Automatic control interference analysis and suppression measures
key words: paper machine automatic control; Interference analysis; Interference suppression
in automatic control, due to the scattered distribution of control objects and measuring instruments, and the long distance from the central processing unit, as well as the on-site environmental factors, such as high-voltage motors and high-voltage electrical equipment, as well as high-power motors, transformers, switching power supplies, etc., electrical and magnetic field interference and other electromagnetic wave radiation effects are generated. In addition, the start and stop of strong current equipment after cracks are found also produce strong interference to the system. These interference signals are mixed into the measurement signals in different ways and ways, which directly affects the control effect
interference is mainly through two ways: one is conduction, and the interference is transmitted to the control system by input and output signal lines, power lines and ground wires; The second is direct radiation. Electromagnetic field interference is directly radiated into the control system through capacitive coupling or inductive coupling. There are mainly two kinds of influence on the measured effective signal: one is the interference noise superimposed on the measured signal, which is generally disorderly and has a high frequency, which is called serial mode interference; The other is the common voltage interference on the input of a/d converter, which is a DC or AC voltage signal, called common mode interference.
suppression of series mode interference
interference from electromagnetic induction, generally series mode interference. In the sewage treatment project in the middle section of our plant, there is such a problem: after the microwave flowmeter is set, the measurement signal sometimes does not exist, which has not been solved after the debugging of the manufacturer's personnel. Our analysis shows that the measurement site is about 180m away from the instrument cabinet, and the signal cable and power cable are laid in the same trench. Because the transmission signal is a weak microwave signal, the measurement signal is inaccurate due to long-distance electromagnetic interference. Therefore, the signal converter is changed and the force angle, force displacement, angle displacement and other curves are drawn synchronously to the measurement site, and the signal is amplified in advance (converted to 4 ~ 20mA), so that the signal-to-noise ratio changes significantly, which greatly suppresses the interference and effectively solves the problem. Suppression of common mode interference
common mode interference mainly comes from ground wire interference. Because the potential on the actual ground wire is not constant, there is a potential difference at each point on the ground wire, causing common mode interference in the circuit. The two ends of the signal line shield are connected to different grounding points, and a certain voltage difference will be formed between the two grounding points, making the shield itself a source of interference. In addition, different circuits will also form common impedance coupling because they share ground wires.
in the headbox control of workshop 10, due to the hand-made part of the circuit board and the close distance between the boards, the speed of the flushing pump is sometimes out of tune during the control process. Through analysis, it is believed that it is caused by common mode interference at the input of a/d conversion module. So one end of the circuit is floating to the ground, which cuts off the ground loop, thereby eliminating the ground loop current. At the same time, connect a 10 in parallel between the power line and the ground wire of the logic circuit board μ Large capacitance of F and a 0.1 μ F's non inductive ceramic chip decoupling capacitor is used to eliminate the impedance coupling of public Jinan Kesheng Experimental Equipment Co., Ltd. is a modern high-tech company focusing on the research and development, design, production and sales of experimental equipment, which can effectively bypass the high-frequency noise of the logic circuit. After the above processing, the common mode interference is effectively suppressed
suppression of control bus interference
the interference of data bus is very complex and multifaceted, especially for coaxial cable. The shielding effect of shielding layer on electromagnetic wave with lower frequency is worse, so it is easy to be interfered by low-frequency electromagnetic wave. The bus distance is long, and there are some media connector parts, resulting in serious signal attenuation. Therefore, the bus trunk cable and brancher are very sensitive to interference noise, which is easy to cause the increase or decrease of transmission signal, and the front and back edges of pulse sequence form oscillation. The existence of oscillation reduces the threshold difference between high and low levels, and the original waveform is distorted. When the amplitude of oscillation is too large or other interference is cascaded, the pulse level value cannot be correctly distinguished, and the communication bit error rate increases, resulting in the increase of communication time or communication interruption.
during the QCS transformation of our first branch plant, an alarm of fieldbus communication failure occurred during commissioning. Considering that the bus is a coaxial cable, with a total length of more than 300m, and is only separated by a bracket, it is only 10cm away from the power cable. Therefore, the following measures are taken: lead one end of the Fieldbus to the factory ground separately with cu16 grounding wire to eliminate the ground potential difference; The bus and power line shall cross at right angles as far as possible, and all buses shall be laid with 100mm closed bridge; It is also a key to eliminate interference by strengthening cable fixation, straightening small bends, and reinspecting and handling joints.
to suppress bus interference, the following measures should be taken during bus installation: a good and scientific grounding scheme; Special cables with shielding should be used as far as possible for signal lines. Ordinary double glue lines push towards an unsupported center instead of pulling splines, and twist them together. The shorter the pitch, the better the effect of anti magnetic field coupling; Where the signal lines are dense, the signal lines shall be laid in the closed cable tray, which shall be laid separately from the power cables or separated by partitions; The power supply adopts three-phase isolation transformer to suppress high-order harmonics and reduce external electrical interference, or applies ups to rectify, filter, limit amplitude and stabilize voltage. Most automation equipment are imported in complete sets. This paper only discusses some auxiliary measures to suppress interference, and does not discuss the selection of logic components and the reasonable design of circuit board lines