Digitizing arm options

[turmite]

Hi guys,
I have been following this thread and ger21 brought up a point of rigidity. I am not a machinist, not good at math, am not an engineer so I have very little to offer. With that said I am curious what your thought are on the materials for the arm itself? Does the arm need to be light weight but rigid or heavy and rigid? If you want light weight and rigid the best material you can use is carbon fiber with either the aluminum or nomex honeycomb core. Stiff stuff and if you have the equipment not too hard to work with.

Mike

ps Gary's idea of mounting the base on a track seems an easier math solution that a rotating base, from a non-math person though! Edited to add, this would allow you to make the size larger in one direction without having to make the arm any longer!

[Graham S]

as there will be no loads as such, just the weight of the arms there should not be much flex, ali tubing would probably be OK but carbon fibre fishing pole (as in continental pole fishing rather than another name for rod) sections might be cool and quite cheap. The machine is essentially floppy rather than rigid, just rigid between flops Other than that the angular resolution of the encoders/pots and the quality of the bearing would seem to be the limiting factors, cool thing to try.

On encoders, they don't have to be expensive, US digital's are OK and sometimes really cheap surplus servos (in small sizes) come with them.

I think the maths will be pretty easy, as soon as you get a program that will keep track of the volts/encoders and store the numbers when you press fire then the code to convert that raw data into a point cloud would be pretty easy.

Graham (the homebrew touch probe guy)

[Cliff_J]

Jason - you're math looks good to me. At first I thought one of the sin should be a cos but on second thought it looks correct. Nothing like publik skool edumakshun. And an OOPIC sounds much easier than a regular PIC since you can probably use variables and other typical high-level language constructs instead of assembly code.

For construction, I think aluminum bar might be the easiest because it can machined as a single assembly without the need for welding or epoxy that would need precise jigs to keep things in alignment. Or maybe short sections of bar machined into a square and circular profile that is stuck in the ends of the tubes for assembly with final maching of the pivots to follow assembly. I'd think a regular ABEC7 rollerblade bearing with its 8mm ID and 21mm OD could be doubled up on an 8mm shaft for good accuracy with low cost. Even with axial preload on the bearings, this is low RPM movement with small amounts of travel so I'd guess wear to be a small issue. A homebrew double angular contact bearing of sorts - yah/nah?

BTW, the machined aluminum on tubes is pretty much how the microscribe is constructed. Its an aluminum base and aluminum upright on the lazy susan pivot but from then on its aluminum ends with graphite/carbon fiber tubes connecting the ends. Some of the Romer/Faro arms look very similar.

Highseas - yes the lack of a wrist joint makes digitizing objects more challenging when there is an undercut or angled recess. But leapfrogging the unit to different points OR creating a turntable to rotate the objects on (with another precision potentiometer to measure the angle of roatation) would be much easier solutions (right now) than trying to add wrist joints because of the math which is considered graduate level robotics study. For my purposes of digitizing automotive interiors I almost need the 7 DOF offered by a Romer/Faro but as I get better using the Microscribe and Rhino this seems to be less and less of an issue.

The hexpod falls into the same math issue. I know Chuck had great ideas on the other forum about building one for machining, but the lack of understanding of the math pushes it outside our collective abilities for the moment. The way things are going though, who knows....we'll have our own ASIMOs serving up cold beers by next summer.

Cliff

[Cliff_J]

Oh, one more thing. We'd probably need to machine in some sort of alignment pin we could use to 'lock' each joint at a specified angle. Then any trimming of the angle reference (whether mechanically on the potentiometer mount or in software) so we start with a known point since our math depends on it.

Cliff

P.S. I figure the .03 degree pots to give .003 inch resolution at 12" so with four joints our resolution would still be within .012" which is pretty awesome, and .007 degree would be .0007" at 12" so the collective error after 4 joints would be .002" which is likely better than the construction will be able to hold.

