Walking robots are intrinsically
slow machines, and machine speed is well known to depend theoretically
on the number of legs the machine has. Therefore, a hexapod can
achieve higher speed than a quadruped, and a hexapod achieves its
highest speed when using a wave gait with a duty factor of
β = 1/2,
that is, using alternating tripods. Although stability is not optimum
when using alternating tripods, a hexapod configuration has been
chosen just to try to increase the machine’s speed. The walking-robot
development is based on certain subsystems developed for the
SILO-4 walking robot. The SILO4 is a quadruped robot developed for
basic research activities and educational purposes. For this reason,
this new walking robot is named SILO-6, referring to its six legs.
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The main tasks of a walking robots body are to support legs and to
accommodate subsystems. Therefore, the body must be big enough to contain the
required subsystems, such as an onboard computer, electronics, drivers, a DGPS and
batteries. The preliminary volumes of these subsystems define the volume of the body
(see Table 1).
Alternating tripods means that two non-adjacent legs on one side
and the central leg on the opposite side alternate in supporting the robot (see videos).
That means that, for a given foot position, the central leg in its support phase is
carrying about half the robots weight, whilst the two collateral legs in their
support phase are carrying about one-fourth of the robots weight. This is
especially significant in traditional hexapod configurations, where legs are placed at the
same distance from the longitudinal axis of the body. If the robot has similar legs,
then the non-central legs will be over-sized, and to optimise the mechanism the central
legs design should differ from that of the rest of the legs. However, using
just one leg design has many advantages in terms of design cost, replacements, modularity
and so on.
Satisfactory force distribution and system homogenisation can be achieved by
shifting the central leg slightly from
the body’s longitudinal axis so that
the central legs support less weight and the corner legs increase their
contribution to supporting the body.
The solution chosen was to select equal legs and situate the central legs in
a forward position with reference to the bodys longitudinal axis. A distance of
about 156 mm was finally selected, because it produces an adequate body shape and reduces
the required torques by about 15.06%
(see Figures 1 and 2).
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Figure 1. Main structure
of the SILO-6 walking robot
Figure 2. Body structure
of the SILO-6 walking robot
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Figure 3. Drawing of the leg
Figure 4. Leg prototype
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Walking robots need leg configurations that provide just contact points with the ground, so a 3-DOF device suffice to accomplish motion. Legs have to be designed to be lightweight
mechanisms and have to support the robot's weight. Therefore, the load carried by
each leg is very heavy and must be supported with the leg in different
configurations. A mammal configuration is the most efficient leg configuration from
the energy point of view (lower torques are required). However, it is not very
efficient in terms of stability. Insect-like legs seem to be more efficient
stability-wise, but power consumption increases extraordinarily in an insect-like
configuration. The idea is to provide a leg configuration that can accomplish its
job with both stability and energy efficiency (a very important factor for outdoor mobile
robots). Development is therefore underway on a leg that can be used in both the
mammal and the
insect
configuration (see pictures). The starting point is to consider the torques the robot has to endure
in the worst-case scenario, an insect configuration. These torques, for the selected
body configuration, have been computed through simulation. One good way to reduce motor size is to use actuators working in
parallel, that is, actuators placed so that two actuators work at the same time to
accomplish motion in a single joint. Simultaneous motions in two joints are also
allowed. This configuration gives the benefit of using small motors.
Therefore, a differential driving mechanism will be used for joints 2 and 3. Figure 3 shows a preliminary design for the leg, and Figure 4 presents the leg prototype. Table 1 presents the legs main features.
Feet can be designed in two basic configurations, a ball fixed to the ankle
or a flat sole with articulated passive joints. The first design is the simplest and
can work for applications in loose terrain if the radius of the ball is big enough. |