from a detection signal e detected by the detection unit 10. Part 11.
[0015]
In FIG. 1 showing the detection unit 10, reference numeral 1 denotes a measurement tube for flowing the fluid 2 to be measured, which is made of an insulator such as ceramics. 4A and 4B are planar electrodes for detecting an electromotive voltage generated in the fluid 2 to be measured. The surface electrodes 4A and AB are insulated from the fluid 2 to be measured by the measuring tube 1 and the measuring tube 1 Embedded in the tube wall of the measuring tube 1 or facing each other.
[0016]
Excitation coils 3A and 3B are arranged on the outer peripheral surface of the measurement tube 1 so that a magnetic field is formed in a direction perpendicular to the axis connecting the surface electrodes 4A and 4B and the tube axis of the measurement tube 1. Has been. The outer peripheral surfaces of the exciting coils 3A and 3B are electrostatically shielded and connected to a ground G that is a reference potential.
[0017]
Guard electrodes 5A and 5B are respectively arranged on the outer circumferences of the surface electrodes 4A and 4B so as to cover them with a predetermined distance and insulation. In addition, at both ends (not shown) of the measuring tube 1, a liquid contact electrode E serving as a reference potential of the fluid 2 to be measured is provided and connected to the ground G.
[0018]
The surface electrodes 4A and 4B are connected to the non-inverting input terminal + of the preamplifiers 6A and 6B, respectively, and the output terminals thereof are connected to the inverting input terminal −. An empty detection signal generation circuit 9 described later is connected between one guard electrode 5A and the output terminal of the preamplifier 6A. The other guard electrode 5B is connected to the output of the preamplifier 6B.
[0019]
8 supplies by the excitation circuit, the exciting coil 3A receives a signal from a control circuit (not shown), the exciting current i _F of the rectangular wave to 3B. The frequency of the excitation current i _F is set to a frequency that is advantageous for distinguishing it from fluid noise, for example, 200 Hz. The reason for this will be described with reference to FIG.
[0020]
FIG. 2 shows an example of the measurement result of the fluid noise described above. The horizontal axis represents frequency, the vertical axis represents noise power dBm, and the flow rate is 2.5 m / sec and 0.5 m / sec. The characteristics of
[0021]
As shown in this figure, the fluid noise generally increases as the flow velocity increases, but tends to attenuate from about 10 Hz regardless of the flow velocity of the fluid to be measured and converge to the -70 dBm level per 200 Hz. For this reason, the excitation frequency is set to 200 Hz or more at which the S / N with the electromotive voltage generated at the flow velocity of the fluid 2 to be measured increases.
[0022]
Returning to FIG. 1 again, the outputs of the preamplifiers 6A and 6B are connected to the input terminals of a differential amplifier 7. The output of the differential amplifier 7 is an analog-to-digital conversion circuit (hereinafter referred to as an ADC) constituting a signal processing block 11. (Referred to as a circuit) 111, the flow measurement processing unit 112, and the input of the ADC circuit 111 which is an input unit of the empty determination processing unit 113.
[0023]
The detection signal e, which is an input signal to the ADC circuit 111, is converted into a digital signal by the ADC circuit 111 and thereafter digitally processed, so that the sampling theorem is satisfied so that the frequency component included in the detection signal e is not lost. It is converted into a sampling frequency that is at least twice as high as the condition, that is, the frequency component included in the detection signal e.
[0024]
For example, the frequency component of the detection signal e is necessary for digital processing without losing the frequency component of the detection voltage, that is, the excitation frequency 200 Hz, the third harmonic component 600 Hz, and the fifth harmonic frequency 1000 Hz. The sampling frequency is set to 2 kHz.
[0025]
Next, detailed settings of each unit will be described. First, details of the sky detection signal generation circuit 9 will be described with reference to FIG. As described above, this circuit 9 is a sine wave sky detection signal oscillation circuit 92 that oscillates a sine wave signal having a sky detection frequency (hereinafter referred to as f1) of 500 Hz which is higher than the excitation frequency (hereinafter referred to as f0) of 200 Hz. The output of the sky detection signal oscillation circuit 92 is connected to the negative input terminal of the amplifier 91 via the resistor R1, and the output terminal of the amplifier 91 is further connected to the guard electrode 5A via the feedback resistor R2 of the amplifier 91. Keep it.
