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WEEK 06: MODES OF CONTROL Sections: On-Off | Proportional | P-Integral | P-Derivative | P-I-D Description. Sometimes called the Two-Position Control or usually, bang-bang. This modes is widely used because it is inexpensive, reliable and easy to adjust and maintain.
In the graph on the right, with a setpoint at 120°F, when the signal [represented by the black bold line] is less than 120°F, the valve is 100% open, or ON. While when the signal is greater than 120°F, the valve is 0% open, i.e., closed, or OFF. Universal Characteristic. Actual temperature or any measured value in an on-off mode of control, tends to oscillate around the setpoint, following a typical sinusoidal wave. This wave creates what is called the Overshoot and the Undershoot, the total of which is generally equal to the Differential Gap. More often than not, the overshoot is not necessarily equal to the undershoot. Differential Gap [D.G.]. Shown as the yellow band in the above graph, it is the smallest range of value the signal must pass through to cause the correcting device to go from one portion to the next. Usually expressed as a percentage of the full scale. The measured value must rise above the setpoint by a certain amount, where the error signal is thus positive, to close the valve or must drop below the setpoint by a ceratin amount, i.e., the error siganl becomes negative, to open the valve. Example Calculations. Given a controller specifications as 60°F to 300°F, and a 3°F to close or open. Compute for the full range, F.R., and the % differential gap, %D.G. Solution: For many On-Off controllers, the differential gap is fixed and usually less than 2% of full scale or full range. Some On-Off controllers, however, have adjustable differential gaps. Adjustable differential gap is advantageous because it help reduce the wear in the correcting device, but disadvantageous because the instantaneous measured value can stay further from the setpoint, the greater the gap. |
The final correcting device [FCD] has only two positions or operating states: the ON and the OFF. Thus, there is no middle ground. [A typical example is flipping a light switch, try manipulating it so that the rocker is barely on center, and you will find out that the lights will either go off or on.]