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Why Do Control Systems Need Signal Isolators?

1. The principle of signal isolator function


In the production process, whether it is a control system or a monitoring system, it may face interference from complex environments, or conditions such as short circuit, overvoltage, unknown pulse and the voltage pulse output may also need to be considered. Therefore, the signal isolator function that protects the lower control loop, weakens the impact of environmental noise, suppresses grounding or equipment interference, has the functions of voltage limiting and rated current, and protects the input, output and communication interfaces of the control system has become an indispensable part of the control system.


The principle of the signal conditioning isolation is to convert the signal of the transmitter or the instrument through the modulation and conversion of the semiconductor device, and then realize the isolation conversion through the light-sensing or magnetic-sensing device, and then implement the demodulation and convert it back to the original signal before isolation. At the same time, the isolated signal power supply is isolated to ensure the absolute independence between the converted signal, power supply and ground. Meantime, the interference signal superimposed on the measured value is filtered, and the signal is matched according to the input and output requirements of the control system. Therefore, isolation, amplification, filtering and matching are the functions of the signal isolator.


Some DCS modules have their own photoelectric isolation function, which can fully meet the normal working requirements in some systems; however, the system cannot always be in normal working state. Once an accident occurs and the device is damaged, 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 according to the actual situation.


2. The classification of signal isolators


Signal isolators are divided into active signal isolators and passive signal isolators.


Active signal isolators are powered by a separate power supply to ensure that the isolators work well. Modules require an active signal on the input side, and on the output side they provide a filtered and amplified signal. According to the situation of application, the input/output and the power supply are isolated from each other. Active signal isolators include three-terminal isolation, input terminal isolation and output terminal isolation.


The passive signal isolator provides an additional and substantial convenience. It does not require additional power supply. The working power of the module is provided through the input or output loop, and its internal circuit consumes very little current, which does not affect the correct transmission of signals. According to the power supply mode of the signal isolator, it is divided into input side power supply, output side power supply, passive feeding, etc.


3. How to choose a signal isolator


The signal isolator is located between the two types of signal conditioning system channels, so in terms of the selection of the isolated signal conditioner, you must first determine the input and output functions, and at the same time, the input and output mode of the isolator should be adapted to the 1:3 mode of the front and rear channels. In addition, there are many important parameters such as accuracy, power consumption, noise, dielectric strength, bus communication function, etc. related to product performance. In short, applicability, reliability, and cost-effectiveness are the main principles for choosing an isolator.

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