Abstract:
The present invention is directed to system and method for determining a fluid level. The system comprises a sensing rod assembly comprised of releasably connectable rod segments. Each rod segment carries a processor and a plurality of inductors. Each inductor generates a position signal responsive to a float signal. A float assembly comprises a housing containing a circuit having an integrally formed coil periodically energizable by a processor to emit the float signal. A control assembly receives the generated position signals and determines the fluid level. The method comprises providing releasably connectable rod segments with inductors that generate a position signal responsive to a float signal, determining a sensing rod assembly length, forming the sensing rod assembly, disposing it within a container, providing a float assembly periodically actuated to generate a float signal, generating position signals responsive to the float signal and analyzing those signals to determine the fluid level.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of provisional Application No. 60/720,132, filed Sep. 23, 2005, the disclosure of which is hereby incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
       [0002]     Not applicable.  
       BACKGROUND OF THE INVENTION  
       [0003]     A number of fluid level sensing devices currently are available for determining the level of a fluid in a container. For example, in the oil and gas industry systems exist for measuring the levels of oil and water in a tank. One type of system, for example, utilizes ultrasonic technology. For this type of system, sound waves are directed from a remote location onto the surface of the fluid whose level is to be measured. These sound waves are reflected back and the time delay between the transmission of the initial waves and the sensed reflected waves indicates the fluid level of the fluid in the tank. Ultrasonic technology has proven inaccurate, however, as it is difficult to accurately predict the tank environment. Factors such as humidity, temperature fluctuations, pressure fluctuations, etc. make ultrasonic technology ill-suited for fluid level measurement.  
         [0004]     A number of systems also have been developed using Reed switch technology. Generally, these devices include a rod assembly having a plurality of discrete Reed switches located along its length. A permanent magnet is provided, generally in the form of a float assembly, that floats on the surface of the fluid whose level is desired to be measured. As the float rises and falls with the fluid level, the magnetic field generated by the permanent magnet causes the Reed switches to close. The state of the Reed switches, being in either an open or closed configuration, indicates the fluid level. See, for example, U.S. Pat. Nos. 3,976,963; 4,730,491; 4,976,146; and 6,571,626. U.S. Pat. Nos. 4,589,282; 5,793,200 and 6,563,306 disclose similar systems but incorporate Hall effect sensors rather than Reed switches.  
         [0005]     Published PCT application WO 97/13122 discloses a sensor system utilizing a rod assembly having a plurality of coils wound along its length. A float assembly having a combination resonator coil and capacitor or ferromagnetic coil interacts with the rod assembly coils to provide position location information from which the fluid level can be determined.  
         [0006]     Systems such as those described above often include an integrally formed rod assembly that may be 20 to 30 feet in length. Transportation and installation of large numbers of rod assemblies having those lengths is difficult and expensive. The total cost of these systems also is affected by the implementation of Reed switches, which are a relatively expensive component. For Reed switch systems, the accuracy of the system is dependant on the number of switches spaced apart along the rod assembly. As such, the more accurate the system, the more costly it becomes.  
         [0007]     In the oil and gas industry, a customer may have thousands of wells for which the fluid levels of multiple fluids are desired to be measured. For example, a typical tank will contain both oil and water, and a customer may desire to know the respective levels of both fluids. Water has a specific gravity of 1, while the specific gravity of crude oil, e.g., from California, Mexico, or Texas, ranges from between about 0.8 to about 0.9 at 60° F. The relative closeness of the specific gravities of oil and water makes simultaneous measurement of these fluid levels difficult.  
         [0008]     Because of the number of wells and the relatively harsh conditions in which the wells are located, an advantageous fluid level indicator system must be easily shipped and installed, have a long lifespan, and require little maintenance. Such a system also must be readily able to accurately determine the fluid levels of multiple fluids contained with a tank. Generated well data must be easily accessible by the customer.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     Disclosed herein is an improved linear position indicator system and method for accurately measuring one or more fluid levels in a container. One feature of the linear position indicator system is a sensing rod assembly formed from connectable rod segments. With this approach, disassembled rod segments may be easily and inexpensively shipped to the field where they may be assembled into a rod assembly of any given length.  
         [0010]     Another feature of the invention is a unique float assembly that floats on the surface of the fluid whose fluid level is to be measured. The float assembly is designed to periodically generate a low amplitude float signal corresponding to the position of the float assembly. Specifically, the float assembly includes a circuit having an integrally formed coil which is energizable, for example, at 3 second intervals, to generate the float signal. A processor is provided within the float assembly to control the generation of the float signals. Sensitive inductors are provided along each rod segment of the sensing rod assembly for detecting the low amplitude float signal and generating a position signal responsive thereto. In a container having a second fluid whose level is to be measured, a second float assembly is provided, which floats on the surface of the second fluid. The second float assembly is configured in the same fashion as the first float assembly but periodically transmits its float signal at a different interval, for example, 3.25 seconds. A plurality of floats could be used in this fashion to detect fluid of varying densities  
         [0011]     A control assembly associated with the sensing rod assembly contains a processor programmed to receive the position signals generated by the inductors and analyzes the signals to determine the level of the fluid within the container. The processor may be programmed for a simple data analysis or more complex data analysis may be implemented to improve the accuracy of the system.  
         [0012]     Also broadly disclosed herein is are methods for determining the fluid level of a fluid contained within a container. One embodiment of the method comprises the steps of: 
        (a) providing a plurality of releasably connectable rod segments, each rod segment extending a given rod segment length and having a plurality of inductors located along said length, each said inductor being responsive to a float signal to generate a position signal;     (d) determining a sensing rod assembly length based on the depth of said container;     (e) forming a sensing rod assembly having said rod assembly length by connecting a plurality of rod segments;     (f) disposing said sensing rod assembly generally vertically within said container;     (g) providing a float assembly that floats on said fluid surface and is vertically movable along said sensing rod assembly, said float assembly comprising a circuit having an integrally formed coil excitable to generate a float signal corresponding to the position of said float assembly;     (g) periodically exciting said float assembly coil to generate said float signal;     (h) sensing said float signal with said inductors to generate a plurality of position signals; and     (i) analyzing said sensed position signals to determine said fluid level of said fluid in said container.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     For a fuller understanding of the nature and advantages of the present invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:  
         [0022]      FIG. 1  shows a perspective view of a linear position indicator system having a pair of float assemblies, the system being disposed within a tank containing two fluids whose fluid levels are to be measured;  
         [0023]      FIG. 2  shows two connected rod segments being inserted within a fiberglass tube;  
         [0024]      FIG. 3  illustrates a portion of a sensing rod assembly embodiment including six rod segments releasably connected together;  
         [0025]      FIG. 4  is an exploded view showing the connector components on two adjacent rod segments;  
