Hexapod (robotics)
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A hexapod robot is a mechanical vehicle whose locomotion is based on three pair of legs. The term commonly refers to robots biologically inspired by Hexapoda locomotion. Hexapod robots are considered to be more stable than bipedal robots because in most cases hexapods are statically stable. Therefore, they do not have to depend on real-time controllers for standing or walking. Nonetheless, it has been demonstrated that at high walking speeds, insects do depend on dynamic factors.
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[edit] Biologically Inspired
Insects are chosen as models because their nervous system is simpler than other animal species. Also, complex behaviours can be attributed to just a few neurons and the pathway between sensory input and motor output is relatively shorter. Insect's walking behaviour and neural architecture are used to improve robot locomotion. Alternatively, biologist use hexapod robots for testing different hypotheses.
Biologically inspired hexapod robots largely depend on the insect specie used as a model. The cockroach and the stick insect are the two most commonly used insect species; both have been ethologically and neurophysiologically extensively studied. At present no complete nervous system is known, therefore, models usually combine different insect models, including those of other insects.
Hexapod robots have some clear limitations compared to their biological counterparts. Their morphology is subject to mechanical constrains, like the lack of effective artificial tarsi and unrealistic insect-like actuators. Furthermore, dynamics between insects and mechanical robots differ significantly because of their mass and size. Nonetheless, hexapod robots have demonstrated the ability to complete tasks that wheeled vehicles have failed.
[edit] Leg Coordination
Leg coordination refers to the mechanism responsible for controlling leg step transitions (stance and swing); assuring that the body will not tumble. Most approaches try to replicate known insect gaits, e.g. tripod or tetrapod gaits. However, other approaches have been used to find stable gaits; for instance, by running genetic algorithms or optimizing walking energy cost function.
Insect gaits are usually obtained by two approaches: the centralized and the decentralized control architectures. Centralized controllers directly specify transitions of all legs, whereas in decentralized architectures, six nodes (legs) are connected in a parallel network; gaits arise by the interaction between neighbouring legs.
[edit] Single Leg Controller
There is no boundary to the complexity of leg morphology. However, legs of those based on insect models usually range from two to six degrees of freedom. Leg segments are typically named after their biologically counterpart; which is similar for most species. From the body to the leg’s tip, segments are known as coxa, femur and tibia; typically the coxa-femur and the femur tibia joint are assumed to be a simple hinge joint. The body-coxa joint model ranges from one to three degrees of freedom, depending on the specie and the thoracic segment that leg resides.
