Automatic tone control for stringed musical instruments

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS With reference to the Drawing, and initially to FIG.
1, an automatic string tension control arrangement 10 is illustrated in connection with one end of a vibrating musical string 12, such as a guitar string.
The other end of the string 12 is wrapped onto a conventional tuning peg 14, which is adjusted initially to tension the string 12 until its open or unfingered musical pitch is at a desired frequency.
A tone arm 16 is rockably mounted about a pivot pin 18 onto the musical instrument.
There is a hook 20 at a first end of the tone arm 16 into which one end of the spring 12 is received.
A bias spring 22 is coupled by means of an attachment 24 to the tone arm 16 on the side opposite the hook 20.
The spring 22 is also coupled to a bias anchor 26 affixed onto the musical instrument to hold the spring 22 under tension that opposes the tension in the string 12.
An adjustment nut 28 and an associated adjustment screw 30 permit adjustment of the spring tension by the musician.
In this case, the lever arms from the pivot pin 18 to the hook 20 and from the pivot pin 18 to the bias spring attachment 24 is a ratio of 1:1 to 3:1.
The tone arm 16 continues beyond the spring attachment 24 to a distal end at which there is attached a shape memory alloy wire traction motor 32.
This motor 32 is formed of a shape memory alloy wire of small diameter formed into one or more strands 34.
In this case, the wire is bent into two strands and extends from an anchor block 36 that is fixed with respect to the instrument, over a pulley 38 mounted on the end of the tone arm 16, and back to the anchor block 36.
A strain gauge 40 is mounted onto a reduced thickness zone 42 on the tone arm 16, this zone being between the pulley 38 and the bias spring attachment 34.
The reduced thickness zone 42 is more flexible than the remainder of the arm 16, and thus the strain gauge 40 is particularly sensitive to changes in the opposing tension forces applied by the string 12 and by the traction motor 32