The ability to predict the onset of failure in mechanical systems is key to the reduction in maintenance costs, downtime, and health hazards in industrial environments. Vibration monitoring can be used to spot mechanical problems, trigger preventative maintenance and diagnose the health of rotating machinery. Accelerometers are used in this role to detect defects in rotating components through analysis of harmonics in the power-frequency distribution. Further, high frequency acoustic emission sensors can be used to determine crack growth in metal components or failures in composite structures. Currently, a variety of MEMS accelerometers and piezoelectric acoustic emission sensors are used in health monitoring that are limited by their size, cost, and performance. Optical fiber sensors are rapidly emerging as viable alternatives to these devices as effective means of health monitoring in harsh environments. These sensors are tolerant to extreme temperature, EMI, shock and vibration, and offer reduced weight and increased accuracy over conventional instrumentation. As a result, these sensors have begun to replace conventional sensors in harsh environment applications. Optical fiber sensors offer much smaller size, reduced weight, ability to operate at temperatures up to 20OO0C, immunity to electromagnetic interference, resistance to corrosive environments, inherent safety within flammable environments, and the ability to multiplex multiple sensors on a single optical fiber. This paper presents the research and development of both low-profile fiber optic-based acoustic emissions sensors for use in norr destructive evaluation systems and fiber optic accelerometers for harsh environment health monitoring applications.
Fiber optic pressure sensors were integrated into the grinding plates of an operational paper pulp mill for real-time monitoring of the pulp grinding process. On-line system monitoring will allow smart, active control of the grinding plates thereby improving the quality and consistency of the pulp produced. Sensors were constructed and calibrated for use in the harsh environment of an operating paper pulp grinder. The sensors were 1.65mm in diameter including titanium housing, and were installed directly into the grooves of the grinding plates. The sensing elements were flushmounted with the wall and exposed to the wood pulp slurry. Nine sensors were calibrated up to 1000psi. During operation, pressure was sampled at 1.0MHz, and pressure spikes up to 175psi were observed. Pressure pulses measured are due to the relative motion between the grooves and channels on two pulp grinding plates. The consistency, size distribution, and quality of paper pulp exiting from the grinder are directly related to the distance between the channels on the two rotating elements. The pressure pulses produced are also proportional to the distance between channels. Therefore, by monitoring pressure fluctuations, grinding elements can be dynamically controlled thereby producing a "smart mill."
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