Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND SYSTEM FACILITATING CONTROL STRATEGY FOR
POWER ELECTRONICS INTERFACE OF DISTRIBUTED GENERATION
RESOURCES
This application is being filed on 21 October 2011, as a PCT international
patent application in the name of PETRA SOLAR, INC., a U.S. national
corporation, applicant for the designation of all countries except the US, and
Hussam Alatrash, a citizen of Jordan and Nasser Kutkut, a citizen of the U.S.,
applicants for the designation of the US only, and claims priority to U.S.
Provisional
Application Serial Number 61/455,556 filed October 22, 2010, the subject
matter of
which is incorporated by reference in its entirety.
FIELD OF THE INVENTION
The present invention relates, in general, to the domain of the generation of
electrical energy. More specifically, the present invention relates to a
method and
system facilitating control strategy for power electronics interfaces (PEI) of
distributed generation resources.
BACKGROUND
A large amount of today's electric power is generated by large-scale,
centralized power plants using fossil fuels, hydropower or nuclear power, and
is
transported over long distances to end-users. In these systems, power flows
from the
central power stations in one direction through the distribution networks to
consumers. Yet, the centralized power generation paradigm has many
disadvantages, including the environmental impact of greenhouse gases and
other
pollutants, transmission losses and inefficiency, growing security of supply
concerns, system sustainability issues, over-consumption, and the high cost of
ongoing upgrades and replacement of transmission and distribution
infrastructure.
Over the past few years, technological innovations, changing economic and
regulatory environments, and shifting environmental and social priorities have
spurred interest in Distributed Generation (DG) systems. Distributed
Generation is a
new model for the power system that is based on the integration of small and
medium-sized generators based on new and renewable energy technologies, such
as
solar, wind, and fuel cells, into the utility grid. All these generators are
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interconnected through a fully interactive intelligent electricity network.
This
revolution will require sophisticated control and communication technologies.
Most DG resources are primarily used to supplement the traditional electric
power systems. For example, these resources can be combined to supply nearby
loads in specific areas with continuous power during disturbances and
interruptions
of the main utility grid. Such a grouping of DG resources with the nearby
loads is
referred to as a micro grid. Micro grids are, generally, self-contained
electrical
ecosystems. In these systems, power is produced, transmitted, consumed,
monitored, and managed on a local scale. In many cases, they can be integrated
into
larger, central grids, but their defining characteristic proves that they can
operate
independently if disconnected from the whole.
Most of the current DG resources developed cannot be employed as a part of
micro grids since these resources are designed as current sources. The output
produced by such power systems is significantly unstable. Further, such a
power
system is not capable of operating in isolation from the main grid to power a
specific
area. The process of using a micro grid in isolation from the main grid to
power a
specific area or a location is known as islanding. Further, the DG resources
cannot
be employed as a part of a micro grid if they are designed as voltage sources.
This is
due to minute differences in instantaneous output voltages that may result in
large
amounts of circulating currents and may damage the DG resources. Micro grids
used in recent times employ synchronous generators that have moderate output
impedance characteristics that allow them to operate in parallel, tied to the
grid, or
isolated from it.
In light of the aforementioned challenges, there lies a need for a system in
which a plurality of the DG resources can be controlled to exhibit improved
characteristics by utilizing a power electronics interface. Further, there
exists a need
for a system in which multiple DG resources are integrated to form a micro
grid. In
this way, multiple DG resources can be controlled in an appropriate manner to
maintain a stable AC power system. Moreover, the system should be able to work
when connected with a larger utility grid (known as grid-tie mode) as well as
separately from a utility grid (known as islanding mode). In addition, the
system
should provide a smooth transition between the grid-tie mode and islanding
mode of
operation.
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SUMMARY OF THE INVENTION
An objective of the present invention is to provide a stable AC power
generation and distribution system based on Distributed Generation (DG)
resources.
In an embodiment of the present invention, a system is provided to facilitate
control of a DG resource by using a Power Electronic Interface (PEI) such that
the
DG resource exhibits predetermined electrical characteristics. The
predetermined
electrical characteristics exhibited by the system are similar to these of a
synchronous generator.
