Xiangzhi Wei

A study on revolute joints in 3D-printed non-assembly mechanisms

Purpose The purpose of this paper is to explore a new design for the journal of revolute joints that can improve the dynamic performance of 3D printed non-assembly mechanisms. Design/methodology/approach The design improves upon previous proposed designs that use drum-shaped journals in place of cylindrical ones. The authors introduce an innovative new worm-shaped journal. The authors also propose a systematic and efficient procedure to identify the best parameter values for defining the exact shape of the journal. Using three different mechanisms for the experiments, the paper constructs 3D computer-aided design (CAD) models using the design as well as cylindrical and drum-shaped designs. The parameters for the optimum geometry for each type of design are determined by dynamic simulation using the CAD system. Actual prototypes of the ideal designs are constructed using a commercial fused deposition modeling (FDM) machine for physical comparisons. Findings This paper shows that in simulations as well in physical models, the proposed design outperforms the previous designs significantly. Research limitations/implications This study was mainly focused on the FDM process, and the authors have not yet explored other processes. One limitation of this approach is that it requires the mechanism to be printed along the axial direction of the revolute joint. Originality/value This paper proposes a new design for the journal in 3D printed revolute joints. A clear advantage of the design is that it can easily be used to replace normal revolute joins in non-assembly models without affecting any other parts of the geometry. Therefore, with relatively little effort, the authors can print non-assembly mechanisms with improved dynamic performance.

An Improved Algorithm for the Automated Design of Large-Scaled Robot Skin

A recent paper titled “On the Problem of the Automated Design of Large-Scaled Robot Skin” (Anghinolfi et al., 2013) published in the IEEE Transactions on Automation Science and Engineering addressed the problem of covering the surface of a humanoid robot with the largest number of nonoverlapping equilateral triangular sensor modules. The problem is eventually approximated by a simpler one: how to find the placement of a given polygon P on an equilateral triangular grid G that contains the largest number of the grid triangles. In this paper, we show how to improve the efficiency of the algorithm presented in that paper. Further, we show that the general problem of filling P with the largest number of disjoint equilateral triangles (all entirely contained in P and all of the same size) is not equivalent to that of finding an optimal placement of P on G. Using this result, we propose an improved heuristic for the original problem of covering the skin of a robot with the largest number of triangular sensor modules.

Design and Rolling Analysis of a Novel Deformable Polyhedron Robot

In this paper, a new rolling robot is proposed. The mechanism of the robot consists of eight links with three degrees of freedom (DOFs). The shape of each link of the robot is an equilateral triangle. The robot realizes its direction switching function by deforming into different modes of planar parallelogram mechanisms (PPM). In any deterministic mode, the robot can roll on the ground. The motion of the robot is studied based on the kinematic and zero moment point (ZMP) analyses. Though the robot has three DOFs, we show that it can realize flexible mobility via direction switching and rolling functions with two DOFs and one DOF, respectively. A prototype robot was manufactured. A series of simulations and experiments done using this prototype is reported, verifying the feasibility of the design.

On the Polygon Containment Problem on an Isometric Grid

This paper addresses the issue of placing a simple polygon (upon translation and rotation) on an isometric triangular grid such that the polygon contains the maximum number of triangles in its closure. This solves the problem left open in two recent papers titled “On the problem of the automated design of large-scale robot skin” and “An improved algorithm for the automated design of large scale robot skin” published in the IEEE Transactions on Automation Science and Engineering . Based on the properties of the grid, an improved algorithm is also presented. We also present some experimental results describing the use of this algorithm.

Optimal uniformly monotone partitioning of polygons with holes

Polygon partitioning is an important problem in computational geometry with a long history. In this paper we consider the problem of partitioning a polygon with holes into a minimum number of uniformly monotone components allowing arbitrary Steiner points. We call this the MUMC problem. We show that, given a polygon with n vertices and h holes and a scan direction, the MUMC problem relative to this direction can be solved in time O(nlogn+hlog^3h). Our algorithm produces a compressed representation of the subdivision of size O(n), from which it is possible to extract either the entire decomposition or just the boundary of any desired component, in time proportional to the output size. When the scan direction is not given, the problem can be solved in time O(K(nlogn+hlog^3h)), where K is the number of edges in the polygons visibility graph. Our approach is quite different from existing algorithms for monotone decomposition. We show that in O(nlogn) time the problem can be reduced to the problem of computing a maximum flow in a planar network of size O(h) with multiple sources and multiple sinks. The problem is then solved by applying any standard network flow algorithm to the resulting network. We also present a practical heuristic for reducing the number of Steiner points.

On Minimum Link Monotone Path Problems

The problem of finding monotone paths between two given points has useful applications in path planning, and in particular, it is useful to look for minimum link paths. We are given a simple polygon P or a polygonal domain D with n vertices and a triplet of input parameters: (s, t, d), where s and t are two points in the plane and d is any direction. We show how to answer a query for the existence of a d-monotone path between s and t inside P (or D) in logarithmic time after preprocessing P in O(En) time, or D in O(En + ERlogR) time, where E is the size of the visibility graph of P (or D), and R is the number of reflex vertices in D. Our approach is based on the novel idea utilizing the dual graph of the trapezoidal map of P (or D). For polygonal domains, our approach uses a trapezoidal map associated with each visibility edge of D, and we show how to compute this large set of trapezoidal maps efficiently. Furthermore, we show how to output a minimum linkd-monotone path between points s and t, for an arbitrary input triplet (s, t, d).

Toward Support-Free 3D Printing: A Skeletal Approach for Partitioning Models

Minimizing support structures is crucial in reducing 3D printing material and time. Partition-based methods are efficient means in realizing this objective. Although some algorithms exist for support-free fabrication of solid models, no algorithm ever considers the problem of support-free fabrication for shell models (i.e., hollowed meshes). In this paper, we present a skeleton-based algorithm for partitioning a 3D surface model into the least number of parts for 3D printing without using any support structure. To achieve support-free fabrication while minimizing the effect of the seams and cracks that are inevitably induced by the partition, which affect the aesthetics and strength of the final assembled surface, we put forward an optimization system with the minimization of the number of partitions and the total length of the cuts, under the constraints of support-free printing angle. Our approach is particularly tailored for shell models, and it can be applicable to solid models as well. We first rigorously show that the optimization problem is NP-hard and then propose a stochastic method to find an optimal solution to the objectives. We propose a polynomial-time algorithm for a special case when the skeleton graph satisfies the requirement that the number of partitioned parts and the degree of each node are bounded by a small constant. We evaluate our partition method on a number of 3D models and validate our method by 3D printing experiments.