National Physical Laboratory

Large Volume Dimensional Metrology

With users in aerospace, automotive, robotics, energy generation and civil engineering, the measurement and control of large objects, parts and assemblies is critical to much of modern life. NPL's research and services in large volume metrology support these diverse communities.

Image courtesy of iStockphoto
Image courtesy of iStockphoto

Overview

Large Volume Metrology (LVM) takes over when items are too big or difficult to move to a separate measuring device such as a Coordinate Measuring Machine. This often means that items must be measured in situ, at the point of manufacture or assembly. This brings a set of challenges not encountered in typical calibration laboratory environments - apart from the physical size (1 m up to hundreds of metres or km) thermal effects, refractive index and fixture stability can disturb measurements.

Laser tracker research

The most common LVM instruments, tracking laser interferometers or laser trackers, can achieve remarkable uncertainties if used in the correct way. However, despite their conceptually simple design (measure two angles and one distance), there are many internal and external factors which affect the accuracy that can be achieved by an operator using a laser tracker. NPL's research with laser trackers is developing best practice guidance and fast approaches to mapping the internal errors of the tracker and determining the likely achievable accuracy in use.

Frequency scanning interferometry research

Frequency Scanning Interferometry (FSI) is a technique used for measuring absolute distances (as opposed to displacements). NPL is using FSI as a traceable measuring technique to deliver high accuracy 3D metrology for LVM applications. The technique will allow for high speed data acquisition from multiple targets in a 3D volume, and use of multiple targets attached to a rigid structure will allow 6 degree of freedom (6DoF) measurements (3D position, 3 angular rotations).

Error mapping of large CMMs and machine tools

NPL and PTB have jointly developed a method of directly measuring the parametric errors of CMMs and machine tools using a single laser tracker and a form of multilateration. This patented technique has the following advantage over other techniques: high accuracy; no physical artefact is required; the method works for any size of machine; the technique is much quicker than conventional approaches. As well as being suited for use with laser trackers, a dedicated high accuracy laser tracker such as those developed by NPL and the laserTRACER from Etalon can achieve even smaller uncertainties.

Image courtesy of iStockphoto
Image courtesy of iStockphoto


NPL software tools

As part of this work NPL has developed a range of sophisticated software tools for solving generic multi-sensor metrology networks and for determining the uncertainty associated with the target locations and other system parameters. The techniques we have developed are applicable to any measurement network involving multiple sensors, be they distance, displacement or angle based, or any combination. This flexibility makes these tools particularly useful for assessing the performance of new classes of large volume dimensional measurement systems such as laser radar, indoor GPS, etc.

These software tools are applicable to many of today's challenging advanced manufacturing and assembly processes in the aerospace, automotive, defence and power generation fields.

EMRP Project 'LUMINAR'

To start to address some of the most urgent and generic problems in LVM, NPL is leading a consortium of European metrology laboratories and universities in a European Metrology Research Programme (EMRP) project into Large Volume Metrology. The project, entitled 'LUMINAR' is developing several new technologies and will demonstrate these in end user environments.

Large Volume Metrology Conference

NPL is a corporate sponsor and NPL members of staff are regular attendees and presenters at the annual Large Volume Metrology Conference (LVMC) event which takes place each autumn.

Research for third parties

As well as the generic large volume research work, NPL also undertakes research studies, measurement campaigns and contract-based research in Large Volume for a range of customers.

Contact: Ben Hughes

Last Updated: 1 Oct 2014
Created: 17 Jan 2011

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