In another embodiment of the present invention, a method is disclosed for
grouping multiple DG units into a micro grid. The multiple DG units of the
micro
grid may be grouped by using a supervisory control agent. The supervisory
control
agent communicates with multiple DG units over a Low Bandwidth Communication
Network (LBCN). Further, the micro grid operates when connected to a main grid
and can also work in an island mode.
In yet another embodiment of the present invention, a method is provided to
create a micro grid hierarchy by integrating multiple DG resources. The micro
grid
hierarchy can be implemented by arranging the multiple micro grids into a
predefined hierarchical architecture.
BRIEF DESCRIPTION OF THE DRAWINGS
Various embodiments of the invention will, hereinafter, be described in
conjunction with the appended drawings provided to illustrate and not to limit
the
invention, wherein like designations denote like elements, and in which
FIG. 1 is an exemplary environment in which various embodiments of the
present invention can be practiced;
FIG. 2 shows a block diagram illustrating system elements implementing
control strategy for a Distributed Generation (DG) unit;
FIG. 3 is a circuit diagram representing a Virtual Electrical Network (VEN),
in accordance with an embodiment of the present invention;
FIG. 4 is an exemplary circuit diagram of a Virtual Electrical Network
(VEN), in accordance with another embodiment of the present invention;
FIG. 5 depicts another exemplary circuit diagram of a Virtual Electrical
Network (VEN), in accordance with yet another embodiment of the present
invention;
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FIG. 6 depicts a block diagram illustrating grouping of DG systems, loads,
and associated controllers into a micro grid, in accordance with an embodiment
of
the present invention; and
FIG. 7 represents an exemplary micro grid hierarchy, in accordance with an
embodiment of the present invention.
DETAILED DESCRIPTION
The present invention discloses a system and a method for facilitating a
control strategy for Power Electronic Interfaces (PEIs) of Distributed
Generation
(DG) resources. Various examples of the DG resources may include, but are not
limited to, Photovoltaic (PV) systems, wind turbines, battery storage, and
fuel cells.
In particular, the present invention focuses on the interaction of one or more
PEIs
with Alternating Current (AC) power distribution systems.
FIG. 1 is an exemplary environment in which various embodiments of the
present invention can be practiced. FIG. 1 is shown to include a generation
station
102, one or more transmission units 104a, 104b, and 104c (collectively
referred to as
transmission units 104), one or more distribution units 106a and 106b
(collectively
referred to as distribution units 106), a micro grid 108, one or more loads
110a and
110b (collectively referred to as loads 110), and a distribution network 112.
Additionally, micro grid 108 further includes one or more PEIs 114a, 114b, and
114c (collectively referred to as PEIs 114), one or more DG resources 116a,
116b,
and 116c (collectively referred to as DG resources 116), and one or more DG
units
118a, 118b, and 118c (collectively referred to as DG units 118). As shown in
FIG.
1, PEI units 114 when combined with DG resources 116 form DG units 118.
In accordance with an embodiment of the present invention, FIG. 1 may
include one or more grid control centers (not shown in FIG. 1). Additionally,
the
grid control center provides supervision and control of generation,
transmission, and
distribution.
As described above, the generation station 102 depends on traditional and
renewable sources, which include, but are not limited to, fossil fuels,
nuclear, hydro,
wind, photovoltaic, and geo-thermal. In addition to the above, the generation
station
102 generates a large-scale power to be distributed to the loads 110 via the
distribution network 112.
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In accordance with an embodiment of the present invention, the power
generated by the generation station 102 is provided to the transmission units
104 to
further transmit the power to the distribution units 106.
In accordance with an embodiment of the invention, the micro grid 108
includes DG units 118 as depicted in FIG. 1. The DG units 118 may depend on
sources such as photovoltaic systems, wind turbines, battery storage, and full
cells.
In accordance with an embodiment of the present invention, the micro grid 108
is
connected to operate in parallel with a utility grid. In accordance with
another
embodiment of the invention, the micro grid 108 may operate in isolation with
the
utility grid.
In accordance with an embodiment of the present invention, the generation
station 102, the transmission units 104, the distribution units 106, the loads
110, and
the distribution network 112 may collectively be referred to as the utility
grid.
FIG. 2 shows a block diagram illustrating system elements implementing
control strategy for a Distributed Generation (DG) unit. As depicted in FIG.
