Dynamic volumetric instrument gauge

This instrument contains a transducer and a sensor that are mounted on the exterior surface of a liquid container or tank. The transducer is tuned by electronic signals to the mechanical resonant frequency of the liquid container or tank. The sensor converts the mechanical vibrations into dynamic electrical signals using electronic components. A phase detector and feedback circuitry force the transducer to track the resonant frequency of the tank as liquid is removed from the tank. The signal output of the feedback control circuitry is converted to display the amount of liquid remaining in the tank. The resonant frequency of the liquid container is a measure of the total mass of the container. This includes the tare weight of the container plus the weight of the liquid in the tank. The tare weight of the tank is a constant. The stiffness of the tank can be considered a constant. The dampening coefficient changes do not adversely effect the accuracy requirements of the instrument. Therefore, the change in resonant frequency of the tank as liquid is removed is nonlinear, but is an analytic function of the liquid remaining in the tank. Zero and span adjustments eliminate the effect of the constants and permit one design to satisfy many requirements. At resonance the amplitude response of the tank is much larger than the noise of the environment. As the intelligence of the instrument to interpret changes in liquid volume is a function of frequency, this adds to the noise immunity. The nonlinear dynamic response of the instrument is an asset in the use of this gauge for fuel tanks and other applications.

BACKGROUND 
1. Field of the Invention 
This invention relates to gauges for measuring liquid volume in liquid 
containers and tanks. More particularly, the invention relates to a 
dynamic gauge that uses signals that permit the installation of 
transducers or sensors on the outside surface of the containers. 
2. Description of Prior Art 
Other systems use capacitance probes, float actuated electro-mechanical 
devices, or differential pressure sensors immersed in the fluid. 
Ultrasonic transducer systems have been inserted in the top of the tank to 
measure liquid level. Similarly, acoustic systems have used transducers 
inserted in the tank to measure the volumetric change as fluid is removed 
from a tank. Strain gauges have been used on the exterior of the tank to 
measure the liquid weight or the volume of liquid remaining in the tank. 
Exterior mounting, or non-invasiveness, is obviously a desirable 
attribute, but strain gauges are expensive and have reliability problems 
even in environments that are only moderately hostile. All invasive 
systems, i.e. those that require the sensor or transducer to be inserted 
int he liquid container, have higher installation costs and usually are 
relatively unreliable, more expensive, and require more maintenance. These 
systems do not lend themselves to multi-tank applications easily. 
SUMMARY OF THE INVENTION 
This invention is the result of analysis to provide the best configuration 
for the liquid volumetric instrument gauges. For analysis and test three 
different liquids were considered. The liquids were gasoline, diesel fuel, 
and water. But, as the specified system depends on the change in weight of 
the tank as liquid is removed, other liquids could be substituted. Tests 
on prototype units, using dedicated interface hardware, software, and a 
personal computer proved earlier design analysis: that a single design 
approach would satisfy most requirements for a low cost and reliable 
liquid volumetric gauge. 
Mounting the transducer and the sensor on the outside surface of the tank 
provided the improvement in reliability, maintainability, and cost of 
installation by at least one order of magnitude over the present 
electro-mechanical level gauges. Ease of installation and initial 
adjustment design also exceeds that of the present competitive gauges. 
Several types of transducers were considered to excite the tank into 
mechanical resonance. Magnetostrictive, piezoelectric, sonarthumper, and 
moving coil transducers were analyzed for the major market requirements. 
All may be used for a variety of applications, but the magnetostrictive, 
piezoelectric, and moving coil transducers have been tested. The 
magnetostrictive devices will be described because of their high 
reliability under adverse environmental conditions. 
An electrical signal applied to a wire coil wound around a magnetostrictive 
material will change the dimensions of the material at the same frequency 
as the applied signal. Mechanically coupled to a structure it can produce 
a vibration in the structure at the same frequency as the electrical 
signal. The frequency of the signal can then be varied until the structure 
of the liquid container resonates. Known components in the electronic 
circuitry can then interpret the resonance frequency in terms of the 
liquid volume remaining in the tank.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 describes a unique embodiment of this invention, wherein a 
transducer 11 and a sensor 12 are mounted on the exterior surface of a 
tank 10. The structural resonance of the tank is measured. The electronic 
circuitry provides the data manipulation to display on a volumetric 
display 24 the volume of fluid remaining in the tank 10. 
This invention essentially consists of automatically tuning the tank 10 to 
its resonant frequency which is an analytic inverse exponential function 
of the volume of fluid remaining in the tank. Phase locked loop feedback 
control circuitry forces the transducer to change its frequency and track 
the resonant frequency received by the sensor. 
This invention is also unique in that dynamic rather than static 
measurement techniques are used. Strain gauges are considered static 
devices. The transducers and sensors used in this invention are dynamic 
devices. Dynamic signals can be defined as those signals which vary as a 
function of time and the intelligence is in the frequency of the signal. 
