Fundamentals of Parametric Constraints


In a parametric drawing, you can add constraints to geometry to ensure that your design conforms to specified requirements.  Parametric drawing can be defined as a technology that is used for designing with constraints.  Constraints are defined as restrictions and associations that are applied to 2D geometry.  They provide a way to enforce requirements when looking at different designs or when making changes in the design phase of a project. 

There are two types of constraints:  geometric and dimensional.  Geometric constraints are used to control the relationships of objects in respect to each other.  Dimensional constraints are used to control the distance, angle, radius, and length values of objects. 

With constraints you can:

  • include formulas and equations within dimensional constraints.
  • maintain design requirements and specifications by constraining the geometry within a drawing.
  • instantly apply multiple geometric constraints to objects.
  • change the value of a variable to make design changes quickly.

It is important to note that you should first apply geometric constraints to determine the shape of a design and then apply dimensional constraints to determine the size of objects in a design.

Parametric constraints have a dedicated ribbon tab that groups the tools and their visibility controls in a logical arrangement.  The Parametric Manager Palette allows tabular editing of names, values, and formulas once dimensional parameters are applied.  This palette can be launched from the ribbon (see Figure 1).

Now that we have an understanding of constraints, we will look at Geometric Constraints followed by Dimensional Constraints, as well as some uses of constraints.

Figure 1: Parametric Palette

Control Parameters with the Parameters Manager

The Parameters Manager lists dimensional constraint parameters, reference parameters, and user variables that you can use to create, edit, and organize.  You can easily create, modify, and delete parameters from the Parameters Manager.  Within the Parameters Manager, you can:

  • click on the name of a dimensional constraint parameter to highlight the constraint in the drawing.
  • double-click a name or expression to edit it.
  • right-click and then click Delete to remove a dimensional constraint parameter or user variable.
  • click a column heading to sort the list of parameters by name, expression, or value.

The Parameters Manager also allows you to define parameter groups in the drawing editor.  A parameter group is basically a collection of named parameters.  It contains a subset of all parameters defined for the current space, but the group can also be empty.  Click the Filter icon to create a group, which displays a filter tree on the left vertical panel of the palette where you can show, hide, or expand the group filters.  Drag and drop the parameters from the grid control into a parameter group.  The Invert Filter check box displays all the parameters that do not belong in the group.  You can filter the display of variables to do the following:

  • All – Displays all associative variables.  No filter is applied.
  • All Used in Expressions – Displays all variables that contain expressions to evaluate a value and variables contained in expressions.
  • Custom group filter – Displays all parameters added in the defined parameter group.

In the Block Editor, the Parameters Manager displays a consolidated view of all parameters and user-defined properties for the block definition.  The Parameters Manager palette includes a grid control with three columns (Name, Expression, Value) by default.  You can add one or more additional columns (Type, Order, Show, or Description) using the shortcut menu.  The Type column affects the formatting of the parameter in the Parameters Manager and Properties palette.  It also affects the value of the parameter when the block is scaled.  The Type cell for User parameters displays a drop-down list of the value types.  The parameter can be sorted and can be set to Show or Hide in the Properties palette.  The value of the parameter resets to 1 when you change the type from String to another type.  Parameters can be renamed; equations and values can be entered or modified.  Changes to a parameter name are immediately updated throughout the table and in the Block Editor.  The parameters for the block definition are organized in the following categories:

  • Action Parameters
  • Dimensional Constraint Parameters
  • Reference Parameters
  • User Parameters
  • Attributes

You can only add user parameters to the table.  When you delete any item from the table, the item is automatically deleted from the block definition.

Geometric Constraints

Geometric constraints are used to determine the relationships between 2D geometric objects or points on objects relative to each other.  When the constrained geometry is edited, the constraints are maintained.  Therefore, you have a method of including design requirements in your drawing by using geometric constraints.

Geometric Constraints contains controls for Coincident (with other object points), Fix (to an absolute location), Horizontal, Vertical, Concentric, Tangent, Parallel, Perpendicular, Collinear, Smooth (join splines), Equal, and Symmetric (matches characteristics about an axis).  See Figure 2. When these are added to the drawing, the first object that you select becomes the master and subsequent elections follow it.

When a constraint is applied, the selected object will automatically adjust to conform to the specified constraint and a gray constraint icon will display near the constrained object.  A small, blue glyph displays with your cursor when you move it over an object that is constrained.  Once a constraint has been applied, only changes to geometry that does not violate the constraints are permitted. 

