New direct scanning strategy improves accuracy and repeatability

Since the launch of the Leica Absolute Tracker ATS600, the system’s ground-breaking direct scanning capability has opened a wide range of new metrology-grade inspection applications – measurements that were previously unfeasible can now be carried out with ease, allowing for greater quality control over large-scale products than ever before possible.

Hexagon’s laser tracker team were contacted during 2022 by a major aerospace customer interested in changing their current measurement process (which involved a tracker and reflector measurement process) to direct scanning with the ATS600. Their current process involved an operator being given a list of surface points on the part, and then bringing a reflector to each of these locations to take a measurement. Points were only recorded once the reflector reached the coordinate. Due to the size of the part, a scaffold and safety equipment is required, which needed to be repositioned multiple times due to the typical size of the parts being measured. This was an extremely time-consuming process, which was the key motivator for investigating the switch to direct scanning. 

The company were however concerned about whether the performance of the ATS600’s direct scanning would reach their tolerance requirements – Hexagon have been open about the convenience of direct scanning coming at the cost of reduced accuracy when compared to reflector inspection. A round of testing was planned with the business’ local Hexagon team and a demonstration unit, with the goal of showing whether or not the reflectorless measurement results would be good enough to meet their tight aerospace tolerances.

During the planned testing, the first measurements performed in parallel with a reflector and with direct scanning did not give matching results. This variability was believed to have been a result of the quality of the surface, which was scratched and had marks from the previous manufacturing process. As the local team believed this was an issue of variability rather than a pure inability to meet the required level of accuracy, they contacted Hexagon’s team of laser tracker experts based in Switzerland, where the Absolute Tracker range is designed and constructed, for assistance.

The team began by constructing a series of experiments, during which a brushed steel plate was measured at various distances (from 2 to 15 metres) and incidences (0° to 60°) to the ATS600. The plate was initially measured with a 1.5” reflector from each distance and incidence, with 21 points taken and used to fit a plane. Subsequent direct scanning measurements showed the same variability issue that had been found by the local team on site with the aerospace customer.