I. Shielding technology in sensor
Including electrostatic shielding, electromagnetic shielding, low-frequency magnetic shielding, thermal shielding, etc.
Electrostatic shielding in sensor:
Which is to make a closed metal container with a material with good conductivity such as copper or aluminum, and connect it to the ground wire, and put the circuit that needs to be shielded in it so that the electric field of the external interference electric field is not Affect the internal circuit, and conversely, the power line generated by the internal circuit cannot escape to affect the external circuit.
It can not only prevent electrostatic interference but also prevent the interference of alternating electric fields. Therefore, the housings of many instruments are made of conductive materials and grounded. Although more and more instruments are made of engineering plastic (ABS), when you open the case, you will still see a layer of grounded metal film pasted on the inner wall of the case, which acts like a metal case. The electrostatic shielding effect.
Low-frequency magnetic shielding in sensor
it is an effective measure to isolate the coupling interference of low-frequency and fixed magnetic fields. There is a magnetic field around any wire or coil that passes current, which may cause magnetic field coupling interference to the signal line of the detection instrument or the instrument. In order to prevent magnetic field coupling interference, we must use high-permeability materials as the shielding layer.
In order to allow low-frequency interference magnetic lines of force to pass through the magnetic shielding layer with small magnetic resistance. So that the circuits inside the low-frequency magnetic shielding layer are protected from low-frequency magnetic field coupling interference. For example, the iron shell of the instrument acts as a low-frequency magnetic shield. If the shell is further grounded, it will also act as an electrostatic shield.
Electromagnetic shielding in sensor
Electromagnetic shielding also uses good conductive metal materials to make shields, shielding boxes, and other different shapes, which enclose 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 an electromagnetic shielding layer with good conductivity, it induces an eddy current of the same frequency on its outer surface, thereby consuming the energy of high-frequency interference.
Secondly, the eddy current will also generate a new magnetic field. According to Lenz’s law, its direction is exactly opposite to the direction of the interference source to offset a part of the energy of the interference magnetic field, so that the circuit inside the electromagnetic shielding layer is protected from high-frequency interference magnetic field. Impact.
Since the nature of radio broadcasting is electromagnetic waves, electromagnetic shielding can also absorb their energy. This is why we cannot receive radio in a car (steel body, but not grounded), and we must pull the radio antenna out of the car.
If you grounded the electromagnetic shielding layer, it can also have an electrostatic shielding effect at the same time. We usually use the copper mesh shielded cable as a transmission line. It can play the role of electromagnetic and electrostatic shielding at the same time when we grounded it.
II. Grounding technology in sensor
Which includes safety grounding, signal grounding, signal source grounding, load grounding, etc.
It is a method to ensure the safety and anti-interference of people and equipment. Reasonable selection of grounding methods is an important measure to suppress capacitive coupling, inductive coupling, and resistance coupling, and to reduce or weaken interference.
The detection system is usually composed of a sensor (primary instrument) and a secondary instrument. In the actual industrial site, because the two are far apart and the signal transmission line is longer, the measured data will jump and cause the error to become larger. To solve this kind of problem, we must follow the one-point grounding principle.
The so-called one-point grounding means that if you use multiple points of grounding in the circuit, the potential of each grounding point may be different, which may cause interference signals in the circuit. Therefore, you should achieve one point of grounding in the circuit as much as possible. If you cannot achieve one point of grounding, try your best to Widen the grounding wire so that the potentials of the grounding points are similar, so as not to form a source of signal interference.
III. Isolation measures in sensor
It includes transformer isolation, photocoupler isolation, etc.
The isolation is a technical measure to destroy the interference path and cut off the coupling channel, so as to suppress the interference.
We mainly used Transformer isolation in transmission channels that transmit alternating signals.
We use Optocoupler isolation in digital interface circuits widely. At present, we use more and more photocouplers in automatic detection systems. In order to improve the system’s ability to resist common-mode interference. The photoelectric coupler is a kind of electric photoelectric coupling device. Its input is electric current, the output is also electric current, but both are electrically insulated. Ensure the electrical isolation of the input circuit and output circuit.
The main features of photocouplers are: high insulation resistance of input and output circuits (greater than 1010Ω), withstand voltage exceeding 1KV; because the transmission of light is one-way, the output signal will not feedback and affect the input end; the input and output circuits are in Electrically complete
Isolation can well solve the isolation and transmission contradiction between different potentials and different logic circuits.
IV. Filtering technology in sensor
Filtering technology is to uses a corresponding form of filter to filter out various interference signals so that the interference signals in the signal transmission process do not enter the detection system. It is one of the most effective measures to suppress interference. Filtering technology is a widely used measure especially for suppressing the interference coupled to the circuit via wires. Connect the filter to the signal transmission channel to filter or attenuate the interference signal. In order to achieve the purpose of improving the signal-to-noise ratio and suppressing interference.
Various filters are one of the effective measures to suppress differential mode interference. The filters commonly used in automatic detection systems are:
When the signal source is a sensor with slow signal changes such as thermocouples and strain gauges, the use of a small, low-cost passive RC low-pass filter will have a better suppression effect on serial mode interference.
AC power filter.
The power network absorbs various high and low-frequency noises. For this, we use LC filters to suppress the noise mixed into the power supply. After inserting the AC power filter, we can attenuate more than tens of times electromagnetic interference. It is best to connect AC power filters in series with the power supplies and other electrical equipment. The main considerations when choosing an AC power filter are: first, the rated current of the filter must be greater than the working current; second, in the foreseeable frequency range, the attenuation coefficient for interference must meet the requirements. When in use, you can choose a power filter that contains one-stage LC or two-stage or even three-stage LC inside according to your needs. It must be well-grounded when in use.
DC power supply filter.
Several circuits often share a DC power supply. In order to avoid the internal resistance of the power supply, mutual interference should add RC or LC decoupling filtering. Other interferences can adopt corresponding measures. Thermal shielding can solve thermal interference; we can use temperature interference, temperature compensation to adapt to temperature changes at the worksite. In addition, in the design process, when selecting electrical components and sensors, we should select devices with adaptability to specific working site conditions.