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Robotics Technology - Mobility

Industrial robots are rarely mobile. Work is generally brought to the robot. A few industrial robots are mounted on tracks and are mobile within their work station. Service robots are virtually the only kind of robots that travel autonomously. Research on robot mobility is extensive. The goal of the research is usually to have the robot navigate in unstructured environments while encountering unforeseen obstacles. Some projects raise the technical barriers by insisting that the locomotion involve walking, either on two appendages, like humans, or on many, like insects. Most projects, however, use wheels or tractor mechanisms. Many kinds of effectors and actuators can be used to move a robot around. Some categories are:

  • legs (for walking/crawling/climbing/jumping/hopping)

  • wheels (for rolling)

  • arms (for swinging/crawling/climbing)

  • flippers (for swimming)


Wheels are the locomotion effector of choice. Wheeled robots (as well as almost all wheeled mechanical devices, such as cars) are built to be statically stable. It is important to remember that wheels can be constructed with as much variety and innovative flair as legs: wheels can vary in size and shape, can consist of simple tires, or complex tire patterns, or tracks, or wheels within cylinders within other wheels spinning in different directions to provide different types of locomotion properties. So wheels need not be simple, but typically they are, because even simple wheels are quite efficient.  Having wheels does not imply holonomicity. 2 or 4-wheeled robots are not usually holonomic. A popular and efficient design involves 2 differentially-steerable wheels and a passive caster.  Differential steering means that the two (or more) wheels can be steered separately (individually) and thus differently. If one wheel can turn in one direction and the other in the opposite direction, the robot can spin in place. This is very helpful for following arbitrary trajectories. Tracks are often used (e.g., tanks).


While most animals use legs to get around, legged locomotion is a very difficult robotic problem, especially when compared to wheeled locomotion.   First, any robot needs to be stable (i.e., not wobble and fall over easily). There are two kinds of stability: static and dynamic. A statically stable robot can stand still without falling over. This is a useful feature, but a difficult one to achieve: it requires that there be enough legs/wheels on the robot to provide sufficient static points of support.  For example, people are not statically stable. In order to stand up, which appears effortless to us, we are actually using active control of our balance, though nerves and muscles and tendons. This balancing is largely unconscious, but must be learned, so that's why it takes babies a while to get it right, and certain injuries can make it difficult or impossible.  With more legs, static stability becomes quite simple. In order to remain stable, the robot's center of gravity (COG) must fall under its polygon of support. This polygon is basically the projection between all of its support points onto the surface. So in a two-legged robot, the polygon is really a line, and the COG cannot be stably aligned with a point on that line to keep the robot upright. However, a three-legged robot, with its legs in a tripod organization, and its body above, produces a stable polygon of support, and is thus statically stable.  But what happens when a statically stable robot lifts a leg and tries to move. Does its COG stay within the polygon of support? It may or may not, depending on the geometry. For certain robot geometries, it is possible (with various numbers of legs) to always stay statically stable while walking. This is very safe, but it is also very slow and energy inefficient.  A basic assumption of the static gait (statically stable gait) is that the weight of a leg is negligible compared to that of the body, so that the total center of gravity (COG) of the robot is not affected by the leg swing. Based on this assumption, the conventional static gait is designed so as to maintain the COG of the robot inside of the support polygon, which is outlined by each support leg's tip position.  The alternative to static stability is dynamic stability which allows a robot (or animal) to be stable while moving. For example, one-legged hopping robots are dynamically stable: they can hop in place or to various destinations, and not fall over. But they cannot stop and stay standing (this is an inverse pendulum balancing problem).  

A statically stable robot can use dynamically-stable walking patterns, to be fast, or it can use statically stable walking. A simple way to think about this is by how many legs are up in the air during the robot's movement (i.e., gait). 6 legs is the most popular number as they allow for a very stable walking gait, the tripod gait . If the same three legs move at a time, this is called the alternating tripod gait. if the legs vary, it is called the ripple gait. A rectangular 6-legged robot can lift three legs at a time to move forward, and still retain static stability. How does it do that? It uses the so-called alternating tripod gait, a biologically common walking pattern for 6 or more legs. In this gait, one middle leg on one side and two non-adjacent legs on the other side of the body lift and move forward at the same time, while the other 3 legs remain on the ground and keep the robot statically stable. Roaches move this way, and can do so very quickly. Insects with more than 6 legs (e.g., centipedes and millipedes), use the ripple gate. However, when they run really fast, they switch gates to actually become airborne (and thus not statically stable) for brief periods of time. 

Statically stable walking is very energy inefficient. As an alternative, dynamic stability enables a robot to stay up while moving. This requires active control (i.e., the inverse pendulum problem). Dynamic stability can allow for greater speed, but requires harder control.  Balance and stability are very difficult problems in control and robotics, so that is why when you look at most existing robots, they will have wheels or plenty of legs (at least 6). Research robotics, of course, is studying single-legged, two legged, and other dynamically-stable robots, for various scientific and applied reasons.  Wheels are more efficient than legs. They also do appear in nature, in certain bacteria, so the common myth that biology cannot make wheels is not well founded. However, evolution favors lateral symmetry and legs are much easier to evolve, as is abundantly obvious. However, if you look at population sizes, insects are most populous animals, and they all have many more than 2 legs.  

The Spider, a Legged Robot

In solving problems, the Spider is aided by the spring quality of its 1 mm steel wire legs. Hold one of its feet in place relative to the body and the mechanism keeps turning, the obstructed motor consuming less than 40 mA while it bends the leg. Let go and the leg springs back into shape. As I write this, the Spider is scrambling up and over my keyboard. Some of its feet get temporarily stuck between keys, springing loose again as others push down. It has no trouble whatsoever with this obstacle, nor with any of the others on my cluttered desk - even though it is still utterly brainless. 

Mobility Limits of the Spider

As the feet rise to a maximum of 2 cm off the floor, a cassette box is about the tallest vertical obstacle that the Spider is able to step onto. Another limitation is slope. When asked to sustain a climb angle of more than about 20 degrees, the Spider rolls over backwards. And even this fairly modest angle (extremely steep for a car, by the way) requires careful gait control, making sure that both rear legs do not lift at the same time. Improvements are certainly possible. Increasing step size would require a longer body (more distance between the legs) and thus a different gear train. A better choice might be more legs, like 10 or 12 on a longer body, but with the same size gear wheels. That would give better traction and climbing ability. And if a third motor is allowed, one might construct a horizontal hinge in the `backbone'. Make a gear shaft the center of a nice, tight hinge joint. Then the drive train will function as before. Using the third motor and a suitable mechanism, the robot could raise its front part to step onto a tall obstacle, somewhat like a caterpillar. But turning on the spot becomes difficult.

Flying and Underwater Robots

Most robots do not fly or swim.  Recently, researchers have been exploring the possibilities and problems involved with flying and swimming robots.




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