Examining the metrology-grade application of Wave Form Digitiser technology 

Abstract

In early 2019 a ground-breaking new system came to market: the Leica Absolute Tracker ATS600. For the first time, a laser tracker that could not only track a spherical mounted reflector but was also uniquely able to scan surfaces directly with no handheld device or part preparation required.

To make this possible, the core expertise of Hexagon was applied to enable direct laser tracker measurement in the most demanding situations. Key challenges in the development of the ATS600 included ensuring good performance on a diverse range of materials and in spite of the shallow angle of incidence with surfaces often faced during measuring tasks, as well as achieving sub-millimetre accuracy without any part preparation. Combined with this was the need to retain the capacity for high-accuracy measurement with reflectors that would allow for properly aligning point clouds captured from multiple stations.

This white paper describes in detail the different strategies applied to meet those requirements. Its focus is on the fundamentals and peculiarities of the Wave Form Digitiser (WFD) technology on which the ATS600 is built. The paper also describes some of the strategies taken to ensure the metrology-grade quality of point cloud data on a wide range of different materials. Information on areas such as signal accumulation and attenuation are also covered, to provide a more complete foundation for understanding the working principles of the ATS600.

Content

Abstract	3
Introduction	4
The Wave Form Digitiser	4
Time-of-flight measurement	4
Signal accumulation	5
Signal delay	5
Dynamic attenuator	6
Angle of incidence	6
Beam width and angular resolution	7
Optimised measurement modes	8
Other key features and characteristics	8
Overview camera and PowerLock	8
Selective scanning	8
Laser safety	8
Accuracy specifications	9
Accessories	10
Applications	10
Summary of benefits	11
References	11

Introduction

As data analysis has become increasingly important in the world of industrial production, manufacturers are applying the tools of metrology in ever more areas. This includes large-scale measurement that is beyond the scope of, or too time-consuming to perform with, traditional metrology equipment. A solution taken up by more and more manufacturers of extremely large parts has been the application of terrestrial laser scanners within the industrial context. Such equipment means sacrificing accuracy and workflow, but for many applications this still represents a better solution than applying industrial metrology tools on a scale for which they are not designed.

However, in most industrial applications, the scanning of only selected areas would be much preferred to the fulldome scanning delivered by terrestrial laser scanners. While the data acquisition aspect of a full-dome scan can be quickly completed, the post-processing required to clean all the unnecessary data and stitch the point cloud as accurately as possible is very time consuming. Some terrestrial scanners even require several separate pieces of software to enable: a) point-cloud acquisition; b) point registration or the combination of multiple point clouds into a common coordinate system; c) comparison with CAD; and d) the production of a usable inspection report.

The Leica Absolute Tracker ATS600 was designed to fill this gap in the market as the first scanner for ultra-largescale measurement that delivers both metrology-grade results and a productive workflow in step with that of existing industrial metrology processes. It was built on the experience gained in developing multiple proven technologies: the Leica Nova MS50, Leica BLK360 and other scan stations that successfully established Wave Form Digitiser technology in geomatics applications; the revolutionary design and motorisation of the Leica Absolute Tracker AT930 and AT960 laser trackers; and the Leica Absolute Tracker AT403 with its PowerLock automatic target recognition functionality when tracking reflectors.

The key challenge in creating the ATS600 was improving the accuracy of WFD technology. The ‘time-of-flight’ measurement concept on which the technology is based has until now typically been used in surveying applications, where tolerances are much less stringent than in industrial metrology. A number of strategies were applied in combination to meet this goal, as will be outlined in the following pages. A straightforward integrated measurement process was also considered vital to the success of the ATS600. The easiest way to achieve this was to ensure the system was compatible with metrology packages already known in the industrial metrology world, so the established Leica Metrology Foundation (LMF) platform became an important part of the development.

It was clear that some potential customers would also require traditional laser tracking capabilities in order to use high-accuracy reflector measurement for specific tasks. This added another layer of complexity to the development of the ATS600.