A rather beginner-level question: How does DCS achieve loop control?
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How does a DCS carry out loop control, and what is the path of the signals?
The simple control mechanism involves the field transmitter converting mechanical variables into 4-20mA signals, which are transmitted to the DCS junction box. From there, they travel via inter-panel cables to the AI analog input interface of the control cabinet; after being processed by the computer, they go to the AO analog output interface of the control cabinet and then to the junction box, from where they are sent to the field control valves.
Field measurement instruments, DCS cabinets, DCS, field applications, pressure, AI modules for display and control on the host computer, AO modules, actuators (regulating valves, etc.), flow rate, liquid level, temperature, switches, DI modules for display and control on the host computer, DO modules, actuators (pumps, etc.)
Which unit provides the power for signal transmission? Is it a dedicated power supply or is it supplied by the DCS cabinet?
For example, in terms of flow regulation control, the on-site flow meter sends a 4-20mA signal (the power supply depends on the requirements of the specific meter; transmitters are generally powered by the DCS). This signal enters the DCS as an AI measurement value, and a deviation calculation is performed in the controller. Based on this deviation value, a PID calculation is carried out, the result of which is sent to the analog operator, and from there to the AO output, which in turn drives the on-site control valve. This is a simple adjustment for one cycle of the control loop.