U.S. patent application number 12/235157 was filed with the patent office on 2009-01-15 for non-destructive evaluation via measurement of magnetic drag force.
This patent application is currently assigned to MagCanica, Inc.. Invention is credited to Ivan J. Garshelis, Stijn P.L. Tollens.
Application Number | 20090013794 12/235157 |
Document ID | / |
Family ID | 38117401 |
Filed Date | 2009-01-15 |
United States Patent
Application |
20090013794 |
Kind Code |
A1 |
Garshelis; Ivan J. ; et
al. |
January 15, 2009 |
NON-DESTRUCTIVE EVALUATION VIA MEASUREMENT OF MAGNETIC DRAG
FORCE
Abstract
Sensors for outputting signals indicative of a magnetic drag
force between a ferromagnetic sample that is or has been in motion
relative to one or more measurement magnets are described. Such
sensors include at least one measurement magnet and a sensing
element operably associated therewith, such that upon or after
exposure of the measurement magnet(s) to a ferromagnetic sample in
motion relative thereto, the sensor can sense the drag force, if
any, or changes therein, experienced by the measurement magnet.
Devices for detecting and/or measuring magnetic drag force that
employ one or more of these sensors are also described, as are
various applications for such devices.
Inventors: |
Garshelis; Ivan J.; (Dalton,
MA) ; Tollens; Stijn P.L.; (San Diego, CA) |
Correspondence
Address: |
Biotechnology Law Group;c/o Portfolioip
P.O. Box 52050
Minneapolis
MN
55402
US
|
Assignee: |
MagCanica, Inc.
San Diego
CA
|
Family ID: |
38117401 |
Appl. No.: |
12/235157 |
Filed: |
September 22, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11588983 |
Oct 27, 2006 |
7437942 |
|
|
12235157 |
|
|
|
|
60731882 |
Oct 30, 2005 |
|
|
|
Current U.S.
Class: |
73/779 |
Current CPC
Class: |
G01R 33/038
20130101 |
Class at
Publication: |
73/779 |
International
Class: |
G01B 7/16 20060101
G01B007/16 |
Claims
1. A magnetic drag force sensor comprising: a. a measurement
magnet; and b. a sensing element operably associated with the
measurement magnet, wherein the sensing element outputs a signal
indicative of a magnetic drag force experienced by the measurement
magnet upon exposure to a ferromagnetic sample in relative motion
to the measurement magnet.
2. A magnetic drag force sensor according to claim 1 wherein the
measurement magnet is selected from the group consisting of an
electromagnet and a permanent magnet.
3. A magnetic drag force sensor according to claim 1 wherein the
sensing element senses physical displacement of the measurement
magnet in response to the magnetic drag force.
4. A magnetic drag force sensor according to claim 1 wherein the
sensing element senses pressure applied to the measurement magnet
in response to the magnetic drag force.
5. A magnetic drag force measurement device, comprising: a. at
least one magnetic drag force sensor according to claim 1
positioned such that the measurement magnet is proximately spaced
from a ferromagnetic sample capable of moving relative to the
measurement magnet; and b. a processor configured to process
signals from the sensing element to determine a parameter of the
magnetic drag force experienced by the measurement magnet.
6. A magnetic drag force measurement device according to claim 5
further comprising a drive for moving a ferromagnetic sample past
the measurement magnet.
7. A magnetic drag force measurement device according to claim 5
further comprising an upstream magnet to achieve sufficient
magnetization of a ferromagnetic sample upon association of the
ferromagnetic sample with the device.
8. A magnetic drag force measurement device according to claim 5
configured to measure magnetic drag force in a ferromagnetic sample
selected from the group consisting of a ferromagnetic strip,
ferromagnetic bar, a ferromagnetic plate, a ferromagnetic wire, and
a ferromagnetic cable.
9. A magnetic drag force measurement device according to claim 5
that comprises first and second magnetic drag force sensors, each
according to claim 1.
10. A magnetic drag force measurement device according to claim 9
wherein the first magnetic drag force sensor is proximately spaced
from one surface of the ferromagnetic sample and the second
magnetic drag force sensor is proximately spaced from a second
surface of the ferromagnetic sample.
11. A magnetic drag force measurement device according to claim 10
wherein the first magnetic drag force sensor is disposed
substantially opposite the second magnetic drag force sensor.
12. A magnetic drag force measurement device according to claim 5
that comprises a plurality of a magnetic drag force sensors each
according to claim 1 spaced concentrically about an opening through
which a ferromagnetic sample of substantially uniform geometric
cross-section can be passed.
13. A magnetic drag force measurement device according to claim 12
wherein the geometric cross-section of the ferromagnetic sample is
selected from the group consisting of a circle, an ovoid shape, and
a polygon.
14. A magnetic drag force measurement device according to claim 5
that comprises a plurality of a magnetic drag force sensors each
according to claim 1.
15. A magnetic drag force measurement device according to claim 14
wherein the magnetic drag force sensors are disposed in an array
selected from the group consisting of a staggered sensor array, a
segmented sensor array, and a staggered, segmented sensor
array.
16. A magnetic drag force measurement device according to claim 15
wherein a sample surface area that can be swept by the measurement
magnets of the array is less than the sum of the surface areas that
can be swept by each of the measurement magnets of the magnetic
drag force sensors in the array.
17. A method for measuring magnetic drag force, comprising: a.
moving a ferromagnetic sample relative to a magnetic drag force
measurement device according to claim 5; and b. measuring the
magnetic drag force experienced by the measurement magnet as a
result of relative motion between the ferromagnetic sample and the
measurement magnet.
18. A method according to claim 17 wherein the measurement of the
magnetic drag force allows evaluation of the ferromagnetic sample's
identity, quality, or position, or position of an inhomogeneity
within the ferromagnetic sample.
19. A method according to claim 17 wherein the ferromagnetic sample
is magnetized prior to passage past the measurement magnet.
20. A magnetic drag force measurement device, comprising: a. a
sample stage; and b. a sensing element operably associated with the
sample stage, wherein the sensing element outputs a signal
indicative of a magnetic drag force experienced by a ferromagnetic
sample upon exposure to a pre-determined magnetic field generated
by a magnet spaced from and in motion relative to the ferromagnetic
sample.
21. A magnetic drag force measurement device according to claim 20
further comprising a processor configured to process signals from
the sensing element to determine a parameter of the magnetic drag
force experienced by the ferromagnetic sample.
22. A magnetic drag force measurement device according to claim 20
further comprising a drive for moving the magnet past the
ferromagnetic sample.
23. A method for measuring magnetic drag force, comprising: a.
moving a magnet that provides a magnetic field of pre-determined
strength relative to a magnetic drag force measurement device
according to claim 20 having a ferromagnetic sample positioned on
the sample stage; and b. measuring the magnetic drag force
experienced by the ferromagnetic sample as a result of relative
movement between the ferromagnetic sample and the magnet.
Description
RELATED APPLICATION
[0001] This application claims priority to, the benefit of, and
incorporates by reference for all purposes the following commonly
owned patent-related documents, each in its entirety: U.S.
provisional patent application Ser. No. 60/731,882, filed 31 Oct.
2005; and U.S. patent application Ser. No. 11/588,983, filed 27
Oct. 2006, of which this application is a continuation.
TECHNICAL FIELD
[0002] This invention concerns sensors and devices suited to
non-destructively evaluate ferromagnetic samples. The sensors used
in these devices sense the magnetic drag force between a sample and
one or more measurement or reference magnets.
BACKGROUND OF THE INVENTION
1. Introduction
[0003] The following description includes information that may be
useful in understanding the present invention. It is not an
admission that any such information is prior art, or relevant, to
the presently claimed inventions, or that any publication
specifically or implicitly referenced is prior art.
2. Background
[0004] Ferromagnetic materials are a predominant or essential
constituent of most of the structures and machines built and
utilized throughout the modern industrial world. By far, most of
the tonnage of the steels and other iron alloys used in these
applications is ferromagnetic. Many alloys of nickel and/or cobalt,
as well as alloys of such rare earth elements as gadolinium,
terbium, dysprosium, and samarium, are also ferromagnetic.
Ferromagnetic structural steels provide unmatched strength-to-cost
ratios, and the magnetic flux conduction capabilities of electrical
steels and related ferromagnetic materials are essential to the
efficient operation of the electrical machines that generate, use,
or transform electricity. Whether selected for particular
mechanical or magnetic characteristics, it is important that one or
more relevant properties of the material be known to meet or exceed
the specified or expected value. To this end, a wide range of
evaluation techniques has been developed. Of special interest are
methods where the evaluation can be performed on the actual
product, rather than on a surrogate sample, ostensibly, but not
assuredly, having sufficiently identical values of the measured
properties to those possessed by the actual product. The practice
of many simple testing techniques results in the sample being
destroyed, marred, or otherwise made unusable, or less usable, for
its original purpose. Thus, much effort has been expended in the
development of non-destructive evaluation (NDE) techniques, wherein
the actual properties of interest, or other properties that
unwaveringly correlate with the properties of interest, can be
measured on the actual product.
[0005] While the quality of the finished product, in regards to the
measured quantity, can be ascertained by one or more already
available techniques, it would be even more useful if the property
of interest could be ascertained in the material from which the
product is subsequently fabricated while at some stage in its
processing where actions to improve the property of interest can
still be taken. For example, detection of scratches, dents, or
other unacceptable surface markings on sheet steels destined to be
used for refrigerator door panels or the like, caused, for example,
by rubbing against an out of place guide during a rolling
operation, would be better made at the rolling mill than during or
after fabrication of the door panel. Undesirable variations in
properties depending on controlled heat treatments, or quenching
conditions of materials produced in continuous form, such as sheet,
strip, wire, cable, ribbon, and the like, are clearly best found
early in the process where corrective measures may be instituted
before the variations reach unacceptable limits and to minimize
wastage.
