Why do control systems require signal isolators?
The principle of signal isolator：
The principle of a signal isolator is to modulate and convert the signals from transmitters or instruments through semiconductor devices, then achieve isolation and conversion through photo-sensing or magnetic-sensing devices, and then demodulate and convert them back to the original signals before isolation. At the same time, the power supply for the isolated signals is isolated and processed. The signals, power supplies, and grounds after isolation are independent. At the same time, interference signals superimposed on the measured values are filtered, and the signals are matched 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 are equipped with photoelectric isolation function, which can fully meet the normal operation requirements in some systems. However, the system may not always be in a normal working state. Once an accident occurs and causes damage to the components, replacing the isolator is more time-saving and cost-effective than replacing the DCS module. Therefore, the instrumentation and control department suggests that it is still necessary to determine whether to install an isolator based on the actual situation.
Classification of Signal Isolators：
Signal isolators are classified into active signal isolators and passive signal isolators.
1，Active signal isolator
The active signal isolator is powered by an independent power supply to ensure its excellent performance. The module requires an active signal at the input side, and it provides filtered and amplified signals at the output side. The isolation between the input/output and the power supply is achieved according to the application requirements. The active signal isolator includes three-terminal isolation, input-end isolation and output-end isolation.
Three-terminal isolation
Three-terminal isolation only requires one power supply, which is isolated from the measurement circuit. Modules using this technology for isolation have all components connected at the input, output, or power terminals not interfering with each other, and the three terminals are also electrically isolated from each other accordingly.
Input isolation
Modules adopting this isolation technology should protect the electronic devices connected to the output side (such as the input card of the controller) from various on-site interferences. Therefore, the input end and the equipotential output end and the power supply part are electrically isolated.
Output isolation
Modules adopting this isolation technology should protect the electronic devices connected on the input side (such as the output card of the controller) from various disturbances in the field. Therefore, the output side and the equipotential input side and the power supply part are electrically isolated.
2，Passive signal isolator
The passive signal isolator offers an additional and substantive convenience. It does not require an extra power supply. The working power of the module is provided through the input or output circuits. The current consumed by its internal circuits is extremely small and does not affect the correct transmission of the signal. According to the power supply mode of the signal isolator, it can be classified as input-side power supply, output-side power supply, passive feed, etc.
Input-side power supply
When this isolation technology is adopted, these modules obtain the required energy from the active input circuit (such as an electromagnetic flowmeter or a control system output card) for signal transmission and electrical isolation. The output side provides processed current signals for control or regulation.
Output-side power supply
When this isolation technology is adopted, these modules obtain the required energy from the active output circuit (supplied by the auxiliary power source from the control system input card) for signal transmission and electrical isolation.
Passive feedthrough isolator
When this isolation technology is adopted, these modules obtain the required energy from the active output circuit for signal transmission and electrical isolation. The passive feedthrough isolator supplies this energy obtained from the output circuit to another passive detection probe (such as a pressure transmitter) connected on the input side. The detection probe emits an active signal by using the provided energy, which is electrically isolated through the passive feedthrough isolator and output from the output side.
How to Select Signal Isolators：
The isolator is located between two system channels. Therefore, when choosing an isolator, one must first determine the input and output functions, and at the same time ensure that the input and output modes of the isolator (voltage type, current type, loop power supply type, etc.) are adapted to the 1:3 mode of the front-end and rear-end channels. In addition, there are many important parameters such as accuracy, power consumption, noise, insulation strength, and bus communication function that are related to the product performance. For example, noise is related to accuracy, power consumption and heat generation are related to reliability. These need to be carefully selected by users. In conclusion, applicability, reliability, and product cost performance are the main principles for selecting isolators.