Minimally intrusive torsional vibration sensing on rotating shafts

Time Interval Measurement System (TIMS) is a common torsional vibration sensing method used for rotating equipment. The technique uses high-speed counters to detect "zero crossings" in a carrier signal that is generated by a multiple pulse per revolution encoder on the shaft. The "zero crossings" are based on the passage timing of discrete intervals from an incremental geometric encoder (i.e., gear, optical encoder) on a rotating shaft. A variety of transducers have been used to sense the encoder interval passages, including Hall effect and reflective light intensity transducers. The encoder and sensor require physical attachment to the rotating shaft and surrounding hardware. Furthermore, the combinations must work in concert with each other. Usually the sensing transducer requires precise positioning with respect to the shaft encoder. The physical attachment and installation of the encoding and sensing devices can range from problematic to extremely difficult depending on the application. To make sensing of torsional vibration on a rotating shaft easier and minimally intrusive a combination of an adhesive backed "zebra" tape and a specially modified laser tachometer is used. The laser tachometer is considerably easier as permits a greater range of standoff, targeting and mounting options. Tests are performed a on mechanical diagnostics test bed with a 30 hp electrical drive connected to a 70 hp load motor by a shaft and couplers to demonstrate the issues. Torsional vibration measurements are acquired with the laser tachometer-"zebra" tape, Hall effect-gear and a precision optical encoder. Results are presented and discussed from the various torsional vibration sensing systems. The application illustrates the ease of set up for the laser tachometer-"zebra tape" combination and the high data quality that can be obtained.