Tip Clearance Measurement Technique for Stationary Gas Turbines Using an Autofocusing Millimeter-Wave Synthetic Aperture Radar

In this paper, an autofocusing radar technique is presented for tip clearance measurements in combustion turbines capable of resolving the edges of a typical blade tip. The clearance is determined by measuring the reflection at the blade tip while passing by the antenna, subsequently focusing the data by means of a matched filter operation and interpreting the phase of the blade edge reflection according to the continuous wave radar principle. For this, an autofocus approach was developed, which minimizes defocusing effects and provides an estimate of the clearance as the first result, which is utilized to overcome the phase ambiguity and, thus, to increase the measurement range. The autofocus algorithm applies a weighted phase gradient of the pointlike blade edge reflection as cost function and sensitive indicator for the focal quality.