Three factors and coupling modes of environmental noise interference to ultrasonic flowmeter

1. Introduction
       Anti-interference ability is an important function for ultrasonic flowmeters, because there are usually various noise sources in the working environment of ultrasonic flowmeters, which will cause different degrees of interference and influence. Interference enters the ultrasonic flowmeter through different coupling methods, causing the measurement results to deviate from the true value, or cause abnormal operation, affecting the normal operation of the ultrasonic flowmeter and causing different degrees of error in the measurement results of the instrument. If we want to ensure the normal operation of the ultrasonic flowmeter, we must understand the source of noise interference and coupling transmission problems, minimize the impact of noise on instrument measurement, and ensure the stability and accuracy of production measurement data.
 
2. Classification of noise and three elements of interference
        There are many classifications of productive noise. Here we can divide it into the following types according to its source:
1. Mechanical noise Noise generated by mechanical impact, friction, and rotation, such as the sound of machine tools, textile machines, electric saws, ball mills, etc.
2. Fluid dynamic noise The sound generated by sudden changes in gas pressure or volume or fluid flow. Such as the sound of air compressors, ventilators, ejectors, boiler exhaust water discharge, air whistles, etc.
3. Electromagnetic noise occurs due to the interaction of alternating forces in the motor, such as the humming sound emitted by generators and transformers.
 
        According to the different distribution of noise over time, noise can be divided into continuous and intermittent noise. Continuous noise can be divided into steady-state noise (sound pressure level fluctuation is less than 5dB) and non-steady-state noise. The pulse noise in the latter (the duration of the sound is less than 0.5 seconds, the interval time is greater than 1 second, and the change of the sound pressure level is greater than 40dB) not only affects the work of the instrument, but also causes great harm to the human body.
        The three elements that affect the normal operation of the ultrasonic flowmeter due to noise interference are the noise source, the susceptible circuit of the ultrasonic flowmeter that is sensitive to noise, and the coupling channel between the two.
 


3.
        Coupling of interference Coupling refers to the way the interference signal enters the ultrasonic flowmeter. When analyzing the interference problem, it is necessary to clarify the interference source, the susceptible circuit and the coupling between the two. The interference source and the susceptible circuit exist objectively and are difficult to eliminate, so it is very important to cut off the coupling. There are several ways to couple interference.
3.1 Inductive coupling Inductive coupling refers to
the existence of mutual inductance between two circuits. When the current in one circuit changes, an induced voltage is caused in the other circuit. Figure 1 is an equivalent circuit diagram of electromagnetic coupling between two circuits. There is mutual inductance between the two circuits, and the mutual inductance coefficient is m. When the interference current ni of circuit 1 changes, an interference voltage nnujmi ω is generated in circuit 2 through electromagnetic coupling. It can be seen that the voltage nu? is proportional to the current n.
3.2 Capacitive coupling
Capacitive coupling refers to the existence of parasitic capacitance between two circuits, which produces electrostatic induction, so that the charge change of one circuit affects the other circuit. In the circuit shown in Figure 2, wire 1 is the interference source, wire 2 is the transmission line of the ultrasonic flowmeter, c1 and c2 are the parasitic capacitance of wires 1 and 2 to the ground, c12 is the parasitic capacitance between wires 1 and 2, and r is the resistance of wire 2 to the ground. When there is an interference voltage u1 in wire 1, the interference voltage generated in wire 2 is
It can be seen that capacitive coupling interference increases with the increase of coupling capacitance c12.
3.3 Leakage current coupling
Leakage current coupling refers to the interference caused by the leakage of high-potential circuit to low-potential circuit through insulation resistance due to poor insulation. The circuit shown in Figure 3 is the equivalent circuit diagram of leakage current coupling. The interference source ne? leaks current to the circuit iz through the leakage resistance mr, and the generated interference voltage
3.4 Common impedance coupling
Common impedance coupling is due to the fact that two or more circuits share an impedance. When current flows through one circuit, interference voltage is generated in the other circuit. There are three types of common impedance coupling.
1) Common impedance coupling of power supply internal resistance. When a power supply is used to power several electronic circuits or sensors, the output current of the high-potential circuit or large current flows through the power supply. Due to the existence of the power supply internal resistance, the voltage drop on the power supply internal resistance is converted into an interference source.
2) Common ground line common impedance coupling. Various signal currents flow through the common ground line of the ultrasonic flowmeter. Due to the impedance of the ground line, interference voltage is formed on the ground line.
3) Common impedance coupling of signal output circuit. When the signal circuit of the ultrasonic flowmeter has several loads, any change in any load will affect other output circuits through the output common impedance coupling.
3.5 Radiated coupling
Radiated coupling refers to the radio device continuously emitting electromagnetic fields. If the instrument is placed in such a transmitting field, it will sense an induced electromotive force proportional to the emitted electromagnetic field and cause interference.
3.6 Conduction coupling
        Conduction coupling refers to the interference caused by the noise picked up by the wire and then transmitted to the ultrasonic flowmeter circuit through the wire. The most common is the power line noise, which induces the alternating magnetic field into the power circuit to form an induced voltage, and then transmits it to various circuits through the power line to cause interference.

IV. Conclusion of the article
In the use of ultrasonic flowmeters, there are many ways for the noise source to couple to the ultrasonic flowmeter, and the boundaries are not obvious. The noise sources are also various, and sometimes the interference appears and disappears. Therefore, the coupling path should be carefully analyzed to eliminate the interference.