Fiber optic sensor for localized vibration measurement

Structural analysis demands the localized vibration study of metallic, ceramic and composite structures. This paper reports the design and development of a vibration sensor to measure the instantaneous amplitude and frequency of vibrations of structures over very small-localized surface area. Optical fiber based displacement sensor is extended as a vibration sensor, based on the consideration that the vibration is a periodic variation of displacement. A fiber optic probe is designed using a pair of well-polished plastic fibers, whose ends are placed parallel to each other. One of the fiber is connected to an LED and the other to a photo-transistor. The probe is placed vertically above the location of the structure where the vibration is to be measured. A small reflecting film is also pasted on the vibrating part of the body. The vibration causes intensity changes to the light received at the phototransistor and the sensor is calibrated by using a standard mechanical vibrator. Experimental studies were carried out to optimise the response of the sensor probe by keeping the fibers at various angular configurations. The detected voltage is digitised and fed to a micro-controller and is interfaced with PC. Instantaneous voltage values are recorded and processed to get the amplitude and frequency values.