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Goniometers are so 2014

 
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MaxRockbin



Joined: 08 Nov 2014
Posts: 185
Location: Portland, OR

PostPosted: Mon Jul 27, 2015 9:58 pm    Post subject: Goniometers are so 2014 Reply with quote

This is purely equipment porn and never likely to be in the hands of mere mortal macro photographers, but it's still something to drool over...

Video:
https://www.youtube.com/watch?v=B756Novxejo

Some interesting numbers:
Travel ranges to 34 mm / 42°
Load capacity to 5 kg
Actuator resolution 40 nm
Min. incremental motion to 0.2 µm
Repeatability to ±0.1 µm

It can also move at 10mm/s which has to be good for something.
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Chris S.
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Joined: 05 Apr 2009
Posts: 2729
Location: Ohio, USA

PostPosted: Mon Jul 27, 2015 10:17 pm    Post subject: Reply with quote

I've been looking at this sort of platform (quite a few varieties exist) for several years. Have been itching to build one for the Bratcam, and program an Arduino to run it. But there are a bunch of things higher on the to do list.

As you say, they are expensive--at least, all the ones I've priced are. This is a large part of why I'd like to build one and launch a how-to into the public domain. I don't actually think it would be all that difficult.

--Chris
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MaxRockbin



Joined: 08 Nov 2014
Posts: 185
Location: Portland, OR

PostPosted: Tue Jul 28, 2015 9:27 am    Post subject: Reply with quote

In a way, the PI platform is a two-fer. Given the accuracy, it could be used for stacking (horizontal or vertical), as well as making very versatile stage. Assuming your subject doesn't mind going for a ride.

Plus, would you rather tell people that you mount your subjects on a goniometer, which sounds like something left over from a pelvic exam, or on a shiny new robot?
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RobertOToole



Joined: 17 Jan 2013
Posts: 234
Location: United States

PostPosted: Wed Jul 29, 2015 5:23 pm    Post subject: Reply with quote

Thanks for sharing the link to youtube.

You know I have seen these type of units before on Newport's site for example but never have seen one in action!

Robert
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g4lab



Joined: 23 May 2008
Posts: 1424

PostPosted: Fri Jul 31, 2015 2:39 pm    Post subject: Reply with quote

https://www.thingiverse.com/thing:46970
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MaxRockbin



Joined: 08 Nov 2014
Posts: 185
Location: Portland, OR

PostPosted: Fri Jul 31, 2015 2:59 pm    Post subject: Reply with quote

Pretty neat!
This delta design though seems kind of limited on the rotations (the hexapod can rotate on 3 axes as well as x,y,z translations).

I did see a design for a do-it-yourself hexapod, but it didn't seem nearly precise enough for macro work. Only $150 in parts though. Companies like this one sell some interesting parts for do-it-yourself roboticists for cheap:
https://www.pololu.com/product/88

Also there's the issue of the driver. There are what are called "G Code" parsers out there (a language for industrial motion control), but I think that's really on a way to specify a path. You still need to tell the hardware what to do with the info.

It's kind of interesting to ponder - especially when all legs can lengthen/contract independently like with the hexapod - exactly how you would program how much to adjust each leg to move and rotate the platform in a specific way. My high school trigonometry book had a final chapter on spherical trigonometry and the only application suggested was celestial navigation. Just hasn't come up a lot for me. But I think that's exactly what you'd need for one of these gizmos....
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 17873
Location: Richland, Washington State, USA

PostPosted: Fri Jul 31, 2015 3:29 pm    Post subject: Reply with quote

Interesting devices!

Quote:
It's kind of interesting to ponder - especially when all legs can lengthen/contract independently like with the hexapod - exactly how you would program how much to adjust each leg to move and rotate the platform in a specific way. My high school trigonometry book had a final chapter on spherical trigonometry and the only application suggested was celestial navigation. Just hasn't come up a lot for me. But I think that's exactly what you'd need for one of these gizmos....

I guess it depends on what you mean by "spherical trigonometry". As defined by Wikipedia, "Spherical trigonometry is the branch of spherical geometry that deals with the relationships between trigonometric functions of the sides and angles of the spherical polygons (especially spherical triangles) defined by a number of intersecting great circles on the sphere." In other words, it relates to what happens when the edges of triangles are forced to lie on the surface of a sphere. Lots of interesting things happen there, like the sum of vertex angles is always greater than 180 degrees.

These devices are simpler than that because all the parts are either straight or planar. So, you can start by figuring out where you want the top platform to be, using your favorite parameterization. From that, use ordinary trig to figure out the 3D coordinates of the centers of the ball joints. Then use the Pythagorean theorem in 3D to figure out the actuator lengths.

With a hexapod, control should be especially simple because there's no redundancy. Within some envelope, any combination of actuator lengths is valid. You don't have to worry about breaking anything, even if you adjust the actuators one at a time.

With the Delta Robot v1 that g4lab pointed to, things seem much more difficult. It looks to me like in that design, with 8 actuators, you would have to move multiple actuators in careful synchronization to avoid stressing the joints.

It's sort of like the difference between adjusting a 4-legged chair and a 3-legged stool, while keeping all legs in contact with the floor.

--Rik
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MaxRockbin



Joined: 08 Nov 2014
Posts: 185
Location: Portland, OR

PostPosted: Fri Jul 31, 2015 4:27 pm    Post subject: Reply with quote

Good point about spherical trigonometry. There is no need to calculate the angles on the surface of a sphere or arc lengths.

Just deltas of x,y,z for the vertices after rotation in 3 axes (which corresponds to a 3 new positions on a great circle centered at the center of the platform) and deltas for simple xyz translations. So you can use regular Cartesian/Pythagorian/Euclidean distance and piece things together - with a little plane trigonometry.

I looked up the Delta Platform on Wikipedia:
https://en.wikipedia.org/wiki/Delta_robot and the object of it seems to be to maintain the orientation of the platform while it moves around. It doesn't rotate.
So that reduces binding issues. They contrast it with the Stewart platform - aka hexapod.
But, unfortunately, not too exciting for use as a macro platform.
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rjlittlefield
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Joined: 01 Aug 2006
Posts: 17873
Location: Richland, Washington State, USA

PostPosted: Sat Apr 16, 2016 1:22 pm    Post subject: Reply with quote

Resurrecting this thread from last year...

I noticed this morning that the creator of the Delta Robot linked earlier by g4lab is now selling a complete kit for a positioning device described as follows:
Quote:
Rotary Stewart Platform v2

CAD $597.92

This is a six degree of freedom motion control platform. It can heave and tilt (up/down), sway and roll (left/right), as well as surge and pitch (forward/back). This is a scale model of the machine found under flight simulators, multi-axis milling machines...

See https://www.marginallyclever.com/product/rotary-stewart-platform-v2 for more information.

--Rik
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elf



Joined: 18 Nov 2007
Posts: 1307

PostPosted: Sun Apr 17, 2016 11:28 am    Post subject: Reply with quote

And if you want to build one, check this link: http://www.instructables.com/id/Arduino-controlled-Rotary-Stewart-Platform/

I would probably build it more like my 3d printer with magnetic ball joints and carbon fiber arms.
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