Resonant frequency tracking system and method for use in a radio frequency (RF) power supply

• US 6,730,894 B2
• Filed: 11/25/2002
• Issued: 05/04/2004
• Est. Priority Date: 07/10/1998
• Status: Expired due to Term

• Current Assignee: Ambrell Corporation (inTEST Corporation)
• Original Assignee: Ameritherm, Inc. (inTEST Corporation)
• Inventors: Thompson, Leslie L., Lincoln, Daniel J., Schwenck, Gary A.
• Primary Examiner(s): Leung, Philip H.
• Application Number:
  US10/302,981
• Publication Number:
  US 20030071034A1
• Time in Patent Office: 526 Days
• Field of Search: 219/663, 219/665, 219/666, 219/667, 219/661, 219/670, 219/650, 219/779, 219/702, 219/715, 219/716, 363/95, 363/97, 363/17, 323/234, 323/265, 323/285, 323/293
• US Class Current: 219/666
• CPC Class Codes: H02M 7/48 using discharge tubes with control electrode or semiconductor devices with control electrode
• CPC Class Codes: H05B 6/04 Sources of current
• CPC Class Codes: H05B 6/06 Control, e.g. of temperature, of power

Abstract

An RF power supply that is capable of tracking rapid changes in the resonant frequency of a load and capable of quickly responding to varying load conditions so as to deliver the desired amount of power. The present invention also provides an RF power supply capable of delivering a wide range of power over a broad frequency range to a load that is remotely located from the power supply. According to one embodiment, the RF power supply includes a direct current (DC) voltage source that provides a DC voltage within a predetermined voltage range; an amplifier, coupled to the DC voltage source, that provides an alternating voltage to a tank circuit connected to an output of the RF power supply; a frequency controller, coupled to the amplifier, to set the frequency of the alternating voltage produced by the amplifier; and a sensor, coupled to the load, to provide a signal to the frequency controller, where the frequency controller sets the frequency of the alternating voltage based on the signal received from the sensor.

Claims

1. A method for heating a workpiece using a radio frequency (RF) field, the method comprising the steps of:

providing an RF signal having a frequency and a first voltage level to a circuit having an initial resonant frequency, wherein the RF signal and the circuit are used in producing the RF field;

determining an estimate of the initial resonant frequency of the circuit after providing the RF signal to the circuit;

setting the frequency of the RF signal to the determined estimate of the resonant frequency;

increasing the voltage level of the RF signal from the first voltage level to a second voltage level after determining the estimate of the initial resonant frequency; and

tracking the resonant frequency of the circuit at least during a portion of a period during which the workpiece is being heated, wherein the tracking step comprises the steps of sensing power applied to the circuit, sensing power reflected from the circuit, and modifying the frequency of the RF signal so that the frequency of the RF signal tracks the resonant frequency.

2. The method of claim 1, wherein the first voltage level is not more than about 10% of the second voltage level.

3. The method of claim 1, wherein the circuit comprises a radiator, wherein the radiator is used to generate an RF field for use in heating the workpiece.

4. The method of claim 3, where the radiator comprises an induction coil.

5. The method of claim 1, wherein the modifying step comprises determining whether to increase or decrease the frequency of the RF signal, wherein the determination is a function of, at the least, the sensed power applied to the circuit and the sensed reflected power.

6. A method for heating a workpiece using a radio frequency (RF) field, comprising the steps of:

applying an RF signal having a frequency and a first voltage level to a circuit having an initial resonant frequency, wherein the RF signal and the circuit are used in producing the RF field;

determining a coarse estimate of the initial resonant frequency of the circuit;

determining a fine estimate of the initial resonant frequency of the circuit after determining the coarse estimate;

setting the frequency of the RF signal to the fine estimate of the resonant frequency; and

tracking the resonant frequency of the circuit at least during a portion of a period during which the workpiece is being heated, wherein the tracking step comprises the steps of sensing power applied to the circuit, sensing power reflected from the circuit, and modifying the frequency of the RF signal so that the frequency of the RF signal tracks the resonant frequency.

