JPH0611370A - Electromagnetic flowmeter - Google Patents

Abstract

PURPOSE:To eliminate fluid noise and sharply reduce the electric current consumption of the title flowmeter by converting the mean value of sampling signals into a digital signal having a period longer than that of an exciting frequency and calculating the flow rate of a fluid by using the digital signal, and then, outputting the calculated flow rate as a flow rate signal. CONSTITUTION:When a flow-frequency exciting current If' is made to flow to exciting coils 11a and 111b, a signal voltage eSO' proportional to the flow rate of a fluid to be measured is outputted to the output terminal of a differential amplifier Q3. The voltage eSO' is sampled just before the current If' changes from positive to negative or from negative to positive and signals held by capacitors C2-C7 are added to each other by means of an addition amplifier Q5. Then the signals are converted into digital signals by means of an A/D converter 24 based on a timing signal ST6 having a period which is three times as long as that of the current If' and the digital signals are outputted to a CPU 23. Therefore, the converter 23 is refreshed in data at a rate of one time per a plurality of exciting periods. In addition, the CPU 23 performs various kinds of processes, such as flow rate calculation, actual flow rate conversion, etc., and outputs processed results to an output circuit 19.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic flow meter which excites a signal voltage generated at a predetermined frequency in response to a flow rate and samples and outputs the signal voltage in units of each cycle in synchronization with the predetermined frequency. In particular, the present invention relates to an electromagnetic flow meter improved so as to improve the SN ratio while reducing current consumption.

[0002]

2. Description of the Related Art A conventional electromagnetic flow meter employs a low frequency excitation method for exciting at a low frequency of several Hz to 10 Hz in order to secure the stability of the zero point. FIG. 4 shows the configuration of such a low-frequency excitation type conventional electromagnetic flowmeter. The outline will be described below with reference to FIG.

Reference numeral 10 is an insulating cylindrical pipe through which the measurement fluid Q flows. A low-frequency exciting current _If is passed from the exciting circuit 12A to the exciting coils 11a and 11b, whereby a magnetic field B is generated and applied to the measurement fluid Q.

The exciting circuit 12A includes a timing circuit 1
A timing signal ST ₁ for switching the exciting current I _f into, for example, a rectangular wave is applied from 3A, and this timing circuit 13A is controlled by a timing signal ST ₂ from a microprocessor (CPU) 14.

On the other hand, the voltage generated by the flow of the measurement fluid Q is detected by the detection electrodes 15a and 15b fixed to the pipe 10 and in contact with the measurement fluid Q. This detection electrode 1
The voltages e _S1 and e _S2 detected by 5a and 15b are applied to the input terminals of the amplifiers Q1 and Q2 having high input impedance, respectively.

The outputs of the amplifiers Q1 and Q2 are applied to the differential amplifier Q3, and the difference between them is calculated and output as the output voltage e _S0 . These amplifiers Q1 and Q2,
The input circuit 16 is configured by the differential amplifier Q3.

The output voltage e _S0 is output to the sample and hold circuit 17 as it is or through the inverting amplifier Q4. The sample and hold circuit 17 includes a switch SW ₁ ,
It is composed of SW _{2 , a} holding capacitor C1, an amplifier Q5, and the like.

The output voltage e _S0 is applied to one end of the switch SW ₁ ,
Alternatively, it is connected to one end of the switch SW ₂ via the inverting amplifier Q4, and the other ends of the switches SW ₁ and SW ₂ are connected to the common potential point COM via the capacitor C1 and also to the input end of the amplifier Q5. There is.

The switches SW ₁ and SW ₂ are sampled with their opening / closing controlled by a timing signal ST ₃ synchronized with the low-frequency exciting current I _f output from the microprocessor 14.

Further, the output voltage of the amplifier Q5 is the low-frequency exciting current I _f output from the microprocessor 14.
The signal is converted into a digital signal by the analog / digital converter 18 whose conversion timing is controlled by the timing signal ST ₄ synchronized with and is output to the microprocessor 14.

The microprocessor 14 uses the input digital signal to execute a flow rate calculation using a built-in program, and outputs the obtained flow rate signal to the output end 20 via the output circuit 19.

Next, the operation of the electromagnetic flowmeter shown in FIG. 4 will be described with reference to the waveform chart shown in FIG. Exciting current I _f the exciting coils 11a of the low frequency with a rectangular waveform as shown in FIG. 5 (a), the flow to 11b, the signal voltage e _S0 proportional to the flow rate of fluid should the output of the differential amplifier Q3 The output voltage e _S0 is output to the end.

