StrumBot: A Strumming Robotic Guitar

Abstract

StrumBot is a novel guitar robot built as a single instrument capable of moving capable of playing most western songs. Mechanisms are designed to enable musical expressivity and minimise acoustic noise. It is desirable for less than 60 dBA of noise at 1 m to be emitted to allow StrumBot to play in intimate venues such as cafés or restaurants without loud motor noises detracting from the musical experience. During typical operation, acoustic noise levels of around 56 dBA are observed.

A parallel selective compliance articulated robotic arm is designed to strum six guitar strings in a similar manner to a human guitarist. Using a robotic arm allows the pick’s reach to be adjusted, with the resulting pluck’s timbre being controllable with a centroid frequency range of ±18%. The pick angle is servo controlled, allowing for the RMS pluck power to be adjusted with a range of 20.1 dB. The arm is capable of strumming at 9.3 strums per second.

Each guitar string has a servo controlled pitch shifter capable of moving one octave in 144 ms. To enable vibrato and pitch bends, every pitch shifter has a servo controlled variable pressure clamp which can adjust a string’s pitch up to two semitones without moving the pitch shifter’s location. The clamps are also capable of muting the guitar string, controlling the sustain time of a pluck between 0.2 and 1.5 seconds, emulating palm muting techniques.

A MIDI input allows commercial or custom controllers to operate StrumBot. Novel note allocation algorithms were created to allow a single MIDI stream of notes to be allocated across the six guitar strings. Latency measurements from MIDI input to string pluck are as low as 40 ms for a best case scenario strum, allowing StrumBot to accompany a live musician.

A relay based loop switcher is incorporated, allowing StrumBot to activate standard commercial guitar pedals based on a MIDI instruction. A human machine interface is also included to allow for diagnostics and calibration without needing to connect StrumBot to a PC for programming. Commercial guitar pickups are used to translate the guitar string’s vibrations into an electrical signal capable of plugging into a standard guitar amplifier.

StrumBot improves upon previous RMI’s by allowing additional expressive opportunities for a composer to utilise. StrumBot can perform slides, vibrato, muting techniques, pitch bends, pluck power variances, timbre control, complex chords and fast strumming patterns and can play in close proximity to people without causing fatigue or annoyance from mechanical noise.