Abstract:
A digitally controlled current to pressure converter (CPC) and method of controlling same is provided. The method of controlling includes the step of periodically imparting symmetrically-opposed movement of a control valve of the CPC to loosen and flush accumulated silt therefrom. More particularly, the method may include the step of periodically introducing a small-amplitude symmetrically-opposed impulse to a controller that actuates a hydraulic control shaft of a three-way rotary valve. Also provided is a method of preventing malfunction due to faulty input or feedback signals received by the CPC, and a method of detecting the health status of multiple CPCs when used in a redundant configuration.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     This patent application is a Divisional of co-pending U.S. patent application Ser. No. 13/489,832, filed Jun. 6, 2012, which is a Continuation of U.S. patent application Ser. No. 12/024,148, filed Feb. 1, 2008, now U.S. Pat. No. 8,215,329, the entire teachings and disclosures of which are incorporated herein by reference thereto. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to positioning control systems for steam and fuel valves and their associated servo mechanisms, and more particularly to current to pressure converters (CPC) that convert an analog current control signal to hydraulic pressure for use therewith. 
     BACKGROUND OF THE INVENTION 
     Many control components in plants, buildings, and other manufacturing facilities utilize hydraulic pressure to position the actuators, control valves, or operating surfaces of these components. Such components include steam control valves, fuel valves, dampers, vanes, etc. One common means of positioning the actuators is to provide a linearly increasing variable hydraulic pressure that acts upon the piston of a linear hydraulic actuator or vane of a rotary cylinder. The opposing force required to counterbalance this variable pressure and thus create proportionality can be in the form of an opposing spring, or hydraulic pressure. 
     While purely hydraulic valving and control systems have been utilized to effectuate the positioning of these control components, modern electronic controls have increased the functionality and flexibility of the system control. Such component, system, and plant controllers typically utilize PLC- or DCS based computing systems to monitor and control the various components within the system. The use of such controllers, therefore, necessitates the use of an interface component that is capable of taking the control signal outputs from such controllers and converting those electronic control signals into hydraulic control signals that can effectuate the positioning and control of the hydraulic actuated components. One such interface control device is known as a current to pressure converter (CPC). 
     A typical CPC is configured to receive an analog 4-20 mA control signal from a system or plant controller. This 4-20 mA control signal is then proportionally converted into a hydraulic output pressure by the CPC. As such, the CPC may be thought of as a electrohydraulic, pressure regulating valve. Such CPCs typically include an internal 3-way valve, actuator, pressure sensor or pressure feedback mechanism, and on-board analog electronics. A cascade control loop is typically employed to achieve closed loop control of pressure. The first control loop compares the input control signal or pressure setpoint to the measured feedback. The difference is then modified by a circuit or algorithm to generate a position demand signal which is the input of the second control loop. The position demand signal is then compared to the measured position and the difference modified by a circuit or control algorithm to produce a drive signal which will open or close the actuator to match the position demand over time. The combined operation of the dual control loops in conjunction with the actuator and valve ensures that the measured feedback matches the setpoint over time. 
     The valve internal to the CPC is a three way control valve. At the center position, the control port is isolated from both the supply and drain. By moving the valve slightly above the center position, the control port is connected to the supply port resulting in an increase in pressure. By moving the valve below the center position, the control port is connected to the drain, resulting in a decrease in pressure. A return spring is provided in the assembly such that in the event of loss of power or an electric fault, the valve will move to the “minimum pressure” position which in most applications is the direction to shut down the turbine. 
     While current CPC&#39;s perform adequately in many applications, the accuracy of such control in some installations may be adversely affected by the thermal drift associated with the analog control circuits within the CPC itself. Further, CPC malfunction has been noted in some systems that do not typically change the positioning of the control component for long periods of time, or in backup CPC&#39;s in systems that utilize a primary and backup regulator to ensure system operation in case of malfunction of the primary CPC. Such malfunctions have been determined to be caused by the build-up of silt and other contaminates that have accumulated on the valve element during a long period of stagnant control. 
