Issue 4

JTAM, Sofia, vol. 44 Issue 4 (2014)

New Exoskeleton Arm Concept Design and Actuation for Haptic Interaction with Virtual Objects

D. Chakarov, I. Veneva, M. Tsveov, T. Tiankov, D. Trifonov
Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 4, Sofia 1113, Bulgaria

In the work presented in this paper the conceptual design and actuation of one new exoskeleton of the upper limb is presented. The device is designed for application where both motion tracking and force feedback are required, such as human interaction with virtual environment or rehabilitation tasks. The choice is presented of mechanical structure kinematical equivalent to the structure of the human arm. An actuation system is selected based on braided pneumatic muscle actuators. Antagonistic drive system for each joint is shown, using pulley and cable transmissions. Force/displacement diagrams are presented of two antagonistic acting muscles. Kinematics and dynamic estimations are performed of the system exoskeleton and upper limb. Selected parameters ensure in the antagonistic scheme joint torque regulation and humanarm range of motion.

JTAM, Sofia, vol. 44 Issue 4 pp. 03-14 (2014), [Full Article]

Meteoroids Interaction with the Earth Atmosphere

Leonid I. Turchak1, Maria I. Gritsevich1,2,3
1Russian Academy of Sciences Dorodnicyn Computing Centre, Vavilov Str. 40, 119333 Moscow, Russia
2Finnish Geodetic Institute, Geodeetininne 2, 02430 Masala, Finland
3Institute of Physics and Technology, Ural Federal University, Mira Str. 19, 620002 Ekaterinburg, Russia

In this study we evaluate meteoroid mass and its other properties based on the observed atmospheric trajectory. With account for aerodynamics, we formulate a problem by introducing key dimensionless parameters in the model, responsible for the drag, mass loss and rotation of meteoroid. The proposed model is suitable to categorize various impact events in terms of meteor survivability and impact damage and thus, to analyze consequences that accompany collisions of cosmic bodies with planetary atmosphere and surface. The different types of events, namely, formation of a massive single crater (Barringer, Lonar Lake), dispersion of craters and meteorites over a large area (Sikhote-Alin), absent of craters and meteorites, but huge damage (Tunguska) are considered as illustrative examples. The proposed approach helps to summarize the data on existing terrestrial impacts and to formulate recommendations for further studies valuable for planetary defence. It also significantly increases chances of successful meteorite recoveries in future. In other words, the study represents a ’cheap’ possibility to probe cosmic matter reaching planetary surface and it complements results of sample-return missions bringing back pristine samples of the materials.

JTAM, Sofia, vol. 44 Issue 4 pp. 15-28 (2014), [Full Article]

Seismic Soil-Tunnels Interaction via BEM Part II. Numerical Results

P. Dineva1, S. Parvanova2, G. Vasilev2, F. Wuttke3
1Institute of Mechanics, Bulgarian Academy of Sciences, Acad. G. Bonchev St., Bl. 4, Sofia 1113, Bulgaria
2Department of Structural Engineering, UACEG, 1, Hristo Smirnenski Blvd, 1046 Sofia, Bulgaria
3Marine and Land Geomechanics and Geotechnics, Christian-Albrechts-University of Kiel, Institute of Applied Geo-science, Germany

The mechanical model for seismic response of unlined and lined tunnels located in a layered half-plane with free surface relief was described in the first part of this work. The computational methodology developed in the first part is based on a combination of both the idea of the global matrix propagator method which allows derivation of a relation between the wave field quantities along different interfaces in the layered half-plane and boundary element method (BEM) using the elastodynamic fundamental solution in frequency domain. The solution of the problem for transient waves is solved by the usage of direct and inverse Fast Fourier Transform (FFT). The aim of this second part is to demonstrate the accuracy and the convergence of the proposed computational tool. Furthermore, subsequent extensive numerical simulations help in establishing the sensitivity of the seismic field along the free surface and near the tunnels to the following key factors: type and characteristics of the incident wave, dynamic tunnels interaction, soil-tunnel interaction, free surface relief, type of the tunnel construction and mechanical properties of the layered half-plane.

JTAM, Sofia, vol. 44 Issue 4 pp. 29-50 (2014), [Full Article]

Interaction between a Permeable Crack and Piezoelectric Screw Dislocations, Line Forces and Line Charges in a Finite Piezoelectric Cylinder

H. P. Song, C. F. Gao
College of Aerospace Engineering, State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics & Astronautics, Nanjing 210016, Chin

The problem of a piezoelectric screw dislocation, line force and line charge around a permeable crack in a finite piezoelectric cylinder is dealt with in this paper. Utilizing the complex function and conformal mapping methods, the closed form solutions of the stress fields and the electric displacement fields are derived. The stress intensity factor and the image force are discussed in detail. The results show that the stress fields are in direct proportion to the line force, but independent of the line charge. The shielding effect produced by the dislocation increases with the increasing of the radius of the piezoelectric cylinder. Moreover, the unstable equilibrium point and the image force are also severely affected by the radius of the piezoelectric cylinder.

JTAM, Sofia, vol. 44 Issue 4 pp. 51-68 (2014), [Full Article]

Vibration-Based Methods for Detecting a Crack in a Simply Supported Beam

Dimitrina Kindova-Petrova
Department of Technical Mechanics, University of Architecture, Civil Engineering and Geodesy, 1 Hristo Smirnenski Blvd, 1046, Sofia, Bulgaria

Variable loadings cause fatigue to the structural elements leading to crack initiation and propagation, which results in a decrease in the fatigue life of the elements. In particular, cracks decrease the stiffness and the natural frequency thus, causing specimens to fail under normal working conditions. This paper presents the application of the vibration-based technique for detecting the location and the size of a fatigue crack in structures. The two presented methods are based on the measurements of the natural frequencies of the cracked beam. The crack is modelled by a rotational spring. The predicted crack depth and location are compared with the actual data obtained from finite element models.

JTAM, Sofia, vol. 44 Issue 4 pp. 69-82 (2014), [Full Article]

Nonlinear Stability of Free Thin Films: Thermocapillary and Van-Der-Waals Effects

S. Tabakova1,2, S. Radev2
1Department of Mechanics, TU – Sofia, branch Plovdiv, 25, Tzanko Djustabanov, BG-4000 Plovdiv, Bulgaria
2Department of Fluid Mechanics, Institute of Mechanics, BAS, Acad. G. Bontchev St., Bl. 4, 1113 Sofia, Bulgaria

In the present work the dynamics of a non-isothermal thin viscous film, with fully mobile interfaces, is studied in the case when the inertial, viscous, capillary, van der Waals and thermocapillary forces are important. The film is laterally bounded by a frame, whose temperature is higher than the environmental one. The stability of the static film shapes is examined numerically by a linear and non-linear analysis. The results show that the film rupture is mostly governed by the dynamics, but it could be delayed or enhanced by the thermocapillary convection and the heat transfer with the surrounding environment.

JTAM, Sofia, vol. 44 Issue 4 pp. 83-96 (2014), [Full Article]