Constraint (computer-aided design)

The shaft and pulleys share a common centerline. The constraints of the key are set in relation to the keyseat.

In engineering design, particularly in the use of computer-aided drafting and design, in the creation of 3D assemblies and multibody systems, the plural term "constraints" refers to demarcations of geometrical characteristics between two or more entities or solid modeling bodies; these delimiters are intentional in defining diverse properties of theoretical physical position and motion, or displacement. In addition, 2D sketches - including the ones used to create extrusions and solid bodies - can also be constrained.

Constraint may be specified for two (or more) entities (e.g. lines) at once. For instance, two lines may be constrained to have equal length or diameter of circles can be set to have the same dimension (e.g. radius or lenght). Moreover, the concept may be applied to solid models to be locked or fixed in a specified space.

The terminology of constraint may vary depending on a CAD program vendor.

Overview

The purpose of constraints in a design is to control and limit the behavior of the entities and bodies in relation to another entity, plane or body. Effective constraints or mates between two or more bodies may exist at the assembly level of these or between two or more entities in defining a sketch, but adding conflicting, unnecessary or redundant constraints may result in an overdefined sketch and an error message.

Types

There are several constraints that may be applied between the entities or bodies depending on their actual natural geometry (may also be referred to as ’’mates’’): collinearity, perpendicularity, tangency, symmetry, coincidency, and parallelity among other ways of establishing the orientation of the entity.

History

The original idea of "constraits" was introduced by Ivan Sutherland in 1975 and is derived from ides employed in Sketchpad system (1963). in his work he stated that a usefulness of a techinical drawing made by a computer programs dwells exactly on their structured nature. Traditional drawings lack this structure and therefore are inferior. In important idea was that a CAD system should maintain structure of a entities (lines, angles, areas etc.) as designer manipulates a geometric model.

In 70s the idea was further extended into three-dimensional spaces (a solid 3D modelling constraints) and in 80s a more generalized constraint-based programming languages idea has emerged and has found some application in CADs software. At least one conceptual prototype built around concepts of constraint maintenance was implemented in 1989.

Example

Ideally, a rod will need to be concentric to a hole drilled through the plate where it will be inserted, so the constraint "concentric" guarantees that the diameter of the rod and the diameter of the hole maintain a common centerline, thus "locking" the manner the rod relates to the hole in the plate; this means that the rod could still slide on either direction since the position of its ends has not been limited. Instance 2 illustrates that the rod may still rotate along its centerline while it slides up or down.

Concentric mating of cylinders, shafts, or rods to a plate.

See also

• Constraint (classical mechanics)
• Geometric constraint solving
• Parametric modeling
• Preliminary design & detailed design

Sources

• Introducing AutoCAD 2010 and AutoCAD LT 2010 (pages 117-122), by George Omura. 2009; 1st. Edition. Wiley Publishing, Inc., Indianapolis, Indiana. ISBN 978-0-470-43867-1 Hard Cover; 384 pages.
• Autodesk® Inventor® 2011 Essentials Plus (pages 312-341), by Daniel T. Banach; Travis Jones; Alan J. Kalameja. 2011; Delmar/Cengage Learning, Autodesk Press. Printed in the United States of America. ISBN 978-1-1111-3527-0; ISBN 1-1111-3527-4. New York.