This paper presents a novel scheme for achieving attitude control of a tumbling combination system in the post-capture phase of a tethered space robot (TSR). Given the combination rotation characteristics, tether force is applied to provide greater control torques for stabilising the attitude. The proposed control scheme involves two attitude controllers, which coordinate the controller of the tether force and thruster force and the controller of single thruster force. The numerical simulations include a comparison between this coordinated control and the traditional thruster control and a sensitivity analysis on initial values of parameters. Simulation results validate the feasibility of the attitude control scheme for a tumbling combination system, and fuel consumption of the attitude control is efficiently reduced using the coordinated control strategies.

In this paper a rolling robot resembling the shape of a triangular-bipyramid is proposed. The robot has three degrees of freedom and is formed by connecting two tripod mechanisms with three spherical joints. By kinematic analysis, the robot can be viewed as a planar four-bar linkage. Further, its dynamic rolling ability is discussed by Zero Moment Point (ZMP) analysis. We show that the robot has the capability to roll, adjust its step length, and switch rolling directions. These functions are verified by a series of simulations with a CAD (computer-aided design) model and experiments with a prototype.

In this paper, we analyze the end-to-end delay performance of a tandem queueing system with mobile queues. Due to state-space explosion, there is no hope for a numerical exact analysis for the joint-queue-length distribution. For this reason, we present an analytical approximation that is based on queue-length analysis. Through extensive numerical validation, we find that the queue-length approximation exhibits excellent performance for light traffic load.

This paper reports a Spanish-American telecollaborative project through which students used Twitter, blogs and podcasts for intercultural exchange over the course of one semester. The paper outlines the methodology for the project including pedagogical objectives, task design, selection of web tools and implementation. Using qualitative and quantitative data collection, the study explored how the application of Web 2.0 facilitated cross-cultural communication. How the use of digital technology affected the way in which the students viewed intercultural learning and peer feedback was examined. The findings showed that students viewed the online exchange as a superb venue for intercultural communication with native speakers. Through social engagements, students not only gained cultural knowledge but also became more aware of their own beliefs and attitudes toward their own culture. In addition, discussions on topics of tangible and intangible cultures afforded the opportunity to raise students’ awareness of cultural norms and practices. Peer feedback helped learners increase lexical knowledge, prevent language fossilization, and acquire native-sounding discourse. The study suggests that allocating sufficient time to complete each task and making personal commitment to online contributions are essential to successful intercultural exchanges.

We present an algorithm for offsetting the workspace obstacles of a circular robot. Our method has two major steps: It finds the raw offset curve for both lines and circular arcs, and then removes the global invalid loops to find the final offset. To generate the raw offset curve and remove global invalid loops, O(n) and O((n+k)log m) computational times are needed respectively, where n is the number of vertices in the original polygon, k is the number of self-intersections and m is the number of segments in the raw offset curve, where m ≤ n. Any local invalid loops are removed before generating the raw offset curve by invoking a pair-wise intersection detection test (PIDT). In the PIDT, two intersecting entities are checked immediately after they are computed, and if the test is positive, portions of the intersecting segments are removed. Our method works for conventional polygons as well as the polygons that contain circular arcs. Our algorithm is simple and very fast, as each sub-process of the algorithm can be completed in linear time except the last one, which is nearly linear. Therefore, the overall complexity of the algorithm is nearly linear. By applying our simple and efficient approach, offsetting obstacles of any shape make it possible to construct a configuration space that ensures optimized motion planning.

Named after the French–Belgian mathematician Eugène Charles Catalan, Catalan's numbers arise in various combinatorial problems [12]. Catalan's triangle, a triangular array of numbers somewhat similar to Pascal's triangle, extends the combinatorial meaning of Catalan's numbers and generalizes them [1,5,11]. A need for a generalization of Catalan's triangle itself arose while conducting a probabilistic analysis of the Asymmetric Simple Inclusion Process (ASIP) — a model for a tandem array of queues with unlimited batch service [7–10]. In this paper, we introduce Catalan's trapezoids, a countable set of trapezoids whose first element is Catalan's triangle. An iterative scheme for the construction of these trapezoids is presented, and a closed-form formula for the calculation of their entries is derived. We further discuss the combinatorial interpretations and applications of Catalan's trapezoids.

In this study, we analyze the influence of passive joint viscous friction (PJVF) on modal space decoupling for a class of symmetric spatial parallel mechanisms (SSPM). The Jacobian matrix relating the platform movements to each passive joint velocity is first gained by vector analysis and the passive joint damping matrix is then derived by applying the Kane method. Next, an analytic formula index measuring the degree of coupling effects between the damping terms in the modal coordinates is proposed using classical modal analysis of dynamic equations in task space. Based on the index, a new optimal design method is found which establishes the kinematics parameters for minimizing the coupling degree of damping and achieves optimal fault tolerance for modal space decoupling when all struts have identical damping and stiffness coefficients in their axial directions. To illustrate the effectiveness of the theory, the new method was used to redesign two configurations of a specific manipulator.

Skellam's name is traditionally attached to the distribution of the difference of two independent Poisson random variables. Many bivariate extensions of this distribution are possible, e.g., through copulas. In this paper, the authors focus on a probabilistic construction in which two Skellam random variables are affected by a common shock. Two different bivariate extensions of the Skellam distribution stem from this construction, depending on whether the shock follows a Poisson or a Skellam distribution. The models are nested, easy to interpret, and yield positive quadrant-dependent distributions, which share the convolution closure property of the univariate Skellam distribution. The models can also be adapted readily to account for negative dependence. Closed form expressions for Pearson's correlation between the components make it simple to estimate the para-meters via the method of moments. More complex formulas for Kendall's tau and Spearman's rho are also provided.

A crucial problem is the risk that a manipulator arm would be damaged by twisting or bending during and after contacting a target satellite. This paper presents a solution to minimize the risk of damage to the arm and thereby enhance contact performance. First, a hand-eye servo controller is proposed as a method for accurately tracking and capturing a target satellite. Next, a motion planning strategy is employed to obtain the best-fit contacting moments. Also, an impedance control law is implemented to increase protection during operation and to ensure more accurate compliance. Finally, to overcome the challenge of verifying algorithms for a space manipulator while on the ground, a novel experimental system with a 6-DOF (degree of freedom) manipulator on a chaser field robot is presented and implemented to capture a target field robot; the proposed methods are then validated using the experimental platform.