Sunday, September 17, 2006


Ugly Stewart Platform

It is built from six 5cc syringes, on platforms of cardboard, attached to both platforms with "universal joints" made of thick copper wire loops. The large platform is a 13cm equilateral triangle, the small platform is a 7cm equilateral triangle.

Needless to say I do not expect to build a functioning reprap upon this. This is a mock-up to see if I had anything close to an understanding of what I was trying to build. For something made of cardboard and held together with wire and masking tape, it is impressively stable! Precisely what I'd hoped for from a Stewart Platform.

The real version will be made with 12cc syringes, on metal or wooden platforms, held together with magnetic ball-and-socket joints. I already have all the freakishly strong magnets I'll need for that, obtained from dead hard drives.

More pictures here and here.

How do you plan on pumping it? And, is there any way of calibrating it, besides using calculus-driven software?
The 12cc syringes come pre-calibrated, with graduations of millimeters. And, while it takes calculus to go backwards from lengths to coordinates, all that is needed to go from coordinates to lengths is simple trigonometry.

Six opposing syringes will be driven by six stepper motors. One of those was one of the things I'd hoped to build this weekend but didn't have time for for various reasons. Electronic complexity will be the price I pay for building a platform without rigid axis systems, I guess.
Isn't it possible to drive the legs directly with the steppers turning nuts on threaded rods? If you extend the axis of each cylinder below the deck far enough, they shouldn't colide; more inertia, though, I suppose.
Any idea on what the accuracy needed in each piston, relative to the desired accuracy in the working space? It seems, looking at the layout here, that the accuracy needs to be much greater than the horizontal accuracies (I.E, the motion of .1 mm on a piston might translate to 1.0 mm position change on the surface.), but vertical accuracy will be pretty much on par with cylinder accuracy.

Am I missing anything?
Adrian Bowyer: The joints would be more complex, and the struts would have to be thicker to avoid bending, but it would definitely work. I've seen milling machines built like that. I picked my design because it's a mechanically simple magnetic sculpture, allows me to build and test the platform without building the actuators first, and fairly insensitive to tiny mistakes in machining; instead of redrilling a hole, just scoot that magnet over 0.3mm to the left...

beaglefury: My understanding is it's the opposite, the platform travels less than the actuators do. Consider the height of a triangle as related to the length of it's sides.
My understanding is it's the opposite, the platform travels less than the actuators do. Consider the height of a triangle as related to the length of it's sides.

This is true for up and down motion, but what about motion left and right, and forward and back?

Imagine two pistons 10 units long, and converging fairly close to the same point on the work surface, separated at the base by 10 units (I.E, they roughly form an equilateral triangle)

The height of the top point will be ~8.86 units high. We want to shift this so it is still ~8.86 units high, but translated 1.00 unit to the left or right. Using simple algebra, this means the piston lengths must be ~9.54 and ~10.54 units long. To shift it one more unit, you'd need to adjust the piston lengths to ~9.16 and ~11.13 units.

In other words, you need about twice the accuracy in your pistons.

It gets a worse if you make the base narrower, for example, if the pistons are 10 units long, and separated by 4 units at the base, the center point will be ~9.79 units high. To translate one unit in either direction requires changing the piston lengths to ~9.85 and ~10.24 (4-8 times the precision needed in the pistons than the target precision.)
try lego penumatic set. get this from ebays. pneumatic cylinder with spring for pumping(1 unit), and several pneumatic cylinder of the same dimension for the actuator leg.
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