Robotic string exciter makes for polyphonic music-making fun

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
Three ways to play: strings can be struck, slapped or sustained


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.

Greg’s Harp

Source: FranklyBuilt

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