     In view of the above, the inventor has recognized a need for a new and improved CPC that overcomes the inaccuracies resulting from thermal drift of the analog control circuits and that ensures continued operation even after extended periods of inactivity that would otherwise result in silt build-up on the valving element leading to malfunction. Embodiments of the present invention provide such a new and improved CPC. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of the above, embodiments of the present invention provide a new and improved current to pressure converter (CPC) that overcomes one or more of the problems existing in the art. More particularly, embodiments of the present invention provide a new and improved CPC that does not suffer from inaccuracies resulting from thermal drift of the analog control components used in some CPC&#39;s. Still further, embodiments of the present invention may also eliminate or greatly reduce the likelihood of CPC malfunction in installations experiencing long periods of inactivity between repositioning of the control valve therein. 
     An embodiment to the present invention includes fully digital processing of the control loop and diagnostic signals, which beneficially reduces the thermal drift associated with the prior analog control systems used to control the CPC. An onboard pressure sensor is also incorporated in one embodiment to provide closed loop control of the output pressure. Such onboard pressure sensor offers improved linearity and accuracy over previous CPC&#39;s that utilized force feedback devices. 
     Improved reliability is provided in one embodiment by including a redundant, dynamic sealing system with an intermediate passage to the hydraulic drain circuit to ensure that the pressure drop across the outboard seal is very low, thereby minimizing the potential for leakage and improving the reliability of the CPC. One embodiment of the present invention also includes provisions for improved redundancy and fault management to ensure failsafe operation in the event of internal component failure. 
     Reliable operation is also provided in embodiments of the present invention through the inclusion of an anti-silting algorithm that will deter the accumulation of fine silting particles. Such accumulation has been problematic and a chronic problem on steam turbines which use the turbine&#39;s lube/oil for the hydraulic supply. Embodiments of this algorithm will introduce a small amplitude, symmetrically opposed, impulse on the position of the rotating valve. This impulse will rotate the valve element very slightly to loosen and flush away any silt that has accumulated on the valve element. In one embodiment, the impulse is of a very short duration, and includes opposed negative, then positive, components. In such an embodiment the result is a near net zero displacement of fluid in the output circuit controlled by the CPC. As such, there is no or only minimal detectable behavior of the output servo during the anti-silting impulse. Such small amplitude, symmetrically opposed impulse may be applied periodically, at fixed time intervals, and can be easily adjusted by the user based on the oil quality of the application. 
     In other embodiments of the present invention, the digital controller may monitor driver current levels, and may increase or decrease the interval between impulses automatically upon the detection of a variance in driver current levels that may indicate the buildup of contamination, or lack thereof, to effectuate a self tuning of the interval based upon actual need. 
     Reliable operation is also provided in embodiments of the present invention through the inclusion of redundant control inputs for either the main control setpoint or the pressure transducer used for closed loop control. Historically operation of the turbine is often adversely impacted by failure of the main controller, wiring between the controller and CPC, or the transducer used for pressure feedback. In the preferred embodiment a second input is provided which can be configured to monitor a 2 nd  controller, or receive a 2 nd  command signal via an independent wiring path from the turbine controller, or a 2 nd  pressure feedback transducer. As such, the user can configure the installation of the CPC for additional robustness to these failure modes, and the logic executed within the CPC will utilize the 2 nd  input signal to maintain operation in the event of failure. 
