# Investigate the Closed-Loop System Function

With the simplification to the open-loop system function in place, you can dive in and find the closed-loop system function of the CD/DVD case study, with full variable substitution. Here’s the closed-loop system function *H*(*s*) with the simplified open-loop system function applied:

With *K** _{a}* = 50, you have a natural frequency of 15.95 rad/s and damping factor of 0.78.

The feedback connection has definitely changed things. There’s no longer a pole at *s* = 0. The roots given *K** _{a}* = 50 are

In [83]: Ka = 50 In [84]: roots([1, 25, Ka*16/pi]) Out[84]: array([-12.5+9.91957201j, -12.5-9.91957201j])

This is a complex conjugate pole pair in the left-half plane, so *H*(*s*) is stable! Check the exact *G*_{0}(*s*) model. After working through the math by hand to find *H*(*s*), you can use PyLab to numerically root the denominator polynomial:

In [85]: roots([1, 1250+25, 25*1250, Ka*4000*10/(2*pi)]) Out[85]: array([-1250.21 +0.j, -12.396+10.047j, -12.396-10.047j])

The complex conjugate root pair for the exact closed-loop model is close to the simplified model. A third real root is included at –1,250 rad/s, again resulting in a time constant that decays very rapidly relative to the complex conjugate poles.

The frequency response, magnitude, and phase plots also play a role in control system design. To plot the frequency response, use w,H=freqs(b,a,w) from the signal package:

In [145]: w = logspace(0,4,500) In [146]: b,a = ssd.position_CD(50,'fb_approx') In [147]: w,H_simple = signal.freqs(b,a,w) In [148]: b,a = ssd.position_CD(50,'fb_exact') In [149]: w,H_exact = signal.freqs(b,a,w) In [151]: semilogx(w,20*log10(abs(H_simple))) In [152]: semilogx(w,20*log10(abs(H_exact)),'r') In [156]: semilogx(w,angle(H_simple)*180/pi) In [157]: semilogx(w,angle(H_exact)*180/pi,'r')

The third pole at 1,250 rad/s (from the exact model) kicks in at about 80 dB of loop attenuation. This has little impact on the closed-loop performance as long as the loop gain isn’t too large.