36
(B) In the case of a stable control
The AT process will fluctuate around the SV.
(1) Calculating PID constant
(2) PID constant calculated
(3) Controlled by the PID
constant set by auto-tuning
(Fig. 8.2-2)
(C) In the case of a large difference between the SV and processing temperature as
the temperature is falling
When AT bias is set to 20 , the
AT process will fluctuate
at the temperature 20
higher than the SV.
(1) Calculating PID constant
(2) PID constant calculated
(3) Controlled by the PID
constant set by auto-tuning
(4) AT bias value
(Fig. 8.2.3)
8.3 Auto-reset (offset correction)
Auto-reset is performed to correct the offset at the point at which PV indication
is stabilized within the proportional band during the PD action.
Since the corrected value is internally memorized, it is not necessary to perform
the auto-reset again as long as the process is the same.
However, when the proportional band is set to 0, the corrected value is cleared.
(Fig. 8.3-1)
Temperature
SV
Offset span
Time
Offset is corrected
Auto-reset is performed
AT starting point
(1)
(2)
(3)
Temperature
SV
Time
AT starting point
(1)
(2)
(3)
(4)
Temperature
SV
Time
Temperature 20
higher than the SV
