Want stepper drivers @ > 2A ?

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Want stepper drivers @ > 2A ?

Postby frob » Tue Feb 07, 2012 2:47 am

I'm just wondering if anyone here might need or want a stepper driver capable of sourcing more than the usual 2A limit of integrated driver IC's such as the a4988 etc. found in Pololu and similar driver modules.
I'm guessing the answer's probably no, as we're usually looking at fairly small motors in lasers / 3d printers, but i though i'd ask just in case.
Surveying the available cheap steppers with decent torque i see a few nema 17's & 23's that need more amperage ( like 7A+) i wouldn't mind using for my build - I though maybe i'm not alone...
If so i'd consider doing a stepper driver module very conservatively rated at 12-50V 10A max, no heatsink. Could even make it Pololu compatible but rather not since that would require 2-sided assembly which is not ideal in my mini-oven.

Any takers?
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Re: Want stepper drivers @ > 2A ?

Postby bdring » Tue Feb 07, 2012 12:19 pm

10A is one hell of a stepper. Most NEMA 23's are happy with about 2.5-3A. At 10Amp you are into the 34 and larger range. You probably want those up in the 80V range too. At that size you can get big resonance problems. The driver needs to deal with that directly at that size. I think that is part of the reason why you don't see too many steppers above the 1200 oz/in range.
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Re: Want stepper drivers @ > 2A ?

Postby frob » Tue Feb 07, 2012 4:51 pm

Did you catch the error?
Most people count sheep to fall asleep.... I count electrons... and then i realized i was missing a few....

The on resistance of the a4988 is .32r(typ) to .43r(max) ; @ 2A : >> this is for each 1/2-bridge, of which there are 2 per phase <<
...each one is driving 2 phases which means I^2R losses are *actually* ~ 5 to 7W !

Temp rise is then:
Utopia : 5W x 32C/W = 164C
Best case scenario: 5W x 65C/W = 325C
Worst case scenario: 6.9W x 115C/W = 791C

The good news is i just found a really nifty little stepper driver IC that's got a typ output RDSon of .105 Ohm, 1/32 micro-stepping and tons of other cool features.
One i especially like is the overtemp warning signal that tells you when its less than 20C away from shutting down from overheating.
And no current sense resistors needed, so no power wasted there.
Another one is the analog Back-EMF output, great for analyzing motor dynamics.
And SPI configuration interface so no jumpers/dip switches necessary..
Plus its much better characterized than the ones Ive seen so far - its *conservatively* rated for 2.2A continuous, 5A @ low duty. These ratings i can trust.
Downside is its 40-50% more expensive and rated 30V, but i think its still a good bargain.
Combined with some better thermal management, i could easily get double or triple the useful *safe* current out of it compared to the $14 a4988 boards.
On the other hand, that extra 40-50% price increase would also cover the 4 dual-mosfets i would need to make an ice-cold 10amp 50V stepper driver, compatible with a4988.
Hard to decide.
Anyone wanna give me their 2 cents ?

Here's a thought - You can get for $10-$20 a super-efficient heatpipe-fan-heatsink for PC CPU's , that can get sub- .5C/W thermal resistance.
What if we had a 4- or 6- channel driver board designed to mate to that ?
With 1C drop from die to slug, and another 2-3 though the board with thermal vias, .5 for solder and thermal gasket...
So maybe 5C/W die to ambient, assuming 40C ambient, run the die at 80C max means safely burning 8W in there.... even 10 or 12 at the outer limits with some margin....
Going backwards from that means... 3W per 1/2-bridge; adjusting max RDSon for temp gives .25 Ohm worst case, .14 typ.
So that's... 3.5 to 4.5 amps continuous; 8 to 10 amps intermittent. Not too shabby! i may just have to try this :)
Suppose a 4-channel unit like this sold for $100 including heatsink - is that attractive ?
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Re: Want stepper drivers @ > 2A ?

Postby frob » Wed Feb 08, 2012 3:25 pm

Oops, i though i quoted myself there but i musta hit edit by mistake... really shoulda had some coffee first.
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Re: Want stepper drivers @ > 2A ?

Postby macona » Thu Feb 09, 2012 10:26 am

10 amps is a bit much. The biggest motors we had at work were NEMA 56 Triple stacks and they ran at 8 amps with 180v drivers.

The biggest 23's I saw were 4 and 5 amps, but they were the exception. Most 34s were 4 amps max. Couple double stacks were 6 to 7. As for 43's they peaked about 7 as well but those also ran on high voltage power supplies. >100v
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Re: Want stepper drivers @ > 2A ?

Postby r691175002 » Thu Feb 09, 2012 7:33 pm

Generally voltage is more useful than amperage. 48V is pretty standard for nema 23's. Anywhere from 3-5A is enough even for large benchtop machines.

The main reason we use the pololu drivers is that they are dirt cheap. As soon as you move into the 50-100$ range there are a lot of compelling drivers from keling/gecko.

