DEX LOG
2019 11
https://github.com/ChrisEberl/Python_DIC
Vision Controller Hello-World
We're currently working to build a computer vision based displacement sensing method for the DEX. Since our machine (or, many machines manufactured by novices / in the public domain) are liable to flex (indeed, nothing is infinitely stiff!), the thought is to measure local displacements of the sample, at the sample, rather than measuring open-loop through the machine's structure.
To spin this up, I've written a small / barebones subpixel template tracker in the browser, in cuttlefish. This is conveniently lightweight - the whole cycle (image collection -> analysis) can happen at ~ 10Hz, which is not splendid, but not diabolical either.
Here we can see a desktop test - I am reading the X- position from my tracking system on to a chart, and moving the tracker on a linear stage. This system resolves ~ 15um, which is not bad for a proof of concept.
2019 10
Re-did the machine last week, now much simpler:
Comparison to Instron 4411 (2019 10)
To see how we do against a real instron, I tested identical samples on the DEX as well as on an Instron '4411' with a 5kN load cell. In the plot below (I'm using cuttlefish to plot the .csv that I saved from Bluehill, the Instron software), the leftmost plot is taken on the 4411, and the lazier slope belongs to the DEX.
While the samples fail around the same load, the difference in elongation is ~ 1.5mm wide: this is almost surely the machine's own deflection, stretch in the belts, etc.
This obviously warrants correction. One way to do this is to build a stiffer machine, however, we will be chasing up the cost and complexity if we do this. Rather, we should throw some more control at it. To start, we can circle back to our attempts at subpixel tracking, or attach a small linear stage directly to our fixturing elements. For this, I am imagining something like the AMS5311, which should do some 12 bits inside of a 2mm throw (for 0.4um resolution). Either can be added to existing systems, given network controllers / modular browser code. Since I want to integrate it elsewhere, it's likely that the camera option comes first.
update the new machine uses a ballscrew transmission, not a belt (as used here) which should elminate most of the creep tested here against the 4411. forward progress has been made with a vision based controller, to improve further, but has not been characterized yet.
Some Force Maths (and a .xlsx file)
- want (?) from ex. nylon dogbone
- for this, w/ motor doing 1.5NM, how much reduction ?
Have this spreadsheet already, that's great. The D683 dogbone is ~ 8mm wide, and it would be great to test parts at 4 layers of 0.2mm - 0.8mm thick. That's skinny, but here we are: these are all under ~ 500N. OK.
For that oomph, with a 22T drive pinion, I'll want a 6:1 reduction and a NEMA23 motor (just a short stack) pushing 1.26NM through to a max. 660N linear force. OK, glad to have checked, nice to know about the N23 - I had been thinking of N17s - and will have to watch about getting that 6:1.
uSSM #3
Theres a few new criteria that ussm-3 aims to achieve:
- Manufactured only using simple shop tools, PLA 3d printers, and a standard bed size (24'x 24') laser cutter.
- Pulling force of around 600N
In order to be laser cuttable, acryllic and delrin were the main two materials considered. Delrin was the go to option to avoid shattering.
This design uses mainly the following adapted gantry system: gantry along with this beam design: beam for rigidity.
The main idea is to attach a beam to the linear axis and use it as a carriage to hold fixturing for tensile testing on the top half and compression testing in the bottom half. The gantry system shown above does not supply enough torque, so an adapted version was made shown below.
The uSSM #3 design takes advantage of the beam design by attaching 4 different beams to a larger "O-face", with webs in the corners to prevent torsion. The larger face itself is split into a bottom and top in order to make assembly easier, make each piece smaller to fit into laser cutter bed, and to have the possibility of changing top for larger specimens. The beam design also utilizes custom joinery talked about in the beam repository. This makes the machine take some time to build, but it allows delrin sheets to be used, allowing many fabs with laser cutters to be able to make this machine.
fixturing
Fixturing is being designed to use mainly 3D printed parts. First up, PLA is being in the jaws to see if it can withstand the 600N load alongside a few other McMaster parts and this load cell
Drawingboard Return
With #2 feeling somewhat unloved ('both overdesigned and underdesigned'), I'm back at the basics for #3. There's a few major selections, and decisions to make:
Material Selection
-> ALU, FR4(G10), Acrylic?
While aluminum is my go-to for machine design, and is ostensibly possible to mill on a shopbot by a motivated user (see jens), there is some hesitation to use it.
| Material | Young's Modulus (GPA) | Specific Young's | Cost for 6mm x 24x24" | Machinability |
|---|---|---|---|---|
| ABS | 2 | ? | 52 | Not Dimensionally Stable, but OK to Machine |
| Nylon 6 | 3 | 2.5 | 130 | Painful |
| HDPE | 1 | ? | 23 | Easy |
| Acetal (Delrin) | 2.8 | ? | 89 | Breezy, also lasers, and non-cracking |
| Cast Acrylic | 3.3 | 2.8 | 46 | Breezy, esp. w/ Lasers |
| 6061 ALU | 69 | 22 | 87 | Breezy with WJ, Painful on Shopbot |
| FR1/CE (Canvas / Phenolic) | 6 | ? | 81 | TBD, probably WJ Pain and Ease on SB |
| FR4/G10 (Fiberglass) | 22 | ? | 98 | Painful on a WJ, Slightly Easier on a Shopbot |
That said, ALU lands pretty well 1 order of magnitude above Canvas Phenolic ('FR1' or 'CE') for strength, while costing a similar amount of dollars. Fiberglass is a nice candidate, so machining G10 is likely a worthwhile experiment. However, both composites have anisotropic-ness and are sensitive to the size of local features (and to localized loads), making them less favourable.
