Milwaukee Servoelectric Guitar is an ingenious experimental instrument. Individual servos for each string control the string tension, with a clever compensator wheel arrangement. The servos are powered by low cost home made amplifiers. The tension / frequency sensing circuit is a novel design incorporating a potentiometer and springs. Solenoids to pick and damp the strings would fully automate this instrument.
The project page is full of information, and contains enough detail to allow you to make your own servo guitar. The Lemur Guitar Robot uses a simpler design with servo positioned slides on each string, with solenoid to picks and dampers. But the Milwaukee design allows you play the guitar by hand at the same time. Incidentally, Gibson's dark fire autotuning guitar robot accomplishes a similar thing, using servo powered string tension control to autotune the guitar. Link: Milwaukee Servoelectric Guitar project page