KTH Industrial Engineering And Management

Suspended forestry machines for sustainable forestry

Abbos Quchqarboyevich Ismoilov

Doctoral thesis in Machine Design Stockholm, Sweden, 2016

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TRITA – MMK 2016:08 KTH School of Industrial ISSN 1400-1179 Engineering and Management ISRN/KTH/MMK/R-16/08-SE SE-100 44 Stockholm ISBN 978-91-7729-227-2 Sweden

Academic thesis, which with the approval of KTH Royal Institute of Technology, will be presented for public review in fulfilment of the requirements for a Doctorate of Technology in Machine Design. The public review is held at KTH Royal Institute of Technology, Brinellvägen 85, floor 3, room B319, at 13:00, January 25, 2017.

Copyright: Abbos Quchqarboyevich Ismoilov, December 2016

Print: US-AB

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Department of Machine Design KTH Royal Institute of Technology S-100 44 StockholmSWEDEN

TRITA – MMK 2016:08 ISSN 1400-1179 ISRN/KTH/MMK/R-16/08-SE ISBN 978-91-7729-227-2 Document type

Thesis

Date

2016-01-25 Author

Abbos Quchqarboyevich Ismoilov

(ismoilov@kth.se)

Supervisor(s)

Ulf Sellgren, Kjell Andersson Sponsor(s)

Skogforsk Title

Suspended forestry machines for sustainable forestry

Abstract

Cut-to-length (CTL) logging is a mechanized two-machine solution. The harvester processes tree stems into smaller logs and a forwarder transports the logs from the logging site to a landing area accessible by trucks. The working machines for CTL logging are heavy and their suspension system is generally rudimentary, basically the only damping is provided by the tires. To meet future demands on operator comfort, sustainable forestry, and climate concerns, significant challenges are to find means for reducing daily vibration dosage, soil damage, and rolling resistance. Paper A proposes a full-scale virtual model of a four-wheeled forwarder concept equipped with two pendulum axels with an actively controlled hydraulic suspension system mounted on each wheel axle. The simulation results are then analyzed to determine the required actuation power. Paper B presents a performance comparison of a six-wheeled medium-sized pendulum-arm suspended forwarder, with three different suspension systems; active, semi-active, and passive. A methodology to optimize and analyze forestry vehicle suspension performance based on multi-body dynamic simulations are proposed and applied for the studied forwarder. Paper C is a model-based investigation of the dynamic behavior of a traditional eight-wheeled bogie type of forwarder with the main focus on identifying critical issues and suggesting criteria for assessing the performance of the machine while traveling on sloped and rough terrain. Paper D investigates the performance of a novel all-wheel-drive pendulum-arm suspended medium-sized forestry machine with passive and active chassis suspensions. The dynamic performance of the pendulum-arm machine concept is quantified with simulations and compared with a “traditional” bogie-machine. Paper E investigates how to model a tracked forwarder and how the performance comparison can be evaluated in multi-body simulation software like Adams ATV. Paper F presents a comparison of the dynamic behavior of forestry machines with different types of passive chassis suspensions from three perspectives: their gentleness to terrain, operator and their potential for improved fuel efficiency. Paper G proposes a 12 degrees-of-freedom multi-body dynamics simulation model of a standard eight-wheeled bogie type of medium-sized forwarder and verifies the simulation model with measured data from the field test that was carried out the actual machine. Paper H reconfigures the model presented in Paper G and compares a medium-sized forwarder equipped with two different track units with the performance of a wheeled and bogie-type of forwarder on hard rough ground, as well as on soft soil. Keywords

CTL-logging, forestry machine, forwarder, machine-ground interaction, multi-body simulations, suspension, track unit, wheels on bogies

Language

English

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ACKNOWLEDGEMENTS

This thesis summarizes my excursion as a doctoral student into the world of forestry machine dynamics.  I am indebted to a number of people and organizations that have all made this endeavor possible.

First and foremost, I wish to express my sincere gratitude to my supervisors Ulf Sellgren and Kjell Andersson for accepting me as a doctoral student in the first place and providing enthusiastic assistance throughout my research, their brilliance in both academic and technical matters has been absolutely crucial for the success of this thesis.

Financing was provided by Erasmus Mundus Action 2 ARCADE in first stage and I am thankful to the coordinators of the project, Francesca Chicco, Mohammad and Bassam Kayal for their support and good collaborations. Financing was followed by Skogforsk – the Forestry Research Institute of Sweden and their support is gratefully acknowledged. I would also like to wish my deepest thanks to Dr. Björn Löfgren for giving me opportunity to participate in full scale field test measurement with actual machines, thus widening my perspective on sustainable vehicle design and providing a valuable framework for my research.

I am indebted to my many fellow doctoral students and senior colleagues at the Department of Machine Design including Patrick Rohlmann, Bertrand Kerres, Mattia Alemani, Gabriele Riva, Moritz Ploss, Bhaskar, Ju Shu, Xuan Sun, Martin Andersson, Katja Gradin, Aftab Ahmad, Abdurasul Pirnazarov, Baha Alhaj Hasan, Pouya, Xinhai Zhang, Didem Gürdür, Daniel Frede and Vicki Derbyshire for their support, motivation, and providing a stimulating and fun filled environment.

My sincere thanks also go to Martin Grimheden and Sulaymon Eshkabilov for giving me opportunity to be involved in Tempus project between Machine design department and universities in Uzbekistan which made my PhD life more vibrant. I would like to extend my thanks to my dear friends at IT Training center in Tashkent.

I am also thankful to MSc students Federico Baez and Praveen Ramachandran from forestry master school at the Department of Machine Design for contributing to the project through their thesis work. Many thanks are extended to Komatsu Forest AB and MSC Software Nordics for the collaboration and the support they have provided.

I would like to express my deepest love to my wife, Shoira and my adorable newborn son, Yunus. They encouraged and motivated me to get things done. Their love and passion brought me a lot of joy and peace into my life and I love them very much.

Last but not the least; I would like to thank my parents, brothers, sisters and relatives back at home for their endless support, encouragement and guidance. My dear parents, thank you for believing in me and supporting me throughout my work. This thesis work is a gift for you from Stockholm.

Stockholm, December 2016

Abbos Quchqarboyev Ismoilov

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LIST OF APPENDED PUBLICATIONS

This thesis consists of a summary and the following appended papers:

Paper A

Abbos Ismoilov, Ulf Sellgren, Kjell Andersson, Björn Löfgren (2014) “Four wheeled active suspended design concept for forestry machines on soft and rough terrain”, Proc. 5th Forest Engineering Conference, September 23-26, 2014, Gerardmer, France.

The author performed the planning, modeling and simulation part, and most of the writing in collaboration with the other authors.

Paper B

Federico Baez, Abbos Ismoilov, Ulf Sellgren, Kjell Andersson, Björn Löfgren (2014) “Multi-objective performance optimization of pendulum-arm suspensions for forestry machines”, Proc. 5th Forest Engineering Conference, September 23-26, 2014, Gerardmer, France.

The author did much of the machine modeling and simulation part, and contributed to the writing.

Paper C

Abbos Ismoilov, Ulf Sellgren, Abdurasul Pirnazarov, Kjell Andersson, Björn Löfgren (2014) “Investigating the dynamic behavior of a mid-sized forestry machine on sloped rough terrain”, Proc. 18th International Conference of the ISTVS, September 22-25, 2014, Seoul, South Korea.

