Today, the society has built an innumerable amount of buildings, with sometimes complex architecture designs. Long-term monitoring, also known as SHM, has a high degree of importance when the structures are subject to heavy physical loads, aging, and other external factors (e.g. corrosion, seismic damage). It involves the installation of many sensors fixed to critical areas in order to track internal strains, microscopic deformations and frequency responses.
Wireless sensors communicate through radios and need a source of energy to operate correctly. To increase the durability of the system, the power consumption of each component – including the accelerometer – should be as low as possible.
A small bias derivation improve the overall reliability of the sensor: each day, they endure thermal variations and frequent switches, generating new errors after each measurement. It is key to perform recalibration and have stable sensors in order to avoid false alarms.
The Appropriate Technology
The number of sensors partially depends on the configuration and on the duration of the monitoring. Piezoceramic sensors were the first to be selected for standard SHM. They have a very large frequency bandwidth, however they cannot measure DC signals. Wired SHM and crash testing are typical applications where piezo sensors are implied.
More recently, MEMS capacitive accelerometers (VC) proved to offer good advantages, due to its simple yet efficient design. They need a very small current to operate, as they function in an open loop configuration. MEMS VC sensors are ideal for wireless long term monitoring, as they offer portability, reliability and durability. Furthermore, piezo and VC sensors can be combined to increase their overall performances.
Colibrys offers the VS1000 series, a family of high performance, open loop vibration sensors. This MEMS product has one of the largest frequency bandwidth, capable of measuring signals typically from DC 0 Hz to 2500 Hz at ±5% FS, and up to 7000 Hz at ±3 dB. The ranges considered for SHM would be ±2g, ±5g, and ±10g. With a spectral density down to 7 µg/√Hz, and a non-linearity of 0.1% FS, the VS1000 is considered as the most accurate vibration sensor made by Colibrys. It is estimated that it can function during 11 months, as long as it is powered by two AA batteries .