or even use double servo knee joints.
I suggest buying a servo, make a leg section with a servo joint, and doing some servo stress testing.
I had about 18 of it’s brother, HSR5990TG. These servos are WONDERFUL (watch your fingers though).
I’ve used up 12 of it to restore my LM Biped Scout (since I figured out that the 645MG will not work with the reverse knee configuration), and sold that and six more to Mike (4mem8). I’ve been asking him how it’s working out.
As for batteries, you don’t need a large capacity, you just need something that can source large current…
LiPo are great for this. The stuff that are used in the RC car world are INSANE. LiPos are rated in C rating for discharge characteristics, where C is the capacity. So for a 25C LiPo with 5000mAH of capacity:
25*5000mA = 125A !!!
But note that the discharge rating is usually not a continuous, but a burst rating of typically 30 seconds (sometimes, LiPos will have two different C ratings, burst and continuous).
Even if the continuous discharge rate is 15C for a 5000mAH capacity battery, that’s still 75 amps of continuous sourcing…
If you’re going the LiPo way, I highly recommend doing what Zenta did, he put a BEC Switcher on it to step down the voltage…
You almost have to go that way since I don’t know of any commercially available LiPos that is 6.0V.
Even though the 5980/5990 is rated up to 7.2V, I’d still run it at 6.0V to keep it safe and cool (although the 5990 is encased with aluminum extrusion for heatsinking).
I’ve been thinking about permanently installing a 7.2/7.4 LiPo pack with the Turnigy BEC step-down, like what Zenta did in his, in my Phoenix.
If you get a 5000mAH 7.4V 2S LiPo pack, you can probably have the Phoenix walking around for an hour continuously before it dumps…
Just a warning though, LiPos DO NOT LIKE UNDERVOLTING. Many of the ESCs (electronic speed control) and BEC DC-to-DC switchers (like the Turnigy one that Zenta uses) has a LiPo cut-off function built it, if you’re operating it raw, I suggest browsing around at Hobby Stores for a LiPo cut-off/warning systems that you can buy that are piggy-backed onto older ESCs that doesn’t have a LiPo cut-off feature in the RC world…
LiPos are a bit dangerous, they are not meant for children, and when they’re damaged, such as being banged too hard in RC cars, it can errupt in flames (I’ve seen a couple of RC cars go up in flames at my local track).
If you want to play it safe, there’s also LiFe (Lithium Iron) cells that are now hitting the market. I think they were originally developed for cordless hand-drills. However, the power density vs. weight isn’t as good as compared to LiPos. But the discharge capability is still awesome, and there are demonstration videos of a drill bit being driven through the LiFe packs and just causing smoke, as opposed to driving a drill bit through Li-Ion (lithium ion, not the same as LiPo, but close) and the Li-Ion pack errupting in flames!!!
Here are some cool videos of LiPos go up in flames (cool to watch, not cool in your robot/RC cars)
youtube.com/watch?v=YCWdnjLqVWw
youtube.com/watch?v=4OsBc8RqSKU
Edit:
Forgot to mention:
Only use chargers that are designed for LiPos to charge LiPos. Often times, LiPo chargers and batteries will have a “balancing” lead in it which is used to balance the voltage (which is an indicator of charge in batteries) between the cells inside…
If a LiPo charger doesn’t come with a balancing port (like my Pulsar 3 Competition Charger), you can buy piggy-back balancers.
If your LiPo does start swelling (from damage, under-volting), the best way to neutralize it is with a bucket full of salt water and dunk your swollen packs in it and leave it in there for a few days…
Hi Nav,
I just saw that you are aiming on the same movement that Sienna is using on her tripod. If you watch the video closely you’ll seen that the legs isn’t simply lifted but it also needs to push the complete bot in to the air so the body will be close to the COG. This will even cost more torque then simply lifting a single leg. If you build a quad you don’t need to push the weight up but you can simply lift one leg. This can be done with less torque. Another thing to consider is that you don’t need dynamics (IK + torque, gravity and stuff) to do the calculations. A much simpler IK solution will do fine with a quad.
Maybe you can go with a tripod and simply add a different body and an extra leg if it won’t work. Or build one leg to run some tests to see if it can jump with half the weight. Just an idea though… I’m not familiar with doing the mechanical math like torque and stuff.
Xan
Cool vids Tom!
Or not, considering I’ve got a few of them at home… :s
Did you ever find a vid about a LiPo exploding because it’s voltage dropped below it’s save voltage?
Xan
As long as you don’t try to recharge the LiPo after heavy voltage drop it should be ok. Hopefully you’ll get a failure from the charger since the voltage isn’t within the safe limits.
But, if you shortcircuit a LiPo… 
Then it will go into flames! My father did that too 
.
Yeah, the higher the energy density, the more potential it carries… Whether it’s sourcing or during a disaster!
How are those BECs working out Zenta? Are those Turnigy BEC Switchers worth buying? I’m thinking of commandeering my 7.2V 6-Cell Sub-C packs from my RC so I’m looking for a good step-down… The 6-Cells are NiMH matched packs for my RC, so they pack a lot of punch (not nearly as much as LiPos though)…
One precaution that I’ve read about cheapo LiPo packs, there was a warning to check to make sure the soldering joints are sound between the cells of cheap LiPo (like the Zippy packs)… One guy at my track had one cell out of each of his two packs go bad and he just mixed and matched the leftover good cell from each of his two pack and made one good one!
Sorry this thred has been a little dead for a while, Ive just been waiting for some cheapo test parts to get a full scale test version going. The last of the servos came today and im hoping that by the weekend i mught have a leg or 2 going. Im just using some pretty basic china made digital ervos for now, these ones
cgi.ebay.com/4x-MG996R-Metal-Gea … 23028a42f7
I know they are not ones sold on here but i jsut needed to play with something before I go spending half a years salery in it:-0
So some videos should come in the next weeks of some of tghe leg design (made of balsa:)
Oh yeah! Let’s see it walk!
Alan KM6VV
4 servos at $12.99 each - it’s hard not to want to buy some of these, regardless the negative performance reviews. I have seen example after example of robot projects that use this servo and it seems to work fine with some exception to moderate performance. The main issue I usually see is the jitter at holding positions. I would have done the same myself getting these servos for the prototype stage before dumping a half years salary on premium servos.
If one is doing something new, it is probably smart to do some proof of concept testing before plunking down the serious $$$. Soon I may purchase some of the el cheapo $3.50-5.00 servos just for the boards in them for a test setup.
$3 to $5 servos? where at?
I “prototype” with some used Futaba 3004 R/C servos. They work (most of them), but of course don’t have the torque necessary. Also run them from a lab adjustable power supply. That way, I can get them to move (up on “blocks”), and the lower toque is less likely to cause damage to servos (and fingers!).
Alan KM6VV
Haven’t ordered from the below site, but the price is right for experimenting with the boards inside the servos.