FIG. 1 displays the general design for a single tank system. The box 
elements in the circuits shown in the drawings are not shown in detail 
because they are all conventional off-the-shelf items known to those 
skilled in the art. The sensor 12 sends its dynamic signals to a suitable 
amplifier, which, in the case of a piezoelectric sensor would be a charge 
amplifier 13. The charge amplifier circuitry includes a bandpass filter 
and a line driver. The charge amplifier circuitry drives a switched 
capacitor filter 14. This filter 14 is controlled by the output of the 
voltage to frequency converter 17 and delivers its output to the phase 
detector 15. This phase detector drives a filter 16. The filter drives the 
voltage to frequency converter 17. The VFC 17, through a divide by 100 
circuit 18 (better known as a "clock divider"), closes the loop to the 
phase detector 15 and drives a transducer driver 19 which drives the 
transducer to vibrate the tank at its resonance frequency. The VFC 17 
drives conventional linearize circuits 20 which have a 
frequency-to-voltage converter driving an analog multiplier, the output of 
which is converted from an analog voltage signal back to a frequency 
signal in a voltage-to-frequency converter. These linearize circuits 20 
convert the structural resonances to frequencies that can drive a 
volumetric display 24 through a counter 22. The flow display 23 is 
inoperative when the residual volume is greater than 20%. The flow display 
and the counter are both provided with clock information by the clock 
circuits 21. 
The ZERO and SPAN (or GAIN) adjustments, which in practice would be made to 
the analog multiplier in the linearize circuits 20, permit a single system 
design to cover a wide range of applications. Similarly, the linearize 
circuits 20 will permit application to a wide range of liquid container 
configurations. 
For many applications, the transducer TX 11 is a magnetostrictive 
component. This type of transducer changes its physical dimensions under 
the influence of a change in magnetic flux. Tests have shown that this 
type of a transducer provides an excellent method of transmitting 
vibrations into a mechanical structure. For some applications the sensor 
12 is a piezoelectric film type component. This is a low cost, reliable, 
and easily installed component. 
FIG. 2 illustrates in schematic form the circuitry required for measuring 
the liquid remaining in a large number of tanks. These tanks may report 
sequentially their tank content or upon interrogation annunciate their 
content. A desk top computer 106 is shown in FIG. 2. This computer is 
augmented with interface cards containing data acquisition, digital signal 
processing, and multiplexing components. Software functions are used to 
track the volume remaining in the multiple tanks and annunciate the 
required data. An elapsed time clock in the computer provides the time 
when each tank was interrogated. The software, interface hardware, and 
computer memory provide the closed loop control and displays required in 
many applications. "ZERO" and "SPAN" (or "GAIN") adjustments, 
corresponding to the similar adjustments described for FIG. 1, are readily 
incorporated in the computer memory. As the intrinsic data is nonlinear 
the accuracy improves exponentially as the tank is emptied. This improves 
the accuracy of liquid flow information acquisition at low levels of 
liquids. For many fuel tank applications this is a requirement and FIG. 4 
illustrates the improvement in accuracy as the remaining fluid volume 
decreases. The number of tanks is limited only by the time required to 
reliably annunciate the quantity of liquid remaining in each tank. To 
improve the reliability in a noisy environment, digital filtering and 
averaging techniques are employed. 
The tank 100 in FIG. 2 is shown rectangular in shape, but may be 
cylindrical as shown in FIG. 1. As this invention describes a volumetric 
gauge, the shape of the tank is irrelevant. The only structural 
requirements of the tank are that the material have relatively high 
stiffness and low dampening. Steel, aluminum, and fiberglass tanks usually 
qualify. A sensor, such as piezoelectric sensor "RX" 101 is required for 
each tank. A charge amplifier "A1" 109 interfaces between the sensor 101 
and a multiplexer (MUX2) 111. A transducer driver 103 drives a transducer 
102. All other components are at the computer location. A multiplexer 
"MUX1" 104 routes the correct driving signal to the required tank through 
the driver 103. An output component 105 couples the signal from the 
computer to the transducer driver 103. The "COMPUTER" 106 stores the 
acquired data in its memory, displays the data on its "MONITOR" 118, and 
prints the data on the "PRINTER" 107. The multiplexer "MUX2" 111 receives 
the dynamic data from each of the tanks upon command from the computer 
interface "C" 110. Sequentially, the sample and hold component 112 
receives the dynamic data from its multiplexer and delivers it upon 
command to the analog to digital converter 113. The A/D converter 113 
receives its sampling frequency from the computer timer through the (FS) 
interface 114. The A/D converter 113, with sufficient samples, converts 
the analog signal and delivers a digital block of data to the digital 
signal processing component "DSP" 115. The DSP 115 Performs all the 
mathematical computations to meet the variety of requirements. This may 
include correlation as a method of tone decoding, digital filtering, and 
linearization of data for display and recording. The DSP 115 interfaces to 
the computer through an input/output component "I/O" 117 and a bus 
interface "C" 116. 
FIG. 3 describes, in schematic form, a low cost preferred embodiment of 
this invention. From the tank 200 to the voltage to frequency converter 
"VFC" 207 the circuitry and components are identical to that of FIG. 1. 
