Modeling and simulation of a six degrees of freedom excavator
Päkkilä, Sami (2017)
Päkkilä, Sami
2017
Automaatiotekniikka
Teknisten tieteiden tiedekunta - Faculty of Engineering Sciences
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Hyväksymispäivämäärä
2017-09-06
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tty-201708241796
https://urn.fi/URN:NBN:fi:tty-201708241796
Tiivistelmä
In this thesis, a simulation model for a Komatsu PC138US-8 excavator was constructed. To verify the performance and validity of the model, two closed-loop control schemes were created, and they were tested with use case scenarios in which an excavator might operate at a work site. The mechanical model of the excavator was assembled in Solid-Works CAD software and imported into MATLAB Simulink environment. The hydrau-lics of the excavator were constructed in Simulink using the excavator’s hydraulics schematic as a reference.
First off, the forward and inverse kinematics were formulated. With these kinematical equations, the excavator’s operational space and joint space can be linked together, providing a foundation for the controller design implemented in this thesis. In addition to the kinematics, the dynamics of the excavator are mandatory in order to construct the desired controller schemes in this thesis. The dynamics were derived using Lagrangian equations of motion using the mass and dimension properties obtained from the CAD model.
The first use case studied in this thesis was a digging and dumping sequence. The refer-ence trajectory was constructed from four separate sections, which were combined to form the full reference trajectory. The digging and dumping sections of the full se-quence were constructed in the Cartesian space, and the bucket turning sections were constructed in the joint space. The full trajectory was then constructed in the joint space. Based on the simulation results, the excavator follows the reference trajectory well. However, fast movement on the trajectory causes the joint angles to lag the reference momentarily leading to small error in the joint value. In addition, minute steady state error is present in the joint values due to the lack of a derivative block in the implement-ed controller.
The second use case studied relied on the use of a hybrid position/force controller in a digging operation, where contact with the ground inflicts external force on the excava-tor end-effector. The x-directional position reference trajectory was given in the Carte-sian space and the external contact force was given as the z-directional force reference. The simulation results show that the position tracking was good as the upper level posi-tion control loop kept the joint error under 0.025 radians during the digging operation. The lower level force control loop followed the force reference trajectory well, but quite large vibration was present in the controller force signal.
First off, the forward and inverse kinematics were formulated. With these kinematical equations, the excavator’s operational space and joint space can be linked together, providing a foundation for the controller design implemented in this thesis. In addition to the kinematics, the dynamics of the excavator are mandatory in order to construct the desired controller schemes in this thesis. The dynamics were derived using Lagrangian equations of motion using the mass and dimension properties obtained from the CAD model.
The first use case studied in this thesis was a digging and dumping sequence. The refer-ence trajectory was constructed from four separate sections, which were combined to form the full reference trajectory. The digging and dumping sections of the full se-quence were constructed in the Cartesian space, and the bucket turning sections were constructed in the joint space. The full trajectory was then constructed in the joint space. Based on the simulation results, the excavator follows the reference trajectory well. However, fast movement on the trajectory causes the joint angles to lag the reference momentarily leading to small error in the joint value. In addition, minute steady state error is present in the joint values due to the lack of a derivative block in the implement-ed controller.
The second use case studied relied on the use of a hybrid position/force controller in a digging operation, where contact with the ground inflicts external force on the excava-tor end-effector. The x-directional position reference trajectory was given in the Carte-sian space and the external contact force was given as the z-directional force reference. The simulation results show that the position tracking was good as the upper level posi-tion control loop kept the joint error under 0.025 radians during the digging operation. The lower level force control loop followed the force reference trajectory well, but quite large vibration was present in the controller force signal.