Using constraints to control sketch geometry

Constraints create logical relationships that control the position or size of geometry. Constraints are defined without using numerical values. The other types of geometry controls, dimensions and parameters, are specialized constraints that use values or evaluated expressions to define numerical relationships in a sketch. Constraints are not required in Abaqus/CAE sketches; however, they can help you preserve the design intent. When you modify a constrained sketch, you must either work within the existing controls, modify them, or delete them to complete your changes.

Related Topics
The Sketcher cursors and preselection
Customizing the Sketcher
Constraining, dimensioning, and parameterizing a sketch

Table 1 lists each constraint type, its symbol, and a brief description of its use.

Table 1. Sketcher constraints.
Constraints Symbol Description
Coincident o Abaqus/CAE moves the selected entities so that they share a common point. The point may be a point on both entities or a projected point that would exist only if one of the entities was extended. If two straight lines are selected, they become colinear.
Concentric   Abaqus/CAE moves the selected arcs or circles to share a common center.
Equal length / Abaqus/CAE makes the selected lines all the same length.
Equal radius — Abaqus/CAE makes the selected arcs or circles the same radius.
Fixed   Abaqus/CAE fixes the radius of an arc or circle, the angle of a line, or the position of a vertex.
Horizontal H Abaqus/CAE aligns the selected line with the X-axis of the sketch. The horizontal constraint is not affected by grid orientation.
Equal distance |o| Abaqus/CAE moves the selected vertex so that it is the same distance from the two vertices or lines used to define the constraint.
Parallel // Abaqus/CAE makes the selected lines parallel.
Perpendicular   Abaqus/CAE makes the selected lines perpendicular.
Symmetry o|o Abaqus/CAE makes the selected entities symmetrical about your chosen line of symmetry.
Tangent || Abaqus/CAE makes the selected entities tangent at their nearest point. At least one of the entities must be curved.
Vertical V Abaqus/CAE aligns the selected line with the Y-axis of the sketch. The horizontal constraint is not affected by grid orientation.

Note:

The behaviors described in Table 1 assume that no other constraints are applied to the selected entities. The presence of other constraints and the constraint solution method selected in the Sketcher Options may affect how Abaqus/CAE moves entities to comply with the constraints.

You can create multiple instances of the same type of constraint in a sketch. You can use the sketch module queries to locate the geometries controlled by a constraint or to list the constraint details for the entire sketch (for more information, see The Query toolset”).

You can use the following techniques to add constraints to a sketch:

Preselecting geometry

If preselection is active, Abaqus/CAE can add some constraints while you create geometry. Coincident, horizontal, perpendicular, tangent, and vertical constraints can be added using the preselection cursor. For more information, see The Sketcher cursors and preselection.

Selecting midpoints of lines or curves

If you select the midpoint of a line or curve as the vertex for additional sketch geometry, Abaqus/CAE also creates an equal distance constraint between the midpoint and the endpoints of the line or curve.

Adding constraints automatically

After you create a sketch, you can use the auto-constrain tool to add constraints to the entire sketch or to a selected group of entities. Use the sketch options to control the constraint types that Abaqus/CAE can add automatically and the allowable tolerances for adding a constraint. For more information, see Customizing the use of constraints in the Sketcher.

Adding individual constraints

After you create sketch geometry, you can select the constraint type and pick the entities to apply the constraint. This method gives you the most control, but it is also the most time consuming. For more information, see Adding individual constraints.

When you add constraints, you must not overconstrain the sketch geometry. Geometry is overconstrained when multiple controls are applied to a single degree of freedom (for more information, see Fully constrained geometry). Abaqus/CAE colors conflicting constraints magenta to indicate an overconstraint condition. You must resolve any overconstraints before you can use a sketch to create a feature or save it with the model database.