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+# A Detailed Look into SEA(Serial Elastic Actuator)
+
+## why we need to know this
+SEA is a commonly used actuator. It is powerful, a lot of research and product are based on SEA. For example, the Rethink Robotcs widely use SEA in their colaborate robot, Hebi Robotics use SEA in their modules as well.
+
+<p align="center">
+  <img width="400" src="https://spectrum.ieee.org/image/MjYxMjM2MQ.jpeg"><br>
+  Rethink Sawyer Robot, All the joints are SEA
+</p>
+
+
+<p align="center">
+  <img width="400" src="http://docs.hebi.us//resources/quickstart/two_actuators.jpg"><br>
+  Hebi Modules
+</p>
+
+More importantly, I am recently dealing with SEA motors from HEBI robotics. I have got a new [Daisy Robot](http://docs.hebi.us/resources/kits/assyInstructions/X-Series_Hexapod.pdf). The Robot is heavy and it trembles a lot while walking. So I want to make it crystal clear that I understand the motor in the very basic level.
+
+This also help understanding the famous branch of actuator.
+
+## What is SEA
+SEA is a mechanical mechanism. It could add to any kind of actuators if properly designed. To put it simple, it attach a spring bwtween the end shaft of the motor, to create elasticity and reduce stiffness of the motor.
+
+## A little bit of history
+The famous [paper from Gill Pratt MIT](http://www.cs.cmu.edu/~cga/legs/jh1c.pdf).
+
+## How does this work and Why this works
+
+## Pros & Cons