[0006] Several currently practiced methods of non-destructive
evaluation are based on magnetic phenomena, typically associated
with the detection of leakage flux arising from locally
inhomogeneous magnetization as might occur across cracks or similar
flaws at or near the surface of the part being tested. One of the
most generally practiced of these methods requires coating of the
part to be inspected with fluorescent dyes mixed with fine magnetic
particles. Successful use of the method requires skilled personnel
trained to both use the equipment and interpret the results. In a
related method, leakage flux is detected by Hall effect or
magnetoresistive field sensors in conjunction with permanent or
electromagnets. These types of field sensors respond to magnetic
fields within highly localized regions of space, hence a plurality
of such sensors is required to inspect the full width of a strip or
other product having dimensions significantly larger than the
sensing range of the test probe. Alternatively, large regions must
be inspected by repeated scanning in raster like fashion, a process
adding significantly to inspection time and cost.
[0007] Electrical steels are often characterized by their magnetic
hysteresis loss, since this property is a major determinant of the
energy and size efficiency of the transformer, motor, or other
device in which the steel is used. Usual methods for measuring
these losses involve clamping a sample strip to the pole surfaces
of a magnetizing yoke, electrically excited by currents from a
computer controlled power supply. The process is both time
consuming and requires relatively sophisticated apparatus.
[0008] Clearly there remains a need in the art for alternative NDE
technologies, and it is the object of this invention to address
this need.
3. Definitions
[0009] Before describing the instant invention in detail, several
terms used in the context of the present invention will be defined.
In addition to these terms, others are defined elsewhere in the
specification, as necessary. Unless otherwise expressly defined
herein, terms of art used in this specification will have their
art-recognized meanings.
[0010] An "array" refers to an organized grouping of two or more
similar or identical components. For example, an "array of
measurement magnets" or "measure magnet array" refers to an array
that includes a plurality of measurement magnets. These magnets may
be arrays in any desired configuration, including segmented arrays
(such that the longitudinal axes of the magnets are on the same
line), staggered arrays (i.e., the longitudinal axes of some of the
magnets are on lines that differ from those formed by the axes of
other of the magnets), and segmented, staggered arrays.
[0011] An "electromagnet" is a piece of wire intended to generate a
magnetic field by the passage of electric current through it.
Typically, the wire is coiled or wound, and an electromagnet is
preferably constructed in such a way as to maximize the strength of
the magnetic field it produces upon passage of an electric current
through the wire. On the other hand, a "permanent" magnet refers to
a magnet made of a material (e.g., an NdFeB alloy) that maintains a
magnetic field with no external help.
[0012] A "ferromagnetic" material is one that readily magnetizes in
the presence of an external magnetic field.
[0013] The terms "measure", "measuring", "measurement" and the like
refer not only to quantitative measurement of a particular
variable, for example, magnetic drag force, but also to qualitative
and semi-quantitative measurements. Accordingly, "measurement" also
includes detection, meaning that merely detecting a change, without
quantification, constitutes measurement.
[0014] A "patentable" process, machine, or article of manufacture
according to the invention means that the subject matter satisfies
all statutory requirements for patentability at the time the
analysis is performed. For example, with regard to novelty,
non-obviousness, or the like, if later investigation reveals that
one or more claims encompass one or more embodiments that would
negate novelty, non-obviousness, etc., the claim(s), being limited
by definition to "patentable" embodiments, specifically exclude the
unpatentable embodiment(s). Also, the claims appended hereto are to
be interpreted both to provide the broadest reasonable scope, as
well as to preserve their validity. Furthermore, if one or more of
the statutory requirements for patentability are amended or if the
standards change for assessing whether a particular statutory
requirement for patentability is satisfied from the time this
application is filed or issues as a patent to a time the validity
of one or more of the appended claims is questioned, the claims are
to be interpreted in a way that (1) preserves their validity and
(2) provides the broadest reasonable interpretation under the
circumstances.
[0015] The term "operably associated" refers to an operable
association between two or more components. For example, a
measurement magnet is "operably associated" with a sensing element
when it is possible for the sensing element to sense the
application of a force applied to the measurement magnet.
[0016] A "plurality" means more than one.
SUMMARY OF THE INVENTION
[0017] One object of this invention is to provide sensors and
devices that rely on the drag force between at least one external
magnet, usually a reference magnet or a measurement magnet, and a
ferromagnetic sample. Another object is to provide methods of
making and using such sensors and devices, for example, in various
applications, including the determination of magnetic hysteresis
loss, to detect and/or determine the presence, size, and/or
location of local inhomogeneities in the structure, composition,
and/or dimensions of ferromagnetic parts or members, and to detect
purposefully instilled patterns of inhomogeneities as markers or
signatures in order to identify a specific item, its orientation,
or location.
[0018] Thus, one aspect of the invention concerns sensors capable
of responding to a magnetic drag force. In general, such a sensor
employs at least one measurement magnet and an operably associated
sensing element (e.g., a load cell). Any suitable electromagnet or
permanent magnet can be used as a measurement magnet, with magnets
that produce consistent, uniform magnetic fields of known strength
being preferred. Similarly, any suitable sensing element can be
used. In general, any such sensing element will detect, or "sense",
a force applied to the measurement magnet and output a signal
indicative of that force. In particular, the sensing element can
detect application of a magnetic drag force to the measurement
magnet (or an array of measurement magnets) upon exposure to a
ferromagnetic sample in relative motion to the measurement magnet
(or measurement magnet array) and output a signal indicative of
that force. Preferred sensing elements include those that sense
physical displacement of a measurement magnet in response to a
magnetic drag force, those that sense the force experienced by the
measurement magnet in response to a magnetic drag force. Typically,
the sensing element will employ one or more load cells, capacitive
force transducers, force-sensing resistors, magnetoelastic force
sensors, or torsional balances to sense the force applied to the
measurement magnet. Preferred load cells include those that
comprise a strain gauge, piezoelectric crystals, or a hydraulic or
pneumatic load cell.
[0019] A closely related aspect of the invention concerns devices
that employ one or more sensors according to the invention, i.e.,
magnetic drag force measurement devices. In general, such devices
include at least one magnetic drag force sensor positioned such
that, in operation, the measurement magnet will be proximately
spaced from a ferromagnetic sample capable of moving relative to
the measurement magnet. Thus, the sensor, the sample, or both may
be moved in relation to the other during operation of the device.
Signals output by the sensor are preferably conditioned by any
suitable electronic circuitry and then recorded or analyzed. In
preferred embodiments, the signals are digitized into a form
suitable for use by a processor configured to process them in order
to determine a parameter of the magnetic drag force experienced by
the measurement magnet. The results of the processing may be stored
in a storage device and/or output to an output device (for example,
a plotter, a computer monitor).
[0020] In order to move the magnetic drag force sensor and/or the
ferromagnetic sample, a device according to the invention
preferably includes a drive system adapted for the particular
application. Moreover, for some applications (e.g., measurement of
hysteresis loss), it is desirable to sufficiently magnetize the
sample, preferably by placing it in a state of known remanent
magnetization, prior to moving the sample relative to the magnetic
drag force sensor. This may be accomplished through the use of one
or more magnets positioned upstream of the measurement magnet(s) of
the magnetic drag force sensor. Alternatively, the sample may first
be exposed to the measurement magnet in a non-sensing mode in order
to establish the desired state of remanent magnetization.
Thereafter, the sample can be moved relative to the magnetic drag
force sensor so that the drag force can be measured. It will also
be appreciated that a magnetic drag force may also be detected
after stopping the relative motion of the magnetic drag force
sensor and sample.
[0021] In addition, or alternatively, a device of the invention may
also include one or more magnets, including one or more measurement
magnets associated with one or more magnetic drag force sensors,
positioned such that upon exposure to a sample, at least one
measurement magnet is proximately spaced from a first surface of
the sample and a second measurement magnet is proximately spaced
from a second, different surface of the sample. For example, in a
device wherein the sample horizontally moves through the device,
one measurement magnet is positioned above and is proximately
spaced from the upper surface of the sample, whereas the other
measurement magnet is positioned below and is proximately spaced
from the lower surface of the sample.
[0022] A device according to the invention can be adapted for
detecting and/or measuring (qualitatively, semi-quantitatively, or
quantitatively) magnetic drag forces in conjunction with
ferromagnetic samples of any shape, size, or composition.
Representative sample shapes include plates, bars, strips, wires,
and cables. In cross-section, such samples may, for example, have a
geometric shape selected from the group consisting of a circle, an
ovoid shape, and a polygon (e.g., a triangle, rectangle, square,
etc.). In many embodiments, the device will be configured to
analyze ferromagnetic samples of substantially uniform geometric
cross-section, while in other embodiments, the devices will be
configured to adapt to different sample sizes, shapes, etc. Samples
may be separate pieces, or they may be one continuous piece.
[0023] Another related aspect of the invention concerns methods of
using the sensors and devices of the invention. In general terms,
such methods involve moving a ferromagnetic sample relative to a
magnetic drag force measurement device and measuring the magnetic
drag force experienced by the measurement magnet of the magnetic
drag force sensor as a result of the relative movement between the
ferromagnetic sample and the measurement magnet. Preferred
applications for such methods include measurement of hysteresis
losses, sample hardness and/or thickness, and material composition
and/or microstructure, and detection of inhomogeneity or other
defects (e.g., internal flaws, surface scratches, etc.) in the
sample. Other applications include determining a material's
orientation, position, and/or identity or source (for example, by
detecting a coded arrangement of non-visible features).
[0024] Each of these general applications (e.g., determining
magnetic hysteresis loss, detection of local inhomogeneities,
detection of purposefully instilled patterns of such
inhomogeneities as markers or signatures, detection of the location
of an inhomogeneity within a sample, determining which portion of a
sample is being analyzed, etc.) has a variety of specific
applications. In many of these, drag force measurement often offers
benefits over other approaches intended to accomplish the same end.
For example, measurement of hysteresis loss in strip samples of
electrical steels (standard size strips, commonly called "Epstein
strips" after the name of the apparatus in which the hystersis
losses of such strips are commonly measured) by drag force
measurement requires less time, utilizes smaller and more
economical apparatus, and can be used to simultaneously detect
local inhomogeneites. It also allows real time measurement of
hysteresis losses during the manufacture of such materials. In
actuality, there are at least two sources of drag force. One of
these is based on the asymmetrical magnetization that arises within
a (homogeneous) sample that is moving, or has been moved, through
the intense field close to a properly oriented permanent magnet.
