NPL Brings Frequency Scanning Interferometry to Large-Scale Manufacturing

Frequency scanning interferometry uses a tunable laser to provide highly accurate distance measurements. It was first developed by researchers at the University of Oxford who were working on the Large Hadron Collider at CERN. They needed a way to measure tiny movements of the particle detectors and there was no existing measurement technology that could provide the required accuracy over the tens of meters of the detector array. The system they developed used columnated laser beams. These had to be individually aligned with a single target. The UK’s National Physical Laboratory (NPL) has taken this state of the art technology and developed it into a much more practical and versatile measurement system, addressing the needs of large-scale manufacturing. The system they have developed enables a single measurement device to simultaneously direct multiple laser beams to many targets, measuring each with extreme accuracy.

Figure 1: The Divergent beam FSI lab at NPL.

Manufacturers of large-scale, high-value products have considerable measurement challenges. Products such as aircraft and wind turbines require coordinate measurements with uncertainties measured in the micro meters, over tens of meters. Laser trackers are widely used since they are the most accurate and practical coordinate measurement systems available for use at these scales. The columnated beam of laser trackers only allows them to measure a single target at any point in time. An interferometric or absolute distance measurement (ADM) technique is used to measure the distance to the target while angular encoders track the direction to the target. Cartesian coordinates are then calculated from this spherical measurement frame.

The accuracy of laser trackers is limited by environmental factors, most notably temperature gradients in the air. When light travels through a temperature gradient, differences in the refractive index cause it to bend slightly. On a hot day, extreme temperature gradients occur over surfaces which are giving off heat, for example where the hot ground has cool air blowing over it. When the gradient is steep enough we can observe the bending of the light, phenomena commonly referred to as a heat haze or a mirage. These effects cannot be observed with the naked eye in normal factor conditions, but they are still the dominant source of error in optical measurements which rely on knowing the direction of a beam of light, i.e. the angular measurements of a laser tracker and also photogrammetric measurements. More accurate measurements can be made using a laser tracker by measuring the target from multiple laser tracker stations and then calculating the coordinates using only the distance measurements, which are much less effecting by the bending of the light. This technique, known as multilateration, greatly increases the time and expense of measurement.

In particular, the aerospace industry requires a more accurate and faster way to measure coordinates at large scales. Reducing the fuel burn of aircraft means improving their aerodynamic efficiency, with manufacturers aiming for natural laminar flow wings in the next generation of civil aircraft. This will require measurements which are approximately an order of magnitude more accurate than what is achievable with a laser tracker. Increasing production rate and reducing manufacturing cost will require a much faster assembly process. Achieving part-to-part interchangeable assembly will also require a step change in large scale measurement accuracy.

NPL’s solution to this challenge is divergent beam FSI. It has the potential to track multiple targets at a frequency of 30Hz and accuracy of tens of micrometers. It is cost-effective because a single laser, detector and controller can be used to measure multiple distances. The laser is split and fiber-channeled to multiple measurement devices, each of which directs beams to all of the targets. The returned light follows the same path back to the detector, housed with the laser. Each distance is then uniquely determined from the returned light using Fourier transform analysis.

“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.”

Prof. Ben Hughes, Principal Research Scientist, NPL

Each divergent beam FSI sensor contains a low-cost illumination laser which is projected through a divergent lens to illuminate the measurement field. The direction of the retroreflective targets is then identified by imaging the returned light on a CMOS camera. The sensor also receives a fiber-channeled FSI laser beam and directs it through a Spatial Light Modulator (SLM) and lens. The direction information is provided to the SLM so that it can generate an individual FSI laser beam directed at each target. The light from the FSI laser, which is returned by the retroreflective targets, is then directed back into the fiber channel to be returned to the central FSI detector. A dichroic mirror directs the light from the two different lasers to the relevant detectors. This technique allows each sensor to measure the distance to multiple targets. Moving targets can be tracked and targets can be added or removed during dynamic measurement. When several sensors are positioned around a collection of targets, their coordinates can be measured using multilateration.

Figure 2: Each sensor can detect the direction of multiple targets and then measure their distance using FSI.

“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.”

Prof. Ben Hughes, Senior Scientist, NPL

Figure 3: NPL uses spherical targets produced from a special glass with a refractive index of two. These are capable of retro reflection—reflecting light directly back to its origin and from any direction.

NPL’s current prototype can measure targets within a 10 x 10 x 5m volume, but they intend to increase this amount. As development continues, NPL aims to license this technology to manufacturers within one to two years.