INFICON STC-2000A Thin Film Deposition Controller Operating Manual - Manual (Page 107)

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STC-2000A DEPOSITION CONTROLLER
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is typically set to equal the slowest time constant in the control loop. The D or Derivative term helps to
anticipate control loop overshoot and negates some of the slowing down effect of the Integral term. This
term is seldom required for systems with time constants faster than 10 seconds. Too much D term can
actually contribute to loop drift and instability.
Determining Control Loop Settings
Using Data Display For Tuning
All the control loop parameters interact to some degree in the overall response of the control loop.
Therefore, there are many combinations of settings that will give equally satisfactory results. Also, the
control loop that is optimized for steady state control will have quite different settings from one requiring
fast control acquisition with minimal overshoot. The graphics available on the RunTime display can
greatly aid the user in determining the control loop settings and performance. The user can observe real
time rate, rate deviation, or control power on this display. By introducing a step response into the control
system and observing the graphical display responses it is quite easy to "tune" the control loop. There are
several ways to introduce a step change. The first is by programming a different deposition rate number
while in control. Another way is to go into Manual Power and ramp the power either up or down, then
come out of manual and go back into control.
Setting The P-I-D Terms
The basic philosophy of tuning a control loop is to isolate each of the P, I, and D parameters as
much as possible. Start off by disabling the I and D terms. The I parameter should be set to 0.0 secs and
the D term to its lowest value (0.0 secs). This isolates just the gain term (P). Start off with a low value of P
and increase it until your system starts to oscillate in response to a step input. Make a note of this number.
Reduce it by 50% as a starting point. You will notice that your desired control point will not be reached by
using this number, (the actual rate will always be below the desired rate). This offset can be eliminated by
setting (to non 0.0) the integral term (I). If you know the time constant of your system, use this number (or
slightly longer) as a starting point. Smaller values of the I term will make the system respond faster, but
may cause oscillations. At this point, you can go back and re-adjust (lower) the P term. Keep in mind that
there is interaction between the P term and the I and D terms.
For most systems, these 2 terms (P and I) will be enough to stabilize your system. If you have a
very slow system, you will want to use the D parameter also. This term can be helpful in controlling the
amount of overshoot you have in your system. If you know what the lag of your system is, set the D term
to it. The D term puts lead into the control loop to compensate for lag in your system. Remember that a
P-I-D control loop is much more difficult to tune than just a P-I loop.
Typical Settings
Typical Sources
Fast responding sources such as electron beam types generally can operate at high gain settings
(20 to 200), fast integration times (.1 to .5 sec), and minimal or no derivative term (0 sec). This is
somewhat material sensitive and also is affected by the use or non-use of source liners. These settings are
also typical of most sputtering applications.
Medium response sources such as resistive type boats, baskets, or filaments typically require
lower gain settings (10 to 100), longer integration times (.5 to 10 sec) and some derivative term (0 to 1 sec).
Slow sources such as Knudsen or induction heated types may require lower gains (1 to 50), longer
integration times (5 to 30 sec), and a derivative term approximately 20% of the integral term (1 to 10 sec).
SECTION 3.XX page 107 of 292