Process Value Processing Branch
(Original Document)
At a Glance
There are two types of process values:
The external process value allows you to input to the loop controller a process value PV which has been processed outside the control loop. This solution is offered for cases where calculation of the process value requires specific or customized functions which are not available in standard process value processing.
Standard Process Value
Block diagram of the standard process value branch.
External Process Value
Block diagram of the external process value branch.
Initialization Overview
Runtime Control Operation
The two serious faults checked by process value processing are parameter and internal calculation errors (division by zero, overflow, etc.).
If
Then
a serious error has been detected
the loop processing switches to fallback state :
  • the value of the calculated process value PV is frozen,
  • the control loop outputs are frozen.
the error disappears
the loop resumes in the previous operating mode, without bumps on the outputs.
there is a serious error on process value processing during a cold start
the loop remains in its initialization position and does not start.
scale values are incorrect (non-integer value) during a cold start
the loop remains in initialization position yet does not start.
due to an error, the loop remains in its initialization position and does not start.
the problem must be eliminated for the loop to restart automatically.
there is an runtime error in the scale values.
processing is performed with the old valid scale values which were overwritten in the current scale parameters. Scale parameters are updated when the check proves to be correct.