Abstract
One of the most popular options in the Structural Health Monitoring field is the tracking of the modal parameters, which are estimated through the frequency response functions of the structure, usually in the form of accelerances, which are computed as the ratio between the measured accelerations and the applied forces. This requires the use of devices capable of synchronously recording accelerations at several points of the structure at high sampling rates and the subsequent computational analysis using the recorded data. To this end, this work presents and validates a new scalable acquisition system based on multiple myRIO devices and digital MEMS (Micro-Electro-Mechanical System) accelerometers, intended for modal analysis of large structures. A simple form of this system was presented by the authors in a previous work, showing that a single board with some accelerometers connected to it got to obtain high quality measurements in both time and frequency domains. Now, a larger system composed by several slave boards connected and synchronized to a master one is presented. Delays lower than 100 ns are found between the synchronised channels of the proposed system. For validation purposes, a case study is presented where the devices are deployed on a timber platform to estimate its modal properties, which are compared with the ones provided by a commercial system, based on analog accelerometers, to show that similar results are obtained at a significantly lower cost.
Highlights
The need for cost-effective systems capable of detecting and recording the dynamical response of large and relevant structures is undeniable
The values obtained with the proposed system and the Frequency Domain Parameter Identification algorithm (FDPI) algorithm are compared to the ones obtained with the reference system and the CF algorithm by means of the relative error, whose expression is shown in Equation (6) and where the symbol x may stand for the natural frequency or the damping ratio
The mode shapes are compared through the known as Modal Assurance Criterion (MAC) [38], whose mathematical expression is shown in Equation (7), which is a coefficient conceived to compare two complex column vectors φi and φk, regardless their scaling and rotation in the complex plane
Summary
The need for cost-effective systems capable of detecting and recording the dynamical response of large and relevant structures is undeniable. To be considered as such, an SHM system requires a set of sensors and a proficient data logger In this sense, the low-cost system presented in the previous paper [18] was said to be scalable and potentially useful to estimate the structural modal properties, but only one device was used to monitor a simple timber beam and estimate its frequency response functions (FRFs). It is based on the wireless interconnection of the Back-end units with both the Front-end unit, devoted to acquiring data from the sensors, and the Processing unit, devoted to handling the registered information. It manages all the Back-end units connected to the system using a Wi-Fi interface and is responsible for configuring the accelerometers connected to each myRIO device, controlling the start and the end of the acquisition, as well as any problem that may arise during the whole process
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