Method and apparatus for leak detection with float excitation and self-calibration

A leak detector including a float having a number of magnets connected to the float. Electromagnetic coils are disposed along a length of the possible range of travel of the float within the leak detector. The coils, when energized, interact with the magnet making the float move-about and settle to a position representing its true equilibrium buoyancy. Optionally, the leak detector includes a self-calibration apparatus that provides a reference signal indicting the fixed position of the self-calibration apparatus, allowing the leak detector to compensate for offset errors.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates, in general, to fluid storage tank monitoring 
and, more particularly, to a float excitation system which efficiently 
minimizes the effects of fluid surface tension, allowing float to settle 
into the fluid to its true buoyancy for high precision leak detection. 
2. Statement of the Problem 
Leaking fluid product storage tanks represent a significant economic and 
environmental concern. Product leaks waste valuable product fluid stored 
in the tanks. Moreover, such leaks can cause water and possibly soil to 
contaminate the product fluid stored in the tank. More importantly, 
leaking fluid product storage tanks result in contamination of the 
surrounding soil and groundwater, which is especially critical when the 
fluid product is fuel, chemicals, or the like. 
Leaks are often so small that volume lost over time is a fraction of the 
storage capacity of the tank. Over time, however, significant quantities 
of product fluid are lost. The slow leak rate makes rapid leak detection 
particularly difficult. The measurement problem is complicated because 
underground storage tanks are particularly harsh environments in which to 
make accurate measurements. For example, temperature changes or changes in 
barometric pressure affect the product fluid level and the measurement 
apparatus to such an extent that the volume change caused by a slow leaks 
is difficult. 
Leak detectors, also called fluid level detectors, usually include a float 
that rides on the surface of the product fluid. The float is attached to 
some form of position transducer that generates a signal indicating the 
position of the float relative to some fixed reference position. The float 
position is monitored over a long period of time to detect leaks. Often, 
the tank is in use (i.e., having product added and removed) during the 
measurement, so the leak detector must distinguish between the slow, 
steady volume change caused by a leak and the more dramatic change caused 
by normal use. 
U.S. Pat. No. 4,850,223 entitled "LEAK DETECTOR", issued on Jul. 25, 1989, 
U.S. Pat. No. 5,156,042 issued on Oct. 20, 1992, and U.S. Pat. No. 
5,209,106 entitled "LEAK DETECTOR FLOAT SYSTEM AND METHOD THEREFOR", 
issued on May 11, 1993, set forth precision tank monitors which discloses 
problems involved in making highly accurate accounting of fluid products 
in storage tanks. These patents set out the problems involved in 
accurately detecting volume changes caused by environmental conditions 
such as temperature variation, changes in barometric pressure, and the 
like. 
One of the most important steps in obtaining correct readings is 
positioning the float as close to its true buoyancy with respect to the 
product fluid before performing a level measurement of the float. The 
following two prior approaches involve mechanical solutions to positioning 
the float. U.S. Pat. No. 5,209,106 discloses a mechanical vibrator placed 
on the probe such that vibrational waves are delivered down the probe to 
break the surface tension existing between the float and the fluid. Once 
the surface tension is broken, the float positions itself within the fluid 
product based upon the buoyancy of the float in the fluid. 
U.S. Pat. No. 5,220,310 issued to Pye on Jun. 15, 1993 pertains to a device 
having a motion-transmitting rod attached to a float. The rod extends 
upwardly through a steel core within a linear variable displacement 
transformer. A motor-driven cam applies a vibrational force that is 
transferred through a mechanical linkage including a pair of tension 
springs to the rod and float. After the vibrational force is removed, the 
float is subjected to three forces: the weight of the float, the float 
buoyancy force of the liquid, and the spring forces. The float responds to 
these forces by hunting up and down to seek an equilibrium. The range of 
the float travel is restricted to the practical range of linear variable 
displacement transformers and the practical length in which springs may be 
attached to the motion-transmitting rod member. 
A need remains in the industry for a robust leak detector that has high 
accuracy and is insensitive to environmental conditions. 
In general, the prior mechanical float excitation methods set forth above 
consume significant power due to the mechanical generation and coupling of 
the excitation signal. Mechanical excitation effectively breaks the 
surface tension of the float, but consumes significant power in the 
process. Because liquid level measurement tools are often powered by 
batteries and must operate reliably over a period of several days or 
weeks, power consumption is critical to long term usefulness and 
reliability. Also, the magnitude of voltage and current used to measure 
fluid levels in storage tanks containing volatile products such as 
gasoline, jet fuel, chemicals, etc. is of concern. A need exists for a 
leak detector with high efficiency and low voltage and operating current. 
Mechanical excitation systems use motors, vibration transducers, and 
mechanical linkages that are subject to wear. Also, complex mechanical 
systems are difficult and expensive to manufacture and repair. Because 
leak detectors are moved often and must operate in harsh environments, it 
is desirable to minimize or eliminate any use of mechanical components to 
position the float. 
Hence, a need exists to provide a system for achieving essentially true 
buoyancy of the float that (1) consumes little power and (2) does not use 
mechanical vibration while still providing a buoyancy system that operates 
in the harsh environment of a liquid storage tank. A further need exists 
to optionally provide self-calibration. 
3. Solution to the Problem: 
The problems set out above are solved by a leak detector that includes an 
energy efficient means for excitation of a float to repeatably position 
the float. The system of the present invention couples a high percentage 
of vibrational energy to the float itself so that power is not lost in 
mechanical linkages and vibration of other components. A leak detector 
that provides positive float movement and settling with respect to the 
surface of the fluid, that uses lower operating voltage and current and 
that increases efficiency and safety. The non-mechanical excitation system 
simplifies construction, transportation, installation, and maintenance of 
the leak detector. By minimizing mechanical linkages, errors caused by 
wear and thermal expansion of the components are limited. Also, by 
including a self-calibration apparatus, errors caused by temperature 
changes and the like can be accounted for making the leak detector highly 
accurate and robust. 