[0026]
Further, the output terminal of the preamplifier 6A is connected to the + input terminal of the amplifier 91 via the voltage dividing resistors R3 and R4, and the output voltage of the preamplifier 6A is about 10 mV level which is the same as the electromotive voltage between the electrodes generated in proportion to the flow velocity. The resistance values of the resistors R1 to R4 are selected and set so as to be applied between the surface electrode 4A and the guard electrode 5A as the sky detection minute signal _vNE .
[0027]
For example, in FIG. 3, since the + end and the − end of the preamplifier 6A are regarded as the same potential, the potential e _A of the surface electrode 4A, the potential v _G of the guard electrode 5A, and the signal applied from the empty detection signal circuit 92 are v _{If N} ,

Since the relationship is established, the resistors R1, R4 100 K.OMEGA, resistors R2, R3 and 1K ohm, as a signal _{v N,} 1V, is applied to 500Hz signal from one end of the resistor R1, the surface electrode 4A and the guard electrode 5A In the meantime, a signal of 10 mV is applied as the empty detection minute signal v _NE (e _A -v _G ).
[0028]
The outline of the operation of the capacitive electromagnetic flow meter set in this way will be described with reference to FIG. When a rectangular wave positive / negative signal having a steady value is applied by the exciting current i _F applied to the exciting coils 3A and 3B, a magnetic field having a substantially similar waveform is applied.
[0029]
Here, when the fluid to be measured 2 flows, an electromotive voltage proportional to the flow velocity of the fluid to be measured 2 is generated, and the preamplifier is connected via the capacitance between the fluid to be measured 2 and the surface electrodes 4A and 4B. Amplified by 6A and 6B, further amplified by the differential amplifier 7, and sent to the subsequent signal processing unit 11 as a detection signal e.
[0030]
As described above, the ADC circuit 111 of the signal processing unit 11 is sampled at a frequency that is 10 times or more the excitation frequency (f0) and sends it as a digital value to the flow rate measurement processing unit 112 and the empty determination processing unit 113. It is processed by the built-in program processing function and outputs a flow rate measurement signal and an empty determination signal.
[0031]
Next, processing in the flow rate measurement processing unit 112 and the empty determination processing unit 113 will be described with reference to FIGS.
[0032]
4A shows the relationship between the excitation current i _F , FIG. 4B shows the sampling pulse s _A of the ADC circuit 111, and FIG. 4C shows the relationship of the detection signal e at this time. The detection signal e has a waveform in which the above-described differential noise (D portion of the waveform in FIG. 4C) is superimposed on the rising and falling portions of the excitation current.
[0033]
This detection signal e is sampled at the timing of the sampling pulse signal s _A of the ADC circuit 111 shown in FIG. 4B of the excitation current, and processed by the flow rate measurement processing unit 112 as a digital value.
[0034]
This processing operation will be described below. As shown in FIG. 4B, the process in the flow measurement processing unit 112 is performed as a value for each period from each value sampled at the timing of S4, S5, S9, and S10, which does not include the differential noise component. A calculation for obtaining (S4 + S5) − (S9 + S10) is performed, and this value is converted into a flow rate signal as an average value of a predetermined measurement cycle, and output. In order to speed up the response, it is output every cycle of the excitation frequency (f0).
[0035]
Next, the operation of the empty determination processing unit 113 will be described. 4 (e) and 4 (f) are outputs obtained as a result of Fourier transform processing of the detection signal e for a fixed time, showing the frequency spectrum E _S (ω), each horizontal axis is frequency (Hz), and the vertical axis is its frequency. The power of the detection signal e is shown.
[0036]
A signal component and a noise component included in the detection signal e are superimposed on the frequency spectrum E _S (ω). The signal components include a fundamental frequency (f0) component of 200 Hz, which is a frequency component included in the rectangular wave excitation current included in the electromotive voltage proportional to the flow velocity, a third harmonic component (this frequency is hereinafter referred to as f3), There is a fifth harmonic component (this frequency is hereinafter referred to as f5).
