Improvement of BTT precision considering transient rotating speed fluctuations

Abstract

A novel method is proposed to correct errors caused by rotational speed fluctuations in the BTT system. The analysis shows that the traditional BTT measurement using the constant speed assumption has low-frequency error due to the integration effect when the rotational speed fluctuation exists. Therefore, a filtering method based on the periodic non-uniform sampling (PNS) theorem is proposed. The details of the PNS-based method were investigated. The results show recommended order of the digital FIR filter is a length of 4 to 6 revolutions. The process of the PNS-based method for the unsteady state is given. Monitoring the condition number of the delayed phase matrix A is recommended when using the PNS-based method in the unsteady state. The effectiveness of the method was verified by a laboratory experiment and a field test. The laboratory experiment results showed that the error was reduced by 30 to 70 μm in the steady state. When the blade vibration was weak, the error was even halved. In the unsteady state, the PNS-based method controlled the error below 200 μm. In the field test, the PNS-based method also achieved the desired effect. The proposed method can significantly improve BTT measurement accuracy, showing great value and the prospect of blade health monitors in turbomachines.