Hey friend
We have spent a lot of time in this series talking about legs and balance and walking and whole body control.. If you ask most people what makes a humanoid robot truly useful in daily life many will point to one thing: its hands.
A robot can have balance and impressive walking ability but if its hands are stiff or fragile or too simple it will struggle to interact with our world. That is why modern humanoid hand design is shifting toward compliance and adaptability and clever mechanical intelligence.
Today let us explore the mechanism design principles behind advanced humanoid hands: underactuation and tendon-driven systems and soft robotics.
Why Compliant Hands Matter
Traditional industrial robot grippers are stiff and precise and powerful great for factories terrible for homes. Humanoid hands need to grasp fragile objects like eggs and glass and fruit. Humanoid hands need to adapt to shapes. Humanoid hands need to work around humans. Humanoid hands need to handle tools and perform tasks.
This requires a shift from rigid precision to controlled compliance.
1. Underactuation: Doing More with Fewer Motors
Underactuation means having motors than degrees of freedom. Humanoid hands with underactuation use mechanical linkages and springs and differential mechanisms so that one motor can control multiple joints.
The advantages of underactuation are that it makes the design significantly lighter and simpler. Underactuation also makes the fingers automatically conform to the object shape. Underactuation is also lower in cost and better in energy efficiency.
There are examples of underactuation in humanoid hands. Many versions of the Shadow Hand and Robotiq grippers use underactuation. The Tesla Optimus and Figure 01 hands use designs in the fingers to achieve impressive dexterity with fewer motors.
The trade-off is reduced control of each joint but for grasping and manipulation in unstructured environments this is often an acceptable compromise for humanoid hands.
2. Tendon-Driven Hands: Mimicking Human Anatomy
Tendon-driven systems route cables from motors located in the forearm or palm to the finger joints, similar to how human hands work. Humanoid hands with tendon-driven systems have motors that can be placed away from the fingers making the hands much lighter.
The benefits of tendon-driven systems are excellent force transmission and compliance. Tendon-driven systems also have a looking finger curling motion.
However there are challenges with tendon-driven systems. Tendons stretch over time. Need careful tension management. There is also friction and backlash in the routing system. Tendon-driven systems require complex maintenance.
This approach is very popular in research and high-end hands because it combines good strength with compliance and low weight at the fingertip.
3. Soft Robotics: The New Frontier
Soft robotics takes compliance to the level by using flexible materials like silicone and elastomers and pneumatic actuators instead of rigid links. Humanoid hands with robotics have fingers made of soft deformable materials that naturally conform to objects.
The key ideas of robotics are pneumatic or hydraulic actuation, sometimes combined with tendons. Soft robotics also has embedded sensors in the material for rich tactile feedback.
The advantages of robotics are that it is extremely safe for human interaction. Soft robotics also has adaptability to irregular shapes. Soft robotics has built-in compliance and shock absorption.
However there are limitations to soft robotics. Soft robotics has precision and force output compared to rigid mechanisms. Soft robotics also has a response time in some designs. There are also durability challenges over periods.
Many cutting-edge research platforms are now combining bone-like structures with soft skin and pads creating hybrid anthropomorphic hands that get the best of both worlds.
Current Best Practices in Humanoid Hands
humanoid projects today tend to use a hybrid approach. Humanoid hands have a core structure for strength. Humanoid hands use underactuation and tendon-driven mechanisms for dexterity and lightness. Humanoid hands use pads and compliant fingertips for gentle grasping and tactile sensing. Humanoid hands use a combination of position control and force/impedance control.
The Tesla Optimus hands for example show influence from all three principles: underactuation for simplicity tendon-like routing for weight savings and compliant materials for safe interaction.
My Personal Take
Hand design might be the area where humanoid robotics still has the room for improvement and also the most exciting innovation potential. I believe the winning hand designs of the 5–10 years will be hybrid compliant systems: mechanically intelligent with soft sensor-rich contact surfaces. They will not try to copy the hand exactly but they will borrow its core principles, adaptability, compliance and efficiency.
When a humanoid can reliably pick up an egg use a hammer hand you a cup of coffee and gently shake your hand we will know we have made real progress, with humanoid hands.