One way to mill boards from a tree is with a chainsaw mill which pulls a chain saw through a large piece of wood.

The chainsaw is typically moved by hand and the user slows down and speeds up depending on how hard it is to cut the wood, which precludes attaching a constant speed drive to move the saw.

Recording the sound of the saw and analyzing its frequency content might be able to be done acoustically, and the chainsaw motion controlled  accordingly – perhaps a good E90.