If a humanoid robot falls, can it self-recover and stand up like a human? What are the underlying technologies involved?

Certainly! Humanoid robots autonomously standing up after falling might sound like science fiction, but many advanced robots, like Boston Dynamics' Atlas, can already do it. Behind this isn't just a simple act of "standing up," but a very complex interplay of technologies.

You can imagine a robot as a "martial arts master" who has practiced countless times how to stand up from a fall. When we humans fall, we instinctively use our hands to brace ourselves, adjust our posture, and then stand up. Robots are similar, but all their movements rely on precise calculations and control.

This complex interplay primarily involves these core technologies:

1. Perception: Figuring out "Where am I? What's my posture?"

After a robot falls, the first thing it needs to do is understand its current state. Unlike us, it doesn't have skin or an inner ear for perception, but it has more precise "tools":

• Inertial Measurement Unit (IMU): This is like the robot's "cerebellum" and "inner ear," containing gyroscopes and accelerometers. It tells the robot whether it's head-down or feet-down, lying flat on its back, or on its side.
• Joint Encoders: Each joint (e.g., knees, elbows) has sensors that precisely tell the "brain" how much that joint is bent. This allows the robot to build a precise mental model of its limb posture.
• Force/Torque Sensors: Some robots also have force sensors on their feet or hands, which can detect how much support force the ground or an object is exerting on them. This is crucial for finding points of support.

2. Planning: Figuring out "How should I stand up?"

Once it knows its posture, the robot's "brain" (central processor) begins to plan. This process is called Motion Planning.

It's not a single, instantaneous step, but a detailed "script." For example:

"I'm currently lying flat on my back. Okay, step one: pull my legs in, use my elbows to push off the ground, and roll onto my stomach. Step two: push up with my hands, raising my body into a kneeling position. Step three: step one leg forward, forming a single-knee kneeling posture. Step four: adjust my center of gravity, use the strength of my legs and waist to 'push off' and stand up!"

This "script" consists of various strategies pre-designed by scientists. The robot selects the optimal one to execute based on its falling posture.

3. Control: Stay steady! "Don't fall again!"

This is the most crucial and challenging step, known as Balance Control.

The core concept is the Center of Mass (CoM). You can think of it as an object's "balance point." Humans can stand steadily because we instinctively maintain our CoM within the support base between our feet.

Throughout the entire process of standing up, the robot must constantly calculate and adjust its CoM.

• Adjusting Posture: When it lifts its body from the ground, its CoM changes. It must quickly swing its arms, twist its waist, and even move its head to "pull" the CoM back to a stable position. This is similar to how we spread our arms to maintain balance when walking on a tightrope.
• Dynamic Adjustment: The entire standing process is dynamic, not a series of static poses. The robot's computer continuously fine-tunes the movements of each joint at hundreds or even thousands of times per second, based on sensor data, to ensure that the CoM remains within a controllable range as it moves upward.

Here's a summary of the entire process:

Imagine the Atlas robot falls:

1. "Oops, I fell!" — The IMU and joint sensors immediately send the falling posture data to the brain.
2. "Don't panic, let me think." — Based on the posture, the brain retrieves an optimal "standing script" from its database.
3. "Initiate!" — The brain sends commands to all the motors (the robot's "muscles") to begin executing the first step of the script, such as curling its body.
4. "Steady, steady!" — As each movement is executed, the balance control system operates at high speed, continuously adjusting the CoM through actions like arm swings to ensure the body doesn't fall again during the upward movement.
5. "Done, I'm up!" — Finally, the robot returns to an upright posture and continues to maintain balance.

Therefore, a robot's ability to stand up on its own is the result of the perfect coordination of three major technologies: Perception, Planning, and Control. It requires not only powerful hardware (motors, sensors) but also an extremely intelligent "brain" (algorithms) to accomplish this seemingly simple yet incredibly complex action.