Exercise U
143
If the figure for Proportional Band obtained by this method is very small (less than
5%), and the Integral Time very large (more than 30s) then the method chosen for
finding approximate tuning values may not be suitable and the best value for
Proportional Band may be greater than 100%.
Change the temperature set point to 0°C, and cool the water in the vessel to 30°C or
less. At this point it is possible to temporarily increase the flow rate through the
vessel to accelerate the cooling process. Return the flow rate to its previous value
before continuing with the exercise.
Set the Proportional Band to 200%, the Integral Time to 60s and the Derivative Time
to 0s.
Create a new results sheet by selecting the
icon in the tool bar of the software.
Return the temperature set point to 40°C
Select the
icon to begin data logging.
Log the temperature as it rises and then settles around the set point value. Logging
should continue until any oscillations have settled- 10 minutes is suggested.
Select the
icon to finish data logging.
Plot a graph of temperature T1 against time and compare the shape of the graph to
that obtained using the values obtained from the On/Off control graph.
If a Proportional Band of greater than 100% gives less oscillation and overshoot than
the previous settings, leave the PID values as they are.
Proportional control sets the band within which the controller will vary time the heater
spends on and off. Outside this band the heater will be always on or always off. If a
slow response is observed, or large oscillations occur in temperature, reduce the
value of P. Investigate the results of the changed value by plotting a graph of the
temperature as it rises from a low value and comparing this to the initial results.
Return the value of P to the basic value obtained from the initial measurements.
Integral control can be adjusted to reduce offset in the temperature. If a significant
offset is observed, reduce the value of I. Investigate the results of any change by
plotting a graph of the temperature as it rises from a low value and comparing this to
the initial results.
Return the value of I to the basic value obtained from the initial measurements.
Derivative control can be adjusted to reduce excessive oscillation. If oscillations are
extreme, reduce the value of D. Investigate the results of the changed value by
plotting a graph of the temperature as it rises from a low value and comparing this to
the initial results.
From the graphs obtained, select values of P, I and D that give the best results.
Perform a final test run, logging the temperature over time as it rises from a low
value. Compare this to the initial graph.
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