Semi-Active Smart Rocks with Flipping Magnets

Scour at bridge foundation accounted for most of the bridge collapses in the United States. Passive smart rocks with embedded permanent magnets have recently been proposed for bridge scour monitoring. Once deployed around a bridge foundation as field agents, properly-designed passive smart rocks can roll to the bottom of a scour hole naturally and their positions are directly correlated to the maximum scour depth, which can be remotely determined by a magnetometer. However, the magnetic field intensity is greatly influenced by the changing orientation of magnets as they rotate in the process of scour. In this study, a permanent magnet is placed inside many turns of an electric coil so that the rotation of the magnet can be controlled by external power supplied to the electric coil. When cast inside a rock, the entire system is referred to as a semi-active smart rock. A prototype semi-active smart rock was designed and tested for its dynamic range of measurements, data repeatability, and differentiability from the effect of other ferromagnetic substances. To ensure free rotation, the permanent magnet is placed inside a solid sphere (inner) and in turn enclosed by another plastic shell (outer) with their gap filled with low viscosity liquid. When the coil is excited with electric current, the magnet and the inner sphere rotate back and forth, according to the time function of the applied current. The total magnetic field intensity produced by the semi-active smart rock and other ferromagnetic substances can be measured by a remote magnetometer. Due to the known time function of applied current, the effect of a rotating permanent magnet can be reliably separated from that of passing-by ferrous objects or another smart rock with embedded magnet in practical applications. This new feature with semi-active smart rocks can lead to improvement in measured data quality and reduction in rock localization error.

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