Patent Publication Number: US-2023137049-A1

Title: Resolver offset detection

Description:
TECHNICAL FIELD 
     The present disclosure relates to detecting and correcting a resolver offset in an electric motor. 
     BACKGROUND 
     Electric vehicles are propelled by a DC high-voltage (HV) battery supplying power to an AC electric motor. An inverter is used to convert the DC power into AC power. A DC bus capacitor connected between positive and negative HV buses may be discharged by applying current on a d-axis of the motor after the vehicle is parked. A resolver offset is characterized once the electric machine is assembled and this offset is used to adjust a rotor position reading in a motor controller such that a current angle can be properly aligned to a desired rotor position, relative to a magnetic circuit. The resolver offset may have some amount of error due to resolver design, measurement method, or current control accuracy. If a resolver offset error occurs, a q-axis current may be produced which in turn produces torque. 
     SUMMARY 
     A vehicle includes an electric machine having a rotor and a stator, a resolver that measures a position of the rotor relative to the stator, and a controller. The controller, based on the position and a resolver offset, injects a first current having only a d-axis component into the electric machine, and responsive to detecting a first motion output of the electric machine being greater than a threshold, adjusts the resolver offset according to a magnitude and direction of the first motion output. 
     A method for controlling an electric machine of a vehicle includes while the vehicle is parked, repeatedly injecting current into the electric machine having only a d-axis component until motion output of a rotor of the electric machine corresponding to the current is less than a threshold. 
     A device, for measuring a resolver offset of a resolver of an electric machine of a vehicle, includes a controller that, responsive to a resolver offset testing condition being met, commands that a first current having only a d-axis component be injected into the electric machine, and responsive to detecting a first motion output of the electric machine being greater than a threshold, adjusts the resolver offset according to a magnitude and direction of the first motion output. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    depicts a possible configuration for an electrified vehicle. 
         FIG.  2    depicts a possible configuration for a vehicle system including power electronics associated with an electric machine. 
         FIG.  3    depicts a flow diagram for a resolver offset measuring and correcting process. 
         FIG.  4    depicts a waveform diagram of an electric machine speed output in response to a d-axis current injection. 
         FIGS.  5 A and  5 B  depict a waveform diagram of an electric machine torque output in response to a d-axis current injection. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure are described herein. It is to be understood, however, that the disclosed embodiments are merely examples and other embodiments can 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 can 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. 
       FIG.  1    depicts an electrified vehicle  112  that may be referred to as a plug-in hybrid-electric vehicle (PHEV). A plug-in hybrid-electric vehicle  112  may comprise one or more electric machines  114  mechanically coupled to a gearbox or hybrid transmission  116 . The electric machines  114  may be capable of operating as a motor and a generator. In addition, the hybrid transmission  116  is mechanically coupled to an engine  118 . The hybrid transmission  116  may be mechanically coupled to a differential  119  that is configured to adjust the speed of drive shafts  120  that are mechanically coupled to drive wheels  122  of the vehicle  112 . The drive shafts  120  may be referred to as the drive axle. In some configurations, a clutch may be disposed between the hybrid transmission  116  and the differential  119 . The electric machines  114  can provide propulsion and slowing capability when the engine  118  is turned on or off. The electric machines  114  may also act as generators and can provide fuel economy benefits by recovering energy that would normally be lost as heat in a friction braking system. The electric machines  114  may also reduce vehicle emissions by allowing the engine  118  to operate at more efficient speeds and allowing the hybrid-electric vehicle  112  to be operated in electric mode with the engine  118  off under certain conditions. An electrified vehicle  112  may also be a battery electric vehicle (BEV). In a BEV configuration, the engine  118  may not be present. In other configurations, the electrified vehicle  112  may be a full hybrid-electric vehicle (FHEV) without plug-in capability. 