[0037]
Further, as noise components, a sky detection minute signal v _NE component of 500 Hz (this frequency is hereinafter referred to as f1) applied for sky detection, and a preamplifier 6A having uniform power intensity in all frequency regions. , The white noise v _NW component from 6B, and the above-described fluid noise component distributed below 200 Hz.
[0038]
Hereinafter, with reference to FIG. 5, a processing procedure in the sky determination processing unit 113 based on the minute signal v _NE for the sky detection superimposed on the detection signal e will be described.
[0039]
(Step 1)
First, the full water state and the empty state of the fluid 2 to be measured are set in the measuring tube 1, the Fourier transform process of the detection signal e is performed, and the empty detection frequency (f 1) component E at the time of the full water from this frequency spectrum E _S (ω). _S (ω) _f1O is obtained. Normally, the full water state is preferably set in a state in which the fluid 2 to be measured without generating an electromotive voltage is stationary, but can be set in a flowing state.
[0040]
Further, the Fourier transform processing (integration) time is a predetermined time (every one cycle of the exciting current), in 10 times the sampling frequency 2kHz the excitation frequency, and sets the detection signal e can 2,000 samples, empty determination processing Set to about 1 second .
[0041]
The frequency spectrum E _S (ω) in the full water state is shown in FIG. In FIG. 4 (e), the peak values observed at 200 Hz, 600 Hz, and 1000 Hz are frequency spectrum components included in the electromotive voltage proportional to the flow velocity, and the fundamental wave component and the third harmonic of the excitation frequency (f0), respectively. The component (f3), the fifth harmonic (f5), and the peak value seen at 500 Hz, the empty detection frequency (f1) component appears.
[0042]
The spectrum components of other frequencies indicate white noise v _NW components from the preamplifiers 6A and 6B, and the above-described fluid noise components are included in the spectrum of frequencies of 200 Hz or less.
[0043]
(Step 2)
Next, the fluid 2 to be measured is caused to flow out, and an empty state is set between the surface electrodes 4A and 4B, and E _S (ω) _{f1E in space} is obtained.
[0044]
The frequency spectrum E _S (ω) in this empty state is shown by a broken line in FIG. The frequency spectrum E _S (ω) at this time is larger than that in FIG.
[0045]
The reason for this will be described. Since the circuit configuration shown in FIG. 3 is an equivalent circuit as shown in FIG. 7, the input voltage of the preamplifier 6A, that is, the detection signal e _A from the one surface electrode 4A and the minute signal v _NE for sky detection, The relationship between the white noise voltage v _Nw of the preamplifier 6A is as shown in the following equation (1).
[0046]
e _A = (1 + jωC1 · Rs) C2 / C1 × (v _NE + v _Nw ) (1)
Here, C1 represents a capacitance value between the surface electrode 4A and the fluid to be measured 2, Rs represents a resistance value of the fluid to be measured 2, and C2 represents a capacitance value between the surface electrode 4A and the guard electrode 5A.
[0047]
When an empty state occurs in the fluid 2 to be measured, the resistance value Rs of the fluid 2 to be measured further increases, and when a foreign substance adheres to the inner wall of the measuring tube 1, the dielectric constant in the fluid 2 to be measured is higher than that of water. Therefore, the value of C1 becomes a smaller value. As a result, the detection signal e _A when empty than when full capacity is greatly amplified.
[0048]
Therefore, the frequency spectrum E _S (ω) at this time is that shown in FIG. 4 (e) when the water is full, but when it is empty, it shows a uniformly large power value as shown in FIG. 4 (f) (broken line). . Similarly, the spectrum component (E _S (ω) _f1E =) H _E at the sky detection frequency f1 in the _spacetime is also a value larger than the spectrum component (E _S (ω) _f1O =) H 2 _O at full water. Will be shown.
[0049]
(Step 3)
Next, in Step 3 of FIG. 5, the sky determination value N _R (ω) _f1 is set to a predetermined value within the following range from the spectrum power value of the sky detection frequency (f1) obtained in Step 1 and Step 2. To do.
[0050]
H _E > N _R (ω) _f1 > H 2 _O (2)
(Step 4)
After this empty determination value N _R (ω) _f1 is set to a predetermined value, normal flow rate measurement is performed by the arithmetic processing described above.
