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routing ready
Jake Read authored
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README.md

MachineKit Stepper Driver

Uses TMC2130
Uses ATSAMD51

Has AS5047 backpack ?

IC Spec'ing

Step Driver

I like the Trinamic TMC2130 and TMC262 - the 2130 has h-bridges built in and is suited to NEMA17 size motors - fairly common. The TMC262 is a very similar chip, but only has pre-driving circuitry for external bridges. This means I can use very similar layouts for two 'sizes' of driver - however, they have different register tables, which is a bit of a bummer. I would be writing twice the software (ish). The other option is to roll a board with the TMC626, using very tiny external mosfets and then later very trivially using much larger mosfets, in the rare case that I want a, say, NEMA 34 or 42. Also, I can roll one off the bat that will be solid for NEMA 23s - normally about 2.5 amps of current. This should be no problem.

OK: so - TMC262 w/ external fets, here we go.

tmc262

Mosfets

I also need: Mosfets then, as I'm going for external option. 8 per board: 2 full bridges. I'll look for arrays - like two or four in a package, this should help keep things small. I like two, for each h-bridge... There is a nice 4.1A continuious / 14A peak DirectFet in a small package - this might be appropriate for a big driver. As it turns out, there's this appropriate-looking full H-bridge in a small package that will drive 8 Amps at 60v. That's enough for even fairly large steppers... It's got a reasonable RDSon and pretty tiny gate charge... so I feel OK about this as a first shot. Bueno. I am ready to get a schematic going and build library parts.

Sense Resistor

Sense resistors generally need to scale with the expected motor current - so I want to find a package size that I can use for large and small motors...

Microcontroller

This stepdriver is part of another project on networked machine control (I am calling this MachineKit, although I may have to change this as I am now realizing that machinekit.com is taken). My favourite new microcontroller for those endpoints is the ATSAMD51, so it's the stand-in. I would also consider the ATSAMD21 (these are both very well supported by 'the community') and the 51 is newer, faster, etc, at a price increase from $2 -> $4. But, in the interests of only dealing with one set of registers, bootloaders, packages, etc - I am just going to not think too hard and pick the ATSAMD51 for this bb, as well. Future firmware-writing self will thank me.

To Encoder or not to Encoder?

Mechaduino is a really excellent project that Sam and I are fond of. They use an AS5047 encoder to resolve more-steps-than-should-be-possible-with-a-stepper. In combining this with the fancy drive technology in the TMC chip, we can basically arrive at a low cost closed-loop motion control solution, so that's pretty rad. I'm familiar with this chip and use it in my brushless motor controller, so it's feeling quite likely I will implement it here as well. My one quam is the added complexity, etc. I could mount it right in dead-center of the board, use side-mount leds for indication and then have a nice clean backplate... (I am trying to avoid a two sided board). I had also thought I would perhaps have the possibility for an external encoder-only board... but, again, less complexity the better. Backplate mounter. OK.

Form Factor

A-la Mechaduino and many other solutions, this beckons to be mounted on the arse-side of the motor it is responsible for... that way, it looks neat, and we can just glue a magnet on the back of the shaft to read position with our encoder. In the future, I really want to also control NEMA8 and NEMA11 motors (because they're cute as heck) but for the immediate future NEMA17s are OK and I can always use a laser-cut adapter to mount the same board onto a NEMA23. OK, again, simplicity wins, we go for the NEMA17 board, but spec drivers to NEMA23 size.

Board Design

OK, I only have two major footprints left to draw / place - the AS5047D and the sense resistors.

Done, now schematick-ing ...

  • power selectors & vccio

  • swd

  • reset line

  • bfc ? 100uF smd ?

  • gates & sense r -> smaller sense r?

Then I have the big, dubious challenge of picking a network interface port for this whole next rev of work. Oh my. Then I will be doing schematic layup and will be on my routing way. Hurray.

Ordering

FETS https://www.digikey.com/product-detail/en/on-semiconductor/FDMQ8203/FDMQ8203CT-ND/3908189 Smaller Reset https://www.digikey.com/product-detail/en/c-k/KMT211NG-HF-LFS/CKN9433CT-ND/1886651 2010 2W Shunt (do 50mOhm) https://www.digikey.com/product-detail/en/vishay-dale/RCWE1020R100FKEA/541-2516-1-ND/5357840 2512 3W Shunt (do 50mOhm) https://www.digikey.com/product-detail/en/bourns-inc/CRA2512-FZ-R100ELF/CRA2512-FZ-R100ELFTR-ND/1775034 100uF Cap https://www.digikey.com/product-detail/en/panasonic-electronic-components/EEE-FTH101XAP/PCE5024CT-ND/2652079