Abstract:
In a constant current circuit, a constant current is caused to flow through a resistor, thereby causing a constant voltage to occur across the resistor. This constant voltage is then superimposed on an output signal of an operational amplifier that is to be fed back to the drain of a field effect transistor, thereby maintaining the same potential in an AC manner between the output terminal of the operational amplifier and the drain of the field effect transistor. In this way, the gate and drain of the field effect transistor is caused to exhibit the same potential in an AC manner, so that no current will occur through the stray capacitance between the gate and drain of the field effect transistor. As a result, similarly to a case of using a feedback capacitor, the input impedance of the field effect transistor can be raised.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a U.S. national phase application under U.S.C. §371 of International Patent Application No. PCT/2007/065262, filed Aug. 3, 2007 and claims the benefit of Japanese Patent Application No. 2006-295744, filed Oct. 31, 2006. The International Application was published in Japanese on May 8, 2008 as International Publication No. WO/2008/053624 under PCT Article 21(2) the contents of which are incorporated herein in their entirety. 
     FIELD OF TECHNOLOGY 
     The present invention relates to a capacitive electromagnetic flowmeter provided with a signal electrode for electrostatic capacitive coupling with a fluid that flows within a measuring tube. 
     BACKGROUND OF THE INVENTION 
     Conventionally, this type of capacitive electromagnetic flowmeter has an excitation coil for producing a magnetic field in a direction that is perpendicular to the direction of flow of the fluid that flows within the measuring tube, and a signal electrode for electrostatic capacitance coupling with a fluid that flows within the measuring tube, provided within the measuring tube, to pick up, through the signal electrode, the electromotive force that is generated in the fluid that flows within the measuring tube due to the magnetic field that is created by the excitation coil. Note that normally a guard electrode for shielding the signal electrode is provided for the signal electrode, and a pair of signal electrodes and the guide electrodes is provided in a direction that is perpendicular to the magnetic field that is produced by the excitation coil. 
     In this type of capacitive electromagnetic flowmeter, the impedance of an insulating portion between a signal electrode and the fluid (the electrostatic capacitive coupling portion) is high, in the order of several dozen to several hundred MΩ. A circuit with high input impedance (a signal acquiring circuit) is necessary to order to pick up the signal (the electromotive force) with high accuracy from the high impedance electrostatic capacitive coupling portion. For example, in order to have the attenuation of the signal be 0.1% at electric static capacitive coupling portion, it is necessary for the input impedance of the signal pickup circuit to be 1000 times the impedance of the electrostatic capacitive coupling portion. A signal pickup circuit such as been proposed in Japanese Unexamined Patent Application Publication H6-241856 as a circuit that fulfills this requirement is illustrated in  FIG. 9 . 
     In  FIG. 9 , Vd is the alternating current electromotive force that is generated in the fluid being measured; Cd is the electrostatic capacitance that is formed between the fluid being measured and a signal electrode  4 ; Q 1  is a field effect transistor; and OP 1  is an operational amplifier. The gate G of the field effect transistor Q 1  is connected to the signal electrode  4 ; the source S is connected to the non-inverting input terminal (+) of the operational amplifier OP 1 ; and the drain D is connected to the power supply VDD (the high voltage point) through a resistor R 3 . 
     A series circuit of a resistor R 1  and a resistor R 2  is connected between a common voltage point COM and the gate G of the field effect transistor Q 1 , and the contact point between the resistor R 1  and the resistor R 2  is connected through a capacitor C 1  to the inverting input terminal (−) of the operational amplifier OP 1 . A bootstrap circuit is structured from these resistors R 1  and R 2  and the capacitor C 1 . 
     The power supply VSS (low voltage point: VSS&lt;VDD) is connected through a resistor R 4  to the source S of the field effect transistor Q 1 , and the output terminal of the operational amplifier OP 1  is not only connected to the drain D of the field effect transistor Q 1  through a capacitor C 2 , and also connected to the inverting input terminal (−) of the operational amplifier OP 1  and the capacitor C 1 . 
