Abstract:
A powertrain controller for a vehicle may include input channels configured to receive start requests for an engine and operating condition data for an electric machine, and output channels configured to provide torque commands for the electric machine. The powertrain controller may further include control logic configured to, in response to receiving a start request for the electric machine while the operating condition data indicates that the electric machine is operating at a torque limit to drive the vehicle, generate torque commands that cause the electric machine to exceed the torque limit.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to modifying torque limits in hybrid vehicles. 
       BACKGROUND 
       [0002]    A hybrid electric vehicle utilizes both an engine and an electric machine to provide torque to the wheels. A disconnect clutch may decouple the engine from the vehicle powertrain to allow the engine to be in an off state while the electric machine is propelling the vehicle. 
       SUMMARY 
       [0003]    A method of controlling a vehicle is provided. The method may include, in response to receiving a start request for an engine while an electric machine is generating torque to drive the vehicle at a torque limit of the electric machine, increasing the torque beyond the torque limit for a predefined duration of time to provide torque to start the engine. 
         [0004]    A vehicle is provided. The vehicle includes an engine, a fraction motor, and a controller. The controller may be configured to, in response to receiving a request for additional torque to start the engine while the fraction motor is operating at a torque limit to satisfy a drive torque command, command the traction motor to increase torque output for a predefined duration of time to satisfy the request for additional torque. 
         [0005]    A powertrain controller for a vehicle is provided. The controller may include input channels configured to receive start requests for an engine and operating condition data for an electric machine, and output channels configured to provide torque commands for the electric machine. The controller may further include control logic configured to, in response to receiving a start request for the electric machine while the operating condition data indicates that the electric machine is operating at a torque limit to drive the vehicle, generate torque commands that cause the electric machine to exceed the torque limit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a schematic diagram of a hybrid electric vehicle; 
           [0007]      FIG. 2  is a graph depicting the relationship between torque and speed during operation of a hybrid electric vehicle; 
           [0008]      FIGS. 3A through 3C  are a series of graphs depicting the relationship between speed, torque, and time during operation of a hybrid electric vehicle; and 
           [0009]      FIG. 4  is a flow chart describing control logic for a powertrain controller of a hybrid electric vehicle. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments may take various and alternative forms. The figures are not necessarily to scale; some features could be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. As those of ordinary skill in the art will understand, various features illustrated and described with reference to any one of the figures may be combined with features illustrated in one or more other figures to produce embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. Various combinations and modifications of the features consistent with the teachings of this disclosure, however, could be desired for particular applications or implementations. 
         [0011]    Referring to  FIG. 1 , a schematic diagram of a hybrid electric vehicle (HEV)  10  is illustrated.  FIG. 1  illustrates representative relationships among several vehicle components. Physical placement and orientation of the components within the vehicle  10  may vary. The vehicle  10  includes a powertrain  12 . The powertrain  12  includes an engine  14  that drives a transmission  16 . As will be described in further detail below, the transmission  16  includes an electric machine such as an electric motor/generator (M/G)  18 , an associated traction battery  20 , a torque converter  22 , and a multiple step-ratio automatic transmission, or gearbox  24 . 
         [0012]    The engine  14  and the M/G  18  are both capable of providing motive power for the HEV  10 . The engine  14  generally represents a power source which may include an internal combustion engine such as a gasoline, diesel, or natural gas powered engine, or a fuel cell. The engine  14  generates an engine power and corresponding engine torque that is supplied to the M/G  18  when a disconnect clutch  26  between the engine  14  and the M/G  18  is at least partially engaged. The M/G  18  may be implemented by any one of a plurality of types of electric machines. For example, the M/G  18  may be a permanent magnet synchronous motor. Power electronics  28  condition direct current (DC) power provided by the battery  20  to the requirements of the M/G  18 , as will be described below. For example, power electronics may provide three phase alternating current (AC) to the M/G  18 . 
         [0013]    The engine  14  may additionally be coupled to a turbocharger  46  to provide an air intake pressure increase, or “boost” to force a higher volume of air into a combustion chamber of the engine  14 . Related to the increased air pressure provided to the engine  14  by the turbocharger  46 , a corresponding increase in the rate of fuel combustion may be achieved. The additional air pressure boost therefore allows the engine  14  to achieve additional output power, thereby increasing engine torque. 
