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Exam 2 Take Home Component

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F11Jonathan

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Unread post Tue Dec 06, 2011 1:37 pm

Exam 2 Take Home Component

I spoke with Professor Negrut and we only have to use the second moment of inertial of 450. We don't need to plot the three inertia cases like in the book.

Here are my results for the positions and velocities of interest, as well as my force versus x3 position curve.
Results.jpg
Results.jpg (130.24 KiB) Viewed 8261 times

The values are very close to what is in the book.

My Fx force looks a lot different from what is in the book though.
Fx.jpg
Fx.jpg (22.62 KiB) Viewed 8261 times


Is this the Fx force that other people are getting?
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F11Wendy

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Unread post Tue Dec 06, 2011 11:56 pm

Re: Exam 2 Take Home Component

Here are my plots, my Fx is consistent with the book.
Plots.jpg
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F11Jonathan

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Unread post Wed Dec 07, 2011 11:07 am

Re: Exam 2 Take Home Component

Hey Wendy, How did you calculate your Fx?

I believe that is the x force in the revolute joint between bodies 1 and 2 in Fig 8.2.3. I do get the same Fx force when I plot that force.

The figure in the book is labeled the x force in the revolute joint between bodies 1 and 4, which for me is the force due to the absolute x constraint between the ground and flywheel.

Is it mislabeled in the book, or are you really plotting the force in the joint between the flywheel and ground?
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F11Wendy

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Unread post Wed Dec 07, 2011 2:23 pm

Re: Exam 2 Take Home Component

I am plotting the force of the ground on the flywheel as labeled in the book. aka., the x-component of the force due to the absolute x-constraint on the flywheel.
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F11Wendy

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Unread post Wed Dec 07, 2011 2:29 pm

Re: Exam 2 Take Home Component

What format should our results file be in? I understand it needs to be columns of position, velocity, acceleration for each body and our lagrange multipliers, but what order should those be in?
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Dan Negrut

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Unread post Fri Dec 09, 2011 6:31 am

Re: Exam 2 Take Home Component

F11Wendy wrote:What format should our results file be in? I understand it needs to be columns of position, velocity, acceleration for each body and our lagrange multipliers, but what order should those be in?

pos1 vel1 acc1 pos2 vel2 acc2 ...(and so on) ... lambda1 lambda2...(and so on)
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F11Carrie

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Unread post Fri Dec 09, 2011 3:25 pm

Re: Exam 2 Take Home Component

Has anyone else had issues with the step size? When I run my code with the given h=0.005, my Newmark integration blows up and I get a singular matrix. This issue is fixed when I switch to h=0.001. Doing this yields plots that look very similar to the ones in the book, but it appears that the frequency of oscillation for my simulation is slightly lower than in the book. For example, the slider position in the book is between 2 and 3 at t=1.0, but mine is around 4. Any ideas?
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Dan Negrut

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Unread post Fri Dec 09, 2011 6:46 pm

Re: Exam 2 Take Home Component

F11Carrie wrote:Has anyone else had issues with the step size? When I run my code with the given h=0.005, my Newmark integration blows up and I get a singular matrix. This issue is fixed when I switch to h=0.001. Doing this yields plots that look very similar to the ones in the book, but it appears that the frequency of oscillation for my simulation is slightly lower than in the book. For example, the slider position in the book is between 2 and 3 at t=1.0, but mine is around 4. Any ideas?


Carrie - try to run to two smaller step-sizes yet in order to see whether the numerical solution seem to be converging. Maybe even he h=0.001 step size is too large.
I'm curious, how long does on you machine take to complete the simulation?
Dan
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F11Graham

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Unread post Sat Dec 10, 2011 5:00 pm

Re: Exam 2 Take Home Component

I'm a little confused by the problem statement. The take home exam states that gravity acts in the negative y which agrees with the book problem statement in section 8.2.2. However, when they switch to the dynamic analysis section, 8.2.4, the book changes the orientation and says that it acts in positive x and this is the section with the plots. Is this just a matter of the orientation of your global coordinate system? Otherwise wouldn't our plots look different from the book plots?
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Dan Negrut

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Unread post Sat Dec 10, 2011 10:36 pm

Re: Exam 2 Take Home Component

F11Graham wrote:I'm a little confused by the problem statement. The take home exam states that gravity acts in the negative y which agrees with the book problem statement in section 8.2.2. However, when they switch to the dynamic analysis section, 8.2.4, the book changes the orientation and says that it acts in positive x and this is the section with the plots. Is this just a matter of the orientation of your global coordinate system? Otherwise wouldn't our plots look different from the book plots?


Graham - this is a good question. I'm traveling right now with very limited internet access (I'm actually in China).
Here's what i suggest to do to figure out which is correct:
- make the gravity be an applied force and indicate this in the adm file. In one case, make the Y component be -9.81 and the X component be zero.
- in the second case switch these values, and otherwise keep everything else the same in the adm file.
- in both cases xP and yP should be zero, since the force gets applied at the center of mass
- apply the external force in the global reference frame
- see which of the two cases gives you results like the ones in the textbook.
Let me know if it makes sense.
I hope this helps.
Dan
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F11Ryan

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Unread post Mon Dec 12, 2011 1:31 am

Re: Exam 2 Take Home Component

Here are my graphs, I can't figure out whats wrong. Any suggestions?
Attachments
graphs.JPG
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F11Wendy

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Unread post Mon Dec 12, 2011 10:12 am

Re: Exam 2 Take Home Component

@Ryan

Make sure you have a consistent set of initial conditions.
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F11Carrie

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Unread post Mon Dec 12, 2011 1:06 pm

Re: Exam 2 Take Home Component

I tried h=0.001 and h=0.0005, the plots are pretty close, but there is a little bit of separation between them by the time t=1. I'm also having an issue similar to Ryan in that the angular velocity for the crank wobbles a little at the beginning of the run and never goes above 30. I've double checked my initial conditions and they I think they're consistent.

One question I did have is that in the book, the constant torque is reported in units of Newtons. Do we need to convert this somehow to N-m?

My code is taking between 6 and 9 seconds to run.

Another thing I have noticed is that my lambda_1 is negative of what is in the book for the x-component of the force associated with the absolute-X condition.
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F11Christopher

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Unread post Mon Dec 12, 2011 5:20 pm

Re: Exam 2 Take Home Component

Is see the exact same results as Ryan. I used the slider in the far left position as my Initial state. That means my x3 was 1.5 and phi1=pi. The rest of the positions follow from this. I set all velocities and accelerations equal to zero except for phi_dot_1 which I set to 30 as the book suggested. I'm not really sure about all the other accelerations but I'm pretty confident in all the positions and accelerations I chose.

My frequency is much lower than the books solution and the angular velocity of the crank seems to diminish over time.

Again, I'm with Ryan. Any other suggestions?
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F11Carrie

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Unread post Mon Dec 12, 2011 6:38 pm

Re: Exam 2 Take Home Component

My code seems to be working better, although there is still a dependence on the step size, and the reaction force is plotted as -lambda.
Results.png
Results.png (73.37 KiB) Viewed 8104 times
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