O2 sensor question

[Garfield Willis]

On Thu, 23 Mar 2000 14:23:21 -0700, "Chad" <chad at dynojet.com> wrote:

>This is one of the full blown five wire O2 sensors.  If you can get ahold of
>the technical information on it from Bosch (Dave: this is where to get the info
>you want if they will send it to you), they have some response graphs that
>characterize the O2 content based on the current coming from the sensor.  The
>bias conditions for these graphs include temperature and reference voltage.
>This is why we needed to control the heater so closely (even though it does
>have a positive temperature coefficient).  Sure, the heater is self regulating
>in that it won't get into a thermal runaway situation, but the drifts in
>temperature mess up the accuracy of the measurement relative to the graphs.

Understood. Since this is at least like the LSU if not in fact the LSU,
this explains why even the LA-3 has to use a PID controller on the
heater. Much more sensitive to temp and heater drive than the NTKs then.
It's important to have both a positive tempco, as well as the right
tempco that gives best self-regulation. I really wonder why on earth
Bosch ever made ANY that had a negative tempco. Puzzling; maybe the
right materials weren't available earlier.

>Maybe you can teach me a little bit about the terminology.  This sensor either
>draws or sources current if the mixture is above or below stoichiometric, is
>this what a 'current-pump' type sensor is?

Normally I would say "yes, exactly". The oxygen pump is operated by a
pumping current, whose polarity is determined by either depleted or
excess O2, and whose current level relates to the amount of depletion or
excess, hence your curves. But from the description, I may be jumping to
some conclusions. You mention "bias conditions" and a reference voltage;
it sounds like those are set and the resultant current is measured. On
the face of it, this sounds a bit different than the way the NTKs work.
The NTKs require a feedback loop which the current pump is part of, and
which is servo'd off the nernst measurement cell. As such, there isn't
any biasing or reference per se. But here again, I might be leaning on
words too heavily. They may infact work very similar and I'm just
getting hung up on the terminology meself.

Gar


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