The author performed the planning, modeling and simulation, and analysis parts, and most of the writing in collaboration with the other authors.

Paper D

Ulf Sellgren, Abbos Ismoilov, Federico Baez, Björn Löfgren, Kjell Andersson (2014) “Model-based verification of a pendulum-arm suspended forwarder for sustainable forestry”, Proc. 18th International Conference of the ISTVS, September 22-25, 2014, Seoul, South Korea.

The author performed the modeling and simulation part, and contributed to analysis and writing in collaboration with the other authors.

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Paper E

Praveen Ramachandran, Abbos Ismoilov, Ulf Sellgren, Björn Löfgren, Kjell Andersson (2014) “Model-based analysis of a tracked forwarder for sustainable forestry”, Proc. 13th ISTVS European Conference, October 21-23, 2015, Rome, Italy.

The author contributed to the planning, modeling and simulation part, in collaboration with the main author, and parts of the writing in collaboration with the main author.

Paper F

Abbos Ismoilov, Ulf Sellgren, Kjell Andersson, Björn Löfgren (2015) “A comparison of novel chassis suspended machines for sustainable forestry”. Journal of Terramechanics, 58 pp. 59-68, Elsevier Ltd

The author performed the planning, modeling and simulation, and analysis parts, and most of the writing in collaboration with the other authors.

Paper G

Abbos Ismoilov, Ulf Sellgren, Kjell Andersson (2016) “Development of a dynamic base model of a bogie suspended forwarder”. IMechE Part K: J Multi-Body Dynamics, SAGE Publishing, first published on September 23 2016, DOI: 10.1177/1464419316671025

The author performed the planning, modeling and simulation, and analysis parts, and most of the writing in collaboration with the other authors.

Paper H

Abbos Ismoilov, Ulf Sellgren, Kjell Andersson, Björn Löfgren (2016) “Model-based performance comparison of tracked forwarders”. Submitted for publication.

The author performed the planning, modeling and simulation, and analysis parts, and most of the writing in collaboration with the other authors.

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CONTENTS

ACKNOWLEDGEMENTS ................................................................................................................ 5

LIST OF APPENDED PUBLICATIONS .......................................................................................... 7

CONTENTS ........................................................................................................................................ 9

1 INTRODUCTION ......................................................................................................................... 11

1.1 Background ............................................................................................................................... 11

1.2 Challenges for sustainable forestry machine development ....................................................... 14

1.3 Off-road machine suspension and traction units ....................................................................... 15

1.4 Objectives and research questions ............................................................................................ 20

2 MODEL-BASED RESEARCH METHODOLOGY ..................................................................... 21

2.1 A model-based research framework ....................................................................................... 21

2.2 The research process ............................................................................................................... 22

2.3 The research method and tools ................................................................................................ 23

3 SUMMARY OF RESULTS AND APPENDED PAPERS ........................................................... 24

4 DISCUSSION AND FUTURE WORK ........................................................................................ 30

5 CONCLUSIONS ........................................................................................................................... 33

6 REFERENCES .............................................................................................................................. 35 

Appended papers

A. Four-wheeled active suspended design concept for forestry machines on soft and rough terrain 39

B. Multi-objective performance optimization of pendulum-arm suspension for forestry machine 47

C. Investigating the dynamic behavior of a mid-sized forestry machine on sloped rough terrain 57

D. Model-based verification of a pendulum-arm suspended forwarder for sustainable forestry 67

E. Model-based analysis of a tracked forwarder for sustainable forestry 75

F. A comparison of novel chassis suspended machines for sustainable forestry 91

G. Development of dynamic base model of a bogie suspended forwarder 103

H. Model-based performance comparison of tracked forwarders 121

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1 INTRODUCTION  

The Brundtland Commission [1] defined sustainable development as “development that meets the need of the present without compromising the ability of future generations to meet their own needs”. A widely accepted definition of sustainable development recognizes three core areas, or pillars, of sustainable development: economic development, social development, and environmental protection. This is also the definition that is used in this thesis.

Almost sixty percent of the land area of Sweden is covered by, what can be considered as, productive forests [2]. Sweden holds almost one percent of the world’s commercial forest area, but provides as much as ten percent of the world production of sawn timber, pulp and paper [2]. Almost 90 percent of the total timber volume coming from North European forests is harvested with the modern and fully mechanized cut-to-length (CTL) logging process [3]. There is an increasing interest in developing productive and sustainable forest management approaches that are based on gentleness for the operators and to the environment. Sustainable forest management requires that the impact from the management operations should not exceed the natural capacity of the sites to renew or repair themselves [4]. It also benefits from improved forestry machine chassis suspensions and ground contact units that reduce soil damage, rolling resistance, and the daily vibration dosages for the machine operators.

1.1 Background

Harvesting systems and methods refer to the harvesting activities, such as felling, processing, loading, and log transportation to the saw or paper mill. Since the early days of North European forestry, cutting tree stems into logs in the forest has been opted as the most rational handling and transportation approach [5]. The CTL method is a two-man and two-machine operation, with a harvester felling, delimbing and bucking trees, and a forwarder loading and carrying the logs from the harvesting area to a roadside landing area (Shown in Figure 1.1). Normal CTL harvester heads can normally treat trees with a diameter less than 900 mm.

Figure 1.1. A typical harvester (left) and a medium‐sized forwarder (right). 

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However, there are other types of harvesting methods as well, like whole-tree logging (WTL) where the entire above-ground portion of the trees are cut and transported to a roadside landing area for delimbing and further transport to a saw mill. WTL is more commonly used in North America where tree sizes often exceed the capacity of the CTL harvester heads. In WTL, skidders transport the entire tree from the felling site by lifting the butt end up from the ground and dragging the tree along the ground [6]. In WTL the terrain is more negatively affected due to the skidding of the felled trees than it is by CTL operations

Most CTL forwarders are eight-wheeled and articulated machines that are equipped with four bogies. The front and rear wagons are connected to each other by a combined articulation and roll joint, which enables relative yaw and roll motion between the front and rear wagons as shown in Figure 1.2. These machines are not supported with any chassis suspension system that can isolate the machine from vibrations caused by operation on rough ground.

 

Figure 1.2. The standard eight wheeled bogie type of forwarder. 

Forest terrain varies from very soft soil to hard ground with steep slopes covered with boulders and stones as shown in Figure 1.3. A large portion of the North European forest soil is a very sensitive bed and it consists of large areas of sand and clay with embedded stones, and some areas are marsh-lands with a very low bearing capacity causing the wheels to slip and loose traction, and potentially also getting stuck. On rough terrain, a loaded forwarder might become unstable due to lateral inertia forces in combination with a high center of gravity.

Ride comfort analysis of modern forestry machines is extremely critical because operators are exposed to a high-level and long-time vibration environment, which is strongly dependent upon machine-terrain interactions and the dynamics of the machine [7]. State-of-the-art construction machines represent a high level of sophistication in several areas, whereas

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forestry machines have rudimentary suspension systems, or are unsuspended with a very light damping that primarily originates from the tires [7], [8]. This leads to high level whole-body vibrations for the operators especially in the lateral and roll directions which also limit the speed of operation on uneven terrain [9]. Directive 2002/44/EEC of the European Parliament limits the daily exposure of workers from risks to their health and safety due to exposure to mechanical hand/arm vibrations and whole body vibration, which defines the minimum safety requirements [10]. Therefore there is a growing interest to develop and assess suspension systems that potentially may enhance the mobility of off-road machines on rough and soft terrain. 

