With the advancement of modern technology, the level of production automation has also increased. In industrial production. Various sensors and automatic detection devices are widely used to monitor various aspects of production, and some also require a computer to control the whole process of production. In such a system, hundreds of different sensors are generally required to convert various non-electrical parameters. The amount of electricity is processed by the computer. However, because there are often a large number of electrical and magnetic interference sources at the production site, they can damage the normal operation of sensors, computers and even the entire detection system. Therefore, anti-interference technology is an important part of the sensor detection system. For those engaged in automatic inspection work. In fact, understanding anti-jamming technology is very necessary.
A useless signal appearing in the circuit of an electronic measuring device is called noise. When noise affects the normal operation of the circuit, the noise is called interference. The formation of interference during signal transmission must have three factors, namely the interference source, the interference path and the receiving circuit with high sensitivity to noise. Therefore, the method of eliminating or attenuating noise interference can take measures against any of these three items. A more common method in the sensor detection circuit is to take corresponding measures on the interference path and the receiving circuit to eliminate or attenuate the noise interference. The following describes several commonly used and effective anti-jamming technologies.
1, shielding technology
Made of metal materials. The circuit to be protected is packaged therein to effectively prevent interference from electric or magnetic fields. 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 electromagnetics, there is no electric field line inside the sealed hollow conductor placed in the electrostatic field, and the internal points are equipotential. Using this principle, a metal container with good conductivity such as copper or aluminum is used as a material to form a closed metal container, which is connected to the ground wire, and the circuit value r to be protected is such that the external interference electric field does not affect its internal circuit, and vice versa. The electric field generated by the internal circuit also does not affect the external circuit. This method is called electrostatic shielding. For example, in the sensing 嚣 measurement circuit, a conductor with a gap is inserted between the primary side and the secondary side of the power transformer, and grounded to prevent electrostatic coupling between the two windings. This method belongs to 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 generate eddy current in the shielding metal, consume the energy that interferes with the magnetic field, and the eddy current magnetic field cancels the high-frequency interference magnetic field, thereby protecting the protected circuit from the high-frequency electromagnetic field. . This shielding method is called electromagnetic shielding. If the electromagnetic shielding layer is grounded, it also has the function of electrostatic shielding. The output cable of the sensor is generally shielded by a copper mesh, which has both electrostatic shielding and electromagnetic shielding. The shielding material must be selected from low-resistance materials with good electrical conductivity, such as copper, aluminum or silver-plated copper.
4, low frequency magnetic shielding
If the disturbance is a low-frequency magnetic field, the eddy current phenomenon is not obvious at this time. The anti-interference effect is not so good only by the above method. Therefore, it is necessary to use a high magnetic permeability material as a shielding layer to limit the low-frequency interference magnetic sensing line to the magnetic field. The inside of the magnetic shield is very small. Protect the protected circuit from low frequency magnetic field coupling interference. 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. If it is further grounded, it will also function as electrostatic shielding and electromagnetic shielding.
Based on the above three commonly used shielding technologies, the composite shielding cable can be used on the side where the interference is serious, that is, the outer layer is a low frequency magnetic shielding layer. The inner layer is the electromagnetic shielding layer. Achieve double shielding. For example, the capacitive sensor's parasitic capacitance is a key problem that must be solved in actual measurement, otherwise its transmission efficiency and sensitivity will be low. The sensor must be electrostatically shielded, and the electrode lead-out line uses double-layer shielding technology, which is generally called 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 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. There are two purposes for grounding: safety and interference suppression. Therefore, the grounding is divided into protective grounding, shielding grounding, and signal grounding. The protective grounding is for safety purposes, and the casing, chassis, etc. of the sensor measuring device are grounded. The grounding resistance is required to be 10 Ω or less. Shield grounding is a low-resistance path from the interference voltage to ground to prevent interference with the measuring device. The grounding resistance should be less than 0.02 ?.
