Overconstraint problems frequently arise in multibody system models containing rigid bodies. Overconstraints in the model can result in nonconvergence, slow convergence, solver small pivot messages, and in some cases an incorrect solution. Often, overconstraint problems are not readily identifiable. For example, even adding flexibility to the model may not completely resolve an overconstraint problem. It is therefore vital that you address overconstraint issues during the modeling phase if possible instead of trying to resolve overconstraint problems afterwards.
Mechanical APDL does not resolve overconstraints automatically. To check for overconstraints, model the multibody mechanism as a rigid mechanism using a rigid body solver.
Following are some hints to help you resolve overconstraint problems:
Perform a degree-of-freedom count in the mechanism. Various methods are available for evaluating the number of free degrees of freedom in a given rigid body mechanism. See Learning More About Multibody Dynamics.
Know the number of constraints for each joint element. In some cases, replacing one type of joint with another may resolve an overconstraint issue. Check the number of constraints for a given joint and replace it with a simpler one if possible. For example, a revolute joint (which imposes five constraints) can possibly be replaced by a cylindrical joint (which imposes only four constraints). For more information, see Joint Element Types.
A translational joint fixes five degrees of freedom while allowing motion in only one direction. You may be able to replace it with a slot joint which allows more free relative degrees of freedom.
The local axes specified at the joint element nodes must be defined properly. Improper definitions result in unanticipated motion or constraints. For example, if you define the four-bar mechanism in Figure 7.1: Overconstrained System: Standard 3D Four-Bar Mechanism in a plane other than one of the global Cartesian planes, verify that the joint coordinate systems for each joint align.
Perform a modal analysis to ensure that appropriate modes are present in the idealized model of the mechanism. Overconstraints can lead to modes that are not usually present in the actual system.
Use more flexible components in the model. Avoid models with only rigid bodies, which can lead to solver difficulties.
Avoid external (user-defined) constraint equations (CE and CP). They may conflict with those generated internally by Mechanical APDL for contact with MPC and the joint elements.
Check the model for redundant boundary conditions.
Do not mix MPC184 Rigid Beam/Link and MPC184 Joint elements implemented using the Lagrange multiplier method with those implemented using the direct elimination method.