Barkhausen noise method for stress and defect detecting in hard steel

A sensor for a system that detects stresses and defects in a metal piece by employing the Barkhausen phenomenon enables static or dynamic testing of such metal pieces as hard steel in which only low levels of Barkhausen noise can be generated or which are configured in one of a large variety of shapes. The sensor includes apparatus that ensures that a uniform time-varying magnetic field can be produced in all the metal pieces of a variety of shapes. The sensor can include an energizing coil assembly which generates the magnetic field in the metal piece having a core constructed from such a material in which a low level of magnetic noise is generated as ferrite. The level of the magnetic noise generated in the ferrite core is so low that, even if the magnetic noise is sensed by the sensing coil assembly, which receives the Barkhausen noise gererated within the metal piece, that noise cannot interfere with the higher levels of Barkhausen noise generated in all ferromagnetic metals, including hard steel. The dynamic sensor has apparatus, such as wheel assemblies, for maintaining an air-gap of a substantially constant depth between the specimen and the energizing and sensing cores. The dynamic sensor can also include an energizing core which can be reoriented relative to the sensor to determine the directional characteristics of stress and defects in the specimen.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to stress and defect identification 
and, in particular, to a sensor for a system that employs magnetic fields 
to identify stress and defects in a metal piece. 
2. Description of the Prior Art 
Stress and structural defects in a metal piece and certain other properties 
of the metal piece can be identified by creating a time-varying magnetic 
field within the metal piece and analyzing the magnetic noise created in 
the metal piece by the magnetic field. In particular, one method employs a 
phenomenon known as the "Barkhausen effect" to identify stresses and 
defects in a metal piece. As is stated in column 2 of U.S. Pat. No. 
3,427,872 ("the Leep et al. patent"), the Barkhausen effect is "defined in 
Webster's New International Dictionary (3rd Edition) as `a series of 
abrupt changes or jumps in the magnetization of a substance when the 
magnetizing field is gradually altered`". The abrupt jumps that occur in 
the magnetization as the intensity of the field is changed can be detected 
as electrical noise by a sensing coil disposed proximate or in contact 
with the metal piece. The noise carried by the electrical leads from the 
coil--commonly referred to as "Barkhausen noise"--can be fed through a 
suitable processing network and, if desired, to a speaker. The level of 
the Barkhausen noise that is generated at a location within a metal piece 
depends in part on the sense, magnitude and direction of the stress at 
that location and the microstructure of the metal. Accordingly, workers in 
the art have attempted to employ the Barkhausen effect and Barkhausen 
noise to identify stresses and defects in and some microstructural 
characteristics of a metal piece. A discussion of the Barkhausen effect 
can be found in columns 1 and 2 of the Leep et al. patent. 
Most systems which employ the Barkhausen effect to identify stresses and 
defects in a metal piece include an energizing coil assembly and a sensing 
coil assembly. The energizing coil is disposed proximate the location of 
the metal piece under examination and is energized with a periodically 
time-varying signal to induce in the metal piece a periodically 
time-varying magnetic field. The time-varying magnetic field in the metal 
piece causes abrupt jumps in the magnetization of the metal piece to 
occur. A sensing coil is disposed near or in contact with the metal piece 
near the same location and detects the abrupt jumps in the magnetization 
and converts those jumps to Barkhausen noise on the electrical conductor 
of the coil. The Barkhausen noise is then fed to circuitry, usually 
located externally of the sensor, which can process the noise in a variety 
of manners, depending upon the type of information to be obtained and 
displayed. Ultimately, the processed Barkhausen noise is fed to a device 
which displays it. A system for energizing the energizing coil assembly 
and processing the Barkhausen noise, and a description of displays of 
Barkhausen noise are described in the Leep et al. patent. 
Known Barkhausen sensors are suggested for use in either static or dynamic 
applications. Static testing involves making a relatively small number of 
inspections of a single piece or of each of a relatively small number of 
pieces. That limitation is imposed by the fact that most sensors used in 
static testing require physical contact between the energizing core of the 
energizing coil assembly and the test piece. Therefore, changing the 
location of inspection requires removing the sensor from the test piece 
and moving it into contact with the next inspection site to prevent rapid 
and undesirable wear of the energizing core that would result from moving 
the sensor from test site to test site while the core is in contact with 
the piece. Dynamic testing is employed in situations where the entire 
surface of a single metal piece must be tested or where more than one 
location of each of a number of test pieces must be inspected. The sensor 
used in dynamic testing does not require physical contact between the 
energizing and sensing coil assemblies and the metal piece. Therefore, 
there can be constant relative movement between the metal pieces under 
examination and the sensor. 
