INFICON Composer Gas Concentration Controller - Manual (Page 83)

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Composer Operating Manual
3.2 Tutorials
The following information is available:
Š Control Loop Fundamentals — Concentration Control, see section 3.2.1 on
page 3-21
.
Š Computing the System’s Time Constant, see section 3.2.1.5 on page 3-25
for Concentration Control or section 3.1.13 on page 3-11 for Bulk Flow.
Š How to Determine Process Parameters, see section 3.2.3 on page 3-28 .
Š What if the NOVRAM’s Battery Fails and I Lose All My Stored Parameters?,
see section 3.2.4 on page 3-32
.
Š Why is a Digital Connection to the System Controller Preferred?, see
section 3.2.5 on page 3-33
.
3.2.1 Control Loop Fundamentals —
Concentration Control
NOTE: Control in the Bulk Flow mode is best described in section 3.1.13 on
page 3-11 .
The function of the control loop parameters is to match the instrument’s reaction
to an error (between the measured concentration and the desired
concentration) to the time related characteristics of the reactor. There are three
adjustable parameters; PROCESS GAIN, TIME CONSTANT and DEAD TIME
used to accomplish this. It is convenient to think of Delivery Systems as falling
into two categories: slow or fast.
Š A slow delivery system, for the purpose of this discussion, has more than a
one second delay (lag) between the control voltage change (into the
Bubbler’s MFC) and the measurement system’s ability to sense that change
has taken place. Most delivery systems have this characteristic. See
section
3.2.1.1, Tuning a Slow Delivery System, on page 3-22 .
Š A fast system, for the purpose of this discussion, is a system that has not
more than a one second dead time between the control voltage change (into
the MFC’s set point) and the measurement system’s ability to sense that
change has taken place. See
section 3.2.1.2, Tuning a Fast Delivery
System, on page 3-23 .
Fast systems effectively use an integrating type controller while slow systems
are better controlled with a PID type. The tuning parameters are affected by
liquid source level, flow rate, delivery system volume, and solid source
condition.
Because the volume to flow rate ratio is typically quite large in current delivery
systems, it is expected that most will be characterized as slow.