# Need some help understanding Zero Moment Points.

1. Jul 11, 2013

### chi-young

So I was reading up "Zero Moment Point -Thirty Five Years of Its Life" by Vukobratic for a presentation, and is said something like this:

So if I am reading this right, it's saying the point where the GRF is acting upon will shift based on the magnitude of the force created by the orientation of the upper portions of the robot. The greater the horizontal component of the Moment M_A, the farther away from the ankle the point will be. (fig d in uploaded diagram).

However, it also goes on to say that

Okay, so if the upper body's movements are greater than can be compensated for by the shifting of the GRF focus point (is that right?), than the foot will experience a torque equivalent to the uncompensated for force? Huh? I'm confused.

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2. Jul 11, 2013

### tiny-tim

hi chi-young! welcome to pf!

i don't understand this

it seems to be confusing moment with moment of inertia

and i don't understand what the "horizontal component of the reaction moment" is supposed to be

forget robots, just consider a crane …

it has a base, supported on four wheels at the corner, and it has an arm that sticks out and has a heavy object on the end

the weight of the whole crane is purely vertical, and the line of action of its resultant (its total) will either pass inside the base or outside it

if it passes outside the base (the "support polygon"), then the moment of this vertical resultant will be in the same direction as the moment of the reaction forces from the ground, and so the crane will rotate

(and if it passes inside the base, then the moment of this vertical resultant will be in the opposite direction to the moment of the reaction forces from the ground, and so the reaction forces can be equal and opposite, and there will be equilibrium)​