50V/5A with decent digital stepping for 50$ would be very nice but there are dozens of stepper drivers available so its not like there is a gap in the market that needs filling.
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Re: Want stepper drivers @ > 2A ?

Postby frob » Fri Feb 10, 2012 4:01 pm

Interesting - i had assumed that as steppers got bigger the currents and voltage both scale in proportion with a good selection of trade-off options possible at any power level -
As you point out, that doesn't seem to hold true.
I did a quick survey of the higher power/ torque motors and that definitely bears it out- although i could find some with high-current and very low inductance that would live happily with a 50V drive all the way up to 2000-4000 oz-in in nema 42+ sizes, they seem to be more the rare exception than the general rule .
The current range really seems to top out @ 8-9A at the extreme, but most are 5 or less; what does increase faster on average is the motor inductance (up to 22mH) so yes higher voltage there is much more desirable than current.
Plus if you're looking to drive a 2000+ oz-in motor your spending at least $100-200 on the motor itself, so surely you can afford another $100 for a commercial high-voltage driver to go with it.
So for a 50v drive 5-7 amps i agree is as much as anyone could really use ;
At 30V i'd say 4-5A is plenty so its never a limiting factor, the voltage will be limiting performance for the larger motors.
I think a 50V 5-7 A model could be made in a Pololu-compatible footprint with a board that's .8"x.8" (slightly wider, since the Pololu's are .8" x .6" ) but with no heatsink or fan necessary.
They could be worth $30 ea. in singles or get a 4-pack for $100. Maybe less once production is rolling.

Scaling voltage above that gets a lot harder than current; doing a 100v or 200V stepper driver is possible only as a discrete design without the benefit of an integrated stepper controller-driver IC as those just cant handle the voltage and though doing a kluge to boost their voltage capacity is not impossible, it gets very messy and tricky so is better then to just start fresh with an FPGA for example. But as you point out there's lots of good product in that range already so its pointless for me to do yet another one. I'd rather invest in a good servo driver design in the higher power space if i did anything there.

I'm still thinking the 30v 3-5 stepper module might make sense as a 4-channel module ( ex.: X-Y-Z+Extruder), but going that route i might as well throw in a cheap USB interface and the other missing essentials to make it into an all-in-one 3D printer solution. But since the FDM process is so slow i doubt you'd really need the torque there; I'd think its more useful in a router or laser design where you can actually use the extra speed or torque.
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I love the smell of Fedex packages in the morning!

Postby frob » Mon Feb 13, 2012 5:59 pm

.. especially when they contain free samples of brand new stepper driver IC's requested a few days prior - shipped direct from China to boot.
I guess that means i have no excuse not to go ahead and make some prototypes :)
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What ive been up to since supper...

Postby frob » Tue Feb 14, 2012 7:53 am

schematic and layout done!

So you can see, this is designed to have all parts on the underside when mounted,
Exposing the groundplane on top to efficiently radiate heat upwards;
the soldermask has cutout about 0.35" x 0.4" directly opposite the driver IC,
where a copper heatsink can be directly soldered to the groundplane- this should really help!

SOTA-STEP-1-TOP.png


SOTA-STEP-1-BOT.png


Also note, there is no pot; adjustments are CPU controlled through the SPI interface,
which replaces the pins used to select the microstep mode, and enable becomes SPI CS.
Otherwise i tried to keep the same pin assignment as the Pololu a3988 module so it shouldn't be too
hard to get an existing control board to interface with it.
Also Vdd is now a 5V output, able to source 8mA- leave unconnected if not needed, do not connect to a power source.
It is likely i will change that to a 2nd HV supply pin as it would be better at high currents and improve the layout a lot.
A number of settings are programmable through the SPI interface, including:
Microstepping - full, half, 1/4, 1/8, 1/6, and 1/32
Max Current : 32 levels on a piecewise exponential curve from 122 mA to 4.75A
sleep, enable, programmable slew rate, switching frequency, jitter (emi reduction)
Overtemp warning, open /shorted motor windings, readback of motor position,
step/dir polarity select...etc..etc...
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Final design to get fabbed ?

Postby frob » Tue Feb 14, 2012 6:41 pm

Playing with it some more over lunch,
i am very pleased with the result now, so i may get these made at this point.
I added some pins on a .1" grid spacing on the as yet unused edges to get the analog back-EMF output, Watchdog reset output, and VDD output for anyone that might want them when doing a custom board.
That opens up the topside groundplane even more so i can stick a bigger , standard size heatsink on the top.
The old VDD pin is now a Vmotor supply pin, so in "backward compatible mode" you either have to mod the control board or not connect that one (don't install a pin there) and solder a wire back to the first VM pin.

SOTAstep1bBOT.png
SOTA advanced 32-microstep 5A motor driver - bottom side, no pins


SOTAstep1bTOP.png
SOTA advanced 32-microstep 5A motor driver - top side, no heatsink
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