Theory of walking

1. Theory of walking Locomotion on ground can be realized with three different basic mechanisms: • slide • lever • wheel or track First two are walking mechanisms

2. 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

3. 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

4. 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)

5. 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

6. 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

7. 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.

8. 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.

9. Periodic gaits • All periodic gaits can be controlled by using one phase variable, which indicates thephase 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

10. 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

11. 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