nuckolls.bob(at)aeroelect
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 Posted: Wed Jan 11, 2012 1:15 am Post subject: Capacitive Fuel Level Sensor - How do they work? |
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| Quote: | I believe the capacitive probe will be linear over its length as
well; it won't "know" about the cross sectional area of the tank at
any given sensing point, just whether or not fuel is present there.
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True for off-the-shelf probes . . . wherein the designer
has no first-hand knowledge of tank geometry for the
proposed installation. It is possible to build a capacitive
probe that presents an output that is not linear with
relation to liquid level. This patent speaks to some
examples:
http://tinyurl.com/6ogp4ve
The advantage may be that a long capacitive probe can extend through
a tank designed such that a float type sensor, on one end or the
other, will 'range out', because when one end of the tank is full,
the other end is still not full, or vice versa. It's also possible,
though difficult, to custom bend a capacitive probe so that there's
more range (flatter slope) in greater cross-sectional areas of the
tank, and less range (steeper slope) in smaller areas of the tank;
that tends to linearize the output. But it's a tough thing to
accomplish with high resolution.
Until the micro-controller came along.
The last trade study I did for HBC spoke to a system with
a self calibrating feature. You put the signal
conditioner for a pure linear probe into a calibrate
mode, put unusable fuel in tank and then tell the
signal conditioner "this is zero fuel". Then fill
the tank with 10% steps for capacity stopping each
time to tall the signal conditioner to "remember
this value as x%" ending up at 100% or full.
When taken out of the calibrate mode, the signal
conditioner does a linear interpolation of the
fuel level across the constellation of calibrated
data points to offer a very close representation
of available fuel irrespective of tank and probe
geometry.
The approach to sensing and signal conditioning
would produce a device that's adaptable to virtually
any airplane and combination of probes.
This produces a gage that is "too accurate" in
that substantial uncertainty in calibration points
is introduced by dielectric constant of various
loads of fuel and temperatures. The "ultimate"
system includes a calibration probe at the bottom
of the tank that is always submerged and offers
the signal conditioner a real-time sample of
variables for that particular load of fuel.
The electronics becomes so simple that it costs
more to build an install a sturdy set of probes
in something like a Hawker 4000 than the
electronics which processes data from those
probes! Bill of materials for the electronics
came in at about $10 in production lots.
| Quote: | In the 80's in process instrumentation industry, there were a number
of well-designed (non-interactive adjustment) signal conditioners,
for under $100, that had three or four linearization adjustments
across an input range.
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I took a whack at such a device for Cessna about
that time. They were proposing a spring cartridge
in the elevator controls that could be 'wound up'
to offer a constant stick-force-per-G under the range
of flight conditions of airspeed and CG. Made my
head get real tight! The last iteration drove
a non linear cam to 'get the curve'. The system
we proposed never made it into the airplane.
Don't recall how they solved the problem.
A couple years later, we (Electro-Mech) did what
I belive is the first micro-processor driven trim
system that flew on the aborted Mooney M30 program.
This did use the lookup table I described
for fuel level signals to drive the pivot location
actuator for a servo/anti-servo tab.
That showed more promise but the project got
canceled (prototype missed target weights by
about 500 pounds!). In any case, it proved the
future utility of software for slaying such
dragons.
Bob . . .
Bob . . .
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