SUMMARY OF THE INVENTION 
Briefly stated, the leak detector of the present invention includes a float 
having one or a plurality of magnets positioned within it. Electromagnetic 
coils are disposed along the full length of the possible range of travel 
of the float within the leak detector. The coils, when current is applied, 
create a magnetic field that interacts with the magnets in the float. By 
controlling the pulse shape, frequency, amplitude, phase and duration of 
the excitation current applied to the coils, the float can be made to 
move-about and to settle to a position representing its essential true 
equilibrium buoyancy. This "true" position is repeatable and thus provides 
a highly accurate measurement of the product fluid level. 
Another feature of the present invention is a self-calibration apparatus 
that is integrated with the leak detector. The self-calibration apparatus 
includes an electromagnet positioned in a fixed location with respect to a 
position sensor. A signal is transmitted to the electromagnet and 
reflected back to the position sensor. Because the electromagnet is in a 
fixed position, any error in the position sensor can be determined and 
compensated for when the position sensor is used to determine the float 
position. The self-calibration apparatus allows the leak detector to 
compensate for environmental changes and provide consistently accurate 
measurements.

DETAILED DESCRIPTION OF THE DRAWING 
1. Overview 
FIG. 1 shows a first embodiment in accordance with the present invention as 
it would be deployed in a fluid product storage tank 10 containing a fluid 
product 20, such as gasoline. An elongated hollow test probe assembly 100 
is inserted extending downwardly into fluid storage tank 10. The fluid 
level, indicated by numeral 30 in FIG. 1, may change in height from full 
to empty from time to time due to product being removed from or stored 
into tank 10. Additionally, tank 10 may have a leak 40 which causes flow 
into or out from tank 10 effecting level 30. Also, fluid level 30 may rise 
or fall due to other physical phenomena such as expansion or contraction 
of fluid product 20 as temperature changes. 
The test probe assembly 100 would be suitable for either a portable test 
instrument being used to periodically test storage tanks or, permanently 
installed for continuous monitoring of a storage tank. Likewise, although 
the present invention is particularly suitable for underground storage 
tanks, above ground storage tanks may also be monitored in accordance with 
the teaching of the present invention. Fluid product 20 may be any fluid 
that is compatible (i.e., not corrosive) with the materials used to build 
test probe assembly 100. 
Test probe assembly 100 is hollow and contains in a lower section 115 a 
central rod 110 affixed by joining member 112 to a support 120. Float 700 
is positioned about central rod 110 so as to float freely up and down in 
the direction of arrow 130 as the fluid level 30 changes. Preferably, 
float 700 can move up and down over the full length of rod 110. It is 
important that up and down motion of float 700 not be restrained. Float 
700 includes a plurality of magnets 720 which serve a variety of useful 
functions described hereinafter. 
Also, optionally affixed at the lower end of rod 110 is a self-calibration 
apparatus 800. The self-calibration apparatus 800 includes several series 
coupled electromagnetic coils (shown in FIG. 8a through 8c) which may be 
selectively activated to provide calibration and compensation information. 
Self-calibration apparatus 800 is described in greater detail hereinafter 
in reference to FIG. 8a through FIG. 8c. 
Test probe assembly 100 has a lower section having a protective sleeve 140 
and a float enclosure 210 which adjoin at support 120. Excitation coils 
200 and 220 (220 shown in FIG. 2) are sandwiched between the float 
enclosure 210 and the protective sleeve 140. As shown in FIG. 1, test 
probe assembly 100 with protective sleeve 140 installed over excitation 
coils 200 and 220 is preferably a single tubular shape from top to bottom, 
which is, for example, one and seven-eighths inches in outside diameter. 
This makes a smooth assembly 100 to install into conventional fuel tank 
pipe risers 60, for example two inches inside diameter without 
obstruction. 
Test probe assembly 100 has an upper compartment 105 in which is sealed and 
secured a set of electronics including control circuitry 150. Control 
circuitry 150 preferably includes a microprocessor. Control circuit 150 is 
coupled by leads 152 through connector 154 and wire harness 160 to 
excitation coils 200 and 220. Control circuit 150 is also coupled through 
leads 156, connector 158 and wire harness 165 to self-calibration 
apparatus 800. Control circuitry 150 delivers electrical analog 
"excitation signals" to excitation coils 200 and 220 and to 
self-calibration apparatus 800. Control circuit 150 is preferably fully 
contained within compartment 105 of the test probe assembly 100. Details 
of compartment 105 and the delivery of wires 152 and 156 may be adapted to 
the needs of a particular application and are not presented in greater 
detail for ease of illustration and description. 
Control circuitry 150 communicates with an external microprocessor 50 to 
receive digital programming instructions and to upload acquired test and 
monitoring data. Communication path 55 may be a wire, optical, radio 
frequency, or other well known data link. External processor 50 may be 
intermittently connected to test probe assembly 100 only during upload 
operations or, may be permanently connected in applications that require 
continuous monitoring. Test probe assembly 100 operates under control of 
control circuitry 150 during normal operation and so can operate 
autonomously from external processor 50. 
In a preferred embodiment, a magnetostrictive linear displacement 
transducer comprising pulse transmitter and receiver elements (not shown) 
is mounted in a fixed position in the upper portion of probe assembly 100. 
Center rod 110 acts as a waveguide for directing measurement pulses to and 
from the transmitter and receiver elements. Magnetostrictive linear 
displacement transducers are well known and can be obtained from MTS 
Systems Corp., Research Triangle park, N.C. 27709 as TEMPSONICS II (R) 
linear displacement transducer or from Balluff Inc., 8125 Holton Drive, 
Florence, Ky. 41042. 
