During the production process, both control and monitoring systems may face interference from complex environments, or situations such as short circuits, overvoltage, and unexplained pulses. Therefore, isolators, which protect the lower control loop, reduce the impact of environmental noise, suppress grounding or equipment interference, limit voltage and current, and protect the input/output and communication interfaces of the control system, have become an indispensable part of the control system.
Principle of signal isolators
The principle of a signal isolator is to modulate and convert the signal from a transmitter or instrument using semiconductor devices, then isolate and convert it using optical or magnetic sensors, and finally demodulate and convert it back to the original signal before isolation. Simultaneously, the power supply to the isolated signal is isolated, ensuring absolute independence between the converted signal, power supply, and ground. It also filters interference signals superimposed on the measured value and matches the signal according to the input and output requirements of the control system. Therefore, isolation, amplification, filtering, and matching are the functions of a signal isolator.
Some DCS modules have built-in opto-isolation capabilities, which are sufficient for normal operation in some systems. However, systems cannot always operate normally. If an accident causes device damage, replacing the isolator is more time-saving and cost-effective than replacing the DCS module. Therefore, it is recommended to determine whether to install an isolator based on the actual situation.
Classification of signal isolators
Signal isolators are divided into active signal isolators and passive signal isolators.
Active signal isolator
Active signal isolators are powered by an independent power supply to ensure optimal operation. The module requires an active signal at the input side and provides a filtered and amplified signal at the output side. Depending on the application, the input/output is isolated from the power supply. Active signal isolators include three-terminal isolation, input isolation, and output isolation.
Three-terminal isolation
Three-terminal isolation requires only one power supply, which is isolated from the measurement circuit. Modules using this isolation technology will not have any interference between their components connected to the input, output, or power supply, and the three terminals are also electrically isolated from each other accordingly.
Input isolation
Modules employing this isolation technology should protect electronic devices connected to the output side (such as the controller's input card) from various field interferences. Therefore, the input terminals and the equipotential output terminals and power supply sections are electrically isolated.
Output isolation
Modules employing this isolation technology should protect electronic devices connected to the input side (such as the controller's output card) from various field interferences. Therefore, the output terminals and the equipotentially connected input terminals and power supply sections are electrically isolated.
passive signal isolator
Passive signal isolators offer an additional and substantial convenience, requiring no external power supply. The module's operating power is provided through the input or output circuit, and its internal circuitry consumes minimal current, not affecting the correct transmission of signals. Based on their power supply method, signal isolators can be categorized as input-side powered, output-side powered, or passively fed.
Input side power supply
When using this isolation technology, these modules draw the necessary energy from the active input circuit (such as an electromagnetic flowmeter or control system output card) for signal transmission and electrical isolation, while the output side provides a processed current signal for control or regulation.
Output power supply
When using this isolation technology, these modules draw the necessary energy from the active output circuit (preferably powered by an auxiliary power supply from the control system input card) for signal transmission and electrical isolation.
passive feeder isolator
When using this isolation technology, these modules obtain the necessary energy from the active output circuit for signal transmission and electrical isolation. The passive power supply isolator also supplies this energy obtained from the output circuit to a passive detection probe (e.g., a pressure transmitter) connected to the input side. The detection probe emits an active signal with the help of the supplied energy, which is electrically isolated by the passive power supply isolator and output from the output side.
How to select a signal isolator
Since isolators are located between two system channels, the first step in selecting an isolator is to determine its input/output functions. Simultaneously, the isolator's input/output mode (voltage type, current type, loop-powered type, etc.) must be compatible with a 1:3 connection between the front and rear channels. Furthermore, many other important parameters affect product performance, such as accuracy, power consumption, noise, insulation strength, and bus communication capabilities. For example, noise is related to accuracy, and power consumption and heat are related to reliability; these require careful selection by the user. In short, suitability, reliability, and cost-effectiveness are the main principles for selecting isolators.
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