Introduction

Constraints (also known as joints) for rigid bodies connect two rigid bodies. The physics constraints are meant to be attached to an Empty object. The constraint then has fields which can be pointed at the two physics-enabled object which will be bound by the constraint. The empty object provides a location and axis for the constraint distinct from the two constrained objects. The location of the entity hosting the physics constraint marks a location and set of axes on each of the two constrained objects. These two anchor points are calculated at the beginning of the animation and their position and orientation remain fixed in the local coordinate system of the object for the duration of the animation. The objects can move far from the constraint object, but the constraint anchor moves with the object. If this feature seems limiting, consider using multiple objects with a non-physics Child of constraint and animate the relative location of the child.

Connect

The quickest way to constrain two objects is to select both and click the Connect button in Object ‣ Rigid Body. This creates a new empty object (named "Constraint") with a physics constraint already attached and pointing at the two selected objects.

Physics Menu

Also you can create Rigid Body Constraint on one of the two constrained objects with Rigid Body Constraint button of the Physics tab in the Properties. This constraint is dependent on the object location and rotation on which it was created. This way, there are no empty object created for the constraint. The role of the empty object is put on this object. The constrained object can be then be set as a Passive type for better driving of the constraint.

Additional parameters appear in the Rigid Body Constraint panel of the Physics tab in the Properties for the selected empty object or the one of the two constrained objects with the created constraint.

Common Options

Reference

Panel:

Physics ‣ Rigid Body Constraint

Settings

Enabled

Specifies whether the constraint is active during the simulation.

Disable Collisions

Allows constrained objects to pass through one another.

Breakable

Allows constraint to break during simulation. Disabled for the Motor constraint. This can be used to simulate destruction.

Threshold

Impulse strength that needs to be reached before the constraint breaks.

Limits

By using limits you can constrain objects even more by specifying a translation/rotation range on/around respectively one axis (see below for each one individually). To lock one axis, set both limits to 0.

Objects

First

First object to be constrained.

Second

Second object to be constrained.

Override Iterations

Allows making constraints stronger (more iterations) or weaker (less iterations) than specified in the rigid body world.

Iterations

Number of constraint solver iterations made per simulation step for this constraint.