Improving a MEMS-based Sensor for Helicopter Gearbox and Rolling Stock Applications

This paper describes work related to improving an accelerometer-based sensor used for monitoring a gear box in a helicopter and for monitoring rolling stock: the locomotives and cars used in trains. The sensor was designed to measure the speed of rotating shafts of up to 5500 RPMs and two versions were developed and successfully passed prototype testing: the first was installed on a pinion gear in a helicopter gear box and proven to work, and the second installed on wheel hubs of locomotives and freight cars of a 100-car train at the National Test Track Center in Pueblo, Colorado. A three-day test included a 10-hour, non-stop, 400-mile test run at a fairly constant 40 mph. The sensor was (and still is) the first to survive, intact and operational after that three-day test. Even so, a manufacturer wanted a more rugged sensor and better signal quality. Improving the ruggedness and signal quality of the sensor comprised two physical actions: relocation of components to address signal degradation and potting to prevent physical dislocation due to shock and vibration. The improvement achieved for the sensor for rolling stock accrue to the sensor used in helicopter gearboxes. The rationale, the methods, and the results of those improvements are presented in the paper. The paper provides an overview and introduction to using micro-electromechanical systems (MEMS), a practical application - a shaft-mounted, helicopter pinion gear, and a description of the test and results of that test. Ruggedizing details and test results are presented - to include comparisons of signals before and after the improving the ruggedness of the sensor and how improvements apply to the helicopter version of the sensor.