Measuring and testing – Dynamometers – Responsive to torque
Reexamination Certificate
2001-09-19
2003-03-18
Noori, Max (Department: 2855)
Measuring and testing
Dynamometers
Responsive to torque
Reexamination Certificate
active
06532832
ABSTRACT:
TECHNICAL FIELD
The need to measure the torque which is transmitted via a shaft exists both for monitoring and control. The need exists within a plurality of different process industries and for other purposes within devices and products. The present invention is a novel type of magnetoelastic torque transducer for contactless measurement of the transmitted torque in a rotating as well as a stationary shaft.
BACKGROUND ART
Generally, 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° to a generatrix to the cylinder surface.
For most modern contactless magnetoelastic torque transducers, the torque measurement is based on the above principle. The torque in these torque transducers is thus measured by measuring the change of the magnetic permeability in a certain direction in the material constituting a transducer shaft.
The most commonly used torque measurement which makes use of this is designed such that, 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 excitation winding. This results in an equally homogenous magnetic flux density, that is, a B-field, in the shaft in unloaded state. When the shaft is loaded, the field configuration 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 based on the above method is disclosed in a number of patent specifications. 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 generatrices to the cylinder surface of the transducer shaft.
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° to a generatrix to the cylinder surface of the transducer shaft.
U.S. Pat. No. 4,823,620 describes the same embodiment as above with respect to the geometrical anisotropy, however with the addition that the surface of the shaft is hardened or carburized for the purpose of reducing the hysteresis in the transducer. Both of these methods have their limitations. The methods presuppose that full rotational symmetry of the anisotropic patterns is achieved. The shaft must also be machined to achieve the desired magnetic anisotropy.
Another magnetoelastic method for measuring the torque in a shaft is clear from EP 0 525 551 A2. The shaft whose torque is to be measured is provided with a circularly polarized magnetoelastic ring which is pressed, shrunk or glued onto the shaft. When applying torque to the transducer shaft, the shaft, and hence the ring, will be distorted. This results in the originally purely azimuthal magnetization being reoriented in a helical pattern with both an azimuthal and an axial component. With the aid of a Hall element, freely mounted relative to the shaft, which is oriented in such a way that the Hall element only senses the axial component of the magnetization, a measure of the torque occurring in the shaft is obtained.
Using a Hall element according to the above entails several disadvantages. On the one hand, a Hall element will measure the field at one point only along the circumference of the ring. When the transducer shaft rotates, and hence also the circularly polarized magnetic ring, the output signal from the Hall element will vary unless the magnetic field of the ring has a perfect rotational symmetry. Another disadvantage is that Hall elements have very small signal levels, which makes the signal processing difficult and makes the signal sensitive to electrical interference. In addition, Hall elements have a temperature drift which is not unessential.
In patent application WO 9722866A1, a torque transducer comprising a shaft with a circularly polarized ring is described. The ring is concentrically surrounded by a stationary tubular shell, freely mounted in relation to the shaft, which shell is provided with a winding for magnetization of the shell with a given frequency. A phase-sensitive detector connected to the winding supplies a signal corresponding to the content of the voltages with even harmonics, induced in the winding, for the purpose of measuring the axial component of the magnetic field.
One difficulty with the above solution is that a large part of the axial magnetic flux, which arises in the ring when loading a torque transducer shaft, will inevitably complete the circuit through the transducer shaft in the case of magnetic transducer shaft material. This then implies that the static magnetic flux mentioned, which is intended for generating even harmonics of the time-varying flux in the shell above it, will substantially not reach to the shell.
An accurate measurement of the transmitted torque may be relatively difficult to achieve in the case of rotating shafts since the accuracy may be influenced if the outer shell does not possess a sufficiently large degree of rotational symmetry. In addition, the choice of transducer shaft material is dependent on the magnetic properties of the material and is, in practice, limited to paramagnetic materials only.
SUMMARY OF THE INVENTION
One object of the present invention is to achieve a torque transducer for which the choice of transducer shaft material is independent of the magnetic properties of the material. Another object of the invention is to design a torque transducer whose transducer shaft may comprise a ferromagnetic material.
Another object of the invention is to arrange a torque transducer by means of which the axial dc field, which arises in the ring when loading the circularly polarized ring, is utilized more efficiently for generating even harmonics of the flux in the magnetic shell.
Still another object is to arrange a shell and a transducer shaft of a torque transducer in such a way that the angular dependence of the transducer signal becomes minimal. An additional object of the invention is to arrange a torque transducer in which the power losses, due to air leakage in the high-reluctance air path outside the excitation winding of the transducer, are reduced.
Yet another object of the invention is to arrange a torque transducer by means of which a good accuracy of measurement can always be obtained, also when the magnetic field in the magnetoelastic ring has a moderate degree of rotational symmetry. When measuring the transmitted torque in, for example, a rotating shaft, it is of great importance that the accuracy of measurement should be independent of any rotational symmetry of the mentioned magnetic field.
These and other objects are fulfilled according to the invention by a torque transducer for measuring the transmitted torque in a rotating as well as a stationary shaft according to claim
1
. Further, some advantageous embodiments of the present invention are described in claims
2
-
14
. The present invention relates to a torque transducer which is arranged such that the transducer shaft is manufactured of an arbitrary material independently of the magnetic properties of the material.
According to the prior art, when applying torque to the transducer shaft, a static magnetic axial flux arises in the transducer ring. The static magnetic axial flux completes the circuit through the air above the ring and through the shaft material below the ring. The proportion of magnetic flux which completes the circuit through the air is approximately proportional to the quotient between the relative magnetic permeability in the air and in the shaft material, resp
Blomkvist Kent
Ling Hans
Shahcheraghi Roozbeh
Sobel Jarl
ABB AB
Dykema Gossett PLLC
Noori Max
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