Walking robot

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Walking robots are special robots that can move with the help of "legs". This is already possible with just one "leg" (hopping). As a rule, however, four-legged, six-legged or two-legged machines are used. There are machines that have even more legs, but these can be traced back to the six-legged variant.

History and background information

"Annie G. galloping", photographic study of movement by Eadweard Muybridge , 1878.

An early subject of investigation were the various horse gait gardens . The photographer Eadweard Muybridge captured the gallop of a horse with a specially developed recording technique and was thus able to provide photographic evidence that in this gait the horse only touches the ground with one hoof at different times and not at all at other times. Although the horse at any single moment in a non- stable is state, it is still stable.

So that walking robots can fulfill their task, the sequence of movements and the reaction to disturbances must be precisely defined. In the case of wheels, these are axial rotations and occasional steering movements, while a machine with “legs” moves several components in a coordinated manner in space and time.

Machines that have at least six legs ( hexipedes , millipedes , arachnids with at least eight legs) always have at least three legs on the ground so that the machine is stable at all times.

Even with four-legged feet , a reasonably stable stance can only be achieved if three legs are on the ground. The example of the animals shows, however, that a high running speed cannot be achieved with them.

Walking and running behavior

Static walking

Static walking is when the center of gravity of a robot is above its feet at all times so that it cannot fall over without the action of an external force.

Dynamic walking and running

Dynamic walking occurs when the center of gravity of a robot can also be outside the area of ​​the feet without the robot falling. In fact, one could speak of a “controlled fall”, as the robot would fall if its movement stopped suddenly.

We speak of dynamic running when the movement required to maintain the speed means that temporarily none of the robot's legs touch the ground.

Static walking robots

The classic walking robot consists of actuators , sensors and a computer control . The "legs" are usually moved by servomotors in such a way that a specified movement program is unwound.

Two-legged static walking robots

The ASIMO robot moves at a maximum speed of 6 km / h, is 1.30 m tall and weighs 52 kg, and it needs a lot of electrical energy for this. One of his special abilities is that he can climb stairs.

Six-legged walking robots

Hexapod robot
Tripod corridor (top view); red leg: standing leg; black leg: unloaded
Tripod gear: running diagram
Tetrapod corridor (top view); red leg: standing leg; black leg: unloaded
Tetrapod corridor: running diagram

Six-legged constructions are an ideal basis for statically stable walking robots. They are therefore suitable for movement on uneven terrain. A distinction is made between two gaits (sequence of leg movements):

  • Tripod gear
  • Tetrapod corridor

In tripod gait, there are three legs on the ground at all times (example: Indian stick insect , with 3 standing and 3 swinging legs).

When walking with the tetrapod, there are always four legs on the ground (4 standing legs, 2 swinging legs).

In walking machines with six orthogonal legs, a distinction is made not only according to the sequence of leg movements but also according to the basic type of movement of the legs:

  • Follow the leader (e.g. tripod gang, tetrapod gang)
  • Circulation walker
  • Weaving walker

Six-legged creatures run as followers. One leg follows the other (in whatever order). Machines can do more. In a circular runner, the three legs on the right side have a common axis of rotation - like the hands of a clock (and the left legs accordingly). The rearmost leg is swung in front of the foremost leg. But how is the rearmost leg supposed to get past the other two legs? It just swings through under the belly (the robot platform).

The loom, too, performs a biologically impossible movement. In the loom, all six legs sit on a common vertical axis in the center of the platform. Each leg can move completely around the whole body (a horizontal telescopic movement makes it possible). The legs move from their rearmost position to the foremost position by being guided around the two other legs on the outside.

When walking on uneven terrain, it is crucial that the robot finds a safe touchdown point within its stride (potential touchdown area of ​​the leg - foothold selection area) without having to deviate too far from its main direction of march.

Dynamic walking robots

Passive dynamic runners

Walking robots that can move without a source of energy are based on a toy invented 150 years ago. It just had to be bumped and could then walk down a small slope on its own. To do this, the toy swings from right to left and swings the leg that has just been lifted forward a little. Then it rocks from left to right and the other leg swings forward.

With this construction, the toy can move energy-efficiently and serve as a starting model for technically more sophisticated walking robots. In the 1980s, Tad McGeer used the principle of the pendulum implemented in this toy to stabilize movements. A complex and slow control system in a computer carried along should no longer make the robot run, but the structure of the musculoskeletal system should stabilize the walking robot without additional action. If the construction of the simple toy is supplemented with a “hip” or with “movable feet”, then such walking robots only need energy to accelerate the moving masses and no longer when braking, as was the case with earlier walking robots.

See also

literature

  • René Steiner: Attractor control for the movement of a simulated two-legged walking machine in three dimensions . dissertation.de, Berlin 1999, ISBN 3-933342-26-0 .
  • Stefan Piekenbrock: Reactive control of a six-legged walking machine in an unknown environment . Shaker Verlag , Aachen 1996, ISBN 3-8265-1313-4 .
  • Winfried Ilg: A biologically motivated adaptive motion control for a four-legged walking machine . Academic V.-G. Aka, Berlin 2001, ISBN 3-89838-245-1 .
  • Martin Pfrommer, Yordan Todorov: Control concepts for the leg control of the Lauron IVb walking robot . Grin Verlag, 2008, ISBN 978-3-640-22283-4 .

Web links