Abstract:
A remotely testable Passive Integrated Transponder (PIT) tag interrogation system is described. The system comprises a remotely activated test PIT tag fixed within the field of the system antenna. By selectively activating the test PIT tag, and determining successful receipt of the test tag identification signal during such activation, remote confirmation of proper operation of the system is achieved. The system also contains components which permit the remote detection of the antenna field by analyzing signals present in the antenna during operation. Remote detection of the natural frequency to which the antenna is tuned is also achieved by analyzing signals present in the antenna during the collapse of the electromagnetic field in the antenna when the antenna drive signal is removed.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of passive integrated transponder tag interrogation systems, and specifically to an apparatus and method for remotely testing such systems. 
     BACKGROUND OF THE INVENTION 
     The use of Passive Integrated Transponder (PIT) tags which may be embedded in or attached to items to be tracked and accounted for has been widespread for some time. These commonly known PIT tag systems generally comprise one or more antenna coils so positioned as to generate a field of radiated electromagnetic energy within which the tagged item or object must pass. As generally deployed, such systems are used to track and/or count animals within which a PIT tag has been subcutaneously embedded or externally affixed. As the PIT tag traverses the radiated field of electromagnetic energy, it is energized in a manner known in the art. The PIT tag uses this energy—which is typically stored in a capacitive element—to power a transmission circuit which emits a unique PIT tag identification signal that is detected by the aforementioned antenna element. The identification signal detected by the antenna element is then transmitted to remote processing equipment which decodes the detected signal and uses this decoded information for the purposes of counting, tracking or otherwise maintaining records pertinent to the population of items or animals being tracked by said system. 
     These commonly known and used PIT tag interrogation systems suffer from a drawback heretofore unremedied in the art, that being the ability to remotely insure proper operation of the interrogation system. This shortcoming is particularly troublesome in applications where the antenna system is located in a remote location from the control point, or in locations which are physically difficult to access, such as, for example, underwater tunnels through which PIT-tagged fish travel for identification and tracking. The present solution for testing such systems is to have a user travel to the antenna location and physically pass a PIT tag through the antenna&#39;s radiated energy field so as to generate a tag signal detectable by the control unit. Successful receipt of the manually passed tag signal confirms that the system is operating properly. This present method of testing is expensive, resource intensive, and time consuming. Further, the difficulty in performing such an operation in fish tunnels placed underwater is obvious. 
     It would therefore be greatly advantageous for an apparatus or method to be developed which would alleviate the above identified shortcomings of the prior art. The present invention provides a solution. 
     SUMMARY OF THE INVENTION 
     The remotely testable PIT tag interrogation system of the present invention generally comprises a receiver/transmitter antenna, an antenna interface unit, a control unit, and a general purpose digital computer, along with associated control cabling, input keyboards, and visual displays. As used herein the terms Passive Integrated Transponder tags and/or PIT tags are used synonymously and are intended to mean any type of passive transponder which emits a signal in response to exposure to a radiated electromagnetic, electrical or magnetic energy field, such as, for example, 134 kilohertz (kHz) transponder tags conforming to the ISO/DIS 11785 standard, or their art recognized equivalent. Such tags are commonly subcutaneously embedded in animals for the purpose of tracking and identifying them in such locations as zoos and farms, or in the wild. Such tags are also used for tagging fish and birds, as well as domestic pets. As is known in the art, and as will become evident from a further reading of the material below, systems such as the one described herein, may be deployed in numerous applications and situations limited only by the imagination of the person of skill in the art. The system of the present invention may therefor be used in any application wherein transponders are placed on objects for tracking such objects as they pass within or through the field of a bi-directional antenna which energizes the transponder and receives identifying signals therefrom. As used herein, the terms object and item are used interchangeably and mean any PIT tagged entity, animate or inanimate. 
     The present invention utilizes a bi-directional antenna which is deployed in a location such that animals or objects equipped with PIT tags will pass through the energy field radiated by the antenna. When passing within the energizing field, the PIT tag is energized by the electromagnetic energy radiated by the antenna. This energization is achieved by charging a storage device in the PIT tag, typically a capacitive element, which then becomes a power source for the PIT tag. The PIT tag utilizes this stored energy to generate a unique tag identification signal which is received by the antenna. The antenna is connected via antenna leads, in a manner commonly known in the art, to an antenna interface unit which demodulates the tag identification signal and converts it into a data signal which is passed to a control unit which logs and tracks the identification signals received. The control unit generally comprises a display and keyboard through which a user may operate the system and view information about the tags being interrogated by the system. 