[foamcutter]

Hi Guys,
I just got through reading this thread from the beginning, WOW!. I am not a math guy either but I used to be a fabrication guy, draw a picture on a napkin and I could build it. Have you thought of using carbon fibre arrow shafts with machined aluminum joints on the ends for the arm? They are cheap, straight usually, and uniform in diameter. Just a thought now I'll also go back to lurking in the corner. That's what I do best. Ron

[jtedmonds]

Great information guys. It still seems to me that the easiest way to resolve the coordinates is to use 4 pots on an articulated arm. The resulting voltage is run through a A/D converter, then the digital information is translated into joint angles. Once we figure out the math it will be a snap to write the program to resolve the point. I think we have identified the correct formulas to translate the data. It's just a matter of putting the puzzle pieces together and finding that first object to digitize! Okay, I will admit there is much to do between now and then, but with the interest we've seen here we have folks from many backgrounds, experience and talents.

I did a rough sketch of a joint design today using aluminum round bar turned down with one side forming an axle that inserts into the other side piece which has the ball bearing. The pot is externally mounted through the second piece connecting to the axle. The sketch hopefully will show it better. I'll try to attach it tomorrow afternoon. I had thought about aluminum bar or tube as segments between the joints, but the carbon fiber arrow shaft sounds interesting.

Maybe a list of objectives would help - if it's not taken as me being to controlling

1. General design - articulated arm, track mounted arm, etc.
2. Math to solve for particular design concept (we have run down the formulas for the rotating arm already - we think).
3. Parts list with contacts - joint material, pots, ADC/microcontroller, computer interface.
4. Shop drawings of parts to be machined - maybe several versions to see which work best and giving options depending on an interested individual's available shop equipment (lathe, mill, etc).
5. Assemby instructions - General.
6. Sampling Software - Freeware - OF COURSE

Let me know what you all think. Maybe each of us should focus on a certain area(s) as we see ourselves being best used. I just know for myself, I have a tendancy to go in too many directions and never ready accomplish my goals. For instance, I would love to design mechanical joints, but of the above list that is the area I have the least experience. I hope this does not sound too pushy , I would just LOVE to have an accurate Homebuilt Digitizer!!

Graham: I'm glad to see you joining in, your homebuilt touch probe website started my mind's gears turning - very interesting. I hope the concept here will intially be easier to run since we have no motors to control and can choose specific points at will. I compare of this device to land surveying for a topo map. You actually shoot the points that descripe the surface best - bottom of a ditch line, top of the slopes, etc.

Sorry to go on and on. I guess it fairly transparent how excited I am about this

[Graham S]

A homebrew angular contact bearing would seem fine to me unless we think of some unusual off the shelf hinge that would work.

Build the arms from whatever suits you I would say.

Adding a wrist does not make the maths much worse, unlike a hexapod the arm is a serial set of joints that work one after the other so it is just addition of vectors. I am fairly sure a general solution could be programmed up where you put in arm lengths and when nessesary offsets (as some arms have the pivot line to the side of the arm for example).

My personal feeling is if you get an arm that will produce some numbers the software will come. Get building!!

Graham

[jtedmonds]

I sat down a few minutes this afternoon and put together a spreadsheet that calculates XYZ of the pointer based on constant segment lengths and supplied joint angles (4) - nothing very fancy. I solved for the resultant vector (in polar coordinates) in the YZ plane and then rotated about the Z axis (to create spherical coordinates). I am attaching as a zip file - it will not let me attach xls file. Let me know what you all think. As you can see, these are fairly simple formulas. If we all agree that these are correct, all I would need to do is write a small VB program to read the angle values from the OOPIC.

After thinking last night about this arm, I believe for me the articulated arm is going to be the best solution. It will be small enough to put on the kitchen table (shop is cold in the winter) or could be enlarged to map the surface of a full scale vehicle.

I think we will need to place a calibration point on the base. This will be used to zero out the pots.

In my mind I see the segment offsets canceling each other out since they would alternate over the three segments. Do I have this pictured wrong?