         [0026]      FIG. 5  is a top view of a rod segment illustrating a plurality of spaced apart RFID transponder coils, a microprocessor, rod segment connector components, and a programming component;  
         [0027]      FIG. 6  is a top view of a bottommost rod segment;  
         [0028]      FIG. 7  is a cross-sectional view taken through the plane  7 - 7  in  FIG. 1  and showing in greater detail a portion of the sensing rod assembly and second float assembly;  
         [0029]      FIG. 8  is a perspective top view of the encapsulate of the second float assembly which includes a circuit, battery, and spacer material;  
         [0030]      FIG. 9  is a perspective bottom view of the encapsulate shown in  FIG. 8 ;  
         [0031]      FIG. 10  is a schematic circuit design of one embodiment of a float assembly;  
         [0032]      FIG. 11  is a schematic circuit design for one embodiment of a rod assembly incorporating RFID transponder coil components;  
         [0033]      FIG. 12  is a schematic circuit design of another embodiment of a rod assembly incorporating circuits having integrally formed coil components; and  
         [0034]      FIG. 13  is a block schematic diagram of one embodiment of a control assembly. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0035]      FIG. 1  illustrates a linear position indicator system shown generally at  10 . System  10  is seen to be disposed within an oil storage tank,  12 , whose outer dimensions are depicted in dashed lines. Tank  12  extends a given depth between a top portion,  14 , and a bottom portion,  16 . Contained within tank  12  are two fluids of differing specific gravities whose fluid levels are desired to be measured. The fluids shown in this example are crude oil and water. The fluid level of the first fluid, oil, is indicated in dashed lines at  18 , while the fluid level of the second fluid, water, is indicated by dashed lines  20 . As was noted above, crude oil has a lower specific gravity than water, so the crude oil present in tank  12  rests upon the water and, thus, has a volume extending between fluid level  18  and fluid level  20  as represented generally at  22 . The water contained within tank  12 , represented generally at  24 , has a volume between fluid level  20  and bottom  16  of tank  12 .  
         [0036]      FIG. 1  shows system  10  disposed in an oil storage tank; however, the system may be disposed within any container designed to contain one or more fluids, with the construction of the container being appropriate for the fluid contained therein. Additionally, the inclusion of only two fluids within tank  12  is intended for explanatory purposes only. Tank  12  may contain additional fluids of differing specific gravities whose fluid levels are desired to be measured.  
         [0037]     Linear position indicator system  10  is seen to include a sensing rod assembly,  26 , which extends substantially vertically within tank  12 . Sensing rod assembly  26  includes a sensing rod (not shown) enclosed within fiberglass tubing,  27 . Tubing  27  extends generally between top  14  and bottom  16  of tank  12  and protects sensing rod assembly  26  from fluids and contaminants contained within tank  12 . Positioned about fiber glass tubing  27  are a pair of float assemblies,  28  and  30 . Float assembly  28  is seen to float on the surface of crude oil  22  at fluid level  18  and includes a signal generating encapsulate,  32 , to which is attached an amount of buoyant material provided in this example as four disks of closed cell foam. Such foam is commercially available from Rogers Corporation of Rogers, Conn. under the trade name NITROPHYL. Float assembly  30  is seen to float on the surface of water  24  at fluid level  20  and includes a signal generating encapsulate,  42 , of the same size, shape and density as encapsulate  32 , and a pair of disks,  44  and  46 . Foam disks  44  and  46  are formed of the same closed cell foam as disks  34 - 40 . Because the specific gravity of water is greater than that of crude oil, only two disks are required to enable encapsulate  30  to float on the surface of water  24 . For a given float assembly, the amount of buoyant material required to enable the float assembly to float on the surface of a given fluid is determined based on the specific gravity of the selected fluid and the density of the encapsulate.  
         [0038]     At one end of tubing  27  is an end cap,  48 , which weights the rod and protects the sensing rod assembly from being damaged by contacting the bottom surface of tank  12 . At the opposite end of tubing  27  is a connector,  50 , which connects sensing rod assembly  26  to a control assembly  60 . As will be described in greater detail below and specifically in connection with  FIG. 13 , control assembly  60  controls the operation of system  10 . In operation, each of the float assemblies  28  and  30  periodically generates a float signal. When a float signal from either of the float assemblies is sensed by sensing rod assembly  26 , sensing rod assembly  26  generates a position signal responsive thereto. Those signals are transmitted to control assembly  60 , which processes the received position signals to accurately determine the position of each of the floats, and correspondingly, the fluid levels of the fluids on which the fluid assemblies float. Control assembly  60  is seen to include a housing,  62 , having an antenna,  64 , for wirelessly transmitting data from tank  12  to another location. The other location may be, for example, a customer&#39;s central location. With such wireless capability, data from a multitude of tanks may be transmitted and monitored at the central location. A solar panel,  66 , with associated charge storage device, is provided as a power supply electrically associated with control assembly  60 . Such a solar panel is advantageous for containers, such as crude oil tanks, which are located outdoors in areas with sufficient amounts of sunlight. Alternatively, control assembly  60  may be powered by any other conventional power supply, such as a battery, or unconventional power supply such as thermal scavenging.  
         [0039]     One of the advantages of system  10  is that rod assembly  26 , including tubing  27  and the sensing rod contained therein, are segmented to enable a custom system of any desired length to be constructed in the field. For example, tubing  27 , may be formed of segments of a given length which are connected together by threads such as those shown at  68  in  FIG. 1 . Turning to  FIG. 2 , the sensing rod assembly,  70 , of system  10  is seen being inserted into tubing  27 , which would occur during installation of system  10  into tank  12 . Components previously identified in  FIG. 1  retain their prior numeration. Sensing rod assembly,  70 , is formed of a plurality of sensing rods, two of which are seen generally in  FIG. 2  at  72  and  74 . Looking at  FIGS. 2 and 5 , it may be seen that rod segment  72  is formed as a printed circuit board (PCB),  76 , extending a given length between a top portion  78  and a bottom portion,  80 . In this embodiment, each rod segment has a length of about 12 inches. This length is intended to be illustrative, only, as the sensing rod assembly may be formed from a plurality of rod segments of any given length. A rod segment having a length of  19  inches is discussed later in connection with  FIG. 12 . Longer or shorter rod segments may be desired in view of cost considerations and the customer&#39;s specific needs.  
         [0040]     PCB  76  has  8  inductors,  82 ,  84 ,  86 ,  88 ,  90 ,  92 ,  94 , and  96 , spaced apart at, for example, 1.5 inch intervals along its length. These inductors may be wirewound, surface mounted Radio Frequency Identification (RFID) transponder coils (ferrite core inductors), which exhibit relatively high sensitivity making them advantageous for sensing low amplitude signals. Such RFID coils, having a part number of 5315TC, are commercially available from Coilcraft, Inc. of Cary, Ill. The read distance for these coils is about 16 to 30 inches, and they are impact resistant and capable of operating at high temperatures. PCB  76  also bears a microprocessor,  98 , located between RFID transponder coils  88  and  90 .  
         [0041]     RFID transponder coils  82  to  96  sense the float signals generated by the float assemblies  28  and  30  ( FIG. 1 ) and each coil will generate a position signal responsive thereto. For example, when a float assembly is located adjacent a particular coil, that coil will generate a position signal having the maximum amplitude that the coil can generate. The greater the distance between the float assembly and a specific coil, the smaller the magnitude of the position signal generated by that coil. When the distance between the float assembly and a coil exceeds the signal sensitivity of the coil, the coil will not detect the signal. Starting at the bottommost PCB of the sensing rod, the signals for all the RFID coils are transmitted from one PCB to the next PCB. The resulting array of position signal data then is transmitted to control system  60 . A programming connector is provided at  100 , which enables a user to easily update the programming of microprocessor  98 .  