     In applications requiring the highest level of reliability, two CPC&#39;s are sometimes applied in a tandem arrangement. In this configuration, failure modes of the turbine control system, the control wiring between the control and the CPC, or failure of the CPC itself can largely be mitigated. In the preferred embodiment the two CPC&#39;s have a status link wired directly from one unit to the other. As such, each CPC knows the operational status of the other and should a fault occur within the CPC in control of the system, the back-up unit can resume control in an extremely short time interval without intervention from the main turbine or plant controller. This minimizes the potential for dynamic transitions which could adversely affect the speed or load of the turbine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
         FIG. 1  is a simplified system level diagram of a typical installation of an embodiment of a CPC constructed in accordance with the teachings of the present invention; 
         FIG. 2  is a cross-sectional illustration of an embodiment of a CPC constructed in accordance with the present invention; 
         FIG. 3  is a hydraulic schematic of the CPC of  FIG. 2 ; 
         FIG. 4  is a simplified CPC controller block diagram illustrating one embodiment of a controlled constructed in accordance with the teachings of the present invention; 
         FIG. 5  is simplified system level diagram of a master/slave CPC installation constructed in accordance with the teachings of the present invention; 
         FIG. 6  is a simplified block diagram illustrating the redundancy switch over logic utilized in an embodiment to the present invention for redundant control inputs; 
         FIG. 7  is a simplified block diagram illustrating the redundancy switch over logic utilized in an embodiment to the present invention for redundant transducer feedback; 
         FIG. 8  is a graphical illustration of the output pressure verses command input available from one embodiment of the present invention; 
         FIG. 9  is a graphical illustration of the symmetrical anti-silting impulse function effectuated by one embodiment to the present invention; and 
         FIG. 10  is a graphical illustration of a single symmetrical anti-silting impulse event utilized in one embodiment of the present invention. 
     
    
    
     While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, and specifically to  FIG. 1 , there is illustrated a typical turbine control system  100  to which embodiments of the present invention are particularly well suited. However, while the following description will utilize this exemplarity installation of an embodiment of a CPC  102  constructed in accordance with the teachings of the present invention, this installation is not meant to be limiting, but will aid in the understanding of the functionality and advantages provided by such a CPC  102 . Other installations and operations of embodiments of the present invention will be recognized by those skilled in the art from the following description, and application thereof are specifically reserved. 
     In such an installation as illustrated in  FIG. 1 , the CPC  102  may interface with a turbine controller  104 , such as a Model 505/505E Steam Turbine Digital Controller available from the assignee of the instant invention. Such a turbine controller  104  is typically microprocessor-based and is designed to operate steam turbines  106 , such as, e.g. single extraction and/or admission steam turbines. Embodiments of the CPC  102  may also interface with a system plant controller  108  to provide, for example, feedback information from the CPC  102  as well as fault information. 
     In operation, the CPC  102  receives command signals from the turbine controller  104  in the form of an analog control signal varying between 4 and 20 mA. The control logic within the CPC  102  processes this control command signal and either increases or decreases the hydraulic pressure to the turbine&#39;s servo system  110 . The servo system  110  is operable to vary a steam control valve  112  to vary the operating speed of the steam turbine  106 . In the system illustrated in  FIG. 1 , the steam turbine  106  is used to drive a load, such as a pump  114 , a generator (not shown), etc. As will be discussed more fully below, when the CPC  102  determines that additional hydraulic pressure is needed, it sources hydraulic fluid from tank  116  via pump  118  and filter  120  to the servo system  110 . When the CPC  102  determines that less hydraulic pressure is needed, the CPC operates to drain hydraulic fluid back to tank  116 . 
     To effectuate such operational control, the CPC  102  includes digital control mounted internal to the housing  122  on a digital electronic assembly (referred to hereinafter as a digital printed circuit board (PCB)  124 ) as may be seen in  FIG. 2 . This digital PCB  124  is protected by a PCB cover  126  and may be accessed by removing the access cover  128  which meets with the housing  122  to form a sealed enclosure. 
     The controller mounted on this digital PCB  124  controls the position of the hydraulic control shaft  130  via a rotary limited angle torque (LAT) actuator  132 . Specifically, the LAT  132  includes a permanent magnet rotor  134  that is directly coupled to the hydraulic control shaft  130 . The position of the rotor  134  is measured by a solid state integrated circuit on the digital PCB  124  which detects the direction of the sensing magnet  136  on the hydraulic control shaft  130 . The H-bridge drive of the LAT  132  is regulated by the microprocessor on the digital PCB  124  to control the position of the hydraulic control shaft precisely to maintain the pressure set point received from the turbine controller  104 . 