In operation, the magnetostrictive linear displacement transducer, under 
control of the microprocessor in control circuit 150, outputs an 
electromagnetic pulse. The electromagnetic pulse travels through center 
rod 110 and is reflected by magnetic fields created by magnets 720 in 
float 700. The reflected electromagnetic pulse travels back through center 
rod 110 and is detected by the receiver elements (not shown), and the 
position of float 700 is calculated from the elapsed time between the 
output pulse and the detected reflected pulse. 
In summary the present invention involves a leak detector designed to fit 
into a fluid product storage tank. The leak detector in accordance with 
the present invention may be permanently or temporarily installed in the 
tank. A linear displacement transducer, which is preferably a 
magnetostrictive transducer, measures the vertical position of the float 
700 with respect to a known, fixed position of a self-calibration 
apparatus 800. Although the preferred embodiment is described in terms of 
a magnetostrictive transducer, any type of linear transducer, or 
combination of transducers, may be adapted to the teachings of the present 
invention. 
2. Float Excitation Apparatus 
In accordance with the present invention, float 700 is made to settle to 
its "true" buoyancy position within fluid level 30 by an "excitation 
apparatus." The excitation apparatus includes control circuit 150, coils 
200 and 220, as well as float 700 and magnets 720 positioned therein. The 
components of the excitation apparatus cooperate to provide power 
efficient control over the motion of float 700 with respect to fluid level 
30. 
2a. Excitation Coils 200 and 220 
FIG. 2 illustrates a perspective view of the lower section 115 of the test 
probe assembly 100. Excitation coils 200 and 220 are exploded away from 
the surface of float enclosure 210 for purposes of illustration, however 
it should be understood that coils 200 and 220 are preferably positioned 
on the outer surface of float enclosure 210 when test probe assembly 100 
is fully assembled. 
Coil 200, comprises a right vertical segment 202, a left vertical segment 
204, top segment 206 and bottom segment 208. Coil element 200 additionally 
has wire leads including start lead 250 and end lead 255. Likewise, coil 
element 220, includes a front vertical segments 222, a back vertical 
segment 224, as well as top and bottom segments 226 and 228. Coil element 
220 additionally has wire start lead 260 and end lead 265. 
Coil element 200 is affixed to float enclosure 210 in such orientation as 
to have the center of top and bottom segments 206 and 208 aligned with 
dashed line 230. In like manner, coil element 220 is affixed to float 
enclosure 210 in such orientation as to have the center of top and bottom 
segments 226 and 228 aligned with dashed line 240. In the preferred 
embodiment, dashed line 230 and dashed line 240 intersect at a ninety 
degree angle, indicated by arrow 275, near a center of float enclosure 
210. 
Vertical segments 202, 204, 222 and 224 are long enough to span the entire 
desired or expected travel distance of float 700 (shown in FIG. 1) within 
float enclosure 210. Top segments 206 and 226 and bottom segments 208 and 
228 are approximately as long as one half the outer circumference of float 
enclosure 210. Top segments 206 and 226 and bottom segments 208 and 228 
are also curved to wrap around float enclosure 210 during assembly. Each 
of the vertical segments 202, 204, 222, and 224 are thus positioned 
approximately ninety degrees from a nearest other vertical segment about 
the surface of float enclosure 210. 
When excitation coils 200 and 220 are affixed to float enclosure 210 as 
described, vertical segments 202 and 204 are positioned 180 degrees from 
each other, and vertical segments 222 and 224 are 180 degrees from each 
other. The vertical segment 204 is overlapped by vertical segment 222 with 
each of the vertical segments 202, 204, 222 and 224 being 90 degrees from 
one another. The designations of right, left, and front, back for vertical 
segments 202, 204, 222, and 224 are for purposes of illustration only, as 
the present invention may be used in any orientation. 
Any suitable adhesive designed to withstand the environment presented by 
fluid product 20 may be used to affix excitation coils to the outer 
surface of float enclosure 210. Epoxy adhesive compound, such as that 
marketed as J B Weld epoxy, is preferably used. A droplet of adhesive (not 
shown in FIG. 2) every four to six inches along each of vertical segments 
202, 204, 222, and 224 is sufficient to secure the elements to the float 
enclosure body. 
Outer tubular protective sleeve 140 (shown in FIG. 1) slides over coils 200 
and 220 once they are attached to float enclosure 210 at the bottom end 
212 in so as to sandwich excitation coils 200 and 220 between float 
enclosure 210 and protective sleeve 140. When protective sleeve 140 is 
fully installed, a top edge of protective sleeve 140 preferably abuts top 
end 214 of float enclosure 210 at junction 218. Connectors 154 (shown in 
FIG. 1) for the wires 250, 255, 260 and 265 are thus protected within the 
protective sleeve 140. 
FIG. 3 shows a side planar view of the assembly shown in FIG. 2 with 
excitation coils 200 and 220 affixed to float enclosure 210. Wire leads 
250, 255, 260 and 265 are ganged together to form wire harness 160. Access 
hole 310 allows the wire leads 250, 255 and 260, 265 of harness 160 to 
pass through float enclosure 210 into inner space 270 for connections to 
connector 154 of the electronics earlier discussed. Wire harness 165 of 
the self-calibration apparatus 800, discussed latter, passes through 
access hole 310 to couple to connector 158. 
FIG. 4, illustrates an exemplary method of manufacturing excitation coils 
200 and 220. Length 410, which is the length of the vertical sections 202, 
204, 222 and 224, are chosen to match the height from top to bottom of 
fluid storage tank 40, or to match the desired vertical travel distance of 
float 700 (shown in FIG. 1). Width 420, which is the width of top and 
bottom sections 206, 226, 208 and 228, is, in a specific example, 2.1 
inches when the outside diameter of float enclosure 210 is 1.375. The coil 
elements 200 and 220 each consists of, for example, 43 turns of #28 gage 
magnet wire and terminated with Teflon coated leads 250, 255 and 260, 265 
respectively. The coil elements are placed so as to overlap each other by 
fifty percent when wrapped around float enclosure tube 210 as is indicated 
in FIG. 2 and FIG. 3. 