     The control unit is typically further connected to a general purpose digital computer which can be used to remotely control the system as well as to collect and process data related to the tags interrogated and identified by the system. Additionally, the control unit, the computer, or both may be used to activate programmable logic control (PLC) devices for triggering other events, such as, for example, gate closures, alarm indications, etc. 
     Generally, PIT tag systems are designed to operate automatically, with little to no operator intervention. Without direct visual observation, it is therefore impossible to determine if the absence of a tag interrogation and identification cycle is due to an absence of animals or objects passing within the antenna&#39;s radiated energy field or due to a system malfunction which is preventing tag interrogation signals from reaching the control unit. To overcome this problem, a remotely controllable PIT tag is mounted at the antenna site within the radiant energy field of the antenna. This remote test PIT tag, under the control of the system control unit and/or the attached general purpose computer, can be selectively operated so that at predetermined times the system may activate the test tag and thereby insure that the system is operating normally. 
     The remotely deployed test tag contains a unique identification code which when received by the system is recognized as the test tag identification code. This test tag is typically remotely activated by a relay, powered from the control unit, but deployed within the remote test tag for selectively activating and deactivating the test tag electronics. Further, varying the position of the test tag within the radiated energy field of the antenna provides an indication of the field strength of the antenna, since, if the remote test tag is fixably located at the fringes of the radiated energy field, and a remote test tag identification signal is successfully received by the control unit, this would indicate that the antenna is operating at peak performance. 
     In addition to the remotely controllable test tag, the system may be supplied with an additional remote test feature which, in the event of a test tag failure, will further isolate the source of the difficulty within the system. Specifically, the antenna interface unit is equipped with circuitry which permits the remote detection of the field produced by the antenna. Such a capability permits the detection of an open antenna coil or a circuitry failure in the antenna drive unit. 
     When operating properly, the antenna interface unit receives a signal from a PIT tag in the antenna field and digitizes it for further transmission to the control unit. The antenna may be located at a variety of distances from the antenna interface unit through antenna cabling commonly known in the art. Further, the antenna interface unit may be either remotely located or co-located with the control unit. The control unit may similarly be located near to, or remote from the general purpose computer, as application requirements dictate. Data transmission within the system may be accomplished over twisted pair cable, fiber optic cable or a combination thereof as a matter of design choice. Of course, the person of skill will recognize that the various data communication methodologies that are employed in the system may be varied to fit the application requirements to which the system is addressed, and these methodologies may be wired or wireless, metallic or optical, or any art recognized combination thereof, as a matter of design choice. 
     Other objects and features of the present invention will become apparent from the following detailed description considering conjunction with the accompanying drawing figures. It is to be understood, however, that the drawings, which are not to scale, are designed solely for the purpose of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. 
    
    
     DESCRIPTION OF THE DRAWING FIGURES 
     In the drawing figures, which are not to scale, and which are merely illustrative, and wherein like reference numerals depict like elements throughout the several views: 
     FIG. 1 is a schematic diagram of a remotely testable PIT tag interrogation system in accordance with the present invention; 
     FIG. 2 is a schematic diagram of the antenna interface unit component of the present invention; 
     FIG. 3 is a schematic representation of the control unit component of the present invention; 
     FIG. 4 is a schematic diagram of the remotely controlled test tag of the present invention; 
     FIG. 5 is a schematic circuit diagram of a preferred cross-over detector circuit deployed in the system of the present invention; and 
     FIG. 6 is an illustration of exemplary signal waveforms obtained during certain diagnostic tests conducted within the system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With initial reference to FIG. 1, there is depicted a schematic diagram of a passive integrated transponder tag interrogation system  10  in accordance with the present invention. An antenna  12  is connected via an antenna interface unit  20  to a control unit  40 . Control unit  40  is generally connected to a general purpose digital computer  60 , such as an IBM compatible personal computer, a mini computer or other device capable of running software programs having functionality of the type further discussed herein. Control unit  40 , however, controls the major functions of the system  10  and maybe operated via a directly connected keyboard  44  and display  42 , providing direct operator control without the need for computer  60 . 