         [0042]     Generally, the fluid levels desired to be measured will range over the entire depth of the container in which linear position indicator system  10  is disposed. As such, the required length of the sensing rod assembly  70  will be determined by the depth of the container. The proper number of rod segments then may be connected together to form a sensing rod assembly commensurate in length to that depth. Clearly, the linear position indicator system may be configured having a sensing rod assembly having a length less than the depth of the container if desired.  FIG. 3  illustrates an assembled sensing rod assembly,  108 , having six rod segments,  110 ,  112 ,  114 ,  116 ,  118 , and  120  connected together to give the sensing rod assembly a total length of about 6 feet. This figure also reveals that each rod segment,  110  to  120 , inclusive, includes  8  RFID transponder coils. The number, type and position of inductors located along each rod segment may be selected to adapt the sensing rod assembly to a given customer&#39;s specific needs. For example, as the skilled artisan will appreciate, additional inductors may easily be added to increase the accuracy of the resulting system but such addition may concomitantly represent additional expense.  
         [0043]     Looking to  FIG. 4 , an exploded view of bottom portion  80  illustrates the releasable connection of rod segments  72  and  74  shown previously in  FIG. 2 . Rod segment  72  again includes PCB  76  and RFID transponder coil  96 . Rod segment  74  is constructed in the same manner as rod segment  72  and is seen to include PCB  126  and RFID transponder coil  128 . The two rod segments are positioned opposite one another so that the top surface,  130 , of PCB  76  faces top surface  132  of PCB  126 . Bottom end  80  of rod segment  72  includes a male connector component  134  having two rows of pins, the end pins of which are shown at  136  and  138 . Male connector component  134  is mounted to a pair of through holes in PCB  76  as indicated at  140  and  142 . The top portion,  144 , of PCB  126  includes a corresponding female connector component  146  mounted to PCB  126  by through holes as at  148  and  150 . Male connector component  134  and female connector component  146  are configured to releasable secure rod segments  72  and  74  together. Every rod segment has the same configuration, with a male connector component at one end and a female connector component at the other. To form a rod assembly, the rod segments are sequentially connected together as shown in  FIG. 4  to form a completed sensing rod assembly such as that shown in  FIG. 3 . Other conventional or customized connector components may be used as convenient or desired to releasably secure the rod segments together. It should be noted that connector components, such as those shown, provide a connection resistant to separation when acted on by vertical forces, i.e., as may be applied when a sensing rod is withdrawn from a container.  
         [0044]      FIG. 5  shows a top view of a standard rod segment,  160 , which is seen to include a PCB,  161 , having a female connector component,  162 , at one end and a male connector component,  164 , at the opposite end. The top surface,  166 , of PCB  161  again includes  8  RFID transponder coils,  168  through  182 , a microprocessor,  184 , and a programming connector,  186 . Once the rod segments are connected together to form a rod assembly, that assembly is inserted within fiberglass tubing, as described previously at  27  in  FIG. 1 . That fiberglass tubing  27  is sealed to protect the sensing rod assembly from exposure to fluids, contaminants, etc. Such sealing is particularly important when a fluid contained in the container is explosive, as in crude oil tanks. To provide additional protection, rod segment  160  is seen to be coated with a potting compound,  188 , such as an acrylic, polyurethane, epoxy, etc.  
         [0045]      FIG. 6  reveals a rod segment,  190 , that is specifically designed to be used as the bottommost rod segment of a sensing rod assembly. Rod segment  190  is seen to include PCB,  192 , RFID transponder coils,  194  through  208 , a microprocessor,  210 , and a programming connector,  212 . At top portion  218 , rod segment  190  includes a male connector component,  216 ; however, at the opposite end,  220 , rod segment  190  includes two arrays of through holes, as at  222  and  224 , but a female connector component is not provided. This connector is not necessary as rod segment  190  is the bottommost rod segment and another rod segment will not be connected to it. Like rod segment  160 , rod segment  190  also is seen to be encased within potting compound,  214 . It should be noted that for this bottommost rod segment, potting compound  214  encases the entire bottom portion  220 . Rod segment  190  also may include a float switch or conductivity sensor (not shown) to detect the ingress of fluids from container  12  if leakage occurs, in order to permit servicing before the fluid reaches the electronic components. Rod segment  190  also includes an additional component, namely, a jumper component,  226 . Jumper component  226  enables communications from control assembly  60  to loop around and passed up the chain of connected rod segments back to control assembly  60 .  
         [0046]     Each rod segment communicates in one direction using an asynchronous serial protocol at 9600 bps. Signals are received at the bottom end of the rod segment and transmitted at the top. In a sensing rod, the signal from control assembly  60  is routed all the way down the sensing rod without amplification to the receiver of the bottom card. Communication then is looped by jumper  226 , as noted above, daisy chained though all rod segments and emerges at the top to return to control assembly  60 .  
         [0047]     Each rod segment powers down, or “sleeps,” as much as possible to conserve energy. It must therefore wake on the leading edge transition of a received communication byte. Thus, typically the rod segment microcontroller wakes on the first edge of a command byte and then stays awake to be interrupted again at the end of this byte. This byte defines the command and begins a command sequence and is followed by a zero byte from the host. The rod segment passes the command byte as quickly as possible to minimize command latency along the set of rod segments. It also starts the command timer from the receipt of this command byte. It then increments the next byte and passes this on also, so this becomes a count of the number of rod segments from the bottom, or the total number of rod segments by the time it returns to control assembly  60 .  
         [0048]     Following the two command bytes, the rod segment reschedules upcoming reading and search activities to avoid conflicts and waits a length of time to ensure any active reading is completed. It then executes the command (e.g., gets reading values, does temperature reading, or self-test) and prepares the result for transmission. Any received data in this time from a lower rod segment is buffered in a circular buffer to ensure that the data is sent in the proper sequence. At the appropriate time, the results and buffered data are transmitted.  
         [0049]     The rod segments are powered by a nominal 3.3 v supply from control assembly  60 . This supply is a current limited, intrinsically safe deign, and the current limiting resistor is on each individual rod segment. Local current limiting saves needing one current limit large enough to serve all rod segments at once, as this current limit might be too large for the required safety. The greatest current consumption occurs when several rod segments are transmitting and receiving a data set up through the sensing rod chain. In order to constrain this peak current, the time of transmission is scheduled so that not all rod segments can transmit at the same time. For example, 5 rod segments may be permitted to transmit simultaneously, and this achieved by buffering the communication at each node and scheduling the start of transmission later in each successive node through the chain. As a result the lowest rod segments complete transmission first, before rod segments further along the chain are active. The result is a wave of data transfer and power consumption traveling along the length of the sensing rod, and a more constant current consumption during this time from control assembly  60 . Thus, a select number of rod segments are active at any time, and the peak current is much lower than if all data was transferred immediately by all rod segments at the same time.  
