Abstract:
The present disclosure includes electrical motor/generator drive systems and methods that significantly reduce inverter direct-current (DC) bus ripple currents and thus the volume and cost of a capacitor. The drive methodology is based on a segmented drive system that does not add switches or passive components but involves reconfiguring inverter switches and motor stator winding connections in a way that allows the formation of multiple, independent drive units and the use of simple alternated switching and optimized Pulse Width Modulation (PWM) schemes to eliminate or significantly reduce the capacitor ripple current.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of Provisional Application Ser. No. 61/245,682 entitled “ELECTRICAL MOTOR/GENERATOR DRIVE APPARATUS AND METHOD”, filed Sep. 25, 2009, which is herein incorporated by reference in its entirety 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0002]    This invention was made with government support under Contract No. DE-AC05-00OR22725 awarded by the U.S. Department of Energy. The government has certain rights in the invention. 
     
    
     THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
       [0003]    None. 
       BACKGROUND OF THE INVENTION 
       [0004]    1. Field of the Invention 
         [0005]    This disclosure relates to electric motors and more specifically to electrical motor drive methods and apparatuses. 
         [0006]    2. Description of the Related Art 
         [0007]    The use of electric vehicles (EVs) has been promoted in recent years as an effective way to reduce crude oil consumption and emissions of harmful pollutants and green-house gasses. EVs broadly include battery powered electric vehicles (BEVs), fuel cell powered vehicles, and hybrid electric vehicles (HEVs). 
         [0008]    HEVs employ an energy storage device such as a battery and an electrical motor drive system that are designed to optimize the energy conversion efficiency of an internal combustion engine. They also capture a portion of the kinetic energy of the vehicle for later use through dynamic braking by the motor during deceleration. An electrical motor drive system may include one or more drive units and each drive unit typically consists of a power inverter and a motor. The inverter either functions to convert a direct-current (DC) voltage to an alternating-current (AC) voltage suitable to operate the motor, or functions as a converter when the motor is operating in power generation mode. Multiple electrical drive units can be used to provide dual-wheel or four-wheel drive capabilities. 
         [0009]    A power inverter in HEVs operates off a DC voltage source, such as a battery, and typically produces a three-phase AC voltage with adjustable frequency and amplitude to control a three-phase AC motor. As an example,  FIG. 1  illustrates a block diagram of a conventional electrical motor drive system  10  including a DC power supply  12 , a DC bus filter capacitor  14 , a three-phase inverter  16 , and a three-phase motor/generator  18  for parallel HEVs or battery electric vehicles BEVs. In the motor  18 , three-phase currents flowing through the stator windings  20  create a rotating electrical field, causing a rotor to spin. 
         [0010]    Power semiconductor devices, such as the insulated-gate-bipolar transistor (IGBT) and diode connected in anti-parallel fashion or the Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFET), are often used for the inverter  16  switches, (S 1 -S 6 ). An electronic controller  22 , based on one or more microprocessors, is typically used to control operations of the electrical motor drive system  10 . A gate driver circuit  24  is also used to convert the low-voltage digital control signals generated by the controller  22  to higher voltage gating signals that are suitable for turning on or off the semiconductor switches (S 1 -S 6 ) and to provide galvanic isolation between the power inverter and the electronic controller. 
         [0011]    The electric motor  18  is controlled by the inverter  16  and powered by the battery  12  to handle the variations in the driving force demands for optimizing the fuel efficiency of a vehicle. For charging the battery  12 , the motor  18  functions as a generator driven by the engine or by the vehicle&#39;s inertia during deceleration and produces an AC voltage, which is converted by the inverter  16  to a DC voltage at a level that is suitable to charge the battery  12 . 
         [0012]    The ratio of conduction to non-conduction time interval for each switch (S 1 -S 6 ) is determined by using a pulse width modulation (PWM) scheme to adjust the amplitude and frequency of the fundamental component of the inverter  16  output voltage. The PWM switching of the motor currents generates a pulsed inverter DC bus current consisting of a DC component and large ripple AC current components, as shown in  FIG. 2  as an example. 
