Monitoring and control in industrial processes require a variety of automated instruments, control systems and actuators. Industrial sites generally need to use two-wire transmission mode of the distributor, both for such as pressure transmitter, temperature transmitter and other primary instruments to provide 24V distribution power, but also to the input current signal acquisition, amplification, operation, and anti-interference processing, and then output isolated current and voltage signals for the secondary instruments or other instruments to use. This type of power distribution equipment used in the field is the signal isolator.
The signal isolator can solve the impact of multi-point grounding to form a "grounding loop", and solve the impact of electromagnetic interference and high-frequency signal infiltration, and the signal isolator has a certain isolation function, in a large range, can effectively protect the post-level equipment from lightning strikes and surges.
The use of signal isolator can solve the problem of voltage drop when the voltage signal is transmitted over a long distance due to the existence of resistance in the wire, resulting in signal distortion. The signal isolator can convert the voltage signal into the current signal to realize the long-distance signal transmission.
Using a signal isolator, it can receive standard or non-standard analog signal input from the field and output it to the control room or DCS in the form of a standard signal to achieve power, input and output isolation. The current signal, millivolt small signal or some special signal transmitted by the field sensor and transmitter cannot be directly sampled, and these signals need to be converted into a certain range of voltage signals to facilitate the collection of field data. The signal isolator can be easily applied to the various signals of the field instrument for conditioning, through isolation, and converted into industrial computers, DCS, PLC and other acceptable standard signals or user specified special signals, in industrial measurement systems, medical electronic equipment, power equipment and other aspects are widely used.
The signal isolator can receive signals from temperature sensors/transmitters, pressure sensors/transmitters, level sensors/transmitters, humidity sensors/transmitters, flow sensors/transmitters, gas sensors/transmitters, light sensors/transmitters, and more.
The following are general applications of signal isolators:
2. Causes of interference and solutions
The signal isolator of Taihua instrument is an amplifier of electrical insulation between input and output. It is widely used in measuring equipment, medical electronic equipment, power equipment and so on. Monitoring and control in industrial processes require the use of a variety of automated instruments, control systems and actuators. After the formation of the system, it is sometimes found that the signal transmission in the instrument and equipment interferes with each other, resulting in system instability and even misoperation. the
In addition to the performance reasons of each instrument and equipment itself, such as poor performance against electromagnetic interference, there are mainly other reasons.
2.1 Multi-point grounding affects the ground loop
Various reasons may be that the system connection has multi-point grounding, so that there may be a potential difference between the signal reference points between the devices, resulting in the formation of a "ground loop" resulting in signal transmission distortion.
According to theoretical analysis and practice, the signal isolation method is used in each process loop to disconnect the process loop without affecting the normal transmission of the process signal, so as to completely solve the problem of signal distortion caused by the ground loop.
2.2 Electromagnetic interference, high-frequency signal infiltration impact
In the industrial process monitoring system, the measurement signal is often unstable. 1. Caused by electromagnetic interference; Second, high-frequency signals may penetrate.
If the current signal output controls the frequency converter, the frequency converter high frequency penetrates into the signal, so that the control often makes the frequency converter and the valve work unstable and abnormal.
Solve electromagnetic interference, high frequency signal infiltration effect. Practice has proved that adding an appropriate isolator between two device signal connections is one of the most effective methods.
2.3 Equipment protection, protect safety, prevent misoperation and accident expansion
When the signal is transmitted over a long distance, when there is a lightning surge or high voltage pulse, it will cause damage to the sampling equipment (DCS, PLC, IPC machine and other valuable equipment). Using isolator, there is a certain isolation voltage withstand, so as to effectively protect the sampling equipment. Sensors are often used in medical electronic devices to measure patient contact. If the equipment fails, the patient will be electrocuted, resulting in a major accident. Therefore, in addition to the basic insulation of the power input part, the probe part in contact with the patient and the internal circuit also need to take insulation measures for double insulation. In power stations, substations, and other monitoring equipment, in case of emergencies, are equipped with double protection systems to improve safety. In view of the phenomenon of power lines touching the ground, short circuit and high common-state current caused by lightning in natural disasters, we can prevent the misoperation of monitoring devices and destructive accidents by electrical insulation measures.
Third, the application of signal isolator
There are many varieties of signal isolators, which are divided into single, double, three, four and one input, two, three, and four output signal distribution functions from the number of input channels. From the power supply mode, it can be divided into loop power supply type and independent power supply type.
3.1 Loop power supply type
Input and output are two wire, easy wiring, it DCS,PLC or display table provided by the power supply is isolated to the two-wire transmitter power distribution, at the same time, the two-wire transmitter generated
The 4-20mA signal is isolated and input to DCS,PLC or display table. It is particularly suitable for field use as a two-wire transmitter, requiring isolated input to DCS,PLC systems or display meters, and the input card of the input device has an internal power supply function, as shown below.
Two wire transmitter
But it has some disadvantages:
1, the isolator is equivalent to a load, after the isolator between the two ends of the isolation there is a voltage drop of not more than 6V, so it will reduce the distribution voltage of the two-wire transmitter, the transmitter is generally required to work 12V power supply.
For example: the power supply is 24V,RL=250Ω, when 20mA, the distribution voltage of the supply wire transmitter UO≈24V-0.02xRL-6≥13V, which generally requires the wire transmitter to work normally under 12V voltage.
2, the transmission accuracy relative to the independent power supply isolator is slightly worse, about 0.2%-0.4%F.S, special attention should be paid to the selection.
3.2 Independent power supply
This is the most commonly used distribution isolator with a two-wire transmitter, which requires independent power supply to the isolator, as shown in the figure below. Its characteristics are:
1, high transmission accuracy, up to 0.1%F.S.
2, the wiring mode is flexible, can be connected to two wire transmitter, three wire transmitter or current source signal, flexible and convenient to use.
3. Complete isolation between power supply, input and output to ensure high anti-interference performance.
Four, the application of one in two out signal distributor
In specific applications, we will often encounter a transmitter signal access to two or more receiving devices, if the series loop is used, the circuit at any point in the loop will cause the instrument on the entire loop no signal, and the sum of load resistance is easy to exceed the load capacity of the transmitter, but also must ensure that the reference point between the two load signals is not the same, as shown below.
According to theoretical analysis and practice, the most ideal solution to solve the above problems by using signal in each process loop is to use signal distributor. Convert one input into two isolated outputs with high precision and strong isolation ability, as shown in the figure below.
(1) The method of taking a voltage signal through a series resistor is based on the assumption that the input impedance of the receiving device is infinite, so the input impedance of the receiving device will inevitably produce an error in the measurement of the signal, and the more the number of parallel devices, the greater the error.
2, the longer the wire, the greater the voltage drop of the line resistance, the greater the impact on the actual voltage signal, therefore, the signal transmission distance can not be too long.
(3) Because the RFI/EMI signal is easily superimposed with the voltage signal, the connection is vulnerable to radio frequency/electromagnetic interference.
The most ideal solution to these problems is to use a signal distributor. Convert one input into two isolated outputs with high precision and strong isolation ability, as shown in the figure below.
① The two output signals can be the same or different, and the transmitter and each receiving device are completely isolated.
The failure of any receiving device will not affect the entire loop or the other receiving device.
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