In an application with one output assigned to heating and another assigned to cooling, each will have a separate proportional parameter. The heating parameter takes effect when the process temperature is lower than the set point, and the cooling parameter takes effect when the process temperature is higher than the set point.
Adjust the proportional band with Heat Proportional Band [`h;Pb] or Cool Proportional Band [`C;Pb] (Operations Page, Loop Menu).
Set Point | Overshoot |
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Temperature | Proportional Band | Droop |
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Proportional plus Integral (PI) Control
The droop caused by proportional control can be corrected by adding integral (reset) control. When the system settles down, the integral value is tuned to bring the temperature or process value closer to the set point. Integral determines the speed of the cor- rection, but this may increase the overshoot at startup or when the set point is changed. Too much integral action will make the system unstable. Integral is cleared when the process value is outside of the proportional band.
Adjust the integral with Time Integral [``ti] (Operations Page, Loop Menu).
Proportional plus Integral plus Derivative (PID) Control
Use derivative (rate) control to minimize the overshoot in a
Derivative action is active only when the process value is within twice the proportional value from the set point.
Adjust the derivative with Time Derivative [``td] (Operations Page, Loop Menu).
Reduced Overshoot
Set Point
Proportional Band
Proportional Band x 2
Temperature | Heating Slows |
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Dead Band
In a PID application the dead bands above and below the set point can save an application’s energy and wear by maintaining process temperature within acceptable ranges.
Proportional action ceases when the process value is within the dead band. Integral action continues to bring the process temperature to the set point.
Using a positive dead band value keeps the two systems from fighting each other.
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When the dead band value is zero, the heating output activates when the temperature drops below the set point, and the cooling output switches on when the temperature exceeds the set point.
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Watlow | • 61 • | Chapter 9 Features |