Patent ID: 8521353

Claim:
A method for monitoring the performance of a subsystem or function of a forest machine, comprising the steps of: performing processing of several tree trunks by a harvester head connected to the forest machine, and in the forest machine: taking measurements during the processing of each trunk to find out a trunk size class specific to the trunk being processed, collecting, during the processing of said trunk, measurement data associated with said trunk and relating to operation of a subsystem or performing of a function, continually collecting said measurement data for several trunks during the processing of said several trunks, making a division of said collected measurement data into several trunk size classes to compensate for processing conditions, wherein the division is based on the trunk size class of the trunk said collected measurement data is associated with, computing at least one characteristic value for said several trunks, wherein the computation takes place on the basis of the measurement data divided into the trunk size classes, to describe the performance of said subsystem or function, repeatedly computing the characteristic value, and storing the time history of the computed characteristic value in the form of a performance history of the forest machine.