Abstract:
A robotic 3D printing system has a six degree of freedom (DOF) robot that holds the platform on which the 3D part is built on. The system uses the dexterity of the 6 DOF robot to move and rotate the platform relative to the 3D printing head, which deposits the material on the platform. The system allows the part build in 3D directly with a simple printing head and depositing the material along the gravity direction. The 3D printing head can be fixed relative to robot base, or moved in the X-Y plane with 2 or 3 DOF, or held by another robot or robots. The robot movement can be calibrated to improve the accuracy and efficiency for high precision 3D part printing.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 61/388,403, which was filed on Sep. 30, 2010 and is entitled “COORDINATED CONTROL OF MULTI-TERMINAL HVDC SYSTEMS.” The complete disclosure of the above-identified patent application is hereby incorporated by reference for all purposes. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure relates to high-voltage, direct current (HVDC) systems, and more particularly to systems and methods for controlling multi-terminal HVDC systems that include a plurality of converter stations. 
       BACKGROUND 
       [0003]    In response to a disruption or disturbance in an HVDC system, it may be necessary to isolate some equipment, such as cables, overhead lines and/or converter stations. The resulting outage(s) may or may not impact the operation of the HVDC system. If the operation is impacted, the HVDC system needs to be moved from an unstable, uneconomical and/or emergency operating point to a stable, economical and non-emergency operating point. The converter station local controls may attempt to restore the power balance in the direct current (DC) grid by changing the DC voltages to achieve a power balance in the lines terminating in the converter station. However, these uncoordinated actions, which are based on local measurements at the converter stations, might not drive system voltages to nominal levels. Rather, the combined individual actions of the converter station local controls might cause the HVDC system to experience prolonged operation at voltages above or below nominal, which may be unstable, uneconomical and/or detrimental to the security of the DC grid. 
         [0004]    Examples of HVDC systems, and methods and systems for controlling HVDC systems, are disclosed in U.S. Pat. Nos. 4,419,591; 6,400,585 and 7,729,142, and in U.S. Patent Application Pub. Nos. 2006/0282239 and 2009/0279328. Examples of multi-terminal HVDC systems are disclosed in U.S. Pat. Nos. 4,419,591 and 7,729,142. The disclosures of these and all other publications referenced herein are incorporated by reference in their entirety for all purposes. 
       SUMMARY 
       [0005]    In some examples, methods for controlling multi-terminal HVDC systems having a plurality of converter stations may include receiving a plurality of measurements from a plurality of measurement units disposed on the HVDC system, identifying from the measurements a disruption within the HVDC system, monitoring the measurements to identify a steady-state disrupted condition for the HVDC system, calculating a new set point for at least one of the plurality of converter stations, and transmitting the new set point to the at least one of the plurality of converter stations. The new set point may be based on the steady-state disrupted condition and the measurements. 
         [0006]    In some examples, a computer readable storage medium may have embodied thereon a plurality of machine-readable instructions configured to be executed by a computer processor to control multi-terminal HVDC systems having a plurality of converter stations. The plurality of machine-readable instructions may include instructions to receive a plurality of measurements from a plurality of measurement units disposed on the HVDC system, instructions to identify from the measurements a disturbance within the HVDC system, instructions to monitor the measurements to identify a steady-state disturbed condition for the HVDC system, instructions to calculate a new set point for at least one of the plurality of converter stations, which new set point may be based on the steady-state disturbed condition and the measurements, and instructions to transmit the new set point to the at least one of the plurality of converter stations. 
         [0007]    In some examples, multi-terminal HVDC systems may include a plurality of converter stations, an HVDC grid interconnecting the plurality of converter stations, a plurality of measurement units disposed within the HVDC system, and a controller communicatively linked to the plurality of measurement units and the plurality of converter stations. The measurement units may be configured to obtain a plurality of time-tagged measurements from the HVDC system. The controller may be configured execute instructions to receive the measurements from the plurality of measurement units, identify from the measurements an outage within the HVDC system, monitor the measurements to identify a steady-state outaged condition for the HVDC system, calculate a new set point for at least one of the plurality of converter stations, which new set point may be based on the steady-state outaged condition and the measurements, and transmit the new set point to the at least one of the plurality of converter stations. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]      FIG. 1  is a simplified single line and block diagram schematic illustration of a representative HVDC system, with the protective DC relays shown. 