Any system employing Barkhausen noise to locate stress and defects within a 
specimen depends, for proper functioning, upon the phenomenon that the 
level of Barkhausen noise generated in a specimen changes significantly 
when the sensor is moved between a location where no stress or defects 
exist and a location where stress or a defect is located. Therefore, 
variations in the level of Barkhausen noise generated in the metal piece 
caused by the examining process itself must be minimized to the point 
where proper analysis of the Barkhausen noise is not prevented. 
Accordingly, the effects on the generated Barkhausen noise due to magnetic 
self-coupling within the Barkhausen sensor, inconsistent magnetic coupling 
between the sensor and the metal piece and variations in the shapes of 
metal pieces must be minimized. 
Magnetic self-coupling can occur between the energizing and sensing coils 
of the sensor. Often, it is necessary to examine a metal piece constructed 
of hard steel, in which only low levels of Barkhausen noise can be 
generated. The term "hard steel", as used herein, means steel having a 
Rockwell C hardness greater than about 50. Exemplary of such steels are 
(i) martensitic steels that have been either quenched or quenched and 
tempered; (ii) fine grained, high strength alloy steels; and (iii) 
carburized steels. Generally, the cores of the energizing coil assemblies 
of conventional Barkhausen sensors are fabricated from iron or steel. A 
relatively high level of magnetic noise is generated within iron and steel 
as the magnetic field induced in the iron or steel is varied. Accordingly, 
a high level of magnetic noise is generated within an iron or steel 
energizing core, which can be sensed by the sensing coil assembly and 
interferes with the low level Barkhausen noise generated in a hard steel 
piece. 
Nonetheless, steel or iron has been used consistently as the material for 
energizing coil assembly cores--even for examining hard steel--for, 
generally, two reasons. First, the periodic signal used to energize the 
energizing coil assembly is of a rather low frequency. Electromagnetic 
devices which operate under low frequency commonly have cores constructed 
of iron or steel. Second, the relatively high noise generated in an iron 
or steel energizing core can interfere with Barkhausen noise generated in 
a test piece only if the energizing core and sensing coil assembly are 
located sufficiently proximate each other to permit electromagnetic 
coupling of the noise between the energizing core and sensing coil 
assembly. When conventional Barkhausen systems have been used to test hard 
steel for stress or defects, they have been used only in experimental 
settings and only on large pieces. Accordingly, the energizing core of the 
sensor has been large and, therefore, spaced a sufficient distance from 
the sensing coil assembly to prevent electromagnetic coupling from 
occurring. 
However, in commercial applications, metal pieces with small 
cross-sectional areas must be examined. Therefore, the sensor used to 
examine those metal pieces must be small and the distance between the 
energizing core and sensing coil assembly must be reduced from that of 
experimental sensors. Destructive electromagnetic coupling occurs between 
the energizing core and sensing coil assembly, which makes it impossible 
to distinguish the low level Barkhausen noise generated in a hard steel 
piece from the high noise generated in the iron or steel energizing core. 
Therefore, there exists a need for a Barkhausen sensor which generates 
lower intrinsic noise than conventional Barkhausen sensors, and that 
permits distinguishing the intrinsic sensor noise from the Barkhausen 
signal obtained from the metal piece. 
Proper magnetic coupling between the sensor and the metal piece can be 
achieved in several ways. A Barkhausen sensor having an energizing core 
that is adapted to contact the metal piece shall be referred to 
hereinafter as a "contact sensor". Since the magnitude of the magnetic 
field induced in a metal piece, and, accordingly, the level of the 
Barkhausen noise generated within the piece, by a contact sensor depends 
on the magnitude of the area of the core of the energizing coil assembly 
that is in contact with the metal piece, the same sensor will provide 
different readings for different magnitudes of contact area, making 
interpretation of the results difficult. Since conventional contact 
sensors have been employed only under experimental conditions, each sensor 
has been adapted for use with metal pieces of only one general shape. When 
used with that shape, a given constant magnitude of contact area is 
achieved. If the sensor were to be used in a commercial setting, the 
magnitude of the areas of the sensor and the metal pieces which are in 
contact with each other would vary with the shape of the metal piece. 
