Abstract:
Apparatus and methods are presented for mitigating overvoltages and limiting reverse motor speeds for motor drive power loss events, in which a first power dissipation circuit is enabled at the motor drive output to limit reverse rotation of a driven motor load when motor drive power is lost, and a second power dissipation circuit in a DC bus circuit is used to mitigate over voltages following restoration of motor drive power.

Description:
BACKGROUND 
     Motor drives are power conversion systems used to provide power to a driven electric motor by converting received input power. The motor load, in turn, may be used in a variety of different applications. In submersible well pumps, a driven pump motor is used to drive a screw or centrifugal type pump, typically to extract fluid from a well. In normal operation, the pump motor turns in a forward direction to pump the fluid upward within the well tube. If the associated motor drive loses power, however, the pump motor will stop rotating, and previously pumped fluid will start draining back down into the well, causing the rotor of the pump motor to spin in the reverse direction. If the motor is constructed with permanent magnets, the reverse rotation of the pump motor creates a back EMF which may lead to significant voltage that can damage the motor and/or degrade components in the motor drive, including a DC bus capacitor at the input of the drive inverter. Screw type pumps in particular can accelerate in the reverse direction to a point where the back EMF creates a significant voltage that is greater than the nominal voltage of the motor. Moreover, reverse rotation of the pump motor allows fluid to drain back down into the well, whereby the pumping work that was done prior to power loss must be redone later when power is restored. A similar situation occurs in motor driven cranes, in which the load on a crane motor may tend to reverse the motor rotation during power loss events. Accordingly, a need remains for techniques and apparatus to protect motor drives and driven motors from damage due to excess back EMF, and to prevent the motor from reaching high reverse speeds for power loss situations. 
     SUMMARY 
     Various aspects of the present disclosure are now summarized to facilitate a basic understanding of the disclosure, wherein this summary is not an extensive overview of the disclosure, and is intended neither to identify certain elements of the disclosure, nor to delineate the scope thereof. Rather, the primary purpose of this summary is to present various concepts of the disclosure in a simplified form prior to the more detailed description that is presented hereinafter. Techniques, systems, and apparatus are disclosed for power conversion in which reverse motor load rotation is controlled or limited by enabling an output dissipation circuit following loss of system input power, and DC bus overvoltages are mitigated by activating a second dissipation circuit after system input power is reinstated. The disclosed concepts can be advantageously employed in pump motor power conversion systems, motor driven crane systems, and other applications in which it is desirable to control unpowered reverse motor rotation and to mitigate the effects of associated back EMF to protect the power conversion system and/or a driven motor. 
     In accordance with one or more aspects of the present disclosure, a power conversion system is provided which includes a DC bus circuit, an inverter and a controller, as well as first and second power dissipation circuits. The first power dissipation circuit is coupled with the system AC output and selectively couples one or more resistive loads between two or more of the AC output terminals to dissipate regenerative power provided to the system from a driven load. The second power dissipation circuit is operable to selectively dissipate power in the DC bus circuit. In certain embodiments, the controller responds to loss of system input power to provide a control signal to activate the first power dissipation circuit in order to dissipate power provided to the AC output from the load. In this manner, reverse motor rotational speed can be limited, thereby reducing the amount of pumping or lifting work previously done in submersible pump and/or crane applications. The controller may also selectively disable operation of the output inverter in response to loss of system power. 
     After input power is restored, the controller in certain embodiments deactivates the first power dissipation circuit, resumes switching operation of the inverter, and activates the second power dissipation circuit to selectively dissipate power in the DC bus circuit. By this operation, the potential adverse effects of excess DC bus voltage can be avoided or mitigated to protect the DC bus capacitance and other power conversion system components. Certain embodiments of the system may include an output filter connected between the inverter and the system AC output, with the first power dissipation circuit connected to two or more AC output terminals following the filter. In certain implementations, the first power dissipation circuit includes a rectifier as well as a switch and a resistor, and the switch may be activated by a control signal from the conversion system controller in certain embodiments to selectively connect the resistor across the output of the rectifier for dissipating regenerative power. In other possible implementations, a contactor or other type of switch can be used to connect one or more resistors across two or more of the output leads to dissipate regenerative power. The first power dissipation circuit, moreover, can be integral to a motor drive type power converter, or can be separately connected between a motor drive AC output and a driven load in a power conversion system. 
