15

not make use of any other feedback to correct or adjust the overall motion of the creature.

Therefore, we will refer to pose control using a fixed PCG as being a form of open loopcontrol.

Our definition is one of convenience and is appropriate since we choose to use pose control as our

fundamental representation. In essence, the local closed loop control at the joints is treated as part

of the system being controlled rather than part of the controller itself.

In this context, closed loop

refers to feedback used to modify the desired joint angle parameters of the PCG.

The transitions between the states of a PCG are based on a fixed hold timeand/or are sensor-

based. A hold-time transition occurs after a specified time has elapsed in the associated state.

A

sensor-based transition

occurs

when

a

particular

binary

sensor

turns

"on",

or

takes

place

immediately if the sensor is already on upon entering the state.

The only form of sensors used in

this work are ground contact sensors on the creature's feet, which are considered to be on when

the foot is in contact with the ground.

(b)

(a)

A periodic PCG is one in

Figure 2.5- Pose control graph structures
(a) cyclic
(b) aperiodic
(c) composite

PCGs may be periodic, aperiodic, or composite(see Figure 2.5).

which the state machine is cyclic.

In this form, the desired poses are perfectly periodic.

The

actual motion and the applied torques need not be so; they do not typically repeat from one cycle to

the next.

As discussed in [vKF94], creatures controlled using periodic PCGs are analogous to

simple windup toys.

An aperiodic PCG is a chain of successive poses, useful for performing a

single motion instance, such as recovering after a fall.

A composite PCG is a more general form

of state machine composed of periodic and aperiodic PCGs.