Abstract:
The weight of liquid in a tank subject to tilting is determined by measuring differences in air pressure in the ullage (the empty volume of the tank above the level of the liquid) and the air pressure produced by the weight of the liquid at the bottom of the tank. The pressures at the bottom of the tank are measured in a plurality of locations, and the differential pressures are combined to compensate for changes in the depth of the liquid when the tank is not level (e.g., when the tank is in an aircraft flying at an attitude other than straight and level). The electronics to measure the pressure and to process the pressure measurements are located remotely form the tanks, thus eliminating the possibility of ignition caused by the electronics when the liquid is fuel or other flammable liquid.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention is related to the measurement of liquid in a tank that is subject to tilting, and, more particularly, is related to the measurement of the weight of the liquid in the tank by determining differences in pressures at multiple locations in the tank. 
   2. Description of the Related Art 
   The measurement of the fuel remaining in the fuel tanks of an aircraft is highly important in order to assure that an aircraft is not flown beyond the limits of the available fuel. Fuel measurement techniques have evolved from the original method of inserting a rod into the tank prior to a flight. A current industry standard for measuring fuel uses capacitive probes, which measure the height of the fuel in a tank by measuring the change in capacitance of concentric tubes. The principle behind this approach is that changing fuel levels change the dielectric constant between the concentric tubes. In particular, a higher fuel level causes a greater capacitance, and a lower fuel causes a lower capacitance. The value of the capacitance of the concentric tubes is very small (e.g., on the order of 50 to 100 picofarads). Therefore, an accurate measurement of such a small capacitance is challenging, especially, since the stray wiring capacitance can far exceed 100 picofarads. Thus, capacitive fuel probes are very expensive and may cost in a range of $600-$2,000 per probe. Furthermore, multiple probes may be required in each tank. Since the capacitive probes are operated electronically, a portion of the electronics are typically located in the fuel tanks, which may present a risk of explosion. Accordingly, a need exists for simpler, safer and less expensive fuel probes, which provide accurate measurements of fuel during aircraft flight. 
   SUMMARY OF THE INVENTION 
   The weight of liquid in a tank that is subject to tilting is determined by measuring differences between the air pressure in the ullage (the empty volume of the tank above the level of the liquid) and the air pressure produced by the weight of the liquid at the bottom of the tank. The pressures at the bottom of the tank are measured in a plurality of locations using differential pressure transducers, and the differential pressures are used to determine the quantity of liquid remaining in the tank. The system determines the liquid quantity even when the tank is tilted. For example, the quantity of liquid remaining in a tank of an aircraft is determined when the aircraft is at an attitude other than straight and level flying. The electronics to measure the pressure and to process the pressure measurements are located remotely from the tank, thus eliminating the possibility of ignition caused by the electronics when the liquid is aviation fuel or another flammable material. 
   The electronics system that measures the pressures includes a differential pressure transducer and an analog-to-digital (A/D) converter module with a serial data output. No electronic components are located in the tank. 
   Unlike the industry standard capacitive probes, which measure the height of the liquid, the systems and methods described herein determine the weight of the liquid in a tank by measuring the pressure caused by the weight of the liquid at a plurality of locations in the tank. Each measurement probe is inserted so that an end proximate to the bottom of the tank is affected by the pressure at the bottom of the tank caused by the weight of the liquid above the end of the probe. The pressure at the bottom of the tank is measured differentially with respect to a probe located in the ullage. Since the ullage probe measures a pressure that is common to the tank, the differential pressure accurately represents the pressure caused by the weight of the liquid. 
   A plurality of measurement probes are located in each tank to compensate for changes in the attitude (e.g., tilting) of the tank. For example, the tank may be tilted by movement of the aircraft (e.g., changes in elevation (pitch), bank and yaw) or by being positioned on an uneven support structure or surface. The pressure measurements from the multiple probes in each tank are combined to account for the tilting of the tank issue to eliminate the need to independently measure the tilt angle of the tank. 
