Patent Abstract:
Embodiments of the invention relate to a flight control system for controlling an aircraft in flight having a backup control system integrated into an active control stick. The actuated control stick may include a processing unit that includes independent and separate hardware and/or software dedicated to the primary control system and the backup control system. For the primary control system, the processing unit may receive a sensed primary control stick signal and communicate with a primary processor, which may be configured to generate a primary control signal. For the backup control system, the processing unit may receive a sensed backup control stick signal and generate a backup control signal. The processing unit may also generate tactile signal for use by the actuated control stick to adjust the feel of a pilot&#39;s control stick.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/654,474, filed Jan. 17, 2007, U.S. Pat. No. 7,878,461, issued on Feb. 1, 2011 and entitled “System and Method for an Integrated Backup Control System,” which claims priority to U.S. Provisional Patent Application 60/759,029, filed Jan. 17, 2006, and entitled, “Integrated Control Stick and Backup Controller” which is assigned to the assignee of the present invention and is hereby incorporated by reference in its entirety. This application is related to co-pending U.S. patent application Ser. No. 11/654,890 filed on Jan. 17, 2007, entitled “Apparatus and Method for a Backup Control System for a Distributed Flight Control System,” which is also assigned to the assignee of the present invention and is hereby incorporated by reference in its entirety. The complete disclosure of each of the above-identified applications is hereby fully incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     Embodiments of the invention relate generally to aircraft flight control systems, and, more specifically, to an integrated backup control system. 
     BACKGROUND OF THE INVENTION 
     With the rapid developments in aircraft technology, ever-increasing flight envelopes, and overall performance, the flight control systems implemented in modern aircraft have become extremely complex. Advanced flight control systems have been developed to address various aircraft characteristics such as flight performance, fuel efficiency, safety, etc. A fly-by-wire flight control system on modern aircraft typically includes a complex set of components including pilot sensors and controls, electronic processor, electronic wiring or data buses, actuators, and control surfaces. 
     In addition to primary control systems and control components, advanced aircraft often require a certain degree of redundancy in the control systems for safety requirements. The redundancy or backup system of a primary control system typically increases as the criticality of a control functions increases. Even with a carefully designed primary flight control system, it may be difficult to completely prevent so-called common mode failures within a control system, where an error or generic fault propagates from the primary control system to the redundant or backup components. A common mode failure also includes a generic fault that impacts all the identical redundant system elements simultaneously. As a consequence, redundant or backup control system may be configured as fully redundant and dissimilar, which unfortunately increases the part count or line-replaceable unit (LRU) count, cost, and weight for an aircraft. This problem can be especially difficult when the control system utilized a distributed fly-by-wire control system, where the actuators on the aircraft include their own servo loop closure electronics at or near the actuator. 
     A backup control system may vary between a completely redundant backup control system, duplicating the components and the performance of the primary control system, and a scaled down or minimum flight control system, reducing performance but saving on weight and cost. Because fully redundant backup control systems are expensive and often excessive, backup systems may be configured as simple as possible, making them robust and reliable while reducing cost and weight. Further, in order to prevent common mode failures, a backup system may be configured as independent and dissimilar, employing separate processors and flight computers for use in the event a failure in the primary control system. 
     On some advanced control systems for military aircraft, an active control stick in the cockpit may be used to actively shape the feel of the control stick by applying force or resistance on the control stick. The “active feel” of the active control stick or computer actuated control stick may be based on pilot inputs, aircraft configuration and flight conditions and may provide a pilot or copilot with improved situational awareness. For example, the improved situational awareness may include better coordination between pilots by electrically coupling the control sticks on each side of the cockpit, similar to the traditional cable driven coupling between control sticks. In addition to the pilot-to-pilot coupling, an active control stick can be made to follow the Autopilot commands so that the stick moves according to the Autopilot command inputs, giving the pilots better awareness of aircraft. 
     Other advanced features of an active control stick may include soft stops within the feel gradient of an active stick, which may be used to indicate various envelop and performance limits. For example, when the aircraft is approaching a stall condition, an active stick could incorporate a soft stop in the feel gradient to give a pilot a cue that he is approaching some predetermined margin (e.g. 15% from stall). Equivalently, a soft stop in the feel gradient could indicate an aircraft load factor limitation or attitude angle limitation. The pilot may then have the option to override such limits if he or she deems that appropriate. Yet another example of an advanced feature of an active control stick could be a variable amplitude and/or frequency stick shaker, which could be implemented so that the amplitude of the shaker function increases as the aircraft gets closer and closer to the stall angle of attack, for example. 