         [0014]    The gearbox  24  may include gear sets (not shown) that are selectively placed in different gear ratios by selective engagement of friction elements such as clutches and brakes (not shown) to establish the desired multiple discrete or step drive ratios. The friction elements are controllable through a shift schedule that connects and disconnects certain elements of the gear sets to control the ratio between a transmission output shaft  38  and the transmission input shaft  34 . The gearbox  24  ultimately provides a powertrain output torque to output shaft  38 . 
         [0015]    As further shown in the representative embodiment of  FIG. 1 , the output shaft  38  is connected to a differential  40 . The differential  40  drives a pair of wheels  42  via respective axles  44  connected to the differential  40 . The differential transmits torque allocated to each wheel  42  while permitting slight speed differences such as when the vehicle turns a corner. Different types of differentials or similar devices may be used to distribute torque from the powertrain to one or more wheels. In some applications, torque distribution may vary depending on the particular operating mode or condition, for example. 
         [0016]    The vehicle  10  further includes a foundation brake system  54 . The system may comprise friction brakes suitable to selectively apply pressure by way of stationary pads attached to a rotor affixed to the wheels. The applied pressure between the pads and rotors creates friction to resist rotation of the vehicle wheels  42 , and is thereby capable of slowing the speed of vehicle  10 . 
         [0017]    When the disconnect clutch  26  is at least partially engaged, power flow from the engine  14  to the M/G  18  or from the M/G  18  to the engine  14  is possible. For example, when the disconnect clutch  26  is engaged, the M/G  18  may operate as a generator to convert rotational energy provided by a crankshaft  30  through M/G shaft  32  into electrical energy to be stored in the battery  20 . The rotational resistance imparted on the shaft through regeneration of energy may be used as a brake to decelerate the vehicle. The disconnect clutch  26  can also be disengaged to decouple the engine  14  from the remainder of the powertrain  12  such that the M/G  18  can operate as the sole drive source for the vehicle  10 . 
         [0018]    Operation states of the powertrain  12  may be dictated by at least one controller. While illustrated by way of example as a single controller, such as a vehicle system controller (VSC)  48 , there may be a larger control system including several controllers. The individual controllers, or the control system, may be influenced by various other controllers throughout the vehicle  10 . For example controllers that may be included within representation of the VSC  48  include a transmission control module (TCM), brake system control module (BSCM), a high voltage battery energy control module (BECM), as well as other controllers in communication which are responsible for various vehicle functions. The at least one controller can collectively be referred to as a “controller” that commands various actuators in response to signals from various sensors. The VSC  48  response may serve to dictate or influence a number of vehicle functions such as starting/stopping engine  14 , operating the M/G  18  to provide wheel torque or recharge the traction battery  20 , select or schedule transmission gear shifts, etc. 
         [0019]    The VSC  48  may further include a microprocessor or central processing unit (CPU) in communication with various types of computer readable storage devices or media. Computer readable storage devices or media may include volatile and nonvolatile storage in read-only memory (ROM), random-access memory (RAM), and keep-alive memory (KAM), for example. KAM is a persistent or non-volatile memory that may be used to store various operating variables while the CPU is powered down. Computer-readable storage devices or media may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by the controller in controlling the engine or vehicle. 
