Saturday, May 12, 2007

RepRap: Homemade Engineering

Sometimes bent steel and alumninum just don't cut it... especially when your mobile robot needs just too much torque to move well (per calculations). Oh well.

So, we depart on some DIY rapid prototyping to help shed weight. The RepRap project is trying to do just that. They have a ways to go, but they have working electronics and mechanics for soft plastic. I definitely can see them moving up to something more engineering grade, such as ABS or Polycarbonate, from CAPA (melts at 80C) and HDPE.

I've decided to embark on my own variant. The current design is very VERY good at what it does. I'm starting with a Darwin and working up.

Planned upgrades follow. Some will be in my Version 1.0 machine. I plan to call it the Hydra.

Electronics: My own design. Theirs is absolutely BRILLIANT to reduce complexity. I'm planning on using the USB-CAN controller I've been working on for communication. This may evolve into a USB-ETHERNET-CAN controller. Drives will be dsPICs. Extruder heads will be whatever I want, depending on complexity. CAN will be my backbone.

Power: I do like their use of an old AT power supply. If my motors don't take 12V, I'll be moving up to

Motors: I'm considering building my DSPIC drive boards as quad half H bridge units with encoder feedback. This would let me (possibly) parallel my connections to drive more load when I build a DC servo motor, move up to BLDC if I want to, and still stick with steppers if that's all I have available. If I do stick with steppers, I will be microstepping for accuracy.

Extruder Head: I'm expecting to have multiple extruder heads. They'll be hanging on a rack to the side. I'm looking into either an allthread or maybe pneumatic lift that's built into the main head. It'll have a dovetail design and be able to "hang up" the heads on the rack to swap. The CAN network will be terminated in the extruder head. I'm thinking that some decently heavy duty spring contacts for the main power (extruder motors, heaters, etc) and a set of four contacts for CAN. CAN supports hot-plugging electrically, so this should work.

Power Transfer: Belts work great. Although they may see some stretch issues. I know they moved to them for speed, but they're also running steppers. I might migrate the design back to precision allthread. For instance, to get 1 inch per second drive rates out of 20TPI allthread, you'd need to spin it at 1500RPM. This DOES concern me. Don't need to be smoking steel. I've looked at precision Acme screws, too expensive really, doubly so for any ballscrew arrangement. Standard acme screws aren't much more expensive than allthread, but they sound like they'll pull even more power than allthread due to the innaccuracies. Needs more research. I will start with a belt system, however.

Main frame: I'll start with the Darwin frame and modify it. I'd build a box around the unit for storage (and cat-proofing), and hopefully some protected runtime. I'd switch out the bottom board for some roughened aluminum plate. I'd mount a thermoelectric cooler under here with a fan. This would allow me to control build area temperature somewhat, which should aid in adhesion or cooling times, allowing faster building. One side will have to morph into my head "rack". I might also need some heater wire underneath, too, for when I need a warm plate.


Mike said...

When you get it built, I want to order an electronics kit from you. Maybe a "service plan" too so I can get the updates to any of the PICs.

I should be all settled in a couple of months. I will be reserving a space in my living room for the Hydra! I was thinking of putting it where everyone else would put their TV.

Vik Olliver said...

Just in case you missed the post in the RepRap blog, we're now extruding polylactic acid. This melts at about 140C, is organically derived and biodegradable.