     A method and computer readable medium with computer executable instructions are provided in accordance with further aspects of the disclosure for mitigating motor drive overvoltage and limiting reverse rotation of a motor load for motor drive power loss events. The method involves disabling operation of the motor drive output inverter and selectively coupling a first resistive load circuit to two or more AC nodes between the inverter and the motor load in response to loss of motor drive system input power in order to limit reverse rotation of the motor load by dissipation of power provided to the motor drive from the motor load. The method further includes selectively disabling the first resistive load circuit and selectively enabling inverter operation to drive the motor load to resume rotation in a forward direction after restoration of motor drive system input power, as well as selectively connecting a second resistor load to dissipate power in a motor drive DC bus circuit. In certain implementations, the second resistive load is thereafter disconnected to resume normal motor drive operation. The selective operation of the first and second resistive loads involves provision of associated control signals from a motor drive controller in certain embodiments. In certain implementations, moreover, operation of the motor drive output inverter is disabled prior to connection of the first resistive load circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following description and drawings set forth certain illustrative implementations of the disclosure in detail, which are indicative of several exemplary ways in which the various principles of the disclosure may be carried out. The illustrated examples, however, are not exhaustive of the many possible embodiments of the disclosure. Other objects, advantages and novel features of the disclosure will be set forth in the following detailed description when considered in conjunction with the drawings, in which: 
         FIG. 1  is a schematic diagram illustrating an exemplary power conversion system for driving a motor load, including first and second power dissipation circuits operated according to control signals from a motor drive controller in accordance with one or more aspects of the present disclosure; 
         FIG. 2  is a schematic diagram illustrating an exemplary motor snubber power dissipation circuit connected between an output filter and a step up transformer, including a three-phase rectifier, an internal DC capacitor, a cooling fan, a switch controlled by a signal from the drive controller, and a load resistor for selective dissipation of regenerative power in certain embodiments of the system of  FIG. 1 ; 
         FIG. 3  is a schematic diagram showing another embodiment of the first power dissipation circuit including a three-phase contactor for selectively connecting load resistors to AC output lines of the system according to a signal from the motor drive controller; 
         FIG. 4  is a flow diagram illustrating an exemplary process for limiting reverse rotation of the motor load and mitigating motor drive overvoltage for power loss events in the power conversion system of  FIG. 1 ; and 
         FIG. 5  is a signal and waveform diagram illustrating graphs of motor drive input power and DC bus voltages, motor speed, and various control signals and states in the power conversion system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the figures, several embodiments or implementations are hereinafter described in conjunction with the drawings, wherein like reference numerals are used to refer to like elements throughout, and wherein the various features are not necessarily drawn to scale. 
       FIG. 1  shows an exemplary power system  2  including an AC input source  4  and a motor drive  10  with an AC output  11  providing power through an optional transformer  8  to drive a motor load  6 . The motor drive  10  includes a rectifier  12 , which can be an active (e.g., switching) rectifier and/or may be a passive rectifier with one or more switches and/or diodes that receive three-phase AC input power from the source  4  and provide a DC bus voltage to a DC bus circuit  14  for use by an output inverter  20 . The DC bus circuit  14  includes a DC capacitance C, which can be a single capacitor component or multiple capacitors connected it in any suitable series, parallel and/or series/parallel configuration to provide a capacitance between upper and lower DC circuit branches  14 A and  14 B, respectively. In addition, as discussed further below, a DC bus snubber circuit is included in the bus circuit  14 , having a switching device  16  connected in series with a load resistance  18  between the upper and lower DC circuit branches  14 A and  14 B of the bus circuit  14 . The DC bus circuit  14  provides a DC voltage as an input to the inverter  20  at first and second DC input terminals  21 , and the inverter selectively connects the DC input terminals  21  with three AC output terminals of an AC output  28  using inverter switching devices  22 ,  23 ,  24 ,  25 ,  26  and  27  operated according to pulse width modulated switching control signals  29  from a motor drive controller  30 . 