   

Figure 1.3. A forwarder on rough hard terrain (left) and effect of repeated passage on soft 

soil (right). 

It is explicit that forest harvesting operation has negative effects on forest soil which also influence the regeneration at the harvesting sites [11]. Forestry machines, especially forwarders, are heavy working machines and the risk that they cause rutting and soil compaction is considerably high. Forest soil with embedded rocks of various sizes and with several root layers have a significantly higher bearing capacity, i.e. the ability of the soil to safely carry the contact pressure from vehicles and working machines, compared to marshland [12], [13]. Ruts, as shown in Figure 1.3, may be formed due to repeated passes of

heavy machines driving along the same path. Excessive soil compaction and rutting restricts the water flow across and through the soils, thus damaging roots of remaining trees in thinning operations and, consequently, reduces the growth rate for remaining and replanted trees. If a soft terrain is trafficked under wet conditions, severe level of compaction occurs and ruts may be formed which will channel subsequent rainfall and increase the erosion potential [14]. In addition, rutting has a significant negative impact on vehicle performance and energy consumption [15].

Forestry in the North European countries is based on sustainable management principles by immediate reforestation after harvesting. According to United Nations Framework Convention on Climate change in Sweden from the year 1990 to 2008, total aggregate greenhouse gas (GHG) emissions, excluding land use, land-use change, and forestry decreased by approximately 10 percent [16]. However mechanized CTL harvesting

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operations remain as one of the potential sources of greenhouse emissions in the Nordic countries [17], [18] due to the substantial consumption of diesel fuel. In the CTL method as discussed earlier, timber is usually harvested mechanically in remote areas and then transported to mills, a process which consumes substantial amounts of fossil fuels, hydraulic oil, and lubricants, thus releasing GHGs into the atmosphere [17], [19]. The magnitude of the GHG emissions caused by motor vehicles is related to the type of fossil fuel used, type of engine and vehicle, and the used emission-control technology [17]. Designing the next generation of forestry machines with reduced rolling resistance, would result in less GHG emissions, since when the rolling resistance is decreased, fuel consumption is also decreased.

On average, the actual economic life of both harvesters and forwarders is more than 10,000 operational hours and it may approach 18,000 hours [20]. When calculating forest machine usage costs, we may distinguish between capital costs, i.e. the cost of the machine itself, running costs such as costs of fuel, oil, labor, maintenance, spare parts and repairs, and financial costs that is the cost of financing the machine over its lifetime [21]. In addition the productivity of the harvesters is particularly affected by the average volume of the felled trees, and the loading/unloading and transport productivity of the forwarders is affected mainly by the two factors haulage distance and machine payload [22].

1.2 Challengesforsustainableforestrymachinedevelopment

Most forwarders are heavy machines, with a machine weight between 15 and 30 tons, and they are normally not equipped with any level control or chassis suspension. Consequently, the risk for rutting and soil compaction during operation is considerably high. There are, thus, engineering and research challenges for machine manufacturers to find means for reducing soil damage, reducing the rolling resistance of wheeled machines, and the peak accelerations as well as the daily vibration dosages for the machine operators, i.e. reducing the peak and daily mean levels of whole-body vibrations.

The main challenges for the manufacturers of forestry machines for CTL logging, which have a very small global market (roughly 3000 machines per year), is to meet the global harvesting competition as well as new customer demands and societal legislations with general purpose machines that:

o continuously increase the harvesting and log transportation productivity,

o reduce the damage of the soil, which has a negative impact on the growth rate in thinning stands,

o reduce fuel consumption,

o reduce daily vibration dosages for the machine operators, when operating on rough terrain.

Several of these challenges have been addressed and tested in last couple of years, but most the activities have been focused on physical prototyping, i.e., come up with an idea or apply a solution from another type of business area, realize it as a full-scale prototype and then test

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the outcome. Some of these attempts and targeted technologies from other applications than forestry, are reviewed in the next sub-chapter.

1.3 Off‐roadmachinesuspensionandtractionunits

The invention of the wheel is often regarded as the greatest invention in the history of human transportation. However, almost half of the surface of the earth is not accessible by conventional wheeled machines, mainly due to a presence of boulders, a too steep terrain, marshland with low bearing capacity, or soil with low shear resistance, such as sand [23]. Furthermore, in harvesting operations, especially in rough terrain, there is a high demand for a large ground clearance that enables transport over large obstacles. Conventional wheeled vehicles have a number of limitations, e.g. they are not suitable for addressing obstacles larger than the wheel radius. Common ways to improve traction and to overcome large obstacles has been to increase the wheel diameter and profile or by replacing wheels with rubber or metal tracks. For mobility reasons it is of vital importance to develop off-road working machines with a tractive unit that give low maximum contact pressures and shear stresses in their interaction with the soil, and also that the contact forces are evenly distributed between all the tractive units. State-of-the-art chassis suspension and damping concepts for off-road machines are presented below.

1.3.1 Continuous tracks

A continuous track, also called tank tread or caterpillar track, which is a continuous band of treads or track plates that is driven by two or more wheels, was invented before the internal combustion engine and tractor was developed [24]. Continuous tracks are commonly used on a variety of vehicles and mobile machines, including bulldozers, excavators, tanks, and tractors, with the main objective to reduce the ground pressure, see Figure 1.4. Tracked vehicles, in general, have a larger contact area compared to rubber tires and is, in many cases, better than an equivalent wheeled vehicle to traverse soft ground with less likelihood of getting stuck due to excessive soil sinkage [25].

 

Figure 1.4. Caterpillar D9 bulldozer with metal tracks (left), and an agricultural tractor with 

rubber tracks (right) [27].  

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Tracked vehicles also potentially reduce soil compaction effects due to the relatively lower ground pressure compared to wheeled machines. Furthermore, tracks are also utilized widely in armored military vehicles that are designed to operate in unfavorable conditions, and to reduce serious soil damages [25]. Tracked off-road vehicles also show good tractive and stability performance, but on the other hand, they have the disadvantage of causing high ground shear, especially at turning maneuvers [26].

A common method to increase the footprint area, i.e. to reduce ground pressure, of a standard wheeled forwarder, especially in terrain with a low bearing capacity, is to mount steel tracks on pairs of bogie wheels, as shown in Figure 1.5 left.

 

Figure 1.5. Tracks on bogie wheels (left), a special‐purpose tracked forwarder (mid), and a 

tracked harvester (right).

A secondary purpose for using steel tracks is to protect the tires when operating in rough and stony terrain. Drawbacks of mounting steel tracks on the rubber wheels are that they are heavy (approximately 800kg extra weight per wheel pair) and that the inflation pressure must be very high (500-600 kPA), which has a negative impact on the machine vibration level. There are also fully tracked machines available on the market, see e.g. the mid and right pictures in Figure 1.5. These tracked machines show good performance on soils with low bearing capacity [28], but not on hard and very rough ground, e.g. they can’t be considered as general purpose machines.

1.3.2 Walking robots

Walking robots have been investigated and developed intensively, especially with the particular purpose to enable space exploration [29], as well as for terrestrial applications [30].