[0051]
(Step 5)
Next, every time the Fourier integration time of 1 second elapses, the sky determination set value N _R (ω) _f1 set in step 3 and the sky detection frequency component E _S (ω) _{f1 of the} frequency spectrum E _S (ω) are obtained. The flow rate measurement is continued if it is less than the empty determination set value N _R (ω) _f1 .
[0052]
(Step 6)
If the result of step 5 is that the sky detection frequency component E _S (ω) _{f1 of} the frequency spectrum E _S (ω) is _{equal to} or greater than N _R (ω) _f1 , it is determined as an empty state.
[0053]
It should be noted that the sky detection signal generation circuit 9 may be configured such that the sky detection minute signal _vNE is applied only during an excitation pause period or the like so as not to affect the flow velocity measurement processing unit 112.
[0054]
Further, as another method for preventing the sky detection minute signal _vNE from influencing the flow velocity measurement processing unit 112, there is a method of improving the S / N by providing a filter for the detection signal e.
[0055]
In this method, as shown in FIG. 8, two ADC circuits 111A and 111B are provided, and in the flow rate measurement system, a filter 114 that cuts off the empty detection minute signal _vNE component from the empty detection signal generating circuit 9 is provided. Is configured so as to be guided to the flow velocity measurement processing unit via the ADC circuit 111B, and each parallel processing is possible, and the signal from the empty detection signal generation circuit 9 is not superimposed on the flow measurement processing unit 112 with high accuracy. Can measure the flow rate.
[0056]
Further, the capacitance value C2 between the surface electrode 4A and the guard electrode 5A is measured in advance, and the frequency (f1) of the minute signal v _NE for sky detection applied from the sky detection signal generator 9 is time-divided into two types. Applying the above, the capacitance value C1 between the surface electrode 4A and the fluid 2 to be measured is obtained by measuring the detection signal e when the water is full and when using the relationship of the above-described equation (1). The resistance value Rs of the fluid 2 to be measured can be measured.
[0057]
In this way, since it is possible to determine whether the resistance value is changed or the capacitance value is changed, it is predicted whether the empty state is caused by a change in the conductivity of the fluid 2 to be measured or due to adhesion of foreign matter or the like. It becomes possible to judge.
[0058]
As described above, according to the embodiment of the present invention, since the known fixed sky detection minute signal _vNE is applied from the outside as the detection signal e, the differential waveform of the detection signal e It is possible to stably perform sky determination independent of distortion.
[0059]
In addition, since the empty determination process is frequency-separated by statistical processing using Fourier transform processing and the S / N ratio can be increased, it is possible to determine a semi-full state between the fluid under measurement 2 being full and empty.
[0060]
In addition, since the excitation frequency is set to be equal to or higher than the frequency at which the frequency component of fluid noise is reduced, stable flow rate measurement and empty determination that are not affected by fluctuations in fluid noise are possible.
[0061]
(Second Embodiment)
Next, the operation of the second embodiment of the present invention will be described with reference to FIG. 1, FIG. 4, and FIG.
[0062]
In the first embodiment, a new sky detection signal v _NE is applied by the sky detection signal generation circuit 9, and the sky spectrum signal E _S (ω) _{f 1} of the frequency (f 1) of this signal is changed to the sky. Although a change in state is detected, in the second embodiment of the present invention, the integrated value E _SN (ω) _Σ of the spectral power of the white noise v _NW component from the preamplifiers 6A and 6B included in the detection signal e Based on this change, the sky determination is performed.
[0063]
The frequency range to be integrated is a range from 200 Hz to 1000 Hz excluding excitation frequency (f0, f3, f5) components.
[0064]
Therefore, the difference in configuration between the second embodiment and the first embodiment is that the detection unit 10 does not have the empty detection signal generation circuit 9, and the connection from the guard electrode 5A is different from the connection from the guard electrode 5B. Similarly, the output of the preamplifier 6A and the negative input terminal are connected. Other configurations are the same as those of the first embodiment.
[0065]
As shown in FIG. 4G, the frequency spectrum E _S (ω) of the Fourier transform output of the detection signal e is the spectrum power indicated by the solid line when full, but the power changes greatly as indicated by the broken line when empty. To do. The reason for this is that the internal capacity value (C1) of the fluid becomes smaller and greatly amplified in the above-described empty state.
