JPH0478127B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to an electromagnetic flowmeter that applies a magnetic field to a fluid to be measured and measures its flow rate, and particularly relates to an electromagnetic flowmeter that applies a magnetic field to a fluid to be measured and measures its flow rate. This invention relates to an electromagnetic flowmeter that eliminates demodulation loss due to delay.

<Prior art> An example of a conventional electromagnetic flowmeter in which a rectangular-wave excitation current is passed through an excitation coil and an alternating current voltage generated in response to the flow rate is demodulated by a synchronization signal synchronized with the excitation current is shown in Fig. 5. Shown below.

10 is a conduit with an insulated inner surface through which the fluid to be measured flows. Electrodes 11a and 11b are arranged in contact with the inner surface of the conduit 10.

An excitation coil 1 is connected to the fluid to be measured via a conduit 10.
A magnetic field is applied from 2. A rectangular wave excitation voltage is applied to the excitation coil 12 from the excitation circuit 13, and an excitation current I _f0 is caused to flow with a response delay due to the inductance of the excitation coil 12. Excitation coil 1
A reference voltage R _V for generating a reference voltage V _R0 is connected between the reference voltage V R0 and the common potential point COM.

On the other hand, the amplifier 14 receives the voltages across the electrodes 11a and 11b, and generates a signal voltage e ₀ at its output terminal. The synchronous rectifier circuit 15 synchronously rectifies the signal voltage e 0 using the synchronous signal S _y0 corresponding to the excitation voltage of the excitation circuit 13 as a reference signal, and inputs the signal voltage e ₀ to the division circuit 16 as an output signal e _s0 . The division circuit 16 calculates the ratio between the reference voltage V _R0 and the output signal e _s0 and outputs the output terminal 1.
7 as a flow rate signal e _f0 .

Next, the conventional electromagnetic flowmeter shown in FIG. 5 and its operation will be explained with reference to FIG. 6.

The excitation circuit 13 applies a rectangular wave excitation voltage to the excitation coil 12, but since the excitation coil 12 has inductance, there is a delay in the response and the excitation voltage is applied to the excitation coil 12.
The waveform of the excitation current I _f0 is as shown in Figure (B). on the other hand,
A magnetic field having a waveform corresponding to this excitation current I _f0 is applied to the fluid to be measured, and a signal voltage e ₀ as shown in FIG. 6C is generated at the output end of the amplifier 14.

The synchronous rectifier circuit 15 synchronously rectifies this signal voltage e ₀ in response to a synchronous signal S _y0 (FIG. 6A) corresponding to the waveform of the excitation voltage, and outputs an output signal e _s0 having a waveform as shown in FIG. 6D. Output. The DC voltage obtained by smoothing this output signal e _s0 is proportional to the flow rate, and the division circuit 16 performs an operation to calculate the ratio of the voltage obtained by rectifying and smoothing the reference voltage V _R0 and this DC voltage. Then, a voltage proportional to the flow rate can be obtained at the output end 17 without being affected by fluctuations in the excitation current.

<Problems to be Solved by the Invention> However, in such a conventional electromagnetic flowmeter, a voltage loss occurs due to the response delay of the excitation current I _f shown as N ₁ in Fig. 6, and when smoothed, the average DC voltage decreases. This tendency becomes more pronounced as the excitation frequency becomes higher or as the time constant of the excitation coil becomes larger.

<Means for Solving the Problems> In order to solve these problems, the present invention provides an excitation means for exciting an excitation coil in synchronization with an excitation signal, and a standard proportional to the excitation current flowing through the excitation coil. A reference voltage generating means that generates a voltage, and a synchronization delay that determines the polarity of the reference voltage using an average voltage obtained by smoothing a predetermined excitation frequency component in the reference voltage as a threshold and generates a delayed synchronization signal corresponding to the response delay of the excitation coil. Amplifying means for amplifying a signal voltage generated in accordance with the flow rate, and synchronous rectifying means for synchronously rectifying the output of the amplifying means using a delayed synchronized signal.

<Example> Hereinafter, an example of the present invention will be described based on the drawings. FIG. 1 is a block diagram showing one embodiment of the present invention.

