work in progress - hex walker

dan wrote:


Dan, They have some pretty big ones in Australia! Some scorpions, the largest arachnids on the planet, can grow to eight inches long.
Even the small ones tend to be considered the "creepiest things on the planet." I have to agree with John that legged robots are not conducive to acceptance by the average person. When they're holding still they look harmless enough, but when they start to move I can see the goosebumps on people's arms.
We're getting back into black window season around here. If you'd like, I can scoop you up a couple and mail them to you. You can study them all you want! <g>
-- Gordon
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Gordon McComb wrote:

the
conducive
like,
all
No need, we've got plenty of spiders around here too. Anyday now, they'll be chasing the squirrels in the back yard. I am lucky that there are a lot of jumping spiders in the backyard. They're small, but fun to watch.
I think the largest spiders drag birds down into their dens to feed the kiddies .... 30 cm across, but couple pounds shy of 400#.
http://www.google.com/custom?q=bird+eating+spiders&sitesearch Truly, the scariest thing about a large spider is those guys have cute pedipalps, but EEEEnormous fangs .... hate to be a bug
http://images.google.com/images?q=spider+fangs
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dan wrote:

Dan, As I know you've found in your research on legged critters, some insects let their bodies slide on the ground when they walk, and others lower themselves during rest. (Also applies to some quadrupeds, like lizards.) I don't think it would kill a walking robot to have a low-friction glide point on its under carriage, if not during the walking phase at least during the stop phase. A rounded piece of UHMW is rugged, cheap, not that heavy, and has self-lubricating properties.
-- Gordon
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Always wanted to do worm drives on a walker for good static power consumption. Of course the trade off is lousy efficiency durring running.

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blueeyedpop wrote:

This would basically work for any geartrain that is not reversible. Most servos don't have that steep of a gear ratio, so they'll backdrive under weight. A rack-and-pinion is not (readily) reversible, is pretty efficient as there is no sliding action (as is the case with a worm gear), and is a natural for converting rotational motion to linear movement. Best of all it's pretty cheap.
A hint of what I've been working on: I have some pull-cord cars I bought at the toy store that have a relatively nice flexible plastic rack gear, about 8" long. The pinion gear it mates with can be fastened to a servo horn, as it has a large hub (with screw holes even!). I cement the rack along the edge of some leg pieces, cut from 1/4" PVC. The legs slide up and down fairly well, and the motors hold a static weight. I've made only one prototype, but I think a full walker could work.
The original intent of the design was for a parallel gripper strong enough to hold a 12 oz. brewski with a standard servo, and not spill it when driving across the living room. (The store-bought cans, not the stuff Mikey (Kap'n Salty) brews in his basement.)
-- Gordon
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Gordon McComb wrote:

holding
This is

others
is
Hi Gordon, you're obviously correct that real animals with cantilevered legs or side-attachment legs [like lizards, etc] can slide on their bellies, but I've never actually seen any hexapod robots using this idea. They tend to hold their bodies up off the ground, and require energy expenditure to do this. OTOH, 2 or 3 years back I ran across a link to a turtle-like robot that slid on its stomach and waved its feet for locomotion, not unlike swimming. Lost the link, however. So, at least one person caught onto that idea.
Another thing along these lines. The newer version of Urbie has wheels on leg-like extensions it can put down. Doesn't walk, but it's a wheeled/tracked hybrid ...
http://robotics.jpl.nasa.gov/tasks/tmr/homepage.html http://robotics.jpl.nasa.gov/tasks/tmr/hybrid.html
I've also thought some about the idea of a hybrid vehicle, but with small motorized wheels attached to the end of regular robot legs. On smooth surfaces you roll, and on rough surfaces you walk over them.
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dan wrote:

Kind of like Shrimp III with the addition of tracks. Build for speed, but can climb over things. I can see that on smooth surfaces of other planets the wheeled motion would mean less battery drain.
Reminds me of the Packbot, but with wheels added to the ends of the outrigger tracks.
-- Gordon
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cantilevered
feet
Aha. By virtue of someone else's post, I found the turtle that slides on its belly .... Quad 4L1.
http://www2.plala.or.jp/k_y_yoshino/eng/index.html
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Hi Dan,
Yes as you say "most" hexapod walkers need to expend energy just holding themselves up. However if you look carefully at
http://xcprod.com/ROBOT/imgp1662-small.jpg you will see that it is standing on its legs (not slumped) without any power applied. This is possible here because the weight of the robot is directed straight through the servo while it is in this stance. If you consider the horizontal bar extending from the body to be a strange looking hip joint (this is the way I look at it) then the knee and leg is in fact under the robot and behaving like that of a vertebrate skeleton as you suggest above.
Regards Sergio Masci
http://www.xcprod.com/titan/XCSB - optimising PIC compiler FREE for personal non-commercial use
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Sergio Masci wrote:

holding
standing
here
servo while

from the

then
a
Yes, with your design this is correct. When it stands up by extending the lower leg segments straight downwards, the weight will still be held by the structure and not the motor energy. This is not true for EH3 or Symapod, however. In those cases, the legs are cantilevered outwards and the body weight is held by motor activation.
http://www.colinmackenzie.net/techlib/robots/symapod.html
If you compare the 2 different situations, you'll see that the EH3 and Symapod designs actually have capability for better ground clearance. This seems to be one of the main disadvantages of the vertebrate design. With the legs rotated under the body, they cannot be lifted as much, so travel over rough terrain is more of a problem. You'll see this if you compare the potential ground clearance for your leg designs versus the other 2 cases.
So there's a tradeoff between the 2 designs. The one uses energy more efficiently, and is also capable of holding enormous body loads, like the case of the dinosaurs, while the other is more versatile with better ground clearance over rough surfaces, but uses more energy to hold the body up, and because of this apparently cannot handle really large body weights.
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Did you look at some of R. Brooks work at MIT AI Lab in the 90s. They did some stuff on behaviu=oural robotics. I think the legged robot was called Hannibal and it used a 'subsumption' architecture.
Your robot looks cool.
Sergio Masci wrote:

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No I haven't but thanks for the info.

Thanks.
Regards Sergio Masci
http://www.xcprod.com/titan/XCSB - optimising PIC compiler FREE for personal non-commercial use
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Sergio Masci wrote:

Sergio, What are those white spacers used on the clear plastic servo brackets? Do they have some kind of flex in them?
-- Gordon
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Hi Gordon,
The spacers are white nylon threaded rod with nylon washers interspersed with nylon nuts. This gives a very rigid assembly. Originally I used just washers but this allowed the assembly to twist slightly under load. I figured this would not be good for the servos. I was going to put bracing between the spacers to increase the rigidity but accidentally found that this configuration of nuts and washers did the job.
Originally I was planning to use a lot of extruded aluminium and steel threaded rods. I decided to try polycarbonate and nylon after reading the "Robot Builder's Bonanza*". The polycarb has allowed me to greatly simplify the design.
*For anyone that doesn't know, the author of the book is "Gordon McComb".
Regards Sergio Masci
http://www.xcprod.com/titan/XCSB - optimising PIC compiler FREE for personal non-commercial use
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