[0066]
Hereinafter, the processing procedure in the sky determination processing unit 113 will be described with reference to FIG. 6 for the data of the frequency spectrum E _S (ω) in FIG.
[0067]
(Step 1, Step 2)
First, in the same manner as in the first embodiment, the state of the fluid 2 to be measured in the measuring tube 1 when it is full and empty is set, and the white spectrum is obtained from the frequency spectrum E _S (ω) after the Fourier transform by the following formula. The integrated value of the noise v _NW and the frequency spectrum E _SN (ω) _Σ are obtained.
[0068]
E _SN (ω) _Σ = E _S (ω) _Σ -E _S (ω) _{0 to f0} -E _S (ω) _{f3 + f5} (3) The integration range of equation (3) is illustrated in FIG. This is within the hatched area of (g).
[0069]
That _{E SN} (ω) _Σ from the frequency 0 entire frequency range of the spectral components Es of f5 (omega) _sigma, and range of component _{Es (ω)} 0~f0 from frequency 0 f0 (200 Hz), the excitation This is obtained by removing the harmonic frequency component Es (ω) _{f3 + f5} of the frequencies f3 and f5.
[0070]
In the above _calculation formula, the value at full water, E _SN (ω) _ΣO (step 1), the value at space time, E _SN (ω) _ΣE (step 2) are obtained.
[0071]
(Step 3)
Next, a predetermined value is set within the following range as the empty determination value N _R (ω) _Σ .
[0072]
_{E SN (ω) ΣE> N} R (ω) Σ> E SN (ω) ΣO ··· (4)
(Step 4)
After the above-described empty determination value N _R (ω) _Σ is set, normal flow rate measurement is performed by the arithmetic processing described above.
[0073]
(Step 5)
Next, every time the Fourier integration time of 1 second elapses, the sky determination set value N _R (ω) _Σ set in step 3 and the spectrum integration value E _SN (ω) of the white noise v _NW obtained by the equation (3) are used. _Σ is compared, and if it is equal to or less than the empty judgment value N _R (ω) _Σ , the flow rate measurement is continued.
[0074]
(Step 6)
If it is determined in step 5 that the sky determination value N _R (ω) _Σ or more, it is determined as an empty state.
[0075]
In the above-described equation (3), the excitation frequency components (f0), (f3), (f5) and components below the excitation frequency (f0) are excluded as the spectral integration value E _SN (ω) _Σ. If the white noise v _NW component from 6A and 6B has a sufficiently large power compared to the excitation frequency component, it is possible to determine without excluding the frequency component of the electromotive voltage proportional to the flow velocity.
[0076]
As described above, according to the second embodiment, it is possible to make an empty determination that is hardly influenced by fluid noise or differential noise. Moreover, it can be realized by adding only software functions and without requiring new components.
[0077]
【The invention's effect】
As described above, according to the present invention, the S / N ratio is high due to the null determination process in the range in which the frequency range of the differential noise and the fluid noise is excluded due to the state change of the frequency spectrum. It is possible to provide a capacitive electromagnetic flow meter that can detect an empty state that is not easily affected by noise.
[Brief description of the drawings]
FIG. 1 is a configuration diagram illustrating an embodiment of the present invention.
FIG. 2 is an explanatory diagram of fluid noise of a low conductivity fluid.
FIG. 3 is an explanatory diagram of an embodiment of a method for applying a sky detection signal.
FIG. 4 is an explanatory diagram of the operation of the present invention.
FIG. 5 is an explanatory diagram of the first embodiment of the present invention.
FIG. 6 is an explanatory diagram of a second embodiment of the present invention.
FIG. 7 is an equivalent circuit diagram for explaining the principle of sky detection.
