Real time flutter monitoring system for turbomachinery

A fiber optic laser probe based system is described for real time monitoring of flutter in rotating turbomachinery. The digital flutter monitoring system is designed for continuous processing of blade tip timing data at a rate of 10 MB/s. A USB2.0 interface provides un-interrupted real time processing of the data. The blade tip arrival times are measured with a 50 MHz oscillator and a 24-bit pipelined counter architecture. A graphical user interface provides on-line interrogation of any blade tip from any light probe sensor. Alternatively, data from all blades can be superimposed into a single composite scatter plot displaying the vibration amplitude of each blade. A hardware platform was developed to simulate a seventy two bladed turbine operating at 15,000 rpm. Blade tip responses from three light probes were generated in an infinite loop, providing reproducible and controlled conditions for testing the vibration monitoring system. Time interval measurements were consistently made with a single count error in a 24-bit count vector. Real time testing was done using a two blade rotor mounted in an evacuated chamber at the Spin Rig Facility at the NASA Glen Research Center. The shaft in this facility was suspended by two radial magnetic bearings and the non-synchronous vibration was communicated to the blades through the magnetic bearing. The shaft motion was much smaller than the blade vibratory amplitude, realistically simulating flutter vibrations. Non-synchronous vibratory amplitudes for the first mode were of the order of twenty mils and for the second mode of the order of a few mils.