Patent Publication Number: US-2019199229-A1

Title: Matrix converter system with current control mode operation

Description:
BACKGROUND 
     The present invention relates generally to matrix converters, and in particular to a current control mode for a matrix converter. 
     Matrix converters are generally used as alternating current (AC)-to-AC converters that receive a multiphase input and produce a multiphase output. Traditional matrix converters operate in voltage control mode (VCM) such that the output voltage is controlled directly based upon the input voltage. In systems that utilize VCM, input inductor-capacitor (LC) circuits are implemented at the input of the matrix converter. The input LC circuit generally includes capacitors connected between each phase of the input. These capacitors create low impedance paths between phases and thus, the matrix converter cannot be controlled to provide short circuits between input phase lines without risking damage to the switching elements. Because the matrix converter cannot create output voltages higher than the line-to-line input voltages, the matrix converter is never able to provide an output voltage that is greater than 86.6% of the input voltage without distortion. 
     SUMMARY 
     In one embodiment, a matrix converter system includes a matrix converter, a generator, a plurality of output capacitors, and a controller. The matrix converter includes a plurality of switches and is connected between a multiphase input and a multiphase output. The plurality of output capacitors are connected between the multiphase output and ground. The generator is connected to the multiphase input and includes internal inductances. The controller is configured to control the plurality of switches to control active current and reactive current from the generator based on the internal inductances of the generator. The active and reactive currents are controlled to charge the plurality of output capacitors. 
     In another embodiment, a matrix converter includes a plurality of switches and is connected to receive power from a generator at a multiphase input of the matrix converter, and wherein a plurality of capacitors are connected to a multiphase output of the matrix converter. A method of controlling the matrix converter includes controlling, by a controller, the plurality of switches to generate reactive current using internal inductances of the generator; controlling, by the controller, the plurality of switches to generate active current using voltage on the multiphase output; and controlling, by the controller, the plurality of switches to control the voltage on the multiphase output based on the active current and the reactive current. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram illustrating a prior art matrix converter system. 
         FIG. 2  is a circuit diagram illustrating a matrix converter system that utilizes a current control mode. 
         FIGS. 3A-3C  are circuit diagrams illustrating a switch matrix, a bidirectional switch, and a damping circuit, respectively, for the matrix converter system of  FIG. 2 . 
         FIG. 4A  is a line diagram illustrating a back electromotive force (EMF) of a generator connected to an input of a matrix converter. 
         FIG. 4B  is a line diagram illustrating a balance between the back EMF of a generator and an input voltage of a matrix converter system. 
         FIG. 4C  is a phasor diagram illustrating vectors for back EMF voltage of a generator, the voltage at the input to a matrix converter, and active and reactive currents from the generator. 
         FIG. 5  is a block diagram illustrating a controller for controlling a matrix converter using a current control mode. 
     
    
    
     DETAILED DESCRIPTION 
     A matrix converter system is disclosed herein that operates in a current control mode (CCM). The matrix converter does not include an inductor-capacitor (LC) input filter and thus, there are no line-to-line capacitors on the input side of the matrix converter. The input of the matrix converter is connected to receive power from a multiphase generator. The generator includes internal inductances on each phase. Capacitors are connected between each output phase of the matrix converter and ground. 
     The matrix converter system is capable of operating in a boost mode. When the voltage on the output capacitors is relatively small, the switches of the converter may be operated to short phase lines to generate energy within the internal inductances of the generator. This is possible due to the absence of the LC circuit at the input, which removes the low impedance line-to-line capacitors. Upon generation of energy within the internal inductances of the generator, the switches of the matrix converter may then be controlled to provide current to charge the output capacitors using the generated energy stored in the inductances of the generator. This way, the output capacitors may be charged to a voltage greater than that of the input voltage. Once the voltage on the output capacitors is sufficient, the matrix converter may be controlled normally to control the output to a load. 
