Mohammad Poursina

• Mamo, Mebaye Belete; Ebbesen, Morten Kjeld & Poursina, Mohammad (2023). Closed-Form Dynamic Model of Planar Multilink Flexible Manipulator. I IEEE, . (Red.), IECON 2023- 49th Annual Conference of the IEEE Industrial Electronics Society. IEEE (Institute of Electrical and Electronics Engineers). ISSN 979-8-3503-3182-0. doi: 10.1109/IECON51785.2023.10312080.
• Mamo, Mebaye Belete; Ebbesen, Morten Kjeld & Poursina, Mohammad (2023). Dynamic Modelling and Simulation of Two-link Flexible Robot Using Rayleigh Beam Theory . I Zhou, Jing (Red.), 2023 11th International Conference on Control, Mechatronics and Automation (ICCMA). IEEE conference proceedings. ISSN 979-8-3503-1568-4. s. 164–170. doi: 10.1109/ICCMA59762.2023.10374812.
• Ho, Alex; Wold, Margrethe; Poursina, Mohammad & Conway, John Thomas (2023). The accuracy of mutual potential approximations in simulations of binary asteroids. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 671. doi: 10.1051/0004-6361/202245552. Vis sammendrag

  Context. Simulations of asteroid binaries commonly use mutual gravitational potentials approximated by series expansions, leading to truncation errors, and also preventing correct computations of force and torque for certain configurations where the bodies have overlapping bounding spheres, such as in the rotational fission model for creating asteroid binaries and pairs. Aims. We address errors encountered when potentials truncated at order two and four are used in simulations of binaries, as well as other errors related to configurations with overlapping bounding spheres where the series diverge. Methods. For this we utilized a recently developed method where the gravitational interaction between two triaxial ellipsoids can be calculated without approximations for any configuration. The method utilizes surface integration for both force and torque calculations, and it is exact for ellipsoidal shapes. We also computed approximate solutions using potentials truncated at second and fourth order, and we compare these with the solutions obtained with the surface integral method. The approximate solutions were generated with the “General Use Binary Asteroid Simulator” (GUBAS). Results. If the secondary is located with its centroid in the equatorial plane of the primary, the error in the force increases as the secondary is moved closer to the primary, but is still relatively small for both second and fourth order potentials. For torque calculations, the errors become more significant, especially if the other body is located close to one of the extended principal axes. On the axes themselves, the second order series approximation fails by 100%. For dynamical simulations of components separated a few primary radii apart, the fourth order approximation is significantly more accurate than the second order. Furthermore, because of larger errors in the torque calculations, the rotational motion is subject to greater inaccuracies than the translational motion. For configurations resembling contact binaries where the bounding spheres overlap, the errors in both force and torque in the initial stages of the simulation are considerable, regardless of the approximation order, because the series diverge. A comparison of the computational efficiency of the force and torque calculations shows that the surface integration method is approximately 82 times and four times slower than the second and fourth order potentials, respectively, but approximately 16 times faster than the order eight potential. Comparing the computation efficiency of full simulations, including the calculations of the equations of motion, shows that the surface integration scheme is comparable with GUBAS when an order four potential is used. Conclusions. The errors generated when mutual gravitational potentials are truncated at second or fourth order lead to larger errors in the rotational than in the translational motion. Using a mathematically exact method for computing forces and torques becomes important when the bodies are initially close and the bounding spheres overlap, in which case both the translational and rotational motion of the bodies have large errors associated with them. For simulations with two triaxial ellipsoids, the computational efficiency of the surface integral method is comparable to fourth order approximations with GUBAS, and superior to eight order or higher.

• Nikravesh, Parviz E. & Poursina, Mohammad (2022). Determination of effective mass for continuous contact models in multibody dynamics. Multibody system dynamics. ISSN 1384-5640. doi: 10.1007/s11044-022-09859-4. Vis sammendrag

  Continuous contact models have been popular in representing contact forces between impacting bodies of a multibody system. These models consider the contact force to be the result of a logical spring-damper element between the contacting bodies that exists for a very short period. The simplified and approximated model for generating the contact force is then assumed to be a mass-spring-damper system. Therefore, three common parameters that these models require are the spring stiffness, damping coefficient, and the so-called effective mass. For systems containing one degree-of-freedom, classical methods based on the kinetic energy have commonly been used to determine the effective mass. However, for multiple-degree-of-freedom multibody systems that contain kinematic joints, the determination of the effective mass has not been adequately addressed in the literature. This paper proposes a simple method for computing the effective mass based on the concept of impulse–momentum balance. This approach is applicable to both constrained and unconstrained equations of motion regardless of the multibody system’s number of degrees-of-freedom.

