RF PRECISION HEATING APPARATUSES AND METHODS

Apparatuses and methods for rapid heating a load having magnetic material(s). In some embodiments, the apparatus includes a source of radio frequency (RF) signals and a power management assembly that receives the RF signals and that increases or decreases power of the RF signals. The apparatus additionally includes directional coupler(s) that measure power of the RF signals received from the power management assembly and power of the RF signals reflected from the load to the at least one directional coupler. The apparatus further includes a control assembly operable to receive the measured powers, determine a temperature of the load based on the measured powers, and send one or more control signals to the power management assembly instructing the power management assembly to increase or decrease power of the RF signals received from the source of RF signals to maintain the determined temperature of the load at a predetermined temperature.

BACKGROUND OF THE INVENTION

The present disclosure is directed to apparatuses and methods for rapid heating of magnetic materials using radio frequencies (RF) with the ability to precisely control the operating temperature of the material by measuring, for example, the reflection coefficient—the ratio of the incident (i.e., forward) and reflected (i.e., reverse) voltage. The apparatuses and methods of the present disclosure may be applied in heating die for molding parts, soldering and other industrial manufacturing areas where precision and control of temperature is demanded, such as medical equipment manufacturing and processing.

Magnetic materials undergo a sharp drop in permeability, ur, (and susceptibility, χ, where μr=1+χ) at the Curie point TC, from a relatively high value, such as 50, to a value of approximately 1. Based on this relationship between urand TC, specific temperatures can be determined. Around the Curie point transition range, the skin depth δ of the magnetic material will increase sharply as μrdecreases. The Curie point transition range includes the Curie point TCand can be described as the temperature range in which the magnetic material transitions between ferromagnetic and paramagnetic phases, or in which permeability urdecreases from about 80% of its initial value until the temperature at which permeability no longer (or no longer significantly) decreases, as shown in the dashed box area ofFIG. 1. The Curie point TCoccurs at the temperature where the slope of permeability versus temperature transitions from increasing negative to decreasing negative, which is about 760° C. inFIG. 1. In other words, the Curie point TCis where the second derivative of the permeability versus temperature goes from negative to positive.

Upon knowing the skin depth and permeability, the inductance of devices utilizing the magnetic material may further be calculated. The skin depth is a function of the frequency, permeability ur, and resistivity ρ of the magnetic material:

ρ=Resistivity of the Material

Using the example where the magnetic material is contained in a solenoid (i.e., a wire coiled N turns around a magnetic material core of cylindrical shape), the solenoid may function as a transformer, where the primary winding is the N turns of wire, and the secondary winding is the magnetic material core itself, which can be treated as a single turn conductor in which the current effectively penetrates to a depth δ. The inductance LSof the solenoid having N turns is a function of the permeability ur, resistivity ρ of the magnetic material, and the number of turns N, length l, and radius R of the solenoid:

Here “A” refers to the effective surface area of the solenoid, typically shown as an annular ring with the outer radius of R and an effective inner radius R−δ. For this geometry:

This equation shows that as the permeability, ur, decreases at the Curie point there is a net decrease in inductance, LS, since urdecreases at a faster rate than the skin depth, δ, and associated area, A, increases.
For δ<<R it can be shown that resistance of this single turn secondary winding is

Further, neglecting the conductor resistance of the wire in the primary N-turn winding, the resistance presented to the input terminals of the N turn primary windings of the solenoid is:

A decrease in urcauses an increase in δ and so the resistance will decrease at the Curie point.

Aside from inductance, the resistance of the single-turn secondary winding formed by the magnetic material core also decreases measurably in the Curie point transition range. Upon knowing the values of these variables, the impedance (resistance and reactance) of devices, such as inductors, using the magnetic material may then be calculated, and may be used to design rapid heating apparatuses that heat up magnetic materials in the Curie temperature range. The heating apparatuses and methods existing in the prior art have drawbacks because they use sum of the incident and reflected voltage waves, which create a standing wave voltage that is measured to control the temperature of the load after it has heated above the Curie point. Unfortunately, the above approach makes it difficult to control temperature above and below the Curie point for at least the reasons discussed below.

