The application of intelligent digital display instrument in petrochemical industry is quite complicated. There are magnetic fields, vibrations, etc. around it. These interferences are difficult to change, but it is very important to try to effectively suppress them. Anti-jamming measures in display meter applications
In production, the measured parameters are often converted into weak low-level voltage signals and transmitted to the display instrument over long distances (sometimes up to several hundred meters or even farther) due to the complexity of the display instrument application environment Strong alternating magnetic field, electric field, vibration, thermal noise, strong radiation, temperature effect, power supply, etc., make electrical interference also added to the display instrument input, plus the instrument's internal power transformers, relays, switches, and power lines Sources of interference affect the measurement. When there is a large disturbance (detection signal interference is mainly a strong magnetic field and electric field: when the interference source is low voltage and high current, then the interference source is mainly magnetic field; when the interference source is high voltage and small current, then the interference source is near Mainly electric field), often through the following methods (such as serial mode interference, common mode interference, etc.) superimposed on the signal line, into the instrument. 1. Electromagnetic induction (magnetic coupling).
In the surrounding spaces of high-power transformers, AC motors, and high-current power grids, there is a strong alternating magnetic field, and the control system (detection, transmission, conversion, adjustment, calculation, execution, auxiliary, display, etc.) The closed loop is in this changing magnetic field will be induced potential, so that the connection between the signal source and instrumentation, the wiring inside the instrument through the magnetic coupling in the circuit to form interference. This electromagnetic induction potential is connected in series with the useful signal. When the signal source is far away from the display instrument, the interference is more prominent. In addition, devices such as high-frequency generators and motors with commutators also generate high-frequency interference. 2. Electrostatic induction (refers to the coupling of electricity).
Electrostatic induction is the result of the interaction of two electric fields. In the opposite two wires, if the potential of one of the two wires changes, the potential of the other wire also changes due to the capacitance change between the wires, and the interference source forms a disturbance in the loop with a capacitive coupling. 3. Additional thermoelectric and chemical potentials.
Due to the thermoelectric potential generated by different metals and the chemical potential generated by metal corrosion, DC electrical interference is formed in the circuit loop. 4. vibration.
In a highly vibrating environment, the conductor generates an induced potential due to its movement in a magnetic field. This interference is in series with the signal and enters the instrument in the form of series-mode interference. 5. Different ground potentials introduce interference.
In the vicinity of high-power electrical equipment, when the insulation performance of the equipment is poor, the introduction of the potential difference of different ground potentials interferes. In the use of the instrument, there are often two or more connection points at the input. Therefore, the potential difference of different grounding points will be introduced into the instrument and meter in the form of common mode interference. This interference occurs on both signal lines at the same time. 6. The source is an unbalanced bridge.
When the bridge power supply is grounded, in addition to the unbalanced voltage (ie signal voltage) of the diagonal of the bridge, both signal lines have a common common-mode interference voltage to the ground. Although the common mode interference is not superimposed on the signal and does not directly affect the instrument, it can form a leakage current through the measurement system to the ground and can directly act on the meter (or amplifier) â€‹â€‹through the coupling of the resistor and cause interference. 7. Some pulse-like interference voltages can be applied to the digital circuit to directly interfere with the outside of the analog circuit. These interference voltages are caused by inductive loads such as switches, motors, and relays, and devices that generate electrical discharges.
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