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 over. 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 over 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 order 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 t rajectory 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 !!!
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