附录Y A Novel Parallel Engraving Machine Based on 6-PUS Mechanism and Related Technologies
Kong Ling-fu amp; Zhang Shi-hui
1.Introduction
Conventional computer engraving machine has played an important role in industries such as machinery machining, printing and dyeing and entertainment, but it has the inherent disadvantages such as cutting tool can be fed only along the fixed guideway, lower degree-of-freedom(DOF) of cutting tool, lower flexibility and mobility for machining etc. Parallel mechanism has the merits such as high mechanical stiffness, high load capacity, high precision, good dynamic performance etc (Huang,1997). According to the characteristics of parallel mechanism, it has been a hot research topic to apply parallel mechanism to the domain of future machining. By applying parallel mechanism to engraving domain, its inherent advantages can be fully exerted and the disadvantages of conventional engraving machine can be overcome or compensated. But as the special structure of parallel mechanism, the related theory and technology during its engraving is very different from that of conventional engraving machine, and it is a undeveloped research topic by now. In addition, with the development of computer network technology, the new concept and method such as network machining and manufacturing has become hot research topic(Huang amp; Mak,2001; Taylor amp; Dalton,2000; Yao amp; Lu,1999). A novel parallel engraving machine with six-axis linkage is proposed in this paper, which uses the 6-PUS parallel mechanism with 6-DOF as the prototype, and some key technologies such as size design, tool path planning, engraving force control and teleoperation are studied on this basis.
2. Confirming of Mechanism Type and Engraving Machinersquo;s Size
2.1 Selection of Mechanism Type and Coordinate System
The selection of mechanism type is the first step for designing novel engraving machine, the following reasons make us select the 6-PUS parallel mechanism for designing our engraving machine. Comparing with traditional mechanism, 6-PUS parallel mechanism uses base platform, three uprights layout and high rigidity framework structure and has the merits such as high modularization, high accuracy and low cost. Its model is shown in Fig.1.
As shown in Fig.1, 6-PUS parallel mechanism consists of base platform, dynamic platform and 6 branch chains with same structure, every branch joins with base platform through prismatic pairs (P), slider of prismatic pairs joins with up end of the fixed length link through universal joint (U) down end of the fixed length link joins with dynamic platform through sphere hinge (S), so it is called 6-PUS parallel mechanism.
Figure 1. The model of 6-PUS parallel mechanism
Figure 2. Coordinate system of 6-PUS parallel engraving mechanism
The coordinate system of 6-PUS parallel engraving mechanism is shown in Fig. 2. In Fig.2, the geometry centers of base platform and dynamic platform plane are supposed as OB and op respectively. In every branch, the centers of prismatic pairs, universal joint and sphere hinge are marked with Ai, Bi, and Ci (i=1,2,hellip;,6) respectively. Coordinate system OB - XBYBZB is fixed on base platform, taking {B} as briefly. The origin of {B} lies on geometry center of base platformrsquo;s up plane, axis ZB is vertical with base platform and directs to up, axis YB directs to angle bisector of the first and second branch lead screw center line, and axis XB can be determined with right-hand rule. Supposing the coordinate system set on dynamic platform is op minus; xpypzp , taking {P} as briefly, its origin lies on geometry center of dynamic platform, the initial state of dynamic platform system is consistent with that of base platform system completely. Supposing the coordinate of op is (00Z) in {B}, this configuration without relative rotation to every axis is the initial configuration of this mechanism, and Z changing with mechanismrsquo;s size. On the basis of 631 coordinate system mentioned, we use influence coefficient theory and the actual parameters of this mechanism to calculate the first and the second order influence coefficient matrix of every branch under different configuration. Then, we can get the first and the second order integrated influence coefficient matrix and of the whole mechanism. The significance and detailed solution process for influence coefficient matrix is omitted here, for more information please refer (Huang et al., 1997).
2.2 Mechanism Performance Analysis Based on Influence Coefficient Matrix
The performance of engraving machine will change with its size. To find out the better size for all the performance indices of both kinematics and dynamics, we obtain a group of mechanisms by changing its parameters. These mechanismsrsquo; length of fixed length links (L) range between 45cm and 55cm (step is 1cm), radius of dynamic platform (R) range between 10cm and 20cm (step is 1cm). Other parameters of the mechanism are unchanging, so we get 121 mechanisms totally.
Taking these mechanisms as research object, we confirm the sample point for every mechanism in its workspace with algorithm PerformanceAnalysis, then calculate the first and the second order influence coefficient matrix in every point. Furthermore, calculate all the performance indices in every sample point and draw all the global performance atlas of 121 mechanisms ultimately. To describe conveniently, we abbreviate the first and the second order integrated influence coefficient matrix and to G and H, and use Gomega; , Homega; and , as the angular velocity submatrix and linear velocity submatrix of the first and the se
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