INFICON Composer Gas Concentration Controller - Manual (Page 84)

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Composer Operating Manual
NOTE: The use of a chart recorder, especially when beginning a new
application, is highly recommended. Connect the recorder output to the
Controller’s output and use it to monitor the response to small changes
in the TARGET CONCENTRATION.
If you do not know if the system is fast or slow, it is straight forward to
measure the delay using the chart recorder. Using manual flow control,
establish a flow rate and allow the measured concentration to become
steady. When the chart recorder pen crosses some convenient reference
point, increase the flow rate a few percent (~5% if possible). Allow the
source to again stabilize. Graph the delay time to determine if the system
is fast or slow. Run the recorder at a chart speed sufficiently fast to
accurately measure time. Dead times greater than 1 second characterize
the source as slow.
3.2.1.1 Tuning a Slow Delivery System
If the response has been characterized as slow (Dead Time is greater than 1
second), review
section 2.3, Control Loop Primer, on page 2-6 and then set the
PID control parameters as follows:
PROCESS GAIN . . . . . . . . . . . . . . K
P
, enter this value into the parameter
TIME CONSTANT . . . . . . . . . . . . . T
1
, enter this value into the parameter
DEAD TIME . . . . . . . . . . . . . . . . . L , enter this value into the parameter
As illustrated in Figure 2-2 on page 2-7 , the control dead time, L , is the time
delay between a change in the Bubbler’s flow setting and a noticeable change
in concentration. The time constant,
T
1
is ( T
0.632
- L ) where T
0.632
is the time
between a change in the Bubbler’s flow setting and the time to achieve 63.2%
of the new equilibrium concentration.
K
P
is then the ratio of the change in concentration divided by the change in flow
setting.
[30]
These values may be adjusted slightly in use to optimize the tuning. The tuning
may change because of process variations. Once determined, TIME
CONSTANT and DEAD TIME usually do not need to be changed unless flow
rates or operating pressure are altered. At higher flow rates, the gas mix turns
over quicker; at higher operating pressures it is slower for the same flow rate.
NOTE: Remember that increasing the value of PROCESS GAIN reduces the
controller’s change for a given concentration error, that is, the system
responds more slowly.
K
p
change in output ( )
change in control signal ( )
-----------------------------------------------------------------
concentration (mole%)
controller output (volt)
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