Wide Ratio O2 meter

[Orin Eman]

> The Bosch LSM-11 sensor:
>  1. There are 3 curves at 650, 750 and 900 C from which we can ESTIMATE
>     the transfer function to air fuel.
>  2. The sensor temp may be deduced from from the internal resistance of
>     the sensor which varies as follows:
> 			650 deg C	120 ohms
> 			750 deg C	 35 ohms
> 			900 deg C	 10 ohms
>  3. Here is the major hangup. How do you measure the internal resistance
>    of the sensor without disturbing its output. A 1979 SAE paper, 790143,
>    shows a parallal resistance method in which a known resistance is put
>    in parallal with sensor and change in resistance is noted. From that
>    change, it is a simple formula to calculate the sensor internal
>    resistance and thus its temperature.

Easy enough.  Single transistor in series with a known resistor
wired parallel to the sensor.  Transisitor off, read voltage, call it
Vs.  Transistor on, read voltage again, call it Vd (for 'divided'
voltage).  If Rs is sensor internal resistance and Rk is known
resistance, then:

Vd = Vs * Rk / (Rk + Rs)

or  Rs = (Vs * Rk / Vd) - Rk
or  Rs = Rk * (Vs/Vd - 1)

>  4. What is not so simple, at least for those of us with no micro
>   design, is how you get a microprocessor to do this: you got to put
>   resistor in parallal, measure resistance and do math- I guess on a
>   rapid sampling basis so do not disturb sensor.

The cost is one IO line to switch the resistor in/out.  The math is easy,
I have multiply/divide routines.  I'd probably rearrange the above
and work with conductance rather than resistance to avoid the divide.

>  6. All of the above has to be rapid enough to get at least 10 samples
>    per sec data or you will miss rapid accels.

10 samples per sec is easy for a PIC running at 10E6 instructions per
second.  I'm doing similar with a 16C73 for Audi Turbo engines...
I'm dumping to RS-232 at 9600 baud, 80 characters 10 times a second and
driving a 40x2 line LCD, AtoD is an MC145051 10 bit serial interface,
11 channels, though I don't use them all.

>  7. The sensor is touchy. I have used alot in racecars and have overlayed
>    many laps of data taken at 100 times per sec using a MOTEC, $3500,
>   interface to the Bosch sensor with an analog out to datalogger. What
>    you see is that it takes several laps for data to stabilize at the
>   final a/f. It needs to be HOT. Thus you need a good heater controller
>   design with supply at > 12 volts.

Hmmm.  Guess we have to do some experimentation!

Orin.