Input sensorless FRF measurements by laser excitation system

The authors have been proposed an analyzing method for vibration testing based on impulse excitation by laser ablation in order to experimentally identify dynamic characteristics of micro devices such as HDD head actuators or MEMS that have in the high frequency region the natural frequencies of a few tens of kilohertz. This paper proposes a method that makes it possible to analyze FRF by only measuring the output (acceleration response) in a laser excitation experiment. This enables the measurement of the force input sensorless. First, the laser excitation force is normalized by Newton's second law using a rigid block. Next, the laser excitation experiment with an object structure having a natural frequency within the high frequency region is conducted. Complex Fourier spectrum obtained by Fourier transforming the measured response is divided by the estimated laser excitation force. Finally, since the trigger position of the response and the time the impulse input is actually applied have errors, phase characteristics of the forceregulated complex Fourier transform is modified by taking the dead time included in the response into account, resulting in the FRF of the structure. The effectiveness of the proposed method is demonstrated by the vibration test with an aluminum block as object structure.