Analog signal conditioning

[garfield at pilgrimhouse.com]

On Sun, 14 Jun 1998 15:32:06 -0400, Chris Conlon <synchris at ricochet.net>
wrote:

>MAP sensor: I'll bet you could get some good arguments going here.
>My feeling is that the cutoff frequency should be related to RPM.
>I prefer to sample MAP at high speed (1KHz or more, depending on
>the natural frequency response of your MAP sensor), and average it
>out over the last N complete firing cycles (2N rotations). You can
>tell by now I'm aggressive ISO good throttle response. If you need
>to pick a fixed frequency, you could probably pick something that
>corresponds to a low RPM, but if you set the cutoff low enough to
>avoid aliasing at low RPM, it may seriously hurt response at higher
>RPMs.
>
>The O2 sensor line is probably more open than most to induced noise,
>but the way it's sampled is very noise-tolerant. If you're running
>a typical system that just wants to see stoich crossings every so
>often, my guess is 5-10Hz should do you ok.

Hmm, your comments on MAP carry also a good point for O2 conditioning
once we get fast-reponse sensing like EGOR/NTK/current-pump type sensors
into play. According to the NTK/SAE paper (SAE #920234; see I learned my
lesson, repeat is EVERY time and you won't get the incessant repeat
questions for the # B), the step response time of these sensors with no
filtering from/in the electronics, is around 300mSec. However, later in
the same paper they seem to suggest that the "small signal" response is
somewhere at least around 50-100Hz, cuz in an example with a 4-cyl
engine running between 3-4Krpm, they say, and I quote:

"Using this lean signal [detection of the UEGO sensor going lean as a
result of misfire on a particular cylinder], WITH the ignition timing
signal, the misfired cylinder also can be distinquished." This suggests
that the sensor responds fast enough to conceivable pick out the AFR on
an individual cylinder basis, and if this is possible at all, I would
think your idea of adapting the filter bandwidth to the rpm is also
applicable/important.

Just an itinerant thot.

Gar