Powering a stepper with A4988 on the go

Hey guys, n00b here (first ever hardware project, I'm a software engineer) :) I am currently in the middle of a time lapse slider project (yes, another one :)), and I'm puzzled about powering it on the go.

I have a Mega board, A4988 as a driver (the red clone) and a NEMA17 stepper (I have a feeling that it is a Wantai 42BYGH W811, but not sure how to check).

Currently (in a prototyping phase) I'm powering it using a 9V battery, but obviously it's not a good option (it's weak and very small capacity). I have two options in my mind: 1. Buy a mobile power bank with big capacity and higher voltage output (e.g. this: http://www.fasttech.com/products/0/10004461/1341206-qidian-1a-1-5a-2a-4a-6-18650-mobile-power-bank has big capacity and I could use 9V 1.5A output) 2. Buy a sealed lead acid battery (afaik, much less capacity)

I would be very happy if I could just use the first option because I could take it with me on the plane, which I doubt I could do with the second option.

Thanks!

Are you satisfied that you don't need to run the motor at 12v or 24v?

...R

Why not use the 18650 Li cells directly? Three should do the job. I use them in robots. Cheap rechargers are readily available. I do not see any advantage to putting a swithching regulator between the batteries and the motor.

Joe

Robin2: Are you satisfied that you don't need to run the motor at 12v or 24v?

Sorry, didn't quite get your comment, did you mean that I should run it at higher voltage than 9v?

[quote author=joe mcd link=topic=218590.msg1596250#msg1596250 date=1392669619] Why not use the 18650 Li cells directly? Three should do the job. I use them in robots. Cheap rechargers are readily available. I do not see any advantage to putting a swithching regulator between the batteries and the motor. [/quote] Well, they are only 3.7V and according to A4988 datasheet, it needs 5-32V. Stacking lithium cells without a regulator is asking for trouble (unless you use protected cells). I have a few of those batteries (I use those in my electronic cigarette).

timtog: Sorry, didn't quite get your comment, did you mean that I should run it at higher voltage than 9v?

Stepper motors produce more torque at higher speeds when powered by higher voltages. I am just wondering if you have satisfied yourself that 9v is sufficient for your project.

...R

What is the resistance of the motor windings and the voltage rating? Alternatively, what are the current and voltage ratings? It is essential to know the motor current draw, in order to properly configure the system.

Well, they are only 3.7V and according to A4988 datasheet, it needs 5-32V. Stacking lithium cells without a regulator is asking for trouble (unless you use protected cells).

All the 18650's that I have are protected. They are used in consumer products like flashlites.

Joe

jremington: What is the resistance of the motor windings and the voltage rating? Alternatively, what are the current and voltage ratings? It is essential to know the motor current draw, in order to properly configure the system.

As I don't know the exact model of the motor, I can't tell you for sure, but according to this: http://reprap.org/wiki/NEMA_17_Stepper_motor, the rated voltage is usually around 3V. TBH, I don't know anything about the coil resistance... The driver can output up to 2A but as it won't be cooled, I think it should be left at 1A, maybe I'll try to put 1.5A.

I'm sure I'm not the first one who wants to power a stepper on the go, what do people usually use?

Robin2: Stepper motors produce more torque at higher speeds when powered by higher voltages. I am just wondering if you have satisfied yourself that 9v is sufficient for your project.

The power bank that I am thinking of buying can give 12V too, but then it will be 1A. I am quite bad with electronics, so I don't know what would be better, say 9v/1.5A or 12v/1A.

For timelapse speed isn’t going to be an issue I think. Power consumption is so setting the
A4988 output current as low as you can and still have it move reliably is going to save
battery life.

For battery operation steppers are, alas, a very poor choice. DC gear motor with
absolute encoder would be able to power down completely between movements.

The A4988 has an enable pin to shut down power to the motor.

For battery operation steppers are, alas, a very poor choice. DC gear motor with absolute encoder would be able to power down completely between movements.

This view is widely held but somewhat misleading. It depends on holding torque. If the position is held even when motor powers down, MAJOR power savings can be acheived. We simply deactivate the motor between movements. Steppers controllers also normally have a 'tuning' potentiometer. Tuning means reducing the current pulse width shorter than the step time. Short pulses at higher voltage lead to crisper stepper action and sometimes power savings.

If the stepper must be powered to hold accurate position (such as CNC applications), then you are correct. If the stepper must move frequently near maximum speed, it is also a power hog.

I have experimented with the inexpensive 28BYJ-48, which has an internal gearbox with plenty of hold. Depending on the application, reductions of 90% power consumption are easily reached with a few extra line of Arduino code.

Joe