Traditional electric drive systems commonly consist of a speed/direction control, an electric motor, and some sort of gear train which is coupled directly to the load. As a result of our research we have discovered that introducing an instrumented elasticity between the gear train’s output and the load vastly improves the quality of the actuator package while paradoxically reducing its cost. This improvement is particularly strong for actuators intended for robots that interact with the natural environment. Among the advantages of Series-Elastic Actuators are that: 

1. Low output impedance and backdrivability, even in hydraulic systems.  That is to say, the dynamic effects of motor inertia and friction are nearly invisible at the output.  In traditional systems, the actuator dynamics often dominate the mechanism dynamics, making it difficult to achieve motions which require high force fidelity.  
2. Shock tolerance is greatly improved.
3. The force transmission fidelity of the gear reduction is no longer important, allowing inexpensive gear reduction to be used. Gears typically transmit position with much higher fidelity than force. The series elasticity serves as a transducer between gear reduction output position and load force, greatly increasing the fidelity of force control. 
4. The required motor force fidelity is drastically reduced, allowing inexpensive motors to be used. It is the motor shaft’s position, not its output torque which is responsible for the generation of load force. As a result, motors with large torque ripple can be used. 
5. Force control stability is improved, even in intermittent contact with hard surfaces. Despite common intuition, the addition of a spring in series with the load makes force control easier, not harder, to achieve. The feedback and feedforward incorporated into a series-elastic actuator’s control system eliminates the resonances caused by the elasticity, much as a trained operator eliminates the pendulum resonance of a large crane’s cable by appropriate modulation of the crane’s position. 
6. Energy can be stored and released in the elasticity, potentially improving efficiency. Animals commonly utilize the elasticity of tendons to store energy in one part of a locomotive cycle and release it in another, with the muscle doing much less work overall than would otherwise be required. Series-elastic actuators may allow the same effect to happen in robots, extending battery life in autonomous robots.
7. The passive impedance of the actuator at high frequencies is more desirable. Traditional actuators project a passive impedance that resembles a large inertia (the motor’s rotor inertia multiplied by the square of the gear ratio) at high frequencies. A series-elastic actuator looks like a spring at high frequencies, which is much more forgiving of collisions and other unexpected interactions. 

A number of robots have been built incorporating series-elastic actuators.  Those at MIT include a 3-dimensional biped ("M2"), two 2-dimensional walking bipeds ("Spring Turkey" and "Spring Flamingo"), the arms of a humanoid robot ("Cog"), an arm for a small planetary rover ("Pebbles"), a small autonomous bipedal robot ("Troody" - resembling the dinosaur Troodon) and a 3 fingered robot hand for experiments on dexterous manipulation. Those at Yobotics include an Agile Robot Arm and a powered orthotic brace.  These robots have all shown the advantages of series elasticity to be significant. In each of these robots, the control circuit, power amplifier, motor, gear-train, series-elasticity, and strain sensor were separate components brought together by significant design effort. We are now working on integrating all of these elements into a single device, the Integrated Series-Elastic Actuator (ISEA)

Series Elastic Actuators were invented by Gill Pratt and Matt Williamson (U.S. patent number 5650704) at the MIT Artificial Intelligence Lab.  Yobotics is now licensing this technology from MIT.  We have produced a few commercially available versions that are being used in our robots and in several robotics labs.    

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