Root Locus Plot

Control System:
Root Locus Plot
By
Dr.K.Hussain
Associate Professor & Head
Dept. of EE, SITCOE

the closed loop Consider the feedback control system shown in Figure,
transfer function is given by:
transfer
k G(s)
R(s) 1 k G(s)H (s)
T(s) 
C(s)

R(s) C(s)G(s)
H (s)
k
 Where k is a constant gain parameter. The poles of the
function are the roots of the characteristicequation given by:
1  k G(s) H (s)  0
Root Locus
The root locus technique was introduced by W.R.Evans in1948 for the analysis
of controlsystem.
It is powerful tool for adjustingthe location of close loop poles to achieve the
desired performanceby varying one or more systemparameters.

Definition of Root Locus
 The root locus is the locus of the characteristic equation of the closed-
loop system as a specific parameter (usually,gain K) is varied from zero
to infinity. For 0 ≤ 𝑘 < ∞ , if 1 kG(s)H (s)  0
and for positive k, this means that a point s which is a point on locus
must satisfy the magnitudeand angle and conditions:
The magnitude condition:
thenG ( s ) H ( s )  -
1
k
| G ( s ) H ( s ) |
1
q  0,1,2,3,....
 The angle condition:
G(s)H (s)  180o
(2q+1)
The angle condition is the point at which the angle of the open loop transfer
function is an odd multiple of 1800.
k

Root Locus Rules
 Rule 1: Root locus is symmetric about the real axis,
which reflects the fact that closed-loop poles appear
in complex conjugate pairs.
 Rule 2: Root locus starts at open-loop poles (when
K= 0) and ends at open loop zeros (when K= ).
If the number of open loop poles is greater than the
number of open-loop zeros, somebranches starting
from finite open-loop poles will terminate at zeros at
infinity.
Rule 3: The number of branches of the
root locus is equal to the number of
closed-loop poles (or roots of the characteristic
equation).

Root Locus Rules
 Rule 4: Along the real axis, the root locus includes all
segments that are to the left of an odd number of
finite real open-loop poles and zeros.
 Rule 5: If number of poles (n) exceeds the number of
zeros (m), then as K, (n - m) branches will
become asymptotic to straight lines. These straight
lines intersect the real axis with angles A at A .
A

Sum of open loop poles - Sum of open loop zeros
No.of open loop poles - No.of open loop zerosn -m
  pi -zi

q 0, 1, 2, 3, ....where
n -m
 180 (2q1)
A

Root Locus Rules
 Rule 6: “Breakaway and/or break-in points” where
the locus between two poles on the real axis leaves
the real axis is called the breakaway point and the
point where the locus between two zeros on the real
axis returns to the real axis is called the break-in
point. The loci leave or return to the real axis at the
maximum gain k of the following equation:
 Rule 7: The departure angle for a pole p ( the arrival
angle for a zero z) can be calculated by slightly
modifying the following equation:
dK/ds=0

Root Locus Rules
The angle of departure p of a locus from a complex pole
is given by:
p = 180 – [sum of the other GH pole angles to the pole
under consideration] + [sum of the GH zero angles to
the pole]
The angle of arrival z of a locus to a complex zero is
given by:
z = 180 + [sum of the other GH pole angles to the zero
under consideration] –
[sum of the GH zero angles to the zero]
Rule 8: If the root locus passes through the imaginary axis
(the stability boundary), the crossing point j and the
corresponding gain K can be found using Routh-Hurwitz
criterion.

Steps to Sketch Root Locus (1/2)
into the
Mark the
open-loop poles and zeros on the complex plane. Use ‘x’ to represent
open-loop poles and ‘o’ to represent the open-loop zeros.
Step#3: Determine the real axis segments that are on the root locus
by applying Rule 4.
Step#4:Determine the number of asymptotes and the corresponding
intersection and angles by applying Rules 2 and 5.
 Step#1: Transform the closed-loop characteristic equation
standard form for sketching root locus:
1  k G(s) H (s)  0
 Step#2: Find the open-loop zeros and the open-loop poles.

Steps to Sketch Root Locus (2/2)
 Step#5: (If necessary) Determine the break-away and break-in points using
Rule 6.
 Step#6: (If necessary) Determine the departure and arrivalangles using
Rule 7.
 Step#7: (If necessary) Determine the imaginary axis crossings using Rule 8.
 Step#8: Use the information from Steps 1-7, sketch the root locus.

Step 7: Root locus is symmetrical about real axis.

Root Locus Plot

More Related Content

What's hot (20)

Similar to Root Locus Plot (20)

More from Hussain K (20)

Recently uploaded (20)

Root Locus Plot