2, the
block diagram illustrates a Distributed Generation unit 202. The DG unit 202
includes a Distributed Generation (DG) resource 204, a Power Electronics
Interface
(PEI) unit 206, an AC current sensor 208, a behavioral controller 210, a
current
feedback controller 212, a power flow controller 214, and short-term energy
storage
216.
The DG resource 204 utilizes one or more sources such as photovoltaic
systems, wind turbines, battery storage, and/or fuel cells for generating
power. In
accordance with an embodiment of the present invention, a control scheme is
implemented using the PEI unit 206 to control the DG resource 204 to obtain a
preferred behavior. The architecture of the PEI unit 206 may vary in
accordance
with specific characteristics of the DG resource 204 connected to it.
In accordance with an embodiment of the present invention, the PEI unit 206
may be programmed to exhibit electrical characteristics similar to a
synchronous
generator. The PEI unit is typically a combination of hardware and software.
The
characteristics are emulated using a Virtual Electrical Network (VEN), which
is
represented as a combination of an AC voltage source (Vs) and a pre-defined
impedance network. Further, various parameters of the AC voltage source (Vs)
such
as amplitude, frequency, and phase may vary in real time to control the
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reactive/active power output of the DG unit 202. This process will be
described in
greater detail in subsequent paragraphs.
A control strategy implemented by the PEI unit 206 is built around a current
feedback loop. The current feedback loop is implemented through the current
sensor
208 and the current feedback controller 212. The output current of the PEI
unit 206
is controlled based on the comparison between current sensed using the current
sensor 208 and a current reference (Iac). The behavioral controller 210
utilizes an
instantaneous measurement of the AC system voltage (Vac) in addition to the
mathematical model of the VEN for calculating instantaneous values for the
output
current reference (Iac). The behavioral controller 210 manipulates the current
reference (Iac) to reproduce the behavior of the VEN, and thereby the behavior
of a
synchronous generator is emulated. The VEN, as described above, will be
explained in detail in conjunction with FIG. 3.
The power flow controller 214 manages energy flow between the DG
resource 204 and the short-term energy storage 216 by continuously modulating
a
power angle to achieve specific power management objectives. One of the power
management objectives may be to create a pre-determined droop relationship
between average active power output of the DG unit 202 and the frequency of
the
AC system voltage (Vac). The power angle is determined by utilizing the phase
difference between the AC system voltage (Vac) and a voltage Vs of the VEN,
which
will be discussed later. In addition, the power flow controller 214 is
responsible for
ensuring that short-term energy storage 216 maintains a satisfactory level of
energy
that allows proper PEI operation and response to system transients.
FIG. 3 is a circuit diagram representing a Virtual Electrical Network (VEN),
in accordance with an embodiment of the present invention. As depicted in FIG.
3,
the circuit diagram includes an AC voltage source (Vs) 302 and impedances 304
and
306 (collectively referred to as impedances). Impedances 304 and 306 may be
referred to as an impedance network.
In accordance with an embodiment of the present invention, the virtual
circuit as shown in FIG. 3 may be implemented through the behavioral
controller
210.
An imaginary circuit such as VEN is designed to include the AC voltage
source (Vs) 302 in combination with the impedance network. Further, the
impedances forming the impedance network are designed in accordance with
desired
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characteristics and the value of the AC voltage source (Vs) 302. The value of
the
AC voltage source (Vs) 302 is determined by the power flow controller 214.
It may be appreciated by a person ordinarily skilled in art that the impedance
network is designed to fulfill one or more objectives. The design should
maintain an
acceptable output voltage quality from the DG unit when supporting a dedicated
load. This includes minimizing voltage distortion in the presence of load
current
harmonics, providing appropriate damping in response to load transients and
minimizing voltage drop at heavy loads. Another objective of the impedance
network relates to creating preferred droop characteristics based on amplitude
and
phase of the voltage (Vac). The preferred droop characteristics result in
increased
reactive power output of the DG unit when the amplitude of the AC system
voltage
(Vac) is decreased. The preferred droop characteristics also result in
increased active
power output of the DG unit when the phase angle of Vs is increased relative
to the
AC system voltage (Vac).