     Antenna  12  is deployed proximate a tunnel or passage  16  through which the tagged items to be tracked must pass. Antenna  12  is powered by antenna interface  20  under the control of control unit  40  and generates an electromagnetic energy field within passage  16 . In a manner known in the art, as an item carrying a PIT tag passes through the energy field generated by antenna  12  (not shown), the PIT tag is energized by the electromagnetic field and in turn emits an identifying signal which is detected by antenna  12  and sent through antenna interface  20  to control unit  40  in a manner discussed further below. While the tunnel or passage  16  is depicted as a generally tubular passage in FIG. 1, it will be recognized that the particular shape and configuration of the passage, channel, tunnel or other structure which leads a PIT tagged item to traverse the electromagnetic field generated by antenna  12  is purely a matter of design choice for the person of skill in the art, it being recognized that the passage  16  may vary in countless ways depending on the application to which the system is applied. Thus, as long as passage  16  is developed in such a manner as to cause the item carrying a PIT tag to pass within the electromagnetic field of antenna  12  in such a manner as to be energized by said electromagnetic field and to emit a signal responsive thereto, the requirements of the inventive system have been met. Similarly, while antenna  12  is depicted as an antenna coil surrounding passage  16 , it will further be recognized by the person of skill in the art that antenna  12  may be configured and deployed in numerous ways such as, by way of none limiting example, flat panel antennas, arrays, antenna rods, dishes, etc. As discussed above in connection with passage  16 , antenna  12  need only be configured and deployed in such a manner as to generate an electromagnetic field of sufficient strength and dimension to energize a PIT tag in a desired location and to detect the signal generated by that PIT tag in response to energization by the antenna. In a preferred embodiment, the antenna is designed for the purpose of reading 134 Kilohertz (kHz) PIT tags conforming to the ISO/DIS 11785 standard. It will of course be recognized by the person of skill that the antenna and PIT tags may be designed to operate at other frequencies and energy levels than that of the preferred ISO standard, as a matter of engineering design choice. 
     Referring once again to FIG. 1, a remote test tag  14  is mounted proximate said antenna  12  on said passage  16 . Remote test tag  14  may be deployed within passage  16 , or placed external to passage  16  as a matter of design choice, provided that the test tag is fixedly mounted within the energizing field generated by antenna  12  for being energized by antenna  12  and for emitting an identifying signal to antenna  12  for processing by antenna interface unit  20  and subsequently by control unit  40 . In a preferred embodiment, remote test tag  14  is mounted outside passage  16  so as to avoid interference with any items passing through channel  16  and also for the added benefit of providing an indication of the field strength of antenna  12 . Specifically, remote test tag  14  is positioned at the fringes of the radiated energy field, thus removing it from the location of maximum field strength of antenna  12 . When so positioned, if remote test tag  14  can successfully be energized by antenna  12 , and antenna  12  can successfully detect a signal generated by remote test tag  14 , this would generally indicate that the antenna is generating an adequate electromagnetic field within channel  16 , where the field strength is at its maximum, and also is sensitive enough to detect PIT tags within channel  16 . Thus, placement of the test tag may vary depending on the field strength of the antenna. 
     Antenna  12  is connected to antenna interface unit  20  via an antenna cable  18 . Antenna cable  18  may be any type of antenna cable known in the art, such as, for example, coaxial cable, twin lead, or other conductor. In a preferred embodiment, antenna  12  comprises a coil of 16 gauge (AWG) copper wire having 16 gauge wire leads directly connected to antenna interface  20 . Such a coil is particularly advantageously deployed in applications where the antenna surrounds a passage  16  having a round, ovoid, rectangular or square cross section, such as those commonly used to direct the passage of tagged fish through the electromagnetic field of antenna  12 . The specific technique of connecting antenna  12  to antenna interface  20  through antenna cable  18  is purely a matter of design choice well within the skill of the routineer in the art of such antennas. 
