Prime Photonics' FOCIS™ system is the most capable blade vibration measurement system available today. This optical system times the passage of each blade and analyzes the data to determine blade vibrations.
FOCIS™ Optical Blade Vibration Monitoring System
The FOCIS™ system is an optical blade vibration monitoring system which measures blade tip timing. It detects the time of arrival of each blade 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 quality blade vibration data that surpass any other method.
The FOCIS™ blade vibration monitoring system displays blade vibration data in real time and offers post-processing analysis capabilities for in-depth understanding of blade dynamics and blade vibration events. The FOCIS™ system measures both synchronous and asynchronous blade vibrations for detection of blade deflection, vibration amplitude and frequency, flutter and stall events.
Campbell Diagram displaying synchronous blade vibration
For more information about the FOCIS™ system, please read Details of FOCIS™ Blade Vibration Monitoring System.
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 can have many causes including imbalance, uneven flow conditions, compressor surge, and more. A number of blade vibration causes create synchronous vibration, meaning that the blades oscillate an integral number of times N per revolution, called an engine order. A condition where the blades oscillate 4 times per revolution is called a 4th engine order resonance or EO 4. Other vibration modes create non-synchronous vibration, meaning that the blade oscillation frequency is not a multiple of the revolution period. A non-synchronous blade vibration would appear from an outside observer to look like a travelling wave. Several blade modes can be superposed at a given engine speed, which complicates the blade vibration analysis.
Design of turbomachinery strives to avoid blade resonance over their operating range. Because blade resonance is a complex phenomenon with multiple causes, every new type of turbomachinery is also tested to ensure that predictions were correct and that vibration modes do not get excited in the normal operating range.
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. The blade vibration measurement system combines the times of passage of every blade at every revolution in front of multiple sensors to determine the frequency and amplitude of the oscillations.
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.