Mindboards

You may find Pulito, the sweeping robot interesting. All it has to do is bump up to the charging station to charge. If I was to replicate this, I'd stick the charger bars top to bottom, instead of side to side, so that it would be able to charge from a completely round charging station, making docking easier. Of course, it'd probably be safest to make this with the Lego Rechargeable battery, less things to go wrong and fry your NXT. That adds another $50+ US unless you already have the Rechargeable battery .

nxtreme wrote:You may find Pulito, the sweeping robot interesting. All it has to do is bump up to the charging station to charge. If I was to replicate this, I'd stick the charger bars top to bottom, instead of side to side, so that it would be able to charge from a completely round charging station, making docking easier. Of course, it'd probably be safest to make this with the Lego Rechargeable battery, less things to go wrong and fry your NXT. That adds another $50+ US unless you already have the Rechargeable battery .

I just thought of something that might work well. How about an inductive charging system, like the kind they use for mobile phones (ex. PowerMat). This way the robot can sit on the charging mat, and when it moves off and returns, it doesn't have to be very accurate as long as it makes it onto the mat.

There are a few hurdles for this one. First, most of the retail mats require "receivers" that replace the battery pack. But that's mainly because they are marketed for lithium mobile phones. But I found one product that's basically a cup that you can drop in loose rechargeable batteries, or put in a whole device (like an battery powered toothbrush).
The other issue is the whether NXT unit itself could get fried by sitting on this thing.

While it is a very cool concept, I just don't see it being very practical. If you were using smaller voltages (i.e. single LiPo cell at 3.7V, or something like this) it might work, albeit with some significant efficiency issues. I haven't seen many inductive charging projects that are made for, or use, 9V though I haven't really looked. I think that the simpler the better, less problems overall .

Inductive charging works by inducing very high magnetic fields which change rapidly. This induces currents in a coil in the receiver and it is really just a standard transformer built in two separate units. Because it lacks the common core of conventional transformers, I expect that the fields will have to be stronger to compensate.

Unless the NXT was shielded in some way there is the possibility of excessive currents being induced in the wiring (the speaker is an obvious example).

Not to say that it would cause permanent damage, but it could lead to some very strange behaviour!