Measuring and testing – Dynamometers – Responsive to torque
Patent
1996-09-04
1997-07-08
Chilcot, Richard
Measuring and testing
Dynamometers
Responsive to torque
73862333, G01L 300
Patent
active
056463561
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a transducer for accurately measuring, in a non-contacting manner and within a large temperature range, the torque acting in a stationary or a rotating shaft.
BACKGROUND ART
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.degree. to the longitudinal axis of the shaft.
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 type of anisotropy, which causes the magnetic flux density to be deflected at an angle away from its natural direction in parallel with the axis of the transducer shaft. In one zone the principal direction of the anisotropy coincides with the principal 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 (in the case of positive magnetostriction), the zone reluctance will therefore decrease or increase if the magnetic flux density has been deflected towards the tensile direction or the compressive direction.
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 is usually measured by creating a time-dependent H-field directed along the shaft and with equal amplitude in both zones, using a primary coil concentric with the shaft. With the aid of two identical secondary coils, centred around each zone, the difference in B-fields between the zones can be 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.
This reluctance-measuring part in many cases constitutes a kind of magnetic yoke to the shaft and will, therefore, in the following be referred to as the "yoke".
To create a high sensitivity to torsional moments, it is required that the anisotropy is 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 axis of the transducer shaft because of the influence of the anisotropy. If this angle is 45.degree. in the zones, the anisotropy is optimal as the B-field is then directed along the principal stress directions of the transducer shaft loaded with torsion.
Of the utmost importance is maintaining a complete rotational symmetry, both with respect to the distribution of the mechanical stress and the magnetic field, in order to prevent a signal variation which is only due to the transducer being rotated in relation to the reluctance-measuring part.
What distinguishes different torque transducers according to the above general description is primarily the method of realizing the anisotropy.
SU 667836 describes a method in which the anisotropy is created purely geometrically in each zone by cutting grooves in the surface of the shaft according to a specific pattern. This pattern consists of a number of mutually parallel lines directed at an angle of 45.degree. to the axis of the transducer shaft.
However, this solution entails an insufficient anisotropy and hence also low sensitivity, since the magnetic field can "creep under" the grooves in a relatively simple manner, unless these grooves are made deep. If the grooves are made deep, however, the stre
REFERENCES:
patent: 4823620 (1989-04-01), Edo et al.
patent: 4891992 (1990-01-01), Kobayashi et al.
patent: 4907462 (1990-03-01), Odama et al.
patent: 5052232 (1991-10-01), Garshelis
patent: 5092182 (1992-03-01), Ikeda et al.
Ling Hans
Sobel Jarl
Uggla Dan J.
Asea Brown Boveri AB
Biegel Ronald
Chilcot Richard
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