         [0013]    Referring now to  FIG. 2 , the current waveforms at the battery I bat , the inverter I inv , the DC bus capacitor I Cbus , and the stator windings i a ,i b , i c  are shown. The root-mean-squared (rms) value of the sum of all the ripple components in the inverter DC-link current can reach up to fifty percent (50%) of the motor current. The inverter DC-bus current can be expressed by equation 1 below. 
         [0000]        i   inv   =I   dc +Σ k=0   ∞ Σ n=1   ∞   I   n,k  sin [2π( nf   sw ±6 kf   m ) t+α   n,k ]  (1)
 
         [0000]    In equation 1, the first term is the DC component and the second term represents the ripple components, which have frequencies of multiples of the switching frequency (f sw ) or their side bands associated with the fundamental frequency (f m ). 
         [0014]    To absorb the large ripple currents, which are detrimental to the battery  12 , and to maintain a near ideal voltage source to the inverter  16 , the inverter  16  generally requires the use of a very high performance DC bus capacitor  14 . The DC bus capacitor  14  is physically located close to the inverter switches (S 1 -S 6 ) when installed. 
         [0015]    Currently available capacitors  14  that can meet the demanding requirements of HEV application are both costly and bulky. Further, their ripple current handling capability drops rapidly as the ambient temperature increases. Thus, a low temperature liquid cooling system is oftentimes needed to operate the inverters in the engine compartment of an HEV, adding cost, weight and complexity to the vehicle. As a result, conventional inverter designs make it difficult to meet cost, volume and lifetime requirements for HEV applications. 
       SUMMARY 
       [0016]    Electrical motor/generator drive systems are disclosed. The drive systems include a battery and a DC bus filter capacitor that is connected in parallel with the battery. An inverter having pairs of switches is connected in parallel with the DC bus filter capacitor. One or more motor/generators, defining at least two sets of stator windings, are coupled to the pairs of inverter switches. Each of the sets of windings can be used independently cause motion of a rotor in one of the motor/generators. The pairs of the switches associated with each set of stator windings define an inverter bridge. Thus each inverter bridge forms an independent drive unit for each stator winding set. 
         [0017]    Control methods for such drive systems are also disclosed. The methods include performing a first step of obtaining plurality of triangular carrier signals, where each of the carrier signals is associated with one of the drive units. Each of the carrier signals configured to have an electrical degree phase shift with respect to each other. Next, a plurality of modulation signals are obtained, where each of the pairs of inverter switches in each drive unit is associated with one of the modulation signals. Thereafter a set of control signals for the inverter switches in the drive units are generated based on comparison of the plurality of modulation signals to one of the plurality of triangular carrier signals associated with the one of the plurality of drive units. 
         [0018]    These and other objects, features and advantages will become apparent to those skilled in the art in view of the following detailed description and accompanying illustrations of multiple examples. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  illustrates an example of a prior art motor control system. 
           [0020]      FIG. 2  illustrates electrical current waveforms in various components of the prior art motor control system of  FIG. 1 . 
           [0021]      FIG. 3  illustrates a first example of a motor control system in accordance with the various embodiments of the invention. 
           [0022]      FIG. 4  illustrates interleaved switching according to the example of  FIG. 3 . 
           [0023]      FIG. 5  illustrates a second example of a motor control system in accordance with the various embodiments of the invention. 
           [0024]      FIG. 6  illustrates a third example of a motor control system in accordance with the various embodiments of the invention. 
           [0025]      FIG. 7  illustrates a fourth example of a motor control system in accordance with the various embodiments of the invention. 
           [0026]      FIGS. 8A and 8B  show a flow chart illustrating the various method steps for performing an electrical motor/generator drive control method in accordance with the various embodiments of the invention. 
           [0027]      FIG. 9  is an X-Y plot of capacitor ripple current as a function of motor phase current for a conventional drive system configuration as in  FIG. 1  and a drive system configured in accordance with the embodiment illustrated in  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    The invention is described with reference to the attached figures, wherein like reference numerals are used throughout the figures to designate similar or equivalent elements. The figures are not drawn to scale and they are provided merely to illustrate the instant invention. Several aspects of the invention are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the invention. One having ordinary skill in the relevant art, however, will readily recognize that the invention can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the invention. The invention is not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the invention. 