FIG. 5 shows an end planar view of excitation coils 200 and 220 wrapped 
around float enclosure 210 showing the vertical sections of the coils 200 
and 220 in their proper orientation. Coil segments 202 and 204 are aligned 
with axis 520, which is perpendicular to a central axis of float enclosure 
210. Coil segments 222 and 224 are aligned with axis 510. Axis 510 is 
substantially perpendicular to axis 520. 
FIG. 6 depicts in schematic form the electrical circuit of excitation coils 
200 and 220. It is to be expressly understood that the number of turns and 
the gage of magnet wire may be altered to fit the desired electrical 
requirements such as drive voltage and drive current. The number of turns 
and wire gage size directly effect the current efficiency of the 
excitation coil system of the present invention, as is well known in the 
field of electromagnet coil design. 
It is to be understood that the above discussion is only for the preferred 
embodiment and that the design and installation of the coils could be 
performed in other designs under the teachings of the present invention. 
For example, the location of the coils can be anywhere within an operative 
electromagnetic range of the float as it travels along the center rod. Any 
source of electromagnetic field could be used to move the float about as 
discussed in the text. 
2b. Float 700 
FIG. 7a through FIG. 7d illustrate various structural and operational 
features of float 700. In FIG. 7a, float 700 is shown in a top perspective 
view showing miniature magnets 720 disposed thereon. Preferably body 705 
is octagon shaped and has sloped sides 710. In a specific example, float 
700 has a 0.5625 inch hole 715 through the center. Magnets 720 are 0.050 
inch diameter by 0.375 inch long in the specific example. Also, magnets 
720 are high performance fully saturated magnets of neodymium iron boron. 
Eight magnets 720 are disposed at approximately 45 degrees from one 
another in holes around the circumference of the float body in the 
specific example. 
Each magnet 720 is positioned with its north pole pointed inwardly so that 
an inside diameter formed by the inner tips of magnets 720 is about 0.400 
inches in the specific example. Alternatively, each magnet 720 could be 
oriented with its south pole pointed inwardly, so long as the polarity of 
each magnet is the same. Each magnet 720 extends out and downwardly at a 
substantially 40 degree angle to the float body lower exterior. A working 
apparatus can be made with the magnet angle in the range of 20 to 60 
degrees with respect to the float body lower exterior. The particular 
angle is chosen to maximize interaction between a magnetic field created 
by magnets 720 and the magnetic field created by excitor coils 200 and 
220. It has been experimentally determined that a 40 degree angle 
maximizes this interaction in the geometries of the specific example, as 
is shown in FIGS. 7b through 7d. 
FIG. 7b is a cross-sectional view of the present invention at the elevation 
of the float of FIG. 7a as it may be in product fluid 20. Vertical 
elements 202, 204 and 222 and 224 of excitation coils 200 and 220 are in 
place on float enclosure 210. Note that due to surface tension of the 
fluid, float 700 is pulled over to one side so as to cause the inner tips 
722 and 724 of two of the magnets 720 to touch center rod 110. In the 
specific example, the 0.400 inch inside diameter of magnets 720 allow 
center rod 110 (which is 0.375 inch outside diameter) to pass through 
float 700 with a 0.025 inch clearance. It is possible that any of the 
eight magnets 720 may touch rod 110 at any given time in this manner. 
It is to be expressly understood that the teachings of the present 
invention are not limited to the above preferred embodiment. Any magnetic 
structure could be incorporated into the float which is operative with the 
production of the electromagnetic field. Likewise, the number, shape, 
location, and orientation of the magnets that are connected to the float 
may be changed so long as they still interact with the excitation coils. 
3. Operation of the Float Excitation Apparatus 
For purposes of illustration float 700 is oriented such that none of 
magnets 720 are aligned with any of the four coil segments 202, 204, 222, 
and 224. When current is applied to coils 200 and 220, an electromagnetic 
field is created within float enclosure 210 and magnets 720 align with the 
coils segments 202, 204,222 and 224. This "self-aligning" feature of the 
present invention ensures uniform, repeatable, and maximum interaction 
between the magnetic field created by excitation coils 200 and 220 and 
magnets 720 in float 700. 
In the example of FIG. 7b, float 700 rotates in the direction of arrow 730 
because outer magnet end 726 is closer to coil element 224 than is outer 
magnet end 728 to coil element 224. Each of the coil elements 202, 204 and 
222 respectively attract the nearest magnet 720 to help align float 700 to 
the electromagnetic field of the coil segments 202, 204, 222 and 224 when 
current is passing through the excitation coils 200 and 220. 
In the preferred embodiment, vertical segments 202, 204, 222 and 224 
positioned at each of the four ninety degree positions, and magnets 720 at 
each of the eight 45 degree positions, guarantee that there will always be 
an interaction between float 700 and excitation coils 200 and 220. The 
maximum rotation the float 700 of the preferred embodiment shall ever 
experience is 22.5 degrees, which is well within the flux patterns of the 
electromagnetic coils 200 and 220 and magnets 720. The directional arrow 
730 is for illustration and it must be understood that float 700 is free 
floating and may rotate in either direction depending on which of the 
magnet end 726 or 728 are in closest proximity with energized coils 
segments 202, 204, 222 and 224. 
