Anti-jamming technology in sensor detection – Turck

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, and this method 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 each point inside it has equal potentials. Using this principle, a metal with outstanding conductivity such as copper or aluminum is used as a material to make a closed metal container, and connect it with the ground wire, so that the circuit value that needs to be maintained is r, so that the external disturbance electric field does not affect its internal circuit, and conversely, the electric field generated by the internal circuit does not affect the external circuit. This method is called electrostatic shielding. For example, in the sensing noise measurement circuit, insert a conductor with a gap between the primary side and the secondary side of the power transformer, and connect it to the ground, which can prevent the electrostatic coupling of the two windings, and this method is attributed to electrostatic shielding.

Anti-jamming technology in sensor detection – Turck

Anti-jamming technology in sensor detection – Turck

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 shielding layer is grounded, it also 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. It is necessary to select low-resistance materials with excellent conductive properties, such as copper, aluminum, or silver-plated copper.

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 by 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 plays the role of low-frequency magnetic shielding. If it is further grounded, it plays the role of electrostatic shielding and electromagnetic shielding together.

Based on the above three commonly used shielding skills, so in the other side where the disturbance is more serious, the composite shielded 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 two layers of shielding. For example, the parasitic capacitance of capacitive sensors is a key problem that needs to be dealt with when measuring in practice, otherwise their transmission power and sensitivity 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. In this way, it is useful to overcome the parasitic capacitance of the sensor during use.

5. Grounding skills

The grounding skill is one of the useful skills for pressing and disturbing, and it is an important guarantee for the shielding skill. Proper grounding can usefully suppress external disturbances, which together 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 input and output of electronic devices, 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 select the appropriate grounding method, and the commonly used grounding methods are one point grounding and multiple points to ground. These two different grounding treatments are given below.

6. A little grounding

In the low-frequency circuit, it is generally recommended to use 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, because the characteristics of each grounding loop are different, and potential differences will occur at different loop closure points, constituting disturbance. To prevent this, it is best to choose a little grounding method.

The sensor and the measuring device 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 series mode disturbance. Therefore, in this case, a little grounding method should also be used.

7. Multi-point grounding

High-frequency circuits are generally recommended to use 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 the plane grounding method can be selected, that is, the multi-point grounding method, the use of an outstanding conductive plane body (such as the selection of a layer in the multilayer circuit board) is connected to the zero potential reference point, and the ground of each high-frequency circuit is connected to the conductive plane body nearby. Because the high-frequency impedance of the conductive plane is very small, the common potential of each potential is basically guaranteed, 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 communicate serial mode disturbances. The common filter circuits in the sensor detection circuit include Rc filter, communication power filter and true current power filter.

Anti-jamming technology in sensor detection – Turck

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