With the advancement of modern technology, the level of production automation is constantly improving. In industrial production, various sensors and automatic detection devices are widely used to monitor all aspects of production. Some systems even require computers to control the entire production process. Such systems typically require hundreds of different sensors to convert various non-electrical parameters into electrical quantities for computer processing. However, because there are often numerous electrical and magnetic interference sources in the production environment, these can disrupt the normal operation of sensors, computers, and even the entire detection system. Therefore, anti-interference technology is a crucial aspect of sensor detection systems, and understanding anti-interference technology is essential for those working in automatic detection.
In electronic measuring devices, useless signals are called noise. When noise affects the normal operation of the circuit, it is called interference. Interference during signal transmission requires three factors: the interference source, the interference path, and a receiving circuit that is highly sensitive to noise. Therefore, methods to eliminate or reduce noise interference can target any one of these three factors. A commonly used method in sensor detection circuits is to take appropriate measures against the interference path and the receiving circuit to eliminate or reduce noise interference. Several commonly used and effective anti-interference techniques are introduced below.
1. Shielding technology
Using a metal container to enclose the circuitry that needs protection can effectively prevent interference from electric or magnetic fields; this method is called shielding. Shielding can be further divided into electrostatic shielding, electromagnetic shielding, and low-frequency magnetic shielding, among others.
2. Electrostatic shielding
According to the principles of electromagnetism, a sealed hollow conductor placed in an electrostatic field has no electric field lines inside, and all points inside are at the same potential. Using this principle, a sealed metal container can be made using highly conductive metals such as copper or aluminum and connected to ground. The circuit to be protected can then be placed within this container, preventing external interference fields from affecting the internal circuitry, and conversely, preventing the electric field generated by the internal circuitry from affecting the external circuitry. This method is called electrostatic shielding. For example, in sensor measurement circuits, inserting a conductor with a gap between the primary and secondary windings of a power transformer and grounding it can prevent electrostatic coupling between the two windings; this method is an example of electrostatic shielding.
3. Electromagnetic shielding
For high-frequency interference magnetic fields, the principle of eddy currents is used to generate eddy currents within the shielding metal, consuming the energy of the interference magnetic field. The eddy current magnetic field cancels out the high-frequency interference magnetic field, thus protecting the circuit from the effects of the high-frequency electromagnetic field. This shielding method is called electromagnetic shielding. If the electromagnetic shielding layer is grounded, it also provides electrostatic shielding. The output cable of the sensor generally uses copper mesh shielding, which provides 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, and the above-mentioned methods are not very effective in suppressing interference. Therefore, it is necessary to use a high-permeability magnetic material as a shielding layer to confine the low-frequency interference magnetic field lines within the magnetic shielding layer with very low magnetic resistance. This protects the circuit from the effects of low-frequency magnetic field coupling interference. This shielding method is generally called low-frequency magnetic shielding. The sheet metal casing of the sensor detection instrument serves as low-frequency magnetic shielding. If it is further grounded, it also serves as electrostatic shielding and electromagnetic shielding.
Based on the three commonly used shielding technologies mentioned above, composite shielded cables can be used in areas with severe interference. These cables consist of an outer low-frequency magnetic shielding layer and an inner electromagnetic shielding layer, achieving double shielding. For example, in actual measurements, the parasitic capacitance of capacitive sensors is a critical issue that must be addressed; otherwise, their transmission efficiency and sensitivity will decrease. Electrostatic shielding of the sensor is necessary, and its electrode leads utilize double-layer shielding technology, generally referred to as drive cable technology. This method effectively overcomes the parasitic capacitance of the sensor during use.
5. Grounding technology
Grounding is an effective technique for suppressing interference and a crucial guarantee for shielding technology. Proper grounding can effectively suppress external interference, improve the reliability of the testing system, and reduce interference factors generated by the system itself. Grounding serves two purposes: safety and interference suppression. Therefore, grounding is divided into protective grounding, shielding grounding, and signal grounding. Protective grounding is for safety purposes; the housing, chassis, and other components of the sensor measuring device must be grounded. The grounding resistance should be below 10 ohms. Shielding grounding creates a low-resistance path between the interference voltage and ground to prevent interference with the measuring device. The grounding resistance should be less than 0.02 ohms.
