The realization of monitoring and control in the industrial production process requires the use of various automated instruments, control systems and actuators. The signal transmission between them includes both small signals to the millivolt level and microampere level, and dozens of volts.
Even large signals of thousands of volts and hundreds of amps; both low-frequency DC signals and high-frequency pulse signals, etc. After forming a system, it is often found that signal transmission between meters and equipment interferes with each other, causing the system to be unstable or even
Misoperation. When this happens, in addition to the performance reasons of each meter and equipment, such as anti-electromagnetic interference, there is a very important factor because of the potential between the signal reference point between the meter and the equipment.
Poor, thus forming a "ground loop" causing distortion during signal transmission. Therefore, to ensure the stable and reliable operation of the system, the "ground loop" problem is a problem that must be solved in the process of system signal processing.
Solutions to "ground loops"
According to theoretical and practical analysis, there are three solutions:
The first solution: all field devices are not grounded, so that all process loops have only one ground point and cannot form a loop. This method seems simple, but it is often difficult to achieve in practical applications, because some equipment requirements must be connected.
The ground can only ensure the measurement accuracy or life safety. Some equipment may form a new ground point due to long-term corrosion and wear or weather.
The second scheme: make the potentials of the two ground points the same, but because the resistance of the ground points is affected by many factors such as geological conditions and climate change, this scheme cannot actually be achieved in practice.
The third solution: use the signal isolation method in each process loop to disconnect the process loop without affecting the normal transmission of the process signal, thereby completely solving the problem of the ground loop.
抗干扰措施 Anti- interference measures of intelligent isolator
Anti-interference design of power supply system
The most serious damage to the normal operation of sensors and instruments is the spike interference of the power grid. The electrical equipment that generates spike interference includes: welding machines, large motors, controllable machines, relay contactors, and inflatable lighting lamps with ballasts.
Even wait. Spike interference can be suppressed by a combination of hardware and software.
(1) There are three common ways to suppress the impact of spike interference with hardware lines:
① An interference controller designed according to the principle of spectrum equalization is connected in series at the input end of the AC power supply of the instrument, and the energy concentrated by the peak voltage is distributed to different frequency bands, thereby reducing its destructiveness;
② Add a super isolation transformer to the input end of the AC power supply of the instrument and use the principle of ferromagnetic resonance to suppress the spike pulse;
③ A varistor is connected in parallel to the input terminal of the instrument's AC power supply, and the resistance value is reduced when the spike pulse arrives to reduce the voltage the instrument receives from the power supply, thereby weakening the influence of interference.
(2) Using software methods to suppress spike interference. For periodic interference, you can use programming to perform time filtering, that is, use a program to control the thyristor to not sample instantaneously, thereby effectively eliminating interference. (3) Using a combination of hardware and software
Watchdog technology suppresses the impact of spikes. Software: Before the timer expires, the CPU accesses the timer once and lets the timer restart. The normal program runs. The timer does not generate overflow pulses.
watchdog will not work. Once the “spike” occurs in the spike interference, the CPU will not access the timer before the timer expires, so the timing signal will appear, which will cause the system reset interrupt and ensure that the smart instrument returns.
Come up with normal procedures.
(4) The power supply is implemented in groups, for example, the driving power of the execution motor is separated from the control power to prevent interference between equipment.
(5) The noise filter can also effectively suppress the interference of the AC servo drive to other equipment. This measure can effectively suppress the above several kinds of interference phenomena.
采用了先进的数字化技术，在对高、低频干扰信号的抑制方面均有着优异表现，即使在大功率变频控制系统中依然能够可靠应用，内部采用数字 The signal intelligent isolator uses advanced digital technology, and has excellent performance in the suppression of high and low frequency interference signals. It can still be reliably applied even in high-power variable frequency control systems.
Many advanced technologies such as chemical adjustment, zero-free and full-scale potentiometers, automatic dynamic calibration zeros, and automatic compensation for temperature drifts, etc., and comply with the fourth type (harsh industrial site) environment stipulated in IEC61000-4-4: 1995 Anti-electromagnetic
Interference requirements, the application of this series of technologies make the stability and reliability of the product scientifically guaranteed. The above technologies are leading the international advanced level.
The signal isolator can be used in combination with the unit combination instrument and DCS, PLC and other systems, and has been widely used in major projects in the oilfield, petrochemical, manufacturing, power, metallurgy and other industries.
Smart isolator power supply
Independently powered isolators:
Requires an independent 20 ~ 35VDC DC power supply. The advantage of this method is high isolation transmission accuracy; complete isolation between power supply, input, and output. Multi-channel system power supply does not need to be isolated to ensure high anti-interference performance.
The input signal can be converted to other types.