This asymmetry arises because of magnetic hysteresis in the sample
material. Another source of drag force is the appearance of local
magnetic "poles" within the sample at the extrema of local
inhomogeneities. The presence of these poles (resulting from local
inhomogeneity) can be sensed by the forces they exert on the
"poles" of the measurement magnet. In most cases, a ferromagnetic
material will have both hysteresis and be imperfectly homogenous
and therefore show both a finite steady state drag force and a
superimposed drag force that varies with position of the material
relative to the measurement magnet.
[0025] Yet another aspect of the invention relates to magnetic drag
force measurement devices for measuring magnetic drag force not
using a sensor having sensing element operably associated with a
measurement magnet, but with a sample stage. In this way, drag
force on the sample is measured. Here, the sensing element outputs
signals indicative of a magnetic drag force experienced by the
ferromagnetic sample upon exposure to a pre-determined magnetic
field generated by a reference magnet (or reference magnet array)
spaced from and in motion relative to the ferromagnetic sample. In
preferred embodiments, additional electronics are associated with
the sensor so that the signals may be processed, analyzed, and used
to generate a meaningful output. In such devices, a drive may be
used to move the proximately spaced reference magnet across some or
all of one or more surfaces of the sample. In operation, a magnetic
drag force is typically measured by moving a proximately spaced
reference magnet (or reference magnet array) that provides a
magnetic field of pre-determined strength in relation to a magnetic
drag force measurement device having a ferromagnetic sample
positioned on the sample stage, wherein the sample stage is
operably associated with one or more force sensing elements.
[0026] Other features and advantages of the invention will be
apparent from the following drawings, detailed description, and
appended claims.
BRIEF DESCRIPTION OF THE FIGURES
[0027] This application contains at least one figure executed in
color. Copies of this application with color drawing(s) will be
provided upon request and payment of the necessary fee.
[0028] FIG. 1 illustrates the physical arrangement of a polarizing
magnet used to create a band having circumferential remanent
magnetization in a ferromagnetic shaft that is rotated about its
longitudinal axis.
[0029] FIG. 2 shows the variation of H, H.sub.cir, and H.sub.rad,
as defined in FIG. 1, on the periphery of a cylindrical
ferromagnetic shaft as a function of position angle .alpha.. All
values are normalized against the maximum value of H, and they all
continue to diminish for values of .alpha. beyond the plot
edges.
[0030] FIG. 3 shows the variation in intensity, orientation, and
polarity of the magnetic field at the shaft surface within the
narrow range of position angles indicated. The length and angular
orientation of any line from the origin to the curve represents the
relative magnitude of H and its direction relative to the actual
radial and circumferential directions. The circle indicates the
relative (isotropic) coercive force, H.sub.c, of the shaft
material.
[0031] FIG. 4 shows two schematic representations of the
distribution of magnetization intensity (M) and direction within
the active zone of magnetization (represented as the shaded area in
panels (a) and (b)) where H>H.sub.c. Panel (a) shows the
symmetrical distribution after the magnet has been brought to its
final position by radial movement towards the shaft. Panel (b)
shows the asymmetry in the distribution after the shaft starts to
rotate, manifesting the need to develop sufficient magnetostatic
energy before M will rotate towards H. M, in the radial direction,
is reduced by the relatively large demagnetizing factor in that
direction (indicated by the relatively short arrows).
[0032] FIG. 5 shows a schematic diagram of the experimental
apparatus described in Example 2. The magnet is attached to the end
of a pendulum supported by instrument bearings in the frame. The
bias torque supplied by the offset weight allows measurement of
drag forces in either direction.
[0033] FIG. 6 shows variation of magnetic drag force with
revolution count for various gaps between a rotating shaft and a
magnetization magnet.
[0034] FIG. 7 shows variation of magnetic drag force with
revolutions on a rotating shaft for various magnetization magnet
widths, with the gap between the shaft and magnetization magnet
being 0.5 mm.
[0035] FIG. 8 shows the progression of changes in the
circumferential magnetization within the "C" section outside the AZ
brought about by continuous shaft rotation.
[0036] FIG. 9 schematically represents a device wherein magnetic
drag force is measured using one or more force sensing elements
operably associated with a ferromagnetic sample resting on a sample
stage (not shown).
[0037] FIG. 10, panel A, shows preferred areas in which the sensors
and devices of the invention can be used for evaluation. Panel B
shows some of the various flaws and defects that can be detected in
certain materials (e.g., sheets of ferromagnetic material) using
the sensors and devices described herein.
[0038] FIG. 11 has four panels, a-d. Each panel shows a strip (36)
capable of being driven in opposite directions (indicated by arrows
39) by a drive mechanism (not shown). Panel (a) illustrates a strip
(36) positioned for analysis by a device of the invention that
employs one measurement magnet (35). Panel (b) shows an embodiment
having two magnets, a measurement magnet (35) and an initialization
magnet (37). Panel (c) shows an illustration of a device according
to the invention that has two measurement magnets (35 and 38)
disposed on opposite sides of the strip (36). In panel (d), a
device having two initialization magnets (37 and 39) and two
measurement magnets (35 and 38) is shown. As will be appreciated,
an initialization magnet may also be referred to as an "upstream"
magnet, in that it is positioned upstream of the measurement magnet
when the sample under test ("SUT", here, a strip (36)) is moved in
one direction, such as in the illustrations in this Figure, from
left to right.
[0039] FIG. 12 contains two photographs ((a) and (b) of a preferred
magnetic drag force measurement device according to the invention.
The photograph in FIG. 12(a) shows a side view of the device,
whereas the photograph in FIG. 12(b) provides a detailed view of
the device's drive mechanism. In this embodiment, the drag force
sensor employs two measurement magnets, one positioned above the
sample, the other below the sample. The magnets are held in
position by a magnet holder.
[0040] FIG. 13 has two panels, (a) and (b). Panel (a) contains two
illustrations. One shows a front view of the magnet holder (55),
which in this embodiment, holds two measurement magnets (50 and 51)
each positioned about a slot through which strip (53) can be moved.
In this embodiment, each of the measurement magnets extends beyond
the slot through which suitably sized strips may be inserted.
Advantageously, such an arrangement provides for more uniform
magnetic fields across the width of a strip as it is driven through
the device. Panel (b) shows a side view of an embodiment of a
device according to the invention having one measurement
magnet.
[0041] FIG. 14 shows an actual plot of load cell output versus time
(position) for a strip evaluated as described in Example 3.
[0042] FIG. 15 shows a schematic arrangement of a strip (130) and a
measurement magnet (132) in a drag force measurement device. In
this Figure, "m" designates the magnetic moment of the measurement
magnet (132) positioned a distance (135) from the strip (130). 134
represents drag forces that may be experienced by the measurement
magnet (132). 133 represents the motion of the strip (130) in the
device.
[0043] FIGS. 16 (a), (b), (c), (d), (e), and (f) plot results from
theoretical modeling described in Example 3. All parameters, except
normalized distance x, are normalized against their maximum
values.
[0044] FIG. 17 plots the hysteresis loops for the materials
indicated (Example 3 describes the experiments that gave rise to
this data), with the peak applied field being .+-.10 kA/m.
[0045] FIG. 18 plots the hysteresis loops for the materials
indicated (Example 3 describes the experiments that gave rise to
this data), with the peak applied field being .+-.10 kA/m.
[0046] FIG. 19 plots the effect of gap between a measurement magnet
and a sample strip on measured drag forces, as measured using a
device as described in Example 3.
[0047] FIG. 20 shows two cartoon views of a portable magnetic drag
force-sensing device according to the invention.
[0048] FIG. 21 shows three views of a magnetic drag force-sensing
device to detect flaws in cables.
[0049] FIG. 22 shows a strip of low carbon steel in which various
"defects" were purposefully instilled at various locations.
[0050] FIG. 23 shows a plot of drag force against time generated by
moving the strip of FIG. 22 in an apparatus according to the
invention, first by moving the strip in one direction, and then,
after a small pause, in the reverse direction.
[0051] FIG. 24 shows a drag force plot for a "stack" of three
strips all having the same nominal length and width dimensions and
cut from the same sheet of material, in which the "middle" strip
had purposefully instilled "defects".
[0052] As those in the art will appreciate, the following detailed
description describes certain preferred embodiments of the
invention in detail, and is thus only representative and does not
depict the actual scope of the invention. Before describing the
present invention in detail, it is understood that the invention is
not limited to the particular aspects and embodiments described, as
these may vary. It is also to be understood that the terminology
used herein is for the purpose of describing particular embodiments
only, and is not intended to limit the scope of the invention
defined by the appended claims.
DETAILED DESCRIPTION
1. Introduction
[0053] This invention is based on the surprising discovery that the
drag torque experienced during the process of magnetizing one or
more polarized circumferential bands on a ferromagnetic shaft
diminishes from its starting value during some number of shaft
revolutions in close proximity to a (circumferentially) thin
magnet. Magnetization of a polarized band on the shaft is
determined to be complete when the drag torque stabilizes.
[0054] Without wishing to be bound to any particular theory, the
following description represents what is believed to be the basis
for this discovery. FIG. 1 shows the physical arrangement used to
magnetically polarize a circumferential band of width w and depth p
on a ferromagnetic shaft (2) together with pertinent geometric
factors. "R" represents the outside radius of the shaft, "G" the
gap between magnet (1) and shaft, "r" the distance between any
point "P" on the periphery of the shaft and the magnet center, and
".alpha.", the position angle for any point P measured in the CW
direction starting from the magnet side. Values for "r", ".phi.",
and ".theta." can be calculated according to the following
formulas:
r=[(R+G).sup.2+R.sup.2-2(R+G)R cos.alpha.].sup.1/2 (1)
.phi.=tan.sup.-1(0.5 tan .theta.) (2)
.theta.=.pi./2-sin.sup.-1(R sin.alpha./r) (3)
Although a real polarizing magnet comprises a 3-dimensional
distribution of magnetic dipoles, it can be represented as a single
equivalent dipole of magnetic moment m. Thus, the intensity of the
field H at any point P on the periphery of the shaft in the plane
of m can be determined using the formula:
H = m r 3 ( 3 cos 2 .theta. + 1 ) 1 / 2 , ( 4 ) ##EQU00001##
[0055] Since H is directed at angle .phi. to r, where
.phi.tan.sup.-10.5 tan .theta., it will have both circumferential
and radial components, as follows:
H.sub.cir=H cos(.alpha.-.theta.-.phi.)
