Torsional Vibration Transduction in a Solid Shaft by MPTs
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In this study, we aim to investigate the feasibility to use MPTs (Magnetostrictive Patch Transducers) for torsional vibration measurement in solid ferromagnetic cylinders. MPTs consisting of thin magnetostrictive patches, permanent magnets and a solenoid coil have been widely used for elastic wave transduction in the ultrasound frequency range  but they have been seldom used for sonic-frequency range vibration measurement, in spite of their unique wireless transduction characteristics. While a MPT was used in Ref.  to perform torsional modal testing in a hollow cylinder or a pipe having relatively small torsional rigidity, no investigation has been reported yet on the use of MPTs in “solid” “ferromagnetic” shafts, common torsional power carrying elements in machines.While we will be mainly focused here on the torsional wave measurement in stationary shafts, the MPT-based torsional measurement can be also applied to rotating shafts. Because the torsional rigidities of solid shafts are much larger than those of hollow cylinders of the same radii, it is important to find optimal MPT configurations, such as the optimal number of rectangular patches to be installed around the surface of a solid shaft. Thereby, we performed numerical investigations and accordingly designed a series of experiments for torsional vibration testing in steel shafts. The actual modal testing experiments with the designed MPTs were found to predict the torsional Eigen-frequencies and Eigen-modes that agree well with the theoretical predictions. Also the relation between the measured vibration signals from MPTs and those from strain gages was checked experimentally and in fact, the experimental result favorably agreed with the theoretical prediction. Potential applications of the MPT-based torsional vibration measurement technique in rotating solid shafts for structural health monitoring are also briefly discussed.