Stepper with good torque/acceleration for a corexy?

Hello
I made a corexy (smartrapcore) that works pretty well.
The only problem is travel acceleration and sometimes printing speed. It looses steps..

My X/Y stepper setup is nema17 (17HS4417) 1,7A - 40.0 N·cm, with drv8825 and RAMPS 1.4.
On Italian subforum a guy told me to try a 17hs4218 1.8A 60mm chassis saying it's really good as nema17 for heavy load.

I have some doubt about this motor..
On reprap nema17 page it is written that a good range for a nema17 inductance is 3 to 8 mH inductance per phase.

I saw on some specs tables that this motor has 49 mH inductance!
Also rotor inertia is 102 g/cm2 while my actual one is around 54..

So I'm asking if this solution is good for a corexy printer.
Corexy need a lot of precision and really fast rotation change due to mixed movement.
I read that speed is not crucial, while acceleration is! So could this motor give me plenty of acceleration/deceleration with these specs??

Thanks everybody
Pietro

Edited 1 time(s). Last edit at 05/23/2016 02:00PM by pieri70.

You won't get very much better than your current motors in Nema 17 size. What current are you running the existing stepper motors at, and what is your steps/mm?

I suspect someone has misread the datasheet and those alternative stepper motors have 4.9mH inductance, not 49mH.

Your loss of steps may be caused by the drivers overheating. Those little plug-in drivers have totally inadequate cooling, because the driver chips are supposed to be soldered down to a much larger area of PCB to carry the heat away.

Another factor is the supply voltage you use. For really high speeds, you need to use 24V power, so that the available torque doesn't drop so rapidly with increasing speed. However, most RAMPS boards are limited to 12V.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks dc42
I'm running @ 0.7-0.8 Vref. 16 micro steps. My drivers have a small ALU heatsink glued on top and two fans blowing at them..
They run little warm.
Also steppers aren't really hot. They get warm of course but you can touch them with the top of your hand without feel any pain.
And yes I power my ramps with an atx power supply with 11.9v and 15A.
Only electronics are on this circuit. Heated bed has its own ps.
So what would you suggest?
Pietro

Edited 2 time(s). Last edit at 05/23/2016 04:21PM by pieri70.

Quote
pieri70
Thanks dc42
I'm running @ 0.7-0.8 Vref. 16 micro steps. My drivers have a small ALU heatsink glued on top and two fans blowing at them..
They run little warm.

Unfortunately, that proves nothing. Heatsinks glued to the top of those chips are completely ineffective, because the heat doesn't transfer well from the chip through the plastic to the heatsink. The heatsinks probably make the cooling worse instead of better, by restricting the airflow to the PCB where it is really needed. Try putting your finger on the PCB instead of the heatsink - but be careful not to burn yourself.

There are two sorts of stepper driver for which the foregoing is not true and heatsinks do help. Once is driver modules using the THB6128 driver, which has a thermal pad on top of the chip. The other is driver modules that mount the DRV8825 chip underneath the PCB and attach the heatsink directly to the top of the PCB.

What value sense resistors do your DRV8825 drivers have? If it is 0.1 ohms, then the current @ 0.7V Vref is 0.8/(5 * 0.1) = 1.6A. At that current, I would expect the driver modules to get really hot, and the stepper motors to get quite hot too.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Yes Vref is half the current for drv8825.
Indeed I moved from 0.8 to 0.7V (0.72) so my current is about 1.44A and 1.7A max current per phase on my nema17. With 0.8V my stepper get hot not a lot but they did..
As I told before the heatsink doesn't warm a lot, but I'll try to touch the pcb next time.
In any case you suggest to change drivers before I change steppers.
Is it possible to find the drivers you indicated in a shape that fits the ramps 1.4 rails?
P

Edited 1 time(s). Last edit at 05/23/2016 04:54PM by pieri70.

Quote
dc42
There are two sorts of stepper driver for which the foregoing is not true and heatsinks do help. Once is driver modules using the THB6128 driver, which has a thermal pad on top of the chip. The other is driver modules that mount the DRV8825 chip underneath the PCB and attach the heatsink directly to the top of the PCB.

The first one you mentioned is too big to fit a ramps 1.4.
The second I think is good solution but is a little pita to set vfer, as it has vref regulation knob onder the bcb



So it's the pcb to get hot, I thought it was the chip..

Also this solution is fancy



So both PCB and Chip are cooled

Edited 2 time(s). Last edit at 05/24/2016 02:43AM by pieri70.