Flywheel/Encoder patterns

[Todd Knighton]

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