[duluthboat]

I spent some time today reading up on some of the many things I don’t understand about this thing. I think I now know what a 00PIC is, and I know a little more about encoders. This may help to keep my eyes from glazing over when I read some of your posts.

I reworked the design, making the joints just large enough to hold the P6500 and .625” diameter carbon tube. The tubes in this model are 24" long only because that’s the size I need, they could be a little longer without being worried about flex. The pointer is in the same vertical plane as the center of the base. I now will try to covert some of this stuff to available items, the rod, bearings, and nuts. The rest requires machining but nothing to tough. Let me know if you see any obvious flaws. I may build a prototype this week.

Jason,
We can put a cal point on the base but I would also like the option to zero xyz on a spot of my choosing, I not sure if this is possible. Is this thing really as simple as it seems? Why isn’t there a low end model like this on the market.

Gary

[jtedmonds]

Very nice model, Gary. I am very impressed. I think you demonstrated what I was trying to say earlier about the segment offsets canceling out.

I was thinking the same thing on the base point (benchmark) option. I think we still will need the calibration point on the base, but I think you are talking about a "benchmark" option to move this thing around large objects. Again I will compare this to surveying. If you are digitizing a large object, you will initially setup the arm at the "000" position. After you "shoot" all the points possible at that location, you will move to the next location. The second location will need to be within reach of the first position and one other known position (a defined point on the object). In surveying this is called taking your backshot so that you have the alignment of the instrument correct. It seems to me that if you built your machine big enough for the majority of the items you will digitize and have a routine in your software to allow you to change positions, you really can digitize anything.

It all does seem fairly simple. I'm sure there will be small surprises, but it will not be from a lack of looking at all sides of the problem. My problem will probably be something stupid like, "I can't find my soldering iron!"

My only thought on the pricing of the low end models is that they are priced at what they think the market will bare. With available technology what it is today and forums such as this one it does open up many options to the home DIY'er. I think this can only be a good thing because it will push the consumer market toward advancement to stay one step ahead of the homebuilder.

[Cliff_J]

The calibration 'ticks' would need to be on each joint, otherwise just a single point wouldn't handle the angle of the pointer segment and the kinematics don't work out as too many variables would be open. A tube the pointer was inserted into may work, but if we had machined indexes into the joints to lock at 90 or 180 degrees a straight edge and simple workworking square could be used to check the calibration reasonably well and would alleviate the need for precision machining on the base and sub-assemblies and their fitment.

Gary, once the machine is calibrated one time mechanically it should not need it again until some future point in time. With the Microscribe (like lots of testing gear) the higher precision model is just one that's been calibrated by a person to fit a tighter standard. So just like a multimeter or oscilloscope that's $200 more than the base model the equipment is the same but its been adjusted better.

I'd imagine the software would be easy enough to program to accomodate this. Once the probe knows its own world coordinates, then using any user defined coordinate system should be easy enough to convert to by a cartesian-sphereical-cartesian function. With the Microscribe and Rhino when you first initialize the probe you click once to define 0,0,0 then a second time for the x-axis and a third time for a y-axis.

I think the low-end is the Microscribe and $3000 is a bargin in the corporate mindset. On the Rhino forums its amusing how many people talk about clients who shrug off Rhino as inferior software because it only costs $900.

Jason, I'll need to look at your spreadsheet tomorrow. It looks to be on the right track but a diagram would go a long ways to visualize what each letter represents. IMO the joint offsets don't really matter as long as they all are parallel and a single common plane intersects the joints so the polar/sphereical conversion works without kinematics. Your list looks good too.

Cliff

[ger21]

I noticed on the Immersion (Microscribe) website that the plugins to use this in different software packages can cost more than the Digitizer itself. Most likely, the software to control this thing will be a standalone app. Instead of having to create polygon Meshes in the software, you get just create the point cloud and get the free points2polys program here: http://www.metris.com/ Look under Paraform.