StepStick

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Please note: StepStick has 0.2 ohm sense resistors instead of Pololu stepper driver boards 0.05 ohm. This limits the current to 1A. See Notes on building for more info.
Crystal Clear action run.png
StepStick

Release status: experimental

CIMG6304.jpg
Description
Revision 0.1
License
unknown
Author
Contributors
Based-on
[[]]
Categories
CAD Models
Eagle
External Link


Introduction

With the recent outage of Pololu stepper driver boards, I've been wanting to build my own, and break my dependency (no offense, I <3 you Pololu!). And after spending a lot of time designing the another board, I figured I could give this a go.

This is an Allegro A4983 / A4988 x4 breakout board for Sanguinololu. It can be snapped apart at the score in case of Allegro failure, and replaced with another or a Pololu. Snap all 4 apart and get a pin-compatible clones for use on boards like RAMPS or Gen7.

Now this is not for the iron wielding solderer. All parts are SMT, and somewhat small - there are some 0402 sized packages. Not to mention the Allegro's thermal pad - a solder pad on the bottom of the chip - can't be soldered without an iron of magic.

That being said, I believe this is an easy to solder board using a toaster oven or hotplate reflow method. There is not too many pads facilitating easy solder paste application using a syringe, and the components should be spaced out enough that a steady hand with fine tweezers can place them. If you've built Sanguinololu with success, perhaps this is the next challenge on your soldering skills adventures. (Take a look at youtube for oven and hotplate reflow methods - not hard at all!)

But if you're not up to the task, stay tuned and keep an eye on this place for a published list of places where you can get this pre-assembled.

Notes on building

The Pololu A4988 Stepper Motor Driver Carrier http://www.pololu.com/catalog/product/1182 is produced on a 2oz copper PCB board. Most PCBs use 1oz copper boards, and Stepstick is designed to use this weight PCB. However, thermal dissipation will be much less than the 2oz copper used on the Pololu carrier. As such, the Stepstick has been designed with a current limit of 1A to suit a 1oz, 2-layer PCB, which should generally be plenty for reprap-type applications. This may be a limiting factor if you plan to use the same electronics for milling, or larger NEMA23 motors, where current draw is likely to be higher. If you are getting the boards produced yourself, you can of course choose how much copper to put in, and hence the thermal characteristics. The Allegro A4988 chip (datasheet available from: http://www.allegromicro.com/Products/Motor-Driver-And-Interface-ICs/Bipolar-Stepper-Motor-Drivers/A4988.aspx ) is capable of 35V and 2A, but this is based on using a 4-layer PCB, so lots of copper to dissipate heat. You can attach heatsinks and have a fan directed at the electronics to improve heat dissipation. The A4988 has a built-in thermal cut-out, so will turn off if it gets too hot.
To increase the current output, you will need to change the value of the sense resistors (S1, S2), and the trimpot (T1) and/or it's resistor (R1). See this thread for more details and suggested values: http://forums.reprap.org/read.php?13,128220

Another consideration is the problem of using x16 microstepping in a low-current application. The Allegro A4988 has a "Low Current Microstepping" mode, enabled by shorting the ROSC pin to ground, R4 in the case of the Stepstick. Nophead discusses the reasons for doing this in this article: http://hydraraptor.blogspot.co.uk/2012/04/stepstuck.html

Nophead has written a number of other very useful articles about the stepstick, and stepper motors and drivers in general:
http://hydraraptor.blogspot.co.uk/2012/04/stepstuck.html
http://hydraraptor.blogspot.co.uk/2009/08/motor-maths.html
http://hydraraptor.blogspot.co.uk/2009/07/lessons-from-a3977.html

If you find any other useful discussions related to the Stepstick, please link them in below.

BOM

This is the Bill of Materials for a standard Stepstick, ie one limited to 1A.

Item Package Value Value Tolerance Voltage Position Note
Capacitor 0402 0.1uf 10% 16V C1, C2, C5, C6 16V capacitor for 12V maximum voltage; use higher voltage capacitors for higher voltage applications, max 35V
Capacitor 0402 0.22uf 10% 16V C4, C7 16V capacitor for 12V maximum voltage; use higher voltage capacitors for higher voltage applications, max 35V
Capacitor 1206 4.7uf 10% 16V C3 16V capacitor for 12V maximum voltage; use higher voltage capacitors for higher voltage applications, max 35V
Motor driver chip QNF IC1 Allegro A4988
Resistor 0805 0.2ohm 0.25W 1% S1,S2 Change these sense resistors to alter current limit
Resistor 0402 10k 10% R4
Resistor 0402 20k 10% R1
Resistor 0402 100k 10% R2, R3
Trimpot 3mm 10k T1 Change trimpot to alter current limit in conjunction with S1, S2

Adjusting and testing the current

You change the current to the motor by adjusting the trimpot. Set the trimpot to minimum to start with, by turning the trimpot fully anti-clockwise. Turn clockwise until the motors are not skipping steps at your target speed and load.

To find out the current that is actually being delivered, follow this advice from nophead http://forums.reprap.org/read.php?13,128220,129335#msg-129335

There is a test point for VREF on the Pololu but it is missing on the Stepstick. Since it is just the wiper of the pot you can measure it there and it is easier as it is a bigger target. I hold the meter probe on the shaft of a metal screwdriver so I can see the value while I am turning the pot. The current will be VREF / (8 * RS).

Where to buy?

  • StepStikc with A4988 and heatsink GADGETS3D
  • ebay.com (tijnekind)[1]
  • Ebay for all sellers
  • UK based seller on eMAKERshop Boards right now, kits coming soon, contact through eMAKERshop
  • Fully assembled and tested boards with heatsink at RepRap.me -- NOTE! 1/8 step only. These are NOT true StepSticks, they have been modified to use the less expensive (and less capable) A4984, not the A4988.
  • Fully assembled boards at AVRThing.com -- NOTE these have the A4983 and .22 ohm sense resistors

Schematic & Board Images

StepStick.Top.jpg

StepStick.Bottom.jpg

StepStick.Schematic.png

Not obvious from this schematics: the trimpot has 10 kΩ. See [2].

Herewith StepStick A4988 picture, its manufactured by SMD factory Unit now, I was tested by using Ramps1.2 + Arduino Atmage1280 + Sprinter Firmware. And it run very good.

Follows video shown that above hardware setup and using 80mm/s FeedRate running test cube printing test.

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<videoflash type="youtube">SjQVLWIA9-A&feature=channel_video_title|320|240</videoflash>

CIMG6304.jpg

CIMG6369.jpg CIMG6378.jpg Stepstick single macro.jpg

EAGLE files

https://github.com/mosfet/StepStick