work in progress - hex walker

work in progress - hex walker
http://xcprod.com/ROBOT
Regards Sergio Masci
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Sergio Masci wrote:

Very sweet, how much do you expect it to weigh? Do you have any walking algorithms?
I've always avoided walking robots because it seems like they have a higher power requirement than a rolling one. With wheels one only needs to move the robot, with legs, you have to lift it as well.
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A few kilos, not sure exactly yet. It will depend on the batteries. I can shed some weight by cutting away some of the base if I need to (drill large holes to make a honeycombe base). Or I could swap the servos for more powerful ones. I just wanted to stop planning and start building :-)

No not yet. I have a few in mind. I'll suspend it off the ground on a tether and experiment by sending it lists of position / time settings, then get it to play the lists.

higher
Yes but I was looking for a control application that emphasised real time control of many motors. I plan to make the step by step build details available once I get it walking. I'll also make the executable available so you can program a PIC yourself without needing an expensive controller.
Regards Sergio Masci
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Sergio Masci wrote:

If money is not much of an object, the NiMH batteries have a very high power density. It is about 6 lbs (5 lbs 13 ounces) for a 12V 7AH lead acid battery. For NiMH, you can get AA batteries that have 2300 mAh (2.3 AH) they weigh about one ounce each. You will need 3 banks of 10 batteries to be 12.5 volts at 6.9 AH and is about 2 lbs (1 lbs 14 ounces), less than a third of the weight.
$60 dollars for the NiMH (assuming $2.00 per battery) $14 dollars for the lead acid battery.
So, for about 4X the money, you can get 1/3 the weight.
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power
Thank you for the info. I have been thinking about NiMH. I have seen 1800mAH for half the price of 2200mAH (from the same manufacturer). I have also been thinking about Li-Ion batteries. There seem to be quite a few sources of "cheap" Li-Ion for mobile phones on eBay. The biggest hurdle here would be a charger.
Actually I wouldn't need 12.5v, 7.2v would be fine - even less weight :-)
Regards Sergio Masci
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Sergio Masci wrote:

batteries.
.................
1800mAH
also been

of
would be a

............
I have 3 walkers, and all use NiMH AA-cells, 1800-2100 mAh. These work well, are light in weight, and will run the smaller bots for quite a while. Gimlee, Nico, and Nico2 ...
http://www.oricomtech.com/projects.htm
Lead-acid batteries are probably much too heavy for a walker, especially given the energy capacity of this type of battery. I would probably go with NiMH "C-cells" rather than use a 7.2AH SLA.
I see a lot of walkers on the net made of machined aluminum, but this is also very heavy for a walker. Heavy base requires stronger servos, and these in turn require larger batteries. It's a vicious circle. Anymore, the first thing I try to do is design a light-weight frame, meaning some kind of plastic instead of aluminum. Gimlee uses an aluminum plate for body, but it's basically underpowered for its weight.
have fun, - dan michaels ===================
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I did a nifty Algo for the EH3-R from Lynx Motion. It does inverse kinematics for each leg, with a sort of "coathanger" gait. It does vectored motion, so it can "crab" in any direction. Added a wireless playstation controller. Lots of floating point math, but some good ideas there.
I put Li Ion batteries on it from Lightflightrc.com , along with a voltage comparator circuit to keep from frying them. LiPoly would be better, but the budget didn't allow at the time.
Videos at http://www.bio-bot.com/bio-botsite/lynxmotion /
I was going to list it on Ebay this weekend, to sponsor the next wave of robotic madness, but now I just don't know...
Mike