     Referring now to FIGS. 1 and 4, remote test tag  14  is shown connected through a remote test tag cable  19  to control unit  40 . Remote test tag  14  is, in a presently preferred embodiment, a standard ISO type B tag comprising standard ISO type B tag circuitry  104 . Test tag  14  is, however, modified to include a remotely controllable relay  102  which may be selectively activated and deactivated by control unit  40  through a remote test tag cable  19 . In use, control unit  40 , either at specifically programmed intervals or under direct user instruction, activates relay  102  in such a manner as to activate tag circuitry  104  on demand. When tag circuit  104  is so activated, electromagnetic field produced by antenna  12  will energize test tag  14  in a manner known in the art. Test tag  14  will, correspondingly, in a manner also known in the art, emit its test tag identification signal. Thus, when control unit  40  remotely activates remote test tag  14  and in turn receives through antenna  12  and antenna interface unit  20  the test tag identification signal emitted by test tag  14 , the integrity of the system has been confirmed and control unit  40  will signal to an operator that the system has successfully been tested. Operator indication may be accomplished through an indication on display  42 , or through a signal sent through data communications cable  50  to computer  60 . It will of course be recognized by the person of skill in the art that the manner of controlling test tag  14  via control unit  40  may be accomplished in numerous ways other than through a direct wire and relay arrangement, as long as control unit  40  has the ability to selectively activate and deactivate tag circuitry  104  at predetermined times selected by the operator of the system  10 . For an ISO type B tag, the relay is operated for 50 to 60 milliseconds, which permits ample time for the tag to emit its identification signal while allowing for the reaction time of the relay. Different relays and tags, and different operating frequencies, will of course necessitate different activation times, as a matter of design choice. 
     With reference now to FIGS. 1 and 2, there is depicted a schematic diagram of an antenna interface unit  20 . Antenna interface unit  20  is a bi-directional subsystem which provides power at a predetermined antenna frequency to antenna  12  via an antenna drive unit  24  which is connected to antenna  12  through antenna cable  18 . Antenna interface unit  20  communicates with control unit  40  via a communications interface unit  22  over control communication cable  32 . Communication interface unit  22  is a bi-directional communications device conforming to EIA standard 485. Interface unit  22  manages the receipt of digital control signals from control unit  40  via communication control cable  32  and also manages the transmission of tag identification signals received by antenna  12  back to control unit  40 . 
     An antenna excitation signal sent by control unit  40  at a predetermined frequency, presently preferred at 134.2 Kilohertz (kHz), is directed to antenna drive  24  by interface unit  22 . Antenna drive unit  24  contains power transistors and MOSFET circuitry known in the art for powering antenna  12  which generates the electromagnetic excitation field within which PIT tags will pass. The particular circuitry of communication interface unit  22  and antenna drive  24  is of a type commonly known in the art and will not be discussed in detail herein, it being well within the abilities of the person of skill in the art to design such systems. 
     In a presently preferred embodiment, the communication control cable  32  is a shielded cable containing four twisted pairs of low capacitance 24 AWG stranded conductors, having aluminum shielding, a capacitance of 12.5 picofarads (pf) per foot, and a nominal impedance of 100 ohms. Of course, it will be recognized that the particular methodology for the transmission and reception of bi-directional signals between antenna interface unit  20  and control unit  40  is a matter of design choice, and can be implemented in any manner known in the art for exchanging analog and/or digital signals. It is, however, preferred that the communication interface and cable conform to the presently known EIA RS 485 communication standard. 
     When the system is active, communication interface  22  receives a 134.2 kHz excitation signal from control unit  40  via communication cable  32 . This excitation signal is in turn fed to antenna drive unit  24  which powers antenna  12  and causes antenna  12  to generate an electromagnetic field having a frequency at or near the excitation frequency of 134.2 kHz, depending on such factors as antenna geometry and tuning, which are easily modified as needed by the person of skill. With continued reference to FIG. 2, when test tag  14  is enabled by control unit  40 , test tag circuitry  104  is charged by the electromagnetic field  200  emitted by antenna  12  and utilizes this charge to emit a tag identification signal  400  which is detected by antenna  12 . Antenna  12  then passes the detected tag identification signal  400  to antenna interface unit  20 . Identification signal  400  is preferably an analog signal generated, in a manner known in the art, in accordance with the ISO type B tag identification convention. It will of course be recognized, however, that the identification signal emitted by any PIT tag detected by the system, including remote test tag  14 , may be generated in any manner known in the art of passive integrated transponder signaling as a matter of design choice. 