   In preferred embodiments, an additional probe is included to independently determine when a tank is approaching an empty state. The end of the additional probe does not extend to the bottom of the tank. Rather, the location of the end of the additional probe is selected to be at a height above the bottom of the tank. The height is selected such that when the level of the liquid in the tank decreases sufficiently that the ullage expands to expose the end of the additional probe, the additional probe also measures common pressure of the tank. Since the pressure applied to the additional probe is measured differentially with respect to the common probe, the differential pressure is zero. The measurement of zero differential pressure by the additional probe is advantageously used to activate a warning indication that the liquid level in the tank is approaching empty. The level of the end of the additional probe can be set to a desired percentage of the maximum liquid height in accordance with a desired safety factor (e.g., in a range from 5-10 percent of the maximum capacity of the tank). 
   The differential pressure probes described herein are less costly than capacitive measurement probes. Furthermore, no electronics are located in the tanks. The system uses pressure sensors which are highly accurate. The combination of the accurate pressure sensors and averaging of the differential measurements from the plurality of probes enables the system to accurately determine the liquid weight. The measurement probes are lightweight and are constructed of materials that will not damage the tanks in the event of a crash or a hard landing. The differential probes are easily configured for different tank sizes and shapes. The differential probes work with all liquid types and do not require temperature compensation. The multiple probes enable compensation for the tilt angle of the tank. The probes are simple to install and easy to maintain and repair. Since no wiring in the tanks is required, the probes are immune to lightning and electromagnetic interference (EMI) and are thus particularly suitable for use in tanks containing fuel or other flammable materials. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Certain embodiments in accordance with the present invention are described below in connection with the accompanying drawing figures in which: 
       FIG. 1  is a perspective illustration of a simplified embodiment of a tank containing a liquid, wherein a portion of the tank removed to show a plurality of pressure measurement probes positioned within the tank, and wherein an upper surface of the liquid is represented by a dashed and dotted line; 
       FIG. 2  illustrates the effect of elevation and bank on the liquid level above each probe; 
       FIG. 3  illustrates a block diagram of an exemplary probe, including a differential pressure sensor, a differential amplifier, and an analog-to-digital (A/D) converter; 
       FIG. 4  illustrates a block diagram of an exemplary probe that further includes a low liquid level detection circuit; and 
       FIG. 5  illustrates a flow diagram of the operation of the fuel probes and the fuel management computer in the exemplary embodiment for measuring fuel remaining in a fuel tank. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  illustrates a tank  100  that may be used for transporting liquid in an aircraft or other machinery subject to movement. For example, the tank  100  may be a fuel tank located within the fuselage or a wing of an aircraft or it may be located within a ground transportation vehicle. In the following description, the tank  100  is located in an aircraft (not shown). Although described herein with respect to the fuel tanks of an aircraft, the invention may advantageously be embodied in other systems for measuring fuel in environments where the tank containing the fuel is subject to tilting. The invention may also be embodied in systems for measuring other liquids, such as, for example, the fire suppression liquids (e.g., water) carried in tanks of firefighting equipment. 
   The tank  100  may have many different configurations in order to conform the tank to the structure of the aircraft or other environment in which the tank is located. For example, the tank  100  may have an irregular shape that conforms to the airfoil and taper of an aircraft wing. In  FIG. 1 , the tank  100  is illustrated as a rectangular parallelepiped (e.g., a conventional box-shaped enclosure) having an upper surface  110 , a lower surface  112 , a forward wall  114 , an aft wall  116 , a left wall  118  and a right wall  120 . Although the lower surface  112  is shown as a continuous plane, it should be understood that the lower surface  112  may include one or more drains (not shown) so that liquid can be removed from the tank  100 . Similarly, the upper surface  110  or one of the other surfaces may include ports (not shown) for adding liquid and for venting vapors. 
   As illustrated in  FIG. 1 , the tank  100  is positioned within the aircraft so that the tank  100  is generally level when the aircraft is flying straight and level (e.g., the aircraft is not banking and has a neutral pitch attitude). A first centerline  130  is positioned parallel to the upper surface  110  and the lower surface  112  and is perpendicular to the forward surface  114 . The first centerline  130  points in the direction of the forward motion of the aircraft. A second centerline  132  is also positioned parallel to the upper surface  110  and the lower surface  112 . The second centerline  132  is orthogonal to the first centerline  130  and is perpendicular to the right surface  120 . The second centerline  132  points in a direction perpendicular to the direction of travel of the aircraft and generally points in the direction of the right wing of the aircraft. In  FIG. 1 , the first centerline  130  and the second centerline  132  are parallel to the earth&#39;s surface and represent the orientation of the tank  100  for 0° elevation (pitch) and 0° bank. Thus, the first centerline  130  is superimposed on a 0° elevation line  134 , and the second centerline  132  is superimposed on a 0° bank line  136 . The 0° elevation line  134  and the 0° bank line  136  are reference lines that define a horizontal plane parallel to the earth&#39;s surface. 