         [0050]     Each rod segment&#39;s microcontroller uses two clock sources, one 32 kHz clock used for a timing reference, and a higher frequency internal clock used as a processor core clock when the processor is active. The latter clock frequency defines some time critical functions such as the serial data rate, the ADC sample timing, and the self test cycles. It is important, therefore, that this clock frequency is fairly accurate and stable. Accuracy is derived by regularly measuring the high frequency clock against the 32 kHz clock and adjusting the high frequency clock to ensure the correct ratio. The 32 kHz clock is used directly as a timer clock for defining the search period and float timing period, etc.  
         [0051]      FIGS. 7-9  illustrate the float assembly of linear position indicator system  10  in greater detail.  FIG. 7  is a cross-sectional view of float assembly  30  taken through the plane  7 - 7  in  FIG. 1 . Components previously identified in  FIG. 1  retain their prior numeration. From this figure it may be seen that float assembly  30  has a generally doughnut shape with a centrally disposed aperture which enables float assembly  30  to encompass sensing rod assembly  26  and move freely along the vertical length of sensing rod assembly  26 . As noted previously, sensing rod assembly  26  includes fiberglass tubing  27  within which is inserted a sensing rod including a plurality of rod segments, such as  240 . The portion of rod segment  240  shown includes PCB  242  and RFID transponder coils  244  and  246 . Float assembly  30  includes signal generating encapsulate  42 , foam disks  44  and  46 , and a cover,  47 .  
         [0052]     Turning to  FIG. 8 , the internal components of encapsulate  42  are shown. Encapsulate  42  includes a PCB  260  one surface,  261 , of which bears a pair of styrophome spacers,  262  and  264 , a power supply,  266 , and a float assembly control circuit,  268 . In this embodiment, power supply  266  is provided as a 3 Volt lithium non-rechargeable battery. Other power supplies may be used. In selecting the power supply, its voltage, shelf-life and weight must all be considered. Control circuit  268  comprises a microprocessor that is connected to battery  266 . Looking back to  FIG. 7 , it may be seen that the components shown in  FIG. 8  are encapsulated in a potting compound,  270 . To form the completed float assembly  30 , foam disks  44  and  46  and cover  47  are secured to three threaded components,  272 ,  274 , and  276 , on PCB  260  ( FIG. 9 ) of encapsulate  42  by three screws, one of which is shown in  FIG. 7 . Screw,  248 , extends through a bore,  250 , in cover  47  and disks  44  and  46  and connects to threaded component  272 . The additional two screws extend through similar bores to connect to the remaining threaded components,  274  and  276 .  
         [0053]     Turning to  FIG. 9 , it may be seen that in addition to the three threaded components, surface,  263 , of PCB  260  bears a multi-layer coil,  278 , formed as a trace, which is connected to control circuit  268 . In this embodiment, coil  278  consists of 5 layers, each layer including  21  turns. A small gauge wire coil may be used in place of the integrally formed PCB coil; however, using a coil as at  278  advantageously reduces the size and weight of encapsulate  42 . Rather than continuously transmitting a signal from encapsulate  42 , control circuit  268  drives coil  278  to periodically generate an approximately 85 kHz square wave. That signal, for example, may be generated about every 3 seconds for a duration of 30 to 40 milliseconds. The signal level is approximately 3V peak to peak. Because encapsulate  42  only periodically generates a low amplitude signal, float assembly  30  exhibits a relatively long lifespan. This feature is particularly advantageous when the linear position indicator system is difficult to access once installed in the container. For the configuration shown, float assembly  30  may have a lifespan of about 5 years with the limiting factor being the shelf-life of the 3 Volt lithium battery. Note that the 3 second drive interval may be changed to meet the specific application requirement, e.g., shorter for quicker response, longer for increased battery life.  
         [0054]     If more than one fluid level is desired to be measured, then additional float assemblies may be used. Any number of additional floats may easily be added to the linear position indicator system  10 . For example,  FIG. 1  illustrates another float assembly  28  measuring the fluid level  18 . When additional float assemblies are added, the timing of the float signals generated are controlled to minimize the overlap of signal sensing by the inductors. For example, circuit  268  of float assembly  30  has been described as energizing coil  278  to generate a float signal about every 3 seconds. The control circuit of a second float assembly may be programmed to energize its coil, for example, every 3.25 seconds. Based on the different timings of the float signals, a rod segment can identify which signals are generated a given float assembly. Alternatively, signals from different float assemblies may be identified, for example, by RF frequency or other coding of the float signal. The timing of float signal generation and sensing is discussed further below in connection with the rod segment embodiments illustrated in  FIGS. 11 and 12 .  
         [0055]      FIG. 10  is a schematic circuit design for one embodiment of a float assembly, such as float assembly  30  in  FIG. 1  and  FIGS. 7-9 . At the far left of  FIG. 10 , the 3.0 V lithium battery,  290 , is shown connected to ground via line  292 . The other side of battery  290  is connected to the voltage input of microprocessor  298  through decoupling capacitor  296 . Power start-up resistor  300  and capacitor  302  are connected to the reset pin of microprocessor  298  via line  304  to pull the reset line high. Microprocessor  298  is connected to ground via line  306 . Microprocessor  298  may be an ultralow-power microprocessor with flash memory available from Texas Instruments of Dallas, Tex. as part number MSP430F1232IPW. The processors disclosed herein generally consist of conventional mixed signal microprocessors; however, as the skilled artisan will appreciate any conventional digital, analog or mixed signal microcontroller may be implemented.  
         [0056]     A 32.768 KHz crystal,  308 , is connected to microprocessor  298  via line  310 . Output from microprocessor  298  to crystal  308  is via line  312 . A JTAG programming connector,  314 , is connected to ground via line  316  and connected to microprocessor  298  via lines  318 ,  320 ,  322 , and  324 . JTAG programming connector  314  enables the programming of flash memory within microprocessor  298 . Connector  314  also is connector to the test pin of microprocessor  298  via line  326  and the reset pin via line  328 . A connector, such as  314 , is commercially available from Samtec, Inc. of New Albany, Ind. as part number TSM-104-01-T-DV. On the right hand side of the  FIG. 10  is a graphical representation of the multi-layer coil described at  278  in connection with  FIG. 9 . That coil,  340  in  FIG. 10 , is seen to include  5  layers as at  342 ,  344 ,  346 ,  348 , and  350 . Coil  340  is connected to microprocessor  298  via lines  352  and  354  including d.c. blocking capacitor  353 .  
         [0057]      FIG. 11  illustrates a schematic circuit design for one embodiment of a rod segment, such as that shown in  FIG. 6 . Across the top of  FIG. 11  is a network of RFID transponder coils connected in series. Each RFID transponder coil includes an inductor in series with a capacitor and a resistor, the resonating capacitor and resistor defining the quality factor of each inductor. For simplicity, the components of each RFID transponder coil have been identified as L, R and C to indicate an inductor, resistor and capacitor, respectively. RFID transponder coils are commercially available, for example, from Coilcraft, Inc. of Cary, Ill. as part number of 5315TC. Each RFID coil transmits an analog signal to ultralow-power microprocessor,  370 , which may be of the same type as microprocessor  298  in  FIG. 10 . The first RFID transponder coil, consisting of L 1 , C 1  and R 1 , connects to microprocessor  370  via line  372 . The second RFID transponder coil connects to microprocessor  370  via line  374 . The remaining RFID transponder coils connect to microprocessor  370  via lines  376 ,  378 ,  380 ,  382 ,  384 , and  386  respectively. At the end of the network of RFID transponder coils is a pair of capacitors,  390  and  392 , which connect to a.c. signal ground. Between R 5  and I 6  are a pair of resistors  394  and  396 , act as a voltage divider which biases the RFID coils at V cc  divided by 2.  
