Measuring and testing – Dynamometers – Responsive to torque
Reexamination Certificate
2000-10-20
2003-06-24
Noori, Max (Department: 2855)
Measuring and testing
Dynamometers
Responsive to torque
Reexamination Certificate
active
06581480
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a sensor arrangement for sensing a torque or flexural force in a shaft and to a method and apparatus for inducing a magnetic field in a shaft for sensing by a sensor arrangement. The invention also relates to shafts incorporating transducer elements and to a method of producing a magnetically calibrated shaft for use in a torque sensor.
BACKGROUND TO THE INVENTION
Magnetic torque sensors are known in the art. They are based on a phenomenon commonly referred to as magnetoelasticity. Known sensors use a magnetized ring secured to a shaft so that torque in the shaft is transmitted to the ring which acts as a transducer element. The sensor also includes one or more magnetic field sensitive elements external to the ring to sense a disturbance of the magnetic field due to the torque resulting from rotation or a torque applied to a non-rotatable shaft. The rotation of any shaft from one end to drive a load at the other end generates torque in the shaft due to differential angular displacement (which may be very small) between the point at which drive is applied and the point at which the load is driven. The same applies to a shaft having one fixed end and another end subject to torque. More particularly to measure this torque the magnetized ring of a magnetic torque sensor has established within it a magnetic field that is essentially confined within the ring. The torque distortion causes magnetic flux to emanate from the ring in proportion to the torque so that the flux can be sensed by one or more external sensing elements.
Various forms of magnetic field sensitive elements have been proposed. One proposal is to sense the small changes in magnetic field due to applied torque, by use of saturable coils (inductors) driven into saturation so that the point of saturation of a coil, with respect to a drive current, depends on the torque-induced external magnetic field associated with the magnetised ring. The points of saturation for the opposite drive polarities in the coil become unbalanced due to the presence of an external field and it is this unbalance that is sensed. Circuitry responsive to the point of saturation is used to develop an output signal representing torque.
Other magnetic field sensing elements have been proposed as is mentioned below.
Thus in general the prior proposed torque sensor comprises two parts: one is the transducer element mounted to the shaft; the other is the sensor element(s) and associated circuitry for driving the elements where necessary and for processing the torque-dependent signals to derive an output representing the measured torque. This other part of the sensor is mountable in a static fashion with the sensor elements closely adjacent to but not contacting the transducer element.
By way of example, a magnetometer using a saturating inductor and the associated circuitry is described in U.S. Pat. No. 5,124,648 (Webb and Brokaw). More detailed information on the mounting of torque sensor coils (inductors) with respect to rotary shafts is to be found in U.S. Pat. No. 5,520,054 (Garshelis). Reference may also be made to a technical paper published by the Society of Automotive Engineers (SAE), “Development of a Non-Contact Torque Transducer for Electric Power Steering Systems”, SAE Technical Paper Series, No. 920707, I. J. Garshelis, K. Whitney and L. May, reprinted from: Sensors and Actuators, 1992 (SP-903), International Congress and Exposition, Detroit, Mich., Feb. 24-28, 1992, pp. 173-182. Further discussion of torque transducers is found in “A Single Transducer for Non-Contact Measurement of the Power, Torque and Speed of a Rotating Shaft”, I. J. Garshelis, C. R. Conto and W. S. Fiegel, SAE Technical Paper Series, No. 950536, reprinted from: Sensors and Actuators (SP-1066), International Congress and Exposition, Detroit, Mich., Feb. 27-Mar. 2, 1995, pp. 57-65, particularly pp. 58-59.
An example of a commercially-offered magnetoelastic torque transducer is that offered under the trade mark TorqStar by the Lebow Products division of Eaton Corporation, 1728 Maplelawn Road, Troy, Mich., U.S.A.
Torque sensing arrangements based on magnetoelasticity are described in related U.S. Pat. Nos. 5,351,555 and 5,465,627 Garshelis, assigned to Magnetoelastic Devices, Inc.) and in U.S. Pat. No. 5,520,059 (Garshelis, assigned to Magnetoelastic Devices, Inc.).
U.S. Pat. Nos. 5,351,555 and 5,465,627 are primarily concerned with torque sensing arrangements described in these patents utilize a magnetised ring or torus secured to the shaft, the torque in which is to be measured, and a non-contacting sensor responsive to changes in the magnetic field external to the ring that arise from stresses in the shaft when the shaft is put under torque. The non-contacting torque sensor may use coil assemblies or other devices sensitive to magnetic fields, e.g. Hall effect devices, magnetoresistive devices and so on.
In these patents there is a single circumferential field generated in the ring which in the no-torque state is entirely contained within the ring. The conditions for the establishment of such a field are discussed in these patents. Emphasis is laid on two practical requirements:
1) the ring is endowed with an effective uniaxial magnetic anisotropy having the circumferential direction as the easy axis
2) the ring is subject to hoop stress, that is a stress tending to expand the ring.
There is also a suggestion in these two patents that the separate ring can be implemented in the form of a ring of suitable material cast on to the shaft by spraying or explosively welded on to the shaft, or a ring of surface modified material formed in the shaft as by ion implantation. None of the suggestions is taught in detail and to the present Applicant's knowledge they have not been put to commercial use.
U.S. Pat. Nos. 5,351,555 and 5,465,627 also give attention to the need to confine the circumferential field within the ring. If the shaft to which the ring is fitted is of low permeability (i.e. paramagnetic), the magnetised transducer ring can be mounted directly to the shaft. However, if the shaft is of high permeability, e.g. ferromagnetic, it is proposed to mount a low permeability spacer between the ring and the shaft. The consideration underlying these proposals appears to be that if the circumferentially magnetised ring were to be mounted directly to a mass of high permeability material, the flux that emanates from the ring under torque would be shunted through the shaft and no useful flux would be available emanating from the ring.
In practice the ring is made with a thin wall thickness, typically 1 mm. for a 20 mm. diameter ring. In order to be pressed onto the shaft and to deal with high torque loads on the shaft, making the ring as thin as possible is advantageous but this has to be balanced against the magnetic field output becoming weaker as the ring becomes thinner. Anothr cost factor to be considered is the need to produce the ring by maching and any special measures taken on the shaft and/or ring to secure the ring against slippage under torque loads add to cost.
U.S. Pat. No. 5,520,059 adds to the disclosure of the two patents discussed above, the concept of the ring transducer having two axially-adjacent zones each having a circumferential magnetisation but the respective magnetisations being of opposite polarity, that is in opposite circumferential directions. More than two zones may be employed of alternating polarity of magnetisation.
There is also a suggestion in U.S. Pat. No. 5,520,059 that a tubular shaft, shown as thin-walled relative to the tube diameter, have a section of the tube itself magnetised to provide a transducer portion of the tube. Specifically the magnetised section has two adjacent zones of opposite polarity magnetisation. However, special measures are taken in this case by inserting a plug into the section of tubular shaft that is to provide the transducer in order to induce hoop stress into that section of the shaft. Furthermore, the magnetised zones should lie axially inward of the ends
Brodey Neill
May Lutz Axel
Owsley John
Fast Technology AG
Fleit Kain
Kain, Jr. Robert C.
Noori Max
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