Abstract:
In a position control system for monitoring the position of a valve, a signal transmitter for generating a signal indicative of a valve position includes a shaft coupled to the valve and rotatable to an azimuth angle indicative of the valve position. The signal transmitter includes a signal flag mounted on the shaft in either a calibration state or an operating state. In the calibrating state, a signal-flag adjuster sets the azimuth angle of the signal flag independently of the shaft or of any other signal flags. In the operating state, the signal flag rotates with the shaft.

Description:
This application claims benefit of provisional application Ser. No. 60/078,185, filed Mar. 16, 1998. 
    
    
     This invention relates to devices for verifying the correct operation of a valve positioner in a process control system, and in particular, to a signal transmitter having adjustable limit switches that move in response to movement of a valve positioner. 
     BACKGROUND 
     In a feedback control system, a controller obtains the value of a controlled variable, compares that value with a setpoint, and adjusts the value of a manipulated variable in order to drive the controlled variable toward the setpoint. In the context of a process control system, adjustment of the manipulated variable generally involves adjusting a valve. For example, if the controlled variable is the level of fluid in a tank having an intake valve and an outlet valve, the manipulated variable can be the volume rate of flow into or out of the tank. Both of these variables are ultimately manipulated by adjusting the position of a valve. A valve for controlling the flow of fluid is thus a critical component in the control of a processing plant. 
     To control a valve, the controller sends a signal to a positioner, which is a mechanical device intimately associated with the valve that moves in response to the signal. When the positioner moves, it changes the position of the valve and hence, the value of the manipulated variable controlled by that valve. This change in the manipulated variable results in a corresponding change in the controlled variable. The controller then measures the value of the controlled variable and, if necessary, sends another signal to the positioner to correct the value of the manipulated variable. This process of measurement, followed by correction on the basis of the measurement, is at the heart of a feedback control system. 
     Unfortunately, it is possible for the controller to send the positioner a signal and for the positioner to do nothing, to move an incorrect amount, or, in the worst case, to move in the wrong direction. The failure of a positioner can, of course, be detected by measuring the value of the controlled variable and observing whether that value is inconsistent with the expected value of the manipulated variable. However, in many processes, there may be significant lag time or dead time. In such processes, it may be some time before the controller realizes that the controlled variable is not changing as expected. During this lag time, significant damage may occur. For example, if the valve controls the flow of coolant in a nuclear power plant, by the time the temperature of the coolant rises, the core temperature may already be dangerously high. 
     It is therefore desirable to detect the failure of a positioner as soon as possible. Because the positioner is typically hidden from view, this is most readily accomplished by having the positioner transmit a signal verifying that it has, indeed, moved to the location specified by the controller. This generally requires a signal transmitter mechanically coupled to the positioner such that when the positioner is in the desired position, an electrical signal is transmitted to the controller, to an alarm panel, or to some other appropriate location. In a typical signal transmitter of this type, a protruding signal flag coupled to the positioner moves into engagement with an electromagnetic switch when the positioner reaches a desired position. 
     A disadvantage of known signal transmitters is the difficulty encountered in adjusting the location of the signal flag to accommodate variations in valve positioners. In known signal transmitters, adjustment of the flag location generally requires access to the top and sides of the signal transmitter. In addition, when the flags are loosened for adjustment, they move relatively freely and are therefore difficult to adjust independently of each other with precision. 
     Because of the difficulty in adjusting the signal flags with precision, the mechanical motion of the positioner needs to be amplified so that small errors in positioning the flags do not result in large errors in the perceived position of the valve. This, in turn, requires that a system of gears having a gear ratio selected to amplify the mechanical motion of the positioner be interposed between the positioner itself and the signal transmitter. This gear system provides yet another source of possible failure, adds to the cost of the signal transmitter, and, because the mechanical resolution of the system is limited by the spacing between the gear teeth, decreases the overall resolution of the signal transmitter. 
     An additional disadvantage of known signal transmitters is that the signal flags are mounted in a manner susceptible to vibration. Exposure to such vibrations can eventually cause the signal flags to become misaligned. As a result, such signal transmitters require frequent maintenance. 
     A position indicating apparatus according to the preamble of claim  1  is known from GB 2 265 204 A. 
