A laser based differential power method for measuring the natural frequency of vibrations in a plastic string

The laser based non-contact and high bandwidth technique was developed to measure the natural frequency of vibrations in a thin string. The string, which is caused to vibrate by a piezoelectric actuator, was located in the middle of a beam of light from a laser. Changes in the power of the laser light resulting from micro-vibrations of the string were measured by a two-quadrant photo sensor. Tennis racket strings were selected for use in this experiment and vibrations were generated over a range of different frequencies to validate the laser measurement system. The results showed that the measured signals, which for analysis were subject to Fast Fourier Transformation, were coincident with the input frequency. Finally, the natural frequencies of strings subjected to different tension force were measured and analyzed to determine the validity of the method. The results showed a relative error of less than 0.6 %