     The hydraulic control shaft  130  rotates within a hydraulic control bushing  138  that is ported to form a three-way rotary valve  140 . This three-way rotary valve  140  controls the hydraulic fluid flow from the supply (not shown) to the control port  142  and from the control port  142  to the drain (not shown). In a preferred embodiment, both the hydraulic control shaft  130  and the hydraulic control bushing  138  are made of stainless steel. This offers precise, reliable, and contamination-tolerant operation on typical oils used for steam turbine lubrication. 
     To provide failsafe operation in the event of component or power failure, a spiral power spring  144  operates the bottom portion of the hydraulic control shaft  130  in the lower cavity  146  of the housing  122 . Access to the spiral power spring  144  is via lower cover  148 . In the event of power failure, the spiral power spring  144  will provide sufficient rotary power to rotate the hydraulic control shaft  130  into a failsafe condition. One embodiment of this failsafe condition couples the control port  142  with the drain. 
     To protect the dry stator  150  a redundant dynamic sealing system  152  is utilized. This redundant dynamic sealing system  152  includes an intermediate passage  154  to the hydraulic drain circuit. This ensures that the pressure drop across the outboard seal  156  is very low, minimizing the potential for leakage and improving the reliability of the CPC  102 . 
     Precise hydraulic pressure control is aided by the inclusion of a pressure transducer  158  that provides the microprocessor with a precise indication of the current hydraulic pressure supplied via control port  142 . This on-board pressure transducer  148  improves the linearity and accuracy of the closed loop control of the output pressure over prior CPC&#39;s that utilized a force feedback device. 
     The simplified hydraulic schematic of  FIG. 3  provides an illustration of the operational connection of the hydraulic control circuit within the CPC  102 . As may be seen, the digital PCB  124  provides a position control signal to the LAT  132  to position the three-way rotary valve  140 . Position feedback for the closed loop control is provided by the sensing magnet  136 . As discussed briefly above, the 3-way rotary valve  140  is designed to couple the control port  142  with either the supply port  160  or the drain port  162  to control the pressure, sensed by pressure transducer  158 , to the servo system  110  in accordance with the control signal received from the turbine controller  104  (see  FIG. 1 ). 
     As may be seen from this hydraulic schematic of  FIG. 3 , when the 3-way rotary valve  140  is in the mid position, the control port  142  is coupled to neither the supply port  160  nor the drain port  162 . In this configuration, the output hydraulic pressure is held constant at control port  142 . If the pressure drops below the set point pressure, the PCB  124  will command the LAT  132  to rotate the valve such that control port  142  is connected to the supply port  160  to increase the pressure in the control port  142  (as sensed by pressure transducer  158 ). If, however the hydraulic pressure at the control port  142  is higher than desired, the control circuitry on the PCB  124  will command the LAT  132  to position the 3-way rotary valve  140  so as to couple the control port  142  to the drain port  162  to lower the pressure at the control port  142 . 
     This dynamic pressure control is controlled by a digital control algorithm  164  executed within the digital PCB  124 , such as that illustrated in simplified block diagram form in  FIG. 4 , to which reference is now made. As may be seen from the simplified block diagram, the digital controller  164  may include a control demand input  166  and, in one embodiment, a redundant control demand input  168 . This or each of these inputs are passed through signal conditioners  170 . The control logic for dealing with two controlled demand inputs will be discussed more fully below. Once this control demand input is determined, it is used in a pressure proportional integral derivative (PID) control loop  172 . As illustrated, this PID control loop  172  receives a control oil pressure feedback from pressure transducer  158  and a feedback position signal from the sensing magnet  136  (not shown). Based on this information, the digital controller  164  controls the position of the valve  140  via the LAT  132 . 
