Electricity: measuring and testing – Magnetic – With means to create magnetic field to test material
Reexamination Certificate
1999-11-09
2001-08-14
Oda, Christine (Department: 2862)
Electricity: measuring and testing
Magnetic
With means to create magnetic field to test material
C324S239000, C702S170000
Reexamination Certificate
active
06275030
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method for describing an electromagnetic signal generated by eddy currents in a conductive material.
Such a method can be used inter alia in determining properties of an electrically conductive measuring object composed of that material. This invention relates in particular to a method for determining properties of an electrically conductive measuring object, wherein:
a. utilizing at least one transmitting antenna, an electromagnetic field changing over time is emitted to the measuring object for generating eddy currents in the object;
b. utilizing at least one receiving antenna, the electromagnetic signal generated by the eddy currents is detected; and
c. on the basis of the detected electromagnetic signal, the properties of the measuring object are determined.
This invention further relates to an apparatus for practising such a method.
2. Description of Related Art
Such a method and apparatus are known from, for instance, U.S. Pat. No. 4,843,319. In this known method and apparatus, by means of a transmitting coil, a pulsated electromagnetic field is generated in the material of the measuring object. This gives rise to time-dependent eddy currents in the material. These eddy currents are detected by means of a receiving coil. The eddy currents, which change with time, cause a changing magnetic flux through the receiving coil, so that an induction voltage prevails across the receiving coil. Utilizing an amplifier, this changing induction voltage can be registered as a function of time. Thus, the electromagnetic signal generated by the eddy currents is detected as a function of the time t.
With the known apparatus, it is stated that the time-dependent behavior of the signal for small times t is determined by a constant logarithmic rate of decay of about 1.5. In other words, the received signal can be described with a signal V(t), to which the following relation applies: d(1n V)/d(1n t)=−1.5.
Beyond a certain critical time, designated &tgr; and which is directly proportional to the square of the thickness of the surface of the material of the object under examination, so that &tgr;=cd
2
, the logarithmic rate of decay falls to a lower value which equals A−2.171n(t). In other words, the following applies where t is greater than &tgr;: d(1n V)/d(1n t)=A−2.171n(t). Here A is determined by the material properties and the geometry of the measuring object. Accordingly, before the thickness of the measuring object can be determined, first the constants c and A are to be determined. Determining the constants c and A is carried out through two measurements on two different test objects of the same material but having different, homogeneous wall thicknesses. This means that for carrying out the method, at all times two mutually different test specimens are to be at hand. Further, in this known method and apparatus, from a single measurement, only the homogeneous wall thickness of the material can be computed. Furthermore, the apparatus is limited to the use of a receiving coil for measuring the signal.
SUMMARY OF THE INVENTION
The invention contemplates a solution to the disadvantages outlined above. Accordingly, an object of the invention is to provide a method and apparatus wherein for detecting the wall thickness of a measuring object, only a single measurement on a test object is to be performed beforehand. Another object of the invention is to make it possible to determine the distribution of wall thicknesses of the material of the measuring object. A further object of the invention is to determine, instead of the wall thicknesses, the permeability and the conductivity of the material of the measuring object. It is even possible to determine the spread in the conductivity or the spread in the relative permeability of the material. In order to provide a basis for carrying out such methods, the method for describing an electromagnetic signal generated by eddy currents in an electrically conductive material is characterized, according to the invention, in that the signal V(t) is described by at least one product of two factors F and G(t), where F is a function of the geometry of the material and the electrical and magnetic properties of the material, and where G(t) is a function of the geometry of the material, the electrical and magnetic properties of the material, the thickness perpendicular to the surface of the material and time.
The method according to the invention, in which said description of the signal is utilized for determining properties of an electrically conductive measuring object, is characterized in that utilizing a predetermined algorithm, parameters
σ
i
μ
i
or parameters to be derived from these parameters of the equation
V
(
t
)
=
∑
i
=
1
n
θ
i
F
i
G
i
(
t
)
(
1
)
or equation to be derived therefrom, with
F
i
=
δ
σ
i
μ
i
(
2
)
G
i
(
t
)
=
t
γ
1
+
α
(
β
t
τ
i
)
m
ⅇ
β
t
τ
i
(
3
)
i
=1, 2
, . . . n
(4)
and
∑
i
=
1
n
θ
i
=
1
(
5
)
are selected such that V(t) according to a predetermined criterion of the algorithm corresponds to the course over time of the detected electromagnetic signal, where &agr;, &bgr;, &ggr;, &dgr; and m are real numbers which are dependent on the geometry of the measuring object, the transmitting antenna and the receiving antenna, as well as on the relative positions of the object, the transmitting antenna and the receiving antenna, &mgr;
i
represents the magnetic permeability of an area i of the measuring object and &sgr;
i
represents the electrical conductivity of the area i of the measuring object, and the areas i (i=1, 2, . . . , n) together generate the detected electromagnetic signal.
When with this method the thickness d
i
of the material is to be determined, it is necessary only once, using a test object, to determine the magnetic permeability and the electrical conductivity of the material. The values of &agr;, &bgr;, &ggr;, &dgr; and m can, in principle, given a known geometry of the transmitting antenna, receiving antenna and the object, be priorly calculated on the basis of a simulation model. When the conductivity or relative permeability is known, it is possible, using the method outlined above, to determine the wall thickness of the material on the basis of the formula &tgr;
i
=&mgr;
i
&sgr;
i
d
i
2
with i=n=1. It is also possible to determine the distribution of wall thicknesses of the material, given a known conductivity and a known relative permeability of the material, with n≧2.
The invention further makes it possible to determine the conductivity and the relative permeability of the material, given a known wall thickness. It is even possible to determine the spread in the conductivity or the spread in the relative permeability of the material, given a known wall thickness.
REFERENCES:
patent: 3597678 (1971-08-01), Fearon
patent: 5339256 (1994-08-01), Levy et al.
patent: 5525903 (1996-06-01), Mandl et al.
patent: 5680042 (1997-10-01), Griffen et al.
Jolly Anthony
Oda Christine
Rontgen Technische Dienst B.V.
Varnum Riddering Schmidt Howlett LLP
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