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Description of five new schemes for electromagnetic flowmeter hardware anti-interference

a. The design of the preamplifier is the primary step to improve the anti-interference ability.
The output signal of the electromagnetic flow sensor is very weak and the internal impedance is high. Therefore, the high input-input impedance, low drift, low noise, and high CRMM preamplifier can meet the requirements of anti-in-phase common mode interference. The preamplifier uses JFET high input impedance voltage buffer, low drift low noise subtractor, precision resistors to carefully match the instrumentation amplifier, and uses input protection technology, common mode voltage bootstrap technology and grounding technology to greatly improve anti-common mode interference. Ability to suppress the effects of zero drift.

b. Frequency compensation technique for simultaneous sampling
Simultaneous sampling and power frequency power frequency monitoring and compensation technology is an effective method to improve the power frequency interference caused by mixed power frequency interference and power frequency power frequency fluctuation in the anti-flow signal potential. Synchronous sampling technology, the sampling pulse width is an integer multiple of the power frequency period, so that the average value of the power frequency interference in the flow signal potential is equal to zero to eliminate the influence of power frequency interference; the frequency fluctuation compensation of the power frequency power supply is to ensure the dynamic fluctuation of the frequency. The excitation power supply and the sampling pulse can be adjusted synchronously, realizing synchronous sampling technology and synchronous excitation technology, and synchronous A/D conversion to reduce the influence of power frequency interference.

c. New excitation technology is an important means to improve the anti-interference ability of electromagnetic flowmeter
The development of electromagnetic flowmeter excitation technology not only weakens the influence of electrode polarization potential, mud interference and flow noise, but also changes the form of power frequency interference, and facilitates synchronous sampling technology to process power frequency interference noise to avoid the influence of power frequency interference. At present, the electromagnetic flow sensor adopts the power frequency frequency synchronous three-value low-frequency rectangular excitation and the double-frequency rectangular wave excitation, thereby improving the overall anti-interference ability of the electromagnetic flowmeter and improving the measurement accuracy and reliability of the electromagnetic flowmeter.

d. Microprocessor system power supply voltage monitoring technology
In the electromagnetic flowmeter, when the power supply transient is undervoltage, the excitation switch pulse action will cause the microprocessor to malfunction and data loss. Therefore, reliable reset circuit and power supply voltage monitoring technology must be adopted. The simplest and most practical method is to use a low-cost power supply with a high-sensitivity power supply voltage monitor to improve the microprocessor system and anti-interference ability. As shown in Figure 4, the microprocessor voltage monitor uses the TL7705CP power supply voltage monitor chip, which has a power supply and a power supply transient undervoltage to generate a reliable reset signal.

e. Adopt new HCMOS series chip technology
Compared with the 74LS series chip, the 74HC series chip technology has a low noise margin of 2.4 times and a high noise tolerance of 2.1 times. The intelligent electromagnetic flowmeter uses 74HC series chips for the whole hardware, which not only reduces the overall power consumption, but also improves the components themselves. The anti-interference ability lays the foundation for the small and lightweight integration of the power flow meter.

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