         [0020]    The VSC  48  communicates with various engine/vehicle sensors and actuators via an input/output (I/O) interface that may be implemented as a single integrated interface that provides various raw data or signal conditioning, processing, and/or conversion, short-circuit protection, and the like. Alternatively, one or more dedicated hardware or firmware chips may be used to condition and process particular signals before being supplied to the CPU. As generally illustrated in the representative embodiment of  FIG. 1 , the VSC  48  may communicate signals to and/or from the engine  14 , the turbocharger  46 , the disconnect clutch  26 , the M/G  18 , the transmission gearbox  24 , torque converter  22 , the torque converter bypass clutch  36 , and the power electronics  28 . Although not explicitly illustrated, those of ordinary skill in the art will recognize various functions or components that may be controlled by the VSC  48  within each of the subsystems identified above. Representative examples of parameters, systems, and/or components that may be directly or indirectly actuated using control logic executed by the controller include fuel injection timing, rate, and duration, throttle valve position, spark plug ignition timing (for spark-ignition engines), intake/exhaust valve timing and duration, front-end accessory drive (FEAD) components such as an alternator, air conditioning compressor, battery charging, regenerative braking, M/G operation, clutch pressures for disconnect clutch  26 , torque converter bypass clutch  36 , and transmission gearbox  24 , and the like. Sensors communicating input through the I/O interface may be used to indicate turbocharger boost pressure, turbocharger rotation speed, crankshaft position, engine rotational speed (RPM), wheel speeds, vehicle speed, engine coolant temperature, intake manifold pressure, accelerator pedal position, ignition switch position, throttle valve position, air temperature, exhaust gas oxygen or other exhaust gas component concentration or presence, intake air flow, transmission gear, ratio, or mode, transmission oil temperature, transmission turbine speed, torque converter bypass clutch status, deceleration, or shift mode, for example. 
         [0021]    The VSC  48  also includes a torque control logic feature. The VSC  48  is capable of interpreting driver requests based on several vehicle inputs. These inputs may include, for example, gear selection (PRNDL), accelerator pedal inputs, brake pedal input, battery temperature, voltage, current, and battery state of charge (SOC). The VSC  48  in turn may issue command signals to the transmission to control the operation of the M/G  18 . 
         [0022]    The M/G  18  is also in connection with the torque converter  22  via shaft  32 . Therefore, the torque converter  22  is also connected to the engine  14  when the disconnect clutch  26  is at least partially engaged. The torque converter  22  includes an impeller fixed to the M/G shaft  32  and a turbine fixed to a transmission input shaft  34 . The torque converter  22  provides a hydraulic coupling between shaft  32  and transmission input shaft  34 . An internal bypass clutch  36  may also be provided such that, when engaged, clutch  36  frictionally or mechanically couples the impeller and the turbine of the torque converter  22 , permitting more efficient power transfer. The torque converter bypass clutch  36  may be operated as a launch clutch to provide smooth vehicle launch. In contrast, when the bypass clutch  36  is disengaged, the M/G  18  may be decoupled from the differential  40  and the vehicle axles  44 . For example, during deceleration the bypass clutch  36  may disengage at low vehicle speeds, providing a torque bypass, to allow the engine to idle and deliver little or no output torque to drive the wheels. 
         [0023]    A driver of the vehicle  10  may provide input at accelerator pedal  50  and create a demanded torque, power, or drive command to propel the vehicle  10 . In general, depressing and releasing the pedal  50  generates an accelerator input signal that may be interpreted by the VSC  48  as a demand for increased power or decreased power, respectively. Based at least upon input from the pedal, the controller  48  may allocate torque commands between each of the engine  14  and/or the M/G  18  to satisfy the vehicle torque output demanded by the driver. The controller  48  may also control the timing of gear shifts within the gearbox  24 , as well as engagement or disengagement of the disconnect clutch  26  and the torque converter bypass clutch  36 . Like the disconnect clutch  26 , the torque converter bypass clutch  36  can be modulated across a range between the engaged and disengaged positions. This may produce a variable slip in the torque converter  22  in addition to the variable slip produced by the hydrodynamic coupling between the impeller and the turbine. Alternatively, the torque converter bypass clutch  36  may be operated as either locked or open without using a modulated operating mode depending on the particular application. 
         [0024]    The driver of vehicle  10  may additionally provide input at brake pedal  52  to create a vehicle braking demand. Depressing brake pedal  52  generates a braking input signal that is interpreted by controller  48  as a command to decelerate the vehicle. The controller  48  may in turn issue commands to cause the application of negative torque to the powertrain output shaft. Additionally or in combination, the controller may issue commands to activate the brake system  54  to apply friction brake resistance to inhibit rotation of the vehicle wheels  42 . The negative torque values provided by both of the powertrain and the friction brakes may be allocated to vary the amount by which each satisfies driver braking demand. 