First, the voltage of the standing wave is measured typically at an electrical length of a quarter wavelength from the load because that is where the peak amplitude will occur when the load impedance is at a minimum, which is above the Curie point. Thus, a generally long interconnect is generally required, such as a 6 foot long coax cable, which adds loss to the apparatus and less flexibility as to where the load is placed. Second, the sum of the incident and reflected voltage waves to detect temperature has limited precision in the Curie point transition range where the reflected wave is often small compared to the incident wave because a small change in an already small voltage is hard to detect when added to a large voltage. What is desired, therefore, are rapid heating apparatuses and/or methods that allow improved temperature control over the whole Curie point transition range with no constraint on the interconnect length between the apparatus and the load, all while remaining easy to design and use.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring toFIG. 2, an RF (radio frequency) heating apparatus80that may be used to adjust and accurately control temperature of a load180in the Curie point transition region is shown. The apparatus80successfully does so through, for example, controlling the reflection coefficient of load180, which has a magnetic material in its core (e.g., a solenoid). In the example shown inFIG. 2, apparatus80includes an RF power source90, a power management assembly94, a directional coupler assembly110, and a control assembly136.

The RF power source90supplies RF signals. A typical frequency of the power source is one of the Industrial, Science and Medicine (ISM) frequency bands (i.e. 6.77 MHz, 13.56 MHz, 26.96 MHz, 40.66 MHz, etc.) and typical power output is around 1 mW to 10 mW (0 dBm to 10 dBm). Examples of suitable RF power sources90include Kyocera AVX, Fairview Microwave, etc. A band pass filter (not shown) can be used to attenuate harmonics of the RF signals if desired.

Power management assembly94is electronically tuned or controlled by control assembly136, such as based on a control voltage, to decrease or increase power of the RF signals from RF power source90. The power management assembly may include an attenuator and/or an amplifier. Examples of suitable electronically tunable or controllable attenuators and amplifiers include Mini Circuits MVA-2000+ and Mini Circuits ZFL-1200G+.

Directional coupler assembly110samples and/or measures the power of RF signals sent or transmitted to load180(also known as the forward or incident power wave P+), and the power of RF signals reflected from load180(also known as reverse or reflected power wave P−). In some examples, directional coupler assembly100may include separate forward and reverse directional couplers. The forward directional coupler samples and/or measures the power of RF signals sent or transmitted to load180, while the reverse directional coupler samples and/or measures the power of RF signals reflected from load180. In other examples, directional coupler assembly100includes a dual directional coupler or bi-directional coupler. In further examples, directional coupler assembly100includes a transformer bi-directional coupler. Examples of suitable directional couplers include Werlatone C5960-12, Mini Circuits ZABDC50-51HP+, etc.

Control assembly or feedback loop136receives the sampled power of RF signals sent or transmitted to load180and the sampled power of RF signals reflected from load180from directional coupler assembly110, determines temperature of load180based on the above sampled powers, and sends one or more control signals (e.g., control voltages) to power management assembly94to increase or decrease power of the RF signals received from RF power source90to maintain the determined temperature at a predetermined temperature, such as 200° C. In the example shown inFIG. 2, the control assembly calculates a reflection coefficient from the above sampled powers and sends one or more control signals to power management assembly94to increase or decrease power of the RF signals received from RF power source90to maintain a reflection coefficient that corresponds to a predetermined temperature. For example, the control assembly may maintain the load at a constant temperature in the Curie point transition range. An example of a suitable control assembly136is further discussed below.

Referring toFIG. 3, illustrative components of control assembly136are shown. Control assembly136includes a first power detector138, a second power detector140, a comparator142, and an amplifier144. First and second power detectors138,140convert the forward and reverse power waves from directional coupler assembly110into voltages Vaand Vb, respectively. Comparator142compares voltages Vaand Vband generates an output voltage based on the comparison. Amplifier144amplifies the output voltage to a control voltage that will control power management assembly94(e.g., to increase or decrease power of the transmitted RF signals from RF source90). In some examples, control assembly136includes a low pass filter146that receives output voltage from the comparator to build a “set-point” voltage that is amplified by amplifier144.

