The current Milwaukee Guitar design uses an off-board computer (a desktop PC) to control the servomotors. A standard USB number pad is connected to the computer which then provides output signals through a Velleman K8055 USB experiment interface board to the servo circuits. This K8055 board provides eight digital outputs and two analog outputs. In order to get three analog voltages, a sample and hold circuit using a CMOS CD4066 quad bilateral switch and two LF 353 high input impedance operational amplifiers was constructed. (schematic)
The KE 8055 was controlled with C program written in Borland C (available free from Borland) executing in the command window on a Microsoft Windows operating system. The program permits tuning of the guitar one string at a time and provides an arbitrary mapping of keys of the keyboard to combinations of string tensions.
A schematic and the current C program will follow.
Ultimately I would like to have an on-Board computer, for example based on the Pic controller integrated circuit line.