F/A feedback oscillations

[Dale Ulan]

...
> corresponding to the point where the root locus crosses the imaginary
> axis. With this approach, the settling time is more like 2 sec. but
> the output F/A ratio will oscillate with a "swing" dependent on the
> gain parameters.
> 
> So what's the bottom line? Well, it would seem that *in part* (1) the
> oscillation in the feedback is due to the desire to greatly improve
> the settling time of the system. (2) By controlling the PI gain factors
> the amplitude of the oscillations are controlled (clearly a function of
> sensor characteristics and engine speed/load).

A standard Zirconia cell also mis-behaves if you try to run the
sensor right at stoich. I'm not sure of the mechanism, but the
cell's output begins to drift around a lot, especially at idle,
so although you control the sensor's real output to say, .5 volts,
the actual A/F will vary. The sensor appears to be more accurate
during the limit cycle oscillation, rather than a steady state.
This may be due to the sensor's temperature sensitivity, or its
sensitivity to carbon monoxide, which tends to bias the sensor
on the rich side.

> So, Dale, if we were to add a differentiation term to help the
> oscillation, would this seem consistent with what you have found in
> other controllers? (also excluding the terms to induce more/faster
> oscillations)

I've only seen it in the GM controller, but it does appear as though
increasing the D term should help stabilize the oscillation, and make
it less dependant on the sensor itself, and rather, with the controller
and the system.
It is worth noting that the oscillation size is fed back, along with
the oscillation frequency, to result in new and better P, I, and D
coefficients... possibly to servo the frequency to where they wanted
it.

-Dale