An Extensible Java Applet for Spatial Linkage Synthesis

A spatial linkage is defined by a workpiece supported by several serial chains. The task of the linkage is defined by specifying rigid positions for the workpiece that approximate a desired workspace. Spatial serial chains can be synthesized and combined to form more complicated linkages. Because there are many serial chains available for this design process, we have developed a Java-based architecture for a computer-aided-design system for spatial linkages that is organized to allow incorporation of individually defined serial chain classes prepared by users/collaborators. Using the RR and TS chains as an example, we present the general features common to dimensional synthesis methodologies, and identify the types of analysis, synthesis, and evaluation routines that are required for displaying, animating, and designing spatial linkages formed from serial chain primitives. We present the basic components of the design system as well as specifications for the serial chain analysis and synthesis classes. An example is provided to demonstrate how the system can be used to perform three position synthesis of spatial RR, TS, and RRTS linkages. INTRODUCTION The focus of commercial computer-aided design (CAD) software has been on tools for shape representation and manipulation, assembly and detail drawing creation, and integration with computer-aided manufacturing (CAM) and analysis packages. This focus is clearly represented in high end systems such ondence to this author (email: [email protected]) 1 as Dassault’s CATIA, and PTC’s Pro/ENGINEER. Mid-range modeling applications such as SolidWorks, AutoCAD, IronCad, and Vellum Solids have export capabilities or additional plugin interfaces to CAM packages such as SurfCam or MasterCam, FEA packages such as Algor or Nastran, and dynamic simulation packages such as WorkingModel or ADAMS. While this software is extremely powerful and versatile, it does not help automate the step in the design process described as the function-to-form transformation. This is the creative phase in the product development cycle where the designer synthesizes the geometric model of individual parts and assemblies to realize the particular function and satisfy the particular requirements. One specific area of design research that has well developed tools for this function-to-form transformation is the theory of mechanism synthesis. Mechanism synthesis theory can be divided into methodologies for type and number synthesis, and dimensional synthesis (Sandor and Erdman 1997). Type synthesis allows the designer to match the input requirements to a space of possible mechanism types, such as gear systems, cam systems, or linkage systems. Once a mechanism type is identified, number synthesis can be used to generate the set of potential topological configurations available for further consideration. For example if an input specification is compatible with a planar linkage, number synthesis would generate a variety of linkage topologies with single and multiple loops. Once a mechanism topology is identified, the designer can use the tools of dimensional synthesis to identify the values of geometric parameters that satisfy the motion requirements. Copyright  2002 by ASME Linkage Design Software For linkages, the dimensional synthesis problem is particularly well defined. The input specification, or task, describes the desired workspace or motion capabilities, and is typically represented by a set of discrete positions. From this description, the geometric constraints associated with the linkage architecture are resolved to determine its physical dimensions. This requires the solution of a complex set of nonlinear equations. These problems range in size and complexity depending on the number of specifications, and the type of mechanism being synthesized. Mechanism synthesis research well established and solution algorithms have been presented for a variety of mechanism architectures and input conditions (see for example, Suh and Radcliffe (1978) and McCarthy (2000)). While there are numerous special purpose algorithms for synthesizing a variety of mechanism architectures, no general purpose design tool exists that integrates them. Each synthesis problem typically has different input specifications and different output requirements. This characteristic has lead to the development of special-purpose software systems for linkage design. Research software has been developed to aide the design of planar (Erdman and Gustafson (1977), Ruth and McCarthy (1997)), spherical (Ruth and McCarthy (1997), Furlong, Vance and Larochelle (1998)), and spatial (Larochelle (1998), Kihonge, Vance and Larochelle (2001)) linkages. These packages have a number of features in common. They all provide an interface for specifying discrete workspace positions, they solve for the dimensions of a single mechanism architecture, and they provide some kind of graphical visualization of the resulting linkage designs. In general, they are developed for specific computing platforms and have no export capabilities. Two commercial packages for planar linkage design are also available. SyMech software (2002) allows the designer to synthesize planar mechanisms from within Pro/E. Wireframe kinematic diagrams are generated that can be used to create parts. WATT software (2002) is a stand-alone package that can synthesize 4, 6 and 8 bar planar linkages and display them as stick models; it has very limited import and export capabilities.