Vibration Measurements by Laser Techniques on Rotating Micro-Tools
Non-contact vibration measurements performed by laser techniques are extremely interesting for applications in all cases where the object under test shall not be perturbed by the physical contact with a sensor. Among the different approaches, the most used is Laser Doppler Velocimetry (LDV), that offers a high resolution and good versatility, and has also been demonstrated for applications to tiny objects. A specific version of LDV is represented by the self-mixing configuration, where the light backscattered by the target is re-injected into the laser diode cavity and mixed locally with the lasing field, yielding a very compact configuration. A non-coherent approach is represented by the triangulation principle, that also allows to measure vibration and displacements of rotating objects.
A specific application case is represented by rotating objects (typically: shafts), where the rotation motion is often accompanied by vibration of the principal rotating axis, and by eccentricity of the bearings/object. The main characteristic of these measurements is the fact that the target surface has a relevant in-plane motion. This may cause time-variable spackle-pattern effects, making the application of LDV techniques difficult. An even more difficult case is represented by tiny cylinder-shaped objects (diameter smaller than 3 mm) rotating at very high speed (up to 30,000 rpm). This is the case of special drill or mandrel tools, and of the micro-EDM (Electrical Discharge Machining) application, where electrodes with a diameter as small as 300 µm can be used.
Julight developed a customized triangulation sensor for the monitoring of vibrations and eccentricity of a tiny (diameter smaller than 1 mm) rotating micro-tool 
 “Vibration Measurements by Laser Techniques on Rotating Micro-Tools”, XIX Congresso Nazionale AIVELA, 1-2 dicembre 2011.