Intrusive acoustic sensor mounting arrangement

An intrusive mounting arrangement for an acoustic sensor includes an acoustic window and a flange assembly for mounting the sensor and window on a fluid tank, the flange assembly having a body portion configured to extend through an opening in the tank wall in a fluid-tight manner into the tank interior, the flange assembly having a body flange that is attachable to the tank wall; the body portion having a fluid passageway therein and a port that permits fluid in the tank to flow into the passageway; the flange assembly further comprising means for retaining the acoustic window in the passageway; the sensor comprising a body that is insertable in the fluid passageway in a fluid-tight manner; the sensor further comprising an active surface acoustically coupled to the window by fluid when the sensor is fully inserted in the passageway.

BACKGROUND OF THE INVENTION 
The invention relates generally to fluid level sensors, such as acoustic 
transducers that emit and receive acoustic pulses. More particularly, the 
invention relates to techniques for mounting ultrasonic fluid level 
sensors or other electronic devices in a container in an intrusive 
arrangement so that the fluid in the container wets the device or 
transducer active surface. 
The use of acoustic transducers for determining fluid levels in containers 
is well known. In one form of use known as intrusive, an acoustic 
transducer is mounted within the container so that the transducer emits 
the acoustic pulses directly into the fluid. Usually, a stillwell is used 
to reduce the effects of the fluid swashing around the transducer, as well 
as to provide a channel for the acoustic waves to follow to the surface of 
the fluid. 
A typical application of an intrusive transducer is with fuel tanks used on 
aircraft. By mounting a transducer at the bottom of a tank, the transducer 
can be used to emit pulses towards the fuel surface. The round trip time 
for the acoustic energy to be reflected hack to the transducer can be 
correlated with the fuel height when the velocity of the acoustic pulses 
in the fuel is known. 
Numerous problems are encountered with the known fuel sensor mounting 
arrangements. Among them is the fact that sensors typically are mounted to 
the tank in such a manner that in order to remove a sensor (such as for 
repair or replacement during routine maintenance the fuel must first be 
removed from the tank. Draining the fuel for simple repair or replacement 
of a sensor is an expensive and time consuming task. In other mounting 
arrangements, the sensors are fixed to the tank wall, thus not only 
requiring draining the fuel hut also an extensive tear down of the fuel 
tank. 
The next generation aircraft are expected to make extensive use of 
composite materials for the wings. In circumstances where the wing also 
serves as the fuel tank, tear down for sensor replacement will not be 
acceptable maintenance practice. 
The need exists, therefore, for an intrusive sensor mounting arrangement 
that permits quick and easy sensor installation and removal without 
needing to drain the fuel prior to sensor removal. The mounting 
arrangement should also provide minimal fuel displacement from the fuel 
tank when a sensor is removed; and the sensor should be installable and 
removable without tank or structural tear down or damage. 
SUMMARY OF THE INVENTION 
In view of the aforementioned problems with previous mounting arrangements, 
the invention contemplates an acoustic sensor mounting apparatus and 
method which, in a preferred embodiment, comprises a flange assembly 
attachable to the tank wall and having a fluid passageway therein, the 
sensor being adapted to mate and unmate with the flange assembly in the 
fluid passageway, the sensor being wetted by fluid when mated to the 
flange assembly, and the flange assembly comprising means for preventing 
fluid loss when the sensor is unmated therefrom. The preferred intrusive 
mounting method comprises the steps of mounting a flange assembly having a 
fluid passageway at a through hole in the fluid tank in a fluid-tight 
manner; sealing the flange assembly passageway to prevent fluid loss when 
the sensor is uncoupled from the flange assembly; and permitting fluid 
flow into the passageway when the sensor is coupled to the flange assembly 
to permit fluid to wet an acoustic element of the sensor. 
These and other aspects and advantages of the present invention will be 
readily understood and appreciated by those skilled in the art from the 
following detailed description of the preferred embodiments as the best 
mode contemplated for carrying out the invention, in view of the 
accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
The present invention contemplates in a general sense a mounting technique 
for positioning an electronic device inside a fluid container such that 
the device can easily be installed and removed without significant loss of 
fluid from the container. The invention is particularly useful for 
intrusive installation of a device within a container such that the device 
is directly exposed to the fluid therein. As an example, the invention has 
been found to be particularly useful for intrusive installation of an 
ultrasonic fuel level sensor inside an aircraft fuel tank. Although the 
preferred embodiments of the invention are described herein with reference 
to such a specific application, it will be readily appreciated that the 
invention can similarly be used in different situations that impose 
intrusive mounting requirements for an electronic device. 
