Saturday, December 17, 2016

Building a low cost, light weight, closed loop, Series Elastic Actuator (SEA)

Now you might ask, what is a SEA, its kind of a servo with dampening.

When you build a robot, you probably want to be able to detect impacts or get the load on a specific joint or actuator and you do not want it to break on high impacts. Here is where a SEA gets handy.

A better description can be found in the conclusions section of: Matthew M. Williamson's paper "Series Elastic Actuator"

"Conventional robotic actuators suff er from a number of problems when it comes to
providing good torque control. A way to address this problem has been presented
in this thesis. If an elastic element is placed in series with the output of an electric
motor, the force control performance of the motor is improved. The motor is isolated
from shock loads, and the e ffects of backlash, torque ripple and friction are filtered
by the elastic element. A further advantage is that the actuator exhibits stable behaviour while in contact with all environments, a quality which has been hard to attain with a conventional
electric motor based actuators. Along with the bene ts of elasticity come disadvan-
tages which include a limit on the maximum force that the actuator can output due
to the mechanical properties of the elastic element, a reduction in the force control
bandwidth due to the low pass nature of the spring, and increased complexity and
bulk in the mechanical design."

In other words, its awesome, with the drawback that it becomes a bit more complex to keep track of the forces applied on your actuator/robot state. But the benefit is significantly lower power consumption as you can store energy in the springs.

An example of an SEA from MIT

With "soft joints" you can almost make the robot walk by it self as seen in this video (passive dynamic walker):

More detailed description on how a SEA works can be seen here:

And finally a very good motivation for using SEA in a walking biped:

To conclude, If you want to build a cheap biped, then you want lower motor power requirements. To achieve that you build it light and you preferably use a SEA. 

Next post will show some different designs of a lightweight 3d printable SEA, I will also make the designs, drivers and BOM available on github.

Part 2 is now available here

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