Abstract:
A position verification apparatus comprising a movable member disposed within and movable with respect to a housing containing the movable member, a means for generating a magnetic field within the housing, a first magnetically responsive means for sensing the position of the movable member and outputting a signal indicative of the position of the movable member, a second magnetically responsive means for sensing the presence of the movable member and outputting a signal indicative of the position of the movable member and a means for comparing the signal output from the first magnetically responsive means and the signal output from the second magnetically responsive means.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to an apparatus for verifying the position of a movable member. More particularly, the present invention is directed to a control element assembly (CEA) position indicator that includes a CEA position verification apparatus. 
     BACKGROUND OF THE INVENTION 
     Nuclear power plants typically include independent shut-down and safe-operations systems that monitor plant operation and evaluate numerous safety-related parameters. In the event one or more measured parameters indicate the existence of an unsafe condition, the shut-down system designed to mitigate the effects of an anticipated transient condition and/or the safe-operation can automatically effect the appropriate remedial action. It is imperative that these safety control systems, known as plant protection systems, operate reliably, and accordingly, it is imperative that all measured and sensed parameters be valid. 
     In the context of nuclear plant protection systems, it is not uncommon to measure a multitude of parameters related to plant operation. These parameters include, for example, temperatures, pressures, flow rates, power density, neutron flux, fluid levels, etc. Other functions of the plant protection system include the status-monitoring of various components including valves, pumps, motors, control devices and generators. 
     Additionally, the plant protection system, under certain defined conditions, may initiate a reactor trip (RT), i.e., the rapid, controlled, and safe shut-down of the reactor by actuating various field systems and remote actuation devices. In the case of a pressurized light water reactor, the shut-down is often accomplished by the dropping of moderating control rods into the reactor core to cause the reactor to become sub-critical. 
     The rod assemblies are comprised of four or twelve rods joined by a spiderlike connecting flange. Each rod assembly is commonly referred to as a control element assembly (CEA). The CEAs are arranged in groups or sub-groups consisting of a minimum of four CEAs. The connecting flange couples the CEA to a control element assembly drive mechanism that controls the movement of the CEA in and out of the reactor core. The control element assembly drive mechanism normally includes a position indicator that senses location of the CEA. As the CEA controls the overall reactor power level and provides the principal means of quickly and safely shutting down the reactor, sensing and monitoring the CEA&#39;s position in a nuclear reactor is imperative. 
     There exist well known systems for sensing and monitoring CEA position. In U.S. Pat. No. 3,656,074, which is assigned to the Assignee of the present invention and incorporated herein by reference, there is described a CEA position sensing apparatus  10 . As shown in FIG. 1, the apparatus includes a CEA represented by a single control rod located within a control rod housing and a control rod drive motor; a permanent magnet physically positioned on the CEA drive shaft; and a position transmitter including a plurality of flux responsive devices and a voltage divider network. 
     As the CEA illustrated in FIG. 1 is represented as a single control rod, only one control rod housing  12  is shown extending upwardly from the top of the reactor  11 . The control rod housing  12  will typically be a nonmagnetic stainless steel tube that is approximately five inches in diameter having a one inch thick wall. The control rod drive shaft is situated within housing  12 , and the control rod itself extends into the main portion of reactor  11 . 
     A control rod drive motor  14  is mounted above a cap which seals the upper end of housing  12 . Through appropriate gearing, the control rod drive motor  14  causes the control rod drive shaft to move axially relative to housing  12 , thus adjusting the position of the CEA relative to the reactor core. The control rod drive shaft of motor  14  communicates with the interior of housing  12  through specially designed seals that are available commercially. 
     Permanent magnet  16  is mounted on the control rod drive shaft. The magnet  16  must be constructed of material capable of withstanding the highly corrosive conditions of the environment to which it will be exposed, as the magnet will be located inside a nuclear reactor. Further, the magnet  16  must be sufficiently strong so that its flux field will bridge the thick stainless steel walls of the housing  12 . 
     As described below, a portion of the position sensing apparatus  10  is mounted on the exterior of the housing  12  and is contained within a separate transmitter housing  18 . The housing  18  comprises an elongated hollow member, generally of tubular form, comprised of a nonmagnetic material which preferably has a high coefficient of thermal conductivity. The upper end of housing  18  is adapted to receive an electrical connector  20 . 
