LIQUID QUANTITY DETERMINING APPARATUS AND METHOD

Abstract: A liquid level sensor system such as for sensing the level of fuel in aircraft fuel taniks (T) includes a plurality of capacitive sensors (S₁,S₂,S₃) which provide output capacitances which are a function of the fractions of the sensors wetted by the fuel. The sensors are positioned in the liquid-containing tank (T) with each sensor having a first and at least a second probe-specific liquid-quantity profile corresponding to a first and at least a second attitude of the tank. The capacitance of the sensors ise determined as function of the length of the respective sensor wetted by the liquid. A memory (M) is provided with a first and second sets of address locations containing liquid-quantity profile information for the sensors for their respective first and second attitudes. The memory (M) is addressed by determining an address offset as function of the attitude of the system, the relative location of the profile information for the sensor, and the fraction of the sensor wetted by the liquid.

Description: The invention relates to apparatus for determining the quantity of a liquid in a liquid-containing tank having at least two possible attitudes relative to the gravity vector, comprising liquid-level sensing probes in the tank, each probe producing for each said attitude of the tank a respective output which varies in dependence on the fraction of it which is immersed in the liquid and which is related to the quantity of liquid in the tank in a manner dependent on the shape of the tank and the relative position of the probe within the tank, determining means responsive to the outputs for determining for each probe an output value representative of the fraction thereof which is immersed in the liquid at a particular measuring time, memory means having a plurality of addressable storage locations each storing liquid-quantity-representing data for a particular output value of a respective one of the probes, addressing means responsive to the particular said attitude of the tank, to the respective positions of the probes in the tank and to the current said output values of each probe for accessing for each probe a respective one of the storage locations and reading the data therein, and output means responsive to all the read data for indicating the quantity of liquid in the tank. The invention also relates to a method for determining the quantity of a liquid in a liquid-containing tank having at least two possible attitudes relative to the gravity vector and liquid-level sensing probes in the tank, each probe producing for each said attitude of the tank a respective output which varies in dependence on the fraction of it which is immersed in the liquid and which is related to the quantity of liquid in the tank in a manner dependent on the shape of the tank and the relative position of the probe within the tank, comprising the steps of storing, in each of a plurality of addressable storage locations, liquid-quantity-representing data corresponding to a particular output value of a respective one of the probes, determining for each probe the output value representative of the fraction thereof which is immersed in the liquid at a particular time, responding to the particular said attitude of the tank, to the respective positions of the probes in the tank and to the current said output values of each probe by accessing for each probe a respective one of the storage locations, and reading the data in each accessed storage location, and indicating the quantity of liquid in the tank in response to all the read data. One such apparatus and method is known from US-A-4 373 390. In such prior apparatus and method, the manner in which the relevant data is obtained from the memory means is complicated and involves the extensive use of a microprocessor. The invention aims to overcome this problem. Accordingly, the apparatus as set forth above characterised in that the storage locations are arranged in groups one group for each of the said attitudes of the tank and are arranged in sub-groups within each said group, one sub-group for each of the probes, with the storage locations being arranged sequentially within each sub-group with one storage location therein for each of the output values of the respective probe, and in that the addressing means locates a particular one of the storage locations in response to each probe output value by means of a first address offset representative of the attitude of the tank so as to locate the corresponding group of storage locations, a second address offset representative of the position of the particular probe in the tank so as to locate the corresponding sub-group, and an address within that sub-group dependent on the probe output value. Similarly, the method as set forth above is characterised by the steps of arranging the storage locations in groups one for each of the said attitudes of the tank, arranging them within sub-groups in each said group, one sub-group for each of the probes, with the storage locations being arranged sequentially within each sub-group with one storage location therein for each of the output values of the respective probe, and locating a particular one of the storage locations in response to each probe output value by means of a first address offset