Anti-interference connection application solution for EDP cable in industrial control equipment
At the moment when the process of industrial automation is accelerating, the stability and reliability of industrial control equipment as the core hub of the production process are crucial. As the key interface for operators to interact with equipment, the display system needs to accurately and stably present various data and image information. EDP (Embedded DisplayPort) line has been widely used in the display connection of industrial control equipment due to its advantages such as high bandwidth and low power consumption. However, the complex electromagnetic environment in industrial scenes poses a severe challenge to the transmission of EDP Cable signals, so it is inevitable to build an efficient anti-interference connection application scheme.
Analysis of industrial electromagnetic interference sources
Interference from high-power equipment
In industrial production workshops, there are many high-power motors, welding machines, transformers and other high-power equipment. Taking high-power motors as an example, during their startup and operation, the current will change dramatically, generating a strong alternating magnetic field. When the EDP Cable is close to such a motor, the alternating magnetic field will induce an electromotive force in the conductor of the EDP Cable, thereby forming an interference current, which is superimposed on the originally transmitted display signal, causing the signal to fluctuate and distort. In severe cases, it may cause the display screen to flicker, the screen will be distorted, or even black. When the welding machine is working, the pulse current generated by instantaneous discharge will cause extremely wide-band electromagnetic radiation, with an interference range of up to several meters, which has a great impact on the surrounding
EDP Cable signal transmission.
High-frequency equipment interference
With the development of industrial technology, various high-frequency devices, such as radio frequency identification (RFID) readers and writers, wireless communication base stations, etc. are widely used in industrial scenarios. When the RFID reader is working, it will emit radio frequency signals of specific frequencies, usually in the frequency band of 13.56MHz, 915MHz, etc. These high-frequency signals are radiated through space and are very easy to couple with the EDP Cable. When the operating frequency of the EDP Cable is close to the frequency of the interference signal, resonance will occur, which will greatly enhance the intensity of the interference signal, seriously damage the integrity of the display signal, affect the normal display of the display interface of the industrial control equipment, and cause the operator to be unable to accurately obtain the equipment operation status information.
Poor grounding interference
Industrial control equipment is usually in a complex grounding network. If the grounding system is not designed reasonably or there is poor grounding, there will be a potential difference between the various parts of the equipment. When the display unit connected by the EDP Cable is inconsistent with the ground potential of the control motherboard, a ground loop current will be generated in the EDP Cable. This current will interfere with signal transmission, causing clutter and noise in the display signal, and causing problems such as stripes and blur on the display screen. For example, in some old industrial plants, due to the aging and corrosion of the grounding line, the grounding resistance increases, and the ground loop interference problem is particularly prominent, seriously affecting the stable operation of industrial control equipment.
Principle of EDP Cable anti-interference technology
Principle of differential signal transmission
The EDP Cable adopts low-voltage differential signal (LVDS) transmission technology to transmit a pair of signals through two differential lines, and the voltage difference between the signals is used to represent the data. Under ideal conditions, the two lines of the differential signal are subject to the same external interference, and the interference signal will appear on the two lines in the form of common mode. The differential amplifier at the receiving end has a strong suppression ability for common-mode signals, which can effectively remove common-mode interference and only amplify differential signals, thereby ensuring accurate signal transmission. For example, when external electromagnetic interference induces interference voltages of equal magnitude and direction in the differential line pair, the differential amplifier will offset this part of the common-mode interference voltage and only amplify the differential voltage representing the data to ensure that the display signal transmitted by the EDP Cable is not affected by interference, providing a stable and clear display screen for industrial control equipment.
Shielding layer anti-interference principle
EDP Cables usually have a metal shielding layer, which is generally composed of copper foil or woven copper mesh. The main function of the shielding layer is to limit the external interference electromagnetic field outside the shielding layer through the principle of electromagnetic induction, preventing it from entering the EDP Cable to interfere with the signal transmission. When the external interference electromagnetic field acts on the shielding layer, an induced current will be generated in the shielding layer, and the induced current will generate a magnetic field in the opposite direction of the external interference electromagnetic field. The two offset each other, thereby achieving shielding protection for the internal signal. For example, in an industrial workshop, when there is a strong electromagnetic interference source around the EDP Cable, the shielding layer can effectively block the interference electromagnetic field, so that the display signal transmitted inside the EDP Cable is not interfered with, and the normal operation of the display system of the industrial control equipment is guaranteed.
Anti-interference connection application design
Key points for cable selection
In industrial control equipment, EDP cables with double-layer shielding structure should be preferred. The inner shielding layer uses copper foil, which tightly wraps the wire core and can effectively shield low-frequency interference; the outer layer uses braided copper mesh to enhance the shielding effect of high-frequency interference and improve the mechanical strength of the cable. The wire core material of the cable should be made of high-purity oxygen-free copper to reduce resistance and reduce attenuation and distortion during signal transmission. For example, for application scenarios with long transmission distances (more than 3 meters), EDP cables with larger core cross-sectional areas (such as more than 0.5mm²) can be selected to ensure that the signal can still maintain sufficient strength during long-distance transmission and reduce the decrease in anti-interference ability caused by attenuation. The outer sheath of the cable should have good wear resistance, corrosion resistance and flexibility to adapt to complex working conditions in industrial environments, such as frequent bending, oil erosion, etc.
