What is Blade Tip Timing?
Blade Tip Timing (BTT) measurements, also known as time of arrival measurements, are the precise measurement of rotor blade tip passing events at a specific angular location. In turbomachinery, variations in blade tip timing can occur due to normal or abnormal rotor blade vibrations, or damage to the blade or rotor.
Optical Blade Tip Timing systems are well accepted by industry to provide the most accurate and precise rotor blade measurements time of arrival measurements. These systems involve a (laser) light source, photodetector/amplifier, and light probe. Probes can be installed to illuminate the rotor blades and capture reflections as the blades pass in proximity to the probe, or pitch-catch probes can be installed which rely on the rotor blades chopping a light signal as the rotor blades pass between the pitch-catch probes.
Uses of Blade Tip Timing
Blade tip timing is used for gas turbine and turbocharger test and evaluation, research and condition monitoring.
Blade tip timing is used primarily as a technique for Blade Vibration Monitoring. By placing several probes around the same rotor, we obtain information on how the blades are vibrating as they rotate. When used for that particular purpose, a typical blade tip timing system uses 5 to 9 optical probes per rotor, depending on the vibration modes which are to be measured.
Blade tip timing can also be used to measure rotor blade health and to detect cracked and damaged blades. This is done by identifying anomalous blade deflections and/or changes to blade vibration signatures. By measuring changes in vibrational amplitude or frequency over time, condition-based blade health monitoring can be performed.
FOCISTM as an Optical Blade Tip Timing System
FOCISTM is the most elaborate, most complete optical blade tip timing system available. Each hardware component is world-class, including high temperature micro-lensed probes, multi-function timing and clearance probes, lower-cost miniature unlensed timing probes, high gain / low noise Sensor Interface Unit (SIU) instrumentation, custom high speed data acquisition and signal processing Data Capture Cards (DCCs). FOCIS™ provides users superior data that users can rely on to make the best decisions.