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3 CD constraints VS ri = rj for spherical joints




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Unread post Mon Nov 21, 2016 2:39 pm

3 CD constraints VS ri = rj for spherical joints

Hi Everyone,
Besides looking at it from an implementation standpoint, is there any reason to choose one over the other? In SimEngine3D we used 3 CD constraints to specify spherical joints but in the written examples (dangling cube) we use ri = rj. Is there any reason to chose one over the other? In both cases we still get three constraint equations.

Dan Negrut

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Unread post Mon Nov 21, 2016 3:23 pm

Re: 3 CD constraints VS ri = rj for spherical joints

ri=rj is not a basic GCon.
By imposing x_ri - x_rj = 0, y_ri - y_rj = 0, and z_ri - z_rj = 0, we ended up using the CD basic GCon three times to essentially enforce your ri=rj.
Being lazy, we want to implement the minimal number of GCon primitives that can be combined in various ways to produce all the high level physical joints that you encounter in practical applications.
I hope this makes sense.

On a different note, here's a question that requires some thinking, and might be a good exam question. Recall that we have the D Gcon. Why can't I enforce a spherical joint (which typically calls for a set of three constraints) by simply using *one* D GCon that demands that the distance between two points is *zero*?


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