The laser beam alignment is one of the most important and critical step in this experimental arrangement The laser vibrometer must be aligned with both the spindle rotation axis and the inductive displacement sensors measuring the displacements in the laser beam direction for valid measurements and for estimating the calibration transfer function When we calculate the calibration function we assume that the system behaves close to linear. Further the inductive displacement sensors measuring the displacements in the cross direction must be perfectly orthogonal to the laser vib- rometer for comparison.
The laser vibrometer was aligned with the spindle axis by looking at the specular reflection from the polished surface The inevitable instrument angular misalignment was minimized by try-and-error which was error which was time- consuming. The actual value of the angular misalignment is unknown and difficult to control now afterwards.