         [0029]      FIG. 3  illustrates a first example of an electrical motor drive system according to the present disclosure. The system  110  includes a battery  112  or any other DC power supply, a DC bus filter capacitor  114 , a three-phase inverter  116  and a three-phase electrical motor/generator  118 . The inverter  116  switches (S a1p , S a1n , S b1p , S b1n , S c1n , S c1p , S a2p , S a2n , S b2p , S b2n , S c2n , and S c2p ) and Y-connected stator windings  120  in the motor  118  are separated into two sets of windings (a 1 , b 1 , c 1 ) and (a 2 , b 2 , c 2 ), where each set of windings includes a winding associated with a phase group. As used herein with respect to stator windings, the term “phase group” refers to windings associated with a same AC signal component of an AC poly-phase signal. 
         [0030]    Further, as shown in  FIG. 3 , each phase group of the stator windings (a 1 , a 2 ), (b 1 , b 2 ), and (c 1 , c 2 ) are co-located. That is, each phase group of stator windings is disposed in a same portion, region, or slot of the stator. For example, the phase group of windings can be disposed next to each other or intertwined, but galvanically isolated. The common points (i.e., the neutral node) of the two sets of the stator windings can be coupled together. Each group of switches (S a1p , S a1n , S b1p , S b1n , S c1n , S c1p ) and (S a2p , S a2n , S b2p , S b2n , S c2n , S c2p ) is connected as an inverter bridge and connects to one set of the motor stator windings  119 , forming an independent drive unit. 
         [0031]    Although the various embodiments are illustrated with a single capacitor  114 , this is for illustrative purposes only. Accordingly, the invention is not limited in this regard. Rather, any bus filter capacitor circuit providing the functionality of a single bus filter capacitor  114  can be used in the various embodiments of the invention. 
         [0032]    An electronic controller  122 , based on one or more microprocessors, is used to control operations of the electrical motor drive system  110 . The electronic controller  122  can take measurements of two motor phase currents (i a  and i c ), rotor position/speed, battery voltage (V bat ), and/or motor speed or torque commands as inputs and carries out a predefined motor control algorithm to generate a set of three-phase modulation signals (v a , v b , v c ). Any known methods can be used for generating the modulation signals. In the configuration illustrated in  FIG. 3 , the controller  122  further performs pulse width modulations by comparing the three-phase modulation signals with two carrier signals that have an electrical degree phase shift between to generates two sets of low-voltage digital control signals (v g     —     a1p , v g     —     a1n, v   g     —     b1p , v g     —     b1n , v g     —     c1n , v g     —     c1p ) and (v g     —     a2p , v g     —     a2n , v g     —     b2p , v g     —     b2n , v g     —     c2n , v g     —     c2p ) for controlling the switching timing of the switches. A gate driver circuit  124  is also used to convert the low-voltage digital control signals generated by the controller  122  to higher voltage gating signals that are suitable for turning-on or turning-off the switches in the inverter  116 . The timing of turning-on and turning-off of the corresponding switches in the two independent drive units is controlled in a way that minimizes the DC bus ripple currents. 
         [0033]    Although the various embodiments are illustrated with switches comprising a bipolar transistor and a diode connected in anti-parallel fashion, this is for illustrative purposes only. Accordingly, the invention is not limited in this regard. Rather, any type of switch element or circuit providing the functionality of the switches illustrated in exemplary embodiments can be used in the various embodiments of the invention. 
         [0034]      FIG. 4  graphically illustrates an exemplary method of controlling switching for two independent drive units in accordance with the various embodiments. In  FIG. 4 , the switch control signals are generated by comparing three-phase modulation signals, (v a , v b , v c ) with two triangular carrier signals, v c     —     a1  for the first drive unit and v c     —     a2  for the second drive unit. In particular,  FIG. 4  illustrates how switch control signals are generated for a first inverter switch pair S a1p ), S a1n  and a second inverter switch pair S a2p , S a2n , associated with a first phase group (“phase a”) of first and second drive units, respectively. The switch control signals for the remainder of the switches associated with the other phases can be generated in a similar fashion. 
