L'objectif des travaux présentés concerne la modélisation des procédés de bûcheronnage actuels et en cours de développement dans le cadre du projet ECOMEF (Éco-concevoir un outil de mécanisation pour le bûcheronnage dans les peuplements feuillus), et ce en vue d'évaluer et d'optimiser les performances (gain de productivité, mais également efficacité énergétique) des nouvelles solutions envisagées au regard des systèmes existants. Après une présentation du cadre de travail, cette publication présente, dans une première partie, une démarche de décomposition fonctionnelle basée sur la méthode SADT, en vue de déterminer des « briques élémentaires » de modélisation des systèmes d'abattage existants. Ainsi, l'objectif général « Modéliser le fonctionnement d'une tête d'abattage» (considéré comme la fonction A-0 de notre diagramme SADT) a été décomposé en quatre fonctions, très proches des phases de travail des machines actuelles, à savoir modéliser le serrage, l'entraînement, l'ébranchage, et le billonnage du tronc. Au vu de cette décomposition et des priorités formalisées au sein du projet, trois modèles ont été identifiés comme indispensables à développer : un modèle d'entraînement du tronc, un modèle de coupe adapté à l'ébranchage par choc ainsi qu'un modèle de contact entre le tronc et les différentes parties de la tête d'abattage. Dans une seconde partie, le modèle d'entraînement est détaillé : les études existantes ainsi que les objectifs de ce modèle sont présentés. Afin de développer chaque modèle, deux démarches sont menées simultanément : le développement d'une approche analytique et la mise en place de campagnes d'essais expérimentaux en vue du recalage et/ou de la caractérisation des modèles. L'approche théorique mise en place pour le modèle d'entraînement est détaillée, et enfin le développement d'un banc d'essai et les expérimentations prévues pour le recalage du modèle sont évoqués, avant de conclure sur les résultats attendus