H.sub.rad=H sin(.alpha.-.theta.-.phi.) (5)
[0056] FIG. 2 shows the variations of H, H.sub.cir, and H.sub.rad
with .alpha. for the arbitrarily chosen gap, G=0.1 R. The manner in
which the field intensity and its direction vary are more clearly
understood from a study of the plot shown in FIG. 3. Analogous
curves for other values of G/R have similar shapes, although the
range of .alpha. where the visible portions of the curve reside
diminishes with decreasing G/R. In those regions of the shaft close
to a strong polarizing magnet, the absolute intensity of H will
exceed the coercive force, H.sub.c, of the shaft material. The
dashed circle in FIG. 3 indicates such a condition.
[0057] In those regions outside the circle shown in FIG. 3, but
within the curve, the local intensity and direction of
magnetization are determined (more so for the circumferential
component, less so for the radial component due to its large
demagnetization factor) by these field characteristics, as
indicated schematically in FIG. 4(a). If the shaft slowly rotates,
e.g., to the position shown in FIG. 4(b), the local magnetization
at points within the shaft near the surface will change
continuously during passage through this active zone (AZ). H may
well (as in FIG. 3) remain above H.sub.c throughout a direction
change of at least 180.degree., and even 240.degree. or more, while
passing through the AZ. Despite never reaching 360.degree., nor is
H constant, which are the characterizing conditions for "rotational
hysteresis", the same types of irreversible magnetization altering
processes occur during passage through the AZ. While H and M are
changing in direction, and may not always be colinear, a hysteresis
loss, E.sub.h=.intg.H.cndot.dM, is induced within affected portions
of the shaft.
[0058] During a complete revolution of the shaft, all portions of
the shaft within the band of axial width w (determined by the
corresponding magnet dimension) and to some penetration depth, p,
inward from the surface of the shaft facing the magnet (determined
by H/H.sub.c) pass through the AZ. Since H/H.sub.c diminishes with
increasing G, and as the effective gap grows with p, E.sub.h will
vary with p according to the formula E.sub.h=f(p). As H/H.sub.c
diminishes with increasing p, so too will E.sub.h, especially as
H/H.sub.c.fwdarw.1, and clearly more precipitously after H drops
below H.sub.c. During each shaft revolution there is then a
hysteretic energy, W.sub.h, that must be dissipated within the
shaft. The magnitude of this energy can be calculated using the
formula:
W h = w 2 .pi. .intg. 0 t E h ( R - p ) p . ( 6 ) ##EQU00002##
This hysteretic energy is supplied by the mechanical work expended
during each revolution: W.sub.m=2.pi.T, where T is the torque
required to rotate the shaft. T is produced by the tangential
component of force, F=T/R, originating from the magnetostatic
interaction between the magnet and the "poles" associated with the
asymmetrical .gradient..cndot.M within the AZ. Equating W.sub.h and
W.sub.m and replacing T by FR yields the equation:
2 .pi. FR = w 2 .pi. .intg. 0 t E h ( R - p ) p p . ( 7 )
##EQU00003##
[0059] An upper limit for F, F.sub.max, may be found by considering
that the magnet is sufficiently strong for H>>H.sub.c to a
depth p.gtoreq.t. Under these conditions E.sub.h will reach a
saturation value, (E.sub.h).sub.sat, throughout the shaft cross
section. If t<<R, equation (7) has the simple solution:
F.sub.max=wt(E.sub.h).sub.sat. (8)
[0060] The maximum drag force is seen to depend on just two
dimensions in addition to (E.sub.h).sub.sat: w, determined by the
magnet, and t, determined by the shaft construction. A fair
estimate for (E.sub.h).sub.sat is simply the rectangular area in
the B-H loop bounded by H.sub.c and B.sub.r (the retentivity of the
shaft material), viz.,
(E.sub.h).sub.sat=2H.sub.c2B.sub.4/4.pi.=H.sub.cB.sub.r/.pi..
[0061] It is instructive to estimate the order of magnitude of the
drag force expected under practical conditions. For example, in a
quenched and tempered alloy steel shaft containing about 95% iron
by weight, H.sub.c.apprxeq.40, Oe and B.sub.r.apprxeq.10 kG, for
which (E.sub.h).sub.sat=127,000 ergs/cm.sup.3, or 12,700 J/m.sup.3,
when w=1 cm, t=0.1 cm, and F.sub.max=0.127 N. The drag torque
associated with hysteretic losses has long been exploited in a
variety of commercial "braking" devices.
[0062] FIGS. 6 and 7 illustrate how drag force changes with
cumulative shaft rotation angle. In general, initial growth in the
drag force results from the development of asymmetry in the
distribution of M within the AZ, as shown in FIG. 4(b). As
demonstrated by FIG. 3, the direction of H changes smoothly within
the AZ, although M is delayed by the energetic impediments to
domain wall motion and anisotropy. This asymmetry grows during
rotation up to the angular width of the AZ, and perhaps somewhat
beyond as the penetration depth of M increases. Thus, the drag
force grows during this rotational period.
[0063] The demagnetizing field associated with the circumferential
magnetization (M.sub.c) that initially appears outside the AZ (see
FIG. 8(a)) reduces the intensity of M.sub.c. This field diminishes
as the region expands circumferentially with further shaft rotation
(see FIG. 8(b)). Eventually this circumferential, but non-uniform,
magnetization extends around to the beginning of the AZ (see FIG.
8(c)). Ever less limited by its own demagnetization, M.sub.c grows
in both intensity and uniformity during subsequent shaft rotations.
The orientation distribution of the field within the AZ, previously
determined solely by the magnet, is altered by a new and growing
field, primarily in the -H.sub.cir direction, from this magnetized
"C" section. The field in the AZ becomes more uniform and more
uniformly directed. With the range of field excursion during
passage through the AZ diminished, E.sub.h decreases. W.sub.h, and
hence W.sub.m, T, and F, are all decreased. An equilibrium point is
reached when, after a sufficient number of revolutions, the
magnetization in the region just leaving the AZ becomes the same as
that in the portion of the shaft just entering the AZ. In this
manner stabilization of the drag force indicates the completion of
the magnetization process.
[0064] Example 2, below, describes a representative device useful
for measuring the drag force on a variety of ferromagnetic shafts
being magnetized with variety of different magnets spaced at
different distances from the shaft.
2. Magnetic Drag Force Sensing
[0065] As described above, successful, complete magnetization of
one or more polarized circumferential bands on a ferromagnetic
shaft is indicated when the magnetic drag torque stabilizes. The
inventors have further discovered that detecting changes in
magnetic drag torque can be used in many applications. Accordingly,
a patentable new class of sensors have been invented. In general,
the sensors of the invention employ at least one measurement magnet
and an operably associated sensing element used to detect force
applied to the measurement magnet. Magnets and sensing element are
described in the following sections.
[0066] A. Magnets
[0067] Any suitable electromagnet or permanent magnet can be used
as a measurement magnet, although magnets that produce consistent,
uniform magnetic fields of known strength are preferred.
Particularly preferred permanent magnets are NdFeB magnets.
Individual measurement magnets may be used, as may measurement
magnet arrays. Suitable magnet arrays include segmented arrays,
staggered arrays, and segmented, staggered arrays. As will be
appreciated, the measurement magnets in a given array may be
configured so that the surface area that they collectively sweep is
less than the sum of the surface areas that is swept by each of
them. Alternatively, the surface area collectively swept by the
measurement magnets may be equivalent to or greater than the
surface areas that are swept by each of them when considered
individually.
[0068] The size, shape, and field strength of any measurement or
reference magnet, or array of measurement or reference magnets,
will vary depending upon the particular application. The design and
selection of suitable magnets for a given application is well
within the ordinary skill in the art.
[0069] B. Sensing Elements
[0070] The sensing element operably associated with a measurement
magnet outputs a signal that is indicative of the magnetic drag
force experienced by the measurement magnet upon exposure to a
ferromagnetic sample in motion (or having previously been in
motion) relative to the measurement magnet. As those in the art
will appreciate, the task of measuring force resides primarily in
sensor design, which can be resolved into two problems: geometric
or physical constraints; and converting the force into a workable
signal. As is known, an applied force can be measured many ways,
including: by balancing the applied force against a standard mass
through a system of levers; measuring the acceleration of a known
mass and using Newton's second law; equalizing the applied force to
a magnetic force generated by the interaction of a current-carrying
coil and a magnet; distributing the applied force on a specific
area to generate pressure, and then measuring the pressure; and
converting the applied force into the deformation of an elastic
element. Below several well-known types of force sensors useful for
force-to-signal conversion are described. Any of these, as well as
any other suitable now known or later developed force-to-signal
measurement device, can be readily adapted for use in practicing
this invention.
[0071] i. Strain Gauge Load Cells
[0072] In general, a strain gauge load cell comprises a metal wire
structure that elastically deforms when subjected to a force. As a
member is stressed, the resulting strain deforms the strain gauge
that is bonded (e.g., by cement or epoxy) or otherwise securely
attached to the member. The resistance of the metal wires changes
as it undergoes strain, which change in resistance is used by a
strain gauge load cell to produce an electrical signal proportional
to the deformation of the member. Small loads are commonly measured
by beam-type strain gage load cells. Ring-type load cells cover a
wider range of loads.
[0073] ii. Piezoelectric Load Cells
[0074] When a force is exerted on certain crystalline materials
(i.e., piezoelectric crystals), electric charges are formed on the
crystal surface in proportion to that force. To make use of
piezoelectric crystals, a charge amplifier is required to integrate
the electric charges to give a signal that is proportional to the
applied force and large enough to measure. Sensors based on
piezoelectric crystals differ from most other sensing techniques in
that they are active sensing elements. No power supply is needed,
and the deformation to generate a signal is very small, which has
the advantage of a high frequency response of the measuring system
without introducing geometric changes to the force-measuring
path.