     The connector  20  provides a means for connecting the position transmitter circuitry to external circuitry in a manner which permits removal of the connector  20  from the housing  18 . The connector  20  also electrically couples the output of the position sensing apparatus  10  to a position indicator  22 . 
     FIGS.  2 ( a ) and  2 ( b ) respectively show top and side views of the position transmitter circuitry located within the housing  18  through cut-away portions of said housing. Shown within the housing  18  is a terminal strip  24  which is mounted to a reed switch position transmitter  26  including a plurality of magnetic flux responsive switches and the components of an incremental potentiometer. The flux responsive devices are shown as reed switches  28  and  28 ′, and the components of the incremental potentiometer are shown as resistors  30 . The reed switches  28 ,  28 ′ and the resistors  30 , are mounted to terminal strip  24  and electrically interconnected by means of standoff and feed through connectors  32 . The reed switches  28 ,  28 ′ are spaced on the terminal strip  24  at uniform incremental distances small enough to insure that at least one of the switches will be actuated from any location of the magnet. 
     In the preferred embodiment, the reed switches are wired in pairs as designated in FIG. 3 by the reference numerals  28 ,  28 ′. The reed switches  28 ,  28 ′ have a length of approximately one inch, and are disposed in serial axial alignment parallel to the path of magnet  16 . The reed switches  28 ,  28 ′ forming the reed switch pairs are spaced apart approximately one inch to provide an arrangement wherein a small overlap of switch actuation will occur, thus reducing the possibility of a CEA position at which no switch would be actuated. As the control rod drive shaft travels axially within control rod housing  12 , switches  28 ,  28 ′ will be sequentially closed at the approach of the field of magnet  16  and opened after the magnetic field passes. The switches are arranged such that serially adjacent switches will be closed as the magnet  16  is intermediate the two switches. 
     With regard to the electrical portion of the position indicating apparatus  10 , FIG. 3 shows a plurality of resistors  30  of the same size and type connected at end points  33  and  34  across the power supply  38  (power supply  38  shown in FIG.  1 ). The resistors  30  form an incremental potentiometer or voltage divider. As discussed above, reed switches  28 ′ are electrically connected in series with each of switches  28 , and are positioned in substantially the same locations as switches  28 . Each of reed switches  28  is connected to a different point or tab on the voltage divider comprising resistors  30 . All of the circuits comprising the series connected switches  28 ,  28 ′ are connected to a signal bus bar having a terminal point  20 . Thus, upon closing of one of the switches  28  and its serially connected back up switch  28 ′, a signal from the incremental potentiometer comprising resistors  30  will be applied to bus bar terminal  20 . The amplitude of this signal indicates which switch of switch pair  28 ,  28 ′ is at that instant subject to the field from magnet  16 . 
     Another system for sensing, monitoring and transmitting an indication of control element assembly (CEA) position is described in U.S. Pat. No. 5,333,160, assigned to Assignee of the present invention and herein incorporated by reference. As shown in FIGS. 4 and 5, the apparatus  100  includes a CEA represented by a single CEA  120  located within housing  140 , a plurality of magnetic circuits  102  comprising at least one magnet  104  and at least one acruate-shaped magnetic path  106 , and at least one position transmitter assembly  108  including a plurality reed switches  28 ,  28 ′ or other magnetically responsive switches and a voltage divider network or incremental potentiometer  29 . 
     The CEA  120 , housing  140 , reed switches  28  and  28 ′, and voltage divider network  29  may take the form of like elements disclosed in U.S. Pat. No. 3,656,074, the explanation of which is incorporated herein by reference. A control rod drive coil stack  142  is mounted on the control rod housing  140 . Magnetic flux from the coil stack  142 , acting through the stainless steel housing  140  causes the CEA  120  located inside the housing  140  to move axially, thus adjusting the position of the CEA relative to the reactor core. As the CEA  120  is withdrawn from the reactor, a control rod extension shaft  150  moves up into an extension shaft housing  190  above the CEA  120  housing. 
     The vertical position of the CEA  120  may be detected by determining the position of the extension shaft  150 . The CEA  120  position transmitter includes a number of magnetic circuits located on the outside of the extension shaft housing  190  at a predetermined number of elevations. For example, the magnetic circuits may be spaced one inch apart over an eleven foot length of the extension shaft housing  190 . Each magnetic circuit has at least one magnet  104 , a carbon steel magnetic path  106  surrounding the extension shaft housing  190 , and at least one longitudinally extending reed switch position assembly  108 . Referring to FIG. 5, there are provided a pair of magnets  104  and a pair of position transmitter assemblies  108  to provide redundant monitoring of CEA&#39;s  120  position for increased reliability. 