           [0009]      FIG. 2  is another simplified single line and block diagram schematic illustration of the representative HVDC system of  FIG. 1 , shown with examples of possible measurements. 
           [0010]      FIG. 3  is a flow chart of a nonexclusive illustrative example of a method for controlling a multi-terminal HVDC system having a plurality of converter stations, such as the HVDC system of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    A representative example of a multi-terminal HVDC system suitable for use with the methods disclosed herein is shown generally at  20  in  FIGS. 1 and 2 . Unless otherwise specified, the HVDC system  20  may, but is not required to, contain at least one of the structure, components, functionality, and/or variations described, illustrated, and/or incorporated herein. The HVDC system  20  includes a plurality of converter stations  22 , an HVDC grid  24 , a plurality of measurement units  26  (shown in  FIG. 2 ) disposed on or within the HVDC system  20 , and a controller  28 . 
         [0012]    The converter stations  22  may include any suitable converter  30  for converting alternating current (AC) power to DC power and/or for converting DC power to AC power. In the converter stations  22  illustrated in  FIGS. 1 and 2 , the converter  30  is a voltage source converter (VSC), such that the HVDC system  20  may be classified as a VSC-based or driven HVDC system (VSC-HVDC). 
         [0013]    A VSC may include, by way of example, two three-phase groups of semiconductor valves in a six-pulse bridge connection. The semiconductor valves may include branches of gate turn on/turn off semiconductor elements, such as insulated gate bipolar transistors (IGBTs), and diodes in anti-parallel connection with these elements. Additional discussion regarding the details of VSCs is found in U.S. Pat. No. 6,259,616, the entire disclosure of which is incorporated by reference for all purposes. In some examples, the converter stations  22  may alternatively or additionally include other types of converters, such as a current source converter (CSC) or other type of converter. As shown in  FIGS. 1 and 2 , the illustrated HVDC system includes four converter stations  22 . However, the systems and methods disclosed herein are also applicable to HVDC systems having two, three, five or even more converter stations. 
         [0014]    The HVDC grid  24 , which interconnects the plurality of converter stations  22 , may include at least one cable circuit or HVDC transmission line  34 . In the example illustrated in  FIGS. 1 and 2 , the HVDC grid  24  includes six HVDC transmission lines interconnecting the four converter stations  22  by way of the illustrated topology. In some examples, the HVDC grid  24  and the HVDC transmission lines  34  may be or include any suitable combination of direct current cables, direct current overhead lines, direct current cables and direct current overhead lines connected in series, direct current fault-current limiting reactors, or the like. 
         [0015]    Some examples of HVDC systems may include at least one switch associated with at least one of the HVDC transmission lines and/or at least one of the converter stations. As shown in  FIG. 1 , each of the converter stations  22  of the HVDC system  20  includes a switch  36  associated with each of the HVDC transmission lines  34  extending from that converter station. The switches may be any suitable current interrupting device, such as a DC circuit or line breaker. With respect to  FIG. 1 , each of the HVDC transmission lines  34  is shown to be protected from faults by a pair of the switches  36  located at the ends of the transmission line. For example, the HVDC transmission line  38  between the first and second ones  40 ,  42  of the converter stations  22  is protected from a fault  44  thereon by a pair of protective relays  46 ,  48 . In particular, in response to the fault  44 , which may be a pole-to-pole or pole-to-ground fault, the relays  46  and  48  will trip and isolate the transmission line  38 , such that the HVDC system  20  will be in the condition illustrated in  FIG. 2 , with the transmission line  38  being isolated. It should be understood that the number and locations of switches  36  illustrated in  FIG. 1  on the HVDC system  20  are for purposes of illustration only, and the HVDC power system  20  may include any suitable number of switches, which may be disposed in any suitable locations. 