Unlike traditional harvesters, walking harvesters can venture over uneven ground, operate on slopes, and move in any direction. With walking harvesters, virtually no obstacles in the forest will be out of reach for the harvester. An example of a walking harvester concept that was realized as a full-scale prototype is the six-legged Plustech Oy harvester shown in Figure 1.6. The main reason for developing this concept was to create a harvester that would be gentler to the ground than wheeled machines. However, field-tests showed severe rutting, du to sliding, when the machine was walking on steep slopes. The machine has a high number of degrees of freedom, which requires many actuators, and thus has a tendency to have high energy losses, combined with a significant control complexity that limits its mobility in rough

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terrain. Moreover, the large number of actuators and sensors makes it expensive, potentially unreliable, and also economically unfeasible [31], [32]. Three copies of the six-legged “Plusjack” walking harvester were made and the prototypes have been used for demonstration and testing purposes only. The last copy of the six-legged walking harvester “Plusjack” was developed in 1999 by the “Plustech” Finnish company and in 2011 the prototype was donated to the Finish Forest Museum [33].

Figure 1.6. The Plusjack walking harvester full‐scale prototype [33]. 

1.3.3 Autonomous live-axle suspended machines

In a live-axle suspended machine, two wheels are connected to the same axle that at its center point is connected to the frame of the machine by a pin joint. Figure 1.7 shows the Elforest

GT-8 diesel-electric hybrid forwarder, which is an unmanned forwarder with live axels. It was designed with one of its goals to be gentle to the ground, e.g. with a reduced machine weight compared to the man-operated forwarders. The GT-8 forwarder is robust and simple to manufacture. Due to the live-axle suspension, the vertical travel is limited, which is a severe drawback in rough terrain. There is also a significant roll-over risk for the machine when it operates on slopes.

Figure 1.7. The live‐axle suspended and unmanned Elforest GT‐8 [34]. 

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1.3.4 Leaf spring and hydro-pneumatic suspensions

A leaf spring can be attached directly to the frame at both ends (see Figure 1.8 - left). Leaf

springs have a number of advantages, such as a simple design, low cost, a good capability to absorb vertical, lateral, and longitudinal forces. Leaf springs have, however, several disadvantages, such as large weight, uncontrolled and varied friction losses in the sliding between the leaves, and a very limited travel, which severely limits its usefulness in rough terrain.

      

Figure 1.8. A leaf spring (left) and a hydro‐pneumatic (right) rear axle suspension of a 

Scania heavy truck [35]. 

The purpose of a hydro-pneumatic suspension system is to provide a sensitive, dynamic and high-capacity suspension that supports ride quality (see Figure 1.8 - right). Nitrogen gas acts like a spring and by balancing a contained fluid volume inside a cylinder, a leveling function is obtained. Nitrogen gas in the suspension sphere is isolated from the hydraulic oil by a rubber membrane. Nitrogen gas can provide more efficient self-leveling to a vehicle compared to a leaf spring and it can easily absorb road roughness while maintaining a good ground clearance in rough terrain. Hydro-pneumatic suspension systems give good comfort and they are easy to control efficiently. But, they have disadvantages, such as high cost, high risk of failure of the hydraulic system, and it also requires a high-performing pneumatic system in addition to the hydraulic system that is already used for the crane operations in both harvesters and forwarders. Some components of a hydro-pneumatic suspension are highly sensitive to the very harsh forest environment.

1.3.5 Bogies

A bogie is a pivot-mounted link structure that is carrying a pair of wheels. Bogies serve as a mechanism of averaging and smoothing the path of the center of gravity when the working machine overcomes an obstacle, as shown in Figure 1.9. Bogies ensure wheel-ground contact

even on rough terrain and bogies take various forms in various modes of transport and bogies are used in trains, trailer, a semi-trailer, in construction machines, forestry machines and etc. Bogies might be a good general solution for keeping the dynamic stability of the machine in different terrains. However, they significantly increase the machine weight, which reduce the load carrying capacity and they, consequently, also increase the relative fuel cost. Even though bogie suspensions are proven to be very robust, the bogie link does not provide

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efficient vibration damping for neither the chassis, nor the operator, and it is not easily upgraded with an active control function.

Figure 1.9. A bogie suspended forwarder. 

1.3.6 Pendulum-arm suspensions

In a pendulum suspended system, each wheel is individually mounted on a link arm that is connected to the main frame with a revolute joint. Usually, a hydraulic cylinder mounted between the arm and the frame provides vibration damping and potentially also a leveling function. Pendulum-arm suspensions may be seen on some forestry machines, e.g. the front wheels on the Eco Log 550D harvesters shown in Figure 1.10 (left).

Figure 1.10. The pendulum‐arm leveling harvester Eco‐Log 550D [36] (left) and the XT28 full‐scale pendulum‐arm suspended forwarder prototype (right) 

The Swedish Forestry Research Institute Skogforsk in collaboration with eXtractor has recently realized a full-scale pendulum arm suspended forwarder prototype, named XT28 shown in Figure 1.10 (right). This prototype uses hydraulic pendulum-arm suspension for each of its six wheels and the pendulum-arm suspension system enables all wheels to adapt individually to the terrain, i.e. it’s an active suspension, and thus makes it potentially easier to pass over large obstacles, i.e. it allows for large vertical wheel travel with good ride comfort. The most important drawback of this type of active suspension system is its significant control complexity.

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1.4 Objectivesandresearchquestions

In the last decades, extensive research dealing with the mobility and trafficability performance of forestry machines has been performed. Many different ideas have been realized as full-scale physical prototypes, such as multi-wheel concepts [31], [37], tracked vehicles, a combination of wheels and tracks [38], [39]. Each concept has its advantages and disadvantages [40]. The present global market for CTL-machines is just around 3000 machines [32]. A significant design constraint is, consequently, to develop a general concept that is gentle to the soil as well as to the operator, and that is productive and good enough for all types of targeted terrains and operations.

As stated above, leaf springs and live axles cannot satisfy forestry machine stability requirements on operations in rough terrain because of the limited vertical travel of these suspension solutions, and the fact that the load is too high. Hydro-pneumatic suspensions suffer from a severe risk of being destroyed by branches and falling objects during operations in harsh forestry terrain, which has a negative impact on machine availability, and thus is not economically sustainable. As discussed in the sub-sections above, the most feasible chassis suspension options for CTL-focused forestry machines, seems to be tracked, bogie, and pendulum arm solutions, but their relative multi-objective performance, i.e. effect on soil, operator and fuel consumption, have not been addressed in previous research.

The main objective of the presented research is to develop efficient, i.e. as simple as possible and as detailed as necessary, multi-body dynamics simulation models of CTL logging machines operating in rough terrain. The scope has been narrowed to the forwarder and the focus of the forwarder models to develop is to enable model-based and simulation-driven research and design as means to assess and compare different machine architectures and technical solutions aimed for reducing the negative explicit impact from machine operations on operator comfort and soil, and thus implicitly also the fuel consumption.

The main objective is decomposed into six interrelated research questions:

RQ1: How does an active pendulum-axle suspension affect forwarder performance on hard rough ground?

RQ2: What is the potential benefit from a pendulum arm suspension to forwarder performance in hard rough terrain?

RQ3: How can we qualitatively and quantitatively compare the performance of forwarder machine concepts with different suspension and traction unit solutions?

