INFICON XTC/3 Thin Film Deposition Controller Operating Manual - Manual (Page 206)

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XTC/3 Operating Manual
Three parameters are determined from the process reaction curve. They are the
steady state gain, K
p
, the dead time, L, and the time constant, T
1
. Several methods
have been proposed to extract the required parameters from the system response
as graphed in
Figure 8-7 . These are: a one point fit at 63.2% of the transition (one
time constant); a two point exponential fit; and a weighted least-square-exponential
fit. From the above information a process is sufficiently characterized so that a
controller algorithm may be customized.
Figure 8-7 Response of process to an open loop step change
(at t=0 control signal is increased)
A controller model used extensively is the PID type, shown in Laplace form in
equation [6] .
[6]
Where
 M(s) = manipulated variable or power
 K
c
= controller gain (the proportional term)
 T
i
= integral time
 T
d
= derivative time
 E(s) = process error
Figure 8-8 represents the controller algorithm and a process with first order lag plus
a dead time. The process block implicitly includes the dynamics of the measuring
devices and the final control elements, in our case the evaporator power supply.
T
1
= t
(0.632)
- L
K
p
= (Change in Output) / (Change in Control Signal)
Point of
Maximum
Slope
1.00 K
p
K
p
0
L t
(0.632)
Time, t
0.632
M s   K
c
1
1
T
i
s
------- T
d
s + +
 
  Es
=