Series Elastic Actuators
Here you can find CAD files, assembly instructions, assembly videos, specs, etc. for fabricating linear Series Elastic Actuator SEA23-23. Feel free to use the files for non commercial purposes and low volume commercial purposes, but note that neither IHMC nor Yobotics, Inc. provides any warranty, guarantee, or liability of anysort whatsoever. The files are AS-IS and will require someone skilled in the art of mechatronics and robotics to derive any value from them. If you are interested in using the actuators for high volume commercial purposes, please contact us.
If you do use these files, please cite the following papers in your publications that benefit from the actuators:
- Pratt, Williamson, Dilworth, Pratt, Ulland, Wright 1995. Stiffness Isn’t Everything Proceedings of ISER ’95, Stanford CA.
- Jerry Pratt, Ben Krupp, and Chris Morse, 2002. Series elastic actuators for high fidelity force control. Industrial Robot Journal, Vol. 29, No. 3, pp. 234-241
These actuators used to be available for sale by Yobotics, Inc., but are no longer. Instead the design has been made open source on these pages. If you do use these designs, please let us know and send us a link that we can add here. If you improve on the designs (which we recommend), please let us know so we can improve on ours. Some things that could use improvement include:
- The shoulder screw holes for attaching the actuator could be made larger and stronger. Occassionally we will have the shoulder screws rip out. However, when bolted down in a joint (such as the hip pitch joint of M2V2), they seem to be plenty strong.
- The linear encoder sensors that measure the position and force of the actuator are not very high resolution. They are 500 line per inch, resulting in 2000 counts per inch quadrature encoding. These are the highest precision encoders that US Digital makes as of 2011. It makes estimating a velocity a little difficult. Having higher resolutoin sensing and/or an encoder on the back of the motor would be good.
- The spring carriage tends to twist when loaded which can sometimes cause problems with the sensors.
- Occasionally the glue holding the spring carriage together will give out on the actuator and you’ll need to reassemble it. Since much of the actuator is glued together, it is hard to fix the actuators. Often, we’ll just replace a broken actuator with a spare and salvage what we can from the broken actuator.
Fabrication and construction is somewhat tricky, but should be feasible for someone versed in custom robot design and construction. The most complex parts are: acquiring the custom ball screw, removing the magnets from the shaft of the motor if you don’t get a motor part set, compressing the springs during assembly, various glueing operations during assembly, being careful not to bend the ball screw during assembly.
Series Elastic Actuators provide many benefits in force control of robots in unconstrained environments. These benefits include high force fidelity, extremely low impedance, low friction, and good force control bandwidth. Series Elastic Actuators employ a novel mechanical design architecture which goes against the common machine design principal of â€œstiffer is betterâ€. A compliant element is placed between the gear train and driven load to intentionally reduce the stiffness of the actuator. A position sensor measures the deflection, and the force output is accurately calculated using Hookeâ€™s Law (F=Kx). A control loop then servos the actuator to the desired output force. The resulting actuator has inherent shock tolerance, high force fidelity and extremely low impedance. These characteristics are desirable in many applications including legged robots, exoskeletons for human performance amplification, robotic arms, haptic interfaces, and adaptive suspensions.
Publications related to SEAs can be found here.