     It is thus an object of the invention to provide a signal transmitter in which the signal flags can be adjusted independently of each other with sufficient precision to eliminate the need for an amplifying gear between the positioner and the signal transmitter. 
     It is a further object of the invention to significantly reduce the sensitivity of the signal flags to vibrations. 
     SUMMARY 
     These objects are achieved by an apparatus according to claim  1 . 
     Further developments of the invention are given in the dependent claims. 
     A signal transmitter incorporating includes a signal flag mounted to a shaft that rotates in azimuth in a manner indicative of the valve position. The signal flag is mounted in either an operating state, in which the signal flag rotates only when the shaft rotates and a calibration state in which the signal flag can be rotated independently of the shaft and any other signal flags mounted thereon. 
     The signal flag has a switch-engaging portion, which engages a switch when the signal flag is rotated to a selected azimuth angle, thereby causing the switch to generate a signal indicative of the azimuth angle of the switch-engaging portion. This azimuth angle is, in turn, indicative of a particular valve position 
     In the preferred embodiment, the signal flag is an annular disk that is coaxial with the shaft and held between first and second surfaces by a variable compressive force. The annular disk has an inner rim with teeth adapted to engage a gear formed thereon and an outer rim having a protrusion extending radially outward from the switch-engaging portion. 
     To specify what valve position is to be associated with a particular azimuth angle, an apparatus embodying the invention includes a signal-flag adjuster fixedly mounted to the shaft and coupled to the signal flag. When the signal flag is mounted in its calibration state, the signal-flag adjuster selectively rotates the signal flag independently of the rotation of the shaft and independently of the rotation of any other signal flags mounted to the shaft. 
     In the preferred embodiment, the signal-flag adjuster includes a rotatable gear for engaging the teeth on the inner rim of the annular disk forming the signal flag. When the signal flag is mounted in its calibration state, rotation of this gear rotates the switching engaging portion of the signal flag independently of the shaft. Typically, the rotating gear is a radially extending portion of the shank of a screw extending parallel to the shaft axis and having a screw head accessible from the outside. Maintenance personnel can therefore perform the necessary calibration without the need to significantly dismantle the signal transmitter. 
     The diameter of the gear extending from the screw is typically smaller than the inner diameter of the annular disk on which the gear teeth are formed. Consequently, a full rotation of the screw (and hence of the gear) results in only a small change in the azimuth angle of the signal engaging portion of the signal flag. This allows the azimuth angle of the signal flag to be adjusted with great precision. 
     To switch between the calibration state and the operating state, an apparatus embodying the invention includes a mechanism for applying a variable compressive force between two surfaces supporting this signal flag. To mount the signal flag in its operating state, the screw is rotated in a first direction which draws the two surfaces closer together, thereby increasing the compressive force on the signal flag and preventing it from rotating relative to the shaft. To mount the signal flag in its calibrating state, the screw is rotated in a second direction, thereby allowing the two surfaces to be drawn apart and reducing the compressive force applied to the signal flag. This allows the signal flag to rotate relative to the shaft and to do so independently of any other signal flags also mounted on the shaft. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following description and apparent from the accompanying drawings, in which like reference characters refer to the same parts throughout the different views. The drawings illustrate principles of the invention and, although not to scale, show relative dimensions. 
     FIG. 1 is a cross-sectional view of a signal transmitter embodying the invention; and 
     FIG. 2 is an exploded view of the signal transmitter of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     An adjustable signal transmitter  10  embodying the invention, as shown in cross-section in FIG.  1  and in an exploded view in FIG. 2, includes a shaft  100  having a shaft axis X extending in a direction perpendicular to the top surface of a stationary mounting platform  160 . The shaft  100  is a generally cylindrical structure having an outer diameter that varies discontinuously along its length. In particular, the shaft  100  includes a positioner-engaging section  110  at its bottom end, a bearing section  120  adjacent to the positioner-engaging section  110 , a pin-engaging section  130  adjacent to the bearing section  120 , and a flag-bearing section  140  at its topmost end and adjacent to the pin-engaging section  130 . 
