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Last Post:
Nov 6, 2009 4:22 PM
Last Post By: lichao_yu_autod...
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Posts:
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Registered:
09/23/08
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DYNAMIC SIMULATION, unknown force, constraint problem
Posted:
Oct 28, 2009 10:57 PM
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Here is a tricky problem
I am trying to determine the spring needed to counterbalance a weight; the basic parts and assemblies are in the attached file.
The idea is that little effort is needed to move the weight to any angle and the when no force (or torque) is present, the weight remains in the position without falling.
I have problems with the unknown force, in the assembly give me a constant force as output and should give me a curve that starts at zero and then starts to rise.
I don’t apply any external force, just the rotating of one of the joint inside the unknown force
In the model just go to Dynamic Simulation, unknown force, the jack should be placed in the central tube and the weight should rotate up to 90 deg
Can someone tell me what I am doing wrong?
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Posts:
10
Registered:
09/23/08
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Re: DYNAMIC SIMULATION, unknown force, constraint problem
Posted:
Oct 28, 2009 11:01 PM
in response to: r_romeo
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Sorry, the files were too big, here are the parts that have to be added in the folder "parts"
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Posts:
10
Registered:
09/23/08
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Re: DYNAMIC SIMULATION, unknown force, constraint problem
Posted:
Oct 29, 2009 7:06 PM
in response to: r_romeo
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still don't know why unknown force remains constant
here is a picture of the placement of force and unknown force setup
HELP PLEASE !!!!!!!
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Posts:
10
Registered:
09/23/08
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Re: DYNAMIC SIMULATION, unknown force, constraint problem
Posted:
Nov 2, 2009 8:23 AM
in response to: r_romeo
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the ideal curve of the force is 1-COS(x) were "x" is the angle of the weigth
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Posts:
83
Registered:
04/29/08
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Re: DYNAMIC SIMULATION, unknown force, constraint problem
Posted:
Nov 3, 2009 7:45 AM
in response to: r_romeo
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Hi Romeo,
In order to counterbalance the weight, the torque generated by spring force should be euqal to that produced by Gravity. After deep investigation, we can find that the value of Spring Force divided by Gravity is constant.Please see attached picture, Gravity/Spring Force=L1/L2, and length of Line of force (L1, L2) is unchangeable during rotation. I think that's why the unknown force generated in your dataset is constant.
Regards,
Jimmy_Yuan
Autodesk Simualtion QA
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Posts:
10
Registered:
09/23/08
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Re: DYNAMIC SIMULATION, unknown force, constraint problem
Posted:
Nov 4, 2009 11:38 AM
in response to: Jimmy_Yuan|Auto...
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thank for your answer Jimmy,
nevertheless if you see the energy balance equation , the variation in the energy in the spring, with the angle as reference is A(1-COS(x)) where A is a constant. This is a known response and is widely used. I was just doing a exercise in balancing in Inventor and I know there is something in my setup of forces in Simulation that is wrong,
Maybe I need an external force apart of gravity, I don't know
Edited by: r_romeo on Nov 4, 2009 11:41 AM
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Posts:
10
Registered:
09/23/08
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Re: DYNAMIC SIMULATION, unknown force, constraint problem
Posted:
Nov 4, 2009 6:10 PM
in response to: r_romeo
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Sorry I forgot to say that this is a known design and in the center of the design it's suppost to be a spring for counterbalancing the load
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Posts:
1
Registered:
11/06/09
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Re: DYNAMIC SIMULATION, unknown force, constraint problem
Posted:
Nov 6, 2009 4:22 PM
in response to: r_romeo
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Romeo,
I think you are mixing the concepts of force and torque. I simplified the model and listed the static equlilibrium equations in the attached picture (forgive my hand drawing and writing). You can see the reaction force is always equal to the gravity mg. However, the reaction torque is changing with the angle Alpha. In DS, you can add an unknow torque to see what you expected.
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