Detection of Cracks in Turborotors—A New Observer Based Method

Abstract

The clear relation between shaft cracks in turborotors and vibration effects measured in bearings can be established by model-based methods very well. Here a new concept has been presented, based on the theory of disturbance rejection control, extended for nonlinear systems and applied on a turborotor. Simulations have been done, showing the theoretical success of this method, especially for reconstructing disturbance forces as inner forces caused by the crack. Calculating the relative crack compliance as the ratio of additional compliance caused by the crack and undamaged compliance a clear statement about the opening and closing, and therefore for the existence of the crack, and about the crack depth is possible. Theoretically it has been shown that it is possible to detect a crack with very small stiffness changes which corresponds to a crack depth of 5 percent of the radius of the rotor.

On fault induced nonlinear rotary response and instability

“…The dynamic response of a rotor-bearing system subjected to the actions of surface crack breathing and disk-induced imbalance can be described using the following equations of motion [8,9,19]:…”

“…Restoring forces realized by clearances in ball bearings and squeeze film dampers, oil films in journal bearings, seals, magnetic forces, friction and stiffening effects in elongated shafts are among possible sources of rotor-dynamic nonlinearity. Local nonlinearity such as bearing clearances [1,2], internal damping [3,4], fluid-film bearing forces [5][6][7] and crack breathing [8,9] can significantly impact the local and global rotary behaviors and bring about nonlinear responses including resonance curve shape change, jump and entrainment phenomena, subharmonic resonance, chaotic vibration and self-excited oscillation. As it is highly involved in dealing with local nonlinearity, procedures of linearization are widely adopted to predict critical speeds and the limiting conditions within which stable operation can be certain and designed for [10].…”

Non-Linear System Identification Using Lumped Parameter Models With Embedded Feedforward Neural Networks

“…o offker et al [3] for developing a technique for detection of cracks in turborotors. The advantage of the method is that the system model is linear and thus well-developed linear system theory is applicable in estimating the states.…”

Quasi-Periodic Vibration of Cracked Rotor on Flexible Bearings

“…Similar work has been done by Rizos and Tsai [11,12]. Collins [13], Armon [14] and SoK !ker [15] contributed their investigations on crack detection in di!erent respects. Seibold [16] used EKF method; a time domain method, to study the localization of cracks.…”