Calibration and Measurement Stability on a Running Line

A vision measurement station is qualified on the day it is commissioned and then lives in a workshop. Keeping it inside its tolerance is a routine, not a product feature: a short verification against a reference every shift, a full recalibration whenever the optical configuration changes, and a control chart on one reference feature that turns slow drift into a visible trend. Most escapes on a running line are traceable to one of four causes — contamination, temperature, vibration and illumination ageing — and each leaves a different signature.

What each disturbance does to the number

• Contamination lowers contrast and adds veiling glare, which shifts apparent edge position. On backlit silhouettes it mainly destroys edge steepness and repeatability.
• Temperature changes both the part and the machine: steel expands about 11.5 µm per metre per kelvin, aluminium about 23, so a 100 mm steel part moves roughly 1.2 µm per kelvin — 10 K of ambient swing is already about 12 µm.
• Vibration blurs the exposure by roughly v · t: 100 mm/s during a 200 µs exposure moves the image about 20 µm, comparable with the object-side pixel of a large-field station.
• LED ageing reduces output and forces longer exposures or higher gain; exposure value trending upward over weeks is the early warning.

Calibration, verification and the difference between them

Calibration establishes scale factors and any distortion model. Verification checks that they still hold, using a known artefact, without changing anything. Confusing the two hides drift: teams recalibrate when a value looks wrong instead of logging the check and watching it move. The workable routine is a one-button artefact check at a fixed interval with pass/fail limits set to a fraction of the tolerance, typically a tenth, logging the value automatically; and a full recalibration only when the configuration changes — camera, lens, aperture, focus, working distance, mounting — or when the check leaves its band.

Detecting drift before it produces bad parts

Measure one reference feature automatically at a fixed cadence and plot it against time. A step change points to a mechanical or configuration event, a steady ramp to thermal drift or wear, and an increase in scatter to contamination, illumination ageing or vibration. Because the reference is measured by the same chain as the product, it captures the whole system — which is exactly what an audit will ask about. Keep the configuration record, the calibration result with residuals, the verification values with timestamps and temperatures, the maintenance actions and the measured data per batch; six categories that make an accepted batch reconstructible months later.

Related reading: building a telecentric measurement station, object-side resolution versus accuracy, and the telecentric lens range. The practical questions on recalibration intervals, records and operator checks are answered below — describe your line and we will suggest a verification interval.