NON-INVASIVE SPEED SENSOR

1 . A method of measuring rotational speed of a shaft, comprising the steps of:

coupling an optical pickup to a shaft speed sensor having an indicator light that pulses proportionally to a rotational speed of a shaft being measured by the speed sensor;

receiving light pulses from the indicator light of the speed sensor; and

determining the rotational speed based on a rate of received light pulses.

2 . The method of claim 1 , wherein the coupling step further comprises coupling the optical pickup to an existing speed sensor at a shaft of a wind turbine.

3 . A wind turbine condition monitoring system, comprising:

a processor;

a digital memory having a program stored thereon and being accessible by the processor; and

a plurality of sensors that provide data to the processor, the sensors including accelerometers and an optical pickup, wherein the processor is operable under control of the program to determine a rotational speed of the wind turbine based on light pulses received by the optical pickup.

4 . A wind turbine condition monitoring system as defined in claim 3 , further comprising an interface circuit coupled between the optical pickup and processor, said interface circuit being operable to output shaped pulses based on input received by the interface circuit from the optical pickup.