     The controller  30  can be implemented using any suitable hardware, processor executed software or firmware, or combinations thereof, wherein an exemplary embodiment of the controller  30  includes one or more processing elements such as microprocessors, microcontrollers, FPGAs, DSPs, programmable logic, etc., along with electronic memory, program memory and signal conditioning driver circuitry, with the processing element(s) programmed or otherwise configured to generate the inverter switching control signals  29  suitable for operating the switching devices of the inverter  20 , as well as to perform other motor drive operational tasks to drive a load. Moreover, computer readable mediums are contemplated with computer executable instructions for implementing the described power converter switching control processes and techniques, which may be stored as program instructions in an electronic memory forming a part of, or otherwise operatively associated with, the controller  30 . The controller  30  also provides a switching control signal  34  to selectively actuate the DC bus snubber switch  16 , and receives one or more feedback signals including a DC bus voltage feedback signal  36  indicating the voltage in the DC bus circuit  14 . 
     The inverter output  28  is connected to ultimately drive the motor load  6 , either directly or through one or more intervening circuits. In the illustrated embodiment, for example, an output filter  38  is connected between the inverter output  28  and the AC output terminals of the motor drive AC output  11 , and an external step up transformer  8  is connected between the motor drive output  11  and the motor load  6 . The output filter  38  in certain embodiments may be an LC filter, an LCL filter, or any other suitable form of a filter circuit configuration, and may be physically separate from the drive  10  or may be integrated therein. The transformer  8  may be an external device as shown, or may be integrated within the motor drive  10  in certain embodiments, and may have any suitable turns ratio, such as a step up transformer in one implementation. A step up transformer may be useful in a submersible pump application in which it is desirable to provide high voltage signals to the motor load  6  in order to mitigate I 2 R losses along a lengthy cable run between the transformer  8  and the motor load  6 . A step up transformer  8  may also be used to match the output of a medium or low voltage motor drive  10  with a motor load  6  of a higher voltage rating, or to otherwise match the drive  10  with the motor  6  in consideration of potentially high voltage drops across a long cable run. In other embodiments, the transformer and/or the output filter  38  may be omitted. 
     As illustrated in  FIG. 1 , moreover, the system includes an output or motor snubber circuit  40 , referred to herein as a first power dissipation circuit or a motor snubber. The power dissipation circuit  40  in certain embodiments is activated or enabled by a control signal  32  from the motor drive controller  30 , although other implementations are possible in which the first power dissipation circuit  40  is actuated independent of the operation of the controller  30 . Likewise, the illustrated controller  30  provides a control signal  34  to actuate the DC bus snubber circuit switch  16  to connect the dissipation resistor  18  across the DC bus, for example, according to the level of the DC bus voltage feedback signal or value  36 , but other implementations are possible in which the DC bus snubber circuit  16 ,  18  is actuated independent of the controller  30 . Moreover, a single controller  30  may be used to provide one or both of the control signals  32  and  34  as well is the inverter switching control signals  29 , or separate controllers or circuits may be used for any or all of the signaling  29 ,  32 ,  34 . 
     Although the system  2  of  FIG. 1  receives a multiphase AC input from the source  4  and provides a multiphase AC output to drive the motor load  6 , the various concepts of the present disclosure are not limited to three-phase implementations on either the input or the output, and other embodiments are possible in which a single phase motor loads  6  or other types of loads are driven by a single phase inverter output and/or other output configurations are possible having more than three phases. In addition, while the illustrated motor drive  10  includes an onboard rectifier  12  receiving AC input power from the source  4 , other embodiments are possible in which the power conversion system receives DC power as an input, wherein the onboard rectifier  12  can be omitted. 
     The first power dissipation circuit (motor snubber)  40  can be any suitable circuitry by which one or more resistive load components can be selectively connected, directly or indirectly, to at least two of the AC output nodes carrying power between the inverter output  28  and the motor load  6 . In the implementation of  FIG. 1 , for instance, the motor snubber  40  is connected to the three output lines between the filter  38  and the transformer  8 . In other possible implementations, the snubber  40  can be connected between the inverter output  28  and the filter  38 , or may instead be connected to the secondary side of the transformer  8 . As mentioned above, moreover, one or more of the filter  38  and the transformer  8  may be omitted in certain embodiments, with the snubber  40  being directly or indirectly connected to the motor drive output  11 . 
