INFICON IC/5 Thin Film Deposition Controller - Manual (Page 51)

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IC/5 Operating Manual
ITAE is insensitive to the initial and somewhat unavoidable errors, but it will weight
heavily any errors occurring late in time. Optimum responses defined by ITAE will
consequently show short total response times and larger overshoots than with
either of the other criteria. It has been found that this criteria is generally most
useful for deposition process control.
INFICON’s Auto-Control-Tune is based on an open loop measurement of the
system’s response. From a step change in the control signal, the response
characteristics of the system are calculated. The experimental determination of
response characteristics is accomplished through two types of 2-point curve fits.
They are determined either quickly, but less accurately (i.e., “Quick Tune”) at an
arbitrary deposition rate, or less quickly, but more accurately (i.e., “Complete
Tune”) near the desired rate setpoint. Since the process response characteristics
depend on the position of the system (i.e. deposition rate for this discussion), the
process response is best measured at the desired operating point of the system.
This measured process information (i.e. process gain, K
p
, time constant, T
1
, and
dead time, L) is used to generate the best fitting PID control loop parameters for
the specific system.
The most satisfactory performance criterion for deposition controllers is the ITAE.
There will be overshoot, but the response time is quick, and the settling time is
short. For all of the above integral performance criteria, controller tuning relations
have been developed to minimize the associated errors. Using manually entered
or experimentally determined process response coefficients, ideal PID controller
coefficients can be readily calculated for the ITAE criteria as shown below.
[13]
[14]
[15]
For slow systems, in order to help avoid controller windup (windup is the rapid
increase in control signal before the system has the chance to respond to the
changed signal), the time period between manipulated variable (control voltage)
changes is lengthened. This allows the system to respond to the previous controller
setting change, and aggressive controller settings can be used. A secondary
advantage is that immunity to process noise is increased since the data used for
control is now comprised of multiple readings instead of a single rate
measurement, taking advantage of the mass integrating nature of the quartz
crystal.
With process systems that respond quickly (short time constant) and with little to
no measurable dead time, the PID controller often has difficulty with the deposition
process noise (beam sweep, fast thermal shorts of melt to crucible, etc.). In these
situations a control algorithm used successfully is an integral/reset type of
controller. This type of controller will always integrate the error, driving the system
K
c
1.36 K
p
   L T
1
  
0.947 –
=
T
i
1.19T
1
  L T
1
  
0.738
=
T
d
0.381T
1
  L T
1
  
0.995
=