         [0035]    As used herein, the term “inverter switch pair” refers to the serial combination of two switch elements in an inverter bridge coupled to a DC power supply, where a common node of the two switch elements is coupled to a stator winding. The term “positive switch” refers to the switch element in the inverter switch pair coupled between a stator winding and a positive or high voltage terminal of a DC power supply. The term “negative switch” refers to the switch element in the inverter switch pair coupled between a stator winding and a negative or low voltage terminal of a DC power supply. 
         [0036]    The generation of the switch control signals for switches S alp , S a2p , S a1n , and S a2n  is as follows. First, a current value of the phase a modulation signal v a  is compared with v c     —     a1 , the carrier signal for the first drive unit. When v a &gt;v c     —     a1 , a control signal corresponding to a digital high or “1” is provided as the switch control signal (v g     —     a1p ) for positive switch (S a1p ). Otherwise, a digital low of “0” (i.e., when v a &lt;v c     —     a1 ) is provided as the switch control signal for switch S a1p . The switch control signal (v g     —     a1n ) for a corresponding negative switch (S a1n ) can then be generated by logically inverting v g     —     a1p . However, the various embodiments are not limited in this regard and switch control signals for the negative switches can be independently generated based on the comparison above. 
         [0037]    Concurrently, the current value of the phase a modulation signal v a  is compared with v c     —     a2 , the carrier signal for the second drive unit. When v a &gt;v c     —     a2 , a control signal corresponding to a digital high or “1” is provided as the switch control signal (v g     —     a2p ) for positive switch (S a2p ). Otherwise, a digital low of “0” (i.e., when v a &lt;v c     —     a2 ) is provided as the switch control signal for switch S a2p . The switch control signal (v g     —     a2n ) for a corresponding negative switch (S a2n ) can then be generated by logically inverting v g     —     a2p . However, the various embodiments are not limited in this regard and switch control signals for the negative switches can be independently generated based on the comparison above. 
         [0038]    Switch control signals for the positive switches associated with other phases can be similarly generated by comparing the respective modulation signal with the corresponding carrier signal. Further, as described above, switch control signals for the negative switches can be obtained by logically inverting the corresponding first switch control signal. In the embodiment illustrated in  FIG. 4 , the two carrier signals are identical except there is a 180 electrical degrees phase shift between the two signals. The resultant inverter DC bus currents of the two units and their combined one can be expressed by equations 2-4 below. 
         [0000]    
       
         
           
             
               
                 
                   
                       
                   
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         [0039]    As shown in the equations above, the amplitude of each combined ripple component can be reduced by introducing a phase shift into the corresponding ripple components of the two drive units, i.e. β n,k ≠α n,k . Moreover, the combined ripple current components can be reduced to zero by introducing a 180 electrical degree phase shift into the corresponding ripple components of the two drive units. Shifting the carrier signals as shown in  FIG. 4  can lead to cancellation of some of the ripple current components while reducing others. The net effect is a significantly reduced DC bus ripple current to be filtered out by the DC bus capacitor. As a result of the reduced DC ripple current, the size of capacitors  114  can be reduced by 55%-75%. Thus, less costly and less bulky capacitors can be used. Further such capacitors typically would not require a low temperature liquid cooling system. 
         [0040]    The control methodology described above with respect to  FIG. 4  is not limited solely to control of a drive system in accordance with  FIG. 3 . Rather, the control methodology can be used with various other configurations in which multiple drive units are being powered by a single DC voltage supply and reduction of DC ripple current is desired. 
