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Dan Negrut

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Unread post Mon Mar 15, 2010 1:53 pm

Office Hours Related

I'll post here any topics related to office hours.

For instance, today i have to leave right at 4 pm (end of official office hours) to pick up kids from school. 
Post any questions you might have on the forum, i'll try to reply tonight.
Dan
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ME751Chris

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Unread post Mon Mar 15, 2010 11:48 pm

Re: Office Hours Related

In office hours we talked about taking ground out and how Ai would be the identity matrix (assuming ground was body i in the constraint equation).  However, isn't it the negative identity matrix (-I)?
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ME751Chris

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Unread post Tue Mar 16, 2010 12:55 am

Re: Office Hours Related

also, pibar will be 6x3 if ground is one of the parts in the constraint eqn? 3 because there are three axes in which omega is described by?
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ME751Chris

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Unread post Tue Mar 16, 2010 1:43 am

Re: Office Hours Related

For the normalization constraint of p, what is it with respect to q?  I know it will be a 1x7, with the first 3 entries being zeros (not dependent on r).  The equation is p'*p-1, so I was thinking it was going to be 2*p'*pdot which is a number....not a 1x4 I was hoping.
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Dan Negrut

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Unread post Tue Mar 16, 2010 6:56 am

Re: Office Hours Related

ME751Chris wrote:In office hours we talked about taking ground out and how Ai would be the identity matrix (assuming ground was body i in the constraint equation).  However, isn't it the negative identity matrix (-I)?


Chris - it should always be I.  Let's talk after class about why this would be -I.  Body "0", the ground, doesn't have any generalized coordinates associated with it.  Its A is I, and stays always constant.
I hope this helps.
dan
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Dan Negrut

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Unread post Tue Mar 16, 2010 6:59 am

Re: Office Hours Related

ME751Chris wrote:also, pibar will be 6x3 if ground is one of the parts in the constraint eqn? 3 because there are three axes in which omega is described by?


3 because the dimension of omega is 3x1.  Recall that pibar is the matrix that multiplies the omega_bar in the time derivative of Phi.  Since Phi is of dimension 6 (5 from revolute joint + 1 driving constraint), pibar will be 6x3.
dan
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Dan Negrut

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Unread post Tue Mar 16, 2010 7:01 am

Re: Office Hours Related

ME751Chris wrote:For the normalization constraint of p, what is it with respect to q?  I know it will be a 1x7, with the first 3 entries being zeros (not dependent on r).  The equation is p'*p-1, so I was thinking it was going to be 2*p'*pdot which is a number....not a 1x4 I was hoping.


think a bit about it, it's 2p', which is a 1x4.  2*p'*pdot  it's the time derivative of the Euler Param normalization constraint.

dan
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ME751Chris

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Unread post Tue Mar 16, 2010 9:18 pm

Re: Office Hours Related

i don't think parsing out the data of a 7x14 matrix will work like we talked about in office today.  Think about the DP1 constraint in which the Ai matrix (assuming body_i is ground) because p would be 0 0 0 0, which when making Ai, would result in a -I matrix...right?
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Dan Negrut

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Unread post Wed Mar 17, 2010 6:40 am

Re: Office Hours Related

ME751Chris wrote:i don't think parsing out the data of a 7x14 matrix will work like we talked about in office today.  Think about the DP1 constraint in which the Ai matrix (assuming body_i is ground) because p would be 0 0 0 0, which when making Ai, would result in a -I matrix...right?


Chris - the p would be [1,0,0,0].  Recall that the norm of p should be 1.0 (the normalization constraint).  As such, you can't have all values in p be zero.
With [1,0,0,0] the A matrix becomes the identity matrix.
dan
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ME751Chris

ME751 Student

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Unread post Tue Apr 13, 2010 9:31 pm

Re: Office Hours Related

In computing the acceleration at time = 0, you used the following in the masspringdamperNR.m file:
accVec(crntStep,1) = (sin(2*crntTime) - c*v_old*v_old - k*x_old^1.5)/m;

How was this found?
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Dan Negrut

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Unread post Wed Apr 14, 2010 6:32 am

Re: Office Hours Related

Chris - see my answer from "Assignment 11" discussion thread.
dan

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