FIG. 8 shows another implementation example of the signal processing unit.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Measurement tube 2 Fluid to be measured 3A, 3B Excitation coil 4A, 4B Surface electrode 5A, 5B Guard electrode 6A, 6B Preamplifier 7 Differential amplifier 8 Excitation circuit 9 Empty detection signal generation circuit 10 Detection part 11 Signal processing part 92 Empty Detection signal oscillator 111, 111A, 111B ADC circuit 112 Flow rate measurement processing unit 113 Empty determination processing unit 114 Filter

Claims (4)

Capacitive type that applies a magnetic field to the fluid to be measured flowing through the measurement tube and detects a signal corresponding to the flow rate of the fluid to be measured through a capacitance formed between the fluid to be measured and a pair of surface electrodes An electromagnetic flow meter,
The capacitive electromagnetic flow meter includes a detection unit that detects a signal corresponding to a flow rate, and a signal processing unit that obtains a flow rate from the detection signal detected by the detection unit,
The detector is
An exciting coil which is arranged opposite to the outer wall of the measuring tube through which the fluid to be measured flows and is supplied with an exciting current of a rectangular wave having a fundamental frequency of 200 Hz or higher;
A pair of surface electrodes and a guard electrode disposed opposite to the outer wall of the measurement tube in a direction orthogonal to the direction of the magnetic field by the excitation coil;
A pair of preamplifiers in which the surface electrode is connected to a non-inverting input, a guard electrode arranged to cover the surface electrode is connected to an inverting input, and the inverting input is connected to an output;
A sine wave signal having a frequency higher than the fundamental frequency of the excitation current is applied at a predetermined signal level between the output of one of the preamplifiers and a guard electrode corresponding to one surface electrode of the preamplifier. An empty detection signal generation circuit;
A differential amplifier for obtaining a difference between outputs of the pair of preamplifiers,
The signal processing unit
An ADC circuit for AD converting the output of the differential amplifier;
A flow rate measurement processing unit for obtaining a flow rate from an output sampled at a predetermined timing excluding differential noise for each cycle of the excitation current with respect to the output of the ADC circuit;
Only the frequency component of the sine wave signal with respect to the output of the ADC circuit is separated / extracted from the fundamental frequency component of the excitation current and its harmonic component, and the frequency component of the sine wave signal thus separated / extracted is extracted. A capacitive electromagnetic flow meter comprising: an empty determination processing unit that compares outputs at a predetermined level and determines an empty state of the fluid to be measured in the measurement pipe. The sky determination processing unit performs a Fourier transform process on the output of the ADC circuit for each period of the excitation current, and separates and extracts only the frequency component of the sine wave signal from the frequency spectrum output that has been subjected to the Fourier transform process. The capacitive electromagnetic flow meter according to claim 1, wherein the capacitive electromagnetic flow meter is configured as described above.   2. The capacitive electromagnetic flow meter according to claim 1, wherein the empty detection signal generation circuit applies a sine wave signal during a pause period of the excitation current. Capacitive type that applies a magnetic field to the fluid to be measured flowing through the measurement tube and detects a signal corresponding to the flow rate of the fluid to be measured through a capacitance formed between the fluid to be measured and a pair of surface electrodes An electromagnetic flow meter,
The capacitive electromagnetic flow meter includes a detection unit that detects a signal corresponding to a flow rate, and a signal processing unit that obtains a flow rate from the detection signal detected by the detection unit,
The detector is
An exciting coil which is arranged opposite to the outer wall of the measuring tube through which the fluid to be measured flows and is supplied with an exciting current of a rectangular wave having a fundamental frequency of 200 Hz or higher;
A pair of surface electrodes and a guard electrode disposed opposite to the outer wall of the measurement tube in a direction orthogonal to the direction of the magnetic field by the excitation coil;
A pair of preamplifiers in which the surface electrode is connected to a non-inverting input, a guard electrode arranged to cover the surface electrode is connected to an inverting input, and the inverting input is connected to an output;
A differential amplifier for obtaining a difference between outputs of the pair of preamplifiers,
The signal processing unit
An ADC circuit for AD converting the output of the differential amplifier;
A flow rate measurement processing unit for obtaining a flow rate from an output sampled at a predetermined timing excluding differential noise for each cycle of the excitation current with respect to the output of the ADC circuit;
The output of the ADC circuit is subjected to a Fourier transform process for each period of the excitation current, and a white noise frequency component of 200 Hz or more excluding the fundamental frequency component and its harmonic component of the excitation current is separated and extracted from the frequency spectrum. compares the integrated value of the white noise frequency component the separating extracted at a predetermined level, characterized in that a said air-size determining an empty status of the fluid to be measured constant processing portion of the measuring tube Capacity type electromagnetic flow meter.

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