The information below was gathered from the 2018 Simcenter Conference - Americas that took place October 15-17 in Detroit, Michigan.
Author: Matt Orzewalla, Structural Engineer
The SHERLOC instrument on the Mars 2020 Rover is mounted adjacent to the rover’s rotary percussive drill. To prevent large vibratory excitations from being transmitted into the instrument, SHERLOC is mounted using a hexapod with wire mesh toroid compliant elements in the struts providing the major compliance and damping sources for the system. These wire mesh elements have a nonlinear load versus deflection behavior. When preloaded in pairs, they behave in a fairly linear fashion for moderate amplitudes, but then stiffen at large amplitudes. Random vibration testing of the isolated SHERLOC system was found to produce higher than usual crest factors for the full level excitation. Investigation of the data from the test revealed that the nonlinear behavior of the struts may be significantly affecting the response of the system. The test configuration was modeled in Simcenter. Nastran was used to compute the random vibration response of the model and the linear modal transient response. To simulate the nonlinear dynamics, Nastran NOLIN1 entries were added to the model to produce the nonlinear portion of the load versus deflection curves for each wire mesh element. The resulting nonlinear dynamic behavior better matched the test data and allowed for further investigation of the system with confidence. The use of NOLIN1 entries provide a powerful and efficient means of modeling nonlinear load versus deflection dynamic behavior in an otherwise linear system. This presentation proves an example use case along with the setup of EPOINT, TF, and NOLIN1 entries used to produce the desired behavior.