       FIG. 1  is a circuit diagram illustrating a prior art matrix converter system  10  that includes matrix converter  12 . Matrix converter  12  includes twelve switches and receives three phases (U, V, W) of input power. Capacitors C uvw  are connected between phases (U, V, W), and each phase (U, V, W) has an inductor L uvw  to form an LC input filter. Each input phase (U, V, W) is connected to each output phase (a, b, c) through matrix converter  12 . The switches of matrix converter  12  are controlled to convert input power on input phases (U, V, W) to output power on the output phases (a, b, c). Each output phase has an inductor L abc . Capacitors C a , C b , and C c  are connected between a respective output phase (a, b, c) and a common node to form an LC filter on the output of matrix converter  12 . Damp circuit  14  is utilized to provide a path for energy stored in input inductors L uvw  and output inductors L abc  upon shutdown of matrix converter  12 . In the embodiment illustrated in  FIG. 1 , system  10  is configured to provide an AC-AC voltage conversion that provides three phase smooth voltage on the output phases (a, b, c). However, in other embodiments, such as motor control applications, a motor can tolerate discontinuous voltages and therefore, motor windings can be directly connected to the output of matrix converter  12 . The internal inductances of the motor can reduce or eliminate the need for inductors L abc . 
     Matrix converter  12  is operated in a voltage control mode (VCM) such that the output voltage on lines (a, b, c) is controlled and limited by the available input voltage on lines (U, V, W). Because of capacitors C uvw , there exists a low impedance path between each of the input phase lines (U, V, W). Because of this, the switches of matrix converter  12  cannot be controlled to provide a short circuit between any of phase lines (U, V, W). If a short circuit were created, the low impedance path created by capacitors C uvw  would create a large current, risking damage to the switches. Because there are no energy storage devices present in system  10  to boost voltage during conversion, the analysis has shown that VCM for system  10  can only achieve, at best, 86.6% of the input voltage at the output. 
       FIG. 2  is a circuit diagram illustrating matrix converter system  20  capable of operation in a current control mode (CCM). Matrix converter system  20  includes matrix converter  22  connected to receive input power from three-phase generator  24  on input phases (u, v, w). Matrix converter  22  include switches S ua , S ub , S uc , S va , S vb , S vc , S wa , S wb , and S wc  (hereinafter “S ua -S wc ”) that each receive control from controller  26 . Capacitors C a , C b , and C c  are connected to the three respective output phases (a, b, c) of matrix converter  22 . Damp circuit  28  (illustrated in further detail in  FIG. 3C ) is connected to the three phase inputs of matrix converter  22 . Generator  24  is connected directly to matrix converter  22  through grounded sheath  30 . An electromagnetic interference (EMI) filter  32  is connected to the output of matrix converter  22  to provide further filtering for system  20 . While illustrated as three input phases (u, v, w) and three output phases (a, b, c), any number of input phases and any number of output phases may be connected through matrix converter  22 . 
     Matrix converter system  20  does not include inductors L u,v,w  and L a,b,c  ( FIG. 1 ) nor does system  20  include capacitors C u,v,w  ( FIG. 1 ) at the input of matrix converter  22 . By eliminating the capacitors at the input of matrix converter  22 , the low impedance paths between phases (u, v, w) on the input side of matrix converter  22  are eliminated. The internal inductances of generator  24  create high impedances and thus, switches S ua -S wc  may be controlled such that phase-to-phase short circuits are created without the risk of generating damaging overcurrents. This allows system  20  to utilize the internal inductances of generator  24  to generate reactive currents, which can be utilized to charge output capacitors C a , C b , and C c . This allows matrix converter system  20  to operate in a boost mode, generating currents to charge capacitors C a , C b , and C c  when the voltage on output phases (a, b, c) is relatively small. Thus, system  20  is able to overcome the 86.6% input-to-output voltage limit of prior art system  10 . 
     Controller  26  may control matrix converter  22  using a pulse-width modulation (PWM) control scheme, for example. Controller  26  may observe the current on input phases (u, v, w) and the voltage on output phases (a, b, c) to control matrix converter  22 . Controller  26  may select a desired output frequency that is independent of the input frequency. For example, the output may be a three-phase AC output at a frequency greater than, or less than, that of generator  24 . Generator  24  may also be a variable frequency generator, such that the frequency is at times greater than the output frequency, and at times less than the output frequency. The output frequency may also be selected to be zero, allowing for matrix converter  22  to generate a DC output from the AC input. 
