Instrument maker Frank Piesik has built a three-string robotic music machine called Greg's Harp where three different actuators excite the strings in different ways to produce sound controlled by a MIDI keyboard.
Along each string are four of motorized frets called moFrets that alter pitch and represent five semi-tones per string. The strings are tuned to major thirds, and have one tone that overlaps so that string one and two share a tone and string two and three share a tone. Each moFret has a dedicated servo and driver, and an algorithm analyses incoming notes and translates them into something that the moFret controller can make playable.
![Three ways to play: strings can be struck, slapped or sustained](https://assets.newatlas.com/dims4/default/c7a3124/2147483647/strip/true/crop/3000x2000+0+0/resize/1440x960!/quality/90/?url=http%3A%2F%2Fnewatlas-brightspot.s3.amazonaws.com%2Fd6%2F7f%2F51f9d0da42328e586031de5d05dc%2Fdsc0815439164586986113247420..jpg)
The strings are excited in three different ways. First, a solenoid strikes the string from below, a set of small motors rocking tape propellers strike the strings when prompted for the second method, and finally an ebow-like actuator provides infinite sustain.
Piesik opted to use piezo pickups connected to a preamp circuit at the bridge for signal output – so that any electromagnetic noise is not registered by the setup – and two Arduino Nano microcontrollers drive the 12 servos, actuators and motors. Meanwhile, a Teensy 3.2 with audio shields interprets incoming MIDI signals from an external keyboard and takes care of audio processing.
It's an interesting build created for a friend that sounds as good as it looks, as you can see in the video below.
Source: FranklyBuilt