representative of the attitude of the tank so as to locate the corresponding group of storage locations, a second address offset representative of the position of the particular probe in the tank so as to locate the corresponding sub-group, and an address within that sub-group dependent on the probe output value. The subject matter of this application is related to that disclosed in co-pending European Patent Application No. 88303383.9 corresponding to U.S. Patent Application Serial No. 042229, filed 24th April 1987, the disclosure of which is incorporated herein by reference. A liquid level sensing apparatus or system embodying the invention, and methods according to the invention for liquid level sensing, will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is schematic block diagram of the system; Figure 2 is an illustrative representation of the memory topology of the memory in the apparatus of Figure 1 in which the left hand column represents an address (in decimal) and the right hand column represents stored liquid quantity information; and Figure 3 is a flow diagram of the addressing sequence of the memory of Figure 1. As shown in Fig. 1 a system control and processing unit 10 provides output control signals to a one-of-N multiplexer 12 and an integrator rate controller 14, which, in turn, provides rate control signals to a bounded integrator 16. The bounded integrator 16 provides symmetrical voltage limits of opposite polarity, for example of +10 and -10 VDC, with the rate of change of the output between these two limits controlled by the integrator rate controller 14 under instruction from the system control and processing unit 10 to provide an interrogation waveform as described in accordance with the above-incorporated co-pending patent application. The multiplexer 12 is of a 1-of-N type under the control of the system control and processing unit 10 and provides the voltage output of the bounded integrator 16 to a selected one of the N output lines. A reference driver unit REF and a plurality of driver units DU₁ ... DU n , of which only DU₁, D₂ and D₃ are illustrated, are connected to the various output lines of the multiplexer 12. The reference driver unit REF is connected to a sensor reference S ref having a predetermined or otherwise known resistance characteristic R ref , and the various driver units DU₁ ... DU₃ are connected to respective sensor probes S₁ ... S₃. The sensor probes S₁ ... S₃ are shown immersed in a liquid L contained in a tank T having an irregular cross-section to illustrate the manner by which nonlinearities are introduced into the output of a sensor probe having generally linear response characteristics. The irregular cross section of the tank T includes a first, vertical step T₁ in the bottom wall of the tank T between the sensor probes S₁ and S₂ and a second, inclined ramp surface T₂ between the sensor probes S₂ and S₃. In the preferred embodiment, the sensor probes S₁ ... S₃ are used to determine fuel quantities in one or more aircraft fuel tanks, which typically have irregular shapes and different attitudes relative to the gravity vector depending upon whether the aircraft is in flight or on the ground. Each of the sensor probes S n has capacitive and resistive characteristics which vary as a function of the environmental characteristics to be measured. For example, the sensor probes S n can be formed as concentric cylindrical capacitive elements in which the capacitance varies in a generally linear manner as a function of the fraction of the sensor probe S n which is wetted by the liquid between the capacitive elements. While the various sensor probes S₁ ... S₃ have a generally linear output characteristic with rising liquid L level, the irregularities in the tank T shape can introduce irregularities in the output of the sensor probes S₁ ... S₃, as explained below. The various sensor probes S n are connected in common circuit with one another and to the input of a transconductance amplifier 18 which converts the current flow of a selected sensor probe S n , or of the sensor reference S ref , to a voltage for amplification by a voltage amplifier 20. A sample and hold circuit 22 accepts the output of the voltage amplifier 20 at selected times during the application of the voltage waveform provided through the multiplexer 12 from the bounded integrator 16. An analog-to-digital converter 24 accepts, under the control of the system control and processing unit 10, the output of the sample and hold circuit 22 and provides a corresponding binary value along a data bus 26 to an input of the system control and processing unit 10. The system control and processing unit 10 may be stored-program-controlled and includes an arithmetic and logic unit ALU as well as various instruction and data registers for carrying out the liquid level quantity determinations as described below. Additionally, the system control and processing unit 10 is connected to a memory, characterized in general form by the reference character M, which includes a plurality of addressable memory locations containing liquid quantity information, as explained more fully below in relation to Fig. 2. The memory M includes an address input port which receives address information from the system control and processing unit 10 through an address bus 28 and provides the