Exercise P
121
Disturbances
Within the software it is possible to introduce disturbances in the temperature inside
the process vessel by passing cold water through the coil inside the vessel:
Create a new results sheet by selecting the
icon in the tool bar of the software.
Select the
icon to begin data logging.
Log for two minutes (with the software taking readings every 10 seconds), then set
the PSV to 50%.
Continue logging for 10 minutes, then set the PSV to 100%.
Continue logging for a further 10 minutes, then set the PSV to 0%.
Log for a final 10 minutes, then select the
icon to finish data logging.
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 (or 10 degrees higher than
the initial set point if a value other than 30 degrees was used).
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 vessel temperature exceeds
approximately 80°C. It is recommended that Set Point values are be kept well below
this value.
Once the temperature set point has been investigated, 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
section. This may be speeded up by passing cold water through the heating/cooling
coil by setting the PSV to 100%, as for investigating disturbances. Return the PSV
setting to 0% after cooling the water to 30°C or less.
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