         [0058]     In the center on the left hand side of  FIG. 11  is a 32.768 KHz crystal,  410 , such as that shown at  308  in  FIG. 10 . Crystal  410  is connected to microprocessor  370  via lines  412  and  414 . Microprocessor  370  is connected to ground via line  416 . To the right of microprocessor  370  is a JTAG programming connector,  422 , connected via lines  424 ,  426 ,  428 , and  430 . Connector  422  is connected to ground via line  432 . Connector  422  also is connected to the microprocessor test pin via line  434  and the reset pin via line  436 . Connector  422  is commercially available, for example, from Molex of Lisle, Ill. as part number 10-89-4142. Across the bottom of  FIG. 11  are a pair of connector components,  444  and  446 . These connector components correspond to the female and male connector components located on opposite ends of a rod segment as shown in  FIG. 5  at  164  and  162 . Female connector component,  444 , is commercially available, for example from Samtec, Inc of Cary, Ill. as part number SDL-105-G-10, while male connector component  446  also is commercially available, for example, from Samtec, Inc. as part number BDL-105-G-E. Connector components  444  and  446  are connected to one another via line  448  and to the reset pin of microprocessor  370  via line  450 . A power start up resistor is provided at  452  with capacitor  453 . Both pins  1  and  2  of female connector component  444  are seen to connect to corresponding pins  1  and  2  of male connector component  446  via line  448 . This redundancy is provided to ensure reliability. Likewise, pins  3  and  9  of female connector component  444  are seen to connect to corresponding pins  3  and  9  of male connector component  446  via line  454 . Pins  4  and  10  of female connector component receive transmissions from microprocessor  370  via line  456 , while male connector component pins  4  and  10  transmit to microprocessor  370  via line  458 . Pins  5  and  6  of female connector component  444  connect to corresponding pins  5  and  6  of male connector component  446  via line  460  and then to power via line  462  through resistor  464 . Pins  7  and  8  of female connector component  444  connect to corresponding pins  7  and  8  of male connector component  446  via line  466  and then to ground via lines  467  and  468 . These connectors provide the communications line to transmit data from the RFID transponder network on each rod segment along adjacent rod segments to control assembly  60  as described previously.  
         [0059]     A two connection female jumper component,  470 , extends across the two pin header of male connector component  446 , which are connected to the TX and RX pins of microprocessor  370 . Such a component is available from Molex of Lisle, Ill. as part number 22-12-2021. This component is required only on the bottommost rod segment of a sensing rod assembly and creates a communications loop, allowing communications to pass down through the connected rod segments from control assembly  60  and back around and up through the connected rod segments to control assembly  60 . Thus, for example, control assembly  60  may generate a signal prompting the RFID transponder coils to sense a float signal. Sensed data from the RFID transponder coils then is passed up from the rod segments back to control assembly  60 .  
         [0060]     In order to identify faulty rod segments, a self-test is provided. Each inductor, L 1  to L 8 , connects to microcontroller  370  on a pin that may be defined as an I/O or as an ADC input. For self-test, the output is driven with a cycle of signal approximately at the inductor resonance rate, the pin is then immediately switched to the ADC measurement mode and a quick sequence of measurement samples is taken. The amplitude of the signal at each of these samples is determined by the resonance frequency, the ring-down rate (related to Q) and the sample times, which are know so long as the clock is stable. A normal range may be developed based on the measured values for operational rod segments. Giving bounds around these values, unexpected inductor resonance behavior indicates a fault.  
         [0061]      FIG. 12  shows an alternative schematic design for a rod  19  inch rod segment having a plurality of inductors integrally formed onto a printed circuit board, in similar fashion as described with respect to coil  278  of float assembly  30 . These coils perform the function of the previously recited RFID transponder coil network shown and described in connection with  FIG. 11 . Looking to the top of  FIG. 12 , an inductor network is seen to include 16 inductors, identified as L 1  through L 16 , inclusive. The voltage generated by each inductor in the presence of an alternating field, e.g., that generated by a float assembly, will be proportional to the number of turns and the area enclosed by one turn of the coil. Each inductor, L 1  to L 16 , encloses an area of 1.5 mm by 10 mm and has 48 turns. Inductors L 1  through L 8  are connected via lines  482  to  496 , respectively, to a first multiplexer,  498 , which is grounded via line  500 . Inductors L 9  through L 16  are connected via lines  512  to  526  to a second multiplexer,  528 , which is at ground as shown at line  530 . Multiplexers  498  and  528  are commercially available, for example, from Maxim Integrated Products, Inc. of Sunnyvale, Calif. as part number MAX4581EUE. Each inductor is paired with a resistor, the resistors being identified as R 1  through R 16 . These resistors are provided for self testing. An a.c. stimulus may provided to these resistors and the resulting signal measured to test inductors L 1  through L 16  and their receive paths. Inductors L 1  through L 16  also are grounded.  
         [0062]     Note that in the  FIG. 11 , no amplification of the RFID transponder coil signals was necessary as the signals generally will be on the order of hundreds of millivolts. The signal generated by inductors L 1  through L 16 , however, will generally be on the order of 1 to 2 millivolts. Because of the weakness of the signals, an amplification circuit,  534 , has been provided which is seen connected to the inductor network through multiplexers,  498  and  528 , via lines  536  and  538 . Between amplification circuit  534  and multiplexer  498  is capacitor  540  connected in parallel with resistor  544 . A pair of capacitors  542  and  546  lead into the inverting input of operational amplifier  560  which is grounded at  562 . Capacitor  548 , at ground as indicated at line  564 , is connected to the non-inverting input of amplifier  560 . This gain stage amplifies the signal to a magnitude of 24 times its initial amplitude. The output of amplifier  560  leads into another gain stage including capacitor  566  and resistor  569 , which is at ground as indicated at  570 , connected to the inverting input of operational amplifier  578 . The non-inverting input of operational amplifier  578  is connected to a capacitor  572  and a resistor  574  at ground as shown at  576 . The output of amplifier  578  is connected to a resistor  580  and capacitor  584  connected in parallel. This is another 24 gain amplification stage. To the left of the amplification stages is a peak detector with a low pass filter including a Schottky diode at  586  connected through resistor  588  to ground at  590 . Diode  586  also is connected to capacitor  592  to ground at  594  and to resistor  596  and capacitor  598  then to ground as at  600 .  
         [0063]     Multiplexers  498  and  528  also are connected via lines  606  and  608  to an inverter,  610 , which is grounded via line  612 . Inverter  610  is connected via line  613  to a microprocessor,  614 , which is connected to ground via line  618  and to Vcc via line  618 , through capacitor  620  to ground via line  622 . Microprocessor  614  may be a 16 bit ultralow-power microprocessor with flash memory available from Texas Instruments of Dallas, Tex. as part number MSP430F1232IPW. Selectors A, B, and C of multiplexer  498  are connected to microprocessor  614  via lines,  626 ,  628 , and  630 , respectively. Likewise, selectors A, B, and C of multiplexer  528  are connected to microprocessor  614  via lines  632 ,  634 , and  636 . With multiplexers  498  and  528 , and inverter  610 , microprocessor  614  can select any inductor from the inductor network, including inductors L 1  through L 16 . Power to inductors L 1  through L 8  is supplied by microprocessor  614  via line  638  through capacitor  640 . Similarly, power is supplied to inductors L 9  through L 16  via line  642  through capacitor  644 .  
         [0064]     Microprocessor  614  also is connected to a 32.768 KHz crystal,  656 , via lines  658  and  660 . Two test inputs are identified at  662  and  664  connected to microprocessor  614  via lines  666  and  668 , respectively. Amplification circuit  534  also is connected to microprocessor  614  via lines  670  and  672 .  
         [0065]     Looking to the right of microprocessor  614 , a JTAG programming connector has been provided at  680 . This connector component may be used to download programming to the flash memory contained within microprocessor  614  and is commercially available, for example, from Molex of Lisle, Ill. as part number 10-89-4082. Connector component  680  is connected to ground via line  862  and Vcc power via line  684  through resistor  685 . On the opposite side of the chip, connector  680  is connected to microprocessor  614  via lines  686 ,  688 ,  690 , and  692 . Connector  680  also is connected to the test pin of microprocessor  614  via line  694 .  
