How should the direct and reverse actions of the controller and control valve be selected in the control loop?
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As the title says. In a specific control loop, how should the positive and negative actions of the controller and those of the control valve be determined? Is only one selected pair of reactions capable of meeting the process requirements? Can the control objective be achieved by using an opposite control action? I hope fellow sailors will not hesitate to share their insights!
I recall that someone discussed this topic; take a look: http://bbs.hcbbs.com/thread-996299-1-1.html The relationships and differences between the forward and reverse actions of regulators (controllers) in control valves, those of positioners, and those of valves
The air-open and air-close (positive or negative action) of the control valve are determined based on the safety requirements of the process. The positive or negative action of the regulator is determined according to the needs of the control loop, for example, to make the control loop a negative feedback control system.
The setting of the positive and negative feedback in the control loop begins with the selection of the actuator, taking into account safety and process requirements; thereafter, the entire control system must form a negative feedback loop.
The air-open and air-close modes of control valves are intended to ensure process safety, right? Is there any relationship between the positive/negative action and the air-open/air-close modes?
Control valves also have direct and reverse action; when determining the mode of operation for system control, it is necessary first to identify the type of action of the control valve. A control valve with its stem moving downward when the signal increases is a direct-acting valve, while one with the opposite behavior is a reverse-acting valve.
Got it. Generally, when the air line is above the valve head, it is of positive action; when it is below, it is of reverse action
Pneumatic control valves are classified as air-open and air-close, based on the need to prevent equipment from overheating or overpressuring in the event of issues with the air supply circuit. Generally, the air for feed and fuel gas is turned on, while the air for discharge is turned off. The positive and negative actions of the controller are determined by the actual operational requirements