INFICON SQM-242 Thin Film Deposition Controller Card Operating Manual - Manual (Page 81)

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SQM-242 Operating Manual
The proportional gain(P) parameter sets the rate at which the control voltage
changes in response to an error signal (see
Figure 8-1 ). Any error in the rate
causes the source control voltage to ramp to a new value. When the source control
voltage increases or decreases to the correct value, the value required to achieve
the desired rate, the error goes to zero and the output remains constant.
Figure 8-1 Proportional Gain
The integral time constant(I) is defined as the time difference between the actual
start of a change in rate and the time at which 63% of the rate step is achieved (see
Figure 8-2 .) It instructs the controller on how much attention to pay to the schedule
of the thickness profile. 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.
Figure 8-2 Time Constant