Regarding interlocking, cascade control, and proportional control.

With cascade and split-range control, you’ll understand it once you look at the PID diagram. You will understand interlocking by combining the PID diagram with the interlocking instructions.

I’ve learned it; it’s a bit difficult to put into words

It is recommended to find a book to read; it’s best for understanding and memorization

It’s difficult to understand interlocking, mainly due to the confusion with control (regulation). There is a significant difference between interlock systems and control systems. Interlock systems are generally based on digital control, with their control outputs being either 0 or 1. The control valves used in interlock systems are usually two-position valves that are controlled by solenoids (although in some cases, solenoids are added to the instrument air circuit of the control valve to achieve interlock functionality). Control valves in control systems are typically controlled by valve positioners, whose inputs are analog signals. For interlock systems that require high levels of reliability, a separate PLC, ESD, or SIS system is needed, separate from the DCS used for control, to ensure the reliability of the interlock system. Also, the primary instruments of the interlock system are usually separated from the control instruments. Interlock instruments are typically digital switches, such as level switches and temperature switches, thereby ensuring the reliability of the interlocks. A cascade control system has two closed loops in its structure; one closed loop is inside the other, and it is referred to as the inner loop or secondary loop ； The other closed loop is external, and is called the outer loop or main loop. The secondary loop is responsible for coarse adjustment during the control process, while the primary loop handles fine adjustment. Cascade control achieves control effects that are difficult to attain with a conventional single-loop control system by leveraging the cooperation of these two loops. Split-range control involves dividing the controller’s output signal into several signal segments; each segment controls a separate control valve, with each valve operating only within a specific portion of the overall range of the controller’s output signal. It is primarily used in systems that employ multiple control methods with logical relationships but share the same control objective, and it is designed to coordinate the operational logic of these different control methods.

3. Step control – For example, the step control of the nitrogen seal at the tank top (comprising a nitrogen seal control valve and an exhaust control valve in two circuits) involves dividing the automatic control range of 0-100 into two sections. The range of 0-50 corresponds to an opening degree of 100-0% for the nitrogen control valve (with the exhaust valve closed at values <50), while the range of 50-100 corresponds to an opening degree of 0-100% for the exhaust valve (with the nitrogen valve closed at values >50). When the pressure inside the tank equals the automatically set pressure, the value 50 is displayed; at this point, the opening degree of the nitrogen seal valve is 0 and that of the exhaust valve is also 0. When the pressure drops below the set value, the nitrogen valve opens (with the exhaust valve remaining closed). When the automatic control display shows 49, the opening degree of the nitrogen seal valve is 2 (when it displays 40, the opening degree is 20, and so on)； Conversely, when 51 is displayed on the automatic control system, the opening degree of the vent valve is 2 (when 60 is displayed on the automatic control system, the opening degree of the nitrogen seal valve is 20, and so on).