Chinese drivers and motors powere issue

Hello everyone!

Have a problem here. Any help would be highly appreciated Looks like my motors are not powerful enough. Or my stepper driver setup is wrong.
What I own: elecfreaks A4988 Stepper Driver, Wantai NEMA17 42BYGHM809 (0.9 deg, 1.7A). Here are the links:
[www.aliexpress.com] and [www.elecfreaks.com].
My home made geared extruder has beld driven 3:1 ratio, hobbed bolt is 5mm dia. I'm trying to print with PLA (3mm filament). Software is Marlin and Pronterface.
When I try to extrude few mm with 100-200 speed, motor sometimes slips few steps or just jams. Tried to push filament by hand and hotend works fine.
My current Vref is set to ~0.67, extruder steps per unit 722 (steps\mm?).
Sorry for a crappy phone video. Here they are:
[youtu.be] and [youtu.be].
Thanks in advence!

Artem

turn up the current on the drivers..... your motor doesnt seem to be getting enough current...

Quote
ekaggrat
turn up the current on the drivers..... your motor doesnt seem to be getting enough current...

Based on the published specs, he's right where he should be. 1.7 amps * 8 * .05 ohms = .68 volts. The question then becomes are the drivers and motors actually to spec. I'd double check the sense resistor to make sure that they are actually .05 ohms. Official Pololus have a .05 ohm sense resistor, which is what the linked to specification states as well. However the image shows StepStick, which the "official" version called for a .2 ohm resistor. With open designs and knockoffs that say they are official, it could be anything...I had some drivers that came with .3 ohm resistor which threw me for a loop until I discovered that variance from the norm.

The chinese steppers also may state they are one spec, but their real spec is something different. What is the measured resistance of one coil on the motor? Is is actually 1.65 ohms? If it's more or less, recompute the current by taking 2.8 volts and dividing it by your measured resistance. That would be the specificed maximum current requirement. The potential for having a motor that wasn't exactly what the spec stated was why I didn't go with any old stepper I found on Alibaba. For that model stepper I've found a variety of different specs, all close, but still different (more than just a rounding error).

One last thing is that you may be trying to drive the stepper faster than what your Arduino can push it. A .9 degree steps requires twice as many steps as a 1.8 degree stepper with the same level of microstepping enabled. You might try reducing that down to 1/8 or 1/4 if you were trying to run it previously at 1/16 microsteps. Or slow down the speed to 1/2 or 1/3 what you were trying originally and slowly increase it until you have problems. Different motors, but I had similar symptoms with my z-axis trying to run it at 240mm/min. Dropping that down to 210 makes them run no problem.

Checked coil resistance. Unfortunately, not even close to stated - 2.4Ohms each. So it's just ~1.2A per coil. So am I overpowering motor?
I've tried to extrude even at 100mm/min, so it will not help. Also, I forgot to mention, that I tired both 1/16 and 1/8 microstepping and didn't notice any significant change.
Thanks for your thoughts anyway!
So I should set Vref to ~0.48V? I've seen somewhere on reprap wiki, that Wade's extruder worked well with A4988 and 0.9dep 1.7A stepper. Now that you confirmed I have right setup on stepper driver, I'm sure stepper are crap.

One more thing - stepper drivers are crap also)) Checked sense resistors, both are marked "R100", which is 0.1 Ohm. So with my current Vref set to ~0.67V, max Amps set to ~0.84A. So it's 30% less of what even my crappy motor can do.
New drivers/motors or resolder resistors and try this drivers?

P.S. I guess crappy drivers are the reason my x/y/z axis a so weak also

Edited 2 time(s). Last edit at 03/21/2014 01:32PM by Sazabi.

Your math for a vRef of .67v giving you .84 amps is correct. But you aren't overpowering the motor, your underpowering it by about 30%.

