Pair-check & additional checks

Physical plausibility

Besides interlocks (i) and CMZ there are other monitoring mechanisms that do not control the sequence and do not secure system integrity, but rather monitor the physical plausibility of states.

These checks ensure that impossible or contradictory states are detected and clearly diagnosed.

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Goal of physical plausibility checks

Plausibility checks answer the question:

Can this signal state even exist in the real world?

They serve to:

• detect faulty sensors

• reveal wiring errors

• make mechanical defects visible

• explicitly test implicit assumptions

The following applies:

A physically impossible state is always an error – regardless of the sequence.

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Pair-Check

Basic principle

The Pair-Check monitors related signals, whose simultaneous occurrence is physically impossible.

Typical examples:

• Cylinder end position front and end position rear active at the same time

• Valve open and closed reported simultaneously

• Axis position reached left and right simultaneously

These signals are defined as pair .

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Behavior of the pair check

If an impossible state is detected:

• the sequence is not continued

• the state is recognized as faulty

• a clear diagnosis is generated

The pair check:

• interprets nothing

• corrects nothing

• does not decide on the sequence

It merely determines that reality contradicts the model.

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Pair check in the model context

The pair check is:

• bound to zones independent of the current state

• not state-dependent like an interlock

• not system-wide like a CMZ

• It thus complements:

Interlock → logical consistency

• CMZ → system integrity

• Pair-Check →

• physical plausibility Other plausibility checks

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In addition to the pair check, further checks can be defined, e.g.:

Processing check

Monitoring that a processing operation only takes place under physically sensible conditions.

Processing only at reached position

Examples:

• Processing only with active tool

• Processing not when the medium is stationary

• Consistency checks

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Monitoring logical relationships between signals.

Feedback without prior actuation

Examples:

• State change without possible cause

• Signal change outside physical time limits

• Automatic diagnosis

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All plausibility checks generate

automatic diagnosis This diagnosis:.

points to the affected zone

• describes the contradictory state

• is independent of the operator

• is not interpretation-dependent

• no manual fault assignment

There are:

• no aggregated messages

• no tacit corrections

• The diagnosis names the impossible state –

not a suspected cause. Distinction from other safety mechanisms

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Pair check & other checks

For clear classification:

• check physical plausibility

  • operate independently of the sequence

  • generate diagnosis

  • checks sequence conditions

• Interlock (i)

  • acts state-dependently

  • protects system integrity

• CMZ

  • prevents any movement

  • Plausibility checks:

do not replace safety

• do not replace interlocks

• do not replace CMZ

• They

complement the model with verifiable reality. Typical mistakes in dealing with plausibility checks

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Using plausibility checks as sequence control

Common mistakes are:

• compensating missing checks with logic

• tolerating impossible states

• suppressing or summarizing diagnosis

• What is physically impossible

Rule of thumb:

must not be silently accepted in the model. Pair check & other plausibility checks:

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Summary

detect impossible states

• make physical contradictions visible

• produce automatic, unambiguous diagnosis

• increase robustness and explainability

• Plausibility checks ensure that

the model fits reality – or the deviation is clearly named. or the deviation is clearly named.