         [0025]    To drive the vehicle with the engine  14 , the disconnect clutch  26  is at least partially engaged to transfer at least a portion of the engine torque through the disconnect clutch  26  to the M/G  18 , and then from the M/G  18  through the torque converter  22  and gearbox  24 . The M/G  18  may assist the engine  14  by providing additional powered torque to turn the shaft  32 . This operation mode may be referred to as a “hybrid mode.” As mentioned above, the VSC  48  may be further operable to issue commands to allocate a torque output of both the engine  14  and the M/G  18  such that the combination of both torque outputs satisfies an accelerator  50  input from the driver. 
         [0026]    To drive the vehicle  10  with the M/G  18  as the sole power source, the power flow remains the same except the disconnect clutch  26  isolates the engine  14  from the remainder of the powertrain  12 . Combustion in the engine  14  may be disabled or otherwise OFF during this time in order to conserve fuel, for example. The traction battery  20  transmits stored electrical energy through wiring  51  to power electronics  28  that may include an inverter. The power electronics  28  convert high-voltage direct current from the battery  20  into alternating current for use by the M/G  18 . The VSC  48  may further issue commands to the power electronics  28  such that the M/G  18  is enabled to provide positive or negative torque to the shaft  32 . This operation where the M/G  18  is the sole motive source may be referred to as an “electric only” operation mode. 
         [0027]    Therefore, it may be advantageous to operate the vehicle  10  in the “electric only” operation mode. However, during an engine restart command from the VSC  48 , drive torque from the M/G  18  may be reduced in order to supply the necessary engine torque to restart the vehicle engine  14 . In at least one embodiment, the VSC  48  may be programmed to increase torque output by the M/G  18  such that the torque output exceeds a drive torque limit of the M/G  18  to provide start torque for the engine  14 . This allows for an extended “electric only” operation mode. 
         [0028]    Additionally, the M/G  18  may operate as a generator to convert kinetic energy from the powertrain  12  into electric energy to be stored in the battery  20 . The M/G  18  may act as a generator while the engine  14  is providing the sole propulsion power for the vehicle  10 , for example. The M/G  18  may also act as a generator during times of regenerative braking in which rotational energy from spinning of the output shaft  38  is transferred back through the gearbox  24  and is converted into electrical energy for storage in the battery  20 . 
         [0029]      FIG. 2  is a graph of an increased torque output by the M/G  18 .  FIG. 2  shows torque in Nm increasing along the y-axis and speed in RPM increasing along the x-axis.  FIG. 2  depicts curves for periods of constant torque and constant power. Modifying a drive torque limit of the M/G  18  allows the M/G  18  to briefly output torque above a maximum drive torque limit. Curve  100  represents an unmodified drive torque limit for drive torque generated by the M/G  18 . The unmodified drive torque limit, as represented by curve  100 , may be a generally conservative maximum drive torque limit. The maximum drive torque limit of the M/G  18  is based on the basic design of the M/G  18 . Likewise, curve  120  represents a maximum drive torque availability for “electric only” operation mode. Curve  120  is representative of the unmodified drive torque limit of curve  100  minus an engine start torque reserved for engine starts or restarts. As depicted by curve  120 , this minimizes the availability of drive torque from the M/G  18  used during “electric only” operation mode. Increasing the maximum drive torque available during “electric only” operation mode without increasing the size of the M/G  18  improves overall fuel economy. 
         [0030]    Curve  140  represents a modified drive torque limit for drive torque generated by the M/G  18 . Because the unmodified drive torque limit, as represented by curve  100 , is generally conservative, a modified drive torque limit, as represented by curve  140 , may be used that accounts for transient bursts of required drive torque. For example, the modified drive torque limit represented by curve  140 , may be used for engine starts and restarts that occur in less than one second. Likewise, curve  160  represents a new maximum drive torque availability for “electric only” operation mode. This is based on the modified drive torque limit, as represented by curve  140 . The new maximum drive torque availability, as represented by curve  160 , equals the modified torque limit, as represented by curve  140 , minus the torque reserved for engine starts and restarts. By increasing the unmodified, steady-state maximum torque limit of curve  100  to account for short transient bursts of required drive torque, more drive torque is available for operation within the “electric only” operation mode. This allows the M/G  18  to provide the sole motive power for a longer duration. Extending the range of the “electric only” operation mode allows for significant improvement in vehicle fuel economy. 
