Importance of locomotion in mobile robotics :
Locomotion is an important factor for any mobile robot to move around in its environment, There are multiple ways to achieve motion. so choosing the best locomotion method based on the robot/application is an important aspect of mobile robot design. we can choose our robot to walk, jump, run, fly, swim, skate, etc based on our application.
Locomotion mechanisms present in biological systems :
Walking: will cause a loss of kinetic energy.
Running: Will cause loss of kinetic energy.
Sliding: Frictional forces
Crawl: Frictional forces
Because of such limitations, mobile robots generally locomote using the wheeled mechanism or using small legs.
Challenges :
Normally the legged locomotion will be having a complex mechanical structure as it requires more degree of freedom.
But in the case of the wheel, the mechanical structure will be simple but they are not efficient for all kinds of terrain.
Wheeled locomotion will be affected by rolling friction. but in the case of legged locomotion as it consists only of point contacts friction won't be having much effect on its operation as compared to wheeled locomotion.
Efficiency :
The efficiency of wheeled locomotion is directly dependent on the environment/terrain qualities. and in legged locomotion, the efficiency depends more on the leg mass and body mass
Factors affecting locomotion :
1. Type of environment
like structure and medium (eg. Water, air, soft/hard ground)
2. Characteristics of contact :
Friction, angle of contact, contact point, etc.
3. Stability :
number and geometry of contact points
Center of gravity
static and dynamic stability
the inclination of terrain.
Legged robots:
Legged robots are more suitable for rough terrain environments as they can traverse obstacles such as gaps steps etc .as such situations are hard for a wheeled robot.
In legged robots, one of the main constrain is the number of legs to get a perfect balance.
for a robot with two legs, its application is very huge but it comes with a big price as to get constant control and balancing the robot with two legs is a complex task.
same time in a robot with 3 legs it will be easier to balance as its center of gravity is distributed within 3 ground contacts.
that is in such a system it doesn't need any movement to maintain its balance.
Legged robots and Degree of freedom(DOF):
Definition for DOF :
The term is widely used to define the motion capabilities of robots.
The number of degrees of freedom is equal to the total number of independent displacements or aspects of motion. A machine may operate in two or
three dimensions but have more than three degrees of freedom.
Leg coordination in legged robots:
Leg coordination is an important concept in multi-legged robots.
as all legs must be in coordination to achieve flawless movements.
The number of possible gaits depends on the number of legs.
Gait: gaits is the sequence of lift and release events for an individual leg
in the case of a mobile robot,
with number of legs=k
and the total number of distinct event sequence=N
N=(2k-1)!
for a robot with 2 legs, the number of distinct events will be
N=(2k-1)!
=3!
=3*2*!
=6
The 6 events will be :
1. Both legs down --> right leg down and left leg up--> both legs down
2.Both legs down --> right leg up left leg down --> both legs down .
3.Both legs down -->both legs up --> both legs down
4.right leg down &left leg up -->both legs up --> right leg down&left leg up
5.right leg up/left leg down --> both leg up --> right leg up &left leg down
6.right leg down & left leg up -->right leg up &left leg down -->right leg down and left leg up
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