Low current Stepper controller

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rurwin
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Low current Stepper controller

Postby rurwin » Thu Jan 26, 2017 7:46 am

Ok so I've got a new project planned and I want a very geared stepper for it. I'm talking tens to one of gearing. So I found these, but at (only) 10:1 I think I'd also like to micro-step them.
http://www.banggood.com/DC-5V-2-Phase-4 ... 53297.html
Model GM1527-10
Voltage 5.0V
Reduction Ratio 1 : 10
NO. of Phases 2 Phases
Excitation Method Bipolar 2-2 Phase
Step Angle 1.8° +/- 7%
Rotation Direction CW/CCW
Detent Torque 50gf.cm (Ref)
Resistance Per Phase 30Ω +/- 7%
Holding Torque 50gf.cm min
Pull Out Torque 40gf.cm min
Max No Load Response 1000HZ Min
Max Slew Speed 800HZ Min

I've also bought some stepper controllers: http://www.banggood.com/3Pcs-3D-Printer ... 52018.html
These use the DRV8825 chip.
However, the current required for that stepper motor is very low: 5V at 30 ohms is about 0.16A. If that is right then the DRV8825 is going to be operating far below its normal current range.

Have I calculated the current correctly, and is the driver likely to be able to control the motor without letting the magic smoke out?

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Re: Low current Stepper controller

Postby Mac The Knife » Thu Jan 26, 2017 8:21 am

I am a little suspicious of the step angle,,,, Is that 1.8 degree before, or after gear reduction?
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Eric
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Re: Low current Stepper controller

Postby Eric » Thu Jan 26, 2017 10:45 am

Since they didn't specify, it's a fair bet the 1.8 degrees is AFTER the gear ratio, not before. Which makes the motor an 18 degree motor, or 20 steps per revolution.

I don't know the needs of your application, but your question about current suggests you may not appreciate how small that motor assembly is. So yes, you're going to be far below the capacity of the 8825 running that motor. The 8825 limits are designed for larger motors with heavier gauge windings and more surface area to dissapate heat.

Take another look at the measurements...at 15mm wide by 22mm long including gears it's about the size of my ring finger from the tip to the first knuckle, with a 3mm shaft sticking out. Your hand may be different, but you get the idea. By comparision, the Nema 17 motors most of you use in printers are 1.8 degree motors 42mm wide with a 5mm shaft.

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KAS
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Re: Low current Stepper controller

Postby KAS » Thu Jan 26, 2017 1:22 pm

Not sure of your size constraints, but nema 8's are somewhat small. Powerful little buggers; something like "8HS15-0604S-PG90" is 90:1

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Re: Low current Stepper controller

Postby Polygonhell » Thu Jan 26, 2017 9:02 pm

Just an FYI in case you intend to the the gear reduced stepper motors for precision work.
IME geared steppers tend to have enough backlash to swamp the reduced step size, the ones I tested were 50:1.

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Re: Low current Stepper controller

Postby Eric » Fri Jan 27, 2017 1:44 am

Planetary gears range from good to excellent on backlash.

Harmonic drive gears have zero backlash and huge ratios. Commercially they're expensive precision items, way above a hobbyist budget. However, there have been a number of evolving 3d printed designs that may be worth looking at, if you have plenty of space and the loads aren't too heavy for plastic.

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Re: Low current Stepper controller

Postby rurwin » Fri Jan 27, 2017 4:09 pm

We'll have to see. TBH, I'm not expecting much for £5. It would be a shame if those numbers were after reduction, but you may very well be right. If so then the torque is going to be the killer. In which case I have a pair of interesting and tiny motors to play with. Might help with another project I have on the back-burner.

It's a pain if the driver board won't work with them; I know just enough electronics to be dangerous. The datasheet says it works below VRef = 1V with degraded accuracy. I don't know what the sense resistor is on those boards, but assuming it's 0.2 ohms as recommended in the datasheet that makes the minimum accurate current 1A. And I need 0.16A, so it won't be accurate by quite a way but it might be usable.

To be honest, I'm going off gearing. Those NEMA8's have steps of 0.02 degrees and backlash of up to 1 degree. That's 50 steps. It's still 1 degree for all of them even down to 5:1. But without gearing, torque becomes an issue.

In the worst case, Banggood also do NEMA17's for £10-20 and NEMA23'a for £10 more. They may be rubbish for that price but at least they'll validate the rest of the design. Once I've proved that, I can spend decent money on it.


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