     In the preferred ISO type B embodiment, identification signal  400  is generated by modulating the excitation energy field  200 . Specifically, for a type B identification tag, the ISO requirement is to set the excitation field at a fixed frequency of 134.2 kHz. As soon as the type B tag receives enough energy, it returns its identification signal at a rate of 4,194 bits per second by modulating the amplitude of the excitation field at a rate of 1 bit per 32 cycles of the excitation field. A bit  0  is represented when the amplitude variation is done in the middle of a 32 cycle bit cell. A bit  1  is represented when no transition occurs in the middle of the bit cell. It will be recognized by the person of skill in the art following the ISO type B requirement that there must be a variation of amplitude at each 32 cycle bit cell in order for the receiver to stay synchronized with the tag identification frame. Variations of the frequencies, cell size and bit transitions will of course be well within the abilities of the person of skill in the art, and may be modified as a matter of design choice and still considered within the scope of the present invention. 
     Referring once again to FIG. 2, a demodulator  28  receives the modulated tag identification signal  400  from antenna  12  and demodulates it, thereby converting it into a signal useable by control unit  40 . Demodulator  28  contains a digitizer (not shown) of a type known in the art which digitizes the tag identification signal and passes it to communication interface  22  for further transmission to control unit  40 . 
     Antenna interface unit  20  further contains a crossover detection circuit  26  that is controllable by control unit  40 . When activated, the crossover detector  26  permits the detection of the voltage signal present on antenna  12  when antenna  12  is driven by antenna drive unit  24 . To achieve this, control unit  40  activates a switch (not shown) which causes the output of crossover detector  26  to be sent through communication interface  22  to control unit  40  rather than the output of demodulator  28 . The switch function may be implemented in firmware in communication interface  22 , or via relay arrangement, or other commonly known technique, as a matter of design choice. Therefore, while the antenna is being driven, it is possible to know if a field is actually being produced by the antenna. This feature allows a system operator to detect an open antenna coil or a failure in the antenna drive system. A presently preferred crossover detection circuit schematic is depicted in FIG. 5, and exemplary output waveforms are depicted in FIG.  6 . 
     Referring now to FIG. 6, there are depicted three exemplary waveforms representing, from top to bottom, an excitation drive signal  90  at 134.2 kHz, the corresponding antenna voltage signal  92  generated by said antenna when driven at that frequency, and an antenna crossover signal  96  emitted by said crossover detector  26  when detecting the antenna voltage signal. The set of signals to the left of dashed line  500  shows that crossover detector circuit  26  will output a square wave  96  having leading and trailing edges corresponding to the zero crossing points of the sinusoidal antenna voltage signal  92  present on antenna  12  as it is being driven by antenna drive circuit  24  in response to an applied square wave drive signal  90  at the preferred system design frequency of 134.2 kHz. Of course it will be recognized by the person of skill that detection of the antenna voltage may be implemented in a number of ways, as long as the detector emits a signal corresponding to the signal present on antenna  12  during operation from which amplitude and frequency may be derived. 
     With reference now to FIGS. 1 and 3, FIG. 3 shows a schematic representation of the control unit  40 . Control unit  40  comprises a microprocessor  46  which is preferably a programmable microprocessor containing, in a manner known in the art, a CPU, memory and a programmable logic unit (not shown). Microprocessor  46  may be controlled via keyboard  44  and may output information via visual display  42  which may be, for example, a backlit liquid crystal display (LCD) or other commonly known visual display device such as, for example, a standard LCD screen or CRT. Microprocessor  46  is also capable of driving visual and audible indicators (not shown) in a manner known in the art. 
     Microprocessor  46  is connected to a mother board  48  via a microprocessor controller cable array  49 . Cable  49  may comprise a series of EIA RS 232 serial cables, as well as direct wiring connections between the microprocessor  46  and other system components. Mother board  48  and microprocessor  46  are powered by a power unit  52  which converts standard 120 volt AC power to the various voltage requirements of the various system components in a manner known in the art. A battery backup unit  58  may also be optionally included. 
