Hi all.
This is a quick pass at the design using an average of the injector
voltage as an analog of the fuel volume.  It compares the air mass
flow to the fuel volume flow and adjusts the fuel flow (via Control on
the 555) to force the fuel to match the air flow. The signals are
summed in the integrator made up of the opamp IOP1 and C5, R8.
  I ran a simulation to make sure it was stable.  The injector pulse
smoothing filter values ( R7 and C7) are approximate, but they
shouldn't be critical.  Since the constant pulse width, one pulse per
rev approach gives a first approximation of the fuel requirements of
the engine, allowing 1 second or more  for the feedback loop to adjust
to the new MAF sensor voltage shouldn't be an issue.
For an actual system, some component values need to be optimized.  I
used 15k and 200nF for the one shot RC time constant.  For a 555, the
RC product of the components is very close to the actual pulse width;
here it's 1.5E4 Ohms x 2E-7 Farads= 3E-3 seconds= 3ms.

That means that when the Control voltage is 2/3 of the supply voltage
(6.66V in my case) the output pulse is 3ms.  This base value can be
easily adjusted by varying R1.

You should be aware that increasing Control voltage above about
0.95xVsupply  will cause strange things to happen.  So  shouldn't make
that voltage part of the normal operating range.  Therefore I would
pick RC to be the middle of the expected injector pulse width range.
That allows Control to adjust the pulse width over a reasonable range.

Also note that I haven't make any attempt to "calibrate" the system.
I have no idea, for instance, what full scale ( 5V? ) on the MAF
sensor output is equal to expected full speed-full throttle air flow.
I don't even know if the MAF sensor output is LINEAR with air mass
flow.  I sure hope so!

Craig Taylor

Thanks for doing this Craig.

Let me get this straight in my feeble mind. VG1 is a fixed frequency
square wave generator correct?

VS23 is the mass airflow sensor correct?

Why not just connect the MAF sensor directly to the control pin?

Paul Lamar


Craig - We need two 555 oneshots, one for each rotor. If we feed both of
them back to the single control (doubling pulse rate) does it need to
change the resistor values?

Victor

We use two 555's for each rotor as we have two injectors for each rotor.
This gives us a complete back up system.

Paul Lamar


Victor, I don't really understand your question.

This isn't a complete design.  It's too simple to work well and needs cleanup.

Craig Taylor