Anti-jamming technology for sensors – we are blessed

1. Shielding skills

Containers are made from metal materials. Wrapping circuits that require maintenance between them can effectively prevent the disturbance of electric or magnetic fields, which is called shielding. Shielding can be divided into electrostatic shielding, electromagnetic shielding and low-frequency magnetic shielding.

2. Electrostatic shielding

According to the principle of electromagnetism, there are no electric field lines inside a closed hollow conductor placed in an electrostatic field, and the internal points of each point are equipotential. With this principle, with copper or aluminum and other conductive metals as materials, make a closed metal container, and connect with the ground wire, the circuit value r of the demand maintenance, so that the external disturbance electric field does not affect its internal circuit, and conversely, the electric field generated by the internal circuit will not affect the external circuit. This method is called electrostatic shielding. For example, in the sensing noise measurement circuit, a conductor with a gap is pierced between the primary side and the secondary side of the power transformer, and it is grounded, which can prevent the electrostatic coupling of the two windings, and this method belongs to electrostatic shielding.

3. Electromagnetic shielding

Regarding the high-frequency disturbing magnetic field, the eddy current principle is used to make the high-frequency disturbing electromagnetic field produce eddy current in the shielded metal, dissipate the energy of the disturbing magnetic field, and the eddy current magnetic field cancels out the high-frequency disturbing magnetic field, so that the maintained circuit is protected from the influence of high-frequency electromagnetic field. This shielding method is called electromagnetic shielding. If the electromagnetic shield is grounded, it has the effect of electrostatic shielding. The output cable of the sensor is generally made of copper mesh shielding, which has both electrostatic shielding and electromagnetic shielding effects. It is necessary to choose low-resistance materials with good conductive function, such as copper, aluminum or silver-plated copper.

Anti-jamming technology for sensors – we are blessed

Anti-jamming technology for sensors – we are blessed

4. Low-frequency magnetic shielding

If the disturbance is a low-frequency magnetic field, the eddy current phenomenon at this time is not very significant, and the anti-disturbance effect is not very good only with the above-mentioned method, so it is necessary to choose the high-permeability material as the shielding layer, so that the low-frequency disturbance magnetic inductance line is confined in the magnetic shielding layer that the magnetic resistance is very small. Protects the circuit to be maintained from low-frequency magnetic field coupling disturbances. This shielding method is generally referred to as low-frequency magnetic shielding. The iron shell of the sensor detection instrument has the effect of low-frequency magnetic shielding. If it is further grounded, it will have the effect of electrostatic shielding and electromagnetic shielding.

According to the above three commonly used shielding skills, so in the other side where the disturbance is more serious, the composite shielding cable can be selected, that is, the outer layer is a low-frequency magnetic shielding layer. The inner layer is an electromagnetic shielding layer. Achieve the effect of double shielding. For example, the parasitic capacitance of capacitive sensors is a key problem that needs to be dealt with when they are measured in practice, otherwise their transmission efficiency and activity will be reduced. It is necessary to electrostatic shield the sensor, and the electrode lead wire is double-shielded technology, which is generally called the drive cable skill. This method can be useful to overcome the parasitic capacitance of the sensor during use.

5. Grounding skills

The grounding skill is one of the useful skills of pressing and disturbing, and it is an important guarantee for the shielding skill. Correct grounding can usefully suppress external disturbances, which can improve the reliability of the test system and reduce the disturbance factors that occur in the system itself. The intent of grounding is twofold: safety and press disturbance. Therefore, grounding is divided into maintenance grounding, shield grounding and signal grounding. Maintenance grounding is intended for safety, and the chassis and chassis of sensor measurement equipment must be grounded. The ground resistance is required to be less than 10Ω. The shield grounding is a low-impedance path between the disturbing voltage and the ground to prevent disturbance of the measurement equipment. The ground resistance should be less than 0.02Ω.

Signal ground is a common wire with zero signal potential for the input and output of an electronic device, which may itself be insulated from the earth. The signal ground wire is divided into analog signal ground wire and digital signal ground wire, the analog signal is generally weaker, so the ground wire requirements are higher: the digital signal is generally stronger, so the ground wire requirements can be lower.

Different sensor detection conditions also have different requirements for grounding methods, it is necessary to choose the appropriate grounding method, and the commonly used grounding methods have a little grounding and more grounding. These two different grounding treatments are given below.

6. A little grounding

In the low-frequency circuit, it is generally advocated to choose a point of grounding, which has a radial grounding wire and a busbar grounding line. Radial grounding is that each functional circuit in the circuit is directly connected with the zero potential reference point with a wire: the bus type grounding is to adopt a high-quality conductor with a certain cross-sectional area as the grounding bus, directly connected to the zero potential point, and the ground of each functional block in the circuit can be connected to the bus nearby. At this time, if multi-point grounding is selected, a plurality of grounding loops will be formed in the circuit, and when the low-frequency signal or pulsed magnetic field passes through these circuits, electromagnetic induction noise will be caused, and due to the different characteristics of each grounding loop, the potential difference will occur at different loop closure points, constituting disturbance. In order to avoid this situation, it is best to choose a little grounding method.

Sensors and measuring devices form a complete detection system, but the two may be far apart. Because the earth current of the industrial site is very complex, so the potential between the grounding point of these two parts of the shell is generally not the same, if the zero potential of the sensor and the measuring equipment is separated from the ground in two places, that is, the two points are grounded, then there will be a larger current flowing through the signal transmission line with very low internal resistance and the voltage drop occurs, causing the series mode disturbance. Therefore, in this case, a little grounding method should also be used.

7. Multi-point grounding

High-frequency circuits generally advocate the use of multi-point grounding. At high frequency, even a small section of ground wire will have a larger impedance voltage drop, plus the effect of distributed capacitance, it is impossible to complete a point of grounding, so a planar grounding method can be selected, that is, multi-point grounding method, use a good conductive plane body (such as choosing a layer in the multi-layer circuit board) to connect to the zero potential reference point, and the ground of each high-frequency circuit is connected to this conductive plane body nearby. Since the high-frequency impedance of the conductive plane is very small, the common potential of each potential is ensured, and the bypass capacitor is added together to reduce the voltage drop. Therefore, in this case, the multi-point grounding method is used.

8. Filtering skills

Filters are one of the useful means to press the AC serial mode to disturb. The common filter circuits in sensor detection circuits include Rc filter, AC power filter, and true current power filter.

Anti-jamming technology for sensors – we are blessed

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