FIGS. 7c and 7d illustrate motion of float 700 while energy is applied to 
excitation coils. In FIG. 7c, right coil segment 202 pulls float 700 to 
the right and downwardly and coil segment 204 pulls float 700 left and 
upwardly. This would be the effect caused by current passing through the 
coil in the direction of arrows 740 and 745. FIG. 7d shows the effect if 
the polarity were reversed on coils segment 202 and 204 in the direction 
of arrow 750 and 755. In this case, float 700 is pulled to the right and 
upwardly by the right segment 202 and left and downwardly by the left 
segment 204. In like manner, coil segments 222 and 224 (not shown in FIGS. 
7c and 7d), cause float 700 to rock back and forth, upward and downward as 
current passes through the coils segment 222 and 224 as polarity was being 
reversed. 
4. Self-Calibration Apparatus 800 
FIG. 8a is a side cross-sectional view of an embodiment of the 
self-calibration apparatus 800 in accordance with the present invention. 
Housing 805 has a cavity 810 and dividers 812 which compartmentize the 
inner housing into four areas. Each of the four areas hold an 
electromagnetic assembly such as core 840 and coil 845. Core 840 has 7 
layers of "E" shaped plates of steel and coil 845 includes 325 turns of 
#33 gage magnet wire comprising coil 845 wrapped around the center arm or 
tine of the "E" shaped steel plates in the preferred embodiment. The 
"open" end of the "E" shaped core is facing a center hole 820 of housing 
805. 
Three other cores 830, 850 and 860 (shown in FIG. 8b) are wound in like 
manner with the open end of the "E" structure facing inwardly around hole 
820. Encapsulant 815 seals electromagnetic assemblies securely in housing 
805. A center hole 820 extends through housing 805 which accepts the 
center rod 110. Securing screws 825, when tightened, permanently holds the 
self-calibration apparatus in position on the center rod 110. 
FIG. 8b is a top cross-sectional view of FIG. 8a showing electromagnetic 
assemblies in cavity 810. Winding 835 is connected in series with winding 
845 by connector 882. Likewise, winding 845 is connected in series to 
winding 855 by connector 884 and further in series to winding 865 of core 
860 by connector 886. 
FIG. 8c shows a schematic diagram of the electromagnetic assemblies of the 
self-calibration apparatus 800. One or more diodes, such as Schottky 
diodes 870 and 875, which parallel each other to provide redundancy and 
extra current handling capacity, are coupled across leads 880 and 888 
before exiting the housing 805 in wire harness 165. Diodes 870 and 875 are 
provided to clamp high voltages which result from an inductive kick when 
power is removed from series coupled coils 835, 845, 855 and 865. Diodes 
870 and 875 are redundant, that is to say each can handle the entire 
expected current, so that self-calibration apparatus 800 can continue to 
operate if one fails. Cathodes of diodes 870 and 875 are electrically 
wired to the positive (+) potential on lead 880. Diodes 870 and 875 also 
serve to discharge the serially coupled electromagnetic coils 835, 845, 
855 and 865 when the operating current is removed from lead 880 and 888. 
The inductive effect of coils 835, 845, 855 and 865 acts to reverse the 
potential across leads 880 and 888 when current is removed so that a 
positive potential appears at the anode side of the diodes. Because 
Schottky diodes 870 and 875 break over in the forward direction at 0.3 
volts, coils 835, 845, 855 and 865 are immediately shorted. Diodes 870 and 
875 must be selected to dissipate the charge stored in coils 835, 845, 855 
and 865 without damage, and so more diodes may be required and coupled in 
parallel as shown in FIG. 8c. This also assures no erroneous magnetic 
fields effecting readings by the magnetostrictive linear displacement 
transducer. 
5. Operation of the Self-Calibration Apparatus 
Coils 835, 845, 855, and 865, when energized, create magnetic fields 
similar to the magnetic fields of magnets 720 on float 700. This feature 
is most useful in the present invention when a magnetostrictive linear 
displacement transducer is used as the level sensing means, as in the 
preferred embodiment. If another type of linear displacement transducer is 
used, self-calibration apparatus 800 should be designed to provide another 
type of signal that is compatible with that linear transducer. What is 
essential is that self-calibration apparatus 800 is positioned at a fixed 
position with respect to the linear displacement transducer and so 
provides a reliable reference position for calibration of the position 
measurement of float 700. 
In operation, the magnetostrictive linear displacement transducer, as 
controlled by the microprocessor of the control electronics 150, outputs a 
measurement pulse and waits for a return pulse. The return pulse is 
essentially the output measurement pulse that echoed or reflected off of 
the magnetic fields created by magnets 720 or self-calibration apparatus 
800. 
A first return pulse is echoed or reflected from magnets 720 in float 700 
as it is floating in the product fluid 20. Hence, the first return pulse 
indicates the position of float 700 and the product fluid level 30 (shown 
in FIG. 1). A measurement of the position of float 700 may involve several 
hundred repetitions of the processes of outputing a measurement pulse and 
measuring the return pulse from float 700. The process of making repeated 
measurements is known as "recirculation". The several hundred measurements 
are averaged or otherwise statistically combined to calculate a single 
measurement value. Preferably, this single measurement value is stored in 
memory in control circuit 150 (shown in FIG. 1). During the measurements 
of float 700, self-calibration apparatus 800 is not energized to prevent 
echoes or reflections of the measurement signal from self-calibration 
apparatus 800 that would confuse or confound the measurements of float 
700. 
Once the several measurements of float 700 are made, self-calibration 
apparatus 800 is energized. A second return pulse is echoed or reflected 
by self-calibration apparatus 800 when the electromagnetic assemblies in 
self-calibration apparatus 800 are energized. Again, several hundred 
measurements are taken using the recirculation method and averaged or 
otherwise statistically combined to calculate a single measurement value. 
Preferably, this single measurement value is stored in memory in control 
circuit 150 (shown in FIG. 1). Thus, two independent measurement readings, 
the first from float 700 and the second from self-calibration apparatus 
800, are taken and stored. 
