Underactuation

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Underactuation is a technical term used in robotics and control theory to describe mechanical devices that have a lower number of actuators than degrees of freedom.

The classic inverted pendulum is an underactuated system: it has four degrees of freedom (two for its support's motion in the horizontal plane, and two for the angular motion of the pendulum), but only two of them (the cart position) are actuated, and the others are only indirectly controlled. Although naturally extremely unstable, this underactuated system is controllable given a suitable feedback control system.

In a device designed for gripping, for example, a greater number of actuators means that the device increases in versatility, but this comes at the cost of size, complexity, cost and weight of the device. Underactuated devices can be more efficient, simpler and more reliable than their fully actuated alternatives. Although the control of underactuated devices is theoretically more complex than that of fully actuated systems, the combination of underactuated control with naturally stable or nearly stable mechanical systems can be very effective, as has been demonstrated by research on bipedal walking robots.

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[edit] Further reading

Saliba, M. & de Silva, C.W. (1991). An Innovative Robotic Gripper for Grasping and Handling Research IEEE Journal of Robotics and Automation, p975-979.