With reference now to FIG. 1, a first embodiment of the invention is 
illustrated in partial cross-section. An ultrasonic transducer or sensor 
assembly is generally indicated with the numeral 10. The transducer 10 is 
mechanically matable with a mounting flange assembly 12. The sensor 
assembly 10 can be conveniently mated with the flange assembly 12 by a 
threaded connection, a bayonet-type connection, or other suitable 
arrangement. The mounting flange 12 is rigidly mounted to a wall 14 of a 
fluid container, such as, for example, a bottom wall portion of an 
aircraft fuel tank. 
The sensor assembly 10 may include a potted electronics portion 16 with a 
pigtail or cable connection 18 for supplying electrical power and signal 
lines. The sensor assembly 10 includes a body 20 that is provided with an 
external nut 22 to facilitate a threaded engagement with the flange 
assembly 12. 
The sensor assembly 10 further includes an acoustic element 24 such as, for 
example, a piezoelectric crystal that is typically provided with a backing 
pad 26 to reduce back reflections. The acoustic element 24 also provides 
an acoustically active surface 28 that is exposed to the fluid in the 
container 14. The active surface 28 may include a bonded material in a 
known manner that protects the crystal from adverse effects of the fluid 
or contamination. When the crystal is energized with a high frequency 
pulse, acoustic pulses are emitted into the tank fluid, reflected from the 
fluid surface, and detected by the sensor 10. 
A resilient seal 30 such as, for example, an O-ring, is positioned in a 
seal groove 32 formed in the sensor body 20. The seal 30 provides a 
fluid-tight interface between the sensor body 20 and the flange assembly 
12 to prevent fluid loss when the sensor assembly 10 is mated with the 
flange assembly 12. The O-ring 30 may be made, for example, of Viton 
material which is fuel compatible. 
The flange assembly 12 is mounted in a through hole 34 formed in the 
container wall 14. The flange assembly 12 is preferably provided with a 
backing flange 36 to help rigidly support the flange assembly in the 
through hole 34. It will be apparent that the flange assembly will be 
mounted in the hole 34 in a fluid-tight manner. This can be accomplished 
by any suitable means 38 such as preferably using an adhesive/epoxy 
bonding agent, or O-rings, gaskets, welding and so on. The flange assembly 
12 also includes a mounting flange 40 that is integral with a flange body 
42 that can be fixedly attached to the container wall 14 such as with 
threaded mounting bolts 44. 
The flange body 42 defines a fluid passageway 46 open to fluid in the 
container. The flange body has a counterbore 48 formed therein near one 
end of the fluid passageway. A second resilient seal 50, such as, for 
example, a Vitron O-ring, is seated in the counterbore 48. The flange 
assembly further retains an acoustic window 52, which may be a solid piece 
of plastic-like material such as ENVEX.TM. type polyimide. The acoustic 
window 52 is used for impedance matching between the sensor active surface 
28 and the fluid in the container. The acoustic window 52 should thus be 
designed with an appropriate thickness and acoustic impedance 
characteristics to achieve good acoustic coupling between the active 
surface 28 and the fluid in the container. The window preferably effects 
an odd multiple of the acoustic pulse quarter wavelength (.lambda./4) with 
an acoustic impedance that approximates the geometric mean of the 
impedances of the active surface and the fluid. According to an important 
aspect of the invention, the acoustic window 52 is acoustically coupled to 
the sensor active surface 28 by a thin film of fluid when the sensor 
assembly 10 is fully mated with the flange assembly 12. 
As illustrated in FIG. 1, the active surface 28 preferably extends slightly 
axially beyond the inner distal end 54 of the sensor body 20 so as to 
engage the acoustic window 52 as the sensor assembly 10 is mated or 
inserted in the flange assembly 12. 
The acoustic window 52 is movably retained in the flange body 42 by one or 
more ring-like wave washers 56. In the fully mated position illustrated in 
FIG. 1, the wave washers 56 are somewhat compressed between the window 52 
and a backup ring 60 held in position with a snap ring 62. The wave 
washers 56 are of sufficient diameter that they do not interfere with the 
acoustic pulses emitted from the sensor 10 through the window 52 into the 
fluid passageway 46. When the sensor assembly 10 is unmated from the 
flange assembly 12, the wave washers exert an axial force on the acoustic 
window 52 and urge it into a position in which it is seated against the 
counterbore seal 50. Whenever the sensor assembly 10 is less than fully 
mated with the flange assembly 12, the window 52 and seal 50 prevent fluid 
from escaping the container through the fluid passageway 46 and through 
hole 34. 
