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Root Locus Design Methods for Feedback
MAE 691
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MATLAB Root Locus
• Consider aTransfer Function Model of the form G(s)=num(s)/den(s): 4(s+2)/(s2 +10s +12)
num= 4*[1 6]
den= [1 10 12]
rlocus(num,den)
-30 -25 -20 -15 -10 -5 0 5
-1.5
-1
-0.5
0
0.5
1
1.5
Root Locus
Real Axis (seconds-1)
Imagin
ary
Axis
(seconds-1
)
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Simple sketching rules for root locus plots
• Get characteristic equation
1+kG(s)H(s)=0
• Place open loop poles of G(s)H(s) on plot with ’x’ (n is number of poles)
• Place open loop zeros of G(s)H(s) on plot with ’o’ (m is number of zeros)
• n-m is the number if infinite closed loop zeros as k-> ∞
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Simple sketching rules for root locus plots (cont.)
• On the real axis: Draw lines to the left from odd numbered real value pole/zero to the next real value pole/zero or ∞ (Note: number from the right side)
• Find the asymptotes • Number= n-m, and general Butterworth pattern
• Location of asymptotes center on real axis
mn
zpm
i
i
n
i
i
A
11
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Simple sketching rules for root locus plots (cont.)
• Find break-in or break-out points on real axis by dk/ds=0 { from 1+kG(s)H(s)=0; k=-1/(G(s)H(s)) }
• Find where the root locus crosses the real axis by substituting s=jω into the characteristic equation
• Root locus is symmetric about the real axis
• Roots go from open-loop poles to open-loop zeros as k is increased from 0 to ∞