Patent ID: 7861592

Claim:
A method of monitoring a rotating coupled blade row with a vibration monitor, said rotating coupled blade row comprising a plurality of blades supported for rotation on a rotor and a shroud structure coupling said blades and defining an outer circumference of said blade row, and said vibration monitor comprising a stationary sensor positioned at said shroud structure and a plurality of sensor targets positioned at circumferentially spaced locations on said shroud structure with said sensor providing a time-dependent output signal responsive to the passage of each said sensor target, comprising the steps of: determining, based on sensor output signals provided from said sensor, the actual arrival time of each said sensor target, and said vibration monitor storing a plurality of sensor output signals as a function of time relative to the periodic rotation of said shroud structure; calculating for each of said sensor output signals a displacement between said actual arrival time and a predetermined expected arrival time for each sensor target to determine displacements of a plurality of different circumferential locations on said shroud structure as a function of time; and deriving a signal characteristic of a vibrational mode of said shroud structure based on said displacements of said circumferential locations on said shroud structure, wherein said signal characteristic of a vibrational mode comprises a recurring vibrational phase, wherein said recurring vibrational phase is a function of a fundamental shroud frequency and a shroud nodal diameter, and wherein said recurring vibrational phase comprises a shroud vibrational frequency, fv, equal to: fv=ff Â±( frÃ—ND ) where: ff=the fundamental frequency of the shroud structure; fr=the rotational frequency of the rotor; and ND=the shroud nodal diameter.