         [0031]    The modified drive torque limit, as represented by curve  140 , acts as a buffer accounting for engine starts and restarts. The amount of torque required for engine starts and restarts may be pre-calculated. Therefore, the steady-state drive torque limit, as represented by curve  100 , may be raised by the pre-calculated torque necessary for engine starts and restarts for short durations. This allows for improved “electric only” operation mode capability. Further, this increases the engine off capability. Increasing the engine off capability offers the flexibility to utilize different engine brake specific fuel consumption maps. Improving the “electric only” operation mode capability and increasing the engine off capability improves fuel economy over a wide range of operating conditions. 
         [0032]      FIGS. 3A through 3C  are a series of graphs depicting the modified drive torque limit during “electric only” operation mode and “hybrid mode.” The graphs measure three different curvatures over a period of five different time intervals. The first graph measures the M/G speed and the engine speed increasing along the y-axis with the time intervals extending along the x-axis. The second graph measures M/G drive torque, engine torque, and disconnect clutch torque increasing along the y-axis with the time intervals extending along the x-axis. The third graph measures the engine torque increasing along the y-axis with the time intervals extending along the x-axis. 
         [0033]    The first graph, referenced as graph A, measures the M/G speed as well as the engine speed over time. Specifically, the first graph compares the behavior of the M/G speed and the engine speed during the “electric only” operation mode and the “hybrid mode.” As noted in the first graph, engine speed reaches peak  200  between T 2  and T 3 . As discussed in more detail below, this peak is consistent with an engine start or restart command due to an accelerator pedal tip-in event. Further, from time interval T 3  through T 4 , the engine speed ramps up reaching peak  220  at T 4 . Peak  220  represents the point at which the disconnect clutch  26  is locked and the engine speed matches the M/G speed. Therefore, from time interval T 4  through T 5 , the engine  14  will be supplying engine torque along with the M/G  18  providing drive torque. When the engine  14  is on, the vehicle  10  will be in the “hybrid mode” operation. 
         [0034]    The second graph, referenced as graph B, depicts torque increasing along the y-axis and time increasing along the x-axis. Dashed line  240  represents the maximum motor torque limit, as modified, to account for a transient burst of demanded torque during engine starts and restarts. Dashed line  260  represents the torque available during “electric only” operation mode. Using the modified maximum torque limit, as represented by line  240 , allows for much more M/G drive torque available during “electric only” operation mode. For example, as a vehicle driver demands impeller torque from the engine at peak  280  between time intervals T 1  and T 2 , the modified maximum motor torque limit allows the M/G  18  to provide the impeller torque demand. 
         [0035]    Dashed line  250  represents the unmodified maximum motor torque limit. As stated above, the unmodified maximum motor torque is a generally conservative limit. This allows the M/G  18  to ramp up to the modified maximum torque limit, as represented by line  240 , for transient bursts during an engine start request. By increasing the unmodified maximum motor torque limit of line  250  to the modified maximum torque limit of line  240 , the vehicle is able to operate in “electric only” operation mode for a longer period of time. 
         [0036]    During time interval T 2  and T 3  the M/G torque will be increased, between peaks  300  and  320 , up to the modified maximum torque limit. The M/G  18  will continue to provide drive torque at the modified maximum torque limit through a relatively small time interval. For example, in order to account for the torque demanded for engine starts and restarts, the M/G  18  will continue to provide drive torque at the modified maximum torque limit for approximately one second. Likewise, during time intervals T 2  and T 3  the disconnect clutch torque may have a complementary curvature as the M/G torque, as described above. The disconnect clutch torque will decrease by the amount of torque demanded from the modified maximum motor torque limit between peaks  380  and  400 . The disconnect clutch torque decreases due to pressure applied to the disconnect clutch in order to account for the engine start command. The additional torque load from the engine drags the disconnect clutch torque negative. This is consistent with a partially closed position of the disconnect clutch. The M/G  18  compensates for the negative torque of the disconnect clutch by applying increased positive torque. This allows the net transmission input shaft torque to remain constant. Between time intervals T 3  and T 4  the M/G  18  will ramp down at  340  and continue to provide drive torque at the maximum torque availability limit represented by dashed line  260 . 
