Building fully flexible tail (several joints) with minimal actuation required

[Squirrelrock]

Originally Posted by Al Skierkiewicz

You may be envisioning the chain which has nearly +/- 180 dregrees of motion at each joint. Think of something that only has +/- 10 degrees. Monkees with prehensile tails can have motion that allows the tail to wrap around a limb, but to duplicate that with mechanical/electrical devices is a challenge. NASA has been working on this long and hard. A friend of mine was experimenting with "muscular motion" by stretching wire across a bladder. Inflate the bladder and the wires are pulled taut.

Is that something like the no-rod pneumatics thing discussed here? (http://www.chiefdelphi.com/forums/sh...ad.php?t=39004)

grr, internet explorer doesn't work well with this interface....

[Madison]

I believe what I'm attaching is an illustration of the idea Al first mentioned, though using rotary rather than linear actuation. There are other methods of course, for implementing both linear and rotary actuation, including employing a simple cam.

It produces motion in a single plane and can curve to both the left and right -- though not both simultaneously.

[mechanicalbrain]

First off the cable wouldn't be attached to each joint. Put a hole on each side of each segment and you only connect the cable to both sides of the last segment.

[Madison]

Originally Posted by mechanicalbrain

First off the cable wouldn't be attached to each joint. Put a hole on each side of each segment and you only connect the cable to both sides of the last segment.

The cable is not attached to any segment but the first and last, it is simply captured by pulleys to keep it close to each segment when in tension or relaxed.

[mechanicalbrain]

Originally Posted by M. Krass

The cable is not attached to any segment but the first and last, it is simply captured by pulleys to keep it close to each segment when in tension or relaxed.

I was responding to Al's post. It doesnt need all those pulleys. You know i keep thinking of those toy robot-like hands that you wore over your own. Each one of your fingers go in a ring and the finger was extended. Ill post a picture.

[mechanicalbrain]

Here look at the cables in the fingers. Keep it simple.

[Not2B]

There was some talk about springs in the structure of the tail.

How about this...

try a piece of thin metal, with eye-hooks bolted on along the length Tie off a string/rope/cable to the tip eye-hook, and run the string through the other eyes (acting like the pulleys Al was talking about.) You'll need / want to play with it a little to get the right look. Cheap, easy, fast, robust, not real precise, not easy to control.... but I think it might get the job done. The metal strip is the body AND the spring.

(My prototype for a compliant big ball grabber for 2004 - never to be used)

[Al Skierkiewicz]

Hey Oz,
The pic shows wires that are on the back of the fingers for opening the hand. Please note how the wires are held in place. Kinda looks like there are pulleys at each joint.

[Gdeaver]

I've seen fiber optic inspection tools that have a range of motion similar to what you want. I've never seen the the guts but I believe they are a stiff rubber or plastic tube with a spring cable inside. The twisting motion is caused by winding and unwinding the spring cable with a nob. One unit had 2 nob's so maybe there was a spring cable inside of a spring cable. That unit had a very wide range of motions. For the older posters how are those things controlled that doctors stick up your b-t ?

[mechanicalbrain]

Originally Posted by Al Skierkiewicz

Hey Oz,
The pic shows wires that are on the back of the fingers for opening the hand. Please note how the wires are held in place. Kinda looks like there are pulleys at each joint.

Yeah it might. It was the closest thing I could find to what i was thinking. I Googled it but it just didn't show. Well I guess the thing I'm trying to describe would be to take jdiwnab's design.
http://www.chiefdelphi.com/forums/at...achmentid=3593
but attach eye bolts along the length to hold the cable to the body.

[sanddrag]

Originally Posted by Al Skierkiewicz

When you pull back on the wire which is attached to the tip, it pulls on the tip until the joint runs out of mechanical play (hits a premade mechanical stop.) Then the next joint flexes until the same thing happens. You don't get the exact motion of a true muscle structure since you are only really using one "muscle" but the approximation is pretty good.

The "hits a premade mechanical stop" part is the reason why my roller chain experiment failed. This is the key factor that I was missing. Thanks for the tip!

Now that we have lots of good ideas of how to make this work, how do we make it look good? Yes it is for a parade float and millions of people will be watching, including judges. Typically, the float is covered in mesh screen and sprayed with expanding foam and the flowers are either stuck into that or glued onto that. One slip joint might be okay (where the moving part fits inside the stationary part so you don't see the "guts" when it pivots) but I certainly don't think it would look good to have these slip joints 6 or 8 times (or more) down the length of the tail.

So, I was thinking to have some sort of flexible skin or fabric. The problem is, the tail is supposed to look smooth, not wrinkled and not scrunched up like a CV boot (or an accordian).

So, how do you make this look good, be able to move, and have the flowers remain intact? The looking good part is just as important as the motion. And if it cannot be made to look good, it will not be built this way.

Some other people working on this tail want it to only swing side to side at one pivot point. I'm trying to convince them that we can do better than that. But I really need to tackle the problem of decoration to convince them, because without a good way to decorate it, it can't be used.

[Madison]

Originally Posted by sanddrag

Now that we have lots of good ideas of how to make this work, how do we make it look good? Yes it is for a parade float and millions of people will be watching, including judges.

The Tournament of Roses? If so, your float will be entirely covered in flowers and I wouldn't be too concerned about visible joints. Clever use of floral cover should camouflage the joints.

That said, if there is genuine concern about visible joints, latex rubber is often used as skin on animatronics. It is first mixed, then poured into molds and it sets pretty quickly.

[sanddrag]

Originally Posted by M. Krass

The Tournament of Roses? If so, your float will be entirely covered in flowers and I wouldn't be too concerned about visible joints. Clever use of floral cover should camouflage the joints.

See, that's the part I need help with - covering the joints with flowers. I'm no floral decoration expert but The internals of the joint must never be visible no matter what position it is in.

So, could you describe this "clever use" a little more?

And yes it is for the Tournament of Roses Parade.

[mechanicalbrain]

Originally Posted by sanddrag

See, that's the part I need help with - covering the joints with flowers. I'm no floral decoration expert but The internals of the joint must never be visible no matter what position it is in.

So, could you describe this "clever use" a little more?

And yes it is for the Tournament of Roses Parade.

LOTS OF FLOWERS Seriously, if you have enough nothing will be visible. The other thing i can think of is a skin. Make sure to use large bloom petals..... WHAT? Half my family are landscapers OK!!!

[Madison]

Originally Posted by sanddrag

So, could you describe this "clever use" a little more?

And yes it is for the Tournament of Roses Parade.

Cool. If you have a means, a latex rubber skin made from just two pieces seems to best solution. That would require making a mold that, potentially, is as big as 9' x 3' x 3' or so. Ultimately it depends on the design of the tail. The two halves can be joined such that the seams are along the anterior and dorsal parts of the tail and the only joint would be at the tail's base -- a static connection.

I don't have any hands-on experience with latex rubber, but it's what Disney uses on its animatronics and they've made some pretty big, highly mobile monsters. A test is always a good idea.

If a large mold is not possible, you'd need to put a shell around each segment -- but the challenge is in being sure the shells do not destroy flowers as they slide in and out of one another as the tail moves. Obviously, you'd want to minimize the number of joints and I think you could manage sufficiently smooth motion with three or four 2' segments along an 8' tail.

You could similarly make several smaller latex skins that join in the middle of each segment rather than at the joint. That will offer flexibility over the joints without squishing flowers over and over again.