[0075] iii. Capacitive Force Transducers
[0076] Transducers that use capacitance variation can also be
applied to measure force. In such sensors, the force is directed
onto a plane whose elastic deflection is detected by a variation of
the capacitance. Highly sensitive force transducers can be
constructed because capacitive transducers accurately sense very
small deflections. An electric circuit converts the capacitance
variations into dc-voltage variations.
[0077] iv. Force-Sensing Resistors (Conductive Polymers)
[0078] Force-sensing resistors utilize the fact that certain
polymer thick-film devices exhibit decreasing resistance with the
increase of an applied force. A force-sensing resistor is made up
of two parts. The first is a resistive material applied to a film.
The second is a set of digitating contacts applied to another film.
The resistive material completes the electrical circuit between the
two sets of conductors on the other film. When a force is applied
to such a sensor, a better connection is made between the contacts;
hence, the conductivity is increased.
[0079] v. Magnetoelastic Force Sensors
[0080] Magnetostrictive transducer devices operate based on the
Joule effect, which states that a ferromagnetic material is
dimensionally altered when subjected to a magnetic field. The
effect is reversible and used in magnetoelastic force sensors: if
an external force produces a strain in a magnetostrictive material,
the material's magnetic state changes proportionally to the applied
stress. An electric circuit converts these magnetic state changes
to a voltage signal for subsequent processing.
[0081] vi. Torsional Balances
[0082] Balancing devices that utilize the deflection of a spring
may also be used to determine forces. Torsional balances are equal
arm scale force measuring devices. They are comprised of horizontal
steel bands instead of pivots and bearings. The principle of
operation is based on force application on one of the arms that
will deflect the torsional spring in proportion to the applied
force.
[0083] vii. Hydraulic Load Cells
[0084] A hydraulic load cell is a device filled with a liquid
(usually oil) that has a pre-load pressure. Application of a force
to the loading member increases the fluid pressure, which is
detected by a pressure transducer.
[0085] viii. Pneumatic Load Cells
[0086] The operating principles of a pneumatic load cell are
similar to those of hydraulic load cells. Again, a force is applied
to one side of a piston or a diaphragm of flexible material and
balanced by pneumatic pressure on the other side. This
counteracting pressure is proportional to the applied force, and is
detected by a pressure transducer.
[0087] C. Devices
[0088] The magnetic drag force sensors of the invention will most
frequently be integrated into a complete device for measuring
magnetic drag force, i.e., a magnetic drag force measurement
device. In general, the magnetic drag force sensor(s) is positioned
such that, in operation, the measurement magnet thereof is
proximately spaced from a ferromagnetic sample capable of moving
relative to the measurement magnet, such that the sensor, the
sample, or both may be moved relative to one another during
operation of the device. Signals output by the sensor are
preferably conditioned by such suitable electronic circuitry as is
required for the particular application, thus allowing it to be
recorded, transmitted, and/or analyzed. In preferred embodiments,
the signals are digitized into a form suitable for use by a
processor configured to process them in order to determine a
parameter of the magnetic drag force experienced by the measurement
magnet. The results of the processing may be stored in a storage
device and/or output to an output device (for example, a plotter, a
computer monitor) interfaced with the device.
[0089] In order to move a magnetic drag force sensor and/or the
ferromagnetic sample, a device according to the invention
preferably includes a drive system adapted for the particular
application. Moreover, for some applications (e.g., measurement of
hysteresis loss), it is desirable to sufficiently magnetize the
sample, preferably by placing it in a state of known remanant
magnetization, prior to moving the sample relative to the magnetic
drag force sensor. This may be accomplished through the use of one
or more magnets positioned upstream of the measurement magnet(s) of
magnetic drag force sensor. Alternatively, the sample may first be
exposed to the measurement magnet in a non-sensing mode in order to
establish the desired state of remanent magnetization. Thereafter,
the sample can be moved relative to the magnetic drag force sensor
so that the drag force can be measured. It will also be appreciated
that a magnetic drag force may also be detected after stopping the
relative motion of the magnetic drag force sensor and sample.
[0090] In addition, or alternatively, a device of the invention may
also include one or more magnets, including one or more measurement
magnets disposed in one or more magnetic drag force sensors,
positioned such that upon exposure to a sample, at least one
measurement magnet is proximately spaced from a first surface of
the sample and a second measurement magnet is proximately spaced
from a second, different surface of the sample. For example, in a
device wherein the sample horizontally moves through the device,
one measurement magnet is positioned above and is proximately
spaced from the upper surface of the sample, whereas the other
measurement magnet is positioned below and is proximately spaced
from the lower surface of the sample.
[0091] A device according to the invention can be adapted for
detecting and/or measuring (qualitatively, semi-quantitatively, or
quantitatively) magnetic drag forces in conjunction with
ferromagnetic samples of any shape, size, or composition.
Representative sample shapes include plates, bars, strips, wires,
and cables. In cross-section, such samples may, for example, have a
geometric shape selected from the group consisting of a circle, an
ovoid shape, and a polygon (e.g., a triangle, rectangle, square,
etc.). In many embodiments, the device will be configured to
analyze ferromagnetic samples of substantially uniform geometric
cross-section, while in other embodiments, the devices will be
configured to adapt to different sample sizes, shapes, etc. Samples
may be separate pieces, or they may be one continuous piece.
[0092] While devices according to the invention have largely been
described as comprising magnetic drag force sensors that include at
least one measurement magnet operably associated with a sensing
element, it will also be appreciated that magnetic drag forces can
also be measured from their effects on the sample, particularly
small samples. In such embodiments, the device typically comprises
a sample stage, or platform, upon which a ferromagnetic sample can
be placed. One or more sensing elements are positioned so that they
are operably associated with the sample stage. As with the other
embodiments of the invention where a sensing element is operably
associated with a measurement magnet, here, the sensing element
also outputs signals indicative of a magnetic drag force, the
difference being that drag force is experienced by the
ferromagnetic sample upon exposure to a pre-determined magnetic
field generated by one or more reference magnets (or a reference
magnet array) proximately spaced from the ferromagnetic sample. In
some embodiments, the reference magnet (or reference magnet array)
may be movable in relation to the sample stage (see FIG. 9, sample
stage not shown), for example, by placement of the reference magnet
PM (26) on a carriage that allows controlled movement of the
reference magnet (or reference magnet array). As shown in FIG. 9,
movement (25) of the reference magnet (26) about a carriage causes
the sample (28) to experience a drag force (32), which can be
sensed by the sensing elements (force sensor elements 30 and 31).
The sample stage (not shown) is preferably made of a material that
is non-magnetic. Similar embodiments, in which the reference magnet
remains stationary and the sample stage is moved at a uniform
speed, are also envisioned.
3. Applications for Magnetic Drag Force Sensing
[0093] NDE is typically incorporated into manufacturing processes
in one of four primary ways: to provide quality control during
manufacture or fabrication; to ensure that an item conforms to
established specifications; to examine plant, equipment, or
components during service, in order to meet regulatory requirements
or to check for evidence of wear or premature failure; or as a
diagnostic tool in research and development. FIG. 10 illustrates
some of the numerous applications for the instant magnetic drag
force sensors. For example, the magnetic drag force sensors and
devices of the invention can be used to measure hysteresis losses,
thickness of material, material composition and/or microstructure,
inhomogeneity or other defects (e.g., internal flaws, surface
scratches, etc.) in samples, and correlated mechanical properties
(e.g., hardness, strength, etc.). Such sensors and devices may also
be used to determine a material's orientation (for example, by
detecting the presence of non-visible features) and for
identification purposes (for example, by detecting a coded
arrangement of non-visible features). As will be the case for other
applications, the particular context in which the method is to be
performed determines the ultimate configuration of the device to be
employed, although in each case they will be used to sense a
detectable drag force parameter, such as a steady state, time rate
of change of the drag force, etc.
[0094] As will be appreciated by those in the art, an important
application of the sensors and devices of the invention is in the
context of measuring hysteresis losses in ferromagnetic materials,
particular electrical steels. Several general device configurations
are schematically shown in FIG. 11. Similar devices employing a
single measurement magnet are described in detail in Example 1,
below. In general, these devices employ at least one magnetic drag
force sensor positioned near the surface of a ferromagnetic sample
to be tested. The sensor comprises one or more measurement magnets
spaced slightly from sample.
[0095] In these embodiments, the measurement magnet(s) is(are)
operably associated with a force sensing element (e.g., a load
cell) such that a drag force, or changes in the drag force,
experienced by the measurement magnet(s) can be measured during or
after the sensor and sample move in relation to each other.
Preferably, the sample is magnetized (i.e., "initialized") before
encountering the measurement magnet(s). Initialization ensures that
the ferromagnetic sample enters the active zone (AZ) of the
measurement magnet(s) in the desired state of magnetization. For
hysteresis loss measurement, the sample should enter the AZ of the
measurement magnet(s) remanently magnetized with a polarity
relative to the first encountered field of the measurement
magnet(s), which depends on which loop area is to be measured. In
particular, it has been discovered that by positioning the
measurement magnet(s) such that its(their) magnetic moment(s)
is(are) normal to the direction of sample travel, the device can be
used for the measurement of the hysteresis loss associated with a
major hysteresis loop. On the other hand, by positioning the
measurement magnet(s) such that its(their) magnetic moment(s)
is(are) parallel to the direction of sample travel, the device can
be used for the measurement of the hysteresis losses associated
with the combination of the major and minor hysteresis loops. Thus,
if desired, the contribution of the minor hysteresis loop can be
determined by using two measurement magnets, preferably one having
its magnetic moment oriented parallel to the sample's direction of
travel, the other, normal to the sample's direction of travel.