18 is an excitation circuit, which includes an excitation signal generator 19 that provides synchronization timing of the excitation current I _f1 , and a comparator 2.
0 and a switch circuit 21. In the switch circuit 21, switches 22 and 23 are formed by C-MOS transistors, each of which has one end connected to +
Voltages E and -E are applied, and each other end is connected to one end of the excitation coil 12. One end of the excitation signal generator 19 is connected to the common connection point COM, and the other end is connected to one end of the input of the comparator 20. A comparator 20 is connected to one end of the reference resistor R _V connected in series between the other end of the excitation coil 12 and the common potential point COM.
The other input end of is connected, and the output end thereof is connected to the gates of switches 22 and 23.

The signal voltage _e1 at the output end of the amplifier 14 is applied to the synchronous rectifier circuit 24, and the synchronously rectified output signal is
e _s1 is smoothed by the smoothing circuit 25 and outputted to the output end 17 via the amplifier 26 and the output circuit 27 as the flow rate signal e _f1 .

The synchronous rectifier circuit 24 is formed of switches 28 and 29 formed of C-MOS transistors, and the switches 28 and 29 receive the delayed synchronous signal S _y1 from the synchronous delay circuit 30 as a reference signal and perform synchronous rectification.

The synchronous delay circuit 30 includes a comparator 31 and a filter 32
The input terminal of the filter 32 and the comparator 31
One end of the input of is the reference voltage across the reference resistor R _V
V _R1 is applied. An average voltage _R1 of the reference voltage V _R1 is obtained at the output end of the filter 32, and the comparator 31 uses this average voltage _R1 as a threshold and outputs the delayed synchronization signal S _y1 at its output end.

Next, the operation of the embodiment shown in FIG. 1 constructed as above will be explained using the waveform diagram shown in FIG. 2.

As shown in FIG. 2A, the excitation signal generator 19 applies a rectangular excitation signal S _ex to the non-inverting input terminal (+) of the comparator 20, and the inverting input terminal (-) of the comparator 20.
A reference voltage proportional to the excitation current I _f1 is applied to
The comparator 20 turns on and off the switch circuit 21 so that the excitation current I _f1 corresponds to the controlled excitation signal S _ex . As a result, the waveform of the excitation current I _f1 becomes
The waveform will look like the one shown in Figure B.

A magnetic field proportional to the excitation current I _f1 is applied to the fluid to be measured, and a signal voltage e ₁ shown in FIG. 2C is generated at the output end of the amplifier 14.

On the other hand, the comparator 31 uses the average voltage _R1 of the reference voltage V _R1 proportional to the excitation current I _f1 as a threshold value, and compares this with the reference voltage V _R1 to generate the delayed synchronization signal S _y1 shown in FIG. 2D. The delayed synchronization signal S _y1 (FIG. 2 D) has a delay from the excitation signal S _ex corresponding to the response delay of the excitation current I _f1 . Synchronous rectifier circuit 24
The signal voltage e ₁ shown in FIG. 2C is synchronously rectified using the delayed synchronization signal S _y1 shown in FIG. 2D as a reference voltage, and output signal e _s1 shown in FIG. 2E is produced. The output signal e _s1 is smoothed by the smoothing circuit 25 and outputted to the output end 17 via the amplifier 26 and the output circuit 27 as a flow rate signal.

As described above, by delaying the phase of the synchronizing signal in response to the response delay of the excitation current, the voltage loss _N1 shown by diagonal lines in FIG. 6 can be avoided, and sensitivity can be improved.

FIG. 3 is a block diagram showing a second embodiment of the invention. This embodiment is an example in which the present invention is applied to an electromagnetic flowmeter configured to flow excitation currents of a plurality of frequencies through an excitation coil 12.

33 is an excitation circuit that supplies excitation current I _f2 of two types of frequencies, high frequency and low frequency, to the excitation coil 12. 34 is a step-down counter, which receives the clock frequency f ₀ from the clock generator 35.
is input to input terminal CL. The step-down counter 34 counts down this, and its output terminal
Output to Q ₁ , Q ₂ ~Q _o . Output terminal Q ₃ , Q ₄ , Q ₅ ,
At _Q12 , pulse outputs having the waveforms shown in Fig. 4 A, B, C, and D are obtained. The respective pulse outputs of the output terminals Q ₃ , Q ₄ , Q ₅ , and Q ₁₂ are logically operated by the logic circuit 36, and the pulse outputs S ₁ and S ₂ enter the switch states shown in FIG. 4, E, F, G, and H, respectively. , S ₃ , and S ₄ are output. For the pulse outputs S ₁ and S ₂ , one ends of switches 37 and 38 constituted by C-MOS transistors are connected to the positive power supply +E and negative power supply -E, respectively, and the other ends are connected to one end of the excitation coil 12 . Pulse output S ₃ ,
_S4 is applied to the gates of switches 39 and 40, each consisting of a C-MOS transistor. One ends of the switches 39 and 40 are respectively connected to a common potential point COM, and the other ends are connected to one end of the excitation coil 12 via diodes 41 and 42.

