Milwaukee Servoelectric Guitar Project

Theory (original design)

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Control System

block diagram of servoguitar functional elements

Note (pitch) selection for the Milwaukee-design servoelectric guitar is by means of a keyboard providing electrical signals to a computer. The computer converts the keyboard signals, indicating a pressed key, to three note signals based on a pre-programmed "palette". For example, the palette may provide for different ones of a set of major, minor, or seventh chords assigned to individual keys arranged in the two dimensions of the keyboard.

The computer converts the note signals, one for each string, into a tension signals that will be provided to the servo motors for tensioning the strings. Each of the tension signals is then corrected by a tuning factor that is updated at periodic intervals when the musician enters a tuning mode to tune the guitar. In the tuning mode, the keyboard allows individual strings to be selected and allows sharpening or flattening of the notes of the strings using the "plus" or "minus" keys.

The tension signals are provided to a digital to analog converter (D/A) which provides three command voltages to three different closed loop servo systems controlling the tension of one individual string. For clarity only one closed loop servo system is shown. Each command signal is provided to a summing node which sums the command signal with a feedback signal from the potentiometer (the latter which is generally inverted to provide "negative" feedback). The result of this sum, termed the "error voltage" is provided to a high gain amplifier which connects directly to the motor whose motion control is tension in the string.

Generally the system works as follows: assume that the string is at a tension value of 23 (indicated by the potentiometer) and that a command tension of 14 is provided from the D/A converter. At the summing node, an error voltage of 23-14=9 is provided to the high gain amplifier. The high gain amplifier multiplies the error voltage by a high gain factor (for example hundred) producing an effective voltage related to the limits of the power supply (typically 12 V). The motor rapidly reduces the tension of the string until the potentiometer value falls to 14 upon which the error voltage drops to zero and the motor stops.

If the motor overshoots slightly (which it can at high gains) the string may be slightly under tensioned, for example to 13. This produces an error voltage of -1 which, when amplified by the amplifier, becomes a high negative voltage value, for example -12 V, causing the motor to rapidly reverse direction to increase the tension of the string. When the gain is properly set, this oscillation dies out quickly with the string ending at a final value very close to 14.

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