Abstract: In traditional blade tip timing (BTT) measurement technology, a once-per-revolution (OPR) sensor is typically used as a reference to obtain the ideal time of arrival (ToA). The measured actual ToA is then compared with the ideal ToA to calculate the blade tip vibration displacement. However, in practical applications, due to the limited space inside the engine, it is often difficult to obtain a true or virtual rotational reference for vibration calculation. Moreover, the computation of the ideal ToA introduces additional errors. Under variable rotational speed conditions, speed fluctuations can cause the calculated blade tip vibration displacement to exhibit trends or steady-state offsets, which severely affect subsequent vibration analyses. To overcome these challenges, this study proposes an OPR-free method that does not require a rotational reference or the computation of ideal ToA to obtain vibration velocity. The method relies on a pair of BTT sensors and the measured actual ToAs, enabling the estimation of blade tip vibration velocity. By replacing traditional vibration displacement with vibration velocity, the proposed method minimizes computational errors and uncertainties in BTT measurements while maintaining accurate identification of blade vibration parameters. More importantly, vibration velocity provides a better characterization of high-frequency components, laying a foundation for high-frequency vibration identification. Finally, based on the intrinsic relationship between vibration displacement and velocity, the blade’s natural frequency can be directly determined without any prior knowledge of engine orders.

Key words: blade tip timing, OPR-free, vibration velocity, parameter identification