Thus, the magnitude of the Barkhausen noise that indicates existence of a 
defect would vary with the shape of the metal piece and, accordingly, 
analysis of the signals received from the sensor would be unduly 
difficult. Therefore, there exists a need for a Barkhausen contact sensor 
which either provides constant contact area between the metal piece and 
the sensor, regardless of the shape of the metal piece, or electrically 
maintains a uniformly varying magnetic field among metal pieces of various 
shapes. 
A Barkhausen sensor having an energizing core that is not adapted to 
contact the metal piece under examination shall be referred to hereinafter 
as "air gap sensor". Use of an air gap sensor requires the existence of an 
air gap between the energizing and sensing coil assemblies and the metal 
piece. Generation by the sensor of intelligible information, in the form 
of Barkhausen noise, depends on the maintenance of an air gap of uniform 
depth throughout the examination area defined by the sensor and upon 
maintenance of an air gap of a constant depth from inspection point to 
inspection point either within the same metal piece or among metal pieces. 
If dynamic or continuous examination of a metal piece or pieces is 
desired, an air gap of a constant depth must be maintained during the 
period of relative movement between the metal pieces under examination and 
the sensor. Accordingly, there is a need for a Barkhausen sensor that 
provides an air gap of a consistent configuration during performance of 
static or dynamic examination. 
Finally, the thickness of the metal piece itself can change the level of 
Barkhausen noise generated within the metal piece. Accordingly, 
examination of metal pieces of different thicknesses can require constant 
calibration of the system that analyzes the Barkhausen noise and makes it 
available for inspection. Because interpreting Barkhausen noise 
differently for different thicknesses of metal pieces is unduly 
inconvenient, it is common to normalize the system each time the thickness 
of the test piece changes by altering the magnitude of the current 
supplied to the energizing coil assembly. The proper current level can be 
determined in advance experimentally for different thicknesses of metal 
pieces. 
Known Barkhausen sensors have not successfully addressed the concerns 
described above. Because of its low intrinsic Barkhausen noise level, it 
has been proposed in Finland Patent No. 51873 to use ferrite as the core 
material for a Barkhausen noise sensor adapted to investigate flat metal 
pieces; however, in that proposal, a single coil serves as both the 
energizing coil and the sensing coil. The result of that arrangement is 
that the sensing coil tends to detect the intrinsic noise of the core 
material rather than the noise induced in the specimen. This is especially 
the case when measuring hard steels in which only low levels of Barkhausen 
noise can be generated. Accordingly, that arrangement cannot be 
successfully used to detect Barkhausen noise in hard steels due to 
magnetic self-coupling occurring within the sensor. 
An article entitled "Measurement of Barkhausen Noise On Moving Steel Sheet 
with Non-Contact Sensor", published in Finland by Markku Pesonen, 
discloses a concept for dynamically measuring Barkhausen noise generated 
in a steel sheet. FIGS. 5.3(a) and (b) of the publication illustrate the 
sensors in diagrammatic form. FIG. 5.3(a) shows a DC sensor having 
energizing coils energized by a DC source and FIG. 5.3(b) shows an AC 
sensor having energizing coils energized from an AC source. Both sensors 
employ identical sets of energizing and sensing coils on both sides of a 
steel sheet to reduce the effect of vibration of the sheet. The DC sensor 
employs two sets of energizing coils, each set of which is energized by a 
DC source of a polarity opposite to that which energizes the remaining 
set. The DC sensor relies on movement of the sheet relative to the coils 
to generate a time varying magnetic field, which causes Barkhausen noise 
to be generated, in the sheet. 
Several problems are experienced through use of the DC sensor shown in FIG. 
5.3(a). First, the DC sensor can be used to test only relatively soft 
metal pieces if the piece must be moved at a slow speed. At slow speeds, 
insufficient Barkhausen noise is generated within hard metal pieces. 
