JPH0450496Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an improvement 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. . Many modern 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 and DC excitation methods. There is. In this type of low-frequency excitation type electromagnetic flowmeter, the excitation current supplied to the excitation coil is
cyclically (at low frequency) between two steady-state values,
The electrochemical The influence of offset voltage based on DC voltage etc. is removed and an output signal corresponding to the fluid flow rate is obtained. In such a low frequency excitation type electromagnetic flowmeter, changes in the excitation current result in errors, so
The excitation current is kept constant by controlling the excitation current so that the detected voltage obtained by detecting the excitation current is equal to the set voltage. By the way, the set voltage is usually obtained from a set voltage source using a Zener diode or the like, but if the Zener voltage fluctuates due to ambient temperature or the like, the excitation current also fluctuates, and the voltage generated between the electrodes is also affected by this. For this reason, it is necessary to use a highly accurate set voltage source that is not affected by ambient temperature, etc., which is expensive.

The present invention provides a set voltage based on a Zener voltage from a set voltage source using a Zener diode, supplies a low frequency excitation current with an amplitude corresponding to the set voltage to an excitation coil of an electromagnetic flowmeter transmitter, and The voltage generated between the pair of electrodes of the electromagnetic flowmeter oscillator is amplified by an amplifier, and the amplified voltage is further inverted by an inverting amplifier, which are switched in synchronization with the excitation current and integrated by an integrator for a fixed period of time. obtaining an integral output related to the difference between the generated voltage that occurs when the excitation current is at one steady value and the generated voltage that occurs when the excitation current is at the other steady value, and resetting this integral output with the Zener voltage; By obtaining an output signal related to the reset time width, a low frequency excitation type electromagnetic flowmeter is realized which can effectively eliminate the influence of fluctuations in the Zener voltage of the set voltage source.

FIG. 1 is a connection diagram showing one embodiment of the electromagnetic flowmeter of the present invention. In FIG. 1, 1 is an electromagnetic flowmeter transmitter, which includes an exciting coil 11 and a pipe line 12 through which fluid flows.
and a pair of electrodes 13 and 14. 2 is an excitation circuit that supplies an excitation current I that turns on and off at a low frequency to the excitation coil 11; In the figure, the excitation current I
The voltage V _i detected by variable resistor RV ₁ and the set voltage V _r
The error amplifier 21 detects the difference between the two, and the output drives the transistor 22 to control the excitation current I so that V _i =V _r . 3 is a set voltage source, which includes a Zener diode 31 connected to the power supply V _b via a resistor R ₁ , and a Zener voltage source.
It has a switch 32 that turns on and off _Vz , and generates a set voltage _Vr that turns on and off at a low frequency. 4 is a signal processing circuit, which includes an amplifier 41 that amplifies the generated voltage V ₁ generated between the electrodes 13 and 14 of the oscillator 1, and an inverting amplifier 42 that inverts the output V ₂ of the amplifier 41;
Amplifier output V ₂ synchronizes with on/off of excitation current I
and an inverting amplifier output _V3 , and a sampling switch 44 that samples the voltage selected by the switch 43 for a certain period of time.
and an integrating capacitor CI connected to the feedback circuit of the operational amplifiers P and P, and the voltage sampled by the sampling switch 44 is applied to the input via the resistor _RI1 , and the voltage sampled by the sampling switch 44 is applied to the input via the reset switch 45 and the resistor RI. The integrator 46 to which the Zener voltage V _z is applied via V ₂ and the output V ₄ of the integrator 46 are monitored, and when V ₄ becomes zero, the output V ₅
A comparator 47 in which
7 and a gate circuit 48 to which the output V ₅ of No. 7 is applied. The gate circuit 48 is a NAND gate G ₁ ,
_G2 and an inverter IV, the output of the NAND gate _G1 drives the reset switch 45, generates a pulse width signal PW at the output end of the inverter IV, and generates a pulse number output N at the output end of the NAND gate _G2 . do. 5 is a pulse generation circuit, which includes an oscillator 51 that oscillates at a frequency f, and divides the output of the oscillator 51 to generate timing pulses P ₁ , P ₂ , P ₃ , P ₄ and a reference pulse.
The pulse P ₁ drives the switch 32, the pulse P ₂ drives the switch 43, _{the pulse P 3 drives the switch 44, and the pulse P 4 drives the} NAND gate G ₁ of the gate circuit 48. Then, the pulse P ₅ is applied to the NAND gate G ₂ of the gate circuit 48, respectively.

