MIDI Self-playing Guitar (Part 2)

This is a continuation of the description of the design of my MIDI guitar. In the first part I explained how I handled the picking of the strings. Here I will explain the process I went through to handle fretting.

I had used solenoids on the organ and thought that would be a good idea for the guitar as well. Space is tight on the neck of a guitar but my plan was to use levers to transfer the motion from the solenoids into the tight confines of the fret board. Here are  pictures of a prototype.

Another advantage of the levers was that I could trade-off stroke length and force by moving the pivot point of the lever. Unfortunately my solenoids didn't have enough inherent stroke and power to both firmly press the string against the fret as well as withdraw far enough such that they did not interfere with the vibrating string. I tried many different pivot positions but none proved to be dependable enough to satisfy me.






While mulling this problem I happened to think of player pianos. They need a long powerful stroke, especially on the lower keys. And they trigger those key presses via a tiny amount of air drawn through holes in a paper roll.  I didn't know how they worked so once again I matriculated at the University of YouTube.





I found that they used pneumatics designed like little bellows. Here are pictures of the bottom and top of one I made to experiment with. In the first picture the large hole on the right goes through to the inside of the pneumatic. When that hole is connected to a vacuum pump it sucks the top of the device down. However, I needed something to open it back up when the vacuum was removed. I thought about various types of springs, both internal and external. Ultimately, though, I decided to use rare earth magnets. That's what the large blind holes on bottom and top are for. I glued magnets into each of those, with common poles facing each other so they repelled. That seemed to do the trick.

One of the advantage of these pneumatics is that their throw and force can be adjusted by simply changing their size. I made mine about  four inches long and one inch wide, which I think is a typical size for pianos. The purple stuff is a synthetic bellows cloth made specifically for player pianos.
It is pretty air-tight, cuts and glues easily, and is thin and flexible enough to fold well. The small hole on the top is to bleed air back into the pneumatic allowing it to expand when the vacuum is no longer connected. The small holes on the bottom are just screw holes drilled for various prototype configurations.


But how do you connect and disconnect the vacuum you might ask. I thought I would use these solenoid operated valves that I got from Adafruit.com for another project that has not yet come to fruition (no pun intended). In player pianos they often use these beautiful little valves that are themselves operated by pneumatics, triggered by the air flowing through the little hole in the paper of the piano roll, but I would have had to used little solenoids, which I didn't have, and which would have cost almost as much as the valves, which I did have. This decision led to a completed device that more or less dwarfs the guitar itself, but more on that later.

Next I had to figure out how the pneumatics would actually push the strings down onto the frets.

I thought a good idea would be to drill holes in the fret board and mount the pneumatics under the neck of the guitar. I then connected a wire and ran it up through the hole and put a little 90-degree bend in the end. To that bend I added a little 3d printed plastic button. Those buttons were kind of noisy so I added a wrap of leather around the button. Good idea, huh? Well, not so much. This scheme had the advantage that as it played it was easy to observe the fretting of the strings. The major disadvantage was that things were so packed together that it was very difficult to fit the wires, and there wasn't room to squeeze in adjusters for the wire lengths. The length of the wire is critical because the button must rise high enough when that pneumatic is not activated so the string doesn't buzz against it, yet pull down far enough to press the string firmly against the fret when the pneumatic is triggered. I spent approximately 142 man-years trying to get them all adjusted, finally considering using a sledge hammer as an adjustment tool, and  ultimately falling back to re-group.

OK, that's it for this episode. In the next one I'll describe my perhaps ungainly but more or less successful solution. Thanks for your interest.