         [0041]    For example,  FIG. 5  illustrates a second example of an electrical motor drive system  510  configured in accordance with an embodiment of the invention. Similar to system  110 , system  510  includes a battery  112 , a DC bus filter capacitor  114 , and a three-phase inverter  116  coupled to a three-phase electrical motor/generator  118 . Further, the switches in inverter  116  and Y-connected stator windings  120  in the motor  118  are separated into two sets (a 1 , b 1 , c 1 ) and (a 2 , b 2 , c 2 ) and each phase group of the stator windings (a 1 , a 2 ), (b 1 , b 2 ), and (c 1 , c 2 ) are co-located. Further, an electronic controller  122  controls the operations of the electrical motor drive system  510  and gate driver circuit  124  is also used to convert the low-voltage digital control signals generated by the controller  122  to higher voltage gating signals that are suitable for turning-on or turning-off the switches in inverter  116 . However, in contrast to system  110 , the neutral nodes of the two sets of the stator windings in system  510  are galvanically isolated. Nonetheless, the timing of turning-on and turning-off of the corresponding switches in the two independent drive units can be controlled in a way that minimizes the DC bus ripple currents by using the methods described above with respect to  FIGS. 3 and 4 . 
         [0042]    In another example,  FIG. 6  illustrates a third example of an electrical motor drive system  610  configured in accordance with an embodiment of the invention. Similar to system  110 , the system  610  includes a battery  112 , a DC bus filter capacitor  114 , and a three-phase inverter  116  coupled to a three-phase electrical motor/generator  118 . The switches in inverter  116  and Y-connected stator windings  120  in the motor  118  are separated into two groups (a 1 , b 1 , c 1 ) and (a 2 , b 2 , c 2 ). An electronic controller  222  controls the operations of the electrical motor drive system  210 . A gate driver circuit  124  is also used to convert the low-voltage digital control signals generated by the controller  122  to higher voltage gating signals that are suitable for turning-on or turning-off the switches in inverter  116 . However, in contrast to system  110 , the two sets of the stator windings in system  610  are spatially separated. That is, the stator windings occupy different stator slots. Further, the neutral nodes of the two sets of the stator windings are not bundled together. Despite these differences, the timing of turning-on and turning-off of the corresponding switches in the two independent drive units can also be controlled in a way that minimizes the DC bus ripple currents by using the methods described above with respect to  FIGS. 3 and 4 . 
         [0043]    In still another example,  FIG. 7  illustrates a fourth example of an electrical motor drive system  710  according to the present disclosure. The system includes a battery  712 , a DC bus capacitor  714 , and two identical, separate drive units, each consisting of a three-phase inverter  716   a  and  716   b  and a three-phase motor/generator  718   a  and  718   b . Each of the motors  718   a  and  718   b  has a set of stator windings  720   a  and  720   b.    
         [0044]    An electronic controller  722  based on one or more microprocessors controls the operations of the electrical motor drive system  710 . Gate driver circuits  724   a  and  724   b  are also used to convert the low-voltage digital control signals generated by the controller  722  to higher voltage gating signals that are suitable for turning-on or turning-off the semiconductor switches in each of the drive units. A phase shift is introduced into the carrier signals for the two drive units as described above with respect to  FIGS. 3 and 4  to reduce the combined inverter (DC) bus ripple current. 
         [0045]    Referring now to  FIG. 8 , an electrical motor/generator drive control method  800  is illustrated. In a first step represented by block  802 , a first, triangular carrier signal v c     —     a1  for a first drive unit is generated. Next, as represented by block  804 , a second, triangular carrier signal v c     —     a2  is generated for a second drive unit that has an electrical degree phase shift in relation to the first carrier signal v c     —     a1 . Next, as represented by block  806 , a set of three-phase modulation signals (v a , v b , v c ) is generated for both drive units by carrying out a predefined motor speed or torque control algorithm. 
