How sensor detection avoids interference – ifm

Sensor detection is known to prevent disturbances. Once disturbed, the sensor may be damaged, the measurement system will be affected, the inaccurate data will affect the future use, the judgment will be biased, and the impact can be large or small, so anti-interference is very important. Today, we're going to review the sensor's anti-disturbance skills. Let's see if we can use these skills to resist disturbances.

Blocking skills.

The container is made of metal material. This method is called shielding, and wrapping around the circuit that needs to be maintained can usefully prevent the disturbance of electric or magnetic fields. Shielding can be divided into electrostatic shielding, electromagnetic shielding, and low-frequency magnetic shielding.

Electrostatic shielding.

According to the electromagnetic principle, there are no electric field lines in the closed hollow conductor in the electrostatic field, and the internal points are equipotential. The enclosed metal container is made of metal with good conductivity such as copper or aluminum, and is connected with the ground wire, so that the external disturbance electric field does not affect the internal circuit, and the electric field generated by the internal circuit does not affect the external circuit. This method is known as electrostatic shielding. For example, in a sensing arrogance measurement circuit, void conductors are inserted into the primary and secondary sides of a power transformer and grounded to prevent electrostatic coupling between the two windings. This method is attributed to electrostatic shielding.

Electromagnetic shielding.

With regard to high-frequency disturbance of magnetic fields, the eddy current principle is used to generate eddy currents in shielded metals, which consume the energy of disturbing the magnetic field. The eddy current magnetic field cancels out the high-frequency turbulent magnetic field and maintains the circuit from the high-frequency electromagnetic field. This shielding method is called electromagnetic shielding. If the electromagnetic shielding layer is grounded, it has the function of electrostatic shielding. The output cable of the sensor is generally shielded with copper mesh, which has the functions of electrostatic shielding and electromagnetic shielding. The shielding material must be a conductive low-resistance material, such as copper, aluminum, or silver-plated copper.

How sensor detection avoids interference – ifm

How sensor detection avoids interference – ifm

Low-frequency magnetic shielding.

If the disturbance is a low-frequency magnetic field, the eddy current phenomenon is not very obvious, only the anti-disturbance effect of the above method is not very good, so it is necessary to use a high permeability material as a shielding layer to confine the low-frequency disturbance magnetic induction line in the magnetic shielding layer with small magnetic resistance. The maintenance circuit is not affected by the coupling disturbance of low-frequency magnetic fields. This shielding method is often referred to as low-frequency magnetic shielding. The iron shell of the sensor detection instrument plays the role of low-frequency magnetic shielding. If further grounded, it will exert the effect of electrostatic and electromagnetic shielding.

Composite shielded cable, that is, according to the above three commonly used shielding skills, the outer layer is a low-frequency magnetic shielding layer. The inner layer is an electromagnetic shielding layer. Complete the double shielding. For example, the parasitic capacitance of a capacitive sensor is a key problem that needs to be solved in practical measurement, otherwise its transmission power and sensitivity will be reduced. It is necessary for the sensor to be electrostatically shielded, and the electrode lead wire is double-shielded, which is generally called the drive cable skill. This method can be useful to overcome the parasitic capacitance of the sensor during use.

Grounding skills.

Grounding skills are one of the useful skills for pressing and disturbing, and it is also an important guarantee for shielding skills. Correct grounding can usefully suppress external disturbances, improve the reliability of the test system, and reduce the disturbance factors of the system itself. Grounding has two intentions: safety and push-to-disturb. Therefore, grounding can be divided into maintenance grounding, shield grounding and signal grounding. In order to start the gauge safely, the housing and chassis of the sensor measuring device should be grounded. The ground resistance should be less than 10W. Shielding grounding is a low-impedance path for the disturbance voltage to the ground to prevent disturbance of the measurement equipment. The ground resistance should be less than 0.02W.

Signal grounding is a common line with zero signal potential for the input and output of electronic devices, which can be insulated from the earth. Signal ground is divided into analog signal ground and digital signal ground. The analog signal is generally weaker and has higher requirements for the ground wire: the digital signal is generally stronger, and the requirements for the ground wire are 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 method has a little grounding and multi-point grounding. Here are two different grounding treatments.

It is generally advocated to use - point grounding wires in low-frequency circuits, including radioactive grounding wires and busbar grounding wires. Radioactive grounding refers to the direct connection of each functional circuit in the circuit to the zero potential reference point: the wire grounding is directly connected to the zero potential point with a high-quality conductor with a certain cross-sectional area as the grounding bus, and the grounding of each functional block in the circuit can be close to the busbar. In this case, if multi-point grounding is selected, multiple grounding circuits will be formed in the circuit. When a low-frequency signal or a pulsed magnetic field signal or a pulsed magnetic field, it causes electromagnetic induced noise. Due to the different characteristics of each ground circuit, the potential difference between the closing points of different circuits can be disturbed. To prevent this, it's best to use a little grounding.

Sensors and measuring devices form a complete detection system, but the problems between the two can be very far apart. The above-ground currents in industrial sites are very complex, and the potentials between the grounding points of the enclosure are very different. If the zero potential of the sensor and the measuring equipment are grounded separately, that is, two points are grounded, the voltage drop occurs in the transmission line of the low-internal resistance signal, resulting in series mode disturbance. Therefore, in this case, a point grounding method should also be used.

Multi-point grounding.

It is generally advocated to have multi-point grounding high-frequency circuits. At high frequencies, even a small section of grounding wire will have a large impedance voltage drop, coupled with the effect of capacitance dispersion, it is impossible to complete a point of grounding. As a result, outstanding conductive planes (e.g., -layer in multilayer circuit boards) can be connected to the zero-potential reference point via a planar ground, with the ground of each high-frequency circuit being close to the conductive plane. Due to the small high-frequency impedance of the conductive plane, the consistency of each potential is fundamentally ensured, and the bypass capacitance is increased to reduce the voltage drop. Therefore, in this case, multi-point grounding is used.

How sensor detection avoids interference – ifm

Sections viewed

Contact

working hours