MS Thesis Presentation by Thomas A. Holst
Monday, April 11, 2005

(Dr. Thomas Kurfess, Chair)

"Spatial Filtering in Microwave Sensor Measurements of Turbine Blades"


In-process turbine measurement and monitoring has been a subject of research since the advent of gas turbines; however, it is difficult because it requires precision measurements to be made at high speeds and temperatures. The measurement of turbine blade tips is especially intriguing because of the potential it holds to greatly increase the efficiency of engine operation and maintenance. Tip-to-casing clearance is one of the major sources of inefficiency in a turbine and monitoring of this clearance would allow active tip-clearance control systems to be implemented. Also, analysis of engine wear through vibration monitoring would provide information needed for more effective engine maintenance and repair.

A sensor recently developed at Georgia Tech could answer this challenge. The sensor operates by measuring the phase change of reflected microwaves to measure blade tip displacement. It is robust even in the harsh turbine environment because it emits 5.8 GHz microwave signals which reflect off blade tips while seeing through any combustion products which might be present in the turbine. And the sensor antenna is a metal-cased ceramic puck with a platinum-palladium-silver resonator on it; this type of antenna is effective even in the heart of a turbine where gas temperatures can reach 1300° C and much vibration exists.

In measurements of the sensor, the beam pattern of a microwave sensor causes a phenomenon called spatial filtering to occur in measurements, which may compromise the precision of measurements. Since the beam is not a thin line reflecting off a single point on the turbine blade, measurements are a weighted average of measurements along the entire surface within the field-of-view of the sensor. The weighting of this average is determined by the signal attenuation in space, the reflection angle of the signal, the direction from the sensor, and other factors. The net effect is a blurred measurement. In measuring turbine blades, only the tip is vital, so the blurring in between blades is not extremely detrimental. However, changing measurement geometry affects the amount of spatial filtering and hence the accuracy of the measurement.

This thesis presents a detailed analysis of this phenomenon and especially its effect on turbine blade tip clearance measurements. A five-factor, full-factorial design of experiments is presented to qualitatively understand the effect of geometric factors on tip measurements. Along with experimentation, a robust, three-dimensional, ray-tracing, electromagnetic model is presented which was developed to further understand spatial filtering and to analyze specific geometric factors in the measurement of turbine blades. The model is designed to be applied in all measurement situations and is presented and compared with actual sensor data and experimentation. In conclusion, the research shows that microwave measurements may still be made to sufficient accuracy even considering the effect of spatial filtering, and by quantifying spatial filtering in measurements, it may be possible in to glean additional useful data from measurements.