Undefined function 'cos' for input arguments of type 'tf'.

I am trying to calculate the laplace transform of the equation below (theta1) but it doesn't seem to recognize neither the cos or cosd function. Here is my code:

theta1=((3/8)*pi)+((1/8)*pi*cos(((pi/.3)*s)+(pi/16)));
thata1s=laplace(theta1);

[Adam Danz]

Check out the documentation describing the first input to that function

https://www.mathworks.com/help/symbolic/laplace.html#d118e158594

Spolier alert: the input is expected to be symbolic, not double (where did you get the "TF" from in the title of your question? - I just realized you're working with a transfer function)

Here are two tutorials on this function

https://www.seas.upenn.edu/~ese216/handouts/Chpt13LaplaceTransformsMATLAB.pdf

https://www.egr.msu.edu/~aviyente/LT_matlab.pdf

Answer 1

[madhan ravi]

syms s % forgot to add this part
theta1=((3/8)*pi)+((1/8)*pi*cos(((pi/.3)*s)+(pi/16)));
theta1s=laplace(theta1);

Answer 2

[Walter Roberson]

An s or z generated by tf('s') or tf('z') is not a symbolic variable but laplace() and ilaplace() can only be used with Symbolic expressions.

If you look in the File Exchange searching for tf2sym then as part of two of the contributions there are functions that can convert tf representation to symbolic representation. sym2tf on the same contributions can convert some symbolic expressions to tf notation.