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

I'm looking to build something similar to the tail of an animal. I would like to have as many joints as possible (think piece of roller chain turned up on it's side) to get a semi free-flowing motion out of it. Here's the catch: I can only use one or two hydraulic cylinders (and any assortment of linkages or cables) to actuate it. If possible, I'd like to get a free flowing motion similar to a snake.

For now, let's say this thing only has to move in the horizontal plane, and only needs to be able to go from straight, to curved in one direction. Let's say it does not need to swing the other way.

I'd just trying to get conceptual ideas if there is any way to do this. If knowing the size helps, hold on to your armrests. I'm looking at a total of approximately 8 feet long give or take a couple.

Any ideas are welcome, as long as they are explained so I can understand. :) Thanks in advance.

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

Sanddrag,
A little study of anatomy will show you that the motion you need can be achieved with cables on both sides of your device. Pull on the cable on one side and the device will naturally curl in that direction. Of course you need to secure the cable at each of the joints but that is easy to figure out. A few springs will allow the "tail" to straighten when the tension is released.

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

Geez, you read a post, decide to reply later, and got to sleep for 8 hours, then Al takes your idea...

Anyway, consider a chain, like roller chain that would go on a sprocket. Put it on a table with the pins perpendicular to the surface of the table (ie if it were on a sprocket and the sprocket was lying flat on the table). With the chain straight, run a cable along the side. Put some sort of guide on each link and attach the cable to the upper link. If you pull on the cable with your actuator, it'll curl in the direction of the cable. Now, run a cable on the other side of the chain and keep your "tail" curled in this direction using a spring. If you were to completely relax the actuator on the first side, the "tail" would naturally curl toward the spring side. You can then use the actuator to straighten it by pulling the cable on its own side. You can now make it curl in both directions using only one actuator. This however, would only get you one big curl, not the snake-like motion you wanted.

You only asked how to curl it in one plane, but I wanted it to go in the perpendicular plane also. To make it curl perpendicular to the table's surface, you'd need the other actuator (which is why I only used one above). Obviously, you'd also need to find a replacement for the chain because it doesn't bend in that direction. If you put the same cable setup in the perpendicular plane, you now have 3-D range of motion.

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

Sorry,
Us old guys don't need as much sleep.

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

Thanks for all the ideas. I think one big smooth curl is okay. I'm going to experiment on a smaller scale as soon as I can. :)

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

Actually you wouldn't attach it to each joint. You have something like described above except drill holes through each piece and connect the cables to the end piece. Ive seen a snake toy similiar to this. It completely freaked people out because its movements were 100% real looking!

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

what if you used springs?

use what you can for hydralics, and then use the springs on the joints to keep it realistic and moving. They should be easy enough to work with minimal actuation.

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

Mechanicalbrain, I think I actually had a toy snake sort of like that at one time or another. But it wasn't actuated, you just held it out and tilted it a little and gravity would make it move. It was a lot like plastic roller chain actually.

Anyway, I tried to make a model this thing with a piece of roller chain (#35) and a rubber band (#33) cut so it would be straight and not in a loop. I tied it to two points on the chain such that when the chain was straight, the rubber band would be under tension. I then tied a piece of string to the end of the chain, and ran it down the other side. I then placed a small zip tie at every link to keep the string and the rubber band close to the chain. Anyway, the results were quite marginal at best. Some links would want to move and some would want to stay. The biggest trend I noticed is that some links would want to move in the opposite direction from the direction the chain was moving as a whole. This resulted in the chain pivoting at only one or two points out of maybe 12. Basically, the chain was attempting to stack on itself every time the string was pulled. For the tail to properly curl (in one direction) I would need to have stops on each "link" so that it could not pivot in what we'll call the negative direction (the other way past being straight). And even then, I'm still not sure it would work, but I think there'd be much better chances.

What I want it to be able to do is go from being straight, to being sort of like this but on it's side.

And now here's the really hard part. This thing's motion has to look good on TV (slow and smooth) and it will be all covered in flowers. There mustn't be anything visible except for flowers (I believe carnations to be exact) so it will need some sort of flexible skin/covering.