BOUZGARROU, Belhassen-Chedli

GAGNOL, Vincent

GOGU, Grigore

HATTON, Benjamin

I-Revues

21e`me Congre`s Franc¸ais de Me´canique Bordeaux, 26 au 30 aouˆt 2013
Modelling of the hardwood harvesting process: feeding
model
B. Hattona, V. Gagnola, B. C. Bouzgarroua, G. Gogua
a. Institut Pascal (UMR 6602 CNRS / UBP / IFMA), Campus de Clermont Ferrand les Ce´zeaux -
CS 20265, 63175 AUBIE`RE cedex
Re´sume´ :
Les travaux relate´s concernent la mode´lisation de proce´de´s de buˆcheronnage au sein du projet ECO-
MEF (E´co-concevoir un outil de me´canisation pour le buˆcheronnage dans les peuplements feuillus),
en vue d’e´valuer et d’optimiser leurs performances (gain de productivite´ et efficacite´ e´nerge´tique).
Cette publication pre´sente une de´marche de de´composition fonctionnelle base´e sur la me´thode SADT,
afin de de´terminer des briques e´le´mentaires de mode´lisation. Ainsi, quatre fonctions permettent de
mode´liser le fonctionnement d’une teˆte de buˆcheronnage, a` savoir mode´liser le contact et le serrage du
tronc par les diffe´rents organes, mode´liser l’entraˆınement, mode´liser l’e´branchage par choc, et mode´liser
le billonnage, les trois premiers mode`les e´nonce´s ayant e´te´ identifie´s comme indispensables. Ensuite,
le mode`le d’entraˆınement est de´taille´. Le de´veloppement d’une approche analytique et la re´alisation de
campagnes d’expe´rimentations en vue du recalage du mode`le sont mene´s simultane´ment. L’approche
the´orique est de´veloppe´e, ainsi que les premiers re´sultats expe´rimentaux et les essais a` venir.
Abstract :
This work deals with modelling and simulation of harvesting processes within the framework of the
ECOMEF project (Eco-design of a mechanized equipment for hardwood harvesting), the purpose being
the assessment and optimization of processes’ performance (productivity and energy efficiency). This
paper presents a function modelling methodology, based on the SADT method, used to determine ”fun-
damental components” of an existing harvesting system model. Thus, four functions are proposed to
model the actual cut-to-length logging process, namely modelling the contact and gripping of the trunk,
modelling stem feeding, modelling delimbing, and modelling bucking, the three first models being iden-
tified as essential to develop. The feeding model is then detailed. The development of an analytical
approach and the establishment of experimental tests for the calibration of the model are conducted
simultaneously. The theoretical approach of the feeding model is developed, and the first experimental
results and tests planned are discussed.
Mots clefs : Harvesting head ; feeding model ; experimental characterization
1 Introduction
This work is a part of the ECOMEF project which aims to develop a tool more specifically designed
to harvest hardwoods.Within the framework of this project, the work presented focuses on model-
ling current harvesting processes and new developed concepts to evaluate and optimize their energy
performance and productivity.
The process that should be modelled is the one mainly used in Europe, that is to say cut-to-length
logging (CTL) [1]. CTL is usually performed with two machines, a forestry harvester and a forwarder.
As presented in [1], a typical CTL operation is composed of felling, and processing trees, processing
being itself composed of delimbing and bucking. More precisely, this paper concentrates on the single-
grip feller-processors which are attachments mounted on the harvester boom, able to process each step
detailed above.
1
21e`me Congre`s Franc¸ais de Me´canique Bordeaux, 26 au 30 aouˆt 2013
2 Function modelling
2.1 Processes to model
To identify the processes that should be modelled in order to simulate CTL as widely as possible in
a mechanical perspective, a top-down approach based on the SADT method has been used. Starting
from an ”A-0” top level which only contains the main goal of this work, namely Modelling the CTL
process : felling and processing (delimbing + bucking), the lower ”A0” level is naturally composed of
four functions that almost represents the different phases of the CTL process (figure 1).
Figure 1 – A0 level of the SADT diagram
We can see that felling and bucking have been gathered together in one single function because from
a mechanical point of view, these two steps boil down to driving the chain saw, even if the conditions
are not exactly similar [2]. The tree is indeed vertical while felling and almost horizontal while bucking
which leads to different stresses in the trunk.
Lower levels are created using the same methodology and the same mechanical point of view. This
leads to the definition of indivisible base components, such as the A21 function Calculating transmitted
and transmissible forces described in this paper and referred later as the feeding model.
2.2 Fundamental components
Using this top-down function analysis, an exhaustive inventory of modelling components has been
obtained. Before trying to adapt existing models and/or trying to develop new ones, fundamental
components should be highlighted. Considering the ECOMEF project as the framework of this study,
it has been decided to focus on the components dealing directly with hardwood specificity. Compared
to softwood, broad-leaved trees have indeed more crooked trunk and bigger branches with more varied
implantation angles, which makes them more difficult to delimb and mechanically more stressful for
the harvesting head.
The components showing feedbacks between each other are also preferred, because closed loops denote
a great source of optimization. Based on this analysis, three components are defined as fundamental
to model CTL : A1 function Modelling trunk gripping, A2 function Modelling trunk feeding and A3
2
21e`me Congre`s Franc¸ais de Me´canique Bordeaux, 26 au 30 aouˆt 2013
function Modelling delimbing. The second part of this paper will focus on the feeding model, more
specifically using feed rollers.
3 Feeding model
