Extract num and den in symbolic expression.
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Bruce Taylor
on 24 Jul 2019
Edited: Walter Roberson
on 26 Jul 2019
The 'tf' function in Control Systems needs numerator and denometer in a vector of decreasing powers of s.
For example a numerator of a*s^2 + b*s + c needs to be num= [a b c];. I have been cutting and pasting the necessary coefficients
but to do this I have to pause my program. Is there a way to extract the appropriate vector(s) programatically?
Thanks,
Bruce Taylor
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Accepted Answer
Star Strider
on 24 Jul 2019
3 Comments
Star Strider
on 24 Jul 2019
I thought you had already isolated the numerator and denominator.
Try this:
syms a b c d e f g s
sys=(a*s^2+b*s+c)/(d*s^3+e*s^2+f*s+g)
sys = subs(sys,{a,b,c,d,e,f,g},{3,6,8,1,2,9,5})
[np,dp] = numden(sys)
n = sym2poly(np)
d = sym2poly(dp)
producing:
sys =
(a*s^2 + b*s + c)/(d*s^3 + e*s^2 + f*s + g)
sys =
(3*s^2 + 6*s + 8)/(s^3 + 2*s^2 + 9*s + 5)
np =
3*s^2 + 6*s + 8
dp =
s^3 + 2*s^2 + 9*s + 5
n =
3 6 8
d =
1 2 9 5
Note that this will only work with numeric coefficients. It will error with symbolic coefficients.
Walter Roberson
on 25 Jul 2019
Star Strider is correct, MATLAB transfer functions, tf() and ss(), cannot support symbolic expressions.
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