     A battery pack or traction battery  124  stores energy that can be used by the electric machines  114 . The traction battery  124  may provide a high voltage direct current (DC) output. A contactor module  123  may include one or more contactors configured to isolate the traction battery  124  from a high-voltage bus  125  when opened and connect the traction battery  124  to the high-voltage bus  125  when closed. The high-voltage bus  125  may include power and return conductors for carrying current over the high-voltage bus  125 . The contactor module  123  may be located in the traction battery  124 . One or more power electronics modules  126  may be electrically coupled to the high-voltage bus  125 . The power electronics modules  126  are also electrically coupled to the electric machines  114  and provide the ability to bi-directionally transfer energy between the traction battery  124  and the electric machines  114 . For example, a traction battery  124  may provide a DC voltage while the electric machines  114  may operate with a three-phase alternating current (AC) to function. The power electronics module  126  may convert the DC voltage to a three-phase AC current to operate the electric machines  114 . In a regenerative mode, the power electronics module  126  may convert the three-phase AC current from the electric machines  114  acting as generators to the DC voltage compatible with the traction battery  124 . 
     In addition to providing energy for propulsion, the traction battery  124  may provide energy for other vehicle electrical systems. The vehicle  112  may include a DC/DC converter module  128  that converts the high voltage DC output from the high-voltage bus  125  to a low-voltage DC level of a low-voltage bus  129  that is compatible with low-voltage loads  131 . An output of the DC/DC converter module  128  may be electrically coupled to an auxiliary battery  130  (e.g., 12V battery) for charging the auxiliary battery  130 . The low-voltage loads  131  may be electrically coupled to the auxiliary battery  130  via the low-voltage bus  129 . One or more high-voltage electrical loads  133  may be coupled to the high-voltage bus  125 . The high-voltage electrical loads  133  may have an associated controller that operates and controls the high-voltage electrical loads  133  when appropriate. Examples of high-voltage electrical loads  133  may be a fan, an electric heating element, and/or an air-conditioning compressor. 
     The electrified vehicle  112  may be configured to recharge the traction battery  124  from an external power source  136 . The external power source  136  may be a connection to an electrical outlet. The external power source  136  may be electrically coupled to a charge station or electric vehicle supply equipment (EVSE)  138 . The external power source  136  may be an electrical power distribution network or grid as provided by an electric utility company. The EVSE  138  may provide circuitry and controls to manage the transfer of energy between the power source  136  and the vehicle  112 . The external power source  136  may provide DC or AC electric power to the EVSE  138 . The EVSE  138  may have a charge connector  140  for coupling to a charge port  134  of the vehicle  112 . The charge port  134  may be any type of port configured to transfer power from the EVSE  138  to the vehicle  112 . The charge port  134  may be electrically coupled to an on-board power conversion module or charger  132 . The charger  132  may condition the power supplied from the EVSE  138  to provide the proper voltage and current levels to the traction battery  124  and the high-voltage bus  125 . The charger  132  may interface with the EVSE  138  to coordinate the delivery of power to the vehicle  112 . The EVSE connector  140  may have pins that mate with corresponding recesses of the charge port  134 . Alternatively, various components described as being electrically coupled or connected may transfer power using a wireless inductive coupling. 
     Electronic modules in the vehicle  112  may communicate via one or more vehicle networks. The vehicle network may include a plurality of channels for communication. One channel of the vehicle network may be a serial bus such as a Controller Area Network (CAN). One of the channels of the vehicle network may include an Ethernet network defined by the Institute of Electrical and Electronics Engineers (IEEE) 802 family of standards. Additional channels of the vehicle network may include discrete connections between modules and may include power signals from the auxiliary battery  130 . Different signals may be transferred over different channels of the vehicle network. For example, video signals may be transferred over a high-speed channel (e.g., Ethernet) while control signals may be transferred over CAN or discrete signals. The vehicle network may include any hardware and software components that aid in transferring signals and data between modules. The vehicle network is not shown in  FIG.  1    but it may be implied that the vehicle network may connect to any electronic module that is present in the vehicle  112 . A vehicle system controller (VSC)  142  may be present to coordinate the operation of the various components. Note that operations and procedures that are described herein may be implemented in one or more controllers. Implementation of features that may be described as being implemented by a particular controller is not necessarily limited to implementation by that particular controller. Functions may be distributed among multiple controllers communicating via the vehicle network. 
