Fluke RUSKA 7252, User Manual

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Theory of Operation
Control Strategy
2
2-9
Alignment of Inner Loop with Outer Loop
Since the control system of the Calibrator is based on the alignment between the inner
and outer loop sensors, there is a procedure that automatically aligns these sensors. It is
possible that during typical operation the alignment between the inner loop and the outer
loop sensor can become miss-aligned. This can cause control problems such as overshoot.
This can be realigned using the Auto-Tune feature. See Chapter 6, Maintenance, for
details on the Auto-Tune functions of the RUSKA 7252.
PID Control
The outer loop pressure controller is a form of PID control executed every 100
milliseconds (10 times per second). The pressure is controlled to an exponential decay
curve.
) ( * ) ( * ) ( * A S
dt
d
d dt A S i A S p V
− + − ∫ + − =
S
= Set-point
A
= Actual
V
= Control Output
d i p , ,
= Control Coefficients
Figures 2-4 show a typical Control mode operation. Pressure in percent of full scale is
plotted versus time to show the change in pressure as the instrument steps from one
control point to the next.
0 5 10 15 20 25
Time - seconds
P
ressu
re
glb05.eps
Figure 2-4. Pressure Control - Normal Mode