What can I write in a MATLAB function block in Simulink?

Hi I am working with Simulink. I wrote a function in Matlab that basically does a circular shift, and I wanted to put it into Simulink by using the MATLAB function block. The thing is that if I write

function y= shift_with0(u,s)
y = circshift(u,s);
y(1:s)=0;
end

no problem and everything is working. But, I wanted to allow as inputs also vectors, so I wrote this function

function y= shift_with0(u,s)
if isvector(s)
    pm=sign(s);
    inegatif = sum(pm(:)==-1);
    s = inegatif;
end
y = circshift(u,s);
y(1:s)=0;
end

In Matlab is working, but the Simulink block not. So, I assume that is the "if" part that Simulink does not accept. In general, could someone clarify for me what I can write/not write in a MATLAB function block? Thanks

[Birdman]

So if s is a vector, you do not want to do any shifting, right?

[Maria]

If s is a vector, I will just count the number of negative entries, and then shift the y by this amount

[KL]

isvector should return 1, even if it had only one element. It actually helps you differentiate between a vector and a matrix. try

 isvector([1 2 3])

and then

 isvector(1)

always read the documentation: https://www.mathworks.com/help/matlab/ref/isvector.html

Also, I do not get what you mean by y(1:s)=0;.

[Maria]

You are right about the isvector. I modified it

function y= shift_with0(u,s)
[n,m] = size(s);
if n>1 || m>1
  pm=sign(s);
  inegatif = sum(pm(:)==-1);
  s = inegatif;
end
y = circshift(u,s);
y(1:s)=0;
end

With y(1:s)=0 I set the components before the "s" point to zero. It is because want to shift indeed, not really to circularly shift. I found this solution that suits me. In front of the values that have been shifted, I want zero. In Matlab I get what I actually want. But Simulink complains. I attach the file.

Answer 1

[Birdman]

Run the attached model and let me know the results.

[Maria]

Also mine is working in this way. But, if I put in "constant" a vector like [-1 -1 0 1 1 ], then Simulink does not work... I guess that the Matlab function block is not compiled in the same way as a standard matlab function.. ?

[Birdman]

Is it not working in the way that you want, or does it give an error?

[Maria]

Sorry, I was not checking also on your function. Your function is working! Wait a sec, so that I can understand and thank you properly

[Birdman]

If it helped, can you accept it?

[Maria]

So, your function is perfectly working! Could you be so nice and tell me why Simulink did not like my function? Was it not doing the same?

[Birdman]

You were overwriting initial s vector, Simulink does not like it. Instead, I defined a new one and solved the issue.

[Maria]

Thank you very much!I am learning a lot from you!

[Maria]

Hi, not sure if I should open a new question or not. I would like to know why, if I look at the output of the matlab function block in matlab, I see a table, where my vector is basically horizontally replicated, for each time sample. Instead, I would have expected to get only two vectors as output: one for the time on the left, and my vector on the right. In case I should open a new question for this, just let me know. Best

[Birdman]

You do not need to. How did you save your data to workspace, just tell me that.

[Maria]

I have used the "to workspace" block because the "scope" was not working. My input signal is a current, that has as many samples as the time, so I was expecting to be able to plot it.

[Birdman]

Save output format should be Structure with time and please specify a reasonable sampling time for your signal, something like 0.001.

[Maria]

In the Data Import/Export, I selected now the format you said, but I do not find the output variable "yout" in the workspace. I attach the figures.

I also took away the "to workspace" and used just a "display".

[Maria]

Okay I do see my output now. Sorry, I am not yet used to Simulink. In yout, I see the time and, inside the structure signals, a matrix "values" that is a matrix 501 x 501

[Birdman]

You can reach values inside struct by the notation

.signals.values(:,1)

1 stands for the column number.

[Maria]

Thank you. I think I understood now what I can expect when I work with constants.

[Birdman]

You are welcome, Maria.

Answer 2

[KL]

If s is a vector, I will just count the number of negative entries, and then shift the y by this amount

Shouldn't you be simply writing,

...
if numel(s)>1
    s = numel(s(s<0));
end
y = circshift(u,s);
...

[Maria]

In the comment of the above answer, I modified the function. The function is fine for me in Matlab, I just do not know why is Simulink not fine with it. So I was wondering if there are some constraints about the command I can put in a MATLAB function block (in Simulink).

[KL]

The difference between matlab and simulink is not only with the block based modelling but also its code generating abilities. The matlab-function block gives you the ability to do that and of course it comes with a restriction. This is why you use extrinsic command for some matlab functions inside simulink.

Unlike matlab, when you assign/reassign you have to mind certain things in simulink environment.

You can read more to understand why this happens. Documentation is the best place to learn how things work.

https://www.mathworks.com/help/simulink/slref/matlabfunction.html

https://www.mathworks.com/help/simulink/ug/when-you-can-reassign-variable-properties-for-c-c-code-generation.html

https://www.mathworks.com/help/simulink/ug/best-practices-for-defining-variables-for-c-c-code-generation.html

[Maria]

Thanks!