         [0016]    The plurality of measurement units  26  are configured to obtain a plurality of measurements from the HVDC system  20  and transmit the measurements to the controller  28 . As shown in  FIG. 2 , at least some of the plurality of measurement units  26  may be disposed within or linked to the converter stations  22 . Nonexclusive illustrative examples of the measurements that may be obtained by the plurality of measurement units  26 , and transmitted to the controller  28 , include discrete value measurements, such as line breaker or converter station switch status information, as well as continuous measurements, such as converter station DC busbar voltages, DC cable currents, power flows, and DC power output from the converter stations. The power flows measured within the HVDC system may include power flows within the DC grid, DC power flows within or into the cables or transmission lines  38 , power flows between at least two of the converter stations  22 , and power flows within, across or through one of the converter stations  22 , which may include variable power losses in at least one of the converter stations. For purposes of illustration, the measurement units  26  illustrated in  FIG. 2  are identified by type, with cable current measurement units being identified by “A,” cable power measurement units being identified by “P,” converter station DC busbar voltage measurement units being identified by “V,” and discrete value measurement units, such as line breaker status value measurement units, being identified by “S.” However, it should be understood that the particular types, location and combination of measurement units shown in  FIG. 2  are for purposes of illustration only, and the HVDC power system  20 , the converter stations  22 , and/or the various ones of the transmission lines  38  may be provided with any suitable number, type, location or combination of measurement units. 
         [0017]    In some examples, the measurements may be time synchronized. For example, the measurements may be marked or provided with a suitable time stamp, which may allow later time-aligning or time-synchronizing of the measurements by the controller. By way of nonexclusive illustrative example, each of the measurement units  26  may time-tag or synchronize the measurements using a suitable time signal, such as network-based time synchronization signal produced within the HVDC system or a signal based on a GPS time signal, which may be received from a GPS satellite  52 . 
         [0018]    The controller  28 , as suggested in  FIGS. 1 and 2 , is communicatively linked to the plurality of converter stations  22  by way of suitable communication links or pathways  54 . In some examples, two or more of the converter stations may be linked to one another by way of suitable communication links or pathways. The plurality of measurement units  26 , as suggested in  FIGS. 1 and 2 , are also communicatively linked to the controller  28  by way of suitable links or pathways, which may, in some examples, correspond to the communication pathways  54  linking the controller to the converter stations. Accordingly, the measurements are sent from or transmitted by the measurement units  26  and received by the controller  28  over suitable communication links or pathways. Furthermore, as will be more fully discussed below, control signals, such as set point information, are sent or transmitted from the controller  28  to the converter stations  22 . Nonexclusive illustrative examples of suitable communication pathways  54  linking the controller  28  to the converter stations  22  and/or the measurement units  26  may include wired, fiber optic, radio-frequency wireless, microwave, power-line carrier, satellite, telephone, cellular telephone, an Ethernet, the internet, or any suitable wide area communication system. 
         [0019]    The following paragraphs describe nonexclusive illustrative examples of methods for controlling multi-terminal HVDC systems having a plurality of converter stations, using the concepts and components discussed above. Although the actions of the following methods may be performed in the order in which they are presented below, it is within the scope of this disclosure for the following actions, either alone or in various combinations, to be performed before and/or after any of the other following actions. A method for controlling a multi-terminal HVDC system having a plurality of converter stations, which may be at least partially carried out by the controller  28  as a processor therein executes instructions, may generally include the controller  28  receiving a plurality of measurements sent from the plurality of measurement units  26 , the controller  28  identifying from the measurements a disruption, such as a disturbance or outage, within the HVDC system  20 , the controller  28  monitoring the measurements to identify a steady-state disrupted condition for the HVDC system  20 , the controller  28  calculating new set points for at least some of the plurality of converter stations  22 , and the controller  28  sending or transmitting the new set points to the converter stations  22 . 
         [0020]    A nonexclusive illustrative example of such methods is discussed below with regard to the flow chart shown in  FIG. 3 . At block  100 , the multi-terminal HVDC system  20  is operating nominally. By nominally, it may be said the that HVDC system is operating normally, with no equipment outages, no equipment limits violated, and in some examples, at a stable, economical and non-emergency operating point. However, the HVDC system may also be operating nominally after a disruption, such as with one or more faulted or isolated components, few or no equipment limits violated, and with the system operating at a stable, economical and non-emergency operating point in view of the disruption. 
