Magnetoelastic contactless torque transducer with a shaft with a

Details Magnetoelastic contactless torque transducer with a shaft with a Magnetoelastic contactless torque transducer with a shaft with a

1995-09-12

1996-11-12

Chilcot, Richard

Measuring and testing

Dynamometers

Responsive to torque

73862333, G01L 100

Patent

active

055742319

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a transducer for measuring, in a contactless manner, the torque acting on a stationary or a rotating shaft. In this context it is important that the shaft of the transducer exhibits anisotropic properties. To achieve a desired anisotropy in the shaft, according to the invention material is utilized which has a microstructure with at least two phases which are anisotropically distributed.


BACKGROUND ART, PROBLEMS

A circular cylindrical shaft which is subjected to a torque is influenced by a pure shear stress. This stress state can be expressed in terms of its principal stresses as a compressive stress and a tensile stress, directed perpendicularly thereto, of the same magnitude. The principal stress directions are inclined at .+-.45 degrees to a generatrix to the cylinder surface.
If, within a measurement range of the shaft, a rotationally symmetrical, homogeneous magnetizing field, that is an H-field, is created with the aid of a surrounding stationary exciting winding, an equally homogeneous magnetic flux density, that is, a B-field, is obtained in the shaft in non-loaded state. When the shaft is loaded, the field plot of the B-field is distorted, which can be detected with the aid of detection windings.
The state of the art as regards the constructive design of torque transducers is disclosed in a number of patent specifications and technical articles. Common to most of these solutions is that two zones are created in the magnetic material, with some form of anisotropy which causes the magnetic flux density to be deflected at an angle away from its natural direction in parallel with the generatrices to the cylinder surface of the transducer shaft. In one zone the principal direction of the anisotropy coincides with the principal stress direction which provides tensile stress. In the other zone, the principal direction coincides with the direction which provides compressive stress.
Because of the magnetoelastic effect, the zone reluctance will therefore decrease or increase, where the magnetic flux density has been deflected towards the tensile direction or the compressive direction in the case of positive magnetostriction. By finally measuring the difference in reluctance between these zones, a measure of the torque is obtained which has little sensitivity to axial forces or bending stress.
The reluctance difference between the zones is usually measured by creating, via a primary coil concentric with the shaft, a time-dependent H-field directed along the shaft and with equal amplitude in both zones. With the aid of two identical secondary coils, one across each zone, the difference in B-fields between the zones is measured. This is achieved in the simplest way by connecting the secondary coils in opposition in such a way that the induced voltages in the respective coil are subtracted from each other. By phase-sensitive rectification of the secondary signal obtained in this way, it is possible, in addition, to distinguish between torsional moments of different directions.
To create a high sensitivity to torsional moments, it is required that the anisotropy be sufficient, such that the difference between the zones becomes as great as possible. A measure of the anisotropy is the angle at which the magnetic field is deflected from the natural direction parallel to the generatrix to the cylinder surface of the transducer shaft because of the influence of the anisotropy. If this angle is 45 degrees in the zones, the anisotropy is maximal as the B-field is then directed along the principal stress directions of the transducer shaft loaded with the torsion.
Of the utmost importance is also to really maintain a complete rotational symmetry, both with respect to the mechanical stress configuration and the magnetic field configuration in order to prevent a signal variation which is only due to the transducer being rotated in relation to the reluctance-measuring part.
According to the state of the art, there are a number of methods of achieving anisotropy, a few of

REFERENCES:
patent: 4627298 (1986-12-01), Sahashi et al.
patent: 4840073 (1989-06-01), Aoki et al.
patent: 4920809 (1990-05-01), Yoshimura et al.
patent: 4972726 (1990-11-01), Yoshimura et al.
patent: 4972728 (1990-11-01), Yoshimura et al.

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