How Can Vibration Help with Structural Health Monitoring

Vibration is a physical phenomenon exists in operational rotating machineries and structures, even when they are in good condition. There are numerous sources of vibration, such as rotating shafts, meshing gear-teeth, rolling bearing elements, rotating electric field, fluid flows, combustion events, structural resonance and angular rotations. Because of its ubiquity, vibration is highly applicable for investigating the operational conditions and status of rotating machinery and structures.

Vibration can be measured through various types of sensors. Based on different types of vibrations, there are sensors designed to measure displacement, velocity and acceleration, with different measuring technologies, such as piezoelectric (PZT) sensors, fibre optic (FO) sensors, microelectromechanical sensors (MEMS), proximity probes, laser Doppler vibrometer and many others. Specially, FO sensors are drawing more attentions as they are passive and provide excellent noise immunity in harsh environment. In BladeSave system, the vibration is captured using this type of sensors.

To analyse vibration signals, one can extract information from both their time series as well as frequency spectrum using Fourier Transform. Vibration signals are usually up to 20 kHz, except for certain vibration resonances that can reach beyond that. In practice, the sampling rate should be carefully chosen, to make sure that the bandwidth containing frequencies of interest are captured. Additionally, the recording length for one measurement should be at least several periods of the lowest speed of the structural vibrating mode.

Ice detection of wind turbine blades is a typical usage of vibration for structure health monitoring. Under normal condition, the blade vibrates in certain modes and a fixed structural natural frequency. However, if the mass of the blade is changed due to ice-accretion, the natural frequency will shift with regard to the added extraneous mass. Below is an example where simulated mass was added on a wind turbine blade, and vibration signals were captured to validate the change of blade vibration pattern.

Figure 1 – Blade with simulated weight
Figure 2 – The frequency shift with added mass on the blade

As shown above, the blade’s natural vibration frequency shifted to lower range with added mass. All in all, vibration analysis is widely applied in condition monitoring and structural health monitoring, as it has advantages such as real-time reaction to the change of health conditions, supports remote condition monitoring, well-established processing and signal analysis methods/algorithms for predictive maintenance and supported by various sensors commercially available for different operational conditions. However, it lacks the ability to conduct fault localisation and it’s difficult to monitoring crack propagation on the structure. That’s where Acoustic Emission (AE), a high frequency elastic wave beyond audible frequency range can be useful. BladeSave is a complete FO system that combines the strengths of AE, vibration and strain, offering full monitoring of the wind turbine blade health. 

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