Exercise L
107
Return the value of P to the basic value obtained from the initial measurements.
Integral control can be adjusted to reduce offset in the controlled fluid level. If a
significant offset is observed after changing the value of I, reduce the value.
Investigate the results of any change by plotting a graph of the fluid level as it rises
from 0mm 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
rapid or large after changing the value of D, reduce the value. Investigate the results
of the changed value by plotting a graph of the fluid level as it rises from 0mm and
comparing this to the initial results.
Leaving the optimised value of D, set the values of P and I to the optimised values
found by experiment. Perform a final test run, logging the fluid level over time as it
rises from 0mm. Compare this to the initial graph.
Feed Rate
Create a new results sheet by selecting the icon in the tool bar of the software.
Select the
icon to begin data logging.
To change the inlet feed rate, reduce the setting on the PSV to 80%.
Wait until the oscillations in fluid level stabilise in magnitude and duration, then select
the
icon to finish data logging.
Repeat the feed rate exercise by reducing the PSV setting in steps of 20% until the
fluid level in the vessel no longer returns to the Set Point level (the drain rate
exceeds the inlet flow rate). For each step create a new results sheet before starting
data logging, and continue logging until the oscillations stabilise.
Effect of Vessel Volume
In the software, set the PSV setting to 0%. This will stop flow into the large process
vessel.
Fully open the drain valve in the base of the large process vessel and allow the water
to drain from the vessel.
Carefully remove the lid on top of the large process vessel, and fit the removable
inner cylinder, pressing it down firmly to seal the bottom edge. This reduces the
effective volume of the process vessel.
Replace the lid and partially close the drain valve.
Repeat the procedure described at the start of the exercise, remembering to create a
new results sheet before commencing data logging.
If there is sufficient laboratory time then the investigations of disturbances, Set Point
value, PID settings and feed rate may be repeated at this reduced volume.
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