CMC results on "free" motions reasonable?

[Daniel Krueger]

Hi everyone,
my colleague and me were currently discussing about CMC simulations. Out of curiosity I created a model of a simple 1-DOF pendulum with a coordinate actuator assigned to the pivot. We simulated 0.5 seconds of free swinging motion using the forward dynamic tool. Free means no excitation on the coordinate actuator and thus no driving torque, only gravity. Afterwards we used this motion sequence as desired kinematics within a CMC simulation. The results were a bit surprising since the CMC tool calculated non-zero excitation values for our coordinate actuator. Can anyone explain this behaviour? I would have expected all excitations to be (near) zero since the gravity more or less produces the desired accelerations cmc is solving for.
Best regards,
Daniel

[jimmy d]

Hi Daniel,

CMC takes the position data, splines it, and differentiates it. The splining and (potential) filtering will mean that that the accelerations are going to be slightly different from your Forward results. CMC then uses the actuator to minimize this difference.

Hope that helps,
-james