Digitizing arm options

[Cliff_J]

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EDIT: For those joining the conversation in this thread a little late, a short summary of the thread has graciously been put up by another poster (Graham S) to help bring you up to speed on what has been discussed. Many ideas have been presented and this synopsis should give a reader a good idea of what we are talking about in any new posts with our jargon and acronyms. Happy reading.

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


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I'd like to model the inside of an automotive interior to machine plastic parts to fit against the countoured surfaces of the sheet metal.

I'm looking at purchasing a digitizing arm as it seems the next most affordable option after using a contact probe on a homemade 3-axis machine. A machine that needs to be setup quite differently since it needs to work 'inside' of boundries.

The Microscribe seems like a decent price at just over $3k, and I've heard the Faro arms are over $10k but instead of a 3ft arm can go to a 12ft arm! And I've seen some other companies market arms too. Anyone know of other affordable options?

I'd love to think that someone has worked out a DIY system for putting affordable 2000 count encoders on a homemade arm with rollerbearing pivots and software to translate the kinematics. But I'm not about to hold my breath.....

Cliff

[jtedmonds]

Originally Posted by Cliff_J I'd love to think that someone has worked out a DIY system for putting affordable 2000 count encoders on a homemade arm with rollerbearing pivots and software to translate the kinematics. But I'm not about to hold my breath..... Cliff

I have never tried one, but after loking at a DIY structured light scanner, I was thinking last night that it would be possible....with great care in the design, parts, and machining. Commercial systems are not in my budget! I want to be able to digitize 1:18th scale die-cast models and import the points into AutoCAD for editing. The electronics would be fairly simple (counting clicks) and the software can easily be done in Visual Basic. I am looking very hard at this avenue. Who knows, it might just work!

[NewCents]

Another 3K Arm with a 50" circular radius...
http://www.remtek.com/remtek/3d_contents.htm

DIY Touch Probe
http://www.indoor.flyer.co.uk/probe.htm

I like the arm idea and I'd be doing the exact same thing you're planning on doing Cliff_J.

[turmite]

Here's another on and they can be had with huge working areas.

http://www.nemi.com/product-pictures...elescoping.htm

Mike

[High Seas]

Wow - those babys aren't cheap!
I wonder -- has vacpress done any more on the DIY "microscribe" varrient? He was going to use mouse bits and build a robotic arm digitizer.
Sounds like a good compromise if you could get close and work the fit after the fact.

On a similar project (car interior) I am planning on using the existing doorskins and dash - digitizing the backsides (the side against the door/firewall), create a point cloud and then go from there - figured the machine would probably run for a couple of days to get just one door done. - but hey - might save some plotting time - mirror the left and right. Here's hoping GM did a fair job building!
Cheers - Jim

[Cliff_J]

Wow, a flurry of activity after a period of hibernation....

I'd thought about using the mouse encoders as well, but have since purchased a used Microscribe off ebay. Its really quite nice, a bit small, but quite nice. Now its just a matter of figuring out how to 'leapfrog' the unit from place to place to capture larger objects.

My biggest concern with any arm was the resolution. Each step needs to be quite precise for the length of the arc covered by an arm. How Faro and Romer pull off .003" resolution on a 12ft arm is beyond me, that's like a super-encoder of 160,000 cpr and with zero backlash!

The Microscribe works for me, but if someone can work out the resolution with maybe a large encoding disk and a regular mouse encoding sensor I'd be open to trying to build my own especially if it could cover a larger working area. Someone with a lathe that could make a 6" or 8" disk with 10,000 slots cut in its perimeter would be a nice start, now you'd be looking at .003" resolution for a 4ft arm. Some ABEC7 bearings and aluminum extrusion and the mechanicals would be done. Now, about that pesky software to translate the rotations of the joints into cartesian co-ordinates...

It seems like its so close to a DIY solution that I can see it already. I fear the reality is I'm missing some of the difficulties in assembling the HW & SW into a usable system.