In some examples, RF heating apparatus80includes one or more amplifiers, such as shown at100and104, to achieve a desired power of RF signals to heat load180. For example, desired power levels may be 10 to 1,000 Watts (40 to 60 dBm), but other power levels are certainly possible. Examples of suitable amplifiers include Mini Circuits LZY-22X+, Mini Circuits ZHL-1A-S+, Mini Circuits LHA-13HLN+, etc.

Additionally, or alternatively, RF heating apparatus80may include an attenuator108disposed between power management assembly94and directional coupler110(or between RF power amplifier104, when included, and directional coupler110). Attenuator108serves two purposes. First, attenuator108helps to reduce power reflected back into the output of second amplifier104, which helps prevent potential damage to upstream components, such as amplifier104, especially when the load possesses a high reflection coefficient (e.g., a short or open circuit). Attenuator108absorbs enough of this reflected power to prevent damage to the second amplifier104, when part of RF heating apparatus80. Second, attenuator108obtains a desired impedance to directional coupler110. Examples of suitable attenuators include Fairview Microwave SAS2N1007-03.

Referring toFIG. 4, an example of RF heating apparatus80is generally indicated at300. RF heating apparatus300includes an RF source302, an electronically tunable attenuator or amplifier304, a first amplifier306, a second amplifier308, a load attenuator310, a dual directional coupler312, a control assembly314, and a plurality of interconnects316. The dual directional coupler may also be referred to as a “reflectometer.” The dual directional coupler can be microstrip, stripline, coax, waveguide, or other types. Dual directional coupler312includes a forward coupled port that samples the incident power and has an output coupling coefficient equal to k. The power output is thus kP1+. Similarly, a reverse coupled port samples the reverse (or reflected) power wave (P1−), typically having the same coupling coefficient k. The power output is thus kP1−.

In some examples, RF heating apparatus includes a load capacitor, series capacitor, or resonating capacitor Cres318to cancel out or at least substantially reduce positive reactance of the inductor, where the reactance of the capacitor is given by:

Because XCis negative, by picking the appropriate value of Cresthe positive reactance, XL, of the inductance of the load can be cancelled out, decreasing the reflection coefficient to a value close to zero. This is known as series resonance when the reactance is zero.

Referring toFIG. 5, another example of RF heating apparatus80is generally indicated at400. RF heating apparatus400is substantially similar to RF heating apparatus300except that RF heating apparatus400includes a forward directional coupler411and a reverse directional coupler413instead of a dual directional coupler. The pair of above directional coupler are used to sample the forward and reverse power waves at the coupled ports. The couplers can be microstrip, stripline, coax, waveguide, or other types. The unused isolated port in each of the above directional couplers are terminated in a load impedance420having an appropriate characteristic impedance for proper operation. In some examples, RF heating apparatus400includes a load capacitor, series capacitor, or resonating capacitor418.

Referring toFIG. 6, another example of RF heating apparatus80is generally indicated at500. RF heating apparatus500is substantially similar to RF heating apparatus300and400except that RF heating apparatus500includes a transformer coupler or transformer bi-directional coupler520instead of a dual directional coupler, a forward directional coupler, or a reverse directional coupler. Transformer coupler520includes a first or input port522, a second or transmitted port524, a third or coupled port526, and a fourth or isolated port528. The forward (incident) power wave enters first port522and is transmitted to second port524, and then onto load180. The forward power wave is sampled at third port526, which is sometimes called the “coupled port” and fed into control assembly514. Fourth port528is the isolated port for the forward power wave. Similarly, the reverse power wave enters second port524and is sampled at fourth port528and fed into control assembly514. One advantage of RF heating apparatus500is that it can be used at lower frequencies than conventional directional couplers that are based on transmission line theory. In some examples, RF heating apparatus500includes a load capacitor, series capacitor, or resonating capacitor518.

