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Derivation of second order system transfer function 
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#1
Jan1711, 11:02 AM

P: 39

Hi,
I am trying to derive the general transfer function for a second order dynamic system, shown below: [tex]\frac{Y(s)}{X(s)}=\frac{K\omega_n^2}{s^2+2\zeta\omega_ns+\omega_n^2}[/tex] In order to do this I am considering a massspringdamper system, with an input force of f(t) that satisfies the following secondorder differential equation: [tex]m\frac{d^2y}{\dt^2}+c\frac{dy}{dt}+ky=f(t)[/tex] Using the following two relationships: [tex]c=2\zeta\omega_nm[/tex] [tex]\frac{k}{m}=\omega_n^2[/tex] I get this: [tex]\frac{d^2y}{dt^2}+2\zeta\omega_n\frac{dy}{dt}+\omega_n^2y=\frac{f(t)}{m }[/tex] [tex]\mathcal{L}\left\{\frac{d^2y}{dt^2}\right\}+2\zeta\omega_n\mathcal{L}\l eft\{\frac{dy}{dt}\right\}+\omega_n\mathcal{L}\left\{y\right\}=\frac{1} {m}\mathcal{L}\left\{f(t)\right\}[/tex] [tex]Y(s)\left[s^2+2\zeta\omega_ns+\omega_n^2\right]=\frac{F(s)}{m}[/tex] [tex]\frac{Y(s)}{F(s)}=\frac{1}{m(s^2+2\zeta\omega_ns+\omega_n^2)}[/tex] Wheras my lecturer has the following in his notes: [tex]\frac{d^2y}{dt^2}+2\zeta\omega_n\frac{dy}{dt}+\omega_n^2y=K\omega_n^2x( t)[/tex] [tex]\mathcal{L}\left\{\frac{d^2y}{dt^2}\right\}+2\zeta\omega_n\mathcal{L}\l eft\{\frac{dy}{dt}\right\}+\omega_n\mathcal{L}\left\{y\right\}=K\omega_ n^2\mathcal{L}\{x(t)\}[/tex] [tex]Y(s)\left[s^2+2\zeta\omega_ns+\omega_n^2\right]=K\omega_n^2X(s)[/tex] [tex]\frac{Y(s)}{X(s)}=\frac{K\omega_n^2}{s^2+2\zeta\omega_ns+\omega_n^2}[/tex] This obvisously gives the correct transfer function. So, from the two approaches, I have come to the conclusion that: [tex]\frac{f(t)}{m}=K\omega_n^2x(t)[/tex] But I do not understand the physical reasoning behind this. Can anyone offer any help with this? Thanks, Ryan 


#2
Jan1711, 06:13 PM

P: 273

That is standard notation. The "trick" is to multiply the right hand side by [tex]\frac{k}{k}[/tex]. As for physical intuition. Perform a unit analysis. You should be able to draw a clear conclusion from that.



#3
Jan1711, 06:48 PM

P: 39

Ah yes, I completely missed that. Although substituting [tex]\frac{k}{m}=\omega_n^2[/tex] leaves the gain of the system as [tex]\frac{1}{k}[/tex] which is then not dimensionless. I thought this transfer function was supposed to be dimensionless?



#4
Jan1711, 08:30 PM

P: 273

Derivation of second order system transfer function
No transfer functions are hardly dimensionless. Transfer functions are the ratio of system [tex]\frac{output}{input}[/tex]. Thus you can see that the transfer function can hold any units as long as it contains the outputinput relationship you are looking for.



#5
Jan1811, 02:33 AM

P: 39

Ok, thanks for your help viscousflow. It is very much appreciated.
Ryan 


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