Patent ID: 7840370

Claim:
A system for monitoring the rotational speed of a gas turbine shaft having a number of electrically conductive spaced features on and/or rotating with shaft, the system including: (i) a magnet for generating a magnetic flux pattern in the path of said spaced features; (ii) two spaced eddy current sensors for sensing the movement of said spaced features past the respective sensors, each said sensor including an eddy current detector element to be positioned adjacent said path to detect magnetic fields generated by eddy currents generated in each said spaced feature as it moves through the flux pattern, and induce a voltage in each sensor output, the voltage varying with variations in the eddy current sensed by the respective sensors so as to create a voltage signal pulse corresponding to the passage of a feature past each sensor; (iii) a timer for determining the time period between voltage signal pulses generated by the same feature as it passes between the two sensors; (iv) distance determination means for determining a measure of the distance between the points at which said features generate a respective signal pulse at each of said sensors; (v) means for updating the distance determination as the distance changes over time; and (vi) speed calculation means for calculating speed from the time period of element (iii) and the distance of element (iv).