         [0021]    At block  102 , the controller  28  receives measurements from the measurement units  26 , such as via the communication pathways  54  illustrated in  FIGS. 1 and 2 . As noted above, in some examples, the measurements may be time-aligned upon or after receipt from the measurement units. For example, after receiving the measurements, the controller  28 , or another piece of equipment within the HVDC system, may time synchronize the measurements by aligning the time-tagged measurement data. 
         [0022]    As noted above, the controller  28  may identify from the measurements a disruption, such as a disturbance or outage, within the HVDC system  20 . Nonexclusive illustrative examples of disruptions may include: a fault on one or more of the transmission lines within the DC grid; one or more of the transmission lines or other equipment of the DC grid having an outage or being isolated, such as in response to a fault; a failure or shutdown of one or more of the converter stations; and an outage of switchgear components in a converter station, such as DC circuit breaker failure, current limiting reactor failure and/or busbar short circuit. As part of identifying a disruption within the HVDC system, the controller  28  may detect a disruption, such as a fault or equipment outage, within the HVDC system  20 , as indicated at block  104 , and the controller  28  may identify the disrupted equipment, as indicated at block  106 . 
         [0023]    In some examples, the controller  28  may identify the disrupted or outaged equipment using switch information. For example, the controller  28  may monitor the status of some or all of the switching equipment in the HVDC system  20 , which may include at least DC breakers, and continuously perform a topology processing that identifies the presence of an outage in the HVDC grid. 
         [0024]    At block  108 , the controller  28  monitors measurements to identify a steady-state disrupted condition for the HVDC system  20 . In some examples, the controller  28  may monitor a power flow within the HVDC system, such as between two or more of the converter stations  22 , and identify the steady-state disrupted condition based on the monitored power flow. For example, the controller may monitor oscillations in the power flow and identify a steady-state condition when the power flow oscillations have negligible magnitudes and/or frequencies. 
         [0025]    In some examples, rather than continuously performing all aspects of the disclosed method on an HVDC system, which may be operating in a transient or variable condition, the controller  28  may monitor the measurements to identify or detect commencement of a post-transient operating condition; that is, where the HVDC system  20  has stabilized into a disrupted, albeit steady-state, operating condition. Thus, although the controller  28  may monitor measurements, such as the power flows in the HVDC grid, during the transient as well as post transient time frame, such as in response to a topology change, the entire method may only be performed when the HVDC system is operating in a non-steady-state or transient condition. 
         [0026]    At block  110 , the controller  28  calculates a new set point for at least one of the of converter stations  22  or, in some examples, for each of the converter stations. For example, the controller  28  may calculate the desired real power, reactive power and/or DC voltage set point for at least one of the converter stations  22  based on measurements of power flows and voltage in the HVDC system  20 . In some examples, at least some of the new set points are based on measurements taken when the system has stabilized into a disrupted, albeit steady-state, operating condition; based on the steady-state topology of the system; and/or based on reliability, stability and economic factors for the HVDC system and/or its components. 
         [0027]    In some examples, the controller  28  may calculate new set points for at least some of the converter stations based on optimal power flow (OPF) techniques, which seek to optimize a global objective by acting on the controllable parameters of various power system equipment. As used herein, optimal power flow may refer to any optimal power flow within the HVDC system, and may include any combination of an optimal power flow determined within the HVDC grid, an optimal power flow determined between two or more converter stations and/or an optimal power flow determined through or across any one or more converter stations. In some examples, an optimal power flow may be determined based on variable power losses in at least one of the converter stations. Thus, the new set points may be calculated and/or adjusted based on, or according to, an optimal power flow that has been determined for or within the HVDC system. 
         [0028]    The OPF technique is based on solving an optimal power flow problem, which is classically formulated as an optimization problem in the form of equation (1). 
         [0000]      min  f ( x,u ) 
         [0000]      subject to  g ( x,y )=0 
         [0000]        h ( x,y )≦0  (1)
 
