US2023262845

Extracted document text. Layout, formulas and symbols may not survive extraction; consult the preserved PDF for the original presentation.

Text extraction covers 15 pages, including 15 transcribed by optical character recognition.

Page 1

US 20230262845A1

as) United States

2) Patent Application Publication (10) Pub. No.: US 2023/0262845 Al

Tour et al.

(43) Pub. Date: Aug. 17, 2023

(54)

(7)

(72)

@1)

(60)

VARIABLE FREQUENCY DRIVE FOR FLASH
JOULE HEATING SYSTEM AND METHOD

Applicants: Universal Matter Inc., Burlington (CA);
William Marsh Rice University,
Houston, TX (US)

Inventors: James Mitchell Tour, Bellaire, TX (US);
Duy X Luong, Houston, TX (US);
Carter Kittrell, Houston, TX (US);
Tyler Cooksey, Houston, TX (US);
Zhiyong Zhang, Houston, TX (US);
Vladimir Mancevski, Austin, TX (US)

Appl. No.: 18/109,457
Filed: Feb. 14, 2023
Related U.S. Application Data

Provisional application No. 63/309,897, filed on Feb.
14, 2022.

100

YA, 102
LL

Publication Classification

(51) Int. Cl.
HOSB 3/00 (2006.01)
HOSB 1/02 (2006.01)
(52) US. CL
CPC neeseesveeee HOSB 3/0023 (2013.01); HOSB 1/02
(2013.01); HOSB 2203/035 (2013.01)
67) ABSTRACT

Systems and methods for flash joule heating carbon with
variable frequency drives, for the production of graphene.
The system includes a flash joule heating system, and a vari-
able frequency drive system for driving the flash joule heat-
ing system, wherein the variable frequency drive system is
coupled to the flash joule heating system, and is configured
to output a pulse-width modulated current. The system and
methods may further include sample temperature feedback,
to adjust the output of variable frequency drive system.

104

L

Variable Frequency

Flash Joule Heating
System

Drive System

Sample
112

Variable Frequency
Controller

Page 2

Patent Application Publication Aug. 17, 2023 Sheet 1 of 10 US 2023/0262845 Al

100
102 104
Flash Joule Heating
System

Variable Frequency

Drive System Sample

112
Variable Frequency sn s”
Controller LO snnnnnntnnnnsneet

FIG. 1

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US 2023/0262845 Al

Aug. 17, 2023 Sheet 2 of 10

Patent Application Publication

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Page 4

Patent Application Publication Aug. 17, 2023 Sheet 3 of 10 US 2023/0262845 Al

302

1500 -

——— DG
PWM

A)

Current

0 1000 2000 3000 4000 5000
Time (ms)

FIG. 3A

Page 5

Patent Application Publication Aug. 17, 2023 Sheet 4 of 10 US 2023/0262845 Al

304
3000
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Time (ms)

FIG. 3B

Page 6

Patent Application Publication Aug. 17, 2023 Sheet 5 of 10 US 2023/0262845 Al

402

Temperature (°C)

Time (s}

FIG. 4A

Page 7

Patent Application Publication Aug. 17, 2023 Sheet 6 of 10 US 2023/0262845 Al

404

Duty Cycle (%)

Time (s)

FIG. 4B

Page 8

Patent Application Publication Aug. 17, 2023 Sheet 7 of 10 US 2023/0262845 Al

902

Production rate
400000
= y= 3E-205¢%00
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FIG. 5A

Page 9

Patent Application Publication Aug. 17, 2023 Sheet 8 of 10 US 2023/0262845 Al

504

FIG. 5B

Page 10

Patent Application Publication Aug. 17, 2023 Sheet 9 of 10 US 2023/0262845 Al

506

FIG. 5C

Page 11

Patent Application Publication Aug. 17, 2023 Sheet 10 of 10 US 2023/0262845 Al

600

ie
Providing a carbon sample

!

Flash joule heating the sample to 604
produce graphene using a i
variable frequency drive system

FIG. 6

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VARIABLE FREQUENCY DRIVE FOR
FLASH JOULE HEATING SYSTEM AND
METHOD

TECHNICAL FIELD

[0001] The following relates generally to flash joule heat-
ing systems and methods, and more particularly to systems
and methods for flash joule heating with variable frequency
drives, with temperature control.