     One possible implementation of the motor snubber  40  is illustrated in  FIG. 2 , including a three-phase passive rectifier  41  connected to the three AC output lines, which rectifies the output power to provide a DC voltage across a capacitor  42 . A transistor or other type of switching device  44  is connected in series with a load resistor  45  in parallel with the capacitor  42 , and a diode  43  is connected with an anode at the node joining the switch  44  and the resistor  45  and a cathode connected to the positive DC node. In this embodiment, moreover, a fan  46  may be included, for example, to provide cooling to inductors of the filter  38  ( FIG. 1 ), although the fan  46  may be omitted in certain implementations. In addition, this embodiment provides for external actuation or enablement of the motor snubber  40 , by the controller  30  providing a control signal  32  to the base of the transistor  44 . Other embodiments are possible in which different types of switching and control are used to activate the snubber  40 , and the circuit  40  may alternatively be self-enabled, for example, using a crowbar circuit as shown in U.S. Pat. No. 7,479,756 to Kasunich, entitled “System and Method for Protecting a Motor Drive Unit from Motor Back EMF Under Fault Conditions”, assigned to the assignee of the present disclosure, the entirety of which is hereby incorporated by reference. 
       FIG. 3  illustrates another possible embodiment of the motor snubber  40 . In this case, a contactor  47  is used to selectively connect resistors  48  directly to the AC lines, with the controller  30  providing a control signal  32  in order to actuate the contactor  47 . In this embodiment, moreover, the contacts  47  are preferably normally closed (NC) such that loss of power to the controller  30  will result in the contacts  47  being closed, thereby connecting the resistors  48  to the power converter output lines. Thus, in normal operation, the controller  30  may assert the signal  32  in a first state that energizes the contactor coil (not shown), thereby opening the contacts  47 , and then change the state of the control signal  32 , whether under active operation of the controller  30 , or due to loss of power to the controller  30 , to close the contacts  47 . In this implementation, moreover, closure of the contacts  47  effectively connects the resistor elements  48  between the output lines, thereby providing a resistive load to dissipate energy regenerated by the connected motor load  6  back toward the inverter  20  ( FIG. 1 ). 
     Referring also to  FIGS. 4 and 5 , a process  50  is illustrated in  FIG. 4  by which motor drive overvoltage conditions may be mitigated or avoided, and reverse rotation of the motor load  6  may be limited or otherwise controlled. As noted above, the activation or enablement of the motor snubber  40  operates to selectively connect a resistive load between the driven motor  6  and the output of the inverter  20 . This is particularly advantageous in situations in which the motor drive  10  loses input power (e.g., the AC supply  4  becomes disconnected or otherwise inoperative). In such a situation, where the driven motor  6  is used in a submerged pumping application to extract fluid from a well, for example, loss of power can lead to motor stoppage followed by rotation in a reverse direction due to the pressure of previously pumped fluid draining back down the well. The inventors have appreciated that providing a load using the motor snubber  40  operates to inhibit reverse direction acceleration of the motor  6 . Consequently, a certain amount of previously performed work by the pump motor  6  can be preserved by effectively slowing down the pump motor  6 . Therefore, once power is restored to the system  2 , less rework needs to be done in many cases. In addition, the connection of the motor snubber  40  can also mitigate the amount of back EMF and corresponding voltage levels at the motor  6  and also in the motor drive  10 , thereby protecting these components from potential overvoltage degradation. 
     The process  50  in  FIG. 4  illustrates one exemplary scenario in which the controller  30  and the power dissipation circuitry  16 ,  18 ,  40  operate to control the motor reverse rotation speed and also to mitigate overvoltage conditions in the motor drive  10  and at the motor  6 . Reference is also made to the various signal and waveform diagrams illustrated in graphs  60 ,  70 ,  80  and  90  of  FIG. 5 . The process  50  is illustrated and described below in the form of a series of acts or events, although the various methods of the disclosure are not limited by the illustrated ordering of such acts or events. In this regard, except as specifically provided hereinafter, some acts or events may occur in different order and/or concurrently with other acts or events apart from those illustrated and described herein in accordance with the disclosure. In addition, not all illustrated steps may be required to implement a process or method in accordance with the present disclosure, and one or more such acts may be combined. The illustrated method  50  and other methods of the disclosure may be implemented in hardware, processor-executed software, or combinations thereof, such as in the exemplary controller  30 , and may be embodied in the form of computer executable instructions stored in a tangible, non-transitory computer readable medium, such as in an electronic memory operatively associated with the controller  130  in one example. 