While the self-calibration measurements are taken, any reflections of the 
measurement signal from float 700 over the entire known range of travel of 
float 700 are electronically "blocked out". In other words, an electronic 
window or time window is created around the expected arrival of the 
reflected measurement pulse from self-calibration apparatus 800, and all 
pulses that are detected outside of this window are ignored. This allows 
an accurate measurement of self-calibration apparatus 800 despite noise of 
reflected signals from float 700. The purpose of the self-calibration 
apparatus data is to give calibration information as to electronic drift 
in any of the components including control circuit 150 or other circuity 
used in the measurement apparatus. 
Because self-calibration apparatus 800 is permanently affixed in a fixed 
position and the exact distance is known between self calibration 
apparatus 800 and the linear displacement transducer, any discrepancy 
between the known position and the measured position of self-calibration 
apparatus 800 calculated from the signal returned by self-calibration 
apparatus 800 must be caused by component or environmental error. Since 
the factors bringing about the error closely or identically affect the 
position measurement of float 700, the reference signal from 
self-calibration apparatus 800 can be determined and used to compensate 
the signal returned by float 700 when measuring the position of float 700. 
Similarly, any distortion caused by thermal effects and electronic drift 
inherent in magnetostrictive and electronic devices can be "zeroed-out" 
once the reference signal from self-calibration apparatus 800 is known. 
In summary, the method of operating the leak detecting float system in 
accordance with the present invention begins by initially positioning 
float 700 in fluid product 20. Excitation coils 200 and 220, for example, 
are used to provide an initial magnetic field in proximity of float 700 to 
abruptly move float 700 to break float 700 from surface tension of fluid 
product 20. Other methods of providing an acceptable magnetic field such 
as using permanent magnets that are moved into proximity with float 700 
may also be used in accordance with the method of the present invention. 
Preferably a final electromagnetic field is provided in proximity of float 
700 to gently move float 700 to a final position near a true buoyancy 
position. 
The position of float 700 is then measured using a magnetostrictive linear 
displacement transducer in the preferred embodiment. This measurement 
process is initiated by transmitting a measurement signal to the float 
through center rod 110. The measurement signal reflects from the magnetic 
field provided by magnets 720 in float 700 to provide a first reflected 
signal. Optionally, a measurement of self-calibration apparatus 800 is 
performed in a similar manner if one is provided. 
The reflected measurement signals from float 700 are detected. An elapsed 
time between the transmission of the measurement signal and detection of 
the signal from float 700 is determined. This first elapsed time indicates 
a raw, uncalibrated position of float 700. Preferably several hundred 
measurements are taken in this manner using the recirculation process, and 
combined to determine a single measurement value. 
The self-calibration apparatus 800 is then energized and second elapsed 
time between the transmission of the measurement signal and the detection 
of the signal reflected by self-calibration apparatus 800 is determined. 
This second elapsed time indicates the position of self-calibration 
apparatus 800 and serves as a calibration measurement. Again, several 
hundred measurements are taken and combined using recirculation. 
Preferably, the measurement of float 700 is stored independently of the 
measurement of self-calibration apparatus 800. 
Once the measurement of float 700 and the calibration measurement are 
known, a compensated position measurement of float 700 is calculated. This 
calculation may occur immediately after the measurements are taken, or 
performed at any convenient time after the measurements are taken when the 
two measurements are stored as in the preferred embodiment. 
6. Control Electronics 
FIG. 9 is a schematic block diagram of control electronics 150 of the 
present invention relating to the control of excitation coils 200 and 220 
and self-calibration apparatus 800. Signals from the microprocessor 900 
are provided on line 901 and are eight-byte digital coding inputed to 
digital to analog (D/A) converted 905 in the preferred embodiment. 
In the preferred embodiment, the eight-byte data signal yields 256 possible 
levels for the analog signal output of D/A converter 905 over line 906 to 
power amplifier 910. Power amplifier 910 is capable of driving all 
circuitry which is attached over the common line 912. The inputs to 
electronic switches 915, 925, 935 and 945 receive signals appearing from 
common line 912 and are further connected back to an analog-to-digital 
(A/D) converter that is integral to microprocessor 900 for monitoring. 
Signals on common line 912 are also provided on wire 880 through connector 
158. 
ON/OFF signals from microprocessor 900 are provided on line 914 and are 
commonly coupled to control inputs or gates of electronic switches 915 and 
920. Electronic switches 915 are preferably solid state devices provided 
in either discrete or integrated circuit form. Note that electronic switch 
915 is a "negative" gate type electronic switch (i.e., a P-channel FET or 
a PNP bipolar transistor), while switch 920 is a "positive" gate type 
electronic switch (i.e., an N-channel FET or a NPN bipolar transistor). In 
other words, the switches 915 and 920 form a complementary pair of 
switches. The complementary pair of switches 915 and 920 each have one 
current carrying electrodes coupled in parallel to output lines 922. 
Likewise, ON/OFF signals from microprocessor 900 provided over lines 924, 
934, and 944 are commonly connected to the control gates of complementary 
pairs of transistors 925 and 930, 935 and 940, and 945 and 950 
respectively. Each switch in each pair of complementary switches has one 
current carrying electrode coupled to a common output to form output lines 
932, 942, and 952. Further, microprocessor control signals over line 958 
connect to a control gate of electronic switch 960. One current carrying 
electrode of switch 660 is coupled to output line 962. 
Line 968 is coupled to ground or common potential through resistor 970. 
Line 968 is also coupled to one of the current carrying electrodes of each 
of electronic switches 920, 930, 940, 950 and 960. Line 968 is further 
coupled back to an A/D converter in microprocessor 900 for monitoring. 