     At selected elevations along the direction of travel of the CEA  120 , a measurement is made by providing a magnetic circuit around the extension shaft housing  190  which senses the presence of the extension shaft  150 , depending on how far the control rod is withdrawn. In the preferred embodiment, the extension shaft  150  is composed of a ferromagnetic material. The strength of the magnetic field in the area of the position transmitter assembly  108  will be dependent on the reluctance of the magnetic circuit, which in turn depends on how much ferromagnetic material is present in the magnetic circuit. Since the ferromagnetic extension shaft  150  moves in the path of the magnetic field, the magnetic field strength at a particular elevation in the area of a particular reed switch  28 ,  28 ′ will be greater if the extension shaft  150  is at or above that elevation, and less if the extension shaft  150  is below that elevation. The sensitivity of the reed switches  28 ,  28 ′ is chosen so that the reed switches  28 ,  28 ′ will close in the stronger magnetic field and open in the lesser field. 
     FIGS.  2 ( a ) and  2 ( b ), respectively, also show top and side cut-away views of a transmitter circuitry located within the housing  190 . Shown within the housing is a terminal strip  24  to which are mounted flux responsive reed switches  28  and  28 ′ and other components of an incremental potentiometer of the position transmitter  108 . The reed switches  28  and  28 ′ are spaced along the terminal strip  24  at uniform incremental distances corresponding with each predetermined elevation of the magnetic circuitry. 
     The other components of the incremental potentiometer  29 , resistors  30 , are mounted to the terminal strip  24  and are electrically interconnected by means of standoff and feed through connectors  32 . 
     Referring to FIG. 3, a schematic diagram for the electrical portion of the position transmitter assembly  108  is shown. A plurality of resistors  30  of the same size and type are connected at end points  33  and  34  across the power supply  38  to form an incremental potentiometer or voltage divider  29 . 
     Standard reed switch position transmitter (RSPT) systems may result in an unnecessary reactor trip if there are erroneous indications of the location of the control element assembly (CEA) sent to the control element assembly calculator (CEAC). Certain malfunctions may cause CEA position locators to incorrectly report the location of the CEA. These malfunctions may consist of the failure of electrical devices or other components of the system. If any component of the system fails, an incorrect rod position will be input to the CEAC, resulting in the generation of a penalty factor, which is input to the core protection calculator. The core protection calculator uses the penalty factor information, along with other parameters to determine if safety limits are exceeded and initiates a reactor trip if limits will be exceeded. Consequently, a position locator system is needed that may reduce the likelihood of a false CEA position signal. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to overcome the above-mentioned and other disadvantages of the prior art by providing a position verification apparatus that independently senses and transmits to the control element assembly calculator (CEAC) a signal indicative of the position of the CEA. 
     It is another object of the present invention to provide a position verification apparatus that may be easily incorporated into existing reed switch position transmitter (RSPT) systems. 
     It is a further object of the present invention to provide a position verification apparatus that may increase the reliability of current RSPT systems. 
     It is a further object of the present invention to provide a position verification apparatus that is inexpensive and easy to install on existing nuclear plants. 
     To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied, this invention may comprise a movable member disposed within and movable with respect to a housing containing the movable member, a means for generating a magnetic field within the housing, a first magnetically responsive means for sensing the position of the movable member within the housing and outputting a signal indicative of the position of the movable member, a second magnetically responsive means for sensing the presence of the movable member and outputting a signal indicative of the position of the movable member and a means for comparing the signal output from the first magnetically responsive means and the signal output from the second magnetically responsive means. 
     In an alternative embodiment, the components of the position transmitting apparatus, the first and second magnetically responsive means and the voltage divider network, may be contained within a transmitter housing positioned adjacent to the housing containing the movable member. The means for generating the magnetic field may include a permanent or an electrically excited magnet. The magnet generates a magnetic field proximate the movable member. Thus, the magnetic field is strongest in the housing at the point immediately adjacent the movable member and closest to the source of the magnetic field. Consequently, as the movable member travels axially within the housing, the intensity of the field strength at various points within the housing varies depending on the location of the movable member. 
