Exercise AA
171
PID Settings and optimisation
To begin tuning it is necessary to set initial values which can be modified to improve
the control results obtained. Follow this simple procedure to find start point values
from which to begin optimisation:
Begin with the process vessel set up as described at the start of this laboratory
sheet. If the apparatus has been used recently before this section of the exercise it
will be necessary to cool the water in the process vessel before beginning this
section. This may be speeded up by increasing the flow rate through the coil and
turning off the heater:
Select the Heater Controller. Set Mode of Operation to Manual and Manual Output to
0%. Apply and close the window.
Select the Pump Controller and set Mode of Operation to Manual. Set the Manual
Output to 100% to allow full flow through the coil. When the temperature of the
process fluid (T1, monitored on the main display screen) has cooled to around 30 °C,
set the Manual Output to 0%. Apply and close the window.
Select the Heater Controller. Set Mode of Operation to Automatic. The Set Point
should be 50°C (or the original Set Point setting if this was higher). Apply and close
the window.
Set the controller to provide On/Off control (set P to 0%, and check that I, D and
Cycle Time are all set to 0s).
Set the temperature Set Point to 40°C (this allows room for overshoot during the
initial tuning process).
Set the controller to Automatic Control, Apply the new settings and close the
controller window.
Select the
icon to begin data logging.
Wait as the temperature rises to the set point and overshoots. Continue logging as
the controller starts the gear pump, increasing the flow rate and reducing the water
temperature. Continue logging until the controller switches off the pump and the
temperature begins to rise once more, then select the
icon to finish data logging.
Plot a graph of time against water temperature at the coil outlet, T3.
From the graph, determine the peak to peak variation, y, between the highest value
of the overshoot and the lowest value of the undershoot.
Calculate the time between these two values, t.
From the values y and t, starting values may be found for P, I and D as follows
P = y / 3
I = t
D = t / 6
Set these values in the software.
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