INFICON MDC-370 Thin Film Deposition Controller - Manual (Page 93)

MDC-370 DEPOSITION CONTROLLER
gone 48 miles because of some traffic earlier on. Our Integral error is 2 miles. If
we want to get back on schedule we need to speed up a bit. If schedule is very
important to us, we will speed up a lot to get back on schedule fast. If schedule is
not important at all we will maintain our speed. The Integral Time constant
instructs the controller on how much attention to pay to the schedule. If we don’t
care what happened in the past and we want zero rate error right now, we don’t
want any Integral feedback. To accomplish that we set the Integral Time constant
to its maximum value, which tells the controller to ignore any past error unless it
lasts for a very long time.
6.3 ESTABLISHING MDC-370 CONTROL LOOP PARAMETERS
As explained above, the MDC utilizes three control loop parameters referred to as
PID parameters; Proportional gain, Integral Time constant and Derivative Time
constant to provide for optimization of the control loop. The MDC provides
default values for each of these parameters.
Default and Range for PID Parameters
Parameter Minimum
value
Maximum
value
Default
value
Proportional gain 1 9999 1000
Integral time constant,
sec.
0 99.9 99.9
Derivative time constant
sec.
0 99.9 0.0
The following table lists some recommended PID values for different types of
deposition sources. These values represent a good starting point and in some
cases may not need to be further modified.
Suggested PID Starting Values for Different Sources
Parameter Electron
Beam Gun
Filament
Boat
Proportional gain 2000 600
Integral time constant,
sec.
99.9 99.9
Derivative time constant
sec.
25.0 75.0
In the MDC-370, the PID parameters are defined at the material level because
different materials often require different PID settings even though they may be
deposited from the same source. Therefore it is usually necessary to establish the
PID parameters for every each material and deposition source.
Tuning the MDC-370 Control Loop 6-3