• Ho, Alex; Wold, Margrethe; Poursina, Mohammad & Conway, John Thomas (2022). Dynamics of asteroid systems post-rotational fission. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 665. doi: 10.1051/0004-6361/202243706. Fulltekst i vitenarkiv
• Sabet, Sahand; Singh, Mohit; Poursina, Mohammad & Nikravesh, Parviz E (2021). A Highly Maneuverable Hybrid Energy-Efficient Rolling/Flying System, 2021 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS). IEEE conference proceedings. ISSN 978-1-6654-1714-3. s. 2485–2490. doi: 10.1109/IROS51168.2021.9636811.
• Sabet, Sahand; Poursina, Mohammad & Nikravesh, Parviz E (2021). Control of Spherical Robots on Uneven Terrains, 2021 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS). IEEE conference proceedings. ISSN 978-1-6654-1714-3. s. 8159–8165. doi: 10.1109/IROS51168.2021.9636543. Vis sammendrag

  Hybrid robots incorporate the advantages of both aerial-only and terrestrial-only vehicles to achieve enhanced mobility and better energy efficiency. Among hybrid vehicles, spherical robots offer the best maneuverability. While operating on uneven surfaces is one of the main benefits of spherical robots, the current literature only covers control of these robots on flat surfaces. This work presents two control algorithms to track a desired trajectory and angular velocity of spherical robots on uneven terrains. The proposed control algorithms can be used when the terrain is known analytically or empirically (i.e., point cloud). By allowing the controller to use empirical information about the terrain profile, this work broadens the implementation of spherical robots in real applications.

• Ho, Alex; Wold, Margrethe; Conway, John Thomas & Poursina, Mohammad (2021). Extended two-body problem for rotating rigid bodies. Celestial mechanics & dynamical astronomy. ISSN 0923-2958. 133. doi: 10.1007/s10569-021-10034-8. Fulltekst i vitenarkiv Vis sammendrag

  A new technique that utilizes surface integrals to find the force, torque and potential energy between two non-spherical, rigid bodies is presented. The method is relatively fast, and allows us to solve the full rigid two-body problem for pairs of spheroids and ellipsoids with 12 degrees of freedom. We demonstrate the method with two dimensionless test scenarios, one where tumbling motion develops, and one where the motion of the bodies resemble spinning tops. We also test the method on the asteroid binary (66391) 1999 KW4, where both components are modelled either as spheroids or ellipsoids. The two different shape models have negligible effects on the eccentricity and semi-major axis, but have a larger impact on the angular velocity along the z-direction. In all cases, energy and total angular momentum is conserved, and the simulation accuracy is kept at the machine accuracy level.

• Dabiri, Arman; Poursina, Mohammad & Machado, Jose Tenreiro (2020). Dynamics and optimal control of multibody systems using fractional generalized divide-and-conquer algorithm. Nonlinear dynamics. ISSN 0924-090X. 102, s. 1611–1626. doi: 10.1007/s11071-020-05954-3.
• Poursina, Mohammad & Nikravesh, Parviz E (2020). Characterization of the Optimal Damping Coefficient in the Continuous Contact Model. Journal of Computational and Nonlinear Dynamics. ISSN 1555-1415. 15(9). doi: 10.1115/1.4047136. Vis sammendrag

  This paper presents an analytical formula to characterize the damping coefficient as a function of system's parameters in a continuous force model of impact. The contact force element consists of a linear damper which is in a parallel connection to a spring with Hertz force-deformation characteristic. Unlike the existing models in which the separation condition is assumed to be at the time at which both zero penetration (deformation) and zero force occur, in this study, only zero contact force is considered as the separation condition. To ensure that the continuous contact model obtains the desired restitution, an optimization process is performed to find the equivalent damping coefficient. The analytical and numerical investigations show that the resulting damping coefficient can be expressed as a function of system's parameters such as the effective mass, penetration speed at the start of the impact, Hertz spring constant, and the coefficient of restitution.