In accordance with an embodiment of the invention, the value of the
impedances is kept constant for the VEN, and the values are determined based
on
the desired characteristics of the output. Further, the value of Vs is kept
dynamic as
the amplitude and phase of Vs with respect to the system voltage Vac will
affect the
active/reactive power output of the DG unit. Also, as discussed above, the
instantaneous value of Vs is determined by the power flow controller 214.
FIGS. 4 and 5 represent exemplary circuit diagrams of Virtual Electrical
Networks (VENs). The diagrams illustrate practical VEN configurations in
accordance with respective embodiments.
In accordance with an embodiment of the present invention, an exemplary
circuit diagram representing a VEN configuration, as shown in FIG. 4, will be
described herein. For a person ordinarily skilled in art, it is understood
that the
implementation details of the circuit as shown in FIG. 4 are similar to that
shown in
the circuit of FIG. 3.
The VEN circuit diagram as illustrated in FIG. 4 includes an AC voltage
source (Vs) in combination with an impedance network. The impedance network
includes a combination of impedances Z1 and Z2 as described in accordance with
FIG. 3. In this embodiment, the VEN circuit diagram, according to FIG. 4,
includes
a series combination of inductance Ls and resistance Ric (similar to Z1
defined in
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FIG. 3). Further, FIG. 4 may include a capacitance Cs (similar to Z2 defined
in FIG.
3).
In accordance with another embodiment of the present invention, an
exemplary circuit diagram representing a VEN configuration as shown in FIG. 5
will
be described herein. For a person ordinarily skilled in art, it is understood
that the
implementation details of the circuit diagram shown in FIG. 5 are similar that
shown
in the circuit diagram of FIG. 3.
The VEN circuit diagram as shown in FIG. 5 includes an AC voltage source
(Vs) in combination with an impedance network. The impedance network includes
a
combination of impedances Z1 and Z2 as described in accordance with FIG. 3. In
this embodiment, according to FIG. 5, the circuit diagram includes a series
combination of inductance Ls1 and a parallel combination of resistance 114c
and
capacitance Cdc (similar to Z1 defined in FIG. 3). In addition to this, FIG. 5
may
include a series combination of capacitance Cs and resistance Rklamp (similar
to Z2
defined in FIG. 3).
In accordance with an embodiment of the present invention, the
configuration of the VEN circuit may vary during operation to optimize the
characteristics of the synchronous generator at various operating conditions.
To a person ordinary skilled in the art, it is understood that the VEN may
vary according to requirements. Varying elements of the circuit may include,
but
are not limited to, one or more voltage sources, one or more current sources,
linear
or non- linear resistive components, capacitive components, and inductive
components.
FIG. 6 depicts a block diagram illustrating grouping of DG systems, loads,
and associated controllers into a micro grid, in accordance with an embodiment
of
the present invention.
A control scheme can be applied to a DG system such that it allows grouping
of a number of DG units, loads, and associated controllers into a micro grid.
The
block diagram as shown in FIG. 6 includes: a utility grid 622; a utility grid
controller
628; and a micro grid 602. The micro grid 602 in turn includes: some
combination
of one or more DG units 604 controlled according to the control scheme
presented
herein; one or more additional DG units 606 controlled in accordance other
control
schemes,; synchronous machine systems 608; a Supervisory Control Agent (SCA)
610; a low bandwidth communication network (LBCN) 624; and a load 626. All
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components may be hardware only, software only, or combinations of hardware
and
software. The load 626 may include one or more of a smart load 630 and a
conventional load 632. Further, the SCA 610 includes a smart switch 612 and an
energy management controller 614. The SCA 610 may further include a load
scheduling controller'. In an embodiment of the present invention, the load
scheduling controller 616 may form a part of the smart load 630 or connected
to the
conventional load 632. The smart switch 612 further includes a smart
controller 618
and sensors 620. In various embodiments of the present invention, the micro
grid
602 and the utility grid 622 may be directly connected to the smart switch 612
such
that the smart switch 612 provides a switching interface between the micro
grid 602
and the utility grid 622.
In accordance with an embodiment of the present invention, all components
as shown in FIG. 6 may be a combination of hardware and software. In
accordance
with another embodiment of the present invention, all components as shown in
the
figure may represent hardware components.
