A Novel 3-D Embedded Module for Displacement Measurement in Metal Structures

Measurements of relative displacement between the components of structural steel frameworks such as those used in large buildings, bridges, ships, and offshore platforms can be used to detect overstress or impending failure. However, current approaches used to measure displacement require complex arrays of optical sensors or strain gauges. We instead present a novel low-cost wireless 3-D embedded module that uses inductive eddy-current sensing with planar printed circuit board (PCB) coils to measure relative displacement between two or more metal plates. The module is a 50-mm cube with four faces formed by the planar PCB inductors that are used for measurement of relative displacement. The other faces and the hollow center of the cube are used for a four-channel inductive sensing IC, a microcontroller with integrated 802.15.4-compatible radio, antenna, micro-USB serial connector, battery, and battery charger/monitor. The addition of temperature and humidity sensors and a microelectromechanical system inertial measurement unit makes the module into a full structural health monitoring system that can also sense environmental factors, inclination, vibration, and shock. The module was evaluated in two axes in a custom test system that allowed controlled relative movement to two orthogonal mild steel plates, as well as inclination of the plate in a different test setup. The results show that a resolution of 0.5 mm can be achieved in both axes when measuring displacement from the planar coils to the steel plates, with negligible interference between coils/plates on the adjacent faces. The displacement measurement is immune to the presence of water between the planar inductors and the steel plates, allowing the module to be used in applications exposed to high humidity and rain.