Instrumentation: [Question of the Week] Week 46, 2011: How are the forward and reverse actions of actuators and regulators defined?

An actuator usually refers to a control valve, which consists of an actuating mechanism and a control mechanism. Actuators are divided into direct-acting and reverse-acting types. Such as pneumatic diaphragm actuators: when a signal is received, they cause the push rod to move downward, which constitutes positive action and results in the valve closing in a pneumatic manner. When a signal is received, it drives the push rod to move upward, which constitutes a reaction that results in the valve operating in an open position.

When the input signal increases (or decreases), and the output of the regulator increases (or decreases) accordingly, it is positive action. Secondly, when the input signal increases (or decreases) and the output of the regulator decreases or (increases), this is known as feedback. The positive and negative actions of the regulator are used to calculate for forming a closed-loop negative feedback system.

Positive action of the actuator – As the air pressure increases, the valve stem moves downward; this is considered positive action. Conversely, as the air pressure increases, the valve stem moves upward, which is negative action. Positive action of the regulator-----The output of the regulator increases as the positive deviation increases. If it is a negative deviation, the situation is reversed ； “Inverse action——the output of the regulator decreases as the positive deviation increases. If it is a negative deviation, the situation is reversed.

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 factor of the control valve: The amplification factor 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.

This post was last edited by denghl on 2012-12-12 at 21:21. Reply 1#: For the actuators in Baghdad, the air-operated mode is of positive action type, while the air-shut mode is of reverse action type. When the controlled parameter (i.e., the signal sent by the transmitter) increases, and the output of the regulator also increases, this is referred to as positive action; if the output of the regulator decreases as the controlled parameter increases, it is called reverse action

This post was last edited by denghl on 2012-12-12 21:21. The actuator consists of an actuating mechanism and a regulating mechanism. An increase in the input signal results in a decrease in the output – this is a feedback regulator; conversely, it is a feedforward actuator. For a regulator, according to the standard rules, as the measured value increases, the output increases; when the amplifier 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.

This post was last edited by denghl on 2012-12-12 21:21. Reply 1# Baghdad 1. Regulator: For a regulator, according to standard rules, as the measured value increases, the output also increases; when the amplifier 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. Direct and reverse action of the actuator: An increase in the inlet air pressure causes the valve stem to move downward, which is considered direct action; the opposite case is reverse action.

This post was last edited by denghl on 2012-12-12 21:21. Definition of the forward/reverse action of actuators: An actuator that operates when air is supplied (or when electricity is applied) has forward action, while one that operates when air is stopped (or when electricity is cut off) has reverse action； That is, when the input signal increases, an effect in which the action is enhanced is a positive effect; the opposite is a negative effect ； Definition of positive and negative action of a regulator: When the parameter being regulated (i.e., the signal sent by the transmitter) increases, an increase in the regulator’s output is referred to as positive action; whereas if the regulator’s output decreases as the parameter being regulated increases, it is called negative action.

This post was last edited by denghl on 2012-12-12 21:21. The direct action of a regulator is such that the greater the deviation between the actual value and the set value, the larger the output current signal; its reverse action is such that the greater the deviation between the actual value and the set value, the smaller the output current signal becomes. Control valves can be positive-acting air-open valves, positive-acting air-close valves, reverse-acting air-open valves, and reverse-acting air-close valves. Positive action refers to the situation where, as the air supply pressure increases, the actuator pushes the valve stem downward, while negative action refers to the situation where, as the air supply pressure increases, the actuator pushes the valve stem upward.

This post was last edited by denghl on 2012-12-12 21:21. When the input signal increases (or decreases), and the output of the regulator increases (or decreases) accordingly, it is called positive action. Secondly, when the input signal increases (or decreases) and the output of the regulator decreases or (increases), this is known as feedback. 1. Pneumatic diaphragm actuator: when a signal is received, it drives the push rod to move downward, which constitutes a positive action and results in the valve closing in a pneumatic manner. 2. When a signal is received, it drives the push rod to move upward, which constitutes a reaction effect and results in the valve operating in an air-open mode.