Page 28 - Introduction to Autonomous Mobile Robots

2 Locomotion

2.1 Introduction

A mobile robot needs locomotion mechanisms that enable it to move unbounded through-
out its environment. But there are a large variety of possible ways to move, and so the selec-
tion of a robot’s approach to locomotion is an important aspect of mobile robot design. In
the laboratory, there are research robots that can walk, jump, run, slide, skate, swim, fly,
and, of course, roll. Most of these locomotion mechanisms have been inspired by their bio-
logical counterparts (see figure 2.1).
There is, however, one exception: the actively powered wheel is a human invention that
achieves extremely high efficiency on flat ground. This mechanism is not completely for-
eign to biological systems. Our bipedal walking system can be approximated by a rolling
d
polygon, with sides equal in length to the span of the step (figure 2.2). As the step size
decreases, the polygon approaches a circle or wheel. But nature did not develop a fully
rotating, actively powered joint, which is the technology necessary for wheeled locomo-
tion.
Biological systems succeed in moving through a wide variety of harsh environments.
Therefore it can be desirable to copy their selection of locomotion mechanisms. However,
replicating nature in this regard is extremely difficult for several reasons. To begin with,
mechanical complexity is easily achieved in biological systems through structural replica-
tion. Cell division, in combination with specialization, can readily produce a millipede with
several hundred legs and several tens of thousands of individually sensed cilia. In man-
made structures, each part must be fabricated individually, and so no such economies of
scale exist. Additionally, the cell is a microscopic building block that enables extreme min-
iaturization. With very small size and weight, insects achieve a level of robustness that we
have not been able to match with human fabrication techniques. Finally, the biological
energy storage system and the muscular and hydraulic activation systems used by large ani-
mals and insects achieve torque, response time, and conversion efficiencies that far exceed
similarly scaled man-made systems.