     The electric machines  114  may be a permanent magnet synchronous motor (PMSM) type machine. A PMSM electric machine includes a rotor and a stator. The stator may include windings for producing a magnetic field to rotate the rotor. Current through the stator windings may be controlled to vary the magnetic field acting on the rotor. The rotor of a PMSM includes permanent magnets that create a magnetic field that interacts with the stator magnetic field to cause rotation of the rotor. The rotor speed may be controlled by the frequency of the magnetic field created by the stator. 
     The electric machines  114  may be comprised of a stator that includes stator windings and a rotor. The rotor may rotate about a central axis relative to the stator. The electric machines  114  may be controlled by flowing a generally sinusoidal current through stator windings. The amplitude and frequency of the current may be varied to control the torque and speed of the rotor. The stator current creates an electromagnetic field that interacts with the permanent magnets that are part of the rotor. This electromagnetic field causes the rotor to rotate. The electric machines  114  may be configured as three-phase machines. That is, the stator windings may include three separate phase windings. To control the electric machines  114 , a three-phase voltage or current waveform is applied to the phase windings. The three-phase waveform is such that each phase signal is separated by a phase difference of 120 degrees. 
     The electric machines  114  may be coupled to the power electronics module  126  via one or more conductors that are associated with each of the phase windings.  FIG.  2    depicts a block diagram of a vehicle system that includes a motor control system. The vehicle  112  may include one or more power electronics controllers  200  configured to monitor and control the power electronics module  126 . The conductors may be part of a wiring harness between the electric machine  114  and the power electronics module  126 . A three-phase electric machine  114  may have three conductors coupled to the power electronics module  126 . The power electronics module  126  may be configured to switch positive and negative terminals of the high-voltage bus  125  to phase terminals of the electric machines  114 . 
     The power electronics module  126  may be controlled to provide sinusoidal voltage and current signals to the electric machine  114 . The frequency of the signals may be proportional to the rotational speed of the electric machine  114 . 
     The controller  200  may be configured to adjust the voltage and current output of the power electronics module  126  at a predetermined switching frequency. The switching frequency may be the rate at which the states of switching devices within the power electronics module  126  are changed. The frequency of the injection voltage may be selected as a predetermined multiple of the switching frequency. 
     The power electronics module  126  may interface with a position/speed feedback device  202  that is coupled to the rotor of the electric machine  114 . For example, the position/speed feedback device  202  may be a resolver or an encoder. The position/speed feedback device  202  may provide signals indicative of a position and/or speed of the rotor of the electric machine  114 . The power electronics  126  may include the power electronics controller  200  that interfaces to the speed feedback device  202  and processes signals from the speed feedback device  202 . The power electronics controller  200  may be programmed to utilize the speed and position feedback to control operation of the electric machine  114 . 
     The power electronics  126  may include power switching circuitry  240  that includes a plurality of switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220 . The switching devices may be Insulated Gate Bipolar Junction Transistors (IGBTs) or other solid-state switching devices. The switching devices may be configured to selectively couple a positive terminal and a negative terminal of the high-voltage bus  125  to each phase terminal or leg (e.g., labeled U, V, W) of the electric machine  114 . Each of the switching devices within the power switching circuitry  240  may have an associated diode  222 ,  224 ,  226 ,  228   230 ,  232  connected in parallel to provide a path for inductive current when the switching device is in a non-conducting state. Each of the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  may have a control terminal for controlling operation of the associated switching device. The control terminals may be electrically coupled to the power electronics controller  200 . The power electronics controller  200  may include associated circuitry to drive and monitor the control terminals. For example, the control terminals may be coupled to the gate input of the solid-state switching devices. 
