Process and apparatus for the homogeneous, electromagnetic induction heating with transverse flux of conducting and non-magnetic flat products

A process and apparatus for inductive heating of flat, thin, conductive, nonmagnetic products of variable dimensions. A plurality of currents are inductively generated in the product in such a way that elementary current arrays are formed in the product in both the longitudinal and lateral directions. Current arrays of local heating heterogeneity, each comprising at least one of the elementary current arrays, are defined, and the intensities of the inductively generated current in the arrays of local heating heterogeneity are controlled as a function of the volume of the array of local heating heterogeneity with which they are associated, so that the average value of power dissipated per unit volume in each array of local heating heterogeneity is approximately the same as for all other arrays. The apparatus specifically includes an inductor with individually controllable coils (poles) arranged to extend longitudinally and laterally over the area of the product to be heated. The positions of the boundaries of the product, together with other product data and desired heating data, are used to control the individual coils of the inductor so as to control the currents induced in the product as a function of the relative positions of the product boundaries and the individual coils.

BACKGROUND OF THE INVENTION 
The present invention relates to a process and apparatus for the 
homogeneous heating of thin, conducting and nonmagnetic products of 
variable dimensions through the use of a transverse electromagnetic flux. 
Processes and devices for the electromagnetic induction heating of thin 
products with transverse flux are known. Such known processes and devices 
insure a relative homogeneity of heating only by the advance of the 
product, which reduces their application to strip. 
Furthermore, in one known system, control of heating as a function of width 
is effected mechanically. Temperature differences generated in the course 
of heating are large and may result in deformations of the product. In 
other known systems, there is no regulation of the homogeneity of heating 
over the width. 
OBJECTS AND BRIEF SUMMARY OF THE INVENTION 
The principal object of the present invention is to effect the homogeneous 
heating at rest of a flat product having two finite dimensions, regardless 
of the magnitude of the dimensions, for example in the course of the 
manufacture of a range of sheet metal. 
Relatively simple arrangements also are provided for the appropriate 
adaptation of the process to a displacement of the sheet or to the use of 
the technique for the heating of a strip, for example. 
In accordance with the invention, heating is obtained by the principle of 
transverse flux, electromagnetic induction as applied to conducting, 
nonmagnetic products. 
The present invention is concerned more particularly with a process for the 
transverse flux, electromagnetic heating of conducting, nonmagnetic, flat 
products in order to obtain a homogeneity of temperature, characterized 
by: 
generating in the product, currents included inside elementary arrays of 
juxtaposed currents, and 
defining arrays of local heating heterogeneity, each comprising at least 
one of the elementary arrays, and 
regulating the intensity of each of the currents as a function of the 
volume of the array of local heating heterogeneity to which such currents 
correspond, in a manner such that the average value of power dissipated 
per unit volume in each array, including the arrays of local heating 
heterogeneity, is the same over the entire product. 
The process according to the invention further includes: 
using means to create an alternating magnetic field, said means being 
designated inductors and consisting of conductors forming current loops; 
and 
controlling the intensities traversing these conductors such that at least 
some of the currents are controlled independently to each other so that 
the control of one with respect to another is a function of at least one 
of the dimensions of the product. 
The process according to the invention further includes, depending on the 
situation: 
determining the position of the product with respect to the inductor and, 
particularly, its boundaries with respect to the inductors; 
defining the rise in temperature to be effected; 
employing a computer; 
providing the temperature rise and position data to the computer to derive 
the values of the current intensities to be circulated in the different 
poles of the inductors as a function of the characteristics of the product 
and the heating desired; 
controlling, by the values of the current intensities calculated and 
through the use of a source which may be variable in frequency, the 
current intensities of each pole or group of poles of the inductors. 
The invention further concerns an apparatus for the electromagnetic 
alternating transverse flux heating of conducting, nonmagnetic, flat 
products in order to obtain a homogeneity of temperature, comprising at 
least one inductor, which consists of conductors forming a lattice of 
current loops and a magnetic circuit reinforcing the effectiveness of the 
apparatus, which effects the process according to the invention and is 
characterized in that it further comprises: 
means to determine the position of the product with respect to the inductor 
and, particularly, the position of its boundaries; 
means to define the rise in temperature to be effected; 
means to observe the temperature of the product; 
means connected to the position determining, temperature defining and 
temperature monitoring means to determine the current intensities to be 
circulated in the different loops of the inductors as a function of the 
characteristics of the product and the heating desired; 
means connected with the current determining means and with the inductors 
capable of generating the current intensities determined in this manner.

