Achieving Stable Walking
To achieve stable walking...

Issues to be address in order to achieve stable walking...

· Not falling down even when the floor is uneven.
· Not falling down even when pushed.
· Being able to walk stable on stairs or slopes.
A. Posture Controls to Achieve Stable Walking

Floor Reaction Control : Firm standing control of the soles of the feet while absorbing floor unevenness.
Target ZMP Control : Control to maintain position by accelerating the upper torso in the direction in which it threatens to fall when the soles of the feet cannot stand firmly.
Foot Planting Location Control : Control using side steps to adjust for irregularities in the upper torso caused by target ZMP control.

* ZMP = Zero Moment Point: The point when total inertial force is 0.
3Position-Control Arangement


When the robot is walking, it is influenced by inertial forces caused by the earth's gravity and the acceleration and deceleration of walking.These combined forces are called the total inertial force. When the robot's foot contacts the ground it is influenced by a reaction from the ground called the floor reaction force.

The intersection of the floor and the axis of the total inertial force has a total inertial force moment of 0, so it is called the Zero Moment Point.The point where the floor reaction force operates is called the floor reaction point.
Basically, an ideal walking pattern is created by the computer and the robot's joints are moved accordingly. The total inertial force of the ideal walking pattern is called the target total inertial force, and the ZMP of the ideal walking pattern is called the target ZMP.
When the robot is maintaining perfect balance while walking, the axes of the target total inertial force and the actual floor reaction are the same. Accordingly, the target ZMP and the center of ground reaction are the same.

When the robot walks across uneven ground, the axes of the target total inertial force and the actual floor reaction force are out of alignment, balance is lost and falling force is generated.
This falling force is comparable to the misalignment of the target ZMP and the center of ground reaction. In short, the misalignment between the target ZMP and the center of ground reaction is the main cause of loss of balance.

When the Honda robot loses its balance and threatens to fall, the following three control systems operate to prevent the fall and allow continued walking.
Floor Reaction Control

The floor reaction control absorbs irregularities in the floor and controls the placement of the soles of the feet when falling is imminent. For example, if the tip of the robot's toe steps on a rock, the actual center of ground reaction shifts to the tip of the toe. The floor reaction control then causes the toe to rise slightly, returning the center of ground reaction to the target ZMP.
Another example would be if something caused the robot to lean forward, the tips of the toes would be lowered, placing more pressure on them and the actual floor reaction action point would be shifted forward, generating a position recovery force. However, because the center of ground reaction cannot exceed the scope of the foot sole contact patch there is a limit to the position recovery force, and if the robot leans too far forward it will fall. forward it will fall.
Target ZMP Control

If the robot leans too far over, the target ZMP control operates to prevent it from falling.As stated above, misalignment of the target ZMP and the actual floor reaction action point generates a falling force. However, the target ZMP control maintains the robot's stability.For example, in the diagram to the left, if the robot starts to fall forward, its walking speed is accelerated forward from the ideal walking pattern. As a result, the target ZMP is shifted rearward from the actual floor reaction action point and a rearward falling force is created which corrects the robot's position.
Foot Planting Location Control

When the target ZMP control operates, the target position of the upper torso shifts in the direction of acceleration. When the next step is taken in the ideal step length, the feet will fall behind the torso. The stepping placement control idealizes the stride to ensure the ideal relationship between torso speed and length of stride is maintained.
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