     To accomplish these active control stick functions, extensive processing may be required by stick control electronics to perform flight calculations and compute specific force loop functions, which dictate the amount and direction of force or resistance to apply to the control stick in a given flight condition. Although these calculations may be performed by the primary flight control electronics, such an arrangement may consume significant computing power needed for the primary control system. As such, typical active control sticks include independent and dedicated electronics and processors capable of performing the necessary computations for the active stick control, leaving the primary flight control electronics free to perform other flight critical functions. 
     SUMMARY OF THE INVENTION 
     Separate processors and flight computers may include significant computing power that may remain unused or underutilized during normal operations. In some cases, other processors or flight electronics may go underutilized in the typical design of an advanced control system. The flight electronics used to drive an active flight control stick may also have go underutilized under normal operating conditions or have excess processing capacity available. 
     In accordance with embodiments of the invention, a distributed backup control system may be integrated with the drive electronics or processing unit of an active control stick. One embodiment of the invention may include a flight control system for controlling an aircraft in flight having a first actuated control stick and a second actuated control stick. Each actuated control stick may include a primary sensor, a backup sensor, and a processing unit having at least one processor configured to generate a tactile signal for the actuated control stick. Each processing unit may includes a set of primary electronics may be coupled to a primary sensor and a set of backup electronics coupled to a backup sensor. The flight control system may also include a primary processor coupled to the sets of primary electronics and may be located external to the processing units. The primary processor may generate a primary control signal for use by aircraft actuators. The sets of backup electronics may be configured to generate backup control signals for use by the aircraft actuators. In the event that the actuators determine that the primary control signal is invalid, the aircraft actuators may be configured to use the backup control signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a flight control system in accordance with one embodiment of the present invention; 
         FIG. 2  schematically illustrates another flight control system in accordance with another example of an embodiment of the present invention; 
         FIG. 3  schematically illustrates a active control stick with electronics in accordance with one embodiment of the present invention; 
         FIG. 4  schematically illustrates an augmented backup control system in accordance with another embodiment of the present invention; and 
         FIG. 5  schematically illustrates another augmented backup control system in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present disclosure will now be described more fully with reference to the Figures in which various embodiments of the present invention are shown. The subject matter of this disclosure may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. 
     In accordance with one embodiment of the present invention, the processors and computing capability of the active control stick may be integrated into the communications for the control systems for the aircraft such that the active control stick may also function as a backup control system processor. The electronics in the active control stick, also referred to as a smart cockpit controller, may be utilized as a backup controller or an integrated sensor data processor. As understood by those of skill in the art, the primary control system may still be configured to meet all the safety requirements in terms of redundancy and monitoring capabilities. Likewise, it should be understood that the active control stick electronics may be configured to meet the same safety requirements as the primary control system or some other level of redundancy. When including an active control stick on an aircraft, embodiments of the invention may take advantage of the computing resources of the active control stick electronics without adding another system to act as a backup control system. A backup control system that is integrated into the active control stick electronics may avoid replication of every element of the primary flight control system while taking advantage of the signal conditioning and processing power of the active control stick electronics unit. Additionally, such an arrangement may be used to configure the backup control system with dissimilar and independent processing and communication features when compared to the primary control system. Although the active control stick may be used by the backup control system, it is contemplated that the active feel of the stick may be unnecessary in the event that the primary control system fails. 
     In accordance with embodiments of the invention, the control stick electronics unit or processing unit may be divided into two separate and independent applications. For example, the control stick processing unit may include a primary partition, for use with the primary control system, and a backup partition, for use with the backup control system. As used herein, the term partition is intended to include physically separate and independent hardware and/or separate and independent software that may be fire-walled. In other words, an active control stick processing unit may include primary and backup applications, that may be independent and separate based on their hardware and/or software. The primary and backup partitions may also represent a sets of electronics in the active control stick processing unit that may be separate and independent based on their hardware and/or software. 
