Tagged: ramps 1.6 mp3dp
February 8, 2019 at 4:58 pm #88207
If you’re feeling adventurous, you can use one wire of the harness as a current-sensing resistor: measure the wire resistance, connect a scope across the wire, and the waveform will give you the current. For example, a 0.25 Ω wire will give you a 250 mV/A = 4 A/V waveform. Note that the wire has nothing in common with circuit “ground”: connecting the ground clip of a line-operated scope probe to the wire will kill the driver chip stone cold dead.
I just might. I have a cheapo scope, but I think I can run it from a battery…
The IR drop varies with the current, so it’s a minimum when the sine wave crosses zero and maximum at the peaks. However, when you’re looking for trouble, don’t start by minimizing the trouble. You’re not showing why something works, but why it might not; always expect reality to be worse than your assumptions.
Yeah, I might try to break it down for all 32 microsteps in my spreadsheet. Just to see how pecimistic this is. I’m guessing the worst step will vary with speed, but be closer to the middle steps.
Accelerating that huge mass hanging on the gantry requires more torque than cruising along at top speed, which means the acceleration limits influence the results. A sufficiently low acceleration may prevent the motor from reaching top speed during a short move, which will screw up all your calculations.
Those things are true, but they are problems on the otherside. Determining what torque is fighting the motor is different than maximizing torque output.
Basically, you cannot determine a hard speed limit from these simple models. What you can get is a better understanding of how speed, torque, and supply voltage interact, which is nothing to sniff at!
Yeah, I understand more than I wanted, for sure. Thanks for dragging me through this.February 8, 2019 at 5:23 pm #88218
Sorry to butt in, Ed, Thank you for getting more pictures of my machine in the magazine again!
I am desperately trying to follow along with you two but you went over my head real fast. So I will just sit quietly while the smart guys talk, try to learn a new word or two, Glean some knowledge. It is fun seeing real engineers discuss an issue.February 8, 2019 at 5:27 pm #88223
I’m with Ryan, but I just updated my contact list for really smart people to add Ed to the collection with Jeff and Ryan and … so many others!February 9, 2019 at 5:10 am #88300
more pictures of my machine in the magazine again
Readers may not recognize it from the bug’s eye view, but the middle assembly is definitely an impressive collection of plastic & bolts!February 9, 2019 at 6:09 am #88303
If I plug in 900mA, I get the cutoff moved to 53mm/s.
Torque is proportional to current: at 0.9 A the motor produces only 60% = 0.9/1.5 of its rated 76 oz·in = 45 oz·in.
Power dissipation in the winding resistance varies as the square of the current, so they’re dissipating only 36% of their rated power. Even at 1.5 A, though, they’d dissipate only 5-ish W and shouldn’t get too warm for PLA.February 9, 2019 at 6:12 am #88305
contact list for really smart people
Nah, I’ve just managed to learn a bit from a whole bunch of mistakes … [sigh]February 9, 2019 at 7:06 am #88315
I’m with Ryan, but I just updated my contact list for really smart people to add Ed to the collection with Jeff and Ryan and … so many others!
Well thank you but I am more of a persistent bastard, than a smart guy. If I can’t figure it out I just keep trying, a little finger crossing, some cussing, occasionally a plug and pray.March 4, 2019 at 5:16 pm #91783
OMG – my head just exploded trying to read the posts on page 2 of this thread… good god, there are some very large brains on this board!!
I’m gonna go back to drooling on myself now. lol.
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