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Sections: On-Off | Proportional | P-Integral | P-Derivative | P-I-D

Proportional Plus Integral Mode of Control

Description. Popularly called the Proportional Plus Reset. The final correcting device's position is determined by the magnitude of the error signal [proportional part] and the time integral of the error signal [integral part]. Time integral is equal to the absolute value of the magnitude multiplied by the time the error persisted.

Principle of Operation. The proportional part positions the valve in proportion to the magnitude of the error signal. The integral part, consisting of a capacitance and a resistance, senses if the offset is still persisting and moves the valve in the same direction, thereby reducing the offset. When the error is zero, the valve stops.

A the capacitor, C, charges, the voltage in the resistance, R, decreases. When the capacitor, C, is fully charged, then the voltage drop at the resistance, V_R, becomes zero, therefore the valve stops.

Time Constant [TC]. The value of the time constant is derived from the relationship of the capacitance and resistance, through the use of transient analysis. When the time constant, TC, is large, i.e., the resistance is also large, the integral part is less effective, thus the controller acts like a typical proportional mode of control. When the time constant is small, i.e., the resistance is small, the control is more effective.

Reset Rate [RR]. This is the reciprocal of the Time Constant. Thus, when the reset rate, RR, is low, there is a large time contant. When the rate, RR, is high, there is a smaller time contant. Too large a reset rate, RR, will cause prolonged oscillations of the signal. The controller will act like a typical On-Off control.

ProportionalProportional-Derivative Mode

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