     The digital controller  164  also includes in an embodiment a service port  174  that interfaces with the CPC supervisory logic  176  via a service port communications module  178 . This service port allows, for example, field programming and diagnostics via a PC or microprocessor-based service tool. The CPC supervisory logic  176  monitors the operation of the CPC and includes outputs for a shutdown relay  180 , an alarm relay or red unit status  182 , a master slave indication  184  where such functionality is provided (see description below regarding  FIG. 5 ), and/or analog output  186  that is capable of driving, for example, a control pressure meter  188 . 
     In the embodiments of the CPC  102  of the present invention that are utilized in a master/slave environment such as that shown in  FIG. 5 , the CPC supervisory logic  176  also includes master/slave inputs  190 . These master/slave inputs  190  may be utilized with external jumpers or relays to establish an initial role for the particular CPC based on installation location. However, even in systems that utilize initial role designations, the CPC supervisory logic  176  includes logic to facilitate control transfer between CPCs upon the detection of a fault of a master or other conditions that may necessitate or make desirable such control transfer. 
     As illustrated in  FIG. 4 , the master indication output  184  includes two status lines that connect to the other CPC(s). The CPC supervisory logic  176  in each CPC  102  monitors these lines. If either CPC  102  fails, or if the signal line fails, the other CPC will take control within a prescribed period of time, i.e. 10 milliseconds. This minimizes any transition bump to the servo system  110  which can occur with prior CPC&#39;s wherein such delay between switching to a new CPC control upon the failure of the master may be 100 milliseconds or more depending upon the computing rate of the main controller. As will be recognized by those skilled in the art, such a long transition time may result in a very large control pressure transient and corresponding changes in speed or load of the controlled plant. 
     A simplified single line illustration of a system  100 ′ utilizing a slave CPC  102 A and associated slave turbine controller  104 A in addition to the master CPC  102 B and associated master turbine controller  104 B is shown in  FIG. 5 . As discussed above, control of the hydraulic pressure to the servo system  110  is provided by the master CPC  102 B until a fault is detected therewith. Upon determination that the master CPC  102 B has failed, the select valve  210  switches hydraulic pressure control to the slave CPC  102 A. In an embodiment of the present invention, the two CPCs  102 A/ 102 B have a status link  212  wired directly from the master indication circuit  184  (see  FIG. 4 ) of one CPC  102 A to the other CPC  102 B. As such, each CPC  102 A/ 102 B knows the operational status of the other CPC  102 B/ 102 A. Should a fault occur within the master CPC  102 B in control of the system  100 ′, the back-up unit or slave CPC  102 A can assume control in an extremely short time interval without intervention from the main turbine or plant controller. This minimizes the potential for dynamic transitions which could adversely affect the speed or load of the turbine  106 . 
       FIG. 6  illustrates a functional block diagram of the redundancy switch over logic  196  utilized in embodiments of the present invention that include multiple control inputs  166 ,  168 . Specifically, each control input  166 ,  168  is monitored by input diagnostics  192 ,  194  to evaluation the reasonableness of the values on each of the two control inputs  166 ,  168 . If the input diagnostics  192 ,  194  determines that one of the input signals  166 ,  168  is considered faulty, e.g., out of range, unstable, etc., it will be voted out of the chain by the redundancy switch over logic  196  and the CPC  102  will continue to operate only based on the non-faulty input. If the redundancy switch over logic  196  determines that both signals are faulty, then the CPC is commanded to a failsafe position and an appropriate alarm or other indication is provided. If both input signals are considered valid, although different, the redundancy switch over logic  196  may simply select one of the two inputs for control, may average the two inputs, take the higher, the lower, or other input logic processing to provide the operating setpoint signal for the CPC  102 . 
     Similar redundancy switch over logic  198  may be utilized along with feedback signal diagnostics  200 ,  202  to evaluate the reasonableness of multiple feedback signals  204 ,  206  in embodiments that utilize multiple feedbacks, e.g., multiple feedback transducers, position sensors, etc. This feedback redundancy switch over logic  198  is illustrated in  FIG. 7 . 
