New direct scanning strategy improves accuracy and repeatability

The specific needs of an aerospace customer led to the development of an innovative new measurement strategy for direct scanning with the Leica Absolute Tracker ATS600

Developing new measurement strategies for the Absolute Tracker range

Technical article - Leica Absolute Tracker ATS600, part 1

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.

The Leica Absolute Tracker ATS600 is already widely used for a variety of applications within the aerospace sector.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.

After looking into the problem, the team developed a new measurement strategy that has been termed surface comparison point measurement. The concept involves taking a grid of closely spaced point measurements and using those results to calculate an average coordinate value for the central point of the grid.

Let’s assume that for a particular target object, 100 discrete points must be measured. The coordinates of those points can be imported to the software and around each one a small but dense grid of reflectorless measurement points will be automatically created. When the user clicks on “Measure”, the ATS600 will scan each point in the grid and then calculate a mean value representing the desired coordinate. This process eliminates outliers caused by scratches on the material. The recommended radius for the grid is between 4 and 8 millimetres, with the larger recommended for longer measurement distances as the laser beam diameter increases.

 

Improving accuracy by a factor of two, with repeatability drastically reduced to only 10 microns

The result is a relatively simple new measurement strategy that offers excellent accuracy and repeatability results, while still delivering significant time savings compared to reflector measurement.

Extensive testing by Hexagon’s laser tracker engineers has shown that this strategy can improve accuracy by a factor of two, with repeatability drastically reduced to only 10 microns. The new measurement can easily be implemented by those metrology software packages having an interface to the ATS600; customers should contact their local Hexagon representative if they are also interested in employing this new measurement strategy.
Measurement results using single-point direct scanning (left) | Results using 0.5 mm x 0.5 mm surface comparison point grids (right)

Based on these results, the aerospace company that initiated the investigation invested in 3 new ATS600 units. Compared to their existing setup and warmup workflows, inspection times will be reduced from 2.5 hours to just 45 minutes.

Additonally, the direct scanning method ensures workers’ safety as working at heights is no longer required. Further to this, the ability of the ATS600 to carry out both reflector and reflectorless measurements allows it to continue performing the current automated wing-joining routines with reflectors, while also being able to switch to new non-contact scanning inspection routines without multiple technology setups and teardowns.