Oxygen sensors

[Andy Harrah]

When I first got interested in O2 sensors, I went down to the Patent Office
to do some research.  Its about 10 miles from me.

I found an interesting GM patent (no. 4,130,095 Dec. 1978) that describes
how they can tell of the sensors are hot enough to be given reliable data.
The model of the O2 sensor is like a battery.  The terminal voltage is a
function of the CO or oxygen content, and the internal resistance is a function
of the temperature.  These things are really high impedance devices.  GM uses
a clever approach that alternates between taking samples with a high impedance
input, and one that loads down the sensor.

I built such a circuit with two TLC25L2C op-amps.  I switches a 100K load
across the sensor to take the 'heat' measurement.  The output of the thing is
an the analog voltage from the sensor, buffered by the op-amp, and a digital
clock at about 100 Hz that indicates whether the output is straight or loaded.
The micro then compares the output between load and unloaded.  If the loaded
voltage is within some fraction of the unloaded voltage, you can assume
the sensor is hot enough to use.

This is how GM controllers know when to go into closed-loop mode.

Hope this is helpful to somebody.../Bill Lewis