2. Loop-powered isolators In practical industrial monitoring systems, DCS, PLC, or other display instruments with cards are increasingly used for internal power supply. Loop-powered isolators often cannot meet the signal requirements of these cards.
Requirements for high isolation transmission accuracy and high voltage requirements for two-wire transmitter power distribution. Therefore, the output loop-powered isolator not only retains the superior performance of the independent power supply-type isolator, but also meets the requirements of the output loop power interface.
的作用原理 How does the smart isolator work?
1: Causes of system interference (1) Ground circulation interference
The realization of monitoring and control in the industrial production process requires the use of various automated instruments, control systems and actuators. The signal transmission between them is ranging from weak signals to millivolts and milliamps; there are also dozens of volts.
Large signals of thousands of volts and hundreds of amps; both low-frequency DC signals and high-frequency pulse signals, etc. After forming a system, it is often found that transmission and interference between instruments and equipment cause mutual instability, causing system instability and even misoperation. Situation
In addition to the performance reasons of each instrument and equipment, such as resistance to electromagnetic interference, there is a very important reason that various instruments and equipment need to be grounded according to requirements and purposes. For example, for safety, the chassis needs to be connected to the ground; in order to make the circuit
For normal operation, the system needs a common reference point; in order to suppress interference and add a shield, the shield also needs to be grounded, but because of the potential difference between the reference points between the instrument and the device (that is, the common location of each device is different), it forms " Ground
"Circulation current" and "ground circulation" are the problems that must be solved in the process of signal processing.
(2) Natural interference
Lightning is a major natural source of interference, and the interference caused by lightning can be transmitted thousands of kilometers away. The time-domain waveform of lightning interference is a large spike on a background of a series of random pulses. Cosmic noise is electricity
Ionizing radiation is constantly changing throughout the day. Solar noise changes dramatically with solar activity. Natural noise will mainly interfere with communication, and lightning energy spike pulses can cause damage to many devices and should be avoided
Or reduce the degree of damage and reduce losses.
(3) Human interference
The root cause of electromagnetic interference is a change in voltage or current in the conductor, that is, a larger dv / dt or di / dt. Dv / dt or di / dt can cause the conductor to generate electromagnetic wave radiation. On the one hand, people can use this feature to achieve specific functions,
For example, infinite communication, radar, or other functions. On the other hand, when an electronic device is working, dv / dt or di / dt in the conductor generates accompanying electromagnetic radiation. No matter what the subjective purpose is, objectively it causes pollution to the electromagnetic environment.
There are also some large-scale equipment (motors, frequency converters) that frequently switch on and off during the production process of factory enterprises. They will also cause some capacitive and inductive interference, which will also affect the normal display or acquisition of instruments. Any voltage and current surge
There will definitely be electromagnetic interference. Digital pulse circuit is a typical interference source. With the widespread application of electronic technology, the situation of electromagnetic pollution will become more and more serious.
2: Methods to resolve various interferences The first three factors of interference are interference sources, sensitive sources, and coupling paths. If one of these three elements is missing, electromagnetic compatibility problems will not exist. So start with these three elements. Find the most convenient solution,
Generally, interference sources and sensitive sources cannot be solved, usually from the coupling path, it is the most commonly used method. Such as adding shielding, filtering and other means. It is the most common and most troublesome to deal with ground circulation. Now this is the topic of discussion.
(1) The first method: all field devices are not grounded, so that all process loops have only one ground point and cannot form a loop. This method seems simple. However, it is often difficult to achieve in practical applications, because some equipment requires grounding.
Can guarantee measurement accuracy or personal safety. Some equipment may form new ground points due to long-term corrosion and wear or weather.
(2) The second method: make the potentials of the two ground points the same (as shown in Figure 1, let V1 = V2), but because the resistance of the ground is affected by many factors such as geological conditions and climate change, this method is actually It can't be done in practice.
(3) The third method: using a signal isolator in each process link to disconnect the process loop without affecting the normal transmission of the process signal, thereby completely solving the problem of the ground loop
3: Use the advantages of intelligent isolators. Use signal isolation methods in each process loop. You can use isolation cards such as DCS or PLC or on-site transmitters with isolation (some equipment can do this). You can also use intelligent isolators. to realise.
In comparison, using an intelligent isolator has the following advantages:
· In most cases, it is cheaper to use a signal isolator + non-isolated card than to use an isolated card
· Smart isolators are superior to isolation cards in terms of isolation ability and anti-electromagnetic interference.
· The intelligent isolator is flexible in application, and it also has functions such as signal conversion, signal distribution and interface conversion, which makes it more convenient to use
通常有单通道、双通道、通道间相互完全独立，构成系统的配置、日常维护更加方便 · Intelligent isolators usually have single channel, dual channel, and the channels are completely independent of each other, which makes the configuration and daily maintenance of the system more convenient.