INFICON SQM-242 Thin Film Deposition Controller Card Operating Manual - Manual (Page 82)

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SQM-242 Operating Manual
The derivative time constant(D) is utilized to compensate for slow responding
sources such as boats and induction heated sources. This value is defined as the
time difference between a change in % power and the start of an actual change in
rate (see
Figure 8-3 .) The Derivative Time constant instructs the controller on how
much attention to pay to the rate of change of a error. A value of zero tells the
controller to ignore the rate of change of the error. A large value tells the controller
that the source is slow and it is going to be hard to get it going and hard to stop it.
So if the rate starts to fall off, give it power, or if the target is quickly approaching,
begin to decrease the power.
Figure 8-3 Dead Time
8.5.1 Fast Source
In general, fast sources are: all electron beam types (unless a hearth liner is
used), some very small filament sources and sputtering sources. If the source
response has been characterized as "FAST", as suggested in the NOTES in
section 8.5 , a integrating type control loop should be established.
With fast sources, the I and D terms can typically be set to 0.1. If satisfactory
control cannot be established using only P, the source is probably not a "fast"
source. The response of a system with too little controller gain (its P value is too
large) is characterized as over damped as shown in
Figure 8-4 . Decrease the P
value until the system oscillates as is shown by the under damped curve. Proper
control is established by an intermediate value that approximates the critically
damped curve.