Determination of Simultaneous Steady-State Movements Using Blade Tip Timing Data

Abstract

Blade tip timing (BTT) includes a number of uncertainties that discourage its use. One of the main ones is the shift in the equilibrium position of the blade tip due to steady (non-oscillatory) bending and/or twisting of the blade, and axial movement of the bladed disk (blisk)-shaft system. This results in a shift in the effective measurement position of the probe relative to the blade chord, resulting in errors in the tip vibration measurement which can translate to a huge error in the corresponding stress estimate, which relies on calibration against finite element (FE) models. Previous experimentally validated research by the authors introduced a method for quantifying steady movement of a single type (axial, lean, or untwist), using BTT data from not more than two probes. In this paper, a development of the previous method is presented that provides a solution for the case of simultaneous types of blade steady movements. Additional probes are used for determining the direction, but these can be placed at any angular positions. The developed method is validated using a BTT simulator of a blisk, and accurate results obtained. The simultaneous axial and lean movements can be accurately determined when the untwist is negligible, and an uncertainty level can be specified when the untwist is not negligible. The untwist itself can be calculated accurately in all cases of simultaneous movements. Guidelines for the use of the method in different scenarios are provided.

An experimentally validated modal model simulator for the assessment of different Blade Tip Timing algorithms

“…DC-noise appears as a steady-state offset in the data that is caused by uncertainty in the position of the sensing point on the blade tip relative to the probe, resulting from probe and/or blade positional errors, or from speed-dependent steady (non-oscillatory) movements of the blisk (axial float, blade lean, and blade untwist) [3]. The authors presented a method in [3][4][5] for determining the steady movements from BTT data, enabling the identification of the true sensing point, and consequently improving the reliability of BTT vibration estimates. AC-noise arises from delays and errors in the sampling and acquisition instruments [6], and also from other significant sources such as blade buffeting [7].…”

“…the inter-blade coupling, blade-disk coupling). It can also allow the study of the effect of steady (non-oscillatory) movements (such as axial float, blade lean and blade untwist [3,25]) on BTT methods, which are the focus of separate papers [3][4][5]. The three BTT methods considered are presented in detail.…”

“…This latter shift is optionally applied to simulate steady movements, which are a source of "DC noise" (Introduction). This was considered by the authors in [3][4][5] and is not considered here. The vector (rgd) ( ) is given by…”