All mechanical elements were designed with flexibility in mind. We wanted to ensure the device could be used to tune any guitar. Thus we needed a way to rigidly attach a motor to a string's peg in any number of locations. We also needed to be able to produce enough torque to turn the peg.
To attach the motor to the peg, we designed fixturing which consisted of a platform to hold the guitar and a series of tracks or slides to adjust the motor into position. An adapter attaches the motor to the peg of the guitar. The final fixturing that was produced offers adjustments in three linear degrees of freedom. To do this, the fixture uses three attached slides, each of which can independently adjust in an x, y, or z direction. Thus we are able to attach to a peg in any spot in the range of our slides. The whole motor fixture can be removed and attached on the opposite large track to access pegs on both side of the guitar head.
The motor needed to be able produce about 3 in-lbs of torque at a relatively slow speed. Also, no motor position feedback was necessary. Thus a DC Gearmotor was selected to turn the peg. The specific motor used was a Pittman GM9000 series gearmotor producing approximately 4 in-lbs at 60 rpm and 12V (gear ratio was 63:1). A flex coupling attached the adapter to the motor shaft to complete the fixture.
A labeled picture (top view) of the fixturing is shown below:
(Another view with tracks visible)
(View with guitar in place)
Solid Model Picture of Fixture
Drawing of Base
Drawing of X-Track
Drawing of Y-Track
Drawing of Motor Plate (Z-Track)
Drawing of Slide Platform