Exercise G
83
Alter the I setting to 50. Observe the changes in fluid level.
Experiment with other values, making notes of the effect on fluid level oscillation and
level control.
With derivative control action, the controller gives an output that is proportional to the
derivative of the rate of change of the error. The output is related only to the rate of
change, not to the magnitude of the error. Derivative control action cannot be used
alone, but must be combined with another action such as proportional control action.
When used with proportional action, derivative control can eliminate excessive
oscillation. It cannot eliminate offset errors inherent in proportional action
Without logging the data:
Change the I setting back to 0 (zero).
Alter the setting for D to 1, and observe the variations in fluid level.
Alter the D setting to 5. Observe the changes in fluid level.
Leaving P and I constant, experiment with different settings of D while making notes
of the effect on fluid level oscillation and level control.
Return the D setting to 0.
Pick two illustrative values for each of the P, I and D settings.
For each of these values, create a new results sheet and start data logging. Set the
new value and log the result until the oscillations stabilise, or for five minutes if this is
longer. Return each parameter to its original value after investigating the two different
settings, and change only one value for each data collection run.
Proportional, integral and derivative control actions may be combined to eliminate
offset, reduce maximum deviation and minimise the frequency of oscillation. Finding
the optimum values of P, I and D for a particular process is often referred to as tuning
or 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:
Create a new results sheet by selecting the
icon in the tool bar of the software.
Set the set point level to 0mm and allow the process vessel to empty.
Set the software to provide On/Off control (set P to 0).
Set the set point to 150mm (this allows room for considerable overshoot during the
initial tuning process).
Select the
icon to begin data logging.
Wait as the fluid level rises to the set point and overshoots. Continue logging as the
controller operates the PSV and the fluid level falls again to below the set point value.
Wait until the fluid level begins to rise once more, then select the
icon to finish
data logging.
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