The outputs of complementary switches 915 and 920 are commonly connected 
over line 922 to lead 250 through connector 154. Likewise, outputs of 925 
and 930 over line 932, 935 and 945 over line 942, and 945 and 950 over 
line 952 connected to leads 255, 260 and 265 respectively through 
connector 154. 
In operation, microprocessor 900 within control circuitry 150 in the 
preferred embodiment controls and monitors all activity in a conventional 
manner by executing program instructions. The amplitude of energy (voltage 
and current) available to excitation coils 200 and 220 is determined by 
power amplifier 910. Power is selectively switched to coils 200 and 220 by 
electronic switches 915, 925, 935 and 945 as "ON" signals appear over line 
914, 924, 934 and 944 respectively. 
Electronic switches 920, 930, 940 and 950 react opposite to the "ON" 
signals by switching off. That is, for each set of complimentary 
electronic switches 915 and 920, 925 and 930, 935 and 940, and 945 and 950 
only one is activated at a time for a given signal on the control lines 
914, 924, 934 and 944 respectively. An "ON" signal the control lines 
914,924, 934 and 944 causes switches 915, 925, 935 and 945 to activate or 
become conductive, while an "OFF" signal would cause switches 920, 930, 
940 or 950 to activate. 
When electronic switches 920, 930, 940 or 950 are activated, a ground 
potential is applied to the excitation coils 200 and 220 over lines 922, 
932, 943 and 952 respectively. While a ground potential is the most 
convenient potential to apply through switches 920, 930, 940 and 950, it 
should be understood that any potential, fixed or variable, might be 
applied rather than ground potential to meet the needs of a particular 
application. 
Electronic switch 960 has an output connected to wire lead 888 over line 
962 through connector 158. Switch 960, upon activation by an "ON" signal 
over line 958 couples the ground potential from line 968 to wire lead 888 
of electromagnetic coil 865. 
It is important to understand the function of monitoring the output energy 
source of the power amplifier 910 over line 912 and the ground potential 
through resister 970 over line 968 by the microprocessor. The signal over 
line 912 gives positive confirmation that a desired energy level has been 
produced by the power amplifier 910. Further, as each of the electronic 
switches 920, 930, 940, 950 or 960 are activated, the current drawn over 
line 968 is measured by the microprocessor. In this way control circuit 
150 can determine if either of excitation coils 200 or 220 has developed 
an "open" wire and is malfunctioning and thereby not giving excitation to 
float 700. Likewise, control circuit 150 can determine if the 
electromagnets of self-calibration apparatus 800 are functioning properly. 
Any level reading of float 700 or self-calibration apparatus 800 while 
experiencing an open wire malfunction, could be properly indicated as 
"flawed" measurements and data not relayed upon, and further to give 
notice for repair. 
To produce a desired frequency of the excitation signal applied to 
excitation coils 200 and 220, electronic switches 915, 925, 935 or 945 are 
toggled on and off under control of microprocessor 900. Pulse duration for 
each pulse in the excitation signal is controlled by controlling the 
elapsed time switches 915, 925, 935 or 945 are activated ON. 
An "in phase" case occurs when coil segments 250 and 260 are coupled to 
power amplifier 910 as ON signals appear over lines 914 and 934, while the 
ground potential coupled to coils segments 255 and 265 as OFF signals 
appear over lines 924 and 944. An "out of phase" case occurs when coil 
segments 250 and 265 are coupled to power amplifier 910 as ON signals 
appear over lines 914 and 944, while the ground potential coupled to coils 
segments 255 and 260 as OFF signals appear over lines 924 and 934. Exactly 
opposite of the above may be selected by reversing which coil segments is 
coupled to power amplifier 910 and which is coupled to ground potential. 
The pulse shape of the excitation signal presented to the excitation coils 
is controlled by D/A converter 905 and power amplifier 910. A square wave 
is generated by switching D/A converter 905 between two fixed amplitudes. 
Alternatively, the pulse may be sloped-shaped with a linear, progressively 
increasing amplitude followed by a linear progressively deceasing 
amplitude output of D/A converter 905 and power amplifier 910. A sine wave 
may be achieved similarly with consideration given to rounding the signal. 
The combination of using various shaped signals at various frequencies, 
and the use of the amplitude, duration, phase and frequency in the 
activating of coils 200 and 220 cause precision excitation of float 700. 
One example of this combined effect is liken to a coin being spun on end. 
It first would rotate rapidly on end an as its inertia diminished, it 
would start to wobble followed by a rolling on its corner edge. Finally 
the coin would wobble increasing in its rotational frequency as its side 
becomes lower and lower to the surface until it settles gently to a stop 
flat on the surface. 
In a preferred embodiment, the excitation signal is designed to draw float 
700 from side to side and back and forth while being drawn up and down, 
giving a warbling effect like the coin mentioned above as it breaks the 
surface tension. An example excitation waveform for one of coils 200 or 
210 is shown in FIG. 11. In FIG. 11, the horizontal axis represents time 
and the vertical axis represents current amplitude in excitation coil 200 
or 220. 
An initial time period 1101 spanning from t.sub.1 to t.sub.2 has a 
square-wave shaped wave of relatively large magnitude and abrupt changes 
in current. The abrupt excitation during time period 1101 serves to break 
float 700 free from surface tension. The phase is switched in the second 
half of each pulse (i.e., the portion of each pulse below the centerline). 
The amplitude steadily diminishes and frequency increases through time 
period 1101. 
As the excitation period advances towards intermediate time period 1102, 
spanning from t.sub.3 to t.sub.4, a more subtle combination of excitation 
signals allows float 700 to go into a quiver settling as the float finds 
its true buoyancy. Here, again, the phase is switched to produce the 
second half of each pulse, but a more gentle slope is used to reduce the 
movement of float 700 as the signals gradually diminish during time period 
1102. 