The switches 37 to 40 are sequentially controlled by each pulse output S ₁ to _{S 4} from the logic circuit 36, and the excitation coil 1
2 is supplied with an excitation current I _f2 having the same waveform as the reference voltage V _R2 generated across the reference resistor R _V shown in FIG.
The excitation current I _f2 has a high frequency component (period 2H) shown in Figure 4 C and a low frequency component (period 2H) shown in Figure 4 D.
2L).

The reference voltage V _R2 is input to the synchronous delay circuit 43,
The synchronization delay circuit 43 converts it into a delayed synchronization signal S _y2 . In the synchronous delay circuit 43, a reference voltage V _R2 is applied to one end of switches 44 and 45 composed of C-MOS transistors, and the other end is applied to filters 46 and 4.
7 is connected. The output terminal _Q12 of the step-down counter 34 is connected to the gates of the switches 44 and 45.
The output (FIG. 4D) is applied, synchronously rectified at a low frequency, and then smoothed by filters 46 and 47. As a result, the outputs of the filters 46 and 47 have an average voltage V _L1 averaged over a period of 2L,
V _L2 is obtained.

These average voltages V _L1 and V _L2 are the output at the output terminal _Q12 of the step-down counter 34 (Fig. 4 D).
The signal is applied to the inverting input terminal (-) of the comparator 50 via switches 48 and 49 of C-MOS transistors controlled by . Using these average voltages V _L1 and V _L2 as thresholds, a reference voltage V _R2 applied to the non-inverting input terminal (+) of the comparator 50 is compared, and a delayed synchronization signal S _y2 is obtained at its output terminal.

On the other hand, the signal voltage obtained at the output terminal of the amplifier 14
e ₂ (Fig. 2 O) is input to the synchronous rectifier circuit 61,
Synchronous rectification is performed by C-MOS transistor switches 62 and 63 controlled by a delayed synchronization signal _Sy2 . The switches 62 and 63 are opened and closed by the delayed synchronization signal S _y2 , and are opened and closed at high speed as shown in FIG. As a result, an output signal e _s2 having a waveform as shown in FIG. 4 is obtained at the output end of the synchronous rectifier circuit 61. Although this output signal e _s2 is demodulated at high speed with a period of 2H, differential noise associated with switching of the excitation current I _f2 indicated by N ₁ and N ₂ is added and is not eliminated. This differential noise is caused by the magnetic field interlinking with the signal circuit, and if the interlinkage area changes due to temperature, for example, it becomes a factor of zero drift.
Note that the negative voltage waveform indicated by N ₃ is the signal voltage e ₂
This occurs in response to the level of DC noise voltage contained in the current, and is not due to a response delay of the excitation current I _f2 . The output signal e _s2 is input to a filter 64 and smoothed, converted into a digital value by an analog-to-digital converter (A/D converter) 65, and input to a microcomputer 66.

Also, the signal voltage obtained at the output terminal of the amplifier 14
e ₂ (Fig. 2 O) is input to the synchronous rectifier circuit 67,
It is synchronously rectified by the C-MOS transistor switches 68 and 69 controlled by the output of the output terminal Q12 of the step-down counter ₃₄ (FIG. 4D), and the fourth
Obtain the output signal e _s3 shown in Figure F. This output signal e _s3
is demodulated at a low speed with a period of 2L, but the differential noises N ₄ and N ₅ that occur due to switching of the excitation current I _f2 cancel each other out, and a stable zero point is obtained. The output signal e _s3 is input to a filter 69, smoothed, digitized by an A/D converter 65, and input to a microcomputer 66.

The microcomputer 66 additively combines the output _s2 of the filter 64 and the output _s3 of the filter 68, and outputs the result to the output terminal 17 via the output circuit 70 as a flow rate signal e _f2 . Note that the microcomputer 66 divides the average voltage (V _L1 + V _L2 ) by the digital value obtained via the A/D converter and the output ( _s2 + _s3 ), thereby reducing the error due to fluctuations in the excitation current I _f2. can be prevented.

