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DERIVATIVE TIME
Derivative action is used to shorten the process response time and helps to stabilize the
process by providing an output based on the rate of change of the process. In effect,
derivative action anticipates where the process is headed and changes the output before
it actually “arrives”. The derivative time is calculated in seconds. Increasing the derivative
time helps to stabilize the response, but too much derivative time coupled with noisy
signal processes, may cause the output to fluctuate too greatly, yielding poor control.
None or too little derivative action usually results in decreased stability with higher
overshoots. No derivative action usually requires a wider proportional and slower integral
times to maintain the same degree of stability as with derivative action. Derivative action is
disabled by setting the time to zero.
Integral time is defined as the time, in seconds, which corrects for any offset (between
setpoint and process variable) automatically over time by shifting the proportioning band.
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 (“MNL.r” initially set to zero) to eliminate steady state errors. The controller
has the feature to prevent integral action when operating outside the proportional band.
This feature is called
“antireset wind-up”
.
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PROPORTIONAL BAND:
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INTEGRAL TIME:
Proportional band is the area around the set point where the controller is actually
controlling the process; the output is at some level other than 100% or 0%.
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MANUAL RESET:
Proportional band is expressed in terms of degree centigrade.
If the proportional band is too narrow an oscillation around the setpoint will result. If the
proportional band is too wide the control will respond in a sluggish manner, could take a
long time to settle at set point and may not respond adequately to upsets.
Virtually no process requires precisely 50% output on single output controls or 0% output
on two output controls. The adjustment called manual reset allows the user to redefine the
output requirement at the setpoint. A proportioning control without manual or automatic
reset will settle out somewhere within the proportioning band but likely not on the setpoint.
34
User Guide
