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simEngine3D - initial velocities

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samuelacuna

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Unread post Wed Nov 16, 2016 5:05 pm

simEngine3D - initial velocities

For my simEngine3D I am close to implementing the N four-bar mechanism benchmark, but the benchmark requires an initial velocity specified at one point of the mechanism.

I am really struggling figuring out how to prescribe initial velocities so that they align with the velocity constraints. I've looked around online and through ADAMs documentation, and it seems like it involves setting up another constraint? Not sure.

Currently I've set the system to start from rest. It solves, but doesn't generate the motion described in the benchmark. Any insights would be helpful
- Samuel
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Dan Negrut

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Unread post Wed Nov 16, 2016 10:51 pm

Re: simEngine3D - initial velocities

Samuel,
To that end, you need to do a velocity analysis.
You will have to compute omega_bar and r_dot (or p_dot and r_dot, if you prefer to go that way).
You are going to have to satisfy the velocity constraints.
Incidentally, you will have less constraints than unknowns, which means that you can choose some velocity/velocities the way you want.
Say you have four kinematics constraints and six velocities. You can choose two velocities and compute the other four using the four constraint equations. This amounts to solving a linear system of dimension 4 by 4.
Other way to go about it (which probably you should not pursue) is to provide an initial guess and find the closest set of velocities that verify the kinematic constraint equations. This is a minimization problem subject to the velocity constraints.
Please let me know if this makes sense.
Dan
P.S. Before you compute the velocities, you will have to have a consistent configuration in positions; i.e., a set of positions that satisfies the kinematic constraint equations at the position level.
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danielpiombino

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Unread post Wed Dec 07, 2016 3:03 pm

Re: simEngine3D - initial velocities

Dan,

I'm confused on how to go about choosing the excess velocities. Can you just pick any velocities at random and assign them arbitrary values? I would think that doing that could wind up in accidentally picking values that do not have a consistent solution. I'm not sure how to go about picking terms and values without already knowing what the initial velocities need to be in order to be consistent.
For the N-bar mechanism I'm pretty sure I know how I could implement initial constraints with f_dot terms that would account for the initial velocities and bring the number of DOF to zero in order to perform the velocity analysis, but that's only because it's a fairly simple system. I wouldn't know how to apply that to something in a more unusual initial position.

Thanks,
-Dan
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Dan Negrut

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Unread post Wed Dec 07, 2016 4:19 pm

Re: simEngine3D - initial velocities

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.
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danielpiombino

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Unread post Wed Dec 07, 2016 4:35 pm

Re: simEngine3D - initial velocities

That makes sense, and it's essentially what I've been doing so far. I was trying to visualize how that would work with a system with more unconstrained DOF's, and I think I was overthinking it. For any unconstrained DOF in the system you would either have an initial velocity, or that initial velocity could be anything you arbitrarily choose.

Thank you!

-Dan
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Dan Negrut

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Unread post Wed Dec 07, 2016 4:47 pm

Re: simEngine3D - initial velocities

Dan,
you don't say that you got it just because the alternative would have been to come and meet with me, do you?
;-)

It's tricky for instance when you have vehicle model with hundreds of degree of freedom and need to come up with a velocity for all of them.
Also, think if you have tires made up of FEA elements - you'd have to deal with a large problem...

Cheers,
Dan

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