Method and System for Measuring Rotor Blade Tip Deflection Using Blade Tip Timing (BTT)

A method for measuring rotor blade tip deflections of turbomachine rotor blades during operation using Blade Tip Timing (BTT), the turbomachine comprising a housing and rotor including a shaft with the rotor blades attached thereto and at least one proximity probe mounted to the housing, the method comprising:

• measuring, by the proximity sensor, a proximity signal caused by a presence of a proximate tip of a moving rotor blade (the “rotor blade tip”);
• determining, by a control module, a Time-of-Arrival (ToA) of the rotor blade tip from the measured signal;
• measuring, by a shaft encoder, a time that the shaft starts to rotate a measurable angular distance and a time the shaft has completed its rotation of the measurable distance (these times are henceforth referred to as zero crossing times);
• storing, in a memory module at least temporarily, timing data indicative of a plurality of ToA measurements of the rotor blade and the zero crossing times;
• determining, by the control module, the shaft Instantaneous Angular Position (IAP) between at least two zero crossing times, which determination comprises:
• expressing, by the control module, the shaft IAP between at least two zero crossing times as a continuous, non-constant IAP mathematical function of time, with unknown function coefficients;
• and
• calculating, by the control module, the unknown function coefficients of the IAP mathematical function using at least the following information:
• the angular distance travelled by the shaft in between the recorded zero crossing times;
• and
• the values of the recorded zero crossing times applicable to the IAP mathematical function;
• determining, by the control module, the shaft IAP at each stored ToA using the IAP mathematical function;
• and
• calculating, by the control module, the rotor blade tip deflection at each ToA using the IAP at each ToA and a stored IAP value representing an expected angular arrival location.