INTRODUCTION

[0002] This disclosure builds on a flash joule heating
synthesis method and compositions thereof described in
Patent Cooperation Treaty Application having International
Publication Number WO 2020/051000 AI to Tour et al.,
having an international publication date of Mar. 12, 2020,
which is incorporated herein by reference in its entirety.

SUMMARY

[0003] Provided are systems and methods for flash joule
heating with variable frequency drives, with temperature
control, for the production of graphene.

[0004] According to an embodiment, disclosed is a system
for the production of graphene. The system includes a flash
joule heating system, and a variable frequency drive system
for driving the flash joule heating system, wherein the vari-
able frequency drive system is coupled to the flash joule
heating system.

[0005] According to some embodiments, the variable fre-
quency drive system comprises a pulse width modulated
output.

[0006] According to some embodiments, the variable fre-
quency drive system comprises an insulated gate bipolar
transistor for switching an output of the variable frequency
drive system.

[0007] According to some embodiments, the system com-
prises a variable frequency controller for varying an output
of the variable frequency drive system.

[0008] According to some embodiments, the flash joule
heating system heats a sample to a maximum temperature
of 3000° C.

[0009] According to some embodiments, the variable fre-
quency drive system outputs a pulse width modulated cur-
rent with a frequency between 100 Hz and 10000 Hz
[0010] According to some embodiments, an output of the
variable frequency drive system is adjusted according to a
feedback signal of a temperature of a sample.

[0011] According to some embodiments, the feedback sig-
nal comprises a temperature measurement of a sample com-
prising the mean value of multiple temperature sensors.
[0012] According to some embodiments, the output of the
variable frequency drive system is adjusted according to a
proportional integral derivative control scheme.

[0013] According to some embodiments, the proportional
integral derivative control scheme is a dynamic proportional
integral derivative control scheme, wherein the proportional
integral derivative parameters are varied according to the
feedback signal.

[0014] Disclosed is a method for the production of gra-
phene, according to an embodiment. The method includes
providing a carbon sample, and applying flash joule heating
to the carbon sample to produce graphene, wherein the flash

Aug. 17, 2023

joule heating step is driven by a variable frequency drive
system.

[0015] According to some embodiments, the variable fre-
quency drive system comprises a pulse width modulated
output.

[0016] According to some embodiments, the sample is
heated to 3000° C.

[0017] According to some embodiments, the variable fre-
quency drive system is driven by a variable frequency drive
controller.

[0018] According to some embodiments, the controller
comprises a temperature sensor coupled to the sample, and
the controller applies a closed loop control scheme to vary
the output of the variable frequency drive system.

[0019] According to some embodiments, the controller
applies a proportional integral derivative control scheme.
[0020] According to some embodiments, the controller
applies a dynamic proportional integral derivative control
scheme, wherein the proportional integral derivative para-
meters are varied according to a feedback signal.

[0021] According to some embodiments, the temperature
sensor comprises multiple temperature sensors, wherein the
outputs of each individual temperature sensor are averaged.
to determine a mean temperature.

[0022] According to some embodiments, the heating step
first comprises a variable frequency drive system duty cycle
of 10%, followed by a variable frequency drive system duty
cycle adjusted according to the feedback signal.

[0023] Aspects and features will become apparent to those
ordinarily skilled in the art, upon review of the following
description of some exemplary embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The drawings included herewith are for illustrating
various examples of articles, methods, and apparatuses of
the present specification. In the drawings:

[0025] FIG. 1 is a system block diagram of a flash Joule
heating system coupled to a variable frequency drive sys-
tem, according to an embodiment;

[0026] FIG. 2 is schematic ofa flash Joule heating system
with IGBTs used for pulse width modulation (PWM)
switching, according to an embodiment;

[0027] FIG. 3A is a graph depicting the current and tem-
perature of a VFD flashing system over time, according to
an embodiment;

[0028] FIG. 3B is a graph depicting the temperature of a
VFD flashing system over time, according to an
embodiment;

[0029] FIG. 4A is a graph of the temperature over time of
a VFD flash Joule heating system, according to an
embodiment:

[0030] FIG. 4B is a graph of the duty cycle over time of a
VFD flash Joule heating system, according to an
embodiment;

[0031] FIG. 5A is a graph of a production rate of a flash
joule heating system, a photograph of a flashing system, and
a photograph of an automated flashing system, according to
an embodiment;

[0032] FIG. 5B is a photograph of a flashing system,
according to an embodiment;

[0033] FIG. 5C is a photograph of an automated flashing
system, according to an embodiment; and

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[0034] FIG. 6 is a flow chart depicting a method of produ-
cing graphene by driving a flash joule heating system with a
variable frequency drive, according to an embodiment.

