JPH0219694Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an improvement in the excitation circuit of a low-frequency excitation type electromagnetic flowmeter.

Generally, an electromagnetic flowmeter is configured to apply a magnetic field perpendicular to the fluid flow direction, simultaneously detect changes in electrical signals in the fluid flow path, and measure the fluid flow rate based on this. . Recent electromagnetic flowmeters use low-frequency excitation methods, such as trapezoidal wave excitation and square wave excitation, which have superior zero point stability compared to AC excitation methods and DC excitation methods using commercial power supply frequencies. It is often used. An example of an excitation circuit using a conventional low frequency excitation method is shown in FIG. In FIG. 1, an AC power source 1 is connected via a transformer 2 to a first rectifying and smoothing circuit 3a and a second rectifying and smoothing circuit 3b. The first rectifying and smoothing circuit 3a has a diode Da and a smoothing capacitor Ca, and rectifies and smoothes the voltage from the AC power supply ₁ to output a positive DC voltage Va 1.
It is connected to the excitation coil 5 of the electromagnetic flowmeter via the switch 4a. The second rectifying and smoothing circuit 3b has a diode Db and a smoothing capacitor Cb, and rectifies and smoothes the voltage from the AC power supply 1 to produce a negative DC voltage.
It outputs Vb ₁ , and its output end is connected to the excitation coil 5 via the second switch 4b. The switches 4a and 4b are waveform diagram A in FIG.
It turns on and off in synchronization with the low-frequency drive pulses Pa and Pb as shown in FIG. When is on, an excitation current Io in the opposite direction flows through the excitation coil 5. As a result, the excitation coil 5 has the following properties as shown in FIG.
A low-frequency excitation current Io whose polarity changes periodically flows. Note that the diodes 6a and 6b connected in parallel to the switches 4a and 4b are connected to the switches 4a and 4b, respectively.
This is to cause the smoothing capacitors Ca and Cb to absorb the energy stored in the excitation coil 5 when the excitation coil 4b is turned off.

By the way, as shown in FIG. 2C, the exciting current Io reaches a constant value after being delayed at the rising and falling portions of the waveform due to the influence of the exciting coil 5 when the switches 4a and 4b are turned on and off. The rising and falling responses are determined by the magnitudes of the DC voltages Va ₁ and Vb ₁ and the capacitances of the smoothing capacitors Ca and _Cb .
When Vb ₁ is small and when the capacitance of Ca and Cb is large, the rising and falling responses become gentle. We want to make the rise and fall responses as fast as possible, but in the circuit shown in Figure 1, the DC voltages Va ₁ ,
Vb ₁ is the product of the excitation current Io and the resistance R of the excitation coil 5 and is small, and the capacitance of the smoothing capacitors Ca and Cb must be increased to reduce power supply ripples, which speeds up the rise and fall responses. I couldn't do that.

The present invention is achieved by connecting first and second capacitors with small capacitances via diodes between the output terminals of the first and second rectifying and smoothing circuits and the first and second switches, respectively. , we have realized an excitation circuit for an electromagnetic flowmeter that can provide an excitation current with small ripples and quick rise and fall responses.

FIG. 3 is a connection diagram showing one embodiment of the excitation circuit of the present invention. The difference between FIG. 3 and the conventional example shown in FIG. 1 is that a first capacitor 7a with a small capacitance is connected to the output terminal of the first rectifying and smoothing circuit 3a via a diode 8a for preventing backflow, and The point where the series circuit of switch 4a and excitation coil 5 is connected in parallel to 7Aa, and the second rectifying and smoothing circuit 3b
A second capacitor 7b with a small capacitance is connected to the output terminal of the capacitor 7b via a diode 8b for preventing backflow, and a series circuit of the switch 4b and the excitation coil 5 is connected in parallel to the capacitor 7b.

The operation of the circuit of the present invention constructed in this way will be explained below with reference to the waveform diagram of FIG. In Fig. 4, A is the waveform of the drive pulse Pa, B is the waveform of the drive pulse Pb, C is the operating waveform of the switch 4a, D is the operating waveform of the switch 4b, E is the waveform of the exciting current Io, and F is the first waveform. The waveform of the output Va ₁ of the rectifying and smoothing circuit 3a is the waveform of the voltage Va ₂ across the capacitor 7a,
H is the waveform of the output Vb ₁ of the second rectifying and smoothing circuit 3b,
2 is the waveform of the voltage Vb ₂ across the capacitor 7b.

