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A different perspective from which to view the operation of magnetoelastic force transducers is described. Unlike the more conventional approach based on the alteration of B-H loop shape by a stress dependent permeability, this perspective starts with the separate examination of the processes by which stress and magnetic field affect magnetization. The purpose of applying magnetic fields is identified as a useful means to observe the independent effects of stress. A transducer of novel construction, conceptually derived by a straightforward application of this perspective, is also described. The concept is supported by data obtained from experimental transducers employing permanent magnets and solid state field sensors in place of the conventional exciting currents and detection coils. The basic device can be readily configured as a transducer for axial, flexural or torsional loadings.

Need for non-contact torque transducers in modern power steering systems is identified. Inherent size and complexity of existing transducer designs is shown to discourage their use in this application. A new magnetoelastic type of transducer of unusually small and simple construction is described. The transducing function is fulfilled by just two elements: a circumferentially magnetized ring affixed at a convenient location on any shaft in the torque path between the steering wheel and the steering box, and a magnetic field sensor mounted in proximity to the ring. Stresses in the ring, associated with the torque transmitted along the shaft, cause the ring magnetization to develop an axial component. The resulting magnetic field in the space around the ring has a polarity and intensity correlated with the sense and magnitude of the torque.

A new type of sensor capable of sensing the direction and velocity of motion of smooth surfaced metallic targets is described. The sensor consists of a small permanent magnet and a magnetic field sensor displaced from each other along the line of motion, each at a small fixed distance from the target surface. Operation is based on the motion dependent location, strength and polarity of magnetic field sources created within proximate target regions via their passage through the field of the magnet. The fields arising from these target regions are detected by the magnetic field sensors, typically Hall effect or magnetoresistive devices. With ferromagnetic targets, a non-volatile memory of the direction of last occurring motion is also provided. Utility of these sensing capabilities is illustrated by descriptions of applications for automatic turn signal canceling and antitheft devices, back-up alarm activation and for engine misfire detection via crankshaft speed variation.

Simple constructional modifications extend the capability of polarized ring torque transducers, when applied to rotating shafts, to include the measurement of power and speed. Gear-like teeth on thin, high permeability rings affixed to the polar regions of the magnetoelastically active ring spatially modulate the magnetic field arising with the torque. Speed is found from the frequency of the ac component of the field sensor signal while torque is found from its amplitude. Drift in quiescent outputs of low cost Hall effect sensors have no effect on these ac signal features. Signals proportional to transmitted power are generated in sensing coils by shaft rotation.

The development of a transducer for sensing the torque on the output shaft of a four speed rear wheel drive automatic transmission is described. Magnetoelastic polarized ring technology was selected based on its independence from shaft properties and its non-contact mode of sensing. The ring and several intermediate sleeves were attached by press fits onto an experimental shaft. The magnetic field arising from the ring with the application of torque was sensed by flux gate sensing elements. Ability to accurately measure the output torque of an engine driven transmission over its full range of torque and speed was demonstrated by dynamometer tests.

The benefits of a magnetoelastic torque sensing system and its application on Formula 1 and CHAMP Car racing driveshafts are identified. In particular, the use of circumferential remanent magnetic bands on the shaft itself has enabled a design that satisfies the application's demanding packaging requirements. The criteria used to determine the suitability of a given shaft material for optimal magnetoelastic characteristics and concurrent mechanical strength are illustrated. Shaft material and geometry are shown to have significant effects on sensor performance. Design considerations for the magnetic field sensor layout and signal conditioning circuit are also presented. The racing driveshaft application is shown to add particular challenges in terms of temperature, packaging, and kinematic tracking. Magnetization and calibration processes associated with field operation of the sensor are discussed.