During a final time period 1103, spanning from t.sub.5 to t.sub.6, the wave 
shape is rounded like a sine wave, amplitude is lower, and frequency 
higher than during initial time period 1101. At the end of time period 
1103, movement of float 700 diminishes to a stop--leaving float 700 fully 
settled at near-perfect equilibrium. At this point, a level reading may be 
taken of float 700 to accurately and repeatably measure its position. 
The preferred sequencing of pulses in the excitation waveform shown in FIG. 
11 is only an example of a possible scheme to indicate the float control 
that the present invention provides. It is possible that a less complex 
excitation scheme could provide adequate performance in a particular 
application. For example, a single pulse shape could be used for all time 
periods 1101, 1102, and 1103. Alternatively, pulse amplitude and or 
frequency could remain constant or vary non-linearly throughout each time 
period. 
It is to be expressly understood that any number of excitation coils 200 or 
220, with appropriate drive circuitry such as 915 and 920, may be used in 
the present invention. For example, two additional sets of excitation 
coils 200 and 220, totaling eight segments, could be cycled on and off 
around the circumference of the float body to create the rotational 
spinning of the float 700. Which in effect, would "screw" or "twist" the 
float into its true buoyant position. Further, the necessity of the float 
enclosure 210 could be foregone and suspend the excitation coils in free 
space around the float 700. The approach shown in the present patent is 
exemplary of one approach an is the preferred embodiment. 
Components illustrated in schematic block diagram form in FIG. 9 represent 
components that are commonly available in a diversity of configurations 
from many manufacturers. Such components are easily connected to one 
another by those skilled in electronics. It is to be expressly noted that 
while construction details for coils shown in FIG. 9, such as the number 
of turns, wire size and wire length have been set forth and discussed for 
electronics shown in FIG. 9, other windings and number of turns may be 
substituted to result in the same function in accordance with the present 
invention. 
FIG. 10 illustrates a side perspective of an alternate embodiment similar 
to the view of the first embodiment shown in FIG. 2. In this embodiment, a 
coil 1010 is wound directly onto float enclosure 210. A process of winding 
coil 1010 begins near a top end 214 of float enclosure 210 with wire start 
lead 1050. Wire start lead 1050 is coupled to beginning length 1020 which 
extends along the desired length of travel of float 700 (shown in FIG. 1). 
Coil 1010 including beginning length 1020 spans a length similar to length 
410 indicated in FIG. 4. 
A first turn 1030, located at a bottom end 212, is wrapped around the outer 
surface of float enclosure 210, preferably in a counterclockwise 
direction. First turn 1030 will wrap over or on top of beginning length 
1020, as do all the other turn or loop of coil 1010. Hence, beginning 
length 1020 is protected from damage by the loops of coil 1010. The 
winding process is continued with each turn or loop of coil 1010 
positioned tightly against the previous turn or loop. The winding process 
is continued until coil 1010 extends near top end 214 of float enclosure 
210. Wire end lead 1055 couples to a last turn or loop in coil 1010 and is 
preferably adjacent to wire start lead 1050. 
Coil 1010 is excited in a manner similar to that described in reference to 
coils 200 and 220, except only one coil need be excited. Power amplifier 
910 (shown in FIG. 9) and complementary switch pairs 915, 920 and 925, 930 
are used to supply power to start lead 1050 and end lead 1055. Current 
passing through coil 1010 creates a magnetic field within float enclosure 
210. 
The magnetic field thus created interacts with magnets 720 in float 700 to 
move float 700 up and down within float enclosure 210. This up and down 
motion causes float 700 to find its true buoyancy. The up and down motion 
of the alternative embodiment should be compared to the back and forth 
rocking motion created by coils 200 and 220 in the preferred embodiment. 
In either the first or second embodiments, float 700 is efficiently 
excited and caused to break any surface tension of float 700 in product 
fluid 20 (shown in FIG. 1) without having to vibrate the entire assembly 
100. 
The magnetic field can be controlled by changing polarity (i.e., direction 
of current flow) and by controlling pulse shape, frequency, amplitude and 
duration of the current flow in coil 1010. A preferred excitation wave 
pattern, illustrated in FIG. 11, begins as a low frequency, high amplitude 
square wave pulse (or series of pulses) sufficient to break float 700 free 
of the effects of surface tension. Subsequent pulses decrease in amplitude 
and pulse shape changes from an abrupt square wave to more gentle shapes 
such as a sloping triangle wave or sine wave. Subsequent pulses also 
increase in frequency to cause float 700 to change from abrupt turbulent 
motion to a gentle quivering stir. Finally, the pulse amplitude diminishes 
to bring float 700 to a gentle fluttering motion as it fully positions 
itself to a tranquil stop at true buoyancy equilibrium. 
It has been found that a leak detector constructed and operated in 
accordance with the teachings of the present invention offers power 
efficiency as well as low operating voltages and currents so as to be 
approved for uses in volatile fluid storage systems. Most importantly, a 
leak detector in accordance with the present invention has been tested and 
shown to provide a probability of detecting a leak and a probability of a 
false reading (i.e., detecting a leak where none exists or not detecting a 
leak when one does exists) which greatly exceed current government 
requirements for accuracy and are attributable in large part to the 
precise and repeatable settling of float 700 at its true buoyancy before 
measurements are taken. 
It is to be expressly understood that the claimed invention is not to be 
limited to the description of the preferred embodiment but encompasses 
other modifications and alterations within the scope and spirit of the 
inventive concept. For example, the overall geometry of the leak detector 
may be modified to have any convenient cross-section shape to meet the 
needs of a particular application. More excitation coils may be used. The 
circuitry used to drive the excitation coils may be modified and still 
provide useful operation in accordance with the present invention. 
Accordingly, these and other modifications are within the scope and spirit 
of the present invention.