addressed data along a data bus 30 to a data input port of the system control and processing unit 10. While the memory M has been shown as a separate functional block from the system control and processing unit 10, the memory M can be combined with the system control and processing unit 10 as an integrated circuit. An output device 32, which may take the form of a multi-digit BCD display, is connected to the system control and processing unit 10 and provides an indication of the liquid level output. While not shown, the output device may also take the form of a printer to provide a hard copy output. The irregular shape of the exemplary tank T shown in Fig. 1 introduces discontinuities and nonlinearities in the output of the various sensor probes S n depending upon their relative location. For example, the sensor probe S₁ will provide a relatively linear output with a rising level of the liquid L from the lowermost portion of the tank T bottom until the level of the liquid L attains and surmounts the step T₁ after which the output of the sensor probe S₁ will change in a discontinuous or near discontinuous manner in accordance with another relationship relative to the rising liquid L level. In contrast to the sensor probe S₁, the sensor probe S₂ will provide a nonlinear output with rising liquid L because of the inclined ramp surface T₂ until the liquid L attains and surmounts the ramp surface T₂ after which the output will be essentially linear. Lastly, the output of the sensor probe S₃ will be essentially linear with rising liquid level through its entire operating length. As can be appreciated, the output of the sensor probes S n and the relative linearity or nonlinearity will be altered by a change in attitude of the tank T. In the context of an aircraft fuel tank, the output of the sensor probes S n will change depending upon whether the aircraft is in a first attitude, e.g., level flight with a slight nose-up pitch or in a second attitude, e.g., on the ground with a slight nose-down pitch. The embodiment of Fig. 1 operates to poll selectively the sensor reference S ref and the various sensor probes S n . For example, the system control and processing unit 10 controls the multiplexer 12 to select the sensor reference S ref and controls the bounded integrator 16 to provide an interrogation voltage waveform, as described in the above-incorporated co-pending application, through the reference driver unit REF to the sensor reference S ref . The current drawn through the sensor reference S ref is converted by the transconductance amplifier 18 to a voltage which, in amplified form through the voltage amplifier 20, is fed to the sample and hold circuit 22 for conversion to a binary value and presentation to the system control and processing unit 10. In a similar manner, the sensor probes S₁ ... S₃ are likewise interrogated to provide the system control and processing unit 10 with information indicating the fraction (e.g. 0.85) of the particular sensor probe S n wetted by the liquid L. As shown in Fig. 2, the memory M of Fig. 1 is illustrated as having thrity contiguous addressable memory locations 0000 to 0029 (decimal) with a first set of fifteen memory locations 0000 to 0014 containing three groups of five memory locations for the sensor probes S₁, S₂, and S₃, and a second set of fifteen memory locations 0015 to 0029 defining an additional three groups of five memory locations for the sensor probes S₁, S₂ and S₃. The first set of memory locations 0000 to 0014 contains fuel quantity information for the sensor probes S₁, S₂ and S₃ when the tank T is in a first attitude corresponding to, for example, when the aircraft is in an on-the-ground attitude, and the second set of memory locations contains fuel quantity information for the sensor probes S₁, S₂ and S₃ when the tank T is in a second attitude corresponding to the aircraft in flight. The three groups of memory locations in the first set, addresses 0000 to 0014, include address group 0000 to 0004 containing corresponding fuel quantity information for the first sensor S₁ when aircraft is in the first attitude, address group 0005 to 0009 containing corresponding fuel quantity information for the second sensor S₂ when aircraft is in the first attitude, and address group 0010 to 0014 containing corresponding fuel quantity information for the third sensor S₃, also when aircraft is in the first attitude. In an analogous manner, the three groups of memory locations in the second set, addresses 0015 to 0029, include address group 0015 to 0019 containing corresponding fuel quantity information for the first sensor S₁ when aircraft is in the second attitude, address group 0020 to 0024 containing corresponding fuel quantity information for the second sensor S₂ when aircraft is in the second attitude, and address group 0025 to 0029 containing corresponding fuel quantity information for the third sensor S₃, also when aircraft is in the second attitude. In the disclosed embodiment, the addresses within each group are sequential; however, the stored fuel quantity information is generally not linear with the sequentially increasing addresses and preferably follows the nonlinearity or discontinuity of the particular tank T configuration, as represented, for example, in address locations 0025 to 0029. The memory topology presented