Flywheel/Encoder patterns

[kleenair at ix.netcom.com]

Todd Knighton wrote:
> 
> Mazda Ebrahimi wrote:
> >
> > This is exactly what I was thinking about when I decided to use 180
> > teeth.  I figured if I use a lower resolution, I would have to calculate
> > RPM (which I have to do anyway), then calculate time elapsed per degree
> > of rotation (which is estimated), and load my counter.  By going to 180
> > teeth, I've eliminated the last two steps, and all events happen once
> > per degree based on an actual signal, not an estimated value.  I realize
> > that this approach also generates more interrupts per revolution, but it
> > seems easier to work with.
> 
> Mazda,
>         Bosch used a similar thing on the C1 brains for 84-89 carrera's,
> however they used 129 tooth flywheel for the 6 cylinder cars and then
> idle air compensator pulse width to come up with a rpm number.
> Basically the thing couted the number of teeth per IAC cycle and that x
> 40 gave the rpms.  This gives a resolution of 40 rpms with a max of
> 10200 rpms.
>         They use the teeth to get a 1.4 degree resolution on the ignition,
> rising or falling.  But they do not use it for fuel.  Your resolution
> would be horrible at low speeds.  Typically an 8 bit number generating 0
> - 255 multiplied by 32 gives you up to 16ms injector pulse width with a
> resolution of .06ms.  In your case at 1000 rpm's you've got 60ms from
> tdc to tdc with 180 teeth that's only .333ms resolution, or using rising
> and falling edge triggers maybe .16666ms, you're still double that of a
> 16bit timer running at 6mhz.  The only reason I say this, is that this
> .06ms resolution sometimes isn't enough at idle for larger injectors, if
> your planning on running a low horsepower application with very small
> injectors, you might be able to get away with it.
> 
> Todd Knighton
> Protomotive Engineering

Thanks Todd, that is a very good point.

The way I was looking at the problem was that I have a maximum window of 
720 degrees to fire the injector.  By using the 180 teeth with rising and 
falling edges, I can control my duty cycle in 0.13 % increments (1/720), 
and I thought this would be satisfactory.  But I can see your point about 
low load AND low RPM conditions posing a problem.  I think I'll be OK on 
my current project, but I'll keep that in mind for the next application.

Best Regards,

Mazda