   As further shown in  FIG. 1 , a dashed and dotted line represents the boundary of the upper surface  140  of the liquid in the tank  100  for a condition when the tank  100  contains approximately 75 percent of a maximum capacity. As illustrated for the embodiment of  FIG. 1 , during straight and level flight, the upper surface  140  of the liquid in the tank  100  is substantially parallel to the upper surface  110 . Thus, for the illustrated embodiment having a flat bottom surface  112 , the liquid has a substantially uniform depth from the upper surface  140  of the liquid to the bottom surface  112  of the tank  100 . In other embodiments (not shown), the bottom surface  112  may be contoured or have an irregular depth. The unfilled portion of the tank  100  above the liquid level  140  and below the upper surface  110  is the ullage  142 . The volume of the ullage  142  increases as the quantity of the liquid in the tank decreases. 
   In the illustrated embodiment, the liquid in the tank  100  is measured by a first differential pressure probe assembly  150 A, a second differential pressure probe assembly  150 B, a third differential pressure probe assembly  150 C, a fourth differential pressure probe assembly  150 D, a fifth differential pressure probe assembly  150 E and a sixth differential pressure probe assembly  150 F. Except as described below with respect to the sixth probe assembly  150 F, the six probe assemblies  150 A-F are substantially similar and are described accordingly. Although described herein with respect to six differential pressure probe assemblies  150 A-F, other tank configurations may require fewer or more differential pressure probes. For example, if a particular tank has irregular upper and lower surfaces to accommodate the contour of a wing or to conform to other requirements, additional differential pressure probes may be advantageously used. 
   Each of the differential pressure probe assemblies  150 A-F comprises a respective differential pressure sensor  160 A-F (labeled “S” in  FIG. 1 ). The pressure sensors  160 A-F generate output signals, which, in the illustrated embodiment, are communicated to a fuel management computer  320  (described below in connection with  FIG. 3 ). The pressure sensor  160 F for the pressure probe assembly  150 F advantageously generates a second output signal, which is communicated to a low fuel alarm  330  (described below in connection with  FIG. 4 ). 
   Each differential pressure sensor  160 A-F has a first port  162  and a second port  164 . The numeric indicators for the first port  162  and the second port are shown only for the first sensor  160 A and the sixth sensor  160 F in  FIG. 1 . Each pressure probe assembly  150 A-F includes a respective measurement probe configured as a respective measurement tube  170 A-F and a respective reference probe configured as a respective reference tube  180 A-F. As indicated for the measurement probes  170 A and  170 F, each measurement tube has a first end  172  coupled to the respective first port  162  of the respective sensor  160 . Each measurement tube  170 A-F passes through the upper surface  110  of the tank  100  and extends toward the bottom surface  112 . A second end  174  of each of the measurement tubes  170 A-F is positioned proximate to the bottom surface  112 . The tank  100  may advantageously include structures (not shown) to retain the second ends  174  of the measurement tubes  170 A-F in fixed positions with respect to the bottom surface  112 . The second ends  174  of the measurement tubes  170 A-F are open and are exposed to the pressure caused by the weight of the liquid above the second ends  174 , which corresponds to the depth of the liquid. Thus, the measurement tubes  170 A-F communicate the pressure proximate to the bottom surface  112  of the tank  100  to the first port  162  of the sensor  160 . 
   As illustrated in  FIG. 1 , the lengths of the measurement tubes  170 A-E coupled to the first through fifth differential pressure sensors  160 A-E are selected so that the respective second ends  174  are positioned close to the bottom surface  112  of the tank  100  so that the second ends  174  are immersed in the liquid until the tank  100  is substantially empty. The length of the measurement tube  170 F coupled to the sixth pressure sensor  160 F is selected so that the second end  174  of the tube  170 F is offset from the bottom surface  112  by a selected distance, shown as the safety offset in  FIG. 1 . The purpose for the safety offset is discussed below in connection with  FIG. 3 . 
