What are the commonly used anti-interference technologies for sensors?

One shielding technology

Including electrostatic shielding, electromagnetic shielding, low-frequency magnetic shielding, thermal shielding, etc.

1. Electrostatic shielding: Electrostatic shielding is a closed metal container made of conductive materials such as copper or aluminum, and connected to a ground wire. The circuit that needs to be shielded is placed inside it, so that the external interference electric field affects the internal circuit. Conversely, the power lines generated by the internal circuit cannot escape the impact of the external circuit. Electrostatic shielding not only prevents static interference, but also prevents interference from alternating electric fields. Therefore, many instrument casings are made of conductive materials and grounded. Although more and more instruments are made of engineering plastic (ABS), when you open the case, you still see a grounded metal film sticking to the inner wall of the case, acting like a metal case. Electrostatic shielding effect.

2. Low frequency magnetic shielding: Low frequency magnetic shielding is an effective measure to isolate the coupling interference between low-frequency magnetic fields and fixed magnetic fields. There is a magnetic field around any wire or coil passing through an electric current, and objectively there is also a magnetic field, which may cause magnetic field coupling interference to the signal line of the testing instrument or instrument. To prevent magnetic field coupling interference, it is necessary to use high permeability materials as shielding layers to allow low-frequency interference magnetic field lines to pass through the magnetic shielding layer with small magnetic resistance, thereby protecting the circuits inside the low-frequency magnetic shielding layer from low-frequency magnetic field coupling interference. For example, the iron shell of the instrument serves as a low-frequency magnetic shield. If the shell is further grounded, it will also serve as an electrostatic shield.

3. Electromagnetic shielding: Electromagnetic shielding also uses good conductive metal materials to make shielding covers, shielding boxes, and other shielding covers of different shapes to surround the protected circuit. The interference object it shields is not an electric field, but a high-frequency (above 40KHz) magnetic field. When the high-frequency magnetic field generated by the interference source encounters a conductive electromagnetic shielding layer, it will induce eddy currents of the same frequency on its outer surface, thereby consuming the energy of high-frequency interference. Secondly, eddy currents also generate a new magnetic field. According to Rentz's law, its direction is exactly opposite to the direction of the interference source to offset a portion of the energy of the interference magnetic field, thereby protecting the circuits inside the electromagnetic shielding layer from the influence of high-frequency interference magnetic fields. influence

Due to the nature of radio broadcasting being electromagnetic waves, electromagnetic shielding can also absorb their energy. That's why we can't receive the radio in the car (steel body, but not grounded), we must unplug the radio antenna from the car. reason

If the electromagnetic shielding layer is grounded, it can also have an electrostatic shielding effect, which will have a better shielding effect on electromagnetic waves. Copper mesh shielded cables, commonly used as transmission lines, can simultaneously serve as electromagnetic and electrostatic shielding when grounded.

II. Grounding Technology

Grounding includes safety grounding, signal grounding, power grounding, load grounding, etc.

Grounding is a method of ensuring the safety and anti-interference of personnel and equipment. Reasonable selection of grounding methods is an important measure to suppress capacitance coupling, inductance coupling, and resistance coupling, and reduce or weaken interferen

The detection system usually consists of sensors (main instruments) and auxiliary instruments. In actual industrial sites, due to the distance between the two and the longer signal transmission lines, measurement data may jump, resulting in larger errors. To solve such problems, we must follow the principle of one point grounding. The so-called one point grounding refers to the use of multiple grounding points in a circuit, where the potential of each grounding point may be different, which may generate interference signals in the circuit. Therefore, it is advisable to achieve a point of grounding in the circuit as much as possible. If a single point of grounding cannot be achieved, please try to widen the grounding wire as much as possible to make the potential of the grounding point similar, in order to avoid forming a signal interference 

Three isolation measures

Isolation includes transformer isolation and optocoupler

Isolation is a technical measure to disrupt interference paths and cut off coupling channels to suppress interference.

Transformer isolation is mainly used for the transmission channel of AC signals.

Optocoupler isolation is widely used in digital interface circuits. Currently, more and more optocouplers are being used in automatic detection systems to improve their ability to resist common mode interference. Optoelectronic coupler is a type of optoelectronic coupling device, where the input is current and the output is also current, but the input and output are electrically insulated. Ensure electrical isolation between input and output circuits.

The main characteristics of optocouplers are: high insulation resistance (greater than 1010 Ω) in the input and output circuits, and withstand voltage exceeding 1KV; Due to the unidirectional transmission of light, the output signal will not feedback and affect the input end; The input and output circuits are in a completely electrically isolated state, which can solve the isolation and transmission contradictions between different potentials and logic circuits.

Four filtration technology

Filtering technology uses corresponding forms of filters to filter out various interference signals, so that interference signals during signal transmission do not enter the detection system. This is one of the most effective measures to suppress interference. Filtering technology is a widely used measure, especially for suppressing interference coupled to circuits through wires. Connect filters in the corresponding frequency band to the signal transmission channel, filtering or attenuating interference signals as much as possible to achieve the goal of improving signal-to-noise ratio and suppressing interference.

Various filters are one of the effective measures to suppress differential mode interference. The commonly used filters in automatic detection systems includ

1. RC filter. When the signal source is sensors with slow signal changes such as thermocouples and strain gauges, using a small, low-cost passive RC low-pass filter will have better suppression effect on serial mode interference.

2. AC power filter. The power network absorbs various high-frequency and low-frequency noise. For this reason, LC filters are often used to suppress noise mixed into the power supply. After inserting an AC power filter between the power supply and the load, electromagnetic interference in the range of several thousand hertz to several tens of megahertz can be attenuated by several tens of times. It is best to connect the AC power filter in series with the power supply of computers, sensors, secondary instruments, and other electrical equipment working in interference environments. The main considerations when selecting an AC power filter are: firstly, the rated current of the filter must be greater than the working current of the electrical equipment; Secondly, within the foreseeable frequency range, the interference attenuation coefficient must meet the requirements. When using, you can choose a power filter with built-in primary LC, secondary LC, or even tertiary LC as needed. It must be well grounded during use.

3. DC power filter. DC power is usually shared by several circuits. To avoid mutual interference between multiple circuits caused by internal resistance of the power supply, RC or LC decoupling filtering should be added to the DC power supply of each circuit. The device can also take corresponding measures for other interferences, such as thermal interference, temperature interference, and light interference. Thermal interference can be solved through thermal shielding; For temperature interference, temperature compensation can be used to adapt to temperature changes on the construction site. In addition, during the design process, when selecting electrical components and sensors, it is necessary to select equipment with stable performance parameters, high reliability, and ability to adapt to specific working site conditions to ensure the reliability and safety of the system after composition