PURPOSE:To enable stable measurement of the longitudinal position, longitudinal vibration displacement, torsional angle and vibrational angle of a shaft for a long time by determining a phase difference between output waveforms of noncontact-type electromagnetic rotational pulse detectors which are provided oppositely to the outer circumferences of a spur gear and a helical gear respectively. CONSTITUTION:A helical gear 8 and a spur gear 9 are provided at the front end of a crank shaft 7, and a turning gear 10 which is a spur gear being equal in the number of teeth to the spur gear 9 is provided at the rear end of the crank shaft 7. Noncontact-type electromagnetic rotational pulse detectors 11, 12 and 13 are provided oppositely to the outer circumferences of the gear 10, the gear 8 and the gear 9 respectively. An instantaneous longitudinal vibration phase difference measuring means comprising a longitudinal vibration phase difference counter 21a, a longitudinal vibration divider 23a and a D/A converter 24a and also an instantaneous torsional vibration phase difference detecting means comprising a torsional vibration phase difference counter 21b, a torsional vibration divider 23b and a D/A converter 24b determine phase differences between output waveforms of the detectors 12 and 13 and the detectors 11 and 13 and output them in an analog amount respectively.
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