     During operation of system  20 , when the output voltage is sufficiently high, the operation of matrix converter  22  may be similar to that of system  10 . However, when the system is starting up, or if the voltage on the output phases (a, b, c) becomes relatively small, controller  26  may control matrix converter  22  in a boost mode in order to boost the voltage on output phases (a, b, c). To do this, controller  26  may control select switches (S ua -S wc ) to provide one or more short circuit paths between input phases (u, v, w). These short circuit paths rely on the high impedances created by the internal inductance of generator  24 . During the short circuit condition, energy is generated within the internal inductances of generator  24 . After a selected time, switches (S ua -S wc ) are controlled to remove the short circuit, and utilize the energy stored in the internal inductances of generator  24  to direct current to charge capacitors C a , C b , and C c , which allows system  20  to create output voltages greater than the input voltages. 
       FIGS. 3A-3B  are circuit diagrams illustrating a switch matrix for matrix converter  22 , a bidirectional switch  40 , and damp circuit  28 , respectively.  FIG. 3A  illustrates an embodiment of matrix converter  22 . As illustrated in  FIG. 3A , matrix converter  22  may have any number (n) of input phases, and any number (m) of output phases. In the embodiment illustrated in  FIG. 2 , matrix converter has three input phases (n=3) and three output phases (m=3). As illustrated in  FIG. 3A , matrix converter system  20  may be configured to generate any number of output phases from any number of input phases. 
       FIG. 3B  illustrates bidirectional switch  40 . In prior art systems, matrix converters included bidirectional switches implemented using insulated gate bipolar transistors (IGBTs), for example. The use of IGBTs required the use of diodes to properly achieve bidirectional power flow. Bidirectional switch  40 , in contrast, uses metal-oxide-semiconductor field-effect transistors (MOSFETs)  42  and  44 , which may be silicon carbide (SiC) MOSFETs, for example. MOSFETs are channel devices that do not require the use of external diodes, as the internal channels of the MOSFET accomplish this task. Thus, bidirectional switch  40  may be accomplished by connecting the drains of MOSFETs  42  and  44 . By using MOSFETs, the conduction loss of the bidirectional switch may be reduced. 
       FIG. 3C  illustrates an embodiment of damp circuit  28 . As seen in  FIG. 2 , and in contrast to system  10  of  FIG. 1 , damp circuit  28  is only connected to the input side of matrix converter  22 . This is due to the absence of inductors L a , L b , and L c  ( FIG. 1 ) from system  20 . Damp circuit  28  is configured such that if there is a disruption of current flow, the energy stored in the internal inductances of generator  24  has a path. Damp circuit  28  includes diodes D 1 -D 7 , capacitor C D , resistor R D , and switch S D . Diodes D 1 -D 6  form a rectifier circuit. The energy stored in the internal inductances of generator  24  upon shutdown of matrix converter  22 , for example, flows through the rectifier circuit and charges capacitor C D . Switch S D  may then be controlled to discharge capacitor C D  through resistor R D . 
       FIGS. 4A-4C  are diagrams illustrating voltage and current relationships within system  20 .  FIG. 4A  is a line diagram illustrating a back electromotive force (EMF)  50  of generator  24  connected to the input of matrix converter  22 .  FIG. 4B  is a line diagram illustrating a balance between the back EMF voltage E x  of generator  24  and input voltage V x  to matrix converter  22 .  FIG. 4C  is a phasor diagram illustrating vectors for back EMF voltage Ė x  of a generator, the voltage {dot over (V)} x  at the input to a matrix converter, and active İ q  and reactive İ d  currents from generator  24 .  FIGS. 4A-4C  will be discussed together. 
     Generator  24  produces back EMF  50  (e uvw ) and has internal inductances L G . Voltage (v uvw ) at the output of generator  24  is provided to matrix converter  22 . The voltage across capacitors Cabs (v abc ) is the output voltage of matrix converter  22 . For CCM, controller  26  operates to control input voltage (v uvw ) to control current through matrix converter  22 . This relationship is seen in  FIG. 4B . Ė x  designates the voltage vector for the voltage of back EMF  50  (e uvw ). {dot over (V)} x  designates the voltage vector for the input voltage (v uvw ) to matrix converter  22 . İ x  designates the current vector for current flowing through internal inductances L G . Thus, to control current flow in input phases (u, v, w), controller  26  may control the input voltage (v uvw ). For example, if the input voltage vector {dot over (V)} x  is equal to the back EMF vector Ė x , no current will be flowing to the input of matrix converter  22 . 
