Design and Calibration of a Hall Effect System for Measurement of Six Degree-of-Freedom Motion within a Stacked Column

This paper presents the design, development and evaluation of a unique non-contact instrumentation system that can accurately measure the interface displacement between two rigid components in six degrees of freedom. The system was developed to allow measurement of the relative displacements between interfaces within a stacked column of brick-like components, with an accuracy of 0.05mm and 0.1 degrees. The columns comprised up to 14 components, each component being a scale model of a graphite brick within an Advanced Gas-cooled Reactor core. A set of 585 of these columns make up the Multi Layer Array which was designed to investigate the response of the reactor core to seismic inputs, with excitation levels up to 1g from 0 to 100Hz. The nature of the application required a compact and robust design capable of accurately recording fully coupled motion in all six degrees of freedom during dynamic testing. The novel design implemented 12 Hall Effect sensors with a calibration procedure based on system identification techniques. The measurement uncertainty was ±0.050mm for displacement and ±0.052 degrees for rotation and the system can tolerate loss of data from two sensors with the uncertainly increasing to only 0.061mm in translation and 0.088 degrees in rotation. The system has been deployed in a research programme that has enabled EDF to present seismic safety cases to the Office for Nuclear Regulation, resulting in life extension approvals for several reactors. The measurement system developed could be readily applied to other situations where the imposed level of stress at the interface causes negligible material strain, and accurate non-contact six degree of freedom interface measurement is required.