In accordance with an embodiment of the invention, the SCA 610 can exist
as a single physical module. In accordance with another embodiment of the
invention, the SCA 610 may exist as a collection of features built into a
number of
discrete systems or sub-systems.
In accordance with an embodiment of the invention, the load 626 may
comprise, but is not limited to, adjustable load, schedule-able load, and
fixed load.
The load 626 may further include one or more of smart loads 630 integrated
with a
load scheduling controller 616 and a conventional load 632. To operate with
the
conventional load 632, an external load controller may be desirable. The
external
load controller may be a part of the SCA 610 or may be a discrete component.
In accordance with an embodiment of the invention, the smart switch 612
functions as an AC connector to isolate the micro grid 602 from the utility
grid 622.
The smart switch 612 may further include a smart controller 618 and sensors
620.
The smart controller 618 is responsible for analyzing measurements of the
micro
grid voltage, the utility grid voltage, and the current flow. The smart
controller 618
further reports results of the analysis to the energy management controller
614 over
the low bandwidth communication network (LBCN) 624. The smart controller 618
also assists in disconnection, synchronization, and interconnection of the
micro grid
602 to the utility grid 622. Further, the smart controller 618 manages
adherence to
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standards at the point of common coupling to the utility grid 622. This may
include
the limitation of reactive power and/or harmonic currents into the main grid
and/or
the prevention of non-intentional islanding (energizing the load outside the
microgrid during a grid brownout/blackout period).
In accordance with an embodiment of the invention, the energy management
controller 614 and the load scheduling controller 616 may perform one or more
functions. The functions may include, but are not limited to: gathering and
sharing
information with individual DG units 604, 606, and the synchronous machine
systems 608, smart switch 612, and the load scheduling controller 616;
gathering
and sharing information with the utility grid controller 628; forecasting of
the
availability of DG resources and availability of the grid; and energy pricing.
The
forecast is based on factors such as current/forecast load, weather
conditions, and
other data obtained locally or from external services. In addition to this,
their
functions may also include: making decisions about transition into and out of
intentional islanding mode of operation, providing load shedding and
prioritization
schedule, prioritizing utilization and recharge of DG resources, responding to
real-
time pricing, and engaging in energy markets.
According to FIG. 6, all communication between the DG PEIs takes place
over the LBCN 624. The low bandwidth communication is preferred over other
modes because it is less expensive and easier to design. In accordance with
one
embodiment of the invention, the LBCN 624 may be implemented as a separate
network which is dedicated towards the control of such systems. In accordance
with
another embodiment of the invention, the LBCN 624 may be implemented using one
or more combinations of Local Area Network (LAN), Wi-Fi, WLAN, power line
communications, and GPRS network.
In accordance with an embodiment of the invention, the system as described
in FIG. 6 can be operated in an island mode by integrating multiple DG units
604
and 606 in parallel with each other to support an AC load. The SCA 610
implements a process of operation in the island mode by controlling the DG
units
604 and 606. Initially, the SCA 610 facilitates disconnection of one or more
micro
grids from the utility grid 622. Thereafter, the DG units 604 and 606
collectively
regulate the micro grid voltage within a tolerable limit. Further, in
accordance with
one embodiment of the invention, the SCA 610 may facilitate load sharing for
the
DG units 604 and 606. In accordance with another embodiment of the invention,
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SCA 610 may also allow the exchange of energy based on a predefined loading
priority schedule without necessitating the requirement of direct
communication
between at least two of the multiple DG units 604 and 606. Further, the SCA
610
influences loading priority and loading distribution by issuing appropriate
commands over the LBCN 624. In accordance with another embodiment of the
invention, the load sharing and exchange of energy based on predefined loading
priority schedule may be performed by the DG units without any involvement of
the
SCA 610.
In accordance with another embodiment of the invention, the system as
described in FIG. 6 may be operated in a grid-connected mode by connecting the
DG units 604 and 606 in parallel to the utility grid 622. The system
illustrated in
FIG. 6 implements the process of operation in grid-connected mode as described
below. The SCA 610 first facilitates the connection of the micro grid 602,
including
the DG units 604 and 606, to the utility grid 622. The DG units 604 and 606
supply
active/reactive power based on predefined default settings. Further, the SCA
610
influences the active/reactive power supplied by each of the DG units 604 and
606
in the micro grid by issuing appropriate commands over the LBCN 624.