       FIG. 4C  is a phasor diagram illustrating the relationship of the back EMF voltage (e uvw ) and the input voltage (v uvw ).  FIG. 4C  includes phasors Ė x , {dot over (V)} x , and İ x . İ x  is the product of the active current phasor İ q  and the reactive current phasor İ d , which are 90° out of phase. The active current phasor İ q  is in phase with the back EMF voltage Ė x . The voltage drop across the internal inductance L G  (jωLİ x ) balances the input voltage {dot over (V)} x  with the back EMF voltage Ė x . The voltage drop across inductor L G  (jωLİ x ) is the result of the voltage drop due to the active current (jωLİ q ) and due to the reactive current (jωLİ d ). Thus, as can be seen in  FIGS. 4B and 4C , {dot over (V)} x  can be controlled to control the current İ x  to matrix converter  22 . As seen in  FIG. 4A , v uvx  can be controlled, in part, by controlling switch matrix  22  to connect v abc  on the output phases (a, b, c) to the input phases (u, v, w). 
       FIG. 5  is a block diagram illustrating a portion of controller  26  for controlling matrix converter  22  using CCM. Controller  26  includes PWM control circuit  60 , magnitude block  62 , voltage regulator  64 , active current regulator  66 , current sensors  68 , transform block  70 , observer  72 , rotators  74  and  76 , reactive current regulator  78 , desaturation block  80 , adder  82 , and transform block  84 . Switches S ua -S wc  of matrix converter  22  are controlled by signals output from PWM control circuit  60 . PWM control circuit  60  implements a matrix converter PWM algorithm such as, for example, space-vector control with triangle comparison, or any other PWM algorithm for matrix converter  22 . 
     The output voltage (v abc ) of matrix converter  22  is sampled by magnitude block  62 . Magnitude block  62  is any analog or digital circuit or device capable of outputting a voltage value (V) indicative of the magnitude of voltage at capacitors C a , C b , and C c . The input voltage (v uvw ) of matrix converter  22  is sampled at the output of generator  24  and provided to observer  72 . Current (i uvw ) is sampled by current sensors  68  at the output of generator  24  and also provided to observer  72 . Observer  72  is a circuit capable of determining an angular position (θ uvw ) of generator  24  based upon the sampled voltage (v uvw ) and current (i uvw ). Sampled current (i uvw ) is also provided to transform circuit  70 . 
     Transform circuit  70  performs an abc-to-dq transformation to convert the three-phase signal into two DC-like signals (i q   s  and i d   s , hereinafter referred to as “static DC signals”). The static DC signals (i q   s  and i d   s ) are provided to rotator  74 , which uses the angular position (θ uvw ) of generator  24  to rotate the two static DC signals (i q   s  and i d   s ) to generate active current reference (i q ) and reactive current reference (i d ). These signals are indicative of the present active and reactive currents in generator  24 . Thus, controller  26  receives a reference output voltage magnitude (V), a reference active current (i q ), and a reference reactive current (i d ). 
     Controller  26  generates a control vector V* that represents a desired output voltage (v abc ). The output voltage magnitude (V) is provided to voltage regulator  64 . Using the control input (V*) and the measured voltage magnitude (V), voltage regulator  64  generates a current indicative of the difference between the output voltage magnitude (V) and the control input (V*). Because the active current (i q ), as illustrated in  FIG. 4C , is in phase with the back EMF voltage of generator  24 , the output of voltage regulator  64  is an active current control signal (i q *). The active current control signal (i q *) is provided to active current regulator  66 . 
     Active current regulator  66  receives the active current feedback (i q ) and the active current control signal (i q *). Active current regulator  66  generates a voltage indicative of the difference between the active current feedback (i q ) and the active current control signal (i q *) and provides the voltage to rotator  76 . 
     PWM control circuit  60  provides an output signal to desaturation block  80 . The output signal is indicative of saturation of the PWM algorithm. For example, if the output voltage (v abc ) has become small compared to the input voltage (v uvw ), the PWM algorithm may no longer be able to control the input voltage (v uvw ) using the output voltage (v abc ), resulting in saturation of the PWM algorithm. In this scenario, matrix converter  22  will need to be controlled to generate some reactive current in generator  24  to increase the output voltage (v abc ) to pull the PWM algorithm out of saturation. Thus, desaturation block  80  is configured to output a control signal indicative of the desired change in reactive current (Δi d *). 