         [0000]    where f(x,u) is the objective function, g(x,y) are the equality constraints, and h(x,y) are the inequality constraints. The vector x contains the voltages and angles of all buses and the vector u contains the set of controllable variables. The vector y is composed of both scheduled p and controllable variables u and is written as: 
         [0000]        y=[u p]   T   (2)
 
         [0000]    The equality constraints g(x,y) include the power flow equations. The inequality constraints h(x,y) include bounds in operational ratings of equipment, such as bus voltage limits, branch flow limits, generation limits, or the like. The set of control variables may include AC system generator voltage, AC system LTC (Load Tap Changer transformer) tap position, AC system phase shifter angle, AC system SVC (Static VAR Compensator) variables, load shedding, DC line flow, or the like. 
         [0029]    The power flow equations of a multi-terminal VSC-HVDC link may be expressed in the form g VSC-HVDC =0, as set out in equation (3), where the new state vector x includes the DC bus voltages. 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0030]    The controlled variables of the multi-terminal VSC-HVDC link may be expressed in the form h VSC-HVDC ≦0 and would normally include limits on the bus voltages and converter maximum P and Q limits, as set out in equation (4). 
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         [0000]    where V AC,i  is the bus voltage on the AC-side of a VSC converter station i, for i=1 . . . N, with N being the number of VSC converter stations; P AC,i  is the real power injection from the AC system into the VSC converter station i; Q AC,i  is the imaginary power injection from the AC system into the VSC converter station  1 ; V DC,k  is the DC-side bus voltage at the k-th DC busbar, for k=1 . . . K, with K being the number of controllable nodes or branches in the DC grid; and P DC,m  is the DC-side real power at the m-th branch in the DC grid, for m=1 . . . M, with M being the number of controllable branches in the DC grid. 
         [0031]    In the general case, the control vector u VSC-HVDC  may be expressed as in equation (5). 
         [0000]    
       
