Rotor blade vibration occurs in all turbomachinery including aircraft engines, steam and gas turbines, compressor systems and turbochargers. Undesired blade vibration can lead to high cycle fatigue (HCF) blade failures, low cycle fatigue (LCF) blade failures, flutter or be an indicator of an equipment health problem such as failing bearings. Understanding blade vibration is essential to validating new turbomachinery designs, making decisions about future designs, and in understanding how to operate equipment efficiently and safely.
Blade Vibration Monitoring Systems
Blade vibration has historically been measured via strain gauges. This requires attaching strain gauges to the blades, which is not ideal because the mass of the strain gauge in itself might introduce perturbations. Strain gauges also require wiring to carry the signal back to the stator portion. Advancements have been made through the use of telemetry which offer an alternative to slip ring assemblies, but strain gauge measurement methods still require instrumentation of the blades and rotor.
Blade tip timing is a non-contact measurement method for determining blade vibration. It relies on the measurement of the time of passage, also called Time Of Arrival (TOA), of every blade. If the blades were not deflecting or vibrating, they would always have the same time of arrival. Because they vibrate, the time of arrival varies. Time of arrival is different for different sensing positions around the rotor, and it is also different at every turn for the same measurement position. Precise timing of blades at several positions around the rotor gives a complete picture of blade vibration.
Due to its non-intrusive nature, blade tip timing is widely used for rotor blade vibration measurements, both as standalone tool and in conjunction with strain gauge-based vibratory analysis.
FOCISTM Optical Blade Vibration Monitoring System
Prime Photonics' FOCIS™ system is an optical blade vibration monitoring system which measures blade tip timing. It detects time of arrival by shining a laser at the blade tip and capturing the reflected light from the passage of every blade. Optical blade tip timing is the most accurate blade tip timing method, allowing time of arrival data that surpass any other method.