Instrument measurement and control system anti-jamming technology

The reliability of instruments and control systems directly affects the safe and stable operation of modern industrial production devices. The system's anti-jamming capability is the key to the reliable operation of the entire system. In the measurement process, if these disturbances are not handled well, it will falsify the measurement results and even make the instrument or computer completely unable to work in severe cases. A lot of practice shows that the anti-jamming performance is a very important issue for various electronic measuring devices. In particular, the widespread application and rapid development of DCS and field bus technologies have effectively eliminated and inhibited various types of interference, and it has become necessary to discuss and solve them. Urgent problems, because the interference can not only cause logic confusion, make the system measurement and control failure, resulting in lower product quality, and even damage the production equipment and cause accidents. Therefore, anti-jamming technology must pay sufficient attention to the design, manufacture, installation, and daily maintenance of the instrument measurement and control system.

Common sources of interference and interference with the system

Since the measurement and control signals are often a kind of weak dc or slowly changing alternating signals, and finally they also need to be transmitted over long distances (sometimes up to hundreds of meters or more), like the magnetic fields generated by high-power motors and other electrical equipment, The presence and changes of electric fields generated by high-voltage electrical equipment, as well as various types of electromagnetic radiation, etc., will be mixed into the measurement and control system in different ways and in different ways. In general, the interference sources of the interference control instrument monitoring and control system mainly include the transient process of the power network and electrical equipment, the radiation interference caused by lightning and other space, and the system external lead interference caused by the system power line, signal lead, and grounding. These interferences are generally divided into two major categories: external interference and internal interference. The detailed analysis is nothing more than radiation, temperature, humidity, vibration, transmission, induction, power supply, and grounding. The following analysis of several common interference mechanisms:

Radiated interference from the space affects the measurement and control system mainly through two paths: one is direct internal radiation to the computer, and the other is interference induced by the circuit; the second is radiation to the computer peripheral equipment and communication network, which is induced by the induction of peripheral equipment and communication lines. interference.

There are two main sources of interference from the transmission: one is through the power supply of the sensor power supply or public signal instrument, that is, the power distribution system is connected to the power distribution network; the other is that the signal line is interfered by space electromagnetic radiation induction, which can cause component damage when severe. , logic errors and large system failures.

The interference from the grounding system is mainly caused by the confusion of the grounding system: the shielded grounding wire of the measurement and control system and the noise coupling interference caused by the chassis grounding wire, signal grounding wire, power ground wire, and AC power ground wire.

From the above, we can conclude that various interference sources (noise sources) produce interference currents (voltages) to the measuring device and the detection system, which need to have three elements at the same time: (1) noise source; (2) noise-sensitive receiving circuit; (3) The source of noise to the circuit between the receiving circuits.

Universal anti-interference technology

Since the formation of noise interference to measurement devices and detection systems requires "three elements", the method of eliminating and reducing noise interference should also take measures against three factors, namely: (1) eliminate or suppress noise sources; (2) intercept interference transmission pathways; (3) The sensitivity of the receiving circuit to noise interference is impaired. The above three measures are all hardware measures. With the application of microcomputers in industrial production, smart sensors and smart meters, more measures and methods for suppressing interference such as digital filtering and digital processing have been applied in software, and the safety of instrument measurement and control systems has been greatly improved. The above measures are usually adopted for isolation, shielding, suppression, grounding protection, and software technology. These techniques are described below.

Isolation includes two kinds of meanings: One is reliable insulation, that is, to ensure that no leakage current occurs between the wires. Therefore, it is required that the withstand voltage level and insulation resistance of the wire insulation material must meet the requirements; the other is reasonable wiring, that is, the signal line is required to be as far as possible. Avoid interference sources, such as when the power line and signal line are laid in parallel, the two must maintain a certain distance, the two should be as vertical as possible, the power line and the signal line should be in different conduits when the conduit is laid. The signal lines with different signal amplitudes should not be put in the same conduit. When using metal wire trunking, different radiator wires, cables and power lines need to be separated by metal partitions. The same multi-core cable should not have different signal lines with different amplitudes.

Shielding and suppression is the use of metal conductors to shield the shielded components, assemblies, circuits and signal lines, mainly used to suppress current noise coupling, play a certain role in magnetic shielding. In addition, using twisted pair instead of two parallel lines is an effective method for suppressing magnetic field interference.

Ground protection refers to protecting equipment and personal safety and suppressing interference through grounding. Generally divided into shielded grounding, intrinsically safe grounding, protective grounding, signal loop grounding, the following are introduced separately: (1) protective grounding is the electrical equipment, power meter normal uncharged metal parts and grounded body to make a good metal connection If the instrument panel is accidentally energized, the grounding short-circuit current is mostly through the grounding resistor. (2) The working grounding is to ensure that the instrument works accurately and reliably. It includes signal loop grounding, shield grounding, and intrinsically safe instrument grounding.

