Page 377 - Mechatronics for Safety, Security and Dependability in a New Era

Ch73-I044963.fm Page 361 Friday, July 28, 2006 1:57 PM
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Friday, July 28,2006
1:57 PM
Ch73-I044963.fm
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Figure 1: Correct stem position (left) and a "hanging" stem (right).

DIAMETER MEASUREMENT PROCESSING
300
250 \

200
]
m S
m
[

r
e
t
e 150
m
a i
D
100
50
0
10
12
0 2 4 6 8 i 10 12 14 16 18 20
Length [m]
Length [m]
Figure 2: Typical diameter profile of a stem. Note the level sections and the drop in diameter.
A typical diameter profile of a stem is presented in Figure 2. The diameter in millimeters is on the verti-
cal axis and the length in meters is on the horizontal axis, so that the bottom of the tree is on the left and
the top is on the right. There are two characteristic unnatural features in the diameter profiles measured
by a harvester. First, there are long level sections in the diameter profile, i.e. the diameter of the stem
seems to remain constant. Second, there are abrupt drops in the profile. Both can be seen in Figure 2.
Since the delimbing knives measure the diameter of the stem, they must follow the surface of the stem as
closely as possible. Moreover, the stem should stay at all times against the frame of the harvester head.
Loss of contact between the stem and the frame of the harvester head causes the distance between the
stem and the frame to be added to the diameter. Tf the stem loses contact with the harvester head (on the
right in Figure 1), the delimbing knives open, giving a diameter measurement that increases towards the
top of the stem. The measuring system of the harvester requires diameters to be monotonically decreas-
ing, and outputs a constant diameter value until the diameter decreases again. The result is a level
section in the diameter profile. When feeding ends and the stem stops, the delimbing knives grasp the
stem with maximum force, forcing the delimbing knives against the stem and the stem against the har-
vester head. This can be seen as the sharp narrowing in the diameter profile if the delimbing knives were
open or if the stem was not against the harvester head. Thus the measurement following a sharp narrow-
ing can be considered to be more accurate than the ones before the narrowing. Other sources of error
with less significance are e.g. branches between the harvester head frame and the stem, hysteresis in the
potentiometers that measure the position of the delimbing knives, incorrect calibration of the measuring
device, and the functioning of the diameter measurement processing algorithm in some special cases.
Different aspects must be taken into account when designing algorithms for processing the diameter
measurements to get more accurate results. Due to the large amount of disturbances that is present in the