Ignition timing reference points

[Stephen Dubovsky]

>>
>>>I was thinking of adding a 256 or 360 slot optical interrupter disk into
>>>my distributor to give double the that number of edges every rev 
>>...
...
>
>I have often wondered about using a resolver on the distributor drive to
>measure the position.  Although the new price is frightening, there is a
>lot of old equipment around with them in at give-away prices.  At least
>near me in Nottingham with its big military surplus dealer. (Anchor
>Surplus).
>
>My worry is whether they would tolerate going round at half crank speed
>for hours and hours.  I know they have top quality bearings but is the
>life adequate?  Its not too much problem digitising the sine and cosine
>output signals.
>-- 

  I have read several responses that say this much accuracy is required, but
Im going to comment on the technical aspects and not the reason to use it...

  Even the 'sealed' optical encoders can have problems.  One is on the end
of an induction motor (for electric vehicles) and microscopic drops of oil
can get in cause false edges.  A REAL pain.  We just have not had good
experiences w/ them.

  Several people wrote me asking about resolvers so...

I believe that are reliable enough to use on a motor.  There are no wiping
contacts (i.e. no brushes) so only the bearings can fail.  Many resolvers
are designed to be put on the end of permanent magnet three phase machines
(at least this is where we find them most common) so you can determine the
field position at any time w/ good accuracy (so we can bulid electronics and
turn them into 'DC brushless' machines).  Since they were designed to mount
to an electric motor, rotational life is very long and they are designed to
turn very fast (have seen them rated well past 12-14k rpm which is our top
speed).  Anyway, the little things should be able to be used to get crank
position w/ good accuracy and still be really reliable.

Some one said they are analog in nature (agreed) so he couldnt use them w/ a
micro (here's where I disagree).  We use an Analog devices R2D converter
(resolver to digital) that gives you a nice 10-14 bit word (depending on
dynamic perf. required) giving absolute poistion.  Not too bad -> 0.35 to
0.02 degrees resolution.  (The R2D we use is about $60)

They are pretty much little rotating transformers.  The is a primary on the
stator (outside part that doesnt turn) that you excite at, say 10kHz.  There
are 2 secondary windings 90deg apart on the rotor.  If you look at the
signals induced on these windings, you get the carrier (at 10k) sin and cos
modulated.  The signals on the rotor are then coupled back to the stator
useing more little transformers that dont care about position.  You have
then put a signal on the rotor, modulated it w/ the rotor angle and gotten
the info back to the stator w/o any electrical contacts.  Cool huh.  Then
you can use some PLLs and counters to get the rotor angle from the modulated
info (or use a R2D w/ everything in one chip). 

Thats my 2 cents worth.  Hope this helps.
SMD