Measuring RPMs from Stator - while not charging...

I sample the Stator signal -  feeding it into an IRQ. The primary reason for this it to synchronize the INA-220 sampling cycle to be in lock step with Stator pulses (Best practice for measuring voltages it seems), but I also use this to determine the Alternators (and derive the engines) RPMs.

This approach for measuring RPMs works very well when we are charging.  However, when the Field is dropped to zero the Stator interrupts  go away and we are all of a sudden 'driving Blind' when it comes to managing the throttle.  Some alternator regulators have a 'Tech' mode that will always apply some small level of field voltage, just enough to allow for enough stator signal to sample RPMs.  Used primary on Diesel engine it does come with a warning that 'extended use' could cause battery overcharging.

Today I went back over the 20 or so Live Run data sets I have collected, looking at when RPMs measurement appears to become stable, and then Amps actually being delivered.  And there seems to be a rather solid pattern:

• A PWM of 2 in all runs (one needed 4) gave sufficient signal to allow for stable RPMs measurement,
• Amperage does not start to be delivered until PWM values of upper 40's (and some times into the 60's).

It was interesting that in all cases RPMs could be sampled with a very low PWM value..  Very low.  And given that actual charging does not appear to start until PWM values of near 50 or greater, there is LOTs of margin.

Now, I also noticed a curious situation in 1/2 the runs.  All runs (with out exception) started given solid RPMs at 2-4, but in about 1/2 of the runs the RPMs became unstable when the PWM value reached the mid to upper 20's.  In fact, the indicated RPMs would drop to 0 until the PWM value had climbed into the  40's when they would again become stable.  Amps would start to be delivered when the PWM  raised another 10 or so.  

Some time I will put a scope to the system and see if I can figure out what is happening during this Blank Period.  But until then, my walk away is:

• We can safely send a small Field PWM value to the alternator to allow for RPM measurements without worry of overcharging the battery.

In MY system, a PWM value of 5 or so looks to be gold.   On other systems one would likely want to verify this number by looking at the DEBUG logs of a few live runs.  But given the above I am going to modify the source to allow for all-the-time RPM measurements   This will mostly help while starting, as well as when stopping.  (Currently stopping runs blind, making a nice slowdown of the motor a little difficult to prevent entering a to-slow situation before fully cutting the throttle).

Of course the PWM value will be selectable, and I will have a way to disable this capability of one wishes.