RQ4: What is the relative performance of bogie-wheeled and tracked forwarders when operating on rough hard ground and on soft soil?

RQ5: What is the relative performance of track units equipped with pendulum arm suspended and unsuspended bogie-mounted road wheels?

RQ6: How can we most efficiently use multi-body dynamics simulation models to predict and compare the performance of forestry machines on hard and soft terrain?

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2 MODEL-BASED RESEARCH METHODOLOGY

Intense competition forces manufacturers and engineers to develop new high-quality products at an increasingly rapid pace. The first step in improving such a process is to model and understand it [41]. A model is a simplified representation of “something”. Basically, in engineering research, models are created and used as means to understand reality and to identify what main aspect(s) to take into account to be able to manage it [42]. A common method in engineering, as well as in research, is to create and elaborate on cognitive, virtual, and physical models, with the purpose to describe properties, study relations, and/or to predict change/behavior. Further on, the term model is here used for a computer-based representation, and simulation is used for imitating the behavior of a real system by constructing and experimenting with a computer-based model of the system [43]. Simulation-driven design is a design process where decisions related to the behavior and the performance of a product design in all major phases of the process are significantly supported by computer-based modeling and simulations and an enabling framework [44].

2.1 A model-based framework for research on forestry machine and terrain interaction

Figure 2.1 shows the basic framework for the model-based and simulation-driven process for forest machine research and development that was previously presented by Pirnazarov [14]. The main objective for the modelling and simulation framework is to aid design decisions with respect to trafficability, mobility, and traction, e.g., machine architectures, soil and terrain types, machine-terrain interaction, traction and suspension control strategies, operator comfort, etc. A study of the mechanics of off-road machine and-terrain interaction is usually referred to as terramechanics, and it relies on geotechnical soil properties and terrain properties, such as spatial description of slopes, obstacles etc.

The product model is a parametric 3D CAD model that allows models of wheeled, tracked, and legged forest machines to be configured. Dependent on the actual design question, either a fine-grain MBS Adams [45] machine model, a medium-grain SimMechanics [46] model, which is a Simulink toolbox from Matworks Inc, or a coarse-grain US Army Waterways Experiment Station (WES) model [47] can be created. The WES model, which is empirically based, can be used to analytically target trafficability and mobility issues, while the SimMechanics model ideally is used to study and compare different principal solutions, such as choice of passive, semi-active, and active suspension and balancing strategies for the chassis, cabin, and/or operator seat. The fine-grain Adams model is most suited to aid detail design of chosen or studied machine architecture when operating on a specific soil in a specific terrain. A physical scale model is viewed as complementary to the virtual models.

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Figure 2.1. A modeling framework for configuring focused system models from 

terramechanics and machine sub‐models, from [14].

2.2 The research process

The research process used to address the research questions presented above contains the following main activities:

Perform a literature review and information search on the state-of-the-art in chassissuspension and traction unit designs in off-road machines in general, and forestrymachines in particular.

Perform a basic theoretical study of vehicle dynamics, ride comfort and lateralstability on an elementary level to build a theoretical base for further studies.

Use a standardized rough hard ground to make simulations and physical full scaletests repeatable and comparable.

Develop a, as simple as possible and as detailed as required, wheeled and trackedmulti-body dynamics simulation models with the modeling and simulation tools MSCAdams for MBD modeling and simulation, Adams ATV for track unit modeling, andMatlab Simulink for control modeling and co-simulation.

Verify and improve the simulation models with data from full vehicle testing of atraditional medium-sized and bogie-suspended forwarder, operating on hard roughterrain.

Develop a standard mid-sized forwarder base model that can be configured withdifferent suspensions and types of traction units.

Perform a model-based study of the relative performance of tracked forwarders onrough hard ground and on soft soil.

Analyze, generalize and present the simulation results.

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2.3 The research method and tools

The main methodology, i.e. the methods and assisting tools, used to enable the research process and to create knowledge on the potential benefits and challenges with different conceptual machine solutions has been to create and experiment with computer-based simulation models. A model is a cognitive tool and to assist a knowledge creation process it is preferably as simple as possible, but it must still include all the major aspects and relations, i.e., it must not be too simple. The chosen type of model to investigate the dynamic operator-related behavior and the machine-ground interaction has been a multi-body dynamics (MBD)model, and the chosen tools were MSC Adams [48] for MBD modeling and simulation,Adams ATV for track unit modeling, and Matlab Simulink for control modeling. To enablecomparison between different design concepts, a model of the standardized hard ground testtrack, the Skogforsk test track in Jälla [49], has been used (see Figure 2.2). Data fromphysical full-scale tests on this test track have also been used to verify and validate thesimulation base model.

Figure 2.2 The Skogforsk test track (left) and a model of the same (right), from [49].  

When investigating and assessing vehicle or working machine dynamics, it is common to analyze and present the generalized motions, velocities, and accelerations in the longitudinal, lateral, and vertical directions of the object, indicated in Figure 2.3 as x, y, and z translational directions, and the corresponding roll, pitch and yaw rotations. This notation is used in this thesis.

Figure 2.3. The six directions of motion. 

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3 SUMMARY OF RESULTS AND APPENDED PAPERS

Paper A: Four wheeled active suspended design concept for forestry machines on soft and rough terrain

This paper, which addresses RQ1, focuses on a four-wheeled forwarder concept that is equipped with two pendulum axels and an actively controlled hydraulic suspension system mounted on each wheel axle. In the paper the most commonly used chassis suspension systems e.g. leaf spring, hydro-pneumatic suspensions, and bogie systems are discussed. Then, a pendulum axle concept is proposed, and its dynamic behavior is investigated with comparison to a traditional bogie suspended system. The new concept machine has a significantly higher load capacity/machine weight ratio compared to existing machines, but the load capacity is lower than traditional medium-sized forwarders.

It is argued that the new forestry machines must be able to handle high loads and to operate in rough terrain and traditional six or eight wheeled machines with pairs of wheels mounted on bogies have very limited chassis damping and there is no active levelling control system. A prerequisite for good trafficability is that the suspension system should enable the working machine to overcome discrete obstacles and to operate on rough and steep slopes. A machine that is able to actively redistribute the load between its wheels might be a good solution to increasing the ability to surpass obstacles. Hence, an evenly distributed weight also evens the contact pressure and tractive shear between ground and all contact units and it can also provide a good ground clearance when operating on rough terrain.

In this study, the multi-objective concept performance of the design is studied with dynamic multi-body simulations and response analysis. A full-scale virtual prototype is modeled with MSC Adams, with the purpose to investigate the dynamic stability and the ability to traverse uneven terrain at relatively high speed. The study shows that the pendulum-axle suspension concept can provide transport at a higher speed on a standard rough hard-ground test track than a traditional bogie-suspended forwarder with less chassis vibrations and a more evenly distribution of the wheel-ground contact force. One observed drawback is that the studied pendulum-axle concept potentially requires large hydraulic power in the hydraulic actuators for levelling the machine and the carried load when travelling on very rough terrain.

Paper B: Multi-objective performance optimization of pendulum-arm suspensions for forestry machines

This paper focuses on analyzing and comparing the performance of active, semi-active, and passive suspension systems, with a pendulum-arm architecture, would provide to forestry machines by studying their implementation in the full-scale XT28 prototype. The XT28, a six-wheeled medium size forwarder prototype with active pendulum arm suspension, is being realized by Extractor AB in collaboration with Skogforsk, the Forestry Research Institute of Sweden. The prototype’s wheel suspension consists of an independent hydraulically actuated

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pendulum arm system. The architecture allows implementation of an active suspension control system.