         
           
             
               
                 
                   
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         [0000]    However, various control philosophies, which may involve control of single or multiple parameters, could be included and/or enabled in a multi-terminal VSC-HVDC link. For example, one control implementation sets the DC-side voltage for one converter station, while setting the real and reactive power flow orders at the rest of converter stations, resulting in a control vector as in equation (6). 
         [0000]    
       
         
           
             
               
                 
                   
                     
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                         - 
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         [0000]    In particular, one VSC converter station, K, has its DC bus voltage V DC,k  controlled, while the rest of the VSC converter stations, i=1, . . . N, i≠K, have their real and reactive power injections P AC,i , Q AC,i  controlled. 
         [0032]    Accordingly, the optimal power flow problem for the multi-terminal VSC-HVDC link may be expressed as in equation (7). 
         [0000]      min  f ( x,u ) 
         [0000]      subject to [ g ( x,y ) g   VSC-HVDC ( x,y )] T =0 
         [0000]      [ h ( x,y ) h   VSC-HVDC ( x,y )] T ≦0  (7)
 
         [0033]    In some examples, as indicated at block  112 , the controller  28  may adjust at least some of the new set points based on a participation factor for at least one of the converter stations  22 . In particular, the controller  28  may adjust the new set point for a particular converter station based on a participation factor assigned to, or associated with, that converter station. As used herein, “participation factor” refers to the degree of participation of a given converter to the required power change for the HVDC system, such as in response to a disruption or equipment outage, such as an outage of one or more of the HVDC transmission lines or converter stations. In some examples, the participation factor for a particular converter station may correspond to the desired percentage pickup of the total change, such that the participation factors for all included converter stations add up to 100% for the entire system. 
         [0034]    Adjustments based on participation factors may be useful in situations such as where energy prices are set for each of the converter stations. For example, the converter stations may be electricity market players such that set point adjustments based on participation factors may be used to move the overall system towards a more economical operating point. 
         [0035]    In some examples, the set points may be adjusted based on the OPF techniques set out above. For example, the set points may be adjusted such that converter participation, or the amount of change in an existing converter power set point, accounts for any losses within the converters. 
         [0036]    At block  114 , the controller  28  transmits, sends or dispatches the calculated new set points to the corresponding converter stations  22 , such as via the communication pathways  54  illustrated in  FIGS. 1 and 2 . In some examples, the new set points may be transmitted in the form of power orders or control signals. Based on the new set points, the converter station controllers may then implement appropriate control actions, such as by setting the real power, the reactive power and/or the DC voltage for the converter station. 
         [0037]    In some examples, as indicated at block  116 , the controller  28  may monitor the response of the HVDC system  20  to the new set points or power orders. In particular, the controller  28  may monitor the measurements to determine from the measurements a system response to the new set points. 
         [0038]    The controller  28  may monitor the system response to the new set points to determine, as indicated at block  118 , whether the system response either violates at least one equipment rating or limit for the HVDC system or has not cleared an equipment limit violation. Nonexclusive illustrative examples of equipment limit violations may include thermal overloads of converters or transmission lines, abnormal voltages, such as undervoltage in converters, and/or excessive current or power flows. 
         [0039]    If any equipment limits for the HVDC system remain violated, the controller returns to block  110  and recalculates and/or adjusts the new set points. If the system response does not violate any equipment limits and/or has cleared all equipment limit violations, the controller  28  may determine that the HVDC system is operating nominally, or at least efficiently, in view of the steady-state disrupted condition. 
         [0040]    The disclosed methods and systems may be used to control the operation of converter stations to bring a multi-terminal HVDC system into a stable operating point following a disruption to the DC grid, such as one involving the outage of cables, overhead lines, or converter stations. In some examples, the methods and systems set or adjust the real and reactive power orders or set points, which may include the DC voltage set points, based on the results of an optimal power flow technique that accounts for the losses in the converters. The inputs to the disclosed methods and systems may include the power injections to the AC system that are made by the remaining converter stations under the post-transient or post-contingency steady-state disrupted conditions. 
         [0041]    Some examples of the systems and methods disclosed herein may provide and/or support coordinated control of multi-terminal HVDC systems, such as where the controller  28  is a central or wide area controller configured to control the HVDC system  20  based on measurements received from throughout the system. Use of a central controller to control the HVDC system based on measurements received from throughout the system, as opposed to local control of the plurality of converter stations based on local measurements, may allow for coordinated control of the converter stations and, correspondingly, of the HVDC system. In particular, the controller  28  may coordinate the operation of the converter stations  22  following a disruption or disturbance to the HVDC system, such as by simultaneously adjusting the real power, reactive power and/or DC voltage set points of a plurality of the converter stations, which may move the HVDC system toward a stable, feasible and/or economical operating point with voltages restored to their nominal values, power balance in the system, and without any equipment limit violations. When optimal power flow techniques are included, the controller  28  may coordinate the operation of the converter stations  22  following a disruption or disturbance to the HVDC system to move the HVDC system toward an optimal operating point in view of the disruption and equipment limits. 
         [0042]    In some examples, at least some of the converter stations  22  may be configured to provide real-time monitoring of the Thevenin&#39;s impedance seen by the converter station such that the distribution of power among the converter stations considers or accounts for AC system strength. 
         [0043]    The disclosed methods and systems may be embodied as or take the form of the methods and systems previously described, as well as of a transitory or non-transitory computer readable medium having computer-readable instructions stored thereon which, when executed by a processor, carry out operations of the disclosed methods and systems. The computer-readable medium may be any medium that can contain, store, communicate, propagate, or transport the program instruction for use by or in connection with the instruction execution system, apparatus, or device and may, by way of example but without limitation, be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium or other suitable medium upon which the program is recorded. More specific examples (a non-exhaustive list) of such a computer-readable medium may include: a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CD-ROM), an optical storage device, a transmission media such as those supporting the Internet or an intranet, or a magnetic storage device. Computer program code or instructions for carrying out operations of the disclosed methods and systems may be written in any suitable programming language provided it allows achieving the previously described technical results. 
         [0044]    It is believed that the disclosure set forth herein encompasses multiple distinct inventions with independent utility. While each of these inventions has been disclosed in its preferred form, the specific embodiments thereof as disclosed and illustrated herein are not to be considered in a limiting sense as numerous variations are possible. The subject matter of the disclosure includes all novel and non-obvious combinations and subcombinations of the various elements, features, functions and/or properties disclosed herein. Similarly, recitation in the disclosure and/or the claims of “a” or “a first” element, or the equivalent thereof, should be understood to include incorporation of one or more such elements, neither requiring nor excluding two or more such elements. 
         [0045]    It is believed that the following claims particularly point out certain combinations and subcombinations that are directed to one of the disclosed inventions and are novel and non-obvious. Inventions embodied in other combinations and subcombinations of features, functions, elements and/or properties may be claimed through amendment of the present claims or presentation of new claims in this or a related application. Such amended or new claims, whether they are directed to a different invention or directed to the same invention, whether different, broader, narrower or equal in scope to the original claims, are also regarded as included within the subject matter of the inventions of the present disclosure.