Release velocity - linear or exponential?

Who knows... in the real piano the dampers are not free, their position is fixed by the position of keys.
I own a digital piano without MIDI note off speed implementation, using hall sensors, small magnets, and a teensy board (something like arduino) tried to make a homemade midi keyboard, the only thing I can tell you is that if MIDI release velocity is set as 1/(key return speed) results are awful.