Brand new noob, robotic tentacles

[jonwhunt]

Hey guys, so glad I found this board.  I'm trying to make a very simple robotic tentacle.  I make high grade mask and prosthetics like seen in Hollywood and I'm trying to make a Davy Jones mask from Pirates of the Carribean.  I need help figuring out how to make the tentacles move around randomly.  I know VERY little about servos and actuators.  I don't think I need anything like 42 servos per tentacle or anything like that.  I just need either servos or actuators that won't burn out from me turning my head and the motion pulling them against the weight of what they are already lifting or moving.  I plan to basically create a lightweight plastic or balsa wood spine and use servos or actuators to pull strings running down each side of the tentacle to make it move from one direction to another.  Then I plan to cover it all in about 1/8th inch of silicone to give it a fleshy feel.  If anyone knows of a good website showing simple robotic snakes or something that would help a ton.  I know just enough to buy too many parts and burn out the ones I would actually need lol.  I believe I would need something like the RAPU unit from http://www.brookshiresoftware.com/ and a lot of remote control car batteries to make it all work.  Thanks for any advice.  Oh yeah, the quiter the better... I don't want to shout over my mask in order for people to hear me lol.

[Hal9000]

One servo pulling a string would probably be ok.

I made a Dr. Octopus arm with some straws stuck to the insides of the segments of the arm.

One side pulled the arm to the left, one pulled the arm to the right. You could probably get away with using 1 servo.

Yeah, that's probably quite a fun project to do. Especially with the silicone *imagines breasts*

[Admin]

Hmmm so this is kind of tough.

What is the tentacle length and weight of the material at that length?

How fast do you need it to move?

How many degrees of freedom (rotatable joints, or DOF) does it need?

Motors often burn out when you apply what is called stall current - the point where the maximum amount of torque is being applied by the motor. This basically means that the motors you were using are definitely too weak for the job.

So how do you know how much torque you need from a motor? Well your tentacles are similar to any typical robot arm, and so I have a tutorial written on how to calculate torque from a robot arm:
http://www.societyofrobots.com/mechanics_statics.shtml
It may seem a bit math intensive at first, but its really just fundamental physics equations.

Hope that helps . . .

[jonwhunt]

What about magnetic stepper motors?  Do they burn out or can they just let the load slip if it becomes to much.  I'd rather have motors that would simply release or slip then burn out if the load became too much.  The longest tentacle would be maybe two feet and weigh hopefully no more then 1.5 pounds tops.  That's including the plastic skeleton and silicone skin.  I want the whole thing to move in a slow fluid motion.  The closer I get to the way it looks in the movie the better.  I was thinking linear actuators for a little while, using a pull motion to shorten the string running down one side or the other, but then I thought it might be easier to use one bi-directional (hope I called it the right thing lol) stepper motor to wind the string in one direction then release it and wind the other way at a later time.  I've really got to take electronics classes at the local community college lol, this project is starting to scare me.  I've got to find the smallest motors (or whatever will work) possible so I can mount several of them to an under-shell (think hard baseball cap with little motors all over it pulling strings and ran off rechargeable batteries).  I only plan to mount the motors that will be pulling the highest part of each tentacle, I hope to mount REALLY small motors/gizmos actually inside the tentacle so that sections operate independant of the next section.  so when one part is being pulled up, the next part of the tentacle is twisting to the left, and maybe the smallest part is curling at the end.  I envy all you guys that actually know what you're talking about lol.

[jonwhunt]

Ok I've hunted online for projects or tutorials on building tentacles or even very small snake robots and I can't find anything other then that Japanese "snake" robot that is huge, and then the article about how the military is using a tentacle on its latest bomb/inspection robot.  If anyone can make a tutorial for me then once I finish this beast of a project I might be able to swing sending one of the completed Davy Jones animatronic masks your way.  It all depends on what my income to expenses ratio is for the next several months lol.  Whatever motors, gears, pulleys, gizmos, rocket launchers, or gophers are used they need to be small enough to move the largest of the tentacles (there will be two that are 20 inches long by 2 to 3 inches in diameter) in three seperate sections... I'm thinking one making the biggest section pull up and a little side to side, then one farther down making the middle section pull and twist, then I would like the tip to almost be able to curl up like its trying to grab a pencil or something... jsut like an octopus tentacle.  Everyone's seen Pirates of the Carribean 2 and knows what Davy Jones looks like, so I need to be able to mount all the gears, motors, etc. to soemthing similiar to a plastic bowl on my head lol.  There will also be soemthing over my face similiar to the Hannibal Lector mask, but without the bars over my mouth    So I will be able to mount motors for the larger tentacles on the front of my face jsut at the corners of my mouth basically.  I want the movement to be slow, fluid, very quite, working off batteris small enough to carry (I'm thinking rechargeable R/C car batteries or even 6v lantern batteries), and it needs to be able to not burn out the motors when I turn my head and the weight pulls against them.  The silicone I skin the project in will weigh at MOST a half pound for the largest tentacle.  Thanks all.

[dunk]

so i don't watch tv so haven't seen the movie but sounds like pneumatics might be a better way of operating this thing than motors.
i'm thinking inflatable tubes inside the tentacle that give it rigidity in certain directions when inflated.

will your hands be free to operate the thing? if so you could miss the complication of having electronics at all and just operate valves and taps.

or maybe i'm not understanding your concept correctly.

dunk.

[jonwhunt]

Yeah you're understanding everything right, unfortunately one hand will be controlling a huge claw (think old lady reach and grab arm thingy that everyone plays with when they're little... pincher on a pole) and the other hand will only have three fingers free because the others will be inside another tentacle.  I think robotics will be the only way to go... and I think I'll have to have the signals programmed into portable control unit that I can wear on my belt under my pirate coat.  If you type Davy Jones in youtube I'm sure a video would pop up.  Well time for me to get back to searching the net for tutorials and such.  Thanks in advance to anyone that comes up with a solution.

[Hal9000]

I still think you should just make it light as possible and use one or two servos.

[Admin]

how about something like this?
http://www.societyofrobots.com/robotforum/index.php?topic=1225.0

or this?
http://www.ece.clemson.edu/crb/labs/biomimetic/elephant.htm
(see video)

[jonwhunt]

Oh man!    That elephant trunk was awesome!  I need something like that, but it doesn't need to be as big, or as sophisticated, or at expensive lol.  I want something just like that but that moves around randomly and is smaller.  I'm hoping that since I want it to be random that I won't have to strap a laptop to me just to keep it moving lol.  So if anyone wants to tackle this project jsut keep in mind it needs to be TINY, silent, and no where nearly as elaborate as that elephant trunk    If no one wants to mess with it that's cool, I've got a local robotics group checking on it for me.