Thermcraft XST-6-0-36-3V - Manual (Page 225)

Installation and Operation Handbook Programmer Operation
2416 Controller 5-7
‘
Hi’ − Deviation High Holdback holds the program back when the process variable
deviates above the setpoint by more than the holdback value.
‘
bAnd’ − Deviation Band Holdback is a combination of the two. It holds the program back
when the process variable deviates either above, or below, the setpoint by more
than the holdback value.
There is a single Holdback Value which applies to the whole program. However, the
Holdback type and whether or not it is enabled, can be applied to the program as a whole, or
individually in each segment.
Power failure
If power is lost and then restored, while a program is running, the behaviour of the
programmer is determined by the setting of the parameter ‘
Pwr.F’ Power fail strategy in
Programmer configuration. This can have one of three settings:
− cont (Continue), rmP.b
(Ramp from PV), or
rSEt (Reset).
If ‘
cont’ is selected, then when power is restored the program continues from where it was
interrupted when power was lost. All parameters, such as the setpoint and time remaining in
the active segment, will be restored to their power-down values. For applications that need to
bring the measured process value to the setpoint as soon as possible, this is the best strategy.
If ‘
rmP.b’ is selected, then when power is restored the setpoint starts at (‘servos to’) the
current measured value, and then ramps to the target setpoint of the active segment at the last
ramp rate used by the program. This strategy provides a smoother recovery. The two
diagrams below illustrate the respective responses, Fig5-2 if power fails during a dwell
segment and Fig5-3 if it fails during a ramp segment.
Figure 5-2 Continue after a power fail Figure 5-3 Ramp back after a power fail
If ‘
rSEt’ is selected, then when power is restored the program terminates.
Setpoint
Ramp
Segment Dwell Segment
Time
t1 t2
Segment dwell
time = t1 + t2
Power off
Power on
Setpoint
Ramp Segment
Time
Power off
Power on -
Servo to new PV level
Target setpoint