Crank Angle Sensor resolution - Re: Old 486 ..

[Diehl, Jeffrey]

This is very helpful.  Let me ask a few questions to make sure I understand.
(6inch skull syndrome)

For every rpm range, you use an appropriate frequency to run a counter.
This counter is started by an event from the cam/flywheel sensor.  So, you
know which tooth triggered the event.  You know that the given frequency
will divide the event into N intervals.  So, you can develop positioning to
any desired resolution.

This relies on the ability of the timer to change frequency as quickly as
the motor can cycle through each of the rpm bands.  Is this a reliable
assumption?  If not, then this system might drift or miss a crank angle
position.  We also assume that we can measure rpm and make it available to
the system before it changes noticeably.  True?  Finally, deciding what
frequency is needed to run the counter at each rpm range can be tedious.  Am
I missing something here?

TIA,
Mike Diehl,
MR-2, '87na

-----Original Message-----
From: Peter Gargano [mailto:peter at ntserver.techedge.com.au]
Sent: May 08, 2000 2:25 PM
To: diy_efi at diy-efi.org
Subject: Crank Angle Sensor resolution - Re: Old 486 ..


Diehl, Jeffrey wrote:
> 
> I went to Electromotive's web page and read up on the TEC II unit.  They
> claim 1/4 degree resolution.  This seems more reasonable than the 1.1
degree
> that I can imagine.  

Remember that it's not the Crank Angle Sensor (CAS) that gives you the 
resolution, but the timer you're using to trigger from the CAS "event".
So, if you have a timer that ticks over at 4 uSec, then at 6,000 RPM
(that's 100 revs/Sec, or 10 mSec/rev) that timer will give you a resolution
of 4/10,000 x 360 degrees = 0.144 degrees.

> I was wondering how you could handle situations where
> you might want to advance the timeing, say, 1.5 degrees at 5600rpm.  

At 6,000 RPM, for 1.5 degrees with the above timer, you need an incremental
count 
of 1.5/0.144 = 10 (giving 1.44 degrees, a difference of 0.06 degrees over
what 
you really wanted). At lower revs the figures are even more accurate.

I mention this because, as others have pointed out, the PC may be good speed
wise, but it needs some additional hardware counter support to be truly
useful for an EFI setup. With hardware timers you can use a slow processor
to
get very good resolution.

Peter
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