INFICON Cygnus Thin Film Deposition Controller - Manual (Page 213)

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Cygnus Operating Manual
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.