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perturbations. Redefining the up vector using a vector fixed to the pelvis as illustrated in Figure

3.26, rather than the torso, is sufficient to overcome this problem.

Figure 3.26 -Pelvis-based up vector balance indicator

Torso servoing ensures that the upper body of the biped remains upright, and provides increased

stability by damping large upper body movements.

However, it should be noted that it does

nothing to prevent the biped from falling over since the legs must still support the upper body. An

example of a fall with torso servoing applied can be seen in Figure 3.27.

There are a number of benefits in addition to a more aesthetic result. Reducing the highly dynamic

oscillations of the torso (a significant portion of the body mass) results in a successful limit cycle

for a larger range of Q^d
. As well, longer stride times and very slow walks become possible.

Figure 3.27 - Falling with torso servoing enabled.