To feed the trunk through the harvesting head, two technologies can be used on single-grip feller-
processors : feed rollers and stroke cylinder.
Stroke feeding is based on a linear hydraulic actuator. Trunk is fed by an alternate movement of the
stroke cylinder, coupled with the gripping/releasing of the trunk alternatively with the upper and the
lower knives. Delimbing forces are often higher, but this solution remains slower than the feed rollers.
When using a harvesting head equipped with feed rollers, the rollers are pressed against the trunk
using a gripping mechanism that can have concentric, lateral or hybrid motion. As explained in [2],
concentric gripping mechanisms are not very suitable for small stems, whereas lateral gripping mecha-
nisms facilitates passing of crooked parts of the trunk but could lack pressing force when processing
large trunks. Of course, the hybrid solutions try to adapt the kinematics of the gripping motion to
the diameter of the trunk, in order to be efficient on the whole range of considered diameters.
3.1 Context
Feeding has been studied using different points of view. In [4], the authors carried out an experimental
study of stud damages to logs, in order to determine which type of roller causes the most.
In [3], the damage of different feed rollers are compared too, but the authors also consider the operatio-
nal efficiency of each roller and establish models of the effective feeding time and the fuel consumption
for three different tree species (pine, birch and spruce).
Even closer to mechanical considerations, [5] and [6] present two experimental studies about penetra-
tion depth and admissible traction force depending on the type of feed roller.
Unfortunately, there is actually neither analytical study of the feeding process, nor model of the
mechanical behaviour of gripping / feeding trunk with rollers. In order to optimize the efficiency of
this process, a model should thus be developed. The purpose is to be able to calculate and then to
optimize, for a given configuration (geometrical, mechanical and dynamical known parameters), the
actual and maximum admissible forces.
3.2 Theoretical approach
The approach presented above is mainly based on the model of non-elastic deformation of the chip by
an asperity of the tool proposed by Klamecki [7]. This model has been initially developed to calculate
the friction coefficient due to the creation of a furrow by a conical asperity travelling through the
chip. Klamecki expresses the transverse and frontal forces on the asperity using the simple relation
F = σ S, F being the force and S the projected area in the considered direction.
For the transverse force T , the considered surface At is half a disk of diameter d (maximum penetration
of the cone into the chip, figure 2), and the stress linking force and area is the yield strength of the
chip material. For the frontal force N , the Af area represents the projection of the cone in a radial
plane, perpendicular to the force (triangle of the height of x and of the base of d). The considered
stress is the strength of the chip material.
The feeding model follows a similar approach. First, the maximum stress admissible by a given wood
species in the gripping direction (perpendicular to grain) needs to be experimentally determined, in
order to link the measured gripping force with the projected area normal to this direction. Then, an
analytical model allows the calculation of the penetration depth and the projected area normal to the
feeding direction (parallel to grain). Finally, the maximum stress admissible in this direction (to be
determined experimentally too) will permit to calculate the maximum transmissible feeding force.
Still being one of the most used type of stud on feed rollers, the elliptical stud or steel spike could be
geometrically obtained by two successive oblique cuts in a rod (figure 3).
3
21e`me Congre`s Franc¸ais de Me´canique Bordeaux, 26 au 30 aouˆt 2013
Figure 2 – Klamecki definition of the
frontal and transverse surfaces Figure 3 – Geometrical definition of an elliptical stud
The parameters used to define the geometry of the elliptical stud are the following :
– p the diameter of the rod
– hp the stud height
– α the top angle
– β the inclination angle of the stud with respect to the vertical
– h the ”free height” of the stud (height of the stud that has not penetrated into the trunk)
Looking at figure 3, the frontal surface Af and transverse surface At can be modelled as portions
of ellipse. Considering a centred ellipse with semi-major axis a and semi-minor axis b, the portions
are delimited by a vertical line with abscissa x = a′ (figure 4), with Af being approximated by a
surface like S1 (when x ∈ [−a ; a′]) and At by a surface like S2 (when x ∈ [a′ ; a]). The areas can be
calculated :
Figure 4 – Portions of ellipse
A[−a;a′] =
ab
2
[
pi + sin
(
2 asin
(
a′
a
))
+ 2 asin
(
a′
a
)]
(1)
A[a′;a] =
ab
2
[
pi − sin
(
2 asin
(
a′
a
))
− 2 asin
(
a′
a
)]
(2)
Starting from (1), the parameters a, a′ and b can be expressed using the geometrical definition pa-
rameters of the stud to analytically express the area of At. The same can be done for Af using (2).
Figure 5 – ”Real” and calculated areas of the frontal and transverse global surfaces
4
21e`me Congre`s Franc¸ais de Me´canique Bordeaux, 26 au 30 aouˆt 2013
Using these formulae for every stud penetrating the trunk, the global frontal and transverse areas Sf
and St can be obtained. To confirm the calculated values, a CAD model of the configuration were
created to obtain reference surfaces and global areas (figure 5). For each area, the relative error never
exceeds 5%.
3.3 Experimental approach
Specific stress in both gripping and feeding directions (perpendicular and parallel to grain) is ex-
perimentally determined by measuring forces and penetration depth (used to calculate areas). To
determine these values, two experiments are carried out, one for each direction. The experimental de-