     A first switching device  210  may selectively couple the HV-bus positive terminal to a first phase terminal (e.g., U) of the electric machine  114 . A first diode  222  may be coupled in parallel to the first switching device  210 . A second switching device  212  may selectively couple the HV-bus negative terminal to the first phase terminal (e.g., U) of the electric machine  114 . A second diode  224  may be coupled in parallel to the second switching device  212 . A third switching device  214  may selectively couple the HV-bus positive terminal to a second phase terminal (e.g., V) of the electric machine  114 . A third diode  226  may be coupled in parallel to the third switching device  214 . A fourth switching device  216  may selectively couple the HV-bus negative terminal to the second phase terminal (e.g., V) of the electric machine  114 . A fourth diode  228  may be coupled in parallel to the fourth switching device  216 . A fifth switching device  218  may selectively couple the HV-bus positive terminal to a third phase terminal (e.g., W) of the electric machine  114 . A fifth diode  230  may be coupled in parallel to the fifth switching device  218 . A sixth switching device  220  may selectively couple the HV-bus negative terminal to the third phase terminal (e.g., W) of the electric machine  114 . A sixth diode  232  may be coupled in parallel to the sixth switching device  220 . 
     The power electronics controller  200  may be programmed to operate the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  to control the voltage and current applied to the phase windings of the electric machine  114 . The power electronics controller  200  may operate the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  so that each phase terminal is coupled to only one of the HV-bus positive terminal or the HV-bus negative terminal at a particular time. 
     Various motor control algorithms and strategies are available to be implemented in the power electronics controller  200 . The power electronics module  126  may also include current sensors  204 . The current sensors  204  may be inductive or Hall-effect devices configured to generate a signal indicative of the current passing through the associated circuit. In some configurations, two current sensors  204  may be utilized and the third phase current may be calculated from the two measured currents. The controller  200  may sample the current sensors  204  at a predetermined sampling rate. Measurement values for the phase currents of the electric machine  114  may be stored in controller memory for later computations. 
     The power electronics module  126  may include one or more voltage sensors. The voltage sensors may be configured to measure an input voltage to the power electronics module  126  and/or one or more of the output voltages of the power electronics module  126 . The voltage sensors may be resistive networks and include isolation elements to separate high-voltage levels from the low-voltage system. In addition, the power electronics module  126  may include associated circuitry for scaling and filtering the signals from the current sensors  204  and the voltage sensors. 
     Under normal operating conditions, the power electronics controller  200  controls operation of the electric machine  114 . For example, in response to torque and/or speed setpoints, the power electronics controller  200  may operate the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  to control the torque and speed of the electric machine  114  to achieve the setpoints. The torque and/or speed setpoints may be processed to generate a desired switching pattern for the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220 . The control terminals of the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  may be driven with Pulse Width Modulated (PWM) signals to control the torque and speed of the electric machine  114 . The power electronics controller  200  may implement various well-known control strategies to control the electric machine  114  using the switching devices such as vector control and/or six-step control. During normal operating conditions, the switching devices  210 ,  212 ,  214 ,  216 ,  218 ,  220  are actively controlled to achieve a desired current through each phase of the electric machine  114 . 
     The power electronics module  126  may further include one or more capacitors connected across the high-voltage bus  125 . For instance, a DC bus capacitor  260  (DC link capacitor) may be connected across the high-voltage bus  125  to maintain the voltage drop between the positive and negative terminals of the high-voltage bus  125 . The DC bus capacitor  260  may be further configured to filter ripple currents generated at battery  124  and stabilize the voltage across the high-voltage bus  125 . Although the DC bus capacitor  260  is illustrated as a single capacitor in  FIG.  2   , it is noted that the present disclosure is not limited thereto and the DC bus capacitor  260  may include a plurality of capacitors under various configurations. When the power electronics module  126  is in operation, the DC bus capacitor  260  is charged such that the voltage across the positive and negative terminals of the high-voltage bus  125  may be maintained. When the vehicle is parked and switched off, the DC bus capacitor  260  may be discharged to discharge the high-voltage bus  125 . The power electronics controller  200  may apply a discharge current only having a d-axis component to create loss in electric machine windings which in turn discharges energy stored in the DC bus capacitor  260  when the vehicle is parked. The discharge current is preferably applied only to the d-axis of the electric machine  114  without any q-axis component such that the DC bus capacitor  260  may be discharged without causing any rotor torque or rotation of the electric machine  114 . 
