LS Dyna

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Nodes translation restriction (only moving forward)

• Yuqian TU
Subscriber

Hello!

Is there a keyword or a way to constrain nodes (either in a shell/solid element or rigid node set) at X and Y directions that nodes are only allowed to move forward. When the nodes want to move backward, the displacement is set to 0.

I was thinking about local coordinate system with rigidwall, but didn’t seem to work.

Thanks a lot!

• Jim Day
Ansys Employee

A couple of possibilities:

1. Attach a stiff *ELEMENT_SEATBELT and an accompanying *ELEMENT_SEATBELT_RETRACTOR such that the seatbelt resists backsliding and is reeled in as the part moves forward.
2. To resist backsliding, a feedback control system of sorts could be employed to apply a resisting force.

For example, here’s an example in which a force is applied to part #1 in accordance with the control law shown below so that it translates to a specified target point.

Force = -gain1 * (displacement – target) – gain2 * velocity

The feedback control system is governed by the following keywords which are used to:

1. define the gains and other pertinent parameters
2. define the feedback control law
3. apply the load

*PARAMETER
R gain1 0.060000
R gain2 0.010000
R target 2.000000
\$
*DEFINE_CURVE_FUNCTION
\$# lcid sidr sfa sfo offa offo dattyp
100 0 0.000 0.000 0.000 0.000 0
\$# function
-gain1*(dx(28)-target) – gain2*vx(28)
\$
*LOAD_RIGID_BODY
\$# pid dof lcid sf cid m1 m2 m3
2 1 100 0.000 0 0 0 0

In *DEFINE_CURVE_FUNCTION dx(28) is used to monitor the x-displacement of node #28 while vx(28) returns the node’s velocity. These responses are updated continuously with the current values as the simulation progresses. The function itself could be a control law that involves any type of model response, such as, nodal forces, accelerations, stresses, etc.

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