As the sensor assembly 10 is fully mated with the flange assembly 12, as in 
FIG. 1, the active surface 28 pushes on the acoustic window and displaces 
it to the position shown in FIG. 1. Fluid is able to flow between the 
active surface 28 and the acoustic window 52 to provide acoustic coupling 
between those elements. In order to facilitate this acoustic coupling, the 
inner end of the flange body 42 is provided with one or more cross ports 
58. The ports 58 are preferably positioned axially behind but adjacent the 
counterbore 48 such that a slight displacement of the acoustic window from 
the counterbore 48 (when the sensor 10 is fully mated with the flange 
assembly 12) permits fluid communication between the container interior 
and the active surface/window interface. Thus, the active surface 28 is 
wetted and acoustically coupled to the acoustic window 52 by the fluid in 
the container. The ports are preferably used because the acoustic window 
is diametrically sized close to the inner diameter size of the counterbore 
48. Thus, fluid flow would be less effective without the ports or other 
fluid path outside the periphery of the window 52. For example, instead of 
ports, channels could be formed in the flange body 42 to permit fluid to 
flow from the container interior to the active surface/window interface. 
Alternatively, the window periphery could be provided with notches (not 
shown) to channel fluid to the interface region. These are but a few 
examples of the many ways of enhancing the wetting action of the active 
surface/window interface. 
As illustrated in FIG. 1, the acoustic window 52 preferably is provided 
with a raised bead or other structure 64 that axially separates the active 
surface 28 and the window 52. For clarity, the amount of separation is 
exaggerated in FIG. 1. The interface between the active surface and window 
in practice will be quite small, for example on the order of less than 
.lambda./10 at a transmit frequency of one megahertz, such that the fluid 
provides a thin film-like acoustic coupling therebetween. With reference 
to FIGS. 2A and 2B, other embodiments are shown for the acoustic window 52 
to facilitate the fluid flow at the interface to the active surface. In 
FIG. 2A, radial serrations 70 are provided in the window interface region 
72. Fluid can thus flow along the serrations to provide the acoustic 
coupling with the sensor 10 active surface. The window region 72 
preferably is the same size as the active surface 28, thus permitting a 
peripheral reinforced region 74 to engage the wave washers 56. In the 
embodiment of FIG. 2B, the window 52 is provided with raised ridges 76 
that separate the active surface 28 from the window 52 just enough to 
permit a fluid interface. Again, the peripheral portion of the window can 
thus be reinforced to engage the wave washers 56. As illustrated in FIG. 
3, the window periphery may be provided with foot extensions 80 against 
which the wave washers 56 exert the sealing displacement force. These 
extensions 80 are notched as at 82 to prevent fluid from being trapped by 
the window 52. 
As stated, the wall 14 may be the bottom tank wall of an aircraft fuel 
tank. By appropriate selection of dimensions, the invention provides a 
mounting arrangement by which the active surface 28 is positioned, when 
the sensor 10 is fully mated with the flange assembly 12, at or near the 
very bottom of the tank yet can be easily wetted for low fuel level 
readings. The sensor 10, of course, can also be easily unmated or removed 
from the flange assembly 12 without significant loss of fluid from the 
container. 
Another important aspect of the present invention is that the acoustically 
active element 24 of the sensor 10 can be self-wetted and acoustically 
coupled by the container fluid to the impedance matching window 52 without 
the need for an acoustic gel at the interface. This wet interface 
technique also avoids the need for a rigid bonded acoustic coupling 
between the active surface 28 and the window 52. This aspect of the 
invention and its advantages can be realized not only with the mounting 
arrangement illustrated herein, but also with a mounting arrangement in 
which the acoustic window is fixedly held in the container 14 (not shown). 
In such an arrangement, of course, the container should be drained before 
removal of the sensor because the window will not be able to slide to a 
fluid blocking position. The fluid acoustic coupling with the impedance 
matching window is still a benefit in such applications since it obviates 
the need for rigid bonding between the active surface and the window. 
While the invention has been shown and described with respect to specific 
embodiments thereof, this is for the purpose of illustration rather than 
limitation, and other variations and modifications of the specific 
embodiments herein shown and described will be apparent to those skilled 
in the art within the intended spirit and scope of the invention as set 
forth in the appended claims.