Interconnects316,416, and516have a characteristic impedance Z0. The interconnects need not possess identical impedance, but a typical impedance may be around 50 ohms. Load180(e.g., a solenoid) has a temperature dependent impedance ZL:

where RLis the resistance and XLis the reactance. The reactance of the solenoid can be calculated as:

Generally, if the load impedance is equal to the characteristic impedance Z0of the apparatus (e.g., the Z0of the interconnects, directional coupler, attenuators, etc.), then all incident power is absorbed by the load and there is no reflected power. In contrast, when the load impedance is not equal to the characteristic impedance, that means power is being reflected off from load180, and there is an associated reflected voltage wave V−. The measured ratio of the reflected voltage wave V−and forward voltage wave V+of load180is called the reflection coefficient, typically represented as Γ or S11. The reflection coefficient can be calculated as follows:

Referring toFIG. 7, an example of control assembly136is shown, which is generally indicated at636. Control assembly636includes a first power detector638and a second power detector640, which convert the forward and reverse power waves from directional coupler assembly110into incident and reflected voltages (Vaand Vb), respectively. Additionally, control assembly636includes a first operational amplifier642that is configured as a comparator. With all resistor values equal (R1through R6), the output voltage Vo1is given by: Vo1=Vh+Va−(Vb+V1). Many power detectors have a square-law type response and thus their output voltage is proportional to the detected power. Voltages Vhand V1are used to start the control assembly from cold temperatures and to set the desired operating temperatures (or predetermined temperatures). Output voltage Vo1is fed into a low pass RC filter646that is composed of R7and C as an integrator to build a “set-point” voltage that is amplified by operational amplifier644to bring the control assembly into a “locked” condition at the desired temperature via feeding a control voltage Vo2into power management assembly94. Attenuation of power management assembly94decreases as control voltage Vo2increases. The power management assembly increases its attenuation as control voltage Vo2decreases, which lowers the power delivered to the load, which results in the load cooling off and returning to desired operating temperature Top. Similarly, if the load begins to cool, the reflection coefficient will decrease and the control assembly will now increase power delivered to the load to increase its temperature back to Top.

Control assembly636uses magnitude of the incident and reflected voltage (or equivalently the magnitude of corresponding reflection coefficient S11, which is referred to as a “scalar system” because it uses only voltage and does not use phase information. In other embodiments, control assembly636includes the relative phase between the incident and reflected waves, which is referred to a “vector system” because it has both amplitude and phase. In further other embodiments (not shown), the control assembly may include analog to digital (A/D) converters to convert the incident and reflected voltages into digital signals. Digital signal processing software can then be used to generate an output voltage Vo2, to control the power management assembly94. In some other embodiments, the control assembly may be in the form of a proportional-integral-derivative controller (PID controller or three-term controller) (not shown), where the controller calculates an error value continuously based on the difference between a desired setpoint (SP) and a measured process variable (PV), and then applies a correction based on three control terms, namely proportional, integral and derivative terms.

Referring toFIGS. 8-9, the load resistance and inductance are mapped against temperature for the RF heating apparatuses of the present disclosure used on a solenoid with an INVAR36 core and operating at 13.56 Mhz. BothFIGS. 8-9show a sharp decrease in both parameters as their core is heated and passes through the Curie point transition region.

Referring toFIG. 10, reflection coefficient of the load varies versus temperature such that the exact temperature can be determined in the transition region about the Curie point because there is a unique value of S11vs. temperature. The magnitude of the reflection coefficient of the example embodiment ranges from 0.2 to 0.53 in the transition region, when the magnetic material in the load passes through the Curie point. In a preferred embodiment, the temperature tuning range is approximately 225 to 250° C. However, different temperature tuning ranges may be used, particularly with other magnetic materials with different Curie temperatures. As discussed, the load has inductance L that has an associated reactance XL. This reactance generally prevents the magnitude reflection coefficient S11from being zero, as indicated inFIG. 10. If the reflection coefficient can be reduced (e.g., close to zero) at cold load temperatures (e.g., below the Curie point), the apparatus may possess greater range of temperature control and in turn offer better performance. As shown inFIG. 10, after adding a series capacitor, Cres, the change in S11now ranges from about 0.03 to 0.6 over the transition region when the magnetic material passes through the Curie point. An example of Top=230 degrees has an associated reflection coefficient magnitude of 0.4.

Referring toFIG. 11, a heating time of 30 seconds is needed for the temperature of the load to increase from 50 to 200 degrees ° C. for a RF heating apparatus with a dual directional coupler, such as shown inFIG. 4.

