Development of a New Pavement Strain Coil Measuring System at CAPTIF

Abstract

The Canterbury Accelerated Pavement Testing Indoor Facility (CAPTIF) has been measuring plastic and elastic strain in unbound pavements for more than 40 years. The systems used for this type of measurement are based around inductive coil sensors. CAPTIF’s current system relies on commercially produced electronic circuit boards for LVDT instrumentation. These boards are being phased out by the manufacturer. This has necessitated a need to develop a replacement system. This paper documents the development of new instrumentation that will replace CAPTIF’s current system. The criteria for the new system was to be equivalent or better than CAPTIF’s current system providing stability for measuring plastic strains over long periods and high resolution when measuring elastic strains from dynamic loading events. The new instrumentation is based around a special purpose integrated circuit for LVDT signal conditioning, the Analog Devices AD698. By developing this instrumentation ‘in house’, CAPTIF has been able to meet the criteria with a significantly lower cost system than in the past. In addition, the new instrumentation is more adaptable to taking portable field measurements because of its simpler setup and programming requirements.