• Sajiv Shah

Idea of a Human Muscle-Like Variable Stiffness Mechanical Actuator

After reading many papers on the development of robots that can be used in collabarotve spaces with humans, I identified many current issues.

  1. Loss of precision

  2. Loss of repeatibility

  3. Loss of speed

These three issues in mechanisms that have built in flexibility, such as series elastic actuators, mainly derive from the fact that an elastic element in a mechanism is not predictable at high speed movements, and can cause unexpected motion.


That said, my mentor and I decided that it would be ideal to create some sort of mechanism where stiffness could be mechanically turned on and off. That way, the robot could act safely when around humans during the work day, and then run precise and fast operations when not around humans.


It is important to note that we previously have seen this done with current feedback loops and Quasi-Direct-Drive actuators, but we believe that a mechanical system is the safest option as it responds much faster and has very little margin for error.


We then finalized on our major question: How do we mechanically change the stiffness of an actuator?


After much pondering, we thought of a perfect model of what we were trying to acheive: the human body and it's muscles.



Human muscles in most situations are compromised of antagonistic pairs. That means two muscles control one joint, and that gives the joint many characteristics. Specifically, it gives it the nature of applying a variant amount of force and having a variable amount of stiffness.


This can be seen in the following two scenarios:

  1. When I slap my brother across the face, I do not clench up and tension the muscles in my arm to a great extent. If i did that, then my brother would probably get hurt, which would definitely end in me getting even more hurt (he always wins our fights). Even though I move my hand with fast speed, I am able to hit the target on his cheek relatively precisely with a good amount of force.

  2. If I would choose rather to punch my brother, I would tense up and clench all my muscles very stiff. Then, while driving my fist at a similar speed to that of when I slap my brother, I would strike him with a great amount of force.

What these two situations demonstrate are exactly what are required from a variable stiffness actuator. The ability to perform mutltiple types of motion with variable stiffness has countless benefits, and the next step is to consider how we can mechanically test and create an actuator that can exhibit the traits of antagonistic muscle pairs in human joints.