The output of the VFC 207 sends its resonance signal to the frequency to 
voltage converter 210. This component has a span adjustment, also known as 
a gain or full scale adjustment. The direct current voltage output of this 
component is connected to a filter invertor 211. This component has a zero 
adjustment. An optional warning signal for low fuel limits is provided for 
some requirements. The output of the filter invertor 211 is then connected 
to the input of a conventional display device, such as an analog meter 212 
or digital display. Analysis of the consumer, marine and industrial market 
requirements have shown that the inverse exponential response of this 
instrument to be an advantage. The accuracy of reading increases 
exponentially as the fluid volume decreases. This is interesting to marine 
vessel operators on long voyages. 
FIG. 4A is a graphical plot of the functional relationship between the 
resultant resonant frequency of the tank and the volume remaining in the 
tank. The frequency is considered the dependant variable. The volume of 
the fluid in the tank is considered the independent variable. This is 
justified as the fluids used in the tanks are relatively incompressible. 
Also, mass is considered equivalent to volume through a constant 
multiplier. The "ZERO" and "SPAN" adjustments compensates for all constant 
value considerations. From the dynamics equation of motion: 
EQU Fo=Mx+Cx+kX 
As the dampening coefficient is considered small and the stiffness is 
considered constant over the range of measurements: 
##EQU1## 
Of is defined as a force developed by an electrical signal applied through 
a transducer which drives the structure (tank) to mechanical resonance. C 
is defined as the dampening coefficient and is considered small. K is 
defined as the stiffness of the structure. Fr is defined as the resonant 
frequency of the tank in Hz. M in this equation is defined for diesel fuel 
at 57.2 pounds per cubic foot. K' in this equation is combined with other 
constants. K" is defined in this equation to convert mass to gallons. K" 
is the one parameter that is adjustable to meet the requirements of a wide 
variety of tanks. The "ZERO" adjustment compensates for the tare weight of 
the tank. The "SPAN" adjustment provides calibration of the full tank. 
FIG. 4B is a schematic diagram of the circuit used to acquire the data 
depicted in FIG. 4B. It is essentially a simplified version of the circuit 
shown in FIG. 2 but without the feedback from the sensor to track the tank 
resonance frequency, since that function is performed by the function 
generator. 
FIG. 5 describes data acquisition circuitry similar to that of FIG. 3, but 
this embodiment significantly reduces the cost of the transducer and 
forces the burden of capability into the design of the electronic 
circuitry. This is provided by the electronics with little increase in 
cost or reduction in reliability. Differing from FIG. 3 the transducer 301 
and the transducer driver 303 are no longer inside the feedback loop 
circuitry. The transducer driver 303 now delivers periodic narrow impulses 
to the transducer 301. The one-shot or impulse logic circuit 304 generates 
these impulses under control of a timer 305. The impulse logic circuit 304 
also controls the inhibit logic 306. The inhibit logic delays the response 
of the charge amplifiers 307 for a predetermined time after the impulse to 
the tank 300. The inhibit logic also precludes the operation of the ample 
and hold 311 components operation for a predetermined time. The charge 
amplifier 307 receives a resonant periodic transient signal from the 
sensor 302. The band-pass filter 308 presents this filtered signal to a 
modified phase locked loop component 309. The output of the phase locked 
loop component 309 is filtered in a filter 310 and sent to a sample and 
hold component 311, which drives a voltage controlled oscillator 312, 
which in turn, closes the loop to the phase detector within the phase 
locked loop circuitry. The voltage controller oscillator 312 in turn 
drives circuitry similar to that of FIG. 3. This consists of components 
313, 314, and 315. 
FIG. 6 describes the embodiment of components and circuitry that will 
satisfy several marine instrumentation requirements. This embodiment is 
identical to that of FIG. 1 from the tank 400 to the voltage to frequency 
converter 408. The tone decoder circuitry 409 provides a novel and low 
cost way to annunciate the volume of fuel remaining in the tank. The 
display panel 410 uses a series of lights to display the volume of fuel. 
In FIG. 6 the circuitry is designed to annunciate an alarm when the fuel 
drops below 2 gallons by flashing both the 0 and 2 gallon lights. Tone 
decoder circuitry 411 also permits annunciation of alarms when the 
signature vibrations of the engine or vessel controls are outside the 
normal range of frequencies for various speeds. A tone decoder also 
provides an alarm when frequencies are present in the water that alarm 
fish. A charge amplifier 412 receives the alarm signals from the sensor 
413. 
FIG. 7 describes an embodiment of instrumentation for dual marine engines. 
The instrumentation includes static quantity parameters such as 
temperature, pressure; with the VOLUMERIC FUEL GAUGE, a dynamic 
measurement device. All the sensor and transducer data is controlled by 
the multiplexer 500. The multiplexer receives address information from on 
of the four momentary contact switches on the front panel. Both front 
panel readouts are frequency counters. Frequency decoders provide alarm 
annunciation.