Unique Musical Instrument Defies Description

Since the first of our ancestors discovered that banging a stick on a hollow log makes a jolly sound, we hominids have been finding new and unusual ways to make music. We haven’t come close to tapping out the potential for novel instruments, but then again it’s not every day that we come across a unique instrument and a new sound, as is the case with this string-plucking robot harp.

Named “Greg’s Harp” after builder [Frank Piesik]’s friend [Gregor], this three-stringed instrument almost defies classification. It’s sort of like a harp, but different, and sort of like an electric guitar, but not quite. Each steel string has three different ways to be played: what [Frank] calls “KickUps”, which are solenoids that strike the strings; an “eBow” coil stimulator; and a small motor with plastic plectra that pluck the strings. Each creates a unique sound at the fundamental frequency of the string, while servo-controlled hoops around each string serve as a robotic fretboard to change the notes. Sound is picked up by piezo transducers, and everything is controlled by a pair of Nanos and a Teensy, which takes care of MIDI duties.

Check out the video below and see if you find the sound both familiar and completely new. We’ve been featuring unique instruments builds forever, from not-quite-violins to self-playing kalimbas to the Theremincello, but we still find this one enchanting.

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Table Held Up By Strings Teaches Physics

If you’ve never heard of a tensegrity structure, you should stop now and watch the video below. In it, [The Action Lab] shows a 3D printed table that is held up only with strings. We didn’t say suspended by strings but held up. Or so it appears. The model is from Thingiverse, but it is one of those things you have to see to believe.

The basic idea is pretty simple. Strings have a lot of tensile strength but collapse under the slightest compressive force. The arrangement of strings puts the force on the center string which is essentially hanging — the force is pulling the string down. The other three strings aren’t just for show, though, they keep the structure from tipping over in any one direction.

There are actually real-life examples of these kinds of structures. The video shows the Skylon at the Festival of Britain as one example and an Australian bridge. The video also makes the point that the human body uses tensegrity, since tendons are very similar to the strings in the model.

This would be a great experiment for a homeschooler or even kids cooped up in quarantine. The print itself doesn’t look very challenging, although the assembly might be a bit tricky.

This isn’t the first structure like this that we’ve seen. If the talk about tendons makes you think this might be useful in robotics, you’d be correct.

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Automatic Rewinder Makes Kite Retrieval A Breeze

So you’ve built a fine kite, taken it to the beach, and let it ride the wind aloft on a spool of line. Eventually it has to come down, and the process of reeling all that line that was so easily paid out is likely a bigger chore than you care to face. What to do?

If you’re like [Matt Bilsky], the answer is simple: build a motorized kite reel to bring it back in painlessly. Of course what’s simple in conception is often difficult to execute, and as the second video below shows, [Matt] went through an extensive design and prototype phase before starting to create parts. Basic questions had to be answered, such as how much torque would be needed to reel in the kite, and what were the dimensions of a standard kite string reel. With that information and a cardboard prototype in hand, the guts of a cordless drill joined a bunch of 3D-printed parts to form the running gear. We really liked the research that went into the self-reversing screw used to evenly wind the string across the spool; who knew that someone could do a doctoral dissertation on yarn-winding? Check out the “Reeler-Inner” in action in the first, much shorter video below.

With some extra power left from the original drill battery, [Matt] feature-crept a bit with the USB charger port and voltmeter, but who can blame him? Personally, we’d have included a counter to keep track of how much line is fed out; something like this printer filament counter might work, as long as you can keep the sand out of it.

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Polar Platform Spins Out Intricate String Art Portraits

We have semi-fond memories of string art from our grade school art class days. We recall liking the part where we all banged nails into a board, but that bit with wrapping the thread around the nails got a bit tedious. This CNC string art machine elevates the art form far above the grammar school level without all the tedium.