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

I cannot tell a lie, I saw this first demonstrated for the tail on Jabba the Hut in Star Wars. Just the right tension and balance on a few springs and a mechanical joystick was able to perform some pretty good motion.

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

To do more than a small curve, you need the wires you are pulling to be able to extend into the gap between the curled up part to the end. Reguarding what Al said, Jabba's tail had covering enough and flab enough so that going from the tip back to the point they were pulling wasn't hard. It was already covered. See the pic below for what I am trying to say. If you try to keep the wire (small line) next to the tail (big line), you will be pulling agenst youself after a little while.

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

why dont you make it overly complicated and use a bunch of servos? but i assume its going to be bigger that what im picturing it, but a bunch of standard RC servos would work really well, you could also get a board too, and make all the servos sync up with eachother

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

Are you building a parade float?

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

Bryan,
Not really. The wires were attached at each joint with a pulley system so that there was no need for wires "out in the air". 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. Similar mechanics were used in the original "Jaws" but had fewer joints and needs far more power.

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

I would think that attaching it to every joint would limit the movement and the amount of curve becuase you would start fighting youself, especially if you what it to curve back like in sanddrag's picture. But I might have a serious misunderstanding of what I'm talking about. It makes sence in my own head.

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

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.

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

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....

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

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.

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

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.

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

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.

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

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.

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

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

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

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)

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

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.

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

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 ?

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

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.

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

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.

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

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.

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

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.

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

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!!! :D

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

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.

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

Casting a large piece is a challenge. Latex also rips easy. There are RTV silicone moilding compound but they're not cheap. Maybe urethane foam sheats? its tough and the edges can be bonded with contact adheasive. It bends pretty good without buckling or creasing.

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

Why not some thick fabric like canvas. You can even paint it so it blends in with the flowers

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

Dudes and Dudettes,
This only needs to last a few hours. McMaster sells latex sheets that you can stretch over the parts, paint and decorate any way you want. If you have no sharp edges it should last for several hours. You could use nylon fabric, and blow some air into it during the parade. The positive pressure will keep it somewhat inflated and virtually hide the joints.

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

The problem is the strains from moving the tail will spread the flowers apart possibly leaving gaps. Such gaps are not allowed in the ToR and they are pretty darned strict about it. I see two ways to deal with this.

One is to use big bunchy flowers that will spring open when they are stretched apart and squeeze together when they are pushed together. So the flowers themselves act as springs. You will probably want to install these at the last minute. I'm not sure just how long before line up you have to be finished, but you'll probably spend a good part of New Years Eve on it.

The other is to use segments that slide over one another and cover the inside segment with a really tough plant material. I'd use Yucca leaves myself, or maybe tree bark. It just has to be plant material, not necessarily flowers per se. If you're clever about it the joints become part of the design (say vertical stripes on the tail) and it looks natural.

What organization are you working with? The Cal Polys perhaps?

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

Yes, I'm working on Cal Poly Pomona's half of the float. (San Luis Obispo builds the other half).

I've done some experimenting and it looks like polyurethane foam (is that right?, because there is also polyethylene foam) may be the material of choice for covering. I'm talking about the same stuff they put in sofa cushions. I'm going to the fabrics store ( :ahh: ) tomorrow to play with a very large piece of it and see if it suits the needs of this project. If it turns out that this is what I will use, does anyone know where I can get lots of it for very cheap. It doesn't matter if it is "pre-owned" as long as the pieces are large and it is in relatively good shape.

Chris, I share your concern for the flowers. That's the exact issue I wanted to address. The plan is for the tail (and the rest of the dragon) to be covered primarily in Carnations. I'm thinking if they are placed with enough density, then then can "open" as you said when the one side of the tail skin stretches/bends into a convex curve. Perhaps there can be a similarly colored organic material underneath so that if we do get any gaps during movement we will not be looking at foam. And perhaps on the side of the tail that will be scrunching into a concave curve the flowers can be placed with less density to allow room for them to "scrunch." I'm just hoping the flowers will be durable enough to do this a few hundred times. Since they are only bumping into one another (and all are soft), I'm thinking it'll work out okay.