     The first magnetically responsive means includes a plurality of switch means such as reed switches. The reed switches are electrically connected to a voltage divider network, and are uniformly spaced apart a predetermined vertical distance such that at least one switch is always energized, closed. The reed switches are electrically coupled to a connector that receives an input signal from the reed switches that is indicative of the position of the movable member. 
     The second magnetically responsive means also comprises a plurality of reed switches spaced apart a predetermined vertical distance. The reed switches comprising this second set of reed switches are electrically coupled to a signal contact which receives a signal from the second set of switches indicative of the position of the movable member. 
     When the position transmitting apparatus is installed on a nuclear plant, the movable member is replaced by a control element assembly (CEA). The control element assembly calculator (CEAC) receives the signal from the signal contact (described above) electrically associated with the first set of switches and that from the signal contact electrically associated with the second set of switches. The CEAC compares the signal from the first set of switches with that received from the second set of switches to determine the value of any penalty factors, if any, that must be generated to control the reactivity of the reactor. If the output from the first set of switches and the second set of switches is inconsistent with regard to the location of the movable member, the signals are ignored. If the output from the first set of switches indicates that the CEA is at a particular location and the output from the second set of switches confirms that the CEA is not at the top, the location of the CEA is assumed to be as indicated by the first set of switches. 
     Finally, the position verification apparatus includes a display for visually representing the position of the movable member. 
     In operation, the switches forming the first set of magnetically responsive switches sequentially open and close as the magnetic field of the magnet becomes intermediate the axial position of a particular switch. The strength of the magnetic field causes the switch to close. The switches are arranged such that at least one switch forming the plurality of switches comprising the first set of switches is always closed. 
     The second set of magnetically responsive switches are located primarily at a position corresponding to the CEA being withdrawn to the top of the core. At least one switch included in this second set of switches will remain closed, activated, as long as the CEA is positioned so that the CEA is outside the reactor core. Once the CEA penetrates the reactor core, at least one switch included in the second switches will remain open, indicating that the CEA is no longer positioned at the top of the core, but is positioned within the core. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and inventive aspects of the present invention will become more apparent upon reading the following detailed description, claims, and drawings, of which the following is a brief description: 
     FIG. 1 shows a prior art control rod position sensing apparatus; 
     FIG.  2 ( a ) is a plan view of the position sensing apparatus shown in FIGS. 1 and 4, wherein a portion of the housing is cutaway to show the included circuitry of the control rod position sensing device; 
     FIG.  2 ( b ) is a side view of the position sensing apparatus shown in FIGS. 1 and 4, wherein a portion of the housing is cutaway to show the included circuitry of the control rod position sensing device; 
     FIG. 3 is a schematic diagram of the electrical circuit of the position sensing apparatus shown in FIGS. 1 and 4; 
     FIG. 4 shows a second prior art control rod position sensing apparatus; 
     FIG. 5 is a perspective view of the internal structure of the apparatus shown in FIG. 4; 
     FIG.  6 ( a ) is a plan view of a control rod position verification apparatus formed in accordance with the teachings of the present invention, wherein a portion of the housing is cutaway to show the included circuitry of the control rod position sensing device; 
     FIG.  6 ( b ) is a side view of a control rod position verification apparatus formed in accordance with the teachings of the present invention, wherein a portion of the housing is cutaway to show the included circuitry of the control rod position sensing device; 
     FIG. 7 is a circuit diagram of the position verification apparatus shown in FIGS.  6 ( a ) and  6 ( b ); and 
     FIG. 8 is a flow chart showing the logic employed by a control element assembly calculator formed in accordance with the teachings of this invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG.  6 ( a ) shows a plan view of a position verification apparatus  200  formed in accordance with the teachings of this invention. The present invention may be incorporated into existing reed switch position transmitters (RSPT) to provide a verification of the position of the CEA indicated by the RSPT. For example, the position verification apparatus  200  may be incorporated into the RSPT systems described herein in FIGS. 1-5. For illustration purposes only, the disclosed embodiment shows the position verification apparatus  200  incorporated as a component part of a RSPT system of the type described in FIG.  1 . In this application, the apparatus  200  is used to verify the position of the CEA as determined by the RSPT system. It will be appreciated, however, that the use of the position verification apparatus  200  is not limited to the disclosed application. 