If your motors are indeed 2.8 volts, and resistance 2.4 ohms, that gives you 2.8/2.4 = ~1.17 amps. 1.17 amps * 8 * .1ohms = .928 volts for your maximum vRef. Since your current is signifiantly different than the rated spec, I'd honestly call into question the voltage as well as producable torque values. Maybe those motors will work. But maybe they are junk too. For the $17+ that they were listed on AliExpress, you could have gotten betters motors from a reputable store that would provide actual service. I ended up paying about $15 each shipped for my 5.5Kg.cm (76 oz-in) steppers here in the US (not sure where you're at).

Your drivers may be perfectly fine. The sense resistor can help limit what the maximum current your driver can produce. As the sense resistor resistance goes down, maximum current goes up, up to the limit of the driver chip (2 amps with a A4988) before it releases it's magic smoke.



Also your steps per mm is off. Or at least I'm not sure how you computed 722 steps / mm. The formula for computing steps/mm starting point for a geared (or pully in your case) extruder is:
(Steps per revolution * microstepping level * output gear) / (input gear * hobbed bolt effective diameter * 3.142)

With your setup if you had 1/16 microsteps, you'd have (400 * 16 * 3) / (1 * 5 * 3.142) = ~1273. If you were doing 1/8 microstepping, that would be (400 * 8 * 3) / (1 * 5 * 3.142) = ~637. If your 5mm hobbed bolt is 5mm before it was hobbed, then the 5 would need to be reduced down to the diameter where the filament is coming into contact.

Once you get your extruder actually extruding reliably, you can tweak the formula by adding one more ratio to the end of the formula. Mark the filament at 120mm before it enters your extruder. Extrude 100mm. Measure the distance between the extruder and your mark and subtract that from 120. That's the amount of filament that was actually extruded. Plug that into the two new formula of:
(Steps per revolution * microstepping level * big gear ratio) / (small gear ratio * hobbed bolt effective diameter * 3.142) * (mm filament desired / mm filament actually extruded)
That ratio accounts for any slight variance in an individual machine's hardware. So for instance if your drive ratio no being EXACTLY 3:1. Or your hobbed bolt being 5mm before it was hobbed making the diameter a little under 5mm.

So if you had 1/16 microstepping, you asked for 100mm of filament and it only consumed 98.6 (underextruding), your calculation would look like: (400 * 16 * 3) / (1 * 5 * 3.142) * (100/98.6) = ~1291
If you had 1/8 microstepping, you asked for 100mm and it used 102.3 (overextruding), then it would look like: (400 * 8 * 3) / (1 * 5 * 3.142) * (100/102.3) = ~622.

Giving props where props are due, the above formulas were adapted from Nophead's Mendel90 manual.

Thanks, cdru!

You're absolutely right. I've mistakenly took current Vref as max Vref.
About steps per mm - I took 722 for testing purposes and forgot to change it. Experimentally I found it to be about 1400, which is close to what you have calculated (with 1/16 microstepping). Big thanks for explaining math for this!
Unfortunately, I'm faaar from US (Kazakhstan) and it doubles motor price (because of shipping fee). And China store sells with free shipping.

0.925V for Vref is the best I was able to achieve at the moment. Significant improvement! Will test more and write results later.

I was digging internet for the info of my steppers and found next info:
rate voltage is 3.06V (well, got this info on another page on wantain seller on aliexpress, also here (not adv) - [www.openimpulse.com]⋅cm/, they say 2.8,but attached datasheet says 3.06V
So looks like I need to tweak my Vref to 1.02. Well, I believe it won't hurt motors much even if I'm wrong. Just a bit of additional heat?

Btw, one more question. I have some problems with extruding. First layer exdrudes just well, second/third also. But than motor starts to slip, just like it can't push filament any further. Actually, it's nearly impossible to push it by hand also. Looks like extrude speed is too fast? In Config.h I have "#define DEFAULT_MAX_FEEDRATE {500, 500, 5, 25} "
Attached is a pic of my extruder. Bolt (ptfe tube inside) screwed in big white ptfe round piece for about 12-15mm.