Tracking laser interferometers

Laser trackers are revolutionising large volume measurement. High speed, long range and small uncertainties are the benefits of this technology. But how well do these instruments really work?

Technology

Over the last two decades, the laser tracker has become a popular measuring tool for users needing to make precision measurements of items that are simply too big to be handled by Coordinate Measuring Machines (CMMs). Laser Trackers are now sold commercially by several manufacturers but, at their heart, they are all based on the same measurement principles.

Fundamentally, a laser tracker is a polar coordinate measuring system – it makes measurements in terms of two angles (horizontal and vertical) and a distance (radius). Simple trigonometry converts these into the usual Cartesian coordinates people are used to dealing with (x, y, z). The distance measurement is performed using either a laser interferometer or an absolute distance metre (ADM) and the two angles are measured using angular scales mounted in the tracker mechanics. The item that is measured directly is the location of a spherically mounted retro-reflector (SMR). Special tooling fixtures for the SMRs allow the measurement of not just individual point locations, but also of surfaces (flatness, level), angles (squareness), surface form and alignment between parts. By combining laser trackers with other measuring systems, such as laser scanners, large objects with complex surfaces can be measured.

Calibration, verification and training

However, as with any measurement device, the best performance can only obtained when the operator is properly trained and understands the measurement principles. Poor tracker location, unstable mounting, bad environment or poorly chosen reference locations can all lead to measurement errors. NPL has identified laser trackers as one of the technologies of large volume metrology where structured, guided learning is essential. NPL has recently launched a large volume metrology training programme.

NPL and PTB have jointly developed a method of directly measuring the parametric errors of CMMs and machine tools using a single laser tracker and a form of multilateration. This patented technique has the following advantage over other techniques: high accuracy; no physical artefact is required; the method work for any size of machine and the technique is much quicker than other processes. NPL uses this technique to offer its on-site CMM/machine tool verification and calibration service.

As with any metrology tool, a laser tracker needs to be calibrated and its performance verified. NPL has set up a service for the calibration and verification of laser trackers (laser wavelength calibration, performance verification to ASME B89 or similar standards).