Direct scanning to simple contrast targets to fit existing industrial processes

As-built documentation is a requirement for many industrial applications where formal documentation of the project installation is necessary, and shipbuilding is no exception. Accurate inspection is essential to ensure that the ship remains straight and does not bend due to small deviations in the manufacturing process, and this inspection requires reliable verification that is capable over the large scales required when it comes to ships.

The use of terrestrial laser scanners provides a large point cloud that is suitable for the construction segment, and this is a widely used solution in the industry for documentation purposes. These devices are however simply not accurate enough for the proper dimensional verification also required in the shipbuilding sector.

Following discussions with a group of industry experts, our engineers began to explore the use of the Leica Absolute Tracker ATS600 for high-accuracy inspection in shipbuilding, as a system to use alongside the hemispherical terrestrial scanner documentation process already well established in the industry.

The concept is based on the unique direct scanning capability of the ATS600 laser tracker, which allows it to measure surface points with metrology-grade accuracy from a distance of up to 60 metres with no need for a target at the point of measurement.

The manufacturing process in shipbuilding involves welding panels to create subassemblies, which are then welded to form a module. Different modules are welded together to form the complete ship. It is crucial to ensure that the ship remains straight throughout this process, and terrestrial laser scanners are currently often used for documenting the final state of the module or entire ship. However, they lack the metrology-grade accuracy of the ATS600, making them unsuitable for validated quality inspection and reporting.

Based on knowledge of this process, a solution was proposed that would combine the direct scanning capabilities of the ATS600 with the use of simple contrast targets commonly used for dimensional registration in the industry. While the ATS600 can certainly measure to a reflector in the traditional laser tracker style – a functionality simply not possible with any type of terrestrial scanner – reflectors are expensive, and therefore unlikely to be an acceptable alternative.

Our experts needed to show that simple, inexpensive black and white contrast targets where measurable with the ATS600, while maintaining the required accuracy level of measurement to within a tolerance of 1 millimetre.

The Leica GZT21 black and white target was tested and found to be a low-cost alternative to reflectors with the ATS600, with accuracy achieved to well within 1 millimetre. Three tests were performed: repeatability tests at 5, 10, and 20 metres in different rotations of the target; a best fit alignment of five contrast targets in five different tracker positions; and a best fit alignment of five 1.5” red-ring reflectors (RRRs) in the same tracker positions for comparison.

Hexagon’s SpatialAnalyzer software offers a feature to automatically extract the centrepoint of the target and adjust the angle of inclination of the scan to ensure the homogeneous distribution of the points on the target. It is recommended to scan the target at a 45-degree angle to achieve the highest accuracy and equally distributed point spacing.

The testing results showed that while reflector measurement is certainly more accurate, measurement to a Leica GZT21 black and white target was consistently within tolerance at all measurement positions. This establishes that the proposed solution is a valid option for achieving accurate point clouds with low cost and high accuracy by combining the direct scanning capabilities of the ATS600 with the use of common targets for accurate registration, with the Leica GZT21 black and white target identified as a low-cost alternative to traditional laser tracker reflectors.

This solution facilitates as-built documentation in shipbuilding while also ensuring accuracy in inspection tasks, without demanding major changes to current process standards. Once established as part of the inspection and documentation process in shipbuilding, the ATS600 can have even more to offer by directly assisting in the assembly process. Using the tracker in build mode, the user can make use of the system’s live DRO to guide the part to a specified position and subsequently weld it in place with much greater reliability than with the current typically manual process.

Figure 1: Results of the repeatability test at various distances – 20 runs per position and target rotation.

Figure 2: Best fit – reference position to measured positions.