Locomotion means how a robot gets from one place to another. A robot can roll on wheels, walk on legs or fly. Each way is good at some jobs and bad at others. This page has a small simulator for every idea: push a robot over ice, balance a walking robot, drive with two wheels, roll through sand, fly over a wall, and pick the best way to move for a job. Everything here is a simulation, so no robot is needed.
To move, a robot has to push against something. A wheel pushes on the ground, a leg pushes on the ground, and a drone propeller pushes on the air. The surroundings push back, and that push moves the robot. This is what “exchanging forces with the environment” means. So the surroundings decide how well a robot can move. First try pushing a robot over different ground. Notice the unit N (newton), which measures how hard something pushes. Then see why no single way of moving wins every time.
Pick a job, or set the sliders by hand. The bars show how well wheels, legs and flying fit that job. Notice that the winner changes whenever the job changes.
Living things have had millions of years to learn how to move well. Humans walk, insects crawl, fish swim and birds fly. Engineers copy these ideas to build robots. This is called biologically inspired robotics. Pick an animal to see the robot it inspired.
Legged robots copy humans (two legs) and animals (four legs or more). Legs can step over things, so legged robots can handle rough ground, and they can also walk, run, jump and keep their balance. The price is complexity. This section builds the idea in five small steps, starting with one single leg.
A leg is a chain of stiff bars (called links) joined by joints that bend. Each joint has its own motor. This is a real robot leg. Click the numbers to learn each part.
The bars cannot change length, so the position of the foot depends only on the three joint angles. Choosing those three angles is how a robot puts its foot where it wants it.
Balance is simple to understand. Every robot has a balance point: the middle of all its weight. It stays up only while that point is straight above ground that a foot is holding. Think of a ruler balanced on a finger: it stays up only while its middle is over the finger. Test it here.
Seen from above, the feet that touch the ground make a shape, and the dashed outline is the safe area. The robot is safe while the dot (the balance point) is inside it. To walk, a robot has to lift feet, which shrinks the safe area, so it must keep moving the dot. Try the three robots and the three modes.
Every leg has joints. Every joint needs a motor and a sensor, and a computer has to coordinate all of them many times a second. Change the number of legs and joints and compare with a simple wheeled robot.
Wheels do not come from nature. People invented them. Trains, cars, bicycles, shopping carts and most wheeled mobile robots use them. A wheeled robot is simple, efficient and fast, and needs only a few joints. Here is a robot with two driven wheels. Change the speed of each wheel and watch how the robot moves. Only two numbers are needed to steer it.
Wheels shine in structured places: rails, roads, workshops, houses and factory floors. These surfaces are flat, hard and smooth, so a wheel just rolls. Pick a surface and see how much push is needed to keep a wheeled robot rolling. Then try driving a legged robot on the same surface.
Move a wheeled robot from a road into sand and the story changes. A bicycle is easy to push on a road and much harder to push through sand. There, walking can work better than pedalling. Click through the grounds from hard to soft and compare the bars. Then test a wheel against a step.
A wheel meets a step. Make the step taller, or the wheel bigger, or the ground grippier. A leg simply lifts a foot and steps up.
Drones (UAVs) fly. Wheeled and many legged robots stay on a flat 2D surface. A drone moves through 3D space, so it has a bigger workspace and more degrees of freedom. Click anywhere in the scene to place a target and see which robots can get there.
No locomotion system is better than all others. The right one depends on application + environment + requirements. Here is the basic idea: a flat, structured place calls for wheels. Rough, uneven ground calls for legs. Movement in 3D calls for flying. Try it on real jobs.