     The stationary mounting platform  160  has an upper surface  162  from which a first switch-mounting bracket  163 , integral with and located at a first edge of the mounting platform  160 , extends upward. A second switch-mounting bracket  164 , also integral with the mounting platform  160  extends upward from the upper surface  162  at a second edge opposite the first edge of the stationary mounting platform  160 . The first switch-mounting bracket  163  supports a first inductive switch  410  having a groove  420  formed in a direction tangent to a circle centered on the shaft axis X. Similarly, the second switch-mounting bracket  164  supports a second inductive switch  430  also having a groove  440  formed in a direction tangent to a circle centered on the shaft axis X. The second inductive switch  430  is axially displaced relative to the first inductive switch  410  such that a plane passing through the groove  420  in the first inductive switch  410  is parallel to, but not coplanar with, a plane passing through the groove  440  in the second inductive switch  430 . 
     A mounting hole  165  formed in the upper surface  162  between the first and second switch-mounting brackets  163 ,  164  extends through the stationary platform  160  and connects the upper surface  165  to a lower surface  166  opposed to the upper surface  165 , the bearing section  120  of the shaft fits into this mounting hole  165 , thereby engaging the shaft  100  to the stationary platform  160 . 
     The bearing section  120  of the shaft  100  has an outer diameter chosen to be large enough to allow it to fit snugly into the mounting hole  165 , thereby ensuring that the shaft axis X remain perpendicular to the upper surface  162  of the mounting platform  160 . The outer diameter of the bearing section  120  is also chosen to be small enough to allow the bearing section  120  to freely rotate about the shaft axis X relative to the platform  160 . 
     As a consequence of its position below the bearing section  120 , the positioner-engaging section  110  of the shaft  100  extends downward from the bearing section  160 , through the lower surface  166  of the platform  160 , to a free end. The diameter of the positioner-engaging section  110  is thus chosen to be less than or equal to that of the bearing section  120  in order to allow the positioner-engaging section  110  to pass through the mounting hole  165 . 
     The positioner-engaging section  110  is mechanically coupled to a valve positioner  20  so that motion of the valve positioner  20  results in rotation, or a change in the azimuth angle, of the positioner-engaging section  110 . This in turn results in rotation, or a change in the azimuth angle, of the entire shaft  100 . 
     In the preferred embodiment, the free end of the positioner-engaging section  110  is bifurcated, or slotted, and the valve positioner  20  has a corresponding protrusion for engaging the slot. Consequently, a rotation of the valve positioner  20  causes a corresponding rotation of the entire shaft  100  relative to the platform  160 . 
     It will be appreciated that the link between the positioner  20  and the shaft  100  is a direct link with no interviewing gears required to amplify the rotation of the positioner  20 . This feature of the invention provides simplicity of construction and enhanced reliability in the operation of the signal transmitter  10 . 
     It will also be appreciated that, depending on the details of the design of the valve with which it is to be used, the signal transmitter  10  can be mechanically coupled to the valve directly, thereby bypassing the valve positioner  20 . 
     The pin-engaging section  130  extends upwardly from the bearing section  120  in the direction away from the mounting hole  165 . The diameter of the pin-engaging section  130  is preferably larger than that of the mounting hole  165  in order to ensure that only the bearing section  120  and the positioner-engaging section  110  are disposed below the upper surface  162  of the platform  160  and that the remainder of the shaft  100  is above the upper surface  162 . 
     A first radial aperture  132  formed on an outer wall  134  of the pin-engaging section  130  connects an axially extending hollow core  131  to the outside of the shaft  100 . Similarly, a second radial aperture  133  formed on the outer wall  134  of the pin-engaging section  130  diametrically opposed from the first radial aperture  132  provides an opening from the axially extending hollow core  131  to the outside of the shaft  100 . 
     The first and second radial apertures  132 ,  133  in the outer wall of the pin-engaging section  130  are aligned with diametrically opposed first and second collar holes  210 ,  220  of similar diameter formed in a pin-engaging collar  200  coaxial with the pin-engaging section  130  of the shaft  100 . The collar  200  forms an annular space  240  between the outer wall  134  of the pin-engaging section  130  and an inner wall  230  of the pin-engaging collar  200 . 
