Hexapod robotics

A hexapod robot is a mechanical vehicle that walks on six legs. Since a robot can be statically stable on three or more legs, a hexapod robot has a great deal of flexibility in how it can move. If legs become disabled, the robot may still be able to walk. Furthermore, not all of the robot’s legs are needed for stability; other legs are free to reach new foot placements or manipulate a payload.

Many hexapod robots are biologically inspired by Hexapoda locomotion. Hexapods may be used to test biological theories about insect locomotion, motor control, and neurobiology.

Hexapod designs vary in leg arrangement. Insect-inspired robots are typically laterally symmetric, such as the RiSE robot at Carnegie Mellon. A radially symmetric hexapod is ATHLETE (All-Terrain Hex-Legged Extra-Terrestrial Explorer) robot at JPL.

Typically, individual legs range from two to six degrees of freedom. Hexapod feet are typically pointed, but can also be tipped with adhesive material to help climb walls or wheels so the robot can drive quickly when the ground is flat.

Most often, hexapods are controlled by gaits, which allow the robot to move forward, turn, and perhaps side-step. Some of the most common gaits are as follows:

Alternating tripod: 3 legs on the ground at a time.
Crawl: move just one leg at a time.

Gaits for hexapods are often stable, even in slightly rocky and uneven terrain.

Motion may also be nongaited, which means the sequence of leg motions is not fixed, but rather chosen by the computer in response to the sensed environment. This may be most helpful in very rocky terrain, but existing techniques for motion planning are computationally expensive.

Biologically inspired
Insects are chosen as models because their nervous system are 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. Insects’ walking behaviour and neural architecture are used to improve robot locomotion. Conversely, biologists can use hexapod robots for testing different hypotheses.

Biologically inspired hexapod robots largely depend on the insect species 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.

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.

Foot coordination
The term “foot coordination” refers to the mechanism responsible for controlling the transition between footsteps; considering that the body does not turn around. Most approaches try to replicate the appearance of known insects, for example the tripod or tetrapod shape. However, other approaches have been used to find stable paces; for example, by launching programs using genetic algorithms or by optimizing the energy of walking.

The walking patterns of insects are usually obtained by two approaches: centralized and decentralized control architectures. The centralized controllers directly specify the transitions of all the legs, whereas in the decentralized architectures, six nodes (legs) are connected in a parallel network; the paces are obtained thanks to the interaction between the neighboring legs.

Controller of the paw
There are no borders to the complexity of the morphology of the paw. However, legs that are built on an insect model usually have between two and six degrees of freedom. Leg segments typically bear the name of their biological counterpart, which are similar for most species. From the body to the end of the paw, the segments bear the names of coxa, femur and tibia ; typically the joints between the coxa and the femur and between the femur and the tibia are considered as simple hinges. The models of the articulation between the body and the coxa comprise between one and three degrees of freedom, depending on the species and the thoracic segment on which the leg is enclosed.

Source from Wikipedia