When we perform an operation to add the outputs _s2 and _s3 , we get the output _s2
It is possible to ensure the long-term stability of output s3 while taking advantage of the high-speed response of _s3 .

In the case of normal operation with small flow rate fluctuations, the frequency of fluctuations in the signal voltage e ₂ included in the output of the amplifier 14 is small, and the output signal e _s2 is passed through the switches 62 and 63 (repetition period 2H) that function as a high-pass filter.
Therefore, there is no response, and a stable low-speed response output signal e _s3 at the zero point by the switches 68 and 69 (repetition period 2L) functioning as a low-pass filter is output as a composite output. On the other hand, regarding low-frequency random noise (hereinafter referred to as flow noise) that increases with the flow velocity generated in slurry fluids, etc., the switch 6, which functions as a low-pass filter,
Due to the presence of switches 8 and 69, the effect is reduced and does not appear as a fluctuation in the output signal e _s3 , and the switches 62 and 69 are switched in synchronization with the high excitation frequency.
63, there is a large frequency difference with the flow noise existing in the low frequency region, and its influence does not appear on the output signal e _s2 . In other words, in the case of normal operation with little variation in flow rate, it is possible to provide an electromagnetic flowmeter that is not easily affected by flow noise while ensuring a stable zero point.

Next, if the flow rate fluctuates suddenly, the output signal
e _s3 does not respond due to the large time constants of switches 68 and 69 that function as low-pass filters, but output signal e _s2 immediately responds and becomes output signal e due to the small time constants of switches 62 and 63 that function as high-pass filters. _s2 is output as the composite output.

In the above description, since flow noise is low-frequency random noise, it is desirable that the synchronization signal as a reference voltage of a synchronous rectification circuit exhibiting high-speed response has a high frequency. However, if you simply increase the higher of the two excitation frequencies, the response delay due to the time constant of the excitation coil will increase the noise _N1 shown with diagonal lines in Figure 6D, decrease the output signal e _s2 , and cause a decrease in sensitivity. invite Therefore, in the embodiment shown in FIG. 3, the synchronous delay circuit 43 uses the delayed synchronous signal S _y2 having a delay corresponding to the response delay of the excitation current as a reference voltage, thereby reducing the sensitivity due to the response delay of the excitation current I _f2 . This allows the higher frequency in double excitation to be increased without deteriorating sensitivity, and the influence of flow noise can be easily removed.

<Effects of the Invention> As described above in detail with the embodiments, according to the present invention, even when there is a delay in the excitation current due to the inductance of the excitation coil of the transmitter, the synchronization signal can be generated in response to this delay. Since the switching timing is delayed and moved to the optimum position, the flow rate can be stably measured without changes in sensitivity, that is, fluctuations in span.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of the present invention;
2 is a waveform diagram showing the waveforms of each part of the embodiment shown in FIG. 1, FIG. 3 is a block diagram showing another embodiment of the present invention, and FIG. 4 is a waveform diagram of each part of the embodiment shown in FIG. FIG. 5 is a block diagram showing the configuration of a conventional electromagnetic flowmeter, and FIG. 6 is a waveform diagram showing waveforms of various parts of the electromagnetic flowmeter shown in FIG. 10... Conduit, 12... Excitation coil, 13,1
8, 33...excitation circuit, 14...amplifier, 15,
24, 61, 67... Synchronous rectifier circuit, 19... Excitation signal generator, 25... Smoothing circuit, 30, 43...
...Synchronization delay circuit, S _y1 , S _y2 ...Delayed synchronization signal,
S _ex ...excitation signal, e ₀ ~ e ₂ ... signal voltage, e _s0 ~ e _s2
...Output signal, e _f0 to e _f2 ...Flow rate signal.

Claims (1)

[Claims] 1. excitation means for exciting an excitation coil in synchronization with an excitation signal; reference voltage generation means for generating a reference voltage proportional to an excitation current flowing through the excitation coil;
a synchronization delay means that determines the polarity of the reference voltage using an average voltage obtained by smoothing a predetermined excitation frequency component in the reference voltage as a threshold and generates a delayed synchronization signal corresponding to a response delay of the excitation coil; An electromagnetic flowmeter comprising: amplifying means for amplifying a signal voltage generated by the amplifying means; and synchronous rectifying means for synchronously rectifying the output of the amplifying means using the delayed synchronized signal.