Sources of variability in the set-up of an indoor GPS

Abstract

An increasing demand for an extended flexibility to model types and production volumes in the manufacture of large-size assemblies has generated a growing interest in the reduction of jigs and fixtures deployment during assembly operations. A key factor enabling and sustaining this reduction is the constantly expanding availability of instruments for dimensional measurement of large-size products. However, the increasing complexity of these measurement systems and their set-up procedures may hinder the final users in their effort to assess whether the performance of these instruments is adequate for pre-specified inspection tasks. In this paper, mixed-effects and fixed-effects linear statistical models are proposed as a tool to assess quantitatively the effect of set-up procedures on the uncertainty of measurement results. This approach is demonstrated on a Metris Indoor GPS system (iGPS). The main conclusion is that more than 99% of the variability in the considered measurements is accounted for by the number of points used in the bundle adjustment procedure during the set-up phase. Also, different regions of the workspace have significantly different error standard deviations and a significant effect on the transient duration of measurement. This is expected to affect adversely the precision and unbiasedness of measurements taken with Indoor GPS when tracking moving objects.

Keywords:

• large scale metrology
• large volume metrology
• distributed coordinate measuring systems
• Indoor GPS
• iGPS
• uncertainty

Acknowledgements

This study is part of the research initiatives of The Bath Innovative Design and Manufacturing Research Centre (IdMRC), which is based in the Department of Mechanical Engineering of the University of Bath and which is supported by the United Kingdom Engineering and Physical Sciences Research Council (EPSRC). In particular, this investigation was carried out within the scope of the IdMRC research theme ‘Metrology, Assembly Systems and Technologies’ (MAST), which is coordinated by Professor Paul Maropoulos.