     The position verification apparatus  200  includes a plurality of magnetically responsive switches  210 , contained within housing  18 . The responsive switches  210 , are mounted to a terminal strip  24  and electrically interconnected by means of standoff and feed through connectors  32 . Care is taken to ensure that the length of the standoff connectors  45  is not so great as to make electrical contact with the inner wall of the housing  18 . Further, the leads of the responsive switches  210  may be insulated to prevent unwanted electrical contact. One of ordinary skill in the art will appreciate that a terminal strip containing a printed circuit board rather than standoff and feed through connectors may be used for component connection. 
     In the preferred embodiment, the position verification apparatus  200  includes five (5) responsive switch pairs  210 ,  210 ′, wherein the switches forming the switch pairs are electrically connected in series. The number of switches included as components of the position verification apparatus  200  will depend on the size, type and magnetic responsiveness of the switches selected and the strength of the magnetic field generated within the housing  18 . 
     Typically, reed switches of the type used in the position verification apparatus  200  have a length of approximately one inch. The switch pairs  210 ,  210 ′ are uniformly spaced apart on the terminal strip  24  at a predetermined interval. In the preferred embodiment, the reed switch pairs  210 ,  210 ′ are electrically connected such that the switches forming the switch pair are located at substantially the same location within the housing  18 . The upper most switch pair  210 ,  210 ′ is located at approximately 154⅛ inches from the bottom edge of the control rod housing  12 . The switch pairs  210 ,  210 ′ are spaced along the housing  18  such that there is approximately 1½ inches of vertical distance between successive switch pairs. The reed switch pairs  210 ,  210 ′ are disposed in serial axial alignment parallel to the path of the control rod. Arranging the switch pairs  210 ,  210 ′ in the disclosed manner provides an arrangement wherein a small overlap of switch actuation will occur, thus reducing the possibility of a CEA position at which no switch would be actuated. As the CEA travels axially within the control rod housing  12 , one or more of the switch pairs  210 ,  210 ′ will be sequentially closed at the approach of the magnet  16  and opened after the magnet  16  passes. The switch pairs  210 ,  210 ′ are arranged such that two adjacent switches will be closed when the magnet  16  is intermediate the two switches, as the field strength causes the switches  210 ,  210 ′ to close. 
     All of the circuits comprising the series connected switches  210  and  210 ′ are connected to signal contacts  27 ,  27 ′. The signal from signal contacts  27 ,  27 ′ and that from the reed switch position transmitter (RSPT) of apparatus  10  are input into a control element assembly calculator (CEAC). The CEAC considers the location of the CEA in determining the value of any penalty factors needed to control the reactivity of the core. The CEAC calculates the penalty factors using algorithms currently known and used in existing RSPT/CEAC systems. 
     In the present invention, the CEAC compares the signal from contact signals  27 ,  27 ′ with that from the RSPT of position indicating apparatus  10 . As illustrated in FIG. 8, the CEAC will calculate any required penalty factors needed to adjust the reactivity of the core only if the information from the RSPT of apparatus  10  as to CEA position is consistent with that received from the position verification apparatus  200 . For instance, if the signal from the RSPT of apparatus  10  indicates that the CEA is at the top of the core, and the signal  27  from the position verification apparatus  200  indicates that the CEA is at top of the core (at least one switch pair  210 ,  210 ′ being closed), the signal is deemed valid, and the CEA position should be used in the CEAC calculation to determine any necessary penalty factors. On the other hand, if the signal from the RSPT of apparatus  10  indicates that the CEA is at the top of the core and the position verification apparatus  200  indicates that the CEA is not at the top of the core (switch pairs  210 ,  210 ′ open), the signal is not valid, and the CEA position will not be used in CEAC calculations in determining penalty factors. Additionally, if the RSPT of apparatus  10  indicates that the CEA is at an intermediate position within the core and the position verification apparatus  200  indicates that the CEA is at the top of the core, the signal is not deemed valid. Likewise, if the signal from the RSPT of apparatus  10  indicates that the rod is at some intermediate position within the core and the signal from the position verification apparatus  200  indicates that the CEA is at some position within the core (switches  210 ,  210 ′ open), the signal is deemed valid, and the CEA position is used by the CEAC in calculating any necessary penalty factors. 
     Tables 1 and 2 illustrate the operation of a CEAC using prior software logic and that of a CEAC designed in accordance with the teaching of this invention. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 PRIOR SOFTWARE LOGIC 
               
