Monday, May 14, 2007

RepRap Hydra: SBC?

So, I decided to look at my previous post about H-bridges to start on V1 of my power board. Standard Reprap runs at 12V, with 1A/stepper roughly.

As this is my own, I decided to try the Freescale MC34921 first. I'm not afraid of surface mount, but this won't work for everyone. I might rebuild with discretes later. However, this does pose a VERY interesting result. I had picked the Freescale as it's fairly flexible, as I can drive DC motors and bipolar steppers with it with no hardware changes. It has high current limits (low heat for a lower amp setup) and current limiting (safety). I was planning one of these per axis so I could do microstepping or servo control. The third driver (2A DC motor or stepper) was just a bonus that would be used for some option switching... or so I thought.

I decided to pair this with a 3.3V dsPIC33FJ128MC706 (might change still, though). Same TQFP-64 10mmx10mm package. Perish the thought that I might mix up which chip goes with which pad! Four PWM generators. I was hoping to use something similar for a "universal driver" quad half-H board. This is close, but I can't uncouple the H bridges. I can, however, set up a PWM to all four (is it really four?) PWM inputs this way. I can even use another timer to provide another PWM output.

Added bonus, I have a 0.6A switching 5V PSU, a 2.5A switching 3.3V PSU, and a 1.5/1.8/2.5V 750mA LDO, all feeding off the same max power system (up to 38V, works fine at 12V).


40MIPS, two 5A dc motor drivers, one 2A DC motor driver or unipolar stepper driver, one dedicated encoder interface, several spare serial interfaces. Add in the Austria Microsystems magnetic encoders (that I can't find who's using due to being down) on the second SPI channel, and I think I MIGHT be able to make a single board servo based Darwin driver if it all doesn't spectacularly melt down on me.

I think I have some thermal calculations ahead of me.


Forrest Higgs said...

"Add in the Austria Microsystems magnetic encoders (that I can't find who's using due to being down)"

I'm using.

How can I help you?

SOI Sentinel said...

Patience is what I require, lots of it :) I'm hoping to have a little time to determine the heat dissipation of the parts.

I already have some nearly free samples of various Austria Microsystems 10 and 12 bit encoders on the way. As I'm using a 16 bit microcontroller, the 10 bit units are perfect (16 bit serial data). I know I'll lose resolution at higher RPM speeds, but oh well. I'll be building small boards for these eventually.

My background is an industrial system integrator, so I'm attacking RepRap from the top down. I've been reading the blogs and the forums as I can. I know I should dive in and join the forums, but I want to get a little farther on this board design first. Suggestions, promptings, or otherwise are welcome.