Page 376 - Mechatronics for Safety, Security and Dependability in a New Era

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measurement signals during stem processing, thus resulting in non-optimal bucking. One criterion of the
grade of a log is the top diameter. If the top diameter is measured incorrectly, the log may be classified
to the wrong grade. Usually this means that a stock becomes a less valuable paper-wood log.
In addition to the optimization of bucking, accurate length and diameter measurements are needed also
for computing the volume of the logs. The volume determines the price of the log, and consequently
inaccurate measurements cause financial loss either to the seller or to the buyer of the wood. The prob-
lem is highly relevant, since measurements done by the harvester were used as the delivery measurement
for 65 % of the timber harvested in Finland in 2002. In standing sales of privately owned forests the
amount is even larger, 87 %. (Metsateho, 2003) In other countries the volume of the wood is measured
on the roadside before transportation, or at the saw mill, because the harvester measurement is not con-
sidered to be as reliable and objective as other methods. If using the harvester measurement became
widely accepted, the cost of this second measurement could be saved.
In this work a soft-sensor approach is used to improve the accuracy of the stem diameter measurement.
Using a soft-sensor means that instead of measuring a process variable directly with one physical sensor,
measurements form several sensors and other knowledge of the process are incorporated using software
to obtain an even more accurate measurement.

MECHANICAL TIMBER HARVESTING

There are different methods for mechanised timber harvesting differing by their philosophy and the ma-
chines needed. In North America the full-tree and tree-length methods are common whereas in
Scandinavia the cut-to-length method is dominant. In the cut-to-length method the trees are felled, de-
limbed and bucked (i.e. cut to logs) with a forest harvester. The harvester is equipped with measuring
devices that measure the length and diameter of the stem. Optimization algorithms choose the bucking
such that the value of the stem is maximized. Once the stems are processed, a forwarder carries the logs
to the roadside for further transportation. A cut-to-length forest harvester can be divided into four main
parts: engine and power transmission, cabin and controls, crane and harvester head. The diesel engine is
used for rotating the supply pumps of work hydraulics and hydrostatic transmission. The supply pump of
work hydraulics delivers hydraulic power to the crane, to the harvester head and to all the auxiliary func-
tions of the machine. The hydrostatic transmission consists of a variable displacement pump, of a
variable displacement hydraulic motor and of mechanical transmission to the wheels. The cabin is
equipped with the controls that are needed for operating the functions of the harvester and with a display
module, which gives the operator information on the harvesting process and on the state of the harvester.

The most complex part of the harvester is the harvester head, which has a large-scale effect on the over-
all timber harvesting efficiency, and on the quality of the harvested timber. Its main functions are
sawing, feeding, delimbing of branches, and measuring log length and diameter profile. Trees are felled
and stems are cut to logs with a hydraulically actuated chain saw. Once sawing is complete, the stem is
fed to a new cutting point with the hydraulic feeding rollers. To prevent the feeding rollers from slip-
ping, the rollers are pressed hard against the stem with a hydraulic cylinder. In front and behind of the
feeding rollers there are delimbing knives, which wrap around the stem. As the stem is fed to the next
cutting point, branches are cut when they meet the delimbing knives. Delimbing knives also prevent the
stem from falling out of harvester head grasp during the feeding operation. The diameter of the stem is
measured using the delimbing knives. The setup is depicted on the left in Figure 1 where the delimbing
knives can be seen holding the stem against the frame of the harvester head. Both delimbing knives are
fitted with a potentiometer that gives a voltage that is proportional to the position of the knife. This
measuring arrangement assumes that the stem stays in contact with the harvester head frame and that the
delimbing knives touch the surface of the stem. If these assumptions do not hold, a measurement error
will be introduced.