08/19/99....
This one is similar to the
timing period jitter but I know what is causing it. Also I'm not
absolutely sure if it is really a problem or not, or if I am being too
picky (probably).... The MAP sensor has enough bandwidth that you
can see the pressure changes in the manifold as different intake values
open and close. 200 mv to 400 mv of ripple is not uncommon.
That is 4% to 8% of ripple (2.32 inches, 19 ADC counts, or a 15% change
in injector pulse width at 2500 RPM !! It really shouldn't ever be
this bad because the ADC is really read close to the same point in time
every timing period.). What concerns me is that when the ECU reads
the ADC it is usually on the steep portion of the slope. A little
jitter here and the ADC readings are going to be moving around quite a
bit. Looking at some of the data I've logged, the MAP ADC values
are always noisy, even under a steady load. The average value is
steady, with a lot of noise riding on it. I have observed a 5 to
10 ADC counts of jitter. This means what is being fed into the ADC
could be as much as twice as bad as that because the the ECU averages the
last two reading together before reporting them. It sure would be
nice if you could feed good data into the ECU, you know 'Garbage In - Garbage
Out'. One thing that may be aggravating this MAP noise is the timing
scatter problem. Fixing that one will help this one because
it will remove the jitter from when the MAP ADC is read. I've been
looking at a way to try to filter the MAP sensor output. The filtering
on the ECU itself is very high frequency so it doesn't help us at all.
The problem with using an filter with a low enough cut off frequency to
filter the intake value pulses will also slow down the real response of
the sensor to load changes. This is true if you use a electronic
filter or a mechanical one (like longer hoses or some restrictions to the
flow). Back to one of these 'Catch 22' things. I don't want
to slow down the response of the MAP sensor because that will affect how
the ECU responds to rapid changes in load but I also want to filter
out a fairly low frequency ripple at the same time. The only way that I
can see to accomplish this is to use some kind of preprocessor on the signal.
Take an ADC and digitize the wave form, average it, maybe apply some more
smart filtering and noise reduction routines, before passing it on to the
ECU. Have been looking at quite a few micro controllers here lately.
Need something with a fast ADC, a couple 16 bit timers, and enough power
to do all I want of it at cruising speeds. It also has to be small
enough to fit inside of the ECU itself (or at least that is what I am shooting
for). It also would be nice if it was something that I can program
fairly easily with having to learn yet another processor. So far
I haven't located my dream chip yet. I would like to do this at the
same time as I do the In Car Programming
modifications.
08/22/99
Thinking more about this, some of this noise may
be just an artifact of the timing period jitter problem. May not
actually do anything with this until the timing period is stable.
Fixing that may make this noise so small to be a non issue (it may be so
small now that it is a non issue anyway.....).
Below are some plots of what I am seeing so far........
The above plots are what I have observed idling around
850 RPM. The lower one is just the MAP ADC data expanded over the
same period of time. You can see at time the MAP ADC values are around
10 ADC counts or higher. (The 'Y' scaling is 10 times so 1,480 is ADC=148).
This plot was taken around 2500 RPM cruising steadily down the freeway. About a 4 ADC count pp here.
This one was taken while the motor was idling around 850 RPM. The falling edge of the trigger event is the 10 deg BTDC point. Shortly after this is when the MAP ADC is read. Slight uncertainty in the reading time and the ADC reading will move up and down the slope. Looking at this plot alone would not explain the noise I see when I log data from the ECU.
These above two were taken with the motor is free running around 2500
RPM....
Just some more MAP noise around 1000 RPM...
These two are not directly related to any problem but are kind of interesting
seeing the relationship between the TPS and MAP. The motor was idling
and the throttle was given a quick jab.