Need help powering my robot with 20 servos.

[MadGadget]

Hi!  I've been an animator and a programmer, now I'm trying to make robots.  I'm making a 4 legged robot, trying to deal with problems as I get to them.  I've got the robot built http://i.imgur.com/jrrcw.jpg and a lot of the software done.  My problem now has to do with power.  I'm using HS-485HB servos and have a SD21 servo controller http://www.robotshop.com/devantech-sd21-servo-controller.html.  I've gotten one leg to work by plugging in just the 4 servos and powered it with 1 6 volt 2000mAh battery.  I'm a little stuck now on how to provide enough power for all 20 servos. 

The robot currently has to be tethered to my computer anyway, so I'm hoping to power it with something I can plug into the wall (the robot isn't big and strong enough to carry enough batteries anyway).  I've though of using a computer power supply, but I'm not sure how to proceed with that.  The power supply I have lying around has 5v and 3.3v outputs.  I'm assuming I can combine two of those to get me the proper voltage, but my lack of electronics knowledge (leading to fear of blowing something up or killing myself) has left me stalled.  If that seems like a good way to go, could someone help me out with how I'd do that?  Either some advice or maybe a good link?  Or is there a better way to get a nice steady 6v supply with enough amperage to power 20 servos? 

Thanks very much!

[Admin]

Not knowing what servos you are using, you can expect each servo to draw about 0.5A on average. So 20 servos is 10A. If you use a battery (or multiple in parallel), make sure they can supply that much current. The cheaper servo types, such as what you're using, won't last long if you go above 6V. If you use tethered power, it'll say on the transformer what the output voltage and current is.

Another option would be to not be tethered, ie just running off of batteries. These two below robots run off my Axon microcontroller (keep in mind I'm financially motivated to sell you one ).

Pumpkin Dance: Introducing the Creeper 2

Self-Transforming Mobile Robot

[klims]

I'm using LiPo's and a UBEC. It's a great combination. Lithium Polymer batteries are much lighter than the NiMh's you are probably running at the minute.

[MadGadget]

Thanks so far guys.  Admin I already have an Axon   I didn't mention that to simplify the problem.  I'm actually going to be using the Axon to talk to my computer, plug sensors into, and send signals to the servo controller. 

I'm really looking for a tethered solution for now.  My first goal is to synchronize all the servos to walk, and most of the processing is happening on my computer so it has to be tethered anyway.  The robot won't have to move far, and I don't want to worry about draining batteries while I'm testing things.  I've been doing google searches for dc transformers but those that output 6v have lower amperage than I need, and the ones with enough amperage are 13+ volts.  I actually already have access to a 13v 10 amp transformer.  Is it possible to safely drop the voltage down to 6 volts? 

I do have 3 NiMh batteries which are 6 volts and 2000mAh.  Maybe I'll try linking them together and see how that goes.  I'm just worried they'll run out too quickly, but I guess it's worth a shot. 

[klims]

If it helps, my system has 16+ servos and I'm running 4 X 900mAh Lithium Polymer packs through the UBEC. From a full charge I get a good hour of running, and closer to 4 or 5 hours while testing. The whole power system cost me about 30 bucks delivered.

If you could get your hands on a decent desk top power supply I think you should be able to get enough amperage for a tethered system. If I remember correctly the systems we had at uni could do about 5 or 6 amp at 5v.

Are you doing lots of quick movements? are there many servos under heavy load?
My biped system pulls between 2-5 amp during normal operation (walking around slowly) but can get as high as 10-12 amps when under strain

[MadGadget]

I might have been making a big deal over nothing.  I connected the 3 batteries I have and tried running all 20 servos and they all seem to be pretty stiff.  I think I must have done a bad test early on that convinced me I wasn't going to have enough power, but it looks like things are working out! 

Klims, what's the purpose of that UBEC thing?  What benefit does it give over just plugging the batteries together and connecting them to the servo controller directly? 

[garrettg84]

http://www.powersupplydepot.com/

I've ordered a few power supplies from them and had good luck.

This one in particular seems to suit your needs as described... 5v @30Amps for $40
http://www.powersupplydepot.com/Power_Supplies/Single_Output_Supplies/5V,_30A_150WATT_POWER_SUPPLY_16017_PS.asp

[Conscripted]

If it helps, my system has 16+ servos and I'm running 4 X 900mAh Lithium Polymer packs through the UBEC. From a full charge I get a good hour of running, and closer to 4 or 5 hours while testing. The whole power system cost me about 30 bucks delivered.

$30 deliviered? Got a link?

Conscripted

[garrettg84]

$30 deliviered? Got a link?

Conscripted

Likely http://www.hobbyking.com/

Hobbyking has all kinds of lipos for CHEAP

Battery 1300mah @ ~$5
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=6519
BEC 6v/5amp 5v/5amp @ ~ $5
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=10312

pro tip - this is going to sound dumb, but let your browser sit for about 2 minutes on the page, hobbyking.com will try to entice you to buy by offering a lower price.

[klims]

UBEC is just a regulator. It will transform your battery voltage to something your servos can handle. Most have two settings, 5v and 6v I think.

I bought 4x Esky Lama 900mAh lipos off ebay for 20. My tip is to find lipos that are used for hobby helicopters, like the esky lama, because you can get some pretty good deals.

Can't remember where I got the ubec from but I remember posting between 10 and 15 bucks a couple years back. Probably an ebay job too.

[MadGadget]

Thanks so much every body.  If I use one of those power supplies that garrett84 linked to that output 5V, will I lose a lot of torque on my servos by supplying less than the 6 or 7 volts recommended?  I'm already a little worried that the cheapish servos I'm using won't be strong enough.