     At  51  in  FIG. 4 , the AC input power to the system  2  is lost. The power loss can be due to any cause, such as failure or disconnection of the AC input source  4  in  FIG. 1  above.  FIG. 5  illustrates a graph  60  showing an AC input voltage curve  62 , where power is lost at time T 1 , and reaches zero before time T 2  in the illustrated example. At  52  in  FIG. 4 , the controller  30  disables switching operation of the inverter  20 , such as by discontinuing provision of the switching control signals  29  in  FIG. 1 . This is illustrated in the graph  90  of  FIG. 5  by the low going transition in the “IGBTS ENABLED” signal  92 . As further noted in the graph  70  of  FIG. 5 , loss of the AC input power begins a decrease in the DC voltage of the bus circuit  14 , where  FIG. 5  provides a graph  70  showing the DC bus voltage waveform  72  which begins decreasing at time T 1  while the inverter switching devices  22 - 27  continue to operate. Thereafter at time T 2 , with the deactivation of the IGBT switching (control signal  92  going low in graph  90 ), the DC bus voltage decreases at a slower rate.  FIG. 5  further provides a graph  80  showing a motor speed curve (RPM)  82 , with the motor speed (in a forward direction) initially being at a steady state value, and decreasing after the power is interrupted at T 1 . 
     The controller  30  actuates the signal  32  at time T 2 A ( FIG. 5 ) to enable the motor snubber  40  ( 53  in  FIG. 4 ), by which the circuit  40  is placed in a first mode to selectively couple at least one resistive load between two or more of the AC output terminals to begin dissipation of power regenerated from the load  60  toward the motor drive  10 . In the example of  FIG. 2  above, actuation of the control signal  32  by the controller  30  turns on the transistor  44 , thereby connecting the resistor  45  across the DC output of the rectifier  41  to dissipate regenerative power provided by the slowing and reversal of the motor  6 . In the case of the contactor-based motor snubber  40  of  FIG. 3 , provision of the control signal  32  allows closure of the contacts  47  in a controlled manner under operation of the controller  30 . Alternatively, the use of normally closed contacts in the contactor  47  will result in the contacts closing to connect the loading resistors  48  to the AC output nodes without further action by the controller  30  if the controller  30  ceases to operate. It is also noted that certain embodiments may employ motor snubber circuitry  40  that is self-enabling, in which case no control signal  32  need be provided by the controller  30  in order to enable the motor snubber  40  in response to loss of system input power. 
     In embodiments in which the controller  30  provides the control signal  32  to enable the motor snubber  40 , moreover, the controller may disable the inverter switching (signal  92  in  FIG. 5 ) and enable the motor snubber (via signal  32 ) contemporaneously, or may alternatively disable the IGBTs after enabling the motor snubber  40 . However, the illustrated example ( FIG. 5 ) advantageously disables operation of the inverter switches  22 - 27  at time T 2  prior to enabling the motor snubber  40  at T 2 A, and the controller  30  may implement the timing of these signal transitions according to a predetermined time in certain embodiments. In other possible implementations, moreover, the controller  30  may gate the transitions of the signals  92 ,  32  according to one or more system operating conditions, such as a measured or estimated motor speed ( 82  in  FIG. 5 ), a measured DC bus voltage (bus voltage  72  in  FIG. 5 , ascertained via feedback signal  36  in  FIG. 1  above), etc. As seen in the graph  80  of  FIG. 5 , moreover, the motor speed continues to slow (in the forward direction) from time T 2 A until a time T 2 B at which the motor stops and ultimately reverses direction (RPM&lt;0 in the figure). As seen in the graph  80 , moreover, activation of the motor snubber  40  at time T 2 A advantageously reduces the amount of reverse direction acceleration in the motor speed  82 , whereas the slope of the curve  82  in the reverse direction would continue at a steeper rate absent the use of the power dissipation circuit  40 . Thus, the reverse motor speed is attenuated by operation of the snubber  40 , thereby advantageously conserving the amount of previously pumped fluid in a deep well application, and also mitigating the amount of back EMF generated by the reverse rotation of the motor load  6 . In this regard, it is noted that very lengthy power interruptions to the system  2  may indeed result in loss of all the previously pumped fluid in a given well, but the regulation or limiting of the reverse direction rotation of the motor load  6  nevertheless mitigates the level of back EMF regenerated by the motor  6 , thereby protecting the motor  6  and/or the motor drive  10  and the components thereof. 
