Output characteristics of two-circle coaxial optical fiber bundle with regard to three-dimensional tip clearance

Endoscopic displacement measurement based on fiber optic bundles

In-line monitoring and routine inspection are essential for using and maintaining complex equipment. The simultaneous implementation of visual positioning and displacement measurement allows the accurate acquisition of characteristics, including object dimensions and mechanical vibrations, while rapidly locking the target position. However, the internal structure of equipment is frequently obscured, making direct visual inspection challenging; therefore, flexible and bendable fiber optic-based endoscopes are extremely valuable in harsh conditions. This study enables all-fiber visual displacement measurement using a single-mode fiber and an imaging fiber bundle. Based on optical triangulation and spot centers extraction method from fiber bundle images, 0.07 mm precision at a measurement distance of 40.12 mm is achieved vertically for rough objects. We demonstrate its surface reconstruction and vibration measurement functions. Factors that affect measurement accuracy, such as light source and object roughness, are also discussed.

Optical-fiber-based dynamic measurement system for 3D tip clearance of rotating blades

This paper proposes a dynamic model for measuring the 3D tip clearance of rotating blades using an isosceles-right-triangle sensing structure consisting of three identical two-circle coaxial optical fiber bundles and a demodulation approach using the ratios of difference signals between any two bundles based on the Taylor expansion principle. The dynamic system comprising the optical fiber probe, hardware circuits, and measurement software is designed and established according to the characteristics of dynamic sensing signals. Finally, the feasibility of the system for the dynamic measurement of 3D tip clearance is experimentally verified using the simulated blades of a rotor test bench.

Performance Comparison of Three Fibre-Based Reflective Optical Sensors for Aero Engine Monitorization

Among the different available optical technologies, fibre bundle-based reflective optical sensors represent an interesting alternative for parameter monitorization in aero engines. Tip clearance is one of the parameters of great concern for engine designers and engineers. In the framework of this optical technology, three fibre-based reflective optical sensors have been compared. Two of them are custom designed and based on the same geometrical fibre arrangement, whereas the third one is commercially available and relies on a different geometrical arrangement of the fibres. Their performance has been compared in clearance measurements carried out during an experimental program followed at a transonic wind tunnel for aero turbines. The custom-designed solution that operates in the most sensitive part of its response curve proved to be by far the most reliable tool for clearance measurements. Its high resolution opens up the possibility to detect small blade features such as cracks, reflectivity changes, etc. that otherwise could not be tracked. These results show that the detection of unexpected features on blade tips may have an important effect on how the clearance is calculated, ultimately giving rise to corrective actions.

Demodulation technique for 3-D tip clearance measurements based on output signals from optical fiber probe with three two-circle coaxial optical fiber bundles

In this work, we propose a new demodulation technique for three-dimensional (3-D) tip clearance measurements using the output signals acquired from three two-circle coaxial optical fiber bundles. This technique is based on the ratio of the difference in the signal intensities between any two sensing units of the optical fiber probe, and we derived the demodulation equations using the second-order Taylor expansion for a three-variable function. We verified the feasibility of the demodulation technique by experiments and demodulation error curves, which indicates that the method is viable for 3-D tip clearance measurements.