Conkey, Andrew P. (2007-08). Vibration sensors utilizing fiber fabry-perot interferometers and permanent magnets. Doctoral Dissertation.
A unique set of vibration sensors was designed that incorporate the fiber Fabry-Perot interferometer (FFPI) and permanent magnets. Feasibility of the design and its advantages over traditional sensors were verified by experiments. The new sensors consist of one that monitors the motion of a body across an air gap with the other designed to respond to the motion while in contact with the vibrating body. The FFPI is the device used to transmit the vibrational response while permanent magnets are used to transfer the vibratory motion to the FFPI. In the non-contacting sensor, a permanent magnet was used as the motive to transfer the vibratory motion to be registered at the FFPI. Although the magnet resulted in a non-linear response, the effect of the non-linearity can be removed by signal processing. The fiber optic gap sensor (FOGS) developed has a sensitivity of 250 mV/mil for a 30 mil gap to 50 mV/mil for a 65 mil gap and a dynamic range up to 820 Hz. An advantage of the FOGS over traditional eddy current based gap sensors (ECP), for measuring rotor vibration, was demonstrated with a test rotor face that had brass strips attached to it. The FOGS response was not affected by the strips whereas the response of the traditional ECP was greatly affected by the strips. This demonstrated the potential of the FOGS for measuring the motion of a coated shaft. The fiber optic vibration sensor (FOVS) had a marginal response as its construction led to spurious signals. The feasibility of it to act as a vibration sensor, however, was demonstrated. Due to problems in maintaining the proper gap between the proof mass and sensor head during assembly the natural frequency of the FOVS was around 31 Hz and not the targeted 10 Hz. The sensors presented were meant to be a first generation. However, access to additional FFPI elements was restricted limiting further development. Improvements to the sensors would be to modify the sensing head, magnet size, and the embedment of the FFPI as well as to employ tighter machining precision and assembly practices.