Self-Seeking Set Point - ABB COMMANDER 360 User Manual

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3.2.4 Self-seeking Set Point – Fig. 3.5
The Self-seeking Set Point function reduces the delay between the end of a program and the beginning of the next
program. The process variable value is used as the program start point and the set point steps up to the process
variable value. This has the effect of changing the overall segment time and maintains a constant ramp rate.
Current Process
Variable Inserted as
Program Start Level
Set
Point
Seg. 1
Program
Status
Start
3.2.5 Retort Function – Fig. 3.6
The Retort function ensures safe operation of retort vessels under fault conditions. If the heat source fails during
a soak segment, the process variable will inevitably fall. When the process variable falls below the holdback
hysteresis value the program is put on HOLD (as for normal operation). The set point then follows the process
variable as it continues to fall (Retort Hold).
Set Point = Process Variable + Hysteresis value
Upon recovery of the heat source, the process is controlled at the new set point value. When the process variable
reaches the set point it is then ramped back to the initial soak value at the rate of the previous ramp (Retort Ramp).
When the soak level is reached the program is released from its hold state and the segment is either completed
or repeated from the beginning, depending on the retort mode selected.
The retort mode is selected in the
Note. For the retort function to operate, either LO or HI-LO hysteresis must be applied to the soak
segments.
Hysteresis applied to
Soak segment
Process Variable
Set Point
1
The program is put on HOLD when the
process variable falls below the holdback
hysteresis
2 The setpoint follows the process variable
as it continues to fall. (SP = PV +
hysteresis). The set point 'tracks the lowest
value of the process variable.
28
Process
Variable
Seg. 2 Seg. 3
Run
Stop
Fig 3.5 Self-seeking Set Point
Ramp/Soak Profile Page
Failure of heat source
1
Seg. 1
Fig 3.6 Retort Function
Current Process Variable
inserted as Program Start Level
Seg. 1 Seg. 2 Seg. 3
Run
Start
.
Program on Hold
4
2
3
Recovery of heat source
Seg. 2
When the process variable reaches the
3
set point, the set point is ramped back to
the soak level at the rate of the previous
ramp.
Upon reaching the soak level, 'HOLD' is
4
released and the segment is completed
or repeated.
Seg.
2
Seg. 1
Run
Stop
Start
Seg. 3
Stop