The next generation of civil aircraft are intended to achieve greater fuel efficiency, with natural laminar flow wings. They will also have to be built more quickly and at reduced cost, necessitating part-to-part interchangeable assembly. Such advances will require a step change in large scale measurement accuracy.

Frequency Scanning Interferometry (FSI), a measurement technique originally developed for the Large Hadron Collider, can provide this accuracy. The UK’s National Physical Laboratory (NPL) has developed an FSI-based coordinate measurement system which promises to allow high accuracy coordinate measurements of multiple targets simultaneously. Could this be the new standard for airframe builders?

The most accurate large scale coordinate measurement systems currently available are laser trackers. These use a columnated laser beam to measure the distance and direction to a single target, allowing the target coordinates to be determined. The measurement of distance is considerably more accurate than the direction. This is due to the way that light bends slightly as it passes through temperature gradients in the air.

In extreme circumstances we can observe this effect as a ‘heat haze’ or mirage. Therefore, more accurate measurements require targets to be measured from three or more laser tracker stations so that the coordinates can be calculated using only the distances; a technique known as multilateration.

However, multilateration either takes a lot more time or requires multiple, very expensive, laser trackers. Also, laser trackers can only measure a single target at a time. Currently, the main alternative is photogrammetry, which can measure multiple targets simultaneously, but its accuracy is generally not as good as a laser tracker. Furthermore, the higher the frequency of measurement, the lower the accuracy.

“The main advantages of combining FSI with multilateration are that the system can be made directly traceable to the SI meter, can calibrate itself as an inherent part of the measurement process and it can provide rigorous measurement uncertainty estimates taking into account environmental conditions such as vibration and air temperature gradients or turbulence,” according to Professor Ben Hughes, Principal Research Scientist at NPL.

NPL’s divergent beam FSI aims to overcome these limitations in a system which can potentially track multiple targets at a frequency of 30 Hz and accuracy of tens of micrometers. FSI allows a single laser, detector and controller to measure multiple distances. The laser is split and fiber channeled to multiple measurement lines before being returned to the single detector. Fourier transform analysis of the returned light then allows the signals from multiple lines to be sorted out so that each distance can be uniquely determined.

This technique was first developed by a team at the University of Oxford to measure movements of the detectors in the Large Hadron Collider at CERN. The system at CERN used columnated laser beams, each of which had to be precisely aligned with a single target and required a detector for each line.

In the NPL system, each fiber channeled laser beam is directed through a Spatial Light Modulator (SLM) and lens. This produces multiple individual laser beams, each directed at a different target. The light reflected from each target is collected by the lens and directed back into the fiber channel. In this way, each sensor measures the distance to multiple targets. By arranging a number of sensors around a group of targets, their coordinates can be measured highly accurately by multilateration. The targets used are glass spheres with a refractive index of two, these always reflect light directly back to its origin, or retro-reflect, and can do this from any direction.

“The idea was prompted by discussions with end-users of portable coordinate measurement systems who often asked us about how they could be sure the system was operating within specification and if it was, how accurate it was or what would the measurement uncertainty be. These are difficult questions to answer in general, so we decided to see if we could develop a system that calibrated itself and provided measurement uncertainty estimates for every coordinate measured,” says Hughes.

The reflected light, returned through the lens, is split before being returned to the FSI detector. This allows the positions of the targets to also be imaged by a CMOS (complementary metal oxide semiconductor) array, like those used in digital cameras. The camera image is used to determine where the targets are in order to direct the laser beams at the targets. A red illumination laser is used to make the targets more visible to the camera. In this way moving targets can be tracked and new targets added to the scene can be included in the measurement.

The current system is able to work within a volume of 10 x 10 x 5 m but it is hoped that this can be extended in the future. NPL hope that this technology will be ready to license to manufacturers within 1-2 years.
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