A non-contact structural health monitoring method based on radio frequency signal analysis
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Structural health monitoring (SHM) is a group of technologies enabling the monitoring of structural motion during external loading. This loading is often caused by earthquakes. SHM provides understanding of the severity and location of damage to a structure without requiring visual structural inspection. The technologies used to perform SHM require input measurements taken from the structure during seismic events.
Many modern implementations of SHM instrumentation networks use accelerometers or strain gauges to obtain data. For parametric SHM methods in particular, this data must be converted to displacement to determine the severity of structural property changes. This necessity means 1ltering must be applied to structural acceleration data, which removes crucial response information that can indicate damage. A method of directly measuring displacement was thus sought.
Research into methods of direct measurement of displacement indicated frequencymodulated continuous wave (FMCW) radar was suitable. This technology uses mixing of transmitted and re2ected signals with time-varying frequency to determine signal time-of-2ight. Simulation of this technology in a 1D channel indicated the method would be precise enough for SHM data collection. Research indicated interstorey drift ratios (IDRs) as small as 0.2% would need to be detected to identify slight damage. Simulation showed that with the use of signal processing techniques, a sub-millimetre target displacement precision could be obtained, which is much smaller than the 5mm displacement resolution requirement found in literature.
A hardware prototype was constructed for experimental testing on a shake table. This prototype was constructed with components that allowed for maximum con1gurability to test radar systems with different input parameters. The table was driven with historical earthquake acceleration data. The mean equivalent IDR error found with the use of cross-correlation and multitaper signal processing methods was 0.05 %, indicating the concept of radar-based SHM was viable.
A pair of structural instrumentation schemes using FMCW radar sensors were devised. The 1rst of these methods requires the placement of two radar transceiver units in adjacent corners of a 2oor, and two corner re2ectors in opposite corners. The second method uses a single transceiver unit placed in the centre of a 2oor, with two corner re2ectors placed in upper adjacent corners. The former method allows for more precise monitoring, while the latter method is suitable for structures with signi1cant centrallylocated structural material or in situations where cost is a limiting factor.
The possibility of using existing wireless local area network (WLAN) hardware for SHM purposes was investigated. It was found that the IEEE 802.11 5GHz band had suitable bandwidth for implementing FMCW radar-based SHM. Simulation of a system using standard-compliant transmission demonstrated that cheap installations of SHM are possible using this method, allowing the use of SHM to become more widespread and thus public safety to be improved.