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
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).
Legs
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. Astatically
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
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
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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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Source : www.electronicsteacher.com
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