Control Strategy Investigation for a Down- Scaled Forwarder Refinement, Testing and Analysis
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DEGREE PROJECT IN MECHANICAL ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2017 Control Strategy Investigation for a Down- Scaled Forwarder Refinement, Testing and Analysis ANQING DUAN KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT Examensarbete MMK 2016:99 MDA 571 Control strategy investigation for a down-scaled forwarder Anqing Duan Godkänt Examinator Handledare 2017-06-15 Jan Wikander Lei Feng Uppdragsgivare Kontaktperson Skogforsk Olle Gelin Sammanfattning Den dominerande metoden för att skörda träd i Norden är kortvirkesmetoden. Kortvirkesmetoden realiseras av två maskiner: en skördare som används för avverkning av träden och en forwarder som används för att transportera stockarna till en väg som är tillgänglig för lastbilar. För att öka produktiviteten är en möjlig lösning att öka forwarderns körhastighet. Detta kommer dock att bidrar till att operatören utsätts för större vibration och att marken utsätts för större däcktryck. För att lösa ovanstående problem har en sexhjulig forwarder med hjulupphängning i pendelarmar tidigare utvecklats där varje hjul styrs individuellt av en motsvarande hydraulmotor. Med tanke på att genomförande av fälttester på den verkliga forwardern skulle bli tids- och resurskrävande, har tidigare en 1: 5 nedskalad forwarder tillverkats för att underlätta forskningen. På grund av storleksbegränsningarna ersattes de hydrauliska motorerna med linjära cylindrar samt gjordes en del andra konstruktionsändringar. Arbetet med denna avhandling är att ytterligare förfina den nedskalade forwardern med fokus på en 3D modellerings- och animeringsmiljö. Styrstrategin som föreslås är baserad på kinematik. Den föreslagna regleralgoritmen kan balansera den simulerade nedskalade forwardern genom att lösa ett linjärt optimeringsproblem i realtid. Begränsad av sensorprestanda och filteralgoritmer testas endast en förenklad kinematikreglering i simuleringen och i den verkliga nerskalade forwardern. Det kan ses i simulering att både pitch- och roll-vinklar kan minskas kraftigt även med den förenklade reglermetoden vid körning genom Skogforsks standardprovbana. Stabilitetsanalysmetoden för den nedskalade forwardern diskuteras också i denna avhandling. Huvudbidraget från detta examensarbets består i att en annan synvinkel presenterats jämfört med föregående arbeten när det gäller förslag till nya fungerande styrstrategier. Master of Science Thesis MMK 2016:99 MDA 571 Control strategy investigation for a down-scaled forwarder Anqing Duan Approved Examiner Supervisor 2017-06-15 Jan Wikander Lei Feng Commissioner Contact person Skogforsk Olle Gelin Abstract The predominant method of trees harvesting in Nordic countries is cut-to-length logging (CTL). CTL is realized by two machines: a harvester which is used for felling the trees, and a forwarder which is used for transporting the logs to a road accessible by trucks. To increase the productivity, one possible solution is to increase the forwarder driving speed. However, this will make the operator exposed to larger vibration and cause bigger tire-ground pressure. To solve the abovementioned problems, Skogforsk developed a six-wheel pendulum arm suspended forwarder with each wheel of the forwarder controlled individually by the corresponding hydraulic motor. Given that conducting the field tests on the real forwarder will be time consuming and inconvenient, a 1:5 down-scaled forwarder was manufactured previously to facilitate the research. Due to the size limitation, the hydraulic motors are replaced with the linear actuators besides a few other design changes. The work of this thesis is to further refine the down-scaled forwarder with the focus on implementing the control system. The forwarder is modeled using Sim which provides a 3D animation modelling environment. The control strategy is proposed in a perspective of kinematic control. The proposed control algorithm can balance the down-scaled forwarder orientation by solving a linear optimization problem in real time. Restricted by the sensors implementation and filter algorithms, only a simplified kinematics control is tested in the simulation and the real down-scaled forwarder. It can be seen in simulation that both the pitch and roll angle can be reduced greatly by driving through the Skoforsk standard test track with the simplified control method. Stability analysis method for the down-scaled forwarder is also discussed in this thesis. The main contribution of this thesis will be indicating a different point of view from the previous work in terms of control strategy proposal. Acknowledgement First of all, I would like to express my gratitude to Ulf Sellgren and Skogforsk rep- resentatives, without whom I will not be able to have this great thesis opportunity. Second, I really appreciate the guidance from my supervisor Lei Feng. In addition to technical details, I think the most important thing I have learned from him is rigorous working attitude towards research. I would also like to thank Staffan Qvarnström for his help on the hardware. Last but not least, I am really thankful for the support from my colleagues Yuchao Li, Zhen Li and Qingxiao An. I will never forget the days when we worked together. Nomenclature Abbreviations Symbol Description WAAV Wheeled actively articulated vehicle LQ Linear quadratic LQG Linear-quadratic-Gaussian PID Proportional–integral–derivative MPC Model predictive control IMU Inertia measurement unit PD Proportional–derivative RMS Root-mean-square ECU Electronic control unit FR Front right FL Front left MR Middle right ML Middle left BR Back right BL Back left CoM Center of mass 10 Contents 1 Introduction 1 1.1 Background and Problem Description . 