Supporting the measurement of large objects on the assembly line
Large volume metrology (LVM) is used 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, such as aircraft assemblies and engines, 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. The physical size (1 m up to hundreds of metres or km) can be influenced by thermal effects, refractive index and fixture stability.
We carry out research studies, measurement campaigns and contract-based research in large volume metrology for a range of customers.
Tracking laser interferometers, or laser trackers, are the most common large volume metrology instruments and 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. Our research 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
Frequency scanning interferometry (FSI) is a technique used for measuring absolute distances, as opposed to displacements, so it can cope with beam-break events and can measure multiple distances simultaneously.
We use FSI as a traceable measuring technique to deliver high accuracy 3D metrology for LVM applications. The technique allows data acquisition from multiple targets in a 3D volume, and use of multiple targets attached to a rigid structure will allow six degree of freedom (6DoF) measurements to be realised (3D position, three angular rotations).
We have 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. 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.