         [0066]     Power start-up resistor  661  and capacitor  663  extend into the rest pin of microprocessor  614 . At the bottom of  FIG. 12  are female and male connector components,  702  and  704 , such as those described in connection with  FIG. 11 . Female connector component  702  is commercially available, for example from Samtec, Inc of Cary, Ill. as part number SDL-105-G-10, while male connector component  704  also is commercially available, for example, from Samtec, Inc. as part number BDL-105-G-E. Connector components  702  and  704  are connected to one another via line  706  and to the reset pin of microprocessor  614  via line  708  through resistor  710 . Both pins  1  and  2  of female connector component  702  are seen to connect to corresponding pins  1  and  2  of male connector component  704  via line  706 . As with the design illustrated in  FIG. 11 , this redundancy is provided to ensure reliability. Likewise, pins  3  and  9  of female connector component  702  are seen to connect to corresponding pins  3  and  9  of male connector component  704  via line  712 . Pins  4  and  10  of female connector component  702  receive transmissions from microprocessor  614  via line  714 , while male connector component pins  4  and  10  transmit to microprocessor  614  via line  718 . Pins  5  and  6  of female connector component  702  connect to corresponding pins  5  and  6  of male connector component  704  via line  720  and then to power via line  722  through resistor  724 . Pins  7  and  8  of female connector component  702  connect to corresponding pins  7  and  8  of male connector component  704  via line  726  and then to ground via lines  728  and  730 . Connectors  702  and  704  provide the communications line to transmit data from the inductor network on each rod segment along adjacent rod segments to control assembly  60  as described previously.  
         [0067]     A two connection female jumper component,  732 , extends across the two pin header of male connector component  704 , which are connected to the TX and RX pins of microprocessor  614 . Such a component is available from Molex of Lisle, Ill. as part number 22-12-2021.  
         [0068]     One of the advantages of system  10  is that the float assemblies, as at  28  and  30 , only periodically generate a low amplitude signal. Also, the sensing rod assembly rod segments are designed to power down to additionally conserve power. In view of the design of the float assemblies and sensing rod assembly, the timing of the generated float signals and the rod segment sensing must be coordinated to ensure accurate fluid level measurement.  
         [0069]     If the float signal generated by a float assembly lasts  40  ms, for example, then to ensure that that float signal is detected, a rod segment must search for the float signal at least every 40 ms, and may be set to search, for example, every 38 ms. The microprocessor can only poll one inductor signal at a time, thus it scans through selecting each inductor in turn for a reading. These are read in sequence in quick succession such that the timing of the last reading is not greatly different from the first. Additionally, the embodiment shown in  FIG. 12  does not have a resonant circuit for each inductor and, thus, there must be a small delay between each switching of the multiplexer and a reading. This is such that the resonance of the one match circuit can settle with the selected inductor, and so the detector LPF can settle to the new value, e.g., at least 100 us for search readings, and at least 200 us for measurement readings, described below, where more accuracy is required. Within the microprocessor, one timer compare register is used to define the repetition rate of the search readings.  
         [0070]     When a float signal is detected, for the  FIG. 11  embodiment each measurement comprises 4 samples taken with an interval of three-fourths of the float assembly signal. Of these 4 samples, the first and 3 rd  are differenced, and the 2 nd  and 4 th  are differenced. This removes any d.c. component and increases the reading accuracy compared to using two quadrature samples only. These differenced values are then combined as root-sum-squares to give the a.c. amplitude independent of phase. For the  FIG. 12  design, each measurement is simply a dc level measured by the microprocessor&#39;s analog-to-digital converter after an appropriate settling time.  
         [0071]     When a float signal is detected, the microprocessor schedules a measurement reading for the next expected float burst time, using the second compare register. If neither the first (oil) nor the second (water) float assemblies currently are being tracked then both will be scheduled, but only one will find a signal at the time of the reading. If one float assembly is already being tracked on this processor then the other one is scheduled. If both float assemblies are being tracked then the search detection is ignored. Search readings are suppressed in the vicinity of the measurement readings, and while a serial port command is pending or active to avoid real time conflicts.  
         [0072]     It is inevitable that the float assembly timing will be slightly different from the rod segment timing due to crystal differences and the measurement timing will slowly drift from the optimum mid-point timing. In order to compensate, the measurement algorithm includes a sample reading before or after the actual measurement reading. The sample reading is alternated before or after on each successive reading. If either sample reading is found to be below the expected level then the next reading interval is shifted earlier or later in order to adjust the next reading time towards the center of the burst.  
         [0073]      FIG. 13  illustrates, in block diagrammatic form, the function of control assembly  60 , shown at the top of sensing rod assembly  26  in  FIG. 1 . Control assembly  60  is in electrical association with sensing rod assembly  26  as indicated at block  740 . Signals from block  740  are transmitted to a microprocessor,  746 , via line  742 . Signals from microprocessor  746  are transmitted to block  740  as indicated at line  744 . Microprocessor  746  includes programmable flash memory  750  associated by line  748 . The output of a real time clock, indicated at block  754 , is shown associated with microprocessor  746  via line  752 . Data may be wireless transmitted to and from control assembly  60  to a remote location as indicated by lines  756  and  758  and RF transceiver block  760 . Transceiver block  760  is connected via line  762  to associated amplification block  764  which is connected by line  766  to associated antenna  768 . Control assembly  60  may be solar powered as indicated at block  770 . Block  770  is connected via line  772  to an associated energy source device at block  774 . Energy from block  774  is subject to power conditioning as indicated at line  776  and block  778  and then transmitted to microprocessor  746  as indicated by bi-directional arrow  780 .  
         [0074]     Control assembly  60  controls the timing of the sensing assembly&#39;s position signal generation, receives and analyzes that signal data to determine the fluid level of fluid contained with the container. As noted previously, the array of position signals generated by the inductors, for example, RFID transponder coils, coils integrally formed on a PCB, etc., generally will exhibit a Gaussian profile, with the peak of the curve indicating the position of the float assembly associated with that curve, and, thus, the corresponding fluid level. This simple analysis will accurately determine the fluid level of the fluid to within about 0.75 inch. Additional data analysis may be employed to determine the fluid level with even greater accuracy.  
         [0075]     A non-linear regression analysis may be implemented as follows. The float assembly coil, such as that shown at  276  in  FIG. 9 , is a set of coplanar, concentric 12-sided polygons. For the purposes of this analysis, the float assembly coil will be oriented so that the coils are located in the x-y plane, with the center located at the origin. The inductor coils on the sensing rod assembly are located along the z-axis. Linear distances are normalized to the inductor coil spacing, so that each inductor coil is one “unit” apart. The location of the float assembly coil relative to the central inductor on a given segment is a distance “d”.  
         [0076]     A few approximations are made to simplify the analysis of the resulting data. Specifically, the polygonal shape of the float assembly coil will be approximated with a circular coil. The N concentric turns will be modeled by a single “lumped” circular coil with radius “a”. The current through the “lumped” coil is the sum of the currents through the individual turns. Finally, the inductors will be modeled by the measurement of the magnetic field at a single zero-dimensional point. 
 
