as I said lately about piecewise function , i have problem ?

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ebi
ebi 2013-10-13
评论： ebi 2013-10-14
i want to model part 2 of a paper i attach it as I said before ( i want define a piecewise function like as picture below that t is variable and we have f(t)= piecewise function and i want to plot it and do fourier illustration of this function and use fft function on it ant plot them) i do it with the help of some of mathworks member to stage of formation of piecwise and plot it on matlab
/matlabcentral/answers/uploaded_files/636/untitled-0000.jpg
and after that i couldn't continue the procedure of my program . and had error.
please give me some information ... thanks
L=input(' define L (lenght of wagon -m) : ');
V=input(' define V (speed of train -km/h) : ');
X1=input(' define X1(distance betwean 2 wagon -m) : ');
X2=input(' define X2(distance betwean 2 middle wheels -m): ');
X=input(' define X (distance betwean 2 near wheels -m) : ');
W=input(' define W (total weight of wagon and passenger -ton) : ');
V=V*1000/3600;disp([' *** V : (speed of train)= ',num2str(V),' m/s'])
disp([' *** L : (lenght of wagon) = ',num2str(L),' m'])
disp([' *** X1: (distance betwean 2 wagon) = ',num2str(X1),' m'])
disp([' *** X2: (distance betwean 2 middle wheels) = ',num2str(X2),' m'])
disp([' *** X : (distance betwean 2 near wheels) = ',num2str(X),' m'])
disp([' *** W : (total weight of wagon) = ',num2str(W),' ton'])
F1=W*9.81/8;
disp([' *** F1: (force of 1 wheel) = ',num2str(F1),' kN'])
t1=X/V;t2=(X+X2)/V;t3=(2*X+X2)/V;t4=(2*X+X2+(L-X2-2*X)+X1)/V;
disp([' *** F1 = ',num2str(F1),' kN'])
disp([' * t1 = ',num2str(t1),' s'])
disp([' * t2 = ',num2str(t2),' s'])
disp([' * t3 = ',num2str(t3),' s'])
disp([' * t4 = ',num2str(t4),' s'])
t1 =0.0001*round(10000*t1);
t2 =0.0001*round(10000*t2);
t3 =0.0001*round(10000*t3);
t4 =0.0001*round(10000*t4);
F1 =0.0001*round(10000*F1);
t = 0:.001:t4;
F = zeros(size(t));
F((t>=0&t<=t1)) = F1;
F((t>t1&t<t2)) = 0;
F((t>=t2&t<=t3)) = F1;
F((t>=t3&t<=t4)) = 0;
plot(t,F)
figure
k=fourier(F);
plot(t,F)
figure
g=fft(F);
plot(abs(g),[0,100000])
----------------------------------- run
define L (lenght of wagon -m) : 24.5
define V (speed of train -km/h) : 270
define X1(distance betwean 2 wagon -m) : 0.6
define X2(distance betwean 2 middle wheels -m): 20
define X (distance betwean 2 near wheels -m) : 1.5
define W (total weight of wagon and passenger -ton) : 100
*** V : (speed of train)= 75 m/s
*** L : (lenght of wagon) = 24.5 m
*** X1: (distance betwean 2 wagon) = 0.6 m
*** X2: (distance betwean 2 middle wheels) = 20 m
*** X : (distance betwean 2 near wheels) = 1.5 m
*** W : (total weight of wagon) = 100 ton
*** F1: (force of 1 wheel) = 122.625 kN
*** F1 = 122.625 kN
* t1 = 0.02 s
* t2 = 0.28667 s
* t3 = 0.30667 s
* t4 = 0.33467 s
Undefined function 'fourier' for input arguments of type 'double'.
Error in Untitled (line 47)
k=fourier(F);

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回答（1 个）

Walter Roberson
Walter Roberson 2013-10-14
编辑：Walter Roberson 2013-10-14
Your F is a numeric vector. You can apply a Fast Fourier Transform (fft) or short time fft or the like to a numeric vector, but you cannot apply a full Fourier transform to a numeric vector. A full Fourier transform is something that can only be applied to symbolic expressions.
http://www.mathworks.com/help/symbolic/fourier.html
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Walter Roberson
Walter Roberson 2013-10-14
You would look back to your previous Question where I showed the symbolic form http://www.mathworks.co.uk/matlabcentral/answers/90014-how-we-can-define-piecewise-function-in-matlab#comment_173844
You can fourier() the result of the subs() that I show there. Be sure to provide the two additional parameters that the symbolic fourier routine calls for.

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