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On-Off |
Proportional |
P-Integral |
P-Derivative |
P-I-D
Proportional-Integral-Derivative Mode of Control
Description.
This is the most popular due to a number of applications and most expensive mode of control. The final correcting device's position is determined by the magnitude of the error signal [proportional part], the time integral of error [integral part] and the time rate of change of error [derivative part]. This mode of control is sually applied for very rapid load changes and long time delay.
Principle of Operation.
The proportional part positions the valve in proportion to the magnitude of the error signal. The derivative part, consists of a capacitance and a resistance. If the error signal changes slowly, the position pot wiper will follow rather slowly, thus the capacitor at the derivative part can catch up and therefore it can charge. If the error signal changes rapidly, the position pot wiper will follow rapidly, thus the capacitor cannot catch up with charging. The integral part, consisting of a capacitance and a resistance but in reverse compared to the P-D part, senses if the offset is still persisting and moves the valve in the same direction, thereby reducing the offset. The output of the integral part is the input tot he derivative part. When the error signal is zero, the valve stops.
Time Constant [TC].
For Derivative TC=0, large errror means a long time of correction. Only the integral part is effective. For a short derivative Time Constant, initial error is small, thus the recovery is earlier. For a long derivative time constant, very small error is created thus the recovery is almost instantaneously. Too much of the derivative part, however, the rate time will be too high and the controller will cause oscilations on the position of the valve.
Proportional-Derivative
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