INFICON XTC/C Thin Film Deposition Controller - Manual (Page 148)

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XTC/C - XTC/2 Operating Manual
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 towards zero error. This technique works well when
there is little or no dead time. If this technique is used on a process with
measurable lag or dead time, then the control loop will tend to be oscillatory due
to the control loop over-compensating the control signal before the system has
a chance to respond.