Cliff

[jtedmonds]

Ciff:
What kind of resolution are you seeing on the unit you purchased? I've been doing quite a bit of research this weekend and am almost thinking a small touch probe unit would be easier to build and more accurate. I thought about rotary absolute encoders on the arm, but they are very expensive and you would need four! The Homemade Touch Probe site looks to fill the bill however.

Jason
South Carolina

[Cliff_J]

The resolution seems to be .001 although if you are not holding your breath or keeping it against a surface it will dance all over the place. Just dragging it across a surface and it will bounce a little. As far as accuracy, its seems to be within about .008" which is better than its spec of .015" for surfaces that I consider to be flat. Unfortunately, I have no precision anything to check its accuracy beyond like the surface of my table saw or a metal striaght edge designed for workworking. For me, ignorance to the contrary is bliss and the unit seems quite precise. For nearly $2k, it'd better be.

Someone posted a link to a site that had a multitude of encoders that were 2000cpr with quadrature, and at only $50 each that seemed cheap. With a 12" arm, that would put the resolution at about .018" which might be good enough.

Cliff

[jtedmonds]

Cliff:

I've been thinking on and researching this subject in my spare time this week. I have been reminded of something I read about servo motors and there operation several years ago. I went back to that book (my wife refers to it as my NERD book) and read that section. Maybe we are looking at the angular sensor issue all wrong. Why not use precision potentiometers in lieu of optic encoders? It appears that they my be both more accurate and more economical. I have emailed Novotechnik for pricing. Their P6500 has a resolution of 0.007 degrees and linearity of +/- 0.05%. The best part would be that there would be no need to count "clicks". This would seem to reduce the overhead of the electronics. If you added an A/D converter or connected the fours pots to a microcontroller (ex. OOPIC) there would be even less overhead on the computer. Maybe I'm making this sound too easy. What do you all think?

[Cliff_J]

Now this discussion is getting interesting, that's an awesome idea. We wouldn't need multi-turn since the arm can only pivot so far without colliding with itself and not having to deal with quadrature and indexed encoder wheels would be a huge plus.

The .007 degrees is awesome. We'd probably want a 16-bit A/D and most of the PICs I've seen are only 10-bit but this wouldn't be too much of an issue to figure out (I don't think) with a seperate A/D chip and a serial link to a PIC. I've been putting off learning PIC programming (for building a standalone DRO and manual CNC control) but this would be as good a reason as any. Plus many other people are proficient at it, we might be able to leverage other's expertise.

Two thumbs up Jason!! I like the idea, no caveats come to mind that haven't been mentioned with respect to optical devices and this would be much simpler.

Cliff

[Cliff_J]

Next, lets talk about finding the location of our pointer. Treating each joint as a seperate entity, each arm segment can only rotate in one plane of motion. Within that plane the length of the segment is fixed thus the x,y co-ordinates for the 'free' end will always fall on an arc equadistant from the 'fixed' end.

So from any position of the tip to any other position of the tip the change in x,y,z would be exactly the same as the orthogonal vector addition of the change in each segments points? Could it be that simple?

Unless I'm omitting something substantial, I think this wouldn't be too hard to pull off with simple trig. Just write some simple software that pulls down the 4 or 5 numbers that represent the angle of each joint and compute the location.

This seems too easy. Hopefully the pots aren't too expensive and this would be simple enough to test using a sound card as a cheap A/D converter to see if a working model could be built 1 or 2 joints at a time with simple unity-gain op-amps to create a makeshift analog MUX for the number of joints and keep the electronics to an absolute minimum. If this worked, this would be awesome!

Cliff

[High Seas]

VacPress! Where are you? You should jump in here too!
I'm very interested in this project - I would build one but have little tech (electronic/digital) abilities to contribute. Vacpress and I had an off-line discuss a while back about a similar project using mouse bits. I like the idea of using soundcards as A/D I need to research that somemore - use it for weather sat decoing but hadn't considered other possibilities. Anxious to see the effort progress! - I'll be lurking on the side.
Jim