     The topmost section of the shaft  100 , namely the flag-bearing section  140 , has a diameter at least as large as the diameter of the pin-engaging section  130  adjacent to it. The flag-bearing section has a top face  141  having a central opening  143  into an axially extending hollow core  146 . The hollow core  146  of the flag-bearing section  140  is aligned with, and has the same diameter as, the hollow core  131  of the pin-engaging section  130 . The hollow core  131  of the pin-engaging section  130  and the hollow core  146  of the flag-bearing section  140  thus form one continuous hollow core into which a cylindrical sleeve  600  is inserted. 
     The sleeve  600  has a top face  605  in which is formed an axially extending threaded hole  610 . The threaded hole  610  intersects a cylindrical passageway  612  extending along a diameter of the sleeve  600  and perpendicular to the axially extending hole  610 . The diameter of the passageway  612  is the same as the diameter of the first and second collar holes  210 ,  220  in the pin-engaging collar  200  and the same as the diameter of the first and second radial apertures  132 ,  133  in the pin-engaging section  130  of the shaft  100 . 
     The sleeve  600  is inserted through the opening in the top face  141  of the flag-bearing section  140  such that the top face  605  of the sleeve  600  is coplanar with the top face  141  of the flag-bearing section  140 . When inserted in this manner, the sleeve  600  extends through the flag-bearing section  140  and into the pin-engaging section  130 . The sleeve  600  is then rotated about the shaft axis X such that the cylindrical passageway  612  aligns with the first and second radial apertures  132 ,  133  in the pin-engaging section  130 . With the cylindrical passageway  612  thus oriented, a locking pin  80  is inserted through the first and second collar holes  210 ,  220  in the collar  200 , the first and second radial apertures  132 ,  133  in the pin-engaging section  130  of the shaft  100 , and through the passageway  612  in the sleeve  600 . In this way, the locking pin  80  extends diametrically across the shaft  100  and locks together the shaft  100  and the collar  200 . 
     The top face  141  of the flag-bearing section  140  further includes a first adjustment-screw hole  142  that extends through the flag-bearing section  140  and opens into the annular space  240  between the outer wall  134  of the pin-engaging section  130  and the inner wall  230  of the pin-engaging collar  200 . The top face of the flag-bearing section further includes a second adjustment-screw hole  144  diametrically opposed from the first adjustment screw hole  142  and likewise extending through the flag-bearing section  140  and opening into the annular space  240 . 
     A cap  300  covering the top face  141  of the flag bearing section  140  includes a central well  305 , best seen in FIG. 1, coaxial with the shaft axis X. The central well  305  has a well floor  307  through which a central screw-hole  309  is formed. A compression spring  310  coaxial with and surrounding an axially extending collar  312  having a flanged end  312  is inserted into the central well  305  with the flanged end  312  of the collar  313  disposed upward and away from the central screw-hole  309 . As a result, the compression spring  310  is held between the well floor  307  and the flanged end  312  of the collar  313 . 
     The cap  300  is positioned over the top face  141  of the flag-bearing section  140  such that the central screw-hole  309  in the well floor  307  is aligned with the threaded hole  610  in the flag-bearing section  140 . An anchoring screw  320  having threads adapted to mate with the threaded hole  610  is then inserted through the central screw hole  309  and screwed into the threaded hole  610 . 
     The cap  300  farther includes a first adjustment-screw hole  342  extending axially through the cap  300  and a second adjustment-screw hole  344  diametrically opposed to the first adjustment screw-hole  342  and likewise extending axially through the cap  300 . The first and second adjustment-screw holes  342 ,  344  are positioned to align with the first and second adjustment-screw holes  142 ,  144  in the flag-bearing section  140 . 
     A first adjustment screw  510  extends axially with its base section  512  disposed in the annular space  240 , its shank  513  passing through the first adjustment-screw hole  142 , a gear section  515  between the shank  513  and the base section  512 , and its head  514  protruding beyond the top face  141  of the flag-bearing section  140  and into the first adjustment-screw hole  342  in the cap  300 . The shank  513  and base section  515  are cylindrical sections having the same diameter. The gear section  515  is a cylindrical section having a diameter greater than that of the shank  513  and teeth formed in its outer wall. Its axial position on the first adjustment screw  510  is chosen such that a plane passing through the groove  420  in the first inductive switch  410  intersects the gear section  515 . 
