Jumping locomotion system based on a multistable tensegrity structure. - In: Mechanical systems and signal processing, ISSN 1096-1216, Bd. 152 (2021), 107384
All known locomotion principles are limited respective to environmental conditions. Often, the occurrence of obstacles or gaps means the break-off for the operating motion systems. For such circumstances, a controllable jumping locomotion is required to cross these barriers. However, this locomotion demands sophisticated requirements to the actuation. The abrupt actuation is commonly realized by high dynamic actuators or complex mechanisms. In this work, a simple solution utilizing the multistability of a compliant tensegrity structure is described. Therefore, a two-dimensional tensegrity structure featuring four stable equilibria is considered. Based on bifurcation analyses a feasible actuation to control the current equilibrium configuration is derived. Changing between selected equilibrium states enables a great difference in potential energy, which yields a jumping motion of the structure. Based on numerical simulations a suitable actuation strategy is chosen to overcome obstacle and steps by jumping forward or backward, respectively. The theoretical approach is examined experimentally with a prototype of the multistable tensegrity structure.
https://doi.org/10.1016/j.ymssp.2020.107384
Reconfiguration of planar quadrilateral linkages utilizing the tensegrity principle. - In: Mechanism and machine theory, Bd. 156 (2021), 104172
The development of reconfigurable planar four-bar linkages by applying the tensegrity principle is considered. Conventional quadrilateral linkages enable two operation modes differing in the kinematic behavior. However, a change between these states is not possible due to the geometric constraints. To enable a reconfiguration between the different modes one-sided limited nonholonomic constraints are introduced in this work. This issue is realized by applying ropes that cannot resist compression. However, to guarantee an appropriate load case in operation a prestress within the mechanism is required. Hence, the linkage is extended to a tensegrity-based mechanism. The structural dynamics are derived using the Lagrange formalism and the structural behavior is evaluated using numerical simulations. Furthermore, a prototype of an exemplary tensegrity-based mechanism is manufactured and experiments regarding the mechanical properties, in particular the reconfiguration, are performed. The results suggest the potential benefit of applying the tensegrity principle within conventional planar four-bar linkages.
https://doi.org/10.1016/j.mechmachtheory.2020.104172
A method to approach constant isotropic permeabilities and demagnetization factors of magneto-rheological elastomers. - In: Journal of magnetism and magnetic materials, ISSN 1873-4766, Bd. 527 (2021), 167742
The use of non-conventional materials is nowadays of much interest in scientific community. Magneto-rheological elastomers are hybrid materials, which in presence of magnetic fields state a change in their mechanical properties. They are composed by an elastomeric matrix with embedded magnetic particles. One of the most attractive features of these materials is that as soon as the magnetic field is removed from the material, the original mechanical properties are completely recovered, with negligible differences in comparison to the original state. This paper focuses on the study of magnetic characteristics of these smart materials, such as relative permeability and demagnetizing factors, for samples with different volume concentration of ferromagnetic particles.
https://doi.org/10.1016/j.jmmm.2021.167742
A vibrissa-inspired highly flexible tactile sensor: scanning 3D object surfaces providing tactile images. - In: Sensors, ISSN 1424-8220, Bd. 21 (2021), 5, 1572, insges. 29 S.
Just as the sense of touch complements vision in various species, several robots could benefit from advanced tactile sensors, in particular when operating under poor visibility. A prominent tactile sense organ, frequently serving as a natural paragon for developing tactile sensors, is the vibrissae of, e.g., rats. Within this study, we present a vibrissa-inspired sensor concept for 3D object scanning and reconstruction to be exemplarily used in mobile robots. The setup consists of a highly flexible rod attached to a 3D force-torque transducer (measuring device). The scanning process is realized by translationally shifting the base of the rod relative to the object. Consequently, the rod sweeps over the object’s surface, undergoing large bending deflections. Then, the support reactions at the base of the rod are evaluated for contact localization. Presenting a method of theoretically generating these support reactions, we provide an important basis for future parameter studies. During scanning, lateral slip of the rod is not actively prevented, in contrast to literature. In this way, we demonstrate the suitability of the sensor for passively dragging it on a mobile robot. Experimental scanning sweeps using an artificial vibrissa (steel wire) of length 50 mm and a glass sphere as a test object with a diameter of 60 mm verify the theoretical results and serve as a proof of concept.
https://doi.org/10.3390/s21051572
Dynamics of a magnetic pendulum in the presence of an oscillating conducting plate. - In: Proceedings in applied mathematics and mechanics, ISSN 1617-7061, Bd. 20 (2021), 1, e202000083, insges. 2 S.
