With the development of technology and the deep implementation of national intelligent manufacturing strategy, displacement sensor measurement and control system is widely used in the field of industrial automation. Displacement sensor used in measurement and control system is mainly used in displacement measurement, position positioning, size measurement and other occasions. The performance of the sensor can directly affect the control accuracy and use effect of mechanical equipment. Because of the strong electric circuit, electric field and magnetic field of mechanical equipment, the measurement accuracy and reliability of displacement measurement and control system are often greatly affected. How to effectively weaken or eliminate the influence of interference factors is the key point to ensure the accuracy and performance of mechanical equipment.
There are large interference sources in the structure of industrial equipment with large variation of voltage and current. According to different interference modes, it can be divided into common mode interference and differential mode interference. Common mode interference (CMI) is the potential difference between signal and ground, which is mainly caused by the series connection of power grid, ground potential difference and common state (same direction) voltage induced by space electromagnetic radiation on signal line. The common mode voltage can be converted into differential mode voltage through asymmetric circuit, which directly affects the measurement and control signal and causes damage to components. Differential mode interference refers to the interference voltage between signal poles, which is mainly formed by coupling induction of space electromagnetic field between signals and conversion of common mode interference by unbalanced circuit. This kind of interference is directly superimposed on the signal and directly affects the measurement and control accuracy.
What are the main interference sources of analog signal in displacement sensor measurement and control system?
1. Radiation interference from space
Space radiation interference is mainly produced by power network, transient process of electrical equipment, lightning, radio broadcasting, television, radar, high frequency induction heating equipment, etc. The interference intensity is related to the field equipment layout and the electromagnetic field generated by the equipment, especially the frequency, which is generally protected by shielded cables or high-voltage relief elements.
2. Interference of system external leads
It is mainly introduced through power supply and signal line, which is usually called conducted interference. There are both common mode interference and differential mode interference. There are many methods to suppress conducted interference, and the most effective one is to use passive filter.
3. Power interference
It is common that the displacement measurement and control system fault is caused by power supply interference in industrial field. In general, the power supply of PLC system is provided by power grid. Due to the wide coverage of power grid, it will be subject to all space electromagnetic interference and the change of internal voltage of power grid. PLC system usually uses isolated power supply, but its isolation performance is not ideal due to the influence of structure design and manufacturing process.
4. Interference of grounding system in disorder
Grounding is a common and effective means to improve the electromagnetic compatibility of electronic equipment. Correct grounding can not only restrain the influence of electromagnetic interference, but also restrain the equipment from sending out interference; while wrong grounding will lead to serious interference signal.
5. Interference from PLC system
The mutual electromagnetic radiation between components and circuits in the system will cause interference, so the PLC system should give priority to the products with good field application performance.
How to solve the signal interference of PLC system effectively?
The ideal state is to select equipment with better isolation performance, select high-quality power supply, power line and signal line, and reasonable wiring and grounding. However, this requires the cooperation of different equipment manufacturers, and the cost is high. More on-site will consider using various methods to do a good job in signal isolation and filtering, to reduce the impact of interference signal on the system. The common treatment methods are as follows:
Add signal isolator;
Add magnetic ring and capacitance;
PLC power supply and isolation transformer;
Switch value, analog signal and digital signal go separately;
The current signal transmission is adopted to improve the anti-interference ability of the system;
PLC input and output signal line needs to use shielding wire, which is suspended at the input and output side and grounded at the PLC side;
The signal line should be far away from strong interference sources, such as frequency converter, high-power silicon rectifier and large power equipment;
The shielding layer of twisted pair shielded wire should be grounded at both ends, but if there is potential difference between the two ends of the cable, the connection current will be generated in the shielding layer. At this time, one end of the cable shielding layer should be grounded.