         [0037]    Utilizing the modified maximum torque limit to account for an increase torque demand event, such as an engine start or restart, allows for a torque buffer  360 . This allows much more drive torque available from the M/G  18  during “electric only” operation mode. Having more drive torque allows for an improved electric drive capability and improves fuel economy over a wide range of operating conditions. Further, since the additional torque is only provided within a relatively small time interval, there is little impact on the lifespan or functionality of the M/G  18 . 
         [0038]    As the vehicle  10  begins to enter “hybrid mode” operation, between time intervals T 4  and T 5 , the drive torque produced by the M/G  18  will ramp down slope  420 . As discussed above, when the vehicle is in the “hybrid” drive mode the engine  14  is providing engine torque to the powertrain  12 . When the engine  14  is providing engine torque to the powertrain  12 , the drive torque produced by the M/G  18  will reduce to zero. Likewise, the torque produced by the disconnect clutch  26  will ramp up slope  440  until it meets the driver demanded impeller torque from the engine  14 . Slope  440  represents a slipping condition of the disconnect clutch. The slipping condition of the disconnect clutch occurs when the turbine shaft is rotating at a faster rate than the impeller shaft. Therefore, the disconnect clutch will be in a locked condition after time interval T 5 , when the impeller shaft speed of rotation meets the turbine shaft speed of rotation. This couples the engine  14  to the powertrain  12 . This increases the driver demanded torque limit at curve  460  between time intervals T 4  and T 5 . This further allows the engine  14  to have a higher driver demand torque limit and produce more output torque. 
         [0039]    The third graph, referenced as graph C, depicts torque increasing along the y-axis and time extending along the x-axis. Line  480  depicts driver demanded impeller torque consistent with an engine start and restart event between time interval T 1  and T 5 . Line  500  represents the modified final delivered impeller torque between time interval T 1  and T 5 . As the engine starts or restarts and the vehicle begins to enter “hybrid” drive operation mode, the final delivered impeller torque peaks at  520  before reaching the demanded impeller torque. Line  510  represents the unmodified final delivered impeller torque between time interval T 1  and T 5 . Line  510  shows the final delivered impeller toque using the unmodified maximum motor torque limit. Comparing lines  500  and  510  shows the availability of more torque during “electric only” operation mode. Therefore using the modified maximum M/G torque limit, as discussed above, allows for increased capability within the “electric only” operation mode. 
         [0040]    Referring to  FIG. 4 , a flowchart depicting the control logic of the VSC  48  is shown. At  540 , the VSC  48  calculates the unmodified maximum drive torque limit. At  560 , the VSC  48  calculates the required drive torque from the M/G  18  necessary for an engine start or restart event. The VSC  48  adds the required drive torque for an engine start at  560  to the unmodified maximum drive torque limit calculated at  540 . This allows for a modified maximum drive torque limit at  560 . At  580 , the VSC  48  determines if an engine start or restart request has been made. If, at  580 , the VSC  48  determines that an engine start or restart request has not been made, then at  600  the VSC  48  may command the vehicle to drive during “electric only” operation mode using the unmodified maximum drive torque limit calculated at  540 . 
         [0041]    Likewise, if at  580 , the VSC  48  determines that an engine start or restart request has been made, then at  620  the VSC  48  may command the vehicle to drive during “electric only” operation mode using the modified maximum drive torque limit. This allows the VSC  48  to account for the extra output torque needed in order to start or restart the vehicle engine  14  as the vehicle exits the “electric only” operation mode. Further, the VSC  48  may only command, at  600 , operation at the modified maximum torque limit for a short duration. Operating at the modified maximum torque limit for a short duration allows the VSC  48  to account for the added torque necessary for engine start or restart requests without modifying the M/G  18 . This allows for an improved fuel economy over a wide range of operating conditions as well as an improved “electric only” operation mode capability. 
         [0042]    While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms encompassed by the claims. The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the disclosure. As previously described, the features of various embodiments may be combined to form further embodiments of the invention that may not be explicitly described or illustrated. While various embodiments could have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, those of ordinary skill in the art recognize that one or more features or characteristics may be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes may include, but are not limited to cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. As such, embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications.