Farhan A. Salem International Journal of Current Engineering and Technology, Vol.3, No.2 (June 2013)
255
Where: T the disturbance torque, all torques including
coulomb friction, substituting and solving gives equation
(6) written in the 7
th
page of this paper:
2.3 Controller selection and design.
Different resources introduce different methodologies and
approaches for mobile robot modeling and controller
design, for instant; in (Bashir M. Y. Nouri ,2005),
introduced and tested Modeling and control of mobile
robot using deadbeat response, by (J. R. Asensio et al,
2002) different control strategies are used and tested for
Modeling and controller design of basic used DC motor
speed control. Since we are most interested in dynamics,
modeling and simulation, we will apply only PID
controller, this control block can be replaced with any
other controller type. PID controllers are ones of most
used to achieve the desired time-domain behavior of many
different types of dynamic plants. The sign of the
controller’s output, will determine the direction in which
the motor will turn. The PID gains (K
P
, K
I,
K
D
) are to be
tuned experimentally to obtain the desired overall desired
response. The PID controller transfer function is given by:
2
2
/
I D P I P I
PID P D D
DD
K K s K s K K K
G K K K s s s
s s K K
2.4 Differential drive Kinematics and dynamics modeling
To characterize the current localization of the mobile robot
in its operational space of evolution a 2D plane (x,y), we
must define first its position and its orientation. Assuming
the angular orientation (direction) of a wheel is defined by
angle θ, between the instant linear velocity of the mobile
robot v and the local vertical axis as shown in Figure
2(a). The linear instant velocity of the mobile robot v , is a
result of the linear velocities of the left driven wheel v
L
and respectively the right driven wheel v
R
. These two
drive velocities v
L
and v
R
are permanently two parallel
vectors and, in the same time, they are permanently
perpendicular on the common mechanical axis of these
two driven wheels.
When a wheel movement is restricted to a 2D plane
(x,y), and the wheel is free to rotate about its axis (x axis),
the robot exhibits preferential rolling motion in one
direction (y axis) and a certain amount of lateral slip (
Figure 2(a)(b)) , the wheel movement ( speed) is the
product of wheel's radius and angular speed and is directly
proportional by the angular velocity of the wheel, and
given by:
Where: φ: wheel angular position. Referring to Figure
2(c) ,while the wheel is following a path and having no
slippery conditions, the velocity of the wheel at a given
time, has two velocity components with respect to
coordinate axes X and Y.
sin cos
0 cos sin
xy
xy
r
Fig.2(a) The differential drive motion
(b) Wheel (c)
(d)
Fig. 2(a)(b)(c) Wheel movement kinematics
Fig. 3 circumference movement of mobile robot (Jaroslav
Hanzel, 2011)
Assuming the mobile robot follows a circular trajectory
shown in Figure 3, and Δs , Δθ and R are the arc distance
traveled by the wheel, and its respective orientation with