Software anti-jamming technology: Industrial complex environment hardware anti-jamming measures are powerless, such as the IPC crash or control error. This will have terrible consequences for production, so it is important to use software anti-jamming measures to avoid and mitigate these accidents. Commonly used software anti-jamming technologies include: real-time control of self-monitoring during software operation, mutual monitoring of real-time control systems, and backup of important data.

Practical application of anti-jamming technology

1. Implement technological transformation, relieve system interference, restore interlocking design of key units 33 tons/hour granulator is the first large-scale extrusion unit in China, designed 294 temperature, pressure, flow, vibration and other interlocking, alarm control In the event of an abnormality, the interlocking control program ensures that the crew safely stops. However, due to lack of experience of designers, poor consideration was taken. After the unit was started, a number of temperature interlocking control malfunctions caused the extruder to stop, causing great pressure on the production of the device. After the technicians dealt with, some of the malfunction points were removed, but the temperature interlock point was frequently misoperation, so the interlock was temporarily cancelled and the alarm was retained. Although the crew can drive, the temporary removal of the 31 interlocking points has brought great pressure and potential safety hazards to the production, and frequent temperature mis-alarms have caused great pressure on the operators. Therefore, as soon as possible to find a solution, restore the interlock placed in front of business leaders and technical personnel. Analyze the design, construction and operation of the control system by organizing technicians. Found that poor design, high temperature control panel, control cable shielding is not good, the secondary instrument is a plastic shell and 220VAC power supply, so that the original signal is weak, a detection element thermocouple measurement circuit MV signal interference is serious, causing the secondary instrument Frequent misuse. After the problem is clearly analyzed, we can solve it by improving the system.

(1) Change the detection element from thermocouple to thermal resistance to enhance signal anti-interference ability.

(2) Change the control cable to shielded cable with less signal interference.

(3) The secondary control indicator is changed from 220V AC power supply to 24V DC power supply to reduce the control panel temperature and signal interference.

(4) The control panel increases the exhaust fan and lowers the temperature.

(5) Organize construction strictly to ensure quality.

Through the above work, the unit's interlock control can be put into use after one year of outage, which plays a key role in the stable operation of the large-scale unit of the company.

2. Adopting isolation and shielding anti-jamming technology to ensure the stable operation of DCS system and eliminating production hazards. The annual output of 200,000 tons of polypropylene device is the first in China to introduce the world's advanced gas-phase mass production process. The advanced control loop in the instrument measurement and control system accounts for A large proportion. However, due to factors such as design and construction, when the device has just been driven, frequent malfunctions of the instrument stop the car, causing adverse effects on the production. The DCS card is also damaged from time to time. After analysis, the system electrical to the instrument control cable shielding And the isolation measures are poor, causing the DC card's DI card to sense 170~200V voltage from time to time, causing DCS logic malfunction and even causing the device to stop. In response to this, more than 30 sets of signal lines that seriously affect production have been added with isolated relay panels and replacement of shielded control cables has achieved significant results.

3. Reduce remote transmission control and eliminate signal interference.

The design of the key compressor control system of a device is the control panel of the site and the control panel of the central control room. The control panel of the central control room is mainly used for the safety interlock logic of the compressor control. In addition to the field control panel, some manual control functions are also designed for on-site startup. Among them, the anti-surge control valve of the compressor can be switched between manual and automatic control mode at the scene. This part controls the connection between the main control room and the site through remote communication transmission. Everything is normal during the debugging phase. However, after the device was driven, due to serious signal interference, the anti-surge control valve of the compressor was cut into the manual mode from time to time, and no one operated the scene, resulting in multiple interlocked parking. After the communication signal lines were updated, the situation improved slightly, but signal interference occurred occasionally, resulting in interlocked parking. After many analysis by the technical personnel, it is believed that due to various reasons, the main device is not standardized enough for cable bridge control cables to be laid, and there is a large amount of interference, which causes the device communication signals to not work well. In view of the actual situation, the disturbance of the compressor control signal mainly affects the anti-surge control valve switching mode. In normal driving, we choose to automatically control the opening of the anti-surge control valve, and when it is abnormal, manually control the anti-surge control valve in the field. Opening degree. After careful deliberation, moving the on-site communication control part to the master control panel is safer and more reliable. After the implementation, the effect is very good. No signal interference has caused the compressor to stop. In the design of compressor control for new installations, we recommend that foreign companies adopt this program and achieve good results. With the joint efforts of all, we have achieved a good performance of the successful start-up of the device.