Regarding the meaning of positive and negative feedback in the PID module parameters of DCS systems

The forward or reverse action of the PID is determined based on whether the valve operates in an open-or-close manner and the process control requirements

Anyway, I memorized that \"positive action\" occurs when the controlled quantity decreases as the valve opens, while \"negative action\" occurs when the controlled quantity increases as the valve opens. As you said, temperature regulation has a reverse effect, so it must control the heat medium; if it’s a cold medium, then the effect is positive.

The term “great master” is truly apt. It clicked all of a sudden. . . . But I’d like to delve deeper into the principles to see if I can offer some insights

The method to be used is determined based on the way the valve opens and closes; for example, if it operates pneumatically, then the reaction mechanism is used

This post was last edited by Hao Liang Zhi Le on 2019-3-21 at 12:22. The purpose of having both positive and negative actions in the controller is to ensure that the loop operates as a negative feedback system. The forward and reverse action of the controller is related to whether the control valve operates on an air-open or air-close basis, as well as to the way in which the control variable affects the controlled variable, that is, to the positive or negative nature of the controlled system.

How to tell: If the parameters you want to control (pressure, flow rate, temperature) increase due to fluctuations and you need to open a valve in order to reduce these values, it is a direct-acting system (for example, if the pressure at the top of the tower is higher than the set value, the valve opens automatically to release gas and lower the pressure; this is direct action). Conversely, if the flow rate increases due to fluctuations, you need to close the valve to reduce the flow rate; this is the reverse effect. In one sentence: if the loop is automatic and the setpoint remains constant, a parallel change in the measured value trend and the output trend indicates positive action, while an opposite change indicates negative action. The simplest model: In water tank level control, controlling the inflow is a feedback mechanism, while controlling the outflow is a feedforward mechanism.

It has nothing to do with whether the valve operates on air opening or air closing; it’s mainly a requirement of the circuit

That’s how I understand it. For example, in a level control system for tower feed, if the liquid level is too high, it is necessary to reduce the valve opening; one level being high and the other low represents a feedback effect. When the liquid level is low, the valve needs to be opened wider; when it’s low, increasing the valve opening size is the opposite approach. In level control, when the liquid level at the outlet of the tower is high, it is necessary to open the valve wider; an increase in one value along with an increase in the other represents a positive signal. When the liquid level is low, the valve needs to be closed more tightly; a decrease in one value along with a decrease in the other also indicates a positive signal. In terms of temperature control, the feed material is used to lower the temperature. If the temperature inside the tower rises, it is necessary to increase the amount of feed and widen the valve opening; an increase in one value along with an increase in the other signifies a positive signal. If the feed is used to raise the temperature, then when the temperature inside the tower rises, the valve opening needs to be reduced; an increase in one value along with a decrease in the other indicates a negative signal

It is based on the most fundamental principle of negative feedback control in classical control theory.