         [0046]    Once the various carrier and modulation signals are generated at blocks  802 - 806 , a first set of switch control signals (v g     —     a1p , v g     —     b1p , v g     —     c1p , v g     —     a1n , v g     —     b1n , v g     —     c1n ) is generated for the first drive unit at block  808 . In particular, the switch control signals are generated by first comparing the modulation signals (v a , v b , v c ) with the first carrier signal v c     —     a1  to obtain (v g     —     a1p , v g     —     b1p , v g     —     c1p ) for the positive switches and then by logically inverting v g     —     a1p , v g     —     b1p  and v g     —     c1p , respectively, to obtain v g     —     a1n , v g     —     b1n , v g     —     c1n ) for the negative switches. Also, a second set of switch control signals (v g     —     a2p , v g     —     b2p , v g     —     c2p , v g     —     a2n , v g     —     b2n , v g     —     c2n ) is generated for the second drive unit by first comparing the modulation signals (v a , v b , v c ) with the second carrier signal v c     —     a2  to obtain (v g     —     a2p , v g     —     b2p , v g     —     c2p ) and then by logically inverting (v g     —     a2p , v g     —     b2p  and v g     —     c2p , respectively, to obtain (v g     —     a2n , v g     —     b2n , v g     —     c2n ) for the second switches as represented by block  810 . Finally in block  812 , the switches of the first and second drive units are controlled with the first set of switch control signals (v g     —     a1p , v g     —     b1p , v g     —     c1p , v g     —     a1n , v g     —     b1n , v g     —     c1n ) and the second set of switch control signals (v g     —     a2p , v g     —     b2p , v g     —     c2p , v g     —     a2n , v g     —     b2n , v g     —     c2n ) respectively. 
         [0047]    Although the various exemplary embodiments are shown as including only two drive units, the various embodiments of the invention are not limited in this regard. Rather, any number of drive units can be used in the various embodiments of the invention. In such configurations, a carrier signal is generated for each of the drive units, where each of the carrier signals are evenly separated in phase. That is, each of the carrier signals is separated in electrical degree phase by an amount 2π/n, where n is the total number of drive units. 
       EXAMPLES 
       [0048]    The following non-limiting Examples serve to illustrate selected embodiments of the invention. It will be appreciated that variations in proportions and alternatives in elements of the components shown will be apparent to those skilled in the art and are within the scope of embodiments of the invention. 
         [0049]      FIG. 9  is an X-Y plot  900  of capacitor ripple current as a function of motor phase current for a conventional drive system configuration as in  FIG. 1  (Curve  902 ) and a drive system configured in accordance with the embodiment illustrated in  FIG. 5  (Curve  904 ). For purposes of  FIG. 9 , the two systems were configured to utilize a DC power supply outputting 300V, a 400 μF capacitor, IGBT switches (as described above), and a three-phase induction motor with two sets of Y-connected stator windings, where the neutral nodes of the windings are not directly coupled. 
         [0050]    Control of the IGBT switches was provided using a TMS320F2812 digital signal processor (DSP) manufactured by Texas Instruments of Dallas, Tex. The DSP was also configured to generate the three modulation signals with a varying amplitude, a frequency of 60 Hz, and separated in phase by 120 degrees. Further, for the configuration in  FIG. 5 , the DSP was configured to generate the two triangular carrier signals with a fixed amplitude, a frequency of 10 kHz, and separated in phase by 180 degrees. 
         [0051]    As shown in  FIG. 9 , the conventional drive system provides a capacitor ripple current that is substantially higher than the capacitor ripple current for the system of  FIG. 5 , as shown by the difference between curves  902  and  904 . In particular,  FIG. 9  shows that the configuration in  FIG. 5  provides a reduction in ripple current between 55% and 75%. That is, the ripple current in the configuration of  FIG. 5  is less than ½ of that of a conventional drive system. 
         [0052]    Other alternatives, modifications, equivalents, and variations will become apparent to those skilled in the art having reviewed the figures and read the foregoing description. Accordingly, the invention is intended to embrace those alternatives, modifications, equivalents, and variations as fall within the broad scope of the appended claims. The technology disclosed and claimed is available for license by the assignee of record. 
         [0053]    Applicants present certain theoretical aspects above that are believed to be accurate that appear to explain observations made regarding embodiments of the invention. However, embodiments of the invention may be practiced without the theoretical aspects presented. Moreover, the theoretical aspects are presented with the understanding that applicants do not seek to be bound by the theory presented. 
         [0054]    While various embodiments of the invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the invention. Thus, the breadth and scope of the invention should not be limited by any of the above described embodiments. Rather, the scope of the invention should be defined in accordance with the following claims and their equivalents. 
         [0055]    Although the invention has been illustrated and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. 
         [0056]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and/or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.” 
         [0057]    Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.