I have to admit that I don't completely understand jimo's algorithm... for example, the first line says "if amps go up and rpm goes down, don't change the duty cycle."
But my understanding is, for example, that if the mill is underloaded (or unloaded) then increasing the duty cycle will increase the amps and decrease the RPM, however, one increment in duty cycle is not likely to get you to the maximum power point and you may need to increment the duty cycle again rather than "don't change the duty cycle." I don't see how this line gets (or keeps) the mill moving toward the maximum power point. My technique will continue to load the mill as long as amps are going up.
(Let me say that when I write "my technique" I only mean the one I'm using, I don't claim invention or ownership!)
The next line says that if the amps go down and the rpm goes up, then increase the duty cycle. How can that happen? Well, if you had just unloaded the mill when you were below the MPP (reduced the duty cycle) then the amps would go down and RPM could go up. My technique would say to reverse the step direction yielding the same effect, the duty cycle would be increased.
A change in wind can not effect this test. At any given duty cycle, if the wind increases, amps will go up so this case wouldn't apply. If the wind decreases, RPM will go down so this case doesn't apply.
The third line says that if the amps go up and RPM goes up to "do the same thing as last time." My technique always does the same thing if amps go up so they yield the same effect.
The fourth line says if the amps goes down and the rpm does down then decrement the duty cycle. Well, if you have incremented and overloaded the mill beyond the MPPT then both the RPM and amps will go down. My technique says to reverse the step direction (always) if amps go down so it will decrement the duty cycle as well. Same effect.
Can a change in wind effect this case? No. At a given duty cycle, if the wind increases the RPMs will go up so this case does not apply. If the wind decreases, RPM will go do and we need to unload the mill; however, the current-only technique does this anyway without the need to check RPM.
... so it looks to me that the current-only technique yield the exact same behavior as the current/RPM technique (except in the first case which doesn't seem correct to me) so I don't see that checking RPM added anything...
Also Jim writes "an algorithm based solely on current measurement would have to change duty relatively slowly to get around the problem of the inertia of the rotating mass."
I don't see that either. My mill will double in speed in about a second so maybe his mill has a lot more inertia than mine. Also, since inertia is the inability of the mill to change RPM, I don't see how checking RPM gets around "the problem" (not that I see what "the problem" is either.) At any rate, I'm running about 5 updates a second with the current-only technique and haven't seen any problems (fingers crossed) yet.
- Ed.