     One embodiment of the present invention may include combining a smart cockpit controller (e.g. control stick with feedback control capability) and a backup processor into one unit. In another embodiment of the present invention, the smart cockpit controller may include a primary partition, which may merely act as communication concentrator and a voter of different digital transmissions, and a backup partition, which includes processing capabilities for the active control stick and the backup control system. In another embodiment of the present invention, the smart cockpit controller may provide a digital or analog signal directly to a separated and stand-alone primary controller unit, such as a primary flight control computer (“FCC”) and the smart cockpit controller may include the backup controller as a backup control system for the FCC. 
     The backup controller may be implemented as a part of the computing functions of the active control stick, such as the pilot force feedback control processor. The backup controller functions may be implemented using advanced electronics and processing or may be implemented using only relatively simple electronic hardware. 
     Some aircraft include a relaxed static stability or include a particular natural dynamic motion of the aircraft which requires active damping (such as Dutch-roll motion via the yaw damper). In such cases, the backup or backup control system may require certain augmentation signals from sensors (e.g. aircraft angular rates) in order to effectively control the aircraft using the backup control system. In one embodiment of the present invention, the smart cockpit controller and its backup controller partition, in order to optimize sensor arrangement at the aircraft level, may utilize augmentation signals from aircraft sensors typically designated for other aircraft functions. 
     In aircraft control systems requiring signal augmentation, embodiments of the invention may integrate backup sensors, such as micro electronic mechanical systems (“MEMS”) technology or other sensor technologies known to those of skill in the art, into the system architecture by integrating the sensors into the smart cockpit controller, to be used by the active control stick itself, the backup control system, and maybe additional aircraft functions external to the smart cockpit controller. Alternatively, it should be understood that dedicated sensors may be used as stand-alone units. Further, sensors may be independent of the smart cockpit controller but provide signals to the multiple aircraft devices, such as the backup control system and standby display instruments, as examples. 
     Referring to  FIG. 1 , a flight control system  100  is schematically shown in accordance with one embodiment of the present invention. As shown, two completely dissimilar processing paths and transmission media provide primary control signals and backup control signals to an aircraft actuator. In some embodiments of the invention, the actuator may include a smart actuator having a remote electronics unit (“REU”) that may be configured to determine if the primary control signal is valid and use the primary control signal over the backup control signal for actuation of the actuator. 
     In  FIG. 1 , redundant sensors  10  and  20  may be configured to receive control inputs from a pilot or copilot, as discussed above. The primary control system includes the sensor  10  and the primary controller  14  connected by a transmission media  12 . The primary control system also includes a transmission media  16 , which connects the primary controller  14  with the smart actuator  30 . Although a smart actuator is shown in the figures, it should be understood that alternative actuator control arrangements may be implemented without deviating from the scope and spirit of the present invention. For example, a centralized Fly-By-Wire control system using Actuator Control Electronics (ACE) units (not shown in the figures), which typically receive their augmented commands from the primary flight control computers could also receive commands from a backup control system or controller integrated with an active control stick. 
     A backup control system is shown in  FIG. 1  including the sensor  20  and the backup controller  24 , which are connected by a transmission media  22 . The backup control system also includes a transmission media  26 , which connects the backup controller with the smart actuator  30 . It should be noted that the primary and backup control systems may be configured independent and dissimilar, as shown in  FIG. 1 . However, it would be apparent to one of ordinary skill in the art that other configurations of the primary and backup control systems could be implemented with the present invention. 
     It should be understood that both primary and backup control systems may include sensors, associated with each input control, such as a rudder pedals or control stick, for example. Additionally, the control systems may receive inputs from the many different types of sensors used in flight control system, including sensing multiple axes on a given control instrument, such as sensing for pitch, roll, and perhaps yaw if necessary on a control stick. Although only one sensor is schematically shown in the figures for simplicity, it should be understood that the and backup control systems may be configured to receive many input signals from sensors, controls, and other devices. 
       FIG. 2  schematically shows an example of a flight control system  200  in accordance with an embodiment of the present invention.  FIG. 2  illustrates a centralized primary control system with redundant primary processors  101 , often called flight control computers (“FCC”). The primary processors  101  may receive inputs or sensor signals from the pilot smart cockpit controller or active control stick  110  and the copilot smart cockpit controller  111 . 