     In the signal pickup circuit  100 , the alternating current electromotive force Vd that is produced in the fluid that is being measured passes through the electrostatic capacitance Cd to be applied to the gate G of the field effect transistor Q 1 . In this case, the field effect transistor Q 1  functions as a source follower, and so the signal that is applied to the gate G appears as a signal with the same voltage at the source S of the field effect transistor Q 1 . This signal is applied to the non-inverting input terminal (+) of the operational amplifier OP 1 . 
     The operational amplifier OP 1  functions as a buffer with an amplification of essentially “1,” and outputs a signal of the same voltage as the signal that is applied to the non-inverting input terminal (+). The output terminal of the operational amplifier OP 1  is connected to the inverting input terminal (−), so the non-inverting input terminal (+) and the inverting input terminal (−) of the operational amplifier OP 1  will be at the same voltage, so that the output terminal of the operational amplifier OP 1  and the gate G of the field effect transistor Q 1  will also be at the same voltage. 
     On the other hand, because the capacitor C 2  is connected between the output terminal of the operational amplifier OP 1  and the drain D of the field effect transistor Q 1 , the same voltage is maintained between the two in terms of alternating current. Consequently, the voltages of the gate G and the drain D of the field effect transistor Q 1  will be identical in terms of alternating current, so that there will be no electric current through the floating capacitance between the two. 
     The field effect transistor Q 1  functions as a source follower, so the voltages between the gate G and the source S will be identical in terms of alternating current, and so there will be no current through the floating capacitance between these two either. 
     The result is that there will be no drop in impedance caused by the floating capacitance in the field effect transistor Q 1 , making it possible to increase the input impedance of the field effect transistor Q 1 . Additionally, the point of contact between the resistors R 1  and R 2  will be essentially the same voltage as the gate G, so that there will be no electric current through the resistor R 1 . Consequently, the input impedance of the signal pickup circuit  100  will be infinitely large, making it possible to pick up, with high accuracy, the electromotive source Vd that is produced in the fluid being measured. 
     Note that the publication by Tamotsu INABA, “Selected Analog Circuits,” Page 30, CQ Publishers, 10 Jan. 1989 can be proposed as a detector circuit for a signal wherein the signal source impedance is high. This reference describes how it is possible to achieve a high input impedance through combining a field effect transistor and a bootstrap circuit in the input portion, and also to use a capacitor to apply a positive feedback. The structure described in this reference suggests a structure wherein a capacitor C 2  (hereinafter termed the “feedback capacitor”) is connected between the output terminal of the operational amplifier OP 1  and the drain D of the field effect transistor Q 1  in the signal pickup circuit  100 , as described above. 
     However, given the conventional signal pickup circuit  100  illustrated in  FIG. 9 , there is the possibility that an electric charge will accumulate in the large capacitance of the feedback capacitor C 2  between the output terminal of the operational amplifier OP 1  and the drain D of the field effect transistor Q 1 . Because of this, it is necessary to use a large capacitance capacitor, which is physically large, as the feedback capacitor C 2 , with not only a problem that the electric circuit will take too much space, but also that it is necessary to house the circuit in a strictly explosion-proof container. 
     The object of the present invention is to provide a capacitive electromagnetic flowmeter wherein it is possible to increase the input impedance of the signal pickup circuit while eliminating the feedback capacitor to reduce the size of the circuit, while also eliminating the need for the explosion-proof enclosure. 
