These studies examine the influence of group size and the passage of time on two characteristics of online communities: dispersion of participation in group discussions and active member turnover from month to month. We used multilevel analysis to examine the dynamics of user contributions to discussions on Reddit, a popular website that hosts large-group discussions, across 30 groups over 6 years. As groups grow in size, participation becomes more highly concentrated among fewer members while turnover decreases. As time passes, participation becomes more widely dispersed while group member turnover increases. An increase in group member turnover appears to be the result of both a maturation effect (as each group ages, turnover increases) as well as a cohort effect (groups formed at a later date have higher turnover than groups formed earlier). We can conclude from these results that as time progresses and groups become larger, they become less community-like, but in different ways.
Robustness, compactness, and portability of tensegrity robots make them suitable candidates for locomotion on unknown terrains. Despite these advantages, challenges remain relating to ease of fabrication, shape morphing (packing-unpacking), and locomotion capabilities. The paper introduces a design methodology for fabricating tensegrity robots of varying morphologies with modular components. The design methodology utilizes perforated links, coplanar (2D) alignment of components and individual cable tensioning to achieve a 3D tensegrity structure. These techniques are utilized to fabricate prism (three-link) tensegrity structures, followed by tensegrity robots in icosahedron (six-link), and shpericon (curved two-link) formation. The methodology is used to explore different robot morphologies that attempt to minimize structural complexity (number of elements) while facilitating smooth locomotion (impact between robot and surface). Locomotion strategies for such robots involve altering the position of center-of-mass (referred to as internal mass shifting) to induce “tip-over.” As an example, a sphericon formation comprising of two orthogonally placed circular arcs with conincident center illustrates smooth locomotion along a line (one degree of freedom). The design of curved links of tensegrity mechanisms facilitates continuous change of the point of contact (along the curve) that results from the tip-over. This contrasts to the sudden and piece-wise continuous change for the case of robots with traditional straight links which generate impulse reaction forces during locomotion. The two resulting robots—the Icosahedron and the Sphericon Tensegrity Robots—display shape morphing (packing-unpacking) capabilities and achieve locomotion through internal mass-shifting. The presented static equilibrium analysis of sphericon with mass is the first step in the direction of dynamic locomotion control of these curved link robots.
Robustness, compactness, and portability of tensegrity robots make them suitable candidates for locomotion on unknown terrains. Locomotion is achieved by breaking symmetry and altering the position of center-of-mass to induce “tip-over”. The design of curved links of tensegrity mechanisms allows continuous change in the point of contact (along the curve) as compared to discontinuities in the traditional straight links (point contact) which induces impulse reaction forces during locomotion. The illustrated curve-link tensegrity robot achieves smooth locomotion through internal mass-shifting. Additionally, this tensegrity robot displays folding and unfolding. Introduced is a design methodology for fabricating tensegrity robots of varying morphologies with modular components created using rapid prototyping techniques, including 3D printing and laser-cutting. The techniques are utilized to fabricate simple tensegrity structures, followed by locomotive tensegrity robots in icosahedron and half-circle arc morphologies.
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