Position Trigger Works Slowly but Vanishes at Speed? Check the Window
A narrow software position window can be crossed entirely between PLC evaluations. The axis can pass the target correctly while the application never sees the condition true.
Original sampled-position example; choose an event mechanism for the required accuracy.
What is happening?
A condition such as position between two limits observes sampled positions. If travel per evaluation exceeds the window width, no sample is guaranteed inside it. Crossing detection or hardware position events may be more appropriate for the application.
Check these five things
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Calculate distance traveled during the evaluation interval.
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Compare that distance with the window width.
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Check task jitter and feedback update timing.
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Review supported crossing or position-event functions.
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Model forward, reverse, and wraparound behavior.
Worked example
At 1000 mm/s, an axis travels 10 mm during a 10 ms evaluation interval. A 2 mm window can be skipped between samples. An offline trace with positions 98 mm and 108 mm illustrates why a 100–102 mm condition never becomes true.
Quick diagnostic reference
| Quantity | Example |
|---|---|
| Travel per evaluation | 10 mm |
| Window width | 2 mm |
| Observed result | Possible missed condition |
The mistake to avoid
Simply widening a window changes trigger uncertainty. A production timing requirement may need a supported hardware event rather than a larger software tolerance.
How to verify the fix
Test maximum supported speed, timing variation, reverse motion, and repeated crossings. Confirm one event per intended crossing in the model and approved validation.
Field-work boundary: These are educational diagnostic guides. Use the exact equipment manuals and approved site procedures. Electrical testing and machinery changes belong to qualified, authorized personnel; control hazardous energy and validate affected functions before release.
Technical reference
Beckhoff: NC PTP motion and position functions. The examples and diagnostic tables above are original teaching material; they do not replace the product manual.