     Similarly, a second adjustment screw  520  diametrically opposed to the first adjustment screw  510  extends axially with its shank  523  passing through both the annular space  240  and through the second adjustment-screw hole  144 , its head  524  protruding beyond the top face  141  of the flag-bearing section  140  and into the second adjustment-screw hole  342  in the cap  300 , and a gear section  525  between the head  524  and the shank  523 . The gear section  525  is a cylindrical section axially displaced relative to the gear section  525  of the first adjustment screw  510 , having a diameter greater than that of the shank  523  and having teeth formed in its outer wall. Its axial position on the second adjustment screw  520  is chosen such that a plane passing through the groove  440  in the second inductive switch  430  intersects the gear section  525 . 
     The collar  200  and the flag-bearing section  140  fixedly engage between them a first signal flag  710  concentric with the shaft  100 . The first signal flag  710  is an annular disk held between a supporting surface  202  on the collar  200  and an engagement surface  145  on the flag-bearing section  140 . The axial location and orientation of the annular disk are such that a plane defined by the annular disk passes through the groove  420  in the first inductive switch  410 , and such that teeth formed along the inner circumference of the annular disk engage with corresponding teeth on the gear section  525  of the second adjustment screw  520 . The annular disk defining the first signal flag  710  is divided into two arcuate sections: a switch-engaging section  714  having an outer diameter of sufficient magnitude to extend the switch-engaging section  714  into the groove  420  of the first inductive switch  410 , and a switch-avoiding section  712  having an outer diameter insufficient to extend into the groove  420  regardless of the azimuth angle of the signal flag  710 . 
     Similarly, the flag-bearing section  140  and the cap  300  engage between them a second signal flag  720  concentric with the shaft  100 . The second signal flag  720  is an annular disk held between a supporting surface  149  on the flag-bearing section  140  and an engagement surface  349  on the cap  300 . The annular disk has an axial location and orientation such that a plane defined by the annular disk passes through the groove  440  in the second inductive switch  430  and such that teeth formed along the inner circumference of the annular disk engage with corresponding teeth on the gear section  515  of the first adjustment screw  510 . The annular disk defining the second signal flag  720  is divided into two arcuate sections: a switch-engaging section  724  having an outer diameter of sufficient magnitude to extend the switch-engaging section  724  into the groove  440  of the second inductive switch  430  (as shown in FIG.  1 ), and a switch-avoiding section  722  having an outer diameter insufficient to extend into the groove  440  regardless of the azimuth angle of the signal flag  720 . 
     As a result of the coupling between the positioner  20  and the positioner-engaging section  110  of the shaft  100 , when the positioner  20  changes the position of a valve, the shaft  100  rotates about the shaft axis X. The first signal flag  710 , because it is fixedly attached to the shaft  100 , also rotates about the shaft axis X. 
     It is apparent from FIG. 2 that as the shaft  100  rotates, the azimuth angle of the switch-engaging section  714  of the first signal flag  710  changes. As a result, the switch-engaging section  714  enters the groove  420  in the first inductive switch  410 . This results in a change in inductance which, in turn, results in the generation of an electrical signal. Because this electrical signal is generated when the shaft  100  is at a selected angular location, and because the selected angular location depends on the motion of the valve positioner  20 , the signal thus generated is representative of a particular valve position. 
     As the shaft  100  continues to rotate, the azimuth angle of the first signal flag continues to change until the switch-engaging section  714  exits the groove  420 . This results in another change in inductance which, in turn, results in the generation of another electrical signal representative of the position of the valve. 
     In the manner described above, the adjustable signal transmitter  10  embodying the invention generates electrical signals verifying that the valve positioner  20  has placed the valve into one of two positions: a first position corresponding to the entry of the switch-engaging section  714  into the groove  420 , and a second position corresponding to the exit of the switch-engaging section  714  from the groove  420 . 
     It will be apparent to one of ordinary skill in the art that the second signal flag  720  cooperates with the second inductive switch  430  in a manner similar to the manner described above in connection with the first signal flag  710 . As the azimuth angle of the second signal flag  720  changes, the switch-engaging section  724  of the second signal flag  720  enters the groove  440  in the second inductive switch  430 . This results in a change in inductance which, in turn, results in the generation of an electrical signal. Because this electrical signal is generated when the shaft  100  is at a selected angular location, and because the selected angular location depends on the motion of the valve positioner  20 , the signal thus generated is representative of a particular valve position. 