1 1.2 Purpose . 2 1.3 Delimitations . 4 1.4 Method Description . 4 1.5 Expected outcomes . 5 2 Frame of Reference 7 2.1 Suspension Systems . 7 2.2 Brief Survey on Active Suspensions . 9 2.3 Wheeled Actively Articulated Vehicle . 9 2.4 Vehicle Model . 12 2.4.1 Dynamic Model . 12 2.4.2 Kinematics Model . 14 2.4.3 Model Environment . 17 3 Modelling Approaches and Control 19 3.1 Simulation Model . 19 3.1.1 Linear Actuator . 19 3.1.2 Pendulum Arm . 21 3.1.3 Test Track . 21 3.1.4 Wheel-Track Interaction . 22 3.2 Control Strategy Proposal . 23 3.2.1 Kinematic Control . 23 3.2.2 Simplified kinematic control: On-off Strategy . 26 3.2.3 PID Control . 28 3.3 Simulation results . 31 4 Implementation 35 4.1 System Overview . 35 4.2 Hardware List . 36 4.3 IMU Filter Algorithm Analysis . 38 5 Results 41 5.1 Simulation results . 41 5.1.1 Tilt Angles . 41 5.1.2 Tire Pressure . 43 5.2 Model verification . 49 6 Future Work and Recommendations 53 6.1 Faced Troubles . 53 6.2 Future Work . 54 Bibliography 57 Appendices Chapter 1 Introduction This chapter introduces the background of the thesis work, describes the problem to be solved, states the purpose and delimitations, and proposes the selected method as well as the expected outcomes. 1.1 Background and Problem Description The predominant harvesting method used in Scandinavian countries is cut-to-length logging (CTL), which is based on a two-machine solution: a harvester and a for- warder [1]. The harvester is used for felling, delimbing and bucking trees while the forwarder carries the logs from the harvesting area to a roadside loading area where they can be picked up by a truck. In order to stay competitive, the Swedish logging industry needs to increase the productivity by 2% to 3% per year [2]. One main solution to improving the produc- tivity is to increase the forestry vehicle driving speed. However, if an off-road vehicle drives through a rough terrain at a high speed, the operator will be exposed to a strong vibration. It has been shown that individual lifetime exposure to the whole- body vibration may play an important part in the evaluation of health effects [3]. In addition, forestry machines are usually very heavy, the usage of them may harm the fragile and irreplaceable landscapes, leading to erosion and soil compaction, which then will affect water absorption and soil permeability [4]. In an endeavor to solve these problems, Skogforsk [5], the forest research institute of Sweden, has built a full-scale forwarder prototype called XT28 as shown in Fig. 1.1. This prototype uses hydraulic pendulum arm suspension for each of its six wheels. The pendulum arm suspension can be controlled individually to drop and lift its corresponding wheel. Therefore, the off-road vehicle will be leveled when driving through the rough terrain with bumps or pits. By doing so, the operator should have better driving comfort since the vehicle can balance itself. Also, the soil damage will be reduced by actively distributing tire-ground pressure among six wheels. 1 CHAPTER 1. INTRODUCTION Figure 1.1: XT28 Prototype To facilitate the research on the XT28 prototype, a physical 1:5 downscaled for- warder (Fig. 1.2) was realized previously [6]. With the help of the down-scaled prototype, it is much more economic and time saving to test specific features or functions instead of driving a full-scaled version. Some relevant experiments have shown how a scaled vehicle system can be used to facilitate the prototyping of vehicle control systems [7]. Figure 1.2: Down-scaled Prototype 1.2 Purpose The general context of this thesis work is to further refine the down-scaled forwarder which has been developed previously, yet without any control system implemented. The specific purpose of this thesis work is to investigate a suitable control strategy and manage to implement it on the down-scaled forwarder so that the active sus- pension system would function to keep the vehicle body as horizontally stable as 2 CHAPTER 1. INTRODUCTION possible while reducing the tire-ground pressure. The control effects of the proposed control strategy shall be verified by driving the down-scaled forwarder through the Skogforsk standard test track. The specification of the test track is shown in Fig. 1.3. Figure 1.3: Skogforsk Test Track Sketch To illustrate the vehicle dynamic behaviour, the coordinate system and naming convention are introduced in Fig.