Electromagnetic String Actuation

Electromagnetic String Actuation

Electromagnetic actuators directly induce the strings to vibration, bypassing the piano's percussive hammer mechanism. One solenoid electromagnet is mounted above each note of the instrument, approximately 1/8" above the strings. (48 notes are covered in the present prototype, easily expanded to 88.) The electromagnetic actuators work on the principle of ferromagnetism, in which a magnetic field exerts a force on the ferromagnetic (steel) piano strings.

Left: Complete MRP system, showing amplifier at bottom and cables running to actuators inside instrument. Right: Electromagnetic actuators above the piano strings.

The generated magnetic field is directly proportional to the current through the actuator. By modulating the current at the natural frequency of the string (or one of its harmonics), the string is induced to vibration without any physical contact.

The actuator system can be installed in any acoustic grand piano. Because tuning varies slightly among instruments, a pickup is placed on the piano soundboard which measures the combined vibrations of every string. A digital phase-locked loop control system keeps the waveform through each actuator aligned in frequency and phase with the natural motion of the corresponding string, reinforcing and amplifying the string's vibration. The sound of the MRP is produced acoustically by the vibration of strings and soundboard; no external speakers are used.

Phase-locked loop system, implemented digitally for each note. The output is frequency-locked with the input, with a fixed phase offset determined by the control input Δφ.

Actuator waveforms are generated by a computer which responds to the actions of a human performer. A specialized amplifier and signal-routing system allows up to 88 notes to be controlled using many fewer audio output channels. For example, a six-channel audio interface will allow up to six simultaneous notes, which can be played on any of the piano strings. Modifying the amplitude, frequency and spectrum of each actuator waveform allows continuous shaping of the dynamics and timbre of the corresponding piano note.

Amplifier and signal routing system. One amplifier channel is used for each actuator, but any (smaller) number of audio output channels can be used.