Trying to replicate matlab cumtrapz() function

Forum|Forum|5 years ago
August 11, 2020
2 replies
3916 views

Hello all,

I am trying to recreate a matlab program in mathcad and the only part I'm struggling with is finding a simple replacement for the matlab cumtrapz() function. It is explained in detail here and here.

So what I have are 2 vectors containing x and y plot points for a shear forces graph. I'd like to generate the moment plot points which involves getting the cumulative integral from one plot point to the next. I'm looking for the simplest way to do this.

A method I've found that appears to work but is a little overly complex is the following:

a := cspline(x, y)

b(x) := interp (a, x, y, c)

c := 0,1..150

f(x) := ∫-b(x) dx (integrate from 0 to x)

and then I can plot f(c) against c

Any help in simplifying is greatly appreciated. Thanks!

* I should note that I am using Prime 4.0 but don't currently have access, so screenshots will work better than files at the moment

F

Fred_Kohlhepp

23-Emerald I

Looks about right! You could combine lines a bit:

Note that your answers may shift slightly depending on your choice of spline function. And you can use units (shear force and position to get moments.)

V

ValeryOchkov

24-Ruby IV

Mathcad + Matlab = Mathlab

W

Werner_E

25-Diamond I

As you are looking for a replacement for Matlabs "cumtrapz()" function - wouldn't simple linear interpolation do the job as well?

https://support.ptc.com/help/mathcad/r6.0/en/index.html#page/PTC_Mathcad_Help%2Flinear_interpolation.html%23

It looks like you are not looking for the vector result of cumtrapz() but rather for a continuous function. Using linear interpolation you might do it like that:

Remark: You shouldn't use the same variable as integration limit AND integration variable, therefore the xi - you may replace it by c or anything you like.

J

jwalk

Author

1-Visitor

Thanks, I think linterp is simpler and achieves the same thing in this case.

I don't know if this is due to the difference between how matlabs cumtrapz() and mathcads linterp() work, but I'm having an issue with my plot matching the output from the matlab program.