[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]

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.
:cheers: Jim

[jtedmonds]

I got the information on the precision potentiometers this morning. I had asked her for a "Budget" price to see if this was an area to continue to research. The pots with the 0.007 degree resolution are approx. $135 each - not that bad considering the other options. The A/D chips are only about $5 plus a few bucks here & there. The cheapest way to determine if this thing will work is probably to build something using cheap pots and switch them out when all looks right and seems to work to get the resolution right.

On to the issue of resolving the points. I pulled out my "Vector Mechanics for Engineers: Statics" textbook this evening. After blowing off the dust I went to the vector introduction section for a brushup. Cliff, I believe you are right. I've thought about this all day and have looked at several "Vector Education" websites. The easiest way to determine the cartesian coordinates may be by converting the Spherical coordinates (distance with 2 angles - base & joint). There are fairly simple formulas for doing this. Seems to me that you could resolve the Cartesian coordinates of the joints until you got to the pointer tip or add the vectors and convert the resultant sphereical coordinates into cartesian for the solution. I think the software to run this thing should be kept as simple as possible. That should not be a problem.

I plan on doing several sketches this weekend - again mostly concerned with the math not the construction. I love a challenge, especially when it's for building something I can't justify going out and buying. Hopefully we can put our heads together and come up with a solution that is cost effective and within the skill level of most home machinists.

Vector Site - Vector Education

[Cliff_J]

I'll have to read up on the vectors, I'm pretty rusty on geometry but thanks for the research. Its been a while since I took a statics or dynamics class and the textbook for the latter was horrible enough to mess up anyone's understanding of more than the homework problems.

I think I'll order some Alps (or other) precision pots from Mouser soon and try to keep the price down to $10-20 each. Then I'll see if I can find some websites on using the soundcard as the A/D solution. Kind of a Kleinbauer approach to build as cheap as possible and then work on the precision for the second generation.

Like I said above I've got a Microscribe we can compare to. If I can build one for 1/10th the price I'll be so stoked as then I can work on building one with a large enough envelope to handle my needs instead of leapfrogging a little guy all over the place. And I could probably build the larger one with the funds from selling the Microscribe on ebay....awesome!!

I called Novotechnik as well and they have the SP2800 which is used as a throttle sensor in CART/Indy/F1 cars with .03 degree repeatability and costs $57 which is one of their least expensive pots that we might consider. I think I'll try the Alps first for the prototype and if we can get that working....

Cliff

[duluthboat]

Hi guys, I’ve been watching from the side lines, mostly because I have nothing to add in the math or electronics area. I may be able to help with the mechanics and or modeling I have looked at the Faro arms longingly but will never have the money. I’ll go back to watching, if I can be of any help just ask.

Gary

[Cliff_J]

Glad to see you're watching Gary.

What are your thoughts on constructing the machine in terms of materials and the joints and so on? And do you have ideas of other contributors on other forums that may be of assistance in the math arena?

Cliff

P.S. I just learned that most PC sound cards are high-pass filtered at around 10Hz or higher. So now we'll need to use an AC waveform or external ADC to find the potentiometer position, but this shouldn't be too hard. I just found some source code for VB that lets me read the output of the ADC in a sound card from an oscilloscope program. One more piece in our little puzzle.

[duluthboat]

The construction would be very similar to Faro, it’s about as simple as it could be. I wish I could find a nice off the shelf joint but it will likely be a machined item. How big are these encoders you’re talking about? I’m hoping the math will not be a big obstacle; we only need 5 axis positioning not 5 axis interpolation. I’m thinking of a pointer and a trigger to record a point then down load the point cloud into Rhino or TurboCad to create a surface. For myself I don’t have a need for a transducer probe.

Gary

[jtedmonds]

I did some reading today. I kept reading the term Forward Kinematics - I don't remember that from my years at Clemson. Forward Kinematics is the term for solving for pointer coordinates of an arm - at least one application. In our case, we will have known distances and angles (4). Inverse Kinematics would be resolving the angles for a target point knowing the point and arm lengths. There appears to be a lot information on the web for this subject. I did a search for Forward Kinematics and found several interesting sites. Digging through the theory for the formulas will be the hard part.

I have been thinking about the construction. I'm assuming we will machine the parts out of aluminum. I don't have a mill, but have a family friend in that business. My mini-lathe should be large enough for the parts I can make. I'm thinking about making a prototype out of very light material to see this thing work and to iron out the bugs. I was thinking that I will blow the dust off my old OOPIC for my test bed A/D converter.

Gary: I believe the P6500 (0.007 degree resolution) pots are approx. 2" in diameter.

I have recently become a HUGE fan of forums such as this. I have been very impressed with how members are very willing to help folks find the solution that best fits their needs. It's interesting to bring folks with similar interests together and pool resources, talent and experience. I think this is the best way to approach challenging projects such as this. I personally just know enough about various things to be dangerous. I also have more time to read & plan than I do actual shop time.

Since I'm new to this forum, I'm assuming a "veteran" will suggest if and when we need to move this to the Open-Source or Projects area.

[duluthboat]

I’ll do a quick and dirty model so we have something to throw rocks at. I’ll use this pot as our encoder.

Gary

[jtedmonds]

Great. I'm looking forward to seeing what you come up with. I'll be working on the math this weekend and digging out the information I have on the OOPIC. However, as Cliff said, we need all the help we can get on the MATH!