Blade shroud vibration monitor

1. 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 ±( fr×ND )

where:

ff=the fundamental frequency of the shroud structure;

fr=the rotational frequency of the rotor; and

ND=the shroud nodal diameter.

2. The method of claim 1 , wherein said step of deriving a signal characteristic of a vibrational mode of said shroud structure includes determining a peak shroud vibration wave during time intervals that are integer multiples of rotor rotation.

3. The method of claim 1 , wherein the number of said sensor targets substantially corresponds to the number of said blades in said blade row.

4. The method of claim 1 , wherein said step of deriving a signal characteristic of a vibrational mode of said shroud structure is performed based on a sampling frequency, fs, equal to:

fs=fr×ys×yt

where:

fr=the rotational frequency of the rotor;

ys=the number of sensors; and

yt=the number sensor targets.

5. The method of claim 4 , wherein the number of sensor targets, yt, equals the number of said blades in said blade row.

6. The method of claim 1 , wherein each of said blades have a shroud portion having opposing front and rear contact surfaces with respect to a rotational direction of said blade row, said shroud portions being arranged in such a way that shroud portions of two adjacent blades are brought into contact with each other at adjacent front and rear contact surfaces during rotor spin-up.

7. A method of predicting a blade structure failure within a blade structure comprising a plurality of blades supported for rotation on a rotor and a shroud structure coupling said blades, comprising the steps of:

determining displacements of a plurality of predetermined circumferential locations on said shroud structure during rotation of said blade row, where said displacements are provided as a function of time relative to the periodic rotation of said shroud structure for time intervals that are integer multiples of rotor rotation; 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 shroud structure includes a plurality of sensor targets defining said predetermined circumferential locations on said shroud structure, and including at least one stationary sensor positioned at said shroud structure and providing a time-dependent output signal responsive to the passage of each said sensor target,

wherein said step of deriving a signal characteristic of a vibrational mode of said shroud structure is performed based on a sampling frequency, fs, equal to:

fs=fr×ys×yt

where:

fr=the rotational frequency of the blade row,

ys=the number of sensors; and

yt=the number sensor targets.

8. The method of claim 7 , wherein the number of sensor targets, yt, equals the number of said blades on said blade row.

9. The method of claim 7 , wherein the number of sensor targets, yt, is less than the number of said blades on said blade row.

10. The method of claim 7 , wherein said signal characteristic of a vibrational mode comprises a recurring vibrational phase.

11. The method of claim 10 , wherein said recurring vibrational phase is a function of a fundamental shroud frequency and a shroud nodal diameter.

12. The method of claim 11 , wherein said recurring vibrational phase comprises a shroud vibrational frequency, fv, equal to:

fv=ff ±( fr×ND )

where:

ff=the fundamental frequency of the shroud structure;

fr=the rotational frequency of the rotor; and

ND=the shroud nodal diameter.

13. The method of claim 12 , wherein said fundamental frequency, ff, comprises a fundamental frequency of said shroud structure shifted up by an amount approximately equal to 1 to 2 Hz multiplied by the nodal diameter.

14. The method of claim 7 , wherein said step of deriving a signal characteristic of a vibrational mode of said shroud structure includes determining a peak shroud vibration wave occurring at time intervals that are integer multiples of rotor rotation.

15. The method of claim 7 , wherein the number of said plurality of circumferential locations substantially corresponds to the number of said blades in said blade row.