Research on the design method of the calibrator based on the electromagnetic flowmeter signal converter

1. Preface:

The calibrator of the electromagnetic flowmeter signal converter is a device for testing and calibrating the performance of the electromagnetic flowmeter signal converter. The existing electromagnetic flowmeter signal converter is composed of a series of high-precision resistor networks, which can generally only perform fixed-point testing and calibration. The parameters such as the amplification factor of converters from different manufacturers are different. Therefore, such a calibrator of the electromagnetic flowmeter signal converter can only perform fixed-point testing and calibration on a specific electromagnetic flowmeter signal converter without monitoring conditions. This limits the application of electromagnetic flowmeters.

Second design idea:

The designed electromagnetic flowmeter signal converter calibrator includes an external electromagnetic flowmeter signal converter to be calibrated and a resistor network that outputs a fluid flow rate signal to be measured by the simulated electromagnetic flowmeter. The excitation current output by the electromagnetic flowmeter signal converter is used to provide power and synchronization signals to obtain the output signal of the simulated electromagnetic flowmeter sensor. The relative size of the analog signal can be adjusted and measured steplessly; and the output signal can be adjusted according to different types of converters, and different flow rate signals and different fluid impedances can be simulated. The relative amplitude of the output signal can be measured by an ordinary digital voltmeter, which can be used to calibrate electromagnetic flowmeter signal converter products from different manufacturers.

Three implementation options:

The present design includes an external electromagnetic flowmeter signal converter to be calibrated and a resistor network that outputs a fluid flow rate signal to be measured by the simulated electromagnetic flowmeter. The excitation current output of the external electromagnetic flowmeter signal converter is connected to the primary input of a transformer via a bidirectional regulator, and the secondary output of the transformer is connected to the input of the resistor network. The output of the resistor network is connected to the input of a freewheeling circuit and the input of a constant current source appliance after passing through a full-wave rectifier, and the output of the constant current source circuit is connected to a sampling resistor. The resistor network simultaneously outputs the fluid flow rate signal to be measured by the simulated electromagnetic flowmeter and is connected to the signal input of the external electromagnetic flowmeter signal converter.

Four-check circuit implementation process:

The circuit schematic is shown in Figure 2

1) The two output ends of the electromagnetic flowmeter excitation current S1 are connected in parallel with the two ends of the bidirectional voltage regulator circuit and the two input ends of the primary coil of the transformer. The secondary coil of the transformer outputs the signal synchronization source and working power supply S2 which are electrically isolated from S1.

2) The two output ends of the transformer secondary coil output signal S2 are respectively connected to the input end of the resistor network and the AC input end of the full-wave rectifier. The DC output end of the full-wave rectifier provides power for the load circuit formed by the adjustable constant current source and the sampling resistor in series, wherein the anode of the full-wave rectifier is connected to the input of the adjustable constant current source and one end of the freewheeling circuit, the cathode of the full-wave rectifier is connected to the other end of the freewheeling circuit, and the other end of the sampling resistor is connected to the output of the constant current source circuit. When the switch SW1 of the freewheeling circuit is disconnected, the current I value A determined by the adjustable constant current source determines the positive and negative amplitude A of the positive and negative alternating current I flowing through the input end of the resistor network, so that the output signal S3 of the resistor network output verifier has positive and negative amplitudes B;

B=KХA

K is the attenuation coefficient, which is determined by the resistance value in the resistor network;

3) The freewheeling circuit composed of a capacitor and a switch SW1 is connected in parallel to the DC output end of the full-wave rectifier. When the switch SW1 of the freewheeling circuit is turned on, the current I value A determined by the adjustable constant current source forms a directly measurable DC voltage signal S4 on the sampling resistor. The positive and negative amplitudes A of the alternating current I of the signal S3 can be obtained through the signal S4, A=S4/R, and R is the resistance of the sampling resistor.

4) When the switch SW1 of the freewheeling circuit is disconnected, the output signal S3 of the checker is determined by the adjustable constant current source current i value A. When the switch SW1 of the freewheeling circuit is closed, the positive and negative amplitudes B of the actual checker output signal S3 are obtained by measuring the signal S4:

B=KXS4/R

The current waveforms of each part during operation are shown in Figure 3.

The output of the full-wave rectifier circuit D2 provides power for U1. U1 is an adjustable constant current source circuit. In this example, LM334 is selected. The current can be adjusted by adjusting R4. The current can be obtained by measuring the voltage across R6. When using an ordinary voltmeter to measure the voltage, switch SW1 should be turned on, and capacitor C1 should be connected to the circuit to play a filtering role, so that the current source circuit can obtain a stable power input, thereby obtaining a voltage across R6.

The voltage is stable and can be measured accurately. After the measurement is completed, disconnect SW1.

In the circuit shown in Figure 2, by adjusting the constant current source current, the current obtained is 2 microamperes to 4 milliamperes, and the output voltage range can be obtained at S3, which is about 0 millivolts to 40 millivolts. The corresponding voltage generated at both ends of the measuring resistor R6 is about 200 microvolts to 400 millivolts, which can be measured by an ordinary digital voltmeter. By adjusting R3 and R4, the device can be adapted to the calibration of electromagnetic flowmeter converters from different manufacturers. When the designed calibrator of the electromagnetic flowmeter signal converter is working, the current waveforms of its various parts are shown in Figure 3.

5. Measurement circuit based on calibrator

The voltage signal at both ends of resistor R6 and the square wave signal on S3 can be measured by using chips such as PIC18LF2520. Thus, the adjustment voltage value and flow rate signal value can be obtained. The circuit block diagram is shown in Figure 4. The hardware design of the circuit mainly includes signal conditioning, A/D conversion, LCD display and peripheral circuits related to the single-chip microcomputer. The voltage signal obtained by the electromagnetic flow sensor is processed by the signal conditioning circuit, converted by the A/D converter, and then processed by the CPU. The corresponding flow rate is displayed using a liquid crystal display . The functions of each module are as follows:

Sensor signal acquisition and signal conditioning circuit: The core and difficulty lies in controlling the polarization voltage to a repetitive and stable value, extracting the weak induced electromotive force, and adjusting it to an appropriate range that can be processed by the subsequent circuit. Due to the idea of ​​automatic tracking feedback control, the signal conditioning circuit is controlled by a single-chip microcomputer.

A/D conversion circuit: In order to ensure the accuracy and stability of measurement, a 16-bit ∑-△ type analog-to-digital converter is used, which has high measurement accuracy and strong anti-interference ability.

MCU related peripheral circuits: clock, reset circuit, keyboard and LCD display. The keyboard input records the initial zero point, and the LCD displays the flow in real time.

Power supply: The power supply part of the entire flow measurement system is powered by a lithium battery with high energy density.

The main program flow chart of the electromagnetic flowmeter software design based on the verifier is shown in Figure 5.

VI. Conclusion

Compared with the existing technology, this design has the following obvious outstanding features and significant advantages: using the excitation current to provide power for the circuit, eliminating the battery required for the usual analog signal generator; using the output resistance network and the constant current circuit in series, the obtained output signal is strictly symmetrical and can simulate the state of the fluid flowing at a constant speed in the pipeline; the output signal is synchronized with the excitation signal, the signal size can be adjusted steplessly and the relative size can be measured, which can adapt to converters from different manufacturers, and is particularly suitable for performance verification in the production process of electromagnetic flowmeter signal converters. The electromagnetic flowmeter designed based on this verifier has been proven to be effective in practice. The measurement accuracy has been greatly improved.