59519 - Minimum Measurable Displacement of an Optical Blade Tip Timing System 

Blade Tip Timing (BTT) began life as a research project in the hands of the major Gas Turbine OEM’s each developing their own capabilities in isolation for a number of years. Various papers show developments as early as the 1960’s with more complex method disclosure throughout the 1980’s followed by significant improvements in capability once high performance desktop PC’s were the norm. The OEM’s with some help from major military customers were encouraged to compare their technology in order to reduce development costs and encourage standards that could be applied across many industries. These, at that time, were totally reliant upon strain gauges and telemetry systems.

Successful applications of BTT on Aero-engines and Industrial Turbines have increased the awareness and advantages of BTT technologies within industry over the past 20 years. As the technology has become more widely adopted, engineering teams intent on applying current BTT technologies are doing so to turbomachinery with increasingly demanding rotational speeds, blade passing frequencies and higher deflection gradients.

In order to consider a system’s capability for a particularly demanding application, engineers require an understanding of what minimum measurable displacement can be measured by a BTT system. The objective of this paper is to investigate common misconceptions with respect to determining the capabilities of a BTT system and provide engineering teams with insight surrounding the niche measurement technique that is BTT measurement.

We outline mathematics involved in processing Time Of Arrival measurements into linear displacements and demonstrate how techniques are utilised to significantly improve the working resolution of a BTT System as opposed to the calculated theoretical resolution. 

The technology which offers the highest level of precision and consequently the lowest minimum measurable displacement is commonly accepted as the optical system. Two contemporary optical BTT measurement systems with different theoretical resolutions based on their system specifications are used to process identical measurement data sets of simulated high speed rotors with equivalent tip speeds of up to 1000m/s and known linear displacement values.

Using measurement data with known Time of Arrival displacement values we can accurately perform measurement system analysis techniques to determine measurement error, deviation and system resolution to present achievable processed values.

In conclusion the measurement data presented provides engineering teams with meaningful outputs to demonstrate the actual minimum measurable displacement of an optimised optical BTT system incorporating systematic error levels and measurement uncertainties.