INDUCTION HEATING METHOD IMPLEMENTED IN A DEVICE INCLUDING MAGNETICALLY COUPLED INDUCTORS

• US 20120199579A1
• Filed: 10/19/2010
• Published: 08/09/2012
• Est. Priority Date: 10/19/2009
• Status: Active Grant

• Current Assignee: Centre National De La Recherche Scientifique, Electricitã© De France, Institut National Polytechnique de Toulouse
• Original Assignee: Centre National De La Recherche Scientifique, Electricitã© De France, Institut National Polytechnique de Toulouse
• Inventors: Neau, Yves, Maussion, Pascal, Pateau, Olivier, Lefevre, Yvan, Ladoux, Philippe, Manot, Gilbert
• Granted Patent: US 9,398,643 B2
• Time in Patent Office: Days
• US Class Current: 219/672
• CPC Class Codes: H05B 6/06 Control, e.g. of temperature, of power
• CPC Class Codes: H05B 6/08 using compensating or balancing arrangements
• CPC Class Codes: H05B 6/104 metal pieces being elongated like wires or bands
• CPC Class Codes: H05B 6/40 Establishing desired heat distribution, e.g. to heat particular parts of workpieces
• CPC Class Codes: H05B 6/44 having more than one coil or coil segment

Abstract

Provided is an induction heating method implemented in a device for heating a metal part, the device including magnetically coupled inductors. Oscillating circuits of the device have at least approximately the same resonance frequency, each inverter is controlled by a control unit to vary amplitude and phase of current passing through the corresponding inductor, the device also including a means for determining said current and an actual temperature profile of said part. The method includes: a) comparing said actual temperature profile with a reference temperature profile and calculating a reference power density profile; b) calculating target currents which the inverters must produce in order for the currents of the inductors to reach suitable target values; c) determining the currents passing through the inductors to compare said currents with said target values and determine correction current deviations, and sending correction instructions to said control units in accordance with said current deviations.

Claims

1. An induction heating method implemented in a device for heating a metal part, the device including magnetically coupled inductors, each inductor being powered by a dedicated inverter associated with a capacitor such as to form an oscillating circuit, said oscillating circuits having at least approximately the same resonance frequency, each inverter being controlled by a control unit such as to vary the amplitude and the phase of the current passing through the corresponding inductor, the device also including means for determining said current as well as means for determining an actual temperature profile of said metal part, said method including comprising the following steps:

a) comparing said actual temperature profile with a reference temperature profile and calculating a profile of the reference power density which the heating device must inject into said part in order to achieve said reference temperature profile;

b) from a matrix of impedances determined by knowledge of the electromagnetic relationships linking said inductors with each other and with said part and by knowledge of vector image functions representing the relationships between the current densities created by the inductors and the currents passing through the inductors, calculating the target currents which the inverters must produce in order for the currents of the inductors to reach target values that are suitable for injecting said reference power density profile into said part;

c) determining the currents passing through the inductors in order to compare them with said target values and to determine current deviations to be corrected, and sending correction instructions to said control units in accordance with said current deviations in order to control the inverters such as to correct the currents passing through the inductors.

2. The heating method according to claim 1, wherein the capacitances of said capacitors are determined, and said matrix of impedances is associated with a vector of the capacitances.

3. The heating method according to claim 1, wherein the initial value of said matrix of impedances is determined for a given initial average temperature of said inductors and of said part, then the matrix of impedances modified for at least one increased value of said average temperature is determined at variable or periodic intervals, and said modified matrix of impedances is used for recalculating said target values.

4. The heating method according to claim 1, wherein after having successively carried out steps (a) and (b), step (c) is carried out at least once in order to reduce the current deviations to be corrected, then steps (a), (b) and (c) are reiterated at least once on updating said actual temperature profile with temperature measurements in different heated zones of the part.

5. The heating method according to claim 1, wherein for the determination by calculation of said target values in step (b), because of knowledge of said vector image functions, image functions of the power densities are calculated according to the spatial characteristics of the zones of the part into which said power densities are injected, and an optimized vector of the target currents to be determined is calculated by minimizing the difference between each of said image functions of the power densities and a reference power density function corresponding to said reference power density profile.