!TODO: beam equations for the above, to size req' depth !TODO: shear forces for the same, !TODO: cost not-mcmaster, and acrylic, some composite like hydrostone w/ fiber
- ... composite vendors at fabclass website
Transmission Design
How Many kNs ?
We want lots of force, with very fine control of position. This means a nice linear transmission. To estimate the forces we might want to see, I wrote a quick table of forces required to rip apart ~ 3mm square (0.001mm^2) samples of a few materials.
| Material | Yield (MPA) | F at Break (N) |
|---|---|---|
| ABS | 40 | 360 |
| Nylon 6 | 70 | 630 |
| HDPE | 15 | 135 |
| 6061 ALU | 310 | 2790 |
| 4140 Stainless | 655 | 5895 |
| 6AL-4V Ti | 950 | 8550 |
Brinell hardness tests range from 10N through to 30kN (for steel and cast iron) but non-ferrous materials normally see 5kN only.
So, a ballpark of ~ 10kN would be ideal - this is a big number - off the bat I'm going to estimate that 5kN will be a more reasonable target. 1kN is enough for a complete set of plastics, but that's only allowing for a realtively small sample.
-> Ballscrews, Belt Rack and Pinion, Rack and Pinion
Generating kNs of force is no easy feat, especially when we want to do it very smoothly while displacing very small amounts.
I will start by mentioning that this is dead easy with ballscrews. With a 1605 ballscrew, (16mm diameter, 5mm per turn) and a NEMA23 with 3Nm of torque, we can generate about 3kN of linear force (per motor) - to land at 5kN total no problem.
However, these are somewhat cumbersome and expensive - and they land in fixed sizes. Towards more parametric machines, we can look at a rack and pinion type axis.
Because tooth geometry very sensitively affects linear-ness of drive, especially where (down below the mm) we will be driving an entire instron test-cycle inside of one tooth-phase, I want to discount a traditional rack and pinion right off.
I am curious about a belt-driven rack, similar to this design.
!TODO: compare by transmission ratios (abstract from motor) and cost of parts. !TODO: belt spec for hight (huge) load belts: tooth shear, and stiffnesses.
Motor Torques
To generate the force required, we're going to need some motor / transmission oomph. Here's a list of typical NEMA size motors, and the torques they can generate. The atkstepper can supply enough current to power any of these.
| Motor | kg | Nm |
|---|---|---|
| N23 56mm | 1.2 | 1.3 |
| N23 100mm | 1.8 | 3.0 |
| N34 68mm | 1.8 | 3.4 |
| N34 156mm | 5.4 | 13.0 |
| N42 150mm | 10.9 | 22.0 |
Racks and Pinions
We'll be using two linear stages (one on either side of the platform), so, from our N34 156mm motor with 13Nm of torque we'll need 26, 5.2 and 2.6mm lever arms respectively for 1, 5 and 10kN total force.
Considering practical limits on pinion diameter (with a shaft of 14mm, and 19 3mm-pitch teeth, we'll have an 18mm diameter pinion - 9mm lever arm) we will only realistically achieve 1.44kN of linear force per motor with direct-drive rack and pinion on a Nema 34 size motor. This makes a 3kN machine, but to add some safety factor we're at at 2kN goal with this approach.
The next step would be to check against tooth shear stress for 3mm pitch.
From reasonable WEDM (time) and Waterjet (washout) limits, I expect the thickness of any fabricated pinion to have a limit around 12mm. To make this all simple, I'll say 10mm. For the waterjet, this is a bit of a stretch (get it?)...
To guess at resolutions, we'll take our sample above in aluminium having a length of 100mm - with elongation at break being 12%, we're interested in a 12mm 'long' stress / strain curve. For 1000 pts on this curve, we're interested in a step size of 12 microns.
From our 18mm diameter pinion having a circumference of 56mm, this means 4667 counts per revolution. In a 200-step motor, we would need 32 microsteps per revolution - most drivers will go to 256 - but microstepping isn't exactly linear. To do this really well, we will want to finish work on closed loop stepper driving, where we can use a 14-bit encoder to control around ~ 4096 counts reliably.
All in, a direct rack and pinion drive can land us at a 2kN machine with some desired resolution, but we're at or near most of the limits here.
Belts
Since I was spinning this up to test on a belt-driven gantry system, I was able to confirm that this is a bad idea. We defeat the elastic stretch of the belt by measuring at the specimen (not through the machine's structural loop) but a meagre GT2 belt profile skips teeth pretty quickly as we approach any tens of kilos of load.
Ball or Leadscrews
Racks aside, a ballscrew is the obvious way to do this. Ballscrews can be had for less than hundereds of dollars. Besides transmitting motion smoothly, their ratios are favourable. For example, with a 5mm pitch ballscrew having an efficiency of 85%, we can drive 14kN with our 13Nm motor - so 28kN for the machine.
For the same ballscrew, to achieve 12 micron resolution we'll only need 416 steps in each rotation - this is easy to get.
Design Spec / Notes
The goal here is to design and build a machine, which can be fabricated in $250k size fab labs, that can generate stress-strain curves for a wide set of materials as well as perform hardness testing.
To ballpark, I'd like to see 100mm diameter plates having a total travel from 0mm separation -> 500mm, this leaves enough room for fixturing etc.
Sam's note: the ballscrews should be 'pulling' in all cases. Against their fixed, driven side. This is a good note, thank you sam.
Phoning Home
Samples are ~ < 1mm thick, and data from Materiom shows loads all falling well under 100N. IMO, the machine should be spec'd somewhere positive of 500N pulling force, so that we can reasonably pull apart big plastic samples. That said, 500N < 5kN.