[Diy_efi] Interesting quandry regarding UEGO patent 6120677

[Perry Harrington]

> 
> They probably don't; unless they're doing the testing carefully, in
> an insulated chamber so that the temperature is stable... I don't
> beliece that they would rely on heater resistance for temperature
> measurement; a thermocouple could be attached for more-reliable
> measurements in a lab.

No where in the document do they refer to temperature, except for
the 800C operating temp.  IIRC, 750-950 are the lower and upper
bounds.  They simply compare cell impedance to heater voltage.  This
implies that they KNOW what the heater voltage means temp wise.  Actually,
it's easier than that.  They aren't doing anything fancy, they are
simply measuring the voltage across the heater.  That would mean
that they are using ~13v to power the heater.  When the heater
reaches it's "saturation" point, the drop remains constant, as
witnessed between 12 and 13v.  So that means the heater is at
full operatin temp at 12v, colder at 11v, and warmer, but neglible
at 13v.

So one could simplify the heater control circuit even more to
follow these graphs.  All you have to do is measure the voltage
across the heater, when it reaches 12v, it's warmed up and you can
start doing your modulation.

> 
> I hinted at that procedure in the Calibration section of the DDL
> project.
> 
> The heater impedance may be a reliable indication of the heater
> temperature; insofar as being repeatable. It is however, not as
> accurate an indication of cell temperature with the sensor in
> exhaust gas flow.

That is correct, but if we assume that the heater is 830c at 12v,
then that's fine.  We aren't interested in the temp of the heater,
but the cell.  If we KNOW that at 12v the cell is at operating
temperature, we DON'T need to measure the cell impedance, just
maintain the heater at 12v.  Does that sound reasonable?

> 
> > The other thing that this graph shows (and the accompanying
> > details) is that the AFR has no effect on cell temp when high PWM
> > frequencies are used to sample the impedance.
> 
> > >From this data I posit this:  You do not have to sense cell
> > >impedance if you
> > can generate a more accurate heater temp transfer function *AND*
> > you use a sufficiently high PWM frequency for controlling the Ip
> > pump, specifically > 1khz.
> 
> Unfortunately, you'd have to run open-loop on heater control until the
> sensor is within its nominal operating temperature range, if you
> only relied on cell impedance for temperature measurement.

You must run the heater open loop anyway.  Perhaps it was your page which
suggested that you supply full power to the heater until the Vs drops
below 1.1v, then you know the sensor is warm.  Then you turn the Ip pump
on 10 seconds after that point.

--Perry

-- 
Perry Harrington			Data Acquisition & Instrumentation, Inc	
perry at dainst dot com					 http://www.dainst.com/

Those who would give up essential liberty to purchase a little temporary safety
deserve neither liberty or safety. Nor, are they likely to end up with either.
                             -- Benjamin Franklin

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