FEA Compliant Modeler > Best Practices for Compliant Model

Below are the general steps necessary in creating a compliant 3DCS model:

Rigid Body Model - create a completed 3DCS model utilizing the standard DCS Moves, Measures and Tolerances. This is called a rigid model. The compliant model will be built on top of the rigid model.

Compliant parts/points - determine which parts in the rigid model will be simulated as compliant (flexible) parts. On these compliant parts create additional DCS points for the clamp, weld, force application locations (some of these points may already be created for the rigid model). Create any additional DCS modeling points in locations on compliant parts where visual bending is desired and additional measurements will be taken. These points can be feature points or coordinate points.

Mesh files - If possible, obtain the mesh files from your FEA Group. Otherwise, using an FEA meshing software, create a mesh file for each of the compliant parts.

Create FEA files - If available, select the FEA Solver in the StiffGen dialog. The solver will be an executable file such as Abaqus.exe, or Nastran.exe. StiffGen can call the solver using a *.bat file as well. This will allow you to generate the stiffness, mass, or load matrices from inside the 3DCS software. Also it will automatically create an updated mesh file that contains a list of ASETs (closest mesh nodes to the modeling points).

Load FEA files - create the Load FEA Data move(s) and load the mesh, stiffness, and mass files. Once loaded a Nominal Build is required to link the ASETs in the mesh file to the modeling points for the parts.

Constrain the parts - This is a case specific operation based on the build sequence. Parts can be fixed in space with LockDOF, clamped to a fixture or clamped to another part. It is important that parts are fully constrained at all steps of the build sequence. Users can combine hard and soft clamps in the same move if they consider necessary.

A best practice is to use three hard clamps at the primary direction locators (datums if applicable) and use soft clamps at the other locations . For simplicity the first three clamps can be in one move and the other clamps in a separate move.

Apply loads - Create the other compliant moves: gravity, join, force application, thermal. All these moves should be applied on fully constrained parts.

Second stage - To relocate the part(s) to the second stage fixture (or another part), follow these steps:

8.1. Unlock all locked points. Unclamp the parts from the first fixture but keep clamped three points (or more if the assembly process requires it). These (three) points should be later used to clamp the part in the next stage (if applicable). The part will relax (springback) relative to these unclamped points. For a 3-DOF stiffness matrix three points that form a plane are necessary to fully constrain the part with a hard clamp. For a 6-DOF stiffness matrix a single point can be clamped with a hard clamp to fully constrain the part. User should pay attention when unclamping so the part does not become underconstrained.

8.2. Unclamp the last points using the Skip Deformation. This option will unclamp the points without deforming the parts. The part/assembly (already relaxed relative to the last clamps), will keep the shape until it is clamped in the next stage.

8.3. Use a rigid body move to transfer the part(s) to the new location (fixture).

8.4. Clamp the part(s) to the second fixture (or target part).

8.5. Create the other compliant moves for this level.

Tips for improving the performance of the compliant model:

1. Use fewer compliant moves if your process allows it. The CM calculation is move based and since DCS builds moves sequentially, more moves will result in longer calculation time. You can shorten the calculation time by having more operations in one move, such as fixing multiple parts in one move.

2. Generate stiffness matrix in the assembly orientation. Although CM supports rotating parts, this requires a transformation of matrix to adjust the orientation of the stiffness matrix, which adds computational cost. If stiffness matrix is set up in the design orientation, the calculation triggered by transforming stiffness matrix can be omitted.

3. Do not create/keep unnecessary points in the compliant part. Only the points used in compliant moves, the measured points, and a certain number of extra points distributed equally (if possible) on the entire surface of the part should be kept. Reducing the number of modeling points will make stiffness matrix smaller and the calculation faster. At the same time, a stiffness matrix that is not representing the part may give inaccurate results (stiffness matrix uses a superelement) .

4. Soft clamp is computationally more intensive compared with LockDOF and hard clamp. Soft clamp is applied by adding extra constraints to the stiffness matrix, while LockDOF and hard clamp are applied by constraining existing DOFs. The stiffness matrix created after applying a soft clamp is larger because of the extra variables, thus more calculation.