• Sabet, Sahand; Poursina, Mohammad; Nikravesh, Parviz E.; Reverdy, Paul & Agha-Mohammadi, Ali-Akbar (2020). Dynamic Modeling, Energy Analysis, and Path Planning of Spherical Robots on Uneven Terrains. IEEE Robotics and Automation Letters. ISSN 2377-3766. 5(4). doi: 10.1109/LRA.2020.3010489. Vis sammendrag

  Spherical robots are generally comprised of a spherical shell and an internal actuation unit. These robots have a variety of applications ranging from search and rescue to agriculture. Although one of the main advantages of spherical robots is their capability to operate on uneven surfaces, energy analysis and path planning of such systems have been studied only for flat terrains. This work introduces a novel approach to evaluate the dynamic equations, energy consumption, and separation analysis of these robots rolling on uneven terrains. The presented dynamics modeling, separation analysis, and energy analysis allow us to simply implement path planning algorithms to find an optimal path. One of the advantages of this work is that these algorithms can be used when either the analytical or the empirical information about the terrain is available.

• Poursina, Mohammad & Nikravesh, Parviz E. (2020). Optimal damping coefficient for a class of continuous contact models. Multibody system dynamics. ISSN 1384-5640. 50, s. 169–188. doi: 10.1007/s11044-020-09745-x. Fulltekst i vitenarkiv Vis sammendrag

  In this study, we develop an analytical formula to approximate the damping coefficient as a function of the coefficient of restitution for a class of continuous contact models. The contact force is generated by a logical point-to-point force element consisting of a linear damper connected in parallel to a spring with Hertz force–penetration characteristic, while the exponent of deformation of the Hertz spring can vary between one and two. In this nonlinear model, it is assumed that the bodies start to separate when the contact force becomes zero. After separation, either the restitution continues or a permanent penetration is achieved. Therefore, this model is capable of addressing a wide range of impact problems. Herein, we apply an optimization strategy on the solution of the equations governing the dynamics of the penetration, ensuring that the desired restitution is reproduced at the time of separation. Furthermore, based on the results of the optimization process along with analytical investigations, the resulting optimal damping coefficient is analytically expressed at the time of impact in terms of system properties such as the effective mass, penetration velocity just before the impact, coefficient of restitution, and the characteristics of the Hertz spring model.

Se alle arbeider i Cristin

• Mamo, Mebaye Belete; Ebbesen, Morten Kjeld & Poursina, Mohammad (2023). Dynamic Modelling and Simulation of Two-link Flexible Robot Using Rayleigh Beam Theory.
• Mamo, Mebaye Belete; Ebbesen, Morten Kjeld & Poursina, Mohammad (2023). Closed-Form Dynamic Model of Planar Multilink Flexible Manipulator.
• Poursina, Mohammad & Nikravesh, Parviz E (2023). Damping Coefficient for Impacts with Residual Deformation at the Time of Separation.
• Wold, Margrethe; Ho, Alex; Poursina, Mohammad & Conway, John Thomas (2022). Two-body interactions with surface integrals.
• Shahbazi, Zahra & Poursina, Mohammad (2021). Protein Kinematics. I Ang, M. H.; Khatib, O. & Siciliano, B. (Red.), Encyclopedia of Robotics. Springer. ISSN 978-3-642-41610-1. doi: 10.1007/978-3-642-41610-1.
• Sabet, Sahand; Sing, Mohit; Poursina, Mohammad & Nikravesh, Parviz E (2021). A Highly Maneuverable Hybrid Energy-Efficient Rolling/Flying System. Vis sammendrag

  Spherical robots are typically comprised of an actuation unit enclosed by a spherical shell. Among nonholonomic systems, spherical robots offer the best maneuverability and lowest energy consumption (due to their omnidirectional movement and single contact point with the ground). This allows them to traverse rough and uneven terrains. Further, using their ability to roll on the ground, they can provide a significantly higher operating time compared to aerial-only robots. Unfortunately, these robots are under-emphasized by researchers compared to other robots (i.e., legged or wheeled robots). Additionally, despite their potential to be used in a multitude of real-world applications, spherical robots have not been successfully adopted by the industry. This is due to the lack of controllability and traversability of the developed designs. In this paper, we introduce a hybrid rolling/flying robot. This design benefits from a flywheel to reduce the effects of the terrain (shocks and vibrations) on the camera and sensors. Our design allows the application of existing control algorithms of drones (such as PX4) on a rolling system. In addition, we propose a dynamics model that can use the point cloud representation of the terrain to simulate the motion of the system with applications in real-time modeling and control.

