THE ITD MODE-SHAPE COHERENCE AND CONFIDENCE FACTOR AND ITS APPLICATION TO SEPARATING EIGENVALUE POSITIONS IN THEZ -PLANE

The properties of a newly defined mode-shape coherence and confidence factor are discussed in detail in connection with the Ibrahim time domain method. It has been found that this factor is able to distinguish three kinds of modes, true physical modes within the frequency range (0~fpi) (corresponding to an angular range (0~pi) of eigenvalue positions in the Z -plane), frequency folded and/or overlapped modes and computational (noise) ones. Making use of this ability gives at least two benefits; (a) it releases the limitation on the userʹs constant delta t1and increases the flexibility in use of all the userʹs constant combinations, and (b) it extends the angular range of eigenvalue positions to a multiple of 2 pi by using a large value of delta t1and makes the identification of eigenvalue positions in the Z -plane more accurate by a sort of zoom process. Digital simulation and measurements on a gearbox were chosen as application examples to illustrate the above advantages.