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is a technical term used in robotics
Robotics is the branch of technology that deals with the design, construction, operation, structural disposition, manufacture and application of robots...
and control theory
Control theory is an interdisciplinary branch of engineering and mathematics that deals with the behavior of dynamical systems. The desired output of a system is called the reference...
to describe mechanical devices that have a lower number of actuator
An actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into some kind of motion. An actuator is the mechanism by which...
s than degrees of freedom
In mechanics, degrees of freedom are the set of independent displacements and/or rotations that specify completely the displaced or deformed position and orientation of the body or system...
The classic inverted pendulum
An inverted pendulum is a pendulum which has its mass above its pivot point. It is often implemented with the pivot point mounted on a cart that can move horizontally and may be called a cart and pole...
is an underactuated system: it has two degrees of freedom (one for its support's motion in the horizontal plane, and one for the angular motion of the pendulum), but only one of them (the cart position) is actuated, and the other is only indirectly controlled. Although naturally extremely unstable, this underactuated system is controllable.
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.
- Saliba, M. & de Silva, C.W. (1991). An Innovative Robotic Gripper for Grasping and Handling Research IEEE Journal of Robotics and Automation, p975-979.