vice and the first results of force as a function of penetration depth obtained for the gripping operation
are presented.
Figure 6 presents the gripping test bench. A portion of a feed roller and a frame supporting the trunk
are inserted into the chuck jaws of a tension/compression testing machine. The machine is then moved
to the contact point between the roller and the stem. Starting from this point, the displacement and
the applied force are measured until the cylindrical surface of the roller touches the trunk.
Figure 6 – Gripping
test bench Figure 7 – Stress curves obtained for each test and average value
Curves of force as a function of penetration depth are then obtained. Knowing the penetration depth,
the global transverse area is computed for each time step using equations (1) and (2). This allows to
plot stress (force divided by transverse area) as a function of penetration depth (figure 7).
Preliminary tests were made with different log diameters. Every tests have been made on logs coming
from the same trunk of green sessile oak. It is then possible to consider that the moisture content and
the specific gravity are not relevant parameters in this case.
The stress curves can then be approximated as horizontal lines. This assumption is based on the
simple formula σ = F ÷ S with F the applied force and S the projected surface normal to the
application direction, σ being equivalent to the maximum admissible stress when pressing the feed
roller against the trunk. Considering that wood has the same mechanical properties for each test, the
maximum admissible stress should be unique. As presented on figure 7, the calculated average stress
value is approximately 18 MPa. This value is positioned between the compressive stress perpendicular
to grain and the hardness values usually found in the literature [10].
This value allows the calculation of the projected area and the penetration depth. Experimental study
of the feeding operation is the next step to be able to compute the maximum transmissible feeding
force. Of course, more tests are needed to determine values for each hardwood species of interest and to
investigate the influence of specific density and moisture content on this maximum admissible stress.
5
21e`me Congre`s Franc¸ais de Me´canique Bordeaux, 26 au 30 aouˆt 2013
4 Conclusions
An analytical model to determine the global projected surfaces of the feed roller was developed. This
model allows to calculate the areas when using elliptical studs. To extend the study to other shapes of
studs, derived analytical formulations or a CAD-based calculation method will be proposed, in order
to help engineers to optimize the feeding process (maximizing the maximum transmissible feeding
force by adapting the roller geometry but also the shape and the pattern of the studs).
Concerning the gripping of the trunk, first tests seem to confirm the trend of a constant relationship
between calculated stress and penetration depth. More experimentations are needed to verify this
hypothesis and to study the influence of several parameters on the admissible stress.
To study the feeding operation, a specific workbench has been designed. The bench allows the ap-
plication of both an adjustable gripping force of the roller against the clamped trunk and a feeding
force, applied to the roller using a hydraulic actuator and causing its rotation. These two forces and
the stroke of the actuator are measured until the total bulk failure to determine the maximum stress
in the feeding direction.
Acknowledgements
This research work is part of FUI ECOMEF project founded by Conseil Regional Auvergne and by
the program Regional competitiveness and employment 2007-2013 (ERDF - Auvergne region). These
organisms are acknowledged for their financial support.
Re´fe´rences
[1] C´upric´, Lj.N., Bajic´, V.V. 2009 Cut-to-length machines. Proceedings of the XIX International
Conference on ”Material Handling, Constructions and Logistics” (ed. S. Bosˇnjak and N. Zrnic´) pp.
109-118, University of Belgrade, Faculty of Mechanical Engineering, Serbia.
[2] Goubet, D., Fauroux, J.C., Gogu, G. 2012 Gripping mechanisms in current wood harvesting ma-
chines. Front. Mech. Eng. 10 p.
[3] Nuutinen, Y., Va¨a¨ta¨inen, K., Asikainen, A., Prinz, R., Heinonen, J. 2010 Operational efficiency
and damage to sawlogs by feed rollers of the harvester head. Silva Fennica 44(1) 121-139
[4] Brunberg, T., Hofsten, H., Jonsson, M. 2007 Stud damage to logs - research and evaluation.
Skogsforsk Results 1 4 p.
[5] Hallonberg, U., Granlund, P., Norden, B. 2004 Need to improve harvester feed systems. Skogsforsk
Results 2 4 p.
[6] Memic, A., Ga¨fvert, M., Vilhelmsson, U. 2008 Matarhjulens dubbintra¨ning i timmer : Underso¨kning
av dubbskador som matarhjulens mo¨nster orsakar p˚a timmer (Harvesting damage on timber caused
by feed rollers : A study of damage, on timber, caused by the feed rollers stud pattern). Student
thesis 46 p., Va¨xjo¨ University, School of Technology and Design, Sweden.
[7] Klamecki, B.E. 1976 Friction Mechanisms in Wood Cutting. Wood Science and Technology 20
209-214
[8] Sveningsson, L. 2011 Dubbskador p˚a Alvestakubb (Stud damages on Alvestakubb). Student thesis
34 p., Swedish University of Agricultural Sciences, Faculty of Forest Sciences, Sweden.
[9] Dahl, D. 2006 Produktionsskillnader och virkesskador med olika typer av matarvalsar (Differences
in productivity and damage on timber with different types of feeding rolls). Students’ reports -
Forest Technology 88 18 p., Swedish University of Agricultural Sciences, Faculty of Forest Sciences,
Sweden.
[10] Kretschmann, D. E. 2010 Wood Handbook - Wood as an Engineering Material - Chapter 05 :
Mechanical Properties of Wood General Technical Report FPL-GTR-190 508 p., United States
Department of Agriculture, Forest Service, Forest Products Laboratory.
6


Modelling of the hardwood harvesting process: feeding model

Abstract

Similar works

Full text

Available Versions

I-Revues