A powerful new feature available in LS-DYNA version 971 is the *SENSOR keyword. To briefly summarize the Keyword Manual description, *SENSOR allows boundary conditions and various model entities to be activated/deactivated based on element, force, and/or node based criteria.
Options are available for modeling very elaborate systems, but we will focus on a very simple example which can be downloaded here: sensor.k. The cards of primary interest are as follows:
The *SENSOR_DEFINE_NODE card instructs LS-DYNA to monitor the distance between two nodes in the global Z direction. The *SENSOR_SWITCH card determines when the result becomes less than a specified value. And when this occurs, the *SENSOR_CONTROL card deactivates the spring element in the model.