gnupic: ADC using an RC circuit instead of a ADC port??
Subject:
Re: ADC using an RC circuit instead of a ADC port??
From:
Declan Moriarty ####@####.####
Date:
2 Sep 2002 09:26:24 -0000
Message-Id: <02090210474900.00822@genius.chateau.dec>
Was it Brent Picasso who wrote on Friday 30 August 2002 23:40:
> Thanks for the great replies..
>
> When you say "low speed input" - what's low speed? do
> you think I could take a measurement within 2ms?
>
> I want to track a pressure sensor to track changes in
> a automobile's manifold pressure.
>
> I want to measure it at for at least every ignition on
> the engine... at 10,000 RPM with a 4 cylinder engine
> that happens every 4ms or so.
10,000 rpm on a 4 cylinder engine? Sounds like a motorbike. Let's do some
basic sums:
10,000 rpm means 5,000 'inlet' strokes for each of 4 cylinders, if it's a 4
stroke, and 10,000 inlet periods for a 2 stroke. You are only using one a/d,
so I gather you are looking for an average, and not trying to track peaks.
You will find very little variation, or accuracy with only one sensor. If
your work is on carburettors only, this may do. This means one inlet stroke
is about 3 mS long, and you may well have variations inside the 3 mS that you
would be interested in.
IIRC, the correct way to do this is to measure just at the manifold entry to
the individual cylinders, which would require 4 a/ds. It is easy to think
that vacuum is constant along the manifold, but this is not in fact the case,
as if it was there would be no airflow. The Honda manifolds in fact have
tapped points fitted to their manifolds to allow setting the tickover using
vacuum guages.
The vacuum in the manifold also has peaks and troughts as dictated by the
resonance of the manifold. On performance machines (At 10,000 rpm, yours
sounds like one) manifold length can be tuned to provide negative vaccuum at
the inlet as the valve opens, and likewise exhaust manifolds can be tuned to
provide negative backpressure at exhaust opening, with resonances in between.
Trapping these would require a fast a/d with several channels.
Don't limit yourself in your electronic hardware unless you want crap. Take a
step beck and think again. Feel free to drop me a line off list if you want
to.
--
Regards,
Declan Moriarty
Applied Researches - Ireland's Foremost Electronic Hardware Genius
A Slightly Serious(TM) Company
Experience is like a comb,
that Life gives you - AFTER all your hair has fallen out!
