QMT Features: April 2017
Shining a light on Large Volume Metrology
Andrew Lewis, LUMINAR* Project Coordinator, NPL, reports on recent outputs from a major European research project

How do you align a particle accelerator as big as the planned successor to the Large Hadron Collider or accurately measure large, next generation aircraft components during manufacture and assembly? Such questions in Large Volume Metrology (LVM) have triggered research by a collaboration of scientists across Europe  to deliver a range of new ideas and prototype measuring systems for use in LVM. The three-year collaborative research project (LUMINAR*) reached its conclusion in summer 2016 with simultaneous demonstration of the new systems and research outputs in a live industrial environment at Airbus, Filton, followed by a two-day workshop.

Large Volume Metrology plays a critical role in quality assurance and manufacturing control in many areas of advanced manufacturing, civil engineering and scientific research. Recently, several issues had been identified requiring further research: a need for measurements (often in harsh environments) of multiple targets simultaneously with accuracy better than photogrammetry; how to guarantee traceability in absolute distance meters (ADMs); achieving better compensation for refractive index and refraction (beam bending) effects; better compensation for thermal expansion at non-standard temperatures; and new issues associated with using LVM tools dynamically (i.e. with moving targets).

The project developed several new tools, techniques and facilities, in parallel which were then tested, inter-compared and then demonstrated at Airbus, Filton, in a live factory environment, before being discussed at a two-day end of project workshop. (Full details available at the project website – see footnote).

The issues of ADM traceability and refractive index compensation were addressed by CNAM and PTB which developed two new absolute distance measuring systems, both incorporating along-the-beam refractive index measurement and traceability to international references. CNAM developed a long-range telemeter operating at 1.55 µm (infra-red) using relatively cheap off-the-shelf optical components (see figure 1). Traceability to the SI is assured through a simple frequency measurement. The absolute distance capability was demonstrated over ranges up to 50 m with resolution and accuracy around 2 µm. Refractive index compensation was achieved using a second wavelength at 785 nm and delivered 500 µm accuracy at 50 m range - this will be improved in future work to closer match the ADM accuracy. This instrument is highly portable and designed for measuring and monitoring of long path lengths such as large aerospace assembly jigs and accelerator components’ positioning. Variations on the instrument can operate up to ~1 km range.

PTB, in collaboration with SIOS Meßtechnik, developed a next generation of 3D laser meter, based on existing laser tracer mechanics with a new design of optics and laser sources. Rather than a single red laser, the PTB device uses two high power lasers and modulation techniques to generate four wavelengths in both the visible (green) and infrared parts of the spectrum. Combinations of these four wavelengths generate signals which can either determine the absolute distance to the target or to compensate for the refractive index of the air along the beam path. Traceability to the SI metre comes through the use of an iodine-stabilised laser. The system is more complex than that of CNAM but offers higher accuracy (refractive index compensated length accuracy 1 µm + 0.1 parts per million; ADM accuracy 0.5 parts per million) and the ability to track moving targets. The PTB device is also shown in figure 1 which shows the CNAM and PTB devices being compared against a reference laser interferometer in the underground 50 m testing bench at GUM. This bench was upgraded during the project with variable temperature control (for simulation of industrial environments) and improved sensors for temperature, humidity and pressure monitoring.

 The development of new systems for multiple target metrology (including those in difficult environments) was tackled by INRIM and NPL. INRIM have developed a system called InPlanT – an in-plane measuring technique designed for operation in harsh environments where the precision metrology has to be separated from the target being tracked yet still has to make coordinate measurements inside the compromised volume. Three linear axes mounted outside the measurement volume carry moving carriages which track an optical target (a glass sphere) within the volume using a projected laser beam. Each measuring head makes a measurement within a single plane (hence the name InPlanT) which is insensitive to thermal or refractive index gradients in the same plane. Combining the three axial positions of the measuring heads gives the 3D coordinate of the target. Within the limited project funding, the INRIM system simulated a large 3D volume during testing at Airbus and achieved accuracies of around 50 µm. The InPlanT system is ideally suited to operation in harsh environments (e.g. radioactive, flammable) as the only items within the hazardous environment are small glass target spheres.

NPL developed a novel coordinate measuring system based on frequency scanning interferometry called OPTIMUM (OPtically Tracked Interferometric Measurement with Uncertainty using Multilateration). The system is analogous to well-known outdoor GNSS except that it operates indoors using infrared laser beams to simultaneously measure the distance to multiple targets from multiple measuring heads. When there are enough targets and measuring heads, the system equations can be solved mathematically to calculate the 3D coordinates of all targets and measuring heads together with uncertainties of those coordinates. The system can track multiple targets simultaneously in 3D – the system has been tested with up to 15 targets so far. The distance measurements are compensated for target vibration using a second laser source and an in-built quantum standard provides traceability to the SI.
 The University of Bath, developed a new hybrid modelling approach which uses a synthesis between live data (thermal and dimensional) measured at a carefully selected sub-set of locations on a 3D structure, and simulated thermal modelling, to achieve a significant improvement on conventional thermal expansion correction for large parts in industrial environments. The live data is used to iteratively correct a finite element model – measurements at a few critical locations are used to determine better corrections across the entirety of the assembly (see figure 4).

 University College London developed several systems for detecting and quantifying refraction effects (see figure 5) in large volumes, especially regarding their effect on photogrammetry (a common LVM technique). These include a multi-spectral photogrammetry target system for detecting refraction effects over long ranges, a digital axicon camera for high precision photogrammetric target measurement and volumetric refraction-enabled photogrammetry analysis software which can take refraction effects into account when performing conventional photogrammetric measurements.

 Research at KIT has focused on improving the ability of LVM to make real-time determination of reliable spatiotemporal uncertainties of arbitrarily moving objects, namely position and orientation, both with respect to time. This has required the development of real-time model of laser tracker kinematics with Bayes filtering as well as fluctuating refractive index model. Testing of the research was performed using a laser tracker with a constellation of four high-accuracy laser tracers used as a reference. The research showed that, in certain situations, an industrial robot’s motion control can be, at times, more accurate that a laser-tracker derived measurement of the robot position.

In summary, the LUMINAR project has advanced the state of the art in high accuracy LVM and generated much interest from a range of potential end users of the new technologies, some of which we expect to be developed into commercial products or services.
For further information, in the first instance please contact the project coordinator, Andrew Lewis, at NPL (andrew.lewis@npl.co.uk )
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