Theory of walking

Locomotion on ground can be realized with three different basic mechanisms:

• slide

• lever

• wheel or track

First two are walking mechanisms

Benefits of walking

Walking is reasonable on soft uneven terrains. Benefits that can then be obtained:

• better mobility

• better stability of the platform

• better energy efficiency

• smaller impact on ground

Facts affecting to the choise of locomotion system

• Terrain on which the robot mainly moves• Operational flexibility needed when working• Power and/or energy efficiency requirements• Payload capacity requirements• Stability • Impact on the environment

Motion control of a walking robot

• The motion control system should control the motion of the body so that leg movements automatically generate the desired body movements.

• Control system needs

- to control gait, ie. the sequence of supporting leg configurations.

- to control foot placement (motion of the non-supporting legs) to find next foothold

- to control body movement on supporting legs

( Mecant) (video)

Gaits• Gait determines the sequence of supporting leg

configurations during the motion• Gaits can be divided in two classes:

- periodic gaits, which repeat the same sequence of supporting leg configurations

- non-periodic or free gaits, which have no periodicity in their gait pattern

• The number of different gaits depends on the number of legs

Walking vs. running

• Motion of a legged robot is called walking if in all time instants at least one leg is supporting the body.

• When legs are not all the time supporting the motion is called running.

• Walking can be statically or dynamically stable.

• Running is always dynamically stable

Stability• Stability means the capability to maintain the body

at standing position on the legs.• Statically stable walking means that the legs can

be freezed and the the motion stopped at any time instant without loosing stability.

• Dynamically stable walking or running means that stability can be maintained only in active motion when legs are moved.

• Statically stable walking can be controlled by using kinematic models. Dynamical walking and running need dynamical models.

Stability margin

• In statistically stable walking the concept of stability margin is used to evaluate risk of loosing stability. Stability margin depends on support leg pattern and vary along a gait sequence.

• Two types of stability margins can be defined:

- Geometrical based

- Energy calculation based• Geometrical stability margins are more simple and

thus mostly used.

Periodic gaits• All periodic gaits can be controlled by using one

phase variable, which indicates the phase of the gait cycle and says when legs are put on transfer state.

• There are very many periodic gates, the number increases with the number of legs.

• However, only few are practical:

- wave gaits

- equal phase gaits

- follow the leader gait

Free gaits

• Control of a free gait is based on logic rules that say when legs are put on transfer state.

• Rules are usually based on certain criteria which depend on the recuirements of the motion.

• One of the most natural criterion is maximizing stability.

• Another one is transferring those legs which are at the end of their stroke

Periodic vs free gait• Periodic gaits are simple and thus often used

• They include, however, certain problems:

- legs are forced to go to support state in a certain order, which may stop the motion if a proper footplace is not available

- stability may be difficult to maintain if critical terrain conditions coincidence with the low stability margin support pattern

• Free gaits offer more flexibility, but are individual in nature and may suffer logic errors