In this paper, which addresses RQ2, a model-based methodology to automate design optimization of forestry machine suspensions with the use of optimization algorithms and multi-body dynamics (MBD) simulation software is proposed. The proposed methodology is applied to optimize active and semi-active control systems, as well as a passive system, for the XT28’s pendulum arm suspension and the performance of the different solutions are compared.

To develop forestry machines that are gentle to operators and terrain, it is essential to develop and implement efficient and robust chassis suspension solutions. The design objectives of the suspension are focused on reducing accelerations of the vehicle’s sprung mass and principally their effects on the operator’s health, and minimizing the amplitude of dynamic tire-ground loads, thus maximizing terrain-friendliness and ride safety.

The suspension control algorithm is modelled in Simulink and the simplified tire-ground interaction model is an Adams sub-model. The multi-objective optimization problem to reduce the variation in the tire-ground contact forces and the operator whole-body vibrations is performed with an evolutionary multi- objective optimization (EMO) genetic algorithm (GA) implemented in the tool Matlab.

The proposed methodology provides a fair and standardized way to compare the performance of the different suspensions. Simulation and optimization results show that a well-designed pendulum arm suspension systems has the potential to significantly improve forestry machine performance in terms of terrain gentleness and whole-body vibration levels, compared to unsuspended systems. Implementation of the passive or semi-active suspension systems would result in vibration dose values safely below the EU Directive 2002/44/EC imposed limit value, but above the imposed action value. The only suspension that showed a potential to provide complete compliance with the directive would be the actively controlled one.

Paper C: Investigating the dynamic behavior of a mid-sized forestry machine on sloped rough terrain

The main purpose of the presented investigation is to suggest criteria for assessing the performance of off-road forestry machines when operating in rough terrain. i.e., to address RQ3.

Since existing bogie suspended machine does not have any chassis suspension system, it is impossible to maintain an even ground contact force between all wheels when operating in rough terrain. The articulated joint that connects the front and rear wagons does not allow any relative vertical motion between the two wagons. When operating on a sloped terrain, this will cause an unfavorable distribution of the machine weight between the wheels. This paper investigates directly critical design issues of a conventional mid- sized bogie suspended forwarder operating in sloped rough terrain and indirectly their impact on the environment and the soil. Specifically, the dynamic generalized motion, velocity, and acceleration, in the

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longitudinal, lateral and vertical directions, are quantified and the dynamic stability performance of the studied forestry machine in sloped terrain is assessed. The chosen approach to study the multi-objective concept performance is to rely on multi-body simulations and response analysis.

Paper D: Model-based verification of a pendulum-arm suspended forwarder for sustainable forestry

This paper, which addresses RQ2, presents a study of a novel all-wheel-drive pendulum-arm suspended medium-sized forestry machine, with several upgrade performance steps, e.g. passive and active chassis suspensions. The main objectives of this research is to study how the daily vibration dosage can be reduced for a forwarder operator with the developed active pendulum arm suspended forwarder in comparison to the existing bogie-wheeled type of forwarder.

An engineering and research challenge is to define a robust and cost-effective chassis suspension that does not have the limitations of present and proposed seat and cabin suspension solutions. A six-wheeled pendulum-arm suspended forwarder has been designed and a full-scale prototype is realized. The dynamic performance of the suspended forwarder is assessed with MBD simulations with of straight motion on the standardized Skogforsk hard-ground test track. The dynamic performance is analyzed with respect to whole-body vibrations in the cabin and compared to the dynamic behavior of a bogie-wheeled forwarder of similar size and load carrying capacity. Two pendulum arm suspension variants are studied - a passive suspension and an actively controlled suspension.

The simulation results show a significant reduction of whole body vibration for the studied six-wheeled pendulum-arm suspended machine. The simulation results show that whole body vibrations dosage can be reduced by 80% with an actively controlled suspension and by 70% with a tuned passive suspension compared to present passive bogie solutions when operating on a standardized test ground. RMS tire-ground contact forces show 59% less with the passive suspension and 69% less with the actively controlled suspension compared to a traditional unsuspended machine.

Paper E: Model-based analysis of a tracked forwarder for sustainable forestry

This paper presents a model-based study of a novel tracked medium-sized forestry machine. There has been continuous research on how to make forest machines more sustainable and environmentally friendly without compromising their productivity. Issues, such as soil damage and vibration dosages for the operators are of high significance in this challenge. Irreversible soil damage caused by forestry machine operations must be significantly reduced and track units can reduce the ground pressure due to an increased contact area.

The Adams All-Terrain Vehicle (ATV) toolkit is add-on to the Adams Car tool which provides modeling templates to build tracked vehicles. The primary objective of the study is to develop a reliable multi-body dynamics simulation model of a tracked forwarder in the

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MSC Adams ATV module. This model will be used to evaluate the traction, handling and ride performance of the forwarder and to perform design analysis and optimization before realizing a full-scale prototype. The paper is thus a pre-study, necessary for dealing with RQ4 & RQ5 in an efficient way. The modeled machine is a standard eight-wheeled bogie type of forwarder with the four bogies replaced with four passively suspended track units reverse engineered and adapted from an off-road military vehicle. The paper briefly summarize how a tracked forwarder can be modeled and simulated using multi-body simulation software like Adams ATV and how the performance parameters can be evaluated.

Tuning the tensioner properties and rotational spring properties are important factors to make the simulation model run properly. There has to be enough tensioner force to ensure smooth operation of a track unit. The stiffness should not be too low resulting in unstable behavior or too high to compromise the ride quality. Too large or small stiffness properties may also cause numerical convergence problems. As the first step in the simulation run process, static equilibrium operation is performed to identify a reasonable value for the torsional spring stiffness for the road wheel. The vertical force exerted by the tracks on the ground has significant effect on soil damage and rutting. A good track solution will exert relatively low and uniform pressure on the ground. The force under the track segment in three directions; vertical, longitudinal and lateral is measured at an instant of time with the help of an Adams ATV macro. The Skogforsk hard and rough ground test track was incorporated in the simulation model as a road file.

Simulations, limited to straight line drive events were performed and the results show that tracks reduce the maximum contact pressure, which makes the tracked forwarder potentially gentler to soft soil than wheeled machines. A result from the presented research is also a methodology for model-based design of tracked forwarders based on using multi-body dynamics simulations with the Adams ATV modeling and simulation tool.

Paper F: A comparison of novel chassis suspended machines for sustainable forestry

The main objectives of this paper, which address RQ3, is to investigate and compare different forestry machine chassis suspension solutions with respect to their potential to reduce rutting of soft soil, i.e. to reduce compaction of soil and damage of roots, reduce the daily vibration dosages for the operators, and to increase driveline efficiency. The performance of various passive chassis suspensions on hard rough terrain is investigated based on multi-body dynamics simulations. The potential of each concept for further automation and performance upgrade is also qualitatively assessed.

In this paper it is argued that the most feasible chassis suspension options for forestry machines, seems to be bogie and pendulum arm suspension solutions. Because other type of chassis suspensions like leaf springs and live axles cannot satisfy the forestry machine stability requirements in rough terrain because of the limited vertical travel and the high load. Hydro-pneumatic suspensions suffer from a severe risk to be destroyed by branches and falling objects during forestry operations, which has a negative impact on the machine availability.

