PID CONTROL EXPLANATIONS
Proportional Band
Proportional band is defined as the “band” (range) the process changes to cause the percent output power to change from 0% to 100%. The band may or may not be centered about the setpoint value depending upon the steady state requirements of the process. The band is shifted by manual offset or integral action (automatic reset) to maintain zero error. Proportional band is expressed as percent of input sensor range.
For T48, the ranges are fixed for the thermocouple and RTD types.
For P48, the range is set by the difference between “dSP2 and dSP1”.
Figure 30, Proportional Band
Example: Thermocouple type T with a temperature range of 600°C is used and is indicated in degrees Celsius with a proportional band of 5%. This yields a band of 600°C X 5% = 30°C.
The proportional band should be set to obtain the best response to a disturbance while minimizing overshoot. Low proportional band settings (high gain) result in quick controller response at expense of stability and increased overshoot. Settings that are excessively low produce continuous oscillations at setpoint. High proportional band settings (low gain) result in a sluggish response with long periods of process “droop”. A proportional band of 0.0% forces the controller into ON/OFF control mode with its characteristic cycling at setpoint (See ON/OFF Control, page 48, for more information).
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Integral Time
Integral time is defined as the time, in seconds, in which the output due to integral action alone equals the output due to proportional action with a constant process error. As long as a constant error exists, integral action repeats the proportional action each integral time. Integral action shifts the center point position of the proportional band to eliminate error in the steady state. The units of integral time are seconds per repeat.
Integral action (also known as “automatic reset”) changes the output power to bring the process to setpoint. Integral times that are too fast (small times) do not allow the process to respond to the new output value. This causes over-compensation and leads to an unstable process with excessive overshoot.
Integral times that are too slow (large times) cause a slow response to steady state errors. Integral action may be disabled by setting the time to zero. If time is set to zero, the previous integral output power value is maintained.
If integral action is disabled, manual reset is available by modifying the output power offset (“OPOF” initially set to zero) to eliminate steady state errors. This parameter appears in unprotected parameter mode when integral time is set to zero. The controller has the feature to prevent integral action when operating outside the proportional band. This prevents “reset wind-up”.
Note: The Proportional band shift due to integral action may itself be “reset” by temporarily setting the controller into the ON/OFF control mode (proportional band =0)
Figure 31, Integral Time