     Mother board  48  provides a point for termination and distribution of the various communication cables used throughout the system. Thus, as mentioned above, the twisted pair connection between control unit  40  and antenna interface  20 , the control wire leads  19  of remote test tag  14 , and the data communication cable  50  connecting the control unit to computer  60  all route through mother board  40 . As mentioned above, the particular wiring and communication schemes mentioned herein are merely representative of the presently preferred embodiments, it being recognized by the person of skill in the art that the particular data communication methods and control wiring schemes may be adopted in any manner known in the art, provided that the herein described signals necessary to operate the system  10  are present. Thus, for example, data communication cables  50  may be RS 232 type DB25 cables, EIA 485 metallic cables, fiber optic cables, or any combination thereof. Alternatively, a wireless communication methodology, utilizing digital or analog radio signals, infrared signals, etc. may also be deployed, as a matter of design choice. 
     With continued reference to FIGS. 1 and 3, control unit  40  is connected to a general purpose digital computer  60  and communicates therewith for the purpose of receiving control information therefrom and for passing information such as received PIT tag ID&#39;s and diagnostic signals from control unit  40 . If desired, control unit  40  may communicate via additional communication line  110  to, for example, an RS 232 device for communication with a remote maintenance computer of other control mechanism (not shown). The system  10  through control unit  40  may also optionally control programmable logic control (PLC) devices through PLC lead  80  under the control of microprocessor  46  through mother board  48 . The modular nature of the system also permits, if desired, a redundant configuration so that two sets of antenna interface units  20 , control units  40 , antennas  12 , and remote tags  14  may be deployed for a single passage  16 , controlled by one or more computers  60 . 
     Referring once again to FIGS. 2,  5  and  6 , an additional diagnostic capability of the system is depicted and herein described. Specifically, it is possible to remotely test the tuning of antenna  12  utilizing crossover detector  26  under the control of control unit  40  and optionally under the remote control of computer  60 . Referring to FIG. 6, there is depicted to the right of dash line  500  the respective excitation drive signal  90 , analog antenna voltage signal  92  and antenna crossover signal  94  described above. These signals result when the antenna drive unit  24  and crossover detector  26  are operated by control unit  40  through communication interface  22  in the following manner. The excitation drive signal to antenna drive  24  is turned off by control unit  40 , thus the excitation drive signal  90  to the right of dash line  500  drops to zero. During normal operation, when driven by an excitation signal, the antenna is forced to oscillate at the excitation frequency, which in the preferred embodiment is 134.2 kHz as per ISO requirements. However, when the excitation drive signal is turned off, the electromagnetic field present at antenna  12  collapses and the antenna voltage  92  will decay at the natural, self resonant frequency of the antenna, which may be derived using the well-known frequency equation:        f   =     1     2      π        LC                                
     As is known in the art, the natural frequency of an antenna is determined by the inductance of the antenna (L) and the total value of the tuning capacitors (C). When the antenna is properly tuned, the natural frequency of the antenna should be 134.2 kHz, the preferred driving frequency of the system. By analyzing the frequency of the decaying voltage waveform  92 —or of the crossover signal  94 —after the excitation drive signal has been turned off, a system operator can determine the natural frequency of the antenna. Specifically, the frequency of the zero crossing (crossover) signal  94  output by the antenna crossover detector  26  while the antenna voltage is decaying corresponds to the resonant frequency of the antenna. Thus, proper tuning of the antenna  12  may be determined remotely utilizing the above-described antenna tuning test. Such a test may be activated under direct operator control by interaction with keyboard  44  connected to control unit  40  or, alternatively, may be implemented under the control of computer  60 , as a matter of design choice. Through the use of feedback techniques known in the art, it is possible to remotely tune the antenna using this frequency information. 
     Thus, it will be readily recognized that the system in accordance with the present invention offers significant advantages over prior art systems in its ability to perform various functions from control points not local to the antenna sub-systems, which are typically field located and difficult to reach. The above-described diagnostics may be preprogrammed into computer  60  and/or control unit  40  for regular and systematic testing of the entire system without specific user intervention. Further, reprogramming, maintenance and functional changes may also be made from local or remote computer locations. Thus, changing operating parameters and performing preliminary troubleshooting from a central point is an inherent feature of the inventive system. Overall maintenance costs can be greatly reduced, since it will only be necessary to dispatch maintenance personnel to remote locations after verification that the equipment has actually malfunctioned, through the diagnostic tests described above. 
     Thus, while there have been shown and described and pointed out fundamental novel features of the invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the disclosed invention may be made by those skilled in the art without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.