What are the main technical types of actuators (the "muscles" and "joints") that drive humanoid robot motion?

Hello! That's an excellent and very vivid question you've asked. The analogy of "muscles" and "joints" immediately captures the essence of humanoid robot movement. In robotics, we call these components Actuators.

Simply put, an actuator is a device that converts energy (such as electrical or hydraulic energy) into mechanical motion. Below, I'll explain the main types of current mainstream technologies in an easy-to-understand way.

1. Electric Drive: The Most Mainstream, Most "Nerve-Controlled Muscle"-Like Method

This is currently the most widely used method. The vast majority of humanoid robots you see, such as Tesla's Optimus, use this type.

• Working Principle: As the name suggests, it uses electricity to drive a motor to rotate. But simple rotation isn't enough; robot joints require precise angles and sufficient force. Therefore, a complete "electric joint" typically includes:

  1. Servo Motor: This is the core. It's not just a simple motor that spins blindly; it has an internal "position sensor" (encoder) that constantly tells the "brain" (controller) its current angle. When the brain commands "turn to 90 degrees," it will precisely turn to 90 degrees and stop, and it will resist external forces to maintain that position. This is key to how robots can perform various poses and maintain balance.
  2. Reducer/Gearbox: Motors typically have very high rotational speeds (e.g., thousands of revolutions per minute) but low force (torque), making them unable to lift arms or legs. A reducer is like a car's gearbox or a bicycle's gears; it uses a complex gear structure to reduce the motor's high speed while amplifying its force dozens or even hundreds of times. This allows the robot to perform powerful movements.

• Advantages:

  • Precise Control: Electrical response is extremely fast, and control precision is very high.
  • Clean, Quiet: No risk of fluid leaks, and relatively quiet during operation.
  • High Efficiency: Energy conversion efficiency is relatively high, saving power.

• Disadvantages:

  • Weak Burst Force/Impact Resistance: Due to rigid connections, if the robot falls, the huge impact force can easily damage the expensive reducer. Unlike human muscles and tendons, which have synovial fluid for cushioning.
  • Power Density Bottleneck: It's currently difficult to achieve the same high force as hydraulics in a similarly compact volume.

• Analogy: A precise "electronic muscle" with built-in sensors, contracting and relaxing accurately via electrical signals.

2. Hydraulic Drive: The "Strongman" of the Robot World

If electric drive is the precise "gymnast," then hydraulic drive is the powerful "weightlifting champion."

• Working Principle: A high-pressure pump forces oil (hydraulic fluid) into a sealed cylinder, pushing a piston. This process can generate immense force. Excavators and cranes you see, with their powerful mechanical arms, are hydraulically driven.

• Notable Example: The famous Boston Dynamics' Atlas robot. You see it run, jump, and do backflips, performing various highly explosive movements, all thanks to the powerful force provided by its hydraulic system.

• Advantages:

  • Immense Force: Extremely high power density, capable of unleashing astonishing force in a small volume.
  • Good Dynamics: Can perform very fast, explosive movements.
  • Natural Compliance: Liquids have some compressibility, making them much more impact-resistant than electric motors with rigid reducers.

• Disadvantages:

  • Complex and "Messy": Requires a complete system of hydraulic pumps, oil lines, valves, and radiators, making the system large, heavy, and noisy. "Leaks and spills" are common, and maintenance is troublesome.
  • High Control Difficulty: Controlling fluid flow and pressure is much more complex than controlling electric current, making it difficult to achieve the same precision as electric motors.
  • High Energy Consumption: To be ready for bursts of power, hydraulic pumps often need to maintain a high-pressure state, like a car idling, which consumes a lot of energy.

• Analogy: Equipping the robot with a power system similar to an excavator's arm, possessing immense strength.

3. Pneumatic Drive: The Lightweight, Fast "Sprinter"

This method is relatively less common but highly effective in specific areas.

• Working Principle: Similar to hydraulics, but the working medium is compressible air instead of incompressible oil. Joints are driven by inflating and deflating "air bladders" or cylinders.

• Application Scenarios: Often used in applications requiring speed, lightness, and high safety for human interaction. Because air is compressible, pneumatic joints inherently possess a "compliant" characteristic; hitting a person won't be as rigid as a metal lump. Some research uses it to create "artificial muscles."

• Advantages:

  • Fast speed, quick response.
  • Compliant, safe, and elastic.
  • Simple, lightweight system, as the working medium is air, there's no need to consider liquid recovery.

• Disadvantages:

  • Poor Control Precision: Due to air's compressibility, it's difficult to keep the joint stable at a precise position; it tends to "float."
  • Low Force: Provides much less force than hydraulics, and even less than electric motors of comparable size.

• Analogy: Equipping the robot with a bunch of quickly inflatable/deflatable "air bladders" to simulate muscles.

In Summary

Currently, electric drive is the absolute mainstream and the future direction of development. Scientists are also working to address its shortcomings, for example, by adding elastic elements like springs between the motor and the joint (this structure is called Series Elastic Actuator - SEA), to mimic human tendons, making robot movements more natural and impact-resistant.

I hope this explanation helps you!