The Displacement Exercise (DEX)

DEX is a small materials testing machine that should be capable of running tensile and compressive tests at up to 600N of force.

Operating Principle

A stress - strain plot is a very useful piece of information when characterizing materials.

To generate these curves, the DEX slowly pulls samples (normally some 'dogbone' shape below) apart, while measuring the amount that it stretches (~ the strain), and the amount of force it exerts as it is stretched (~ the stress). These types of machine are common in industry, commonly referred to by their leading brand name 'Instron', or as 'Universal Testing Machines'.

Hardware

The DEX is an open-source piece of materials testing equipment. The machine can be manufactured by anyone with access to a laser cutter with at least a 24x12" bed, and nearly any FDM 3D Printer. A bill of materials of purchased parts required to complete the machine is below, totaling some ~ $500 USD.

The machine is made largely from laser cut delrin and 3D printed (commodity FDM) parts.

More detailed documentation will follow.

CAD

CAD for the machine is available in this Repo, under cad/fusion - the .f3z file is a Fusion 360 parametric model of the machine. To source parts, consult the BOM below.

BOM

Part numbers are from McMaster Carr unless otherwise linked.

... choice of sheet material: mill w/ composite, or laser w/ acrylic or delrin,

Control and Interface

DEX runs a squidworks controller. The dex branches of cuttlefish and ponyo contain code that is known to work with the machine. Again, more documentation for these controllers is coming, for now - consult the repositories.

Comparison to Instron 4411

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.

Vision in the Loop

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.

vd

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.

Vision on the Machine

To get after this, next step here is to integrate a camera onto the DEX mechanically. The previous work taught me that (1) strong lighting is important, as is clarity of target-against-background. Some prep will be worth it. This is a fitting (haha) time to re-design the fixturing for the DEX, as the targets / camera / fixturing system will all need to agree with one another.

Testing Notes

The D683 ASTM Dogbones:

Roadmap

2019-10-17

One complete DEX exists in the MIT CBA shop, and has been validated against an Instron 4411. Current efforts are twofold:

(1) Working with the materiom project (whose mission: to provide open data on how to develop bio-inspired materials for a circular economy), and the fablab at cic, we are replicating the current design outside of the CBA shop, and building documentation while we do.

(2) To improve the machine's test accuracy, we are integrating CV processing into the control architecture, to use subpixel tracking. Further controller improvements from the squidworks project will also bring rate control on-board.

Notes

- Where to build logs go? This repo -> gitlab, mtm.cba, etc.
- Future integration of Temperature and Humidity Sensors
- Higher quality integration of load-cell amplifier
- Developing control for Cyclic Loading, Fatigue, etc.
- Developing direct upload / websocket-to-database tools
- Dec. 5: controllers -> CIC and London
- Cuttlefish: manipulation, understanding, computing on test datasets in the browser.

TODO

- encapsulate branches for ponyo, cuttlefish.
- bs ?
- put rpi setup back on desk with dex, branch, close
- revisit with cameras, rate control