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blueeyedpop wrote:

wireless
ideas
voltage
but the

of
Hi Mike, I notice you don't believe in small AVI files ;-). I downloaded maytag, and it looks pretty much as I expected! Oh, man. [actually, my HD ran out of space this morning and I spent half the day doing file maintainance. Bummer].
At any rate, I was going to mention that Sergio's hexagonal leg attachment design was probably going to involve a lot of trig-math, which you no doubt have been enmeshed in with EH3. Do you think that design is superior to the usual hexapod leg-attachment scheme? It's probably on the same level of verstaility as a synchrodrive, turn on a dime, etc ....
http://www.visi.com/~dc/synchro /
but I imagine you need to solve a lot of maths before it can take even a couple of simple steps. No ????
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Yes, lots of math, but lots of control. Almost like having 6 robotic arms all doing inverse kinematics. In my math, I inclluded the ability to configure the legs for any rotation relitive to the body, so it would also work on an inline hex as well. It is basicly like a synchro in it's ability to simply re vector, though a bit more flexable in that it can jump vectors in 90 degree increments a lot faster, though there is little utility there.
As far as what a "usual " leg attachment is, I do not pay enough attention to R/C servo hexapods to know. His design is similar to Lynx's and if I recall, Colin Mackenzie's as well. It differs from Crust Crawler's in that the hip of the crust crawler uses a linkake and cantilever arrangement.
Mike
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that
on a

even
arms
also
ability
vectors
there.
attention
I
that
arrangement.
Regards "usual" leg attachment, it wasn't clear, but I was actually referring to the usual scheme of attaching the front and rear legs on the sides of the beast, inline with the middles, rather than at equal angles around the perimeter, as in EH3 and Sergio's. With the inline arrangement, it's easy to get straight-ahead walking. With the equal angle perimeter arrangement, you have to solve a lot of trig to get straightline walking, although turning is highly versatile. I also noted Colin's Twitchie/etc several years ago.
http://www.colinmackenzie.net/techlib/robots/images/rotatingwalker.gif
http://www.colinmackenzie.net /
It's a nice challenge to do all the maths, but I was wondering whether you think you really gain much by going to the trouble, as compared to using the simple inline leg arrangement, where the maths is relatively trivial?
Also, when you get to more complicated gait movements, it seems the computational complexity would keep increasing. Eg, if the bot is on a sideways slope, and attempting to maintain the body level, all of the foot trajectories will be different, etc. In the same situation with the inline arrangement, pretty much all the legs do exactly the same thing, one after the other, so once you figure out what to do with the front leg, all the others follow. Randy Dumse recently commented on the complexity of doing the inverse kinematics over on yahoo TRaCy. I think he was referring to EH3, also.
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Mike and I have gone back and forth on the walkers. He'll do a revision, then I will, and so on. Yes, I probably was talking about the EH-3, but specifically the EH-3I inline (as opposed to the EH-3R the round body). The extra trouble of doing all the math allows articulated movement of each leg. For instance see the movie of the inline walking at 45 degree angle:
http://www.lynxmotion.com/images/video/eh3/tripangl.mpg
It can also walk forward
http://www.lynxmotion.com/images/video/eh3/tripfw.mpg
and sideways crab style. http://www.lynxmotion.com/images/video/eh3/tricw.mpg
What I'm working on now is getting it to rotate while walking. Eventually, I should be able to get it to circle something while pointing at it, a sort of stalker manuever.
You can't get these kinds of life like motion without dealing with each leg as an independent entity.
--
Randy M. Dumse
www.newmicros.com
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The math isn't that bad, in that once you solve for one leg, you solve for all. Once I have an active sensor on there, I may go to something more reflexive. As it stands, any walker without feedback is walking only because the surface it is on accidentally happens to match its expectations. Reflexive walking would require an "ingrained" sense of matching desired direction and leg sensor feedback to an appropriate servo response. While inverse kinematics is clearly over processing, it does allow one to make precise motions and could be an interesting training tool for a more sophisticated learning algorithm, representing an ideal fitness function.
In my maths, I have ground height in the calcs, so it is one step away for setting individual ground heights for each leg. I wanted to build a software package that was extensable to inline or odd leg arrangements. I eventually plan to do a 10 legged scorpion, with each leg on a side being of different length and orientation to the body. Again, my maths are not far off. When I do the scorpion, i will likely implement one smaller processor per leg, like an TiniARM or Plug A Pod, and link them via CANbus. At this time, I will start considering the neural calcs and feedback sensors, where I will need a lot more munber crunching. I will also be doing it with DC servo motors, not RC servos.
My maths are ground track accurate, with very little scrub. To do the maths for an inline or a straight line walker is no different. I got impacted at work, so I handed further development off to Randy Dumse of NMI, since it is a great showcase for the capabilities of the ServoPod. He is building in rotation tangental to a point on a plane. This will require the legs to follow an arc or psuedo arc of different radii depending on leg position relitive to the body and the center of the arc.
One feature of the math is the gait waveform tappinge different legs into the same waveform, but at different phases allows expansion of the gait. By amplifying the height of the gait, I can make the legs more or less scary looking as they cycle through. It can mince or stomp.
The thing here is that I did a lot of this simply because I could. I wasn't putting an undue burden onthe ServoPod. I didn't have to resort to serial comms ro some servo accessory, and I could do 20K plus floating point operations per second without having to get clever with my code. Since then, Randy has optimized my code, because that is the sort of thing that bugs him, but I wasn't against hasing 20% of my processor cycles in the interest of getting things done.
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blueeyedpop wrote:

What language do you program that math in?
m
--
"In theory, there is no difference between theory and practice. In
practice, there is."
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also
ability
vectors
there.
challenges. (or a way to develop artificial neural networks)

I am working is IsoMax, a superset of FORTH.
It is pretty easy. It uses postfix notation, which takes a bit of getting used to, but it is worth it. The ServoPod has 26 register based PWM channels which makes things easy too.
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Sergio Masci wrote:

tether
get it

Gary Parker and his students at Connecticut College have been doing some interesting work using Genetic Algorithms to evolve efficient gaits for hexapods. They've also looked at adaptive learning for systems in which, for example, one leg is damaged or otherwise subparr. Perhaps you can find some useful information at the following URL:
http://cs.conncoll.edu/Parker/_papers.html
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Thank you I will take a look.
Regards Sergio Masci
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mlw wrote:

You only have to lift some of the legs. The rest of the mass does not have to rise.
m
--
"In theory, there is no difference between theory and practice. In
practice, there is."
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I. Myself wrote:

walking
higher
move
not
Actually, most hexapod walkers have to expend some energy just holding themselves up, and will slump down when you turn off the power. This is one of the prices you pay for having cantilevered legs. Eg, get down on the floor and extend your arms & legs out away from your body, and try holding yourself up with only your fingertips and toes touching the floor. One of the advantages of vertebrate skeleton design [except for some amphibians and reptiles] is that vertebrate legs are "rotated" under the body so the bones+skeleton directly hold more of the weight and the muscles can do less work.
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dan wrote:

Maybe cantilevered legs are not a good design for larger robots just as they are not for larger animals? I suspect a reptile rests on its tummy when not running about? Perhaps the design is only suitable for small robots.
The other problem I see with this design in larger robots is the surface area it takes up.
Also in a domesticated robot a rather large mechanical spider scampering down the passage would be rather scary :)
A domestic robot needs to be "cute" and innocuous looking.
It should also be silent or at least have a nice motor sound.
-John
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JGCASEY wrote:

This is exactly the point. When is the last time you saw a 400 kg spider? [heaven forbid]. An 80-tonne dino with cantilevered legs is probably physically impossible, especially when the femurs of those guys were somewhat wider and taller than a man.
BTW, you might be interested that crocs can gallop ... pretty phenomenal to watch ...
http://www.flmnh.ufl.edu/cnhc/cbd-gb6.htm http://reptilis.net/crocodylia/moving.html

You notice that most of the japanese android designs are rather non-threatening looking.
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