BEMF with TCS problem - Model Train Forum

pokey_toaster · 11-10-2019, 02:27 AM

I have two locomotives with Kato motors (one Kato the other Athearn) that I use to load coal trains. They have TCS TX4 decoders. I use live loads and only need the BEMF to maintain HO person walking speed while loading the train so the BEMF is set to turn off at speed step 15. I usually load at step 4. I'm using a Digitrax throttle and the locomotives will creep along fine making perfect loads but when I accelerate away from the loadout, first one then the other will appear to drop several speed steps as the BEMF cuts out. Even though both are set to turn BEMF off at speed step 15 they don't slow down at the same speed step on the throttle. I'm using Digitrax consisting and not advanced consisting, basic speed control (not tables) and program with Decoder Pro. Any ideas to eliminate the slow down when the BEMF turns off would be appreciated.

Paul

gregc · 11-10-2019, 06:12 AM

isn't this a momentum issue?

pokey_toaster · 11-10-2019, 11:39 AM

I don't think it's momentum. As I'm accelerating, each locomotive will suddenly decelerate then start to accelerate again. All the time I am advancing the throttle. The deceleration occurs around 12 of 99 on the throttle which is close the 15 of 128 on the decoder where the BEMF is supposed to turn off.

Paul

wvgca · 11-10-2019, 12:07 PM

turn off the bemf entirely, it will show where the problem is ..

pokey_toaster · 11-10-2019, 12:19 PM

The decoders are set up so F6 toggles BEMF on and off. With it off they run fine but are not able to maintain the slow consistent speed needed to load the train.

Paul

gregc · 11-10-2019, 02:11 PM

back EMF momentarily stops providing power to the motor in order to measure the voltage (EMF) generated by the motor. BEMF is a measure of the motor rpm.

in this mode, the decoder drives the motor with whatever voltage is necessary so that the motor speed (rpm) corresponds to the speed setting from the controller.

in a consist, one loco can help the other, the rpm of the motor isn't solely dependent on the voltage the decoder supplies. i'll guess this can be more severe at lower speeds and some decoders BEMF algorithms deal with it better than others

pokey_toaster · 11-19-2019, 09:48 PM

Update. I put them in an advanced consist so I can turn the BEMF on and off as needed. In an advanced consist, I don't need to select each locomotive, it is just one keystroke. It's not ideal but works better. I just turn it on when loading the train or spotting the first couple cars in the dumper.
I'm still interested to hear any suggestions for eliminated the hesitation when the BEMF turns off.
Paul

Severn · 11-20-2019, 08:57 AM

I think "bemf done right" -- the sampling of the voltage which in my mind puts the motors in "coast" or (possibly) "brake" mode to generate a current by which to measure the voltage, which then as folks say relates directly to RPM -- ought to be imperceptible to the user... yes?

gregc · 11-20-2019, 12:26 PM

Quote:

Originally Posted by Severn

-- ought to be imperceptible to the user... yes?

i think just a few msec, if that. enough time to disable (set to zero) the PWM output voltage, read an ADC and restore PWM

Severn · 11-20-2019, 01:12 PM

I just want to crow a bit with some background-- somewhat off topic but topical.

I got interested in this BEMF stuff which I'd never heard of before a couple years back with O gauge. I bought some parts, arduino and all that. I read and read online. I understood the concept but not see how to make it work -- not being a hardware person I was a bit lost and didn't understand some of the advanced explanation much at all.

Finally I realized all I needed to do was measure the voltage on the adc pin of the arduino if I divided the voltage appropriately from the can motor (and had an appropriate motor controller) to not blow up the arduino which is 5 volts.

While I not understanding all that -- I did understand the simplicity of reading the rotations off the motor directly with some kind of IR sensor etc... and so implemented that and made a very simple RPM reader with it.

Then having the light bulb moment somewhat later as I just described I managed to get a very simple BEMF working on the same set up -- and can run them parallel.

It works OK -- not so great at the low or high end but matches closely with directly RPM reader "in the middle".

However there's a definite lag that's audible when you coast the motor to get the voltage going. I played around with this but not in a serious way and never got it to go away completely.

That's why I think a better implementation would be more successful and you'd not see any visible lags or jerkiness in the engine. I suspect a full on hardware approach might do the trick and the pauses or coasts can be minimized to the point of not being audible to the human ear with no visible effects on the engine. Perhaps there's some concept of "super sampling" or something that allows lots of micro-pauses to somehow give you the RPM reading etc...

Anyway blah blah blah ... I just wonder about the various decoders out there and how good they do the job. On my one soundtraxx current generation you cannot hear a thing as far as all this goes and performs very good at slow speeds too. I have never seen any jerkiness and so on.

I also wonder at this point why the sound decoder folks don't provide a "1 revolution pin input" for RPM sampling provided by some other device .... seems like that's the way to go in the end and hobbyists might enjoy providing the little bit of hardware to do it.