The mark is correct; the sampled position is late
A printed strip travels at 1000 mm/s. A mark sensor changes state, but the application reads it 4 ms later. The strip has moved another 4 mm before the software sample. At 250 mm/s, the same delay represents 1 mm. A correction that worked at low speed can drift at production speed without any change in the printed marks.
Position registration connects a physical event with a coordinate. Hardware capture can latch encoder position at the event without waiting for an ordinary PLC input scan. The complete error still includes sensor response, wiring/input delay, capture hardware and the measurement point.
Keep the event and its product together
For each capture, record validity, captured coordinate, event sequence or timestamp and product identity. A valid number from yesterday's or the previous carton's event is unsuitable for the current correction. This is especially important when several products occupy the conveyor at once.
Define an expected mark window in position or time. An event outside that window might be a false mark, a double detection or the wrong product. Missing marks need a defined action: reject the product, stop registration-controlled processing or use a bounded fallback that the process design permits.
position_error_mm = actual_capture_mm - expected_capture_mm
sampling_distance_mm = speed_mm_per_second * delay_secondsChoose the correction sign from the physical coordinate definition. Test it with a known early and late mark. A mathematically plausible sign can make every correction push the error farther away.
Touch-probe concepts
PLCopen touch-probe operations describe arming and retrieving a captured event, with exact trigger inputs and data formats defined by the vendor and hardware. Some devices latch drive position; others supply timestamped events for controller reconstruction. Do not assume ordinary input wiring automatically provides a hardware capture path.
TryPLC's ST runtime does not execute MC_TouchProbe. This chapter's calculations and scaling bench are mathematical exercises. A vendor lab must validate arming, capture validity, cancellation and rearming with the installed module.
Bound correction before applying it
A strip feeder might compare a captured mark with its expected position and adjust the next feed length. Clamp the accepted correction according to the machine/process design, log rejected corrections and avoid a single implausible sensor edge producing a large move.
If correction changes a cam phase or a synchronized axis relationship, account for the resulting velocity and acceleration demands. An instantaneous coordinate jump can demand dynamics the mechanics cannot deliver. Apply the correction through a documented phase/update mechanism with a permitted transition.
Registration improves a measurement relationship; it does not repair roller slip or guarantee product restraint. A sensor on the material provides different evidence from the motor encoder driving it. Record both when diagnosing speed-dependent errors.
Worked investigation
Assume the mark should appear at coordinate 200 mm and capture is 203 mm. The measured error is +3 mm in that coordinate system. Whether the next feed should increase or decrease depends on where the cutter is and how the feed coordinate is defined. Draw the machine and direction before choosing the sign.
Now assume software delay is 4 ms at 1000 mm/s. Its 4 mm contribution is larger than the observed 3 mm. That does not prove the delay is the sole cause: the expected coordinate or other delays may offset it. Measure the event path and compare low/high-speed data instead of fitting one constant to a single result.
Exercises
Calculate delay distance at 500, 1000 and 1500 mm/s for a 4 ms delay: 2, 4 and 6 mm. Design tests for one missing mark, two marks in a window and a capture belonging to an earlier product. For each, state whether a correction is allowed and which diagnostic record survives.
Use the scaling bench to translate a captured count into millimetres. Then add a home offset and explain why both expected and captured positions must use the same coordinate definition.
References
PLCopen motion control covers motion and capture block families. Capture capability, latency and wiring must be checked against the specific sensor, input module and drive manuals.