     A resolver offset is characterized once the electric machine is assembled and this resolver offset is used to adjust the rotor position reading in the power electronics controller  200  so that current angle can be properly aligned to the desired rotor position, relative to a magnetic circuit. In other words, the power electronics controller  200  needs an accurate resolver offset value to precisely apply the discharge current to the d-axis only without incurring any q-axis component. However, there may be a certain amount of error in the resolver offset in each electric machine affecting the position reading of the rotor position by the power electronics controller  200 . In case that a resolver offset error occurs, a q-axis current may be produced which in turn produces torque on the electric machine  114 . Torque or motion is undesirable when the vehicle is parked. The present disclosure proposes systems and methods for measuring and correcting the resolver offset error. 
     Referring to  FIG.  3   , an example flow diagram for a process  300  for measuring and correcting resolver offset is illustrated. With continuing reference to  FIGS.  1  to  2   , the process  300  may be implemented via the vehicle  112  by the power electronics controller  200  in one example. In an alternative example, the process  300  may be implemented via a standalone device attached to the vehicle  112  under essentially the same concept. For simplicity, the following description will be made with reference to the power electronics controller  200  of the vehicle  112 . At operation  302 , the power electronics controller  200  detects whether a resolver offset testing condition is met. The resolver offset condition may include various situations. For instance, the power electronics controller  200  may be configured to perform the resolver offset measurement process  300  each time the vehicle is keyed-off, and/or put in park. Alternatively, the process  300  may be also performed on a spinning electric machine  114 . Responsive to detecting the measurement condition is met, the process proceeds to operation  304  and the power electronics controller  200  injects a current only having a d-axis component based on the current rotor position measured by the feedback device  202 . The power electronics controller  200  further measures a speed and/or torque output (motion output) from the electric machine  114 . If the resultant speed/torque output from the electric machine in response to the d-axis current injection is below a predefined threshold indicating the resolver offset error is within a tolerable amount, the process returns to operation  302 . Otherwise, if the speed/torque output is above the threshold, the process proceeds to operation  308  and the power electronics controller  200  adjusts the resolver offset using the speed/torque output as measured and the magnitude of injection current. There are a few ways to adjust the resolver offset based on the output from the electric machine  114 . As an example, a look-up table (not shown) indicative of a corresponding relationship between the injection current magnitude, the output recorded, and the count adjustment for the resolver offset may be provided to the power electronics controller  200 . Alternatively, the power electronics controller  200  may use a predefined algorithm to determine the adjustment amount using magnitude of the injection current and the output data from the electric machine  114  as input presented in the following equation as an example: 
       Adjust_count= f (Id_magnitude,Motor_output) 
     At operation  310 , the power electronics controller  200  re-injects the d-axis-only current based on the resolver offset as adjusted and measures the speed/torque output. Here, since the vehicle is still off or parked, the motor position should still be the same. If the latest measured speed/torque output reduces to within the threshold indicative of the adjusted resolver offset having a tolerable amount of error, the process proceeds to operation  314  and the power electronics controller  200  records the current resolver offset as adjusted. Otherwise, if the speed/torque output is still beyond the threshold, the process proceeds to operation  316  to verify if the newly measured speed/torque output is less or greater than the previously measured speed/torque output. If the magnitude of the newly measured output is less than the magnitude of the previously measured output, indicative of an accuracy improvement of the resolver offset after the adjustment, the process returns to operation  308  to repeat the adjustment process until the output from the electric machine  114  is within the threshold. Here, the power electronics controller  200  may be further configured to adjust the magnitude of the d-axis injection current at operation  318 , For instance, the power electronics controller  200  may increase the magnitude of the injection current to compensate for the reduced output magnitude such that the next speed/torque output may be more observable. To compensate for the increased d-axis current magnitude, the power electronics controller  200  may be further configured to adjust (e.g., increase) the threshold used at operation  312  accordingly. The power electronics controller  200  may be further configured to normalize the value of the resultant speed/torque outputs from the electric machine before the comparison at operation  316 . Otherwise, if the newly measured output is greater than the previously measured output, resolver offset as adjusted is less accurate than the pre-adjusted status, the process proceeds to operation  320  and the power electronics controller  200  flags an error and outputs the error to a vehicle user. For instance, a message may be output to the vehicle user via a human-machine interface to ask the user to schedule a repair. 
