i found out more when looking for vaibration midi sensors.
look at the STRIMIDILATOR pdf on “new interfaces for musical expression” convention page.
the problem with this design is that he uses to diffrent sensors to “snese” the string values.
one is a simple linear transducer the problem with this
transducer is
Connecting the linear transducer however influences the
behavior of the string. Initially the linear transducers also
contained a spring inside, which caused the transducer to
pull at the string. As this was an undesired effect, I removed
this spring from the transducer. Even with the spring removed,
the transducer still influences the string in such a
way that the string can no longer vibrate freely.
this si unacceptable for the harp losses its acoustic abilities.
as for the vibration sensor this design used
For detecting the vibration of the string, another solution
had to be found. I chose for a conventional way of translating
vibrations into an electronic signal: a coil that one can find
on any electric guitar.
http://www.music.mcgill.ca/musictech/nime/onlineproceedings/TOC.html -the convention
http://www.music.mcgill.ca/musictech/nime/onlineproceedings/Papers/NIME03_Baalman.pdf -direct link to the pdf
http://www.nescivi.de/research/nmi.html -research page (other intresting stuff)