     SUMMARY OF THE INVENTION 
     In order to achieve this object, capacitive electromagnetic flowmeter includes an excitation coil for producing a magnetic field in a direction that is perpendicular to the direction of flow of a fluid that flows within a measuring tube; a signal electrode, provided within the measuring tube, for picking up an electromotive force that is produced in the fluid that flows within the measuring tube due to the magnetic field that is produced by the excitation coil, through electrostatic capacitive coupling with the fluid that flows within the measuring tube; a first field effect transistor that has a gate input of the electromotive force that is picked up by the signal electrode; an operational amplifier wherein the output of the source of the first field effect transistor is applied to the non-inverting input terminal thereof; a first and a second feedback path to feedback, to the inverting input terminal of the operational amplifier and to the drain of the first field effect transistor, the output signal from the output terminal of the operational amplifier; a voltage maintaining circuit for maintaining the voltages of the gate and the drain of the first field effect transistor at the same voltage, in terms of alternating current through superimposing a predetermined constant voltage, generated by a flow of an electric current, onto the output signal of the operational amplifier that is fed back to the drain of the first field effect transistor; a series connection circuit of a first and a second resistor, connected between the gate of the first field effect transistor and a common voltage point; and a capacitor that is connected between the contact point of the first and second resistors and the inverting input terminal of the operational amplifier. 
     Given the present invention, the voltages of the gate and the drain of the first field effect transistor are maintained at the same voltage, in terms of alternating current, by superimposing a predetermined fixed voltage that is generated by the flow of the electric current onto the output signal from the operational amplifier that is fed back to the drain of the first field effect transistor, enabling the feedback capacitor between the output terminal of the operational amplifier and the drain of the first field effect transistor to be eliminated, making it possible to eliminate the need for containment in an explosion-proof container, and enabling the input impedance of the signal pickup circuit to be increased, in a circuit with a small surface area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram of a signal pickup circuit for a capacitive electromagnetic flowmeter according to an embodiment according to the present invention. 
         FIG. 2  is a circuit diagram of a signal pickup circuit for a capacitive electromagnetic flowmeter according to another embodiment according to the present invention. 
         FIG. 3  is a characteristic chart illustrating the result of actually measuring the relationship between the signal voltage I/O ratio and the frequency in the circuit of  FIG. 2 . 
         FIG. 4  is a circuit diagram illustrating a constant current circuit used as voltage maintaining means. 
         FIG. 5  is a circuit diagram illustrating another example of a constant current circuit used as voltage maintaining means. 
         FIG. 6  is a circuit diagram illustrating a further example of a constant current circuit used as voltage maintaining means. 
         FIG. 7  is a circuit diagram illustrating a fourth example of voltage maintaining means. 
         FIG. 8A  is a longitudinal sectional diagram of a capacitive electromagnetic flowmeter according to the present invention, and  FIG. 8B  is a cross-sectional diagram along the section I-I in  FIG. 8A . 
         FIG. 9  is a circuit diagram illustrating a signal pickup circuit for a conventional capacitive electromagnetic flowmeter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described in detail below based on the drawings. A capacitive electromagnetic flowmeter according to an embodiment according to the present invention will be described using  FIG. 1 ,  FIG. 8A , and  FIG. 8B . 
     In  FIGS. 8A and 8B ,  1  illustrate a measuring tube, a non-magnetic pipe  2  (such as, for example, a pipe made out of stainless steel with an insulating resin lining  3  on the inside thereof),  4  is a signal electrode, and  5  is a guard electrode for shielding the signal electrode  4 . Two sets of signal electrodes  4  and guard electrodes  5  are provided facing each other in the resin lining  3 . 
     An excitation coil  6  for producing a magnetic field in a direction that is perpendicular to the direction of flow of the fluid that is flowing within the measuring tube  1  is provided coiled around a core  7 . The signal electrodes  4  and the guard electrodes  5  are provided in a direction that is perpendicular to the magnetic field that is produced by the excitation coil  6 . Note that a capacitive electromagnetic flowmeter of this type of structure is proposed in the U.S. Pat. No. 4,631,969. The electromotive force that is picked up by the signal electrode  4  is provided to the signal pickup circuit  200  illustrated in  FIG. 1 . 