The output voltage e _S0 is sampled just before the exciting current I _f is switched by the timing signal ST ₃ (FIG. 5 (b)) synchronized with the exciting current I _f output from the microprocessor 14 and analogized. / Is output to the digital converter 18.

The analog / digital converter 18 is a timing signal ST output from the microprocessor 14.
₄ (FIG. 5C), the timing signal ST ₃ (FIG.
It is converted into a digital signal at the conversion time T _c in the same repetition cycle as in (b) and is output to the microprocessor 14.

Instead of the sample and hold circuit 17, a sample and hold circuit 21 as shown in FIG. 6 may be used. In this case, filter for damping that is composed of a switch SW _1, resistor to the other end of the SW ₂ R1 and the capacitor C2 is Yes is added, thereby to average the error component at each timing.

[0016]

However, in the electromagnetic flowmeter of low frequency excitation as described above, the excitation frequency is about several Hz to 10 Hz, while the fluid noise generated by the fluid is 1 / f (f Indicates a frequency) characteristic, and fluid noise increases as the frequency becomes lower. For this reason, the excitation frequency and the frequency characteristic band of the fluid noise are approximated to each other, which has a drawback that a large output fluctuation is exhibited.

Therefore, recently, attempts have been made to improve the S / N by increasing the excitation frequency. Since the corner frequency of fluid noise is several tens of Hz, the excitation frequency is selected to be about 100 Hz or higher.

However, if the excitation frequency is simply increased, current consumption in the digital circuit increases, and power is supplied from the load side to the converter as a current signal of 4 mA to 20 mA, especially using two transmission lines. In a so-called two-wire type electromagnetic flowmeter of the type described above, the power consumption increases, which makes it difficult to realize. In this case, even if the C-MOS type low power consumption type element is used, the consumption current increases remarkably.

[0019]

According to the present invention, as a structure for solving the above-mentioned problems, a signal voltage generated in response to a flow rate by being excited at a predetermined frequency is synchronized with the above predetermined frequency in each cycle. In an electromagnetic flow meter that samples and outputs in units of, a calculating unit that calculates an average value of sampling signals sampled over a plurality of previous cycles, and the average value is digitally recorded at a cycle longer than the cycle of the previous excitation frequency. An analog / digital conversion means for converting into a signal is provided, and the converted digital signal is used to execute a flow rate calculation and output as a flow rate signal.

[0020]

[Operation] The calculating means calculates an average value of sampling signals sampled over a plurality of sampling cycles. The analog / digital conversion means converts this average value into a digital signal with a cycle longer than the cycle of the excitation frequency.

Then, the flow rate calculation is executed by using the converted digital signal to output as a flow rate signal. As a result, it is possible to realize an electromagnetic flow meter that removes fluid noise and consumes less power.

[0022]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of one embodiment of the present invention. The parts having the same functions as those of the conventional electromagnetic flow meter shown in FIG. 4 are designated by the same reference numerals and the description thereof will be appropriately omitted.

An exciting current I _f ′ having a frequency of about 100 Hz is supplied from the exciting circuit 12B controlled by the timing signal ST ₁ ′ from the timing circuit 13B to the exciting coils 11a and 11b, and the magnetic field B is applied to the measurement fluid Q. 'Is being applied.

On the other hand, the signal voltage generated on the detection electrodes 15a and 15b is impedance-converted by the input circuit 16 and output to the sample-hold circuit 22 as an output voltage e _S0 ′.

The sample and hold circuit 22 includes switches SW _2P , SW _3P , SW _4P , and switches SW _2N , S.
It is applied to one end of each of W _3N and SW _4N . The other ends of these switches are connected to the common potential point COM via capacitors C2 and C7, respectively, and are also connected to one ends of the resistors R2 to R7.

The other ends of the resistors R2, R3 and R4 are all connected to the non-inverting input end (+) of the summing amplifier Q5, and the resistor R
The other ends of 5, R6 and R7 are all connected to the inverting input terminal (-) of the summing amplifier Q5.

The opening and closing of these switches are controlled by the timing signal ST ₅ output from the microprocessor 23. Timing signal ST in this case
₅ is a pair for each excitation period, for example, SW _2P and SW _2N ,
SW _3P and SW _3N , etc. are opened and closed as a pair.

The hold voltage of the sample and hold circuit 22 is output to the analog / digital converter 24. The analog / digital converter 24 is the microprocessor 23.
The conversion timing is controlled by the timing signal ST ₆ output from the device and converted into a digital signal.

FIG. 3 shows a detailed internal structure of the analog / digital converter 24. The output signal of the summing amplifier Q5 is held in the capacitor C ₀ via the switch SW ₀ , and this hold voltage is opened / closed by the switch SW ₀ controlled by the timing signal ST ₆ to the analog / digital conversion section via the amplifier Q6. Is output.

