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Session 1438

Teaching Mechanism Design Using Constraint-Based Design Tools in a CAD System

Laurence J. Logue Southern Polytechnic State University

Abstract

This paper describes the use of a CAD system with parametric design capabilities to synthesize and analyze planar mechanisms. The techniques used have been developed and used in a third- year course entitled Dynamics of Machines. Geometric and dimensional constraints are applied to the skeleton diagram of a linkage created using standard graphical synthesis methods. The constraints used to emulate the behavior of different types of joints are discussed. The constraints are applied to the skeleton diagram in a way that allows simulation of the operation of the mechanism. The paths of key points such as coupler points can be plotted as part of the simulation. The use of algebraic constraints to impose relations between parameters is also discussed. All of the techniques described will be demonstrated in the presentation.

1. Introduction

Graphical techniques have long been a necessary tool for the mechanism designer. Synthesis techniques based on geometric constructions are traditionally followed by analysis which include finding extreme positions as well as drawing the mechanism in a large number of positions throughout its full range of motion. 1 This form of position analysis can then be followed by graphical analysis of velocities and accelerations. Complete analysis requires that the process be repeated at each position throughout the motion. The use of CAD systems in this graphical approach greatly reduces the time and labor required and should encourage the designer to explore alternative solutions more extensively. Specifically, constraint-based, or parametric design tools make the process easier and quicker by eliminating the need to re-draw elements of the mechanism, even when changing their dimensions.

Constraint-based design is a term used to describe a technique which allows the designer to experiment with design changes while maintaining a set of imposed limits or constraints. It is also used to describe the technique of varying constraints in order to assess their effect on the function of the system being designed. Constraints may include factors such as size, weight, relationships between elements, types of materials, cost, and function. In this context, designing linkages for a particular purpose, we will focus on the sizes of elements and relationships between those elements.

The constraints we will consider can be divided into three basic categories: geometric;

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Logue, L. J. (2000, June), Teaching Mechanism Design Using Constraint Based Design Tools In A Cad System Paper presented at 2000 Annual Conference, St. Louis, Missouri. 10.18260/1-2--8753