Is your laser tracker still in specification? The 2021 interim test explained

A laser tracker can hold its specification for years, or it can drift after a knock in transit without anyone noticing. The revised laser tracker standards published in 2021 give users a better way to find out which, without sending the instrument away.

Why trackers need checking

Laser trackers are now the standard tool for measuring and assembling large components, and their basic design, a range-measuring unit on a two-axis gimbal, has changed little in thirty years [1]. What has changed is how much work they are trusted with.

Their accuracy depends on many things: how well the instrument compensates for air conditions, thermal expansion of the tracker and its stand, thermal distortion of the part itself, and the internal alignment of the gimbal axes and angle encoders. Work at the UK's National Physical Laboratory identified the internal mechanical and optical alignments and the angular scales as the error sources that ultimately limit what a tracker can achieve [2]. On the shop floor, airflow, temperature, vibration and floor movement add to this [3].

What the standards test

ASME B89.4.19 and ISO 10360-10 are the two main performance standards. Their core test measures a known reference length in many positions and orientations and compares the error against the manufacturer's maximum permissible error (MPE) [2]. NIST researchers showed that these tests need to be sensitive to small tilts, offsets and eccentricities inside the tracker, because those misalignments produce systematic errors in the measured angles and ranges [4].

The 2021 interim test

The most useful change for users is the new interim test in ASME B89.4.19-2021 and ISO 10360-10:2021. According to NIST, it is more comprehensive and more sensitive to systematic errors than the interim checks in the previous editions, and it is designed to be run by users in the field. It has two parts: one detects geometric errors in the tracker, and the other checks the inclination sensor [5].

A practical routine

• Run the interim test on a regular schedule and after any shipping, drop or major temperature change, and keep the results with your measurement records.
• Record air temperature, part temperature and warm-up time alongside every job. They are the first things an auditor or customer will ask about.
• For large structures, consider measuring from several tracker stations. Multi-station networks are one of the main ways the literature reports improving large-volume accuracy [1].

References

1. Muralikrishnan, B., Phillips, S. and Sawyer, D. (2016). Laser trackers for large-scale dimensional metrology: a review. Precision Engineering, 44, 13–28.
2. NPL (2012). ASME B89.4.19 standard for laser tracker verification: experiences and optimisations. International Journal of Metrology and Quality Engineering.
3. ResearchGate record and citing work on external influences on laser tracker measurement.
4. Muralikrishnan, B. et al. (2009). ASME B89.4.19 performance evaluation tests and geometric misalignments in laser trackers. J. Res. NIST, 114.
5. Motta de Souza, M., Muralikrishnan, B., Lee, V. and Sawyer, D. (2023). Laser tracker interim testing per the ASME B89.4.19-2021 and ISO 10360-10:2021 standards. NIST IR 8469.