Doubts about mathematical model of a pendulum

[Liliana]

Hello
I have a problem and i hope you can help me

I am study a pendulum, when i get its mathematical model i find from it the transfer function, an in the denomonaor i get (s^2-1) so, when next i want to find a transfer function for frequence i.e. G(jw) = Re(w) + Im(w) i get am imaginary part is equal to zero.
Methematiciliy my work is good, but my problem is : it is correct to get imaginary part equal to zero.

Plezse help me
thanks

 

[mdanaie]

help me please!

In your model you have not included any losses therefore you have a transfer fuction with two poles at s=+j and -j . If the imaginary part is zero but the phase is zero before w=1 and -180 for w>1 therfore it is no problem

 

[Liliana]

Re: help me please!

When i want to draw faze characteristics: faze(w) = Im(w)/Re(w) but my Im=0 so my faze all time is equal to zero. Is it right or there is a problem.


P.S. from here i get Im=0;

My tranfer function is:

G(s) = 1 / (s^2-1)

putting s = jw

from here G(jw) = 1 / (-w^2 -1)

so: Re = 1 / (-w^2 -1)
Im = 0;