Vibration tests

Vibrations are mainly measured using specific sensors, the most common being accelerometers. Accelerometers convert acceleration (g) into an electrical signal. This signal is then recorded and processed to obtain vibration parameters. A velocimeter (or speed sensor) can also measure vibration speed. The displacement sensor is used to measure the relative displacement between two points.

The sensor signal is analysed by a data acquisition system (DAQ) and spectrum analysis software (fast Fourier transform or FFT) to identify resonance frequencies, amplitude and other vibration characteristics. Common units of measurement are:

• acceleration (m/s² or g)
•  velocity (mm/s or inch/s)
• displacement (mm or inch)

In engineering and modal analysis, the vibration of a structure is often broken down into natural modes of vibration, characterised by a natural frequency and a specific modal shape. Although there is no universal classification of the ‘5 modes’, here are five fundamental concepts or types used to describe vibratory motion:

• Free (or natural) vibration: movement that occurs after initial excitation, without any continuous external force (determines natural frequencies).
• Forced vibration: movement maintained by a continuous external force or excitation (e.g., motor imbalance).
• Damped vibration: vibration whose amplitude decreases over time due to energy dissipation (friction, air resistance).
• Undamped vibration: theoretical model where energy is not dissipated and the amplitude remains constant.
• Resonance: critical case of forced vibration where the excitation frequency is equal to or very close to a natural frequency of the structure, leading to a potentially destructive amplification of the amplitude.

Vibration testing is essential in many fields (aerospace, automotive, electronics, etc.) and serves several crucial purposes:

• Design and robustness validation to ensure that the product or structure can withstand the dynamic stresses it will encounter in its real environment (transport, machine operation, earthquake, etc.) without structural or functional failure.
•  Identifying resonance frequencies in order to determine the critical frequencies of the system. Knowing these frequencies allows the design to be modified so that they do not coincide with the operating frequencies.
•  Product qualification is used to demonstrate that it complies with the industrial, military or environmental standards specified for its category (qualification or accelerated ageing tests).
• Diagnostics and predictive maintenance (on rotating machines) measure vibrations during operation in order to detect mechanical faults (misalignment, imbalance, bearing failure, excessive play) before they cause catastrophic failure.