in Fig. 2 is for the purpose of illustration only and actual memory addressing is typically accomplished in hexadecimal. In order to obtain fuel quantity information, the interrogation circuit of Fig. 1 provides an information value which is representative of the fraction of the sensor probe S n wetted by the liquid as disclosed in the above-incorporated co-pending application. This value combined with information representative of attitude of the fuel tank and the particular sensor probe S n provides an address for the memory M and provides the desired liquid level output information in accordance with the flow diagram of Fig. 3. As shown therein, the sequence is initiated by storing the value zero for variables N and ADDR where N is the identity of the sensor and ADDR the memory address. A query is then presented as to the system attitude and where the system attitude, in the context of the best mode, indicates an airborne attitude, the sequence branches to the leftward operational path. Conversely, where the system attitude is ground based, the sequence branches to the rightward operational path. If the system is in an airborne attitude, the system memory offset of fifteen is added to the variable ADDR. Thereafter the variable N is incremented and an address group offset that is a function of the sensor probe number and the number of address locations in each group (i.e., five) is added to the variable ADDR. The fraction of the N th sensor probe wetted by the liquid is then determined (or recalled from a memory location) and multiplied by the number of address locations in the group with the result added to variable ADDR. The resulting cumulative address variable is then used to address the memory M and the readout liquid quantity information stored to a variable Q n . A specific example will now be considered. The status of the system is first determined, in this case whether the aircraft is on the ground or in the air. Ground corresponds to an address offset of 0, air corresponds to an address offset of 15. Then the sensor whose value is desired is selected. In this case, Sensor 1 has an address offset of 0, Sensor 2 has an address offset of 5, and Sensor 3 has an address offset of 10. Finally, the height of fuel at each sensor is measured in a linear fashion. The resulting value is a fraction from 0 to 1 which is multiplied by 5 (in this example - the actual value is dependent on the actual table construction). For purposes of example, assume that Sensor 1 is covered 0.4, Sensor 2 is covered by 0.6, and Sensor 3 is covered by 0.7. Also assume that the system determines that it is in the air. The address is computed by: address of data = (system offset)+(probe offset)+(probe fluid height) In the example: Sensor 1 data address = 15+0+5*0.4 = 17 address 17 data = 13 volume units, where each volume unit is 3.785 litres (1 United States gallon) Sensor data address = 15+5+5*0.6 = 23 address 23 data = 10 volume units Sensor 3 data address = 15+10+5*0.7 = 28.5 address 28 data = 6 volume units address 29 data = 12 volume units therefore address 28.5 data = 9 volume units using linear interpolation The liquid quantity is the sum of the individual quantities. Thus in this example, the total liquid quantity is 13+10+9 = 32 volume units. The sequence of Fig. 3 thus advantageously provides an instruction sequence for obtaining fuel quantity information as a function of summed value of the system offset, the sensor probe offset, and a value representative of the number of address locations in each address group multiplied by the wetted fraction of the sensor probe. The memory topology shown in Fig. 2 is illustrative only and, for reasons of exposition, does not represent an actual memory topology. In actual practice, a several hundred or thousand contiguous memory address locations would be provided to store corresponding liquid level or quantity information so that fine gradations of liquid level or quantity information would be available for the smallest changes in the fractions of the sensor probe wetted by the liquid. If desired, where a numeric address is ascertained, as described above, and does not precisely correspond to an existing address, the next lower or higher address can be used or interpolation between tabular values used. As can be appreciated, the method and apparatus described provides for the accurate determination of liquid quantities in a liquid containing tank where the relationship between liquid level and liquid quantity is nonlinear and/or of a discontinuous nature and which the attitude of the liquid containing tank and probes can vary relative to the gravity vector. Thus it will be appreciated from the above that as a result of the present invention, a highly effective apparatus and method for determining liquid levels is provided by which the principal objective, among others, is completely fulfilled. It will be equally apparent and is contemplated that modification and/or changes may be made in the illustrated embodiment without departure from the invention. Accordingly, it is expressly intended that the foregoing description and accompanying drawings are illustrative of preferred embodiments only, not limiting, and that the true scope of the present invention will be determined by reference to the appended claims and their legal equivalent.