Examples of metal pieces which must be moved at a low rate of speed 
include odd shaped pieces which simply cannot be moved rapidly. Further, 
variations in the speed of movement of the metal sheet under observation 
change the "frequency" of the apparent time-varying field and introduces 
substantial variation in the generated Barkhausen noise that is not due to 
the presence or absence of stress or defects. Finally, the AC and DC 
sensors shown in FIGS. 5.3(a) and (b) are suitable only for inspection of 
flat, relatively thin metal sheets, which is a serious limitation on their 
use. Accordingly, the DC and AC sensors do not provide Barkhausen sensors 
that are practical for use during dynamic testing. 
Accordingly, there exists a need for a sensor that minimizes magnetic 
coupling within the sensor. Further, there is a need for a sensor that 
provides proper coupling between the sensor and metal pieces of differing 
shapes and thicknesses. 
SUMMARY OF THE INVENTION 
The present invention provides contact and air gap sensors that can be used 
to statically or dynamically detect stress and defects in metal pieces of 
a large variety of microstructures, compositions, sizes and shapes. 
The sensors are particularly useful in nondestructively inspecting 
relatively small pieces of hard steel, which represent a large proportion 
of metal pieces that must be tested in a commercial setting. Further, the 
prolems of magnetic self-coupling and maintenance of proper magnetic 
coupling between the sensor and the test piece have been so minimized that 
only one set of criteria for determining defects from measured Barkhausen 
noise levels need be established and used for all shapes and sizes of 
metal pieces of the same composition and microstructure. 
The present invention provides sensors for use with a system that provides 
information related to the stress and defects in a metal piece by 
generating a magnetic field in the metal piece. The sensor includes an 
energizing coil assembly including an electrical conductor wound around a 
ferrite core. The energizing coil assembly is adapted to receive a 
periodic time-varying electrical signal which will induce in the metal 
piece, when the ferrite core is located proximate the metal piece, a 
time-varying magnetic field. The sensor also includes a sensing coil 
assembly including a second electrical coil which is so disposed relative 
to the energizing coil assembly and, when the ferrite core is located 
proximate the metal piece, to the metal piece as to permit the sensing 
coil assembly to detect noise generated within the metal piece by the 
induced magnetic field. The sensing coil of the present invention is 
physically and electrically independent of the energizing coil assembly. 
Preferably, for static testing, the ferrite core and the sensing coil 
assembly are adapted to make contact with the metal piece. Preferably, the 
magnetic field inducing apparatus includes a pair of contacts defined by 
the ferrite core. The contacts are so oriented in shape that uniform 
contact can be maintained between the ferrite core and the metal piece. 
The contacts can be rounded to permit a constant magnitude of contact area 
between the energizing coil assembly and metal pieces of a variety of 
shapes. 
Preferably, the static sensor also includes apparatus associated with at 
least one of the energizing coil assemblies and the sensing coil assembly 
for ensuring that uniform contact is maintained between the metal piece 
and the sensing coil assemblies. Preferably, the ensuring apparatus 
includes a device for urging the sensing coil assembly into contact with 
the metal piece when the energizing coil assembly is in contact with the 
metal piece and is an elastic, nonmagnetic member. 
Preferably, the air gap sensor of the present invention that is adapted for 
use as a dynamic test instrument includes apparatus for maintaining an air 
gap of substantially constant depth between the ferrite core and the metal 
piece when there exists movement of the ferrite core relative to the metal 
piece. The air gap sensor can include apparatus for reorienting the 
ferrite core relative to the metal piece to reorient the induced magnetic 
field within the metal piece. The dynamic sensor can include a housing for 
containing the ferrite core and a table mounted for rotation with respect 
to the housing to which the ferrite core is mounted. The maintaining 
apparatus can be a pair of wheels that so cooperate with the ferrite core 
that the sensor can be rolled along the metal piece to dynamically test it 
for stress and defects. Further, the maintaining apparatus can be a 
spacing member constructed of a suitable nonmagnetic, wear-resistant 
material --such as Teflon.RTM., nylon or austenitic stainless steel--that 
is disposed between the energizing and sensing coil assemblies and is in 
contact with the metal piece during normal use of the sensor. Also, the 
maintaining apparatus can establish an air cushion between the metal piece 
and the sensor, which is mounted to a stationary object. 
The present invention also provides a sensor capable of inducing in the 
test piece at least two magnetic fields, each oriented in a different 
direction from that of the other.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1-5 show a static sensor 10, a preferred embodiment of the present 
invention. Static sensor 10 is particularly adapted for testing where only 
a relatively small number of readings need to be taken since sensor 10 
should be disengaged from one test site before it is moved to a different 
test site. Sensor 10 includes a housing 12 which contains an energizing 
coil assembly 14 and a sensing coil assembly 16 suitably mounted therein 
by conventional means (not shown). 
