TLS and GB-RAR Measurements of Vibration Frequencies and Oscillation Amplitudes of Tall Structures: An Application to Wind Towers

Abstract

This article presents a methodology for the monitoring of tall structures based on the joint use of a terrestrial laser scanner (TLS), configured in line scanner mode, and a ground-based real aperture radar (GB-RAR) interferometer. The methodology provides both natural frequencies and oscillation amplitudes of tall structures. Acquisitions of the surface of the tall structure are performed by the TLS with a high sampling rate: each line scan provides an instantaneous longitudinal section. By interpolating the points of each line, oscillation profiles are estimated with a much better precision than each single point. The amplitude and frequency of the main oscillation mode of the whole structure are derived from the TLS profiles. GB-RAR measurements are used to measure the vibration frequencies of higher oscillation modes which are not caught by the TLS due its lower precision in the measurement of displacements. In contrast, the high spatial resolution of TLS measurements provides an accurate description of oscillation amplitude along the tower, which cannot be caught by the GB-RAR, due to its poorer spatial resolution. TLS and GB-RAR acquisitions are simultaneous. The comparison with the analytical solution for oscillation modes demonstrates that the proposed methodology can provide useful information for structural health monitoring (SHM). The methodology does not require the use of targets on the structure and it can be applied during its normal use, even in presence of dynamic loads (wind, traffic vibrations, etc.). A test was carried out on a wind tower where the synergistic use of TLS and GB-RAR made it possible to fully describe the spectral properties of the tower and at the same time measure the amplitude of the first oscillation mode along the tower with a high spatial resolution.

Health Monitoring of Civil Infrastructures by Subspace System Identification Method: An Overview

“…The SSI is a numerically reliable algorithm and it estimates models with good quality, particularly for multivariable systems [85,86]. The state-space form of the equation of motion can be written as Equation (2).…”

Application of Terrestrial Laser Scanning (TLS) in the Architecture, Engineering and Construction (AEC) Industry

“…Deformation measurement is focused on changes in the relative position of a structure, which requires the collection scanning data at a certain interval to maintain periodic monitoring of the structural response by comparing data at different time points. Various case studies have been carried out on the monitoring of engineering structures, including buildings [ 104 , 145 ], dams [ 113 , 122 , 126 , 130 ], bridges [ 59 , 100 , 105 , 106 , 108 , 109 , 110 , 116 , 118 , 119 , 121 , 123 , 124 , 128 , 137 , 144 ], tunnels [ 57 , 80 , 103 , 107 , 114 , 115 , 117 , 125 , 127 , 129 , 131 , 132 , 134 , 135 , 136 , 139 , 140 ], stations [ 99 , 143 ], foundation pits [ 186 ], pipe racks [ 52 ], towers [ 101 ], and many others. In addition, another main direction of studies for deformation measurement is to perform structural health monitoring of infrastructures that are in service for a long time, especially for masonry [ 99 , 1...…”

Wind Turbine Tower Deformation Measurement Using Terrestrial Laser Scanning on a 3.4 MW Wind Turbine

“…The transition between the concrete and the steel towers was visible at a height of around 57 m. The second sharp bend can be explained by the tower's geometry: the steel tower increased slightly in diameter before thinning out. Since a hybrid tower was used in this study, which has a more complex geometry compared to the regular steel towers, the method of fitting a third-degree polynomial through the data as described in [12] cannot be used here. By fitting other curves through the data points, a measurement of the tower's deformation in the axial direction would be possible; however, this study was focused on presenting the contour line.…”

“…Terrestrial laser scanning (TLS) is a known technique to monitor the deformation of large-scale objects from a single access point, without the need for reflective markers, from a measurement distance of over 150 m. TLS has been used to study the deformation of, for example, bridges, dams, and towers [8,9]. Schill and Eichhorn [10], Mitka et al [11], as well as Artese and Nico [12] applied TLS to study the deformation of a wind turbine tower. They used laser scanners in line-scanning mode, scanning vertically along the tower axis.…”