Referring toFIG. 12, an example of a method1000of heating a load having one or more magnetic materials via RF signals is shown. At1002, RF signals are transmitted from a RF power source to heat the load. At1004, power of RF signals transmitted to load is measured, such as via one or more directional couplers. At1006, power of RF signals reflected from the load is measured, such as via the one or more directional couplers.

At1008, temperature of the load is determined based on the measured power of RF signals transmitted to the load and the measured power of RF signals reflected from the load. In some examples, the reflection coefficient is calculated from the measured power of RF signals transmitted to the load and the measured power of RF signals reflected from the load at1012and the temperature is determined based on that calculated reflection coefficient, such as by determining the temperature that corresponds to the calculated reflection coefficient (e.g.,FIG. 10). At1010, the power of RF signals transmitted to the load is adjusted to maintain the determined temperature at a predetermined temperature, which may, for example, be within a Curie point transition range. In some examples, the power of RF signals transmitted to the load is adjusted to maintain a reflection coefficient that corresponds to the predetermined temperature.

Method1000may include one or more additional steps, such as at least substantially reducing or cancelling out positive reactance of the load via, for example, a load capacitor. Additionally, or alternatively, method1000may include a step of absorbing at least a substantial portion of the RF signals from the load, such as via an attenuator. AlthoughFIG. 11shows particular steps for method1000, other examples of the method may add, omit, replace, repeat, and/or modify one or more steps. Additionally, the steps shown inFIG. 11may be performed in any suitable order and two or more steps may be performed concurrently or simultaneously.

The RF heating apparatuses and methods of the present disclosure use the ratio of incident and reflected power waves (which can be converted into the ratio of the incident and reflected voltage waves) and provides various benefits over prior art RF heating apparatuses and methods. First, the RF heating apparatuses and methods of the present disclosure does not require for a specific length of interconnect because the ratio of the incident and reflected power waves is constant regardless of the distance between the measurement circuit and the load. Second, calculating the reflection coefficient of the load provides increased precision in determining temperature in the Curie point transition range. Finally, the RF heating apparatuses and methods of the present disclosure allows for temperature control both above and below the Curie point.

Example Features: This section describes additional aspects and features of the apparatuses and methods for rapid heating of magnetic materials with the ability to precisely control the operating temperature of the material, presented without limitation as a series of paragraphs, some or all of which may be alphanumerically designated for clarity and efficiency. Each of these paragraphs can be combined with one or more other paragraphs, and/or with disclosure from elsewhere in this application in any suitable manner. Some of the paragraphs below expressly refer to and further limit other paragraphs, providing, without limitation, examples of some of the suitable combinations.

A. An apparatus for precision temperature control, the apparatus comprising:

a radio frequency power source;

an amplifier;

an attenuator;

a directional coupler, wherein the directional coupler has a first coupled port that samples the incident power wave and a second coupled port that samples the reflected power wave from the load;

and a feedback loop, wherein the feedback loop has a first and second detector and the loop compares the incident power wave to the reflected power wave.

A1. The apparatus of paragraph A, wherein the feedback loop further converts the incident and reflected power waves into incident and reflected voltages or currents.
A2. The apparatus of paragraph A1, wherein the feedback loop further compares the voltages with a reference and outputs a control voltage.
B. A method for precision temperature control, the method comprising:

supplying a radio frequency signal that generates an output power;

amplifying and adjusting the output power;

reflecting a portion of the output power back;

drawing a portion of the output power;

sampling the drawn and reflected power waves; and

comparing the drawn and reflected power waves.

B1. The method of paragraph B, further comprising comparing the ratio of the drawn and reflected power waves.

It will be appreciated that the invention is not restricted to the particular embodiment that has been described, and that variations may be made therein without departing from the scope of the invention as defined in the appending claims, as interpreted in accordance with principles of prevailing law, including the doctrine of equivalents or any other principle that enlarges the enforceable scope of a claim beyond its literal scope. Unless the context indicates otherwise, a reference in a claim to the number of instances of an element, be it a reference to one instance or more than one instance, requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated. The word “comprise” or a derivative thereof, when used in a claim, is used in a nonexclusive sense that is not intended to exclude the presence of other elements or steps in a claimed structure or method.