     Controller  26  also generates a reactive current control signal (i d *) indicative of a desired reactive current. This may be done while matrix converter system  20  is operating in a boost mode, for example, such as during startup of generator  24 . Adder  82  is used to adjust the reactive current control signal (i d *) based on the desired change in reactive current (Δi d *) from desaturation block  80 . The output of adder  82  is provided to reactive current regulator  78 . Reactive current regulator  78  generates a voltage indicative of the difference between the reactive current feedback (i d ) and the reactive current control signal from adder  82 , and provides the voltage to rotator  76 . 
     Rotator  76  receives the two static voltages from regulators  66  and  78 . Rotator  78  also receives a control angular position (θ abc ). Controller  26  uses the control angular position (θ abc ) to control the frequency of the output voltage (v abc ). Rotator  76  outputs two static voltages and provides the two static voltages to transform block  84 . Transform block  84  performs a dq-to-abc transformation on the two static voltages to generate three-phase signals for PWM control circuit  60 . PWM control circuit  60  then uses the three-phase signals to control matrix converter  22 . 
     As discussed above, if the desired reactive current is non-zero, PWM control circuit  60  may control some of switches S ua -S wc  to create line-to-line short circuits in order to generate the desired amount of reactive current. Then, to supply the desired amount of active current, PWM control circuit  60  may control switches S ua -S wc  to control the input voltage (v uvw ) based on the output voltage (v abc ) in order to control the active current. 
     Matrix converter system  20  has several advantages over matrix converter system  10 . By controlling matrix converter system  20  using CCM, the input LC circuit may be removed, and the output inductors may also be removed. This reduces the size and weight of the matrix converter system. Matrix converter system  20  is also able to operate in a boost mode, allowing output voltage operation above 86.6% of the input voltage which has been a major drawback in previous matrix converter system design. 
     Discussion of Possible Embodiments 
     The following are non-exclusive descriptions of possible embodiments of the present invention. 
     A matrix converter system includes a matrix converter, a generator, a plurality of output capacitors, and a controller. The matrix converter includes a plurality of switches and is connected between a multiphase input and a multiphase output. The plurality of output capacitors are connected between the multiphase output and ground. The generator is connected to the multiphase input and includes internal inductances. The controller is configured to control the plurality of switches to control active current and reactive current from the generator based on the internal inductances of the generator. The active and reactive currents are controlled to charge the plurality of output capacitors. 
     The system of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     A further embodiment of the foregoing system, wherein the controller comprises a pulse-width modulation circuit configured to execute a pulse width modulation algorithm to generate switch control signals for the plurality of switches to control the active current and the reactive current from the generator. 
     A further embodiment of any of the foregoing systems, wherein the controller is further configured to generate a desired voltage magnitude and a desired reactive current, and wherein the controller further includes a first regulator configured to receive the desired voltage magnitude and an observed voltage magnitude, wherein the observed voltage magnitude is a magnitude of a voltage on the multiphase output, and wherein the first regulator circuit is configured to output a desired active current. 
     A further embodiment of any of the foregoing systems, wherein the pulse-width modulation circuit is further configured to output a signal indicative of saturation of the pulse-width modulation algorithm, and wherein the controller further includes a desaturation module configured to generate a reference delta reactive current based on the signal indicative of saturation; an adder configured to output a reactive current command based on the desired reactive current and the reference delta reactive current; a second regulator configured to receive the reactive current command and an observed reactive current, and generate a reactive current voltage; and a third regulator configured to receive the desired reactive current and an observed active current, and generate an active current voltage. 
     A further embodiment of any of the foregoing systems, wherein the multiphase input is a three-phase input, and wherein the controller further includes an observer configured to determine a generator phase angle; a first converter configured to convert a sensed three-phase current on the three-phase input into two static direct current signals; and a first rotator configured to output the observed active current and the observed reactive current based on the two static direct current signals and the generator phase angle. 
