Rigid bodies are widely used for numerical simulation of multibody dynamic applications. The definition of a rigid body is similar to the definition of a rigid surface constraint, except that all the elements are defined by target elements.
The motion of the rigid body is governed by the degrees of freedom at a pilot node which allows accurate representation of the geometry, mass, and rotary inertia of the rigid body. Each rigid body is associated with one pilot node only. The pilot node can be one of the nodes on a target element or a node at any arbitrary location. It can be connected to point mass, follower (FOLLW201), and deformable elements.
For transient analysis you should locate the pilot node at the gravity center of the rigid body if the center of mass is known or can be approximated. You can add a point mass element, MASS21, to the gravity center.
If overconstraint is a concern, the relaxation method (KEYOPT(11) = 1 on the target element) can be applied to a rigid body when there is no contact defined.
Similar to a rigid surface constraint, a rigid body can experience thermal expansion when KEYOPT(12) = 1 on the target element: each target node expands along the line that connects to the pilot node. For more information, see Defining Surface-Based Constraints
See also Modeling Rigid Bodies in a Multibody Analysis in the Multibody Analysis Guide.
You can model contact between two rigid bodies by specifying a contact pair consisting of a contact surface on one rigid body and a target surface on another rigid body. These contact and target elements should overlap the existing contact elements defined for the rigid bodies. Use either the augmented Lagrange algorithm or penalty algorithm (KEYOPT(2) on the contact elements) to avoid overconstraint between rigid-body constraints and contact constraints.
You cannot use the multipoint constraint (MPC) algorithm (KEYOPT(2) = 2) with bonded or no-separation contact behavior (KEYOPT(12)) to connect two rigid bodies. Doing so would cause the model to be overconstrained. Instead, add a third rigid body which connects the first two rigid bodies at their pilot nodes.
MPC bonded contact between a flexible body and a rigid body is allowed. For this case, the contact surface of the MPC bonded contact pair must belong to the flexible body. Otherwise, the MPC bonded constraints and rigid-body constraints would be redundant.
It is possible to model two rigid bodies that are connected or overlapping through shared rigid-body nodes or pilot nodes. To prevent overconstraint, the program internally merges the two rigid bodies into one rigid body and considers the second pilot node as a regular rigid-body node.