Armfield Instruction Manual
162
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.
If time permits, create a new results sheet and repeat the optimisation process with
the gear pump set to a speed of 50% to circulate (stir or mix) the water in the process
vessel. Return the gear pump setting to 0%.
Effect of Process Lag
Process lag may be investigated using the holding tube provided with the PCT40.
Disconnect the tube from the outlet of the coil (labelled T3), and instead connect the
large diameter holding tube. Connect the other end to the temperature sensor T4,
mounted at the back of the plinth to the left of the large process vessel. Connect the
outlet of this fitting to the tube removed from the coil and direct the other end to drain.
The process variable monitored by the PID controller will now be the temperature at
the end of the holding tube, T4, although the temperature at the end of the coil, T3, is
also logged. To change the process variable in the software, select the PID control
box (labelled ‘Control’) on the mimic diagram screed. From the process variable
drop-down box, select T4. Leave the control variable as ‘Heater’ and the control
action as ‘Reverse’. The PID values may be left at their optimised values from the
previous section of the exercise.
Repeat the investigations of disturbances and set point change, now monitoring T4
instead of T3. Note any difference in initial speed of response and the time within
which the system settles. If time allows, the PID optimisation process may be
repeated with the holding tube in place, to investigate any difference in optimum
values due to process lag.
Results
For easy identification or results, it is suggested that each results sheet is renamed
with a descriptive title (e.g. ‘Flow Rate’, ‘Integral Setting’, etc.). The entire workbook
should be saved with a suitable filename for future reference (e.g. PCT40 Exercise
Y).
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 optimisations for PID level
control. Compare the results obtained using PID temperature control against those
using a simple on/off controller (as in Exercise W) and time proportioning control
(Exercise X). Discuss the suitability of PID control for regulating process temperature.
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