   As indicated for the reference probes  180 A and  180 F, each of the reference tubes  180 A-F has a respective first end  182  coupled to the second port  164  of the sensor  160 . The reference tubes  180 A-F also pass through the upper surface  110  of the tank  100 ; however, the reference tubes  180 A-F extend only a short distance into the tank  100 . In particular, a respective second end  184  of each reference  180 A-F opens into the ullage  142  proximate the upper surface  110 . Thus, the reference tubes  180 A-F transfer the pressure in the ullage  142  of the tank  100  to the second ports  164  of the sensors  160 A-F. In an unpressurized tank, the pressure in the ullage  142  is generally equal to the ambient pressure of the atmosphere at the altitude of the aircraft. In a pressurized tank, the pressure in the ullage is the pressure applied to the tank by a pressure source (not shown). In another embodiment (not shown), the plurality of reference tubes  180 A-F may be replaced by a single reference tube that provides a common reference pressure to the second ports  164  of all of the sensors  160 . 
   In  FIG. 1 , the measurement tubes  170 A-F and the reference tubes  180 A-F are shown as continuous tubes from the respective first ends  172 ,  182  at the sensors  160 A-F to the respective second ends  174 ,  184  in the tank  100 . In certain embodiments, the portions of the tubes within the tank  100  are advantageously fixed within the tank  100 , and the external portions of the tubes  170 ,  180  are coupled to the internal portions by couplers or other suitable interconnection devices (not shown). 
     FIG. 3  illustrates a block diagram of the differential pressure sensor  160 A. In the illustrated embodiment, the second through fifth differential pressure sensors  160 B-E are substantially similar to the first differential pressure sensor  160 A and are also represented by  FIG. 3 . The sixth differential pressure sensor  160 F includes an additional function and is described below with respect to  FIG. 4 . 
   The first differential pressure sensor  160 A comprises a differential pressure transducer  300 . The transducer measures a difference between the pressures applied to two input ports. For example, as shown in  FIG. 3 , the transducer  300  generates signals responsive to pressures at a first input  302  and a second input  304 . A differential amplifier  310  receives the signals from the transducer  300  and produces an analog output signal representing a difference between the pressures on the two inputs  302 ,  304 . The analog output signal from the amplifier  310  is applied as an analog input to an analog-to-digital (A/D) converter  312 . The A/D converter  312  produces an output signal that represents the measured pressure differential. In a particularly preferred embodiment, the output signal from the differential pressure sensor  160 A is a serial output signal. In certain advantageous embodiments, the transducer  300  comprises a pair of absolute pressure sensors, such as, for example, ported MPX4250AP pressure sensors available from Motorola, Inc., which produce output voltages in a range from 0 to approximately 4.7 volts in response to pressures from 0 to 250 kilopascals (kPa). One sensor in the pair produces an output voltage that represents the pressure at the first input  302 , and the other sensor in the pair produces an output voltage that represents the pressure at the second input  304 . The analog output of the differential amplifier  310  is responsive to a difference between the two voltages. Other pressure sensors can also be used. For example, sensors having different pressure measurement ranges may be used to accommodate different ranges of liquid depths in the tank  100 . In an exemplary embodiment, the A/D converter  312  comprises a Sigma-Delta A/D converter, such as, for example, an AD7715 A/D converter available from Analog Devices, which produces a serial output signal representing the digitized value of the analog signal from the differential amplifier  310 . Other types of A/D converters may also be used. 
   As further shown in  FIG. 3 , the serial output signal from the differential pressure sensor  160 A is provided as an input signal to the fuel management computer  320 , which is advantageously located in the cockpit of the aircraft or in an electronics bay. The operation of the fuel management computer  320  with respect to the serial output signal from the differential pressure sensor  160 A is described below with respect to  FIG. 5 . 