Claims: Apparatus for determining the quantity of a liquid in a liquid-containing tank (T) having at least two possible attitudes relative to the gravity vector, comprising N liquid-level sensing probes (S₁ ... S n ) in the tank (T), each probe (S₁ ... S n ) producing for each said attitude of the tank (T) a respective output which varies in dependence on the fraction of it which is immersed in the liquid and which is related to the quantity of liquid in the tank (T) in a manner dependent on the shape of the tank (T) and the relative position of the probe within the tank (T), determining means (12,18,20,22) responsive to the outputs for determining for each probe an output value representative of the fraction thereof which is immersed in the liquid at a particular measuring time, memory means (M) having a plurality of addressable storage locations each storing liquid-quantity-representing data for a particular output value of a respective one of the probes (S₁ ... S n ), addressing means (28) responsive to the particular said attitude of the tank (T), to the respective positions of the probes in the tank (T) and to the current said output values of each probe for accessing for each probe a respective one of the storage locations and reading the data therein, and output means responsive to all the read data for indicating the quantity of liquid in the tank (T), characterised in that the storage locations are arranged in groups one group for each of the said attitudes of the tank and are arranged in sub-groups within each said group, one sub-group for each of the probes (S₁ ... S n ), with the storage locations being arranged sequentially within each sub-group with one storage location therein for each of the output values of the respective probe (S₁ ... S n ), and in that the addressing means (28) locates a particular one of the storage locations in response to each probe output value by means of a first address offset representative of the attitude of the tank so as to locate the corresponding group of storage locations, a second address offset representative of the position of the particular probe (S₁ ... S n ) in the tank (T) so as to locate the corresponding sub-group, and an address within that sub-group dependent on the probe output value. Apparatus according to claim 1, characterised by accumulator means for summing, for a particular measuring time, the liquid quantity values obtained from the addressed storage locations respective to each of the probes (S₁.......S n ). Apparatus according to claim 1 or 2, characterised in that the determining means comprises stored-program controlled processor means (10) for determining a value representative of the fraction of each of said probes (S₁ ... S n ) immersed in the liquid, the processor means also incorporating the addressing means (28). A method for determining the quantity of a liquid in a liquid-containing tank (T) having at least two possible attitudes relative to the gravity vector and N liquid-level sensing probes (S₁ ... S n ) in the tank T, each probe (S₁ ... S n ) producing for each said attitude of the tank a respective output which varies in dependence on the fraction of it which is immersed in the liquid and which is related to the quantity of liquid in the tank in a manner dependent on the shape of the tank and the relative position of the probe within the tank, comprising the steps of storing, in each of a plurality of addressable storage locations, liquid-quantity-representing data corresponding to a particular output value of a respective one of the probes (S₁.....S n ), determining for each probe the output value representative of the fraction thereof which is immersed in the liquid at a particular time, responding to the particular said attitude of the tank (T), to the respective positions of the probes in the tank (T) and to the current said output values of each probe by accessing for each probe a respective one of the storage locations, and reading the data in each accessed storage location, and indicating the quantity of liquid in the tank (T) in response to all the read data, characterised by the steps of arranging the storage locations in groups one for each of the said attitudes of the tank, arranging them within sub-groups in each said group, one sub-group for each of the probes (S₁ ... S n ), with the storage locations being arranged sequentially within each sub-group with one storage location therein for each of the output values of the respective probe (S₁ ... S n ), and locating a particular one of the storage locations in response to each probe output value by means of a first address offset representative of the attitude of the tank so as to locate the corresponding group of storage locations, a second address offset representative of the position of the particular probe (S₁ ... S n ) in the tank (T) so as to locate the corresponding sub-group, and an address within that sub-group dependent on the probe output value.

Inventors: MAIER, LAWRENCE CARL
Applicants: SIMMONDS PRECISION PRODUCTS INC.
Application year: 1988
Main classification: G01F23
IPC codes: G01F23, G01F3