6. The heating method according to claim 1, wherein an inverter having, in comparison with the other inverters, the highest current in the case of a current inverter or the highest voltage in the case of a voltage inverter is chosen as the reference inverter and shift angles are introduced in the controls of the other inverters with respect to a control angle of the reference inverter.

7. The heating method according to claim 6, wherein the reference inverter is adjusted with a duty cycle equal to ⅔

, in order to reduce the harmonic interference created by this inverter on its neighbours.

8. The heating method according to claim 6, wherein the RMS value of the current in said reference inverter is adjusted by acting on a DC power supply which powers the inverters.

11. The heating method according to claim 3, wherein after having successively carried out steps (a) and (b), step (c) is carried out at least once in order to reduce the current deviations to be corrected, then steps (a), (b) and (c) are reiterated at least once on updating said actual temperature profile with temperature measurements in different heated zones of the part.

12. The heating method according to claim 3, wherein for the determination by calculation of said target values in step (b), because of knowledge of said vector image functions, image functions of the power densities are calculated according to the spatial characteristics of the zones of the part into which said power densities are injected, and an optimized vector of the target currents to be determined is calculated by minimizing the difference between each of said image functions of the power densities and a reference power density function corresponding to said reference power density profile.

13. The heating method according to claim 3, wherein an inverter having, in comparison with the other inverters, the highest current in the case of a current inverter or the highest voltage in the case of a voltage inverter is chosen as the reference inverter and shift angles are introduced in the controls of the other inverters with respect to a control angle of the reference inverter.

14. The heating method according to claim 13, wherein the reference inverter is adjusted with a duty cycle equal to ⅔

, in order to reduce the harmonic interference created by this inverter on its neighbors.

15. The heating method according to claim 13, wherein the RMS value of the current in said reference inverter is adjusted by acting on a DC power supply which powers the inverters.

16. The heating method according to claim 4, wherein for the determination by calculation of said target values in step (b), because of knowledge of said vector image functions, image functions of the power densities are calculated according to the spatial characteristics of the zones of the part into which said power densities are injected, and an optimized vector of the target currents to be determined is calculated by minimizing the difference between each of said image functions of the power densities and a reference power density function corresponding to said reference power density profile.

17. The heating method according to claim 4, wherein an inverter having, in comparison with the other inverters, the highest current in the case of a current inverter or the highest voltage in the case of a voltage inverter is chosen as the reference inverter and shift angles are introduced in the controls of the other inverters with respect to a control angle of the reference inverter.

18. The heating method according to claim 16, wherein an inverter having, in comparison with the other inverters, the highest current in the case of a current inverter or the highest voltage in the case of a voltage inverter is chosen as the reference inverter and shift angles are introduced in the controls of the other inverters with respect to a control angle of the reference inverter.

19. The heating method according to claim 7, wherein the RMS value of the current in said reference inverter is adjusted by acting on a DC power supply which powers the inverters.

9. An induction heating device comprising:

magnetically coupled inductors, each inductor being associated with a capacitor in order to form an oscillating circuit, said oscillating circuits having at least approximately the same resonance frequency;

inverters, each powering a dedicated inductor, each inverter being controlled by a control unit in such a way as to vary the amplitude and the phase of the current passing through the corresponding inductor;

further comprising;

means of determination of the currents passing through the inductors as well as means of determination of an actual temperature profile of a metal part heated by the device;

means of comparison of said actual temperature profile with respect to a reference temperature profile;

means of calculating a reference power density profile that the heating device must inject into said part in order to achieve said reference temperature profile;

means of calculating, based on knowledge of a matrix of the impedances, target currents that the inverters must deliver in order that the inductor currents reach appropriate target values for injecting said reference power density profile into said part;

means of comparison of the currents passing through the inductors with respect to said target values, capable of determining current deviations to be corrected, and means of processing said current deviations capable of generating correction instructions sent to said control units for controlling the inverters in such a way as to correct the currents passing through the inductors.

10. The induction heating device according to claim 9, wherein the inverters are powered by the same current source or voltage source power supply, and in which wherein said means of comparison of said determined currents passing through the inductors include comparator units each receiving determined parameters of a current passing through an inductor and parameters of the corresponding target values and each being connected to a unit for processing said current deviations, one of said comparator units furthermore receiving parameters representative of what said power supply delivers and its associated processing unit being adapted to generate regulation instructions sent to said power supply in order to modify the current or the voltage that it delivers.