7. The method of claim 6, further comprising the step of increasing the voltage level of the RF signal from the first voltage level to a second voltage level after determining the fine estimate of the resonant frequency.

8. The method of claim 6, wherein the first voltage level is not more than about 10% of the second voltage level.

9. The method of claim 6, wherein the modifying step comprises determining whether to increase or decrease the frequency of the RF signal, wherein the determination is a function of, at the least, the sensed power applied to the circuit and the sensed reflected power.

10. In a radio frequency (RF) heating system for heating a material that is capable of being heated when exposed to an RF field, wherein the RF heating system comprises a circuit having a radiator for producing an RF Field and a resonant frequency that changes while the material is being heated, a method for heating the material, comprising the steps of:

providing to the circuit a signal having a frequency, thereby causing the radiator to produce the RF field; and

tracking the resonant frequency of the circuit, at least during a portion of a period during which the material is being heated by the RF heating system, so that the frequency of the signal follows the resonant frequency of the circuit, wherein the tracking step comprises the steps of;

(a) measuring the amount of power applied to the circuit and/or the amount of power reflected from the circuit;

(b) determining whether the resonant frequency is increasing or decreasing, wherein the determination is a function of, at the least, the measured amount of power applied to the circuit or the measured amount of power reflected from the circuit and a previously measured amount of forward power to the circuit or reflected power from the circuit;

(c) increasing the frequency of the signal if in step (b) it was determined that the resonant frequency is increasing; and

(d) decreasing the frequency of the signal if in step (b) it was determined that the resonant frequency is decreasing.

11. The method of claim 10, wherein the tracking step comprises the step of measuring both the amount of power applied to the circuit and the amount of power reflected from the circuit.

12. The method of claim 11, wherein the determination as to whether the resonant frequency is increasing or decreasing is a function of, at the least, the measured amount of power applied to the circuit and the measured amount of power reflected from the circuit.

13. The method of claim 11, wherein the tracking step further comprises the step of determining a ratio of the amount of power applied to the circuit to the amount of power reflected from the circuit, and wherein the determination as to whether the resonant frequency of the circuit is increasing or decreasing is based, at least in part, on the ratio.

14. The method of claim 11, wherein the tracking step further comprises the step of determining a ratio of the amount of power reflected from the circuit to the amount of power applied to the circuit, and wherein the determination as to whether the resonant frequency of the circuit is increasing or decreasing is based, at least in part, on the ratio.

15. A method of using an RF heating system to heat a material, wherein the RF heating system comprises a circuit having a resonant frequency, comprising the steps of:

(a) determining an estimate of the resonant frequency of the circuit and applying an RF signal to the circuit, wherein the frequency of the RF signal is substantially equal to the estimated resonant frequency of the circuit;

(b) measuring the forward power to the circuit;

(c) measuring the reflected power from the circuit;

(d) determining a ratio of the reflected power to the forward power;

(e) decreasing the frequency of the RF signal applied to the circuit after performing steps (b) and (c);

(f) after performing step (e) then performing the steps of;

(g) measuring the forward to the circuit;

(h) measuring the reflected power from the circuit; and

(i) determining the ratio of the reflected power to the forward power;

(j) comparing the ratio determined in step (i) to the ratio determined in step (d);

(k) increasing the frequency of the RF signal if, based on the comparison, it is determined that the ratio determined in step (i) is greater than the ratio determined in step (d); and

(l) decreasing the frequency of the RF signal if, based on the comparison, it is determined that the ratio determined in step (i) is less than the ratio determined in step (d).