[0096] Now, with reference to FIG. 11(b), to measure a major loop
area only using a device having an upstream magnet (UM; 37) and a
measurement magnet (MM; 35) disposed facing the same surface (but
different locations at any given instant) of the sample (SUT; 36),
both UM and MM will have their magnetic moments normal to the
direction of motion and to the sample surface. The UM and MM have
the same polarity orientation. As the leading edge of the SUT
(presumed to have not been previously magnetized, but its magnetic
state is not significant in any event) enters the AZ of the UM, it
is subjected first to an increasingly intense field of one
polarity, followed by a decrease in intensity and, after crossing
zero, to an increasing field of the opposite polarity, followed by
a decrease towards zero as it recedes from the AZ. The SUT thus
leaves the AZ of the UM in a remanent magnetic state. As the
leading edge of the SUT (and all subsequent points) enters the AZ
of the MM, it first encounters an increasingly intense field of
opposite polarity to its remanence. Its remanent magnetization is
first reduced by this field (reaching zero at the coercive field),
then increased in intensity until reaching technical saturation at
the peak intensity of this field. During further motion, it passes
through the region of decreasing field intensity, with the
magnetization continuously relaxing, reaching remanence at zero
field. During further motion, points on the SUT experience an
increasingly intense field of opposite polarity. Again the
magnetization is first reduced, again crossing zero at the coercive
field and rising to technical saturation of opposite polarity at
the peak field intensity. During further motion, points on the SUT
experience a continuous reduction in field intensity approaching
zero as they leave the AZ. Since the magnetization at each affected
point in the SUT started from remanence and ends at remanence of
the same polarity, having been subjected to saturation of each
polarity, it has traversed a single major hysteresis loop. The net
drag force on the measurement magnet is thus equal to the product
of the area of this loop and the cross sectional area of the
SUT.
[0097] Again with reference to FIG. 11(d), the use of one or more
magnets (CUM (39) and CMM (38)) positioned below the SUT (36) and
opposite the corresponding magnet(s) (37 and 35, respectively)
disposed proximate to the opposite surface of the SUT, can provide
compensating attractive forces. If used, the drag force on the CMM
may also be measured and added to the drag force measured on the
MM. Alternatively, the MM and CMM may be physically connected to
the same force measurement system. As will be appreciated,
embodiments that reduce the attractive force between measurement
and/or upstream magnets and the sample being tested may simplify
the magnet support system and prevent distortion of thin SUTs. Such
configurations will also reduce the intensity of field components
normal to the direction of motion, thereby more nearly reproducing
the uniaxial fields used in conventional hysteresis loss
measurement systems.
[0098] As shown in FIG. 11 and as described herein, one or more
upstream magnets (UM and CUM, 37 and 39, respectively) may be
employed to "initialize" a ferromagnetic sample, i.e., place it
into a desired state of remanent magnetization. Of course,
initialization can also be achieved by bidirectional motion of the
sample in relation to the measurement magnet. The magnetic moment
of the measurement magnet will preferably be disposed parallel or
normal, depending upon application, to the direction of motion of
the sample, and combinations of measurement magnets wherein the
magnetic moment of one is oriented normal to, and another is
oriented such that its magnetic moment is oriented parallel to, the
sample's direction of motion, can also be used. Similarly, one or
more magnets placed on opposite sides of the sample can serve to
reduce or eliminate the attractive force.
[0099] Another series of applications for the sensors and devices
of the invention is in the context of detecting inhomogeneity and
defects in ferromagnetic materials for various shapes and sizes,
from small strips of materials to large plates, wires, cables,
bars, etc. These methods may be applied during manufacturing
processes, or in the field, for example, to test for wear and
fatigue of structurally critical components.
[0100] As will be appreciated, one or more markers that disrupt the
homogeneity of some portion of a ferromagnetic material may be
intentionally introduced in order to provide a "signature" that
enables the material to be identified, its orientation to be
determined, etc. Such markers can be introduced into a sample, for
example, by heat-treating small areas of the sample at specific
locations, preferably in a specific pattern that can later be
detected in a signal obtained by measuring magnetic drag force in
at least the region(s) where the marker was earlier introduced.
Other methods of introducing markers include shot peening, etching,
scratching, or otherwise scoring a surface, forming one or more
holes or cavities in the material, introducing an inhomogeneity
into one or more specific locations in a material during the
manufacturing process (e.g., by emplacing materials having chemical
compositions or properties that differ from the material from which
the sample is otherwise formed), etc. Indeed, any method suitable
for the introduction of a flaw or defect that locally alters a
magnetic property of the material can be used for this purpose.
[0101] Yet another application for this instant invention is the
context of monitoring manufacturing processes, even in real time.
For example, by monitoring magnetic drag force during a production
process, material homogeneity can be assessed, as can material
quality. In addition, the instant invention can be used to monitor
whether a particular production process, or portions thereof, is
functioning properly. For example, if a ferromagnetic material
produced by a rolling, stamping, or other forming process is
required to have a certain degree of surface smoothness, for
example, use of a device according to the invention can be used to
monitor that metric (here, degree of surface smoothness), and also
to detect, for example, when a roller upstream of the drag force
measurement sensor may be damaged or worn, whether some portion of
the stamping surface of a tool has become damaged or worn, etc.
EXAMPLES
[0102] The following Examples are provided to illustrate certain
aspects, embodiments, and applications of the present invention,
and to aid those of skill in the art in practicing the invention.
These Examples are in no way to be considered to limit the scope of
the invention in any manner.
Example 1
Detecting Magnetic Drag Force Using a Single Measurement Magnet
[0103] This example describes one preferred embodiment of the
invention. In this embodiment the force resisting the motion of a
ferromagnetic member through the intense field close to a permanent
magnet is measured by the equal and opposite reaction force on the
permanent magnet. This force tends to "drag" the magnet in the
direction of motion of the ferromagnetic member.
[0104] Photographs of apparatus incorporating this embodiment is
shown in FIGS. 12(a) and (b). The essential features are depicted
in the schematic diagram of FIGS. 13 (a) and (b). As illustrated in
FIG. 13(b), the drag force measuring device (90) employs a single
permanent magnet (PM; 100) suspended at the distal end (102) of a
pendulum (104) which can rotate freely about shaft (108). The
pendulum is biased to bear slightly against the load cell (110) in
the absence of any drag (tangential) force on the magnet. This bias
prevents the pendulum from losing contact with the load cell. The
output signal of the load cell is generally fed through a cable to
a meter, recorder, and/or data acquisition system (not shown), as
required. The ferromagnetic sample under test, SUT, here shown as a
strip (106), is maintained at a fixed distance (109) from the
magnet by guides (not shown). The SUT (106) is driven in the
directions shown (112) by any suitable means, here shown as rollers
(114).
[0105] The actual value of drag force is determined as one-half the
difference between the output forces measured while the strip (106)
moves first in one direction, then the other. In this way, there is
no need to know the actual value of the deliberately applied bias
force, nor do any unknown biasing influences affect the measurement
accuracy.
[0106] A typical plot of the output signal from the load cell as a
cold rolled steel strip sample (25.5 mm wide, 1.63 mm thick and 300
mm long) was moved first in one direction, then, after a brief
pause, in the other direction, is shown in FIG. 14. The speed of
motion of the SUT for this plot was approximately 4 mm/s. The
dimensions of the measurement magnet were 50.8 mm by 12.7 mm by
3.175 mm, and the magnetic moment was assumed to be approximately
1920 ergs/gauss.
[0107] While the instantaneous drag force shows variations
associated with the imperfect homogeneity of the sample, the
remarkable symmetry of those portions of the plot (FIG. 14)
corresponding to each direction of motion of the SUT is immediately
apparent. Thus features A, B, C, D, E, and F on the left side of
the pause region (501) are seen to have excellent correspondence
with features A', B', C', D', E', and F', respectively, on the
right side of the pause region. The drag force computed as one-half
the vertical displacement of corresponding features seen on the two
sides of the pause region is found to be closely the same for all
such features. In common practice, wherein such plots are made by
sampling and digitizing the signal from the load cell, the computed
average value of load cell signal at all samples between salient
features, such as A and F, is subtracted from a similarly obtained
average for all samples between corresponding features, such as A'
and F'. The drag force is found simply as one-half of the
difference between these averages.
Example 2
Apparatus for Measuring Magnetic Drag Force on Rotating
Ferromagnetic Shafts
[0108] This example describes a preferred embodiment of the
invention that relates a rotating ferromagnetic shaft. A schematic
diagram of the apparatus is illustrated in FIG. 5. The shaft was
made out of 300M steel (0.43% C, 1.8% Ni, 1.6% Si, 0.8% Cr, 0.4%
Mo, 0.07% V, Bal Fe) with an outside radius, R, of 17.5 mm and a
wall thickness, t, of 2.5 mm. The coercive force of the material is
.+-.39 Oe. The measurement magnet was of NdFeB type with an energy
product of 38MGO, and had dimensions of 2 in. by 0.5 in. by 0.125
in. The load cell was manufactured by Futek (model L2338). The
shaft was rotated at slow speed (16.6 rpm) by coupling to a
synchronous gear head motor and at high speed (2000 rpm) with a
variable speed motor using an O-ring belt.
[0109] Measured values of the drag force for the first 22
revolutions of the shaft (characterized in FIG. 5) are shown in
FIG. 6 for a variety of gaps between the magnet and the shaft
surface. A similar plot for a variety of magnet sizes is shown in
FIG. 7. In this embodiment, a polarizing magnet experiences a
reaction force that is equal in amplitude to the drag force on a
ferromagnetic shaft being magnetized. Such an apparatus can measure
this force using a wide range of shaft sizes.
[0110] As the data in FIG. 6 shows, the drag force develops during
the early part of the first revolution, reaches a peak before the
end of that revolution, and diminishes thereafter to a stable value
within 5-10 complete shaft revolutions. Thus, drag force
measurement can be employed to determine when in this process for
instilling a circumferentially magnetized band within a shaft that
magnetization has reached a stable value. The data also
demonstrates that the drag force stabilizes sooner and at higher
values with smaller gaps between the magnetization magnet and the
shaft.
[0111] FIG. 7 also shows that the limiting value of drag force
increases somewhat more quickly than magnet width, reflecting the
greater average field intensity developed by wider magnets. At
rotational speeds of greater than about 40 rpm, the final drag
force values started to grow continuously with increasing speed, an
expected consequence of eddy currents.
