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Optical, non-contact resonance testing using laser sensors

Resonance analysis seeks to identify the natural frequencies or resonant modes of an object. When an object is subjected to vibrations at or near its natural frequencies, it tends to vibrate with larger amplitudes. This effect allows for detecting structural issues, defects, or weaknesses. By analyzing the frequency response, engineers can gain insights into the structural characteristics of the object being tested, including its stiffness and mechanical properties, damping properties and overall integrity.

Laser Doppler vibration measurement is an advanced optical technology that offers several advantages in the context of resonance testing:

• High precision: Laser-based measurement systems provide extremely accurate and precise data, enabling engineers to detect even subtle changes in an object's vibrational behavior.
• Non-contact: Laser vibrometers do not physically touch the object being tested, minimizing interference and ensuring that measurements do not alter the object's properties. This is crucial when testing delicate and sensitive components or even hot objects where sensors cannot be applied.

• Wide frequency range: Laser vibrometers can measure a wide range of frequencies, making them suitable for low-frequency up to high-frequency (GHz) resonance analysis.

• Remote sensing: Laser vibrometers can be used to measure vibrations from a distance, allowing engineers to assess components in situ without the need for direct physical access. This is indispensable in danger zones like high voltage or explosive areas.

• Rapid data acquisition: Laser vibrometers can capture data quickly with both analog and digital data transfer, thus enabling real-time analysis and immediate feedback during testing.

Testing resonances of hard disk drives and precision parts

As a case study, hard disk drives – with their ever growing storage densities and shorter access times – require extremely high levels of stability as regards the read/write head’s location and positioning in relation to the disk drive interface. The flying height is a compromise between competing effects. A lower flying height enables better local resolution for read/write operations and thus a higher data density; meanwhile the risk of collisions with the medium grows at the same time. The flying height is just a few nanometers and very much depends on the ambient pressure due to the aerodynamic bearing. The aerodynamic bearing, however, has resonances that depend on the ambient pressure too and that may lead to instabilities.

Since the measurement process is both non-contact and non-intrusive, in this situation using laser vibrometers is the only way of measuring the response behaviour of the read/write head including its suspension following dynamic excitation. When performing resonance test measurements with single-point and scanning vibrometers, the frequency spectrum of the read/write head’s deflection is measured as a function of the ambient pressure. You can use this to identify critical conditions and then make constructive changes. The goal of the optimization process is to develop read/write units that respond robustly to resonances caused by aerodynamic excitation.