Armfield Instruction Manual
188
Set the PSV to Automatic Control.
Wait as the pressure P3 rises to the set point and overshoots. Continue logging as
the controller adjusts the PSV, increasing the flow rate and thus affecting the
differential pressure across the orifice. Continue logging until the controller starts to
close the PSV and the pressure begins to change once more, then select the
icon to finish data logging.
Plot a graph of pressure P3 against time.
From the graph, determine the peak to peak variation, y, between the highest value
of the overshoot and the lowest value of the undershoot.
Calculate the time between these two values, t.
From the values y and t, starting values may be found for P, I and D as follows
P = y / 3
I = t
D = t / 6
Set these values in the software.
Create a new results sheet by selecting the
icon in the tool bar of the software.
Select the
icon to begin data logging.
Log the pressure as it rises and then settles around the set point value. Logging
should continue until any oscillations have settled.
Select the
icon to finish data logging.
From these new results, plot a graph of pressure P3 against time and observe the
shape of the graph.
Proportional control sets the gain of the controller. If a slow response is observed, or
large oscillations occur in temperature, reduce the value of P. Investigate the results
of the changed value by plotting a graph of the pressure as it rises from a low value
and comparing this to the initial results.
Integral control can be adjusted to reduce offset in the pressure. If a significant offset
is observed, reduce the value of I. Investigate the results of any change by plotting a
graph of the pressure as it rises from a low value 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
extreme, reduce the value of D. Investigate the results of the changed value by
plotting a graph of the pressure differential as it increases from a low value and
comparing this to the initial results.
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