Rotor health monitoring and online damage detection are increasingly gaining the interest of aircraft engine manufacturers. This is primarily due to the need to improve safety during operation as well as to lower maintenance costs. Applied techniques for detecting damage in and monitoring the health of rotors are essential for engine safety, reliability, and life prediction. A few years ago, the United States set the ambitious goal of reducing the fatal accident rate for commercial aviation by 80 percent within 10 years (ref. 1). In turn, NASA, in collaboration with the Federal Aviation Administration and other federal agencies, universities, and airline and aircraft industries, responded by developing the Aviation Safety Program. The program provides research and technology products needed to help the aerospace industry improve aviation safety. Researchers within NASA Glenn Research Center’s Optical Instrumentation and Nondestructive Evaluation Technology Branch are developing propulsion-system-specific technologies intended to detect damage prior to catastrophe under the propulsion health management task.
Sensors that locally “ride” on the rotor are deemed a necessary choice for sensitive, high-resolution structural health monitoring of turbine engines because of the limited resolution of vibration-based methodologies. One such system being developed utilizes either traditional ultrasonic transducers or robust piezoelectric patches for real-time, continuous monitoring of acoustic emissions in the hopes of recognizing damage prior to catastrophe. In order to implement such a system, concerns regarding broadband data transfer from the rotor (location of the sensor) to the stator (location of signal-processing equipment) need to be addressed. As a result, a new1-MHz broadband wireless slip ring was designed and fabricated at the NASA Glenn Research Center. The wireless communication between the rotor and the stator is accomplished through capacitive coupling. The antennas are fashioned as a ring-within-a-ring setup. In the current form, both the modulator and the demodulator use a 3-V battery supply, although both can function within the range of 2 to 7 V. Current draws at 3 V are 25 mA for the modulator and 10 mA for the demodulator. Future systems will aim to eliminate battery usage by power harvesting. Power harvesting converts ambient energy (such as normal structural vibrations and temperature gradients) into electrical power.
Prototype wireless slip ring, showing modulator (top), wireless slip rings (center), and analog demodulator (bottom).
Tests were conducted at Glenn under static conditions in order to document the signal accuracy of the system after modulation and demodulation. Results showed that the wireless slip ring was able to accurately recreate the input signal, generated with a pulse-function generator, in the frequency range of approximately 375 Hz to 1 MHz. Success was achieved for sinusoidal waves as well as square waves with amplitudes ranging from 50 mV to 1 V. The preceding figure shows the results for an 800-kHz sine wave input. The next step for verification tests involves subscale rotor tests, followed by further hardware refinements and finally full-scale tests in turbine engines.
Comparison of input and output signals.
Last updated: October 12, 2006
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