Investigation of laser doppler vibrometry

number: 
863
English
Degree: 
Author: 
Ashraf Tariq Moslem
Supervisor: 
Dr. Moayad Aziz
year: 
2004
Abstract:

Vibration detection and measurement is an increasingly important sector of nowadays technology. New methods are being developed to compensate many problems and complexities encountered with traditional old techniques. The laser Doppler shift technique has become recently one of the cutting edge techniques after many years of development and research while still ongoing. Since the importance of vibration anemometry, this project was initiated to design and construct a laser Doppler vibrometer for detecting and measuring object vibrations, remotely with the use of a laboratory standard He-Ne laser. This device depends on theory of the Doppler shift effect encountered in sounds and applied to light waves. The setup involves the Michelson Interferometer theme as the optical arrangement to achieve the desired optical mixing condition. Aside of the optical components used, a detection circuit has been built at which the back reflected signal off an object falls and then transferred to an oscilloscope to be viewed. Unfortunately, this work lacks of a major component to resolve the frequency that the object is vibrating at. Nevertheless the results showed a positive response to the increasing and decreasing of the vibration frequency and instances recorded at four different frequencies from 500Hz to 1.5KHz. In addition, another technique has been experimented that is the direct detection technique, which depends on the back-reflected laser light off an object that is suffering vibration. This suggested technique is rather simple and needs no special sophisticated instruments to aid the measurement process. The same detection circuit that has been built for the Doppler shift technique was also used to detect the reflected signal from the object when studying the direct detection technique. This technique has proven its validity over an appreciable range from DC level to 2KHz. The direct detection has been applied in four different settings to study the varying response of objects. For some readings, the range did not start from DC level, it rather started a few hertz after and reached up to 1KHz. One reading though started from DC level and extended to 2KHz when using small lightweight mirror to reflect the laser beam off the vibrating object. Nevertheless, this extended reading showed some drop in the signal amplitude level to 6 volts while the other readings range above 10 volts except for the fourth reading set when using a small piece of Aluminum plate to reflect the laser beam with very poor signal quality and worked only over 50Hz to 500Hz. This work showed very important indispensable key component that is; the use of a photoreflective surface which does not depend on the metal reflectivity in most cases especially when using direct detection in back reflection mode.