Hi Wenxuan, 1. When you load a mesh or an analytical object, it's by default in family 255, a reserved and automatically fixed family. You probably changed your mesh into a family that does not have any motion prescription, then in that case it follows the "normal physics" like all the particles you load in, including falling under gravity. If you wish to fix that family, just use *SetFamilyFixed*.
2. This is more complicated. First, we have to understand that in DEME, strictly speaking, objects work with their principal frames of inertia (centered at CoM, having a diagonal inertia matrix). This is a current limitation due to how the inertia matrix-defining APIs are designed: you can only input the principal moments of inertia. But this also means that setting the frame correctly is not necessary if you completely prescribe the motion of that object (a.k.a. this object does not respond to physics), and you don't care about getting a torque reading. However, in your case, I suggest you do so, since you'll prescribe a complex motion, and standardizing the frame to use at least the CoM frame can save you some head-scratching. I suggest that you make it such that in the mesh file you load in, the (0,0,0) point is right at the CoM of this mesh; but if not, you can adjust the CoM and principal axes with *InformCentroidPrincipal* after loading in the mesh. After that is taken care of, you need to combine the two rotations into a rotation about this object's own CoM and a linear translation of this object's CoM (because family motion prescription allows for setting these two motions only). Use your MBD knowledge to do that. I don't guarantee the correctness of this, but some back-of-the-envelop calculation seems to put the angular velocity to be (w2×cos(w1×t), w2×sin(w1×t), w1), and the linear velocity to be 5×(w2×cos(w2×t)×sin(w1×t)+sin(w2×t)×w1×cos(w1×t), -w2×cos(w2×t)×cos(w1×t)+sin(w2×t)×w1×sin(w1×t), 0). Again, please verify yourself. After that, prescribe the object's family's motion using it. But if this is too complicated and you can write your code to calculate the location and quaternion of this object at any given *t *(which for some people is easier than mathematically deriving the analytical expression for the linear and rotational motions), then you can set this object fixed, then use the tracker methods *SetPos *and *SetOriQ *to enforce the location and rotation of this object *at each time step*. It should achieve the same assuming this object is not moving at a very high velocity. Thank you, Ruochun On Thursday, June 27, 2024 at 2:49:43 PM UTC+8 [email protected] wrote: > Hi, > I want to achieve a composite rotational motion of a geometric body, > as follows > Motion 1: Rotate at a speed of w1 rad/s, with the rotation axis being the > z-axis > Motion 2: Rotate at a speed of w2 rad/s, with a rotation vector of [1 * > cos (w1 * time), 1 * sin (w1 * time), 0], and an action point of [0, 0, 5], > as shown in attached figure. > [image: rotating_Chute.png] > > > However, the following issues were encountered: > 1. After using DEMTracker to track geometry, even if I don't add any > motion, the geometry will continue to drop down. How can I fix the geometry? > 2. How to achieve the composite rotational motion mentioned above? > > Best regards, > Wenxuan Xu > -- You received this message because you are subscribed to the Google Groups "ProjectChrono" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/projectchrono/71201812-4db1-4c8d-9783-d0ca4e3ecedcn%40googlegroups.com.
