EGOR Output Signal Survey

[bcroe at juno.com]

It appears all the design problems of a practical
wideband OX sensor circuit have been solved.
That leaves the question of just what to do with
the output.  I am proposing some ideas here.
Your feedback would be greatly appreciated 
before things are finalized.

The first item is the Rcal resistor, the resistor 
built into each wideband sensor at the factory
to cancel differences between sensors.  A simple 
circuit has been designed to use this feature, and
I think it should be included in any group design.

Next is the output voltage.  A curve seen in some 
equipment reads like
    A/F             Volts
    22:1           3.5
14.57:1         3.0     stoich
  10.5:1          2.0

A circuit from the list runs about 0.5 V lower
than above, but otherwise the same.  My ckt
runs 0.2 V lower than above.  I propose we use 
a design that can produce any of the above by
changing a couple of resistors.  

I have a reason for running mine at 2.8 V stoich 
instead of 3 V.  The most accuracy is desired 
around stoich, so my circuit is intended to be 
dead on at that point.  Calibration and other 
errors come into effect as you move away from 
stoich.  To minimize errors, the stoich output V 
is the same as my reference power supply, and 
my reference power supply is 2.8 V.  The reason
for that is to allow use of newer parts, eliminating
a couple dozen other parts.

My next item is a real time display, a readout.
I propose a bar graph with a center LED to 
indicate operation at stoich.  As operation 
moves more rich, a column of green LEDs would
light forming a bar growing away from stoich.
For lean operation, a column of red LEDs would
grow in the opposite direction.  The length of the
bar would show how far you are from stoich, and
the color indicates in which direction.

     RRRRRRRRRRSGGGGGGGGGG

If we have a readout, how should it be calibrated,
and to what resolution?  I would suggest 10 
divisions in either direction might be good for a
dashboard gauge, but up to 20 each way might 
be better for wide range tuning and operation.

What are meaningful divisions?  The A/F to
voltage curve is pretty badly curved, but is 
that even what we want to display?  Here is
how I see it.  Imagine we have this box of air
and add a bit of fuel.  We get a voltage reading
from our sensor when we burn the fuel.  Every 
time we add a larger amount of fuel (increase 
injector pulse width), Ip and our Vout shift a 
proportional amount.  A nice linear function of 
fuel increase to voltage.  So why is the A/F 
curve so UN straight?  BECAUSE the function 
described above IS NOT A/F, IT IS F/A.  So I
propose a linear display of the voltage out is
really what we need, to relate tuning to output.
We can MARK it in A/F, but the markings will 
not be linearly spaced.

Allright, there is one more factor.  That almost 
perfectly linear delta fuel to delta voltage
relation, shifts as you pass stoich, to a slightly 
different very linear relation.  I propose that the 
gain (number of divisions per volt) BE SLIGHTLY 
DIFFERENT on the 2 sides of stoich.  So now a 
given increase in fuel will cause the SAME 
number of divisions movement of the bar, from 
one side of the display to the other.  Circuit wise 
this function is almost free. 

Something else we could add is a switch for
free air calibration.  This would normally be 
off scale, but it would reduce gain to put an
indication at 22:1.

There is a warmup time approaching a minute
for the sensor.  I suggest the display be blanked
(turned off) during warm up.   Besides giving an
indication of operation, it allows the considerable
LED current to be "stolen" from the sensor 
heater circuit, saving power, heat, and heat sinks.
The cost is one transistor. 

One more output that might be useful would 
have a 1 volt output range, reading 0 to 1 volt as 
we moved from 15:1 to 14:1 A/F.  The idea is to
look like a conventional narrow band OX sensor.

All these outputs could exist at the same time, or
just put in parts for the ones you want.  I await 
your opinions on the subject.  Bruce Roe 
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