Chapter 12

12.2 Model from Open Loop Frequency Response

12.2.1 Phase Margin vs. Damping ratio

Consider again the closed-loop system in Figure 12‑1. The closed-loop transfer function is that of the standard 2nd order system with the DC gain equal to 1, shown in Equation 12‑1. The closed-loop system is type 1 – one integrator in G(s). Let us now consider the open-loop system frequency response of that system:

G(jω)=ω2njω(jω+2ζωn)G(jω)=ω2njω(jω+2ζωn)

Equation 12‑14

Let us find the system Phase Margin, ΦmΦm, defined by Equation 11‑4. To find the frequency of crossover, the open-loop gain in Equation 12‑15 is set to 1 (0dB), as per the definition of the Phase Margin, ΦmΦm, shown in Figure 12‑8.

It will be shown that the Phase Margin,ΦmΦm, relates to the closed-loop system transient performance (time-domain). This relationship forms the basis of the classical controller design in the frequency domain.

ω=ωcpω=ωcp
|G(jωcp)|=1|G(jωcp)|=1

Equation 12‑15

ω2nωcpω2cp+4ζ2ω2n=1ω2nωcpω2cp+4ζ2ω2n=1
ω4n=ω2cp(ω2cp+4ζ2ω2n)ω4n=ω2cp(ω2cp+4ζ2ω2n)
ω4cp+4ζ2ω2nω2cpω4n=0ω4cp+4ζ2ω2nω2cpω4n=0

Equation 12‑16

Fig. 12-8: Definition of the Phase Margin

The formula for a quadratic solution is applied:

ax4+bx2+c=0ax4+bx2+c=0
Δ=b24ac=Δ=b24ac=
16ζ4ω4n+4ω4n=4ω4n(4ζ4+1)16ζ4ω4n+4ω4n=4ω4n(4ζ4+1)
Δ=2ω2n(4ζ4+1)Δ=2ω2n(4ζ4+1)
x=4ζ2ω2n+2ω2n(4ζ4+1)2=x=4ζ2ω2n+2ω2n(4ζ4+1)2=
2ζ2ω2n+ω2n(4ζ4+1)2ζ2ω2n+ω2n(4ζ4+1)
ω2cp=x=2ζ2ω2n+ω2n(4ζ4+1)ω2cp=x=2ζ2ω2n+ω2n(4ζ4+1)
(ωcpωn)2=2ζ2+(4ζ4+1)(ωcpωn)2=2ζ2+(4ζ4+1)

Equation 12‑17

Phase margin ΦmΦm can now be found:

Φm=1800+GH(ωcp)=1800900tan1(ωcp2ζωn)Φm=1800+GH(ωcp)=1800900tan1(ωcp2ζωn)
=900tan1(12ζ(2ζ2+4ζ4+1))=900tan1(12ζ(2ζ2+4ζ4+1))
tan(900α)=1tanαtan(900α)=1tanα
tan(Φm)=2ζ2ζ2+4ζ4+1tan(Φm)=2ζ2ζ2+4ζ4+1
Φm=tan1(2ζ2ζ2+4ζ4+1)Φm=tan1(2ζ2ζ2+4ζ4+1)
Equation 12‑18

This relationship looks quite complicated, however, when plotted in Figure 12‑9, a very simple approximation becomes obvious:

Φm100.ζζ0.01.ΦmΦm100.ζζ0.01.Φm Equation 12‑19
Fig. 12-9: Phase Margin vs. Damping Ratio

For Phase Margins between 0 and 15 degrees, and between 55 and 60 degrees, this approximation is very accurate. For Phase Margins between 15 and 55 degrees, as shown in Figure 12‑9, the actual value of the damping ratio is below the straight line approximation and Equation 12‑19 can be slightly modified:

Φm100.ζ+50ζ0.01.(Φm50)Φm100.ζ+50ζ0.01.(Φm50) Equation 12‑20

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Introduction to Control Systems Copyright © by Malgorzata Zywno is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.