Exercise U
141
Disturbances
Within the software it is possible to introduce disturbances in the temperature inside
the process vessel by changing the flowrate of water through the vessel:
In the software, select the
icon to create a new results sheet.
Select the
icon to begin data logging.
In the software, increase the PSV setting by 20%. Open the drain valve at the base
of the process vessel enough to balance the flow in and out of the vessel, so that the
fluid level in the vessel remains constant.
Log the temperature for 10 minutes.
Increase the PSV setting by a further 20%, and open the drain valve more to
maintain the fluid level within the vessel.
Continue to log the temperature for another 10 minutes, and then select the
icon
to finish data logging.
Set the PSV back to its original setting and close the drain valve.
Temperature Set Point
Investigate the effect of changing the set point temperature:
Create a new results sheet by selecting the
icon.
In the software, alter the temperature set point to 40°C. If the initial Set Point was not
30°C then a value 10°C higher than the initial value should be used instead.
Select the
icon to begin data logging.
Log the temperature for 10 minutes, and then select the
icon to finish data
logging.
Repeat the procedure, altering the set point and recording the change in temperature
over time. Suggested values for the temperature set point value are 50 and 60°C, but
more values may be attempted if laboratory time allows. Remember to create a new
results sheet for each set of data. Note that the heater circuit includes a safety cut-
out that will shut off power to the heater if the temperature in the process vessel
exceeds approximately 80°C. For this reason set point values should be kept well
below this temperature.
Return the temperature set point to its original value.
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
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