Hey friend
After spending a lot of time talking about balance and movement in humanoids it is time to talk about the parts that make them move the actuators.
I remember when I first opened up a servo motor and was surprised at how small the actual motor was compared to the big gearbox around it. This made me realize that the actuator is an important part of a humanoid robot.
Today lets talk about the three common types of actuators used in humanoids.
1. The Heart: Electric Motors
Most humanoid robots use motors. These motors are efficient and easy to control.
A motor has a curve that shows how much torque it can produce at different speeds:
- When the motor is not moving it can produce a lot of torque.
- As the motor starts moving, the torque it can produce decreases.
- If the motor is moving fast it can hardly produce any torque.
This curve is very important. If your robot needs to move strongly you use the motor at the low speed end of the curve. If your robot needs to move you use the motor at the high speed end of the curve.
2. The Gearbox Problem
Motors usually spin fast and do not produce enough torque for robot joints. So we add gearboxes to them.
There are three types of gearboxes used in humanoids:
A. Traditional Servo
This type of gearbox is used in many smaller robots.
- It has a high gear ratio, which means it can produce a lot of torque at speed.
- It can be stiff and hard to move. It is not very good at moving smoothly.
B. Harmonic Drive
This type of gearbox is used in high-end humanoids like Tesla Optimus.
- It is very precise. Can produce a lot of torque.
- It is heavy and can be hard to move.
C. Quasi-Direct Drive
This is a type of gearbox that is used in some advanced robots.
- It has a gear ratio, which means it can produce a lot of torque at high speed.
- It is very good at moving and can be easily pushed by hand.
Torque-Speed Curves in Real Robots
Let me explain why this is important with an example:
- A high-ratio gearbox has a tall and narrow curve, which means it can produce a lot of torque at low speed.
- A quasi-direct drive gearbox has a wider curve, which means it can produce a lot of torque at different speeds.
This is why many researchers like to use quasi-direct drive gearboxes in the legs of humanoids. When a robot is walking its legs need to move at speeds and a quasi-direct drive gearbox can help it do that.
Key Trade-offs Every Humanoid Designer Faces
- Precision vs Compliance: High-ratio gearboxes are precise but stiff. Low-ratio gearboxes are compliant but less precise.
- Weight vs Torque: Heavy gearboxes can produce torque but are harder to move.
- Efficiency vs Safety: Stiff actuators can be dangerous. Compliant ones are safer.
- Cost vs Performance: Harmonic drive gearboxes are expensive but quasi-direct drive gearboxes require good motors and careful engineering.
My Personal Take
I think quasi-direct drive gearboxes are very good for legs and ankles. They make the robot feel more alive. When you can push the joint and it moves back easily you know the robot is safe and natural.
I also understand why some robots use harmonic drive gearboxes. Sometimes you need that precision.
The future of humanoids I believe, is in using a mix of types of gearboxes. You use high-precision gearboxes where you need them and compliant gearboxes where you need them to be safe and natural.
Understanding how actuators work and how they affect the movement of a robot is very important. It helps you appreciate why adding parts, like toes or elastic elements can make a robot move more naturally.