 For a circular loop of radius a carrying current I, the magnetic field along the central axis a distance z from the center of the loop is:  
             H   =       Ia   2       2   ⁢       (       a   2     +     z   2       )       3   /   2                   Equation   ⁢           ⁢   1             
 
 The measured voltage on the ADC should follow the following form:  
                     ⁢     ADC   =     K       (       a   2     +     z   2       )       3   /   2                   Equation   ⁢           ⁢   2             
        where K˜Ia 2  
 
 Making the following substitutions:  
       y   =     1     ADC     2   /   3             
       C   =     K     2   /   3           
 
 Equation 2 reduces to:  
         1   /   y     =     C       a   2     +     z   2             
  z   2   +a   2   =Cy    Equation 3 
       
 
         [0078]     For simplicity, assume the peak coil measurement occurs at sample 0 and the coil spacing is 1 “linear unit.” Assume that the coil is actually located at position d. The three primary coil measurements generate the data shown in Table I.  
                                             TABLE I                                   Distance from coil to       Coil   Measurement   Translated   loop                                −1   ADC[−1]   y[−1] = ADC[−1] −2/3     1 + d       0   ADC[0]   y[0]   D       1   ADC[1]   y[1]   1 − d                  
 
 Substitute the measured points into Equation 3: 
 