     As illustrated in  FIG. 8 , the digital controller  164  provides multiple adjustments that greatly increase the type of installations and functionality within each installation of the CPC  102  of the present invention. As shown in the output pressure versus command input scaling graphical illustration of  FIG. 8 , an adjustment can be made of the minimum pressure level and the maximum pressure level. The minimum pressure level adjustment sets the level of output pressure. Adjusting the minimum pressure level changes all points of the output pressure uniformly. That is, adjusting the minimum level sets the minimum travel of the servo system  110  (see  FIG. 1 ). The maximum pressure level adjustment sets the maximum output pressure when the output command control signal is at 20 milliamps. Increasing this level increases the slope of the line  208  and the position of the output servo system for each value of the input signal. 
     The PID control loop  172  settings may also be adjusted to tune the dynamic performance of the CPC  102 . The proportional gain may be adjusted to set the amount of gain (proportional action). In one embodiment ten percent gain is used. As will be recognized by those skilled in the art, a high gain provides a fast response time, but can cause instability. The integral gain may also be adjusted to set the stability (integration action) of the PID control loop  172 . This stability cooperates with the proportional gain setting to provide stable operation. Finally, a derivative component of the PID control loop  172  may also be adjusted to set the amplitude of the output dither. 
     As discussed above, failures of CPC&#39;s in installations that utilize redundant or backup CPC&#39;s or in systems that do not vary the hydraulic output for extended periods of time have been determined to be a result of the accumulation of fine silting particles. These failures are particularly acute in steam turbine applications such as that shown in  FIG. 1  wherein the turbine&#39;s lubricating oil is used as the hydraulic control fluid. To overcome this problem, an embodiment of the CPC  102  of the present invention include a symmetrical anti-silting impulse function as part of its control logic. Specifically, the digital controller  164  includes an algorithm which introduces a small amplitude, symmetrically opposed, impulse on the position of the three-way rotary valve  140 . This small impulse will cause a rotation of the three-way rotary valve  140  very slightly in both directions. 
     As illustrated in  FIG. 9 , these impulses may occur at fixed or periodic times. The selection of the interval may be set or varied based upon the particular installation and the amount of silt and contamination typically associated therein. These small impulses are effective in loosening and flushing away any silt that has accumulated on the valve element during the period of inactivity. The plot of  FIG. 9  shows an impulse interval set at five hours. However, this interval may be manually or automatically varied within the CPC  102 . 
     In one embodiment the automatic variation of the anti-silting impulse is based upon a detection of a deviation in the driver current levels needed to effectuate movement of the three-way rotary valve  140 . Increased driver current requirements are an indication of the build-up of contamination on the valve. When such a condition is detected, the frequency of anti-silting impulses may be increased. Similarly, if the driver current is not sensed as being at a level that might indicate contamination build-up on the valving element, the anti-silting interval may be extended so as to prolong the life of the internal bearings and seals. 
     As illustrated in  FIG. 10 , each anti-silting impulse event is of very short duration and very low amplitude. Indeed, the use of the digital controller  164  allows adjustment of various parameters controlling the anti-silting impulse, such as the amplitude, frequency, and duration thereof. With regard to the amplitude of the anti-silting impulse, typically a one percent impulse is sufficient to flush contaminants from the CPC  102 . However, this may be adjusted as required to achieve the anti-silting benefit within the parameters of the system performance. In one embodiment amplitudes up to five percent can be set either at the factory or in the field via the service tool interface discussed above. The frequency of the anti-silting impulse may also be adjusted as discussed above. While  FIG. 9  illustrated an interval of five hours, other embodiments may utilize the impulse only once per day, although other installations may require impulses generated at a frequency from, for example, one minute to three months. This interval may also be set at the factory or in the field. Finally, the duration of each anti-silting impulse may also be varied. Depending on overall system performance issues, durations from four milliseconds to one hundred milliseconds may be utilized, with a typical duration of forty milliseconds being sufficient to loosen the silt without causing undue motion of the servo mechanism  110 . However, longer or shorter durations may be set as required or desired. 
     All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.