[Diy_efi] Re: MegaWideband digital WBO2

[Bernd Felsche]

Perry Harrington tapped away at the keyboard with:

> > Not in the 210. My firewall address is 203.59.*.*
> 
> Upstream.

You were blocking AlterNet in Sacramento? :-)

> > Now... back to the topic; you are aware that the apparent
> > resistance of the pump cell will change with temperature?
> > That will change the current that actually flows through it will
> > vary. The EMF presented by the pump cell will vary depending on the
> > difference in EGO in the exhaust stream and the measurement cavity.

> So the bias current required to compensate the Vs will vary with temp?

Apparently. According to a couple of patents that deal with
measuring the cell temperatures.

> Just like the heater, correct?  Can we assume that the cell temps
> are the same as the heater temp?  The design is very critical of the

Only approximately.

> Heater temp.  The schematic will be redesigned to put the sense
> resistor on the ground of the FET and simply sense the voltage directly
> there.  The design measures the impedance of the Heater in realtime,

I was going to suggest that. Clamp the voltage (in case you get a
short-circuit) with a Zener unless the chip's input is sufficiently
protected internally.

> controlling the current to the sensor in order to keep the heater at
> 800C.  I assume 800C is the target temp?  According to the figures

Target temperature should be above that of the exhaust gas.
Minimum operating temperature depends on sensor type.

> So, if we know the cell temp, then we know what amount of current
> is required to bias the pump in order for Vs to equal 450mv?

You only know the heater temperature, approximately.

> The temp bias would cancel out, since we try to regulate the temp tightly.
> Am I missing something here?

Cell and heater temperature may be different, especially if you're
no longer providing power to the heater. To work properly, the
sensor's cells must be hotter than the exhaust gas.

> BTW, if you have a look at the design I did away with the switched
> polarity H-bridge that you are using.  Instead I offset ground and
> switch the high side, this gives 2.5v cell voltage and requires
> less components.  The mCU can sink 20ma on the PWM lines, so no
> external logic is required.

That's another way to skin the cat. :-) There were other reasons to
implement it using the AVR. The chip used at the design stage could
also not source the necessary current for direct drive. A later one
(ATmega8) can. I'm too busy with other stuff at the moment to revise
the design based on updated components and fresher ideas.

> I think I will add a relay to simultaneously switch the Vs/Ip
> ground and enable, this way I don't have to compensate for voltage
> drop and there is no possibility of engaging the Ip pump if the
> chip is wedged.

That was one of the reasons for an external chip.
I wanted to be able stop current running by default.

A watchdog reset should kick in on the uC before damage is done;
resetting all inputs and outputs to high-impedance. That _should_
work.

> Further modifications may see a 12v quick heat mode to get the
> heater to operating temp quicker.

Connect directly to the battery (via fuse!) and measure the battery
voltage as well as the "down-stream" voltage.

See the antique DDL circuit at
	http://bernd.felsche.org/tech/EFI/DDL/DDL1.html

This means that you're not running the heater current through the
regulator.  The LM317 won't put out any useful current if the input
voltage is too low (below 10.34 + dropout of about 1.5). Expect a
12V battery to get down to 8V during cranking.

Regulate to the desired ratio of the two measured inputs. Battery
voltage won't change so quickly (unless you have thin power supply
wires) as to screw up regulation.

You can only measure the voltage drop through the heater while the
FET is on full anyway; measure the battery voltage when it's off.

I've kept an old NB-O2 sensor to see how the side-effects PWM of the
heater can be minimised - beats damaging an NTK WB during
experimentation.

> > See the bibliography on my DDL project page
> 
> I didn't see a bibliography.
> 
> > 	http://bernd.felsche.org/tech/EFI/DDL/DDL.html

Sorry; forgot that I moved that to a sub-page on the sensor...
	http://bernd.felsche.org/tech/EFI/DDL/Sensor.html
-- 
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