Abstract:
A liquid characterization and level sensor with a coaxial probe attached to a closed loop servo circuitry combined with a DSP and novel algorithms to scan, lock and track signals to ascertain the level and purity of fluid in a container.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     A claim of benefit is made to U.S. Provisional Application Ser. No. 60/859,756 filed Nov. 17, 2006, the contents of which are incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to methods and apparatus to detect liquid levels in containers. 
     BACKGROUND OF THE INVENTION 
     Detecting fluid levels is important for a variety of reasons. Marine and aviation applications require accurate measurements of fuel to ensure sufficient supplies to reach intended destinations. Aviation applications are exceptionally important to monitor the fuel levels in multiple tanks to ensure proper balance of levels to impart the least impact on a plane&#39;s aerodynamics, which can be significantly affected by changes in a plane&#39;s three-dimensional center of gravity. 
     Another important measurement function is to ascertain the presence of, and amount of, any contaminants in fuel to ensure the safe and proper operation of engines operated with the fuel. Entry of contaminants into an operating engine can lead to severe performance problems and even engine failure. A means to constantly monitor the presence and amount of contaminants, particularly water, is an essential component of any fuel measurement system. 
     An accurate, reliable and safe method of measuring the amount of liquid in a container is essential. Applications include fuel tanks containing volatile liquids, although the invention described herein can accommodate a wide range of liquids, regardless of their volatility characteristics. Other parameters that must be ascertained with accuracy and consistency are the type of fuel and the contamination content, if any. A further consideration is a need for hardware that meets the EMI, ESD and Interface requirements of a container, such as an aviation fuel tank, in its environment in a safe manner. 
     Prior radar technology includes methods to scan, lock on and track targets. The basic approach is to transmit a signal that scans targets, perform gating on a receiver and select targets to lock onto and track. Analysis of the received signal can then be used to determine the distance (range) of the target and to perform signature recognition to define the type of target and its characteristics. Combining radar technology with transmission line theory through a shielded cable solves the problems attendant with sensing liquid levels in containers, particularly those used in the aviation field. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, a coaxial probe (tube) is connected to a power source and circuitry designed to provide time domain reflectometric readings to determine the level of, and presence of, impurities in a liquid. Also called Micropower Impulse radar, the system involves sending a very low microwave pulse along a wave-guide, a probe. At least part of the pulse is reflected at the fluid being measured. The transmitted and reflected pulse times are measured and used as the basis for calculating the fluid&#39;s level in a container. 
     In another aspect of the invention, the coaxial probe connected to the circuitry enables the apparatus to meet all aircraft intrinsic safety requirements such as low electromagnetic interference sensitivity. The apparatus works exceptionally well in a very noisy environment while creating very little noise out. By using a coaxial probe, the apparatus is hermetically sealed and requires very low voltage to operate. 
     In a further aspect of the invention, a closed loop configuration is used to scan, track and lock onto transmitted and reflected pulses. The propagation speed of the microwave pulse through a material can be used to ascertain the identity of the material through which the pulse is traveling. The propagation speed is directly related to the material&#39;s dielectric constant. Materials with different dielectric constants will result in different propagation speeds. Thus, the apparatus can be used to measure fluid and/or gas contamination. The transit time of the pulse is used to measure the dielectric constant also known as the material&#39;s relative permittivity. 
     A further aspect of the invention is to employ a digital signal processor (DSP) to substantially reduce signal-to-noise ratio and improve signal clarity with a novel algorithm to scan, lock and track fluid levels in a container. 
     These and other objects will be apparent from a reading of the following summary and detailed description along with a review of the appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of the circuitry according to one embodiment of the invention. 
         FIG. 2  is a block diagram and flowchart of the DSP integrate and limit algorithms according to one embodiment of the invention. 
         FIG. 3  is a block diagram and flowchart of DSP algorithms using a low pass/high pass filter according to another embodiment of the invention. 
         FIG. 4   a  is a flow chart of the calculations made to convert reflected signals into measurements of height, weight and temperature of a monitored fluid according to one embodiment of the invention.  FIG. 4   b  shows measured receiver data and results calculated there from according one embodiment of the invention. 
         FIG. 5  is shows a series of coaxial probes attached to a central processing unit. 
         FIG. 6  is a graphical representation/plot of a recorded pulse showing the presence of air and fuel in a fuel tank half full with fuel. 
         FIG. 7  is a graphical representation/plot of a recorded pulse showing the presence of air in an empty fuel tank. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , in one aspect of the invention, the diagram illustrates the basic implementation of the analog transmit and receive channels. A digital signal processor (DSP) used to close the scan, lock and track loops and analyze the data is shown. The DSP is particularly beneficial in this application as it provides a superior signal-to-noise ratio and enhanced fidelity or image clarity, provides very accurate crystal-based timing and is independent of temperature variations as compared to prior art analog circuitry. More specifically, with respect to the transmitter component of the invention, a square wave pulse from about 1 to about 3 MHz is generated by a clock in the DSP and is buffered by passing through amplifier  12 . The transmission pulse is delayed by delay  14  to synchronize the transmitter and receiver. Delay  14  may be analog or digital and fixes the leading edge of the transmit signal with respect to the receiver being gated. 