( d+ 1) 2   +a   2   =Cy[− 1]  Equation 4 
 
 d   2   +a   2   =Cy[ 0]  Equation 5 
 
(1− d ) 2   +a   2   =Cy[ 1]  Equation 6 
 
 In these equations, y[i] are known, and d, a, and C are unknown. The goal is to solve for d as a function of known constants or variables. 
 
 Adding Equation 4 and Equation 6: 
 
2 d   2 +2+2 a   2   =Cy[− 1]+ Cy[ 1]
 
               d   2     =           Cy   ⁡     [     -   1     ]       +     Cy   ⁡     [   1   ]         2     -     a   2     -   1             Equation   ⁢           ⁢   7             
 
 Subtract Equation 5 from Equation 7:  
           Cy   ⁡     [   0   ]       -     a   2       =           Cy   ⁡     [   1   ]       +     Cy   ⁡     [     -   1     ]         2     -   1   -     a   2           
         C   ⁡     (       y   ⁡     [   1   ]       +     y   ⁡     [     -   1     ]       +     2   ⁢     y   ⁡     [   0   ]           )       =   2       
 
             C   =     2       y   ⁡     [     -   1     ]       +     y   ⁡     [   1   ]       -     2   ⁢     y   ⁡     [   0   ]                     Equation   ⁢           ⁢   8             
 
 C is now known. Subtract the Equation 5 from Equation 4: 
 
2 d+ 1= Cy[− 1]− Cy[ 0]
 
             d   =         Cy   ⁡     [     -   1     ]       -     Cy   ⁡     [   0   ]       -   1     2             Equation   ⁢           ⁢   9             
 
 From Equation 5, 
 
 a=√{square root over (Cy[0]−d 2 )}   Equation 10 
 
 Using the above calculations, the control assembly can perform a curve fit that increases the accuracy of the fluid level measurement by a factor of approximately 5 times.