[Diy_efi] About MAP sensing and it's jitter

[Alexei Pavlov]

sravet at arm.com wrote:
> 
> "Tomas Sokorai Sch." wrote:
> > On Wednesday 14 January 2004 14:57, steve ravet wrote:
> > > There's an article on the gmecm page about how GM implements lag filters
> > > for O2, MAP, etc.  It's always a good idea to filter based on your
> > > sample rate.  In the case of MAP you can do it with a small orifice
> > > restriction in the line to the MAP sensor, or electrically, or in
> > > software.
> > >
> > > --steve
> >
> > Thanks!, I'm already using a filter like that on the coolant temp. input. But
> > it has a fixed filter coefficient (. I guess that to implement such filter on
> > a signal with a sampling rate dependant on the engine speed, it could be a
> > good idea to lower the coefficient proportionally with the RPM increase?
> 
> Sampling based on engine position complicates the filtering.  To keep it
> simple you could filter based on the slowest sample rate (at idle),
> unless that introduces too much lag.

That's why I use first hardware filtering with a fixed sampled rate,
and then a second software stage which changes with RPM.
This is a very common way to do on digital systems. An average
calculation
is the same as a 1nd order filtering at the cut frequency equal sampling
rate divied by the averaging sample number (average of two samples is
a freq/2 filtering).
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