One reason so many mobile robots use wheels is that the act of walking, though it seems natural to us, is a lot more complicated than you might think.  It involves a complex interaction of balance and movement.

Mark Raibert of the Massachusetts institute of technology laboratory (Cambridge, USA) has been developing robots that run, walk, and hop in one leg.  One of the advantages of robots with adjustable legs is that they can move across terrain that wheeled robots cannot, such as uneven and pitted surfaces.

There are two basic types of arms used for robots. Cartesian arms are built on a rectangular frame and cannot reach outside of their own frame. They are suited for pick-and-place operations, such as on an assembly line.

Revolute arms have at least one elbow joint, so they can bend in many directions and approach a target from different angles, making them useful for tasks such as spray painting the curved surface of cars.

Everybody has an ideal shape and weight

Whenever you encounter robotic systems, it’s interesting to note their varying physical attributes.  They may have light or touch sensors similar to our own senses.  They may even have torsos, arms, legs, or hands.  Whatever their parts, whether robots look like us or more like a box or like a bug depends on what functions they are designed to fulfill.

The choice of shape, size, material, and even style depends first on function.  A VCR could be round, but it would be more expensive to manufacture and probably would not fit into your home entertainment system very well. A vending machine could be small, but a small machine has to be refilled often. Plastics are used for their lighter weight, metals are used for their grater strength, smoke detectors could be red or green squares or triangles but are styled to be unobtrusive in the home.

Weight is another important factor in designing a robot. An inexperienced designer might make the mistake of building a robot too heavy to move itself or it’s payload, or too heavy to allow any speed or maneuverability.  You want to make a robot stable and strong enough to support all its parts. Robotic engineers have to balance the needs for lighter weight and grater strength, for example when building robots to travel deep into the ocean or off into space.

Some robots need to be able to move around to perform their functions.  For example there are rover robots designed to explore the surface of other planets and factory robots that move freight.  Many robots even those meant to look like humans, use wheels to get around. The wheels themselves vary in size, shape, and material according to the surface over which the robot is meant to travel.  For instance, the wheels you use for sandy surfaces would be different from those used for rocky, muddy or smooth surfaces.