Unit 3 - Mobility

The mechanical structure of a mobile robot consists of one or more rigid bodies equipped with a locomotion system

Wheeled mobile robots

Base (chassis)
_Wheel_s (providing motion with respect to ground)
Eventually trailers, also equipped with wheels, connected to base by means of revolute joints

Legged mobile robots

Fixed Wheel

Can rotate about an axis going through center of wheel and orthogonal to wheel plane
Constant orientation with respect to chassis (rigidly attached)

Steerable Wheel

Caster Wheel

Vertical axis not going through center of wheel (displaced by constant offset)
Automatic swivel rapidly aligning with direction of motion of chassis
Supporting point for static balance without affecting mobility of chassis (commonly used in shopping carts and wheelchairs)

The 3 types of conventional wheels with their respective icons

Two fixed wheels with common axis of rotation, separately controlled
One or more passive caster wheels, whose function is to keep robot statically balanced
Can rotate on the spot, without moving midpoint between wheels, provided that angular velocities of the two wheels are equal and opposite

A differential-drive mobile robot

Three aligned steerable wheels synchronously driven by only two motors through mechanical coupling (chain or transmission belt)
First motor controls rotation of wheels about horizontal axis (vehicle traction)
Second motor controls rotation of wheels about vertical axis (vehicle steering)
Often, third motor used to rotate independently upper part of chassis (turret) with respect to lower part (orienting directional sensor or recovering orientation error)

A synchro-drive mobile robot

Two fixed wheels mounted on rear axle, driven by single motor controlling their traction)
Steerable front wheel, driven by another motor
Alternatively, two passive rear wheels and front wheel providing traction and steering

A tricycle mobile robot

Two fixed wheels mounted on rear axle
Two steerable wheels mounted on front axle
One motor provides (front or rear) traction
Another motor changes orientation of front wheels with respect to chassis
Two front wheels must have different orientation (avoiding slippage)
Internal wheel slightly more steered with respect to external one (Ackermann steering device)

A car-like mobile robot

Three caster wheels usually arranged in symmetric pattern
Their traction velocities are independently driven
Can move instantaneously in any Cartesian direction as well as re-orient itself on the spot

An omnidirectional mobile robot with three independently driven caster wheels

Fixed wheel with passive rollers placed along external rim
Axis of rotation of each roller typically inclined by 45° with respect to plane of wheel
Vehicle equipped with four mecanum wheels mounted in pairs on two parallel axles is also omnidirectional

A Mecanum (or Swedish) wheel

Balance

A three-wheel robot is statically balanced as long as its center of mass falls inside the support triangle defined by the contact points between wheels and ground
Robots with more than three wheels have a support polygon (typically easier to guarantee balance)
When robot moves on uneven terrain, a suspension system is needed to maintain contact between each wheel and ground

Workspace

Workspace of mobile robot potentially unlimited
Local mobility of non-omnidirectional mobile robot always reduced
A tricycle robot cannot move instantaneously in a direction parallel to rear wheel axle, yet can be manoeuvered so as to obtain net displacement in that direction at end of motion
Number of robot’s DOFs (number of admissible instantaneous motions) lower than number of its configuration variables

Manipulator mounted on mobile base, combining dexterity of articulated arm with unlimited mobility of base
Complex design related to robot’s static and dynamic mechanical balance, as well as to actuation of the two systems

A mobile manipulator obtained by mounting an anthropomorphic arm on a differential-drive vehicle