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Direct tach read has got to better than BEMF especially (DCC approach here) at the low end of the motor speed, and at the high end because it's hard to sample the generated voltage in the time windows or there's not much being generated at the lower speeds. (opinion based on -- i now have a very primitive bemf get up at home... i mean it appears to work in the most basic ways and i have a tach reader i built also -- and that's what i see on the speed/rpm matching when i sample them together).

Further drifting off topic...

Given that, I do wonder why HO manufactures or actually the DCC encoder folks don't offer the option for tach readers. What I see in my huge (sarcasm) samples of new DCC engines, is the same overall design with the motors centrally located in the model, and long shafts out either end to the front and rear wheels.

And it just seems there's lot of places a reader could be put if one wanted do it. All they'd need then is to offer a pin or two on the board to read the ticks, and a CSV value or two in the firmware to config... or so I imagine.

(and then either leave it up the to the intrepid hobbier to supply all the parts for it, OR sell a harness)
 

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The BEMF that Jon Z. developed was quite sophisticated. You are right, at very low speeds, the back-EMF is minimal, and it's hard to control low speeds. What Jon did was to actually detect commutator transitions at low speed to improve the low speed performance. The result is, the Lionel (and ERR) back-EMF drives have excellent low speed performance.
 

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I've guessed there is cottage industry of knowledge among the folks in the biz on this topic. Well Jon Z was in the biz at least...

I have no idea how'd you read commutator transitions -- except perhaps by long hours staring at the scope and seeing a pattern. Maybe its obvious to those in know hardware wise -- I am not one of those people.

For own efforts, I have not hooked up a scope to look at what mine is really doing.

But it does appear to be working -- and again I just want to emphasize. It took me several years to understand what the thing was and how one could read it in something like a small CPU -- you know in my free time here and there as a non-hardware person.

All it is in my case is a voltage divider off the motor into the adc of the arduino. Then a tiny bit of code to spin and coast the motor and read the voltage. [note: Im using MTH motors and 20v power supply, h-bridge board from .. pololu, etc...]

This appears to be enough to get the generator effect from the bemf and then I can measure RPM with using the various formulas and motor measurements i made separately.

In comparison to the tach which I built from an IR set up similar to the mth concept-- the results are good. In the "middle" of the speed is about 1% or so similar value most samples, at the "ends" it drifts off. [right now the tach is on the same arduino at the same time]


During bench only operation at this time -- I can also hear the motor surging a little during bemf sampling, and while I played with the code a little, I can't get it to go completely away.

I didn't try super hard and as I said I didn't look into the scope at all, but wonder if this points to a more solid hw solution or other super secret techniques.

I also read elsewhere that braking the motor can be used in this area and this may be another aspect to explore. I only spin it and coast it right now. I could brake it. I haven't tried this... but wondered about it.

Having said all that I'm just about sure a tach is so much easier, why bother!
 

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Jon Z was not only "in the business", he is a very gifted engineer. Until recently, he was the Chief Technical Officer for Lionel, and many of the current products carry his stamp.

I don't know that you would want to brake the motor for the drive motor. As it turns out, I actually use dynamic braking for my Super-Chuffer to get better definitions of the smoke chuffs.
 
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