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In this study traditional eight-wheeled bogie type of medium-sized forwarder model is adapted with combination of pendulum suspension systems. In one concept only front bogies are substituted with pendulum suspension systems and in other concept the machines is suspended with six-wheel pendulum arm.

Based on multi-body dynamics simulations the dynamic behavior of three new medium-sized forwarder concepts travelling on standardized rough hard ground have been simulated, and the dynamic response of each concept has been compared with the behavior of a forwarder that is representative for most mid-sized forwarders seen in CTL logging. It is shown that a passive pendulum arm suspension can reduce the lateral accelerations in a passively suspended cabin with 50% compared to traditional bogie machines when travelling in rough hard terrain.

Paper G: Development of a dynamic base model of a bogie suspended forwarder

This paper presents a twelve degrees-of-freedom multi-body dynamics simulation model of a standard eight-wheeled bogie type of medium-sized forwarder with articulated steering which is equipped with pairs of wheels mounted on bogies. The main objective of this study is to build a computationally efficient multi-body dynamics base model of the forwarder and to verify the simulation model with measured data from a field tests that was carried out with the actual machine operating on a standardized hard ground test track. The validated simulation model is targeted to be used as a design base model that can be configured with different types of traction units to predict traction unit and soil interaction and machine dynamics. How to develop a new road file and how to import CAD geometries and define their properties for MBD model in MBD tool Adams from MSC software is illustrated in this paper.

A field test was conducted by Skogforsk, with the purpose to gain important sensor data for front and rear wagon motions, when traversing the rough hard ground test track. In order to capture all necessary frame motion signals from the front and rear wagons, a gyroscope sensor was attached with screws to the rear axle’s lid. A second gyroscope unit was placed under the operator seat inside the cabin. The measured data assist proper tuning and verification of the multi-body dynamic simulation model, which is targeted to be used as a reference model for future research. This particular research addresses thus RQ6 and is a pre-requisite for addressing RQ4 and RQ6.

Model-based design relies on focused models that are as simple as possible, but not too simple. It is shown that a configuration of seven rigid subsystems and eight flexible tires represented with the simple and computer efficient Fiala tire model enables the forwarder dynamic simulation model to be used to predict the roll and lateral motions of a forwarder operating on hard and rough terrain.

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Paper H: Model-based comparison of tracked forwarders

The focus of this study is to analyze and compare the dynamic performance on hard rough terrain of the standard existing eight-wheeled bogie type forwarder with two conceptually different forwarder track units, namely a track unit with pendulum arm suspended ground wheels and bogie suspended ground wheels. This study addresses RQ4 and RQ5. The main target is to assess the performance of tracked forwarders on hard rough terrain and to compare with the performance of a standard wheeled bogie type of solution. Furthermore, a secondary task is to compare the performance of different tracked forwarder concepts on very soft flat terrain, i.e., to address RQ5. For the hard rough terrain simulations, a model of the standard Skogforsk test track was used with focus on dynamic performance, and more specifically the roll and pitch rates, and the vertical acceleration. A type of sandy loam, consisting of 16% clay, 22% silt, and 62% sand, was used in the soft soil simulations.

It is clear that tracked forwarders are superior to wheeled machines on very soft and/or wet terrain and in this study the specific focus is on the wagon roll behavior, when operating on hard and modestly rough terrain. Based on MBS-simulations, maximum pitch and roll angles, and root mean square values for roll and pitch angles and vertical translation accelerations in the time domain are presented and compared for all models.

The simulations clearly indicate that that the tracked forwarder has a lower impact on the soil than the wheeled forwarder due to its significantly larger footprint area. Furthermore, it is shown that a tracked forwarder equipped with pendulum-arm suspended road wheels is slightly gentler for the operator on hard rough terrain and significantly better than the bogie-type of track unit.

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4 DISCUSSION AND FUTURE WORK

It is of outmost importance to have predictive, as well as explorative, models and tools that enable efficient development and engineering of the next generation of forestry machines, machines that are targeted to cause less damage to the soil and the root carpet, as well as geeing significantly more gentle to the machine operators in terms of whole body vibrations and mental strain. In science and engineering, research is to a large extent assisted by model construction and performing experiments with the models, such as mental models, virtual models, reduced physical models, and full-scale models. Model-based research can be viewed as a method in which exploration and learning is supported by a combination of theory-based modeling and experimental simulations with a purpose to explore or to predict the effects from prescribed causes [50].

In this research, the multibody dynamics system (MBS) simulation tool Adams from MSC Software is used to assess the dynamic behavior of forestry machines equipped with different types of chassis suspension systems and traction units. The main focus for simulations was their explicit impact on the operator and soil, and their implicit impact on the environment in terms of rolling resistance and thus fuel consumption. Tools for multibody dynamics (MBD) enable modeling and simulating the dynamic behavior of complex assemblies and they help to investigate how a system performs at a prescribed range of operations. With MBD, engineers can potentially create and test virtual prototypes of mechanical systems in a fraction of the time and cost required to build and test physical prototypes and to predict the dynamic behavior of moving parts, and how loads and forces are distributed throughout a mechanical system [51].

In order to estimate and compare the multi-criteria performance of studied machine concepts, the nine characteristic properties shown in Table 1 have been chosen, based on reasoning, and used. These properties represent different sustainability aspects of machine gentleness, i.e. how gentle a concept is to the ground, to the operator, and to the environment, or more specifically the engine emissions. The effect on the ground depends on the total machine weight, how this weight is distributed between the traction units, and how the total traction force is distributed between the traction units. i.e., the lower the machine weight, the more even the weight and the traction force is distributed, the less negative impact the machine will have on the ground. The forwarder operator is negatively affected by whole-body vibrations and the operator of the harvester by the mental strain. The dynamics related effects, i.e. the whole-body vibrations have been focused in this research, and not the mental strain or other health aspects. The whole-body vibrations can be managed with different means, such as chassis suspensions, cab suspensions, seat suspensions, and combinations of those three. Furthermore, suspension can be passive, active or semi-active/semi-passive. Since the presented research was focused on effects from chassis suspension and traction unit design, the gentleness to the operator has been simplified to the dynamic behavior of a passively suspended cabin, and more specifically the vertical, lateral, and roll motions of the cab.

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Table 4.1.  Targeted performance characteristics. 

Sustainability requirement Performance property Gentle to the ground Total machine weight

Maximum/mean wheel normal force Maximum/mean wheel tractive force

Gentle to the operator Vertical cab motion Later cab motion

Cab rollEnabler of active suspension

Gentle to the air (fuel consumption) Rolling resistance on soft soil Distribution of tractive force between traction units

More detailed analyses of the machine-soil interaction and especially the negative effects on soil and roots, require that the normal and shear contact forces are transformed to detailed distributions contact pressure and shear stresses.

It can also be argued that none of the two studied track units are concepts and all design parameter values, such as dimensions, number of and diameter of the road wheels, placement of the sprocket etc., are based on engineering judgement and not performance optimized in any way. It is quite clear that both track unit concepts can be improved. But, it is quite clear, based on the simulated results, that operation on hard rough terrain benefits from pivot mounted track units. It is also clear that the performance of a bogie tracked unit operating on soft soil would benefit from being a fixed mounted with the frame. This design dilemma for the bogie tracked concept is probably solvable, but it comes with an extra function, such as on-off function, which adds complexity and cost.

