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Aug 28 2018

Analysis of Eight Anti-jamming Technologies in Sensor Detection

Summary

        This paper summarizes the eight anti-jamming technologies in sensor detection, and analyzes the details of various anti-jamming technologies, such as working effects, applicable scenarios, operating procedures and internal principles. The beginning of the article explains the extent to which sensors are widely used in modern life and the problems that sensors use in real life, which straightforwardly identifies the importance of anti-jamming techniques in sensor detection. It also focuses on different details of various anti-jamming technologies.

Article coreAnalysis and explanation of anti-jamming technologiesPurposeUse anti-jamming technologies to protect production
Article nameAnalysis of eight anti-jamming technologies in sensor detectionCatagoryTechnology
ApplicationTechnology,IC chips,Industrial production,Control systemKeywordsSensor,production, interfence



Catalogs  

Ⅰ  Shielding Technology
Ⅱ  Electrostatic Shielding
Ⅲ  Electromagnetic Shielding
Ⅳ  Low-frequency magnetic shielding
Ⅴ  Grounding technology
Ⅵ  Single-point Grounding
  Multi-point Grounding
Ⅷ  Filtering technique


Introduction  

        With the advancement of modern technology, various sensors and automatic detection devices are widely used in industrial production to monitor various production links. A computer is also needed to control the entire process of production in which hundreds of different sensors are generally required to convert different non-electrical parameters for computer processing.   

        Electrical or magnetic interference are often large at the production site which may break the normal work of sensor and computer and even the entire inspection system. Therefore, the anti-jamming technology is an important part of the sensor detection system. Furthermore, it is necessary for people engaged in automatic inspection to understand anti-jamming technology.

        The unwanted signal appearing in the circuit of the electronic measuring device is called noise, and when the noise affects the normal operation of the circuit, the noise is called interference. Forming interference during signal transmission must have three factors, namely the interference source, the interference path and the receiving circuit with high sensitivity to noise. Ways to reduce or eliminate noise interference can take action against any of these three parts. Taking measures against the interference path and the receiving circuit are the more common methods in the sensor detection circuit.

sensor


Details

        The following describes several commonly used and effective anti-jamming technologies.

 Ⅰ. Shielding Technology

  Made of metal materials, the container can effectively prevent the interference of electric field or magnetic field by wrapping the circuit which needs to be protected, this method is called shielding technology, which has divided into electrostatic shielding, electromagnetic shielding and low frequency magnetic shielding and so on.

shielding technology


       Ⅱ. Electrostatic Shielding

According to the principle of electromagnetism, a sealed hollow conductor placed in the electrostatic field has no electric field line inside, and the potentials at the internal points are equal. Based on this principle, we can make a closed metal container by Metallic materials with good conductivity such as copper or aluminum, placing the circuit needing protection inside and connecting it to the ground wire, so that the external interference electric field does not affect its internal circuit. In turn, the electric field generated by the internal circuit does not affect the external circuit. This method is called electrostatic technology. 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 then grounded, to prevent electrostatic coupling between the two windings. Such method belongs to electrostatic shielding.


Ⅲ. Electromagnetic Shielding

Electromagnetic Shielding

According to the eddy current principle, the high-frequency interference electromagnetic field generates an eddy current in the shielding metal, which would consume the energy that  interferes the magnetic field. That is, using the eddy current magnetic field to offset the high-frequency interference magnetic field, thereby protecting the protected circuit from the high-frequency electromagnetic field. Such method is called the electromagnetic shielding. It also has the function of electrostatic shielding if the electromagnetic shielding layer is grounded.  The output cable of the sensor is generally shielded by a copper mesh, which has both electrostatic shielding and electromagnetic shielding. Low-resistance materials with good electrical conductivity, such as copper, aluminum or silver-plated copper would be selected as the shielding material.


 Ⅳ.Low-frequency magnetic shielding

        The eddy current phenomenon is not obvious if the interference comes from a low-frequency magnetic field, and the anti-interference effect is not ideal if only using the above method. Therefore, to limit the low frequency interference magnetic line to the inside of the magnetic shield with small magnetic reluctance, and to keep the protected circuits from the low-frequency magnetic field, the shielding layer must use materials of high magnetic permeability. This shielding method is generally referred to as low frequency magnetic shielding. The metal housing of the sensor inspection instrument acts as a low frequency magnetic shield. It also plays the role of electrostatic shielding and electromagnetic shielding when grounded.

Based on the above three common shielding technologies, a composite shielded cable can be used in places where interference is severe, that is, the outer layer is a low-frequency magnetic shielding layer, and the inner layer is an electromagnetic shielding layer, which achieves double shielding. For example, the parasitic capacitance is a key problem that must be solved when a capacitive sensor is actually working, otherwise, its transmission efficiency and sensitivity should become lower. So the sensor must be electrostatically shielded, the electrode lead-out line uses double-layer shielding technology, which is generally called the drive cable technology, to effectively overcome the parasitic capacitance of the sensor.


