An electronic system that includes a capacitive proximity sensor is under development as a prototype of instrumentation systems for real-time monitoring of vibrations of turbine blades and shafts. Because vibrations are caused by stresses that can induce fatigue and/or are sometimes associated with damage, monitoring of vibrations can provide information needed to detect damage, detect incipient fatigue failures, and alter turbine operating parameters to prevent or postpone failures. The design of this system overcomes the frequency-response and spatial-resolution limitations of prior capacitive-sensor-based turbine-monitoring systems. Its resolution is comparable to that of optical-sensor-based turbine-monitoring systems used in testing turbines at temperatures below their operating temperatures; however, unlike optical sensors, capacitive sensors can withstand high turbine operating temperatures [in some cases, >2,000 °F (>1,100 °C)].
The capacitive proximity sensor in this system (see Figure 1) includes N (N = 6 in the figure) stripe electrodes insulated from, and placed within recesses in, a larger ground electrode. In a fully developed version, the electrode surfaces would be flush with each other and the sensor would be mounted in a turbine so that the electrode surfaces would be flush with the surface of the rub strip: hence, in a fully developed version, the electrode surfaces would be curved at the radius of the rub strip. The sensors are excited with a DC bias between 100 and 200 V. Each of the non-grounded electrodes could be connected to individual signal processing-circuits as in prior turbine-monitoring systems; instead, in this system, all of the non-grounded electrodes are electrically connected together and the resulting composite signal is applied to a single wide-band preamplifier. Hence, the sensing and preamplification portions of this system are simpler than those of prior systems.
The circumferential distance between adjacent non-grounded electrodes and the total circumferential extent of the electrode array are less than the circumferential distance between adjacent turbine blades. Consequently, a single blade passes completely by all the electrodes before the next blade arrives, and the output of the preamplifier is a burst of N pulses corresponding to the passage of the blade by each of the N non-grounded electrodes (see Figure 2).
The output signal of the preamplifier is digitized and then processed to measure the heights of the pulses and the times of arrival of the pulses in the burst. The height of each pulse is a direct measure of the distance between the blade tip and the sensor surface. The differences among intervals between times of arrival are measures of high-frequency vibration that manifests itself as circumferential oscillation at the blade tip. The intervals between subsequent bursts are taken as measures of the times of arrival of adjacent blades.
The design of the preamplifier is such as to suppress much of the noise at frequencies below the blade-passage frequency. This low-frequency noise is attributable to vibrations of sensor cables and other spurious phenomena. To reduce low-frequency noise further, in digital signal processing, a signal generated by fitting a curve to the preamplifier output when no blade is present is subtracted from the preamplifier output when blades are present.
This work was done by Wayne C. Haase of Aerogage Corp. and Michael J. Drumm of ExSell Inc. for Glenn Research Center. For further information, please contact Dr. George Y. Baaklini at (216) 433-6016 or
Inquiries concerning rights for the commercial use of this invention should be addressed to
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Refer to LEW-17180.