reading the crank

[David Doddek]

Lets take a look at a typical example of reading the crank for degrees.

Say that you have a crank trigger wheel with 4 teeth.  One being longer than
the rest for sync reasons.  The leading edge of each trigger point is 90
degrees before TDC for some cylinder (4 or 8 cyl engine).  Now lets say that
you want a delay of 60 degrees so that you will have 90-60=30 deg BTDC
timing.  Lets also assume that you have a 16 bit counter with a resolution
of 1 count per microsecond.  
Measure the time between the leading edges of the two triggers before the
desired timing event = delta t in microseconds.  You know that during this
time, the crank rotated 90 degrees.  Relating these two quantities gives :

   delta t / 90  = microseconds per degree  (mpd)   (1)

Multiply (1) by the desired delay in degrees (60):

   delta t (uS)
  --------------- *  60 deg  = delay time in microseconds
      90 deg

So thus if you were to start timing from the instant that the trigger pulse
occured, you will have an event at 30 deg BTDC.  This event would then be
used to turn off the coil.

To get RPM from this information, we know that one revolution takes four
times  our delta t ,the time it took for our 90 degree crank segment.  Since
we have one revolution in some amount of microseconds and we want some
number of revolutions in one minute, we need to convert.  Our only incorrect
term is time, so remember that there are 1 million microseconds in a second
and 60 seconds in a minute.  

This gives 60,000,000 uS / Min  (2) 

 Multiply (2) by our microseconds per rev and:

   60,000,000 (us)           1 rev
  -------------      *  --------------    =  RPM
     Min                 delta t  * 4 (us)

Of course there is more to this such as calculating when to turn on the coil
and determining the sync of the crankshaft from the given triggers.  Also it
is up to the individual to come up with a clever way of doing the math in
the simplest form.  (Especially when using assembler, 60,000,000 is not a
fun number).

I hope that this was of help and answered some questions and I was not just
babling.

By the way this is the way that the Yamaha FZR 600 reads the crank,  3 short
pulses and 1 long.  And this is a very dynamic engine, so dont flame on only
having 4 triggers per rev please.

David J. Doddek                                          |pantera at dris.com
Owner SGD Electronics & Development Engr for Caterpillar |h 309 685-7965
Formula SAE Team Sidewinder 94-95                        |w 309 578-2931
89 T-bird SC,  69 Fairlane w/SGD EFI                     |fx 217 428-4686
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