The notes here are being sequenced automatically by Cakewalk playing a midi file and directing the melody to the servo-string through a $5 USB/Midi cable and directing the harmony, bass and percussion through the computer sound card. I also tried serveral rock standards which worked well, but I wasn't sure how much, on a video, would be fair use.
This design uses two Arduios, a first Arduino that receives MIDI data and interprets it to a note within the one octave obtainable by the string. The second Arduio converts the note number to a tension of the string, allows tuning of the string (adjusting a table that converts note to string tension) and handles the control algorithm for the servo motors. For a four string design, three Arduinos would be used, one for MIDI decoding and two each handling two strings. The servo amplifier is the same as used in previous designs.
The second Arduino also provides autotuning to slowly correct any string drift over time. The autotuning uses a variation of autocorrelation called average magnitude difference to detect string tuning opportunistically between notes. Thanks to Keith McMillen of Keith McMillen Instruments for this suggestion. To make this work, the Arduion timers need to be reset which, in part, is why the midi functions are moved to another machine which can use the Arduino serial library for simple Midi communication.
An important mechanical innovation in this design is the use of a crank arm and lever which is both more compact and has bounded motion when the servo loop fails for fewer broken strings.
As with previous designs, the elements are mounted on a one inch square steel tube for rigidity and simplicity of fabrication
A linear slide potentiometer is used to sense string tension as a function of the stretching of a small spring in series with the guitar string. This spring decreases sensitivity to guitar neck flexure and temperature effects.
The bridge must has some mass to provide good sustain, but must allow the string to move about 3/16 inches. An aluminum wheel with a groove provides both.
A repurposed relay coil with a strong magnet tuck on the side is used to sense single string vibrations for autotuning.
The MIDI input incorporates an operational amplifier that would probably not be needed in many applications. It is used here so that the USB connector for the boot loader can remain attached. The USB cable injects current into the Rx pin (like a 1 kohm pull-up resistor) which the op amp can sink.
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