Could everyone help check if this working mechanism is reversed?

No problems were found; this reaction is the one associated with the PID control, that is, when PV > SP. The PID output for the valve decreases, and similarly, PV…

FO is positive acting; the flow control valve is also positive acting, so the overall effect is positive acting – it’s a mistake in the design specification.

1. The direct-acting and reverse-acting nature of the valve has nothing to do with FO or FC; FO can be either direct-acting or reverse-acting. Air-operated valve: It requires compressed air to open, and it closes when the air supply is cut off. (FC) Air-operated valve: It requires compressed air to close; the valve opens when the air supply is cut off. (FO) Direct action: When the actuator moves downward as the pressure of the actuating gas increases, this is referred to as direct action. Reaction: The upward movement of the valve stem by the actuator as the pressure of the actuating gas increases is called reaction. 2. The direct and reverse actions of the controller in the system loop are related to FO and FC. Direct Control Action: When the input (error PV-SV) increases, the output (MV) increases, which is a positive action ； In simple terms, as PV increases, MV increases as well; when PV decreases, MV also decreases. Reverse Control Action: When the input (deviation PV-SV) increases, the output (MV) decreases, which is a positive action ； In simple terms, when PV increases, MV decreases; when PV decreases, MV increases. A single-loop system consists of four components: the regulator, the transmitter, the valve, and the process being controlled. For the loop to be controllable, it is necessary to maintain negative feedback, which means that the product of these four components must be negative. Both the transmitter and the controlled object are positive, FO is negative, and FC is positive. Therefore, it is concluded that the control effect should be positive. 3. For a cascade control system to operate properly, both the primary and secondary loops must have negative feedback. In a cascade control system, the choice of how the secondary controller operates is determined based on requirements such as process safety; after selecting whether the control valve should operate in an open-or-close manner, it is decided in accordance with the principle of forming a secondary feedback system with the secondary loop. Therefore, the operating mode of the secondary controller is related to the characteristics of the secondary object and whether the control valve operates in air-open or air-close mode. In a cascade control system, the choice of how the main controller operates is determined entirely by the process conditions, and it has nothing to do with whether the control valve is of the air-open or air-close type, nor with the operation mode of the secondary controller. In other words, it is sufficient to select a main controller whose operating direction is opposite to that of the main process variable, based on the characteristics of that variable. Therefore, if it is the secondary loop of cascade control, the control effect remains positive.

Thank you for your advice. However, the information provided in the design indicates a reverse orientation – could this be considered a design error?

This involves the issue of defining the forward and reverse actions of the controller. Basic definition: Deviation e ↑→→ output u ↑, positive action ； Deviation definition: Deviation e = measured value z – given r; as deviation e increases, output u decreases, as a reaction ； Extended definition: Direct action: Measuring z↑→→produces output u↑, direct action ； The output moves in the same direction as the change in the measurement signal ； Given r↓→→output u↑, positive effect ； The output moves in the opposite direction to the change in the given signal ； Extended definition: Reaction: Measuring z↑→→output u↓, reaction ； The output moves in the opposite direction to the change in the measurement signal ； Given r↓→→output u↓, positive action ； The output moves in the same direction as the change in the given signal ； The definition mentioned above is the instrumentation industry’s definition of the forward and reverse action of controllers. This definition is currently used in the vast majority of DCS systems. The system definitions are different, with the main difference lying in the definition of deviation. The system deviation is defined as: deviation e = given r – measured z; thus, the two definitions for the controller’s positive and negative actions differ by a ‘–’ sign. Currently, some DCS manufacturers have taken note of this difference and unified the two definitions into a system definition; to the best of my knowledge, the controller forward/reverse action in Siemens’ PCS7 uses this system definition. Therefore, regarding the role of the flow controller in this flow control system, according to the definitions provided by most DCS manufacturers, it should be a positive-action controller.

Could you provide the entire diagram, including this cascade main circuit?