MECHANICAL ENGINEER
Series Elastic Actuator (MIT 2.12)
About the Project
This project was made for MIT's 2.12 Introduction to Robotics class. The theme of the class was collaborative robotics, so our final project was a robotic arm designed to aid people living with disabilities with common household tasks. Our arm had to be able to perform two tasks: open a heavy drawer alongside a human, and help wrap a towel around them. In order to make the system safe, we were required to design a series elastic actuator (SEA) to add an element of compliance to the assembly. My role on the team was to design and manufacture the SEA.
Design Requirements
The design requirements for our SEA were that it had to be able to withstand the maximum amount of torque that would be applied to it while completing the task. It also had to have at least 10 degrees of compliance when the max expected force was applied at the end of the robot's arm.
Overall Design
The overall design consists of a motor and gear reduction, an output shaft, and an elastic element connecting the input and output together. Encoders were placed on the motor and the output shaft, allowing us to measure their positions. By designing an elastic element with a known stiffness and comparing the encoder values, we were able to sense the amount of torque being applied to the elastic element, and therefore the force being applied to the end of the arm. We used this force sensing to program in safety mechanisms so that the robot would back off if the force at the tip exceeded a certain value.
Elastic Element
We made a custom elastic element out of spring steel. The flexure's overall geometry was inspired by the SEA used in Rethink Robotics' Baxter robot. I used SolidWorks FEA to tweak the specific thicknesses of the elastic element to ensure that we were able to reach our goal deflection amount without yielding when the maximum expected torque was applied. Once the part was designed, I manufactured it using a water jet. Two elastic elements were placed in series to increase the overall compliance of the system.
Results
Our group's robot successfully performed all of the required tasks and placed first in the class competition. I was also awarded the Most Valuable Engineer Award for my work on designing and manufacturing the series elastic actuator. A video of the arm in action can be seen below.
Images
Video
