CHAPTER 5 MOVE IT! LOCOMOTION Shamsul Huda Rehana Nipa What Is - - PowerPoint PPT Presentation

CHAPTER 5

MOVE IT! LOCOMOTION

Shamsul Huda Rehana Nipa

What Is LOCOMOTION?

• Locomotion refers to the way a robot moves

from place to place.

• The term comes from the Latin word ‘locus’

meaning place and the English word ‘motion’ for movement. Basically, movement from place to place.

• Moving is a challenge for everyone as to why it

requires a significant increase in ‘brain power.’

• A creature that move has to avoid collisions,

falling over/down/under and also being run

• ver. They must use their brain in order to stay

away from danger.

Effectors & Actuators

• Different types of effectors and actuators can be

used to move a robot which includes:

• Legs-for walking/running/crawling/jumping, etc.
• Wheels-for rolling
• Arms-for swinging/crawling/climbing
• Wings-for flying
• Flippers-for swimming

Stability

• Most animals use their legs to get around places but for

a robot, it is more complicated compared to wheels because of a thing called stability. It is harder to stay stable on legs than it is on wheels.

• Robots need to be stable so therefore they cannot

wobble, lean or fall over easily, in order to get their job done.

• There are two kinds of stability- static and dynamic.

Static Stability

• A statically stable robot can stand still without

falling over; it can be static as well as stable. This feature is very useful but the robot would need enough legs/wheels in order to provide sufficient static points of support to keep it from falling over. (The more legs it has the more statically stable it is).

• If a robot can walk while staying balanced all the

time, it is called statically stable walking.

• Static stability is safe but inefficient.

Dynamic Stability

• To be dynamically stable, the body must actively

move to remain stable. For ex. a one-legged hopping robot are dynamically stable, the reason is because the can hop to various places & not fall

• ver but if they tried to stop & stand still, they

wouldn't be able to.

• Balancing one-legged robots or objects is called the

inverse pendulum problem. Similar to how a person tries to balance a stick on their finger. Our brain solves this problem whenever we stand, & so must the robots if it's dynamically stable.

Moving and Gaits

• Gait is the particular way a robot moves, including the
• rder in which it lifts and lowers its legs and place its

feet on the ground.

• Desirable robot gaits have the following properties:
• Stability, speed, energy efficiency, robustness, and

simplicity.

• We humans, with our two legs, more complicated

walking, and slower running, are a minority relative to most animals.

• Most animals have six legs and arthropods have six or

more, while majority have four, making it easier for them to move.

• Six legs allow for multiple walking gaits.
• Tripod gait
• Statically stable.
• Two groups of three legs.
• Three stay on the ground, other three lift and move.
• If the sets of three legs are altering, the gait is called the

alternating tripod gait. It produces quite efficient walking.

• Almost all six-legged robots are endowed with the

alternating tripod gait, because it satisfies most of the desirable gait properties that we have talked about.

• Ripple gait
• Alternating tripod gait can be used in form of this gait,

when a robot has six or more legs. It ripples down the length of the body. (ex. Centipedes)

Alternating Tripod Gait

Genghis, the popular six-legged walking robot.

Wheels and Steering

• Due to efficiency and comparative simplicity of

control, wheels are the locomotion effector of choice in robotics.

• Generally, robots with wheels are designed to be

statically stable, however, not necessarily holonomic, meaning they cannot control all of their available degrees of freedom (DOF).

• To recall, Degrees of Freedom
• Translational (3D space - x,y,z)
• Rotational (roll, yaw, pitch)

• Having Multiple wheels means there are multiple

ways in which those wheels can be controlled.

• The

ability to drive wheels separately and independently, through use of separate motors, is called a differential drive.

• Similarly, being able to steer wheels independently

is called differential steering.

A popular and efficient design for wheeled robots involves two differentially steerable driven wheels and a passive caster for balance.

Staying On The Path vs. Getting There

• In robot locomotion, we possibly can be concerned

with getting the robot to a destination or having it to follow a specific route aka trajectory.

• In fact, following a given trajectory is harder than

having to get to a particular place simply using any path.

• Because of some robot’s holonomic constraints, it is

not likely for them to follow some paths.

• For other robots, those paths can be followed only if

it’s allowed to stop, change directions, and continue moving.

• A complex method that involves penetrating

throughout all the possible paths & evaluating them to find which will satisfy the requirements, is called trajectory planning or aka motion planning.

• Optimal trajectory is when we think it’s

essential to find the best (shortest/safest/most efficient, etc.) for our robot.

Wrap It Up !

• Moving around takes brains, or at least some nontrivial processing.
• Stability can be static or dynamic.
• Static stability is safe but inefficient, dynamic stability requires

computation.

• The number of legs is important. Two-legged walking is hard but slow;

walking start to get easier with four legs, & much more so with six or higher.

• Alternating tripod & ripple gaits are popular when six or more legs are

available.

• Wheels are not boring, and drive control is not trivial. There are many

wheel designs & drive designs to choose from.

• Differential drive & steering are the preferred options in mobile robotics.
• Following a specific path or trajectory is hard, as is computing a specific

path/trajectory that has particular properties (shortest, safest, etc.).

• Getting to a destination is not the same as following a specific path.

Let’s Try Moving Efficiently !!!