Critique my power system--Please!
January 5, 2012
Edit. I got into a discussion with Chris the Carpenter about pricing and alternate parts sources, and found an interesting item on Ebay that should work out. See the bottom of my post for more.
I'm in the planning and parts procurement stage of a relatively (for me at least) complicated robot project.
I've got a 4 motor Rover 5, and a Dagu 4 channel motor controller. I've been testing and developing with a Mega 2560. I'm thinking of putting a red back spider controller into the final build, but first I want to be sure it doesn't outgrow the mega family.
So far, I've been powering the Mega, Rover 5 and a handful, (8 so far. Probably will end up at 12.) RC type servos. with a 7.5 volt, 3 amp, and 5 volt 3 amp set of wall warts.
Today, I'll be placing an order for THIS SBC. (Chosen because it gets its power from a barrel jack.)
Ultimately, I hope to turn this into a ROS platform. But my next step after transferring my developement platform over to the Zotac SBC, is to make it all mobile by bolting it to the Rover 5 and POWERING the whole thing.
Here's what I had in mind.
I don't want to be bothered with multiple battery packs. I've thought about using some lithium technology or other--and I've done some research, but I figure, if Apple and Dell can build self-immolating computers, so can I. Therefore, I'm thinking of a single 14.4v NIMH pack.
The best deal on a battery I've found so far is this.
It's a Roomba battery repair kit. which means, until it actually goes into a Roomba battery shell, it's nothing more than a 12 NIMH C cell battery pack--and considerably cheaper than just about any other 12 cell NIMH pack I've seen. Even the AA packs, which I'd use in order to save some weight.
Now, I figure I have to provide 12 volts to the SBC, 7 to the motors, and Arduino, and 5 to the servos. If I use a Spider Controller, I'll let that provide power to the servos. If I stick with a Mega, or some other micro contrtoller, I'll work that out later.
Here's how I plan to get my regulation.
This device for 12 volts for the SBC.
It can provide 12 volts to the SBC, whether its input voltage goes considerably higher or lower. The battery voltage will be monitored, (by a method I haven't given much thought to yet I'll admit.) So I won't let it get too low--but at least I have that option.
For the Arduino and servo supply I'm going to use a similar device--but it's a little simpler, and cheaper, since I'll never let the batteries get anywhere near too low to be stepped down to 7 volts.
The downside of this whole thing as I see it, are weight of the battery pack, and the fact that those DC-DC coverters are going to need some active cooling.
SO, does anyone have any comments as to how well this battery pack and these two DC-DC converters would work out for a Rover 5/SBC/Arduino based computer? The batteries and converters are my next parts order--as soon as I'm reasonably certain they will work out.
Chris mentioned that he gets voltage converters that are an order of magnitude less expensive than the ones I found on Robot Shop. The one he reccomended was nice--but not quite what I'm looking for. But a little more searching turned up this:
This device is intriguing because it has a 5 volt output, and a 3 volt output and two other channels, one of which is bucked (lowered) and the other is boosted.
There are some very specific input requirements for the buck and boost channels. The boosted supply can not be lower than the input voltage +3v, and the boosted bucked can not be higher than the bucked output - 3 volts.
If I boost an 8.4v nimh pack to 12vdc and buck that output to 7vdc--I think I have one board that will handle all my power needs, and a smaller, cheaper, lighter battery pack.
All I have to do now is figure out how to get a heat sink onto it.