how to produce sine wave with exactly same values for turning Points (Peaks and Valleys) ?

Hello everybody ,

i want to produce forces as sine waves with different forms. The problem That when i am producing it not all the peaks, even for sine wave with constant amplitude , are not exactly the same Values

Example

FR = 4000*sin(tt);

The peaks must be all the time = 4000 , but the problem that it is not like that some would be 3999.998 other would be 3999.996 and so on . and the same for Valleys .

This is for simple example of sine wave .

And it is making Problem for Rainflow counting because the counter can't simply realise that both are the same Values and both must be 4000 and both must be counted as Cycles!

for simple sine wave it is easy to realise the Problem but with more complicated waves it is hard. as this function

FR(l) = sum(7000*sin(1*pi*([1:2:71])*tt(l))./([1:2:71]));

Is there any way to fix this Problem ?

Thank you for Helping !

Answer 1

[Cris LaPierre]

You need to set your input to sine such that the points of tt land exactly at the peaks and valleys. That means having your spacing such that the points and occur in tt.

tt = 0:pi/2:10*pi;
FR = 4000*sin(tt);

You can adjust the increment. Just make sure is always a multiple of it.

In your second code snippet, you've changed things slightly, and we don't know what tt is.

[khaled elbatawy]

Thank you for your Help !

i have used also sind () it is producing exact values even Reaching exact Zero for tt=180 in opposite of sin(pi)

so it would be

tt = 0:0.1:1440 ;
FR = 4000*sin(tt);

Have you any Knowledge or experience with rainflow counting in Matlab ?

for the other Function

tt = 0:0.001:10
FR(l) = sum(7000*sin(1*pi*([1:2:71])*tt(l))./([1:2:71]));

[Cris LaPierre]

For FR, do you mean to use sind?

Also, what is the value of tt(l)? Is this supposed to be tt(1)?

I'm not familiar with rainflow counting, but one idea I have would be to explicitly insert the pi/2 increments you want into the array.

val = pi*([1:2:71]);
pks = 0:pi/2:10;

num = sort([val*tt(l) pks]);
den = sort([val pks]);
FR(l) = sum(7000*sin(num)./den);

You'd have to adjust the actual values in pks to be what you need.

[khaled elbatawy]

Yes, i mean to use , Sorry i have written it wrong above

FR = 4000*sind(tt);

tt should be the time

tt = 0:0.1:1440 ;

instead of pi here i have used the angle as degree , for example 8*pi*180/pi,

The whole problem of rainflow counting that the signal has not exact the same Values of Peaks or turning Points So that giving wrong counting cycles .

I tried your code but giving me error if using tt that the Matrix dimensions must agree.

And without tt giving me FR = NaN

Exactly that is the solution of the whole problem as you said exactly

[Cris LaPierre]

Waht about your rainflow equation that uses FR(l) and tt(l)? Instead of the letter "l" do you mean to use the number "1"?

tt = 0:0.001:10
FR(l) = sum(7000*sin(1*pi*([1:2:71])*tt(l))./([1:2:71]));
   ^                                    ^

[khaled elbatawy]

No sorry it was not clear i am using it in for loop as follow , it is letter L

tt = 0:0.01:10 ;
for l=1:length(tt)
    
     FR(l) = sum(7000*sin(0.125*([1:2:31])*tt(l))./([1:2:31]));
 
end

[Cris LaPierre]

Ah, ok. Minor detail :)

You don't need the for loop. This returns a result for each value of tt.

tt = 0:0.01:10;
FR = sum(7000*sin(pi*[1:2:71]'*tt)./[1:2:71]');

This takes advantage of implicit expanstion and linear algebra to perform the calculation in a single line of code.

[khaled elbatawy]

Thank you for advice .

It is really helpful Informations.

I am just new with matlab so i am trying still to find the best way

And how should i now to the actual values in turning Points or Peaks ?the same code above ?

[Cris LaPierre]

Yes, see my earlier reply about how get results for the peaks. It's not just the value of tt you have to be worried about. It's tt*pi*[1:2:71].