Chinese Ramps 1.6

This topic contains 67 replies, has 12 voices, and was last updated by  Jeff 8 months, 1 week ago.

Viewing 8 posts - 61 through 68 (of 68 total)
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  • #88207

    Jeffeb3
    Participant

    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.

    #88218

    Ryan
    Keymaster

    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.

     

    #88223

    BT
    Participant

    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!

    #88300

    Ed Nisley
    Participant

    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!

    #88303

    Ed Nisley
    Participant

    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.

    #88305

    Ed Nisley
    Participant

    contact list for really smart people

    Nah, I’ve just managed to learn a bit from a whole bunch of mistakes … [sigh]

    #88315

    Ryan
    Keymaster

    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.

    #91783

    Jeff
    Participant

    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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