The operation of the electromagnetic flowmeter of the present invention constructed as described above will be explained below with reference to the waveform diagram of FIG. First, the pulse generating circuit 5 divides the oscillation output of the oscillator 51 with a frequency f and generates pulses P ₁ , P ₂ , P ₃ , and P ₄ at the timings shown in FIG. 2 A, B, C, and D, respectively. Reference pulse as shown in Figure 2
Generates _P5 . When the switch 32 of the set voltage source 3 is turned on and off by the low frequency pulse _P1 ,
When the Zener voltage _Vz is turned on and off, the set voltage _Vr applied to the excitation circuit 2 has a waveform as shown in FIG. 2E. As a result, the excitation coil 11 of the transmitter 1 is supplied with an excitation current I that turns on and off at a low frequency as shown in FIG.
As shown in Fig. 2, the generated voltage V ₁ of No. 14 becomes a voltage in which the offset voltage V _pf is superimposed on the signal voltage V _s corresponding to the fluid flow rate during the period when the excitation current I is off, and the excitation current I During the period when is off, only the offset voltage V _pf is present. On the other hand, the changeover switch 43 is switched by a pulse _P2 synchronized with the on/off of the excitation current I, and when the excitation current I is off, it is connected to the output side a of the amplifier 41, and the offset voltage is
A voltage obtained by amplifying V _pf is selected, and when the excitation current I is on, it is connected to the output side b of the inverting amplifier 42,
After amplifying the sum of the signal voltage _Vs and the offset voltage _Vpf , the inverted voltage is selected. The voltage selected by the changeover switch 43 is applied to an integrator 46 via a sampling switch 44 driven by a pulse _P3 . Therefore, the integrator 46 outputs the output of the amplifier 41 related to the offset voltage V _pf as shown in FIG.
V ₂ is integrated in the negative direction for a fixed time t _s , and then the output V ₃ of the inverting amplifier 42, which is related to the sum of the signal voltage V _s and the offset voltage V _pf , is integrated in the positive direction for a fixed time t _s . Therefore, the output V ₄ of the integrator 46 after integrating the inverting amplifier output V ₃ for a certain period of time has a value related only to the signal voltage V _s with the offset voltage V _pf removed. The output _V5 of the comparator 47 which monitors the integrator output _V4 is as shown in FIG. 2, and when _V4 becomes positive, the output _V5 becomes high level. The NAND gate _G1 of the gate circuit 48 produces an output that turns on the reset switch ₄₅ when a pulse _P4 synchronized with the excitation current I is applied when the comparator output V5 is at a high level. When the reset switch 45 is turned on, the zener voltage _Vz is applied to the integrator 46 via the resistor _RI2 , and the integrator output _V4
reset at a constant slope. As a result, when the integrator output V ₄ becomes zero, the output V ₅ of the comparator 47
is inverted and becomes low level, so the gate circuit 48
The output of NAND gate _G1 becomes high level,
Turn off the reset switch 45. The reset time width Δt during which the reset switch 45 is on is such that the integrator output V ₄ at the start of reset is the signal voltage.
Since it is proportional only to V _s , it is given by the following equation.