             
          
           
               
                 RSPT Ladder Indicates 
                 RSPT Ladder Indicates CEA at 
                   
               
               
                 CEA at Top 
                 Intermediate Position 
                 Consequences  
               
               
                   
               
               
                 Yes 
                   
                 1 
               
               
                   
                 Yes 
                 2  
               
               
                   
               
               
                 Consequence 1: Signal is valid and CEA position should be used in downstream calculations. (Since CEA is at top of the core no adverse consequences will result).  
               
               
                 Consequence 2: Signal is valid and CEA position should be used in downstream calculations. Even if the CEA is actually at top of the core, a malfunction in the signal would result in the indicated CEA position being used in downstream calculations and potentially result in unnecessary reactor trip.  
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 MODIFIED SOFTWARE LOGIC 
               
             
          
           
               
                   
                 RSPT 
                   
                   
                   
               
               
                 RSPT 
                 Ladder 
               
               
                 Ladder 
                 Indicates 
                 Additional 
               
               
                 Indicates 
                 CEA at 
                 Contact 
                 Additional 
               
               
                 CEA 
                 Intermediate 
                 Switches 
                 Contact 
               
               
                 at Top 
                 Position 
                 Closed 
                 Switches Open 
                 Consequences  
               
               
                   
               
               
                 Yes 
                   
                 Yes 
                   
                 1 
               
               
                 Yes 
                   
                   
                 Yes 
                 2 
               
               
                   
                 Yes 
                 Yes 
                   
                 2 
               
               
                   
                 Yes 
                   
                 Yes 
                 1  
               
               
                   
               
               
                 Consequence 1: Signal is valid and CEA position should be used in downstream calculations.  
               
               
                 Consequence 2: Signal is not valid and CEA position should not be used in downstream calculations. By incorporating this logic in the software, unnecessary reactor trips can be avoided due to signal malfunctions.  
               
             
          
         
       
     
     FIG. 7 shows an electrical schematic for the position verification apparatus  200  and position indicating apparatus  10 . The voltage divider network forming an element of the present invention is identical to that shown in FIG. 3, the discussion of which is incorporated herein by reference. With regard to the position verification apparatus  200 , as discussed above, all of the circuits comprising the series connected switches  210 ,  210 ′ are connected to the signal contacts  27 ,  27 ′. Thus, upon the closing of one of the switches  210  and its serially connected back-up switch  210 ′, the signal from the signal contacts  27 ,  27 ′ will be applied to the CEAC. As discussed above, the CEAC compares the signal from the contact signal  27  to determine the value, if any, of penalty factors needed to adjust the reactivity of the reactor. 
     The addition of the contact signal from contact signals  27 ,  27 ′ in the present invention permits verification of the information provided to the CEAC concerning the position of the CEA as indicated by the RSPT of apparatus  10 . The present invention, thus, may reduce the number of unnecessary reactor trips associated with faulty CEA position indicators. 
     There are a variety of configurations which may be employed to fabricate position verification apparatus  200 . For example, the system  200  need not include the redundant reed switch  210 ′, nor does the position sensing apparatus  10  require the redundant switch  28 ′. These redundant switches  210 ′ and  28 ′ provide a back-up should one switch of the switch pair fail in the closed position. Thus, the disclosed embodiments are given to illustrate the invention. However, they are not intended to limit the scope and spirit of the invention. Therefore, the invention should be limited only by the appended claims. 
     Preferred embodiments of the present invention have been disclosed. A person of ordinary skill in the art would realize, however, that certain modifications would come within the teachings of this invention. Therefore, the following claims should be studied to determine the true scope and content of the invention.