Signal ground is the common line for zero signal potential between the input and output of an electronic device; it may be insulated from the earth. Signal ground lines are further divided into analog signal ground lines and digital signal ground lines. Analog signals are generally weaker, so the requirements for the ground line are higher; digital signals are generally stronger, so the requirements for the ground line are lower.
Different sensor detection conditions require different grounding methods, so it is essential to select an appropriate grounding method. Common grounding methods include single-point grounding and multi-point grounding. The following describes these two different grounding procedures.
6. Single-point grounding
In low-frequency circuits, single-point grounding is generally recommended. This can be either radial grounding or busbar grounding. Radial grounding involves directly connecting each functional circuit to a zero-potential reference point using a conductor. Busbar grounding uses a high-quality conductor with a certain cross-sectional area as the grounding busbar, directly connected to the zero-potential point, allowing the ground of each functional block in the circuit to be connected to the nearest busbar. If multi-point grounding is used, multiple grounding 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 grounding loop, potential differences will occur at different loop closure points, causing interference. To avoid this situation, single-point grounding is the best approach.
The sensor and measuring device constitute a complete detection system, but they may be far apart. Because the ground current in industrial environments is highly complex, the potentials between the grounding points of these two components are generally different. If the zero potential of the sensor and measuring device are grounded at two separate points (i.e., two-point grounding), a large current will flow through the low-resistance signal transmission line, causing a voltage drop and resulting in common-mode interference. Therefore, in this case, a single-point grounding method should be used.
7. Multiple grounding points
For high-frequency circuits, multi-point grounding is generally recommended. At high frequencies, even a short ground wire will have a significant impedance voltage drop. Combined with the effect of distributed capacitance, single-point grounding is impossible. Therefore, a planar grounding method, i.e., multi-point grounding, can be used. This involves connecting a good conductive plane (such as one layer of a multilayer circuit board) to a zero-potential reference point, and then connecting the ground of each high-frequency circuit to the nearest conductive plane. Since the high-frequency impedance of the conductive plane is very small, it essentially ensures a consistent potential at each point. Furthermore, bypass capacitors can be added to reduce voltage drop. Therefore, multi-point grounding is essential in this situation.
8. Filtering techniques
Filters are one of the effective means of suppressing AC crosstalk interference. Common filtering circuits in sensor detection circuits include Rc filters, AC power supply filters, and true current power supply filters.
The applications of these filtering circuits are described below.
RC filter
When the signal source is a sensor with slow signal change, such as a thermocouple or strain gauge, a small-sized, low-cost passive Rc filter can effectively suppress cross-mode interference. However, it should be noted that Rc filters reduce cross-mode interference at the cost of sacrificing system response speed.
AC power filter
Power networks absorb various high- and low-frequency noises, and Lc filters are commonly used to suppress noise mixed into the power supply.
DC power filter
DC power supplies are often shared by several circuits. To avoid mutual interference between the circuits due to 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.
Optocoupler technology
An optocoupler is an electro-optical-electrical coupling device consisting of a light-emitting diode (LED) and a phototransistor. Its input and output are electrically isolated. Therefore, besides its use in optoelectronic control, this device is increasingly used to improve a system's common-mode interference immunity. When a drive current flows through the LED in the optocoupler, the phototransistor becomes light-saturated. Its emitter outputs a high-level signal, thus achieving signal transmission. This way, even if there is interference in the input circuit, as long as it is within the threshold, it will not affect the output.
Noise suppression in pulse circuits
If interference noise exists in the pulse circuit, the input pulse can be differentiated and then integrated, and a threshold voltage of a certain amplitude can be set so that signals below the threshold voltage are filtered out. For analog signals, A/D conversion can be used first, and then this method can be used to filter out noise.
When using these anti-interference technologies, we must choose them according to the actual situation. Blindly using them is strictly prohibited; otherwise, not only will the anti-interference purpose be failed, but other adverse effects may also occur.