     As the shaft  100  continues to rotate, the azimuth angle of the second signal flag  720  continues to change until the switch-engaging section  724  exits the groove  440 . This results in another change in inductance which, in turn, results in the generation of another electrical signal representative of the position of the valve. 
     In the manner described above, the adjustable signal transmitter  10  embodying the invention generates two additional electrical signals verifying that the valve positioner  20  has placed the valve into one of two additional positions: a first position corresponding to the entry of the switch-engaging section  724  into the groove  440 , and a second position corresponding to the exit of the switch-engaging section  724  from the groove  440 . 
     The valve position that generates any one of the foregoing signals is determined by the azimuth, or circumferential angle of the switch-engaging sections  714 ,  724  relative to the shaft  100 . For example, suppose that one wishes to ensure that when the valve is in a fully open position, the first inductive switch  410  generates a signal. Under these circumstances, it is necessary to ensure that the switch-engaging section  714  of the first signal flag  710  engages the groove  420 . This can be accomplished using a calibration procedure in which one places the valve in the open position and then adjusts the azimuth angle of the first signal flag  710  so that, with the valve in the open position, the switch-engaging element  714  engages the groove  420  in the first inductive switch  410 . The ease with which the azimuth angle of the first and second signal flags  710 ,  730  is adjusted, as set forth below, is a significant advantage of the invention. 
     The first step in adjusting the azimuth angle of either or both signal flags is to loosen the anchoring screw  320 . This moves the cylindrical sleeve  600  downward, and thereby relieves the pressure holding the first signal flag  710  between the collar  200  and the flag-bearing section  140  and the pressure holding the second signal flag  720  between the cap  300  and the flag bearing section  140 . However, even with the anchoring screw  320  loosened, the compression spring  310  still exerts a downward force on the well floor  307 . As a result of this downward force, the azimuthal motion of the first and second signal flags  710 ,  720  is constrained. 
     With the anchoring screw  320  thus loosened, the next step in adjusting the azimuth angle of the first signal flag  710  is to engage the head  524  of the second adjustment screw  520  by inserting a screwdriver, or similar tool, into the second adjustment screw hole  344  in the cap  300 . One then twists the head  524  to rotate the second adjustment screw  524  in a direction corresponding to the desired change in the azimuth angle. As the second adjustment screw  524  rotates, the teeth on the gear section  525  engage the teeth on the first signal flag  710 , thereby rotating, and hence changing the azimuth angle of, the first signal flag  710 . Because the gear section  525  is a smaller diameter gear than the inner diameter of the first signal flag, a complete revolution of the gear section  525  changes the azimuth angle of the signal flag  710  by only a small amount As a result, it is relatively easy to change the azimuth angle by very small amounts. 
     The foregoing method of adjusting the azimuth angle of the first signal flag by turning an adjustment screw allows for great precision in the choice of azimuth angle. Additionally, because the motion of the force exerted by the compressed spring  310  constains the rotation of the first signal flag, the signal flag rotates only with the adjustment screw is turned; there is no rebound or residuary change in azimuth angle caused by either disengaging the screwdriver from the head  524  or by re-tightening the anchoring screw  320  at the end of the calibration procedure. 
     It will be apparent to one of ordinary skill in the art that to adjust the azimuth angle of the second signal flag  720 , and to do so independently of the azinuth angle of the first signal flag  710 , one proceeds as described above in connection with the first signal flag  710 , with the exception that one twists the head  514  of the first adjustment screw  510  rather than the second adjustment screw  520 . 
     The last step in the calibration procedure is to re-tighten the anchoring screw  320 . This has the effect of drawing the sleeve  600  up toward the cap  300 . Since the locking pin  80  passes through the sleeve  600 , this also has the effect of drawing the locking pin  80  upward. This in turn causes the collar  200  to be drawn up tightly against the flag-bearing section  140  of the shaft  100 , thereby securely fixing the first signal flag  710  between the collar and the flag-bearing section  140 . In a similar manner, re-tightening the anchoring screw  320  causes the flag-bearing section  140  to be drawn up tightly against the cap  300 , thereby securely fixing the second signal flag  720  between the cap  300  and the flag-bearing section  140 . 
     It will thus be seen that the invention efficiently attains the objects set forth above. Since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense.