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A method (and corresponding apparatus) for monitoring gas turbine blades. The output from an eddy current sensor monitoring the movement of turbine blades past the sensor is processed to determine when the signal train from the sensor omits a signal or pulse corresponding to one of the shaft's full

A method (and corresponding apparatus) for monitoring gas turbine blades. The output from an eddy current sensor monitoring the movement of turbine blades past the sensor is processed to determine when the signal train from the sensor omits a signal or pulse corresponding to one of the shaft's full complement of blades. The signal processor compares sensed blade periods with average blade periods.

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1. A method for monitoring gas turbine blades on a rotatable shaft of a gas turbine where the blade tips can move relative to each other, comprising the steps of: a. producing a signal train comprising a sequence of signals or pulses, wherein each signal or pulse corresponds to the passage of a blad

1. A method for monitoring gas turbine blades on a rotatable shaft of a gas turbine where the blade tips can move relative to each other, comprising the steps of: a. producing a signal train comprising a sequence of signals or pulses, wherein each signal or pulse corresponds to the passage of a blade past a sensor;b. determining a blade time period between successive signals or pulses;c. calculating an average blade period over a plurality of consecutive blade time periods; andd. determining a ratio of a determined blade period to the average blade period. 2. A method according to claim 1, for determining whether or not the signal train omits a signal or pulse corresponding to one of the gas turbine shaft's full complement of blades. 3. A method according to claim 1, wherein the determined ratio of a blade period to the average blade period is compared to a number of pre-determined values of said ratio. 4. A method according to claim 3, wherein the plurality of consecutive blade periods over which the average blade period is calculated does not include the determined blade period, and wherein the pre-determined values of said ratio are bounded by the possible values of the equations: RatioM⁢⁢AX=AV×D+1+2×jAV+M-2×jandRatioM⁢⁢IN=AV×D+1-2×jAV+M+2×jwhere: RatioMAX is the upper boundary value of the ratio, RatioMIN is the lower boundary value of the ratio; AV is the number of signal pulses or peaks over which the average blade period is calculated; M is the number of missing pulses or peaks over a single complete revolution; D is the number of missing pulses or peaks in the determined blade period; and j is the maximum value of blade tip movement relative to nominal blade position expressed as a fraction of the nominal blade separation. 5. A method according to claim 3, wherein the plurality of consecutive blade periods over which the average blade period is calculated includes the determined blade period, and wherein the pre-determined values of said ratio are bounded by the possible values of the equations: RatioM⁢⁢AX=AV×D+1+2×jAV+M-D-2×j⁢.RatioM⁢⁢IN=AV×D+1-2×jAV+M-D+2×jwhere: RatioMAX is the upper boundary value of the ratio, RatioMIN is the lower boundary value of the ratio; AV is the number of signal pulses or peaks over which the average blade period is calculated; M is the number of missing pulses or peaks over a single complete revolution; D is the number of missing pulses or peaks in the determined blade period; and j is the maximum value of blade tip movement relative to nominal blade position expressed as a fraction of the nominal blade separation. 6. A method according to claim 1, wherein the average blade period is determining by averaging a series of blade periods signals each representative of a series of measure blade period signals, predicting a value for a first signal from a historical blade period signal value, generating a first calculated blade period signal from the predicted value of the first measurement signal; comparing the measured signal to its predicted value period, and: if the measured signal is within a pre-determined range of acceptable values, using the first measured blade signal to predict a value for a second measured signal blade period;if the measured signal is outside the pre-determined range of acceptable values, using the first predicted value to predict a second measured signal,and generating a second calculated blade period signal from the predicted value of the second measured signal. 7. A method according to claim 1, further comprising the step of monitoring the amplitude of the signals or pulses. 8. A method according to claim 1, for monitoring health of a gas turbine. 9. A method according to claim 1, wherein said gas turbine is an industrial gas turbine. 10. A method according to claim 1, wherein the gas turbine is a jet engine. 11. A method according to claim 1, wherein the blades are compressor blades. 12. A system for monitoring gas turbine blades on a rotatable shaft of a gas turbine where the blade tips can move relative to each other, comprising: a. means for producing a signal train comprising a sequence of signals or pulses, wherein each signal or pulse corresponds to passage of a blade past a sensor;b. means for determining a blade time period between successive signals or pulses;c. means for calculating an average blade period over a plurality of consecutive blade time periods; andd. means for determining a ratio of a determined blade period to the average blade period. 13. A system according to claim 12, for determining whether or not the signal train omits a signal or pulse corresponding to one of the gas turbine shaft's full complement of blades. 14. A system according to claim 12, wherein the determined ratio of a blade period to the average blade period is compared to a number of pre-determined values of said ratio. 15. A system according to claim 12, wherein the plurality of consecutive blade periods over which the average blade period is calculated does not include the determined blade period, and wherein the pre-determined values of said ratio are bounded by the possible values of the equations: RatioM⁢⁢AX=AV×D+1+2×jAV+M-2×jandRatioM⁢⁢IN=AV×D+1-2×jAV+M+2×jwhere: RatioMAX is the upper boundary value of the ratio; RatioMIN is the lower boundary value of the ratio; AV is the number of signal pulses or peaks over which the average blade period is calculated; M is the number of missing pulses or peaks over a single complete revolution; D is the number of missing pulses or peaks in the determined blade period; and j is the maximum value of blade tip movement relative to the nominal blade position expressed as a fraction of the nominal blade separation. 16. A system according to claim 12, wherein the plurality of consecutive blade periods over which the average blade period is calculated includes the determined blade period, and wherein the pre-determined values of said ratio are bounded by the possible values of the equations: RatioM⁢⁢AX=AV×D+1+2×jAV+M-D-2×j⁢.RatioM⁢⁢IN=AV×D+1-2×jAV+M-D+2×jwhere: RatioMAX is the upper boundary value of the ratio; RatioMIN is the lower boundary value of the ratio; AV is the number of signal pulses or peaks over which the average blade period is calculated; M is the number of missing pulses or peaks over a single complete revolution; D is the number of missing pulses or peaks in the determined blade period; and j is the maximum value of blade tip movement relative to the nominal blade position expressed as a fraction of the nominal blade separation. 17. A system according to claim 12, further comprising means for monitoring the amplitude of the pulses or peaks. 18. A system according to claim 12, for monitoring health of a gas turbine. 19. A system according to claim 12, for monitoring the health of an industrial gas turbine. 20. A system according to claim 12, for monitoring the health of a jet engine. 21. A data processor programmed to perform the steps of the method of claim 1.