MegaSquirt Driveability Question
Bruce Bowling
bbowling at earthlink.net
Fri Oct 26 00:19:40 GMT 2001
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Good question.
Going over each sensor, the worst one to lose is the MAP sensor. If this
goes out, then the fuel computation gets whacked out and the engine will
die. This one is also hard to detect a failure, since it bounces all over
the place in normal driving, and a default setting is hard to determine
(i.e cruising, idle, etc). If one did detect a failure then the software
could in theory resort to an alpha-n setup to get you home. This is not in
the code, but could be added if one wanted to. However, the MAP sensor on
MegaSquirt is mounted right on the PCB, so it is somewhat protected
compared to direct underhood mounting. This is a rugged unit from Mot
designed for automotive use, so I give it a high MTBF rate.
For the TPS, this is only used for acceleration enrichment - you can live
without this to get the car home.
For the Coolant or MAT sensor, it depends on the failure mode. If it opens
up, then the voltage divider circuit will pull the corresponding ADC
channel up to +5 volts (upper rail). This failure mode for the coolant
sensor is like a really cold day (- 40 degrees F) and the mixture will run
very rich, but the car will still run. I think that this is the most likely
failure mode. If the sensor shorts out, then the ADC channel will see near
zero volts, the the opposite will occur. For wacked-out sensor readings,
the results will also be whacked-out - not much can be done about this,
unless one wants to put a slow tracking filter in the software to detect
and provide alternate values if something goes wrong - this is not in the
code right now, but could be added.
It should be noted that the MAP, coolant, and MAT sensors all go thru a
lookup table - for instance, the MAP sensor ADC channel, when it generates
a new reading, passes this number as a offset into the KPAFACTOR.inc lookup
table - the result of this lookup is a pressure in direct KPA units. For
the 8-bit ADC channel, there are 256 entries. If one wanted to "guard"
against extreme values - i.e like 0 KPA or 115KPA for the MAP, one can
simply put in different values in the lookup table at these end points
which keeps the car running, i.e. like 50 KPA on the extremes. No
additional computation is required with this setup. My car has extreme
values of 70 degrees for the MAT table, and 160 degrees for the coolant
table - I can unplug the sensors and the car will run just fine, due to
these endpoint values.
There are numerous other ways to detect and treat errors. I hope that
people take the embedded code and add their own fail-safe modes.
- Bruce
>------------------------------
>
>Date: Thu, 25 Oct 2001 07:37:24 -0700 (PDT)
>From: Kenny Bauman <kennybauman at yahoo.com>
>Subject: MegaSquirt Driveability question
>
>First I want to say thanks to the design team for
>making the MegaSquirt possible and available.
>
>Second I wonder hows the unit does when dealing with a
>sensor failure? I know there are lots of possible
>failure modes, but for example how is the driveability
>with the TPS disconected?
>
>__________________________________________________
>Do You Yahoo!?
>Make a great connection at Yahoo! Personals.
>http://personals.yahoo.com
>- ----------------------------------------------------------------------------
>To unsubscribe from diy_efi, send "unsubscribe diy_efi" (without the quotes)
>in the body of a message (not the subject) to majordomo at lists.diy-efi.org
- Bruce
---------------------------------------------
Bruce A. Bowling
bbowling at earthlink.net
http://home.earthlink.net/~bbowling
---------------------------------------------
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Content-Type: text/html; charset="us-ascii"
<html>
Good question.<br><br>
Going over each sensor, the worst one to lose is the MAP sensor. If this
goes out, then the fuel computation gets whacked out and the engine will
die. This one is also hard to detect a failure, since it bounces all over
the place in normal driving, and a default setting is hard to determine
(i.e cruising, idle, etc). If one did detect a failure then the software
could in theory resort to an alpha-n setup to get you home. This is not
in the code, but could be added if one wanted to. However, the MAP sensor
on MegaSquirt is mounted right on the PCB, so it is somewhat protected
compared to direct underhood mounting. This is a rugged unit from Mot
designed for automotive use, so I give it a high MTBF rate.<br><br>
For the TPS, this is only used for acceleration enrichment - you can live
without this to get the car home. <br><br>
For the Coolant or MAT sensor, it depends on the failure mode. If it
opens up, then the voltage divider circuit will pull the corresponding
ADC channel up to +5 volts (upper rail). This failure mode for the
coolant sensor is like a really cold day (- 40 degrees F) and the mixture
will run very rich, but the car will still run. I think that this is the
most likely failure mode. If the sensor shorts out, then the ADC
channel will see near zero volts, the the opposite will occur. For
wacked-out sensor readings, the results will also be whacked-out - not
much can be done about this, unless one wants to put a slow tracking
filter in the software to detect and provide alternate values if
something goes wrong - this is not in the code right now, but could
be added.<br><br>
It should be noted that the MAP, coolant, and MAT sensors all go thru a
lookup table - for instance, the MAP sensor ADC channel, when it
generates a new reading, passes this number as a offset into the
KPAFACTOR.inc lookup table - the result of this lookup is a pressure in
direct KPA units. For the 8-bit ADC channel, there are 256 entries. If
one wanted to "guard" against extreme values - i.e like 0 KPA
or 115KPA for the MAP, one can simply put in different values in the
lookup table at these end points which keeps the car running, i.e. like
50 KPA on the extremes. No additional computation is required with
this setup. My car has extreme values of 70 degrees for the MAT table,
and 160 degrees for the coolant table - I can unplug the sensors and the
car will run just fine, due to these endpoint values. <br><br>
There are numerous other ways to detect and treat errors. I hope that
people take the embedded code and add their own fail-safe
modes.<br><br>
- Bruce<br><br>
<br><br>
<blockquote type=cite class=cite cite>------------------------------<br><br>
Date: Thu, 25 Oct 2001 07:37:24 -0700 (PDT)<br>
From: Kenny Bauman <kennybauman at yahoo.com><br>
Subject: MegaSquirt Driveability question<br><br>
First I want to say thanks to the design team for<br>
making the MegaSquirt possible and available.<br><br>
Second I wonder hows the unit does when dealing with a<br>
sensor failure? I know there are lots of possible<br>
failure modes, but for example how is the driveability<br>
with the TPS disconected?<br><br>
__________________________________________________<br>
Do You Yahoo!?<br>
Make a great connection at Yahoo! Personals.<br>
<a href="http://personals.yahoo.com/" eudora="autourl">http://personals.yahoo.com</a><br>
-
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<dl></blockquote>
<x-sigsep><p></x-sigsep>
</dl>- Bruce<br><br>
---------------------------------------------<br>
Bruce A. Bowling<br>
bbowling at earthlink.net<br>
<a href="http://home.earthlink.net/~bbowling" eudora="autourl">http://home.earthlink.net/~bbowling</a>
<br>
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