from symbolic to tf_object

[thefantasticme]

hi
i have a polynomial expression: Q(s)=4*S^2+1, defined as “symbolic” but I need it as “tf-object” (defined as a transfer function) to be used in the matlab command "lsim", anyone knows how to do it?
thanks
gabi

 

[electricpete]

I may be misunderstanding you're question, but here are some basics:

A transfer function is reprented by a numerator polynomial and denominator polynomial. A polynomial is a vector identifying the coefficients from highest order down to 0 order (constant)

For your example:
num = [4 0 1]
den = [1]
lsim(num,den,u,t)
(see help for u,t)

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[thefantasticme]

Actually I do want to use to function lsim, and that’s why I need to transform Q(s) [i.e.: Q(s)=4*s^2+1] from "sym-object" to "tf-object".
Why do I have Q(s) as a "sym-object"? well, i had to made several previous calculus and as I a have 3 variables y need to define s as symbolic

 

[xnuke]

Try sym2poly or coeffs to return the polynomial coefficients, then follow electricpete's help above.

xnuke
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