     The signal pickup circuit  200  differs from the conventional signal pickup circuit  100 , illustrated in  FIG. 9 , in the point that a constant current circuit CT 1  is provided instead of the feedback capacitor C 2 . The constant current circuit CT 1  achieves the same role as the feedback capacitor C 2  ( FIG. 9 ), and functions as voltage maintaining means for causing the voltages of the drain D and the gate G of the field effect transistor Q 1  to be identical, in terms of alternating current. Note that the other structures of the signal pickup circuit  200  are identical to those in the signal pickup circuit  100  ( FIG. 9 ), so explanations thereof are omitted. 
     The constant current circuit CT 1  is structured from a field effect transistor Q 2  and a resistor R 5 . In the constant current circuit CT 1 , one end of the resistor R 5  is connected to the output terminal of an operational amplifier OP 1 , and the other end of the resistor R 5  is connected to the drain D of the field effect transistor Q 1  and to the source S of the field effect transistor Q 2 . Additionally, the gate G of the field effect transistor Q 2  is connected to one end of the resistor R 5 , and the drain D is connected to the power supply VDD. 
     In the signal pickup circuit  200 , an alternating current electromotive force Vd that is produced in the fluid that is measured and that is picked up by the signal electrode  4  is applied through an electrostatic capacitance Cd to the gate G of the field effect transistor Q 1 . In this case, the field effect transistor Q 1  functions as a source follower, so a signal appears at the source S of the field effect transistor Q 1  that is of the same voltage as the signal that is applied to the gate G. This signal is applied to the non-inverting input terminal (+) of the operational amplifier OP 1 . 
     The operational amplifier OP 1  functions as a buffer with an amplification of essentially 1, to output a signal with the same voltage as the signal that is applied to the non-inverting input terminal (+). Additionally, the output signal of the operational amplifier OP 1  is connected to the inverting input terminal (−), so the non-inverting input terminal (+) and the inverting input terminal (−) of the operational amplifier OP 1  will be at identical voltages, so that the output terminal of the operational amplifier OP 1  and the gate G of the field effect transistor Q 1  will also be at identical voltages. 
     On the other hand, the resistor R 5  in the constant current circuit CT 1  is connected between the output of the operational amplifier OP 1  and the drain D of the field effect transistor Q 1 , so the output signal from the operational amplifier OP 1  is fed back to the drain D of the field effect transistor Q 1  through the resistor R 5 . 
     At this time, a constant voltage is produced at between the ends of the resistor R 5  by the constant current that flows in the resistor R 5 . The constant voltage that is produced between the two ends of the resistor R 5  is superimposed on the output signal from the operational amplifier OP 1  that is fed back to the drain D of the field effect transistor Q 1 . The same voltage is maintained, in terms of an alternating current, between the output terminal of the operational amplifier OP 1  in the drain D of the field effect transistor Q 1  thereby. 
     As a result, the voltages at the gate G and the drain D of the field effect transistor Q 1  will be identical, in terms of an alternating current, and thus no current caused by the floating capacitance therebetween will be produced. Consequently, it is possible to increase the input impedance of the field effect transistor Q 1  in the same manner as in the case wherein the feedback capacitor C 2  was used. 
     As can be understood from the circuit operation of this type, in the signal pickup circuit  200  of the present example, a constant voltage that is superimposed onto the output signal from the operational amplifier OP 1  is produced by the constant current that flows in the resistor R 5 , and thus there is no problem with the accumulation of electric charge with a large capacitance in the feedback capacitor C 2  as there is in the conventional signal pickup circuit  100 . Furthermore, the constant current circuit CT 1  that is structured from the field effect transistor Q 2  and the resistor R 5 , when compared with the physically-large large capacity feedback capacitor C 2 , needs only a circuit of a small surface area, and there is no need for containment in an explosion-proof container. 
