matlab - octave control package tf -

using octave/matlab control toolbox:

octave.exe:1> pkg load control 

i define same transfer function in 2 different ways:

octave.exe:2> = tf('1/(s + 1)')  transfer function 'a' input 'u1' output ...   y1:  1/(s + 1)  continuous-time model. octave.exe:3> b = 1 / (tf('s') + 1)  transfer function 'b' input 'u1' output ...          1  y1:  -----       s + 1  continuous-time model. 

and evaluate @ s = j:

octave.exe:4> a(1) ans =  0 + 1i octave.exe:5> b(1) ans =  0.50000 - 0.50000i 

why these different!?

i think way define a incorrect. not sure why doesn't error out when run command, it's not how should define transfer function. if consider following:

>> = tf(1,[1 1])  transfer function 'a' input 'u1' output ...          1  y1:  -----       s + 1  continuous-time model. >> a(1) ans =  0.50000 - 0.50000i >> b = 1/(tf('s')+1)  transfer function 'b' input 'u1' output ...          1  y1:  -----       s + 1  continuous-time model. >> b(1) ans =  0.50000 - 0.50000i >> c = tf('1/(s+1)')  transfer function 'c' input 'u1' output ...   y1:  1/(s+1)  continuous-time model. >> c(1) ans =  0 + 1i >> s = tf('s')  transfer function 's' input 'u1' output ...   y1:  s  continuous-time model. >> d = 1/(s+1)  transfer function 'd' input 'u1' output ...          1  y1:  -----       s + 1  continuous-time model. >> d(1) ans =  0.50000 - 0.50000i 

you'll notice how c in example (a in yours) not displayed same other transfer function, it's on 1 line. maybe it's treating input 1/(s+1) string? don't know.

anyway, point 3 other ways of defining transfer function correct , equivalent, , give same , correct result.