Linear Actuators Stewart Platform

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Re: Linear Actuators Stewart Platform

Postby lasersafe1 » Fri Jan 13, 2012 1:34 am

Hi Bart,

I did quite a bit of research on Stewart platforms last year. As you have already mentioned, they are indeed capable of superior rigidity over standard gantry machines. Yes, EMC2 is already capable of running them too. But you failed to mention the most important part. In a linear system where a Z axis is mounted on top of a moving X axis which is mounted on a moving Y axis (like the Gantry systems), the backlash and other errors are cumulative. In the Stewart platform, they are independent and once measured, they can easily be removed by software. One of my tasks at work will be to build a highly accurate Stewart platform that is miniaturized to move a Pockels cell with high precsion with X Y Z pitch roll and yaw indepently controlled. It will be about 4 inches tall. I plan on using picomotors with encoder feedback. Rather than worry about control systems, I will use a simple single board computer running EMC2 into which I can serially command positions of all 6 axis. Similar units are sold for $68,000, but I know I can build one for under $5K with sub-micron precision.
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Re: Linear Actuators Stewart Platform

Postby bdring » Fri Jan 13, 2012 3:00 am

I see very small ones in some magazine all the time. I think it is Laser Focus world. They claim similar result to what you are talking about. I should get some long, long awaited lead screw nuts that I need to try my simple machine.
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Re: Linear Actuators Stewart Platform

Postby lasersafe1 » Fri Jan 13, 2012 1:23 pm

Yes, the ones in LFW are the super expensive ones. It's crazy, they charge $40K for the platform and $28K for the controller. I guess they saw the niche market at NASA or something and think taxpayer money is endless. Those days are coming to an end (I hope).
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Re: Linear Actuators Stewart Platform

Postby Enraged » Sun Jan 15, 2012 6:56 pm

Heres a thought: if you make a 3 axis stewart platform with Makerslide, how much harder would it be to mount an extruder head from Makerbot, and then you have a 3D printer? Consider the MK6 stepstruder which Makerbot sells complete for $165. Obviously the software would need to be worked out, but from a hardware perspective, it would be fairly inexpensive to build.
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Re: Linear Actuators Stewart Platform

Postby TLHarrell » Mon Jan 16, 2012 4:46 am

That'd solve some of the issues with printing overhangs... nice idea.
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Re: Linear Actuators Stewart Platform

Postby akonneker » Mon Jan 16, 2012 7:17 am

While I've been thinking about how to make something along the lines of Bart's Stewart platform idea I've been stymied about how to connect the three vertical posts to some horizontal beams to complete the frame. One of the ideas I've had just recently is to do away with all those tricksy non-90 degree angles and just use 4 sets of parallel arms, similar to the Adept Quattro pick-and-place robot. See:



I think it's interesting to note that the extra arm gives you a rotational degree of freedom for the end effector. Wouldn't be particularly useful for plotting or 3d printing, but it seems like it might be useful for pick-and-place operations. I know there is another thread about the openPnP project on the forum already, but having a Delta-style system might be a nice complement to the gantry-style system that's being developed. My extensive youtube research has led me to believe that Delta robots seem to have found a nice niche in pancake stacking, muffin packaging, and other sundry pick and place operations.
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Re: Linear Actuators Stewart Platform

Postby canadianavenger » Mon Jan 16, 2012 6:51 pm

Why not use a rectangle, with one arm/post mounted in the centre of one of the long sides, and the other two in the corners of the opposite rail? All you would need is a plate to hold the 60 degree angle for the two posts mounted in the corners, which should be easy enough to cut on your laser. [not that hard to make by hand either]
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Re: Linear Actuators Stewart Platform

Postby Enraged » Mon Jan 16, 2012 10:29 pm

TLHarrell wrote:That'd solve some of the issues with printing overhangs... nice idea.


You would also only be limited for height by the length of the makerslide and the structural strength of the plastic, which means you could print some fairly large models.
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Re: Linear Actuators Stewart Platform

Postby Enraged » Wed Jan 25, 2012 10:10 pm

well someone already did my idea:



and for the rod end / tie rod end links, tie rods for RC cars are cheap, and available in everything from plastic to titanium.

Or, these guys used universal joints and a similar axis setup:


and here is a good read: http://kvarc.extra.hu/step/motor/emc/emckinematics.html
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Re: Linear Actuators Stewart Platform

Postby bdring » Thu Jan 26, 2012 1:49 pm

That is similar to the design concepts I show earlier in the thread. There was quite a bit of talk at ORD Camp among the 3D printer people about using these types of machines. The consensus was, that it would be best if the machine was very fast. The quality of the prints goes up quickly if the moves with the extruder is off is very fast. The stepper extruders do a better job of stopping the ooze, but a fast rapid is important too. Keeping the work piece completely stationary was also mentioned as a good feature of the design. It is hard to move a large printed piece fast without issues. If you go this this route as a 3D printer, you might consider a fast motor and a multi start lead screw.
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