Energizing coil assembly 14 includes an electrical conductor 18 which is 
wound around ferrite base 20. A pair of ferrite legs 22 and 24 are 
disposed within fittings 26 and 28, respectively, of housing 12. 
Preferably, base 20 and legs 22 and 24 are constructed, for instance, from 
such a manganese oxide based ferrite material as that sold under the 
trademarks PHILIPS 3C6 or 3C8 by Philips A. G. Seats 30 and 32 are adapted 
to be in contact with ends 34 and 36, respectively, of ferrite base 20. 
Legs 22 and 24 define rounded contacts 38 and 40, respectively, which are 
adapted to contact the metal piece under examination. Legs 22 and 24 are 
held by friction within fittings 26 and 28 and can be removed to reshape 
contacts 38 and 40 and then replaced within fittings 26 and 28. Conductor 
18 receives a periodically time-varying signal from any suitable 
conventional source. When contacts 38 and 40 are in contact with a metal 
piece, the periodic electrical signal induces a magnetic field in ferrite 
base 20 and ferrite legs 22 and 24, which causes the metal piece to become 
magnetized between contacts 38 and 40 of legs 22 and 24. The varying 
magnetic field generated by energizing coil assembly 14 causes abrupt 
jumps in the magnetization of the metal piece, known as the Barkhausen 
effect, to occur. The rounded contacts 38 and 40 of legs 22 and 24 permit 
point contact to be established between legs 22 and 24 and metal pieces of 
a variety of shapes. 
Sensing coil assembly 16 includes an electrical coil 42 disposed within 
container 12 and wound around a ferrite sensing core 44. Sensing core 44 
is secured to an elastic silicone member 50, which is held by friction 
within fitting 46 of case 12. Contact 48 of core 44 is rounded to permit 
point contact between sensor 10 and metal pieces of a variety of shapes. 
Core 44 can be removed from fitting 46, to reshape contact 48 of core 44, 
and subsequently replaced. Member 50 urges core 44 away from passage base 
52 and into contact with the metal piece under examination, but cannot 
expel core 44 completely from within fitting 46. The abrupt magnetic jumps 
occurring in the metal piece due to the magnetic field established by 
energizing coil assembly 14 induce electrical Barkhausen noise in coil 42 
of sensing coil assembly 16 when contact 48 is in contact with the metal 
piece. The Barkhausen noise induced in coil 42 can be processed and 
displayed in any known desired fashion. 
FIGS. 6, 7 and 8 show sensor 100, an alternate embodiment of the present 
invention. Sensor 100 is identical to sensor 10, with the exception that 
core legs 22 and 24 and core 44 of sensor 100 define sections each of 
which includes two opposed converging sides defining rounded contact 
surfaces 112, 114 and 116, respectively. Contacts 112, 114 and 116, 
facilitate testing the teeth and roots of metal gears, as is shown in 
FIGS. 6 and 7. 
To use sensor 10, contacts 38, 40 and 48 are brought into contact with the 
metal piece. A button 80, located on the exterior of case 12, is depressed 
to begin a test cycle. Depression of button 80 causes a predetermined 
number of cycles of the energizing electrical signal to be imposed across 
energizing coil assembly 14. Energizing coil assembly 14 causes a 
periodically time-varying magnetic field to be established in the metal 
piece. Sensing coil assembly 16 detects the magnetization jumps generated 
within the metal piece and converts them to an electrical signal. A 
conven-tional averaging circuit averages the level of the Barkhausen noise 
generated during the test cycle and provides the averaged signal to 
circuitry which processes and displays the noise in any desired known 
fashion. 
Generally, sensor 100 is used in the same manner as that employed with 
sensor 10. The length of the test cycle should be chosen to ensure that a 
statistically sound value of Barkhausen noise is obtained. 
An indicator light can be provided to indicate that a test cycle is in 
progress. 
FIGS. 9 through 16 show sensor 200--another preferred embodiment of the 
present invention. Sensor 200 is particularly useful for dynamic testing, 
which is desirable where a very large number of locations is tested. 
Sensor 200 includes a housing assembly 202 which houses a magnetic 
assembly 204. 
