3.2 PairCheck – contradiction check as a safety principle
Idea
A cylinder can physically only be in one place at a time.
Either front or rear.
But if both end-position sensors report a signal at the same time, there is a contradiction – and thus potentially a dangerous fault.
Selmo detects this case automatically through the PairCheck.
The PairCheck is a logic integrated into every zone that checks whether two or more feedback signals exclude each other.
Procedure
Cylinder 1 has two feedbacks: Front and Rear.
In the zone description it is specified that these signals belong together – a so-called PairCheck group.
During runtime the system continuously compares their states.
If both are TRUE or both are FALSE, the PairCheck detects the logical error.
The result:
The zone automatically sets an interlock.
The automatic release is lost.
A red diagnostic appears in the HMI:
“PairCheck Error – Contradictory feedback.”
Benefit
This completely eliminates a common source of error.
It is impossible for a cylinder to be “seen” in two positions at the same time or to fail unnoticed.
The PairCheck is therefore an integrated safety monitor, not an additional module.
