42172Re: [SeattleRobotics] Looking to share an order of Polymorph (Polycaprolactone/ShapeLock/themo set plastic/low melting-temperature polymer)
- Mar 1, 2010David
Thank you for all those figures.
10-15km is 2-3 million steps. That requires resonably good engineering. 10
hrs for Wild-Eagle is maybe 20,000 steps. I'm sure polymorph would have no
problems with that.
To me, I think it depends on why you're building the robot. I suppose most
people's anwer would be to learn - for their own interest or to advance
robotics research. So the robots that you've had on display are retired.
They may be working harder than they did when they had a job but they are no
longer active for your learning. (Of course, if you build a robot for, say,
a museum display then its job is to move all day, every day and it must be
built to do that. But only one in a thousand home-brew or university robots
has that sort of life.)
So my guess is still that polymorph hinges are perfectly good for 99.9% of
> A pertinant question is how much energy does it take to flex a polymorphEnergy or torque? The hinge is springy so any energy you put into it to flex
it is recovered when it straightens. I kept feeling the hinge while it was
being flexed for its 10,000 trials and it didn't get warm so I guess that it
wastes little energy.
A friend suggested that if it begins to fail, it will be self-healing.
Cracks can't propagate because the extra energy loss at the crack tip will
raise the temperature and soften the polymorph, hence healing it. It's a
nice idea if it's true.
Of course, the next question is, how do you recover the stored energy? How
do you make use of the springyness? If the hinge is connected to a servo
(via a typical model-aircraft style wire con-rod) then the servo will
probably be unable to make use of that stored energy. I guess it will depend
on the design.
The torque needed to bend the hinge is very roughly 0.15kg.cm/radian. The
hinge region is 0.65mm thick less than 1mm long and 10mm wide. I suppose it
all scales roughly linearly. (Or will it go up as the square of the
thickness? - it's a long time since I learned about bending beams.)
If a hinge typically bends 1rad during a stride then 0.15kg.cm is about a
twentieth of the torque of a standard servo. If you've designed the leg
properly then maybe that 0.15kg.cm can be used to partially support the
weight of the robot.
> It may be desirable in say a foot which needs to deform to absorb footdownOtherwise? You mean why bother with a polymorph design rather than the
> loads, but otherwise?
standard hexapod legs where the servos act as the joints? Standard hexapod
legs may be the best design but I feel too few robots explore other
possibilities. Standard hexapods are a cliche. Cliches aren't neccessarily
wrong but it's always good to question them. Homebrew and university robots
are for learning and you don't learn much about the physical design of a
robot from Yet Another Standard Hexapod.
If you're using polymorph then (I think) you have to use model-aircraft
style con-rods. They don't have to be wire con-rods; they could be part of
the single piece polymorph leg. So the new design has two independent
(1) it uses con-rods
(2) it's made of polymorph
Con-rods have certain advantages. You can put the servos in the body - not
on the joints. You can use the full rotation of the servo: if a typical
servo can manage almost 180deg but the leg only swings 60deg then the
con-rod can give you almost a 3:1 mechanical advantage. You can arrange the
servos to give "unusual" motions to the leg. (I find it hard to design
linkages like that from scratch so I've written a two-servo linkage
editor/simulator. I'll upload it if people are interested.)
Is polymorph worth using? I don't have any experience. As I've often said
here before, robots are for learning. If you're learning then you don't know
what you're doing; possibly no-one knows. Engineering is for when you do
know what you're doing. Therefore 99.9% of robots should *not* be
engineered. They should have a design life of hours, not years. Robots
should be made of popsicle sticks and hot-glue. Polymorph *may* be a good
material for lash-ups. If you don't like the design, it's cheap and easy to
soften the polymorph and change it. How often have you thought "well, the
design looked good on paper but I wish I'd made that part 1cm longer and now
it's too much trouble to change it"?
(I know that a lot of people like standing at their milling machine making
beautifully crafted/engineered designs. I enjoy it too - it's a form of
zen-like meditative relaxation. But that's a separate issue from making
Personally, I spent many years working with crabs and thinking what
fantastic robots they are. The few polymorph robots on the web have a
wonderful organic look. It's probably a great material for making robot
As I asked before:
> If you were making aThe answer is not obvious to me.
> 3 DOF one-piece polymorph leg, what sort of design would you choose?
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