DETAILED DESCRIPTION 
The process according to the invention comprises the generation of currents 
(m) in the product, the currents being within arrays having dimensions and 
configurations resulting from the spatial variations of alternating 
magnetic fields to which the product is exposed. The intensities of the 
currents in each array are controlled such that the average value of the 
power dissipated per unit volume in each array is the same over the entire 
product. 
Local heating homogeneity within each current array is insured by 
conduction and depends directly on the size of the array. 
The boundaries (ends and edges), in general, are not compatible with a 
predetermined spatial distribution of the magnetic field, as the 
dimensions of the product are variable or the expansion due to heating 
causes an appreciable variation of said dimensions. At the boundaries, 
therefore, the elementary current arrays generated are not always those 
which would exist in the case of an infinite product. 
For an identical magnetic field current of excitation (b), the average 
power per unit volume dissipated in one of these boundary arrays is 
different from that dissipated in an infinite product. Moreover, certain 
arrays close to the boundary arrays may be perturbed. Such arrays are 
therefore referred to herein as arrays of local heating heterogeneity. 
In an infinite product, an array of local heating heterogeneity always 
merges with the elementary array of induced current (m). In other words, 
there are no edge induced distrubances and the arrays are all homogeneous 
as discussed herein. 
According to the present process for heating finite products, in order to 
obtain the same average value of the heating power per unit volume in the 
elementary arrays of the boundaries as in the reset of the product, arrays 
of local heating heterogeneity along the boundary, each comprising one or 
several juxtaposed elementary arrays, are defined. The control of the 
power dissipated in each array of local heterogeneity is effected by the 
regulation of the intensity of current loops (b) facing this array of 
local heterogeneity thus defined. 
According to a particular embodiment, each array of local heating 
heterogeneity is defined by an elementary array. The current loops of the 
inductor not facing the product, i.e., those inductor loops having no part 
of the product directly over or under them, are extinguished or 
deenergized. 
An apparatus embodying the process according to the invention comprises: 
means (A) to generate an alternating magnetic field, preferably inductors, 
that comprises conductors which form current loops traversed by variable 
current intensities and magnetic circuits enhancing the effectiveness of 
the device; and, depending on the specific case: 
means (B) to determine the position of the product with respect to the 
inductor and, in particular, the position of its boundaries; 
means (C) to define the rise in temperature to be effected; 
means (D) to monitor the temperature of the product; 
means (E) connected with the aforementioned means to determine the current 
intensities to be circulated in the different "loops" of the inductors as 
a function of the characteristics of the product (F) and the heating 
effect desired; 
means (G) possibly connected with the latter and with the inductors, 
capable of generating the intensities determined in this manner. 
In a preferred form of embodiment of the invention, the heating device 
comprises identical, horizontal inductors (A1 and A2) facing each other, 
placed on both sides of the product (F) to be heated (FIG. 1). Each of the 
inductors comprises conductor windings (1) of a square configuration, 
placed regularly in accordance with an identical polar pitch in two 
orthogonal directions. In each of these directions, at each given instant, 
the current loops (b) of the conductor windings formed in this manner 
constitute a succession of alternating North and South magnetic poles 
(FIGS. 2 and 3). The closure of magnetic fluxes, to reinforce the 
efficiency of the apparatus, is insured by a magnetic circuit (2), 
possibly of a laminated construction. This closure may be effected in one 
of the aforementioned directions as illustrated, or both, if desired. 
Closure in a single direction renders the control of the variation of the 
profile of the field in the orthogonal direction simpler, as the 
interactions between poles of two lines parallel to the direction to the 
closures are weaker (FIG. 4). 
The size of the pole is conventionally determined as a function of the 
maximum heating power per unit volume to be obtained, the thermal 
conductivity of the product and the maximum temperature difference 
permissible in the product during heating. However, the temperature 
differences in the product may be reduced, at the termination of the 
heating, by a reduction of the power per unit volume to which the 
termination differences are proportional in the first case. 
