Banner Sensors - Analysis of the Eight Anti-Interference Technologies in Sensor Detection

1. Shielding technology

Containers are made of metal materials. Wrapping the circuit that needs to be protected in it can effectively prevent the interference 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 the internal points of each point are equipotential. With this principle, a closed metal container is made of a metal with good conductivity such as copper or aluminum, and connected with the ground wire, so that the external interference electric field does not affect its internal circuit, and in turn, the electric field generated by the internal circuit does not affect the external circuit. This method is known as electrostatic shielding. For example, in the sensor measurement circuit, a conductor with a gap is inserted between the primary side and the secondary side of the power transformer, and it is grounded to prevent the electrostatic coupling of the two windings, which is an electrostatic shielding.

3. Electromagnetic shielding

For the high-frequency interference magnetic field, the eddy current principle is used to make the high-frequency interference electromagnetic field produce eddy current in the shielded metal, consume the energy of the interference magnetic field, and the eddy current magnetic field cancels out the high-frequency interference magnetic field, so that the protected 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 shielded with copper mesh, which has both electrostatic and electromagnetic shielding. The shielding material must be a low-resistance material with good conductivity, such as copper, aluminum or silver-plated copper.

4. Low-frequency magnetic shielding

If the interference is a low-frequency magnetic field, the eddy current phenomenon is not very obvious at this time, and the anti-interference effect is not very good only by the above method, so it is necessary to use high magnetic permeability materials as a shielding layer, so as to limit the low-frequency interference magnetic inductance line to the magnetic shielding layer with very little magnetic resistance. Protects the protected circuit from low-frequency magnetic field coupling interference. 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 at the same time. Based on the above three commonly used shielding technologies, in other parties where the interference is more serious, composite shielded cables can be used, 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 must be solved in actual measurement, otherwise their transmission efficiency and sensitivity will be reduced. The sensor must be electrostatically shielded, and the electrode leads are double-shielded, commonly known as drive cable technology. In this way, the parasitic capacitance of the sensor during use can be effectively overcome.

5. Grounding technology

Grounding technology is one of the effective technologies to suppress interference, and it is an important guarantee for shielding technology. Proper grounding can effectively suppress external interference, and at the same time improve the reliability of the test system and reduce the interference factors generated by the system itself. The purpose of grounding is twofold: safety and suppression of interference. Therefore, grounding is divided into protective grounding, shielding grounding and signal grounding. Protective grounding is for the purpose of safety, and the casing, chassis, etc. of the sensor measuring device must be grounded. The ground resistance is required to be less than 10?. Shielding grounding is a low-impedance path between the interfering voltage and the ground to prevent interference with the measurement device. The ground resistance should be less than 0.02?. A 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 ground. 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 the way of grounding, and the appropriate grounding method must be selected. These two different grounding treatments are given below.

6. A little grounding

In low-frequency circuits, 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 grounding is to use 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 adopted, a plurality of grounding loops will be formed in the circuit, and when low-frequency signals or pulsed magnetic fields pass through these circuits, electromagnetic induction noise will be caused, and due to the different characteristics of each grounding loop, potential difference will be generated at different loop closure points, forming interference. To avoid this, it's best to use a little grounding. Sensors and measuring devices form a complete detection system, but they can be far apart. If the zero potential of the sensor and the measuring device is grounded in two places, that is, two points are grounded, there will be a larger current flowing through the signal transmission line with very low internal resistance to produce a voltage drop, resulting in serial mode interference. Therefore, a little grounding method should also be used in this case.

7. Multi-point grounding

Multi-point grounding is generally recommended for high-frequency circuits. At high frequency, even a small section of ground wire will have a large impedance voltage drop, coupled with the effect of distributed capacitance, it is impossible to achieve a point of grounding, so a planar grounding mode can be adopted, that is, a multi-point grounding mode, a good conductive plane body (such as a layer of multi-layer circuit board) is used to connect 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 consistency of each potential is basically guaranteed, and the bypass capacitor is added to reduce the voltage drop. Therefore, in this case, a multi-point grounding method is used.

8. Filtering technology

Filters are one of the effective means to suppress AC serial-mode interference. The common filter circuits in sensor detection circuits include Rc filter, AC power filter, and true current power filter. The following describes the applications of these types of filter circuits.

1) RC filter: When the signal source is a thermocouple, strain gauge and other sensors with slow signal changes, the use of small size, low-cost passive Rc filter will have a better suppression effect on the series-mode interference. However, it should be mentioned that the Rc filter is used to reduce serial mode interference at the expense of the system's response speed.

2) AC power filter: The power supply network absorbs various high-frequency and low-frequency noises, and LC filters are commonly used to suppress the noise mixed into the power supply.

3) DC power filter: DC power supply is often shared by several circuits, in order to avoid mutual interference caused by several circuits through the internal resistance of the power supply, Rc or Lc decoupling filter should be added to the DC power supply of each circuit to filter out low-frequency noise. Photocoupling technology: photocoupler is a kind of electrical-optical-electric coupling device, which is composed of light-emitting diode and phototriode packaging, and its input and output are electrically insulated, so this device is used more and more to improve the anti-common-mode interference ability of the system in addition to photoelectric control. When a driving current flows through the light-emitting diode in the optical coupling, the phototriode is saturated with light. Its emitter outputs a high level, so as to achieve the purpose of signal transmission. This allows even if there is interference in the input loop. As long as it's within the threshold, it doesn't affect the output. Sound suppression in pulsed circuits, if there is interference noise in pulsed circuits. The input pulses can be differentiated, reintegrated, and then set a threshold voltage of such amplitude that signals below the threshold voltage are filtered out. For analog signals, you can use A/D conversion first. This method is then used to filter out the noise.

When we use these anti-interference technologies, we should choose according to the actual situation. It must not be used blindly, otherwise it will not only fail to achieve the purpose of anti-interference, but may also have other adverse effects.

Banner Sensors - Analysis of the Eight Anti-Interference Technologies in Sensor Detection

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