Measuring and testing – Dynamometers – Responsive to torque
Reexamination Certificate
2002-08-09
2004-09-21
Noori, Max (Department: 2855)
Measuring and testing
Dynamometers
Responsive to torque
Reexamination Certificate
active
06792817
ABSTRACT:
The present invention relates to an apparatus and method for magnetizing a shaft, and more particularly to an apparatus and method for circumferentially magnetizing magnetoelastic torque transducer shafts for providing a measure of torque applied to the shaft.
BACKGROUND
In the control of systems having rotating drive shafts, the amount of torque applied to the drive shaft is an important parameter for control feedback. Therefore, the sensing and measurement of torque in an accurate, reliable and inexpensive manner has been a primary objective. For this purpose, non-contacting magnetoelastic torque transducers have been provided.
Magnetoelastic torque transducers commonly have two features—1) a shaft which is ferromagnetic and magnetostrictive; and 2) a means for detecting or sensing the measure of torque applied to the shaft. Ferromagnetism ensures the existence of magnetic domains within the shaft and magnetostriction allows the orientation of magnetization within each domain to be altered by the stress associated with applied torque.
Torque transducers based on the magnetoelastic response to torque induced mechanical stresses require an internal remanent magnetization of a controlled profile. One type of such transducer comprises a cylindrical shaft having bands of magnetization wherein the magnetization is circumferentially directed. The bands may be either a physically separate component applied to a shaft, e.g. a ring or collar affixed to the shaft to perform the active element function, or one or more magnetoelastic regions integrated into the axial length of the shaft.
Operation of a transducer for the measurement of torque applied to a shaft requires the shaft to be magnetically polarized in a substantially purely circumferential direction. A common method of magnetizing a transducer shaft includes the use of polarizing magnets. A typical arrangement of shaft and polarizing magnets is illustrated in
FIGS. 1A and 1B
, which show an arrangement of polarizing magnets
90
,
92
and shaft
94
for simultaneously creating two magnetically contiguous polarized regions
96
,
98
. The number of sources of polarizing fields will in general be the same as the number of polarized regions being created. The polarizing magnets
90
,
92
are held close to the shaft surface
94
while the shaft
94
is rotated on its axis in either direction in the magnetic field produced externally to the shaft from the dipole-type magnetic source of the polarizing magnets
90
,
92
. With this technique, it is difficult to control the magnetization profile. In addition, as a practical matter, it is extremely difficult to magnetize a shaft by conventional magnetization methods using polarizing magnets to a depth greater than about 1-2 mm because it is difficult to generate a strong enough magnetic field so far from the magnetic field source, due to the change in reluctance caused by the air gap between the magnet and shaft to be magnetized.
Moreover, the use of external polarizing magnets may result in uneven magnetization where the transducer material deepest within the shaft is insufficiently magnetized, leading to degraded transducer performance, such as reduced short term and long term sensitivity and the creation of “hot-spots”—nonuniformity in the transducer response. This technique is also difficult to optimize, configure and control.
With hollow shafts of large diameter, cooperating internal as well as external polarizing magnets also may be required to obtain a uniform, full-depth polarization of the active region(s), thereby increasing the cost of the apparatus.
An alternative method of magnetizing a shaft includes providing a current in an axial direction near the shaft, directly through the shaft or through a coaxial conductor passed through the central hole of the shaft. In torque transducers of the present invention where the active region is of generally limited axial extent and is to be located at some desirable axial position along the shaft, conventional methods involving the conduction of electrical currents through the entire shaft or through coaxial conductors passing through hollow shafts are unsuitable. Unlike conventional apparatus and methods, the apparatus and method of the present invention magnetizes a length of a shaft of limited axial extent in a substantially purely circumferential direction and throughout the entire depth or thickness of the length of the shaft or width of magnetic zone wanted.
SUMMARY
The scope of the invention is determined solely by the appended claims and their equivalents and is not affected to any degree by the statements within this summary.
The invention provides a method and apparatus for circumferentially magnetizing the active regions of torque transducer shafts for the measurement of torque applied to a shaft, preferably in an automotive steering mechanism. Specifically, the method and apparatus of the present invention address the disadvantages of conventional apparatus and methods of magnetizing torque transducer shafts by providing an apparatus and method that ensures substantially complete magnetization of the active regions of the transducer shaft.
In accordance with one aspect of the present invention, at least three spaced-apart conductor couplers are provided having internal diameters sized so as to circumferentially contact the exterior diameter of the part of the shaft. The couplers substantially surround the outer circumference of the shaft to be magnetized. The contact points of the two outer couplers are coincident with the axial ends of each of circumferential magnetic bands to be provided on the transducer shaft. The center coupler contact point is coincident with the common center of the circumferential magnetic bands to be provided on the shaft. The outer conductor couplers are in at least electrical contact with a two-part inner electrical current conduction tube or conventional conductor and the shaft. The central conductor coupler is in at least electrical contact with an outer electrical current conduction tube or conventional conductor and the shaft.
In one embodiment, at least one high-intensity electric current pulse is applied to the two ends of an outer electrical conduction tube, and directed through the walls of an outer electrical current conduction tube to the central coupler disposed with in the outer electrical current conduction tube. The current radially enters the shaft at a substantially 90° degree angle to the axis of the shaft and is forced axially along the length of the shaft portion comprising the bands of the active region of the transducer in the directions of the outer couplers. The current flow produces a circumferential magnetic field inside the shaft, which leaves the material magnetized after removal of the current. The current exits the shaft through the outer couplers and two inner electrical current conduction tubes. The apparatus of the present invention injects the current in an inherently axisymmetric manner and produces an inherently circumferential remanent magnetization in the transducer shaft. The high-intensity of the current pulse ensures that the transducer material is magnetized throughout its thickness.
In another embodiment, the at least one high-intensity electric current pulse is applied to the ends of the inner conduction tubes and is directed through the inner current conduction tubes to the outer couplers and radially to the shaft. In this embodiment, the current is forced axially along the lengths of the bands of the active region of the transducer shaft in the direction of the center coupler. The current then exits the shaft through the center coupler to the outer current conduction tube.
In yet another embodiment, a decaying alternating current pulse, the first mode of which flows opposite of the apriori applied high-intensity pulse in each band, is then injected to stabilize the magnetization.
REFERENCES:
patent: 3932112 (1976-01-01), Garshelis
patent: 3939448 (1976-02-01), Garshelis
patent: 3959751 (1976-05-01), Garshelis
patent: 3961297 (1976-06-01), Garsheli
Moore William T.
Viola Jeffrey L.
Brinks Hofer Gilson & Lione
Noori Max
Visteon Global Technologies Inc.
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