Long Legs


The reason why hobby humanoids are so unstable and need those big feet is that they are short and their center of mass is at around 20cm height. meaning that upon any perturbation, they will very quickly loose balance and reach an angle where a disastrous full fall can not be prevented.  T=0.897 sec/cycle means that controlling the instability requires reaction at an order of T/4 = 0.22 Sec. Thats about the time it takes the servo to run through 45Deg. If that servo is on the hip of the humanoid, moving the whole leg whose length is about 15cm it can spread that leg about 10cm on the ground. Seems like enough to keep balance but I prefer to do our experiments in a less stressful predisposition.

If the CM is brought up to 35cm, T/4 = 0.297 Sec. Hence more time to react to instability. The downside is that when building this structure from Bioloid plastic parts, there’s now an additional flexibility and springiness to everything.

The reason for double jointed knees is so that the speed of folding and extending those knees is about double the speed of moving the whole leg from the torso.

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