DETAILED DESCRIPTION

[0035] Various apparatuses or processes will be described
below to provide an example of each claimed embodiment.
No embodiment described below limits any claimed embo-
diment and any claimed embodiment may cover processes
or apparatuses that differ from those described below. The
claimed embodiments are not limited to apparatuses or pro-
cesses having all of the features of any one apparatus or
process described below or to features common to multiple
or all of the apparatuses described below.

[0036] A description of an embodiment with several com-
ponents in communication with each other does not imply
that all such components are required. On the contrary, a
variety of optional components are described to illustrate
the wide variety of possible embodiments of the present
disclosure.

[0037] Further, although process steps, method steps,
algorithms or the like may be described (in the disclosure
and / or in the claims) in a sequential order, such processes,
methods and algorithms may be configured to work in alter-
nate orders. In other words, any sequence or order of steps
that may be described does not necessarily indicate a
requirement that the steps be performed in that order. The
steps of processes described herein may be performed in any
order that is practical. Further, some steps may be performed
simultaneously.

[0038] When a single device or article is described herein,
it will be readily apparent that more than one device / article
(whether or not they cooperate) may be used in place of a
single device / article. Similarly, where more than one
device or article is described herein (whether or not they
cooperate), it will be readily apparent that a single device /
article may be used in place of the more than one device or
article.

[0039] The following relates generally to flash joule heat-
ing systems and methods, and more particularly to systems
and methods for flash joule heating with variable frequency
drives, with temperature control, for the production of gra-
phene from carbon samples.

[0040] The systems and methods described herein apply a
variable frequency drive system (VFD) to deliver the vol-
tage across a carbon sample subjected to flash joule heating.
Flash joule heating this carbon sample may produce gra-
phene of varying forms. The process of inverting DC vol-
tage to variable voltage variable frequency AC voltage in an.
inverter section of a VFD may be referred to as pulse width
modulation (PWM). Pulse-width modulation is a widely
used means of controlling power, for example, for provision
to large induction motors. PMW provides a generally regu-
lar, (e.g. in some examples, sinusoidal) current output, to
control frequency and voltage. PWM VFDs are efficient
and typically provide high levels of performance. VFDs
described herein may utilize insulated gate bipolar transis-
tors or IGBTs for the generation of their variable frequency
drive output.

[0041] Referring first to FIG. 1, pictured therein is a block
diagram 100 of a variable frequency drive system 102
coupled to a flash joule heating system 104, for driving the
flash joule heating system 104. As described in International
Publication Number WO 2020/051000 A1, the application
of a flash joule heating system 104 for the production of

Aug. 17, 2023

graphene may provide for graphene with specific favorable
properties. The flash joule heating system 104 may be dri-
ven by a supply of electrical current. Supplies of electrical
currents with differing characteristics (e.g. time variation, or
current) may result in differing graphene production
characteristics.

[0042] Flash joule heating system 104 comprises an elec-
trical system, configured to be driven by an electrical current
source, for heating a carbon containing sample (e.g. sample
112). In some examples, sample 112 may be conductive, and
may be substantially non-graphene carbon. Flash joule heat-
ing system 104 heats the sample 112 using resistive electric
heating, wherein a current is discharged through the sample
112, heating the sample 112. The heating characteristics of
system 104 may vary according to the character of the cur-
tent discharged through sample 112.

[0043] Variable frequency drive system 102 comprises an
electrical system for delivering electrical current with a par-
ticular character, magnitude, and time variation to flash
joule heating system 104. For example, variable frequency
drive system 102 may provide a pulse width modulated out-
put current, with a certain peak current, frequency, duty
cycle and other characteristics.

[0044] Coupled to variable frequency drive system 102 is
controller 114. Controller 114 may send control signals to
system 102 to adjust the output of system 102. For example,
controller 114 may provide a periodic control signal, which
the output of system 102 may be matched to. Such a periodic
control signal may comprise a sinusoidal control signal, a
square wave signal, or any other signal known in the art.
Such periodic control signals may vary over time. For exam-
ple, the period of the signal may continuously increase or
decrease.

