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J
JBlackhole16-Pearl
16-Pearl
May 7, 2019
Solved

Find() works but not solve

  • May 7, 2019
  • 2 replies
  • 3212 views

To all experts,

 

I am breaking down an investigation I am hoping to do to simple blocks. I am looking at creating “dummy” bi-modal PSDs to meet a specific rms value. See attached mcad15 sheet

If I specify the values fn, zeta1, A1, alpha and p is there a way of Mathcad finding the zeta2 value required to get a given rms value? A solve is returning “no solution found” but it works with a Find(). I believe one cannot use Find() in a program

 

Ultimately, I would like to create a number of f dummy PSDs with the same (or near) rms value by changing the parameters. For example, I would like to specify a series of alpha values and get a vector of rms value

 

Thanks

Regards

JXB

Best answer by Werner_E

The symbolic "solve" tries to give you an exact result, so it first tries to integrate symbolically which it obviously is not capable of - thats the reason it finds no solution.

 

Fortunately you are looking just for numeric results and so you definitely should use Mathcads numeric methods like the solve block you had implemented or one of the two flavors of the "root" command:

B1.PNG

You sure can use a solve block or the "root" command in a program. You should turn them into a function of the variables you wish to change and in case of the solve block you may even add the guess value as argument of that function.

The attached worksheet shows how to do that and somehow I got the impression that the function zeta.1 based on the root command works faster than zeta.2 based on the solve block.

B2.PNG

2 replies

W
Werner_E25-Diamond IAnswer
25-Diamond I
May 7, 2019

The symbolic "solve" tries to give you an exact result, so it first tries to integrate symbolically which it obviously is not capable of - thats the reason it finds no solution.

 

Fortunately you are looking just for numeric results and so you definitely should use Mathcads numeric methods like the solve block you had implemented or one of the two flavors of the "root" command:

B1.PNG

You sure can use a solve block or the "root" command in a program. You should turn them into a function of the variables you wish to change and in case of the solve block you may even add the guess value as argument of that function.

The attached worksheet shows how to do that and somehow I got the impression that the function zeta.1 based on the root command works faster than zeta.2 based on the solve block.

B2.PNG

    J
    JBlackhole16-PearlAuthor
    16-Pearl
    May 8, 2019

    Thanks a lot for the solution. Another block sorted.

    L
    LucMeekes23-Emerald IV
    23-Emerald IV
    May 8, 2019

    This cannot be solved symbolically. But... Are you sure your definition of H is correct?

    In the denominators you are adding r1^4 to r1^2. That should throw an error if r1 has units (I suppose it hasn't, because r1 is the ratio of f and fn), but still...

    Needless to say: if that first term under the root would be (1-r1)^2 or (1-r1^2) the integral would easily solve symbolically.

     

    Success!
    Luc

     

     

    J
    JBlackhole16-PearlAuthor
    16-Pearl
    May 8, 2019

    Appreciate I put this together quickly last night but it seems correct to me. The "famous" transmissibility for a 1-DoF model

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