RMS of Piecewise Function

Hello Community,

i try to calculate the root mean square of a piecwise function.

I had different approaches, but all them don't work.

1. Approach

syms t
y = piecewise(0<=t<=2,-sin(2*pi/8*t));
rms = rms(y);

Error Message: Error using sym/subsindex (line 953)

Invalid indexing or function definition. Indexing must follow MATLAB indexing. Function arguments must be symbolic variables, and function body must be sym expression.

2. Approach

syms t
y = piecewise(0<=t<=2,-sin(2*pi/8*t));
t = 0:0.001:2;
z = subs(y);
m = double(z);
rms = rms(m);

Error Message: Array indices must be positive integers or logical values.

I don't know any other approach.

I hope, I expressed my problem appropriatly.

Thanks for your help!

Smitty

Answer 1

[Sam Chak]

@Smitty

Can you check if this is the expected RMS value over the interval ?

t = linspace(0, 2, 2001);
y = - sin(2*pi/8*t);
v = rms(y)

v = 0.7071

plot(t, y)

[Smitty]

Yes, that is the expected RMS. My problem is to get it for Functions, that I define with "Piecewise" so that I can combine Rectangle und Sinus Signals.

[Smitty]

It just came to my mind, that i can just implement the RMS by myself.

syms t
y = piecewise(0<=t<=2,sin(2*pi/8*t));
T = 2;
RMS = sqrt(1/T*int(y^2,0,T));

That works fine.