24 v supply, 2.5 A motors and Stepsticks = trouble?

Hi!
I'm just finishing my printer and i have what i gather is a somewhat unusual combination of hardware.
Basically, i'm running a RAMPS 1.4 with Stepsticks, a 24 V power supply and Wantai 42BYGHW811 2.5 A 1.25 ohm stepper motors. The extruder is a Greg's hinged accesible with a J-head IVB 0.5 mm hot end.

I'm having a bit of trouble with my extruder (and occasionally Z) motor stalling.
I get what seems to be stalling when doing longer passes of extrusion. The gears look like they oscillate around a point accompanied by the motor rattling, but when i nudge the big gear slightly, it seems to help and continues on it's way. At really low speed (5mm/min) it seems to go away.

I think there is either a mechnical or motor drive problem, or a combination of both. Tomorrow i'll tear down the extruder and check that the filament path is ok. I'll also check that my home made hobbed bolt is up to job. I guess if the filament wanders out of the groove it would cause a big increase in resistance and could possibly cause the motor to stall.

One problem is that I don't quite understand how my supply voltage/motor current rating will affect the actual current setting of the stepsticks. I've read nopheads excellent post on motor maths, but haven't digested it fully yet...

The stepsticks are limited to about 1 A as is. I gather If i was running 12 V i would probably need to change the sense resistors to 0,1 ohms to get adequate current out of the stepsticks. Does this still apply at 24 V or will the increased voltage make up for the lower current?


On an unrelated note I shorted out R4 on the stepsticks since reading stepstuck. This was a big improvement, and even visually noticable at low speed. Before it was more akin to macrostepping. If my calculations are right i would need a bit more than 128 µs off time, and the 10k resistor in place was about an order of magnitude off...

The basic problem is 2.5A motors. They should be around 1.65A for best torque from a Pololu / stepstick.

Lower sense resistors will help and the current isn't affected by the supply voltage. You will probably need to add a heatsink.

Higher voltage is better for high speed, but the extruder motor doesn't need to move fast. It does make the off time problem worse though. 128us is impractical because it is in the audio range so the best value is 0 to enable low current microstepping mode.

Edited 1 time(s). Last edit at 08/06/2012 03:59AM by nophead.

---

[www.hydraraptor.blogspot.com]

Thank you for the clarification, then i know what steps i need to take to correct the problem.
I'll start by replacing the sense resistors on the extruder driver to 0,1 ohm, if that's not enough i'll have to look for another motor for the extruder.

I already have heatsinks on the drivers, but i'll see if i can't add adittional cooling on the extruder board, perhaps a tiny heatsink on the underside of the vias. I've got a spare thermistor i'm using to monitor the extruder stepstick temperature.

The other axes seem to be ok or close to ok. If i run into problems there swapping the sense resistors and upping the current slightly is likely enough to fix any stalling.

Thanks again for helping Nophead, and for your wonderful blog posts! They really help with the details that are most often overlooked elsewhere.