Installation and Operation Handbook
Programmer Operation
5.4
Running A Program Using The Run/Hold Button
If you are using a four (4) program version of the controller, you must first select the number of the program that
you want to run. Do this in the ‘
run
’ list - see the previous topic,
Running a program from the Run list.
Then:
RUN / HOLD
button
Press once to run a program (RUN light on)
Press again to hold a program (HOLD light on)
Press again to cancel hold and continue running (HOLD light off, RUN light
on)
Press and hold in for two seconds to reset a program (RUN and HOLD
lights off).
5.5
Automatic Behaviour
The preceding topics explain how to operate the programmer manually.
The following topics cover aspects of its automatic behaviour:
Servo
,
Holdback
and
Power Failure
.
5.5.1
Servo
When a program is RUN, the setpoint can start either from the initial controller setpoint, or from the process value.
Whichever it is, the starting point is called the ‘servo’ point and you set it up in configuration. When the program
starts, the transition of the setpoint to its starting point is called ‘servoing’.
The normal method is to servo to the process value, because this will produce a smooth and bumpless start to the
program. However, if you want to guarantee the time period of the first segment, you should set the controller to
servo to its setpoint.
5.5.2
Holdback
As the setpoint ramps up, or down (or dwells), the measured value may lag behind, or deviate from, the setpoint
by an undesirable amount. ‘Holdback’ is available to freeze the program at its current state, should this occur.
The action of Holdback is the same as a deviation alarm. It can be enabled, or disabled. Holdback has
two
parameters - a
value
and a
type
.
If the error from the setpoint exceeds the set ‘holdback’ value, then the Holdback feature, if enabled, will
automatically freeze the program at its current point and flash the HOLD light. When the error comes within the
holdback value, the program will resume normal running.
There are
four
different Holdback types. The choice of type is made by setting a parameter when creating a
program, and may be one of the following:
−
‘
OFF
’
−
Disables
Holdback
−
therefore no action is taken.
‘
Lo
’
−
Deviation Low Holdback
holds the program back when the process variable deviates
below
the setpoint
by more than the holdback value.
‘
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.
i
Note:
The RUN/HOLD button can be disabled, either when ordering the controller, or
subsequently in configuration. This will force you to operate the programmer from the ‘run’
list all the time. The main advantage of this method is that it will reduce the chance of
accidentally changing the state of a program.
HA025041 issue 12 Aug 14 5-5
Summary of Contents for XST-2-0-12-1V1-E28
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