Instrumentation: 【One Question per Week】Week 17 of 2011: Selecting the forward and reverse action of a regulator

This post was last edited by denghl on 2012-12-21 08:26. In accordance with the safety principles of production processes, the type of switch is determined first, and then, based on the characteristics of the controlled object, its positive or negative action is decided； Finally, in accordance with the principle that the product of the polarities of the static gain coefficients of the three components that make up the system must be positive, the forward and reverse actions of the regulator are determined.

This post was last edited by denghl on 2012-12-21 08:27. In accordance with the safety principles of production processes, the type of switch is determined first, and then, based on the characteristics of the controlled object, its positive or negative action is decided； Finally, in accordance with the principle that the product of the polarities of the static gain coefficients of the three components that make up the system must be positive, the forward and reverse actions of the regulator are determined. Specifically as follows: First, understand how the forward and reverse action directions of the controlled object, control valve, and regulator are defined The forward and reverse action directions of the controlled variable are defined as follows: a variable for which the controlled variable increases as the control variable increases is considered to have forward action ； Conversely, those in which the dependent variable decreases as the independent variable increases are considered to be reactive. The operating direction of a control valve is determined by its air-open or air-close type. A gas open valve operates in the forward direction, while a gas close valve operates in the reverse direction. If the input error signal of a regulator is defined as the measured value minus the set value, then a regulator whose output increases as the error increases is called a positive-action regulator ； Conversely, a regulator whose output signal decreases as the deviation increases is called a negative feedback regulator. To select the forward or reverse action of the regulator, the following two rules apply: 1. Air-operated control valves are assigned +A, while air-shut control valves are assigned -A ； 2. When the control valve is opened, the regulated parameter increases to +B and decreases to -B. Then when A.B=+, the regulator selects reverse action ； When A.B = -, the regulator operates in the positive direction.

This post was last edited by denghl on 2012-12-21 08:28. Setting: Gas-open control valve is +A, gas-close control valve is -A； When the control valve is opened, the regulated parameter increases to +B and decreases to -B; in this case, if A·B equals “+”, a regulator with reverse action should be used, while if A·B equals “–”, a regulator with direct action should be used

This post was last edited by denghl on 2012-12-21 08:28. Reply to 1#: In the PID control loop of Baghdad, the forward and reverse actions are not the same as those of the regulator; they are distinct concepts. Firstly, a pressure-operated valve is one that opens when a signal pressure is applied, and remains completely closed in the absence of such a signal ； The air control valve closes when a signal pressure is applied, and stays fully open in the absence of such a signal. From the perspective of the control system, a positive action is selected for the PID loop output when it is a air-operated open valve, while negative action is chosen when it is an air-operated closed valve. The positive or negative action of a regulator can be determined in a simple way: when PV (measured value) > SV (set value) and MV (valve position) needs to increase, it is positive action ； Conversely, when PV (measured value) > SV (set value), MV (valve position value) needs to be reduced, which is a reactive action. I’m not sure if this is correct; please forgive me, moderator. I’ll check the answer later

When KC*KV*KO is negative, negative feedback occurs; the amplification factor k0 is positive. When KV is at its maximum value, it corresponds to a positive state, while a minimum value corresponds to a negative state. Based on this, it can be determined whether the KC regulator operates in a positive or negative mode

This post was last edited by denghl on 2012-12-21 08:29. ① Gas-open control valves are marked +A, while gas-close control valves are marked -A； ②When the control valve is opened, the regulated parameter increases to +B and decreases to -B; in this case, if A·B equals “+”, a regulator with reverse action should be used, while if A·B equals “–”, a regulator with direct action should be used

This post was last edited by denghl on 2012-12-21 08:29, based on the implementation of negative feedback for a closed-loop system. First, two rules are established: ① Gas-open control valves are set to +A, while gas-close control valves are set to -A ； ②When the control valve is opened wider, the controlled parameter increases to +B and decreases to -B; in this case, A·B = “+”. If a reverse action is desired, the regulator should be selected so that A·B = “–”. In Figure 2-10, the controlled parameter is the liquid level. When the valve operates in an air-open mode, it corresponds to +A; if the valve is opened wider, the liquid level decreases, resulting in -B. Therefore, (+A)·(-B) = “–”, which means a positive action regulator should be used. http://www.tj88.cn/webedit/uploadfile/20100608155411780.jpg

This post was last edited by denghl on 2012-12-21 08:30. For a single-loop control system, the entire loop must form a closed-loop negative feedback system. For a simple single-loop system, it is required that the product of the amplification factors of the control valve and the regulator be negative; in other words, one of them must be positive while the other must be negative. When the control valve has a direct-acting actuator or an inverse-acting actuator, the regulator selects inverse action. When the control valve has a direct-acting reverse actuator or a reverse-acting direct actuator, the regulator is selected to be of the direct-acting type.

This post was last edited by denghl on 2012-12-21 08:31. In a single-loop control system, negative feedback control can be achieved as long as the product of the amplifier coefficient Kc of the regulator, the amplifier coefficient Kv of the control valve, and the amplifier coefficient Ko of the controlled object is positive. The signs for the regulator, control valve, and plant gain are specified as follows. 1. The sign of the regulator’s amplification coefficient: For regulators, according to standard rules, as the measured value increases, the output also increases; when the regulator amplification coefficient Kc is negative, this is referred to as positive action. As the measured value increases, the output decreases; when Kc is positive, this is referred to as feedback. 2. The sign of the amplification coefficient of the control valve: The amplification coefficient Kv of a control valve is defined as positive for air-operated open valves, and negative for air-operated closed valves. 3. The sign of the object amplification factor The object amplification factor Ko is defined as follows: if the control variable increases, the controlled variable also increases, then Ko is positive ； As the control variable increases, the controlled variable decreases, and Ko is negative. It can be seen that the method for determining the forward and reverse action of a regulator in a single-loop control system is as follows: first, determine the sign of the plant gain Ko; then, based on whether the control valve is of the air-open or air-close type, determine the sign of the control valve gain Kv; finally, since the product of Kc, Kv, and Ko must be positive, the action mode of the regulator can be determined

In a single-loop control system, negative feedback control can be achieved as long as the product of the amplifier coefficient Kc of the regulator, the amplifier coefficient Kv of the control valve, and the amplifier coefficient Ko of the controlled object is positive. The signs for the regulator, control valve, and plant gain are specified as follows. 1. The sign of the regulator’s amplification coefficient: For regulators, according to standard rules, as the measured value increases, the output also increases; when the regulator amplification coefficient Kc is negative, this is referred to as positive action. As the measured value increases, the output decreases; when Kc is positive, this is referred to as feedback. 2. The sign of the amplification coefficient of the control valve: The amplification coefficient Kv of a control valve is defined as positive for air-operated open valves, and negative for air-operated closed valves. 3. The sign of the object amplification factor The object amplification factor Ko is defined as follows: if the control variable increases, the controlled variable also increases, then Ko is positive ； As the control variable increases, the controlled variable decreases, and Ko is negative. It can be seen that the method for determining the forward and reverse action of a regulator in a single-loop control system is as follows: first, determine the sign of the plant gain Ko; then, based on whether the control valve is of the air-open or air-close type, determine the sign of the control valve gain Kv; finally, since the product of Kc, Kv, and Ko must be positive, the mode of operation of the regulator can be determined.