Luc Sibille said: (by the date of Mon, 25 Oct 2010 19:51:42 +0200) > Hi all, > > I am still investigating the different moment-rotation contact laws in > Yade (version 0.50 and last update version). > > I have two interrogations: > > 1/ First in every case relative particle rotations is totaly recomputed > at each time step from the initial orientation with a code like that: > > Quaternionr delta((rbp1.ori * (initialOrientation1.conjugate())) * > (initialOrientation2 * (rbp2.ori.conjugate()))); > > is it safe to make such a computation, where after a long time > simulation, relative rotation can become very large more than 2*pi > whereas there is after a correction for the computed relative rotation > if it is greater than 2*pi.
Remember that this is a relative rotation. Not rotation of both spheres. I was doing tests, like rotating whole beam (a line of spheres) around various axes, and the beam still behaved properly, even though it was doing hundreds of 2*pi rotations. I mean - it was working, because whole sample was rotating, so relative rotations between spheres remained very small. If you have a sample, where in a single contact, one of the spheres "rotates away", then indeed troubles are possible. That needs checking. > In addition: > > Just to note that this formula will "fail" if the relative rotation of a > > particle will be more than 180 degrees (the quaternion will take it the > > other way around, as it goes always the shortest way). > > > I imagined a priori to compute this relative rotation incrementally (as > the shear force), to avoid to deal with great relative rotation. What is > your opinion? > why did you choose this formula and not an incremental > formula? It is simpler. Just calculate the rotation difference. If you want to track 2 pi rotations, you can store previously calculated angle, and compare it with a new one. When you spot a 2*pi jump in values, then increment some integer counter of 'total rotations'. In fact I am doing that in lattice model, so it works if I spin a rod around another rod several times, and then I can see it unspinning back the exact same amount of full 2*pi rotations. I recall that Vaclav has also implemented a 'total distance, no increments' approach on shear force. > 2/ Then I rediscovered that aa.angle() can return nan, as commented in > source file, it happens when the quaternion is close to identity. Please > apologise for my lack of culture, but what does represent physicaly a > quaternion close to identity? hmm. All quaternions that we use are identity quaternions: a^2+b^2+c^2+d^2=1. If that condition is not kept, then it is not a rotation, but just some random quaternions. To explain by analogy: think about complex numbers: a^2+b^2=1 means that we are using only complex numbers from a circle of radius=1. If that condition is not kept, then it is just some random complex number. So perhaps you mean, that if a quaternion is not a unit quaternion then angle()==nan. This is correct, - you cannot have an angle if quaternion does not represent rotation. - you cannot have an angle if a^2+b^2+c^2+d^2 is different from one. -- Janek Kozicki http://janek.kozicki.pl/ | _______________________________________________ Mailing list: https://launchpad.net/~yade-dev Post to : [email protected] Unsubscribe : https://launchpad.net/~yade-dev More help : https://help.launchpad.net/ListHelp
