[DeTomaso] Fuel Gage Linearizer

Sat Nov 1 15:56:52 EDT 2008

Hi guys,

        I have been away from the list for a while and was searching through
the archives.  I came across a message about a circuit I designed a couple
of years ago.  Being an instrumentation engineer for 3 decades, one of my
pet peeves developed over the years is the non-linearity of automotive fuel
gages.  The fuel gage in most cars is really a fuel level gage and not a
fuel quantity gage.  Since the fuel level transducer is usually a wire-wound
potentiometer actuated by a float at the end of an arm, the potentiometer
output is proportional to the angle of the float arm which is a sinusoidal
function.  Most of the automotive fuel level pots I have examined have a
linear output with respect to rotation angle of the shaft.  So, the fuel
level with respect to the output of the potentiometer becomes a function of
the length of the float arm and the sin of it's angle.  In other words the
measurement, if uncorrected, is non-linear.  This is sometimes corrected for
by manufacturing a non-linear pot that linearizes the sinusoidal output in
the range of interest.  Remember, that is for fuel LEVEL.  When you
translate level into quantity. The shape of the fuel tank becomes the next
source of non-linearity.  Obviously if you had a cone shaped tank, one inch
of fuel level near the top is a lot more than quantity 1 inch at the bottom.
Depending on the irregularity of the cross-sectional area of the tank as the
fuel level rises, the equation for quantity Vs. level can get quite
complicated.  The simplest way to correct the situation and still be
adaptable to different tank shapes and sizes is to use a micro-controller
and a calibrated look up table to translate the level into actual quantity.
The design I came up with for my car uses a PIC microcontroller that has on
board an EEPROM to store the look-up table.  Operationally, the fuel level
potentiometer is connected to my circuit which reads the resistance and
"looks up" the correct corresponding fuel quantity. It then drives the gage
with the correct voltage to indicate the actual quantity.  The setup
requires calibrating the tank at 1 gallon intervals which means draining the
tank and filling it one gallon at a time.  Since I was only building one of
these circuits for myself, I did not write any code to have the micro
perform the calibration automatically, meaning, that I recorded ADC output
on my computer, generated the lookup table with Excel and manually entered
the lookup table into the micro.  I did the same thing for the one I built
for Chuck Buthala.  But that meant he had to perform the calibration, email
me the data, and I programmed his look up table and sent the box to him.

        If you guys are interested in something like this, I can do 1 of 2
things.  I can put together a kit consisting of a PCB and a parts list with
a schematic and a wiring diagram.  Or, I can build you one and send it to
you with installation instructions.  But unless you are familiar with PIC
microcontrollers and own a programmer, you would have to send me the data
and I would have to program it and send it back to you.  In order for me to
write the code for an auto calibration routine, I would have to have enough
commitments to justify the time spent.  Bear in mind, I'm not trying to make
money here, just offering this to my Pantera brothers if they are
interested.  If there is enough interest, I will figure out a price based on
the responses I get.

        Cheers,

               Fast Eddie  (#2100)

P.S.  If my explanation is either too complicated or too simple, please let
me know and I will clarify any questions you may have. 

Edward A. Nauman\

C.E.O.

Smart Litez Inc

Palmdale, CA. 93551

(661) 305-7636

>The circuit was designed by Ed Nauman.  He built one for me and it works
great.  The gizmo works with a chip which uses the sending unit resistance,
compares it to a look up table in the memory and >sends the correct signal
to the gage.  You have to go though a calibration cycle to set it up.  There
is an article in the POCA news about two years ago on the circuit.  Ed if
your listening I hope my >description is correct.

>"www.ProvaMo.com" <pantera007 at sbcglobal.net
<http://list.realbig.com/mailman/listinfo/detomaso> > wrote:   It was not
"Me" Chuck that Ohm'ed, but I did test one gallon at a time, the
>ohming was, I think, someone in the Nordic region perhaps.

>Chuck

-----Original Message-----
From: detomaso-bounces at realbig.com
<http://list.realbig.com/mailman/listinfo/detomaso>
[mailto:detomaso-bounces at realbig.com
<http://list.realbig.com/mailman/listinfo/detomaso> ]On
Behalf Of Mark McWhinney
Sent: Tuesday, July 24, 2007 10:41 AM
To: 'Garth Rodericks'; detomaso at realbig.com
<http://list.realbig.com/mailman/listinfo/detomaso> 
Subject: Re: [DeTomaso] Incorrect gas gauge

Some of the fancier aftermarket gauges allow for multiple calibration
points. I think little Larry has that. If he is lurking, may he can fill
us in.

As I recall, Chuck did the one-gallon at a time work to record the ohm
readings.

-----Original Message-----
From: detomaso-bounces at realbig.com
<http://list.realbig.com/mailman/listinfo/detomaso>
[mailto:detomaso-bounces at realbig.com
<http://list.realbig.com/mailman/listinfo/detomaso> ] On
Behalf Of Garth Rodericks
Sent: Tuesday, July 24, 2007 10:33 AM
To: detomaso at realbig.com
<http://list.realbig.com/mailman/listinfo/detomaso> 
Subject: [DeTomaso] Incorrect gas gauge

Just another reference point for the non-calibrated nature of our gas
gauges.

When mine is full it registers Full.
40 miles after fill up it registers at the 3/4 mark, or slightly below.
But I can drive for quite a long time between 1/2 tank and Empty.
And, at Empty the warning light comes on.

I have not tested how far I can go after the warning light comes on,
however the most I've
been able to add at a fill up was 19.5 gallons.

Now, I recall a year or two back that someone posted that they made a
simple electronic
converter to calibrate the fuel gauge. I think they took ohm readings from
the sender as they
filled the tank 1 gallon at a time, then created a mapping table to send
an accurate reading
to the gauge. Does anyone remember who that was? I recall they were
planning to make it
available for sale, but I never saw anything more on it. Does anyone have
more info? I'd like
one.

Cheers!
Garth
#4033

YMMV,
Garth
#4033

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