QMT Features: November 2010
How to quickly check your laser trackers for measurement accuracy, understanding laser tracker targets and speeding up large scale assembly of composite structures - some of the topics at LVMC'10.

A major theme of this year’s Large Volume Measurement Conference, LVMC’10, was  understanding laser trackers. Held at Chester racecourse on 2-3rd November, LVMC is hosted by Airbus and is the only European event solely dedicated to portable and large volume 3D measurement technology.

Ben Hughes, (ben.hughes@npl.co.uk)  from the UK’s National Physical Laboratory, proposed a new technique to determine the systematic errors of laser trackers aimed at endusers who might want to know how accurate their system really was without getting  into complex and time consuming testing.  Presenting on the theme: “Determining laser tracker alignment errors using a network measurement,”

Ben Hughes outlined a procedure which took into account tracker misalignments, offsets, angle encoder errors.  Based on a network of fixed points, measurements are taken  from a number of different tracker positions. All parameters are obtained simultaneously from the solution of a network measurement and uncertainties are calculated. Software mathematically fits the observed range and angle data to the error model. No specialist hardware is required, no calibrated artefact needed and  no prior knowledge of error parameters. All this is completed quirky and easily within 1 hour.

To verify the accuracy of the NPL method, it was tested against the B89.4.19 2006 laser tracker testing standard using a manufacturer’s error model. It was demonstrated that the technique can correct an instrument to within MPE. The new testing approach is seen as a first step in incorporating tracker measurements in a framework of traceable dimensional metrology.

Laser tracker targets
Developing the theme of the quality of laser tracker measurement,  “Understanding Laser Tracker Targets” by Ken Steffey (Ken.Steffey@faro.com), from Faro Technologies, looked at how the precision of the targets used can determine the outcome.   Laser tracker targets are complex, mechanical structures of precision optics, precious metals, high-performance adhesives and near-perfect geometry. The job for the laser tracker target is a simple one: to return the laser beams exactly as they are sent from the tracker.

In practice, it can be difficult to to maintain the required build tolerances and manufacturing processes for consistent production of these opto-mechanical systems.  “It is critical that every target is evaluated by sophisticated instruments to validate  the individual performance,“  said Ken Steffey. “It is important that the laser tracker operators understand the various specifications  of tracker targets and how deviations can contribute to poor tracker performance or errors in the measurements.”

Regardless of the type of laser tracker target used - cateyes, repeatability targets or  spherically mounted retroflectors  (SMR)  - some simple best practice  can minimise any errors.  Ken Steffey outlined some  of these in relation to SMR targets. For example, when measuring always  keep the SMR oriented in the same direction: the easiest practice is to keep the serial number  of the logo facing up at all times.   This practice minimises the effect of SMR runout errors such as poor centring and dihederal angle errors. Another is, when placing the SMR into nests or precision tooling, rotate the SMR back and forth a few degrees  before taking the point. The movement in the nest will push  away any metallic dust or other particles that can cause the SMR  not to set in the nest accurately.

Considering the potential of the SMR characteristics changing due to use, abuse, manufacturing defects or poor design, it is in the best interest of the operator that the SMR is quickly checked  before every critical measurement job. In the event that the SMR does fall, the same test can be used to verify that it was not damaged  and the measurement session can continue.

Measurement assisted  determinant assembly (MADA)   of large composite structures was presented by Jody Muelaner (www.muelaner.com)  from The Lab for  Integrated Metrology Applications, University of Bath.  Using an example of a novel wingbox design and production process, Jody Muelaner illustrated how MADA can facilitate lean production of aerospace structures. His thesis is that the aerospace industry has not benefited from the  significant reductions  in production cost and cycle time that can result form greater assembly efficiency, part-to-part  interchangeability and the use of flexible automation. This is due largely to the very high accuracies  required across large scale assemblies. The essence of MADA is that it makes possible, through the application of measurement assisted processes and design for manufacture, to economically produce all of the large components with relatively slack tolerances and still achieve part-to-part.

MADA gives a complete and generic set of design principles and production techniques. The fundamental principal of MADA is that large components can be economically produced with inaccurate interfaces and hole positions and these can be accurately measured.  Bespoke smaller components can subsequently  be produced to high accuracy with respect to these measurements. The complete assembly can then be put together with part-to-part holes and interfaces facilitating determinant assembly.

The MADA approach, said Jody Muelaner, is  a more radical departure from from the conventional aerospace assembly process with greatly reduced process step, no requirement for monolithic tooling solutions and almost all the work being carried out at the component manufacturing stage to allow rapid assembly.l
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