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A base PCG producing an open-loop walk of a few steps is more

compensating perturbations.

likely to be successfully stabilized than one that falls in the first step.

Figure 3.7 shows a base PCG used to generate a forward walking motion for the simplest human

model along with its equivalent graphical representation.

The base PCG has right-left symmetry,

although PCG asymmetry can also be quite useful aswill be demonstrated

perturbations presented in Section 6.2.

by

the

turning

The base PCG consists of six states.

The poses in states S1 and S2 are identical.

The poses in

states S4, S5, and S6 are identical to those of states S1, S2 and S3 respectively, with the left and

right sides exchanging roles. Having a total of only four unique poses, the controller describes a

simpler motion than a typical human walk.

This relatively small number of poses simplifies the

specification of a base PCG as well as the creation of suitable parametric perturbations.

Sensor-based transitions are necessary to synchronize the biped's motion to the external world.

The base PCG of Figure 3.7 uses such transitions to move from S1 to S2 and from S4 to S5.

A

transition occurs when the swing foot for the current step (i.e. the next stance foot) contacts the

ground. If this has already happened before the state is entered, the transition occurs immediately.

Actively ensuring ground contact at fixed points in the walk cycle is important in certain situations

such as when starting from an initial resting state.

While the sensors are typically no longer

necessary once a walking limit cycle is reached, changing from one limit cycle to another can

require sensor-based synchronization again.