Analytical Methods for Transonic Compressor Rotor Blade Tip Clearance Measurements Using Capacitive Probes

Abstract A method is presented to improve the capability of proximity sensing capacitive probes used in rotor blade tip clearance (BTC) measurements. This research presents the development and result of a post-processing method used to calibrate capacitive probe instrumentation that will be used to passively analyze BTC of a transonic axial fan under high-speed and high-transient loading. The method attempts to address limitations currently placed on BTC measurements due to poor a SNR. Each revolution’s raw capacitive probe signal is uniformly resampled to a synchronizing once-per-revolution signal, normalizing the data. This reduces timing inaccuracies in the sampling frequency inherent to the data acquisition system and transfers data into a spatial domain removing time dependence and rotational speed variance, which in turn improves the SNR. A Fast Fourier Transform (FFT) is used to identify acceptable lowpass filter passbands that are then adaptively applied to this resampled data based on tip clearance distance. This produces increased precision and dynamic range in capacitive probe calibration compared to existing BTC calibration methods. Use of the proposed method gives the potential ability to operate capacitive probes at larger ranges and in poor SNR environments and gives a possible application in monitoring rotor blade integrity, at a low, additional computation time cost.