In this case, the timing signal ST ₆ gives a timing for converting to every hold circuit every period when the signal is held. In this way, the digital signal converted by the analog / digital converter 24 is output to the microprocessor 23, where various processes such as flow rate calculation, damping process, rate limit process, and actual flow rate conversion are performed, and the output circuit It is output to 19.

Next, the above operation will be specifically described with reference to the waveform chart shown in FIG. A low-frequency exciting current I _f ′ having a rectangular waveform as shown in FIG.
When it is applied to a and 11b, the signal voltage e _S0 ′ proportional to the flow rate of the measurement fluid is output to the output terminal of the differential amplifier Q3.

This output voltage e _S0 ′ is controlled by the switches SW _2P and SW _2N whose opening and closing are controlled by the timing signal ST ₅ (FIG. 2B) which is synchronized with the exciting current I _f ′ output from the microprocessor 23. , Timing signal S
Switch S whose opening and closing is controlled by T ₅ (FIG. 2 (c))
Switches SW _4P and S whose opening and closing are controlled by W _3P and SW _{3N and} by timing signal ST ₅ (FIG. 2 (d))
The signals sampled immediately before the exciting current _If 'is switched to positive or negative by W _4N and held in the capacitors C2 to C7 are added by the adding amplifier Q5.

Then, the added signal is converted into a digital signal by the analog / digital converter 24 by the timing signal ST _{6 having} a cycle which is three cycles longer than the cycle of the exciting current I _f ′, as shown in FIG. It is output to the microprocessor 23. Therefore, in the analog / digital converter 24, the data is refreshed once for a plurality of excitation periods.

In the embodiment shown in FIG. 1, the algebraic sum of sample values of each excitation period is calculated and analog / digital conversion is performed after a plurality of cycles have passed. However, instead of the algebraic sum, as shown in FIG. A low-pass filter may be used to make the filter time constant sufficiently larger than the excitation period. In this case, as shown in FIG. 1, a large number of pairs of switches are not required and the circuit is simplified.

Further, as the signal synchronous rectification method, a configuration having an inverting amplifier Q4 shown in FIG. 4 or a microprocessor may be used for the calculation. Further, the timing for operating the analog / digital converter may be controlled by the microprocessor by distinguishing the magnitude of the flow rate signal, the presence / absence of flow rate, etc.

In the above description, an example in which a liquid contact type electrode is used has been described, but the present invention is not limited to this, and for example, a capacitance detection type electromagnetic flow meter coupled to a measurement fluid via capacitance is also used. The present invention can be similarly applied.

[0037]

As described above in detail with reference to the embodiments, according to the present invention, it is possible to significantly reduce the influence of fluid noise by exciting at a high frequency, and
Since the period of digital conversion is made large, it is possible to greatly reduce the current consumption, and in particular, the two-wire system in which power is supplied from the load side as a current signal of 4 mA to 20 mA using two transmission lines. The effect is large when applied to an electromagnetic flowmeter.

Further, since the analog / digital conversion time can be set to a multiple of the excitation period, it is easy to obtain the necessary resolution. Further, since the flow rate calculation by the microprocessor can be performed slowly and the clock frequency can be reduced, the power consumption of the logic circuit section can be reduced.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention.

FIG. 2 is a waveform diagram for explaining the operation of the embodiment shown in FIG.

FIG. 3 is a configuration diagram showing a specific configuration of analog / digital conversion shown in FIG.

FIG. 4 is a block diagram showing a configuration of a conventional electromagnetic flow meter.

5 is a waveform diagram illustrating the operation of the electromagnetic flow meter shown in FIG.

FIG. 6 is a configuration diagram showing a modification of the sample and hold circuit shown in FIG.

[Explanation of symbols]

 10 Pipe 11a, 11b Excitation coil 12A, 12B Excitation circuit 13A, 13B Timing circuit 14,23 Microprocessor 17,22 Sample and hold circuit 18,24 Analog / digital converter

Claims (1)

[Claims] 1. An electromagnetic flow meter which excites a signal voltage generated at a predetermined frequency in response to a flow rate and samples and outputs the signal voltage in synchronization with the predetermined frequency in each cycle as a unit,
The conversion means comprises an arithmetic means for calculating an average value of sampling signals sampled over a plurality of the cycles, and an analog / digital conversion means for converting the average value into a digital signal at a cycle longer than the cycle of the excitation frequency. An electromagnetic flowmeter characterized by executing a flow rate calculation using the digital signal and outputting it as a flow rate signal.