Help with 6 DOF table

SteveW719

Has anyone ever built a 6 DOF motion table? I am looking to build a small one for experimentation and I would love to see some plans of the geometries of the actuator positioning and the type of joints (universal?) where the actuators join to the base and table. Mechanics first then hopefully I can get into the electronics.

Thanks for any help.

ElectricAye

SteveW719 wrote: »

.... Mechanics first then hopefully I can get into the electronics.

...

I strongly suggest you look at what your electronic and computational needs will be while you are planning the mechanical aspects. Of course, much depends on what sort of accuracies you need and whether it's okay for the different axes of rotation to intersect at a center or if it's okay for such axes to be offset, etc. But what you must keep in mind is that, unless you design things properly, your various DOF's can become dependent on each other - say you want to roll one axis and one axis only but, oops, that's affecting another axis at the same time, etc. If you aren't careful, you can get that sort of coupling of DOFs. If your electronics can handle it and you've got a computer to back out such dependencies, it might be okay, but the computational problems can get unmanageable really fast, and I've seen at least one case where a multimillion dollar project came to a grinding halt because it turned out there were no computers anywhere on earth able to handle exactly those kinds of coupling problems in real time. You'll want to think about this problem in a very integrated way.

Graham Stabler

Steve,

I am guessing you are referring to a a Stewart platform or Hexapod which use parallel kinematics for high speed and low inertia. Parallel kinematics is a hobby of mine or at least it was:

http://www.indoor.flyer.co.uk/kinematics.htm

As far as the mechanics goes you are in the wrong forum, may I suggest the cnc zone, there are many builders of all kind of machines over there and probably a section especially for parallel kinematics, there is also this website which may provide inspiration:

http://www.parallemic.org/

If you just want to play around there are examples of delta robots (three axis parallel machines with rotary actuators) on youtube based on RC servos, you could do something similar.

It is funny that ElectricAye mentions DOFs interfering with each other, with a parallel machine they all do and all of the time, that's the point! For 6DOF it is more difficult than for simple machines but not impossible. The real issue is building the machine to match the mathematics and then doing calibration to make corrections to the mathematics to match reality. One method used is simulated annealing as I recall but I know no more about it than its name.

Note on my website there is a pdf, in that is an example of how to create the formula for simple machines, you can see that if you just assume all works perfectly (with no corrections etc) then it is not so bad, no matrices just Pythagoras.

I'm not sure if a propeller can handle a hexapod or not, really no idea.

Graham

SteveW719

Thanks for the reply. You are right at this point it is more mechanical so maybe the machine type forums would be more help. I am more comfortable with the electronics so I gravitated here. I will check out the CNC zone.