06-10-2012, 12:01 PM
Seminar Report Robot Leg Mechanisms
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Abstract
This paper describes designs of the leg drive mechanisms, hardware architecture and the
leg control methods for walking machines. The body of knowledge that applies to mobile
wheeled robots is quite well developed. However, autonomous walking vehicles are still
relatively new, and the body of knowledge concerning their development is not as well
defined. The difficulty factor in building a legged robot is also considerably higher than
that for a wheeled robot.
In this report a brief introduction is given in beginning on limitations of wheeled and
tracked wheeled robots in rocky and hilly terrains and stepped and discontinues paths.
The human leg and walking gait, control system, the musculo-skeletal system and the
central nervous system have been discussed.
Brief guidelines for the design of leg mechanisms have been presented through the study
of various joints, links, sensors and degrees of freedoms in legs. Then a study of various
robot leg mechanisms is made to identify their applications and advantages.
At the end this paper describes mechanism and control of leg-wheel hybrid mobile robot.
Legged locomotion has high adaptability for rough terrain, and wheeled locomotion
posses speed and efficiency. A new locomotion mechanism that combines legs and
wheels is proposed.
Introduction
Traditionally, most mobile robots have been equipped with wheels. The wheel is easy to
control and direct. It provides a stable base on which a robot can maneuver and is easy to
build. One of the major drawbacks of the wheel, however, is the limitation it imposes on
the terrain that can be successfully navigated. A wheel requires a relatively flat surface on
which to operate. Rocky or hilly terrain, which might be found in many applications as
forestry, waste clean-up and planetary exploration, imposes high demands on a robot and
precludes the use of wheels. A second approach to this problem would be to use tracked
wheel robots. For many applications this is acceptable, especially in very controlled
environments. However, in other instances the environment cannot be controlled or
predicted and a robot must be able to adapt to its surroundings. Such a surrounding can
be places where robots would have to step over the obstacles such as a surface where
pipes are running and where they have to move on discontinuous terrain like steps.
Research into legged robotics promises to overcome these difficulties.
Human Walk
Human walk is the most efficient bipedal walk known. It uses a dynamic walking gait. At
any given condition, i.e. at any walking speed and step rate, a human chooses the most
energy efficient gait for locomotion. In steady state, walking is symmetric and periodic.
The physical structure of a human body consists of the musculo-skeletal system for the
physical realization of the walking gait and the central nervous system for the
optimization and control of the gait. The skeletal system consists of the bones which are
actuated by the muscles on the skeletal system. The sole and knees are designed to
minimize the ground impact forces by use of soft tissue at the sole and also by bending of
the knees.
Study of Robot Leg Mechanisms
In comparison with the industrial manipulators, the task of building an adaptable,
autonomous walking machine is more difficult. Walking machines have more active
degrees of freedom (DOF) than industrial robots. To enlarge the work-space of the legend,
and thus enhance the machine’s ability to adapt to the terrain, each leg should have
at least 3 DOF, which results in a total of 12 DOF for a quadruped or 18 DOF for a
hexapod. All those joints must be controlled adequately in real time. This also means that
the hardware and software systems must meet more critical requirements than those
formulated for industrial robot controllers. Moreover, fully autonomous vehicles use only
on-board controllers and so those controllers have to be miniaturized to an utmost extent.
Mechanical structure of a walking machine should not only imitate the leg structure of
living creatures (e.g., insects, spiders), but should also take into account the actuating
systems properties (e.g., size, weight and power of the motors) and constraints (e.g., size
of the body and the leg work-space).
The need for a general solution to the problem of robot legs design that can be used either
by two-, four- or six-legged vehicles, is clear. However the ability to meet this need has
been hampered by the lack of adequate joint mechanisms and controls. Joint technology
is a key problem in the development of such vehicles, because hip and ankle joints
require, at a minimum, pitch and yaw motion about a common center with remote
location of actuation sources analogous to our muscles and joints. The lack of simple,
compact, cost-effective and reliable actuator packages has also been a major stumbling
block in current designs. Ineffective joint design leads to unwieldy vehicles that
compensate for the instability of their simple joints by means of additional legs.
Effectiveness of leg joints relating to the walking.
It is found that the walking was not affected even if there were no fingers and that the
roots of the fingers and heel are more important for supporting the body weight. As far as
the ankle joint and walking function are concerned, if the ankle joints were fixed
• there will be a lack of contact feeling with ground surface and the fore-and-aft
• standing still is difficult if eyes were closed and
• when side crossing a sloped surface, the feeling of contact with ground surface
stability will be weak.
Sensors relating to the walking.
Humans have three senses for sensing the equilibrium. One is the sensor to sense
acceleration by ear drum; the second one is the sensor to sense the tipping rate by
semicircular canals and the third one is the sensor to sense the angles of joints movement,
angle acceleration, muscular strength, pressure feeling of foot sole and skin. We also
have the visual sensor which complements and alternates the sense of equilibrium
mentioned above and also manages the walking information. Basing on this information,
it can be concluded that a robot in its system needs a G-sensor.
Leg Mechanisms
If one stops to consider for a moment it becomes immediately evident that the earth is
literally crawling with walking machines. The locomotion of all organisms in nature is
produced by a system of levers. In some cases, such as the caterpillar, the whole animal
can be considered a single lever but, nevertheless, the motion can be reduced to a levered
system. The fact that nature has confined herself to levered machines should be extremely
significant to the designer of off-road equipment.
The successful design of a legged robot depends to a large extent on the leg design
chosen. Since all aspects of walking are ultimately governed by the physical limitations
of the leg, it is important to select a leg that will allow for a maximum range of motion
and that will not impose unnecessary constraints on the walking gait chosen. The first
stage of the leg design process therefore consists of a search for an optimal leg design.