In accordance with another embodiment of the invention, the DG units 604
and 606 may be able to achieve seamless transition between the grid-connected
mode and the island mode. Further, the process of achieving seamless
transition
from the island mode to the grid-connected mode may influence voltage
amplitude,
frequency, and phase of the DG units 604 and 606 by issuing commands from the
SCA 610 to the PEIs of each of the DG units 604 and 606 over the LBCN 624.
Accordingly, the SCA 610 connects the micro grid 602 to the utility grid 622
when
voltage, amplitude, frequency, and phase of the output of the DG units 604 and
606
are satisfactorily synchronized.
FIG. 7 represents a micro grid hierarchy, in accordance with an embodiment
of the present invention. FIG. 7 includes a system 702, one or more child
micro
grids 704 and 706 (collectively referred to as child micro grids), and a
parent micro
grid 708.
A micro grid hierarchy represents an arrangement of multiple micro grids
into a predefined hierarchical architecture. The micro grid hierarchy is
created by
assigning pre-determined parent-child relationships between the SCAs of
different
micro grids, as discussed above. The predetermined parent-child relationships
are
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based on various factors which may include, but not limited to, the size of
micro
grids, topology, and geographical location within the system. Further, the
relationships may be pre-assigned or dynamically modified in real time to
adapt to
varying operating conditions.
In accordance with an embodiment of the invention, different micro grids in
the micro grid hierarchy may be enlisted as a child micro gird or a parent
micro grid.
Each of the child micro grids may be enlisted as a member of a parent micro
grid,
thereby creating a parent-child relationship between the SCAs of these micro
grids.
It is understood by a person ordinarily skilled in art that a parent micro
grid may
further behave as a child of even larger micro grids, as illustrated in
conjunction
with an example below.
As depicted in FIG. 7, the system 702 behaves as a parent micro grid for the
child micro grids 704 and 706. However, at the same time, the system 702
behaves
as a child micro grid for the parent micro grid 708. The parent-child
relationship as
discussed above allows the parent SCA to treat the child micro grid as a
generic DG
resource. The parent SCA may use DG communication protocols and data models
to gather data from the child micro-grid, and subsequently issue commands to
supervise its operation. Further, the child SCA is responsible for collecting
data
from its member systems, presenting aggregated data to the parent SCA. The
child
SCA also analyzes commands issued by respective parent SCA, and may issue
commands to its member systems ensuring a proper response to the parent micro-
grid.
The formation of the micro grid hierarchy allows the distribution of
intelligence throughout the utility grid that includes a main grid and the
hierarchy of
micro grids. The distribution helps in avoiding reliance on a centralized
energy
management controller that requires massive data collection, processing,
decision-
making, and communication resources. Further, avoiding reliance oti the
centralized
energy management controller also eliminates the possibility of a single point
of
failure. The formation of a micro grid hierarchy further allows sectioning of
the
system and assists in the formation of intentional islands at different levels
of the
hierarchy.
The method and the system facilitating control strategy for power electronics
interface of distributed generation resources, or any of its components, as
described
in the present invention, may be embodied in the form of an embedded
controller.
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Typical examples of embedded controllers include a general-purpose computer, a
programmable microprocessor, a micro controller, a peripheral integrated
circuit
element, ASIC's (Application Specific Integrated Circuit), PLC's (Programmable
Logic Controller), and other devices or arrangements of devices that are
capable of
implementing the steps that constitute the method for the present invention.
The embedded controller executes a set of instructions (or program
instructions) that are stored in one or more storage elements to process the
input data.
These storage elements can also hold data or other information, as desired,
and may
be in the form of an information source or a physical memory element present
in the
processing machine. The set of instructions may include various commands that
instruct the processing machine to perform specific tasks such as the steps
that
constitute the method for the present invention. The set of instructions may
be in the
form of a software or firmware program. Further, the software or firmware may
be
in the form of a collection of separate programs, a program module with a
large
program, or a portion of a program module.
While various embodiments of the invention have been illustrated and
described, it will be clear that the invention is not limited only to these
embodiments.
Numerous modifications, changes, variations, substitutions, and equivalents
will be
apparent to those skilled in the art, without departing from the spirit and
scope of the
invention.
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