Ⅴ. Grounding technology

        Grounding technology is one of the effective techniques to suppress interference and 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. To ensure safety and to suppress interference, grounding is divided into protective grounding, shield grounding, and signal grounding. The grounding protection is to ensure safety, so casing and chassis of the sensor measuring device should be grounded and the grounding resistance should be 10 Ω or less. Shield grounding can prevent the voltage from forming a low-resistance path to the ground, so that to prevent interference from the measuring device, and its grounding resistance should be less than 0.02 Ω.

 Signal grounding refers to the common line of the zero signal potential of the input and output of the electronic device, which maybe insulated from the earth. The signal ground line is further divided into analog signal ground lines and digital signal ground lines. The analog signal is generally weak, so its requirement to the ground line is high. The digital signal is generally strong, so its requirement for ground line can be lower.

 Different sensor detecting conditions also have different requirements to the grounding method, so It is necessary to select appropriate grounding methods. Single-point grounding and multi-points grounding are the Common grounding methods. Detailed treating measures are as follows.


 Ⅵ. Single-point Grounding

        In the low frequency circuit, it is generally recommended to use single-point grounding, which divided into radiation grounding and bus-bar grounding. Radiation grounding is the direct connection of each functional circuit to the zero potential reference point in the circuit. Bus-bar grounding is to use a high-quality conductor with a certain cross-sectional area as the grounding busbar to directly connected to the zero potential point, to which the functional blocks in the circuit can be connected. Multiple ground loops will be formed in the circuit in from of multi-points grounding , and electromagnetic induction noise will be generated when low-frequency signals or pulsed magnetic fields pass through these loops. Due to the different characteristics of each ground loop, different potential difference will be generated at the point of different closed loops to form interference. Single-point grounding should be chose to avoid this condition.

 Single-point Grounding

 Single-point Grounding

A complete inspection system can be formed by Sensors and measuring devices, but the two can be quite different. The earth current of the industrial site is very complicated, so the potential of the points where this two parts are connected to earth are generally different. If the zero potential of the sensor and the measuring device are grounded separately at different places, that is, two-point grounding, a large current will flow through the signal transmission line that with a low internal resistance to cause a voltage drop, resulting in series mode interference. Therefore,  single-point grounding should be the option in this case.


Ⅶ. Multi-point Grounding

Multi-point grounding is generally recommended for high-frequency circuits. In the high-frequency circuit, even a small piece of ground wire will have a large impedance voltage drop, and the role of distributed capacitance, it is impossible to achieve single-point grounding, so planar grounding, also called multi-point grounding, can be used. Use a good conductive planar body (such as using the multilayer circuit boards) to connect to the zero potential reference point, and each high frequency circuit is connected to the conductive planar body adjacent. Because the high-frequency impedance of the conductive plane is very small, the uniformity of the potential at each place is basically guaranteed. What’s more, the voltage drop can be reduced by adding a bypass capacitor, so multi-point grounding is suitable for this situation.

Multi-point Grounding


      Ⅷ. Filtering technique

The filter is one of the effective methods to suppress AC series mode interference. Filter circuits commonly found in sensor detection circuit include Rc filters, AC power supply filters, and DC power supply filter.

The applications of these kinds of filter circuits are described below.

Rc Filters

When the signal source is a sensor with slow signal change such as thermocouple or strain gauge,   the series mode interference can be preferably suppressed by a small-volume, low-cost passive Rc filter. However, it should be mentioned that the Rc filter reduces the serial-mode interference at the expense of the system response speed

 Rc Filters

a)Connection of Single RC Filter and Amplifier      b)Double RC Filter

c)Low Pass Filter Graphic Symbols          d)  Frequency Characteristics

  AC Supply Filter

 The power network absorbs various noises of high or low frequency, and Lc filter is  commonly used to suppress the noise that mixing into the power supply.

Internal circuit of AC Supply Filter

Internal circuit of AC Supply Filter


Internal circuit of AC Supply Filter

Internal circuit of AC Supply Filter

DC Power Filter

DC power supply is often shared by several circuits. To avoid interference between several circuits caused by the internal resistance of power supply, an Rc or Lc decoupling filter should be added to each DC power supply to remove low frequency noise.

Internal circuit of DC Power Filter

Internal circuit of DC Power Filter

Photoelectric Coupling Technology

Optocouplers are working from electricity to light and then from light to electricity. Consisted of a light-emitting diode and a Photoelectric triode, Its input and output are electrically insulated. Therefore, in addition to being used for photoelectric control, this device is now being used more and more to improve the system's anti-common-mode interference capability. When a driving current flows through the light-emitting diode in the optical coupler, the Photoelectric triode is saturated with light. Its emitter output is high, thus achieving the purpose of signal transmission. Even if there is interference in the input loop, it will not affect the output as long as it is within the threshold.

Internal circuit of DC Power Filter Internal circuit of DC Power Filter    Photoelectric Coupling Technology

       Noise Suppression in Pulse Circuits

 If there is interference noise in the pulse circuit. the input pulse can be differentiated and then integrated, and a threshold voltage of a certain amplitude can be set to filter out the signals  that smaller than the threshold voltage. For analog signals, A/D conversion can be used first, and then filter out the noise by this method.


 When using these anti-jamming technologies, we should based on actual conditions, do not use it blindly, or it will not achieve the purpose and even cause other adverse effects.


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