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

• 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

• The angular motion of each wheel is determined by the rate of change of its angular position (i.e. θL and θR ).

ly.respective ls,right whee and lef oflocity angular ve are and R

L

RR

LL

rvrv

RRLL ;

),,( ),( yxRL

Forward Kinematics

Backward Kinematics

Differential Drive (3)

• 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.

l

vv

vv

vvlR

vl

Rvl

R

lr

rl

rl

rl

;2

2 ;

2

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 WheelsDrive WheelsCastor WheelCastor Wheel