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77

More formally, to control walking speed, Q
~
d, a new value for Qdis computed each step by

adding a velocity feedback term to a nominal value of Qdas follows:

Q
~
d=Qbiasd+kp[!](vd-v)

( 5.1)

where

Qbiasd

vd

v

kp

is a nominal bias value


is a desired speed


is a measure of the speed of the current step

is a proportional feedback gain

The bias value is required because the required Qdfor a stationary walk (i.e. in-place) is not

necessarily 0. The feedback term adjusts Q
~
din the appropriate direction (more forward or more

upright) by an amount proportional to the error in speed.

We choose to use the distance travelled

by the pelvis over the current step as a velocity measure for vandvd.


The speed of the walk can be varied using either vdor Qbiasd, since both contribute a constant term

Fixing vdand varying Qbiasdis akin to choosing a nominal speed and

to the overall equation.

varying the actual speed about that point by leaning more forward or more upright.


To prevent excessively large accelerations that could cause a fall, the feedback term is limited in

magnitude:

IMAGE Imgs/thesis.final.w6157.gif


DQmax,

[!]

where DQmaxis the maximum allowable change in Q
~
dfrom Qbiasd

[CONVERTED BY MYRMIDON]