Mechatronics 1
Week 11
Learning Outcomes
• By the end of week 11 session, students
will understand some sorts of mobile robot
and locomotion of wheeled robots.
Course Outline
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•
•
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Legged robot.
Wheeled robot.
Holonomic & Nonholonomic robots.
Locomotion.
Mobile Robots
• Legged Robot, it uses legs to travel from 1
location to another.
• Wheeled Robot, it uses wheels to travel
from 1 location to another.
Wheeled Robot
• Holonomic : When the wheels
rotate in the forward direction
and then backward to its
previous angular position, the
robot will arrive in the same
location.
• Non Holonomic : When the
wheels rotate in the forward
direction and then backward
to its previous angular
position, the robot will not
necessarily arrive in the same
location.
Holonomic System (1)
• Roll motion can be achieved by rotating a wheel
about its axis.
• If a lateral slip (a slip in the wheel axis direction)
exists, the actual motion is the resultant
produced by the roll motion and the lateral slip
direction.
Holonomic System (2)
• For a robot with several wheels, a motion can only
possible if all wheel axes intersect in a single point, i.e.
Instantaneous Centre of Curvature (ICC).
• Each wheel’s velocity must be consistent with the
requirement to maintain a rigid rotation about the ICC.
Differential Drive (1)
• A Differential drive system has 2 wheels rotating about a
common axis, in which each wheel has its own motor to
drive the wheel independently.
• A pose of robot is determined by the position of the robot
(x,y) and its orientation (Φ).
• The independent drive allows a robot to move along a
specified trajectory.
• A robot with differential drive system needs a supporting
wheel(s), e.g. a castor wheel.
Differential Drive (2)
• The linear velocity of each wheel is given
by
vL   L r
vR   R r
L and R are angular ve locity of lef and right whee ls, respective ly.
• The angular motion of each wheel is
determined by the rate of change of its
angular position (i.e. θL and θR ).
L  L ; R  R
( L , R )
Forward Kinematics
Backward Kinematics
( x, y, )
Differential Drive (3)
l
l


  R    vl ;   R    vr
2
2


vr  vl
l vl  vr 
R
; 
2 vl  vr 
l
• Notes :
– vl, vr, ω, and R are functions of time.
– Special cases of vl = vr , vl = -vr .
– If l = 0  not a differential drive system.
Differential Drive (4)
Robot
Forward Kinematics :
The angular rate difference
between both wheels determines
position & orientation of robot.
Inverse Kinematics :
The position and orientation of a
robot determines the angular rate
difference between both wheels.
Non Holonomic Robot
Drive Wheels
Castor Wheel