Harnett Lab - Page 14 of 22 - At the University of Louisville J. B. Speed School of Engineering

New grant: Combining Soft Materials with Mechanical Parts

The lab has a new grant from the Kentucky Science and Engineering Foundation: “Combining Soft Materials with Mechanical Parts for Robotic and Human Health Applications.” We will install functional fibers in laser-cut and 3D printed parts using a modified sewing machine. Above: video of the current machine installing high strength Kevlar fiber in a plastic sheet, a process that we will develop to work with thicker fibers in the funded work. In related news, I hit the road with the embroidery machine this summer. The video above shows it stitching a design by Steve Ceron in the Organic Robotics Lab, directed by Rob Shepherd at Cornell. Such fibers are often used to control the expansion of robotic actuators, for example wrapping inflatable soft robotic “fingers” to make them bend instead of puff up. With the support from KSEF, we should be able to do more with these stiff fibers and also soft, stretchy and fuzzy materials–including some newly developed threads from the summer that are pushing the limits of the machine we have now.

Below: Kevlar fiber couched to a plastic sheet, in one of Steve’s layouts.

Below: polymer fibers under development; functional fibers spooled up and ready for stitching.

Snap to it

Our lab has a new paper out in collaboration with the Berfield group about a windowpane-shaped microstructure that has two stable shapes. Tom Lucas (who graduated with a Ph.D. from our group in 2014) and Jaz pointed a heat gun at the MEMS devices one day, trying to flatten them by thermal expansion. This led to the discovery that the structure would flip from one to the other by aiming compressed air at it. Thanks to these experiments and Dan Porter’s model, we know that larger devices flip at lower airflow velocities.

The best applications for these bistable microelectromechanical (MEMS) devices are situations where power is limited, because they don’t need power to hold their shape. For this reason, bistable devices are candidates for radiofrequency (RF) switches in mobile phone applications, where it’s important to conserve the battery. The difference between this windowpane device and most other bistable devices is that the windowpane curls out of the plane of the silicon wafer due to residual stress. Usually MEMS makers try to cancel out residual stress, which is the enemy of predictable in-plane motion. Also different: the device in this paper would probably still be bistable if completely removed from the wafer.

The windowpane-shaped device in the paper could be used as a passive marker for peak airflows. Our group has also switched bistable devices using electric currents. With electrical contacts, it could potentially make a low-power electronic switch using significantly less wafer area than a planar device.

Strings attached

Whether it’s superstrings in physics or the first violin in a symphony orchestra, strings run the universe. Invisible strings control everything from creepy marionettes to the direction of the global economy. Without them, we would lack conduits for mechanical forces and fodder for cheesy metaphors.

Strings. They form the fabric of human society and the clothing we all hope you’re wearing right now.

When it comes to engineering, how can strings help? There are plenty of cable-driven and articulated designs where automated string installation would greatly speed up the build. Conductive, shrinking, and optical fibers add functions desktop 3D printers are not yet able to provide. These are the reasons we have been working on methods to insert strings and fibers into 3D printed and laser cut parts.

Manual string installation from YouTube assembly videos of articulated 3D printed parts

(a) Fold-flat bike helmet assembly https://youtu.be/DVzoognroCY?t=28s

(b) eNable prosthetic hand assembly https://youtu.be/5HVwC3RnWXk?t=46m22s

(c) Articulated dragon model assembly https://youtu.be/pEerHkxMN2w?t=9m22s

How about some ideas from nature? Tendons come to mind, but here is something weirder: the mysterious extinct animal Dinomischus of the Burgess Shale used strings at its core. Were they muscle fibers? Intriguing but unlikely, say paleontologists. Was their only function to keep the stomach in place, just like bungees keep a zorber centered? And how did they grow? Who knows for sure. Only three Dinomischus fossils have been found.

Screen Shot 2016-05-28 at 3.18.14 PM

The strings or “suspensory fibers” are labeled “Sus. Fb.” in this image from

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This semester’s ECE 412 projects are on another level

April 25 was demo day in the ECE 412 (Embedded Systems) course I taught this semester, and we had the biggest batch ever. 17 teams presented projects ranging from musical instruments to games to wheeled robots.

Clockwise from top left: Skittles sorter, guitar auto-strummer, disturbing metal creature probably found in the depths of LVL1, and capacitive-touch LED-equipped piano.

This spring’s project quality led to mild swearing from the instructor. Students had about three weeks to do these projects and many were working in lab on the weekend before (or early morning of) the 8am Monday demo. New additions for spring 2016 included TWI/I2C code from Eugene Rockey that will enable students to add powerful sensors in future years. We benefited from fantastic TA’s (Eugene, plus Troy Kremer), Ben Douglas on grading/lab, Tom Carroll’s parts procurement, the growing base of previous code for the A3BUs donated by Atmel, the nearby resources of FirstBuild, the Engineering Garage, and LVL1, and possibly the fact that the 17 teams were each named after delicious local restaurants.

Stringing some wires

We have been dealing with bistable structures across different size scales. A common question is, how can we detect their state electronically. This project uses machine-sewable conductive thread to add an electronic switch to bendable compliant beams in a cm-scale structure. The beam material is 0.125 mm thick plastic film: thick enough to have some “snap” (video here) yet thin enough that a sewing needle easily punches through. In figure (a), silver-plated nylon conductive thread is suspended across a U-shaped cutout that has already had a conductive thread sewn down the middle. When the beam is down, the cutout moves away from the suspended threads, opening the circuit, and when it’s up, the threads are in contact. We can detect the beam position by connecting the switch to a digital input as shown in figure (b), it’s very crisp. In figure (c), there are a few different length beams that snap at different angles (paper here). The embroidered pattern needed to be aligned to these laser-cut beams, and to do that a pair of “+” shaped alignment marks helped line up the needle. Those marks can be seen on the middle beam, except the left one is stitched over so it’s hidden. This flexible design is a big improvement over a previous iteration (video here) where we had soldered on switches that kept flying off.