6.1.3 Coupling Constraint

Coupling constraints couple the motion of a surface to that of a reference point (RP). The RP point has 6 dof. You have to restrain the dof that are not used to constrain the surface, or else it will produce negative pivots during the solution. Say you want to couple U1 of the RP to a surface, so that a load P1 (or displacement U1) will be distributed (applied) equally to the entire surface. In this case, the remaining dof of the RP must be constrained, because you are using only U1 to couple to the surface and you should not let the remaining dofs loose. There are two cases:

If applying a load to the RP

If you are applying a load to the RP, then you create a load and apply the load to the RP on the unrestrained dof (see figure below). Next, you create a new BC on the RP (see figure below) and use it to restrain the dof that you did not use to apply the load. See also [3, Example 7.5].
\includegraphics[width=.5\textwidth ]{./AbqImages/ApplyLoad2RP.PNG} \includegraphics[width=.5\textwidth ]{./AbqImages/ApplyDisp2LoadedRP.PNG}

If applying a displacement to the RP

If you are applying a displacement to the RP, constrain the remaining displacements right there:
\includegraphics[width=.5\textwidth ]{./AbqImages/ApplyDisp2RP.PNG}


# displacement applied to RP
Model.DisplacementBC(amplitude=UNSET, createStepName='ApplyStrain', 
    distributionType=UNIFORM, fieldName='', fixed=OFF, localCsys=None, 
    name='Load-RP', region=Instance.sets['Set-RP'], 
    u2=SET, u3=SET, ur1=SET, ur2=SET, ur3=SET)