Exercise L
105
Alter the P setting to 1%. Observe the changes in fluid level.
Experiment with other values, making notes of the effect on fluid level oscillation and
level control.
Return the P setting to 5%.
With integral control action, the controller gives an output that is proportional to the
time integral of the error. Integral control action can potentially be used alone to
control a process, but is normally used in conjunction with proportional action. It can
be used with proportional action to eliminate offset. It can also cause higher
maximum deviation and a longer response time than with proportional action alone.
Without logging the data:
Alter the setting for Integral Time Constant (I) to 1s, and observe the variations in
fluid level.
Alter the I setting to 10s. Observe the changes in fluid level.
Experiment with other values of I, making notes of the effect on fluid level oscillation
and level control.
Return the I setting to 0s.
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:
Alter the setting for D to 1s, and observe the variations in fluid level.
Alter the D setting to 5s. 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 0s.
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.
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