When building a model that will be used in an Adams simulation, an important consideration is how to represent interface points within the structure. An interface point is a node that will have an applied joint or force in the Adams program. Keep in mind that, in Adams, the forces can only be applied to interface points.
The number of interface points used will determine the number of constraint modes for the model. Constraint modes are the static shapes assumed by the component when one degree of freedom of an interface point is given a unit deflection while holding all other interface degrees of freedom fixed. The number of constraint modes is equal to the number of degrees of freedom of all interface points. (For 3D models, the interface points have 6 DOF; therefore, each interface point has 6 constraint modes.)
You must pay special attention to modeling interface points for these reasons:
An interface point must have six degrees of freedom (except for 2D elements).
Force (applied directly or via a joint) should be applied to the structure by distributing it over an area rather than applying it at a single node.
If there is no node in the structure where you can apply the force or joint in Adams (for example, a pin center), you need to create a geometric location for that point.
Use the following guidelines to determine the best way to model the interface points for your structure:
To ensure that all your loads will be projected on the deformation modes in the Adams simulation, you must define all nodes where you are going to apply a joint or a force as interface points.
Interface points in Mechanical APDL must always have six degrees of freedom, except for 2D elements. If your model consists of solid elements, use constraint equations or a spiderweb of beam elements to ensure that the interface node has 6 degrees of freedom.
A good practice for modeling interface points is to reinforce the area using beam elements or constraint equations. Using one of these techniques will distribute the force over an area rather than applying it to a single node, which would be unrealistic.
If you use a spiderweb of beam elements, use a high stiffness and a small mass for the beams. Otherwise, you will alter the stiffness and mass of your model, which could result in eigenmodes and frequencies that do not represent the original model.
If you use constraint equations, we recommend using contact elements and the internal multipoint constraint (MPC) algorithm (see Surface-based Constraints in the Contact Technology Guide) to attach the interface node. As an alternative to the MPC method, you may use constraint equation commands such as CE and CERIG (for example, CERIG,
INDEPEND,DEPEND,UXYZ, whereDEPENDis the interface node). (Avoid the RBE3 command since problems can occur with the independent degrees of freedom.) If you use constraint equations, mesh the interface point with a MASS21 element (use KEYOPT(3) = 0) that has small (negligible) inertias.
Do not define interface points that lie next to each other and are connected by constraint equations or short beams. This type of connection would require too many eigenmodes and result in a model that is not well conditioned.
There are three different ways that you may attempt to attach an interface point to a structure. The first two methods demonstrate valid methods of attachment. The third method is a poor method of attachment that should not be used.
Each method depicted in the figure is described below.
Surface-based constraints using rigid surface constraint connects to the interface point to the structure. This method is recommended (and preferred) because:
No MASS21 element is necessary.
A spiderweb of beams is connecting the interface point to the structure. This method is acceptable but outdated:
Force is distributed over an area.
No MASS21 element is necessary (because the beams supply the six degrees of freedom).
Moment loads are transmitted.
One beam is used to connect the interface point to the structure. This is not recommended because:
The force is applied to the structure at a single node.
Solid elements do not have rotational degrees of freedom. Therefore, moments will not be properly transmitted from the interface point to the structure (a spiderweb scheme should be used).