   The first input  302  of the differential pressure transducer  300  is coupled to the first end  172  of the measurement tube  170 A via the first port  162 . The second input  304  of the transducer  300  is coupled to the first end  182  of the reference tube  180 A via the second port  164 . Thus, the pressure transducer  300  receives a first pressure P 1  from the bottom  112  of the tank  100  at the second end  174  of the measurement tube  170 A and receives a second pressure P 2  from the ullage  142  of the tank  100  at the second end  184  of the reference tube  180 A. The first pressure P 1  is the pressure caused by the weight of the liquid above the second end  174  of the measurement tube  170 A plus the pressure in the ullage  142 . Thus, by measuring the differential pressure (e.g., P 2 -P 1 ), the common pressure in the ullage  142  is canceled, and the output signal produced by the transducer  300  represents the pressure caused by the weight of the liquid above the second end  174  of the measurement tube  170 A. 
   The weight of the liquid is directly proportional to the height of the liquid surface  140  above the second end  174  of a measurement tube  170 -F. Accordingly, the output signal generated by each transducer  300  represents the height of the liquid surface  140  above the second end  174  of the respective measurement tube  170 A-E. During straight and level flight, the liquid depth corresponds to the percentage of liquid in the tank  100 . As the level of the liquid decreases, the differential pressure sensed by each transducer  300  decreases as the weight of the liquid above the second ends  174  of the respective measurement tubes  170 A-F decreases. Accordingly, the quantity of liquid remaining in the tank  100  is readily determined by calibrating the measured pressures to the quantity of liquid. 
   The six pressure sensors  150 A-F are substantially similar; however, as discussed above with respect to  FIG. 1 , the position of the second end  174  of the measurement tube  170 F of the sixth pressure sensor  150 F is positioned farther from the bottom surface  112  than the respective positions of the second ends  174  of the respective measurement tubes  170 A-E of the other five sensors  150 A-E. In particular, the measurement tube  170 F is truncated so that the second end  174  is located at a height above the bottom surface  112  that is selected to cause an empty tank indication when approximately 5-10 percent of the maximum liquid weight remains in the tank. The height of the second end  174  of the measurement tube  170 F is identified as the safety offset in  FIG. 1 . As the liquid level in the tank  100  decreases during operation of the aircraft, the second end  174  of the measurement tube  170 F will be exposed to the ullage  142  before the second ends  174  of the other measurement tubes  170 A-E. When this condition occurs, the second end  174  of the measurement tube  170 F and the second end  184  of the reference tube  180 F are exposed to substantially the same pressure. 
   As shown in  FIG. 4 , the corresponding differential pressure sensor  160 F advantageously includes a zero-differential detection circuit  340 , which is coupled to the output of the differential amplifier  310 . The detection circuit  340  senses when the output of the differential amplifier  310  corresponds to a substantially zero pressure differential between the two tubes, which occurs with the second end  174  of the measurement tube  170 F is exposed to the ullage  142 . When the detection circuit  340  detects this condition, the detection circuit  340  generates an output signal that represents zero pressure. The output signal from the detection circuit  340  is advantageously provided to the fuel management computer  320  in addition to the digital pressure values. The fuel management computer  320  is advantageously configured to respond to the zero pressure indication and provide one or more indications to the operator of the aircraft (e.g., the pilot) to inform the operator that the liquid in the tank  100  is approaching empty. For example, the fuel management computer  320  may activate the low fuel warning alarm  330 . 
   In particularly preferred embodiments, the sixth differential pressure sensor  160 F is advantageously provided with a secondary source of electrical power so that the sixth sensor  160 F continues to receive power even if the power provided to the other sensors  160 A-E is interrupted. Additionally, the sixth sensor  160 F advantageously provides a secondary output signal that is activated when the liquid level decreases below safety offset height shown in  FIG. 1 . For example, the secondary output signal is advantageously coupled to the low fuel alarm  330 , which comprises one or more of a warning light, an audio warning device, or the like, to inform the operator of the aircraft of the low fuel condition even if the fuel management computer  320  malfunctions as a result of a power interruption or other fault. 
