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ptc-47483821-Visitor
1-Visitor
September 24, 2012
Solved

Using solve block to solve 4 bar linkage FBD / Geometry

  • September 24, 2012
  • 2 replies
  • 7456 views

I'm trying to solve a system of equations for a 4 bar linkage for all the geometry and the forces. For some reason the comparitor functions are not actually constraining the system as they should. For example all angles inside a triangle should add to 180deg (pi). ProE does this. MathCad solve block allows this is float slightly off (therefore giving incorrect geometry).

I modeled it in ProE and locked down the geometry I wanted fixed. The main input is 1 of the angles and everything else should be solveable from that.

I've attached 2 MathCad files. 1 for the geometry (using law of cosines and basic trig). Another for the FBD using primarily law of cosines. And a 3rd trying to solve the geometry using only angles and basic trig.

I also attached the FBD sketched out...

Anyone have any advice (specifically on how to "lock down" constraints, if possible)?

Thanks,

Pete

Best answer by AlanStevens

Pete Johnson wrote:

I decided to put in the entire FBD and see if it could solve it (20 unknowns).

It worked and gives result similar to what we've measured in an actual machine.

Very good, but note that you have used 24 equations to solve for 20 unknowns. You can safely remove, say, the last two equations for theta12 and theta14, and the last two with theta 23 on the LHS (all in the 4 bar linkage section) and still get the same result.

Alan

2 replies

A
AlanStevens19-Tanzanite
19-Tanzanite
September 24, 2012

Anyone have any advice (specifically on how to "lock down" constraints, if possible)?

Thanks,

Pete

Assign the angles to the solution vector! See attached.

Alan

P
ptc-47483821-VisitorAuthor
1-Visitor
September 25, 2012

Thanks Alan.

I'm still struggling with CTL_4_bar_FBD_solve6.mcdx

c+f should always equal 1019.6 but the solver allows it to change from it's constrained number.

likewise with a+d which I want to "lock down" at 800.24.

Maybe I should stop bothering to find the lengths of the triangles using law of cosines and just use the method I have in CTL_4_bar_dims_angles2.mcdx which you helped me with?

Thanks again!

Pete

A
AlanStevens19-Tanzanite
19-Tanzanite
September 25, 2012

Pete Johnson wrote:

Thanks Alan.

I'm still struggling with CTL_4_bar_FBD_solve6.mcdx

c+f should always equal 1019.6 but the solver allows it to change from it's constrained number.

likewise with a+d which I want to "lock down" at 800.24.

Pete

I don't know quite what's happening, but when I recalculate your sheet I get exactly the values you expect! See attached (All I've done in the attached is to use 'separate regions' to see things more clearly, and then recalculated!

Alan

V
24-Ruby IV
ValeryOchkov24-Ruby IV
24-Ruby IV
September 26, 2012

I hope this Group about bar linkage

http://communities.ptc.com/groups/kinematic-models-in-mathcad

wil be interesting and usefuul to you.

P
ptc-47483821-VisitorAuthor
1-Visitor
October 2, 2012

Valery (or anyone else),

I would like to solve the FBD system of equations iteratively, changing some of the input constants (Fcg) over a range and plot the results.

I tried creating a For loop and mathcad doesn't allow a solve block inside a program.

I also tried simply making the input constant a range variable, i.e. Fcg:=10000..15000

Can someone point me in the right direction as to how to do this?

Thanks, Pete

V
24-Ruby IV
ValeryOchkov24-Ruby IV
24-Ruby IV
October 2, 2012

Pete Johnson wrote:

I would like to solve the FBD system of equations iteratively, changing some of the input constants (Fcg) over a range and plot the results.

Solution(Fcg):=Find(...)

and you can use this function in your program with different values of Fcg

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