What does the forward and reverse action of a control valve regulator refer to?
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I have always been unclear about the forward and reverse actions of the actuator in pneumatic diaphragm control valves; seeking an explanation.
This seems to be the difference between open-air and closed-air types
Regulator action: The output increases as the deviation (measurement – setpoint) increases, which is positive action; otherwise it is negative action； Pneumatic diaphragm control valve: An increase in the input signal causes the valve stem to extend, which is a positive action; the opposite is a negative action.
The positive and negative actions of a control regulator are defined in terms of the input and output; the difference between the input signal and the setpoint signal is known as the error. When this error increases, the output increases, and this is characteristic of a positive-action regulator; conversely, it is the case for a negative-action regulator. For control valves, when the input increases and the valve stem moves downward, it is a direct-acting control valve; otherwise, it is an inverse-acting control valve. In a control system, the entire control system should be negative feedback.