A pendulum with an attached permanent magnet moving near a conductor is a typical experiment for the demonstration of electromagnetic braking. When the conductor itself moves, it can transfer energy to the pendulum. We study a simple but exact analytical model where the conductor is a horizontally unbounded flat plate. For this geometry, eddy currents and induced Lorentz force due to the motion of a magnetic dipole are known analytically in the quasistatic limit. A vertical oscillation of such a horizontal plate located beneath the magnet is considered. In this setup, the vertical position of the pendulum is an equilibrium point when the magnetic moment of the magnet is perpendicular to its plane of motion. Depending on the strength of the magnetic dipole moment, the frequency and amplitude of the plate as well as the distance between plate and magnet, the plate oscillation can destabilize the equilibrium. The stability limits for weak electromagnetic coupling are computed analytically using the harmonic balancing method. For stronger coupling, the stability limits are obtained numerically using Floquet analysis. Chaotic motions with finite amplitudes are also found.
https://doi.org/10.1002/pamm.202000083
Characterization of thin flexure hinges for precision applications based on first eigenfrequency. - In: Microactuators, microsensors and micromechanisms, (2021), S. 15-24
Flexure hinges with small cross-section heights are state of the art in numerous precision engineering applications due to their capability for smooth and repeatable motion. However, the high sensitivity to manufacturing influences represents a challenge. We propose a characterization method for flexure hinges based on the measurement of the free oscillation, to enable the consideration of manufacturing influences in the early stages of the design process. Three semi-circular flexure hinges with different cross-section heights and highly accurate geometry were investigated experimentally to compare them with three theoretical modeling approaches. The results for the three flexure hinge specimens showed small deviations to the predicted values from the models which is in agreement with the results of dimensional measurements. With each modeling approach, a deviation of the minimal notch height from the nominal value can be calculated. This value, in turn, can be used as manufacturing allowance for subsequent manufacturing of compliant mechanisms using the same manufacturing method. An exemplary compliant parallel-crank mechanism proves the applicability of the concept to compliant mechanisms with multiple flexure hinges.
https://doi.org/10.1007/978-3-030-61652-6_2
Mathematical model of a linear motor controlled by a periodic magnetic field considering dry and viscous friction. - In: Applied mathematical modelling, Bd. 89 (2021), S. 1155-1162
The paper deals with a drive concept that uses the controllable mechanical properties of a magnetorheological fluid (MRF). The biologically inspired operating principle is based on crawling using anisotropic friction, as of worms, and non-Newtonian fluids, as of snails. The MRF located between a slider and two slide-blocks is functionally relevant for the drive system to generate a translational motion. A controlled magnetic field is utilized to change the friction conditions in the drive system by varying the properties of the MRF. An extended friction model takes the dry friction into account, along with the viscous friction. In this case, apart from the ratio of the coefficients of viscous friction, it is necessary to introduce two more parameters: the ratio of the coefficients of dry friction in the absence and presence of the magnetic field, and the ratio of the characteristic forces of viscous and dry frictions. These parameters allow refining the mathematical model that governs the behavior of a linear motor. Using asymptotic methods of non-linear mechanics, an expression for the average velocity of the slider is obtained for the case when the friction force is assumed to be small in comparison with the driving force of the slide-blocks. The theoretical results are verified experimentally on a prototype.
https://doi.org/10.1016/j.apm.2020.08.021
Kinematic analysis of a rolling tensegrity structure with spatially curved members. - In: Meccanica, ISSN 1572-9648, Bd. 56 (2021), 4, S. 953-961
In this work, a tensegrity structure with spatially curved members is applied as rolling locomotion system. The actuation of the structure allows a variation of the originally cylindrical shape to a conical shape. Moreover, the structure is equipped with internal movable masses to control the position of the center of mass of the structure. To control the locomotion system a reliable actuation strategy is required. Therefore, the kinematics of the system considering the nonholonomic constraints are derived in this paper. Based on the resulting insight in the locomotion behavior a feasible actuation strategy is designed to control the trajectory of the system. To verify this approach kinematic analyses are evaluated numerically. The simulation data confirm the path following due to an appropriate shape change of the tensegrity structure. Thus, this system enables a two-dimensional rolling locomotion.
https://doi.org/10.1007/s11012-020-01199-x
Kinematika koles i koncepcii šassi dlja senapravlennych :
The kinematics of wheels and chassis concepts for omnidirectional robots. - In: Problems of mechanics, ISSN 1512-0740, (2020), No. 4(81), Seite 21-33
Kinematika sistemy robot-pricep s Mekanum kolesami :
Kinematics of a robot-trailer system with Mecanum wheels. - In: Problems of mechanics, ISSN 1512-0740, (2020), No. 2(79), Seite 13-20