     At  54  in  FIG. 4 , the controller  30  (MCB or main control board) may stop normal operation at T 3  in  FIG. 5  (illustrated by the main control board running signal  94 ), for example, due to the input power loss. It is noted that the length of time during which input power is discontinued may dictate whether or not the controller  30  stops running. 
     At T 4  in  FIG. 5 , the AC input power is restored ( 55  in  FIG. 4 ), as seen in the graph  60  with the increase in the AC voltage  62  and the corresponding increase in the DC bus voltage  72  (graph  70 ). Since the inverter  20  remains inoperative and the motor snubber  40  remains activated at this point, the reverse motor speed ( 82  in  FIG. 5 ) continues as before. At some point (T 4 A in  FIG. 5 ), the main control board (controller  30 ) begins running (indicated as a rising edge in the signal  94  in  FIG. 5 ), and the controller  30  disables the motor snubber via signal  32  at time T 4 B ( 56  in  FIG. 4 ). In addition, the controller resumes switching operation of the inverter  20  at  57  in  FIG. 4  (T 5  in  FIG. 5 ) and enables the DC bus snubber circuit  16 ,  18  via control signal  34  ( 58  in  FIG. 4 ). As seen in  FIG. 5 , the IGBTs  22 - 27  may be enabled at roughly the same time as the DC bus snubber circuit is activated via signal  34 , although not a strict requirement of all the possible implementations of the concepts of the present disclosure. Moreover, while the illustrated example provides for deactivation of the motor snubber  40  at time T 4 B prior to activation of the DC bus snubber  16 ,  18  and the inverter  20  at T 5 , these actions may be taken contemporaneously or different sequences can be used in various implementations. 
     In the illustrated example, deactivation of the motor snubber  40  initially causes the reverse rotation of the motor  62  increase (acceleration downward in the RPM curve  82  of  FIG. 5 ), and this trend is reversed when the switching operation of the inverter  20  is resumed at T 5 . Thereafter, the reversed motor slows and eventually stops, and subsequently begins rotating in the forward direction. As further seen in  FIG. 5 , moreover, resumption of the IGBT operation at T 5  increases the DC bus voltage  72 , and the DC snubber  16 ,  18  operates to mitigate the amount of excess DC bus voltage, thereby preventing or mitigating degradation of the DC bus capacitor C and other components of the motor drive  10 . At T 6  in  FIG. 5 , moreover, the DC bus snubber circuit  16 ,  18  is deactivated ( 59  in  FIG. 4 ), and the system  2  resumes normal operation thereafter. As noted above, the controller  30  in certain embodiments may employ the DC bus snubber  16 ,  18  based at least in part on the feedback signal or value  36  indicating the DC bus voltage level. For example, the circuit  16 ,  18  may be activated in one implementation based on an increase of the DC bus voltage ( 72  in  FIG. 5 ) above a first threshold value. In certain embodiments, moreover, the controller  30  may release or deactivate the snubber circuitry  16 ,  18  once the DC bus voltage  72  thereafter decreases below the same or a second threshold value. In one possible implementation, moreover, the controller  30  may provide a pulse width modulated control signal  34  for closing the DC bus snubber switch  16 , for example, with a pulse width of the control signal  34  being controlled according to the amount of excess DC bus voltage beyond the threshold value. In other possible implementations, the activation and deactivation of the DC bus voltage snubber circuit  16 ,  18  may be implemented by separate circuitry, or may be self-activated without use of any control signal  34  from the controller  30 . 
     The above examples are merely illustrative of several possible embodiments of various aspects of the present disclosure, wherein equivalent alterations and/or modifications will occur to others skilled in the art upon reading and understanding this specification and the annexed drawings. In particular regard to the various functions performed by the above described components (assemblies, devices, systems, circuits, and the like), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component, such as hardware, processor-executed software, or combinations thereof, which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the illustrated implementations of the disclosure. In addition, although a particular feature of the disclosure 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. Also, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in the detailed description and/or in the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.