     The pilot active control stick  110  may include a control stick  114 , a primary sensor  140 , a backup sensor  142 , a primary partition  130  for the active stick control functions, and a backup partition  120  for the backup control function. The primary partition  130  receives input signals from the control stick  114  via the primary sensor  140 . The active control stick  110  may include additional sensors, the number of which may be a function of the overall aircraft level system redundancy requirements. For example, the primary sensor  130  may represent multiple redundant physical sensor elements, such as linear-variable-displacement transducers (LVDT) or rotary-variable-displacement-transducers (RVDT) or other type of sensors. 
     Likewise, the backup partition  120  for the backup control function receives input signals from the control stick  114  via the backup sensor  142 . Again, the sensor  142  may represent a single sensor or multiple sensors depending on the overall backup control system architecture for a given aircraft. 
     The copilot active control stick  111  may include a control stick  115 , a primary sensor  141 , a backup sensor  143 , a primary partition  131  for the active stick control functions, and a backup partition  121  for the backup control function. The primary partition  131  receives input signals from the control stick  115  via the primary sensor  141 . Likewise, the backup partition  121  receives input signals from the control stick  115  and the backup sensor  143 . 
     The primary partitions  130  and  131  of the smart cockpit controllers  110  and  111  may be simply configured to pass the primary sensor signals in an analog format to the primary processors  101  for processing and signal output by the primary flight controller  101 . Alternatively, primary partition  130  could process the analog signals form the sensor  140 . For example, the primary partition  130  may validate and vote on the redundant primary sensor signals from the sensors  140  and pass a validated pilot control position signal to the primary control system processors  101  in a digital format. The primary controller  101  may take the pilot control inputs and process the inputs in accordance with the aircraft level control laws. For example, the pilot input may include a control surface position for an aileron or other control surface. The processing of the pilots surface position command may include data from various other type of sensors in the aircraft, such as air data and inertial reference data. The primary partition  130  may also receive redundant control signals  104  from the primary processors  101 . The signals  104  may include parameters for the basic stick force gradient, any possible soft stops or activation command of a pilot awareness function, such as a stick shaker, and may be used by the primary partition  130  to adjust the feel characteristics of the control stick for the pilot. The primary partition  131  may function in the same way to provide signals to the primary processors  101  and adjust the feel of the control stick  115  using the signals  104 . 
     It should be noted that multiple processors or processing units  101  may be used as the primary flight control computers as shown in  FIG. 2 . As understood by those of skill in the art, these multiple processors may be packaged in individual enclosures, often referred to as flight control computers (“FCCs”). The processors may also be combined together in one or more enclosures, in which the enclosures are often called control channels. Regardless, each processing element may be divided into a self-checking pair of processors, called a command-monitor type of architecture. Equivalently, a triplex architecture in the flight control computers, in which three processors compute their own commands which are then voted for a mid-value or average, may be employed for the primary control system. The primary control system may include various levels of redundancy and self-monitoring as understood by those of skill in the art. 
     Alternatively, the backup control system may be constructed as a single string design, where there is only a series of signal and processing paths without any parallel monitoring within the backup system itself. Although such a backup control system arrangement would not include any self-monitoring, the backup control system may be monitored by the primary control system by sending the backup control signal, as received by a smart actuator, back to the primary control system. The primary control system may then compare the backup control signal to a backup controller model within the primary control system as discussed in related co-pending U.S. patent application filed on Jan. 17, 2006, entitled “Apparatus and Method for a Backup Control System for a Distributed Flight Control System.” Any discrepancy of such comparison can be announced to the pilots for them to take the appropriate action. 
     The backup control system shown in  FIG. 2  may effectively act as a “hot spare” for the primary control system. The backup system may be implemented through the active control sticks  110  and  111  and the backup partitions  120  and  121 . In the event that the primary control system experiences a generic fault, the backup partitions  120  and  121  shown in  FIG. 2  may be configured to perform the necessary functions to drive the “active feel” of the control sticks and to process the inputs from the control sticks  114  and  115  for the backup control system. The backup system may also be employed in the event that the aircraft experiences a fault, such as a total loss of electrical power to the primary control system. It should be understood that the backup control system may have an independent power source, allowing the backup control system to survive some aircraft level faults. It is also contemplated that the control stick may revert to passive devices in the event of a generic fault in the primary control system in order to reserve all processing power in the control stick electronics for the backup control system. 
