Thermcraft XST-6-0-36-3V - Manual (Page 224)

Programmer Operation Installation and Operation Handbook
5-6 2416 Controller
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
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).
Note: The RUN/HOLD button can be disabled, either when ordering the controller, or
subsequently in configuration. This will force you to operate the program 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.
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.
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.
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.