     The output of delay  14  is inverted at a high speed digital logic gate (inverting amplifier)  16 , the output of which is buffered by high speed transistor  17 . Transistor  17  acts as a switch that turns on when a positive or relatively high voltage, e.g., 3.3 volts, is received and turns off when the voltage is low or ground. In one embodiment, inverter  14  puts out either ground or 3+ volts. The output of transistor  17  is converted to a pulse by a capacitor  18  that functions as an open circuit for the slow speed signal or rising edge of the pulse wave, and as a short circuit for the high speed signal or falling edge. In this manner, capacitor  18  prevents passage of the low speed signal. 
     Capacitor  18  drives the shielded probe which forms a transmission line driven from the source impedance R 7 . The high speed edge continues along the transmit line into a probe  22 , which in one embodiment is shielded, where the source, characteristic and load impedances are optimized to produce large, clearly delineated reflections of various points in a fuel container such as the container bottom (a) and container top (b). 
     The propagation time of the wave forms or reflections are directly dependent on the dielectric constant of the material being traversed. For example, the propagation time through air will be faster than through fuel because of the differences between the dielectric constants of air and fuel. Additionally, the propagation time will be affected by the amount of fuel or other fluid in the container. A higher fuel content will lead to a slower return pulse. 
     The depth and type of liquid and size of the container dictate the minimum propagation time and what frequency should be used for the transmitter and receiver rates. The frequency must allow time for the propagation of the transmit signal to the bottom of the container and return to the receiver. In one embodiment, the maximum frequency is used thus allowing integration of the maximum number of returns for good signal to noise ratio. The 1-3 MHz transmit cycle is constantly repeated in a substantially constant manner. 
     Turning to the receiver, the clock output is low pass filtered via resistor  30  and added to the dc signal from amplifier  42  so that the combined signal sets the threshold for comparator  28 . The amplifier output is inverted similar to the inversion performed by inverter  16  except without the preceding delay. The output is buffered by high speed transistor  29  and converted to a pulse by capacitor  26  that couples the high speed falling edge of the waveform to turn a switch  24  on and off. Switch  24  may be implemented by either a Fet switch  68  or a diode switch  70 . The result is a receiver gate signal synchronized to the transmit pulse but shifted in time depending on the dc signal from amplifier  42 . 
     Once switch  24  is closed, voltage sampling begins at probe  22 . A charge pump  23  is incorporated into the system to follow the transmitted and reflected signals. The sampling switch is closed for relatively short periods of time (&lt;1 nanosecond) and shifted in picoseconds over about a 200 to about a 500 ms cycle depending on the length of the tube and reflection times. Multiple samples are measured at almost the same time spot to insure a high signal to noise ratio. The charge pump includes a capacitor  60  and a resistor  62 . Whether the charge pump is charged is dependent on the voltage of the received reflections. If positive voltages are received, the pump is charged. 
     By sampling multiple times at the same spot, the relatively high frequency reflection is converted to a low frequency dc voltage signal that accommodates the limitations of the DSP. By reducing the frequency, rather than having to take measurements in nanoseconds or picoseconds, measurements can be made in milliseconds. 
     With this configuration, sampling can be conducted in the same spot relative to the transmit pulse for a relatively long period of time by being shifted (at a picoseconds rate) each time the switch is turned on over this incrementally increasing time differential. Sampling at other spots is accomplished by incrementally increasing the time gap between adjacent switch operation events. DSP  72  initiates the process of setting the time gaps. 
     An initial command is sent by receiver gate  75  to an amplifier  42  via an integrate and limit algorithm  90  and a 12 bit pulse width modulator (PWM)  94 . The dc voltage passes to resistor  36  and a 2 pole low pass filter  37  that includes capacitors  34  and  40  and resistor  38 . The voltage is then buffered by amplifier  42 . The output voltage of amplifier  42  passes to a resistor  32  and is summed with the output of resistor  30 , the sum of which passes to comparator  28 . The first command sent results in the initial voltage into resistor  32  being 0 so that there is no time delay effect on the initial signal to switch  24 . 
     The output of comparator  28 , which is inverted, passes to switch  44  and resistor  46 . Simultaneously with the exception of the delay produced by delay  14 , the transmit line pulse travels to switch  48  and resistor  50 . The outputs from resistors  46  and  48  are summed and pass through a 2 pole low pass filter  55  that includes capacitors  52  and  56  and resistor  54 . The filtered dc signal is amplified by amplifier  58  and enters DSP  72  via a 12 bit a/d converter. The digitized signal is summed with the command from receiver gate control  75  and processed by the integrate and limit algorithm  90 . 