Δt=k ₁ V _s t _s /V _z ...(1) However, k ₁ : proportionality constant or signal voltage V _s is proportional to fluid flow rate F and excitation current I, and excitation current I is equal to Zener voltage V Since it is proportional to _z , the reset time width _Δt is as follows: Δt=k ₁ k ₂ FV _{z t s} /V _z =k ₃ t _s F ……(2) However, k ₂ : Constant of proportionality k ₃ = k ₁ ·k ₂ and is not affected by fluctuations in the Zener voltage _Vz . Therefore, inverter IV of gate circuit 48
A pulse width signal PW having a period T and a pulse width Δt as shown in FIG. Since the period T is proportional to 1/f of the oscillation frequency f of the oscillator 51, and the constant integration time _ts is also proportional to 1/f, the duty ratio Δt/T of the pulse width signal PW is Δt _{/T=k3k4fF/k5f=kF...(3} ) _However , _k4 , _k5 : proportionality _constant k= _k3k4 / _k5 , which is proportional to the flow rate F, and the oscillation of _the oscillator 51 Not affected by frequency f.

In addition, the number N of pulses that the reference pulse _P5 passes through the NAND gate _G2 of the gate circuit 48 in the time Δt is N=k ₆ fΔt=k ₃ k ₄ k ₆ F (4), which is also proportional to the flow rate F. , is not affected by the oscillation frequency f of the oscillator 51. In this case, since the number N of pulses passing through the NAND gate _G2 is an integer value, an error of up to 1/N×100% occurs. Therefore, if the frequency k ₆ f of the reference pulse P ₅ is made high enough,
The error becomes sufficiently small. Further, as shown in FIG. 3, a reference pulse _P5 is applied to the comparator 47 via the capacitor _C1 and the resistor _R3 ,
The above-mentioned error can be eliminated by synchronizing the operation of P5 with the reference pulse _P5 .

In addition, in the above description, the case where the excitation current I that turns on and off at a low frequency is supplied from the excitation circuit 2 is illustrated, but
As shown in FIG. 3, switches 23, 24, 25,
26 may be used to switch the exciting current I between forward and reverse directions. In this case, the Zener voltage V _z is given directly or as a divided voltage as the set voltage V _r .
Further, in the above description, the case where the non-inverting input terminal (+) of the operational amplifier OP constituting the integrator 46 is connected to the reference point has been exemplified, but as shown in _FIG .
Add R ₇ and voltage divider resistor RV ₂ , integrator output V ₄
voltage divider resistor RV ₂ so that it is slightly negative
By adjusting , it is possible to avoid large integration errors that occur when the flow rate is zero for a long time.

As described above, the present invention provides a low frequency excitation type electromagnetic flowmeter that is not affected by fluctuations in the Zener voltage of the set voltage source.

[Brief explanation of the drawing]

Fig. 1 is a connection diagram showing one embodiment of the electromagnetic flowmeter of the invention, Fig. 2 is a waveform diagram for explaining its operation, and Figs. 3 and 4 show other embodiments of the electromagnetic flowmeter of the invention. FIG. 1... Electromagnetic flow meter transmitter, 2... Excitation circuit, 3
...Setting voltage source, 4...Signal processing circuit, 5...Pulse generation circuit.

Claims (1)

[Scope of utility model registration request] An electromagnetic flowmeter transmitter, an excitation circuit that supplies a low-frequency excitation current with an amplitude corresponding to a set voltage to an excitation coil of the electromagnetic flowmeter transmitter, and a Zener diode that generates the set voltage based on the Zener voltage. A set voltage source to be used, a voltage obtained by amplifying the voltage generated between the pair of electrodes of the electromagnetic flowmeter oscillator using an amplifier, and a voltage obtained by inverting this amplified voltage using an inverting amplifier are synchronized with the excitation current for a certain period of time. a Zener voltage from the set voltage source is applied to the integrator via a reset switch; A low frequency excitation type electromagnetic flowmeter comprising a circuit for resetting the flow rate and generating an output signal related to the reset time width.