[Diy_efi] Interesting quandry regarding UEGO patent 6120677

[Bernd Felsche]

Perry Harrington tapped away at the keyboard with:

> Hello all,
> 
> I was reviewing the said patent and have a interesting quandry.
> 
> The figures 5A and 5B are very interesting, as they graph pump &
> EMF element impedance against heater voltage.

> The obvious reason to do impedance sensing on the EMF cell is to
> control the heater more accurately.  However the patent leaves out
> some very important information.  Namely the temp transfer curve
> for the EMF cell.  One could infer they assume that the heater

TANSTAAFL. :-)

> temp is the same as the EMF temp.  This also implies that the
> impedance of the heater is not an accurate measurement of the
> heater temp.

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.

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.

> 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.

-- 
/"\ Bernd Felsche - Innovative Reckoning, Perth, Western Australia
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