Exercise AA
173
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.
Holding Tube
The system may be modified by the addition of a holding tube after the process fluid
exits the coil. A suitable tube has been provided with the PCT40 base unit.
In the Pump Control, set the Mode of Operation to Manual and the Manual Output to
0%.
In the Heater Control, set the Mode of Operation to Manual and the Manual Output to
0%.
Disconnect the flexible drain tube from the exit to the coil, and connect the holding
tube. Connect the other end of the holding tube to the fitting for temperature sensor
T4, situated behind the large process vessel.
Repeat the investigations of process fluid flow rate and water bath temperature
described earlier in this exercise, but this time controlling the exit temperature for the
holding tube, T4. The pump controller should be reset to use T4 as the process
variable instead of T3 (select T3 from the drop-down selection box). Remember to
create a new results sheet for each set of results.
If laboratory time permits then the investigations of disturbances may also be
repeated.
When you have finished, set the gear pump to 0% and the heater power to 0%.
Switch off SOL 1.
Results
For easy identification or results, it is suggested that each results sheet is renamed
with a descriptive title (e.g. ‘T1 Set Point’, etc.). The entire workbook should be saved
with a suitable filename for future reference (e.g. PCT40 Exercise AA).
Each set of data not already graphed as part of the procedure should be plotted on a
graph of temperature T3 against time.
Make a comparison of each variation in the experimental conditions against the
graph of the initial set of results, noting the difference in response. Comment on the
results obtained by varying the values of P, I and D, and on the ease of optimisation
of the process. If the level control exercises have already been performed, compare
the ease of optimisation for temperature control with the optimisation for PID level
control. Discuss the suitability of PID control for regulating process temperature.
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