   When the attitude of the aircraft is changed so that the aircraft is no longer flying straight and level, the orientation of the tank  100  changes accordingly. For example,  FIG. 2  illustrates the orientation of the tank  100  when the aircraft is descending and banking to the right. The 0° elevation line  134  and the 0° bank line  136  remain in a horizontal reference plane parallel to the earth&#39;s surface as in  FIG. 1 . Since the tank  100  is tilting forward and to the right, the first centerline  130  in  FIG. 2  is not aligned with the 0° elevation line  134 , and the second centerline  132  is not aligned with the 0° bank line  136 . Rather, the first centerline  130  is offset below the 0° elevation line  134  by a pitch angle, and the second centerline  132  is offset below the 0° bank line  136  by a bank angle. 
   Assuming the aircraft is in a steady descent and bank, the liquid surface  140  remains level with respect to the earth&#39;s surface. Thus, the liquid surface  140  remains in a horizontal plane parallel to the plane defined by the 0° elevation line  134  and the 0° bank line  136 . As a result, the depth of the liquid in the tank  100  varies within the tank  100 . For example, the depth of the liquid at the forward, right corner  220  of the tank  100  is substantially increased over the liquid depth in the corresponding corner in  FIG. 1 . The liquid depth at the aft left corner  222  of the tank  100  is substantially decreased below the liquid depth in the corresponding corner of  FIG. 1 . 
   Since the liquid depth varies across the bottom surface  112  when the aircraft is not in a level flight attitude as shown in  FIG. 2 , the depth of the liquid above the measurement tube  170  of a single probe assembly  150  does not accurately represent the quantity of liquid in the tank  100 . However, the pressure values from the first five probe assemblies  150 A-E in  FIGS. 1 and 2  are used to determine the liquid quantity. In particular, as illustrated by an activity block  510  in a flow diagram  500  in  FIG. 5 , the differential pressure transducers  300  in the probe assemblies  150 A-F periodically sample the differential pressures at a selected sampling rate (e.g., one sample per second or per other suitable interval). In an activity block  520 , the transducers  300  generate the digital representations of the measured pressures and output the values to the fuel management computer  320 . In an activity block  530 , the fuel management computer  320  receives and stores the digital pressure values. In an activity block  540 , the fuel management computer  320  adjusts the pressure values to compensate for any differences in the shape of the tank  100 , which may, for example, cause the respective second end  174  of the measurement tube  170  of certain probe asemblies  150  to be positioned at a different depths with respect to the liquid surface  140  even when the tank  100  is in the level condition shown in  FIG. 1 . 
   After performing any needed compensation of the pressure values, the fuel management computer  320  uses the pressure values from the five probe assemblies  150 A-E in an activity block  550  to obtain a value representing the total weight of the liquid remaining in the tank  100 . The total weight may be calculated from the measured pressures. Alternatively, the pressure values may be used to index a lookup table or other stored data to determine a weight of liquid corresponding to the measured pressure values. In an activity block  560 , the fuel management computer  320  calculates the quantity of liquid remaining in the tank  100  using the total weight determined in the block  550  and the density of the liquid, and displays a value that represents the liquid quantity. 
   Although described herein as a fuel management computer  320 , the computer  320  is advantageously adaptable to the management of other liquids. The calculations performed in the block  560  are adjusted in accordance with the density of a particular liquid to determined the quantity of liquid in the tank  100   
   The differential probes described herein are relatively inexpensive and are highly accurate. In one advantageous embodiment, the combination of the accurate pressure sensors and the plurality of probes enables the system to determine the liquid weight within the tank  100  even when the tank  100  is tilted (e.g., the tank is in an aircraft that is banking). The measurement probes are lightweight and are constructed of materials that will not damage the tanks in the event of a crash or a hard landing. The differential probes are easily configured for different tank sizes. The differential probes work with all liquid types. Since the pressure measurements are differential and are determined by the weight of the liquid, the system does not require temperature compensation. The multiple probes compensate for the tilt angle of the tank. The probes are simple to install and easy to maintain and repair. Since no wiring in the tanks is required, the probes are immune to lightning and electromagnetic interference (EMI). Unlike a capacitive probe, the location of the electronics with respect to the tank does not affect the performance of the system. 
   One skilled in art will appreciate that the foregoing embodiments are illustrative of the present invention. The present invention can be advantageously incorporated into alternative embodiments while remaining within the spirit and scope of the present invention, as defined by the appended claims.