Housing assembly 202 includes a housing frame 206. A wheel plate 208 is 
mounted to the outside surface of each housing frame side panel 210 by 
four nut and bolt assemblies 212 which are assembled through openings 
formed in wheel plates 208 and side panels 210. Openings 214 formed in 
wheel plates 208 are elongated and permit vertical repositioning of each 
wheel plate 208 with respect to side panels 210. 
A wheel assembly 256 is mounted to each wheel plate 208. Accordingly, 
vertical adjustment of wheel plate 208 with respect to housing frame 206 
achieves adjustment of the distance between base plate 244 and the metal 
piece in contact with wheel assembly 256. Each wheel assembly 256 includes 
a wheel mount 220 which is secured to wheel plate 208 by a threaded wheel 
shaft 216. Shaft 216 is threaded into a threaded opening 222 formed in 
wheel mount 220 and opening 218 of wheel plate 208. Wheel mount 220 
defines a recess 223 which receives head 221 of shaft 216. A rubber ring 
225 is disposed around the outer surface of wheel mount 220 between flange 
226 defined by wheel mount 220 and wheel plate 208. Rubber ring 225 
dampens a substantial portion of the mechanical vibrations created when 
wheel assemblies 256 are rolling on a metal piece as it is being examined. 
Rubber ring 225 defines annual flanges 227 between which a ball bearing 
224 is disposed on rubber ring 225. A wheel can be mounted on ball bearing 
224. However, in sensor 200 a rubber tire 230 is disposed around the outer 
ring 228 of bearing 224 to eliminate the need for a wheel. Tire 230 aids 
in dampening mechanical vibrations created by wheel assemblies 256 as 
sensor 200 is used. 
Magnet assembly 204 includes energizing coil assembly 232 and sensing coil 
assembly 234. Energizing coil assembly 232 includes energizing core 236 
constructed of ferrite. An electrical conductor is wound around leg 238 of 
energizing core 236 to form an energizing coil 240. Legs 242 of energizing 
core 236 are secured within base 244 of housing frame 206. Legs 246 are 
secured to legs 242 of energizing core 236. Legs 246 provide structural 
support for energizing core 236. 
Sensing coil assembly 234 includes a cylindrical sensing potcore 248 
constructed of ferrite material within which an electrical conductor is 
wrapped to form sensing coil 250. Sensing coil assembly 234 is secured 
within an opening formed in base plate 244. 
Sensor 200 operates in the same manner as sensor 10 except that a uniform 
magnetic field is maintained in the metal piece by wheel assemblies 256. 
Wheel assemblies 256 maintain an air gap of a substantially constant depth 
between the metal piece and sensing and energizing cores 248 and 236, 
respectively. Sensor 200 is adapted to roll along a metal piece on wheel 
assemblies 256 to continuously create Barkhausen noise along the surface 
of the metal piece. Sensing coil assembly 234 converts the Barkhausen 
noise to electrical noise which is provided along cable 258, which also 
provides magnetizing current for energizing coil assembly 232 and noise 
communication to the Barkhausen system. Sensor 200 can be used to 
investigate any metal piece which spans the distance between wheel 
assemblies 256. Continuous scanning can be achieved by either moving 
sensor 200 along the metal piece, by moving the metal piece with respect 
to sensor 200, or a combination of both. The dimensions of sensor 200 
depend in part on the size of the metal pieces to be tested. If the 
dimensions of sensor 200 must be made very small, the two wheel assemblies 
256 can be replaced by one wheel assembly or by a sheet of nonmagnetic, 
wear-resistant material secured to the bottom surface of base plate 244 
and adapted to contact the metal piece. Alternately, sensor 10 could be 
converted to a dynamic sensor by encasing in epoxy the exposed ends of 
members 22, 24 and 44, shown best in FIG. 3, to provide a spacing member 
adapted to contact the metal piece. In such a case, the encased ends of 
member 22, 24 and 44 would define a shape similar to that shown in FIG. 8. 
FIGS. 17 through 22 show a dynamic sensor 300, a further preferred 
embodiment of the present invention. Sensor 300 is particularly useful in 
applications where it is inconvenient or impossible to reorient the 
Barkhausen sensor with respect to the metal piece under observation to 
reorient the magnetic field induced in the metal piece by the energizing 
coil of the sensor. The energizing coil assembly of sensor 300 is mounted 
for rotation with respect to the housing sensor 300. 
