I. INTRODUCTION
IN past few years, fractional order calculus has been
substantially used to model the dynamics of many real-life
systems. For example, application of fractional order
derivatives can be found in viscoelastic materials [1],
electrical networks [2]etc. The use of fractional order
derivative helps us to determine the system more accurately
than its integer order realisation. Earlier due to lack of
available methods, a fractional order system was used to be
approximated as an integer order model. At the present time,
there are many available numerical techniques which are used
to approximate the fractional order derivatives and integrals
[3] .For example, optimal approximation of the fundamental
linear fractional order transfer function using a distribution of
the relaxation time function [4]. Earlier there was a simple but
efficient method was using ousterloop method of rational
approximation of fractional order transfer function [5]. One of
the advantages of using fractional-order controllers for a
linear control system is that with more accurate dynamic
modeling of system, a more robust control design is
possible.[6] The basic characteristics of the relative stability
and the transient performances of a closed loop control system
are directly related to the location of the closed loop roots of
the characteristics equation in s plane. It is frequently
necessary to adjust one or more system parameters in order to
obtain suitable root locations. So it becomes necessary to
check the position of closed poles if the system parameters are
varied. In case of fractional order system it becomes more
important than integer order system to use root locus
techniques to determine the stability and responses of the
closed loop system. Because unlike integer order system we
cannot apply Routh Howritz stability criterion in the
fractional order systems. Merrikh-Bayat Farshad et al
presented an extension of root locus method in fractional order
system[7]. Still that work was not complete and does not
include fractional order systems having complex zero or/and
complex pole. This is the goal of this paper. In this paper we
will analytically show how to draw the root locus of any
fractional order closed loop system of commensurate type.
Moreover detailed analysis of stability and transient response
by observing the root locus of the system is also given in this
paper . We should note that not only the features of the root
locus(asymptotes, break away and break in points) changes in
FOS, but also the condition of stability in FOS is different
from integer order systems