Example 3
Determining Hysteresis Loss by Measuring Drag Force Using a Single
Measurement Magnet
1. Abstract.
[0112] This example describes another preferred embodiment, wherein
a magnetic drag force measurement device according to the invention
(as schematically illustrated in FIG. 15) is used to measure
hysteresis loss in a ferromagnetic strip. Thus, this example also
describes novel methods for determining hysteresis losses,
particularly in thin strips of soft magnetic materials. These
methods are based on the measurement of a drag force that arises
with movement of a thin sample strip through the strong field
existing in the space near a measurement magnet (here, a permanent
magnet). Not associated with macro eddy currents, the drag force is
shown to originate from the magnetic hysteresis of the material,
having in fact an amplitude equal to the product of hysteresis loss
and the area of the sample cross section. Correlation within 18%
with measurements made by conventional methods is shown for a wide
range of experimental materials.
2. Introduction.
[0113] Hysteresis loss is a defining characteristic of electrical
steels, and strongly influences the energy efficiency and
functionality of the products in which such materials are used.
Hysteresis loss varies greatly with the elemental composition of
the particular steel, the thermal and mechanical fabrication
processes used to produce the steel, and the direction of
magnetization (Dupre, et al. (2000), J. Magn. Mag. Mat., vol. 215,
p. 112). Hysteresis loss measurement, therefore, is routinely
practiced both during the development of such materials and to
ensure the consistent quality of finished products.
[0114] Although varying significantly in detail (see De Wulf, et
al. (2003), J. Appl. Phys., vol. 93, p. 8543; De Wulf, et al.
(2000), J. Appl. Phys., vol. 89, p. 5239), conventional methods
determine hysteresis loss from the enclosed area of a sample's B-H
loop. This is typically obtained by concurrent measurements of an
applied field, H, slowly varying between desired limits, and the
resulting induction, B, in a sample of known cross sectional area,
A. In contrast, the method described in this example, while also
requiring knowledge of A, depends only on the measurement of a
mechanical force. As will be shown, this method, in addition to its
applicability to standard strip samples, offers an opportunity for
the continuous measurement of hysteresis loss, in real time, during
the manufacture of product in wire, strip, sheet, and even bar
form.
3. Theory.
[0115] In using a device as schematically illustrated in FIG. 15,
the specimen under test, or SUT (130), is maintained at a small,
fixed distance (135), from a magnetic dipole of moment, m,
typically a permanent measurement magnet, PM (132). Both the SUT
(130) and the PM (132) are constrained to disallow the mutual
attractive force, F.sub.a, to bring them into contact. The SUT
(130) is made to move in a direction parallel to m at some
convenient, not necessarily constant, velocity, but slowly enough
to avoid the corrupting influence of eddy currents. As described
herein, a "drag" force, F.sub.d (134), originating from the
magnetic hysteresis of the material, resists the motion of the SUT.
Since the SUT may be large, is in motion, and is subjected to a
variety of associated forces, the measurement of F.sub.d is more
conveniently made by its reaction on the measurement magnet, PM
(132), which therefore is supported in such manner as to both
rigidly resist F.sub.a and provide for the measurement of F.sub.d.
Of course, other device configurations are also possible for
measuring this drag force, including measuring the drag force on
the sample under test.
[0116] To simplify the analytical treatment, PM is assumed to be a
single dipole. Also, the SUT, though having a finite cross
sectional area, A, is assumed to have negligible dimensions normal
to m and, in the plane of its surface facing PM, normal to the
direction of motion. By thus implying that the distance to the
dipole is large compared to these SUT dimensions, the intensity of
the dipole field, H, at points within the SUT effectively varies
only with longitudinal position of the SUT in relation to PM. The
SUT is also assumed to extend far enough in both longitudinal
directions so that its ends are situated in regions of vanishingly
small H. Although H includes components normal to m, the shape
anisotropy of the SUT limits the effects of these components on the
magnetization orientation. It is nevertheless recognized that
F.sub.a derives from the normal component of magnetization. Thus,
the longitudinal component of H, and the history of exposure to
this component, are the significant determinants of the intensity
and polarity of the local magnetization, M, within the SUT.
Following from Cullity's derivation (B. D. Cullity, Introduction to
Magnetic Materials (Addison-Wesley, Reading, Mass., 1972) p. 614),
H at a point P within the SUT, at a distance Gx from the central
location of m, is readily shown to be:
H = m G 3 ( 1 + x 2 ) 3 / 2 ( 3 x 2 1 + x 2 + 1 ) 1 / 2 , ( 9 a )
##EQU00004##
directed at an angle, .beta.=tan.sup.-1 (0.5/x)+tan.sup.-1 (1/x),
to m. Its longitudinal component is then found from
H L = H cos .beta. = H ( x 2 - 0.5 ) ( x 4 + 1.25 x 2 + 0.25 ) 1 /
2 . ( 9 b ) ##EQU00005##
Equations (9a) and (9b) show H.sub.L to depend only on m, G and the
normalized distance, x, to P. FIG. 16 shows H.sub.L, normalized
against its maximum value at x=0, plotted against x. Several
features of this plot should be noted. First, H.sub.L is
symmetrical around x. Second, the peak negative H.sub.L is 20.2% of
its peak positive value, H.sub.p+, and occurs at 1.225 G. Third,
H.sub.L crosses zero at x=0.707 G. Fourth, H.sub.L at x=+6 G is
than about 0.01 of its peak value; thus, significant changes in M
are limited to locations between about plus or minus 5 G, the
active zone (AZ) in this device.
[0117] The SUT is assumed to have arrived at the position shown in
FIG. 15 by motion from left to right, and that in so doing, at
least the portion shown, passed under a second, identical PM
(PM2--not shown), also separated from the SUT by G, and located
greater than 12 G upstream (in the device illustrated in FIG. 15,
to the left of PM) of the permanent measurement magnet. An element
of material of infinitesimal length, dx, at position 1 in FIG. 16a,
while presently located where the fields from both permanent
magnets (PM1 and PM2) are near zero, will previously have been
exposed to the peak negative field, H.sub.p-, from PM2, assumed to
be sufficient to result in technical saturation. Thus, when
reaching position 1, the start of the AZ, this element of material
will be at negative remanence, -M.sub.r, indicated as point 1 on
the hysteresis loop in FIG. 16b, and transcribed to a plot of M vs.
x in FIG. 16c. During further rightward motion of the SUT, a
distance sufficient for the element originally at 1 to arrive at 2,
the location of H.sub.p, M within this element will grow along the
path indicated 1.fwdarw.2 in FIGS. 16b and 16c. During further
motion to the right, H.sub.L falls to zero and M returns to
-M.sub.r along path 2.fwdarw.3 (FIG. 16(a), 16(b), and 16(c)),
thereby completing the traversal of a minor hysteresis loop. The
continuously moving element then experiences a steep growth in
H.sub.L of opposite polarity, reaching H.sub.p+ at 4, relaxing to
zero at 5, a growth to H.sub.p at 6, and again approaching zero at
7, the end of the AZ. If moved slowly enough for quasistatic
conditions to prevail, M within the element follows these field
variations, reaching positive saturation at point 4, +M.sub.r at
point 5, negative saturation at point 6, and returns to its
starting value of -M.sub.r at point 7, thereby completing traversal
of a major hysteresis loop.
[0118] At each position within AZ, the sample has a magnetic
moment, MAdx, and by virtue of the field gradient, dH/dx at that
location, it experiences a longitudinal force dF=MAdxdH/dx.
Variation in dH/dx with position is shown in FIG. 16d, and
variation in dF (plotted as dF/dx) is shown in FIG. 16e. Since, at
any one instant, there are elements of like size at every location
in the AZ, the sum of these elemental forces comprises a net force
acting on the sample, which can be described as:
F = .intg. F = .intg. MA x H x = A .intg. M H ( 10 )
##EQU00006##
Since the AZ contains elements having magnetizations representative
of traversal of both the minor loop 1.fwdarw.2.fwdarw.3 and the
major loop 3.fwdarw.4.fwdarw.5.fwdarw.6.fwdarw.7, F in equation
(10) clearly derives from the total area of both loops. FIG. 16f
shows the cumulative sum of the elemental forces to the left of
each point within the AZ. The existence of a finite final sum, F,
as clearly shown in FIG. 16f, reflects the asymmetry of the plots
in FIG. 16 (c0 and 16(e,) asymmetries that arise from the
hysteretic M-H functions of the sample material. F is seen to be a
repulsive force, acting to resist the motion of the sample. The
reaction on the measurement magnet is in the opposite direction,
tending to drag it along in the direction of the motion, hence its
appropriate appellation, "drag force", F.sub.d.
4. Experiments.
[0119] Both drag force and conventional hysteresis loss
measurements were performed on strip samples, 25 mm wide and 280 mm
long, of materials characterized in Table 1. Table 1:
Identification of materials tested, together with measurement
results and correlation assessment.
[0120] The magnetic drag force measurement device used to conduct
these experiments was essentially the same device as described in
Example 1. Also, the device was modified to hold the sample strip
at fixed distances ranging from between 0.25 mm and 5.1 mm under
the
TABLE-US-00001 TABLE 1 Identification of materials tested, together
with measurement results and correlation assessment. ID Material
Condition Thickness Major Minor Minor/Major Major + Minor By
F.sub.d Diff. % A Black Nickel As Rec'd 0.254 mm 992.8 J/m.sup.3
80.6 J/m.sup.3 8.11% 1073.4 J/m.sup.3 1037.0 J/m.sup.3 -3.4 B
AISI1010 Steel Cold Rolled 0.127 5450.4 698.0 12.81 6148.4 6336.2
3.1 C AISI1010 Steel Annealed 0.125 1450.1 285.2 19.67 1735.3
1952.3 12.5 D AISI1010 Steel Cold Rolled 0.254 6317.3 492.1 7.79
6809.4 6798.6 -0.2 E AISI1010 Steel Annealed 0.250 1240.1 59.7 4.81
1299.8 1324.9 1.9 F AISI1010 Steel Cold Rolled 0.381 6745.1 744.8
11.04 7489.9 7137.0 -4.7 G AISI1010 Steel Annealed 0.370 1439.8
198.2 13.77 1638.0 1393.6 -14.9 X FeSi NO 0.500 505.5 124.7 24.67
630.2 518.2 -17.8 Y FeSi GO 75 deg 0.235 372.4 173.3 46.54 545.7
531.9 -2.5 Z FeSi GO 0 deg 0.288 155.3 39.2 25.24 194.5 199.5
2.6
pendulum-mounted measurement magnet, and it was equipped with a
small gear head motor and driving rollers that allowed the sample
strip to be moved in either horizontal direction at speeds ranging
from 1.6 to 7 mm/s. The measurement magnet was a 50.8 mm long (and
thus extended well beyond the edges of the narrower sample strips),
12.7 mm wide (normal to the strip surface), and 3.17 mm thick
(longitudinal).