Proper tire modelling is essential for performing high quality simulations of the ride and handling characteristics of vehicles in general, and of off-road wheeled machines in particular. A proper simulation model of a machine with large off-road tires rolling on non-deformable terrain requires a lot of tire characteristic properties in the form of side-force versus slip angle curves. Many different tire models have been developed and validated for passenger cars, but they are, however, not applicable for large tires. The Fiala tire model does not take into account the speed dependency of wheel stiffness and damping. For simulations of a low and constant speed case, the accuracy of the simulation result is much higher than if it runs with a varying speed. The Fiala tire model is chosen in the studies that have been performed, since the main target results were the vertical contact forces and the lateral and roll motions of the cabin, when the forwarder was operating on hard rough terrain, i.e. not on soft and fairly flat soil.

Throughout this thesis work, the Skogforsk hard and rough ground test track has been used for most of the simulations. The test track is a simplified terrain representation, i.e. a model, of rough forest terrain, and it enables reproducible full-scale tests to be performed, but it is obviously not real forest terrain. Comparisons between the dynamic performances of different machine concepts when traversing this standard test have proved to be reliable, but the

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comparisons are only valid for the tests track used. The relative performance may be different in other types of real or simplified terrains.

The simulated vertical, lateral, roll and pitch motions for the rigid-body dynamics wheeled bogie type of forwarder, equipped with the flexible Fiala tire model, correlated very well in the low-frequency range with the measured data from field-tests performed on the well-defined Skogforsk hard and rough test track driving at constant speed of 0.5 m/s. There are, however, uncertainties in how the simulated dynamic behavior in the medium- and high-frequency ranges is affected by the simplifications imposed by rigid component assumption and the simplified tire used in MBD models.

The suspension system connects the wheels with the frame and consequently contributes to the overall stability of the machine and potentially filters the vibrations and shocks caused by the dynamic interaction between the uneven ground and the traction units. In this thesis, the main focus was on chassis suspensions. Development of novel forestry machines must, most likely, find modular solutions that balance the benefits and disadvantages of chassis, cabin, and seat suspensions. Further issues that should be focused on in the following research are without priority:

Perform simulations and analyze the results for cases with a variety of soft soils 

Perform and analyze a wider range of field tests. Perform a detailed investigation of which tire model that is most efficient for different

off-road mobility cases.  

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5 CONCLUSIONS

The presented research is here concluded as answers to the research questions formulated in chapter 1.4, and future challenges that have been identified.

The main objective is decomposed into six interrelated research questions:

RQ1: How does an active pendulum-axle suspension affect forwarder performance on hard rough ground?

An active pendulum axle suspension has some advantages compared to a traditional bogie-wheeled forwarder. The studied machine concept has a significantly higher load capacity/machine weight ratio compared to existing machines, which potentially can be used to increase the load capacity and thus the productivity, or to reduce the ground pressure. Another advantage is that the vertical motion will be smaller for a pendulum-axle machine than a bogie-wheeled machine operating on very rough terrain, because the wheel width normally is significantly larger than the distance between a wheel pair on a bogie. The drawback is that a pendulum-axle forwarder requires very substantial actuation forces for stabilizing the load which will induce large lateral motion and thus large inertia forces that must be handled by high-power actuators.

RQ2: What is the potential benefit from a pendulum arm suspension to forwarder performance in hard rough terrain?

A six-wheeled forwarder with each wheel mounted on a passively suspended pendulum arm can significantly reduce the large motions of the machine compared to a bogie-wheeled machine. A passively pendulum-arm suspended forwarder has the potential to reduce the operator vibration dose value (VDV) with almost 80% and the total tire-ground normal force with almost 60% compared to traditional bogie-wheeled forwarders. A pendulum-arm suspended machine enables implementation of an active suspension that can reduce whole-body vibrations during transport in rough terrain at low speed and also enhance stability by providing a levelling function.

RQ3: How can we qualitatively and quantitatively compare the performance of forwarder machine concepts with different suspension and traction unit solutions?

Requirements on the next generation of forestry machines are that they are gentler to the operator, in terms of whole-body vibrations, cause less damage to the forest soil, and have significantly less specific fuel consumption, i.e. fuel consumption per harvested volume. These targets are multidimensional and to some extent contradictory, which implies that they should not be aggregated into a single scalar performance quantity, but that the performance in each of the three dimensions should be managed separately. Furthermore, to enable repeatable results, it is advisable to use a standardized test ground, such as the Skogforsk test ground, for physical and virtual comparisons of the dynamic performance on hard and rough ground. By applying the proposed multidimensional sustainability properties, it is shown that a passively pendulum-arm suspended forwarder is a

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significantly more operator and soil sustainable design compared to traditional bogie-wheeled forwarders.

RQ4: What is the relative performance of bogie-wheeled and tracked forwarders when operating on rough hard ground and on soft soil?

It is quite clear that an articulated forwarder equipped with four track units that are pivot mounted on the front and rear frames shows a comparable dynamic behavior on hard and rough terrain as an eight-wheeled forwarder equipped with wheels mounted pair-wise on bogies. Furthermore, a tracked forwarder with pendulum-arm passively suspended road wheels in the track units can be tuned to give less vertical, lateral and roll vibrations in the front frame than an eight-wheeled forwarder. A forwarder equipped with track units with bogie-mounted road wheels, on the other hand, shows larger vibration amplitudes than a traditional bogie-wheeled forwarder. Tracked forwarders are potentially much better on very soft, or sensitive, soil than wheeled machines, with the pendulum-arm type of track unit significantly superior to the bogie-type of track unit.

RQ5: What is the relative performance of track units equipped with pendulum arm suspended and unsuspended bogie-mounted road wheels?

The pendulum-arm type of track unit is significantly superior to the bogie-type of track unit on hard rough terrain as well as on very soft soil with low bearing capacity. The best dynamic performance from tracked forwarders on hard rough ground was achieved with the track units pivot mounted on the frames. Pivot mounted track units with pendulum-arm mounted road wheels also showed a superior performance on very soft soil. The track unit with bogie-mounted road wheels showed a large pivot rotation of the pivot-mounted track unit when a tractive torque was applied, resulting in a significant reduction of the real contact area.

RQ6: How can we most efficiently use multi-body dynamics simulation models to predict and compare the performance of forestry machines on hard and soft terrain?

A predictive method, physical or virtual, should preferably be as simple as possible, but it must also be as detailed as required. The level of detail and accuracy required is context dependent. Model-based methods gives repeatable results, which is valuable when investigating cause and effect relations and when qualitatively and/or quantitatively comparing different machine configurations. Comparisons of logged sensor data from physical tests with full-scale bogie-wheeled machines traversing the Skogforsk standard hard ground test track, with replicated multi- body model simulation results clearly shows that rigid-body models with flexible tires can effectively be used to assess the dynamic behavior relevant for cabin vibrations and machine-ground contact behavior. Comparison of full-scale field test data and MBD-simulation results clearly show that the Fiala model, the simplest and thus least computer demanding tire-ground interaction model, is sufficient when primarily low-medium frequency vertical, lateral and roll motions are targeted, which is the case when reduction of whole-body operator vibrations are in focus.

 

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