     The operations of the process  300  may be applied to various situations. Referring to  FIG.  4   , an example waveform diagram of an electric machine speed output in response to a d-axis current injection is illustrated. Responsive to detecting a resolver offset measurement condition being met, the power electronics controller  200  may inject a d-axis current having a magnitude of 250 A into the electric machine  114 . In the present example, the d-axis current is injected momentarily (i.e., a pulse current). A waveform  402  represents a resultant motor speed output in response to the  250 A d-axis current. As illustrated in  FIG.  4   , the power electronics controller  200  may record a resultant motor speed output having a peak value of more than +5 rad/s in a forward direction which is above a predefined threshold (e.g., +/−1 rad/s). The power electronics controller  200  may determine an adjustment amount using the magnitude of the motor speed output. In the present example, the +5 rad/s speed responsive to 250 A current injection may correspond to 40 counts resolver offset adjustment in the positive direction (e.g., 496 counts per revolution). Having the adjustment amount and direction determined, the power electronics controller  200  may apply the adjustment and re-inject the d-axis current pulse accordingly. As illustrated by waveform  404 , the resultant motor speed output after the adjustment has a significantly less magnitude of −1.2 rad/s in the reverse direction, which is still beyond the threshold. The power electronics controller  200  may continue to adjust the resolver offset by 7 counts in the negative direction and perform the current injection again until the resultant speed output is within the threshold as represented by a waveform  406  having a maximum magnitude of +0.5 rad/s in the forward direction. 
     Referring to  FIGS.  5 A and  5 B , waveform diagrams of electric machine torque output in response to a d-axis current injection are illustrated. In the present example, a persistent  100 A d-axis current is injected into a spinning electric machine  114  and the resultant torque output responsive to the current injection may be measured by the power electronics controller  200 . It is noted that although the present example is described with reference to a rotating electric machine, the same concept may also be applied to a stationary electric machine. As illustrated with reference to  FIG.  5 A , no current injection is performed between to until ti when the  100 A d-axis current is injected into the electric machine  114 . Before the current injection, a motor drag torque of approximately −13 Nm may be recorded by the power electronics controller  200 . After the current injection, the torque output of the electric machine  114  drops to −35 Nm and is recorded. In other words, the power electronics controller  200  measured a −22 Nm torque as a result of the  100 A d-axis current injection, beyond the torque threshold (e.g., +/−5 Nm). The power electronics controller  200  may adjust the resolver offset by 29 counts in the negative direction corresponding to the  100 A d-axis current injection and −22 Nm torque output, and then reperform the measurement. As illustrated by a waveform  504  in  FIG.  5 B , the torque output remains essentially unchanged at around −13 Nm after the torque injection at ti as a result of the resolver offset adjustment. 
     The processes, methods, or algorithms disclosed herein can be deliverable to/implemented by a processing device, controller, or computer, which can include any existing programmable electronic control unit or dedicated electronic control unit. Similarly, the processes, methods, or algorithms can be stored as data and instructions executable by a controller or computer in many forms including, but not limited to, information permanently stored on non-writable storage media such as Read Only Memory (ROM) devices and information alterably stored on writeable storage media such as floppy disks, magnetic tapes, Compact Discs (CDs), Random Access Memory (RAM) devices, and other magnetic and optical media. The processes, methods, or algorithms can also be implemented in a software executable object. Alternatively, the processes, methods, or algorithms can be embodied in whole or in part using suitable hardware components, such as Application Specific Integrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs), state machines, controllers or other hardware components or devices, or a combination of hardware, software and firmware components. 
     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 can be made without departing from the spirit and scope of the disclosure. 
     As previously described, the features of various embodiments can be combined to form further embodiments 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 can 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 strength, durability, life cycle, 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 can be desirable for particular applications.