     Referring now to  FIG. 2 , with respect to the DSP operation and algorithms, the amplifier  58  dc output represents the time between when the transmit pulse begins and the receiver gate is started. This time is compared to the time commanded by receiver gate control  75 . The difference forms the error signal for the scan, lock and track loops. The error is input into the loop compensation (sets the loop bandwidth) shown in  FIG. 2 . 
     The signal processing begins at start  110 . A timer  112  controls the operation. If the timer is ready, the a/d conversion begins at  114 . If not, the system loops back and tries again until timer  112  is ready. Once the a/d conversion is complete, the signal from amplifier  58  (represented as v p ) is put through a summation step  116  with the voltage command signal from receiver gate control  75  (represented as v r ). The result V in  is multiplied by a constant at step  118  and added at step  120  to the immediately preceding voltage output Vout- 1  to produce Vout. The magnitude of the resulting Vout is checked at step  122  to determine its magnitude. If the magnitude is greater than a preselected limit, the Vout is set to the selected limit at step  126 . If the Vout is less than the selected limit, is passes to the PWM  94 . Vout also loops back and is delayed at step  124  to be added to the next V in . 
     The output of algorithm  90  is used to set the width of the PWM  94  DSP output and controls the time the receiver gate is started (opened) and ended (closed). The loop will drive the error to zero and thus track any or all return propagation times from the transmit time to the bottom of the container. In one illustrative embodiment, in scan mode, the receiver gate controller  75  output begins at zero time (transmit) and increases until the fluid level and the bottom of the tank/container are detected. Once these times are known, the mode can be commanded to change to lock and track the propagation time (distance) with respect to a) the top of the fuel level, b) through the fluid between the top of the fluid and the bottom of the tank, and c) the bottom of the container only. 
     Referring again to  FIG. 1 , the low frequency signals are buffered by amplifier  66  and input to the DSP  72  and into a 12 bit a/d converter  73 . The amount of filtering and amplification can be varied as is known in the art to optimize the tracking analysis. Once the signal is digitized, it is processed through a low pass/high pass filter to shape the signal and remove any noise. 
     Referring to  FIG. 3 , the filter process begins at start  160 . A timer  162  controls the operation. If the timer is ready, the a/d conversion begins at  164 . If not, the system loops back and tries again until timer  162  is ready. After the conversion, a check is made as to whether the a/d conversion is complete at step  166 . If not, the system loops back and continues the conversion. Once the conversion is complete, the digitized signal represented as Vin is multiplied by a constant K 3  at step  170 . The result is put through a unit delay at  174  to produce K 3  Vin- 1 . Initially Vin is put through a magnitude check at step  172 . If the magnitude is greater than a preselected maximum value, the Vin is set to the Vmax and exists to pulse shapers  76  and  78  as Vout. If not, if passes to the pulse shapers  76  and  78  as Vout. Vout also loops back and is multiplied by constant K 2  at step  176  and put through a unit delay at  178  to become Vout- 1 . K 3 Vin is then added to Vout- 1  and subtracted from K 3 Vin- 1 . The result is again checked for magnitude at step  172  and passed out as Vout. 
     Filters  76  and  78  (shown in  FIG. 1 ) are used for gain adjustments to keep the returns in a linear region for analysis. The filters shape the pulses and pass the shaped pulses to comparators  86  and  88 , respectively. Hi and low references  80  are put into the comparators. A transmit pulse is obtained (+) signal when the transmit pulse is larger than the high reference. A bottom tank pulse is obtained when the signal is lower than the low reference. The time between the two values is proportional to the amount of fuel in the tank. The more fuel, the larger the time separation will be. 
     Algorithm filter  92  (also shown in  FIG. 1 ) is used to calculate the propagation time and determine the dielectric constant, the temperature and height and weight of the fluid, which is a function of the fluid. The range of these parameters is then used to determine if any harmful contaminates are present. 
       FIG. 4A  shows the calculations necessary to convert the signals into measurements of the height, weight and temperature of the fluid being monitored. Measured receiver data and the results calculated thereof are shown in  FIG. 4B . 
     Referring to  FIG. 5 , a series of TDR fuel probes can be used to monitor fuel levels in one or more fuel tanks found in a conventional aircraft wing. In one embodiment as shown, two independent systems having a plurality of probes connected to independent power sources and independent fuel quantity indicators are used to monitor fuel levels in separate fuel tanks such as left and right fuel tanks positioned in left and right wings, respectively, of a conventional aircraft. 
     Referring now to  FIGS. 6 and 7 , plots are shown of readings taken from tanks with the novel coaxial probe system.  FIG. 6  shows readings taken from a tank half full with fuel. The points on the plot representing the air component and fuel or fluid component of the tank are labeled for clarity. For illustrative purposes,  FIG. 7  is provided to show readings taken from the same tank when essentially empty. 
     While the present invention has been described in connection with several embodiments thereof, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from the true spirit and scope of the present invention. Accordingly, it is intended by the appended claims to cover all such changes and modifications as come within the true spirit and scope of the invention.