Blade Tip Timing Measurement Using Forward Rearward Probes
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Solution Overview
Problem
Existing blade tip timing (BTT) methods for analyzing rotating blades in gas turbine engines face challenges such as undersampling due to low sampling rates and noise from probe movement, which limits accurate vibration measurement and stress analysis.
Innovation Solution
A method using forward and rearward blade tip timing datasets and a once per revolution dataset to calculate the instantaneous position of the blade stagger angle axis and tip stagger angle, allowing for the measurement of blade tip axial displacement, which can be caused by centrifugal and aerodynamic forces, without requiring probes to be fixed to the blades.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If blade tip timing probes are mounted on the engine casing to avoid telemetry systems, then the measurement setup becomes simpler, but the sampling rate is determined by rotational frequency which is below the Nyquist frequency for vibrations of interest, causing undersampling and aliasing
Solution Approach 1:
The patent introduces a blade marker as an intermediary element that rotates with the blade. This marker provides a reference signal that enables the system to distinguish between rotational frequency and vibration frequency, allowing accurate vibration measurement even when the probe sampling rate is determined by rotational frequency. The marker acts as a mediator between the probe system and the blade vibration, resolving the aliasing problem.
2Measurement precision
If multiple timing probes are used to perform useful vibration analysis, then vibration measurement capability is improved, but the data becomes intrinsically noisy due to probe movement caused by mounting restrictions and casing thickness
Solution Approach 1:
The patent uses a blade marker that creates a copy of the blade's rotational and vibrational motion. Instead of directly measuring blade tip position with probes mounted on the casing, the system measures the position of the marker which replicates the blade motion. This copying approach eliminates the noise introduced by probe movement and mounting restrictions, as the marker moves perfectly with the blade without being affected by casing vibrations.
3Device complexity
If the sampling rate is determined by rotational frequency to simplify the measurement system, then the system complexity is reduced, but the sampling rate falls below the Nyquist frequency for vibrations of interest, leading to aliasing
Solution Approach 1:
The patent implements a feedback mechanism where the blade marker position measurements are used to continuously update and refine the vibration analysis. The marker provides a reference signal that feeds back into the measurement system, allowing the processing algorithm to distinguish between rotational motion and vibrational motion. This feedback enables reliable vibration measurement even at low sampling rates by using the marker information to correct for aliasing effects.
Data Source
AI summary
Methods are provided for: (i) measuring the position of the blade stagger angle axis for one or more blades of a row of blades attached to a rotor, (ii) measuring the blade tip stagger angle for one or more such blades, and (iii) measuring the blade tip axial displacement for one or more such blades. The methods use forward and rearward blade tip timing datasets for successive rotations of the blades from two axially spaced blade tip timing probes. The forward probe is forward of the rearward probe along the axial direction of the rotor. The blade tip timing datasets allow the times of arrival of the blades at the respective probes to be measured. The methods also use a once per revolution dataset for the successive rotations of the blades. The once per revolution dataset allows the angular velocity of the blades to be measured.