INFICON Composer Gas Concentration Controller - Manual (Page 48)

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Composer Operating Manual
2.3 Control Loop Primer
The instrumental advances in measurement speed, precision and reliability
would not be complete without a means of translating this improved information
into improved process control. For a CVD process, this means keeping the
reactor’s inlet concentration as close as possible to the target concentration.
The purpose of a control loop is to take the information flow from the
measurement system and to make gas flow corrections that are appropriate to
the characteristics of the particular precursor source. When properly operating,
the control system translates small errors in the controlled parameter, or
concentration, into the appropriate corrections in the manipulated parameter,
flow. The controller’s ability to quickly and accurately measure and then react
appropriately to the small changes keeps the process from deviating very far
from the target concentration.
The most commonly chosen controller model for converting error into action, is
called PID. In the PID model, P stands for proportional, I stands for integral and
D stands for derivative action. Certain aspects of this model will be examined
in detail a little further on.
Knowledge of the responses of the precursor source can be found by
repetitively observing the system response to a disturbance under a particular
set of controller settings. After observing the response, new controller
parameters are estimated and then tried again until satisfactory control is
obtained. Control, when it is finally optimized, essentially matches the
parameters of the controller model to the characteristics of the precursor
delivery system.
In general, it is not possible to characterize all processes exactly; some
approximation must be applied. The most common is to assume that the
dynamic characteristics of the process can be represented by a first-order lag
time plus a dead time. The Laplace transform for this model (conversion to the
s domain) is approximated as:
[22]
Output
Input
------------------
K
p
L –
s
------
 
 
exp
T1s 1 +
----------------------------- =