Despite constant research on Blade Tip-Timing (BTT) for decades, the major
limitation of this turbomachinery blade vibration measurement technique in
terms of insufficient data sampling is still not overcome. Complex vibrations
cannot reliably be analysed and, to tackle this issue, an innovative data
analysis approach to increasing the amount of useful data was developed.
Instead of only being used for determining vibration parameters on the basis of
times of blade arrival at a sensor, complete waveforms produced by stationary
casing-mounted sensors are processed. Such waveforms usually hold multiple
data points of the blade vibration trajectory, yet these data points are hidden
and not directly accessible when eyeing the sensor output. Their analysis
thus provides new opportunities in respect of the following: characterising
multi-mode vibrations occurring within the same instant of time; detecting
non-linear vibration responses arising from excitation of engine components
with non-linear behaviour; ascertaining axial, radial, and tangential blade
positions during operation.
After an introduction to state-of-the-art BTT, the novel concept of investigating
blade vibration in turbomachinery is presented on the principle of
Blade Tip-Timing (BTT) and clearance sensor waveform analysis (BLASMA),
which was filed for patent protection by Rolls-Royce. It is shown that the
modulation of the sensor output by blade vibration can offer additional information
compared with under-sampled time-of-arrival (TOA) data from
traditional BTT applications. The sensor data can help to not only dispel
doubts on the validity of statements on blade vibration but also lessen the
dependence on contact-based strain gauges measurements for producing reference
data. A numerical and experimental study was conducted to evaluate
the merit of sensor waveform analysis for describing multi-harmonic blade
vibration. Results are carefully discussed in this work and future strategies
for proceeding with BLASMA are proposed.