The frequency of the power supply of the apparatus conforms to two 
objectives: 
an appreciable improvement of yield in the case where the industrial 
frequency is not adopted; 
electromagnetic support of the product treated, each of which may be of 
different thickness, resistivity and specific gravity. Adjustments of the 
frequency may thus be necessary to take into account variations of these 
parameters. 
The aforedescribed variation of the magnetic field further provides a 
stable support of the product between the inductors. 
Displacement of the product with respect to the inductors may be obtained 
by varying the profile of the magnetic field (reduction of intensity in 
the direction of the displacement) or by the addition of windings to 
constitute a linear, triphase motor, the latter devices being known in 
themselves. 
The position of the product with respect to the inductors is known, for 
example by its entry position and the displacements effected. 
From the position of the product (B, FIG. 5), in particular the position of 
its boundary with respect to the poles of the inductor, and from the 
characteristics of the product, a computer (E) derives the value of the 
current intensities that must be passed through the poles to obtain 
homogeneous heating. These current intensities are substantially equal 
over the major portion of the product; they are different only for the 
poles close to the boundaries of the product. In the case of products much 
longer than wide, the embodiment may be simplified by controlling only the 
series of poles parallel to the edges of the product, with the relative 
variations in intensity thus concerning only two or three rows along each 
side edge of the product. 
From the value of the intensities calculated, the current intensities in 
each pole or group of poles are regulated by a suitable control device (G) 
connected to a source (S), the frequency of which may be variable. 
The rise in temperature desired may be obtained from entry in the computer 
(E) of a stored record of the temperature desired (C) and a measurement of 
the actual temperature (D) of the product, which values are compared by 
the computer (E). 
In another embodiment (FIG. 5A), a function generator develops the mean 
desired temperature function of the product with respect to time and this 
temperature function is stored or used as it is developed. The computer 
(E) compares this temperature function record (C) with a temperature of 
the product calculated by the summation or integration of the heating 
already effected, to furnish the current intensity parameters required to 
obtain the temperature function desired. 
To complement this approach, the calculated temperature may be compared 
with a measure of the actual temperature of the product to avoid slow 
integration drifting, or actual temperature may be used for direct control 
thus providing an automatic adaption of the mathematic heating model used 
by the computer. 
In a form of embodiment of the invention more adapted to the heating of 
strip, the inductors consist of poles (3) of an elongated configuration 
(FIGS. 6 and 7), which, at a given instant, are alternatingly north and 
south. 
At the outlet of the inductors (FIG. 8) a temperature sensor (4) placed to 
face each of the poles, permits the regulation of the current of the 
corresponding pole as a function of an assigned temperature (C). In this 
manner, variations in the width and the position of the product (F) are 
taken into account implicitly. Poles which do not face the product are 
extinquished (deenergized). 
As an alternative to using temperature sensors to sense the actual product 
temperature at each transverse pole location, a computer may be used to 
determine the different current intensities required to obtain a correct 
transverse heating profile and, in response to a single temperature sensor 
and the computed intensity required for the profile, the total level of 
current intensities may be controlled. 
In a preferred form of embodiment, the products treated are rectangular. 
The length and the width of the product are inputs in the principal 
computer. As the principal axis of the product is preferably parallel to 
the heating device, knowledge of the position of one of the points of the 
product, for example the center, with respect to the heating device makes 
it possible to determine completely the position of the product 
(particularly that of its boundaries) with respect to the inductor. 
For this purpose, upon its arrival, the product is placed symmetrically 
with respect to the two perpendicular axes known. The displacement of the 
product is effected by the successive extinction of rows of adjacent 
transverse poles, thus step by step, by a distance equal to a polar step. 
The computer is incremented at each extinction and thereby yields the 
position of the center at each instant. 
The increase in temperature is known, for example, by the integration as a 
function of time of the ratio of power per unit volume (determined by the 
computer) to the specific heat. It may be verified by measuring the 
temperature of the product with a contact thermometer. 
It should be understood, finally, that the present invention has been 
described and illustrated only in connection with preferred examples and 
that equivalent substitutions may be made in its constituting elements 
without otherwise departing from the scope of the invention.