16. A method of using an RF heating system to heat a material, wherein the RF heating system comprises a circuit having a resonant frequency, comprising the steps of:

(a) determining an estimate of the resonant frequency of the circuit and applying an RF signal to the circuit, wherein the frequency of the RF signal is substantially equal to the estimated resonant frequency of the circuit;

(b) measuring the forward power to the circuit;

(c) measuring the reflected power from the circuit;

(d) determining a ratio of the forward power to the reflected power;

(e) decreasing the frequency of the RF signal applied to the circuit by an offset amount after performing steps (b) and (c);

(f) after performing step (e) then performing the steps of;

(g) measuring the forward to the circuit;

(h) measuring the reflected power from the circuit; and

(i) determining the ratio of the forward power to the reflected power;

(j) comparing the ratio determined in step (i) to the ratio determined in step (d);

(k) increasing the frequency of the RF signal if, based on the comparison, it is determined that the ratio determined in step (i) is less than the ratio determined in step (d); and

(l) decreasing the frequency of the RF signal if, based on the comparison, it is determined that the ratio determined in step (i) is greater than the ratio determined in step (d).

17. In an RF heating system for heating a material, wherein the RF heating system comprises a circuit having a radiator for heating the material and a resonant frequency that changes while the material is being heated, a method for efficiently heating the material, comprising the steps of:

placing the material close enough to the radiator so that an electromagnetic field produced by the radiator will heat the material;

providing a signal to the circuit, the signal having a frequency;

measuring the power applied to the circuit;

measuring the power reflected from the circuit; and

tracking the changes in the resonant frequency of the circuit while the material is being heated by either increasing or decreasing the frequency of the signal by a offset amount so that the frequency of the signal follows the resonant frequency of the circuit, wherein the decision as to whether to increase or decrease the frequency of the signal is based, at least in part, on measurements of both the forward and reflected power.

18. An RF heating system for heating a material, comprising:

a circuit, wherein the circuit has a resonant frequency that changes while the material is being heated;

a signal generator that generates a signal having a frequency;

an amplifier coupled between the circuit and the signal generator that amplifies the signal, wherein the amplified signal is provided to the circuit;

a forward power sensor that senses the forward power to the circuit;

a reflected power sensor that senses the reflected power from the circuit; and

a processor coupled to the forward power sensor, the reflected power sensor, and the signal generator, wherein the processor controls the signal generator so that the frequency of the signal generated by the signal generator tracks the resonant frequency of the circuit while the material is being heated, and wherein the processor uses the sensed forward and reflected power in controlling the signal generator.

19. The heating system of claim 18, wherein the circuit includes a radiator for producing an electromagnetic field when the amplified signal is provided to the circuit, where the field is used to heat the material.

20. The heating system of claim 19, wherein the radiator comprises an induction coil.

21. A method for tracking the resonant frequency of a circuit comprising the steps of:

(1) generating a signal having a frequency (F);

(2) providing the signal to the circuit;

(3) setting a direction flag to either a first value or a second value;

(4) measuring the power applied to the circuit or the power reflected from the circuit;

(5) if the direction flag is set to the first value, then decrease the frequency of the signal provided to the circuit to a new frequency (Fnew), where Fnew equals F minus an offset amount;

(6) if the direction flag is set to the second value, then increase the frequency of the signal provided to the circuit to a new frequency (Fnew), where Fnew equals F plus an offset amount;

(7) measuring the power applied to the circuit or the power reflected from the circuit;

(8) determining which of F and Fnew appears closer to the resonant frequency of the circuit, wherein the determination is based at least in part on the applied or reflected power as measured in steps (4) and (7);

(9) if F appears closer to the resonant frequency of the circuit and the direction flag is set to the first value, then set the direction flag to the second value and increase the frequency of the signal provided to the circuit;

(10) if F appears closer to the resonant frequency of the circuit and the direction flag is set to the second value, then set the direction flag to the first value and decrease the frequency of the signal provided to the circuit.

22. The method of claim 21, wherein the offset amount substantially equals (0.1%) (F).