• Sabet, Sahand; Poursina, Mohammad & Nikravesh, Parviz E (2021). Control of Spherical Robots on Uneven Terrains. Vis sammendrag

  Hybrid robots incorporate the advantages of both aerial-only and terrestrial-only vehicles to achieve enhanced mobility and better energy efficiency. Among hybrid vehicles, spherical robots offer the best maneuverability. While operating on uneven surfaces is one of the main benefits of spherical robots, the current literature only covers control of these robots on flat surfaces. This work presents two control algorithms to track a desired trajectory and angular velocity of spherical robots on uneven terrains. The proposed control algorithms can be used when the terrain is known analytically or empirically (i.e., point cloud). By allowing the controller to use empirical information about the terrain profile, this work broadens the implementation of spherical robots in real applications.

• Ho, Alex; Wold, Margrethe; Conway, John Thomas & Poursina, Mohammad (2021). Dynamics of Asteroid Binary Systems through the Use of Surface Integrals.
• Ho, Alex; Wold, Margrethe; Poursina, Mohammad & Conway, John Thomas (2023). Modeling asteroid binary systems with the full two-body problem using surface integrals. Universitetet i Agder. ISSN 978-82-8427-149-1. Fulltekst i vitenarkiv Vis sammendrag

  An asteroid binary system, where two asteroids are in mutual orbit, is important to study as it can provide knowledge of the history of the asteroid population. The most important mechanism to form asteroid binaries in the near-Earth population, and for asteroids with diameters less than 10 km, is rotational fission. Rotational fission occurs when a rubble pile asteroid, which can be thought of as a collection of rocks held together by gravity, reaches a critical spin rate and the rubble pile starts to shed mass. Studying the dynamics of asteroid binaries allows one to better understand how they have evolved. However, due to their non-spherical shapes, one has to take into account both the translational and rotational motion of asteroids, which is known as the full two-body problem. The study of the full two-body problem is a challenge as the mutual gravitational potential between two non-spherical bodies cannot be expressed analytically. Previous studies have used approximations to model the mutual potential between two asteroids. However, these approximations often suffer from inaccuracies when the bodies are close to each other, and also from truncation errors. In this thesis, we make use of a new method to determine the mutual potential, between two asteroids, with the use of surface integrals. We apply this method to study the dynamics of the 1999 KW4 binary system, where both bodies are modeled as ellipsoids. With the use of an order nine Runge-Kutta method, the system energy and angular momentum are conserved to the 11th decimal digit. One of the advantages of the surface integration method is that the results are valid even if the bodies are close to each other. We make use of this advantage to study the dynamics of asteroid systems formed by rotational fission, as the two bodies are very close to each other in the initial formation stages. We consider ellipsoidal bodies for the simulations. Six models are considered, three where the secondary takes different densities and three where we change the shape of the secondary. The simulations show that more than 80\% of the simulations result in the two bodies colliding. The secondary is more likely to escape the gravitational pull of the primary, forming an asteroid pair, and experience secondary fission, if the secondary has a higher density than the primary, or has a more elongated shape. We also compare the rotation periods of the bodies from the simulations with the ones from observations of asteroid binaries and pairs. The rotation periods from the simulations match very well with the rotation periods of observed asteroid pairs. The surface integration scheme can yield exact values to the mutual gravitational potential between two ellipsoidal bodies. This method can therefore be used to determine the accuracy of methods that approximates the mutual potential between two ellipsoids. We compare the surface integration scheme with an approach that expands the mutual potential with the use of inertia integrals. The differences in the gravitational force and torque, between the two methods, are less than 1\% if the bodies are separated by $2-3$ times the radius of the primary. If the bodies are almost touching, however, the differences can exceed 100\% if the shape of the primary becomes elongated. The discrepancies in the torques are typically an order magnitude larger than the difference in the forces.

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