OS-E: 4015 Optimization of MBD System Level Response

MBD system level responses can be displacement, velocity, acceleration, force, or some predefined function of those four quantities. MBD system level responses can be optimized/constrained along with other structural responses of flexible bodies in MBD systems.

Model Files

Before you begin, copy the file(s) used in this example to your working directory.

mbdsystemlvlopt.fem

Model Description

The example shown here is intended to minimize the mass of a system. Constraints are imposed on the stress of a flexible body and the velocity of a point. Notable points include the following.

System Level Responses

In order for the system level responses to be made available to the optimization process, MBREQE or MBREQM entries must be defined so that the MBD solver can generate the appropriate output.

$------|--MID--|--GID--|
MARKER 55      9929617
$------|--RSID-|--RID--|--ITEM-|--MID--|
MBREQM  99     999     VEL     55

Then RID (999 in this case) must be referenced in DRESP1.

$------|-------|-------|-------|-------|-------|-------|-------|-------|
DRESP1  3       VELO    MBVEL   MBREQM          TX      MIN     999

It is not necessary to reference the requests defined by MBREQM/MBREQE in the subcase to make the requested responses available for the optimization. RSID (99 in this case) of MBREQM can be an arbitrary positive integer.

DVGRID Including Joint Location

Changing the length of a body is a common method of achieving desired system level responses. This change in shape can be defined using DVGRIDs. In this case, the DVGRIDs need to be defined carefully, as it is necessary to ensure that the joints remain in a valid configuration while changing the length of the bodies.

Figure 1. A Common Mistake in Defining Shape Change that Involves Change of Joint Locations

In Figure 1, Nodes A and B must remain coincident after applying the shape perturbation.

The correct configuration after applying shape perturbation vector should be:

Figure 2. Correct Way of Defining Shape Change that Involves Change of Joint Locations

Results

The optimization result should look like Figure 3.