Process Value Processing Branch

At a Glance

There are two types of process values:

Standard process value,
External process value.

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.

The first order filtering function has a gain coefficient.
There are four thresholds for the Alarms on process value block with a hysteresis of 1% of the full scale.
Two input formats may be used: unipolar or bipolar.
There is no bump when switching to simulated mode; the initial simulation value retained is the last process value read.
The function generator has scaling built-in.
It is possible to limit the process value to the scale limits.

External Process Value

Block diagram of the external process value branch.

Initialization Overview

On start-up, the associated data is first updated before the launch of the first process of this branch.
If the process value input address is not set, processing is performed on the simulated value initially set to zero.
On initialization, there is a consistency check on the configuration entered. If there is a problem with the configuration, the loop remains in initialization state.

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.