Inspired by a string art maker we recently feature, [Bart Dring] decided to tackle the problem without using an industrial robot to dispense the thread. Using design elements from his recent coaster-creating polar plotter, he built a large, rotating platform flanked by a thread handling mechanism. The platform rotates the circular “canvas” for the portrait, ringed with closely spaced nails, following G-code generated offline. A combination of in and out motion of the arm and slight rotation of the platform wraps the thread around each nail, while rotating the platform pays the thread out to the next nail. Angled nails cause the thread to find its own level naturally, so no Z-axis is needed. The video below shows a brief glimpse of an additional tool that seems to coax the threads down, too. Mercifully, [Bart] included a second fixture to drill the hundreds of angled holes needed; the nails appear to be inserted manually, but we can think of a few fixes for that.

We really like this machine, both in terms of [Bart]’s usual high build-quality standards and for the unique art it creates. He mentions several upgrades before he releases the build files, but we think it’s pretty amazing as is.

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Hurdy-Gurdy Gets Modernized With MIDI Upgrades

If you’ve never heard a hurdy-gurdy before, you’re in for a treat. Not many people have, since they’re instruments which are uncommon outside of some eastern European communities. Think of a violin that replaces the bow with a hand-cranked wheel, and adds some extra strings that function similar to drones on a bagpipe. The instrument has been around for hundreds of years, but now it’s been given an upgrade via the magic of MIDI.

All of these new features come from [Barnaby Walters] who builds hurdy-gurdys by hand but has recently been focusing on his MIDI interface. The interface can do pitch-shifting polyphony, which allows the instrument to make its own chords and harmonies. It also has a hybrid poly synthesizer, which plays completely different sounds, and can layer them on top of one another. It can also split the keyboard into two instruments, where the top half plays one sound and the bottom half another. It’s an interesting take on an interesting instrument, and the video is definitely worth a look.

The hurdy-gurdy isn’t a commonly used instrument for hacking compared to something like drums or the violin, of course. In fact we had to go back over ten years to find any other articles featuring the hurdy-gurdy, the Furby Gurdy. It was an appropriately named instrument.

Thanks to [baldpower] for the tip!

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One String, One Print, One Harp

To exclude musical instruments in the overflowing library of possibility that 3D printing enables would be a disservice to makers and musicians everywhere. For the minds over at [Makefast Workshop], an experimental idea took shape: a single stringed harp.

The TuneFast Harp needed enough notes for a full octave, robust enough to handle the tension of the string, a single tuning mechanism and small enough to print. But how to produce multiple notes on a harp out of only one string? V-grooved bearings to the rescue! The string zig-zags around the bearings acting as endpoints that rotate as its tuned, while the rigid PLA printing filament resists deforming under tension.

After a bit of math and numerous iterations — ranging from complete reconfigurations of part placements to versions using sliding pick mechanisms using magnets! — a melodic result!

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Twisted String Actuators

[Travis] tells us about a neat actuator concept that’s as old as dirt. It’s capable of lifting 7kg when powered by a pager motor, and the only real component is a piece of string.

The concept behind the twisted string actuator, as it’s known to academia, is as simple as putting a motor on one end of a piece of string, tying the other end off to a load, and putting a few twists in the string. It’s an amazingly simple concept that has been known and used for thousands of years: ballistas and bow-string fire starters use the same theory.

Although the concept of a twisted string actuator is intuitively known by anyone over the age of six, there aren’t many studies and even fewer projects that use this extremely high gear ratio, low power, and very cheap form of linear motion. A study from 2012 (PDF) put some empirical data behind this simple device. The takeaway from this study is that tension on the string doesn’t matter, and more strands or larger diameter strands means the actuator shrinks with a fewer number of turns. Fewer strands and smaller diameter strands take more turns to shrink to the same length.

As for useful applications of these twisted string actuators, there are a few projects that have used these systems in anthropomorphic hands and elbows. No surprise there, really; strings don’t take up much space, and they work just like muscles and tendons do in the human body.

Thanks [ar0cketman] for the link.