     A further embodiment of any of the foregoing systems, wherein the controller further includes a second rotator configured to output a static voltage reference based on the reference active voltage, the reference reactive voltage, and a reference phase angle; and a second converter configured to convert the static voltage reference into a three-phase reference voltage, wherein the pulse-width modulation algorithm utilizes the three-phase reference voltage to generate the switch control signals. 
     A further embodiment of any of the foregoing systems, wherein the reference phase angle is selected based on a desired output frequency of the matrix converter. 
     A further embodiment of any of the foregoing systems, wherein each of the plurality of switches is a bidirectional switch comprising first and second power metal-oxide-semiconductor field-effect transistors. 
     A further embodiment of any of the foregoing systems, further comprising an energy damp circuit connected only to the multiphase input. 
     A further embodiment of any of the foregoing systems, wherein there are no capacitors connected between any phases of the multiphase input. 
     A method of controlling a matrix converter, wherein the matrix converter includes a plurality of switches and is connected to receive power from a generator at a multiphase input of the matrix converter, and wherein a plurality of capacitors are connected to a multiphase output of the matrix converter, the method including controlling, by a controller, the plurality of switches to generate reactive current using internal inductances of the generator; controlling, by the controller, the plurality of switches to generate active current using voltage on the multiphase output; and controlling, by the controller, the plurality of switches to control the voltage on the multiphase output based on the active current and the reactive current. 
     The method of the preceding paragraph can optionally include, additionally and/or alternatively, any one or more of the following features, configurations and/or additional components: 
     A further embodiment of the foregoing method, wherein controlling, by the controller, the plurality of switches to generate reactive current includes controlling the plurality of switches to provide short circuit connections between phases of the multiphase input. 
     A further embodiment of any of the foregoing methods, wherein controlling, by the controller, the plurality of switches to control the voltage on the multiphase output includes controlling, by a pulse-width modulation circuit, the plurality of switches using a pulse-width modulation algorithm. 
     A further embodiment of any of the foregoing methods, wherein controlling, by the controller, the plurality of switches to control the voltage on the multiphase output further includes generating, by the controller, a desired voltage magnitude and a desired reactive current; sampling, by the controller, an observed voltage magnitude on the multiphase output of the matrix converter; receiving, by a first regulator, the desired voltage magnitude and the observed voltage magnitude; and generating, by the first regulator, a desired active current. 
     A further embodiment of any of the foregoing methods, wherein controlling, by the controller, the plurality of switches to control the voltage on the multiphase output further includes generating, by the pulse-width modulation circuit, a signal indicative of saturation of the pulse-width modulation algorithm; generating, by a desaturation module, a reference delta reactive current based on the signal indicative of saturation; generating, by an adder, a reactive current command based on the desired reactive current and the reference delta reactive current; generating, by a second regulator, a reactive current voltage using the reactive current command and an observed reactive current; and generating, by a third regulator, an active current voltage using the desired reactive current and an observed active current. 
     A further embodiment of any of the foregoing methods, wherein the multiphase input is a three-phase input, and wherein controlling, by the controller, the plurality of switches to control the voltage on the multiphase output further includes determining, by an observer, a generator phase angle; sensing a three-phase current on the three-phase input; converting, by a first converter, the sensed three-phase current into two static direct current signals; and generating, by a first rotator, the observed active current and the observed reactive current based on the two static direct current signals and the generator phase angle. 
     A further embodiment of any of the foregoing methods, wherein the multiphase output is a three-phase output, and wherein controlling, by the controller, the plurality of switches to control the voltage on the multiphase output further includes generating, by a second rotator, a static voltage reference based on the reference active voltage, the reference reactive voltage, and a reference phase angle; and converting, by a second converter, the static voltage reference into a three-phase reference voltage, wherein the pulse-width modulation algorithm utilizes the three-phase reference voltage to generate the switch control signals. 
     A further embodiment of any of the foregoing methods, wherein the reference phase angle is selected based on a desired output frequency of the matrix converter. 
     A further embodiment of any of the foregoing methods, wherein there are no capacitors connected between any phases of the multiphase input. 
     A further embodiment of any of the foregoing methods, wherein each of the plurality of switches is a bidirectional switch comprising first and second power metal-oxide-semiconductor field-effect transistors. 
     While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.