Sensor 300 includes a housing assembly 302, which includes a stationary 
portion 304 and a rotatable portion 306. Portion 306 may be manually 
rotated within stationary housing 304 to a new position and is there held 
in place by set screw 331. Wheel assemblies 308 are identical to wheel 
assemblies 256 of sensor 200 and are secured to sensor 300 in the same 
manner as wheel assemblies 256 are secured to sensor 200. 
Rotatable housing 306 contains an energizing coil assembly 310 and a 
sensing coil assembly 312, both of which can be identical to sensing coil 
assembly 234 and energizing coil assembly 236, respectively, of sensor 
200. Coil assemblies 310 and 312 are mounted to base plate 314 of 
rotatable housing 306 in the same manner as coil assemblies 232 and 234 
are mounted to base 244 of sensor 200. Legs 316 are secured to legs 320 of 
energizing coil assembly 310. 
A guide wheel assembly 322 is mounted to the front of sensor 300 to 
facilitate rotation of sensor 300 with respect to the metal piece under 
investigation, if desired. Fork 324 of assembly 322 is mounted for 
rotation about a vertical axis to a wheel mounting 326 which is secured to 
stationary housing 304. Wheel 328 is mounted for rotation on a shaft 330 
between the legs of fork 324. Vibrations of the wheel assembly 322 have 
been damped in the same way as in the wheel assembly 256. Wheel assembly 
322 is identical to wheel assembly 308 with the exception that it includes 
a second wheel mount. 
Sensor 300 can be used in a fashion that is identical to use of sensor 200. 
However, if the directional characteristics of stress in a metal piece are 
desired to be determined or if the orientation of the energizing magnetic 
field within the metal piece is desired to be reoriented for some other 
reason, sensor 300 facilitates such reorientation. Guide wheel assembly 
322 can be used to change the direction of travel of sensor 300 over the 
metal piece as the metal piece and sensor 300 move with respect to each 
other to reorient the energizing magnetic field with respect to the metal 
piece or to measure a full circle by rotating sensor 300 over one spot. 
Further, rotatable housing 306 can be rotated with respect to the 
stationary housing 304 to reorient the energizing core of energizing coil 
assembly 310 with respect to the motion of the metal piece relative to 
sensor 300, and therefore reorient the energizing magnetic field. Further, 
the position of rotatable housing 306 can be adjusted relative to 
stationary housing 304 to adjust the depth of the air gap between the 
metal piece and sensor 300. 
Generally, as is known in the art, the resolution of the Barkhausen system 
depends on the frequency of the magnetic field induced in the metal piece 
and the relative speed between the sensor and the piece. Generally, at 
least one cycle of the magnetic field must be able to occur over the area 
of a defect in a metal piece before the system can detect the defect. The 
following table is offered as a general guideline for selecting the system 
magnetizing frequency: 
______________________________________ 
MAGNETIZING FREQUENCY (HZ.) 
HIGHEST RELATIVE 
SPEED-(meters/second) 
2 cm. resolution 
4 cm. resolution 
______________________________________ 
1 50 25 
5 250 125 
10 500 250 
______________________________________ 
The electrical noise caused by the mechanical vibration of the rolling 
wheels of sensors 200 and 300 also depends on the relative speed between 
the metal piece and the sensor. The Barkhausen system must filter this low 
frequency noise caused by the mechanical vibration. As a general guide, 
the system must eliminate all frequencies below 4 kHz for a sensor rolling 
at 1 m/sec., 10 kHz for a sensor rolling at 5 m/sec., and 20 kHz for a 
sensor rolling at 10 m/sec. 
As is shown in FIG. 23, sensors 200 and 300 can be adapted to the 
inspection of moving steel sheets, if the vibrations of the wheel are 
eliminated by a deflection drum 500 made of nonmagnetic material. In such 
an application, if the sheet is moving at a high speed, it is convenient 
to maintain the air gap with means other than wheels. In any application 
where relative movement of the sensor and the test piece occurs at high 
speed, proper spacing between the energizing and sensing coil assemblies 
and the test piece can be accomplished by suitably creating an air cushion 
between the sensor and the test piece. In such an application, the sensor 
would be rigidly mounted to a stationary object.