EMC design and protection of industrial control chassis

When we use electronic devices, electromagnetic interference will be generated. EMC (Electromagnetic compatibility) phenomenon can be seen everywhere in our lives. For example, when we are watching TV, if the mobile phone is placed next to the TV, the mobile phone signal will interfere with the TV signal and form snow flakes when a call comes in. With the widespread use of daily electronic devices, the EMC design of electronic devices is becoming increasingly important.

Most developed countries in the world have established EMC standards. Electronic products sold to these countries or regions must comply with the corresponding EMC standards and pass their certification. Among them, the more well-known certifications include the FCC certification of the United States, the VCCI certification of Japan, the CE certification of the European Union, and the 3C certification of our mainland China.

Next we will explain how to carry out EMC design for an excellent industrial computer.

Whether it is an industrial computer or a commercial computer, the first thing to consider in EMC design is the EMI (electromagnetic interference) design of the board itself. Therefore, electromagnetic radiation is greatly reduced in board design. However, due to the increase in processor frequency, relying solely on the EMI design of the board itself cannot fully meet the requirements of EMC certification. Therefore, the EMC design of the industrial computer chassis has become more important. This article also focuses on the EMC design of the chassis.

Before explaining the EMC design of industrial computer chassis, let's first look at the EMC design of commercial PC chassis. Since commercial PCs are used in home environments and are in contact with the human body for a longer time, the requirements for EMC design are also very strict.

Although most famous PC brands have passed electromagnetic compatibility certification in various countries due to their global marketing, the EMC certification of commercial PCs and industrial computers have different focuses due to their different uses. The EMC certification of commercial PCs is more inclined to consider the radiation amount of the PC itself to the human body, so the materials and chassis design are based on this consideration. The application environment of industrial computers is more complex, and the scope of EMC certification is also wider. In addition to considering the electromagnetic radiation interference of industrial computers, preventing various external factors from interfering with the internal equipment of industrial computers is also an EMC design point.

In short, the EMC design of commercial PCs is internal, while the EMC design of industrial computers is bidirectional.

Below we will explain the difference between internal and two-way communication by comparing it with commercial PCs:

1. EMC design of heat dissipation port

First of all, let's make a clear concept. Whether it is for internal or external use, the best way to deal with electromagnetic radiation is to use a fully sealed metal box to shield the electromagnetic radiator, and the thicker the metal layer used for shielding, the better. In this way, the electromagnetic shielding effect is the best, both for internal and external use. However, due to factors such as heat dissipation and external expansion devices, neither commercial PCs nor industrial computers can be strictly sealed. An important point about the chassis design to prevent electromagnetic leakage is the good balance between heat dissipation and electromagnetic leakage. It is necessary to fully dissipate heat and take into account the design of electromagnetic compatibility. What we usually see on commercial PCs is the heat dissipation aperture above, which is round and porous.

But in fact, the design standards for preventing electromagnetic leakage have many restrictions on the size and shape of the heat dissipation apertures. Research experiments have shown that square apertures are more effective in preventing electromagnetic leakage than circular apertures.

2. EMC design at chassis joints

According to the theory, when the distance between two contact points on the chassis is greater than 0.25xλ (wavelength symbol), harmful electromagnetic radiation will easily leak out between the two points. Therefore, the design of the chassis joints is also an important indicator for examining the EMC design foundation.
The thickness of the chassis steel plate of commercial PCs is only 0.6mm, and the strength of the steel plate is limited. It is difficult to ensure that the chassis itself will not deform after being stressed. In addition, since most commercial PCs are now vertical chassis, it is difficult to ensure that the side opening panels are tightly connected at the joints. In order to overcome these phenomena, some high-end commercial PCs have EMI shrapnel designs at the chassis joints.

However, the use environment of industrial computers is complex and changeable. Some environments are highly vibrating. The EMI shrapnel design is prone to lose the elasticity of the shrapnel in a vibrating environment. The chassis material used in industrial computers is 1.2mm high-quality steel plate, which has excellent anti-deformation properties. Since the chassis of industrial computers are mostly horizontal, the weight of the top steel plate itself and the characteristics of the steel material can ensure close contact between the chassis walls. Excellent industrial computer chassis can also pass CE, FCC and other EMC certifications without using EMI shrapnel. If there are some environments with extremely strict electromagnetic compatibility, such as the military industry, we will recommend customers to use more reliable metal conductive cloth at the chassis seams.

The joint between the board and the chassis is also a place that is extremely prone to electromagnetic leakage. Since commercial PCs have fewer expansion cards, few brands use EMI shrapnel design in this place. There are multiple boards inserted inside the industrial computer, and whether the board and the chassis are in close contact is an important indicator of whether the electromagnetic compatibility is excellent. Therefore, well-made industrial computers will use EMI shrapnel to ensure that the joint between the board and the chassis is tight and reliable.

3. Special design of chassis surface

Commercial PC chassis are mostly made of galvanized steel plates, and considering the effectiveness of absorbing radiation inside the chassis, the inside of the chassis is not painted. Therefore, the galvanized steel plates on the inner wall of the chassis are conductive and easier to absorb electromagnetic radiation, which also gives commercial PCs an inherent advantage in electrostatic protection and electromagnetic shielding. This type of commercial PC chassis is fine for ordinary use, but it is very inappropriate for industrial control environments.

First, industrial computers are sometimes used in highly corrosive environments, such as sea salt, automobile exhaust on highways, and gas from chemical plants. Therefore, the inner and outer surfaces of the chassis must be treated with anti-corrosion. The following figure shows the chassis of Advantech's industrial computer after the outer surface and inner wall of the chassis have been treated with anti-corrosion paint.

Secondly, sometimes computers work in a vibrating environment. In some special cases, the vibration can cause certain electrical components in the chassis to come into contact with the inner surface of the chassis. Since the inner wall of the chassis is conductive, it can easily cause a short circuit and burn the equipment.

The internal painting of industrial computers makes the inner wall of the chassis non-conductive, which can prevent such phenomena from happening. However, as the saying goes, success or failure depends on Xiao He, it is precisely because of these non-conductive inner walls of the chassis that the EMC design of industrial computers is more challenging. Since the inner wall of the industrial computer is also painted, the surface of the chassis is non-conductive and cannot form an equipotential body like a commercial PC. Therefore, it is necessary to leave unpainted contact surfaces at some positions to form a conductor and form an equipotential body on the entire chassis surface. The processing process is relatively complicated.