Humanoid robots balance by constantly measuring how their bodies are tilted, where their weight is, and how the ground pushes back on their feet. This matters because a two-legged robot has a narrow base of support compared with a wheeled robot. If its center of mass moves too far outside its feet, gravity creates a tipping effect.
Robots such as Atlas, Honda ASIMO, and Tesla Optimus use fast sensing and control to stay upright while standing, walking, or recovering from a push.
A balance controller acts like a rapid feedback loop. Sensors such as an inertial measurement unit detect tilt and angular motion, while foot sensors estimate ground reaction forces. The robot then adjusts hip, knee, and ankle joint angles to move its center of mass back over its support area.
Many humanoid robots use zero moment point control, which keeps the effective tipping point inside the foot contact region.
Key Facts
- Balance is stable when the center of mass projection stays inside the support polygon formed by the feet.
- Torque from gravity can cause tipping: τ = rF sin θ.
- An IMU measures motion using accelerometers and gyroscopes.
- Ground reaction force is the force from the floor that pushes upward on the robot's feet.
- Newton's second law guides motion control: F = ma.
- Zero moment point control tries to keep the net tipping moment at the ground equal to zero within the foot area.
Vocabulary
- Center of mass
- The average location of an object's mass, where gravity can be treated as acting on the whole object.
- Support polygon
- The area on the ground enclosed by the robot's feet or contact points.
- Inertial measurement unit
- A sensor package that measures acceleration and rotation to estimate tilt and motion.
- Ground reaction force
- The force that the ground applies to a robot's foot in response to the robot pushing on the ground.
- Zero moment point
- The point on the ground where the net tipping moment from gravity and motion is zero.
Common Mistakes to Avoid
- Thinking balance only depends on standing still is wrong because walking robots must also control acceleration, foot placement, and momentum.
- Confusing center of mass with the robot's geometric center is wrong because heavy parts like motors and batteries can shift the center of mass.
- Ignoring the feet is wrong because foot size, contact points, and ground reaction forces determine the support polygon and tipping risk.
- Assuming sensors fix balance by themselves is wrong because sensor data must be used by a controller to command joint torques and angle corrections.
Practice Questions
- 1 A robot has its center of mass 0.90 m above the ground. If its center of mass projection is 0.08 m from the edge of its foot support area, how much farther sideways can it shift before it reaches the tipping boundary?
- 2 A 60 kg humanoid robot stands still on both feet. What is the total upward ground reaction force if g = 9.8 m/s²? If the weight is shared equally, what force is on each foot?
- 3 A robot is pushed gently from the side and begins to lean left. Explain how its IMU, foot force sensors, and hip, knee, and ankle joints work together to prevent a fall.