[0045] Controller 114 may comprise a computing device,
microcontroller, single board computer, programmable
logic controller, signal generator, PID controller or any
other controller type known in the art which may supply a
control signal to drive system 102 to adjust and/or modulate
the output of heating system 104, to optimize graphene pro-
duction from initial sample 112. Controller 114 may be
coupled, directly or indirectly, to switching components of
system 104, such as transistors, relays, or other controllable
switches.

[0046] In some examples, controller 114 may be coupled
to sample 112, to receive temperature feedback 116. Tem-
perature feedback 116 may allow controller 114 to modulate
the output of system 102, to alter the heating rate or condi-
tions of heating system 104. Temperature feedback 116 may
comprise a thermocouple, infrared pyrometer, emission
spectroscopy or other spectroscopy-based temperature mea-
surement, or any other thermal measurement method or
mechanism suitable for measuring temperatures up to
3000° C. in some embodiments. In some examples, feed-
back 116 may comprise the average value of multiple tem-
perature measurements from multiple sensors. In such
examples, this average value may more accurately corre-
spond to the average temperature of sample 112, due to
uneven heat distribution across the sample.

[0047] Referring now to FIG. 2, shown therein is an elec-
trical schematic 200 of a variable frequency drive system
202 coupled to a flash Joule heating system 204, according
to an embodiment. Drive system 202 may correspond to
system 102 and system 204 may correspond to system
104. Variable frequency drive 202 is driven by a 480 VAC
input source, converted to 360 VAC through the application
of a transformer 210. The 360 VAC current is then rectified

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using a plurality of diodes 214, and capacitor 216. The out-
put of system 204 comprises a pulse width modulated out-
put, with IGBTs (¢.g. IGBTS 206) used for pulse width
modulation (PWM) switching. It may be advantageous to
use Insulated Gate Bipolar Transistors (IGBTs) for the
high current, high frequency switching, as other solid-state
relays and/or switching components may not provide the
speed as well as current rating for the operation of the
flash joule heating system 204 for the production of
graphene.

[0048] In other embodiments, other configurations or cir-
cuit designs may be present within the variable frequency
drive system. For example, such a system may include a
single or a plurality of integrated circuits, according to
some embodiments.

[0049] Referring now to FIG. 3, FIG. 3A comprises graph
302 and FIG. 3B comprises graph 304. At graph 302, shown
therein is the current profile of DC flash vs PWM flash for
joule heating-based production of graphene. The pulse
sequence of the PWM flash is shown in the inset. At graph
304 of FIG. 3, pictured therein is a temperature profile of the
same DC and PWM flash.

[0050] In some examples wherein DC flash methods are
applied, pretreatment (e.g. the application of lower current
or voltage flashes) is needed to avoid violent outgassing
from the sample that could result in the explosion of the
sample. After a few pretreatments of lower voltage and cur-
rent, the typical, full power DC flash may be applied, with a
peak current of more than 1500 A. In contrast, VFD system-
based methods, lower voltage pretreatment is not necessary,
and the peak current output during the flashing process may
be less than 1000 A, reducing the flashing time and the risk
of explosion and equipment failure, while increasing the
energy efficiency and production throughput.

[0051] As seen in 304, temperature profile shows the DC
flash has higher temperature with 3000° C. peak and
<500 ms flash duration. The PWM flash method peaks at
1600° C. but has a longer duration of ~2 s. However, when
comparing the graphene quality produced by each respec-
tive method using Raman spectroscopy, the two graphene
samples are similar spectroscopically. This result indicates
that the lower temperature of the VFD method may be com-
pensated by longer flashing time. Even though the DC flash
seems to have shorter duration, the production throughput
using DC is much lower than with a PWM flash due to the
addition of pretreatment flashes needed in the DC method.
[0052] In some examples, a VFD frequency may prefer-
ably be in the range of 100-10000 Hz with a duty cycle
range of 0-50% to optimize graphene production. The
VFD flash may employ a soft start, wherein a low duty
cycle output is first provided to slowly heat the sample to
remove some of volatile compounds from within the
sample.

[0053] In some examples, the systems and methods
described herein may apply a temperature control system
using Proportional, Integral, Derivative (PID) control
schemes, state space control schemes, or any other closed.
loop control scheme known in the art.

