The use environment of the controller is complex, and the electromagnetic interference will have a serious impact on the performance of the product. Therefore, the EMC design of products can meet the requirements of alarm control. This paper systematically analyses the interference that gas alarm controller may encounter in the course of using, and gives anti-interference measures, so as to improve the electromagnetic compatibility of gas alarm controller. The combustible gas alarm controller can real-time detect the concentration of combustible gas in the workplace environment. When the concentration exceeds the alarm value set, the controller sends out alarm signal and provides control output interface. Only by reasonably designing its EMC performance can the alarm protection function of the equipment be guaranteed to play its best performance.
Electromagnetic disturbance causes the reliability of the concentration acquisition function of the controller to decline. The interference source is transmitted to the controller through power supply, signal line or control cable. It may lead to host reset, abnormal display, and even false alarm. There are two ways of disturbance on the power line of the controller: common mode and differential mode. Common-mode interference exists between any relative earth of the power supply or between the middle line and the earth. Common mode interference (CMI) can also radiate to the adjacent space while differential mode does not. Therefore, CMI is more likely to cause electromagnetic interference than differential mode. The main design includes sub-circuit design, circuit board design, shielding structure design, signal/power line filtering and grounding mode design. In the circuit design, filter and winding magnetic ring are added to the power module and signal line transmission interface to enhance the input and output filter circuit. At the same time, grounding capacitance is added to the main chip and memory chip to improve the anti-interference of the transmission signal. The controller adopts distributed intelligent control circuit, and the host computer is composed of display board and circuit board.
Each circuit board has a CPU control. The display board realizes the display function of concentration and detector information. The circuit board realizes the collection of detector concentration and the confirmation function of alarm failure status. The two circuit boards are communicated by CAN to realize the information interaction. The chassis of the controller is wall-mounted, and the AC and DC power conversion module is installed in the chassis.
The power supply is a switching power supply, which generates various noises of its own and forms a strong electromagnetic interference source. So a shield is added outside the power module to avoid its influence on the signal. In the overall structure design, the panel structure avoids the gap or the gap as small as possible, and the PMMA plate is added between the metal plate and the sensitive device as the auxiliary function of electrostatic isolation. Conducted emission current will appear on the power supply line of the controller, and the interference will propagate along the power supply line, which may lead to abnormal operation of the host.
The power line is filtered by magnetic ring, as far as possible away from all kinds of signal cables. In the immunity test of fast transient impulse group, the fast transient signal contains a wide spectrum, which can be easily transmitted into the control circuit in a common mode mode mode. Usually in the small signal circuit of communication port and control port, TVS tube and corresponding grounding protection are used. It also reduces the distributed capacitance of common-mode inductors, enhances the common-mode signal filtering of input circuits (adding common-mode capacitors or inserting loss-type ferrite magnetic rings, etc.) to improve the anti-interference performance of the system. When the electromagnetic radiation on the signal line of the controller exceeds the standard, a filter and a filter winding ferrite magnetic ring are added to reduce the unnecessary high frequency components (mainly common mode) on the signal line, thereby reducing the electromagnetic radiation of the cable, or preventing the cable from receiving space electromagnetic interference as an antenna, and conducting it into the cabinet. The contact effect between the chassis and the ground is enhanced by grinding the bottom of the chassis. The anti-electric transient interference ability of the signal line and the power line is enhanced by increasing the wiring between the circuit board inside the chassis and the wiring between the filter and the power line and the chassis under the power conversion module. The performance of power supply, signal and control ports of the controller is evaluated when they are disturbed by repetitive fast transient pulses, and the immunity to various transient disturbances from operational transient processes (e.g., interruption inductive load, thermostatic element relay contact bounce, etc.
) is verified. According to the test method specified in GB 16838, the electromagnetic interference test is applied to the sample under the conditions shown in Table 1. The test equipment shall meet the requirements of GB 16838. Capacitive coupling clamp is used for AC/DC power supply port test.
The test voltage is applied to the protective ground through the coupling/decoupling network. The inner power module is equipped with a winding magnetic ring, and the outer power terminal is equipped with a 1000P ceramic dielectric capacitance between the fire wire, the zero wire and the ground. For the test of other connecting wires, capacitive coupling clamp and clamped magnetic ring are adopted. Observations and records during the experiment are shown in Table 2. Check the applicability of the controller to the electrostatic discharge caused by the contact of persons and objects with static electricity. During the test, the sample should be kept under normal monitoring. The test equipment shall meet the requirements of GB 16838. According to the test method specified in GB 16838, the electromagnetic interference test of the sample and the coupling plate under the conditions shown in Table 3 is carried out. The state of the sample was observed and recorded during the test. Observations and records during the experiment are shown in Table 4. Starting from the practical work, this paper introduces the EMC design method of combustible gas alarm controller in detail. This method has universal significance.
In the design, the main consideration is the poor ability of the controller itself to resist external interference.
The anti-interference ability of the controller is strengthened in circuit design, circuit board design, shielding structure, signal/power line filtering and grounding mode design.