     As described above, given the present embodiment, the low current circuit CT 1 , as the voltage maintaining means, superimposes a predetermined constant voltage onto the output signal from the operational amplifier OP 1  to feedback to the drain of the field effect transistor Q 1 , to maintain the voltages of the gate and the drain of the field effect transistor Q 1  at the same voltage, in terms of alternating current. This makes it possible to increase the input impedance of the field effect transistor Q 1  without producing a current, through a floating capacitance, between the gate and the drain of the field effect transistor Q 1 . In the present example, the constant voltage that is superimposed on the output signal from the operational amplifier OP 1  is produced by a flow of electric current. 
     That is, while in the conventional feedback capacitor a constant voltage is produced by the electric charge that is accumulated in the feedback capacitor, in the present embodiment, this is produced through the flow of an electric current, rather than by an accumulated charge. 
     The constant current circuit CT 1 , as the voltage maintaining means, is provided with a third resistor that is connected, on the one end thereof, to the output terminal of an operational amplifier and is connected, on the other end thereof, to the drain of the first field effect transistor, and provided with a field effect transistor Q 2  wherein the gate is connected to one end of the resistor R 5 , the source is connected to the other end of the resistor R 5 , and the drain is connected to the high-voltage point. In the constant current circuit CT 1 , a constant voltage is produced in the resistor R 5  by the constant current that flows in the resistor R 5 , where the constant voltage that is generated in the resistor R 5  is superimposed onto the output signal from the operational amplifier OP 1  that is fed back to the drain of the field effect transistor Q 1 . 
     A signal pickup circuit according to the present invention will be explained using  FIG. 2 . In the signal pickup circuit  200 ′ according to the embodiment, a resistor R 6  is connected in series with a capacitor C 1  between the inverting input terminal (−) of the operational amplifier OP 1  and the contact point between the resistor R 1  and the resistor R 2 . 
     In the signal pickup circuit  200 ′, the resistor R 6  functions as a resistor for preventing oscillation. That is, in the relationship between the frequency and the ratio between the input signal and the output signal (the I/O frequency characteristics), the peak value of the resonance point is reduced, so as to have the effect of preventing the oscillation that occurs when a signal with the frequency at the point of resonance is inputted. Note that while in the present example, the resistor R 6  is connected to the inverting input terminal (−) side of the operational amplifier OP 1 , it may instead be connected to the point of connection between the resistor R 1  and the resistor R 2 . 
       FIG. 3  illustrates the effect of an actual measurement of the relationship between the I/O ratio of the signal voltage and the frequency depending on whether or not the resistor R 6  is present. In the measurement results, inserting the resistor R 6  improved the peak by approximately ⅕ dB when compared to the case wherein there is no resistor R 6 . It can be seen from this as well that the peak value at the resonance point is reduced through the use of the oscillation preventing resistor R 6 . 
     Note that while, in the embodiments described above, a constant current circuit CT 1  structured from the field effect transistor Q 2  and the resistor R 5  was used as the voltage maintaining means, a variety of other variations may be considered for the constant current circuit that can be used for the voltage maintaining means. 
     For example, as illustrated in  FIG. 4 , a constant current circuit CT 2  may be structured from a field effect transistor Q 3  and from resistors R 7  and R 8 . Furthermore, as is illustrated in  FIG. 5 , a constant current circuit CT 3  may be structured from a transistor TR 1  and resistors R 5 , R 9 , and R 10 . Furthermore, as is illustrated in  FIG. 6 , a constant current circuit CT 4  may be structured from a transistor TR 1 , resistors R 5  and R 11 , and a Zener diode ZD 1 . 
     Additionally, a constant current circuit need not necessarily be used for the voltage maintaining means. For example, as illustrated in  FIG. 7 , a forward voltage that is produced by a diode D 1  through a current that flows through a resistor R 12  may be superimposed on the output voltage from the operational amplifier OP 1  to be fed back to the drain D of the field effect transistor Q 1 . The diode D 1  will produce a constant forward voltage Vd even if the electric current that flows is not a constant current.