[0054] Referring now to FIG. 4, shown therein is graph
402 in FIG. 4A and graph 404 in FIG. 4B, each showing
temperature control of a flash joule heating process, with
dynamic PID. Graph 402 shows temperature profile of the
flash joule heating process with a 1600° C. target tempera-
ture PID controlled. Graph 404 shows duty cycle that
changes, corresponding to the measured temperature fluc-

Aug. 17, 2023

tuation (e.g. using feedback such as feedback 116), to main-
tain the target temperature.

[0055] In some examples, the VFD system allows for
high-speed temperature control with dynamic PID. In such
examples, the PID parameters change with the temperature
and the voltage of a capacitor, modifying the characteristics
of the current output of the VFD system. For example, PID
parameters may vary to produce a soft start, heating the
sample at low temperature and subsequently, provide for
an aggressive flash of current toward the end of the flash
joule heating process to compensate for falling voltage.
[0056] In the example of FIG. 4, the duty cycle of the flash
joule heating process is initialized at 10% for a soft start.
When the temperature increases and fluctuates, the duty
cycle changes accordingly to maintain the target tempera-
ture, according to a temperature feedback signal (e.g. feed-
back 116). Toward the end of the flash joule heating process,
the duty cycle is increased to compensate for the falling
temperature and voltage. The duty cycle may be continu-
ously adjusted according to temperature and/or capacitor
voltage feedback.

[0057] The systems and methods described herein may
comprise temperature readings which are not representative
of the average temperature of the sample. Such measure-
ments may be adjusted or calibrated to account for this dis-
crepancy. In some examples, multiple IR pyrometers may be
employed to collect multiple measurements, to determine an
average measured temperature, which may more closely
correspond to an overall average temperature.

[0058] The systems and methods described herein may be
employed for any other flash heating process and/or config-
uration that includes the production of other 2D materials,
and/or material phase changes.

[0059] The systems and methods described herein may be
applied to perform lab scale synthesis of materials such as
flash joule heating sourced graphene.

[0060] The systems and methods described herein may be
used to heat a carbon sample to a maximum temperature of
3000° C., according to some embodiments.

[0061] Referring now to FIG. 5, FIG. SA comprises graph
502, FIG. SB comprises photograph 504 and FIG. 5C com-
prises photograph 506.

[0062] Shown at graph 502 is the production rate using a
flash Joule heating process.

[0063] Shown at photograph 504 is an example of a
scaled-up flashing system for the production of graphene.
The system comprises a total capacitance of 0.624 F with a
500 VDC rating, and is capable of flashing 10 g of metal-
lurgy coke per batch. In photograph 504, “1” corresponds to
6 capacitor banks, each has 8 capacitors with 13 mF and
500 V capacitance, “B2” corresponds to a switching control-
ler (e.g. analogous to variable frequency controller 114),
“B3” corresponds to kill switches, “B4” corresponds to an
inductor with a 1 mH inductance and “BS” corresponds to
the flashing chamber.

[0064] Shown at photograph 506 is an example of an auto-
mated system capable of flashing 5.7 g batch of metallurgy
coke per batch and reloading the new sample with ~20 s
cycling. The system of photograph 506 may correspond to
system 100 and/or the system of FIG. 2. “C1” corresponds
to a sample hoop that can hold 50 samples, “C2” corre-
sponds to an automation frame, “C3” corresponds to a flash-
ing jig, “C4” corresponds to a sample ramp, “C5” corre-
sponds to a sample tray, “C6” corresponds to an
automation controller, “C7” corresponds to a connector to