With some geometric constraints, you can specify constraint points on objects instead of selecting the objects.  This is similar to object snaps; however, the locations are limited to center points, mid points, endpoints, and insertion points.

Multiple geometric constraints can be applied automatically and manually.  If you want to apply all essential geometric constraints to a drawing automatically, you can use the AUTOCONSTRAIN command with the objects that you select. 

Even though a geometric constraint cannot be modified, you can delete it and apply a different one.  This option is available from the shortcut menu.  You can also delete all constraints from a selection using the command DELCONSTRAINT.

Figure 2: Geometric Constraints

Dimensional Constraints

Dimensional constraints are used to control the proportions and size of a design.  They can constrain distances between objects, sizes of arcs and circles, and angles between objects.  If the value of a dimensional constraint is changed, all the constraints that are in the object are evaluated.  The affected objects are automatically updated.  You can also add constraints directly to segments within a polyline as if the objects were separate.

It is important to note that dimensional constraints are different from dimension objects.  Dimensional constraints drive the angle or size or objects; however, dimensions are driven by objects.  Dimensional constraints are used in the design phase of a drawing, whereas dimensions are usually created in the documentation phase. 

Dimensional constraints can be created in the following forms:  dynamic constraints and annotational constraints (see Figure 3). Dynamic constraints are ideal for normal parametric design and drawing tasks.  They display a fixed dimension style, maintain the same size when zooming in or out, position textual information automatically, and do not display when the drawing is plotted.  Annotational constraints are more useful when you want dimensional constraints to change size when zooming in or out, display individually with layers, and display when the drawing is plotted.

There are also reference constraints, which are driven dimensional constraints and can be either dynamic or annotational.  Reference constraints do not control the associated geometry but instead report a measurement similar to a dimension object.  This is a convenient way to display measurements that you would otherwise have to calculate.  Textual information in reference constraints will always be shown in parentheses.  The reference property can be set in the Properties palette to convert a dynamic or annotational constraint to a reference constraint (see Figure 4). However, you cannot change a reference constraint back to a dimensional constraint if this would overconstrain the geometry.

Figure 3: Dimensional constraints

Figure 4: Reference constraints

Design using Constraints

When you are creating or changing a design, a drawing will be in one of three states.

  • Fully constrained – all relevant geometric and dimensional constraints are applied to the geometry.  For a fully constrained set of objects, you will need to include at least one Fix constraint to lock the location of the geometry.
  • Unconstrained – no constraints are applied to any geometry.
  • Underconstrained – some constraints are applied to the geometry.

Please note that AutoCAD® Architecture prevents you from applying any constraints that result in an overconstrained condition.

For designing with constraints, there are two basic methods:

  1. Work in an underconstrained drawing and make changes as you go.
  2. Create and fully constrain a drawing first.  In this way, you can control the design by relaxing and replacing geometric constraints and changing values in dimensional constraints.

Your choice of method will depend on your design practices.  Both are equally effective depending on your needs.

Use Constraints with Blocks and Xrefs

You can apply constraints between the following:

  • An object within a block reference and an object within a drawing.
  • An object within a block reference and an object within a different block reference (not within the same block reference).
  • The insertion point of an xref and a block or an object, but not to any objects within xrefs.

When constraints are applied to block references, you can automatically select objects contained within the block.  You do not need to press Ctrl for sub-object selection.  Adding constraints to a block reference can many times cause it to rotate or move as a result.

When you apply constraints to dynamic blocks, the display of their dynamic grips is suppressed.  You can still use the Properties Palette to change the values in a dynamic block, but the constraints must first be removed from the dynamic block in order to redisplay the dynamic grips.  Constraints can be used in block definitions, which results in creating dynamic blocks.  You can control the shape and size of dynamic blocks directly from within the drawing.

Remove or Relax Constraints

You can cancel the effects of constraints when you need to make design changes by using one of two methods.

  1. Delete each of the constraints individually and then later apply new constraints.  While the cursor hovers over a geometric constraint icon, you can use the Delete key or the shortcut menu to delete the constraint.
  2. Relax the constraints temporarily on selected objects to allow you to make the changes.  With a grip selected or when you specify options during an editing command, use the Ctrl key to alternate between relaxing constraints and maintaining constraints (see Figure 5).

Relaxed constraints are not maintained during editing.  If possible, constraints are restored automatically when the editing process is complete.  Constraints that are no longer valid are removed.

Figure 5: Relaxing constraints


Parametric constraints are a great tool with many design possibilities.  They can be used to great lengths when designing in 2D within AutoCAD Architecture.  How can parametric constraints work best for you?  There’s only one way to find out—start using them today!

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