The signal ground is the common line of the zero signal potential of the input and output of the electronic device, which itself may be insulated from the earth. The signal ground line is further divided into an analog signal ground line and a digital signal ground line. The analog signal is generally weak, so the ground line requirement is high: the digital signal is generally strong, so the ground line requirement can be lower.
Different sensor detection conditions also have different requirements for the grounding method. The appropriate grounding method must be selected. The common grounding method has a little grounding and multi-clicking. These two different grounding treatments are given below.
6, a little ground
It is generally recommended to use a grounding in the low frequency circuit, which has a radial grounding wire and a busbar grounding wire. Radial grounding is the direct connection of each functional circuit in the circuit with the zero potential reference point: the bus-type 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, the ground of each functional block in the circuit. Can be connected to the bus. At this time, if multi-point grounding is used, multiple ground loops will be formed in the circuit. When low-frequency signals or pulsed magnetic fields pass through these loops, electromagnetic induction noise will be generated. Due to the different characteristics of each ground loop, the loops are closed in different loops. A potential difference is generated at the point to form an interference. To avoid this, it is best to use a grounding method.
The sensor and measuring device form a complete detection system, but the two may be far apart. Because the earth current in the industrial site is very complicated, the potential between the two parts of the outer casing is generally different. If the zero potential of the sensor and the measuring device are grounded at two places, that is, two points are grounded, there will be A large current flows through a signal transmission line with a very low internal resistance to cause a voltage drop, causing series mode interference. Therefore, a 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 frequencies, even a small piece of ground wire will have a large impedance drop, and with the effect of distributed capacitance, it is impossible to achieve a little grounding. Therefore, a planar grounding method, that is, a multi-point grounding method can be used, and a good The conductive planar body (such as one of the multilayer circuit boards) is connected to the zero potential reference point, and the ground of each high frequency circuit is closely connected to the conductive planar body. Since the high-frequency impedance of the conductive plane body is small, the potential of each potential is basically ensured, and a bypass capacitor is added to reduce the voltage drop. Therefore, this situation uses a multi-point grounding method.
8, filtering technology
The filter is one of the effective means to suppress AC series mode interference. Filter circuits commonly found in sensor detection circuits include Rc filters, AC line filters, and true current source filters. The application of these kinds of filter circuits is described below.
1, RC filter
When the signal source is a sensor with slow signal change such as thermocouple or strain gauge, the use of a small-volume, 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 reduces the serial-mode interference at the expense of the system response speed.
2, AC power filter
The power network absorbs various high and low frequency noises, and the common Lc filter is used to suppress the noise mixed into the power supply.
3, DC power supply filter
The DC power supply is often shared by several circuits. In order to avoid interference between several circuits caused by the internal resistance of the power supply, an Rc or Lc decoupling filter should be added to the DC power supply of each circuit to filter out low frequency noise.
4. Photoelectric coupling technology
An optocoupler is an electro-optical-electric coupling device consisting of a light-emitting diode and a phototransistor package. The input and output are electrically insulated, so that the device is used for photoelectric control. It is now being used more and more to improve the system's ability to resist common mode interference. When a driving current flows through the light emitting diode in the optical coupler, the phototransistor is saturated with light. Its emitter output is high, thus achieving the purpose of signal transmission. This is even if there is interference in the input loop. As long as it is within the threshold, it will not affect the output.
Beep suppression in 5 pulse circuits
If there is interference noise in the pulse circuit. The input pulse can be differentiated and then integrated, and then a threshold voltage of a certain amplitude is set so that a signal smaller than the threshold voltage is filtered out. For analog signals, A/D conversion can be used first. This method is used to filter out noise.
When using these anti-interference technologies, we should choose according to the actual situation. Do not use it blindly, otherwise it will not achieve the purpose of anti-interference, and there may be other adverse effects.
TDC2000 systems have been in operation for more than 25 years and will be supported by Honeywell in the future. However, ageing workforce and a greater demand for energy savings, environmental protection and improved operational effectiveness require users to look closely at different solutions for technology refresh and upgrades.
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