Thursday, May 17, 2007

Hydra: Mechanical head ideas

Well, sitting here eating a late lunch, and I decide to ponder the major point of the Hydra variant. I want multiple heads and I don't want to pay for them.

The way I decided to do this is to have a rack along one or two sides depending on if you want single or dual head capability. Each head has a dovetail-like slot on it, with a stop at the top. One on each side of the head. One is the hanger for storage. The other is active slide with power and data spring contacts. The carriage trolley needs either a solid solenoid or a motor drive on it. To pick up a head, the appropriate (open) trolley slot drops. It rolls over to the head and aligns the slot and holder. Raising the slot engages the head. When the head hits the positive slot at the top, we are aligned and should have a repeatable position.

There are other ways to do it. This method needs constant power. Perhaps an "expanding" dovetail would hold the unit better and not need power or a brake to lock the head in place through power dips, but also would need another actuator.

The trolley head holders are not meant to run constantly, just when the heads are being replaced. If a fourth axis is needed, such as for milling or inspection, it should be integrated into the head. Same for the polymer pumps and other material supply methods. Looks like I'll have to get acquainted with CAD again.

Wednesday, May 16, 2007

RepRap Hydra: A real drive board

Just a short note, looks from the datasheet that the MC34921 drive chip is transistor based. They only list voltage drops for the bridges. The package is rated at 2W dissipation, but the math makes me think it'll be pushing 9W if everything is on full blast. I'll still design the board around that, but it means I can't use it as a RepRap basic SBC.

I'm scaling back to a more basic DC drive (maybe make it a dual chip unit as a bipolar drive) and perhaps combining a MC33486 with dual IRFZ44ZS low side MOSFETs. This should let me drive 10A constant, and 35A intermittent. It's been on my list for a while for other motors. It's FAR bigger than RepRap needs, but this has several advantages. One, I have other projects that I meant a board like this for anyway. Two, this loosens the size requirements for RepRap servo drives, so they can get bigger (anyone want a 1m3 build area?). Three, the heat dissipation with smaller motors may alleviate any need for a heat sink.

Note to self: add links later, too tired right now.

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.

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.