Hey friend
I want to tell you about something that really surprised me when I was learning about humanoid robots. The best robots are not the ones that’re super stiff.
In fact some of the advanced robots actually add “softness” to their joints. At first I thought this was an idea. Why would you make a machine that is supposed to be precise less rigid?. Then I saw some demos and it all made sense.
Today we are talking about mechanisms and Series Elastic Actuators. These are two important ideas that make humanoid robots safer more efficient and more like humans.
So why do rigid robots have problems?
Traditional robots with parts are very precise but they have some big downsides.
- They feel every bump and shock which’s bad for the robots durability
- They can be dangerous around people because if they make a mistake it can hurt someone
- They waste a lot of energy trying to move because they are so stiff
- They have trouble walking on uneven surfaces because they cannot adapt
But nature has already figured out how to solve this problem. Our muscles and tendons are flexible so they can. Store energy. Now designers of humanoid robots are starting to do the thing.
What are compliant mechanisms?
Compliant mechanisms are parts that can move or transmit force because they are flexible not because they’re rigid.
- They can be made from plastics or metals that can bend
- They can be like springs
- They can be inspired by the shells of shrimp
These mechanisms can bend or deform a bit when they are under load, which gives the robot a natural way to absorb shocks and adapt to its environment.
Series Elastic Actuators are a game changer.
They work by putting a spring between the motor and the joint.
So the power goes from the motor to the gearbox to the spring to the joint.
This simple idea gives the robot a lot of advantages.
- It can absorb shocks. If the foot hits the ground or the arm bumps into something the spring compresses and absorbs the impact.
- It can. Release energy like the tendons in our legs which makes walking more efficient.
- It is safer because if the robot accidentally hits someone the spring gives way and reduces the force.
- It can control force better because it can measure how much the spring is compressed and use that to control the force it applies.
There are already some real-world examples of humanoid robots that use Series Elastic Actuators.
- Some research robots and Unitree robots use them in their legs to walk efficiently and adapt to terrain.
- Boston Dynamics uses compliance in their Atlas robot to make it more stable and able to recover from falls.
- Some advanced legged robots combine Series Elastic Actuators with other technologies to get the best of both worlds: precision and softness.
Even Tesla Optimus and Figure 01 are starting to use compliant designs in their lower bodies to improve walking and safety.
Course there are some trade-offs.
- Adding compliance can make the robots position less accurate
- The spring adds weight and complexity
- Control becomes more challenging
But good designs can tune the stiffness of the spring and combine it with control to make it work.
My personal take on this is that rigidity is not as important as we thought.
Adding controlled softness through mechanisms and Series Elastic Actuators is a really smart move. It makes robots more like humans using elasticity for efficiency, safety and adaptability.
This idea connects to a lot of topics we have already talked about.
- It works well with contact mechanics and friction management
- It complements the energy-based view from Lagrangian mechanics
- It makes torque control and force control more practical
- It pairs perfectly with reinforcement learning for robust walking
We have already seen how nature uses flexible exoskeletons and elastic tendons. Series elastic actuators are our way of learning from nature.
I think the next generation of humanoid robots will rely heavily on this kind of controlled compliance not because it looks cool but because it simply works better in the world.