Wed Dec 07, 2016 4:19 pm by Dan Negrut

Dan - that's a very good question.

Here's how people go about it in the general case.

Imagine you have 9 constraints and have a mechanism that has two bodies. If you go to r-w formulation (instead of r-p), it means that you need to specify for your two bodies a set of 12 velocities.

You already have 9 constraints (the set of velocity kinematic constraint equations). That means that the 12 velocities must satisfy 9 conditions right off the bat. Which means that you are free to specify 3 additional velocity conditions.

There are many ways in which you can specify these 3 conditions. The simplest way, is to impose 3 values for 3 of the 12 velocities that you need to come up with based on how you want the mechanism to start at t=0. You would do this only at the beginning of the simulation, to come up with a consistent set of initial conditions. Thereafter, the dynamics solution takes care of everything.

Does this make sense?

Keep in mind that you don't necessarily need to specify the value of 3 of the 12 velocities. For instance, you might consider prescribing 3 additional GCONs at t=0, just to get you going with a good/consistent set of initial conditions. You would have to make these additional/helper GCONs disappear right after the velocity analysis at t=0.

Let me know if this doesn't make sense. If it doesn't, stop by tomorrow morning before 10:30 am - I can explain in 10 mins what i tried to type in here.

Dan

P.S. If you go the r-p formulation, first find r-w, then use the G transformation to get p_dot once you have w.