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A Series Elastic Actuator is an actuator which has a an elastic element such as a spring in series with its output. This means that, in contrast to most actuators, the output of a series elastic actuator is designed to give when pressed upon. This is useful in situations where the exact position of an actuator is less important than the force it exerts. Perhaps the most common example of this is that most biological actuators such as the muscles in the human body are force-based, springy and soft rather hard and position controlled. In biomimetic robotics, it is desirable to emulate these qualities and therefore the Series Elastic Actuator was invented.
Fundamentally, force control is a feedback loop around the force of the actuator rather than some other measured quantity. To accomplish this, it is necessary to measure the output force of the actuator. This can be done in a number of ways, but is most easily done by introducing a spring or other elastic element which deflects linearly with force and measuring the deflection of that. Then, through Hooke's Law, the force may be calculated. The difficulty in this is that many force-sensing applications actually want very little deflection to occur under different forces, so in load cells the spring constant is very high and therefore the deflection under expected forces is very slight. In contrast, in a series elastic actuator this elasticity is desired and designed for. Therefore, the force measurement can be easily integrated into the elastic element.