Mechanical System Overview
The guitar player is composed of two mechanical systems that interact to play a range of 29 musical notes. A plucking mechanism with six independently controlled picks is mounted over the body of the guitar and a fingering mechanism with an array of 23 fingertips is mounted over the first four frets of the fingerboard.
Figure 1: Overall view of the player guitar showing the guitar mounted on the electrical enclosure and the plucking and fingering mechanisms mounted above the guitar.
Guitar Mounting and Electrical Enclosure
The guitar is mounted horizontally with the strings parallel to the mounting surface and is supported at the end of the neck and at two points on the upper part of the guitar body (see Figure 1 above). The guitar rests on rubber isolation pads and is clamped in place by small adjustable toggle clamps. This mounting arrangement is secure and offers easy adjustment of the guitar position relative to the actuation mechanisms. The rubber mounts help isolate the guitar vibration from the rest of the system where vibration could produce unwanted noise. The guitar mounting board forms the top plate of the electrical enclosure that houses the control and power electronics for the guitar player. The enclosure has two fixed panels on its short ends and two removable panels with magnetic latches on its long ends. The front panel holds the main power switch, reset button, and vent grating for cooling. The rear panel holds the power receptacle, MIDI receptacle and the cooling fan. The removable side panels allow easy access to the control and power circuitry. Figure 2 shows the front, rear and open side panels of the enclosure.
Figure 2: The electronics that control and power the guitar player are protected in a wood enclosure on top of which the guitar is mounted. The removable side panels allow access to the electronics and the front and rear panels hold the switches and the power and MIDI receptacles.
The plucking mechanism mimics the action of a harpsichord. In a harpsichord, the pick is driven upward by the force of the player striking a key on the keyboard. The pick then strikes the string and causes it to vibrate. When the key is released, the pick passes by the string to its original position without re-plucking the string. The guitar plucking mechanism uses a similar plucking action, but was design for double-acting use, meaning that the string is plucked both on the up-stoke and down-stroke. As shown in Figure 3, the six picks are mounted on linear bearings that are actuated by two opposed solenoids through a rocker-arm assembly.
Figure 3: Views of the plucking mechanism showing the double-acting rocker-arm assembly actuated by two opposed solenoids. The rocker arms move small Teflon picks vertically to vibrate the strings.
By toggling the opposed solenoids between their up and down states, the rocker arms force the picks past the guitar strings, causing the strings to vibrate. The rocker arms are stopped at the end of their travel by two opposed push-button switches. These spring-loaded switches provide much quieter operation than solenoids alone. Because the plucking mechanism is mounted vertically, the actuators must be kept latched during operation to prevent the pick from dropping onto the strings and causing unwanted damping. The solenoids must be kept on continuously to provide this latching force, so the switches also serve the important function of dropping the latching current through the solenoids to a level that is acceptable for continuous duty. This important function is discussed further in the electronics section.
Finger Board Operation
An array of 23 small plastic fingertips press the strings down to the frets to play the range of notes covered by the first four frets of the guitar. Each fingertip is mounted on a linear bearing, which is actuated by a solenoid through a horizontal lever arm (see Figure 4).
Figure 4: Views of the fingering mechanism showing the lever arms actuated by an array of solenoids to press 23 small fingertips against the fingerboard of the guitar.
Small compression springs return the fingertips to their resting position above the strings where they do not interfere with the vibration of the strings. The compactness of the array of fingertips poses significant challenges for actuation. The lever arms were mounted at an angle in order to reach the center strings and the solenoid mounts were staggered vertically to pack them together as compactly as possible. The linkage between the solenoid and lever arm was made with a threaded rod and clevis joint on the lever arm to provide easy mechanical adjustment in the height of the fingertips and the throw of the solenoid. This fine adjustment is necessary to ensure that the fingertips provide adequate holding force when actuated and clearance of the strings in the off state.