Also, I'm wondering what to use for the spring. For right now, I'm thinking of making it with the spring down one side and the cable down the other such that the spring is under minimal tension when the tail is approximately straight and when the cable is pulled the tail curves toward the cable side and the spring extends. The cable will follow a pulley at each joint so it does not make a straight line out in space between both attachment points (fixed and actuated). The goal is to have the cable roughly follow the contour of the tail. I'm thinking an elastic rope of sorts will be better than a metal spring. Do you think something like they use for bungee jumping would be good? Or does that stretch too easily or not easily enough? I've never encountered it in person.

Also:

M. Krass,
Why the double pulleys in your drawing instead of just a single one?

Thanks everyone for the excellent ideas.

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

There are many types of foam. Closed cell and open cell. There are many grades of stiffness You may get buckling with some. I was thinking of closed cell urathane foam. I use it to insulate water tank. Look at Mcmaster-Carr to see the selection. One other thought. There are many types of flexible round air ducting. The internal spring wire uould keep the tail round and buckling should not be a problem. There allot of types, plastice outer jacket aluminized mylar outer jacket and foil jacket. The best choice maybe aluminum flexible duct. The dryer duct stuff. It comes in 4,6,7,8". A local HVAC supplier that is well stocked would have a selection for you to look at.

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

Sanddrag,
The foam of which you speak does come in various thicknesses and in 1" I have seen it on rolls. It cuts well with a a straight razor, BTW.

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

Be sure to put some pictures up for us! :D

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

Perhaps if I detail a bit more about how what I sketched might work, the reason I included so many pulleys might become clearer.

The tail can swing to both the left and right in a single plane, rather than simply going from straight to curved in a single direction and is actuated by rotary motion. The circle at the leftmost end is a drum rather than a pulley so as to give and take cable out from both sides of the tail, making the device use a single cable rather than two. The drum would only have to spin some fraction of a rotation to give or take the change in cable length along a side. As I mentioned previously, there are multitudinous other ways this same give and take can be accomplished, including through the use of a cam that constantly rotates, pushing and then pulling a bar that tugs or releases two separate cables.

At the opposite end, a pair of tension springs fastened to each end of this cable would, optimally, maintain constant cable tension whether the cable be on the inside or outside of the curve.

Each joint has two pulleys per side rather than one simply as a means of capturing that cable whether it is on the inside of outside of the curve. One set keeps the cable near to the tail structure when it's on the inside of a curve. The second set, similarly, keeps the cable away from the tail structure when it's on the outside of the curve -- simply so you don't risk entangling the cable within the joints.

You could simply place the two pulleys directly on opposite sides of the cable from each other, engaging one or the other, but not both; but by wrapping the cable through the two pulleys instead, you ensure that it is always engaged for both possible conditions.

Does that make sense? It's not nearly as complicated as I pretend.

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

Makes perfect sense. Maddison, your design is very good; I Like it a lot. You have great talent lurking within you. If the tail is required to curve both ways, I will definitely experiment with something like that. For now, I think the tail only needs to curl one way, so I'll be experimenting with running a cable down one side and a spring down the other.

Does anyone have any ideas for what to use as cheap pulleys in my mockup design? (I'm going for something maybe a couple feet long and made probably out of wood) I to use something I alreadyu have or can buy at a local store like a hardware or office store. I just want to try the idea before going full force into an actual full scale design.

Also, does anyone have any suggestions in the way of a spring/bungee on the real thing? (For now let's say it will be cable down one side and spring donw the other and it will go from straight to a 180 degree curve)

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

sandrag,

i was fooling around with some wireway chain and fishing line (yes, we use fishing line in our manufacturing process) and i was able to make the wireway coil simply by tying the line to the last link running it through the wireway, and giving it a good tug. its motion is more like a tentacle than a tail, it can only curl in one direction, and it isn't very strong (obviously the moment arm is very small; less than an inch), but it would be very easy to do if you're limited in the fabrication department.

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

I finished building my own wood and PVC and duct tape prototype to prove the concept. It is about 2 and a half or 3 feet long. For the most part, it works. I put in mechanical stops, but they are too limiting; I want more curl than it currently has, but in designing the real thing that will be no biggie to change. The prototype turned out very sloppy since I didn't make a single measurement in the whole fabrication process. But, it works. If you hold the end link horizontal and let the rest hang out in air, it will slightly bow down because the sloppy manner in which it is constructed. But the links do curl fairly nicely one at a time.

