Judgment on the positive and negative effects of the flow control regulator
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Let’s first talk about how I made my judgment; everyone, please share if you have any different opinions. PV: Measured value; SP: Set value; OP: Output. 1. When PV > SP, OP increases as a result ； When PV > SP, OP decreases, which is a reaction. (Note: The positive and negative action of the regulator has nothing to do with whether the valve is open-air or closed-air.) The air-on/air-off setting is related only to the DCS output channel configuration; for air-on valves, the DCS output channel is configured in forward mode, with 0% corresponding to 4 mA ； If it is a gas shut-off valve, the configuration should be set to reverse mode, with 100% corresponding to 4 mA. 2. I don’t know how others make their decisions, but following the first method for configuring the forward/reverse operation of the DCS control circuit generally works every time; disagreements only arise in the case of flow control circuits. According to the first rule, when traffic PV > SP, the valve must be closed, which means OP decreases; therefore, it is definitely a reactive effect. But as a result, all the flow regulators are operating in reverse, which makes me feel uneasy.
Isn’t it obvious as long as the loop forms negative feedback?
“When the traffic PV is greater than SP, the valve must be closed, which means OP decreases; therefore, it is definitely a reactive effect. ”This statement is not accurate; when the flow rate PV is greater than SP, the valve closes. However, OP does not necessarily decrease – it depends on whether you choose an electrically open-type or electrically closed-type valve, or a pneumatically open-type (pneumatically closed-type) valve. Therefore, both positive and negative reactions are possible.
1 PID module: When the error (PV-SP) increases, the PID output is in direct mode; when the error (PV-SP) decreases, the PID output is in reverse mode. 2 AO module: 4mA-20mA corresponds to 0%-100% in direct mode, and 4mA-20mA corresponds to 100%-0% in reverse mode. 3 If you change the AO operation mode to reverse, then the flow control PID module can also be set to direct mode, which will give you more confidence: lol 4 I think as long as the valve is fully open when OP is at 100% and fully closed when OP is at 0%, it doesn’t matter what the direction of operation is
Determination of the forward and reverse action of the regulator in a single-parameter control system: (1) It is assumed that a gas-open control valve is “+A”, while a gas-close valve is “-A””； (2) When the control valve is opened, the controlled parameter increases to “+B”; when it decreases to “-B”, then AB=“+”, and the regulator is set with reverse action ； AB=“——” indicates a positive action of the regulator.
In a cascade control system, the operation mode of the secondary regulator is the same as that of the selection method in a simple control system； The operating mode of the main regulator can be determined as follows: when the primary and secondary variables increase (or decrease), if the direction of action of the control valve is the same, the main regulator should be selected with a \"reverse\" action ； Otherwise, select “Positive: Effect.”
You’re right; that’s how I judge it as well. Flow regulation is always reactive, that’s correct.
Use the on-air and off-air configurations of DCS for output; when configuring the regulator’s direct or reverse action, it can be considered as follows: if the input increases, the output should also increase, which is direct action, while if the input decreases, the output should decrease, which is reverse action. If it is a single regulator, then consideration must be given to air-on and air-off functions. I’ve never worked with PLCs, so I’m not sure if there are preset parameters for air-on/Air-off mode.
A PLC combined with a PID module can be used for control; it’s essentially a regulator, and the principle is the same – there is an option to choose between positive and negative feedback...
Find a book on self-control to read; whatever is discussed there are all mature techniques.