New inductive speed transducer

A new inductive transducer, using the technique of a series resonance bridge for accurate and sensitive measurement of angular speed, is described. This technique is essentially a contactless method of speed measurement, in which only a thin slotted aluminium disc need be attached to the rotating member. The slotted disc rotates in the airgap of a laminated steel core. Eddy currents induced in the aluminium disc oppose the magnetic field of a coil wound on the laminated steel core, and cause an apparent variation of inductance in a cyclic manner at a rate proportional to the angular speed. The variation of this inductance is sensed by a series-resonance bridge circuit and is used to modulate the amplitude of a carrier voltage, producing an electrical response whose frequency is a measure of the angular speed of the rotating member. This inductive transducer imposes negligible load on the rotating member under test, and offers a simple lowcost and compact device which can replace typical digital or pulse tachometers for incremental motion control or in phase-locked loop control systems or sensing applications. Furthermore, with modifications it is appropriate for use in remote speed sensing. The design details and experimental results of this transducer are presented.