Now here's my new question: For best performance, shoudl the chain (tail) be constructed like attachment 1 or attachment 2? Currently, my prototype is like #1 and I'm thinking maybe I should redo it to be like #2. Any thoughts?

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

For simplicity, the second drawing is probably the best design. It'll be easier to make several segments that are alike than to make some quantity each of two different designs.

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

In the first picture, the two blue segments can remain parallel with each other but offset by means of the red segment angling. It is like a geometric transversal. This isn't good. So I suppose I'll go with the second picture if we decide to actually make a fully articulated tail.

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

I think that number two will give a more authentic look and are easier to add mechanical stops to the motion. The first one may accordian on itself if something should go wrong. (Mr. Murphy wispers in my ear, spitting in the process he wets my shirt and the side of my face. Never forget, Murphy.)

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

You haven't mentioned the total length of the tail yet. 8 to 10 ' is what I envision. Close? Now would be a good time to get a estimate of the weight of the flowers. If the entire tail is articulated, then you maybe running into some structural issues. If the strength - weight becomes an issue you could make say the first 4' ridged and the last part of the tail articulated. A 10' articulated tail may exceed the limits of plywood unless you help it.

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

Well, we built the tail. It's not quite done yet but it's getting there. It is six links total and made from 2x2" by 3/16 or 1/4" wall steel box tubing and it has UHMW in every joint to help it move freely. Even with it all the way assembled (10-11' out there) it moves very freely at the first joint. I think it can definitely take another 100lbs on it without a problem.

I'll try to get some pics soon.

So, right now, we have a beautiful freely moving set of joined links. In fact, they move a bit too freely. Somewhere something went wrong so that they don't stop rotating at 15 degrees at each joint like they were supposed to. Some links will move a full 90 degrees in relation to the other one. So, we need stops. But rather than building in a hard stop (because welding on/around greased metal is not good), I was wondering if I could put stops on the cable instead. It is 1/4" aircraft cable that runs through loops all along the tail. And I was thinking maybe I could put a cable clamp at the correct position on the cable so that when the cable is pulled, the clamp hits the loop, and then that segment of cable cannot move anymore, and then the segment of cable on the next link will will move until it's stop hits that loop, and so on. Do you think it will work?

Also, right now we plan to have another cable running down the other side with a garage door spring at the end to provide the counter force to bring the tail back to nearly straight. Does that sound okay?

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

That sounds like it should work just fine. Be advised that the mechanical stops need to take some pretty big forces if the tail is moving fast. Try to think that you might have a windy day and what that might do to the tail. You don't need to weld in mechanical stops, you can clamp something onto the tubing. You might also be able to attach cables internal to the tail diagonally across the joints. (from left to right) These would be slack until the joint reaches the angle you want and then go taut to stop motion.

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

Well, it's almost done. The limiting cables are in and the acuation cables are in and the cylinder is mounted and hooked up. We just have some tweaking to do, like keeping good tension on the actuation cables for well controlled movement. We did a cable along each side and the cylinder is affixed to a lever arm such that when one cable is pulled, the other is released. (a lot of the above stuff isn't shows in the pic). No springs currently but we may need to add some somewhere. Once we get it all dialed in, it looks like it will work just fine.

Anyway, sorry it took so long to get a pic up. Been coming home very late from working on the float. Well, without further delay, here it is :)

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

Try to see if you can get a good video up, either of it almost done, or in real action. I can't always catch the Tournament of Roses parade.

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

Sanddrag,
It looks realy cool! Very robust and made to last. I hope it works as planned. Hey, what is your name anyway?

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

How about covering the whole tail in that black corrugated drainage pipe they sell at Home Depot? While the 4" stuff may be too thin, 6" looks (from the photo) to be just about right. It is stiff enough to hold its own and not get damaged from the knobs sticking out of the tail mechanism, felxible enough to bend just right, and it won't bend enough that the folwers will fall off or show gaps. Just thinking out loud.

Don