PCB layout optimization
In the circuit board design of industrial control equipment, the interface circuit of the EDP Cable should be placed as close as possible to the interface between the display chip and the display screen, shortening the routing length of the EDP Cable on the PCB board and reducing the chance of interference coupling in the signal transmission path. The differential pair routing of the EDP Cable should be kept short and straight, avoiding 90° right-angle turns, and using 45° angles or arc transitions to reduce signal reflections. The differential pairs should be kept evenly spaced, and the differential impedance should be strictly controlled within the range of 100Ω±10%. The relevant parameters should be accurately determined through PCB stacking design and line width and line spacing calculation tools (such as Polar SI9000). For example, the high-speed differential routing layer of the EDP Cable is set between two ground plane layers to form a "GND - Signal - GND" sandwich structure, using the ground plane to provide a good return path for the signal, reduce electromagnetic radiation, and enhance anti-interference capabilities. At the same time, avoid arranging other high-speed signal lines, clock lines, and other interference-prone lines around the EDP Cable routing to prevent signal crosstalk.
Shielding and grounding measures
At the connection point between the EDP Cable and the equipment, ensure that the shielding layer is reliably grounded. For the case of using a connector for connection, the metal shell of the connector should be in good contact with the metal chassis of the equipment, and the metal shell should be connected to the chassis ground plane through multiple grounding vias, and the grounding resistance should be less than 0.1Ω. During the wiring process of the EDP Cable, the shielding layer is connected to the ground plane of the PCB through vias at a certain distance (such as 10-15cm) to form multi-point grounding and reduce the induced voltage on the shielding layer. For EDP Cables that pass through different electromagnetic environment areas, metal wire troughs or metal hoses can be used for secondary shielding protection. The metal wire trough or hose should be reliably grounded and electrically connected to the shielding layer of the EDP Cable to further enhance the anti-interference ability. For example, in an industrial automation production line, passing the EDP Cable through a metal wire trough and grounding both ends of the wire trough to the equipment chassis can effectively block strong electromagnetic interference in the surrounding area, ensure that the EDP Cable stably transmits display signals, and provide operators with accurate equipment operation display images.
Filter circuit design
On the signal transmission path of the EDP Cable, filter circuits are added near the signal source and the receiving end. At the transmitting end, a π-type filter circuit composed of capacitors and inductors can be used. The capacitors are ceramic capacitors with good high-frequency characteristics (such as 0.1μF, 0.01μF), and the inductors are common-mode inductors to filter out high-frequency clutter and common-mode interference in the signal. At the receiving end, a filter circuit is also set to perform secondary filtering on signals that may be mixed with interference after long-distance transmission. For example, by integrating a dedicated signal filter chip in the EDP Cable interface circuit, the signal is accurately filtered using the filtering algorithm and circuit structure inside the chip to further improve the anti-interference performance. At the same time, the parameters of the filter circuit are reasonably set, and the cutoff frequency, passband gain and other parameters of the filter circuit are optimized according to the frequency range of the EDP Cable transmission signal and the spectrum characteristics of the industrial field interference signal, to ensure that the filter circuit can effectively filter out the interference signal without causing attenuation or distortion to the normal display signal.
Solution implementation and test verification
Implementation process
During the manufacturing process of industrial control equipment, the installation and wiring of EDP cables are strictly carried out in accordance with the anti-interference connection application solution. First, the purchased EDP cables are quality inspected to check whether the shielding layer is complete, whether the wire core conductivity is good, and whether the cable sheath is damaged. During the equipment assembly stage, according to the PCB layout design, the EDP cable is accurately connected to the display chip, display interface and related circuit modules to ensure that the connection is firm and the shielding layer is reliably grounded. During the EDP cable wiring process, follow the wiring rules to avoid cable crossing and entanglement, and provide secondary shielding protection for cables passing through different areas. After assembly, the equipment is preliminarily powered on to check whether the display system can work normally and observe whether the display screen is abnormal.
Test method
Use professional electromagnetic compatibility (EMC) test equipment to conduct comprehensive tests on industrial control equipment equipped with anti-interference connection solutions. In the conducted emission test, a linear impedance stabilization network (LISN) is used to measure the strength of the interference signal conducted by the EDP cable to the power port when it is working to ensure that it meets relevant industrial standards (such as the limits specified in CISPR 11). In the radiation emission test, the device is placed in an anechoic chamber, and a spectrum analyzer and a receiving antenna are used to measure the radiation interference intensity generated by the EDP Cable to evaluate its electromagnetic impact on the surrounding environment. Through the electrostatic discharge immunity test (ESD), the impact of human electrostatic discharge on the device is simulated to test the reliability of the EDP Cable anti-interference solution under electrostatic shock, ensuring that the display system can still work normally and the display screen is normal when the device is subjected to ±8kV contact discharge and ±15kV air discharge.
Performance evaluation
Through comparative tests, the performance improvement effect of the anti-interference connection application solution is evaluated. In the same industrial electromagnetic interference environment, the display systems of industrial control equipment that do not use the anti-interference solution and those that use the solution are tested respectively. Compare the clarity and stability of the display screen, and record the number and duration of abnormal phenomena such as screen distortion, flickering, and stripes. For example, in tests in an industrial workshop environment with densely populated high-voltage equipment, devices that did not adopt an anti-interference solution experienced screen distortion five times per hour on average, with each duration being about five seconds. However, after eight hours of testing, devices that adopted an anti-interference connection solution did not experience any screen distortion, flickering, or other anomalies, and the display remained clear and stable at all times, effectively ensuring that operators could accurately monitor the operating status of the equipment and improving the reliability and stability of industrial control equipment in complex electromagnetic environments.
Author: AMY_Li