     In order to optimize aircraft control, the backup control signals from each backup partitions may be fed to the cross-side backup partition via communication links  126  and  127  for processing. For example, the backup control signals may be scaled and summed together and the sum of the two signals may be limited to the maximum value allowed by a single controller. In this way, the input from both pilot controllers will be included in the aircraft level command computation. The backup partition  120 , may be configured to transmit the backup control signals to the left side actuator channels. Likewise, the backup partition  121  may transmit the backup control signals to the right side actuator channels. As understood by those of skill in the art, the terms left and right should not be limited to channels physically located on the left and right side of an aircraft, but rather the terms left and right may indicate the source of command data for a given actuator channel. 
     The health of the overall backup control paths may be monitored on a continuous basis during the normal operation, so that its availability and even accuracy can be verified even when not in use. If all of the primary control command sources become unavailable or in the event of a general fault in the primary control system renders it unavailable, the control system  200  may be configured to switch to the backup control signals generated by the backup control system. If smart actuators are used, the smart actuators may be configured to automatically switch between the primary and backup control signals if the primary control signal is determined to be invalid or absent. 
     It should be understood that the processor units  101  shown in  FIG. 2  may be configured to perform other aircraft functions, such as outputting signals for cockpit display, such as crew alerting system (CAS) or maintenance signals. 
     In an alternative embodiment of the invention, the primary partition  130  and the backup partition  120  of the control stick  110  and the primary partition  131  and  121  of the control stick  111  may include software partitions within the active stick controller software instead of being physically separated partitions. The partitions may also be kept strictly physically isolated to minimize the possibility of one partition affecting the operation of another partition. Additionally, the control stick  110  and the control stick  111 , including the primary partitions and the backup partitions, may be configured as a dissimilar design compared to the primary controller. For example, the dissimilarity may be based on hardware and/or software. The dissimilarity between the primary controllers and the control sticks  110  and  111  may also include using different signal processing algorithms and different aircraft level control laws between the primary and the backup control systems. 
       FIG. 3  schematically illustrates an active control stick  400  in accordance with one embodiment of the invention. The active control stick  400 , as shown in  FIG. 3 , includes a control stick  410 , and primary and backup sensors  412  and  414 . The active control stick  400  is also shown with a primary partition  420  and a backup partition  430 . As discussed with reference to  FIG. 2 , the primary partition may form a component of the primary control system in different ways. For example the primary partition may include a voter and signal verification device or may function as a simple primary transmission path for transmitting an analog signal from the control stick sensors to the FCC. 
     The backup partition  430  may be configured to include active control stick electronics and backup control system electronics. As shown in  FIG. 3 , the backup partition  430  includes a demodulator and analog to digital converter  432 , a processor  434 , and force feedback electronics  436 . The backup partition may also include a data bus receiver and transmitter device  438  for communicating with other backup control system components and with the FCC for other information, such as force gradients and soft stops, etc. The backup control system may utilize the existing active control stick electronics  432 ,  434 ,  436 , and  438  as a backup system solution for the primary control system. It should be noted the above description may be further simplified if the active stick function, for example, is not required in the backup control mode of the flight control system. The processor  434  may alternatively be implemented by using analog electronics and the data bus interface  438  may also be implemented by using analog signal drivers. 
     As mentioned above, in aircraft designed with a relaxed static stability or in aircraft with a particular natural dynamic motion of the aircraft that requires damping (such as Dutch-roll motion via the yaw damper) the backup or backup control system requires certain augmentation signals from backup sensors (e.g. aircraft angular rates) in order to properly control the aircraft.  FIG. 4  illustrates one embodiment of an augmented backup control system  500 . 
     As shown in  FIG. 4 , the backup control system  500  includes the backup controllers  522  and  542  positioned within the active control sticks  510  and  530 . The backup controller  522  receives input signals from the control stick  512  and the backup sensor  514 . The backup controller  542  receives input signals from the control stick  532  and the backup sensor  534 . In accordance with this embodiment, the MEMS or other type of backup rate or acceleration sensors  520  provides augmentation signals to the backup controller  522  and the MEMS sensors  540  provides augmentation signals to the backup controller  542 . 