[0121] In operation, the samples were first moved back and forth
such that the central 250 mm of each 280 mm strip passed once in
each direction under the measurement magnet. This ensured that all
portions of a strip not in the AZ were placed in the (negative)
remanent state without the need for a second magnet upstream of the
measurement magnet. Each sample strip was then positioned to allow
the central 80 mm to pass once in each direction under the
measurement magnet while the horizontal force on the magnet was
measured and recorded. Forward and reverse motions were used to
eliminate the effect of possible components of the attractive force
due to imperfect parallelism between m and the direction of sample
strip motion. The pendulum was biased to always exert a force in
one direction on the load cell; F.sub.d then being taken as
0.5.times. the difference between the average forces measured in
each direction. Limiting the measurements to a relatively small
central region of the SUT prevented its ends from getting close
enough to the measurement magnet to develop significant parasitic
forces.
[0122] Quasistatic hysteresis loss associated with both major and
minor loops was measured in a double yoke, small size single sheet
tester (SST)(De Wulf, et al. (2003), J. Magn. Magn. Mat., vol. 254,
p. 70) using a current mode excitation with a constant dH/dt of 1
(kA/m)/s. Major and minor B-H loops for the 3 Si-steel samples are
shown in FIG. 17, with similar loops for the blackened nickel and a
low carbon steel, in both cold-rolled and annealed conditions,
shown in FIG. 18. The results of both conventional and magnetic
drag force measurements are listed in Table 1, above.
[0123] FIG. 19 shows the effect of varying the spacing between
measurement magnet and the various sample strips. As these results
show, the magnetic drag force initially increases with decreasing
gap for all of the test specimens, with all except strip Z reaching
limiting values near 1 mm gaps. The data scatter seen for this
sample strip suggests that the accurate measurement of very low
drag forces (about 1.1 mN) may be beyond the capability of the load
cell (5 N range) utilized.
5. Discussion.
[0124] Hysteresis losses determined by magnetic drag force
measurement was seen to match within 18% those determined by a
conventional method. This unexpectedly close correlation for
materials having a wide range of magnetic and geometric
characteristics indicates that neither normal field components nor
the demagnetizing fields arising from the large values of dM/dx
(FIG. 16@) existing within some portions of the active zone have
significant effects. The sluggish dependence on gap was also not
unexpected, since peak field excursions of just a few times the
coercivity are usually sufficient to develop the major portion of
major loop areas. These encouraging results demonstrate the utility
of this approach for assessing hysteresis losses in electrical
steels. While the use of a single measurement magnet oriented with
its magnetic moment parallel to the direction of strip travel in
the device allows a determination of hysteresis losses attributable
to the combination of both the major and minor hysteresis loops,
use of a separate device having a second magnetic drag force sensor
having its measurement magnet, equivalent to the first in terms of
field strength, dimensions, etc. but oriented normal to the
direction of sample travel (from which losses attributable to major
hysteresis loop only, with only negligible contributions from the
minor loop), will allow the hysteresis losses attributable to each
of the major and minor loops to be separated and accurately
determined, if desired. Alternatively, other embodiments, for
example, those that include measurement magnets placed on both
sides of the sample, preferably opposite one another (see FIG. 11)
will substantially reduce the normal force and allow this method to
be applied to thicker samples.
Example 4
Portable Device for Measuring Magnetic Drag Force
[0125] This example provides a description of a preferred
embodiment of the invention that can be used, for example, to
detect defects in large plates. See FIG. 20. This device (150) is a
small three-wheeled (152) machine that can be propelled manually by
pushing on the handles (154). The magnetic drag force sensor is
housed within a cavity (157) in the body of the device (150). The
magnetic drag force sensor has a pendulum (159) that holds a
measurement magnet (156), which is disposed at the bottom end of
the pendulum (159). Two sensing elements (170) engage the pendulum
(159), which can pivot about a shaft (160). As this device is moved
at a uniform speed across a large ferromagnetic surface (e.g., a
submarine hull), changes in the magnetic drag force can be
sensed.
Example 5
Device for Measuring Magnetic Drag Force in Conjunction with Moving
Cables
[0126] This example provides a description of a preferred
embodiment of the invention that can be used, for example, to
detect flaws in a cable, for example, a ski-lift cable. See FIG.
21. In this embodiment, the cable (180) passes through a stationary
magnetic drag force sensor (182) that comprises a ring magnet (184)
operably associated with two force sensing elements (186, 188). As
the cable moves through the magnetic drag force sensor, flaws in
the cable can be detected in real-time.
Example 6
Detection of Hidden Flaws Using Drag Force Measurements
[0127] This example describes the ability of the sensors and
devices of the invention to detect hidden flaws in a ferromagnetic
sample. These results are illustrated in FIGS. 22, 23, and 24. FIG.
22 shows a strip of low carbon steel in which various "defects"
were purposefully instilled. The figure shows the dimensions of the
strip and the locations and dimensions of three drilled holes (193,
194, and 195) and three abrasively cut slots (191, 192, and 196).
The holes went completely through the strip, while the slots had a
maximum depth of 0.30 mm (80%) of the strip thickness. FIG. 23
shows a plot of the drag force against time (hence of position
along the strip), as the strip was moved first in one direction,
and then, after a small pause (197), in the reverse direction,
between a pair of identical measurement magnets (while held at a
small constant distance below a measurement magnet) mounted in the
previously described apparatus. The variation in drag force with
position clearly shows both the relative size and location of the
instilled defects (191-196). The portions of the plot on the left
side of the paused region are seen to closely mirror those on the
right side of this region, with the variations in drag force on
each side clearly reflecting the defects instilled in the strip.
Depending on the direction of motion, the magnitude of the drag
force associated with each defect is seen to have a sharp decrease
(or increase) followed by a similarly sharp increase (or decrease).
These changes in drag force are believed to arise from the magnetic
poles that form at the longitudinal extremes of each defect in
response to the inhomogeneous magnetization existing between the
bulk of the strip and the regions within the defects where the
material is absent. Depending on the pole orientation of the
measurement magnet, either a north or south pole will form at one
end of the defect and an opposite pole will form at the other. The
repulsive force between the upstream pole of the measurement magnet
and the approaching pole from the defect is the source of the first
occurring drag force peak and the repulsive force between the
downstream pole of the measurement magnet and the receding pole
from the defect is the source of the second occurring peak. The
vertical displacement of the two regions of the plot shown in FIG.
23 is indicative of the magnetic hysteresis loss of the strip being
tested.
[0128] Subsequently, two strips, neither having any deliberately
instilled defects and each having the same nominal length and width
dimensions and having been cut from the same sheet of material as
the original strip, were cemented, one on each side of the
defect-containing strip. FIG. 24 shows the drag force plot for this
"stack" of strips when tested in the same apparatus, and in the
same manner as the previously described defect-containing strip.
Although the actual defects in the center strip of this stack were
no longer visible, their readily identifiable signatures (191-196)
are clearly seen in the drag force plot of the stack.
[0129] Data collected from comparisons of the peak amplitudes and
other features of the drag force signature of well characterized
defects instilled in standardized test strips with the resultant
signatures when such strips are "buried" at known depths from the
surface of stacks of reasonably flawless strips will enable
determinations to be made of the type, size and depth of hidden
flaws in bulk materials. Scanning the surface of the part to be
examined with described combinations of measurement magnets, force
sensors and supplementary magnets, together with processors to
capture, process and store both the output signals from the force
sensors and signals indicative of the corresponding location on the
surface of the part, can thus provide a simple and economical means
for the nondestructive detection of the presence of structural or
compositional anomalies within the part being examined.
[0130] All of the articles and methods disclosed and claimed herein
can be made and executed without undue experimentation in light of
the present disclosure. While the articles and methods of this
invention have been described in terms of preferred embodiments, it
will be apparent to those of skill in the art that variations may
be applied to the articles and methods without departing from the
spirit and scope of the invention. All such variations and
equivalents apparent to those skilled in the art, whether now
existing or later developed, are deemed to be within the spirit and
scope of the invention as defined by the appended claims.
[0131] All patents, patent applications, and publications mentioned
in the specification are indicative of the levels of those of
ordinary skill in the art to which the invention pertains. All
patents, patent applications, and publications are herein
incorporated by reference in their entirety for all purposes and to
the same extent as if each individual publication was specifically
and individually indicated to be incorporated by reference in its
entirety for any and all purposes.
[0132] The invention illustratively described herein suitably may
be practiced in the absence of any element(s) not specifically
disclosed herein. Thus, for example, in each instance herein any of
the terms "comprising", "consisting essentially of", and
"consisting of" may be replaced with either of the other two terms.
The terms and expressions which have been employed are used as
terms of description and not of limitation, and there is no
intention that in the use of such terms and expressions of
excluding any equivalents of the features shown and described or
portions thereof, but it is recognized that various modifications
are possible within the scope of the invention claimed. Thus, it
should be understood that although the present invention has been
specifically disclosed by preferred embodiments and optional
features, modification and variation of the concepts herein
disclosed may be resorted to by those skilled in the art, and that
such modifications and variations are considered to be within the
scope of this invention as defined by the appended claims.
* * * * *