When the switch 4a is on and the switch 4b is off, a constant excitation current Io is flowing through the excitation coil 5 in the positive direction (in the direction of the arrow in the figure). When the switch 4a is turned off in this state, the current flowing in the exciting coil 5 flows through the diode 6b, and the energy stored in the exciting coil 5 is transferred to the capacitor 7b. The response at this time is determined by the resonance frequency of the inductance of the excitation coil 5 and the capacitance of the capacitor 7b, and since the capacitance of the capacitor 7b is small, the positive excitation current Io falls quickly. Next, when the switch 4b is turned on, the energy stored in the capacitor 7b is returned to the excitation coil 5, and an excitation current Io in the opposite direction rises. This rise also depends on the inductance of the excitation coil 5 and the capacitor 7b.
The speed is determined by the capacity. When the steady state is reached, the diode 8b is turned on, and a constant value of excitation current is supplied to the excitation coil 5 in the opposite direction by the output voltage _Vb1 of the second rectifying and smoothing circuit 3b. When the switch 4b is turned off in this state, the excitation coil 5
The current flowing through the diode 6a flows through the diode 6a, transferring the energy stored in the excitation coil 5 to the capacitor 7a, which has a small capacity. The response at this time is determined by the inductance of the excitation coil 5 and the capacitance of the capacitor 7a, and the excitation current Io in the opposite direction and falls quickly. Next, when the switch 4a is turned on, the energy stored in the capacitor 7a is returned to the excitation coil 5, and an excitation current Io in the positive direction rises. This rise is also determined by the inductance of the excitation coil 5 and the capacitance of the capacitor 7a and becomes faster. When the steady state is reached, the diode 8a is turned on, and a constant value of exciting current Io is supplied to the exciting coil 5 in the positive direction by the output voltage Va ₁ of the first rectifying and smoothing circuit 3a. In this way, a low-frequency excitation current Io whose polarity changes periodically flows through the excitation coil 5.

Note that the values of the output voltages Va ₁ and Va ₂ of the first and second rectifying and smoothing circuits 3a and 3b in a steady state are the product (IoxR) of the excitation current Io and the resistance R of the excitation coil 5.
and the forward voltage Vd of the diodes 8a and 8b. Further, the value of the excitation current Io in the steady state is stabilized by controlling the power supply voltage of the AC power supply 1, etc. In this case, as the AC power supply 1, one having a switching regulator or the like for both AC and DC use is used. In addition, the first and second switches 4
When using transistors as a and 4b,
Since the voltage charged to the first and second capacitors 7a and 7b becomes higher, the circuit of FIG.
It is necessary to use a high current PNP type transistor. Furthermore, as shown in FIG. 5, if the secondary winding of the transformer 2 is made independent and the first and second switches 4a and 4b are provided on the common side of the excitation coil 5, the switches 4a and 4b can be used as high withstand voltage switches. , high current NPN transistors Q ₁ and Q ₂ can be used. Moreover, PNP transistors for high voltage and high current use are expensive and difficult to obtain, whereas NPN transistors for high voltage and high current use have the advantage of being inexpensive and easy to obtain. In FIG. 5, Q ₃ and Q ₄ are transistors that are Darlington connected to Q ₁ and Q ₂ , respectively. Furthermore, as shown in FIG.
If it is also provided on the common side of the excitation coil 5, the arm current flowing through the shield S of the transformer 2 can be reduced.

As explained above, in the excitation circuit of the present invention, first and second capacitors of small capacitance are connected between the first and second rectifying and smoothing circuits and the first and second switches for switching the excitation current, respectively. When the excitation current falls, the energy stored in the excitation coil is transferred to the capacitor, and when the excitation current rises, the energy stored in the capacitor is returned to the excitation coil, so that the response to the rise and fall of the excitation current is In addition, since the capacity of the smoothing capacitor in the rectifying and smoothing circuit remains large, the power supply ripple can be sufficiently reduced.

[Brief explanation of drawings]

Figure 1 is a connection diagram showing an example of a conventional excitation circuit.
Fig. 2 is a waveform diagram for explaining its operation, Fig. 3 is a connection diagram showing one embodiment of the excitation circuit of the present invention, Fig. 4 is a waveform diagram for explaining its operation, and Figs. 5 and 6. FIG. 2 is a connection diagram showing another embodiment of the excitation circuit of the present invention. 1...AC power supply, 2...Transformer, 3a, 3b
... Rectifier smoothing circuit, 4a, 4b ... Switch, 5
... Excitation coil, 6a, 6b ... Diode, 7
a, 7b...capacitor with small capacity, 8a, 8
b...Reverse current prevention diode, Q ₁ , Q ₂ ... NPN type transistor for high voltage and high current.

Claims (1)

[Scope of utility model registration request] a first rectifying and smoothing circuit that rectifies and smoothes an AC power source to obtain a positive DC voltage; a second rectifying and smoothing circuit that rectifies and smoothes the AC power source to obtain a negative DC voltage; and the first rectifying and smoothing circuit. a first capacitor with a small capacitance connected via a diode to the output terminal of the second rectifying and smoothing circuit; a second capacitor with a small capacitance connected via a diode to the output terminal of the second rectifying and smoothing circuit; means for connecting a series circuit of a first switch and an excitation coil of the electromagnetic flowmeter in parallel to the capacitor; and means for connecting a series circuit of a second switch and the excitation coil to the second capacitor in parallel. , a diode connected in parallel to the first switch in a direction in which a current flows in a direction opposite to that of the current flowing through the first switch; and a diode connected in parallel to the second switch and flowing through the second switch An excitation circuit for an electromagnetic flowmeter that includes a diode connected in the direction in which current flows in the opposite direction to the current.