     The augmentation signals from the MEMS sensors  520  and  540 , which may be configured to provide aircraft attitude, angular rate, and linear acceleration data, may also be utilized by the standby instrument displays or as backup inputs into the primary displays. Such an arrangement may allow a part number reduction at the aircraft level, which, in turn, may save weight and cost of the aircraft. The primary partitions  518  and  538  may receive signals from the control sticks  512  and  532  and the primary sensors  516  and  536 . The primary control system may be implemented as discussed above or as known by those of skill in the art. The combination of sensors within the active control stick may serve to optimize the total amount of backup sensors at the aircraft level. Further, the MEMS sensors may be configured, as shown in  FIG. 4 , to serve multiple aircraft functions, such as input signals for standby displays and augmentation signals for a backup control system, while optimizing the utilization of MEMS sensors. 
     Upon computing a backup control signal, the backup controller  522  of the active control stick  510  may be configured to provide the backup control signals to the aircraft actuators. As shown in  FIG. 4 , the backup controller  522  provides backup control signals to left side smart actuators  550 . Likewise, the backup controller  542  provides backup control signals to right side smart actuators  560 . The smart actuators may include remote electronics units as shown in the figure. As would be apparent to those of skill in the art, the backup controllers  522  and  542  may be cross-linked or otherwise configured. Furthermore, the sensors  520  and  540  may be installed as individual and separate units in the aircraft. 
       FIG. 5  illustrates an alternative embodiment of an augmented backup control system where the standby instruments  670  and  680 , as with the sensors  520  and  540  shown in  FIG. 4 , may provide the required augmentation sensor data to the backup controller in aircraft designed with a relaxed static stability or in aircraft with a particular natural dynamic motion. As shown in  FIG. 5 , the backup control system  600  includes the backup controllers  620  and  640  positioned within the active control sticks  610  and  630 . The backup controller  620  receives input signals from the control stick  612  and the backup sensor  614 . The backup controller  640  receives input signals from the control stick  632  and the backup sensor  634 . 
     In accordance with the embodiment of the invention shown in  FIG. 5 , the sensor signals computed by the standby instruments may be forwarded to the backup controller. As shown in  FIG. 5 , the standby instrument  670  is configured to provide both backup controllers  620  and  640  with augmentation signals. The standby instrument  680  is configured to provide both the backup controllers  620  and  640  with augmentation signals. 
     As such, the flight data provided to the standby instruments  670  and  680  may be utilized for multiple aircraft functions, displaying the flight data for the standby displays and providing augmentation signals to the backup controllers. It should also be understood that the flight data from backup sensors as shown in  FIGS. 4 and 5  may be used by the active control stick in order to process any force feed back computations for the control stick. 
     The primary partitions  618  and  638  may receive signals from the control sticks  612  and  632  and the primary sensors  616  and  636 . The primary control system may be implemented as discussed above or, alternatively, as known by those of skill in the art. The combination of sensors within the active control stick may serve to optimize the total amount of sensors at the aircraft level. Further, the MEMS or other type of backup sensors may be configured, as shown in  FIG. 4 , to serve multiple aircraft functions, such as input signals for standby displays and augmentation signals for a backup control system, while optimizing the utilization of MEMS sensors. 
     Upon computing a backup control signal, the backup controller  620  of the active control stick  610  may be configured to provide the backup control signals to the aircraft actuators. As shown in  FIG. 5 , the backup controller  620  provides backup control signals to left side smart actuators  650 . Likewise, the backup controller  640  provides backup control signals to right side smart actuators  660 . The smart actuators may include remote electronics units as shown in the figure. Although smart actuators are shown in  FIGS. 4 and 5 , other actuators known to those of skill in the art may be used. Also, as would be apparent to those of skill in the art, the backup controllers  620  and  640  may cross-linked or otherwise configured without deviating from the scope and spirit of the present invention. 
     The foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations are possible in view of the above teachings. While the embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to best utilize the invention, various embodiments with various modifications as are suited to the particular use are also possible. The scope of the invention is to be defined only by the claims appended hereto, and by their equivalents.

Technology Classification (CPC): 1