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the flashing system, and “C8” corresponds to a compressed
air nozzle.
[0065] From the discovery publication of the flash Joule
heating process, graphene production rate has doubled
each 9 weeks on average (Gram-scale bottom-up flash gra-
phene synthesis. Nature, 577(7792), pp.647-651, Luong,
D.X., Bets, K.V., Algozeeb, W.A., Stanford, M.G., Kittrell,
C., Chen, W., Salvatierra, R.V., Ren, M., McHugh, E.A.,
Advincula, PA. and Wang, Z., 2020.). This exponential
growth in production rate is driven by two additions: a
higher capacity flashing system which capable of flashing
10 g of metallurgy coke each batch and an automated sam-
ple reloading system.
[0066] The flashing system may be powered by 48 capa-
citors at total of 0.624 F capacitance. Each 8 capacitors may
be assembled into a capacitor bank and controlled by a
solid-state relay. 6 capacitor banks may be coupled to a
kill switch before wiring the banks to the sample jig. To
slow down the current for more stable electrical control, a
1 mH inductor is put in series with the circuit. The carbon
sample may then be flashed in the controlled environment
flashing chamber or in ambient atmosphere, producing
graphene.
[0067] The automated system pictured in photograph 506
is controlled by a customized LabVIEW program. Once the
sample is loaded inside the flashing jig, the flashing system
will commence the flash. After the flash is done, the sample
is dropped into a collecting tray and the new sample will be
loaded into the flashing jig.
[0068] The high rate of automation is possible as the
VFD-based flash heating method allows for fast cycling
time with a single flash without the need for pre-flashing,
as described above in reference to DC-based flashing
methods.
[0069] Referring now to FIG. 6, shown therein is a flow
chart detailing a method 600 of producing graphene.
Method 600 comprises steps 602 and 604.
[0070] At step 602, a sample is provided for conversion to
graphene. The sample may comprise graphite or other forms
of carbon.
[0071] At step 604, the sample is flash joule heated to pro-
duce graphene, the flash joule heating driven using a vari-
able frequency drive system.
[0072] In some examples of method 600, the variable fre-
quency drive system provides a pulse width modulated out-
put. In some examples of method 600, the variable fre-
quency drive system is driven by a variable frequency
controller.
[0073] The variable frequency drive system of method
600 may correspond to system 102 or 202 in some embodi-
ments. The flash joule heating system of method 600 may
correspond to system 104 or 204 in some embodiments.
[0074] While the above description provides examples of
one or more apparatus, methods, or systems, it will be
appreciated that other apparatus, methods, or systems may
be within the scope of the claims as interpreted by one of
skill in the art.

1. A system for the production of graphene, the system
comprising:

a flash joule heating system; and

avariable frequency drive system for driving the flash joule

heating system;

Aug. 17, 2023

wherein the variable frequency drive system is coupled to

the flash joule heating system.

2. The system of claim 1, wherein the variable frequency
drive system comprises a pulse width modulated output.

3. The system of claim 1, wherein the variable frequency
drive system comprises an insulated gate bipolar transistor for
switching an output of the variable frequency drive system.

4. The system of claim 1, wherein the system comprises a
variable frequency controller for varying an output of the vari-
able frequency drive system.

5. The system of claim 1, wherein the flash joule heating
system heats a sample toa maximum temperature of 3000° C.

6. The system of claim 1, wherein the variable frequency
drive system outputs a pulse width modulated current with a
frequency between 100 Hz and 10000 Hz.

7. The system of claim 1, wherein an output of the variable
frequency drive system is adjusted according to a feedback
signal ofa temperature of a sample.

8. The system of claim 7, wherein the feedback signal com-
prises a temperature measurement ofa sample comprising the
mean value of the output of multiple temperature sensors.

9. The system of claim 7, wherein the output of the variable
frequency drive system is adjusted according toa proportional
integral derivative control scheme.

10. The system of claim 9, where the proportional integral
derivative control scheme is a dynamic proportional integral
derivative control scheme, wherein the proportional integral
derivative parameters are varied according to the feedback
signal.

11. A method for the production of graphene, the method
comprising:

providing a carbon sample; and

applying flash joule heating to the carbon sample to pro-

duce graphene;

wherein the flash joule heating step is driven by a variable

frequency drive system.

12. The method of claim 11, wherein the variable frequency
drive system comprises a pulse width modulated output.

13. Themethod of claim 11, wherein the sample is heated to
a maximum temperature of 3000° C.

14. The method of claim 11, wherein the variable frequency
drive system is driven by a variable frequency drive
controller.

15. The method of claim 14, wherein the controller com-
prises a temperature sensor coupled to the sample, and the
controller applies a closed loop control scheme to vary the
output of the variable frequency drive system according to
feedback from the temperature sensor.

16. The method of claim 15, wherein the controller applies a
proportional integral derivative control scheme.

17. The method ofclaim 15, wherein the controller applies a
dynamic proportional integral derivative control scheme,
wherein the proportional integral derivative parameters are
varied according to the feedback.

18. The method of claim 15, wherein the temperature sen-
sor comprises multiple temperature sensors, wherein the out-
puts of each individual temperature sensor are averaged to
determine a mean temperature.

19. The method of claim 15, wherein the heating step first
comprises a variable frequency drive system duty cycle of
10%, followed by a variable frequency drive system duty
cycle adjusted according to the feedback.

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